Cocaine-induced immediate-early gene expression in the nucleus accumbens: roles of separate cAMP sensors
Immediate-early gene (IEG) induction after administration of amphetamine or cocaine has been used to trace the signaling pathways that mediate neuronal plasticity required for the short- and long-term behavioral effects of these psychostimulants. We recently reported that a novel cyclic AMP (cAMP)-dependent Rap guanine nucleotide exchange factor-2 (RapGEF2)-ERK signaling pathway is required for Egr-1 induction in D1 medium-spiny neurons (MSNs) of the nucleus accumbens (NAc) after cocaine treatment, and that its deletion from the NAc neurons attenuates cocaine-induced locomotor sensitization and conditioned place preference (CPP). However, the cell type-specific neuronal mechanisms underlying this effect remain unclear. In this study, we used Cre-LoxP technology and a novel Cre-amplifier transgene to generate conditional RapGEF2 knockout mice targeting D1-MSNs and investigated the functional role of RapGEF2 in cocaine reward. Deletion of RapGEF2 in D1-MSNs blocked cocaine-induced ERK phosphorylation (p-ERK) and Egr-1 induction. D1-MSN-specific RapGEF2 deletion did not affect intravenous cocaine self-administration, nor did it affect Fos induction by cocaine, prompting us to examine more closely the role of metabotropic (cAMP-dependent) signaling to IEGs after cocaine administration. We used a battery of D1-MSN-specific genetic interventions targeting cAMP signaling, including Drd1-Cre::Rap1A/Bfl/fl mice, and AAV injection of a Cre-dependent catalytically active phosphodiesterase (PDE4D3-cat) or a Cre-dependent protein kinase A-inhibitor (PKI) in the NAc of Drd1-Cre mice, to explore further the underlying cAMP dependence of IEG induction by acute and chronic cocaine administration, and the cAMP sensors required. Rap1 is reported as a necessary component for both RapGEF2- and PKA-dependent ERK activation, but a requirement for Rap in Fos induction by cocaine has not been examined. Deletion of Rap1A/B in D1-MSNs blocked cocaine-induced p-ERK and Egr-1 expression, but not c-Fos, supporting the idea that the RapGEF2-Rap1-ERK pathway specifically regulates Egr-1, not c-Fos, expression. D1-specific expression of the PDE4D3-cat ablated up-regulation of both Egr-1 and Fos in NAc after cocaine administration, demonstrating that induction of both IEGs requires cAMP elevation in D1-MSNs. Notably, inhibition of PKA activity via AAV-mediated expression of PKI-alpha in D1-MSNs blocked both c-Fos and Egr-1 induction. Thus, acute or chronic cocaine administration activates at least two cAMP-dependent signaling pathways in D1-MSNs: a PKA-Fos pathway and a RapGEF2-ERK-Egr-1 pathway. The finding that PKA also activates the ERK-Egr-1 signaling pathway by convergence on Rap1, and concomitantly activates c-Fos independently of Rap1, may underlie selective effects of metabotropic activation of RapGEF2 and PKA activation by cAMP on cocaine-dependent behaviors in mice.