Brainstem cholinergic cell loss causes slowly-developing motor impairments

Pedunculopontine nucleus (PPN) cholinergic degeneration is a feature of Parkinson's disease (PD), but it is difficult to isolate the symptoms and circuit alterations caused by cholinergic cell loss in PD, where multiple nuclei are simultaneously degenerating. Here we develop a novel mouse model of brainstem cholinergic cell loss to define the timeline of symptom development and map physiological adaptations within the brainstem. Mild motor symptoms, including balance and motor learning deficits reminiscent of early-stage PD, developed after cholinergic lesion with an unexpected long delay. Ex vivo whole-cell recordings showed that cholinergic cell loss increased excitatory synaptic drive onto glutamatergic PPN neurons and increased their intrinsic ability to generate sustained action potential output. In parallel, fiber photometry experiments revealed a slight increase in glutamatergic sensory responsiveness that closely tracked the delayed onset of behavioral symptoms. Together, these results reveal that brainstem cholinergic cell loss triggers the reorganization of motor-related brainstem circuits, providing critical insight into the consequences of non-dopaminergic degeneration in PD.