Similar to ON cells, OFF RGC light responses are AMPA and NMDA receptor dependent and, in some mammalian species, the NMDAR contribution is substantial (Manookin et al., 2010). ON cells, however, express a different subtype of NMDAR than OFF cells. Anatomical

and physiological evidence suggests that ON cells have a significant fraction of GluN2B-containing NMDARs, while OFF cell NMDARs are GluN2A containing (Kalbaugh et al., 2009; Manookin et al., 2010; Zhang and Diamond, 2009). Additionally, NMDAR activation may be differentially regulated at OFF synapses (Sagdullaev et al., 2006). To determine whether the AMPAR ratio in OFF cells is modulated by NMDARs, we began by measuring the RI of OFF cell light-evoked EPSCs. OFF PFT�� research buy cell EPSCs displayed a dependence on both CP-AMPARs and CI-AMPARs

in about an equal ratio selleck compound to that of ON RGCs (Figures 2A–2C; RI = 0.56 ± 0.073; n = 7). However, when we activated NMDARs on these cells, we found no change in the current at either +40mV or −60mV after 20 min. Accordingly, the I-V relationship or RI remained the same (RI = 0.57 ± 0.089; p = 0.57), indicating that AMPARs in OFF cells are not regulated by NMDARs in the same manner as those in ON cells. In summary, these data suggest that NMDAR-mediated AMPAR plasticity is exclusively expressed in synapses of the ON pathway. We examined whether the dichotomy between the ON and OFF pathways was preserved in cells that receive both types of inputs. ON-OFF cells form synapses with presynaptic bipolar cells in both the ON and OFF pathway. If there is a pathway-specific difference in the expression

of AMPAR plasticity, it should be possible to compare differences in the ON and OFF responses within these cells. Initially, the amount of rectification in the I-V relationship of the EPSC from both the ON and OFF pathway was nearly equal (Figure 3; n = 9; RI = 0.55 ± 0.7 and 0.50 ± 0.08, respectively; p = 0.62) and similar to that of ON and OFF RGCs. However, we found that activating NMDARs on these cells selectively modulated the ON response, while leaving the OFF response unchanged. The mean RI was reduced to 0.33 ± 0.06 (p = 0.0006) for ON responses but only to 0.48 ± 0.08 (p = 0.53) for OFF responses in the same cells. These data provide evidence for a pathway-specific PAK6 regulation of synaptic AMPARs and functionally distinct roles for NMDARs in the ON and OFF pathway. As AMPAR plasticity is expressed equally in the ON responses of both ON and ON-OFF cells, and as synapses in ON and OFF cells have been reported to be anatomically and physiologically similar to those in ON-OFF cells (Kalbaugh et al., 2009; Zhang and Diamond, 2009), we combined results from ON synapses of ON and ON-OFF cells throughout the rest of the study. Ca2+ influx is a common trigger of NMDAR-mediated synaptic plasticity (Bear and Malenka, 1994; Cull-Candy et al., 2006; Sun and June Liu, 2007).