Targeted overexpression of CRH receptor subtype 1 in central amygdala neurons: effect on alcohol-seeking behavior

Corticotropin releasing factor and drug seeking in substance use disorders: Preclinical evidence and translational limitations

The neuropeptide, corticotropin releasing factor (CRF), has been an enigmatic target for the development of medications aimed at treating stress-related disorders. Despite a large body of evidence from preclinical studies in rodents demonstrating that CRF receptor antagonists prevent stressor-induced drug seeking, medications targeting the CRF-R1 have failed in clinical trials. Here, we provide an overview of the abundant findings from preclinical rodent studies suggesting that CRF signaling is involved in stressor-induced relapse. The scientific literature that has defined the receptors, mechanisms and neurocircuits through which CRF contributes to stressor-induced reinstatement of drug seeking following self-administration and conditioned place preference in rodents is reviewed. Evidence that CRF signaling is recruited with repeated drug use in a manner that heightens susceptibility to stressor-induced drug seeking in rodents is presented. Factors that may determine the influence of CRF signaling in substance use disorders, including developmental windows, biological sex, and genetics are examined. Finally, we discuss the translational failure of medications targeting CRF signaling as interventions for substance use disorders and other stress-related conditions. We conclude that new perspectives and research directions are needed to unravel the mysterious role of CRF in substance use disorders.

Yohimbine as a pharmacological probe for alcohol research: a systematic review of rodent and human studies

Alcohol use disorder (AUD) is a significant public health concern, contributing to a myriad of social, psychological, and physiological issues. Despite substantial efforts within the alcohol research field, promising preclinical findings have failed to translate to clinical use, highlighting the necessity to develop safe and effective pharmacological probes with the ability to be used in preclinical and clinical research. Yohimbine, an α2 adrenergic receptor antagonist, is a well-validated pharmacological tool that has been widely employed in alcohol studies to evaluate noradrenergic activation. This scoping systematic review examines published literature in rodent and human studies involving the use of yohimbine relevant to alcohol research. We conducted a systematic literature review of MEDLINE, Embase, Web of Science Core Collection, CINAHL, PsycInfo, and Cochrane Central Register of Controlled Trials to identify: (1) Experimental Characteristics and Methodology, (2) Sex Differences, (3) Neurochemical Systems and Brain Regions, and (4) Discussion of Applications for Medication Development. Sixty-seven (62 preclinical and 5 clinical) studies were identified meeting the stated criteria, comprising extensive evidence supporting the use of yohimbine as a safe, titratable pharmacological agent for translational alcohol research. Support for the use of yohimbine as a fully translational tool, however, is hindered by limited available findings from human laboratory studies, as well as a dearth of studies examining sex differences in yohimbine's mechanistic actions. Additional consideration should be given to further translational modeling, ideally allowing for parallel preclinical and clinical assessment of yohimbine, methodological assessment of neurochemical systems and brain regions.

Central amygdala corticotropin-releasing factor neurons promote hyponeophagia but do not control alcohol drinking in mice

Corticotropin-releasing factor (CRF) signaling in the central nucleus of the amygdala (CeA) plays a critical role in rodent models of excessive alcohol drinking. However, the source of CRF acting in the CeA during alcohol withdrawal remains to be identified. In the present study, we hypothesized that CeA CRF interneurons may represent a behaviorally relevant source of CRF to the CeA increasing motivation for alcohol via negative reinforcement. We first observed that Crh mRNA expression in the anterior part of the mouse CeA correlates positively with alcohol intake in C57BL/6J males with a history of chronic binge drinking followed by abstinence and increases upon exposure to chronic intermittent ethanol (CIE) vapor inhalation. We then found that chemogenetic activation of CeA CRF neurons in Crh-IRES-Cre mouse brain slices increases gamma-aminobutyric acid (GABA) release in the medial CeA, in part via CRF1 receptor activation. While chemogenetic stimulation exacerbated novelty-induced feeding suppression (NSF) in alcohol-naïve mice, thereby mimicking the effect of withdrawal from CIE, it had no effect on voluntary alcohol consumption, following either acute or chronic manipulation. Furthermore, chemogenetic inhibition of CeA CRF neurons did not affect alcohol consumption or NSF in chronic alcohol drinkers exposed to air or CIE. Altogether, these findings indicate that CeA CRF neurons produce local release of GABA and CRF and promote hyponeophagia in naïve mice, but do not drive alcohol intake escalation or negative affect in CIE-withdrawn mice. The latter result contrasts with previous findings in rats and demonstrates species specificity of CRF circuit engagement in alcohol dependence.

Opposite effects of stress on effortful motivation in high and low anxiety are mediated by CRHR1 in the VTA

Individuals frequently differ in their behavioral and cognitive responses to stress. However, whether motivation is differently affected by acute stress in different individuals remains to be established. By exploiting natural variation in trait anxiety in outbred Wistar rats, we show that acute stress facilitates effort-related motivation in low anxious animals, while dampening effort in high anxious ones. This model allowed us to address the mechanisms underlying acute stress-induced differences in motivated behavior. We show that CRHR1 expression levels in dopamine neurons of the ventral tegmental area (VTA)-a neuronal type implicated in the regulation of motivation-depend on animals' anxiety, and these differences in CRHR1 expression levels explain the divergent effects of stress on both effortful behavior and the functioning of mesolimbic DA neurons. These findings highlight CRHR1 in VTA DA neurons-whose levels vary with individuals' anxiety-as a switching mechanism determining whether acute stress facilitates or dampens motivation.

Epigenome-wide association study and multi-tissue replication of individuals with alcohol use disorder: evidence for abnormal glucocorticoid signaling pathway gene regulation

Alcohol use disorder (AUD) is a chronic debilitating disorder with limited treatment options and poorly defined pathophysiology. There are substantial genetic and epigenetic components; however, the underlying mechanisms contributing to AUD remain largely unknown. We conducted the largest DNA methylation epigenome-wide association study (EWAS) analyses currently available for AUD (total N = 625) and employed a top hit replication (N = 4798) using a cross-tissue/cross-phenotypic approach with the goal of identifying novel epigenetic targets relevant to AUD. Results show that a network of differentially methylated regions in glucocorticoid signaling and inflammation-related genes were associated with alcohol use behaviors. A top probe consistently associated across all cohorts was located in the long non-coding RNA growth arrest specific five gene (GAS5) (p < 10-24). GAS5 has been implicated in regulating transcriptional activity of the glucocorticoid receptor and has multiple functions related to apoptosis, immune function and various cancers. Endophenotypic analyses using peripheral cortisol levels and neuroimaging paradigms showed that methylomic variation in GAS5 network-related probes were associated with stress phenotypes. Postmortem brain analyses documented increased GAS5 expression in the amygdala of individuals with AUD. Our data suggest that alcohol use is associated with differential methylation in the glucocorticoid system that might influence stress and inflammatory reactivity and subsequently risk for AUD.

A balancing act: the role of pro- and anti-stress peptides within the central amygdala in anxiety and alcohol use disorders

The central nucleus of the amygdala (CeA) is widely implicated as a structure that integrates both appetitive and aversive stimuli. While intrinsic CeA microcircuits primarily consist of GABAergic neurons that regulate amygdala output, a notable feature of the CeA is the heterogeneity of neuropeptides and neuropeptide/neuromodulator receptors that it expresses. There is growing interest in the role of the CeA in mediating psychopathologies, including stress and anxiety states and their interactions with alcohol use disorders. Within the CeA, neuropeptides and neuromodulators often exert pro- or anti- stress actions, which can influence anxiety and alcohol associated behaviours. In turn, alcohol use can cause adaptions within the CeA, which may render an individual more vulnerable to stress which is a major trigger of relapse to alcohol seeking. This review examines the neurocircuitry, neurochemical phenotypes and how pro- and anti-stress peptide systems act within the CeA to regulate anxiety and alcohol seeking, focusing on preclinical observations from animal models. Furthermore, literature exploring the targeting of genetically defined populations or neuronal ensembles and the role of the CeA in mediating sex differences in stress x alcohol interactions are explored.

Driving the Downward Spiral: Alcohol-Induced Dysregulation of Extended Amygdala Circuits and Negative Affect

Alcohol use disorder (AUD) afflicts a large number of individuals, families, and communities globally. Affective disturbances, including stress, depression, and anxiety, are highly comorbid with AUD, contributing in some cases to initial alcohol use and continued use. Negative affect has a particularly strong influence on the withdrawal/abstinence stage of addiction as individuals with AUD frequently report stressful events, depression, and anxiety as key factors for relapse. Treatment options for negative affect associated with AUD are limited and often ineffective, highlighting the pressing need for preclinical studies examining the underlying neural circuitry driving AUD-associated negative affect. The extended amygdala (EA) is a set of brain areas collectively involved in generating and regulating affect, and extensive research has defined a critical role for the EA in all facets of substance use disorder. Here, we review the expansive historical literature examining the effects of ethanol exposure on the EA, with an emphasis on the complex EA neural circuitry driving negative affect in all phases of the alcohol addiction cycle. Specifically, this review focuses on the effects of alcohol exposure on the neural circuitry in 2 key components of the EA, the central nucleus of the amygdala and the bed nucleus of the stria terminalis. Additionally, future directions are proposed to advance our understanding of the relationship between AUD-associated negative affect and neural circuitry in the EA, with the long-term goal of developing better diagnostic tools and new pharmacological targets aimed at treating negative affect in AUD. The concepts detailed here will serve as the foundation for a companion review focusing on the potential for the endogenous cannabinoid system in the EA as a novel target for treating the stress, anxiety, and negative emotional state driving AUD.

Oligomeric Receptor Complexes and Their Allosteric Receptor-Receptor Interactions in the Plasma Membrane Represent a New Biological Principle for Integration of Signals in the CNS

G protein-coupled receptors (GPCRs) not only exist as monomers but also as homomers and heteromers in which allosteric receptor-receptor interactions take place, modulating the functions of the participating GPCR protomers. GPCRs can also form heteroreceptor complexes with ionotropic receptors and receptor tyrosine kinases modulating their function. Furthermore, adaptor proteins interact with receptor protomers and modulate their interactions. The state of the art is that the allosteric receptor-receptor interactions are reciprocal, highly dynamic and substantially alter the signaling, trafficking, recognition and pharmacology of the participating protomers. The pattern of changes appears to be unique for each heteromer and can favor antagonistic or facilitatory interactions or switch the G protein coupling from e.g., Gi/o to Gq or to beta-arrestin signaling. It lends a new dimension to molecular integration in the nervous system. Future direction should be aimed at determining the receptor interface involving building models of selected heterodimers. This will make design of interface-interfering peptides that specifically disrupt the heterodimer possible. This will help to determine the functional role of the allosteric receptor-receptor interactions as well as the integration of signals at the plasma membrane by the heteroreceptor complexes, vs. integration of the intracellular signaling pathways. Integration of signals also at the plasma membrane seems crucial in view of the hypothesis that learning and memory at a molecular level takes place by reorganization of homo and heteroreceptor complexes in the postsynaptic membrane. Homo and heteroreceptor complexes are in balance with each other, and their disbalance is linked to disease. Targeting heteroreceptor complexes represents a novel strategy for the treatment of brain disorders.

Proteomic and transcriptional profiling of rat amygdala following social play

Social play is the most characteristic form of social interaction which is necessary for adolescents to develop proper cognitive, emotional, and social competency. The information available on neural substrates and the mechanism involved in social play is limited. This study characterized social play by proteomic and transcriptional profiling studies. Social play was performed on male Sprague Dawley rats on postnatal day 38 and protein and gene expression in the amygdala was determined following behavioral testing. The proteomic analysis led to the identification of 170 differentially expressed proteins (p ≤ 0.05) with 67 upregulated and 103 downregulated proteins. The transcriptomic analysis led to the identification of 188 genes (FDR ≤ 0.05) with 55 upregulated and 133 downregulated genes. DAVID analysis of gene/protein expression data revealed that social play altered GABAergic signaling, glutamatergic signaling, and G-protein coupled receptor (GPCR) signaling. These data suggest that the synaptic levels of GABA and glutamate increased during play. Ingenuity Pathway Analysis (IPA) confirmed these alterations. IPA also revealed that differentially expressed genes/proteins in our data had significant over representation of neurotransmitter signaling systems, including the opioid, serotonin, and dopamine systems, suggesting that play alters the systems involved in the regulation of reward. In addition, corticotropin-releasing hormone signaling was altered indicating that an increased level of stress occurs during play. Overall, our data suggest that increased inhibitory GPCR signaling in these neurotransmitter pathways occurs following social play as a physiological response to regulate the induced level of reward and stress and to maintain the excitatory-inhibitory balance in the neurotransmitter systems.

Impact of Genetic Variation on Stress-Related Ethanol Consumption

BACKGROUND

The effect of stress on alcohol consumption in humans is highly variable, and the underlying processes are not yet understood. Attempts to model a positive relationship between stress and increased ethanol (EtOH) consumption in animals have been only modestly successful. Our hypothesis is that individual differences in stress effects on EtOH consumption are mediated by genetics.

METHODS

We measured alcohol consumption, using the drinking-in-the-dark (DID) paradigm in females from 2 inbred mouse strains, C57BL/6J (B6) and DBA/2J (D2), and 35 of their inbred progeny (the BXD family). A control group was maintained in normal housing and a stress group was exposed to chronic mild stress (CMS), consisting of unpredictable stressors over 7 weeks. These included predator, social, and environmental perturbations. Alcohol intake was measured over 16 weeks in both groups during baseline (preceding 5-week period), CMS (intervening 7-week period), and post-CMS (final 4-week period).

RESULTS

We detected a strong effect of CMS on alcohol intake. A few strains demonstrated CMS-related increased alcohol consumption; however, most showed decreased intake. We identified 1 nearly significant quantitative trait locus on chromosome 5 that contains the neuronal nitric oxide synthase gene (Nos1). The expression of Nos1 is frequently changed following alcohol exposure, and variants in this gene segregating among the BXD population may modulate alcohol intake in response to stress.

CONCLUSIONS

The results we present here represent the first study to combine chronic stress and alcohol consumption in a genetic reference population of mice. Differences in susceptibility to the effects of stressful environments vis-à-vis alcohol use disorders would suggest that the differences have at least some basis in genetic constitution. We have also nominated a likely candidate gene underlying the large individual differences in effects of stress on alcohol consumption.