The role of endogenous opioid peptides in physiological and pharmacological reward responses--a survey of present-day knowledge.

Endogenous opioid peptides (EOP) participate in a variety of physiological and pharmacological responses that can be recognized by: increased concentrations of EOP in the cerebrospinal fluid; a combination of euphoria and analgesia; and suppression of the responses by specific opiate antagonists. Application of these responses is exemplified in acupuncture, in self-stimulation via hypothalamic electrodes, and in food and fluid consumption. For example, intake of sweet solutions exerts a biphasic effect: in the beginning pain threshold is elevated, but later on tolerance to morphine-induced analgesia develops. A supply of 2% NaCl also stimulates fluid intake as well as raising the pain threshold. Stress-induced analgesia is due to coupled release of ACTH and beta-endorphin from the pituitary. In man, large variations in pain sensitivity may be related to the level of intrinsic EOP.