RGS2 Regulates Cocaine Self‐Administration through Midbrain Dopamine D2 Autoreceptors

Abstract

IntroductionRepeated exposure to drugs of abuse alters the transmission of the dopaminergic system including dopamine D2 autoreceptors (D2ARs). Reduced midbrain D2ARs is associated with enhanced motivation for drugs and increased drug intake. Additionally, drug self‐administration disrupts the ability of D2ARs to inhibit dopamine release in the nucleus accumbens (NAcc) and abolishes midbrain D2AR‐mediated G protein signaling. However, what regulates midbrain D2AR function remains unclear. The family of regulator of G protein signaling (RGS) proteins negatively modulates many GPCR signaling by accelerating GTP hydrolysis and terminating G protein signaling. Among all the subtypes of RGS proteins, we previously have shown that RGS2 is expressed in midbrain dopaminergic neurons where D2ARs are located. Importantly, we have demonstrated that RGS2 is a negative modulator of dopamine D2 receptor‐mediated G protein signaling in neuroblastoma 2a cells. We hypothesize that RGS2 in dopamine neurons may regulate D2AR activity and thus drug self‐administration. Thus, this project will determine whether RGS2 regulates cocaine self‐administration by modulating D2AR function in midbrain dopaminergic neurons.MethodsMale TH‐Cre Long Evans rats (~10 weeks old) were injected bilaterally into the VTA with an AAV expressing a loss‐of‐function RGS2 mutant with N‐terminus‐deletion (RGS2x1– 79) or empty control vector. Three weeks post‐virus infusion, infection was verified using immunocytochemistry and Western blotting. Immunohistochemistry assays were used to visualize the co‐localization of mCherry‐tagged RGS2x1‐79 virus and tyrosine hydroxylase in the VTA of rats. Western blotting was performed on VTA tissue lysates to measure levels of HA‐tagged RGS2x1‐79 and RGS2.Three weeks post‐viral infusion, baseline locomotor activity was measured. Intravenous catheters were then implanted and rats were trained to self‐administer cocaine. Multiple facets of cocaine self‐administration behaviors were measured including behavioral economic assessment of motivation to take cocaine (threshold protocol), extinction training and cue/cocaine‐primed reinstatement of cocaine seeking. In a separate cohort of rats, cocaine‐induced locomotor activity was measured three weeks post‐viral infusion.To understand the role of RGS2 in regulating midbrain D2AR‐mediated G protein signaling, fast‐scan cyclic voltammetry was used to measure D2AR‐mediated inhibition of dopamine release in the NAcc of rats virally infused with the RGS2x1–79 mutant or empty control virus. Brain slices containing the NAcc were stimulated with increasing doses of D2/D3 agonist quinpirole and dopamine release was measured.ResultsWe found that inhibiting RGS2 activity by overexpressing a loss‐of‐function RGS2x1–79 mutant in the VTA of rats significantly reduced motivation for cocaine and decreased cocaine‐primed reinstatement of cocaine seeking. Further, overexpression of the RGS2x1–79 mutant significantly reduced cocaine‐induced locomotor activity. The present study provides first time evidence that RGS2 plays a critical role in cocaine self‐administration by regulating midbrain D2AR function.Support or Funding InformationNIH DA042862, DA006634, AA025532This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.