Growth hormone release in the rat: effects of somatostatin and thyrotropin-releasing factor

Morphine sulfate (MS) and pentobarbital (PB) stimulate growth hormone (GH) release in the rat in vivo, but not from enzymatically dissociated anterior pituitary cells in vitro. Somatostatin and thyrotropin-releasing factor (TRF) inhibit the in vivo release of GH induced by MS, with 50% inhibition at ca. 2.3 and 4.6 mug/100 g BW, respectively. Somatostatin and TRF similarly inhibit PB-induced GH release. Prostaglandin E2 stimulates GH release both in vivo and in vitro. Both of these responses are inhibited by somatostatin (50% inhibition at ca. 10 mug/100g BW), but neither is altered by TRF (100 mug/100g BW). Both normal and hypophysectomized rats receiving MS exhibited a rapid vibration of the tail immediately after administration of TRF (30 mug/100g BW). Structural analogs of TRF with low TSH-releasing activity did not inhibit GH release nor induced tail vibration in MS-treated rats. Pyroglutamyl-3-methyl-histidyl-prolinamide, with 8 times the hypophysiotropic potency of TRF, is similarly more potent than TRF in inhibiting GH release and inducing tail vibration in MS-treated rats. These results suggest the following: 1) MS and PB act at a central nervous system (CNS) site to release GH; 2) TRF may act at a CNS site to inhibit MS- and PB-induced GH release; 3) somatostatin has direct pituitary effects on inhibition of GH, but a CNS site of action cannot be excluded; and 4) TRF stimulates tail motor activity in MS-treated rats.