, 2000; see also Li et al., 2008), Vardi and colleagues subsequently attributed this result to cross-reactivity of the “T4” antibody and showed that NKCC1 found in the outer plexiform layer of the retina is expressed in horizontal cells (Zhang et al., 2007). The lack of NKCC1 in rod DBCs is also consistent with the demonstration that the chloride reversal potentials are the same at the dendritic and axonal ends of rod DBCs in retinal slices (Satoh et al., 2001), which argues against chloride transport in opposite

directions at these locations. Based on this evidence, we predicted that inactivation of KCC2 in WT mice should shift the chloride equilibrium potential to a more positive value and reduce the amplitude of DBC light response and the ERG b-wave. Indeed, intraocular injections of a KCC2 blocker reduced dark sensitivity, operational range, learn more and Rmax,dark of WT rod-driven b-waves (Figure 3 and Figure 4E), closely resembling reductions seen in GABACR−/− mice. Furthermore, the effects of the KCC2 blockade were see more not restored

by exogenous GABA, which is consistent with a disrupted chloride gradient. Another prerequisite for our hypothesis is the existence of a tonic hyperpolarizing GABA-mediated current. GABACRs are particularly well suited for this function due to lack of GABA-dependent desensitization (Amin and Weiss, 1994). Tonic GABACR-mediated currents have been observed in goldfish bipolar cell terminals (Hull et al., 2006 and Jones and Palmer, 2009) but have not been observed previously in mammalian rod DBCs. To directly document this current, we conducted patch-clamp recordings from rod DBCs in retinal slices. We supplemented our slices with a low concentration of exogenous GABA (5 μM) in the bath to replace GABA lost by perfusion, following the experimental paradigm reviewed in Glykys and Mody (2007). These experiments revealed the presence of a tonic GABAergic current of ∼7 pA in WT rod DBCs that was antagonized by TPMPA (Figures 5A and 5B). This current was absent in GABACR−/− rod DBCs. We next tested whether this GABACR-mediated sustained current could change the rod DBC Purple acid phosphatases resting membrane potential. When we measured the resting membrane potential in

current clamp with zero holding current, TPMPA depolarized the resting potential of WT rod DBCs but had no effect on GABACR−/− rod DBCs ( Figure 5C). This tonic GABACR-mediated conductance is expected to change the input resistance of WT rod DBCs and reduce the degree of depolarization caused by light-dependent synaptic inputs. To test this idea, we determined the input resistance in WT rod DBCs by measuring voltage changes in response to current injection in the presence or absence of TPMPA. As predicted, blocking GABACRs in WT rod DBCs increased the slope of the current-voltage (I-V) plot and, consequently, the rod DBC input resistance (Figures 5D and 5E). In contrast, TPMPA did not significantly alter the input resistance of GABACR−/− rod DBCs.