EFFECT OF OMEGA-CONOTOXIN ON CHOLINERGIC AND TACHYKININERGIC EXCITATORY NEUROTRANSMISSION TO THE CIRCULAR MUSCLE OF THE GUINEA-PIG COLON

The aim of this study was to compare the stimulus-response characteris tics of the cholinergic and tachykininergic excitatory transmission to the circular muscle of the guinea-pig proximal colon and their suscep tibility to inhibition by the N-type calcium channel blocker omega-con otoxin (CTX). All experiments were performed in the presence of guanet hidine (3 mu M), indomethacin (10 mu M), L-nitroarginine (L-NOARG, 30 mu M) and apamin (0.1 mu M). In the presence of the tachykinin recepto r antagonists, FK 888 (10 mu M) and GR 94800 (3 mu M), to block NK1 an d NK2 receptors, respectively, electrical field stimulation (EFS) prod uced frequency-dependent atropine- (1 mu M) sensitive contractions. In the presence of atropine (1 mu M), EFS produced tachykininergic contr actions which were abolished by the combined administration of FK 888 (10 mu M) and GR 94 800 (3 mu M). The maximal responses produced by ch olinergic and tachykininergic neurotransmission ranged between 80 and 100% of the maximal contractile response to 80 mM KCl. The frequency o f 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 effi cient in producing twitch contractions in response to single pulse EFS . Furthermore, cholinergic transmission was more efficient than tachyk ininergic transmission in producing contraction in response to short p eriods of EFS. CTX (0.1 mu M for 30 min) produced a large and comparab le rightward shift of the cholinergic and tachykininergic frequency-re sponse curve (19 and 17 fold increase in the frequency of stimulation producing 50% of the maximal response, respectively) and markedly depr essed (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 ag onist, methacholine (0.1-0.3 mu M) and those produced by the tachykini n NK1 and NK2 receptor selective agonists [Sar(9)] substance P sulfone and [beta Ala(8)] 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 colo n are activated at comparable intensities of nerve stimulation and are both inhibited, in a qualitatively and quantitatively comparable mann er, by CTX at the prejunctional level. These findings are consistent w ith the idea that acetylcholine and tachykinins are co-released from t he same population of enteric motoneurones which innervate the circula r muscle of the colon.