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

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 omega-conotoxin (omega-CTX) GVIA (3-300 nM), with an apparent IC50 of 8.6 nM. Superfusion of omega-CTX MVIIC (500 nM) also suppressed the amplitude of slow IPSPs, but both omega-agatoxin IVA (100 nM) and nicardipine (1-10 mu M) were ineffective. The hyperpolarization induced by exogenous noradrenaline was not affected by omega-CTX GVIA (100 nM). 3. In contrast to the slow IPSP, the amplitude of the cholinergic fast excitatory postsynaptic potential (EPXP) was partially inhibited, but not abolished, by omega-CTX GVIA (0.1-1 mu M). Furthermore, omega-agatoxin IVA (0.1-1 mu M) or omega-CTX MVIIC (0.1-1 mu M) also affected the fast EPSP, but nicardipine (1-10 mu M) was ineffective. In combination, omega-CTX GVIA (100 nM) and omega-agatoxin IVA (100 nM) inhibited the fast EPSP by 74 +/- 6%; the residual fast EPSP was not affected by omega-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 mu M). The depolarization induced by exogenous acetylcholine was not affected by either omega-CTX GVIA (100 nM), omega-agatoxin IVA (100 nM) or omega-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.