The actions of opiates in the rat substantia nigra: An electrophysiological analysis

Single unit recording and micropressure ejection techniques were used to investigate the actions of opiates on dopaminergic and non-dopaminergic neurons in the rat substantia nigra. Systemic administration of morphine, 1 to 4 mg/kg, led to a naloxone-reversible increase in firing rate of all zona compacta dopaminergic (ZC) neurons examined (n=10). In a specifically defined subpopulation of non-dopaminergic nigral zona reticulata (ZR) neurons, systemically administered morphine led to a naloxone reversible decrease in activity (n=9). D-Ala²-d-leu⁵ (DADL)-enkephalin, when applied directly onto ZC neurons by micropressure ejection techniques, had no effect on their firing rate. In contrast, micropressure ejection of DADL enkephalin onto ZR neurons produced a decrease in firing rate which was blocked by systemically administered naloxone. Morphine sulfate applied by pressure ejection onto both ZC and ZR neurons produced mixed results which were not always blocked by naloxone. These results suggest that one of the mechanisms by which opiates increase dopaminergic neurotransmission is through disinhibition of dopaminergic neurons in the substantia nigra.