Neuronal Grin2c expression and the cocaine engram: a medial prefrontal cortex substrate for conditioned place preference
Learned associations between drugs of abuse and drug-associated stimuli are thought to be encoded by physical alterations, termed engrams, within activity-dependent neuronal ensembles. However, the molecular mechanisms that support drug-memory engrams remain largely unknown. The goal of our study was to assess transcriptional alterations as candidate engrams within medial prefrontal cortex (mPFC) ensembles selectively activated by exposure to a cocaine-paired context following cocaine conditioned place preference (CPP). Using the immediate early gene Fos to identify strongly activated neurons, we found that mPFC was strongly activated by exposure to the cocaine-paired context, but not to the unpaired saline control context. We then used a combination of fluorescence activated cell sorting (FACS) and subsequent qPCR to identify gene targets upregulated in the Fos-positive mPFC ensemble neurons relative to the Fos-negative mPFC neurons following context re-exposure. Of the genes examined, the most strongly induced in the Fos-expressing neurons was Grin2c, which encodes the NR2C subunit of the NMDA receptor. We confirmed this finding using RNAscope in situ hybridization. To assess a causal role for Grin2c in cocaine-induced CPP, we used AAV-mediated, neuron-selective expression of Grin2c microRNA to knockdown Grin2c expression in mPFC neurons, which reduced cocaine CPP and facilitated CPP extinction over repeated tests. Our results indicate that neuronal Grin2c expression in the mPFC plays a role in associative learning underlying cocaine CPP. Future studies are needed to determine whether Grin2c expression plays a broader role within engrams underlying other forms of learning and memory.