Role of the entopeduncular nucleus in caudate nucleus-induced suppression of intralaminar thalamic unit responses in the cat

Suppression by caudate nucleus stimulation of nonspecific sensory responses in the intralaminar thalamus could potentially be mediated by either of two major pathways, through the entopeduncular nucleus or through the pars reticulata of the substantia nigra. The role of the entopeduncular nucleus was investigated in the chloralose-anesthetized cat by (i) comparison of the effects of conditioning stimuli, applied to either the entopeduncular nucleus or the caudate nucleus, on somatic-evoked single-unit responses in the intralaminar thalamus and (ii) acute interruption of caudate nucleus outflow pathways. Entopeduncular stimulation duplicated caudate-induced suppression of intralaminar responses, invariably at lower stimulus thresholds, and, unlike caudate stimulation, often evoked excitatory responses preceding suppression. Lesions of the entopeduncular nucleus greatly attenuated caudate-induced suppression, whereas selective interruption of the caudatonigral pathway had no significant effect. Thus, caudate-induced suppression of intralaminar responses is mediated primarily by the entopeduncular nucleus and may involve activation of entopeduncular efferent fibers which directly or indirectly inhibit intralaminar neurones. These findings suggest that differential behavioral functions may be subserved by caudate outflows through the entopeduncular nucleus and substantia nigra.