Correlation between locomotor stimulation and the electrophysiological effects of low doses of morphine on substantia nigra dopamine neurons. I. Acute drug administration.

Parallel experiments were done to determine whether the behavioral effects of low doses of morphine correlated with changes in the electrophysiological activity of two subpopulations of mesencephalic dopamine neurons in rats. Acute administration of morphine sulfate (0.01 or 0.20 mg/kg i.v.) produced a naloxone antagonizable increase in locomotion and a corresponding increase in the discharge rates of Type A dopamine neurons. Morphine sulfate led to a relatively long-latency (120-200 sec) loss of spontaneous discharge activity in the majority of Type B dopamine neurons and this was blocked by a high dose of naloxone (5.0 mg/kg). Naloxone (0.10 or 5.0 mg/kg i.v.), combined with morphine, led to a paradoxical behavioral suppression. These data suggest that morphine produces opposite effects on two functionally distinct subtypes of neurons in the substantia nigra pars compacta and that behavioral output may reflect an interaction between these changes in discharge activity in mesencephalic dopamine pathways.