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.
