Normal cognition in Parkinson's disease may involve hippocampal cholinergic compensation: An exploratory PET imaging study with [18F]-FEOBV

Subregional analysis of the amygdala, thalamus, and hypothalamus at the pre-decline stage in Parkinson's disease with later cognitive impairment

Cognitive decline in Parkinson's disease (PD) significantly impacts patients' quality of life, yet early detection remains challenging. While structural brain abnormalities in cortical regions have been widely documented using magnetic resonance imaging (MRI), subcortical regions have received less analytical attention despite their potential role as early biomarkers. This study investigated changes in specific subregions of the amygdala, thalamus, and hypothalamus in patients with PD before cognitive decline development. We analyzed MRI data from 163 participants (97 healthy controls [HC] and 66 patients with PD) from the Parkinson's Progression Markers Initiative database. The patients with PD were classified based on cognitive status during a four-year follow-up: 21 who developed cognitive impairment (PDCI) and 45 who maintained normal cognition (PDNC). Cognitive function was assessed using the Montreal Cognitive Assessment and domain-specific tests. The PDCI group showed significantly lower corrected brain volumes in specific subregions of the amygdala (left basal nucleus), thalamus (bilateral lateral geniculate nuclei, right medial dorsal nucleus, and right anterior pulvinar nucleus), and hypothalamus (bilateral anterior-superior and left superior tubular parts) compared to that of HC. A significant difference between the PDCI and PDNC groups was observed only in the left lateral geniculate nucleus. In contrast, widespread structural changes were observed in cortical regions in the PDCI group, which showed stronger correlations with memory, attention, executive function, and visuospatial abilities. Hazard ratio analysis confirmed that structural changes in multiple cortical regions were significant predictors of cognitive decline. Although structural alterations were observed in subcortical regions, cortical changes demonstrated stronger associations with cognitive decline. These findings suggest that structural abnormalities may appear in the cerebral cortex before the stage proposed by conventional α-synuclein propagation models, potentially involving multiple mechanisms beyond α-synuclein, including global neural circuit dysfunction, disruption of neurotransmitter systems, breakdown of compensatory mechanisms, and coexisting pathologies (beta-amyloid and tau proteins). This study provides insights into early brain changes in PD and emphasizes the need for a comprehensive approach considering multiple mechanisms in early diagnosis and intervention strategies for PD-related cognitive impairment.

Cortical α4β2-nicotinic acetylcholine receptors and cognitive decline in Parkinson's disease

BackgroundAutopsy and in vivo molecular imaging studies suggest altered binding of the α4β2-nicotinic cholinergic receptor (α4β2-nAChR) with cognitive dysfunction in Parkinson's disease (PD).ObjectiveTo determine the relationship between cortical and hippocampal binding of the α4β2-nAChR with [18F]XTRA PET, a high-affinity radiotracer that enables quantification of α4β2-nAChR in these regions, and cognitive function in individuals with PD.MethodsIndividuals with PD (N = 32) and age-similar, controls without PD or dementia (N = 10) completed a cognitive assessment and one 90-min, [18F]XTRA PET scan. Metabolite-corrected arterial input function radioactivity time-activity curves were generated to obtain total distribution volume (VT) across 12 regions of interest (ROIs). [18F]XTRA binding was compared 1) between controls and people with PD and 2) between controls, persons with PD with normal cognition (PD-NC), and persons with PD with MCI (PD-MCI).Results[18F]XTRA binding was higher in the occipital cortex of the combined group of PD participants compared to age-similar controls. No regions showed lower binding in PD. VT with, but not without, partial volume correction was different between controls, PD-NC, and PD-MCI groups, and this was driven by higher binding in PD-MCI compared to controls. Regression of regional VT on cognitive domain T-scores, adjusting for age, showed that worse performance in visual-spatial memory tasks was associated with higher VT in the precuneus and the entire parietal cortex.ConclusionsHigher α4β2-nAChR binding in posterior cortical regions is found in PD and associated with worse visual perception and memory, possibly due to lower receptor occupancy by endogenous acetylcholine.

Heartbeat signature for predicting motor and non-motor involvement among nonparkinsonian LRRK2 G2019S mutation carriers

PURPOSE

Increased beat-to-beat heart rate variability (HRV) is a feature of patients with Parkinson's disease (PD) who carry the G2019S mutation in the LRRK2 gene (LRRK2-PD). Since LRRK2 mutations have incomplete penetrance, HRV changes preceding PD conversion would likely be observed only in a subset of LRRK2 non-manifesting carriers (NMC). We aimed to assess HRV in a subgroup of NMC with distinctive characteristics of LRRK2-PD, identified through clustering analysis.

METHODS

HRV measures derived from 300 normal heartbeat intervals extracted from the electrocardiograms of 25 NMC, 32 related non-carriers (RNC), 27 unrelated healthy controls, and 14 patients with LRRK2-PD were analyzed. Clinical symptoms were evaluated using questionnaires and scales, and three NMC subgroups were identified using a k-means cluster analysis on the basis of the deceleration capacity of heart rate (DC) and Rényi entropy. Standard and advanced HRV measures were compared using multiple regression analysis, controlling for age, sex, and mean heart rate.

RESULTS

Beat-to-beat HRV markers were significantly increased in a subgroup of seven NMC (NMC2, 28%) compared with RNC and controls. Increased irregularity and DC were also verified in the NMC2 compared with controls, and were typical traits in both the NMC2 and RNC. Overall, the HRV profile of NMC2 was comparable to that of patients with LRRK2-PD. NMC2 further exhibited greater motor and non-motor traits than the other NMC, RNC, and controls.

CONCLUSIONS

Our results confirmed that HRV characteristics of LRRK2-PD are also found in a subset of NMC displaying clinical traits of LRRK2-PD. Further research is needed to clarify whether higher HRV represents a LRRK2-PD prodromal manifestation.

Memory network and cognitive reserve are associated with preserved and stimulated cholinergic neurotransmission

The cholinergic system plays a central role in cognition and neural function, and, in Alzheimer disease (AD) and Lewy body disease (LBD), it has profound implications for cognitive impairment and dementia. The cholinergic forebrain pathway, innervating the neocortex and limbic system, is crucial for learning, memory, and other essential aspects of cognition and plays a wider role in promoting neuronal plasticity. Given the neuroplasticity processes characterizing the cholinergic regions, this system may be sensitive to modulatory phenomena such as cognitive reserve (CR). The concept of CR has been introduced to account for the fact that individual clinical presentation might be milder than expected based on neuropathology. This mismatch can be explained by individual brain reserve (BR) buildup on life experiences, lifestyles, and neurobiologic factors that are associated with resilience. Sparse findings exist suggesting that the CR might result in an increased or preserved function of the cholinergic system in AD patients, and compensatory mechanisms in the early stages of LBD. The limited availability of effective treatment for neurodegenerative dementia emphasizes the importance of CR and BR, as they play a major role in delaying or slowing disease onset and progression. This chapter will describe the involvement of the cholinergic system in neurodegenerative diseases and the tools for the in vivo assessment, focusing specifically on the evidence suggesting the possibility of its modulation by CR.

Cholinergic Basal Forebrain Integrity and Cognition in Parkinson's Disease: A Reappraisal of Magnetic Resonance Imaging Evidence

Cognitive impairment is a well-recognized and debilitating symptom of Parkinson's disease (PD). Degradation in the cortical cholinergic system is thought to be a key contributor. Both postmortem and in vivo cholinergic positron emission tomography (PET) studies have provided valuable evidence of cholinergic system changes in PD, which are pronounced in PD dementia (PDD). A growing body of literature has employed magnetic resonance imaging (MRI), a noninvasive, more cost-effective alternative to PET, to examine cholinergic system structural changes in PD. This review provides a comprehensive discussion of the methodologies and findings of studies that have focused on the relationship between cholinergic basal forebrain (cBF) integrity, based on T1- and diffusion-weighted MRI, and cognitive function in PD. Nucleus basalis of Meynert (Ch4) volume has been consistently reduced in cognitively impaired PD samples and has shown potential utility as a prognostic indicator for future cognitive decline. However, the extent of structural changes in Ch4, especially in early stages of cognitive decline in PD, remains unclear. In addition, evidence for structural change in anterior cBF regions in PD has not been well established. This review underscores the importance of continued cross-sectional and longitudinal research to elucidate the role of cholinergic dysfunction in the cognitive manifestations of PD. © 2024 The Author(s). Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

Anticholinergic Medication Burden and Cognitive Subtypes in Parkinson's Disease without Dementia

OBJECTIVE

Cognitive changes are heterogeneous in Parkinson's disease (PD). This study compared whether anticholinergic burden drives differences in cognitive domain performance and empirically-derived PD-cognitive phenotypes.

METHOD

A retrospective chart review contained participants (n = 493) who had idiopathic PD without dementia. Participants' medications were scored (0-3) and summed based on the anticholinergic cognitive burden scale (ACBS). We examined the ACBS' relationship to five cognitive domain composites (normative z-scores) and three (K-means clustering based) cognitive phenotypes: cognitively intact, low executive function (EF), and predominately impaired EF/memory. Analyses included Spearman correlations, analysis of covariance, and Pearson chi-squared test.

RESULTS

Overall, phenotypes did not differ in anticholinergic burden, and (after false-discovery-rate corrections) no cognitive domains related. When comparing those above and below the clinically relevant ACBS cutoff (i.e., score ≥3), no significant phenotype or domain differences were found.

CONCLUSIONS

Anticholinergic medication usage did not drive cognitive performance in a large clinical sample of idiopathic PD without dementia.

Parkinson's disease CA2-CA3 hippocampal atrophy is accompanied by increased cholinergic innervation in patients with normal cognition but not in patients with mild cognitive impairment

Although brain cholinergic denervation has been largely associated with cognitive decline in patients with Parkinson's disease (PD), new evidence suggests that cholinergic upregulation occurs in the hippocampus of PD patients without cognitive deficits. The specific hippocampal sectors and potential mechanisms of this cholinergic compensatory process have been further studied here, using MRI volumetry and morphometry coupled with molecular imaging using the PET radiotracer [18F]-Fluoroethoxybenzovesamicol ([18F]-FEOBV). Following a thorough screening procedure, 18 participants were selected and evenly distributed in three groups, including cognitively normal PD patients (PD-CN), PD patients with mild cognitive impairment (PD-MCI), and healthy volunteers (HV). Participants underwent a detailed neuropsychological assessment, structural MRI, and PET imaging with [18F]-FEOBV. Basal forebrain Ch1-Ch2 volumes were measured using stereotaxic mapping. Hippocampal subfields were automatically defined using the MAGeT-Brain segmentation algorithm. Cholinergic innervation density was quantified using [18F]-FEOBV uptake. Compared with HV, both PD-CN and PD-MCI displayed significantly reduced volumes in CA2-CA3 bilaterally. We found no other hippocampal subfield nor Ch1-Ch2 volume differences between the three groups. PET imaging revealed higher [18F]-FEOBV uptake in CA2-CA3 of the PD-CN compared with HV or PD-MCI. A positive correlation was observed between cognitive performances and [18F]-FEOBV uptake in the right CA2-CA3 subfield. Reduced volume, together with increased [18F]-FEOBV uptake, were observed specifically in the CA2-CA3 hippocampal subfields. However, while the volume change was observed in both PD-CN and PD-MCI, increased [18F]-FEOBV uptake was present only in the PD-CN group. This suggests that a cholinergic compensatory process takes place in the atrophied CA2-CA3 hippocampal subfields and might underlie normal cognition in PD.

Cholinergic changes in Lewy body disease: implications for presentation, progression and subtypes

Cholinergic degeneration is significant in Lewy body disease, including Parkinson's disease, dementia with Lewy bodies, and isolated REM sleep behaviour disorder. Extensive research has demonstrated cholinergic alterations in the CNS of these disorders. More recently, studies have revealed cholinergic denervation in organs that receive parasympathetic denervation. This enables a comprehensive review of cholinergic changes in Lewy body disease, encompassing both central and peripheral regions, various disease stages and diagnostic categories. Across studies, brain regions affected in Lewy body dementia show equal or greater levels of cholinergic impairment compared to the brain regions affected in Lewy body disease without dementia. This observation suggests a continuum of cholinergic alterations between these disorders. Patients without dementia exhibit relative sparing of limbic regions, whereas occipital and superior temporal regions appear to be affected to a similar extent in patients with and without dementia. This implies that posterior cholinergic cell groups in the basal forebrain are affected in the early stages of Lewy body disorders, while more anterior regions are typically affected later in the disease progression. The topographical changes observed in patients affected by comorbid Alzheimer pathology may reflect a combination of changes seen in pure forms of Lewy body disease and those seen in Alzheimer's disease. This suggests that Alzheimer co-pathology is important to understand cholinergic degeneration in Lewy body disease. Thalamic cholinergic innervation is more affected in Lewy body patients with dementia compared to those without dementia, and this may contribute to the distinct clinical presentations observed in these groups. In patients with Alzheimer's disease, the thalamus is variably affected, suggesting a different sequential involvement of cholinergic cell groups in Alzheimer's disease compared to Lewy body disease. Patients with isolated REM sleep behaviour disorder demonstrate cholinergic denervation in abdominal organs that receive parasympathetic innervation from the dorsal motor nucleus of the vagus, similar to patients who experienced this sleep disorder in their prodrome. This implies that REM sleep behaviour disorder is important for understanding peripheral cholinergic changes in both prodromal and manifest phases of Lewy body disease. In conclusion, cholinergic changes in Lewy body disease carry implications for understanding phenotypes and the influence of Alzheimer co-pathology, delineating subtypes and pathological spreading routes, and for developing tailored treatments targeting the cholinergic system.

Integrity of the nucleus basalis of meynert and self-reported cognitive dysfunction during wearing-off periods in parkinson's disease

BACKGROUND

Cognition in Parkinson's Disease can be impacted by the wearing-off phenomenon which results from changes in dopaminergic tone throughout the day. Given the well-established role of the cholinergic basal forebrain in cognition, we hypothesized that the Nucleus Basalis of Meynert may support cognitive processes during wearing-off periods. Specifically, we evaluated whether worsening of cognitive symptoms during wearing-off is more likely to occur with structural degeneration of the Nucleus Basalis of Meynert.

METHODS

Cognitive wearing-off was evaluated via the Movement Disorders Society Non-Motor Fluctuation Assessment Questionnaire in 33 Parkinson's Disease participants undergoing evaluation for deep brain stimulation. Pre-operative diffusion MRIs were used to measure brain diffusion metrics of the Nucleus Basalis of Meynert and control regions (caudate and putamen).

RESULTS

The number of cognitive symptoms which worsened during OFF periods positively correlated with mean diffusivity (ρ = 0.561, p = 0.0007) and generalized fractional anisotropy (ρ=-0.447, p = 0.009) within the Nucleus Basalis of Meynert but not in the caudate or putamen. Meanwhile, stable cognitive symptoms, and ON-state cognitive performance as measured by the DRS-2 did not correlate with Nucleus Basalis of Meynert metrics. Correlations were corrected for age, sex, scanner type, disease duration, education and LEDD.

CONCLUSIONS

Our study suggests that reduced structural integrity of the Nucleus Basalis of Meynert is associated with worsening of participant-reported cognitive deficits during OFF periods, but not overall cognitive functioning in the ON-state. These findings support the hypothesis that structural integrity of the cholinergic Nucleus Basalis of Meynert may provide resilience to cognitive worsening during dopamine-related wearing-off.

Impaired cholinergic integrity of the colon and pancreas in dementia with Lewy bodies

Dementia with Lewy bodies is characterized by a high burden of autonomic dysfunction and Lewy pathology in peripheral organs and components of the sympathetic and parasympathetic nervous system. Parasympathetic terminals may be quantified with 18F-fluoroetoxybenzovesamicol, a PET tracer that binds to the vesicular acetylcholine transporter in cholinergic presynaptic terminals. Parasympathetic imaging may be useful for diagnostics, improving our understanding of autonomic dysfunction and for clarifying the spatiotemporal relationship of neuronal degeneration in prodromal disease. Therefore, we aimed to investigate the cholinergic parasympathetic integrity in peripheral organs and central autonomic regions of subjects with dementia with Lewy bodies and its association with subjective and objective measures of autonomic dysfunction. We hypothesized that organs with known parasympathetic innervation, especially the pancreas and colon, would have impaired cholinergic integrity. To achieve these aims, we conducted a cross-sectional comparison study including 23 newly diagnosed non-diabetic subjects with dementia with Lewy bodies (74 ± 6 years, 83% male) and 21 elderly control subjects (74 ± 6 years, 67% male). We obtained whole-body images to quantify PET uptake in peripheral organs and brain images to quantify PET uptake in regions of the brainstem and hypothalamus. Autonomic dysfunction was assessed with questionnaires and measurements of orthostatic blood pressure. Subjects with dementia with Lewy bodies displayed reduced cholinergic tracer uptake in the pancreas (32% reduction, P = 0.0003) and colon (19% reduction, P = 0.0048), but not in organs with little or no parasympathetic innervation. Tracer uptake in a region of the medulla oblongata overlapping the dorsal motor nucleus of the vagus correlated with autonomic symptoms (rs = -0.54, P = 0.0077) and changes in orthostatic blood pressure (rs = 0.76, P < 0.0001). Tracer uptake in the pedunculopontine region correlated with autonomic symptoms (rs = -0.52, P = 0.0104) and a measure of non-motor symptoms (rs = -0.47, P = 0.0230). In conclusion, our findings provide the first imaging-based evidence of impaired cholinergic integrity of the pancreas and colon in dementia with Lewy bodies. The observed changes may reflect parasympathetic denervation, implying that this process is initiated well before the point of diagnosis. The findings also support that cholinergic denervation in the brainstem contributes to dysautonomia.

A multiple hits hypothesis for memory dysfunction in Parkinson disease

Cognitive disorders are increasingly recognized in Parkinson disease (PD), even in early disease stages, and memory is one of the most affected cognitive domains. Classically, hippocampal cholinergic system dysfunction was associated with memory disorders, whereas nigrostriatal dopaminergic system impairment was considered responsible for executive deficits. Evidence from PD studies now supports involvement of the amygdala, which modulates emotional attribution to experiences. Here, we propose a tripartite model including the hippocampus, striatum and amygdala as key structures for cognitive disorders in PD. First, the anatomo-functional relationships of these structures are explored and experimental evidence supporting their role in cognitive dysfunction in PD is summarized. We then discuss the potential role of α-synuclein, a pathological hallmark of PD, in the tripartite memory system as a key mechanism in the pathogenesis of memory disorders in the disease.

A cholinergic contribution to postural control and freezing of gait in Parkinson's disease

This scientific commentary refers to ‘Cholinergic system correlates of postural control changes in Parkinson's disease freezers’ by Roytman et al. (https://doi.org/10.1093/brain/awad134).

Atrophy of the Cholinergic Basal Forebrain can Detect Presynaptic Cholinergic Loss in Parkinson's Disease

OBJECTIVES

Structural imaging of the cholinergic basal forebrain may provide a biomarker for cholinergic system integrity that can be used in motor and non-motor outcome studies in Parkinson's disease. However, no prior studies have validated these structural metrics with cholinergic nerve terminal in vivo imaging in Parkinson's disease. Here, we correlate cholinergic basal forebrain morphometry with the topography of vesicular acetylcholine transporter in a large Parkinson's sample.

METHODS

[18 F]-Fluoroethoxybenzovesamicol vesicular acetylcholine transporter positron emission tomography was carried out in 101 non-demented people with Parkinson's (76.24% male, mean age 67.6 ± 7.72 years, disease duration 5.7 ± 4.4 years). Subregional cholinergic basal forebrain volumes were measured using magnetic resonance imaging morphometry. Relationships were assessed via volume-of-interest based correlation analysis.

RESULTS

Subregional volumes of the cholinergic basal forebrain predicted cholinergic nerve terminal loss, with most robust correlations occurring between the posterior cholinergic basal forebrain and temporofrontal, insula, cingulum, and hippocampal regions, and with modest correlations in parieto-occipital regions. Hippocampal correlations were not limited to the cholinergic basal forebrain subregion Ch1-2. Correlations were also observed in the striatum, thalamus, and brainstem.

INTERPRETATION

Cholinergic basal forebrain morphometry is a robust predictor of regional cerebral vesicular acetylcholine transporter bindings, especially in the anterior brain. The relative lack of correlation between parieto-occipital binding and basal forebrain volumes may reflect the presence of more diffuse synaptopathy in the posterior cortex due to etiologies that extend well beyond the cholinergic system. ANN NEUROL 2023;93:991-998.

Imaging in Movement Disorders

OBJECTIVE

This article reviews commonly used imaging modalities in movement disorders, particularly parkinsonism. The review includes the diagnostic utility, role in differential diagnosis, reflection of pathophysiology, and limitations of neuroimaging in the setting of movement disorders. It also introduces promising new imaging modalities and describes the current status of research.

LATEST DEVELOPMENTS

Iron-sensitive MRI sequences and neuromelanin-sensitive MRI can be used to directly assess the integrity of nigral dopaminergic neurons and thus may reflect disease pathology and progression throughout the full range of severity in Parkinson disease (PD). The striatal uptake of presynaptic radiotracers in their terminal axons as currently assessed using clinically approved positron emission tomography (PET) or single-photon emission computed tomography (SPECT) imaging correlates with nigral pathology and disease severity only in early PD. Cholinergic PET, using radiotracers that target the presynaptic vesicular acetylcholine transporter, constitutes a substantial advance and may provide crucial insights into the pathophysiology of clinical symptoms such as dementia, freezing, and falls.

ESSENTIAL POINTS

In the absence of valid, direct, objective biomarkers of intracellular misfolded α-synuclein, PD remains a clinical diagnosis. The clinical utility of PET- or SPECT-based striatal measures is currently limited given their lack of specificity and inability to reflect nigral pathology in moderate to severe PD. These scans may be more sensitive than clinical examination to detect nigrostriatal deficiency that occurs in multiple parkinsonian syndromes and may still be recommended for clinical use in the future to identify prodromal PD if and when disease-modifying treatments become available. Multimodal imaging to evaluate underlying nigral pathology and its functional consequences may hold the key to future advances.

Mapping Cholinergic Synaptic Loss in Parkinson's Disease: An [18F]FEOBV PET Case-Control Study

BACKGROUND

Cholinergic degeneration is strongly associated with cognitive decline in patients with Parkinson's disease (PD) but may also cause motor symptoms and olfactory dysfunction. Regional differences are striking and may reflect different PD related symptoms and disease progression patterns.

OBJECTIVE

To map and quantify the regional cerebral cholinergic alterations in non-demented PD patients.

METHODS

We included 15 non-demented PD patients in early-moderate disease stage and 15 age- and sex-matched healthy controls for [18F]FEOBV positron emission tomography imaging. We quantitated regional variations using VOI-based analyses which were supported by a vertex-wise cluster analysis. Correlations between imaging data and clinical and neuropsychological data were explored.

RESULTS

We found significantly decreased [18F]FEOBV uptake in global neocortex (38%, p = 0.0002). The most severe reductions were seen in occipital and posterior temporo-parietal regions (p < 0.0001). The vertex-wise cluster analysis corroborated these findings. All subcortical structures showed modest non-significant reductions. Motor symptoms (postural instability and gait difficulty) and cognition (executive function and composite z-score) correlated with regional [18F]FEOBV uptake (thalamus and cingulate cortex/insula/hippocampus, respectively), but the correlations were not statistically significant after multiple comparison correction. A strong correlation was found between interhemispheric [18F]FEOBV asymmetry, and motor symptom asymmetry of the extremities (r = 0.84, p = 0.0001).

CONCLUSION

Cortical cholinergic degeneration is prominent in non-demented PD patients, but more subtle in subcortical structures. Regional differences suggest uneven involvement of cholinergic nuclei in the brain and may represent a window to follow disease progression. The correlation between asymmetric motor symptoms and neocortical [18F]FEOBV asymmetry indicates that unilateral cholinergic degeneration parallels ipsilateral dopaminergic degeneration.

PET Imaging of Cholinergic Neurotransmission in Neurodegenerative Disorders

As a neuromodulator, the neurotransmitter acetylcholine plays an important role in cognitive, mood, locomotor, sleep/wake, and olfactory functions. In the pathophysiology of most neurodegenerative diseases, such as Alzheimer disease (AD) or Lewy body disorder (LBD), cholinergic receptors, transporters, or enzymes are involved and relevant as imaging targets. The aim of this review is to summarize current knowledge on PET imaging of cholinergic neurotransmission in neurodegenerative diseases. For PET imaging of presynaptic vesicular acetylcholine transporters (VAChT), (-)-¹⁸F-fluoroethoxybenzovesamicol (¹⁸F-FEOBV) was the first PET ligand that could be successfully translated to clinical application. Since then, the number of ¹⁸F-FEOBV PET investigations on patients with AD or LBD has grown rapidly and provided novel, important findings concerning the pathophysiology of AD and LBD. Regarding the α4β2 nicotinic acetylcholine receptors (nAChRs), various second-generation PET ligands, such as ¹⁸F-nifene, ¹⁸F-AZAN, ¹⁸F-XTRA, (-)-¹⁸F-flubatine, and (+)-¹⁸F-flubatine, were developed and successfully translated to human application. In neurodegenerative diseases such as AD and LBD, PET imaging of α4β2 nAChRs is of special value for monitoring disease progression and drugs directed to α4β2 nAChRs. For PET of α7 nAChR, ¹⁸F-ASEM and ¹¹C-MeQAA were successfully applied in mild cognitive impairment and AD, respectively. The highest potential for α7 nAChR PET is seen in staging, in evaluating disease progression, and in therapy monitoring. PET of selective muscarinic acetylcholine receptors (mAChRs) is still in an early stage, as the development of subtype-selective radioligands is complicated. Promising radioligands to image mAChR subtypes M1 (¹¹C-LSN3172176), M2 (¹⁸F-FP-TZTP), and M4 (¹¹C-MK-6884) were developed and successfully translated to humans. PET imaging of mAChRs is relevant for the assessment and monitoring of therapies in AD and LBD. PET of acetylcholine esterase activity has been investigated since the 1990s. Many PET studies with ¹¹C-PMP and ¹¹C-MP4A demonstrated cortical cholinergic dysfunction in dementia associated with AD and LBD. Recent studies indicated a solid relationship between subcortical and cortical cholinergic dysfunction and noncognitive dysfunctions such as balance and gait in LBD. Taken together, PET of distinct components of cholinergic neurotransmission is of great interest for diagnosis, disease monitoring, and therapy monitoring and to gain insight into the pathophysiology of different neurodegenerative disorders.

No Dopamine Agonist Modulation of Brain [18F]FEOBV Binding in Parkinson's Disease

The [¹⁸F]fluoroethoxybenzovesamicol ([¹⁸F]FEOBV) positron emission tomography (PET) ligand targets the vesicular acetylcholine transporter. Recent [¹⁸F]FEOBV PET rodent studies suggest that regional brain [¹⁸F]FEOBV binding may be modulated by dopamine D2-like receptor agents. We examined associations of regional brain [¹⁸F]FEOBV PET binding in Parkinson's disease (PD) subjects without versus with dopamine D2-like receptor agonist drug treatment. PD subjects (n = 108; 84 males, 24 females; mean age 68.0 ± 7.6 [SD] years), mean disease duration of 6.0 ± 4.0 years, and mean Movement Disorder Society-revised Unified PD Rating Scale III 35.5 ± 14.2 completed [¹⁸F]FEOBV brain PET imaging. Thirty-eight subjects were taking dopamine D2-like agonists. Vesicular monoamine transporter type 2 [¹¹C]dihydrotetrabenazine (DTBZ) PET was available in a subset of 54 patients. Subjects on dopamine D2-like agonists were younger, had a longer duration of disease, and were taking a higher levodopa equivalent dose (LED) compared to subjects not taking dopamine agonists. A group comparison between subjects with versus without dopamine D2-like agonist use did not yield significant differences in cortical, striatal, thalamic, or cerebellar gray matter [¹⁸F]FEOBV binding. Confounder analysis using age, duration of disease, LED, and striatal [¹¹C]DTBZ binding also failed to show significant regional [¹⁸F]FEOBV binding differences between these two groups. Chronic D2-like dopamine agonist use in PD subjects is not associated with significant alterations of regional brain [¹⁸F]FEOBV binding.

Cholinergic system changes in Parkinson's disease: emerging therapeutic approaches

In patients with Parkinson's disease, heterogeneous cholinergic system changes can occur in different brain regions. These changes correlate with a range of clinical features, both motor and non-motor, that are refractory to dopaminergic therapy, and can be conceptualised within a systems-level framework in which nodal deficits can produce circuit dysfunctions. The topographies of cholinergic changes overlap with neural circuitries involved in sleep and cognitive, motor, visuo-auditory perceptual, and autonomic functions. Cholinergic deficits within cognition network hubs predict cognitive deficits better than do total brain cholinergic changes. Postural instability and gait difficulties are associated with cholinergic system changes in thalamic, caudate, limbic, neocortical, and cerebellar nodes. Cholinergic system deficits can involve also peripheral organs. Hypercholinergic activity of mesopontine cholinergic neurons in people with isolated rapid eye movement (REM) sleep behaviour disorder, as well as in the hippocampi of cognitively normal patients with Parkinson's disease, suggests early compensation during the prodromal and early stages of Parkinson's disease. Novel pharmacological and neurostimulation approaches could target the cholinergic system to treat motor and non-motor features of Parkinson's disease.

Cholinergic systems, attentional-motor integration, and cognitive control in Parkinson's disease

Dysfunction and degeneration of CNS cholinergic systems is a significant component of multi-system pathology in Parkinson's disease (PD). We review the basic architecture of human CNS cholinergic systems and the tools available for studying changes in human cholinergic systems. Earlier post-mortem studies implicated abnormalities of basal forebrain corticopetal cholinergic (BFCC) and pedunculopontine-laterodorsal tegmental (PPN-LDT) cholinergic projections in cognitive deficits and gait-balance deficits, respectively. Recent application of imaging methods, particularly molecular imaging, allowed more sophisticated correlation of clinical features with regional cholinergic deficits. BFCC projection deficits correlate with general and domain specific cognitive deficits, particularly for attentional and executive functions. Detailed analyses suggest that cholinergic deficits within the salience and cingulo-opercular task control networks, including both neocortical, thalamic, and striatal nodes, are a significant component of cognitive deficits in non-demented PD subjects. Both BFCC and PPN-LDT cholinergic projection systems, and striatal cholinergic interneuron (SChI), abnormalities are implicated in PD gait-balance disorders. In the context of experimental studies, these results indicate that disrupted attentional functions of BFCC and PPN-LDT cholinergic systems underlie impaired gait-balance functions. SChI dysfunction likely impairs intra-striatal integration of attentional and motor information. Thalamic and entorhinal cortex cholinergic deficits may impair multi-sensory integration. Overt degeneration of CNS systems may be preceded by increased activity of cholinergic neurons compensating for nigrostriatal dopaminergic deficits. Subsequent dysfunction and degeneration of cholinergic systems unmasks and exacerbates functional deficits secondary to dopaminergic denervation. Research on CNS cholinergic systems dysfunctions in PD requires a systems-level approach to understanding PD pathophysiology.