Effects of thalamic midline nuclei stimulation upon lateral geniculate transmission in rats: inhibition of principal cells and excitation of internuncial cells.

Abstract

Effects of low-frequency stimulation of thalamic midline nuclei (MID) upon unitary activities of principal (P) and internuncial (I) cells of the lateral geniculate body were examined in urethane-anesthetized rats. P-cells responsed to single shocks to the optic tract (OT) with single spikes at short latencies (initial spike, IS), followed by a phase of inhibition which lasted for more than 150 msec and ended with a burst of grouped discharges (late discharge, LD). Excitability of P-cells, assessed with the response probability of IS to testing OT shocks, was reduced for about 300 msec after single shock stimulation of MID. Usually this excitability depression became evident about 20 msec after MID shocks and was maximum from 50 to 100 msec. LDs were made more vigorous when MID shocks were given almost simultaneously with testing OT shocks. This was interpreted that MID stimulation induced an inhibition in P-cells and it summed with that induced by OT stimulation. That the MID-induced inhibition of P-cells was due to IPSPs was proved by the quasi-intracellular recording. I-cells, the units fired repetitively by single OT shocks and have been presumed to be inhibitory neurons acting upon P-cells, were found to receive from MID stimulation an excitatory effect followed by an inhibitory one of long duration. It was suggested that the primary inhibition of P-cells by low-frequency stimulation of MID would be a direct consequence of the primary excitation of I-cells.