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Presynaptic calcium channels mediating synaptic transmission in submucosal neurones of the guinea-pig caecum

Shauna M. C. Cunningham, Satoshi Mihara and Hideho Higashi

1. Intracellular recording techniques were used to examine the voltage-activated calcium channels mediating neurotransmitter release from nerve terminals of extrinsic, sympathetic origin and intrinsic (enteric) origin innervating submucosal neurones of the guinea-pig caecum.

2. The noradrenergic slow inhibitory postsynaptic potential (IPSP) was abolished by superfusion of -conotoxin (-CTX) GVIA (3-300 nM), with an apparent IC₅₀ of 8·6 nM. Superfusion of -CTX MVIIC (500 nM) also suppressed the amplitude of slow IPSPs, but both -agatoxin IVA (100 nM) and nicardipine (1-10 µM) were ineffective. The hyperpolarization induced by exogenous noradrenaline was not affected by -CTX GVIA (100 nM).

3. In contrast to the slow IPSP, the amplitude of the cholinergic fast excitatory postsynaptic potential (EPSP) was partially inhibited, but not abolished, by -CTX GVIA (0·1-1 µM). Furthermore, -agatoxin IVA (0·1-1 µM) or -CTX MVIIC (0·1-1 µM) also affected the fast EPSP, but nicardipine (1-10 µM) was ineffective. In combination, -CTX GVIA (100 nM) and -agatoxin IVA (100 nM) inhibited the fast EPSP by 74 ± 6 %; the residual fast EPSP was not affected by -CTX MVIIC (100 nM). The fast EPSP was completely abolished by low Ca²⁺, high Mg²⁺ Krebs solution or Krebs solution containing Co²⁺ (2 mM) and Cd²⁺ (400 µM). The depolarization induced by exogenous acetylcholine was not affected by either -CTX GVIA (100 nM), -agatoxin IVA (100 nM) or -CTX MVIIC (100 nM).

4. Taken together, these results suggest that, in the submucosal plexus of the guinea-pig caecum, release of noradrenaline from extrinsic nerve terminals is regulated by N-type calcium channels, whereas release of acetylcholine from intrinsic nerve terminals involves several types of calcium channel.

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