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.
Biology, Animal Physiology, Biology, Veterinary Science
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign