Sex differences in 50 kHz call subtypes emitted during tickling-induced playful behaviour in rats

Abstract

Abstract‘Tickling’ induces positive affective states in laboratory rats as evidenced by the production of 50-kHz ultrasonic vocalisations (USVs), although this has mostly been investigated in males. Juvenile rats emit distinctive 50-kHz USV subtypes. Frequency-modulated (FM) 50-kHz USVs are thought to be associated with positive affect and flat 50-kHz USVs with social communication. FM and flat USVs are produced by both sexes during tickling, but it is unclear whether these calls are produced in relation to particular play-related behaviours, and whether USV subtypes are used in a sexually dimorphic manner during tickling. We tested the hypotheses that FM USVs are associated with tickle-induced play behaviours in a sex-specific way, and that flat USVs are associated with non-play activities. Rats were allocated to one of two treatment groups: tickling (tickled, n = 16/sex) or no hand contact (control, n = 16/sex). Play behaviours (hopping, darting and hand approaches) and FM and flat USVs emitted during the testing session were quantified for each rat, with the frequency of FM and flat USVs made in anticipation of, and during, each behaviour analysed. In females, play behaviours were associated with more flat USVs than in males (before and during; p < 0.001), irrespective of treatment. FM USVs were paired with hopping and darting (before and during; p < 0.001), and in anticipation of hand approaches (p < 0.001) in both tickled females and males compared to controls (both sexes) suggesting that FM USVs are linked with play behaviour. The higher call rate of flat USVs paired with play behaviour in females suggests that there may be sex differences in the role of flat USVs during play. This result is evidence of sex differences in tickle-induced behaviours and has implications for our understanding of the function of different USVs in juvenile female and male rats.

Simultaneous recording of ultrasonic vocalizations and sniffing from socially interacting individual rats using a miniature microphone

“…We also found small but statistically significant differences in the spectral characteristics of the recorded vocalizations across the various social contexts. Overall, these results add to multiple recent studies demonstrating that rat USVs are age-sex- and social status-dependent, and hence may convey social information during social interactions 41–45 .…”

Tickling of rats is associated with positive affective states, but does not replicate juvenile social play regarding the involvement of central oxytocin and vasopressin systems

“…Our previous study showed that one subtype of 50 kHz USVs ( at) were paired with scampering behaviours in females more than males, while frequency-modulated subtype calls were paired with scampering behaviours in both tickled males and females. This suggests that there is a temporal link between tickle-induced vocalisations and solitary play behaviours [ 20 ].…”

“…In this study we investigated whether the hypothalamic oxytocin and vasopressin systems are active during a playful tickling stimulus by quantifying Fos-ir in oxytocin and vasopressin neurons via immunohistochemical double labelling. As we have previously provided evidence of sex differences in tickle-induced behaviours [ 20 ], we also compared double labelling between male and female juvenile Wistar rats. Tickling elicited more 50 kHz USVs and scampering behaviours than controls, with females expressing higher levels of both behaviours.…”

Simultaneous recording of ultrasonic vocalizations and sniffing from socially interacting individual rats using a miniature microphone

“…Overall, these results add to multiple recent studies demonstrating that rat USVs are age, sex, and social status dependent and hence may convey social information during social interactions. 41 , 42 , 43 , 44 , 45 …”