Juvenile social experience and differential age-related changes in the dendritic morphologies of subareas of the prefrontal cortex in rats.

Juvenile social interactions have been shown to influence the dendritic complexity of neurons in the prefrontal cortex (PFC). In particular, social play induces pruning of the cells in the medial prefrontal cortex (mPFC), whereas interacting with multiple partners, whether those interactions involve play or not, increases the complexity of cells in the orbital frontal cortex (OFC). Previous studies suggest that these changes differ in their stability during adulthood. In the present study, rats were reared in groups of either four (quads) or two (pairs) and the brains of the rats from each rearing condition were then harvested at 60 days (i.e., shortly after sexual maturity) and 100 days (i.e., fully adult). The rats housed with multiple partners had more complex neurons of the OFC at 60 days and this complexity declined to a comparable level to that of pair housed rats by 100 days. In contrast, the play-induced changes of the mPFC remained similar at both ages. These findings suggest that the changes in the PFC induced by different social experiences in the juvenile period differ in how long they are maintained in adulthood. Differences in the functions regulated by the OFC and the mPFC are considered with regard to these differences in the stability of juvenile-induced neural changes.

Effects of prenatal exposure to valproic acid on the development of juvenile-typical social play in rats.

Autism is a severe neurodevelopmental disorder characterized by qualitative impairments in social behavior, communication, and aberrant repetitive behaviors. A major focus of animal models of autism has been to mimic the social deficits of the disorder. The present study assessed whether rats exposed prenatally to valproic acid (VPA) show deficits in social play as juveniles that are consistent with the social deficits observed in autism. Dams were exposed to an acute dose of VPA on gestational day 12.5. Later, the playful interactions and associated ultrasonic vocalizations of the juveniles were examined. It was predicted that VPA-treated rats should play less than the controls. Characteristic of neurobehavioral insult at this early age, the VPA-treated juveniles showed significant increases in the frequency of body shakes and sexual mounting, but played at the same frequency as the controls. However, when playing, they were less likely to use tactics that facilitated bodily contact and vocalized less. These data suggest that prenatal VPA exposure disrupts some aspects of being able to communicate effectively and engage partners in dynamic interactions - deficits that are consistent with those observed in autism.

Are 50-kHz calls used as play signals in the playful interactions of rats? II. Evidence from the effects of devocalization.

During playful interactions, juvenile rats emit many 50-kHz ultrasonic vocalizations, which are associated with a positive affective state. In addition, these calls may also serve a communicative role - as play signals that promote playful contact. Consistent with this hypothesis, a previous study found that vocalizations are more frequent prior to playful contact than after contact is terminated. The present study uses devocalized rats to test three predictions arising from the play signals hypothesis. First, if vocalizations are used to facilitate contact, then in pairs of rats in which one is devocalized, the higher frequency of pre-contact calling should only be present when the intact rat is initiating the approach. Second, when both partners in a playing pair are devocalized, the frequency of play should be reduced and the typical pattern of playful wrestling disrupted. Finally, when given a choice to play with a vocal and a non-vocal partner, rats should prefer to play with the one able to vocalize. The second prediction was supported in that the frequency of playful interactions as well as some typical patterns of play was disrupted. Even though the data for the other two predictions did not produce the expected findings, they support the conclusion that, in rats, 50-kHz calls are likely to function to maintain a playful mood and for them to signal to one another during play fighting.

Domestication and diversification: a comparative analysis of the play fighting of the Brown Norway, Sprague-Dawley, and Wistar laboratory strains of (Rattus norvegicus).

Laboratory strains of rats are a commonly used subject to study play behavior. Recent research has shown that play in one laboratory strain of rat (e.g., Long-Evans hooded) differs in a number of ways from its wild counterparts. These findings suggest that domestication affects some aspects of play behavior. However, there are multiple strains of laboratory rats, which have been domesticated through different lineages all derived from wild rats and it cannot be assumed that all domestic strains are identical in their play. Therefore, the aim of this study was to compare the play behavior of three other strains of laboratory rats (e.g., Wistar, Sprague-Dawley, and Brown Norway). All strains were similar to each other as they all engaged in high frequencies of play, tolerated similar interanimal distances before initiating playful defense and displayed similar acrobatic capacities, suggesting domestication produces some common changes in play and other factors that influence play. However, strains differed significantly from one another in the use of tactics that promote bodily contact during play. Indeed, in this regard, some strains were more similar to wild rats than others, suggesting that some domestication-induced changes are either unique or more prominent in some laboratory strains than others. Such a mosaic pattern of transformation not only offers the possibility of using strain differences to characterize the genetic factors contributing to different facets of play, but also cautions researchers from making rat-general conclusions from studies on any one strain.

The effects of chronic photoperiod shifting on the physiology of female Long-Evans rats.

As the prevalence of shift work is increasing, it is important to elucidate the impact that shift work has on health. Because of the alternating work schedules present in rotating shift work and working at night, shift workers are in a chronic state of circadian disruption. Animal models of circadian disruption are useful because they offer more experimental control than the largely correlational human shift work studies. The effects of chronic circadian disruption on food preference, glucose tolerance, corticosterone secretion, and performance in a stress-inducing task were investigated in female Long-Evans rats. A 64-day photoperiod shifting paradigm was used to induce circadian disruption. Surprisingly, neither the photoperiod shifted animals, nor the control animals demonstrated a preference for either an unhealthy or healthy diet. Nor was there a difference between the groups in weight gained during photoperiod shifting. However, the photoperiod shifted rats gained significantly more weight than control animals, without eating more food during discriminative fear conditioning to context (DFCTC). Surprisingly, chronic photoperiod shifting appeared to facilitate retention in the DFCTC task. The photoperiod shifted animals also had increased serum glucose values during fasting and after a glucose challenge test. The photoperiod shifted animals only had elevated corticosterone during the final two phases of photoperiod shifting. This study demonstrates that chronic photoperiod shifting elicits weight gain when exposed to a stressful event and impairs glucose tolerance in the same individual.