Effects of temperature and mode of delivery on responses to gonadotropin-releasing hormone by superfused frog pituitaries.

An in vitro superfusion system was used to examine the effects of temperature on the responsiveness of frog, Rana pipiens, hemipituitaries to chronic superfusion with gonadotropin-releasing hormone (GnRH). The effects of pulsatile as opposed to continuous delivery of GnRH on luteinizing hormone (LH) and follicle-stimulating hormone (FSH) secretion were also studied at 30 degrees. Secretion of both gonadotropins (FSH and LH) increased significantly in a dose-dependent manner in response to a brief exposure to 0.1-1000 ng/ml (nM) GnRH at both 18 and 32 degrees, but the magnitude of response at each dose was temperature dependent. Moreover, the temporal pattern of the chronic response to a standard dose of GnRH (100 ng/ml) also varied markedly between 10 and 32 degrees: Hemipituitaries superfused at 10 or 18 degrees showed greatly attenuated responsiveness to GnRH after the first hour of GnRH treatment, whereas responsiveness was maintained or even continued to increase for many hours at higher temperatures. These temperature effects may be related to seasonal changes in reproductive activity. Pulsatile administration of GnRH was more effective in inducing elevated gonadotropin (GtH) secretion during the first few hours of superfusion, but continuous superfusion with GnRH was more effective at maintaining elevated GtH levels over long periods. The results support and extend previous studies by indicating that, unlike the situation in many mammals and birds, the frog pituitary is highly resistant to desensitization by even high doses of GnRH, and in fact, self-priming and maintained responsiveness to GnRH is better supported by chronic treatment than by pulsatile delivery of GnRH.