Interaction of tonic immobility and dexamethasone in the modulation of hippocampal 5-HT activity in rabbits.

The immobility reaction (animal hypnosis) in the rabbit is related to basal levels of corticosterone and to modifications in 5-HT activity in the brain. In order to investigate the relationship between adrenocortical function, brain 5-HT activity and immobility response in rabbits, the susceptibility to the reaction and 5-HT and 5-HIAA levels in the hippocampus were studied in animals whose pituitary-adrenal function was altered by chronic DEX treatment. It was still possible to induce immobility in animals treated with DEX. The reduction in duration observed under the present testing schedule (two sessions of immobility separated by a one week interval) did not occur in the treated animals. There was a reduction in 5-HT turnover as a consequence of the immobility response and this confirms previous findings in other brain areas of the same species. In animals pretreated with DEX, so as to drastically reduce ACTH and corticosterone, not only was a reduction in 5-HT turnover prevented, but there was also an increase in 5-HT and 5-HIAA levels when immobility was induced. These results are discussed in terms of the influence of the pituitary-adrenal system in the neuroendocrine and behavioural aspects of the immobility reaction.