Summary. The pattern and opioidergic control of the secretion of gonadotrophins in prepubertal heifer calves were examined. Ten age-matched Hereford heifer calves were weighed and a blood sample was taken every 2 weeks from 2 to 25 weeks of age and then weekly until 60 weeks of age. At 60 weeks, a fertile bull was introduced and at 75 weeks of age pregnancy diagnosis was performed by transrectal ultrasonography. At 4, 12, 18, 24 and 32 weeks of age, the opioid antagonist naloxone was injected (i.v., n = 5; 1 mg kg−1 body weight) each hour for 12 h. Control heifers received sterile saline at the same ages. Blood samples were collected every 12 min for the 12 h treatment and serum samples were analysed for luteinizing hormone (LH) and follicle-stimulating hormone (FSH). Samples taken once every 2 weeks from 2 to 60 weeks were analysed for LH, FSH and oestradiol, and weekly samples were taken for progesterone determination. There was no effect of naloxone on the age at puberty, which was 56·2 ± 0·7 weeks at a body weight of 388·5 ± 8·0 kg. The mean age at conception was 63·4 ± 0·5 weeks. On the basis of samples taken every other week, serum concentrations of LH were high at 10 weeks and between 40 and 60 weeks of age. From the periods of intensive blood collection, the early rise in mean serum concentrations of LH appeared later at 12 and 18 weeks of age and was caused by a rise in LH pulse amplitude. Serum FSH concentrations were increased between 20 and 22 weeks and oestradiol concentrations at 22, 56 and 58 weeks of age. At 4 weeks, naloxone increased mean LH concentrations, pulse amplitude and pulse frequency (P < 0·01) and thereafter only decreased LH pulse amplitude at 18 weeks (P < 0·05) and increased LH pulse frequency at 24 weeks (P < 0·05). The FSH secretion was pulsatile at all ages and naloxone only increased FSH pulse amplitude at 4 weeks.
From these data we conclude that (i) there is an early transient increase in gonadotrophin secretion in prepubertal heifers, (ii) significant opioidergic inhibition of gonadotrophin secretion occurs only in very young heifers and (iii) a decrease in endogenous opioid inhibition of LH secretion, particularly LH pulse amplitude, allows for the early rise in LH secretion.
Keywords: puberty; gonadotrophins; naloxone; heifer