Effect of ω-conotoxin on cholinergic and tachykininergic excitatory neurotransmission to the circular muscle of the guinea-pig colon

Abstract

The aim of this study was to compare the stimulus-response characteristics of the cholinergic and tachykininergic excitatory transmission to the circular muscle of the guinea-pig proximal colon and their susceptibility to inhibition by the N-type calcium channel blocker ω-conotoxin (CTX). All experiments were performed in the presence of guanethidine (3 μM), indomethacin (10 μM), L-nitroarginine (L-NOARG, 30 μM) and apamin (0.1 μM). In the presence of the tachykinin receptor antagonists, FK 888 (10 μM) and GR 94800 (3 μM), to block NK₁ and NK₂ receptors, respectively, electrical field stimulation (EFS) produced frequency-dependent atropine- (1 μM) sensitive contractions. In the presence of atropine (1 μM), EFS produced tachykininergic contractions which were abolished by the combined administration of FK 888 (10 μM) and GR 94 800 (3 μM). The maximal responses produced by cholinergic and tachykininergic neurotransmission ranged between 80 and 100% of the maximal contractile response to 80 mM KCI. The frequency of stimulation, pulse width and voltage required to produce 50% of the maximal cholinergic and tachykininergic contraction were not different from each other, although cholinergic transmission appeared more efficient in producing twitch contractions in response to single pulse EFS. Furthermore, cholinergic transmission was more efficient than tachykininergic transmission in producing contraction in response to short periods of EFS.

CTX (0.1 μM for 30 min) produced a large and comparable rightward shift of the cholinergic and tachykininergic frequency-response curve (19 and 17 fold increase in the frequency of stimulation producing 50% of the maximal response, respectively) and markedly depressed (51 and 43% inhibition, respectively) the maximal concentrations response. CTX failed to affect the contraction of the colon produced by submaximally effective concentrations of the muscarinic receptor agonist, methacholine (0.1–0.3 μM) and those produced by the tachykinin NK₁ and NK₂ receptor selective agonists [Sar⁹] substance P sulfone and [\Ala⁸] neurokinin A (4–10) (1–3 nM).

The present findings demonstrate that the cholinergic and tachykininergic components of the excitatory transmission to the circular muscle of the guinea-pig colon are activated at comparable intensities of nerve stimulation and are both inhibited, in a qualitatively and quantitatively comparable manner, by CTX at the prejunctional level. These findings are consistent with the idea that acetylcholine and tachykinins are co-released from the same population of enteric motoneurones which innervate the circular muscle of the colon.