Next, we will address how excitation spreads laterally between ON CBCs to provide insight into the mechanisms that initiate and propagate stage III waves. Voltage-clamp recordings showed that ON CBCs fall into two groups that receive excitatory input via distinct mechanisms. In one
group (II), stage III waves as well as focal glutamate applications on axon terminals appear to activate cation-nonselective conductances (Figures 6 and 7). In vision, ON CBCs hyperpolarize to glutamate release from cones via dendritically Bortezomib localized mGluRs coupled to Trpm1 channels (Koike et al., 2010, Masu et al., 1995 and Morgans et al., 2009). Based on our results, the most parsimonious conclusion is that during development group II ON CBCs express iGluRs on their axon terminals. While expression of these receptors needs to be confirmed and is likely transient, there is some evidence that even in mature circuits a subset of ON CBCs may utilize iGluRs (Kamphuis et al., 2003 and Pang et al., 2012). Wave-associated
spillover of glutamate into the extrasynaptic space (Blankenship et al., 2009 and Firl et al., 2013) combined with the expression of axonal http://www.selleckchem.com/products/VX-770.html iGluRs would provide a direct excitatory link between neighboring ON CBCs. In another group (I) of ON CBCs, gap junctions mediate depolarizations during waves and in response to focal glutamate applications (Figures 6 and 7). Which neurons form these gap junctions with ON CBCs? Candidates have to depolarize during the ON phase of stage III waves and be activated by glutamate. In mature circuits, ON CBCs are known to couple to AII ACs, which express iGluRs (Hartveit
and Veruki, 1997, Kolb and Famiglietti, 1974, Mills and Massey, 1995, Veruki and Hartveit, 2002 and Zhou and Dacheux, 2004). In addition to participating in visual processing, these electrical connections are involved in the generation of patterned spontaneous activity in retinas with Ketanserin photoreceptor degeneration (Borowska et al., 2011 and Trenholm et al., 2012). Among the diffuse ACs we recorded, four were morphologically identified as AII ACs. Each depolarized during the ON phase of stage III waves. Aside from excitatory coupling to ON CBCs, AII ACs likely participate in glycinergic crossover inhibition of OFF CBCs and OFF RGCs. In addition to AII ACs, ON CBCs may be coupled to other ACs (Farrow et al., 2013) and/or each other (Arai et al., 2010). Thus, gap junctions provide a second lateral excitatory link, either direct or via intermediate ACs, among ON CBCs. Both forms of excitatory input are recruited by glutamate released from ON CBCs, which we propose forms the basis for the generation and coordinated lateral propagation of stage III waves.