Mazet, Bruno

Effects of nitric oxide (NO) generated from sodium nitroprusside (SNP) on neurons of mouse superior mesenteric ganglion (SMG) were studied in vitro using intracellular recording techniques. SNP solutions caused a membrane hyperpolarization in the majority (64%) of the neurons tested or a hyperpolarization followed by a depolarization in 8% of the neurons tested. The SNP-induced hyperpolarization persisted in a low-Ca2+ high-Mg2+ solution, indicating a direct effect of NO on the postsynaptic membrane. The hyperpolarizing effect of SNP was reduced or abolished by oxyhemoglobin. Electrical stimulation of the colonic nerves evoked a late slow excitatory postsynaptic potential (late sEPSP) in a population of neurons in normal Krebs solution. The amplitude of the late sEPSP was significantly enhanced in the presence of NG-nitroL-arginine, a NO synthase inhibitor. The results, particularly those observed with the NO synthase inhibitor, suggest that endogenous NO was released in the mouse SMG by repetitive nerve stimulation and that it modulated slow synaptic transmission, presumably by a direct action on ganglionic neurons.

Nitric oxide (NO) hyperpolarizes intestinal smooth muscle cells. This study was designed to determine the mechanism whereby NO activates KCachannels of circular smooth muscle of the rabbit colon. Transmural biopsies of the rabbit colon were stained for NADPH-diaphorase. Freshly dispersed circular smooth muscle cells were studied in the whole cell configuration, as well as in on-cell and excised inside-out patch recording configurations, while KCacurrent and the activity of KCachannels, respectively, were monitored. NADPH-diaphorase-positive nerve fibers were found in both muscle layers. NO (1%) increased whole cell net outward current by 79% and hyperpolarized resting membrane voltage from −59 to −73 mV ( n = 8 cells, P < 0.01). In the on-cell patch recording configuration, NO (0.5% or 1%) in the bath increased NPoof KCachannels; charybdotoxin (125 nM) in the pipette solution blocked this effect. In the excised inside-out patch recording configuration, NO (1%) had no effect on NPoof KCachannels. In the on-cell patch recording configuration, methylene blue (1 μM) or cystamine (5 mM) in the bath solution decreased the effect of NO (1%) on NPoof KCachannels. NPowas increased by 8-bromo-cGMP (8-BrcGMP; 1 mM), a cGMP analog, and zaprinast (100 μM), an inhibitor of cGMP phosphodiesterase. These data suggest that NO increased whole cell outward K+current by activating KCachannels through a cGMP pathway.