PROJECT SUMMARY Optogenetic methods have been used extensively to expand our knowledge of the normal and pathological roles of the basal ganglia and related brain areas involved in Parkinson’s disease (PD). This revolutionary tech- nology has, so far, been restricted to studies in rodent models of parkinsonism, while technical constraints have largely limited the use of optogenetic experimentation in non-human primates (NHPs). The scarcity of optoge- netics studies in NHPs is not limited to PD-related research. A critical limitation to the expansion of optogenetics in primates is the lack of non-invasive methods to monitor opsin expression in vivo. Postmortem histology re- mains the only reliable method to confirm the correct targeting of brain regions and level of opsin expression in NHPs. Evidently, this method does not allow for in vivo corrections of experimental approaches. Expansion of the use of optogenetics in NHPs require the development of methods to monitor the temporal course and level of opsin expression, so that insufficient expression levels can be detected and corrected in vivo, thus avoiding costly experimental failures, and reducing the number of NHPs needed for research. We propose a non-invasive approach to monitor opsin expression repeatedly in vivo, based on positron imaging tomography (PET) imaging in NHPs. We will use an innovative version of the excitatory opsin channelrhodopsin-2 (ChR2), in which the opsin is fused to the ligand binding domain of the human estrogen receptor alpha, creating the novel opsin “ChRERa”. The radiolabeled fluorinated estradiol analog, [18F]-fluoroestradiol ([18F]-FES), binds strongly to ChRERa, and the brain distribution of [18F]-FES can be visualized with PET. We will use ChRERa and re- peated [18F]-FES PET scans to monitor opsin expression in Rhesus monkeys longitudinally. Using viral vectors, we will express ChRERa in the motor portion of the striatum, and then conduct several [18F]-FES PET scans over the next 12 months to describe the time course of PET detectable opsin expression. In parallel, we will examine if the PET-identified pattern of ChRERa expression corresponds to the presence of functional opsins, by conducting extracellular recordings during light activation. Finally, we will examine the correspondence be- tween the opsin expression pattern found in the [18F]-FES PET scans and the opsin distribution identified with post-mortem histology. These studies will establish a fundamental technology to evaluate in vivo the distribution of opsins in the NHP. In alignment with the purpose of this FOA PA-21-219 (“Joint NINDS/NIMH Exploratory Neuroscience Research Grant”), these studies have the long-term goal of using ChRERa and [18F]-FES PET scans in our NHP optogenetic studies of the pathophysiology of parkinsonism. More broadly, validation of a non-invasive in vivo method to monitor opsin expression could help accelerate reliable use of optogenetics for the study of NHP models of neur...