Isolation rearing in the rat disrupts the hippocampal response to stress

Both human schizophrenia and the effects of isolation rearing in rats produce deficits in hippocampal and cortical functioning. This study was concerned with identifying changes associated with altered neuronal function in the rat hippocampus following isolation rearing. Rats were isolated from weaning at 21 days postnatal for 6 weeks and the hippocampal sensitivity to isolation rearing and stress were studied using c-fos immunohistochemistry and in vivo microdialysis. Isolation rearing altered neuronal activity measured by Fos-like immunoreactivity in the specific brain areas as measured by either increased or reduced expression. Basal neuronal activity in the ventral CA1 hippocampus in isolation-reared rats was notably higher compared to group-reared rats but markedly lower Fos-like immunoreactivity was found in the central and basolateral nuclei of the amygdala. Exposure to stress produced differential effects on neuronal activity in isolation-reared rats between the dorsal and ventral hippocampus, with increased Fos-like immunoreactivity in the dorsal hippocampus but lower Fos-like immunoreactivity in the ventral hippocampus compared to group-reared rats. These results indicate that isolation rearing may alter the relationship between hippocampal neuronal function in the dorsal and ventral hippocampus. An in vivo microdialysis study showed that systemically administered parachloroamphetamine (2.5 mg/kg, i.p.) enhanced extracellular 5-hydroxytryptamine (5-HT) in the dorsal hippocampus in group-reared but not in isolation-reared rats. Restraint stress had no effect on hippocampal extracellular 5-HT in group-reared rats but reduced levels in isolation-reared rats during the period of restraint. Inescapable mild footshock produced a marked increase in extracellular hippocampal 5-HT in group-reared but not isolation-reared rats. Overall the results provide extensive evidence that isolation rearing alters presynaptic 5-HT hippocampal function and that the neuronal response to stress is altered by isolation. Isolation rearing in the rat alters hippocampal function, including the serotonergic system, leading to changes in neurotransmitter systems in other brain areas. These changes may model aspects of human neurodevelopmental disorders such as schizophrenia.