Effect of striatal stimulation on cellular activities of medial thalamic neurons studied in rats

Medial thalamic cells responded to stimulation of the striatum in either lightly anaesthetized or chronically implanted awake rats. Orthodromic and antidromic short-latency excitatory responses were associated with a pause in spontaneous activity followed by bursting activity. The thalamic distribution of the different types of response is given; the main nuclei involved were parafascicularis, centrum medianum, centralis lateralis, habenula and lateralis posterior. When observed with intracellular electrodes the pause in spontaneous activity was accompanied by a long hyperpolarisation. The possibility that this effect could be due either to inhibition or to disfacilitation is examined. Electrophysiological evidence is given to show that a direct striatothalamic pathway does not exist. Different relayed pathways which could be responsible for the observed responses are proposed and their possible role is discussed on the basis of anatomical and electrophysiological findings obtained in cats and rats. A role for a pathway making a cortical detour is suggested.