In addition to distinctions in timing, ON RGCs tend to receive more excitation than inhibition and OFF RGCs more inhibition than excitation. Similar patterns of synaptic inputs to ON and OFF RGCs are elicited by light stimulation in mature retinal circuits (Murphy and Rieke, 2006 and Pang et al., 2003) and

differences PD-1/PD-L1 inhibitor 2 in excitation/inhibition ratios of ON and OFF RGCs persist after photoreceptor degeneration (Margolis et al., 2008 and Yee et al., 2012). This suggests that key circuits in the inner retina, particularly those mediating ON-to-OFF crossover inhibition, are established prior to vision, maintained following its loss, and play an important role in patterning both spontaneous Lumacaftor solubility dmso and light-evoked RGC activity. Because inhibition stereotypically precedes excitation to OFF RGCs during stage III waves and light-evoked spike trains of OFF RGCs are shaped by disinhibition (Manookin et al., 2008 and Murphy and Rieke, 2006), we tested the contribution of postinhibitory rebound to the delayed bursting

of OFF RGCs (Figure S1). Unlike in mature OFF RGCs (Margolis and Detwiler, 2007), we found that rebound depolarizations following somatic current injections rarely elicited spikes at P11–P13 and observed no differences in the intrinsic excitability of ON and OFF RGCs (Myhr et al., 2001). With the caveat that somatic current injections may not adequately capture the influence of dendritic 4-Aminobutyrate aminotransferase inhibition (Gidon and Segev, 2012), we therefore conclude that offset excitatory synaptic inputs account for the sequential spiking of ON and OFF RGCs. Asynchronous excitation of RGCs suggested

that ON and OFF CBCs, which provide input to ON and OFF RGCs, respectively, participate differently in stage III waves. Indeed, we found that during the ON phase of each wave ON CBCs depolarize, whereas OFF CBCs hyperpolarize (Figure 2). In conjunction with the timing of RGC EPSCs, these data imply that OFF CBCs release glutamate as their voltage returns to baseline following transient hyperpolarizations. The ability of CBCs to continuously vary neurotransmission as a function of voltage relies on the specialized release machinery of ribbon synapses (Matthews and Fuchs, 2010). The importance of ribbon synapses to stage III waves is underlined by the observation that these waves first appear as synaptic ribbons are being assembled in the IPL (Fisher, 1979), a period that is predated by conventional glutamate release from CBCs (Johnson et al., 2003). The mechanisms by which an OFF CBC’s return to baseline voltage without appreciable overshoot (Figure 2) is translated into a phasic EPSC in an OFF RGC are discussed in the supplement (Supplemental Discussion).