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

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.

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.

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.

It might be plausible that chronic morphine exposure might alter empathy for pain throughaffective and not sensory pain pathways.