Differential effect of motor cortex stimulation on unit activities in the ventral posterior lateral thalamus in cats

Although motor cortex stimulation (MCS) is being increasingly used to treat chronic refractory neuropathic pain in humans, its mechanisms of action remain elusive. Studies in animals have suggested the involvement of subcortical structures, in particular, the thalamus. Most of these studies have been performed in rats, a species presenting significant differences in thalamic anatomy and function relative to primates, in particular, a very limited number of thalamic GABA interneurons. The aim of this study was to investigate the effects of MCS on single-unit activities of the thalamic ventral posterior lateral (VPL) nucleus in cats, which contains substantial numbers of GABA interneurons. Spontaneous and evoked activities of VPL units were studied before and after MCS. Motor cortex stimulation induced significant depression of the wide-dynamic-range (WDR) cells' firing rate, concomitant with activity enhancement of nonnociceptive (NN) units. More than half of WDR cells showed a significant decrease in the firing rate, while a similar proportion of NN units exhibited the opposite after-effect. Maximal firing attenuation of WDR cells occurred when the MCS location matched somatotopically their receptive field. Repetition of MCS runs led to an accentuation of WDR depression. After peripheral stimulation, evoked activity in each cell showed MCS effects similar to those observed in spontaneous activity. These data demonstrate a selective top-down inhibition by MCS of nonspecific nociceptive (WDR) cells, enhanced by somatotopic concordance and stimulation repetition, in parallel to facilitation of NN cells. These 2 outcomes may play a role in the complex analgesic effect of MCS observed in neuropathic pain conditions.