Anxiety and physical condition may affect responses to tickling in male wistar rats.

The tickling paradigm is a well-established model used to elicit positive emotional states in rats, but there is little evidence on how pre-existing characteristics (e.g. anxiety and 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 batch) was handled. During handling, frequency modulated ultrasonic vocalisation (FM USV) production and approach behaviour in the form of hand-following (HF) was measured. All animals were run through the elevated plus maze (EPM) as a measure of anxiety during the pre (EPM1) and post-(EPM2) handling phases. There were significant batch effects in body weight, anxiety and tickling responses. In batch 1 (B1), 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 batch 2 (B2), treatment had no effect on FM USVs (F1,94= 0.44, p = 0.508). However, there was no difference in HF between the batches (F1,188 = 0, p = 0.948). Compared with B1, B2 tickled animals produced fewer FM USVs (F1,94 = 6.98, p < 0.001), with all animals showing increased anxiety (EPM1; F1,188 = 51.22, p < 0.001 and EPM2; F1,188 = 51.22, p < 0.001), a lower starting body weight (F1,188 = 50.25, p < 0.001) and reduced average daily gain (F1,188 = 35.95, p = 0.008). FM USV production showed an inverse relationship with anxiety (EPM1; F1,188 = 10.82, p = 0.001 and EPM2; F1,188 = 21.58, p < 0.001) and a relationship with physical condition (start weight; F1,188 = 5.73, p = 0.018 and average daily gain; F1,188 = 11.40, p < 0.001). This is the first study to investigate the effects of pre-existing characteristics on responses to tickling. Despite being sourced from the same supplier, rats showed pre-existing differences in anxiety and physical condition which may have affected responses to the tickling protocol.