Effects of hypothalamic paraventricular nucleus lesion on the cold-stimulated TSH responses to 5-HT in male rats.

The effects of lesion of the hypothalamic paraventricular nuclei (PVNx), the main thyrotrophic area, on the cold-stimulated thyrotropin (TSH) responses to intracerebroventricular (i.c.v.) 5-HT were studied in male rats. PVNx significantly attentuated the cold-stimulated TSH levels, but significantly affected neither hypothalamic thyrotropin-releasing hormone nor somatostatin content. Serum T3 levels were significantly decreased 8 days after PVNx. Irrespective of the lesion (sham or PVNx), 5-HT infusion (9 micrograms per rat) into the posterior third ventricle attenuated markedly the cold-stimulated TSH levels, whereas infusion into the anterior third ventricle did not. Bilateral 5-HT infusions (2 micrograms per side) into the hypothalamic dorsomedial nuclei significantly decreased serum TSH, but bilateral infusions into the posterior hypothalamic nuclei were without effect. Sham-lesion and PVNx decreased serum prolactin levels without affecting the stimulation of prolactin secretion by i.c.v. 5-HT. These results suggest that the inhibitory effect of i.c.v. 5-HT on TSH secretion and its stimulatory action on prolactin secretion are only partially dependent on the PVN.