GABA acts on rod DBCs through two types of GABA receptors, GABAAR and GABACR, which are both chloride channels (e.g., Chávez et al., 2010, Lukasiewicz and Shields, 1998 and McCall et al., 2002). Therefore, we examined b-wave responses in mice in which individual GABA receptor expression was eliminated or pharmacologically blocked. We first analyzed GABACR knockout (GABACR−/−) mice ( McCall et al., 2002) and found that they display a phenotype strikingly similar to that of D1R−/− mice ( Figures

2A and 2B). GABACR−/− and D1R−/− mice had both a substantial reduction in dark sensitivity (∼40% and ∼55%, respectively) and a compression of the operational range. In contrast, blocking GABAAR-mediated input pharmacologically (there is no knockout available that removes all GABAARs RNA Synthesis inhibitor from DBCs) did not affect either dark sensitivity or operational range of rod-driven b-waves ( Figure 2B). These data reveal that GABACRs regulate the light sensitivity of rod-driven DBCs and raise the possibility that the effect of the D1R knockout may be explained by an altered GABACR-mediated input onto rod-driven DBCs. We also note that these GABACR-mediated

effects on rod DBCs cannot be explained by altered rod photoreceptor synaptic output because rods do not express GABACRs ( Enz et al., 1995). In reciprocal experiments, we measured ERG responses after intraocular injections of GABA (Figures 2C and 2D). As has been reported previously, GABA increased b-wave amplitudes (Naarendorp to and Sieving, 1991 and Robson et al., 2004). Despite this amplitude increase, GABA did not affect Ku-0059436 mw the sensitivity or operational range of b-wave responses in WT mice (Figure 2D). Although GABA injections into D1R−/− mice also increased b-wave amplitudes, in this case both the light sensitivity and operational range of b-waves were restored

to those observed in WT animals (these phenomena were phenocopied in WT mice after pharmacological block of D1R; Figure S1). These data show that the lack of D1R-mediated signaling can be completely masked by exogenous GABA, consistent with a role for D1R in modulating a GABAergic input onto rod-driven DBCs. Interestingly, intraocular injections of glycine, which normally provides lateral feedback onto rod DBC axons via chloride currents through glycine receptor channels, reduced the b-wave sensitivity functions in both WT and D1R−/− mice, rather than restoring the loss of sensitivity and operational range in the D1R knockout, as in the case of GABA injections into the eyes of D1R−/− mice ( Figure S3). This suggests that the GABAC-dependent mechanism revealed in this study is specific and implies a difference in ionic microenvironments surrounding GABACR and glycine receptors. Our hypothesis that D1R mediates a GABAergic input onto rod-driven DBCs predicts that pharmacological blockade of GABACRs should not further reduce b-wave sensitivity in D1R−/− mice.