Serotonin and tryptophan hydroxylase in isolated hypothalamic and brain stem nuclei of rats exposed to acute and repeated immobilization stress.

The effect of acute and repeated immobilization stress on serotonin concentration and tryptophan hydroxylase activity in some isolated hypothalamic and brain stem nuclei was measured using a special microdissection technique and sensitive isotopic-enzymatic microassays. In acutely immobilized rats serotonin concentration was increased in the median eminence, ventromedial and dorsomedial nuclei. In repeatedly immobilized rats increased serotonin concentration was recorded in the dorsomedial nucleus immediately after seven, and in the median eminence after forty consecutive daily exposures to immobilization. Decreased tryptophan hydroxylase activity was found in the suprachiasmatic nucleus after seven exposures to immobilization stress. Acute and repeated immobilization stress failed to produce any changes of serotonin concentration in the isolated brain stem nuclei studied, and of tryptophan hydroxylase activity in the dorsal raphe nucleus and n. centralis superior. The increased tryptophan hydroxylase activity observed without any changes in serotonin concentration in the locus coeruleus after the 7th immobilization may suggest an increased synthesis and release rate of serotonin in serotonergic nerve terminals in this area. The changes of serotonin concentration in some hypothalamic nuclei under the first exposure of rats to stress indicate the involvement of serotonin in the activation of the pituitary-adrenocortical system as well as in other neuroendocrine reactions initiated in the hypothalamus during acute stress. On the basis of the results presented, the presumed role of the serotonergic system in the regulation of pituitary-adrenocortical stress response in repeatedly stressed rats has not been established. The reported response of brain tryptophan hydroxylase to the release of endogenous corticosterone could not be confirmed in our experiments.