Neural and neurochemical mechanisms of pain inhibition.

Recent studies which suggest the existence of an endogenous neural substrate of pain inhibition are reviewed. Electrical stimulation of some areas of the medial brain stem in laboratory rats has produced a dramatic degree of analgesia. Such stimulation-produced analgesia is partially blocked by an opiate antagonist drug. This observation has proved seminal to those searching for endogenous opiate-like neurochemicals, the opioid peptides (enkephalins and endorphins). Particular attention is focused on studies of stimulation-produced analgesia, the underlying anatomy and physiology of endogenous analgesia systems elucidated by such work, and the relation of these findings to discoveries about opiate receptors and opioid peptides. Very recent work suggesting that certain forms of stress are natural triggers for activating endogenous analgesia mechanisms is described. Although certain stressors cause analgesia mediated by opioids, other stressors cause analgesia by neurochemically different means. Thus, multiple analgesia substrates, opioid and nonopioid, appear to exist.