Abstract
Retinal ganglion cells (RGCs) receive glutamatergic input from bipolar cells through NMDA- and AMPA-type glutamate receptors. Both GluA2-containing, Ca(2+)-impermeable AMPA receptors (CI-AMPARs) and GluA2-lacking, Ca(2+)-permeable AMPA receptors (CP-AMPARs) contribute to light-evoked responses in ON RGCs; however, specific roles for each subtype are not well understood. Here, we present evidence that light intensity determines the subtype of AMPAR that is activated during the synaptic response in ON RGCs. Using current voltage analysis of the EPSC we show that light intensities near RGC threshold, intensities that travel through the well described primary rod pathway, evoke synaptic currents that are preferentially mediated by CP-AMPARs. Synaptic responses evoked by spontaneous release of transmitter from bipolar cell terminals also preferentially activate CP-AMPARs. Conversely, higher light intensities, most likely carried by secondary rod pathways, activate CI-AMPARs. The same pattern of CP-AMPAR and CI-AMPAR activation was observed in mice containing only functional rods, suggesting that the recruitment of CI-AMPARs at higher light intensity does not require cone stimulation. When glutamate spillover was induced by blocking transporters with TBOA, both the near threshold and spontaneous EPSCs contained a significant CI-AMPAR component. We propose that CI-AMPARs are activated by "spillover" of synaptic glutamate only during bright illumination, or when glutamate uptake is blocked. Glutamate may spill over to more distant sites at the same synapse, or perhaps as far as neighboring synapses. Together, our data suggest that the spatial organization of AMPARs at ON RGCs synapses allows for selective, intensity-dependent activation of AMPARs with distinct subunit composition.