Infection of male rats with Toxoplasma gondii results in enhanced delay aversion and neural changes in the nucleus accumbens core

Latent toxoplasmosis, Cytomegalovirus, and Herpes Simplex Virus infections and risk of motorcycle accidents: A case-control study in a county with a high rate of motorcycle injuries in Iran

BACKGROUND

Road traffic injuries (RTIs) are among the most important issues worldwide. Several studies reported that infection with the neurotropic parasite Toxoplasma gondii (T. gondii) increased the risk of car accidents. In this study, our objective was to investigate the possible associations among latent T. gondii, Cytomegalovirus (CMV), and Herpes Simplex Virus (HSV) infections with the risk of motorcycle accidents in Jahrom (Fars Province), which is a county with a high rate of motorcycle accidents in Iran.

METHODS

In the setting of a case-control study; 176 motorcyclist men, including 88 survivors of motorcycle accidents and 88 motorcyclist without accidents, were considered as case and control groups, respectively. Rates of latent infections with T. gondii, CMV, and HSV were assessed by an enzyme-linked immunosorbent assay (ELISA).

RESULTS

Eleven of 88 (12.5%) in the case group and 22 of 88 (25.0%) in controls were positive for anti-T. gondii IgG antibodies, this difference was statistically significant (OR = 0.42; CI: 0.19-0.95, p = 0.03). The general seroprevalence of CMV (94.3% in the case group vs. 87.5% in the control group, OR = 2.37; CI: 0.78-7.13, p = 0.12) and HSV (63.6% in the case group vs. 62.5% in the control group, OR = 1.05; CI: 0.57-1.94, p = 0.87) were not significantly different between the case and control groups.

CONCLUSIONS

Although latent toxoplasmosis has been associated with traffic accidents in recent reports, we found a negative association between latent toxoplasmosis and motorcycle accidents among survivors of these accidents. As such, latent CMV and HSV infections did not differ significantly between the cases compared to the control groups.

Evaluation of aversive behavior in Rattus norvegicus experimentally infected by two distinct strains of Toxoplasma gondii (ME49 and VEG): study of epigenetic markers

BACKGROUND

Behavioral changes in Rattus norvegicus infected with two strains of Toxoplasma gondii (ME49 and VEG) were investigated.

METHODS

Rats were evaluated for motor activity and aversion or attraction to cat urine 60 days after infection. After euthanasia, arginine-vasopressin gene methylation in the central nervous system was evaluated.

RESULTS

A significant difference was observed in the methylation of the arginine-vasopressin promoter gene between rats infected with the ME49 and VEG strains.

CONCLUSIONS

Although differences were not observed in many parameters, significant differences were observed in the methylation of the arginine-vasopressin promoter gene in rats infected with the two studied strains.

Behavioral biology of Toxoplasma gondii infection

Toxoplasma gondii is a protozoan parasite with a complex life cycle and a cosmopolitan host range. The asexual part of its life cycle can be perpetually sustained in a variety of intermediate hosts through a combination of carnivory and vertical transmission. However, T. gondii produces gametes only in felids after the predation of infected intermediate hosts. The parasite changes the behavior of its intermediate hosts by reducing their innate fear to cat odors and thereby plausibly increasing the probability that the definitive host will devour the infected host. Here, we provide a short description of such parasitic behavioral manipulation in laboratory rodents infected with T. gondii, along with a bird's eye view of underpinning biological changes in the host. We also summarize critical gaps and opportunities for future research in this exciting research area with broad implications in the transdisciplinary study of host-parasite relationships.

Behavioral Manipulation by Toxoplasma gondii: Does Brain Residence Matter?

The protozoan parasite Toxoplasma gondii infects a wide range of intermediate hosts. The parasite produces brain cysts during the latent phase of its infection, in parallel to causing a loss of innate aversion in the rat host towards cat odors. Host behavioral change presumably reflects a parasitic manipulation to increase predation by definitive felid hosts, although evidence for increased predation is not yet available. In this opinion piece, we propose a neuroendocrine loop to explain the role of gonadal steroids in the parasitized hosts in mediating the behavioral manipulation. We argue that the presence of tissue cysts within the host brain is merely incidental to the behavioral change, without a necessary or sufficient role.

Medial Amygdala Arginine Vasopressin Neurons Regulate Innate Aversion to Cat Odors in Male Mice

Aversion to environmental cues of predators is an integral part of defensive behaviors in many prey animals. It enhances their survival and probability of future reproduction. At the same time, animals cannot be maximally defended because imperatives of defense usually trade-off with behaviors required for sexual reproduction like display of dominance and production of sexual pheromones. Here, we approach this trade-off through the lens of arginine vasopressin (AVP) neurons within the posterodorsal medial amygdala (MePD) of mice. This neuronal population is known to be involved in sexual behaviors like approach to sexually salient cues. We show that chemogenetic partial ablation of this neuronal population increases aversion to predator odors. Moreover, overexpression of AVP within this population is sufficient to reduce aversion to predator odors. The loss of fear of the predator odor occurs in parallel with increased recruitment of AVP neurons within the MePD. These observations suggest that AVP neurons in the medial aspect of the extended amygdala are a proximate locus for the reduction in innate fear during life stages dominated by reproductive efforts.

Toxoplasma gondii, Suicidal Behavior, and Intermediate Phenotypes for Suicidal Behavior

Within the general literature on infections and suicidal behavior, studies on Toxoplasma gondii (T. gondii) occupy a central position. This is related to the parasite's neurotropism, high prevalence of chronic infection, as well as specific and non-specific behavioral alterations in rodents that lead to increased risk taking, which are recapitulated in humans by T. gondii's associations with suicidal behavior, as well as trait impulsivity and aggression, mental illness and traffic accidents. This paper is a detailed review of the associations between T. gondii serology and suicidal behavior, a field of study that started 15 years ago with our publication of associations between T. gondii IgG serology and suicidal behavior in persons with mood disorders. This "legacy" article presents, chronologically, our primary studies in individuals with mood disorders and schizophrenia in Germany, recent attempters in Sweden, and in a large cohort of mothers in Denmark. Then, it reviews findings from all three meta-analyses published to date, confirming our reported associations and overall consistent in effect size [ranging between 39 and 57% elevation of odds of suicide attempt in T. gondii immunoglobulin (IgG) positives]. Finally, the article introduces certain links between T. gondii and biomarkers previously associated with suicidal behavior (kynurenines, phenylalanine/tyrosine), intermediate phenotypes of suicidal behavior (impulsivity, aggression) and state-dependent suicide risk factors (hopelessness/dysphoria, sleep impairment). In sum, an abundance of evidence supports a positive link between suicide attempts (but not suicidal ideation) and T. gondii IgG (but not IgM) seropositivity and serointensity. Trait impulsivity and aggression, endophenotypes of suicidal behavior have also been positively associated with T. gondii seropositivity in both the psychiatrically healthy as well as in patients with Intermittent Explosive Disorder. Yet, causality has not been demonstrated. Thus, randomized interventional studies are necessary to advance causal inferences and, if causality is confirmed, to provide hope that an etiological treatment for a distinct subgroup of individuals at an increased risk for suicide could emerge.

Testosterone Acts Within the Medial Amygdala of Rats to Reduce Innate Fear to Predator Odor Akin to the Effects of Toxoplasma gondii Infection

Rats infected with the protozoan Toxoplasma gondii exhibit a reduced aversion to cat odor. This behavioral change is thought to increase trophic transmission of the parasite. Infected male rats also show a greater testicular synthesis of testosterone and epigenetic change in arginine vasopressin within the medial amygdala. Here, we show that exogenous supply of testosterone within MeA of uninfected castrates recapitulates reduction in innate fear akin to behavioral change attributed to the parasite. We also show that castration post establishment of chronic infection precludes changes in fear and medial amygdala arginine vasopressin in the infected male rats. These observations support the role of gonadal hormones and pursuant neuroendocrine changes in mediating the loss of fear in the infected rats. This work also demonstrates that testosterone acting specifically within the medial amygdala sufficiently explains reduced defensive behaviors often observed during the appetitive component of reproductive behaviors.

Driving us mad: the association of Toxoplasma gondii with suicide attempts and traffic accidents - a systematic review and meta-analysis

Unnatural causes of death due to traffic accidents (TA) and suicide attempts (SA) constitute a major burden on global health, which remained stable in the last decade despite widespread efforts of prevention. Recently, latent infection with Toxoplasma gondii (T. gondii) has been suggested to be a biological risk factor for both TA and SA. Therefore, a systematic search concerning the relationship of T. gondii infection with TA and/or SA according to PRISMA guidelines in Medline, Pubmed and PsychInfo was conducted collecting papers up to 11 February 2019 (PROSPERO #CRD42018090206). The random-effect model was applied and sensitivity analyses were subsequently performed. Lastly, the population attributable fraction (PAF) was calculated. We found a significant association for antibodies against T. gondii with TA [odds ratio (OR) = 1.69; 95% confidence interval (CI) 1.20-2.38, p = 0.003] and SA (OR = 1.39; 95% CI 1.10-1.76, p = 0006). Indication of publication bias was found for TA, but statistical adjustment for this bias did not change the OR. Heterogeneity between studies on SA was partly explained by type of control population used (ORhealthy controls = 1.9, p < 0.001 v. ORpsychiatric controls = 1.06, p = 0.87) and whether subjects with schizophrenia only were analysed (ORschizophrenia = 0.87, p = 0.62 v. ORvarious = 1.8, p < 0.001). The association was significantly stronger with higher antibody titres in TA and in studies that did not focus on schizophrenia subjects concerning SA. PAF of a T. gondii infection was 17% for TA and 10% for SA. This indicates that preventing T. gondii infection may play a role in the prevention of TA or SA, although uncertainty remains whether infection and outcome are truly causally related.

Downregulation of the Central Noradrenergic System by Toxoplasma gondii Infection

Toxoplasma gondii is associated with physiological effects in the host. Dysregulation of catecholamines in the central nervous system has previously been observed in chronically infected animals. In the study described here, the noradrenergic system was found to be suppressed with decreased levels of norepinephrine (NE) in brains of infected animals and in infected human and rat neural cells in vitro The mechanism responsible for the NE suppression was found to be downregulation of dopamine β-hydroxylase (DBH) gene expression, encoding the enzyme that synthesizes norepinephrine from dopamine, with downregulation observed in vitro and in infected brain tissue, particularly in the dorsal locus coeruleus/pons region. The downregulation was sex specific, with males expressing reduced DBH mRNA levels whereas females were unchanged. Rather, DBH expression correlated with estrogen receptor in the female rat brains for this estrogen-regulated gene. DBH silencing was not a general response of neurons to infection, as human cytomegalovirus did not downregulate DBH expression. The noradrenergic-linked behaviors of sociability and arousal were altered in chronically infected animals, with a high correlation between DBH expression and infection intensity. A decrease in DBH expression in noradrenergic neurons can elevate dopamine levels, which provides a possible explanation for mixed observations of changes in this neurotransmitter with infection. Decreased NE is consistent with the loss of coordination and motor impairments associated with toxoplasmosis. Further, the altered norepinephrine synthesis observed here may, in part, explain behavioral effects of infection and associations with mental illness.

Moderation of the relationship between Toxoplasma gondii seropositivity and trait impulsivity in younger men by the phenylalanine-tyrosine ratio

Previously, we reported that Toxoplasma gondii (T. gondii)-seropositivity is associated with higher impulsive sensation seeking in younger men. As dopaminergic and serotonergic signaling regulate impulsivity, and as T. gondii directly and indirectly affects dopaminergic signaling and induces activation of the kynurenine pathway leading to the diversion of tryptophan from serotonin production, we investigated if dopamine and serotonin precursors or the tryptophan metabolite kynurenine interact with the T. gondii-impulsivity association. In 950 psychiatrically healthy participants, trait impulsivity scores were related to T. gondii IgG seropositivity. Interactions were also identified between categorized levels of phenylalanine (Phe), tyrosine (Tyr), Phe:Tyr ratio, kynurenine (Kyn), tryptophan (Trp) and Kyn:Trp ratio, and age and gender. Only younger T. gondii-positive men with a high Phe:Tyr ratio, were found to have significantly higher impulsivity scores. There were no significant associations in other demographic groups, including women and older men. No significant effects or interactions were identified for Phe, Tyr, Kyn, Trp, or Kyn:Trp ratio. Phe:Tyr ratio, therefore, may play a moderating role in the association between T. gondii seropositivity and impulsivity in younger men. These results could potentially lead to individualized approaches to reduce impulsivity, based on combined demographic, biochemical and serological factors.

Effects of stress or infection on rat behavior show robust reversals due to environmental disturbance

Background: The behavior of animals is intricately linked to the environment; a relationship that is often studied in laboratory conditions by using environmental perturbations to study biological mechanisms underlying the behavioral change.  Methods: This study pertains to two such well-studied and well-replicated perturbations, i.e., stress-induced anxiogenesis and Toxoplasmagondii -induced loss of innate fear. Here, we demonstrate that behavioral outcomes of these experimental manipulations are contingent upon the ambient quality of the wider environment where animal facilities are situated. Results: During late 2014 and early 2015, a building construction project started adjacent to our animal facility. During this phase, we observed that maternal separation stress caused anxiolysis, rather than historically observed anxiogenesis, in laboratory rats. We also found that Toxoplasma gondii infection caused an increase, rather than historically observed decrease, in innate aversion to predator odors in rats. Conclusion: These observations suggest that effects of stress and Toxoplasma gondii are dependent on variables in the environment that often go unreported in the published literature.

Effects of stress or infection on rat behavior show robust reversals due to environmental disturbance

Background: The behavior of animals is intricately linked to the environment; a relationship that is often studied in laboratory conditions by using environmental perturbations to study biological mechanisms underlying the behavioral change.  Methods: This study pertains to two such well-studied and well-replicated perturbations, i.e., stress-induced anxiogenesis and Toxoplasma-induced loss of innate fear. Here, we demonstrate that behavioral outcomes of these experimental manipulations are contingent upon the ambient quality of the wider environment where animal facilities are situated. Results: During late 2014 and early 2015, a building construction project started adjacent to our animal facility. During this phase, we observed that maternal separation stress caused anxiolysis, rather than historically observed anxiogenesis, in laboratory rats. We also found that Toxoplasma infection caused an increase, rather than historically observed decrease, in innate aversion to predator odors in rats. Conclusion: These observations suggest that effects of stress and Toxoplasma are dependent on variables in the environment that often go unreported in the published literature.

Toxoplasma-induced changes in host risk behaviour are independent of parasite-derived AaaH2 tyrosine hydroxylase

Toxoplasma gondii infects a broad range of hosts and can establish chronic infections with the formation of brain cysts. Infected animals show altered risk behaviour which has been suggested to increase capture probability of hosts, and thus enhance parasite transmission. It has been proposed that the ability of Toxoplasma cysts to secrete tyrosine hydroxylase could mediate these behavioural alterations. We tested the involvement of secreted tyrosine hydroxylase, coded by the parasite AaaH2 gene, in the development of alterations in mouse behaviour, by generating an AaaH2 deletion mutant parasite strain and testing its influence on behaviour. We found that both mice infected with wild type or AaaH2 mutant strains showed changes in risk behaviour. We confirmed these findings using factor analysis of the behaviour, which revealed that behavioural changes happened along a single dimension, and were observed in both infected groups. Furthermore, we developed a new behavioural paradigm in which animals are unpredictably trapped, and observed that both groups of infected animals perceive trapping but fail to adjust their behaviour to avoid further trapping. These results demonstrate that parasite-secreted AaaH2 TH is neither necessary for the generation of risky behaviour nor for the increased trappability observed during chronic Toxoplasma infection.

Humans with latent toxoplasmosis display altered reward modulation of cognitive control

Latent infection with Toxoplasma gondii has repeatedly been shown to be associated with behavioral changes that are commonly attributed to a presumed increase in dopaminergic signaling. Yet, virtually nothing is known about its effects on dopamine-driven reward processing. We therefore assessed behavior and event-related potentials in individuals with vs. without latent toxoplasmosis performing a rewarded control task. The data show that otherwise healthy young adults with latent toxoplasmosis show a greatly diminished response to monetary rewards as compared to their non-infected counterparts. While this selective effect eliminated a toxoplasmosis-induced speed advantage previously observed for non-rewarded behavior, Toxo-positive subjects could still be demonstrated to be superior to Toxo-negative subjects with respect to response accuracy. Event-related potential (ERP) and source localization analyses revealed that this advantage during rewarded behavior was based on increased allocation of processing resources reflected by larger visual late positive component (LPC) amplitudes and associated activity changes in the right temporo-parietal junction (BA40) and left auditory cortex (BA41). Taken together, individuals with latent toxoplasmosis show superior behavioral performance in challenging cognitive control situations but may at the same time have a reduced sensitivity towards motivational effects of rewards, which might be explained by the presumed increase in dopamine.

Toxoplasma gondii infection and testosterone congruently increase tolerance of male rats for risk of reward forfeiture

Decision making under risk involves balancing the potential of gaining rewards with the possibility of loss and/or punishment. Tolerance to risk varies between individuals. Understanding the biological basis of risk tolerance is pertinent because excessive tolerance contributes to adverse health and safety outcomes. Yet, not much is known about biological factors mediating inter-individual variability in this regard. We investigate if latent Toxoplasma gondii infection can cause risk tolerance. Using a rodent model of the balloon analogous risk task, we show that latent T. gondii infection leads to a greater tolerance of reward forfeiture. Furthermore, effects of the infection on risk can be recapitulated with testosterone supplementation alone, demonstrating that greater testosterone synthesis by the host post-infection is sufficient to change risk tolerance. T. gondii is a frequent parasite of humans and animals. Thus, the infection status can potentially explain some of the inter-individual variability in the risky decision making.

Infection of male rats with Toxoplasma gondii induces effort-aversion in a T-maze decision-making task

Rats chronically infected with protozoan Toxoplasma gondii exhibit greater delay aversion in an inter-temporal task. Moreover T. gondii infection also results in dendritic atrophy of basolateral amygdala neurons. Basolateral amygdala is reported to bias decision making towards greater effortful alternatives. In this context, we report that T. gondii increases effort aversion in infected male rats. This host-parasite association has been widely studied in the context of loss of innate fear in the infected males. It is suggested that reduced fear towards predators reflects a parasitic behavioral manipulation to enhance trophic transmission of T. gondii. Observations reported here extend this paradigm away from a monolithic change in fear and towards a multi-dimensional change in decision making.

Can the behaviour of threespine stickleback parasitized with Schistocephalus solidus be replicated by manipulating host physiology?

Sticklebacks infected by the parasitic flatworm Schistocephalus solidus show dramatic changes in phenotype, including a loss of species-typical behavioural responses to predators. The timing of host behaviour change coincides with the development of infectivity of the parasite to the final host (a piscivorous bird), making it an ideal model for studying the mechanisms of infection-induced behavioural modification. However, whether the loss of host anti-predator behaviour results from direct manipulation by the parasite, or is a by-product (e.g. host immune response) or side-effect of infection (e.g. energetic loss), remains controversial. To understand the physiological mechanisms that generate these behavioural changes, we quantified the behavioural profiles of experimentally infected fish and attempted to replicate these in non-parasitized fish by exposing them to treatments including immunity activation and fasting, or by pharmacologically inhibiting the stress axis. All fish were screened for the following behaviours: activity, water depth preference, sociability, phototaxis, anti-predator response and latency to feed. We were able to change individual behaviours with certain treatments. Our results suggest that the impact of S. solidus on the stickleback might be of a multifactorial nature. The behaviour changes observed in infected fish may be due to the combined effects of modifying the serotonergic axis, the lack of energy, and the activation of the immune system.