Studies on the Mechanism of Growth Hormone and Thyrotropin Responses to Somatostatin Antiserum in Anesthetized Rats*

An iv administration of 1 ml sheep antiserum to somatostatin (anti-SS) resulted in marked increases of both serum GH and TSH, with a peak 10--20 min after administration in male rats anesthetized with urethane or pentobarbital. Administration of anti-SS had no effect on serum PRL. Ablation of the basal medial hypothalamus abolished the rises of both serum GH and TSH after anti-SS administration. Intravenous injection of 1 ml rabbit antiserum to TRH (anti-TRH) decreased serum TSH levels 15 min after injection, whereas injection of normal rabbit serum did not affect TSH levels. Serum TSH levels did not rise after injection of anti-SS in rats pretreated with anti-TRH. On the other hand, pretreatment with anti-TRH did not affect the basal serum GH levels nor the anti-SS-induced GH release. The enhanced secretion of GH and TSH after anti-SS injections was not blocked by pretreatment with indomethacin, an inhibitor of prostaglandin synthesis. The following conclusions were made: 1) both GH and TSH responses to anti-SS require an intact basal medial hypothalamus; (2) TSH response to anti-SS is mediated by hypothalamic TRH; and 3) the GH response may be mediated by hypothalamic GH-releasing hormone which is not TRH or prostaglandins.