Aspartate may be an excitatory transmitter mediating visual excitation of "sustained" but not "transient" cells in the cat retina: iontophoretic studies in vivo.

Although the excitatory amino acids, aspartate and glutamate are present in large quantities in the layers of the mammalian retina where the bipolar and amacrine cells make contact with the retinal ganglion cells, it was not known whether these amino acids are the actual neurotransmitters which excite the retinal ganglion cells. To answer this L-aspartate, L-glutamate and the recently discovered powerful and selective antagonist for the N-methyl-D-aspartate receptor, 2-amino-5-phosphonovalerate, were applied iontophoretically to the "sustained" and the "transient" classes of retinal ganglion cells in the optically intact eye of anaesthetised cats. The visually-driven excitation of all "sustained" cells was significantly suppressed by 2-amino-5-phosphonovalerate, whereas that of "transient" cells was not. L-aspartate enhanced the visually-driven excitation and increased the spontaneous firing rare of all "sustained" cells but not of "transient" cells and these effects were blocked by 2-amino-5-phosphonovalerate. The results with L-glutamate were inconclusive. It is suggested that L-aspartate may be an excitatory transmitter mediating the visual response at the receptor field centre of "sustained" retinal ganglion cells, but that excitation of "transient" retinal ganglion cells is mediated by a different transmitter.