Modulation of single cell responses and neuronal interactions produced by iontophoresis of glutamate in adult cat visual cortex.

Single neuron responses and interneuronal coordinations after microiontophoretic injections of L-glutamic acid (glutamate) were examined in cats striate cortex with an aid of multielectrode extracellular recording and crosscorrelation analysis. The effects of 15 min long stimulation with glutamate were compared with spontaneous mutability observed within at least 1.5 h. A mutability coefficient was suggested for quantitative data analysis. Spontaneous changes were analyzed in 144 neuronal pairs in two anesthetized and seven pretrigeminal cats. Differences in spontaneous mutability between these groups were not significant. Fluctuations of crosscorrelograms were smaller than fluctuations of poststimulus time histograms (PST). The type of correlogram and receptive field preferred orientation and direction never changed. The effect of glutamate injection was studied in 157 neuronal pairs in four anesthetized and eight pretrigeminal cats. Excitatory responses were observed in 69% of neurons, inhibitory in 18%. The excitatory response to glutamate was accompanied with enhanced visual response in 47% of cases and with reduced visual response in 18%. Crosscorrelogram strength was reduced in 43% of pairs with excitatory response to glutamate and increased only in 23%. After the termination of iontophoresis a persistent increase of the shared input coordination was observed in two pairs in anesthetized animals and in three pairs in pretrigeminal cats. The appearance of direct excitatory coordination was found in one case in pretrigeminal cats. Alterations of PST histograms did not exceed the range of spontaneous mutability. We infer that in a small percent of neuronal pairs the short glutamate stimulation can affect interneuronal links even in adult cat.