An apparent suicidal feline attraction and risk behavioural profile among untreated infected individuals was clearly evident here. Under natural conditions, these are all behavioural traits likely to increase predation rate by the definitive host and hence completion of the parasites life cycle (Webster 2001
), either through increased risk behaviours (presence in areas with evidence of cat presence), conspicuousness (increased activity or still and exposed), or through decreased attention on predator avoidance (grooming in exposed areas) relative to their untreated uninfected counterparts (, ). Treatment of infected rats with, in order of decreasing efficacy: HAL, PD and VAL, reduced these predator-risk behavioural traits. Therefore, our results lend support to the hypothesis that the anti-psychotic and mood stabilizing activity of some medications used in the treatment of schizophrenia and human affective disorder may be augmented through their inhibitory impact upon T. gondii
in infected individuals
In terms of potential mechanistic explanations, the treatments, at least PD and HAL, may function by directly minimizing T. gondii
replication and invasion of host brain cells, as has been demonstrated in vitro
(Jones-Brando et al. 2003
). Indeed, our unpublished observations on T. gondii
immunohistochemical staining of tissue sections throughout the brains of these experimental rats indicate that the frequencies of T. gondii
exposed animals showing immunohistochemically positive neurons and glial cells was reduced following drug treatment, with a superiority of HAL over PD and VAL (S. Weis & I. C. Llenos 2005, personal communication). Such anti-T. gondii
activity may be related, at least in part, to the calcium inhibitory properties of these drugs. T. gondii
tachyzoites require calcium in order to invade host cells, and this invasion is inhibited by calcium channel blockers and calmodulin antagonists, including trifluoperazine, another phenothiazine anti-psychotic (Pezzella et al. 1997
). Accordingly, HAL and VAL, are each capable of inhibiting calcium transport through cellular ion channels (Itoa et al. 1996
; Johannessen 2000
Another, not mutually exclusive, explanation for the effects of T. gondii
and drug treatment on feline avoidance behaviour relates to their potentially neuromodulatory impact, either directly or indirectly (Blanchard et al. 1990
; Berdoy et al. 2000
; Torrey & Yolken 2003
). The reaction by potential prey to cat stimuli is used to study the neurological basis of anxiety and the mechanisms of anxiolytic (anxiety relieving) drugs, and such studies have found, for example that blocking the normally anxiogenic NMDA receptors in the amygdala also causes laboratory rats to ‘fearlessly’ approach areas treated with cat urine (Adamec et al. 1999
). Likewise, while exposure of rats to predator odours induces fast wave activity in the dentate gyrus of the hippocampus (File et al. 1993
; Hogg & File 1994
), such a response can be blocked by serotonin antagonists (Blanchard et al. 1990
), or alternatively by the presence in mice of another protozoan, Eimeria vermiformins
(Kavaliers & Colwell 1994
), which could suggest a similar neuromodulatory action of certain protozoan parasites including T. gondii
. Indeed, T. gondii
infection is known to be associated with raised dopamine levels in particular (Stibbs 1985
; Flegr et al. 2003
). Dopamine levels are also often raised within patients with schizophrenia (Torrey & Yolken 2003
). Thus, as HAL is a dopamine D2 antagonist (Seeman 1980
), one could propose that its superior therapeutic impact here may be through a combination of both its ability to inhibit T. gondii
replication and to reduce, directly and indirectly, dopamine levels. This may contrast to the restricted anti-parasitic, rather than neuromodulatory, action of PD. Likewise, while the mechanisms by which VAL may exert mood-stabilizing effects are not yet fully elucidated, recent studies have found little evidence that these are mediated by its effects on serotonin or dopamine receptors (Delva et al. 2002
). Thus, one may postulate that the relatively lowered success of VAL in decreasing the parasite-induced behavioural alterations observed here may also be restricted to those achieved through its direct inhibition on parasite replication alone, albeit less efficaciously than PD.
Our results raise several important theoretical and applied implications, from, for example providing further support for the theory of T. gondii as a causative agent in some cases of schizophrenia, to predicting that such drugs may be expected to be particularly effective in individuals with schizophrenia who are also infected with T. gondii. It may thus be worth investigating the T. gondii inhibitory effects of other anti-psychotics, especially the second-generation agents (olanzapine, clozapine, quetiapine, ziprasidone, risperidone and aripiprazole), as well as the effectiveness of alternative anti-T. gondii treatments, such as pyrimethamine with clindamycin, co-trimoxazole, or ponazuril, as adjunct therapies for schizophrenia. These potential treatments could also prove particularly valuable as prophylactic treatments for groups at serious risk of developing psychiatric disorders in later life as a result of T. gondii infection. Moreover, the results here, particularly with HAL, suggest that further research into anti-psychotics may provide potential alternatives for anti-T. gondii drug treatment, where new anti-bradyzoite treatments, in particular, are imperative.
However, the behavioural changes observed among the uninfected, but treated, rats indicate that serious consideration must be given to possible side-effects of such treatments on human behaviour: all three drug treatments caused uninfected rats to behave, albeit to a much reduced extent, in a similar way to infected untreated rats, in terms of feline attraction and activity levels, with further mild behavioural alterations, such as increased water and food consumption. Many of these behavioural alternations may also relate to the neuromodulatory action of such chemotherapeutics, as discussed above. Indeed, any anxiolytic drug effects may well be predicted to result in treated, but uninfected, rats being less averse to the cat smell, and also potentially more active, than their untreated uninfected counterparts, as was observed here (Blanchard et al. 1990
). The potential generalizability of such side-effects is certainly worthy of further inter-host specific research. In terms of the current study, the mild drug-induced feline attraction observed here among uninfected rats may, moreover, detract from the drug-induced reductions in the infection-related increases in entrances and duration spent within cat areas.
Our results, as is always the case in research, raise as many questions as they do answers. What will be important to elucidate now is, for example, why any potential effect of T. gondii on host behaviour, in particular in terms of the clinical outcome of human behaviour, may differ between individuals. Potential key factors may relate to inherent differences in individual genetic predisposition, the state of the immune system, the time of exposure (e.g. infections in the first trimester of pregnancy may differ from those in the third trimester; and/or prenatal infection may differ from postnatal), the duration of exposure (e.g. humans live longer than the average rodent intermediate host), and/or the part(s) of the brain affected. From the perspective of the parasite, key factors may relate to the dose, the source of infection (i.e. oocyst or cyst stage consumption), the genotype of the infecting strain, and/or even an interaction with other infectious agents.
In summary, our results to date do demonstrate that the behavioural changes associated with T. gondii can be effectively reduced by those anti-psychotic drug treatments previously demonstrated to inhibit parasite replication in vitro. This may provide further evidence for a potential role of T. gondii in the aetiology of schizophrenia, and specifically here that the actions of anti-psychotics may work in part via parasite inhibition. Clinical trials based on these findings are warranted, including those perhaps of anti-psychotic drugs with patients separated into those with and without additional T. gondii infection. Such trials could lead to improved prognosis and potentially new medication combinations and therapeutic modalities for the treatment of both toxoplasmosis and severe psychiatric disorders.