Role of Opioid System in Empathy-like Behaviours in Rats

Background: Empathy is defined as the ability to simulate the mental states of others. Recent studies havedemonstrated empathy-like behaviors in other animals including rats and mice. The objective of the currentstudy was to evaluate the effect of acute administration of morphine and naloxone on cognition andnociception changes following observing conspecifics undergoing nociceptive stimulus.Methods: Adult male Wistar rats were used (n = 8 for each group). One cagemate received formalin injectioninto the hindpaw five times within a nine-day period and the other cagemate observed the pain while beingpretreated with saline, morphine, or naloxone [10 mg/kg, intraperitoneal (i.p.)]. Pain behaviors, anxiety-likebehaviour, locomotion, balance and muscle strength were evaluated in the observer animals.Findings: Observing a cagemate in pain increased anxiety-like behavior and reduced thermal pain threshold in theobserver animals. Administration of morphine reversed these effects and naloxone did not affect the responses.Conclusion: Results of the current study reveal an important role for opioid receptors (ORs) in empathy forpain, so that activation of this system dampens the empathy-like responses.