Pathophysiological signatures of functional connectomics in parkinsonian and dyskinetic striatal microcircuits

Stimulus encoding by specific inactivation of cortical neurons

Neuronal ensembles are groups of neurons with correlated activity associated with sensory, motor, and behavioral functions. To explore how ensembles encode information, we investigated responses of visual cortical neurons in awake mice using volumetric two-photon calcium imaging during visual stimulation. We identified neuronal ensembles employing an unsupervised model-free algorithm and, besides neurons activated by the visual stimulus (termed "onsemble"), we also find neurons that are specifically inactivated (termed "offsemble"). Offsemble neurons showed faster calcium decay during stimuli, suggesting selective inhibition. In response to visual stimuli, each ensemble (onsemble+offsemble) exhibited small trial-to-trial variability, high orientation selectivity, and superior predictive accuracy for visual stimulus orientation, surpassing the sum of individual neuron activity. Thus, the combined selective activation and inactivation of cortical neurons enhances visual encoding as an emergent and distributed neural code.

Reversal of pathological motor behavior in a model of Parkinson's disease by striatal dopamine uncaging

Motor deficits observed in Parkinson's disease (PD) are caused by the loss of dopaminergic neurons and the subsequent dopamine depletion in different brain areas. The most common therapy to treat motor symptoms for patients with this disorder is the systemic intake of L-DOPA that increases dopamine levels in all the brain, making it difficult to discern the main locus of dopaminergic action in the alleviation of motor control. Caged compounds are molecules with the ability to release neuromodulators locally in temporary controlled conditions using light. In the present study, we measured the turning behavior of unilateral dopamine-depleted mice before and after dopamine uncaging. The optical delivery of dopamine in the striatum of lesioned mice produced contralateral turning behavior that resembled, to a lesser extent, the contralateral turning behavior evoked by a systemic injection of apomorphine. Contralateral turning behavior induced by dopamine uncaging was temporarily tied to the transient elevation of dopamine concentration and was reversed when dopamine decreased to pathological levels. Remarkably, contralateral turning behavior was tuned by changing the power and frequency of light stimulation, opening the possibility to modulate dopamine fluctuations using different light stimulation protocols. Moreover, striatal dopamine uncaging recapitulated the motor effects of a low concentration of systemic L-DOPA, but with better temporal control of dopamine levels. Finally, dopamine uncaging reduced the pathological synchronization of striatal neuronal ensembles that characterize unilateral dopamine-depleted mice. We conclude that optical delivery of dopamine in the striatum resembles the motor effects induced by systemic injection of dopaminergic agonists in unilateral dopamine-depleted mice. Future experiments using this approach could help to elucidate the role of dopamine in different brain nuclei in normal and pathological conditions.

Insulin decreases epileptiform activity in rat layer 5/6 prefrontal cortex in vitro

Accumulating evidence indicates that insulin-mediated signaling in the brain may play important roles in regulating neuronal function. Alterations to insulin signaling are associated with the development of neurological disorders including Alzheimer's disease and Parkinson's disease. Also, hyperglycemia and insulin resistance have been associated with seizure activity and brain injury. In recent work, we found that insulin increased inhibitory GABA_A -mediated tonic currents in the prefrontal cortex (PFC). In this work, we used local field potential recordings and calcium imaging to investigate the effect of insulin on seizure-like activity in PFC slices. Seizure-like events (SLEs) were induced by perfusing the slices with magnesium-free artificial cerebrospinal fluid (ACSF) containing the proconvulsive compound 4-aminopyridine (4-AP). We found that insulin decreased the frequency, amplitude, and duration of SLEs as well as the synchronic activity of PFC neurons evoked by 4-AP. These insulin effects were mediated by the PI3K/Akt signaling pathway and mimicked by gaboxadol (THIP), a δ GABA_A receptor agonist. The effect of insulin on the number of SLEs was partially blocked by L-655,708, an inverse agonist with high selectivity for GABA_A receptors containing the α5 subunit. Our results suggest that insulin reduces neuronal excitability by an increase of GABAergic tonic currents. The physiological relevance of these findings is discussed.

Exercise improves behavioral dysfunction and inhibits the spontaneous excitatory postsynaptic current of D2-medium spiny neurons

The abnormal function of striatal medium spiny neurons (MSNs) leads to the excitation-inhibition imbalance of the basal ganglia, which is an important pathogenic factor of Parkinson's disease (PD). Exercise improves the dysfunction of basal ganglia through neuroprotective and neuroreparative effects, which may be related to the functional changes of expresses D2 receptors MSNs (D2-MSNs). In this study, D2-Cre mice were selected as the research objects, the PD model was induced by unilateral injection of 6-hydroxydopamine (6-OHDA) in the striatum, and the 4-week treadmill training method was used for exercise intervention. Using optogenetics and behavioral tests, we determined that the average total movement distance of PD and PD + Ex groups was significantly lower than that of the Control group, while that of the PD + Ex and PD + Laser groups was significantly higher than that of the PD group, and the two intervention methods of exercise and optogenetic-stimulation of the D2-MSNs had basically similar effects on improving the autonomic behavior of PD mice. To further investigate the cellular mechanisms, whole-cell patch clamp recordings were carried out on D2-MSNs. We found that exercise decreased the frequency and amplitude of spontaneous excitatory postsynaptic current (sEPSC) and increased the paired-pulse radio of D2-MSNs while leaving basic electrophysiological properties of MSNs unaffected. Combined with behavioral improvement and enhanced D2R protein expression, our findings suggest the inhibited sEPSC of D2-MSNs may contribute to the behavioral improvement after exercise.

Dimensionality reduction and recurrence analysis reveal hidden structures of striatal pathological states

A pipeline is proposed here to describe different features to study brain microcircuits on a histological scale using multi-scale analyses, including the uniform manifold approximation and projection (UMAP) dimensional reduction technique and modularity algorithm to identify neuronal ensembles, Runs tests to show significant ensembles activation, graph theory to show trajectories between ensembles, and recurrence analyses to describe how regular or chaotic ensembles dynamics are. The data set includes ex-vivo NMDA-activated striatal tissue in control conditions as well as experimental models of disease states: decorticated, dopamine depleted, and L-DOPA-induced dyskinetic rodent samples. The goal was to separate neuronal ensembles that have correlated activity patterns. The pipeline allows for the demonstration of differences between disease states in a brain slice. First, the ensembles were projected in distinctive locations in the UMAP space. Second, graphs revealed functional connectivity between neurons comprising neuronal ensembles. Third, the Runs test detected significant peaks of coactivity within neuronal ensembles. Fourth, significant peaks of coactivity were used to show activity transitions between ensembles, revealing recurrent temporal sequences between them. Fifth, recurrence analysis shows how deterministic, chaotic, or recurrent these circuits are. We found that all revealed circuits had recurrent activity except for the decorticated circuits, which tended to be divergent and chaotic. The Parkinsonian circuits exhibit fewer transitions, becoming rigid and deterministic, exhibiting a predominant temporal sequence that disrupts transitions found in the controls, thus resembling the clinical signs of rigidity and paucity of movements. Dyskinetic circuits display a higher recurrence rate between neuronal ensembles transitions, paralleling clinical findings: enhancement in involuntary movements. These findings confirm that looking at neuronal circuits at the histological scale, recording dozens of neurons simultaneously, can show clear differences between control and diseased striatal states: "fingerprints" of the disease states. Therefore, the present analysis is coherent with previous ones of striatal disease states, showing that data obtained from the tissue are robust. At the same time, it adds heuristic ways to interpret circuitry activity in different states.

Synaptic determinants of cholinergic interneurons hyperactivity during parkinsonism

Parkinson’s disease is a neurodegenerative ailment generated by the loss of dopamine in the basal ganglia, mainly in the striatum. The disease courses with increased striatal levels of acetylcholine, disrupting the balance among these modulatory transmitters. These modifications disturb the excitatory and inhibitory balance in the striatal circuitry, as reflected in the activity of projection striatal neurons. In addition, changes in the firing pattern of striatal tonically active interneurons during the disease, including cholinergic interneurons (CINs), are being searched. Dopamine-depleted striatal circuits exhibit pathological hyperactivity as compared to controls. One aim of this study was to show how striatal CINs contribute to this hyperactivity. A second aim was to show the contribution of extrinsic synaptic inputs to striatal CINs hyperactivity. Electrophysiological and calcium imaging recordings in Cre-mice allowed us to evaluate the activity of dozens of identified CINs with single-cell resolution in ex vivo brain slices. CINs show hyperactivity with bursts and silences in the dopamine-depleted striatum. We confirmed that the intrinsic differences between the activity of control and dopamine-depleted CINs are one source of their hyperactivity. We also show that a great part of this hyperactivity and firing pattern change is a product of extrinsic synaptic inputs, targeting CINs. Both glutamatergic and GABAergic inputs are essential to sustain hyperactivity. In addition, cholinergic transmission through nicotinic receptors also participates, suggesting that the joint activity of CINs drives the phenomenon; since striatal CINs express nicotinic receptors, not expressed in striatal projection neurons. Therefore, CINs hyperactivity is the result of changes in intrinsic properties and excitatory and inhibitory inputs, in addition to the modification of local circuitry due to cholinergic nicotinic transmission. We conclude that CINs are the main drivers of the pathological hyperactivity present in the striatum that is depleted of dopamine, and this is, in part, a result of extrinsic synaptic inputs. These results show that CINs may be a main therapeutic target to treat Parkinson’s disease by intervening in their synaptic inputs.

Translational neuronal ensembles: Neuronal microcircuits in psychology, physiology, pharmacology and pathology

Multi-recording techniques show evidence that neurons coordinate their firing forming ensembles and that brain networks are made by connections between ensembles. While “canonical” microcircuits are composed of interconnected principal neurons and interneurons, it is not clear how they participate in recorded neuronal ensembles: “groups of neurons that show spatiotemporal co-activation”. Understanding synapses and their plasticity has become complex, making hard to consider all details to fill the gap between cellular-synaptic and circuit levels. Therefore, two assumptions became necessary: First, whatever the nature of the synapses these may be simplified by “functional connections”. Second, whatever the mechanisms to achieve synaptic potentiation or depression, the resultant synaptic weights are relatively stable. Both assumptions have experimental basis cited in this review, and tools to analyze neuronal populations are being developed based on them. Microcircuitry processing followed with multi-recording techniques show temporal sequences of neuronal ensembles resembling computational routines. These sequences can be aligned with the steps of behavioral tasks and behavior can be modified upon their manipulation, supporting the hypothesis that they are memory traces. In vitro, recordings show that these temporal sequences can be contained in isolated tissue of histological scale. Sequences found in control conditions differ from those recorded in pathological tissue obtained from animal disease models and those recorded after the actions of clinically useful drugs to treat disease states, setting the basis for new bioassays to test drugs with potential clinical use. These findings make the neuronal ensembles theoretical framework a dynamic neuroscience paradigm.

Striatal neuronal ensembles reveal differential actions of amantadine and clozapine to ameliorate mice L-DOPA-induced dyskinesia

Amantadine and clozapine have proved to reduce abnormal involuntary movements (AIMs) in preclinical and clinical studies of L-DOPA-Induced Dyskinesias (LID). Even though both drugs decrease AIMs, they may have different action mechanisms by using different receptors and signaling profiles. Here we asked whether there are differences in how they modulate neuronal activity of multiple striatal neurons within the striatal microcircuit at histological level during the dose-peak of L-DOPA in ex-vivo brain slices obtained from dyskinetic mice. To answer this question, we used calcium imaging to record the activity of dozens of neurons of the dorsolateral striatum before and after drugs administration in vitro. We also developed an analysis framework to extract encoding insights from calcium imaging data by quantifying neuronal activity, identifying neuronal ensembles by linking neurons that coactivate using hierarchical cluster analysis and extracting network parameters using Graph Theory. The results show that while both drugs reduce LIDs scores behaviorally in a similar way, they have several different and specific actions on modulating the dyskinetic striatal microcircuit. The extracted features were highly accurate in separating amantadine and clozapine effects by means of principal components analysis (PCA) and support vector machine (SVM) algorithms. These results predict possible synergistic actions of amantadine and clozapine on the dyskinetic striatal microcircuit establishing a framework for a bioassay to test novel antidyskinetic drugs or treatments in vitro.

Dopamine D2 and Adenosine A2A Receptors Interaction on Ca2+ Current Modulation in a Rodent Model of Parkinsonism

Adenosine A1 and A2A receptors are expressed in striatal projection neurons (SPNs). A1 receptors are located in direct (dSPN) and indirect SPNs (iSNP). A2A receptors are only present in iSPNs. Dopamine D2 receptors are also expressed in iSPNs and interactions between D2 and A2A receptors have received attention. iSPNs activity increases during parkinsonism (PD) and A2A receptors may be responsible by enhancing Ca2+ currents (iCa2+). Therefore, A2A receptors blockade is a therapeutic approach. We asked whether A2A receptors need the interaction with D2 receptors (D2R) to exert their actions. By using isolated and identified iSPNs to avoid indirect influences, we show that D2R action habilitates A2A receptors (A2AR) modulation. iCa2+ through voltage gated Ca2+ channels (CaV) was used as a signal to observe this interaction. Voltage-clamp recordings in acutely dissociated iSPNs, current-clamp recordings in slices and calcium imaging in transgenic A2A-Cre mice, showed that D2R reduction in iCa2+ endows A2AR to restore iCa2+ on iSPNs showing an antagonistic interaction between D2 and A2A receptors. A2A receptors were blocked by the antagonist istradefylline, however, this blockade differed in control and dopamine-depleted iSPNs: istradefylline reduced D2R modulation in parkinsonian animals as compared to controls. Calcium imaging recordings show that istradefylline occludes D2R actions in the parkinsonian circuitry and this effect depends on the order of drugs application. Thus, while D2 activation enables A2A receptors action, blockade of A2AR induces a reduction in the action of D2 agonists, confirming a complex interaction.

Summary Statement

A2A receptor required previous D2 receptor activation to modulate Ca2+ currents. Istradefylline decreases pramipexole modulation on Ca2+ currents. Istradefylline reduces A2A + neurons activity in striatial microcircuit, but pramipexole failed to further reduce neuronal activity.

Long-term stability of cortical ensembles

Neuronal ensembles, coactive groups of neurons found in spontaneous and evoked cortical activity, are causally related to memories and perception, but it is still unknown how stable or flexible they are over time. We used two-photon multiplane calcium imaging to track over weeks the activity of the same pyramidal neurons in layer 2/3 of the visual cortex from awake mice and recorded their spontaneous and visually evoked responses. Less than half of the neurons remained active across any two imaging sessions. These stable neurons formed ensembles that lasted weeks, but some ensembles were also transient and appeared only in one single session. Stable ensembles preserved most of their neurons for up to 46 days, our longest imaged period, and these 'core' cells had stronger functional connectivity. Our results demonstrate that neuronal ensembles can last for weeks and could, in principle, serve as a substrate for long-lasting representation of perceptual states or memories.

Morphological and Calcium Signaling Alterations of Neuroglial Cells in Cerebellar Cortical Dysplasia Induced by Carmustine

Cortical dysplasias are alterations in the organization of the layers of the brain cortex due to problems in neuronal migration during development. The neuronal component has been widely studied in experimental models of cortical dysplasias. In contrast, little is known about how glia are affected. In the cerebellum, Bergmann glia (BG) are essential for neuronal migration during development, and in adult they mediate the control of fine movements through glutamatergic transmission. The aim of this study was to characterize the morphology and intracellular calcium dynamics of BG and astrocytes from mouse cerebellum and their modifications in a model of cortical dysplasia induced by carmustine (BCNU). Carmustine-treated mice were affected in their motor coordination and balance. Cerebellar dysplasias and heterotopias were more frequently found in lobule X. Morphology of BG cells and astrocytes was affected, as were their spontaneous [Ca²⁺]ⁱ transients in slice preparation and in vitro.

Configuration and dynamics of dominant inspiratory multineuronal activity patterns during eupnea and gasping generation in vitro

The pre-Bötzinger complex (preBötC), located within the ventral respiratory column, produces inspiratory bursts in varying degrees of synchronization/amplitude. This wide range of population burst patterns reflects the flexibility of the preBötC neurons, which is expressed in variations in the onset/offset times of their activations and their activity during the population bursts, with respiratory neurons exhibiting a large cycle-to-cycle timing jitter both at the population activity onset and at the population activity peak, suggesting that respiratory neurons are stochastically activated before and during the inspiratory bursts. However, it is still unknown whether this stochasticity is maintained while evaluating the coactivity of respiratory neuronal ensembles. Moreover, the preBötC topology also remains unknown. In this study, by simultaneously recording tens of preBötC neurons and using coactivation analysis during the inspiratory periods, we found that the preBötC has a scale-free configuration (mixture of not many highly connected nodes, hubs, with abundant poorly connected elements) exhibiting the rich-club phenomenon (hubs more likely interconnected with each other). PreBötC neurons also produce multineuronal activity patterns (MAPs) that are highly stable and change during the hypoxia-induced reconfiguration. Moreover, preBötC contains a coactivating core network shared by all its MAPs. Finally, we found a distinctive pattern of sequential coactivation of core network neurons at the beginning of the inspiratory periods, indicating that, when evaluated at the multicellular level, the coactivation of respiratory neurons seems not to be stochastic.NEW & NOTEWORTHY By means of multielectrode recordings of preBötC neurons, we evaluated their configuration in normoxia and hypoxia, finding that the preBötC exhibits a scale-free configuration with a rich-club phenomenon. preBötC neurons produce multineuronal activity patterns that are highly stable but change during hypoxia. The preBötC contains a coactivating core network that exhibit a distinctive pattern of coactivation at the beginning of inspirations. These results reveal some network basis of inspiratory rhythm generation and its reconfiguration during hypoxia.

Nicotine Increases Spontaneous Glutamate Release in the Rostromedial Tegmental Nucleus

The rostromedial tegmental nucleus (RMTg) is a bilateral structure localized in the brainstem and comprise of mainly GABAergic neurons. One of the main functions of the RMTg is to regulate the activity of dopamine neurons of the mesoaccumbens pathway. Therefore, the RMTg has been proposed as a modulator of the reward system and adaptive behaviors associated to reward learning. The RMTg receives an important glutamatergic input from the lateral habenula. Also, it receives cholinergic inputs from the laterodorsal and pedunculopontine tegmental nuclei. Previously, it was reported that nicotine increases glutamate release, evoked by electric stimulation, in the RMTg nucleus. However, the mechanisms by which nicotine induces this effect were not explored. In the present work, we performed electrophysiological experiments in brainstem slices to study the effect of nicotine on spontaneous excitatory postsynaptic currents recorded from immunocytochemically identified RMTg neurons. Also, we used calcium imaging techniques to explore the effects of nicotine on multiple RMTg neurons simultaneously. We found that nicotine promotes the persistent release of glutamate through the activation of α7 nicotinic acetylcholine receptors present on glutamatergic afferents and by a mechanism involving calcium release from intracellular stores. Through these mechanisms, nicotine increases the excitability and synchronizes the activity of RMTg neurons. Our results suggest that the RMTg nucleus mediates the noxious effects of the nicotine, and it could be a potential therapeutic target against tobacco addiction.

Spontaneous Activity of Neuronal Ensembles in Mouse Motor Cortex: Changes after GABAergic Blockade

The mouse motor cortex exhibits spontaneous activity in the form of temporal sequences of neuronal ensembles in vitro without the need of tissue stimulation. These neuronal ensembles are defined as groups of neurons with a strong correlation between its firing patterns, generating what appears to be a predetermined neural conduction mode that needs study. Each ensemble is commonly accompanied by one or more parvalbumin expressing neurons (PV+) or fast spiking interneurons. Many of these interneurons have functional connections between them, helping to form a circuit configuration similar to a small-world network. However, rich club metrics show that most connected neurons are neurons not expressing parvalbumin, mainly pyramidal neurons (PV-) suggesting feed-forward propagation through pyramidal cells. Ensembles with PV+ neurons are connected to these hubs. When ligand-gated fast GABAergic transmission is blocked, temporal sequences of ensembles collapse into a unique synchronous and recurrent ensemble, showing the need of inhibition for coding cortical spontaneous activity. This new ensemble has a duration and electrophysiological characteristics of brief recurrent interictal epileptiform discharges (IEDs) composed by the coactivity of both PV- and PV+ neurons, demonstrating that GABA transmission impedes its occurrence. Synchronous ensembles are clearly divided into two clusters one of them lasting longer and mainly composed by PV+ neurons. Because an ictal-like event was not recorded after several minutes of IEDs recording, it is inferred that an external stimulus and/or fast GABA transmission are necessary for its appearance, making this preparation ideal to study both the neuronal machinery to encode cortical spontaneous activity and its transformation into brief non-ictal epileptiform discharges.

Dopamine D1 Receptors Regulate Spines in Striatal Direct-Pathway and Indirect-Pathway Neurons

BACKGROUND

Dopamine transmission is involved in the maintenance of the structural plasticity of direct-pathway and indirect-pathway striatal projection neurons (d-SPNs and i-SPNs, respectively). The lack of dopamine in Parkinson's disease produces synaptic remodeling in both types of SPNs, reducing the length of the dendritic arbor and spine density and increasing the intrinsic excitability. Meanwhile, the elevation of dopamine levels by levodopa recovers these alterations selectively in i-SPNs. However, little is known about the specific role of the D1 receptor (D1R) in these alterations.

METHODS

To explore the specific role of D1R in the synaptic remodeling of SPNs, we used knockout D1R mice (D1R^-/- ) and wild-type mice crossed with drd2-enhanced green fluorescent protein (eGFP) to identify d-SPNs and i-SPNs. Corticostriatal slices were used for reconstruction of the dendritic arbors after Lucifer yellow intracellular injection and for whole-cell recordings in naïve and parkinsonian mice treated with saline or levodopa.

RESULTS

The genetic inactivation of D1R reduces the length of the dendritic tree and the spine density in all SPNs, although more so in d-SPNs, which also increases their spiking. In parkinsonian D1R^-/- mice, the spine density decreases in i-SPNs, and this spine loss recovers after chronic levodopa.

CONCLUSIONS

D1R is essential for the maintenance of spine plasticity in d-SPNs but also affects i-SPNs, indicating an important crosstalk between these 2 types of neurons. © 2020 International Parkinson and Movement Disorder Society.

Oxytocin prevents neuronal network pain-related changes on spinal cord dorsal horn in vitro

Recently, oxytocin (OT) has been studied as a potential modulator of endogenous analgesia by acting upon pain circuits at the spinal cord and supraspinal levels. Yet the detailed action mechanisms of OT are still undetermined. The present study aimed to evaluate the action of OT in the spinal cord dorsal horn network under nociceptive-like conditions induced by the activation of the N-methyl-d-aspartate (NMDA) receptor and formalin injection, using calcium imaging techniques. Results demonstrate that the spontaneous Ca²⁺-dependent activity of the dorsal horn cells was scarce, and the coactivity of cells was mainly absent. When NMDA was applied, high rates of activity and coactivity occurred in the dorsal horn cells; these rates of high activity mimicked the activity dynamics evoked by a neuropathic pain condition. In addition, although OT treatment increased activity rates, it was also capable of disrupting the conformation of coordinated activity previously consolidated by NMDA treatment, without showing any effect by itself. Altogether, our results suggest that OT globally prevents the formation of coordinated patterns previously generated by nociceptive-like conditions on dorsal horn cells by NMDA application, which supports previous evidence showing that OT represents a potential therapeutic alternative for the treatment of chronic neuropathic pain.

Mouse corticospinal system comprises different functional neuronal ensembles depending on their hodology

BACKGROUND

Movement performance depends on the synaptic interactions generated by coherent parallel sensorimotor cortical outputs to different downstream targets. The major outputs of the neocortex to subcortical structures are driven by pyramidal tract neurons (PTNs) located in layer 5B. One of the main targets of PTNs is the spinal cord through the corticospinal (CS) system, which is formed by a complex collection of distinct CS circuits. However, little is known about intracortical synaptic interactions that originate CS commands and how different populations of CS neurons are functionally organized. To further understand the functional organization of the CS system, we analyzed the activity of unambiguously identified CS neurons projecting to different zones of the same spinal cord segment using two-photon calcium imaging and retrograde neuronal tracers.

RESULTS

Sensorimotor cortex slices obtained from transgenic mice expressing GCaMP6 funder the Thy1 promoter were used to analyze the spontaneous calcium transients in layer 5 pyramidal neurons. Distinct subgroups of CS neurons projecting to dorsal horn and ventral areas of the same segment show more synchronous activity between them than with other subgroups.

CONCLUSIONS

The results indicate that CS neurons projecting to different spinal cord zones segregated into functional ensembles depending on their hodology, suggesting that a modular organization of CS outputs controls sensorimotor behaviors in a coordinated manner.

Electrophysiological characterization of glucose sensing neurons in the hypothalamic arcuate nucleus of male rats

The arcuate nucleus (ARC), located at the base of hypothalamus, contains two main populations of neurons involved in the regulation of food intake and energy expenditure. The NPY neurons are orexigenic and their activation stimulates food intake while the activation of POMC neurons promote the opposite effect. Several works have tried to identify these neurons based on their electrophysiological and pharmacological characteristics. However, the classification of ARC neurons is still inconclusive. In this work, glucose concentrations were changed within at physiological range, and the response of rat ARC neurons to this stimulus was used to identify them. Subsequently, the cells were classified on the basis of their passive and active electrophysiological properties. Finally, calcium imaging experiments were done to study the response of ARC neurons populations changing glucose concentrations. We found that NPY and putative POMC neurons can be distinguished based on their electrophysiological properties such as input resistance and firing pattern. Calcium imaging experiments confirmed the diversity of ARC neurons.

Synchronized activation of striatal direct and indirect pathways underlies the behavior in unilateral dopamine-depleted mice

For more than three decades it has been known, that striatal neurons become hyperactive after the loss of dopamine input, but the involvement of dopamine (DA) D1‐ or D2‐receptor‐expressing neurons has only been demonstrated indirectly. By recording neuronal activity using fluorescent calcium indicators in D1 or D2 eGFP‐expressing mice, we showed that following dopamine depletion, both types of striatal output neurons are involved in the large increase in neuronal activity generating a characteristic cell assembly of particular neurons that dominate the pattern. When we expressed channelrhodopsin in all the output neurons, light activation in freely moving animals, caused turning like that following dopamine loss. However, if the light stimulation was patterned in pulses the animals circled in the other direction. To explore the neuronal participation during this stimulation we infected normal mice with channelrhodopsin and calcium indicator in striatal output neurons. In slices made from these animals, continuous light stimulation for 15 s induced many cells to be active together and a particular dominant group of neurons, whereas light in patterned pulses activated fewer cells in more variable groups. These results suggest that the simultaneous activity of a large dominant group of striatal output neurons is intimately associated with parkinsonian symptoms.

Parkinson's disease research: adopting a more human perspective to accelerate advances

Parkinson's disease (PD) affects 1% of the population over 60 years old and, with global increases in the aging population, presents huge economic and societal burdens. The etiology of PD remains unknown; most cases are idiopathic, presumed to result from genetic and environmental risk factors. Despite 200 years since the first description of PD, the mechanisms behind initiation and progression of the characteristic neurodegenerative processes are not known. Here, we review progress and limitations of the multiple PD animal models available and identify advances that could be implemented to better understand pathological processes, improve disease outcome, and reduce dependence on animal models. Lessons learned from reducing animal use in PD research could serve as guideposts for wider biomedical research.

Calcium currents in striatal fast-spiking interneurons: dopaminergic modulation of CaV1 channels

BACKGROUND

Striatal fast-spiking interneurons (FSI) are a subset of GABAergic cells that express calcium-binding protein parvalbumin (PV). They provide feed-forward inhibition to striatal projection neurons (SPNs), receive cortical, thalamic and dopaminergic inputs and are coupled together by electrical and chemical synapses, being important components of the striatal circuitry. It is known that dopamine (DA) depolarizes FSI via D₁-class DA receptors, but no studies about the ionic mechanism of this action have been reported. Here we ask about the ion channels that are the effectors of DA actions. This work studies their Ca²⁺ currents.

RESULTS

Whole-cell recordings in acutely dissociated and identified FSI from PV-Cre transgenic mice were used to show that FSI express an array of voltage gated Ca²⁺ channel classes: Ca_V1, Ca_V2.1, Ca_V2.2, Ca_V2.3 and Ca_V3. However, Ca_V1 Ca²⁺ channel carries most of the whole-cell Ca²⁺ current in FSI. Activation of D₁-like class of DA receptors by the D₁-receptor selective agonist SKF-81297 (SKF) enhances whole-cell Ca²⁺ currents through Ca_V1 channels modulation. A previous block of Ca_V1 channels with nicardipine occludes the action of the DA-agonist, suggesting that no other Ca²⁺ channel is modulated by D₁-receptor activation. Bath application of SKF in brain slices increases the firing rate and activity of FSI as measured with both whole-cell and Ca²⁺ imaging recordings. These actions are reduced by nicardipine.

CONCLUSIONS

The present work discloses one final effector of DA modulation in FSI. We conclude that the facilitatory action of DA in FSI is in part due to Ca_V1 Ca²⁺ channels positive modulation.

Differential Synaptic Remodeling by Dopamine in Direct and Indirect Striatal Projection Neurons in Pitx3-/- Mice, a Genetic Model of Parkinson's Disease

In toxin-based models of Parkinson's disease (PD), striatal projection neurons (SPNs) exhibit dendritic atrophy and spine loss concurrent with an increase in excitability. Chronic l-DOPA treatment that induces dyskinesia selectively restores spine density and excitability in indirect pathway SPNs (iSPNs), whereas spine loss and hyperexcitability persist in direct pathway SPNs (dSPNs). These alterations have only been characterized in toxin-based models of PD, raising the possibility that they are an artifact of exposure to the toxin, which may engage compensatory mechanisms independent of the PD-like pathology or due to the loss of dopaminergic afferents. To test all these, we studied the synaptic remodeling in Pitx3^-/- or aphakia mice, a genetic model of PD, in which most of the dopamine neurons in the substantia nigra fail to fully differentiate and to innervate the striatum. We made 3D reconstructions of the dendritic arbor and measured excitability in identified SPNs located in dorsal striatum of BAC-Pitx3^-/- mice treated with saline or l-DOPA. Both dSPNs and iSPNs from BAC-Pitx3^-/- mice had shorter dendritic trees, lower spine density, and more action potentials than their counterparts from WT mice. Chronic l-DOPA treatment restored spine density and firing rate in iSPNs. By contrast, in dSPNs, spine loss and hyperexcitability persisted following l-DOPA treatment, which is similar to what happens in 6-OHDA WT mice. This indicates that dopamine-mediated synaptic remodeling and plasticity is independent of dopamine innervation during SPN development and that Pitx3^-/- mice are a good model because they develop the same pathology described in the toxins-based models and in human postmortem studies of advanced PD.SIGNIFICANCE STATEMENT As the only genetic model of Parkinson's disease (PD) that develops dyskinesia, Pitx3^-/- mice reproduce the behavioral effects seen in humans and are a good system for studying dopamine-induced synaptic remodeling. The studies we present here establish that the structural and functional synaptic plasticity that occur in striatal projection neurons in PD and in l-DOPA-induced dyskinesia are specifically due to modulation of the neurotransmitter dopamine and are not artifacts of the use of chemical toxins in PD models. In addition, our findings provide evidence that synaptic plasticity in the Pitx3^-/- mouse is similar to that seen in toxin models despite its lack of dopaminergic innervation of the striatum during development. Pitx3^-/- mice reproduced the alterations described in patients with advanced PD and in well accepted toxin-based models of PD and dyskinesia. These results further consolidate the fidelity of the Pitx3^-/- mouse as a PD model in which to study the morphological and physiological remodeling of striatal projection neurons by administration of l-DOPA and other drugs.

Cortical stimulation relieves parkinsonian pathological activity in vitro

Previously, we have shown that chemical excitatory drives such as N-methyl-d-aspartate (NMDA) are capable of activating the striatal microcircuit exhibiting neuronal ensembles that alternate their activity producing temporal sequences. One aim of this work was to demonstrate whether similar activity could be evoked by delivering cortical stimulation. Dynamic calcium imaging allowed us to follow the activity of dozens of neurons with single-cell resolution in mus musculus brain slices. A train of electrical stimuli in the cortex evoked network activity similar to the one induced by bath application of NMDA. Previously, we have also shown that the dopamine-depleted striatal microcircuit increases its spontaneous activity generating dominant recurrent ensembles that interrupt the temporal sequences found in control microcircuits. This activity correlates with parkinsonian pathological activity. Several cortical stimulation protocols such as transcranial magnetic stimulation reduce motor signs of Parkinsonism. Here, we show that cortical stimulation in vitro temporarily eliminates the pathological activity from the dopamine-depleted striatal microcircuit by turning off some neurons that sustain this activity and recruiting new ones that allow transitions between network states, similar to the control circuit. When cortical stimulation is given in the presence of L-DOPA, parkinsonian activity is eliminated during the whole recording period. The present experimental evidence suggests that cortical stimulation such as that generated by transcranial magnetic stimulation, or otherwise, may allow reduce L-DOPA dosage.

L-DOPA Oppositely Regulates Synaptic Strength and Spine Morphology in D1 and D2 Striatal Projection Neurons in Dyskinesia

Dopamine depletion in Parkinson's disease (PD) produces dendritic spine loss in striatal medium spiny neurons (MSNs) and increases their excitability. However, the synaptic changes that occur in MSNs in PD, in particular those induced by chronic L-3,4-dihydroxyphenylalanine (L-DOPA) treatment, are still poorly understood. We exposed BAC-transgenic D1-tomato and D2-eGFP mice to PD and dyskinesia model paradigms, enabling cell type-specific assessment of changes in synaptic physiology and morphology. The distinct fluorescence markers allowed us to identify D1 and D2 MSNs for analysis using intracellular sharp electrode recordings, electron microscopy, and 3D reconstructions with single-cell Lucifer Yellow injections. Dopamine depletion induced spine pruning in both types of MSNs, affecting mushroom and thin spines equally. Dopamine depletion also increased firing rate in both D1- and D2-MSNs, but reduced evoked-EPSP amplitude selectively in D2-MSNs. L-DOPA treatment that produced dyskinesia differentially affected synaptic properties in D1- and D2-MSNs. In D1-MSNs, spine density remained reduced but the remaining spines were enlarged, with bigger heads and larger postsynaptic densities. These morphological changes were accompanied by facilitation of action potential firing triggered by synaptic inputs. In contrast, although L-DOPA restored the number of spines in D2-MSNs, it resulted in shortened postsynaptic densities. These changes in D2-MSNs correlated with a decrease in synaptic transmission. Our findings indicate that L-DOPA-induced dyskinesia is associated with abnormal spine morphology, modified synaptic transmission, and altered EPSP-spike coupling, with distinct effects in D1- and D2-MSNs.

Functional comparison of corticostriatal and thalamostriatal postsynaptic responses in striatal neurons of the mouse

Synaptic inputs from cortex and thalamus were compared in electrophysiologically defined striatal cell classes: direct and indirect pathways' striatal projection neurons (dSPNs and iSPNs), fast-spiking interneurons (FS), cholinergic interneurons (ChINs), and low-threshold spiking-like (LTS-like) interneurons. Our purpose was to observe whether stimulus from cortex or thalamus had equivalent synaptic strength to evoke prolonged suprathreshold synaptic responses in these neuron classes. Subthreshold responses showed that inputs from either source functionally mix up in their dendrites at similar electrotonic distances from their somata. Passive and active properties of striatal neuron classes were consistent with the previous studies. Cre-dependent adeno-associated viruses containing Td-Tomato or eYFP fluorescent proteins were used to identify target cells. Transfections with ChR2-eYFP driven by the promoters CamKII or EF1.DIO in intralaminar thalamic nuclei using Vglut-2-Cre mice, or CAMKII in the motor cortex were used to stimulate cortical or thalamic afferents optogenetically. Both field stimuli in the cortex or photostimulation of ChR2-YFP cortical fibers evoked similar prolonged suprathreshold responses in SPNs. Photostimulation of ChR2-YFP thalamic afferents also evoked suprathreshold responses. Differences previously described between responses of dSPNs and iSPNs were observed in both cases. Prolonged suprathreshold responses could also be evoked from both sources onto all other neuron classes studied. However, to evoke thalamostriatal suprathreshold responses, afferents from more than one thalamic nucleus had to be stimulated. In conclusion, both thalamus and cortex are capable to generate suprathreshold responses converging on diverse striatal cell classes. Postsynaptic properties appear to shape these responses.

Inflammatory cytokine-induced changes in neural network activity measured by waveform analysis of high-content calcium imaging in murine cortical neurons

During acute neuroinflammation, increased levels of cytokines within the brain may contribute to synaptic reorganization that results in long-term changes in network hyperexcitability. Indeed, inflammatory cytokines are implicated in synaptic dysfunction in epilepsy and in an array of degenerative and autoimmune diseases of the central nervous system. Current tools for studying the impact of inflammatory factors on neural networks are either insufficiently fast and sensitive or require complicated and costly experimental rigs. Calcium imaging offers a reasonable surrogate for direct measurement of neuronal network activity, but traditional imaging paradigms are confounded by cellular heterogeneity and cannot readily distinguish between glial and neuronal calcium transients. While the establishment of pure neuron cultures is possible, the removal of glial cells ignores physiologically relevant cell-cell interactions that may be critical for circuit level disruptions induced by inflammatory factors. To overcome these issues, we provide techniques and algorithms for image processing and waveform feature extraction using automated analysis of spontaneous and evoked calcium transients in primary murine cortical neuron cultures transduced with an adeno-associated viral vector driving the GCaMP6f reporter behind a synapsin promoter. Using this system, we provide evidence of network perturbations induced by the inflammatory cytokines TNFα, IL1β, and IFNγ.

Historical perspective of cell transplantation in Parkinson’s disease

Cell grafting has been considered a therapeutic approach for Parkinson’s disease (PD) since the 1980s. The classical motor symptoms of PD are caused by the loss of dopaminergic neurons in the substantia nigra pars compacta, leading to a decrement in dopamine release in the striatum. Consequently, the therapy of cell-transplantation for PD consists in grafting dopamine-producing cells directly into the brain to reestablish dopamine levels. Different cell sources have been shown to induce functional benefits on both animal models of PD and human patients. However, the observed motor improvements are highly variable between individual subjects, and the sources of this variability are not fully understood. The purpose of this review is to provide a general overview of the pioneering studies done in animal models of PD that established the basis for the first clinical trials in humans, and compare these with the latest findings to identify the most relevant aspects that remain unanswered to date. The main focus of the discussions presented here will be on the mechanisms associated with the survival and functionality of the transplants. These include the role of the dopamine released by the grafts and the capacity of the grafted cells to extend fibers and to integrate into the motor circuit. The complete understanding of these aspects will require extensive research on basic aspects of molecular and cellular physiology, together with neuronal network function, in order to uncover the real potential of cell grafting for treating PD.

Amyloid β Peptide-Induced Changes in Prefrontal Cortex Activity and Its Response to Hippocampal Input

Alterations in prefrontal cortex (PFC) function and abnormalities in its interactions with other brain areas (i.e., the hippocampus) have been related to Alzheimer Disease (AD). Considering that these malfunctions correlate with the increase in the brain's amyloid beta (Aβ) peptide production, here we looked for a causal relationship between these pathognomonic signs of AD. Thus, we tested whether or not Aβ affects the activity of the PFC network and the activation of this cortex by hippocampal input stimulation in vitro. We found that Aβ application to brain slices inhibits PFC spontaneous network activity as well as PFC activation, both at the population and at the single-cell level, when the hippocampal input is stimulated. Our data suggest that Aβ can contribute to AD by disrupting PFC activity and its long-range interactions throughout the brain.