Serotonergic involvement in stress-induced ACTH release

We investigated the involvement of serotonin (5-HT) and 5-HT receptors in mediation of stress-induced ACTH secretion in adult male rats, which were pretreated by 5-HT antagonists before restraint-, ether-, cold swim-stress or endotoxin. All stressors potently increased plasma ACTH. Lesion of 5-HT neurons with 5, 7-dihydroxytryptamine injected intracerebroventricularly, into the paraventricular nucleus or into the raphe nuclei, inhibited the restraint stress-induced ACTH response by 50%. Restraint increased the content of 5-HT and its metabolite 5-hydroxyindole acetic acid, in the raphe nuclei, whereas the other stressors had no such effect. Pretreatment with the 5-HT1A receptor antagonist WAY 100635 inhibited the restraint stress- and endotoxin-induced ACTH secretion by 50%. The 5-HT1+2 antagonist methysergide or the 5-HT2 antagonist ketanserin inhibited the restraint- or ether stress-induced ACTH response, and eliminated the endotoxin-induced ACTH response. The 5-HT2 receptor antagonist LY 53857 blocked only the endotoxin-induced ACTH response. Pretreatment with the 5-HT3 receptor antagonist ondansetrone had no effect on stress-stimulated ACTH secretion. The 5-HT3+4 receptor antagonist tropisetrone inhibited the restraint- and ether stress-induced response. The ACTH response to swim stress was not affected by any of the antagonists used. It is concluded that the 5-HT1A, the 5-HT2A and the 5-HT2C receptor, but not the 5-HT3 receptor are involved in the stress-induced ACTH secretion. An involvement of the 5-HT4 receptor is possible. Furthermore, that serotonergic neurons in the raphe nuclei are activated during restraint stress, and that these neurons and neurons in PVN of the hypothalamus, are important for the mediation of the restraint stress-induced ACTH response.