The relationships between gonadotropin-releasing hormone, luteinizing hormone, and testosterone secretion in the male sheep

Two separate studies were conducted to elucidate the relationships between gonadotropin-releasing hormone (GnRH), luteinizing hormone (LH), and testosterone (T) in the ram. In the first study, the relationships between GnRH, LH, and T secretion were examined in intact rams in May (during the non-breeding season), September (breeding season), and March (transition between breeding and non-breeding season). A very high coincidence between GnRH and LH pulses was observed in the ram across the season. Individual LH pulses in May and September occurred at relatively regular intervals, whereas in March groups of pulses were separated by relatively long intervals without pulses. No significant effect of season on GnRH pulse parameters was found, but there was a significant effect of season on LH and T pulse amplitude and on mean T concentrations. LH pulse amplitude was highest in March and T pulse amplitude was highest in September. The ratio of LH/GnRH pulse amplitude was significantly higher in March than in September or May. The ratio of T/LH pulse amplitude was highest in September. Release of LH in response to exogenous GnRH also varied significantly with month, being higher in March than in September or May. These results illustrate significant seasonal changes in responsiveness of the pituitary to endogenous GnRH in the absence of observable changes in GnRH pulse parameters and significant seasonal changes in responsiveness of the testis to LH.In the second study, using combination of intravenous infusion (to mimic the pulse) with subcutaneous implantation of T (to mimic the basal level), the effectiveness of a constant versus a pulsatile pattern of T in suppressing gonadotropin secretion was compared in castrated rams. Mean LH was suppressed more by constant T than by pulsatile T. LH inter-pulse interval was increased more by constant T than by pulsatile T. However, both constant and pulsatile patterns of T failed to affect follicle-stimulating hormone (FSH) secretion. These results provide, for the first time, evidence that a constant pattern of T is more effective than a quasi-physiological pulsatile pattern of T in suppressing LH secretion in the ram.U of I OnlyETDs are only available to UIUC Users without author permissio