The similar past pain experience evokes both observational contagious pain and consolation in stranger rat observers.

Laboratory rodents have been shown to have an ability to recognize the injury site and negative emotional state of their conspecifics in pain, resulting in empathic consoling behaviors and observational contagious pain (OCP). However, these empathic responses have been shown to be familiarity-dependent. In this report, we further explored whether the past pain experience could evoke empathic response in stranger observers. In our rodent model, two types of empathic response have been identified from naive cagemate observer (COnaive) during and after a priming dyadic social interaction (PDSI) with a cagemate demonstrator in pain (CDpain): the consolation and OCP. Consolation is represented by allolicking and allogrooming behaviors toward the CDpain, while the OCP is represented by a long-term mechanical pain hypersensitivity. The current results showed that: (1) neither the consolation nor OCP could be identified in the naive noncagemate observer (NCOnaive) during and after a PDSI with a noncagemate demonstrator in pain (NCDpain); (2) nor were the two types of empathic response seen in the NCO, who had just experienced acute pain (NCOpainexp), during and after a PDSI with a naive unfamiliar conspecific (NCDnaive). However, both the consolation and OCP were dramatically identified in the NCOpainexp during and after a PDSI with a NCD in pain (NCDpain). The current results demonstrated that the past pain experience can evoke both consolation and OCP in stranger rat observers when witnessing a conspecific in pain, implicating that the processing of empathy for pain can be modulated by past negative mood experience.