The microbiome regulates neuronal function and “fear extinction” learning [2] Bonus 2013 article — Parasite makes mice lose fear of cats permanently

These GEITP pages have frequently described the growing appreciation of our intestinal microbiome (bacteria, fungi and viruses) and our increasing appreciation of the brain-gut-microbiome axis. Which species predominate in the microbiota — and what metabolism takes place in the intestine — can have direct impact on health and disease, including our mental behavior. Authors [see attached article & editorial] describe that mice, lacking a particular complex microbiota, exhibit an altered fear-associated behavior, changes in gene expression in cells in the brain, and alterations in the firing patterns and rewiring ability of neurons…!! It is well known that all organisms are constantly updating their responses to environmental cues, throughout their lifetimes; this process of behavioral adaptation is, of course, driven by underlying cellular and molecular changes in the brain.

Authors [see attached article] investigated how changes in the gut microbiota affect one specifc adaptation: fear conditioning. First, the authors trained mice to associate a sound with an electric shock — and how strongly that association was formed. The association was seen to develop normally, both in control mice and in microbiota-deficient mice (i.e. those treated with antibiotics to deplete their gut microbiota). Authors then performed an “extinction task”, in which they repeatedly played the tone without an electric shock; this measures the rate at which the animals “updated their behavior” (such an adaptation indicates the fear response has been extinguished). The microbiota-deficient mice were unable to update their response, showing persistent fearful behavior long after control animals had adapted. The same phenomenon was found to occur in mice that had been raised germ-free in sterile isolators (i.e. they had never developed a normal healthy gut microbiome).

Authors then performed nucleus RNA-sequencing in individual cells of the medial prefrontal cortex of the brain; significant changes in gene expression were found in excitatory neurons, glial cells, and other cell-types. Transcranial two-photon imaging showed that deficits in extinction learning — after manipulation of the microbiota in adult mice — were associated with “defective learning-related remodeling” of postsynaptic dendritic spines, and decreased activity in cue-encoding neurons in the medial prefrontal cortex. Furthermore, selective reestablishment of the microbiota revealed a “limited neonatal developmental window” in which microbiota-derived signals can restore normal extinction learning in adulthood.

Lastly, unbiased metabolomics analysis identified four metabolites that were significantly down-regulated in germ-free mice; these same metabolites have been reported to be related to neuropsychiatric disorders in humans and mouse models — strongly suggesting that microbiota-derived chemicals might directly affect brain function and behavior. Together, these data indicate that fear extinction learning requires microbiome-derived signals — both during early postnatal neurodevelopment and in adult mice. These EXCITING BREAKTHROUGH findings have implications for our understanding of how diet, infection, exposure to drugs (especially antibiotics?), and lifestyle can influence brain health and subsequent susceptibility to neuropsychiatric disorders…!! 😊 [See the accompanying article below, from a study six years ago.] 😉


Nature 24 Oct 2019; 574: 543-548 & editorial pp 488-489

And here is an additional news item (from 6 years ago), pointed out by Elizabeth Kopras, that can now be explained by the 2019 Nature article [above]:

Parasite makes mice lose fear of cats permanently..!!

Behavioral changes persist after Toxoplasma infection is cleared.

· Eliot Barford

18 September 2013 Nature News


Mice infected with toxoplasmosis lose their instinctive fear for the smell of cats — and the parasite’s effects may be permanent.

A parasite that infects up to one-third of people around the world may have the ability to permanently alter a specific brain function in mice, according to a study published in PLoS ONE today1.

Toxoplasma gondii (a one-celled parasitic eukaryote) is known to remove rodents’ innate fear of cats. The new research shows that even months after infection, when parasites are no longer detectable, the effect remains. This raises the possibility that the microbe causes a permanent structural change in the brain.

The microbe is a single-celled pathogen that infects most types of mammal and bird, causing a disease called toxoplasmosis. But its effects on rodents are unique; most flee cat odor, but infected ones are mildly attracted to it.

This is thought to be an evolutionary adaptation to help the parasite complete its life cycle: Toxoplasma can sexually reproduce only in the cat gut, and for it to get there, the pathogen’s rodent host must be eaten.

In humans, studies have linked Toxoplasma infection with behavioral changes and schizophrenia. One work found an increased risk of traffic accidents in people infected with the parasite2; another found changes in responses to cat odor3. People with schizophrenia are more likely than the general population to have been infected with Toxoplasma, and medications used to treat schizophrenia may work in part by inhibiting the pathogen’s replication.

Schizophrenia is thought to involve excess activity of the neurotransmitter dopamine in the brain. This has bolstered one possible explanation for Toxoplasma’s behavioural effect: the parasite establishes persistent infections by means of microscopic cysts that grow slowly in brain cells. It can increase those cells’ production of dopamine, which could significantly alter their function. Most other suggested mechanisms also rely on the presence of cysts.
Persistent trait

Research on Toxoplasma has mainly used the North American Type II strain. Wendy Ingram, a molecular cell biologist at the University of California, Berkeley, and her colleagues investigated the effects of two other major strains, Type I and Type III, on mouse behavior. They found that within three weeks of infection with either strain, mice lost all fear of cat odor — showing that the behavioral shift is a general trait of Toxoplasma.

More surprising was the situation four months after infection. The Type I pathogen that the researchers used had been genetically modified to provoke an effective immune response, allowing the mice to overcome the infection. After four months, it was undetectable in the mouse brain, indicating that no more than 200 parasite cells remained. “We actually expected that Type I wouldn’t be able to form cysts, and therefore wouldn’t be able to cause the behavior change,” explains Ingram.

But that was not the case: the mice remained as unperturbed by cat odor as they had been at three weeks. “Long after we lose the ability to see it in the brain, we still see its behavioral effect,” says geneticist Michael Eisen, also at Berkeley.

This suggests that the behavioral change could be due to a specific, hard-wired alteration in brain structure, which is generated before cysts form and cannot be reversed. The finding casts doubt on theories that cysts or dopamine cause the behavioural changes of Toxoplasma infections.
Mind over matter

Joanne Webster, a parasite epidemiologist at Imperial College London who co-discovered the fear-negating effects of Toxoplasma in rats4, highlights the worrying implication that if the behavioral changes of Toxoplasma-caused schizophrenia are fixed, treatments that are intended to target cysts might have no effect. However, she notes that mice are not the best model for Toxoplasma infection in humans, because they experience more severe symptoms and complications. Webster uses rats in her research.

Ingram says that her group is using mice because of the better genetic tools available to help to uncover the mechanism behind behavioral changes. However, she is not yet convinced of the link between Toxoplasma infections and schizophrenia. Her findings may actually weaken that link, because they seem to provide evidence against the dopamine hypothesis.

She notes that Toxoplasma infections are common around the world, but their prevalence varies by region, whereas schizophrenia rates are consistent at around 1% globally. Furthermore, it is possible that the increased rate of Toxoplasma infections among people with schizophrenia is caused by them being more likely to pick up the parasite, rather than by the parasite causing schizophrenia.

Nature doi:10.1038/nature.2013.13777


1. Ingram, W. M., Goodrich, L. M., Robey. E. A. & Eisen, M. B. PLoS ONE 8, e75246 (2013).

2. Flegr, J., Havlícek, J., Kodym, P., Malý, M. & Smahel, Z. BMC Infect. Dis. 2, 11 (2002).

3. Flegr, J., Lenochová, P., Hodný, Z. & Vondrová, M. PLoS Neglect. Trop. Dis. 5, e1389 (2011).

4. Berdoy, M., Webster, J. P. & Macdonald, D. W. Proc. R. Soc. B 267, 1591–1594 (2000).

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