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Release of GH is stimulated by TRH in chickens. However, for 60 min following a priming injection of TRH, a second injection of TRH is unable to provoke further GH release. TRH binds to the plasma membranes of the pituitary caudal lobe, in which somatotrophs predominate, although the magnitude of [³H]3-methyl-histidine²-TRH ([³H]Me-TRH) binding was reduced (by 25–50%) 30 min after the i.v. administration of a dose of TRH (5 µg/kg maximally effective in provoking GH release. A significant reduction in [³H]Me-TRH binding to caudal lobe membranes was also observed within 15 min of TRH administration and was maintained for at least 60 min. Control levels of [³H]Me-TRH binding were restored 2 h after TRH injection, coincident with the restoration of GH responsiveness to TRH challenge. The suppression of [³H]Me-TRH binding to pituitary membranes 30 min after in-vivo TRH administration was dose related, whereas the maximal (10 min) GH response to TRH was biphasic. The suppression of [³H]Me-TRH binding to chicken pituitary membranes was due to direct pituitary actions of TRH and could be induced by a 30-min exposure to 100 nM TRH in vitro.

These results demonstrate that avian pituitary TRH-binding sites differ greatly from mammalian ones in the timing of the onset and duration of down-regulation. Pituitary TRH-binding sites in birds are rapidly and transiently down-regulated following TRH administration in vivo, coincident with the period of GH refractoriness to TRH challenge.

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				Y.-M. Sun, R. P. Millar, H. Ho, M. C. Gershengorn, and N. Illing Cloning and Characterization of the Chicken Thyrotropin-Releasing Hormone Receptor Endocrinology, August 1, 1998; 139(8): 3390 - 3398. [Abstract] [Full Text] [PDF]