Diurnal rhythm of plasma immunoreactive beta-endorphin and its relationship to sleep stages and plasma rhythms of cortisol and prolactin.

To determine the diurnal rhythm of plasma beta-endorphin (beta-End), 10 healthy male volunteers between the ages of 20 and 32 yr were studied in a sleep laboratory setting for a 24-h period. Blood samples were taken through an indwelling catheter at 0800, 1000, 1400, 1800, 2200, 2300, and 2400 h and then half-hourly until 0730 h, and they were then assayed for total beta-End-like immunoreactivity (ir beta-End), PRL, and cortisol. Subjects were habituated by spending the night before the study in the sleep laboratory with an indwelling catheter and electrodes for sleep recording in place. There was a clear diurnal variation of ir beta-End, with lowest levels between 2200 and 0330 h and highest levels between 0400 and 1000 h. There was no evidence for direct entrainment with sleep stage. There was a close correlation with cortisol levels, suggesting a similar secretory pattern for beta-End and PRL in only 5 of the subjects. Because the beta-End antiserum used has a cross-reactivity of 30% with beta-lipotropin gel permeation chromatography was done on extracts of plasma taken at 1400, 2400, 0400, and 0730 h for each subject. The peak in the beta-End elution position showed a clear diurnal variation similar to that of ir beta-End.