Play Behavior In Juvenile Rats Research Articles

Sex differences in the structure and function of the vasopressin system in the ventral pallidum are associated with the sex-specific regulation of social play behavior in juvenile rats

Vasopressin (AVP) regulates various social behaviors, often in sex-specific ways, including social play behavior, a rewarding behavior displayed primarily by juveniles. Here, we examined whether and how AVP acting in the brain's reward system regulates social play behavior in juvenile rats. Specifically, we focused on AVP signaling in the ventral pallidum (VP), a brain region that is a part of the reward system. First, we examined the organization of the VP-AVP system in juvenile rats and found sex differences, with higher density of both AVP-immunoreactive fibers and AVP V1a receptor (V1aR) binding in males compared to females while females show a greater number of V1aR-expressing cells compared to males. We further found that, in both sexes, V1aR-expressing cells co-express a GABA marker to a much greater extent (approx. 10 times) than a marker for glutamate. Next, we examined the functional involvement of V1aR-expressing VP cells in social play behavior. We found that exposure to social play enhanced the proportion of activated V1aR-expressing VP cells in males only. Finally, we showed that infusion of a specific V1aR antagonist into the VP increased social play behaviors in juvenile male rats while decreasing these behaviors in juvenile female rats. Overall, these findings reveal structural and functional sex differences in the AVP-V1aR system in the VP that are associated with the sex-specific regulation of social play behavior.

The medial preoptic area and its projections to the ventral tegmental area and the periaqueductal gray are activated in response to social play behavior in juvenile rats.

The medial preoptic area (MPOA) is well known for its role in sexual and maternal behaviors. This region also plays an important role in affiliative social behaviors outside reproductive contexts. We recently demonstrated that the MPOA is a central nucleus in which opioids govern highly rewarding social play behavior in adolescent rats. However, the neural circuit mechanisms underlying MPOA-mediated social play remain largely unresolved. We hypothesized that the MPOA unites a complementary neural system through which social play induces reward via a projection to the ventral tegmental area (VTA) and reduces a negative affective state through a projection to the periaqueductal gray (PAG). To test whether the two projection pathways are activated in response to social play behavior, we combined retrograde tract tracing with immediate early gene (IEG) expression and immunofluorescent labeling to identify opioid-sensitive projection pathways from the MPOA to VTA and PAG that are activated after performance of social play. Retrograde tracer, fluoro-gold (FG), was microinjected into the VTA or PAG. IEG expression (i.e., Egr1) was assessed and triple immunofluorescent labeling for mu opioid receptor (MOR), Egr1, and FG in the MPOA was performed after social play. We revealed that play animals displayed an increase in neurons double labeled for Egr1 + FG and triple labeled for MOR + Egr1 + FG in the MPOA projecting to both the VTA and PAG when compared to no-play rats. The increased activation of projection neurons that express MORs from MPOA to VTA or PAG after social play suggests that opioids may act through these projection pathways to govern social play. (PsycInfo Database Record (c) 2023 APA, all rights reserved).

Social Play Behavior Is Critical for the Development of Prefrontal Inhibitory Synapses and Cognitive Flexibility in Rats.

Sensory driven activity during early life is critical for setting up the proper connectivity of the sensory cortices. We ask here whether social play behavior, a particular form of social interaction that is highly abundant during postweaning development, is equally important for setting up connections in the developing prefrontal cortex (PFC). Young male rats were deprived from social play with peers during the period in life when social play behavior normally peaks [postnatal day 21-42] (SPD rats), followed by resocialization until adulthood. We recorded synaptic currents in layer 5 cells in slices from medial PFC of adult SPD and control rats and observed that inhibitory synaptic currents were reduced in SPD slices, while excitatory synaptic currents were unaffected. This was associated with a decrease in perisomatic inhibitory synapses from parvalbumin-positive GABAergic cells. In parallel experiments, adult SPD rats achieved more reversals in a probabilistic reversal learning (PRL) task, which depends on the integrity of the PFC, by using a more simplified cognitive strategy than controls. Interestingly, we observed that one daily hour of play during SPD partially rescued the behavioral performance in the PRL, but did not prevent the decrease in PFC inhibitory synaptic inputs. Our data demonstrate the importance of unrestricted social play for the development of inhibitory synapses in the PFC and cognitive skills in adulthood and show that specific synaptic alterations in the PFC can result in a complex behavioral outcome.SIGNIFICANCE STATEMENT This study addressed the question whether social play behavior in juvenile rats contributes to functional development of the prefrontal cortex (PFC). We found that rats that had been deprived from juvenile social play (social play deprivation - SPD) showed a reduction in inhibitory synapses in the PFC and a simplified strategy to solve a complex behavioral task in adulthood. Providing one daily hour of play during SPD partially rescued the cognitive skills in these rats, but did not prevent the reduction in PFC inhibitory synapses. Our results demonstrate a key role for unrestricted juvenile social play in PFC development and emphasize the complex relation between PFC circuit connectivity and cognitive function.

Characterization of juvenile play in rats: importance of sex of self and sex of partner.

BackgroundJuvenile social play is observed in many mammalian species, and its disruption in several neuropsychiatric disorders has greatly increased interest in understanding the origins and sources of variability in this behavior.MethodsWe quantified social play behavior in juvenile rats and investigated the impact of sex and familiarity of the play partner. Sex differences in play behavior were investigated by comparing males and females from either same- or mixed-sex pairs with data pooled over 12 days of analysis. Whether play was altered based on the sex of the play partner was assessed using a paired analysis to compare play with a same- or opposite-sex play partner for both males and females. Additionally, a repeated measures design was utilized to determine whether play changed with increasing age. On postnatal day 33, a novel play partner was introduced. We used a repeated measures analysis to compare postnatal day 33 with the previous day. These approaches were used to assess the effects of age, sex, sex of partner, and familiarity of partner on total social play behavior as well as how play was broken down into components, such as pouncing, pinning, chasing, and boxing.ResultsThere were sex differences in total frequency of play, and specific parameters of play behavior, such as chasing, pouncing, pinning, and boxing. Additionally, males significantly altered their play behavior in response to the sex of their play partner, whereas females were more sensitive to the familiarity of the play partner.ConclusionsThis study provides critical groundwork for uncovering factors that regulate social play behavior and can be used to guide future mechanistic based work.

Dynamic changes in extracellular release of GABA and glutamate in the lateral septum during social play behavior in juvenile rats: Implications for sex-specific regulation of social play behavior

Social play is a motivated and rewarding behavior that is displayed by nearly all mammals and peaks in the juvenile period. Moreover, social play is essential for the development of social skills and is impaired in social disorders like autism. We recently showed that the lateral septum (LS) is involved in the regulation of social play behavior in juvenile male and female rats. The LS is largely modulated by GABA and glutamate neurotransmission, but their role in social play behavior is unknown. Here, we determined whether social play behavior is associated with changes in the extracellular release of GABA and glutamate in the LS and to what extent such changes modulate social play behavior in male and female juvenile rats. Using intracerebral microdialysis in freely behaving rats, we found no sex difference in extracellular GABA concentrations, but extracellular glutamate concentrations are higher in males than in females under baseline conditions and during social play. This resulted in a higher glutamate/GABA concentration ratio in males vs. females and thus, an excitatory predominance in the LS of males. Furthermore, social play behavior in both sexes is associated with significant increases in extracellular release of GABA and glutamate in the LS. Pharmacological blockade of GABA-A receptors in the LS with bicuculline (100ng/0.5μl, 250ng/0.5μl) dose-dependently decreased the duration of social play behavior in both sexes. In contrast, pharmacological blockade of ionotropic glutamate receptors (NMDA and AMPA/kainate receptors) in the LS with AP-5+CNQX (2mM+0.4mM/0.5μl, 30mM+3mM/0.5μl) dose-dependently decreased the duration of social play behavior in females, but did not alter social play behavior in males. Together, these data suggest a role for GABA neurotransmission in the LS in the regulation of juvenile social play behavior in both sexes, while glutamate neurotransmission in the LS is involved in the sex-specific regulation of juvenile social play behavior.

Endogenous vasopressin, innate anxiety, and the emission of pro-social 50-kHz ultrasonic vocalizations during social play behavior in juvenile rats

Although the involvement of the neuropeptide arginine vasopressin (AVP) in rodent social interaction is already extensively characterized, little is known about its role in social communication. Rats communicate in the ultrasonic range by means of ultrasonic vocalizations (USV). Depending on developmental stage and affective state, rats emit various distinct types of USV, with appetitive 50-kHz USV being induced by positive social interactions, like juvenile social play, probably serving an affiliative communicative function, namely to (re)establish or induce social proximity. In rats and mice selectively bred for low (LAB) and high (HAB) anxiety-related behavior, the emission of isolation-induced distress USV during maternal deprivation as pups correlates with innate high levels of hypothalamic AVP availability. Moreover, male LAB and HAB rats express deficits in social approach towards conspecifics, together with high and/or abnormal forms of aggression when confronted with harmless opponents, possibly due to a lack of social communication skills. The aim of this study was therefore (1) to investigate and characterize social play behavior and concomitant pro-social 50-kHz USV emission in male and female, juvenile LAB and HAB rats and to compare them to non-selected Wistar (NAB) rats; and (2) to link these findings pharmacologically to the central AVP system via applying an AVP 1a receptor (V1aR) antagonist (0.75 μg; Manning compound) or synthetic AVP (1 ng) into the lateral ventricle of male juvenile NAB rats. Our results show that reduced social play behavior in highly anxious male and female, juvenile HAB rats is accompanied by low amounts of pro-social 50-kHz USV, as compared to respective LAB and NAB rats, possibly reflecting a lack of positive affective states in expectation of or following social interactions in these individuals. Secondly, although synthetic AVP did not alter social play behavior and pro-social 50-kHz USV, we demonstrated for the first time that a blockade of the central AVP system not only reduces juvenile social play behavior, but at the same time pro-social 50-kHz USV emission rates, indicating an involvement of the social neuropeptide in regulating affiliative communication in rodents.

Sex-specific modulation of juvenile social play by vasopressin.

Social play activities among juveniles are thought to contribute to the development of social and emotional skills in humans and animals. Conversely, social play deficits are observed in developmental neuropsychiatric disorders. Importantly, many of these disorders show sex differences in incidence, course of the disease, and severity of symptoms. We hypothesized that sex differences in the neural systems controlling social behavior can contribute to these differences. We therefore studied the involvement of the sexually dimorphic vasopressin and oxytocin systems, which have been implicated in these disorders, in juvenile social play behavior. Single-housed 5-week-old juvenile male and female rats were exposed in their home cage to an age-and sex-matched novel conspecific for 10 min, and social play behaviors were recorded. We found no consistent sex differences in duration or elements of social play in vehicle-treated rats. However, intracerebroventricular injection of the specific vasopressin 1a receptor (V1aR) antagonist (CH2)5Tyr(Me(2))AVP significantly reduced social play behaviors in males while increasing them in females. Intracerebroventricular injection of the specific oxytocin receptor antagonist des-Gly-NH2,d(CH2)5[Tyr(Me)(2),Thr(4)]OVT did not alter social play in either sex. To locate the effects of V1aR blockade on social play, we targeted the lateral septum, a sexually dimorphic brain region showing denser vasopressin fibers in males than in females and an abundant expression of V1aR in both sexes. Surprisingly, blockade of V1aR in the lateral septum increased social play behaviors in males, but decreased them in females. These findings suggest sex- and brain region-specific roles for vasopressin in the regulation of social play behavior in juvenile rats.

Serotonin, motivation, and playfulness in the juvenile rat

The effects of the selective 5HT(1A) agonist 8-OH-DPAT were assessed on the play behavior of juvenile rats. When both rats of the test pair were comparably motivated to play, the only significant effect of 8-OH-DPAT was for play to be reduced at higher doses. When there was a baseline asymmetry in playful solicitation due to a differential motivation to play and only one rat of the pair was treated, low doses of 8-OH-DPAT resulted in a collapse of asymmetry in playful solicitations. It did not matter whether the rat that was treated initially accounted for more nape contacts or fewer nape contacts, the net effect of 8-OH-DPAT in this model was for low doses of 8-OH-DPAT to decrease a pre-established asymmetry in play solicitation. It is concluded that selective stimulation of 5HT(1A) receptors changes the dynamic of a playful interaction between two participants that are differentially motivated to play. These results are discussed within a broader framework of serotonergic involvement in mammalian playfulness.

The effects of selective breeding for infant ultrasonic vocalizations on play behavior in juvenile rats

For over 25 generations, two lines of rats (High and Low USV lines) have been selectively bred for extreme rates of infantile (45 kHz) ultrasonic vocalizations (USV) in response to maternal separation at postnatal day (P)10. High and low line juveniles (P30–P40) were socially isolated and allowed to play in same-sex sibling pairs for 10 min per day over three days. Measures of play were nape contacts and pinning. Other social and nonsocial behaviors were also scored during the three sessions; two of these, 55 kHz USV and walk-overs, were statistically associated with play. Compared to the Random control line, both High and Low line juveniles showed deficits in play behavior. In the High line, play initiatory behavior (nape contacts) was reduced, but pinning, USV and walk-overs were relatively unchanged. In contrast, nape contacts, pinning, USV and walk-overs were all reduced in Low line juveniles compared to Random line controls. The results suggest that selection for extremes of infant USV rates has produced temperamental differences that are expressed in juvenile play in the High and Low USV lines.

Effects of neonatal handling and maternal separation on rough-and-tumble play in the rat.

The extent to which brief daily handling and longer periods of separation from the mother during the first 2 weeks of life can affect play behavior in juvenile rats was assessed. Rat pups were separated from the mother for either 15 min daily (handling) or for 3 hr daily (maternal separation), and play was observed as juveniles. Overall levels of playfulness were not affected by either manipulation, although certain aspects of playful responsiveness were affected in males, but not females. In particular, the pattern of responsiveness to playful contacts was feminized in both handled and separated male rats. Activity in a novel open field at 15 days of age was increased in both males and females from the separated group, but not in the handled animals, as were the number of rears exhibited during the play bouts. These data suggest that early rearing experiences can have subtle gender-dependent effects on some aspects of play in juvenile rats and that the underlying mechanism(s) responsible for these effects may differ from those associated with other effects reported for handling and maternal separation.

D2 dopamine receptor involvement in the rough-and-tumble play behavior of juvenile rats.

By using dorsal contacts and pinning to quantify play behavior in juvenile rats, it was found that the D2 agonist, quinpirole, reduced both measures of play at doses greater than 0.05 mg/kg. Eticlopride, a D2 antagonist, also reduced both measures of play and blocked the reduction caused by quinpirole. The effect of quinpirole on play was largely unaffected by concurrent administration of either a D1 agonist (SKF 38393) or a D1 antagonist (SCH 23390), suggesting that D1 and D2 receptors are functionally independent with respect to play behavior. Quinpirole also reduced overall activity, suggesting that the effects on play may not be selective to neural circuitry responsible for play behavior. Although low doses of quinpirole (0.001--0.03 mg/kg) had a tendency to increase pinning, this effect was not very robust. These data suggest that D2 dopamine receptors may not have a major role in the control of play behavior in juvenile rats.

D₂ dopamine receptor involvement in the rough-and-tumble play behavior of juvenile rats.

D₂ dopamine receptor involvement in the rough-and-tumble play behavior of juvenile rats.

Sequential analysis of social play behavior in juvenile rats: effects of morphine

The effects of morphine on social play behavior in juvenile rats were investigated using sequential analysis. Social play behavior of 21-day-old rats treated with 1.0 mg/kg of morphine or saline was analyzed for 15 min. Frequencies and durations of measures of social play behavior, such as pinning, boxing/wrestling and following/chasing were significantly increased after treatment with morphine. Social behaviors not related to play, such as social exploration and crawling over/under were slightly decreased, while social grooming and non-social behavior were not affected by morphine treatment. Using sequential analysis, the dissociation between social behaviors related and unrelated to play was confirmed. Pinning and boxing/wrestling were highly significantly associated. In addition, crawling over/under significantly often preceded social grooming, and an association between social exploration and non-social behavior was found as well. Pinning and boxing/wrestling appeared to be negatively associated with non-social behaviors, social exploration and crawling over/under. A negative association between social grooming and social exploration was also found. Upon treatment with morphine, no major effects on the sequential structure of social behavior were observed: pinning and boxing/wrestling appeared more associated with following/chasing, and the negative association between pinning and boxing/wrestling on the one hand and social exploration on the other was enhanced. Thus, morphine slightly increased the coherence of social play behavior. It is concluded that morphine exerts its effects on social play behavior by increasing social play behavior as a whole rather than by changing its structure, suggesting a key role for opioid systems in the regulation of social play behavior.

Influence of environmental factors on social play behavior of juvenile rats

The effects of light level and familiarity to the testing environment on social behaviors related and unrelated to play were investigated in juvenile rats accustomed to dim light conditions. Pinning, a measure characteristic for social play in rats, was completely suppressed under intense light conditions. Following/chasing and boxing/wrestling, social behaviors related to play, were also decreased under intense light. Of the measures of social behavior not related to play, contact behavior was decreased under intense light whereas social exploration was hardly affected. Levels of social exploration and following/chasing gradually declined during the 15-min test period. Frequency of contact behavior decreased, whereas duration increased with time. Under dim light conditions, unfamiliarity to the test cage suppressed pinning and boxing/wrestling but not the other social behaviors in the first part of the test period. These findings show that social behavior in juvenile rats, as in adult rats, can be influenced by light level and familiarity to the test cage. Social behaviors related and unrelated to play seem to be differentially influenced by environmental stimuli.

Social play alters regional brain opioid receptor binding in juvenile rats

An in vivo autoradiographic procedure was employed to visualize local changes in brain opioid receptor occupancy in juvenile rats. This procedure is based on the assumption that released endogenous ligand will exclude exogenously applied tracer, in this case [ 3H]diprenorphine, from opioid receptors. Increases in availability of opioid peptides will then result in decreased opioid receptor binding. From behavioral studies there is ample evidence that opioid systems are involved in the regulation of social play behavior in juvenile rats. In the present study, changes in regional brain opioid activity as a result of social isolation-induced social play behavior were monitored. Twenty-one-day-old rats were socially isolated for 0, 3.5 or 24 h prior to testing, and tested alone or in a dyadic encounter. After behavioral testing, [ 3H]diprenorphine was administered and the brain was prepared for autoradiography. Social isolation caused increases in social behavior (dyadic encounters) but not in non-social behavior (singly tested animals). Modest differences in brain opioid receptor binding due to social isolation, social play behavior, or an interaction of the two, were found in claustrum, nucleus accumbens, globus pallidus, paraventricular and arcuate nuclei of the hypothalamus, and the dorsolateral and paratenial thalamic nuclei. These results support the notion that opioid systems are involved in the regulation of social play behavior. In addition, the observation of changes in opioid binding in areas involved in reward processes, adds evidence to the hypothesis that opioid systems are involved in the regulation of the rewarding aspects of social plah in juvenile rats.

Social play in juvenile rats: A decade of methodological and experimental research

The social play behavior of juvenile rats was originally described nearly a century ago, but research methods have only recently included the controlled laboratory investigation of psychobiological variables. This review covers the experimental literature of social play or play fighting behavior in juvenile laboratory rats reported during the last decade. Innovative measures for quantifying social play are described; hormonal, pharmacological, and neurological variables are examined; and interpretative concepts of social play are discussed. The current emphasis on measures and procedures as well as the limited scope of current research effort suggests a formative stage of research development.

Developmental analyses of social play behavior in juvenile rats

Juvenile rats normally engage in rough-and-tumble social play behavior. Casual and informal reports of social play have been superseded by more detailed observations, including longitudinal studies of the developmental function from preweanling to young adult stages. Social play is first seen at 15 or 16 days of age. It then increases in frequency and vigor to a peak level at approximately 36 days of age. At puberty, social play behaviors of males merge into situation-specific behaviors related to copulation and aggression. In females, social play continues into sexual maturity. This review compares the available ontogenetic descriptions of social play in juvenile rats and specifies a number of informational gaps in the literature.