SEVERAL TYPES OF CA2-1-IMMUNOLABELED RAT RETINAL GANGLION NEURONS( CHANNELS MEDIATE GLUTAMATERGIC SYNAPTIC RESPONSES TO ACTIVATION OF SINGLE THY)

dissociated cell culture from the postnatal rat retina was established to characterize the synapses formed by retinal ganglion neurons (RGNs ) in vitro. An antibody against Thy-1.1 was used to preselect putative RGNs for pair patch-clamp recording with the principal aim of identif ying the released transmitter(s) and estimating the role of different types of voltage-activated Ca2+ channels in evoked transmitter release . The population of Thy-1+ neurons was heterogeneous. Staining pattern s, soma-dendritic geometries and axon length displayed variations that could be related to basic electrophysiological properties, such as am plitudes of voltage-activated Na+ currents (I-Na(V)), action potential size and capacity for repetitive discharge. Out of 73 coupled connect ions, 33 pairs were glutamatergic. With no exception, these connection s were formed by the axons of strongly labeled Thy-1+ neurons with lar ge I-Na(V) (typically >2 nA) and repetitive firing over a broad curren t range, Such neurons were classified as RGNs, Forty out of 73 coupled pairs were GABAergic. These connections were always formed by weakly stained Thy-1+ neurons with small I-Na(V) (typically <2 nA) and very l imited capacity for repetitive discharge. Such neurons were tentativel y classified as displaced amacrine cells. Evoked EPSCs in response to RGN activation were completely blocked by low concentrations of Cd2+ o r Gd3+, omega-CgTx-GVIA (5 mu M) reduced EPSCs to 67 +/- 29%, omega-Ag aTx-IVA (200 nM) had no effect, and nifedipine (15 mu M) enhanced the evoked EPSCs. Our experiments indicate that (1) the transmitter releas ed by RGNs is glutamate and (2) the major part of synaptic glutamate r elease is governed by a novel toxin-resistant Ca2+ channel. The result s further suggest that the characteristic phenotype of RGNs is well ma intained in dissociated cell culture, In conjunction with electrophysi ological tests Thy-1+ labeling can be used for RGN identification.