Persistent changes in behaviour and brain serotonin during ageing in rats subjected to infant nasal virus infection

Suckling rats were infected intranasally with the temperature-sensitive mutant G41 strain of vesicular stomatitis virus. The rats survived but demonstrated lifelong learning deficits in the Morris maze and impaired exploratory behaviour in the open field test. When examined at 18 months of age they had a severe loss of neurons in the medial and dorsal raphe nuclei in the brain stem and reduced levels of serotonin and its metabolite 5-hydroxyindole acetic acid in the cerebral neocortex and hippocampus. The levels of noradrenaline, dopamine, homovanillic acid, 3,4-dihydroxyphenylacetic acid, choline acetyltransferase and glutamate decarboxylase were largely unaffected. The permanent disturbance in brain serotonin metabolism did not cause any histological changes in the cerebral cortex. Thus there were no neurofibrillary tangles or amyloid plaques as has been reported as a late effect of chemically induced lesion to the cholinergic system in the rat brain. It is concluded that the brain serotonergic system is especially vulnerable to an episode of virus attack along olfactory pathways and that the neurochemical and behavioural alterations caused by such an episode persist during a major part of the animal's life span.