Dissecting functional subgroups and closed-loop circuits between the pedunculopontine nucleus and the basal ganglia - Abstract The pedunculopontine nucleus (PPN) interacts with the basal ganglia to enable smooth movement and is a target for deep brain stimulation (DBS) in Parkinson's disease (PD). PD is the most common neurodegenerative motor disorder, characterized by resting tremor, rigidity, slow movement, and postural instability. Caudal-targeted DBS stimulation improves gait impairment in both patients and rodent PD models whereas rostral-targeted DBS worsens it in rodent PD models. Effective PPN-targeted DBS may be attributed to increasing alpha oscillations in the caudal PPN generated by closed-loop circuits and increasing neuronal activity across the PPN. Contradictory outcomes of PPN-targeted DBS in both patients and rodent models suggest functional heterogeneity within the PPN and differences between its rostral and caudal subregions. To understand how DBS differentially modulates movement within the PPN, it is critical to dissect the functional subpopulations within the PPN and determine how they communicate with the basal ganglia. In this proposal, I focus on the cholinergic PPN neurons whose selective degeneration in PD also correlates with gait impairment. I will use ex vivo whole-cell patch clamp electrophysiology, optogenetics, calcium imaging, and retro bead labeling (1) to characterize the intrinsic electrophysiological properties and identify a molecular marker that differentiates rostral and caudal cholinergic PPN neurons in adult mice, (2) to characterize and comprehensively map the regional connectivity of inhibitory synaptic inputs to the PPN, and (3) to identify closed-loop circuits between the inhibitory basal ganglia nuclei and the PPN. Our preliminary data show that the two major inhibitory basal ganglia nuclei, the substantia nigra pars reticulata (SNr) and globus pallidus externus (GPe), differentially project to the rostral and caudal PPN in axon imaging. Using optogenetics with whole cell patch clamp and calcium imaging, we can confirm the functional connectivity of these anatomical axonal projections and characterize inhibitory currents from the SNr and GPe on the PPN. Using immunostaining, I will test the differential expression of five proteins selectively expressed in the PPN compared to other brain regions to identify a molecular marker that can be used as a tool to genetically access the rostral and caudal PPN. Using optogenetics and retrobead-labeling, I will determine whether SNr- and GPe-projecting PPN neurons also receive inhibitory input from the respective basal ganglia nuclei, forming a closed-loop circuit. This proposal will comprehensively characterize and map the regional connectivity of the SNr and GPe to the cholinergic PPN and identify closed-loop circuits that can help us understand how the PPN interacts with the basal ganglia to modulate movement and how its degeneration underlies motor deficits in PD. These findings will guide new pharmacological targets and DBS-targeting of the PPN circuit in PD patients.
