Maternal separation alters maternal care, but has minor effects on behavior and brain opioid peptides in adult offspring

Sex differences in hippocampal structural plasticity and glycosaminoglycan disaccharide levels after neonatal handling

In this study we investigated the effects of a neonatal handling protocol that mimics the handling of sham control pups in protocols of neonatal exposure to brain insults on dendritic arborization and glycosaminoglycan (GAG) levels in the developing brain. GAGs are long, unbranched polysaccharides, consisting of repeating disaccharide units that can be modified by sulfation at specific sites and are involved in modulating neuronal plasticity during brain development. In this study, male and female Sprague-Dawley rats underwent neonatal handling daily between post-natal day (PD)4 and PD9, with brains analyzed on PD9. Neuronal morphology and morphometric analysis of the apical and basal dendritic trees of CA1 hippocampal pyramidal neurons were carried out by Golgi-Cox staining followed by neuron tracing and analysis with the software Neurolucida. Chondroitin sulfate (CS)-, Hyaluronic Acid (HA)-, and Heparan Sulfate (HS)-GAG disaccharide levels were quantified in the hippocampus by Liquid Chromatography/Mass Spectrometry analyses. We found sex by neonatal handling interactions on several parameters of CA1 pyramidal neuron morphology and in the levels of HS-GAGs, with females, but not males, showing an increase in both dendritic arborization and HS-GAG levels. We also observed increased expression of glucocorticoid receptor gene Nr3c1 in the hippocampus of both males and females following neonatal handling suggesting that both sexes experienced a similar stress during the handling procedure. This is the first study to show sex differences in two parameters of brain plasticity, CA1 neuron morphology and HS-GAG levels, following handling stress in neonatal rats.

Sex-dependence and comorbidities of the early-life adversity induced mental and metabolic disease risks: Where are we at?

Early-life adversity (ELA) is a major risk factor for developing later-life mental and metabolic disorders. However, if and to what extent ELA contributes to the comorbidity and sex-dependent prevalence/presentation of these disorders remains unclear. We here comprehensively review and integrate human and rodent ELA (pre- and postnatal) studies examining mental or metabolic health in both sexes and discuss the role of the placenta and maternal milk, key in transferring maternal effects to the offspring. We conclude that ELA impacts mental and metabolic health with sex-specific presentations that depend on timing of exposure, and that human and rodent studies largely converge in their findings. ELA is more often reported to impact cognitive and externalizing domains in males, internalizing behaviors in both sexes and concerning the metabolic dimension, adiposity in females and insulin sensitivity in males. Thus, ELA seems to be involved in the origin of the comorbidity and sex-specific prevalence/presentation of some of the most common disorders in our society. Therefore, ELA-induced disease states deserve specific preventive and intervention strategies.

Maternal separation: Does it hold the potential to model consequences of postpartum depression?

The postpartum period is a sensitive time where women are especially vulnerable to develop postpartum depression (PPD), with 10%-15% of women affected. This review investigates whether the maternal separation (MS) paradigm in rodents holds the potential to help to understand mothers suffering from PPD. MS is a well-established stress model to investigate effects on infants, whereas effects on the dam are often overlooked. The database PubMed was searched for studies investigating effects of daily MS within the first weeks after parturition on dams in rats and mice and compared to findings in PPD mothers. MS was categorized as brief MS (5-45 min) with or without handling of pups and long MS (3-4 h and longer). MS alters maternal care, depressive-like behavior, anxiety, and aggression; leads to alterations in neuronal gene expression; and affects hormone and neurotransmitter levels similar to observations in PPD patients. Even though there are disparities between human and rodent mothers, with some results differing in directionality, as well as the reason for separation (self-induced in PPD, externally induced in MS), the overall effects found on neurobiological, hormonal, and behavioral levels mostly coincide. Thus, the MS paradigm can add relevant knowledge to existing PPD animal models, further advancing the study of PPD.

Predictable maternal separation confers adult stress resilience via the medial prefrontal cortex oxytocin signaling pathway in rats

Early-life stress is normally thought of as a major risk for psychiatric disorders, but many researchers have revealed that adversity early in life may enhance stress resilience later in life. Few studies have been performed in rodents to address the possibility that exposure to early-life stress may enhance stress resilience, and the underlying neural mechanisms are far from being understood. Here, we established a "two-hit" stress model in rats by applying two different early-life stress paradigms: predictable and unpredictable maternal separation (MS). Predictable MS during the postnatal period promotes resilience to adult restraint stress, while unpredictable MS increases stress susceptibility. We demonstrate that structural and functional impairments occur in glutamatergic synapses in pyramidal neurons of the medial prefrontal cortex (mPFC) in rats with unpredictable MS but not in rats with predictable MS. Then, we used differentially expressed gene (DEG) analysis of RNA sequencing data from the adult male PFC to identify a hub gene that is responsible for stress resilience. Oxytocin, a peptide hormone, was the highest ranked differentially expressed gene of these altered genes. Predictable MS increases the expression of oxytocin in the mPFC compared to normal raised and unpredictable MS rats. Conditional knockout of the oxytocin receptor in the mPFC was sufficient to generate excitatory synaptic dysfunction and anxiety behavior in rats with predictable MS, whereas restoration of oxytocin receptor expression in the mPFC modified excitatory synaptic function and anxiety behavior in rats subjected to unpredictable MS. These findings were further supported by the demonstration that blocking oxytocinergic projections from the paraventricular nucleus of the hypothalamus (PVN) to the mPFC was sufficient to exacerbate anxiety behavior in rats exposed to predictable MS. Our findings provide direct evidence for the notion that predictable MS promotes stress resilience, while unpredictable MS increases stress susceptibility via mPFC oxytocin signaling in rats.

Effect of early life social adversity on drug abuse vulnerability: Focus on corticotropin-releasing factor and oxytocin

Early life adversity can set the trajectory for later psychiatric disorders, including substance use disorders. There are a host of neurobiological factors that may play a role in the negative trajectory. The current review examines preclinical evidence suggesting that early life adversity specifically involving social factors (maternal separation, adolescent social isolation and adolescent social defeat) may influence drug abuse vulnerability by strengthening corticotropin-releasing factor (CRF) systems and weakening oxytocin (OT) systems. In adulthood, pharmacological and genetic evidence indicates that both CRF and OT systems are directly involved in drug reward processes. With early life adversity, numerous studies show an increase in drug abuse vulnerability measured in adulthood, along a concomitant strengthening of CRF systems and a weakening of OT systems. Mechanistic studies, while relatively few in number, are generally consistent with the theme that strengthened CRF systems and weakened OT systems mediate, at least in part, the link between early life adversity and drug abuse vulnerability. Establishing a direct role of CRF and OT in mediating the relation between early life social stressors and drug abuse vulnerability will inform clinical researchers and practitioners toward the development of intervention strategies to reduce risk among those suffering from early life adversities. This article is part of the special issue on 'Vulnerabilities to Substance Abuse'.

Impact of preweaning stress on long-term neurobehavioral outcomes in Sprague-Dawley rats: Differential effects of barren cage rearing, pup isolation, and the combination

Two developmental stressors were compared in preweaning rats exposed to either one stressor or both. Stressors were barren cage rearing or maternal separation (pup isolation). 40 gravid Sprague-Dawley CD/IGS rats were randomly assigned to two cage conditions: standard (Std) cage or barren cage (Bar), 20 litters/condition throughout gestation and lactation. After delivery, litters were randomly culled to 4 males and 4 females. The second stressor was maternal separation: Two male/female pairs per litter were isolated from their dam 4 h/day (Iso) and two pairs were not (Norm). Hence, there were 4 conditions: Std-Norm, Std-Iso, Bar-Norm, and Bar-Iso. One pair/litter/stress condition received the following: elevated zero-maze (EZM), open-field, swim channel, Cincinnati water maze, conditioned fear, and open-field with methamphetamine challenge. The second pair/litter/condition received the light-dark test, swim channel, Morris water maze, forced swim, and EZM with diazepam challenge. Barren rearing reduced EZM time-in-open, whereas isolation rearing reduced open-field activity in males and increased it in females. Effects on straight channel swimming were minor. In the Cincinnati water maze test of egocentric learning, isolation rearing increased errors whereas barren cage housing reduced errors in combination with normal rearing. Barren cage with maternal separation (pup isolation) increased Cincinnati water maze escape latency but not errors. Barren cage housing reduced hyperactivity in response to methamphetamine. Isolation rearing increased time in open in the EZM after diazepam challenge. Trends were seen in the Morris water maze. These suggested that barren cage and isolation rearing in combination reduced latency on acquisition on days 1 and 2 in males, whereas females had increased latency on days 2 and 3. Combined exposure to two developmental stressors did not induce additive or synergistic effects, however the data show that these stressors had long-term effects with some evidence that the combination of both caused effects when either stressor alone did not, but synergism was not observed.

Behavioral Profiling in Early Adolescence and Early Adulthood of Male Wistar Rats After Short and Prolonged Maternal Separation

Early-life stress and its possible correlations to genes, environment, and later health outcomes can only be studied retrospectively in humans. Animal models enable the exploration of such connections with prospective, well-controlled study designs. However, with the recent awareness of replicability issues in preclinical research, the reproducibility of results from animal models has been highlighted. The present study aims to reproduce the behavioral effects of maternal separation (MS) previously observed in the multivariate concentric square field^TM (MCSF) test. A second objective was to replicate the adolescent behavioral profiles previously described in the MCSF test. Male rats, subjected to short or prolonged MS or standard rearing, were subjected to behavioral testing in early adolescence and early adulthood. As seen in previous studies, the behavioral effects of MS in the MCSF were small at both tested time points. When tested in early adolescence, the animals exhibited a similar behavioral profile as previously seen, and the finding of adolescent behavioral types was also reproduced. The distribution of animals into the behavioral types was different than in the initial study, but in a manner consistent with developmental theories, as the current cohort was younger than the previous. Notably, the Shelter seeker behavioral type persisted through development, while the Explorer type did not. The lack of basal behavioral effect after MS is in line with the literature on this MS paradigm; the working hypothesis is that the prolonged MS gives rise to a phenotype predisposed to negative health outcomes but which is not apparent without additional provocation.

Maternal separation impairs long term-potentiation in CA3-CA1 synapses in adolescent female rats

Mother-infant interactions influence the development of physiology and behavior during the first weeks after birth. As an adverse early life experience, maternal separation (MS) produces behavioral and neuroendocrine functions disorders associated with the hippocampus. Considering the critical role of long-term potentiation (LTP) in learning and memory, we investigated whether MS affects LTP in adolescent female rats. In this study, female rat pups were exposed to daily 3-h (MS180) or 15-min (MS15) periods of maternal separation on postnatal days (PND) 1-14 and control offspring remained with the dams all the time before weaning. Extracellular evoked field excitatory postsynaptic potentials (fEPSPs) were recorded in the stratum radiatum of the CA1 area of the slice at 28-35 days of age. Our results indicate that a significant difference existed in the magnitude of LTP between the control group and MS180 group, but the MS15 group was not different from control. In conclusion, these findings suggest that MS may impair LTP induction in the CA1 area of the hippocampus in adolescent female rats.

Glyphosate and glyphosate-based herbicide exposure during the peripartum period affects maternal brain plasticity, maternal behaviour and microbiome

Glyphosate is found in a large array of non-selective herbicides such as Roundup® (Monsanto, Creve Coeur, MO, USA) and is by far the most widely used herbicide. Recent work in rodent models suggests that glyphosate-based herbicides during development can affect neuronal communication and result in altered behaviours, albeit through undefined mechanisms of action. To our knowledge, no study has investigated the effects glyphosate or its formulation in herbicide on maternal behaviour and physiology. In the present study, relatively low doses of glyphosate (5 mg kg-1  d-1 ), Roundup® (5 mg kg-1  d-1 glyphosate equivalent), or vehicle were administered by ingestion to Sprague-Dawley rats from gestational day (GD) 10 to postpartum day (PD)22. The treatments significantly altered licking behaviour toward pups between PD2 and PD6. We also show in the dams at PD22 that Roundup exposure affected the maturation of doublecortin-immunoreactive new neurones in the dorsal dentate gyrus of the hippocampus of the mother. In addition, the expression of synaptophysin was up-regulated by glyphosate in the dorsal and ventral dentate gyrus and CA3 regions of the hippocampus, and down-regulated in the cingulate gyrus. Although a direct effect of glyphosate alone or its formulation on the central nervous system is currently not clear, we show that gut microbiota is significantly altered by the exposure to the pesticides, with significant alteration of the phyla Bacteroidetes and Firmicutes. This is the first study to provide evidence that glyphosate alone or in formulation (Roundup) differentially affects maternal behaviour and modulates neuroplasticity and gut microbiota in the mother.

Early life stress and the propensity to develop addictive behaviors

There is a vast literature on effects of early life manipulations in rodents much of which is aimed at investigating the long-term consequences related to emotion and cognition in adulthood. Less is known about how these manipulations affect responses reflective of alcohol (AUD) and substance (SUD) use disorders. The purpose of this paper is to review the literature of studies that employed early life manipulations and assessed behavioral responses to psychoactive substances, specifically alcohol, opiates, and stimulants, in rodents. While the findings with alcohol are more limited and mixed, studies with opiates and stimulants show strong support for the ability of these manipulations to enhance behavioral responsivity to these substances in line with epidemiological data. Some outcomes show sex differences. The mechanisms that influence these enduring changes may reflect epigenetic alterations. Several studies support a role for altered DNA methylation (and other epigenetic mechanisms) as biological responses to early environmental insults. The chemical changes induced by DNA methylation affect transcriptional activity of DNA and thus can have a long-term impact on the individual's phenotype. Such effects are particularly robust when they occur during sensitive periods of brain development (e.g., first postnatal weeks in rodents). We review this emerging literature as it relates to the known neurobiology of AUDs and SUDs and suggest new avenues of research. Such findings will have implications for the treatment and prevention of AUDs and SUDs and could provide insight into factors that support resiliency.

Challenges to the parental brain: Neuroethological and translational considerations

Extending from research documenting adaptive parental responses in nonthreatening contexts, the influences of various neuroethological and physiological challenges on effective parenting responses are considered in the current review. In natural habitats, rodent family units are exposed to predators, compromised resources, and other environmental stressors that disrupt HPA axis functions. With the additional physiological demands associated with caring for offspring, alterations in stress-related neuroendocrine responsiveness contribute to adaptive responses in many challenging contexts. Some environmental contexts, however, such as restricted nesting resources, result in disrupted maternal responses that have a negative impact on offspring wellbeing. Additionally, parental dysregulation associated with exposure to environmental chemicals or pharmacological substances, also compromise maternal responses with effects that often extend to future generations. Continued preclinical and clinical research elucidating parental responses to various stressors and physiological disruptors is necessary to provide valuable translational information identifying threats to effective parenting outcomes.

Effects of neonatal inhalation exposure to ultrafine carbon particles on pathology and behavioral outcomes in C57BL/6J mice

Background

Recent epidemiological studies indicate early-life exposure to air pollution is associated with adverse neurodevelopmental outcomes. Previous studies investigating neonatal exposure to ambient fine and ultrafine particles have shown sex specific inflammation-linked pathological changes and protracted learning deficits. A potential contributor to the adverse phenotypes from developmental exposure to particulate matter observed in previous studies may be elemental carbon, a well-known contributor to pollution particulate. The present study is an evaluation of pathological and protracted behavioral alterations in adulthood following subacute neonatal exposure to ultrafine elemental carbon. C57BL/6J mice were exposed to ultrafine elemental carbon at 50 μg/m³ from postnatal days 4–7 and 10–13 for 4 h/day. Behavioral outcomes measured were locomotor activity, novel object recognition (short-term memory), elevated plus maze (anxiety-like behavior), fixed interval (FI) schedule of food reward (learning, timing) and differential reinforcement of low rate (DRL) schedule of food reward (impulsivity, inability to inhibit responding). Neuropathology was assessed by measures of inflammation (glial fibrillary-acidic protein), myelin basic protein expression in the corpus callosum, and lateral ventricle area.

Results

Twenty-four hours following the final exposure day, no significant differences in anogenital distance, body weight or central nervous system pathological markers were observed in offspring of either sex. Nor were significant changes observed in novel object recognition, elevated plus maze performance, FI, or DRL schedule-controlled behavior in either females or males.

Conclusion

The limited effect of neonatal exposure to ultrafine elemental carbon suggests this component of air pollution is not a substantial contributor to the behavioral alterations and neuropathology previously observed in response to ambient pollution particulate exposures. Rather, other more reactive constituent species, organic and/or inorganic, gas-phase components, or combinations of constituents may be involved. Defining these neurotoxic components is critical to the formulation of better animal models, more focused mechanistic assessments, and potential regulatory policies for air pollution.

Electronic supplementary material

The online version of this article (10.1186/s12989-019-0293-5) contains supplementary material, which is available to authorized users.

Limited developmental neurotoxicity from neonatal inhalation exposure to diesel exhaust particles in C57BL/6 mice

Background

Recent epidemiological studies indicate early-life exposure to pollution particulate is associated with adverse neurodevelopmental outcomes. The need is arising to evaluate the risks conferred by individual components and sources of air pollution to provide a framework for the regulation of the most relevant components for public health protection. Previous studies in rodent models have shown diesel particulate matter has neurotoxic potential and could be a health concern for neurodevelopment. The present study shows an evaluation of pathological and protracted behavioral alterations following neonatal exposure to aerosolized diesel exhaust particles (NIST SRM 1650b). The particular behavioral focus was on temporal control learning, a broad and fundamental cognitive domain in which reward delivery is contingent on a fixed interval schedule. For this purpose, C57BL/6 J mice were exposed to aerosolized NIST SRM 1650b, a well-characterized diesel particulate material, from postnatal days 4–7 and 10–13, for four hours per day. Pathological features, including glial fibrillary-acidic protein, myelin basic protein expression in the corpus callosum, and ventriculomegaly, as well as learning alterations were measured to determine the extent to which NIST SRM 1650b would induce developmental neurotoxicity.

Results

Twenty-four hours following exposure significant increases in glial-fibrillary acidic protein (GFAP) in the corpus callosum and cortex of exposed male mice were present. Additionally, the body weights of juvenile and early adult diesel particle exposed males were lower than controls, although the difference was not statistically significant. No treatment-related differences in males or females on overall locomotor activity or temporal learning during adulthood were observed in response to diesel particulate exposure.

Conclusion

While some sex and regional-specific pathological alterations in GFAP immunoreactivity suggestive of an inflammatory reaction to SRM 1650b were observed, the lack of protracted behavioral and pathological deficits suggests further clarity is needed on the developmental effects of diesel emissions prior to enacting regulatory guidelines.

Electronic supplementary material

The online version of this article (10.1186/s12989-018-0287-8) contains supplementary material, which is available to authorized users.

Sex-dependent impact of early-life stress and adult immobilization in the attribution of incentive salience in rats

Early life stress (ELS) induces long-term effects in later functioning and interacts with further exposure to other stressors in adulthood to shape our responsiveness to reward-related cues. The attribution of incentive salience to food-related cues may be modulated by previous and current exposures to stressors in a sex-dependent manner. We hypothesized from human data that exposure to a traumatic (severe) adult stressor will decrease the attribution of incentive salience to reward-associated cues, especially in females, because these effects are modulated by previous ELS. To study these factors in Long-Evans rats, we used as an ELS model of restriction of nesting material and concurrently evaluated maternal care. In adulthood, the offspring of both sexes were exposed to acute immobilization (IMO), and several days after, a Pavlovian conditioning procedure was used to assess the incentive salience of food-related cues. Some rats developed more attraction to the cue predictive of reward (sign-tracking) and others were attracted to the location of the reward itself, the food-magazine (goal-tracking). Several dopaminergic markers were evaluated by in situ hybridization. The results showed that ELS increased maternal care and decreased body weight gain (only in females). Regarding incentive salience, in absolute control animals, females presented slightly greater sign-tracking behavior than males. Non-ELS male rats exposed to IMO showed a bias towards goal-tracking, whereas in females, IMO produced a bias towards sign-tracking. Animals of both sexes not exposed to IMO displayed an intermediate phenotype. ELS in IMO-treated females was able to reduce sign-tracking and decrease tyrosine hydroxylase expression in the ventral tegmental area and dopamine D1 receptor expression in the accumbens shell. Although the predicted greater decrease in females in sign-tracking after IMO exposure was not corroborated by the data, the results highlight the idea that sex is an important factor in the study of the long-term impact of early and adult stressors.

Few long-term consequences after prolonged maternal separation in female Wistar rats

Environmental factors during the early-life period are known to have long-term consequences for the adult phenotype. An intimate interplay between genes and environment shape the individual and may affect vulnerability for psychopathology in a sex-dependent manner. A rodent maternal separation model was here used to study the long-term effects of different early-life rearing conditions on adult behavior, HPA axis activity and long-term voluntary alcohol intake in female rats. Litters were subjected to 15 min (MS15) or 360 min (MS360) of daily maternal separation during postnatal day 1–21. In adulthood, the behavioral profiles were investigated using the multivariate concentric square field™ (MCSF) test or examined for HPA axis reactivity by cat-odor exposure with subsequent characterization of voluntary alcohol intake and associated changes in HPA axis activity. Adult female MS360 offspring showed mostly no, or only minor, effects on behavior, HPA axis reactivity and long-term alcohol intake relative to MS15. Instead, more pronounced effects were found dependent on changes in the natural hormonal cycle or by the choice of animal supplier. However, changes were revealed in corticosterone load after long-term alcohol access, as females subjected to MS360 had higher concentrations of fecal corticosterone. The present findings are in line with and expand on previous studies on the long-term effects of maternal separation in female rats with regard to behavior, HPA axis activity and voluntary alcohol intake. It can also be a window into further studies detailing how early-life experiences interact with other risk and protective factors to impact the adult phenotype and how possible sex differences play a role.

An overview of maternal separation effects on behavioural outcomes in mice: Evidence from a four-stage methodological systematic review

Early life stress (ELS) developmental effects have been widely studied by preclinical researchers. Despite the growing body of evidence from ELS models, such as the maternal separation paradigm, the reported results have marked inconsistencies. The maternal separation model has several methodological pitfalls that could influence the reliability of its results. Here, we critically review 94 mice studies that addressed the effects of maternal separation on behavioural outcomes. We also discuss methodological issues related to the heterogeneity of separation protocols and the quality of reporting methods. Our findings indicate a lack of consistency in maternal separation effects: major studies of behavioural and biological phenotypes failed to find significant deleterious effects. Furthermore, we identified several specific variations in separation methodological procedures. These methodological variations could contribute to the inconsistency of maternal separation effects by producing different degrees of stress exposure in maternal separation-reared pups. These methodological problems, together with insufficient reporting, might lead to inaccurate and unreliable effect estimates in maternal separation studies.

Naltrexone Prevents in Males and Attenuates in Females the Expression of Behavioral Sensitization to Ethanol Regardless of Maternal Separation

Maternal separation alters the activity of the opioid system, which modulates ethanol-induced stimulation and behavioral sensitization. This study examined the effects of an opioid antagonist, naltrexone (NTX), on the expression of behavioral sensitization to ethanol in adult male and female mice submitted to maternal separation from postnatal days (PNDs) 2 to 14. Whole litters of Swiss mice were either not separated [animal facility rearing (AFR)] or separated from their mothers for 3 h [long maternal separation (LMS)]. Starting on PND 90, male and female AFR and LMS mice received daily i.p. injections of saline (SAL) or ethanol (EtOH, 2.2 g/kg) for 21 days. Locomotor activity was assessed in cages containing photoelectric beams, once a week, to examine the development of behavioral sensitization. Five days after the end of the chronic treatment, animals were submitted to four locomotor activity tests spaced by 48 h, to assess the expression of behavioral sensitization. In all tests, animals received two i.p. injections with a 30-min interval and were then assessed for locomotor response to different treatment challenges, which were: SAL/SAL, SAL/EtOH (2.2 g/kg), NTX 2.0 mg/kg (NTX2)/EtOH, and NTX 4.0 mg/kg (NTX4)/EtOH. Regardless of maternal separation, EtOH-treated male and female mice displayed increased locomotor responses to EtOH during the 21-day treatment, indicating the development of behavioral sensitization. In the SAL/EtOH challenge, EtOH-treated LMS and AFR male and female mice exhibited higher locomotor activity than their SAL-treated counterparts, indicating the expression of sensitization. The coadministration of either dose of NTX blocked the expression of locomotor sensitization in both AFR and LMS male mice with a history of EtOH sensitization. In females, a significant attenuation of EtOH sensitization was promoted by both NTX doses, while still maintaining an augmented stimulant response to EtOH. Importantly, maternal separation did not interfere in this phenomenon. These results indicate that expression of behavioral sensitization was importantly modulated by opioidergic mechanisms both in male and female mice and that maternal separation did not play a major role in either development or expression of this EtOH sensitization.

Early maternal deprivation enhances voluntary alcohol intake induced by exposure to stressful events later in life

In the present study, we aimed to assess the impact of early life stress, in the form of early maternal deprivation (MD, 24 h on postnatal day, pnd, 9), on voluntary alcohol intake in adolescent male and female Wistar rats. During adolescence, from pnd 28 to pnd 50, voluntary ethanol intake (20%, v/v) was investigated using the two-bottle free choice paradigm. To better understand the relationship between stress and alcohol consumption, voluntary alcohol intake was also evaluated following additional stressful events later in life, that is, a week of alcohol cessation and a week of alcohol cessation combined with exposure to restraint stress. Female animals consumed more alcohol than males only after a second episode of alcohol cessation combined with restraint stress. MD did not affect baseline voluntary alcohol intake but increased voluntary alcohol intake after stress exposure, indicating that MD may render animals more vulnerable to the effects of stress on alcohol intake. During adolescence, when animals had free access to alcohol, MD animals showed lower body weight gain but a higher growth rate than control animals. Moreover, the higher growth rate was accompanied by a decrease in food intake, suggesting an altered metabolic regulation in MD animals that may interact with alcohol intake.

Early maternal separation: a rodent model of depression and a prevailing human condition

The early life of most mammals is spent in close contact with the mother, and for the neonate, early maternal separation is a traumatic event that, depending on various conditions, may shape its behavioral and neurochemical phenotype in adulthood. Studies on rodents demonstrated that a very brief separation followed by increased maternal care may positively affect the development of the offspring but that prolonged separation causes significant amounts of stress. The consequences of this stress (particularly the hyperreactivity of the HPA (hypothalamic-pituitary-adrenal) axis are expressed in adulthood and persist for life. Maternal separation in rodents, particularly rats, was used as a model for various psychotic conditions, especially depression. The most popular separation procedure of a 3-h daily separation from the second to the 12th postpartum day yields a depression model of high construct and predictive validity. The results of studies on maternal separation in rats and monkeys prompt a discussion of the consequences of traditional procedures in the maternity wards of developed countries where attention is focused on the hygiene of the neonates and not on their psychological needs. This alternate focus results in a drastic limitation of mother-infant contact and prolonged periods of separation. It is tempting to speculate that differences in the course and severity of various mental disorders, which are usually less prevalent in underdeveloped countries than in developed countries (as noted by Kraepelin), may be related to different modes of infant care. Only recently has so-called kangaroo mother care (establishing mother-infant skin-to-skin contact immediately after birth) become popular in developed countries. In addition to its instant benefits for the neonates, this procedure may also be beneficial for the mental health of the offspring in adulthood.

Litter and sex effects on maternal behavior and DNA methylation of the Nr3c1 exon 17 promoter gene in hippocampus and cerebellum

Early life events can alter gene expression through DNA methylation. The methylation status of the exon 17 promoter of the glucocorticoid receptor (Nr3c1 gene) in hippocampus associates with frequency of pup licking. Much of this work was conducted with male rats. Because dams more frequently lick male pups, this may contribute to sex differences in phenotypes through DNA methylation. Modifying litter gender composition (LGC), in which offspring of single-sex litters are compared to mixed-sex litters, alters maternal behavior. Previously, we demonstrated that LGC and sex affected pup licking times as well as anxiety and hippocampal DNA methylation of the Nr3c1 exon 17 promoter gene in adolescence. Now, we expand upon this work by examining effects in cerebellum and measuring mRNA levels. We also re-assessed DNA methylation in hippocampus using pyrosequencing and re-analyzed pup licking with the more commonly used frequency measure. Litters, culled to 8 pups on postnatal day 1 (PN1), were assigned to one of three conditions: all male (n = 10), all female (n = 12), or half of each sex (n = 20). Licking was rated on PN4, 7, and 10. On PN35, hippocampal and cerebellar samples were obtained. Single-sex males were licked the least and mixed-sex males, the most. Hippocampal Nr3c1 mRNA levels were lowest in mixed females with no LGC or Sex effects in DNA methylation. Cerebellar DNA methylation levels were lowest in mixed males with no effect on mRNA levels. Maternal pup licking associated with DNA methylation of the Nr3c1 exon 17 promoter gene in cerebellum and with hippocampal mRNA.

Sex-dependent effects of an early life treatment in rats that increases maternal care: vulnerability or resilience?

Early life stress (ELS) in rodents has profound long-term effects that are partially mediated by changes in maternal care. ELS not only induces “detrimental” effects in adulthood, increasing psychopathology, but also promotes resilience to further stressors. In Long-Evans rats, we evaluated a combination of two procedures as a model of ELS: restriction of bedding during the first post-natal days and exposure to a “substitute” mother. The maternal care of biological and “substitute” mothers was measured. The male and female offspring were evaluated during adulthood in several contexts. Anxiety was measured by the elevated plus-maze (EPM), acoustic startle response (ASR) and forced swim test (FST). In other group of animals, novelty-seeking was measured (activity in an inescapable novel environment, preference for novel environments and exploration of novel objects). Plasmatic ACTH and corticosterone in basal conditions and in response to stress were also measured. Cognitive impulsivity was assessed by a delay-discounting paradigm, and impulsive action, attention and compulsive-like behavior by a five choice serial reaction time task (5CSRTT). ELS decreased pup body weight and increased the care of the biological mother; however, the “substitute” mother did not exhibit overt maltreatment. A mixture of “detrimental” and “beneficial” effects was shown. In the 5CSRTT, attention was impaired in both genders, and in females, ELS increased compulsive-like behavior. Novel object exploration was only increased by ELS in males, but the preference for novel spaces decreased in both genders. Baseline anxiety (EPM and ASR) and recognition memory were not affected. Unexpectedly, ELS decreased the ACTH response to novelty and swim stress and increased active coping in the FST in both genders. Cognitive impulsivity was decreased only in females, but impulsive action was not affected. The enhancement in maternal care may “buffer” the effects of ELS in a context-dependent manner.

Early handling effect on female rat spatial and non-spatial learning and memory

This study aims at providing an insight into early handling procedures on learning and memory performance in adult female rats. Early handling procedures were started on post-natal day 2 until 21, and consisted in 15 min, daily separations of the dams from their litters. Assessment of declarative memory was carried out in the novel-object recognition task; spatial learning, reference- and working memory were evaluated in the Morris water maze (MWM). Our results indicate that early handling induced an enhancement in: (1) declarative memory, in the object recognition task, both at 1h and 24h intervals; (2) reference memory in the probe test and working memory and behavioral flexibility in the "single-trial and four-trial place learning paradigm" of the MWM. Short-term separation by increasing maternal care causes a dampening in HPA axis response in the pups. A modulated activation of the stress response may help to protect brain structures, involved in cognitive function. In conclusion, this study shows the long-term effects of a brief maternal separation in enhancing object recognition-, spatial reference- and working memory in female rats, remarking the impact of early environmental experiences and the consequent maternal care on the behavioral adaptive mechanisms in adulthood.

Is the rodent maternal separation model a valid and effective model for studies on the early-life impact on ethanol consumption?

RATIONALE

Early-life events can cause long-term neurobiological and behavioural changes with a resultant effect upon reward and addiction processes that enhance risk to develop alcohol use disorders. Maternal separation (MS) is used to study the mediating mechanisms of early-life influences in rodents. In MS studies, the pups are exposed to maternal absence during the first postnatal weeks. The outcome of MS experiments exhibits considerable variation and questions have been raised about the validity of MS models.

OBJECTIVES

This short review aims to provide information about experimental conditions that are important to consider when assessing the impact of early-life environment on voluntary ethanol consumption.

RESULTS

The results from currently used MS protocols are not uniform. However, studies consistently show that longer separations of intact litters predispose for higher ethanol consumption and/or preference in adult male rats as compared to shorter periods of MS. Studies using individual pup MS paradigms, other controls, low ethanol concentrations, adult females or examining adolescent consumption reported no differences or inconsistent results.

CONCLUSIONS

There is no "a rodent MS model", there are several models and they generate different results. The compiled literature shows that MS is a model of choice for analysis of early-life effects on voluntary ethanol consumption but there are examples of MS paradigms that are not suitable. These studies emphasize the importance to carefully designed MS experiments to supply the optimal conditions to definitely test the research hypothesis and to be particulate in the interpretation of the outcome.

Individual differences and social influences on the neurobehavioral pharmacology of abused drugs

The interaction of drugs with biologic targets is a critical area of research, particularly for the development of medications to treat substance use disorders. In addition to understanding these drug-target interactions, however, there is a need to understand more fully the psychosocial influences that moderate these interactions. The first section of this review introduces some examples from human behavioral pharmacology that illustrate the clinical importance of this research. The second section covers preclinical evidence to characterize some of the key individual differences that alter drug sensitivity and abuse vulnerability, related primarily to differences in response to novelty and impulsivity. Evidence is presented to indicate that critical neuropharmacological mechanisms associated with these individual differences involve integrated neurocircuits underlying stress, reward, and behavioral inhibitory processes. The third section covers social influences on drug abuse vulnerability, including effects experienced during infancy, adolescence, and young adulthood, such as maternal separation, housing conditions, and social interactions (defeat, play, and social rank). Some of the same neurocircuits involved in individual differences also are altered by social influences, although the precise neurochemical and cellular mechanisms involved remain to be elucidated fully. Finally, some speculation is offered about the implications of this research for the prevention and treatment of substance abuse.

The Effects of Maternal Separation on Adult Methamphetamine Self-Administration, Extinction, Reinstatement, and MeCP2 Immunoreactivity in the Nucleus Accumbens

The maternal separation (MS) paradigm is an animal model of early life stress. Animals subjected to MS during the first 2 weeks of life display altered behavioral and neuroendocrinological stress responses as adults. MS also produces altered responsiveness to and self-administration (SA) of various drugs of abuse including cocaine, ethanol, and amphetamine. However, no studies have yet examined the effects of MS on methamphetamine (METH) SA. This study was performed to examine the effects of MS on the acquisition of METH SA, extinction, and reinstatement of METH-seeking behavior in adulthood. Given the known influence of early life stress and drug exposure on epigenetic processes, we also investigated group differences in levels of the epigenetic marker methyl CpG binding protein 2 (MeCP2) in the nucleus accumbens (NAc) core. Long-Evans pups and dams were separated on postnatal days (PND) 2-14 for either 180 (MS180) or 15 min (MS15). Male offspring were allowed to acquire METH SA (0.05 mg/kg/infusion) in 15 2-h daily sessions starting at PND67, followed by extinction training and cue-induced reinstatement of METH-seeking behavior. Rats were then assessed for MeCP2 levels in the NAc core by immunohistochemistry. The MS180 group self-administered significantly more METH and acquired SA earlier than the MS15 group. No group differences in extinction or cue-induced reinstatement were observed. MS15 rats had significantly elevated MeCP2-immunoreactive cells in the NAc core as compared to MS180 rats. Together, these data suggest that MS has lasting influences on METH SA as well as epigenetic processes in the brain reward circuitry.

Emotional valence and context of social influences on drug abuse-related behavior in animal models of social stress and prosocial interaction

RATIONALE

Social factors are important determinants of drug dependence and relapse.

OBJECTIVES

We reviewed pre-clinical literature examining the role of social experiences from early life through the development of drug dependence and relapse, emphasizing two aspects of these experiences: (1) whether the social interaction is appetitive or aversive and (2) whether the social interaction occurs within or outside of the drug-taking context.

METHODS

The models reviewed include neonatal care, isolation, social defeat, chronic subordination, and prosocial interactions. We review results from these models in regard to effects on self-administration and conditioned place preference established with alcohol, psychostimulants, and opiates.

RESULTS

We suggest that in general, when the interactions occur outside of the drug-taking context, prosocial interactions are protective against drug abuse-related behaviors, whereas social stressors facilitate these behaviors. By contrast, positive or negative social interactions occurring within the drug-taking context may interact with other risk factors to enhance or inhibit these behaviors.

CONCLUSIONS

Despite differences in the nature and complexity of human social behavior compared to other species, the evolving animal literature provides useful models for understanding social influences on drug abuse-related behavior that will allow for research on the behavioral and biological mechanisms involved. The models have contributed to understanding social influences on initiation and maintenance of drug use, but more research is needed to understand social influences on drug relapse.

Altered locomotor and stereotyped responses to acute methamphetamine in adolescent, maternally separated rats

RATIONALE

Neonatal maternal separation (MS) has been used to model the effects of early life stress in rodents. MS alters behavioral responses to a variety of abused drugs, but few studies have examined its effects on methamphetamine sensitivity.

OBJECTIVES

We sought to determine the effects of MS on locomotor and stereotyped responses to low-to-moderate doses of methamphetamine in male and female adolescent rats.

METHODS

Male and female rat pups were subjected to 3 h per day of MS on postnatal days (PN) 2-14 or a brief handling control procedure during the same period. During adolescence (approximately PN 40), all rats were tested for locomotor activity and stereotyped behavior in response to acute methamphetamine administration (0, 1.0, or 3.0 mg/kg, s.c.).

RESULTS

MS rats of both sexes exhibited increased locomotor activity in a novel environment, relative to handled controls. MS increased the locomotor response to methamphetamine (METH), and this effect occurred at different doses for male (3.0 mg/kg) and female (1.0 mg/kg) rats. MS also increased stereotyped behavior in response to METH (1.0 mg/kg) in both sexes.

CONCLUSIONS

MS enhances the locomotor response to METH in a dose- and sex-dependent manner. These results suggest that individuals with a history of early life stress may be particularly vulnerable to the psychostimulant effects of METH, even at relatively low doses.

Early life manipulations alter learning and memory in rats

Much research shows that early life manipulations have enduring behavioral, neural, and hormonal effects. However, findings of learning and memory performance vary widely across studies. We reviewed studies in which pre-weaning rat pups were exposed to stressors and tested on learning and memory tasks in adulthood. Tasks were classified as aversive conditioning, inhibitory learning, or spatial/relational memory. Variables of duration, type, and timing of neonatal manipulation and sex and strain of animals were examined to determine if any predict enhanced or impaired performance. Brief separations enhanced and prolonged separations impaired performance on spatial/relational tasks. Performance was impaired in aversive conditioning and enhanced in inhibitory learning tasks regardless of manipulation duration. Opposing effects on performance for spatial/relational memory also depended upon timing of manipulation. Enhanced performance was likely if the manipulation occurred during postnatal week 3 but performance was impaired if it was confined to the first two postnatal weeks. Thus, the relationship between early life experiences and adulthood learning and memory performance is multifaceted and decidedly task-dependent.

Neuropeptides as mediators of the early-life impact on the brain; implications for alcohol use disorders

The brain is constantly exposed to external and internal input and to function in an ever-changing environment we are dependent on processes that enable the brain to adapt to new stimuli. Exposure to postnatal environmental stimuli can interfere with vital adaption processes and cause long-term changes in physiological function and behavior. Early-life alterations in brain function may result in impaired ability to adapt to new situations, in altered sensitivity to challenges later in life and thereby mediate risk or protection for psychopathology such as alcohol use disorders (AUD). In clinical research the studies of mechanisms, mediators, and causal relation between early environmental factors and vulnerability to AUD are restricted and attempts are made to find valid animal models for studies of the early-life influence on the brain. This review focuses on rodent models and the effects of adverse and naturalistic conditions on peptide networks within the brain and pituitary gland. Importantly, the consequences of alcohol addiction are not discussed but rather neurobiological alterations that can cause risk consumption and vulnerability to addiction. The article reviews earlier results and includes new data and multivariate data analysis with emphasis on endogenous opioid peptides but also oxytocin and vasopressin. These peptides are vital for developmental processes and it is hypothesized that early-life changes in peptide networks may interfere with neuronal processes and thereby contribute the individual vulnerability for AUD. The summarized results indicate a link between early-life rearing conditions, opioids, and ethanol consumption and that the ethanol-induced effects and the treatment with opioid antagonists later in life are dependent on early-life experiences. Endogenous opioids are therefore of interest to further study in the early-life impact on individual differences in vulnerability to AUD and treatment outcome.

Differences in basal and ethanol-induced levels of opioid peptides in Wistar rats from five different suppliers

One major cause for discrepancies in results from animal experimental studies is the use of different animal strains and suppliers. We have previously reported that Wistar rats from five different suppliers display profound differences in ethanol intake and behavior. One of the neurobiological processes that could be underlying these differences is the endogenous opioid system, which has been implicated in the rewarding and reinforcing effects of alcohol. We therefore hypothesized that the differences between the supplier groups would also be evident in the endogenous opioid system. Radioimmunoassay was used to determine the levels of the opioid peptides Met-enkephalin-Arg(6)Phe(7) and dynorphin B in several brain areas of ethanol-drinking and ethanol naïve Wistar rats from five different suppliers. In the ethanol naïve animals, differences between the supplier groups were found in the pituitary gland, hypothalamus, frontal cortex, dorsal striatum and hippocampus. In the ethanol-drinking rats, differences were found in the same structures, with the addition of medial prefrontal cortex and substantia nigra. Correlations between ethanol intake and peptide levels were also found in several of the areas examined. The structures in which differences were found have all been implicated in the transition from drug use to addiction and these differences may lead to different propensities and vulnerability to this transition. Because the endogenous opioids have been suggested to be involved in a number of neurobiological disorders the results do not only have implications for research on alcohol or drug addiction, but many other fields as well.

Effects of stress on alcohol drinking: a review of animal studies

RATIONALE

While stress is often proposed to play a significant role in influencing alcohol consumption, the relationship between stress and alcohol is complex and poorly understood. Over several decades, stress effects on alcohol drinking have been studied using a variety of animal models and experimental procedures, yet this large body of literature has generally produced equivocal results.

OBJECTIVES

This paper reviews results from animal studies in which alcohol consumption is evaluated under conditions of acute/sub-chronic stress exposure or models of chronic stress exposure. Evidence also is presented indicating that chronic intermittent alcohol exposure serves as a stressor that consequently influences drinking.

RESULTS

The effects of various acute/sub-chronic stress procedures on alcohol consumption have generally been mixed, but most study outcomes suggest either no effect or decreased alcohol consumption. In contrast, most studies indicate that chronic stress, especially when administered early in development, results in elevated drinking later in adulthood. Chronic alcohol exposure constitutes a potent stressor itself, and models of chronic intermittent alcohol exposure reliably produce escalation of voluntary alcohol consumption.

CONCLUSIONS

A complex and dynamic interplay among a wide array of genetic, biological, and environmental factors govern stress responses, regulation of alcohol drinking, and the circumstances in which stress modulates alcohol consumption. Suggestions for future directions and new approaches are presented that may aid in developing more sensitive and valid animal models that not only better mimic the clinical situation, but also provide greater understanding of mechanisms that underlie the complexity of stress effects on alcohol drinking.

Maternal separation altered behavior and neuronal spine density without influencing amphetamine sensitization

We studied the long-term influence of maternal separation (MS) on periadolescent behavior, adult amphetamine (AMPH) sensitization, and structural plasticity in the corticolimbic regions in rats. Male and female pups, separated daily for 3h from the dam during postnatal day 3-21, were tested for periadolescent exploratory, emotional, cognitive, and social behaviors. The development and persistence of drug-induced behavioral sensitization were tested by repeated AMPH administration and a challenge, respectively. The spine density was examined in the nucleus accumbens (NAc), the medial prefrontal cortex (mPFC), and the orbital frontal cortex (OFC) from Golgi-Cox stained neurons. The results showed that MS enhanced anxiety-like behavior in males. MS abolished the sex difference in playful attacks observed in controls with resultant feminization of male play behavior. Furthermore, the probability of complete rotation defense to face an attack was decreased in females. AMPH administration resulted in the development of behavioral sensitization that persisted at least for two weeks. Sensitization was not influenced by MS. MS increased the spine density in the NAc, the mPFC, and the OFC. Repeated AMPH administration increased the spine density in the NAc and the mPFC, and decreased it in the OFC. MS blocked the drug-induced alteration in these regions. In sum, MS during development influenced periadolescent behavior in males, and structurally reorganized cortical and subcortical brain regions without affecting AMPH-induced behavioral sensitization.

Effect of complete maternal and littermate deprivation on morphine-induced Fos-immunoreactivity in the adult male rat brain

Previous research has demonstrated that rats reared in isolation from their dam and littermates show altered behavioral responsiveness to both natural and drug-mediated rewards. This study examined the effects of complete maternal deprivation through the use of artificial rearing on neural activation after acute morphine exposure in adulthood. Male rats were either artificially reared (AR) or maternally reared (MR) from postnatal day 5 to 21. In adulthood (4 mo old), rats received a single injection of morphine sulfate (10 mg/kg) or equivolume saline 2 h before perfusion and brain extraction. Neural activation was quantified using Fos immunohistochemistry. Analyses of several brain regions revealed a consistent pattern of differences between AR and MR rats. Specifically, relative to MR rats, AR rats showed significantly greater morphine-induced Fos-immunoreactivity in brain regions associated with the mesocorticolimbic "reward" pathway. These results support the hypothesis that functional activity in reward neurocircuitry can be altered by early life experience.

Immediate and enduring effects of neonatal isolation on maternal behavior in rats

Previously, we showed that neonatal isolation (1-hisolation/day from dam, litter, and nest on PND 2-9) facilitates cocaine self-administration and increases extracellular dopamine responses in ventral striatum after stimulant administration in adulthood. Recent studies suggest that enduring alterations in neurobehavioral responses associated with early life manipulations reflect changes in maternal behavior. Thus, we sought to determine if neonatal isolation alters maternal care and if dams with neonatal isolation experience as pups showed differential maternal care towards their pups. In Experiment 1, litters were assigned to one of three conditions: neonatal isolation, handled (5-min separation of dam from litter), or non-handled (no separation). Maternal behaviors were rated on PND 2-9 for 60-min immediately following reunion of mother and litter. In Experiment 2, female rats with or without neonatal isolation experience were assigned to either the neonatal isolation or non-handled litter condition and maternal behaviors rated. Dams of isolated and handled litters spent more time licking pups and less time picking up pups to put outside the nest than dams of non-handled litters. Further, dams of isolated and handled vs. non-handled litters showed less non-maternal behaviors of burrowing and grooming. Neonatal isolation-experienced dams with isolated litters failed to increase pup-licking and decrease non-maternal behaviors. Rather, these dams picked up pups to place outside the nest more than non-handled-experienced dams. Neonatal isolation alters maternal behavior that, in turn, may shape neurobehavioral responses of offspring including effects on maternal care. Such changes may reflect epigenetic effects resulting from changes in maternal behavior.

Differential effects of repeated long and brief maternal separation on behaviour and neuroendocrine parameters in Wistar dams

Repeated, prolonged maternal separation has been suggested to model the development of a depression-like syndrome in rats. The long separations from the pups have been proposed to be stressful for the dams, which in turn could mediate the changes seen in adult offspring. In the present study we investigated whether prolonged maternal separation really is stressful for rat dams by studying parameters known to be affected by long-term stress such as spontaneous motor activity, anxiety-like behaviour, adrenal gland weight and plasma corticosterone levels. Dams were separated from their litter for either 4 h (MS240) or 15 min (MS15) on eight random days during postnatal day 1-14, or left undisturbed (animal facility reared, AFR). After weaning MS240 dams showed decreased peripheral activity and habituated slower in horizontal activity. On the contrary, MS15 dams showed more peripheral activity and less rearing activity compared to both AFR and MS240 dams when habituated to the testing apparatus, suggesting that MS15 dams are more anxious. The adrenal glands from MS15 dams weighed significantly less and plasma corticosterone levels were significantly higher compared to AFR and MS240 dams. These results suggest that repeated brief maternal separations from pups could be stressful for rat mothers, whereas prolonged separations are not. Since these results are in contrast to the current notion that the short separation procedure may be considered as a safe milieu, whereas the prolonged separations have been suggested to be stressful for both dams and pups, further studies in this field are warranted.

Effects of maternal separation on the neurobehavioral development of newborn Wistar rats

Animal models of neonatal stress, like maternal separation, may provide important correlation with human stress-related disorders. Early maternal deprivation has been shown to cause several short- and long-term neurochemical and behavioral deficits. Little is known about the early neurobehavioral development after postnatal stress. The aim of the present study was to investigate the development of reflexes and motor coordination in male and female pups subjected to maternal deprivation. Pups were removed from their mothers from postnatal day 1-14, for 3h daily. Somatic development (weight gain, eye opening, ear unfolding, incisor eruption) and reflex development was tested during the first 3 weeks. The appearance of the following reflexes was investigated: crossed extensor, grasping, placing, gait, righting and sensory reflexes, and negative geotaxis. Timely performance of negative geotaxis, righting and gait were also tested daily during the first 3 weeks. Motor coordination and open-field tests were performed on postnatal weeks 3-5 (rotarod, elevated grid-walk, footfault, rope suspension, inclined board and walk initiation tests). The results revealed that a 3-h-long daily maternal separation did not lead to a marked delay or enhancement in reflex development and motor coordination. A subtle enhancement was observed in the appearance of hindlimb grasp and gait reflexes, and a better performance in footfault test in male rats suffering from maternal deprivation. In contrast, female maternally deprived (MD) rats displayed a slight delay in forelimb grasp and air righting reflex appearance, and surface righting performance. Open-field activity was not changed in maternally deprived rats. In summary, our present observations indicate that maternal deprivation does not induce drastic changes in early neurodevelopment, therefore, further research is needed to determine the onset of behavioral alterations in subject with maternal deprivation history. Gender differences described in this study could help to understand how gender-specific differences in early life experience-induced stress-related disorders appear in adult life.

Maternal separation stress in male mice: long-term increases in alcohol intake

RATIONALE

In rodents, prolonged maternal separation has been used as a model of developmentally early environmental stress to influence adult drug intake.

OBJECTIVES

The aim of the present study was to evaluate the long-term effects of prolonged maternal separation on alcohol consumption using two different self-administration procedures in mice: operant alcohol self-administration vs. three-bottle choice.

MATERIALS AND METHODS

From postnatal day (PND) 1 to 14, pups were separated from the dam (maternal separation, MS) daily for 180 min or were left undisturbed, only handled during cage cleaning (animal facility rearing, AFR). On PND 60, they were assigned to one of two experimental manipulations: either a three-bottle choice or operant oral alcohol self-administration. In the three-bottle choice procedure, mice were given access to 6% or 10% alcohol or 0.05% saccharin solution for 2 h/day for 10 days. In the second experiment, mice were reinforced for nose poking by delivery of oral alcohol (6% or 10% in saccharin) or 0.05% saccharin solutions during daily 30-min sessions. Following the acquisition phase, "break points" were determined. Later, mice were allowed 1 h access to the reinforcing solution with no dosage limitation.

RESULTS

In the three-bottle choice procedure, MS mice showed higher alcohol intake than AFR at the 10% alcohol concentration. In the operant alcohol self-administration, MS mice achieved higher alcohol intake than AFR at the concentrations 6% and 10% during the 1-h session.

CONCLUSIONS

The results demonstrate the long-term consequences of MS on alcohol intake in male mice, suggesting early life stress as a risk factor for alcohol consumption and abuse.

Effects of maternal strain on ethanol responses in reciprocal F1 C57BL/6J and DBA/2J hybrid mice

Variations in maternal behavior, either occurring naturally or in response to experimental manipulations, have been shown to exert long-lasting consequences on offspring behavior and physiology. Despite previous research examining the effects of developmental manipulations on drug-related phenotypes, few studies have specifically investigated the influence of strain-based differences in maternal behavior on drug responses in mice. The current experiments used reciprocal F1 hybrids of two inbred mouse strains (i.e. DBA/2J and C57BL/6J) that differ in both ethanol (EtOH) responses and maternal behavior to assess the effects of maternal environment on EtOH-related phenotypes. Male and female DBA/2J and C57BL/6J mice and their reciprocal F1 hybrids reared by either DBA/2J or C57BL/6J dams were tested in adulthood for EtOH intake (choice, forced), EtOH-induced hypothermia, EtOH-induced activity and EtOH-induced conditioned place preference (CPP). C57BL/6J and DBA/2J mice showed differences on all EtOH responses. Consistent with previous reports that maternal strain can influence EtOH intake, F1 hybrids reared by C57BL/6J dams consumed more EtOH during forced exposure than did F1 hybrids reared by DBA/2J dams. Maternal strain also influenced EtOH-induced hypothermic responses in F1 hybrids, producing differences in hybrid mice that paralleled those of the inbred strains. In contrast, maternal strain did not influence EtOH-induced activity or CPP in hybrid mice. The current findings indicate that maternal environment may contribute to variance in EtOH-induced hypothermia and EtOH intake, although effects on EtOH intake appear to be dependent upon the type of EtOH exposure.

The impact of postnatal environment on opioid peptides in young and adult male Wistar rats

Early environmental influences can change the neuronal development and thereby affect behavior in adult life. The aim in the present study was to thoroughly examine the impact of early environmental factors on endogenous opioids by using a rodent maternal separation (MS) model. The endogenous opioid peptide system is not fully developed at birth, and short- and/or long-term alterations may occur in these neural networks in animals exposed to manipulation of the postnatal environment. Rat pups were subjected to one of five rearing conditions; 15 min (MS15) litter (l) or individual (i), 360 min (MS360) l or i daily MS, or housed under normal animal facility rearing (AFR) conditions during postnatal days 1-21. Measurements of immunoreactive (ir) Met-enkephalin-Arg6Phe7 (MEAP) and dynorphin B (DYNB) peptide levels in the pituitary gland and in a number of brain areas, were performed at three and 10 weeks of age, respectively. MS-induced changes were more pronounced in ir MEAP levels, especially in individually separated rats at three weeks of age and in litter-separated rats at 10 weeks of age. The enkephalin and dynorphin systems have different developmental patterns, dynorphin appearing earlier, which may point at a more sensitive enkephalin system during the early postnatal weeks. The results provide evidence that opioid peptides are sensitive for early environmental factors and show that the separation conditions are critical and also result in changes manifesting at different time points. MS-induced effects were observed in areas related to stress, drug reward and dependence mechanisms. By describing effects on opioid peptides, the study addresses the possible role of a deranged endogenous opioid system in the previously described behavioral consequences of MS.

Early maternal separation and sex differences in the aversive effects of amphetamine in adult rats

Early life stress increases vulnerability to psychostimulant abuse, sometimes in a sex-dependent manner. These effects are presumably produced by modulation of neurobiological reward mechanisms; however, drug intake is also influenced by sensitivity to the drug's aversive properties, and little is known about the effects of early life stress on stimulant aversion. To assess this possibility, Sprague-Dawley rat litters experienced daily separation from the dam for 3 h (MS) or 15 min (H) on post-natal days 2-14; control litters (AFR) experienced twice-weekly routine animal facility care only. As adults, the animals were tested for conditioned taste aversions (CTA) induced by 1.5, 2.0 or 3.0 mg/kg d-amphetamine administered intraperitoneally over three acquisition trials followed by six drug-free extinction trials (n=7-8/sex/dose/rearing group). All groups acquired significant aversions compared to their vehicle-treated controls, but differential rearing had no effect on CTA acquisition or extinction. Collapsed across rearing groups, females exhibited significantly stronger aversions and slower extinction than their male counterparts at the low 1.5 mg/kg dose, and unlike the males, failed to fully extinguish relative to their vehicle controls at 1.5 and 2.0 mg/kg. These data underscore the importance of sex differences in assays of stimulant abuse liability, and further support the hypothesis that the effects of post-natal stress on the reinforcing efficacy of psychostimulants may be predominantly due to modulation of reward mechanisms.

Maternal deprivation and handling modify the effect of the dopamine D3 receptor agonist, BP 897 on morphine-conditioned place preference in rats

RATIONALE

Maternal deprivation and handling can lead to a vulnerability to opiate dependence. However, the involvement of the dopamine D3 receptors has not been investigated.

OBJECTIVES

This study analysed the effects of a selective partial D3 receptor agonist, BP 897, on morphine-conditioned place preference (CPP) in deprived and handled rats.

MATERIALS AND METHODS

The effects of BP 897 were studied on the expression and the extinction of morphine CPP. Quantitative autoradiography of D2, D3 receptors and immunoautoradiography of dopamine transporter were performed in some saline- and morphine-treated rats 24 h after the place preference test.

RESULTS

Morphine (5 mg/kg) induced a more prolonged morphine CPP in deprived and handled rats than in control animals. BP 897 (0.5 or 2 mg/kg) enhanced the expression of morphine conditioning in control rats. Same doses did not change morphine conditioning in deprived rats. BP 897 (2 mg/kg) suppressed morphine CPP in handled rats. An increase in basal D2 receptor density in the mesencephalon of handled rats, which was suppressed after morphine CPP, was observed. A decrease in D2 receptor levels in morphine-treated deprived rats occurred in the nucleus accumbens.

CONCLUSIONS

This study shows that maternal deprivation and handling induced a prolonged morphine CPP, and different changes of D2/D3 receptor functioning revealed after morphine CPP. Early manipulations of infant-mother relationships may have different consequences on the balance of opioidergic and dopaminergic neurotransmission and may be of interest to reveal pharmacological properties of dopamine receptor partial agonists or antagonists potentially useful for therapeutic applications.

Long maternal separation accelerates behavioural sensitization to ethanol in female, but not in male mice

Early life stress is associated with dysfunction of the hypothalamic-pituitary-adrenal (HPA) axis, and with aspects involved in drug abuse. In this study, we investigated the effects of brief (BMS) and long maternal separation (LMS) on the HPA axis response and behavioural sensitization to ethanol (EtOH) in male and female mice. From PND 2 to 14, pups were subjected to daily maternal separation for 15 min (BMS) or 180 min (LMS) or no separated, only handled during cage cleaning (animal facility rearing-AFR). As adults, animals were treated every other day with saline (SAL) or EtOH (2.2g/kg), i.p., for 10 days, and immediately after each administration, their locomotor response was evaluated for 15 min. Forty-eight hours after the 5th administration, all animals were challenged with saline, followed 48 h later, by an EtOH challenge. Corticosterone (CORT) plasma levels were determined 3 times: basal, after the 1st administration and after the EtOH challenge. LMS females showed higher CORT levels than BMS females at basal, but not in response to acute or chronic EtOH administration. The CORT response to EtOH was more robust in LMS and BMS male than AFR male mice. Repeated EtOH treatment induced behavioural sensitization in all groups of male mice. In females, LMS induced a faster sensitization, although BMS females also exhibited behavioural sensitization (4th day and 5th day of treatment, respectively). In conclusion, LMS and BMS produced gender-dependent effects. In females, LMS and BMS facilitated the development of behavioural sensitization, but in the LMS group this effect occurred faster, which may represent increased vulnerability to drug abuse. Moreover, LMS females showed higher basal CORT levels compared to BMS. In males, LMS and BMS increased the CORT response to EtOH but did not modify behavioural sensitization. Therefore, we postulate that LMS female mice exhibited a faster development of behavioural sensitization, but CORT levels were not involved with this effect.