Sodium action potentials are not required for light-evoked release of GABAor glycine from retinal amacrine cells

Although most CNS neurons require sodium action potentials (Na-APs) for nor mal stimulus-evoked release of classical neurotransmitters, many types of r etinal and other sensory neurons instead use only graded potentials for neu rotransmitter release. The physiological properties and information process ing capacity of Na AP-producing neurons appear significantly different from those of graded potential neurons. To classify amacrine cells in this dich otomy, we investigated whether Na-APs. which are often observed in these ce lls, are required for functional light-evoked release of inhibitory neurotr ansmitters from these cells. We recorded light-evoked inhibitory postsynapt ic currents (IPSCs) from retinal ganglion cells, neurons directly postsynap tic to amacrine cells. and applied mt to block Na-APs. In control solution, TTX application always led to partial suppression of the light-evoked IPSC . To isolate release from glycinergic amacrine cells. we used either bicucu lline, a GABA(A) receptor antagonist, or picrotoxin. a GABA(A) and GABA(C) receptor antagonist. rm application only partially suppressed the glycinerg ic IPSC. To isolate release from GABAergic amacrine cells, we; used the gly cine receptor blocker strychnine. TTX application only partially suppressed the light-evoked GABAergic IPSC. Glycinergic and GABAergic amacrine cells did not obviously differ in the usage of Na-APs for release. These observat ions, in conjunction with previous studies of other retinal neurons, indica te that amacrine cells, taken as a class, are the only type of retinal neur on that uses both Na-AP-dependent and -independent modes for light-evoked r elease of neurotransmitters. These results also provide evidence for anothe r parallel between the properties of retinal amacrine cells and olfactory b ulb granule cells.