The type 5 metabotropic glutamate receptor (mGluR5) plays a critical role in various aspects of drug addiction. Pharmacological or genetic inactivation of mGluR5 receptors reduces drug reward, reinforcement, and relapse- like behaviors. On the other hand, pharmacological potentiation of mGluR5 function via positive allosteric modulators (PAMs) produces pro-cognitive effects such as facilitated extinction learning and reversal of drug- induced cognitive impairments. However, various challenges and limitations to pharmacological activation of mGluR5 receptors significantly limit its utility as a research tool for detailed investigation of mGluR5 function in drug addiction. Such limitations include poor spatiotemporal control and lack of cell-type specificity of receptor activation, non-selectivity of orthosteric mGluR5 agonists, and suboptimal physiochemical properties of mGluR5 PAMs. These technical limitations can be surmounted by the successful development of optogenetic tools that allow for precise spatiotemporal and cell-type specific activation of mGluR5 receptor signaling. We have recently developed and performed a preliminary characterization of a lentiviral vector that expresses light- activated mGluR5 receptor (OptoXR-mGluR5). The expression of OptoXR-mGluR5 is under the control of Ca2+/calmodulin-dependent kinase II? (CaMKII?) promoter, allowing for selective expression in cortical glutamatergic neurons, and the vector also encodes enhanced yellow fluorescent protein (eYFP) reporter protein for visualization of expression. Our preliminary characterization of this lentivirus in the rat cerebral cortex in vivo demonstrates its cell-type specificity and ability to acutely activate mGluR5 signaling. However, the effects of OptoXR-mGluR5 activation in animal models of addiction-related behaviors and cognitive dysfunction have not yet been assessed. In addition, the ability of repeated stimulation of OptoXR-mGluR5 to consistently activate mGluR5 signaling, which would be necessary for behavioral studies conducted over the course of days or weeks, has not yet been explored. Therefore, the overarching goals of the studies proposed in this application are to examine the behavioral effects of acute OptoXR-mGluR5 activation in a rodent model of drug-induced cognitive dysfunction, and to develop novel tools for optogenetic activation of mGluR5 signaling that are amenable to behavioral paradigms involving repeated stimulation. These goals will be achieved under two independent Specific Aims. In Specific Aim 1, we will assess the ability of acute activation of OptoXR-mGluR5 to reverse drug-induced cognitive dysfunction. In Specific Aim 2, we will develop novel optogenetic tools for repeated activation of mGluR5 signaling. Successful development of optogenetic tools for mGluR5 activation will allow for an unprecedented level of investigation into the role of these receptors n various aspects of drug addiction. Optogenetic tools for mGluR5 activation may also provide insight into pathophysiological mechanisms and novel therapeutic avenues for other neuropsychiatric disorders in which mGluR5 receptors are implicated, including Alzheimer's disease, Fragile X syndrome, and schizophrenia.