Muscarinic modulation of nitrergic neurotransmission in guinea-pig gastric fundus

Muscarinic receptor activation by (4-Hydroxy-2-butynyl)-1-trimethylammonium-m-chlorocarbanilate chloride (McN-A-343) was investigated both on NADPH-d staining and on electrically induced responses in guinea-pig gastric fundus. McN-A-343 (10 mumol L-1) significantly increased the optical density of NADPH-d positive neurones, while blockade of nitric oxide synthase with N-omega-nitro-L-arginine (L-NA) decreased it, suggesting facilitation of nitric oxide (NO) production. Electrical field stimulation (EFS; 2 Hz, 0.2 ms, supramaximal current intensity, 10 s train duration) elicited on-contraction followed by off-relaxation in the circular muscle strips. McN-A-343 (10 mumol L-1) transformed the EFS-evoked response from on-contraction into on-relaxation, which was neurogenic, tetrodotoxin-sensitive and hexamethonium-resistant. L-NA partly reduced the EFS-evoked relaxation, revealing two components: a nitrergic and a non-nitrergic one. The effect of McN-A-343 on the amplitude of the EFS-evoked relaxation was not changed by the M-3 receptor antagonist para-fluoro-hexahydro-sila-difenidol hydrochloride, but was significantly enhanced by M-1 receptor blockade with telenzepine. In the presence of telenzepine, the L-NA-dependent nitrergic component of the EFS-induced relaxation predominates. We suggest that cholinergic receptor activation has a dual effect on nitrergic neurotransmission: (i) stimulation of NOS by muscarinic receptor(s) different from M-1 and M-3 subtype, (ii) prejunctional inhibition of NO-mediated relaxation via M-1 receptors. In addition, M-1 receptors may facilitate the non-nitrergic relaxation.