Tachykinin NK1 but not NK2 receptors mediate non‐cholinergic excitatory junction potentials in the circular muscle of guinea‐pig colon
Authors: Zagorodnyuk Vladimir , Santicioli Paolo , Maggi Carlo Alberto 
Published: July 2012

The effect of tachykinin NK1 and NK2 receptor antagonists on noncholinergic excitatory junction potentials (e.j.ps) evoked by electric field stimulation (EFS) in the circular muscle of the guinea‐pig proximal colon was investigated by means of a sucrose‐gap technique.
In the presence of 1 μm atropine, submaximal EFS (10 Hz, 20–30 V, 0.5 ms pulse width, 1 s train duration) evoked an inhibitory junction potential (i.j.p.) followed by e.j.p. with superimposed action potentials (APs) and contraction. Addition of either NG‐nitro‐l‐arginine (l‐NOARG, 0.1 mm) or apamin (0.1 μm) inhibited the evoked i.j.p. and the combined administration of the two agents almost abolished it. In the presence of both l‐NOARG and apamin, an atropine‐resistant e.j.p. was the only electrical response evoked by EFS in 50% of cases and a small i.j.p. (10% of original amplitude) followed by e.j.p. was evident in the remainder.
In the presence of l‐NOARG and apamin, the tachykinin NK1 receptor antagonists, (±)‐CP 96,345 and GR 82,334 (10 nm − 3 μm) concentration‐dependently inhibited the atropine‐resistant e.j.p. and accompanying contraction evoked by EFS. EC50 values were: 0.77 μm (e.j.p. inhibition) and 0.22 μm (inhibition of contraction) for (±)‐CP 96,345; 0.61 μm (e.j.p. inhibition) and 0.20 μm (inhibition of contraction) for GR 82,334. The tachykinin NK2 receptor antagonists, MEN 10,376 (up to 3 μm) and SR 48,968 (up to 1 μm) had no effect on the atropine‐resistant e.j.p. MEN 10,376 (3 μm) but not SR 48,968 produced a slight inhibition of the evoked contraction.
(±)‐CP 96,345 (3 μm) and GR 82,334 (3 μm) markedly reduced (81 and 89% inhibition, respectively) the atropine‐resistant e.j.p. in the absence of l‐NOARG and apamin, without affecting the i.j.p. MEN 10,376 (3 μm) and SR 48,968 (1 μm) had no significant effect on noncholinergic i.j.p. and e.j.p. evoked in the absence of apamin and l‐NOARG.
The electrical and mechanical responses to the NK1 receptor agonist [Sar9]substance P (SP) sulfone were blocked by (±)‐CP 96,345 (3 μm) or GR 82,334 (3 μm) which, at the same concentration, failed to affect the responses to the NK2 receptor agonist [βAla8] neurokinin A (NKA) (4–10). In contrast, MEN 10,376 (3 μm) or SR 48,968 (1 μm) blocked the response to [βAla8]NKA(4–10) without affecting the response to [Sar9]SP sulfone.
In the presence of l‐NOARG and apamin, and in the absence of atropine, EFS of low pulse width (0.02–0.03 ms, other parameters as above) produced cholinergic e.j.ps and contraction which were unaffected by GR 82,334 (3 μm). (±)‐CP 96,345 (3 μm) produced 24% reduction in the area of the atropine‐sensitive e.j.p. without affecting the peak amplitude of e.j.p. or contraction.
These findings demonstrate that the noncholinergic e.j.ps and accompanying contraction of the circular muscle of the guinea‐pig colon are produced through activation of intramural tachykininergic nerves and that the resultant smooth muscle response is almost entirely mediated through NK1 receptors.