Effect of GABA‐ergic mechanisms on synaptosomal NO synthesis and the nitrergic component of NANC relaxation in rat ileum

Background  γ‐Aminobutyric acid (GABA) acts on specific neural receptors [A, B and C(Aρ)] to modulate gastrointestinal function. The precise role of GABA receptor activation in the regulation of presynaptic nitric oxide (NO) synthesis in nerve terminals is unknown. Methods  Rat ileal nerve terminals were isolated by differential centrifugation. Nitric oxide synthesis was analysed using a L‐[³H]arginine assay. In vitro studies were performed under non‐adrenergic non‐cholinergic (NANC) conditions on isolated ileal segments. Key Results  γ‐Aminobutyric acid inhibited NO synthesis significantly (n = 6, P < 0.05) [(fmol mg⁻¹ min⁻¹) control: 27.7 ± 1.5, 10⁻⁶ mol L⁻¹: 19.7 ± 1.3; 10⁻⁵ mol L⁻¹: 17.5 ± 3.0]. This effect was antagonized by the GABA A receptor antagonist bicuculline and the GABA C receptor antagonist (1,2,5,6‐tetrahydropyridin‐4‐yl)methylphosphinic acid (TPMPA), but not by the GABA B receptor antagonist SCH 50911. The GABA A receptor agonist muscimol [(fmol mg⁻¹ min⁻¹) control: 27.6 ± 1.0, 10⁻⁶ mol L⁻¹: 19.1 ± 1.7, n = 5, P < 0.05] and the GABA C receptor agonist cis‐4‐aminocrotonic acid (CACA) [(fmol mg⁻¹ min⁻¹) control: 29.5 ± 3.2, 10⁻³ mol L⁻¹: 20.3 ± 2.5, n = 6, P < 0.05], mimicked the GABA‐effect, whereas the GABA B agonist baclofen was ineffective. Bicuculline reversed the inhibitory effect of muscimol, TPMPA antagonized the effect of CACA. In functional experiments the GABA A and C receptor agonists reduced the NANC relaxation induced by electrical field stimulation in rat ileum by about 40%. After NOS‐inhibition by Nε‐nitro‐l ‐arginine methyl ester (l ‐NAME) the GABA A receptor agonist had no effect, whereas the GABA C receptor agonist still showed a residual response. Conclusions & Inferences  γ‐Aminobutyric acid inhibits neural NO synthesis in rat ileum by GABA A and GABA C(Aρ) receptor‐mediated mechanisms.