Voltage-gated sodium channel, Nav 1.8, regulates calcium dynamics in the dorsal root ganglion induced by cold nociceptive stimulus

Nociceptive pain responses are mediated by neurons in the dorsal root ganglion. Certain voltage-gated sodium channels, such as Nav 1.8, are necessary for action potential firing and sensing pain. Prior work has demonstrated that interfering with voltage-gated calcium channel activity can also reduce acute and chronic pain. Furthermore, it is reported that dorsal root ganglion calcium response to electric-field stimulation was inhibited by voltage-gated sodium channel blockers. Here we aim to explore the intracellular calcium dynamics in dorsal root ganglion neurons in response to cold pain via intravital confocal imaging. Genetically encoded calcium indicators, such as the GCaMP family of proteins, have been widely used to assess neuron excitability, using fluorescence due to calcium flux as a surrogate for neuron depolarization. We utilized Thy1-GCaMP6f transgenic mice to image calcium transients in live animals or freshly-isolated dorsal root ganglion neurons to identify neuron activity. Measurement of calcium transients demonstrated response to a TRPV1 agonist, capsaicin, or a voltage-gated sodium channel agonist, veratridine, and this response could be inhibited by capsazepine: a TRPV1 antagonist; carbamazepine: a pan-voltage gated sodium channel blocker; or A-803467: a Nav 1.8 inhibitor. We demonstrated through intravital imaging that the cold paw swab increases calcium transients in dorsal root ganglion neurons, and this effect was inhibited by voltage-gated calcium channel blockers. We further demonstrated by in vivo imaging that the cold-pain induced calcium transients in dorsal root ganglion neurons is blocked by A-803467. These data demonstrate that imaging can be used to visualize and measure cold pain signaling in dorsal root ganglion neurons due to L-type voltage-gated calcium channel activity triggered by Nav 1.8 activation.