Early separation of young rats from the mother and the development of play fighting

Evaluating the control: minipump implantation and breathing behavior in the neonatal rat

We evaluated genioglossus (GG) gross motoneuron morphology, electromyographic (EMG) activities, and respiratory patterning in rat pups allowed to develop without interference (unexposed) and pups born to dams subjected to osmotic minipump implantation in utero (saline-exposed). In experiment 1, 48 Sprague-Dawley rat pups (Charles-River Laboratories), ages postnatal day 7 (P7) through postnatal day 10 (P10), were drawn from two experimental groups, saline-exposed (n = 24) and unexposed (n = 24), and studied on P7, P8, P9, or P10. Pups in both groups were sedated (Inactin hydrate, 70 mg/kg), and fine-wire electrodes were inserted into the GG muscle of the tongue and intercostal muscles to record EMG activities during breathing in air and at three levels of normoxic hypercapnia [inspired CO2 fraction (FiCO2 ): 0.03, 0.06, and 0.09]. Using this approach, we assessed breathing frequency, heart rate, apnea type, respiratory event types, and respiratory stability. In experiment 2, 16 rat pups were drawn from the same experimental groups, saline-exposed (n = 9) and unexposed (n = 7), and used in motoneuron-labeling studies. In these pups a retrograde dye was injected into the GG muscle, and the brain stems were subsequently harvested and sliced. Labeled GG motoneurons were identified with microscopy, impaled, and filled with Lucifer yellow. Double-labeled motoneurons were reconstructed, and the number of primary projections and soma volumes were calculated. Whereas pups in each group exhibited the same number (P = 0.226) and duration (P = 0.093) of respiratory event types and comparable motoneuron morphologies, pups in the implant group exhibited more central apneas and respiratory instability relative to pups allowed to develop without interference.

Maternal-infant separation impedes changes in feeding behavior during estrous cycle of rats

Traumatic and stressful events during childhood are associated with the development of eating disorders. We conducted an animal study to test if association stress in childhood affects ingestive behavior later in life by using female rats that have an adjusted estrous cycle. First, electrical impedance of the vagina was conducted to test estrous cycle adjustment. Second, the effects of 6 h per day maternal separation from birth to weaning, which models a psychologically stressful experience in childhood, was used to test feeding behavior during an ovarian cycle in female adult rats with matched estrous cycles. Food and water intake in maternal separated and non-separated rats was measured in each estrous phase. Non-separated rats showed periodical changes, but maternal separated rats showed no significant changes in food and water intake during an estrous cycle. An opposing tendency for food and water intake was seen between maternal separated and non-separated rats. These observations suggest that electrical impedance of the vagina showed the highest value in the estrous phase of rats housed in a reversed light-dark cycle, and maternal separation was found to disturb changes in feeding behavior during the estrous cycle.

The effects of social deprivation on levels of social play in the laboratory rat Rattus norvegicus

Previous studies on a variety of species suggest the existence of a 'rebound' effect in social play: young animals may show heightened levels of play, following temporary deprivation of opportunity for social interaction. The present experiment investigated this phenomenon in greater detail, by measuring the effect of different types of social deprivation on levels of social play in prepubescent laboratory rats Rattus norvegicus. In one deprivation condition, subjects were denied whole-body contact, but not other forms of social interaction; in another condition, subjects experienced more extensive isolation. Both kinds of deprivation produced equally large post-deprivation increases in social play, suggesting that the important factor in producing the rebound effect is the absence of opportunity for whole-body contact - the predominant component of "rough and tumble" social play.

Social influences on neurobiology and behavior: epigenetic effects during development

The quality of the social environment can have profound influences on the development and activity of neural systems with implications for numerous behavioral and physiological responses, including the expression of emotionality. Though social experiences occurring early in development may be particularly influential on the developing brain, there is continued plasticity within these neural circuits amongst juveniles and into early adulthood. In this review, we explore the evidence derived from studies in rodents which illustrates the social modulation during development of neural systems, with a particular emphasis on those systems in which a long-term effect is observed. One possible explanation for the persistence of dynamic changes in these systems in response to the environment is the involvement of epigenetic mechanisms, and here we discuss recent studies which support the role of these mechanisms in mediating the link between social experiences, gene expression, neurobiological changes, and behavioral variation. This literature raises critical questions about the interaction between neural systems, the concordance between neural and behavioral changes, sexual dimorphism in effects, the importance of considering individual differences in response to the social environment, and the potential of an epigenetic perspective in advancing our understanding of the pathways leading to variations in mental health.

Adolescent opiate exposure in the female rat induces subtle alterations in maternal care and transgenerational effects on play behavior

The non-medical use of prescription opiates, such as Vicodin(®) and MSContin(®), has increased dramatically over the past decade. Of particular concern is the rising popularity of these drugs in adolescent female populations. Use during this critical developmental period could have significant long-term consequences for both the female user as well as potential effects on her future offspring. To address this issue, we have begun modeling adolescent opiate exposure in female rats and have observed significant transgenerational effects despite the fact that all drugs are withdrawn several weeks prior to pregnancy. The purpose of the current set of studies was to determine whether adolescent morphine exposure modifies postpartum care. In addition, we also examined juvenile play behavior in both male and female offspring. The choice of the social play paradigm was based on previous findings demonstrating effects of both postpartum care and opioid activity on play behavior. The findings revealed subtle modifications in the maternal behavior of adolescent morphine-exposed females, primarily related to the amount of time females' spend nursing and in non-nursing contact with their young. In addition, male offspring of adolescent morphine-exposed mothers (MOR-F1) demonstrate decreased rough and tumble play behaviors, with no significant differences in general social behaviors (i.e., social grooming and social exploration). Moreover, there was a tendency toward increased rough and tumble play in MOR-F1 females, demonstrating the sex-specific nature of these effects. Given the importance of the postpartum environment on neurodevelopment, it is possible that modifications in maternal-offspring interactions, related to a history of adolescent opiate exposure, plays a role in the observed transgenerational effects. Overall, these studies indicate that the long-term consequences of adolescent opiate exposure can impact both the female and her future offspring.

The meaning of weaning: influence of the weaning period on behavioral development in mice

Maternal care during the first week postpartum has long-term consequences for offspring development in rodents. However, mother-infant interactions continue well beyond this period, with several physiological and behavioral changes occurring between days 18 and 28 PN. In the present study, we investigate the long-term effects on offspring behavior of being weaned at day 21 PN versus day 28 PN. We found that male and female offspring engage in higher initial levels of social interaction if weaned at day 28 PN, as well as sexually dimorphic changes in exploratory behavior. Females who were themselves weaned earlier also appeared to wean their own pups earlier. Sex-specific effects of weaning age were found on levels of oxytocin and vasopressin V1a receptor density in the hypothalamus, central nucleus of the amygdala and nucleus accumbens. These results indicate that altering weaning age in mice may be a useful model for investigating the development of sexual dimorphism in neurobiology and behavior.

Maternal deprivation by early weaning increases corticosterone and decreases hippocampal BDNF and neurogenesis in mice

We previously demonstrated that early weaning increases anxiety and neuroendocrine stress responses in rats and mice. In addition, early-weaned mice show precocious myelin formation, especially in the amygdala, suggesting that these mice are vulnerable to psychological stress. In the present experiments, we examined corticosterone response after early weaning and how early weaning affects hippocampal neurotrophic factor and neurogenesis, which have been linked to depressive behavior in human and animals models. When the mice were weaned at PD14, both male and female mice showed higher corticosterone levels up to 48h after weaning. In contrast, after standard weaning, corticosterone levels returned to the baseline within 2h. Early-weaned males, but not females, had less brain-derived neurotrophic factor (BDNF) protein in the hippocampus at 3 weeks of age than standard-weaned mice. Neural stem cells were labeled with bromodeoxyuridine (BrdU) injections at 2, 3, or 5 weeks of age, and assayed at 3, 5, and 8 weeks of age, respectively. Early-weaned males had fewer BrdU immunoreactive cells in the dentate gyrus at 3, 5, and 8 weeks. In early-weaned females, fewer BrdU-positive cells were observed only at 5 weeks. Double-staining with BrdU and the neuron markers NeuN and Tuj1 demonstrated that neurogenesis was lower in early-weaned mice at 5 weeks of age. These results suggest that lack of mother-infant interaction during the late lactation period leads to an increase in corticosterone synthesis for 2 days and a decrease in BDNF synthesis in males; moreover, this lack of interaction transiently inhibits hippocampal cell proliferation and survival in both males and females, although the effects were more pronounced in males.

Behavioural and neurochemical consequences of early weaning in rodents

Among all mammalian species, pups are highly dependent on their mother not only for nutrition, but also for physical interaction. Therefore, disruption of the mother-pup interaction changes the physiology and behaviour of pups. We review how maternal separation in the early developmental period brings about changes in the behaviour and neuronal systems of the offspring of rats and mice. Early weaning in mice results in adulthood a persistent increase in anxiety-like and aggressive behaviour. The early-weaned mice also show higher hypothalamic-pituitary-adrenal activity in response to novelty stress. Neurochemically, the early-weaned male mice, but not female mice, show precocious myelination in the amygdala, decreased brain-derived neurotrophic factor protein levels in the hippocampus and prefrontal cortex, and reduced bromodeoxyuridine immunoreactivity in the dentate gyrus. Because higher corticosterone levels are persistently observed up to 48 h when the mice are weaned on postnatal day 14, the exposure of the developing brain to higher corticosterone levels may be one of the effects of early weaning. These results suggest that deprivation of the mother-infant interaction during the late lactating period results in behavioural and neurochemical changes in adulthood and that these stress responses are sexually dimorphic (i.e. the male is more vulnerable to early weaning stress).

Early weaning decreases play-fighting behavior during the postweaning developmental period of Wistar rats

We examined the influence of early weaning on the development of play-fighting behaviors and anxiety status in Wistar rats. Pups were divided into two groups, those weaned at postnatal day (PD) 16 (early-weaned group) and those weaned at PD30 (normally weaned group), and were housed in pairs of the same sex. Playful interactions were measured for each pair once a week from 4 to 7 weeks of age. Thereafter, during early adulthood, all the rats were subjected to the elevated plus-maze test. The frequencies of pinning and playful attack were less in the early-weaned group than in the normally weaned group. In the elevated plus-maze test, rat pups in the early-weaned group had higher anxiety levels. The results showed that deprivation of mother-pup interactions during the preweaning period decreases affiliative interactions between cage mates, including play-fighting behaviors during the postweaning developmental period, and increases anxiety levels during early adulthood.

Fostering and environmental enrichment ameliorate anxious behavior induced by early weaning in Balb/c mice

Postnatal stimuli affect many aspects of physiological and behavioral development. In mice, earlier weaning augments anxiety, putatively as a result of removing mother-pup interactions during the weaning period. Here, we examined the ameliorating effects of social and environmental enrichment on anxiety related to early weaning. Mice weaned at postpartum day 14 were fostered by virgin females, who displayed some nursing behavior during the 1-week fostering period. In elevated plus-maze tests, 10-week-old pups reared with a foster mother spent more time in the open arms than early-weaned mice, and entered into the open arms at a rate between that of normally- and early-weaned mice. Subsequently, the mice from each rearing group were transferred into either standard housing or housing enriched with toys that were changed periodically. Elevated plus-maze tests were conducted again when the mice were 18 and 26 weeks old. The enriched environment increased the duration of time spent in the open arms, but the magnitude of the effect varied with the rearing condition. Furthermore, mice that lived in the enriched environment showed lower activity than those kept in standard housing. These results suggest that fostering after early weaning attenuates increases in anxiety levels, and maternal care during this period may be important in the development of an offspring's emotionality. Environmental stimuli in adulthood may act to blunt the effects deprivation in early life.

Early weaning augments neuroendocrine stress responses in mice

Weaning is one of the most important events in early mammalian life. We investigated the influence of early weaning on the development of neuroendocrine responses to stress in mice. Our study indicated that precocious weaning augments anxiety and aggressiveness in mice, and the lack of mother-pup interaction from postnatal days 15-21 may account for this phenomenon. A litter of Balb/cA mice was divided into two groups, with one group weaned at postnatal day 14 (early weaned) and the other at day 21 (normally weaned). Baseline levels of corticosterone, corticosterone response to the moderate stress of exposure to the elevated plus maze test, and hippocampal glucocorticoid receptor (GR) mRNA expression levels in the mice were assayed at the age of 3, 5, and 8 weeks. At 8 weeks, basal corticosterone levels in early-weaned males were higher than those of normally weaned males. Also at 8 weeks, the early-weaned mice showed a higher anxiety level in the elevated plus maze test, and responded with greater levels of corticosterone secretion than the normally weaned mice. GR expression in the early-weaned mice was higher at 3 weeks but lower at 8 weeks. These GR changes were observed only in male mice. These results suggest that early weaning increases anxiety and enhances neuroendocrine responses to stress and thus modulates the development of the neuroendocrine stress system.

Effects of early weaning on anxiety and autonomic responses to stress in rats

Environmental stimuli affect various aspects of the early physical and behavioral development in rats. One of the most important events in the early stage of life is weaning, and we recently reported that precocious weaning augments anxiety and aggressiveness in rats and mice. In the present study, we investigated the autonomic responses to stress in two groups of rats: the early-weaned group (weaned at 16 days of age), and the normally weaned group (weaned at 30 days) as a control. First, the early and normally weaned rats were subjected to an elevated plus-maze test to assess their anxiety levels. It was confirmed that early-weaned male rats, but not the females, showed a lower frequency of entry into and shorter duration of stay in the open arms of the maze compared to the normally weaned rats. Subsequently, the two groups were either placed in a novel clean cage or exposed to an unfamiliar conspecific, and their heart rates and core body temperatures were monitored to evaluate their autonomic stress responses. There was an exacerbation of autonomic responses, such as stress-induced hyperthermia and tachycardia, and an alternation of behavioral responses, including increased sniffing, and decreased grooming and resting. These effects of early weaning were significant only in males. In contrast, when rats encountered an unfamiliar individual, no significant differences were observed between the two groups in either sex. This suggests that stimuli emanating from an unfamiliar intruder were too intense to detect the augmentation of stress responses in the early-weaned rat. The results of the present study demonstrate that precocious weaning augments, not only behavioral but also autonomic responses, to stressful conditions with sexually dimorphic patterns, i.e. more profoundly in males than in females.

Early weaning deprives mouse pups of maternal care and decreases their maternal behavior in adulthood

Weaning is one of the most important events in the early stage of life, and recently we have found that precocious weaning augments anxiety and aggressiveness in mice. Maternal behavior has been reported to be transmitted from one generation to the next; that is, female pups that received intensive maternal care showed higher maternal behavior in their adulthood. In the present study, the following three experiments were conducted to understand maternal behavior transmission in early-weaned mice that were separated from the dam on postnatal day 14. First, the maternal behavior observed from the postpartum day 15 to 21, which was deprived in the early-weaned mice, were analyzed. Mothers spent 3% of their time on licking/grooming and arched-back nursing of their pups on postpartum day 15, and the time spent on these behaviors was gradually decreased until postpartum day 21; however, they spent 50% of their time attending to their pups through postpartum days 15-21. Simultaneously, the behavior of the pups was monitored, and it was found that the early-weaned mice had higher activity and lower resting behavior over the period from postnatal day 15 to 21. Secondly, the early- and normally weaned female mice were subjected to an elevated plus maze test at the age of 8 weeks to assess their anxiety level. The early-weaned mice showed a lower frequency of entering the open arms, and a shorter duration of time spent within them, as compared to the normally weaned mice, suggesting that early-weaned females had a higher anxiety level. In the third experiment, the two groups of female mice were paired with adult male mice for 2 weeks, and the mother's maternal behavior was analyzed. The early-weaned female mice showed lower frequency of licking/grooming and arched-back nursing of their pups as compared to the normally weaned mice, whereas the time of mother-off pups and attending to pups were not different between groups. These results suggest that early-weaning manipulation deprives offspring of a certain level of maternal care, and as a consequence, the offspring show higher anxiety levels and lower maternal behavior in their own adulthood.

Multidimensional structure of anxiety-related behavior in early-weaned rats

Early environmental stimuli affect various aspects of physical and behavioral development. Weaning is one of the most important events in the early stage of life, and recently we have found that precocious weaning augments anxiety and aggressiveness in mice. Here, we report the presence of virtually identical phenomena in rats. To understand the multidimensional structure of anxiety-related behavior, the influence of early weaning upon behavior in adulthood was investigated using three behavioral tests: the elevated plus-maze test, the hole-board test and the open-field test. Two groups of rats were prepared. One was weaned from the dam at 16 days of age (early-weaned group) and the other at 30 days (normally weaned group) as a control. Both groups were subjected to the three tests at 8-10 weeks of age. The elevated plus-maze test revealed lower frequency of entry to and shorter duration of stay in the open arms in the early-weaned animals. In the hole-board test, the early-weaned rats showed lower frequency and shorter duration of head dipping into the holes. And in the open-field test, the early-weaned rats tended to stay at the central square for a shorter period and to defecate more frequently. The behavioral parameters of the three tests were combined and subjected to principal component analysis (PCA). The factorial scores for six extracted factors were compared between the early-weaned and normally weaned groups, and it was revealed that the early-weaned rats had a lower score in Factor 1 (non-anxious exploration) and Factor 5 (risk assessment behavior). Taken together, these results suggest that the time of weaning had a considerable impact on behavioral development, particularly with respect to anxiety-related behaviors.

Febrile convulsions affect ultrasonic vocalizations in the rat pup

Febrile convulsions are common among preschool children and are studied experimentally with hyperthermia in rat pups. Although heat-induced convulsions (HC+) can affect behaviors in adulthood, to date no one has examined the effects on behavior in the neonatal period. We examined the effects of HC+ on ultrasonic vocalizations (USVs) and mother-pup interactions in P10- and P12-aged rat pups. On P7, 34 pups were subjected to HC+ (32 controls, HC-) and behaviors were observed on P10 and P12. For mother-pup interactions, a trend for a sex x treatment interaction was found, with HC- females and HC+ males interacting more often with their dam. Further, HC+ pups vocalized more than controls (HC-) and male pups vocalized more than females. Interestingly, females and HC+ pups displayed less efficient USVs. However, mother-pup interactions did not significantly differ between HC+ and HC- pups, despite greater vocalization. Thus, dams differentiate among their pups, perhaps through USVs.

Early weaning induces anxiety and aggression in adult mice

Early environmental stimuli have been shown to affect many aspects of physiological and behavioral development in humans and other mammals. In this study, we investigated the effects of early weaning on behavioral traits in adulthood. Male and female Balb/c mice were divided into two groups, one weaned from the dam at 14 days of age (early-weaned group) and the other weaned at 21 days of age (normally weaned group), as a control. At 8 and 22 weeks of age, animals of both groups were subjected to the plus maze test to assess their anxiety levels. The early-weaned mice showed lower frequency of entry into the open arms of the maze. Although a subsequent isolation-induced aggression test revealed no clear differences between the two groups, when males from each of the two weaning groups were regrouped after 1 month of isolation, the early-weaned animals showed a great number of wounds on their tails and hindquarters. These results suggest that the deprivation of mother-pup interaction from 14 through 21 postnatal days augments anxiety and aggressiveness.

Episodic neonatal hypoxia evokes executive dysfunction and regionally specific alterations in markers of dopamine signaling

Perinatal ischemic-anoxic and prolonged anoxic insults lead to impaired dopaminergic signaling and are hypothesized to contribute, at least in part, to the pathogenesis of disorders of minimal brain dysfunction such as attention-deficit hyperactivity disorder. We hypothesized that subtle intermittent hypoxic insults, occurring during a period of critical brain development, are also pathogenic to dopaminergic signaling, thereby contributing to behavioral and executive dysfunction. Between postnatal days 7 and 11, rat pups were exposed to either 20-s bursts of isocapnic hypoxic gas, compressed air, or were left undisturbed with the dam. On postnatal days 23 pups were instrumented with electroencephalographic/electromyographic electrodes and sleep-wake architecture was characterized. Locomotor activity was assessed between postnatal days 35 and 38, learning, and working memory evaluated between postnatal days 53 and 64. Rats were killed on postnatal day 80 and tyrosine hydroxylase, vesicular monoamine transporter, dopamine transporter, and dopamine D1 receptors were quantified in the prefrontal cortex, primary sensorimotor cortex, and precommissural striatum by Western blot analyses. Post-hypoxic pups spent less time awake and more time in rapid-eye-movement sleep during the lights-on phase of the circadian cycle, were hyperlocomotive, and expressed impaired working memory. Striatal expression of vesicular monoamine transporter and D1 receptor proteins were increased in post-hypoxic rats, consistent with depressed dopaminergic signaling. These observations lead to the intriguing hypothesis that intermittent hypoxia occurring during a period of critical brain development evokes behavioral and neurochemical alterations that are long lasting, and consistent with disorders of minimal brain dysfunction.

The Influence of Early Weaning on Aggressive Behavior in Mice

Early postnatal experiences have been shown to have a tremendous influence on behavior development. In this study, we focused on weaning time and investigated the effects of early weaning on adulthood aggressive behavior in mice with the resident-intruder test. Early weaning resulted in decreased sideways threat and tail rattle. In addition, the frequency of attack bites was more variable in the early weaned group, although the mean frequency did not differ from the normally weaned group. The results of the present study suggest that aggressive behavior is also affected by early weaning manipulation.

Postnatal stress of early weaning exacerbates behavioral outcome in prenatal alcohol-exposed juvenile rats

Some of the behavioral deficits caused by prenatal or postnatal alcohol exposure have been demonstrated to be ameliorated by environmental manipulations such as handling or environmental enrichment. This experiment, in contrast, investigated whether behavioral deficits due to prenatal alcohol exposure could be exacerbated by a stressful experience, early weaning. Pregnant dams were given either a liquid diet with 35% of the calories derived from alcohol, a liquid diet without alcohol to control for any effects of the liquid diet administration, or ad libitum food and water. Half of each litter were weaned at 15 days of age (early weaning) and half were weaned at 21 days of age (normally weaned). Offspring were weighed, tested for activity in an open field at 18 days of age, and trained to find a hidden platform in the Morris water maze at 22-24 days of age. Alcohol-exposed subjects who were weaned early were more impaired in spatial navigation ability than any other group. Similarly, the combination of early weaning and prenatal alcohol exposure caused the slowest growth. All subjects exposed to alcohol, regardless of weaning condition, had greater latencies to find the platform than those from the two control groups. There was no synergistic effect of alcohol and stress on activity levels, but all early-weaned females were more active than normally weaned females; males did not show this effect. Thus, environmental stressors such as early weaning can compound detrimental symptoms of prenatal alcohol exposure. These results have implications for the understanding of the effects of the environment on neuronal plasticity.

Análise motivacional, causal e funcional da brincadeira em duas espécies de roedores

A brincadeira é um fenômeno bastante comum em indivíduos jovens de diferentes espécies, principalmente mamíferos. O objetivo do presente artigo foi fazer uma revisão sobre pesquisas realizadas com roedores, mais especificamente hamsters dourados (Mesocricetus auratus) e ratos albinos (Rattus norvegicus). Esses animais apresentam os mais altos índices de complexidade de brincadeira e são os mais estudados em laboratório nessa área, entre os roedores. Com base nos artigos da literatura pesquisada, conclui-se que a brincadeira: a) é um sistema motivacional próprio que apresenta características que o diferencia de outros sistemas motivacionais; b) é modulada pelo sexo e idade do indivíduo e dos parceiros envolvidos na interação social; e c) o significado funcional pode estar relacionado com a preparação do indivíduo para viver em ambientes sociais e treinamento físico. Nesse sentido, a brincadeira pode ser relevante para se compreender aspectos do desenvolvimento comportamental e social.

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.

delta-Opioid modulation of social interactions in juvenile mice weaned at different ages

The environmental stimulus of weaning has been shown to affect both the developmental expression of social behavior and the maturation of the opioid delta-receptors' subpopulation in altricial rodents. The aim of this study was to address both these issues by using the social interaction paradigm. Separate groups of male and female mice were randomly assigned to three different weaning ages -- early (Wean-15), regular (Wean-20), and delayed (Wean-25) -- and assessed when 30 days old under intraperitoneal administration of the selective delta-opioid agonist SNC80 (0, 0.1, or 0.3 mg/kg). Wean-15 male and female subjects were much more involved in investigating the partner as well as the cage environment compared to the regular Wean-20 group. An increased social investigation was also found as a consequence of delayed weaning in the female group. The neurobehavioral changes induced by the manipulation of weaning age were also reflected in an altered responsivity to the effects of SNC80 administration. The drug-induced increase in the expression of investigative and affiliative social interactions was further magnified by early weaning. A delayed weaning time was instead associated with reduced sensitivity to the drug, which suggests a delayed maturation of the system. As a whole, the present results indicate that the time of weaning is able to markedly affect the expression of social interactions of adolescent mice by possibly exerting a direct modulatory role on the development of the still plastic delta-opioid system.

Effect of maternal separation on feeding behavior of rats in later life

Effects of maternal separation on feeding behavior, particularly on rebound hyperphagia, in adult rats were examined. Time-restricted scheduled feeding (2 h per day for 6 days), was given at the age of 3, 6, 9 or 12 weeks in rats that were maternal separated from postnatal days (PD) 1-21 and control rats. Following the time-restricted scheduled feeding, rats were fed freely for 24 h (rebound hyperphagia). Body weight, daily normal food consumption and food consumption during time-restricted scheduled feeding and rebound hyperphagia were measured. Body weight of 3-week-old maternally separated rats were less than those of control rats. There was no significant difference in normal daily food consumption. Food consumption during rebound hyperphagia was significantly increased in 6- to 9-week-old female maternally separated rats, but there was no difference observed in males. Postnatal maternal separation enhanced rebound hyperphagia of female rats in later life. These results indicate that postnatal maternal separation made rats more vulnerable to the development of abnormal feeding behavior in response to food restriction in later life.

The developmental psychobiology of behavioural plasticity in mice: the role of social experiences in the family unit

Small perturbations of young animals' sensory experience or hormonal milieu have been shown to alter ontogenetic pathways and to potentially produce huge effects on CNS functioning and behaviour later in life. From a social point of view, variables such as the expression of affiliative bonding and of playful interactions among littermates, the quantity/quality of maternal care, or episodes of maternal or sibling deprivation during critical phases in development, seem to interfere as epigenetic factors with the rigidly ordered temporal sequences of events that occur during the ontogenesis of CNS. This leads to the onset of adaptive neurodevelopmental changes, which are observable within a continuum that encompasses both "normal" individual variability and potential behavioural disorganisation, which in turn will probably be related to profound alteration in the establishment of adult social competence. The present review summarises the more recent work in mice dealing with short-term, as well as long-term modifications, in naturally occurring species-typical social and non-social responses as a function of the early manipulation of social characteristics of the family unit (such as litter gender composition and time of weaning). These analyses were carried out on infant animals, i.e. during the ontogenetic stage of the establishment of social bonding, as well as on pre-pubertal and adult mice and on lactating adult females. Critical issues, such as the respective roles of sibling-sibling and dam-offspring interactions in the shaping of "sibling effects", are also addressed. Overall, these studies indicate that, within their natural range of variation, early patterns of social stimulation are powerful determinants of subsequent behaviour of developing altricial rodents, and confirm that early social life events warrant attention because they can strongly affect neurobehavioural development. Evidence of a relationship between social events occurring during early rearing (i.e. when dramatic transitions in neuroendocrine and neurochemical CNS systems occur) and individual behavioural variability in the infant and adult response to the effects of psychostimulants abused by humans is presented. A better understanding of the mechanisms that mediate such remarkable plasticity might have great psychobiological as well as clinical importance, especially when considering the issue of vulnerability to drug abuse.

Developmental factors influencing aggression. Animal models and clinical correlates

Clearly, models of developmentally induced aggression in animals can give us important insights into the factors inducing aggression in children. Several such models have been produced, and the neurochemical substrates eliciting the aggressive behavior have been identified. In many cases, the serotonergic system is involved. In the future, these animal models may also prove useful in identifying appropriate treatments.

Periodic maternal deprivation alters stress response in adult offspring: potentiates the negative feedback regulation of restraint stress-induced adrenocortical response and reduces the frequencies of open field-induced behaviors

The effects of periodic maternal deprivation (PMD) treatment on the adrenocortical stress response and on open-field behavior in adult offspring were investigated. Sprague-Dawley rat pups were deprived of mothers daily for 4.5 h during the first 3 weeks of life. PMD treatment resulted in lower corticosterone levels during restraint stress later in life. The result of dexamethasone suppression test indicated that PMD treatment caused a potentiation of the negative feedback function of adrenocortical response. These effects of PMD were not accompanied by an increased density of the hippocampal glucocorticoid receptor which has been reported to be induced in neonatal handling treatment (brief 15-min maternal deprivation). Serotonin (5-HT)-2 and beta-adrenergic binding sites were also examined in cerebral cortex and no change of binding capacities were induced by PMD treatment. In the open-field test, PMD treatment decreased the number of ambulations and rearings but did not affect a frequency of defecation. From these results, it is suggested that PMD treatment leads rats to be insensitive to environmental stimuli in adulthood.

The effects of early social isolation on the motivation for social play in juvenile rats

Fifteen-day-old rats were subjected to one of three housing conditions: mother-and-peer (family), peer, and isolation conditions. At 24 days of age, all subjects were rehoused individually. In Experiment 1, play behaviors were monitored in like-raised pairs. Despite their gross lack of social experience, isolation-reared subjects did not exhibit a deficit in frequencies of rough-and-tumble play. It is concluded that the fundamental motivation for rough-and-tumble play is relatively independent of prior learning in rats. Indeed, their elevated dorsal contacts suggested that isolation-raised subjects may have higher appetitive motivation for play. In Experiment 2, the levels of social motivation were compared between family- and isolation-raised animals, using a T-maze. The isolation-raised animals made reliably more choices for social interaction reward over food reward than family-raised animals. Although inconclusive, the results from the two experiments suggest that prolonged social isolation increases the appetitive motivation for social play.

Involvement of opioid and dopaminergic systems in isolation-induced pinning and social grooming of young rats

Pinning, as a measure for play, and social grooming were simultaneously studied in juvenile rats. Short-term social isolation increased both behavioural responses. This increase was attenuated by the opioid antagonist naltrexone, whilst the opiate, morphine, and the opioid peptide beta-endorphin, increased the responses. Pinning was more sensitive to the effects of naltrexone, whilst beta-endorphin stimulated particularly social grooming. Small doses of the dopaminergic drug, apomorphine, decreased both pinning and grooming behaviour of the short-term isolated rats. Some of the effects were partially antagonized by the dopamine antagonist haloperidol, and the neurolepticum-like peptide, desenkephalin-gamma-endorphin (DE-gamma-E). A small dose of haloperidol and DE-gamma-E stimulated social grooming in particular, whilst a larger dose of haloperidol decreased pinning and social grooming. It is concluded that both opioid and dopaminergic systems are implicated in the increase of pinning and social grooming induced by short-term social isolation. The differences in sensitivity of pinning and social grooming for opioid and dopaminergic drugs and peptides are discussed in relation to possible differences in the neural systems underlying both social activities.