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

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 less than 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 less than 0.05) in 4-month-old bulls than in bulls at 7, 8 and 9 months (0.92, 0.37, 0.31, 0.38 micrograms/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.(ABSTRACT TRUNCATED AT 250 WORDS)