Search results
Found 12763 matches for
Impaired intracellular trafficking defines early Parkinson's disease.
Parkinson's disease (PD) is an insidious and incurable neurodegenerative disease, and represents a significant cost to individuals, carers, and ageing societies. It is defined at post-mortem by the loss of dopamine neurons in the substantia nigra together with the presence of Lewy bodies and Lewy neurites. We examine here the role of α-synuclein and other cellular transport proteins implicated in PD and how their aberrant activity may be compounded by the unique anatomy of the dopaminergic neuron. This review uses multiple lines of evidence from genetic studies, human tissue, induced pluripotent stem cells, and refined animal models to argue that prodromal PD can be defined as a disease of impaired intracellular trafficking. Dysfunction of the dopaminergic synapse heralds trafficking impairment.
Somatodendritic dopamine release in midbrain
Midbrain dopamine (DA) neurons of the substantia nigra (SN) and adjacent ventral tegmental area (VTA) fall into two main categories of cells, which were originally classified by their anatomical location in the SN and are consequently referred to as dorsal- and ventral-tier neurons (Fallon et al, 1978). These cells can be distinguished by their morphological characteristics, including dendritic arbor, somatic size, major efferent projections, and biochemistry (Fallon et al., 1978; Gerfen et al, 1987a,b). The primary DA cell type in the SN is the ventral-tier cell, which has a large pyramidal cell body (Fig. lA) and extends dendrites laterally along the band of cell bodies in the SN pars compacta (SNc) and ventrally into the pars reticulata (SNr) (Fig. 1B,C). Smaller DA cells are also found in the dorsal tier of the SNc; these dorsal-tier cells are the predominant cell type in the adjacent VTA. Via the median forebrain bundle, DA neurons of the SNc project primarily to the dorsal striatum, whereas those of the VTA project to the nucleus accumbens (ventral striatum), as well as to prefrontal cortex and other mesolimbic structures (Fallon and Moore, 1978; Fallon et al., 1978). The nigrostriatal DA system is essential for motor facilitation by the basal ganglia, whereas the mesolimbic DA system participates in motivation, including reward. A characteristic of DA neurons in both SN and VTA is the somatodendritic release of DA (Björkland and Lindvall, 1975; Groves et al., 1975; Geffen et al., 1976; Nieoullon et al., 1977); there is evidence for release from somata (Jaffe et al., 1998) and from dendrites (Geffen et al., 1976; Rice et al., 1994). Importantly, release in the SN is exclusively somatodendritic, but that in the VTA is not: the SN receives no identified synaptic DA input or axon coUateralization (Juraska et al., 1977; Wassef et al., 1981), whereas the VTA receives DA input from its own axon collaterals, as well as minor input from DA axons from the SNc (Deutch et al., 1988; Bayer and Pickel, 1990). This review will focus primarily on studies of somatodendritic DA release in the SNc, with comparison to release in VTA, as well as to release from nigrostriatal axons in dorsal striatum. We review the methods used to study somatodendritic DA release (Section 2), its proposed functions (Section 3), the regulation of extracellular DA concentration by uptake (Section 4), proposed mechanisms of release (Section 5), and receptor regulation of release by synaptic and non-synaptic input (Section 6).
Gating of dopamine transmission by calcium and axonal N-, Q-, T- and L-type voltage-gated calcium channels differs between striatal domains.
KEY POINTS: The voltage-gated Ca(2+) channels (VGCCs) that catalyse striatal dopamine transmission are critical to dopamine function and might prime subpopulations of neurons for parkinsonian degeneration. However, the VGCCs that operate on mesostriatal axons are incompletely defined; previous studies encompassed channels on striatal cholinergic interneurons that strongly influence dopamine transmission. We define that multiple types of axonal VGCCs operate that extend beyond classic presynaptic N/P/Q channels to include T- and L-types. We reveal differences in VGCC function between mouse axon types that in humans are vulnerable versus resistant to Parkinson's disease. We show for the first time that this is underpinned by different sensitivity of dopamine transmission to extracellular Ca(2+) and by different spatiotemporal intracellular Ca(2+) microdomains. These data define key principles of how Ca(2+) and VGCCs govern dopamine transmission in the healthy brain and reveal differences between neuron types that might contribute to vulnerability in disease. ABSTRACT: The axonal voltage-gated Ca(2+) channels (VGCCs) that catalyse dopamine (DA) transmission are incompletely defined. Yet, they are critical to DA function and might prime subpopulations of DA neurons for parkinsonian degeneration. Previous studies of VGCCs will have encompassed those on striatal cholinergic interneurons, which strongly influence DA transmission. We identify which VGCCs on DA axons govern DA transmission, we determine their dynamic properties and reveal an underlying basis for differences between the caudate putamen (CPu) and nucleus accumbens (NAc). We detected DA release evoked electrically during nicotinic receptor blockade or optogenetically by light activation of channel rhodopsin-expressing DA axons in mouse striatal slices. Subtype-specific VGCC blockers indicated that N-, Q-, T- and L-VGCCs govern DA release in CPu, but in NAc, T and L-channels are relatively silent. The roles of the most dominant channels were inversely frequency-dependent, due to low-pass filtering of DA release by Ca(2+)-dependent relationships between initial release probability and short-term plasticity. Ca(2+) concentration-response curves revealed that differences between CPu and NAc were due to greater underlying Ca(2+) sensitivity of DA transmission from CPu axons. Functions for 'silent' L- and T-channels in NAc could be unmasked by elevating extracellular [Ca(2+)]. Furthermore, we identified a greater coupling between BAPTA-sensitive, fast Ca(2+) transients and DA transmission in CPu axons, and evidence for endogenous fast buffering of Ca(2+) in NAc. These data reveal that a range of VGCCs operate dynamically on DA axons, depending on local driving forces. Furthermore, they reveal dramatic differences in Ca(2+) handling between axonal subpopulations that show different vulnerability to parkinsonian degeneration.
Ni(2+) affects dopamine uptake which limits suitability as inhibitor of T-type voltage-gated Ca(2+) channels.
Neuronal T-type voltage-gated Ca(2+) channels are reported to have physiological roles that include regulation of burst firing, Ca(2+) oscillations, and neurotransmitter release. These roles are often exposed experimentally by blocking T-type channels with micromolar Ni(2+). We used Ni(2+) to explore the role of axonal T-type channels in dopamine (DA) release in mouse striatum, but identified significant off-target effects on DA uptake. Ni(2+) (100 μM) reversibly increased electrically evoked DA release and markedly extended its extracellular lifetime, detected using fast-scan cyclic voltammetry. Prior inhibition of the DA transporter (DAT) by cocaine (5 μM) occluded the facilitatory action of Ni(2+) on DA release and conversely, allowed Ni(2+) to inhibit release, presumably through T-channel inhibition. Ni(2+) further prolonged the timecourse of DA clearance suggesting further inhibition of DA uptake. In summary, Ni(2+) has major effects on DA transmission besides those due to T-channels that likely involve inhibition of the DAT.
Impaired intracellular trafficking defines early Parkinson's disease
Parkinson's disease (PD) is an insidious and incurable neurodegenerative disease, and represents a significant cost to individuals, carers, and ageing societies. It is defined at post-mortem by the loss of dopamine neurons in the substantia nigra together with the presence of Lewy bodies and Lewy neurites. We examine here the role of α-synuclein and other cellular transport proteins implicated in PD and how their aberrant activity may be compounded by the unique anatomy of the dopaminergic neuron. This review uses multiple lines of evidence from genetic studies, human tissue, induced pluripotent stem cells, and refined animal models to argue that prodromal PD can be defined as a disease of impaired intracellular trafficking. Dysfunction of the dopaminergic synapse heralds trafficking impairment.
The impact of a parkinsonian lesion on dynamic striatal dopamine transmission depends on nicotinic receptor activation.
Dopamine function is disturbed in Parkinson's disease (PD), but whether and how release of dopamine from surviving neurons is altered has long been debated. Nicotinic acetylcholine receptors (nAChRs) on dopamine axons powerfully govern dopamine release and could be critical contributing factors. We revisited whether fundamental properties of dopamine transmission are changed in a parkinsonian brain and tested the potentially profound masking effects of nAChRs. Using real-time detection of dopamine in mouse striatum after a partial 6-hydroxydopamine lesion and under nAChR inhibition, we reveal that dopamine signals show diminished sensitivity to presynaptic activity. This effect manifested as diminished contrast between DA release evoked by the lowest versus highest frequencies. This reduced activity-dependence was underpinned by loss of short-term facilitation of dopamine release, consistent with an increase in release probability (Pr). With nAChRs active, the reduced activity-dependence of dopamine release after a parkinsonian lesion was masked. Consequently, moment-by-moment variation in activity of nAChRs may lead to dynamic co-variation in dopamine signal impairments in PD.
Substance P Weights Striatal Dopamine Transmission Differently within the Striosome-Matrix Axis.
The mammalian striatum has a topographical organization of input-output connectivity, but a complex internal, nonlaminar neuronal architecture comprising projection neurons of two types interspersed among multiple interneuron types and potential local neuromodulators. From this cellular melange arises a biochemical compartmentalization of areas termed striosomes and extrastriosomal matrix. The functions of these compartments are poorly understood but might confer distinct features to striatal signal processing and be discretely governed. Dopamine transmission occurs throughout striosomes and matrix, and is reported to be modulated by the striosomally enriched neuromodulator substance P. However, reported effects are conflicting, ranging from facilitation to inhibition. We addressed whether dopamine transmission is modulated differently in striosome-matrix compartments by substance P.We paired detection of evoked dopamine release at carbon-fiber microelectrodes in mouse striatal slices with subsequent identification of the location of recording sites with respect to μ-opioid receptor-rich striosomes. Substance P had bidirectional effects on dopamine release that varied between recording sites and were prevented by inhibition of neurokinin-1 receptors. The direction of modulation was determined by location within the striosomal-matrix axis: dopamine release was boosted in striosome centers, diminished in striosomal-matrix border regions, and unaffected in the matrix. In turn, this different weighting of dopamine transmission by substance P modified the apparent center-surround contrast of striosomal dopamine signals. These data reveal that dopamine transmission can be differentially modulated within the striosomal-matrix axis, and furthermore, indicate a functionally distinct zone at the striosome-matrix interface, which may have key impacts on striatal integration.
Astrocytic striatal GABA transporter activity governs dopamine release and shows maladaptive downregulation in early parkinsonism
SummaryStriatal dopamine (DA) is critical for action and learning. Recent data show DA release is under tonic inhibition by striatal GABA. Ambient striatal GABA tone on striatal projection neurons can be governed by plasma membrane GABA uptake transporters (GATs) on astrocytes. However, whether striatal GATs and astrocytes determine DA output are unknown. We reveal that DA release in mouse dorsolateral striatum, but not nucleus accumbens core, is governed by GAT-1 and GAT-3. These GATs are partly localized to astrocytes, and are enriched in dorsolateral striatum compared to accumbens core. In a mouse model of early parkinsonism, GATs were downregulated and tonic GABAergic inhibition of DA release augmented, with corresponding attenuation of GABA co-release from dopaminergic axons. These data define previously unappreciated and important roles for GATs and astrocytes in determining DA release in striatum, and reveal a maladaptive plasticity in early parkinsonism that impairs DA output in vulnerable striatal regions.HighlightsGABA transporters set the level of GABA inhibition of DA output in dorsal striatumAstrocytes facilitate DA release levels by limiting tonic GABA inhibitionTonic inhibition of DA release is augmented in a mouse model of early parkinsonismDA and GABA co-release are reduced in a mouse model of early parkinsonism
Lights, fiber, action! A primer on in vivo fiber photometry.
Fiber photometry is a key technique for characterizing brain-behavior relationships in vivo. Initially, it was primarily used to report calcium dynamics as a proxy for neural activity via genetically encoded indicators. This generated new insights into brain functions including movement, memory, and motivation at the level of defined circuits and cell types. Recently, the opportunity for discovery with fiber photometry has exploded with the development of an extensive range of fluorescent sensors for biomolecules including neuromodulators and peptides that were previously inaccessible in vivo. This critical advance, combined with the new availability of affordable "plug-and-play" recording systems, has made monitoring molecules with high spatiotemporal precision during behavior highly accessible. However, while opening exciting new avenues for research, the rapid expansion in fiber photometry applications has occurred without coordination or consensus on best practices. Here, we provide a comprehensive guide to help end-users execute, analyze, and suitably interpret fiber photometry studies.
Impairment of Macroautophagy in Dopamine Neurons Has Opposing Effects on Parkinsonian Pathology and Behavior.
Parkinson's disease (PD) is characterized by the death of dopamine neurons in the substantia nigra pars compacta (SNc) and accumulation of α-synuclein. Impaired autophagy has been implicated and activation of autophagy proposed as a treatment strategy. We generate a human α-synuclein-expressing mouse model of PD with macroautophagic failure in dopamine neurons to understand the interaction between impaired macroautophagy and α-synuclein. We find that impaired macroautophagy generates p62-positive inclusions and progressive neuron loss in the SNc. Despite this parkinsonian pathology, motor phenotypes accompanying human α-synuclein overexpression actually improve with impaired macroautophagy. Real-time fast-scan cyclic voltammetry reveals that macroautophagy impairment in dopamine neurons increases evoked extracellular concentrations of dopamine, reduces dopamine uptake, and relieves paired-stimulus depression. Our findings show that impaired macroautophagy paradoxically enhances dopamine neurotransmission, improving movement while worsening pathology, suggesting that changes to dopamine synapse function compensate for and conceal the underlying PD pathogenesis, with implications for therapies that target autophagy.
Dopamine neuron-derived IGF-1 controls dopamine neuron firing, skill learning, and exploration.
Midbrain dopamine neurons, which can be regulated by neuropeptides and hormones, play a fundamental role in controlling cognitive processes, reward mechanisms, and motor functions. The hormonal actions of insulin-like growth factor 1 (IGF-1) produced by the liver have been well described, but the role of neuronally derived IGF-1 remains largely unexplored. We discovered that dopamine neurons secrete IGF-1 from the cell bodies following depolarization, and that IGF-1 controls release of dopamine in the ventral midbrain. In addition, conditional deletion of dopamine neuron-derived IGF-1 in adult mice leads to decrease of dopamine content in the striatum and deficits in dopamine neuron firing and causes reduced spontaneous locomotion and impairments in explorative and learning behaviors. These data identify that dopamine neuron-derived IGF-1 acts as a regulator of dopamine neurons and regulates dopamine-mediated behaviors.
Inhibition of striatal dopamine release by the L-type calcium channel inhibitor isradipine co-varies with risk factors for Parkinson’s
AbstractCa2+entry into nigrostriatal dopamine (DA) neurons and axons via L-type voltage-gated Ca2+channels (LTCCs) contributes respectively to pacemaker activity and DA release, and has long been thought to contribute to vulnerability to degeneration in Parkinson’s disease. LTCC function is greater in DA axons and neurons from substantia nigra pars compacta than from ventral tegmental area, but this is not explained by channel expression level. We tested the hypothesis that LTCC-control of DA release is governed rather by local mechanisms, focussing on candidate biological factors known to operate differently between types of DA neurons and/or be associated with their differing vulnerability to parkinsonism, including biological sex, α-synuclein, DA transporters (DATs), and calbindin-D28k (Calb1). We detected evoked DA releaseex vivoin mouse striatal slices using fast-scan cyclic voltammetry, and assessed LTCC support of DA release by detecting the inhibition of DA release by the LTCC inhibitors isradipine or CP8. Using genetic knockouts or pharmacological manipulations we identified that striatal LTCC support of DA release depended on multiple intersecting factors, in a regionally and sexually divergent manner. LTCC function was promoted by factors associated with Parkinsonian risk, including male sex, α-synuclein, DAT, and a dorsolateral co-ordinate, but limited by factors associated with protection i.e. female sex, glucocerebrosidase activity, Calb1, and ventromedial co-ordinate. Together, these data show that LTCC function in DA axons, and isradipine effect, are locally governed and suggest they vary in a manner that in turn might impact on, or reflect, the cellular stress that leads to parkinsonian degeneration.Abstract Figure
CLR01 protects dopaminergic neurons in vitro and in mouse models of Parkinson's disease.
Parkinson's disease (PD) affects millions of patients worldwide and is characterized by alpha-synuclein aggregation in dopamine neurons. Molecular tweezers have shown high potential as anti-aggregation agents targeting positively charged residues of proteins undergoing amyloidogenic processes. Here we report that the molecular tweezer CLR01 decreased aggregation and toxicity in induced pluripotent stem cell-derived dopaminergic cultures treated with PD brain protein extracts. In microfluidic devices CLR01 reduced alpha-synuclein aggregation in cell somas when axonal terminals were exposed to alpha-synuclein oligomers. We then tested CLR01 in vivo in a humanized alpha-synuclein overexpressing mouse model; mice treated at 12 months of age when motor defects are mild exhibited an improvement in motor defects and a decreased oligomeric alpha-synuclein burden. Finally, CLR01 reduced alpha-synuclein-associated pathology in mice injected with alpha-synuclein aggregates into the striatum or substantia nigra. Taken together, these results highlight CLR01 as a disease-modifying therapy for PD and support further clinical investigation.
Diabetes Causes Dysfunctional Dopamine Neurotransmission Favoring Nigrostriatal Degeneration in Mice.
BACKGROUND: Numerous studies indicate an association between neurodegenerative and metabolic diseases. Although still a matter of debate, growing evidence from epidemiological and animal studies indicate that preexisting diabetes increases the risk to develop Parkinson's disease. However, the mechanisms of such an association are unknown. OBJECTIVES: We investigated whether diabetes alters striatal dopamine neurotransmission and assessed the vulnerability of nigrostriatal neurons to neurodegeneration. METHODS: We used streptozotocin-treated and genetically diabetic db/db mice. Expression of oxidative stress and nigrostriatal neuronal markers and levels of dopamine and its metabolites were monitored. Dopamine release and uptake were assessed using fast-scan cyclic voltammetry. 6-Hydroxydopamine was unilaterally injected into the striatum using stereotaxic surgery. Motor performance was scored using specific tests. RESULTS: Diabetes resulted in oxidative stress and decreased levels of dopamine and its metabolites in the striatum. Levels of proteins regulating dopamine release and uptake, including the dopamine transporter, the Girk2 potassium channel, the vesicular monoamine transporter 2, and the presynaptic vesicle protein synaptobrevin-2, were decreased in diabetic mice. Electrically evoked levels of extracellular dopamine in the striatum were enhanced, and altered dopamine uptake was observed. Striatal microinjections of a subthreshold dose of the neurotoxin 6-hydroxydopamine in diabetic mice, insufficient to cause motor alterations in nondiabetic animals, resulted in motor impairment, higher loss of striatal dopaminergic axons, and decreased neuronal cell bodies in the substantia nigra. CONCLUSIONS: Our results indicate that diabetes promotes striatal oxidative stress, alters dopamine neurotransmission, and increases vulnerability to neurodegenerative damage leading to motor impairment. © 2020 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.
Inhibition of striatal dopamine release by the L-type calcium channel inhibitor isradipine co-varies with risk factors for Parkinson's.
Ca2+ entry into nigrostriatal dopamine (DA) neurons and axons via L-type voltage-gated Ca2+ channels (LTCCs) contributes, respectively, to pacemaker activity and DA release and has long been thought to contribute to vulnerability to degeneration in Parkinson's disease. LTCC function is greater in DA axons and neurons from substantia nigra pars compacta than from ventral tegmental area, but this is not explained by channel expression level. We tested the hypothesis that LTCC control of DA release is governed rather by local mechanisms, focussing on candidate biological factors known to operate differently between types of DA neurons and/or be associated with their differing vulnerability to parkinsonism, including biological sex, α-synuclein, DA transporters (DATs) and calbindin-D28k (Calb1). We detected evoked DA release ex vivo in mouse striatal slices using fast-scan cyclic voltammetry and assessed LTCC support of DA release by detecting the inhibition of DA release by the LTCC inhibitors isradipine or CP8. Using genetic knockouts or pharmacological manipulations, we identified that striatal LTCC support of DA release depended on multiple intersecting factors, in a regionally and sexually divergent manner. LTCC function was promoted by factors associated with Parkinsonian risk, including male sex, α-synuclein, DAT and a dorsolateral co-ordinate, but limited by factors associated with protection, that is, female sex, glucocerebrosidase activity, Calb1 and ventromedial co-ordinate. Together, these data show that LTCC function in DA axons and isradipine effect are locally governed and suggest they vary in a manner that in turn might impact on, or reflect, the cellular stress that leads to parkinsonian degeneration.
Targeted Activation of Cholinergic Interneurons Accounts for the Modulation of Dopamine by Striatal Nicotinic Receptors.
Striatal dopamine (DA) is a major player in action selection and reinforcement. DA release is under strong local control by striatal ACh acting at axonal nicotinic ACh receptors (nAChRs) on DA axons. Striatal nAChRs have been shown to control how DA is released in response to ascending activity from DA neurons, and they also directly drive DA release following synchronized activity in a small local cholinergic network. The source of striatal ACh has been thought to arise solely from intrinsic cholinergic interneurons (ChIs), but recent findings have identified a source of cholinergic inputs to striatum from brainstem nuclei, the pedunculopontine nucleus (PPN) and laterodorsal tegmentum (LDT). Here, we used targeted optogenetic activation alongside DA detection with fast-scan cyclic voltammetry to test whether ChIs alone and/or brainstem afferents to the striatum can account for how ACh drives and modulates DA release in rat striatum. We demonstrate that targeted transient light activation of rat striatal ChIs drives striatal DA release, corroborating and extending previous observations in mouse to rat. However, the same light stimulation targeted to cholinergic brainstem afferents did not drive DA release, and nor did it modulate DA release activated subsequently by electrical stimulation, whereas targeted activation of ChIs did so. We were unable to obtain any evidence for DA modulation by PPN/LDT stimulation. By contrast, we could readily identify that striatal ChIs alone are sufficient to provide a source of ACh that powerfully regulates DA via nAChRs.
Calbindin-D28K Limits Dopamine Release in Ventral but Not Dorsal Striatum by Regulating Ca2+ Availability and Dopamine Transporter Function.
The calcium-binding protein calbindin-D28K, or calb1, is expressed at higher levels by dopamine (DA) neurons originating in the ventral tegmental area (VTA) than in the adjacent substantia nigra pars compacta (SNc). Calb1 has received attention for a potential role in neuroprotection in Parkinson's disease. The underlying physiological roles for calb1 are incompletely understood. We used cre-loxP technology to knock down calb1 in mouse DA neurons to test whether calb1 governs axonal release of DA in the striatum, detected using fast-scan cyclic voltammetry ex vivo. In the ventral but not dorsal striatum, calb1 knockdown elevated DA release and modified the spatiotemporal coupling of Ca2+ entry to DA release. Furthermore, calb1 knockdown enhanced DA uptake but attenuated the impact of DA transporter (DAT) inhibition by cocaine on underlying DA release. These data reveal that calb1 acts through a range of mechanisms underpinning both DA release and uptake to limit DA transmission in the ventral but not dorsal striatum.
An axonal brake on striatal dopamine output by cholinergic interneurons.
Depolarization of axons is necessary for somatic action potentials to trigger axonal neurotransmitter release. Here we show that striatal cholinergic interneurons (ChIs) and nicotinic receptors (nAChRs) on mouse dopamine axons interrupt this relationship. After nAChR-mediated depolarization, dopamine release by subsequent depolarization events was suppressed for ~100 ms. This suppression was not due to depletion of dopamine or acetylcholine, but to a limited reactivation of dopamine axons after nAChR-mediated depolarization, and is more prominent in dorsal than in ventral striatum. In vivo, nAChRs predominantly depressed dopamine release, as nAChR antagonism in dorsal striatum elevated dopamine detected with optic-fiber photometry of dopamine sensor GRABDA2m and promoted conditioned place preference. Our findings reveal that ChIs acting via nAChRs transiently limit the reactivation of dopamine axons for subsequent action potentials in dopamine neurons and therefore generate a dynamic inverse scaling of dopamine release according to ChI activity.