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Calcium channel subtypes differ at two types of cholinergic synapse in lumbar sympathetic neurones of guinea-pigs

David R. Ireland, Philip J. Davies and Elspeth M. McLachlan

The involvement of different presynaptic Ca²⁺ channels in transmission at 'weak' (subthreshold) and 'strong' (suprathreshold) synapses was investigated in guinea-pig paravertebral ganglia isolated in vitro. Selective Ca²⁺ channel antagonists were used to block excitatory synaptic currents evoked by stimulating single preganglionic axons.

The N-type Ca²⁺ channel blocker, -conotoxin GVIA (100 nM), reduced peak synaptic conductance by similar amounts at weak synapses (by 39 ± 6 %) and strong synapses (34 ± 6 %).

The P-type Ca²⁺ channel blocker, -agatoxin IVA (40 nM), significantly reduced transmitter release at weak synapses (by 42 ± 6 %) but had only a small effect at strong synapses (reduced by 6 ± 2 %).

Blockers of Q-, L- or T-type Ca²⁺ channels had no significant effects on peak synaptic conductance at either type of synapse.

We conclude that the two functionally distinct types of preganglionic terminal in sympathetic ganglia which synapse on the same neurone differ in their expression of particular types of voltage-dependent Ca²⁺ channels. Both types utilize N-type channels and channels resistant to blockade by specific antagonists, but Ca²⁺ entry through P-type channels makes a substantial contribution to acetylcholine release only at weak synapses.

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