Abstract
Purpose: :
Starburst amacrine cells (SACs) in rabbit retina have been shown previously to release ACh and GABA in a calcium-dependent manner. This study investigated the calcium channel subtypes that control the release of ACh and GABA from SACs in functionally mature rabbit retina.
Methods: :
Dual patch-clamp recordings were made from pairs of SACs and direction selective ganglion cells (DSGCs) in mature rabbit retinal wholemounts to detect the release of ACh and GABA from SACs onto DSGCs. Bath-perfusion of selective blockers of N and P/Q calcium channels was used to distinguish the role of calcium channel subtypes in the release of ACh and GABA.
Results: :
Stimulating SACs with depolarizing voltage steps evoked outward GABAergic and inward nicotinic currents in the DSGC, which could be isolated at the holding potential +15mV and -60mV (near ECl), respectively. The GABA responses consisted of two components, a transient component which peaked at the onset of the presynaptic depolarizing pulse, and a more sustained current which outlasted the depolarizing pulse by 300-400 ms. Bath perfusion of ω-Agatoxin IVA, a selective P/Q channel blocker, abolished the initial transient GABAergic peak. The remaining prolonged GABA component was blocked by ω-Conotoxin GVIA, a selective N channel blocker. The nicotinic response was composed of a large transient current at the onset of presynaptic voltage pulse and, sometimes, a small and sustained component. The amplitude of the postsynaptic nicotinic current could be enhanced by repetitive presynaptic stimulation, indicating a facilitation of cholinergic transmission by presynaptic calcium accumulation. ω-Conotoxin GVIA blocked the majority (~85%) of the peak nicotinic current, whereas ω-Agatoxin IVA reduced the nicotinic current amplitude by less than 35%. Application of ω-Conotoxin GVIA and ω-Agatoxin IVA together invariably blocked both GABAergic and nicotinic transmissions completely.
Conclusions: :
N- and P/Q calcium channel types differentially mediated GABA and ACh releases from functionally mature SACs. The large transient GABA release appeared to be mediated selectively by P/Q channels, whereas the prolonged component was mediated by N channels. The release of ACh was predominantly mediated by N type channels, though P/Q type channels also appeared to play a role. Notably, the contribution of N type channels to ACh and GABA releases appeared to involve a secondary amplification process, which facilitated calcium accumulation and ACh release.
Keywords: retina: proximal (bipolar, amacrine, and ganglion cells) • retinal connections, networks, circuitry • neurotransmitters/neurotransmitter systems