Social encounter with a novel partner in adolescent rats: activation of the central endocannabinoid system

The endocannabinoid system is critically involved in the modulation of affect, motivation, and emotion. Here, we investigated the hypothesis that changes in the content of endocannabinoid levels might underlie adaptation to positive social conditions during adolescence. To this aim, separate pairs of adolescent (postnatal days 32-35) male Wistar rats were allowed to interact in a neutral cage under two different testing conditions, i.e. familiar (FAM) and non-familiar (NFAM) social partners. We found that adolescent rats that encountered a NFAM partner spent significantly more time Sniffing and Following the companion than subjects exposed to a FAM partner, whereas no changes in levels of rough-and-tumble play were observed. Notably, the NFAM social encounter significantly increased striatal anandamide (AEA) levels compared to both non-social controls and animals that encountered a FAM partner. Changes in AEA levels appeared to be region-specific, since no changes were observed in the other brain regions analysed, neither were they observed in the activity of the AEA-hydrolase (FAAH) nor in the content of the other major endocannabinoid 2-arachidonylglycerol. In addition, animals that encountered a NFAM partner tended to explore less extensively the illuminated compartment of the light-dark box when compared to animals that had previously encountered a FAM companion. In conclusion, striatal AEA levels seem to participate in the emotional arousal resulting from a NFAM social encounter in adolescent rats, and to be particularly important for coping response to novel social contexts.