Neonatal isolation alters LTP in freely moving juvenile rats: sex differences

Sex-related differences in somatic plasticity and possible role of ERK1/2: An in-vivo study of young-adult rats

The present study investigates sex differences in hippocampal functions in the context of synaptic plasticity, which is the cellular basis of learning and memory, and differences in the mitogen-activated protein kinase (MAPK) pathway that accompanies plasticity in young-adult rats. The long-term potentiation (LTP) and long-term depression (LTD) were induced by stimulating the perforant pathway (PP) and field potentials composed of the field excitatory post-synaptic potential (fEPSP) and population spike (PS) were recorded from the dentate gyrus (DG). Following the completion of the electrophysiological recordings, the hippocampi were removed bilaterally, and the protein and gene expression levels of the extracellular signal-regulated kinase 1/2 (ERK1/2), c-Jun N-terminal kinase (JNK) and P38-MAPK were determined by Western blot analysis and real-time PCR, respectively. No significant difference was found in synaptic and neuronal function before (basal) and after high-frequency stimulation between male and female rats. Nevertheless, female, but not male, rats were able to express long term depression at the PP - DG synapses, suggesting that sex differences in plasticity are stimulation paradigm specific. MAPK1 expression was higher in males and MAPK3 expression was higher in females, but these differences disappeared after induction of plasticity in both sexes. While the expression of MAPK8 is influenced by sex, independent of the induction of plasticity, MAPK14 expression was down regulated by plasticity induction in females, but not males. No effect of sex, HFS and LFS on total and phosphorylated levels of MAPKs was found except phosphorylated ERK1/2. Phosphorylation of ERK1/2 was up regulated after LFS in male rats but did not change in female rats. These findings indicate that LFS-induced plasticity is differentially modulated between sexes, probably as a result of increased activation of ERK1/2 in male rats.

Sex differences in vulnerability to addiction

This article reviews evidence for sex differences in vulnerability to addiction with an emphasis on the neural mechanisms underlying these differences. Sex differences in the way that the gonadal hormone, estradiol, interacts with the ascending telencephalic dopamine system results in sex differences in motivated behaviors, including drug-seeking. In rodents, repeated psychostimulant exposure enhances incentive sensitization to a greater extent in females than males. Estradiol increases females' motivation to attain psychostimulants and enhances the value of drug related cues, which ultimately increases their susceptibility towards spontaneous relapse. This, along with females' dampened ability to alter decisions regarding risky behaviors, enhances their vulnerability for escalation of drug use. In males, recent evidence suggests that estradiol may be protective against susceptibility towards drug-preference. Sex differences in the actions of estradiol are reviewed to provide a foundation for understanding how future research might enhance understanding of the mechanisms of sex differences in addiction-related behaviors, which are dependent on estradiol receptor (ER) subtype and the region of the brain they are acting in. A comprehensive review of the distribution of ERα, ERβ, and GPER1 throughout the rodent brain are provided along with a discussion of the possible ways in which these patterns differentially regulate drug-taking between the sexes. The article concludes with a brief discussion of the actions of gonadal hormones on the circuitry of the stress system, including the hypothalamic pituitary adrenal axis and regulation of corticotropin-releasing factor. Sex differences in the stress system can also contribute to females' enhanced vulnerability towards addiction.

Long-Term Impact of Early-Life Stress on Hippocampal Plasticity: Spotlight on Astrocytes

Adverse experiences during childhood are among the most prominent risk factors for developing mood and anxiety disorders later in life. Early-life stress interventions have been established as suitable models to study the neurobiological basis of childhood adversity in rodents. Different models such as maternal separation, impaired maternal care and juvenile stress during the postweaning/prepubertal life phase are utilized. Especially within the limbic system, they induce lasting alterations in neuronal circuits, neurotransmitter systems, neuronal architecture and plasticity that are further associated with emotional and cognitive information processing. Recent studies found that astrocytes, a special group of glial cells, have altered functions following early-life stress as well. As part of the tripartite synapse, astrocytes interact with neurons in multiple ways by affecting neurotransmitter uptake and metabolism, by providing gliotransmitters and by providing energy to neurons within local circuits. Thus, astrocytes comprise powerful modulators of neuronal plasticity and are well suited to mediate the long-term effects of early-life stress on neuronal circuits. In this review, we will summarize current findings on altered astrocyte function and hippocampal plasticity following early-life stress. Highlighting studies for astrocyte-related plasticity modulation as well as open questions, we will elucidate the potential of astrocytes as new targets for interventions against stress-induced neuropsychiatric disorders.

Sex-specific features of spine densities in the hippocampus

Previously, we found that in dissociated hippocampal cultures the proportion of large spines (head diameter ≥ 0.6 μm) was larger in cultures from female than from male animals. In order to rule out that this result is an in vitro phenomenon, we analyzed the density of large spines in fixed hippocampal vibratome sections of Thy1-GFP mice, in which GFP is expressed only in subpopulations of neurons. We compared spine numbers of the four estrus cycle stages in females with those of male mice. Remarkably, total spine numbers did not vary during the estrus cycle, while estrus cyclicity was evident regarding the number of large spines and was highest during diestrus, when estradiol levels start to rise. The average total spine number in females was identical with the spine number in male animals. The density of large spines, however, was significantly lower in male than in female animals in each stage of the estrus cycle. Interestingly, the number of spine apparatuses, a typical feature of large spines, did not differ between the sexes. Accordingly, NMDA-R1 and NMDA-R2A/B expression were lower in the hippocampus and in postsynaptic density fractions of adult male animals than in those of female animals. This difference could already be observed at birth for NMDA-R1, but not for NMDA-R2A/B expression. In dissociated embryonic hippocampal cultures, no difference was seen after 21 days in culture, while the difference was evident in postnatal cultures. Our data indicate that hippocampal neurons are differentiated in a sex-dependent manner, this differentiation being likely to develop during the perinatal period.

Deficit of Long-Term Potentiation Induction, but Not Maintenance, in the Juvenile Hippocampus after Neonatal Proinflammatory Stress

CA3-CA1 long-term potentiation (LTP) in the hippocampal slices from juvenile Wistar rats was studied to reveal factors potentially contributing to different sensitivity to neonatal proinflammatory stress (NPS). NPS was induced by intra-peritoneal injections of bacterial lipopolysaccharide (LPS) to neonatal rats (two injections of LPS, or saline in the control group, consecutively on postnatal days 3 and 5 [PND3 and PND5]). In females, a significant effect of NPS on hippocampus development was associated with modifications of long-term synaptic plasticity, the synapses becoming more resistant to LTP induction. LTP deficit in the slices of the NPS group was not associated with a decrease in LTP maintenance, since late LTP generally corresponded to early LTP magnitude, similar in all groups. Moreover, partial correlation revealed significantly higher residual LTP 1 h after high-frequency stimulation in the NPS groups compared to the corresponding value of early LTP in the control groups, suggesting improved consolidation. Both effects were evident in NPS females. A number of males responded to NPS similarly to females, while others were relatively resistant to NPS exposure, a significant increase in variability of LTP magnitude being revealed in NPS males compared to respective females and the control groups. We suggest that postnatal development of long-term plasticity after NPS is similar in animals of both sexes; however, additional specific factor(s) may promote a relative resistance of the male brain.

Modulation of the Hypothalamic-Pituitary-Adrenal Axis by Early Life Stress Exposure

Exposure to stress during critical periods in development can have severe long-term consequences, increasing overall risk on psychopathology. One of the key stress response systems mediating these long-term effects of stress is the hypothalamic-pituitary-adrenal (HPA) axis; a cascade of central and peripheral events resulting in the release of corticosteroids from the adrenal glands. Activation of the HPA-axis affects brain functioning to ensure a proper behavioral response to the stressor, but stress-induced (mal)adaptation of the HPA-axis' functional maturation may provide a mechanistic basis for the altered stress susceptibility later in life. Development of the HPA-axis and the brain regions involved in its regulation starts prenatally and continues after birth, and is protected by several mechanisms preventing corticosteroid over-exposure to the maturing brain. Nevertheless, early life stress (ELS) exposure has been reported to have numerous consequences on HPA-axis function in adulthood, affecting both its basal and stress-induced activity. According to the match/mismatch theory, encountering ELS prepares an organism for similar ("matching") adversities during adulthood, while a mismatching environment results in an increased susceptibility to psychopathology, indicating that ELS can exert either beneficial or disadvantageous effects depending on the environmental context. Here, we review studies investigating the mechanistic underpinnings of the ELS-induced alterations in the structural and functional development of the HPA-axis and its key external regulators (amygdala, hippocampus, and prefrontal cortex). The effects of ELS appear highly dependent on the developmental time window affected, the sex of the offspring, and the developmental stage at which effects are assessed. Albeit by distinct mechanisms, ELS induced by prenatal stressors, maternal separation, or the limited nesting model inducing fragmented maternal care, typically results in HPA-axis hyper-reactivity in adulthood, as also found in major depression. This hyper-activity is related to increased corticotrophin-releasing hormone signaling and impaired glucocorticoid receptor-mediated negative feedback. In contrast, initial evidence for HPA-axis hypo-reactivity is observed for early social deprivation, potentially reflecting the abnormal HPA-axis function as observed in post-traumatic stress disorder, and future studies should investigate its neural/neuroendocrine foundation in further detail. Interestingly, experiencing additional (chronic) stress in adulthood seems to normalize these alterations in HPA-axis function, supporting the match/mismatch theory.

Influence of trait anxiety on the effects of acute stress on learning and retention of the passive avoidance task in male and female mice

The influence of anxiety on the effects of acute stress for the acquisition and retention of passive avoidance conditioned task was evaluated in male and female mice. Animals were categorized as high-, medium-, and low-anxiety according to their performance in the elevated plus-maze test. Subsequently, half of the mice in each group were exposed to an acute stressor and assayed in an aversive conditioning test two days later. Exposure to restraint stress before inhibitory avoidance conditioning had a differential impact on the conditioned response of males and females according to their trait anxiety. The acute stressor significantly altered the conditioned response of mice with a high-anxiety level. The long-term effect of the stressor varied for each sex; high-anxiety stressed males showed an enhanced conditioned response with respect to their controls, whereas high-anxiety stressed females presented an impaired performance. These results lead us to believe that the characterization of individuality is an important factor in understanding the interaction between stress and memory for each sex; the trait anxiety of our animals modulated the effects of stress on the conditioned response so that males and females performed in contrasting manners to the same environmental stimuli and experimental conditions.

Electrophysiological insights into the enduring effects of early life stress on the brain

Increasing evidence links exposure to stress early in life to long-term alterations in brain function, which in turn have been linked to a range of psychiatric and neurological disorders in humans. Electrophysiological approaches to studying these causal pathways have been relatively underexploited. Effects of early life stress on neuronal electrophysiological properties offer a set of potential mechanisms for these susceptibilities, notably in the case of epilepsy. Thus, we review experimental evidence for altered cellular and circuit electrophysiology resulting from exposure to early life stress. Much of this work focuses on limbic long-term potentiation, but other studies address alterations in electrophysiological properties of ion channels, neurotransmitter systems, and the autonomic nervous system. We discuss mechanisms which may mediate these effects, including influences of early life stress on key components of brain synaptic transmission, particularly glutamate, GABA and 5-HT receptors, and influences on neuroplasticity (primarily neurogenesis and synaptic density) and on neuronal network activity. The existing literature, although small, provides strong evidence that early life stress induces enduring, often robust effects on a range of electrophysiological properties, suggesting further study of enduring effects of early life stress employing electrophysiological methods and concepts will be productive in illuminating disease pathophysiology.

Nutrition and neurodevelopment: mechanisms of developmental dysfunction and disease in later life

Nutrition plays a central role in linking the fields of developmental neurobiology and cognitive neuroscience. It has a profound impact on the development of brain structure and function and malnutrition can result in developmental dysfunction and disease in later life. A number of diseases, including schizophrenia, may be related to neurodevelopmental insults such as malnutrition, hypoxia, viruses or in utero drug exposure. Some of the most significant findings on nutrition and neurodevelopment during the last three decades, and especially during the last few years, are discussed in this review. Attention is focused on the underlying cellular and molecular mechanisms by which diet exerts its effects. Randomized intervention studies have revealed important effects of early nutrition on later cognitive development, and recent epidemiological findings show that both genetics and environment are risk factors for schizophrenia. Particularly important is the effect of early nutrition on development of the hippocampus, a brain structure important in establishing learning and memory, and hence for cognitive performance. A major aim of future research should be to elucidate the molecular mechanisms underlying nutritionally-induced impairment of neurodevelopment and specifically to determine the mechanisms by which early nutritional experience affects later cognitive performance. Key research objectives should include: (1) increased understanding of mechanisms underlying the normal processes of ageing and neurodegenerative disorders; (2) assessment of the role of susceptibility genes in modulating the effects of early nutrition on neurodevelopment; and (3) development of nutritional and pharmaceutical strategies for preventing and/or ameliorating the adverse effects of early malnutrition on long-term programming.

Sex influences on the neurobiology of learning and memory

In essentially every domain of neuroscience, the generally implicit assumption that few, if any, meaningful differences exist between male and female brain function is being challenged. Here we address how this development is influencing studies of the neurobiology of learning and memory. While it has been commonly held that males show an advantage on spatial tasks, and females on verbal tasks, there is increasing evidence that sex differences are more widespread than previously supposed. Differing performance between the sexes have been observed on a number of common learning tasks in both the human and animal literature, many neither purely spatial nor verbal. We review sex differences reported in various areas to date, while attempting to identify common features of sexually dimorphic tasks, and to place these differences in a neurobiological context. This discussion focuses on studies of four classes of memory tasks for which sex differences have been frequently reported: spatial, verbal, autobiographical, and emotional memory. We conclude that the female verbal advantage extends into numerous tasks, including tests of spatial and autobiographical abilities, but that a small but significant advantage may exist for general episodic memory. We further suggest that for some tasks, stress evokes sex differences, which are not normally observed, and that these differences are mediated largely by interactions between stress and sex hormones.

Memory impairments and hippocampal modifications in adult rats with neonatal isolation stress experience

Early life events have profound consequences. Our research demonstrates that the early life stress of neonatal isolation (1-h individual isolation on postnatal days 2-9) in rats has immediate and enduring neural and behavioral effects. Recently, we showed neonatal isolation impaired hippocampal-dependent context conditioned fear in adult rats. We now expand upon this finding to test whether neonatal isolation impairs performance in inhibitory avoidance and in the non-aversive, hippocampal-dependent object recognition task. In addition to assessments of hippocampal-dependent memory, we examined if neonatal isolation results in cellular alterations in the adult hippocampus. This was measured with antibodies that selectively label calpain-mediated spectrin breakdown product (BDP), a marker of cytoskeletal modification that can have neuronal consequences. Neonatally isolated male and female rats showed impaired performance in both memory tasks as well as elevated BDP levels in hippocampal immunoblot samples. In tissue sections stained for BDP, the cytoskeletal fragmentation was localized to pyramidal neurons and their proximal dendrites. Interestingly, the hippocampal samples also exhibited reduced staining for the postsynaptic marker, GluR1. Neonatal isolation may render those neurons involved in memory encoding to be vulnerable to calpain deregulation and synaptic compromise as shown previously with brain injury. Together with our prior research showing enhanced striatal-dependent learning and neurochemical responsivity, these results indicate that the early experience of neonatal isolation causes enduring yet opposing region-specific neural and behavioral alterations.

Hippocampal long-term potentiation is reduced in mature compared to young male rats but not in female rats

Aging is associated with a decline in cognitive function which could be due to reduced synaptic plasticity. Hippocampal long-term potentiation (LTP) is an activity-dependent form of increased transmission efficacy at synapses that is considered the basis for some forms of learning and memory. We studied the N-methyl-d-aspartic acid (NMDA) receptor-dependent LTP in the CA1 region of hippocampus in young (2 months) and mature (8 months) male and female rats. We have found that in young male rats the tetanus increased the magnitude of excitatory post-synaptic potentials to 204+/-10% of basal while in mature male rats the magnitude of the LTP was significantly lower reaching only 153+/-11% of basal. This decrease did not occur in female rats. Similar changes occurred in the content of the NMDA receptor subunits NR1 and NR2A in hippocampus. The amount of both subunits was reduced significantly (15-16%) in hippocampus of 8-month-old compared with 2-month-old male rats. This decrease was not observed in female rats. Moreover, there is a significant correlation between the content of NR1 subunit and the magnitude of the potentiation. These data suggest that some of the neurobiological changes induced in hippocampus by aging are different in males and females.

Long-term behavioral consequences of brief, repeated neonatal isolation

Rats subjected to stressful stimuli during the stress hyporesponsive period exhibit varied neuroendocrine and behavioral changes as neonates, adolescents and adults. The current work examined the effects of neonatal isolation stress, using a within-litter design, on adult anxiety-related behavior and endocrine stress reactivity. Neonatal rats were isolated daily for 1 h from postnatal day (P) 4 to 9, a manipulation previously shown to induce hypothalamic-pituitary-adrenal (HPA) responses on P9 (Knuth, E.D., Etgen, A.M. (2005) Corticosterone secretion induced by chronic isolation in neonatal rats is sexually dimorphic and accompanied by elevated ACTH. Horm Behav 47:65-75.). Control animals were either handled briefly or left undisturbed (with-dam). Adult rats were tested for anxiety-related behavior using the elevated plus maze and open field, and for endocrine responses following restraint stress. Neonatal isolation decreased center exploration of the open field following 1 h restraint, including decreased time in the center compared to with-dam or handled controls and decreased center entries and distance traveled in the center compared to with-dam controls. It also decreased time in and entries into the open arms of the elevated plus maze compared to handled controls, suggesting enhanced anxiety-related behavior. Neonatal isolation had no effect on basal or restraint-induced levels of ACTH or corticosterone. These findings indicate that neonatal isolation may enhance anxiety-related behaviors, especially in response to stress, without altering HPA function.

Stress effects in the hippocampus: synaptic plasticity and memory

It is now well-documented that exposures to uncontrollable (inescapable and unpredictable) stress in adulthood can have profound effects on brain and behavior. Converging lines of evidence from human and animal studies indicate that stress interferes with subsequent performances on a variety of hippocampal-dependent memory tasks. Animal studies further revealed that stress impedes ensuing induction of long-term potentiation (LTP) in the hippocampus. Because the hippocampus is important for key aspects of memory formation and because LTP has qualities congruent to an information storage mechanism, it is hypothesized that stress-induced modifications in hippocampal plasticity contribute to memory impairments associated with stress. Recent studies provide evidence that the amygdala, a structure important in stress- and emotion-related behaviors, plays a necessary role in the emergence of stress-associated changes in hippocampal LTP and memory. Early life stress also alters hippocampal plasticity and memory in a manner largely consistent with effects of adult stress exposure. This review focuses on endocrine-system-level mechanisms of stress effects in the hippocampus, and how stress, by altering the property of hippocampal plasticity, can subsequently influence hippocampal memory.

Effect of early isolation on signal transfer in the entorhinal cortex–dentate–hippocampal system

Deprivation of socio-sensory interactions during early life impairs brain function in adulthood. In previous investigations we showed that early isolation severely affects neuron development in several structures of the hippocampal region, including the entorhinal cortex. In the present study we investigated the effects of early isolation on signal processing along the entorhinal cortex-dentate-CA3-CA1 system, a major memory circuit of the hippocampal region. Male and female guinea-pigs were assigned at 6-7 days of age to either a social or an isolated environment. At 90-100 days of age the animals were anesthetized and field potentials were recorded from the entorhinal cortex-dentate-CA3-CA1 circuit, driven by dorsal psalterium commissural volleys. Analysis of the input-output function in the different structures showed that in isolated males there was a small reduction in the input-output function of the population excitatory postsynaptic potential and population spike evoked in layer II of the entorhinal cortex. No changes occurred in isolated females. In isolated males and females there was a reduction in the input-output function of the population excitatory postsynaptic potential and population spike evoked in the dentate gyrus, CA3 and CA1, but this effect was larger in males. In isolated males, but not in females, the population spike/population excitatory postsynaptic potential ratio was reduced in all investigated structures, indicating that in males the size of the discharged neuron population was reduced more than due to the decreased input. Results show that isolation reduces the synaptic function in the whole entorhinal cortex-dentate gyrus-CA3-CA1 system. While the entorhinal cortex was moderately impaired, the dentate-hippocampal system was more severely affected. The impairment in the signal transfer along the entorhinal cortex-dentate gyrus-CA3-CA1 system was heavier in males, confirming the larger susceptibility of this sex to early experience. This work provides evidence that malfunctioning of a major hippocampal network may underlie the learning deficits induced by impoverished surroundings during early life.

Ethanol prevents NMDA receptor reduction by maternal separation in neonatal rat hippocampus

We measured the effects of ethanol on glutamate receptor levels in the hippocampus of neonatal Wistar rats using a vapor chamber model. Two control groups were used; a normal suckle group and a maternal separation group. Levels of NMDA receptors were not significantly altered in ethanol-treated animals compared to the normal suckle control group, as shown by [3H]MK-801 binding and Western blot analysis. However, MK-801 binding and NR1 subunit immunoreactivity were greatly reduced in the hippocampus of separation control animals. Neither ethanol treatment nor maternal separation altered levels of GluR1 or GluR2(4). These results have serious implications for the importance of maternal contact for normal brain development.

A selective increase in phosphorylation of cyclic AMP response element-binding protein in hippocampal CA1 region of male, but not female, rats following contextual fear and passive avoidance conditioning

Cyclic AMP response element-binding protein (CREB), a transcription factor on which multiple signal transduction pathways converge, has been implicated in long-term memory. We examined whether the sex difference in the performance of contextual fear or passive avoidance conditioning is associated with a change in the activation of CREB in the hippocampus, a neural structure important for long-term memory. The activation of CREB in different subregions within the hippocampus in male and female rats was determined immunohistochemically with an antibody that specifically recognizes the phosphorylated form of CREB (pCREB). With respect to the freezing time in contextual fear conditioning and the step-through latency in passive avoidance conditioning, male rats exhibited better performance than female rats. Phosphorylation of CREB (% pCREB) as revealed by the ratio of the pCREB-immunoreactive (pCREB-ir) cell number to the CREB-immunoreactive cell number was increased in the CA1 region, but not in CA3, CA4, or in the dentate gyrus following training for both types of conditioning in males. In females, such an increase in % pCREB was not found in any hippocampal subregion at any time after conditioning or by increasing the intensity of foot shock. Orchidectomy in males did not alter either the performance of contextual conditioning or conditioning-induced CREB phosphorylation in CA1. The close relationship between behavioral performance and CREB phosphorylation in the CA1 region suggests that hippocampal CREB is involved in the sex difference in some forms of learning and memory.

Impaired fear memories are correlated with subregion-specific deficits in hippocampal and amygdalar LTP

Inbred mouse strains have different genetic backgrounds that likely influence memory and long-term potentiation (LTP). LTP, a form of synaptic plasticity, is a candidate cellular mechanism for some forms of learning and memory. Strains with impaired fear memory may have selective LTP deficits in different hippocampal subregions or in the amygdala. The authors assessed fear memory in 4 inbred strains: C57BL/6NCrlBR (B6), 129S1/SvImJ (129), C3H/HeJ (C3H), and DBA/2J (D2). The authors also measured LTP in the hippocampal Schaeffer collateral (SC) and medial perforant pathways (MPP) and in the basolateral amygdala. Contextual and cued fear memory, and SC and amygdalar LTP, were intact in B6 and 129, but all were impaired in C3H and D2. MPP LTP was similar in all 4 strains. Thus, SC, but not MPP, LTP correlates with hippocampus-dependent contextual memory expression, and amygdalar LTP correlates with amygdala-dependent cued memory expression, in these inbred strains.

Effect of early isolation on the synaptic function in the dentate gyrus and field CA1 of the guinea pig

We previously reported that neonatal isolation shapes neuron morphology remarkably in the dentate gyrus and hippocampus of the guinea pig, a precocial rodent whose brain is at an advanced stage of maturation at birth. The aim of the present work was to investigate the effects of early isolation on the physiology of the hippocampal trisynaptic circuit. Male and female guinea pigs were assigned at 6-7 days of age to either a social or an isolated environment. After 90-100 days, the animals were anesthetized and electrophysiological experiments were carried out. The monosynaptic response evoked by medial perforant path stimulation in the dentate gyrus (DG) and the following response trisynaptically evoked in field CA1 by the DG-CA3 system were evaluated with several stimulus protocols: (1) current source-density (CSD) analysis; (2) input/output function; (3) paired-pulse potentiation (PPP); and (4) long-term potentiation (LTP). Isolated animals exhibited a reduction in the magnitude of the current sinks in the middle molecular layer and granule cell layer of the DG and in the input/output function of the granule cell population excitatory postsynaptic potential (EPSP) and population spike (PS) over a wide range of stimuli. The latter effect was larger in males. The ratio between the PS and EPSP of the granule cells was reduced in isolated compared to control males, but the opposite occurred in females. Isolation affected PPP of the granule cell response in males only, causing a larger facilitation of the PS. No isolation-related effects were found in the magnitude of the LTP of the DG response in either sex. Isolated animals exhibited a reduction in the current sinks in stratum radiatum and stratum pyramidale of field CA1 and in the input/output function of the EPSP and PS of field CA1. These effects were larger in males. The results show that early isolation causes a reduction in the synaptic function of the DG-CA3-CA1 system, driven by perforant path volleys. The isolation-induced impairment in signal processing along the hippocampal network suggests that the outcome of early isolation may be an impairment in the memory functions in which the entorhinal-hippocampal system plays a key role.

Gender specific effect of neonatal handling on stress reactivity of adolescent rats

Early neonatal handling of rat pups produces dampened hypothalamic-pituitary-adrenal axis reactivity to stress in adult male offspring. However, less is known about whether there is a similar effect for females. Although, most studies of neonatal handling have examined subsequent effects during adulthood, adolescence is an important developmental stage for stress responsivity. To address these issues, the effect of neonatal handling on the endocrine stress response and brain activity of male and female rats was determined in response to acute restraint stress during adolescence. Consistent with previous findings in adult males, neonatal handling reduced restraint stress-induced hormone levels in adolescent males. However, in contrast, we found elevated plasma hormone concentrations in handled females. A gender-specific handling effect on brain activity was also evident, with significantly increased stress-induced activation of the posterior cingulate cortex of handled females, as measured by c-fos mRNA expression. The striking gender difference in the effect of early neonatal handling provides evidence that this must be considered as an important variable in subsequent stress responsivity induced by early manipulations.

State markers of depression in sleep EEG: dependency on drug and gender in patients treated with tianeptine or paroxetine

Tianeptine enhances while paroxetine inhibits serotonin reuptake into neurons; however, both show an antidepressive action. A subgroup of 38 depressed patients from a drug trial comparing the efficacy of tianeptine with that of paroxetine was studied with regard to their effects on sleep regulation, especially in relation to treatment response. We recorded sleep EEGs at day 7 and day 42 after the start of treatment with either compound, which allows measurement of changes due to the antidepressive medication in relation to the duration of treatment. Spectral analysis of the non-REM sleep EEG revealed a strong decline in the higher sigma frequency range (14-16 Hz) in male treatment responders independent of medication, whereas nonresponders did not show marked changes in this frequency range independent of gender. The patients receiving paroxetine showed less REM sleep and more intermittent wakefulness compared to the patients receiving tianeptine. REM density after 1 week of treatment was a predictor of treatment response in the whole sample. Psychopathological features with regard to the score in single items of the HAMD revealed predictive markers for response, some of which were opposite in the gender groups, especially those related to somatic anxiety. Changes in REM density were inversely correlated to the changes in HAMD in the paroxetine, but not the tianeptine, group. Our data suggest the importance of taking gender into account in the study of the biological effects of drugs. The study further points to the importance of the higher sigma frequency range in the sleep EEG of non-REM sleep and REM density as a marker of treatment response.

Neonatal social isolation alters both maternal and pup behaviors in rats

The development of emotional behavior is dependent on the early experiences of the infant and the quality of maternal care. In these experiments, the effects of social isolation during the preweaning period on both pup behavior and maternal responsivity were examined. In the first study, the number of ultrasonic vocalizations (USVs) emitted after brief maternal separation was measured in neonatal rats with differing histories of social isolation. The social isolation procedure consisted of 5 days of daily separation from the dam and littermates for either 3 or 6 hr. At both ages tested, socially isolated pups vocalized significantly less than control pups. In the second study, the effects of prior isolation either daily for 5 previous days (Chronic Isolation) or for 4 hr prior to testing (Acute Isolation) were examined in a T-maze choice test. Pup vocalizations in the presence of the dam and dams' maternal behavior were assessed. When the dam was confined to the start box or during the maternal free access period, both Chronic and Acute Isolates vocalized less than pups that had never left the home nest. Dams spent more time with and licked and groomed more frequently and for a longer time both Chronic and Acute Isolates compared to pups that had always been with dams in the home nest. These results suggest that early isolation experience can alter subsequent responses to separation stress in neonatal rats and that maternal behavior is sensitive to the prior experiences of offspring.

Antidepressant mechanisms: functional and molecular correlates of excitatory amino acid neurotransmission

Specific targeting of the serotonergic and noradrenergic systems for the development of antidepressant compounds has resulted in drugs with more favourable side-effect profiles but essentially no greater efficacy than those compounds discovered more than 40 years ago. Alternative targets are now being considered in the hope that they will have a faster onset of action and be useful for those patients currently unresponsive to conventional treatments. Excitatory amino acid neurotransmission has been attributed various roles in both normal and abnormal brain function. The N-methyl-D-aspartate receptor in particular has long been postulated to play a role in the formation of memories. Major depressive disorder is characterised by alterations in cognitive function, as well as affect. Although there is evidence that early adverse events and stress can have a causal influence on depression, the underlying neurobiology of the disorder is poorly understood. This review will document current evidence for the involvement of excitatory amino acid neurotransmission in the pathophysiology of the affective disorders. The preclinical literature suggests that both electroconvulsive stimulation and antidepressant drugs can affect hippocampal long-term potentiation and the expression of excitatory amino acid receptor subtypes. Exposing animals to stress, including the kind that produces learned helplessness, can also affect synaptic plasticity in the hippocampus. There is clinical evidence that patients with chronic depression have structural brain abnormalities, including hippocampal atrophy, and a preliminary study has shown that an N-methyl-D-aspartate receptor antagonist may have antidepressant efficacy.

Neonatal isolation alters stress hormone and mesolimbic dopamine release in juvenile rats

Rat pups were individually isolated from the mother and nest for 1 h/day from postnatal days (PND) 2 to 9 and tested as juveniles (PND 26-30) compared to nonhandled (NH) controls. In response to 1 h of restraint stress, NH rats increased locomotor activity and dopamine (DA) levels, but neonatally isolated (ISO) rats did not. Both groups had increased plasma corticosterone levels in response to restraint, but corticosterone levels were higher in ISO than in NH. Brain allopregnanolone (3alpha,5alpha-THP) levels also increased in response to stress, but NH and ISO did not differ. Sex of the rats was not a factor for any of the measures except plasma corticosterone levels, where females had higher levels than males. These data indicate that the effects of neonatal isolation persist postweaning and that the effects are most evident in response to stress as opposed to under baseline conditions.

Increased extracellular release of hippocampal NE is associated with tetanization of the medial perforant pathway in the freely moving adult male rat

The induction of long-term potentiation (LTP) within the dentate gyrus of the hippocampal formation is modulated by many afferent influences from a number of subcortical structures known to be intimately involved in hippocampal-dependent learning and memory. It has been demonstrated in slice and anesthetized preparations that norepinephrine (NE) is one of these major neuromodulators involved in the induction of LTP. However, the majority of these studies have not been conducted in the freely moving animal. Recently, we developed surgical procedures and instrumentation techniques to simultaneously record electrophysiological and neurochemical data from the hippocampal formation. The present study uses these techniques to examine the underlying neurochemical changes in the hippocampus associated with the induction of hippocampal dentate LTP in the freely moving adult rat. These findings establish baseline levels of NE that can be used to evaluate the impact of various tetanization paradigms as well as the effect of a variety of insults on hippocampal plasticity.

Social isolation stress during the third week of life has age-dependent effects on spatial learning in rats

Despite extensive research on the relationship between acute stress and hippocampal function in adults, little is known about the short- and long-term effects of prolonged juvenile stress on learning, memory, and other hippocampal functions. This experiment investigated whether spatial learning would be altered in juvenile and adult rats previously exposed to a chronic stressor: 6 h of social isolation (SI) daily at 15-21 days of age. SI was found to increase circulating plasma levels of corticosterone (CORT) and allopregnanolone (3-alpha,5-alpha-pregnan-20-one; 3,5-THP) at 1 h after separation on the fourth day, indicating that the isolation was an effective stressor. When tested as juveniles (post-natal (PN) 22-24), spatial learning was impaired on the Morris water maze in the previously isolated subjects compared to non-isolated controls. However, when tested as adults (PN 92-94), subjects previously exposed to SI during the third week of life demonstrated more rapid learning of the task than controls. These results are discussed in light of research on the effects of CORT on the developing hippocampus.

Effects of prenatal protein malnutrition and neonatal stress on CNS responsiveness

Maturation of the nervous system and consequent behavior depends in part on prenatal nutritional factors and postnatal environmental stimulation. In particular, the hypothalamus and the hippocampus are two important CNS areas that are vulnerable to such pre- and postnatal manipulations. Therefore, the present study was undertaken to explore the effects of both prenatal protein malnutrition and neonatal isolation stress on hypothalamic and hippocampal functioning in infant rats. Specifically, we assessed the levels of plasma corticosterone, as well as dopamine, serotonin and their metabolites in both the hypothalamus and hippocampus in rat pups that had been prenatally malnourished (6% casein diet) and isolated from nest, dam, and siblings for 1 h daily during postnatal days (PND) 2 through 8. We found that on PND 9 malnourished pups weighed less, had smaller hypothalami and a suppressed corticosterone response to acute and chronic isolation stress. However, their dopamine metabolism in the hypothalamus was increased following acute isolation on PND 9 as seen in isolated controls. Prenatal protein malnutrition also resulted in a significant elevation in serotonin in both brain areas, increased 5HIAA in the hypothalamus, and decreased dopamine in the hippocampus. Repeated isolation caused a reduction in 5HIAA in both brain parts, but only in control pups. These pre- and postnatal challenges may each cause a specific pattern of modifications in the CNS and, in combination, may be additive, particularly in the hypothalamic-pituitary-adrenal (HPA) stress response and the serotonergic functioning in both the hypothalamus and hippocampus, a finding with important clinical implications.

Effects of neonatal corticosterone treatment on maze performance and HPA axis in juvenile rats

Previous research has indicated that administering corticosterone to dams' drinking water for 21 days produced persistent alterations in physiology and behavior. We investigated whether 4 days of corticosterone exposure would have similar effects, and whether greater effects would be found when corticosterone was administered early in neonatal life than later in neonatal life. Sprague-Dawley dams were given either corticosterone (250 microg/ml) in their water bottles for postnatal days (PND) 5-9 (early corticosterone treatment: ECT), PND 13-17 (late corticosterone treatment: LCT) or no treatment (NT). At the end of treatment, corticosterone levels were higher in pups of corticosterone drinking dams. However, at weaning, ECT and LCT pups had lower basal corticosterone levels than NT pups. As juveniles, ECT pups learned to navigate to a visible and then to a nonvisible platform in a Morris water maze more quickly than did LCT and NT pups. Among females, ECT pups had higher corticosterone release in response to stress than LCT and NT pups. There were no differences in hippocampal corticosteroid receptor levels among the groups. The pattern of results is similar to, but not identical to, that found for pups exposed to corticosterone for 21 days. The results also suggest that there is a critical or sensitive period for corticosterone treatment in that early treatment was more effective than later treatment.

Postnatal stress selectively upregulates striatal N-methyl-D-aspartate receptors in male rats

Early life events have been thought to contribute towards vulnerability to drug addiction later in life. In the present investigation, the effect of daily neonatal maternal isolation stress on NMDA channel activity was studied. [3H]MK-801 binding was measured in several brain regions from neonatally isolated (ISO) and nonhandled (NH) adult male and female rats. Maximal [3H]MK-801 binding in the caudate-putamen of male ISO rats was 58% higher compared to same sex NH rats. Unlike male rats, maximal [3H]MK-801 binding in the caudate-putamen of female ISO rats was lower than female NH rats. No other brain region showed any significant difference in maximal [3H]MK-801 binding between ISO and NH male and female rats, respectively. There was no effect of pup isolation on the binding affinity (K(d) value) in either sex. Repeated maternal isolation is associated with alterations in the NMDA channel activity in the caudate-putamen of adult rats, and may be responsible for the augmentation in the addictive behavior reported.

The N-methyl-D-aspartate receptor, synaptic plasticity, and depressive disorder. A critical review

The roles of the N-methyl-D-aspartate (NMDA) receptor and NMDA receptor-mediated synaptic plasticity are reviewed in the context of depressive disorder and its treatment. The mode of action of antidepressant treatment is poorly understood. Animal studies have suggested that many antidepressant drugs show activity at the NMDA receptor and that NMDA antagonists have antidepressant profiles in preclinical models of depression. A post-mortem study in humans has suggested that certain binding characteristics of the NMDA receptor may be down-regulated in the brains of suicide victims. "Depressogenic" stressors in animals and chronic administration of antidepressant agents perturb NMDA-dependent synaptic plasticity in the hippocampus.

Neonatal stress alters LTP in freely moving male and female adult rats

We previously reported that neonatal isolation stress significantly changes measures of hippocampal long-term potentiation (LTP) in male and female juvenile rats, i.e., at 30 days of age. The changes in dentate granule population measures, i.e., excitatory postsynaptic potential (EPSP) and population spike amplitude (PSA), evoked by tetanization of the medial perforant pathway, indicated that juvenile rats exposed to neonatal isolation exhibit different enhancement profiles with respect to both the magnitude and duration of LTP in a sex-specific manner. Isolated males showed a significantly greater enhancement of LTP, while female "isolates" showed significantly longer LTP duration when compared to all other groups. The present study was designed to determine whether the effects of the neonatal isolation stress paradigm endures into adulthood. Rats isolated from their mothers for 1 h per day during postnatal days 2-9 were surgically prepared at 70-90 days of age, with stimulating and recording electrodes placed in the medial perforant pathway and the hippocampal dentate gyrus, respectively. Prior to tetanization, no significant effect of sex or treatment was obtained for baseline measures of EPSP slope or PSA. In order to rule out baseline differences in hippocampal cell excitability in female adult rats, we measured the response of dentate granule cells for one estrus cycle and found no pretetanization enhancement in the evoked response in either controls or previously stressed rats. Following tetanization, there was a significant treatment and sex effect. During the induction of LTP, PSA values were significantly enhanced in both isolated males and females and had significantly longer LTP duration when compared to the unhandled control group. Additionally, we observed that females took longer to reach baseline levels than males. Taken together, these results indicate that repeated infant isolation stress enhances LTP induction and duration in both males and females. These results indicate that infant stress alters hippocampal neuroplasticity in such a way that its effect endures into adulthood.

Effects of formalin pain on hippocampal c-Fos expression in male and female rats

Immediate early genes are crucial intermediates in a cascade linking membrane stimulation to long-term alterations of neuronal activity. In the present experiment, we performed immunohistochemistry for c-Fos to determine the effects of persistent pain on cells of the hippocampus of male and female rats. Animals were subcutaneously injected with formalin (50 microl, 10%) and perfused: 2 h later, time 2; 24 h later, time 24; 24 h later after 20 min of the open-field test, time 24/OF. Controls were left undisturbed. In control, c-Fos was higher in females than in males in all hippocampal fields. In males at time 2, formalin increased c-Fos in the dentate gyrus (DG) and CA3 fields; at time 24, c-Fos returned to the control level; at time 24/OF, c-Fos was higher than in control in the DG, but not in the other fields. In the formalin-treated females at time 2 and at time 24, c-Fos levels were lower, or tended to be lower, than in control in all hippocampal fields; at time 24/OF, c-Fos levels in the DG were higher than in control and in males. In conclusion, persistent pain had different effects on c-Fos in the hippocampal subfields, depending on the time after treatment and the sex of the subject.

Ontogeny of gender-specific responsiveness to stress and glucocorticoids in the rat and its determination by the neonatal gonadal steroid environment

The neuroendocrine response to stress in the rat displays gender-specific characteristics resulting from both sex hormone-dependent organization of neuroendocrine regulatory mechanisms and the modulatory action of circulating gonadal steroids. To define the role of gonadal steroid-mediated brain differentiation in the emergence of sex-specific differences in pituitary-adrenal function, and the necessity of physiological gonadal secretions for the manifestation of these differences, we examined the ontogeny of diurnal and stress-induced corticosterone (B) secretion, and suppressibility of the latter by dexamethasone (DEX) in intact male and female rats, and in animals that were subject to neonatal manipulations of the gonadal steroid environment (orchidectomy in males and neonatal estrogenization in females). Further, gene expression of corticosteroid receptors (MR and GR), corticotropin-releasing hormone (CRH) and arginine-vasopressin (AVP) under basal conditions, and following adrenalectomy (ADX) and chronic supplementation with high doses of B, were investigated in adult male and female rats, and individuals of both sexes which have been exposed to alterations of the gonadal steroid milieu during early development. The results demonstrate that: i) gender-specific differences in basal and stress-induced adrenocortical secretion are present at birth, but are still maleable by neonatal alterations of the gonadal steroid environment; ii) gender-specific dichotomy in the sensitivity of the secretory stress response to glucocorticoid feedback becomes fully manifest in adulthood; iii) sex differences in basal adrenocortical secretion become fully expressed only in the presence of intact gonads, whereas, once established by the neonatal hormonal milieu, differential sensitivity of the stress response to glucocorticoids persists in the absence of functioning gonads; iv) neonatal hormone manipulations alter sex-specific characteristics of CRH, AVP, MR and GR gene expression in the brain, and the changes persist in adulthood independently of gonadal secretions; v) regulation of CRH gene expression by glucocorticoids displays gender-specific patterns which are probably established during the period of sex hormone-dependent brain organization and their manifestation does not require physiological gonadal secretions in adulthood.

Hippocampal neurochemical and electrophysiological measures from freely moving rats

This paper describes surgical and recording procedures that have been developed which permit the simultaneous monitoring of levels of select neurochemicals (via microdialysis) and measures of dentate-evoked field potentials within the hippocampal formation of freely moving adult rats. To test and evaluate these procedures, they were employed to examine changes in hippocampal neurochemistry and neuronal excitability associated with the establishment and maintenance of hippocampal long-term potentiation (LTP). Measures of hippocampal norepinephrine (NE) and glutamate levels along with measures of the dentate granule cell population spike amplitude (PSA) were obtained before, during, and after tetanization of the medial perforant path using two separate tetanization paradigms. Results obtained using these new procedures in several animals indicated that changes in NE and glutamate levels were strongly correlated with increases in the dentate granule cell PSA measure obtained following tetanization. The results indicate that this newly developed procedure can be effectively used to directly examine the relationship between neurochemical and neurophysiological changes associated with hippocampal neuroplasticity.

Sexual differentiation of the vertebrate brain: principles and mechanisms

A wide variety of sexual dimorphisms, structural differences between the sexes, have been described in the brains of many vertebrate species, including humans. In animal models of neural sexual dimorphism, gonadal steroid hormones, specifically androgens, play a crucial role in engendering these differences by masculinizing the nervous system of males. Usually, the androgen must act early in life, often during the fetal period to masculinize the nervous system and behavior. However, there are a few examples of androgen, in adulthood, masculinizing both the structure of the nervous system and behavior. In the modal pattern, androgens are required both during development and adulthood to fully masculinize brain structure and behavior. In rodent models of neural sexual dimorphism, it is often the aromatized metabolites of androgen, i.e., estrogens, which interact with estrogen receptors to masculinize the brain, but there is little evidence that aromatized metabolites of androgen play this role in primates, including humans. There are other animal models where androgens themselves masculinize the nervous system through interaction with androgen receptors. In the course of masculinizing the nervous system, steroids can affect a wide variety of cellular mechanisms, including neurogenesis, cell death, cell migration, synapse formation, synapse elimination, and cell differentiation. In animal models, there are no known examples where only a single neural center displays sexual dimorphism. Rather, each case of sexual dimorphism seems to be part of a distributed network of sexually dimorphic neuronal populations which normally interact with each other. Finally, there is ample evidence of sexual dimorphism in the human brain, as sex differences in behavior would require, but there has not yet been any definitive proof that steroids acting early in development directly masculinize the human brain.

Corticosterone release in response to repeated, short episodes of neonatal isolation: evidence of sensitization

Repeated isolation of neonatal rats produces persistent changes in physiology and behavior. In Experiment 1, we examined changes in plasma corticosterone (CORT) levels as a possible mechanism for the effects of isolation. Pups that were isolated from their mother and the nest for 1 h per day on postnatal days (PND) 2-9 were compared to control litters of pups that were either nonhandled or handled but not isolated. On PND 2, compared to nonhandled pups, handled pups had elevated CORT levels that returned to baseline levels within 30 to 60 min of return to the home cage. No significant elevation of CORT levels were found in handled pups on PND 9. The CORT levels of isolated pups were over twice those of nonhandled pups on PND 2 and four times those of nonhandled pups on PND 9. In Experiment 2, we investigated whether the increased CORT release in response to isolation on PND 9 was the result of the pups' treatment on the previous six days as against an effect of maturation. Plasma CORT levels were measured in rat pups that were either isolated, handled or nonhandled on PNDs 2-8 during the conditions of isolation, handling and nonhandling on PND 9. There were no differences among the groups in basal plasma levels of CORT. Handling on PND 9 did not result in elevated CORT levels in any of the groups. All three groups showed a significant increase in plasma CORT levels after isolation on PND 9. However, the CORT response to isolation of pups previously isolated on PND 2-8 were significantly higher than pups that were either handled or nonhandled on PNDs 2-8. Thus, daily episodes of isolation potentiate the hypothalamic-pituitary-adrenal response to stress.