Potentiation of retinal horizontal cell responses to L-glutamate by D-aspartate

Type H1 horizontal cells depolarize when either L-glutamate or L-aspartate is added at millimolar concentrations to Ringer superfused over the photoreceptor side of the isolated goldfish retina. When applied in the presence of D-aspartate, however, L-glutamate depolarizes H1 cells at nearly 15-fold lower concentrations. Despite the fact that L-aspartate and L-glutamate are equally capable of producing depolarizations of H1 cells when applied alone, the effects of L-aspartateare not potentiated by either D-aspartate or D-glutamate. Since D-aspartate seems also to enhance the effect of neurotransmitter released by cone photoreceptors, these results are consistent with the possibility that L-glutamate is released by cones as a neurotransmitters.