Repeated morphine exposure activates synaptogenesis and other neuroplasticity-related gene networks in the dorsomedial prefrontal cortex of male and female rats.

Background: Opioid abuse is a chronic disorder likely involving stable neuroplastic modifications. While a number of molecules contributing to these changes have been identified, the broader spectrum of genes and gene networks that are affected by repeated opioid administration remain understudied.

Methods: We employed Next-Generation RNA-sequencing (RNA-seq) followed by quantitative chromatin immunoprecipitation to investigate changes in gene expression and their regulation in adult male and female rats' dorsomedial prefrontal cortex (dmPFC) after a regimen of daily injection of morphine (5.0 mg/kg; 10 days). Ingenuity Pathway Analysis (IPA) was used to analyze affected molecular pathways, gene networks, and associated regulatory factors. A complementary behavioral study evaluated the effects of the same morphine injection regimen on locomotor activity, pain sensitivity, and somatic withdrawal signs.

Results: Behaviorally, repeated morphine injection induced locomotor hyperactivity and hyperalgesia in both sexes. 90 % of differentially expressed genes (DEGs) in morphine-treated rats were upregulated in both males and females, with a 35 % overlap between sexes. A substantial number of DEGs play roles in synaptic signaling and neuroplasticity. Chromatin immunoprecipitation revealed enrichment of H3 acetylation, a transcriptionally activating chromatin mark. Although broadly similar, some differences were revealed in the gene ontology networks enriched in females and males.

Conclusions: Our results cohere with findings from previous studies based on a priori gene selection. Our results also reveal novel genes and molecular pathways that are upregulated by repeated morphine exposure, with some common to males and females and others that are sex-specific.