Chronic Exposure to Morphine Leads to a Reduced Affective Pain Response in the Presence of Hyperalgesia in an Animal Model of Empathy

Background: Empathy is the capability to represent the mental and emotional states of other subjects.Previous studies have demonstrated a possible correlation between morphine addiction and altered empathyresponse in morphine-addicted subjects. This study was performed to evaluate the effect of chronic morphineexposure as an animal model of morphine addiction on empathic changes in affective and sensory pain.Methods: Adult male Wistar rats (3 months old) were used for the current study. Animals were grouped incages of two (n = 8 for each group) and one animal was selected as the pain observer group. Pain observeranimals received either saline or morphine (10 mg/kg, twice daily for 8 days). At ninth day, formalin [50 µg,5%, subcutaneous (SC)] was injected into the hindpaw of the cagemate and placed inside the cage. Elevatedplus maze (EPM) and open field test (OFT) were recruited to evaluate anxiety; hot plate and tail flick testswere used to assay sensory pain. Conditioned place aversion (CPA) was also measured as indicator ofaffective pain component.Findings: Chronic morphine exposure led to a reduced level of anxiety in EPM and OFT assays. An opioidinduced hyperalgesia was observed in the sensory pain assays, while there was a reduced affective pain in theCPA paradigm in morphine-treated animals.Conclusion: It might be plausible that chronic morphine exposure might alter empathy for pain throughaffective and not sensory pain pathways