Attenuation of pain reactivity by caudate nucleus stimulation in monkeys

The effect of caudate nucleus stimulation on reactivity to painful stimuli was investigated in Macaca speciosa monkeys chronically implanted with electrodes in the right caudate nucleus. The force with which subjects escaped from electrocutaneous leg shock was used as a measure of pain reactivity and was decreased by caudate stimulation. Escape threshold and latencies were not influenced by the brain stimulation. Decreased escape force was obtained only when 50 msec trains of caudate stimulation preceded 20 msec trains of leg shock by 0-100 msec. Pain reactivity was not affected if brain stimulation followed leg shock or if leg shock followed brain stimulation by more than 100 msec. Intershock response distributions indcated that direct motor inhibition was not responsible for the depression of escape force, and the effectiveness of a restricted range of caudate-leg stimulation intervals ruled out generalized effects on arousal. The results indicate that the effect of caudate stimulation is to reduce the affective components of pain elicited by noxious electrocutaneous stimuli. The time course of this caudate effect parallels that previously reported for the caudate-induced depression of evoked activity in the non-specific somatosensory projections of the reticular formation and thalamus.