Changes in firing rate and pattern of GABAergic neurons in subregions of the substantia nigra pars reticulata in rat models of Parkinson's disease

Comparative mapping of single-cell transcriptomic landscapes in neurodegenerative diseases

INTRODUCTION

Alzheimer's disease (AD), Dementia with Lewy bodies (DLB), and Parkinson's disease (PD) represent a spectrum of neurodegenerative disorders (NDDs). Here, we performed the first direct comparison of their transcriptomic landscapes.

METHODS

We profiled the whole transcriptomes of NDD cortical tissue by snRNA-seq. We used computational analyses to identify common and distinct differentially expressed genes (DEGs), biological pathways, vulnerable and disease-driver cell subtypes, and alteration in cell-to-cell interactions.

RESULTS

The same vulnerable inhibitory neuron subtype was depleted in both AD and DLB. Potentially disease-driving neuronal cell subtypes were present in both PD and DLB. Cell-cell communication was predicted to be increased in AD but decreased in DLB and PD. DEGs were most commonly shared across NDDs within inhibitory neuron subtypes. Overall, we observed the greatest transcriptomic divergence between AD and PD, while DLB exhibited an intermediate transcriptomic signature.

DISCUSSION

These results help explain the clinicopathological spectrum of this group of NDDs and provide unique insights into the shared and distinct molecular mechanisms underlying the pathogenesis of NDDs.

Sleep disorders cause Parkinson's disease or the reverse is true: Good GABA good night

BACKGROUND

Parkinson's disease (PD) is a progressive neurodegenerative brain disease due to degeneration of dopaminergic neurons (DNs) presented with motor and non-motor symptoms. PD symptoms are developed in response to the disturbance of diverse neurotransmitters including γ-aminobutyric acid (GABA). GABA has a neuroprotective effect against PD neuropathology by protecting DNs in the substantia nigra pars compacta (SNpc). It has been shown that the degeneration of GABAergic neurons is linked with the degeneration of DNs and the progression of motor and non-motor PD symptoms. GABA neurotransmission is a necessary pathway for normal sleep patterns, thus deregulation of GABAergic neurotransmission in PD could be the potential cause of sleep disorders in PD.

AIM

Sleep disorders affect GABA neurotransmission leading to memory and cognitive dysfunction in PD. For example, insomnia and short sleep duration are associated with a reduction of brain GABA levels. Moreover, PD-related disorders including rigidity and nocturia influence sleep patterns leading to fragmented sleep which may also affect PD neuropathology. However, the mechanistic role of GABA in PD neuropathology regarding motor and non-motor symptoms is not fully elucidated. Therefore, this narrative review aims to clarify the mechanistic role of GABA in PD neuropathology mainly in sleep disorders, and how good GABA improves PD. In addition, this review of published articles tries to elucidate how sleep disorders such as insomnia and REM sleep behavior disorder (RBD) affect PD neuropathology and severity. The present review has many limitations including the paucity of prospective studies and most findings are taken from observational and preclinical studies. GABA involvement in the pathogenesis of PD has been recently discussed by recent studies. Therefore, future prospective studies regarding the use of GABA agonists in the management of PD are suggested to observe their distinct effects on motor and non-motor symptoms.

CONCLUSION

There is a bidirectional relationship between the pathogenesis of PD and sleep disorders which might be due to GABA deregulation.

Targeting parvalbumin-expressing neurons in the substantia nigra pars reticulata restores motor function in parkinsonian mice

The activity of substantia nigra pars reticulata (SNr) neurons, the main output structure of basal ganglia, is altered in Parkinson's disease (PD). However, neither the underlying mechanisms nor the type of neurons responsible for PD-related motor dysfunctions have been elucidated yet. Here, we show that parvalbumin-expressing SNr neurons (SNr-PV+) occupy dorsolateral parts and possess specific electrophysiological properties compared with other SNr cells. We also report that only SNr-PV+ neurons' intrinsic excitability is reduced by downregulation of sodium leak channels in a PD mouse model. Interestingly, in anesthetized parkinsonian mice in vivo, SNr-PV+ neurons display a bursty pattern of activity dependent on glutamatergic tone. Finally, we demonstrate that chemogenetic inhibition of SNr-PV+ neurons is sufficient to alleviate motor impairments in parkinsonian mice. Overall, our findings establish cell-type-specific dysfunction in experimental parkinsonism in the SNr and provide a potential cellular therapeutic target to alleviate motor symptoms in PD.

Altered parabrachial nucleus nociceptive processing may underlie central pain in Parkinson's disease

The presence of central neuropathic pain in Parkinson's disease suggests that the brain circuits that allow us to process pain could be dysfunctional in the disorder. However, there is to date no clear pathophysiological mechanism to explain these symptoms. In this work, we present evidence that the dysfunction of the subthalamic nucleus and/or substantia nigra pars reticulata may impact nociceptive processing in the parabrachial nucleus (PBN), a low level primary nociceptive structure in the brainstem, and induce a cellular and molecular neuro-adaptation in this structure. In rat models of Parkinson's disease with a partial dopaminergic lesion in the substantia nigra compacta, we found that the substantia nigra reticulata showed enhanced nociceptive responses. Such responses were less impacted in the subthalamic nucleus. A total dopaminergic lesion produced an increase in the nociceptive responses as well as an increase of the firing rate in both structures. In the PBN, inhibited nociceptive responses and increased expression of GABA_A receptors were found following a total dopaminergic lesion. However, neuro-adaptations at the level of dendritic spine density and post-synaptic density were found in both dopaminergic lesion groups. These results suggest that the molecular changes within the PBN following a larger dopaminergic lesion, such as increased GABA_A expression, is a key mechanism to produce nociceptive processing impairment, whilst other changes may protect function after smaller dopaminergic lesions. We also propose that these neuro-adaptations follow increased inhibitory tone from the substantia nigra pars reticulata and may represent the mechanism generating central neuropathic pain in Parkinson's disease.

Disrupted basal ganglia output during movement preparation in hemiparkinsonian mice is consistent with behavioral deficits

Parkinsonian motor deficits are associated with elevated inhibitory output from the basal ganglia (BG). However, several features of Parkinson's disease (PD) have not been accounted for by this simple "classical rate model" framework, including the observation in patients with PD that movements guided by external stimuli are less impaired than otherwise identical movements generated based on internal goals. Is this difference due to divergent processing within the BG itself or due to the recruitment of extra-BG pathways by sensory processing? In addition, surprisingly little is known about precisely when, in the sequence from selecting to executing movements, BG output is altered by PD. Here, we address these questions by recording activity in the substantia nigra pars reticulata (SNr), a key BG output nucleus, in hemiparkinsonian mice performing a well-controlled behavioral task requiring stimulus-guided and internally specified directional movements. We found that hemiparkinsonian mice exhibited a bias ipsilateral to the side of dopaminergic cell loss that was stronger when movements were internally specified rather than stimulus guided, consistent with clinical observations in patients with Parkinson's disease. We further found that changes in parkinsonian SNr activity during movement preparation were consistent with the ipsilateral behavioral bias, as well as its greater magnitude for internally specified movements. Although these findings are inconsistent with some aspects of the classical rate model, they are accounted for by a related "directional rate model" positing that SNr output phasically overinhibits motor output in a direction-specific manner. These results suggest that parkinsonian changes in BG output underlying movement preparation contribute to the greater deficit in internally specified than stimulus-guided movements.NEW & NOTEWORTHY Movements of patients with Parkinson's disease are often less impaired when guided by external stimuli than when generated based on internal goals. Whether this effect is due to distinct processing in the basal ganglia (BG) or due to compensation from other motor pathways is an open question with therapeutic implications. We recorded BG output in behaving parkinsonian mice and found that BG activity during movement preparation was consistent with the differences between these forms of movement.

Dopaminergic denervation impairs cortical motor and associative/limbic information processing through the basal ganglia and its modulation by the CB1 receptor

The basal ganglia (BG) are involved in cognitive/motivational functions in addition to movement control. Thus, BG segregated circuits, the sensorimotor (SM) and medial prefrontal (mPF) circuits, process different functional domains, such as motor and cognitive/motivational behaviours, respectively. With a high presence in the BG, the CB1 cannabinoid receptor modulates BG circuits. Furthermore, dopamine (DA), one of the principal neurotransmitters in the BG, also plays a key role in circuit functionality. Taking into account the interaction between DA and the endocannabinoid system at the BG level, we investigated the functioning of BG circuits and their modulation by the CB1 receptor under DA-depleted conditions. We performed single-unit extracellular recordings of substantia nigra pars reticulata (SNr) neurons with simultaneous cortical stimulation in sham and 6-hydroxydopamine (6-OHDA)-lesioned rats, together with immunohistochemical assays. We showed that DA loss alters cortico-nigral information processing in both circuits, with a predominant transmission through the hyperdirect pathway in the SM circuit and an increased transmission through the direct pathway in the mPF circuit. Moreover, although DA denervation does not change CB1 receptor density, it impairs its functionality, leading to a lack of modulation. These data highlight an abnormal transfer of information through the associative/limbic domains after DA denervation that may be related to the non-motor symptoms manifested by Parkinson's disease patients.

Intraoperative Microelectrode Recordings in Substantia Nigra Pars Reticulata in Anesthetized Rats

The Substantia Nigra pars reticulata (SNr) is a promising target for deep brain stimulation (DBS) to treat the gait and postural disturbances in Parkinson’s disease (PD). Positioning the DBS electrode within the SNr is critical for the development of preclinical models of SNr DBS to investigate underlying mechanisms. However, a complete characterization of intraoperative microelectrode recordings in the SNr to guide DBS electrode placement is lacking. In this study, we recorded extracellular single-unit activity in anesthetized rats at multiple locations in the medial SNr (mSNr), lateral SNr (lSNr), and the Ventral Tegmental Area (VTA). Immunohistochemistry and fluorescently dyed electrodes were used to map neural recordings to neuroanatomy. Neural recordings were analyzed in the time domain (i.e., firing rate, interspike interval (ISI) correlation, ISI variance, regularity, spike amplitude, signal-to-noise ratio, half-width, asymmetry, and latency) and the frequency domain (i.e., spectral power in frequency bands of interest). Spike amplitude decreased and ISI correlation increased in the mSNr versus the lSNr. Spike amplitude, signal-to-noise ratio, and ISI correlation increased in the VTA versus the mSNr. ISI correlation increased in the VTA versus the lSNr. Spectral power in the VTA increased versus: (1) the mSNr in the 20–30 Hz band and (2) the lSNr in the 20–40 Hz band. No significant differences were observed between structures for any other feature analyzed. Our results shed light on the heterogeneity of the SNr and suggest electrophysiological features to promote precise targeting of SNr subregions during stereotaxic surgery.

The Pathophysiology of Tics; An Evolving Story

Background:

Tics, defined as quick, rapid, sudden, recurrent, non-rhythmic motor movements or vocalizations are required components of Tourette Syndrome (TS) - a complex disorder characterized by the presence of fluctuating, chronic motor and vocal tics, and the presence of co-existing neuropsychological problems. Despite many advances, the underlying pathophysiology of tics/TS remains unknown.

Objective:

To address a variety of controversies surrounding the pathophysiology of TS. More specifically: 1) the configuration of circuits likely involved; 2) the role of inhibitory influences on motor control; 3) the classification of tics as either goal-directed or habitual behaviors; 4) the potential anatomical site of origin, e.g. cortex, striatum, thalamus, cerebellum, or other(s); and 5) the role of specific neurotransmitters (dopamine, glutamate, GABA, and others) as possible mechanisms (Abstract figure).

Methods:

Existing evidence from current clinical, basic science, and animal model studies are reviewed to provide: 1) an expanded understanding of individual components and the complex integration of the Cortico-Basal Ganglia-Thalamo-Cortical (CBGTC) circuit - the pathway involved with motor control; and 2) scientific data directly addressing each of the aforementioned controversies regarding pathways, inhibition, classification, anatomy, and neurotransmitters.

Conclusion:

Until a definitive pathophysiological mechanism is identified, one functional approach is to consider that a disruption anywhere within CBGTC circuitry, or a brain region inputting to the motor circuit, can lead to an aberrant message arriving at the primary motor cortex and enabling a tic. Pharmacologic modulation may be therapeutically beneficial, even though it might not be directed toward the primary abnormality.

Activation and blockade of α2-adrenoceptors in the prelimbic cortex regulate anxiety-like behaviors in hemiparkinsonian rats

At present, whether α₂-adrenoceptors in the prelimbic cortex (PrL) are involved in Parkinson's disease-related anxiety is unclear. We examined the effects of PrL α₂-adrenoceptors on anxiety-like behaviors in rats with unilateral 6-hydroxydopamine lesions of the medial forebrain bundle. Compared to the sham operation, the lesion induced anxiety-like responses as measured by the open field test and elevated plus-maze test. Intra-PrL injection of the α₂-adrenoceptor agonist clonidine (1.25, 2.5 or 5 μg/rat) produced anxiolytic effects in sham-operated and lesioned rats. Furthermore, intra-PrL injection of the α₂-adrenoceptor antagonist idazoxan (1, 2 or 4 μg/rat) induced anxiogenic effects in two groups of rats. The effective doses produced by clonidine and idazoxan in lesioned rats were higher than those in sham-operated rats. Neurochemical results showed that intra-PrL injection of clonidine (5 μg/rat) or idazoxan (4 μg/rat) decreased or increased dopamine (DA) and noradrenaline (NA) and serotonin (5-HT) levels in the medial prefrontal cortex (mPFC) and amygdala in sham-operated and lesioned rats, respectively. These results suggest that α₂-adrenoceptors in the PrL are involved in the regulation of anxiety-like behaviors, which is attributable to changes in DA, NA and 5-HT levels in the mPFC and amygdala after activation and blockade of α₂-adrenoceptors.

Controversies Surrounding the Pathophysiology of Tics

Tics are sudden, rapid, recurrent, nonrhythmic motor movements or vocalizations (phonic productions) that are commonly present in children and are required symptoms for the diagnosis of Tourette syndrome. Despite their frequency, the underlying pathophysiology of tics/Tourette syndrome remains unknown. In this review, we discuss a variety of controversies surrounding the pathophysiology of tics, including the following: Are tics voluntary or involuntary? What is the role of the premonitory urge? Are tics due to excess excitatory or deficient inhibition? Is it time to adopt the contemporary version of the cortico-basal ganglia-thalamocortical (CBGTC) circuit? and Do we know the primary abnormal neurotransmitter in Tourette syndrome? Data from convergent clinical and animal model studies support complex interactions among the various CBGTC sites and neurotransmitters. Advances are being made; however, numerous pathophysiologic questions persist.

Proliferation of Inhibitory Input to the Substantia Nigra in Experimental Parkinsonism

The substantia nigra pars reticulata (SNr) is one of the output nuclei of the basal ganglia (BG) and plays a vital role in movement execution. Death of dopaminergic neurons in the neighboring nucleus, the substantia nigra pars compacta (SNc), leads to Parkinson's disease. The ensuing dopamine depletion affects all BG nuclei. However, the long-term effects of dopamine depletion on BG output are less characterized. In this in vitro study, we applied electrophysiological and immunohistochemical techniques to investigate the long-term effects of dopamine depletion on GABAergic transmission to the SNr. The findings showed a reduction in firing rate and regularity in SNr neurons after unilateral dopamine depletion with 6-OHDA, which we associate with homeostatic mechanisms. The strength of the GABAergic synapses between the globus pallidus (GP) and the SNr increased but not their short-term dynamics. Consistent with this observation, there was an increase in the frequency and amplitude of spontaneous inhibitory synaptic events to SNr neurons. Immunohistochemistry revealed an increase in the density of vGAT-labeled puncta in dopamine depleted animals. Overall, these results may suggest that synaptic proliferation can explain how dopamine depletion augments GABAergic transmission in the SNr.

Cellular and Synaptic Dysfunctions in Parkinson's Disease: Stepping out of the Striatum

The basal ganglia (BG) are a collection of interconnected subcortical nuclei that participate in a great variety of functions, ranging from motor programming and execution to procedural learning, cognition, and emotions. This network is also the region primarily affected by the degeneration of midbrain dopaminergic neurons localized in the substantia nigra pars compacta (SNc). This degeneration causes cellular and synaptic dysfunctions in the BG network, which are responsible for the appearance of the motor symptoms of Parkinson’s disease. Dopamine (DA) modulation and the consequences of its loss on the striatal microcircuit have been extensively studied, and because of the discrete nature of DA innervation of other BG nuclei, its action outside the striatum has been considered negligible. However, there is a growing body of evidence supporting functional extrastriatal DA modulation of both cellular excitability and synaptic transmission. In this review, the functional relevance of DA modulation outside the striatum in both normal and pathological conditions will be discussed.

Parkinson's disease and pain: Modulation of nociceptive circuitry in a rat model of nigrostriatal lesion

Parkinson's disease (PD) is a neurodegenerative disorder that causes progressive dysfunction of dopaminergic and non-dopaminergic neurons, generating motor and nonmotor signs and symptoms. Pain is reported as the most bothersome nonmotor symptom in PD; however, pain remains overlooked and poorly understood. In this study, we evaluated the nociceptive behavior and the descending analgesia circuitry in a rat model of PD. Three independent experiments were performed to investigate: i) thermal nociceptive behavior; ii) mechanical nociceptive behavior and dopaminergic repositioning; and iii) modulation of the pain control circuitry. The rat model of PD, induced by unilateral striatal 6-hydroxydopamine (6-OHDA), did not interfere with thermal nociceptive responses; however, the mechanical nociceptive threshold was decreased bilaterally compared to that of naive or striatal saline-injected rats. This response was reversed by apomorphine or levodopa treatment. Striatal 6-OHDA induced motor impairments and reduced dopaminergic neuron immunolabeling as well as the pattern of neuronal activation (c-Fos) in the substantia nigra ipsilateral (IPL) to the lesion. In the midbrain periaqueductal gray (PAG), 6-OHDA-induced lesion increased IPL and decreased contralateral PAG GABAergic labeling compared to control. In the dorsal horn of the spinal cord, lesioned rats showed bilateral inhibition of enkephalin and μ-opioid receptor labeling. Taken together, we demonstrated that the unilateral 6-OHDA-induced PD model induces bilateral mechanical hypernociception, which is reversed by dopamine restoration, changes in the PAG circuitry, and inhibition of spinal opioidergic regulation, probably due to impaired descending analgesic control. A better understanding of pain mechanisms in PD patients is critical for developing better therapeutic strategies to improve their quality of life.

Acute dopamine receptor blockade in substantia nigra pars reticulata: a possible model for drug-induced Parkinsonism

Dopamine (DA) depletion modifies the firing pattern of neurons in the substantia nigra pars reticulata (SNr), shifting their mostly tonic firing toward irregularity and bursting, traits of pathological firing underlying rigidity and postural instability in Parkinson's disease (PD) patients and animal models of Parkinsonism (PS). Drug-induced Parkinsonism (DIP) represents 20-40% of clinical cases of PS, becoming a problem for differential diagnosis, and is still not well studied with physiological tools. It may co-occur with tardive dyskinesia. Here we use in vitro slice preparations including the SNr to observe drug-induced pathological firing by using drugs that most likely produce it, DA-receptor antagonists (SCH23390 plus sulpiride), to compare with firing patterns found in DA-depleted tissue. The hypothesis is that SNr firing would be similar under both conditions, a prerequisite to the proposal of a similar preparation to test other DIP-producing drugs. Firing was analyzed with three complementary metrics, showing similarities between DA depletion and acute DA-receptor blockade. Moreover, blockade of either nonselective cationic channels or Ca_v3 T-type calcium channels hyperpolarized the membrane and abolished bursting and irregular firing, silencing SNr neurons in both conditions. Therefore, currents generating firing in control conditions are in part responsible for pathological firing. Haloperidol, a DIP-producing drug, reproduced DA-receptor antagonist firing modifications. Since acute DA-receptor blockade induces SNr neuron firing similar to that found in the 6-hydroxydopamine model of PS, output basal ganglia neurons may play a role in generating DIP. Therefore, this study opens the way to test other DIP-producing drugs. NEW & NOTEWORTHY Dopamine (DA) depletion enhances substantia nigra pars reticulata (SNr) neuron bursting and irregular firing, hallmarks of Parkinsonism. Several drugs, including antipsychotics, antidepressants, and calcium channel antagonists, among others, produce drug-induced Parkinsonism. Here we show the first comparison between SNr neuron firing after DA depletion vs. firing found after acute blockade of DA receptors. It was found that firing in both conditions is similar, implying that pathological SNr neuron firing is also a physiological correlate of drug-induced Parkinsonism.

Chronic exposure to a glyphosate-containing pesticide leads to mitochondrial dysfunction and increased reactive oxygen species production in Caenorhabditis elegans

Glyphosate-containing herbicides are among the most widely-used in the world. Although glyphosate itself is relatively non-toxic, growing evidence suggests that commercial herbicide formulations may lead to increased oxidative stress and mitochondrial inhibition. In order to assess these mechanisms in vivo, we chronically (24h) exposed Caenorhabditis elegans to various concentrations of the glyphosate-containing herbicide TouchDown (TD). Following TD exposure, we evaluated the function of specific mitochondrial electron transport chain complexes. Initial oxygen consumption studies demonstrated inhibition in mid- and high-TD concentration treatment groups compared to controls. Results from tetramethylrhodamine ethyl ester and ATP assays indicated reductions in the proton gradient and ATP levels, respectively. Additional studies were designed to determine whether TD exposure resulted in increased reactive oxygen species (ROS) production. Data from hydrogen peroxide, but not superoxide or hydroxyl radical, assays showed statistically significant increases in this specific ROS. Taken together, these data indicate that exposure of Caenorhabditis elegans to TD leads to mitochondrial inhibition and hydrogen peroxide production.

Nigral injection of a proteasomal inhibitor, lactacystin, induces widespread glial cell activation and shows various phenotypes of Parkinson's disease in young and adult mouse

Proteinaceous inclusions, called Lewy bodies, are used as a pathological hallmark for Parkinson's disease (PD). Lewy bodies contain insoluble α-synuclein (aSyn) and many other ubiquitinated proteins, suggesting a role for protein degradation system failure in the PD pathogenesis. Indeed, proteasomal dysfunction has been linked to PD but commonly used in vivo toxin models, such as 6-OHDA or MPTP, do not have a significant effect on the proteasomal system or protein aggregation. Therefore, we wanted to study the characteristics of a proteasomal inhibitor, lactacystin, as a PD model on young and adult mice. To study this, we performed stereotactic microinjection of lactacystin above the substantia nigra pars compacta in young (2 month old) and adult (12-14 month old) C57Bl/6 mice. Motor behavior was measured by locomotor activity and cylinder tests, and the markers of neuroinflammation, aSyn, and dopaminergic system were assessed by immunohistochemistry and HPLC. We found that lactacystin induced a Parkinson's disease-like motor phenotype 5-7 days after injection in young and adult mice, and this was associated with widespread neuroinflammation based on glial cell markers, aSyn accumulation in substantia nigra, striatal dopamine decrease, and loss of dopaminergic cell bodies in the substantia nigra and terminals in the striatum. When comparing young and adult mice, adult mice were more sensitive for dopaminergic degeneration after lactacystin injection that further supports the use of adult mice instead of young when modeling neurodegeneration. Our data showed that lactacystin is useful in modeling various aspects of Parkinson's disease, and taken together, our findings emphasize the role of a protein degradation deficit in Parkinson's disease pathology, and support the use of proteasomal inhibitors as Parkinson's disease models.

Transient Activation of GABAB Receptors Suppresses SK Channel Currents in Substantia Nigra Pars Compacta Dopaminergic Neurons

Dopaminergic (DA) neurons in the substantia nigra pars compacta (SNc) are richly innervated by GABAergic neurons. The postsynaptic effects of GABA on SNc DA neurons are mediated by a mixture of GABAA and GABAB receptors. Although activation of GABAA receptors inhibits spike generation, the consequences of GABAB receptor activation are less well characterized. To help fill this gap, perforated patch recordings were made from young adult mouse SNc DA neurons. Sustained stimulation of GABAB receptors hyperpolarized SNc DA neurons, as previously described. However, transient stimulation of GABAB receptors by optical uncaging of GABA did not; rather, it reduced the opening of small-conductance, calcium-activated K+ (SK) channels and increased the irregularity of spiking. This modulation was attributable to inhibition of adenylyl cyclase and protein kinase A. Thus, because suppression of SK channel activity increases the probability of burst spiking, transient co-activation of GABAA and GABAB receptors could promote a pause-burst pattern of spiking.

Diversity and development of local inhibitory and excitatory neurons associated with dopaminergic nuclei

For regulation of voluntary movement and motivation the midbrain dopaminergic system receives input from a variety of brain regions. Often this input is mediated by local non-dopaminergic neurons within or closely associated with the dopaminergic nuclei. In addition to the dopaminergic neurons, some of these non-dopaminergic neurons also send functionally important output from the ventral midbrain to forebrain targets. The aim of this review is to introduce subtypes of GABAergic and glutamatergic neurons, which are located in the dopaminergic nuclei or the adjacent brainstem and are important for the regulation of the dopaminergic pathways. In addition, we discuss recent studies beginning to reveal mechanisms for their development, which may hold the key to understanding the diversity of these neurons.

Additional noradrenergic depletion aggravates forelimb akinesia and abnormal subthalamic nucleus activity in a rat model of Parkinson's disease

AIMS

This study aims to identify the contribution of additional noradrenergic depletion to forelimb akinesia and abnormal subthalamic nucleus (STN) firing activity in Parkinson's disease (PD).

MAIN METHODS

Forelimb akinesia behaviors were tested in awake rats with noradrenergic N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4) lesions, unilateral 6-hydroxydopamine (6-OHDA) lesions in the substantia nigra pars compacta (SNc) and with combined 6-OHDA and DSP-4 lesions. STN extracellular single-unit and local field potential (LFP) activities were examined in the animals that were anesthetized with urethane.

KEY FINDINGS

The adjusting steps and the contralateral touches of rats in the forelimb akinesia behavior tests were markedly inhibited by a further noradrenergic lesion with DSP-4 in 6-OHDA+DSP-4-lesioned group when compared with those of 6-OHDA-lesioned animals (P<0.05 for all comparisons). Meanwhile, the neuronal firing pattern of STN also changed significantly towards more bursty in 6-OHDA + DSP-4-lesioned group (P <0 .05). Compared with 6-OHDA-lesioned animals, an additional noradrenergic lesion increased the 0.3-2.5 Hz oscillatory activity and the spike power of STN neurons (P < 0.01 for both comparisons), and strengthened the synchronized oscillation between subthalamic neuronal firing and LFP activity in 6-OHDA + DSP-4-lesioned group (P < 0.01).

SIGNIFICANCE

The results provide evidence to support the correlation between noradrenergic depletion and the further exaggerated dysfunction of STN electrical activity in PD and suggest that an aberrant noradrenergic system might play a specific role in the motor deficits of PD.

Electrophysiological evidence for functionally distinct neuronal populations in the human substantia nigra

The human substantia nigra (SN) is thought to consist of two functionally distinct neuronal populations—dopaminergic (DA) neurons in the pars compacta subregion and GABA-ergic neurons in the pars reticulata subregion. However, a functional dissociation between these neuronal populations has not previously been demonstrated in the awake human. Here we obtained microelectrode recordings from the SN of patients undergoing deep brain stimulation (DBS) surgery for Parkinson's disease as they performed a two-alternative reinforcement learning task. Following positive feedback presentation, we found that putative DA and GABA neurons demonstrated distinct temporal dynamics. DA neurons demonstrated phasic increases in activity (250–500 ms post-feedback) whereas putative GABA neurons demonstrated more delayed and sustained increases in activity (500–1000 ms post-feedback). These results provide the first electrophysiological evidence for a functional dissociation between DA and GABA neurons in the human SN. We discuss possible functions for these neuronal responses based on previous findings in human and animal studies.

L-DOPA-induced dyskinesia in a rat model of Parkinson's disease is associated with the fluctuational release of norepinephrine in the sensorimotor striatum

L-3,4-dihydroxyphenylalanine (L-DOPA)-induced dyskinesia (LID) is the most common complication of standard L-DOPA therapy for Parkinson's disease experienced by most parkinsonian patients. LID is associated with disruption of dopaminergic homeostasis in basal ganglia following L-DOPA administration. Norepinephrine (NE) is another important catecholaminergic neurotransmitter that is also believed to be involved in the pathogenesis of LID. This study compared NE release in the ipsilateral sensorimotor striatum of dyskinetic and nondyskinetic 6-hydroxydopamine-lesioned hemiparkinsonian rats treated chronically with L-DOPA. After L-DOPA injection, the time-course curves of NE levels in the sensorimotor striatum were significantly different between dyskinetic and nondyskinetic rats. Several metabolic kinetic parameters of NE levels were also differentially expressed between the two groups. In comparison with nondyskinetic rats, the ΔCmax of NE was significantly higher in dyskinetic rats, whereas Tmax and t1/2 of NE were significantly shorter. Intrastriatal perfusion of NE into the lesioned sensorimotor striatum revealed a moderate dyskinesia in dyskinetic rats, which was similar to the dyskinetic behavior after L-DOPA administration. The L-DOPA-related dyskinetic behavior was inhibited significantly by a further pretreatment of noradrenergic neurotoxin N-​(2-​chloroethyl)​-​N-​ethyl-​2-​bromobenzylamine or intrastriatal administration of the α2 -adrenoceptor antagonist idazoxan, accompanied by significant changes in metabolic kinetic parameters of NE in the sensorimotor striatum. The results provide evidence to support the correlation between abnormal NE neurotransmission and the induction of LID and suggest that the aberrant change of the quantitative and temporal releasing of NE in the sensorimotor striatum might play an important role in the pathogenesis of LID.

The inhibitory microcircuit of the substantia nigra provides feedback gain control of the basal ganglia output

Dysfunction of the basal ganglia produces severe deficits in the timing, initiation, and vigor of movement. These diverse impairments suggest a control system gone awry. In engineered systems, feedback is critical for control. By contrast, models of the basal ganglia highlight feedforward circuitry and ignore intrinsic feedback circuits. In this study, we show that feedback via axon collaterals of substantia nigra projection neurons control the gain of the basal ganglia output. Through a combination of physiology, optogenetics, anatomy, and circuit mapping, we elaborate a general circuit mechanism for gain control in a microcircuit lacking interneurons. Our data suggest that diverse tonic firing rates, weak unitary connections and a spatially diffuse collateral circuit with distinct topography and kinetics from feedforward input is sufficient to implement divisive feedback inhibition. The importance of feedback for engineered systems implies that the intranigral microcircuit, despite its absence from canonical models, could be essential to basal ganglia function. DOI: http://dx.doi.org/10.7554/eLife.02397.001.

Short- and long-term unilateral 6-hydroxydopamine lesions in rats show different changes in characteristics of spontaneous firing of substantia nigra pars reticulata neurons

The unilateral 6-hydroxydopamine (6-OHDA) lesion of the medial forebrain bundle induces hemiparkinsonism in rats and is a well established animal model of Parkinson's disease. In this study, we assessed the spontaneous activity of substantia nigra pars reticulata (SNr) neurons in unilateral 6-OHDA- or sham-treated rats. Extracellular single cell recordings revealed a bilaterally decreased firing rate in short-term 6-OHDA-lesioned rats (8-10 weeks post lesion) while no rate differences were evident in long-term lesioned animals (5-8 months post lesion) in vivo under chloral hydrate anaesthesia. However, firing pattern of the SNr neurons (indicated by interspike interval (ISI) histogram parameters: coefficient of variation, skewness and kurtosis) was significantly altered only after long-term lesion: 53.8 % of the recorded cells in the ipsilateral 6-OHDA-lesioned SNr fired in a bursting pattern (compared to 5.9-16.7 % in contralateral SNr or sham controls). Additionally, behavioural effects of the lesion were assessed 4 weeks post lesion by the forelimb adjusting stepping test. A decreased number of adjusting steps with the contralateral forepaw, as well as an increased performance with the ipsilateral paw was found for the 6-OHDA-lesioned rats as compared to sham controls. Furthermore, stepping values were negatively correlated with the ISI parameters after long-term lesion, while there were no correlations with the short-term groups. Firing rate was not correlated regardless of the time frame. In conclusion, long-term changes in firing pattern may represent a neuronal correlate of the 6-OHDA-induced hemiparkinsonism and may be useful for the interpretation of 6-OHDA-induced motor deficits and compensatory mechanisms as well.

Exposure to glyphosate- and/or Mn/Zn-ethylene-bis-dithiocarbamate-containing pesticides leads to degeneration of γ-aminobutyric acid and dopamine neurons in Caenorhabditis elegans

Previous studies demonstrate a positive correlation between pesticide usage and Parkinson's disease (PD), which preferentially targets dopaminergic (DAergic) neurons. In order to examine the potential relationship between two common pesticides and specific neurodegeneration, we chronically (24 h) or acutely (30 min) exposed two Caenorhabditis elegans (C. elegans) strains to varying concentrations (LC(25), LC(50) or LC(75)) of TouchDown(®) (TD) as percent active ingredient (glyphosate), or Mancozeb(®) (MZ) as percent active ingredient (manganese/zinc ethylene-bis-dithiocarbamate). Furthermore, to more precisely model environmental exposure, worms were also exposed to TD for 30 min, followed by 30-min incubation with varying MZ concentrations. Previous data from out lab suggested general neuronal degeneration using the worm strain NW1229 (pan-neuronal//green fluorescent protein (GFP) construct). To determine whether distinct neuronal groups were preferentially affected, we specifically used EG1285 (GABAergic neurons//GFP construct) and BZ555 (DAergic neurons//GFP construct) worms to verify GABAergic and DAergic neurodegeneration, respectively. Results indicated a statistically significant decrease, when compared to controls (CN), in number of green pixels associated with GABAergic neurons in both chronic (*P < 0.05) and acute (*P < 0.05) treatment paradigms. Analysis of the BZ555 worms indicated a statistically significant decrease (*P < 0.05) in number of green pixels associated with DAergic neurons in both treatment paradigms (chronic and acute) when compared to CN. Taken together, our data suggest that exposure to TD and/or MZ promotes neurodegeneration in both GABAergic and DAergic neurons in the model organism C. elegans.

The effects of nigrostriatal dopamine depletion on the thalamic parafascicular nucleus

Neuronal loss in Parkinson's disease (PD) is seen in a number of brain regions in addition to the substantia nigra (SN). Among these is the thalamic parafascicular nucleus (PF), which sends glutamatergic projections to the striatum and receives GABAergic inputs from the SN. Recent data suggest that lesions of nigrostriatal dopamine axons cause a loss of PF neurons, which has been interpreted to suggest that the PF cell loss seen in PD is secondary to dopamine denervation. However, the extent of a PF dopamine innervation in the rat is unclear, and it is possible that PF cell loss in parkinsonism is independent of nigrostriatal dopamine degeneration. We characterized the dopamine innervation of the PF in the rat and determined if 6-hydroxydopamine SN lesions cause PF neuron degeneration. Dual-label immunohistochemistry revealed that almost all tyrosine hydroxylase-immunoreactive (TH-ir) axons in the PF also expressed dopamine-beta-hydroxylase and were therefore noradrenergic or adrenergic. Moreover, an antibody directed against dopamine revealed only very rare PF dopaminergic axons. Retrograde-tract tracing-immunohistochemistry did not uncover an innervation of the PF from midbrain dopamine neurons. Nigrostriatal dopamine neuron lesions did not elicit degeneration of PF cells, as reflected by a lack of FluoroJade C staining. Similarly, neither unilateral 6-OHDA lesions of nigrostriatal axons nor the dorsal noradrenergic bundle decreased the number of PF neurons or the number of PF neurons retrogradely-labeled from the striatum. These data suggest that the loss of thalamostriatal PF neurons in Parkinson's Disease is a primary event rather than secondary to nigrostriatal dopamine degeneration.

Differential effects of activating D1 and D2 receptors on electrophysiology of neostriatal neurons in a rat model of Parkinson's disease induced by paraquat and maneb

Neostriatum plays an important role in the pathophysiology of Parkinson's disease (PD). However, the changes of sensitivity of dopamine receptors of neostriatal neurons in PD have been less addressed in vivo. In the present study, systemic exposure to paraquat and maneb induced Parkinsonian symptoms and neuronal loss of substantia nigra pars compacta. Using single-unit recording methods, three types of neostriatal neurons were recorded including medium spiny-like neurons, large aspiny-like neurons and fast-spiking interneurons. In the exposed rats, increased firing activity of neostriatal neurons was revealed when compared to control rats. Following D1 receptor agonist, SKF38393 and D2 receptor agonist, LY171555 iontophoretically administrated respectively, effects of increase and decrease in firing activity were both observed in neostriatal neurons. However, stronger inhibitory effects of activating D1 receptors and weaker excitatory effects of activating D2 receptors were found in the exposed rats as compared to controls. It indicated that differential changes of sensitivity of D1 and D2 receptors in Parkinson's disease were related to the modulation of the imbalance between D1-receptor-dependent striatonigral direct pathway and D2-receptor-dependent striatopallidal indirect pathway. Our results illustrate the electrophysiological changes of in vivo neostriatal neurons in Parkinson's disease, thereby providing insight into the regulatory mechanisms of dopamine-mediated physiology.