Novelty-related behavior of young and adult dopamine transporter knockout rats: Implication for cognitive and emotional phenotypic patterns

Attention deficit hyperactivity disorder (ADHD) is a neuropsychiatric disorder characterized by a developmentally inappropriate, pervasive and persistent pattern of severe inattention, hyperactivity and impulsivity. Despite onset in early childhood, ADHD may continue into adulthood with substantial impairment in social, academic and occupational functioning. A new animal model of this disorder was developed in rats with genetic deletion of the dopamine transporter (DAT) gene (dopamine transporter knockout rats; DAT-KO rats). We analyzed the behavior of DAT-KO rats for a deeper phenotypical characterization of this model. We first tested rats of the 3 genotypes at different ages (preadolescent, adolescent and adult), in a novelty-seeking test using a black/white box (Experiment 1). After that, we tested adult rats in a novelty-preference test using a 3-chamber apparatus with different shapes (Experiment 2). Experiment 1: as evidenced by analysis of time spent in the novel environment, adult DAT heterozygous (DAT-HET) rats show an increased curiosity-driven exploration compared with wild-type (WT) controls while DAT-KO rats did not recognize novelty. The locomotor activity data show a minimal difference between genotypes at adolescent age while the preadolescent and adult DAT-KO rats have significantly increased activity rate compared with WT and DAT-HET subjects. Experiment 2: in this case, due to more clearly evident spatial differences, time spent in novel environment was not significantly different among genotypes. During first 10 minutes, DAT-KO rats showed a decreased hyperactivity, apparently related to curiosity and attention to the new environments. In conclusion, DAT-KO rats may show some inattention while more novelty-seeking traits appear in DAT-HET rats.

Internet Addiction in adolescence: Neurobiological, psychosocial and clinical issues

Despite it has not been formally included in DSM-5 as a disorder, 'Internet addiction (IA)' has become a worldwide issue. It can be broadly defined as a non-chemical, behavioral addiction, which involves human-machine interaction. We pinpoint it as an "instrumental" form of social interaction (i.e. mediated by machines), a notion that appears useful for the sake of possible preclinical modeling. The features of Internet use reveals as addictive when this comes at the expense of genuine real-life sociability, with an overlap towards the hikikomori phenomenon (i.e., extreme retreat to one's own room). Due to the specific neuro-developmental plasticity in adolescence, IA poses risks to youths' mental health, and may likely produce negative consequences in everyday life. The thwarted development of adolescents' identity, self-image and adaptive social relationships is discussed: the IA adolescents often suffer loss of control, feelings of anger, symptoms of distress, social withdrawal, and familial conflicts. Further, more severe clinical conditions are also associated to IA, such as dysthymic, bipolar, affective, social-anxiety disorders, as well as major depression. This paper overviews the literature on IA, from neuro-biological, psycho-social and clinical standpoints, taking into account recent debates on diagnostic criteria, nosographic label and assessment tools. Neuroimaging data and neurochemical regulations are illustrated with links to pathogenetic hypotheses, which are amenable to validation through innovative preclinical modeling.

Early postnatal behavioral changes in the Mecp2-308 truncation mouse model of Rett syndrome

In a mouse model of Rett syndrome (RTT) which expresses a truncated form of methyl-CpG-binding protein 2 (Mecp2) gene (Mecp2-308), we performed a neurobehavioral evaluation across the life span, starting from soon after birth till adulthood. A focus was made on those developmental phases and behavioral domains which have not been previously investigated. The results evidenced subtle anomalies on postnatal days (pnds) 3 to 9 (so-called presymptomatic phase) in spontaneous movements by hemizygous neonatal male mice. Specifically as early as pnd 3, mutant pups exhibited more intense curling and more side responses and on pnd 9 more pivoting and head rising behaviors than wild type (wt) littermates. A significant decrease in ultrasonic vocalization rate, also emerged in Mecp2-308 pups. The same mice were also characterized by increased anxiety-like behaviors (open-field and zero-maze tests) during the early symptomatic phase, in the absence of changes in cognitive passive-avoidance task and rotarod performances. Upon the clearly symptomatic stage, 5-month-old Mecp2-308 mice were also associated with reduced spontaneous home-cage motor activity, motor coordination impairments (rotarod and dowel tests), and a more marked profile of D-amphetamine (10 mg/kg) released stereotyped behavioral syndrome than wt mice. Present results provide an interesting timeline of the progression of symptoms in the Mecp2-308 model and emphasize the need for increased attention to the presymptomatic phase which may be especially informative in mouse models of human neurodevelopmental disorders. This analysis has provided evidence of precocious behavioral markers of RTT and has identified an early developmental window of opportunities on which potential therapies could be investigated.

Increased impulsive behavior and risk proneness following lentivirus-mediated dopamine transporter over-expression in rats' nucleus accumbens

Multiple theories have been proposed for sensation seeking and vulnerability to impulse-control disorders [Zuckerman M, Kuhlman DM (2000) Personality and risk-taking: Common biosocial factors. J Pers 68:999-1029], and many of these rely on a dopamine system deficit. Available animal models reproduce only some behavioral symptoms and seem devoid of construct validity. We used lentivirus tools for over-expressing or silencing the dopamine transporter (DAT) and we evaluated the resulting behavioral profiles in terms of motivation and self-control. Wistar adult rats received stereotaxic inoculation of a lentivirus that allowed localized intra-accumbens delivery of a DAT gene enhancer/silencer, or the green fluorescent protein, GFP. These animals were studied for intolerance to delay, risk proneness and novelty seeking. As expected, controls shifted their demanding from a large reward toward a small one when the delivery of the former was increasingly delayed (or uncertain). Interestingly, in the absence of general locomotor effects, DAT over-expressing rats showed increased impulsivity (i.e. a more marked shift of demanding from the large/delayed toward the small/soon reward), and increased risk proneness (i.e. a less marked shift from the large/uncertain toward the small/sure reward), compared with controls. Rats with enhanced or silenced DAT expression did not show any significant preference for a novel environment. In summary, consistent with literature on comorbidity between attention-deficit/hyperactivity disorder and pathological gambling, we demonstrate that DAT over-expression in rats' nucleus accumbens leads to impulsive and risk prone phenotype. Thus, a reduced dopaminergic tone following altered accumbal DAT function may subserve a sensation-seeker phenotype and the vulnerability to impulse-control disorders.

1H MRS-detectable metabolic brain changes and reduced impulsive behavior in adult rats exposed to methylphenidate during adolescence

Administration of methylphenidate (MPH, Ritalin) to children affected by attention deficit hyperactivity disorder (ADHD) is an elective therapy, which however raises concerns for public health, due to possible persistent neuro-behavioral alterations. We investigated potential long-term consequences at adulthood of MPH exposure during adolescence, by means of behavioral and brain MRS assessment in drug-free state. Wistar adolescent rats (30- to 44-day-old) were treated with MPH (0 or 2 mg/kg once/day for 14 days) and then left undisturbed until adulthood. Levels of impulsive behavior were assessed in the intolerance-to-delay task: Food-restricted rats were tested in operant chambers with two nose-poking holes, delivering one food pellet immediately, or five pellets after a delay whose length was increased over days. MPH-exposed animals showed a less marked shifting profile from the large/late to the small/soon reward, suggesting reduced basal levels of impulsivity, compared to controls. In vivo MRI-guided 1H MRS examinations at 4.7 T in anaesthetised animals revealed long-term biochemical changes in the dorsal striatum (STR), nucleus accumbens (NAcc), and prefrontal cortex (PFC) of MPH-exposed rats. Notably, total creatine and taurine, metabolites respectively involved in bioenergetics and synaptic efficiency, were up-regulated in the STR and conversely down-regulated in the NAcc of MPH-exposed rats. A strong correlation was evident between non-phosphorylated creatine in the STR and behavioral impulsivity. Moreover, unaltered total creatine and increased phospho-creatine/creatine ratio were detected in the PFC, suggesting improved cortical energetic performance. Because of this enduring rearrangement in the forebrain function, MPH-exposed animals may be more efficient when faced with delay of reinforcement. In summary, MPH exposure during adolescence produced enduring MRS-detectable biochemical modifications in brain reward-related circuits, which may account for increased self-control capacity of adult rats.

Sub-neurotoxic neonatal anoxia induces subtle behavioural changes and specific abnormalities in brain group-I metabotropic glutamate receptors in rats

Anoxia in the first week of life can induce neuronal death in vulnerable brain regions usually associated with an impairment of cognitive function that can be detected later in life. We set-up a model of subneurotoxic anoxia based on repeated exposures to 100% nitrogen during the first 7 days of post-natal life. This mild post-natal exposure to anoxia specifically modified the behaviour of the male adult rats, which showed an attention deficit and an increase in anxiety, without any impairment in spatial learning and any detectable brain damage (magnetic resonance imaging and histological analysis). Post-anoxic rats showed a reduction in the expression of group-I metabotropic glutamate receptors (i.e. mGlu1 and mGlu5 receptors) in the hippocampus and cerebral cortex, whereas expression of the mGlu 2/3 receptors, the NR1 subunit of NMDA receptors, and the GluR1 subunit of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptors was unchanged. mGlu1 and mGlu5 receptor signalling was also impaired in postanoxic rats, as revealed by a reduced efficacy of the agonist (1S,3R)-1-Aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) to stimulate polyphosphoinositide hydrolysis in hippocampal slices. We conclude that rats subjected to subneurotoxic doses of anoxia during the early post-natal life develop behavioural symptoms that are frequently encountered in the inattentive subtype of the attention deficit hyperactivity disorder, and that group-I mGlu receptors may be involved in the pathophysiology of these symptoms.

Windows of vulnerability to psychopathology and therapeutic strategy in the adolescent rodent model

Adolescence comes in association with puberty, when maturation and rearrangement of major neurotransmitter pathways and functions are still taking place. The neurobiological processes occurring in the brain during this developmental period have been so far poorly investigated. Yet, it is during adolescence that some major neuropsychiatric disorders may become evident, including ADHD, schizophrenia, and drug abuse. Moreover, the age-related neurobehavioural plasticity renders adolescents particularly vulnerable to the consequences of psychoactive drug exposure. In this view, there is an increased likelihood that addiction will develop when psychoactive drug use starts early during adolescence. From all these observations adolescence emerges as a critical phase in development. In the present review, we focus on recent neurobiological characterization of adolescent rats and mice. As for vulnerability to addictive behaviour, nicotine exposure during adolescence dose-dependently down-regulated levels of AMPA GluR2/3 subunits in the striatum, suggesting a reduced neurobehavioural plasticity in adult subjects. Comparable exposure during adulthood had opposite effects. It was found consistently that exposure to nicotine during adolescence, but not similar exposure in the post-adolescent period, increased the expression of specific subunits of the acetylcholine receptor in adult rats, thus enhancing the reinforcing efficacy of nicotine in a self-administration paradigm. The present data identified a specific age-window, characterized by long-term effects on behavioural and neurochemical indexes, of vulnerability. With respect to potential therapeutic approaches in ADHD, we studied the adolescent spontaneously-hypertensive-rat (SHR) in an intolerance-to-delay operant-behaviour paradigm. The model was further validated by the finding that impulsivity was reduced by chronic methylphenidate administration. Impulsive SHR animals were characterized by reduced cannabinoid CB1 receptor density in the prefrontal cortex. Interestingly, an acute cannabinoid agonist increased levels of self-control behaviour in these animals. The present data suggest that pharmacological modulation of the cannabinoid system might improve some behavioural anomalies seen in ADHD. In conclusion, modelling the adolescent phase in rats and mice appears to be useful for the investigation of determinants of vulnerability to addiction and to other early-onset neuropsychiatric disorders.

Chronic treatment with imipramine reverses immobility behaviour, hippocampal corticosteroid receptors and cortical 5-HT1A receptor mRNA in prenatally stressed rats

Prenatal stress in the rat induces enhanced reactivity of the hypothalamus-pituitary-adrenal (HPA) axis, disturbances in a variety of circadian rhythms and increased anxiety-like behaviour. Such abnormalities parallel those found in human depressed patients. Prenatally stressed (PS) rats could represent, therefore, an interesting animal model for the evaluation of the efficacy of pharmacotherapeutic intervention in psychiatric disorders that has often been addressed using control animals. In the present study, PS and non-stressed rats were chronically treated with the tricyclic antidepressant imipramine (10 mg/kg i.p. for 21 days) and assessed in the forced swim test. Glucocorticoid receptor binding sites in the hippocampus were measured and 5-HT(1A) receptor mRNA levels in the frontal cortex were also assessed. PS rats were characterised by increased immobility in the forced swim test, reduced hippocampal corticosteroid receptor binding and increased levels of cortical 5-HT(1A) mRNA. All these parameters were significantly reversed by chronic imipramine treatment. Conversely, no significant effects were observed for non-stressed rats. All these effects are consistent with the expected pharmacotherapy of depression-like abnormalities in PS rats. These results further indicate that PS rats are a relevant animal model of depression.

Social withdrawal, neophobia, and stereotyped behavior in developing rats exposed to neonatal asphyxia

Perinatal asphyxia is a concern for public health and may promote subtle neuropsychiatric disorders. Anoxic insults to neonatal rats cause long-lasting neurobehavioral deficits. In the present study, we focussed on changes in emotional behaviors as a consequence of neonatal asphyxia in Wistar rats. Newborn pups (24 h after birth) underwent a single 30-min exposure to a 100% N2 atmosphere (or air). The offspring was tested for a) locomotor and exploratory activity with or without a d-amphetamine challenge (0, 1, or 2 mg/kg) on postnatal day (pnd) 15; b) social interactions and novelty seeking during adolescence; c) levels of the brain-derived neurotrophic factor (BDNF). In the open-field test (pnd 15), N2-exposed pups injected with the high (2 mg/kg) amphetamine dose exhibited reduced levels of locomotor hyperactivity, and a more marked involvement in stereotyped behaviors. Individual differences emerged in the locomotor response to the novelty-seeking test: two subgroups of rats (separated on the basis of the median value) showed either arousal/attraction or avoidance/inhibition in response to free-choice novelty. The N2-exposed group showed a more marked novelty-induced avoidance and inhibition. Time devoted to allogrooming and play-soliciting behaviors was reduced, whereas object exploration was increased. Levels of BDNF were reduced in the striatum of N2-exposed rats, suggesting poorer synaptic performance of dopamine pathways. In conclusion, these findings suggest an increased risk of developing social withdrawal, neophobia and behavioral stereotypies (common symptoms found in schizophrenia and autism) as a consequence of neonatal asphyxia in preterm humans.

[Impact of socio-environmental factors on physiological processes of brain damage recovery: contribution of animal models]

This review presents the experimental approaches more widely used in animal models to investigate possible strategies aimed to stimulate plasticity in the nervous system and possibly to increase spontaneous recovery from functional/neurological diseases such as neonatal anoxia. Methods used in laboratory rodents are briefly described. Attention is focused on possible enrichments of the social and physical environment during the development.

Experimentally induced aggressive behavior in subjects with 3,4-methylenedioxy-methamphetamine ("Ecstasy") use history: psychobiological correlates

PURPOSE

Objective measures of experimentally induced aggressiveness were evaluated in 12 male 3,4-methylenedioxy-methamphetamine (MDMA, "Ecstasy") users, in comparison with 20 healthy male subjects.

METHODS

All the subjects were preliminarily submitted to DSM-IV interviews and Buss-Durkee Hostility Inventory (BDHI). During a laboratory task, the Point Subtraction Aggression Paradigm (PSAP), subjects earned monetary reinforcers with repeated button presses, and were provoked by the subtraction of money that was attributed to a fictitious other participant. Subjects could respond by ostensibly subtracting money from the fictitious subject (the aggressive response). Escape responses were also possible protecting the counter from monetary subtractions.

RESULTS

Money-earning responses were not different in Ecstasy users and controls; aggressive responses were significantly higher in Ecstasy users in comparison with control subjects (F=20.74, P<.001). Baseline adrenocorticotropic hormone (ACTH) and cortisol (CORT) levels were higher in Ecstasy users than in controls. No difference was found in norepinephrine (NE) and epinephrine (EPI) basal levels of the two groups. During the experimentally induced aggressiveness, plasma ACTH concentrations increased significantly less and NE and EPI levels, together with heart rate (HR), increased significantly more in Ecstasy users than in healthy subjects. Despite ACTH-blunted responses, CORT did not increase differently from controls in Ecstasy users. PSAP aggressive responses positively correlated with catecholamines and CORT changes, BDHI Direct Aggression and Irritability scores, both in Ecstasy users and controls. A significant correlation was found between Ecstasy exposure extent and aggressive responses (r=.78, P<.001).

IMPLICATIONS

Our findings suggest that Ecstasy users have higher outward-directed aggressiveness than healthy subjects. Aggressiveness in MDMA subjects seems to be associated more with MDMA pharmacological effects than with personality traits: Nevertheless, a premorbid psychobiological proneness to aggressive behavior cannot be excluded. Increased catecholamines reactivity, basal hypothalamus-pituitary-adrenal (HPA) axis hyperactivity, and blunted ACTH responses could be due to MDMA action on monoaminergic pathways and adrenal function.

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.

[Psychobiologic risk factors and vulnerability to psychostimulants in adolescents and animal models]

Adolescence is associated with an increased risk of developing drug abuse/dependence. During this ontogenetic phase, brain and hormonal systems are still undergoing crucial maturational rearrangements, which take place together with significant modifications in psychosocial development. However, the neurohormonal and behavioural facets of adolescence have been poorly investigated in relation to the vulnerability to psychostimulants such as MDMA ("ecstasy") and amphetamine. Novelty-seeking, a temperamental/behavioural trait that is typical of this age period, might substantially contribute to psychobiological vulnerability to drugs. In animal models of periadolescence, the search for novel stimuli and sensations actually shares a common neurobiological substrate (the reward-related brain mesolimbic pathways) with psychostimulants. Periadolescent mice are characterized by an unbalanced and "extremes-oriented" behaviour and by elevated levels of novelty-seeking. A deeper understanding of psychostimulant effects during adolescence, and the interaction between genetic, neurobiologic, psychosocial, and environmental factors, will lead to earlier and more effective prevention strategies.

Novelty seeking in periadolescent mice: sex differences and influence of intrauterine position

In rodents, beside basic sex differences, a certain degree of within-gender phenotypic variation can also be provided in utero by hormones from adjacent fetuses. We investigated novelty-seeking behavior in two groups of male and female mice from know intrauterine position: 2M (between males) and 0M (between females). Subjects were assessed during periadolescence (postnatal days 33--43), an ontogenetic phase, which is characterized by an elevated expression of this novelty-seeking behavior. Periadolescent mice underwent a familiarization session for 3 consecutive training days with one side of a two-chamber apparatus. On testing day 4, the opening of a partition, which allowed mice to freely move from the familiar compartment to a novel one, produced an increased behavioral arousal in all animals. Marked sex differences were found, with females being in general more active than males, whereas the latter showed significantly higher levels of novelty seeking than females. Uterine position failed to affect the profile of novelty preference in females, whereas within the male group 2M subjects expressed a marked profile of novelty seeking. The differential titers of sex hormones reported to characterize the 0M and 2M condition early in fetal development are suggested to account for the individual variability in the seeking for novelty within the male group during puberty.

Striatal dopamine sensitization to D-amphetamine in periadolescent but not in adult rats

The neurobiological and behavioral facets of adolescence have been poorly investigated in relation to the vulnerability to psychostimulants. Periadolescent (33-43 days) and adult (>70 days) Sprague-Dawley rats underwent a 3-day treatment history with D-amphetamine (AMPH) at 0, 2, or 10 mg/kg (once a day). After a short 5-day-long withdrawal interval, freely moving animals were challenged with a 2-mg/kg AMPH dose and their behavior as well as in vivo intrastriatum dopamine (DA) release in the CNS were assessed. Microdialysis data indicated that AMPH-history periadolescent rats showed a prominent sensitization of AMPH-stimulated DA release, whereas no such change was found in adult subjects. As expected, acute AMPH administration strongly reduced time spent lying still and increased levels of cage exploration in animals of both ages. A treatment history of high AMPH dosage was associated with a marked sensitization of the exploratory behavior in adults, whereas it induced a quite opposite profile in periadolescents. The latter group only was also characterized by a compulsive involvement in the stereotyped head-bobbing response. These results indicate that differently from adults, marked alterations in neurobiological target mechanisms are observed in rats around periadolescence as a consequence of a quite mild regimen of intermittent AMPH exposure. Thus, a neurobiological substrate for an age-related increased vulnerability towards the addictive risks of these drugs is suggested.

Attractivity and social preferences in mice (Mus musculus domesticus): the role of prepubertal sexual segregation and of precocious weaning

Mice (Mus musculus domesticus) were raised (Postnatal Day 15 to 25) in single- or mixed-sex litters and precociously (Day 15) or regularly (Day 25) weaned. When they were faced as adults with a basic social choice--between two stimulus mice raised in litters of different sex composition but both of the same sex as the chooser--mice raised in mixed-sex litters were preferred. In the sociosexual choice-between a male and a female, both from the single- or the mixed-sex group--the opposite-sex preference was expressed. Both these preferences were abolished by the sexual segregation of the choosers. This variable hardly affected potential mate choice--between two stimulus mice both of the opposite sex of the chooser but raised in litters of different sex composition. Data indicate that socially mediated behavioral plasticity has a major role in the early shaping of adult individual differences both in attractive stimulus properties and in sociosexual preferences.

Neuroendocrine correlates of depression in abstinent heroin-dependent subjects

The functions of the central alpha-adrenergic, serotoninergic and dopaminergic systems were investigated in 28 heroin-dependent subjects 6-8 weeks after detoxification, and in 22 healthy control subjects (group C). Fourteen heroin-dependent subjects with depressive comorbidity (group A), and 14 heroin-dependent subjects without other Axis I and II pathologies (group B) were included among abstinent substance abusers. Norepinephrine (NE) function was evaluated by growth hormone (GH) responses to acute stimulation with clonidine (clon); serotonin (5-HT) function by prolactin (PRL) and cortisol (CORT) responses to acute stimulation with D-fenfluramine (D-fen) and dopamine (DA) function by GH and PRL responses to acute administration of bromocriptine (brom). Central NE activity, as measured by the GH-clon test, seems to be well preserved both in A and B subjects. PRL and CORT responses to D-fen were significantly blunted both in A subjects and in B subjects, in comparison with control subjects (C); the PRL response in A subjects was significantly lower than in B subjects. The DA system of B subjects was found unimpaired; in contrast, a significantly higher GH response to brom in A subjects (depressed) could express D2 post-synaptic receptor hypersensitivity and, therefore, decreased pre-synaptic DA release. In sum, the study of central monoamine function revealed an alteration only of the 5-HT system in detoxified heroin-dependent subjects without psychiatric comorbidity, which might be a trait character of these subjects, possibly involved in the pathogenesis of the disorder. A more significant impairment of 5-HT function and the hypersensitivity of post-synaptic DA receptors in A subjects suggests that specific biological correlates of psychiatric comorbidity may characterize substance abuser subtypes.

A unique hormonal and behavioral hyporesponsivity to both forced novelty and d-amphetamine in periadolescent mice

The identification of critical ontogenetic periods of increased vulnerability to the effects of drugs of abuse could have a great psychobiological and clinical-therapeutical importance. Potential age-related differences in the response of the hypothalamic-pituitary-adrenal (HPA) axis to both stress and psychostimulants has been tested here in an animal model of adolescence. Periadolescent (PND 33-43) and Adult (PND>60) mice of both sexes were injected with d-amphetamine (AMPH, 0, 2, or 10 mg/kg i.p.) and immediately faced with a mild psychological stress experience, i.e. placement in a novel environment. A detailed time-course analysis of both hormonal and behavioral profiles was performed, with animals being sacrificed for trunk-blood collection at different time-points during the test (before the injection, NT group; 15, 30, or 120 min after the injection). Basal corticosterone (CORT) levels (NT group) were consistently higher in periadolescents than in adults. As a whole, a marked increment of blood CORT levels was found in mice of both ages exposed to forced novelty. However, important age-related differences were also observed, with Saline-injected periadolescents still exhibiting elevated levels of locomotion at the end of the 120-min test session and failing to show the increasing profile of CORT release over the baseline that was typical of adults. Upon an AMPH 2 administration, periadolescents exhibited a much lower profile of locomotor hyperactivity than adults, and also failed to show an increase across the course of the session in CORT release, that was observed in adults. When treated with the high AMPH 10 dose, a marked locomotor hyperactivity was found in periadolescents, which however showed much lower levels of the stereotyped licking and gnawing behavior, that was typical of adults. The present results suggest a unique profile of integrated behavioral and physiological hyporesponsivity in mice during periadolescence. The latter also represents a very useful model for the study of the issue of psychobiological risk factors involved in vulnerability to drugs of abuse in human adolescents.

Paradoxical effects of D-amphetamine in infant and adolescent mice: role of gender and environmental risk factors

The psychostimulant D-amphetamine (AMPH) increases generalised activity in adult subjects, while exerting a paradoxical "calming effect" in children with Attention-deficit Hyperactivity Disorder (AD/HD). A number of animal models have been developed to characterise the neurobiological basis of this AMPH action. In this line, the present review summarises recent work on the effects of AMPH on behavioural and physiological parameters in developing mice with a special emphasis on the role of gender and environmental risk factors. Behavioural and neuroendocrine responses to AMPH administration (0, 1, or 3 mg/kg, IP) and their relation to changes in the environment, represented by social stimuli, were studied in infant CD-1 mouse pups of both sexes at three different developmental ages (3, 8, or 18 postnatal (pnd) days). Mouse pups were assessed either in baseline condition or following 24 h maternal deprivation. AMPH exerted a paradoxical effect on CORT secretion only in maternally deprived subjects while affecting behaviour mainly in deprived female subjects, which showed a generalised shift to the left in the dose-response curve to this drug. Unwanted perseverative motor effects and possible dependence states represent side effects of AMPH administration. Further knowledge on these aspects comes from another set of studies where a shortened conditioned place preference (CPP) paradigm was employed to assess the reinforcing properties of AMPH (0, 1, 3.3, or 10 mg/kg) in developing mice on 14-17, 21-24, and 28-31 pnd. Data indicate that AMPH-CPP develops early, mice being able, already at two weeks of age, to acquire a place preference that relies on adult-like sensory, motor, and associative capacities. AMPH-CPP appears earlier in females, compared to males. A detailed analysis of acute D-amphetamine effects evidenced that the drug produces a dose-dependent increase in locomotor activity and in several responses (including stereotypes). These effects appear much larger at both post weaning stages than in preweanlings and are significantly more pronounced in females than in males. Overall these data suggest that AMPH action is dependent on the baseline level of activity and indicate a strong role of gender in the effects of this drug measured early on during development, with females showing greater sensitivity to this drug. A better understanding of AMPH action during the early ontogenetic phases, particularly its interaction with environmental factors, might extend our knowledge on the neurobiological basis of AD/HD, possibly improving the clinical efficacy of psychostimulant drugs.

Psychobiological risk factors for vulnerability to psychostimulants in human adolescents and animal models

Adolescence is associated with an increased risk of developing drug abuse/dependence. During this ontogenetic phase, brain and hormonal systems are still undergoing crucial maturational rearrangements, which take place together with significant modifications in psychosocial development. However, the neurohormonal and behavioral facets of adolescence have been poorly investigated in relation to the vulnerability to psychostimulants such as MDMA ("Ecstasy") and amphetamine (AMPH). Novelty-seeking, a temperamental/behavioral trait that is typical of this age period, might substantially contribute to both psychological and psychobiological vulnerability. In humans, an elevated score of novelty-sensation seeking and a derangement of monoaminergic function were both associated with late adolescence MDMA users compared to controls. In animal models of periadolescence, the search for novel stimuli and sensations actually shares a common neurobiological substrate (the reward-related brain mesolimbic pathways) with psychostimulants. The present review summarises recent work in mice, which indicates that periadolescent subjects are characterized by an unbalanced and "extremes-oriented" behavior and by elevated novelty-seeking compared to adults. Repeated and intermittent administration of cocaine or AMPH was associated with the development of a prominent locomotor sensitization in periadolescents, which failed to exhibit the marked sensitization of the stereotyped behavioral syndrome--possibly associated with poor welfare--that was typical of adults. A unique profile of integrated behavioral and physiological hyporesponsivity to both forced novelty and acute AMPH administration during periadolescence was also found. As a whole, these results, together with previous work on this topic, suggest that periadolescents may be more "protected" from AMPH-related aversive properties, and perhaps more vulnerable to the experience of internal states of reward, than older animals. Thus, the present animal model of adolescence seems to represent a reliable and useful method for the investigation of vulnerability to a variety of habit-forming agents or emotional experiences whose positive reinforcing properties may rely on common neurobiological substrates. A deeper understanding of psychostimulant effects during adolescence on the complex interaction between genetic, neurobiologic, psychosocial, and environmental factors will lead to earlier and more effective prevention and treatment.

Behavioral and hormonal effects of partner familiarity in periadolescent rat pairs upon novelty exposure

In periadolescent rats, social interactions are typically characterized by elevated levels of playful and affiliative behavior. Aim of the present study was to assess the behavioral and hormonal effects of partner familiarity upon the separation and reunion in a novel environment of established pairs of periadolescent subjects. At weaning (post-natal day, PND 21), Sprague-Dawley rats were pair housed with a non-sibling subject of the same age and sex. On PND 35, the members of each pair were separated for a 24-h period, and randomly assigned to different experimental groups: (1) sacrificed before separation; (2) sacrificed immediately after the isolation period; (3-4) placed individually in a novel cage for 30 min either in low-light or in high-light conditions; (5-6) reunited for 30 min in a novel cage either with their previous cagemate (familiar, FAM); or (7-8) with an unfamiliar rat (UNF) of the same age and sex, in either light conditions. During reunion, the occurrence of social and non-social behaviors was scored. Blood samples were collected at the end of the session from all groups and assayed for corticosterone (CORT). The separation of the two members of an established pair did not affect baseline CORT levels. Upon reunion, the presence of a conspecific exerted a significant buffering effect on the novelty-induced increase in CORT levels. Such an effect of the social companion appeared more marked in males than in females, and in FAM compared to UNF pairs. Interestingly, FAM rats also expressed a significantly higher amount of social investigation and play-soliciting behavior compared to UNF animals. Behavioral results, together with previous data, suggest that periadolescent rats housed in established pairs develop a sort of amicable relationship. The overall CORT output measured at the end of the session is also in line with this interpretation. As a whole, these findings indicate that periadolescence is a time period during rat development, during which social variables play a very important role in modulating both behavioral and physiological responses to novelty in a fashion that does not completely overlap with data on adult subjects.

Elevated novelty seeking and peculiar d-amphetamine sensitization in periadolescent mice compared with adult mice

Novelty seeking as well as amphetamine sensitization were studied in adult (postnatal day "PND" > 60) and periadolescent (PND 3343) mice treated with saline or amphetamine (2 or 10 mg/kg once per day) for 3 days in a familiar environment. After a 48-hr wash-out period, mice were challenged with either saline or amphetamine (2 mg/kg) in the same environment. When given a choice, animals showed a preference for a novel environment, an effect more marked in periadolescents. Acute amphetamine strongly increased novelty seeking in adults, whereas it had an opposite effect in periadolescents. Adult mice in the chronic amphetamine 2 group showed a conditioned preference for the drug-paired compartment, whereas an aversion characterized adult mice in the amphetamine 10 group. Periadolescents in the latter group exhibited a greater sensitization of the locomotor response, but did not show the compulsive licking typical of adults. This appears to be a useful model to study psychobiological risk factors involved in vulnerability to addiction during human adolescence.

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.

Elevated novelty seeking and peculiar d-amphetamine sensitization in periadolescent mice compared with adult mice

Novelty seeking as well as amphetamine sensitization were studied in adult (postnatal day "PND" > 60) and periadolescent (PND 3343) mice treated with saline or amphetamine (2 or 10 mg/kg once per day) for 3 days in a familiar environment. After a 48-hr wash-out period, mice were challenged with either saline or amphetamine (2 mg/kg) in the same environment. When given a choice, animals showed a preference for a novel environment, an effect more marked in periadolescents. Acute amphetamine strongly increased novelty seeking in adults, whereas it had an opposite effect in periadolescents. Adult mice in the chronic amphetamine 2 group showed a conditioned preference for the drug-paired compartment, whereas an aversion characterized adult mice in the amphetamine 10 group. Periadolescents in the latter group exhibited a greater sensitization of the locomotor response, but did not show the compulsive licking typical of adults. This appears to be a useful model to study psychobiological risk factors involved in vulnerability to addiction during human adolescence.

Evaluation of unconditioned novelty-seeking and d-amphetamine-conditioned motivation in mice

Following repeated association between psychostimulant drugs and a distinct environment, contextual cues acquire the ability to elicit a conditioned approach response. Further, both rats and mice have a natural drive to seek for the experience of novelty, and a previously unknown environment is able to elicit an unconditioned approach response. Both the experience of novelty and amphetamine (AMPH)-conditioned effects have been associated in rodents with the activation of brain meso-limbic dopaminergic pathways. This study assessed the relative strength of AMPH-conditioned and novelty-induced unconditioned motivations in mice. During the pretreatment period, mice were randomly assigned to three different treatment history groups, and received d-AMPH (0, 2, or 10 mg/kg i.p. once/day) injections for 3 days in the presence of a familiar environment. Following a 48-h washout from the last drug injection, animals were placed in the familiar and pretreatment-paired environment and challenged with either SAL (to evaluate conditioning) or a standard AMPH dose (2 mg/kg, to assess either acute drug effects or carryover influences of each animal's treatment history with the same drug). Following the opening of a partition, animals showed both a clear-cut preference for a novel environment as well as a marked novelty-induced hyperactivity. Interestingly, when mice were tested in a drug-free state in this free-choice paradigm, they expressed neither conditioning to the drug-associated environment nor carry-over effects on the novelty-induced hyperactivity profile. On the other hand, mice injected with AMPH showed a mixed profile, with AMPH 2 treatment history mice showing a conditioned preference for the familiar and drug-paired environment, whereas AMPH 10 animals preferred to spend more time in the novel compartment. Both AMPH doses were associated with an increased locomotion, whereas only the AMPH 10 dose resulted in a stereotyped behavioral syndrome, possibly reminiscent of an aversive "poor welfare" condition. Thus, as a function of the drug dosage, differential positive or negative incentive properties are suggested to be evoked by the AMPH-conditioned environment. In conclusion, a reliable and useful experimental paradigm has been developed to investigate the issue of vulnerability to a variety of habit-forming agents or emotional experiences whose positive reinforcing properties may rely on a common neurobiological mechanism.

A description of the ontogeny of mouse agonistic behavior

The development of agonistic behavior was characterized in outbred Swiss CD-1 male Mus domesticus. At weaning (postnatal day [PND] 21), mice were housed either individually or as male pairs. Social encounters were carried out between dyads of initially unfamiliar same-age and same-housing subjects every 3rd day, from PND 23 to 47. The majority of both offensive and defensive elements had their onset around PND 29. Overall, their expression increased around puberty (i.e., on PND 35), which also represented the peak of an inverted U-shaped profile for the frequency of the "ambivalent" tail rattling behavior. A stability of dominance-submission relationships over development appeared, and early short latencies to display either the first crouched posture (subordinate) or the first attack (dominant) turned out to be possible predictors of adult social status. Ongoing individual housing was associated with a greater expression and an earlier onset of fighting behavior.

Sexual segregation in infant mice: behavioural and neuroendocrine responses to d-amphetamine administration

Individual differences arise from both genetic and epigenetic factors. The aim of this study was to test whether pups raised in distinct socio-sexual conditions would show different behavioural and neuroendocrine responses to d-amphetamine (AMPH) administration upon placement in a novel environment. This issue was addressed by testing infant CD-1 mouse pups of both sexes at three different developmental ages [3, 8, or 18 postnatal (PND) days]. These pups were raised from birth in all-male, all-female, or mixed-sex litters. AMPH effects were assessed as a function of the hypothalamic-pituitary-adrenal (HPA) axis activational state using litters that were either maternally deprived for 24 h (DEP) or normally kept with the dam (NDEP). A concomitant maternal behaviour score carried out on selected postpartum days showed that mothers taking care of all-male litters were more often involved in Active nursing than those rearing the mixed-sex ones, whereas the latter were found more often Laying still out of the nest. Basal and stress-induced corticosterone (CORT) secretion was increased in unisexually reared pups following maternal deprivation, an effect limited to PND 3. In general, neuroendocrine and behavioural responses to AMPH were found to be dissociated and were affected by sexual segregation only in conjunction with maternal deprivation. On PND 3, AMPH injection (1 or 3 mg/kg, i.p.) decreased CORT secretion in deprived unisexually reared subjects without affecting their behaviour. As a whole, behavioural changes due to unisexual rearing were limited to female subjects. On PND 8, unisexually reared females showed, upon maternal deprivation, a generalized shift to the left in the dose-response curve to AMPH for Crossing behaviour, while on PND 18 AMPH-induced stereotypies were considerably reduced in sexually segregated females, especially following maternal deprivation. Thus, maternal deprivation appeared to "sensitize" the monoaminergic system to an AMPH challenge. The individual behavioural and neuroendocrine profiles shown in response to a stressful challenge suggest that changes in social stimulation early during development might produce subtle shifts in the function of selected central monoaminergic systems.

Prior cocaine exposure in different environments affects the behavioral responses of mouse dams

The present study evaluated the behavioral repertoire of lactating CD-1 mouse dams when tested in an environment associated with cocaine in the premating period. Virgin females were randomly assigned for conditioning (8-day long schedule with four cocaine or saline injections administered every other day) to three experimental groups: i) Coc-Test females received one injection of cocaine (5 or 20 mg/kg IP) in the testing chamber and saline 24 h later in the home cage, ii) Coc-Home females received one injection of the same doses of cocaine in the home cage and saline 24 h later in the testing chamber, and iii) Sal-Sal control females received one saline injection in both environments. All females underwent a 25-day long wash out period during which they were mated. Their behavior was subsequently scored in the testing chamber on postpartum day 2 (drug-free state) in the presence of 3 pups from their own litter (single 15-min session). As a whole, Coc-Test dams appeared to be more involved in pup-directed activities such as pup-nosing and nest-building when compared with the Coc-Home group. In addition, non-pup directed behaviors, such as crossing, self-grooming, and rearing were higher in Coc-Test group than in other groups. The opposite was true for stereotyped gnawing activity. A measure of females' body weight gain revealed that Coc-Test 20 group was significantly higher than other groups particularly during the postpartum phase. On postpartum day 6, lactating dams were injected with the low cocaine dose (5mg/kg) and their response to a male intruder was assessed in the testing chamber (single 5-min session). A higher number of Coc-Test 5 dams showed the on nest and the upright offensive postures compared to the corresponding Coc-Home 5 group, whereas a significant higher number of Sal-Sal females showed the on top posture with respect to the Coc-Test 20 group. Oxytocin levels measured after the behavioral test showed a tendency, even if not significant, to be higher in the hypothalamus of Coc-Test dams. As a whole, the present results suggest that the alterations in maternal behavior here observed as well as the trend underlined by oxytocin values, cannot be ascribed to carry-over effects of cocaine administration in the pre-mating phase per se; rather, they seem to represent a conditioned response to the distinct environment previously associated with the drug experience.

On mouse pups and their lactating dams: behavioral consequences of early exposure to oxazepam and interacting factors

Behavioral analysis in animal models appears to be a valuable and sensitive tool for detecting subtle alterations in CNS function, which can be produced by early exposure to small perturbations of sensory experience, hormonal milieu, or exposure to psychotropic agents devoid of major teratogenic potential. Concerning anxiolytics, the more recent work in mice, which is here summarized, was carried out by putting the emphasis on changes in naturally occurring species-typical social responses as a function of early exposure to benzodiazepines. For adult females, on the behavior expressed during the early postpartum period, whereas for infant subjects, on the ontogenetic stage of the establishment of social bonding. Critical issues such as the choice of fostering procedure and rearing conditions are also addressed. Evidences of relationships between changes in social milieu taking place during early rearing-i.e., when dramatic transitions in the neurochemical target system occur- and the adult behavioral response to challenges with BDZ agents are presented. These data strengthen the notion that the modes of reaction of adult animals to the joint influence of physiological and environmental (stimulus) variables are under the influence of events in early ontogenesis. Therefore, a better understanding of the mechanisms-as unveiled by an appropriate use of drug tools-that mediate such a plasticity might have considerable psychobiological and clinical-therapeutical relevance.

Affiliation in periadolescent rats: behavioral and corticosterone response to social reunion with familiar or unfamiliar partners

The aim of this study was to investigate whether the social relationship between periadolescent rats, which is characterized by a high expression of play behavior, resembles attachment or whether it reflects a nonspecific social attraction between peers. Sprague-Dawley rats have been kept in pairs with a conspecific of the same age and sex from the time of weaning (PND 21). On PND 34, the members of each pair were separated for a 24-h period and randomly assigned to one of three groups: a) immediately sacrificed (NT = nontreated); b) reunited for 30 min with their previous cagemate (FAM); or c) with an unfamiliar conspecific of the same age and sex (UNF). During the reunion, the occurrence of both social and nonsocial behaviors was scored. Overall, periadolescent UNF animals spent more time in social investigation than FAM rats, which were conversely more involved in cage-oriented activities. In addition, higher levels of rough-and-tumble play were expressed in encounters between UNF subjects than between FAM rats. Finally, within the female group, UNF rats were more involved in play soliciting and less in digging activity than FAM animals. Blood corticosterone levels, which were measured at the end of the separation period, were higher in females than in males and were significantly lower than following reunion in a novel environment. The degree of familiarity did not affect hormonal changes. Results suggest that periadolescent rats do not form attachment relationships with each other. However, partner familiarity appears to be an important variable affecting social relationships.

Limited changes of mouse maternal care after prenatal oxazepam: dissociation from pup-related stimulus perception

There is evidence that activity at the GABA/benzodiazepine receptor complex in specific brain areas might be enhanced during rodent motherhood. We tested the hypothesis that the manipulation of this neurochemical system by prenatal benzodiazepine exposure affects typical behavioral responses of lactating mouse dams. Outbred CD-1 mouse fetuses were administered either oxazepam (OX, 15 mg/kg) or vehicle twice a day on embryonic days 12-16 and fostered at birth to untreated dams. Female offspring were subsequently mated at the young adult stage. In a first experiment, the behavioural repertoire of the two groups of lactating females was scored (single 10-min session) from postpartum days 3 to 18. When compared with VEH dams, OX females showed a shorter duration of pup-sniffing at 7-10 days and enhanced crouching behaviour when pups had reached the age of 14-18 days. In addition, OX-treated dams used more cotton for nest construction than the controls. The two female groups were differentiated only in the presence of their offspring. In a second experiment aimed at investigating possible OX-induced changes in pup-stimulus perception, the same lactating females were challenged in sequence on postpartum day 8 with three different patterns of pup-related cues consisting of: three 8-day-old live male pups (LP), three same-age dead pups DP, or three dead pups accompanied by pre-recorded ultrasounds (DPU). In the absence of carry-over effects of prenatal dam treatment, LP stimuli elicited a higher frequency of sniffing and digging than the others, whereas the level of licking, and grooming was reduced. In conclusion, the present results indicate that the slight alteration in maternal care resulting from prenatal OX treatment can be dissociated from changes in pup-related stimulus perception.

Cocaine sensitization in periadolescent and adult rats

Periadolescent rats have been reported to be affected differentially by catecholaminergic agents when compared with younger or adult animals. The present study evaluated the behavioral responsivity of periadolescent (34- to 39-day-old) and adult (60- to 70-day-old) Sprague-Dawley rats of both sexes to i.p. cocaine (Coc) administration (0, 10 or 20 mg/kg, once daily for 4 days). All animals received injections of both saline and Coc every day paired with a different context, with one-half of the animals receiving the drug in the home cage (Coc-Home) and the other half in the testing chamber (Coc-Test). Forty-eight hours after the last drug injection, all animals were challenged with 10 mg/kg i.p. of Coc, and their behavior in the test chamber was scored. As expected, acute Coc induced a prominent increase in a number of behaviors, and this response profile was less marked in periadolescent relative to adult animals. In contrast, Coc-Test animals of both ages showed a clear behavioral sensitization relative to the chronic saline group. No evidence of carry-over effects was found in Coc-Home animals. Females were in general more sensitive than males to acute Coc effects. The development of behavioral sensitization to Coc was a function of age-specific alterations in sensitivity to psychostimulants. Periadolescent rats of both sexes showed sensitization to the locomotor activating effects (matrix crossings) of Coc, whereas a consistent sensitization profile for both stereotyped head scanning and focused sniffing activities were found in adults but not in periadolescents. Chronic Coc reduced body weight and food consumption, particularly in adult males, whereas it did not affect periadolescent patterns. No evidence of sensitization to Coc was found in the hormonal parameters considered.

Sibling effects on the behavior of infant mouse litters (Mus domesticus)

We investigated whether the number of same- or other-sex littermates had a graded effect on the occurrence of early solitary play and social play by mouse (Mus domesticus) family units. Sixty litters, reduced at birth to 5 different sex ratios (6 males, 5 males and 1 female, 3 males and 3 females, 1 male and 5 females, and 6 females) were scored in 15-min sessions on postnatal Days 18 and 21. An increasing trend with age was found for run, pounce, popcorn, self-groom, and explore episodes. Type of family unit influenced the occurrence of specific social interactions: One male-5 female and 6 female litters showed more social play than similar male litters. Litters with a balanced sex ratio showed higher exploration than isosexual litters. The results extend previous reports of both social and solitary play in developing laboratory mice and, in contrast with rat data, indicate a marked female primacy in playful social behavior.

Limited effects of ozone exposure during pregnancy on physical and neurobehavioral development of CD-1 mice

Only a few studies have attempted to assess in laboratory rodents the maternal toxicity and behavioral changes in offspring caused by prenatal exposure to ozone (O3). In particular, no data are available concerning the behavioral development of mouse offspring after maternal exposure, despite the fact that increasing use is made of this species in behavioral teratology studies for reasons both of economy and in order to increase the effectiveness of subsequent higher-tier studies (e.g., of treatment-genotype interactions). In the present work, female CD-1 mice were exposed during pregnancy (Days 7-17) to different O3 concentrations (0, 0.4, 0.8, or 1.2 ppm); to avoid confounding by postnatal maternal effects, all litters were assigned shortly after birth to foster dams neither treated nor handled during pregnancy. The dams' food and water intake and body weight gain were depressed in a concentration-dependent fashion. Tolerance to these effects developed during continuing exposure; such tolerance was faster in the case of food than water intake. Several measures of reproductive performance, such as proportion of pregnancies carried to term, litter size, sex ratio, frequency of stillbirth, and neonatal mortality, failed to show differences between control and O3 animals. Postnatal body weight gain was slightly but significantly depressed in the 1.2 ppm offspring. Otherwise, the somatic development of O3 pups was indistinguishable from that of controls, save for a delay in eye opening; this effect, however, failed to show a significant concentration dependence. Negative results were obtained in a wide range of assessments concerning (i) the development of various reflexes and responses ("Fox battery") from birth to Day 18; (ii) ultrasonic emissions on Postnatal Days 3, 7, and 11; and (iii) activity, habituation, response to an unfamiliar object, and hyperactivity produced by a monoaminergic stimulant (d-amphetamine) at 60-61 days. The present data differ from those of a previous study on rats raised by their biological mothers after gestational exposure to O3 (1 and 1.5 ppm), which showed a substantial impairment in somatic and neurobehavioral development (R. Kavlock, E. Meyer, and C. T. Grabowski, 1980, Toxicol. Lett. 5, 3-9). This difference, be it due to species factors, to postnatal maternal effects, or to the time of occurrence of maximal O3 effects (e.g., on food and water intake) after the onset of exposure and before adaptation or tolerance, may provide significant cues for the understanding of O3 effects in pregnant and developing organisms.

Induction of maternal behavior by mouse neonates: influence of dam parity and prenatal oxazepam exposure

The aim of the present report was to investigate the influence of pup stimulus properties and female parity on mouse maternal behavior. Outbred CD-1 mouse pups, prenatally exposed to either the vehicle (VEH) or oxazepam (OX, 15 mg/kg twice/day on pregnancy days 12-16) and fostered to untreated dams at birth, were offered as a stimulus on postnatal days 4, 6, and 8 to four groups of females that differed in maternal experience: virgin, experienced virgin females, primiparae, and biparae. Maternal behavior was observed during a 15-min session each day. Virgin females were less involved in crouching behavior than primiparae. Pups, age, and prenatal oxazepam showed interactive effects on maternal care, particularly by increasing licking and nest-building activities and decreasing still-out behavior. Moreover, dams receiving younger pups showed high levels of both locomotor activity and rearing. The present findings point to the need for a better understanding of mother-pup interactions in studies aimed at characterizing drug and toxicant effects on both animal and human development.

d-amphetamine conditioned place preference in developing mice: relations with changes in activity and stereotypies

Conditioned place preference (CPP) with both visual and tactile cues, hyperactivity, and stereotypies produced by d-amphetamine (1-10 mg/kg ip, single dose) were studied in CD-1 mice at 2, 3, and 4 weeks from birth. CPP was shown from the youngest age onward in female mice and from 3 weeks in male mice. Hyperactivity was much more pronounced in postweanlings (3 and 4 weeks) than in preweanlings. Stereotypies (at 3.3 and 10 mg/kg) occurred from the youngest age and tended to peak at 3 weeks. Stereotypies may indicate a sickness experience or "poor welfare" (G.J. Mason, 1991; A. Wall, R.E. Hinson, E. Schmidt, C. Johnston, & A. Streather, 1990) due to an aversive component of amphetamine's action. Therefore, the delayed development of fully fledged amphetamine CPP, relative to cocaine CPP (G. Laviola, G. Dell'Omo, E. Alleva, & G. Bignami, 1992), may be due to an age-dependent diminution of the positive hedonic value of the former drug by negative effects that are minimal or absent in the case of the latter drug.

Behavioral and hormonal responses to stress in the newborn mouse: effects of maternal deprivation and chlordiazepoxide

These studies investigated behavioral and hormonal responses to stress in developing mice. Experiment 1 examined the effects of 24-hr maternal deprivation on corticosterone (CORT) secretion and ultrasonic vocalization (UVZ) rate in 4-, 8-, and 12-day-old mice. At these ages, exposure to a novel environment resulted in minimal changes in CORT secretion. Maternal deprivation increased pups' CORT secretion in an age-dependent fashion but did not affect their UVZ rate. The aim of experiment 2 was to test the effects of chlordiazepoxide (CDP), an anxiolytic compound, on CORT secretion and UVZ in both normally reared and in maternally deprived 8-day-old mice. CDP administration elevated CORT secretion in a dose-dependent fashion, producing larger CORT increases in deprived (DEP) animals. CDP affected UVZ only in nondeprived (NDEP) animals: UVZ rate was decreased by high CDP doses. Overall, these findings demonstrate that the infant mouse shows a period of stress hyporesponsiveness similar to the rat and that maternal presence contributes to inhibit adrenocortical activity. CDP administration, but not novelty exposure, increased CORT secretion in 8-day-old normally reared mice suggesting that during the stress hyporesponsive period, the HPA axis is capable of responding only to specific stimuli. Changes in HPA axis activity and UVZ rate resulting from maternal deprivation and/or CDP challenge do not seem to be directly related.

d-amphetamine conditioned place preference in developing mice: relations with changes in activity and stereotypies

Conditioned place preference (CPP) with both visual and tactile cues, hyperactivity, and stereotypies produced by d-amphetamine (1-10 mg/kg ip, single dose) were studied in CD-1 mice at 2, 3, and 4 weeks from birth. CPP was shown from the youngest age onward in female mice and from 3 weeks in male mice. Hyperactivity was much more pronounced in postweanlings (3 and 4 weeks) than in preweanlings. Stereotypies (at 3.3 and 10 mg/kg) occurred from the youngest age and tended to peak at 3 weeks. Stereotypies may indicate a sickness experience or "poor welfare" (G.J. Mason, 1991; A. Wall, R.E. Hinson, E. Schmidt, C. Johnston, & A. Streather, 1990) due to an aversive component of amphetamine's action. Therefore, the delayed development of fully fledged amphetamine CPP, relative to cocaine CPP (G. Laviola, G. Dell'Omo, E. Alleva, & G. Bignami, 1992), may be due to an age-dependent diminution of the positive hedonic value of the former drug by negative effects that are minimal or absent in the case of the latter drug.

Prenatal cocaine potentiates the effects of morphine in adult mice

Prenatal cocaine exposure has been reported to result in abnormal neurobehavioral development, both in animals and humans. In this study, outbred CD-1 mice were exposed in utero to cocaine hydrochloride administered daily as i.p. injections to dams from day 10 of gestation to day 16, at the dose 0, 5 or 50 mg/kg. Cocaine did not alter duration of pregnancy while it decreased the difference in maternal body weight from days 10 to 16 in the dams receiving the higher dose of cocaine. The body weight of the offspring from birth to 15 days of age and the physical maturation were not affected by prenatal cocaine exposure. The development of the response to strong tactile stimulation was either slightly delayed in the 5 mg/kg group or markedly accelerated in the 50 mg/kg group. At adulthood, animals were assessed for behavioral responses to a novel environment, for response to painful stimulation (hot-plate test set at 55 +/- 1 degree C), and for the effects of a single morphine injection (30 mg/kg, i.p.). Data showed that in the absence of prenatal cocaine exposure effects, morphine increased the time spent in inactivity, while it decreased rearing, grooming and bar-holding behaviors. In the case of sniffing, morphine increased this behavior, except in the 5 mg/kg cocaine group. Moreover, morphine administration induced the expected increase of locomotion, irrespective of prenatal condition. With respect to pain reactivity, prenatal cocaine exposure resulted in an increase of licking latency in the 5 mg/kg group.(ABSTRACT TRUNCATED AT 250 WORDS)

Affiliation and neophobia in developing mice prenatally exposed to oxazepam

In the framework of the well-known regulatory role of the GABA-benzodiazepine (BZ) mechanisms of the brain in response to social and environmental challenges, we tested the hypothesis that developmental manipulation of this neurochemical system by prenatal BZ exposure can affect the social repertoire and neophobia level in infant mice. Outbred CD-1 mice were treated with either vehicle or oxazepam (15mg/kg p.o. to the dam, twice a day on Days 12-16 of foetal life), and fostered at birth to untreated dams. Two males plus two females were observed in the home cage, in the presence of a novel object, on Postnatal Days 16, 20 and 24. The expression of social and non-social behaviours increased with age. Prenatal oxazepam exposure reduced sex differences by having a more pronounced effect on males than on females; it also produced fairly specific, though subtle, behavioural changes. Oxazepam exposed mice appeared more involved than prenatal controls in behaviours related to the achievement and maintenance of a "passive" proximity with littermates. By contrast, they performed less active investigation and solicitation of littermates, and non-social behaviours such as locomotor-rotational play, cage exploration, and maintenance activities; they also showed a reduced frequency of approaching and making contact with the novel object. Analysis of social interactions of infant mice seems to represent a sensitive tool for early detection of subtle developmental changes in the CNS.

Early exposure to aluminium affects eight-arm maze performance and hippocampal nerve growth factor levels in adult mice

Aluminium has been implicated in the etiology of several human pathologies but, to date, little attention has been given to the potential damage caused by gestational exposure to aluminium or to any long-term effect of early exposure. Aluminium sulphate (200 mg/kg) was administered intraperitoneally to pregnant female mice (days 10 to 13 of gestation), and male progeny was assessed in a radial eight-arm maze task at 70 days (20 daily sessions, 50 microliters water as reward). Overall, aluminium-exposed mice performed less efficiently in the radial maze than the vehicle-exposed animals. Neurobiological data showed that aluminium mice also had a higher level of nerve growth factor in the hippocampal formation when compared to controls.