Nitric oxide synthase inhibitors enhance mechanosensitive Ca2+ influx in cultured dorsal root ganglion neurons

Nitric oxide (NO) can have opposite effects on peripheral sensory neuron se nsitivity depending on the concentration and source of NO, and the experime ntal setting. The aim of this study was to determine the role of endogenous NO production in the regulation of mechanosensitive Ca2+ influx of dorsal root ganglion (DRG) neurons. Adult mouse DRG neurons were grown in primary culture for 2-5 days, loaded with Fura-2, and tested for mechanically media ted changes in [Ca2+], by fluorescent ratio imaging. In the presence of the NOS inhibitors L-NAME, TRIM, or 7-NI, but not the inactive analogue U-NAME , peak [Ca2+], transients to mechanical stimulation were increased more tha n 2-fold. Neither La3+ (25 muM), an inhibitor of voltage activated Ca2+ cha nnels, or tetrodotoxin (TTX, 1 muM), a selective inhibitor of voltage-gated Naf channels, had an effect on mechanically activated [Ca2+], transients u nder control conditions. However, in the presence of L-NAME, both La3+ and TTX partially blocked the [Ca2+], response. Addition of Gd3+, a blocker of mechanosensitive cation channels and L-type Ca2+ channels, at a concentrati on (100 muM) that markedly inhibited the mechanical response under control conditions, only partially inhibited the response in the presence of L-NAME . The combination of either La3+ or TTX with Gd3+ caused near complete inhi bition of mechanically stimulated [Ca2+], transients in the presence of l.- NAME. We conclude that focal mechanical stimulation of DRG neurons causes C a2+ influx occurs primarily through mechanosensitive cation channels under control conditions. In the presence of NOS inhibitors, additional Ca2+ infl ux occurs, through voltage-sensitive Ca2+ channels. These results suggest t hat endogenously produced NO in cultured DRG neurons decreases mechanosensi tivity by inhibiting voltage-gated Na+ and Ca2+ channels. (C) 2001 Elsevier Science B.V. All rights reserved.