A major problem in treating cocaine addiction is the long lasting vulnerability to relapse, even after months of abstinence. This persistent vulnerability suggests that long lasting neuroadaptations contribute to relapse behavior. Understanding these persistent adaptations is critical to the development of therapeutic agents. Our lab uses a rat model of cocaine addiction, termed the incubation model, in which cue-induced cocaine craving progressively intensifies (incubates) over the first 2 months of withdrawal from extended access cocaine self-administration. We focus on the nucleus accumbens (NAc), a brain region critically involved in the reinforcing properties of drugs of abuse. We showed previously that Ca2+-permeable AMPARs (CP-AMPARs) accumulate in excitatory synapses onto medium spiny neurons of the NAc after 3-4 weeks of withdrawal and then persist for months. Once they accumulate in NAc synapses, these CP-AMPARs mediate the expression of incubated cue-induced cocaine craving. Therefore mechanisms regulating CP-AMPARs are potential therapeutic targets. While our previous work has gathered detailed information on AMPAR plasticity during incubation, little is known about how NMDARs or structural alterations to dendritic spines contribute. Prior studies examining these parameters have typically used non-contingent cocaine regimens which are not directly useful for assessing cocaine craving. The objective of this proposal is to characterize spine morphology and glutamate receptor-mediated Ca2+ signaling in NAc dendritic spines at a withdrawal time before CP-AMPARs accumulate (WD15) and a time after CP-AMPAR accumulation (WD35), to identify plasticity at the single spine level that is associated with CP-AMPAR accumulation. My central hypothesis is that incubation is accompanied by dendritic spine remodeling that involves the formation of spines that contain CP-AMPARs but not NMDARs. This hypothesis will be tested by pursuing two specific aims: 1) Characterize dendritic spine density and morphology in medium spiny neurons (MSNs) during incubation of cocaine craving. Single NAc neurons will be filled with Lucifer yellow, imaged with confocal microscopy, and analyzed with NeuronStudio software. 2) Determine if cocaine and control rats differ in NMDAR and AMPAR mediated Ca2+ influx at the individual spine level in NAc MSN. 2-photon Ca2+ imaging with concurrent electrophysiological measurements will be used to analyze the contribution of CP-AMPARs and NMDARs to Ca2+ signaling in NAc spines on WD15 or WD35 from saline or cocaine self-administration. Caged NMDA and AMPA compounds will be used to dissect functional contributions of NMDARs and CP-AMPARs at the single spine level. These studies will provide a novel window on cocaine-induced neuroadaptations at the single spine level and further our understanding of relationships between glutamate receptor and spine plasticity. While this work is underway, I will participate in a Training Plan that employs coursework, mentoring, and collaborative interactions to develop the non-bench skills needed to reach my goal of becoming a PI in an academic setting.
Environmental cues previously associated with drug use are powerful triggers for relapse in abstinent cocaine users. The proposed research is relevant to NIH's mission of reducing the burden of illness as it characterizes both functional and structural neuronal modifications contributing to enhanced cue- induced cocaine craving during withdrawal. The results have translational significance as the findings from these studies will likely provide targets for the development of therapeutic agents.