Actions of narcotics on brain dopamine metabolism and their relevance for "psychomotor" effects

A review is given about the effects of narcotic analgesics, particularly of morphine, on the dopamine metabolism in the corpus striatum and about the relations of these effects to motility and "psychomotor" phenomena. In rats, acute doses of morphine decrease the dopaminergic neurotransmission in brain, without blocking postsynaptic dopamine receptors. Chronic treatment of rats with morphine reverses these acute effects of morphine and induces symptoms of an increased dopaminergic neurotransmission in brain. In mice and cats on the other hand, acute doses of morphine apparently increase dopaminergic neurotransmission. The effects of morphine on striatal dopamine metabolism seem to be a model well suited to study opioid-specific effects on a cellular level. Furthermore, they might also be responsible for some narcotic-specific effects on behaviour observed in animals and man.