[Endocrine function of the testicle in the Mexican crossbred ram at different times of the year and its opioidergic control during anoestrus].

This work was carried out with the objective of studying the endocrine function of the testes in the Mexican Crossbred Ram (Suffolk x Hampshire) during the different seasons of the year in a latitude of 19 degrees 13' N. Also studied was the opioidergic control of this gland during anoestrus. For this purpose 8 rams were used, from which 3 ml of jugular blood were obtained three times a week to study the seasonal fluctuations in plasma testosterone. The experiment was carried out from May 1993 until June 1994. The androgen was determined by radioimmunoassay. To study the opioidergic control of androgen secretion during seasonal anoestrous (May-June) four of the rams were selected at random and treated with 0.5 mg naloxone at 12 h intervals during 3 continuous weeks. It was observed that blood testosterone was lowest during long days, and concentration was highest during the breeding season. Chronic naloxone medication during anoestrus resulted in a significant increase in blood testosterone (P < 0.001), an effect that was evident after the first week of treatment. When naloxone administration was discontinued, blood testosterone levels returned to basal. With this experiment it was demonstrated that an opioid antagonist such as naloxone administered in low doses, is capable of altering plasma testosterone concentration in the anoestrus ram, giving further support to endogenous opioids as important modulators of reproduction in seasonal breeders such as sheep.