Site-dependent action of intracerebroventricular 5-hydroxytryptamine on the cold-stimulated thyrotropin secretion in male rats.

The effects of central 5-hydroxytryptamine (5-HT) infusions on the cold-stimulated thyrotropin (TSH) levels were studied in male rats. Stainless-steel cannulas were implanted stereotaxically into the anterior or the posterior third ventricle or just lateral to the hypothalamic paraventricular nuclei bilaterally 7 days before experiments. Infusion of 5-HT (4.5 and 9 micrograms/rat) into the posterior third ventricle attenuated significantly the cold-stimulated TSH levels. Inversely, infusion of 5-HT (9 micrograms/rat) into the anterior third ventricle augmented significantly the TSH cold response. Bilateral 5-HT infusions into the vicinity of the hypothalamic paraventricular nuclei did not affect the TSH cold response. Serum prolactin levels increased significantly after 5-HT administration into the anterior and the posterior third ventricle, but no consistent effect on growth hormone (GH) levels could be detected. Infusion of 5-HT into the anterior and the posterior third ventricle decreased body temperature irrespective of the observed hormonal response to 5-HT. The results are in favor of a dual and possibly site-dependent role for 5-HT in the regulation of the cold-stimulated TSH secretion in the rat. The opposite effects of 5-HT on the TSH cold response may result from the predominant inhibition of either the thyrotropin-releasing hormone or the somatostatin-secreting cell groups.