Peptide transmitters: a unifying hypothesis for euphoria, respiration, sleep, and the action of lithium.

Actions of morphine include analgesia, sleep, euphoria, and depression of respiration. Transmitter or modulator substances in the brain that have actions similar to morphine may control these functions in man. This hypothesis proposes that enkephalin is a controlling neurotransmitter and its binding to opiate receptors determines mood state as well as influencing respiratory and sleep patterns. Lithium may act through modification of the opiate receptor affinity for an endogenous morphine-like substance. The theory predicts blocking action of naloxone in mania and in most drug-induced euphorias. It implies a new chemical pathophysiological basis for the phenomenology of mental illness.