Substantia Nigra Pars Compacta Dopamine Neurons Research Articles

Calpain inhibition reduces NMDA receptor rundown in rat substantia nigra dopamine neurons

Repeated activation of N-Methyl-d-aspartate receptors (NMDARs) causes a Ca2+-dependent reduction in NMDAR-mediated current in dopamine (DA) neurons of the substantia nigra pars compacta (SNc) in one week old rats; however, a Ca2+-dependent regulatory protein has not been identified. The role of the Ca2+-dependent cysteine protease, calpain, in mediating NMDAR current rundown was investigated. In brain slices from rats aged postnatal day 7–9 (‘P7’), bath application of either of the membrane permeable calpain inhibitors, N-Acetyl-L-leucyl-L-leucyl-L-norleucinal (ALLN, 20 μM) or MDL-28170 (30 μM) significantly reduced whole-cell NMDAR current rundown. To investigate the role of the calpain-2 isoform, the membrane permeable calpain-2 inhibitor, Z-Leu-Abu-CONH-CH2-C6H3 (3, 5-(OMe)2 (C2I, 200 nM), was applied; C2I application significantly reduced whole cell NMDAR current rundown. Interestingly, ALLN but not C2I significantly reduced rundown of NMDA-EPSCs. These results suggest the calpain-2 isoform mediates Ca2+-dependent regulation of extrasynaptic NMDAR current in the first postnatal week, while calpain-1 might mediate rundown of synaptic NMDAR currents. One week later in postnatal development, at P12-P16 (‘P14’), there was significantly less rundown in SNc-DA neurons, and no significant effect on rundown of either Ca2+ chelation or treatment with the calpain inhibitor, ALLN, suggesting that the rundown observed in SNc-DA neurons from two week-old rats might be Ca2+-independent. In conclusion, Ca2+-dependent rundown of extrasynaptic NMDAR currents in SNc DA neurons involves calpain-2 activation, but Ca2+- and calpain-2-dependent NMDAR current rundown is developmentally regulated.

S40. MEDIAL SEPTUM ACTIVATION PRODUCES OPPOSITE EFFECTS ON DOPAMINE NEURON ACTIVITY IN THE VENTRAL TEGMENTAL AREA AND SUBSTANTIA NIGRA PARS COMPACTA IN MAM VERSUS NORMAL RATS

BackgroundDisruptions in dopamine (DA) signaling are central to the pathophysiology of several major psychiatric disorders, including schizophrenia. Thus, discovery of novel therapeutic approaches that normalize DA signaling is a major focus of research. One pathway that is critical for DA regulation originates from the ventral subiculum (vSub) of the hippocampus and controls ventral tegmental area (VTA) DA neuron activity. A potent regulator of hippocampal function is the medial septum (MS); a sub-region of the basal forebrain that widely innervates the hippocampus, including the vSub, via cholinergic, GABAergic, and glutamatergic projections, drives hippocampal theta rhythms, and affects goal-directed learning and memory. Despite this, it has never been determined if the MS is an afferent regulator of the midbrain DA system, and therefore may be a novel therapeutic treatment target for DA-related disorders.MethodsEffects of MS activation (NMDA, 0.75 µg/ 0.2 µL) were examined in intact and methylazoxymethanol acetate- treated (MAM) male Sprague-Dawley rats using anesthetized single unit DA recordings in the VTA and substantia nigra pars compacta (SNc) and locomotor behavior in an open field following systemic amphetamine (0.75 mg/kg).ResultsMS activation produced a prolonged 71% increase in the number of spontaneously active DA cells in the VTA, and an opposing 40% decrease in the number of active DA cells in the SNc, compared to vehicle infusions. These effects were mediated by the vSub and ventral pallidum as local infusion of TTX (1 µM /0.5 µL) and bicuculline (1 ng/ 0.5 µL), respectively, reversed DA population activity changes in both regions. MS activation also decreased the locomotor response to amphetamine (49% reduction in distance traveled during peak ambulation compared to vehicle). MS activation-induced changes in both DA population activity and amphetamine-induced hyperlocomotion were selectively mediated by different neurotransmitter populations from MS to vSub as infusion of scopolamine (8 µg/1.0 µL) into the vSub selectively prevented DA population activity changes in the VTA. In contrast, infusion of bicuculline (12.5 ng/0.5 µL) selectively prevented DA population activity changes in the SNc and the decrease in amphetamine-induced hyperlocomotion. In MAM rats, MS activation produced opposite effects on DA population activity versus controls as it decreased VTA DA activity by 51% and increased SNc DA activity by 47%. This was accompanied by a similar reversal in amphetamine-induced hyperlocomotion, with MS activation increasing locomotion in MAM animals (113% increase in distance traveled during peak ambulation compared to control animals). The reversal in behavioral output is likely due to disrupted GABAergic projections from MS to vSub as bicuculline infusion into vSub prevented the increase in locomotor behavior.DiscussionThese data indicate that the MS differentially regulates both VTA and SNc DA neuron activity and behavioral output, but via distinct pathways, and that this regulation is disrupted in a well-validated animal model of schizophrenia. Therefore, this suggests that in normal animals the MS might activate the VTA to increase information processing while delaying action via SNc inhibition. In contrast, the inhibition of VTA and activation of SNc in MAM rats might promote a rapid response without adequate processing of information.

Regulator of G‐protein Signaling 6 (RGS6) Modulates Dopamine D2 Autoreceptor Function in Mice

The dopaminergic system plays a critical role in the regulation of movement, motivation and cognition and its dysfunction is linked to various neuropsychiatric disorders including Parkinson's Disease. Presynaptic dopamine D2 autoreceptors modulate dopamine signaling in the brain by attenuating its further synthesis and release following the initial synaptic release of dopamine. Consistent with this function, previous studies have demonstrated that pharmacological activation of D2 autoreceptors by quinpirole inhibits presynaptic dopamine release and suppresses locomotion, effects not observed in DAT‐Cre; D2Rfl/fl mice that lack the D2 autoreceptor. RGS6 is a member of the RGS protein family that functions as a GTPase‐activating protein for Gai/o subunits, thereby controlling the intensity and duration of signaling through Gai/o‐linked GPCRs like the D2R. Interestingly, RGS6 is exclusively expressed in dopaminergic neurons of the substantia nigra pars compacta (SNc) of mice and humans, whose loss causes Parkinson's disease, and RGS6 deficient mice exhibit an age‐dependent loss of SNc and striatal dopamine content as well as Parkinson‐like motor dysfunction. Therefore, we hypothesized that SNc dopamine neuron loss in RGS6 deficient mice might result from a lifetime of exaggerated pre‐synaptic D2 autoreceptor signaling culminating in the accumulation of cytotoxic dopamine. To test this hypothesis, we assessed D2 autoreceptor function both in vitro using mouse striatal slices and in vivo. First, we compared the ability of quinpirole to reduce forskolin‐stimulated PKA‐mediated phosphorylation of tyrosine hydroxylase (Ser40) in acute striatal slices derived from WT and RGS6−/− mice. RGS6 deficiency led to exaggerated quinpirole‐mediated inhibition of tyrosine hydroxylase phosphorylation compared to that observed in WT striatal slices. Consistent with these in vitro findings we also found that when 3 mo. WT and RGS6−/− mice, which do not yet display differences in dopamine content, were treated with quinpirole that RGS6−/− mice were more susceptible to quinpirole‐induced suppression of locomotion compared to WT animals despite the mild hyperactivity that has been reported in young RGS6−/− mice. Furthermore, quinpirole also suppressed unsupported rearing in RGS6−/− mice but not WT mice. Together these results demonstrate that RGS6 functions as a critical regulator of D2 autoreceptors in SNc dopamine neurons to modulate dopamine‐dependent motor behaviors. Therefore, loss of RGS6 would be expected to promote hyperactive D2 autoreceptor signaling in these neurons which may underlie their loss in both RGS6−/− mice and humans with Parkinson's.Support or Funding InformationNIH CA161882, MJ Fox 11551, Iowa Cardiovascular Interdisciplinary Research Fellowship T32HL007121.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Role for VGLUT2 in selective vulnerability of midbrain dopamine neurons.

Parkinson's disease is characterized by the loss of dopamine (DA) neurons in the substantia nigra pars compacta (SNc). DA neurons in the ventral tegmental area are more resistant to this degeneration than those in the SNc, though the mechanisms for selective resistance or vulnerability remain poorly understood. A key to elucidating these processes may lie within the subset of DA neurons that corelease glutamate and express the vesicular glutamate transporter VGLUT2. Here, we addressed the potential relationship between VGLUT expression and DA neuronal vulnerability by overexpressing VGLUT in DA neurons of flies and mice. In Drosophila, VGLUT overexpression led to loss of select DA neuron populations. Similarly, expression of VGLUT2 specifically in murine SNc DA neurons led to neuronal loss and Parkinsonian behaviors. Other neuronal cell types showed no such sensitivity, suggesting that DA neurons are distinctively vulnerable to VGLUT2 expression. Additionally, most DA neurons expressed VGLUT2 during development, and coexpression of VGLUT2 with DA markers increased following injury in the adult. Finally, conditional deletion of VGLUT2 made DA neurons more susceptible to Parkinsonian neurotoxins. These data suggest that the balance of VGLUT2 expression is a crucial determinant of DA neuron survival. Ultimately, manipulation of this VGLUT2-dependent process may represent an avenue for therapeutic development.

Cocaine Selectively Reorganizes Excitatory Inputs to Substantia Nigra Pars Compacta Dopamine Neurons.

Substantia nigra pars compacta (SNc) dopamine neurons and their targets are involved in addiction and cue-induced relapse. However, afferents onto SNc dopamine neurons themselves appear insensitive to drugs of abuse, such as cocaine, when afferents are collectively stimulated electrically. This contrasts with ventral tegmental area (VTA) dopamine neurons, whose glutamate afferents react robustly to cocaine. We used an optogenetic strategy to isolate identified SNc inputs and determine whether cocaine sensitivity in the mouse SNc circuit is conferred at the level of three glutamate afferents: dorsal raphé nucleus (DR), pedunculopontine nucleus (PPN), and subthalamic nucleus (STN). We found that excitatory afferents to SNc dopamine neurons are sensitive to cocaine in an afferent-specific manner. A single exposure to cocaine in vivo led to PPN-innervated synapses reducing the AMPA-to-NMDA receptor-mediated current ratio. In contrast to work in the VTA, this was due to increased NMDA receptor function with no change in AMPA receptor function. STN synapses showed a decrease in calcium-permeable AMPA receptors after cocaine, but no change in the AMPA-to-NMDA ratio. Cocaine also increased the release probability at DR-innervated and STN-innervated synapses, quantified by decreases in paired-pulse ratios. However, release probability at PPN-innervated synapses remained unaffected. By examining identified inputs, our results demonstrate a functional distribution among excitatory SNc afferent nuclei in response to cocaine, and suggest a compelling architecture for differentiation and separate parsing of inputs within the nigrostriatal system.SIGNIFICANCE STATEMENT Prior studies have established that substantia nigra pars compacta (SNc) dopamine neurons are a key node in the circuitry that drives addiction and relapse, yet cocaine apparently has no effect on electrically stimulated excitatory inputs. Our study is the first to demonstrate the functional impact of a drug of abuse on synaptic mechanisms of identified afferents to the SNc. Optogenetic dissection of inputs originating from dorsal raphé, pedunculopontine, and subthalamic nuclei were tested for synaptic modifications following in vivo cocaine exposure. Our results demonstrate that cocaine differentially induces modifications to SNc synapses depending on input origin. This presents implications for understanding dopamine processing of motivated behavior; most critically, it indicates that dopamine neurons selectively modulate signal reception processed by afferent nuclei.

Evidence of functional duplicity of Nestin expression in the adult mouse midbrain

Whether or not neurogenesis occurs in the adult substantia nigra pars compacta (SNc) is an important question relevant for developing better treatments for the motor symptoms of Parkinson's disease (PD). Although controversial, it is generally believed that dividing cells here remain undifferentiated or differentiate into glia, not neurons. However, there is a suggestion that Nestin-expressing neural precursor cells (NPCs) in the adult SNc have a propensity to differentiate into neurons, which we sought to confirm in the present study. Adult (>8-weeks old) transgenic NesCreERT2/GtROSA or NesCreERT2/R26eYFP mice were used to permanently label Nestin-expressing cells and their progeny with β-galactosidase (β-gal) or enhanced yellow fluorescent protein (eYFP), respectively. Most β-gal+ or eYFP+ cells were found in the ependymal lining of the midbrain aqueduct (Aq) and in the midline ventral to Aq. Smaller but significant numbers were in the periaqueductal gray (PAG), the ventral tegmental area (VTA), and in SNc. Low-level basal proliferation was evidenced by a modest increase in number of β-gal+ or eYFP+ cells over time, fewer β-gal+ or eYFP+ cells when mice were administered the anti-mitotic agent Cytarabine, and incorporation of the proliferation marker bromodeoxyuridine (BrdU) in a very small number of β-gal+ cells. No evidence of migration was found, including no immunoreactivity against the migration markers doublecortin (DCX) or polysialic acid neural cell adhesion molecule (PSA-NCAM), and no dispersal of β-gal+ or eYFP+ cells through the midbrain parenchyma over time. However, β-gal+ or eYFP+ cells did increase in size and express higher levels of mature neuronal genes over time, indicating growth and neuronal differentiation. In mice whose SNc dopamine neurons had been depleted with 6-hydroxy-dopamine, a model of PD, there were ~2-fold more β-gal+ cells in SNc specifically, although the proportion that were also NeuN+ was not affected. Remarkably, as early as 4days following putative Nestin-expression, many β-gal+ or eYFP+ cells had mature neuronal morphology and were NeuN+. Furthermore, mature neuronal β-gal+ cells were immunoreactive against the self-renewal or pluripotency marker sex determining region Y-box 2 (Sox2). Overall, our data support the notion that some Nestin-expressing, presumably NPCs, have a limited capacity for proliferation, no capacity for migration, and a propensity to generate new neurons within the microenvironment of the adult midbrain. However, our data also suggest that significant numbers of extant midbrain neurons express Nestin and other classical neurogenesis markers in contexts that are presumably not neurogenic. These findings foreshadow duplicitous roles for Nestin and other molecules that are traditionally associated with neurogenesis in the adult midbrain, which should be considered in future PD research.

Mesocortical Dopamine Phenotypes in Mice Lacking the Sonic Hedgehog Receptor Cdon

Motivated behaviors and many psychopathologies typically involve changes in dopamine release from the projections of the ventral tegmental area (VTA) and/or the substantia nigra pars compacta (SNc). The morphogen Sonic Hedgehog (Shh) specifies fates of midbrain dopamine neurons, but VTA-specific effects of Shh signaling are also being uncovered. In this study, we assessed the role of the Shh receptor Cdon in the development of VTA and SNc dopamine neurons. We find that Cdon is expressed in the proliferating progenitor zone of the embryonic ventral midbrain and that the number of proliferating cells in this region is increased in mouse Cdon−/− embryos. Consistent with a role of Shh in the regulation of neuronal proliferation in this region, we find that the number of tyrosine hydroxylase (TH)-positive neurons is increased in the VTA of Cdon−/− mice at birth and that this effect endures into adulthood. In contrast, the number of TH-positive neurons in the SNc is not altered in Cdon−/− mice at either age. Moreover, adult Cdon−/− mice have a greater number of medial prefrontal cortex (mPFC) dopamine presynaptic sites, and increased baseline concentrations of dopamine and dopamine metabolites selectively in this region. Finally, consistent with increased dopamine function in the mPFC, we find that adult Cdon−/− mice fail to exhibit behavioral plasticity upon repeated amphetamine treatment. Based on these data, we suggest that Cdon plays an important role encoding the diversity of dopamine neurons in the midbrain, influencing both the development of the mesocortical dopamine pathway and behavioral outputs that involve this neural circuitry.

A WNT1-regulated developmental gene cascade prevents dopaminergic neurodegeneration in adult En1+/− mice

The protracted and age-dependent degeneration of dopamine (DA)-producing neurons of the Substantia nigra pars compacta (SNc) and ventral tegmental area (VTA) in the mammalian midbrain is a hallmark of human Parkinson's Disease (PD) and of certain genetic mouse models of PD, such as mice heterozygous for the homeodomain transcription factor Engrailed 1 (En1+/− mice). Neurotoxin-based animal models of PD, in contrast, are characterized by the fast and partly reversible degeneration of the SNc and VTA DA neurons. The secreted protein WNT1 was previously shown to be strongly induced in the neurotoxin-injured adult ventral midbrain (VM), and to protect the SNc and VTA DA neurons from cell death in this context. We demonstrate here that the sustained and ectopic expression of Wnt1 in the SNc and VTA DA neurons of En1+/Wnt1 mice also protected these genetically affected En1 heterozygote (En1+/−) neurons from their premature degeneration in the adult mouse VM. We identified a developmental gene cascade that is up-regulated in the adult En1+/Wnt1 VM, including the direct WNT1/β-catenin signaling targets Lef1, Lmx1a, Fgf20 and Dkk3, as well as the indirect targets Pitx3 (activated by LMX1A) and Bdnf (activated by PITX3). We also show that the secreted neurotrophin BDNF and the secreted WNT modulator DKK3, but not the secreted growth factor FGF20, increased the survival of En1 mutant dopaminergic neurons in vitro. The WNT1-mediated signaling pathway and its downstream targets BDNF and DKK3 might thus provide a useful means to treat certain genetic and environmental (neurotoxic) forms of human PD.

Nigrostriatal Dopamine Acting on Globus Pallidus Regulates Sleep.

Lesions of the globus pallidus externa (GPe) produce a profound sleep loss (∼45%) in rats, suggesting that GPe neurons promote sleep. As GPe neuronal activity is enhanced by dopamine (DA) from the substantia nigra pars compacta (SNc), we hypothesized that SNc DA via the GPe promotes sleep. To test this hypothesis, we selectively destroyed the DA afferents to the caudoputamen (CPu) using 6-hydroxydopamine and examined changes in sleep-wake profiles in rats. Rats with 80-90% loss of SNc neurons displayed a significant 33.7% increase in wakefulness (or sleep reduction). This increase significantly correlated with the extent of SNc DA neuron loss. Furthermore, these animals exhibited sleep-wake fragmentation and reduced diurnal variability of sleep. We then optogenetic-stimulated SNc DA terminals in the CPu and found that 20-Hz stimulation from 9 to 10 PM increased total sleep by 69% with high electroencephalograph (EEG) delta power. We finally directly optogenetic-stimulated GPe neurons and found that 20-Hz stimulation of the GPe from 9 to 10 PM increased total sleep by 66% and significantly increased EEG delta power. These findings elucidate a novel circuit for DA control of sleep and the mechanisms of abnormal sleep in BG disorders such as Parkinson's disease and Huntington's disease.

Organization of Monosynaptic Inputs to the Serotonin and Dopamine Neuromodulatory Systems

Serotonin and dopamine are major neuromodulators. Here, we used a modified rabies virus to identify monosynaptic inputs to serotonin neurons in the dorsal and median raphe (DR and MR). We found that inputs to DR and MR serotonin neurons are spatially shifted in the forebrain, and MR serotonin neurons receive inputs from more medial structures. Then, we compared these data with inputs to dopamine neurons in the ventral tegmental area (VTA) and substantia nigra pars compacta (SNc). We found that DR serotonin neurons receive inputs from a remarkably similar set of areas as VTA dopamine neurons apart from the striatum, which preferentially targets dopamine neurons. Our results suggest three major input streams: a medial stream regulates MR serotonin neurons, an intermediate stream regulates DR serotonin and VTA dopamine neurons, and a lateral stream regulates SNc dopamine neurons. These results provide fundamental organizational principles of afferent control for serotonin and dopamine.

Balance between the proximal dendritic compartment and the soma determines spontaneous firing rate in midbrain dopamine neurons.

Midbrain dopamine (DA) neurons are slow intrinsic pacemakers that require the elaborate composition of many ion channels in the somatodendritic compartments. Understanding the major determinants of the spontaneous firing rate (SFR) of midbrain DA neurons is important because they determine the basal DA levels in target areas, including the striatum. As spontaneous firing occurs synchronously at the soma and dendrites, the electrical coupling between the soma and dendritic compartments has been regarded as a key determinant for the SFR. However, it is not known whether this somatodendritic coupling is served by the whole dendritic compartments or only parts of them. In the rat substantia nigra pars compacta (SNc) DA neurons, we demonstrate that the balance between the proximal dendritic compartment and the soma determines the SFR. Isolated SNc DA neurons showed a wide range of soma size and a variable number of primary dendrites but preserved a quite consistent SFR. The SFR was not correlated with soma size or with the number of primary dendrites, but it was strongly correlated with the area ratios of the proximal dendritic compartments to the somatic compartment. Tetrodotoxin puff and local Ca(2+) perturbation experiments, computer simulation, and local glutamate uncaging experiments suggest the importance of the proximal dendritic compartments in pacemaker activity. These data indicate that the proximal dendritic compartments, not the whole dendritic compartments, play a key role in the somatodendritic balance that determines the SFR in DA neurons.

M5Receptor Activation Produces Opposing Physiological Outcomes in Dopamine Neurons Depending on the Receptor's Location

Of the five muscarinic receptor subtypes, the M5 receptor is the only one detectable in midbrain dopaminergic neurons, making it an attractive potential therapeutic target for treating disorders in which dopaminergic signaling is disrupted. However, developing an understanding of the role of M5 in regulating midbrain dopamine neuron function has been hampered by a lack of subtype-selective compounds. Here, we extensively characterize the novel compound VU0238429 and demonstrate that it acts as a positive allosteric modulator with unprecedented selectivity for the M5 receptor. We then used VU0238429, along with M5 knock-out mice, to elucidate the role of this receptor in regulating substantia nigra pars compacta (SNc) neuron physiology in both mice and rats. In sagittal brain slices that isolate the SNc soma from their striatal terminals, activation of muscarinic receptors induced Ca2+ mobilization and inward currents in SNc dopamine neurons, both of which were potentiated by VU0238429 and absent in M5 knock-out mice. Activation of M5 also increased the spontaneous firing rate of SNc neurons, suggesting that activation of somatodendritic M5 increases the intrinsic excitability of SNc neurons. However, in coronal slices of the striatum, potentiation of M5 with VU0238429 resulted in an inhibition in dopamine release as monitored with fast scan cyclic voltammetry. Accordingly, activation of M5 can lead to opposing physiological outcomes depending on the location of the receptor. Although activation of somatodendritic M5 receptors on SNc neurons leads to increased neuronal firing, activation of M5 receptors in the striatum induces an inhibition in dopamine release.

Similar Roles of Substantia Nigra and Ventral Tegmental Dopamine Neurons in Reward and Aversion

Dopamine neurons in the ventral tegmental area (VTA) are implicated in affective functions. However, it is unclear to what extent dopamine neurons in substantia nigra pars compacta (SNc) play such roles. TH-Cre transgenic mice received adeno-associated viral vectors encoding channelrhodopsin2 (ChR2), halorhodopsin (NpHR), or control vector into the VTA or SNc, resulting in selective expression of these opsins in dopamine neurons. Mice with ChR2 learned instrumental responding to deliver photostimulation into the VTA or SNc and also sought for the compartment where they received photostimulation (i.e., operant place preference). Operant place preference scores were highly correlated with self-stimulation responses. In contrast, mice with NpHR avoided the compartment where they received photostimulation into the VTA, SNc, or dorsal striatum, whereas control mice did not. These observations suggest that the excitation and inhibition of SNc dopamine neurons elicit positive and negative affective effects, respectively, similar to those of VTA dopamine neurons.

Nucleus accumbens high-frequency stimulation selectively impacts nigrostriatal dopaminergic neurons

High-frequency stimulation of the nucleus accumbens, also known as deep brain stimulation (DBS), is currently used to alleviate obsessive compulsive symptoms when pharmacotherapy is ineffective. However, the mechanism by which DBS achieves its therapeutic actions is not understood. Imaging studies and the actions of dopaminergic drugs in untreated patients suggest that the dopamine (DA) system likely plays a role in the pathophysiology of obsessive compulsive disorder. Therefore, we examined whether DBS would impact the DA system as a potential component of its therapeutic actions. The activity of DA neurons in the substantia nigra pars compacta (SNc) and ventral tegmental area (VTA) were recorded in anesthetized rats under high-frequency stimulation. DA neuron activity was measured in terms of number of neurons firing, average firing rate and firing pattern. DBS of the nucleus accumbens core did not significantly affect VTA activity or discharge pattern. On the other hand, DBS caused a potent decrease in the number of SNc DA neurons firing spontaneously. Such an effect could contribute to the disruption of pathological habit formation in the SNc-dorsal striatal projection system that may have therapeutic implications for the treatment of obsessive compulsive disorder.

Deficits in dopaminergic transmission precede neuron loss and dysfunction in a new Parkinson model.

The pathological end-state of Parkinson disease is well described from postmortem tissue, but there remains a pressing need to define early functional changes to susceptible neurons and circuits. In particular, mechanisms underlying the vulnerability of the dopamine neurons of the substantia nigra pars compacta (SNc) and the importance of protein aggregation in driving the disease process remain to be determined. To better understand the sequence of events occurring in familial and sporadic Parkinson disease, we generated bacterial artificial chromosome transgenic mice (SNCA-OVX) that express wild-type α-synuclein from the complete human SNCA locus at disease-relevant levels and display a transgene expression profile that recapitulates that of endogenous α-synuclein. SNCA-OVX mice display age-dependent loss of nigrostriatal dopamine neurons and motor impairments characteristic of Parkinson disease. This phenotype is preceded by early deficits in dopamine release from terminals in the dorsal, but not ventral, striatum. Such neurotransmission deficits are not seen at either noradrenergic or serotoninergic terminals. Dopamine release deficits are associated with an altered distribution of vesicles in dopaminergic axons in the dorsal striatum. Aged SNCA-OVX mice exhibit reduced firing of SNc dopamine neurons in vivo measured by juxtacellular recording of neurochemically identified neurons. These progressive changes in vulnerable SNc neurons were observed independently of overt protein aggregation, suggesting neurophysiological changes precede, and are not driven by, aggregate formation. This longitudinal phenotyping strategy in SNCA-OVX mice thus provides insights into the region-specific neuronal disturbances preceding and accompanying Parkinson disease.

Environmental and behavioral modulation of the number of substantia nigra dopamine neurons in adult mice

BackgroundRecent evidence indicates that hypothalamic neurons acquire or lose the capacity to synthesize and release dopamine (DA) in response to environmental stimuli, and this has functional and behavioral consequences for adult rats. We have evidence that neuronal activity, including that driven by afferent input, regulates acquisition and loss of the DA phenotype by substantia nigra pars compacta (SNc) neurons in adult mice.Hypotheses The aims of the present study were to determine whether the environment or behavior regulates the number of SNc DA neurons in adult mice, and whether this is mediated by afferent input.MethodsAdult mice were subject to two different environments/behaviors: “mating” for 1 week or “environment enrichment” (EE) for 2 weeks; then the numbers of tyrosine hydroxylase (TH, the rate limiting enzyme in DA synthesis) immunopositive (TH+) and immunonegative (TH−) SNc neurons were counted.ResultsMore TH+ neurons were present in mated males whereas less TH+ neurons were present in mated females. Also, more TH+ neurons were present in EE males, and this increase was completely abolished by concurrent local infusion of GABAA receptor antagonists.ConclusionsThe number of DA neurons in the adult SNc is not fixed, but readily increases and decreases in response to environmental stimuli and/or behaviors. These changes are mediated by afferent input relaying information about the environment or behavior to SNc neurons.

MPP+‐dependent inhibition of Ih reduces spontaneous activity and enhances EPSP summation in nigral dopamine neurons

1-Methyl-4-phenylpyridinium (MPP(+) ), a potent parkinsonizing agent in primates and rodents, is a blocker of mitochondrial complex I, therefore MPP(+) -induced parkinsonism is believed to depend largely on mitochondrial impairment. However, it has recently been proposed that other mechanisms may participate in MPP(+) -induced toxicity. We tackled this issue by probing the effects of an acute application of MPP(+) on substantia nigra pars compacta (SNc) dopamine (DA) neurons. The effects of MPP(+) on SNc DA neurons in acute midbrain slices were investigated with electrophysiology techniques. MPP(+) (50 μM) was able to (i) hyperpolarize SNc DA neurons by ∼6 mV; (ii) cause an abrupt and marked (over 50%) reduction of the spontaneous activity; and (iii) inhibit the hyperpolarization-activated inward current (Ih ). MPP(+) shifted Ih activation curve towards negative potentials by ∼11 mV both in Wistar rats and in C57/BL6 mice. Inhibition was voltage- and concentration-dependent (Imax = 47%, IC50 = 7.74 μM). MPP(+) slowed Ih activation kinetics at all potentials. These effects were not dependent on (i) block of mitochondrial complex I/fall of ATP levels; (ii) activation of type 2 DA receptor; and (iii) alteration of cAMP metabolism. Finally, MPP(+) -dependent inhibition of Ih facilitated temporal summation of evoked EPSPs in SNc DA, but not in CA1 hippocampal neurons. Reduced functionality of Ih in SNc DA neurons, via increased responsiveness towards synaptic excitation, might play a role in MPP(+) -induced parkinsonism and, possibly, in the pathogenesis of human Parkinson's.

The energy cost of action potential propagation in dopamine neurons: clues to susceptibility in Parkinson's disease

Dopamine neurons of the substantia nigra pars compacta (SNc) are uniquely sensitive to degeneration in Parkinson's disease (PD) and its models. Although a variety of molecular characteristics have been proposed to underlie this sensitivity, one possible contributory factor is their massive, unmyelinated axonal arbor that is orders of magnitude larger than other neuronal types. We suggest that this puts them under such a high energy demand that any stressor that perturbs energy production leads to energy demand exceeding supply and subsequent cell death. One prediction of this hypothesis is that those dopamine neurons that are selectively vulnerable in PD will have a higher energy cost than those that are less vulnerable. We show here, through the use of a biology-based computational model of the axons of individual dopamine neurons, that the energy cost of axon potential propagation and recovery of the membrane potential increases with the size and complexity of the axonal arbor according to a power law. Thus SNc dopamine neurons, particularly in humans, whose axons we estimate to give rise to more than 1 million synapses and have a total length exceeding 4 m, are at a distinct disadvantage with respect to energy balance which may be a factor in their selective vulnerability in PD.

Whole-Brain Mapping of Direct Inputs to Midbrain Dopamine Neurons

Recent studies indicate that dopamine neurons in the ventral tegmental area (VTA) and substantia nigra pars compacta (SNc) convey distinct signals. To explore this difference, we comprehensively identified each area's monosynaptic inputs using the rabies virus. We show that dopamine neurons in both areas integrate inputs from a more diverse collection of areas than previously thought, including autonomic, motor, and somatosensory areas. SNc and VTA dopamine neurons receive contrasting excitatory inputs: the former from the somatosensory/motor cortex and subthalamic nucleus, which may explain their short-latency responses to salient events; and the latter from the lateral hypothalamus, which may explain their involvement in value coding. We demonstrate that neurons in the striatum that project directly to dopamine neurons form patches in both the dorsal and ventral striatum, whereas those projecting to GABAergic neurons are distributed in the matrix compartment. Neuron-type-specific connectivity lays a foundation for studying how dopamine neurons compute outputs.

Activity‐dependent regulation of the dopamine phenotype in substantia nigra neurons

Degeneration of dopamine (DA) neurons in the substantia nigra pars compacta (SNc) causes the motor symptoms of Parkinson's disease. The development of cell-replacement therapies for Parkinson's disease motor symptoms is hampered by poor acquisition and retention of the DA phenotype by endogenous and transplanted neurons. Factors which regulate the DA phenotype in the adult SNc are, therefore, keenly sought. Transcription of the rate-limiting enzyme in DA synthesis, tyrosine hydroxylase, and possibly other DA genes, is known to be regulated by changes in membrane potential and intracellular Ca²⁺. Furthermore, emerging evidence indicates DA gene transcription is sensitive to fast membrane potential changes and intracellular Ca²⁺ transients, that is, those associated with normal rates and patterns of neuronal activity. In other words, the DA phenotype is activity-dependent. In this review, we highlight the importance of spatiotemporal Ca²⁺ dynamics for regulating gene expression in cells, and the possible role of fast Ca²⁺ dynamics associated with normal rates and patterns of neuronal activity. We review evidence supporting activity- and Ca²⁺-dependent regulation of the DA phenotype in cells, including SNc neurons, as well as knowledge about the molecular pathways intervening between intracellular Ca²⁺ and TH gene expression. We describe the electrophysiology of SNc DA neurons, emphasizing features that may regulate DA gene expression. We conclude by bringing together this information in a model of how neuronal activity might regulate the DA phenotype in SNc neurons.