Growth Hormone Secretion Induced by Thyrotropin-Releasing Hormone in Adult Chickens: Evidence of Dose-Dependent Induction of either Refractoriness or Sensitization

Abstract

Thyrotropin-releasing hormone (TRH) has been previously demonstrated to both stimulate growth hormone (GH) secretion and induce refractoriness to subsequent TRH provocation in vivo in chickens. Challenges with TRH at intervals of 1 h evoked different patterns of GH release in anesthetized adult chickens dependent upon TRH dosage. Initially, the administration of TRH at 10 µg/kg increased the plasma concentration of GH. However, the second and subsequent, through to the sixth challenges with TRH did not affect GH release. Plasma concentrations of GH were elevated by repeated injections of TRH (1.0 µg/kg). The magnitude of the response was reduced for the second and third challenges with full recovery of the GH response to the fifth challenge and an enhanced response to the sixth challenge. Similar GH secretory responses to TRH (at 0.1 µg/kg) were observed with the first through fourth challenge but the magnitude of response was again increased for the fifth and sixth challenge. At a lower dose (0.01 µg/kg), TRH did not initially affect plasma concentrations of GH. However, GH release was increased by the third challenge, and subsequent TRH administration evoked progressively larger increases in plasma concentration of GH. It would appear that only high doses of TRH induce refractoriness to subsequent challenge, while lower doses presensitize the pituitary to TRH. If TRH challenges at a dose which provokes the complete refractoriness (10 µg/kg) are administered at intervals of 2, 3 or 4 h, the GH secretory response is consistent for each challenge. The refractoriness to TRH is thus overcome within a period of 2 h.