Transcriptional regulation of ventral hippocampus-nucleus accumbens circuit excitability drives cocaine seeking.

Ventral hippocampus (vHPC) CA1 pyramidal neurons send glutamatergic projections to nucleus accumbens (NAc), and this vHPC-NAc circuit mediates cocaine seeking and reward, but it is unclear whether vHPC-NAc neuron properties are modulated by cocaine exposure to drive subsequent behavior. The immediate early gene transcription factor FosB/ΔFosB is induced throughout the brain by cocaine and is critical for cocaine seeking, but its function in vHPC-NAc neurons is not understood. We now show that circuit-specific knockout of FosB/ΔFosB in vHPC-NAc neurons impaired cocaine reward expression and forced abstinence-induced seeking. We also found that vHPC-NAc excitability was decreased by experimenter-administered repeated cocaine and cocaine self-administration, and this cocaine-induced excitability decrease was mediated by ΔFosB expression. To uncover the mechanism of this change in circuit function, we used circuit-specific translating ribosome affinity purification to assess cocaine-induced, FosB/ΔFosB-dependent changes in gene expression in vHPC-NAc. We found that cocaine causes a FosB/ΔFosB-dependent increase in the expression of calreticulin, an endoplasmic reticulum-resident calcium-buffering protein. Calreticulin expression mediated vHPC-NAc excitability and was necessary for cocaine reward. These findings uncover a noncanonical mechanism by which cocaine increases calreticulin in vHPC leading to decreased vHPC-NAc excitability and drives cocaine seeking and reward.