Papers of the Week

Morphine and other synthetic opioids are widely prescribed to treat pain. Prolonged morphine exposure can paradoxically enhance pain sensitivity in humans and nociceptive behavior in rodents. To better understand the molecular mechanisms underlying opioid-induced hyperalgesia, we investigated changes in microRNA (miRNA) composition of small extracellular vesicles (sEVs) from the serum of mice after a morphine treatment paradigm that induces hyperalgesia. We observed significant differential expression of 18 miRNAs in sEVs from morphine-treated mice of both sexes compared with controls. Several of these miRNAs were bioinformatically predicted to regulate cyclic AMP response element binding protein (CREB), a well characterized transcription factor implicated in pain and drug addiction. We confirmed the binding and repression of Creb mRNA by miR-155 and miR-10a. We tested if serum-derived sEVs from morphine-treated mice could elicit nociceptive behavior in naïve recipient mice. Intrathecal injection of 1 μg sEVs did not significantly impact basal mechanical and thermal thresholds in naïve recipient mice. However, prophylactic 1 μg sEV administration in recipient mice resulted in faster resolution of complete Freund’s adjuvant-induced mechanical and thermal inflammatory hypersensitivity. Other behaviors assayed following administration of these sEVs were not impacted, including sEV-conditioned place preference and locomotor sensitization. These results indicate that morphine regulation of serum sEV composition can contribute to analgesia and suggest a potential for sEVs to be a nonopioid therapeutic intervention strategy to treat pain. SIGNIFICANCE STATEMENT: A mouse model of opioid-induced hyperalgesia was used to show that chronic morphine treatment causes differential microRNA packaging into small extracellular vesicles (sEVs) present in the serum of mice. Two of these sEV microRNAs can downregulate CREB expression, and administration of these sEVs attenuates pain hypersensitivity in recipient mice. These studies position sEVs as a potential pain therapeutic and highlight changes underlying opioid-induced hyperalgesia, shedding light on a phenomenon with unclear pathophysiology.