T C. Napier - US grants

PROJECT SUMMARY/ABSTRACT Dopamine agonists that activate D2 and D3 receptors (D2R, D3R), e.g., pramipexole, are highly effective therapy for the motor dysfunction of Parkinson's disease (PD). However, a significant subpopulation of treated individuals exhibit impulse control disorders (ICDs). To model this phenomenon, we implemented a novel probability discounting task using intracranial self-stimulation (ICSS) as the positive reinforcer and demonstrated that in PD-like rats, chronic pramipexole increases risk-taking. Emerging are the concepts that D2R/D3R activation engages the Akt/GSK-3¿ signaling pathway, and this cascade is involved in the trafficking and function of ionotropic glutamate receptors (AMPAR/NMDAR). The mechanisms that underlie the propensity of chronic pramipexole to dysregulate reward-motivation, especially in the parkinsonian state, are unknown, but the rich literature from the drug addiction field demonstrates that chronic treatment with indirect dopamine agonists strengthen synapses by increasing surface expression of AMPAR/NMDAR. Linking these concepts together for brain regions that regulate impulsivity, and are preserved in the parkinsonian brain state (e.g., the nucleus accumbens, NAc), will provide new insights into the cause of pramipexole-induced ICDs. Targeting this unmet need, the overall objective of this R21 project is to identify the anatomical substrates and underlying mechanisms by which pramipexole enhances impulsive behavior in a rat model of PD. As pramipexole has a high D3R/D2R affinity ratio, the central hypotheses are that chronic exposure to a therapeutically relevant dose of pramipexole enhances discounting by engaging D3R, Akt/GSK-3¿ pathways and AMPAR/NMDAR trafficking in the NAc, and blocking these targets will disrupt discounting. These hypotheses will be tested in two Specific Aims. For Specific Aim 1, chronic treatment with pramipexole will be used to enhance discounting, and administration of a D3R-selective antagonist, PG01037, will be used to demonstrate that D3R are necessary. We hypothesize that PG01037, given systemically or directly injected into the NAc, will block pramipexole-induced discounting. Specific Aim 2 will demonstrate that GSK-3 signaling and AMPAR/NMDAR in the NAc are necessary for pramipexole-enhanced discounting. We hypothesize that AMPAR/NMDAR surface expression will increase, and pGSK-3¿ (inactive) will decrease after chronic pramipexole and blockade of GSK-3 or AMPAR/NMDAR in the NAc will mitigate pramipexole- enhanced discounting. The proposed research is innovative because it reflects a novel concept regarding the adaptive consequence of chronic D3R activation in PD, and a novel model in which to validate this concept. The research is significant because it will substantially enhance our understanding of ICD neuropathology during dopamine agonist therapy for PD. Outcomes will provide a critical foundation for an R01 application for multidisciplinary, mechanistic evaluations of the neuronal consequences of such therapy, and for future development of therapeutic protocols for PD patients that provide motor benefits while avoiding ICDs.

? DESCRIPTION (provided by applicant): Parkinson's disease (PD) is devastating neurological disorder that affects ~7 million people globally.41 Its etiology is unclear, but both genetic and environmental factors are contributors. Gut dysfunction may be involved; e.g., there is an upregulation/aggregation of a-synuclien (a-syn) in the enteric nervous system prior to a clinical diagnosis of PD. As aggregated a-syn within nigral dopaminergic neurons is a brain hallmark of those who die with PD, gut a-syn may provide a `preclinical' biomarker for this disease. Abuse of the psychostimulant methamphetamine (meth) increases the chance of subsequently developing PD above non-abusers by 76%.8,9 Given that the number of meth users worldwide has recently surged to 33 million,56 these findings predict that the incidence of PD may also surge in the near future, especially in rural regions where meth is a popular abused drug. The biological link between meth abuse and PD is not known and thus therapy that reduces this risk does not exist. The link is difficult to study in humans due in part to the lengthy delay that ofte occurs between meth abuse and the diagnosis of PD. To help decifer the link, we will study a rat model of human drug-taking for which we are experts, meth self-administration (SA). We have revealed that meth SA results in mild `subclinical' PD-like pathology in the gut and brain. To indicate if meth abuse can promote the development of PD, we will challenge these rats with a sub-threshold environmental insult (i.e., low doses of rotenone) that when sufficiently robust, can induce PD-like pathology. The overall objective of this R21 proposal is to identify gut/brain pathological factors that underlie the increased risk to develop PD that are imposed by meth abuse. The central hypothesis is that meth SA renders animals vulnerable, so that a mild environmental insult that normally would not result in PD-like state is now sufficient to induce PD-like motor deficits, and PD-like brain and gut pathology. We predict that discovering proteins engaged in this potentiation will provide insights for biomarkers and therapeutic targets. This hypothesis will be tested in two Specific Aims. For Aim 1, we will use rats to determine rotenone treatments that induce PD-like motor deficits and tissue pathology as well as one that is sub-threshold to these effects. We will measure factors that are altered by meth and rotenone, e.g., dopaminergic lesions, mitochondrial stress, and a-syn. In Aim 2, we will determine if meth SA increases vulnerability for PD-like consequences. Accordingly, we will ascertain if a sub-threshold dose of rotenone in rendered sufficient in meth SA rats to induce PD-like motor deficits and brain/gut pathology. The proposed research is innovative for the studies present a new and substantially different way of mechanistically linking meth abuse and PD. The research is significant because it will substantially enhance our understanding of meth abuse as a risk factor for PD. Outcomes will form the foundation of an R01 application for multidisciplinary, mechanistic evaluations of meth exposure that enhance the vulnerability for developing PD and will determine which mechanisms have potential as therapeutic targets.

SUMMARY Cocaine abuse is prevalent in HIV-infected individuals. Drug abstinence greatly improves the health and well-being of the comorbid individual, but relapse to drug-taking remains exceptionally high in this vulnerable population. In the brain, the actions of HIV-associated proteins and cocaine may overlap on glutamatergic pathways and the innate immune system. Glutamate transmission clearly regulates relapse and a role for inflammation in addiction is becoming appreciated. Thus, relapse-related neuropathology in the comorbid condition is likely to be vastly different from that described for cocaine abuse alone. The overarching objective of this research program is to decipher neurobiological mechanisms that underpin abstinence and relapse to cocaine-taking during HIV-infection. This objective requires valid animal models of the human comorbid condition. Intravenous (i.v.) self-administration protocols are the ?gold standard? facsimile of human drug- taking. To our knowledge, we are one of only two laboratories world-wide that have implemented these protocols for cocaine in a rodent model of human HIV/AIDS, and there are no studies using a comorbid model to study relapse to cocaine-taking. Thus, the interactions between HIV infection and cocaine use on the neurobiological underpinnings of relapse are completely unexplored. Moreover, i.v. does not emulate the routes of cocaine administration used by humans. To fill these gaps, the objectives of this R21 project are to implement a new, clinically relevant model of the human comorbidity, and to use this model to explore immunological and glutamatergic mechanisms that underlie relapse to cocaine-taking in the context of HIV. The model will be Fischer 344 HIV-1 transgenic (Tg) rats that self-administer vaporized cocaine via inhalation. Our overarching hypothesis is that behavioral and brain indices of relapse, i.e., cocaine- seeking and increased glutamatergic synaptic strength, respectively, will be enhanced in Tg rats with a history of inhaled cocaine. We will test this hypothesis and meet our project objectives by experiments grouped into two Specific Aims. Aim 1 will establish relapse-testing in HIV-1 Tg rats. To optimize contingent cocaine-taking, we will implement inhalation delivery of vaporized cocaine. This method emulates a delivery route and rapid brain entry of cocaine, used by human cocaine addicts, and avoids potential immunological complications imposed by surgery and chronic indwelling catheters needed for i.v. delivery. A cue-reactivity protocol will be used to indicate vulnerability to relapse. We hypothesize that cue-reactivity will be greater in cocaine self-administering Tg rats than either condition alone. Aim 2 will determine the contribution of HIV-1 proteins to neuropathology associated with cocaine-seeking. Over activity of glutamatergic AMPA receptor (AMPAR)-mediated transmission within the nucleus accumbens core (NAc) is linked to cocaine-seeking. We reveal here that abstinence from i.v. cocaine self-administration by male Tg rats greatly exaggerated excitability of NAc neurons. HIV-1 toxic proteins promote glutamatergic excitotoxicity and TNF? regulates AMPAR. These outcomes help guide the working hypotheses that inflammation-promoted, AMPAR-mediated increase in NAc synaptic strength are exaggerated in drug-seeking cocaine self-administering Tg rats. These strategies will provide the first behavioral and neurobiological characterization of relapse to cocaine in the context of chronic HIV-1 proteins, and outcomes will form the platform for identifying relapse reduction targets to be explored in a future R01 application.