Stereological analysis of cholinergic neurons within bilateral pedunculopontine nuclei in health and when affected by Parkinson's disease

During Parkinson's disease (PD), loss of brainstem-based pedunculopontine nucleus' (PPN) cholinergic neurons induces progressive postural-gait disability (PGD). PPN-deep brain stimulation inconsistently alleviates PGD, due to stereotactic targeting inaccuracies resulting from insufficiently detailed human PPN anatomical descriptions. Relatedly, rodent studies show rostro-caudal clustering of PPN-cholinergic neurons, reflecting functional sub-territories. We applied unbiased cerebro-bilateral 3-dimensional (3-D) stereology to post-mortem PPNs from PD versus neurological-control cases, to estimate total numbers of cholinergic neurons and describe their rostro-caudal distribution. Given ambiguous descriptions of the PPN's confines, we utilized two complimentary definitions of the PPN's anatomical boundaries. The first was based on the structure's gross anatomy, by considering the nucleus as a recognizable "channel" enclosed by distinct white matter fiber tracts (WMFT) encompassing the medial lemniscus, central tegmental tract and decussation of the superior cerebellar peduncle. Second, the PPN was recognized by its histological architecture, as a dense collection of cholinergic neurons (the "Ch5" group) that were immunoreactive for choline acetyltransferase (ChAT), the enzyme responsible for biosynthesis of the neurotransmitter acetylcholine. Many such ChAT-immunoreactive neurons were dispersed within the traversing tracks and hence the PPN's Ch5-based outlining method permitted their stereological capture while also allowing distinction between the PPN's two subnuclei, namely the pars compacta (PPNc) and pars dissipata (PPNd), based on subnuclei-specific cholinergic cytoarchitectural organization. We further reconstructed template data as 3-D renders, revealing gross morphological differences between control and PD-affected PPNs. PD brains revealed significant PPN cholinergic neuronal loss, particularly affecting the PPNd. Control cases showed bimodal clustering of cholinergic neurons, prominently affecting left-sided PPNs. Most PD cases revealed more severe cholinergic neuronal loss in right-sided PPNs, potentially driving symptom lateralization. Our study provides a comprehensive cholinergic cytoarchitectural atlas of the human PPN in health versus during PD.