Tickling-induced 50-kHz ultrasonic vocalization is individually stable and predicts behaviour in tests of anxiety and depression in rats.

Manipulation of juvenile rats in a way that mimics the rough-and-tumble play and resembles tickling elicits 50-kHz ultrasonic vocalizations (USVs) that have been proposed as a measure of positive affect. In the present experiments the stability of the 50-kHz USV response (chirping) over 1.5 months of daily manipulation and the effect of tickling was studied. By the second week of tickling rats of both sexes developed a level of 50-kHz USVs that remained individually characteristic. During tickling the rats also emitted low levels of 22-kHz USVs. No correlation was found between the two types of USVs. In tests used in anxiety and depression research, tickling on its own had an anxiolytic effect in many experimental settings. Significantly lower levels of [(35)S]GTPgammaS binding to the dopamine-activated receptor-G protein complex in striatum and serotonin transporter levels in the frontal cortex were found in female control rats as compared to males. These differences were eliminated by tickling. Rats which expressed high level of chirping (HC-rats) were similar to low-chirping (LC) rats in anxiety measures but had lower activity in an exploration test and lower sucrose preference. LC-rats adopted more active coping strategies in the forced swimming test. These findings suggest that there are individually characteristic 50-kHz USV response levels to tickling in rats, and that HC- and LC-rats are similar with regard to anxiety levels but have different coping strategies to novelty. The anxiolytic-like changes in behaviour that were brought about by tickling could be mediated by changes in dopamine- and serotonergic systems.