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Key Points: We demonstrated optical activation of primary somatosensory afferents with high selectivity to fast-conducting fibres by means of adeno-associated virus 9 (AAV9)-mediated gene transduction in dorsal root ganglion (DRG) neurons. AVV9 expressing green fluorescent protein showed high selectivity and transduction efficiency for fast-conducting, large-sized DRG neurons. Compared with conventional electrical stimulation, optically elicited volleys in primary afferents had higher sensitivity with stimulus amplitude, but lower sensitivity with stimulus frequency. Optically elicited dorsal root volleys activated postsynaptic neurons in the segmental spinal pathway. This proposed technique will help establish the causal relationships between somatosensory afferent inputs and neural responses in the CNS as well as behavioural outcomes in higher mammals where transgenic animals are not available., Abstract: Previously, fundamental structures and their mode of action in the spinal reflex circuit were determined by confirming their input-output relationship using electrophysiological techniques. In those experiments, the electrical stimulation of afferent fibres was used as a core element to identify different types of reflex pathways; however, a major disadvantage of this technique is its non-selectivity. In this study, we investigated the selective activation of large-diameter afferents by optogenetics combined with a virus vector transduction technique (injection via the sciatic nerve) in non-transgenic male Jcl:Wistar rats. We found that green fluorescent protein gene transduction of rat dorsal root ganglion (DRG) neurons with a preference for medium-to-large-sized cells was achieved using the adeno-associated virus 9 (AAV9) vector compared with the AAV6 vector (P = 0.021). Furthermore, the optical stimulation of Channelrhodopsin 2 (ChR2)-expressing DRG neurons (transduced by AAV9) produced compound action potentials in afferent nerves originating from fast-conducting nerve fibres. We also confirmed that physiological responses to different stimulus amplitudes were comparable between optogenetic and electrophysiological activation. However, compared with electrically elicited responses, the optically elicited responses had lower sensitivity with stimulus frequency. Finally, we showed that afferent volleys evoked by optical stimulation were sufficient to activate postsynaptic neurons in the spinal reflex arc. These results provide new ways for understanding the role of sensory afferent input to the central nervous system regarding behavioural control, especially when genetically manipulated animals are not available, such as higher mammals including non-human primates., (© 2019 The Authors. The Journal of Physiology published by John Wiley & Sons Ltd on behalf of The Physiological Society.)