Short-loop inhibition of thyrotrophin-releasing hormone-induced growth hormone secretion in fowl

The i.c.v. administration of 0.1 or 10 micrograms ovine (o)GH to 12- to 16-week-old hypothyroid chickens of a sex-linked dwarf (SLD) strain suppressed the basal plasma GH concentrations, measured 24 h afterwards. The GH response of the oGH-injected SLDs to TRH was suppressed, in a dose-related way, in comparison with that induced by TRH in birds given control injections (10 micrograms) of bovine serum albumin (BSA). Basal circulating concentrations of GH in euthyroid K strain birds of the same age were even lower than in the SLDs following injection of 10 micrograms oGH, and were not further reduced by oGH administration. The GH response to TRH in the K strain birds injected i.c.v. with 0.1 or 10 micrograms oGH was, nevertheless, suppressed in comparison with the BSA-injected K strain controls. The i.c.v. administration of oGH also suppressed circulating concentrations of LH and the LH response to TRH in the K strain birds. Twenty-four hours after i.c.v. administration of oGH (10 micrograms), the somatostatin (SRIF) content in the medial basal hypothalamus of 8-week-old euthyroid cockerels was greater than that in BSA (10 micrograms)-injected controls. At the same time, the binding of [3H]3-methyl-histidine2-TRH to the pituitary caudal and cephalic lobes of GH-injected birds was less than that in the controls. These results suggest that GH regulation in avian species is partly mediated by an inhibitory short-loop mechanism (mediated by hypothalamic SRIF and a down-regulation of pituitary TRH-binding sites) that suppresses basal and secretagogue-induced GH release.