OPIOIDERGIC REGULATION OF GONADOTROPIN-SECRETION IN THE EARLY PREPUBERTAL BULL CALF

The control of gonadotrophin secretion during the early developmental period in the bull is poorly understood, opioidergic regulation may be involved. The opioid antagonist naloxone was administered to bull calves (i.v. n = 5; 1 mg kg-1 body weight h-1) for 12 h at 4, 12, 18, 24 and 32 weeks of age. Control animals (n = 5) received sterile saline solution. During the treatment period, 4 ml blood samples were collected every 12 min. The calves were also weighed and blood samples collected every two weeks from 2 to 48 weeks of age. Scrotal circumference was measured from 20 to 48 weeks of age to determine age at puberty. Serum samples were assayed for concentrations of LH, FSH, testosterone and oestradiol. On the basis of blood samples taken every two weeks, serum concentrations of LH increased to a peak at six weeks of age and then decreased; serum concentrations of FSH reached a peak at 20 weeks of age. Serum concentrations of testosterone and oestradiol increased with scrotal circumference from 20 to 48 weeks of age. On the basis of blood samples taken every 12 min for 12 h, we concluded that the early rise in serum LH concentrations was due to an increase in LH pulse frequency. LH pulse amplitude decreased from 4 to 18 weeks of age and FSH pulse amplitude from 4 to 32 weeks of age. Naloxone treatment increased LH pulse frequency at 4 and 24 weeks of age but not at 12, 18 or 32 weeks of age. LH pulse amplitude was depressed and mean serum concentrations of LH were increased by naloxone at 4 weeks of age. At 12 weeks of age, LH pulse amplitude and mean and basal serum LH concentrations were increased by naloxone. At 18 weeks, only mean concentrations of LH were increased by naloxone. The only effect of naloxone on FSH secretion was to increase FSH pulse amplitude at 24 weeks of age. We conclude that there is opioidergic inhibition of LH secretion in the young bull calf, but that between 12 and 18 weeks of age a decrease in opioidergic inhibition on LH pulse frequency contributes to the overall increase in mean circulating concentrations of LH at this time.