Dopaminergic Dependency of Cholinergic Pallidal Neurons.

We employed the whole-cell patch-clamp method and ChAT-Cre mice to study the electrophysiological attributes of cholinergic neurons in the external globus pallidus. Most neurons were inactive, although approximately 20% displayed spontaneous firing, including burst firing. The resting membrane potential, the whole neuron input resistance, the membrane time constant and the total neuron membrane capacitance were also characterized. The current-voltage relationship showed time-independent inward rectification without a "sag". Firing induced by current injections had a brief initial fast adaptation followed by tonic firing with minimal accommodation. Intensity-frequency plots exhibited maximal average firing rates of about 10 Hz. These traits are similar to those of some cholinergic neurons in the basal forebrain. Also, we examined their dopamine sensitivity by acutely blocking dopamine receptors. This action demonstrated that the membrane potential, excitability, and firing pattern of pallidal cholinergic neurons rely on the constitutive activity of dopamine receptors, primarily D2-class receptors. The blockade of these receptors induced a resting membrane potential hyperpolarization, a decrease in firing for the same stimulus, the disappearance of fast adaptation, and the emergence of a depolarization block. This shift in physiological characteristics was evident even when the hyperpolarization was corrected with D.C. current. Neither the currents that generate the action potentials nor those from synaptic inputs were responsible. Instead, our findings suggest, that subthreshold slow ion currents, that require further investigation, are the target of this novel dopaminergic signaling.

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

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.

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.

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.

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.

Firing Differences Between Adult Intralaminar Thalamo-striatal Neurons.

Differences in the intrinsic properties of intralaminar thalamo-striatal neurons such as expressing low-threshold-spikes (LTS) or after hyperpolarizing potentials (AHPs) of different duration have been attributed to different maturation stages. However, two morphological types: "diffuse" and "bushy" have been described. Therefore, we explored whether electrophysiological differences persist in adult mice using whole cell recordings. Some recorded neurons were identified by intracellular labeling with biocytin and double labeling with retrograde or anterograde tracings using Cre-mice. We classified these neurons by their AHPs during spontaneous firing. Neurons with long duration AHPs, with fast and slow components, were mostly found in the parafascicular (Pf) nucleus. Neurons with brief AHPs were mainly found in the central lateral (CL) nucleus. However, neurons with both AHPs were found in both nuclei in different proportions. Firing frequency adaptation differed between these neuron classes: those with prolonged AHPs exhibited firing frequency adaptation with fast and slow time constants whereas those with brief AHPs were slow adapters. Neurons with more prolonged AHPs had significant higher input resistances than neurons with brief AHPs. Both cell classes could fire in two modes: trains of single action potentials at depolarized potentials or high frequency bursts on top of LTS at more hyperpolarized potentials. LTS were probably generated by Cav3 calcium channels since they were blocked by the selective antagonist TTA-P2. About 11% of neurons with brief AHPs and 55% of neurons with prolonged AHPs do not show LTS and bursts, even when potassium currents are blocked.

Activation of parvalbumin-expressing neurons reconfigures neuronal ensembles in murine striatal microcircuits.

The striatum is the largest entrance to the basal ganglia. Diverse neuron classes make up striatal microcircuit activity, consisting in the sequential activation of neuronal ensembles. How different neuron classes participate in generating ensemble sequences is unknown. In control mus musculus brain slices in vitro, providing excitatory drive generates ensemble sequences. In Parkinsonian microcircuits captured by a highly recurrent ensemble, a cortical stimulus causes a transitory reconfiguration of neuronal groups alleviating Parkinsonism. Alternation between neuronal ensembles needs interconnectivity, in part due to interneurons, preferentially innervated by incoming afferents. One main class of interneuron expresses parvalbumin (PV+ neurons) and mediates feed-forward inhibition. However, its more global actions within the microcircuit are unknown. Using calcium imaging in ex vivo brain slices simultaneously recording dozens of neurons, we aimed to observe the actions of PV+ neurons within the striatal microcircuit. PV+ neurons in active microcircuits are 5%-11% of the active neurons even if, anatomically, they are <1% of the total neuronal population. In resting microcircuits, optogenetic activation of PV+ neurons turns on circuit activity by activating or disinhibiting, more neurons than those actually inhibited, showing that feed-forward inhibition is not their only function. Optostimulation of PV+ neurons in active microcircuits inhibits and activates different neuron sets, resulting in the reconfiguration of neuronal ensembles by changing their functional connections and ensemble membership, showing that neurons may belong to different ensembles at different situations. Our results show that PV+ neurons participate in the mechanisms that generate alternation of neuronal ensembles, therefore provoking ensemble sequences.

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.

Localization of chloride co-transporters in striatal neurons.

The ionic driving force for the chloride-permeable GABAA receptor is subject to spatial control and distribution of chloride transporters. NKCC1 and KCC2 are mostly expressed in neurons in a specific manner. In the striatum, the localization of these transporters in identified neurons is unknown. In this study, the expression of these transporters was found to be different between projection neurons and interneurons. NKCC1 immunoreactivity was observed in the soma of adult BAC-D1-eGFP+ and D2-eGFP+ striatal projection neurons (SPNs). KCC2 was not expressed in either projection neuron and immunoreactivity to this transporter was observed only in the neuropile. However, NKCC1 and KCC2 co-transporters were not localized in intracellular biocytin-injected dendrites of SPNs of the direct or indirect pathways (D1-SPNs and D2-SPNs). Experiments with PV Cre transgenic mice transfected with Cre-dependent adeno-associated viruses containing tdTomato in the striatum showed a cell-type-specific distribution of KCC2 chloride transporter co-expression associated with PV interneurons. Thus, depolarizing actions of GABA responses in adult projection neurons can be explained by the expression and somatic localization of the NKCC1 transporters. A somato/dendritic distribution of KCC2 expression was observed only in striatal interneurons and corresponds to the hyperpolarizing action of GABA recorded in these cells. This correlates the different roles for GABA actions in striatal neuronal excitability with the expression of specific chloride transporters.

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.

Differences in synaptic integration between direct and indirect striatal projection neurons: Role of CaV 3 channels.

Different corticostriatal suprathreshold responses in direct and indirect striatal projection neurons (SPNs) of rodents have been reported. Responses consist in prolonged synaptic potentials of polysynaptic and intrinsic origin, in which voltage-gated Ca2 ⁺ currents play a role. Recording simultaneous Ca2 ⁺ imaging and voltage responses at the soma, while activating the corticostriatal pathway, we show that encoding of synaptic responses into trains of action potentials (APs) is different in SPNs: firing of APs in D1-SPNs increase gradually, in parallel with Ca2 ⁺ entry, as a function of stimulus intensity. In contrast, D2-SPNs attain a maximum number of evoked spikes at low stimulus intensities, Ca2 ⁺ entry is limited, and both remain the same in spite of increasing stimulus strength. Stimulus needs to reach certain intensity, to have propagated Ca2 ⁺ potentials to the soma plus a sudden step in Ca2 ⁺ entry, without changing the number of fired APs, phenomena never seen in D1-SPNs. Constant firing in spite of changing stimulus, suggested the involvement of underlying inactivating potentials. We found that Caᵥ3 currents contribute to Ca2+ entry in both classes of SPNs, but have a more notable effect in D2-SPNs, where a low-threshold spike was disclosed. Blockade of CaV 3 channels retarded the steep rise in firing in D2-SPNs. Inhibition block increased the number of spikes fired by D2-SPNs, without changing firing in D1-SPNs. These differences in synaptic integration enable a biophysical dissimilarity: dendritic inhibition appears to be more relevant for D2-SPNs. This may imply distinctions in the set of interneurons affecting each SPN class.

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 Cav3 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.

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: CaV1, CaV2.1, CaV2.2, CaV2.3 and CaV3. However, CaV1 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 CaV1 channels modulation. A previous block of CaV1 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 CaV1 Ca2+ channels positive modulation.

Insulin Regulates GABAA Receptor-Mediated Tonic Currents in the Prefrontal Cortex.

Recent studies, have shown that insulin increases extrasynaptic GABAA receptor-mediated currents in the hippocampus, causing alterations of neuronal excitability. The prefrontal cortex (PFC) is another brain area which is involved in cognition functions and expresses insulin receptors. Here, we used electrophysiological, molecular, and immunocytochemical techniques to examine the effect of insulin on the extrasynaptic GABAA receptor-mediated tonic currents in brain slices. We found that insulin (20-500 nM) increases GABAA-mediated tonic currents. Our results suggest that insulin promotes the trafficking of extrasynaptic GABAA receptors from the cytoplasm to the cell membrane. Western blot analysis and immunocytochemistry showed that PFC extrasynaptic GABAA receptors contain α-5 and δ subunits. Insulin effect on tonic currents decreased the firing rate and neuronal excitability in layer 5-6 PFC cells. These effects of insulin were dependent on the activation of the PI3K enzyme, a key mediator of the insulin response within the brain. Taken together, these results suggest that insulin modulation of the GABAA-mediated tonic currents can modify the activity of neural circuits within the PFC. These actions could help to explain the alterations of cognitive processes associated with changes in insulin signaling.

Models of Short-Term Synaptic Plasticity.

We focus on dynamical descriptions of short-term synaptic plasticity. Instead of focusing on the molecular machinery that has been reviewed recently by several authors, we concentrate on the dynamics and functional significance of synaptic plasticity, and review some mathematical models that reproduce different properties of the dynamics of short term synaptic plasticity that have been observed experimentally. The complexity and shortcomings of these models point to the need of simple, yet physiologically meaningful models. We propose a simplified model to be tested in synapses displaying different types of short-term plasticity.

Activation of Glutamatergic Fibers in the Anterior NAc Shell Modulates Reward Activity in the aNAcSh, the Lateral Hypothalamus, and Medial Prefrontal Cortex and Transiently Stops Feeding.

Although the release of mesoaccumbal dopamine is certainly involved in rewarding responses, recent studies point to the importance of the interaction between it and glutamate. One important component of this network is the anterior nucleus accumbens shell (aNAcSh), which sends GABAergic projections into the lateral hypothalamus (LH) and receives extensive glutamatergic inputs from, among others, the medial prefrontal cortex (mPFC). The effects of glutamatergic activation of aNAcSh on the ingestion of rewarding stimuli as well as its effect in the LH and mPFC are not well understood. Therefore, we studied behaving mice that express a light-gated channel (ChR2) in glutamatergic fibers in their aNAcSh while recording from neurons in the aNAcSh, or mPFC or LH. In Thy1-ChR2, but not wild-type, mice activation of aNAcSh fibers transiently stopped the mice licking for sucrose or an empty sipper. Stimulation of aNAcSh fibers both activated and inhibited single-unit responses aNAcSh, mPFC, and LH, in a manner that maintains firing rate homeostasis. One population of licking-inhibited pMSNs in the aNAcSh was also activated by optical stimulation, suggesting their relevance in the cessation of feeding. A rewarding aspect of stimulation of glutamatergic inputs was found when the Thy1-ChR2 mice learned to nose-poke to self-stimulate these inputs, indicating that bulky stimulation of these fibers are rewarding in the sense of wanting. Stimulation of excitatory afferents evoked both monosynaptic and polysynaptic responses distributed in the three recorded areas. In summary, we found that activation of glutamatergic aNAcSh fibers is both rewarding and transiently inhibits feeding. SIGNIFICANCE STATEMENT: We have established that the activation of glutamatergic fibers in the anterior nucleus accumbens shell (aNAcSh) transiently stops feeding and yet, because mice self-stimulate, is rewarding in the sense of wanting. Moreover, we have characterized single-unit responses of distributed components of a hedonic network (comprising the aNAcSh, medial prefrontal cortex, and lateral hypothalamus) recruited by activation of glutamatergic aNAcSh afferents that are involved in encoding a positive valence signal important for the wanting of a reward and that transiently stops ongoing consummatory actions, such as licking.

Pathophysiological signatures of functional connectomics in parkinsonian and dyskinetic striatal microcircuits.

A challenge in neuroscience is to integrate the cellular and system levels. For instance, we still do not know how a few dozen neurons organize their activity and relations in a microcircuit or module of histological scale. By using network theory and Ca(2+) imaging with single-neuron resolution we studied the way in which striatal microcircuits of dozens of cells orchestrate their activity. In addition, control and diseased striatal tissues were compared in rats. In the control tissue, functional connectomics revealed small-world, scale-free and hierarchical network properties. These properties were lost during pathological conditions in ways that could be quantitatively analyzed. Decorticated striatal circuits disclosed that corticostriatal interactions depend on privileged connections with a set of highly connected neurons or "hubs". In the 6-OHDA model of Parkinson's disease there was a decrease in hubs number; but the ones that remained were linked to dominant network states. l-DOPA induced dyskinesia provoked a loss in the hierarchical structure of the circuit. All these conditions conferred distinct temporal sequences to circuit activity. Temporal sequences appeared as particular signatures of disease process thus bringing the possibility of a future quantitative pathophysiology at a histological scale.

Diverse Short-Term Dynamics of Inhibitory Synapses Converging on Striatal Projection Neurons: Differential Changes in a Rodent Model of Parkinson's Disease.

Most neurons in the striatum are projection neurons (SPNs) which make synapses with each other within distances of approximately 100 µm. About 5% of striatal neurons are GABAergic interneurons whose axons expand hundreds of microns. Short-term synaptic plasticity (STSP) between fast-spiking (FS) interneurons and SPNs and between SPNs has been described with electrophysiological and optogenetic techniques. It is difficult to obtain pair recordings from some classes of interneurons and due to limitations of actual techniques, no other types of STSP have been described on SPNs. Diverse STSPs may reflect differences in presynaptic release machineries. Therefore, we focused the present work on answering two questions: Are there different identifiable classes of STSP between GABAergic synapses on SPNs? And, if so, are synapses exhibiting different classes of STSP differentially affected by dopamine depletion? Whole-cell voltage-clamp recordings on SPNs revealed three classes of STSPs: depressing, facilitating, and biphasic (facilitating-depressing), in response to stimulation trains at 20 Hz, in a constant ionic environment. We then used the 6-hydroxydopamine (6-OHDA) rodent model of Parkinson's disease to show that synapses with different STSPs are differentially affected by dopamine depletion. We propose a general model of STSP that fits all the dynamics found in our recordings.

KV7 Channels Regulate Firing during Synaptic Integration in GABAergic Striatal Neurons.

Striatal projection neurons (SPNs) process motor and cognitive information. Their activity is affected by Parkinson's disease, in which dopamine concentration is decreased and acetylcholine concentration is increased. Acetylcholine activates muscarinic receptors in SPNs. Its main source is the cholinergic interneuron that responds with a briefer latency than SPNs during a cortical command. Therefore, an important question is whether muscarinic G-protein coupled receptors and their signaling cascades are fast enough to intervene during synaptic responses to regulate synaptic integration and firing. One of the most known voltage dependent channels regulated by muscarinic receptors is the KV7/KCNQ channel. It is not known whether these channels regulate the integration of suprathreshold corticostriatal responses. Here, we study the impact of cholinergic muscarinic modulation on the synaptic response of SPNs by regulating KV7 channels. We found that KV7 channels regulate corticostriatal synaptic integration and that this modulation occurs in the dendritic/spines compartment. In contrast, it is negligible in the somatic compartment. This modulation occurs on sub- and suprathreshold responses and lasts during the whole duration of the responses, hundreds of milliseconds, greatly altering SPNs firing properties. This modulation affected the behavior of the striatal microcircuit.

Modulation of direct pathway striatal projection neurons by muscarinic M4-type receptors.

Models of basal ganglia (BG) function posit a dynamic balance between two classes of striatal projection neurons (SPNs): direct pathway neurons (dSPNs) that facilitate movements, and indirect pathway neurons (iSPNs) that repress movement execution. Two main modulatory transmitters regulate the output of these neurons: dopamine (DA) and acetylcholine (ACh). dSPNs express D1-type DA, M1-and M4-type ACh receptors, while iSPNs express D2-type DA and M1-type ACh receptors. Actions of M1-, D1-, and D2-receptors have been extensively reported, but we still ignore most actions of muscarinic M4-type receptors. Here, we used whole-cell recordings in acutely dissociated neurons, pharmacological tools such as mamba-toxins, and BAC D(1 or 2)-eGFP transgenic mice to show that activation of M4-type receptors with bath applied muscarine enhances Ca(2+)-currents through CaV1-channels in dSPNs and not in iSPNs. This action increases excitability of dSPNs after both direct current injection and synaptically driven stimulation. The increases in Ca(2+)-current and excitability were blocked specifically by mamba toxin-3, suggesting mediation via M4-type receptors. M4-receptor activation also increased network activity of dSPNs but not of iSPNs as seen with calcium-imaging techniques. Moreover, actions of D1-type and M4-type receptors may add to produce a larger enhancement of excitability of dSPNs or, paradoxically, oppose each other depending on the order of their activation. Possible implications of these findings are discussed.