Nitric oxide modulates Ca2+ channels in dorsal root ganglion neurons innervating rat urinary bladder

The effect of a nitric oxide (NO) donor on high-voltage-activated Ca2+ chan nel currents (I-Ca) was examined using the whole cell patch-clamp technique in L-6-S-1 dorsal root ganglion (DRG) neurons innervating the urinary blad der. The neurons were labeled by axonal transport of a fluorescent dye, Fas t Blue, injected into the bladder wall. Approximately 70% of bladder affere nt neurons exhibited tetrodotoxin (TTX)-resistant action potentials (APs), and 93% of these neurons were sensitive to capsaicin, while the remaining n eurons had TTX-sensitive spikes and were insensitive to capsaicin. The peak current density of nimodipine-sensitive L-type Ca2+ channels activated by depolarizing pulses (0 mV) from a holding potential of -60 mV was greater i n bladder afferent neurons with TTX-resistant APs (39.2 pA/pF) than in blad der afferent neurons with TTX-sensitive APs (28.9 pA/pF), while the current density of omega -conotoxin GVIA-sensitive N-type Ca2+ channels was simila r (43-45 pA/pF) in both types of neurons. In both types of neurons, the NO donor, S-nitroso-N-acetylpenicillamine (SNAP) (500 muM), reversibly reduced (23.4-26.6%) the amplitude of I-Ca elicited by depolarizing pulses to 0 mV from a holding potential of -60 mV. SNAP-induced inhibition of I-Ca was re duced by 90% in the presence of omega -conotoxin GVIA but was unaffected in the presence of nimodipine, indicating that NO-induced inhibition of I-Ca is mainly confined to N-type Ca2+ channels. Exposure of the neurons for 30 min to 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ, 10 muM), an inhibi tor of NO-stimulated guanylyl cyclase, prevented the SNAP-induced reduction in I-Ca. Extracellular application of 8-bromo-cGMP (1 mM) mimicked the eff ects of NO donors by reducing the peak amplitude of I-Ca (28.6% of reductio n). Action potential configuration and firing frequency during depolarizing current pulses were not altered by the application of SNAP (500 mM) in bla dder afferent neurons with TTX-resistant and -sensitive APs. These results indicate that NO acting via a cGMP signaling pathway can modulate N-type Ca 2+ channels in DRG neurons innervating the urinary bladder.