Moreover, our data suggest that rod DBCs take a 2-fold advantage from maintaining large chloride gradients. The well-established role of this gradient is to enable strong, stimulus-dependent, transient GABAergic

feedback inhibition from amacrine cells (Chávez et al., 2010 and Tachibana and Kaneko, Tyrosine Kinase Inhibitor Library 1987), which adjusts the amplitude and kinetics of rod DBC light-evoked or electrically evoked responses (Eggers and Lukasiewicz, 2006 and Roska et al., 2000). We now argue that the same chloride gradient also sensitizes their light responses via small sustained currents. Interestingly, the same chloride channel, GABACR, is used in both cases (though GABAAR is used for the dynamic feedback as well), which requires the transient GABACR-dependent current

mediating the dynamic feedback to be significantly larger than the sustained current. This is entirely consistent with observations learn more made by us and by others (Naarendorp and Sieving, 1991 and Robson et al., 2004) that increasing extracellular GABA by intraocular injections increases rod DBC light-response amplitudes, indicating that GABA is bound only to a fraction of GABACRs in the dark. Another point raised in our study relates to the cellular origin of the dopamine-dependent GABA release. The light dependency of GABA staining in horizontal cells abolished in D1R−/− mice makes these cells a potential candidate. Horizontal cells have long been known to contain GABA ( Figure S4; FKBP Deniz et al., 2011, Guo et al., 2010, Schwartz, 1987, Vardi et al., 1994 and Wässle and Chun, 1989), but the role of GABA release from horizontal cells, at least for the rod circuit, remains poorly understood. For instance, the recently reported inhibitory feedback from these cells onto rod terminals does not appear to rely on GABA ( Babai and Thoreson, 2009). Horizontal cells display the strongest D1R immunostaining in the mouse retina ( Figure 1E) and express D1R in close proximity to the processes of dopaminergic amacrine cells ( Figure S4). The hyperpolarizing light responses of horizontal

cells are also known to be regulated by dopamine via D1-type receptors ( Hankins and Ikeda, 1994, Knapp et al., 1990, Mangel and Dowling, 1985 and Yang et al., 1988). Furthermore, depolarization of horizontal cells favors GABA release in isolated cells ( Schwartz, 1987), and dopamine, acting via D1R, shifts the membrane potential of horizontal cells to more depolarized values ( Hankins and Ikeda, 1994). Combined with the observation that dendrites of rod DBCs have robust GABACR-mediated currents, these properties of horizontal cells allow the following interpretation of our GABA immunostaining data. We suggest that horizontal cells in D1R−/− mice release less GABA than horizontal cells in WT mice under all illumination conditions used in our study.