VTA Glutamate and Orexin Involvement in Cue Reinstatement of Drug Seeking

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DESCRIPTION (provided by applicant): This proposal explores the role of orexin and glutamate afferents to the ventral tegmental area (VTA) in Pavlovian cue-driven reinstatement of cocaine seeking. In this animal model of drug relapse, rats are trained to self-administer cocaine plus a conditioned stimulus (CS) by pressing a lever, then are extinguished on this behavior. On later test days, lever presses are rewarded with the cocaine-associated CS, but no cocaine. Most animals robustly reinstate extinguished drug seeking behavior, which is thought to model relapse in recovering human addicts exposed to Pavlovian drug cues. VTA dopamine projections to forebrain are crucial for drug-associated cues to promote drug seeking, yet the inputs controlling cue-related VTA firing are poorly understood. Orexin and glutamate both promote drug seeking and dopamine cell firing in VTA, and medial prefrontal cortex (mPFC) projects glutamatergically to VTA and is necessary for CS reinstatement of drug seeking. Moreover, orexin facilitates excitation of VTA dopamine neurons by glutamate inputs from mPFC. Here, we propose employing a combination of anatomical, immunohistochemical, pharmacological, and behavioral techniques to examine three related questions. 1) Are mPFC or other VTA glutamate afferents Fos activated by cocaine CSs, and is this activation related to cue-triggered cocaine seeking? 2) Are glutamate, orexin, or co-activation of the two in VTA necessary for cocaine-associated cues to trigger reinstatement of cocaine seeking? 3) Is simultaneous activation of mPFC glutamate and orexin inputs to VTA necessary for CS reinstatement? To address these questions, we use Fos and tract tracing techniques, microinjections of orexin and glutamate antagonists in VTA, and simultaneous inactivation of mPFC and contralateral antagonism of VTA orexin receptors. Many people want to quit using drugs, yet repeatedly relapse when they try to quit. One risk factor is exposure to environmental cues previously associated with drugs, so understanding how the brain processes these cues is crucial for development of therapies to prevent relapse. Here we use multidisciplinary neuroscience techniques to examine the neural circuitry of cue-triggered reinstatement of drug seeking, furthering our understanding of the brain substrates of addiction.