Parasites trick their rat hosts into being eaten by cats
by Alasdair Wilkins / 8/18/11

“The single-celled parasite Toxoplasma gondii infects rats, but it needs to be inside a cat’s digestive system in order to reproduce. The parasite actually alters the brain of its rat host so that it won’t be afraid of cats. Specifically, Stanford researchers discovered that Toxoplasma affects the rat’s brain so that the fear centers of the brain no longer respond to cat odors. Even more crazily, it appears that Toxoplasma makes the rat brain think it’s sexually attracted to the cat odor. Those factors are likely more than enough to get rats hanging around dangerously close to cats, and thus gives the parasite a chance to complete its reproductive cycle.

The parasites appear to be very precise in their alterations – the rats still function normally in all areas not directly related to the fear of cats. Researcher Patrick House explains: “These findings support the idea that in the rat, Toxoplasma is shifting the emotional salience of the detection of the cat. They also suggest that fear and attraction might lie on the same spectrum, or at least that the emotional processing of fear and attraction are not entirely unrelated.”

We don’t know how the parasite has this remarkable effect. Previous research indicates that Toxoplasma tends to enter the rat’s brain and take up residence near the amygdala, a part of the brain heavily involved in fear and other emotional responses. Somehow, Toxoplasma is causing certain subsections of the amygdala to decrease the fear response to cat odor. And it might not be only rats who are affected by this. A third of all humans carry Toxoplasma, and we don’t really have a clear grasp on what – if anything – these parasites might do to the human brain. There’s some evidence that Toxoplasma is linked to incidents of schizophrenia in humans, but what we don’t know still far outweighs what we do.”

How Your Cat Is Making You Crazy

“No one would accuse Jaroslav Flegr of being a conformist. A self-described “sloppy dresser,” the 53-year-old Czech scientist has the contemplative air of someone habitually lost in thought, and his still-youthful, square-jawed face is framed by frizzy red hair that encircles his head like a ring of fire. Certainly Flegr’s thinking is jarringly unconventional. Starting in the early 1990s, he began to suspect that a single-celled parasite in the protozoan family was subtly manipulating his personality, causing him to behave in strange, often self-destructive ways. And if it was messing with his mind, he reasoned, it was probably doing the same to others.

The parasite, which is excreted by cats in their feces, is called Toxoplasma gondii (T. gondii or Toxo for short) and is the microbe that causes toxoplasmosis—the reason pregnant women are told to avoid cats’ litter boxes. Since the 1920s, doctors have recognized that a woman who becomes infected during pregnancy can transmit the disease to the fetus, in some cases resulting in severe brain damage or death. T. gondii is also a major threat to people with weakened immunity: in the early days of the AIDS epidemic, before good antiretroviral drugs were developed, it was to blame for the dementia that afflicted many patients at the disease’s end stage. Healthy children and adults, however, usually experience nothing worse than brief flu-like symptoms before quickly fighting off the protozoan, which thereafter lies dormant inside brain cells—or at least that’s the standard medical wisdom.

But if Flegr is right, the “latent” parasite may be quietly tweaking the connections between our neurons, changing our response to frightening situations, our trust in others, how outgoing we are, and even our preference for certain scents. And that’s not all. He also believes that the organism contributes to car crashes, suicides, and mental disorders such as schizophrenia. When you add up all the different ways it can harm us, says Flegr, “Toxoplasma might even kill as many people as malaria, or at least a million people a year.” An evolutionary biologist at Charles University in Prague, Flegr has pursued this theory for decades in relative obscurity. Because he struggles with English and is not much of a conversationalist even in his native tongue, he rarely travels to scientific conferences. That “may be one of the reasons my theory is not better known,” he says. And, he believes, his views may invite deep-seated opposition. “There is strong psychological resistance to the possibility that human behavior can be influenced by some stupid parasite,” he says. “Nobody likes to feel like a puppet. Reviewers [of my scientific papers] may have been offended.”

But after years of being ignored or discounted, Flegr is starting to gain respectability. Psychedelic as his claims may sound, many researchers, including such big names in neuroscience as Stanford’s Robert Sapolsky, think he could well be onto something. Flegr’s “studies are well conducted, and I can see no reason to doubt them,” Sapolsky tells me. Indeed, recent findings from Sapolsky’s lab and British groups suggest that the parasite is capable of extraordinary shenanigans. T. gondii, reports Sapolsky, can turn a rat’s strong innate aversion to cats into an attraction, luring it into the jaws of its No. 1 predator. Even more amazing is how it does this: the organism rewires circuits in parts of the brain that deal with such primal emotions as fear, anxiety, and sexual arousal. “Overall,” says Sapolsky, “this is wild, bizarre neurobiology.” Another academic heavyweight who takes Flegr seriously is the schizophrenia expert E. Fuller Torrey, director of the Stanley Medical Research Institute, in Maryland. “I admire Jaroslav for doing [this research],” he says. “It’s obviously not politically correct, in the sense that not many labs are doing it. He’s done it mostly on his own, with very little support. I think it bears looking at. I find it completely credible.”

What’s more, many experts think T. gondii may be far from the only microscopic puppeteer capable of pulling our strings. “My guess is that there are scads more examples of this going on in mammals, with parasites we’ve never even heard of,” says Sapolsky. Familiar to most of us, of course, is the rabies virus. On the verge of killing a dog, bat, or other warm-blooded host, it stirs the animal into a rage while simultaneously migrating from the nervous system to the creature’s saliva, ensuring that when the host bites, the virus will live on in a new carrier.

But aside from rabies, stories of parasites commandeering the behavior of large-brained mammals are rare. The far more common victims of parasitic mind control—at least the ones we know about—are fish, crustaceans, and legions of insects, according to Janice Moore, a behavioral biologist at Colorado State University. “Flies, ants, caterpillars, wasps, you name it—there are truckloads of them behaving weirdly as a result of parasites,” she says. Consider Polysphincta gutfreundi, a parasitic wasp that grabs hold of an orb spider and attaches a tiny egg to its belly. A wormlike larva emerges from the egg, and then releases chemicals that prompt the spider to abandon weaving its familiar spiral web and instead spin its silk thread into a special pattern that will hold the cocoon in which the larva matures. The “possessed” spider even crochets a specific geometric design in the net, camouflaging the cocoon from the wasp’s predators.

Flegr himself traces his life’s work to another master of mind control. Almost 30 years ago, as he was reading a book by the British evolutionary biologist Richard Dawkins, Flegr was captivated by a passage describing how a flatworm turns an ant into its slave by invading the ant’s nervous system. A drop in temperature normally causes ants to head underground, but the infected insect instead climbs to the top of a blade of grass and clamps down on it, becoming easy prey for a grazing sheep. “Its mandibles actually become locked in that position, so there’s nothing the ant can do except hang there in the air,” says Flegr. The sheep grazes on the grass and eats the ant; the worm gains entrance into the ungulate’s gut, which is exactly where it needs to be in order to complete the circle of life. “It was the first I learned about this kind of manipulation, so it made a big impression on me,” Flegr says. After he read the book, Flegr began to make a connection that, he readily admits, others might find crazy: his behavior, he noticed, shared similarities with that of the reckless ant. For example, he says, he thought nothing of crossing the street in the middle of dense traffic, “and if cars honked at me, I didn’t jump out of the way.” He also made no effort to hide his scorn for the Communists who ruled Czechoslovakia for most of his early adulthood. “It was very risky to openly speak your mind at that time,” he says. “I was lucky I wasn’t imprisoned.” And during a research stint in eastern Turkey, when the strife-torn region frequently erupted in gunfire, he recalls being “very calm.” In contrast, he says, “my colleagues were terrified. I wondered what was wrong with myself.”

His bewilderment continued until 1990, when he joined the biology faculty of Charles University. As it happened, the 650-year-old institution had long been a world leader in documenting the health effects of T. gondii, as well as developing methods for detecting the parasite. In fact, just as Flegr was arriving, his colleagues were searching for infected individuals on whom to test their improved diagnostic kits, which is how he came to be asked one day to roll up his sleeve and donate blood. He discovered that he had the parasite—and just possibly, he thought, the key to his baffling self-destructive streak. He delved into T. gondii’s life cycle. After an infected cat defecates, Flegr learned, the parasite is typically picked up from the soil by scavenging or grazing animals—notably rodents, pigs, and cattle—all of which then harbor it in their brain and other body tissues. Humans, on the other hand, are exposed not only by coming into contact with litter boxes, but also, he found, by drinking water contaminated with cat feces, eating unwashed vegetables, or, especially in Europe, by consuming raw or undercooked meat. Hence the French, according to Flegr, with their love of steak prepared saignant—literally, “bleeding”—can have infection rates as high as 55 percent. (Americans will be happy to hear that the parasite resides in far fewer of them, though a still substantial portion: 10 to 20 percent.) Once inside an animal or human host, the parasite then needs to get back into the cat, the only place where it can sexually reproduce—and this is when, Flegr believed, behavioral manipulation might come into play

Researchers had already observed a few peculiarities about rodents with T. gondii that bolstered Flegr’s theory. The infected rodents were much more active in running wheels than uninfected rodents were, suggesting that they would be more-attractive targets for cats, which are drawn to fast-moving objects. They also were less wary of predators in exposed spaces. Little, however, was known about how the latent infection might influence humans, because we and other large mammals were widely presumed to be accidental hosts, or, as scientists are fond of putting it, a “dead end” for the parasite. But even if we were never part of the parasite’s life cycle, Flegr reasoned, mammals from mouse to man share the vast majority of their genes, so we might, in a case of mistaken identity, still be vulnerable to manipulations by the parasite.

In the Soviet-stunted economy, animal studies were way beyond Flegr’s research budget. But fortunately for him, 30 to 40 percent of Czechs had the latent form of the disease, so plenty of students were available “to serve as very cheap experimental animals.” He began by giving them and their parasite-free peers standardized personality tests—an inexpensive, if somewhat crude, method of measuring differences between the groups. In addition, he used a computer-based test to assess the reaction times of participants, who were instructed to press a button as soon as a white square popped up anywhere against the dark background of the monitor. The subjects who tested positive for the parasite had significantly delayed reaction times. Flegr was especially surprised to learn, though, that the protozoan appeared to cause many sex-specific changes in personality. Compared with uninfected men, males who had the parasite were more introverted, suspicious, oblivious to other people’s opinions of them, and inclined to disregard rules. Infected women, on the other hand, presented in exactly the opposite way: they were more outgoing, trusting, image-conscious, and rule-abiding than uninfected women. The findings were so bizarre that Flegr initially assumed his data must be flawed. So he tested other groups—civilian and military populations. Again, the same results. Then, in search of more corroborating evidence, he brought subjects in for further observation and a battery of tests, in which they were rated by someone ignorant of their infection status. To assess whether participants valued the opinions of others, the rater judged how well dressed they appeared to be. As a measure of gregariousness, participants were asked about the number of friends they’d interacted with over the past two weeks. To test whether they were prone to being suspicious, they were asked, among other things, to drink an unidentified liquid.

The results meshed well with the questionnaire findings. Compared with uninfected people of the same sex, infected men were more likely to wear rumpled old clothes; infected women tended to be more meticulously attired, many showing up for the study in expensive, designer-brand clothing. Infected men tended to have fewer friends, while infected women tended to have more. And when it came to downing the mystery fluid, reports Flegr, “the infected males were much more hesitant than uninfected men. They wanted to know why they had to do it. Would it harm them?” In contrast, the infected women were the most trusting of all subjects. “They just did what they were told,” he says. Why men and women reacted so differently to the parasite still mystified him. After consulting the psychological literature, he started to suspect that heightened anxiety might be the common denominator underlying their responses. When under emotional strain, he read, women seek solace through social bonding and nurturing. In the lingo of psychologists, they’re inclined to “tend and befriend.” Anxious men, on the other hand, typically respond by withdrawing and becoming hostile or antisocial. Perhaps he was looking at flip sides of the same coin.

Closer inspection of Flegr’s reaction-time results revealed that infected subjects became less attentive and slowed down a minute or so into the test. This suggested to him that Toxoplasma might have an adverse impact on driving, where constant vigilance and fast reflexes are critical. He launched two major epidemiological studies in the Czech Republic, one of men and women in the general population and another of mostly male drivers in the military. Those who tested positive for the parasite, both studies showed, were about two and a half times as likely to be in a traffic accident as their uninfected peers.

When I met Flegr for the first time, last September, at his office on the third floor of Charles University’s Biological Sciences building, I was expecting something of a wild man. But once you get past the riotous red hair, his style is understated. Thin and slight of build, he’s soft-spoken, precise with his facts, and—true to his Toxo status—clad in old sneakers, faded bell-bottom jeans, and a loose-fitting button-up shirt. As our conversation proceeds, I discover that his latest findings have become—to quote Alice in Wonderland—“curiouser and curiouser,” which may explain why his forehead has the deep ruts of a chronic worrier, or someone perpetually perplexed. He’s published some data, he tells me, that suggest infected males might have elevated testosterone levels. Possibly for that reason, women shown photos of these men rate them as more masculine than pictures of uninfected men. “I want to investigate this more closely to see if it’s true,” he says. “Also, it could be women find infected men more attractive. That’s something else we hope to test.”

Meanwhile, two Turkish studies have replicated his studies linking Toxoplasma to traffic accidents. With up to one-third of the world infected with the parasite, Flegr now calculates that T. gondii is a likely factor in several hundred thousand road deaths each year. In addition, reanalysis of his personality-questionnaire data revealed that, just like him, many other people who have the latent infection feel intrepid in dangerous situations. “Maybe,” he says, “that’s another reason they get into traffic accidents. They don’t have a normal fear response.”

It’s almost impossible to hear about Flegr’s research without wondering whether you’re infected—especially if, like me, you’re a cat owner, favor very rare meat, and identify even a little bit with your Toxo sex stereotype. So before coming to Prague, I’d gotten tested for the parasite, but I didn’t yet know the results. It seemed a good time to see what his intuition would tell me. “Can you guess from observing someone whether they have the parasite—myself, for example?,” I ask. “No,” he says, “the parasite’s effects on personality are very subtle.” If, as a woman, you were introverted before being infected, he says, the parasite won’t turn you into a raving extrovert. It might just make you a little less introverted. “I’m very typical of Toxoplasma males,” he continues. “But I don’t know whether my personality traits have anything to do with the infection. It’s impossible to say for any one individual. You usually need about 50 people who are infected and 50 who are not, in order to see a statistically significant difference. The vast majority of people will have no idea they’re infected.”

Still, he concedes, the parasite could be very bad news for a small percentage of people—and not just those who might be at greater risk for car accidents. Many schizophrenia patients show shrinkage in parts of their cerebral cortex, and Flegr thinks the protozoan may be to blame for that. He hands me a recently published paper on the topic that he co-authored with colleagues at Charles University, including a psychiatrist named Jiri Horacek. Twelve of 44 schizophrenia patients who underwent MRI scans, the team found, had reduced gray matter in the brain—and the decrease occurred almost exclusively in those who tested positive for T. gondii. After reading the abstract, I must look stunned, because Flegr smiles and says, “Jiri had the same response. I don’t think he believed it could be true.” When I later speak with Horacek, he admits to having been skeptical about Flegr’s theory at the outset. When they merged the MRI results with the infection data, however, he went from being a doubter to being a believer. “I was amazed at how pronounced the effect was,” he says. “To me that suggests the parasite may trigger schizophrenia in genetically susceptible people.”

One might be tempted to dismiss the bulk of Flegr’s work as hokum—the fanciful imaginings of a lone, eccentric scholar—were it not for the pioneering research of Joanne Webster, a parasitologist at Imperial College London. Just as Flegr was embarking on his human trials, Webster, then a freshly minted Ph.D., was launching studies of Toxo-infected rodents, reasoning, just as Flegr did, that as hosts of the parasite, they would be likely targets for behavioral manipulation. She quickly confirmed, as previous researchers had shown, that infected rats were more active and less cautious in areas where predators lurk. But then, in a simple, elegant experiment, she and her colleagues demonstrated that the parasite did something much more remarkable. They treated one corner of each rat’s enclosure with the animal’s own odor, a second with water, a third with cat urine, and the last corner with the urine of a rabbit, a creature that does not prey on rodents. “We thought the parasite might reduce the rats’ aversion to cat odor,” she told me. “Not only did it do that, but it actually increased their attraction. They spent more time in the cat-treated areas.” She and other scientists repeated the experiment with the urine of dogs and minks, which also prey on rodents. The effect was so specific to cat urine, she says, that “we call it ‘fatal feline attraction.’”

She began tagging the parasite with fluorescent markers and tracking its progress in the rats’ bodies. Given the surgically precise way the microbe alters behavior, Webster anticipated that it would end up in localized regions of the brain. But the results defied expectations. “We were quite surprised to find the cysts—the parasite’s dormant form—all over the brain in what otherwise appeared to be a happy, healthy rat,” she says. Nonetheless, the cysts were most abundant in a part of the brain that deals with pleasure and in another area that’s involved in fear and anxiety (post-traumatic stress disorder affects this region of the brain). Perhaps, she thought, T. gondii uses a scattershot approach, disseminating cysts far and wide, enabling a few of them to zero in on the right targets.

To gain more clarity on the matter, she sought the aid of the parasitologist Glenn McConkey, whose team at the University of Leeds was probing the protozoan’s genome for signs of what it might be doing. The approach brought to light a striking talent of the parasite: it has two genes that allow it to crank up production of the neurotransmitter dopamine in the host brain. “We never cease to be amazed by the sophistication of these parasites,” Webster says. Their findings, reported last summer, created immediate buzz. Dopamine is a critical signaling molecule involved in fear, pleasure, and attention. Furthermore, the neurotransmitter is known to be jacked up in people with schizophrenia—another one of those strange observations about the disease, like its tendency to erode gray matter, that have long puzzled medical researchers. Antipsychotic medicine designed to quell schizophrenic delusions apparently blocks the action of dopamine, which had suggested to Webster that what it might really be doing is thwarting the parasite. Scientists had already shown that adding the medicine to a petri dish where T. gondii is happily dividing will stunt the organism’s growth. So Webster decided to feed the antipsychotic drug to newly infected rats to see how they reacted. Lo and behold, they didn’t develop fatal feline attraction. Suddenly, attributing behavioral changes to the microbe seemed much more plausible.

As the scientific community digested the British team’s dopamine discoveries, Robert Sapolsky’s lab at Stanford announced still more attention-grabbing news. The neuroscientist and his colleagues found that T. gondii disconnects fear circuits in the brain, which might help to explain why infected rats lose their aversion to cat odor. Just as startling, reports Sapolsky, the parasite simultaneously is “able to hijack some of the circuitry related to sexual arousal” in the male rat—probably, he theorizes, by boosting dopamine levels in the reward-processing part of the brain. So when the animal catches a whiff of cat scent, the fear center fails to fully light up, as it would in a normal rat, and instead the area governing sexual pleasure begins to glow. “In other words,” he says, “Toxo makes cat odor smell sexy to male rats.” The neurobiologist Ajai Vyas, after working with Sapolsky on this study as a postdoctoral student, decided to inspect infected rats’ testicles for signs of cysts. Sure enough, he found them there—as well as in the animals’ semen. And when the rat copulates, Vyas discovered, the protozoan moves into the female’s womb, typically infecting 60 percent of her pups, before traveling on up to her own brain—creating still more vehicles for ferrying the parasite back into the belly of a cat.

Could T. gondii be a sexually transmitted disease in humans too? “That’s what we hope to find out,” says Vyas, who now works at Nanyang Technological University, in Singapore. The researchers also discovered that infected male rats suddenly become much more attractive to females. “It’s a very strong effect,” says Vyas. “Seventy-five percent of the females would rather spend time with the infected male.”

After I return from Prague, Flegr informs me that he’s just had a paper accepted for publication that, he claims, “proves fatal feline attraction in humans.” By that he means that infected men like the smell of cat pee—or at least they rank its scent much more favorably than uninfected men do. Displaying the characteristic sex differences that define many Toxo traits, infected women have the reverse response, ranking the scent even more offensive than do women free of the parasite. The sniff test was done blind and also included urine collected from a dog, horse, hyena, and tiger. Infection did not affect how subjects rated these other samples. “Is it possible cat urine may be an aphrodisiac for infected men?,” I ask. “Yes. It’s possible. Why not?” says Flegr. I think he’s smiling at the other end of the phone line, but I’m not sure, which leaves me wondering. When I ask Sapolsky about Flegr’s most recent research, he says the effects Flegr is reporting “are incredibly cool. However, I’m not too worried, in that the effects on humans are not gigantic. If you want to reduce serious car accidents, and you had to choose between curing people of Toxo infections versus getting people not to drive drunk or while texting, go for the latter in terms of impact.”

In fact, Sapolsky thinks that Toxo’s inventiveness might even offer us some benefits. If we can figure out how the parasite makes animals less fearful, he says, it might give us insights into how to devise treatments for people plagued by social-anxiety disorder, phobias, PTSD, and the like. “But frankly,” he adds, “this mostly falls into the ‘Get a load of this, can you believe what nature has come up with?’ category.”

Webster is more circumspect, if not downright troubled. “I don’t want to cause any panic,” she tells me. “In the vast majority of people, there will be no ill effects, and those who are affected will mostly demonstrate subtle shifts of behavior. But in a small number of cases, [Toxo infection] may be linked to schizophrenia and other disturbances associated with altered dopamine levels—for example, obsessive-compulsive disorder, attention-deficit hyperactivity disorder, and mood disorders. The rat may live two or three years, while humans can be infected for many decades, which is why we may be seeing these severe side effects in people. We should be cautious of dismissing such a prevalent parasite.” The psychiatrist E. Fuller Torrey agrees—though he came to this viewpoint from a completely different angle than either Webster or Flegr. His opinion stems from decades of research into the root causes of schizophrenia. “Textbooks today still make silly statements that schizophrenia has always been around, it’s about the same incidence all over the world, and it’s existed since time immemorial,” he says. “The epidemiology literature contradicts that completely.” In fact, he says, schizophrenia did not rise in prevalence until the latter half of the 18th century, when for the first time people in Paris and London started keeping cats as pets. The so-called cat craze began among “poets and left-wing avant-garde Greenwich Village types,” says Torrey, but the trend spread rapidly—and coinciding with that development, the incidence of schizophrenia soared.

Since the 1950s, he notes, about 70 epidemiology studies have explored a link between schizophrenia and T. gondii. When he and his colleague Robert Yolken, a neurovirologist at Johns Hopkins University, surveyed a subset of these papers that met rigorous scientific standards, their conclusion complemented the Prague group’s discovery that schizophrenic patients with Toxo are missing gray matter in their brains. Torrey and Yolken found that the mental illness is two to three times as common in people who have the parasite as in controls from the same region. Human-genome studies, both scientists believe, are also in keeping with that finding—and might explain why schizophrenia runs in families. The most replicated result from that line of investigation, they say, suggests that the genes most commonly associated with schizophrenia relate to the immune system and how it reacts to infectious agents. So in many cases where the disease appears to be hereditary, they theorize, what may in fact be passed down is an aberrant or deficient immune response to invaders like T. gondii.

Epstein-Barr virus, mumps, rubella, and other infectious agents, they point out, have also been linked to schizophrenia—and there are probably more as yet unidentified triggers, including many that have nothing to do with pathogens. But for now, they say, Toxo remains the strongest environmental factor implicated in the disorder. “If I had to guess,” says Torrey, “I’d say 75 percent of cases of schizophrenia are associated with infectious agents, and Toxo would be involved in a significant subset of those.”

Just as worrisome, says Torrey, the parasite may also increase the risk of suicide. In a 2011 study of 20 European countries, the national suicide rate among women increased in direct proportion to the prevalence of the latent Toxo infection in each nation’s female population. According to Teodor Postolache, a psychiatrist and the director of the Mood and Anxiety Program at the University of Maryland School of Medicine, a flurry of other studies, several conducted by his own team, offers further support of T. gondii’s link to higher rates of suicidal behavior. These include investigations of general populations as well as groups made up of patients with bipolar disorder, severe depression, and schizophrenia, and in places as diverse as Turkey, Germany, and the Baltimore / Washington area. Exactly how the parasite may push vulnerable people over the edge is yet to be determined. Postolache theorizes that what disrupts mood and the ability to control violent impulses may not be the organism per se, but rather neurochemical changes associated with the body’s immune response to it. “As far-fetched as these ideas may sound,” says Postolache, “the American Foundation for Suicide Prevention was willing to put money behind this research.” Given all the nasty science swirling around this parasite, is it time for cat lovers to switch their allegiance to other animals?

Even Flegr would advise against that. Indoor cats pose no threat, he says, because they don’t carry the parasite. As for outdoor cats, they shed the parasite for only three weeks of their life, typically when they’re young and have just begun hunting. During that brief period, Flegr simply recommends taking care to keep kitchen counters and tables wiped clean. (He practices what he preaches: he and his wife have two school-age children, and two outdoor cats that have free roam of their home.) Much more important for preventing exposure, he says, is to scrub vegetables thoroughly and avoid drinking water that has not been properly purified, especially in the developing world, where infection rates can reach 95 percent in some places. Also, he advises eating meat on the well-done side—or, if that’s not to your taste, freezing it before cooking, to kill the cysts.

As concerns about the latent infection mount, however, experts have begun thinking about more-aggressive steps to counter the parasite’s spread. Inoculating cats or livestock against T. gondii might be one way to interrupt its life cycle, offers Johns Hopkins’ Robert Yolken. Moving beyond prevention to treatment is a taller order. Once the parasite becomes deeply ensconced in brain cells, routing it out of the body is virtually impossible: the thick-walled cysts are impregnable to antibiotics. Because T. gondii and the malaria protozoan are related, however, Yolken and other researchers are looking among antimalarial agents for more-effective drugs to attack the cysts. But for now, medicine has no therapy to offer people who want to rid themselves of the latent infection; and until solid proof exists that Toxo is as dangerous as some scientists now fear, pharmaceutical companies don’t have much incentive to develop anti-Toxo drugs. Yolken hopes that will change. “To explain where we are in Toxo research today,” he says, “the analogy I always give is the ulcer bacteria. We first needed to find ways of treating the organism and showing that the disease went away when you did that. We will have to show that when we very effectively treat Toxoplasma, some portion of psychiatric illness goes away.”

But T. gondii is just one of an untold number of infectious agents that prey on us. And if the rest of the animal kingdom is anything to go by, says Colorado State University’s Janice Moore, plenty of them may be capable of tinkering with our minds. For example, she and Chris Reiber, a biomedical anthropologist at Binghamton University, in New York, strongly suspected that the flu virus might boost our desire to socialize. Why? Because it spreads through close physical contact, often before symptoms emerge—meaning that it must find a new host quickly. To explore this hunch, Moore and Reiber tracked 36 subjects who received a flu vaccine, reasoning that it contains many of the same chemical components as the live virus and would thus cause the subjects’ immune systems to react as if they’d encountered the real pathogen.

The difference in the subjects’ behavior before and after vaccination was pronounced: the flu shot had the effect of nearly doubling the number of people with whom the participants came in close contact during the brief window when the live virus was maximally contagious. “People who had very limited or simple social lives were suddenly deciding that they needed to go out to bars or parties, or invite a bunch of people over,” says Reiber. “This happened with lots of our subjects. It wasn’t just one or two outliers.” Reiber has her eye trained on other human pathogens that she thinks may well be playing similar games, if only science could prove it. For example, she says, many people at the end stages of AIDS and syphilis express an intense craving for sex. So, too, do individuals at the beginning of a herpes outbreak. These may just be anecdotal accounts, she concedes, but based on her own findings, she wouldn’t be surprised if these urges come from the pathogen making known its will to survive.

“We’ve found all kinds of excuses for why we do the things we do,” observes Moore. “‘My genes made me do it.’ ‘My parents are to blame.’ I’m afraid we may have reached the point where parasites may have to be added to the laundry list of excuses.” She has a point. In fact, I’ve been wondering whether T. gondii might in some small way be contributing to my extreme extroversion—why I can’t resist striking up conversations everywhere I go, even when I’m short of time or with strangers I’ll never see again. Then it occurs to me that cysts in my brain might be behind my seesaw moods or even my splurges on expensive clothes. Maybe, I think with mounting conviction, the real me would have displayed better self-control, had I not been forced to swim upstream against the will of an insidious parasite.

With my feline pal Pixie on my lap (for the record, she’s an outdoor cat), I call to get the results of my Toxo test. Negative. I don’t have the latent infection. I call to tell Flegr the good news. Even though I’m relieved, I know my voice sounds flat. “It’s strange to admit,” I say, “but I think I’m a little disappointed.” He laughs. “People who have cats often feel that way, because they think the parasite explains why they behave this way or that,” he says. “But,” I protest, “you thought the same way.” Then it hits me. I may have dodged T. gondii, but given our knack for fooling ourselves—plus all those parasites out there that may also be playing tricks on our minds—can anyone really know who’s running the show?”




“The study’s results so far show that most Greenpoint backyard samples contain lead levels higher than the EPA’s recommended 400 ppm. Image: Franziska Landes


“…some 92 percent of Greenpoint backyards have at least one sample that exceeds the lead level that the EPA designates as safe for residential soil. Some yards contain seven or eight times more lead than they should — higher than the levels found in some polluted Peruvian mining communities Landes has studied…”

“This chart shows the number of children per 1,000 expected to have blood lead levels exceeding five micrograms per deciliter, the level at which the CDC recommends taking action. Source: NYC Dept of Health, 2015”


“Lead is a potent neurotoxin that accumulates in soft tissues and bone over time. Lead poisoning was documented in ancient Rome, Greece, and China. Lead(II) acetate (also known as sugar of lead) was used by the Roman Empire as a sweetener for wine, and some consider this to be the cause of the dementia that affected many of the Roman Emperors

Lead affects almost every organ and system in the body, targeting primarily the central nervous system but also the cardiovascular system, kidneys, and the immune system. Long-term exposure will cause significant impairment to the nervous system, severely damage the brain and kidneys and, in cases of exposure to high lead levels, ultimately cause death…”

Greenpoint, Brooklyn


“Lead accumulates and makes its home in our bones, where the body prefers to store it. This is done in an effort by the body to protect vital organs. From there, it is released into the blood stream… Meaning, lead can continue to be released into the blood long after exposure… Lead begins leaching into their bodies from the breakdown of bone tissue. Symptoms like fatigue and brain fog begin to occur due to lead toxicity…

It has also been found that exposure to lead early in life may cause neurodegeneration in later life. Among the many neurocognitive effects of lead are: brain damage, mental retardation, memory loss, vision loss, behavior problems, antisocial behavior, and even violence…”

“Vehicles using leaded gasoline that contaminated cities’ air decades ago have increased aggravated assault in urban areas”


-“Lead/pb causes mental regression in adults.  Lack of motor control, partial to full paralysis, coma, and death can be attributed to lead/pb.
-Lead/pb is proven to cause cancer.
-Lead/pb can make your kidneys stop working.
-Lead/pb causes confusion, dizziness, forgetfulness, emotional disorder like self doubt, lack of self confidence.
-Lead/pb can make you lose your balance when you are trying to walk.
-Lead/pb causes blurred vision.
-Lead can make you see things that are not there.
-Lead/pb can cause stuttering, slurred speech, dyslexia.  Lead/pb can make it difficult to talk out loud.
-Lead/pb makes your sense of smell go away.
-Lead/pb can make your teeth fall out earlier in life.
-Lead/pb can make you experience horrible anger.”


“NOTE: Potential hot spots of lead hazards in housing are identified based on indicators, not lead monitoring data. Because local data on lead contamination are generally unavailable, Scorecard relies on housing and demographic indicators to identify areas with housing that has a high risk of lead hazards. Scientific studies have demonstrated that housing built prior to 1950 and households with income below the poverty threshold have an elevated risk of lead contamination. Scorecard uses data from the 2000 U.S. Census for both of these risk factors to estimate potential lead hazards in housing.”

Lead neurotoxicity in children / Brain, Volume 126, Issue 1, 1 January 2003
by Theodore I. Lidsky & Jay S. Schneider

“The direct neurotoxic actions of lead include apoptosis, excitotoxicity, influences on neurotransmitter storage and release processes, mitochondria, second messengers, cerebrovascular endothelial cells, and both astroglia and oligodendroglia. Although all of lead’s toxic effects cannot be tied together by a single unifying mechanism, lead’s ability to substitute for calcium [and perhaps zinc (Bressler and Goldstein, 1991)] is a factor common to many of its toxic actions. For example, lead’s ability to pass through the blood–brain barrier (BBB) is due in large part to its ability to substitute for calcium ions (Ca2+). Experiments with metabolic inhibitors suggest that back‐transport of lead via the Ca‐ATPase pump plays an important role in this process (Bradbury and Deane, 1993). More direct evidence for the role of the Ca‐ATPase pump in the transport of lead into the brain has been provided by in vitro studies of brain capillary endothelial cells, the primary constituent of the BBB (Kerper and Hinkle, 1997a, b)…

Red Hook,Brooklyn

Apoptosis (programmed cell death) can be induced by a variety of stimuli. Apoptosis occurs when a cell activates an internally encoded suicide programme as a response to either intrinsic or extrinsic signals. One of the better characterized apoptotic cascade pathways has mitochondrial dysfunction as its initiator. Mitochondrial dysfunction initiated by the opening of the mitochondrial transition pore leads to mitochondrial depolarization, release of cytochrome C, activation of a variety of caspases and cleavage of downstream death effector proteins, and ultimately results in apoptotic cell death. While a variety of stimuli can trigger opening of the mitochondrial transition pore and cause apoptosis, a sustained intracellular increase in Ca2+ is one of the better‐known triggers; accumulation of lead is another. Lead disrupts calcium homeostasis, causing a marked accumulation of calcium in lead‐exposed cells (Bressler and Goldstein, 1991; Bressler et al., 1999). Lead, in nanomolar concentrations, also induces mitochondrial release of calcium (Silbergeld, 1992), thus initiating apoptosis…

Lead accumulates in and damages mitochondria (Anderson et al., 1996), the organelles mediating cellular energy metabolism. Haem biosynthesis, a function of normal mitochondrial activity, is affected by lead, with disruptive effects on synaptic transmission in the brain (see below, Indirect neurotoxic effects of lead). However, decreased mitochondrial functioning also can transform ordinarily benign synaptic transmission mediated by glutamate into neuron‐killing excitotoxicity (Beal et al., 1993)…”

How to stop lead poisoning / Feb 22nd 2018

“Lead has proved to be such a useful, malleable metal that it turns up everywhere, from water pipes to window flashing and printing type. It went into car batteries and petrol additives. It also helped make bright pigments, used to paint walls, metalwork and toys. Yet lead is also a poison, and its ubiquity makes it a pernicious one (see article). In the worst cases it causes comas, convulsions and death. More often it acts insidiously. It is a menace to toddlers, who are most likely to ingest contaminated dust and paint chips. Their brains are especially vulnerable. Only years after exposure are the results apparent in lower IQs, behavioural disorders and learning disabilities.

Far Rockaway, Queens

The dangers of lead have long been known. America banned it from paint 40 years ago, and by the late 1990s leaded petrol had been phased out in almost all rich countries. But the effects linger. Half a million American children are diagnosed with lead poisoning. The situation is more alarming in the poor world, where the use of lead-based paints is spreading. Curbing lead poisoning more than pays for itself. There is little excuse for poor countries to repeat the mistakes of rich ones.

The Romans did themselves no good by using lead for water pipes and sometimes even as a food sweetener. In 1786 Benjamin Franklin wrote a letter to a friend noting how the use of lead in distilleries had caused North Carolina to complain against New England Rum “that it poison’d their People, giving them the Dry Bellyach, with a Loss of the Use of their Limbs.”

“Members of the Michigan National Guard deliver water, filters, replacement cartridges and water test kits to Flint residents. Photo: Maj. Joe Cannon / U.S. National Guard via Flickr Creative Commons.”

In 2015 the Institute for Health Metrics and Evaluation, a research institute in Seattle, estimated that exposure to lead globally caused about 500,000 deaths that year and 12% of developmental disabilities, such as cerebral palsy and epilepsy. Another estimate is that lead poisoning costs Africa $135bn a year in lost output, the equivalent of 4% of GDP. The most urgent task is to stop putting more lead into the environment. As people in Asia and Africa become richer, they start to spruce up their homes. But the paint they use, even from pots labelled “lead-free”, often contains it. And they lack facilities to recycle lead batteries properly.

“National Guard members distribute water to citizens in Flint.”

It is neither difficult nor expensive to stop using lead. All countries should ban lead in paint. There should be no exemptions for industrial use, because the contamination spreads and industrial paint inevitably finds its way into the consumer market. Yet only four sub-Saharan African countries have formally enacted bans and local manufacturers are often unaware of the harm that lead causes.

The next step is to find and remove more of the lead introduced decades ago, particularly in rich countries. This will not be cheap, especially when the clean-up involves replacing lead pipes, as it often does in America. But the costs are worth it. The Pew Charitable Trusts, an NGO, reckons that every dollar spent on “lead abatement”—painting over old painted walls or removing flaking woodwork—saves at least $17 in medical and special-education costs, and lost productivity.

“Lee Anne Walters of Flint, Mich., pours gallons of bottled water into a bucket and pan to warm up for her twin sons to take a weekly bath. Her son, Gavin, 4, looking on, has been diagnosed with lead poisoning.”

In America investigations are typically carried out only in known cases of lead poisoning. However, children should not be used to test dangerous living conditions. It would be better to test older houses before problems appear. Cities and states need to make sure that landlords carry out remedial work. When poor owners cannot afford to fix their homes, the government should help as a prophylactic to save money on health care and education later. Charities that seek to help sick children and poor countries can contribute, too. There is no need to poison so many young minds.”




“Excitotoxicity is the pathological process by which nerve cells are damaged or killed by excessive stimulation by neurotransmitters such as glutamate and similar substances. This occurs when receptors for the excitatory neurotransmitter glutamate (glutamate receptors) are overactivated by glutamatergic storm.”
“One of the key mechanisms of glutamate-induced neuronal cell injury is in the release of glutamate by microglial cells (13, 14). Microglial cells are essentially the resident immune cells of the brain and nervous system. Once the free pool of glutamate increases, glutamate receptors, such as NMDA are over-stimulated. This then causes a massive influx of calcium ions into cells, resulting in neuronal cell damage, and cell death.”
“Reading these studies, the reader will find discussion of damaged or overly sensitive glutamate receptors, and malfunctioning glutamate transport. The reader will also find discussion of “glutamate pools” where excess glutamate is stored – and then sometimes released to cause brain damage. Rarely mentioned, however, is the role that ingestion of excess amounts of processed free glutamate (MSG) might play in producing these diseases; and if mentioned at all, it is by researchers outside of the United States.”

“Neuroradiologists may encounter, on a daily basis, a challenging diversity of neurologic disorders, including stroke, trauma, epilepsy, and even neurodegenerative conditions, such as Huntington disease, AIDS dementia complex, and amyotrophic lateral sclerosis (1), but this spectrum of disease is not usually thought of as sharing the same mechanism of neuronal injury and death. These and a growing list of other neurologic disorders are now understood to share a final common destructive metabolic pathway called excitotoxicity (2, 3), which has been the focus of intense investigative efforts in the neurosciences over the past several decades (3–31).

Excitotoxicity refers to an excessive activation of neuronal amino acid receptors. The specific type of excitotoxicity triggered by the amino acid glutamate is the key mechanism implicated in the mediation of neuronal death in many disorders. The discovery of excitotoxic injury is a major clue in the search for answers to such fundamental questions as why neurons die in disease states and what is the precise or critical mechanism of neuronal death.

This overview introduces and reviews some of the major concepts of glutamate excitotoxicity. Familiarity with this intriguing pathologic process will enhance the understanding of the neuroimaging changes of many neuropathologic processes, facilitate a conceptual model for some of the newer treatment strategies for some disorders (ie, stroke and trauma), and perhaps cultivate new directions for neuroimaging. The organization of this article follows the excitotoxic process from the formation of glutamate to neuronal death.

Glutamate excitotoxicity is a broad and rapidly evolving field of study with many important nuances that have necessarily been oversimplified or, unfortunately, omitted from this review for the sake of reasonable brevity. The reader is referred to the exponentially expanding literature and some of the sources listed in the reference section for additional information.

Glutamate is an amino acid and one of a group of amino acid neurotransmitters in the brain, although it is the principal excitatory neurotransmitter. More basically, amino acids (Fig 1) consist of a central carbon atom (α carbon) bonded to a carboxyl group (COOH) and an amino group (NH3). A distinctive side chain (R group), which characterizes each amino acid, links to the α carbon. Glutamate (Fig 2) consists of the side chain CH2CH2COO (COOH ending [γ carboxyl group] for glutamic acid) attached to the α carbon, while the closely related glutamine (Fig 3A and B) is created from glutamate with ammonia added at the γ carboxyl group by glutamine synthetase, forming the CH2CH2CONH2 side chain R group. Cerebral glutamate is derived solely from endogenous sources; mainly from α ketoglutarate, which is a product of the Krebs cycle (citric acid cycle, TCA [tricarboxylic acid] cycle).

fig 1.
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fig 1. Amino acid. Typical amino acid consists of a central alpha carbon () that is bonded to a carboxyl group (COOH) on one side and an amino group (H3N) on the other side. Each amino acid is characterized by a distinctive molecular group (R) or side chain attached to the α carbon.

fig 2. Glutamate. The side chain, or R group, of glutamic acid is CH2CH2COOH. The carboxyl group of the side chain is designated the γ carboxyl group, which becomes fully ionized at neutral pH and is therefore frequently written with a negative charge (COO). The term glutamate (instead of glutamic acid) is used to indicate this negative charge or ionized state at physiological pH.

fig 3.
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fig 3. A, Glutamine. This closely related amino acid is formed from glutamate with the addition of an amino group at the γ carboxyl of the side chain. B, This formation of an amide linkage at the γ carboxyl group requires the enzyme glutamine synthetase and the process is adenosine triphosphate (ATP)-dependent. This reaction is also a major mechanism for the detoxification of cerebral ammonia.

The neuronal glutamate considered here acts as a neurotransmitter, which is the method of communication between neurons. This interaction between neurons may be either excitatory or inhibitory. The major excitatory amino acid neurotransmitters are glutamate and aspartate, while GABA (γ-aminobutyric acid), glycine (aminoacetic acid), and taurine are inhibitory.

The processing and transport of glutamate (Fig 4) within the neuron are highly organized and coordinated interactions among multiple cytoplasmic organelles resembling the frenetically detailed but choreographed mosaic activities of an ant farm (32). Glutamate, like other neurosecretory substances, is initially synthesized by the endoplasmic reticulum and then transported to the Golgi apparatus for additional processing. Emerging from the opposite surface of the Golgi apparatus and wrapped inside a vesicular (bilipid) membrane, glutamate is then transported down the axon via a complex system of microtubules. Antegrade motion down the axon on the microtubules is mediated by molecules called motor kinesin, whereas cytoplasmic dynein generates retrograde motion. Mitochondria also accompany these transport molecules, providing the required energy. Upon reaching the axonal tip (Fig 5), the vesicle with the enclosed glutamate merges with the presynaptic membrane by the process called exocytosis to release the glutamate into the synaptic space between neurons. The vesicular membrane is then recycled and transported back up the neuronal axon in a retrograde fashion via the microtubular network. The synaptic glutamate is finally freed to interact with specific receptor sites on the postsynaptic membrane of the adjacent neuron to initiate an important cascade of molecular events within that neuron (Fig 6).

fig 4.
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fig 4. Neuronal glutamate processing and transport. Glutamate is processed by the endoplasmic reticulum and Golgi apparatus in preparation for fast axonal transport, which also requires other transport proteins and mitochondria. When glutamate emerges from the “trans” face of the Golgi apparatus, it is encapsulated inside a neurosecretory vesicle, which consists of a bilipid membrane. These vesicles are transported down the axon along microtubule tracks to be deposited at the tip of the axon near the presynaptic membrane. Waves of axonal membrane depolarization would trigger the release of the glutamate into the synaptic space by exocytosis, which is exhibited by the merging of the neurosecretory vesicles with the postsynaptic membrane to free the packaged glutamate. The empty vesicle would then be recycled back to the neuronal body by retrograde transport along the microtubular tracks (adapted from [32])

fig 5.
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fig 5. A–C,Electron micrographs show neurosecretory vesicles releasing neurotransmitter molecules by exocytosis on the presynaptic membrane. Numerous small neurotransmitter substances can be seen in the synaptic space (open arrows). These neurotransmitters will then settle on and activate receptors on the postsynaptic membrane. Dark circles by straight solid arrows represent vesicles filled with neurotransmitters. Light partial circles by curved solid arrows represent the vesicle merging with the presynaptic membrane and releasing neurotransmitter into the synaptic space (adapted from [32])

fig 6.
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fig 6. Pictorial display of the neurotransmitter glutamate (orange) released into the synaptic space and docking with the glutamate receptor site on the postsynaptic membrane. The activation of the glutamate receptor then opens the ion channel coupled to the receptor, allowing the passage of extracellular calcium (yellow) into the intracellular cytosol, which in turn triggers a series of biochemical events (adapted from Schornak S, BNI Q 11:1995)
fig 7. Ionotropic and metabotropic receptors. The ionotropic receptors NMDA (purple) and AMPA (red) are directly coupled to ion channels. The metabotropic receptors (blue and orange) activate intermediary molecules such as G protein affecting multiple cytoplasmic enzymes to produce molecules, such as IP3, that increase cytosol calcium concentrations. Also depicted are modulatory substances, such as spermine, which facilitate calcium influx, and receptor complex inhibitors, such as zinc, magnesium, and PCP. L-2-amino-4-phosphonopriopionic acid (L-AP4) and aminocyclopentyl dicarboxylic acid (ACPD) receptors are classified as metabotropic, as they are coupled to intermediary G proteins (G) that activate phosphodiesterase (PDE) for L-AP4 receptors and form inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG) from phosphatidylinositol 4,5-bisphosphate (PIP3) for the ACPD receptors via phospholipase C. 2-amino-3-phosphonopriopionic acid (AP3) and quinoxaline-2,3-dione (NBQX) are antagonists for ACPD and AMPA receptors, respectively.

Postsynaptic Anatomy

The interaction of glutamate with the postsynaptic membrane requires a review of glutamate receptors (Fig 7). The two main types of glutamate receptors are ionotropic and metabotropic. Ionotropic receptors are directly coupled to membrane ion channels. The metabotropic receptors are coupled to intermediary compounds, such as G protein, and modulate intracellular second messengers, such as inositol-1,4,5-trisphosphate (IP3), calcium, and cyclic nucleotides. The directly coupled ionotropic receptor, which is the primary consideration of this review, can be further subdivided into three subtypes: NMDA (N-methyl-D-aspartate), AMPA (a-amino-3-hydroxy-5-methyl-4-isoxazolepropionate), and kainate. These subtypes are named for their selective chemical agonists, which resemble glutamate but do not naturally exist in the brain. This nomenclature may seem awkward, but it offers the convenience and consistency of grouping the various cerebral and cerebellar receptors according to their responses to the chemical tools that are used to evaluate or stimulate them.

Excessive accumulation of intracellular calcium is the key observed process leading to neuronal death or injury, and the NMDA receptors activate channels that allow the influx of extracellular calcium (and sodium). Overstimulation of this type of glutamate receptor would then lead to neuronal calcium overload. Some types of AMPA and kainate receptors can contribute to intracellular calcium overload because their coupled membrane ion channels are at least partially permeable to calcium.

The influx of calcium and sodium from glutamate receptor stimulation results in membrane depolarization, which can also activate voltage-dependent calcium channels. These other calcium channels then allow further calcium influx, aggravating the intracellular calcium overload initiated by overstimulation of the glutamate receptors and opening of the associated ion channels. The four main types of voltage-dependent calcium channels considered here are named for their specific properties: T (transient current), N (found in neurons), L (long duration current, large conductance channels), and P (found in Purkinje cells of the cerebellum). The L channel is not the most prevalent type but it disproportionately contributes to calcium-mediated neuronal injury because of the prolonged calcium influx that occurs with activation of this voltage-dependent conduit.

Multiple modulatory sites, however, complicate some of the ionotropic receptors. It may be helpful to think of the receptors as a receptor-channel complex with the receptor closely linked to and controlling the adjacent ion channel. Modulatory sites are separate areas on the receptor and the channel in which other molecules can influence the function of the receptor site or channel. On the NMDA receptors, glycine (once considered a laboratory contaminant) acts as a required coagonist. Hydrogen ions (a reflection of pH) suppress receptor activation. Polyamines, such as spermine, however, can relieve proton block and potentiate NMDA receptor activation in a pH-dependent fashion. The NMDA receptor channels are affected by multiple factors, including magnesium (which blocks the channel), zinc (positive and negative modulator), and multiple drugs, such as dizocilpine and phencyclidine ([PCP, “angel dust”] channel blocker) (Fig 8).

fig 8.
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fig 8. Schematic representation of NMDA, AMPA, and kainate receptors as receptor–channel complexes. Glutamate docks with the receptor, which opens the coupled channel to allow the intracellular influx of extracellular calcium. Other molecules (such as magnesium, zinc, and PCP) can influence receptor function by interacting with several receptor and channel modulatory sites


Glutamate excitotoxic calcium overload can be appreciated from the perspective of the normal mechanisms of neuronal calcium homeostasis. Intracellular cytosolic free calcium is maintained at very low concentrations (micromolar) relative to free extracellular calcium. Plasma membrane calcium transporters regulate this cytosolic free calcium concentration. Membrane transporters in general have been classified as antiporters, symporters, and ATP-coupled active transporters (Fig 9). These transporters are membrane protein compounds that are coupled to energy sources and change the distribution of substrate ions or molecules across a membrane. Antiporters and symporters are called secondary transporters because they use the energy from an existing ion gradient to drive the passage of another ion or molecule in the same (symporter) or opposite (antiporter) direction across a membrane as the energizing ion. Movement through the neuronal membrane is achieved by a change in the conformation of the protein-substrate complex. Complexing with two or more substrates is required to initiate conformational transitions in antiporters (opposite direction coupling) and symporters (same direction coupling). The primary transporters (ATP-coupled active transporter) couple a chemical reaction to the protein conformational transitions that supply the metabolic energy required to generate concentration gradients of substrate ions across the membrane. Calcium is controlled by the antiporter and plasma-membrane calcium pump (PMCA). The antiporter, which has a low affinity but high transport capacity for calcium, moves calcium out of the neuron by a sodium–calcium exchange mechanism. The sodium gradient across the membrane drives this exchanger. The PMCA, on the other hand, has a high affinity but low transport capacity for calcium. This active pump transports one Ca2+ for each ATP hydrolyzed. One distinguishing feature of the PMCA is the enhanced activation of the pump by binding Ca2+/calmodulin, which results in a 20- to 30-fold increase in the affinity of the substrate Ca2+ site (33).

fig 9.
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fig 9. Membrane channels and transporters. The ion channels are pathways to allow the diffusion of ions across the cell membrane. These channels can be opened or closed by changes in membrane voltage, associated ligands, and so on. Excessive intracellular calcium is detrimental to neuronal health, and a calcium gradient is maintained across the neuronal membrane mostly by three main types of transporters. The antiporters use an existing ion gradient (usually established by active or ATP-dependent transport) to transport calcium ions (green) in the opposite direction of the energizing ion (ie, sodium). Symporters transport calcium in the same direction of the energizing ion (blue). Both antiporter and symporters are considered secondary transporters because they derive energy from an existing gradient. The ATP-coupled active transporter is considered a primary transporter that uses ATP to affect the transmembrane movement of calcium to establish its gradient (adapted from [32])

The mitochondrion and endoplasmic reticulum are also significant sources of calcium stores. An antiporter mechanism maintains high calcium concentrations in the mitochondria by moving free calcium from the cytosol to the mitochondria while an antiporter and an ATP-dependent active pump sequester the endoplasmic reticulum calcium (Fig 10).

fig 10.
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fig 10. Calcium homeostasis. Membrane transporter (antiporter and ATP-dependent transporter) maintain a much higher extracellular calcium (small circles) concentration than the cytosol. The endoplasmic reticulum and mitochondria are important sources of intracellular extracytosolic calcium. These internal sources of calcium can be released into the cytosol when provoked by specific agents, such as the second messenger (IP3) actions on the endoplasmic reticulum (adapted from [32])

Glutamate receptor overstimulation increases intracellular calcium by directly opening ion channels and secondarily affecting calcium homeostatic mechanisms. As mentioned, the initial glutamate receptor opening of the sodium/calcium channels not only allows the influx of calcium but also causes membrane depolarization. The depolarization would in turn activate the voltage-dependent calcium channels, which would further increase the intracellular calcium levels. The decreased sodium gradient across the cell membrane caused by the glutamate receptor–coupled channels, however, would also reduce the ability of the sodium gradient–dependent antiporter to remove intracellular calcium. Superimposed disorders that decrease ATP production (ie, hypoxia, neurodegenerative disorders, etc) would adversely affect the activity of the ATP-dependent calcium transporters as well as the energy-dependent sodium potassium pump, which would then also affect the transmembrane sodium gradient and therefore the antiporter function.

Intracellular Toxic Events

The accumulation of high intracellular calcium levels triggers a cascade of membrane, cytoplasmic, and nuclear events leading to neurotoxicity. Elevation of the intracellular calcium, however, appears to be a complex issue, because inducing similar intracellular calcium levels by using a metabolic inhibitor such as cyanide or membrane depolarization with potassium causes less permanent neuronal damage than with glutamate. The glutamate-induced elevated calcium levels proceed to overactivate a number of enzymes, including protein kinase C, calcium/cadmodulin-dependent protein kinase II, phospholipases, proteases, phosphatases, nitric oxide synthase, endonucleases, and ornithine decarboxylase (Fig 11). Some of these enzymes can also produce positive feedback loops to accelerate the downward spiral toward neuronal death (Fig 12). Activation of phospholipase A, for example, would generate platelet-activating factor and arachidonic acid and its metabolites. Platelet-activating factor directly contributes to the excitotoxic cascade by increasing glutamate release. Arachidonic acid inhibits reuptake of glutamate from the synaptic space, leading to further activation of glutamate receptors and more arachidonic acid formation. Increased arachidonic acid levels form oxygen free radicals, which activate phospholipase A, leading to more arachidonic acid formation. These enzymes and the generated feedback loops rapidly lead to neuronal self-digestion by protein breakdown, free radical formation, and lipid peroxidation.

fig 11.
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fig 11. Excessive accumulation of intracellular calcium caused by overactivation of the glutamate receptor sites stimulates multiple enzymes, which are involved in normal neuronal development and function, resulting in damage to the cell membrane, cytoskeleton, and DNA.

fig 12. Abnormally increased activation of some enzymes, such as phospholipase A, can cause an intracellular feedback cycle of events, leading to cell death.

Another important activated enzyme is nitric oxide synthase, which forms nitric oxide. Nitric oxide performs a variety of normal biological functions but the excessively stimulated NMDA receptors will produce abnormally increased levels of nitric oxide and superoxide ions. These substances may react and form peroxynitrite, which is extremely toxic, resulting in neuronal death. Nitric oxide can damage DNA as well as inhibit mitochondrial respiration, which in turn would create more free radicals and cause additional membrane depolarization. The nitric oxide–initiated neurotoxic cascades are important components of the mechanism of cell death in many neurodegenerative disorders, including Huntington disease (34–54).

Excessive Glutamate Accumulation

The key process that triggers the entire excitotoxic cascade is the excessive accumulation of glutamate in the synaptic space. This can be achieved by altering the normal cycling of intracranial glutamate (Fig 13) to increase the release of glutamate into the extracellular space or to decrease glutamate uptake/transport from the synaptic space, or by frank spillage of glutamate from injured neurons.

fig 13.
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fig 13. Neuronal glutamate that is released into the synaptic space is normally removed from the synaptic space by adjacent glial cells, in which the glutamate is converted to the closely related glutamine, which can then readily diffuse back into the neuron. Glutamine is converted back to glutamate in the neuron.

fig 14. Diagram shows sequence of events occurring in cerebral ischemia leading to neuronal death. (Free radical formation and lipase activation are also related to the increased intracellular calcium, although the two processes are not directly connected to the increased calcium by arrows in this schematic.)

Trauma is a blunt mechanism that massively elevates the extracellular glutamate levels. Normal extracellular glutamate concentration is about 0.6 μmol/L. Substantial neuronal excitotoxic injury occurs with glutamate concentrations of 2 to 5 μmol/L. Traumatic injury to neurons can produce disastrous results with the exposure of the normal intracellular glutamate concentrations of about 10 μmol/L to the extracellular space. Mechanical injury to a single neuron, therefore, puts all of the neighboring neurons at risk. Significant collateral injury occurs to surrounding neurons from this type of glutamate release. One recent therapeutic strategy is to immediately treat persons with injuries to the head or spinal column with glutamate receptor blockers to minimize the spread of neuronal death beyond the immediate physically disrupted neurons.

Several mechanisms of excess glutamate accumulation probably come into play in ischemia (Fig 14). Abnormal release of glutamate from its storage sites in neuronal vesicles is at least one factor. A feedback loop is generated as this released glutamate stimulates additional glutamate release. Ischemia also causes energy failure that impairs the reuptake by glutamate transporters. These transporters behave as symporters, which rely on the sodium gradient across cell membranes to move glutamate against its concentration gradients into the cell. The sodium gradient, however, is maintained by an energy-dependent pump that fails in ischemia. Such failure not only affects glutamate transport out of the synaptic space but also causes the transporters to run backward, becoming a source of extracellular glutamate rather than a sink for it. Ischemia deprives the neurons of oxygen and glucose, resulting in energy failure; however, energy failure itself is not particularly toxic to neurons. Neural toxicity occurs with the resultant activation of the cascade of glutamate receptor–dependent mechanisms. If these receptors are blocked by appropriate antagonists, the neurons can survive a period of deprivation of oxygen and metabolic substrate. This is the rationale for the recent development and trial of glutamate receptor blockers to treat acute ischemic events (55–66). While an infarcted zone cannot be salvaged, the hope is to prevent surrounding damage to the at-risk adjacent penumbra.

These receptor blockers may also be critical in the developing arena of interventional and pharmacologically related attempts to reestablish perfusion to acutely ischemic areas of the brain. Tissue reperfusion and increased oxygen concentrations to ischemic areas without concurrent halting of the excitotoxic cascade either at the receptor or intracellular levels may increase rather than decrease neuronal damage by providing additional free radicals in the form of superoxide anions as well as by increasing the intracellular cytosol calcium levels by stimulating the release of mitochondrial calcium stores.

Acceptance of the significant role of mitochondria in neuronal death and excitotoxicity is reflected in the rapidly expanding literature on this topic during the past decade (26, 49, 67–81). Preliminary investigations into the mechanisms of mitochondrial calcium homeostasis have already inspired several therapeutic neuroprotective strategies.

A number of drugs have been developed and used in an attempt to interrupt, influence, or temporarily halt the glutamate excitotoxic cascade toward neuronal injury (82–88). One strategy is the “upstream” attempt to decrease glutamate release. This category of drugs includes riluzole, lamotrigine, and lifarizine, which are sodium channel blockers. The commonly used nimodipine is a voltage-dependent channel (L-type) blocker. Attempts have also been made to affect the various sites of the coupled glutamate receptor itself. Some of these drugs include felbamate, ifenprodil, magnesium, memantine, and nitroglycerin. These “downstream” drugs attempt to influence such intracellular events as free radical formation, nitric oxide formation, proteolysis, endonuclease activity, and ICE-like protease formation (an important component in the process leading to programmed cell death, or apoptosis). Apoptosis occurs as part of the complex process of neuronal death, but many investigators believe that excitotoxicity and apoptosis are essentially different mechanisms that have intersecting influences (27, 56, 89–106).

Neuroradiologic Observations

Routine neuroimaging studies reflect the sequela of glutamate excitotoxic damage. The spreading involvement of adjacent brain tissue beyond the immediate area of insult in trauma and infarction is one example, and the delayed presentation of these conditions is another. The insidious brain parenchymal loss noted on imaging studies in patients with AIDS dementia is also thought to be related to glutamate excitotoxic injury.

The development of spectroscopy at 0.5 T has provided an intriguing opportunity to observe the combined glutamate and glutamine (glx) peak in vivo (107). Delineation of this peak is enabled by the coalescence of the multiplets of glx at this field strength (Figs 15 and 16). The collapse of the γ and β multiplet resonances can be briefly summarized by the ratio of δ/J (δ = chemical shift, J = spin-spin coupling constant), which governs the spectral appearance of a strongly coupled multiplet structure such as glx. The spin-spin coupling constant is determined by molecular structure and is independent of field strength, whereas the chemical shift between coupled spins is a linear function of field strength. Therefore, as the ratio of δ/J approaches zero (or decreases), the multiplet collapses toward a single resonance line.

fig 15.
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fig 15. MR spectrum of a normal frontal lobe obtained at 1.5 T with a single-voxel point-resolved spectroscopy (PRESS) technique at 1500/41 (TR/TE).
fig 16. MR spectrum of the same frontal lobe as in figure 15 obtained at 0.5 T with a single-voxel PRESS technique at 1500/41. Note the more conspicuous glx peak.

Interesting clinical observations are then possible and have been made with the ability to examine the glx peak. Glutamate excitotoxicity has been the implicated mechanism of neuronal injury in mesial temporal sclerosis with consistent, strong supportive experimental data (108–121). Elevated glx peaks have been observed in the hippocampi of these patients (Fig 17A and B) (122). Mitochondrial disorders, such as MELAS (mitochondrial myopathy, encephalopathy, lactacidosis, and stroke), are functionally ischemic, although there is no hypoperfusion but rather an inability to utilize the available oxygen. Imaging findings may be variable, but spectroscopic sampling of apparently normal-appearing areas of the brain by routine imaging can reveal not only the expected lactate peaks but also an elevated glx peak (Fig 18A and B). Even neurodegenerative disorders, such as Huntington disease, have been evaluated by this technique, and elevated glx peaks have been observed in the basal ganglia (123–125).

fig 17.
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fig 17. A, Coronal T2-weighted MR image of a patient with right mesial temporal sclerosis. B, MR spectrum obtained from a voxel (indicated in A) centered in the region of the patient’s right hippocampal formation shows an elevated glx peak.

fig 18.
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fig 18. Proton-density–weighted MR image in a patient with MELAS shows bilateral abnormal signal intensity changes at the periphery of the occipital lobes. B, MR spectrum obtained at 0.5 T shows elevated glx and lactate peaks from a sampling of the normal-appearing right frontal lobe of same patient (voxel placement indicated in A).


Glutamate excitotoxicity is the final common pathway resulting in neuronal injury for many seemingly unrelated disorders, including ischemia, trauma, seizures, hypoglycemia, hypoxia, and even some neural degenerative disorders. Familiarity with this process is important for neuroradiologists because of its central position in many of the disorders encountered in daily practice. This area has been one of the most intensely investigated fields in the neurosciences over the past several decades, and the information generated from this work will clearly influence our basic understanding of many neurologic disorders.”


  • 1 Presented in part at the annual meeting of the American Society of Neuroradiology, San Diego, May 1999.


  1. Waggie KS, Kahle PJ, Tolwani RJ. Neurons and mechanisms of neuronal death in neurodegenerative diseases: a brief review. Lab Anim Sci 1999;49:358-362
  2. Olney J. Neurotoxicity of excitatory amino acids. In: McGeer E, Olney J, McGeer P, eds. Kainic Acid as a Tool in Neurobiology. New York: Raven Press; 1978:95–121
  3. Olney JW. Role of excitotoxins in developmental neuropathology. APMIS Suppl1993;40:103-112
  4. Budd SL. Mechanisms of neuronal damage in brain hypoxia/ischemia: focus on the role of mitochondrial calcium accumulation. Pharmacol Ther 1998;80:203-229
  5. Lipton SA, Nicotera P. Calcium, free radicals and excitotoxins in neuronal apoptosis. Cell Calcium 1998;23:165-171
  6. Siegel GJ, Agranoff BW, Albers RW, Molinoff PB, eds. Basic Neurochemistry: Molecular, Cellular, and Medical Aspects. 5th ed. New York: Raven Press; 1994
  7. Aramburu J, Rao A, Klee CB. Calcineurin: from structure to function. Curr Top Cell Regul 2000;36:237-295
  8. Kader A, Frazzini VI, Solomon RA, Trifiletti RR. Nitric oxide production during focal cerebral ischemia in rats. Stroke 1993;24:1709-1716
  9. Choi D. Antagonizing excitotoxicity: a therapeutic strategy for stroke? Mount Sinai J Med 1998;65:133-138
  10. Gagliardi RJ. Neuroprotection, excitotoxicity and NMDA antagonists. Arq Neuropsiquiatr 2000;58:583-588
  11. Prost RW, Mark LP, Mewissen M, Li SJ. Detection of glutamate/glutamine resonances by 1H magnetic resonance spectroscopy at 0.5 tesla. Magn Reson Med 1997;37:615-618
  12. Sutula T, Koch J, Golarai G, Watanabe Y, McNamara JO. NMDA receptor dependence of kindling and mossy fiber sprouting: evidence that the NMDA receptor regulates patterning of hippocampal circuits in the adult brain. J Neurosci 1996;16:7398-7406
  13. Mark LP, Prost R, Yetkin Z, Haughton VM. An MR Spectroscopic Study of Glutamate in Patients with Temporal Lobe Seizures. Presented at the 33rd annual meeting of the American Society of Neuroradiology, Chicago, 1995
  14. Reynolds NC, Prost RW, Mark LP. Evidence for Excitotoxicity: Comparisons of Proton Magnetic Resonance Spectroscopy in Huntington’s Disease, Idiopathic Dystonia and Progressive Supranuclear Palsy. Presented at the 5th International Congress of Parkinson’s Disease and Movement Disorders, New York, 1998
  • Received March 26, 2001, American Society of Neuroradiology