National Institute of Drug Abuse IRP–NIH, Baltimore, USA
Learned associations between drugs and environment play an important role in addiction and are thought to be encoded within specific patterns of sparsely distributed neurons called neuronal ensembles. Cellular imaging of the immediate early gene c-fos and its protein product Fos has been used to identify sparsely distributed neurons that were strongly activated during conditioned drug behaviors, including drug self-administration and context- and cue-induced reinstatement of drug seeking. We used Fos and the c-fos promoter to demonstrate causal roles for Fos-expressing neuronal ensembles in prefrontal cortex and nucleus accumbens in conditioned drug behaviors. I will be describing work using our Daun02 inactivation procedure to ablate specific neuronal ensembles and induce selective disruption of drug-related memories. We also used fluorescence-activated cell sorting (FACS) of Fos-expressing neurons and slice electrophysiology in transgenic Fos-GFP rats to identify unique molecular and synaptic alterations that are induced only within behaviorally activated neuronal ensembles and not in the surrounding less activated neurons. In general, we observe alterations only in the 1% of neurons within neuronal ensembles that were strongly activated and shown to mediate conditioned drug behaviors, with little or no alterations in the surrounding less activated neurons. We recently developed Fos-Tet-Cre transgenic rats that induce Cre recombinase only in neurons that were strongly activated within a 6-hour time window following systemic injections of tetracycline. We used these rats to demonstrate reactivation of similar ventromedial prefrontal cortex neuronal ensembles activated on two different days, along with optogenetic inhibition of these ensembles to demonstrate their roles in context-induced reinstatement of cocaine seeking. We are using these rats to assess the roles of our previously described molecular and synaptic alterations in Fos-expressing neuronal ensembles in conditioned drug behaviors.