Stress adaptation and hypersensitivity in 5-HT neuronal systems after repeated foot shock

The relationship between adaptation to stress and change in sensitivity of the 5-hydroxytryptamine (5-HT) neuronal system was studied in rats exposed to repeated foot shock stress for up to 10 days. Although hypolocomotion, freezing behavior and loss of weight were observed after in the initial stress, relief from these behavioral changes developed by the 3rd and persisted for another 7 days, indicating the development of stress adaptation. Following an IP injection of 5-methoxy-N,N-dimethyltryptamine (5-MeODMT), rats exposed to the stress for 10 days, but not for 5 days, displayed enhanced forepaw treading, tremor and Straub tail compared to control rats. These results suggest that the hypersensitivity of the 5-HT system after repeated stress may be in part related to the neuronal mechanism of stress adaptation. However, since hypersensitivity was not observed after exposure for 5 days, when adaptation was maximal, it is proposed that the 5-HT system may participate in the maintenance of adaptation rather than its development. On the other hand, no change in 5-HT1, 5-HT1a and 5-HT2 receptor binding assays was found after chronic stress, suggesting that the hypersensitivity of 5-HT system may not be accompanied with changes in the numbers of 5-HT receptor binding sites. The results of beta-adrenergic receptor binding determined simultaneously were also discussed with reference to previous reports of stress-induced reduction in beta-adrenergic receptor density.