Summary. Administration of naloxone (100 mg i.v.; approximately 1·21 mg/kg body weight0·75) to 10 intact calves (24 weeks of age) caused an acute release of LH that was similar in amplitude and duration to spontaneous discharges of LH that occur at the same age. The naloxone-induced release of LH was abolished in 9/10 calves (intact and castrated) treated with oestradiol-17β. To determine the ontogeny of opioid control of secretion of LH, 12 calves were randomly assigned to receive saline or naloxone (1·21 mg/kg body weight0·75, i.v.) at 3, 5, 7, 9, 11, 13, 17 and 21 weeks of age. At each age, blood was collected at 10-min intervals for 4 h and saline or naloxone was administered (i.v.) after collection of the 120-min sample. Before administration of naloxone, plasma LH values increased with age (P < 0·01) but did not differ between the control and naloxone groups (age × treatment, P > 0·05). Administration of naloxone caused concentrations of plasma LH to increase at 3, 11, 13, 17 and 21 weeks of age (treatment × time, P < 0·001). Concentrations of LH (saline vs naloxone, ng/ml) reached a maximum within 20 min after treatment at Weeks 3 (0·3 vs 1·2), 11 (0·6 vs 2·6), 13 (0·6 vs 3·7), 17 (1·1 vs 2·6), and within 40 min after treatment at Week 21 (1·0 vs 3·5). Based on these results, it is concluded that endogenous opioids inhibit the secretion of LH during infancy and the later prepubertal periods of life in bulls but not around the time of the spontaneous increase in secretion of LH that marks the end of the infantile period.
Keywords: naloxone; LH; puberty; bulls