Further studies of serotonergic activity in the regulation of the cold stimulated thyrotropin secretion in male rats.

Serotonergic agonists and antagonists were used to study the role of 5-HT in the regulation of thyrotropin (TSH) secretion in male rats. When given peripherally, the agonists like 5-HT, quipazine and m-chlorophenylpiperazine (mCPP) decreased dose-dependently the cold-stimulated TSH-secretion. The action of 5-HT was antagonized by metergoline but not by ketanserin. The effect of quipazine was counteracted by both antagonists. Small intraperitoneal doses of ketanserin seemed to be stimulatory on the TSH secretion while high doses of both ketanserin and metergoline clearly decreased the cold-stimulated TSH levels. Infusion of quipazine into the 3rd ventricle inhibited significantly the TSH cold-response whereas 5-HT and mCPP did not. The action of quipazine was only partially antagonized by ketanserin pretreatment. 5,7-dihydroxytryptamine (5,7-DHT) was used to make serotonergic lesions into different regions of the brain. The TSH lowering effects of intraperitoneal quipazine and mCPP were potentiated in the rats lesioned in the 3rd ventricle or bilaterally into the posterior hypothalamus but not in the rats lesioned bilaterally into the anterior hypothalamus. Hence serotonergic activity in the vicinity of the posterior part of the 3rd ventricle seems to decrease the cold-stimulated TSH secretion. Both 5-HT1 and 5-HT2 receptors appear to participate in this regulation.