In vivo optogenetic activation of Na1.8 cutaneous nociceptors and their responses to natural stimuli.

Optogenetic methods that utilize expression of the light-sensitive protein channelrhodopsin-2 (ChR2) in neurons have enabled selective activation of specific subtypes or groups of neurons to determine their functions. Using a transgenic mouse model in which neurons natively expressing Na1.8 (a tetrodotoxin-resistant voltage-gated sodium channel) also express the light-gated channel ChR2, we have been able to determine the functional properties of Na1.8-expressing cutaneous nociceptors of the glabrous skin in vivo. Most (44 of 53) of the C-fiber nociceptors isolated from mice were found to be responsive to blue (470 nm) light. Response characteristics, including conduction velocity and responses to mechanical stimuli, were comparable between nociceptors isolated from and control mice. Interestingly, while none of the non-light-responsive C-fibers were sensitive to heat or cold, nearly all (77%) light-sensitive fibers were excited by mechanical and thermal stimuli, suggesting that Na1.8 is predominantly expressed by C-fiber nociceptors that are responsive to multiple stimulus modalities. The ability to activate peripheral nociceptors with light provides a method of stimulation that is noninvasive, does not require mechanical interruption of the skin, and accesses receptive fields that might be difficult or impossible to stimulate with standard stimuli while allowing repeated stimulation without injuring the skin. Transgenic mice that express the blue light-sensitive protein channelrhodopsin2 (ChR2) in nociceptive nerve fibers that contain voltage-gated sodium channel Na1.8 were used to determine functional properties of these afferent fibers. Electrophysiological recordings in vivo revealed that most nociceptive fibers that possess Na1.8 are C-fiber nociceptors that respond to multiple stimulus modalities. Furthermore, responses evoked by blue light stimulation were comparable to those elicited by noxious mechanical, heat, and cold stimuli.