Striatal interneurones: chemical, physiological and morphological characterization

Selective ablation of striatal striosomes produces the deregulation of dopamine nigrostriatal pathway

The striatum is a complex structure in which the organization in two compartments (striosomes and matrix) have been defined by their neurochemical profile and their input-output connections. The striosomes receive afferences from the limbic brain areas and send projections to the dopamine neurons of the substantia nigra pars compacta. Thereby, it has been suggested that the striosomes exert a limbic control over the motor function mediated by the surrounding matrix. However, the functionality of the striosomes are not completely understood. To elucidate the role of the striosomes on the regulation of the nigral dopamine neurons, we have induced specific ablation of this compartment by striatal injections of the neurotoxin dermorphin-saporin (DS) and dopamine neurotransmission markers have been analyzed by immunohistochemistry. The degeneration of the striosomes resulted in a nigrostriatal projections imbalance between the two striatal compartments, with an increase of the dopamine neurotransmission in the striosomes and a decrease in the matrix. The present results highlight the key function of the striosomes for the maintenance of the striatal dopamine tone and would contribute to the understanding of their involvement in some neurological disorders such as Huntington’s disease.

M(4) muscarinic receptors and locomotor activity regulation

M(4) muscarinic receptors (M(4) MR) represent a subfamily of G-protein coupled receptors serving a substantial role in spontaneous locomotor activity regulation, cognition and modulation of cholinergic system. With increasing body of literature discussing the role of M(4) MR some controversies arose. Thus, we try here to summarize the current evidence regarding the M(4) MR, with the special focus on their role in Locomotor activity control. We review the molecular function of M(4) MR in specific brain areas implicated in locomotor regulation, and shortly in other CNS processes that could be connected to locomotor activity. We also focus on brain areas implicated in locomotor activity biorhythm changes like suprachiasmatic nucleus, subparaventricular zone posterior hypothalamic area, striatum and thalamus. Gender-related aspects and differences in locomotor activity in males and females are discussed further.

Acetylcholine Neurotransmitter Receptor Densities in the Striatum of Hemiparkinsonian Rats Following Botulinum Neurotoxin-A Injection

Cholinergic neurotransmission has a pivotal function in the caudate-putamen, and is highly associated with the pathophysiology of Parkinson's disease. Here, we investigated long-term changes in the densities of the muscarinic receptor subtypes M₁, M₂, M₃ (mAchRs) and the nicotinic receptor subtype α₄β₂ (nAchRs) in the striatum of the 6-OHDA-induced hemiparkinsonian (hemi-PD) rat model using quantitative in vitro receptor autoradiography. Hemi-PD rats exhibited an ipsilateral decrease in striatal mAchR densities between 6 and 16%. Moreover, a massive and constant decrease in striatal nAchR density by 57% was found. A second goal of the study was to disclose receptor-related mechanisms for the positive motor effect of intrastriatally injected Botulinum neurotoxin-A (BoNT-A) in hemi-PD rats in the apomorphine rotation test. Therefore, the effect of intrastriatally injected BoNT-A in control and hemi-PD rats on mAchR and nAchR densities was analyzed and compared to control animals or vehicle-injected hemi-PD rats. BoNT-A administration slightly reduced interhemispheric differences of mAchR and nAchR densities in hemi-PD rats. Importantly, the BoNT-A effect on striatal nAchRs significantly correlated with behavioral testing after apomorphine application. This study gives novel insights of 6-OHDA-induced effects on striatal mAchR and nAchR densities, and partly explains the therapeutic effect of BoNT-A in hemi-PD rats on a cellular level.

Target Interneuron Preference in Thalamocortical Pathways Determines the Temporal Structure of Cortical Responses

Sensory processing relies on fast detection of changes in environment, as well as integration of contextual cues over time. The mechanisms by which local circuits of the cerebral cortex simultaneously perform these opposite processes remain obscure. Thalamic “specific” nuclei relay sensory information, whereas “nonspecific” nuclei convey information on the environmental and behavioral contexts. We expressed channelrhodopsin in the ventrobasal specific (sensory) or the rhomboid nonspecific (contextual) thalamic nuclei. By selectively activating each thalamic pathway, we found that nonspecific inputs powerfully activate adapting (slow-responding) interneurons but weakly connect fast-spiking interneurons, whereas specific inputs exhibit opposite interneuron preference. Specific inputs thereby induce rapid feedforward inhibition that limits response duration, whereas, in the same cortical area, nonspecific inputs elicit delayed feedforward inhibition that enables lasting recurrent excitation. Using a mean field model, we confirm that cortical response dynamics depends on the type of interneuron targeted by thalamocortical inputs and show that efficient recruitment of adapting interneurons prolongs the cortical response and allows the summation of sensory and contextual inputs. Hence, target choice between slow- and fast-responding inhibitory neurons endows cortical networks with a simple computational solution to perform both sensory detection and integration.

Differential and Overlapping Pattern of Foxp1 and Foxp2 Expression in the Striatum of Adult Mouse Brain

Genetic mutations of FOXP1 and FOXP2 are associated with neurodevelopmental diseases. It is important to characterize the cell types that express Foxp1 and Foxp2 in the brain. Foxp1 and Foxp2 are expressed at high levels in the striatum of mouse brains. There are two populations of striatal projection neurons (SPNs), dopamine D1 receptor (D1R)-expressing striatonigral neurons and D2 receptor (D2R)-expressing striatopallidal neurons. In addition to SPNs, there are different types of striatal interneurons. Here, we quantitatively analyze the expression pattern of Foxp1 and Foxp2 with respect to specific cell types of projection neurons and interneurons in the striatum of adult mouse brains. Double immunostaining and in situ hybridization showed that Foxp1 and Foxp2 were specifically expressed in SPNs, but not in interneurons. For Foxp1, 50-57% of Foxp1-positive neurons co-expressed D1R mRNA, and 45-52% of Foxp1-positive neurons co-expressed D2R mRNA in the striatum at rostrocaudal levels. For Foxp2, 65-77% of Foxp2-positive neurons co-expressed D1R mRNA, and 21-26% of Foxp2-positive neurons co-expressed D2R mRNA in the striatum at rostrocaudal levels. Neither Foxp1 nor Foxp2 was found to co-localize with parvalbumin, somatostatin, nNOS, calretinin and ChAT in interneurons of the striatum. Moreover, none of parvalbumin-, somatostatin-, nNOS-, and calretinin-positive interneurons co-expressed Foxp1 or Foxp2 in the cerebral cortex. As Foxp1 and Foxp2 can form heterodimers for transcriptional regulation, the differential and overlapping expression pattern of Foxp1 and Foxp2 in SPNs implicates coordinate and distinct roles of Foxp1 and Foxp2 in developmental construction and physiologic functions of striatal circuits in the brain.

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 electrophysiological properties of D1 and D2 spiny projection neurons in the mouse nucleus accumbens core

The striatum consists of the dorsal (caudate/putamen) and the ventral (nucleus accumbens) regions. The nucleus accumbens is further divided into a core and shell. Both the dorsal and ventral striatum contain populations of spiny projection neurons, which make up 95% of the neurons within the striatum. SPNs are canonically categorized into those that express the D1-type dopamine receptor (D1 SPNs) and those that express the D2-type dopamine receptor (D2 SPNs). D1 and D2 SPNs differ with respect to both synaptic inputs and projection targets. In the dorsal striatum, it is well established that these populations of SPNs differ in terms of their electrophysiological and morphological properties. However, there remains a gap in our knowledge of the electrophysiological properties of SPNs in the nucleus accumbens core. To evaluate the differential properties of these SPNs, we performed whole-cell recordings from D1 and D2 SPNs in BAC transgenic mice in which D1 SPNs fluoresce red and D2 SPNs fluoresce green. The two SPN subtypes did not differ in terms of their time constant, capacitance, resting membrane potential, or tonic current. However, D2 SPNs displayed heightened inhibitory postsynaptic current (IPSC) and miniature excitatory PSC frequency as compared with D1 SPNs. Furthermore, D2 SPNs displayed decreased rheobase, increased excitability as measured by firing rates to depolarizing current injections, increased inward rectification, increased input resistance, and decreased dendritic complexity compared to D1 SPNs. Our results demonstrate a dichotomy in the electrophysiological properties of D1 and D2 SPNs in the nucleus accumbens core, which contributes to our knowledge of ventral striatal circuitry.

Striatal Cholinergic Interneurons in a Knock-in Mouse Model of L-DOPA-Responsive Dystonia

Striatal cholinergic dysfunction is a common phenotype associated with various forms of dystonia in which anti-cholinergic drugs have some therapeutic benefits. However, the underlying substrate of striatal cholinergic defects in dystonia remain poorly understood. In this study, we used a recently developed knock-in mouse model of dopamine-responsive dystonia (DRD) with strong symptomatic responses to anti-cholinergic drugs, to assess changes in the prevalence and morphology of striatal cholinergic interneurons (ChIs) in a model of generalized dystonia. Unbiased stereological neuronal counts and Sholl analysis were used to address these issues. To determine the potential effect of aging on the number of ChIs, both young (3 months old) and aged (15 months old) mice were used. For purpose of comparisons with ChIs, the number of GABAergic parvalbumin (PV)-immunoreactive striatal interneurons was also quantified in young mice. Overall, no significant change in the prevalence of ChIs and PV-immunoreactive cells was found throughout various functional regions of the striatum in young DRD mice. Similar results were found for ChIs in aged animals. Subtle changes in the extent and complexity of the dendritic tree of ChIs were found in middle and caudal regions of the striatum in DRD mice. Additional immunohistochemical data also suggested lack of significant change in the expression of striatal cholinergic M1 and M4 muscarinic receptors immunoreactivity in DRD mice. Thus, together with our previous data from a knock-in mouse model of DYT-1 dystonia (Song et al., 2013), our data further suggest that the dysregulation of striatal cholinergic transmission in dystonia is not associated with major neuroplastic changes in the morphology or prevalence of striatal ChIs.

Highlights-

There is no significant change in the number of striatal ChIs in young and aged mice model of DRD

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There is no significant change in the prevalence of striatal GABAergic PV-containing interneurons in the striatum of young mice models of DRD

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Subtle morphological changes in the dendritic arborization of striatal ChIs are found in the middle and caudal tiers of the striatum in young mice models of DRD

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The levels of both M1 and M4 muscarinic receptors immunoreactivity are not significantly changed in the striatum of DRD mice

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Major changes in the prevalence and morphology of striatal ChIs are unlikely to underlie striatal cholinergic dysfunction in DRD

Expression and localization of CB1R, NAPE-PLD, and FAAH in the vervet monkey nucleus accumbens

Extensive rodent literature suggests that the endocannabinoid (eCB) system present in the nucleus accumbens (NAc) modulates dopamine (DA) release in this area. However, expression patterns of the cannabinoid receptor type 1 (CB1R), the synthesizing enzyme N-acyl phosphatidylethanolamine phospholipase D (NAPE-PLD), and the degradation enzyme fatty acid amide hydrolase (FAAH) in the NAc have not yet been described in non-human primates. The goal of this study is therefore to characterize the expression and localization of the eCB system within the NAc of vervet monkeys (Chlorocebus sabaeus) using Western blots and immunohistochemistry. Results show that CB1R, NAPE-PLD, and FAAH are expressed across the NAc rostrocaudal axis, both in the core and shell. CB1R, NAPE-PLD, and FAAH are localized in medium spiny neurons (MSNs) and fast-spiking GABAergic interneurons (FSIs). Dopaminergic projections and astrocytes did not express CB1R, NAPE-PLD, or FAAH. These data show that the eCB system is present in the vervet monkey NAc and supports its role in the primate brain reward circuit.

Identification and Characterization of a Novel Spontaneously Active Bursty GABAergic Interneuron in the Mouse Striatum

The recent availability of different transgenic mouse lines coupled with other modern molecular techniques has led to the discovery of an unexpectedly large cellular diversity and synaptic specificity in striatal interneuronal circuitry. Prior research has described three spontaneously active interneuron types in mouse striatal slices: the cholinergic interneuron, the neuropeptide Y-low threshold spike interneuron, and the tyrosine hydroxylase interneurons (THINs). Using transgenic Htr3a-Cre mice, we now characterize a fourth population of spontaneously active striatal GABAergic interneurons termed spontaneously active bursty interneurons (SABIs) because of their unique burst-firing pattern in cell-attached recordings. Although they bear some qualitative similarity in intrinsic electrophysiological properties to THINs in whole-cell recordings, detailed analysis revealed significant differences in many intrinsic properties and in their morphology. Furthermore, all previously identified striatal GABAergic interneurons have been shown to innervate striatal spiny projection neurons (SPNs), contributing to the suggestion that the principal function of striatal GABAergic interneurons is to provide feedforward inhibition to SPNs. Here, very surprisingly, paired recordings show that SABIs do not innervate SPNs significantly. Further, optogenetic inhibition of striatal Htr3a-Cre interneurons triggers barrages of IPSCs in SPNs. We hypothesize that these IPSCs result from disinhibition of a population of GABAergic interneurons with activity that is constitutively suppressed by the SABIs. We suggest that the SABIs represent the first example of a striatal interneuron-selective interneuron and, further, that their existence, along with previously defined interneuronal networks, may participate in the formation of SPN ensembles observed by others.SIGNIFICANCE STATEMENT Before ∼2010, the main function of the three known subtypes of striatal GABAergic interneurons was assumed to mediate feedforward inhibition of the spiny neurons (SPNs). During the past decade, we and others have described several novel populations of striatal GABAergic interneurons and their synaptic connections and have shown that striatal interneurons and SPNs interact through extensive and highly cell-type-specific connections that form specialized networks. Here, we describe a novel population of striatal GABAergic interneuron and provide several lines of evidence suggesting that it represents the first interneuron-selective interneuron in striatum. Striatal interneurons and their synaptic connections are suggested to play an important role in the formation of ensembles of striatal SPNs interconnected by inhibitory axon collaterals.

A new outlook on cholinergic interneurons in Parkinson's disease and L-DOPA-induced dyskinesia

Traditionally, dopamine (DA) and acetylcholine (ACh) striatal systems were considered antagonistic and imbalances or aberrant signaling between these neurotransmitter systems could be detrimental to basal ganglia activity and pursuant motor function, such as in Parkinson's disease (PD) and L-DOPA-induced dyskinesia (LID). Herein, we discuss the involvement of cholinergic interneurons (ChIs) in striatally-mediated movement in a healthy, parkinsonian, and dyskinetic state. ChIs integrate numerous neurotransmitter signals using intrinsic glutamate, serotonin, and DA receptors and convey the appropriate transmission onto nearby muscarinic and nicotinic ACh receptors to produce movement. In PD, severe DA depletion causes abnormal rises in ChI activity which promote striatal signaling to attenuate normal movement. When treating PD with L-DOPA, hyperkinetic side effects, or LID, develop due to increased striatal DA; however, the role of ChIs and ACh transmission, until recently has been unclear. Fortunately, new technology and pharmacological agents have facilitated understanding of ChI function and ACh signaling in the context of LID, thus offering new opportunities to modify existing and discover future therapeutic strategies in movement disorders.

Altered dopaminergic regulation of the dorsal striatum is able to induce tic-like movements in juvenile rats

Motor tics are sudden, repetitive, involuntary movements representing the hallmark behaviors of the neurodevelopmental disease Tourette’s syndrome (TS). The primary cause of TS remains unclear. The initial observation that dopaminergic antagonists alleviate tics led to the development of a dopaminergic theory of TS etiology which is supported by post mortem and in vivo studies indicating that non-physiological activation of the striatum could generate tics. The striatum controls movement execution through the balanced activity of dopamine receptor D1 and D2-expressing medium spiny neurons of the direct and indirect pathway, respectively. Different neurotransmitters can activate or repress striatal activity and among them, dopamine plays a major role. In this study we introduced a chronic dopaminergic alteration in juvenile rats, in order to modify the delicate balance between direct and indirect pathway. This manipulation was done in the dorsal striatum, that had been associated with tic-like movements generation in animal models. The results were movements resembling tics, which were categorized and scored according to a newly developed rating scale and were reduced by clonidine and riluzole treatment. Finally, post mortem analyses revealed altered RNA expression of dopaminergic receptors D1 and D2, suggesting an imbalanced dopaminergic regulation of medium spiny neuron activity as being causally related to the observed phenotype.

Baseline striatal and nigral interneuronal protein levels in two distinct mice strains differ in accordance with their MPTP susceptibility

Epidemiological studies reveal an ethnicity-based bias in prevalence of Parkinson's disease (PD), deriving from the differences that exist between Caucasians and African or Asian populations. Experimental mice models provide a scope to analyse the cellular mechanisms of differential susceptibility to PD. C57BL/6J mice, for instance, are more susceptible to MPTP-induced Parkinsonism whereas CD-1 mice are resistant. In PD-pathogenesis, interneuronal contribution is also likely, although they comprise only 5-10% of the striatal cells. The interneurons harbour calcium binding proteins, like calretinin (Cal-R) and parvalbumin (PV), which are crucial in Ca²⁺ homeostasis for preventing calcium-induced excitotoxicity. GAD-67-immunoreactive interneurons are the other prominent set of GABAergic interneurons. In PD, dopamine loss up-regulates GAD-67 expression in striatal projection neurons and other basal ganglia circuit. We studied the possible contribution of interneurons in determining variable susceptibility by assessing the expression of calretinin, PV and GAD-67 in both striatum and substantia nigra pars compacta (SNpc) in two distinct mice strains, i.e. C57BL/6J and CD-1 under normal conditions, using unbiased stereology for quantification of immunoreactive cells and immunoblotting. The vulnerable C57BL/6J had lesser basal parvalbumin expression in both nigra and striatum whereas the calretinin levels were low only in the striatum. GAD-67 expression showed no perceptible differences in the striatum or SNpc of either of the strains. Differential expression of calcium buffering/binding proteins under normal physiological condition proffers a role for interneurons in the differential susceptibility to PD. Thus, even the baseline susceptibility indices i.e. without using the neurotoxin; can provide vital mechanistic insights into PD pathogenesis.

Disrupted striatal neuron inputs and outputs in Huntington's disease

Huntington's disease (HD) is a hereditary progressive neurodegenerative disorder caused by a CAG repeat expansion in the gene coding for the protein huntingtin, resulting in a pathogenic expansion of the polyglutamine tract in the N-terminus of this protein. The HD pathology resulting from the mutation is most prominent in the striatal part of the basal ganglia, and progressive differential dysfunction and loss of striatal projection neurons and interneurons account for the progression of motor deficits seen in this disease. The present review summarizes current understanding regarding the progression in striatal neuron dysfunction and loss, based on studies both in human HD victims and in genetic mouse models of HD. We review evidence on early loss of inputs to striatum from cortex and thalamus, which may be the basis of the mild premanifest bradykinesia in HD, as well as on the subsequent loss of indirect pathway striatal projection neurons and their outputs to the external pallidal segment, which appears to be the basis of the chorea seen in early symptomatic HD. Later loss of direct pathway striatal projection neurons and their output to the internal pallidal segment account for the severe akinesia seen late in HD. Loss of parvalbuminergic striatal interneurons may contribute to the late dystonia and rigidity.

Parvalbumin Interneurons of the Mouse Nucleus Accumbens are Required For Amphetamine-Induced Locomotor Sensitization and Conditioned Place Preference

To determine the requirement for parvalbumin (PV) expressing GABAergic interneurons of the nucleus accumbens (NAc) in the behavioral adaptations induced by amphetamine (AMPH), we blocked synaptic vesicle release from these neurons using Cre-inducible viral expression of the tetanus toxin light chain in male and female PV-Cre mice. Silencing PV+ interneurons of the NAc selectively inhibited the expression of locomotor sensitization following repeated injections of AMPH and blocked AMPH-induced conditioned place preference (CPP). AMPH induced significantly more expression of the activity-dependent gene Fos in both D1 and D2 dopamine receptor-expressing medium spiny neurons (MSNs) of the NAc of PV+ interneuron silenced mice, suggesting a function for PV+ interneuron-mediated MSN inhibition in the expression of AMPH-induced locomotor sensitization and CPP. These data show a requirement for PV+ interneurons of the NAc in behavioral responses to AMPH, and they raise the possibility that modulation of PV+ interneuron function may alter the development or expression of psychostimulant-induced behavioral adaptations.

Anatomy, Physiology, and Clinical Syndromes of the Basal Ganglia: A Brief Review

Movement disorders typically arise from dysfunction of the basal ganglia (BG), cerebellum, or both. The BG-a group of deep, subcortical structures-form complex circuits that shape motor control and motor learning, as well as limbic and associative functions. In this article, we summarize the anatomy and physiology of the BG and cerebellum, and briefly highlight the clinical syndromes that may arise in the context of their injury or dysfunction.

Excitatory extrinsic afferents to striatal interneurons and interactions with striatal microcircuitry

The striatum constitutes the main input structure of the basal ganglia and receives two major excitatory glutamatergic inputs, from the cortex and the thalamus. Excitatory cortico- and thalamostriatal connections innervate the principal neurons of the striatum, the spiny projection neurons (SPNs), which constitute the main cellular input as well as the only output of the striatum. In addition, corticostriatal and thalamostriatal inputs also innervate striatal interneurons. Some of these inputs have been very well studied, for example the thalamic innervation of cholinergic interneurons and the cortical innervation of striatal fast-spiking interneurons, but inputs to most other GABAergic interneurons remain largely unstudied, due in part to the relatively recent identification and characterization of many of these interneurons. In this review, we will discuss and reconcile some older as well as more recent data on the extrinsic excitatory inputs to striatal interneurons. We propose that the traditional feed-forward inhibitory model of the cortical input to the fast-spiking interneuron then inhibiting the SPN, often assumed to be the prototype of the main functional organization of striatal interneurons, is incomplete. We provide evidence that the extrinsic innervation of striatal interneurons is not uniform but shows great cell-type specificity. In addition, we will review data showing that striatal interneurons are themselves interconnected in a highly cell-type-specific manner. These data suggest that the impact of the extrinsic inputs on striatal activity critically depends on synaptic interactions within interneuronal circuitry.

Long-term administration of Greek Royal Jelly decreases GABA concentration in the striatum and hypothalamus of naturally aged Wistar male rats

Royal Jelly (RJ) is a unique substance obtained from bees that has been used widely in European and Asian traditional medicine for its potential to prevent signs of aging through its antioxidative, anti-inflammatory, anti-hyperglycemic and anti-hypercholesterolemic properties. We recently reported an enhancement in spatial memory along with changes in monoaminergic transmission in aged rats after chronic RJ administration. Here, we aim to further explore the action of RJ on central nervous system activity by examining levels of amino acids in selected brain structures of aged male Wistar rats following 2-months of Greek RJ administration. RJ powder was previously chemically characterized and given orally (50 or 100 mg of powder/kg b.w./day) by gastric gavage. The concentrations of amino acids (alanine, aspartic acid, gamma-aminobutyric acid, glutamic acid, histidine and taurine) in the brain regions examined (prefrontal cortex, hippocampus, striatum and hypothalamus) were quantified using HPLC. We also examined basic biochemical parameters of renal and hepatic activity, as damage of these organs could potentially explain the changes in brain function and behavior. Upon biochemical examination, a decrease in the concentration of gamma-aminobutyric acid was observed in both the striatum and hypothalamus. Liver and kidney functions were not changed by chronic RJ-administration. Our results provide insight toward understanding the mechanism of action of RJ and its effects on neurotransmission in the central nervous system.

Photobiomodulation reduces gliosis in the basal ganglia of aged mice

This study explored the effects of long-term photobiomodulation (PBM) on the glial and neuronal organization in the striatum of aged mice. Mice aged 12 months were pretreated with PBM (670 nm) for 20 minutes per day, commencing at 5 months old and continued for 8 months. We had 2 control groups, young at 3 months and aged at 12 months old; these mice received no treatment. Brains were aldehyde-fixed and processed for immunohistochemistry with various glial and neuronal markers. We found a clear reduction in glial cell number, both astrocytes and microglia, in the striatum after PBM in aged mice. By contrast, the number of 2 types of striatal interneurons (parvalbumin⁺ and encephalopsin⁺), together with the density of striatal dopaminergic terminals (and their midbrain cell bodies), remained unchanged after such treatment. In summary, our results indicated that long-term PBM had beneficial effects on the aging striatum by reducing glial cell number; and furthermore, that this treatment did not have any deleterious effects on the neurons and terminations in this nucleus.

Cholinergic interneurons in the rat striatum modulate substitution of habits

Behavioural flexibility is crucial for adaptive behaviour, and recent evidence suggests that cholinergic interneurons of the striatum play a distinct role. Previous studies of cholinergic function have focused on strategy switching by the dorsomedial or ventral striatum. We here investigated whether cholinergic interneurons in the dorsolateral striatum play a similar role at the level of switching of habitual responses. Because the dorsolateral striatum is particularly involved in habitual responding, we developed a habit substitution task that involved switching habitual lever‐press responses to one side to another. We first measured the effect of cholinergic activation in the dorsolateral striatum on this task. Chemogenetic activation of cholinergic interneurons caused an increase in the response rate for the substituted response that was significantly greater than the increase normally seen in control animals. The increase was due to burst‐like responses with shorter inter‐press intervals. However, there was no effect on inhibiting the old habit, or on habitual responding that did not require a switch. There was also no effect on lever‐press performance and its reversal before lever‐press responses became habitual. Conversely, neurochemically specific ablation of cholinergic interneurons did not significantly change habitual responding or response substitution. Thus, activation –but not ablation –of cholinergic interneurons in the dorsolateral striatum modulates expression of a new habit when an old habit is replaced by a new one. Together with previous work, this suggests that striatal cholinergic interneurons facilitate behavioural flexibility in both dorsolateral striatum in addition to dorsomedial and ventral striatum.

Distinct Corticostriatal GABAergic Neurons Modulate Striatal Output Neurons and Motor Activity

The motor cortico-basal ganglion loop is critical for motor planning, execution, and learning. Balanced excitation and inhibition in this loop is crucial for proper motor output. Excitatory neurons have been thought to be the only source of motor cortical input to the striatum. Here, we identify long-range projecting GABAergic neurons in the primary (M1) and secondary (M2) motor cortex that target the dorsal striatum. This population of projecting GABAergic neurons comprises both somatostatin-positive (SOM⁺) and parvalbumin-positive (PV⁺) neurons that target direct and indirect pathway striatal output neurons as well as cholinergic interneurons differentially. Notably, optogenetic stimulation of M1 PV⁺ and M2 SOM⁺ projecting neurons reduced locomotion, whereas stimulation of M1 SOM⁺ projecting neurons enhanced locomotion. Thus, corticostriatal GABAergic projections modulate striatal output and motor activity.

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Long-range GABAergic projections from the motor cortex directly innervate the striatum

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M1 and M2 long-range SOM⁺ and PV⁺ neurons differentially innervate striatal neurons

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Striatal cholinergic neurons are innervated mainly by M1 SOM⁺ projecting neurons

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Motor cortex PV⁺ and SOM⁺ projecting neurons differentially modulate locomotion

Evidence for encephalopsin immunoreactivity in interneurones and striosomes of the monkey striatum

In this study, we examined the cellular distribution of encephalopsin (opsin 3; OPN3) expression in the striatum of non-human primates. In addition, because of our long standing interest in Parkinson's disease and neuroprotection, we examined whether parkinsonian (MPTP; 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) insult and/or photobiomodulation (670 nm) had any impact on encephalopsin expression in this key area of the basal ganglia. Striatal sections of control naïve monkeys, together with those that were either MPTP- and/or photobiomodulation-treated were processed for immunohistochemistry. Our results revealed two populations of striatal interneurones that expressed encephalopsin, one of which was the giant, choline acetyltransferase-containing, cholinergic interneurones. The other population had smaller somata and was not cholinergic. Neither cell group expressed the calcium-binding protein, parvalbumin. There was also rich encephalopsin expression in a set of terminals forming striosome-like patches across the striatum. Finally, we found that neither parkinsonian (MPTP) insult nor photobiomodulation had any effect on encephalopsin expression in the striatum. In summary, our results revealed an extensive network of encephalopsin containing structures throughout the striatum, indicating that external light is in a position to influence a range of striatal activities at both the interneurone and striosome level.

Differential coding of reward and movement information in the dorsomedial striatal direct and indirect pathways

The direct and indirect pathways of the basal ganglia have long been thought to mediate behavioral promotion and inhibition, respectively. However, this classic dichotomous model has been recently challenged. To better understand neural processes underlying reward-based learning and movement control, we recorded from direct (dSPNs) and indirect (iSPNs) pathway spiny projection neurons in the dorsomedial striatum of D1-Cre and D2-Cre mice performing a probabilistic Pavlovian conditioning task. dSPNs tend to increase activity while iSPNs decrease activity as a function of reward value, suggesting the striatum represents value in the relative activity levels of dSPNs versus iSPNs. Lick offset-related activity increase is largely dSPN selective, suggesting dSPN involvement in suppressing ongoing licking behavior. Rapid responses to negative outcome and previous reward-related responses are more frequent among iSPNs than dSPNs, suggesting stronger contributions of iSPNs to outcome-dependent behavioral adjustment. These findings provide new insights into striatal neural circuit operations.

Classically the direct and indirect pathways of basal ganglia are understood to have opposing roles in movement and reward learning, but recent work suggests a more complicated view. Here the authors further study indirect and direct pathway neurons, in the context of a probabilistic reward task.

Regulation of Striatal Neuron Activity by Cyclic Nucleotide Signaling and Phosphodiesterase Inhibition: Implications for the Treatment of Parkinson's Disease

Cyclic nucleotide phosphodiesterase (PDE) enzymes catalyze the hydrolysis and inactivation of cyclic nucleotides (cAMP/cGMP) in the brain. Several classes of PDE enzymes with distinct tissue distributions, cyclic nucleotide selectivity, and regulatory factors are highly expressed in brain regions subserving cognitive and motor processes known to be disrupted in neurodegenerative diseases such as Parkinson's disease (PD). Furthermore, small-molecule inhibitors of several different PDE family members alter cyclic nucleotide levels and favorably enhance motor performance and cognition in animal disease models. This chapter will explore the roles and therapeutic potential of non-selective and selective PDE inhibitors on neural processing in fronto-striatal circuits in normal animals and models of DOPA-induced dyskinesias (LIDs) associated with PD. The impact of selective PDE inhibitors and augmentation of cAMP and cGMP signaling on the membrane excitability of striatal medium-sized spiny projection neurons (MSNs) will be discussed. The effects of cyclic nucleotide signaling and PDE inhibitors on synaptic plasticity of striatonigral and striatopallidal MSNs will be also be reviewed. New data on the efficacy of PDE10A inhibitors for reversing behavioral and electrophysiological correlates of L-DOPA-induced dyskinesias in a rat model of PD will also be presented. Together, these data will highlight the potential of novel PDE inhibitors for treatment of movement disorders such as PD which are associated with abnormal corticostriatal transmission.

Parvalbumin-producing striatal interneurons receive excitatory inputs onto proximal dendrites from the motor thalamus in male mice

In rodents, the dorsolateral striatum regulates voluntary movement by integrating excitatory inputs from the motor-related cerebral cortex and thalamus to produce contingent inhibitory output to other basal ganglia nuclei. Striatal parvalbumin (PV)-producing interneurons receiving this excitatory input then inhibit medium spiny neurons (MSNs) and modify their outputs. To understand basal ganglia function in motor control, it is important to reveal the precise synaptic organization of motor-related cortical and thalamic inputs to striatal PV interneurons. To examine which domains of the PV neurons receive these excitatory inputs, we used male bacterial artificial chromosome transgenic mice expressing somatodendritic membrane-targeted green fluorescent protein in PV neurons. An anterograde tracing study with the adeno-associated virus vector combined with immunodetection of pre- and postsynaptic markers visualized the distribution of the excitatory appositions on PV dendrites. Statistical analysis revealed that the density of thalamostriatal appositions along the dendrites was significantly higher on the proximal than distal dendrites. In contrast, there was no positional preference in the density of appositions from axons of the dorsofrontal cortex. Population observations of thalamostriatal and corticostriatal appositions by immunohistochemistry for pathway-specific vesicular glutamate transporters confirmed that thalamic inputs preferentially, and cortical ones less preferentially, made apposition on proximal dendrites of PV neurons. This axodendritic organization suggests that PV neurons produce fast and reliable inhibition of MSNs in response to thalamic inputs and process excitatory inputs from motor cortices locally and plastically, possibly together with other GABAergic and dopaminergic dendritic inputs, to modulate MSN inhibition.

Accumbal D2 cells orchestrate innate risk-avoidance according to orexin signals

Excitation of accumbal D2 cells governs vital actions, including avoidance of learned risks, but the origins of this excitation and roles of D2 cells in innate risk-avoidance are unclear. Hypothalamic neurons producing orexins (also called hypocretins) enhance innate risk-avoidance via poorly understood neurocircuits. We describe a direct orexin→D2 excitatory circuit and show that D2 cell activity is necessary for orexin-dependent innate risk-avoidance in mice, thus revealing an unsuspected hypothalamus-accumbens interplay in action selection.

GABA and Glutamate Synaptic Coadaptations to Chronic Ethanol in the Striatum

Alcohol (ethanol) is a widely used and abused drug with approximately 90% of adults over the age of 18 consuming alcohol at some point in their lifetime. Alcohol exerts its actions through multiple neurotransmitter systems within the brain, most notably the GABAergic and glutamatergic systems. Alcohol's actions on GABAergic and glutamatergic neurotransmission have been suggested to underlie the acute behavioral effects of ethanol. The striatum is the primary input nucleus of the basal ganglia that plays a role in motor and reward systems. The effect of ethanol on GABAergic and glutamatergic neurotransmission within striatal circuitry has been thought to underlie ethanol taking, seeking, withdrawal and relapse. This chapter reviews the effects of ethanol on GABAergic and glutamatergic transmission, highlighting the dynamic changes in striatal circuitry from acute to chronic exposure and withdrawal.

MeCP2 Expression in a Rat Model of Risky Decision Making

Many neuropsychiatric disorders are associated with abnormal decision making involving risk of punishment, but the underlying molecular basis remains poorly understood. Methyl CpG-binding protein 2 (MeCP2) is an epigenetic factor that regulates transcription by directly binding to methylated DNA. Here, we evaluated MeCP2 expression in the context of risk-taking behaviors using the Risky Decision-making Task (RDT), in which rats make discrete choices between a small "safe" food reward and a large "risky" food reward accompanied by varying probabilities of punishment. In Experiment 1, expression of MeCP2 as assessed by immunoblotting in the medial prefrontal cortex (mPFC), but not the striatum, was inversely correlated with the degree of preference for the large, risky reward (risk taking) seven days after the last RDT test. In Experiment 2, MeCP2 expression 90 min after RDT testing, assessed using immunohistochemistry, was suppressed in both the dorsal mPFC (dmPFC) and nucleus accumbens compared to home cage controls, indicating that MeCP2 expression is modulated by RDT performance. Additional experiments revealed that RDT performance increased expression of MeCP2 phosphorylated at Ser421 (associated with neuronal activity and activation of gene expression) in dmPFC principal neurons. Finally, as in Experiment 1, lower expression of MeCP2 in the ventral mPFC was associated with greater risk taking under baseline conditions. Together, these findings indicate a complex regulatory role of MeCP2 in risky decision making, and suggest that epigenetic factors may be an important component of the molecular mechanisms underlying such decision-making processes.

Progress in developing transgenic monkey model for Huntington's disease

Huntington's disease (HD) is a complex neurodegenerative disorder that has no cure. Although treatments can often be given to relieve symptoms, the neuropathology associated with HD cannot be stopped or reversed. HD is characterized by degeneration of the striatum and associated pathways that leads to impairment in motor and cognitive functions as well as psychiatric disturbances. Although cell and rodent models for HD exist, longitudinal study in a transgenic HD nonhuman primate (i.e., rhesus macaque; HD monkeys) shows high similarity in its progression with human patients. Progressive brain atrophy and changes in white matter integrity examined by magnetic resonance imaging are coherent with the decline in cognitive behaviors related to corticostriatal functions and neuropathology. HD monkeys also express higher anxiety and irritability/aggression similar to human HD patients that other model systems have not yet replicated. While a comparative model approach is critical for advancing our understanding of HD pathogenesis, HD monkeys could provide a unique platform for preclinical studies and long-term assessment of translatable outcome measures. This review summarizes the progress in the development of the transgenic HD monkey model and the opportunities for advancing HD preclinical research.

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.

Dysregulation of Corticostriatal Connectivity in Huntington's Disease: A Role for Dopamine Modulation

Aberrant communication between striatum, the main information processing unit of the basal ganglia, and cerebral cortex plays a critical role in the emergence of Huntington’s disease (HD), a fatal monogenetic condition that typically strikes in the prime of life. Although both striatum and cortex undergo substantial cell loss over the course of HD, corticostriatal circuits become dysfunctional long before neurons die. Understanding the dysfunction is key to developing effective strategies for treating a progressively worsening triad of motor, cognitive, and psychiatric symptoms. Cortical output neurons drive striatal activity through the release of glutamate, an excitatory amino acid. Striatal outputs, in turn, release γ-amino butyric acid (GABA) and exert inhibitory control over downstream basal ganglia targets. Ample evidence from transgenic rodent models points to dysregulation of corticostriatal glutamate transmission along with corresponding changes in striatal GABA release as underlying factors in the HD behavioral phenotype. Another contributor is dysregulation of dopamine (DA), a modulator of both glutamate and GABA transmission. In fact, pharmacological manipulation of DA is the only currently available treatment for HD symptoms. Here, we review data from animal models and human patients to evaluate the role of DA in HD, including DA interactions with glutamate and GABA within the context of dysfunctional corticostriatal circuitry.

Immunoreactivities of calbindin‑D28k, calretinin and parvalbumin in the somatosensory cortex of rodents during normal aging

Calbindin-D28k (CB), calretinin (CR) and parvalbumin (PV), which regulate cytosolic free Ca²⁺ concentrations in neurons, are chemically expressed in γ-aminobutyric acid (GABA)ergic neurons that regulate the degree of glutamatergic excitation and output of projection neurons. The present study investigated age-associated differences in CB, CR and PV immunoreactivities in the somatosensory cortex in three species (mice, rats and gerbils) of young (1 month), adult (6 months) and aged (24 months) rodents, using immunohistochemistry and western blotting. Abundant CB-immunoreactive neurons were distributed in layers II and III, and age-associated alterations in their number were different according to the species. CR-immunoreactive neurons were not abundant in all layers; however, the number of CR-immunoreactive neurons was the highest in all adult species. Many PV-immunoreactive neurons were identified in all layers, particularly in layers II and III, and they increased in all layers with age in all species. The present study demonstrated that the distribution pattern of CB-, CR- and PV-containing neurons in the somatosensory cortex were apparently altered in number with normal aging, and that CB and CR exhibited a tendency to decrease in aged rodents, whereas PV tended to increase with age. These results indicate that CB, CR and PV are markedly altered in the somatosensory cortex, and this change may be associated with normal aging. These findings may aid the elucidation of the mechanisms of aging and geriatric disease.

Striatal fast-spiking interneurons selectively modulate circuit output and are required for habitual behavior

Habit formation is a behavioral adaptation that automates routine actions. Habitual behavior correlates with broad reconfigurations of dorsolateral striatal (DLS) circuit properties that increase gain and shift pathway timing. The mechanism(s) for these circuit adaptations are unknown and could be responsible for habitual behavior. Here we find that a single class of interneuron, fast-spiking interneurons (FSIs), modulates all of these habit-predictive properties. Consistent with a role in habits, FSIs are more excitable in habitual mice compared to goal-directed and acute chemogenetic inhibition of FSIs in DLS prevents the expression of habitual lever pressing. In vivo recordings further reveal a previously unappreciated selective modulation of SPNs based on their firing patterns; FSIs inhibit most SPNs but paradoxically promote the activity of a subset displaying high fractions of gamma-frequency spiking. These results establish a microcircuit mechanism for habits and provide a new example of how interneurons mediate experience-dependent behavior.

From biting fingernails to the daily commute, habits are sets of actions that can be completed almost without thinking and that are difficult to change or stop. Behavioral neuroscientists refer to habits as “stimulus-response” behaviors, and know that forming a new habit requires a region deep within the brain called the dorsolateral striatum. Indeed, in this region, the outgoing neurons – which make up 95% of the cells - respond differently to incoming signals in mice that have learned habits compared to non-habitual mice. However a question remained: what exactly was producing these differences?

O’Hare et al. have now found, unexpectedly, that the answer resides not in the 95% of outgoing neurons, but rather in a rare type of cell known as the fast-spiking interneuron. This cell is connected to many others and it appears to act like a conductor, orchestrating the previously identified changes in the output neurons. These findings were made using mice that had been trained to press a lever for a sugar pellet reward. Habit was measured by how long mice kept pressing even if they had just been allowed to eat their fill of pellets and the test lever was no longer dispensing pellets. Habitual mice continue to press the lever in this circumstance, while other mice do not.

O’Hare et al. found that inactivating the “conductor” cell made the output neurons respond in the opposite way to how they normally respond in habitual mice. Further experiments showed that fast-spiking interneurons were also more easily activated in habitual mice. To test whether this putative “conductor” cell was necessary for habitual behaviors, a technique known as chemogenetics was used to turn down its activity in habitual mice. Indeed, reducing activity in the conductor cell blocked the habitual behavior.

While some habits are a helpful and economical way to get through daily life, habits are also thought to be corrupted in a number of diseases such as neurodegenerative diseases, addictions and compulsions. Identifying this specific, yet rare, cell as a critical part of maintaining habits points out a new target to consider for therapies. Further work is needed before such treatments might become available to treat habit-related disorders; though O'Hare et al. are now taking steps in this direction by trying to work out how the fast-spiking interneuron changes its own activity when a habit is formed.

Selective Sparing of Striatal Interneurons after Poly (ADP-Ribose) Polymerase 1 Inhibition in the R6/2 Mouse Model of Huntington's Disease

Poly (ADP-ribose) polymerases (PARPs) are enzymes that catalyze ADP-ribose units transfer from NAD to their substrate proteins. It has been observed that PARP-1 is able to increase both post-ischemic and excitotoxic neuronal death. In fact, we have previously shown that, INO-1001, a PARP-1 inhibitor, displays a neuroprotective effect in the R6/2 model of Huntington’s disease (HD). In this study, we investigated the effects of PARP-1-inhibition on modulation of phosphorylated c-AMP response element binding protein (pCREB) and CREB-binding protein (CBP) localization in the different striatal neuronal subsets. Moreover, we studied the neurodegeneration of those interneurons that are particularly vulnerable to HD such as parvalbuminergic and calretininergic, and of other subclasses of interneurons that are known to be resistant, such as cholinergic and somatostatinergic interneurons. Transgenic mice were treated with INO-1001 (10 mg/Kg daily) starting from 4 weeks of age. Double-label immunofluorescence was performed to value the distribution of CBP in ubiquitinated Neuronal intranuclear inclusions (NIIs) in the striatum. INO-1001-treated and saline-treated brain sections were incubated with: goat anti-choline acetyl transferase; goat anti-nitric oxide synthase; mouse anti-parvalbumin and mouse anti-calretinin. Morphometric evaluation and cell counts were performed. Our study showed that the PARP inhibitor has a positive effect in sparing parvalbumin and calretinin-containing interneurons of the striatum, where CREB was upregulated. Moreover, INO-1001 promoted CBP localization into the nuclei of the R6/2 mouse. The sum of our data corroborates the previous observations indicating PARP inhibition as a possible therapeutic tool to fight HD.

Background activity and visual responsiveness of caudate nucleus neurons in halothane anesthetized and in awake, behaving cats

This study focuses on the important question whether brain activity recorded from anesthetized, paralyzed animals is comparable to that recorded from awake, behaving ones. We compared neuronal activity recorded from the caudate nucleus (CN) of two halothane-anesthetized, paralyzed and two awake, behaving cats. In both models, extracellular recordings were made from the CN during static and dynamic visual stimulation. The anesthesia was maintained during the recordings by a gaseous mixture of air and halothane (1.0%). The behaving animals were trained to perform a visual fixation task. Based on their electrophysiological properties, the recorded CN neurons were separated into three different classes: phasically active (PANs), high firing (HFNs), and tonically active (TANs) neurons. Halothane anesthesia significantly decreased the background activity of the CN neurons in all three classes. The anesthesia had the most remarkable suppressive effect on PANs, where the background activity was consistently under 1 spike/s. The analysis of these responses was almost impossible due to the extremely low activity. The evoked responses during both static and dynamic visual stimulation were obvious in the behaving cats. On the other hand, only weak visual responses were found in some neurons of halothane anesthetized cats. These results show that halothane gas anesthesia has a marked suppressive effect on the feline CN. We suggest that for the purposes of the visual and related multisensory/sensorimotor electrophysiological exploration of the CN, behaving animal models are preferable over anesthetized ones.

Gamma Oscillations in the Rat Ventral Striatum Originate in the Piriform Cortex

Local field potentials (LFPs) recorded from the human and rodent ventral striatum (vStr) exhibit prominent, behaviorally relevant gamma-band oscillations. These oscillations are related to local spiking activity and transiently synchronize with anatomically related areas, suggesting a possible role in organizing vStr activity. However, the origin of vStr gamma is unknown. We recorded vStr gamma oscillations across a 1.4 mm² grid spanned by 64 recording electrodes as male rats rested and foraged for rewards, revealing a highly consistent power gradient originating in the adjacent piriform cortex. Phase differences across the vStr were consistently small (<15°) and current source density analysis further confirmed the absence of local sink-source pairs in the vStr. Reversible occlusions of the ipsilateral (but not contralateral) nostril, known to abolish gamma oscillations in the piriform cortex, strongly reduced vStr gamma power and the occurrence of transient gamma-band events. These results imply that local circuitry is not a major contributor to gamma oscillations in the vStr LFP and that piriform cortex is an important driver of gamma-band oscillations in the vStr and associated limbic areas.SIGNIFICANCE STATEMENT The ventral striatum (vStr) is an area of anatomical convergence in circuits underlying motivated behavior, but it remains unclear how its inputs from different sources interact. A major proposal about how neural circuits may switch dynamically between convergent inputs is through temporal organization reflected in local field potential (LFP) oscillations. Our results show that, in the rat, the mechanisms controlling gamma-band oscillations in the vStr LFP are primarily located in the in the adjacent piriform cortex rather than in the vStr itself, providing a novel interpretation of previous rodent work on gamma oscillations in the vStr and related circuits and an important consideration for future work seeking to use oscillations in these areas as biomarkers for behavioral and neurological disorders.

Cholinergic Interneurons Use Orbitofrontal Input to Track Beliefs about Current State

When conditions change, organisms need to learn about the changed conditions without interfering with what they already know. To do so, they can assign the new learning to a new "state" and the old learning to a previous state. This state assignment is fundamental to behavioral flexibility. Cholinergic interneurons (CINs) in the dorsomedial striatum (DMS) are necessary for associative information to be compartmentalized in this way, but the mechanism by which they do so is unknown. Here we addressed this question by recording putative CINs from the DMS in rats performing a task consisting of a series of trial blocks, or states, that required the recall and application of contradictory associative information. We found that individual CINs in the DMS represented the current state throughout each trial. These state correlates were not observed in dorsolateral striatal CINs recorded in the same rats. Notably, DMS CIN ensembles tracked rats' beliefs about the current state such that, when states were miscoded, rats tended to make suboptimal choices reflecting the miscoding. State information held by the DMS CINs also depended completely on the orbitofrontal cortex, an area that has been proposed to signal environmental states. These results suggest that CINs set the stage for recalling associative information relevant to the current environment by maintaining a real-time representation of the current state. Such a role has novel implications for understanding the neural basis of a variety of psychiatric diseases, such as addiction or anxiety disorders, in which patients generalize inappropriately (or fail to generalize) between different environments.

SIGNIFICANCE STATEMENT

Striatal cholinergic interneurons (CINs) are thought to be identical to tonically active neurons. These neurons have long been thought to have an important influence on striatal processing during reward-related learning. Recently, a more specific function for striatal CINs has been suggested, which is that they are necessary for striatal learning to be compartmentalized into different states as the state of the environment changes. Here we report that putative CINs appear to track rats' beliefs about which environmental state is current. We further show that this property of CINs depends on orbitofrontal cortex input and is correlated with choices made by rats. These findings could provide new insight into neuropsychiatric diseases that involve improper generalization between different contexts.

Dysregulation of striatal projection neurons in Parkinson's disease

The loss of nigrostriatal dopamine (DA) is the primary cause of motor dysfunction in Parkinson's disease (PD), but the underlying striatal mechanisms remain unclear. In spite of abundant literature portraying structural, biochemical and plasticity changes of striatal projection neurons (SPNs), in the past there has been a data vacuum from the natural human disease and its close model in non-human primates. Recently, single-cell recordings in advanced parkinsonian primates have generated new insights into the altered function of SPNs. Currently, there are also human data that provide direct evidence of profoundly dysregulated SPN activity in PD. Here, we review primate recordings that are impacting our understanding of the striatal dysfunction after DA loss, particularly through the analysis of physiologic correlates of parkinsonian motor behaviors. In contrast to recordings in rodents, data obtained in primates and patients demonstrate similar major abnormalities of the spontaneous SPN firing in the alert parkinsonian state. Furthermore, these studies also show altered SPN responses to DA replacement in the advanced parkinsonian state. Clearly, there is yet much to learn about the striatal discharges in PD, but studies using primate models are contributing unique information to advance our understanding of pathophysiologic mechanisms.

Progression and Persistence of Neurotoxicity Induced by MDMA in Dopaminergic Regions of the Mouse Brain and Association with Noradrenergic, GABAergic, and Serotonergic Damage

The amphetamine-related drug 3,4-methylenedioxymethamphetamine (MDMA) is known to induce neurotoxic damage in dopaminergic regions of the mouse brain. In order to characterize how the number of administrations influenced the severity of MDMA-induced dopaminergic damage and to describe the localization and persistence of this damage, we evaluated the changes in tyrosine hydroxylase (TH) and dopamine transporter (DAT) in different regions of the mouse brain. Moreover, we investigated whether dopaminergic damage was associated with noradrenergic, GABAergic, and serotonergic damage, by evaluating the changes in noradrenaline transporter (NET), glutamic acid decarboxylase-67 (GAD-67), and serotonin transporter (SERT). Mice received 14, 28, or 36 MDMA administrations (10 mg/kg twice a week) and were sacrificed at different time points (postnatal days 85, 110, 138, or 214) for immunohistochemical evaluation. Mice receiving 28 administrations showed reduced levels of DAT-positive fibers in caudate-putamen (CPu) and medial prefrontal cortex (mPFC) and reduced levels of TH-positive nigral neurons. These mice also displayed increased NET-positive hippocampal fibers, reduced GAD-67-positive neurons in CPu and hippocampus, and reduced GAD-67-positive fibers in mPFC. Similar effects of MDMA on DAT, TH, and GAD-67 were found in mice receiving 36 administrations, which also displayed reduced levels of striatal, cortical, and hippocampal TH-immunoreactive fibers. The reductions in dopaminergic markers and GAD-67 persisted at 3 months after MDMA discontinuation. Finally, MDMA never modified the levels of SERT. These results provide further insight into the localization and persistence of MDMA-induced dopaminergic damage and show that this effect may associate with GABAergic but not noradrenergic or serotonergic damage.

Loss and remodeling of striatal dendritic spines in Parkinson's disease: from homeostasis to maladaptive plasticity?

In Parkinson's disease (PD) patients and animal models of PD, the progressive degeneration of the nigrostriatal dopamine (DA) projection leads to two major changes in the morphology of striatal projection neurons (SPNs), i.e., a profound loss of dendritic spines and the remodeling of axospinous glutamatergic synapses. Striatal spine loss is an early event tightly associated with the extent of striatal DA denervation, but not the severity of parkinsonian motor symptoms, suggesting that striatal spine pruning might be a form of homeostatic plasticity that compensates for the loss of striatal DA innervation and the resulting dysregulation of corticostriatal glutamatergic transmission. On the other hand, the remodeling of axospinous corticostriatal and thalamostriatal glutamatergic synapses might represent a form of late maladaptive plasticity that underlies changes in the strength and plastic properties of these afferents and the resulting increased firing and bursting activity of striatal SPNs in the parkinsonian state. There is also evidence that these abnormal synaptic connections might contribute to the pathophysiology of L-DOPA-induced dyskinesia. Despite the significant advances made in this field over the last thirty years, many controversial issues remain about the striatal SPN subtypes affected, the role of spine changes in the altered activity of SPNs in the parkinsonisn state, and the importance of striatal spine plasticity in the pathophysiology of L-DOPA-induced dyskinesia. In this review, we will examine the current state of knowledge of these issues, discuss the limitations of the animal models used to address some of these questions, and assess the relevance of data from animal models to the human-diseased condition.

The Nucleus Accumbens: Mechanisms of Addiction across Drug Classes Reflect the Importance of Glutamate Homeostasis

The nucleus accumbens is a major input structure of the basal ganglia and integrates information from cortical and limbic structures to mediate goal-directed behaviors. Chronic exposure to several classes of drugs of abuse disrupts plasticity in this region, allowing drug-associated cues to engender a pathologic motivation for drug seeking. A number of alterations in glutamatergic transmission occur within the nucleus accumbens after withdrawal from chronic drug exposure. These drug-induced neuroadaptations serve as the molecular basis for relapse vulnerability. In this review, we focus on the role that glutamate signal transduction in the nucleus accumbens plays in addiction-related behaviors. First, we explore the nucleus accumbens, including the cell types and neuronal populations present as well as afferent and efferent connections. Next we discuss rodent models of addiction and assess the viability of these models for testing candidate pharmacotherapies for the prevention of relapse. Then we provide a review of the literature describing how synaptic plasticity in the accumbens is altered after exposure to drugs of abuse and withdrawal and also how pharmacological manipulation of glutamate systems in the accumbens can inhibit drug seeking in the laboratory setting. Finally, we examine results from clinical trials in which pharmacotherapies designed to manipulate glutamate systems have been effective in treating relapse in human patients. Further elucidation of how drugs of abuse alter glutamatergic plasticity within the accumbens will be necessary for the development of new therapeutics for the treatment of addiction across all classes of addictive substances.

Learning new sequential stepping patterns requires striatal plasticity during the earliest phase of acquisition

Animals including humans execute motor behavior to reach their goals. For this purpose, they must choose correct strategies according to environmental conditions and shape many parameters of their movements, including their serial order and timing. To investigate the neurobiology underlying such skills, we used a multi-sensor equipped, motor-driven running wheel with adjustable sequences of foothold pegs on which mice ran to obtain water reward. When the peg patterns changed from a familiar pattern to a new pattern, the mice had to learn and implement new locomotor strategies in order to receive reward. We found that the accuracy of stepping and the achievement of water reward improved with the new learning after changes in the peg-pattern, and c-Fos expression levels assayed after the first post-switch session were high in both dorsolateral striatum and motor cortex, relative to post-switch plateau levels. Combined in situ hybridization and immunohistochemistry of striatal sections demonstrated that both enkephalin-positive (indirect pathway) neurons and substance P-positive (direct pathway) neurons were recruited specifically after the pattern switches, as were interneurons expressing neuronal nitric oxide synthase. When we blocked N-methyl-D-aspartate (NMDA) receptors in the dorsolateral striatum by injecting the NMDA receptor antagonist, D-2-amino-5-phosphonopentanoic acid (AP5), we found delays in early post-switch improvement in performance. These findings suggest that the dorsolateral striatum is activated on detecting shifts in environment to adapt motor behavior to the new context via NMDA-dependent plasticity, and that this plasticity may underlie forming and breaking skills and habits as well as to behavioral difficulties in clinical disorders.

M1 muscarinic activation induces long-lasting increase in intrinsic excitability of striatal projection neurons

The dorsolateral striatum is critically involved in movement control and motor learning. Striatal function is regulated by a variety of neuromodulators including acetylcholine. Previous studies have shown that cholinergic activation excites striatal principal projection neurons, medium spiny neurons (MSNs), and this action is mediated by muscarinic acetylcholine subtype 1 receptors (M₁) through modulating multiple potassium channels. In the present study, we used electrophysiology techniques in conjunction with optogenetic and pharmacological tools to determine the long-term effects of striatal cholinergic activation on MSN intrinsic excitability. A transient increase in acetylcholine release in the striatum by optogenetic stimulation resulted in a long-lasting increase in excitability of MSNs, which was associated with hyperpolarizing shift of action potential threshold and decrease in afterhyperpolarization (AHP) amplitude, leading to an increase in probability of EPSP-action potential coupling. The M₁ selective antagonist VU0255035 prevented, while the M₁ selective positive allosteric modulator (PAM) VU0453595 potentiated the cholinergic activation-induced persistent increase in MSN intrinsic excitability, suggesting that M₁ receptors are critically involved in the induction of this long-lasting response. This M₁ receptor-dependent long-lasting change in MSN intrinsic excitability could have significant impact on striatal processing and might provide a novel mechanism underlying cholinergic regulation of the striatum-dependent motor learning and cognitive function. Consistent with this, behavioral studies indicate that potentiation of M₁ receptor signaling by VU0453595 enhanced performance of mice in cue-dependent water-based T-maze, a dorsolateral striatum-dependent learning task.

Circadian Plasticity of Mammalian Inhibitory Interneurons

Inhibitory interneurons participate in all neuronal circuits in the mammalian brain, including the circadian clock system, and are indispensable for their effective function. Although the clock neurons have different molecular and electrical properties, their main function is the generation of circadian oscillations. Here we review the circadian plasticity of GABAergic interneurons in several areas of the mammalian brain, suprachiasmatic nucleus, neocortex, hippocampus, olfactory bulb, cerebellum, striatum, and in the retina.

Diacylglycerol kinase ε localizes to subsurface cisterns of cerebellar Purkinje cells

Following activation of Gq protein-coupled receptors, phospholipase C yields a pair of second messengers: diacylglycerol (DG) and inositol 1,4,5-trisphosphate. Diacylglycerol kinase (DGK) phosphorylates DG to produce phosphatidic acid, another second messenger. Of the DGK family, DGKε is the only DGK isoform that exhibits substrate specificity for DG with an arachidonoyl acyl chain at the sn-2 position. Recently, we demonstrated that hydrophobic residues in the N-terminus of DGKε play an important role in targeting the endoplasmic reticulum in transfected cells. However, its cellular expression and subcellular localization in the brain remain elusive. In the present study, we investigate this issue using specific DGKε antibody. DGKε was richly expressed in principal neurons of higher brain regions, including pyramidal cells in the hippocampus and neocortex, medium spiny neurons in the striatum and Purkinje cells in the cerebellum. In Purkinje cells, DGKε was localized to the subsurface cisterns and colocalized with inositol 1,4,5-trisphosphate receptor-1 in dendrites and axons. In dendrites of Purkinje cells, DGKε was also distributed in close apposition to DG lipase-α, which catalyzes arachidonoyl-DG to produce 2-arachidonoyl glycerol, a major endocannabinoid in the brain. Behaviorally, DGKε-knockout mice exhibited hyper-locomotive activities and impaired motor coordination and learning. These findings suggest that DGKε plays an important role in neuronal and brain functions through its distinct neuronal expression and subcellular localization and also through coordinated arrangement with other molecules involving the phosphoinositide signaling pathway.

Calcium-binding protein, secretagogin, characterizes novel groups of interneurons in the rat striatum

In the rat striatum numerous secretagogin (SCGN) positive neurons were scattered. They were heterogeneous in their morphological and chemical properties. We examined the colocalization of SCGN with known four interneuron markers, parvalbumin (PV), calretinin (CR), nitric oxide synthase (NOS) and choline acetyl transferase (ChAT). 60-70% of SCGN positive striatal neurons contained either PV or CR or ChAT, but none contained NOS. On the other hand the remaining 30-40% expressed none of these markers, most of which were GAD positive. The present study indicates that there are hitherto unknown groups of striatal interneurons in the rat striatum.