RGS2 Modulates Cocaine Self‐administration by Controlling Dopamine D2 Autoreceptor Activity
Authors: Luessen Deborah , Kim Susy , Sun Haiguo , Ferris Mark J. , McCool Brian A. , Chen Rong 
Published: April 2020

Introduction

Repeated exposure to drugs of abuse reduces midbrain dopamine (DA) D2 autoreceptor (D2AR) levels, which is associated with enhanced motivation for drug and escalation of drug intake. Notably, drug self‐administration disrupts the ability of D2ARs to inhibit DA release in the nucleus accumbens (NAc) 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 GPCR signaling by accelerating GTP hydrolysis and terminating G protein signaling. Among all the subtypes of RGS proteins, we previously showed that RGS2 is expressed in midbrain dopaminergic neurons where D2ARs are located. Importantly, we demonstrated that RGS2 is a negative modulator of D2R‐mediated G protein signaling in neuroblastoma 2a cells. We hypothesize that RGS2 in DA neurons may regulate D2AR activity and drug self‐administration. Thus, this project will determine whether RGS2 regulates cocaine self‐administration by modulating D2AR function in midbrain dopaminergic neurons.

Methods

Viral microinjections

Male and female TH‐Cre Long Evans rats (~10 weeks old) were injected into bilateral VTA with an AAV expressing a RGS2 mutant with N‐terminus‐deletion (RGS2ΔN) or empty control vector. Three weeks post‐virus infusion, infection was verified using immunocytochemistry and Western blotting.

Behavioral Assays

Three weeks post‐viral infusion, baseline locomotor activity was measured. Intravenous catheters were 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.

Functional Characterization

In vivo microdialysis experiments were performed in the NAc to measure extracellular DA levels elicited by cocaine or amphetamine challenge. Further, fast‐scan cyclic voltammetry was used to measure D2AR‐mediated inhibition of DA release and potassium Kv1.2 regulation of DA release in NAc slices of rats virally infused with the RGS2ΔN or empty control virus.

Results

We found that inhibiting RGS2 activity by overexpressing RGS2ΔN in the VTA of rats significantly reduced motivation for cocaine and cocaine‐primed reinstatement of cocaine seeking. Cocaine‐induced locomotor activity was also reduced by overexpression of RGS2ΔN and was paralleled by blunted extracellular DA levels in the NAc after psychostimulant exposure. Further, RGS2ΔN overexpression significantly increased D2AR‐mediated inhibition of DA release and reduced electrically‐evoked DA release via upregulation of Kv1.2 channels in the NAc. Additionally, RGS2ΔN overexpression reduced D2AR‐mediated inhibition of locomotor activity; however, baseline and D1R‐stimulated locomotor activity were not altered. 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 Information

This work is supported by NIH R01DA042862, P50DA006634, F31AA025532