IN-VITRO THYROTROPIN-RELEASING ACTIVITY OF CORTICOTROPIN-RELEASING HORMONE-FAMILY PEPTIDES IN COHO SALMON, ONCORHYNCHUS-KISUTCH

Investigations of hypothalamic regulation of fish thyrotropin (TSH) se cretion and subsequent thyroid activity have been impeded by the lack of a reliable assay for TSH. Using a recently developed radioimmunoass ay (RIA) for coho salmon TSH we employed an in vitro pituitary cell cu lture technique to examine regulation of TSH secretion by corticotropi n-releasing hormone (CRH) family peptides [ovine CRH (oCRH), carp urot ensin I (UI), and frog sauvagine (SV)] as well as thyrotropin-releasin g hormone (TRH), salmon growth hormone-releasing hormone (sGHRH). and salmon gonadotropin-releasing hormone (sGnRH). At concentrations of 0. 01 to 100 nM, TRH, sGHRH, and sGnRH did not stimulate TSH secretion fr om coho salmon pituitary cells. However, at these same concentrations, both oCRH and SV caused a significant and concentration-dependent inc rease in TSH secretion; whereas, UI was highly stimulatory at all conc entrations tested. In a related experiment we examined the effect of a lpha-helical CRF(9-41) on oCRH-stimulated TSH release by pituitary cel ls. alpha-Helical CRF(9-41) is an analogue of CRH that has been shown by others to antagonize the adrenocorticotropic hormone (ACTH)-releasi ng activity of CRH in goldfish. Preincubation of cells with 1 mu M alp ha-helical CRF(9-41) for 4 h caused a significant suppression of the T SH-releasing activity of oCRH at 1.0 and 10 nM concentrations. The res ults of these experiments demonstrate the potency of a CRH-like peptid e in the hypothalamic regulation of TSH in fish and reveal similaritie s in the inhibition of the response of both the thyroid and interrenal axis of fish to alpha-helical CRF(9-41). (C) 1998 Academic Press.