Caffeine stimulates beta-endorphin release in blood but not in cerebrospinal fluid.

Plasma beta-endorphin and prolactin profiles were obtained from groups of unstressed, adult male rats. The infusion of caffeine (20 mg/kg) via a chronic, indwelling intra-atrial cannula results in a prompt and sustained (2-2.5 h) rise In plasma beta-endorphin levels. The infusion of the opiate antagonist naloxone causes a modest (40%) decrease in plasma beta-endorphin and blunts the elevation in plasma beta-endorphin following caffeine administration. In contrast, plasma prolactin levels were unchanged following caffeine administration and were decreased by treatment with naloxone. Caffeine treatment did not effect CSF beta-endorphin levels or the release of beta-endorphin from hemipituitaries incubated in vitro.