Development of play fighting in kindling-prone (FAST) and kindling-resistant (SLOW) rats: how does the retention of phenotypic juvenility affect the complexity of play?

Rats selectively bred for susceptibility to amygdala kindling (FAST) have been shown to retain neural and behavioral features of the juvenile phase into adulthood. In contrast, rats selectively bred for resistance to amygdala kindling (SLOW) are neurobehaviorally more typically adult. The development of play fighting in male and female rats of both selected lines was studied. Given the apparent association of juvenility and play often noted in the literature for mammals in general, it was predicted that the FAST rats should be more playful and be more likely to retain the juvenile tactics of play that lead to more prolonged and complex patterns of social contact. As expected, FAST rats initiated more playful attacks and were more likely to defend against attacks than SLOW rats as both juveniles and adults. Unexpectedly, however, both selected lines exhibited patterns of defense that reduced the likelihood of complex and prolonged social contact. Importantly, the two selected lines did so by very different means. The FAST rats did so by avoiding contact whereas the SLOW rats did so by responding in an adult-typical manner that blocks contact. That is, the FAST rats exaggerated the changes typically occurring at puberty whereas the SLOW rats, at all ages, responded in a more adult manner. These data suggest that the different components of play fighting do not change uniformly with changes in the neurobehavioral underpinnings of juvenility.