Nitric oxide promotes GABA release by activating a voltage-independent Ca(2+) influx pathway in retinal amacrine cells.

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 Ca(2+) 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 Ca(2+) and buffering of cytosolic Ca(2+) both inhibit the response to NO. In Ca(2+) imaging experiments, we confirm that NO increases cytosolic Ca(2+) in amacrine cell processes by activating a Ca(2+) influx pathway. Neither the increase in sPSC frequency nor the cytosolic Ca(2+) elevations are dependent upon Ca(2+) release from stores. NO also enhances evoked GABAergic responses. Because voltage-gated Ca(2+) channel function is not altered by NO, the increased evoked response is likely due to the combined effect of voltage-dependent Ca(2+) influx adding to the NO-dependent, voltage-independent, Ca(2+) influx. Insight into the identity of the Ca(2+) 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 Ca(2+) elevations. These data suggest that NO production in the inner retina will enhance Ca(2+)-dependent GABA release from amacrine cells by activating TRPC channel(s).NEW & NOTEWORTHY Our research provides evidence that nitric oxide (NO) promotes GABAergic output from retinal amacrine cells by activating a likely transient receptor potential canonical-mediated Ca(2+) influx pathway. This NO-dependent mechanism promoting GABA release can be voltage independent, suggesting that, in the retina, local NO production can bypass the formal retinal circuitry and increase local inhibition.