Inhibition of AMPA receptors (AMPARs) containing transmembrane AMPAR regulatory protein γ-8 with JNJ-55511118 shows preclinical efficacy in reducing chronic repetitive alcohol self-administration

TARP ɣ-8 is a target of ethanol that regulates self-administration and relapse in mice

Background

Behavioral pathologies that characterize alcohol use disorder (AUD) are driven by the powerful reinforcing, or rewarding, properties of the drug. We have shown that glutamate AMPA receptor (AMPAR) activity is both necessary and sufficient for alcohol (ethanol) reinforcement. Transmembrane AMPAR regulatory protein (TARP) γ-8 is an essential auxiliary protein that regulates AMPAR expression and activity; however, the role of TARP ɣ-8 in AUD or other forms of addiction remains largely unexplored.

Objectives

This study investigated the mechanistic role of TARP γ-8 in operant ethanol self-administration (model of primary reinforcement) and cue-induced reinstatement of ethanol-seeking behavior (model of conditioned reinforcement) using TARP ɣ-8 heterozygous null (+/-) mice. To determine if TARP ɣ-8 signaling is targeted by ethanol use, we evaluated protein expression of TARP γ-8, GluA1, CaMKII, and PSD-95 following ethanol self-administration.

Results

A battery of tests evaluating food and water intake, taste reactivity, anxiety-like behavior, and object recognition memory showed no fundamental behavioral deficits in TARP γ-8 (+/-) mice, and no differences in response to acute ethanol or home-cage drinking as compared to wild-types. However, TARP γ-8 (+/-) mice exhibited significantly reduced acquisition and escalation of operant ethanol self-administration and reduced cue-induced reinstatement of ethanol-seeking behavior, with no differences in parallel sucrose-only controls. In wild-type mice, ethanol self-administration increased TARP γ-8 expression in the amygdala, nucleus accumbens, and hippocampus, and increased GluA1 expression in the amygdala and prefrontal cortex, compared to sucrose controls.

Conclusion

These findings highlight the specificity of TARP ɣ-8 regulation of ethanol reinforcement mechanisms and identify this crucial AMPAR auxiliary protein as a target of ethanol in reward-related brain regions, highlighting its potential for development of novel pharmacotherapies for AUD.

Distinct sex differences in ethanol consumption and operant self-administration in C57BL/6J mice with uniform regulation by glutamate AMPAR activity

Introduction

Considering sex as a biological variable (SABV) in preclinical research can enhance understanding of the neurobiology of alcohol use disorder (AUD). However, the behavioral and neural mechanisms underlying sex-specific differences remain unclear. This study aims to elucidate SABV in ethanol (EtOH) consumption by evaluating its reinforcing effects and regulation by glutamate AMPA receptor activity in male and female mice.

Methods

C57BL/6J mice (male and female) were assessed for EtOH intake under continuous and limited access conditions in the home cage. Acute sensitivity to EtOH sedation and blood clearance were evaluated as potential modifying factors. Motivation to consume EtOH was measured using operant self-administration procedures. Sex-specific differences in neural regulation of EtOH reinforcement were examined by testing the effects of a glutamate AMPA receptor antagonist on operant EtOH self-administration.

Results

Female C57BL/6J mice exhibited a time-dependent escalation in EtOH intake under both continuous and limited access conditions. They were less sensitive to EtOH sedation and had lower blood levels post-EtOH administration (4 g/kg) despite similar clearance rates. Females also showed increased operant EtOH self-administration and progressive ratio performance over a 30-day baseline period compared to males. The AMPAR antagonist GYKI 52466 (0-10 mg/kg, IP) dose-dependently reduced EtOH-reinforced lever pressing in both sexes, with no differences in potency or efficacy.

Discussion

These findings confirm that female C57BL/6J mice consume more EtOH than males in home-cage conditions and exhibit reduced acute sedation, potentially contributing to higher EtOH intake. Females demonstrated increased operant EtOH self-administration and motivation, indicating higher reinforcing efficacy. The lack of sex differences in the relative effects of GYKI 52466 suggests that AMPAR activity is equally required for EtOH reinforcement in both sexes.

Ethanol self-administration targets GluA2-containing AMPA receptor expression and synaptic activity in the nucleus accumbens in a manner that drives the positive reinforcing properties of the drug

RATIONALE

The positive reinforcing effects of alcohol (ethanol) drive repetitive use and contribute to alcohol use disorder (AUD). Ethanol alters the expression of glutamate AMPA receptor (AMPAR) subunits in reward-related brain regions, but the extent to which this effect regulates ethanol's reinforcing properties is unclear.

OBJECTIVE

This study investigates whether ethanol self-administration changes AMPAR subunit expression and synaptic activity in the nucleus accumbens core (AcbC) to regulate ethanol's reinforcing effects in male C57BL/6 J mice.

RESULTS

Sucrose-sweetened ethanol self-administration (0.81 g/kg/day) increased AMPAR GluA2 protein expression in the AcbC, without effect on GluA1, compared to sucrose-only controls. Infusion of myristoylated Pep2m in the AcbC, which blocks GluA2 binding to N-ethylmaleimide-sensitive fusion protein (NSF) and reduces GluA2-containing AMPAR activity, reduced ethanol-reinforced responding without affecting sucrose-only self-administration or motor activity. Antagonizing GluA2-lacking AMPARs, through AcbC infusion of NASPM, had no effect on ethanol self-administration. AcbC neurons receiving projections from the basolateral amygdala (BLA) showed increased sEPSC area under the curve (a measurement of charge transfer) and slower decay kinetics in ethanol self-administering mice as compared to sucrose. Optogenetic activation of these neurons revealed an ethanol-enhanced AMPA/NMDA ratio and significantly reduced paired-pulse ratio, suggesting elevated GluA2 contributions specifically within the BLA➔AcbC pathway.

CONCLUSIONS

Ethanol use upregulates GluA2 protein expression in the AcbC and AMPAR synaptic activity in AcbC neurons receiving BLA projections and enhances synaptic plasticity directly within the BLA➔AcbC circuit. GluA2-containing AMPAR activity in the AcbC regulates the positive reinforcing effects of ethanol through an NSF-dependent mechanism, highlighting a potential therapeutic target in AUD.

Sex Differences in Home-Cage Ethanol Drinking and Operant Self-Administration in C57BL/6J Mice with Equivalent Regulation by Glutamate AMPAR Activity

Introduction

Considering sex as a biological variable (SABV) in preclinical research can enhance understanding of the neurobiology of alcohol use disorder (AUD). However, the behavioral and neural mechanisms underlying sex-specific differences remain unclear. This study aims to elucidate SABV in ethanol (EtOH) consumption by evaluating its reinforcing effects and regulation by glutamate AMPA receptor activity in male and female mice.

Methods

C57BL/6J mice (male and female) were assessed for EtOH intake under continuous and limited access conditions in the home cage. Acute sensitivity to EtOH sedation and blood clearance were evaluated as potential modifying factors. Motivation to consume EtOH was measured using operant self-administration procedures. Sex-specific differences in neural regulation of EtOH reinforcement were examined by testing the effects of a glutamate AMPA receptor antagonist on operant EtOH self-administration.

Results

Female C57BL/6J mice exhibited a time-dependent escalation in EtOH intake under both continuous and limited access conditions. They were less sensitive to EtOH sedation and had lower blood levels post-EtOH administration (4 g/kg) despite similar clearance rates. Females also showed increased operant EtOH self-administration and progressive ratio performance over a 30-day baseline period compared to males. The AMPAR antagonist GYKI 52466 (0-10 mg/kg, IP) dose-dependently reduced EtOH-reinforced lever pressing in both sexes, with no differences in potency or efficacy.

Discussion

These findings confirm that female C57BL/6J mice consume more EtOH than males in home-cage conditions and exhibit reduced acute sedation, potentially contributing to higher EtOH intake. Females demonstrated increased operant EtOH self-administration and motivation, indicating higher reinforcing efficacy. The lack of sex differences in the relative effects of GYKI 52466 suggests that AMPAR activity is equally required for EtOH reinforcement in both sexes.

Operant alcohol self-administration targets GluA2-containing AMPA receptor expression and synaptic activity in the nucleus accumbens in a manner that drives the positive reinforcing properties of the drug

Rationale : The positive reinforcing effects of alcohol (ethanol) drive its repetitive use and contribute to alcohol use disorder (AUD). Ethanol alters the expression of glutamate AMPA receptor (AMPAR) subunits in reward-related brain regions, but the extent to which this effect regulates ethanol's reinforcing properties is unclear. Objective: This study investigates whether ethanol self-administration changes AMPAR subunit expression and synaptic activity in the nucleus accumbens core (AcbC) to regulate ethanol's reinforcing effects in male C57BL/6J mice. Results: Sucrose-sweetened ethanol self-administration (0.81 g/kg/day) increased AMPAR GluA2 protein expression in the AcbC, without effect on GluA1, compared to sucrose-only controls. Infusion of myristoylated Pep2m in the AcbC, which blocks GluA2 binding to N-ethylmaleimide-sensitive fusion protein (NSF) and reduces GluA2-containing AMPAR activity, reduced ethanol-reinforced responding without affecting sucrose-only self-administration or motor activity. Antagonizing GluA2-lacking AMPARs, through AcbC infusion of NASPM, had no effect on ethanol self-administration. AcbC neurons receiving projections from the basolateral amygdala (BLA) showed increased sEPSC area under the curve (a measurement of charge transfer) and slower decay kinetics in ethanol self-administering mice as compared to sucrose. Optogenetic activation of these neurons revealed an ethanol-enhanced AMPA/NMDA ratio and significantly reduced paired-pulse ratio, suggesting elevated GluA2 contributions specifically within the BLA→AcbC pathway. Conclusions: Ethanol use upregulates GluA2 protein expression in the AcbC and AMPAR synaptic activity in AcbC neurons receiving BLA projections and enhances synaptic plasticity directly within the BLA→AcbC circuit. GluA2-containing AMPAR activity in the AcbC regulates the positive reinforcing effects of ethanol through an NSF-dependent mechanism, highlighting a potential therapeutic target in AUD.

Roles of AMPA receptors in social behaviors

As a crucial player in excitatory synaptic transmission, AMPA receptors (AMPARs) contribute to the formation, regulation, and expression of social behaviors. AMPAR modifications have been associated with naturalistic social behaviors, such as aggression, sociability, and social memory, but are also noted in brain diseases featuring impaired social behavior. Understanding the role of AMPARs in social behaviors is timely to reveal therapeutic targets for treating social impairment in disorders, such as autism spectrum disorder and schizophrenia. In this review, we will discuss the contribution of the molecular composition, function, and plasticity of AMPARs to social behaviors. The impact of targeting AMPARs in treating brain disorders will also be discussed.

Prefrontal cortex glutamatergic adaptations in a mouse model of alcohol use disorder

Alcohol use disorder (AUD) produces cognitive deficits, indicating a shift in prefrontal cortex (PFC) function. PFC glutamate neurotransmission is mostly mediated by α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid-type ionotropic receptors (AMPARs); however preclinical studies have mostly focused on other receptor subtypes. Here we examined the impact of early withdrawal from chronic ethanol on AMPAR function in the mouse medial PFC (mPFC). Dependent male C57BL/6J mice were generated using the chronic intermittent ethanol vapor-two bottle choice (CIE-2BC) paradigm. Non-dependent mice had access to water and ethanol bottles but did not receive ethanol vapor. Naïve mice had no ethanol exposure. We used patch-clamp electrophysiology to measure glutamate neurotransmission in layer 2/3 prelimbic mPFC pyramidal neurons. Since AMPAR function can be impacted by subunit composition or plasticity-related proteins, we probed their mPFC expression levels. Dependent mice had higher spontaneous excitatory postsynaptic current (sEPSC) amplitude and kinetics compared to the Naïve/Non-dependent mice. These effects were seen during intoxication and after 3-8 days withdrawal, and were action potential-independent, suggesting direct enhancement of AMPAR function. Surprisingly, 3 days withdrawal decreased expression of genes encoding AMPAR subunits (Gria1/2) and synaptic plasticity proteins (Dlg4 and Grip1) in Dependent mice. Further analysis within the Dependent group revealed a negative correlation between Gria1 mRNA levels and ethanol intake. Collectively, these data establish a role for mPFC AMPAR adaptations in the glutamatergic dysfunction associated with ethanol dependence. Future studies on the underlying AMPAR plasticity mechanisms that promote alcohol reinforcement, seeking, drinking and relapse behavior may help identify new targets for AUD treatment.

Negative modulation of AMPA receptors bound to transmembrane AMPA receptor regulatory protein γ-8 blunts the positive reinforcing properties of alcohol and sucrose in a brain region-dependent manner in male mice

RATIONALE

The development and progression of alcohol use disorder (AUD) are widely viewed as maladaptive neuroplasticity. The transmembrane alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptor (AMPAR) regulatory protein γ8 (TARP γ-8) is a molecular mechanism of neuroplasticity that has not been evaluated in AUD or other addictions.

OBJECTIVE

To address this gap in knowledge, we evaluated the mechanistic role of TARP γ-8 bound AMPAR activity in the basolateral amygdala (BLA) and ventral hippocampus (vHPC) in the positive reinforcing effects of alcohol, which drive repetitive alcohol use throughout the course of AUD, in male C57BL/6 J mice. These brain regions were selected because they exhibit high levels of TARP γ-8 expression and send glutamate projections to the nucleus accumbens (NAc), which is a key nucleus in the brain reward pathway.

METHODS AND RESULTS

Site-specific pharmacological inhibition of AMPARs bound to TARP γ-8 in the BLA via bilateral infusion of the selective negative modulator JNJ-55511118 (0-2 µg/µl/side) significantly decreased operant alcohol self-administration with no effect on sucrose self-administration in behavior-matched controls. Temporal analysis showed that reductions in alcohol-reinforced response rate occurred > 25 min after the onset of responding, consistent with a blunting of the positive reinforcing effects of alcohol in the absence of nonspecific behavioral effects. In contrast, inhibition of TARP γ-8 bound AMPARs in the vHPC selectively decreased sucrose self-administration with no effect on alcohol.

CONCLUSIONS

This study reveals a novel brain region-specific role of TARP γ-8 bound AMPARs as a molecular mechanism of the positive reinforcing effects of alcohol and non-drug rewards.

Small-animal PET study for noninvasive quantification of transmembrane AMPA receptor regulatory protein -8 (TARP -8) in the brain

Transmembrane AMPA receptor regulatory protein γ-8 (TARP γ-8) mediates various AMPA receptor functions. Recently, [¹¹C]TARP-2105 was developed as a PET ligand for TARP γ-8 imaging. We performed a full kinetic analysis of [¹¹C]TARP-2105 using PET with [¹¹C]TARP-2105 for the first time. The distribution volume (V_T), which is a macro parameter consisting of the K₁-k₄ rate constants in the two-tissue compartment model analysis, exhibited the following rank order: hippocampus (1.4 ± 0.3) > amygdala (1.0 ± 0.2) > frontal cortex (0.9 ± 0.2) > striatum (0.8 ± 0.2) ≫ cerebellum (0.5 ± 0.1) ≈ thalamus (0.5 ± 0.1) > pons (0.4 ± 0.1 mL/cm³). These heterogenous V_T values corresponded with the order of biological distribution of TARP γ-8 in the brain. To validate the reference tissue model, the binding potential (BP_ND) of [¹¹C]TARP-2105 for TARP γ-8 was estimated using general methods (SRTM, MRTM0, Logan reference model, and ratio method). These BP_NDs based on reference models indicated excellent correlation (R² > 0.9) to the indirect BP_NDs based on 2TCM with moderate reproducibility (%variability ≈ 10). PET with [¹¹C]TARP-2105 enabled noninvasive BP_ND estimation and visual mapping of TARP γ-8 in the living rat brain.

Postsynaptic signaling at glutamatergic synapses as therapeutic targets

Dysregulation of glutamatergic synapses plays an important role in the pathogenesis of neurological diseases. In addition to mediating excitatory synaptic transmission, postsynaptic glutamate receptors interact with various membrane and intracellular proteins. They form structural and/or signaling synaptic protein complexes and thereby play diverse postsynaptic functions. Recently, several postsynaptic protein complexes have been associated with various neurological diseases and hence, have been characterized as important therapeutic targets. Moreover, novel small molecules and therapeutic peptides targeting and modulating the activities of these protein complexes have been discovered, some of which have advanced through preclinical translational research and/or clinical studies. This article describes the recent investigation of eight key protein complexes associated with the postsynaptic ionotropic and metabotropic glutamate receptors as therapeutic targets for central nervous system diseases.

Shortening time for access to alcohol drives up front-loading behavior, bringing consumption in male rats to the level of females

Background

Incentives to promote drinking (“happy hour”) can encourage faster rates of alcohol consumption, especially in women. Sex differences in drinking dynamics may underlie differential health vulnerabilities relating to alcohol in women versus men. Herein, we used operant procedures to model the happy hour effect and gain insight into the alcohol drinking dynamics of male and female rats.

Methods

Adult male and female Wistar rats underwent operant training to promote voluntary drinking of 10% (w/v) alcohol (8 rats/sex). We tested how drinking patterns changed after manipulating the effort required for alcohol (fixed ratio, FR), as well as the length of time in which rats had access to alcohol (self-administration session length). Rats were tested twice within the 12 h of the dark cycle, first at 2 h (early phase of the dark cycle, “early sessions”) and then again at 10 h into the dark cycle (late phase of the dark cycle, “late sessions”) with an 8-h break between the two sessions in the home cage.

Results

Adult females consumed significantly more alcohol (g/kg) than males in the 30-min sessions with the FR1 schedule of reinforcement when tested late in the dark cycle. Front-loading of alcohol was the primary factor driving higher consumption in females. Changing the schedule of reinforcement from FR1 to FR3 reduced total consumption. Notably, this manipulation had minimal effect on front-loading behavior in females, whereas front-loading behavior was significantly reduced in males when more effort was required to access alcohol. Compressing drinking access to 15 min to model a happy hour drove up front-loading behavior, generating alcohol drinking patterns in males that were similar to patterns in females (faster drinking and higher intake).

Conclusions

This strategy could be useful for exploring sex differences in the neural mechanisms underlying alcohol drinking and related health vulnerabilities. Our findings also highlight the importance of the time of testing for detecting sex differences in drinking behavior.

Voluntary alcohol drinking is higher in adult female rats compared to adult male rats. This sex difference is most pronounced in the later phase of the dark cycle, and when the operant effort is minimal (when 1 lever press gives 1 reward: fixed ratio 1, FR1).

Higher alcohol intake in females is primarily due to “front-loading”, or the rapid consumption of alcohol within the first 5 min of access.

Increasing the effort required to obtain alcohol from FR1 to FR3 dampens front-loading drinking behavior, resulting in similar levels of total intake in males and females.

Compressing the time of access to 15 min drives up front-loading to such a degree that rats end up consuming more alcohol in total than they do in 30-min sessions. In males, this increase in drinking is large enough that it eliminates the sex difference in total alcohol intake.