Thyroid hormones regulate the expression of somatostatin receptor subtypesin the rat pituitary

Circulating TSH levels are increased in hypothyroidism and suppressed in hy perthyroidism. On the other hand, the hypothalamic hormone somatostatin sup presses basal and TRH-induced TSH release, an effect which is enhanced by t hyroid hormones. To investigate whether the effects of thyroid hormones on TSH secretion may be mediated in part through alterations in the gene expre ssion of pituitary somatostatin receptors (SSTR), 3-week-old male Sprague-D awley rats were rendered hypothyroid with antithyroid drugs for 3 weeks. To tal RNA extracted from anterior pituitaries were analysed for SSTR mRNA lev els, using Northern blot hybridization. Compared to controls, hypothyroid r ats had significantly lower pituitary mRNA levels of SSTR1 and SSTR2 (p < 0 .0001 for both, n = 16); the reductions could be prevented by T-4 supplemen tation (3 3 mu g/100 g body weight/day i.p.). In vitro studies using GH(4)C (1) rat pituitary cells showed that the addition of T-3 10(-8) M to cells c ultured in charcoal-stripped bovine calf serum resulted in significant incr eases in mRNA levels of SSTR1 (p < 0.0001; n = 7) and the two transcripts o f SSTR2 (p < 0.0005; n = 7). The increase for SSTR1 showed no further incre ase with higher doses of T-3, but was time-dependent and could be seen cons istently after 8 h of incubation. We conclude that thyroid hormones regulat e the gene expression of SSTR subtypes in the pituitary, via a direct actio n on anterior pituitary cells. Changes in SSTR gene expression may contribu te to the increase in circulating TSH levels in hypothyroidism.