BNST specific mGlu5 receptor knockdown regulates sex-dependent expression of negative affect produced by adolescent ethanol exposure and adult stress

Enhanced fear extinction through infralimbic perineuronal net digestion: The modulatory role of adolescent alcohol exposure

Perineuronal nets (PNNs) are specialized components of the extracellular matrix that play a critical role in learning and memory. In a Pavlovian fear conditioning paradigm, degradation of PNNs affects the formation and storage of fear memories. This study examined the impact of adolescent intermittent ethanol (AIE) exposure by vapor inhalation on the expression of PNNs in the adult rat prelimbic (PrL) and infralimbic (IfL) subregions of the medial prefrontal cortex. Results indicated that following AIE, the total number of PNN positive cells in the PrL cortex increased in layer II/III but did not change in layer V. Conversely, in the IfL cortex, the number of PNN positive cells decreased in layer V, with no change in layer II/III. In addition, the intensity of PNN staining was significantly altered by AIE exposure, which narrowed the distribution of signal intensity, reducing the number of high and low intensity PNNs. Given these changes in PNNs, the next experiment assessed the effects of AIE and PNN digestion on extinction of a conditioned fear memory. In Air control rats, digestion of PNNs by bilateral infusion of Chondroitinase ABC (ChABC) into the IfL cortex enhanced fear extinction and reduced contextual fear renewal. In contrast, both fear extinction learning and contextual fear renewal remained unchanged following PNN digestion in AIE exposed rats. These results highlight the sensitivity of prefrontal PNNs to adolescent alcohol exposure and suggest that ChABC-induced plasticity is reduced in the IfL cortex following AIE exposure.

BNST CRF receptor type 1 modulates mechanical hypersensitivity induced by adolescent alcohol exposure in adult female mice

RATIONALE

Alcohol exposure during adolescence has been linked to long-lasting behavioral consequences, contributing to the development of alcohol use disorder. Negative affect and chronic pain during alcohol withdrawal are critical factors influencing problematic alcohol use and relapse. Our previous research demonstrated that adolescent intermittent ethanol (AIE) vapor exposure elicits sex-specific negative affect-like behavior in adult mice following stress exposure. Additionally, AIE induces persistent mechanical hypersensitivity, which is accompanied by increased activation of corticotropin-releasing factor receptor type 1 (CRFR1) neurons in the dorsolateral bed nucleus of the stria terminalis (dlBNST).

OBJECTIVES

This study extends previous work by investigating plasma corticosterone levels and CRFR1 protein expression in the dlBNST following restraint stress exposure in adult mice with an AIE history. We also aim to explore the role of dlBNST CRFR1 signaling in mediating negative affect-like behavior and mechanical hypersensitivity.

RESULTS

Female mice exhibited elevated plasma corticosterone levels compared to males following restraint stress. Moreover, females with AIE history showed higher expression of CRFR1 protein in the dlBNST compared to air controls. Antagonism of CRFR1 in the dlBNST blocked AIE-induced mechanical hypersensitivity in adult females but did not affect stress-induced negative affect-like behavior. In alcohol-naïve females, intra-dlBNST administration of a CRFR1 agonist induced mechanical hypersensitivity.

CONCLUSIONS

These findings provide new insights into the neurobiological mechanisms underlying stress-induced negative affect and pain-related behavior, both influenced by a history of adolescent alcohol exposure. The results suggest that CRFR1 antagonists warrant further investigation for their potential in addressing alcohol-related chronic pain.

Enhanced Fear Extinction Through Infralimbic Perineuronal Net Digestion: The Modulatory Role of Adolescent Alcohol Exposure

Perineuronal nets (PNNs) are specialized components of the extracellular matrix that play a critical role in learning and memory. In a Pavlovian fear conditioning paradigm, degradation of PNNs affects the formation and storage of fear memories. This study examined the impact of adolescent intermittent ethanol (AIE) exposure by vapor inhalation on the expression of PNNs in the adult rat prelimbic (PrL) and infralimbic (IfL) subregions of the medial prefrontal cortex. Results indicated that following AIE, the total number of PNN positive cells in the PrL cortex increased in layer II/III but did not change in layer V. Conversely, in the IfL cortex, the number of PNN positive cells decreased in layer V, with no change in layer II/III. In addition, the intensity of PNN staining was significantly altered by AIE exposure, which narrowed the distribution of signal intensity, reducing the number of high and low intensity PNNs. Given these changes in PNNs, the next experiment assessed the effects of AIE and PNN digestion on extinction of a conditioned fear memory. In Air control rats, digestion of PNNs by bilateral infusion of Chondroitinase ABC (ChABC) into the IfL cortex enhanced fear extinction and reduced contextual fear renewal. In contrast, both fear extinction learning and contextual fear renewal remained unchanged following PNN digestion in AIE exposed rats. These results highlight the sensitivity of prefrontal PNNs to adolescent alcohol exposure and suggest that ChABC-induced plasticity is reduced in the IfL cortex following AIE exposure.

An escape-enhancing circuit involving subthalamic CRH neurons mediates stress-induced anhedonia in mice

Chronic predator stress (CPS) is an important and ecologically relevant tool for inducing anhedonia in animals, but the neural circuits underlying the associated neurobiological changes remain to be identified. Using cell-type-specific manipulations, we found that corticotropin-releasing hormone (CRH) neurons in the medial subthalamic nucleus (mSTN) enhance struggle behaviors in inescapable situations and lead to anhedonia, predominately through projections to the external globus pallidus (GPe). Recordings of in vivo neuronal activity revealed that CPS distorted mSTN-CRH neuronal responsivity to negative and positive stimuli, which may underlie CPS-induced behavioral despair and anhedonia. Furthermore, we discovered presynaptic inputs from the bed nucleus of the stria terminalis (BNST) to mSTN-CRH neurons projecting to the GPe that were enhanced following CPS, and these inputs may mediate such behaviors. This study identifies a neurocircuitry that co-regulates escape response and anhedonia in response to predator stress. This new understanding of the neural basis of defensive behavior in response to predator stress will likely benefit our understanding of neuropsychiatric diseases.

Sustained antidepressant effects of ketamine metabolite involve GABAergic inhibition-mediated molecular dynamics in aPVT glutamatergic neurons

Despite the rapid and sustained antidepressant effects of ketamine and its metabolites, their underlying cellular and molecular mechanisms are not fully understood. Here, we demonstrate that the sustained antidepressant-like behavioral effects of (2S,6S)-hydroxynorketamine (HNK) in repeatedly stressed animal models involve neurobiological changes in the anterior paraventricular nucleus of the thalamus (aPVT). Mechanistically, (2S,6S)-HNK induces mRNA expression of extrasynaptic GABAA receptors and subsequently enhances GABAA-receptor-mediated tonic currents, leading to the nuclear export of histone demethylase KDM6 and its replacement by histone methyltransferase EZH2. This process increases H3K27me3 levels, which in turn suppresses the transcription of genes associated with G-protein-coupled receptor signaling. Thus, our findings shed light on the comprehensive cellular and molecular mechanisms in aPVT underlying the sustained antidepressant behavioral effects of ketamine metabolites. This study may support the development of potentially effective next-generation pharmacotherapies to promote sustained remission of stress-related psychiatric disorders.

Long-lasting mechanical hypersensitivity and CRF receptor type-1 neuron activation in the BNST following adolescent ethanol exposure

BACKGROUND

Adolescent alcohol use can produce long-lasting alterations in brain function, potentially leading to adverse health outcomes in adulthood. Emerging evidence suggests that chronic alcohol use can increase pain sensitivity or exacerbate existing pain conditions, but the potential neural mechanisms underlying these effects require further investigation. Here, we evaluate the impact of chronic ethanol vapor on mechanical sensitivity over the course of acute and protracted withdrawal in adolescent and adult male and female mice, and its potential association with alterations in corticotropin-releasing factor (CRF) signaling within the bed nucleus of the stria terminalis (BNST).

METHODS

Adolescent and adult male and female mice underwent intermittent ethanol vapor exposure on 4 days/week for 2 weeks. Mechanical thresholds were evaluated 5 h and 7, 14, 21, and 28 d after cessation of ethanol exposure using the von Frey test. For mice with a history of adolescent ethanol exposure, brains were collected for in situ RNAscope processing after the final test. Messenger RNA expression of c-Fos, Crfr1, and Crf in the BNST subregions was examined.

RESULTS

Exposure to intermittent ethanol vapor induced persistent mechanical hypersensitivity during withdrawal in both adolescent and adult mice. Notably, the effect was more transient in mice exposed to ethanol during adulthood, resolving by day 28 after ethanol exposure. Furthermore, both male and female mice with a history of adolescent ethanol exposure exhibited increased activation of CRF receptor type 1 (CRFR1) neurons within the dorsolateral BNST.

CONCLUSIONS

Our results support the conclusion that intermittent ethanol exposure can induce mechanical hypersensitivity, potentially through the activation of BNST CRFR1 neurons. These findings provide a basis for future studies aimed at evaluating specific subpopulations of BNST neurons and their contribution to pain in individuals with a history of alcohol use.

Adolescent alcohol disrupts development of noradrenergic neurons in the nucleus of the tractus solitarius and enhances stress behaviors in adulthood in mice in a sex specific manner

Alcohol use disorders (AUDs) are common mental health issues worldwide and can lead to other chronic diseases. Stress is a major factor in the development and continuation of AUDs, and adolescent alcohol exposure can lead to enhanced stress-responsivity and increased risk for AUD development in adulthood. The exact mechanisms behind the interaction between adolescence, stress, and alcohol are not fully understood and require further research. In this regard, the nucleus of the tractus solitarius (NTS) provides dense norepinephrine projections to the extended amygdala, providing a key pathway for stress-related alcohol behaviors. While NTS norepinephrine neurons are known to be alcohol sensitive, whether adolescent alcohol disrupts NTS-norepinephrine neuron development and if this is related to altered stress-sensitivity and alcohol preference in adulthood has not previously been examined. Here, we exposed male and female C57Bl/6J mice to the commonly used adolescent intermittent ethanol (AIE) vapor model during postnatal day 28-42 and examined AIE effects on: 1) tyrosine hydroxylase (TH) mRNA expression in the NTS across various ages (postnatal day 21-90), 2) behavioral responses to acute stress in the light/dark box test in adulthood, 3) NTS TH neuron responses to acute stress and ethanol challenges in adulthood, and 4) ethanol conditioned place preference behavior in adulthood. Overall the findings indicate that AIE alters NTS TH mRNA expression and increases anxiety-like behaviors following acute stress exposure in a sex-dependent manner. These mRNA expression and behavioral changes occur in the absence of AIE-induced changes in NTS TH neuron sensitivity to either acute stress or acute alcohol exposure or changes to ethanol conditioned place preference.

The role of mGlu receptors in susceptibility to stress-induced anhedonia, fear, and anxiety-like behavior

Stress and trauma exposure contribute to the development of psychiatric disorders such as post-traumatic stress disorder (PTSD) and major depressive disorder (MDD) in a subset of people. A large body of preclinical work has found that the metabotropic glutamate (mGlu) family of G protein-coupled receptors regulate several behaviors that are part of the symptom clusters for both PTSD and MDD, including anhedonia, anxiety, and fear. Here, we review this literature, beginning with a summary of the wide variety of preclinical models used to assess these behaviors. We then summarize the involvement of Group I and II mGlu receptors in these behaviors. Bringing together this extensive literature reveals that mGlu5 signaling plays distinct roles in anhedonia, fear, and anxiety-like behavior. mGlu5 promotes susceptibility to stress-induced anhedonia and resilience to stress-induced anxiety-like behavior, while serving a fundamental role in the learning underlying fear conditioning. The medial prefrontal cortex, basolateral amygdala, nucleus accumbens, and ventral hippocampus are key regions where mGlu5, mGlu2, and mGlu3 regulate these behaviors. There is strong support that stress-induced anhedonia arises from decreased glutamate release and post-synaptic mGlu5 signaling. Conversely, decreasing mGlu5 signaling increases resilience to stress-induced anxiety-like behavior. Consistent with opposing roles for mGlu5 and mGlu2/3 in anhedonia, evidence suggests that increased glutamate transmission may be therapeutic for the extinction of fear learning. Thus, a large body of literature supports the targeting of pre- and post-synaptic glutamate signaling to ameliorate post-stress anhedonia, fear, and anxiety-like behavior.

Sex differences and hormonal regulation of metabotropic glutamate receptor synaptic plasticity

Striking sex differences exist in presentation and incidence of several psychiatric disorders. For example, major depressive disorder is more prevalent in women than men, and women who develop alcohol use disorder progress through drinking milestones more rapidly than men. With regards to psychiatric treatment responses, women respond more favorably to selective serotonin reuptake inhibitors than men, whereas men have better outcomes when prescribed tricyclic antidepressants. Despite such well-documented biases in incidence, presentation, and treatment response, sex as a biological variable has long been neglected in preclinical and clinical research. An emerging family of druggable targets for psychiatric diseases, metabotropic glutamate (mGlu) receptors are G-protein coupled receptors broadly distributed throughout the central nervous system. mGlu receptors confer diverse neuromodulatory actions of glutamate at the levels of synaptic plasticity, neuronal excitability, and gene transcription. In this chapter, we summarize the current preclinical and clinical evidence for sex differences in mGlu receptor function. We first highlight basal sex differences in mGlu receptor expression and function and proceed to describe how gonadal hormones, notably estradiol, regulate mGlu receptor signaling. We then describe sex-specific mechanisms by which mGlu receptors differentially modulate synaptic plasticity and behavior in basal states and models relevant for disease. Finally, we discuss human research findings and highlight areas in need of further research. Taken together, this review emphasizes how mGlu receptor function and expression can differ across sex. Gaining a more complete understanding of how sex differences in mGlu receptor function contribute to psychiatric diseases will be critical in the development of novel therapeutics that are effective in all individuals.

Regulation of glutamate signaling in the extended amygdala by adolescent alcohol exposure

Adolescence is a critical period for brain development and behavioral maturation, marked by increased risk-taking behavior and the initiation of drug use. There are significant changes in gray matter volume and pruning of synapses along with a shift in excitatory to inhibitory balance which marks the maturation of cognition and decision-making. Because of ongoing brain development, adolescents are particularly sensitive to the detrimental effects of drugs, including alcohol, which can cause long-lasting consequences into adulthood. The extended amygdala is a region critically implicated in withdrawal and negative affect such as anxiety and depression. As negative affective disorders develop during adolescence, the effects of adolescent alcohol exposure on extended amygdala circuitry needs further inquiry. Here we aim to provide a framework to discuss the existing literature on the extended amygdala, the neuroadaptations which result from alcohol use, and the intersection of factors which contribute to the long-lasting effects of this exposure.