Molecular Mechanisms of Ethanol Reinforcement

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DESCRIPTION (provided by applicant): The primary goal of this application is to characterize the involvement of metabotropic glutamate receptor subtype-5 receptors (mGluR5) in alcohol's reinforcing effects. Preliminary evidence indicates that chronic alcohol drinking selectively increases mGluR5 mRNA expression in the nucleus accumbens (NAcb) of rats, which is consistent with chronic receptor inhibition by alcohol in a brain region that is fundamental to development of addiction. Experiments in Aim 1 of this project will more fully characterize the involvement of mGluR5 in alcohol reinforcement. These studies will determine if operant alcohol self-administration alters mGluR5 protein expression and if mGluR5 receptor activity functionally regulates ethanol reinforcement in rats. Other preliminary data indicate that microinjection of an mGluR5 antagonist in the NAcb decreases the reinforcing effects of ethanol. Studies in Aim 2 will extend this observation by determining the functional significance of mGluR5 receptor activity in limbic brain regions that express mGluR5. Experiments will determine the effects of site-specific infusion of an mGluR5 antagonist in the ventral tegmental area (VTA), NAcb (core and shell), or medial prefrontal cortex (mPFC) on ethanol reinforced responding. These studies will determine if mGluR5 regulation of alcohol reinforcement is brain-region specific. Evidence indicates that ethanol inhibits mGluR5 function in vitro through a PKC-dependent mechanism. Similarly, our preliminary data indicate that mGluR5 blockade decreases ethanol self-administration in wildtype mice but has no effect in mice carrying a null mutation for PKC-epsilon. Thus, specific Aim 3 will characterize a potential cell- signaling based mechanism in mGluR5 regulation of alcohol reinforcement. Using PKC-epsilon knockout and wildtype mice, experiments will determine if mGluR5 blockade decreases ethanol reinforcement in a PKC-epsilon dependent manner. These studies will examine the effects of mGluR antagonists on alcohol and sucrose reinforced responding, and on ethanol-induced changes in locomotor activity to determine behavioral specificity. Finally, chronic ethanol exposure alters numerous molecular events in the brain including the gene transcription factor cyclic AMP- responsive element binding protein (CREB), which has been implicated in addictive behavior. Activation of mGluR5 increases p-CREB levels via a PKC dependent mechanism, whereas chronic ethanol drinking decreases p-CREB levels in the NAcb. Since ethanol is known to inhibit mGluR5 activity through a PKC-dependent mechanism, these findings suggest that self-administered ethanol may decrease p-CREB through an mGluR5/PKC (possibly PKCepsilon) dependent mechanism. Experiments in Aim 4 will determine if ethanol-induced changes in mGluR5 and p-CREB expression are dependent on PKCe, and the extent to which this is modulated by a history of chronic operant ethanol self-administration. Another study will determine if ethanol inhibits mGluR5-mediated changes in p-CREB in a PKCepsilon dependent manner. These studies, examine the functional linkage between mGluR5 and PKCepsilon in the mediation of chronic ethanol effects on transcriptional regulation, which has implications for adaptive changes in addiction. These findings will aid development of pharmacotherapeutics to treat problems associated with alcoholism.