Reduced cholinergic olfactory centrifugal inputs in patients with neurodegenerative disorders and MPTP-treated monkeys

Olfactory impairment is a common feature of neurodegenerative diseases such as Parkinson's disease (PD), Alzheimer's disease (AD) and dementia with Lewy bodies (DLB). Olfactory bulb (OB) pathology in these diseases shows an increased number of olfactory dopaminergic cells, protein aggregates and dysfunction of neurotransmitter systems. Since cholinergic denervation might be a common underlying pathophysiological feature, the objective of this study was to determine cholinergic innervation of the OB in 27 patients with histological diagnosis of PD (n = 5), AD (n = 14), DLB (n = 8) and 8 healthy control subjects. Cholinergic centrifugal inputs to the OB were clearly reduced in all patients, the most significant decrease being in the DLB group. We also studied cholinergic innervation of the OB in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated monkeys (n = 7) and 7 intact animals. In MPTP-monkeys, we found that cholinergic innervation of the OB was reduced compared to control animals (n = 7). Interestingly, in MPTP-monkeys, we also detected a loss of cholinergic neurons and decreased dopaminergic innervation in the horizontal limb of the diagonal band, which is the origin of the centrifugal cholinergic input to the OB. All these data suggest that cholinergic damage in the OB might contribute, at least in part, to the olfactory dysfunction usually exhibited by these patients. Moreover, decreased cholinergic input to the OB found in MPTP-monkeys suggests that dopamine depletion in itself might reduce the cholinergic tone of basal forebrain cholinergic neurons.