Ghrelin Induces Place Preference for Social Interaction in the Larger Peer of a Male Rat Pair.

Social interaction is important for survival in most social species including humans. To ensure social activities, individuals experience reward from social interaction, generating a powerfully reinforcing process. Here we hypothesized that reward from social interaction in a juvenile male rat pair may be enhanced by ghrelin, a circulating hormone that has been shown to enhance reward from other natural (e.g. food, sex) as well as artificial reinforcers (e.g. alcohol and other drugs of abuse). To this end, we assessed the impact of ghrelin and a ghrelin antagonist on preference for a chamber previously paired to the presence of a social partner in a conditioned place preference paradigm. We found that ghrelin increased and a ghrelin antagonist decreased preference for social interaction, but only in the heavier partner in a social pair. In addition, we found that administered ghrelin induced a positive association between preference for social interaction and body weight difference within socially interacting pairs, where larger ghrelin treated rats preferred social interaction, whereas smaller ghrelin treated rats avoided it, which raises the question if ghrelin could have a role in implementing social hierarchies in rats. In summary, we conclude that ghrelin signaling increases the reward from social interaction in a manner that reflects the degree of divergence in body weight between the social pair.