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Z.J. Zhou, J. Zheng; Differential Control of ACh and GABA Release by N and P/Q Type Ca Channels in Starburst Amacrine Cells . Invest. Ophthalmol. Vis. Sci. 2006;47(13):2673.
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We have previously shown that developing starburst amacrine cells (SACs) release both ACh and GABA onto each other by a Ca–dependent mechanism. However, it is unclear how a single SAC controls the vesicular release of two different transmitters. This study investigated whether SACs regulate ACh and GABA releases differently.
Double patch–clamp recordings were made from pairs of displaced SACs in neonatal rabbit retinal wholemounts. Electrophysiological and pharmacological analysis was used to determine the relative contributions of Ca channel subtypes to ACh and GABA release by SACs.
Depolarizing voltage steps applied to one SAC evoked postsynaptic currents in the neighboring SAC under dual voltage–clamp recording. The postsynaptic currents were outward at 0 mV and identified as GABA–A currents. They consisted of two components: a large fast component which appeared and disappeared quickly at the onset and offset of the presynaptic voltage pulse, respectively, and a slow component, which lasted 1.5–2 s after the termination of the presynaptic voltage pulse. The fast component was preferentially blocked by the P/Q/ type Ca channel blocker agatoxin G4A, whereas the prolonged component was blocked by the N type Ca channel blocker ω–conotoxin G6A, but not by agatoxin G4A. At –70 mV (ECl), the postsynaptic currents were inward and mediated by nicotinic receptors. These currents usually had a slow onset and prolonged offset, similar, in kinetics, to the slow GABAergic component, though a fast but samll nicotinic component was occasionally observable. Omega–conotoxin G6A blocked the nicotinic responses but left the fast GABA responses intact. The N type Ca channel also seemed to be coupled to additional Ca regulatory mechanisms and mediated a prolonged Ca response in SACs. The temporal property of this Ca response matched closely the burst duration of a spontaneous retinal wave.
ACh release from developing SACs is mediated predominantly by N type Ca channels, while a large, fast portion of the GABA release is mediated by the P/Q type Ca channels. It seems likely that ACh and GABA are released from different synaptic vesicles, and that N and P/Q types of Ca channels are coupled differently to these vesicles.
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