Transcriptomics of environmental enrichment and cocaine reveals roles for retinoic acid signaling and ΔFosB in addiction

Environmental enrichment produces protective addiction and depression phenotypes as a non-drug, non-genetic, and non-surgical manipulation in animals. Rats reared in the enriched condition self-administer less cocaine and display antidepressant-like behavior. To understand the molecular mechanisms of environmental enrichment, high-throughput next generation RNA sequencing technology was used to investigate the differential regulation of gene expression by environmental enrichment and cocaine. In the results, more than 14,000 transcripts were indentified and quantified. Pathway analysis of RNA-seq results highlighted the important roles of retinoic acid (RA) signaling pathway and transcription factors. Transcripts in the RA pathway are significantly regulated by environmental enrichment and cocaine. To study the causal relationship between RA and behaviors, adeno-associated viral-mediated shRNA was used to knockdown the expression Cyp26b1 in the nucleus accumbens (NAc) shell. The results revealed that increasing RA signaling by knocking down the RA degradation enzyme, Cyp26b1, increased cocaine self-administration, and produced anxiolytic and depression-like effects. RA is involved in two intracellular pathways by binding to two different binding proteins, Crabp2 mediated RAR-RXR activation and Fabp5 mediated PPAR-RXR activation. In order to manipulate the two intracellular pathways, the expression of two RA binding proteins was decreased in the NAc shell. Knocking down Crabp2 resulted in anxiogenic and depression-like effects, while knocking down Fabp5 reduced cocaine-self-administration. Furthermore, upstream regulator analysis of genes involved in the RA pathway revealed that the RA signaling pathway could be regulated by AP-1 transcription factors, which are also significantly regulated by environmental enrichment and cocaine in the RNA-seq results. One of the AP-1 transcription factors, ΔFosB, is rapidly induced by stress and drugs and ΔFosB protein accumulates with repeated stimulation. Our results demonstrate that rats reared in an enriched environment show high tonic levels of △FosB, similar to rats exposed to repeated stress or cocaine. In addition, overexpression of ΔFosB in the NAc shell mimics the protective addiction phenotype of environmental enrichment. Moreover, the tonic level of ΔFosB protein is negatively correlated with the transcriptional response of some △FosB target genes. With these results, we formalized a tonic/phasic model where high tonic levels of ΔFosB inhibit the phasic response to stress or drugs. To sum up, the tonic/phasic action of ΔFosB-regulated retinoic acid signaling may underlie the protective addiction and depression phenotypes produced by environmental enrichment.