Circadian, circhoral and seasonal variation in patterns of gonadotrophin secretion in geldings.

Blood samples were obtained from 5 mixed-breed, long-term castrated geldings during five 24-h periods between May 1984 and April 1985. Blood samples were collected, beginning at 09:00 h, at 15-min intervals for 8 h and hourly for the remaining 16 h. Plasma concentrations of LH and FSH were determined by RIA. Seasonal changes in hormone concentrations and frequency and amplitude of secretory pulses were evaluated. No diurnal variation in either LH or FSH secretion was observed: however, marked circhoral fluctuations in LH and FSH secretion were noted. Mean LH and FSH concentrations in these long-term castrated geldings were comparable to those measured in intact stallions. The mean LH concentration significantly varied with time of year, with a peak value in late April and a nadir in November. FSH pulse frequency also significantly varied with season, with a peak in August and a nadir in May. The amplitude of secretory episodes of LH and FSH did not fluctuate with season. We conclude that: (1) the long-term castrated gelding does exhibit pulsatile gonadotrophin secretion, and (2) gonadotrophin secretion is subject to seasonal variation in the absence of gonadal steroids.