Research Component 1: Limbic Glutamatergic Circuits in Ethanol Self-administration
UNC Alcohol Research Center
Molecular and Circuit Pathogenesis of Alcohol Addiction
Research Component 1: Limbic Glutamatergic Circuits in Ethanol Self-adminstration
Primary Investigator: Dr. Clyde Hodge
Co-Investigators: Dr. Zoe McElligott
The fundamental behavioral process of reinforcement reflects the tendency of all animals, human and non-human, to repeat actions that produce a desired outcome. Accordingly, reinforcement mechanisms underlie the repetitive nature of alcohol seeking-behavior during the binge/intoxication stage of addiction. Although multiple neural systems regulate reinforcement processes, drugs of abuse gain long-lasting control over behavior, in part, by engaging glutamatergic α-amino-3-hydroxy-5-methyl-4-isooxazole receptor (AMPAR) mechanisms of synaptic plasticity in brain reward pathways. Supporting this concept, we have discovered that AMPAR activity is a target of self-administered alcohol that, in turn, regulates the positive reinforcing effects of the drug. Specifically, binge-like operant alcohol self-administration (SA) is associated with a shift toward GluA2-lacking Ca2+-permeable (CP-AMPAR) activation (increased GluA1-S831 phosphorylation and reduced GluA2 expression) in the basolateral amygdala (BLA). Preliminary optogenetic data show that these adaptations are associated with increased AMPAR synaptic activity in nucleus accumbens core (AcbC) neurons receiving projections from BLA. Importantly, we have also shown that amygdala AMPAR activity is required for the positive reinforcing effects of alcohol8, and activation of AMPARs, systemically or in amygdala, promotes escalated alcohol SA. This convergence of molecular, physiological, and bi-directional behavioral data support the overall hypothesis that: CP-AMPAR activity in the BLAàAcbC pathway functionally regulates the positive reinforcing effects of alcohol. Three separate but integrated aims are proposed:
Specific Aim 1. Investigate effects of binge-like operant alcohol self-administration on AMPAR subunit phosphorylation and composition. AMPAR subunit composition has profound effects on synaptic and behavioral plasticity, and addiction. It is unknown if alcohol’s positive reinforcing effects are associated with adaptations in AMPAR subunit expression or activation. To address this gap in knowledge, we propose to evaluate effects of binge-like operant alcohol SA on AMPAR (GluA1 / GluA2) protein expression and phosphorylation (pGluA1-S831 and S845) in BLA and AcbC subregions from male and female (M/F) C57BL/6J mice. With support from the Scientific Core, we will use an fMRI / optogenetic approach to elucidate alcohol-induced alterations in functional connectivity of the BLA and AcbC, with follow-up evaluation of AMPAR subunit expression. We predict that alcohol SA will increase pGluA1-S831 expression in BLA and AcbC with reduced GluA2 expression, consistent with upregulated CP-AMPAR activity. These studies will identify novel molecular adaptations associated with the reinforcing effects of alcohol, which underlie repetitive alcohol use during the binge/intoxication stage of addiction.
Specific Aim 2. Examine alterations in AMPAR synaptic activity induced by binge-like operant alcohol self-administration. Activity-dependent plasticity at glutamatergic synapses transduces experiences into enduring changes in brain and behavioral functions17. Preliminary data show that alcohol SA increases AMPAR function in BLA projecting AcbC neurons, and upregulates GluA2-lacking CP-AMPAR phosphorylation in BLA, suggesting enhanced plasticity in the BLAàAcbC circuit. This could strengthen synapses and be a molecular correlate of the enhanced importance of alcohol to the organism. To examine alcohol-induced adaptations in CP-AMPAR synaptic activity, we will evaluate synaptic properties of BLA neurons projecting to AcbC, and use optogenetics to test BLAàAcbC circuit function in M/F C57BL/6J mice. We predict increased CP-AMPAR signaling in BLA and BLAàAcbC circuit of alcohol self-administering mice as measured by kinetics in m/sEPSCs, rectification indices, and response to NASPM. This work will increase understanding of how binge-like alcohol SA alters molecular mechanisms of plasticity within specific circuitry.
Specific Aim 3. Evaluate mechanistic regulation of alcohol reinforcement by CP-AMPARs. The ability to function as a reinforcer is a fundamental property of all drugs of abuse18. Growing evidence indicates that GluA2-lacking CP-AMPAR activity is a critical substrate of drug reward5,14. However, AMPAR subunit regulation of alcohol reinforcement in brain reward pathways has not been explored. Based on strong preliminary data implicating CP-AMPAR activity in alcohol reinforcement, we propose site-specific pharmacological studies to determine if CP-AMPAR activity in the BLA or AcbC is necessary for alcohol reinforcement and/or sufficient for escalated binge-like SA in M/F C57BL/6J mice. A complementary AAV approach will express a dominant negative form of the AMPAR GluA1 subunit (GluA1ct) to determine if activity dependent GluA1 trafficking in the BLA or AcbC regulates the reinforcing effects of alcohol. These studies will elucidate novel molecular mechanism(s) of alcohol’s reinforcing effects, which has potential to lead to new therapeutic strategies for treating behavioral pathologies associated with alcohol addiction.
The goal of the UNC ARC is to increase understanding of molecular and cellular pathogenesis in alcoholism. To address this goal, Research Component 1 seeks to elucidate novel molecular targets of initial binge-like alcohol SA that, in turn, gain control over reinforcement processes to drive repetitive use and escalated intake, two of the most significant behavioral pathologies in alcohol addiction.