Seasonal changes in the relationships between secretion of gonadotropin-releasing hormone, luteinizing hormone, and testosterone in the ram

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).(ABSTRACT TRUNCATED AT 250 WORDS)