Ventral tegmental area astrocytes orchestrate avoidance and approach behavior

Increased excitatory amino acid transporter 2 levels in basolateral amygdala astrocytes mediate chronic stress-induced anxiety-like behavior

JOURNAL/nrgr/04.03/01300535-202506000-00024/figure1/v/2024-08-05T133530Z/r/image-tiff The conventional perception of astrocytes as mere supportive cells within the brain has recently been called into question by empirical evidence, which has revealed their active involvement in regulating brain function and encoding behaviors associated with emotions. Specifically, astrocytes in the basolateral amygdala have been found to play a role in the modulation of anxiety-like behaviors triggered by chronic stress. Nevertheless, the precise molecular mechanisms by which basolateral amygdala astrocytes regulate chronic stress-induced anxiety-like behaviors remain to be fully elucidated. In this study, we found that in a mouse model of anxiety triggered by unpredictable chronic mild stress, the expression of excitatory amino acid transporter 2 was upregulated in the basolateral amygdala. Interestingly, our findings indicate that the targeted knockdown of excitatory amino acid transporter 2 specifically within the basolateral amygdala astrocytes was able to rescue the anxiety-like behavior in mice subjected to stress. Furthermore, we found that the overexpression of excitatory amino acid transporter 2 in the basolateral amygdala, whether achieved through intracranial administration of excitatory amino acid transporter 2 agonists or through injection of excitatory amino acid transporter 2-overexpressing viruses with GfaABC1D promoters, evoked anxiety-like behavior in mice. Our single-nucleus RNA sequencing analysis further confirmed that chronic stress induced an upregulation of excitatory amino acid transporter 2 specifically in astrocytes in the basolateral amygdala. Moreover, through in vivo calcium signal recordings, we found that the frequency of calcium activity in the basolateral amygdala of mice subjected to chronic stress was higher compared with normal mice. After knocking down the expression of excitatory amino acid transporter 2 in the basolateral amygdala, the frequency of calcium activity was not significantly increased, and anxiety-like behavior was obviously mitigated. Additionally, administration of an excitatory amino acid transporter 2 inhibitor in the basolateral amygdala yielded a notable reduction in anxiety level among mice subjected to stress. These results suggest that basolateral amygdala astrocytic excitatory amino acid transporter 2 plays a role in in the regulation of unpredictable chronic mild stress-induced anxiety-like behavior by impacting the activity of local glutamatergic neurons, and targeting excitatory amino acid transporter 2 in the basolateral amygdala holds therapeutic promise for addressing anxiety disorders.

Ventral tegmental area interneurons revisited: GABA and glutamate projection neurons make local synapses

The ventral tegmental area (VTA) contains projection neurons that release the neurotransmitters dopamine, GABA, and/or glutamate from distal synapses. VTA also contains GABA neurons that synapse locally on to dopamine neurons, synapses widely credited to a population of so-called VTA interneurons. Interneurons in cortex, striatum, and elsewhere have well-defined morphological features, physiological properties, and molecular markers, but such features have not been clearly described in VTA. Indeed, there is scant evidence that local and distal synapses originate from separate populations of VTA GABA neurons. In this study, we tested whether several markers expressed in non-dopamine VTA neurons are selective markers of interneurons, defined as neurons that synapse locally but not distally. Challenging previous assumptions, we found that VTA neurons genetically defined by expression of parvalbumin, somatostatin, neurotensin, or Mu-opioid receptor project to known VTA targets including nucleus accumbens, ventral pallidum, lateral habenula, and prefrontal cortex. Moreover, we provide evidence that VTA GABA and glutamate projection neurons make functional inhibitory or excitatory synapses locally within VTA. These findings suggest that local collaterals of VTA projection neurons could mediate functions prior attributed to VTA interneurons. This study underscores the need for a refined understanding of VTA connectivity to explain how heterogeneous VTA circuits mediate diverse functions related to reward, motivation, or addiction.

Astrocytes in Rodent Anxiety-Related Behavior: Role of Calcium and Beyond

Anxiety is a physiological, emotional response that anticipates distal threats. When kept under control, anxiety is a beneficial response, helping animals to maintain heightened attention in environments with potential dangers. However, an overestimation of potential threats can lead to an excessive expression of anxiety that, in humans, may evolve into anxiety disorders. Pharmacological treatments show variable efficacy among patients, highlighting the need for more efforts to better understand the pathogenesis of anxiety disorders. Mounting evidence suggests that astrocytes, a type of glial cells, are active partners of neurons in brain circuits and in the regulation of behaviors under both physiological and pathological conditions. In this review, I summarize the current literature on the role of astrocytes from different brain regions in modulating anxious states, with the goal of exploring novel cerebral mechanisms to identify potential innovative therapeutic targets for the treatment of anxiety disorders.

Cross-species dissection of the modular role of the ventral tegmental area in depressive disorders

Depressive disorders, including major depressive disorder (MDD), represent one of the most prevalent set of disorders worldwide. MDD is characterized by a range of cognitive, behavioral, and neurobiological changes that contribute to the vast array of symptom profiles that make this disorder particularly difficult to treat. A multitude of established evidence suggests a role for the dopamine system, stemming in part from the ventral tegmental area (VTA), in mediating symptoms and behavioral changes that underlie depression. Developments in cutting-edge technologies in pre-clinical models of depressive phenotypes, such as retrograde tracing, electrophysiological recordings, immunohistochemistry, and molecular profiling, have allowed a deeper characterization of singular VTA neuron molecular, physiological, and projection properties. These developments have highlighted that the VTA is not a homogenous cell population but instead comprises vast cellular diversity that underscores its modular role across various functions related to reward processing, aversion, salience processing, learning and motivation. In this review, we begin by introducing the various cell types and brain regions that comprise the VTA circuitry. Then, we introduce the role of the VTA in reward processing as it compares to aversion processing. Next, we characterize distinct neural pathways within the VTA circuitry to understand the effects of chronic social and non-social stress and tie together how these neurobiological changes manifest into specific behavioral phenotypes. Finally, we relate these preclinical findings to clinical findings to parse the heterogeneity of depressive phenotypes and explain the efficacy of recent novel pharmacological interventions that may target the VTA in MDD.

Intersect between brain mechanisms of conditioned threat, active avoidance, and reward

Active avoidance is a core behavior for human coping, and its excess is common across psychiatric diseases. The decision to actively avoid a threat is influenced by cost and reward. Yet, threat, avoidance, and reward have been studied in silos. We discuss behavioral and brain circuits of active avoidance and the interactions with fear and threat. In addition, we present a neural toggle switch model enabling fear-to-anxiety transition and approaching reward vs. avoiding harm decision. To fully comprehend how threat, active avoidance, and reward intersect, it is paramount to develop one shared experimental approach across phenomena and behaviors, which will ultimately allow us to better understand human behavior and pathology.

Ventral tegmental area interneurons revisited: GABA and glutamate projection neurons make local synapses

The ventral tegmental area (VTA) contains projection neurons that release the neurotransmitters dopamine, GABA, and/or glutamate from distal synapses. VTA also contains GABA neurons that synapse locally on to dopamine neurons, synapses widely credited to a population of so-called VTA interneurons. Interneurons in cortex, striatum, and elsewhere have well-defined morphological features, physiological properties, and molecular markers, but such features have not been clearly described in VTA. Indeed, there is scant evidence that local and distal synapses originate from separate populations of VTA GABA neurons. In this study we tested whether several markers expressed in non-dopamine VTA neurons are selective markers of interneurons, defined as neurons that synapse locally but not distally. Challenging previous assumptions, we found that VTA neurons genetically defined by expression of parvalbumin, somatostatin, neurotensin, or mu-opioid receptor project to known VTA targets including nucleus accumbens, ventral pallidum, lateral habenula, and prefrontal cortex. Moreover, we provide evidence that VTA GABA and glutamate projection neurons make functional inhibitory or excitatory synapses locally within VTA. These findings suggest that local collaterals of VTA projection neurons could mediate functions prior attributed to VTA interneurons. This study underscores the need for a refined understanding of VTA connectivity to explain how heterogeneous VTA circuits mediate diverse functions related to reward, motivation, or addiction.

Characterization of the Astrocyte Calcium Response to Norepinephrine in the Ventral Tegmental Area

Astrocytes from different brain regions respond with Ca2+ elevations to the catecholamine norepinephrine (NE). However, whether this noradrenergic-mediated signaling is present in astrocytes from the ventral tegmental area (VTA), a dopaminergic circuit receiving noradrenergic inputs, has not yet been investigated. To fill in this gap, we applied a pharmacological approach along with two-photon microscopy and an AAV strategy to express a genetically encoded calcium indicator in VTA astrocytes. We found that VTA astrocytes from both female and male young adult mice showed a strong Ca2+ response to NE at both soma and processes. Our results revealed that Gq-coupled α1 adrenergic receptors, which elicit the production of IP3, are the main mediators of the astrocyte response. In mice lacking the IP3 receptor type-2 (IP3R2-/- mice), we found that the astrocyte response to NE, even if reduced, is still present. We also found that in IP3R2-/- astrocytes, the residual Ca2+ elevations elicited by NE depend on the release of Ca2+ from the endoplasmic reticulum, through IP3Rs different from IP3R2. In conclusion, our results reveal VTA astrocytes as novel targets of the noradrenergic signaling, opening to new interpretations of the cellular and molecular mechanisms that mediate the NE effects in the VTA.

Melanin-concentrating hormone attenuates the hedonic feeding induced by orexin-A in the ventral tegmental area of high-fat diet male mice

Objective

The ventral tegmental area (VTA), a pivotal hub in the brain's reward circuitry, receives inputs from the lateral hypothalamic area (LHA). However, it remains unclear whether melanin-concentrating hormone (MCH) and orexin-A (OX-A) neurons in the LHA exert individual or cooperative influence on palatable food consumption in the VTA. This study aims to investigate the modulatory role of MCH and OX-A in hedonic feeding within the VTA of high-fat diet (HFD) mice.

Methods

Male mice were subjected to an 8-week high-fat diet. To visualize the projections from the LHA to VTA, we employed fluorescent gold retrograde tracing combined with immunofluorescence staining. Immunofluorescence staining or enzyme-linked immunosorbent assay was used to detect the activity of the VTA neurons, expression of OX-A or MCH in the LHA, as well as the activity of their receptors (OXR1 and MCHR1) in the VTA following a sucrose preference test. Single-unit extracellular electrical discharge recordings were conducted to assess the effects of OX-A and MCH on VTA neurons in HFD mice. Additionally, chemogenetic inhibition of MCH neurons and immunofluorescence staining were utilized to observe the regulatory roles of MCH in changes of hedonic feeding induced by OX-A in HFD mice.

Results

Sucrose intake resulted in lower activation of VTA neurons in the HFD mice compared to CON mice, while OX-Aergic and MCHergic neurons project from the LHA to the VTA. Although sucrose intake increased the expression of OX-A and MCH in HFD mice, it led to diminished activation of OXR1-positive and MCHR1-positive VTA neurons compared to CON mice. Extracellular single-unit recording revealed that MCH significantly suppressed the firing rate of OX-A-responsive neurons in the VTA. MCH attenuated the hedonic feeding response induced by OX-A in HFD mice, and administration of MCHR1 antagonist (SNAP94847) significantly potentiated the effect of OX-A. Chemogenetic inhibition of MCH neurons improved the activity of OXR1-expressing neurons, which could be reversed by pretreatment with an OXR1 antagonist (SB334867). Furthermore, chemogenetic inhibition of MCH enhanced hedonic feeding behavior, which was counteracted by SB334867 treatment in HFD mice.

Conclusion

Melanin-concentrating hormone could attenuate the hedonic feeding behavior induced by orexin-A in the VTA of HFD mice.

Rapid modulation of striatal cholinergic interneurons and dopamine release by satellite astrocytes

Astrocytes are increasingly appreciated to possess underestimated and important roles in modulating neuronal circuits. Astrocytes in striatum can regulate dopamine transmission by governing the extracellular tone of axonal neuromodulators, including GABA and adenosine. However, here we reveal that striatal astrocytes occupy a cell type-specific anatomical and functional relationship with cholinergic interneurons (ChIs), through which they rapidly excite ChIs and govern dopamine release via nicotinic acetylcholine receptors on subsecond timescales. We identify that ChI somata are in unexpectedly close proximity to astrocyte somata, in mouse and human, forming a "soma-to-soma" satellite-like configuration not typically observed for other striatal neurons. We find that transient depolarization of astrocytes in mouse striatum reversibly regulates ChI excitability by decreasing extracellular calcium. These findings reveal a privileged satellite astrocyte-interneuron interaction for striatal ChIs operating on subsecond timescales via regulation of extracellular calcium dynamics to shape downstream striatal circuit activity and dopamine signaling.

Long-term intake of thermo-induced oxidized oil results in anxiety-like and depression-like behaviors: involvement of microglia and astrocytes

Frequent consumption of fried foods has been strongly associated with a higher risk of anxiety and depression, particularly among young individuals. The existing evidence has indicated that acrylamide produced from starchy foods at high temperatures can induce anxious behavior. However, there is limited research on the nerve damage caused by thermo-induced oxidized oil (TIOO). In this study, we conducted behavioral tests on mice and found that prolonged consumption of TIOO led to significant anxiety behavior and a tendency toward depression. TIOO primarily induced these two emotional disorders by affecting the differentiation of microglia, the level of inflammatory factors, the activation of astrocytes, and glutamate circulation in brain tissue. By promoting the over-differentiation of microglia into M1 microglia, TIOO disrupted their differentiation balance, resulting in an up-regulation of inflammatory factors (IL-1β, IL-6, TNF-α, NOS2) in M1 microglia and a down-regulation of neuroprotective factors IL-4/IL-10 in M2 microglia, leading to nerve damage. Moreover, TIOO activated astrocytes, accelerating their proliferation and causing GFAP precipitation, which damaged astrocytes. Meanwhile, TIOO stimulates the secretion of the BDNF and reduces the level of the glutamate receptor GLT-1 in astrocytes, leading to a disorder in the glutamate-glutamine cycle, further exacerbating nerve damage. In conclusion, this study suggests that long-term intake of thermo-induced oxidized oil can trigger symptoms of anxiety and depression.

Selective disruption of mTORC1 and mTORC2 in VTA astrocytes induces depression and anxiety-like behaviors in mice

Dysfunction of the mechanistic target of rapamycin (mTOR) signaling pathway is implicated in neuropsychiatric disorders including depression and anxiety. Most studies have been focusing on neurons, and the function of mTOR signaling pathway in astrocytes is less investigated. mTOR forms two distinct complexes, mTORC1 and mTORC2, with key scaffolding protein Raptor and Rictor, respectively. The ventral tegmental area (VTA), a vital component of the brain reward system, is enrolled in regulating both depression and anxiety. In the present study, we aimed to examine the regulation effect of VTA astrocytic mTOR signaling pathway on depression and anxiety. We specifically deleted Raptor or Rictor in VTA astrocytes in mice and performed a series of behavioral tests for depression and anxiety. Deletion of Raptor and Rictor both decreased the immobility time in the tail suspension test and the latency to eat in the novelty suppressed feeding test, and increased the horizontal activity and the movement time in locomotor activity. Deletion of Rictor decreased the number of total arm entries in the elevated plus-maze test and the vertical activity in locomotor activity. These data suggest that VTA astrocytic mTORC1 plays a role in regulating depression-related behaviors and mTORC2 is involved in both depression and anxiety-related behaviors. Our results indicate that VTA astrocytic mTOR signaling pathway might be new targets for the treatment of psychiatric disorders.

Activation and depression of neural and hemodynamic responses induced by the intracortical microstimulation and visual stimulation in the mouse visual cortex

Objective. Intracortical microstimulation (ICMS) can be an effective method for restoring sensory perception in contemporary brain-machine interfaces. However, the mechanisms underlying better control of neuronal responses remain poorly understood, as well as the relationship between neuronal activity and other concomitant phenomena occurring around the stimulation site.Approach. Different microstimulation frequencies were investigatedin vivoon Thy1-GCaMP6s mice using widefield and two-photon imaging to evaluate the evoked excitatory neural responses across multiple spatial scales as well as the induced hemodynamic responses. Specifically, we quantified stimulation-induced neuronal activation and depression in the mouse visual cortex and measured hemodynamic oxyhemoglobin and deoxyhemoglobin signals using mesoscopic-scale widefield imaging.Main results. Our calcium imaging findings revealed a preference for lower-frequency stimulation in driving stronger neuronal activation. A depressive response following the neural activation preferred a slightly higher frequency stimulation compared to the activation. Hemodynamic signals exhibited a comparable spatial spread to neural calcium signals. Oxyhemoglobin concentration around the stimulation site remained elevated during the post-activation (depression) period. Somatic and neuropil calcium responses measured by two-photon microscopy showed similar dependence on stimulation parameters, although the magnitudes measured in soma was greater than in neuropil. Furthermore, higher-frequency stimulation induced a more pronounced activation in soma compared to neuropil, while depression was predominantly induced in soma irrespective of stimulation frequencies.Significance. These results suggest that the mechanism underlying depression differs from activation, requiring ample oxygen supply, and affecting neurons. Our findings provide a novel understanding of evoked excitatory neuronal activity induced by ICMS and offer insights into neuro-devices that utilize both activation and depression phenomena to achieve desired neural responses.

Brain-Region-Specific Genes Form the Major Pathways Featuring Their Basic Functional Role: Their Implication in Animal Chronic Stress Model

The analysis of RNA-Sec data from murine bulk tissue samples taken from five brain regions associated with behavior and stress response was conducted. The focus was on the most contrasting brain region-specific genes (BRSG) sets in terms of their expression rates. These BRSGs are identified as genes with a distinct outlying (high) expression rate in a specific region compared to others used in the study. The analysis suggested that BRSG sets form non-randomly connected compact gene networks, which correspond to the major neuron-mediated functional processes or pathways in each brain region. The number of BRSGs and the connection rate were found to depend on the heterogeneity and coordinated firing rate of neuron types in each brain region. The most connected pathways, along with the highest BRSG number, were observed in the Striatum, referred to as Medium Spiny Neurons (MSNs), which make up 95% of neurons and exhibit synchronous firing upon dopamine influx. However, the Ventral Tegmental Area/Medial Raphe Nucleus (VTA/MRN) regions, although primarily composed of monoaminergic neurons, do not fire synchronously, leading to a smaller BRSG number. The Hippocampus (HPC) region, on the other hand, displays significant neuronal heterogeneity, with glutamatergic neurons being the most numerous and synchronized. Interestingly, the two monoaminergic regions involved in the study displayed a common BRSG subnetwork architecture, emphasizing their proximity in terms of axonal throughput specifics and high-energy metabolism rates. This finding suggests the concerted evolution of monoaminergic neurons, leading to unique adaptations at the genic repertoire scale. With BRSG sets, we were able to highlight the contrasting features of the three groups: control, depressive, and aggressive mice in the animal chronic stress model. Specifically, we observed a decrease in serotonergic turnover in both the depressed and aggressive groups, while dopaminergic emission was high in both groups. There was also a notable absence of dopaminoceptive receptors on the postsynaptic membranes in the striatum in the depressed group. Additionally, we confirmed that neurogenesis BRSGs are specific to HPC, with the aggressive group showing attenuated neurogenesis rates compared to the control/depressive groups. We also confirmed that immune-competent cells like microglia and astrocytes play a crucial role in depressed phenotypes, including mitophagy-related gene Prkcd. Based on this analysis, we propose the use of BRSG sets as a suitable framework for evaluating case-control group-wise assessments of specific brain region gene pathway responses.

Activation and depression of neural and hemodynamic responses induced by the intracortical microstimulation and visual stimulation in the mouse visual cortex

Objective . Intracortical microstimulation can be an effective method for restoring sensory perception in contemporary brain-machine interfaces. However, the mechanisms underlying better control of neuronal responses remain poorly understood, as well as the relationship between neuronal activity and other concomitant phenomena occurring around the stimulation site. Approach . Different microstimulation frequencies were investigated in vivo on Thy1-GCaMP6s mice using widefield and two-photon imaging to evaluate the evoked excitatory neural responses across multiple spatial scales as well as the induced hemodynamic responses. Specifically, we quantified stimulation-induced neuronal activation and depression in the mouse visual cortex and measured hemodynamic oxyhemoglobin and deoxyhemoglobin signals using mesoscopic-scale widefield imaging. Main results . Our calcium imaging findings revealed a preference for lower-frequency stimulation in driving stronger neuronal activation. A depressive response following the neural activation preferred a slightly higher frequency stimulation compared to the activation. Hemodynamic signals exhibited a comparable spatial spread to neural calcium signals. Oxyhemoglobin concentration around the stimulation site remained elevated during the post-activation (depression) period. Somatic and neuropil calcium responses measured by two-photon microscopy showed similar dependence on stimulation parameters, although the magnitudes measured in soma was greater than in neuropil. Furthermore, higher-frequency stimulation induced a more pronounced activation in soma compared to neuropil, while depression was predominantly induced in soma irrespective of stimulation frequencies. Significance . These results suggest that the mechanism underlying depression differs from activation, requiring ample oxygen supply, and affecting neurons. Our findings provide a novel understanding of evoked excitatory neuronal activity induced by intracortical microstimulation and offer insights into neuro-devices that utilize both activation and depression phenomena to achieve desired neural responses.

Astrocytes in Pain Perception: A Systems Neuroscience Approach

Astrocytes play an active role in the function of the brain integrating neuronal activity and regulating back neuronal dynamic. They have recently emerged as active contributors of brain's emergent properties such as perceptions. Here, we analyzed the role of astrocytes in pain perception from the lens of systems neuroscience, and we do this by analyzing how astrocytes encode nociceptive information within brain processing areas and how they are key regulators of the internal state that determines pain perception. Specifically, we discuss the dynamic interactions between astrocytes and neuromodulators, such as noradrenaline, highlighting their role in shaping the level of activation of the neuronal ensemble, thereby influencing the experience of pain. Also, we will discuss the possible implications of an "Astro-NeuroMatrix" in the integration of pain across sensory, affective, and cognitive dimensions of pain perception.

Brain-wide circuit-specific targeting of astrocytes

Astrocytes are integral components of brain circuitry. They enwrap synapses, react to neuronal activity, and regulate synaptic transmission. Astrocytes are heterogeneous and exhibit distinct features and functions in different circuits. Selectively targeting the astrocytes associated with a given neuronal circuit would enable elucidation of their circuit-specific functions but has been technically challenging to date. Recently, we constructed anterograde transneuronal viral vectors based on yellow fever vaccine YFV-17D. Among them, the replication-incompetent YFVΔNS1-Cre can selectively turn on reporter genes in postsynaptic neurons if the viral gene NS1 is expressed in postsynaptic neurons. Here we show that without exogenous expression of NS1 at the postsynaptic sites, locally injected YFVΔNS1-Cre selectively turns on reporter genes in astrocytes in downstream brain regions. The targeting of astrocytes can occur across the whole brain but is specific for the neuronal circuits traced. Therefore, YFVΔNS1-Cre provides a tool for selective genetic targeting of astrocytes to reveal their circuit-specific roles.

Local extracellular K+ in cortex regulates norepinephrine levels, network state, and behavioral output

Extracellular potassium concentration ([K+]e) is known to increase as a function of arousal. [K+]e is also a potent modulator of transmitter release. Yet, it is not known whether [K+]e is involved in the neuromodulator release associated with behavioral transitions. We here show that manipulating [K+]e controls the local release of monoaminergic neuromodulators, including norepinephrine (NE), serotonin, and dopamine. Imposing a [K+]e increase is adequate to boost local NE levels, and conversely, lowering [K+]e can attenuate local NE. Electroencephalography analysis and behavioral assays revealed that manipulation of cortical [K+]e was sufficient to alter the sleep-wake cycle and behavior of mice. These observations point to the concept that NE levels in the cortex are not solely determined by subcortical release, but that local [K+]e dynamics have a strong impact on cortical NE. Thus, cortical [K+]e is an underappreciated regulator of behavioral transitions.

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.

Interleukin-10 enhances activity of ventral tegmental area dopamine neurons resulting in increased dopamine release

Dopamine transmission from the ventral tegmental area (VTA) to the nucleus accumbens (NAc) regulates important aspects of motivation and is influenced by the neuroimmune system. The neuroimmune system is a complex network of leukocytes, microglia and astrocytes that detect and remove foreign threats like bacteria or viruses and communicate with each other to regulate non-immune (e.g neuronal) cell activity through cytokine signaling. Inflammation is a key regulator of motivational states, though the effects of specific cytokines on VTA circuitry and motivation are largely unknown. Therefore, electrophysiology, neurochemical, immunohistochemical and behavioral studies were performed to determine the effects of the anti-inflammatory cytokine interleukin-10 (IL-10) on mesolimbic activity, dopamine transmission and conditioned behavior. IL-10 enhanced VTA dopamine firing and NAc dopamine levels via decreased VTA GABA currents in dopamine neurons. The IL-10 receptor was localized on VTA dopamine and non-dopamine cells. The IL-10 effects on dopamine neurons required post-synaptic phosphoinositide 3-kinase activity, and IL-10 appeared to have little-to-no efficacy on presynaptic GABA terminals. Intracranial IL-10 enhanced NAc dopamine levels in vivo and produced conditioned place aversion. Together, these studies identify the IL-10R on VTA dopamine neurons as a potential regulator of motivational states.

Astrocytes: Role in pathogenesis and effect of commonly misused drugs in the HIV infected brain

The roles of astrocytes as reservoirs and producers of a subset of viral proteins in the HIV infected brain have been studied extensively as a key to understanding HIV-associated neurocognitive disorders (HAND). However, their comprehensive role in the context of intersecting substance use and neurocircuitry of the reward pathway and HAND has yet to be fully explained. Use of methamphetamines, cocaine, or opioids in the context of HIV infection have been shown to lead to a faster progression of HAND. Glutamatergic, dopaminergic, and GABAergic systems are implicated in the development of HAND-induced cognitive impairments. A thorough review of scientific literature exploring the variety of mechanisms in which these drugs exert their effects on the HIV brain and astrocytes has revealed marked areas of convergence in overexcitation leading to increased drug-seeking behavior, inflammation, apoptosis, and irreversible neurotoxicity. The present review investigates astrocytes, the neural pathways, and mechanisms of drug disruption that ultimately play a larger holistic role in terms of HIV progression and drug use. There are opportunities for future research, therapeutic intervention, and preventive strategies to diminish HAND in the subset population of patients with HIV and substance use disorder.

Sex-specific adaptations to VTA circuits following subchronic stress

Dysregulation of the mesolimbic reward circuitry is implicated in the pathophysiology of stress-related illnesses such as depression and anxiety. These disorders are more frequently diagnosed in females, and sex differences in the response to stress are likely to be one factor that leads to enhanced vulnerability of females. In this study, we use subchronic variable stress (SCVS), a model in which females are uniquely vulnerable to behavioral disturbances, to investigate sexually divergent mechanisms of regulation of the ventral tegmental area by stress. Using slice electrophysiology, we find that female, but not male mice have a reduction in the ex vivo firing rate of VTA dopaminergic neurons following SCVS. Surprisingly, both male and female animals show an increase in inhibitory tone onto VTA dopaminergic neurons and an increase in the firing rate of VTA GABAergic neurons. In males, however, this is accompanied by a robust increase in excitatory synaptic tone onto VTA dopamine neurons. This supports a model by which SCVS recruits VTA GABA neurons to inhibit dopaminergic neurons in both male and female mice, but males are protected from diminished functioning of the dopaminergic system by a compensatory upregulation of excitatory synapses.

Induction of astrocytic Slc22a3 regulates sensory processing through histone serotonylation

Neuronal activity drives alterations in gene expression within neurons, yet how it directs transcriptional and epigenomic changes in neighboring astrocytes in functioning circuits is unknown. We found that neuronal activity induces widespread transcriptional up-regulation and down-regulation in astrocytes, highlighted by the identification of Slc22a3 as an activity-inducible astrocyte gene that encodes neuromodulator transporter Slc22a3 and regulates sensory processing in the mouse olfactory bulb. Loss of astrocytic Slc22a3 reduced serotonin levels in astrocytes, leading to alterations in histone serotonylation. Inhibition of histone serotonylation in astrocytes reduced the expression of γ-aminobutyric acid (GABA) biosynthetic genes and GABA release, culminating in olfactory deficits. Our study reveals that neuronal activity orchestrates transcriptional and epigenomic responses in astrocytes while illustrating new mechanisms for how astrocytes process neuromodulatory input to gate neurotransmitter release for sensory processing.

Transcriptomic and Macroscopic Architectures of Multimodal Covariance Network Reveal Molecular-Structural-Functional Co-alterations

Human cognition is usually underpinned by intrinsic structure and functional neural co-activation in spatially distributed brain regions. Owing to lacking an effective approach to quantifying the covarying of structure and functional responses, how the structural-functional circuits interact and how genes encode the relationships, to deepen our knowledge of human cognition and disease, are still unclear. Here, we propose a multimodal covariance network (MCN) construction approach to capture interregional covarying of the structural skeleton and transient functional activities for a single individual. We further explored the potential association between brain-wide gene expression patterns and structural-functional covarying in individuals involved in a gambling task and individuals with major depression disorder (MDD), adopting multimodal data from a publicly available human brain transcriptomic atlas and 2 independent cohorts. MCN analysis showed a replicable cortical structural-functional fine map in healthy individuals, and the expression of cognition- and disease phenotype-related genes was found to be spatially correlated with the corresponding MCN differences. Further analysis of cell type-specific signature genes suggests that the excitatory and inhibitory neuron transcriptomic changes could account for most of the observed correlation with task-evoked MCN differences. In contrast, changes in MCN of MDD patients were enriched for biological processes related to synapse function and neuroinflammation in astrocytes, microglia, and neurons, suggesting its promising application in developing targeted therapies for MDD patients. Collectively, these findings confirmed the correlations of MCN-related differences with brain-wide gene expression patterns, which captured genetically validated structural-functional differences at the cellular level in specific cognitive processes and psychiatric patients.

Differential Effects of Astrocyte Manipulations on Learned Motor Behavior and Neuronal Ensembles in the Motor Cortex

Although motor cortex is crucial for learning precise and reliable movements, whether and how astrocytes contribute to its plasticity and function during motor learning is unknown. Here, we report that astrocyte-specific manipulations in primary motor cortex (M1) during a lever push task alter motor learning and execution, as well as the underlying neuronal population coding. Mice that express decreased levels of the astrocyte glutamate transporter 1 (GLT1) show impaired and variable movement trajectories, whereas mice with increased astrocyte Gq signaling show decreased performance rates, delayed response times, and impaired trajectories. In both groups, which include male and female mice, M1 neurons have altered interneuronal correlations and impaired population representations of task parameters, including response time and movement trajectories. RNA sequencing further supports a role for M1 astrocytes in motor learning and shows changes in astrocytic expression of glutamate transporter genes, GABA transporter genes, and extracellular matrix protein genes in mice that have acquired this learned behavior. Thus, astrocytes coordinate M1 neuronal activity during motor learning, and our results suggest that this contributes to learned movement execution and dexterity through mechanisms that include regulation of neurotransmitter transport and calcium signaling.SIGNIFICANCE STATEMENT We demonstrate for the first time that in the M1 of mice, astrocyte function is critical for coordinating neuronal population activity during motor learning. We demonstrate that knockdown of astrocyte glutamate transporter GLT1 affects specific components of learning, such as smooth trajectory formation. Altering astrocyte calcium signaling by activation of Gq-DREADD upregulates GLT1 and affects other components of learning, such as response rates and reaction times as well as trajectory smoothness. In both manipulations, neuronal activity in motor cortex is dysregulated, but in different ways. Thus, astrocytes have a crucial role in motor learning via their influence on motor cortex neurons, and they do so by mechanisms that include regulation of glutamate transport and calcium signals.

Ventral tegmental area astrocytes modulate cocaine reward by tonically releasing GABA

Addictive drugs increase ventral tegmental area (VTA) dopamine (DA) neuron activity through distinct cellular mechanisms, one of which involves disinhibition of DA neurons by inhibiting local GABA neurons. How drugs regulate VTA GABA neuron activity and drive addictive behaviors remains poorly understood. Here, we show that astrocytes control VTA GABA neuron activity in cocaine reward via tonic inhibition in mice. Repeated cocaine exposure potentiates astrocytic tonic GABA release through volume-regulated anion channels (VRACs) and augments tonic inhibition of VTA GABA neurons, thus downregulating their activities and disinhibiting nucleus accumbens (NAc) projecting DA neurons. Attenuation of tonic inhibition by either deleting Swell1 (Lrrc8a), the obligatory subunit of VRACs, in VTA astrocytes or disrupting δ subunit of GABAA receptors in VTA GABA neurons reduces cocaine-evoked changes in neuron activity, locomotion, and reward behaviors in mice. Together, our findings reveal the critical role of astrocytes in regulating the VTA local circuit and cocaine reward.

A new role for astrocytes in the story of cocaine abuse

In this issue of Neuron, Yang et al.¹ highlight a hitherto unknown action of cocaine in VTA circuitry. They found that chronic cocaine use increased tonic inhibition selectively onto GABA neurons through Swell1 channel-dependent GABA release from astrocytes, leading to disinhibition-mediated hyperactivity in DA neurons and addictive behavior.

Activity-dependent induction of astrocytic Slc22a3 regulates sensory processing through histone serotonylation

Neuronal activity drives global alterations in gene expression within neurons, yet how it directs transcriptional and epigenomic changes in neighboring astrocytes in functioning circuits is unknown. Here we show that neuronal activity induces widespread transcriptional upregulation and downregulation in astrocytes, highlighted by the identification of a neuromodulator transporter Slc22a3 as an activity-inducible astrocyte gene regulating sensory processing in the olfactory bulb. Loss of astrocytic Slc22a3 reduces serotonin levels in astrocytes, leading to alterations in histone serotonylation. Inhibition of histone serotonylation in astrocytes reduces expression of GABA biosynthetic genes and GABA release, culminating in olfactory deficits. Our study reveals that neuronal activity orchestrates transcriptional and epigenomic responses in astrocytes, while illustrating new mechanisms for how astrocytes process neuromodulatory input to gate neurotransmitter release for sensory processing.

The role of astrocytes in behaviors related to emotion and motivation

Astrocytes are present throughout the brain and intimately interact with neurons and blood vessels. Three decades of research have shown that astrocytes reciprocally communicate with neurons and other non-neuronal cells in the brain and dynamically regulate cell function. Astrocytes express numerous receptors for neurotransmitters, neuromodulators, and cytokines and receive information from neurons, other astrocytes, and other non-neuronal cells. Among those receptors, the main focus has been G-protein coupled receptors. Activation of G-protein coupled receptors leads to dramatic changes in intracellular signaling (Ca²⁺ and cAMP), which is considered a form of astrocyte activity. Methodological improvements in measurement and manipulation of astrocytes have advanced our understanding of the role of astrocytes in circuits and have begun to reveal unexpected functions of astrocytes in behavior. Recent studies have suggested that astrocytic activity regulates behavior flexibility, such as coping strategies for stress exposure, and plays an important role in behaviors related to emotion and motivation. Preclinical evidence suggests that impairment of astrocytic function contributes to psychiatric diseases, especially major depression. Here, we review recent progress on the role of astrocytes in behaviors related to emotion and motivation.

Extracellular S100B inhibits A-type voltage-gated potassium currents and increases L-type voltage-gated calcium channel activity in dopaminergic neurons

Parkinson's disease (PD) is associated with an increase in secreted S100B within the midbrain and cerebrospinal fluid. In addition, S100B overexpression in mice accelerates the loss of substantia nigra pars compacta dopaminergic (DA) neurons, suggesting a role for this protein in PD pathogenesis. We found that in the mouse SNc, S100B labeled astrocytic processes completely envelop the somata of tyrosine hydroxylase (TH) expressing DA neurons only in male mice. These data suggest that an increase in S100B secretion by astrocytes within the midbrain could play a role in DA dysfunction during early PD. We therefore asked if acute exposure to extracellular S100B alters the activity of identified TH expressing DA neurons in primary mouse midbrain cultures. Acute exposure to 50 pM S100B specifically inhibited A-type voltage-gated potassium currents in TH+ , but not TH- neurons. This was accompanied by ~2-fold increases in the frequency of both intrinsic firing, as well as L-type voltage-gated calcium channel (VGCC)-mediated calcium fluxes only in TH+ neurons. Further, exposure to 100 μM 4-aminopyridine (4-AP), an A-type voltage-gated potassium channel inhibitor, mimicked the S100B mediated increase in intrinsic firing and L-type VGCC-mediated calcium fluxes in TH+ neurons. Taken together, our finding that extracellular S100B alters the activity of native DA neurons via an inhibition of A-type voltage-gated potassium channels has important implications for understanding the pathophysiology of early PD.

Astrocytes mediate long-lasting synaptic regulation of ventral tegmental area dopamine neurons

The plasticity of glutamatergic transmission in the ventral tegmental area (VTA) represents a fundamental mechanism in the modulation of dopamine neuron burst firing and phasic dopamine release at target regions. These processes encode basic behavioral responses, including locomotor activity, learning and motivated behaviors. Here we describe a hitherto unidentified mechanism of long-term synaptic plasticity in mouse VTA. We found that the burst firing in individual dopamine neurons induces a long-lasting potentiation of excitatory synapses on adjacent dopamine neurons that crucially depends on Ca²⁺ elevations in astrocytes, mediated by endocannabinoid CB1 and dopamine D2 receptors co-localized at the same astrocytic process, and activation of pre-synaptic metabotropic glutamate receptors. Consistent with these findings, selective in vivo activation of astrocytes increases the burst firing of dopamine neurons in the VTA and induces locomotor hyperactivity. Astrocytes play, therefore, a key role in the modulation of VTA dopamine neuron functional activity.

Hippocampal astrocytes modulate anxiety-like behavior

Astrocytes can affect animal behavior by regulating tripartite synaptic transmission, yet their influence on affective behavior remains largely unclear. Here we showed that hippocampal astrocyte calcium activity reflects mouse affective state during virtual elevated plus maze test using two-photon calcium imaging in vivo. Furthermore, optogenetic hippocampal astrocyte activation elevating intracellular calcium induced anxiolytic behaviors in astrocyte-specific channelrhodopsin 2 (ChR2) transgenic mice (hGFAP-ChR2 mice). As underlying mechanisms, we found ATP released from the activated hippocampal astrocytes increased excitatory synaptic transmission in dentate gyrus (DG) granule cells, which exerted anxiolytic effects. Our data uncover a role of hippocampal astrocytes in modulating mice anxiety-like behaviors by regulating ATP-mediated synaptic homeostasis in hippocampal DG granule cells. Thus, manipulating hippocampal astrocytes activity can be a therapeutic strategy to treat anxiety.

Dopamine neurotransmission in the VTA regulates aversive memory formation and persistence

Dopamine (DA) neurons in the ventral tegmental area (VTA) innervating several limbic and neocortical regions of the mammalian brain have long been implicated in motivation, rewarding and aversive behaviors, and memory processing. Recently, we demonstrated that somatodendritic release of DA in the VTA regulates the formation and maintenance of appetitive long-term memories (LTM). However, less is known about the impact of DA neurotransmission in the VTA on aversive LTM. Here, we describe the modulation of negative-valence memories by D1/D5-type DA (D1R)-receptor-mediated neurotransmission in the VTA. As aversive stimuli elicit both active and passive behavioral responses, we used two single-trial aversive training protocols: inhibitory avoidance task and conditioned place aversion. We bilaterally microinfused SCH23390, an antagonist of D1R, into the VTA immediately after training and found that DA neurotransmission in the VTA modulates LTM consolidation and persistence of aversive experiences. Together with previous findings demonstrating that D1R-mediated DA neurotransmission in the medial prefrontal cortex and hippocampus is involved in the formation and persistence of LTM for aversive events, our present results indicate that memory processing of environmental stimuli with negative-valence depends on the integration of information mediated by D1R activation in both the VTA region and in selected downstream target areas.

Regulation of glutamate homeostasis in the nucleus accumbens by astrocytic CB1 receptors and its role in cocaine-motivated behaviors

Cannabinoid type 1 receptors (CB1Rs) orchestrate brain reward circuitry and are prevalent neurobiological targets for endocannabinoids and cannabis in the mammalian brain. Decades of histological and electrophysiological studies have established CB1R as presynaptic G-protein coupled receptors (GPCRs) that inhibit neurotransmitter release through retrograde signaling mechanisms. Recent seminal work demonstrates CB1R expression on astrocytes and the pivotal function of glial cells in endocannabinoid-mediated modulation of neuron-astrocyte signaling. Here, we review key facets of CB1R-mediated astroglia regulation of synaptic glutamate transmission in the nucleus accumbens with a specific emphasis on cocaine-directed behaviors.

The Emerging Role of Astrocytic Autophagy in Central Nervous System Disorders

Astrocytes act as "housekeeping cells" for maintaining cerebral homeostasis and play an important role in many disorders. Recent studies further highlight the contribution of autophagy to astrocytic functions, including astrogenesis, the astrocytic removal of neurotoxins or stressors, and astrocytic polarization. More importantly, genetic and pharmacological approaches have provided evidence that outlines the contributions of astrocytic autophagy to several brain disorders, including neurodegeneration, cerebral ischemia, and depression. In this study, we summarize the emerging role of autophagy in regulating astrocytic functions and discuss the contributions of astrocytic autophagy to different CNS disorders.

Control of complex behavior by astrocytes and microglia

Evidence that glial cells influence behavior has been gaining a steady foothold in scientific literature. Out of the five main subtypes of glial cells in the brain, astrocytes and microglia have received an outsized share of attention with regard to shaping a wide spectrum of behavioral phenomena and there is growing appreciation that the signals intrinsic to these cells as well as their interactions with surrounding neurons reflect behavioral history in a brain region-specific manner. Considerable regional diversity of glial cell phenotypes is beginning to be recognized and may contribute to behavioral outcomes arising from circuit-specific computations within and across discrete brain nuclei. Here, we summarize current knowledge on the impact of astrocyte and microglia activity on behavioral outcomes, with a specific focus on brain areas relevant to higher cognitive control, reward-seeking, and circadian regulation.

In Utero Electroporation for Manipulation of Specific Neuronal Populations

The complexity of brain functions is supported by the heterogeneity of brain tissue and millisecond-scale information processing. Understanding how complex neural circuits control animal behavior requires the precise manipulation of specific neuronal subtypes at high spatiotemporal resolution. In utero electroporation, when combined with optogenetics, is a powerful method for precisely controlling the activity of specific neurons. Optogenetics allows for the control of cellular membrane potentials through light-sensitive ion channels artificially expressed in the plasma membrane of neurons. Here, we first review the basic mechanisms and characteristics of in utero electroporation. Then, we discuss recent applications of in utero electroporation combined with optogenetics to investigate the functions and characteristics of specific regions, layers, and cell types. These techniques will pave the way for further advances in understanding the complex neuronal and circuit mechanisms that underlie behavioral outputs.

Repeated cocaine or methamphetamine treatment alters astrocytic CRF2 and GLAST expression in the ventral midbrain

Dopamine neurons in the substantia nigra (SN) and ventral tegmental area (VTA) play a central role in the reinforcing properties of abused drugs including methamphetamine and cocaine. Chronic effects of psychostimulants in the SN/VTA also involve non-dopaminergic transmitters, including glutamate and the stress-related peptide corticotropin-releasing factor (CRF). In the SN/VTA, astrocytes express a variety of membrane-bound neurotransmitter receptors and transporters that influence neurotransmission. CRF receptor type 2 (CRF2) activity in the VTA is important for stress-induced relapse and drug-seeking behaviour, but the localization of its effects is incompletely understood. Here, we first identified CRF2 transcript in astrocytes of the SN/VTA using RNA-Seq in Aldh1l1;NuTRAP mice and confirmed it using in situ hybridization (RNAscope) in wild-type mice. We then used immunofluorescence to quantify the astrocytic marker protein S100β, glial-specific glutamate/aspartate transporter GLAST, and CRF2 in the SN/VTA following 12 days of treatment (i.p.) with methamphetamine (3 mg/kg), cocaine (10 mg/kg), or saline. We observed a significant decrease in GLAST immunofluorescence in brains of psychostimulant treated mice compared with saline controls. In addition, we observed increased labelling of CRF2 in drug treated groups, a decrease in the number of S100β positive cells, and an increase of co-staining of CRF2 with both S100β and tyrosine hydroxylase (dopamine neurons). Our results suggest a significant interaction between CRF2, GLAST, and astrocytes in the midbrain that emerges with repeated exposure to psychostimulants. These findings provide rationale for future investigation of astrocyte-based strategies for altering cellular and circuit function in response to stress and drug exposure.

Reduced phasic dopamine release and slowed dopamine uptake occur in the nucleus accumbens after a diet high in saturated but not unsaturated fat

High-fat diets are linked with obesity and changes in dopamine neurotransmission. Mounting evidence shows that saturated fat impacts dopamine neurons and their terminal fields, but little is known about the effect a diet high in unsaturated fat has on the dopamine system. This study sought to determine whether fat type, saturated vs. unsaturated, differentially affected body weight, blood glucose regulation, locomotor behavior, and control of dopamine release and uptake at dopamine neuron terminals in the nucleus accumbens (NAc). C57BL/6 mice were fed a control diet or a nutrient-matched diet high in saturated fat (SF), unsaturated flaxseed oil (Flax) or a blend of the two fats. After 6-weeks, mice from each high-fat diet group gained significantly more weight than Controls, but the group fed Flax gained less weight than the SF group and had fasting blood glucose levels similar to Controls. Ex-vivo fast scan cyclic voltammetry revealed the SF group also had significantly slower synaptic dopamine clearance and a reduced capacity for phasic dopamine release in the nucleus accumbens (NAc), but the Flax and Blend groups resembled Controls. These data show that different types of dietary fat have substantially different effects on metabolic phenotype and influence how dopamine terminals in the NAc regulate dopamine neurotransmission. Our data also suggests that a diet high in unsaturated fat may preserve normal metabolic and behavioral parameters as well as dopamine signaling in the NAc.

Advances in optogenetic studies of depressive-like behaviors and underlying neural circuit mechanisms

BACKGROUNDS

The neural circuit mechanisms underlying depression remain unclear. Recently optogenetics has gradually gained recognition as a novel technique to regulate the activity of neurons with light stimulation. Scientists are now transferring their focus to the function of brain regions and neural circuits in the pathogenic progress of depression. Deciphering the circuitry mechanism of depressive-like behaviors may help us better understand the symptomatology of depression. However, few studies have summarized current progress on optogenetic researches into the neural circuit mechanisms of depressive-like behaviors.

AIMS

This review aimed to introduce fundamental characteristics and methodologies of optogenetics, as well as how this technique achieves specific neuronal control with spatial and temporal accuracy. We mainly summarized recent progress in neural circuit discoveries in depressive-like behaviors using optogenetics and exhibited the potential of optogenetics as a tool to investigate the mechanism and possible optimization underlying antidepressant treatment such as ketamine and deep brain stimulation.

METHODS

A systematic review of the literature published in English mainly from 2010 to the present in databases was performed. The selected literature is then categorized and summarized according to their neural circuits and depressive-like behaviors.

CONCLUSIONS

Many important discoveries have been made utilizing optogenetics. These findings support optogenetics as a powerful and potential tool for studying depression. And our comprehension to the etiology of depression and other psychiatric disorders will also be more thorough with this rapidly developing technique in the near future.

Astroglia in the Vulnerability and Maintenance of Alcohol Use Disorders

Changes induced in the morphology and the multiplicity of functional roles played by astrocytes in brain regions critical to the establishment and maintenance of alcohol abuse suggest that they make an important contribution to the vulnerability to alcohol use disorders. The understanding of the relevant mechanisms accounting for that contribution is complicated by the fact that alcohol itself acts directly on astrocytes altering their metabolism, gene expression, and plasticity, so that the ultimate result is a complex interaction of various cellular pathways, including intracellular calcium regulation, neuroimmune responses, and regulation of neurotransmitter and gliotransmitter release and uptake. The recent years have seen a steady increase in the characterization of several of the relevant mechanisms, but much remains to be done for a full understanding of the astrocytes' contribution to the vulnerability to alcohol dependence and abuse and for using that knowledge in designing effective therapies for AUDs.

Astrocytes in Addictive Disorders

Astroglia are key regulators of synaptic function, playing central roles in homeostatic ion buffering, energy dynamics, transmitter uptake, maintenance of neurotransmitter pools, and regulation of synaptic plasticity through release of neuroactive chemicals. Given the myriad of crucial homeostatic and signaling functions attributed to astrocytes and the variety of neurotransmitter receptors expressed by astroglia, they serve as prime cellular candidates for establishing maladaptive synaptic plasticity following drug exposure. Initial studies on astroglia and addiction have placed drug-mediated disruptions in the homeostatic regulation of glutamate as a central aspect of relapse vulnerability. However, the generation of sophisticated tools to study and manipulate astroglia have proven that the interaction between addictive substances, astroglia, and relapse-relevant synaptic plasticity extends far beyond the homeostatic regulation of glutamate. Here we present astroglial systems impacted by drug exposure and discuss how changes in astroglial biology contribute to addiction biology.

Astrocytes and neuropsychiatric symptoms in neurodegenerative diseases: Exploring the missing links

Neurodegenerative diseases (NDs) are characterized by primary symptoms, such as cognitive or motor deficits. In addition, the presence of neuropsychiatric symptoms (NPS) in patients with ND is being increasingly acknowledged as an important disease feature. Yet, their neurobiological basis remains unclear and mostly centered on neurons while overlooking astrocytes, which are crucial regulators of neuronal function underlying complex behaviors. In this opinion article, we briefly review evidence for NPS in ND and discuss their experimental assessment in preclinical models. We then present recent studies showing that astrocyte-specific dysfunctions can lead to NPS. Because many astrocyte alterations are also observed in ND, we suggest that they might underlie ND-associated NPS. We argue that there is a need for dedicated preclinical studies assessing astrocyte-based therapeutic strategies targeting NPS in the context of ND.

Opioid-Induced Molecular and Cellular Plasticity of Ventral Tegmental Area Dopamine Neurons

Opioid drugs are highly valued as potent analgesics; however, there are significant risks associated with long-term use because of their abuse liability. Opioids cause changes in ventral tegmental area (VTA) gene expression and cell activity that have been linked to addiction-related behaviors in rodent models. Here, we focus on VTA dopamine (DA) neurons and review the cellular, structural, and synaptic plasticity changes induced by acute and chronic opioid exposure. We also discuss many avenues for future research including determination of whether opioid neuroadaptations are specific for subpopulations of VTA DA neurons. A better understanding of the molecular adaptations within the cells and circuits that drive opioid abuse is crucial for the development of better treatments for substance use disorders and to create novel, safer pain-relieving therapeutics.

Astrocytic Regulation of Neural Circuits Underlying Behaviors

Astrocytes, characterized by a satellite-like morphology, are the most abundant type of glia in the central nervous system. Their main functions have been thought to be limited to providing homeostatic support for neurons, but recent studies have revealed that astrocytes actually actively interact with local neural circuits and play a crucial role in information processing and generating physiological and behavioral responses. Here, we review the emerging roles of astrocytes in many brain regions, particularly by focusing on intracellular changes in astrocytes and their interactions with neurons at the molecular and neural circuit levels.

Astrocytes and Behavior

Animal behavior was classically considered to be determined exclusively by neuronal activity, whereas surrounding glial cells such as astrocytes played only supportive roles. However, astrocytes are as numerous as neurons in the mammalian brain, and current findings indicate a chemically based dialog between astrocytes and neurons. Activation of astrocytes by synaptically released neurotransmitters converges on regulating intracellular Ca²⁺ in astrocytes, which then can regulate the efficacy of near and distant tripartite synapses at diverse timescales through gliotransmitter release. Here, we discuss recent evidence on how diverse behaviors are impacted by this dialog. These recent findings support a paradigm shift in neuroscience, in which animal behavior does not result exclusively from neuronal activity but from the coordinated activity of both astrocytes and neurons. Decoding how astrocytes and neurons interact with each other in various brain circuits will be fundamental to fully understanding how behaviors originate and become dysregulated in disease.

Magnetic Resonance Spectroscopy in the Ventral Tegmental Area Distinguishes Responders to Suvorexant Prior to Treatment: A 4-Week Prospective Cohort Study

Background: The ventral tegmental area (VTA; a dopaminergic nucleus) plays an important role in the sleep-wake regulation system including orexin system. In addition to neuronal activity, there is increasing evidence for an important role of glial cells (i.e., astrocytes and microglia) in these systems. The present study examined the utility of magnetic resonance spectroscopy (MRS) for detecting neural and/or glial changes in the VTA to distinguish responders from non-responders before treatment with the orexin receptor antagonist suvorexant. Methods: A total of 50 patients were screened and 9 patients were excluded. The remaining 41 patients with insomnia who have or not a psychiatric disease who were expected to receive suvorexant treatment were included in this study. We compared MRS signals in the VTA between responders to suvorexant and non-responders before suvorexant use. Based on previous reports, suvorexant responders were defined as patients who improved ≥3 points on the Pittsburgh Sleep Quality Index after 4 weeks of suvorexant use. MRS data included choline (reflects non-specific cell membrane breakdown, including of glial cells) and N-acetylaspartate (a decrease reflects neuronal degeneration). Results: Among 41 examined patients, 20 patients responded to suvorexant and 21 patients did not. By MRS, the choline/creatine and phosphorylcreatine ratio in the VTA was significantly high in non-responders compared with responders (p = 0.039) before suvorexant treatment. There was no difference in the N-acetylaspartate/creatine and phosphorylcreatine ratio (p = 0.297) between the two groups. Conclusions: Changes in glial viability in the VTA might be used to distinguish responders to suvorexant from non-responders before starting treatment. These findings may help with more appropriate selection of patients for suvorexant treatment in clinical practice. Further, we provide novel possible evidence for a relationship between glial changes in the VTA and the orexin system, which may aid in the development of new hypnotics focusing on the VTA and/or glial cells.

The temporal pattern of Intracortical Microstimulation pulses elicits distinct temporal and spatial recruitment of cortical neuropil and neurons

OBJECTIVE

The spacing or distribution of stimulation pulses of therapeutic neurostimulation waveforms-referred to here as the Temporal Pattern (TP)-has emerged as an important parameter for tuning the response to deep-brain stimulation and intracortical microstimulation (ICMS). While it has long been assumed that modulating the TP of ICMS may be effective by altering the rate coding of the neural response, it is unclear how it alters the neural response at the neural network level. The present study is designed to elucidate the neural response to TP at the network level.

APPROACH

We use in vivo two-photon imaging of ICMS in mice expressing the calcium sensor Thy1-GCaMP or the glutamate sensor hSyn-iGluSnFr to examine the layer II/III neural response to stimulations with different TPs. We study the neuronal calcium and glutamate response to TPs with the same average frequency (10Hz) and same total charge injection, but varying degrees of bursting. We also investigate one control pattern with an average frequency of 100Hz and 10X the charge injection.

MAIN RESULTS

Stimulation trains with the same average frequency (10 Hz) and same total charge injection but distinct temporal patterns recruits distinct sets of neurons. More-than-half (60% of 309 cells) prefer one temporal pattern over the other. Despite their distinct spatial recruitment patterns, both cells exhibit similar ability to follow 30s trains of both TPs without failing, and they exhibit similar levels of glutamate release during stimulation. Both neuronal calcium and glutamate release train to the bursting TP pattern (~21-fold increase in relative power at the frequency of bursting. Bursting also results in a statistically significant elevation in the correlation between somatic calcium activity and neuropil activity, which we explore as a metric for inhibitory-excitatory tone. Interestingly, soma-neuropil correlation during the bursting pattern is a statistically significant predictor of cell preference for TP, which exposes a key link between inhibitory-excitatory tone. Finally, using mesoscale imaging, we show that both TPs result in distal inhibition during stimulation, which reveals complex spatial and temporal interactions between temporal pattern and inhibitory-excitatory tone in ICMS.

SIGNIFICANCE

Our results may ultimately suggest that TP is a valuable parameter space to modulate inhibitory-excitatory tone as well as distinct network activity in ICMS. This presents a broader mechanism of action than rate coding, as previously thought. By implicating these additional mechanisms, TP may have broader utility in the clinic and should be pursued to expand the efficacy of ICMS therapies.

Chronic optogenetic manipulation of basolateral amygdala astrocytes rescues stress-induced anxiety

Chronic exposure to stressors can disrupt normal brain function and induce anxiety-like behavior and neurobiological alterations in the basolateral amygdala (BLA). Here, we showed that unpredictable chronic mild stress (UCMS) induced anxiety-like behavior, lowered glutamatergic neuronal activity and reactive astrocytes in the BLA. Using optogenetic tools, we found that activation of BLA glutamatergic neurons did not rescue anxiety-like behavior in stressed mice. In contrast, however, optogenetic activation of the BLA astrocytes relieved stress-induced anxiety, and, interestingly, chronic optogenetic manipulation fully restored the UCMS-induced behavioral and neurobiological dysfunctions, including anxiety-like behavior, lower c-Fos expression in the BLA, S100 overexpression in the BLA, and higher serum corticosterone concentration. Thus, our findings suggest that chronic manipulation of BLA astrocytes is a potential therapeutic intervention target for pathological anxiety.

Maternal deprivation induces persistent adaptations in putative dopamine neurons in rat ventral tegmental area: in vivo electrophysiological study

Early life aversive experiences can trigger persistent deficits in neuronal signaling within the mesolimbic pathway, most notably in the dopamine (DA) neurons of the ventral tegmental area (VTA). The identity of such cellular mechanisms currently appears as an important issue. To address this concern, we investigated whether early life maternal deprivation (MD) would affect the electrical activity of VTA DA neurons, via in vivo extracellular single-unit recording. Male Wistar rats were deprived of their dams for 3 h per day from postnatal days (PND) 1-14. Thereafter, the adult animals (PND 70-80) were tested for the discharge activity of putative VTA DA neurons. The VTA DA neurons displayed a decrease in firing rate and an increase in the variability of baseline discharge activity in deprived animals. MD also caused a decrease in burst firing of VTA DA neurons compared to control subjects. In summary, early life MD induces a hypoactive VTA DA system, which may contribute to lifespan psychopathologies.