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Retinal amacrine cells express nitric oxide (NO) synthase and produce NO, making NO available to regulate the function of amacrine cells. Here we test the hypothesis that NO can alter the GABAergic synaptic output of amacrine cells. We investigate this using whole cell voltage clamp recordings and Ca2+ imaging of cultured chick retinal amacrine cells. When recording from amacrine cells receiving synaptic input from other amacrine cells, we find that NO increases GABAergic spontaneous postsynaptic current (sPSC) frequency. This increase in sPSC frequency does not require the canonical NO receptor, soluble guanylate cyclase, or presynaptic action potentials. However, removal of extracellular Ca2+ and buffering of cytosolic Ca2+ both inhibit the response to NO. In Ca2+ imaging experiments, we confirm that NO increases cytosolic Ca2+ in amacrine cell processes by activating a Ca2+ influx pathway. Neither the increase in sPSC frequency nor the cytosolic Ca2+ elevations are dependent upon Ca2+ release from stores. NO also enhances evoked GABAergic responses. Because voltage-gated Ca2+ channel function is not altered by NO, the increased evoked response is likely due to the combined effect of voltage-dependent Ca2+ influx adding to the NO-dependent, voltage-independent, Ca2+ influx. Insight into the identity of the Ca2+ influx pathway is provided by the transient receptor potential canonical (TRPC) channel inhibitor clemizole, which prevents the NO-dependent increase in sPSC frequency and cytosolic Ca2+ elevations. These data suggest that NO production in the inner retina will enhance Ca2+ -dependent GABA release from amacrine cells by activating TRPC channel(s). [ABSTRACT FROM AUTHOR]

GABAergic signaling from amacrine cells (ACs) is a fundamental aspect of visual signal processing in the inner retina. We have previously shown that nitric oxide (NO) can elicit release of GABA independently from activation of voltage-gated Ca 2+ channels in cultured retinal ACs. This voltage-independent quantal GABA release relies on a Ca 2+ influx mechanism with pharmacological characteristics consistent with the involvement of the transient receptor potential canonical (TRPC) channels TRPC4 and/or TRPC5. To determine the identity of these channels, we evaluated the ability of NO to elevate dendritic Ca 2+ and to stimulate GABA release from cultured ACs under conditions known to alter the function of TRPC4 and 5. We found that these effects of NO are phospholipase C dependent, have a biphasic dependence on La 3+ , and are unaffected by moderate concentrations of the TRPC4-selective antagonist ML204. Together, these results suggest that NO promotes GABA release by activating TRPC5 channels in AC dendrites. To confirm a role for TRPC5, we knocked down the expression of TRPC5 using CRISPR/Cas9-mediated gene knockdown and found that both the NO-dependent Ca 2+ elevations and increase in GABA release are dependent on the expression of TRPC5. These results demonstrate a novel NO-dependent mechanism for regulating neurotransmitter output from retinal ACs. NEW & NOTEWORTHY Elucidating the mechanisms regulating GABAergic synaptic transmission in the inner retina is key to understanding the flexibility of retinal ganglion cell output. Here, we demonstrate that nitric oxide (NO) can activate a transient receptor potential canonical 5 (TRPC5)-mediated Ca 2+ influx, which is sufficient to drive vesicular GABA release from retinal amacrine cells. This NO-dependent mechanism can bypass the need for depolarization and may have an important role in processing the visual signal by enhancing retinal amacrine cell GABAergic inhibitory output.