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 empathy response in morphine-addicted subjects. this study was performed to evaluate the effect of chronic morphine exposure 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 in cages of two (n = 8 for each group) and one animal was selected as the pain observer group. pain observer animals 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. elevated plus maze (epm) and open field test (oft) were recruited to evaluate anxiety; hot plate and tail flick tests were used to assay sensory pain. conditioned place aversion (cpa) was also measured as indicator of affective pain component. findings: chronic morphine exposure led to a reduced level of anxiety in epm and oft assays. an opioid-induced hyperalgesia was observed in the sensory pain assays, while there was a reduced affective pain in the cpa paradigm in morphine-treated animals. conclusion: it might be plausible that chronic morphine exposure might alter empathy for pain through affective and not sensory pain pathways.