Evidence of gonadal steroid-independent changes in activity of the central LH-releasing hormone pulse generator in developing bull calves

Abstract

ABSTRACT To ascertain whether temporal changes in activity of the hypothalamo-pituitary axis in prepubertal bulls may occur independently of shifts in sensitivity to steroid feedback, the acute post-castration rise in serum gonadotrophins was monitored in bull calves castrated at monthly intervals from 4 to 9 months of age. Since a major feature of the gonadotrophin profiles of developing bulls is a change in LH pulse frequency early in life, pulsatile LH secretion after castration was used as an index of activity of the central LH-releasing hormone (LHRH) pulse generator. Relative to the day of castration (day 0) bull calves (n = 4) were bled at 20-min intervals for 8 h on day −3 and at 10-min intervals for 4 h on days 3, 5 and 7. During the first week after castration, 4-month-old bulls showed a higher (P<0·05) frequency of LH pulses compared with bulls at 8 and 9 months (1·13, 0·88 and 0·75 pulses/h respectively; pooled s.e.m.= 0·13). Mean LH levels before castration were higher (P<0·05) in 4-month-old bulls than in bulls at 7, 8 and 9 months (0·92, 0·37, 0·31, 0·38 μg/l respectively; pooled s.e.m. = 0·12). After castration mean LH levels did not differ with age. Mean FSH levels did not differ among age groups either before or after castration. Increased serum LH levels in 4-month-old bulls confirmed the transient rise in LH secretion that occurs at this time in developing bull calves. Age-related differences in LH pulse frequency observed after castration suggested that in prepubertal bulls changes in activity of the central LHRH pulse generator can occur independently of steroid feedback mechanisms. J. Endocr. (1986) 111, 67–73