Dopaminergic inhibition from substantia nigra of caudate neurons activated by cortical stimulation

Studies were performed to elucidate the role of dopamine originating in the pars compacta of the substantia nigra (SN) on caudate nucleus (CN) neurons receiving input from the motor cortex using cats anesthetized with alpha-chloralose. Activation with cortical stimulation was observed in 50 CN neurons, five out of which also excited by SN stimulation. Conditioning stimuli applied to the SN, 30 msec preceding the test stimulus to the cortex, produced a significant inhibition of spike generation upon cortical stimulation in 28 out of 45 neurons tested and did not affect the remaining 17 neurons. The mean spike number of the 45 neurons was significantly reduced with SN conditioning stimulation. When dopamine up to 200 nA was iontophoretically applied to CN neurons, there was a significant inhibition of the spike generation with cortical stimulation in 9 out of 16 neurons. The mean spike number of the 16 neurons upon cortical stimulation was significantly reduced during dopamine application. In addition, a close correlation was observed between the effects of SN conditioning stimulation and iontophoretic dopamine. These results suggest that dopamine derived from the SN produces an inhibition of CN neurons receiving input from the motor cortex.