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

摘要

class="enumerated" style="list-style-type:decimal">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.The noradrenergic slow inhibitory postsynaptic potential (IPSP) was abolished by superfusion of ω-conotoxin (ω-CTX) GVIA (3-300 nM), with an apparent IC50 of 8.6 nM. Superfusion of ω-CTX MVIIC (500 nM) also suppressed the amplitude of slow IPSPs, but both ω-agatoxin IVA (100 nM) and nicardipine (1-10 μM) were ineffective. The hyperpolarization induced by exogenous noradrenaline was not affected by ω-CTX GVIA (100 nM).In contrast to the slow IPSP, the amplitude of the cholinergic fast excitatory postsynaptic potential (EPSP) was partially inhibited, but not abolished, by ω-CTX GVIA (0.1-1 μM). Furthermore, ω-agatoxin IVA (0.1-1 μM) or ω-CTX MVIIC (0.1-1 μM) also affected the fast EPSP, but nicardipine (1–10 μM) was ineffective. In combination, ω-CTX GVIA (100 nM) and ω-agatoxin IVA (100 nM) inhibited the fast EPSP by 74 ± 6 %; the residual fast EPSP was not affected by ω-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 ω-CTX GVIA (100 nM), ω-agatoxin IVA (100 nM) or ω-CTX MVIIC (100 nM).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.