Roles of aspartate and glutamate in synaptic transmission in rabbit retina. II. Inner plexiform layer

Intracellular recordings were obtained from amacrine and ganglion cells in the superfused, isolated retina-eyecup of the rabbit. Both L-aspartate and L-glutamate displayed excitatory actions on the activity of the vast majority of amacrine and ganglion cells studied. However, these agents occasionally appeared to inhibit the responses of the inner retinal neurons by producing hyperpolarization of the membrane potential and blockage of the ligh-evoked responses. In either case, the effects of aspartate and glutamate were indistinguishable. The glutamate analogues kainate and quisqualate produced strong excitatory effects on the responses of amacrine and ganglion cells at concentrations some 200-fold less than those needed to obtain similar effects with aspartate or glutamate. The aspartate analogue, n-methyl DL-aspartate (NMDLA), also produced strong excitatory effects but was approximately three times less potent than kainate or quisqualate. On one occasion, we encountered a ganglion cell that was depolarized by kainate, but hyperpolarized by NMDLA. The glutamate antagonist α-methyl glutamate and the aspartate antagonist α-amino adipate effectively blocked the responses of amacrine and ganglion cells. However, on any one cell, one antagonist was always clearly more potent than the other. We examind the actions of the glutamate analogue 2-amino-4-phosphonobutyrate (APB) on the responses of inner retinal neurons and found that it selectively abolished all 'on' activity in the inner retina. These data support the hypothesis that on-bipolar cells subserve the 'on' activity of amacrine and ganglion cells. Our data suggest that aspartate and glutamate are excitatory transmitters in the inner retina, possibly being released from bipolar cell axon terminals in the inner plexiform layer.