Socially-induced brain 'fertilization': play promotes brain derived neurotrophic factor transcription in the amygdala and dorsolateral frontal cortex in juvenile rats

The transcriptome of playfulness is sex-biased in the juvenile rat medial amygdala: a role for inhibitory neurons

Social play is a dynamic behavior known to be sexually differentiated; in most species, males play more than females, a sex difference driven in large part by the medial amygdala (MeA). Despite the well-conserved nature of this sex difference and the importance of social play for appropriate maturation of brain and behavior, the full mechanism establishing the sex bias in play is unknown. Here, we explore "the transcriptome of playfulness" in the juvenile rat MeA, assessing differences in gene expression between high- and low-playing animals of both sexes via bulk RNA-sequencing. Using weighted gene co-expression network analysis (WGCNA) to identify gene modules combined with analysis of differentially expressed genes (DEGs), we demonstrate that the transcriptomic profile in the juvenile rat MeA associated with playfulness is largely distinct in males compared to females. Of the 13 play-associated WGCNA networks identified, only two were associated with play in both sexes, and very few DEGs associated with playfulness were shared between males and females. Data from our parallel single-cell RNA-sequencing experiments using amygdala samples from newborn male and female rats suggests that inhibitory neurons drive this sex difference, as the majority of sex-biased DEGs in the neonatal amygdala are enriched within this population. Supporting this notion, we demonstrate that inhibitory neurons comprise the majority of play-active cells in the juvenile MeA, with males having a greater number of play-active cells than females, of which a larger proportion are GABAergic. Through integrative bioinformatic analyses, we further explore the expression, function, and cell-type specificity of key play-associated modules and the regulator "hub genes" predicted to drive them, providing valuable insight into the sex-biased mechanisms underlying this fundamental social behavior.

Finding biomarkers of experience in animals

At a time when there is a growing public interest in animal welfare, it is critical to have objective means to assess the way that an animal experiences a situation. Objectivity is critical to ensure appropriate animal welfare outcomes. Existing behavioural, physiological, and neurobiological indicators that are used to assess animal welfare can verify the absence of extremely negative outcomes. But welfare is more than an absence of negative outcomes and an appropriate indicator should reflect the full spectrum of experience of an animal, from negative to positive. In this review, we draw from the knowledge of human biomedical science to propose a list of candidate biological markers (biomarkers) that should reflect the experiential state of non-human animals. The proposed biomarkers can be classified on their main function as endocrine, oxidative stress, non-coding molecular, and thermobiological markers. We also discuss practical challenges that must be addressed before any of these biomarkers can become useful to assess the experience of an animal in real-life.

Social play behavior shapes the development of prefrontal inhibition in a region-specific manner

Experience-dependent organization of neuronal connectivity is critical for brain development. We recently demonstrated the importance of social play behavior for the developmental fine-tuning of inhibitory synapses in the medial prefrontal cortex in rats. When these effects of play experience occur and if this happens uniformly throughout the prefrontal cortex is currently unclear. Here we report important temporal and regional heterogeneity in the impact of social play on the development of excitatory and inhibitory neurotransmission in the medial prefrontal cortex and the orbitofrontal cortex. We recorded in layer 5 pyramidal neurons from juvenile (postnatal day (P)21), adolescent (P42), and adult (P85) rats after social play deprivation (between P21 and P42). The development of these prefrontal cortex subregions followed different trajectories. On P21, inhibitory and excitatory synaptic input was higher in the orbitofrontal cortex than in the medial prefrontal cortex. Social play deprivation did not affect excitatory currents, but reduced inhibitory transmission in both medial prefrontal cortex and orbitofrontal cortex. Intriguingly, the reduction occurred in the medial prefrontal cortex during social play deprivation, whereas the reduction in the orbitofrontal cortex only became manifested after social play deprivation. These data reveal a complex interaction between social play experience and the specific developmental trajectories of prefrontal subregions.

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.

Play therapy in children with autism: Its role, implications, and limitations

Play is a pleasurable physical or mental activity that enhances the child’s skills involving negotiation abilities, problem-solving, manual dexterity, sharing, decision-making, and working in a group. Play affects all the brain's areas, structures, and functions. Children with autism have adaptive behavior, adaptive response, and social interaction limitations. This review explores the different applications of play therapy in helping children with autism disorder. Play is usually significantly impaired in children with autism. Play therapy is mainly intended to help children to honor their unique mental abilities and developmental levels. The main aim of play therapy is to prevent or solve psychosocial difficulties and achieve optimal child-healthy growth and development. Play therapy helps children with autism to engage in play activities of their interest and choice to express themselves in the most comfortable ways. It changes their way of self-expression from unwanted behaviors to more non-injurious expressive behavior using toys or activities of their choice as their words. Play therapy also helps those children to experience feeling out various interaction styles. Every child with autism is unique and responds differently. Therefore, different types of intervention, like play therapy, could fit the differences in children with autism. Proper evaluation of the child is mandatory to evaluate which type fits the child more than the others. This narrative review revised the different types of play therapy that could fit children with autism in an evidence-based way. Despite weak evidence, play therapy still has potential benefits for patients and their families.

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.

The role of pre-pubertal training history on hippocampal neurotrophic factors and glucocorticoid receptor protein levels in adult male rats

Neurotrophic factors play an integral role in hippocampal plasticity, and interaction with HPA axis components, especially glucocorticoid receptors (GR), can mediate the structural and functional changes. In the present work, we investigated the long-term effects of combined exercise training (CET) and voluntary physical activity in an enriched environment (EE) in the pre-pubertal period on hippocampal neurotrophic factors and GR. For this purpose, a longitudinal study was designed. After three weeks, all rats were kept in the standard cages without any interventions until adulthood. Western blot analysis revealed a significant increase in hippocampal BDNF and VEGF protein levels in both EE and CET groups (P < 0.001), along with an increase in GR protein levels. In addition, EE decreased serum corticosterone levels compared to CET (P < 0.05). Serum insulin-like growth factor-1 (IGF-1) levels did not demonstrate remarkable changes between groups. Training interventions during sensitive developmental periods may produce profound and long-lasting effects on the hippocampus, at least in part by interactive effects of neurotrophic factors cascades and GR.

The downstream effects of forced exercise training and voluntary physical activity in an enriched environment on hippocampal plasticity in preadolescent rats

During critical periods of brain development, exercise-induced physical fitness may greatly impact the brain structure and function. Nevertheless, forced and intensive physical activities may display negative effects, particularly in the pre-pubertal period. Preadolescent rats were exposed to an enriched environment and combined exercise training for three consecutive weeks in the present study. There was a large cage with enriching stimuli and voluntary physical activity opportunities as an enriched environment (EE). The combined exercise training (CET) consisted of aerobic and resistance training programs. The protein levels of corticosterone (CORT), glucocorticoid receptors (GR_s), insulin-like growth factor-1 (IGF-1), brain-derived neurotrophic factor (BDNF), and vascular endothelial growth factor (VEGF) were assessed using Enzyme-linked immunosorbent assay and western blotting. Cresyl violet staining was also used to evaluate the number of cells in the hippocampus. While GR_s levels were significantly increased in both EE and CET groups (P < 0.001), decreased CORT levels were found in enriched rats (P < 0.05). Moreover, elevated BDNF levels were found in the EE (P < 0.01) and CET (P < 0.05) groups. Similarly, VEGF significantly increased in the EE (P < 0.01) and CET (P < 0.05) animals. However, IGF-1 levels were high only in trained rats (P < 0.05). The number of cells also significantly increased in the DG and CA1 region of the hippocampus after each intervention (P < 0.001). These findings clarified that combined exercise training and voluntary physical activity in an enriched environment during the preadolescent period might promote the downstream plasticity effects on the hippocampus.

Proteomic and transcriptional profiling of rat amygdala following social play

Social play is the most characteristic form of social interaction which is necessary for adolescents to develop proper cognitive, emotional, and social competency. The information available on neural substrates and the mechanism involved in social play is limited. This study characterized social play by proteomic and transcriptional profiling studies. Social play was performed on male Sprague Dawley rats on postnatal day 38 and protein and gene expression in the amygdala was determined following behavioral testing. The proteomic analysis led to the identification of 170 differentially expressed proteins (p ≤ 0.05) with 67 upregulated and 103 downregulated proteins. The transcriptomic analysis led to the identification of 188 genes (FDR ≤ 0.05) with 55 upregulated and 133 downregulated genes. DAVID analysis of gene/protein expression data revealed that social play altered GABAergic signaling, glutamatergic signaling, and G-protein coupled receptor (GPCR) signaling. These data suggest that the synaptic levels of GABA and glutamate increased during play. Ingenuity Pathway Analysis (IPA) confirmed these alterations. IPA also revealed that differentially expressed genes/proteins in our data had significant over representation of neurotransmitter signaling systems, including the opioid, serotonin, and dopamine systems, suggesting that play alters the systems involved in the regulation of reward. In addition, corticotropin-releasing hormone signaling was altered indicating that an increased level of stress occurs during play. Overall, our data suggest that increased inhibitory GPCR signaling in these neurotransmitter pathways occurs following social play as a physiological response to regulate the induced level of reward and stress and to maintain the excitatory-inhibitory balance in the neurotransmitter systems.

The Power of Play: A Pediatric Role in Enhancing Development in Young Children

Children need to develop a variety of skill sets to optimize their development and manage toxic stress. Research demonstrates that developmentally appropriate play with parents and peers is a singular opportunity to promote the social-emotional, cognitive, language, and self-regulation skills that build executive function and a prosocial brain. Furthermore, play supports the formation of the safe, stable, and nurturing relationships with all caregivers that children need to thrive.Play is not frivolous: it enhances brain structure and function and promotes executive function (ie, the process of learning, rather than the content), which allow us to pursue goals and ignore distractions.When play and safe, stable, nurturing relationships are missing in a child's life, toxic stress can disrupt the development of executive function and the learning of prosocial behavior; in the presence of childhood adversity, play becomes even more important. The mutual joy and shared communication and attunement (harmonious serve and return interactions) that parents and children can experience during play regulate the body's stress response. This clinical report provides pediatric providers with the information they need to promote the benefits of play and and to write a prescription for play at well visits to complement reach out and read. At a time when early childhood programs are pressured to add more didactic components and less playful learning, pediatricians can play an important role in emphasizing the role of a balanced curriculum that includes the importance of playful learning for the promotion of healthy child development.

Toward a Neuroscientific Understanding of Play: A Dimensional Coding Framework for Analyzing Infant-Adult Play Patterns

Play during early life is a ubiquitous activity, and an individual’s propensity for play is positively related to cognitive development and emotional well-being. Play behavior (which may be solitary or shared with a social partner) is diverse and multi-faceted. A challenge for current research is to converge on a common definition and measurement system for play – whether examined at a behavioral, cognitive or neurological level. Combining these different approaches in a multimodal analysis could yield significant advances in understanding the neurocognitive mechanisms of play, and provide the basis for developing biologically grounded play models. However, there is currently no integrated framework for conducting a multimodal analysis of play that spans brain, cognition and behavior. The proposed coding framework uses grounded and observable behaviors along three dimensions (sensorimotor, cognitive and socio-emotional), to compute inferences about playful behavior in a social context, and related social interactional states. Here, we illustrate the sensitivity and utility of the proposed coding framework using two contrasting dyadic corpora (N = 5) of mother-infant object-oriented interactions during experimental conditions that were either non-conducive (Condition 1) or conducive (Condition 2) to the emergence of playful behavior. We find that the framework accurately identifies the modal form of social interaction as being either non-playful (Condition 1) or playful (Condition 2), and further provides useful insights about differences in the quality of social interaction and temporal synchronicity within the dyad. It is intended that this fine-grained coding of play behavior will be easily assimilated with, and inform, future analysis of neural data that is also collected during adult–infant play. In conclusion, here, we present a novel framework for analyzing the continuous time-evolution of adult–infant play patterns, underpinned by biologically informed state coding along sensorimotor, cognitive and socio-emotional dimensions. We expect that the proposed framework will have wide utility amongst researchers wishing to employ an integrated, multimodal approach to the study of play, and lead toward a greater understanding of the neuroscientific basis of play. It may also yield insights into a new biologically grounded taxonomy of play interactions.

Positive Social Interactions in a Lifespan Perspective with a Focus on Opioidergic and Oxytocinergic Systems: Implications for Neuroprotection

In recent years, a growing interest has emerged in the beneficial effects of positive social interactions on health. The present work aims to review animal and human studies linking social interactions and health throughout the lifespan, with a focus on current knowledge of the possible mediating role of opioids and oxytocin. During the prenatal period, a positive social environment contributes to regulating maternal stress response and protecting the fetus from exposure to maternal active glucocorticoids. Throughout development, positive social contact with the caregiver acts as a “hidden regulator” and promotes infant neuroaffective development. Postnatal social neuroprotection interventions involving caregiver–infant physical contact seem to be crucial for rescuing preterm infants at risk for neurodevelopmental disorders. Attachment figures and friendships in adulthood continue to have a protective role for health and brain functioning, counteracting brain aging. In humans, implementation of meditative practices that promote compassionate motivation and prosocial behavior appears beneficial for health in adolescents and adults. Human and animal studies suggest the oxytocinergic and opioidergic systems are important mediators of the effects of social interactions. However, most of the studies focus on a specific phase of life (i.e., adulthood). Future studies should focus on the role of opioids and oxytocin in positive social interactions adopting a lifespan perspective.

Social Reward and Empathy as Proximal Contributions to Altruism: The Camaraderie Effect

Natural selection favors individuals to act in their own interests, implying that wild animals experience a competitive psychology. Animals in the wild also express helping behaviors, presumably at their own expense and suggestive of a more compassionate psychology. This apparent paradox can be partially explained by ultimate mechanisms that include kin selection, reciprocity, and multilevel selection, yet some theorists argue such ultimate explanations may not be sufficient and that an additional "stake in others" is necessary for altruism's evolution. We suggest this stake is the "camaraderie effect," a by-product of two highly adaptive psychological experiences: social motivation and empathy. Rodents can derive pleasure from access to others and this appetite for social rewards motivates individuals to live together, a valuable psychology when group living is adaptive. Rodents can also experience empathy, the generation of an affective state more appropriate to the situation of another compared to one's own. Empathy is not a compassionate feeling but it has useful predictive value. For instance, empathy allows an individual to feel an unperceived danger from social cues. Empathy of another's stance toward one's self would predict either social acceptance or ostracism and amplify one's physiological sensitivity to social isolation, including impaired immune responses and delayed wound healing. By contrast, altruistic behaviors would promote well-being in others and feelings of camaraderie from others, thereby improving one's own physiological well-being. Together, these affective states engender a stake in others necessary for the expression of altruistic behavior.

The Psycho-Neurology of Cross-Species Affective/Social Neuroscience: Understanding Animal Affective States as a Guide to Development of Novel Psychiatric Treatments

During the past half century of research with preclinical animal models, affective neuroscience has helped identify and illuminate the functional neuroanatomies and neurochemistries of seven primary process, i.e., genetically provided emotional systems of mammalian brains. All are subcortically localized, allowing animal models to guide the needed behavioral and neuroscientific analyses at levels of detail that cannot be achieved through human research, including modern brain imaging. They consist of the following neuronal processes: SEEKING/Enthusiasm, RAGE/Anger, FEAR/Anxiety, sexual LUST/Passion, maternal CARE/Nurturance, separation-distress PANIC/Grief and PLAY/Social Joy. Several of these systems figure heavily in social bonding. I will focus here especially on the genesis of depression. Its genesis is significantly influenced by (i) sustained overactivity of the separation-distress PANIC system reflecting severed social bonds and the excessive "psychological pain" of loneliness that can, if sustained, lead to a downward cascade known as psychological despair, and (ii) the despair phase that follows the acute PANIC response, which is characterized by abnormally low activity of the SEEKING, the so-called brain reward networks, leading to amotivational states that characterize depression. Depressive affect is promoted by such brain affective mechanisms of social attachments and social loss as well as diminished arousability of the SEEKING system, leading to chronic dysphoria. To understand why depression feels so bad, we must understand the neural mechanisms that mediate such social feelings.

The neurobiology of social play and its rewarding value in rats

In the young of many mammalian species, including humans, a vigorous and highly rewarding social activity is abundantly expressed, known as social play behaviour. Social play is thought to be important for the development of social, cognitive and emotional processes and their neural underpinnings, and it is disrupted in pediatric psychiatric disorders. Here, we summarize recent progress in our understanding of the brain mechanisms of social play behaviour, with a focus on its rewarding properties. Opioid, endocannabinoid, dopamine and noradrenaline systems play a prominent role in the modulation of social play. Of these, dopamine is particularly important for the motivational properties of social play. The nucleus accumbens has been identified as a key site for opioid and dopamine modulation of social play. Endocannabinoid influences on social play rely on the basolateral amygdala, whereas noradrenaline modulates social play through the basolateral amygdala, habenula and prefrontal cortex. In sum, social play behaviour is the result of coordinated activity in a network of corticolimbic structures, and its monoamine, opioid and endocannabinoid innervation.

Rough-and-Tumble Play and the Development of the Social Brain

Social play—that is, play directed toward others—is a readily recognizable feature of childhood. In nonhuman animals, social play, especially seemingly competitive rough-and-tumble play or play fighting, has been the most studied of all forms of play. After several decades of study, researchers of play fighting in laboratory rats have pieced together the rudiments of the neural mechanisms that regulate the expression of this behavior in the mammalian brain. Furthermore, the understanding of the organization, development, and neural control of play in rats has provided a model with which to examine how the experiences accrued during play fighting can lead to organizational changes in the brain, especially those areas involved in social behavior.

Social conditioned place preference in the captive ground squirrel (Ictidomys tridecemlineatus): Social reward as a natural phenotype

Social behaviors of wild animals are often considered within an ultimate framework of adaptive benefits versus survival risks. By contrast, studies of laboratory animals more typically focus on affective aspects of behavioral decisions, whether a rodent derives a rewarding experience from social encounter, and how this experience might be initiated and maintained by neural circuits. Artificial selection and inbreeding have rendered laboratory animals more affiliative and less aggressive than their wild conspecifics, leaving open the possibility that social reward is an artifact of domestication. We compared social behaviors of wild and captive population of juvenile 13-lined ground squirrels (Ictidomys tridecemlineatus), the latter being 2nd- and 3rd-generation descendants of wild individuals. At an age corresponding to emergence from the burrow, postnatal day (PD) 38, captive squirrels engaged in vigorous social approach and play and these juvenile behaviors declined significantly by PD 56. Similarly, young wild squirrels expressed social proximity and play; affiliative interactions declined with summer's progression and were replaced by agonistic chasing behaviors. Social conditioned place preference testing (conditioned PDs 40-50) indicated that adolescent squirrels derived a rewarding experience from social reunion. Our results support the contention that undomesticated rodents have the capacity for social reward and more generally suggest the possibility that positive affective experiences may support group cohesion, social cooperation, and altruism in the wild.

“It’s not Just Camp!”

The purpose of this philosophical hermeneutic inquiry was to understand the meaning of children’s cancer camps for the child with cancer and the family. Six childhood cancer families and 5 cancer camp counselors were interviewed, in order to bring understanding to this topic. Findings from this research revealed that camp means different things for different families, and that much is at play in the cancer camp experience: the healing and developmental power of play, finding acceptance and fit, grief as something to live with versus “get over,” storytelling as a means of reshaping and understanding traumatic experiences, and the solidarity of the community as one that creates intense, healing bonds. Children’s cancer camps, we conclude, should be considered a necessity, versus a luxury, and could even be thought of as a psychosocial intervention for some children and families. Barriers such as structure of funding and access to resources are present and likely due to the separateness of camps from hospital programs.

The development of lasting impairments: a mild pediatric brain injury alters gene expression, dendritic morphology, and synaptic connectivity in the prefrontal cortex of rats

Apart from therapeutic discovery, the study of mild traumatic brain injury (mTBI) has been focused on two challenges: why do a majority of individuals recover with little concern, while a considerable proportion suffer with persistent and often debilitating symptomology; and, how do mild injuries significantly increase risk for an early-onset neurodegeneration? Owing to a lack of observable damage following mTBI, this study was designed to determine if there were changes in neuronal morphology, synaptic connectivity, and epigenetic patterning that could contribute to the manifestation of persistent neurological dysfunction. Prefrontal cortex tissue from male and female rats was used for Golgi-Cox analysis along with the profiling of changes in gene expression (BDNF, DNMT1, FGF2, IGF1, Nogo-A, OXYR, and TERT) and telomere length (TL), following a single mTBI or sham injury in the juvenile period. Golgi-Cox analysis of dendritic branch order, dendritic length, and spine density demonstrate that an early mTBI increases complexity of pyramidal neurons in the mPFC. Furthermore, there are also substantial changes in the expression levels of the seven genes of interest and TL following a single mild injury in this brain region. The results from the neuroanatomical measures and changes in gene expression indicate that the mTBI disrupts normal pruning processes that are typically underway at this point in development. In addition, there are significant interactions between the social environment and epigenetic processes that work in concert to perpetuate neurological dysfunction.

Juvenile play experience primes neurons in the medial prefrontal cortex to be more responsive to later experiences

Juvenile play behavior in rats promotes later behavioral flexibility and appears to do so by modifying the neural systems that regulate the animal's response to unexpected challenges. For example, the experience of play has been shown to prune the dendritic arbor of the cells in the medial prefrontal cortex (mPFC), part of the brain's executive control system. The objective of the present study was to determine if the play-induced changes in the mPFC promotes greater plasticity to experiences later in life. In order to test this possibility, exposure to nicotine was used as the secondary experience given later in life, as it has been shown to produce later changes to the morphology of mPFC pyramidal neurons. Animals were either paired with three same-sex peers (play condition) or one adult (no play condition) during their juvenile period. As young adults, half of the rats from each condition were exposed to repeated injections of nicotine and the other half to injections of saline. The neural plasticity of the mPFC was measured by changes in length and branching of dendrites. Neural changes induced separately by play and by nicotine were consistent with previously published findings. The novel finding was that the cells in the mPFC exhibit a greater response to exposure to nicotine if the rats first had play experience. These findings suggest that juvenile play experiences enhance the plasticity of some neural systems.

Affective neuronal selection: the nature of the primordial emotion systems

Based on studies in affective neuroscience and evolutionary psychiatry, a tentative new proposal is made here as to the nature and identification of primordial emotional systems. Our model stresses phylogenetic origins of emotional systems, which we believe is necessary for a full understanding of the functions of emotions and additionally suggests that emotional organizing systems play a role in sculpting the brain during ontogeny. Nascent emotional systems thus affect cognitive development. A second proposal concerns two additions to the affective systems identified by Panksepp. We suggest there is substantial evidence for a primary emotional organizing program dealing with power, rank, dominance, and subordination which instantiates competitive and territorial behavior and is an evolutionary contributor to self-esteem in humans. A program underlying disgust reactions which originally functioned in ancient vertebrates to protect against infection and toxins is also suggested.

Double helix: reciprocity between juvenile play and brain development

This review summarizes what is presently known about the function, sexual differentiation, and neural circuitry of juvenile rough-and-tumble play. Juvenile rough-and-tumble play is a unique motivated behavior that is widespread throughout the mammalian order and usually occurs more often in males. Immediate early gene studies indicate that cortical and subcortical circuits, many of which are sensitive to sex steroid hormones, mediate juvenile play. Sex differences in rough-and-tumble play are controlled in part by neonatal exposure to androgens or their estrogenic metabolites. Studies indicate that testicular androgens during play are also necessary to stimulate male-like levels of play initiation. The resemblance of rough-and-tumble play to aggression and sexual behavior has led some to question whether male-typical adult behavior is contingent upon the experience of play. Attempts to control the amount of play through social isolation show that social experience during adolescence is critical for male-typical adult behaviors to be expressed. This well-established finding, together with evidence that play induces neural plasticity, supports the hypothesis that juvenile play contributes to male-typical brain development that ultimately enables the expression of adult social and reproductive behavior.

Octodon degus. A useful animal model for social-affective neuroscience research: basic description of separation distress, social attachments and play

A challenge for social-affective neuroscience programs is to identify simple and yet valid animal models for studying the expression of basic social emotions and their role during different developmental windows, from infancy to adulthood. For example, although laboratory rats are useful for studying juvenile social interactions, they are not ideal for studying infant attachment bonds. Here, we evaluate current understanding of the social behavior of Octodon degus, a diurnal precocial rodent, to elucidate the value of this species as a model for social-affective neuroscience research. After a synopsis of species-specific characteristics and brain susceptibility to changes of social environment, our behavioral findings on degu social proclivities are summarized. We then discuss why this pre-clinical model provides a valuable addition to the commonly available animal models for the study of human psychopathology.

Affective neuroscience of the emotional BrainMind: evolutionary perspectives and implications for understanding depression

Cross-species affective neuroscience studies confirm that primary-process emotional feelings are organized within primitive subcortical regions of the brain that are anatomically, neurochemically, and functionally homologous in all mammals that have been studied. Emotional feelings (affects) are intrinsic values that inform animals how they are faring in the quest to survive. The various positive affects indicate that animals are returning to “comfort zones” that support survival, and negative affects reflect “discomfort zones” that indicate that animals are in situations that may impair survival. They are ancestral tools for living - evolutionary memories of such importance that they were coded into the genome in rough form (as primary brain processes), which are refined by basic learning mechanisms (secondary processes) as well as by higher-order cognitions/thoughts (tertiary processes). To understand why depression feels horrible, we must fathom the affective infrastructure of the mammalian brain. Advances in our understanding of the nature of primary-process emotional affects can promote the development of better preclinical models of psychiatric disorders and thereby also allow clinicians new and useful ways to understand the foundational aspects of their clients' problems. These networks are of clear importance for understanding psychiatric disorders and advancing psychiatric practice.

The influences of environmental enrichment, cognitive enhancement, and physical exercise on brain development: can we alter the developmental trajectory of ADHD?

Attention-deficit/Hyperactivity Disorder (ADHD) is characterized by a pervasive pattern of developmentally inappropriate inattentive, impulsive and hyperactive behaviors that typically begin during the preschool years and often persist into adulthood. The most effective and widely used treatments for ADHD are medication and behavior modification. These empirically-supported interventions are generally successful in reducing ADHD symptoms, but treatment effects are rarely maintained beyond the active intervention. Because ADHD is now generally thought of as a chronic disorder that is often present well into adolescence and early adulthood, the need for continued treatment throughout the lifetime is both costly and problematic for a number of logistical reasons. Therefore, it would be highly beneficial if treatments would have lasting effects that remain after the intervention is terminated. This review examines the burgeoning literature on the underlying neural determinants of ADHD along with research demonstrating powerful influences of environmental factors on brain development and functioning. Based upon these largely distinct scientific literatures, we propose an approach that employs directed play and physical exercise to promote brain growth which, in turn, could lead to the development of potentially more enduring treatments for the disorder.

Uncovering the molecular basis of positive affect using rough-and-tumble play in rats: a role for insulin-like growth factor I

Positive emotional states have been shown to confer resilience to depression and anxiety in humans, but the molecular mechanisms underlying these effects have not yet been elucidated. In laboratory rats, positive emotional states can be measured by 50-kHz ultrasonic vocalizations (hedonic USVs), which are maximally elicited by juvenile rough-and-tumble play behavior. Using a focused microarray platform, insulin-like growth factor I (IGFI) extracellular signaling genes were found to be upregulated by hedonic rough-and-tumble play but not depressogenic social defeat. Administration of IGFI into the lateral ventricle increased rates of hedonic USVs in an IGFI receptor (IGFIR)-dependent manner. Lateral ventricle infusions of an siRNA specific to the IGFIR decreased rates of hedonic 50-kHz USVs. These results show that IGFI plays a functional role in the generation of positive affective states and that IGFI-dependent signaling is a potential therapeutic target for the treatment of depression and anxiety.

Molecular and genetic substrates linking stress and addiction

Drug addiction is one of the top three health concerns in the United States in terms of economic and health care costs. Despite this, there are very few effective treatment options available. Therefore, understanding the causes and molecular mechanisms underlying the transition from casual drug use to compulsive drug addiction could aid in the development of treatment options. Studies in humans and animal models indicate that stress can lead to both vulnerability to develop addiction, and increased drug taking and relapse in addicted individuals. Exposure to stress or drugs of abuse results in long-term adaptations in the brain that are likely to involve persistent alterations in gene expression or activation of transcription factors, such as the cAMP Response Element Binding (CREB) protein. The signaling pathways controlled by CREB have been strongly implicated in drug addiction and stress. Many potential CREB target genes have been identified based on the presence of a CRE element in promoter DNA sequences. These include, but are not limited to CRF, BDNF, and dynorphin. These genes have been associated with initiation or reinstatement of drug reward and are altered in one direction or the other following stress. While many reviews have examined the interactions between stress and addiction, the goal of this review was to focus on specific molecules that play key roles in both stress and addiction and are therefore posed to mediate the interaction between the two. Focus on these molecules could provide us with new targets for pharmacological treatments for addiction.

Brain-derived neurotrophic factor and its intracellular signaling pathways in cocaine addiction

Cocaine addiction is one of the severest health problems faced by western countries, where there is an increasing prevalence of lifelong abuse. The most challenging aspects in the treatment of cocaine addiction are craving and relapse, especially in view of the fact that, at present, there is a lack of effective pharmacological treatment for the disorder. What is required are new pharmacological approaches based on our current understanding of the neurobiological bases of drug addiction. Within the context of the behavioral and neurochemical actions of cocaine, this paper considers the contribution of brain-derived neurotrophic factor (BDNF) and its main intracellular signaling mechanisms, including mitogen-activated protein kinase/extracellular signal-regulated protein kinase (MAPK/ERK) and phosphatidylinositol 3-kinase (PI3K), in psychostimulant addiction. Repeated cocaine administration leads to an increase in BDNF levels and enhanced activity in the intracellular pathways (PI3K and MAPK/ERK) in the reward-related brain areas, which applies especially several days following withdrawal. It has been hypothesized that these neurochemical changes contribute to the enduring synaptic plasticity that underlies sensitized responses to psychostimulants and drug-conditioned memories leading to compulsive drug use and frequent relapse after withdrawal. Nevertheless, increased BDNF levels could also have a role as a protection factor in addiction. The inhibition of the intracellular pathways, ERK and PI3K, leads to a disruption in sensitized responses and conditioned memories associated with cocaine addiction and suggests new, potential therapeutic strategies to explore in the dependence on psychostimulants.

Sensory experience determines enrichment-induced plasticity in rat auditory cortex

Our previous studies demonstrated that only a few days of housing in an enriched environment increases response strength and paired-pulse depression in the auditory cortex of awake and anesthetized rats [Engineer, N.D., Percaccio, C.R., Pandya, P.K., Moucha, R., Rathbun, D.L., Kilgard, M.P., 2004. Environmental enrichment improves response strength, threshold, selectivity, and latency of auditory cortex neurons. J Neurophysiol. 92, 73-82 and Percaccio, C.R., Engineer, N.D., Pruette, A.L., Pandya, P.K., Moucha, R., Rathbun, D.L., Kilgard, M.P., 2005. Environmental enrichment increases paired-pulse depression in rat auditory cortex. J Neurophysiol. 94, 3590-3600]. Multiple environmental and neurochemical factors likely contribute to the expression of this plasticity. In the current study, we examined the contribution of social stimulation, exercise, auditory exposure, and cholinergic modulation to enrichment-induced plasticity. We recorded epidural evoked potentials from awake rats in response to tone pairs and noise bursts. Auditory evoked responses were not altered by social stimulation or exercise. Rats that could hear the enriched environment, but not interact with it, exhibited enhanced responses to tones and increased paired-pulse depression. The degree to which enrichment increased response strength and forward masking was not reduced after a ventricular injection of 192 IgG-saporin. These results indicate that rich auditory experience stimulates physiological plasticity in the auditory cortex, despite persistent deficits in cholinergic activity. This conclusion may be beneficial to clinical populations with sensory gating and cholinergic abnormalities, including individuals with autism, schizophrenia, and Alzheimer's disease.

Neuroevolutionary sources of laughter and social joy: modeling primal human laughter in laboratory rats

Rats make abundant 50 kHz ultrasonic vocalizations (USVs) when they play and exhibit other positive social interactions. This response can be dramatically increased by tickling animals, especially when directed toward bodily areas toward which animals direct their own play solicitations (e.g., nape of the neck). The analysis of this system indicates that the response largely occurs in positive, playful social situations, and may index willingness for social engagement, similar to human infantile laughter, which may mature into productive adult socio-sexual behaviors. There are now enough formal similarities between rat 50 kHz USVs and human laughter, to realistically hypothesize that they are neurally and functionally homologous at the subcortical level of brain organization. To help contrast this behavior with human laughter, the available evidence concerning neural organization of human laughter is summarized from brain imaging and neuropsychological perspectives. Thus, a study of 50 kHz USVs in rats may offer an animal model for studying some of the fundamental properties of laughter circuitry in humans, and the brain mechanisms that facilitate positive social engagement, in the mammalian brain. It is proposed that further study of this phenomenon may provide a theoretical as well as empirical handle on the sources of social joy within the mammalian brain.

Neurotrophin levels and behaviour in BALB/c mice: impact of intermittent exposure to individual housing and wheel running

This study assessed the effects of intermittent individual housing on behaviour and brain neurotrophins, and whether physical exercise could influence alternate individual-housing-induced effects. Five-week-old BALB/c mice were either housed in enhanced social (E) or standard social (S) housing conditions for 2 weeks. Thereafter they were divided into six groups and for 6 weeks remained in the following experimental conditions: Control groups remained in their respective housing conditions (E-control, S-control); enhanced individual (E-individual) and standard individual (S-individual) groups were exposed every other day to individual cages without running-wheels; enhanced running-wheel (E-wheel) and standard running-wheel (S-wheel) groups were put on alternate days in individual running-wheel cages. Animals were assessed for activity in an automated individual cage system (LABORAS) and brain neurotrophins analysed. Intermittent individual housing increased behavioural activity and reduced nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) levels in frontal cortex; while it increased BDNF level in the amygdala and BDNF protein and mRNA in hippocampus. Besides normalizing motor activity and regulating BDNF and NGF levels in hippocampus, amygdala and cerebellum, physical exercise did not attenuate reduction of cortical NGF and BDNF induced by intermittent individual housing. This study demonstrates that alternate individual housing has significant impact on behaviour and brain neurotrophin levels in mice, which can be partially altered by voluntary physical exercise. Our results also suggest that some changes in neurotrophin levels induced by intermittent individual housing are not similar to those caused by continuous individual housing.

Emotional tagging of memory formation--in the search for neural mechanisms

Memory-related areas, such as the hippocampus, should be able to sort out the more significant from the less relevant aspects of an experience in order to transform only the earlier into long-term memory. We have recently suggested the Emotional Tagging concept, according to which the activation of the amygdala in emotionally arousing events mark the experience as important and aids in enhancing synaptic plasticity in other brain regions. Here, we review evidence from both human and animal studies that lend support to the Emotional Tagging hypothesis and to the central role the amygdala may play in its formation. We further speculate on potential neural mechanisms that may underlie emotional tagging. Long-term memory formation is considered to involve lasting alterations in synaptic efficacy, known as synaptic plasticity. It has been suggested that two factors are crucial for obtaining a synapse-specific long-term plasticity: (a) the successful activation of a synapse-specific, protein synthesis-independent tag, and (b) the activation of synapse-non-specific protein synthesis. The activation of protein synthesis can then induce lasting plasticity only in those synapses marked by a tag. Interestingly and relevant to the Emotional Tagging hypothesis, it has been recently shown that the activation of the amygdala could transform transient into long-lasting plasticity. These recent findings seem to fit well with the Emotional Tagging hypothesis. It seems reasonable to assume that the activation of the amygdala triggers neuromodulatory systems, which in turn reduce the threshold for the activation of the synaptic tag, and by this facilitate the transformation of early- into late-phase memory.

"Laughing" rats and the evolutionary antecedents of human joy?

Paul MacLean's concept of epistemics-the neuroscientific study of subjective experience-requires animal brain research that can be related to predictions concerning the internal experiences of humans. Especially robust relationships come from studies of the emotional/affective processes that arise from subcortical brain systems shared by all mammals. Recent affective neuroscience research has yielded the discovery of play- and tickle-induced ultrasonic vocalization patterns ( approximately 50-kHz chirps) in rats may have more than a passing resemblance to primitive human laughter. In this paper, we summarize a dozen reasons for the working hypothesis that such rat vocalizations reflect a type of positive affect that may have evolutionary relations to the joyfulness of human childhood laughter commonly accompanying social play. The neurobiological nature of human laughter is discussed, and the relevance of such ludic processes for understanding clinical disorders such as attention deficit hyperactivity disorders (ADHD), addictive urges and mood imbalances are discussed.