Endogenous morphines and pain.

The involvement of endogenous morphines (enkephalins and endorphins) in the regulation of pain is demonstrated by the following experimental evidence: (a) their analgesic activities; (b) their distribution in the central nervous systems; (c) the effects of their modifiers, especially of their antagonists, on nociceptive reactions and (or) on various types of analgesia; (d) rare modifications of their brain levels in pain and (or) analgesic states. Besides the well-known facts, the following items are particularly stressed: the functional roles of hypothalamic structures and of the pituitary, the effects of antagonists, the variety of analgesia following noxious and (or) stressful stimuli, genetic and environmental factors, endogenous antinociceptive substances other than opioids, relations with biogenic amines. As a whole, endogenous morphines apparently filter the particular important sensory input represented by nociception and control the reactions to pain, allowing for adjusted behaviour, if the stimuli are avoidable, or for prevention or at least delay of exhaustion if the stimuli are unavoidable.