In adolescent rats, 50-kHz vocalizations are most evident during tickling and rough-and-tumble play. The following experiments evaluated whether 50-kHz vocalizations reflect positive social affect by determining (1) if tickling is a rewarding event, (2) if social or isolate housing conditions differentially influence the response (since housing condition has been found to effect the reward magnitude of social encounters), and (3) if drugs that work on μ-opiate receptors, which has been hypothesized to control positive social affect, modulate tickling. Tickling was positively reinforcing as demonstrated by elevated operant behavior, conditioned place preference, and approach measures. A significant negative correlation between vocalization rate and approach latency measures was found. Social housing reduced tickle-induced vocalizations and approach speeds compared to isolate housing. Naloxone (1 mg/kg) increased vocalization in the socially housed rats and decreased it in isolated Subjects (Ss). These findings suggest that tickling can be used to induce positive social affect in rodents, and that it is modulated by endogenous opioids. © 2001 Elsevier Science Inc.
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