Epac2 in midbrain dopamine neurons contributes to cocaine reinforcement via enhancement of dopamine release

Abstract

Chronic exposure to drugs of abuse results in an upregulation of cAMP signaling in the mesolimbic dopamine system, a molecular adaptation thought to be critically involved in the development of drug dependence. Exchange protein directly activated by cAMP (Epac2) is a major cAMP effector abundantly expressed in the brain. However, it remains unknown whether Epac2 contributes to cocaine reinforcement. Here, we report that Epac2 in the mesolimbic dopamine system promotes cocaine reinforcement via facilitation of dopamine release. Conditional knockout of Epac2 from midbrain dopamine neurons (Epac2-cKO) and the selective Epac2 inhibitor ESI-05 decreased cocaine self-administration in mice under both fixed-ratio and progressive-ratio reinforcement schedules and across a broad range of cocaine doses. In addition, Epac2 in midbrain dopamine neurons robustly facilitated dopamine release in the nucleus accumbens in vitro and in vivo. This mechanism is central to the behavioral effects of Epac2 disruption, as chemogenetic activation of ventral tegmental area (VTA) dopamine neurons increased dopamine release and reversed the impairment of cocaine self-administration in Epac2-cKO mice. Conversely, chemogenetic inhibition of VTA dopamine neurons reduced dopamine release and cocaine self-administration in wild-type mice. Epac2-mediated facilitation of dopamine release may therefore represent a novel and powerful mechanism that contributes to cocaine reinforcement.