COMPLEX ACTIONS OF SEROTONERGIC AGONISTS ON COLD-STIMULATED TSH SECRETION IN MALE-RATS

The effects of serotonergic activation on cold-stimulated thyrotropin (TSH) and prolactin secretion were studied in male rats. Peripheral injections of both 8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT), a 5-HT1A receptor agonist, and 1-(3-chlorophenyl)piperazine (m-CPP), a 5-HT1 agonist, decreased TSH levels. The action of 8-OH-DPAT was antagonized by (±)-pindolol, which is known to have 5-HT1 antagonist activity, but not by metergoline or ketanserin. The action of m-CPP was antagonized by ketanserin but not by metergoline. TSH levels were not affected by a 5-HT3 receptor agonist, 2-methyl-5-HT, or by a 5-HT3 antagonist, MDL 72222. Infusion of 8-OH-DPAT into the anterior third ventricle increased TSH levels; 5-HT tended to increase TSH levels, but the effect was not significant. Inversely, infusion of 5-HT, 8-OH-DPAT or m-CPP into the posterior third ventricle decreased TSH levels. The action of 5-HT was counteracted by metergoline, ketanserin and (±)-pindolol. Unexpectedly, m-CPP infusion into the anterior third ventricle also inhibited TSH secretion. The prolactin-elevating effects of 5-HT, 8-OH-DPAT and m-CPP were neither consistent nor site-specific. In conclusion, stimulation of both 5-HT1 and 5-HT2 receptors may inhibit TSH secretion, but the exact mechanism underlying the site-dependent action of 5-HT and 8-OH-DPAT on TSH secretion remains to be identified. © 1990.