Changes in the beta-endorphin content of discrete hypothalamic nuclei during the estrous cycle of the rat

Concentrations of beta-endorphin were measured in discrete brain nuclei of 4-day cycling rats on each day of the estrous cycle. On the afternoon of proestrus beta-endorphin levels were significantly higher in median eminence and suprachiasmatic nucleus, and lower in arcuate nucleus, when compared to levels found on other days of the cycle. These changes coincided with the peak of plasma prolactin, which was blocked by prior administration of naloxone.