Basal forebrain neurons in the dementia of Parkinson disease

Region-specific Cerebral Metabolic Alterations in Parkinson's Disease Patients With/without Mild Cognitive Impairment

N-acetylaspartate (NAA), choline (Cho) and creatine (Cr) are brain metabolites involved in some key neuronal functions within the brain, such as cognitive function. The aim of this study was to investigate whether Parkinson's disease (PD) with different cognitive status induces regional brain metabolite differences. 38 diagnosed PD patients, including 18 PD patients with normal cognitive (PDN), 20 PD subjects with cognitive impairment (PDMCI) and 25 healthy controls (HC) participated in this study. All subjects underwent a single-voxel proton MR spectroscopy (1H-MRS) on a 3T scanner. 1H-MRS were obtained from bilateral PCC, left thalamus and PFC regions in all subjects, respectively. Region-specific cerebral metabolic alterations existed in PD patients with different cognitive status. PDMCI patients showed a significant reduction of NAA, Cho and tCr in the PCC and left thalamus, compared to healthy controls; whereas lower levels of NAA and Cho in thalamus were found in PDN patients. Moreover, Cho and tCr levels were positively correlated with MMSE scores. Both NAA and tCr in PCC levels were positively correlated with MMSE and MoCA scores. The combination of thalamic and PCC metabolites showed a 75.6% accuracy in distinguishing PDMCI patients from PDN patients. This study provides preliminary evidence that thalamic, PCC and PFC neurometabolic alterations occur in PD patients with cognition decline. Findings of this study indicate that NAA and tCr abnormalities in PCC and thalamus might be used as a biomarker to track cognitive decline in Parkinson's disease in clinical settings.

The basal forebrain cholinergic system as target for cell replacement therapy in Parkinson's disease

In recent years there has been a renewed interest in the basal forebrain cholinergic system as a target for the treatment of cognitive impairments in patients with Parkinson's disease, due in part to the need to explore novel approaches to treat the cognitive symptoms of the disease and in part to the development of more refined imaging tools that have made it possible to monitor the progressive changes in the structure and function of the basal forebrain system as they evolve over time. In parallel, emerging technologies allowing the derivation of authentic basal forebrain cholinergic neurons from human pluripotent stem cells are providing new powerful tools for the exploration of cholinergic neuron replacement in animal models of Parkinson's disease-like cognitive decline. In this review, we discuss the rationale for cholinergic cell replacement as a potential therapeutic strategy in Parkinson's disease and how this approach can be explored in rodent models of Parkinson's disease-like cognitive decline, building on insights gained from the extensive animal experimental work that was performed in rodent and primate models in the 1980s and 90s. Although therapies targeting the cholinergic system have so far been focused mainly on patients with Alzheimer's disease, Parkinson's disease with dementia may be a more relevant condition. In Parkinson's disease with dementia, the basal forebrain system undergoes progressive degeneration and the magnitude of cholinergic cell loss has been shown to correlate with the level of cognitive impairment. Thus, cell therapy aimed to replace the lost basal forebrain cholinergic neurons represents an interesting strategy to combat some of the major cognitive impairments in patients with Parkinson's disease dementia.

Parkinson disease psychosis: from phenomenology to neurobiological mechanisms

Parkinson disease (PD) psychosis (PDP) is a spectrum of illusions, hallucinations and delusions that are associated with PD throughout its disease course. Psychotic phenomena can manifest from the earliest stages of PD and might follow a continuum from minor hallucinations to structured hallucinations and delusions. Initially, PDP was considered to be a complication associated with dopaminergic drug use. However, subsequent research has provided evidence that PDP arises from the progression of brain alterations caused by PD itself, coupled with the use of dopaminergic drugs. The combined dysfunction of attentional control systems, sensory processing, limbic structures, the default mode network and thalamocortical connections provides a conceptual framework to explain how new incoming stimuli are incorrectly categorized, and how aberrant hierarchical predictive processing can produce false percepts that intrude into the stream of consciousness. The past decade has seen the publication of new data on the phenomenology and neurobiological basis of PDP from the initial stages of the disease, as well as the neurotransmitter systems involved in PDP initiation and progression. In this Review, we discuss the latest clinical, neuroimaging and neurochemical evidence that could aid early identification of psychotic phenomena in PD and inform the discovery of new therapeutic targets and strategies.

A review of the neurotransmitter system associated with cognitive function of the cerebellum in Parkinson's disease

The dichotomized brain system is a concept that was generalized from the 'dual syndrome hypothesis' to explain the heterogeneity of cognitive impairment, in which anterior and posterior brain systems are independent but partially overlap. The dopaminergic system acts on the anterior brain and is responsible for executive function, working memory, and planning. In contrast, the cholinergic system acts on the posterior brain and is responsible for semantic fluency and visuospatial function. Evidence from dopaminergic/cholinergic imaging or functional neuroimaging has shed significant insight relating to the involvement of the cerebellum in the cognitive process of patients with Parkinson's disease. Previous research has reported evidence that the cerebellum receives both dopaminergic and cholinergic projections. However, whether these two neurotransmitter systems are associated with cognitive function has yet to be fully elucidated. Furthermore, the precise role of the cerebellum in patients with Parkinson's disease and cognitive impairment remains unclear. Therefore, in this review, we summarize the cerebellar dopaminergic and cholinergic projections and their relationships with cognition, as reported by previous studies, and investigated the role of the cerebellum in patients with Parkinson's disease and cognitive impairment, as determined by functional neuroimaging. Our findings will help us to understand the role of the cerebellum in the mechanisms underlying cognitive impairment in Parkinson's disease.

Cholinergic basal forebrain system degeneration underlies postural instability/gait difficulty and attention impairment in Parkinson's disease

BACKGROUND AND PURPOSE

The specific pathophysiological mechanisms underlying postural instability/gait difficulty (PIGD) and cognitive function in Parkinson's disease (PD) remain unclear. Both postural and gait control, as well as cognitive function, are associated with the cholinergic basal forebrain (cBF) system.

METHODS

A total of 84 PD patients and 82 normal controls were enrolled. Each participant underwent motor and cognitive assessments. Diffusion tensor imaging was used to detect structural abnormalities in the cBF system. The cBF was segmented using FreeSurfer, and its fiber tract was traced using probabilistic tractography. To provide information on extracellular water accumulation, free-water fraction (FWf) was quantified. FWf in the cBF and its fiber tract, as well as cortical projection density, were extracted for statistical analyses.

RESULTS

Patients had significantly higher FWf in the cBF (p < 0.001) and fiber tract (p = 0.021) than normal controls, as well as significantly lower cBF projection in the occipital (p < 0.001), parietal (p < 0.001) and prefrontal cortex (p = 0.005). In patients, a higher FWf in the cBF correlated with worse PIGD score (r = 0.306, p = 0.006) and longer Trail Making Test A time (r = 0.303, p = 0.007). Attentional function (Trail Making Test A) partially mediated the association between FWf in the cBF and PIGD score (indirect effect, a*b = 0.071; total effect, c = 0.256; p = 0.006).

CONCLUSIONS

Our findings suggest that degeneration of the cBF system in PD, from the cBF to its fiber tract and cortical projection, plays an important role in cognitive-motor interaction.

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.

Cortical Brain Injury Causes Retrograde Degeneration of Afferent Basal Forebrain Cholinergic Neurons via the p75NTR

Traumatic brain injury (TBI) elicits neuronal loss at the site of injury and progressive neuronal loss in the penumbra. However, the consequences of TBI on afferent neurons projecting to the injured tissue from distal locations is unknown. Basal forebrain cholinergic neurons (BFCNs) extend long projections to multiple brain regions including the cortex, regulate many cognitive functions, and are compromised in numerous neurodegenerative disorders. To determine the consequence of cortical injury on these afferent neurons, we used the fluid percussion injury model of traumatic brain injury and assessed the effects on BFCN survival and axon integrity in male and female mice. Survival or death of BF neurons can be regulated by neurotrophins or proneurotrophins, respectively. The injury elicited an induction of proNGF and proBDNF in the cortex and a loss of BFCNs ipsilateral to the injury compared with sham uninjured mice. The p75NTR knock-out mice did not show loss of BFCN neurons, indicating a retrograde degenerative effect of the cortical injury on the afferent BFCNs mediated through p75NTR. In contrast, locus ceruleus neurons, which also project throughout the cortex, were unaffected by the injury, suggesting specificity in retrograde degeneration after cortical TBI. Proneurotrophins (proNTs) provided directly to basal forebrain axons in microfluidic cultures triggered retrograde axonal degeneration and cell death, which did not occur in the absence of p75NTR. This study shows that after traumatic brain injury, proNTs induced in the injured cortex promote BFCN axonal degeneration and retrograde neuron loss through p75NTR.

Noradrenergic and cholinergic systems take centre stage in neuropsychiatric diseases of ageing

Noradrenergic and cholinergic systems are among the most vulnerable brain systems in neuropsychiatric diseases of ageing, including Alzheimer's disease, Parkinson's disease, Lewy body dementia, and progressive supranuclear palsy. As these systems fail, they contribute directly to many of the characteristic cognitive and psychiatric symptoms. However, their contribution to symptoms is not sufficiently understood, and pharmacological interventions targeting noradrenergic and cholinergic systems have met with mixed success. Part of the challenge is the complex neurobiology of these systems, operating across multiple timescales, and with non-linear changes across the adult lifespan and disease course. We address these challenges in a detailed review of the noradrenergic and cholinergic systems, outlining their roles in cognition and behaviour, and how they influence neuropsychiatric symptoms in disease. By bridging across levels of analysis, we highlight opportunities for improving drug therapies and for pursuing personalised medicine strategies.

Direct Enhancement Effect of Hippocampal Cholinergic Neurostimulating Peptide on Cholinergic Activity in the Hippocampus

The cholinergic efferent network from the medial septal nucleus to the hippocampus is crucial for learning and memory. This study aimed to clarify whether hippocampal cholinergic neurostimulating peptide (HCNP) has a rescue function in the cholinergic dysfunction of HCNP precursor protein (HCNP-pp) conditional knockout (cKO). Chemically synthesized HCNP or a vehicle were continuously administered into the cerebral ventricle of HCNP-pp cKO mice and littermate floxed (control) mice for two weeks via osmotic pumps. We immunohistochemically measured the cholinergic axon volume in the stratum oriens and functionally evaluated the local field potential in the CA1. Furthermore, choline acetyltransferase (ChAT) and nerve growth factor (NGF) receptor (TrkA and p75NTR) abundances were quantified in wild-type (WT) mice administered HCNP or the vehicle. As a result, HCNP administration morphologically increased the cholinergic axonal volume and electrophysiological theta power in HCNP-pp cKO and control mice. Following the administration of HCNP to WT mice, TrkA and p75NTR levels also decreased significantly. These data suggest that extrinsic HCNP may compensate for the reduced cholinergic axonal volume and theta power in HCNP-pp cKO mice. HCNP may function complementarily to NGF in the cholinergic network in vivo. HCNP may represent a therapeutic candidate for neurological diseases with cholinergic dysfunction, e.g., Alzheimer's disease and Lewy body dementia.

Cholinergic nucleus 4 grey matter density is associated with apathy in Parkinson's disease

Objective: The generation and maintenance of goal-directed behavior is subserved by multiple brain regions that receive cholinergic inputs from the cholinergic nucleus 4 (Ch4). It is unknown if Ch4 degeneration contributes to apathy in Parkinson's disease (PD). Method: We analyzed data from 106 pre-surgical patients with PD who had brain MRIs and completed the Frontal Systems Behavior Scales (FrSBe). Eighty-eight patients also completed the Beck Depression Inventory-2nd Edition. Cholinergic basal forebrain grey matter densities (GMD) were measured by applying probabilistic maps to T1 MPRAGE sequences processed using voxel-based morphometry methods. We used linear and hierarchical regression modelling to examine the association between Ch4 GMD and the FrSBe Apathy subscale scores. We used similar methods to assess the specificity of this association and potential associations between Ch4 target regions and apathy. Results: Ch4 GMD (p = .021) and Ch123 GMD (p = .032) were significantly associated with Apathy subscale scores on univariate analysis. Ch4 GMD, but not Ch123 GMD, remained significantly associated with apathy when adjusting for age, sex, levodopa equivalent doses, and disease duration. Centromedial amygdala GMD, which receives cholinergic inputs from Ch4, was also associated with apathy. Ch4 GMD was not associated with depression or disinhibition, nor was it associated with executive dysfunction when adjusting for clinical and demographic variables. Conclusions: Ch4 GMD is specifically associated with apathy in PD. Ch4 degeneration results in cholinergic denervation of multiple cortical and limbic regions, which may contribute to the cognitive and emotional-affective processing deficits that underlie the behavioral symptoms of apathy.

Determinants of approved acetylcholinesterase inhibitor response outcomes in Alzheimer's disease: relevance for precision medicine in neurodegenerative diseases

Acetylcholinesterase inhibitors (ChEI) are the global standard of care for the symptomatic treatment of Alzheimer's disease (AD) and show significant positive effects in neurodegenerative diseases with cognitive and behavioral symptoms. Although experimental and large-scale clinical evidence indicates the potential long-term efficacy of ChEI, primary outcomes are generally heterogeneous across outpatient clinics and regional healthcare systems. Sub-optimal dosing or slow tapering, heterogeneous guidelines about the timing for therapy initiation (prodromal versus dementia stages), healthcare providers' ambivalence to treatment, lack of disease awareness, delayed medical consultation, prescription of ChEI in non-AD cognitive disorders, contribute to the negative outcomes. We present an evidence-based overview of determinants, spanning genetic, molecular, and large-scale networks, involved in the response to ChEI in patients with AD and other neurodegenerative diseases. A comprehensive understanding of cerebral and retinal cholinergic system dysfunctions along with ChEI response predictors in AD is crucial since disease-modifying therapies will frequently be prescribed in combination with ChEI. Therapeutic algorithms tailored to genetic, biological, clinical (endo)phenotypes, and disease stages will help leverage inter-drug synergy and attain optimal combined response outcomes, in line with the precision medicine model.

Inconsistencies in atlas-based volumetric measures of the human nucleus basalis of Meynert: A need for high-resolution alternatives

The nucleus basalis of Meynert (nbM) is the major source of cortical acetylcholine (ACh) and has been related to cognitive processes and to neurological disorders. However, spatially delineating the human nbM in MRI studies remains challenging. Due to the absence of a functional localiser for the human nbM, studies to date have localised it using nearby neuroanatomical landmarks or using probabilistic atlases. To understand the feasibility of MRI of the nbM we set our four goals; our first goal was to review current human nbM region-of-interest (ROI) selection protocols used in MRI studies, which we found have reported highly variable nbM volume estimates. Our next goal was to quantify and discuss the limitations of existing atlas-based volumetry of nbM. We found that the identified ROI volume depends heavily on the atlas used and on the probabilistic threshold set. In addition, we found large disparities even for data/studies using the same atlas and threshold. To test whether spatial resolution contributes to volume variability, as our third goal, we developed a novel nbM mask based on the normalized BigBrain dataset. We found that as long as the spatial resolution of the target data was 1.3 mm isotropic or above, our novel nbM mask offered realistic and stable volume estimates. Finally, as our last goal we tried to discern nbM using publicly available and novel high resolution structural MRI ex vivo MRI datasets. We find that, using an optimised 9.4T quantitative T₂^⁎ ex vivo dataset, the nbM can be visualised using MRI. We conclude caution is needed when applying the current methods of mapping nbM, especially for high resolution MRI data. Direct imaging of the nbM appears feasible and would eliminate the problems we identify, although further development is required to allow such imaging using standard (f)MRI scanning.

Ultrastructural and biochemical classification of pathogenic tau, α-synuclein and TDP-43

Intracellular accumulation of abnormal proteins with conformational changes is the defining neuropathological feature of neurodegenerative diseases. The pathogenic proteins that accumulate in patients' brains adopt an amyloid-like fibrous structure and exhibit various ultrastructural features. The biochemical analysis of pathogenic proteins in sarkosyl-insoluble fractions extracted from patients' brains also shows disease-specific features. Intriguingly, these ultrastructural and biochemical features are common within the same disease group. These differences among the pathogenic proteins extracted from patients' brains have important implications for definitive diagnosis of the disease, and also suggest the existence of pathogenic protein strains that contribute to the heterogeneity of pathogenesis in neurodegenerative diseases. Recent experimental evidence has shown that prion-like propagation of these pathogenic proteins from host cells to recipient cells underlies the onset and progression of neurodegenerative diseases. The reproduction of the pathological features that characterize each disease in cellular and animal models of prion-like propagation also implies that the structural differences in the pathogenic proteins are inherited in a prion-like manner. In this review, we summarize the ultrastructural and biochemical features of pathogenic proteins extracted from the brains of patients with neurodegenerative diseases that accumulate abnormal forms of tau, α-synuclein, and TDP-43, and we discuss how these disease-specific properties are maintained in the brain, based on recent experimental insights.

Basal Forebrain Impairment: Understanding the Mnemonic Function of the Septal Region Translates in Therapeutic Advances

The basal forebrain is one of the three major brain circuits involved in episodic memory formation together with the hippocampus and the diencephalon. The dysfunction of each of these regions is known to cause anterograde amnesia. While the hippocampal pyramidal neurons are known to encode episodic information and the diencephalic structures are known to provide idiothetic information, the contribution of the basal forebrain to memory formation has been exclusively associated with septo-hippocampal cholinergic signaling. Research data from the last decade broadened our understanding about the role of septal region in memory formation. Animal studies revealed that septal neurons process locomotor, rewarding and attentional stimuli. The integration of these signals results in a systems model for the mnemonic function of the medial septum that could guide new therapeutic strategies for basal forebrain impairment (BFI). BFI includes the disorders characterized with basal forebrain amnesia and neurodegenerative disorders that affect the basal forebrain. Here, we demonstrate how the updated model of septal mnemonic function can lead to innovative translational treatment approaches that include pharmacological, instrumental and behavioral techniques.

Anticholinergic Medication Burden in Parkinson's Disease Outpatients

BACKGROUND

Individuals with Parkinson's disease (PD) may be especially vulnerable to future cognitive decline from anticholinergic medications.

OBJECTIVE

To characterize anticholinergic medication burden, determine the co-occurrence of anticholinergic and cholinesterase inhibitors, and to assess the correlations among anticholinergic burden scales in PD outpatients.

METHODS

We studied 670 PD outpatients enrolled in a clinic registry between 2012 and 2020. Anticholinergic burden was measured with the Anticholinergic Cognitive Burden Scale (ACB), Anticholinergic Drug Scale (ADS), Anticholinergic Risk Scale (ARS), and Drug Burden Index-Anticholinergic component (DBI-Ach). Correlations between scales were assessed with weighted kappa coefficients.

RESULTS

Between 31.5 to 46.3% of PD patients were taking medications with anticholinergic properties. Among the scales applied, the ACB produced the highest prevalence of medications with anticholinergic properties (46.3%). Considering only medications with definite anticholinergic activity (scores of 2 or 3 on ACB, ADS, or ARS), the most common anticholinergic drug classes were antiparkinsonian (8.2%), antipsychotic (6.4%), and urological (3.3%) medications. Cholinesterase inhibitors and medications with anticholinergic properties were co-prescribed to 5.4% of the total cohort. The most highly correlated scales were ACB and ADS (κ= 0.71), ACB and ARS (κ= 0.67), and ADS and ARS (κ= 0.55).

CONCLUSION

A high proportion of PD patients (20%) were either taking antiparkinsonian, urological, or antipsychotic anticholinergic medications or were co-prescribed anticholinergic medications and cholinesterase inhibitors. By virtue of its detection of a high prevalence of anticholinergic medication usage and its high correlation with other scales, our data support use of the ACB scale to assess anticholinergic burden in PD patients.

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.

Cognition and serotonin in Parkinson's disease

Cognitive impairment affects up to 80% of patients with Parkinson's disease (PD) and is associated with poor quality of life. PD cognitive dysfunction includes poor working memory, impairments in executive function and difficulty in set-shifting. The pathophysiology underlying cognitive impairment in PD is still poorly understood, but there is evidence to support involvements of the cholinergic, dopaminergic, and noradrenergic systems. Only rivastigmine, an acetyl- and butyrylcholinesterase inhibitor, is efficacious for the treatment of PD dementia, which limits management of cognitive impairment in PD. Whereas the role of the serotonergic system in PD cognition is less understood, through its interactions with other neurotransmitters systems, namely, the cholinergic system, it may be implicated in cognitive processes. In this chapter, we provide an overview of the pharmacological, clinical and pathological evidence that implicates the serotonergic system in mediating cognition in PD.

Neuromodulation of cognition in Parkinson's disease

Neuromodulation is a widely used treatment for motor symptoms of Parkinson's disease (PD). It can be a highly effective treatment as a result of knowledge of circuit dysfunction associated with motor symptoms in PD. However, the mechanisms underlying cognitive symptoms of PD are less well-known, and the effects of neuromodulation on these symptoms are less consistent. Nonetheless, neuromodulation provides a unique opportunity to modulate motor and cognitive circuits while minimizing off-target side effects. We review the modalities of neuromodulation used in PD and the potential implications for cognitive symptoms. There have been some encouraging findings with both invasive and noninvasive modalities of neuromodulation, and there are promising advances being made in the field of therapeutic neuromodulation. Substantial work is needed to determine which modulation targets are most effective for the different types of cognitive deficits of PD.

Retrograde Axonal Transport of Neurotrophins in Basal Forebrain Cholinergic Neurons

Axonal transport is key for the survival and function of all neurons. This process is especially important in basal forebrain cholinergic neurons due to their extremely long and diffuse axonal projections. These neurons are critical for learning and memory and degenerate rapidly in age-related neurodegenerative disorders like Alzheimer's and Parkinson's disease. The vulnerability of these neurons to age-related neurodegeneration may be partially attributed to their reliance on retrograde axonal transport for neurotrophic support. Unfortunately, little is known about the molecular biology underlying the retrograde transport dynamics of these neurons due to the difficulty associated with their maintenance in vitro. Here, we outline a protocol for culturing primary rodent basal forebrain cholinergic neurons in microfluidic chambers, devices designed specifically for the study of axonal transport in vitro. We outline protocols for labeling neurotrophins and tracking neurotrophin transport in these neurons. Our protocols can also be used to study axonal transport in other types of primary neurons such as cortical and hippocampal neurons.

Data-driven identification of diagnostically useful extrastriatal signal in dopamine transporter SPECT using explainable AI

This study used explainable artificial intelligence for data-driven identification of extrastriatal brain regions that can contribute to the interpretation of dopamine transporter SPECT with ¹²³I-FP-CIT in parkinsonian syndromes. A total of 1306 ¹²³I-FP-CIT-SPECT were included retrospectively. Binary classification as ‘reduced’ or ‘normal’ striatal ¹²³I-FP-CIT uptake by an experienced reader served as standard-of-truth. A custom-made 3-dimensional convolutional neural network (CNN) was trained for classification of the SPECT images with 1006 randomly selected images in three different settings: “full image”, “striatum only” (3-dimensional region covering the striata cropped from the full image), “without striatum” (full image with striatal region removed). The remaining 300 SPECT images were used to test the CNN classification performance. Layer-wise relevance propagation (LRP) was used for voxelwise quantification of the relevance for the CNN-based classification in this test set. Overall accuracy of CNN-based classification was 97.0%, 95.7%, and 69.3% in the “full image”, “striatum only”, and “without striatum” setting. Prominent contributions in the LRP-based relevance maps beyond the striatal signal were detected in insula, amygdala, ventromedial prefrontal cortex, thalamus, anterior temporal cortex, superior frontal lobe, and pons, suggesting that ¹²³I-FP-CIT uptake in these brain regions provides clinically useful information for the differentiation of neurodegenerative and non-neurodegenerative parkinsonian syndromes.

Antimuscarinic Anticholinergic Medications in Parkinson Disease: To Prescribe or Deprescribe?

The relative importance of antimuscarinic anticholinergic medications for Parkinson's disease (PD) declined after the introduction of levodopa, such that anticholinergic medications are now much more likely to be prescribed for clinical indications other than parkinsonism. Recent studies have found an association between anticholinergic medication exposure and future risk of dementia in older individuals and those with PD. These findings provide a further reason to avoid the use of anticholinergic medications to treat motor symptoms of PD. More importantly, they raise the question of whether one of the goals of PD treatment should be to deprescribe all medications with anticholinergic properties, regardless of their indication, to reduce dementia risk. In this review, we discuss the use of anticholinergic medications in PD, the evidence supporting the association between anticholinergic medications and future dementia risk, and the potential implications of these findings for clinical care in PD.

Basal forebrain cholinergic system in the dementias: Vulnerability, resilience, and resistance

The basal forebrain cholinergic neurons (BFCN) provide the primary source of cholinergic innervation of the human cerebral cortex. They are involved in the cognitive processes of learning, memory, and attention. These neurons are differentially vulnerable in various neuropathologic entities that cause dementia. This review summarizes the relevance to BFCN of neuropathologic markers associated with dementias, including the plaques and tangles of Alzheimer's disease (AD), the Lewy bodies of diffuse Lewy body disease, the tauopathy of frontotemporal lobar degeneration (FTLD-TAU) and the TDP-43 proteinopathy of FTLD-TDP. Each of these proteinopathies has a different relationship to BFCN and their corticofugal axons. Available evidence points to early and substantial degeneration of the BFCN in AD and diffuse Lewy body disease. In AD, the major neurodegenerative correlate is accumulation of phosphotau in neurofibrillary tangles. However, these neurons are less vulnerable to the tauopathy of FTLD. An intriguing finding is that the intracellular tau of AD causes destruction of the BFCN, whereas that of FTLD does not. This observation has profound implications for exploring the impact of different species of tauopathy on neuronal survival. The proteinopathy of FTLD-TDP shows virtually no abnormal inclusions within the BFCN. Thus, the BFCN are highly vulnerable to the neurodegenerative effects of tauopathy in AD, resilient to the neurodegenerative effect of tauopathy in FTLD and apparently resistant to the emergence of proteinopathy in FTLD-TDP and perhaps also in Pick's disease. Investigations are beginning to shed light on the potential mechanisms of this differential vulnerability and their implications for therapeutic intervention.

Locus Coeruleus Pathology Indicates a Continuum of Lewy Body Dementia

BACKGROUND

Lewy body dementia (LBD) has two main phenotypes of LBD, Parkinson's disease dementia (PDD) and dementia with Lewy bodies (DLB), separated by the 'one-year-rule'. They also show different symptom profiles: core DLB features include fluctuating cognition, REM-sleep behaviur disorder, and visual hallucinations. These symptoms are sometimes present in PDD, representing an intermediate 'PDD-DLB' phenotype.

OBJECTIVE

DLB-like features may reflect deficits in the functions of the noradrenergic nucleus locus coeruleus (LC). Therefore, we compared the LC in the LBD phenotypes, PD, and controls.

METHODS

38 PD, 56 PDD, 22 DLB, and 11 age-matched control cases from the Parkinson's UK tissue bank were included. LC tissue sections were immunostained for tyrosine-hydroxylase (TH), α-synuclein, tau, and amyloid-β. TH-neurons were quantified and pathologic burden calculated by %-coverage method.

RESULTS

The LC shows a stepwise reduction in neuron count from controls, PD, PDD, to DLB. PDD-DLB cases showed an intermediate clinical phenotype that was reflected pathologically. Cell counts were significantly reduced in DLB compared to PDD after correction for demographic factors. LC degeneration contributed significantly to the onset of all DLB symptoms. While α-synuclein was not significantly different between PDD and DLB cases, DLB exhibited significantly less tau pathology.

CONCLUSION

DLB and DLB-like symptoms represent noradrenergic deficits resulting from neuronal loss in the LC. PDD and DLB are likely to represent a clinical continuum based on the presence or absence of DLB-like symptoms mirrored by a pathological continuum in the LC.

Olfaction, cholinergic basal forebrain degeneration, and cognition in early Parkinson disease

INTRODUCTION

Impaired olfaction and reduced cholinergic nucleus 4 (Ch4) volume both predict greater cognitive decline in Parkinson's disease (PD). We examined the relationship between olfaction, longitudinal change in cholinergic basal forebrain nuclei and their target regions, and cognition in early PD.

METHODS

We analyzed a cohort of 97 PD participants from the Parkinson's Progression Markers Initiative with brain MRIs at baseline, 1 year, 2 years, and 4 years. Using probabilistic maps, regional grey matter density (GMD) was calculated for Ch4, cholinergic nuclei 1, 2, and 3 (Ch123), and their target regions.

RESULTS

Baseline University of Pennsylvania Smell Identification Test score correlated with change in GMD of all regions of interest (all p < 0.05). Rate of change of Ch4 GMD was correlated with rate of change of Ch123 (p = 0.034), cortex (p = 0.001), and amygdala GMD (p < 0.001), but not hippocampus GMD (p = 0.38). Rate of change of Ch123 GMD was correlated with rate of change of cortex (p = 0.001) and hippocampus (p < 0.001), but not amygdala GMD (p = 0.133). In a linear regression model including change in GMD of all regions of interest and age as predictors, change in cortex GMD (βˆ_slope= 38.2; 95 % CI: [0.47, 75.9]) and change in hippocampus GMD (βˆ_slope= 24.8; 95 % CI: [0.80, 48.8]) were significant predictors of Montreal Cognitive Assessment score change over time.

CONCLUSION

Impaired olfaction is associated with degeneration of the cholinergic basal forebrain and bilateral cortex, amygdala, and hippocampus in PD. The relationship between impaired olfaction and cognitive decline may be mediated by greater atrophy of the cortex and hippocampus.

The Biology and Pathobiology of Glutamatergic, Cholinergic, and Dopaminergic Signaling in the Aging Brain

The elderly population is growing worldwide, with important health and socioeconomic implications. Clinical and experimental studies on aging have uncovered numerous changes in the brain, such as decreased neurogenesis, increased synaptic defects, greater metabolic stress, and enhanced inflammation. These changes are associated with cognitive decline and neurobehavioral deficits. Although aging is not a disease, it is a significant risk factor for functional worsening, affective impairment, disease exaggeration, dementia, and general disease susceptibility. Conversely, life events related to mental stress and trauma can also lead to accelerated age-associated disorders and dementia. Here, we review human studies and studies on mice and rats, such as those modeling human neurodegenerative diseases, that have helped elucidate (1) the dynamics and mechanisms underlying the biological and pathological aging of the main projecting systems in the brain (glutamatergic, cholinergic, and dopaminergic) and (2) the effect of defective glutamatergic, cholinergic, and dopaminergic projection on disabilities associated with aging and neurodegenerative disorders, such as Alzheimer's and Parkinson's diseases. Detailed knowledge of the mechanisms of age-related diseases can be an important element in the development of effective ways of treatment. In this context, we briefly analyze which adverse changes associated with neurodegenerative diseases in the cholinergic, glutaminergic and dopaminergic systems could be targeted by therapeutic strategies developed as a result of our better understanding of these damaging mechanisms.

Transcranial Magnetic Stimulation as a Diagnostic and Therapeutic Tool in Various Types of Dementia

Dementia is recognized as a healthcare and social burden and remains challenging in terms of proper diagnosis and treatment. Transcranial magnetic stimulation (TMS) is a diagnostic and therapeutic tool in various neurological diseases that noninvasively investigates cortical excitability and connectivity and can induce brain plasticity. This article reviews findings on TMS in common dementia types as well as therapeutic results. Alzheimer’s disease (AD) is characterized by increased cortical excitability and reduced cortical inhibition, especially as mediated by cholinergic neurons and as documented by impairment of short latency inhibition (SAI). In vascular dementia, excitability is also increased. SAI may have various outcomes, which probably reflects its frequent overlap with AD. Dementia with Lewy bodies (DLB) is associated with SAI decrease. Motor cortical excitability is usually normal, reflecting the lack of corticospinal tract involvement. DLB and other dementia types are also characterized by impairment of short interval intracortical inhibition. In frontotemporal dementia, cortical excitability is increased, but SAI is normal. Repetitive transcranial magnetic stimulation has the potential to improve cognitive function. It has been extensively studied in AD, showing promising results after multisite stimulation. TMS with electroencephalography recording opens new possibilities for improving diagnostic accuracy; however, more studies are needed to support the existing data.

Cortical connectivity of the nucleus basalis of Meynert in Parkinson's disease and Lewy body dementias

Parkinson's disease dementia (PDD) and dementia with Lewy bodies (DLB) are related conditions that are associated with cholinergic system dysfunction. Dysfunction of the nucleus basalis of Meynert (NBM), a basal forebrain structure that provides the dominant source of cortical cholinergic innervation, has been implicated in the pathogenesis of both PDD and DLB. Here we leverage the temporal resolution of magnetoencephalography with the spatial resolution of MRI tractography to explore the intersection of functional and structural connectivity of the NBM in a unique cohort of PDD and DLB patients undergoing deep brain stimulation of this structure. We observe that NBM-cortical structural and functional connectivity correlate within spatially and spectrally segregated networks including: (i) a beta band network to supplementary motor area, where activity in this region was found to drive activity in the NBM; (ii) a delta/theta band network to medial temporal lobe structures encompassing the parahippocampal gyrus; and (iii) a delta/theta band network to visual areas including lingual gyrus. These findings reveal functional networks of the NBM that are likely to subserve important roles in motor control, memory and visual function, respectively. Furthermore, they motivate future studies aimed at disentangling network contribution to disease phenotype.

Meynert nucleus-related cortical thinning in Parkinson's disease with mild cognitive impairment

Background

Cognitive impairment in Parkinson's disease (PD) involves the cholinergic system and cholinergic neurons, especially the nucleus basalis of Meynert (NBM/Ch4) located in the basal forebrain (BF). We analyzed associations between NBM/Ch4 volume and cortical thickness to determine whether the NBM/Ch4-innervated neocortex shows parallel atrophy with the NBM/Ch4 as disease progresses in PD patients with cognitive impairment (PD-MCI).

Methods

We enrolled 35 PD-MCI patients, 48 PD patients with normal cognition (PD-NC), and 33 age- and education-matched healthy controls (HCs), with all participants undergoing neuropsychological assessment and structural magnetic resonance imaging (MRI). Correlation analyses between NBM/Ch4 volume and cortical thickness and correlation coefficient comparisons were conducted within and across groups.

Results

In the PD-MCI group, NBM/Ch4 volume was positively correlated with cortical thickness in the bilateral posterior cingulate, parietal, and frontal and left insular regions. Based on correlation coefficient comparisons, the atrophy of NBM/Ch4 was more correlated with the cortical thickness of right posterior cingulate and precuneus, anterior cingulate and medial orbitofrontal lobe in PD-MCI versus HC, and the right medial orbitofrontal lobe and anterior cingulate in PD-NC versus HC. Further partial correlations between cortical thickness and NBM/Ch4 volume were significant in the right medial orbitofrontal (PD-NC: r=0.3, P=0.045; PD-MCI: r=0.51, P=0.003) and anterior cingulate (PD-NC: r=0.41, P=0.006; PD-MCI: r=0.43, P=0.013) in the PD groups and in the right precuneus (r=0.37, P=0.04) and posterior cingulate (r=0.46, P=0.008) in the PD-MCI group.

Conclusions

The stronger correlation between NBM/Ch4 and cortical thinning in PD-MCI patients suggests that NBM/Ch4 volume loss may play an important role in PD cognitive impairment.

The diagonal band of Broca in health and disease

The diagonal band of Broca (DBB) contains the second largest cholinergic cell group in the human brain, known as the nucleus of the vertical limb of the DBB (nvlDBB). It has major projections to the hippocampus, but it is often underinvestigated, partly due to its ill-defined anatomical boundaries and hence the difficulty of reliable sampling. In this chapter, we have reviewed the historical literature to reestablish the anatomy of the nvlDBB, distinguishing it from neighboring basal forebrain cholinergic nuclei. Although varying degrees of neuronal loss in the nvlDBB have been reported in a range of neurological disorders, and in the aged brain, the significant nvlDBB cholinergic neuronal loss reported in Lewy body dementias is of particular interest. Retrograde tracer study in rodents has demonstrated reciprocal connections between the DBB and the hippocampal CA2 subfield, an area particularly susceptible to Lewy pathologies. Previous functional studies have demonstrated that the nvlDBB is particularly involved in memory retrieval, a cognitive domain severely affected in Lewy body disorders. Based on these observations, we propose an anatomical and functional connection between the cholinergic component of the nvlDBB (Ch2) and the hippocampal CA2.

Functional Subdivisions of Magnocellular Cell Groups in Human Basal Forebrain: Test-Retest Resting-State Study at Ultra-high Field, and Meta-analysis

The heterogeneous neuronal subgroups of the basal forebrain corticopetal system (BFcs) have been shown to modulate cortical functions through their cholinergic, gamma-aminobutyric acid-ergic, and glutamatergic projections to the entire cortex. Although previous studies suggested that the basalo-cortical projection system influences various cognitive functions, particularly via its cholinergic component, these studies only focused on certain parts of the BFcs or nearby structures, leaving aside a more systematic picture of the functional connectivity of BFcs subcompartments. Moreover, these studies lacked the high-spatial resolution and the probability maps needed to identify specific subcompartments. Recent advances in the ultra-high field 7T functional magnetic resonance imaging (fMRI) provided potentially unprecedented spatial resolution of functional MRI images to study the subdivision of the BFcs. In this study, the BF space containing corticopetal cells was divided into 3 functionally distinct subdivisions based on functional connection to cortical regions derived from fMRI. The overall functional connection of each BFcs subdivision was examined with a test-retest study. Finally, a meta-analysis was used to study the related functional topics of each BF subdivision. Our results demonstrate distinct functional connectivity patterns of these subdivisions along the rostrocaudal axis of the BF. All three compartments have shown consistent segregation and overlap at specific target regions including the hippocampus, insula, thalamus, and the cingulate gyrus, suggesting functional integration and separation in BFcs.

Modeling of [18F]FEOBV Pharmacokinetics in Rat Brain

PURPOSE

[¹⁸F]Fluoroethoxybenzovesamicol ([¹⁸F]FEOBV) is a radioligand for the vesicular acetylcholine transporter (VAChT), a marker of the cholinergic system. We evaluated the quantification of [¹⁸F]FEOBV in rats in control conditions and after partial saturation of VAChT using plasma and reference tissue input models and test-retest reliability.

PROCEDURE

Ninety-minute dynamic [¹⁸F]FEOBV PET scans with arterial blood sampling were performed in control rats and rats pretreated with 10 μg/kg FEOBV. Kinetic analyses were performed using one- (1TCM) and two-tissue compartmental models (2TCM), Logan and Patlak graphical analyses with metabolite-corrected plasma input, reference tissue Patlak with cerebellum as reference tissue, standard uptake value (SUV) and SUV ratio (SUVR) using 60- or 90-min acquisition. To assess test-retest reliability, two dynamic [¹⁸F]FEOBV scans were performed 1 week apart.

RESULTS

The 1TCM did not fit the data. Time-activity curves were more reliably estimated by the irreversible than the reversible 2TCM for 60 and 90 min as the influx rate K_i showed a lower coefficient of variation (COV, 14-24 %) than the volume of distribution V_T (16-108 %). Patlak graphical analysis showed a good fit to the data for both acquisition times with a COV (12-27 %) comparable to the irreversible 2TCM. For 60 min, Logan analysis performed comparably to both irreversible models (COV 14-32 %) but showed lower sensitivity to VAChT saturation. Partial saturation of VAChT did not affect model selection when using plasma input. However, poor correlations were found between irreversible 2TCM and SUV and SUVR in partially saturated VAChT states. Test-retest reliability and intraclass correlation for SUV were good.

CONCLUSION

[¹⁸F]FEOBV is best modeled using the irreversible 2TCM or Patlak graphical analysis. SUV should only be used if blood sampling is not possible.

Resting state activity and connectivity of the nucleus basalis of Meynert and globus pallidus in Lewy body dementia and Parkinson's disease dementia

Parkinson's disease dementia (PDD) and dementia with Lewy bodies (DLB) are two related diseases which can be difficult to distinguish. There is no objective biomarker which can reliably differentiate between them. The synergistic combination of electrophysiological and neuroimaging approaches is a powerful method for interrogation of functional brain networks in vivo. We recorded bilateral local field potentials (LFPs) from the nucleus basalis of Meynert (NBM) and the internal globus pallidus (GPi) with simultaneous cortical magnetoencephalography (MEG) in six PDD and five DLB patients undergoing surgery for deep brain stimulation (DBS) to look for differences in underlying resting-state network pathophysiology. In both patient groups we observed spectral peaks in the theta (2–8 Hz) band in both the NBM and the GPi. Furthermore, both the NBM and the GPi exhibited similar spatial and spectral patterns of coupling with the cortex in the two disease states. Specifically, we report two distinct coherent networks between the NBM/GPi and cortical regions: (1) a theta band (2–8 Hz) network linking the NBM/GPi to temporal cortical regions, and (2) a beta band (13–22 Hz) network coupling the NBM/GPi to sensorimotor areas. We also found differences between the two disease groups: oscillatory power in the low beta (13–22Hz) band was significantly higher in the globus pallidus in PDD patients compared to DLB, and coherence in the high beta (22–35Hz) band between the globus pallidus and lateral sensorimotor cortex was significantly higher in DLB patients compared to PDD. Overall, our findings reveal coherent networks of the NBM/GPi region that are common to both DLB and PDD. Although the neurophysiological differences between the two conditions in this study are confounded by systematic differences in DBS lead trajectories and motor symptom severity, they lend support to the hypothesis that DLB and PDD, though closely related, are distinguishable from a neurophysiological perspective.

Nigrostriatal Degeneration in the Cognitive Part of the Striatum in Parkinson Disease Is Associated With Frontomedial Hypometabolism

PURPOSE

The present study investigated possible associations between cortical dysfunction/degeneration as measured by F-FDG PET and nigrostriatal degeneration according to the specific I-FP-CIT binding ratio (SBR) in striatal subregions defined by striato-cortical anatomical connectivity in Parkinson disease (PD) patients.

MATERIALS AND METHODS

The study included 41 patients (61.4 ± 12.8 years) with PD-typical reduction of striatal FP-CIT SBR and no sign of atypical parkinsonian syndrome on FDG PET. FP-CIT SBR was determined separately in the cognitive (composite of executive and limbic) and sensorimotor part of the striatum according to the Oxford-GSK-Imanova Striatal Connectivity Atlas. Scaled FDG uptake was tested voxelwise for correlation with FP-CIT SBR (familywise error corrected P < 0.05).

RESULTS

A large cluster (17.6 mL) of significant correlation of scaled FDG uptake with FP-CIT SBR in the cognitive part of the striatum, corrected for SBR in the sensorimotor part, was detected in the bilateral medial frontal cortex and the anterior cingulate cortex (partial correlation coefficient R = 0.767); small clusters were detected in ipsilateral caudate and ipsilateral thalamus. There was a small contralateral occipital cluster (3.0 mL) of significant correlation between FDG uptake and sensorimotor SBR corrected for cognitive SBR (R = 0.709).

CONCLUSIONS

The correlation between nigrostriatal degeneration in the cognitive striatum and reduced cerebral glucose metabolism in the medial parts of the frontal cortex including the anterior cingulate suggests that nigrostriatal degeneration is specifically involved in the pathogenesis of cognitive deficits associated with medial frontal dysfunction such as impaired inhibitory control.

Neuropathology of dementia in patients with Parkinson's disease: a systematic review of autopsy studies

BACKGROUND

Dementia is a common, debilitating feature of late Parkinson's disease (PD). PD dementia (PDD) is associated with α-synuclein propagation, but coexistent Alzheimer's disease (AD) pathology may coexist. Other pathologies (cerebrovascular, transactive response DNA-binding protein 43 (TDP-43)) may also influence cognition. We aimed to describe the neuropathology underlying dementia in PD.

METHODS

Systematic review of autopsy studies published in English involving PD cases with dementia. Comparison groups included PD without dementia, AD, dementia with Lewy bodies (DLB) and healthy controls.

RESULTS

44 reports involving 2002 cases, 57.2% with dementia, met inclusion criteria. While limbic and neocortical α-synuclein pathology had the strongest association with dementia, between a fifth and a third of all PD cases in the largest studies had comorbid AD. In PD cases with dementia, tau pathology was moderate or severe in around a third, and amyloid-β pathology was moderate or severe in over half. Amyloid-β was associated with a more rapid cognitive decline and earlier mortality, and in the striatum, distinguished PDD from DLB. Positive correlations between multiple measures of α-synuclein, tau and amyloid-β were found. Cerebrovascular and TDP-43 pathologies did not generally contribute to dementia in PD. TDP-43 and amyloid angiopathy correlated with coexistent Alzheimer pathology.

CONCLUSIONS

While significant α-synuclein pathology is the main substrate of dementia in PD, coexistent pathologies are common. In particular, tau and amyloid-β pathologies independently contribute to the development and pattern of cognitive decline in PD. Their presence should be assessed in future clinical trials where dementia is a key outcome measure.

TRIAL REGISTRATION NUMBER

CRD42018088691.

Lower volume, more impairment: reduced cholinergic basal forebrain grey matter density is associated with impaired cognition in Parkinson disease

OBJECTIVE

A major contributor to dementia in Parkinson disease (PD) is degeneration of the cholinergic basal forebrain. This study determined whether cholinergic nucleus 4 (Ch4) density is associated with cognition in early and more advanced PD.

METHODS

We analysed brain MRIs and neuropsychological test scores for 228 newly diagnosed PD participants from the Parkinson's Progression Markers Initiative (PPMI), 101 healthy controls from the PPMI and 125 more advanced PD patients from a local retrospective cohort. Cholinergic basal forebrain nuclei densities were determined by applying probabilistic maps to MPRAGE T1 sequences processed using voxel-based morphometry methods. Relationships between grey matter densities and cognitive scores were analysed using correlations and linear regression models.

RESULTS

In more advanced PD, greater Ch4 density was associated with Montreal Cognitive Assessment (MoCA) score (β=14.2; 95% CI=1.5 to 27.0; p=0.03), attention domain z-score (β=3.2; 95% CI=0.8 to 5.5; p=0.008) and visuospatial domain z-score (β=7.9; 95% CI=2.0 to 13.8; p=0.009). In the PPMI PD cohort, higher Ch4 was associated with higher scores on MoCA (β=9.2; 95% CI=1.9 to 16.5; p=0.01), Judgement of Line Orientation (β=20.4; 95% CI=13.8 to 27.0; p<0.001), Letter Number Sequencing (β=16.5; 95% CI=9.5 to 23.4; p<0.001) and Symbol Digit Modalities Test (β=41.8; 95% CI=18.7 to 65.0; p<0.001). These same relationships were observed in 97 PPMI PD participants at 4 years. There were no significant associations between Ch4 density and cognitive outcomes in healthy controls.

CONCLUSION

In de novo and more advanced PD, lower Ch4 density is associated with impaired global cognition, attention and visuospatial function.

Bilateral Deep Brain Stimulation of the Nucleus Basalis of Meynert for Parkinson Disease Dementia: A Randomized Clinical Trial

Importance

Deep brain stimulation of the nucleus basalis of Meynert (NBM DBS) has been proposed as a treatment option for Parkinson disease dementia.

Objective

To evaluate the safety and potential symptomatic effects of NBM DBS in patients with Parkinson disease dementia.

Design, Setting, and Participants

A randomized, double-blind, crossover clinical trial evaluated the results of 6 patients with Parkinson disease dementia who were treated with NBM DBS at a neurosurgical referral center in the United Kingdom from October 26, 2012, to July 31, 2015. Eligible patients met the diagnostic criteria for Parkinson disease dementia, had motor fluctuations, were appropriate surgical candidates aside from the coexistence of dementia, were age 35 to 80 years, were able to give informed consent, had a Mini-Mental State Examination score of 21 to 26, had minimal atrophy seen on results of brain magnetic resonance imaging, and lived at home with a caregiver-informant.

Interventions

After surgery, patients were assigned to receive either active stimulation (bilateral, low-frequency [20 Hz] NBM DBS) or sham stimulation for 6 weeks, followed by the opposite condition for 6 weeks.

Main Outcomes and Measures

The primary outcome was the difference in scores on each item of an abbreviated cognitive battery (California Verbal Learning Test-II, Wechsler Adult Intelligence Scale-III digit span, verbal fluency, Posner covert attention test, and simple and choice reaction times) between the 2 conditions. Secondary outcomes were exploratory and included differences in scores on standardized measurements of cognitive, psychiatric, and motor symptoms and resting state functional magnetic resonance imaging.

Results

Surgery and stimulation were well tolerated by all 6 patients (all men; mean [SD] age, 65.2 [10.7] years), with no serious adverse events during the trial. No consistent improvements were observed in the primary cognitive outcomes or in results of resting state functional magnetic resonance imaging. An improvement in scores on the Neuropsychiatric Inventory was observed with NBM DBS (8.5 points [range, 4-26 points]) compared with sham stimulation (12 points [range, 8-38 points]; median difference, 5 points; 95% CI, 2.5-8.5 points; P = .03) and the preoperative baseline (13 points [range, 5-25 points]; median difference, 2 points; 95% CI, -8 to 5.5 points; P = .69).

Conclusions and Relevance

Low-frequency NBM DBS was safely conducted in patients with Parkinson disease dementia; however, no improvements were observed in the primary cognitive outcomes. Further studies may be warranted to explore its potential to improve troublesome neuropsychiatric symptoms.

Trial Registration

clinicaltrials.gov Identifier: NCT01701544.

Scopolamine and Medial Frontal Stimulus-Processing during Interval Timing

Neurodegenerative diseases such as Parkinson's disease (PD), dementia with Lewy bodies (DLB), and Alzheimer's disease (AD) involve loss of cholinergic neurons in the basal forebrain. Here, we investigate how cholinergic dysfunction impacts the frontal cortex during interval timing, a process that can be impaired in PD and AD patients. Interval timing requires participants to estimate an interval of several seconds by making a motor response, and depends on the medial frontal cortex (MFC), which is richly innervated by basal forebrain cholinergic projections. Past work has shown that scopolamine, a muscarinic cholinergic receptor antagonist, reliably impairs interval timing. We tested the hypothesis that scopolamine would attenuate time-related ramping, a key form of temporal processing in the MFC. We recorded neuronal ensembles from eight mice during performance of a 12-s fixed-interval timing task, which was impaired by the administration of scopolamine. Consistent with past work, scopolamine impaired timing. To our surprise, we found that time-related ramping was unchanged, but stimulus-related activity was enhanced in the MFC. Principal component analyses revealed no consistent changes in time-related ramping components, but did reveal changes in higher components. Taken together, these data indicate that scopolamine changes stimulus processing rather than temporal processing in the MFC. These data could help understand how cholinergic dysfunction affects cortical circuits in diseases such as PD, DLB, and AD.

The corticotopic organization of the human basal forebrain as revealed by regionally selective functional connectivity profiles

The cholinergic basal forebrain (CBF), comprising different groups of cortically projecting cholinergic neurons, plays a crucial role in higher cognitive processes and has been implicated in diverse neuropsychiatric disorders. A distinct corticotopic organization of CBF projections has been revealed in animal studies, but little is known about their organization in the human brain. We explored regional differences in functional connectivity (FC) profiles within the human CBF by applying a clustering approach to resting-state functional magnetic resonance imaging (rs-fMRI) data of healthy adult individuals (N = 85; 19-85 years). We further examined effects of age on FC of the identified CBF clusters and assessed the reproducibility of cluster-specific FC profiles in independent data from healthy older individuals (N = 25; 65-89 years). Results showed that the human CBF is functionally organized into distinct anterior-medial and posterior-lateral subdivisions that largely follow anatomically defined boundaries of the medial septum/diagonal band and nucleus basalis Meynert. The anterior-medial CBF subdivision was characterized by connectivity with the hippocampus and interconnected nodes of an extended medial cortical memory network, whereas the posterior-lateral subdivision was specifically connected to anterior insula and dorsal anterior cingulate components of a salience/attention network. FC of both CBF subdivisions declined with increasing age, but the overall topography of subregion-specific FC profiles was reproduced in independent rs-fMRI data of healthy older individuals acquired in a typical clinical setting. Rs-fMRI-based assessments of subregion-specific CBF function may complement established volumetric approaches for the in vivo study of CBF involvement in neuropsychiatric disorders.

Molecular Imaging of Dementia With Lewy Bodies

Dementia with Lewy bodies (DLB) is the second most common cause of neurodegenerative dementia. The core clinical features of DLB include fluctuating cognition, visual hallucinations, rapid eye movement sleep behavior disorder, and parkinsonism. Molecular imaging is a powerful tool to assess the brain function in vivo. In this chapter, we reviewed the positron emission tomography, single-photon emission computed tomography, and [¹²³I]-metaiodobenzylguanidine scintigraphy studies evaluating the pathological processes underlying DLB, including altered brain metabolism and neurotransmitter pathways, abnormal protein aggregation, and neuroinflammation. These techniques can aid in the differential diagnosis of DLB (versus Alzheimer's disease and related dementia) and in the monitoring disease progression and treatment efficacy of disease-modifying drugs. Furthermore, we explored the limitations of current imaging biomarkers and future directions, particularly focusing on the vital need for tracers that have high affinity for alpha-synuclein.

Molecular Imaging of the Cholinergic System in Parkinson's Disease

One of the first identified neurotransmitters in the brain, acetylcholine, is an important modulator that drives changes in neuronal and glial activity. For more than two decades, the main focus of molecular imaging of the cholinergic system in Parkinson's disease (PD) has been on cognitive changes. Imaging studies have confirmed that degeneration of the cholinergic system is a major determinant of dementia in PD. Within the last decade, the focus is expanding to studying cholinergic correlates of mobility impairments, dyskinesias, olfaction, sleep, visual hallucinations and risk taking behavior in this disorder. These studies increasingly recognize that the regional topography of cholinergic brain areas associates with specific functions. In parallel with this trend, more recent molecular cholinergic imaging approaches are investigating cholinergic modulatory functions and contributions to large-scale brain network functions. A novel area of research is imaging cholinergic innervation functions of peripheral autonomic organs that may have the potential of future prodromal diagnosis of PD. Finally, emerging evidence of hypercholinergic activity in prodromal and symptomatic leucine-rich repeat kinase 2 PD may reflect neuronal cholinergic compensation versus a response to neuro-inflammation. Molecular imaging of the cholinergic system has led to many new insights in the etiology of dopamine non-responsive symptoms of PD (more "malignant" hypocholinergic disease phenotype) and is poised to guide and evaluate future cholinergic drug development in this disorder.

Review: Revisiting the human cholinergic nucleus of the diagonal band of Broca

Although the nucleus of the vertical limb of the diagonal band of Broca (nvlDBB) is the second largest cholinergic nucleus in the basal forebrain, after the nucleus basalis of Meynert, it has not generally been a focus for studies of neurodegenerative disorders. However, the nvlDBB has an important projection to the hippocampus and discrete lesions of the rostral basal forebrain have been shown to disrupt retrieval memory function, a major deficit seen in patients with Lewy body disorders. One reason for its neglect is that the anatomical boundaries of the nvlDBB are ill defined and this area of the brain is not part of routine diagnostic sampling protocols. We have reviewed the history and anatomy of the nvlDBB and now propose guidelines for distinguishing nvlDBB from other neighbouring cholinergic cell groups for standardizing future clinicopathological work. Thorough review of the literature regarding neurodegenerative conditions reveals inconsistent results in terms of cholinergic neuronal loss within the nvlDBB. This is likely to be due to the use of variable neuronal inclusion criteria and omission of cholinergic immunohistochemical markers. Extrapolating from those studies showing a significant nvlDBB neuronal loss in Lewy body dementia, we propose an anatomical and functional connection between the cholinergic component of the nvlDBB (Ch2) and the CA2 subfield in the hippocampus which may be especially vulnerable in Lewy body disorders.

On Cell Loss and Selective Vulnerability of Neuronal Populations in Parkinson's Disease

Significant advances have been made uncovering the factors that render neurons vulnerable in Parkinson's disease (PD). However, the critical pathogenic events leading to cell loss remain poorly understood, complicating the development of disease-modifying interventions. Given that the cardinal motor symptoms and pathology of PD involve the loss of dopamine (DA) neurons of the substantia nigra pars compacta (SNc), a majority of the work in the PD field has focused on this specific neuronal population. PD however, is not a disease of DA neurons exclusively: pathology, most notably in the form of Lewy bodies and neurites, has been reported in multiple regions of the central and peripheral nervous system, including for example the locus coeruleus, the dorsal raphe nucleus and the dorsal motor nucleus of the vagus. Cell and/or terminal loss of these additional nuclei is likely to contribute to some of the other symptoms of PD and, most notably to the non-motor features. However, exactly which regions show actual, well-documented, cell loss is presently unclear. In this review we will first examine the strength of the evidence describing the regions of cell loss in idiopathic PD, as well as the order in which this loss occurs. Secondly, we will discuss the neurochemical, morphological and physiological characteristics that render SNc DA neurons vulnerable, and will examine the evidence for these characteristics being shared across PD-affected neuronal populations. The insights raised by focusing on the underpinnings of the selective vulnerability of neurons in PD might be helpful to facilitate the development of new disease-modifying strategies and improve animal models of the disease.

A Review of the Pedunculopontine Nucleus in Parkinson's Disease

The pedunculopontine nucleus (PPN) is situated in the upper pons in the dorsolateral portion of the ponto-mesencephalic tegmentum. Its main mass is positioned at the trochlear nucleus level, and is part of the mesenphalic locomotor region (MLR) in the upper brainstem. The human PPN is divided into two subnuclei, the pars compacta (PPNc) and pars dissipatus (PPNd), and constitutes both cholinergic and non-cholinergic neurons with afferent and efferent projections to the cerebral cortex, thalamus, basal ganglia (BG), cerebellum, and spinal cord. The BG controls locomotion and posture via GABAergic output of the substantia nigra pars reticulate (SNr). In PD patients, GABAergic BG output levels are abnormally increased, and gait disturbances are produced via abnormal increases in SNr-induced inhibition of the MLR. Since the PPN is vastly connected with the BG and the brainstem, dysfunction within these systems lead to advanced symptomatic progression in Parkinson's disease (PD), including sleep and cognitive issues. To date, the best treatment is to perform deep brain stimulation (DBS) on PD patients as outcomes have shown positive effects in ameliorating the debilitating symptoms of this disease by treating pathological circuitries within the parkinsonian brain. It is therefore important to address the challenges and develop this procedure to improve the quality of life of PD patients.

Recent advances in cholinergic imaging and cognitive decline-Revisiting the cholinergic hypothesis of dementia

PURPOSE OF REVIEW

Although the cholinergic hypothesis of dementia provided a successful paradigm for the development of new drugs for dementia, this hypothesis has waned in popularity. Cholinergic brain imaging may provide novel insights into the viability of this hypothesis.

RECENT FINDINGS

Cholinergic receptor and forebrain volumetric studies suggest an important role of the cholinergic system in maintaining brain network integrity that may deteriorate with cognitive decline in Alzheimer disease (AD) and Lewy body disorders (LBD). Bidirectional changes in regional receptor expression may suggest the presence of compensatory responses to neurodegenerative injury. Cholinergic system changes are more complex in LBD because of additional subcortical degenerations compared to AD. Cholinergic-dopaminergic interactions affect attentional, verbal learning and executive functions, and impairments in these two transmitter systems may jointly increase the risk of dementia in Parkinson disease.

SUMMARY

The cholinergic hypothesis is evolving from a primary focus on memory toward expanded cognitive functions modulated by regionally more complex and interactive brain networks. Cholinergic network adaptation may serve as a novel research target in neurodegeneration.

Amyloid-Beta and Phosphorylated Tau Accumulations Cause Abnormalities at Synapses of Alzheimer's disease Neurons

Amyloid-beta (Aβ) and hyperphosphorylated tau are hallmark lesions of Alzheimer's disease (AD). However, the loss of synapses and dysfunctions of neurotransmission are more directly tied to disease severity. The role of these lesions in the pathoetiological progression of the disease remains contested. Biochemical, cellular, molecular, and pathological studies provided several lines of evidence and improved our understanding of how Aβ and hyperphosphorylated tau accumulation may directly harm synapses and alter neurotransmission. In vitro evidence suggests that Aβ and hyperphosphorylated tau have both direct and indirect cytotoxic effects that affect neurotransmission, axonal transport, signaling cascades, organelle function, and immune response in ways that lead to synaptic loss and dysfunctions in neurotransmitter release. Observations in preclinical models and autopsy studies support these findings, suggesting that while the pathoetiology of positive lesions remains elusive, their removal may reduce disease severity and progression. The purpose of this article is to highlight the need for further investigation of the role of tau in disease progression and its interactions with Aβ and neurotransmitters alike.

Chronic treatment with carvacrol improves passive avoidance memory in a rat model of Parkinson's disease

ABSTRACT The present study investigated the effects of carvacrol on motor and memory deficits as well as hyperalgesia in the 6-OHDA-lesioned rat model of Parkinson's disease. The animals were subjected to unilateral microinjection of 6-OHDA into the medial forebrain bundle and treated with carvacrol (25, 50 and 100 mg/kg, ip) for six weeks after surgery. The 6-OHDA-lesioned rats showed contralateral rotations towards the lesion side, which was accompanied by learning and memory deficits in a passive avoidance test and a decrease in tail withdrawal latency in a tail flick test at the end of week 6. The results also showed that treatment with carvacrol at a dose of 25 mg/kg ameliorated memory deficits, with no effect on rotations and hyperalgesia in lesioned rats. In conclusion, carvacrol improves memory impairments in rats with Parkinson's disease; therefore, it may serve as an adjunct therapy for the alleviation of memory deficits in Parkinson's disease patients.

Management of Psychosis in Parkinson's Disease: Emphasizing Clinical Subtypes and Pathophysiological Mechanisms of the Condition

Investigation into neuropsychiatric symptoms in Parkinson's disease (PD) is sparse and current drug development is mainly focused on the motor aspect of PD. The tight association of psychosis with an impaired quality of life in PD, together with an important underreporting of this comorbid condition, contributes to its actual insufficient assessment and management. Furthermore, the withdrawal from access to readily available treatment interventions is unacceptable and has an impact on PD prognosis. Despite its impact, to date no standardized guidelines to the adequate management of PD psychosis are available and they are therefore highly needed. Readily available knowledge on distinct clinical features as well as early biomarkers of psychosis in PD justifies the potential for its timely diagnosis and for early intervention strategies. Also, its specific characterisation opens up the possibility of further understanding the underlying pathophysiological mechanisms giving rise to more targeted therapeutic developments in the nearer future. A literature review on the most recent knowledge with special focus on specific clinical subtypes and pathophysiological mechanisms will not only contribute to an up to date practical approach of this condition for the health care providers, but furthermore open up new ideas for research in the near future.

Chronic MPTP administration regimen in monkeys: a model of dopaminergic and non-dopaminergic cell loss in Parkinson's disease

Parkinson's disease (PD) is a progressive neurodegenerative disorder clinically characterized by cardinal motor deficits including bradykinesia, tremor, rigidity and postural instability. Over the past decades, it has become clear that PD symptoms extend far beyond motor signs to include cognitive, autonomic and psychiatric impairments, most likely resulting from cortical and subcortical lesions of non-dopaminergic systems. In addition to nigrostriatal dopaminergic degeneration, pathological examination of PD brains, indeed, reveals widespread distribution of intracytoplasmic inclusions (Lewy bodies) and death of non-dopaminergic neurons in the brainstem and thalamus. For that past three decades, the MPTP-treated monkey has been recognized as the gold standard PD model because it displays some of the key behavioral and pathophysiological changes seen in PD patients. However, a common criticism raised by some authors about this model, and other neurotoxin-based models of PD, is the lack of neuronal loss beyond the nigrostriatal dopaminergic system. In this review, we argue that this assumption is largely incorrect and solely based on data from monkeys intoxicated with acute administration of MPTP. Work achieved in our laboratory and others strongly suggest that long-term chronic administration of MPTP leads to brain pathology beyond the dopaminergic system that displays close similarities to that seen in PD patients. This review critically examines these data and suggests that the chronically MPTP-treated nonhuman primate model may be suitable to study the pathophysiology and therapeutics of some non-motor features of PD.