Behavioural assessment of mice lacking D1A dopamine receptors

Distinct basal ganglia contributions to learning from implicit and explicit value signals in perceptual decision-making

Metacognitive evaluations of confidence provide an estimate of decision accuracy that could guide learning in the absence of explicit feedback. We examine how humans might learn from this implicit feedback in direct comparison with that of explicit feedback, using simultaneous EEG-fMRI. Participants performed a motion direction discrimination task where stimulus difficulty was increased to maintain performance, with intermixed explicit- and no-feedback trials. We isolate single-trial estimates of post-decision confidence using EEG decoding, and find these neural signatures re-emerge at the time of feedback together with separable signatures of explicit feedback. We identified these signatures of implicit versus explicit feedback along a dorsal-ventral gradient in the striatum, a finding uniquely enabled by an EEG-fMRI fusion. These two signals appear to integrate into an aggregate representation in the external globus pallidus, which could broadcast updates to improve cortical decision processing via the thalamus and insular cortex, irrespective of the source of feedback.

Separation of Channels Subserving Approach and Avoidance/Escape at the Level of the Basal Ganglia and Related Brainstem Structures

The basal ganglia have the key function of directing our behavior in the context of events from our environment and/or our internal state. This function relies on afferents targeting the main input structures of the basal ganglia, entering bids for action selection at the level of the striatum or signals for behavioral interruption at the level of the subthalamic nucleus, with behavioral reselection facilitated by dopamine signaling. Numerous experiments have studied action selection in relation to inputs from the cerebral cortex. However, less is known about the anatomical and functional link between the basal ganglia and the brainstem. In this review, we describe how brainstem structures also project to the main input structures of the basal ganglia, namely the striatum, the subthalamic nucleus and midbrain dopaminergic neurons, in the context of approach and avoidance (including escape from threat), two fundamental, mutually exclusive behavioral choices in an animal's repertoire in which the brainstem is strongly involved. We focus on three particularly well-described loci involved in approach and avoidance, namely the superior colliculus, the parabrachial nucleus and the periaqueductal grey nucleus. We consider what is known about how these structures are related to the basal ganglia, focusing on their projections toward the striatum, dopaminergic neurons and subthalamic nucleus, and explore the functional consequences of those interactions.

The Subthalamic Nucleus: A Hub for Sensory Control via Short Three- Lateral Loop Connections with the Brainstem?

The subthalamic nucleus (STN) is classically subdivided into sensori-motor, associative and limbic regions, which is consistent with the involvement of this structure in not only motor control, but also in cognitive and emotional tasks. However, the function of the sensory inputs to the STN's sensori-motor territory is comparatively less well explored, although sensory responses have been reported in this structure. There is still a paucity of information regarding the characteristics of that subdivision and its potential functional role in basal ganglia processing and more widely in associated networks. In this perspective paper, we summarize the type of sensory stimuli that have been reported to activate the STN, and describe the complex sensory properties of the STN and its anatomical link to a sensory network involving the brainstem, characterized in our recent work. Analyzing the sensory input to the STN led us to suggest the existence of previously unreported threelateral subcortical loops between the basal ganglia and the brainstem which do not involve the cortex. Anatomically, these loops closely link the STN, the substantia nigra pars reticulata and various structures from the brainstem such as the superior colliculus and the parabrachial nucleus. We also discuss the potential role of the STN in the control of sensory activity in the brainstem and its possible contribution to favoring sensory habituation or sensitization over brainstem structures to optimize the best selection of action at a given time.

Nigral neuropathology of Parkinson's motor subtypes coincide with circuitopathies: a scoping review

The neuropathological substrates of Parkinson’s disease (PD) patients with motor subtypes tremor-dominance (TD), non-tremor dominance (nTD), postural instability and gait difficulty (PIGD), and akinetic-rigid (AR) are not completely differentiated. While extensive pathological research has been conducted on neuronal tissue of PD patients, data have not been discussed in the context of mechanistic circuitry theories differentiating motor subtypes. It is, therefore, expected that a more specific and tailored management of PD symptoms can be accomplished by understanding symptom-specific neuropathological mechanisms with the detail histology can provide. This scoping review gives an overview of the literature comparing TD and nTD PD motor subtypes by clarify observed pathology with underlying physiological circuitry theories. Studies using an array of pathological examination techniques have shown significant differences between TD and nTD PD subtypes. nTD PD patients show higher neuronal loss, gliosis, extraneuronal melanin deposits, and neuroaxonal dystrophy in multiple subregions of the substantia nigra (SN) related to the overactivity of the indirect motor loop. TD patients show more severe cell loss specifically in medial SN subdivisions, and have damage in the retrorubral field A-8 that projects to the dorsolateral striatum and ventromedial thalamus in the direct motor loop. Pathological studies are consistent with neuroimaging data and support contemporary mechanistic circuitry theories of PD motor symptom genesis. Further multimodal neuroimaging and histological studies are required to validate and expand upon these findings.

Differential Dopamine Receptor-Dependent Sensitivity Improves the Switch Between Hard and Soft Selection in a Model of the Basal Ganglia

The problem of selecting one action from a set of different possible actions, simply referred to as the problem of action selection, is a ubiquitous challenge in the animal world. For vertebrates, the basal ganglia (BG) are widely thought to implement the core computation to solve this problem, as its anatomy and physiology are well suited to this end. However, the BG still display physiological features whose role in achieving efficient action selection remains unclear. In particular, it is known that the two types of dopaminergic receptors (D1 and D2) present in the BG give rise to mechanistically different responses. The overall effect will be a difference in sensitivity to dopamine, which may have ramifications for action selection. However, which receptor type leads to a stronger response is unclear due to the complexity of the intracellular mechanisms involved. In this study, we use an existing, high-level computational model of the BG, which assumes that dopamine contributes to action selection by enabling a switch between different selection regimes, to predict which of D1 or D2 has the greater sensitivity. Thus, we ask, Assuming dopamine enables a switch between action selection regimes in the BG, what functional sensitivity values would result in improved action selection computation? To do this, we quantitatively assessed the model's capacity to perform action selection as we parametrically manipulated the sensitivity weights of D1 and D2. We show that differential (rather than equal) D1 and D2 sensitivity to dopaminergic input improves the switch between selection regimes during the action selection computation in our model. Specifically, greater D2 sensitivity compared to D1 led to these improvements.

The Signaling and Pharmacology of the Dopamine D1 Receptor

The dopamine D1 receptor (D1R) is a Gα_s/olf-coupled GPCR that is expressed in the midbrain and forebrain, regulating motor behavior, reward, motivational states, and cognitive processes. Although the D1R was initially identified as a promising drug target almost 40 years ago, the development of clinically useful ligands has until recently been hampered by a lack of suitable candidate molecules. The emergence of new non-catechol D1R agonists, biased agonists, and allosteric modulators has renewed clinical interest in drugs targeting this receptor, specifically for the treatment of motor impairment in Parkinson's Disease, and cognitive impairment in neuropsychiatric disorders. To develop better therapeutics, advances in ligand chemistry must be matched by an expanded understanding of D1R signaling across cell populations in the brain, and in disease states. Depending on the brain region, the D1R couples primarily to either Gα_s or Gα_olf through which it activates a cAMP/PKA-dependent signaling cascade that can regulate neuronal excitability, stimulate gene expression, and facilitate synaptic plasticity. However, like many GPCRs, the D1R can signal through multiple downstream pathways, and specific signaling signatures may differ between cell types or be altered in disease. To guide development of improved D1R ligands, it is important to understand how signaling unfolds in specific target cells, and how this signaling affects circuit function and behavior. In this review, we provide a summary of D1R-directed signaling in various neuronal populations and describe how specific pathways have been linked to physiological and behavioral outcomes. In addition, we address the current state of D1R drug development, including the pharmacology of newly developed non-catecholamine ligands, and discuss the potential utility of D1R-agonists in Parkinson's Disease and cognitive impairment.

Striatal indirect pathway mediates exploration via collicular competition

The ability to suppress actions that lead to a negative outcome and explore alternative actions is necessary for optimal decision making. Although the basal ganglia have been implicated in these processes^1-5, the circuit mechanisms underlying action selection and exploration remain unclear. Here, using a simple lateralized licking task, we show that indirect striatal projection neurons (iSPN) in the basal ganglia contribute to these processes through modulation of the superior colliculus (SC). Optogenetic activation of iSPNs suppresses contraversive licking and promotes ipsiversive licking. Activity in lateral superior colliculus (lSC), a region downstream of the basal ganglia, is necessary for task performance and predicts lick direction. Furthermore, iSPN activation suppresses ipsilateral lSC, but surprisingly excites contralateral lSC, explaining the emergence of ipsiversive licking. Optogenetic inactivation reveals inter-collicular competition whereby each hemisphere of the superior colliculus inhibits the other, thus allowing the indirect pathway to disinhibit the contralateral lSC and trigger licking. Finally, inactivating iSPNs impairs suppression of devalued but previously rewarded licking and reduces exploratory licking. Our results reveal that iSPNs engage the competitive interaction between lSC hemispheres to trigger a motor action and suggest a general circuit mechanism for exploration during action selection.

Population-specific neuromodulation prolongs therapeutic benefits of deep brain stimulation

Symptoms of neurological diseases emerge through the dysfunction of neural circuits whose diffuse and intertwined architectures pose serious challenges for delivering therapies. Deep brain stimulation (DBS) improves Parkinson’s disease symptoms acutely but does not differentiate between neuronal circuits, and its effects decay rapidly if stimulation is discontinued. Recent findings suggest that optogenetic manipulation of distinct neuronal subpopulations in the external globus pallidus (GPe) provides long-lasting therapeutic effects in dopamine-depleted (DD) mice. We used synaptic differences to excite parvalbumin-expressing GPe neurons and inhibit lim-homeobox-6–expressing GPe neurons simultaneously using brief bursts of electrical stimulation. In DD mice, circuit-inspired DBS provided long-lasting therapeutic benefits that far exceeded those induced by conventional DBS, extending several hours after stimulation. These results establish the feasibility of transforming knowledge of circuit architecture into translatable therapeutic approaches.

Phosphodiesterase as a Target for Cognition Enhancement in Schizophrenia

Schizophrenia is a severe mental disorder that affects more than 1% of the population worldwide. Dopamine system dysfunction and alterations in glutamatergic neurotransmission are strongly implicated in the aetiology of schizophrenia. To date, antipsychotic drugs are the only available treatment for the symptoms of schizophrenia. These medications, which act as D2-receptor antagonist, adequately address the positive symptoms of the disease, but they fail to improve the negative symptoms and cognitive impairment. In schizophrenia, cognitive impairment is a core feature of the disorder. Therefore, the treatment of cognitive impairment and the other symptoms related to schizophrenia remains a significant unmet medical need. Currently, phosphodiesterases (PDEs) are considered the best drug target for the treatment of schizophrenia since many PDE subfamilies are abundant in the brain regions that are relevant to cognition. Thus, this review aims to illustrate the mechanism of PDEs in treating the symptoms of schizophrenia and summarises the encouraging results of PDE inhibitors as anti-schizophrenic drugs in preclinical and clinical studies.

Restoration of Noradrenergic Function in Parkinson's Disease Model Mice

Dysfunction of the central noradrenergic and dopaminergic systems is the primary neurobiological characteristic of Parkinson’s disease (PD). Importantly, neuronal loss in the locus coeruleus (LC) that occurs in early stages of PD may accelerate progressive loss of dopaminergic neurons. Therefore, restoring the activity and function of the deficient noradrenergic system may be an important therapeutic strategy for early PD. In the present study, the lentiviral constructions of transcription factors Phox2a/2b, Hand2 and Gata3, either alone or in combination, were microinjected into the LC region of the PD model VMAT2 Lo mice at 12 and 18 month age. Biochemical analysis showed that microinjection of lentiviral expression cassettes into the LC significantly increased mRNA levels of Phox2a, and Phox2b, which were accompanied by parallel increases of mRNA and proteins of dopamine β-hydroxylase (DBH) and tyrosine hydroxylase (TH) in the LC. Furthermore, there was considerable enhancement of DBH protein levels in the frontal cortex and hippocampus, as well as enhanced TH protein levels in the striatum and substantia nigra. Moreover, these manipulations profoundly increased norepinephrine and dopamine concentrations in the striatum, which was followed by a remarkable improvement of the spatial memory and locomotor behavior. These results reveal that over-expression of these transcription factors in the LC improves noradrenergic and dopaminergic activities and functions in this rodent model of PD. It provides the necessary groundwork for the development of gene therapies of PD, and expands our understanding of the link between the LC-norepinephrine and dopamine systems during the progression of PD.

Corticostriatal control of defense behavior in mice induced by auditory looming cues

Animals exhibit innate defense behaviors in response to approaching threats cued by the dynamics of sensory inputs of various modalities. The underlying neural circuits have been mostly studied in the visual system, but remain unclear for other modalities. Here, by utilizing sounds with increasing (vs. decreasing) loudness to mimic looming (vs. receding) objects, we find that looming sounds elicit stereotypical sequential defensive reactions: freezing followed by flight. Both behaviors require the activity of auditory cortex, in particular the sustained type of responses, but are differentially mediated by corticostriatal projections primarily innervating D2 neurons in the tail of the striatum and corticocollicular projections to the superior colliculus, respectively. The behavioral transition from freezing to flight can be attributed to the differential temporal dynamics of the striatal and collicular neurons in their responses to looming sound stimuli. Our results reveal an essential role of the striatum in the innate defense control.

Optogenetic manipulation of a value-coding pathway from the primate caudate tail facilitates saccadic gaze shift

In the primate basal ganglia, the caudate tail (CDt) encodes the historical values (good or bad) of visual objects (i.e., stable values), and electrical stimulation of CDt evokes saccadic eye movements. However, it is still unknown how output from CDt conveys stable value signals to govern behavior. Here, we apply a pathway-selective optogenetic manipulation to elucidate how such value information modulates saccades. We express channelrhodopsin-2 in CDt delivered by viral vector injections. Selective optical activation of CDt-derived terminals in the substantia nigra pars reticulata (SNr) inhibits SNr neurons. Notably, these SNr neurons show inhibitory responses to good objects. Furthermore, the optical stimulation causes prolonged excitation of visual-saccadic neurons in the superior colliculus (SC), and induces contralateral saccades. These SC neurons respond more strongly to good than to bad objects in the contralateral hemifield. The present results demonstrate that CDt facilitates saccades toward good objects by serial inhibitory pathways through SNr.

Synaptic and cellular plasticity in Parkinson's disease

Parkinson's disease (PD) is a progressive neurodegenerative disease, which causes a tremendous socioeconomic burden. PD patients are suffering from debilitating motor and nonmotor symptoms. Cardinal motor symptoms of PD, including akinesia, bradykinesia, resting tremor, and rigidity, are caused by the degeneration of dopaminergic neurons in the substantia nigra pars compacta. In addition, decreased amounts of dopamine (DA) level in the basal ganglia induces numerous adaptive changes at the cellular and synaptic levels in the basal ganglia circuits. These cellular and synaptic adaptations are believed to underlie the emergence and propagation of correlated, rhythmic pattern of activity throughout the interconnected cortico-basal ganglia-thalamocortical network. The widespread pathological pattern of brain activity is closely linked to the devastating motor symptoms of PD. Accumulating evidence suggests that both dopaminergic degeneration and the associated abnormal cellular and circuit activity in the basal ganglia drive the motor symptoms of PD. In this short review I summarize the recent advances in our understanding of synaptic and cellular alterations in two basal ganglia nuclei (i.e. the striatum and the subthalamic nucleus) following a complete loss of DA, and in our conceptual understanding of the cellular and circuit bases for the pathological pattern of brain activity in parkinsonian state.

Targeting the cholinergic system in Parkinson's disease

Motor control in the striatum is an orchestra played by various neuronal populations. Loss of harmony due to dopamine deficiency is considered the primary pathological cause of the symptoms of Parkinson’s disease (PD). Recent progress in experimental approaches has enabled us to examine the striatal circuitry in a much more comprehensive manner, not only reshaping our understanding of striatal functions in movement regulation but also leading to new opportunities for the development of therapeutic strategies for treating PD. In addition to dopaminergic innervation, giant aspiny cholinergic interneurons (ChIs) within the striatum have long been recognized as a critical node for balancing dopamine signaling and regulating movement. With the roles of ChIs in motor control further uncovered and more specific manipulations available, striatal ChIs and their corresponding receptors are emerging as new promising therapeutic targets for PD. This review summarizes recent progress in functional studies of striatal circuitry and discusses the translational implications of these new findings for the treatment of PD.

Multiple neuronal circuits for variable object-action choices based on short- and long-term memories

At each time in our life, we choose one or few behaviors, while suppressing many other behaviors. This is the basic mechanism in the basal ganglia, which is done by tonic inhibition and selective disinhibition. Dysfunctions of the basal ganglia then cause 2 types of disorders (difficulty in initiating necessary actions and difficulty in suppressing unnecessary actions) that occur in Parkinson's disease. The basal ganglia generate such opposite outcomes through parallel circuits: The direct pathway for initiation and indirect pathway for suppression. Importantly, the direct pathway processes good information and the indirect pathway processes bad information, which enables the choice of good behavior and the rejection of bad behavior. This is mainly enabled by dopaminergic inputs to these circuits. However, the value judgment is complex because the world is complex. Sometimes, the value must be based on recent events, thus is based on short-term memories. Or, the value must be based on historical events, thus is based on long-term memories. Such memory-based value judgment is generated by another parallel circuit originating from the caudate head and caudate tail. These circuit-information mechanisms allow other brain areas (e.g., prefrontal cortex) to contribute to decisions by sending information to these basal ganglia circuits. Moreover, the basal ganglia mechanisms (i.e., what to choose) are associated with cerebellum mechanisms (i.e., when to choose). Overall, multiple levels of parallel circuits in and around the basal ganglia are essential for coordinated behaviors. Understanding these circuits is useful for creating clinical treatments of disorders resulting from the failure of these circuits.

The expression of DARPP-32 in adult male zebra finches (Taenopygia guttata)

Although the catecholaminergic circuitry in the zebra finch brain has been well studied, there is little information regarding the postsynaptic targets of dopamine. To answer this question, we looked at overall patterns of immunoreactivity for DARPP-32 (a dopamine and cAMP-regulated phosphoprotein, present mostly in dopaminoceptive neurons) in adult male zebra finches. Our results demonstrated that as in mammals and other avian species, DARPP-32 expression was highest in both medial and lateral striatum. Interestingly, a specific pattern of immunoreactivity was observed in the song control system, with 'core' song control regions, that is, LMANcore (lateral magnocellular nucleus of the anterior nidopallium), RA (nucleus robustus arcopallialis) and HVC being less immunoreactive for DARPP-32 than 'shell' areas such as LMANshell, RAcup, AId (intermediate arcopallium) and HVCshelf. Our results suggest that whereas dopamine may modulate the shell pathways at various levels of the AFP, dopaminergic modulation of the core pathway occurs mainly through Area X, a basal ganglia nucleus. Further, secondary sensory cortices including the perientopallial belt, Fields L1 and L3 had higher DARPP-32-immunoreactivity than primary sensory cortical areas such as the pallial basolateral nucleus, entopallium proper and Field L2, corresponding to somatosensory, visual and auditory systems, respectively. We also found DARPP-32-rich axon terminals surrounding dopaminergic neurons in the ventral tegmental area-substantia nigra complex which in turn project to the striatum, suggesting that there may be a reciprocal modulation between these regions. Overall, DARPP-32 expression appears to be higher in areas involved in integrating sensory information, which further supports the role of this protein as a molecular integrator of different signal processing pathways.

Adapting the flow of time with dopamine

The modulation of interval timing by dopamine (DA) has been well established over decades of research. The nature of this modulation, however, has remained controversial: Although the pharmacological evidence has largely suggested that time intervals are overestimated with higher DA levels, more recent optogenetic work has shown the opposite effect. In addition, a large body of work has asserted DA's role as a "reward prediction error" (RPE), or a teaching signal that allows the basal ganglia to learn to predict future rewards in reinforcement learning tasks. Whether these two seemingly disparate accounts of DA may be related has remained an open question. By taking a reinforcement learning-based approach to interval timing, we show here that the RPE interpretation of DA naturally extends to its role as a modulator of timekeeping and furthermore that this view reconciles the seemingly conflicting observations. We derive a biologically plausible, DA-dependent plasticity rule that can modulate the rate of timekeeping in either direction and whose effect depends on the timing of the DA signal itself. This bidirectional update rule can account for the results from pharmacology and optogenetics as well as the behavioral effects of reward rate on interval timing and the temporal selectivity of striatal neurons. Hence, by adopting a single RPE interpretation of DA, our results take a step toward unifying computational theories of reinforcement learning and interval timing. NEW & NOTEWORTHY How does dopamine (DA) influence interval timing? A large body of pharmacological evidence has suggested that DA accelerates timekeeping mechanisms. However, recent optogenetic work has shown exactly the opposite effect. In this article, we relate DA's role in timekeeping to its most established role, as a critical component of reinforcement learning. This allows us to derive a neurobiologically plausible framework that reconciles a large body of DA's temporal effects, including pharmacological, behavioral, electrophysiological, and optogenetic.

Increase in Glutamatergic Terminals in the Striatum Following Dopamine Depletion in a Rat Model of Parkinson's Disease

Dopaminergic neuron degeneration is known to give rise to dendrite injury and spine loss of striatal neurons, however, changes of intrastriatal glutamatergic terminals and their synapses after 6-hydroxydopamine (6OHDA)-induced dopamine (DA)-depletion remains controversial. To confirm the effect of striatal DA-depletion on the morphology and protein levels of corticostriatal and thalamostriatal glutamatergic terminals and synapses, immunohistochemistry, immuno-electron microscope (EM), western blotting techniques were performed on Parkinson's disease rat models in this study. The experimental results of this study showed that: (1) 6OHDA-induced DA-depletion resulted in a remarkable increase of Vesicular glutamate transporter 1 (VGlut1) + and Vesicular glutamate transporter 2 (VGlut2)+ terminal densities at both the light microscope (LM) and EM levels, and VGlut1+ and VGlut2+ terminal sizes were shown to be enlarged by immuno-EM; (2) Striatal DA-depletion resulted in a decrease in both the total and axospinous terminal fractions of VGlut1+ terminals, but the axodendritic terminal fraction was not significantly different from the control group. However, total, axospinous and axodendritic terminal fractions for VGlut2+ terminals declined significantly after striatal DA-depletion. (3) Western blotting data showed that striatal DA-depletion up-regulated the expression levels of the VGlut1 and VGlut2 proteins. These results suggest that 6OHDA-induced DA-depletion affects corticostriatal and thalamostriatal glutamatergic synaptic inputs, which are involved in the pathological process of striatal neuron injury induced by DA-depletion.

Cell-type-specific interrogation of CeA Drd2 neurons to identify targets for pharmacological modulation of fear extinction

Behavioral and molecular characterization of cell-type-specific populations governing fear learning and behavior is a promising avenue for the rational identification of potential therapeutics for fear-related disorders. Examining cell-type-specific changes in neuronal translation following fear learning allows for targeted pharmacological intervention during fear extinction learning, mirroring possible treatment strategies in humans. Here we identify the central amygdala (CeA) Drd2-expressing population as a novel fear-supporting neuronal population that is molecularly distinct from other, previously identified, fear-supporting CeA populations. Sequencing of actively translating transcripts of Drd2 neurons using translating ribosome affinity purification (TRAP) technology identifies mRNAs that are differentially regulated following fear learning. Differentially expressed transcripts with potentially targetable gene products include Npy5r, Rxrg, Adora2a, Sst5r, Fgf3, Erbb4, Fkbp14, Dlk1, and Ssh3. Direct pharmacological manipulation of NPY5R, RXR, and ADORA2A confirms the importance of this cell population and these cell-type-specific receptors in fear behavior. Furthermore, these findings validate the use of functionally identified specific cell populations to predict novel pharmacological targets for the modulation of emotional learning.

Abnormal Effective Connectivity of the Anterior Forebrain Regions in Disorders of Consciousness

A number of studies have indicated that disorders of consciousness result from multifocal injuries as well as from the impaired functional and anatomical connectivity between various anterior forebrain regions. However, the specific causal mechanism linking these regions remains unclear. In this study, we used spectral dynamic causal modeling to assess how the effective connections (ECs) between various regions differ between individuals. Next, we used connectome-based predictive modeling to evaluate the performance of the ECs in predicting the clinical scores of DOC patients. We found increased ECs from the striatum to the globus pallidus as well as from the globus pallidus to the posterior cingulate cortex, and decreased ECs from the globus pallidus to the thalamus and from the medial prefrontal cortex to the striatum in DOC patients as compared to healthy controls. Prediction of the patients' outcome was effective using the negative ECs as features. In summary, the present study highlights a key role of the thalamo-basal ganglia-cortical loop in DOCs and supports the anterior forebrain mesocircuit hypothesis. Furthermore, EC could be potentially used to assess the consciousness level.

A meta-analytic approach to genes that are associated with impaired and elevated spatial memory performance

Spatial memory deficits are a common hallmark of psychiatric conditions, possibly due to a genetic predisposition. Thus, unravelling the relationship between genes and memory might suggest novel therapeutic targets and pathogenetic pathways. Genetic deletions are known to lead to memory deficits (post-deletion "forgetfulness" genes, PDF), or, in few instances to improve spatial memory (post-deletion "hypermnesic" genes, PDH). To assess this topic, we performed a meta-analytic approach on memory behavior in knock-out mice. We screened 300 studies from PubMed and retrieved 87 genes tested for possible effects on spatial memory. This database was crossed with the Allen Brain Atlas (brain distribution) and the Enrichr (gene function) databases. The results show that PDF genes have higher expression level in several ventral brain structures, particularly the encephalic trunk and in the hypothalamus. Moreover, part of these genes are implicated in synaptic functions. Conversely, the PDH genes are associated to G-protein coupled receptors downstream signalling. Some candidate drugs were also found to interfere with some of the PDH genes, further suggesting that this approach might help in identifying drugs to improve memory performance in psychiatric conditions.

Propagation of beta/gamma rhythms in the cortico-basal ganglia circuits of the parkinsonian rat

Much of the motor impairment associated with Parkinson’s disease is thought to arise from pathological activity in the networks formed by the basal ganglia (BG) and motor cortex. To evaluate several hypotheses proposed to explain the emergence of pathological oscillations in parkinsonism, we investigated changes to the directed connectivity in BG networks following dopamine depletion. We recorded local field potentials (LFPs) in the cortex and basal ganglia of rats rendered parkinsonian by injection of 6-hydroxydopamine (6-OHDA) and in dopamine-intact controls. We performed systematic analyses of the networks using a novel tool for estimation of directed interactions (nonparametric directionality, NPD). We used a “conditioned” version of the NPD analysis that reveals the dependence of the correlation between two signals on a third reference signal. We find evidence of the dopamine dependency of both low-beta (14–20 Hz) and high-beta/low-gamma (20–40 Hz) directed network interactions. Notably, 6-OHDA lesions were associated with enhancement of the cortical “hyperdirect” connection to the subthalamic nucleus (STN) and its feedback to the cortex and striatum. We find that pathological beta synchronization resulting from 6-OHDA lesioning is widely distributed across the network and cannot be located to any individual structure. Furthermore, we provide evidence that high-beta/gamma oscillations propagate through the striatum in a pathway that is independent of STN. Rhythms at high beta/gamma show susceptibility to conditioning that indicates a hierarchical organization compared with those at low beta. These results further inform our understanding of the substrates for pathological rhythms in salient brain networks in parkinsonism.

NEW & NOTEWORTHY We present a novel analysis of electrophysiological recordings in the cortico-basal ganglia network with the aim of evaluating several hypotheses concerning the origins of abnormal brain rhythms associated with Parkinson’s disease. We present evidence for changes in the directed connections within the network following chronic dopamine depletion in rodents. These findings speak to the plausibility of a “short-circuiting” of the network that gives rise to the conditions from which pathological synchronization may arise.

Sucrose withdrawal induces depression and anxiety-like behavior by Kir2.1 upregulation in the nucleus accumbens

Dieting induces depression and anxiety among other emotional symptoms. Animal models indicate that repeated access to palatable foods such as sugar induces depression and anxiety-like behavior when the food is no longer available. However, the neurobiological mechanisms of how dietary restriction influences mood have not been fully understood. We used the two-bottle sucrose choice paradigm as an overeating and withdrawal model. Withdrawal after lengthy sucrose overeating elicited depression and anxiety-like behavior, which was reversed by sucrose reinstatement. In the nucleus accumbens (NAc) of sucrose withdrawal animals, dopamine levels and cAMP response element binding protein (CREB) activity were significantly reduced, while the inwardly rectifying K⁺ channel, Kir2.1, was significantly elevated. In addition, overexpression of Kir2.1 selectively in neurons expressing dopamine D1 receptors was sufficient to induce negative mood-linked behavior in the absence of sucrose overeating experience. As elevated K⁺ channels reduce neuronal excitability, a sucrose withdrawal-induced increase in Kir2.1 expression is able to decrease NAc activity, which provides a cellular basis for depression and anxiety-like behavior in animals.

Locus Coeruleus and Dopamine-Dependent Memory Consolidation

Most everyday memories including many episodic-like memories that we may form automatically in the hippocampus (HPC) are forgotten, while some of them are retained for a long time by a memory stabilization process, called initial memory consolidation. Specifically, the retention of everyday memory is enhanced, in humans and animals, when something novel happens shortly before or after the time of encoding. Converging evidence has indicated that dopamine (DA) signaling via D₁/D₅ receptors in HPC is required for persistence of synaptic plasticity and memory, thereby playing an important role in the novelty-associated memory enhancement. In this review paper, we aim to provide an overview of the key findings related to D₁/D₅ receptor-dependent persistence of synaptic plasticity and memory in HPC, especially focusing on the emerging evidence for a role of the locus coeruleus (LC) in DA-dependent memory consolidation. We then refer to candidate brain areas and circuits that might be responsible for detection and transmission of the environmental novelty signal and molecular and anatomical evidence for the LC-DA system. We also discuss molecular mechanisms that might mediate the environmental novelty-associated memory enhancement, including plasticity-related proteins that are involved in initial memory consolidation processes in HPC.

The Dopamine D5 Receptor Is Involved in Working Memory

Pharmacological studies indicate that dopamine D₁-like receptors (D₁ and D₅) are critically involved in cognitive function. However, the lack of pharmacological ligands selective for either the D₁ or D₅ receptors has made it difficult to determine the unique contributions of the D₁-like family members. To circumvent these pharmacological limitations, we used D₅ receptor homozygous (-/-) and heterozygous (+/-) knockout mice, to identify the specific role of this receptor in higher order cognitive functions. We identified a novel role for D₅ receptors in the regulation of spatial working memory and temporal order memory function. The D₅ mutant mice acquired a discrete paired-trial variable-delay T-maze task at normal rates. However, both [Formula: see text] and [Formula: see text] mice exhibited impaired performance compared to [Formula: see text] littermates when a higher burden on working memory faculties was imposed. In a temporal order object recognition task, [Formula: see text] exhibited significant memory deficits. No D₅-dependent differences in locomotor functions and interest in exploring objects were evident. Molecular biomarkers of dopaminergic functions within the prefrontal cortex (PFC) revealed a selective gene-dose effect on Akt phosphorylation at Ser473 with increased levels in [Formula: see text] knockout mice. A trend toward reduced levels in CaMKKbeta brain-specific band (64 kDa) in [Formula: see text] compared to [Formula: see text] was also evident. These findings highlight a previously unidentified role for D₅ receptors in working memory function and associated molecular signatures within the PFC.

Regulator of G protein signaling 14 (RGS14) is expressed pre- and postsynaptically in neurons of hippocampus, basal ganglia, and amygdala of monkey and human brain

Regulator of G protein signaling 14 (RGS14) is a multifunctional signaling protein primarily expressed in mouse pyramidal neurons of hippocampal area CA2 where it regulates synaptic plasticity important for learning and memory. However, very little is known about RGS14 protein expression in the primate brain. Here, we validate the specificity of a new polyclonal RGS14 antibody that recognizes not only full-length RGS14 protein in primate, but also lower molecular weight forms of RGS14 protein matching previously predicted human splice variants. These putative RGS14 variants along with full-length RGS14 are expressed in the primate striatum. By contrast, only full-length RGS14 is expressed in hippocampus, and shorter variants are completely absent in rodent brain. We report that RGS14 protein immunoreactivity is found both pre- and postsynaptically in multiple neuron populations throughout hippocampal area CA1 and CA2, caudate nucleus, putamen, globus pallidus, substantia nigra, and amygdala in adult rhesus monkeys. A similar cellular expression pattern of RGS14 in the monkey striatum and hippocampus was further confirmed in humans. Our electron microscopy data show for the first time that RGS14 immunostaining localizes within nuclei of striatal neurons in monkeys. Taken together, these findings suggest new pre- and postsynaptic regulatory functions of RGS14 and RGS14 variants, specific to the primate brain, and provide evidence for unconventional roles of RGS14 in the nuclei of striatal neurons potentially important for human neurophysiology and disease.

Rodents and humans are able to detect the odour of L-Lactate

L-Lactate (LL) is an essential cellular metabolite which can be used to generate energy. In addition, accumulating evidence suggests that LL is used for inter-cellular signalling. Some LL-sensitive receptors have been identified but we recently proposed that there may be yet another unknown G-protein coupled receptor (GPCR) sensitive to LL in the brain. Olfactory receptors (ORs) represent the largest family of GPCRs and some of them are expressed outside the olfactory system, including brain, making them interesting candidates for non-olfactory LL signalling. One of the “ectopically” expressed ORs, Olfr78 in mice (Olr59 in rats and OR51E2 in humans), reportedly can be activated by LL. This implies that both rodents and humans should be able to detect the LL odour. Surprisingly, this has never been demonstrated. Here we show that mice can detect the odour of LL in odour detection and habituation-dishabituation tasks, and discriminate it from peppermint and vanilla odours. Behaviour of the Olfr78 null mice and wildtype mice in odour detection task was not different, indicating that rodents are equipped with more than one LL-sensitive OR. Rats were also able to use the smell of LL as a cue in an odour-reward associative learning task. When presented to humans, more than 90% of participants detected a smell of LL in solution. Interestingly, LL was perceived differently than acetate or propionate—LL was preferentially reported as a pleasant sweet scent while acetate and propionate were perceived as repulsive sour/acid smells. Subjective perception of LL smell was different in men and women. Taken together, our data demonstrate that both rodents and humans are able to detect the odour of LL. Moreover, in mice, LL perception is not purely mediated by Olfr78. Discovery of further LL-sensitive OR might shed the light on their contribution to LL signalling in the body.

Cell-specific pallidal intervention induces long-lasting motor recovery in dopamine-depleted mice

The identification of distinct cell types in the basal ganglia has been critical to our understanding of basal ganglia function and the treatment of neurological disorders. The external globus pallidus (GPe) is a key contributor to motor suppressing pathways in the basal ganglia, yet its neuronal heterogeneity has remained an untapped resource for therapeutic interventions. Here we demonstrate that optogenetic interventions that dissociate the activity of two neuronal populations in the GPe, elevating the activity of parvalbumin (PV)-expressing GPe neurons over that of Lim homeobox 6 (Lhx6)-expressing GPe neurons, restores movement in dopamine-depleted mice and attenuates pathological activity of basal ganglia output neurons for hours beyond stimulation. These results establish the utility of cell-specific interventions in the GPe to target functionally distinct pathways, with the potential to induce long-lasting recovery of movement despite the continued absence of dopamine.

Ethanol-Sensitive Pacemaker Neurons in the Mouse External Globus Pallidus

Although ethanol is one of the most widely used drugs, we still lack a full understanding of which neuronal subtypes are affected by this drug. Pacemaker neurons exert powerful control over brain circuit function, but little is known about ethanol effects on these types of neurons. Neurons in the external globus pallidus (GPe) generate pacemaker activity that controls basal ganglia, circuitry associated with habitual and compulsive drug use. We performed patch-clamp recordings from GPe neurons and found that bath application of ethanol dose-dependently decreased the firing rate of low-frequency GPe neurons, but did not alter the firing of high-frequency neurons. GABA or glutamate receptor antagonists did not block the ethanol effect. The GPe is comprised of a heterogeneous population of neurons. We used Lhx6-EGFP and Npas1-tdTm mice strains to identify low-frequency neurons. Lhx6 and Npas1 neurons exhibited decreased firing with ethanol, but only Npas1 neurons were sensitive to 10 mM ethanol. Large-conductance voltage and Ca²⁺-activated K⁺ (BK) channel have a key role in the ethanol effect on GPe neurons, as the application of BK channel inhibitors blocked the ethanol-induced firing decrease. Ethanol also increased BK channel open probability measured in single-channel recordings from Npas1-tdTm neurons. In addition, in vivo electrophysiological recordings from GPe showed that ethanol decreased the firing of a large subset of low-frequency neurons. These findings indicate how selectivity of ethanol effects on pacemaker neurons can occur, and enhance our understanding of the mechanisms contributing to acute ethanol effects on the basal ganglia.

The Cognitive Role of the Globus Pallidus interna; Insights from Disease States

The motor symptoms of both Parkinson's disease and focal dystonia arise from dysfunction of the basal ganglia, and are improved by pallidotomy or deep brain stimulation of the Globus Pallidus interna (GPi). However, Parkinson's disease is associated with a greater degree of basal ganglia-dependent learning impairment than dystonia. We attempt to understand this observation in terms of a comparison of the electrophysiology of the output of the basal ganglia between the two conditions. We use the natural experiment offered by Deep Brain Stimulation to compare GPi local field potential responses in subjects with Parkinson's disease compared to subjects with dystonia performing a forced-choice decision-making task with sensory feedback. In dystonic subjects, we found that auditory feedback was associated with the presence of high gamma oscillations nestled on a negative deflection, morphologically similar to sharp wave ripple complexes described in human rhinal cortex. These were not present in Parkinson's disease subjects. The temporal properties of the high gamma burst were modified by incorrect trial performance compared to correct trial performance. Both groups exhibited a robust low frequency response to 'incorrect' trial performance in dominant GPi but not non-dominant GPi at theta frequency. Our results suggest that cellular processes associated with striatum-dependent memory function may be selectively impaired in Parkinson's disease even if dopaminergic drugs are administered, but that error detection mechanisms are preserved.

WAVE1 in neurons expressing the D1 dopamine receptor regulates cellular and behavioral actions of cocaine

Wiskott-Aldrich syndrome protein (WASP) family verprolin homologous protein 1 (WAVE1) regulates actin-related protein 2/3 (Arp2/3) complex-mediated actin polymerization. Our previous studies have found WAVE1 to be inhibited by Cdk5-mediated phosphorylation in brain and to play a role in the regulation of dendritic spine morphology. Here we report that mice in which WAVE1 was knocked out (KO) in neurons expressing the D1 dopamine receptor (D1-KO), but not mice where WAVE1 was knocked out in neurons expressing the D2 dopamine receptor (D2-KO), exhibited a significant decrease in place preference associated with cocaine. In contrast to wild-type (WT) and WAVE1 D2-KO mice, cocaine-induced sensitized locomotor behavior was not maintained in WAVE1 D1-KO mice. After chronic cocaine administration and following withdrawal, an acute cocaine challenge induced WAVE1 activation in striatum, which was assessed by dephosphorylation. The cocaine-induced WAVE1 dephosphorylation was attenuated by coadministration of either a D1 dopamine receptor or NMDA glutamate receptor antagonist. Upon an acute challenge of cocaine following chronic cocaine exposure and withdrawal, we also observed in WT, but not in WAVE1 D1-KO mice, a decrease in dendritic spine density and a decrease in the frequency of excitatory postsynaptic AMPA receptor currents in medium spiny projection neurons expressing the D1 dopamine receptor (D1-MSNs) in the nucleus accumbens. These results suggest that WAVE1 is involved selectively in D1-MSNs in cocaine-evoked neuronal activity-mediated feedback regulation of glutamatergic synapses.

The functional logic of corticostriatal connections

Unidirectional connections from the cortex to the matrix of the corpus striatum initiate the cortico-basal ganglia (BG)-thalamocortical loop, thought to be important in momentary action selection and in longer-term fine tuning of behavioural repertoire; a discrete set of striatal compartments, striosomes, has the complementary role of registering or anticipating reward that shapes corticostriatal plasticity. Re-entrant signals traversing the cortico-BG loop impact predominantly frontal cortices, conveyed through topographically ordered output channels; by contrast, striatal input signals originate from a far broader span of cortex, and are far more divergent in their termination. The term 'disclosed loop' is introduced to describe this organisation: a closed circuit that is open to outside influence at the initial stage of cortical input. The closed circuit component of corticostriatal afferents is newly dubbed 'operative', as it is proposed to establish the bid for action selection on the part of an incipient cortical action plan; the broader set of converging corticostriatal afferents is described as contextual. A corollary of this proposal is that every unit of the striatal volume, including the long, C-shaped tail of the caudate nucleus, should receive a mandatory component of operative input, and hence include at least one area of BG-recipient cortex amongst the sources of its corticostriatal afferents. Individual operative afferents contact twin classes of GABAergic striatal projection neuron (SPN), distinguished by their neurochemical character, and onward circuitry. This is the basis of the classic direct and indirect pathway model of the cortico-BG loop. Each pathway utilises a serial chain of inhibition, with two such links, or three, providing positive and negative feedback, respectively. Operative co-activation of direct and indirect SPNs is, therefore, pictured to simultaneously promote action, and to restrain it. The balance of this rival activity is determined by the contextual inputs, which summarise the external and internal sensory environment, and the state of ongoing behavioural priorities. Notably, the distributed sources of contextual convergence upon a striatal locus mirror the transcortical network harnessed by the origin of the operative input to that locus, thereby capturing a similar set of contingencies relevant to determining action. The disclosed loop formulation of corticostriatal and subsequent BG loop circuitry, as advanced here, refines the operating rationale of the classic model and allows the integration of more recent anatomical and physiological data, some of which can appear at variance with the classic model. Equally, it provides a lucid functional context for continuing cellular studies of SPN biophysics and mechanisms of synaptic plasticity.

Dopamine Receptors: Novel Insights from Biochemical and Genetic Studies

Dopamine receptors are targets for drugs with antipsychotic potency, and they are also the primary target in the treatment of Parkinson’s disease. Molecular cloning has identified five genes that code for dopamine receptors. These receptors belong in two functionally distinct classes of G-protein-coupled receptors, known as the D1 class of receptors (D1 and D5) and the D2 class of receptors (D2, D3, and D4). The diversity of dopamine receptor subtypes that belong to the same functional class, their high degree of structural similarity, and the lack of antagonists with selectivity for each of the individual subtypes have challenged studies on the function of the individual receptor subtypes. This review focuses on the recent progress made with studies on the expression and function of D1, D2, and D3 receptors. It summarizes results of studies that suggest that these receptor proteins are expressed in monomeric and oligomeric forms and reviews results of a growing number of gene-targeting studies that begin to illustrate major differences in the phenotypes of D1-, D2-, and D3-mutant mice.

Reappraising striatal D1- and D2-neurons in reward and aversion

The striatum has been involved in complex behaviors such as motor control, learning, decision-making, reward and aversion. The striatum is mainly composed of medium spiny neurons (MSNs), typically divided into those expressing dopamine receptor D1, forming the so-called direct pathway, and those expressing D2 receptor (indirect pathway). For decades it has been proposed that these two populations exhibit opposing control over motor output, and recently, the same dichotomy has been proposed for valenced behaviors. Whereas D1-MSNs mediate reinforcement and reward, D2-MSNs have been associated with punishment and aversion. In this review we will discuss pharmacological, genetic and optogenetic studies that indicate that there is still controversy to what concerns the role of striatal D1- and D2-MSNs in this type of behaviors, highlighting the need to reconsider the early view that they mediate solely opposing aspects of valenced behaviour.

The role of the dopamine D1 receptor in social cognition: studies using a novel genetic rat model

Social cognition is an endophenotype that is impaired in schizophrenia and several other (comorbid) psychiatric disorders. One of the modulators of social cognition is dopamine, but its role is not clear. The effects of dopamine are mediated through dopamine receptors, including the dopamine D1 receptor (Drd1). Because current Drd1 receptor agonists are not Drd1 selective, pharmacological tools are not sufficient to delineate the role of the Drd1. Here, we describe a novel rat model with a genetic mutation in Drd1 in which we measured basic behavioural phenotypes and social cognition. The I116S mutation was predicted to render the receptor less stable. In line with this computational prediction, this Drd1 mutation led to a decreased transmembrane insertion of Drd1, whereas Drd1 expression, as measured by Drd1 mRNA levels, remained unaffected. Owing to decreased transmembrane Drd1 insertion, the mutant rats displayed normal basic motoric and neurological parameters, as well as locomotor activity and anxiety-like behaviour. However, measures of social cognition like social interaction, scent marking, pup ultrasonic vocalizations and sociability, were strongly reduced in the mutant rats. This profile of the Drd1 mutant rat offers the field of neuroscience a novel genetic rat model to study a series of psychiatric disorders including schizophrenia, autism, depression, bipolar disorder and drug addiction.

A single-neuron tracing study of arkypallidal and prototypic neurons in healthy rats

The external globus pallidus (GP) is known as a relay nucleus of the indirect pathway of the basal ganglia. Recent studies in dopamine-depleted and healthy rats indicate that the GP comprises two main types of pallidofugal neurons: the so-called "prototypic" and "arkypallidal" neurons. However, the reconstruction of complete arkypallidal neurons in healthy rats has not been reported. Here we visualized the entire axonal arborization of four single arkypallidal neurons and six single prototypic neurons in rat brain using labeling with a viral vector expressing membrane-targeted green fluorescent protein and examined the distribution of axon boutons in the target nuclei. Results revealed that not only the arkypallidal neurons but nearly all of the prototypic neurons projected to the striatum with numerous axon varicosities. Thus, the striatum is a major target nucleus for pallidal neurons. Arkypallidal and prototypic GP neurons located in the calbindin-positive and calbindin-negative regions mainly projected to the corresponding positive and negative regions in the striatum. Because the GP and striatum calbindin staining patterns reflect the topographic organization of the striatopallidal projection, the striatal neurons in the sensorimotor and associative regions constitute the reciprocal connection with the GP neurons in the corresponding regions.

Aberrant white matter networks mediate cognitive impairment in patients with silent lacunar infarcts in basal ganglia territory

Silent lacunar infarcts, which are present in over 20% of healthy elderly individuals, are associated with subtle deficits in cognitive functions. However, it remains largely unclear how these silent brain infarcts lead to cognitive deficits and even dementia. Here, we used diffusion tensor imaging tractography and graph theory to examine the topological organization of white matter networks in 27 patients with silent lacunar infarcts in the basal ganglia territory and 30 healthy controls. A whole-brain white matter network was constructed for each subject, where the graph nodes represented brain regions and the edges represented interregional white matter tracts. Compared with the controls, the patients exhibited a significant reduction in local efficiency and global efficiency. In addition, a total of eighteen brain regions showed significantly reduced nodal efficiency in patients. Intriguingly, nodal efficiency-behavior associations were significantly different between the two groups. The present findings provide new aspects into our understanding of silent infarcts that even small lesions in subcortical brain regions may affect large-scale cortical white matter network, as such may be the link between subcortical silent infarcts and the associated cognitive impairments. Our findings highlight the need for network-level neuroimaging assessment and more medical care for individuals with silent subcortical infarcts.

Differentiation of forebrain and hippocampal dopamine 1-class receptors, D1R and D5R, in spatial learning and memory

Activation of prefrontal cortical (PFC), striatal, and hippocampal dopamine 1-class receptors (D1R and D5R) is necessary for normal spatial information processing. Yet the precise role of the D1R versus the D5R in the aforementioned structures, and their specific contribution to the water-maze spatial learning task remains unknown. D1R- and D5R-specific in situ hybridization probes showed that forebrain restricted D1R and D5R KO mice (F-D1R/D5R KO) displayed D1R mRNA deletion in the medial (m)PFC, dorsal and ventral striatum, and the dentate gyrus (DG) of the hippocampus. D5R mRNA deletion was limited to the mPFC, the CA1 and DG hippocampal subregions. F-D1R/D5R KO mice were given water-maze training and displayed subtle spatial latency differences between genotypes and spatial memory deficits during both regular and reversal training. To differentiate forebrain D1R from D5R activation, forebrain restricted D1R KO (F-D1R KO) and D5R KO (F-D5R KO) mice were trained on the water-maze task. F-D1R KO animals exhibited escape latency deficits throughout regular and reversal training as well as spatial memory deficits during reversal training. F-D1R KO mice also showed perseverative behavior during the reversal spatial memory probe test. In contrast, F-D5R KO animals did not present observable deficits on the water-maze task. Because F-D1R KO mice showed water-maze deficits we tested the necessity of hippocampal D1R activation for spatial learning and memory. We trained DG restricted D1R KO (DG-D1R KO) mice on the water-maze task. DG-D1R KO mice did not present detectable spatial memory deficit, but did show subtle deficits during specific days of training. Our data provides evidence that forebrain D5R activation plays a unique role in spatial learning and memory in conjunction with D1R activation. Moreover, these data suggest that mPFC and striatal, but not DG D1R activation are essential for spatial learning and memory.

Circuit-dependent striatal PKA and ERK signaling underlies rapid behavioral shift in mating reaction of male mice

The selection of reward-seeking and aversive behaviors is controlled by two distinct D1 and D2 receptor-expressing striatal medium spiny neurons, namely the direct pathway MSNs (dMSNs) and the indirect pathway MSNs (iMSNs), but the dynamic modulation of signaling cascades of dMSNs and iMSNs in behaving animals remains largely elusive. We developed an in vivo methodology to monitor Förster resonance energy transfer (FRET) of the activities of PKA and ERK in either dMSNs or iMSNs by microendoscopy in freely moving mice. PKA and ERK were coordinately but oppositely regulated between dMSNs and iMSNs by rewarding cocaine administration and aversive electric shocks. Notably, the activities of PKA and ERK rapidly shifted when male mice became active or indifferent toward female mice during mating behavior. Importantly, manipulation of PKA cascades by the Designer Receptor recapitulated active and indifferent mating behaviors, indicating a causal linkage of a dynamic activity shift of PKA and ERK between dMSNs and iMSNs in action selection.

The metabotropic glutamate receptor 5 role on motor behavior involves specific neural substrates

Background

The metabotropic glutamate receptor 5 (mGluR5) is involved in various brain functions, including memory, cognition and motor behavior. Regarding locomotor activity, we and others have demonstrated that pharmacological antagonism of mGluR5 promotes hyperkinesia in mice. Moreover, increased locomotor activity can also be observed in mice following the genetic deletion of mGluR5. However, it is still unclear which specific brain substrates contribute to mGluR5-mediated regulation of motor function.

Results

Thus, to better understand the role of mGluR5 in motor control and to determine which neural substrates are involved in this regulation we performed stereotactic microinfusions of the mGluR5 antagonist, MPEP, into specific brain regions and submitted mice to the open field and rotarod apparatus. Our findings indicate that mGluR5 blockage elicits distinct outcomes in terms of locomotor activity and motor coordination depending on the brain region injected with mGluR5 antagonist. MPEP injection into either the dorsal striatum or dorsal hippocampus resulted in increased locomotor activity, whereas MPEP injection into either the ventral striatum or motor cortex resulted in hypokinesia. Moreover, MPEP injected into the olfactory bulb increased the distance mice traveled in the center of the open field arena. With respect to motor coordination on the rotarod, injection of MPEP into the motor cortex and olfactory bulb elicited decreased latency to fall.

Conclusions

Taken together, our data suggest that not only primarily motor neural substrates, but also limbic and sensory structures are involved in mGluR5-mediated motor behavior.

Electronic supplementary material

The online version of this article (doi:10.1186/s13041-015-0113-2) contains supplementary material, which is available to authorized users.

Homeostatic regulation of excitatory synapses on striatal medium spiny neurons expressing the D2 dopamine receptor

Striatal medium spiny neurons (MSNs) are contacted by glutamatergic axon terminals originating from cortex, thalamus and other regions. The striatum is also innervated by dopaminergic (DAergic) terminals, some of which release glutamate as a co-transmitter. Despite evidence for functional DA release at birth in the striatum, the role of DA in the establishment of striatal circuitry is unclear. In light of recent work suggesting activity-dependent homeostatic regulation of glutamatergic terminals on MSNs expressing the D2 DA receptor (D2-MSNs), we used primary co-cultures to test the hypothesis that stimulation of DA and glutamate receptors regulates the homeostasis of glutamatergic synapses on MSNs. Co-culture of D2-MSNs with mesencephalic DA neurons or with cortical neurons produced an increase in spines and functional glutamate synapses expressing VGLUT2 or VGLUT1, respectively. The density of VGLUT2-positive terminals was reduced by the conditional knockout of this gene from DA neurons. In the presence of both mesencephalic and cortical neurons, the density of synapses reached the same total, compatible with the possibility of a homeostatic mechanism capping excitatory synaptic density. Blockade of D2 receptors increased the density of cortical and mesencephalic glutamatergic terminals, without changing MSN spine density or mEPSC frequency. Combined blockade of AMPA and NMDA glutamate receptors increased the density of cortical terminals and decreased that of mesencephalic VGLUT2-positive terminals, with no net change in total excitatory terminal density or in mEPSC frequency. These results suggest that DA and glutamate signaling regulate excitatory inputs to striatal D2-MSNs at both the pre- and postsynaptic level, under the influence of a homeostatic mechanism controlling functional output of the circuit.

A direct GABAergic output from the basal ganglia to frontal cortex

The basal ganglia (BG) are phylogenetically conserved subcortical nuclei necessary for coordinated motor action and reward learning¹. Current models postulate that the BG modulate cerebral cortex indirectly via an inhibitory output to thalamus, bidirectionally controlled by the BG via direct (dSPNs) and indirect (iSPNs) pathway striatal projection neurons^2–4. The BG thalamic output sculpts cortical activity by interacting with signals from sensory and motor systems⁵. Here we describe a direct projection from the globus pallidus externus (GP), a central nucleus of the BG, to frontal regions of the cerebral cortex (FC). Two cell types make up the GP-FC projection, distinguished by their electrophysiological properties, cortical projections and expression of choline acetyltransferase (ChAT), a synthetic enzyme for the neurotransmitter acetylcholine (ACh). Despite these differences, ChAT+ cells, which have been historically identified as an extension of the nucleus basalis (NB), as well as ChAT− cells, release the inhibitory neurotransmitter GABA (γ-aminobutyric acid) and are inhibited by iSPNs and dSPNs of dorsal striatum. Thus GP-FC cells comprise a direct GABAergic/cholinergic projection under the control of striatum that activates frontal cortex in vivo. Furthermore, iSPN inhibition of GP-FC cells is sensitive to dopamine 2 receptor signaling, revealing a pathway by which drugs that target dopamine receptors for the treatment of neuropsychiatric disorders can act in the BG to modulate frontal cortices.