Tickling of rats does not replicate juvenile social play with regards to the involvement of central oxytocin and vasopressin systems

Tickling of rats is a widely used positive handling habituation technique that was modelled on conspecific social rough and tumble play. The behavioural response to tickling is associated with the ascending mesolimbic dopamine system and the somatosensory cortex, however, any hormonal response to tickling has yet to be explored. We tested the hypothesis that, as in conspecific play, oxytocin and vasopressin neurons in the paraventricular nucleus (PVN) and supraoptic nucleus (SON) are involved in regulating the behavioural response to tickling in female and male juvenile Wistar rats. These neurons are associated with the reward circuitry and are thought to play a vital role in coding the rewarding nature of prosocial behaviours. Rats received either tickling (tickled, n=16/sex) or no hand contact (controls, n=16/sex). Play behaviours (hand approaches and scampering), 50 kHz USVs and 22 kHz USVs were quantified. Tickled rats called more than controls and tickled females produced significantly more 50 kHz USVs than tickled male or control rats. Treatment had a significant effect on solitary play behaviours as tickled rats made more scampers compared to controls. Immediate early gene expression (Fos immunoreactivity; Fos-ir) in oxytocinergic and vasopressinergic neurons of the PVN and SON was quantified. There was no effect of tickling on the number of Fos-ir in magnocellular oxytocin or vasopressin neurons in the PVN or SON. However, in the parvocellular oxytocin and vasopressin neurons in the PVN we found that tickled rats, regardless of sex, had lower expression of Fos-ir. Together our findings suggest 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.
All work was performed under ARRIVE guidelines and following local ethical review. Research supported by BBSRC (BB/PO13759/1 and BBS/E/RL/230001C).