A New DREADD Facilitates the Multiplexed Chemogenetic Interrogation of Behavior

Activation of Metabotropic Glutamate Receptor 3 Modulates Thalamo-accumbal Transmission and Rescues Schizophrenia-Like Physiological and Behavioral Deficits

BACKGROUND

Polymorphisms in the gene encoding for metabotropic glutamate receptor 3 (mGlu3) are associated with an increased likelihood of schizophrenia diagnosis and can predict improvements in negative symptoms following treatment with antipsychotics. However, the mechanisms by which mGlu3 can regulate brain circuits involved in schizophrenia pathophysiology are not clear.

METHODS

We employed selective pharmacological tools and a variety of approaches including whole-cell patch-clamp electrophysiology, slice optogenetics, and fiber photometry to investigate the effects of mGlu3 activation on phencyclidine (PCP)-induced impairments in thalamo-accumbal transmission and sociability deficits. A chemogenetic approach was used to evaluate the role of thalamo-accumbal transmission in PCP-induced sociability deficits.

RESULTS

We first established that PCP treatment augmented excitatory transmission onto dopamine D1 receptor-expressing medium spiny neurons (D1-MSNs) in the nucleus accumbens (NAc) and induced sociability deficits. Our studies revealed a selective increase in glutamatergic synaptic transmission from thalamic afferents to D1-MSNs in the NAc shell. Chemogenetic silencing of thalamo-accumbal inputs rescued PCP-induced sociability deficits. Pharmacological activation of mGlu3 normalized PCP-induced impairments in thalamo-accumbal transmission and sociability deficits. Mechanistic studies revealed that mGlu3 activation induced robust long-term depression at synapses from the thalamic projections onto D1-MSNs in the NAc shell.

CONCLUSIONS

These data demonstrate that activation of mGlu3 decreases thalamo-accumbal transmission and thereby rescues sociability deficits in mouse modeling schizophrenia-like symptoms. These findings provide novel insights into the NAc-specific mechanisms and suggest that agents modulating glutamatergic signaling in the NAc may provide a promising approach for treating negative symptoms in schizophrenia.

The parasubthalamic nucleus refeeding ensemble delays feeding initiation and hastens water drinking

The parasubthalamic nucleus (PSTN) is activated by refeeding after food deprivation and several PSTN subpopulations have been shown to suppress feeding. However, no study to date directly addressed the role of PSTN neurons activated upon food access in the control of ensuing food consumption. Here we identify consumption latency as a sensitive behavioral indicator of PSTN activity, and show that, in hungry mice, the ensemble of refeeding-activated PSTN neurons drastically increases the latency to initiate refeeding with both familiar and a novel, familiar food, but does not control the amount of food consumed. In thirsty mice, this ensemble also delays sucrose consumption but accelerates water consumption, possibly reflecting anticipatory prandial thirst, with again no influence on the amount of fluid consumed. We next sought to identify which subpopulations of PSTN neurons might be driving these latency effects, using cell-type and pathway-specific chemogenetic manipulations. Our results suggest a prominent role of PSTN Tac1 neurons projecting to the central amygdala in the hindrance of feeding initiation. While PSTN Crh neurons also delay the latency of hungry mice to ingest familiar foods, they surprisingly promote the consumption of novel, palatable substances. Furthermore, PSTN Crh neurons projecting to the bed nucleus of the stria terminalis accelerate rehydration in thirsty mice. Our results demonstrate the key role of endogenous PSTN activity in the control of feeding and drinking initiation and delineate specific circuits mediating these effects, which may have relevance for eating disorders.

Chemogenetic Signaling in Space and Time: Considerations for Designing Neuroscience Experiments Using DREADDs

The use of designer receptors exclusively activated by designer drugs (DREADDs) has led to significant advances in our understanding of the neural circuits that govern behavior. By allowing selective control over cellular activity and signaling, DREADDs have become an integral tool for defining the pathways and cellular phenotypes that regulate sleep, pain, motor activity, goal-directed behaviors, and a variety of other processes. In this review, we provide a brief overview of DREADDs and discuss notable discoveries in the neurosciences with an emphasis on circuit mechanisms. We then highlight methodological approaches to achieve pathway specific activation of DREADDs. Finally, we discuss spatial and temporal constraints of DREADDs signaling and how these features can be incorporated into experimental designs to precisely dissect circuits of interest.

Updated Toolbox for Assessing Neuronal Network Reconstruction after Cell Therapy

Cell therapy has proven to be a promising treatment for a range of neurological disorders, including Parkinson Disease, drug-resistant epilepsy, and stroke, by restoring function after brain damage. Nevertheless, evaluating the true effectiveness of these therapeutic interventions requires a deep understanding of the functional integration of grafted cells into existing neural networks. This review explores a powerful arsenal of molecular techniques revolutionizing our ability to unveil functional integration of grafted cells within the host brain. From precise manipulation of neuronal activity to pinpoint the functional contribution of transplanted cells by using opto- and chemo-genetics, to real-time monitoring of neuronal dynamics shedding light on functional connectivity within the reconstructed circuits by using genetically encoded (calcium) indicators in vivo. Finally, structural reconstruction and mapping communication pathways between grafted and host neurons can be achieved by monosynaptic tracing with viral vectors. The cutting-edge toolbox presented here holds immense promise for elucidating the impact of cell therapy on neural circuitry and guiding the development of more effective treatments for neurological disorders.

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.

Unlocking the potential of adeno-associated virus in neuroscience: a brief review

Adeno-associated virus (AAV) has emerged as a pivotal tool in neuroscience research, owing to its remarkable versatility and efficiency in delivering genetic material to diverse cell types within the nervous system. This mini review aims to underscore the advanced applications of AAV vectors in neuroscience and their profound potential to revolutionize our understanding of brain function and therapeutic interventions for neurological disorders. By providing a concise overview of the latest developments and strategies employing AAV vectors, this review illuminates the transformative role of AAV technology in unraveling the complexities of neural circuits and paving the way for innovative treatments. Through elucidating the multifaceted capabilities of AAV-mediated gene delivery, this review underscores its pivotal role as a cornerstone in contemporary neuroscience research, promising remarkable insights into the intricacies of brain biology and offering new avenues for therapeutic intervention.

Protein-Based Tools for Studying Neuromodulation

Neuromodulators play crucial roles in regulating neuronal activity and affecting various aspects of brain functions, including learning, memory, cognitive functions, emotional states, and pain modulation. In this Account, we describe our group's efforts in designing sensors and tools for studying neuromodulation. Our lab focuses on developing new classes of integrators that can detect neuromodulators across the whole brain while leaving a mark for further imaging analysis at high spatial resolution. Our lab also designed chemical- and light-dependent protein switches for controlling peptide activity to potentially modulate the endogenous receptors of the neuromodulatory system in order to study the causal effects of selective neuronal pathways.

Synthetic G protein-coupled receptors for programmable sensing and control of cell behavior

Synthetic receptors that mediate antigen-dependent cell responses are transforming therapeutics, drug discovery, and basic research. However, established technologies such as chimeric antigen receptors (CARs) can only detect immobilized antigens, have limited output scope, and lack built-in drug control. Here, we engineer synthetic G protein-coupled receptors (GPCRs) capable of driving a wide range of native or nonnative cellular processes in response to user-defined antigen. We achieve modular antigen gating by engineering and fusing a conditional auto-inhibitory domain onto GPCR scaffolds. Antigen binding to a fused nanobody relieves auto-inhibition and enables receptor activation by drug, thus generating Programmable Antigen-gated G protein-coupled Engineered Receptors (PAGERs). We create PAGERs responsive to more than a dozen biologically and therapeutically important soluble and cell surface antigens, in a single step, from corresponding nanobody binders. Different PAGER scaffolds permit antigen binding to drive transgene expression, real-time fluorescence, or endogenous G protein activation, enabling control of cytosolic Ca 2+ , lipid signaling, cAMP, and neuronal activity. Due to its modular design and generalizability, we expect PAGER to have broad utility in discovery and translational science.

Dentate gyrus is needed for memory retrieval

The hippocampus is crucial for acquiring and retrieving episodic and contextual memories. In previous studies, the inactivation of dentate gyrus (DG) neurons by chemogenetic- and optogenetic-mediated hyperpolarization led to opposing conclusions about DG's role in memory retrieval. One study used Designer Receptors Exclusively Activated by Designer Drugs (DREADD)-mediated clozapine N-oxide (CNO)-induced hyperpolarization and reported that the previously formed memory was erased, thus concluding that denate gyrus is needed for memory maintenance. The other study used optogenetic with halorhodopsin induced hyperpolarization and reported and dentate gyrus is needed for memory retrieval. We hypothesized that this apparent discrepancy could be due to the length of hyperpolarization in previous studies; minutes by optogenetics and several hours by DREADD/CNO. Since hyperpolarization interferes with anterograde and retrograde neuronal signaling, it is possible that the memory engram in the dentate gyrus and the entorhinal to hippocampus trisynaptic circuit was erased by long-term, but not with short-term hyperpolarization. We developed and applied an advanced chemogenetic technology to selectively silence synaptic output by blocking neurotransmitter release without hyperpolarizing DG neurons to explore this apparent discrepancy. We performed in vivo electrophysiology during trace eyeblink in a rabbit model of associative learning. Our work shows that the DG output is required for memory retrieval. Based on previous and recent findings, we propose that the actively functional anterograde and retrograde neuronal signaling is necessary to preserve synaptic memory engrams along the entorhinal cortex to the hippocampal trisynaptic circuit.

Recent advances in neural mechanism of general anesthesia induced unconsciousness: insights from optogenetics and chemogenetics

For over 170 years, general anesthesia has played a crucial role in clinical practice, yet a comprehensive understanding of the neural mechanisms underlying the induction of unconsciousness by general anesthetics remains elusive. Ongoing research into these mechanisms primarily centers around the brain nuclei and neural circuits associated with sleep-wake. In this context, two sophisticated methodologies, optogenetics and chemogenetics, have emerged as vital tools for recording and modulating the activity of specific neuronal populations or circuits within distinct brain regions. Recent advancements have successfully employed these techniques to investigate the impact of general anesthesia on various brain nuclei and neural pathways. This paper provides an in-depth examination of the use of optogenetic and chemogenetic methodologies in studying the effects of general anesthesia on specific brain nuclei and pathways. Additionally, it discusses in depth the advantages and limitations of these two methodologies, as well as the issues that must be considered for scientific research applications. By shedding light on these facets, this paper serves as a valuable reference for furthering the accurate exploration of the neural mechanisms underlying general anesthesia. It aids researchers and clinicians in effectively evaluating the applicability of these techniques in advancing scientific research and clinical practice.

Unraveling the Neural Circuits: Techniques, Opportunities and Challenges in Epilepsy Research

Epilepsy, a prevalent neurological disorder characterized by high morbidity, frequent recurrence, and potential drug resistance, profoundly affects millions of people globally. Understanding the microscopic mechanisms underlying seizures is crucial for effective epilepsy treatment, and a thorough understanding of the intricate neural circuits underlying epilepsy is vital for the development of targeted therapies and the enhancement of clinical outcomes. This review begins with an exploration of the historical evolution of techniques used in studying neural circuits related to epilepsy. It then provides an extensive overview of diverse techniques employed in this domain, discussing their fundamental principles, strengths, limitations, as well as their application. Additionally, the synthesis of multiple techniques to unveil the complexity of neural circuits is summarized. Finally, this review also presents targeted drug therapies associated with epileptic neural circuits. By providing a critical assessment of methodologies used in the study of epileptic neural circuits, this review seeks to enhance the understanding of these techniques, stimulate innovative approaches for unraveling epilepsy's complexities, and ultimately facilitate improved treatment and clinical translation for epilepsy.

Devaluation of response-produced safety signals reveals circuits for goal-directed versus habitual avoidance in dorsal striatum

Active avoidance responses (ARs) are instrumental behaviors that prevent harm. Adaptive ARs may contribute to active coping, whereas maladaptive avoidance habits are implicated in anxiety and obsessive-compulsive disorders. The AR learning mechanism has remained elusive, as successful avoidance trials produce no obvious reinforcer. We used a novel outcome-devaluation procedure in rats to show that ARs are positively reinforced by response-produced feedback (FB) cues that develop into safety signals during training. Males were sensitive to FB-devaluation after moderate training, but not overtraining, consistent with a transition from goal-directed to habitual avoidance. Using chemogenetics and FB-devaluation, we also show that goal-directed vs. habitual ARs depend on dorsomedial vs. dorsolateral striatum, suggesting a significant overlap between the mechanisms of avoidance and rewarded instrumental behavior. Females were insensitive to FB-devaluation due to a remarkable context-dependence of counterconditioning. However, degrading the AR-FB contingency suggests that both sexes rely on safety signals to perform goal-directed ARs.

Impacts of sex differences on optogenetic, chemogenetic, and calcium-imaging tools

Technical innovation in neuroscience introduced powerful tools for measuring and manipulating neuronal activity via optical, chemogenetic, and calcium-imaging tools. These tools were initially tested primarily in male animals but are now increasingly being used in females as well. In this review, we consider how these tools may work differently in males and females. For example, we review sex differences in the metabolism of chemogenetic ligands and their downstream signaling effects. Optical tools more directly alter depolarization or hyperpolarization of neurons, but biological sex and gonadal hormones modulate synaptic inputs and intrinsic excitability. We review studies demonstrating that optogenetic manipulations are sometimes consistent across the rodent estrous cycle but within certain circuits; manipulations can vary across the ovarian cycle. Finally, calcium-imaging methods utilize genetically encoded calcium indicators to measure neuronal activity. Testosterone and estradiol can directly modulate calcium influx, and we consider these implications for interpreting the results of calcium-imaging studies. Together, our findings suggest that these neuroscientific tools may sometimes work differently in males and females and that users should be aware of these differences when applying these methods.

Experience shapes chandelier cell function and structure in the visual cortex

Detailed characterization of interneuron types in primary visual cortex (V1) has greatly contributed to understanding visual perception, yet the role of chandelier cells (ChCs) in visual processing remains poorly characterized. Using viral tracing we found that V1 ChCs predominantly receive monosynaptic input from local layer 5 pyramidal cells and higher-order cortical regions. Two-photon calcium imaging and convolutional neural network modeling revealed that ChCs are visually responsive but weakly selective for stimulus content. In mice running in a virtual tunnel, ChCs respond strongly to events known to elicit arousal, including locomotion and visuomotor mismatch. Repeated exposure of the mice to the virtual tunnel was accompanied by reduced visual responses of ChCs and structural plasticity of ChC boutons and axon initial segment length. Finally, ChCs only weakly inhibited pyramidal cells. These findings suggest that ChCs provide an arousal-related signal to layer 2/3 pyramidal cells that may modulate their activity and/or gate plasticity of their axon initial segments during behaviorally relevant events.

Molecular basis of opioid receptor signaling

Opioids are used for pain management despite the side effects that contribute to the opioid crisis. The pursuit of non-addictive opioid analgesics remains unattained due to the unresolved intricacies of opioid actions, receptor signaling cascades, and neuronal plasticity. Advancements in structural, molecular, and computational tools illuminate the dynamic interplay between opioids and opioid receptors, as well as the molecular determinants of signaling pathways, which are potentially interlinked with pharmacological responses. Here, we review the molecular basis of opioid receptor signaling with a focus on the structures of opioid receptors bound to endogenous peptides or pharmacological agents. These insights unveil specific interactions that dictate ligand selectivity and likely their distinctive pharmacological profiles. Biochemical analysis further unveils molecular features governing opioid receptor signaling. Simultaneously, the synergy between computational biology and medicinal chemistry continues to expedite the discovery of novel chemotypes with the promise of yielding more efficacious and safer opioid compounds.

Preclinical long-term safety of intraspinal transplantation of human dorsal spinal GABA neural progenitor cells

Human pluripotent stem cell (hPSC)-derived neurons have shown promise in treating spinal cord injury (SCI). We previously showed that hPSC-derived dorsal spinal γ-aminobutyric acid (GABA) neurons can alleviate spasticity and promote locomotion in rats with SCI, but their long-term safety remains elusive. Here, we characterized the long-term fate and safety of human dorsal spinal GABA neural progenitor cells (NPCs) in naive rats over one year. All grafted NPCs had undergone differentiation, yielding mainly neurons and astrocytes. Fully mature human neurons grew many axons and formed numerous synapses with rat neural circuits, together with mature human astrocytes that structurally integrated into the rat spinal cord. The sensorimotor function of rats was not impaired by intraspinal transplantation, even when human neurons were activated or inhibited by designer receptors exclusively activated by designer drugs (DREADDs). These findings represent a significant step toward the clinical translation of human spinal neuron transplantation for treating SCI.

Current Status and Future Strategies for Advancing Functional Circuit Mapping In Vivo

The human brain represents one of the most complex biological systems, containing billions of neurons interconnected through trillions of synapses. Inherent to the brain is a biochemical complexity involving ions, signaling molecules, and peptides that regulate neuronal activity and allow for short- and long-term adaptations. Large-scale and noninvasive imaging techniques, such as fMRI and EEG, have highlighted brain regions involved in specific functions and visualized connections between different brain areas. A major shortcoming, however, is the need for more information on specific cell types and neurotransmitters involved, as well as poor spatial and temporal resolution. Recent technologies have been advanced for neuronal circuit mapping and implemented in behaving model organisms to address this. Here, we highlight strategies for targeting specific neuronal subtypes, identifying, and releasing signaling molecules, controlling gene expression, and monitoring neuronal circuits in real-time in vivo Combined, these approaches allow us to establish direct causal links from genes and molecules to the systems level and ultimately to cognitive processes.

Advanced neurobiological tools to interrogate metabolism

Engineered neurobiological tools for the manipulation of cellular activity, such as chemogenetics and optogenetics, have become a cornerstone of modern neuroscience research. These tools are invaluable for the interrogation of the central control of metabolism as they provide a direct means to establish a causal relationship between brain activity and biological processes at the cellular, tissue and organismal levels. The utility of these methods has grown substantially due to advances in cellular-targeting strategies, alongside improvements in the resolution and potency of such tools. Furthermore, the potential to recapitulate endogenous cellular signalling has been enriched by insights into the molecular signatures and activity dynamics of discrete brain cell types. However, each modulatory tool has a specific set of advantages and limitations; therefore, tool selection and suitability are of paramount importance to optimally interrogate the cellular and circuit-based underpinnings of metabolic outcomes within the organism. Here, we describe the key principles and uses of engineered neurobiological tools. We also highlight inspiring applications and outline critical considerations to be made when using these tools within the field of metabolism research. We contend that the appropriate application of these biotechnological advances will enable the delineation of the central circuitry regulating systemic metabolism with unprecedented potential.

Ventral striatal islands of Calleja neurons bidirectionally mediate depression-like behaviors in mice

The ventral striatum is a reward center implicated in the pathophysiology of depression. It contains islands of Calleja, clusters of dopamine D3 receptor-expressing granule cells, predominantly in the olfactory tubercle (OT). These OT D3 neurons regulate self-grooming, a repetitive behavior manifested in affective disorders. Here we show that chronic restraint stress (CRS) induces robust depression-like behaviors in mice and decreases excitability of OT D3 neurons. Ablation or inhibition of these neurons leads to depression-like behaviors, whereas their activation ameliorates CRS-induced depression-like behaviors. Moreover, activation of OT D3 neurons has a rewarding effect, which diminishes when grooming is blocked. Finally, we propose a model that explains how OT D3 neurons may influence dopamine release via synaptic connections with OT spiny projection neurons (SPNs) that project to midbrain dopamine neurons. Our study reveals a crucial role of OT D3 neurons in bidirectionally mediating depression-like behaviors, suggesting a potential therapeutic target.

Pathway specific interventions reveal the multiple roles of ventral hippocampus projections in cognitive functions

Since the 1950s study of Scoville and Milner on the case H.M., the hippocampus has attracted neuroscientists' attention. The hippocampus has been traditionally divided into dorsal and ventral parts, each of which projects to different brain structures and mediates various functions. Despite a predominant interest in its dorsal part in animal models, especially regarding episodic-like and spatial cognition, recent data highlight the role of the ventral hippocampus (vHPC), as the main hippocampal output, in cognitive processes. Here, we review recent studies conducted in rodents that have used advanced in vivo functional techniques to specifically monitor and manipulate vHPC efferent pathways and delineate the roles of these specific projections in learning and memory processes. Results highlight that vHPC projections to basal amygdala are implicated in emotional memory, to nucleus accumbens in social memory and instrumental actions and to prefrontal cortex in all the above as well as in object-based memory. Some of these hippocampal projections also modulate feeding and anxiety-like behaviours providing further evidence that the "one pathway-one function" view is outdated and future directions are proposed to better understand the role of hippocampal pathways and shed further light on its connectivity and function.

Research trends in chemogenetics for neuroscience in recent 14 years: A bibliometric study in CiteSpace

BACKGROUND

Chemogenetics has been widely adopted in Neuroscience. Neuroscience has become a hot research topic for scientists. Therefore, the purpose of this study is to explore the current status and trends in the global application of chemogenetics in neuroscience over the last 14 years via CiteSpace.

METHODS

Publications related to chemogenetics in neuroscience were retrieved from the Science Citation Index-Extended Web of Science from 2008 to 2021. We used CiteSpace to analyze publications, citations, cited journals, countries, institutions, authors, cited authors, cited references, and keywords.

RESULTS

A total of 947 records were retrieved from 2008 to 2021 on February 21, 2022. The number and rate of publications and citations increased significantly. Journal of Neuroscience was the most cited journal, and BRAIN RES BULL ranked first in the centrality of cited journals. The United States of America (USA) had the highest number of publications among the countries. Takashi Minamoto was the most prolific author and Armbruster BN ranked the first among authors cited. The first article in the frequency ranking of the references cited was published by Roth BL. The keyword of "nucleus accumben (NAc)" had the highest frequency. The top 3 keywords with the strongest citation bursts include "transgenic mice," "cancer," and "blood-brain barrier."

CONCLUSION

The period 2008 to 2021 has seen a marked increase in research on chemogenetics in neuroscience. The application of chemogenetics is indispensable for research in the field of neuroscience. This bibliometrics study provides the current situation and trend in chemogenetic methods in neuroscience in recent 14 years, which may help researchers to identify the hot topics and frontiers for future studies in this field.

Advancing the preclinical study of comorbid neuroHIV and substance use disorders: Current perspectives and future directions

Human immunodeficiency virus (HIV) remains a persistent public health concern throughout the world. Substance use disorders (SUDs) are a common comorbidity that can worsen treatment outcomes for people living with HIV. The relationship between HIV infection and SUD outcomes is likely bidirectional, making clear interrogation of neurobehavioral outcomes challenging in clinical populations. Importantly, the mechanisms through which HIV and addictive drugs disrupt homeostatic immune and CNS function appear to be highly overlapping and synergistic within HIV-susceptible reward and motivation circuitry in the central nervous system. Decades of animal research have revealed invaluable insights into mechanisms underlying the pathophysiology SUDs and HIV, although translational studies examining comorbid SUDs and HIV are very limited due to the technical challenges of modeling HIV infection preclinically. In this review, we discuss preclinical animal models of HIV and highlight key pathophysiological characteristics of each model, with a particular emphasis on rodent models of HIV. We then review the implementation of these models in preclinical SUD research and identify key gaps in knowledge in the field. Finally, we discuss how cutting-edge behavioral neuroscience tools, which have revealed key insights into the neurobehavioral mechanisms of SUDs, can be applied to preclinical animal models of HIV to reveal potential, novel treatment avenues for comorbid HIV and SUDs. Here, we argue that future preclinical SUD research would benefit from incorporating comorbidities such as HIV into animal models and would facilitate the discovery of more refined, subpopulation-specific mechanisms and effective SUD prevention and treatment targets.

Ketamine rescues anhedonia by cell-type and input specific adaptations in the Nucleus Accumbens

Ketamine's role in providing a rapid and sustained antidepressant response, particularly for patients unresponsive to conventional treatments, is increasingly recognized. A core symptom of depression, anhedonia, or the loss of enjoyment or interest in previously pleasurable activities, is known to be significantly alleviated by ketamine. While several hypotheses have been proposed regarding the mechanisms by which ketamine alleviates anhedonia, the specific circuits and synaptic changes responsible for its sustained therapeutic effects are not yet understood. Here, we show that the nucleus accumbens (NAc), a major hub of the reward circuitry, is essential for ketamine's effect in rescuing anhedonia in mice subjected to chronic stress, a critical risk factor in the genesis of depression in humans. Specifically, a single exposure to ketamine rescues stress-induced decreased strength of excitatory synapses on NAc D1 dopamine receptor-expressing medium spiny neurons (D1-MSNs). By using a novel cell-specific pharmacology method, we demonstrate that this cell-type specific neuroadaptation is necessary for the sustained therapeutic effects of ketamine. To test for causal sufficiency, we artificially mimicked ketamine-induced increase in excitatory strength on D1-MSNs and found that this recapitulates the behavioral amelioration induced by ketamine. Finally, to determine the presynaptic origin of the relevant glutamatergic inputs for ketamine-elicited synaptic and behavioral effects, we used a combination of opto- and chemogenetics. We found that ketamine rescues stress-induced reduction in excitatory strength at medial prefrontal cortex and ventral hippocampus inputs to NAc D1-MSNs. Chemogenetically preventing ketamine-evoked plasticity at those unique inputs to the NAc reveals a ketamine-operated input-specific control of hedonic behavior. These results establish that ketamine rescues stress-induced anhedonia via cell-type-specific adaptations as well as information integration in the NAc via discrete excitatory synapses.

A synaptic amplifier of hunger for regaining body weight in the hypothalamus

Restricting caloric intake effectively reduces body weight, but most dieters fail long-term adherence to caloric deficit and eventually regain lost weight. Hypothalamic circuits that control hunger drive critically determine body weight; yet, how weight loss sculpts these circuits to motivate food consumption until lost weight is regained remains unclear. Here, we probe the contribution of synaptic plasticity in discrete excitatory afferents on hunger-promoting AgRP neurons. We reveal a crucial role for activity-dependent, remarkably long-lasting amplification of synaptic activity originating from paraventricular hypothalamus thyrotropin-releasing (PVHTRH) neurons in long-term body weight control. Silencing PVHTRH neurons inhibits the potentiation of excitatory input to AgRP neurons and diminishes concomitant regain of lost weight. Brief stimulation of the pathway is sufficient to enduringly potentiate this glutamatergic hunger synapse and triggers an NMDAR-dependent gaining of body weight that enduringly persists. Identification of this activity-dependent synaptic amplifier provides a previously unrecognized target to combat regain of lost weight.

Ligand and G-protein selectivity in the κ-opioid receptor

The κ-opioid receptor (KOR) represents a highly desirable therapeutic target for treating not only pain but also addiction and affective disorders1. However, the development of KOR analgesics has been hindered by the associated hallucinogenic side effects2. The initiation of KOR signalling requires the Gi/o-family proteins including the conventional (Gi1, Gi2, Gi3, GoA and GoB) and nonconventional (Gz and Gg) subtypes. How hallucinogens exert their actions through KOR and how KOR determines G-protein subtype selectivity are not well understood. Here we determined the active-state structures of KOR in a complex with multiple G-protein heterotrimers-Gi1, GoA, Gz and Gg-using cryo-electron microscopy. The KOR-G-protein complexes are bound to hallucinogenic salvinorins or highly selective KOR agonists. Comparisons of these structures reveal molecular determinants critical for KOR-G-protein interactions as well as key elements governing Gi/o-family subtype selectivity and KOR ligand selectivity. Furthermore, the four G-protein subtypes display an intrinsically different binding affinity and allosteric activity on agonist binding at KOR. These results provide insights into the actions of opioids and G-protein-coupling specificity at KOR and establish a foundation to examine the therapeutic potential of pathway-selective agonists of KOR.

Bioelectricity in Developmental Patterning and Size Control: Evidence and Genetically Encoded Tools in the Zebrafish Model

Developmental patterning is essential for regulating cellular events such as axial patterning, segmentation, tissue formation, and organ size determination during embryogenesis. Understanding the patterning mechanisms remains a central challenge and fundamental interest in developmental biology. Ion-channel-regulated bioelectric signals have emerged as a player of the patterning mechanism, which may interact with morphogens. Evidence from multiple model organisms reveals the roles of bioelectricity in embryonic development, regeneration, and cancers. The Zebrafish model is the second most used vertebrate model, next to the mouse model. The zebrafish model has great potential for elucidating the functions of bioelectricity due to many advantages such as external development, transparent early embryogenesis, and tractable genetics. Here, we review genetic evidence from zebrafish mutants with fin-size and pigment changes related to ion channels and bioelectricity. In addition, we review the cell membrane voltage reporting and chemogenetic tools that have already been used or have great potential to be implemented in zebrafish models. Finally, new perspectives and opportunities for bioelectricity research with zebrafish are discussed.

Connectivity of the Brain in the Light of Chemogenetic Modulation of Neuronal Activity

Connectivity is the coordinated activity of the neuronal networks responsible for brain functions; it is detected based on functional magnetic resonance imaging signals that depend on the oxygen level in the blood (blood oxygen level-dependent (BOLD) signals) supplying the brain. The BOLD signal is only indirectly related to the underlying neuronal activity; therefore, it remains an open question whether connectivity and changes in it are only manifestations of normal and pathological states of the brain or they are, to some extent, the causes of these states. The creation of chemogenetic receptors activated by synthetic drugs (designer receptors exclusively activated by designer drugs, DREADDs), which, depending on the receptor type, either facilitate or, on the contrary, inhibit the neuronal response to received physiological stimuli, makes it possible to assess brain connectivity in the light of controlled neuronal activity. Evidence suggests that connectivity is based on neuronal activity and is a manifestation of connections between brain regions that integrate sensory, cognitive, and motor functions. Chemogenetic modulation of the activity of various groups and types of neurons changes the connectivity of the brain and its complex functions. Chemogenetics can be useful in reconfiguring the pathological mechanisms of nervous and mental diseases. The initiated integration, based on the whole-brain connectome from molecular-cellular, neuronal, and synaptic processes to higher nervous activity and behavior, has the potential to significantly increase the fundamental and applied value of this branch of neuroscience.

Plastic and stimulus-specific coding of salient events in the central amygdala

The central amygdala (CeA) is implicated in a range of mental processes including attention, motivation, memory formation and extinction and in behaviours driven by either aversive or appetitive stimuli1-7. How it participates in these divergent functions remains elusive. Here we show that somatostatin-expressing (Sst+) CeA neurons, which mediate much of CeA functions3,6,8-10, generate experience-dependent and stimulus-specific evaluative signals essential for learning. The population responses of these neurons in mice encode the identities of a wide range of salient stimuli, with the responses of separate subpopulations selectively representing the stimuli that have contrasting valences, sensory modalities or physical properties (for example, shock and water reward). These signals scale with stimulus intensity, undergo pronounced amplification and transformation during learning, and are required for both reward and aversive learning. Notably, these signals contribute to the responses of dopamine neurons to reward and reward prediction error, but not to their responses to aversive stimuli. In line with this, Sst+ CeA neuron outputs to dopamine areas are required for reward learning, but are dispensable for aversive learning. Our results suggest that Sst+ CeA neurons selectively process information about differing salient events for evaluation during learning, supporting the diverse roles of the CeA. In particular, the information for dopamine neurons facilitates reward evaluation.

Faded neural projection from the posterior bed nucleus of the stria terminalis to the lateral habenula contributes to social signaling deficit in male BTBR mice as a mouse model of autism

BTBR T⁺ Itpr3^tf/J (BTBR) mice display several behavioral characteristics, including social deficits resembling the core symptoms of human autism. Atypical social behaviors include sequential processes of assembled cognitive-behavior components, such as recognition, investigatory assessment, and signaling response. This study aimed to elucidate the neural circuits responsible for the regulation of the social signaling response, as shown by scent marking behavior in male mice. We first assessed the recognition and investigatory patterns of male BTBR mice compared to those of C57BL/6 J (B6) mice. Next, we examined their scent-marking behavior as innate social signaling responses adjusted to a confronted feature of social stimuli and situations, along with the expression of c-Fos as a marker of neuronal activity in selected brain areas involved in the regulation of social behavior. The function of the targeted brain area was confirmed by chemogenetic manipulation. We also examined the social peptides, oxytocin and vasopressin neurons of the major brain regions that are associated with the regulation of social behavior. Our data indicate that male BTBR mice are less responsive to the presentation of social stimuli and the expression of social signaling responses, which is paralleled by blunted c-Fos responsivity and vasopressin neurons morphological changes in selected brain areas, including the posterior bed nucleus of the stria terminalis (pBnST) and lateral habenula (LHb) in BTBR mice. Further investigation of LHb function revealed that chemogenetic inhibition and activation of LHb activity can induce a change in scent marking responses in both B6 and BTBR mice. Our elucidation of the downstream LHb circuits controlling scent marking behavior indicates intact function in BTBR mice. The altered morphological characteristics of oxytocin neurons in the paraventricular nucleus of the hypothalamus and vasopressin-positive neurons and axonal projections in the pBnST and LHb appear to underlie the dysfunction of scent marking responses in BTBR mice. (300/300 words).

A general method for chemogenetic control of peptide function

Natural or engineered peptides serve important biological functions. A general approach to achieve chemical-dependent activation of short peptides will be valuable for spatial and temporal control of cellular processes. Here we present a pair of chemically activated protein domains (CAPs) for controlling the accessibility of both the N- and C-terminal portion of a peptide. CAPs were developed through directed evolution of an FK506-binding protein. By fusing a peptide to one or both CAPs, the function of the peptide is blocked until a small molecule displaces them from the FK506-binding protein ligand-binding site. We demonstrate that CAPs are generally applicable to a range of short peptides, including a protease cleavage site, a dimerization-inducing heptapeptide, a nuclear localization signal peptide, and an opioid peptide, with a chemical dependence up to 156-fold. We show that the CAPs system can be utilized in cell cultures and multiple organs in living animals.

Calcium signaling in astrocytes and gliotransmitter release

Glia are as numerous in the brain as neurons and widely known to serve supportive roles such as structural scaffolding, extracellular ionic and neurotransmitter homeostasis, and metabolic support. However, over the past two decades, several lines of evidence indicate that astrocytes, which are a type of glia, play active roles in neural information processing. Astrocytes, although not electrically active, can exhibit a form of excitability by dynamic changes in intracellular calcium levels. They sense synaptic activity and release neuroactive substances, named gliotransmitters, that modulate neuronal activity and synaptic transmission in several brain areas, thus impacting animal behavior. This "dialogue" between astrocytes and neurons is embodied in the concept of the tripartite synapse that includes astrocytes as integral elements of synaptic function. Here, we review the recent work and discuss how astrocytes via calcium-mediated excitability modulate synaptic information processing at various spatial and time scales.

Neurotensin neurons in the extended amygdala control dietary choice and energy homeostasis

Obesity is a global pandemic that is causally linked to many life-threatening diseases. Apart from some rare genetic conditions, the biological drivers of overeating and reduced activity are unclear. Here, we show that neurotensin-expressing neurons in the mouse interstitial nucleus of the posterior limb of the anterior commissure (IPAC), a nucleus of the central extended amygdala, encode dietary preference for unhealthy energy-dense foods. Optogenetic activation of IPAC^Nts neurons promotes obesogenic behaviors, such as hedonic eating, and modulates food preference. Conversely, acute inhibition of IPAC^Nts neurons reduces feeding and decreases hedonic eating. Chronic inactivation of IPAC^Nts neurons recapitulates these effects, reduces preference for sweet, non-caloric tastants and, furthermore, enhances locomotion and energy expenditure; as a result, mice display long-term weight loss and improved metabolic health and are protected from obesity. Thus, the activity of a single neuronal population bidirectionally regulates energy homeostasis. Our findings could lead to new therapeutic strategies to prevent and treat obesity.

Pathway-specific GABAergic inhibition contributes to the gain of resilience against anorexia-like behavior of adolescent female mice

Anorexia nervosa is one of the most debilitating mental illnesses that emerges during adolescence, especially among females. Anorexia nervosa is characterized by severe voluntary food restriction and compulsive exercising, which combine to cause extreme body weight loss. We use activity-based anorexia (ABA), an animal model, to investigate the neurobiological bases of vulnerability to anorexia nervosa. This is a Mini-Review, focused on new ideas that have emerged based on recent findings from the Aoki Lab. Our findings point to the cellular and molecular underpinnings of three ABA phenomena: (1) age-dependence of ABA vulnerability; (2) individual differences in the persistence of ABA vulnerability during adolescence; (3) GABAergic synaptic plasticity in the hippocampus and the prefrontal cortex that contributes to the suppression of the maladaptive anorexia-like behaviors. We also include new data on the contribution to ABA vulnerability by cell type-specific knockdown of a GABA receptor subunit, α4, in dorsal hippocampus. Although the GABA system recurs as a key player in the gain of ABA resilience, the data predict why targeting the GABA system, singularly, may have only limited efficacy in treating anorexia nervosa. This is because boosting the GABAergic system may suppress the maladaptive behavior of over-exercising but could also suppress food consumption. We hypothesize that a sub-anesthetic dose of ketamine may be the magic bullet, since a single injection of this drug to mid-adolescent female mice undergoing ABA induction enhances food consumption and reduces wheel running, thereby reducing body weight loss through plasticity at excitatory synaptic inputs to both excitatory and inhibitory neurons. The same treatment is not as efficacious during late adolescence but multiple dosing of ketamine can suppress ABA vulnerability partially. This caveat underscores the importance of conducting behavioral, synaptic and molecular analyses across multiple time points spanning the developmental stage of adolescence and into adulthood. Since this is a Mini-Review, we recommend additional literature for readers seeking more comprehensive reviews on these subjects.

Hypothalamic astrocytes control systemic glucose metabolism and energy balance

The hypothalamus is key in the control of energy balance. However, strategies targeting hypothalamic neurons have failed to provide viable options to treat most metabolic diseases. Conversely, the role of astrocytes in systemic metabolic control has remained largely unexplored. Here, we show that obesity promotes anatomically restricted remodeling of hypothalamic astrocyte activity. In the paraventricular nucleus (PVN) of the hypothalamus, chemogenetic manipulation of astrocytes results in bidirectional control of neighboring neuron activity, autonomic outflow, glucose metabolism, and energy balance. This process recruits a mechanism involving the astrocytic control of ambient glutamate levels, which becomes defective in obesity. Positive or negative chemogenetic manipulation of PVN astrocyte Ca2+ signals, respectively, worsens or improves metabolic status of diet-induced obese mice. Collectively, these findings highlight a yet unappreciated role for astrocytes in the direct control of systemic metabolism and suggest potential targets for anti-obesity strategy.

AgRP neurons control structure and function of the medial prefrontal cortex

Hypothalamic agouti-related peptide and neuropeptide Y-expressing (AgRP) neurons have a critical role in both feeding and non-feeding behaviors of newborn, adolescent, and adult mice, suggesting their broad modulatory impact on brain functions. Here we show that constitutive impairment of AgRP neurons or their peripubertal chemogenetic inhibition resulted in both a numerical and functional reduction of neurons in the medial prefrontal cortex (mPFC) of mice. These changes were accompanied by alteration of oscillatory network activity in mPFC, impaired sensorimotor gating, and altered ambulatory behavior that could be reversed by the administration of clozapine, a non-selective dopamine receptor antagonist. The observed AgRP effects are transduced to mPFC in part via dopaminergic neurons in the ventral tegmental area and may also be conveyed by medial thalamic neurons. Our results unmasked a previously unsuspected role for hypothalamic AgRP neurons in control of neuronal pathways that regulate higher-order brain functions during development and in adulthood.

Neuronal circuitry for recognition memory of object and place in rodent models

Rats and mice are used for studying neuronal circuits underlying recognition memory due to their ability to spontaneously remember the occurrence of an object, its place and an association of the object and place in a particular environment. A joint employment of lesions, pharmacological interventions, optogenetics and chemogenetics is constantly expanding our knowledge of the neural basis for recognition memory of object, place, and their association. In this review, we summarize current studies on recognition memory in rodents with a focus on the novel object preference, novel location preference and object-in-place paradigms. The evidence suggests that the medial prefrontal cortex- and hippocampus-connected circuits contribute to recognition memory for object and place. Under certain conditions, the striatum, medial septum, amygdala, locus coeruleus and cerebellum are also involved. We propose that the neuronal circuitry for recognition memory of object and place is hierarchically connected and constructed by different cortical (perirhinal, entorhinal and retrosplenial cortices), thalamic (nucleus reuniens, mediodorsal and anterior thalamic nuclei) and primeval (hypothalamus and interpeduncular nucleus) modules interacting with the medial prefrontal cortex and hippocampus.

Seizing Control of Neuronal Activity: Chemogenetic Applications in Epilepsy

The fundamental commonality across pharmacotherapies for the epilepsies is the modulation of neuronal excitability. This poses a clear challenge—patterned neuronal excitation is essential to normal function, thus disrupting this activity leads to side effects. Moreover, the efficacy of current pharmacotherapy remains incomplete despite decades of drug development. Approaches that allow for the selective targeting of critical populations of cells and particular pathways in the brain have the potential to both avoid side effects and improve efficacy. Chemogenetic methods, which combine the selective expression of designer receptors with designer drugs, have rapidly grown in use in the neurosciences, including in epilepsy. This review will briefly highlight the history of chemogenetics, their applications to date in epilepsy, and the potential (and potential hurdles to overcome) for future translation.

The limitations of investigating appetite through circuit manipulations: are we biting off more than we can chew?

Disordered eating can underpin a number of debilitating and prevalent chronic diseases, such as obesity. Broader advances in psychopharmacology and biology have motivated some neuroscientists to address diet-induced obesity through reductionist, pre-clinical eating investigations on the rodent brain. Specifically, chemogenetic and optogenetic methods developed in the 21st century allow neuroscientists to perform in vivo, region-specific/projection-specific/promoter-specific circuit manipulations and immediately assess the impact of these manipulations on rodent feeding. These studies are able to rigorously conclude whether a specific neuronal population regulates feeding behaviour in the hope of eventually developing a mechanistic neuroanatomical map of appetite regulation. However, an artificially stimulated/inhibited rodent neuronal population that changes feeding behaviour does not necessarily represent a pharmacological target for treating eating disorders in humans. Chemogenetic/optogenetic findings must therefore be triangulated with the array of theories that contribute to our understanding of appetite. The objective of this review is to provide a wide-ranging discussion of the limitations of chemogenetic/optogenetic circuit manipulation experiments in rodents that are used to investigate appetite. Stepping into and outside of medical science epistemologies, this paper draws on philosophy of science, nutrition, addiction biology and neurophilosophy to prompt more integrative, transdisciplinary interpretations of chemogenetic/optogenetic appetite data. Through discussing the various technical and epistemological limitations of these data, we provide both an overview of chemogenetics and optogenetics accessible to non-neuroscientist obesity researchers, as well as a resource for neuroscientists to expand the number of lenses through which they interpret their circuit manipulation findings.

Neuropeptides and small-molecule amine transmitters: cooperative signaling in the nervous system

Neuropeptides are expressed in cell-specific patterns throughout mammalian brain. Neuropeptide gene expression has been useful for clustering neurons by phenotype, based on single-cell transcriptomics, and for defining specific functional circuits throughout the brain. How neuropeptides function as first messengers in inter-neuronal communication, in cooperation with classical small-molecule amine transmitters (SMATs) is a current topic of systems neurobiology. Questions include how neuropeptides and SMATs cooperate in neurotransmission at the molecular, cellular and circuit levels; whether neuropeptides and SMATs always co-exist in neurons; where neuropeptides and SMATs are stored in the neuron, released from the neuron and acting, and at which receptors, after release; and how neuropeptides affect 'classical' transmitter function, both directly upon co-release, and indirectly, via long-term regulation of gene transcription and neuronal plasticity. Here, we review an extensive body of data about the distribution of neuropeptides and their receptors, their actions after neuronal release, and their function based on pharmacological and genetic loss- and gain-of-function experiments, that addresses these questions, fundamental to understanding brain function, and development of neuropeptide-based, and potentially combinatorial peptide/SMAT-based, neurotherapeutics.

Prefrontal cortical circuits in anxiety and fear: an overview

Pathological anxiety is among the most difficult neuropsychiatric diseases to treat pharmacologically, and it represents a major societal problem. Studies have implicated structural changes within the prefrontal cortex (PFC) and functional changes in the communication of the PFC with distal brain structures in anxiety disorders. Treatments that affect the activity of the PFC, including cognitive therapies and transcranial magnetic stimulation, reverse anxiety- and fear-associated circuit abnormalities through mechanisms that remain largely unclear. While the subjective experience of a rodent cannot be precisely determined, rodent models hold great promise in dissecting well-conserved circuits. Newly developed genetic and viral tools and optogenetic and chemogenetic techniques have revealed the intricacies of neural circuits underlying anxiety and fear by allowing direct examination of hypotheses drawn from existing psychological concepts. This review focuses on studies that have used these circuit-based approaches to gain a more detailed, more comprehensive, and more integrated view on how the PFC governs anxiety and fear and orchestrates adaptive defensive behaviors to hopefully provide a roadmap for the future development of therapies for pathological anxiety.

Modulating the Excitability of Olfactory Output Neurons Affects Whole-Body Metabolism

Metabolic state can alter olfactory sensitivity, but it is unknown whether the activity of the olfactory bulb (OB) may fine tune metabolic homeostasis. Our objective was to use CRISPR gene editing in male and female mice to enhance the excitability of mitral/tufted projection neurons (M/TCs) of the OB to test for improved metabolic health. Ex vivo slice recordings of MCs in CRISPR mice confirmed increased excitability due the targeted loss of Kv1.3 channels, which resulted in a less negative resting membrane potential (RMP), enhanced action potential (AP) firing, and insensitivity to the selective channel blocker margatoxin (MgTx). CRISPR mice exhibited enhanced odor discrimination using a habituation/dishabituation paradigm. CRISPR mice were challenged for 25 weeks with a moderately high-fat (MHF) diet, and compared with littermate controls, male mice were resistance to diet-induced obesity (DIO). Female mice did not exhibit DIO. CRISPR male mice gained less body weight, accumulated less white adipose tissue, cleared a glucose challenge more quickly, and had less serum leptin and liver triglycerides. CRISPR male mice consumed equivalent calories as control littermates, and had unaltered energy expenditure (EE) and locomotor activity, but used more fats for metabolic substrate over that of carbohydrates. Counter to CRISPR-engineered mice, by using chemogenetics to decrease M/TC excitability in male mice, activation of inhibitory designer receptors exclusively activated by designer drugs (DREADDs) caused a decrease in odor discrimination, and resulted in a metabolic profile that was obesogenic, mice had reduced EE and oxygen consumption (VO2). We conclude that the activity of M/TC projection neurons canonically carries olfactory information and simultaneously can regulate whole-body metabolism.SIGNIFICANCE STATEMENT The olfactory system drives food choice, and olfactory sensitivity is strongly correlated to hunger and fullness. Olfactory function thereby influences nutritional balance and obesity outcomes. Obesity has become a health and financial crisis in America, shortening life expectancy and increasing the severity of associated illnesses. It is expected that 51% of Americans will be obese by the year 2030. Using CRISPR gene editing and chemogenetic approaches, we discovered that changing the excitability of output neurons in the olfactory bulb (OB) affects metabolism and body weight stabilization in mice. Our results suggest that long-term therapeutic targeting of OB activity to higher processing centers may be a future clinical treatment of obesity or type II Diabetes.

Optogenetics at the presynapse

Optogenetic actuators enable highly precise spatiotemporal interrogation of biological processes at levels ranging from the subcellular to cells, circuits and behaving organisms. Although their application in neuroscience has traditionally focused on the control of spiking activity at the somatodendritic level, the scope of optogenetic modulators for direct manipulation of presynaptic functions is growing. Presynaptically localized opsins combined with light stimulation at the terminals allow light-mediated neurotransmitter release, presynaptic inhibition, induction of synaptic plasticity and specific manipulation of individual components of the presynaptic machinery. Here, we describe presynaptic applications of optogenetic tools in the context of the unique cell biology of axonal terminals, discuss their potential shortcomings and outline future directions for this rapidly developing research area.

Regulation of Neural Circuitry under General Anesthesia: New Methods and Findings

General anesthesia has been widely utilized since the 1840s, but its underlying neural circuits remain to be completely understood. Since both general anesthesia and sleep are reversible losses of consciousness, studies on the neural-circuit mechanisms affected by general anesthesia have mainly focused on the neural nuclei or the pathways known to regulate sleep. Three advanced technologies commonly used in neuroscience, in vivo calcium imaging, chemogenetics, and optogenetics, are used to record and modulate the activity of specific neurons or neural circuits in the brain areas of interest. Recently, they have successfully been used to study the neural nuclei and pathways of general anesthesia. This article reviews these three techniques and their applications in the brain nuclei or pathways affected by general anesthesia, to serve as a reference for further and more accurate exploration of other neural circuits under general anesthesia and to contribute to other research fields in the future.

General and Specific Aversive Modulation of Active Avoidance Require Central Amygdala

Three studies provide evidence that the central nucleus of the amygdala, a structure with a well-established role in conditioned freezing, is also required for conditioned facilitation of instrumental avoidance in rats. First, the immediate early gene c-Fos was measured following the presentation of a previously shock-paired tone in subjects trained either on an unsignaled avoidance task or not (in addition to tone only presentations in naïve controls). Significantly elevated expression of c-Fos was found in both the avoidance trained and Pavlovian trained conditions relative to naïve controls (but with no difference between the two trained conditions). In a subsequent study, intracranial infusions of muscimol into the central amygdala significantly attenuated the facilitation of shock-avoidance by a shock-paired Pavlovian cue relative to pre-operative responding. The final study used a virogenetic approach to inhibit the central amygdala prior to testing. This treatment eliminated the transfer of motivational control over shock-avoidance by both a shock-paired Pavlovian stimulus, as well as a cue paired with a perceptually distinct aversive event (i.e., klaxon). These findings provide compelling support for a role of central amygdala in producing aversive Pavlovian-instrumental transfer.

Coordination chemogenetics for activation of GPCR-type glutamate receptors in brain tissue

Direct activation of cell-surface receptors is highly desirable for elucidating their physiological roles. A potential approach for cell-type-specific activation of a receptor subtype is chemogenetics, in which both point mutagenesis of the receptors and designed ligands are used. However, ligand-binding properties are affected in most cases. Here, we developed a chemogenetic method for direct activation of metabotropic glutamate receptor 1 (mGlu1), which plays essential roles in cerebellar functions in the brain. Our screening identified a mGlu1 mutant, mGlu1(N264H), that was activated directly by palladium complexes. A palladium complex showing low cytotoxicity successfully activated mGlu1 in mGlu1(N264H) knock-in mice, revealing that activation of endogenous mGlu1 is sufficient to evoke the critical cellular mechanism of synaptic plasticity, a basis of motor learning in the cerebellum. Moreover, cell-type-specific activation of mGlu1 was demonstrated successfully using adeno-associated viruses in mice, which shows the potential utility of this chemogenetics for clarifying the physiological roles of mGlu1 in a cell-type-specific manner.

Cell-type-specific activation of receptors is desirable for elucidating their roles in tissues or animals. Here, the authors developed a chemogenetic method for direct activation of mGlu1, a GPCR-type glutamate receptor subtype, and demonstrate its use in mouse brain tissue.

GABAergic interneurons' feedback inhibition of dorsal raphe-projecting pyramidal neurons of the medial prefrontal cortex suppresses feeding of adolescent female mice undergoing activity-based anorexia

Anorexia Nervosa (AN) is characterized by voluntary food restriction, excessive exercise and extreme body weight loss. AN is particularly prevalent among adolescent females experiencing stress-induced anxiety. We used the animal model, activity-based anorexia (ABA), which captures these characteristics of AN, to reveal the neurobiology underlying individual differences in AN vulnerability. Dorsal raphe (DR) regulates feeding and is recruited when coping inescapable stress. Through chemogenetic activation, we investigated the role of mPFC pyramidal neurons projecting to DR (mPFC→DR) in adolescent female mice's decision to eat or exercise following ABA induction. Although the DREADD ligand C21 could activate 44% of the mPFC→DR neurons, this did not generate significant group mean difference in the amount of food intake, compared to control ABA mice without chemogenetic activation. However, analysis of individuals' responses to C21 revealed a significant, positive correlation between food intake and mPFC→DR neurons that co-express cFos, a marker for neuronal activity. cFos expression by GABAergic interneurons (GABA-IN) in mPFC was significantly greater than that for the control ABA mice, indicating recruitment of GABA-IN by mPFC→DR neurons. Electron microscopic immunohistochemistry revealed that GABAergic innervation is 60% greater for the PFC→DR neurons than adjacent Layer 5 pyramidal neurons without projections to DR. Moreover, individual differences in this innervation correlated negatively with food intake specifically on the day of C21 administration. We propose that C21 activates two antagonistic pathways: (1) PFC→DR pyramidal neurons that promote food intake; and (2) GABA-IN in the mPFC that dampen food intake through feedback inhibition of mPFC→DR neurons.

Advancements in the Quest to Map, Monitor, and Manipulate Neural Circuitry

Neural circuits and the cells that comprise them represent the functional units of the brain. Circuits relay and process sensory information, maintain homeostasis, drive behaviors, and facilitate cognitive functions such as learning and memory. Creating a functionally-precise map of the mammalian brain requires anatomically tracing neural circuits, monitoring their activity patterns, and manipulating their activity to infer function. Advancements in cell-type-specific genetic tools allow interrogation of neural circuits with increased precision. This review provides a broad overview of recombination-based and activity-driven genetic targeting approaches, contemporary viral tracing strategies, electrophysiological recording methods, newly developed calcium, and voltage indicators, and neurotransmitter/neuropeptide biosensors currently being used to investigate circuit architecture and function. Finally, it discusses methods for acute or chronic manipulation of neural activity, including genetically-targeted cellular ablation, optogenetics, chemogenetics, and over-expression of ion channels. With this ever-evolving genetic toolbox, scientists are continuing to probe neural circuits with increasing resolution, elucidating the structure and function of the incredibly complex mammalian brain.

Local modulation by presynaptic receptors controls neuronal communication and behaviour

Central nervous system neurons communicate via fast synaptic transmission mediated by ligand-gated ion channel (LGIC) receptors and slower neuromodulation mediated by G protein-coupled receptors (GPCRs). These receptors influence many neuronal functions, including presynaptic neurotransmitter release. Presynaptic LGIC and GPCR activation by locally released neurotransmitters influences neuronal communication in ways that modify effects of somatic action potentials. Although much is known about presynaptic receptors and their mechanisms of action, less is known about when and where these receptor actions alter release, especially in vivo. This Review focuses on emerging evidence for important local presynaptic receptor actions and ideas for future studies in this area.

Chemogenetics of cell surface receptors: beyond genetic and pharmacological approaches

Cell surface receptors transmit extracellular information into cells. Spatiotemporal regulation of receptor signaling is crucial for cellular functions, and dysregulation of signaling causes various diseases. Thus, it is highly desired to control receptor functions with high spatial and/or temporal resolution. Conventionally, genetic engineering or chemical ligands have been used to control receptor functions in cells. As the alternative, chemogenetics has been proposed, in which target proteins are genetically engineered to interact with a designed chemical partner with high selectivity. The engineered receptor dissects the function of one receptor member among a highly homologous receptor family in a cell-specific manner. Notably, some chemogenetic strategies have been used to reveal the receptor signaling of target cells in living animals. In this review, we summarize the developing chemogenetic methods of transmembrane receptors for cell-specific regulation of receptor signaling. We also discuss the prospects of chemogenetics for clinical applications.