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 antago nist naloxone (0.5 mg kg(-1) i.v.) on plasma LH and testosterone conce ntrations and the possible influence of season and of gonadal steroids were investigated. To determine quantitative as well as qualitative c hanges in gonadotrophin release, LH concentrations were measured by ra dioimmunoassay 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 Decem ber: P< 0.01; August versus December: P< 0.05); plasma testosterone co ncentrations were highest in August (August versus May: P< 0.05, Augus t versus December: P< 0.001). The basal bioactive LH concentration and the ratio of bioactive: immunoreactive LH in stallions were highest i n May. Therefore, in addition to seasonal changes in quantitative LH s ecretion, the bioactivity of LH in the circulation also undergoes seas onal variations. Bioactive LH concentrations and the bioactive:immunor eactive ratio in geldings were higher than in stallions. Naloxone caus ed a significant increase in LH release in stallions in August and Dec ember (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 n aloxone (P< 0.001). In August and December, the bioactive:immunoreacti ve 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 af ter naloxone injection in May. The naloxone-induced LH release was fol lowed by a significant increase in plasma testosterone concentrations in stallions in August (P< 0.001) and December (P< 0.05). In conclusio n, 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 duri ng the breeding season. By affecting LH release, endogenous opioids, a t least in part, regulate seasonal changes in reproductive activity in the stallion.