The actions of
glycine on the
NMDA receptor-mediated synaptic responses 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 stimulation, and were measured using whole-cell patch-clamp techniques. Increasing bath
glycine concentrations to 10 microM had little effect on the amplitude of the puff-evoked EPSCs, indicating either that synaptic
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, reduced the
ganglion cell responses to
NMDA puffs, and reduced the
potassium 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 site. The IC50 values for 5,7-DCK were increased with stronger
potassium puffs or light stimuli, suggesting that synaptic
glycine levels increased with the strength of the stimuli. EPSCs measured in ON-OFF
ganglion cells at light ON and OFF were reduced by 5,7-DCK. For dim light stimuli, the IC50 values were lower for the OFF EPSCs compared to the ON EPSCs, indicating that
glycine concentrations were different at the times of OFF and ON activity. Estimates of synaptic
glycine concentrations suggest that for dim light stimuli, concentrations at the OFF synapses were not saturated, but concentrations at the ON synapses were saturated.