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 have demonstrated empathy-like behaviors in other animals including rats and mice. The objective of the current study was to evaluate the effect of acute administration of morphine and naloxone on cognition and nociception changes following observing conspecifics undergoing nociceptive stimulus. METHODS: Adult male Wistar rats were used (n = 8 for each group). One cagemate received formalin injection into the hindpaw five times within a nine-day period and the other cagemate observed the pain while being pretreated with saline, morphine, or naloxone [10 mg/kg, intraperitoneal (i.p.)]. Pain behaviors, anxiety-like behaviour, 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 the observer 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 for pain, so that activation of this system dampens the empathy-like responses.