Exploring sex differences in the response to tickling in juvenile Wistar rats

Abstract

Positive welfare is considered not simply the absence of suffering, but also the presence of positive experiences. ‘Tickling’ induces positive affective states in laboratory rats as evidenced by the production of 50 kHz ultrasonic vocalisations (USVs). Few studies have investigated the effect of tickling on females, and whether there is a sex difference in response to tickling and the involvement of the neuroendocrine system. Here I investigated whether there are sex differences in behavioural responses to tickling in juvenile Wistar rats due to sex-specific neural regulation of positive affective states. I tested the following hypotheses that: (i) specific elements of the behavioural response to tickling (including USVs) differ between sexes; (ii) 50 kHz USV subtypes are produced in a temporally specific manner during tickling, and they may be paired with different behaviours observed during tickling; (iii) the behavioural response to tickling is regulated by brain regions and neural circuits that are associated with social play in rats, and this is sex specific; (iv) oxytocin and vasopressin neurons in the paraventricular nucleus (PVN) and supraoptic nucleus (SON) are involved in the behavioural response to tickling in a sex specific manner; and (v) 50 kHz USV production in response to tickling is related to measures of motivation for the reward of tickling, and this is greater in females. In experiment 1, rats were tickled or had no hand contact during two minutes of testing per day for 10 days. Play-related behaviours (hand approaches and solitary play) and 50 kHz USVs were quantified. Female rats made more 50 kHz USVs after 10, but not five, days of tickling. Tickled rats made more trill 50 kHz USVs, regardless of sex, while female rats emitted more flat 50 kHz USVs than males after 10 days of testing. Both treatment and sex affected flat 50 kHz USVs, with females producing more flat 50 kHz USVs and regardless of sex, tickled rats producing more trill 50 kHz USVs than controls. Females paired more flat 50 kHz USVs with scampering and approaches, while tickled rats of both sexes paired more trills with those behaviours. Tickled rats elicited more solitary play scampering than control rats, and this was greatest in females. Tickling did not increase hand approaches, and control females approached the hand more than tickled rats. Double-labelled immunohistochemistry was used to quantify c-fos expression (a neuronal activity marker) in oxytocinergic and vasopressinergic neurons that are known to be involved in social behaviours (e.g. social conspecific play) and reward pathways. Tickling did not increase Fos immunoreactivity in the social behaviour, reward or mid- or hind-brain brain regions. Control rats had more correlated Fos immunoreactivity between regions of the social behaviour network and reward pathway. In the PVN, tickled rats, regardless of sex, had lower numbers of Fos positive parvocellular, but not magnocellular, oxytocin and vasopressin neurons. There was no effect of tickling on Fos immunoreactivity in oxytocin and vasopressin SON neurons. In experiment 2, rats were tickled or had no hand contact during two minutes of testing per day for five days, followed by testing in a modified runway test for two days to measure motivation for tickling, and a modified preference test to measure preference for the hand over a novel object. Tickling did not affect the time taken to traverse the runway, but tickled rats made more total and trill 50 kHz USVs while traversing the runway. Tickling did not change preference for a novel object or the hand, but control rats made more visits to the novel object. In summary, there are sex differences in the behavioural response to tickling, but this appears to be dependent on the number of days of tickling and specific to individuals. Tickling did not increase neuronal activity in key regions associated with social behaviours and reward and this may be a consequence of individual responses to tickling. Alternatively, tickling, while rewarding, was not perceived as a social stimulus. This is an important consideration for future tickling studies as tickling should not necessarily be construed as mimicking rat social conspecific play and responses to tickling by both sexes should be considered.