Seasonal Changes in the Relationships between Secretion of Gonadotropin-Releasing Hormone, Luteinizing Hormone, and Testosterone in the Ram1

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Abstract
In this study we examined the relationship between GnRH, LH, and testosterone (T) in intact rams during the nonbreeding season, the breeding season, and the transition between breeding and nonbreeding season. Blood was collected from the hypophyseal portal and jugular veins at 10-min intervals for 12 h from 3 groups of rams in May, September, and March. 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 (p < 0.04) and T (p < 0.001) pulse amplitude and on mean T concentrations (p < 0.001). LH pulse amplitude was highest in March, and T pulse amplitude was highest in September. Simple indices of pituitary and testicular responsiveness were obtained by calculating the ratios of LH to GnRH and T to LH. The ratio of LH to GnRH pulse amplitude was significantly higher in March than in September (p < 0.01) or May (p < 0.05). The ratio of T to LH pulse amplitude was highest (p < 0.01) in September. Release of LH in response to exogenous GnRH also varied significantly (p < 0.01) with month, being higher in March than in September (p < 0.01) or May (p < 0.05). There was no significant (p > 0.05) correlation between GnRH and LH pulse amplitude at any time of the year, but a significant (p < 0.05) correlation between LH and T pulse amplitude was observed in March and May. The amplitudes of individual GnRH, LH, or T pulses were not significantly correlated (p > 0.05) with either the interval from the previous pulse or the interval to the succeeding pulse. These results illustrate significant seasonal changes in the pattern of GnRH and LH pulses, in responsiveness of the pituitary to endogenous GnRH in the absence of observable changes in mean GnRH pulse parameters, and in responsiveness of the testis to LH.

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