This NRSA application is for support of my graduate training, including a translational research project focused on impulse control disorders (ICDs) in Parkinson's disease (PD). Pramipexole (PPX) is a dopamine (DA) D3 receptor (D3R)-preferring agonist used to treat motor dysfunctions in PD. A significant subset of treated patients (e.g., 13.6%1) develop ICDs, i.e., behavioral addictions like pathological gambling, hypersexuality, compulsive shopping and binge eating. Our lab has established a novel probability discounting task in rats that measures "risk-taking" by using intracranial self-stimulation (ICSS) as the positive reinforcer. Using this model, I plan to ascertain the role of the D3R in PPX-induced discounting. Thereafter, I plan to determine if the D3R-mediated effects involve intracellular mechanisms that are known to regulate learning as well as drug/behavioral addictions, i.e., trafficking of the glutamatergic receptor AMPA. Accordingly, my overall hypotheses are that D3R activation by PPX leads to increased risk-taking, which may be more pronounced in PD-like rats due to dysregulated dopamine systems, and that the neurobiology of this effect involves D3R-mediated increases in surface expression of AMPA receptors (Rs) in brain regions that govern impulsivity. I propose three Specific Aims to test these hypotheses. Rats with 6-hydroxydopamine-induced lesions of the dorsolateral striatum and controls will be tested. The Napier lab has already demonstrated that these PD-like rats acquire and perform ICSS-mediated probability discounting.
In Aim 1, I will evaluate the effects of PPX at a low dose suprathreshold to improving motor deficits in the forelimb step task in the probability discounting task. It is my prediction that PPX will enhance risk-taking in PD-like rats, but not in control rat due to increased vulnerability of the PD brain state.
In Aim 2, I will co-administer selective D3R antagonists with PPX, and predict that development and expression of PPX-induced risk-taking will be blocked.
In Aim 3, I will evaluate D3R-mediated cell signaling that governs synaptic plasticity. I propose that D3R activation enhances AMPAR trafficking to the cell surface through an Akt/GSK-3b signaling pathway. I will assess the effects of PPX treatment on surface expression of AMPARs, as well as levels of activated Akt and GSK-3b signaling using a modified Western blot protocol ongoing in the Napier lab. I predict an increase in the efficacy of this signaling pathway in PD-like rats. In summary, this training opportunity will allow me to use a novel preclinical model of ICDs to pioneer the signal transduction pathways that are associated with PPX- induced risk-taking behaviors. Therefore, my extensive training not only will include highly sophisticated techniques, but experimental design for a wide range of applications. This exciting research, along with my training program, will provide me all the necessary skill sets for a future career in medications development for neuropathologies, especially those where diseases of motor function co-occur with dysregulation of mental health.
The proposed research is relevant to public health and advances NIH's mission because it pertains to the pursuit of fundamental knowledge about the nature of impulse control disorders co-occurring with Parkinson's disease. Understanding the neurobiology of impulse control disorders should lead to better treatments that are devoid of these devastating side effects. Completion of the proposed studies will move the field vertically by furthering the field in both PD and ICDs and providing a novel mechanism underlying the development of these behaviors.