Effects of caudate nucleus stimulation on substantia nigra cell activity in monkey

The present study provides evidence that the saccadic signals in the caudate nucleus (caudate) are transmitted to the substantia nigra pars reticulata (SNr). We inserted two microelectrodes into the caudate and SNr of monkeys trained to perform saccade tasks. After identifying the functional characteristics of a SNr neuron recorded, we stimulated the caudate (single pulse, < 100 microA) to see whether its discharge rate changed. Among 138 SNr cells tested, 60 showed responses to stimulation of the caudate: inhibition only (n = 21), inhibition-excitation (n = 17), excitation only (n = 9), and excitation-inhibition (n = 13). The latencies were 9.0-32.5 ms (mean 16.7 ms) for the initial inhibitory responses and 6.5-35.0 ms (mean 16.7 ms) for the initial excitatory responses. Pars compacta cells (n = 10) were unresponsive. The effect of caudate stimulation was selective in terms of (1) functional type of SNr cells, (2) location of SNr cells, and (3) stimulation site within the caudate. Functional type of SNr cells: saccadic, visual, expectation-related cells were more responsive than auditory, mouth/hand/arm movement-related, and reward-related cells. Many of the cells whose functional characteristics were unidentified responded to the caudate stimulation. The preferential effects were seen among the functional subtypes: cells related to memory-guided saccades, not visually guided saccades; cells with conditioned visual responses, not simple visual responses. Location of SNr cells: the stimulus effects were seen preferentially in cells in the central part of the SNr, not in the dorsal part. Stimulus site: stronger effects, whether inhibition or excitation, were obtained when the stimulation was applied to the head-body transitional zone where visuooculomotor cells were clustered. Behaviorally contingent correlation of spike activity was found between the caudate-nigral pair of cells. For example when a SNr cell with memory-contingent saccadic activity was inhibited by the caudate stimulation, a caudate cell at or close to the stimulation site may show memory-contingent saccadic activity with a similar movement field.