Plasma beta-endorphin, pain thresholds and anxiety levels across the human menstrual cycle

Nine normally cycling women and seven other women employing oral contraception were tested during five phases (menstrual, follicular, ovulatory, luteal and premenstrual) of their menstrual cycle. The procedure consisted of administration of an anxiety inventory and determination of pain detection and pain thresholds in response to electric shock and the cold pressor task. Venipunctures were also performed and the plasma of normally menstruating women later assayed for beta-endorphin. Analyses revealed that the variance but not the mean levels in peripheral beta-endorphin levels significantly differed (p less than 0.01) across the menstrual cycle with the greatest amount of variance found during the ovulatory phase and the least during the luteal phase. The high variance during the period around ovulation was due to several subjects having extremely elevated beta-endorphin levels which possibly may have resulted from the occurrence of ovulation. Furthermore, a significant positive correlation between anxiety levels and beta-endorphin levels was found only during the menstrual phase. The absence of findings concerning cyclic variation in pain thresholds is contrary to earlier reports and indicates that such a phenomenon may be dependent upon the paradigm employed.