Early life modulators and predictors of adult synaptic plasticity

Stress in adolescence as a first hit in stress-related disease development: Timing and context are crucial

The two-hit stress model predicts that exposure to stress at two different time-points in life may increase or decrease the risk of developing stress-related disorders later in life. Most studies based on the two-hit stress model have investigated early postnatal stress as the first hit with adult stress as the second hit. Adolescence, however, represents another highly sensitive developmental window during which exposure to stressful events may affect programming outcomes following exposure to stress in adulthood. Here, we discuss the programming effects of different types of stressors (social and nonsocial) occurring during adolescence (first hit) and how such stressors affect the responsiveness toward an additional stressor occurring during adulthood (second hit) in rodents. We then provide a comprehensive overview of the potential mechanisms underlying interindividual and sex differences in the resilience/susceptibility to developing stress-related disorders later in life when stress is experienced in two different life stages.

Postnatal Exposure to Brief Hypoxia Alters Brain VEGF Expression and Capillary Density in Adult Mice

Perinatal hypoxia leads to changes in cerebral angiogenesis and persistent structural and functional changes in the adult brain. It may also result in greater vulnerability to subsequent challenges. We investigated the effect of postnatal day 2 (P2) hypoxic preconditioning on adult brain capillary density and brain vascular endothelial growth factor (VEGF) expression in mice. P2 mice were exposed to hypoxia (5% O2) in a normobaric chamber for 2 h then returned to normoxia while their littermates remained in normoxia (P2 control). After 2-6 months, they were euthanised and their brains were removed for capillary density determination. Another set of animals (P2 hypoxic mice and P2 controls) were euthanised at 2, 10, 23, and 60 days after birth and brain VEGF expression was assessed by western blot. Adult brain capillary density was significantly increased in the P2 hypoxic mice when compared to the P2 control mice. Additionally, VEGF expression appeared to be elevated in the P2-hypoxia mice when compared to the P2-control mice at all time points, and VEGF levels in P2-hypoxia mice declined with age similarly to P2-control mice. These data demonstrate that transient early-postnatal hypoxic stress leads to an increase in capillary density that persists in the adult, possibly due to increased VEGF expression. These results might be explained by epigenetic factors in the VEGF gene.

Neuronal changes and cognitive deficits in a multi-hit rat model following cumulative impact of early life stressors

Perinatal protein malnourishment (LP) is a leading cause for mental and physical retardation in children from poor socioeconomic conditions. Such malnourished children are vulnerable to additional stressors that may synergistically act to cause neurological disorders in adulthood. In this study, the above mentioned condition was mimicked via a multi-hit rat model in which pups born to LP mothers were co-injected with polyinosinic:polycytidylic acid (Poly I:C; viral mimetic) at postnatal day (PND) 3 and lipopolysaccharide (LPS; bacterial mimetic) at PND 9. Individual exposure of Poly I:C and LPS was also given to LP pups to correlate chronicity of stress. Similar treatments were also given to control pups. Hippocampal cellular apoptosis, β III tubulin catastrophe, altered neuronal profiling and spatial memory impairments were assessed at PND 180, using specific immunohistochemical markers (active caspase 3, β III tubulin, doublecortin), golgi studies and cognitive mazes (Morris water maze and T maze). Increase in cellular apoptosis, loss of dendritic arborization and spatial memory impairments were higher in the multi-hit group, than the single-hit groups. Such impairments observed due to multi-hit stress mimicked conditions similar to many neurological disorders and hence, it is hypothesized that later life neurological disorders might be an outcome of multiple early life hits.This article has an associated First Person interview with the first author of the paper.

Impact of Maternal Separation on Dopamine System and its Association with Parkinson's Disease

As a type of stress, maternal separation (MS) has been one of the most widely used models in neuropsychiatric research. An increasing number of studies has found that MS not only affects the function of the hypothalamic-pituitary-adrenal axis and hippocampal 5-hydroxytryptamine system, but also causes dysfunction of the central dopamine (DA) system and increases the susceptibility of dopaminergic neurons to pathogenic factors of Parkinson's disease (PD), for instance, 6-hydroxydopamine, thus impairing motor function. We reviewed the impact of MS on the DA system and its correlation with PD and found the following: (1) discrepant effects of MS on the DA system have been reported; (2) MS is a good model to study the impact of stress on the occurrence and development of PD, however, unified modeling criteria of MS are required; (3) correlation between MS and PD may involve the impact of MS on the DA system, which however is not the only connection; (4) intervening measures can block pathways between MS and PD, which provides reference for the prevention of PD in specific populations such as left-behind children.

Association between dopamine receptor D2 Taq IA gene polymorphism and persistent postural-perceptual dizziness

BACKGROUND:

Persistent postural-perceptual dizziness (PPPD) is a chronic dizziness, its pathogenesis is unknown by now.

OBJECTIVE:

To study the relationship between the DRD2 gene TaqIA polymorphisms and PPPD, and further to explore the molecular mechanism underlying this disease.

METHODS:

43 patients diagnosed with PPPD and 45 randomly selected cases (matched by age and sex) were included in the study and control group, respectively. DRD2 gene TaqIA polymorphisms were detected in all participants by polymerase chain reaction (PCR)combined with the restriction fragment length polymorphism (RFLP) method.

RESULTS:

In the study group, frequencies of the A1 and A2 TaqIA alleles (65.1% and 34.9%, respectively) were significantly different to those in the control group (46.7% and 53.3%, respectively; P < 0.05). The allele frequency in the study group for the A1/A1 genotype was 34.9%, for A1/A2 was 60.5%, and for A2/A2 was 4.6%, all of which were significantly higher than the control group (24.4%, 44.5%. and 31.1%, respectively; P < 0.01).

CONCLUSIONS:

Our findings indicate that the DRD2 TaqIA A1 allele is possibly the susceptibility polymorphism for PPPD, and that the A2/A2 genotype has a potentially protective role for PPPD. However, larger independent studies are required for further validation.

Object, spatial and social recognition testing in a single test paradigm

Animals have the ability to process information about an object or a conspecific's physical features and location, and alter its behavior when such information is updated. In the laboratory, the object, spatial and social recognition are often studied in separate tasks, making them unsuitable to study the potential dissociations and interactions among various types of recognition memories. The present study introduced a single paradigm to detect the object and spatial recognition, and social recognition of a familiar and novel conspecific. Specifically, male and female Sprague-Dawley adult (>75 days old) or preadolescent (25-28 days old) rats were tested with two objects and one social partner in an open-field arena for four 10-min sessions with a 20-min inter-session interval. After the first sample session, a new object replaced one of the sampled objects in the second session, and the location of one of the old objects was changed in the third session. Finally, a new social partner was introduced in the fourth session and replaced the familiar one. Exploration time with each stimulus was recorded and measures for the three recognitions were calculated based on the discrimination ratio. Overall results show that adult and preadolescent male and female rats spent more time exploring the social partner than the objects, showing a clear preference for social stimulus over nonsocial one. They also did not differ in their abilities to discriminate a new object, a new location and a new social partner from a familiar one, and to recognize a familiar conspecific. Acute administration of MK-801 (a NMDA receptor antagonist, 0.025 and 0.10 mg/kg, i.p.) after the sample session dose-dependently reduced the total time spent on exploring the social partner and objects in the adult rats, and had a significantly larger effect in the females than in the males. MK-801 also dose-dependently increased motor activity. However, it did not alter the object, spatial and social recognitions. These findings indicate that the new triple recognition paradigm is capable of recording the object, spatial location and social recognition together and revealing potential sex and age differences. This paradigm is also useful for the study of object and social exploration concurrently and can be used to evaluate cognition-altering drugs in various stages of recognition memories.

Social Interactions and Indirect Genetic Effects on Complex Juvenile and Adult Traits

Most animal species are social in one form or another, yet many studies in rodent model systems use either individually housed animals or ignore potential confounds caused by group housing. While such social interaction effects on developmental and behavioral traits are well established, the genetic basis of social interactions has not been researched in as much detail. Specifically, the effects of genetic variation in social partners on the phenotype of a focal individual have mostly been studied at the phenotypic level. Such indirect genetic effects (IGEs), where the genotype of one individual influences the phenotype of a second individual, can have important evolutionary and medically relevant consequences. In this chapter, we give a brief outline of social interaction effects, and how systems genetics approaches using recombinant inbred populations can be used to investigate indirect genetic effects specifically, including maternal genetic effects. We discuss experimental designs for the study of IGEs and show how indirect genetic loci can be identified that underlie social interaction effects, their mechanisms, and consequences for trait variation in focal individuals.

Antipsychotic-induced sensitization and tolerance: Behavioral characteristics, developmental impacts, and neurobiological mechanisms

Antipsychotic sensitization and tolerance refer to the increased and decreased drug effects due to past drug use, respectively. Both effects reflect the long-term impacts of antipsychotic treatment on the brain and result from the brain's adaptive response to the foreign property of the drug. In this review, clinical evidence of the behavioral aspect of antipsychotic sensitization and tolerance is selectively reviewed, followed by an overview of preclinical literature that examines these behavioral characteristics and the related pharmacological and nonpharmacological factors. Next, recent work on the developmental impacts of adolescent antipsychotic sensitization and tolerance is presented and recent research that delineates the neurobiological mechanisms of antipsychotic sensitization and tolerance is summarized. A theoretical framework based on "drug learning and memory" principles is proposed to account for the phenomena of antipsychotic sensitization and tolerance. It is maintained that antipsychotic sensitization and tolerance follow basic principles of learning or acquisition ("induction") and memory ("expression"). The induction and expression of both effects reflect the consequences of associative and nonassociative processing and are strongly influenced by various pharmacological, environmental, and behavioral factors. Drug-induced neuroplasticity, such as functional changes of striatal dopamine D2 and prefrontal serotonin (5-HT)2A receptors and their mediated signaling pathways, in principle, is responsible for antipsychotic sensitization and tolerance. Understanding the behavioral characteristics and neurobiological underpinnings of antipsychotic sensitization and tolerance has greatly enhanced our understanding of mechanisms of antipsychotic action, and may have important implications for future drug discovery and clinical practice.

The scent of stress: environmental challenge in the peripartum environment of mice affects emotional behaviours of the adult offspring in a sex-specific manner

Early adverse experiences are known to influence the risk of developing psychiatric disorders later. To shed further light on the development of laboratory mice, we systematically examined the influence of a prenatal or postnatal olfactory stressor, namely unfamiliar male mouse faeces, presented to pregnant or nursing mouse dams. Maternal and offspring behaviours were then examined. Maternal behaviours relative to controls revealed changes in nest building by the pregnant dams exposed to the unfamiliar faeces. There were no differences among groups on pup retrieval or exploration by the dams. Behavioural phenotyping of male and female offspring as adults included measures of exploration, anxiety, social and depressive-like behaviours. Additionally, serum corticosterone was assessed as a marker of physiological stress response. Group differences were dependent on the sex of the adult offspring. Males raised by dams that were stressed during pregnancy presented elevated emotionality as indicated by increased numbers of faecal boluses in the open field paradigm. Consistent with the effects of prenatal stress on the males only the prenatally stressed females had higher body weights than their respective controls. Indeed, males in both experimental groups had higher circulating corticosterone levels. By contrast, female offspring of dams exposed to the olfactory stressor after parturition were more anxious in the O-maze as indicated by increased latencies in entering the exposed areas of the maze. These findings emphasize the necessity for researchers to consider the pre- and postnatal environments, even of mice with almost identical genetic backgrounds, in designing experiments and interpreting their data.

Differential effects of acute amphetamine and phencyclidine treatment and withdrawal from repeated amphetamine or phencyclidine treatment on social interaction and social memory in rats

Although animal models based on amphetamine (AMPH) or phencyclidine (PCP) treatment have been used extensively to study the neurobiological and behavioral characteristics of schizophrenia, there are conflicting reports regarding their validity in modeling the negative symptoms and cognitive deficits of schizophrenia. The present study examined how acute AMPH or PCP treatment (Experiment 1) and withdrawal from repeated AMPH treatment (Experiment 2) or PCP treatment (Experiment 3) affects social behavior and social recognition memory in male Sprague-Dawley rats. Each subject was tested on two consecutive days. On the first day, the rats were tested four times (5 min/each) at 10-min intervals with the same partner rat (termed "AAAA" day). One day later, the rats were tested with the previous partner in the first three sessions and with a new partner rat in the final session (termed "AAAB" day). The results show that acute AMPH treatment (1.5 mg/kg, sc) significantly reduced the time spent on social interaction, but did not affect social recognition on the first day. Acute AMPH only disrupted social recognition on the second day of drug testing. In contrast, acute PCP treatment (2.0 mg/kg, sc) had no effect on time spent on social interaction, but did significantly disrupt social recognition on both days. Withdrawal from repeated AMPH (3.0 mg/kg/day for 7 days, ip) or PCP (5.0 mg/kg/twice daily for 7 days, ip) treatment did not affect social interaction or social recognition, indicating a lack of long-term detrimental effect of repeated AMPH or PCP treatment. These results suggest that acute AMPH treatment at a low dose (1.5 mg/kg) may be useful in modeling social withdrawal symptoms of schizophrenia, whereas acute PCP treatment at a similar dose range (2.0 mg/kg) may be useful in modeling the social cognitive deficit of schizophrenia.

Differential effects of intermittent versus continuous haloperidol treatment throughout adolescence on haloperidol sensitization and social behavior in adulthood

Animal work on the behavioral effects of antipsychotic treatment suggests that different dosing regimens could affect drug sensitivity differently, with an intermittent treatment regimen tending to cause a sensitization effect, while a continuous treatment causing a tolerance. In this study, we explored how haloperidol (HAL) sensitization induced throughout adolescence and tested in adulthood was differentially impacted by these two dosing regimens in the conditioned avoidance response (CAR) test. We also examined how these two dosing regiments affected social interaction and social memory in adulthood. Male adolescent Sprague-Dawley rats were treated with HAL via either osmotic minipump (HAL-0.25 CONT; 0.25 mgkg(-1)day(-1), n = 14) or daily injection (HAL-0.05 INT; 0.05 mgkg(-1)day(-1) injection, sc, n = 14), or sterile water (n = 14) from postnatal days (PND) 44 to 71. HAL sensitization was assessed in a challenge test in which all rats were injected with HAL (0.025 and 0.05 mg/kg, sc) on PND 80 and PND 82. Two days later, half of the rats from each group (n = 7/group) were assayed in two 4-trial social interaction tests in which a subject rat was given four 5-min social encounters with a familiar or novel juvenile rat (PND 35-40) at 10 min intervals. Another half were tested in a quinpirole-induced hyperlocomotion assay to assess the potential HAL-induced change in D2-mediated function. Results show that only the intermittent dosing group under the HAL 0.05 mg/kg challenge showed a robust sensitization effect as rats in this group made significantly fewer avoidance responses than those in the vehicle and HAL-0.25 CONT groups. Adolescent HAL treatment did not affect social behavior and social memory, as rats from all 3 groups exhibited a similar level of social interaction and showed a similar level of sensitivity to the change of social stimuli. Similarly, adolescent HAL treatment also did not produce a long-lasting change in D2 function, as all 3 groups exhibited a similar level of increase in motor activity under quinpirole challenge. These findings suggest that HAL sensitization is a dosing-specific phenomenon. It is more likely to be seen under an intermittent dosing regimen than under a continuous dosing one. The findings that the intermittent HAL treatment did not impair social functioning and did not alter D2 function suggest a dissociation between drug-induced alterations in drug sensitivity and those in social function and neuroreceptors.

What makes a good mother? Implication of inter-, and intrastrain strain "cross fostering" for emotional changes in mouse offspring

Currently, the mouse represents the preferred model organism among mammals used for animal studies. Due to a great availability of mutant strains it represents a standard method to analyze in vivo the effects of targeted gene manipulations. While this - at least in theory - represents a valuable tool to elucidate the pathophysiology of certain human diseases, there are several caveats which need to be considered working with animals. In our study we aimed at elucidating, how a widely established breeding strategy, i.e. the use of "foster mothers" to save the survival of compromised mouse pups for ongoing experiments, per se, affects the emotional phenotype of the fostered offspring. Since it is a popular method to use outbred strains like NMRI to do this job, we sought to evaluate the potential effects of such an artificial postnatal condition and compare either offspring nurtured by their biological mothers or two different strains of foster mothers. Hence we analysed changes in maternal care and later on the emotional behaviour of male and female C57BL/6 mice reared by (i) their biological C57BL/6 mothers, (ii) C57BL/6 foster mothers and (iii) NMRI foster mothers in a behavioural test battery. In addition we assessed corticosterone levels as indicator for stress-physiological changes. Besides clear differences in maternal behaviour, our study indicates an altered emotional state (i.e. differences in anxiety and depressive-like features) in mice reared by different "categories" of mothers, which emphasizes the importance to embed such perinatal conditions in the evaluation of animal-deriving data.

On the causes of early life experience effects: evaluating the role of mom

Early life experiences are thought to have long-lasting effects on cognitive, emotional, and social function during adulthood. Changes in neuroendocrine function, particularly the hypothalamic-pituitary-adrenal (HPA) axis, contribute to these systems-level behavioral effects. In searching for causal mechanisms underlying these early experience effects, pioneering research has demonstrated an important role for maternal care in offspring development, and this has led to two persistent ideas that permeate current research and thinking: first, environmental impact on the developing infant is mediated through maternal care behavior; second, the more care that a mother provides, the better off her offspring. While a good beginning, the reality is likely more complex. In this review, we critically examine these ideas and propose a computationally-motivated theoretical framework, and within this framework, we consider evidence supporting a hypothesis of maternal modulation. These findings may inform policy decisions in the context of child health and development.

Facilitation of fear extinction by the 5-HT(1A) receptor agonist tandospirone: possible involvement of dopaminergic modulation

Fear extinction-based exposure treatment is an important component of psychotherapy for anxiety disorders such as posttraumatic stress disorder (PTSD). Recent studies have focused on pharmacological approaches combined with exposure therapy to augment extinction. In this study, we elucidated the therapeutic potential of the serotonin 1A (5-HT(1A) ) receptor agonist tandospirone compared with the effects of the N-methyl-D-aspartate partial agonist D-cycloserine (DCS), focusing on the possible involvement of dopaminergic mechanisms. We used a rat model of juvenile stress [aversive footshock (FS)] exposure during the third postnatal week (3wFS). The 3wFS group exhibited extinction deficit reflected in sustained fear-related behavior and synaptic dysfunction in the hippocampal CA1 field and medial prefrontal cortex (mPFC), which are responsible for extinction processes. Tandospirone administration (5 mg/kg, i.p.) before and after the extinction trials ameliorated both the behavioral deficit and synaptic dysfunction, i.e., synaptic efficacy in the CA1 field and mPFC associated with extinction training and retrieval, respectively, was potentiated in the tandospirone-treated 3wFS group. Extracellular dopamine release in the mPFC was increased by extinction retrieval in the non-FS control group. This facilitation was not observed in the 3wFS group; however, tandospirone treatment increased cortical dopamine levels after extinction retrieval. DCS (15 mg/kg, i.p.) also ameliorated the extinction deficit in the 3wFS group, but impaired extinction in the non-FS control group. These results suggest that tandospirone has therapeutic potential for enhancing synaptic efficacy associated with extinction processes by involving dopaminergic mechanisms. Pharmacological agents that target cortical dopaminergic systems may provide new insights into the development of therapeutic treatments of anxiety disorders, including PTSD.

Unraveling the time domains of corticosteroid hormone influences on brain activity: rapid, slow, and chronic modes

Brain cells are continuously exposed to corticosteroid hormones, although the levels vary (e.g., after stress). Corticosteroids alter neural activity via two receptor types, mineralocorticoid (MR) and glucocorticoid receptors (GR). These receptors regulate gene transcription but also, as we now know, act nongenomically. Via nongenomic pathways, MRs enhance and GRs suppress neural activity. In the hypothalamus, inhibitory GR effects contribute to negative feedback regulation of the stress axis. Nongenomic MR actions are also important extrahypothalamically and help organisms to immediately select an appropriate response strategy. Via genomic mechanisms, corticosteroid actions in the basolateral amygdala and ventral-most part of the cornu ammonis 1 hippocampal area are generally excitatory, providing an extended window for encoding of emotional aspects of a stressful event. GRs in hippocampal and prefrontal pyramidal cells increase surface expression of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors and strengthen glutamatergic signaling through pathways partly overlapping with those involved in long-term potentiation. This raises the threshold for subsequent induction of synaptic potentiation and promotes long-term depression. Synapses activated during stress are thus presumably strengthened but protected against excitatory inputs reaching the cells later. This restores higher cognitive control and promotes, for example, consolidation of stress-related contextual information. When an organism experiences stress early in life or repeatedly in adulthood, the ability to induce synaptic potentiation is strongly reduced and the likelihood to induce depression enhanced, even under rest. Treatment with antiglucocorticoids can ameliorate cellular effects after chronic stress and thus provide an interesting lead for treatment of stress-related disorders.

Early life hypoxic or hypoxic/hypercapnic stress alters acute ventilatory sensitivity in adult mice

In this study we investigated the effect of early life conditioning (hypoxia ± hypercapnia) on adult acute ventilatory sensitivity to hypoxia and hypercapnia. Mice were exposed to either hypoxia (5% O(2)) or hypoxia/hypercapnia (5% O(2)/8% CO(2)) in a normobaric chamber for 2 h at postnatal day 2 (P2), and then returned to normoxia. At 3 months of age, hypoxic ventilatory response (HVR) and hypercapnic ventilatory response (HCVR) were measured using a plethysmograph system. Results showed that HVR was significantly decreased in the P2-hypoxia mice but not in the P2 hypoxia/hypercapnia mice as compared to the P2-normoxic mice, respectively. However, HCVR was significantly decreased in the P2 hypoxia-hypercapnia group but not in the P2-hypoxia group. These data suggest early postnatal hypoxic stress vs. hypoxic/hypercapnic stress plays different roles in fetal programming of the respiratory control system as shown by altered adult acute ventilatory sensitivity.

The link between stress and feeding behaviour

Exposure to stress is inevitable, and it may occur, to varying degrees, at different phases throughout the lifespan. The impact of stress experienced in later life has been well documented as many populations in modern society experience increasing socio-economic demands. The effects of stress early in life are less well known, partly as the impact of an early exposure may be difficult to quantify, however emerging evidence shows it can impact later in life. One of the major impacts of stress besides changes in psychosocial behaviour is altered feeding responses. The system that regulates stress responses, the hypothalamo-pituitary-adrenal axis, also regulates feeding responses because the neural circuits that regulate food intake converge on the paraventricular nucleus, which contains corticotrophin releasing hormone (CRH), and urocortin containing neurons. In other words the systems that control food intake and stress responses share the same anatomy and thus each system can influence each other in eliciting a response. Stress is known to alter feeding responses in a bidirectional pattern, with both increases and decreases in intake observed. Stress-induced bidirectional feeding responses underline the complex mechanisms and multiple contributing factors, including the levels of glucocorticoids (dependent on the severity of a stressor), the interaction between glucocorticoids and feeding related neuropeptides such as neuropeptide Y (NPY), alpha-melanocyte stimulating hormone (α-MSH), agouti-related protein (AgRP), melanocortins and their receptors, CRH, urocortin and peripheral signals (leptin, insulin and ghrelin). This review discusses the neuropeptides that regulate feeding behaviour and how their function can be altered through cross-talk with hormones and neuropeptides that also regulate the hypothalamo-pituitary-adrenal axis. In addition, long-term stress induced alterations in feeding behaviour, and changes in gene expression of neuropeptides regulating stress and food intake through epigenetic modifications will be discussed. This article is part of a Special Issue entitled 'SI: Central Control of Food Intake'.

Olfactory classical conditioning in neonatal mouse pups using thermal stimuli

Mouse models are increasingly used to investigate genetic contributions to developmental disorders in children, especially newborns. In particular, early cognitive assessment in newborn mice is critical to evaluate pediatric drug efficacy and toxicity. Unfortunately, methods for behavioral tests in newborn mice are scarce. Therefore, developing such tests for newborn mice is a priority challenge for neurogenetics and pharmacological research. The aim of the present study was to develop a conditioning method well suited to high-throughput cognitive screening in newborn mice. To this end, we developed an odor-preference conditioning test using ambient temperature as an unconditioned stimulus (US) and artificial odors as conditioned stimuli (CS). First, we showed that mouse pups move toward the thermoneutral temperature when offered a choice between a thermoneutral and cold environment, thus showing thermotaxis. Second, we conducted a classical conditioning paradigm in pups aged six to ten days. In terms of central nervous system development, this period corresponds to extreme prematurity to early post-term period in humans. During acquisition, the pups were alternatively exposed to odor CS paired with either cold or warm temperatures. Immediately after acquisition, the pups underwent a two-odor choice test, which showed preference for the odor previously paired with the warm temperature, thus showing conditioning. The proposed paradigm is easy to conduct, and requires modest experimenter interference. The method is well suited for high-throughput screening of early associative disorders in newborn mice.

Early postnatal stress alters extracellular signal-regulated kinase signaling in the corticolimbic system modulating emotional circuitry in adult rats

The present study elucidated whether early life stress alters the extracellular signal-regulated kinase (ERK) pathway that underlies fear retrieval and fear extinction based on a contextual fear conditioning paradigm, using a juvenile stress model. Levels of phospho-ERK (pERK), the active form of ERK, increased after fear retrieval in the hippocampal CA1 region but not in the medial prefrontal cortex (mPFC). ERK activation in the CA1 following fear retrieval was not observed in adult rats who received aversive footshock (FS) stimuli during the second postnatal period (2wFS), which exhibited low levels of freezing. In fear extinction, pERK levels in the CA1 were increased by repeated extinction trials, but they were not altered after extinction retrieval. In contrast, pERK levels in the mPFC did not change during extinction training, but were enhanced after extinction retrieval. These findings were compatible in part with electrophysiological data showing that synaptic transmission in the CA1 field and mPFC was enhanced during extinction training and extinction retrieval, respectively. ERK activation in the CA1 and mPFC associated with extinction processes did not occur in rats that received FS stimuli during the third postnatal period (3wFS), which exhibited sustained freezing behavior. The repressed ERK signaling and extinction deficit observed in the 3wFS group were ameliorated by treatment with the partial N-methyl-D-aspartate receptor agonist D-cycloserine. These findings suggest that early postnatal stress induced the downregulation of ERK signaling in distinct brain regions through region-specific regulation, which may lead to increased behavioral abnormalities or emotional vulnerabilities in adulthood.

Calcyon upregulation in adolescence impairs response inhibition and working memory in adulthood

Calcyon regulates activity-dependent internalization of α-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate (AMPA) glutamate receptors and long-term depression of excitatory synapses. Elevated levels of calcyon are consistently observed in brains from schizophrenic patients, and the calcyon gene is associated with attention-deficit hyperactivity disorder. Executive function deficits are common to both disorders, and at least for schizophrenia, the etiology appears to involve both heritable and neurodevelopmental factors. Here, we show with calcyon-overexpressing Cal(OE) transgenic mice that lifelong calcyon upregulation impairs executive functions including response inhibition and working memory, without producing learning and memory deficits in general. As response inhibition and working memory, as well as the underlying neural circuitry, continue to mature into early adulthood, we functionally silenced the transgene during postnatal days 28-49, a period corresponding to adolescence. Remarkably, the response inhibition and working memory deficits including perseverative behavior were absent in adult Cal(OE) mice with the transgene silenced in adolescence. Suppressing the calcyon transgene in adulthood only partially rescued the deficits, suggesting calcyon upregulation in adolescence irreversibly alters development of neural circuits supporting mature response inhibition and working memory. Brain regional immunoblots revealed a prominent downregulation of AMPA GluR1 subunits in hippocampus and GluR2/3 subunits in hippocampus and prefrontal cortex of the Cal(OE) mice. Silencing the transgene in adolescence prevented the decrease in hippocampal GluR1, further implicating altered fronto-hippocampal connectivity in the executive function deficits observed in the Cal(OE) mice. Treatments that mitigate the effects of high levels of calcyon during adolescence could preempt adult deficits in executive functions in individuals at risk for serious mental illness.

Effects of birth asphyxia on neonatal hippocampal structure and function in the spiny mouse

Studies of human neonates, and in animal experiments, suggest that birth asphyxia results in functional compromise of the hippocampus, even when structural damage is not observable or resolves in early postnatal life. The aim of this study was to determine if changes in hippocampal function occur in a model of birth asphyxia in the precocial spiny mouse where it is reported there is no major lesion or infarct. Further, to assess if, as in human infants, this functional deficit has a sex-dependent component. At 37 days gestation (term=39 days) spiny mice fetuses were either delivered immediately by caesarean section (control group) or exposed to 7.5min of in utero asphyxia causing systemic acidosis and hypoxia. At 5 days of age hippocampal function was assessed ex vivo in brain slices, or brains were collected for examination of structure or protein expression. This model of birth asphyxia did not cause infarct or cystic lesion in the postnatal day 5 (P5) hippocampus, and the number of proliferating or pyknotic cells in the hippocampus was unchanged, although neuronal density in the CA1 and CA3 was increased. Protein expression of synaptophysin, brain-derived neurotrophic factor (BDNF), and the inositol trisphosphate receptor 1 (IP(3)R1) were all significantly increased after birth asphyxia, while long-term potentiation (LTP), paired pulse facilitation (PPF), and post-tetanic potentiation (PTP) were all reduced at P5 by birth asphyxia. In control P5 pups, PPF and synaptic fatigue were greater in female compared to male pups, and after birth asphyxia PPF and synaptic fatigue were reduced to a greater extent in female vs. male pups. In contrast, the asphyxia-induced increase in synaptophysin expression and neuronal density were greater in male pups. Thus, birth asphyxia in this precocial species causes functional deficits without major structural damage, and there is a sex-dependent effect on the hippocampus. This may be a clinically relevant model for assessing treatments delivered either before or after birth to protect this vulnerable region of the developing brain.

Persistent changes in ability to express long-term potentiation/depression in the rat hippocampus after juvenile/adult stress

BACKGROUND

The ventral hippocampus (VH) was recently shown to express lower magnitude long-term potentiation (LTP) compared with the dorsal hippocampus (DH). Exposure to acute stress reversed this difference, and VH slices from stressed rats expressed larger LTP than that produced in the DH, which was reduced by stress. Stressful experience in adolescence has been shown to produce long-lasting effects on animal behavior and on ability to express LTP/long-term depression (LTD) of reactivity to afferent stimulation in the adult. We are interested in possible interactions between juvenile and adult stress in their effects of adult plasticity.

METHODS

We studied the effects of a composite juvenile (28-30 days) stress, followed by a reminder stressful experience in the young adult (60 days) rat, on the ability to produce LTP and LTD in CA1 region of slices of the VH and DH.

RESULTS

Juvenile or adult stress produced a transient decrease in ability to express LTP in DH and a parallel increase in LTP in VH. Stress in the young adult after juvenile stress produced a striking prolongation of the DH/VH disparity with respect to the ability to express both LTP and LTD into the adulthood of the rat.

CONCLUSIONS

These results have important implications for the impact of juvenile stress on adult neuronal plasticity and on the understanding the functions of the different sectors of the hippocampus.

Physical exercise during the adolescent period of life increases hippocampal parvalbumin expression

In order to investigate whether physical exercise during development would promote changes the calcium-binding protein parvalbumin (PV) expression in the hippocampal formation, we performed an immunostaining study after an aerobic exercise program in rats during adolescent period of life. Wistar rats were submitted to daily exercise program in a treadmill between postnatal day 21 and 60. Running time and speed were gradually increased during the subsequent days until 18m/min for 60min. After the aerobic exercise program, animals of all groups were killed and PV immunostaining procedures were performed. The results showed significant increase of protein level in the hippocampal formation and PV-immunoreactive neurons in CA1 and CA2/CA3 regions of rats submitted to exercise when compared with control rats. This finding indicates that aerobic exercise program during adolescent period promotes neuroplastic changes in hippocampal formation.

Enduring neurobehavioral effects of early life trauma mediated through learning and corticosterone suppression

Early life trauma alters later life emotions, including fear. To better understand mediating mechanisms, we subjected pups to either predictable or unpredictable trauma, in the form of paired or unpaired odor-0.5 mA shock conditioning which, during a sensitive period, produces an odor preference and no learning respectively. Fear conditioning and its neural correlates were then assessed after the sensitive period at postnatal day (PN)13 or in adulthood, ages when amygdala-dependent fear occurs. Our results revealed that paired odor-shock conditioning starting during the sensitive period (PN8–12) blocked fear conditioning in older infants (PN13) and pups continued to express olfactory bulb-dependent odor preference learning. This PN13 fear learning inhibition was also associated with suppression of shock-induced corticosterone, although the age appropriate amygdala-dependent fear learning was reinstated with systemic corticosterone (3 mg/kg) during conditioning. On the other hand, sensitive period odor-shock conditioning did not prevent adult fear conditioning, although freezing, amygdala and hippocampal 2-DG uptake and corticosterone levels were attenuated compared to adult conditioning without infant conditioning. Normal levels of freezing, amygdala and hippocampal 2-DG uptake were induced with systemic corticosterone (5 mg/kg) during adult conditioning. These results suggest that the contingency of early life trauma mediates at least some effects of early life stress through learning and suppression of corticosterone levels. However, developmental differences between infants and adults are expressed with PN13 infants' learning consistent with the original learned preference, while adult conditioning overrides the original learned preference with attenuated amygdala-dependent fear learning.

Age matters

The age of an experimental animal can be a critical variable, yet age matters are often overlooked within neuroscience. Many studies make use of young animals, without considering possible differences between immature and mature subjects. This is especially problematic when attempting to model traits or diseases that do not emerge until adulthood. In this commentary we discuss the reasons for this apparent bias in age of experimental animals, and illustrate the problem with a systematic review of published articles on long-term potentiation. Additionally, we review the developmental stages of a rat and discuss the difficulty of using the weight of an animal as a predictor of its age. Finally, we provide original data from our laboratory and review published data to emphasize that development is an ongoing process that does not end with puberty. Developmental changes can be quantitative in nature, involving gradual changes, rapid switches, or inverted U-shaped curves. Changes can also be qualitative. Thus, phenomena that appear to be unitary may be governed by different mechanisms at different ages. We conclude that selection of the age of the animals may be critically important in the design and interpretation of neurobiological studies.

The contribution of psychosocial stress to the obesity epidemic: an evolutionary approach

The Thrifty Gene hypothesis theorizes that during evolution a set of genes has been selected to ensure survival in environments with limited food supply and marked seasonality. Contemporary environments have predictable and unlimited food availability, an attenuated seasonality due to artificial lighting, indoor heating during the winter and air conditioning during the summer, and promote sedentariness and overeating. In this setting the thrifty genes are constantly activated to enhance energy storage. Psychosocial stress and sleep deprivation are other features of modern societies. Stress-induced hypercortisolemia in the setting of unlimited food supply promotes adiposity. Modern man is becoming obese because these ancient mechanisms are efficiently promoting a positive energy balance. We propose that in today's plentifully provisioned societies, where sedentariness and mental stress have become typical traits, chronic activation of the neuroendocrine systems may contribute to the increased prevalence of obesity. We suggest that some of the yet unidentified thrifty genes may be linked to highly conserved energy sensing mechanisms (AMP kinase, mTOR kinase). These hypotheses are testable. Rural societies that are becoming rapidly industrialized and are witnessing a dramatic increase in obesity may provide a historical opportunity to conduct epidemiological studies of the thrifty genotype. In experimental settings, the effects of various forms of psychosocial stress in increasing metabolic efficiency and gene expression can be further tested.

Axotomy-induced up-regulation of tumor necrosis factor-alpha in the dorsal root ganglia

OBJECTIVES

Neuropathic pain is a chronic pain syndrome associated with drug, injury or disease-induced damage or destruction of sensory afferent fibers of the dorsal root ganglia (DRG). Although the exact underlying pathologic mechanisms are not known, pro-inflammatory cytokines such as tumor necrosis factor-alpha (TNF-alpha) are recognized as potential modulators of peripheral and central nervous system inflammatory responses. They play a crucial role in injury and the pathologic development of chronic pain syndromes such as neuropathic pain.

METHODS

Twenty-four rats were divided into a naive control (n=6), sham (surgery exposing sciatic nerve, n=6), and peripheral nerve lesion group (unilateral axotomy of sciatic nerve, n=12).

RESULTS

The results of this study demonstrate a transient up-regulation of TNF-alpha expression within ipsi- and contralateral DRG following complete unilateral sciatic nerve axotomy as confirmed by immunohistochemistry, reverse transcriptase-polymerase chain reaction (RT-PCR) and real-time PCR. Elevated expression of TNF-alpha was noted to occur within the first 7 days post-axotomy, which subsequently normalized to baseline levels by day 14. This transient up-regulation was also associated with a switch in cellular source from predominant satellite cell expression at baseline to that involving satellite cells and abundant numbers of sensory neurons.

DISCUSSION

These results support the role of TNF-alpha in the upstream cascade of cellular events involved in the underlying pathogenesis of neuropathic pain. Strategies targeting the early attenuation of TNF-alpha within the DRG during the first week post-injury may have significant clinical impact in preventing the downstream cascade of events involved in the underlying cellular pathology of neuropathic pain.

Evaluation of effects of previous exposure to an acute stressor before testing for depression-like behaviours in mice

Test batteries are an essential and broadly used tool for behavioural phenotyping, especially with regard to mouse models of particular diseases, such as depression. Facing the problem of an often limited number of mutant animals, it therefore seems crucial to develop and optimise such test batteries in terms of an ideal throughput of subjects. This study aimed to characterize several common stressors, which are used for the investigation of depressive-like features with regard to their capability of each of them to affect performance in a subsequent behavioural test. Here we investigated swim-, restraint- and footshock-stress in male C57/BL6 mice, focusing on post-stress corticosterone elevations as well as potential effects on the behavioural level. The stressors increased circulating corticosterone levels when assessed 1 h after exposure. On the behavioural level, no test interactions could be detected, which suggests, that in general, combining these test conditions in experiments with a restricted availability of animals seems to be rather unproblematic.

Dissociated gender-specific effects of recurrent seizures on GABA signaling in CA1 pyramidal neurons: role of GABA(A) receptors

Early in development, the depolarizing GABA(A)ergic signaling is needed for normal neuronal differentiation. It is shown here that hyperpolarizing reversal potentials of GABA(A)ergic postsynaptic currents (E(GABA)) appear earlier in female than in male rat CA1 pyramidal neurons because of increased potassium chloride cotransporter 2 (KCC2) expression and decreased bumetanide-sensitive chloride transport in females. Three episodes of neonatal kainic acid-induced status epilepticus (3KA-SE), each elicited at postnatal days 4 (P4)-P6, reverse the direction of GABA(A)ergic responses in both sexes. In males, 3KA-SE trigger a premature appearance of hyperpolarizing GABA(A)ergic signaling at P9, instead of P14. This is driven by an increase in KCC2 expression and decrease in bumetanide-sensitive chloride cotransport. In 3KA-SE females, E(GABA) transiently becomes depolarizing at P8-P13 because of increase in the activity of a bumetanide-sensitive NKCC1 (sodium potassium chloride cotransporter 1)-like chloride cotransporter. However, females regain their hyperpolarizing GABA(A)ergic signaling at P14 and do not manifest spontaneous seizures in adulthood. In maternally separated stressed controls, a hyperpolarizing shift in E(GABA) was observed in both sexes, associated with decreased bumetanide-sensitive chloride cotransport, whereas KCC2 immunoreactivity was increased in males only. GABA(A) receptor blockade at the time of 3KA-SE or maternal separation reversed their effects on E(GABA). These data suggest that the direction of GABA(A)-receptor signaling may be a determining factor for the age and sex-specific effects of prolonged seizures in the hippocampus, because they relate to normal brain development and possibly epileptogenesis. These effects differ from the consequences of severe stress.

Neonatal tactile stimulation enhances spatial working memory, prefrontal long-term potentiation, and D1 receptor activation in adult rats

Environmental stimuli during neonatal periods play an important role in the development of cognitive function. In this study, we examined the long-term effects of neonatal tactile stimulation (TS) on spatial working memory (SWM) and related mechanisms. We also investigated whether TS-induced effects could be counteracted by repeated short periods of maternal separation (MS). Wistar rat pups submitted to TS were handled and marked transiently per day during postnatal days 2-9 or 10-17. TS/MS pups were stimulated in the same way as TS pups and then individually separated from their mother for 1h/day. Their nontactile stimulated (NTS) siblings served as controls. In adulthood, TS and TS/MS rats showed better performance in two versions of the delayed alternation task and superior in vivo long-term potentiation of the hippocampo-prefrontal cortical pathway when compared with controls. Furthermore, there were more doses of A77636 (a selective dopamine D1 agonist) to significantly improve SWM performance in TS and TS/MS rats than in NTS rats, suggesting that activation of prefrontal D1 receptors in TS and TS/MS rats is more optimal for SWM function than in NTS rats. MS did not counteract TS-induced effects because no significant difference was found between TS/MS and TS animals. These data indicate that in early life, external tactile stimulation leads to long-term facilitative effects in SWM-related neural function.

Neonatal transection of the corpus callosum affects rotational side preference in adult Swiss mice

In order to test the hypothesis that the ontogenetic development of the corpus callosum (CC) contributes to the establishment of behavioral lateralization, the rotatory behavior in the free-swimming test was studied in male Swiss mice that were subjected to mid-sagittal transection of the CC during the first postnatal day. At adulthood, 16 Acallosal and 30 Sham-operated mice were submitted to 3 sessions of the free-swimming rotatory test (diameter of the recipient=21 cm; session duration=5 min; inter-test interval=48 h). The number and direction of 30 degrees turns were recorded. Our results indicate that transected animals became progressively more lateralized than Sham ones from the first to the third session. This difference between groups was explained mainly by the more pronounced reduction in turning activity to the non-preferred side along the sessions that was observed in Acallosal mice. Our results give further support to the hypothesis that the normal development of the CC is related to the establishment of brain asymmetries in general and behavioral lateralization in particular.

Adult aggression during an initial social encounter: effects of neonatal anoxia and relation to juvenile open-field activity

In male Long-Evans hooded rats, we examined: (1) combined effects of neonatal anoxia and novelty exposure on aggression during adulthood; (2) open-field activity before juvenility as a predictor for adult aggression. Litters of neonates were exposed to either 100% N2 gas (Anoxia) or room air (Control) for 25 min on postnatal Day 1 (P1). Within each of the Anoxia and Control conditions, one half of the neonates were individually exposed to a non-home cage for 3 min daily during P2-21 (Novel: N(Anoxia)=15; N(Control)=13) while the other half remained in the home cage (Home: N(Anoxia)=15; N(Control)=13). Prior to the onset of juvenility (P25), open-field activity was measured during four 20-s trials. At the onset of adulthood (P100-101), we measured the occurrence of biting during four 5-min sessions of social interaction between pairs of rats. Neonatal anoxia and novelty exposure had contrasting effects on adult aggression with the former increasing aggression and the latter having no statistically significant effect. The open-field measures before the onset of juvenility were significantly correlated with the occurrence of biting behavior during adulthood, suggesting that activity in a novel environment observed very briefly in early life may serve as a predictor for adult aggression.