Diffusion tensor imaging marks dopaminergic and serotonergic lesions in the Parkinsonian monkey

Increased Expression of Alpha-, Beta-, and Gamma-Synucleins in Brainstem Regions of a Non-Human Primate Model of Parkinson’s Disease

Parkinson’s disease (PD) is characterized by cell loss in the substantia nigra and the presence of alpha-synuclein (α-syn)-containing neuronal Lewy bodies. While α-syn has received major interest in the pathogenesis of PD, the function of beta- and gamma-synucleins (β-syn and γ-syn, respectively) is not really known. Yet, these proteins are members of the same family and also concentrated in neuronal terminals. The current preclinical study investigated the expression levels of α-, β-, and γ-synucleins in brainstem regions involved in PD physiopathology. We analyzed synuclein expression in the substantia nigra, raphe nuclei, pedunculopontine nucleus, and locus coeruleus from control and parkinsonian (by MPTP) macaques. MPTP-intoxicated monkeys developed a more or less severe parkinsonian score and were sacrificed after a variable post-MPTP period ranging from 1 to 20 months. The expression of the three synucleins was increased in the substantia nigra after MPTP, and this increase correlates positively, although not very strongly, with cell loss and motor score and not with the time elapsed after intoxication. In the dorsal raphe nucleus, the expression of the three synucleins was also increased, but only α- and γ-Syn are linked to the motor score and associated cell loss. Finally, although no change in synuclein expression was demonstrated in the locus coeruleus after MPTP, we found increased expression levels of γ-Syn, which are only correlated with cell loss in the pedunculopontine nucleus. Altogether, our data suggest that these proteins may play a key role in brainstem regions and mesencephalic tegmentum. Given the involvement of these brain regions in non-motor symptoms of PD, these data also strengthen the relevance of the MPTP macaque model of PD, which exhibits pathological changes beyond nigral DA cell loss and α-synucleinopathy.

Diffusion Tensor Imaging-Based Studies at the Group-Level Applied to Animal Models of Neurodegenerative Diseases

The understanding of human and non-human microstructural brain alterations in the course of neurodegenerative diseases has substantially improved by the non-invasive magnetic resonance imaging (MRI) technique of diffusion tensor imaging (DTI). Animal models (including disease or knockout models) allow for a variety of experimental manipulations, which are not applicable to humans. Thus, the DTI approach provides a promising tool for cross-species cross-sectional and longitudinal investigations of the neurobiological targets and mechanisms of neurodegeneration. This overview with a systematic review focuses on the principles of DTI analysis as used in studies at the group level in living preclinical models of neurodegeneration. The translational aspect from in-vivo animal models toward (clinical) applications in humans is covered as well as the DTI-based research of the non-human brains' microstructure, the methodological aspects in data processing and analysis, and data interpretation at different abstraction levels. The aim of integrating DTI in multiparametric or multimodal imaging protocols will allow the interrogation of DTI data in terms of directional flow of information and may identify the microstructural underpinnings of neurodegeneration-related patterns.

Breathing new life into neurotoxic-based monkey models of Parkinson's disease to study the complex biological interplay between serotonin and dopamine

Numerous clinical studies have shown that the serotonergic system also degenerates in patients with Parkinson's disease. The causal role of this impairment in Parkinson's symptomatology and the response to treatment remains to be refined, in particular thanks to approaches allowing the two components DA and 5-HT to be isolated if possible. We have developed a macaque monkey model of Parkinson's disease exhibiting a double lesion (dopaminergic and serotonergic) thanks to the sequential use of MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) and MDMA (3,4-methylenedioxy-N-methamphetamine) (or MDMA prior MPTP). We characterized this monkey model by multimodal imaging (PET, positron emission tomography with several radiotracers; DTI, diffusion tensor imaging), behavioral assessments (parkinsonism, dyskinesia, neuropsychiatric-like behavior) and post-mortem analysis (with DA and 5-HT markers). When administrated after MPTP, MDMA damaged the 5-HT presynaptic system without affecting the remaining DA neurons. The lesion of 5-HT fibers induced by MDMA altered rigidity and prevented dyskinesia and neuropsychiatric-like symptoms induced by levodopa therapy in MPTP-treated animals. Interestingly also, prior MDMA administration aggravates the parkinsonian deficits and associated DA injury. Dystonic postures, action tremor and global spontaneous activities were significantly affected. All together, these data clearly indicate that late or early lesions of the 5-HT system have a differential impact on parkinsonian symptoms in the macaque model of Parkinson's disease. Whether MDMA has an impact on neuropsychiatric-like symptoms such as apathy, anxiety, depression remains to be addressed. Despite its limitations, this toxin-based double-lesioned monkey model takes on its full meaning and provides material for the experimental study of the heterogeneity of patients.

Early limbic microstructural alterations in apathy and depression in de novo Parkinson's disease

BACKGROUND

Whether structural alterations underpin apathy and depression in de novo parkinsonian patients is unknown. The objectives of this study were to investigate whether apathy and depression in de novo parkinsonian patients are related to structural alterations and how structural abnormalities relate to serotonergic or dopaminergic dysfunction.

METHODS

We compared the morphological and microstructural architecture in gray matter using voxel-based morphometry and diffusion tensor imaging coupled with white matter tract-based spatial statistics in a multimodal imaging case-control study enrolling 14 apathetic and 13 nonapathetic patients with de novo Parkinson's disease and 15 age-matched healthy controls, paired with PET imaging of the presynaptic dopaminergic and serotonergic systems.

RESULTS

De novo parkinsonian patients with apathy had bilateral microstructural alterations in the medial corticostriatal limbic system, exhibiting decreased fractional anisotropy and increased mean diffusivity in the anterior striatum and pregenual anterior cingulate cortex in conjunction with serotonergic dysfunction. Furthermore, microstructural alterations extended to the medial frontal cortex, the subgenual anterior cingulate cortex and subcallosal gyrus, the medial thalamus, and the caudal midbrain, suggesting disruption of long-range nondopaminergic projections originating in the brainstem, in addition to microstructural alterations in callosal interhemispheric connections and frontostriatal association tracts early in the disease course. In addition, microstructural abnormalities related to depressive symptoms in apathetic and nonapathetic patients revealed a distinct, mainly right-sided limbic subnetwork involving limbic and frontal association tracts.

CONCLUSIONS

Early limbic microstructural alterations specifically related to apathy and depression emphasize the role of early disruption of ascending nondopaminergic projections and related corticocortical and corticosubcortical networks which underpin the variable expression of nonmotor and neuropsychiatric symptoms in Parkinson's disease. © 2019 International Parkinson and Movement Disorder Society.

Hyperpolarization-Activated Cyclic Nucleotide-Gated Channels: An Emerging Role in Neurodegenerative Diseases

Neurodegenerative diseases such as Parkinson’s disease (PD), Alzheimer’s disease (AD), amyotrophic lateral sclerosis (ALS), and spinal muscular atrophy (SMA) are chronic, progressive, and age-associated neurological disorders characterized by neuronal deterioration in specific brain regions. Although the specific pathological mechanisms underlying these disorders have remained elusive, ion channel dysfunction has become increasingly accepted as a potential mechanism for neurodegenerative diseases. Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels are encoded by the HCN1-4 gene family and conduct the hyperpolarization-activated current (I_h). These channels play important roles in modulating cellular excitability, rhythmic activity, dendritic integration, and synaptic transmission. In the present review, we first provide a comprehensive picture of the role of HCN channels in PD by summarizing their role in the regulation of neuronal activity in PD-related brain regions. Dysfunction of I_h may participate in 1-methyl-4-phenylpyridinium (MPP⁺)-induced toxicity and represent a pathogenic mechanism in PD. Given current reports of the critical role of HCN channels in neuroinflammation and depression, we also discussed the putative contribution of HCN channels in inflammatory processes and non-motor symptoms in PD. In the second section, we summarize how HCN channels regulate the formation of β-amyloid peptide in AD and the role of these channels in learning and memory. Finally, we briefly discuss the effects of HCN channels in ALS and SMA based on existing discoveries.

Image correction for diffusion tensor imaging of Rhesus monkey thoracic spinal cord

BACKGROUND

The relatively tiny spinal cord of non-human primate (NHP) causes increased challenge in diffusion tensor imaging (DTI) post-processing. This study aimed to establish a reliable correction strategy applied to clinical DTI images of NHP.

METHODS

Six normal and partial spinal cord injury (SCI) rhesus monkeys underwent 3T MR scanning. A correction strategy combining multiple iterations and non-rigid deformation was used for DTI image post-processing. Quantitative evaluations were then conducted to investigate effects of distortion correction.

RESULTS

After correction, longitudinal geometric distortion, global distortion, and residual distance errors were all significantly decreased (P < 0.05). Fractional anisotropy at the injured site was remarkably lower than that at the contralateral site (P = 0.0488) and was substantially lower than those at the adjacent superior (P = 0.0157) and inferior (P = 0.0128) areas at the same side.

CONCLUSIONS

Our image correction strategy can improve the quality of the DTI images of NHP thoracic cords, contributing to the development of SCI preclinical research.

Pathophysiology of dyskinesia and behavioral disorders in non-human primates: the role of serotonergic fibers

The MPTP monkey model of Parkinson's disease (PD) has allowed huge advances regarding the understanding of the pathological mechanisms of PD and L-DOPA-induced adverse effects. Among the main findings were the imbalance between the efferent striatal pathways in opposite directions between the hypokinetic and hyperkinetic states of PD. In both normal and parkinsonian monkeys, the combination of behavioral and anatomical studies has allowed the deciphering of the cortico-basal ganglia circuits involved in both movement and behavioral disorders. A major breakthrough has then been made regarding the hypothesis of the involvement of serotonergic fibers in the conversion of L-DOPA to dopamine when dopaminergic neurons are dying and to release it, in an uncontrolled manner, as serotonergic neurons are deprived from the machinery required for buffering dopamine from the synaptic cleft. The crucial involvement of serotonergic fibers underlying L-DOPA-induced dyskinesia (LID) has been demonstrated in both rodent and monkey models of PD, in which dyskinesia induced by L-DOPA is abolished following lesion of the serotonergic system. Moreover, the role of serotonergic fibers goes well beyond dyskinesia, as lesioning of such serotonergic fibers by MDMA in the monkey also decreased other L-DOPA-induced adverse effects such as impulsive compulsive behaviors and visual hallucinations. The same pathological mechanism, i.e., an imbalance between serotonin and dopamine terminals may, therefore, favor L-DOPA-induced adverse effects according to the basal ganglia territory it inhabits. Further non-human primate studies will be needed to demonstrate the role of such a pathological mechanism in both movement and behavioral disorders driven by L-DOPA therapy but also to determine the causal link between serotonin lesions and the expression of non-motor symptoms like apathy, depression and anxiety, frequently observed in PD patients.