Seasonal changes in the endocrine responsiveness of the pituitary and testes of male sheep in relation to their patterns of gonadotropic hormone and testosterone secretion

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Abstract
Pituitary and testicular endocrine responses to exogenous gonadotropin releasing hormone (GnRH) and luteinizing hormone (LH), respectively, were assessed for adult rams in an investigation of the regulation of seasonal changes in the patterns of episodic LH and testosterone secretion. Concurrent variations in testis size and in circulating levels of follicle stimulating hormone (FSH) and prolactin (PRL) were also examined. On 10 occasions throughout the year, serum hormone levels were assessed over 6- to 8-h periods during which time rams were left untreated (day 1) or were injected (iv) with single doses of either 10 μg synthetic GnRH (day 2) or 30 μg NIH-LH-S18 (day 3); blood samples were collected from the jugular vein at 10- or 20-min intervals. Testicular redevelopment during the summer, as indicated by increasing testis diameter measurements, was associated with increases in mean FSH level and was preceded by a springtime rise in mean PRL level; "spontaneously" occurring LH pulses and those produced in response to GnRH treatment were relatively large during this period. Increases in the magnitude of testosterone elevations in response to both endogenously and exogenously produced LH pulses occurred in August. Mean testosterone levels were elevated fourfold in the fall as a consequence of relatively frequent and small LH pulses stimulating a more responsive testis to produce more frequent and larger testosterone elevations; endogenous LH pulses, however, did not appear to stimulate the testes maximally at this time. Testicular regression occurred during the winter months and was characterized initially by decreases in testosterone levels without obvious changes in the pattern of pulsatile LH release and subsequently by a decrease in testicular responsiveness to relatively infrequent LH pulses which led to further decreases in testosterone levels and smaller testes. Results demonstrate that alterations in pituitary and testicular responsiveness to stimulatory hormones account in part for seasonal changes in the magnitude of LH and testosterone secretory pulses in adult rams.