Social play consists of reciprocal physical interactions between conspecifics with many features conserved across species, including the propensity for males to engage in play more frequently and with higher physical intensity. Animal models, such as the laboratory rat, reveal that the underlying neural circuitry of play is subject to sexual differentiation during a critical period early in life. In this review, we discuss the developmental processes that produce distinct neural nodes which modulate both shared and sex-specific aspects of play with a focus on the medial amygdala, lateral septum, and prefrontal cortex. While the cellular mechanisms determining sex differences in play are beginning to be uncovered, the ultimate advantages of play continue to be debated.

Play is common among the young of many mammalian species. How that play is exhibited results from a dynamic interplay between genetic framework and experiential influences that, in turn, operate on hard-wired brain systems. One approach towards understanding how genes and environment interact with brain substrates to yield a particular playful phenotype is to take advantage of inbred strains of rats that come with a known genetic identity and assess the effects of varying early social experiences and targeted neurobiological interventions on rats of these strains. This paper primarily summarizes research utilizing the F344 inbred strain, a rat that consistently plays less than most other strains.

Social play is a highly rewarding and motivated behaviour displayed by juveniles of many mammalian species. We hypothesized that the orexin/hypocretin (ORX) system is involved in the expression of juvenile social play behaviour because this system is interconnected with brain regions that comprise the social behaviour and mesocorticolimbic reward networks. We found that exposure to social play increased recruitment of ORX-A neurons in juvenile rats. Furthermore, central administration of ORX-A decreased social play duration, while central blockade of ORX-1 receptors differentially altered social play duration in juvenile rats with low versus high baseline levels of social play (increasing social play in low baseline social play individuals and decreasing social play in high baseline social play individuals). Together, our results provided the first evidence of a role for the ORX system in the modulation of juvenile social play behaviour.