Increase of beta-endorphin concentrations in the plasma and pituitary neuro-intermediate lobe of the rat on the afternoon of proestrus.

Beta-endorphin (beta-EP) concentrations in the plasma and the anterior and neuro-intermediate lobes of the pituitary (AP and NIL) were quantitated by radioimmunoassay (RIA) and gel filtration chromatography in female rats at 1000, 1400, and 1900 h on the day of proestrus and diestrus day-1. There were no significant changes in beta-EP in the plasma, AP, or NIL on diestrus day-1. On proestrus, beta-EP in the plasma and NIL, but not the AP, increased significantly from 1000 to 1400 h and returned to basal levels by 1900 h. The time course of this increase of beta-EP in the NIL and plasma is consistent with the temporal sequence of the prolactin and gonadotropin surges on the afternoon of proestrus, suggesting that beta-EP in the NIL may be involved in the regulation of these neuroendocrine events.