Interactions between nutritional and opioidergic pathways in the control of LH secretion in male sheep

Abstract

Our aim was to determine the role of opioidergic processes in the effects of nutrition on the secretion of LH pulses in the mature male sheep. In the first of three experiments, adult Merino rams were acclimatised to a maintenance diet and then allocated to one of three dietary groups ( n = 5): continuation of the maintenance diet (Group M); reduction to half of the maintenance allocation (Group HM); or supplementation of the maintenance diet with lupin grain (Group HD). An initial administration of naloxone (2 mg/kg body weight, i.v.) was followed at 40-min intervals by three further administrations (1 mg/kg). Blood was sampled every 20 min for 12 h before the initial naloxone administration and then for a further 6 h. LH pulse frequency after naloxone treatment was significantly higher in Group HD than in Group HM ( P < 0.05). The second study tested whether the response to naloxone depended on calcium status. We used 22 adult Merino rams in two consecutive experiments, one in which the rams were fed a maintenance diet, and one in which the rams were fed with the maintenance diet plus 1 kg lupin grain for 5 weeks. In both experiments, rams were allocated to groups that received one of the following treatments: (a) 0.02 g/kg calcium borogluconate + 0.2 mg/kg naloxone hydrochloride (Nal + Ca 2+; n = 6); (b) 0.2 mg/kg naloxone hydrochloride (Nal; n = 6); (c) 0.02 g/kg calcium borogluconate (Ca 2+; n = 5); (d) 0.1 ml/kg NaCl 0.9% (Saline; n = 5). All treatments were given as a single i.v. administration daily for 5 days. Blood was sampled every 20 min for 24 h during the acclimatization period (Day 0) and on the last day (Day 5) of treatment. In the first study (under maintenance), none of the treatments affected LH pulse frequency. In the second study (the lupin-supplemented rams), LH pulse frequency was significantly increased ( P < 0.05) by the administration of naloxone + Ca 2+, naloxone alone and Ca 2+ alone. Overall, rams on a low plane of nutrition showed the smallest response to naloxone, suggesting that an opioidergic mechanism is not involved in the suppressive effect of restricted nutrition on the gonadotrophic axis. Rather, because testosterone secretion was increased on the high plane of nutrition, the LH responses to naloxone are better explained by the effects of testosterone on opioidergic mechanisms. Finally, we failed to observe any interaction between opioids and calcium in the control of LH secretion.