Calcium channels controlling acetylcholine release in the guinea-pig isolated anterior pelvic ganglion: an electropharmacological study

Abstract

An electropharmacological analysis of the type(s) of calcium channel controlling neurotransmitter release in preganglionic sympathetic nerve terminals in the guinea-pig anterior pelvic ganglion has been carried out. Conventional intracellular recording techniques were used to record excitatory postsynaptic potentials as a measure of neurotransmitter release. Excitatory postsynaptic potentials were abolished by hexamethonium (30–100 μM) and are therefore mediated by acetylcholine acting at nicotinic receptors. In studies of more than 150 cells, the N-type calcium channel blocker ω-conotoxin GVIA (100–300 nM) failed to block the initiation of the nerve impulse by the excitatory postsynaptic potential. In single-cell studies, ω-conotoxin GVIA (1 μM) sometimes altered the configuration of the excitatory postsynaptic potential/cell body nerve action potential complex, but on only one occasion was the excitatory postsynaptic potential reduced below the threshold required to initiate the action potential. Nifedipine (10 μM), ω-agatoxin IVA (100 nM) and ω-conotoxin MVIIC (300 nM), applied alone or in combination with ω-conotoxin GVIA (300 nM), were also ineffective. However, excitatory postsynaptic potentials evoked by trains of stimuli (0.1–0.5 Hz) were markedly reduced or abolished by the non-specific calcium channel blocker ω-grammotoxin SIA (300 nM). When trains of stimuli were delivered at higher frequencies (4 Hz), the block induced by ω-grammotoxin SIA could be overcome, and excitatory postsynaptic potentials were able to initiate action potentials even when ω-conotoxin GVIA, ω-agatoxin IVA and ω-conotoxin MVIIC were also present. The calcium channel(s) controlling acetylcholine release was (were) blocked by low concentrations of cadmium ions (30 μM) at all stimulation frequencies studied (0.1–50 Hz). Thus, the dominant calcium channels controlling acetylcholine release in sympathetic ganglia are not the L, N, P or Q types. At low frequencies of stimulation, ω-grammotoxin SIA-sensitive calcium channels play a dominant role in acetylcholine release, but at higher stimulation frequencies yet another pharmacologically distinct calcium channel (or subtype) supports neurotransmitter release.