Diurnal Rhythm of Plasma Immunoreactive β-Endorphin and Its Relationship to Sleep Stages and Plasma Rhythms of Cortisol and Prolactin*

Abstract

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 [prolactin], 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 ACTH. There was significant correlation of levels of ir .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.