Cellular mechanisms underlying the effects of an early experience on cognitive abilities and affective states

Effects of early life adversities upon memory processes and cognition in rodent models

Exposure to stressors in early postnatal life induces long-lasting modifications in brainfunction.Thisplasticity,an essential characteristic of the brain that enables adaptation to the environment, may also induce impairments in some psychophysiological functions, including learning and memory. Early life stress (ELS) has long-term effects on thehypothalamic-pituitary-adrenal axisresponse to stressors, and has been reported to lead toneuroinflammation,altered levelsof neurotrophic factors, modifications inneurogenesis andsynaptic plasticity,with changes in neurotransmitter systems and network functioning. In this review, we focus on early postnatal stress in animal models and their effects on learning and memory.Many studies have reported ELS-induced impairments in different types of memories, including spatial memory, fear memory, recognition (both for objects and social) memory, working memory and reversal learning. Studies are not always in agreement, however, no effects, or sometimes facilitation, being reported, depending on the nature and intensity of the early intervention, as well as the age when the outcome was evaluated and the sex of the animals. When considering processes occurring after consolidation, related with memory maintenance or modification, there are a very reduced number of reports. Future studies addressing the mechanisms underlying memory changes for ELS should shed some light on the understanding of the different effects induced by stressors of different types and intensities on cognitive functions.

The impact of environmental and social factors on learning abilities: a meta-analysis

Since the 1950s, researchers have examined how differences in the social and asocial environment affect learning in rats, mice, and, more recently, a variety of other species. Despite this large body of research, little has been done to synthesize these findings and to examine if social and asocial environmental factors have consistent effects on cognitive abilities, and if so, what aspects of these factors have greater or lesser impact. Here, we conducted a systematic review and meta-analysis examining how different external environmental features, including the social environment, impact learning (both speed of acquisition and performance). Using 531 mean-differences from 176 published articles across 27 species (with studies on rats and mice being most prominent) we conducted phylogenetically corrected mixed-effects models that reveal: (i) an average absolute effect size |d| = 0.55 and directional effect size d = 0.34; (ii) interventions manipulating the asocial environment result in larger effects than social interventions alone; and (iii) the length of the intervention is a significant predictor of effect size, with longer interventions resulting in larger effects. Additionally, much of the variation in effect size remained unexplained, possibly suggesting that species differ widely in how they are affected by environmental interventions due to varying ecological and evolutionary histories. Overall our results suggest that social and asocial environmental factors do significantly affect learning, but these effects are highly variable and perhaps not always as predicted. Most notably, the type (social or asocial) and length of interventions are important in determining the strength of the effect.

Serotonin depletion during the postnatal developmental period causes behavioral and cognitive alterations and decreases BDNF level in the brain of rats

A survey of the literature indicates that the developmental disruptions in serotonin (5-HT) levels can influence the brain development and the function. To the best of our knowledge, so far, there are a few studies about the effects of developmental period 5-HT depletion on cognition and behavior of adult male and female rats. Therefore, in the present study, we examined the effects of postnatal days (PND 10-20) administration of para-chlorophenylalanine (PCPA, 100 mg/kg, s.c) a 5-HT synthesis inhibitor, on anxiety-related behaviors, pain sensitivity, short-term recognition memory, and hippocampal and prefrontal cortex (PFC) brain-derived neurotrophic factor (BDNF) mRNA expression in adult male and female rats. Novel object recognition memory (NORM) and behavioral parameters (anxiety-like behaviors and pain sensitivity) were evaluated in early adulthood and after that, the hippocampi and PFC of the rat's brain were removed for the determination of BDNF mRNA expression. Our results indicated that the postnatal period administration of PCPA impaired short-term NORM. The postnatal developmental period treatment with PCPA also increased anxiety-like behaviors in the open field and elevated plus maze (EPM) tests. Postnatal PCPA treatment increased pain sensitivity in the hot plate test in both male and female rats, especially in female animals. In addition, postnatal days serotonin depletion decreased BDNF level in the hippocampus and PFC of both male and female rats. These findings demonstrate that serotonin plays the main role in neurodevelopment, cognitive functions, and behavior. Therefore, serotonergic system dysregulation during the developmental periods may have more adverse influences on the brain development of rats.

Neonatal handling increases neurogenesis, BDNF and GR in the hippocampus favoring memory acquisition in rats

Early life is a critical period for the development of the central nervous system (CNS) when the brain undergoes functional organization, neuronal proliferation and migration. This study aimed to evaluate influences and possible interactions of the neonatal handling (NH) on morphologic, biochemical and molecular markers in the hippocampus, as well as on Mu opioid receptors (MOR) immunoreactivity when adolescent rats were exposed to morphine. On postnatal day (PND) 1, male pups were assigned to two experimental groups: unhandled (UH) or neonatal handling (NH), whose procedure was applied from PND2 to PND9. On PND 50, animals were submitted to memory behavioral test, anesthesia and euthanasia for blood collection and hippocampus removal. Animals exposed to NH showed: i) increased levels of proBDNF and brain-derived neurotrophic factor (BDNF); ii) increased memory performance; iii) decreased lipid peroxidation (LP) in plasma and hippocampus; iv) increased antioxidant defenses; v) increased glucocorticoids receptor (GR) levels. Interestingly, our data showed a positive correlation between BDNF and working memory after NH procedure (r² = 0.73; P = 0.006). Animals submitted to NH showed an increased per se of MOR immunoreactivity regardless of morphine exposure, while this increasing was also observed in the UH group after morphine exposure, even in a small extent. NH beneficial influence during early stage of life can be reflected during the development of the puppies, enhancing memory performance, preventing oxidative events and improving molecular targets in hippocampus. Further experimental studies in addition to clinical ones are needed to validate NH protocol as a therapeutic tool.

Adulthood behavioral and neurodevelopmental effects of being raised byan ambivalent mother in rats: what does not kill you makes you stronger

BACKGROUND/AIM

This study aimed to investigate the effects of early adverse life events and being raised by an ambivalent mother on rats.

MATERIALS AND METHODS

The rats were separated into four groups: 1) the control group (n = 12), which was raised under standard care; 2) the early handling (EH) group, which was raised using an EH model (n = 16); 3) the early deprivation (ED) group, which was raised using an ED model (n = 13), and 4) the ambivalent mother (AM) group, which spent 3 h/day with a "fake mother" (n = 17). When they became adults, their anxiety levels, depressive-like behaviors, and memory functions were measured using the elevated plus maze test, the forced swim test, and the novel object recognition test, respectively. Their neurodevelopment was evaluated by measuring the brain-derived neurotrophic factor (BDNF) levels in the prefrontal cortex, the dentate gyrus, and the cerebellum via ELISA.

RESULTS

The rats in the ED and AM groups exhibited less anxiety and depressive-like behavior than those in the control and EH groups, particularly in females. There was no significant difference between the groups in memory function or brain BDNF levels.

CONCLUSION

Severe and ambivalent early adverse life events may decrease anxiety and depressive-like behavior in adult rats.

How postnatal insults may program development: studies in animal models

During the postnatal period, the nervous system is modified and shaped by experience, in order to adjust it to the particular environment in which the animal will live. This plasticity, one of the most remarkable characteristics of the nervous system, promotes adaptive changes, but it also makes brain more vulnerable to insults. This chapter will focus on the effects of interventions during the postnatal development in animal models of neonatal handling (usually up to 15 min of handling) and maternal separation (usually at least for 3 h). Sex-specific changes and effects of prepubertal stress such as social isolation later on in life were also considered. These interventions during development induce long-lasting traces in the pups' nervous system, which will be reflected in changes in neuroendocrine functions, including the hypothalamus-pituitary-adrenal and hypothalamus-pituitary-gonadal axes; anxiety and cognitive performance; and feeding, sexual, and social behavior. These enduring changes may be adaptive or maladaptive, depending on the environment in which the animal will live. The challenge researchers facing now is to determine how to reverse the deleterious effects that may result from early-life stress exposure.

Neonatal handling: an overview of the positive and negative effects

As one of the first rodent models designed to investigate the effects of early-life experiences, the neonatal handling paradigm has helped us better understand how subtle changes in the infant environment can powerfully drive neurodevelopment of the immature brain in typical or atypical trajectories. Here, we review data from more than 50 years demonstrating the compelling effects of neonatal handling on behavior, physiology, and neural function across the lifespan. Moreover, we present data that challenge the classical view of neonatal handling as an animal model that results only in positive/beneficial outcomes. Indeed, the overall goal of this review is to offer the suggestion that the effects of early-life experiences-including neonatal handling-are nuanced rather than unidirectional. Both beneficial and negative outcomes may occur, depending on the parameters of testing, sex of the subject, and neurobehavioral system analyzed.

Neonatal handling alters the structure of maternal behavior and affects mother-pup bonding

During early life, a mother and her pups establish a very close relationship, and the olfactory learning of the nest odor is very important for the bond formation. The olfactory bulb (OB) is a structure that plays a fundamental role in the olfactory learning (OL) mechanism that also involves maternal behavior (licking and contact). We hypothesized that handling the pups would alter the structure of the maternal behavior, affect OL, and alter mother-pup relationships. Moreover, changes in the cyclic AMP-response element binding protein phosphorylation (CREB) and neurotrophic factors could be a part of the mechanism of these changes. This study aimed to analyze the effects of neonatal handling, 1 min per day from postpartum day 1 to 10 (PPD 1 to PPD 10), on the maternal behavior and pups' preference for the nest odor in a Y maze (PPD 11). We also tested CREB's phosphorylation and BDNF signaling in the OB of the pups (PPD 7) by Western blot analysis. The results showed that handling alters mother-pups interaction by decreasing mother-pups contact and changing the temporal pattern of all components of the maternal behavior especially the daily licking and nest-building. We found sex-dependent changes in the nest odor preference, CREB and BDNF levels in pups OB. Male pups were more affected by alterations in the licking pattern, and female pups were more affected by changes in the mother-pup contact (the time spent outside the nest and nursing).

Sex and litter effects on anxiety and DNA methylation levels of stress and neurotrophin genes in adolescent rats

Maternal care variations associate with DNA methylation of the glucocorticoid receptor gene, Nr3c1, in hippocampus at a nerve-growth factor-inducible protein 1 binding site. Epigenetic regulation of brain-derived neurotrophin factor is affected by early stress. These systems contribute to anxiety and fear. Early stress has sex-dependent effects perhaps reflecting sex differences in maternal care. Altering litter gender composition affects maternal behavior and DNA methylation levels of another gene in hippocampus and nucleus accumbens (NAc). We now test if DNA methylation levels of Nr3c1, Egr1, and Bdnf differ by litter composition or sex. Rats from mixed- or single-sex litters were tested for anxiety and fear on postnatal day 35. Brain tissues were collected and analyzed using direct sequencing methods. Females showed hypermethylation of Nr3c1 of hippocampal DNA and litter composition modified sex effects on methylation of Egr1 in NAc. Few differences were seen for Bdnf. LGC modified some sex differences in behavior.

Differential effects of repeated immobilization stress in early vs. late postnatal period on stress-induced corticosterone response in adult rats

This study was performed in order to determine how immobilization stress in the early postnatal period or in the late postnatal period affects growth in the developing rat, and the response of the hypothalamus-pituitary-adrenocortical (HPA) axis in adult rats subjected to subsequent novel stresses. In addition, the effects of maternal deprivation (MD) within the same period of exposure to immobilization stress were also examined. We used two different types of immobilization stress and two different types of MD: immobilization stress for 30min per day from postnatal day 7 (P7) to P13 (IS-E group); immobilization stress for 30min from P15 to P21 (IS-L group); MD for 30min per day from P7 to P13 (MD-E group); and MD for 30min per day from P15 to P21 (MD-L group). The IS-E group showed a significant reduction in body weight that was maintained until at least P40 when compared with the control group. On the other hand, the IS-L group showed a significant reduction in body weight at only postnatal day (P) 20 when compared with the control group. Furthermore, the IS-E group showed a larger HPA response to novel stress than the IS-L and control groups in adulthood. The MD-E group showed a significant reduction in body weight that was maintained until at least P20 when compared with the control group, but did not show a larger HPA response to novel stress, except at T30 (30min after exposure to novel stress) than the control group in adulthood. The MD-L group did not show a significant reduction in body weight or increased HPA response when compared with control rats. These results suggest that repeated immobilization stress, but not MD, in the early postnatal period induces long-term effects on growth and HPA response to novel stress in adulthood.

Early life manipulations alter learning and memory in rats

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

Differential effects of social defeat in rats with high and low locomotor response to novelty

We compared the response to repeated social defeat in rats selected as high (HR) and low (LR) responders to novelty. In experiment 1, we investigated the behavioral and neuroendocrine effects of repeated social defeat in HR-LR rats. By the last defeat session, HR rats exhibited less passive-submissive behaviors than LR rats, and exhibited higher corticosterone secretion when recovering from defeat. Furthermore, in the forced swim test, while HR defeated rats spent more time immobile than their undefeated controls, LR rats' immobility was unaffected by defeat. In experiment 2, we compared the effects of repeated social defeat on body, adrenal, thymus, and spleen weights in HR-LR rats; moreover, we compared the effects of repeated social defeat on stress related molecules gene expression in these two groups of rats. Our results show that HR rats exhibited a decrease in thymus weight after repeated social defeat that was not present in LRs. Analyses of in situ hybridization results found HR-LR differences in 5-HT(2a) mRNA levels in the parietal cortex and 5-HT(1a) mRNA levels in the dorsal raphe. Moreover, LR rats had higher glucocorticoid receptor (GR) mRNA expression than HR rats in the dentate gyrus, and repeated social defeat decreased this expression in LR rats to HR levels. Finally, hippocampal mineralcorticoid receptor (MR)/GR ratio was reduced in HR rats only. Taken together, our results show a differential response to social defeat in HR-LR rats, and support the HR-LR model as a useful tool to investigate inter-individual differences in response to social stressors.

Neonatal handling impairs spatial memory and leads to altered nitric oxide production and DNA breaks in a sex specific manner

Early life events lead to behavioral and neurochemical changes in adulthood. The aim of this study is to verify the effects of neonatal handling on spatial memory, nitric oxide (NO) production, antioxidant enzymatic activities and DNA breaks in the hippocampus of male and female adult rats. Litters of rats were non-handled or handled (10 min/day, days 1-10 after birth). In adulthood they were subjected to a Morris water maze or used for biochemical evaluations. Female handled rats showed impairment in spatial learning. They also showed decreased NO production, while no effects were observed in these parameters in male rats. No effects were observed on the number of hippocampal NADPH diaphorase positive cells. In the Comet Assay, male handled rats showed increased DNA breaks index when compared to non-handled ones. We conclude that neonatal handling impairs learning performance in a sex-specific manner, what may be related to NO decreased levels.

5-HT1A receptor gene C -1019 G polymorphism and amygdala volume in borderline personality disorder

Alterations of amygdala structure and function have been repeatedly described in patients with borderline personality disorder (BPD). The aim of our study was to determine whether a functional polymorphism of the 5-hydroxytryptamine(1A) receptor (5-HTR(1A)) gene C -1019 G (identity number: rs6295 G/C) is associated with structural changes of the amygdala in patients with BPD. Twenty-five right-handed female inpatients with BPD according to DSM IV and 25 healthy controls matched for age, sex, handedness and educational status were enrolled. Brain volumetry of the amygdala was performed with a 1.5-T Magnetom Vision apparatus (Siemens, Erlangen, Germany) and analyzed by the software program 'BRAINS'. Patients who have the 5-HTR(1A) gene G allele had significantly smaller amygdala volumes than C/C genotype carriers (P = 0.02). While no difference of allelic distribution between patients and controls was detected, the described effect of 5-HTR(1A) genotype on amygdala volume was found for the whole group of patients, as well as in the subgroup of patients with comorbid major depression (P = 0.004) but not in controls. In contrast to these subgroups of BPD patients who had significant amygdala volume differences, the mean amygdala volume of the whole group of BPD patients was not significantly different from that of controls. In summary, our study provides first evidence that 5-HTR(1A) gene C -1019 G polymorphism is associated with structural changes in the limbic system of BPD patients, a finding that might be disease related and might contribute to explanation of previous discrepant results regarding amygdala volume changes in BPD. Future research is recommended to clarify possible interactions between this functional polymorphism and symptoms, course and treatment responses in this disorder.

Sex- and age-dependent effects of early postnatal sibling deprivation on spatial learning and memory in adult rats

In this study, we investigated the effects of early postnatal sibling deprivation (EPSD) on spatial learning and memory in adult rats. Litters were culled to one pup with its mother on postnatal day (PN) 1 or 7 and their spatial learning and memory ability were examined with Morris water maze in adult. EPSD on PN1 improved, but on PN7 impaired performance of the spatial learning task in adult female rats. However, EPSD did not have any effect on the spatial learning ability in adult male rats.

The neurobiology of autism

Improving clinical tests are allowing us to more precisely classify autism spectrum disorders and diagnose them at earlier ages. This raises the possibility of earlier and potentially more effective therapeutic interventions. To fully capitalize on this opportunity, however, will require better understanding of the neurobiological changes underlying this devastating group of developmental disorders. It is becoming clear that the normal trajectory of neurodevelopment is altered in autism, with aberrations in brain growth, neuronal patterning and cortical connectivity. Changes to the structure and function of synapses and dendrites have also been strongly implicated in the pathology of autism by morphological, genetic and animal modeling studies. Finally, environmental factors are likely to interact with the underlying genetic profile, and foster the clinical heterogeneity seen in autism spectrum disorders. In this review we attempt to link the molecular pathways altered in autism to the neurodevelopmental and clinical changes that characterize the disease. We focus on signaling molecules such as neurotrophin, Reelin, PTEN and hepatocyte growth factor, neurotransmitters such as serotonin and glutamate, and synaptic proteins such as neurexin, SHANK and neuroligin. We also discuss evidence implicating oxidative stress, neuroglial activation and neuroimmunity in autism.

Repeated immobilization stress in the early postnatal period increases stress response in adult rats

Repeated immobilization stress tests in the early postnatal period were performed to determine the effects on the growth of developing rats as well as the response of the HPA axis to subsequent novel stress in adulthood. In addition, effects of maternal deprivation (MD) with the same period of the exposure to immobilization stress were also examined. We used 2 different types of immobilization stress and 2 different types of MD: immobilization stress for 30 min/day from postnatal day 7 (P7) to P13 (IS7-13 group); immobilization stress for 30 min on P7 (IS7 group); MD for 30 min/day from P7 to P13 (MD7-13 group); and MD for 30 min on P7 (MD7 group). Body weights were lower in the IS7-13 group than in the control group from P10 to P50, although body weight gain in the MD7-13 group was only transiently affected. Stress-induced corticosterone levels in the IS7-13 group were higher than in the control group and did not return to baseline levels until at least 120 min after the termination of stress, whereas temporal variations of stress-induced corticosterone levels did not differ between the IS, MD7-13, MD7, and control groups. Repeated immobilization stress in the early postnatal period induced long-term effects on the growth of developing rats and stress response of the HPA axis to the novel stress in adulthood, although a single immobilization stress, periodic MD, or a single MD had little effect.

Cellular mechanisms underlying the effect of a single exposure to neonatal handling on neurotrophin-3 in the brain of 1-day-old rats

Neurotrophin-3 (NT-3) has an important role in brain development and is thus a good candidate molecule to be involved in the cellular mechanisms mediating the effects of early experiences on the brain. In the present work we employed the model of neonatal handling, which is known to affect the ability of the adult organism to respond to stressful stimuli, and determined its effects on NT-3 levels in the rat hippocampus and cortex 2, 4 and 8 h after handling on postnatal day 1. We also recorded maternal behavior during the 8 h following handling. At both the 4 and 8 h time-points there was an increase in NT-3 positive cells in field 1 of Ammon's horn (CA1 area of the hippocampus) and parietal cortex of the handled animals. In the parietal cortex NT-3 levels increased with time following handling: at 8 h there were more NT-3 positive cells than at 4 h. During the 4 h following the end of handling, handled pups were subject to more maternal licking, indicating that the more intense maternal care could underlie the handling-induced increase in NT-3. In the hippocampus, the handling induced increase in NT-3 was cancelled by inhibition of N-methyl-D-aspartate (NMDA), AMPA/kainate, or GABA-A receptors, as well as L-type voltage-gated Ca(2+) channels. It thus appears that neonatal handling activates these neurotransmitter receptors and channels, leading to increased intracellular Ca(2+) and increased NT-3 expression. NT-3 can then activate downstream effectors and exert its morphogenetic actions and thus imprint the effects of handling on the brain.

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

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

Effects of brief and long maternal separations on the HPA axis activity and the performance of rats on context and tone fear conditioning

Previous studies show that early life events result in neurobehavioural alterations that may be either beneficial or detrimental to the stress response. Given the close relationship between corticosterone secretion and mnemonic processes, the purpose of the present study was to investigate the effects of brief (BMS, 15 min) and long maternal separations (LMS, 180 min) on memory tasks in adult rats, assessed by context and tone fear conditioning. At adulthood, males were evaluated for behavioural and hormonal reaction to the training environment, being tested for context fear conditioning; tone fear conditioning; and learning curve in the context fear conditioning, in which rats were daily re-exposed to the context, followed by a brief footshock and in the last day of the experiment (day 5) animals were exposed to the context. Corticosterone and ACTH plasma levels were determined in naïve rats (basal) or 5, 25 or 45 min after each test. Peak ACTH and corticosterone levels were similar among the groups after context fear conditioning; however, levels of CTL rats remained elevated for a longer time. In the learning curve of context fear conditioning, both BMS and LMS rats exhibited less freezing behaviour than CTL rats, without differences in hormone secretion. There was neither an association between activity of the HPA axis and performance on memory tasks nor different activational properties of the tasks on the HPA axis between BMS and LMS rats, i.e., both manipulations lead to similar performance in hippocampus-dependent and independent memory tasks.

Early life modulators and predictors of adult synaptic plasticity

Early life experience can induce long-lasting changes in brain and behaviour that are opposite in direction, such as enhancement or impairment in regulation of stress response, structural and functional integrity of the hippocampus, and learning and memory. To explore how multiple early life events jointly determine developmental outcome, we investigated the combined effects of neonatal trauma (anoxia on postnatal day 1, P1) and neonatal novelty exposure (P2-21) on adult social recognition memory (3 months of age) and synaptic plasticity in the CA1 of the rat hippocampus (4.5-8 months of age). While neonatal anoxia selectively reduced post-tetanic potentiation (PTP), neonatal novel exposure selectively increased long-term potentiation (LTP). No interaction between anoxia and novelty exposure was found on either PTP or LTP. These findings suggest that the two contrasting neonatal events have selective and distinct effects on two different forms of synaptic plasticity. At the level of behaviour, the effect of novelty exposure on LTP was associated with increased social memory, and the effect of anoxia on PTP was not accompanied by changes in social memory. Such a finding suggests a bias toward the involvement of LTP over PTP in social memory. Finally, we report a surprising finding that an early behavioural measure of emotional response to a novel environment obtained at 25 days of age can predict adult LTP measured several months later. Therefore, individual differences in emotional responses present during the juvenile stage may contribute to adult individual differences in cellular mechanisms that underlie learning and memory.

Postnatal stimulation of the pups counteracts prenatal stress-induced deficits in hippocampal neurogenesis

BACKGROUND

Prenatal stress constitutes a developmental risk factor for later psychopathology. The behavioral disorders are sustained by neurobiological alterations including long-term reduction of hippocampal neurogenesis; its deregulation has been involved in cognitive impairments, mood disorders and addiction. A major goal is to define periods in development and strategies for intervening to prevent the effects of early stressful events. We investigated the ability of a postnatal infantile stimulation to prevent prenatal stress-induced alteration in hippocampal neurogenesis.

METHODS

The influence of postnatal handling on prenatal stress-induced changes in hippocampal neurogenesis was examined in 4 and 26 month-old male rats. Three distinct phases of the neurogenesis were studied: proliferation, survival and neuronal differentiation.

RESULTS

Prenatal stress reduced hippocampal cell proliferation all throughout life. Furthermore, the survival rate of newborn cells, the number of immature neurons and the number of differentiated new neurons were reduced in young and old prenatally-stressed rats. All those deleterious effects were counteracted by neonatal handling.

CONCLUSIONS

These data show that finer aspects of brain shaping can be rewired by environmental influences occurring at sensitive phase of development. They also suggest that infantile stimulation may reverse the appearance of behavioral disorders induced by early life stress.

Effect of neonatal handling on serotonin 1A sub-type receptors in the rat hippocampus

Serotonin 1A sub-type receptors play an important role in the etiopathogenesis of depression, which is known to occur more often in females than males. Early experiences can be a predisposing factor for depression; however, the underlying cellular processes remain unknown. In an effort to address such issues, we employed neonatal handling, an experimental model of early experience, which has been previously shown to render females more vulnerable to display enhanced depression-like behavior in response to chronic stress, while it increases the ability of males to cope. In rat pre-pubertal (30 days of age) and adult (90 days) hippocampus, of both males and females, the effect of neonatal handling on serotonin 1A sub-type receptor mRNA and protein levels was determined by in situ hybridization and immunohistochemistry, respectively, while the number of binding sites was determined by in vitro autoradiography using [(3)H]8-hydroxy-2(di-n-propylamino)tetralin as the ligand. Our results revealed a significant sex difference in serotonin 1A sub-type receptor mRNA, protein and binding sites, with females having higher levels than males. Handling resulted in statistically significant decreased numbers of cells positive for serotonin 1A sub-type receptor mRNA or protein, as well as [(3)H]8-hydroxy-2(di-n-propylamino)tetralin binding sites in the area 4 of Ammon's horn and dentate gyrus of both pre-pubertal males and females. In adult animals the number of serotonin 1A sub-type receptor mRNA positive cells was increased as a result of handling in the area 1 of Ammon's horn, area 4 of Ammon's horn and dentate gyrus of males, while it was decreased only in the area 4 of Ammon's horn of females. Furthermore, the number of serotonin sub-type 1A receptor immunopositive cells, as well as [(3)H]8-hydroxy-2(di-n-propylamino)tetralin binding sites was increased in the area 1 of Ammon's horn, area 4 of Ammon's horn and dentate gyrus of handled males, whereas it was decreased in these same brain areas in the handled females. We can thus infer that neonatal handling results in alterations in postsynaptic serotonergic neurotransmission, which may contribute to the sex dimorphic effects of handling as to the vulnerability toward depression-like behavior in response to chronic stressful stimuli.

Cellular mechanisms underlying an effect of “early handling” on pCREB and BDNF in the neonatal rat hippocampus

Early experiences have long-term effects on brain function and behavior. However, the precise mechanisms involved still remain elusive. In an effort to address this issue, we employed the model of "early handling", which is known to affect the ability of the adult organism to respond to stressful stimuli, and determined its effects on hippocampal pCREB and BDNF 2, 4, and 8 h later. 8 h following "handling" on postnatal day 1, there was an increase in pCREB and BDNF positive cells in the hippocampus, a brain area which is a specific target of "handling". On the other hand, vehicle injection resulted in decreased pCREB and BDNF in both handled and non-handled animals 2 and 4 h later. The "handling"-induced increase of pCREB and BDNF was cancelled by inhibition of NMDA, AMPA/kainate, GABA-A, 5-HT1A or 5-HT2A/C receptors, as well as L-type voltage-gated Ca(2+) channels. It thus appears that "early handling" activates these neurotransmitter receptors, leading to increased intracellular Ca(2+), phosphorylation of the transcription factor CREB, and increased BDNF expression. BDNF can then exert its morphogenetic effects and thus "imprint" the effects of "handling" on the brain.