Readily evoked forelimb movement in rat via optogenetic stimulation of motor cortex

Virally transfected optogenetics is emerging as a powerful tool for studying neural circuits and behavior in non-genetically tractable animals. A basic test of this tool is the ability to evoke movement by cortical stimulation. While evoked movements have been reported in transgenic mice, there is concern that optogenetic stimulation has not readily evoked skeletal movements in rat or monkey. Here we show that optogenetics can drive limb movements in rat. We injected AAV vectors with channelrhodopsin-2 into deep layers of rat primary motor cortex (M1). Subsequent optical stimulation of M1 elicted robust limb movements in anesthetized animals; anesthesia rules out the possibility that movement is an indirect behavioral response to the stimulation. Histology shows expression in corticospinal neurons, and movement may have been mediated by direct activation of these cells. Our results suggest two factors that may be critical for optogenetically evoking movements in virally transfected animals: a sparse (<; 5%) pattern of distribution of transfected neurons, including corticospinal neurons, and a high power density of stimulation (>1200 mW/mm2) in the deep layers (at least 1.1 mm below cortical surface).