Relationships between responses to tickling, play behaviour and physical condition in male juvenile rats

The tickling paradigm is a well-established model used to elicit positive emotional states in rats, but there is little investigation into the model’s relationship with play behaviour and how pre-existing characteristics (eg physical condition) influence the response of individuals to tickling. 64 adolescent (age 28 days) male Wistar rats, (split into two batches) were housed in pairs and assigned to treatment (tickling) or control (neutral handling). One animal within each cage (n = 32, 16 per cohort) was handled. During handling, frequency modulated ultrasonic vocalisation (FM USV) production and approach behaviour in the form of hand-following (HF) was measured. Prior to handling, home cage play was recorded for 5 minutes. There were significant cohort effects in tickling responses, play and body weight. In cohort 1 (C1), tickled animals produced more FM USVs (F1,89.1 36.18, p < 0.001) and showed increased HF compared with controls (F1,88.7 = 16.46, p < 0.001), both measures indicating that tickled animals were in a more positive emotional state. In cohort 2 (C2), treatment had no effect on FM USVs (F1,94= 0.44, p = 0.508). However, there was no difference in HF between batches (F1,188 = 0, p = 0.948) with tickled animals also showing increased solitary play (F1,94= 5.03, p = 0.027). Compared with C1, C2 produced fewer tickle-induced FM USVs (F1,94 = 6.98, p = 0.01), had a lower starting body weight (F1,188 = 50.25, p < 0.001), reduced average daily gain (F1,188 = 35.95, p = 0.008) and also showed less solitary and social play (solitary; F1,94= 5.74, p = 0.019: social; F1,94= 3.37, p = 0.069). These results suggest some degree of dissociation between tickling responses and play behaviour; whilst solitary play was increased by tickling, social play was not. Measures of the tickling response were also dissociated as whilst USVs and HF responded to treatment, only USVs were sensitive to a cohort effect. Ultimately, this suggests that the relationship between tickling and play behaviour is more complex than previously described.