Pre-synaptic dopaminergic compensation after moderate nigrostriatal damage in non-human primates

Positive relation between dopamine neuron degeneration and metabolic connectivity disruption in the MPTP plus probenecid mouse model of Parkinson's disease

The clinical manifestation of Parkinson's disease (PD) appears when neurodegeneration is already advanced, compromising the efficacy of disease-modifying treatment approaches. Biomarkers to identify the early stages of PD are therefore of paramount importance for the advancement of the therapy of PD. In the present study, by using a mouse model of PD obtained by subchronic treatment with the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and the clearance inhibitor probenecid (MPTPp), we identified prodromal markers of PD by combining in vivo positron emission tomography (PET) imaging and ex vivo immunohistochemistry. Longitudinal PET imaging of the dopamine transporter (DAT) by [18F]-N-(3-fluoropropyl)-2β-carboxymethoxy-3β-(4-iodophenyl) nortropane ([18F]-FP-CIT), and brain glucose metabolism by 2-deoxy-2-[18F]-fluoroglucose ([18F]-FDG) were performed before MPTPp treatment and after 1, 3, and 10 MPTPp administrations, in order to assess relation between dopamine neuron integrity and brain connectivity. The results show that in vivo [18F]-FP-CIT in the dorsal striatum was not modified after the first administration of MPTPp, tended to decrease after 3 administrations, and significantly decreased after 10 MPTPp administrations. Post-mortem immunohistochemical analyses of DAT and tyrosine hydroxylase (TH) in the striatum showed a positive correlation with [18F]-FP-CIT, confirming the validity of repeated MPTPp-treated mice as a model that can reproduce the progressive pathological changes in the early phases of PD. Analysis of [18F]-FDG uptake in several brain areas connected to the striatum showed that metabolic connectivity was progressively disrupted, starting from the first MPTPp administration, and that significant connections between cortical and subcortical regions were lost after 10 MPTPp administrations, suggesting an association between dopamine neuron degeneration and connectivity disruption in this PD model. The results of this study provide a relevant model, where new drugs that can alleviate neurodegeneration in PD could be evaluated preclinically.

Dopamine Signaling in Substantia Nigra and Its Impact on Locomotor Function-Not a New Concept, but Neglected Reality

The mechanistic influences of dopamine (DA) signaling and impact on motor function are nearly always interpreted from changes in nigrostriatal neuron terminals in striatum. This is a standard practice in studies of human Parkinson's disease (PD) and aging and related animal models of PD and aging-related parkinsonism. However, despite dozens of studies indicating an ambiguous relationship between changes in striatal DA signaling and motor phenotype, this perseverating focus on striatum continues. Although DA release in substantia nigra (SN) was first reported almost 50 years ago, assessment of nigral DA signaling changes in relation to motor function is rarely considered. Whereas DA signaling has been well-characterized in striatum at all five steps of neurotransmission (biosynthesis and turnover, storage, release, reuptake, and post-synaptic binding) in the nigrostriatal pathway, the depth of such interrogations in the SN, outside of cell counts, is sparse. However, there is sufficient evidence that these steps in DA neurotransmission in the SN are operational and regulated autonomously from striatum and are present in human PD and aging and related animal models. To complete our understanding of how nigrostriatal DA signaling affects motor function, it is past time to include interrogation of nigral DA signaling. This brief review highlights evidence that changes in nigral DA signaling at each step in DA neurotransmission are autonomous from those in striatum and changes in the SN alone can influence locomotor function. Accordingly, for full characterization of how nigrostriatal DA signaling affects locomotor activity, interrogation of DA signaling in SN is essential.

Investigating the potential effects of α-synuclein aggregation on susceptibility to chronic stress in a mouse Parkinson's disease model

BACKGROUND

Parkinson's disease (PD) is a motor disorder characterized by the degeneration of dopaminergic neurons, putatively due to the accumulation of α-synuclein (α-syn) in Lewy bodies (LBs) in Substantia Nigra. PD is also associated with the formation of LBs in brain areas responsible for emotional and cognitive regulation such as the amygdala and prefrontal cortex, and concurrent depression prevalence in PD patients. The exact link between dopaminergic cell loss, α-syn aggregation, depression, and stress, a major depression risk factor, is unclear. Therefore, we aimed to explore the interplay between sensitivity to chronic stress and α-syn aggregation.

METHODS

Bilateral injections of α-syn preformed fibrils (PFFs) into the striatum of C57Bl/6 J mice were used to induce α-syn aggregation. Three months after injections, animals were exposed to chronic social defeat stress.

RESULTS

α-syn aggregation did not affect stress susceptibility but independently caused increased locomotor activity in the open field test, reduced anxiety in the light-dark box test, and increased active time in the tail suspension test. Ex vivo analysis revealed modest dopaminergic neuron loss in the substantia nigra and reduced dopaminergic innervation in the dorsal striatum in PFFs injected groups. α-Syn aggregates were prominent in the amygdala, prefrontal cortex, and substantia nigra, with minimal α-syn aggregation in the raphe nuclei and locus coeruleus.

CONCLUSIONS

Progressive bilateral α-syn aggregation might lead to compensatory activity increase and alterations in emotionally regulated behavior, without affecting stress susceptibility. Understanding how α-syn aggregation and degeneration in specific brain structures contribute to depression and anxiety in PD patients requires further investigation.

Nigral-specific increase in ser31 phosphorylation compensates for tyrosine hydroxylase protein and nigrostriatal neuron loss: Implications for delaying parkinsonian signs

Compensatory mechanisms that augment dopamine (DA) signaling are thought to mitigate onset of hypokinesia prior to major loss of tyrosine hydroxylase (TH) in striatum that occurs in Parkinson's disease. However, the identity of such mechanisms remains elusive. In the present study, the rat nigrostriatal pathway was unilaterally-lesioned with 6-hydroxydopamine (6-OHDA) to determine whether differences in DA content, TH protein, TH phosphorylation, or D1 receptor expression in striatum or substantia nigra (SN) aligned with hypokinesia onset and severity at two time points. In striatum, DA and TH loss reached its maximum (>90%) 7 days after lesion induction. However, in SN, no DA loss occurred, despite ∼60% TH loss. Hypokinesia was established at 21 days post-lesion and maintained at 28 days. At this time, DA loss was ∼60% in the SN, but still of lesser magnitude than TH loss. At day 7 and 28, ser31 TH phosphorylation increased only in SN, corresponding to less DA versus TH protein loss. In contrast, ser40 TH phosphorylation was unaffected in either region. Despite DA loss in both regions at day 28, D1 receptor expression increased only in lesioned SN. These results support the concept that augmented components of DA signaling in the SN, through increased ser31 TH phosphorylation and D1 receptor expression, contribute as compensatory mechanisms against progressive nigrostriatal neuron and TH protein loss, and may mitigate hypokinesia severity.

Perturbation of serine enantiomers homeostasis in the striatum of MPTP-lesioned monkeys and mice reflects the extent of dopaminergic midbrain degeneration

Loss of dopaminergic midbrain neurons perturbs l-serine and d-serine homeostasis in the post-mortem caudate putamen (CPu) of Parkinson's disease (PD) patients. However, it is unclear whether the severity of dopaminergic nigrostriatal degeneration plays a role in deregulating serine enantiomers' metabolism. Here, through high-performance liquid chromatography (HPLC), we measured the levels of these amino acids in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated monkeys and MPTP-plus-probenecid (MPTPp)-treated mice to determine whether and how dopaminergic midbrain degeneration affects the levels of serine enantiomers in various basal ganglia subregions. In addition, in the same brain regions, we measured the levels of key neuroactive amino acids modulating glutamatergic neurotransmission, including L-glutamate, glycine, l-aspartate, d-aspartate, and their precursors l-glutamine, L-asparagine. In monkeys, MPTP treatment produced severe denervation of nigrostriatal dopaminergic fibers (⁓75%) and increased the levels of serine enantiomers in the rostral putamen (rPut), but not in the subthalamic nucleus, and the lateral and medial portion of the globus pallidus. Moreover, this neurotoxin significantly reduced the protein expression of the astrocytic serine transporter ASCT1 and the glycolytic enzyme GAPDH in the rPut of monkeys. Conversely, concentrations of d-serine and l-serine, as well as ASCT1 and GAPDH expression were unaffected in the striatum of MPTPp-treated mice, which showed only mild dopaminergic degeneration (⁓30%). These findings unveil a link between the severity of dopaminergic nigrostriatal degeneration and striatal serine enantiomers concentration, ASCT1 and GAPDH expression. We hypothesize that the up-regulation of d-serine and l-serine levels occurs as a secondary response within a homeostatic loop to support the metabolic and neurotransmission demands imposed by the degeneration of dopaminergic neurons.

Dystonia and Parkinson's disease: Do they have a shared biology?

Parkinsonism and dystonia co-occur across many movement disorders and are most encountered in the setting of Parkinson's disease. Here we aim to explore the shared neurobiological underpinnings of dystonia and parkinsonism through the clinical lens of the conditions in which these movement disorders can be seen together. Foregrounding the discussion, we briefly review the circuits of the motor system and the neuroanatomical and neurophysiological aspects of motor control and highlight their relevance to the proposed pathophysiology of parkinsonism and dystonia. Insight into shared biology is then sought from dystonia occurring in PD and other forms of parkinsonism including those disorders in which both can be co-expressed simultaneously. We organize these within a biological schema along with important questions to be addressed in this space.

DOPA pheomelanin is increased in nigral neuromelanin of Parkinson's disease

Neuromelanin (NM) in dopaminergic neurons of human substantia nigra (SN) has a melanic component that consists of pheomelanin and eumelanin moieties and has been proposed as a key factor contributing to dopaminergic neuron vulnerability in Parkinson's disease (PD). While eumelanin is considered as an antioxidant, pheomelanin and related oxidative stress are associated with compromised drug and metal ion binding and melanoma risk. Using postmortem SN from patients with PD or Alzheimer's disease (AD) and unaffected controls, we identified increased L-3,4-dihydroxyphenylalanine (DOPA) pheomelanin and increased ratios of dopamine (DA) pheomelanin markers to DA in PD SN compared to controls. Eumelanins derived from both DOPA and DA were reduced in PD group. In addition, we report an increase in DOPA pheomelanin relative to DA pheomelanin in PD SN. In AD SN, we observed unaltered melanin markers despite reduced DOPA compared to controls. Furthermore, synthetic DOPA pheomelanin induced neuronal cell death in vitro while synthetic DOPA eumelanin showed no significant effect on cell viability. Our findings provide insights into the different roles of pheomelanin and eumelanin in PD pathophysiology. We anticipate our study will lead to further investigations on pheomelanin and eumelanin individually as biomarkers and possibly therapeutic targets for PD.

Impact of age on the rotenone-induced sporadic Parkinson's disease model using Drosophila melanogaster

Rotenone, a naturally occurring toxin, has been used to induce sporadic Parkinson's disease (PD) in Drosophila melanogaster for decades. However, the age of flies varies considerably between studies in this model. To investigate the impact of age on the rotenone-induced PD model, we collected male flies at the age of 1, 5, 7, and 10 days post-eclosion, respectively. Then, flies were immediately exposed to a feeding medium supplemented with 250 μM rotenone for seven days. The motor ability of Drosophila was detected by negative geotaxis assay, and the number of dopamine (DA) neurons and tyrosine hydroxylase (TH) expression levels were evaluated. The results showed that both the motor deficits and mortality increased with age. The flies older than five days showed typical PD features, including the loss of DA neurons, decreased TH expression levels, and decreased locomotive ability. However, 1-day-old flies displayed an unstable motor deficit and little TH expression changes after seven days of rotenone exposure. Lastly, after 7 days of exposure to rotenone, the death rate of flies rapidly increased with increasing starting age. The death rates of 1-, 5-, 7-, and 10-days old flies were 10.0%, 22.8%, 41.5%, and 50.4%, respectively. The findings of this study suggest that age is a crucial factor impacting the Drosophila PD model. This information provides a reference for the age selection to use this model for future studies.

Effects of monotherapy with a monoamine oxidase B inhibitor on motor symptoms in Parkinson's disease are dependent on frontal function

BACKGROUND

Monotherapy with monoamine oxidase B (MAO-B) inhibitors enhances the level of endogenous dopamine in treatment for Parkinson's disease (PD) and provides some benefits. Certain neuropsychiatric functions are also regulated by central dopaminergic activity.

AIM

To investigate the relationship of the efficacy of monotherapy with MAO-B inhibitors on motor symptoms in PD with baseline cognitive function.

PATIENTS AND METHODS

Outcomes were examined for 27 consecutive drug-naïve PD patients who received initial treatment with a MAO-B inhibitor (selegiline: 11, rasagiline: 16). Selegiline was titrated to an optimal dose. The dose of rasagiline was fixed at 1 mg/day. Motor symptoms were assessed using the Movement Disorder Society-Unified Parkinson's Disease Rating Scale part III before treatment and after the efficacy reached a plateau within 19 weeks after drug initiation, and the % improvement in motor symptoms was calculated. Pre-treatment cognitive function was assessed using the Montreal Cognitive Assessment (MoCA) and Frontal Assessment Battery (FAB). Correlations of % improvement in motor symptoms and baseline cognitive assessments were examined using Spearman correlation coefficients and multiple regression analysis.

RESULTS

In all patients, the mean % improvement in motor symptoms was 46.5% (range 0-83.3%). Spearman correlation coefficients showed the % improvement in motor symptoms was correlated with FAB (r = 0.631, p < 0.001). In multiple regression analysis with patient background factors as independent variables, only FAB was associated with improvement in motor symptoms in the MAO-B group.

CONCLUSION

Better FAB scores predict a significant improvement in motor symptoms with treatment with MAO-B inhibitors, suggesting high activity of endogenous dopamine.

Neurotoxicological Profiling of Paraquat in Zebrafish Model

Paraquat is a polar herbicide protecting plant products against invasive species, it requires careful manipulation and restricted usage because of its harmful potentials. Exposure to paraquat triggers oxidative damage in dopaminergic neurons and subsequently causes a behavioral defect in vivo. Thereby, persistent exposure to paraquat is known to increase Parkinson's disease risk by dysregulating dopaminergic systems in humans. Therefore, most studies have focused on the dopaminergic systems to elucidate the neurotoxicological mechanism of paraquat poisoning, and more comprehensive neurochemistry including histaminergic, serotonergic, cholinergic, and GABAergic systems has remained unclear. Therefore, in this study, we investigated the toxicological potential of paraquat poisoning using a variety of approaches such as toxicokinetic profiles, behavioral effects, neural activity, and broad-spectrum neurochemistry in zebrafish larvae after short-term exposure to paraquat and we performed the molecular modeling approach. Our results showed that paraquat was slowly absorbed in the brain of zebrafish after oral administration of paraquat. In addition, paraquat toxicity resulted in behavioral impairments, namely, reduced motor activity and led to abnormal neural activities in zebrafish larvae. This locomotor deficit came with a dysregulation of dopamine synthesis induced by the inhibition of tyrosine hydroxylase activity, which was also indirectly confirmed by molecular modeling studies. Furthermore, short-term exposure to paraquat also caused simultaneous dysregulation of other neurochemistry including cholinergic and serotonergic systems in zebrafish larvae. The present study suggests that this neurotoxicological profiling could be a useful tool for understanding the brain neurochemistry of neurotoxic agents that might be a potential risk to human and environmental health.

Motor and non-motor circuit disturbances in early Parkinson disease: which happens first?

For the last two decades, pathogenic concepts in Parkinson disease (PD) have revolved around the toxicity and spread of α-synuclein. Thus, α-synuclein would follow caudo-rostral propagation from the periphery to the central nervous system, first producing non-motor manifestations (such as constipation, sleep disorders and hyposmia), and subsequently impinging upon the mesencephalon to account for the cardinal motor features before reaching the neocortex as the disease evolves towards dementia. This model is the prevailing theory of the principal neurobiological mechanism of disease. Here, we scrutinize the temporal evolution of motor and non-motor manifestations in PD and suggest that, even though the postulated bottom-up mechanisms are likely to be involved, early involvement of the nigrostriatal system is a key and prominent pathophysiological mechanism. Upcoming studies of detailed clinical manifestations with newer neuroimaging techniques will allow us to more closely define, in vivo, the role of α-synuclein aggregates with respect to neuronal loss during the onset and progression of PD.

Dual Roles of Microglia in the Basal Ganglia in Parkinson's Disease

With the increasing age of the population, the incidence of Parkinson’s disease (PD) has increased exponentially. The development of novel therapeutic interventions requires an understanding of the involvement of senescent brain cells in the pathogenesis of PD. In this review, we highlight the roles played by microglia in the basal ganglia in the pathophysiological processes of PD. In PD, dopaminergic (DAergic) neuronal degeneration in the substantia nigra pars compacta (SNc) activates the microglia, which then promote DAergic neuronal degeneration by releasing potentially neurotoxic factors, including nitric oxide, cytokines, and reactive oxygen species. On the other hand, microglia are also activated in the basal ganglia outputs (the substantia nigra pars reticulata and the globus pallidus) in response to excess glutamate released from hyperactive subthalamic nuclei-derived synapses. The activated microglia then eliminate the hyperactive glutamatergic synapses. Synapse elimination may be the mechanism underlying the compensation that masks the appearance of PD symptoms despite substantial DAergic neuronal loss. Microglial senescence may correlate with their enhanced neurotoxicity in the SNc and the reduced compensatory actions in the basal ganglia outputs. The dual roles of microglia in different basal ganglia regions make it difficult to develop interventions targeting microglia for PD treatment.

Upregulating β-hexosaminidase activity in rodents prevents α-synuclein lipid associations and protects dopaminergic neurons from α-synuclein-mediated neurotoxicity

Sandhoff disease (SD) is a lysosomal storage disease, caused by loss of β-hexosaminidase (HEX) activity resulting in the accumulation of ganglioside GM2. There are shared features between SD and Parkinson's disease (PD). α-synuclein (aSYN) inclusions, the diagnostic hallmark sign of PD, are frequently found in the brain in SD patients and HEX knockout mice, and HEX activity is reduced in the substantia nigra in PD. In this study, we biochemically demonstrate that HEX deficiency in mice causes formation of high-molecular weight (HMW) aSYN and ubiquitin in the brain. As expected from HEX enzymatic function requirements, overexpression in vivo of HEXA and B combined, but not either of the subunits expressed alone, increased HEX activity as evidenced by histochemical assays. Biochemically, such HEX gene expression resulted in increased conversion of GM2 to its breakdown product GM3. In a neurodegenerative model of overexpression of aSYN in rats, increasing HEX activity by AAV6 gene transfer in the substantia nigra reduced aSYN embedding in lipid compartments and rescued dopaminergic neurons from degeneration. Overall, these data are consistent with a paradigm shift where lipid abnormalities are central to or preceding protein changes typically associated with PD.

Mesencephalic and extramesencephalic dopaminergic systems in Parkinson's disease

Neurodegeneration of the nigrostriatal dopaminergic system and concurrent dopamine (DA) deficiency in the basal ganglia represent core features of Parkinson's disease (PD). Despite the central role of DA in the pathogenesis of PD, dopaminergic systems outside of the midbrain have not been systematically investigated for Lewy body pathology or neurodegeneration. Dopaminergic neurons show a surprisingly rich neurobiological diversity, suggesting that there is not one general type of dopaminergic neuron, but rather a spectrum of different dopaminergic phenotypes. This heterogeneity on the cellular level could account for the observed differences in susceptibility of the dopaminergic systems to the PD disease process. In this review, we will summarize the long history from the first description of PD to the rationally derived DA replacement therapy, describe the basal neuroanatomical and neuropathological features of the different dopaminergic systems in health and PD, explore how neuroimaging techniques broadened our view of the dysfunctional dopaminergic systems in PD and discuss how dopaminergic replacement therapy ameliorates the classical motor symptoms but simultaneously induces a new set of hyperdopaminergic symptoms.

Behavioral tests predicting striatal dopamine level in a rat hemi-Parkinson's disease model

Parkinson's disease (PD) is a frequent neurodegenerative disease causing bradykinesia, tremor, muscle rigidity and postural instability. Although its main pathology is progressive dopaminergic (DArgic) neuron loss in the substantia nigra, motor deficits are thought not to become apparent until most DArgic neurons are lost, probably due to compensatory mechanisms that overcome the decline of DA level in the striatum. Even in animal PD models, it is difficult to detect motor deficits when most DArgic neurons are functional. In this study, we performed various behavioral tests (apomorphine-induced rotation, cylinder, forepaw adjustment steps (FAS), beam walking, rota-rod, and open-field), using 6-hydroxydopamine (OHDA) and lipopolysaccharide (LPS)-induced hemi-PD model rats with various striatal DA levels, to find the best way to predict the DA level from earlier disease stages. Different from the 6-OHDA-induced model, reduction in the striatal DA levels in the LPS-model was less significant. Among the behavioral tests, data from cylinder and FAS tests, which evaluate forelimb movements, best correlated with decline of the DA level. They also correlated well with decreased body weight gain. The beam and apomorphine tests showed less significant correlation than the cylinder and FAS tests. Open-field and rota-rod tests were not useful. Expressional levels of mRNA encoding tyrosine hydroxylase (TH), a marker of DArgic neurons, correlated well with the DA level. Metabotropic glutamate receptor 4 mRNA expression correlated with the striatal DA level and may be related to compensatory mechanisms. These results suggest that motor impairments of PD should be evaluated by forelimb movements, or hands and forearms in clinical settings, rather than movement of the body or large joints. The combination of cylinder and FAS tests may be the best to evaluate the rat PD models, in which many DArgic neurons survive.

Ketamine Does Not Exert Protective Properties on Dopaminergic Neurons in the Lactacystin Mouse Model of Parkinson's Disease

Parkinson’s disease (PD) is an age-related neurodegenerative condition characterized by a progressive loss of dopaminergic neurons in the substantia nigra pars compacta (SNpc). A loss of proteasome function participates to the pathogenesis of PD, leading to the development of rodent models in which a proteasome inhibitor is applied to the nigrostriatal pathway. We recently characterized the intranigral lactacystin (LAC) mouse model, leading to nigrostriatal degeneration, motor dysfunction and alpha-synuclein accumulation. In the present study, we compared the effect of two commonly used anesthetics for generating animal models of PD—i.e., ketamine (KET) and isoflurane (ISO)—on the vulnerability of mouse dopaminergic neurons to proteasome inhibition-induced degeneration. Both anesthetics have the potential to affect the susceptibility of the nigrostriatal pathway for toxin-induced degeneration, and are known to modulate dopamine (DA) homeostasis. Yet, their impact on nigrostriatal degeneration in the proteasome inhibition model has not been evaluated. Unilateral injection with LAC in the SNpc of mice induced motor impairment and significantly reduced the number of dopaminergic cells to ~55%, irrespective of the anesthetic used. However, LAC-induced striatal DA depletion was slightly affected by the choice of anesthetic, resulting in a significant increase in DA turnover in the ISO- but not in KET-treated mice. These results suggest that the extent of nigrostriatal dopaminergic neural loss caused by LAC is not influenced by the choice of anesthetic, and that compared to other PD models, KET is not neuroprotective in the LAC model.

Animal models of early-stage Parkinson's disease and acute dopamine deficiency to study compensatory neurodegenerative mechanisms

Parkinson's disease is a common neurodegenerative disease characterized by a widely variety of motor and non-motor symptoms. While the motor deficits are only visible following a severe dopamine depletion, neurodegenerative process and some non-motor symptoms are manifested years before the motor deficits. Importantly, chronic degeneration of dopaminergic neurons leads to the development of compensatory mechanisms that play roles in the progression of the disease and the response to anti-parkinsonian therapies. The identification of these mechanisms will be of great importance for improving our understanding of factors with important contributions to the disease course and the underlying adaptive process. To date, most of the data obtained from animal models reflect the late, chronic, dopamine-depleted states, when compensatory mechanisms have already been established. Thus, adequate animal models with which researchers are able to dissect early- and late-phase mechanisms are necessary. Here, we reviewed the literature related to animal models of early-stage PD and pharmacological treatments capable of inducing acute dopamine impairments and/or depletion, such as reserpine, haloperidol and tetrodotoxin. We highlighted the advantages, limitations and the future prospective uses of these models, as well as their applications in the identification of novel agents for treating this neurological disorder.

Deletion of lynx1 reduces the function of α6* nicotinic receptors

The α6 nicotinic acetylcholine receptor (nAChR) subunit is an attractive drug target for treating nicotine addiction because it is present at limited sites in the brain including the reward pathway. Lynx1 modulates several nAChR subtypes; lynx1-nAChR interaction sites could possibly provide drug targets. We found that dopaminergic cells from the substantia nigra pars compacta (SNc) express lynx1 mRNA transcripts and, as assessed by co-immunoprecipitation, α6 receptors form stable complexes with lynx1 protein, although co-transfection with lynx1 did not affect nicotine-induced currents from cell lines transfected with α6 and β2. To test whether lynx1 is important for the function of α6 nAChRs in vivo, we bred transgenic mice carrying a hypersensitive mutation in the α6 nAChR subunit (α6L9′S) with lynx1 knockout mice, providing a selective probe of the effects of lynx1 on α6* nAChRs. Lynx1 removal reduced the α6 component of nicotine-mediated rubidium efflux and dopamine (DA) release from synaptosomal preparations with no effect on numbers of α6β2 binding sites, indicating that lynx1 is functionally important for α6* nAChR activity. No effects of lynx1 removal were detected on nicotine-induced currents in slices from SNc, suggesting that lynx1 affects presynaptic α6* nAChR function more than somatic function. In the absence of agonist, lynx1 removal did not alter DA release in dorsal striatum as measured by fast scan cyclic voltammetry. Lynx1 removal affected some behaviors, including a novel-environment assay and nicotine-stimulated locomotion. Trends in 24-hour home-cage behavior were also suggestive of an effect of lynx1 removal. Conditioned place preference for nicotine was not affected by lynx1 removal. The results show that some functional and behavioral aspects of α6-nAChRs are modulated by lynx1.

Compensatory mechanisms in Parkinson's disease: Circuits adaptations and role in disease modification

The motor features of Parkinson's disease (PD) are well known to manifest only when striatal dopaminergic deficit reaches 60-70%. Thus, PD has a long pre-symptomatic and pre-motor evolution during which compensatory mechanisms take place to delay the clinical onset of disabling manifestations. Classic compensatory mechanisms have been attributed to changes and adjustments in the nigro-striatal system, such as increased neuronal activity in the substantia nigra pars compacta and enhanced dopamine synthesis and release in the striatum. However, it is not so clear currently that such changes occur early enough to account for the pre-symptomatic period. Other possible mechanisms relate to changes in basal ganglia and motor cortical circuits including the cerebellum. However, data from early PD patients are difficult to obtain as most studies have been carried out once the diagnosis and treatments have been established. Likewise, putative compensatory mechanisms taking place throughout disease evolution are nearly impossible to distinguish by themselves. Here, we review the evidence for the role of the best known and other possible compensatory mechanisms in PD. We also discuss the possibility that, although beneficial in practical terms, compensation could also play a deleterious role in disease progression.

Microglia may compensate for dopaminergic neuron loss in experimental Parkinsonism through selective elimination of glutamatergic synapses from the subthalamic nucleus

Parkinson's disease (PD) symptoms do not become apparent until most dopaminergic neurons in the substantia nigra pars compacta (SNc) degenerate, suggesting that compensatory mechanisms play a role. Here, we investigated the compensatory involvement of activated microglia in the SN pars reticulata (SNr) and the globus pallidus (GP) in a 6-hydroxydopamine-induced rat hemiparkinsonism model. Activated microglia accumulated more markedly in the SNr than in the SNc in the model. The cells had enlarged somata and expressed phagocytic markers CD68 and NG2 proteoglycan in a limited region of the SNr, where synapsin I- and postsynaptic density 95-immunoreactivities were reduced. The activated microglia engulfed pre- and post-synaptic elements, including NMDA receptors into their phagosomes. Cells in the SNr and GP engulfed red fluorescent DiI that was injected into the subthalamic nucleus (STN) as an anterograde tracer. Rat primary microglia increased their phagocytic activities in response to glutamate, with increased expression of mRNA encoding phagocytosis-related factors. The synthetic glucocorticoid dexamethasone overcame the stimulating effect of glutamate. Subcutaneous single administration of dexamethasone to the PD model rats suppressed microglial activation in the SNr, resulting in aggravated motor dysfunctions, while expression of mRNA encoding glutamatergic, but not GABAergic, synaptic elements increased. These findings suggest that microglia in the SNr and GP become activated and selectively eliminate glutamatergic synapses from the STN in response to increased glutamatergic activity. Thus, microglia may be involved in a negative feedback loop in the indirect pathway of the basal ganglia to compensate for the loss of dopaminergic neurons in PD brains.

Repulsive Guidance Molecule a (RGMa) Induces Neuropathological and Behavioral Changes That Closely Resemble Parkinson's Disease

Repulsive guidance molecule member a (RGMa) is a membrane-associated or released guidance molecule that is involved in axon guidance, cell patterning, and cell survival. In our previous work, we showed that RGMa is significantly upregulated in the substantia nigra of patients with Parkinson's disease. Here we demonstrate the expression of RGMa in midbrain human dopaminergic (DA) neurons. To investigate whether RGMa might model aspects of the neuropathology of Parkinson's disease in mouse, we targeted RGMa to adult midbrain dopaminergic neurons using adeno-associated viral vectors. Overexpression of RGMa resulted in a progressive movement disorder, including motor coordination and imbalance, which is typical for a loss of DA release in the striatum. In line with this, RGMa induced selective degeneration of dopaminergic neurons in the substantia nigra (SN) and affected the integrity of the nigrostriatal system. The degeneration of dopaminergic neurons was accompanied by a strong microglia and astrocyte activation. The behavioral, molecular, and anatomical changes induced by RGMa in mice are remarkably similar to the clinical and neuropathological hallmarks of Parkinson's disease. Our data indicate that dysregulation of RGMa plays an important role in the pathology of Parkinson's disease, and antibody-mediated functional interference with RGMa may be a disease modifying treatment option.SIGNIFICANCE STATEMENT Parkinson's disease (PD) is a neurodegenerative disease characterized by severe motor dysfunction due to progressive degeneration of mesencephalic dopaminergic (DA) neurons in the substantia nigra. To date, there is no regenerative treatment available. We previously showed that repulsive guidance molecule member a (RGMa) is upregulated in the substantia nigra of PD patients. Adeno-associated virus-mediated targeting of RGMa to mouse DA neurons showed that overexpression of this repulsive axon guidance and cell patterning cue models the behavioral and neuropathological characteristics of PD in a remarkable way. These findings have implications for therapy development as interfering with the function of this specific axon guidance cue may be beneficial to the survival of DA neurons.

Loss of Homeostasis in the Direct Pathway in a Mouse Model of Asymptomatic Parkinson's Disease

The characteristic slowness of movement in Parkinson's disease relates to an imbalance in the activity of striatal medium spiny neurons (MSNs) of the direct (dMSNs) and indirect (iMSNs) pathways. However, it is still unclear whether this imbalance emerges during the asymptomatic phase of the disease or if it correlates with symptom severity. Here, we have used in vivo juxtacellular recordings and transgenic mice showing MSN-type-specific expression of fluorescent proteins to examine striatal imbalance after lesioning dopaminergic neurons of the substantia nigra. Multivariate clustering analysis of behavioral data discriminated 2 groups of dopamine-lesioned mice: asymptomatic (42 ± 7% dopaminergic neuron loss) and symptomatic (85 ± 5% cell loss). Contrary to the view that both pathways have similar gain in control conditions, dMSNs respond more intensely than iMSNs to cortical inputs in control animals. Importantly, asymptomatic mice show significant functional disconnection of dMSNs from motor cortex without changes in iMSN connectivity. Moreover, not only the gain but also the timing of the pathways is altered in symptomatic parkinsonism, where iMSNs fire significantly more and earlier than dMSNs. Therefore, cortical drive to dMSNs decreases after partial nigrostriatal lesions producing no behavioral impairment, but additional alterations in the gain and timing of iMSNs characterize symptomatic rodent parkinsonism.

SIGNIFICANCE STATEMENT

Prevailing models of Parkinson's disease state that motor symptoms arise from an imbalance in the activity of medium spiny neurons (MSNs) from the direct (dMSNs) and indirect (iMSNs) pathways. Therefore, it is hypothesized that symptom severity and the magnitude of this imbalanced activity are correlated. Using a mouse model of Parkinson's disease, we found that behaviorally undetectable nigrostriatal lesions induced a significant disconnection of dMSNs from the motor cortex. In contrast, iMSNs show an increased connectivity with the motor cortex, but only after a severe dopaminergic lesion associated with an evident parkinsonian syndrome. Overall, our data suggest that the lack of symptoms after a partial dopaminergic lesion is not due to compensatory mechanisms maintaining the activity of both striatal pathways balanced.

Chronic Nicotine Mitigates Aberrant Inhibitory Motor Learning Induced by Motor Experience under Dopamine Deficiency

Although dopamine receptor antagonism has long been associated with impairments in motor performance, more recent studies have shown that dopamine D2 receptor (D2R) antagonism, paired with a motor task, not only impairs motor performance concomitant with the pharmacodynamics of the drug, but also impairs future motor performance once antagonism has been relieved. We have termed this phenomenon "aberrant motor learning" and have suggested that it may contribute to motor symptoms in movement disorders such as Parkinson's disease (PD). Here, we show that chronic nicotine (cNIC), but not acute nicotine, treatment mitigates the acquisition of D2R-antagonist-induced aberrant motor learning in mice. Although cNIC mitigates D2R-mediated aberrant motor learning, cNIC has no effect on D1R-mediated motor learning. β2-containing nicotinic receptors in dopamine neurons likely mediate the protective effect of cNIC against aberrant motor learning, because selective deletion of β2 nicotinic subunits in dopamine neurons reduced D2R-mediated aberrant motor learning. Finally, both cNIC treatment and β2 subunit deletion blunted postsynaptic responses to D2R antagonism. These results suggest that a chronic decrease in function or a downregulation of β2-containing nicotinic receptors protects the striatal network against aberrant plasticity and aberrant motor learning induced by motor experience under dopamine deficiency.

SIGNIFICANCE STATEMENT

Increasingly, aberrant plasticity and aberrant learning are recognized as contributing to the development and progression of movement disorders. Here, we show that chronic nicotine (cNIC) treatment or specific deletion of β2 nicotinic receptor subunits in dopamine neurons mitigates aberrant motor learning induced by dopamine D2 receptor (D2R) blockade in mice. Moreover, both manipulations also reduced striatal dopamine release and blunt postsynaptic responses to D2R antagonists. These results suggest that chronic downregulation of function and/or receptor expression of β2-containing nicotinic receptors alters presynaptic and postsynaptic striatal signaling to protect against aberrant motor learning. Moreover, these results suggest that cNIC treatment may alleviate motor symptoms and/or delay the deterioration of motor function in movement disorders by blocking aberrant motor learning.

Prolonged Dysfunction of Astrocytes and Activation of Microglia Accelerate Degeneration of Dopaminergic Neurons in the Rat Substantia Nigra and Block Compensation of Early Motor Dysfunction Induced by 6-OHDA

Progressive degeneration of dopaminergic neurons in the substantia nigra (SN) is the underlying cause of Parkinson’s disease (PD). The disease in early stages is difficult to diagnose, because behavioral deficits are masked by compensatory processes. Astrocytic and microglial pathology precedes motor symptoms. Besides supportive functions of astrocytes in the brain, their role in PD is unrecognized. Prolonged dysfunction of astrocytes could increase the vulnerability of dopaminergic neurons and advance their degeneration during aging. The aim of our studies was to find out whether prolonged dysfunction of astrocytes in the SN is deleterious for neuronal functioning and if it influences their survival after toxic insult or changes the compensatory potential of the remaining neurons. In Wistar rat model, we induced activation, prolonged dysfunction, and death of astrocytes by chronic infusion of fluorocitrate (FC) into the SN, without causing dopaminergic neuron degeneration. Strongly enhanced dopamine turnover in the SN after 7 days of FC infusion was induced probably by microglia activated in response to astrocyte stress. The FC effect was reversible, and astrocyte pool was replenished 3 weeks after the end of infusion. Importantly, the prolonged astrocyte dysfunction and microglia activation accelerated degeneration of dopaminergic neurons induced by 6-hydroxydopamine and blocked the behavioral compensation normally observed after moderate neurodegeneration. Impaired astrocyte functioning, activation of microglia, diminishing compensatory capability of the dopaminergic system, and increasing neuronal vulnerability to external insults could be the underlying causes of PD. This animal model of prolonged astrocyte dysfunction can be useful for in vivo studies of glia–microglia–neuron interaction.

Pioglitazone Attenuates Neuroinflammation and Promotes Dopaminergic Neuronal Survival in the Nigrostriatal System of Rats after Diffuse Brain Injury

Increasing evidence suggests that traumatic brain injury (TBI) may raise the risk of developing late-onset Parkinson's disease (PD). Recently, the peroxisome proliferation-activated receptor gamma (PPARγ) agonist pioglitazone has been demonstrated to be neuroprotective in animal models of neurodegeneration. The present study investigates the vulnerability of the nigrostriatal system after TBI, and intervention with pioglitazone treatment. Adult male Sprague-Dawley rats were subjected to sham or moderate midline fluid percussion brain injury (mFPI), followed by an intraperitoneal injection of 10 mg/kg pioglitazone or vehicle beginning 30 min after the injury and subsequently every 24 h for 5 days. Following injury, pro-inflammatory cytokines and chemokine were acutely increased in the striatum and substantia nigra within 6 h. Dopaminergic axonal damage and microglial activation were revealed using immunohistochemistry in the medial forebrain bundle at 1 day post-injury. Microglial activation identified by Iba1 and OX-6 immunostaining was persistently increased in the substantia nigra pars compacta 7 to 28 days post-injury. Further, brain injury induced significant dopaminergic neuronal loss, which was quantified by tyrosine hydroxylase immunostaining and retrograde fluorescent tracer fluorogold labeling in the nigra at 28 days. Loss of neurons was accompanied by increased extracellular dopamine (DA) turnover in the striatum, indicating enhanced dopaminergic activity in functional compensation after nigrostriatal damage. Strikingly, pioglitazone treatment greatly attenuated microglial activation and improved dopaminergic neuronal survival in the nigrostriatal system, which may promote locomotor recovery. These results suggest that interventions that attenuate secondary inflammation could be a feasible therapeutic treatment to improve outcome after TBI.

Preclinical Evidence for a Role of the Nicotinic Cholinergic System in Parkinson's Disease

One of the primary deficits in Parkinson's disease (PD) is the loss of dopaminergic neurons in the substantia nigra pars compacta which leads to striatal dopaminergic deficits that underlie the motor symptoms associated with the disease. A plethora of animal models have been developed over the years to uncover the molecular alterations that lead to PD development. These models have provided valuable information on neurotransmitter pathways and mechanisms involved. One such a system is the nicotinic cholinergic system. Numerous studies show that nigrostriatal damage affects nicotinic receptor-mediated dopaminergic signaling; therefore therapeutic modulation of the nicotinic cholinergic system may offer a novel approach to manage PD. In fact, there is evidence showing that nicotinic receptor drugs may be useful as neuroprotective agents to prevent Parkinson's disease progression. Additional preclinical studies also show that nicotinic receptor drugs may be beneficial for the treatment of L-dopa induced dyskinesias. Here, we review preclinical findings supporting the idea that nicotinic receptors are valuable therapeutic targets for PD.

Pharmacogenetics of antipsychotic-induced movement disorders as a resource for better understanding Parkinson's disease modifier genes

Antipsychotic-induced movement disorders are major side effects of antipsychotic drugs among schizophrenia patients, and include antipsychotic-induced parkinsonism (AIP) and tardive dyskinesia (TD). Substantial pharmacogenetic work has been done in this field, and several susceptibility variants have been suggested. In this paper, the genetics of antipsychotic-induced movement disorders is considered in a broader context. We hypothesize that genetic variants that are risk factors for AIP and TD may provide insights into the pathophysiology of motor symptoms in Parkinson’s disease (PD). Since loss of dopaminergic stimulation (albeit pharmacological in AIP and degenerative in PD) is shared by the two clinical entities, genes associated with susceptibility to AIP may be modifier genes that influence clinical expression of PD motor sub-phenotypes, such as age at onset, disease severity, or rate of progression. This is due to their possible functional influence on compensatory mechanisms for striatal dopamine loss. Better compensatory potential might be beneficial at the early and later stages of the PD course. AIP vulnerability variants could also be related to latent impairment in the nigrostriatal pathway, affecting its functionality, and leading to subclinical dopaminergic deficits in the striatum. Susceptibility of PD patients to early development of l-DOPA induced dyskinesia (LID) is an additional relevant sub-phenotype. LID might share a common genetic background with TD, with which it shares clinical features. Genetic risk variants may predispose to both phenotypes, exerting a pleiotropic effect. According to this hypothesis, elucidating the genetics of antipsychotic-induced movement disorders may advance our understanding of multiple aspects of PD and it clinical course, rendering this a potentially rewarding field of study.

[Functional properties of dopaminergic neurons obtained from fibroblasts of a patient with PARK2 form of Parkinson's disease]

OBJECTIVE

To carry out a functional analysis of dopamine transporter (DAT) in autosomal recessive Parkinson's disease caused by mutations in the PARK2 gene.

MATERIAL AND METHODS

Cultures of dopaminergic neurons were obtained from fibroblasts of a patient with PARK2 form of Parkinson's disease and a healthy donor with the use of the cell reprogramming technology. DAT expression in both cell cultures was assessed at the RNA and protein levels, and DAT activity was tested with the use of the fluorescent dopamine analogue ASP+.

RESULTS AND CONCLUSION

In the cells with PARK2 mutations, the level of DAT expression was significantly higher than in normal neurons, but the intensity of ASP+ capture by mutant dopaminergic neurons was 25% down from normal neurons. For the study of competitive inhibition of DAT, dopamine was added to the incubation medium containing ASP+: it was shown that dopamine binding by the normal cells was almost twice as much relative to PARK2 mutant neurons. Therefore, dopaminergic neurons carrying mutations in the PARK2 gene are characterized by functional failure of dopamine transport systems. One of cell mechanisms of compensation of this defect seems to be an early increase of expression of the DAT transporter protein.

Norepinephrine transporter inhibition with desipramine exacerbates L-DOPA-induced dyskinesia: role for synaptic dopamine regulation in denervated nigrostriatal terminals

Pharmacological dopamine (DA) replacement with Levodopa [L-dihydroxyphenylalanine (L-DOPA)] is the gold standard treatment of Parkinson's disease (PD). However, long-term L-DOPA treatment is complicated by eventual debilitating abnormal involuntary movements termed L-DOPA-induced dyskinesia (LID), a clinically significant obstacle for the majority of patients who rely on L-DOPA to alleviate PD-related motor symptoms. The manifestation of LID may in part be driven by excessive extracellular DA derived from L-DOPA, but potential involvement of DA reuptake in LID severity or expression is unknown. We recently reported that in 6-hydroxydopamine (6-OHDA)-lesioned striatum, norepinephrine transporter (NET) expression increases and may play a significant role in DA transport. Furthermore, L-DOPA preferentially inhibits DA uptake in lesioned striatum. Therefore, we hypothesized that desipramine (DMI), a NET antagonist, could affect the severity of LID in an established LID model. Whereas DMI alone elicited no dyskinetic effects in lesioned rats, DMI + L-DOPA-treated rats gradually expressed more severe dyskinesia compared with L-DOPA alone over time. At the conclusion of the study, we observed reduced NET expression and norepinephrine-mediated inhibition of DA uptake in the DMI + L-DOPA group compared with L-DOPA-alone group in lesioned striatum. LID severity positively correlated with striatal extracellular signal-regulated protein kinase phosphorylation among the three treatment groups, with increased ppERK1/2 in DMI + L-DOPA group compared with the L-DOPA- and DMI-alone groups. Taken together, these results indicate that the combination of chronic L-DOPA and NET-mediated DA reuptake in lesioned nigrostriatal terminals may have a role in LID severity in experimental Parkinsonism.

Long-term nicotine treatment down-regulates α6β2* nicotinic receptor expression and function in nucleus accumbens

Long-term nicotine exposure induces alterations in dopamine transmission in nucleus accumbens that sustain the reinforcing effects of smoking. One approach to understand the adaptive changes that arise involves measurement of endogenous dopamine release using voltammetry. We therefore treated rats for 2-3 months with nicotine and examined alterations in nAChR subtype expression and electrically evoked dopamine release in rat nucleus accumbens shell, a region key in addiction. Long-term nicotine treatment selectively decreased stimulated α6β2* nAChR-mediated dopamine release compared with vehicle-treated rats. It also reduced α6β2* nAChRs, suggesting the receptor decline may contribute to the functional loss. This decreased response in release after chronic nicotine treatment was still partially sensitive to the agonist nicotine. Studies with an acetylcholinesterase inhibitor demonstrated that the response was also sensitive to increased endogenous acetylcholine. However, unlike the agonists, nAChR antagonists decreased dopamine release only in vehicle- but not nicotine-treated rats. As antagonists function by blocking the action of acetylcholine, their ineffectiveness suggests that reduced acetylcholine levels partly underlie the dampened α6β2* nAChR-mediated function in nicotine-treated rats. As long-term nicotine modifies dopamine release by decreasing α6β2* nAChRs and their function, these data suggest that interventions that target this subtype may be useful for treating nicotine dependence. Long-term nicotine treatment decreases dopamine (DA) transmission in the mesolimbic dopaminergic system. Our data suggest this may involve a decrease in α6β2* nicotinic receptor expression and function. These changes may play a key role in nicotine reward and dependence.

Perspective: Identification of genetic variants associated with dopaminergic compensatory mechanisms in early Parkinson's disease

Parkinson's disease (PD) is slowly progressive, and heterogeneity of its severity among individuals may be due to endogenous mechanisms that counterbalance the striatal dopamine loss. In this perspective paper, we introduce a neuroimaging-genetic approach to identify genetic variants, which may contribute to this compensation. First, we briefly review current known potential compensatory mechanisms for premotor and early disease PD, located in the striatum and other brain regions. Then, we claim that a mismatch between mild symptomatic disease, manifested by low motor score on the Unified PD Rating Scale (UPDRS), and extensive Nigro-Striatal (NS) degeneration, manifested by reduced uptake of [¹²³I]FP-CIT, is indicative of compensatory processes. If genetic variants are associated with the severity of motor symptoms, while the level of striatal terminals degeneration measured by ligand uptake is taken into account and controlled in the analysis, then these variants may be involved in functional compensatory mechanisms for striatal dopamine deficit. To demonstrate feasibility of this approach, we performed a small “proof of concept” study (candidate gene design) in a sample of 28 Jewish PD patients, and preliminary results are presented.

Methamphetamine-induced neurotoxicity disrupts naturally occurring phasic dopamine signaling

Methamphetamine (METH) is a highly addictive drug that is also neurotoxic to central dopamine (DA) systems. Although striatal DA depletions induced by METH are associated with behavioral and cognitive impairments, the link between these phenomena remains poorly understood. Previous work in both METH-pretreated animals and the 6-hydroxydopamine model of Parkinson's disease suggests that a disruption of phasic DA signaling, which is important for learning and goal-directed behavior, may be such a link. However, previous studies used electrical stimulation to elicit phasic-like DA responses and were also performed under anesthesia, which alters DA neuron activity and presynaptic function. Here we investigated the consequences of METH-induced DA terminal loss on both electrically evoked phasic-like DA signals and so-called 'spontaneous' phasic DA transients measured by voltammetry in awake rats. Not ostensibly attributable to discrete stimuli, these subsecond DA changes may play a role in enhancing reward-cue associations. METH pretreatment reduced tissue DA content in the dorsomedial striatum and nucleus accumbens by ~55%. Analysis of phasic-like DA responses elicited by reinforcing stimulation revealed that METH pretreatment decreased their amplitude and underlying mechanisms for release and uptake to a similar degree as DA content in both striatal subregions. Most importantly, characteristics of DA transients were altered by METH-induced DA terminal loss, with amplitude and frequency decreased and duration increased. These results demonstrate for the first time that denervation of DA neurons alters naturally occurring DA transients and are consistent with diminished phasic DA signaling as a plausible mechanism linking METH-induced striatal DA depletions and cognitive deficits.

Dopamine transporter loss in 6-OHDA Parkinson's model is unmet by parallel reduction in dopamine uptake

The dopamine transporter (DAT) regulates synaptic dopamine (DA) in striatum and modulation of DAT can affect locomotor activity. Thus, in Parkinson's disease (PD), DAT loss could affect DA clearance and locomotor activity. The locomotor benefits of L-DOPA may be mediated by transport through monoamine transporters and conversion to DA. However, its impact upon DA reuptake is unknown and may modulate synaptic DA. Using the unilateral 6-OHDA rat PD model, we examined [(3)H]DA uptake dynamics in relation to striatal DAT and tyrosine hydroxylase (TH) protein loss compared with contralateral intact striatum. Despite >70% striatal DAT loss, DA uptake decreased only ∼25% and increased as DAT loss approached 99%. As other monoamine transporters can transport DA, we determined if norepinephrine (NE) and serotonin (5-HT) differentially modulated DA uptake in lesioned striatum. Unlabeled DA, NE, and 5-HT were used, at a concentration that differentially inhibited DA uptake in intact striatum, to compete against [(3)H]DA uptake. In 6-OHDA lesioned striatum, DA was less effective, whereas NE was more effective, at inhibiting [(3)H]DA uptake. Furthermore, norepinephrine transporter (NET) protein levels increased and desipramine was ∼two-fold more effective at inhibiting NE uptake. Serotonin inhibited [(3)H]DA uptake, but without significant difference between lesioned and contralateral striatum. L-DOPA inhibited [(3)H]DA uptake two-fold more in lesioned striatum and inhibited NE uptake ∼five-fold more than DA uptake in naïve striatum. Consequently, DA uptake may be mediated by NET when DAT loss is at PD levels. Increased inhibition of DA uptake by L-DOPA and its preferential inhibition of NE over DA uptake, indicates that NET-mediated DA uptake may be modulated by L-DOPA when DAT loss exceeds 70%. These results indicate a novel mechanism for DA uptake during PD progression and provide new insight into how L-DOPA affects DA uptake, revealing possible mechanisms of its therapeutic and side effect potential.

Compensatory regulation of dopamine after ablation of the tyrosine hydroxylase gene in the nigrostriatal projection

The tyrosine hydroxylase (TH; EC 1.14.16.2) is a rate-limiting enzyme in the dopamine synthesis and important for the central dopaminergic system, which controls voluntary movements and reward-dependent behaviors. Here, to further explore the regulatory mechanism of dopamine levels by TH in adult mouse brains, we employed a genetic method to inactivate the Th gene in the nigrostriatal projection using the Cre-loxP system. Stereotaxic injection of adeno-associated virus expressing Cre recombinase (AAV-Cre) into the substantia nigra pars compacta (SNc), where dopaminergic cell bodies locate, specifically inactivated the Th gene. Whereas the number of TH-expressing cells decreased to less than 40% in the SNc 2 weeks after the AAV-Cre injection, the striatal TH protein level decreased to 75%, 50%, and 39% at 2, 4, and 8 weeks, respectively, after the injection. Thus, unexpectedly, the reduction of TH protein in the striatum, where SNc dopaminergic axons innervate densely, was slower than in the SNc. Moreover, despite the essential requirement of TH for dopamine synthesis, the striatal dopamine contents were only moderately decreased, to 70% even 8 weeks after AAV-Cre injection. Concurrently, in vivo synthesis activity of l-dihydroxyphenylalanine, the dopamine precursor, per TH protein level was augmented, suggesting up-regulation of dopamine synthesis activity in the intact nigrostriatal axons. Collectively, our conditional Th gene targeting method demonstrates two regulatory mechanisms of TH in axon terminals for dopamine homeostasis in vivo: local regulation of TH protein amount independent of soma and trans-axonal regulation of apparent L-dihydroxyphenylalanine synthesis activity per TH protein.

Functional reorganization of the presynaptic dopaminergic terminal in parkinsonism

Whether dopamine (DA) release is compensated during the presymptomatic phase of Parkinson's disease (PD) is controversial. Here we use in vivo voltammetry in the parkinsonian rat and an electrical stimulation protocol established to fatigue nigrostriatal dopaminergic (DAergic) neurons to investigate the plasticity of DA-release mechanisms. Amplitudes of evoked voltammetric signals recorded in intact rat striata decreased with repetitive, high-frequency stimulation (60 Hz, every 5 min/60 min). Strikingly, DA levels were maintained during an identical "fatiguing" protocol in 6-hydroxydopamine-lesioned (<40% denervation) striata in the absence of enhanced DA synthesis. In contrast, more severely lesioned striata (>55% denervation) also appeared to sustain DA release, however, this was demonstrated in the presence of enhanced synthesis. Sustained release was replicated in intact animals after irreversible blockade of the dopamine transporter (DAT) via RTI-76, implicating neuronal uptake as a trigger. We further demonstrate through kinetic analysis that lesions and compromised uptake target a "long-term" (time constant of minutes) presynaptic depression, which underlies the maintenance of release. Taken together, our findings identify a denervation-induced maintenance of DA release that was independent of activated synthesis and driven by altered uptake. This novel neuroadaptation may contribute to early preclinical normalization of function and help resolve discrepant findings regarding compensatory changes in DA release during progression of the parkinsonian state.

Sub-regional differences and mechanisms of the short-term plasticity of dopamine overflow in striatum in mice lacking alpha-synuclein

Mice lacking the pre-synaptic protein alpha-synuclein (α-syn) demonstrate enhanced facilitation of dopamine (DA) overflow in dorsal striatum following repeated, high-frequency burst stimulation of the dopaminergic pathways. Dorsal striatum is most vulnerable to neurodegeneration in Parkinson's disease. The role of α-syn in facilitation of DA overflow in the ventral striatum, which is less vulnerable to neurodegeneration, is unknown. We investigated the link between the absence of α-syn and the plasticity of DA overflow in the dorsal and ventral striatum by in vivo voltammetry and the possible mechanisms of modulation of the plasticity of DA overflow. We show that the facilitation of DA overflow following paired-burst stimulation is significantly enhanced in the dorsolateral but not in the ventral striatum of mice lacking α-syn. Re-uptake inhibitor, GBR12909, completely eliminated the facilitation of DA overflow regardless of the presence of α-syn in both dorsal and ventral striatum, indicating that re-uptake is critical for maintenance of paired-burst facilitation (PBF). Inhibition of D2 autoreceptors by haloperidol decreased PBF only in mice lacking α-syn. However, the basal function of D2 autoreceptors tested by paired-pulse depression of DA overflow was not different between the lines. Therefore, alterations in the D2 autoreceptor system do not explain the different effect of haloperidol on PBF in mice with and without α-syn. This indicates that neither re-uptake nor D2 autoreceptors differentiate the PBF between the genotypes. We propose that modification of DA vesicles in α-syn knockout mice, as reported in several studies, may be a factor underlying the enhanced PBF in these mice.

Methamphetamine neurotoxicity decreases phasic, but not tonic, dopaminergic signaling in the rat striatum

Neurotoxic doses of methamphetamine (METH) are known to cause depletions in striatal dopamine (DA) tissue content. However, the effects of METH-induced insults on dopaminergic neurotransmission are not fully understood. Here, we employed fast-scan cyclic voltammetry at a carbon-fiber microelectrode in the anesthetized rat striatum to assess the effects of a neurotoxic regimen of METH on phasic and tonic modes of dopaminergic signaling and underlying mechanisms of DA release and uptake. Extracellular DA was electrically evoked by stimulation of the medial forebrain bundle mimicking tonic and phasic firing patterns for dopaminergic cells and was monitored simultaneously in both the dorsomedial and dorsolateral striatum. Kinetic analysis of evoked recordings determined parameters describing DA release and uptake. Striatal DA tissue content was quantified by high performance liquid chromatography with electrochemical detection. METH-pretreatment (four doses of 7.5 or 10.0 mg/kg s.c.) induced DA depletions of ∼ 40% on average, which are reported in both striatal subregions. METH pre-treatment significantly decreased the amplitude of signals evoked by phasic, but not tonic, stimulation. Parameters for DA release and uptake were also similarly reduced by ∼ 40%, consistent with effects on evoked phasic-like responses and DA tissue content. Taken together, these results suggest that METH-pretreatment selectively diminishes phasic, but not tonic, dopaminergic signaling in the dorsal striatum.

Not all partial dopamine D(2) receptor agonists are the same in treating schizophrenia. Exploring the effects of bifeprunox and aripiprazole using a computer model of a primate striatal dopaminergic synapse

Species differences in physiology and unique active human metabolites contribute to the limited predictive value of preclinical rodent models for many central nervous system (CNS) drugs. In order to explore possible drivers for this translational disconnect, we developed a computer model of a dopaminergic synapse that simulates the competition among three agents and their binding to pre- and postsynaptic receptors, based on the affinities for their targets and their actual concentrations. The model includes presynaptic autoreceptor effects on neurotransmitter release and modulation by presynaptic firing frequency and is calibrated with actual experimental data on free dopamine levels in the striatum of the rodent and the primate. Using this model, we simulated the postsynaptic dopamine D(2) receptor activation levels of bifeprunox and aripiprazole, two relatively similar dopamine D(2) receptor agonists. The results indicate a substantial difference in dose-response for the two compounds when applying primate calibration parameters as opposed to rodent calibration parameters. In addition, when introducing the major human and rodent metabolites of aripiprazole with their specific pharmacological activities, the model predicts that while bifeprunox would result in a higher postsynaptic D(2) receptor antagonism in the rodent, aripiprazole would result in a higher D(2) receptor antagonism in the primate model. Furthermore, only the highest dose of aripiprazole, but not bifeprunox, reaches postsynaptic functional D(2) receptor antagonism similar to 4 mg haloperidol in the primate model. The model further identifies a limited optimal window of functionality for dopamine D(2) receptor partial agonists. These results suggest that computer modeling of key CNS processes, using well-validated calibration paradigms, can increase the predictive value in the clinical setting of preclinical animal model outcomes.

α6ß2* and α4ß2* nicotinic receptors both regulate dopamine signaling with increased nigrostriatal damage: relevance to Parkinson's disease

Nicotinic receptors (nAChRs) are important modulators of dopaminergic transmission in striatum, a region critical to Parkinson's disease. The nAChRs mainly involved are the α6β2* and α4β2* subtypes. Lesion studies show that the α6β2* receptor is decreased to a much greater extent with nigrostriatal damage than the α4β2* subtype raising the question whether this latter nAChR population is more important with increased nigrostriatal damage. To address this, we investigated the effect of varying nigrostriatal damage on α6β2* and α4β2* receptor-modulated dopamine signaling using cyclic voltammetry. This approach offers the advantage that changes in dopamine release can be observed under different neuronal firing conditions. Total single-pulse-evoked dopamine release decreased in direct proportion to declines in the dopamine transporter and dopamine uptake. We next used α-conotoxinMII and mecamylamine to understand the role of the α4β2* and α6β2* subtypes in release. Single-pulse-stimulated α6β2* and α4β2* receptor dopamine release decreased to a similar extent with increasing nigrostriatal damage, indicating that both subtypes contribute to the control of dopaminergic transmission with lesioning. Total burst-stimulated dopamine release also decreased proportionately with nigrostriatal damage. However, the role of the α4β2* and α6β2* nAChRs varied with different degrees of lesioning, suggesting that the two subtypes play a unique function with burst firing, with a somewhat more prominent and possibly more selective role for the α6β2* subtype. These data have important therapeutic implications because they suggest that drugs directed to both α4β2* and α6β2* nAChRs may be useful in the treatment of neurological disorders such as Parkinson's disease.

Methamphetamine treatment causes delayed decrease in novelty-induced locomotor activity in mice

Methamphetamine (METH) is a psychostimulant that causes damage to dopamine (DA) axons and to non-monoaminergic neurons in the brain. The aim of the present study was to investigate short- and long-term effects of neurotoxic METH treatment on novelty-induced locomotor activity in mice. Male BALB/c mice, 12-14 weeks old, were injected with saline or METH (i.p., 7.5 mg/kg x 4 times, every 2 h). Behavior and neurotoxic effects were assessed at 10 days, 3 and 5 months following drug treatment. METH administration caused marked decreases in DA levels in the mouse striatum and cortex at 10 days post-drug. However, METH did not induce any changes in novelty-induced locomotor activity. At 3 and 5 months after treatment METH-exposed mice showed significant recovery of DA levels in the striatum and cortex. In contrast, these animals demonstrated significant decreases in locomotor activity at 5 months in comparison to aged-matched control mice. Further assessment of METH toxicity using TUNEL staining showed that the drug induced increased cell death in the striatum and cortex at 3 days after administration. Taken together, these data suggest that delayed deficits in novelty-induced locomotor activity observed in METH-exposed animals are not due to neurodegeneration of DA terminals but to combined effects of METH and age-dependent dysfunction of non-DA intrinsic striatal and/or corticostriatal neurons.