EVIDENCE FOR GLYCINE MODULATION OF EXCITATORY SYNAPTIC INPUTS TO RETINAL GANGLION-CELLS

The actions of glycine on the NMDA receptor-mediated synaptic response s of ganglion cells were studied in the tiger salamander retinal slice , Ganglion cell excitatory postsynaptic currents (EPSCs) were elicited either by exciting bipolar cells with potassium puffs or by light sti mulation, and were measured using whole-cell patch-clamp techniques. I ncreasing bath glycine concentrations to 10 mu M had little effect on the amplitude of the puff-evoked EPSCs, indicating either that synapti c glycine concentrations were saturating or that the added glycine was buffered by uptake mechanisms. However, 5,7-dichlorokynurenic acid (5 ,7-DCK), an antagonist for the glycine site on the NMDA receptor, redu ced the ganglion cell responses to NMDA puffs, and reduced the potassi um puff- and light-elicited EPSCs. The IC50 values for 5,7-DCK became larger with increasing glycine concentrations, but not with increasing NMDA concentrations, indicating that 5,7-DCK acted at the glycine sit e. The IC50 values for 5,7-DCK were increased with stronger potassium puffs or light stimuli, suggesting that synaptic glycine levels increa sed with the strength of the stimuli. EPSCs measured in ON-OFF ganglio n cells at light ON and OFF were reduced by 5,7-DCK. For dim light sti muli, the IC50 values were lower for the OFF EPSCs compared to the ON EPSCs, indicating that glycine concentrations were different at the ti mes of OFF and ON activity. Estimates of synaptic glycine concentratio ns suggest that for dim light stimuli, concentrations at the OFF synap ses were not saturated, but concentrations at the ON synapses were sat urated.