Early maternal deprivation reduces the expression of BDNF and NMDA receptor subunits in rat hippocampus

Maternal deprivation and adolescent cannabinoid exposure impact hippocampal astrocytes, CB1 receptors and brain-derived neurotrophic factor in a sexually dimorphic fashion

We have recently reported that early maternal deprivation (MD) for 24 h [postnatal day (PND) 9-10] and/or an adolescent chronic treatment with the cannabinoid agonist CP-55,940 (CP) [0.4 mg/kg, PND 28-42] in Wistar rats induced, in adulthood, diverse sex-dependent long-term behavioral and physiological modifications. Here we show the results obtained from investigating the immunohistochemical analysis of CB1 cannabinoid receptors, glial fibrillary acidic protein (GFAP) positive (+) cells and brain-derived neurotrophic factor (BDNF) expression in the hippocampus of the same animals. MD induced, in males, a significant increase in the number of GFAP+ cells in CA1 and CA3 areas and in the polymorphic layer of the dentate gyrus (DG), an effect that was attenuated by CP in the two latter regions. Adolescent cannabinoid exposure induced, in control non-deprived males, a significant increase in the number of GFAP+ cells in the polymorphic layer of the DG. MD induced a decrease in CB1 expression in both sexes, and this effect was reversed in males by the cannabinoid treatment. In turn, the drug "per se" induced, in males, a general decrease in CB1 immunoreactivity, and the opposite effect was observed in females. Cannabinoid exposure tended to reduce BDNF expression in CA1 and CA3 of females, whereas MD counteracted this trend and induced an increase of BDNF in females. As a whole, the present results show sex-dependent long-term effects of both MD and juvenile cannabinoid exposure as well as functional interactions between the two treatments.

Testing the cumulative stress and mismatch hypotheses of psychopathology in a rat model of early-life adversity

BACKGROUND

In the present study, we tested both the cumulative stress and the mismatch hypothesis of psychopathology. For this purpose the combined effects of early-life adversity and later-life stress exposure on behavioral markers of psychosis susceptibility were studied in male Wistar rats.

METHOD

Experiment I: rat pups divided on the basis of the levels of their maternal care experience in low, medium or high maternal care groups, were reared post-weaning in groups (Exp. IA) or in social isolation (Exp. IB) and tested at adulthood under basal conditions or after an acute corticosterone (CORT) administration. Maternal care levels were assessed by measuring the dam's licking and grooming (LG) the first postnatal week of life. Experiment II: rat pups exposed as neonates to daily sessions of 8h of maternal separation (MS) on postnatal days 3, 4 and 5 either altogether in their home cage (HOME SEP) or alone in a novel environment (NOVEL SEP), were reared post-weaning in groups and tested at adulthood under basal conditions. Adult testing included behaviors marking psychosis susceptibility: apomorphine-induced gnawing (APO-gnawing), acoustic startle response and its modulation by a prepulse stimulus (PPI). The behavior of the Medium LG offspring was used as baseline reference for all the three experiments.

RESULTS

Experiment I: Low maternal LG history alone had limited effects on the behavior of Wistar offspring, although increased acoustic startle and increased PPI, at high prepulse intensity levels, were observed. When low maternal LG history was combined with post-weaning social isolation, basal APO-gnawing was decreased and PPI increased, compared to High LG and Med LG offspring. This reflects attenuated psychosis susceptibility. High LG offspring reared in isolation displayed, however, the highest APO-gnawing and the lowest PPI levels among rats reared in social isolation, which is indicative for increased psychosis susceptibility. These findings support the mismatch hypothesis. For demonstration of the cumulative stress hypothesis an injection of CORT in the adult Low LG offspring was required that increased APO-gnawing and reduced PPI. This CORT-induced PPI disruption was greatly enhanced after additional isolation rearing. The High LG group, either socially housed or reared in isolation, was resistant to the acute effects of CORT at adulthood. Experiment II: MS increased psychosis susceptibility only in NOVEL SEP rats that had experienced MS in the context of early social isolation. These individuals displayed increased adult APO-gnawing and reduced PPI, if reared post-weaning in a condition that does not match with their early life social environment (i.e. group housing). This finding supports the mismatch hypothesis.

CONCLUSION

The outcome of environmental manipulations on developmental programming of psychosis susceptibility depends on the interplay of early-life adversity and later-life stressors in a manner that supports the mismatch hypothesis. However, evidence for the cumulative stress hypothesis arises if vulnerable individuals are exposed in later life additionally to excess of the stress hormone CORT.

Exposure to prenatal psychobiological stress exerts programming influences on the mother and her fetus

BACKGROUND/AIMS

Accumulating evidence from a relatively small number of prospective studies indicates that exposure to prenatal stress profoundly influences the developing human fetus with consequences that persist into childhood and very likely forever.

METHODS

Maternal/fetal dyads are assessed at ∼20, ∼25, ∼31 and ∼36 weeks of gestation. Infant assessments begin 24 h after delivery with the collection of cortisol and behavioral responses to the painful stress of the heel-stick procedure and measures of neonatal neuromuscular maturity. Infant cognitive, neuromotor development, stress and emotional regulation are evaluated at 3, 6 12 and 24 months of age. Maternal psychosocial stress and demographic information is collected in parallel with infant assessments. Child neurodevelopment is assessed with cognitive tests, measures of adjustment and brain imaging between 5 and 8 years of age.

RESULTS

Psychobiological markers of stress during pregnancy, especially early in gestation, result in delayed fetal maturation, disrupted emotional regulation and impaired cognitive performance during infancy and decreased brain volume in areas associated with learning and memory in 6- to 8-year-old children. We review findings from our projects that maternal endocrine alterations that accompany pregnancy and influence fetal/infant/child development are associated with decreased affective responses to stress, altered memory function and increased risk for postpartum depression.

CONCLUSIONS

Our findings indicate that the mother and her fetus both are influenced by exposure to psychosocial and biological stress. The findings that fetal and maternal programming occur in parallel may have important implications for long-term child development and mother/child interactions.

Unpredictable chronic stress in juvenile or adult rats has opposite effects, respectively, promoting and impairing resilience

We evaluated the effects of early maternal deprivation (MD; age 7-14 days) alone or in combination with unpredictable chronic stress (UCS; MDUN; 28-84 days) on anxiety and learning in 90 days old adult rats. We hypothesized that exposure to both stressors (MDUN) would be more detrimental than exposure to one or neither. Unexpectedly, adult rats from the MDUN group did not differ from control animals, whereas adult MD animals exhibited impaired avoidance learning. We next investigated the effect of juvenile-onset (30-90 days) versus adult-onset (60-90 days) stress on avoidance learning in adulthood (90 days). We found that adult-onset chronic stress impaired avoidance learning and memory whereas juvenile-onset stress did not. Thus, the results again indicate that juvenile exposure to UCS induces resilience rather than impairment.

A single episode of maternal deprivation impairs the motivation for cocaine in adolescent mice

RATIONALE

Early-life adverse events, like maternal deprivation (MD), have been associated with the later development of mood and anxiety disorders. Scarce data are available describing behavioural and endocrine alterations in maternally deprived (DEP) animals during the periadolescent period. We hypothesize that a single episode of MD early in life would alter reward function and lead to a long-lasting behavioural and neuroendocrine changes during adolescence.

OBJECTIVES

Our aim was to evaluate the effects of a single episode of MD in CD1 adolescent mice (postnatal day 35) on a range of tests for anxiety- and depression-related behaviours (open field, elevated plus maze and tail suspension test). We further assess whether these effects could affect cocaine self-administration behaviour. In order to correlate behavioural and neuroendocrine responses to stress, brain-derived neurotrophic factor (BDNF) levels were assessed in brain structures related to emotional and cognitive processes.

RESULTS

During the cocaine self-administration, the time required for achieving the acquisition criteria was significantly increased and the breaking point values in progressive schedule were significantly reduced in DEP adolescent mice, suggesting impairment in rewarding functions. The behavioural tests also confirm an increase in anxiety- and depression-related behaviours in DEP adolescent mice. The results on BDNF level indicated a decrease in response to MD in amygdala and hippocampus, confirming the behavioural data.

CONCLUSIONS

Our findings demonstrated for the first time that a single episode of early MD can impair the motivation for cocaine consumption in adolescent mice and can be associated with anxiety- and depressive-like behaviour.

Gut memories: towards a cognitive neurobiology of irritable bowel syndrome

The brain and the gut are engaged in continual crosstalk along a number of pathways collectively termed the 'brain-gut axis'. Over recent years it has become increasingly clear that dysregulation of the axis at a number of levels can result in disorders such as irritable bowel syndrome (IBS). With recent advances in neuroimaging technologies, insights into the neurobiology of IBS are beginning to emerge. However the cognitive neurobiology of IBS has remained relatively unexplored to date. In this review we summarise the available data on cognitive function in IBS. Moreover, we specifically address three key pathophysiological factors, namely; stress, immune activation and chronic pain, together with other factors involved in the manifestation of IBS, and explore how each of these components may impact centrally, what neurobiological mechanisms might be involved, and consider the implications for cognitive functioning in IBS. We conclude that each factor addressed could significantly impinge on central nervous system function, supporting the view that future research efforts must be directed towards a detailed assessment of cognitive function in IBS.

Neurobiological markers of familial risk for depression

Major depression is associated with a wide range of neurobiological disturbances, including anomalies in the structure and function of cortical and subcortical gray matter and dysregulation of the HPA axis. In this chapter, we review research demonstrating that many of these abnormalities are also present in never-depressed offspring of adults with recurrent depression and discuss how such findings might reflect dysfunctional neuroregulatory systems that precede the onset of this disorder. We also briefly discuss candidate genes and environmental factors that have been posited to be directly involved in the transmission of neural and HPA-axis abnormalities from depressed parents to their offspring, and we review how, by obtaining a better understanding of the neurobiological markers of depression risk, we can facilitate the development of targeted strategies for the prevention and treatment of major depression.

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

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

The hippocampus, neurotrophic factors and depression: possible implications for the pharmacotherapy of depression

Depression is a prevalent, highly debilitating mental disorder affecting up to 15% of the population at least once in their lifetime, with huge costs for society. Neurobiological mechanisms of depression are still not well known, although there is consensus about interplay between genetic and environmental factors. Antidepressant medications are frequently used in depression, but at least 50% of patients are poor responders, even to more recently discovered medications. Furthermore, clinical response only occurs following weeks to months of treatment and only chronic treatment is effective, suggesting that actions beyond the rapidly occurring effect of enhancing monoaminergic systems, such as adaptation of these systems, are responsible for the effects of antidepressants. Recent studies indicate that an impairment of synaptic plasticity (neurogenesis, axon branching, dendritogenesis and synaptogenesis) in specific areas of the CNS, particularly the hippocampus, may be a core factor in the pathophysiology of depression. The abnormal neural plasticity may be related to alterations in the levels of neurotrophic factors, namely brain-derived neurotrophic factor (BDNF), which play a central role in plasticity. As BDNF is repressed by stress, epigenetic regulation of the BDNF gene may play an important role in depression. The hippocampus is smaller in depressed patients, although it is unclear whether smaller size is a consequence of depression or a pre-existing, vulnerability marker for depression. Environmental stressors triggering activation of the hypothalamic-pituitary-adrenal axis cause the brain to be exposed to corticosteroids, affecting neurobehavioural functions with a strong downregulation of hippocampal neurogenesis, and are a major risk factor for depression. Antidepressant treatment increases BDNF levels, stimulates neurogenesis and reverses the inhibitory effects of stress, but this effect is evident only after 3-4 weeks of administration, the time course for maturation of new neurons. The ablation of hippocampal neurogenesis blocks the behavioural effects of antidepressants in animal models. The above findings suggest new possible targets for the pharmacotherapy of depression such as neurotrophic factors, their receptors and related intracellular signalling cascades; agents counteracting the effects of stress on hippocampal neurogenesis (including antagonists of corticosteroids, inflammatory cytokines and their receptors); and agents facilitating the activation of gene expression and increasing the transcription of neurotrophins in the brain.

Prenatal maternal anxiety and early childhood temperament

The consequences of exposure to prenatal maternal anxiety for the development of child temperament were examined in a sample of 120 healthy, 2-year-old children. Prenatal maternal state and pregnancy-specific anxiety (PSA) were measured five times during pregnancy, and maternal state anxiety was measured again at 2 years post partum. Child temperament was measured at 2 years using the Early Childhood Behavior Questionnaire. The relationship between the trajectory of maternal anxiety across gestation and negative affectivity was evaluated using hierarchical linear growth curve modeling. Higher maternal PSA between 13 and 17 weeks of gestation was associated with increased negative temperament in the children. This association could not be explained by postnatal maternal anxiety, demographic, or obstetric factors. Prenatal maternal state anxiety was not associated with child temperament. These findings demonstrate that PSA early in gestation has a distinctive influence on the developing fetus.

Maternal pregnancy-specific anxiety is associated with child executive function at 6-9 years age

Because fetal brain development proceeds at an extremely rapid pace, early life experiences have the potential to alter the trajectory of neurodevelopment, which may increase susceptibility for developmental and neuropsychiatric disorders. There is evidence that prenatal maternal stress and anxiety, especially worries specifically related to being pregnant, influence neurodevelopmental outcomes. In the current prospective longitudinal study, we included 89 women for whom serial data were available for pregnancy-specific anxiety, state anxiety, and depression at 15, 19, 25, 31, and 37 weeks gestation. When the offspring from the target pregnancy were between 6 and 9 years of age, their executive function was assessed. High levels of mean maternal pregnancy-specific anxiety over the course of gestation were associated with lower inhibitory control in girls only and lower visuospatial working memory performance in boys and girls. Higher-state anxiety and depression also were associated with lower visuospatial working memory performance. However, neither state anxiety nor depression explained any additional variance after accounting for pregnancy-specific anxiety. The findings contribute to the literature supporting an association between pregnancy-specific anxiety and cognitive development and extend our knowledge about the persistence of this effect until middle childhood.

Chronic variable physical stress during the peripubertal-juvenile period causes differential depressive and anxiogenic effects in the novelty-seeking phenotype: functional implications for hippocampal and amygdalar brain-derived neurotrophic factor and the mossy fibre plasticity

Experimentally naive rats show variance in their locomotor reactivity to novelty, some displaying higher (HR) while others displaying lower (LR) reactivity, associated with vulnerability to stress. We employed a chronic variable physical stress regimen incorporating intermittent and random exposures of physical stressors or control handling during the peripubertal-juvenile period to assess interactions between stress and the LRHR phenotype in depressive- and anxiety-like behaviors on the forced swim and social interaction tests, respectively. A decrease in immobility in the forced swim test along with a decrease in social contact in the social interaction test were observed in the juvenile HRs, coupled with increases in brain-derived neurotrophic factor (BDNF) mRNA in the hippocampus and in the basolateral amygdala with chronic variable physical stress. In contrast, an increase in immobility in the forced swim test and a decrease in social contact was observed in the LR counterparts coupled with an increase in the BDNF mRNA in the basolateral amygdala following chronic variable physical stress. Furthermore, chronic physical stress led to increased H3 and H4 acetylation at the P2 and P4 promoters of the hippocampal BDNF gene in the HR rats that is associated with increased suprapyramidal mossy fibre (SP-MF) terminal field volume. In contrast, chronic variable physical stress led to decreased H4 acetylation at the P4 promoter, associated with decreased SP-MF volume in the LR rats. These findings show dissociation in depressive- and anxiety-like behaviors following chronic variable physical stress in the juvenile HR animals that may be mediated by increased levels of BDNF in the hippocampus and in the amygdala, respectively. Moreover, chronic variable physical stress during the peripubertal-juvenile period results in opposite effects in depressive-like behavior in the LRHR rats by way of inducing differential epigenetic regulation of the hippocampal BDNF gene that, in turn, may mediate mossy fibre sprouting.

Stress during development alters anxiety-like behavior and hippocampal neurotransmission in male and female rats

Epidemiological data indicate that early stress increases vulnerability to psychiatric disorders, including anxiety and depression. In the present study we sought to investigate the long-term behavioral and neurochemical consequences of increased and sustained corticosterone levels induced by a 24 h bout of maternal deprivation (DEP) imposed on postnatal day 11 (DEP11). As adults, animals were exposed to the elevated plus maze for assessment of anxiety-like behavior and corticosterone response to this challenge, or decapitated for determination of monoamines and amino acid neurotransmitters content in the hippocampus by HPLC method. The results showed that DEP11 male and female rats displayed increased time in the central hub of the maze and more risk assessment behavior, reflecting increased anxiety-like behavior; in addition, these animals continuously secreted corticosterone in response to the behavioral test until the latest time-point, e.g., 60 min post-stress. In males, maternal deprivation increased aspartate and glutamate levels and reduced taurine levels compared to non-deprived (NDEP) rats. DEP11 females displayed reduced noradrenaline, aspartate and GABA levels compared to NDEP counterparts. These results indicate that maternal deprivation at 11 days of age produced changes in hippocampal neurotransmission that may mediate the increased anxiety-like behavior observed in male and female deprived rats. This article is part of a Special Issue entitled 'Anxiety and Depression'.

Lesion-induced synaptic plasticity in the somatosensory cortex of prenatally stressed rats

Prenatal stress exposure causes long-lasting impairments of the behavioral and neuroendocrine responses to later stressors of the offspring. Although mechanisms underlying these effects remain largely unknown, abnormalities in the neuronal plasticity might be responsible for neurobiological alterations. This study used the whisker-to-barrel pathway as a model system to investigate the effects of prenatal stress on lesion-induced plasticity of neurons. Pregnant rats were subjected to immobilization stress during the trigeminal neurogenesis period, corresponding to gestational days 12 to 17, for three hours a day. After birth, the middle row (C) whisker follicles of pups from the control and stressed groups were electrocauterized. Ten days later, tangentially sectioned cortical hemispheres were stained with cytochrome oxidase histochemistry to calculate the volumes of each barrel row (A-E) in both lesioned and intact sides of the cortex, using stereological methods. The adrenal to body weight ratios were significantly increased in stressed animals, when compared to the controls. The pattern and total volume of the barrel subfield remained unaltered, but the lesion-induced map plasticity index, calculated as the D/C ratio, decreased in stressed animals. In addition, the BDNF (Brain Derived Neurotrophic Factor), NT-3 (neurotrophin-3) and the cyclic AMP response element binding protein (CREB) phosphorylation levels in tissue homogenates of the barrel cortices were measured using the ELISA method. In prenatally stressed animals, the BDNF and NT-3 levels were reduced on the lesioned side, but significant CREB activation was observed on the intact side of the barrel cortex. Taken together, the results show that prenatal stress exposure negatively affects critical period plasticity by reducing the expansion of active barrels following peripheral whisker lesion. These changes arise independent of CREB phosphorylation and appear to be mediated by reduced levels of neurotrophins.

Impact of early adverse experience on complexity of adult-generated neurons

New neurons continue to be generated in the dentate gyrus (DG) region of the hippocampus throughout adulthood, and abnormal regulation of this process has emerged as an endophenotype common to several psychiatric disorders. Previous research shows that genetic risk factors associated with schizophrenia alter the maturation of adult-generated neurons. Here, we investigate whether early adversity, a potential environmental risk factor, similarly influences adult neurogenesis. During the first 2 weeks of postnatal life, mice were subject to repeated and unpredictable periods of separation from their mothers. When the mice reached adulthood, pharmacological and retroviral labelling techniques were used to assess the generation and maturation of new neurons. We found that adult mice that were repeatedly separated from their mothers early in life had similar rates of proliferation in the DG, but had fewer numbers of cells that survived and differentiated into neurons. Furthermore, neurons generated in adulthood had less complex dendritic arborization and fewer dendritic spines. These findings indicate that early adverse experience has a long-lasting impact on both the number and the complexity of adult-generated neurons in the hippocampus, suggesting that the abnormal regulation of adult neurogenesis associated with psychiatric disorders could arise from environmental influence alone, or from complex interactions of environmental factors with genetic predisposition.

Circulating levels of brain-derived neurotrophic factor: correlation with mood, cognition and motor function

Brain-derived neurotrophic factor (BDNF) is the most widely distributed neurotrophin in the CNS, where it plays several pivotal roles in synaptic plasticity and neuronal survival. As a consequence, BDNF has become a key target in the physiopathology of several neurological and psychiatric diseases. Recent studies have consistently reported altered levels of BDNF in the circulation (i.e., serum or plasma) of patients with major depression, bipolar disorder, Alzheimer's disease, Huntington's disease and Parkinson's disease. Correlations between serum BDNF levels and affective, cognitive and motor symptoms have also been described. BDNF appears to be an unspecific biomarker of neuropsychiatric disorders characterized by neurodegenerative changes.

Long term sex-dependent psychoneuroendocrine effects of maternal deprivation and juvenile unpredictable stress in rats

We have analysed the long-term psychoneuroendocrine effects of maternal deprivation (MD) [24 h at postnatal day (PND) 9] and/or exposure to chronic unpredictable stress (CUS) during the periadolescent period (PND 28 to PND 43) in male and female Wistar rats. Animals were tested in the elevated plus maze (EPM, anxiety) at PND 44 and in two memory tests, spontaneous alternation and novel object recognition (NOT) in adulthood. The expression of hippocampal glucocorticoid (GR) and mineralocorticoid (MR) receptors, as well as of synaptophysin, neural cell adhesion molecule and brain-derived neurotrophic factor, was analysed by in situ hybridisation in selected hippocampal regions. Endocrine determinations of leptin, testosterone and oestradiol plasma levels were carried out by radioimmunoassay. Young CUS animals showed decreased anxiety behaviour in the EPM (increased percentage of time and entries in the open arms) irrespective of neonatal treatment. Memory impairments were induced by the two stressful treatments as was revealed by the NOT, with males being most clearly affected. Although each stressful procedure, when considered separately, induced different (always decrements) effects on the three synaptic molecules analysed and affected males and females differently, the combination of MD and CUS induced an unique disruptive effect on the three synaptic plasticity players. MD induced a long-term significant decrease in hippocampal GR only in males, whereas CUS tended to increase MR in males and decrease MR in females. Both neonatal MD and periadolescent CUS induced marked reductions in testosterone and oestradiol in males, whereas MD male animals also showed significantly decreased leptin levels. By contrast, in females, none of the hormones analysed was altered by any of the stressful procedures. Taking our data together in support of the 'two-hit' hypothesis, MD during neonatal life and/or exposure to CUS during the periadolescent period induced a permanent deficit in memory, which was accompanied by a decrement in markers for hippocampal plasticity. The long-term effects on body weight and hormone levels, particularly among males, might reflect sex-dependent lasting metabolic alterations as well as an impaired reproductive function.

Social influences on neurobiology and behavior: epigenetic effects during development

The quality of the social environment can have profound influences on the development and activity of neural systems with implications for numerous behavioral and physiological responses, including the expression of emotionality. Though social experiences occurring early in development may be particularly influential on the developing brain, there is continued plasticity within these neural circuits amongst juveniles and into early adulthood. In this review, we explore the evidence derived from studies in rodents which illustrates the social modulation during development of neural systems, with a particular emphasis on those systems in which a long-term effect is observed. One possible explanation for the persistence of dynamic changes in these systems in response to the environment is the involvement of epigenetic mechanisms, and here we discuss recent studies which support the role of these mechanisms in mediating the link between social experiences, gene expression, neurobiological changes, and behavioral variation. This literature raises critical questions about the interaction between neural systems, the concordance between neural and behavioral changes, sexual dimorphism in effects, the importance of considering individual differences in response to the social environment, and the potential of an epigenetic perspective in advancing our understanding of the pathways leading to variations in mental health.

Reduced anxiety-like behavior and central neurochemical change in germ-free mice

BACKGROUND

There is increasing interest in the gut-brain axis and the role intestinal microbiota may play in communication between these two systems. Acquisition of intestinal microbiota in the immediate postnatal period has a defining impact on the development and function of the gastrointestinal, immune, neuroendocrine and metabolic systems. For example, the presence of gut microbiota regulates the set point for hypothalamic-pituitary-adrenal (HPA) axis activity.

METHODS

We investigated basal behavior of adult germ-free (GF), Swiss Webster female mice in the elevated plus maze (EPM) and compared this to conventionally reared specific pathogen free (SPF) mice. Additionally, we measured brain mRNA expression of genes implicated in anxiety and stress-reactivity.

KEY RESULTS

Germ-free mice, compared to SPF mice, exhibited basal behavior in the EPM that can be interpreted as anxiolytic. Altered GF behavior was accompanied by a decrease in the N-methyl-D-aspartate receptor subunit NR2B mRNA expression in the central amygdala, increased brain-derived neurotrophic factor expression and decreased serotonin receptor 1A (5HT1A) expression in the dentate granule layer of the hippocampus.

CONCLUSIONS & INFERENCES

We conclude that the presence or absence of conventional intestinal microbiota influences the development of behavior, and is accompanied by neurochemical changes in the brain.

Association of the brain-derived neurotrophic factor Val66Met polymorphism with hippocampus volumes in drug-free depressed patients

OBJECTIVES

Val66Met BDNF gene polymorphism is shown to affect the function of mature BDNF and mature BDNF plays an important role in the hippocampal neurogenesis and neuronal survival.

METHODS

A relationship of Val66Met BDNF gene polymorphism and hippocampal volumes in 33 MDD patients and 40 healthy controls is investigated. Region of interest analysis was conducted on the images acquired via MRI.

RESULTS

Depressed patients had smaller left hippocampal volumes compared to healthy controls. The diagnosis of MDD was not significantly related to hippocampal volumes among Met carriers; however, among Val homozygotes depressed patients had significantly smaller left hippocampal volumes compared to controls. Although both right and left hippocampal volumes showed nearly significant correlation with the duration of illness, this correlation reached (negative) significant levels only in the right hippocampal volume of the Val homozygotes.

CONCLUSIONS

Val homozygote genotype may serve as a vulnerability factor in MDD regarding hippocampal volume loss. This finding can be considered as a supportive evidence for the neurotrophic factor hypothesis of depression.

Maternal separation as a model of brain-gut axis dysfunction

RATIONALE

Early life stress has been implicated in many psychiatric disorders ranging from depression to anxiety. Maternal separation in rodents is a well-studied model of early life stress. However, stress during this critical period also induces alterations in many systems throughout the body. Thus, a variety of other disorders that are associated with adverse early life events are often comorbid with psychiatric illnesses, suggesting a common underlying aetiology. Irritable bowel syndrome (IBS) is a functional gastrointestinal disorder that is thought to involve a dysfunctional interaction between the brain and the gut. Essential aspects of the brain-gut axis include spinal pathways, the hypothalamic pituitary adrenal axis, the immune system, as well as the enteric microbiota. Accumulating evidence suggest that stress, especially in early life, is a predisposing factor to IBS.

OBJECTIVE

The objective of this review was to assess and compile the most relevant data on early life stress and alterations at all levels of the brain gut axis.

RESULTS

In this review, we describe the components of the brain-gut axis individually and how they are altered by maternal separation. The separated phenotype is characterised by alterations of the intestinal barrier function, altered balance in enteric microflora, exaggerated stress response and visceral hypersensitivity, which are all evident in IBS.

CONCLUSION

Thus, maternally separated animals are an excellent model of brain-gut axis dysfunction for the study of disorders such as IBS and for the development of novel therapeutic interventions.

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.

Early life stress enhances behavioral vulnerability to stress through the activation of REST4-mediated gene transcription in the medial prefrontal cortex of rodents

There is growing evidence suggesting that early life events have long-term effects on the neuroendocrine and behavioral developments of rodents. However, little is known about the involvement of early life events in the susceptibility to subsequent stress exposure during adulthood. The present study characterized the effect of maternal separation, an animal model of early life adversity, on the behavioral response to repeated restraint stress in adult rats and investigated the molecular mechanism underlying behavioral vulnerability to chronic stress induced by the maternal separation. Rat pups were separated from the dams for 180 min per day from postnatal day 2 through 14 (HMS180 rats). We found that, as young adults, HMS180 rats showed a greater hypothalamic-pituitary-adrenal axis response to acute restraint stress than nonseparated control rats. In addition, repeatedly restrained HMS180 rats showed increased depression-like behavior and an anhedonic response compared with nonrestrained HMS180 rats. Furthermore, HMS180 rats showed increased expression of REST4, a neuron-specific splicing variant of the transcriptional repressor REST (repressor element-1 silencing transcription factor), and a variety of REST target gene mRNAs and microRNAs in the medial prefrontal cortex (mPFC). Finally, REST4 overexpression in the mPFC of neonatal mice via polyethyleneimine-mediated gene transfer enhanced the expression of its target genes as well as behavioral vulnerability to repeated restraint stress. In contrast, REST4 overexpression in the mPFC of adult mice did not affect depression-like behaviors after repeated stress exposure. These results suggest that the activation of REST4-mediated gene regulation in the mPFC during postnatal development is involved in stress vulnerability.

The roles of BDNF in the pathophysiology of major depression and in antidepressant treatment

Neurotrophic factors are critical regulators of the formation and plasticity of neuronal networks. Brain-derived neurotrophic factor (BDNF) is abundant in the brain and periphery, and is found in both human serum and plasma. Animal studies have demonstrated that stress reduces BDNF expression or activity in the hippocampus and that this reduction can be prevented by treatment with antidepressant drugs. A similar change in BDNF activity occurs in the brain of patients with major depression disorder (MDD). Recently, clinical studies have indicated that serum or plasma BDNF levels are decreased in untreated MDD patients. Antidepressant treatment for at least four weeks can restore the decreased BDNF function up to the normal value. Therefore, MDD is associated with impaired neuronal plasticity. Suicidal behavior can be a consequence of severe impaired neuronal plasticity in the brain. Antidepressant treatment promotes increased BDNF activity as well as several forms of neuronal plasticity, including neurogenesis, synaptogenesis and neuronal maturation. BDNF could also play an important role in the modulation of neuronal networks. Such a neuronal plastic change can positively influence mood or recover depressed mood. These alterations of BDNF levels or neuronal plasticity in MDD patients before and after antidepressant treatment can be measured through the examination of serum or plasma BDNF concentrations. BDNF levels can therefore be useful markers for clinical response or improvement of depressive symptoms, but they are not diagnostic markers of major depression.

Voluntary exercise and palatable high-fat diet both improve behavioural profile and stress responses in male rats exposed to early life stress: role of hippocampus

Childhood trauma induced by adverse early life experience is associated with increased risk of psychological disorders in adulthood. Disruption of normal development has been shown to affect hippocampal morphology and function, influencing adaptations to stress. Here we investigated whether palatable food and/or exercise would ameliorate the behavioural responses following early life stress in rats. Rats were subjected to 15 (S15) or 180 (S180) minutes separation from dams on postnatal days 2-14. After weaning, rats were assigned to either receive chow (C), high-fat diet (HFD), voluntary exercise (running, R), or combined HFD and R for 11 weeks. In addition to anxiety- and depression-like behaviours, response to restraint stress was measured. Glucocorticoid receptor (GR), brain-derived neurotrophic factor (BDNF) and 5-hydroxytryptamine receptor 1A (5HT1A) receptor mRNA in the hippocampus were measured. S180 rats had similar body weight to S15, however their plasma insulin concentrations were double those of S15 rats when consuming HFD; adding exercise reduced plasma insulin. Anxiety-like behaviour in S180 rats, measured using Light Dark test (LDT) and Elevated Plus Maze (EPM) were ameliorated by the provision of HFD, R or HFD+R. A similar effect was observed on depression-like behaviour assessed by forced swim test (FST), with less time being spent immobile. Exposure to early-life stress during development was associated with significant reductions in hippocampal GR, 5HT1A receptor and BDNF mRNA, and these changes were normalized in S180 rats provided with HFD or exercise. Prolonged maternal separation resulted in exacerbated hyperinsulinemia when consuming HFD suggesting that these rats are metabolically disadvantaged. In summary, voluntary exercise alone or in combination with HFD produced beneficial effects on both behaviour and metabolic outcomes in rats exposed to early life stress.

Beneficial Effects of Tianeptine on Hippocampus-Dependent Long-Term Memory and Stress-Induced Alterations of Brain Structure and Function

Tianeptine is a well-described antidepressant which has been shown to prevent stress from producing deleterious effects on brain structure and function. Preclinical studies have shown that tianeptine blocks stress-induced alterations of neuronal morphology and synaptic plasticity. Moreover, tianeptine prevents stress from impairing learning and memory, and, importantly, demonstrates memory-enhancing properties in the absence of stress. Recent research has indicated that tianeptine works by normalizing glutamatergic neurotransmission, a mechanism of action that may underlie its effectiveness as an antidepressant. These findings emphasize the value in focusing on the mechanisms of action of tianeptine, and specifically, the glutamatergic system, in the development of novel pharmacotherapeutic strategies in the treatment of depression.

Promoter specific alterations of brain-derived neurotrophic factor mRNA in schizophrenia

The brain-derived neurotrophic factor (BDNF) gene contains multiple 5' promoters which generate alternate transcripts. Previously, we found that pan-BDNF mRNA and protein are reduced in the dorsolateral prefrontal cortex (DLPFC) from patients with schizophrenia. In this study, we determined which of the four most abundant and best characterized BDNF alternate transcripts, I-IX, II-IX, IV-IX, and VI-IX are altered in schizophrenia. Using a cohort from the NIMH, USA, we found that BDNF II-IX mRNA was significantly reduced in the DLPFC of patients with schizophrenia, and we replicated this finding using a second cohort from Sydney, Australia. Moreover, we show that BDNF protein expression [including prepro ( approximately 32 kDa), pro ( approximately 28 kDa) and mature ( approximately 14 kDa) BDNF] is reduced in the DLPFC of patients with schizophrenia. We next determined the regional specificity of the BDNF mRNA reduction by measuring BDNF transcripts in the parietal cortex and hippocampus and found no significant changes. The effect of antipsychotics on BDNF alternate transcript expression was also examined and we found no relationship between BDNF mRNA expression and antipsychotic use. As schizophrenic patients are often prescribed antidepressants which can up-regulate expression of BDNF, we investigated the relationship between antidepressant treatment and BDNF transcript expression. All four BDNF transcripts were significantly up-regulated in schizophrenic patients treated with antidepressants. Moreover, we found significant reductions in BDNF transcripts II-IX and IV-IX in the parietal cortex and VI-IX in the hippocampus of patients with schizophrenia who did not have a history of treatment with antidepressants. This suggests that down-regulation of at least one out of four major BDNF transcripts occurs in various brain regions of patients with schizophrenia, particularly in the DLPFC which appears to have the most robust BDNF deficit in schizophrenia.

Gestational stress influences cognition and behavior

The developmental origins of disease or fetal programming model predicts that early exposures to threat or adverse conditions have lifelong consequences that result in harmful outcomes for health. The vast majority of the studies in support of the programming model in human beings are retrospective and most rely on surrogate measures of early experience such as birth weight or preterm birth. Recently, a small number of prospective studies have been reported that have documented the developmental consequences of exposures to stressful intrauterine conditions. These studies of gestational stress have clearly shown that fetal exposures to psychosocial and/or biological markers of adversity have significant and largely negative consequences for fetal, infant and child neurological development. Fetal exposure to stress, especially early in gestation, results in delayed fetal maturation and impaired cognitive performance during infancy and results in decreased brain volume in areas associated with learning and memory in children. The accumulating evidence supports the conclusion that fetal exposure to stress profoundly influences the nervous system, with consequences that persist into childhood and perhaps beyond.

Brain CB₂ Receptors: Implications for Neuropsychiatric Disorders

Although previously thought of as the peripheral cannabinoid receptor, it is now accepted that the CB₂ receptor is expressed in the central nervous system on microglia, astrocytes and subpopulations of neurons. Expression of the CB₂ receptor in the brain is significantly lower than that of the CB₁ receptor. Conflicting findings have been reported on the neurological effects of pharmacological agents targeting the CB₂ receptor under normal conditions. Under inflammatory conditions, CB₂ receptor expression in the brain is enhanced and CB₂ receptor agonists exhibit potent anti-inflammatory effects. These findings have prompted research into the CB₂ receptor as a possible target for the treatment of neuroinflammatory and neurodegenerative disorders. Neuroinflammatory alterations are also associated with neuropsychiatric disorders and polymorphisms in the CB₂ gene have been reported in depression, eating disorders and schizophrenia. This review will examine the evidence to date for a role of brain CB₂ receptors in neuropsychiatric disorders.

Epigenetics and the biological basis of gene x environment interactions

OBJECTIVE

Child and adolescent psychiatry is rife with examples of the sustained effects of early experience on brain function. The study of behavioral genetics provides evidence for a relation between genomic variation and personality and with the risk for psychopathology. A pressing challenge is that of conceptually integrating findings from genetics into the study of personality without regressing to arguments concerning the relative importance of genomic variation versus nongenomic or environmental influences.

METHOD

Epigenetics refers to functionally relevant modifications to the genome that do not involve a change in nucleotide sequence. This review examines epigenetics as a candidate biological mechanism for gene x environment interactions, with a focus on environmental influences that occur during early life and that yield sustained effects on neural development and function.

RESULTS

The studies reviewed suggest that epigenetic remodeling occurs in response to the environmental activation of cellular signalling pathways associated with synaptic plasticity, epigenetic marks are actively remodeled during early development in response to environmental events that regulate neural development and function, and epigenetic marks are subject to remodeling by environmental influences even at later stages in development.

CONCLUSION

Epigenetic remodeling might serve as an ideal mechanism for phenotypic plasticity--the process whereby the environment interacts with the genome to produce individual differences in the expression of specific traits.

Interactions between age, stress and insulin on cognition: implications for Alzheimer's disease

There is much interest in understanding the mechanisms responsible for interactions among stress, aging, memory and Alzheimer's disease. Glucocorticoid secretion associated with early life stress may contribute to the variability of the aging process and to the development of neuro- and psychopathologies. Maternal separation (MS), a model of early life stress in which rats experience 3 h of daily separation from the dam during the first 3 weeks of life, was used to study the interactions between stress and aging. Young (3 months) MS rats showed an altered hypothalamic-pituitary-adrenal (HPA) axis reactivity, depressive-like behavior in the Porsolt swimming test and cognitive impairments in the Morris water maze and new object recognition test that persisted in aged (18 months) rats. Levels of insulin receptor, phosphorylated insulin receptor and markers of downstream signaling pathways (pAkt, pGSK3 beta, pTau, and pERK1 levels) were significantly decreased in aged rats. There was a significant decrease in pERK2 and in the plasticity marker ARC in MS aged rats compared with single MS or aged rats. It is interesting to note that there was a significant increase in the C99 : C83 ratio, A beta levels, and BACE1 levels the hippocampus of MS aged rats, suggesting that in aged rats subjected to early life stress, there was an increase in the amyloidogenic processing of amyloid precursor protein (APP). These results are integrated in a tentative mechanism through which aging interplay with stress to influence cognition as the basis of Alzheimer disease (AD). The present results may provide the proof-of-concept for the use of glucocorticoid-/insulin-related drugs in the treatment of AD.

Tianeptine: an antidepressant with memory-protective properties

The development of effective pharmacotherapy for major depression is important because it is such a widespread and debilitating mental disorder. Here, we have reviewed preclinical and clinical studies on tianeptine, an atypical antidepressant which ameliorates the adverse effects of stress on brain and memory. In animal studies, tianeptine has been shown to prevent stress-induced morphological sequelae in the hippocampus and amygdala, as well as to prevent stress from impairing synaptic plasticity in the prefrontal cortex and hippocampus. Tianeptine also has memory-protective characteristics, as it blocks the adverse effects of stress on hippocampus-dependent learning and memory. We have further extended the findings on stress, memory and tianeptine here with two novel observations: 1) stress impairs spatial memory in adrenalectomized (ADX), thereby corticosterone-depleted, rats; and 2) the stress-induced impairment of memory in ADX rats is blocked by tianeptine. These findings are consistent with previous research which indicates that tianeptine produces anti-stress and memory-protective properties without altering the response of the hypothalamic-pituitary-adrenal axis to stress. We conclude with a discussion of findings which indicate that tianeptine accomplishes its anti-stress effects by normalizing stress-induced increases in glutamate in the hippocampus and amygdala. This finding is potentially relevant to recent research which indicates that abnormalities in glutamatergic neurotransmission are involved in the pathogenesis of depression. Ultimately, tianeptine’s prevention of depression-induced sequelae in the brain is likely to be a primary factor in its effectiveness as a pharmacological treatment for depression.

Inverse correlation of brain and blood BDNF levels in a genetic rat model of depression

There is accumulating evidence that brain-derived neurotrophic factor (BDNF) plays a critical role in the pathophysiology of depression. Decreased serum levels have been reported in major depression, and a correlation between BDNF reduction and the severity of the disease was found. Moreover, in post-mortem hippocampal tissue, increased levels of BDNF immunoreactivity have been reported in subjects treated with antidepressants compared to untreated subjects. These findings indicate parallel changes in brain and serum BDNF levels during depression. BDNF has been measured in selected brain areas in several animal models. In investigations between Flinders Sensitive Line (FSL) and Flinders Resistant Line (FRL) rats, a genetic rat model of depression, no differences were found in BDNF levels in the frontal cortex and hippocampus, areas believed to be core brain regions in depression. However, to our knowledge brain and serum BDNF levels have never been reported in parallel for any psychiatric disease model. Therefore, we examined the levels of BDNF in whole blood, serum, cerebrospinal fluid (CSF), hippocampus, and frontal cortex in male FSL and FRL rats. BDNF levels in serum and whole blood of FSL rats were significantly increased compared to FRL rats. In contrast, in the hippocampus the BDNF level was significantly decreased in FSL compared to FRL rats while no differences were found in the frontal cortex and CSF. The differential regulation of the BDNF levels in hippocampus, serum, and whole blood in FSL/FRL rats adds to the hypothesis that neurotrophic factors are related to the pathophysiology of depression.

Reduced hippocampal activity in youth with posttraumatic stress symptoms: an FMRI study

OBJECTIVE

Youth who experience interpersonal trauma and have posttraumatic stress symptoms (PTSS) develop cognitive deficits that impact their development. Our goal is to investigate the function of the hippocampus in adolescents with PTSS during a memory processing task.

METHODS

Twenty-seven adolescents between the ages of 10-17 years (16 with PTSS and 11 healthy controls) encoded and retrieved visually presented nouns (Verbal Declarative Memory Task) while undergoing fMRI scanning.

RESULTS

The PTSS group demonstrated reduced activation of the right hippocampus during the retrieval component of the task. Further, severity of symptoms of avoidance and numbing correlated with reduced left hippocampal activation during retrieval.

CONCLUSIONS

Decreased activity of the hippocampus during a verbal memory task may be a neurofunctional marker of PTSS in youth with history of interpersonal trauma. The results of this study may facilitate the development of focused treatments and may be of utility when assessing treatment outcome for PTSS.

Gene-chemical interactions in the developing mammalian nervous system: Effects on proliferation, neurogenesis and differentiation

The orderly formation of the nervous system requires a multitude of complex, integrated and simultaneously occurring processes. Neural progenitor cells expand through proliferation, commit to different cell fates, exit the cell cycle, generate different neuronal and glial cell types, and new neurons migrate to specified areas and establish synaptic connections. Gestational and perinatal exposure to environmental toxicants, pharmacological agents and drugs of abuse produce immediate, persistent or late-onset alterations in behavioral, cognitive, sensory and/or motor functions. These alterations reflect the disruption of the underlying processes of CNS formation and development. To determine the neurotoxic mechanisms that underlie these deficits it is necessary to analyze and dissect the complex molecular processes that occur during the proliferation, neurogenesis and differentiation of cells. This symposium will provide a framework for understanding the orchestrated events of neurogenesis, the coordination of proliferation and cell fate specification by selected genes, and the effects of well-known neurotoxicants on neurogenesis in the retina, hippocampus and cerebellum. These three tissues share common developmental profiles, mediate diverse neuronal activities and function, and thus provide important substrates for analysis. This paper summarizes four invited talks that were presented at the 12th International Neurotoxicology Association meeting held in Jerusalem, Israel during the summer of 2009. Donald A. Fox described the structural and functional alterations following low-level gestational lead exposure in children and rodents that produced a supernormal electroretinogram and selective increases in neurogenesis and cell proliferation of late-born retinal neurons (rod photoreceptors and bipolar cells), but not Müller glia cells, in mice. Lisa Opanashuk discussed how dioxin [TCDD] binding to the arylhydrocarbon receptor [AhR], a transcription factor that regulates xenobiotic metabolizing enzymes and growth factors, increased granule cell formation and apoptosis in the developing mouse cerebellum. Alex Zharkovsky described how postnatal early postnatal lead exposure decreased cell proliferation, neurogenesis and gene expression in the dentate gyrus of the adult hippocampus and its resultant behavioral effects. Bernard Weiss illustrated how environmental endocrine disruptors produced age- and sex-dependent alterations in synaptogenesis and cognitive behavior.

Maternal deprivation induces a rapid decline in circulating leptin levels and sexually dimorphic modifications in hypothalamic trophic factors and cell turnover

Pathological outcomes, including metabolic and endocrine disturbances, of maternal deprivation (MD) in Wistar rats depend on gender and the timing of deprivation during development. We analyzed the effect of MD between postnatal days 9 and 10, a critical period in hypothalamic development, on circulating hormones and local production of trophic factors involved in this process, as well as on markers of cell turnover and maturation. Males and females were studied 12 and 24 h after MD and 12 h (MD36) after returning the dam to her pups. Circulating corticosterone levels were increased and glucose and leptin levels decreased throughout the study in both sexes. Hypothalamic mRNA levels of leptin receptor increased significantly at MD24 in both sexes, normalizing in females at MD36, but not in males. In male rats insulin-like growth factor mRNA levels were significantly decreased at MD24 and brain derived neurotrophic factor mRNA levels decreased at MD12 and MD24, with both trophic factors unaffected in females. In males cell proliferation was significantly decreased at MD36, as were the glial structural proteins, glial fibrillary acidic protein and vimentin. In females, nestin levels decreased significantly at MD24. These results indicate that MD differently affects trophic factors and cell-turnover in the hypothalamus of males and females, which may underlie the sex differences seen in the endocrine and metabolic outcome.

Epigenetics and the environmental regulation of the genome and its function

There are numerous examples in psychology and other disciplines of the enduring effects of early experience on neural function. In this article, we review the emerging evidence for epigenetics as a candidate mechanism for these effects. Epigenetics refers to functionally relevant modifications to the genome that do not involve a change in nucleotide sequence. Such modifications include chemical marks that regulate the transcription of the genome. There is now evidence that environmental events can directly modify the epigenetic state of the genome. Thus studies with rodent models suggest that during both early development and in adult life, environmental signals can activate intracellular pathways that directly remodel the "epigenome," leading to changes in gene expression and neural function. These studies define a biological basis for the interplay between environmental signals and the genome in the regulation of individual differences in behavior, cognition, and physiology.

Reduced peripheral brain-derived neurotrophic factor mRNA levels are normalized by antidepressant treatment

Consistent data coming from biochemical studies have evidenced a brain-derived neurotrophic factor (BDNF) serum reduction in depressed patients compared to controls and a restoration following antidepressant treatment. However, to date, no study has evaluated whether BDNF synthesis in leukocytes could contribute to such modulation. Therefore, in this study, we analysed BDNF mRNA levels in leukocytes from 21 depressed patients prior to and during escitalopram treatment and from 23 control subjects showing that BDNF mRNA levels were decreased in drug-free depressed patients and that 12 wk escitalopram treatment was able to reverse this deficit. Interestingly, changes in BDNF mRNA levels paralleled BDNF serum increase during antidepressant treatment, and were correlated with symptoms improvement. Our results indicate that BDNF serum modulation observed in depressed patients is associated with BDNF synthesis alteration in leukocytes and suggest that these peripheral cells might play an active role in the mechanisms of action of antidepressants.