Evidence for sex differences in behavioural and neural correlates to tickling in young Wistar rats. 

Positive welfare is considered to be not simply the absence of suffering but also the presence of positive experiences. ‘Tickling’ has been shown to induce positive affective states in laboratory rats as evidenced by the production of appetitive 50kHz ultrasonic vocalisations (USVs). However, few tickling studies have used female rats, thus whether females and males respond differently to tickling is not well understood. It was hypothesised that female rats would exhibit different behavioural responses and neural correlates to tickling than male rats due to sex-specific neural regulation of positive affective states. Wistar rats (n=32/sex) were placed in an arena for 2 min/day for 10 days. Tickled animals (n=16/sex) received alternating 15 seconds of tickling and 15 seconds of rest over 2 minutes. Control animals (n=16/sex) were placed in the arena for 2 minutes; the rat received no hand contact. Play behaviours (hops and darts) and 50kHz USV production were quantified. After 10 days of testing, whole brain was collected for analysis of the neural correlates underpinning observed behavioural responses. Double-labelled immunohistochemistry was used to quantify c-fos (a marker of cellular activity) expression in oxytocinergic neurons of the paraventricular nucleus of the hypothalamus. Behavioural data were analysed using a general linear model and Tukey pair-wise comparison. A multiple regression analysis was used to identify behaviours that explained neuronal activity in key brain regions. It was found that tickled rats, regardless of sex, performed more hops (p=0.006) and darts (p<0.001) than control rats. Female tickled rats produced significantly more 50kHz USVs than any other group (p<0.001). 50kHz USV production in tickled females significantly explained c-fos immunoreactivity in dorsal parvocellular oxytocinergic neurons in PVN (p=0.005), but not in tickled males (p=0.663). Whereas, c-fos immunoreactivity in medial parvocellular oxytocinergic neurons was explained by 50 kHz USV production in tickled males (p=0.040). These results provide evidence for a sex-specific behavioural and neural response to tickling in adolescent rats. Tickling was successful in inducing higher levels of 50kHz USVs in females and this may be explained by activity of parvocellular oxytocinergic neuronal populations in PVN. Elucidating the neurobiological basis of positive affect will develop our understanding of the relationship between tickling and positive affect.