Abstract
Incubation of cocaine craving, a translationally relevant model for the persistence of drug craving during abstinence, ultimately depends on strengthening of nucleus accumbens core (NAcc) synapses through synaptic insertion of homomeric GluA1 Ca(2+)-permeable AMPA receptors (CP-AMPARs). Here we tested the hypothesis that CP-AMPAR upregulation results from a form of homeostatic plasticity, previously characterized in vitro and in other brain regions, that depends on retinoic acid (RA) signaling in dendrites. Under normal conditions, ongoing synaptic transmission maintains intracellular Ca(2+) at levels sufficient to suppress RA synthesis. Prolonged blockade of neuronal activity results in disinhibition of RA synthesis, leading to increased GluA1 translation and synaptic insertion of homomeric GluA1 CP-AMPARs. Using slice recordings, we found that increasing RA signaling in NAcc medium spiny neurons (MSN) from drug-naïve rats rapidly upregulates CP-AMPARs. This is observed only in MSN expressing the D1 dopamine receptor. In MSN recorded from rats that have undergone incubation of craving, we observe CP-AMPAR upregulation in D1 MSN (but not D2 MSN) and the effect of exogenous RA application is occluded in these D1 MSN. Instead, interruption of RA signaling in the slice normalizes the incubation-associated elevation of synaptic CP-AMPARs. Paralleling this in vitro finding, interruption of RA signaling in the NAcc of 'incubated rats' normalizes elevated cue-induced cocaine seeking back to non-incubated levels. These results suggest that RA signaling becomes tonically active in the NAcc during cocaine withdrawal and, by maintaining elevated CP-AMPAR levels, contributes to the incubation of cocaine craving.