Activation patterns of vasopressinergic and oxytocinergic brain regions following social play exposure in juvenile male and female rats

Social play is a highly rewarding and motivated behaviour predominately displayed by juveniles and expressed by nearly all mammalian species. Prior work suggested that the vasopressin (AVP) and oxytocin (OT) systems can regulate the expression of social play in sex-specific ways. In the present study, we investigated whether there are sex differences in the recruitment of vasopressinergic and oxytocinergic brain regions following social play exposure in juvenile rats. Single-housed rats were allowed to play, in their home cage, with an age- and sex-matched unfamiliar conspecific for 10 minutes, or received similar handling but no partner. Double-labelled fluorescence immunohistochemistry for Fos and either AVP or OT was completed in adjacent series of tissues to determine recruitment of AVP- and OT-immunoreactive neurones in response to social play. Exposure to social play did not increase recruitment of AVP or OT neurones in the supraoptic (SO) or paraventricular hypothalamic nuclei of either sex compared to the no-play control condition. Interestingly, there was a robust sex difference in SO recruitment, irrespective of social play condition, with males exhibiting twice the recruitment of SO-AVP and SO-OT neurones compared to females. Lastly, exposure to social play increased recruitment of the posterior bed nuclei of the stria terminalis (pBST) and the posterodorsal medial amygdalar nucleus (MEApd) compared to the no-play control condition, and this effect was most pronounced in females. Our findings revealed sex differences in the recruitment of brain regions (i) independent of play condition (ie, SO) possibly representing a sex difference in the baseline levels of AVP and OT signalling required for typical functioning and (ii) specific to play condition (ie, pBST, MEApd). In sum, the present study provides further evidence that the neural substrates underlying social play behaviour are sex-specific.