Opioidergic regulation of gonadotrophin secretion in the early prepubertal bull calf

Abstract

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.(ABSTRACT TRUNCATED AT 250 WORDS)