The postpubertal change in the playful defense of male rats depends upon neonatal exposure to gonadal hormones

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.

Neonatal MeCP2 is important for the organization of sex differences in vasopressin expression

Several neurodevelopmental disorders are marked by atypical Methyl-CpG-binding protein 2 (MeCP2) expression or function; however, the role of MeCP2 is complex and not entirely clear. Interestingly, there are sex differences in some of these disorders, and it appears that MeCP2 has sex-specific roles during development. Specifically, recent data indicate that a transient reduction in MeCP2 within developing amygdala reduces juvenile social play behavior in males to female-typical levels. These data suggest that MeCP2 within the amygdala is involved in programming lasting sex differences in social behavior. In the present study, we infused MeCP2 or control siRNA into the amygdala of male and female rats during the first three days of postnatal life in order to assess the impact of a transient reduction in MeCP2 on arginine vasopressin (AVP), a neural marker that is expressed differentially between males and females and is linked to a number of social behaviors. The expression of AVP, as well as several other genes, was measured in two-week old and adult animals. Two-week old males expressed more AVP and galanin mRNA in the amygdala than females, and a transient reduction in MeCP2 eliminated this sex difference by reducing the expression of both gene products in males. A transient reduction in MeCP2 also decreased androgen receptor (AR) mRNA in two-week old males. In adulthood, control males had more AVP-immunoreactive (AVP-ir) cells than females in the centromedial amygdala (CMA), bed nucleus of the stria terminalis (BST) and in the fibers that project from these cells to the lateral septum (LS). A transient reduction in MeCP2 eliminated this sex difference. Interestingly, there were no lasting differences in galanin or AR levels in adulthood. Reducing MeCP2 levels during development did not alter estrogen receptorα, neurofilament or Foxg1. We conclude that a transient reduction in MeCP2 expression in the developing male amygdala has a transient impact on galanin and AR expression but a lasting impact on AVP expression, highlighting the importance of MeCP2 in organizing sex differences in the amygdala.

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.

The brain as the engine of sex differences in the organization of movement in rats

Sex differences in the kinematic organization of non-reproductive behavior are often relegated to byproducts of sex differences in body morphology. We review evidence showing not only that male and female rats organize their posture and stepping differently during a variety of actions, but that these differences arise from sex differences in the organization of movement in the central nervous system (CNS). Indeed, the expression and choice of sex-typical patterns of movement can be altered by CNS injury. The pattern of hormonal regulation of these sex differences is also not organized as commonly held theory would predict. As expected, males castrated shortly after birth are female-typical in their motor organization. Females ovariectomized at birth, however, are male-typical in their patterns of movement. Thus, female-typical patterns of movement organization are not the default form, but rather are dependent on the effects of gonadal steroids to feminize the developing CNS. The implications of these findings are discussed with regards to our understanding of the evolution of sex differences in CNS anatomy and behavior both for animals and humans.

Ontogenetic interaction between social relationships and defensive burying behavior in the rat

The present experiments clarify sexual and social relationship factors related to the development of defensive burying behavior in rats. Rats were raised in isolation, or in a variety of pairs differing in sex, age or familiarity during the juvenile and post-juvenile period. In Experiment 1, decreased burying behavior was found in both male and female rats during the juvenile stage when they were reared in isolation, or with an adult female, or for males reared with a same-age female. In Experiment 2, female rats isolated during the juvenile stage who were reared after the juvenile stage with a same-sex, non-isolated rat, showed as much burying behavior as rats reared with a littermate; this was not found for male rats. When both male and female rats isolated during the juvenile stage were reared with each other after isolation, they maintained reduced burying behavior in adulthood. These sex differences in the effect of different social groupings are likely due to the differences in social relationships during the juvenile and after puberty, when social dominance relationships emerge in male rats. In Experiment 3, the effects of social dominance relationships on burying behavior were investigated in male rats. Subordination increased the freezing tendency as a passive defense, while social tension accompanied with rearing with an adult male produced decreased burying behavior as a proactive defense. These findings suggest that affiliative relationships involving playful contacts activate and maintain burying behavior, but familiarity is not a significant factor, while dominance relationships modulate the patterns of burying behavior.

Cortisol controls the pubertal development of agonistic behavior in male golden hamsters via type II corticosteroid receptors

In male golden hamsters, agonistic behavior undergoes a pubertal transition from play fighting to adult aggression. Previous studies have shown that this aspect of behavioral development is associated with pubertal increases in glucocorticoids and that daily social stress or injections of a synthetic glucocorticoid accelerate the transition. The goals of this study were to confirm the effects of cortisol on the development of agonistic behavior and to investigate the role of type II corticosteroid receptors in this process. First, animals treated with cortisol during early puberty [from postnatal days 31 (P-31) to P-36] showed an accelerated transition from play fighting to adult aggression. In a second experiment, the behavioral effects of cortisol were blocked by a co-treatment with a type II corticosteroid receptor antagonist. These findings are the first to show a facilitating role for type II corticosteroid receptors in the pubertal development of a social behavior. As such, these findings provide new insights into the neuroendocrine mechanisms controlling behavioral development during puberty.

Play fighting in androgen-insensitivetfm rats: Evidence that androgen receptors are necessary for the development of adult playful attack and defense

The frequency of playful attack and the style of playful defense, are modifiable by gonadal steroids and change after puberty in male and female rats. The present study examined the play behavior exhibited by testicular feminized mutation (tfm)-affected males, who are insensitive to androgens but can bind estrogens aromatized from androgens, to determine the relative contributions of androgens and estrogens to the age-related changes in play behavior. tfm males did not exhibit a decrease in playful attack with age and were more likely to maintain the use of complete rotations, a juvenile form of playful defense, into adulthood. tfm males did however, show age related changes in the use of partial rotations and upright postures, two other forms of playful defense, that were similar to normal males. These data suggest that the development of play fighting and defense in males is dependent on both androgen- and estrogen-receptor-mediated effects.

Changes in the pattern of exploratory behavior are associated with the emergence of social dominance relationships in male rats

This study examined the effect of the establishment of dominance relationships and subordination on exploratory behavior for both postpubertal and adult male rats. Prior to an open field test, subjects were housed either in isolation (IS) or in littermate pairs (PS) with mild dominance relationships without overt victory or defeat, or in pairs with clear hierarchical relationships as dominants (DOM) or subordinates (SUB). Stretch-attend postures and entries into the center area of the open-field were measured as an index of passive and active exploratory behavior, respectively, and crossings in the peripheral area were counted as activity. SUB rats, both postpubertal and adult, displayed less activity and lower levels of active exploratory behavior, whereas adult IS rats showed higher levels of active exploratory behavior compared to the other groups. Furthermore, both DOM and PS rats exhibited a more passive pattern of exploratory behavior in adulthood than in postpuberty. Thus the results show that an increase in the active exploratory pattern is inhibited by the establishment of social relationships among adult rats, while a decrease in activity is a primarily effect of subordination. The capacity to change exploratory patterns following subordination is found even in the postpubertal stage when adultlike social relationships have not yet appeared.

Quantifying presynaptic terminals at the light microscope level in intact and deafferented central nervous structures

Quantification of presynaptic terminals often requires laborious techniques that involve tissue preparation for ultrastructural analysis. Modern preembedding immunohistochemical techniques provide a high morphological resolution at the light microscope level, thus allowing us to identify immunostained presynaptic boutons using specific antibodies. When absolute density of boutons (D(a)) is analysed for comparison between control and deafferented nervous tissue, quantification may be distorted due to tissue shrinkage that follows deafferentiation. The magnitude of this effect must be, therefore, estimated to correct quantitative data. Using the superior colliculus (SC) as a model, an easily applicable protocol to quantify the density of small size labelled particles in control and deafferented nervous tissue is described. This protocol was used to analyse the effect of neonatal and adult enucleation on the adult pattern of cholinergic input to the rat SC. Statistical treatment of data demonstrated that neonatal enucleation caused a drastic increase in bouton density in the visual collicular layers, stratum zonale (SZ) and stratum griseum superficiale (SGS). The same lesion carried out in adult animals caused an increase in the bouton density exclusively in the SZ.

Social play development in pre-weaning olfactory deprived or stimulated rats

The effect of neonatal unilateral restriction or increased olfactory stimulation upon the development of social play behavior in Wistar strain rats was investigated. Pups were unilaterally thermocauterized in one of the nares on postnatal day 3 (experiment 1), or exposed to amyl acetate from 3 to 19 days of age (experiment 2). Unilateral odor deprivation significantly increased the frequency of boxing, wrestling and pinning in rats of both sexes. However, amyl acetate exposure significantly decreased the frequency of wrestling and pinning in females with no effects in males. The alterations in the development of social play behavior are concurrent with gross physical olfactory bulb modifications that may reflect a significant interference with the integration and transmission of olfactory stimuli to central structures such as septal, entorhinal, cingulate and midline frontal cortices subserving social play behavior. Data suggest that early in life brain mechanisms underlying social play behavior are sensitive to manipulation of neonatal olfactory cues and may participate in the modulation of this behavior.

The adolescent brain and age-related behavioral manifestations

To successfully negotiate the developmental transition between youth and adulthood, adolescents must maneuver this often stressful period while acquiring skills necessary for independence. Certain behavioral features, including age-related increases in social behavior and risk-taking/novelty-seeking, are common among adolescents of diverse mammalian species and may aid in this process. Reduced positive incentive values from stimuli may lead adolescents to pursue new appetitive reinforcers through drug use and other risk-taking behaviors, with their relative insensitivity to drugs supporting comparatively greater per occasion use. Pubertal increases in gonadal hormones are a hallmark of adolescence, although there is little evidence for a simple association of these hormones with behavioral change during adolescence. Prominent developmental transformations are seen in prefrontal cortex and limbic brain regions of adolescents across a variety of species, alterations that include an apparent shift in the balance between mesocortical and mesolimbic dopamine systems. Developmental changes in these stressor-sensitive regions, which are critical for attributing incentive salience to drugs and other stimuli, likely contribute to the unique characteristics of adolescence.

Neonatal cryoanesthesia affects the morphology of the visual cortex in the adult rat

Hypothermia is often employed as an anesthetic for manipulations of neonatal animals. We previously reported [J. Nuñez, J.M. Juraska, The effect of neonatal cryoanesthesia on the gross size of the splenium of the corpus callosum, Dev. Psychobiol. 30 (1997) 259; J. Nuñez, J.M. Juraska, The size of the splenium of the rat corpus callosum: influence of hormones, sex ratio and neonatal cryoanesthesia, Dev. Psychobiol., in press.] that the area of the splenium of the adult rat corpus callosum negatively correlated with the time neonatal pups spent in the cold. In the present study, we examined the effect of exposure to neonatal hypothermia (0, 15, 30 and 60 min at 4 degreesC, within 2 h of birth) on the thickness and volume of the binocular (Oc1B) and monocular (Oc1M) fields of the adult rat visual cortex. The number of neurons and glia in layer 2/3 in these areas was also assessed in the 0 and 60 min cold-exposed groups. Cryoanesthesia significantly decreased the length, thickness, volume and number of neurons and glia in both Oc1M and Oc1B. We also observed significant effects of cold exposure on the thickness of individual cortical layers, with some layers affected more than others after differential exposure times. Sex differences (males greater than females) were observed in a number of measures after varying amounts of time in cold and in the absence of cold exposure, in confirmation of previous work from our lab [S.N.M. Reid, J.M. Juraska, Sex differences in the gross size of the rat neocortex, J. Comp. Neurol. 321 (1992) 442-447; S.N.M. Reid, J.M. Juraska, Sex differences in neuron number in the binocular area of the rat visual cortex, J. Comp. Neurol. 321 (1992) 448-455.]. The present study points to the potential confound of using cryoanesthesia on neonatal animals to study cortical structure or function.