The influence of GABAergic inhibitory processes on the receptive field structure of X and Y cells in cat dorsal lateral geniculate nucleus (dLGN).

Visually elicited inhibitory processes, underlying the receptive field structure of cells in layers A and A1 of the cat dorsal lateral geniculate nucleus (dLGN), have been examined by a combination of visual neurophysiological and iontophoretic techniques. Discrete visual stimulation of both centre and surround mechanisms, produced a powerful suppression of the elevated background discharge levels induced by iontophoretic application of an excitatory amino acid. These observations are consistent with the activation of a postsynaptic inhibitory input, a view supported by the fact that the suppressive effects were blocked by iontophoretic application of bicuculline, an antagonist of GABA, a putative inhibitory transmitter in the dLGN. These inhibitory effects were always elicited by the opposite phase of a flashed stimulus to that eliciting responses associated with the receptive field region. That is 'on' inhibitory effects were elicited from 'off' excitatory regions and 'off' inhibitory effects from 'on' excitatory regions. Plotting the responses of dLGN cells to flashed stimuli of increasing diameter, before and during iontophoretic application of bicuculline, revealed a marked reduction of the surround antagonism of centre response in the presence of the drug. During bicuculline application, surround antagonism of centre responses was at a level associated with that seen in retinal ganglion cells. Annular stimuli of internal and external diameter selected to be just outside the centre-surround border, insofar as they gave pure surround responses, were observed in the presence of bicuculline to elicit strong centre responses, as well as surround responses. These observations indicate that GABAergic inhibitory processes generate the enhanced centre-surround antagonism associated with dLGN cells and serve to increase the contrast between the two sets of mechanisms at the centre-surround border. The present conclusions apply equally to 'X' and 'Y' cells.