The participation of substantia nigra zona compacta and zona reticulata neurons in morphine suppression of caudate spontaneous neuronal activities in the rat

Spontaneous, unitary activity from the caudate nucleus, substantia nigra zona reticulata and zona compacta were recorded extracellularly from lightly anesthetized rats. Intravenous injection of morphine (5 mg/kg) produced a depression of both caudate and reticulata discharges with a concomitant increase in the compacta activities. These effects were reversed by naloxone (1 mg/kg, i.v.). Microinjection of morphine (25, 50, and 100 μg) directly into the substantia nigra zona compacta induced a naloxone-reversible depression of caudate activity. Direct nigral application of strychnine (5 μg), a glycine antagonist, resulted in a moderate decrease in caudate activity and an increase in compacta discharges, but no appreciable effect on the reticulata neurons. Morphine (5 mg/kg, i.v.) further enhanced the strychnine effects on both caudate and compacta cells, while depressing the reticulata neurons. These results suggest that morphine may suppress caudate spontaneous activity via an activation of the nigrostriatal neurons and an inactivation of a novel striatonigral pathway. The latter may incorporate the reticulata neurons, which may exert a tonic inhibition on the compacta cells, possibly using glycine as the putative neurotransmitter.