Influenza A virus infection and NMDA receptor function : A behavioral and molecular study of relevance for schizophrenia
Abstract: Schizophrenia is a neuropsychiatric disorder characterized by positive and negative symptoms, as well as cognitive dysfunctions. There is no single cause of schizophrenia, instead complex combinations of genetic and environmental factors may contribute to the disorder, including exposure to viral infections or other environmental insults during early life. Discoveries during the last 30 years have provided increasing evidence for a glutamatergic hypofunction in schizophrenia. This hypothesis arose from the finding that non-competitive N-methyl-d-aspartate (NMDA) receptor antagonists, such as ketamine and phencyclidine (PCP) can reproduce schizophrenia-like symptoms including cognitive dysfunctions in man. This observation, has led to a search for schizophrenia-related animal models based on NMDA-receptor blockade. The main aim of these studies was to investigate if exposure to influenza A virus during early life can result in cognitive dysfunctions during adult life. In addition, to allow studies on the potential role of NMDA receptor systems for cognition, a novel NMDA receptor hypofunction model was developed. Both acute and repeated treatment with NMDA receptor antagonists is known to produce a dose-related spectrum of motor dysfunctions that interfere with cognitive performance in rodents. For this reason, PCP was examined in a dose-dependent study to establish the dose-range of cognitive impairments versus motor side effects. Repeated administration of the 0.5 mg/kg dose of PCP impaired spatial learning and long-term memory without affecting non-spatial learning. The PCP-induced impairments in learning and memory were prevented by concomitant treatment with the atypical antipsychotic drug clozapine (0.5 mg/kg), but not with the typical antipsychotic haloperidol (0.05 mg/kg). To avoid testing with the drug on board , the effects of repeated administration of PCP on cognitive and social behavior were examined 24 h after the final dose of PCP. In addition, this study examined the effect of such treatment on the expression of two genes involved in neuronal plasticity and learning. Repeated doses of PCP (1 and 2 mg/kg) produced long-term impairments in spatial learning and working memory performance in the water maze task, without any apparent sensorimotor deficits. Furthermore, mice treated with 2 mg/kg of PCP spent less time in active social interaction, and showed increased non-social and aggressive behaviors. These behavioral effects were associated with altered expression of the genes encoding Arc and spinophilin. The alterations in Arc transcripts were observed in areas associated with spatial learning such as the CA1 region of the hippocampus and in the retrosplenial cortex. To examine if exposure to a maternal infection affects gene expression in the offspring, pregnant C57BL/6 mice were instilled intranasally with influenza A/WSN/33 virus on day 14 of gestation. Differential gene expression in the brains of the offspring could be detected at postnatal days 90 and 280, but not earlier. Thus, a maternal influenza A virus infection can give rise to alterations in gene expression that become apparent only after a considerable period of latency. A subsequent study examined the effects of an influenza A virus infection in 3-4-day old C57BL/6 or immunodeficient (Tap1-/-) mice. This infection led to transient changes in expression of several genes associated with schizophrenia in C57BL/6 mice while Tap1-/- mice exhibited more enduring changes. Whereas infected C57BL/6 mice were deficient only in the long-term memory task, infected Tap1-/- mice showed deficits in working memory performance. The olfactory tract provides an interesting route of invasion for studies of viruses to the brain. Therefore, mice were injected with influenza A virus into the olfactory bulbs at 5-6 weeks of age. Sixteen to eighteen weeks after the infection these mice showed impaired spatial learning and reduced anxiety-like behavior. Moreover, genes encoding synaptic regulatory proteins were differentially expressed in the amygdala, the hypothalamus and the cerebellum in these mice. Finally, to link early life virus infection to possible changes in NMDA receptor function, a subchronic dose of PCP (2 mg/kg) was given to control mice and mice infected on day 3. The marked impairment in cognition by subchronic PCP was confirmed in control mice. Virus infected mice tended to be more susceptible to these PCP-induced deficits in cognition and showed a clear difference in the pattern of motor activity compared to control mice. In conclusion this thesis shows that (i) repeated low dose of PCP impairs cognitive functions and is reversed by clozapine; (ii) repeated administration of PCP can produce long-term effects on cognition, social behavior and gene expression after termination of the treatment; (iii) influenza A virus infections can cause alterations in the expression of (schizophrenia-associated) genes and behavior. The effect of the infection appears to be influenced by the timing and route of infection; and (iv) mice infected with a virus during early life appear more susceptible to repeated PCP treatment in adulthood.
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