Relationships between play and responses to tickling in male juvenile rats

International audience

English. Play is a putatively positive experience and of key interest to the study of affective state in animals. Rats produce 50 kHz ultrasonic vocalisation (USVs) during positive experiences, including social play and tickling. The tick-ling paradigm is intended to mimic social play resulting in positively valanced ultrasonic vocalisation (USV)production. We tested two hypotheses on the relationship between tickling and play: that tickling would increaseplay behaviour or that play behaviour would increase in anticipation of tickling, and that tickling would sharesome specific properties of play (rebound and emotional contagion of unexposed cage mates). Male Wistar rats(N = 64, with 32 rats/cohort) of 28 days of age were housed in pairs with one rat assigned to be tickled and oneas the non-tickled control. Production of 50 kHz USVs and hand-following behaviour was measured. Prior tohandling, solitary and social play was recorded for 5 min in the home cage. A two-day break in tickling was usedto assess a potential rebound increase in responses to tickling. Only one rat within each cage was handled toassess emotional contagion through changes in the behaviour of the cage-mate. Solitary but not social play increased prior to tickling relative to controls (p= 0.01). There were marked differences between cohorts;tickled rats in C2 produced less 50 kHz USVs than those in C1 (p= 0.04) and overall, C2 rats played less thanrats in C1 (social p= 0.04 and solitary p < 0.001) and had a lighter start weight on arrival (p= 0.009) compared with cohort 1 (C1). In C1, there was evidence of rebound in USV production (p< 0.001) and a contagious effect of tickling reflected by increased hand-following in cage mates (p= 0.02). We found a positive relationship between start weight and USV responses to tickling (Rs= 0.43,p< 0.001), suggesting that the divergence in USV production may be due to developmental differences between cohorts. The results suggest that the relationship between tickling and play is complex in that tickling only affected solitary and not social play, and that tickling responses showed rebound and contagion effects on cage-mates which were specific to cohort responses to tickling.