Changes in beta-endorphin-like-immunoreactivity in the blood plasma of ewes during estrous cycle and in anestrous ewes under stress condition.

In the light of evidence for endogenous opioid peptide involvement in the modulation of pituitary hormone release and in the body's overall response to stress, the aim of the present study was to determine the beta-endorphin-like immunoreactivity (beta-END-LI) in the blood plasma of ewes during different phases of the estrous cycle and in anestrous ewes subjected to acute or intermittent prolonged footshocks. The highest concentration of beta-END-LI was found in the blood plasma during the luteal phase and lowest one during the follicular stage at proestrus. Nearly equal concentrations of beta-END-LI were determined at estrus and metestrus; their values were over 10 times lower than those observed in the luteal phase. Acute stress caused transient increase of beta-END-LI in the blood plasma of anestrous ewes, with a peak after 15-30 min of footshock application and changes in the molar ratio of beta-LPH to beta-endorphin. No apparent increase in mean daily levels of beta-END-LI was noted in ewes subjected to prolonged footshocking; however, on the day after stimulation beta-END-LI concentration decreased below control values. These results indicate that: 1. secretion of beta-END-LI varies during different phases of the estrous cycle, 2. acute stress is a potent activator for beta-END-LI secretion, 3. no apparent increase of beta-END-LI in the blood plasma of ewes subjected to prolonged stress concomitant with accumulation of this material in pituitary (Polkowska and Przekop, 1988) supports the idea, that prolonged stress augments the synthesis of beta-END-LI but not its release.