Dopaminergic Fibers Research Articles

Microbial modulation via cross-fostering prevents the effects of pervasive environmental stressors on microglia and social behavior, but not the dopamine system.

Environmental toxicant exposure, including air pollution, is increasing worldwide. However, toxicant exposures are not equitably distributed. Rather, low-income and minority communities bear the greatest burden, along with higher levels of psychosocial stress. Both air pollution and maternal stress during pregnancy have been linked to neurodevelopmental disorders such as autism, but biological mechanisms and targets for therapeutic intervention remain poorly understood. We demonstrate that combined prenatal exposure to air pollution (diesel exhaust particles, DEP) and maternal stress (MS) in mice induces social behavior deficits only in male offspring, in line with the male bias in autism. These behavioral deficits are accompanied by changes in microglial morphology and gene expression as well as decreased dopamine receptor expression and dopaminergic fiber input in the nucleus accumbens (NAc). Importantly, the gut-brain axis has been implicated in ASD, and both microglia and the dopamine system are sensitive to the composition of the gut microbiome. In line with this, we find that the composition of the gut microbiome and the structure of the intestinal epithelium are significantly shifted in DEP/MS-exposed males. Excitingly, both the DEP/MS-induced social deficits and microglial alterations in males are prevented by shifting the gut microbiome at birth via a cross-fostering procedure. However, while social deficits in DEP/MS males can be reversed by chemogenetic activation of dopamine neurons in the ventral tegmental area, modulation of the gut microbiome does not impact dopamine endpoints. These findings demonstrate male-specific changes in the gut-brain axis following DEP/MS and suggest that the gut microbiome is an important modulator of both social behavior and microglia.

Intraperitoneal injection of iron dextran induces peripheral iron overload and mild neurodegeneration in the nigrostriatal system in C57BL/6 mice.

Elevated iron levels in the affected areas of brain are linked to several neurodegenerative diseases including Parkinson's disease (PD). This study investigated the influence of peripheral iron overload in peripheral tissues, as well as its entry into the brain regions on lysosomal functions. The survival of dopaminergic neurons in the nigrostriatal system and motor coordination were also investigated. An intraperitoneal injection of iron dextran (FeDx) mouse model was established. Western blot was used to detect iron deposition and lysosomal functions in the liver, spleen, hippocampal (HC), striatum (STR), substantia nigra (SN) and olfactory bulb (OB). Iron in serum and cerebrospinal fluid (CSF) was determined by an iron assay kit. Immunofluorescence and immunohistochemical staining were applied to detect dopaminergic neurons and fibers. Motor behavior was evaluated by gait analysis. Iron was deposited consistently in the liver and spleen, and serum iron was elevated. While iron deposition occurred late in the HC, STR and SN, without apparently affecting CSF iron levels. Although cathepsin B (CTSB), cathepsin D (CTSD), glucocerebrosidase (GCase) and lysosome integrated membrane protein 2 (LIMP-2) protein levels were dramatically up-regulated in the liver and spleen, they were almost unchanged in the brain regions. However, CTSB was up-regulated in acute iron-overloaded OB and primary cultured astrocytes. The number of dopaminergic neurons in the SN remained unchanged, and mice did not exhibit significant motor incoordination. Intraperitoneal injection of FeDx in mice induces largely peripheral iron overload while not necessarily sufficient to cause severe disruption of the nigrostriatal system.

Identification and evaluation of midbrain specific longevity-related genes in exceptionally long-lived but healthy mice.

Brain aging is a complex biological process that is affected by both genetic background and environment. The transcriptomic analysis of aged human and rodent brains has been applied to identify age-associated molecular and cellular processes for which intervention could possibly restore declining brain functions induced by aging. However, whether these age-associated genetic alterations are indeed involved in the healthy aging of the brain remains unclear. We herein characterized a naturally occurring, extremely long-lived (34 months of age) but healthy mouse group retaining well-preserved motor functions. Strikingly, these long-lived mice maintained tyrosine hydroxylase expression and dopaminergic fiber densities, even in the presence of persistent neuroinflammation and expression of aging markers. Combined with Endeavor gene prioritization, we identified the following midbrain-specific longevity-associated genes in the midbrain of these mice: aimp2, hexb, cacybp, akt2, nrf1, axin1, wwp2, sp2, dnajb9, notch, traf7, and lrp1. A detailed biochemical analysis of the midbrain of these long-lived mice confirmed the increased expression of Nrf1 and the activation of Akt1 and 2. Interestingly, dopaminergic neuroprotective and age-associated E3 ubiquitin ligase parkin expression was retained at high levels in the aforementioned midbrains, possibly supporting the suppression of its toxic substrates AIMP2 and PARIS. In contrast, the 24-month-old mice with dopaminergic neurite deficits failed to maintain parkin expression in the midbrain. AIMP2-induced cytotoxicity, mitochondrial stress, and neurite toxicity can be prevented by overexpression of parkin, Akt1, and Nrf1 in SH-SY5Y and PC12 cells, and basal expression of parkin, Akt1, and Nrf1 is required for maintenance of mitochondrial function and neurite integrity in PC12 cells. Taken together, this longevity-associated pathway could be a potential target of intervention to maintain nigrostriatal dopaminergic fibers and motor ability to ensure healthy longevity.

Regional Anesthesia In Patients With Parkinson's Disease

Parkinson's disease is a degenerative disease of the central nervous system, which occurs in 1% of the population over 60 years of age. This disease is caused by the loss of dopaminergic nerve fibers in the basal ganglia of the brain due to an immune response. Parkinsonism is the name given to a clinical syndrome consisting of disorders of voluntary movement (hypokinesia), stiffness, and tremors. The typical pathological picture is the destruction of nerve cells containing dopamine in the substantia nigra of the basal ganglia. The selection of anesthetic techniques in patients with Parkinson's disease plays an important role in minimizing complications and patient morbidity and mortality. Anesthetic concerns in patients with Parkinson's disease are the presence or absence of interactions with anti-Parkinson's drugs that the patient is taking, decreased organ function due to old age, such as decreased cardiovascular, respiratory, and autonomic nerve function. In this case, the patient with Parkinson's disease will undergo repositioning of the left femoral head due to the dislocation. We chose regional anesthesia because it has several advantages over general anesthesia. The results during and after surgery were good, there were no complications and the patient returned to the room.

Upregulation of Ca2+-binding proteins contributes to VTA dopamine neuron survival in the early phases of Alzheimer’s disease in Tg2576 mice

BackgroundRecent clinical and experimental studies have highlighted the involvement of Ventral Tegmental Area (VTA) dopamine (DA) neurons for the early pathogenesis of Alzheimer’s Disease (AD). We have previously described a progressive and selective degeneration of these neurons in the Tg2576 mouse model of AD, long before amyloid-beta plaque formation. The degenerative process in DA neurons is associated with an autophagy flux impairment, whose rescue can prevent neuronal loss. Impairments in autophagy can be the basis for accumulation of damaged mitochondria, leading to disturbance in calcium (Ca2+) homeostasis, and to functional and structural deterioration of DA neurons.MethodsIn Tg2576 mice, we performed amperometric recordings of DA levels and analysis of dopaminergic fibers in the Nucleus Accumbens – a major component of the ventral striatum precociously affected in AD patients – together with retrograde tracing, to identify the most vulnerable DA neuron subpopulations in the VTA. Then, we focused on these neurons to analyze mitochondrial integrity and Apoptosis-inducing factor (AIF) localization by electron and confocal microscopy, respectively. Stereological cell count was also used to evaluate degeneration of DA neuron subpopulations containing the Ca2+-binding proteins Calbindin-D28K and Calretinin. The expression levels for these proteins were analyzed by western blot and confocal microscopy. Lastly, using electrophysiology and microfluorometry we analyzed VTA DA neuron intrinsic properties and cytosolic free Ca2+ levels.ResultsWe found a progressive degeneration of mesolimbic DA neurons projecting to the ventral striatum, located in the paranigral nucleus and parabrachial pigmented subnucleus of the VTA. At the onset of degeneration (3 months of age), the vulnerable DA neurons in the Tg2576 accumulate damaged mitochondria, while AIF translocates from the mitochondria to the nucleus. Although we describe an age-dependent loss of the DA neurons expressing Calbindin-D28K or Calretinin, we observed that the remaining cells upregulate the levels of Ca2+-binding proteins, and the free cytosolic levels of Ca2+ in these neurons are significantly decreased. Coherently, TUNEL-stained Tg2576 DA neurons express lower levels of Calbindin-D28K when compared with non-apoptotic cells.ConclusionOverall, our results suggest that the overexpression of Ca2+-binding proteins in VTA DA neurons might be an attempt of cells to survive by increasing their ability to buffer free Ca2+. Exploring strategies to overexpress Ca2+-binding proteins could be fundamental to reduce neuronal suffering and improve cognitive and non-cognitive functions in AD.

Morphology of the “prefrontal” nidopallium caudolaterale in the long-distance night-migratory Eurasian blackcap (Sylvia atricapilla)

Migrating birds have developed remarkable navigational capabilities to successfully master biannual journeys between their breeding and wintering grounds. To reach their intended destination, they need to calculate navigational goals from a large variety of natural directional and positional cues to set a meaningful motor output command. One brain area, which has been associated with such executive functions, is the nidopallium caudolaterale (NCL), which, due to its striking similarities in terms of neurochemistry, connectivity and function, is considered analogous to the mammalian prefrontal cortex. To establish a baseline for further analyses elucidating the neuronal correlates underlying avian navigation, we performed quantitative and qualitative analyses of dopaminergic fibres in the brains of long-distance night-migratory Eurasian blackcaps (Sylvia atricapilla). We identified four regions in the caudal telencephalon, each of which was characterized by its specific dopaminergic innervation pattern. At least three of them presumably constitute subareas of the NCL in Eurasian blackcaps and could thus be involved in integrating navigational input from different sensory systems. The observed heterogeneity and parcellation of the NCL subcompartments in this migratory species could be a consequence of the special demands related to navigation.

Chlorogenic Acid: a Polyphenol from Coffee Rendered Neuroprotection Against Rotenone-Induced Parkinson’s Disease by GLP-1 Secretion

Parkinson's disease (PD) is a chronic motor disorder, characterized by progressive loss of dopaminergic neurons. Numerous studies suggest that glucagon-like peptide-1 (GLP-1) secretagogue has a neuroprotective role in PD models. The present study evaluated potential of coffee bioactive compounds in terms of their ability to bind GPR-40/43 and tested the neuroprotective effect of best candidate on rotenone-induced PD mice acting via GLP-1 release. In silico molecular docking followed by binding free energy calculation revealed that chlorogenic acid (CGA) has a strong binding affinity for GPR-40/43 in comparison to other bioactive polyphenols. Molecular dynamics simulation studies revealed stable nature of GPR40-CGA and GPR43-CGA interaction and also provided information about the amino acid residues involved in binding. Subsequently, in vitro studies demonstrated that CGA-induced secretion of GLP-1 via enhancing cAMP levels in GLUTag cells. Furthermore, in vivo experiments utilizing rotenone-induced mouse model of PD revealed a significant rise in plasma GLP-1 after CGA administration (50mg/kg, orally for 13weeks) with concomitant increase in colonic GPR-40 and GPR-43 mRNA expression. CGA treatment also prevented rotenone-induced motor and cognitive impairments and significantly restored the rotenone-induced oxidative stress. Meanwhile, western blot results confirmed that CGA treatment downregulated rotenone-induced phosphorylated alpha-synuclein levels by upregulating PI3K/AKT signaling and inactivating GSK-3β through the release of GLP-1. CGA treatment ameliorated rotenone-induced dopaminergic nerve degeneration and alpha-synuclein accumulation in substantia nigra and augmented mean density of dopaminergic nerve fibers in striatum. These findings demonstrated novel biological function of CGA as a GLP-1 secretagogue. An increase in endogenous GLP-1 may render neuroprotection against a rotenone mouse model of PD and has the potential to be used as a neuroprotective agent in management of PD.

Neuroprotective effect of quercetin against rotenone-induced neuroinflammation and alterations in mice behavior.

Various studies suggested that neuroinflammation leads to the development of several neurodegenerative disorders like Parkinson's disease (PD), Alzheimer's disease (AD), andHuntington's disease (HD). Rotenone is an organic pesticide and potent inhibitor of complex I of electron transport chain widely used to develop the PD model. Numerous studies reported rotenone toxicity in the dopaminergic system but very few studies are available on rotenone-induced glial cell activation and subsequent neurodegeneration and alterations in various types of behavior. Therefore, the present study was designed to explore the effect of rotenone on neuroinflammation and its deleterious effect on the behavior of mice, and also how these effects can be protected through quercetin. Quercetin, a natural flavonoid having strong antioxidant and anti-inflammatory properties, is found in vegetables and fruits. The finding of the study indicated that rotenone 5 mg/kg body weight for 60 days through oral gavage leads to the release of inflammatory markers in blood serum, astrocytes activation in substantia nigra and hippocampus, and subsequently decreased density of dopaminergic fibers in the striatum. Rotenone also altered the memory of the mice as indicated by decreased spontaneous alteration in Y-maze and T-maze tests and reduction in exploration time in novel object recognition, increased immobility time in theforced swim testand reduced muscular strength. Co-treatment of quercetin 30 mg/kg/day through oral gavage for 60 days along with rotenone significantly reversed all these adverse effects, suggesting that quercetin could reduce neuroinflammation, and improve memory, and cognitive function.

Enhancement of Motor Cortical Gamma Oscillations and Sniffing Activity by Medial Forebrain Bundle Stimulation Precedes Locomotion.

The medial forebrain bundle (MFB) is a white matter pathway that traverses through mesolimbic structures and includes dopaminergic neural fibers ascending from the ventral tegmental area (VTA). Since dopaminergic signals represent hedonic responses, electrical stimulation of the MFB in animals has been used as a neural reward for operant and spatial tasks. MFB stimulation strongly motivates animals to rapidly learn to perform a variety of behavioral tasks to obtain a reward. Although the MFB is known to connect various brain regions and MFB stimulation dynamically modulates animal behavior, how central and peripheral functions are affected by MFB stimulation per se is poorly understood. To address this question, we simultaneously recorded electrocorticograms (ECoGs) in the primary motor cortex (M1), primary somatosensory cortex (S1), and olfactory bulb (OB) of behaving rats while electrically stimulating the MFB. We found that MFB stimulation increased the locomotor activity of rats. Spectral analysis confirmed that immediately after MFB stimulation, sniffing activity was facilitated and the power of gamma oscillations in the M1 was increased. After sniffing activity and motor cortical gamma oscillations were facilitated, animals started to move. These results provide insight into the importance of sniffing activity and cortical gamma oscillations for motor execution and learning facilitated by MFB stimulation.

Celsr3 Inactivation in the Brainstem Impairs Rubrospinal Tract Development and Mouse Behaviors in Motor Coordination and Mechanic-Induced Response

Inactivation of Celsr3 in the forebrain results in defects of longitudinal axonal tracts such as the corticospinal tract. In this study, we inactivated Celsr3 in the brainstem using En1-Cre mice (Celsr3 cKO) and analyzed axonal and behavioral phenotypes. Celsr3 cKO animals showed an 83% reduction of rubrospinal axons and 30% decrease of corticospinal axons in spinal segments, associated with increased branching of dopaminergic fibers in the ventral horn. Decreases of spinal motoneurons, neuromuscular junctions, and electromyographic signal amplitude of the biceps were also found in mutant animals. Mutant mice had impaired motor coordination and defective response to heavy mechanical stimulation, but no disability in walking and food pellet handling. Transsynaptic tracing demonstrated that rubrospinal axons synapse on spinal neurons in the deep layer of the dorsal horn, and mechanical stimulation of hindpaws induced strong calcium signal of red nuclei in control mice, which was less prominent in mutant mice. In conclusion, Celsr3 regulates development of spinal descending axons and the motor network in cell and non-cell autonomous manners, and the maturation of the rubrospinal system is required for motor coordination and response to mechanical stimulation.

Striatal synaptic bioenergetic and autophagic decline in premotor experimental parkinsonism.

Synaptic impairment might precede neuronal degeneration in Parkinson’s disease. However, the intimate mechanisms altering synaptic function by the accumulation of presynaptic α-synuclein in striatal dopaminergic terminals before dopaminergic death occurs, have not been elucidated. Our aim is to unravel the sequence of synaptic functional and structural changes preceding symptomatic dopaminergic cell death. As such, we evaluated the temporal sequence of functional and structural changes at striatal synapses before parkinsonian motor features appear in a rat model of progressive dopaminergic death induced by overexpression of the human mutated A53T α-synuclein in the substantia nigra pars compacta, a protein transported to these synapses. Sequential window acquisition of all theoretical mass spectra proteomics identified deregulated proteins involved first in energy metabolism and later, in vesicle cycling and autophagy. After protein deregulation and when α-synuclein accumulated at striatal synapses, alterations to mitochondrial bioenergetics were observed using a Seahorse XF96 analyser. Sustained dysfunctional mitochondrial bioenergetics was followed by a decrease in the number of dopaminergic terminals, morphological and ultrastructural alterations, and an abnormal accumulation of autophagic/endocytic vesicles inside the remaining dopaminergic fibres was evident by electron microscopy. The total mitochondrial population remained unchanged whereas the number of ultrastructurally damaged mitochondria increases as the pathological process evolved. We also observed ultrastructural signs of plasticity within glutamatergic synapses before the expression of motor abnormalities, such as a reduction in axospinous synapses and an increase in perforated postsynaptic densities. Overall, we found that a synaptic energetic failure and accumulation of dysfunctional organelles occur sequentially at the dopaminergic terminals as the earliest events preceding structural changes and cell death. We also identify key proteins involved in these earliest functional abnormalities that may be modulated and serve as therapeutic targets to counterbalance the degeneration of dopaminergic cells to delay or prevent the development of Parkinson’s disease.

Evaluation of ABT-888 in the amelioration of α-synuclein fibril-induced neurodegeneration.

The accumulation of α-synuclein inclusions in vulnerable neuronal populations pathologically defines Lewy body diseases including Parkinson’s disease. Recent pre-clinical studies suggest poly(ADP-ribose) polymerase-1 activation and the subsequent generation of poly(ADP-ribose) polymer represent key steps in the formation of toxic α-synuclein aggregates and neurodegeneration. Several studies suggest that the inhibition of poly(ADP-ribose) polymerase-1 activity via the poly(ADP-ribose) polymerase-1/2 small molecule inhibitor ABT-888 (Veliparib), a drug in clinical trials for different cancers, may prevent or ameliorate α-synuclein fibril-induced aggregation, inclusion formation and dopaminergic neurodegeneration. Herein, we evaluated the effects of poly(ADP-ribose) polymer on α-synuclein fibrillization in vitro, the effects of ABT-888 on the formation of fibril-seeded α-synuclein inclusions in primary mouse cortical neurons and the effects of an in-diet ABT-888 dosage regimen with the intracranial injection of α-synuclein fibrils into the mouse dorsal striatum. We found that poly(ADP-ribose) polymer minimally but significantly increased the rate of spontaneously formed α-synuclein fibrils in vitro. Machine-learning algorithms that quantitatively assessed α-synuclein inclusion counts in neurons, both in primary cultures and in the brains of fibril-injected mice, did not reveal differences between ABT-888- and vehicle-treated groups. The in-diet administered ABT-888 molecule demonstrated outstanding brain penetration in mice; however, dopaminergic cell loss in the substantia nigra caused by α-synuclein fibril injections in the striatum was similar between ABT-888- and vehicle-treated groups. α-Synuclein fibril-induced loss of dopaminergic fibres in the dorsal striatum was also similar between ABT-888- and vehicle-treated groups. We conclude that additional pre-clinical evaluation of ABT-888 may be warranted to justify further exploration of ABT-888 for disease modification in Lewy body diseases.

The Organization of Somatostatin-Immunoreactive Cells in the Visual Cortex of the Gerbil.

Somatostatin (SST) is widely expressed in the brain and plays various, vital roles involved in neuromodulation. The purpose of this study is to characterize the organization of SST neurons in the Mongolian gerbil visual cortex (VC) using immunocytochemistry, quantitative analysis, and confocal microscopy. As a diurnal animal, the Mongolian gerbil provides us with a different perspective to other commonly used nocturnal rodent models. In this study, SST neurons were located in all layers of the VC except in layer I; they were most common in layer V. Most SST neurons were multipolar round/oval or stellate cells. No pyramidal neurons were found. Moreover, 2-color immunofluorescence revealed that only 33.50%, 24.05%, 16.73%, 0%, and 64.57% of SST neurons contained gamma-aminobutyric acid, calbindin-D28K, calretinin, parvalbumin, and calcium/calmodulin-dependent protein kinase II, respectively. In contrast, neuropeptide Y and nitric oxide synthase were abundantly expressed, with 80.07% and 75.41% in SST neurons, respectively. Our immunocytochemical analyses of SST with D1 and D2 dopamine receptors and choline acetyltransferase, α7 and β2 nicotinic acetylcholine receptors suggest that dopaminergic and cholinergic fibers contact some SST neurons. The results showed some distinguishable features of SST neurons and provided some insight into their afferent circuitry in the gerbil VC. These findings may support future studies investigating the role of SST neurons in visual processing.

Age-related LRRK2 G2019S Mutation Impacts Microglial Dopaminergic Fiber Refinement and Synaptic Pruning Involved in Abnormal Behaviors.

Mutations in the leucine-rich repeat kinase 2 (LRRK2) gene are the most frequent cause of autosomal dominant Parkinson's disease (PD), producing psychiatric and motor symptoms. We conducted this study to explore whether microglial dopaminergic (DAergic) fiber refinement and synaptic pruning are involved in the abnormal behavioral phenotypes of carriers of the LRRK2 G2019S mutation, by employing young and middle-aged PD model mice. The results revealed a characteristic late-onset hyperactivity and a progressive decline in the motor coordination of the LRRK2 G2019S mutation mice. LRRK2 G2019S mutation-induced aberrant microglial morphogenesis, with more branches and junctions per cell, resulted in excessive microglial refinement of dopaminergic (DAergic) fibers. Moreover, aberrant synaptic pruning distinctly impacted the prefrontal cortex (PFC) and dorsal striatum (DS), with significantly higher spine density in the PFC but the opposite effects in the DS region. Furthermore, LRRK2 G2019S mutation remodeled the inflammatory transcription landscape of microglia, rendering certain cerebral areas highly susceptible to microglial immune response. These findings indicate that LRRK2 G2019S mutation induces the production of inflammatory cytokines and mediates abnormal microglial morphogenesis and activity, resulting in abnormal phagocytosis, synaptic pruning and loss of DAergic fibers during aging, and, eventually, PD-related behavioral abnormalities.

Further Characterization of Intrastriatal Lipopolysaccharide Model of Parkinson's Disease in C57BL/6 Mice.

Parkinson’s disease (PD) is the most common movement disorder, characterized by progressive degeneration of the nigrostriatal pathway, which consists of dopaminergic cell bodies in substantia nigra and their neuronal projections to the striatum. Moreover, PD is associated with an array of non-motor symptoms such as olfactory dysfunction, gastrointestinal dysfunction, impaired regulation of the sleep-wake cycle, anxiety, depression, and cognitive impairment. Inflammation and concomitant oxidative stress are crucial in the pathogenesis of PD. Thus, this study aimed to model PD via intrastriatal injection of the inflammagen lipopolysaccharide (LPS)to investigate if the lesion causes olfactory and motor impairments, inflammation, oxidative stress, and alteration in synaptic proteins in the olfactory bulb, striatum, and colon. Ten µg of LPS was injected unilaterally into the striatum of 27 male C57BL/6 mice, and behavioural assessment was conducted at 4 and 8 weeks post-treatment, followed by tissue collection. Intrastriatal LPS induced motor impairment in C57BL/6 mice at 8 weeks post-treatment evidenced by reduced latency time in the rotarod test. LPS also induced inflammation in the striatum characterized by increased expression of microglial marker Iba-1 and astrocytic marker GFAP, with degeneration of dopaminergic neuronal fibres (reduced tyrosine hydroxylase immunoreactivity), and reduction of synaptic proteins and DJ-1 protein. Additionally, intrastriatal LPS induced inflammation, oxidative stress and alterations in synaptic proteins within the olfactory bulb, although this did not induce a significant impairment in olfactory function. Intrastriatal LPS induced mild inflammatory changes in the distal colon, accompanied by increased protein expression of 3-nitrotyrosine-modified proteins. This model recapitulated the major features of PD such as motor impairment and degeneration of dopaminergic neuronal fibres in the striatum, as well as some pathological changes in the olfactory bulb and colon; thus, this model could be suitable for understanding clinical PD and testing neuroprotective strategies.

Pathogenesis and Treatment of Parkinson's Disease

Neurological disorders in humans can be modeled in animals using standardized procedures that recreate specific pathogenic events and their behavioral outcomes. The development of animal models of Parkinson’s disease (PD) is important to test new neuroprotective agents and strategies. Such animal models of PD have to mimic, at least partially, a Parkinson-like pathology and should reproduce specific features of the human disease. PD is characterized by massive degeneration of dopaminergic neurons in the substantia nigra, the loss of striatal dopaminergic fibers and a dramatic reduction of the striatal dopamine levels. The formation of cytoplasmic inclusion bodies (Lewy bodies) in surviving dopaminergic neurons represents the most important neuropathological feature of PD. Furthermore, the massive striatal dopamine deficiency causes easily detectable motor deficits in PD patients, including bradykinesia, rigidity, and resting tremor, which are the cardinal symptoms of PD. Over the years, a broad variety of experimental models of PD were developed and applied in diverse species. This review focuses on the two most common “classical” toxin-induced PD models, the 6-hydroxy-dopamine (6-OHDA model) and the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) model. Both neurotoxins selectively and rapidly destroy catecholaminergic neurons, whereas in humans the PD pathogenesis follows a progressive course over decades. This discrepancy reflects one important and principal point of weakness related to most animal models. This review discusses the most important properties of 6-OHDA and MPTP, their modes of administration, and critically examines advantages and limitations of selected animal models. The new genetic and environmental toxin models of PD (e.g. rotenone, paraquat, maneb) are discussed elsewhere in this “special issue.”

Subthalamic nucleus deep brain stimulation induces sustained neurorestoration in the mesolimbic dopaminergic system in a Parkinson's disease model

BackgroundDeep brain stimulation (DBS) of the subthalamic nucleus (STN) is an established therapeutic principle in Parkinson's disease, but the underlying mechanisms, particularly mediating non-motor actions, remain largely enigmatic. Objective/hypothesisThe delayed onset of neuropsychiatric actions in conjunction with first experimental evidence that STN-DBS causes disease-modifying effects prompted our investigation on how cellular plasticity in midbrain dopaminergic systems is affected by STN-DBS. MethodsWe applied unilateral or bilateral STN-DBS in two independent cohorts of 6-hydroxydopamine hemiparkinsonian rats four to eight weeks after dopaminergic lesioning to allow for the development of a stable dopaminergic dysfunction prior to DBS electrode implantation. ResultsAfter 5 weeks of STN-DBS, stimulated animals had significantly more TH+ dopaminergic neurons and fibres in both the nigrostriatal and the mesolimbic systems compared to sham controls with large effect sizes of gHedges = 1.9–3.4. DBS of the entopeduncular nucleus as the homologue of the human Globus pallidus internus did not alter the dopaminergic systems. STN-DBS effects on mesolimbic dopaminergic neurons were largely confirmed in an independent animal cohort with unilateral STN stimulation for 6 weeks or for 3 weeks followed by a 3 weeks washout period. The latter subgroup even demonstrated persistent mesolimbic dopaminergic plasticity after washout. Pilot behavioural testing showed that augmentative dopaminergic effects on the mesolimbic system by STN-DBS might translate into improvement of sensorimotor neglect. ConclusionsOur data support sustained neurorestorative effects of STN-DBS not only in the nigrostriatal but also in the mesolimbic system as a potential factor mediating long-latency neuropsychiatric effects of STN-DBS in Parkinson's disease.

MicroPET imaging of vesicular monoamine transporter 2 revealed the potentiation of (+)-dihydrotetrabenazine on MPTP-induced degeneration of dopaminergic neurons

IntroductionVesicular monoamine transporter 2 (VMAT2) has been associated with the risk of PD. Genetic reduction of VMAT2 level is reported to increase the vulnerability for dopaminergic neurodegeneration. In this study, by using in vivo microPET imaging with a VMAT2 radioligand [18F]fluoropropyl-(+)-dihydrotetrabenazine ([18F]FP-(+)-DTBZ), we investigated the enhanced role of inhibiting VMAT2 in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced loss of dopaminergic neurons. MethodsThe (+)-α-dihydrotetrabenazine ((+)-DTBZ, an inhibitor of VMAT2, 5 mg/kg), or MPTP (low dose (ld): 10 mg/kg, high dose (hd): 30 mg/kg) or both of them were intraperitoneally injected into C57BL/6 mice for 5 or 10 consecutive days. MicroPET imaging with [18F]FP-(+)-DTBZ was performed to test the dopaminergic neuronal integrity. [18F]FP-(+)-DTBZ uptake in striatum was quantified as standardized uptake value (SUV). The pathological changes in the striata and substantia nigra were confirmed by measuring the DA contents and immunohistochemical staining of tyrosine hydroxylase (TH). ResultsIn vivo imaging results showed that the striatal SUVs of both DTBZ&MPTPld and MPTPhd groups were substantially declined compared to the baseline. Moreover, the striatal uptakes of [18F]FP-(+)-DTBZ in DTBZ&MPTPld and MPTPhd groups were obviously lower than the control, DTBZ group and MPTPld group. Notably, the decrease of the striatal uptake in the DTBZ&MPTPld/10d group was more serious than the DTBZ&MPTPld/5d group and comparable to the MPTPhd group. Consistently, the ratios of DA metabolites to DA in DTBZ&MPTPld/10d and MPTPhd mice were significantly increased. The correlation analysis showed that SUVs were highly correlated to the striatal dopaminergic fiber density and TH-positive dopaminergic neuron number in the substantia nigra. ConclusionsMicroPET brain imaging with [18F]FP-(+)-DTBZ noninvasively revealed that (+)-DTBZ co-administration significantly aggravated the neurotoxicity of MPTP to dopaminergic neurons, suggesting that inhibition of VMAT2 may be related to the pathogenesis of PD and tracing VMAT2 activity with PET imaging is of potential value in monitoring PD progression.

Disruption of brainstem monoaminergic fibre tracts in multiple sclerosis as a putative mechanism for cognitive fatigue: a fixel-based analysis

In multiple sclerosis (MS), monoaminergic systems are altered as a result of both inflammation-dependent reduced synthesis and direct structural damage. Aberrant monoaminergic neurotransmission is increasingly considered a major contributor to fatigue pathophysiology. In this study, we aimed to compare the integrity of the monoaminergic white matter fibre tracts projecting from brainstem nuclei in a group of patients with MS (n = 68) and healthy controls (n = 34), and to investigate its association with fatigue. Fibre tracts integrity was assessed with the novel fixel-based analysis that simultaneously estimates axonal density, by means of ‘fibre density’, and white matter atrophy, by means of fibre ‘cross section’. We focused on ventral tegmental area, locus coeruleus, and raphe nuclei as the main source of dopaminergic, noradrenergic, and serotoninergic fibres within the brainstem, respectively. Fourteen tracts of interest projecting from these brainstem nuclei were reconstructed using diffusion tractography, and compared by means of the product of fibre-density and cross-section (FDC). Finally, correlations of monoaminergic axonal damage with the modified fatigue impact scale scores were evaluated in MS. Fixel-based analysis revealed significant axonal damage – as measured by FDC reduction – within selective monoaminergic fibre-tracts projecting from brainstem nuclei in MS patients, in comparison to healthy controls; particularly within the dopaminergic-mesolimbic pathway, the noradrenergic-projections to prefrontal cortex, and serotoninergic-projections to cerebellum. Moreover, we observed significant correlations between severity of cognitive fatigue and axonal damage within the mesocorticolimbic tracts projecting from ventral tegmental area, as well as within the locus coeruleus projections to prefrontal cortex, suggesting a potential contribution of dopaminergic and noradrenergic pathways to central fatigue in MS. Our findings support the hypothesis that axonal damage along monoaminergic pathways contributes to the reduction/dysfunction of monoamines in MS and add new information on the mechanisms by which monoaminergic systems contribute to MS pathogenesis and fatigue. This supports the need for further research into monoamines as therapeutic targets aiming to combat and alleviate fatigue in MS.

Neuroprotective Potential of a Small Molecule RET Agonist in Cultured Dopamine Neurons and Hemiparkinsonian Rats.

Background:Parkinson’s disease (PD) is a progressive neurological disorder where loss of dopamine neurons in the substantia nigra and dopamine depletion in the striatum cause characteristic motor symptoms. Currently, no treatment is able to halt the progression of PD. Glial cell line-derived neurotrophic factor (GDNF) rescues degenerating dopamine neurons both in vitro and in animal models of PD. When tested in PD patients, however, the outcomes from intracranial GDNF infusion paradigms have been inconclusive, mainly due to poor pharmacokinetic properties.Objective:We have developed drug-like small molecules, named BT compounds that activate signaling through GDNF’s receptor, the transmembrane receptor tyrosine kinase RET, both in vitro and in vivo and are able to penetrate through the blood-brain barrier. Here we evaluated the properties of BT44, a second generation RET agonist, in immortalized cells, dopamine neurons and rat 6-hydroxydopamine model of PD.Methods:We used biochemical, immunohistochemical and behavioral methods to evaluate the effects of BT44 on dopamine system in vitro and in vivo.Results:BT44 selectively activated RET and intracellular pro-survival AKT and MAPK signaling pathways in immortalized cells. In primary midbrain dopamine neurons cultured in serum-deprived conditions, BT44 promoted the survival of the neurons derived from wild-type, but not from RET knockout mice. BT44 also protected cultured wild-type dopamine neurons from MPP+-induced toxicity. In a rat 6-hydroxydopamine model of PD, BT44 reduced motor imbalance and seemed to protect dopaminergic fibers in the striatum.Conclusion:BT44 holds potential for further development into a novel, possibly disease-modifying, therapy for PD.