Inward currents evoked by retinal stimulation in a ganglion cell were inhibited by glutamate antagonists. Stimulus pulses produced current artifacts denoted by vertical arrows. (A) Inward currents evoked by a current pulse of 10 milliseconds and 45 μA were blocked by the AMPA/KA antagonist NBQX (10 μM). However, as shown in the second row of A, this block was overcome by increasing the duration of the current to 20 milliseconds. After wash-out, the response to 10 milliseconds and 45 μA recovered (this response is duplicated and shown as the control response in B). (B) Application of the NMDA antagonist MK801 (100 μM) inhibited the response evoked by 10 milliseconds and 45 μA, but, as shown in the second row of B, this inhibition was overcome by increasing the current pulse to 20 milliseconds and 90 μA. Addition of NBQX in the continued presence of MK801 blocked the remaining inward current (lower right trace).