LIGAND-GATED CURRENTS OF ALPHA-GANGLION-CELLS AND BETA-GANGLION-CELLS IN THE CAT RETINAL SLICE

1. We studied the receptor pharmacology of the ligand-gated currents of ON- and OFF- alpha and beta ganglion cells in a cat retinal slice preparation using the whole cell recording variation of the patch-clamp technique. Cat retinal slices were cut in N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid (HEPES) buffer and incubated in a bicarbonate-buffered solution. Ganglion cells were voltage clamped at -70 mV in HEPES-buffered Ringer solution. The pipette solution contained a low concentration of Cl- to distinguish mixed cationic from Cl--mediated conductances, and Lucifer yellow (0.5%) was included for identification of the cell type. 2. In Ringer solution containing 1.2 mM Mg2+, current-voltage (I- V) curves of responses to the excitatory amino acid agonist (EAA) N-methyl-D-aspartate (NMDA) (200 mu M) revealed a J-shaped function. In Mg2+-free Ringer solution containing 200 mu M Cd2+ to block synaptic transmission, NMDA (200 mu M) elicited an inward current 5-8 times larger at -70 mV. In both conditions I-V curves of the NMDA-induced currents reversed near 0 mV. These results suggest that there are NMDA EAA receptors present directly on the dendrites of alpha and beta ganglion cells. Responses to NMDA were blocked by +/-2-amino-7-phosphonoheptanoic acid (AP7) (200 mu M). 3. In Ringer solution containing 200-1,000 mu M Cd2+ to block synaptic transmission, both ON- and OFF- alpha and beta cells responded to kainic acid (10-50 mu M), alpha-amino-3-hydroxy-5-methylisoxazole-4-proprionic acid (AMPA) (20-70 mu M), and quisqualic acid (0.1-30 mu M) with inward currents that reversed near 0 mV. These responses were blocked by the quinoxaline EAA antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) (10 mu M). The metabotropic agonists 1-aminocyclopentane-1,3-dicarboxylic acid (ACPD) (25 mu M) and L-2-amino-4-phosphonobutyric acid (L-APB) (50 mu M) in the presence of Cd2+ evoked little or no response for all cells tested. 4. In the presence of Cd2+, alpha and beta cells responded to gamma-aminobutyric acid (GABA) (200 mu M) and glycine (200 mu M) with inward currents that reversed near -35 mV, the calculated chloride equilibrium potential E(cl). Responses to GABA and glycine were both strongly desensitizing. (+)Bicuculline methyl chloride (20 mu M) blocked an average of 90% of the inward current evoked by 200 mu M GABA on all ganglion cell types. Baclofen (10-100 mu M) had no effect on resting currents or on voltage-gated Ca2+ currents. Strychnine (1 mu M) blocked glycine-evoked currents by an average of 94%. 5. We conclude that the excitatory and inhibitory ionotropic amino acid receptors present on ON- and OFF- alpha and beta cells are very similar. This suggests that some of the differences observed in the response properties of these cells may be due to different properties of their presynaptic inputs or differences in their voltage-gated channels.