Circadian variation of beta-endorphin-like immunoreactivity in neurointermediate pituitary.

Immunoreactive beta-endorphin-like activity (-IR) and ACTH-IR were determined in rat pituitary lobes and plasma at 4-hr intervals over a 24-hr period. In addition, we measured beta-endorphin-IR in discrete hypothalamic nuclei, and plasma corticosterone levels at the same time points. Significant and replicable circadian variation was detected only in concentrations of beta-endorphin-IR in neurointermediate pituitary and in plasma corticosterone concentrations.