Thanks to the Pincus-Magaziner Family Undergraduate Research grant, I was afforded the opportunity to continue researching the neurobiological mechanisms of drug addiction in the lab of Dr. Heath Schmidt. The goal of this study was to determine the physiological relevance of endogenous glucagon-like peptide-1 receptor (GLP- 1R) signaling in the VTA for cocaine reinstatement. A growing body of evidence suggests that GLP-1 receptors could serve as a potential molecular target for drugs aimed to prevent craving and relapse. GLP-1 is a metabolic factor produced in the small intestine and nucleus tractus solitarus (NTS) of the hindbrain. GLP-1R are expressed throughout the brain, including the ventral tegmental area (VTA) which plays an important role in motivation and reward. Our preliminary data show that administration of a GLP-1R agonist into the VTA attenuates the reinstatement of cocaine-seeking behavior, but the role of endogenous VTA GLP-1R signaling has remained relatively unexplored. It was shown that NTS GLP-1-expressing neurons project monosynaptically to the VTA and our studies show that cocaine activates GLP-1-expressing neurons in the NTS. Thus, we hypothesized that selective activation of NTS GLP-1-expressing projections to the VTA would attenuate the reinstatement of cocaine-seeking behavior.
We modulated central endogenous GLP-1 signaling using designer receptors exclusively activated by designer drugs (DREADDS), which can be activated in vivo using the inert compound clozapine-N-oxide (CNO). Male Sprague-Dawley rats were given intracerebral infusions of adeno-associated virus (AAV) expressing a Cre-recombinase-dependent neural activating DREADD [AAV-hSyn-DIO-hM3D(Gq)-mCherry] directly into the NTS. Additionally, canine adenovirus-2 expressing Cre-recombinase [CAV-Cre] was injected directly into the VTA and they were implanted with jugular catheters. CAV-2 is a retrograde virus that infects axon terminals, and thus allowed for DREADD expression only in NTS neurons that project to the VTA.
After seven days of recovery, each rat was placed in an operant chamber and allowed to lever-press for intravenous cocaine infusions (0.25 mg/infusion) for 21 days. Cocaine taking behavior was then extinguished by replacing the cocaine with saline. The ability of CNO to attenuate cocaine priming-induced reinstatement was assessed immediately following a 0, 0.1, or 1.0 mg/kg i.p. injection of CNO 30 minutes prior to an i.p. injection of cocaine (10 mg/kg ). We found a significant decrease in active lever pressing (i.e. cocaine-seeking behavior) in a dose dependent manner, giving support to our original hypothesis. Finally, immunohistochemistry was performed on coronal sections of the brain to assess and verify viral expression.
Piloting a project this summer has allowed me to fully immerse myself in preclinical drug addiction research in a way that I was unable to do in the past. Under the mentorship of graduate student Nicole Hernandez, I was able to learn a variety of lab procedures and techniques that will aid me when pursuing independent course work in BBB399 and BBB499 in the future. For example, I improved my proficiency in jugular catheter implantation surgeries, i.p. injections, and learned how to perform a stereotactic brain surgery for viral injections. Pursuing research this summer has not only confirmed my interest in neuroscience, but given me invaluable skills that will aid me as a future physician. Thank you to CURF, Dr. Schmidt, and the entire Schmidt lab for the tremendous experience.