Quality not quantity: Deficient juvenile play experiences lead to altered medial prefrontal cortex neurons and sociocognitive skill deficits.

Reduced play experience over the juvenile period leads to adults with impoverished social skills and to anatomical and physiological aberrations of the neurons found in the medial prefrontal cortex (mPFC). Even rearing rats from high-playing strains with low-playing strains show these developmental consequences. In the present study, we evaluated whether low-playing rats benefit from being reared with higher playing peers. To test this, we reared male Fischer 344 rats (F344), typically thought to be a low-playing strain, with a Long-Evans (LE) peer, a relatively high-playing strain. As juveniles, F344 rats reared with LE rats experienced less play and lower quality play compared to those reared with another F344. As adults, the F344 rats reared with LE partners exhibited poorer social skills and the pyramidal neurons of their mPFC had larger dendritic arbors than F344 rats reared with same-strain peers. These findings show that being reared with a more playful partner does not improve developmental outcomes of F344 rats, rather the discordance in the play styles of F344 and LE rats leads to poorer outcomes.

Play fighting and the development of the social brain: The rat's tale.

The benefits gained by young animals engaging in play fighting have been a subject of conjecture for over a hundred years. Progress in understanding the behavioral development of play fighting and the underlying neurobiology of laboratory rats has produced a coherent model that sheds light on this matter. Depriving rats of typical peer-peer play experience during the juvenile period leads to adults with socio-cognitive deficiencies and these are correlated with physiological and anatomical changes to the neurons of the prefrontal cortex, especially the medial prefrontal cortex. Detailed analysis of juvenile peer play has shown that using the abilities needed to ensure that play fighting is reciprocal is critical for attaining these benefits. Therefore, unlike that which was posited by many earlier hypotheses, play fighting does not train specific motor actions, but rather, improves a skill set that can be applied in many different social and non-social contexts. There are still gaps in the rat model that need to be understood, but the model is well-enough developed to provide a framework for broader comparative studies of mammals from diverse lineages that engage in play fighting.

Using the 'stranger test' to assess social competency in adult female Long Evans rats reared with a Fischer 344 partner.

Rats reared with limited access to a play partner during the juvenile period develop into adults with impairments in various cognitive, emotional, and social skills. The present study assesses the consequences of play deprivation on adult social skills in female Long Evans (LE) rats that were reared with a low-playing Fischer 344 rat over the juvenile period. As adults, their social skills were assessed using the stranger paradigm, by pairing the deprived LE rats with a novel LE partner in a neutral arena. While the deprived rat engages its partner in play there were alterations in key aspects of play, such as reduced pinning and a longer latency to begin playing, that suggest there are impairments in the social ability of the deprived rat. Most notable were the changes in the behaviour of the typically reared partner, a reduction in the amount of play it initiated and fewer actions that produced reciprocal and prolonged interactions. The changes in the behaviour of the normally reared partner suggest that it detected subtle changes in the play deprived LE rats. These findings support the hypothesis that peer-peer play experiences during the juvenile period are important for the development of socio-cognitive skills.

Development of ultrasonic calls in rat pups follows similar patterns regardless of isolation distress.

Ultrasonic vocalizations (USVs) emitted by rat pups have been extensively studied in the context of isolation from their mother and littermates. The heightened call rate of isolation-induced USVs indicates increased anxiety, whereas other acoustic parameters, such as frequency and call duration, are thought to be useful in reflecting developmental changes, which by weaning have developed into their adult form. However, it is possible that stress can affect the quality as well as the quantity of calls, and that as the pups become more mobile, the effects of the stress may change. Therefore, in the present study rats were tested in a test arena either in isolation or with littermates, so as to assess the effects of isolation stress on the developmental changes reported in the literature. Rat pups of both sexes were tested on multiple days from postnatal day (PND) 7 to weaning. Overall, the main changes in the frequency, duration, and types of calls were accounted for by age. The effects of isolation were minor, as were sex differences. This study indicates that the development of USVs in infant rats is a robust process and seemingly resistant to the effects of isolation-induced stress.

Sex differences in rat decision-making: The confounding role of extraneous feeder sampling between trials.

Although male and female rats appear to perform differently in some tasks, a clear picture of sex differences in decision-making has yet to develop. This is in part due to significant variability arising from differences in strains and tasks. The aim of this study was to characterize the effects of sex on specific response elements in a reinforcement learning task so as to help identify potential explanations for this variability. We found that the primary difference between sexes was the propensity to approach feeders out of the task context. This extraneous feeder sampling affects choice on subsequent trials in both sexes by promoting a lose-shift response away from the last feeder sampled. Female rats, however, were more likely to engage in this extraneous feeder sampling, and therefore exhibited a greater rate of this effect. Once trials following extraneous sampling were removed, there were no significant sex differences in any of the tested measures. These data suggest that feeder approach outside of the task context, which is often not recorded, could produce a confound in sex-based differences of reinforcement sensitivity in some tasks.