Modulation of 5-HT release by dynorphin mediates social deficits during opioid withdrawal

Neural circuits and therapeutic mechanisms of empathic pain

Empathy is the capacity to understand and share the experiences of others. This ability fosters connections between individuals, enriching the fabric of our shared world. One notable example is empathy for the pain of others. Such experiences facilitate the identification of potential dangers, both for oneself and for others. Neuroimaging studies have helped to pinpoint brain regions that modulate empathic pain. Recently, there has also been a surge in studies exploring the neural mechanisms of empathic pain in rodent models. Neuropsychiatric disorders such as autism, psychosis, and schizophrenia often exhibit empathy deficits. Targeting the modulation of empathic pain holds potential for alleviating core symptoms in these patients. Interestingly, empathy research may also benefit pain management, leading to new approaches for understanding the negative emotions associated with pain. This review summarizes recent advances in neuroimaging for the study of empathic pain, outlines the underlying neurocircuit mechanisms, describes therapeutic strategies, and explores promising avenues for future research. This article is part of the Special Issue on "Empathic Pain".

BLA KOR inputs to the BNST regulate social stress-escalated alcohol consumption

Background

Aversive social experiences can lead to escalated drug consumption and increase the risk of relapse to drug seeking. Individuals who consume alcohol to alleviate the effects of social stress are more likely to develop an alcohol use disorder (AUD). Repeated social defeat stress (SDS) enhances the rewarding and reinforcing effects of alcohol. However, the neural mechanisms that underlie social stress-escalated alcohol drinking are not well understood. Here we explored the role of the dynorphin/kappa opioid receptor (Dyn/KOR) system in regulating social stress-escalated alcohol consumption.

Methods

Male and female mice were subjected to repeated SDS for 10 days following which they were left undisturbed in their home cages. They were then subject to intermittent access (IA) two-bottle choice alcohol consumption procedure. The effects of systemic and BNST-specific KOR antagonism using the selective KOR antagonist NorBNI on stress-escalated drinking were evaluated. Using chemogenetic approaches in Oprk1-Cre mice, we examined the role of KOR expressing cells in the basolateral amygdala (BLA KORs ) and BLA KOR -BNST pathway in social stress-escalated alcohol consumption.

Results

Repeated SDS increased alcohol consumption and preference in both males and females. Systemic KOR antagonism attenuated SDS-escalated alcohol consumption in both males and females. BNST -specific KOR antagonism also attenuated stress-escalated drinking in males. Finally, selective chemogenetic activation of BLA KORs and BKA KOR -BNST pathway attenuated social stress-escalated alcohol consumption in both sexes.

Conclusion

Our results suggest a significant role for BLA KOR projections to the BNST in regulating social stress-escalated alcohol consumption. Our results provide further evidence that the Dyn/KOR system maybe a viable target for medications development to tareat comorbid stress and AUD.

5-HT2C receptors in the nucleus accumbens constrain the rewarding effects of MDMA

MDMA is a promising adjunct to psychotherapy and has well-known abuse liability, although less than other amphetamine analogs. While the reinforcing dopamine (DA)-releasing properties of MDMA are on par with methamphetamine (METH), MDMA is a far more potent serotonin (5-HT) releaser, via the 5-HT transporter (SERT). MDMA-mediated 5-HT release in a major reward center, the nucleus accumbens (NAc), drives prosocial behaviors via 5-HT1BR activation. We hypothesized that this prosocial mechanism contributes to the reduced reinforcing properties of MDMA compared to METH and used a platform of assays to predict the balance of prosocial and abuse-linked effects of (R)-MDMA, a novel entactogen in clinical development. NAc DA release, measured by GRAB-DA photometry in vivo, increased in proportion to MDMA (7.5 and 15 mg/kg, i.p.) and METH (2 mg/kg i.p.)-conditioned place preference (CPP). Using conditional knockouts (cKOs) for DAT and SERT, microdialysis, and photometry, we found that MDMA-released 5-HT limited MDMA-released DA through actions in the NAc, rather than at ventral tegmental area DAergic cell bodies. SERT cKO reduced the MDMA dose required for CPP three-fold. This enhanced MDMA-CPP and increased DA release were replicated by intra-NAc infusion of either a 5-HT reuptake inhibitor (escitalopram) to prevent MDMA interaction with SERT, or a 5-HT2CR antagonist (SB242084), but not by the 5-HT1BR antagonist NAS-181. These data support separate mechanisms for the low abuse potential versus prosocial effect of MDMA. Using this platform of assays, (R)-MDMA is predicted to have prosocial effects and low abuse potential.

Genetically encoded sensors for in vivo detection of neurochemicals relevant to depression

Depressive disorders are a common and debilitating form of mental illness with significant impacts on individuals and society. Despite the high prevalence, the underlying causes and mechanisms of depressive disorders are still poorly understood. Neurochemical systems, including serotonin, norepinephrine, and dopamine, have been implicated in the development and perpetuation of depressive symptoms. Current treatments for depression target these neuromodulator systems, but there is a need for a better understanding of their role in order to develop more effective treatments. Monitoring neurochemical dynamics during depressive symptoms is crucial for gaining a better a understanding of their involvement in depressive disorders. Genetically encoded sensors have emerged recently that offer high spatial-temporal resolution and the ability to monitor neurochemical dynamics in real time. This review explores the neurochemical systems involved in depression and discusses the applications and limitations of current monitoring tools for neurochemical dynamics. It also highlights the potential of genetically encoded sensors for better characterizing neurochemical dynamics in depression-related behaviors. Furthermore, potential improvements to current sensors are discussed in order to meet the requirements of depression research.

The nucleus accumbens in reward and aversion processing: insights and implications

The nucleus accumbens (NAc), a central component of the brain's reward circuitry, has been implicated in a wide range of behaviors and emotional states. Emerging evidence, primarily drawing from recent rodent studies, suggests that the function of the NAc in reward and aversion processing is multifaceted. Prolonged stress or drug use induces maladaptive neuronal function in the NAc circuitry, which results in pathological conditions. This review aims to provide comprehensive and up-to-date insights on the role of the NAc in motivated behavior regulation and highlights areas that demand further in-depth analysis. It synthesizes the latest findings on how distinct NAc neuronal populations and pathways contribute to the processing of opposite valences. The review examines how a range of neuromodulators, especially monoamines, influence the NAc's control over various motivational states. Furthermore, it delves into the complex underlying mechanisms of psychiatric disorders such as addiction and depression and evaluates prospective interventions to restore NAc functionality.

Dynorphin modulates motivation through a pallido-amygdala cholinergic circuit

The endogenous opioid peptide dynorphin and its receptor κ-opioid receptor (KOR) have been implicated in divergent behaviors, but the underlying mechanisms remain elusive. Here we show that dynorphin released from nucleus accumbens dynorphinergic neurons exerts powerful modulation over a ventral pallidum (VP) disinhibitory circuit, thereby controlling cholinergic transmission to the amygdala and motivational drive in mice. On one hand, dynorphin acts postsynaptically via KORs on local GABAergic neurons in the VP to promote disinhibition of cholinergic neurons, which release acetylcholine into the amygdala to invigorate reward-seeking behaviors. On the other hand, dynorphin also acts presynaptically via KORs on dynorphinergic terminals to limit its own release. Such autoinhibition keeps cholinergic neurons from prolonged activation and release of acetylcholine, and prevents perseverant reward seeking. Our study reveals how dynorphin exquisitely modulate motivation through cholinergic system, and provides an explanation for why these neuromodulators are involved in motivational disorders, including depression and addiction.

VTA μ-opioidergic neurons facilitate low sociability in protracted opioid withdrawal

Opioids initiate dynamic maladaptation in brain reward and affect circuits that occur throughout chronic exposure and withdrawal that persist beyond cessation. Protracted withdrawal is characterized by negative affective behaviors such as heightened anxiety, irritability, dysphoria, and anhedonia, which pose a significant risk factor for relapse. While the ventral tegmental area (VTA) and mu-opioid receptors (MORs) are critical for opioid reinforcement, the specific contributions of VTAMOR neurons in mediating protracted withdrawal-induced negative affect is not fully understood. In our study, we elucidate the role of VTAMOR neurons in mediating negative affect and altered brain-wide neuronal activities following opioid exposure and withdrawal in male and female mice. Utilizing a chronic oral morphine administration model, we observe increased social deficit, anxiety-related, and despair-like behaviors during protracted withdrawal. VTAMOR neurons show heightened neuronal FOS activation at the onset of withdrawal and connect to an array of brain regions that mediate reward and affective processes. Viral re-expression of MORs selectively within the VTA of MOR knockout mice demonstrates that the disrupted social interaction observed during protracted withdrawal is facilitated by this neural population, without affecting other protracted withdrawal behaviors. Lastly, VTAMORs contribute to heightened neuronal FOS activation in the anterior cingulate cortex (ACC) in response to an acute morphine challenge, suggesting their unique role in modulating ACC-specific neuronal activity. These findings identify VTAMOR neurons as critical modulators of low sociability during protracted withdrawal and highlight their potential as a mechanistic target to alleviate negative affective behaviors associated with opioid withdrawal.

Kappa Opioid Receptor Antagonism Restores Phosphorylation, Trafficking and Behavior induced by a Disease Associated Dopamine Transporter Variant

Aberrant dopamine (DA) signaling is implicated in schizophrenia, bipolar disorder (BPD), autism spectrum disorder (ASD), substance use disorder, and attention-deficit/hyperactivity disorder (ADHD). Treatment of these disorders remains inadequate, as exemplified by the therapeutic use of d-amphetamine and methylphenidate for the treatment of ADHD, agents with high abuse liability. In search for an improved and non-addictive therapeutic approach for the treatment of DA-linked disorders, we utilized a preclinical mouse model expressing the human DA transporter (DAT) coding variant DAT Val559, previously identified in individuals with ADHD, ASD, or BPD. DAT Val559, like several other disease-associated variants of DAT, exhibits anomalous DA efflux (ADE) that can be blocked by d-amphetamine and methylphenidate. Kappa opioid receptors (KORs) are expressed by DA neurons and modulate DA release and clearance, suggesting that targeting KORs might also provide an alternative approach to normalizing DA-signaling disrupted by perturbed DAT function. Here we demonstrate that KOR stimulation leads to enhanced surface trafficking and phosphorylation of Thr53 in wildtype DAT, effects achieved constitutively by the Val559 mutant. Moreover, these effects can be rescued by KOR antagonism of DAT Val559 in ex vivo preparations. Importantly, KOR antagonism also corrected in vivo DA release as well as sex-dependent behavioral abnormalities observed in DAT Val559 mice. Given their low abuse liability, our studies with a construct valid model of human DA associated disorders reinforce considerations of KOR antagonism as a pharmacological strategy to treat DA associated brain disorders.

Substance Addiction Rehabilitation Drugs

The relapse rate of substance abusers is high, and addiction rehabilitation adjunct drugs need to be developed urgently. There have been numerous reports on blocking the formation of substance addiction, but studies on drugs that can alleviate withdrawal symptoms are very limited. Both the dopamine transporter (DAT) hypothesis and D3 dopamine receptor (D3R) hypothesis are proposed. DAT activators reduce the extracellular dopamine level, and D3R antagonists reduce the neuron's sensitivity to dopamine, both of which may exacerbate the withdrawal symptoms subsequently. The D3R partial agonist SK608 has biased signaling properties via the G-protein-dependent pathway but did not induce D3R desensitization and, thus, may be a promising drug for the withdrawal symptoms. Drugs for serotoninergic neurons or GABAergic neurons and anti-inflammatory drugs may have auxiliary effects to addiction treatments. Drugs that promote structural synaptic plasticity are also discussed.

Neuroscience in addiction research

The prevention and treatment of addiction (moderate to severe substance use disorder-SUD) have remained challenging because of the dynamic and complex interactions between multiple biological and social determinants that shape SUD. The pharmacological landscape is ever changing and the use of multiple drugs is increasingly common, requiring an unraveling of pharmacological interactions to understand the effects. There are different stages in the trajectory from drug use to addiction that are characterized by distinct cognitive and emotional features. These are directed by different neurobiological processes that require identification and characterization including those that underlie the high co-morbidity with other disorders. Finally, there is substantial individual variability in the susceptibility to develop SUD because there are multiple determinants, including genetics, sex, developmental trajectories and times of drug exposures, and psychosocial and environmental factors including commercial determinants that influence drug availability. Elucidating how these factors interact to determine risk is essential for identifying the biobehavioral basis of addiction and developing prevention and treatment strategies. Basic research is tasked with addressing each of these challenges. The recent proliferation of technological advances that allow for genetic manipulation, visualization of molecular reactions and cellular activity in vivo, multiscale whole brain mapping across the life span, and the mining of massive data sets including multimodality human brain imaging are accelerating our ability to understand how the brain functions and how drugs influence it. Here, we highlight how the application of these tools to the study of addiction promises to illuminate its neurobiological basis and guide strategies for prevention and treatment.

The role of brain serotonin signaling in excessive alcohol consumption and withdrawal: A call for more research in females

Alcohol Use Disorder (AUD) is a leading cause of death and disability worldwide, but current treatments are insufficient in fully addressing the symptoms that often lead to relapses in alcohol consumption. The brain's serotonin system has been implicated in AUD for decades and is a major regulator of stress-related behaviors associated with increased alcohol consumption. This review will discuss the current literature on the association between neurobiological adaptations in serotonin systems and AUD in humans as well as the effectiveness of serotonin receptor manipulations on alcohol-related behaviors like consumption and withdrawal. We will further discuss how these findings in humans relate to findings in animal models, including a comparison of systemic pharmacological manipulations modulating alcohol consumption. We next provide a detailed overview of brain region-specific roles for serotonin and serotonin receptor signaling in alcohol-related behaviors in preclinical animal models, highlighting the complexity of forming a cohesive model of serotonin function in AUD and providing possible avenues for more effective therapeutic intervention. Throughout the review, we discuss what is known about sex differences in the sequelae of AUD and the role of serotonin in these sequelae. We stress a critical need for additional studies in women and female animals so that we may build a clearer path to elucidating sex-specific serotonergic mechanisms and develop better treatments.

Laurent R, Pomrenze MB, Boroon P, Heifets B, Smith M, Malenka RC. MDMA enhances empathy-like behaviors in mice via 5-HT release in the nucleus accumbens

MDMA (3,4-methylenedioxymethamphetamine) is a psychoactive drug with powerful prosocial effects. While MDMA is sometimes termed an "empathogen," empirical studies have struggled to clearly demonstrate these effects or pinpoint underlying mechanisms. Here, we paired the social transfer of pain and analgesia-behavioral tests modeling empathy in mice-with region-specific neuropharmacology, optogenetics, and transgenic manipulations to explore MDMA's action as an empathogen. We report that MDMA, given intraperitoneally or infused directly into the nucleus accumbens (NAc), robustly enhances the social transfer of pain and analgesia. Optogenetic stimulation of 5-HT release in the NAc recapitulates the effects of MDMA, implicating 5-HT signaling as a core mechanism. Last, we demonstrate that systemic MDMA or optogenetic stimulation of NAc 5-HT inputs restores deficits in empathy-like behaviors in the Shank3-deficient mouse model of autism. These findings demonstrate enhancement of empathy-related behaviors by MDMA and implicate 5-HT signaling in the NAc as a core mechanism mediating MDMA's empathogenic effects.

Mindfulness-induced self-transcendence promotes universal love with consequent effects on opioid misuse

In addition to its health benefits, mindfulness has been theorized in classical contemplative frameworks to elicit self-transcendent experiences as a means of promoting universal love and compassion. Increasing feelings of love may be especially clinically relevant for the treatment of opioid misuse, in that addictive use of opioids dysregulates neurobiological processes implicated in the experience of love. Here we tested these hypotheses in a secondary analysis (n = 187) of data from a randomized clinical trial of Mindfulness-Oriented Recovery Enhancement (MORE) versus supportive psychotherapy for comorbid opioid misuse and chronic pain. At pre- and post-treatment, participants completed a measure of state self-transcendence immediately following a laboratory-based mindfulness task. Through 9-month follow-up, we assessed changes in universal love and opioid misuse. Participants also completed ecological momentary assessments of opioid craving during the 8-week study interventions and for the following month. Compared to supportive psychotherapy, participants in MORE reported significantly greater increases in mindfulness-induced self-transcendence, which mediated the effect of MORE on increased feelings of universal love. In turn, increases in universal love significantly predicted decreased opioid craving and lower odds opioid misuse through 1- and 9-month follow-ups, respectively. Findings suggest mindfulness-induced self-transcendence may promote feelings of universal love, with possible downstream benefits on reducing addictive behavior.

Genetic polymorphism in HTR2A rs6313 is associated with internet addiction disorder

Introduction

Internet addiction disorder (IAD) has grown into public health concern of global proportions. Previous studies have indicated that individuals with IAD may exhibit altered levels of serotonin and dopamine, which are known to play crucial roles in depression, anxiety, impulsivity, and addiction. Therefore, polymorphisms in the receptors that mediate the effects of serotonin and dopamine and affect their functional states as well as their activities are suspect. In this study, we aimed to investigate the association between IAD and rs6313 (T102C) polymorphism in the serotonin 2A receptor (5-HT2A) gene, (HTR2A).

Methods

Twenty patients with IAD and twenty healthy controls (HCs) were included in this study. Young's Internet Addiction Test (IAT), Self-Rating Anxiety Scale, Self-Rating Depression Scale, Yale-Brown Obsessive-Compulsive Scale (Y-BOCS), Barratt Impulse Scale, Pittsburgh Sleep Quality Index (PSQI), and Social Support Rating Scale (SSRS) were used to assess the severity of internet addiction, mental status, impulsive traits, sleep quality, and social support. Genotyping was performed to identify rs6313 polymorphisms in the HTR2A gene of all participants.

Results

The frequencies of the C and T alleles of HTR2A T102C were 28% and 72% in the IAD group and 53% and 47% in the HCs group, respectively, indicating that the differences between these two groups were significant. No significant difference was observed in the distribution of the CC, CT, and TT genotypes of HTR2A gene T102C between the IAD and the HCs groups. Additionally, there was no difference in the distribution of the frequencies of the HTR2A gene T102C CC and CT+TT genotypes between the two groups. However, the distribution between the TT and CC+CT genotypes showed an apparent statistical difference in the HTR2A gene T102C between the two groups. Correlation analysis indicated that the IAT score was positively correlated with the Y-BOCS and BIS scores for the CC+CT genotype in patients with IAD. Moreover, the IAT score was positively correlated with the PSQI score in patients with IAD carrying the TT genotype.

Conclusion

The present study demonstrates that rs6313 in HTR2A is associated with IAD, and that the T allele of rs6313 in HTR2A may be a risk factor for IAD.

Making Sense of Psychedelics in the CNS

For centuries, ancient lineages have consumed psychedelic compounds from natural sources. In the modern era, scientists have since harnessed the power of computational tools, cellular assays, and behavioral metrics to study how these compounds instigate changes on molecular, cellular, circuit-wide, and system levels. Here, we provide a brief history of psychedelics and their use in science, medicine, and culture. We then outline current techniques for studying psychedelics from a pharmacological perspective. Finally, we address known gaps in the field and potential avenues of further research to broaden our collective understanding of physiological changes induced by psychedelics, the limits of their therapeutic capabilities, and how researchers can improve and inform treatments that are rapidly becoming accessible worldwide.

Role of α1-GABAA receptors in the serotonergic dorsal raphe nucleus in models of opioid reward, anxiety, and depression

BACKGROUND

The serotonin (5-hydroxytryptamine (5-HT))-mediated system plays an important role in stress-related psychiatric disorders and substance abuse. Our previous studies showed that stress and drug exposure can modulate the dorsal raphe nucleus (DRN)-5-HT system via γ-aminobutyric acid (GABA)A receptors. Moreover, GABAA receptor-mediated inhibition of serotonergic DRN neurons is required for stress-induced reinstatement of opioid seeking.

AIM/METHODS

To further test the role of GABAA receptors in the 5-HT system in stress and opioid-sensitive behaviors, our current study generated mice with conditional genetic deletions of the GABAA α1 subunit to manipulate GABAA receptors in either the DRN or the entire population of 5-HT neurons. The GABAA α1 subunit is a constituent of the most abundant GABAA subtype in the brain and the most highly expressed subunit in 5-HT DRN neurons.

RESULTS

Our results showed that mice with DRN-specific knockout of α1-GABAA receptors exhibited a normal phenotype in tests of anxiety- and depression-like behaviors as well as swim stress-induced reinstatement of morphine-conditioned place preference. By contrast, mice with 5-HT neuron-specific knockout of α1-GABAA receptors exhibited an anxiolytic phenotype at baseline and increased sensitivity to post-morphine withdrawal-induced anxiety.

CONCLUSIONS

Our data suggest that GABAA receptors on 5-HT neurons contribute to anxiety-like behaviors and sensitivity of those behaviors to opioid withdrawal.

Therapeutic mechanisms of psychedelics and entactogens

Recent clinical and preclinical evidence suggests that psychedelics and entactogens may produce both rapid and sustained therapeutic effects across several indications. Currently, there is a disconnect between how these compounds are used in the clinic and how they are studied in preclinical species, which has led to a gap in our mechanistic understanding of how these compounds might positively impact mental health. Human studies have emphasized extra-pharmacological factors that could modulate psychedelic-induced therapeutic responses including set, setting, and integration-factors that are poorly modelled in current animal experiments. In contrast, animal studies have focused on changes in neuronal activation and structural plasticity-outcomes that are challenging to measure in humans. Here, we describe several hypotheses that might explain how psychedelics rescue neuropsychiatric disease symptoms, and we propose ways to bridge the gap between human and rodent studies. Given the diverse pharmacological profiles of psychedelics and entactogens, we suggest that their rapid and sustained therapeutic mechanisms of action might best be described by the collection of circuits that they modulate rather than their actions at any single molecular target. Thus, approaches focusing on selective circuit modulation of behavioral phenotypes might prove more fruitful than target-based methods for identifying novel compounds with rapid and sustained therapeutic effects similar to psychedelics and entactogens.

Mu Opioid Receptor-Positive Neurons in the Dorsal Raphe Nucleus Are Impaired by Morphine Abstinence

BACKGROUND

Chronic opioid exposure leads to hedonic deficits and enhanced vulnerability to addiction, which are observed and even strengthen after a period of abstinence, but the underlying circuit mechanisms are poorly understood. In this study, using both molecular and behavioral approaches, we tested the hypothesis that neurons expressing mu opioid receptors (MORs) in the dorsal raphe nucleus (DRN) are involved in addiction vulnerability associated with morphine abstinence.

METHODS

MOR-Cre mice were exposed to chronic morphine and then went through spontaneous withdrawal for 4 weeks, a well-established mouse model of morphine abstinence. We studied DRN-MOR neurons of abstinent mice using 1) viral translating ribosome affinity for transcriptome profiling, 2) fiber photometry to measure neuronal activity, and 3) an opto-intracranial self-stimulation paradigm applied to DRN-MOR neurons to assess responses related to addiction vulnerability including persistence to respond, motivation to obtain the stimulation, self-stimulation despite punishment, and cue-induced reinstatement.

RESULTS

DRN-MOR neurons of abstinent animals showed a downregulation of genes involved in ion conductance and MOR-mediated signaling, as well as altered responding to acute morphine. Opto-intracranial self-stimulation data showed that abstinent animals executed more impulsive-like and persistent responses during acquisition and scored higher on addiction-like criteria.

CONCLUSIONS

Our data suggest that protracted abstinence to chronic morphine leads to reduced MOR function in DRN-MOR neurons and abnormal self-stimulation of these neurons. We propose that DRN-MOR neurons have partially lost their reward-facilitating properties, which in turn may lead to increased propensity to perform addiction-related behaviors.

Neurogenetics and Epigenetics of Loneliness

Loneliness, an established risk factor for both, mental and physical morbidity, is a mounting public health concern. However, the neurobiological mechanisms underlying loneliness-related morbidity are not yet well defined. Here we examined the role of genes and associated DNA risk polymorphic variants that are implicated in loneliness via genetic and epigenetic mechanisms and may thus point to specific therapeutic targets. Searches were conducted on PubMed, Medline, and EMBASE databases using specific Medical Subject Headings terms such as loneliness and genes, neuro- and epigenetics, addiction, affective disorders, alcohol, anti-reward, anxiety, depression, dopamine, cancer, cardiovascular, cognitive, hypodopaminergia, medical, motivation, (neuro)psychopathology, social isolation, and reward deficiency. The narrative literature review yielded recursive collections of scientific and clinical evidence, which were subsequently condensed and summarized in the following key areas: (1) Genetic Antecedents: Exploration of multiple genes mediating reward, stress, immunity and other important vital functions; (2) Genes and Mental Health: Examination of genes linked to personality traits and mental illnesses providing insights into the intricate network of interaction converging on the experience of loneliness; (3) Epigenetic Effects: Inquiry into instances of loneliness and social isolation that are driven by epigenetic methylations associated with negative childhood experiences; and (4) Neural Correlates: Analysis of loneliness-related affective states and cognitions with a focus on hypodopaminergic reward deficiency arising in the context of early life stress, eg, maternal separation, underscoring the importance of parental support early in life. Identification of the individual contributions by various (epi)genetic factors presents opportunities for the creation of innovative preventive, diagnostic, and therapeutic approaches for individuals who cope with persistent feelings of loneliness. The clinical facets and therapeutic prospects associated with the current understanding of loneliness, are discussed emphasizing the relevance of genes and DNA risk polymorphic variants in the context of loneliness-related morbidity.

Alcohol inhibits sociability via serotonin inputs to the nucleus accumbens

Social interaction is a core component of motivational behavior that is perturbed across multiple neuropsychiatric disorders, including alcohol use disorder (AUD). Positive social bonds are neuroprotective and enhance recovery from stress, so reduced social interaction in AUD may delay recovery and lead to alcohol relapse. We report that chronic intermittent ethanol (CIE) induces social avoidance in a sex-dependent manner and is associated with hyperactivity of serotonin (5-HT) neurons in the dorsal raphe nucleus (DRN). While 5-HTDRN neurons are generally thought to enhance social behavior, recent evidence suggests that specific 5-HT pathways can be aversive. Using chemogenetic iDISCO, the nucleus accumbens (NAcc) was identified as one of 5 regions that were activated by 5-HT DRN stimulation. We then employed an array of molecular genetic tools in transgenic mice to show that 5-HT DRN inputs to NAcc dynorphin neurons drive social avoidance in male mice after CIE by activating 5-HT2C receptors. NAcc dynorphin neurons also inhibit dopamine release during social interaction, reducing the motivational drive to engage with social partners. This study reveals that excessive serotonergic drive after chronic alcohol can promote social aversion by inhibiting accumbal dopamine release. Drugs that boost brain serotonin levels may be contraindicated for individuals with AUD.

Advances in the characterization of negative affect caused by acute and protracted opioid withdrawal using animal models

Opioid use disorder (OUD) is a chronic brain disease which originates from long-term neuroadaptations that develop after repeated opioid consumption and withdrawal episodes. These neuroadaptations lead among other things to the development of a negative affect, which includes loss of motivation for natural rewards, higher anxiety, social deficits, heightened stress reactivity, an inability to identify and describe emotions, physical and/or emotional pain, malaise, dysphoria, sleep disorders and chronic irritability. The urge for relief from this negative affect is one of major causes of relapse, and thus represents a critical challenge for treatment and relapse prevention. Animal models of negative affect induced by opioid withdrawal have recapitulated the development of a negative emotional state with signs such as anhedonia, increased anxiety responses, increased despair-like behaviour and deficits in social interaction. This research has been critical to determine neurocircuitry adaptations during chronic opioid administration or upon withdrawal. In this review, we summarize the recent literature of rodent models of (i) acute withdrawal, (ii) protracted abstinence from passive administration of opioids, (iii) withdrawal or protracted abstinence from opioid self-administration. Finally, we describe neurocircuitry involved in acute withdrawal and protracted abstinence. This article is part of the Special Issue on "Opioid-induced changes in addiction and pain circuits".

Alcohol inhibits sociability via serotonin inputs to the nucleus accumbens

Social interaction is a core component of motivational behavior that is perturbed across multiple neuropsychiatric disorders, including alcohol use disorder (AUD). Positive social bonds are neuroprotective and enhance recovery from stress, so reduced social interaction in AUD may delay recovery and lead to alcohol relapse. We report that chronic intermittent ethanol (CIE) induces social avoidance in a sex-dependent manner and is associated with hyperactivity of serotonin (5-HT) neurons in the dorsal raphe nucleus (DRN). While 5-HT DRN neurons are generally thought to enhance social behavior, recent evidence suggests that specific 5-HT pathways can be aversive. Using chemogenetic iDISCO, the nucleus accumbens (NAcc) was identified as one of 5 regions that were activated by 5-HT DRN stimulation. We then employed an array of molecular genetic tools in transgenic mice to show that 5-HT DRN inputs to NAcc dynorphin neurons drive social avoidance in male mice after CIE by activating 5-HT 2C receptors. NAcc dynorphin neurons also inhibit dopamine release during social interaction, reducing the motivational drive to engage with social partners. This study reveals that excessive serotonergic drive after chronic alcohol can promote social aversion by inhibiting accumbal dopamine release. Drugs that boost brain serotonin levels may be contraindicated for individuals with AUD.

Morphine exposure during adolescence induces enduring social changes dependent on adolescent stage of exposure, sex, and social test

Drug exposure during adolescence, when the 'reward' circuitry of the brain is developing, can permanently impact reward-related behavior. Epidemiological studies show that opioid treatment during adolescence, such as pain management for a dental procedure or surgery, increases the incidence of psychiatric illness including substance use disorders. Moreover, the opioid epidemic currently in the United States is affecting younger individuals raising the impetus to understand the pathogenesis of the negative effects of opioids. One reward-related behavior that develops during adolescence is social behavior. We previously demonstrated that social development occurs in rats during sex-specific adolescent periods: early to mid-adolescence in males (postnatal day (P)30-40) and pre-early adolescence in females (P20-30). We thus hypothesized that morphine exposure during the female critical period would result in adult sociability deficits in females, but not males, and morphine administered during the male critical period would result in adult sociability deficits in males, but not females. We found that morphine exposure during the female critical period primarily resulted in deficits in sociability in females, while morphine exposure during the male critical period primarily resulted in deficits in sociability primarily in males. However, depending on the test performed and the social parameter measured, social alterations could be found in both sexes that received morphine exposure at either adolescent stage. These data indicate that when drug exposure occurs during adolescence, and how the endpoint data are measured, will play a large role in determining the effects of drug exposures on social development.

Preclinical evidence for the use of the atypical antipsychotic, brexpiprazole, for opioid use disorder

Opioid addiction is characterized by adaptations in the mesolimbic dopamine system that occur during chronic opioid use. Alterations in dopaminergic transmission contribute to pathological drug-seeking behavior and other symptoms associated with opioid withdrawal following drug discontinuation, making drug abstinence challenging and contributing to high rates of relapse among those suffering from substance use disorder. Recently, the use of dopamine partial agonists has been proposed as a potential strategy to restore dopaminergic signalling during drug withdrawal, while avoiding the adverse side effects associated with stronger modulators of dopaminergic transmission. We investigated the effects of the atypical antipsychotic brexpiprazole, which is a partial agonist at dopamine D2 and D3 receptors, in a mouse model of opioid dependence. The development of opioid dependence in mice is characterized by locomotor sensitization, analgesic tolerance, opioid-induced hyperalgesia, and drug-seeking behavior. We set up four paradigms to model the effects of brexpiprazole on each of these adaptations that occur during chronic opioid use in male and female C57BL/6J mice. Concomitant treatment of brexpiprazole during chronic morphine administration attenuated the development of locomotor sensitization. Brexpiprazole treatment abolished morphine place preference and blocked reinstatement of this behavior following extinction. Brexpiprazole treatment did not alter morphine analgesia, nor did it impact the development of morphine tolerance. However, brexpiprazole treatment did prevent the expression of opioid-induced hyperalgesia in a tail-withdrawal assay, while failing to improve somatic withdrawal symptoms. Altogether, these results provide preclinical evidence for the efficacy of brexpiprazole as a modulator of dopamine-dependent behaviors during opioid use and withdrawal.

Cycles of addiction and loneliness

In this issue of Neuron, Pomrenze and colleagues¹ report a novel mechanism behind sociability deficits in mice during protracted withdrawal from morphine. Dorsal raphe dynorphin neurons terminating in the nucleus accumbens suppress local serotonin release through kappa opioid receptors. These findings likely have important clinical implications.