Action Selection and Flexible Switching Controlled by the Intralaminar Thalamic Neurons

In relentless pursuit of the white whale: A role for the ventral midline thalamus in behavioral flexibility and adaption?

The reuniens (Re) nucleus is located in the ventral midline thalamus. It has fostered increasing interest, not only for its participation in a variety of cognitive functions (e.g., spatial working memory, systemic consolidation, reconsolidation, extinction of fear or generalization), but also for its neuroanatomical positioning as a bidirectional relay between the prefrontal cortex (PFC) and the hippocampus (HIP). In this review we compile and discuss recent studies having tackled a possible implication of the Re nucleus in behavioral flexibility, a major PFC-dependent executive function controlling goal-directed behaviors. Experiments considered explored a possible role for the Re nucleus in perseveration, reversal learning, fear extinction, and set-shifting. They point to a contribution of this nucleus to behavioral flexibility, mainly by its connections with the PFC, but possibly also by those with the hippocampus, and even with the amygdala, at least for fear-related behavior. As such, the Re nucleus could be a crucial crossroad supporting a PFC-orchestrated ability to cope with new, potentially unpredictable environmental contingencies, and thus behavioral flexibility and adaption.

Changes in dorsomedial striatum activity during expression of goal-directed vs. habit-like cue-induced cocaine seeking

A preclinical model of cue exposure therapy, cue extinction, reduces cue-induced cocaine seeking that is goal-directed but not habit-like. Goal-directed and habitual behaviors differentially rely on the dorsomedial striatum (DMS) and dorsolateral striatum (DLS), but the effects of cue extinction on dorsal striatal responses to cue-induced drug seeking are unknown. We used fiber photometry in rats trained to self-administer cocaine paired with an audiovisual cue to examine how dorsal striatal intracellular calcium and extracellular dopamine activity differs between goal-directed and habit-like cue-induced cocaine seeking and how it is impacted by cue extinction. After minimal fixed-ratio training, rats showed enhanced DMS and DLS calcium responses to cue-reinforced compared to unreinforced lever presses. After rats were trained on goal-promoting fixed ratio schedules or habit-promoting second-order schedules of reinforcement, different patterns of dorsal striatal calcium and dopamine responses to cue-reinforced lever presses emerged. Rats trained on habit-promoting second-order schedules showed reduced DMS calcium responses and enhanced DLS dopamine responses to cue-reinforced lever presses. Cue extinction reduced calcium responses during subsequent drug seeking in the DMS, but not in the DLS. Therefore, cue extinction may reduce goal-directed behavior through its effects on the DMS, whereas habit-like behavior and the DLS are unaffected.

Chemogenetic activation of mammalian brain neurons expressing insect Ionotropic Receptors by systemic ligand precursor administration

Chemogenetic approaches employing ligand-gated ion channels are advantageous regarding manipulation of target neuronal population functions independently of endogenous second messenger pathways. Among them, Ionotropic Receptor (IR)-mediated neuronal activation (IRNA) allows stimulation of mammalian neurons that heterologously express members of the insect chemosensory IR repertoire in response to their cognate ligands. In the original protocol, phenylacetic acid, a ligand of the IR84a/IR8a complex, was locally injected into a brain region due to its low permeability of the blood-brain barrier. To circumvent this invasive injection, we sought to develop a strategy of peripheral administration with a precursor of phenylacetic acid, phenylacetic acid methyl ester, which is efficiently transferred into the brain and converted to the mature ligand by endogenous esterase activities. This strategy was validated by electrophysiological, biochemical, brain-imaging, and behavioral analyses, demonstrating high utility of systemic IRNA technology in the remote activation of target neurons in the brain.

Protocol for highly selective transgene expression through the flip-excision switch system by using a unilateral spacer sequence in rodents

We present a protocol to induce Cre-dependent transgene expression in specific cell types in the rat brain, suppressing a leak expression in off-target cells, by using a flip-excision switch system with a unilateral spacer sequence. We describe steps for construction of transfer plasmids, preparation of adeno-associated viral vectors, intracranial injection, and detection of transgene expression. Our protocol provides a useful strategy for a better understanding of the structure and function of specific cell types in the complex neural circuit. For complete details on the use and execution of this protocol, please refer to Matsushita et al.1.

Anatomical identification of a corticocortical top-down recipient inhibitory circuitry by enhancer-restricted transsynaptic tracing

Despite the importance of postsynaptic inhibitory circuitry targeted by mid/long-range projections (e.g., top-down projections) in cognitive functions, its anatomical properties, such as laminar profile and neuron type, are poorly understood owing to the lack of efficient tracing methods. To this end, we developed a method that combines conventional adeno-associated virus (AAV)-mediated transsynaptic tracing with a distal-less homeobox (Dlx) enhancer-restricted expression system to label postsynaptic inhibitory neurons. We called this method "Dlx enhancer-restricted Interneuron-SpECific transsynaptic Tracing" (DISECT). We applied DISECT to a top-down corticocortical circuit from the secondary motor cortex (M2) to the primary somatosensory cortex (S1) in wild-type mice. First, we injected AAV1-Cre into the M2, which enabled Cre recombinase expression in M2-input recipient S1 neurons. Second, we injected AAV1-hDlx-flex-green fluorescent protein (GFP) into the S1 to transduce GFP into the postsynaptic inhibitory neurons in a Cre-dependent manner. We succeeded in exclusively labeling the recipient inhibitory neurons in the S1. Laminar profile analysis of the neurons labeled via DISECT indicated that the M2-input recipient inhibitory neurons were distributed in the superficial and deep layers of the S1. This laminar distribution was aligned with the laminar density of axons projecting from the M2. We further classified the labeled neuron types using immunohistochemistry and in situ hybridization. This post hoc classification revealed that the dominant top-down M2-input recipient neuron types were somatostatin-expressing neurons in the superficial layers and parvalbumin-expressing neurons in the deep layers. These results demonstrate that DISECT enables the investigation of multiple anatomical properties of the postsynaptic inhibitory circuitry.

Sensory Reinforced Corticostriatal Plasticity

BACKGROUND

Regional changes in corticostriatal transmission induced by phasic dopaminergic signals are an essential feature of the neural network responsible for instrumental reinforcement during discovery of an action. However, the timing of signals that are thought to contribute to the induction of corticostriatal plasticity is difficult to reconcile within the framework of behavioural reinforcement learning, because the reinforcer is normally delayed relative to the selection and execution of causally-related actions.

OBJECTIVE

While recent studies have started to address the relevance of delayed reinforcement signals and their impact on corticostriatal processing, our objective was to establish a model in which a sensory reinforcer triggers appropriately delayed reinforcement signals relayed to the striatum via intact neuronal pathways and to investigate the effects on corticostriatal plasticity.

METHODS

We measured corticostriatal plasticity with electrophysiological recordings using a light flash as a natural sensory reinforcer, and pharmacological manipulations were applied in an in vivo anesthetized rat model preparation.

RESULTS

We demonstrate that the spiking of striatal neurons evoked by single-pulse stimulation of the motor cortex can be potentiated by a natural sensory reinforcer, operating through intact afferent pathways, with signal timing approximating that required for behavioural reinforcement. The pharmacological blockade of dopamine receptors attenuated the observed potentiation of corticostriatal neurotransmission.

CONCLUSION

This novel in vivo model of corticostriatal plasticity offers a behaviourally relevant framework to address the physiological, anatomical, cellular, and molecular bases of instrumental reinforcement learning.

Changes in dorsomedial striatum activity mediate expression of goal-directed vs. habit-like cue-induced cocaine seeking

A preclinical model of cue exposure therapy, cue extinction, reduces cue-induced cocaine seeking when drug seeking is goal-directed but not habitual. Goal-directed and habitual behaviors differentially rely on the dorsomedial striatum (DMS) and dorsolateral striatum (DLS), but the effects of cue extinction on dorsal striatal responses to cue-induced drug seeking are unknown. We used fiber photometry to examine how dorsal striatal intracellular calcium and extracellular dopamine activity differs between goal-directed and habitual cue-induced cocaine seeking and how it is impacted by cue extinction. Rats trained to self-administer cocaine paired with an audiovisual cue on schedules of reinforcement that promote goal-directed or habitual cocaine seeking had different patterns of dorsal striatal calcium and dopamine responses to cue-reinforced lever presses. Cue extinction reduced calcium and dopamine responses during subsequent drug seeking in the DMS, but not in the DLS. Therefore, cue extinction may reduce goal-directed behavior through its effects on the DMS, whereas habitual behavior and the DLS are unaffected.

Reinstating olfactory bulb-derived limbic gamma oscillations alleviates depression-like behavioral deficits in rodents

Although the etiology of major depressive disorder remains poorly understood, reduced gamma oscillations is an emerging biomarker. Olfactory bulbectomy, an established model of depression that reduces limbic gamma oscillations, suffers from non-specific effects of structural damage. Here, we show that transient functional suppression of olfactory bulb neurons or their piriform cortex efferents decreased gamma oscillation power in limbic areas and induced depression-like behaviors in rodents. Enhancing transmission of gamma oscillations from olfactory bulb to limbic structures by closed-loop electrical neuromodulation alleviated these behaviors. By contrast, silencing gamma transmission by anti-phase closed-loop stimulation strengthened depression-like behaviors in naive animals. These induced behaviors were neutralized by ketamine treatment that restored limbic gamma power. Taken together, our results reveal a causal link between limbic gamma oscillations and depression-like behaviors in rodents. Interfering with these endogenous rhythms can affect behaviors in rodent models of depression, suggesting that restoring gamma oscillations may alleviate depressive symptoms.

Highly selective transgene expression through the flip-excision switch system by using a unilateral spacer sequence

The flip-excision switch (FLEX) system with an adeno-associated viral (AAV) vector allows expression of transgenes in specific cell populations having Cre recombinase. A significant issue with this system is non-specific expression of transgenes in tissues after vector injection. We show here that Cre-independent recombination events in the AAV genome carrying the FLEX sequence occur mainly during the production of viral vectors in packaging cells, which results in transgene expression in off-target populations. Introduction of a relatively longer nucleotide sequence between two recognition sites at the unilateral side of the transgene cassette, termed a unilateral spacer sequence (USS), is useful to suppress the recombination in the viral genome, leading to the protection of non-specific transgene expression with enhanced gene expression selectivity. Our FLEX/USS system offers a powerful strategy for highly specific Cre-dependent transgene expression, aiming at various applications for structural and functional analyses of target cell populations.

The thalamus is the causal hub of intervention in patients with major depressive disorder: Evidence from the Granger causality analysis

Major depressive disorder (MDD) is the leading mental disorder and afflicts more than 350 million people worldwide. The underlying neural mechanisms of MDD remain unclear, hindering the accurate treatment. Recent brain imaging studies have observed functional abnormalities in multiple brain regions in patients with MDD, identifying core brain regions is the key to locating potential therapeutic targets for MDD. The Granger causality analysis (GCA) measures directional effects between brain regions and, therefore, can track causal hubs as potential intervention targets for MDD. We reviewed literature employing GCA to investigate abnormal brain connections in patients with MDD. The total degree of effective connections in the thalamus (THA) is more than twice that in traditional targets such as the superior frontal gyrus and anterior cingulate cortex. Altered causal connections in patients with MDD mainly included enhanced bottom-up connections from the thalamus to various cortical and subcortical regions and reduced top-down connections from these regions to the THA, indicating excessive uplink sensory information and insufficient downlink suppression information for negative emotions. We suggest that the thalamus is the most crucial causal hub for MDD, which may serve as the downstream target for non-invasive brain stimulation and medication approaches in MDD treatment.

Catecholaminergic cell type-specific expression of Cre recombinase in knock-in transgenic rats generated by the Combi-CRISPR technology

BACKGROUND

Cell groups containing catecholamines provide a useful model to study the molecular and cellular mechanisms underlying the morphogenesis, physiology, and pathology of the central nervous system. For this purpose, it is necessary to establish a system to induce catecholaminergic group-specific expression of Cre recombinase. Recently, we introduced a gene cassette encoding 2A peptide fused to Cre recombinase into the site between the C-terminus and translational termination codons of the rat tyrosine hydroxylase (TH) open reading frame by the Combi-CRISPR technology, which is a genomic editing method to enable an efficient knock-in (KI) of long DNA sequence into a target site. However, the expression patterns of the transgene and its function as well as the effect of the mutation on the biochemical and behavioral phenotypes in the KI strains have not been characterized yet.

NEW METHOD

We aimed to evaluate the usefulness of TH-Cre KI rats as an experimental model for investigating the structure and function of catecholaminergic neurons in the brain.

RESULTS

We detected cell type-specific expression of Cre recombinase and site-specific recombination activity in the representative catecholaminergic groups in the TH-Cre KI rat strains. In addition, we measured TH protein levels and catecholamine accumulation in the brain regions, as well as motor, reward-related, and anxiety-like behaviors, indicating that catecholamine metabolism and general behavior are apparently normal in these KI rats.

CONCLUSIONS

TH-Cre KI rat strains produced by the Combi-CRISPR system offer a beneficial model to study the molecular and cellular mechanics for the morphogenesis, physiology, and pathology of catecholamine-containing neurons in the brain.

Structural and functional organization of the midline and intralaminar nuclei of the thalamus

The midline and intralaminar nuclei of the thalamus form a major part of the "limbic thalamus;" that is, thalamic structures anatomically and functionally linked with the limbic forebrain. The midline nuclei consist of the paraventricular (PV) and paratenial nuclei, dorsally and the rhomboid and nucleus reuniens (RE), ventrally. The rostral intralaminar nuclei (ILt) consist of the central medial (CM), paracentral (PC) and central lateral (CL) nuclei. We presently concentrate on RE, PV, CM and CL nuclei of the thalamus. The nucleus reuniens receives a diverse array of input from limbic-related sites, and predominantly projects to the hippocampus and to "limbic" cortices. The RE participates in various cognitive functions including spatial working memory, executive functions (attention, behavioral flexibility) and affect/fear behavior. The PV receives significant limbic-related afferents, particularly the hypothalamus, and mainly distributes to "affective" structures of the forebrain including the bed nucleus of stria terminalis, nucleus accumbens and the amygdala. Accordingly, PV serves a critical role in "motivated behaviors" such as arousal, feeding/consummatory behavior and drug addiction. The rostral ILt receives both limbic and sensorimotor-related input and distributes widely over limbic and motor regions of the frontal cortex-and throughout the dorsal striatum. The intralaminar thalamus is critical for maintaining consciousness and directly participates in various sensorimotor functions (visuospatial or reaction time tasks) and cognitive tasks involving striatal-cortical interactions. As discussed herein, while each of the midline and intralaminar nuclei are anatomically and functionally distinct, they collectively serve a vital role in several affective, cognitive and executive behaviors - as major components of a brainstem-diencephalic-thalamocortical circuitry.

The central medial thalamic nucleus facilitates bilateral movement execution in rats

Intralaminar thalamic nuclei, including the central medial nucleus (CMT), have been classically implicated in the control of attentional functional states such as sleep-wake transitions. In rodents, the CMT innervates large cortical and subcortical areas bilaterally, including sensorimotor regions of the cortex and striatum, but its contribution to motor function, which regularly develops in faster temporal scales than attentional states, is still far from being completely understood. Here, by using a novel behavioral protocol to evaluate bilateral coordination in rats, combined with electrophysiological recordings and optogenetic manipulations, we studied the contribution of the CMT to motor control and coordination. We found that optogenetic stimulation of the central region of the CMT produced bilateral recruitment of neural activity in the sensorimotor cortex and striatum. The same type of stimulations produced a significant increase in bilateral movement coordination of the forelimbs accompanied by a decrease in movement trajectory variability. Optogenetic inactivation of the CMT did not affect motor execution but significantly increased execution times, suggesting less interest in the task. Altogether, our results indicate that brief CMT activations create windows of synchronized bilateral cortico-striatal activity, suitable to facilitate motor coordination in temporal scales relevant for motor execution. Significance Statement The central medial thalamic nucleus (CMT) has been classically implicated in attentional processes. However, it also innervates large motor cortico-striatal regions, but its participation in motor control and coordination is still not well understood. Here, by combining a novel behavioral protocol with optogenetic manipulations, we have found that brief CMT activations create windows of synchronized bilateral cortico-striatal activity, suitable to facilitate motor coordination in temporal scales relevant for motor execution.

Where Actions Meet Outcomes: Medial Prefrontal Cortex, Central Thalamus, and the Basal Ganglia

Medial prefrontal cortex (mPFC) interacts with distributed networks that give rise to goal-directed behavior through afferent and efferent connections with multiple thalamic nuclei and recurrent basal ganglia-thalamocortical circuits. Recent studies have revealed individual roles for different thalamic nuclei: mediodorsal (MD) regulation of signaling properties in mPFC neurons, intralaminar control of cortico-basal ganglia networks, ventral medial facilitation of integrative motor function, and hippocampal functions supported by ventral midline and anterior nuclei. Large scale mapping studies have identified functionally distinct cortico-basal ganglia-thalamocortical subnetworks that provide a structural basis for understanding information processing and functional heterogeneity within the basal ganglia. Behavioral analyses comparing functional deficits produced by lesions or inactivation of specific thalamic nuclei or subregions of mPFC or the basal ganglia have elucidated the interdependent roles of these areas in adaptive goal-directed behavior. Electrophysiological recordings of mPFC neurons in rats performing delayed non-matching-to position (DNMTP) and other complex decision making tasks have revealed populations of neurons with activity related to actions and outcomes that underlie these behaviors. These include responses related to motor preparation, instrumental actions, movement, anticipation and delivery of action outcomes, memory delay, and spatial context. Comparison of results for mPFC, MD, and ventral pallidum (VP) suggest critical roles for mPFC in prospective processes that precede actions, MD for reinforcing task-relevant responses in mPFC, and VP for providing feedback about action outcomes. Synthesis of electrophysiological and behavioral results indicates that different networks connecting mPFC with thalamus and the basal ganglia are organized to support distinct functions that allow organisms to act efficiently to obtain intended outcomes.

Neural correlates of multidimensional motor outputs in an excitatory parafascicular-zona incerta circuit

The parafascicular nucleus (Pf) in medial thalamus is interconnected with prefrontal cortex and basal ganglia. Though much research has determined its importance in cognitive regulation of behaviour, its projections to regions in subthalamus remain less known. Such connections include those to zona incerta (ZI), located immediately dorsal to subthalamic nuclei (STN) regulating motor output, and whose role in a motor context is only beginning to be investigated. We thus examined circuits from parafascicular (Pf) thalamus to ZI, and its activity during locomotion and spontaneous behaviours in mice. We found that a distinct group of CaMKIIα-positive excitatory parafascicular neurons, separated from VGLUT2-positive excitatory neurons, project widely into ZI, more than adjacent STN. Our results from fibre photometry and decoding with general linear model (GLM) indicate that PF-ZI pathways do not specifically correlate with amount of locomotion or movement velocity, but instead show more specified activity during relative directional changes of movements observed in turning, sniffing behaviours. These results hint at the PF-ZI pathway having a distinct role in directing action specificity and have implications for subcortical bases in dimensional control of behaviours.

Thalamostriatal System Controls the Acquisition, Performance, and Flexibility of Learning Behavior

The dorsal striatum (DS) is a key structure of the basal ganglia circuitry, which regulates various types of learning processes and flexible switching of behavior. Intralaminar thalamic nuclei (ILNs) provide the main source of thalamostriatal inputs to the DS and constitute multiple nuclear groups, each of which innervates specific subdivisions of the striatum. Although the anatomical and electrophysiological properties of thalamostriatal neurons have been previously characterized, the behavioral and physiological functions of these neurons remain unclarified. Two representative thalamostriatal cell groups in the parafascicular nucleus (PF) and the central lateral nucleus (CL) are located in the caudal and rostral regions of the ILNs in rodents. Recently, the behavioral roles of these thalamostriatal cell groups have been investigated by the use of genetic and pharmacological manipulation techniques. In the current review, we summarize behavioral studies on thalamostriatal neurons, showing the key roles of these neurons in different learning processes, such as the acquisition, performance, and flexibility of behavior.

Hypoactive Thalamic Crh+ Cells in a Female Mouse Model of Alcohol Drinking After Social Trauma

BACKGROUND

Comorbid stress-induced mood and alcohol use disorders are increasingly prevalent among female patients. Stress exposure can disrupt salience processing and goal-directed decision making, contributing to persistent maladaptive behavioral patterns; these and other stress-sensitive cognitive and behavioral processes rely on dynamic and coordinated signaling by midline and intralaminar thalamic nuclei. Considering the role of social trauma in the trajectory of these debilitating psychopathologies, identifying vulnerable thalamic cells may provide guidance for targeting persistent stress-induced symptoms.

METHODS

A novel behavioral protocol traced the progression from social trauma to the development of social defensiveness and chronically escalated alcohol consumption in female mice. Recent cell activation-measured as cFos-was quantified in thalamic cells after safe social interactions, revealing stress-sensitive corticotropin-releasing hormone-expressing (Crh+) anterior central medial thalamic (aCMT) cells. These cells were optogenetically stimulated during stress-induced social defensiveness and abstinence-escalated binge drinking.

RESULTS

Crh+ aCMT neurons exhibited substantial activation after social interactions in stress-naïve but not in stressed female mice. Photoactivating Crh+ aCMT cells dampened stress-induced social deficits, whereas inhibiting these cells increased social defensiveness in stress-naïve mice. Optogenetically activating Crh+ aCMT cells diminished abstinence-escalated binge alcohol drinking in female mice, regardless of stress history.

CONCLUSIONS

This work uncovers a role for Crh+ aCMT neurons in maladaptive stress-induced social interactions and in binge drinking after forced abstinence in female mice. This molecularly defined thalamic cell population may serve as a critical stress-sensitive hub for social deficits caused by exposure to social trauma and for patterns of excessive alcohol drinking in female populations.

Midline Thalamic Damage Associated with Alcohol-Use Disorders: Disruption of Distinct Thalamocortical Pathways and Function

The thalamus, a significant part of the diencephalon, is a symmetrical and bilateral central brain structure. The thalamus is subdivided into three major groups of nuclei based on their function: sensorimotor nuclei (or principal/relay nuclei), limbic nuclei and nuclei bridging these two domains. Anatomically, nuclei within the thalamus are described by their location, such as anterior, medial, lateral, ventral, and posterior. In this review, we summarize the role of medial and midline thalamus in cognition, ranging from learning and memory to flexible adaptation. We focus on the discoveries in animal models of alcohol-related brain damage, which identify the loss of neurons in the medial and midline thalamus as drivers of cognitive dysfunction associated with alcohol use disorders. Models of developmental ethanol exposure and models of adult alcohol-related brain damage and are compared and contrasted, and it was revealed that there are similar (anterior thalamus) and different (intralaminar [adult exposure] versus ventral midline [developmental exposure]) thalamic pathology, as well as disruptions of thalamo-hippocampal and thalamo-cortical circuits. The final part of the review summarizes approaches to recover alcohol-related brain damage and cognitive and behavioral outcomes. These approaches include pharmacological, nutritional and behavioral interventions that demonstrated the potential to mitigate alcohol-related damage. In summary, the medial/midline thalamus is a significant contributor to cognition function, which is also sensitive to alcohol-related brain damage across the life span, and plays a role in alcohol-related cognitive dysfunction.

Retrograde Transgene Expression via Neuron-Specific Lentiviral Vector Depends on Both Species and Input Projections

For achieving retrograde gene transfer, we have so far developed two types of lentiviral vectors pseudotyped with fusion envelope glycoprotein, termed HiRet vector and NeuRet vector, consisting of distinct combinations of rabies virus and vesicular stomatitis virus glycoproteins. In the present study, we compared the patterns of retrograde transgene expression for the HiRet vs. NeuRet vectors by testing the cortical input system. These vectors were injected into the motor cortex in rats, marmosets, and macaques, and the distributions of retrograde labels were investigated in the cortex and thalamus. Our histological analysis revealed that the NeuRet vector generally exhibits a higher efficiency of retrograde gene transfer than the HiRet vector, though its capacity of retrograde transgene expression in the macaque brain is unexpectedly low, especially in terms of the intracortical connections, as compared to the rat and marmoset brains. It was also demonstrated that the NeuRet but not the HiRet vector displays sufficiently high neuron specificity and causes no marked inflammatory/immune responses at the vector injection sites in the primate (marmoset and macaque) brains. The present results indicate that the retrograde transgene efficiency of the NeuRet vector varies depending not only on the species but also on the input projections.

Convergence Circuit Mapping: Genetic Approaches From Structure to Function

Understanding the complex neural circuits that underpin brain function and behavior has been a long-standing goal of neuroscience. Yet this is no small feat considering the interconnectedness of neurons and other cell types, both within and across brain regions. In this review, we describe recent advances in mouse molecular genetic engineering that can be used to integrate information on brain activity and structure at regional, cellular, and subcellular levels. The convergence of structural inputs can be mapped throughout the brain in a cell type-specific manner by antero- and retrograde viral systems expressing various fluorescent proteins and genetic switches. Furthermore, neural activity can be manipulated using opto- and chemo-genetic tools to interrogate the functional significance of this input convergence. Monitoring neuronal activity is obtained with precise spatiotemporal resolution using genetically encoded sensors for calcium changes and specific neurotransmitters. Combining these genetically engineered mapping tools is a compelling approach for unraveling the structural and functional brain architecture of complex behaviors and malfunctioned states of neurological disorders.

Interpreting the role of the striatum during multiple phases of motor learning

The synaptic pathways in the striatum are central to basal ganglia functions including motor control, learning and organization, action selection, acquisition of motor skills, cognitive function, and emotion. Here, we review the role of the striatum and its connections in motor learning and performance. The development of new techniques to record neuronal activity and animal models of motor disorders using neurotoxin, pharmacological, and genetic manipulations are revealing pathways that underlie motor performance and motor learning, as well as how they are altered by pathophysiological mechanisms. We discuss approaches that can be used to analyze complex motor skills, particularly in rodents, and identify specific questions central to understanding how striatal circuits mediate motor learning.

Prefrontal deep projection neurons enable cognitive flexibility via persistent feedback monitoring

Cognitive flexibility, the ability to alter strategy according to changing stimulus-response-reward relationships, is critical for updating learned behavior. Attentional set-shifting, a test of cognitive flexibility, depends on the activity of prefrontal cortex (PFC). It remains unclear, however, what role PFC neurons play to support set-shifting. Using optogenetics and two-photon calcium imaging, we demonstrate that medial PFC activity does not bias sensorimotor responses during set-shifting, but rather enables set-shifting by encoding trial feedback information, a role it has been known to play in other contexts. Unexpectedly, the functional properties of PFC cells did not vary with their efferent projection targets. Instead, representations of trial feedback formed a topological gradient, with cells more strongly selective for feedback information located further from the pial surface, where afferent input from the anterior cingulate cortex was denser. These findings identify a critical role for deep PFC projection neurons in enabling set-shifting through behavioral feedback monitoring.

Rostral Intralaminar Thalamus Engagement in Cognition and Behavior

The thalamic rostral intralaminar nuclei (rILN) are a contiguous band of neurons that include the central medial, paracentral, and central lateral nuclei. The rILN differ from both thalamic relay nuclei, such as the lateral geniculate nucleus, and caudal intralaminar nuclei, such as the parafascicular nucleus, in afferent and efferent connectivity as well as physiological and synaptic properties. rILN activity is associated with a range of neural functions and behaviors, including arousal, pain, executive function, and action control. Here, we review this evidence supporting a role for the rILN in integrating arousal, executive and motor feedback information. In light of rILN projections out to the striatum, amygdala, and sensory as well as executive cortices, we propose that such a function enables the rILN to modulate cognitive and motor resources to meet task-dependent behavioral engagement demands.

Temporal Prediction Signals for Periodic Sensory Events in the Primate Central Thalamus

Prediction of periodic event timing is an important function for everyday activities, while the exact neural mechanism remains unclear. Previous studies in nonhuman primates have demonstrated that neurons in the cerebellar dentate nucleus and those in the caudate nucleus exhibit periodic firing modulation when the animals attempt to detect a single omission of isochronous repetitive audiovisual stimuli. To understand how these subcortical signals are sent and processed through the thalamocortical pathways, we examined single-neuron activities in the central thalamus of two macaque monkeys (one female and one male). We found that three types of neurons responded to each stimulus in the sequence in the absence of movements. Reactive-type neurons showed sensory adaptation and gradually waned the transient response to each stimulus. Predictive-type neurons steadily increased the magnitude of the suppressive response, similar to neurons previously reported in the cerebellum. Switch-type neurons initially showed a transient response, but after several cycles, the direction of firing modulation reversed and the activity decreased for each repetitive stimulus. The time course of Switch-type activity was well explained by the weighted sum of activities of the other types of neurons. Furthermore, for only Switch-type neurons the activity just before stimulus omission significantly correlated with behavioral latency, indicating that this type of neuron may carry a more advanced signal in the system detecting stimulus omission. These results suggest that the central thalamus may transmit integrated signals to the cerebral cortex for temporal information processing, which are necessary to accurately predict rhythmic event timing.SIGNIFICANCE STATEMENT Several cortical and subcortical regions are involved in temporal information processing, and the thalamus will play a role in functionally linking them. The present study aimed to clarify how the paralaminar part of the thalamus transmits and modifies signals for temporal prediction of rhythmic events. Three types of thalamic neurons exhibited periodic activity when monkeys attempted to detect a single omission of isochronous repetitive stimuli. The activity of one type of neuron correlated with the behavioral latency and appeared to be generated by integrating the signals carried by the other types of neurons. Our results revealed the neuronal signals in the thalamus for temporal prediction of sensory events, providing a clue to elucidate information processing in the thalamocortical pathways.

The Contribution of Thalamic Nuclei in Salience Processing

The brain continuously receives diverse information about the external environment and changes in the homeostatic state. The attribution of salience determines which stimuli capture attention and, therefore, plays an essential role in regulating emotions and guiding behaviors. Although the thalamus is included in the salience network, the neural mechanism of how the thalamus contributes to salience processing remains elusive. In this mini-review, we will focus on recent advances in understanding the specific roles of distinct thalamic nuclei in salience processing. We will summarize the functional connections between thalamus nuclei and other key nodes in the salience network. We will highlight the convergence of neural circuits involved in reward and pain processing, arousal, and attention control in thalamic structures. We will discuss how thalamic activities represent salience information in associative learning and how thalamic neurons modulate adaptive behaviors. Lastly, we will review recent studies which investigate the contribution of thalamic dysfunction to aberrant salience processing in neuropsychiatric disorders, such as drug addiction, posttraumatic stress disorder (PTSD), and schizophrenia. Based on emerging evidence from both human and rodent research, we propose that the thalamus, different from previous studies that as an information relay, has a broader role in coordinating the cognitive process and regulating emotions.

Methamphetamine seeking after prolonged abstinence is associated with activated projections from anterior intralaminar nucleus of thalamus to dorsolateral striatum in female rats

Methamphetamine (Meth) seeking progressively increases after cessation from drug self-administration (incubation of Meth craving). We have previously shown that both dorsomedial and dorsolateral striatum (DMS and DLS) play critical roles in this incubation in male rats. Moreover, our recent anatomical tracing study examined afferent projections into DMS and demonstrated a novel role of projections from anterior intralaminar nucleus of thalamus (AIT) to DMS in incubation of Meth craving in male rats. Here we investigated projection-specific activation of afferent glutamate projections into DLS associated with incubated Meth seeking in female rats. We trained female rats to self-administer Meth (6-h/d for 10 d). On abstinence day 12, we injected cholera toxin subunit B (CTb, a retrograde tracer) unilaterally into DLS. On abstinence day 26, we tested rats for relapse to Meth seeking and measured Fos (a neuronal activity marker), and double-labeling of CTb and Fos in anterior cingulate cortex, anterior insula cortex, orbitofrontal cortex, basolateral amygdala, AIT, and parafascicular nuclei of thalamus. We observed neuronal activation in both cortical and thalamic regions associated with incubated Meth seeking. At the circuit level, AIT➔DLS projections were strongly activated, followed by other corticostriatal projections. Overall our results suggest that AIT to DLS may play a role in Meth seeking after prolonged abstinence in female rats.

Remembrance of things practiced with fast and slow learning in cortical and subcortical pathways

The learning of motor skills unfolds over multiple timescales, with rapid initial gains in performance followed by a longer period in which the behavior becomes more refined, habitual, and automatized. While recent lesion and inactivation experiments have provided hints about how various brain areas might contribute to such learning, their precise roles and the neural mechanisms underlying them are not well understood. In this work, we propose neural- and circuit-level mechanisms by which motor cortex, thalamus, and striatum support motor learning. In this model, the combination of fast cortical learning and slow subcortical learning gives rise to a covert learning process through which control of behavior is gradually transferred from cortical to subcortical circuits, while protecting learned behaviors that are practiced repeatedly against overwriting by future learning. Together, these results point to a new computational role for thalamus in motor learning and, more broadly, provide a framework for understanding the neural basis of habit formation and the automatization of behavior through practice.

Pseudotyped lentiviral vectors for tract-targeting and application for the functional control of selective neural circuits

A lentiviral vector strategy for efficient gene transfer through retrograde axonal transport provides a powerful approach for studying the neural circuit mechanisms that mediate higher level functions of the central nervous system. Pseudotyping of human immunodeficiency virus type-1 with different types of fusion glycoproteins (FuGs), which are composed of segments of rabies virus glycoprotein (RV-G) and vesicular stomatitis virus glycoprotein (VSV-G), enhances the efficiency of retrograde gene transfer in both rodent and non-human primate brains. These pseudotyped lentiviral vectors are classified into two groups, highly efficient retrograde gene transfer (HiRet) and neuron-specific retrograde gene transfer (NeuRet) vectors, based on their properties of gene transduction. Combinatorial use of the pseudotyped vectors with various molecular tools for manipulating neural circuit functions (such as the cell targeting, synaptic silencing, and optogenetic or chemogenetic approaches) enables us to control the function of specific neural circuits, thus leading to a deeper understanding of the mechanism underlying various nervous system functions.

Operant self-stimulation of thalamic terminals in the dorsomedial striatum is constrained by metabotropic glutamate receptor 2

Dorsal striatal manipulations including stimulation of dopamine release and activation of medium spiny neurons (MSNs) are sufficient to drive reinforcement-based learning. Glutamatergic innervation of the striatum by the cortex and thalamus is a critical determinant of MSN activity and local regulation of dopamine release. However, the relationship between striatal glutamatergic afferents and behavioral reinforcement is not well understood. We evaluated the reinforcing properties of optogenetic stimulation of thalamostriatal terminals, which are associated with vesicular glutamate transporter 2 (Vglut2) expression, in the dorsomedial striatum (DMS), a region implicated in goal-directed behaviors. In mice expressing channelrhodopsin-2 (ChR2) under control of the Vglut2 promoter, optical stimulation of the DMS reinforced operant lever-pressing behavior. Mice also acquired operant self-stimulation of thalamostriatal terminals when ChR2 expression was virally targeted to the intralaminar thalamus. Stimulation trains that supported operant responding evoked dopamine release in the DMS and excitatory postsynaptic currents in DMS MSNs. Our previous work demonstrated that the presynaptic G protein-coupled receptor metabotropic glutamate receptor 2 (mGlu₂) robustly inhibits glutamate and dopamine release induced by activation of thalamostriatal afferents. Thus, we examined the regulation of thalamostriatal self-stimulation by mGlu₂. Administration of an mGlu_2/3 agonist or an mGlu₂-selective positive allosteric modulator reduced self-stimulation. Conversely, blockade of these receptors increased thalamostriatal self-stimulation, suggesting that endogenous activation of these receptors negatively modulates the reinforcing properties of thalamostriatal activity. These findings demonstrate that stimulation of thalamic terminals in the DMS is sufficient to reinforce a self-initiated action, and that thalamostriatal reinforcement is constrained by mGlu₂ activation.

Modular output circuits of the fastigial nucleus for diverse motor and nonmotor functions of the cerebellar vermis

The cerebellar vermis, long associated with axial motor control, has been implicated in a surprising range of neuropsychiatric disorders and cognitive and affective functions. Remarkably little is known, however, about the specific cell types and neural circuits responsible for these diverse functions. Here, using single-cell gene expression profiling and anatomical circuit analyses of vermis output neurons in the mouse fastigial (medial cerebellar) nucleus, we identify five major classes of glutamatergic projection neurons distinguished by gene expression, morphology, distribution, and input-output connectivity. Each fastigial cell type is connected with a specific set of Purkinje cells and inferior olive neurons and in turn innervates a distinct collection of downstream targets. Transsynaptic tracing indicates extensive disynaptic links with cognitive, affective, and motor forebrain circuits. These results indicate that diverse cerebellar vermis functions could be mediated by modular synaptic connections of distinct fastigial cell types with posturomotor, oromotor, positional-autonomic, orienting, and vigilance circuits.

Activation of the Rostral Intralaminar Thalamus Drives Reinforcement through Striatal Dopamine Release

Glutamatergic projections of the thalamic rostral intralaminar nuclei of the thalamus (rILN) innervate the dorsal striatum (DS) and are implicated in dopamine (DA)-dependent incubation of drug seeking. However, the mechanism by which rILN signaling modulates reward seeking and striatal DA release is unknown. We find that activation of rILN inputs to the DS drives cholinergic interneuron burst-firing behavior and DA D2 receptor-dependent post-burst pauses in cholinergic interneuron firing. In vivo, optogenetic activation of this pathway drives reinforcement in a DA D1 receptor-dependent manner, and chemogenetic suppression of the rILN reduces dopaminergic nigrostriatal terminal activity as measured by fiber photometry. Altogether, these data provide evidence that the rILN activates striatal cholinergic interneurons to enhance the pursuit of reward through local striatal DA release and introduce an additional level of complexity in our understanding of striatal DA signaling.

Cover et al. identify a glutamatergic thalamostriatal pathway that locally elicits striatal dopamine release to drive reward-related behavior in mice.

A Novel Birthdate-Labeling Method Reveals Segregated Parallel Projections of Mitral and External Tufted Cells in the Main Olfactory System

A fundamental strategy in sensory coding is parallel processing, whereby unique, distinct features of sensation are computed and projected to the central target in the form of submodal maps. It remains unclear, however, whether such parallel processing strategy is employed in the main olfactory system, which codes the complex hierarchical odor and behavioral scenes. A potential scheme is that distinct subsets of projection neurons in the olfactory bulb (OB) form parallel projections to the targets. Taking advantage of the observation that the distinct projection neurons develop at different times, we developed a Cre-loxP-based method that allows for birthdate-specific labeling of cell bodies and their axon projections in mice. This birthdate tag analysis revealed that the mitral cells (MCs) born in an early developmental stage and the external tufted cells (TCs) born a few days later form segregated parallel projections. Specifically, the latter subset converges the axons onto only two small specific targets, one of which, located at the anterolateral edge of the olfactory tubercle (OT), excludes widespread MC projections. This target is made up of neurons that express dopamine D1 but not D2 receptor and corresponds to the most anterolateral isolation of the CAP compartments (aiCAP) that were defined previously. This finding of segregated projections suggests that olfactory sensing does indeed involve parallel processing of functionally distinct submodalities. Importantly, the birthdate tag method used here may pave the way for deciphering the functional meaning of these individual projection pathways in the future.

A repeated molecular architecture across thalamic pathways

The thalamus is the central communication hub of the forebrain and provides the cerebral cortex with inputs from sensory organs, subcortical systems and the cortex itself. Multiple thalamic regions send convergent information to each cortical region, but the organizational logic of thalamic projections has remained elusive. Through comprehensive transcriptional analyses of retrogradely labeled thalamic neurons in adult mice, we identify three major profiles of thalamic pathways. These profiles exist along a continuum that is repeated across all major projection systems, such as those for vision, motor control and cognition. The largest component of gene expression variation in the mouse thalamus is topographically organized, with features conserved in humans. Transcriptional differences between these thalamic neuronal identities are tied to cellular features that are critical for function, such as axonal morphology and membrane properties. Molecular profiling therefore reveals covariation in the properties of thalamic pathways serving all major input modalities and output targets, thus establishing a molecular framework for understanding the thalamus.

What, If, and When to Move: Basal Ganglia Circuits and Self-Paced Action Initiation

Deciding what to do and when to move is vital to our survival. Clinical and fundamental studies have identified basal ganglia circuits as critical for this process. The main input nucleus of the basal ganglia, the striatum, receives inputs from frontal, sensory, and motor cortices and interconnected thalamic areas that provide information about potential goals, context, and actions and directly or indirectly modulates basal ganglia outputs. The striatum also receives dopaminergic inputs that can signal reward prediction errors and also behavioral transitions and movement initiation. Here we review studies and models of how direct and indirect pathways can modulate basal ganglia outputs to facilitate movement initiation, and we discuss the role of cortical and dopaminergic inputs to the striatum in determining what to do and if and when to do it. Complex but exciting scenarios emerge that shed new light on how basal ganglia circuits modulate self-paced movement initiation.

Accumbal Adenosine A2A Receptors Enhance Cognitive Flexibility by Facilitating Strategy Shifting

The deficits of cognitive flexibility (including attentional set-shifting and reversal learning) concomitant with dysfunction of the striatum are observed in several neuropsychiatric disorders. Rodent and human studies have identified the striatum [particularly the dorsomedial striatum (DMS) and nucleus accumbens (NAc)] as the critical locus for control of cognitive flexibility, but the effective neuromodulator and pharmacological control of cognitive flexibility remains to be determined. The adenosine A_2A receptors (A_2ARs) are highly enriched in the striatopallidal neurons where they integrate dopamine and glutamate signals to modulate several cognitive behaviors, but their contribution to cognitive flexibility control is unclear. In this study, by coupling an automated operant cognitive flexibility task with striatal subregional knockdown (KD) of the A_2AR via the Cre-loxP strategy, we demonstrated that NAc A_2AR KD improved cognitive flexibility with enhanced attentional set-shifting and reversal learning by decreasing regressive and perseverative errors, respectively. This facilitation was not attributed to mnemonic process or motor activity as NAc A_2AR KD did not affect the visual discrimination, lever-pressing acquisition, and locomotor activity, but was associated with increased attention and motivation as evident by the progressive ratio test (PRT). In contrast to NAc A_2ARs, DMS A_2ARs KD neither affected visual discrimination nor improved set-shifting nor reversal learning, but promoted the effort-related motivation. Thus, NAc and DMS A_2ARs exert dissociable controls of cognitive flexibility with NAc A_2ARs KD selectively enhancing cognitive flexibility by facilitating strategy shifting with increased motivation/attention.

Central thalamic inactivation impairs the expression of action- and outcome-related responses of medial prefrontal cortex neurons in the rat

The mediodorsal (MD) and adjacent intralaminar (IL) and midline nuclei provide the main thalamic input to the medial prefrontal cortex (mPFC) and are critical for associative learning and decision-making. MD neurons exhibit activity related to actions and outcomes that mirror responses of mPFC neurons in rats during dynamic delayed non-match to position (dDNMTP), a variation of DNMTP where start location is varied randomly within an open octagonal arena to avoid confounding behavioral events with spatial location. To test whether the thalamus affects the expression of these responses in mPFC, we inhibited the central thalamus unilaterally by microinjecting muscimol at doses and sites found to affect decision-making when applied bilaterally. Unilateral inactivation reduced normalized task-related responses in the ipsilateral mPFC without disrupting behavior needed to characterize event-related neuronal activity. Our results extend earlier findings that focused on delay-related activity by showing that central thalamic inactivation interferes with responses related to actions and outcomes that occur outside the period of memory delay. These findings are consistent with the broad effects of central thalamic lesions on behavioral measures of reinforcement-guided responding. Most (7/8) of the prefrontal response types affected by thalamic inactivation have also been observed in MD during dDNMTP. These results support the hypothesis that MD and IL act as transthalamic gates: monitoring prefrontal activity through corticothalamic inputs; integrating this information with signals from motivational and sensorimotor systems that converge in thalamus; and acting through thalamocortical projections to enhance expression of neuronal responses in the PFC that support adaptive goal-directed behavior.

A note on retrograde gene transfer efficiency and inflammatory response of lentiviral vectors pseudotyped with FuG-E vs. FuG-B2 glycoproteins

Pseudotyped lentiviral vectors give access to pathway-selective gene manipulation via retrograde transfer. Two types of such lentiviral vectors have been developed. One is the so-called NeuRet vector pseudotyped with fusion glycoprotein type E, which preferentially transduces neurons. The other is the so-called HiRet vector pseudotyped with fusion glycoprotein type B2, which permits gene transfer into both neurons and glial cells at the injection site. Although these vectors have been applied in many studies investigating neural network functions, it remains unclear which vector is more appropriate for retrograde gene delivery in the brain. To compare the gene transfer efficiency and inflammatory response of the NeuRet vs. HiRet vectors, each vector was injected into the striatum in macaque monkeys, common marmosets, and rats. It was revealed that retrograde gene delivery of the NeuRet vector was equal to or greater than that of the HiRet vector. Furthermore, inflammation characterized by microglial and lymphocytic infiltration occurred when the HiRet vector, but not the NeuRet vector, was injected into the primate brain. The present results indicate that the NeuRet vector is more suitable than the HiRet vector for retrograde gene transfer in the primate and rodent brains.

Motor skills mediated through cerebellothalamic tracts projecting to the central lateral nucleus

The cerebellum regulates complex animal behaviors, such as motor control and spatial recognition, through communication with many other brain regions. The major targets of the cerebellar projections are the thalamic regions including the ventroanterior nucleus (VA) and ventrolateral nucleus (VL). Another thalamic target is the central lateral nucleus (CL), which receives the innervations mainly from the dentate nucleus (DN) in the cerebellum. Although previous electrophysiological studies suggest the role of the CL as the relay of cerebellar functions, the kinds of behavioral functions mediated by cerebellothalamic tracts projecting to the CL remain unknown. Here, we used immunotoxin (IT) targeting technology combined with a neuron-specific retrograde labeling technique, and selectively eliminated the cerebellothalamic tracts of mice. We confirmed that the number of neurons in the DN was selectively decreased by the IT treatment. These IT-treated mice showed normal overground locomotion with no ataxic behavior. However, elimination of these neurons impaired motor coordination in the rotarod test and forelimb movement in the reaching test. These mice showed intact acquisition and flexible change of spatial information processing in the place discrimination, Morris water maze, and T-maze tests. Although the tract labeling indicated the existence of axonal collaterals of the DN-CL pathway to the rostral part of the VA/VL complex, excitatory lesion of the rostral VA/VL did not show any significant alterations in motor coordination or forelimb reaching, suggesting no requirement of axonal branches connecting to the VL/VA complex for motor skill function. Taken together, our data highlight that the cerebellothalamic tracts projecting to the CL play a key role in the control of motor skills, including motor coordination and forelimb reaching, but not spatial recognition and its flexibility.

The Cognitive Thalamus as a Gateway to Mental Representations

Historically, the thalamus has been viewed as little more than a relay, simply transferring information to key players of the cast, the cortex and hippocampus, without providing any unique functional contribution. In recent years, evidence from multiple laboratories researching different thalamic nuclei has contradicted this idea of the thalamus as a passive structure. Dated models of thalamic functions are being pushed aside, revealing a greater and far more complex contribution of the thalamus for cognition. In this Viewpoints article, we show how recent data support novel views of thalamic functions that emphasize integrative roles in cognition, ranging from learning and memory to flexible adaption. We propose that these apparently separate cognitive functions may indeed be supported by a more general role in shaping mental representations. Several features of thalamocortical circuits are consistent with this suggested role, and we highlight how divergent and convergent thalamocortical and corticothalamic pathways may complement each other to support these functions. Furthermore, the role of the thalamus for subcortical integration is highlighted as a key mechanism for maintaining and updating representations. Finally, we discuss future areas of research and stress the importance of incorporating new experimental findings into existing knowledge to continue developing thalamic models. The presence of thalamic pathology in a number of neurological conditions reinforces the need to better understand the role of this region in cognition.

Discretion for behavioral selection affects development of habit formation after extended training in rats

As training progresses, animals show a transition from goal-dependent behavior to goal-independent behavior (habitual responses). Habit formation is influenced by several factors, including the amount of training and action-outcome contingency. However, it remains unknown whether and how discretion for behavioral selection influences habit formation. To this end, we trained male rats in two types of two-alternative forced-choice task: visual association and nonvisual association tasks. In the first type of task, rats learned the association between reward and a visual cue, the position of which was randomly changed per trial so that rats had to make a judgmental decision about which choice delivered the reward in each trial (discreet judgment group); in the second type of task, the rats learned that a reward was delivered after either choice following task initiation (uncontrolled judgment group). To test the sensitivity to contingency manipulation, the extinction tests were conducted in short- and long-term trained groups, with the result that the overtrained rats in the uncontrolled judgment group, but not the other three groups, showed less sensitivity. To further investigate the reward sensitivity in the long-term trained groups from another perspective, we continuously and periodically altered the reward size for each trial. The rats of the discreet judgment group changed intertrial intervals depending on reward size, while this tendency was weaker in the uncontrolled judgment group. These results suggest that discreet judgment maintained goal-directed rat behavior, whereas uncontrolled judgment led to the development of habit-like behavior.

Enhancement of the transduction efficiency of a lentiviral vector for neuron-specific retrograde gene delivery through the point mutation of fusion glycoprotein type E

BACKGROUND

Pseudotyping of a lentiviral vector with fusion glycoproteins composed of rabies virus glycoprotein (RVG) and vesicular stomatitis virus glycoprotein (VSVG) segments achieves high gene transfer efficiency through retrograde transport in the nervous system. In our previous study, we determined the junction of RVG/VSVG segments of glycoproteins that enhances the transduction efficiency of the neuron-specific retrograde gene transfer (NeuRet) vector (termed fusion glycoprotein type E or FuG-E).

NEW METHOD

We aimed to optimize the amino acid residue at position 440 in the membrane-proximal region of FuG-E to improve the efficiency of retrograde gene transfer in the brain.

RESULTS

We constructed variants of FuG-E with 18 kinds of single amino acid substitutions at residue 440 to generate lentiviral vectors pseudotyped with these variants, and tested in vivo gene transfer of the vectors in the mouse brain. The FuG-E (P440E) variant, in which proline was substituted by glutamate at residue 440 in FuG-E, showed the greatest retrograde gene transfer efficiency in the brain, bearing the property of the NeuRet vector. The FuG-E (P440E) pseudotype also displayed efficient retrograde gene transfer in the common marmoset brain.

COMPARISON WITH EXISTING METHODS

The NeuRet vector with the FuG-E (P440E) variant demonstrated higher retrograde gene transfer efficiency into different neural pathways compared with the parental FuG-E vector.

CONCLUSIONS

The FuG-E (P440E) pseudotype provides a powerful tool to investigate neural circuit mechanisms underlying various brain functions and for gene therapy trials of neurological and neurodegenerative diseases.

A competitive model for striatal action selection

The direct and indirect pathway striatal medium spiny neurons (dMSNs and iMSNs) have long been linked to action selection, but the precise roles of these neurons in this process remain unclear. Here, we review different models of striatal pathway function, focusing on the classic "go/no-go" model which posits that dMSNs facilitate movement while iMSNs inhibit movement, and the "complementary" model, which argues that dMSNs facilitate the selection of specific actions while iMSNs inhibit potentially conflicting actions. We discuss the merits and shortcomings of these models and propose a "competitive" model to explain the contribution of these two pathways to behavior. The "competitive" model argues that rather than inhibiting conflicting actions, iMSNs are tuned to the same actions that dMSNs facilitate, and the two populations "compete" to determine the animal's behavioral response. This model provides a theoretical explanation for how these pathways work together to select actions. In addition, it provides a link between action selection and behavioral reinforcement, via modulating synaptic strength at inputs onto dMSNs and iMSNs. Finally, this model makes predictions about how imbalances in the activity of these pathways may underlie behavioral traits associated with psychiatric disorders. Understanding the roles of these striatal pathways in action selection may help to clarify the neuronal mechanisms of decision-making under normal and pathological conditions.