The synchronising influence of Substantia Innominata on the thalamus of the cat

We examined the stimulating effect of Substantia Innominata pars anterior ( SIa), during the waking state, on the 'central' part of the Mediodorsal nuc leus of the thalamus (MD), combining electrophysiological and anatomical te chniques in restrained, undrugged, unanaesthetized cats. Thalamic MD units were recorded, after electrical stimulation of the Substantia Innominata. a t 1 Hz, with a single pulse or short trains of four pulses. Responses were studied by poststimulus histograms. In about 64 of the 84 recorded MD neuro nes (76%), stimulation of the Substantia Innominata, during the waking star e. induced a brief cell excitation, followed first by prolonged inhibition of firing and then by a strong excitatory rebound discharge; after this com es a second sequence of inhibition and excitation, of decreasing amplitude. After stimulation of the Substantia Innominata, the MD units tended to sta rt a repetitive discharge at 4-7 Hz. To investigate the connections of Subs tantia Innominata cells upon the areas where MD units were recorded we inje cted horseradish peroxidase wheat germ agglutinin (WGA-HRP), combined with immunohistochemistry for glutamic acid decarboxylase (GAD) and choline acet yl transferase (ChAT). Of the total population of retrogradely labelled cel ls in the Substantia Innominata 53% were GAD positive while less than 16% w ere ChAT positive. The GAD positive MD-projecting cells in the Substantia I nnominata were triangular to fusiform and small to medium in size. These fi ndings indicate that GABAergic input from the Substantia Innominata may con tribute to increasing the hyperpolarizing inhibitory pressure on MD cells i n the 'central' part during slow wave sleep (SWS).