24-hour pattern of immunoreactive beta-endorphin and adrenocorticotropic hormone (ACTH) in rat plasma and tissues.

The levels of beta-endorphin and adrenocorticotropic hormone (ACTH) were measured by radioimmunoassays in rat plasma, pituitary, hypothalamus and pineal at different times of the day. Significant diurnal variations in the levels of immunoreactive beta-endorphin were detected in the plasma and hypothalamus. Immunoreactive ACTH levels varied significantly during the darklight cycle in the posterior lobe of the pituitary, and in the hypothalamus, but not in the plasma. The hormone levels were high during the dark, except in the posterior pituitary, where the highest levels were detected during the light period. Gel chromatography showed that both beta-endorphin and ACTH immunoreactivities in plasma and tissues consist of multiple components.