Nitric oxide synthase inhibitors enhance mechanosensitive Ca(2+) influx in cultured dorsal root ganglion neurons.

Nitric oxide (NO) can have opposite effects on peripheral sensory neuron sensitivity depending on the concentration and source of NO, and the experimental setting. The aim of this study was to determine the role of endogenous NO production in the regulation of mechanosensitive Ca(2+) 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 mediated changes in [Ca(2+)](i) by fluorescent ratio imaging. In the presence of the NOS inhibitors L-NAME, TRIM, or 7-NI, but not the inactive analogue D-NAME, peak [Ca(2+)](i) transients to mechanical stimulation were increased more than 2-fold. Neither La(3+) (25 microM), an inhibitor of voltage activated Ca(2+) channels, or tetrodotoxin (TTX, 1 microM), a selective inhibitor of voltage-gated Na(+) channels, had an effect on mechanically activated [Ca(2+)](i) transients under control conditions. However, in the presence of L-NAME, both La(3+) and TTX partially blocked the [Ca(2+)](i) response. Addition of Gd(3+), a blocker of mechanosensitive cation channels and L-type Ca(2+) channels, at a concentration (100 microM) that markedly inhibited the mechanical response under control conditions, only partially inhibited the response in the presence of L-NAME. The combination of either La(3+) or TTX with Gd(3+) caused near complete inhibition of mechanically stimulated [Ca(2+)](i) transients in the presence of L-NAME. We conclude that focal mechanical stimulation of DRG neurons causes Ca(2+) influx occurs primarily through mechanosensitive cation channels under control conditions. In the presence of NOS inhibitors, additional Ca(2+) influx occurs through voltage-sensitive Ca(2+) channels. These results suggest that endogenously produced NO in cultured DRG neurons decreases mechanosensitivity by inhibiting voltage-gated Na(+) and Ca(2+) channels.