Degeneration of the cholinergic system in individuals with subjective cognitive decline: A systematic review

BACKGROUND

Subjective cognitive decline (SCD) is a risk factor for future cognitive impairment and dementia. It is uncertain whether the neurodegeneration of the cholinergic system is already present in SCD individuals. We aimed to review the current evidence about the association between SCD and biomarkers of degeneration in the cholinergic system.

METHOD

Original articles were extracted from three databases: Pubmed, Web of Sciences, and Scopus, in January 2023. Two researchers screened the studies independently.

RESULTS

A total of 11 research articles were selected. SCD was mostly based on amnestic cognitive complaints. Cholinergic system biomarkers included neuroimaging markers of basal forebrain volume, functional connectivity, transcranial magnetic stimulation, or biofluid. The evidence showed associations between basal forebrain atrophy, poorer connectivity of the cholinergic system, and SCD CONCLUSIONS: Degenerative changes in the cholinergic system can be present in SCD. Subjective complaints may help when identifying individuals with brain changes that are associated with cognitive impairment. These findings may have important implications in targeting individuals that may benefit from cholinergic-target treatments at very early stages of neurodegenerative diseases.

Structural connections between the noradrenergic and cholinergic system shape the dynamics of functional brain networks

Complex cognitive abilities are thought to arise from the ability of the brain to adaptively reconfigure its internal network structure as a function of task demands. Recent work has suggested that this inherent flexibility may in part be conferred by the widespread projections of the ascending arousal systems. While the different components of the ascending arousal system are often studied in isolation, there are anatomical connections between neuromodulatory hubs that we hypothesise are crucial for mediating key features of adaptive network dynamics, such as the balance between integration and segregation. To test this hypothesis, we estimated the strength of structural connectivity between key hubs of the noradrenergic and cholinergic arousal systems (the locus coeruleus [LC] and nucleus basalis of Meynert [nbM], respectively). We then asked whether the strength of structural LC and nbM inter-connectivity was related to individual differences in the emergent, dynamical signatures of functional integration measured from resting state fMRI data, such as network and attractor topography. We observed a significant positive relationship between the strength of white-matter connections between the LC and nbM and the extent of network-level integration following BOLD signal peaks in LC relative to nbM activity. In addition, individuals with denser white-matter streamlines interconnecting neuromodulatory hubs also demonstrated a heightened ability to shift to novel brain states. These results suggest that individuals with stronger structural connectivity between the noradrenergic and cholinergic systems have a greater capacity to mediate the flexible network dynamics required to support complex, adaptive behaviour. Furthermore, our results highlight the underlying static features of the neuromodulatory hubs can impose some constraints on the dynamic features of the brain.

Decreased ipsilateral [¹²³I]iododexetimide binding to cortical muscarinic receptors in unilaterally 6-hydroxydopamine lesioned rats

INTRODUCTION

Dysfunction of the cholinergic neurotransmitter system is present in Parkinson's disease, Parkinson's disease related dementia and dementia with Lewy bodies, and is thought to contribute to cognitive deficits in these patients. In vivo imaging of the cholinergic system in these diseases may be of value to monitor central cholinergic disturbances and to select cases in which treatment with cholinesterase inhibitors could be beneficial. The muscarinic receptor tracer [(123)I]iododexetimide, predominantly reflecting M1 receptor binding, may be an appropriate tool for imaging of the cholinergic system by means of SPECT. In this study, we used [(123)I]iododexetimide to study the effects of a 6-hydroxydopamine lesion (an animal model of Parkinson's disease) on the muscarinic receptor availability in the rat brain.

METHODS

Rats (n=5) were injected in vivo at 10-13 days after a confirmed unilateral 6-hydroxydopamine lesion. Muscarinic receptor availability was measured bilaterally in multiple brain areas on storage phosphor images by region of interest analysis.

RESULTS

Autoradiography revealed a consistent and statistically significant lower [(123)I]iododexetimide binding in all examined neocortical areas on the ipsilateral side of the lesion as compared to the contralateral side. In hippocampal and subcortical areas, such asymmetry was not detected.

CONCLUSIONS

This study suggests that evaluation of muscarinic receptor availability in dopamine depleted brains using [(123)I]iododexetimide is feasible. We conclude that 6-hydroxydopamine lesions induce a decrease of neocortical muscarinic receptor availability. We hypothesize that this arises from down regulation of muscarinic postsynaptic M1 receptors due to hyperactivation of the cortical cholinergic system in response to dopamine depletion.

ADVANCES IN KNOWLEDGE

In rats, dopamine depletion provokes a decrease in neocortical muscarinic receptor availability, which is evaluable by [(123)I]iododexetimide imaging.

IMPLICATIONS FOR PATIENT CARE

This study may further underline the role of a dysregulated muscarinic system in patients with Lewy body disorders.