Nitric oxide (NO) synthesis was examined in intact longitudinal muscle-myenteric plexus preparations of the guinea pig ileum by determining the formation of [³H]citrulline during incubation with [³H]arginine. Spontaneous [³H]citrulline production after 30 min was 80-90 dpm/mg, which constituted ~1% of the tissue radioactivity. Electrical stimulation (10 Hz) led to a threefold increase in [³H]citrulline formation. Removal of calcium from the medium or addition of N^G-nitro-L-arginine strongly inhibited both spontaneous and electrically induced production of [³H]citrulline. TTX reduced the electrically induced but not spontaneous [³H]citrulline formation. The electrically induced formation of [³H]citrulline was diminished by (+)-tubocurarine and mecamylamine and enhanced by scopolamine, which suggests that endogenous ACh inhibits, via muscarinic receptors, and stimulates, via nicotinic receptors, the NO synthesis in the myenteric plexus. The GABA_A receptor agonist muscimol and GABA also reduced the electrically evoked formation of [³H]citrulline, whereas baclofen was without effect. Bicuculline antagonized the inhibitory effect of GABA. It is concluded that nitrergic myenteric neurons are equipped with GABA_A receptors, which mediate inhibition of NO synthesis.

nitric oxide synthase; -aminobutyric acid; GABA_A receptor; nicotinic receptor

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