Alterations in 8-hydroxy-2-(dipropylamino)tetralin-induced neuroendocrine responses after 5,7-dihydroxytryptamine-induced denervation of serotonergic neurons.

In the present study, we examined denervation-induced changes in the sensitivity of hypothalamic postsynaptic serotonin1A (5-HT1A) receptor function with respect to changes in the dose-dependent elevation in plasma hormones [adrenocorticotropic hormone (ACTH), corticosterone, prolactin, oxytocin, prolactin, renin and vasopressin] by the 5-HT1A agonist 8-hydroxy-2-(dipropylamino)tetralin (8-OH-DPAT). Rats received intracerebroventricular (i.c.v.) injections of the serotonin neurotoxin 5,7-dihydroxytryptamine (5,7-DHT) or vehicle (0.1% ascorbate in saline) 3 weeks before challenge with increasing doses of 8-OH-DPAT (0, 10, 50 or 200 micrograms/kg s.c.). The effectiveness of 5,7-DHT-induced destruction of serotonergic neurons was confirmed by a 93% reduction in [3H]paroxetine-labeled 5-HT uptake sites in the hypothalamus. No changes in basal levels of ACTH, corticosterone, oxytocin, prolactin, renin and vasopressin were observed in rats that received i.c.v. 5,7-DHT injections. The dose-response curves for 8-OH-DPAT-induced elevations of plasma corticosterone and prolactin levels were shifted to the left in rats treated with 5,7-DHT, whereas no significant difference in the ACTH dose-response curve was observed between rats treated with vehicle and rats treated with 5,7-DHT. In contrast, the maximal oxytocin response to 8-OH-DPAT was attenuated in rats treated with 5,7-DHT. A 5,7-DHT-induced decline in the synthesis of oxytocin could explain this phenomenon. Although 8-OH-DPAT did not increase plasma levels of renin or vasopressin in rats treated with vehicle, 8-OH-DPAT produced an elevation (75%) in plasma renin concentration but not in vasopressin levels in rats that received i.c.v. injections of 5,7-DHT. No change was observed in [3H]8-OH-DPAT labeled 5-HT1A receptors in the hypothalamus. In summary, denervation of hypothalamic serotonergic nerve terminals produces supersensitivity of some neuroendocrine responses to 8-OH-DPAT independent of changes in the density of hypothalamic 5-HT1A receptors.