THE CONTRIBUTION OF GABA-MEDIATED INHIBITION TO RESPONSE PROPERTIES OF NEURONS IN THE NUCLEUS OF THE OPTIC TRACT IN THE RAT

The contribution of GABA-mediated inhibition to the generation of dire ctional selectivity of neurons in the nucleus of the optic tract (NOT) and the dorsal terminal nucleus of the accessory optic system (DTN) w as examined in anaesthetized rats by iontophoretic application of the GABA(A) receptor antagonist bicuculline methiodide. Spontaneous and vi sually evoked NOT-DTN cell activities were always increased by bicucul line application. The directional selectivity of NOT-DTN cells to slow ly moving whole-field stimuli, expressed as the direction index, was r educed for most neurons. However, the difference between firing rates during stimulus movements in the preferred and in the non-preferred di rection did not change systematically. On average, this difference was not significantly affected in the majority of the neurons, although b icuculline more strongly increased the activity during movement in the preferred or non-preferred direction in some of the neurons. These re sults indicate that directionally selective neurons in the rat NOT-DTN receive GABAergic inhibition which is most likely tonic and independe nt of the stimulus direction.