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

Abstract

Tickling of rats is a widely used positive handling habituation technique that was modelled on conspecific social (rough and tumble) play. Any neuroendocrine response to tickling has yet to be explored. We aimed to test the hypothesis that as in conspecific play, oxytocin and vasopressin neurons in the paraventricular nucleus (PVN) and supraoptic nucleus (SON) of the hypothalamus are involved in regulating the behavioural response to tickling in female and male juvenile Wistar rats. Rats (n=32/sex) received either tickling (Tickled, n=16/sex) or no hand contact (Controls, n=16/sex). Play behaviours (hand approaches and scampering), 50 kHz and 22 kHz ultrasonic vocalisations (USVs) were quantified. Immediate early gene expression (Fos immunoreactivity; Fos-ir) in oxytocinergic and vasopressinergic neurons of the PVN and SON was quantified. We found that tickled rats called more than controls and that tickled females produced more 50 kHz USVs than tickled male or control rats. In the parvocellular PVN we found that tickled rats, regardless of sex, had lower expression of Fos-ir in oxytocin and vasopressin neurons. In contrast, there was no effect of tickling on the number of Fos-ir magnocellular oxytocin or vasopressin neurons in the PVN or SON. Our findings provide the first evidence that tickling does not replicate the involvement of the central oxytocin and vasopressin systems previously reported following social play in juvenile rats. This is an important consideration for future interpretation of central oxytocin and vasopressin in response to tickling.