A central goal in biology has focused on the ability to manipulate a defined population of cells within a model to determine the correlation between modulation and functional output. Ultimately, the technique utilized will ensure that the level of control will be precise, temporally realistic, genetically tangible and reversible. Devising a procedure that can modify a defined group of functionally related neurons both reversibly and rapidly creates a scenario where one can examine the effects of suppression or enhancement of neuro- and synaptic transmission and could establish a correlation between an electrophysiological outcome to a behavioral phenotype.
While numerous innovative approaches have attempted to establish a link between cellular activity and development, plasticity and behavior by modulating a specific group of neurons, studies were often confronted with a multitude of limitations that were intrinsic to the experimental approach. Classical methods faced obstacles in either kinetic, construct or pharmacological design that often rendered the targeted cellular population irreversibly or incompletely modified. The discovery and utilization of light-sensitive opsins, coupled with traditional genetic approaches and innovation within the optogenetic field have revealed that the previously encountered spatial, temporal and reversibility restraints are essentially eliminated.
Taking this into consideration, we hypothesized that the targeted expression and light activation of vertebrate rhodopsin (vRh) within the cerebellum would not only alter the characteristic, cellular firing pattern but would also exert a visible and distinct change in motor behavior. In order to examine the physiological and behavioral effects of vRh expression and photoactivation in vivo, we had to first establish a transgenic mouse line whereby the expression of vRh was exclusively driven to Purkinje neurons. Subsequent steps included the development of a surgical procedure that would allow for the targeted delivery of light to the cerebellar vermis via an optrode as well as the in vivo electrophysiological and behavioral outcomes of light application in both transgenic and wild type littermates.
Our results revealed that vRh expression was restricted to the soma and proximal dendrites of Purkinje neurons in a punctate pattern. The in vivo data revealed that light activation of vRh in vermal Purkinje neurons significantly reduced the firing frequency in a manner similar to baclofen application. Furthermore, we also discovered that a brief pulse of light (26 sec) sufficiently induced a loss of motor coordination and balance in positive transgenic mice.