Exploring sex differences in the response to tickling in juvenile Wistar rats
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Date
07/06/2022Author
Tivey, Emma
Metadata
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.