A major challenge for the clinical treatment of cocaine addiction is the propensity for abstinent users to relapse upon re-exposure to environmental cues previously associated with cocaine. This vulnerability to relapse may increase after the acute drug withdrawal stage. Similarly, cue-induced cocaine seeking in rats intensifies or "incubates" during the first months of withdrawal. This is mediated by an increase in GluA2-lacking, Ca2+ permeable (CP)-AMPARs in the nucleus accumbens (NAc). CP-AMPARs represent a very small percentage of the AMPARs in the NAc of drug-naive rats but accumulate in this region in association with the incubation of cue-induced seeking. These CP-AMPARs exhibit higher conductance than other AMPARs and are therefore likely to make NAc neurons more responsive to drug-related cues;in fact, blocking CP-AMPARs prevents the expression of incubated seeking. Thus, identifying the mechanism that leads to elevation of CP-AMPARs after cocaine withdrawal may help identify potential therapeutic targets for the treatment of cue-induced craving and relapse. In other brain regions, group I metabotropic glutamate receptor (mGluR) activation leads to the selective internalization of postsynaptic CP-AMPARs. While group I mGluRs do not seem to internalize AMPARs in the NAc, this has only been assessed in the NAc of drug-naive rodents, where CP-AMPAR levels are very low. Importantly, several studies report decreased group I mGluR levels in the NAc during withdrawal from repeated cocaine exposure. My central hypothesis is that group I mGluR activation selectively internalizes CP-AMPARs in the NAc and that CP-AMPARs accumulate during prolonged withdrawal from cocaine self- administration due to a decrease in group I mGluR levels in the NAc. I also hypothesize that group I mGluR activation in the NAc will prevent the expression of incubated cue-induced seeking by selectively internalizing CP-AMPARs in these cocaine-experienced rats.
Aim 1 will use cultured NAc neurons, which express CP- AMPARs, to test the working hypothesis that group I mGluR activation leads to the selective internalization of CP-AMPARs. Immunocytochemical experiments will determine if surface expression of CP-AMPARs is altered by acute and long-term group I mGluR activation or inhibition.
Aim 2 will test the working hypothesis that levels of group I mGluRs in the NAc decrease during prolonged withdrawal from extended access cocaine self- administration, and that pharmacological activation of NAc group I mGluRs in vivo reduces cue-induced cocaine seeking. Rats will self-administer cocaine or saline (controls) and biochemical studies will determine if group I mGluR surface, synaptic or extrasynaptic levels decrease in the NAc of "incubated" rats. Behavioral studies will determine if acute intra-NAc injection of a group I mGluR agonist reduces incubated cue-induced cocaine seeking and internalizes CP-AMPARs. While these studies are underway, I will participate in a Training Plan that employs coursework, individual mentoring, and collaborative interactions to develop the non- bench skills needed to reach my goal of becoming a principal investigator in an academic setting.
This proposal uses a rat model of addiction to study the cellular mechanisms that lead to intensification of cue- induced cocaine craving, a common trigger for relapse, after cocaine withdrawal. Our results may identify a molecular target (group I metabotropic glutamate receptors) for the development of medications to reduce cue- induced craving in abstinent cocaine addicts.
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