A suite of engineered mice for interrogating psychedelic drug actions.

Psychedelic drugs like lysergic acid diethylamide (LSD) and psilocybin have emerged as potentially transformative therapeutics for many neuropsychiatric diseases, including depression, anxiety, post-traumatic stress disorder, migraine, and cluster headaches. LSD and psilocybin exert their psychedelic effects via activation of the 5-hydroxytryptamine 2A receptor (HTR2A). Here we provide a suite of engineered mice useful for clarifying the role of HTR2A and HTR2A-expressing neurons in psychedelic drug actions. We first generated Htr2a-EGFP-CT-IRES-CreERT2 mice (CT:C-terminus) to independently identify both HTR2A-EGFP-CT receptors and HTR2A-containing cells thereby providing a detailed anatomical map of HTR2A and identifying cell types that express HTR2A. We also generated a humanized Htr2a mouse line and an additional constitutive Htr2A-Cre mouse line. Psychedelics induced a variety of known behavioral changes in our mice validating their utility for behavioral studies. Finally, electrophysiology studies revealed that extracellular 5-HT elicited a HTR2A-mediated robust increase in firing of genetically-identified pyramidal neurons--consistent with a plasma membrane localization and mode of action. These mouse lines represent invaluable tools for elucidating the molecular, cellular, pharmacological, physiological, behavioral, and other actions of psychedelic drugs in vivo.

Neural circuits regulating prosocial behaviors.

Positive, prosocial interactions are essential for survival, development, and well-being. These intricate and complex behaviors are mediated by an amalgamation of neural circuit mechanisms working in concert. Impairments in prosocial behaviors, which occur in a large number of neuropsychiatric disorders, result from disruption of the coordinated activity of these neural circuits. In this review, we focus our discussion on recent findings that utilize modern approaches in rodents to map, monitor, and manipulate neural circuits implicated in a variety of prosocial behaviors. We highlight how modulation by oxytocin, serotonin, and dopamine of excitatory and inhibitory synaptic transmission in specific brain regions is critical for regulation of adaptive prosocial interactions. We then describe how recent findings have helped elucidate pathophysiological mechanisms underlying the social deficits that accompany neuropsychiatric disorders. We conclude by discussing approaches for the development of more efficacious and targeted therapeutic interventions to ameliorate aberrant prosocial behaviors.

Midbrain projection to the basolateral amygdala encodes anxiety-like but not depression-like behaviors.

Anxiety disorders are complex diseases, and often co-occur with depression. It is as yet unclear if a common neural circuit controls anxiety-related behaviors in both anxiety-alone and comorbid conditions. Here, utilizing the chronic social defeat stress (CSDS) paradigm that induces singular or combined anxiety- and depressive-like phenotypes in mice, we show that a ventral tegmental area (VTA) dopamine circuit projecting to the basolateral amygdala (BLA) selectively controls anxiety- but not depression-like behaviors. Using circuit-dissecting ex vivo electrophysiology and in vivo fiber photometry approaches, we establish that expression of anxiety-like, but not depressive-like, phenotypes are negatively correlated with VTA → BLA dopamine neuron activity. Further, our optogenetic studies demonstrate a causal link between such neuronal activity and anxiety-like behaviors. Overall, these data establish a functional role for VTA → BLA dopamine neurons in bi-directionally controlling anxiety-related behaviors not only in anxiety-alone, but also in anxiety-depressive comorbid conditions in mice.

Systemic enhancement of serotonin signaling reverses social deficits in multiple mouse models for ASD.

Autism spectrum disorder (ASD) is a common set of heterogeneous neurodevelopmental disorders resulting from a variety of genetic and environmental risk factors. A core feature of ASD is impairment in prosocial interactions. Current treatment options for individuals diagnosed with ASD are limited, with no current FDA-approved medications that effectively treat its core symptoms. We recently demonstrated that enhanced serotonin (5-HT) activity in the nucleus accumbens (NAc), via optogenetic activation of 5-HTergic inputs or direct infusion of a specific 5-HT1b receptor agonist, reverses social deficits in a genetic mouse model for ASD based on 16p11.2 copy number variation. Furthermore, the recreational drug MDMA, which is currently being evaluated in clinical trials, promotes sociability in mice due to its 5-HT releasing properties in the NAc. Here, we systematically evaluated the ability of MDMA and a selective 5-HT1b receptor agonist to rescue sociability deficits in multiple different mouse models for ASD. We find that MDMA administration enhances sociability in control mice and reverses sociability deficits in all four ASD mouse models examined, whereas administration of a 5-HT1b receptor agonist selectively rescued the sociability deficits in all six mouse models for ASD. These preclinical findings suggest that pharmacological enhancement of 5-HT release or direct 5-HT1b receptor activation may be therapeutically efficacious in ameliorating some of the core sociability deficits present across etiologically distinct presentations of ASD.

Selective filtering of excitatory inputs to nucleus accumbens by dopamine and serotonin.

The detailed mechanisms by which dopamine (DA) and serotonin (5-HT) act in the nucleus accumbens (NAc) to influence motivated behaviors in distinct ways remain largely unknown. Here, we examined whether DA and 5-HT selectively modulate excitatory synaptic transmission in NAc medium spiny neurons in an input-specific manner. DA reduced excitatory postsynaptic currents (EPSCs) generated by paraventricular thalamus (PVT) inputs but not by ventral hippocampus (vHip), basolateral amygdala (BLA), or medial prefrontal cortex (mPFC) inputs. In contrast, 5-HT reduced EPSCs generated by inputs from all areas except the mPFC. Release of endogenous DA and 5-HT by methamphetamine (METH) and (±)3,4-methylenedioxymethamphetamine (MDMA), respectively, recapitulated these input-specific synaptic effects. Optogenetic inhibition of PVT inputs enhanced cocaine-conditioned place preference, whereas mPFC input inhibition reduced the enhancement of sociability elicited by MDMA. These findings suggest that the distinct, input-specific filtering of excitatory inputs in the NAc by DA and 5-HT contribute to their discrete behavioral effects.

Input-specific modulation of murine nucleus accumbens differentially regulates hedonic feeding.

Hedonic feeding is driven by the "pleasure" derived from consuming palatable food and occurs in the absence of metabolic need. It plays a critical role in the excessive feeding that underlies obesity. Compared to other pathological motivated behaviors, little is known about the neural circuit mechanisms mediating excessive hedonic feeding. Here, we show that modulation of prefrontal cortex (PFC) and anterior paraventricular thalamus (aPVT) excitatory inputs to the nucleus accumbens (NAc), a key node of reward circuitry, has opposing effects on high fat intake in mice. Prolonged high fat intake leads to input- and cell type-specific changes in synaptic strength. Modifying synaptic strength via plasticity protocols, either in an input-specific optogenetic or non-specific electrical manner, causes sustained changes in high fat intake. These results demonstrate that input-specific NAc circuit adaptations occur with repeated exposure to a potent natural reward and suggest that neuromodulatory interventions may be therapeutically useful for individuals with pathologic hedonic feeding.

Dissecting neural mechanisms of prosocial behaviors.

Prosocial behaviors are essential for group cooperation, which enrich life experience and enhance survival. These complex behaviors are governed by intricate interactions between numerous neural circuits functioning in concert. Impairments in prosocial interactions result from disruptions of this coordinated brain activity and are a prominent feature of several pathological conditions including autism spectrum disorder, depression and addiction. Here we highlight recent studies that use advanced techniques to anatomically map, monitor and manipulate neural circuits that influence prosocial behavior. These recent findings provide important clues to unravel the complexities of the neural mechanisms that mediate prosocial interactions and offer insights into new strategies for the treatment of aberrant social behavior.

Loss of the neural-specific BAF subunit ACTL6B relieves repression of early response genes and causes recessive autism.

Synaptic activity in neurons leads to the rapid activation of genes involved in mammalian behavior. ATP-dependent chromatin remodelers such as the BAF complex contribute to these responses and are generally thought to activate transcription. However, the mechanisms keeping such "early activation" genes silent have been a mystery. In the course of investigating Mendelian recessive autism, we identified six families with segregating loss-of-function mutations in the neuronal BAF (nBAF) subunit ACTL6B (originally named BAF53b). Accordingly, ACTL6B was the most significantly mutated gene in the Simons Recessive Autism Cohort. At least 14 subunits of the nBAF complex are mutated in autism, collectively making it a major contributor to autism spectrum disorder (ASD). Patient mutations destabilized ACTL6B protein in neurons and rerouted dendrites to the wrong glomerulus in the fly olfactory system. Humans and mice lacking ACTL6B showed corpus callosum hypoplasia, indicating a conserved role for ACTL6B in facilitating neural connectivity. Actl6b knockout mice on two genetic backgrounds exhibited ASD-related behaviors, including social and memory impairments, repetitive behaviors, and hyperactivity. Surprisingly, mutation of Actl6b relieved repression of early response genes including AP1 transcription factors (Fos, Fosl2, Fosb, and Junb), increased chromatin accessibility at AP1 binding sites, and transcriptional changes in late response genes associated with early response transcription factor activity. ACTL6B loss is thus an important cause of recessive ASD, with impaired neuron-specific chromatin repression indicated as a potential mechanism.

Distinct neural mechanisms for the prosocial and rewarding properties of MDMA.

The extensively abused recreational drug (±)3,4-methylenedioxymethamphetamine (MDMA) has shown promise as an adjunct to psychotherapy for treatment-resistant psychiatric disease. It is unknown, however, whether the mechanisms underlying its prosocial therapeutic effects and abuse potential are distinct. We modeled both the prosocial and nonsocial drug reward of MDMA in mice and investigated the mechanism of these processes using brain region-specific pharmacology, transgenic manipulations, electrophysiology, and in vivo calcium imaging. We demonstrate in mice that MDMA acting at the serotonin transporter within the nucleus accumbens is necessary and sufficient for MDMA's prosocial effect. MDMA's acute rewarding properties, in contrast, require dopaminergic signaling. MDMA's prosocial effect requires 5-HT1b receptor activation and is mimicked by d-fenfluramine, a selective serotonin-releasing compound. By dissociating the mechanisms of MDMA's prosocial effects from its addictive properties, we provide evidence for a conserved neuronal pathway, which can be leveraged to develop novel therapeutics with limited abuse liability.

5-HT release in nucleus accumbens rescues social deficits in mouse autism model.

Dysfunction in prosocial interactions is a core symptom of autism spectrum disorder. However, the neural mechanisms that underlie sociability are poorly understood, limiting the rational development of therapies to treat social deficits. Here we show in mice that bidirectional modulation of the release of serotonin (5-HT) from dorsal raphe neurons in the nucleus accumbens bidirectionally modifies sociability. In a mouse model of a common genetic cause of autism spectrum disorder-a copy number variation on chromosome 16p11.2-genetic deletion of the syntenic region from 5-HT neurons induces deficits in social behaviour and decreases dorsal raphe 5-HT neuronal activity. These sociability deficits can be rescued by optogenetic activation of dorsal raphe 5-HT neurons, an effect requiring and mimicked by activation of 5-HT1b receptors in the nucleus accumbens. These results demonstrate an unexpected role for 5-HT action in the nucleus accumbens in social behaviours, and suggest that targeting this mechanism may prove therapeutically beneficial.

Author Correction: Midbrain circuit regulation of individual alcohol drinking behaviors in mice.

The original version of this Article contained an error in the spelling of the author Scott Edwards, which was incorrectly given as Scott Edward. This has now been corrected in both the PDF and HTML versions of the Article.

Midbrain circuit regulation of individual alcohol drinking behaviors in mice.

Alcohol-use disorder (AUD) is the most prevalent substance-use disorder worldwide. There is substantial individual variability in alcohol drinking behaviors in the population, the neural circuit mechanisms of which remain elusive. Utilizing in vivo electrophysiological techniques, we find that low alcohol drinking (LAD) mice have dramatically higher ventral tegmental area (VTA) dopamine neuron firing and burst activity. Unexpectedly, VTA dopamine neuron activity in high alcohol drinking (HAD) mice does not differ from alcohol naive mice. Optogenetically enhancing VTA dopamine neuron burst activity in HAD mice decreases alcohol drinking behaviors. Circuit-specific recordings reveal that spontaneous activity of nucleus accumbens-projecting VTA (VTA-NAc) neurons is selectively higher in LAD mice. Specifically activating this projection is sufficient to reduce alcohol consumption in HAD mice. Furthermore, we uncover ionic and cellular mechanisms that suggest unique neuroadaptations between the alcohol drinking groups. Together, these data identify a neural circuit responsible for individual alcohol drinking behaviors.

Gating of social reward by oxytocin in the ventral tegmental area.

The reward generated by social interactions is critical for promoting prosocial behaviors. Here we present evidence that oxytocin (OXT) release in the ventral tegmental area (VTA), a key node of the brain's reward circuitry, is necessary to elicit social reward. During social interactions, activity in paraventricular nucleus (PVN) OXT neurons increased. Direct activation of these neurons in the PVN or their terminals in the VTA enhanced prosocial behaviors. Conversely, inhibition of PVN OXT axon terminals in the VTA decreased social interactions. OXT increased excitatory drive onto reward-specific VTA dopamine (DA) neurons. These results demonstrate that OXT promotes prosocial behavior through direct effects on VTA DA neurons, thus providing mechanistic insight into how social interactions can generate rewarding experiences.

Basal forebrain projections to the lateral habenula modulate aggression reward.

Maladaptive aggressive behaviour is associated with a number of neuropsychiatric disorders and is thought to result partly from the inappropriate activation of brain reward systems in response to aggressive or violent social stimuli. Nuclei within the ventromedial hypothalamus, extended amygdala and limbic circuits are known to encode initiation of aggression; however, little is known about the neural mechanisms that directly modulate the motivational component of aggressive behaviour. Here we established a mouse model to measure the valence of aggressive inter-male social interaction with a smaller subordinate intruder as reinforcement for the development of conditioned place preference (CPP). Aggressors develop a CPP, whereas non-aggressors develop a conditioned place aversion to the intruder-paired context. Furthermore, we identify a functional GABAergic projection from the basal forebrain (BF) to the lateral habenula (lHb) that bi-directionally controls the valence of aggressive interactions. Circuit-specific silencing of GABAergic BF-lHb terminals of aggressors with halorhodopsin (NpHR3.0) increases lHb neuronal firing and abolishes CPP to the intruder-paired context. Activation of GABAergic BF-lHb terminals of non-aggressors with channelrhodopsin (ChR2) decreases lHb neuronal firing and promotes CPP to the intruder-paired context. Finally, we show that altering inhibitory transmission at BF-lHb terminals does not control the initiation of aggressive behaviour. These results demonstrate that the BF-lHb circuit has a critical role in regulating the valence of inter-male aggressive behaviour and provide novel mechanistic insight into the neural circuits modulating aggression reward processing.

KCNQ channel openers reverse depressive symptoms via an active resilience mechanism.

Less than half of patients suffering from major depressive disorder, a leading cause of disability worldwide, achieve remission with current antidepressants, making it imperative to develop more effective treatment. A new therapeutic direction is emerging from the increased understanding of natural resilience as an active stress-coping process. It is known that potassium (K(+)) channels in the ventral tegmental area (VTA) are an active mediator of resilience. However, no druggable targets have been identified to potentiate active resilience mechanisms. In the chronic social defeat stress model of depression, we report that KCNQ-type K(+) channel openers, including FDA-approved drug retigabine (ezogabine), show antidepressant efficacy. We demonstrate that overexpression of KCNQ channels in the VTA dopaminergic neurons and either local infusion or systemic administration of retigabine normalized neuronal hyperactivity and depressive behaviours. These findings identify KCNQ as a target for conceptually novel antidepressants that function through the potentiation of active resilience mechanisms.

Essential Role of Mesolimbic Brain-Derived Neurotrophic Factor in Chronic Social Stress-Induced Depressive Behaviors.

BACKGROUND: Previous work has shown that chronic social defeat stress (CSDS) induces increased phasic firing of ventral tegmental area (VTA) dopamine (DA) neurons that project to the nucleus accumbens (NAc) selectively in mice that are susceptible to the deleterious effects of the stress. In addition, acute optogenetic phasic stimulation of these neurons promotes susceptibility in animals exposed to acute defeat stress. These findings are paradoxical, as increased DA signaling in NAc normally promotes motivation and reward, and the influence of chronic phasic VTA firing in the face of chronic stress is unknown. METHODS: We used CSDS with repeated optogenetic activation and pharmacologic manipulations of the mesolimbic VTA-NAc pathway to examine the role of brain-derived neurotrophic factor (BDNF) and DA signaling in depressive-like behaviors. We measured BDNF protein expression and DA release in this model. RESULTS: Pharmacologic blockade of BDNF-tyrosine receptor kinase B (TrkB) signaling, but not DA signaling, in NAc prevented CSDS-induced behavioral abnormalities. Chronic optogenetic phasic stimulation of the VTA-NAc circuit during CSDS exacerbated the defeat-induced behavioral symptoms, and these aggravated symptoms were also normalized by BDNF-TrkB blockade in NAc. The aggravated behavioral deficits induced by phasic stimulation of the VTA-NAc pathway were blocked as well by local knockdown of BDNF in VTA. CONCLUSIONS: These findings show that BDNF-TrkB signaling, rather than DA signaling, in the VTA-NAc circuit is crucial for facilitating depressive-like outcomes after CSDS and they establish BDNF-TrkB signaling as a pathologic mechanism during periods of chronic stress.

Excitatory transmission at thalamo-striatal synapses mediates susceptibility to social stress.

Postsynaptic remodeling of glutamatergic synapses on ventral striatum (vSTR) medium spiny neurons (MSNs) is critical for shaping stress responses. However, it is unclear which presynaptic inputs are involved. Susceptible mice exhibited increased synaptic strength at intralaminar thalamus (ILT), but not prefrontal cortex (PFC), inputs to vSTR MSNs following chronic social stress. Modulation of ILT-vSTR versus PFC-vSTR neuronal activity differentially regulated dendritic spine plasticity and social avoidance.

Locus-specific epigenetic remodeling controls addiction- and depression-related behaviors.

Chronic exposure to drugs of abuse or stress regulates transcription factors, chromatin-modifying enzymes and histone post-translational modifications in discrete brain regions. Given the promiscuity of the enzymes involved, it has not yet been possible to obtain direct causal evidence to implicate the regulation of transcription and consequent behavioral plasticity by chromatin remodeling that occurs at a single gene. We investigated the mechanism linking chromatin dynamics to neurobiological phenomena by applying engineered transcription factors to selectively modify chromatin at a specific mouse gene in vivo. We found that histone methylation or acetylation at the Fosb locus in nucleus accumbens, a brain reward region, was sufficient to control drug- and stress-evoked transcriptional and behavioral responses via interactions with the endogenous transcriptional machinery. This approach allowed us to relate the epigenetic landscape at a given gene directly to regulation of its expression and to its subsequent effects on reward behavior.

Enhancing depression mechanisms in midbrain dopamine neurons achieves homeostatic resilience.

Typical therapies try to reverse pathogenic mechanisms. Here, we describe treatment effects achieved by enhancing depression-causing mechanisms in ventral tegmental area (VTA) dopamine (DA) neurons. In a social defeat stress model of depression, depressed (susceptible) mice display hyperactivity of VTA DA neurons, caused by an up-regulated hyperpolarization-activated current (I(h)). Mice resilient to social defeat stress, however, exhibit stable normal firing of these neurons. Unexpectedly, resilient mice had an even larger I(h), which was observed in parallel with increased potassium (K(+)) channel currents. Experimentally further enhancing Ih or optogenetically increasing the hyperactivity of VTA DA neurons in susceptible mice completely reversed depression-related behaviors, an antidepressant effect achieved through resilience-like, projection-specific homeostatic plasticity. These results indicate a potential therapeutic path of promoting natural resilience for depression treatment.

Stress and CRF gate neural activation of BDNF in the mesolimbic reward pathway.

Mechanisms controlling release of brain-derived neurotrophic factor (BDNF) in the mesolimbic dopamine reward pathway remain unknown. We report that phasic optogenetic activation of this pathway increases BDNF amounts in the nucleus accumbens (NAc) of socially stressed mice but not of stress-naive mice. This stress gating of BDNF signaling is mediated by corticotrophin-releasing factor (CRF) acting in the NAc. These results unravel a stress context-detecting function of the brain's mesolimbic circuit.