Involvement of endogenous opioids in the regulation of LH and testosterone release in the male horse.

To investigate the involvement of endogenous opioids in the regulation of gonadotrophin release in male horses, effects of the opioid antagonist naloxone (0.5 mg kg-1 i.v.) on plasma LH and testosterone concentrations and the possible influence of season and of gonadal steroids were investigated. To determine quantitative as well as qualitative changes in gonadotrophin release, LH concentrations were measured by radioimmunoassay and by an in vitro bioassay. Experiments were performed in May, August and December. In stallions, basal LH secretion in May and August was significantly higher than in December (May versus December: P < 0.01; August versus December: P < 0.05); plasma testosterone concentrations were highest in August (August versus May: P < 0.05, August versus December: P < 0.001). The basal bioactive LH concentration and the ratio of bioactive:immunoreactive LH in stallions were highest in May. Therefore, in addition to seasonal changes in quantitative LH secretion, the bioactivity of LH in the circulation also undergoes seasonal variations. Bioactive LH concentrations and the bioactive:immunoreactive ratio in geldings were higher than in stallions. Naloxone caused a significant increase in LH release in stallions in August and December (P < 0.001); no significant increase was found in May (P = 0.06). In geldings, naloxone did not induce any changes in LH secretion; in stallions, a highly significant correlation was observed between basal testosterone concentrations and the LH increment after injection of naloxone (P < 0.001). In August and December, the bioactive:immunoreactive ratio increased significantly (P < 0.05) after injection of naloxone in stallions, indicating a preferential release of LH molecules with high bioactivity. The bioactive:immunoreactive ratio did not change after naloxone injection in May. The naloxone-induced LH release was followed by a significant increase in plasma testosterone concentrations in stallions in August (P < 0.001) and December (P < 0.05). In conclusion, endogenous opioid systems are involved in the regulation of LH and testosterone secretion in stallions. These mechanisms undergo seasonal changes: their activity is increased during winter and decreased during the breeding season. By affecting LH release, endogenous opioids, at least in part, regulate seasonal changes in reproductive activity in the stallion.