The Effect of Alterations in the Pituitary-Thyroid Axis on Hypothalamic Content and in Vitro Release of Somatostatin-Like Immunoreactivity*

Abstract

The role of somatostatin (SRIF) in the regulation of thyroid homeostasis was studied by measuring hypothalamic SRIF-like immunoreactivity (SRIF-LI) content and in vitro release in hypothyroid (T₃-treated and untreated), euthyroid, and T₃-treated normal rats. Hypothalamic SRIF-LI content was decreased in hypothyroid rats and was restored to euthyroid levels by T₃ treatment. SRIF-LI release from hypothalami of hypothyroid rats was decreased compared to that in euthyroid controls under basal and stimulated (60 mM K⁺ or 10^-6 M dopamine) in vitro conditions. The release of SRIF-LI from hypothalami of hypothyroid rats treated with T₃ was restored to euthyroid levels. The release of SRIF-LI from normal rat hypothalami was unaffected by TRH or TSH but was stimulated by T₃. These results suggest that T₃ exerts its negative feedback effect on pituitary TSH release via stimulation of hypothalamic SRIF release as well as by a direct pituitary effect, and that the elevated TSH levels seen in primary hypothyroidism may result in part from a decrease in the tonic inhibitory effect of hypothalamic SRIF. Pancreatic, but not antral or colonic, SRIF-LI was increased in hypothyroid rats, and the levels were reduced by T₃ treatment. The changes cannot be explained by alterations in food intake and may represent primary effects of the hypothyroid state.