The initiation and maintenance of organized movement through the basal ganglia is strongly influenced by its feed-forward and feedback inhibitory architecture. The substantia nigra pars compacta (SNc) and pedunculopontine nucleus (PPN) contribute to the overall output of the basal ganglia. Neurons in both structures degenerate in Parkinson's Disease, resulting in impaired motion. While treatments such as deep brain stimulation in the PPN (Snijders et al., 2016), and the implantation of stem cells into the SNc (Sonntag et al., 2018) have both met with variable success, their potential efficacy is constrained by a fundamental lack of knowledge about the circuitry of these two nuclei. The research proposed here will generate new insights into the function of inhibitory circuitry in these two nuclei and represents the first step toward a full understanding of the local and extended basal ganglia circuits which control organized motion. My long-term goal is to develop an independent research program focused on identifying cellular and network interactions that underlie basal ganglia control of motion. The overall objective of this K99/R00 application is to determine the extent to which local functional connectivity between genetically-defined subpopulations modulates basal ganglia output. My central hypothesis is that inhibition onto SNc and PPN neurons sculpts basal ganglia output by modulating excitatory gain. This hypothesis is based on preliminary two-photon uncaging, calcium imaging, optogenetic experiments, morphological reconstructions, and computational modeling. The rationale for this research is that once the circuit connectivity of the PPN and SNc is functionally mapped, we can begin to define the connections by which the basal ganglia select actions and control coordinated motion. To achieve my overall objective, I will work with my mentor, Dr. Zayd Khaliq and co-mentor, Dr. Chris McBain to learn and implement multi-channel optogenetic techniques and the simultaneous use of spatially-specific optogenetics with two photon glutamate uncaging and calcium imaging. These new techniques, in combination with my computational modeling and electrophysiological experience will allow me to complete my specific aims. During the mentored phase, I will complete aims 1 by performing functional tests of inhibitory inputs onto SNc dopamine neurons, including a comparison of the strength and location of inhibition from the striatal patch (striosome) compartments and the striatal matrix. In aim 2, I will test the functional consequences of dendrite-specific inhibition on the excitatory gain of SNc dopamine neurons. During the independent phase, I will utilize the same techniques to investigate the inhibitory circuitry of the PPN. In aim 3, I will perform functional tests of inhibitory inputs to the glutamatergic neurons of the PPN which have been identified with rabies tracing. In aim 4, I will define the intrinsic and genetic characteristics of a projection...