GluN2B subunit deletion reveals key role in acute and chronic ethanol sensitivity of glutamate synapses in bed nucleus of the stria terminalis

BNST GluN2D-containing NMDARs contribute to ethanol intake but not negative affective behaviors in female mice

Alcohol use disorder (AUD) is a chronic, relapsing disease, highly comorbid with anxiety and depression. The bed nucleus of the stria terminalis (BNST), and Crh + neurons in this region are thought to play a key role in chronic ethanol-induced increases in volitional ethanol intake. This role has been hypothesized to be driven by emergent BNST-dependent negative affective behaviors. Indeed, we report here that in female mice undergoing a home cage chronic drinking forced abstinence model (CDFA), excitatory transmission undergoes time-dependent upregulation in BNST Crh + cells. Excitatory NMDA receptors (NMDARs) are a major target of ethanol, and chronic ethanol exposure has been shown to regulate NMDAR function and expression. GluN2D subunit-containing NMDARs have emerged as a target of interest due to their limited distribution and potential roles in affective behavior. We find that knockdown of dorsal BNST (dBNST) GluN2D expression significantly decreases ethanol intake in female, but not male, mice. While BNST Grin2b expression was significantly increased in protracted abstinence following CDFA, no differences in Grin2d expression were observed in dBNST or specifically in dBNST Crh + neurons. Finally, to determine the impact of GluN2D expression on negative affective behaviors, open field, elevated zero maze, and forced swim tasks were used to measure anxiety- and depressive-like behaviors in constitutive and conditional BNST GluN2D knockout mice. Surprisingly, we find that deletion of GluN2D fails to alter negative affect in ethanol-naïve female mice. Together, these data suggest a role for BNST GluN2D-containing NMDARs in ethanol drinking behaviors but not abstinence from ethanol, highlighting potential sex differences and behavioral specificity in the context of AUD behaviors. Overall, these data further suggest roles for BNST synaptic signaling in volitional ethanol intake that are partially independent of actions on affective behavior.

Limb-Clasping Response in NMDA Receptor Palmitoylation-Deficient Mice

Proper regulation of N-methyl-D-aspartate-type glutamate receptor (NMDA receptor) expression is responsible for excitatory synaptic functions in the mammalian brain. NMDA receptor dysfunction can cause various neuropsychiatric disorders and neurodegenerative diseases. Posttranslational protein S-palmitoylation, the covalent attachment of palmitic acid to intracellular cysteine residues via thioester bonds, occurs in the carboxyl terminus of GluN2B, which is the major regulatory NMDA receptor subunit. Mutations of three palmitoylatable cysteine residues in the membrane-proximal cluster of GluN2B to non-palmitoylatable serine (3CS) lead to the dephosphorylation of GluN2B Tyr1472 in the hippocampus and cerebral cortex, inducing a reduction in the surface expression of GluN2B-containig NMDA receptors. Furthermore, adult GluN2B 3CS homozygous mice demonstrated a definite clasping response without abnormalities in the gross brain structure, other neurological reflexes, or expression levels of synaptic proteins in the cerebrum. This behavioral disorder, observed in the GluN2B 3CS knock-in mice, indicated that complex higher brain functions are coordinated through the palmitoylation-dependent regulation of NMDA receptors in excitatory synapses.

Pituitary Adenylate Cyclase-Activating Polypeptide (PACAP) of the Bed Nucleus of the Stria Terminalis Mediates Heavy Alcohol Drinking in Mice

Alcohol use disorder (AUD) is a complex psychiatric disease characterized by periods of heavy drinking and periods of withdrawal. Chronic exposure to ethanol causes profound neuroadaptations in the extended amygdala, which cause allostatic changes promoting excessive drinking. The bed nucleus of the stria terminalis (BNST), a brain region involved in both excessive drinking and anxiety-like behavior, shows particularly high levels of pituitary adenylate cyclase-activating polypeptide (PACAP), a key mediator of the stress response. Recently, a role for PACAP in withdrawal-induced alcohol drinking and anxiety-like behavior in alcohol-dependent rats has been proposed; whether the PACAP system of the BNST is also recruited in other models of alcohol addiction and whether it is of local or nonlocal origin is currently unknown. Here, we show that PACAP immunoreactivity is increased selectively in the BNST of C57BL/6J mice exposed to a chronic, intermittent access to ethanol. While pituitary adenylate cyclase-activating polypeptide (PACAP) type 1 receptor-expressing cells were unchanged by chronic alcohol, the levels of a peptide closely related to PACAP, the calcitonin gene-related neuropeptide, were found to also be increased in the BNST. Finally, using a retrograde chemogenetic approach in PACAP-ires-Cre mice, we found that the inhibition of PACAP neuronal afferents to the BNST reduced heavy ethanol drinking. Our data suggest that the PACAP system of the BNST is recruited by chronic, voluntary alcohol drinking in mice and that nonlocally originating PACAP projections to the BNST regulate heavy alcohol intake, indicating that this system may represent a promising target for novel AUD therapies.

Functional Evaluation of a Novel GRIN2B Missense Variant Associated with Epilepsy and Intellectual Disability

Epilepsy, a neurological condition, is widely prevalent among individuals with intellectual disability (ID). It is well established that N-methyl-D-aspartate (NMDA) receptors play an important role in both epilepsy and ID. Autosomal dominant mutations in the GRIN2B gene, which encodes the GluN2B subunit of the NMDA receptor, have been reported to be associated with epilepsy and ID. However, the underlying mechanism of this association is not well-understood. In this study, we identified a novel GRIN2B mutation (c.3272A > C, p.K1091T) in a patient with epilepsy and ID. The proband was a one year and ten months old girl. GRIN2B variant was inherited from her mother. We further investigated the functional consequences of this mutation. Our findings revealed that the p.K1091T mutation created a Casein kinase 2 phosphorylation site. Using recombinant NMDA receptors containing the GluN2B-K1091T along with GluN1 in HEK 293T cells, we observed significant defects in its interactions with postsynaptic density 95. It is accompanied by reduced delivery of the receptors to the cell membrane and a decrease in glutamate affinity. Moreover, primary neurons expressing GluN2B-K1091T also exhibited impaired surface expression of NMDA receptors, a reduction in dendritic spine number and excitatory synaptic transmission. In summary, our study reports a novel GRIN2B mutation and provides functional characteristics of this mutation in vitro, thereby contributing to the understanding of GRIN2B variants in epilepsy and ID.

Whole-brain connectivity to the bed nucleus of the stria terminalis calretinin-expressing interneurons in male mice

The bed nucleus of the stria terminalis (BNST) is a neuropeptide-enriched brain region that modulates a wide variety of emotional behaviours and states, including stress, anxiety, reward and social interaction. The BNST consists of diverse subregions and neuronal ensembles; however, because of the high molecular heterogeneity within BNST neurons, the mechanisms through which the BNST regulates distinct emotional behaviours remain largely unclear. Prior studies have identified BNST calretinin (CR)-expressing neurons, which lack neuropeptides. Here, employing virus-based cell-type-specific retrograde and anterograde tracing systems, we mapped the whole-brain monosynaptic inputs and axonal projections of BNST CR-expressing neurons in male mice. We found that BNST CR-expressing neurons received inputs mainly from the amygdalopiriform transition area, central amygdala and hippocampus and moderately from the medial preoptic area, basolateral amygdala, paraventricular thalamus and lateral hypothalamus. Within the BNST, plenty of input neurons were primarily located in the oval and interfascicular subregions. Furthermore, numerous BNST CR-expressing neuronal boutons were observed within the BNST but not in other brain regions, thus suggesting that these neurons are a type of interneuron. These results will help further elucidate the neuronal circuits underlying the elaborate and distinct functions of the BNST.

Chronic Ethanol Exposure Modulates Periaqueductal Gray to Extended Amygdala Dopamine Circuit

The bed nucleus of the stria terminalis (BNST) is a component of the extended amygdala that regulates motivated behavior and affective states and plays an integral role in the development of alcohol-use disorder (AUD). The dorsal subdivision of the BNST (dBNST) receives dense dopaminergic input from the ventrolateral periaqueductal gray (vlPAG)/dorsal raphe (DR). To date, no studies have examined the effects of chronic alcohol on this circuit. Here, we used chronic intermittent ethanol exposure (CIE), a well-established rodent model of AUD, to functionally interrogate the vlPAG/DR-BNST dopamine (DA) circuit during acute withdrawal. We selectively targeted vlPAG/DR^DA neurons in tyrosine hydroxylase-expressing transgenic adult male mice. Using ex vivo electrophysiology, we found hyperexcitability of vlPAG/DR^DA neurons in CIE-treated mice. Further, using optogenetic approaches to target vlPAG/DR^DA terminals in the dBNST, we revealed a CIE-mediated shift in the vlPAG/DR-driven excitatory-inhibitory (E/I) ratio to a hyperexcitable state in dBNST. Additionally, to quantify the effect of CIE on endogenous DA signaling, we coupled optogenetics with fast-scan cyclic voltammetry to measure pathway-specific DA release in dBNST. CIE-treated mice had significantly reduced signal half-life, suggestive of faster clearance of DA signaling. CIE treatment also altered the ratio of vlPAG/DR^DA-driven cellular inhibition and excitation of a subset of dBNST neurons. Overall, our findings suggest a dysregulation of vlPAG/DR to BNST dopamine circuit, which may contribute to pathophysiological phenotypes associated with AUD.SIGNIFICANCE STATEMENT The dorsal bed nucleus of the stria terminalis (dBNST) is highly implicated in the pathophysiology of alcohol-use disorder and receives dopaminergic inputs from ventrolateral periaqueductal gray/dorsal raphe regions (vlPAG/DR). The present study highlights the plasticity within the vlPAG/DR to dBNST dopamine (DA) circuit during acute withdrawal from chronic ethanol exposure. More specifically, our data reveal that chronic ethanol strengthens vlPAG/DR-dBNST glutamatergic transmission while altering both DA transmission and dopamine-mediated cellular inhibition of dBNST neurons. The net result is a shift toward a hyperexcitable state in dBNST activity. Together, our findings suggest chronic ethanol may promote withdrawal-related plasticity by dysregulating the vlPAG/DR-dBNST DA circuit.

GRIN2B-related neurodevelopmental disorder: current understanding of pathophysiological mechanisms

The GRIN2B-related neurodevelopmental disorder is a rare disease caused by mutations in the GRIN2B gene, which encodes the GluN2B subunit of NMDA receptors. Most individuals with GRIN2B-related neurodevelopmental disorder present with intellectual disability and developmental delay. Motor impairments, autism spectrum disorder, and epilepsy are also common. A large number of pathogenic de novo mutations have been identified in GRIN2B. However, it is not yet known how these variants lead to the clinical symptoms of the disease. Recent research has begun to address this issue. Here, we describe key experimental approaches that have been used to better understand the pathophysiology of this disease. We discuss the impact of several distinct pathogenic GRIN2B variants on NMDA receptor properties. We then critically review pivotal studies examining the synaptic and neurodevelopmental phenotypes observed when disease-associated GluN2B variants are expressed in neurons. These data provide compelling evidence that various GluN2B mutants interfere with neuronal differentiation, dendrite morphogenesis, synaptogenesis, and synaptic plasticity. Finally, we identify important open questions and considerations for future studies aimed at understanding this complex disease. Together, the existing data provide insight into the pathophysiological mechanisms that underlie GRIN2B-related neurodevelopmental disorder and emphasize the importance of comparing the effects of individual, disease-associated variants. Understanding the molecular, cellular and circuit phenotypes produced by a wide range of GRIN2B variants should lead to the identification of core neurodevelopmental phenotypes that characterize the disease and lead to its symptoms. This information could help guide the development and application of effective therapeutic strategies for treating individuals with GRIN2B-related neurodevelopmental disorder.

NLRP3 deficiency decreases alcohol intake controlling anxiety-like behavior via modification of glutamatergic transmission in corticostriatal circuits

BACKGROUND

Alcohol use disorders result from repeated binge and chronic alcohol consumption followed by negative effects, such as anxiety, upon cessation. This process is associated with the activation of NLRP3 inflammasome-mediated responses. However, whether and how inhibition of the NLRP3 inflammasome alters alcohol intake and anxiety behavior remains unclear.

METHODS

A combination of drinking-in-the-dark and gavage was established in NLRP3-knockout and control mice. Behavior was assessed by open-field and elevated plus maze tests. Binge alcohol drinking was measured at 2 h and 4 h. A 2 h/4 h/24 h voluntary drinking was determined by a two-bottle choice paradigm. Western blotting and ELISA were applied to examine the levels of the NLRP3 inflammasome and- inflammatory factors, such as IL-1β and TNF-α. Nissl staining was used to measure neuronal injury. The electrophysiological method was used to determine glutamatergic transmission in corticostriatal circuits. In vivo optogenetic LTP and LTD were applied to control the function of corticostriatal circuits on the behavior of mice. MCC950 was used to antagonize the NLRP3 inflammasome.

RESULTS

The binge alcohol intake was decreased in NLRP3 KO mice compared to the control mice. During alcohol withdrawal, NLRP3 deficiency attenuated anxiety-like behavior and neuronal injury in the mPFC and striatum. Moreover, we discovered that glutamatergic transmission to striatal neurons was reduced in NLRP3 KO mice. Importantly, in vivo optogenetic induction of long-term potentiation (LTP) of corticostriatal circuits reversed the effects of NLRP3 deficiency on glutamatergic transmission and anxiety behavior. We also demonstrated that optogenetic induction of LTD decreased anxiety-like behavior and caused a reduction in glutamatergic transmission. Interestingly, NLRP3 deficiency or inhibition (MCC950 injection) attenuated the anxiety-like behavior, but it did not prevent DID + gavage paradigm-induced a persistent enhancement of drinking in a two-bottle choice at 2 and 4 days into withdrawal.

CONCLUSION

Our results demonstrate that NLRP3 deficiency decreases binge alcohol intake and anxiety-like behavior through downregulation of glutamatergic transmission in corticostriatal circuits, which may provide an anti-inflammatory target for treating alcohol use disorders.

Vesicular glutamate transporter 2-containing neurons of the centrally-projecting Edinger-Westphal nucleus regulate alcohol drinking and body temperature

Previous studies in rodents have repeatedly demonstrated that the centrally-projecting Edinger-Westphal nucleus (EWcp) is highly sensitive to alcohol and is also involved in regulating alcohol intake and body temperature. Historically, the EWcp has been known as the main site of Urocortin 1 (Ucn1) expression, a corticotropin-releasing factor-related peptide, in the brain. However, the EWcp also contains other populations of neurons, including neurons that express the vesicular glutamate transporter 2 (Vglut2). Here we transduced the EWcp with adeno-associated viruses (AAVs) encoding Designer Receptors Exclusively Activated by Designer Drugs (DREADDs) to test the role of the EWcp in alcohol drinking and in the regulation of body temperature. Activation of the EWcp with excitatory DREADDs inhibited alcohol intake in a 2-bottle choice procedure in male C57BL/6J mice, whereas inhibition of the EWcp with DREADDs had no effect. Surprisingly, analysis of DREADD expression indicated Ucn1-containing neurons of the EWcp did not express DREADDs. In contrast, AAVs transduced non-Ucn1-containing EWcp neurons. Subsequent experiments showed that the inhibitory effect of EWcp activation on alcohol intake was also present in male Ucn1 KO mice, suggesting that a Ucn1-devoid population of EWcp regulates alcohol intake. A final set of chemogenetic experiments showed that activation of Vglut2-expressing EWcp neurons inhibited alcohol intake and induced hypothermia in male and female mice. These studies expand on previous literature by indicating that a glutamatergic, Ucn1-devoid subpopulation of the EWcp regulates alcohol consumption and body temperature.

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.

Agmatine requires GluN2B-containing NMDA receptors to inhibit the development of neuropathic pain

A decarboxylated form of L-arginine, agmatine, preferentially antagonizes NMDArs containing Glun2B subunits within the spinal cord and lacks motor side effects commonly associated with non-subunit-selective NMDAr antagonism, namely sedation and motor impairment. Spinally delivered agmatine has been previously shown to reduce the development of tactile hypersensitivity arising from spinal nerve ligation. The present study interrogated the dependence of agmatine’s alleviation of neuropathic pain (spared nerve injury (SNI) model) on GluN2B-containing NMDArs. SNI-induced hypersensitivity was induced in mice with significant reduction of levels of spinal GluN2B subunit of the NMDAr and their floxed controls. Agmatine reduced development of SNI-induced tactile hypersensitivity in controls but had no effect in subjects with reduced levels of GluN2B subunits. Ifenprodil, a known GluN2B-subunit-selective antagonist, similarly reduced tactile hypersensitivity in controls but not in the GluN2B-deficient mice. In contrast, MK-801, an NMDA receptor channel blocker, reduced hypersensitivity in both control and GluN2B-deficient mice, consistent with a pharmacological pattern expected from a NMDAr antagonist that does not have preference for GluN2B subtypes. Additionally, we observed that spinally delivered agmatine, ifenprodil and MK-801 inhibited nociceptive behaviors following intrathecal delivery of NMDA in control mice. By contrast, in GluN2B-deficient mice, MK-801 reduced NMDA-evoked nociceptive behaviors, but agmatine had a blunted effect and ifenprodil had no effect. These results demonstrate that agmatine requires the GluN2B subunit of the NMDA receptor for inhibitory pharmacological actions in pre-clinical models of NMDA receptor-dependent hypersensitivity.

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

Adolescent alcohol use is one of the strongest predictors for the development of an alcohol use disorder (AUD). Notably, this period of risk coincides with the development of affective disorders, which disproportionately impact and drive problematic drinking behavior in women. Stress is a particularly salient factor that drives relapse during periods of abstinence. Previous work in our lab has shown that adolescent intermittent ethanol vapor (AIE) produces sex-dependent changes in glutamatergic activity in the bed nucleus of the stria terminalis (BNST) and behavioral outcomes following acute restraint stress in adulthood. In females, AIE disrupts group 1 metabotropic glutamate (mGlu1/5) receptor activity and enhances anhedonia-like behavior. The current study site-specifically knocked down mGlu5 receptors in the BNST of male and female Grm5loxp mice, exposed them to AIE, and observed the interaction of AIE and stress on negative affect-like behaviors in adulthood. These negative affect-like behaviors included the novelty-induced hypophagia task following acute restraint stress, open field activity, and contextual fear conditioning. Overall, we replicated our previous findings that AIE enhanced anhedonia-like activity in the novelty-induced hypophagia task in females and fear acquisition in males. The primary effect of BNST-mGlu5 receptor knockdown was that it independently enhanced anhedonia-like activity in females. Correlation analyses revealed that behavior in these paradigms showed poor interdependence. These results indicate that preclinical models of negative affective-like states encompass distinct features that may have independent, clinically relevant mechanisms. Further, modulating mGlu5 receptors is a prospective treatment target for females experiencing anhedonic-like states that make them susceptible to alcohol relapse.

Functional anatomy of the bed nucleus of the stria terminalis-hypothalamus neural circuitry: Implications for valence surveillance, addiction, feeding, and social behaviors

The bed nucleus of the stria terminalis (BNST) is a medial basal forebrain structure that modulates the hypothalamo-pituitary-adrenal (HPA) axis. The heterogeneous subnuclei of the BNST integrate inputs from mood and reward-related areas and send direct inhibitory projections to the hypothalamus. The connections between the BNST and hypothalamus are conserved across species, promote activation of the HPA axis, and can increase avoidance of aversive environments, which is historically associated with anxiety behaviors. However, BNST-hypothalamus circuitry is also implicated in motivated behaviors, drug seeking, feeding, and sexual behavior. These complex and diverse roles, as well its sexual dimorphism, indicate that the BNST-hypothalamus circuitry is an essential component of the neural circuitry that may underlie various psychiatric diseases, ranging from anorexia to anxiety to addiction. The following review is a cross-species exploration of BNST-hypothalamus circuitry. First, we describe the BNST subnuclei, microcircuitry and complex reciprocal connections with the hypothalamus. We will then discuss the behavioral functions of BNST-hypothalamus circuitry, including valence surveillance, addiction, feeding, and social behavior. Finally, we will address sex differences in morphology and function of the BNST and hypothalamus.

NMDA receptors in the CeA and BNST differentially regulate fear conditioning to predictable and unpredictable threats

Considerable work demonstrates that Pavlovian fear conditioning depends on N-methyl-D-aspartate (NMDA) receptor-dependent plasticity within the amygdala. In addition, the bed nucleus of the stria terminalis (BNST) has also been implicated in fear conditioning, particularly in the expression of fear to poor predictors of threat. We recently found that the expression of backward (BW) fear conditioning, in which an auditory conditioned stimulus (CS) follows a footshock unconditioned stimulus (US), requires the BNST; the expression of forward (FW) fear conditioning was not disrupted by BNST inactivation. However, whether NMDA receptors within the BNST contribute to the acquisition of fear conditioning is unknown. Moreover, the central nucleus of the amygdala (CeA), which has extensive connections with the BNST, is critically involved in FW conditioning, however whether it participates in BW conditioning has not been explored. Here we test the specific hypothesis that the CeA and the BNST mediate the acquisition of FW and BW fear conditioning, respectively. Adult female and male rats were randomly assigned to receive bilateral infusions of the NMDA receptor antagonist, D,L-2-amino-5-phosphonovalerate (APV), into the CeA or BNST prior to FW or BW fear conditioning. We found that intra-CeA APV impaired the acquisition of both FW and BW conditioning, whereas intra-BNST APV produced selective deficits in BW conditioning. Moreover, APV in the BNST significantly reduced contextual freezing, whereas CeA NMDA receptor antagonism impeded early but not long-lasting contextual fear. Collectively, these data reveal that CeA and BNST NMDA receptors have unique roles in fear conditioning.

Alcohol drinking alters stress response to predator odor via BNST kappa opioid receptor signaling in male mice

Maladaptive responses to stress are a hallmark of alcohol use disorder, but the mechanisms that underlie this are not well characterized. Here, we show that kappa opioid receptor signaling in the bed nucleus of the stria terminalis (BNST) is a critical molecular substrate underlying abnormal stress responses to predator odor following heavy alcohol drinking. Exposure to predator odor during protracted withdrawal from intermittent alcohol drinking resulted in enhanced prefrontal cortex (PFC)-driven excitation of prodynorphin-containing neurons in the BNST. Furthermore, deletion of prodynorphin in the BNST and chemogenetic inhibition of the PFC-BNST pathway restored abnormal responses to predator odor in alcohol-exposed mice. These findings suggest that increased corticolimbic drive may promote abnormal stress behavioral responses to predator odor during protracted withdrawal. Various nodes of this PFC-BNST dynorphin-related circuit may serve as potential targets for potential therapeutic mediation as well as biomarkers of negative responses to stress following heavy alcohol drinking.

Adolescent alcohol exposure produces sex differences in negative affect-like behavior and group I mGluR BNST plasticity

Adolescent alcohol exposure increases the risk of developing alcohol use disorders (AUDs), yet the mechanisms responsible for this vulnerability remain largely unknown. One potential target for alcohol-induced changes is the circuitry that modulates negative affect and stress, two sexually dependent drivers of alcohol relapse. The bed nucleus of the stria terminalis (BNST) is a sexually dimorphic region that critically regulates negative affective- and stress-induced relapse. Group I metabotropic glutamate receptors (mGluR) are a target of interest due to their regulation of stress, anxiety behaviors, and BNST plasticity. The current studies investigate sex-dependent sensitivity to the effects of adolescent intermittent ethanol vapor exposure (AIE) on negative affect during acute and protracted alcohol withdrawal and following stress in adulthood. This work also assessed whether BNST group I mGluR-mediated long-term depression (LTD) was disrupted at these timepoints. During acute withdrawal, AIE altered LTD induced by the group I mGluR antagonist DHPG in females, but not males. During adulthood, stress unmasked persistent changes in DHPG-induced LTD and behavior that were not present under basal conditions. Females with an AIE history demonstrated enhanced negative affective-like behavior in the novelty-induced hypophagia test following restraint stress-a phenotype that could be blocked with systemic mGluR5 allosteric antagonism via MTEP. Conversely, males with an AIE history demonstrated elevated freezing in a contextual fear conditioning paradigm. These studies demonstrate long-lasting, sex-dependent phenotypes produced by AIE and suggest pharmaceutical interventions for alcohol use and comorbid disorders may be more effective if designed with sex differences in mind.

Dose-dependent induction of CPP or CPA by intra-pVTA ethanol: Role of mu opioid receptors and effects on NMDA receptors

The neurobiological mechanisms underlying alcohol motivational properties are still not fully understood, however, the mu-opioid receptors (MORs) have been evidenced as central elements in the manifestation of the alcohol reinforcing properties. Drug-associated environmental stimuli can trigger alcohol relapse and promote alcohol consumption whereby N-methyl-d-aspartate (NMDA) receptors play a pivotal role. Here we sought to demonstrate, for the first time, that ethanol induces conditioned place preference or aversion (CPP or CPA) when administered locally into the ventral tegmental area (VTA) and the associated role of MORs. We further analyzed the changes in the expression and mRNA levels of GluN1 and GluN2A subunits in designated brain areas. The expression of CPP or CPA was characterized following intra-VTA ethanol administration and we showed that either reinforcing (CPP) or aversive (CPA) properties are dependent on the dose administered (ranging here from 35 to 300 nmol). Furthermore, the critical contribution of local MORs in the acquisition of CPP was revealed by a selective antagonist, namely β-Funaltrexamine. Finally, modifications of the expression of NMDA receptor subunits in the Nucleus Accumbens (NAc) and Hippocampus after ethanol-induced CPP were analyzed at the proteomic and transcriptomic levels by western blot and In Situ Hybridation RNAscope techniques, respectively. Results showed that the mRNA levels of GluN2A but not GluN1 in NAc are higher after ethanol CPP. These novel results pave the way for further characterisation of the mechanisms by which ethanol motivational properties are associated with learned environmental cues.

BNST GluN2D-Containing NMDA Receptors Influence Anxiety- and Depressive-like Behaviors and ModulateCell-Specific Excitatory/Inhibitory Synaptic Balance

Excitatory signaling mediated by NMDARs has been shown to regulate mood disorders. However, current treatments targeting NMDAR subtypes have shown limited success in treating patients, highlighting a need for alternative therapeutic targets. Here, we identify a role for GluN2D-containing NMDARs in modulating emotional behaviors and neural activity in the bed nucleus of the stria terminalis (BNST). Using a GluN2D KO mouse line (GluN2D-/-), we assessed behavioral phenotypes across tasks modeling emotional behavior. We then used a combination of ex vivo electrophysiology and in vivo fiber photometry to assess changes in BNST plasticity, cell-specific physiology, and cellular activity profiles. GluN2D-/- male mice exhibit evidence of exacerbated negative emotional behavior, and a deficit in BNST synaptic potentiation. We also found that GluN2D is functionally expressed on corticotropin-releasing factor (CRF)-positive BNST cells implicated in driving negative emotional states, and recordings in mice of both sexes revealed increased excitatory and reduced inhibitory drive onto GluN2D-/- BNST-CRF cells ex vivo and increased activity in vivo Using a GluN2D conditional KO line (GluN2Dflx/flx) to selectively delete the subunit from the BNST, we find that BNST-GluN2Dflx/flx male mice exhibit increased depressive-like behaviors, as well as altered NMDAR function and increased excitatory drive onto BNST-CRF neurons. Together, this study supports a role for GluN2D-NMDARs in regulating emotional behavior through their influence on excitatory signaling in a region-specific manner, and suggests that these NMDARs may serve as a novel target for selectively modulating glutamate signaling in stress-responsive structures and cell populations.SIGNIFICANCE STATEMENT Excitatory signaling mediated through NMDARs plays an important role in shaping emotional behavior; however, the receptor subtypes/brain regions through which this occurs are poorly understood. Here, we demonstrate that loss of GluN2D-containing NMDARs produces an increase in anxiety- and depressive-like behaviors in mice, deficits in BNST synaptic potentiation, and increased activity in BNST-CRF neurons known to drive negative emotional behavior. Further, we determine that deleting GluN2D in the BNST leads to increased depressive-like behaviors and increased excitatory drive onto BNST-CRF cells. Collectively, these results demonstrate a role for GluN2D-NMDARs in regulating the activity of stress-responsive structures and neuronal populations in the adult brain, suggesting them as a potential target for treating negative emotional states in mood-related disorders.

Regulation of NMDA Receptor Plasticity in the BNST Following Adolescent Alcohol Exposure

Persistent alterations in synaptic plasticity and neurotransmission are thought to underlie the heightened risk of adolescent-onset drinkers to develop alcohol use disorders in adulthood. The bed nucleus of the stria terminalis (BNST) is a compelling region to study the consequences of early alcohol, as it is innervated by cortical structures which undergo continued maturation during adolescence and is critically involved in stress and negative affect-associated relapse. In adult mice, chronic ethanol induces long-term changes in GluN2B-containing NMDA receptors (NMDARs) of the BNST. It remains unclear, however, whether the adolescent BNST is susceptible to such persistent alcohol-induced modifications and, if so, whether they are preserved into adulthood. We therefore examined the short- and long-term consequences of adolescent intermittent ethanol exposure (AIE) on NMDAR transmission and plasticity in the BNST of male and female mice. Whole-cell voltage clamp recordings revealed greater glutamatergic tone in the BNST of AIE-treated males and females relative to air-controls. This change, which corresponded to an increase in presynaptic glutamate release, resulted in altered postsynaptic NMDAR metaplasticity and enhanced GluN2B transmission in males but not females. Only AIE-treated males displayed upregulated GluN2B expression (determined by western blot analysis). While these changes did not persist into adulthood under basal conditions, exposing adult males (but not females) to acute restraint stress reinstated AIE-induced alterations in NMDAR metaplasticity and GluN2B function. These data demonstrate that adolescent alcohol exposure specifically modifies NMDARs in the male BNST, that the plastic changes to NMDARs are long-lasting, and that they can be engaged by stress.

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.

Chronic intermittent ethanol exposure dysregulates a GABAergic microcircuit in the bed nucleus of the stria terminalis

Neuroadaptations in brain regions that regulate emotional and reward-seeking behaviors have been suggested to contribute to pathological behaviors associated with alcohol-use disorder. One such region is the bed nucleus of the stria terminalis (BNST), which has been linked to both alcohol consumption and alcohol withdrawal-induced anxiety and depression. Recently, we identified a GABAergic microcircuit in the BNST that regulates anxiety-like behavior. In the present study, we examined how chronic alcohol exposure alters this BNST GABAergic microcircuit in mice. We selectively targeted neurons expressing corticotropin releasing factor (CRF) using a CRF-reporter mouse line and combined retrograde labeling to identify BNST projections to the ventral tegmental area (VTA) and lateral hypothalamus (LH). Following 72 h of withdrawal from four weekly cycles of chronic intermittent ethanol (CIE) vapor exposure, the excitability of a sub-population of putative local CRF neurons that did not project to either VTA or LH (CRF^non-VTA/LH neurons) was increased. Withdrawal from CIE also increased excitability of non-CRF BNST neurons that project to both LH and VTA (BNST^non-CRF-proj neurons). Furthermore, both populations of neurons had a reduction in spontaneous EPSC amplitude while frequency was unaltered. Withdrawal from chronic alcohol was accompanied by a significant increase in spontaneous IPSC frequency selectively in the BNST^non-CRF-proj neurons. Together, these data suggest that withdrawal from chronic ethanol dysregulates local CRF-GABAergic microcircuit to inhibit anxiolytic outputs of the BNST which may contribute to enhanced anxiety during alcohol withdrawal and drive alcohol-seeking behavior. This article is part of the special issue on 'Neuropeptides'.

Increased Alcohol-Drinking Induced by Manipulations of mGlu5 Phosphorylation within the Bed Nucleus of the Stria Terminalis

The bed nucleus of the stria terminalis (BNST) is part of the limbic-hypothalamic system important for behavioral responses to stress, and glutamate transmission within this region has been implicated in the neurobiology of alcoholism. Herein, we used a combination of immunoblotting, neuropharmacological and transgenic procedures to investigate the role for metabotropic glutamate receptor 5 (mGlu5) signaling within the BNST in excessive drinking. We discovered that mGlu5 signaling in the BNST is linked to excessive alcohol consumption in a manner distinct from behavioral or neuropharmacological endophenotypes that have been previously implicated as triggers for heavy drinking. Our studies demonstrate that, in male mice, a history of chronic binge alcohol-drinking elevates BNST levels of the mGlu5-scaffolding protein Homer2 and activated extracellular signal-regulated kinase (ERK) in an adaptive response to limit alcohol consumption. Male and female transgenic mice expressing a point mutation of mGlu5 that cannot be phosphorylated by ERK exhibit excessive alcohol-drinking, despite greater behavioral signs of alcohol intoxication and reduced anxiety, and are insensitive to local manipulations of signaling in the BNST. These transgenic mice also show selective insensitivity to alcohol-aversion and increased novelty-seeking, which may be relevant to excessive drinking. Further, the insensitivity to alcohol-aversion exhibited by male mice can be mimicked by the local inhibition of ERK signaling within the BNST. Our findings elucidate a novel mGluR5-linked signaling state within BNST that plays a central and unanticipated role in excessive alcohol consumption.SIGNIFICANCE STATEMENT The bed nucleus of the stria terminalis (BNST) is part of the limbic-hypothalamic system important for behavioral responses to stress and alcohol, and glutamate transmission within BNST is implicated in the neurobiology of alcoholism. The present study provides evidence that a history of excessive alcohol drinking increases signaling through the metabotropic glutamate receptor 5 (mGlu5) receptor within the BNST in an adaptive response to limit alcohol consumption. In particular, disruption of mGlu5 phosphorylation by extracellular signal-regulated kinase within this brain region induces excessive alcohol-drinking, which reflects a selective insensitivity to the aversive properties of alcohol intoxication. These data indicate that a specific signaling state of mGlu5 within BNST plays a central and unanticipated role in excessive alcohol consumption.

The intersection of stress and reward: BNST modulation of aversive and appetitive states

The bed nucleus of the stria terminalis (BNST) is widely acknowledged as a brain structure that regulates stress and anxiety states, as well as aversive and appetitive behaviours. The diverse roles of the BNST are afforded by its highly modular organisation, neurochemical heterogeneity, and complex intrinsic and extrinsic circuitry. There has been growing interest in the BNST in relation to psychopathologies such as anxiety and addiction. Although research on the human BNST is still in its infancy, there have been extensive preclinical studies examining the molecular signature and hodology of the BNST and their involvement in stress and reward seeking behaviour. This review examines the neurochemical phenotype and connectivity of the BNST, as well as electrophysiological correlates of plasticity in the BNST mediated by stress and/or drugs of abuse.

Chronic Intermittent Ethanol Exposure Selectively Increases Synaptic Excitability in the Ventral Domain of the Rat Hippocampus

Many studies have implicated hippocampal dysregulation in the pathophysiology of alcohol use disorder (AUD). However, over the past twenty years, a growing body of evidence has revealed distinct functional roles of the dorsal (dHC) and ventral (vHC) hippocampal subregions, with the dHC being primarily involved in spatial learning and memory and the vHC regulating anxiety- and depressive-like behaviors. Notably, to our knowledge, no rodent studies have examined the effects of chronic ethanol exposure on synaptic transmission along the dorsal/ventral axis. To that end, we examined the effects of the chronic intermittent ethanol vapor exposure (CIE) model of AUD on dHC and vHC synaptic excitability. Adult male Long-Evans rats were exposed to CIE or AIR for 10 days (12 h/day; targeting blood ethanol levels of 175-225 mg%) and recordings were made 24 h into withdrawal. As expected, this protocol increased anxiety-like behaviors on the elevated plus-maze and successive alleys test. Extracellular recordings revealed marked CIE-associated increases in synaptic excitation in the CA1 region that were exclusively restricted to the ventral domain of the hippocampus. Western blot analysis of synaptoneurosomal fractions revealed that the expression of two proteins that regulate synaptic strength, GluA2 and SK2, were dysregulated in the vHC, but not the dHC, following CIE. Together, these findings suggest that the ventral CA1 region may be particularly sensitive to the maladaptive effects of chronic ethanol exposure and provide new insight into some of the neural substrates that may contribute to the negative affective state that develops during withdrawal.

Genetic loss of GluN2B in D1-expressing cell types enhances long-term cocaine reward and potentiation of thalamo-accumbens synapses

Transient upregulation of GluN2B-containing NMDA receptors (R) in the nucleus accumbens (NAc) is proposed as an intermediate to long-term AMPAR plasticity associated with persistent cocaine-related behaviors. However, cell type- and input-specific contributions of GluN2B underlying lasting actions of cocaine remain to be elucidated. We utilized GluN2B cell type-specific knockouts and optogenetics to deconstruct the role of GluN2B in cocaine-induced NAc synaptic and behavioral plasticity. While reward learning was unaffected, loss of GluN2B in D1 dopamine receptor-expressing cells (D1) led to prolonged retention of reward memory. In control mice, prefrontal cortex (PFC)-D1(+) NAc AMPAR function was unaffected by cocaine exposure, while midline thalamus (mThal)-D1(+) NAc AMPAR function was potentiated but diminished after withdrawal. In D1-GluN2B^-/- mice, the potentiation of mThal-D1(+) NAc AMPAR function persisted following withdrawal, corresponding with continued expression of cocaine reward behavior. These data suggest NAc GluN2B-containing NMDARs serve a feedback role and may weaken reward-related memories.

Synaptic Plasticity in the Bed Nucleus of the Stria Terminalis: Underlying Mechanisms and Potential Ramifications for Reinstatement of Drug- and Alcohol-Seeking Behaviors

The bed nucleus of the stria terminalis (BNST) is a component of the extended amygdala that shows significant changes in activity and plasticity through chronic exposure to drugs and stress. The region is critical for stress- and cue-induced reinstatement of drug-seeking behaviors and is thus a candidate region for the plastic changes that occur in abstinence that prime addicted patients for reinstatement behaviors. Here, we discuss the various forms of long-term potentiation (LTP) and long-term depression (LTD) in the rodent BNST and highlight the way that these changes in excitatory transmission interact with exposure to alcohol and other drugs of abuse, as well as other stressors. In addition, we highlight potential areas for future research in this area, including investigating input- and cell-specific bidirectional changes in activity. As we continue to accrue foundational knowledge in the mechanisms and effects of plasticity in the BNST, molecular targets and treatment strategies that are relevant to reinstatement behaviors will also begin to emerge. Here, we briefly discuss the effects of catecholamine receptor modulators on synaptic plasticity in the BNST due to the role of norepinephrine in LTD and dopamine on the short-term component of LTP as well as the role that signaling at these receptors plays in reinstatement of drug- and alcohol-seeking behaviors. We hope that insights gained on the specific changes in plasticity that occur within the BNST during abstinence from alcohol and other drugs of abuse will provide insight into the biological underpinnings of relapse behavior in human addicts and inform future treatment modalities for addiction that tackle this complex biological problem.

Dorsal BNST α2A-Adrenergic Receptors Produce HCN-Dependent Excitatory Actions That Initiate Anxiogenic Behaviors

Stress is a precipitating agent in neuropsychiatric disease and initiates relapse to drug-seeking behavior in addicted patients. Targeting the stress system in protracted abstinence from drugs of abuse with anxiolytics may be an effective treatment modality for substance use disorders. α_2A-adrenergic receptors (α_2A-ARs) in extended amygdala structures play key roles in dampening stress responses. Contrary to early thinking, α_2A-ARs are expressed at non-noradrenergic sites in the brain. These non-noradrenergic α_2A-ARs play important roles in stress responses, but their cellular mechanisms of action are unclear. In humans, the α_2A-AR agonist guanfacine reduces overall craving and uncouples craving from stress, yet minimally affects relapse, potentially due to competing actions in the brain. Here, we show that heteroceptor α_2A-ARs postsynaptically enhance dorsal bed nucleus of the stria terminalis (dBNST) neuronal activity in mice of both sexes. This effect is mediated by hyperpolarization-activated cyclic nucleotide-gated cation channels because inhibition of these channels is necessary and sufficient for excitatory actions. Finally, this excitatory action is mimicked by clozapine-N-oxide activation of the G_i-coupled DREADD hM4Di in dBNST neurons and its activation elicits anxiety-like behavior in the elevated plus maze. Together, these data provide a framework for elucidating cell-specific actions of GPCR signaling and provide a potential mechanism whereby competing anxiogenic and anxiolytic actions of guanfacine may affect its clinical utility in the treatment of addiction.SIGNIFICANCE STATEMENT Stress affects the development of neuropsychiatric disorders including anxiety and addiction. Guanfacine is an α2A-adrenergic receptor (α2A-AR) agonist with actions in the bed nucleus of the stria terminalis (BNST) that produces antidepressant actions and uncouples stress from reward-related behaviors. Here, we show that guanfacine increases dorsal BNST neuronal activity through actions at postsynaptic α2A-ARs via a mechanism that involves hyperpolarization-activated cyclic nucleotide gated cation channels. This action is mimicked by activation of the designer receptor hM4Di expressed in the BNST, which also induces anxiety-like behaviors. Together, these data suggest that postsynaptic α2A-ARs in BNST have excitatory actions on BNST neurons and that these actions can be phenocopied by the so-called "inhibitory" DREADDs, suggesting that care must be taken regarding interpretation of data obtained with these tools.

Ketamine administration during a critical period after forced ethanol abstinence inhibits the development of time-dependent affective disturbances

Forced abstinence from chronic two bottle-choice ethanol drinking produces the development of negative affective states in female C57BL/6J mice. We previously reported that this disrupted behavior is acutely reversed by administration of ketamine 30 min-prior to testing. Here we assessed whether ketamine can be used as an inoculant against the development of abstinence- dependent affective disturbances. In parallel, we examined the impact of ketamine administration on long-term potentiation (LTP) in the bed nucleus of the stria terminalis (BNST), a region implicated in affective disturbances. We administered ketamine (3 mg/kg i.p.) to female C57BL/6J mice with a history of chronic ethanol drinking at either the onset, two, or 6 days- post-abstinence and observed its impact on affective behavior in the elevated plus maze (EPM), the Novelty Suppressed Feeding Test (NSFT), and the Forced Swim Test (FST). In addition, we assessed BNST synaptic plasticity with field potential electrophysiology two to 3 weeks into abstinence. We found that early abstinence was associated with disrupted behavior on the EPM. Ketamine administered at the onset of forced abstinence prevented both the deficit in early EPM behavior, and the delayed deficits in NSFT and FST. However, ketamine administered either two or 6 days post-abstinence failed to prevent the abstinence-induced affective disturbances. To begin to explore potential alterations in neural circuit activity that accompanies these actions of ketamine, we assessed the impact of ketamine administration at the onset of forced abstinence and measured LTP induction in the BNST. We find that early ketamine administration persistently increased the capacity for LTP within the BNST. These findings suggest a critical period at the onset of forced abstinence in which ketamine inoculation can prevent the development of affective disturbances, in part by enhancing plasticity within the BNST.

A Role for Prefrontal Cortical NMDA Receptors in Murine Alcohol-Heightened Aggression

Alcohol is associated with nearly half of all violent crimes committed in the United States; yet, a potential neural basis for this type of pathological aggression remains elusive. Alcohol may act on N-methyl-d-aspartate receptors (NMDARs) within cortical circuits to impede processing and to promote aggression. Here, male mice were characterized as alcohol-heightened (AHAs) or alcohol non-heightened aggressors (ANAs) during resident-intruder confrontations after self-administering 1.0 g/kg alcohol (6% w/v) or water. Alcohol produced a pathological-like pattern of aggression in AHAs; these mice shifted their bites to more vulnerable locations on the body of a submissive animal, including the anterior back and ventrum after consuming alcohol. In addition, through immunoblotting, we found that AHAs overexpressed the NMDAR GluN2D subunit in the prefrontal cortex (PFC) as compared to ANAs while the two phenotypes expressed similar levels of GluN1, GluN2A and GluN2B. After identifying several behavioral and molecular characteristics that distinguish AHAs from ANAs, we tested additional mice for their aggression following preferential antagonism of GluN2D-containing NMDARs. In these experiments, groups of AHAs and ANAs self-administered 1.0 g/kg alcohol (6% w/v) or water before receiving intraperitoneal (i.p.) doses of ketamine or memantine, or infusions of memantine directly into the prelimbic (PLmPFC) or infralimbic medial PFC (ILmPFC). Moderate doses of IP ketamine, IP memantine, or intra-PLmPFC memantine increased aggression in AHAs, but only in the absence of alcohol. Prior alcohol intake blocked the pro-aggressive effects of ketamine or memantine. In contrast, only memantine, administered systemically or intra-PLmPFC, interacted with prior alcohol intake to escalate aggression in ANAs. Intra-ILmPFC memantine had no effect on aggression in either AHAs or ANAs. In sum, this work illustrates a potential role of GluN2D-containing NMDARs in the PLmPFC in alcohol-heightened aggression. GluN2D-containing NMDARs are highly expressed on cortical parvalbumin-containing interneurons, suggesting that, in a subset of individuals, alcohol may functionally alter signal integration within cortical microcircuits to dysregulate threat reactivity and promote aggression. This work suggests that targeting GluN2D-NMDARs may be of use in reducing the impact of alcohol-related violence in the human population.

Bidirectional and long-lasting control of alcohol-seeking behavior by corticostriatal LTP and LTD

Addiction is proposed to arise from alterations in synaptic strength via mechanisms of long-term potentiation (LTP) and depression (LTD). However, the causality between these synaptic processes and addictive behaviors is difficult to demonstrate. Here we report that LTP and LTD induction altered operant alcohol self-administration, a motivated drug-seeking behavior. We first induced LTP by pairing presynaptic glutamatergic stimulation with optogenetic postsynaptic depolarization in the dorsomedial striatum, a brain region known to control goal-directed behavior. Blockade of this LTP by NMDA-receptor inhibition unmasked an endocannabinoid-dependent LTD. In vivo application of the LTP-inducing protocol caused a long-lasting increase in alcohol-seeking behavior, while the LTD protocol decreased this behavior. We further identified that optogenetic LTP and LTD induction at cortical inputs onto striatal dopamine D1 receptor-expressing neurons controlled these behavioral changes. Our results demonstrate a causal link between synaptic plasticity and alcohol-seeking behavior and suggest that modulation of this plasticity may inspire a therapeutic strategy for addiction.

Opiate dependence induces cell type-specific plasticity of intrinsic membrane properties in the rat juxtacapsular bed nucleus of stria terminalis (jcBNST)

RATIONALE

Drugs of abuse can alter circuit dynamics by modifying synaptic efficacy and/or the intrinsic membrane properties of neurons. The juxtacapsular subdivision of the bed nucleus of stria terminalis (jcBNST) has unique connectivity that positions it to integrate cortical and amygdala inputs and provide feed-forward inhibition to the central nucleus of the amygdala (CeA), among other regions. In this study, we investigated changes in the synaptic and intrinsic properties of neurons in the rat jcBNST during protracted withdrawal from morphine dependence using a combination of conventional electrophysiological methods and the dynamic clamp technique.

RESULTS

A history of opiate dependence induced a form of cell type-specific plasticity characterized by reduced inward rectification associated with more depolarized resting membrane potentials and increased membrane resistance. This cell type also showed a lower rheobase when stimulated with direct current (DC) pulses as well as a decreased firing threshold under simulated synaptic bombardment with the dynamic clamp. Morphine dependence also decreased excitatory postsynaptic potential amplification, suggesting the downregulation of the persistent Na⁺ current (I _NaP).

CONCLUSION

These findings show that a history of morphine dependence leads to persistent cell type-specific plasticity of the passive membrane properties of a jcBNST neuronal population, leading to an overall increased excitability of such neurons. By altering the activity of extended amygdala circuits where they are embedded, changes in the integration properties of jcBNST neurons may contribute to emotional dysregulation associated with drug dependence and withdrawal.

Corticotropin-Releasing Factor (CRF) and Addictive Behaviors

Drug addiction is a complex disorder that is characterized by compulsivity to seek and take the drug, loss of control in limiting intake of the drug, and emergence of a withdrawal syndrome in the absence of the drug. The transition from casual drug use to dependence is mediated by changes in reward and brain stress functions and has been linked to a shift from positive reinforcement to negative reinforcement. The recruitment of brain stress systems mediates the negative emotional state produced by dependence that drives drug seeking through negative reinforcement mechanisms, defined as the "dark side" of addiction. In this chapter we focus on behavioral and cellular neuropharmacological studies that have implicated brain stress systems (i.e., corticotropin-releasing factor [CRF]) in the transition to addiction and the predominant brain regions involved. We also discuss the implication of CRF recruitment in compulsive eating disorders.

Alcohol induces input-specific aberrant synaptic plasticity in the rat dorsomedial striatum

Accumulated evidence suggests that the dorsomedial striatum (DMS) of the basal ganglia plays an essential role in pathological excessive alcohol consumption. The DMS receives multiple glutamatergic inputs. However, whether and how alcohol consumption distinctly affects these excitatory afferents to the DMS remains unknown. Here, we used optogenetics to selectively activate the rat medial prefrontal cortex (mPFC) and basolateral amygdala (BLA) inputs in DMS slices, and measured the effects of alcohol consumption on glutamatergic transmission in these corticostriatal and amygdalostriatal circuits. We found that excessive alcohol consumption increased AMPA receptor- and NMDA receptor (NMDAR)-mediated neurotransmission, as well as the GluN2B/NMDAR ratio, at the corticostriatal input to the DMS. The probability of glutamate release was increased selectively at the amygdalostriatal input. Interestingly, we discovered that paired activation of the mPFC and BLA inputs using dual-channel optogenetics induced robust long-term potentiation (LTP) of the corticostriatal input to the DMS. Taken together, these results indicate that excessive alcohol consumption potentiates glutamatergic transmission via a postsynaptic mechanism for the corticostriatal input and via a presynaptic mechanism for the amygdalostriatal input. These changes may in turn contribute to pathological alcohol consumption.

The bed nucleus of the stria terminalis in drug-associated behavior and affect: A circuit-based perspective

The bed nucleus of the stria terminalis was first described nearly a century ago and has since emerged as a region central to motivated behavior and affective states. The last several decades have firmly established a role for the BNST in drug-associated behavior and implicated this region in addiction-related processes. Whereas past approaches used to characterize the BNST have focused on a more general role of this region and its subnuclei in behavior, more recent work has begun to reveal its elaborate circuitry and cellular components. Such recent developments are largely owed to methodological advances, which have made possible efforts previously deemed intractable, such as tracing of long-range cell-type specific projections and identifying functional efferent and afferent connections. In this review, we integrate earlier foundational work with more recent and advanced studies to construct a broad overview of the molecular neurocircuitry of the BNST in drug-associated behavior and affect. This article is part of the Special Issue entitled "Alcoholism".

Alcohol and basal ganglia circuitry: Animal models

Brain circuits that include the cortex and basal ganglia make up the bulk of the forebrain, and influence behaviors related to almost all aspects of affective, cognitive and sensorimotor functions. The learning of new actions as well as association of existing action repertoires with environmental events are key functions of this circuitry. Unfortunately, the cortico-basal ganglia circuitry is also the target for all drugs of abuse, including alcohol. This makes the circuitry susceptible to the actions of chronic alcohol exposure that impairs circuit function in ways that contribute to cognitive dysfunction and drug use disorders. In the present review, we describe the connectivity and functions of the associative, limbic and sensorimotor cortico-basal ganglia circuits. We then review the effects of acute and chronic alcohol exposure on circuit function. Finally, we review studies examining the roles of the different circuits and circuit elements in alcohol use and abuse. We attempt to synthesize information from a variety of studies in laboratory animals and humans to generate hypotheses about how the three circuits interact with each other and with the other brain circuits during exposure to alcohol and during the development of alcohol use disorders. This article is part of the Special Issue entitled "Alcoholism".

Glutamate plasticity woven through the progression to alcohol use disorder: a multi-circuit perspective

Glutamate signaling in the brain is one of the most studied targets in the alcohol research field. Here, we report the current understanding of how the excitatory neurotransmitter glutamate, its receptors, and its transporters are involved in low, episodic, and heavy alcohol use. Specific animal behavior protocols can be used to assess these different drinking levels, including two-bottle choice, operant self-administration, drinking in the dark, the alcohol deprivation effect, intermittent access to alcohol, and chronic intermittent ethanol vapor inhalation. Importantly, these methods are not limited to a specific category, since they can be interchanged to assess different states in the development from low to heavy drinking. We encourage a circuit-based perspective beyond the classic mesolimbic-centric view, as multiple structures are dynamically engaged during the transition from positive- to negative-related reinforcement to drive alcohol drinking. During this shift from lower-level alcohol drinking to heavy alcohol use, there appears to be a shift from metabotropic glutamate receptor-dependent behaviors to N-methyl-D-aspartate receptor-related processes. Despite high efficacy of the glutamate-related pharmaceutical acamprosate in animal models of drinking, it is ineffective as treatment in the clinic. Therefore, research needs to focus on other promising glutamatergic compounds to reduce heavy drinking or mediate withdrawal symptoms or both.

A Rare Variant Identified Within the GluN2B C-Terminus in a Patient with Autism Affects NMDA Receptor Surface Expression and Spine Density

NMDA receptors (NMDARs) are ionotropic glutamate receptors that are crucial for neuronal development and higher cognitive processes. NMDAR dysfunction is involved in a variety of neurological and psychiatric diseases; however, the mechanistic link between the human pathology and NMDAR dysfunction is poorly understood. Rare missense variants within NMDAR subunits have been identified in numerous patients with mental or neurological disorders. We specifically focused on the GluN2B NMDAR subunit, which is highly expressed in the hippocampus and cortex throughout development. We analyzed several variants located in the GluN2B C terminus and found that three variants in patients with autism (S1415L) or schizophrenia (L1424F and S1452F) (S1413L, L1422F, and S1450F in rodents, respectively) displayed impaired binding to membrane-associated guanylate kinase (MAGUK) proteins. In addition, we observed a deficit in surface expression for GluN2B S1413L. Furthermore, there were fewer dendritic spines in GluN2B S1413L-expressing neurons. Importantly, synaptic NMDAR currents in neurons transfected with GluN2B S1413L in GluN2A/B-deficient mouse brain slices revealed only partial rescue of synaptic current amplitude. Functional properties of GluN2B S1413L in recombinant systems revealed no change in receptor properties, consistent with synaptic defects being the result of reduced trafficking and targeting of GluN2B S1413L to the synapse. Therefore, we find that GluN2B S1413L displays deficits in NMDAR trafficking, synaptic currents, and spine density, raising the possibility that this mutation may contribute to the phenotype in this autism patient. More broadly, our research demonstrates that the targeted study of certain residues in NMDARs based on rare variants identified in patients is a powerful approach to studying receptor function.SIGNIFICANCE STATEMENT We have used a "bedside-to-bench" approach to investigate the functional regulation of NMDA receptors (NMDARs). Using information from deep sequencing of patients with neurological or psychiatric disorders, we investigated missense variants identified in the intracellular C-terminal domain of the GluN2B NMDAR subunit. We found several variants that displayed altered properties. In particular, one variant identified in a patient with autism, human GluN2B S1415L, displayed reduced surface expression and binding to PSD-95. Furthermore expression of GluN2B S1415L (S1413L in mouse) showed a deficit in rescue of synaptic NMDAR currents and fewer dendritic spines, consistent with other reports of spine abnormalities being associated with autism. More broadly, we demonstrate that using patient data is an effective approach to probing the structure/function relationship of NMDARs.

Re-exposure to morphine-associated context facilitated long-term potentiation in the vSUB-NAc glutamatergic pathway via GluN2B-containing receptor activation

The glutamatergic projection from the ventral subiculum of the hippocampus (vSUB) to the nucleus accumbens (NAc) shell has been reported to play a key role in drug-related behavior. The GluN2B subunit of N-methyl-D-aspartate receptors (NMDARs) in the NAc can be selectively elevated after the retrieval of drug-conditioned memory. However, whether the increased GluN2B-containing NMDARs (GluN2B-NMDARs) are able to alter the synaptic plasticity of the vSUB-NAc glutamatergic pathway remains unclear. Here, we found that the long-term potentiation (LTP) in the vSUB-NAc pathway was facilitated and the GluN2B subunit protein level was elevated in synaptoneurosomes of the NAc shell, but not in the core, following morphine-induced conditioned place preference (CPP) expression in rats. The facilitated LTP was prevented by the GluN2B-NMDAR antagonist RO25-6981. Also, a neurochemical disconnection following microinjection of RO25-6981 into the NAc shell, plus microinfusion of GABA agonist baclofen and muscimol into the contralateral vSUB prevented the expression of morphine-induced CPP. These findings suggest that the retrieval of drug-associated memory potentiated synaptic plasticity in the vSUB-NAc pathway, which was dependent on GluN2B-NMDAR activation in the NAc shell. These findings provide a new explanation for the mechanisms that underlie the morphine-associated-context memory. The GluN2B-NMDARs may be regarded as a potential target for erasing morphine-related memory.

Chronic intermittent alcohol disrupts the GluN2B-associated proteome and specifically regulates group I mGlu receptor-dependent long-term depression

N-Methyl-d-aspartate receptors (NMDARs) are major targets of both acute and chronic alcohol, as well as regulators of plasticity in a number of brain regions. Aberrant plasticity may contribute to the treatment resistance and high relapse rates observed in alcoholics. Recent work suggests that chronic alcohol treatment preferentially modulates both the expression and subcellular localization of NMDARs containing the GluN2B subunit. Signaling through synaptic and extrasynaptic GluN2B-NMDARs has already been implicated in the pathophysiology of various other neurological disorders. NMDARs interact with a large number of proteins at the glutamate synapse, and a better understanding of how alcohol modulates this proteome is needed. We employed a discovery-based proteomic approach in subcellular fractions of hippocampal tissue from chronic intermittent alcohol (CIE)-exposed C57Bl/6J mice to gain insight into alcohol-induced changes in GluN2B signaling complexes. Protein enrichment analyses revealed changes in the association of post-synaptic proteins, including scaffolding, glutamate receptor and PDZ-domain binding proteins with GluN2B. In particular, GluN2B interaction with metabotropic glutamate (mGlu)1/5 receptor-dependent long-term depression (LTD)-associated proteins such as Arc and Homer 1 was increased, while GluA2 was decreased. Accordingly, we found a lack of mGlu1/5 -induced LTD while α1 -adrenergic receptor-induced LTD remained intact in hippocampal CA1 following CIE. These data suggest that CIE specifically disrupts mGlu1/5 -LTD, representing a possible connection between NMDAR and mGlu receptor signaling. These studies not only demonstrate a new way in which alcohol can modulate plasticity in the hippocampus but also emphasize the utility of this discovery-based proteomic approach to generate new hypotheses regarding alcohol-related mechanisms.

Reduced ethanol drinking following selective cortical interneuron deletion of the GluN2B NMDA receptors subunit

N-Methyl-d-aspartate receptors (NMDAR) are involved in the regulation of alcohol drinking, but the contribution of NMDAR subunits located on specific neuronal populations remains incompletely understood. The current study examined the role of GluN2B-containing NMDARs expressed on cortical principal neurons and cortical interneurons in mouse ethanol drinking. Consumption of escalating concentrations of ethanol was measured in mice with GluN2B gene deletion in either cortical principal neurons (GluN2B^CxNULL) or interneurons (GluN2B^InterNULL), using a two-bottle choice paradigm. Results showed that GluN2B^InterNULL, but not GluN2B^CxNULL, mice consumed significantly less ethanol, at relatively high concentrations, than non-mutant controls. In a second paradigm in which mice were offered a 15% ethanol concentration, without escalation, GluN2B^CxNULL mice were again no different from controls. These findings provide novel evidence for a contribution of interneuronal GluN2B-containing NMDARs in the regulation of ethanol drinking.

Do specific NMDA receptor subunits act as gateways for addictive behaviors?

Addiction to alcohol and drugs is a major social and economic problem, and there is considerable interest in understanding the molecular mechanisms that promote addictive drives. A number of proteins have been identified that contribute to expression of addictive behaviors. NMDA receptors (NMDARs), a subclass of ionotropic glutamate receptors, have been of particular interest because their physiological properties make them an attractive candidate for gating induction of synaptic plasticity, a molecular change thought to mediate learning and memory. NMDARs are generally inactive at the hyperpolarized resting potentials of many neurons. However, given sufficient depolarization, NMDARs are activated and exhibit long-lasting currents with significant calcium permeability. Also, in addition to stimulating neurons by direct depolarization, NMDARs and their calcium signaling can allow strong and/or synchronized inputs to produce long-term changes in other molecules (such as AMPA-type glutamate receptors) which can last from days to years, binding internal and external stimuli in a long-term memory trace. Such memories could allow salient drug-related stimuli to exert strong control over future behaviors and thus promote addictive drives. Finally, NMDARs may themselves undergo plasticity, which can alter subsequent neuronal stimulation and/or the ability to induce plasticity. This review will address recent and past findings suggesting that NMDAR activity promotes drug- and alcohol-related behaviors, with a particular focus on GluN2B subunits as possible central regulators of many addictive behaviors, as well as newer studies examining the importance of non-canonical NMDAR subunits and endogenous NMDAR cofactors.

Changes in the Adult GluN2B Associated Proteome following Adolescent Intermittent Ethanol Exposure

Adolescent alcohol use is the strongest predictor for alcohol use disorders. In rodents, adolescents have distinct responses to acute ethanol, and prolonged alcohol exposure during adolescence can maintain these phenotypes into adulthood. One brain region that is particularly sensitive to the effects of both acute and chronic ethanol exposure is the hippocampus. Adolescent intermittent ethanol exposure (AIE) produces long lasting changes in hippocampal synaptic plasticity and dendritic morphology, as well as in the susceptibility to acute ethanol-induced spatial memory impairment. Given the pattern of changes in hippocampal structure and function, one potential target for these effects is the ethanol sensitive GluN2B subunit of the NMDA receptor, which is known to be involved in synaptic plasticity and dendritic morphology. Thus we sought to determine if there were persistent changes in hippocampal GluN2B signaling cascades following AIE. We employed a previously validated GluN2B-targeted proteomic strategy that was used to identify novel signaling mechanisms altered by chronic ethanol exposure in the adult hippocampus. We collected adult hippocampal tissue (P70) from rats that had been given 2 weeks of AIE from P30-45. Tissue extracts were fractionated into synaptic and non-synaptic pools, immuno-precipitated for GluN2B, and then analyzed using proteomic methods. We detected a large number of proteins associated with GluN2B. AIE produced significant changes in the association of many proteins with GluN2B in both synaptic and non-synaptic fractions. Intriguingly the number of proteins changed in the non-synaptic fraction was double that found in the synaptic fraction. Some of these proteins include those involved in glutamate signaling cytoskeleton rearrangement, calcium signaling, and plasticity. Disruptions in these pathways may contribute to the persistent cellular and behavioral changes found in the adult hippocampus following AIE. Further, the robust change in non-synaptic proteins suggests that AIE may prime this signaling pathway for future ethanol exposures in adulthood.

Intersubunit interactions at putative sites of ethanol action in the M3 and M4 domains of the NMDA receptor GluN1 and GluN2B subunits

BACKGROUND AND PURPOSE

The NMDA receptor is an important target of alcohol action in the brain. Recent studies in this laboratory have demonstrated that alcohol-sensitive positions in the intersubunit interfaces of the M3 and M4 domains of GluN1 and GluN2A subunits interact with respect to ethanol sensitivity and receptor kinetics and that alcohol-sensitive positions in the M domains of GluN2A and GluN2B subunits differ. In this study, we tested for interactions among alcohol-sensitive positions at the M domain intersubunit interfaces in GluN1/GluN2B NMDA receptors.

EXPERIMENTAL APPROACH

We used whole-cell patch-clamp recording in tsA201 cells expressing tryptophan substitution mutants at ethanol-sensitive positions in the GluN1 and GluN2B NMDA receptor subunits to test for interactions among positions.

KEY RESULTS

Six pairs of positions in GluN1/GluN2B significantly interacted to regulate ethanol inhibition: Gly(638) /Met(824) , Gly(638) /Leu(825) , Phe(639) /Leu(825) , Phe(639) /Gly(826) , Met(818) /Phe(637) and Val(820) /Phe(637) . Tryptophan substitution at Met(824) or Leu(825) in GluN2B did not alter ethanol sensitivity but interacted with positions in the GluN1 M3 domain to regulate ethanol action, whereas tryptophan substitution at Gly(638) , which is the cognate of an ethanol-sensitive position in GluN2A, did not alter ethanol sensitivity or interact with positions in GluN1. Two and three pairs of positions interacted to regulate glutamate steady-state and peak current EC50 , respectively, and one pair interacted with respect to macroscopic desensitization.

CONCLUSIONS

Despite highly-conserved M domain sequences and similar ethanol sensitivity in the GluN2A and GluN2B subunits, the manner in which these subunits interact with the GluN1 subunit to regulate ethanol sensitivity and receptor kinetics differs.

Glutamatergic plasticity and alcohol dependence-induced alterations in reward, affect and cognition

INTRODUCTION

Alcohol dependence is characterized by a reduction in reward threshold, development of a negative affective state, and significant cognitive impairments. Dependence-induced glutamatergic neuroadaptations in the neurocircuitry mediating reward, affect and cognitive function are thought to underlie the neural mechanism for these alterations. These changes serve to promote increased craving for alcohol and facilitate the development of maladaptive behaviors that promote relapse to alcohol drinking during periods of abstinence.

OBJECTIVE

To review the extant literature on the effects of chronic alcohol exposure on glutamatergic neurotransmission and its impact on reward, affect and cognition.

RESULTS

Evidence from a diverse set of studies demonstrates significant enhancement of glutamatergic activity following chronic alcohol exposure. In particular, up-regulation of GluN2B-containing NMDA receptor expression and function is a commonly observed phenomenon that likely reflects activity-dependent adaptive homeostatic plasticity. However, this observation as well as other glutamatergic neuroadaptations are often circuit and cell-type specific.

DISCUSSION

Dependence-induced alterations in glutamate signaling contribute to many of the symptoms experienced in addicted individuals and can persist well into abstinence. This suggests that they play an important role in the development of behaviors that increase the probability for relapse. As our understanding of the complexity of the neurocircuitry involved in the addictive process has advanced, it has become increasingly clear that investigations of cell-type and circuit-specific effects are required to gain a more comprehensive understanding of the glutamatergic adaptations and their functional consequences in alcohol addiction.

CONCLUSION

While pharmacological treatments for alcohol dependence and relapse targeting the glutamatergic system have shown great promise in preclinical models, more research is needed to uncover novel, possibly circuit-specific, therapeutic targets that exhibit improved efficacy and reduced side effects.

The Human BNST: Functional Role in Anxiety and Addiction

The consequences of chronic stress on brain structure and function are far reaching. Whereas stress can produce short-term adaptive changes in the brain, chronic stress leads to long-term maladaptive changes that increase vulnerability to psychiatric disorders, such as anxiety and addiction. These two disorders are the most prevalent psychiatric disorders in the United States, and are typically chronic, disabling, and highly comorbid. Emerging evidence implicates a tiny brain region-the bed nucleus of the stria terminalis (BNST)-in the body's stress response and in anxiety and addiction. Rodent studies provide compelling evidence that the BNST plays a central role in sustained threat monitoring, a form of adaptive anxiety, and in the withdrawal and relapse stages of addiction; however, little is known about the role of BNST in humans. Here, we review current evidence for BNST function in humans, including evidence for a role in the production of both adaptive and maladaptive anxiety. We also review preliminary evidence of the role of BNST in addiction in humans. Together, these studies provide a foundation of knowledge about the role of BNST in adaptive anxiety and stress-related disorders. Although the field is in its infancy, future investigations of human BNST function have tremendous potential to illuminate mechanisms underlying stress-related disorders and identify novel neural targets for treatment.

Alcohol Elicits Functional and Structural Plasticity Selectively in Dopamine D1 Receptor-Expressing Neurons of the Dorsomedial Striatum

Addiction is thought to be a maladaptive form of learning and memory caused by drug-evoked aberrant synaptic plasticity. We previously showed that alcohol facilitates synaptic plasticity in the dorsomedial striatum (DMS), a brain region that drives goal-directed behaviors. The majority of DMS cells are medium spiny neurons (MSNs) that express dopamine D1 receptors (D1Rs) or D2 receptors (D2Rs), which drive "Go" or "No-Go" behaviors, respectively. Here, we report that alcohol induces cell type-specific synaptic and structural plasticity in the DMS. Using mice that express a fluorescence marker to visualize D1R or D2R MSNs, we show that repeated cycles of systemic administration of alcohol or alcohol consumption induces a long-lasting increase in AMPAR activity specifically in DMS D1R but not in D2R MSNs. Importantly, we report that alcohol consumption increases the complexity of dendritic branching and the density of mature mushroom-shaped spines selectively in DMS D1R MSNs. Finally, we found that blockade of D1R but not D2R activity in the DMS attenuates alcohol consumption. Together, these data suggest that alcohol intake produces profound functional and structural plasticity events in a subpopulation of neurons in the DMS that control reinforcement-related learning.

SIGNIFICANCE STATEMENT

Alcohol addiction is considered maladaptive learning and memory processes. Here we unraveled a long-lasting cellular mechanism that may contribute to the memory of alcohol-seeking behaviors. Specifically, we found that alcohol consumption produces a long-lasting enhancement of channel activity and persistent alterations of neuronal morphology in a part of the brain (DMS) that controls alcohol-drinking behaviors. Furthermore, we show that these alterations occur only in a subpopulation of neurons that positively control reward and reinforcement of drugs of abuse. Finally, we report that blocking the activity of this neuronal population reduces alcohol intake. As such synaptic and structural changes are the cellular hallmarks of learning and memory, and these neuroadaptations may drive the development of pathological heavy alcohol consumption.