Tianeptine attenuates stress-induced morphological changes in the hippocampus

Alpha-synuclein-induced stress sensitivity renders the Parkinson's disease brain susceptible to neurodegeneration

A link between chronic stress and Parkinson's disease (PD) pathogenesis is emerging. Ample evidence demonstrates that the presynaptic neuronal protein alpha-synuclein (asyn) is closely tied to PD pathogenesis. However, it is not known whether stress system dysfunction is present in PD, if asyn is involved, and if, together, they contribute to neurodegeneration. To address these questions, we assess stress axis function in transgenic rats overexpressing full-length wildtype human asyn (asyn BAC rats) and perform multi-level stress and PD phenotyping following chronic corticosterone administration. Stress signaling, namely corticotropin-releasing factor, glucocorticoid and mineralocorticoid receptor gene expression, is also examined in post-mortem PD patient brains. Overexpression of human wildtype asyn leads to HPA axis dysregulation in rats, while chronic corticosterone administration significantly aggravates nigrostriatal degeneration, serine129 phosphorylated asyn (pS129) expression and neuroinflammation, leading to phenoconversion from a prodromal to an overt motor PD phenotype. Interestingly, chronic corticosterone in asyn BAC rats induces a robust, twofold increase in pS129 expression in the hypothalamus, the master regulator of the stress response, while the hippocampus, both a regulator and a target of the stress response, also demonstrates elevated pS129 asyn levels and altered markers of stress signalling. Finally, defective hippocampal stress signalling is mirrored in human PD brains and correlates with asyn expression levels. Taken together, our results link brain stress system dysregulation with asyn and provide evidence that elevated circulating glucocorticoids can contribute to asyn-induced neurodegeneration, ultimately triggering phenoconversion from prodromal to overt PD.

Unpredictable chronic mild stress shows neuronal remodeling in multipolar projection neurons of hippocampal complex in postnatal chicks

The hippocampal complex of birds is a narrow-curved strip of tissue that plays a crucial role in learning, memory, spatial navigation, and emotional and sexual behavior. This study was conducted to evaluate the effect of unpredictable chronic mild stress in multipolar neurons of 3-, 5-, 7-, and 9-week-old chick's hippocampal complex. This study revealed that chronic stress results in neuronal remodeling by causing alterations in dendritic field, axonal length, secondary branching, corrected spine number, and dendritic branching at 25, 50, 75, and 100 µm. Due to stress, the overall dendritic length was significantly retracted in 3-week-old chick, whereas no significant difference was observed in 5- and 7-week-old chick, but again it was significantly retracted in 9-week-old chick along with the axonal length. So, this study indicates that during initial days of stress exposure, the dendritic field shows retraction, but when the stress continues up to a certain level, the neurons undergo structural modifications so that chicks adapt and survive in stressful conditions. The repeated exposure to chronic stress for longer duration leads to the neuronal structural disruption by retraction in the dendritic length as well as axonal length. Another characteristic which leads to structural alterations is the dendritic spines which significantly decreased in all age groups of stressed chicks and eventually leads to less synaptic connections, disturbance in physiology, and neurology, which affects the learning, memory, and coping ability of an individual.

Antidepressant mechanisms of ketamine: a review of actions with relevance to treatment-resistance and neuroprogression

Concurrent with recent insights into the neuroprogressive nature of depression, ketamine shows promise in interfering with several neuroprogressive factors, and has been suggested to reverse neuropathological patterns seen in depression. These insights come at a time of great need for novel approaches, as prevalence is rising and current treatment options remain inadequate for a large number of people. The rapidly growing literature on ketamine's antidepressant potential has yielded multiple proposed mechanisms of action, many of which have implications for recently elucidated aspects of depressive pathology. This review aims to provide the reader with an understanding of neuroprogressive aspects of depressive pathology and how ketamine is suggested to act on it. Literature was identified through PubMed and Google Scholar, and the reference lists of retrieved articles. When reviewing the evidence of depressive pathology, a picture emerges of four elements interacting with each other to facilitate progressive worsening, namely stress, inflammation, neurotoxicity and neurodegeneration. Ketamine acts on all of these levels of pathology, with rapid and potent reductions of depressive symptoms. Converging evidence suggests that ketamine works to increase stress resilience and reverse stress-induced dysfunction, modulate systemic inflammation and neuroinflammation, attenuate neurotoxic processes and glial dysfunction, and facilitate synaptogenesis rather than neurodegeneration. Still, much remains to be revealed about ketamine's antidepressant mechanisms of action, and research is lacking on the durability of effect. The findings discussed herein calls for more longitudinal approaches when determining efficacy and its relation to neuroprogressive factors, and could provide relevant considerations for clinical implementation.

Mu opioid receptors on hippocampal GABAergic interneurons are critical for the antidepressant effects of tianeptine

Tianeptine is an atypical antidepressant used in Europe to treat patients who respond poorly to selective serotonin reuptake inhibitors (SSRIs). The recent discovery that tianeptine is a mu opioid receptor (MOR) agonist has provided a potential avenue for expanding our understanding of antidepressant treatment beyond the monoamine hypothesis. Thus, our studies aim to understand the neural circuits underlying tianeptine's antidepressant effects. We show that tianeptine induces rapid antidepressant-like effects in mice after as little as one week of treatment. Critically, we also demonstrate that tianeptine's mechanism of action is distinct from fluoxetine in two important aspects: (1) tianeptine requires MORs for its chronic antidepressant-like effect, while fluoxetine does not, and (2) unlike fluoxetine, tianeptine does not promote hippocampal neurogenesis. Using cell-type specific MOR knockouts we further show that MOR expression on GABAergic cells-specifically somatostatin-positive neurons-is necessary for the acute and chronic antidepressant-like responses to tianeptine. Using central infusion of tianeptine, we also implicate the ventral hippocampus as a potential site of antidepressant action. Moreover, we show a dissociation between the antidepressant-like phenotype and other opioid-like phenotypes resulting from acute tianeptine administration such as analgesia, conditioned place preference, and hyperlocomotion. Taken together, these results suggest a novel entry point for understanding what circuit dysregulations may occur in depression, as well as possible targets for the development of new classes of antidepressant drugs.

Neuroplasticity and depression: Rewiring the brain's networks through pharmacological therapy (Review)

In modern society, depression is one of the most common mental illness; however, its pathophysiology is not yet fully understood. A great body of evidence suggests that depression causes changes in neuroplasticity in specific regions of the brain which are correlated to symptom severity, negative emotional rumination as well as fear learning. Depression is correlated with atrophy of neurons in the cortical and limbic brain regions that control mood and emotion. Antidepressant therapy can exhibit effects on neuroplasticity and reverse the neuroanatomical changes found in depressed patients. The investigation of fast-acting agents that reverse behavioral and neuronal deficiencies of chronic depression, especially the glutamate receptor antagonist NMDA ketamine, and the cellular mechanisms underlying the rapid antidepressant actions of ketamine and related agents are of real interest in current research. Actual medication such as serotonin (5-HT) selective reuptake inhibitor (SSRI) antidepressants, require weeks to months of administration before a clear therapeutic response. The current review aimed to underline the negative effects of depression on neuroplasticity and present the current findings on the effects of antidepressant medication.

Fluoxetine exerts subregion/layer specific effects on parvalbumin/GAD67 protein expression in the dorsal hippocampus of male rats showing social isolation-induced depressive-like behaviour

The molecular background of depression is intensively studied in terms of alterations of inhibitory circuits, mediated by gamma aminobutyric acid (GABA) signalization. We investigated the effects of chronic social isolation (CSIS) and chronic fluoxetine (Flx) treatment (15 mg/kg/day) (3 weeks), on Parvalbumin (PV) and GAD67 expression in a layer-specific manner in rat dorsal hippocampal subregions. CSIS-induced depressive- and anxiety-like behaviours were confirmed with decrease in sucrose preference and increase in marble burying during behavioural testing, while Flx antagonized these effects. CSIS altered PV expression in stratum pyramidale (SP) of dorsal cornu ammonis 1 (dCA1) and stratum radiatum (SR) of dCA3. Flx antagonized this effect, and boosted PV expression in SP of the entire dCA and the dorsal dentate gyrus (dDG), as well as in the SR of dCA1/CA3. CSIS showed no significant effects on GAD67 expression, while Flx boosted its expression within the SR of the entire CA and SO of the dCA3. A correlation between SP of dCA1 and SR of dCA3 with regard to PV changes, implicates their possible role in the inhibitory circuit alterations. Flx-induced increase in GAD67 expression, specifically in SR of the entire dHIPP, may impose its involvement in the cell metabolic processes. Strong negative correlation between GAD67 and sucrose preference following Flx-treatment of CSIS rats was revealed. PV + cells of the SP layer of dCA1 and CA2 could be a potential target for the antidepressant action of Flx, while strong effect of Flx on GAD67 expression in the SR should be more extensively studied.

Neuroplasticity and the Biological Role of Brain Derived Neurotrophic Factor in the Pathophysiology and Management of Depression

Depression is a mental illness that can have serious implications if left untreated. Studies involving a neurotrophic factor called brain derived neurotrophic factor (BDNF) and its associated signaling pathways have solidified our understanding of the pathophysiology of depression. The objective of this literature review is to gain a better understanding of the mechanism by which reduced levels of BDNF are implicated in depression and how antidepressants facilitate the treatment of depression by increasing BDNF levels. The specific approach is to learn about the key involvements of BDNF and its receptor TrkB (tropomyosin receptor kinase B) and how their interactions and subsequent intracellular signaling cascades bring about enhanced neuroplastic changes. In this literature review, we searched for past review articles focusing on BDNF. We collected data using PubMed and created a summary of our findings. The results showed that stress and depression through the reduction of BDNF levels contribute to neuroplastic changes while antidepressants through enhanced BDNF levels are able to generate positive neuroplastic outcomes and thereby help resolve depressive symptoms. In this paper, we will delve into how a better understanding of the neural circuitry involving BDNF will enable us to both understand how current antidepressants work in the limbic regions of the brain as well as search for novel rapid-acting antidepressants to use in clinical practice.

Corticotropin releasing hormone receptor CRHR1 gene is associated with tianeptine antidepressant response in a large sample of outpatients from real-life settings

Polymorphisms of genes involved in the hypothalamic-pituitary-adrenocortical (HPA) axis have been associated with response to several antidepressant treatments in patients suffering of depression. These pharmacogenetics findings have been reported from independent cohorts of patients mostly treated with selective serotonin reuptake inhibitors, tricyclic antidepressant, and mirtazapine. Tianeptine, an atypical antidepressant, recently identified as a mu opioid receptor agonist, which prevents and reverses the stress induced by glucocorticoids, has been investigated in this present pharmacogenetics study. More than 3200 Caucasian outpatients with a major depressive episode (MDE) from real-life settings were herein analyzed for clinical response to tianeptine, a treatment initiated from 79.5% of the subjects, during 6-8 weeks follow-up, assessing polymorphisms targeting four genes involved in the HPA axis (NR3C1, FKPB5, CRHR1, and AVPR1B). We found a significant association (p < 0.001) between CRHR1 gene variants rs878886 and rs16940665, or haplotype rs878886*C-rs16940665*T, and tianeptine antidepressant response and remission according to the hospital anxiety and depression scale. Analyses, including a structural equation model with simple mediation, suggest a moderate effect of sociodemographic characteristics and depressive disorder features on treatment response in individuals carrying the antidepressant responder allele rs8788861 (allele C). These findings suggest direct pharmacological consequences of CRHR1 polymorphisms in the antidepressant tianeptine response and remission, in MDE patients. This study replicates the association of the CRHR1 gene, involved in the HPA axis, with (1) a specificity attributed to treatment response, (2) a lower risk of chance finding, and in (3) an ecological situation.

Antidepressant modulation of isolation and restraint stress effects on brain chemistry and morphology

Stress elicits adaptive responses from the brain, but it can also lead to maladaptive consequences. For example, stress can precipitate mental illness, including depression. Prolonged stress also causes damage to neurons in the hippocampus. Antidepressant drugs must be evaluated, not only for their ability to potentiate adaptive responses, but also to inhibit maladaptive consequences of stress. Ongoing research in our laboratory has compared the atypical tricyclic antidepressant, tianeptine, with the typical tricyclics, desipramine and imipramine, with respect to the effects of isolation and repeated restraint stress. Tianeptine and desipramine similarly attenuated isolation stress-induced increases in locus coeruleus and midbrain tyrosine hydroxylase mRNA levels and isolation-stress induced decreases in preproenkephalin mRNA levels in striatum and nucleus accumbens. However, tianeptine and imipramine differed in their effects in the cerebral cortex and hippocampus on 5HT2, and 5HT1A receptor levels but, surprisingly, produced similar effects on levels of the serotonin transporter labelled with [3H] paroxetine. Tianeptine also prevented stress-induced reductions in the length and number of branchpoints of dendrites of CA3 pyramidal neurons in hippocampus; comparison with effects of typical tricyclics are ongoing. Tianeptine also blocked effects of corticosterone treatment to reduce branching and length of CA3 dendrites. These actions of tianeptine may be due to interactions between 5HT and excitatory amino acids in the mossy fiber terminals on CA3 pyramidal neurons. Taken together, these results indicate that tianeptine has unique properties compared to some other antidepressant drugs, but shares in common with those drugs the ability to attenuate stress effects on tyrosine hydroxylase gene expression and on the serotonin transporter. It remains to be seen whether these actions are the basis of a common antidepressant action.

Beyond the monoamine hypothesis: mechanisms, molecules and methods

The first effective antidepressants (monoamine oxidase inhibitors and tricyclic antidepressants) relied on their ability to augment serotonin and noradrenaline levels at the synapse. Forty years later, the same biological model led to the supremacy of the serotonergic hypothesis to explain not only the pathophysiology of depressive illness, but also the neuropharmacological basis for obsessive compulsive disorder, phobias, posttraumatic stress disorder, and even generalized anxiety disorder. It could be argued that the blinkered view of depression as a solely serotonergic phenomenon has not only restrained and limited research into other potential systems, but has also slowed down the discovery of putative antidepressant drugs. While some might argue that the hypothalamic-pituitary-adrenal (HPA) axis explains an individual’s sensitivity to depression, there are others who equally claim that the most likely explanations are to be found in the neuropsychopharmacology of the immune system or even through reductions in hippocampal volume. There is a richness of possibilities regarding the mechanisms for antidepressant activity embracing theoretical, pharmacological and clinical data. However, the methods by which putative antidepressants are assessed and their clinical efficacy demonstrated are not always robust. That current clinical comparisons of antidepressants rarely show major differences in efficacy between existing molecules could be taken as an indication that “all drugs are the same” or perhaps, more insightfully, as an indication that the ubiquitous Hamilton depression (HAM-D) rating scales are not sensitive to inter-drug differences, even though pronounced pharmacodynamic differences between molecules are easily demonstrated. Any advances in the development of new antidepressants will have to find not only original compounds but also unique psychometric tests by which the drugs can be assessed in a sensitive, reliable, and valid manner.

Interventions after acute stress prevent its delayed effects on the amygdala

Stress is known to elicit contrasting patterns of plasticity in the amygdala and hippocampus. While chronic stress leads to neuronal atrophy in the rodent hippocampus, it has the opposite effect in the basolateral amygdala (BLA). Further, even a single episode of acute stress is known to elicit delayed effects in the amygdala. For example, 2 h of immobilisation stress has been shown to cause a delayed increase in dendritic spine density on BLA principal neurons 10 days later in young rats. This is paralleled by higher anxiety-like behaviour at the same delayed time point. This temporal build-up of morphological and behavioural effects 10 days later, in turn, provides a stress-free time window of intervention after exposure to acute stress. Here, we explore this possibility by specifically testing the efficacy of an anxiolytic drug in reversing the delayed effects of acute immobilisation stress. Oral gavage of diazepam 1 h after immobilisation stress prevented the increase in anxiety-like behaviour on the elevated plus-maze 10 days later. The same post-stress intervention also prevented delayed spinogenesis in the BLA 10 days after acute stress. Surprisingly, gavage of only the vehicle also had a protective effect on both the behavioural and synaptic effects of stress 10 days later. Vehicle gavage was found to trigger a significant rise in corticosterone levels that was comparable to that elicited by acute stress. This suggests that a surge in corticosterone levels, caused by the vehicle gavage 1 h after acute stress, was capable of reversing the delayed enhancing effects of stress on anxiety-like behaviour and BLA synaptic connectivity. These findings are consistent with clinical reports on the protective effects of glucocorticoids against the development of symptoms of post-traumatic stress disorder. Taken together, these results reveal strategies, targeted 1 h after stress, which can prevent the delayed effects of a brief exposure to a severe physical stressor.

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Acute immobilisation stress increases anxiety and BLA spinogenesis 10 days later.

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Oral gavage of diazepam 1 h after stress prevents both these delayed effects.

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Oral gavage of vehicle also has a similar protective effect on anxiety and spines.

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Vehicle-gavage administration leads to an increase in levels of corticosterone.

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This post-stress corticosterone surge may have prevented stress-effects 10 days later.

Tianeptine antagonizes the reduction of PV+ and GAD67 cells number in dorsal hippocampus of socially isolated rats

Adult male rats exposed to chronic social isolation (CSIS) show depressive- and anxiety-like behaviors and reduce the numbers of parvalbumin-positive (PV+) interneurons in the dorsal hippocampus. We aimed to determine whether tianeptine (Tian), administered during the last three weeks of a six-week-social isolation (10 mg/kg/day), may reverse CSIS-induced behavioral changes and antagonize the CSIS-induced reduction in the number of PV+ interneurons. We also studied whether Tian affects the GABA-producing enzyme GAD67+ cells, in Stratum Oriens (SO), Stratum Pyramidale (SP), Stratum Radiatum (SR) and Stratum Lacunosum Moleculare (LM) of CA1-3, as well as in molecular layer-granule cell layer (ML-GCL) and Hilus (H) of the dentate gyrus (DG). CSIS-induced reduction in the number of PV+ cells was layer/subregion-specific with the greatest decrease in SO of CA2. Reduction in the number of PV+ cells was significantly higher than GAD67+ cells, indicating that PV+ cells are the main target following CSIS. Tian reversed CSIS-induced behavior phenotype and antagonized the reduction in the number of PV+ and GAD67+ cells in all subregions. In controls, Tian led to an increase in the number of PV+ and GAD67+ cells in SP of all subregions and PV+ interneurons in ML-GCL of DG, while treatment during CSIS, compared to CSIS alone, resulted with an increase of PV+ interneurons in SO and SP CA1, SP CA2/CA3 and ML-GCL DG with simultaneous increase in GAD67+ cells in all CA1, LM CA2, SO/SR/LM CA3. Data show that Tian offers protection from CSIS via modulation of the dorsal hippocampal GABAergic system.

Tianeptine, an atypical pharmacological approach to depression

The introduction of the first antidepressants in the 50s of the 20th century radically changed the treatment of depression, while providing information on pathophysiological aspects of this disease. New antidepressants drugs (agomelatine, tianeptine, vortioxetine) are providing data that give rise to pathophysiological hypotheses of depression that differ from the classic monoaminergic theory. In this sense, tianeptina, an atypical drug by its mechanism of differential action, contributes to clarify that in depression there is more than monoamines. Thus, tianeptine does not modify the rate of extracellular serotonin, so it does not increase or decrease the reuptake of serotonin. Chronic administration of tianeptine does not alter the density or affinity of more than a hundred classical receptors related to depression. Recently, a weak action of tianeptine on Mu opioid receptors has been described that could explain the release of dopamine in the limbic system and its participation in the modulation of glutamatergic mechanisms. These mechanisms support the hypothesis of the possible mechanism of action of this antidepressant. Tianeptine is an antidepressant, with anxiolytic properties, that can improve somatic symptoms. Tianeptine as a glutamatergic modulator, among other mechanisms, allows us to approach depression from a different point of view than other antidepressants.

Electroconvulsive seizures influence dendritic spine morphology and BDNF expression in a neuroendocrine model of depression

BACKGROUND

Electroconvulsive therapy (ECT) is a rapid and effective treatment for major depressive disorder. Chronic stress-induced depression causes dendrite atrophy and deficiencies in brain-derived neurotrophic factor (BDNF), which are reversed by anti-depressant drugs. Electroconvulsive seizures (ECS), an animal model of ECT, robustly increase BDNF expression and stimulate dendritic outgrowth.

OBJECTIVE

The present study aims to understand cellular and molecular plasticity mechanisms contributing to the efficacy of ECS following chronic stress-induced depression.

METHODS

We quantify Bdnf transcript levels and dendritic spine density and morphology on cortical pyramidal neurons in mice exposed to vehicle or corticosterone and receiving either Sham or ECS treatment.

RESULTS

ECS rescues corticosterone-induced defects in spine morphology and elevates Bdnf exon 1 and exon 4-containing transcripts in cortex.

CONCLUSIONS

Dendritic spine remodeling and induction of activity-induced BDNF in the cortex represent important cellular and molecular plasticity mechanisms underlying the efficacy of ECS for treatment of chronic stress-induced depression.

A comprehensive review of genetic and epigenetic mechanisms that regulate BDNF expression and function with relevance to major depressive disorder

Major depressive disorder (MDD) is a mood disorder that affects behavior and impairs cognition. A gene potentially important to this disorder is the brain derived neurotrophic factor (BDNF) as it is involved in processes controlling neuroplasticity. Various mechanisms exist to regulate BDNF's expression level, subcellular localization, and sorting to appropriate secretory pathways. Alterations to these processes by genetic factors and negative stressors can dysregulate its expression, with possible implications for MDD. Here, we review the mechanisms governing the regulation of BDNF expression, and discuss how disease-associated single nucleotide polymorphisms (SNPs) can alter these mechanisms, and influence MDD. As negative stressors increase the likelihood of MDD, we will also discuss the impact of these stressors on BDNF expression, the cellular effect of such a change, and its impact on behavior in animal models of stress. We will also describe epigenetic processes that mediate this change in BDNF expression. Similarities in BDNF expression between animal models of stress and those in MDD will be highlighted. We will also contrast epigenetic patterns at the BDNF locus between animal models of stress, and MDD patients, and address limitations to current clinical studies. Future work should focus on validating current genetic and epigenetic findings in tightly controlled clinical studies. Regions outside of BDNF promoters should also be explored, as should other epigenetic marks, to improve identification of biomarkers for MDD.

Neural basis of major depressive disorder: Beyond monoamine hypothesis

The monoamine hypothesis has been accepted as the most common hypothesis of major depressive disorder (MDD) for a long period because of its simplicity and understandability. Actually, most currently used antidepressants have been considered to act based on the monoamine hypothesis. However, an important problem of the monoamine hypothesis has been pointed out as follows: it fails to explain the latency of response to antidepressants. In addition, many patients with MDD have remained refractory to currently used antidepressants. Therefore, monoamine-alternate hypotheses are required to explain the latency of response to antidepressants. Such hypotheses have been expected to contribute to identifying hopeful new therapeutic targets for MDD. Past studies have revealed that the volume of the hippocampus is decreased in patients with MDD, which is likely caused by the failure of the hypothalamic-pituitary-adrenal axis and following elevation of glucocorticoids. Two hypotheses have been proposed to explain the volume of the hippocampus: (i) the neuroplasticity hypothesis; and (ii) the neurogenesis hypothesis. The neuroplasticity hypothesis explains how the hippocampal volume is decreased by the morphological changes of hippocampal neurons, such as the shortening length of dendrites and the decreased number and density of spines. The neurogenesis hypothesis explains how the hippocampal volume is decreased by the decrease of neurogenesis in the hippocampal dentate gyrus. These hypotheses are able to explain the latency of response to antidepressants. In this review, we first overview how the neuroplasticity and neurogenesis hypotheses have been developed. We then describe the details of these hypotheses.

Stress and Loss of Adult Neurogenesis Differentially Reduce Hippocampal Volume

BACKGROUND

Hippocampal volume loss is a hallmark of clinical depression. Chronic stress produces volume loss in the hippocampus in humans and atrophy of CA3 pyramidal cells and suppression of adult neurogenesis in rodents.

METHODS

To investigate the relationship between decreased adult neurogenesis and stress-induced changes in hippocampal structure and volume, we compared the effects of chronic unpredictable restraint stress and inhibition of neurogenesis in a rat pharmacogenetic model.

RESULTS

Chronic unpredictable restraint stress over 4 weeks decreased total hippocampal volume, reflecting loss of volume in all hippocampal subfields and in both dorsal and ventral hippocampus. In contrast, complete inhibition of adult neurogenesis for 4 weeks led to volume reduction only in the dentate gyrus. With prolonged inhibition of neurogenesis for 8 or 16 weeks, volume loss spread to the CA3 region, but not CA1. Combining stress and inhibition of adult neurogenesis did not have additive effects on the magnitude of volume loss but did produce a volume reduction throughout the hippocampus. One month of chronic unpredictable restraint stress and inhibition of adult neurogenesis led to atrophy of pyramidal cell apical dendrites in dorsal CA3 and to neuronal reorganization in ventral CA3. Stress also significantly affected granule cell dendrites.

CONCLUSIONS

The findings suggest that adult neurogenesis is required to maintain hippocampal volume but is not responsible for stress-induced volume loss.

Repeated shock stress facilitates basolateral amygdala synaptic plasticity through decreased cAMP-specific phosphodiesterase type IV (PDE4) expression

Previous studies have shown that exposure to stressful events can enhance fear memory and anxiety-like behavior as well as increase synaptic plasticity in the rat basolateral amygdala (BLA). We have evidence that repeated unpredictable shock stress (USS) elicits a long-lasting increase in anxiety-like behavior in rats, but the cellular mechanisms mediating this response remain unclear. Evidence from recent morphological studies suggests that alterations in the dendritic arbor or spine density of BLA principal neurons may underlie stress-induced anxiety behavior. Recently, we have shown that the induction of long-term potentiation (LTP) in BLA principal neurons is dependent on activation of postsynaptic D1 dopamine receptors and the subsequent activation of the cyclic adenosine 5'-monophosphate (cAMP)-protein kinase A (PKA) signaling cascade. Here, we have used in vitro whole-cell patch-clamp recording from BLA principal neurons to investigate the long-term consequences of USS on their morphological properties and synaptic plasticity. We provided evidence that the enhanced anxiety-like behavior in response to USS was not associated with any significant change in the morphological properties of BLA principal neurons, but was associated with a changed frequency dependence of synaptic plasticity, lowered LTP induction threshold, and reduced expression of phosphodiesterase type 4 enzymes (PDE4s). Furthermore, pharmacological inhibition of PDE4 activity with rolipram mimics the effects of chronic stress on LTP induction threshold and baseline startle. Our results provide the first evidence that stress both enhances anxiety-like behavior and facilitates synaptic plasticity in the amygdala through a common mechanism of PDE4-mediated disinhibition of cAMP-PKA signaling.

Circuit and synaptic mechanisms of repeated stress: Perspectives from differing contexts, duration, and development

The current review is meant to synthesize research presented as part of a symposium at the 2016 Neurobiology of Stress workshop in Irvine California. The focus of the symposium was "Stress and the Synapse: New Concepts and Methods" and featured the work of several junior investigators. The presentations focused on the impact of various forms of stress (altered maternal care, binge alcohol drinking, chronic social defeat, and chronic unpredictable stress) on synaptic function, neurodevelopment, and behavioral outcomes. One of the goals of the symposium was to highlight the mechanisms accounting for how the nervous system responds to stress and their impact on outcome measures with converging effects on the development of pathological behavior. Dr. Kevin Bath's presentation focused on the impact of disruptions in early maternal care and its impact on the timing of hippocampus maturation in mice, finding that this form of stress drove accelerated synaptic and behavioral maturation, and contributed to the later emergence of risk for cognitive and emotional disturbance. Dr. Scott Russo highlighted the impact of chronic social defeat stress in adolescent mice on the development and plasticity of reward circuity, with a focus on glutamatergic development in the nucleus accumbens and mesolimbic dopamine system, and the implications of these changes for disruptions in social and hedonic response, key processes disturbed in depressive pathology. Dr. Kristen Pleil described synaptic changes in the bed nuclei of the stria terminalis that underlie the behavioral consequences of allostatic load produced by repeated cycles of alcohol binge drinking and withdrawal. Dr. Eric Wohleb and Dr. Ron Duman provided new data associating decreased mammalian target of rapamycin (mTOR) signaling and neurobiological changes in the synapses in response to chronic unpredictable stress, and highlighted the potential for the novel antidepressant ketamine to rescue synaptic and behavioral effects. In aggregate, these presentations showcased how divergent perspectives provide new insights into the ways in which stress impacts circuit development and function, with implications for understanding emergence of affective pathology.

Sex differences in stress regulation of arousal and cognition

There are sex differences in the prevalence and presentation of many psychiatric disorders. For example, posttraumatic stress disorder (PTSD) and major depression are more common in women than men, and women with these disorders present with more hyperarousal symptoms than men. In contrast, attention deficit hyperactivity disorder (ADHD) and schizophrenia are more common in men than women, and men with these disorders have increased cognitive deficits compared to women. A shared feature of the aforementioned psychiatric disorders is the contribution of stressful events to their onset and/or severity. Here we propose that sex differences in stress responses bias females towards hyperarousal and males towards cognitive deficits. Evidence from clinical and preclinical studies is detailed. We also describe underlying neurobiological mechanisms. For example, sex differences in stress receptor signaling and trafficking in the locus coeruleus-arousal center are detailed. In learning circuits, evidence for sex differences in dendritic morphology is provided. Finally, we describe how evaluating sex-specific mechanisms for responding to stress in female and male rodents can lead to better treatments for stress-related psychiatric disorders.

Bidirectional relationship of mast cells-neurovascular unit communication in neuroinflammation and its involvement in POCD

Postoperative cognitive dysfunction (POCD) has been hypothesized to be mediated by surgery-induced neuroinflammation, which is also a key element in the pathobiology of neurodegenerative diseases, stroke, and neuropsychiatric disorders. There is extensive communication between the immune system and the central nervous system (CNS). Inflammation resulting from activation of the innate immune system cells in the periphery can impact central nervous system behaviors, such as cognitive performance. Mast cells (MCs), as the"first responders" in the CNS, can initiate, amplify, and prolong other immune and nervous responses upon activation. In addition, MCs and their secreted mediators modulate inflammatory processes in multiple CNS pathologies and can thereby either contribute to neurological damage or confer neuroprotection. Neuroinflammation has been considered to be linked to neurovascular dysfunction in several neurological disorders. This review will provide a brief overview of the bidirectional relationship of MCs-neurovascular unit communication in neuroinflammation and its involvement in POCD, providing a new and unique therapeutic target for the adjuvant treatment of POCD.

Chronic stress and hippocampal dendritic complexity: Methodological and functional considerations

The current understanding of how chronic stress impacts hippocampal dendritic arbor complexity and the subsequent relationship to hippocampal-dependent spatial memory is reviewed. A surge in reports investigating hippocampal dendritic morphology is occurring, but with wide variations in methodological detail being reported. Consequently, this review systematically outlines the basic neuroanatomy of relevant hippocampal features to help clarify how chronic stress or glucocorticoids impact hippocampal dendritic complexity and how these changes occur in parallel with spatial cognition. Chronic stress often leads to hippocampal CA3 apical dendritic retraction first with other hippocampal regions (CA3 basal dendrites, CA1, dentate gyrus, DG) showing dendritic retraction when chronic stress is sufficiently robust or long lasting. The stress-induced reduction in hippocampal CA3 apical dendritic arbor complexity often coincides with impaired hippocampal function, such as spatial learning and memory. Yet, when chronic stress ends and a post-stress recovery period ensues, the atrophied dendritic arbors and poor spatial abilities often improve. However, this process differs from a simple reversal of chronic stress-induced deficits. Recent reports suggest that this return to baseline-like functioning is uniquely different from non-stressed controls, emphasizing the need for further studies to enhance our understanding of how a history of stress subsequently alters an organism's spatial abilities. To provide a consistent framework for future studies, this review concludes with an outline for a quick and easy reference on points to consider when planning chronic stress studies with the goal of measuring hippocampal dendritic complexity and spatial ability.

Differential effects of tianeptine on the dorsal hippocampal volume of rats submitted to maternal separation followed by chronic unpredictable stress in adulthood

Early maternal separation (MS) may produce lasting effects in the dorsal hippocampus (DH) that can change its response to chronic stress in adulthood. Chronic stress affects DH morphology and function, but tianeptine (an anti-depressant) can reverse the stress-induced morphological impairments. Morphologic alterations of hippocampus can affect contextual memory. Therefore, we evaluated the effect of tianeptine in MS and chronically stressed rats on: 1) volume of the DH and its areas using stereology and 2) hippocampal-dependent memory using a fear conditioning test. Male Wistar rats were subjected to daily MS for 4.5 h between postnatal days (PND) 1-21, or to animal facility rearing (AFR). Between (PND) days 50 and 74, rats were exposed to chronic unpredictable stress and were treated daily with tianeptine (10 mg/kg) or vehicle, providing eight groups: AFR-unstressed/vehicle (n = 5 for stereology, n = 18 for fear conditioning test); AFR unstressed/tianeptine (n = 6 and n = 10); AFR-chronic stress/vehicle (n = 6 and n = 14); AFR-chronic stress/tianeptine (n = 6 and n = 10), MS-unstressed/vehicle (n = 5 and n = 19), MS-unstressed/tianeptine (n = 6 and n = 10), MS-chronic stress/vehicle (n = 6 and n = 18), and MS-chronic stress/tianeptine (n = 6 and n = 10). MS-chronic stress/tianeptine rats showed a diminished CA1 area than the corresponding MS-unstressed/tianeptine rats. The combination of stressors produced a freezing response similar to those of the control group during postconditioning. During retrieval, MS led to a diminished freezing response compared to the AFR-unstressed groups. Tianeptine had no effect on freezing behavior. Our results show that tianeptine can affect the CA1 area volume differently depending on the nature and quantity of stressors but cannot alter freezing to context.

Review : Stress, Antidepressant Treatments, and Neurotrophic Factors: Molecular and Cellular Mechanisms

Repeated stress or an excess of glucocorticoids can exacerbate neuronal damage in response to insults and, in severe cases, can lead to neuronal atrophy and death. These effects are thought to be related to the actions of stress and glucocorticoids on glutamate function, neuronal metabolism, and the generation of cytotoxic free radicals. Recent studies demonstrate that the regulation of neurotrophic factors may contribute to the actions of stress on neuronal function. Acute or chronic stress decreases the expression of brain derived neurotrophic factor, the most abundant neurotrophin in the brain, in specific regions of the hippocampus, and other forebrain regions. In addition, chronic stress increases the expression of neurotrophin-3 in certain regions of the hippocampus and may, thereby, help to protect these regions from the neurotoxic effects of chronic stress. The deleterious effects of stress may contribute to psy chiatric illnesses, such as depression, that can be precipitated or worsened by stress and that are often characterized by hypercortisolism. Electroconvulsive seizure therapy, as well as antidepressant drugs, increase the expression of brain derived neurotrophic factor and its receptor, trkB, in the brain, demon strating that neurotrophins are a target of antidepressant treatments. These findings outline a role of neurotrophic factors in the etiology and treatment of certain psychiatric illnesses. The Neuroscientist 1:351-360, 1995

Positive Modulation of Pink Nelumbo nucifera Flowers on Memory Impairment, Brain Damage, and Biochemical Profiles in Restraint Rats

Due to the crucial role of oxidative stress in the stress-induced memory deficit, the benefit of substance possessing antioxidant effect is focused. Since no data are available, we aimed to determine the effect of Nelumbo nucifera flowers extract on spatial memory and hippocampal damage in stressed rats. Male Wistar rats, weighing 250-350 g, were orally given N. nucifera extract at doses of 10, 10, and 200 mg·kg(-1) 45 minutes before the exposure to 12-hour restraint stress. The spatial memory and serum corticosterone were assessed at 7 and 14 days of study period. At the end of study, acetylcholinesterase (AChE), monoamine oxidase type A and monoamine oxidase type B (MAO-A and MAO-B), oxidative stress status, neuron density, and Ki67 expression in hippocampus were also assessed. The results showed that N. nucifera extract decreased memory deficit and brain damage, serum corticosterone, oxidative stress status, AChE, and MAO-A and MAO-B activities but increased neuron density and Ki67 expression in hippocampus. These suggested that the improved oxidative stress status, adult neurogenesis, and cholinergic and monoaminergic functions might be responsible for the protective effect against stress-related brain damage and dysfunction of the extract. Therefore, N. nucifera extract is the potential neuroprotective and memory enhancing agent. However, further researches are still required.

Cognitive disruptions in stress-related psychiatric disorders: A role for corticotropin releasing factor (CRF)

This article is part of a Special Issue "SBN 2014". Stress is a potential etiology contributor to both post-traumatic stress disorders (PTSD) and major depression. One stress-related neuropeptide that is hypersecreted in these disorders is corticotropin releasing factor (CRF). Dysregulation of CRF has long been linked to the emotion and mood symptoms that characterize PTSD and depression. However, the idea that CRF also mediates the cognitive disruptions observed in patients with these disorders has received less attention. Here we review literature indicating that CRF can alter cognitive functions. Detailed are anatomical studies revealing that CRF is poised to modulate regions required for learning and memory. We also describe preclinical behavioral studies that demonstrate CRF's ability to alter fear conditioning, impair memory consolidation, and alter a number of executive functions, including attention and cognitive flexibility. The implications of these findings for the etiology and treatment of the cognitive impairments observed in stress-related psychiatric disorders are described.

The influence of stress and gonadal hormones on neuronal structure and function

This article is part of a Special Issue "SBN 2014". The brain is highly plastic, allowing us to adapt and respond to environmental and physiological challenges and experiences. In this review, we discuss the relationships among alterations in dendritic arborization, spine morphology, and behavior due to stress exposure, endogenous hormone fluctuation, or exogenous hormonal manipulation. Very few studies investigate structure-function associations directly in the same cohort of animals, and there are notable inconsistencies in evidence of structure-function relationships in the prefrontal cortex and hippocampus. Moreover, little work has been done to probe the causal relationship between dendritic morphology and neuronal excitability, leaving only speculation about the adaptive versus maladaptive nature of experience-dependent dendritic remodeling. We propose that future studies combine electrophysiology with a circuit-level approach to better understand how dendritic structure contributes to neuronal functional properties and behavioral outcomes.

Effects of fluoxetine on memory under forced treadmill exercise conditions in male rats

Background:

Studies show inconsistent effects of forced exercise on cognitive processes. These differences are probably due to the stress of coercion in forced exercise. Because fluoxetine is used to treat complications caused by stress, this study aimed to evaluate the effects of fluoxetine on memory in rats under forced treadmill exercise.

Materials and Methods:

Experimental groups were the control, the control exercise, the fluoxetine, and the fluoxetine exercise. The exercise program was treadmill running at 22 m/min, 0° inclination for 50 min/day, 6 days/week, for 4 weeks. Fluoxetine (5 mg/kg) was injected 30 min before treadmill. Morris water maze and passive avoidance learning tests were used for evaluation of memory. Acquisition phase of both tests were performed before interventions and memory was evaluated 1-day and 1-week after the last session of exercise and treatments.

Results:

Our data showed that forced exercise impaired performance in passive avoidance learning test (P < 0.05 and P < 0.01, 1-day and 1-week after the last session of exercise and treatments, respectively). Spatial memory was only impaired after 1-week in the exercise group. Fluoxetine improved spatial memory after 1-day in the control group. However, it had no significant effects on memory in the exercise group.

Conclusion:

The data correspond to the possibility that forced treadmill exercise can cause stress, and thereby cause damage to memory. The present results suggest that although fluoxetine may improve memory in intact rats but it cannot prevent damages that are caused by forced exercise.

Effects of doxepin on brain-derived neurotrophic factor, tumor necrosis factor alpha, mitogen-activated protein kinase 14, and AKT1 genes expression in rat hippocampus

BACKGROUND

It has been suggested that doxepin in addition to enhancement of noradrenaline and serotonin levels may have neuroprotective effects. Therefore, this study investigated the effect of doxepin on gene expression of brain-derived neurotrophic factor (BDNF), tumor necrosis factor alpha (TNF-α), mitogen-activated protein kinase 14 (MAPK14), and serine-threonine protein kinase AKT1 in rat hippocampus.

MATERIALS AND METHODS

Male rats were divided randomly into three groups: Control, doxepin 1 mg/kg, and doxepin 5 mg/kg. Rats received an i.p injection of doxepin for 21 days. Then the hippocampi were dissected for the measurement of the expression of BDNF, TNF-α, MAPK14, and AKT1 genes.

RESULTS

Our results showed no significant effects of doxepin on gene expression of BDNF, TNF-α, MAPK14, and AKT1 genes in the hippocampus.

CONCLUSIONS

These results did not show significant effects of doxepin on the genes that affect the neuronal survival in intact animals. However, more studies need to be done, especially in models associated with neuronal damage.

Sex differences in stress-related psychiatric disorders: neurobiological perspectives

Stress is associated with the onset and severity of several psychiatric disorders that occur more frequently in women than men, including posttraumatic stress disorder (PTSD) and depression. Patients with these disorders present with dysregulation of several stress response systems, including the neuroendocrine response to stress, corticolimbic responses to negatively valenced stimuli, and hyperarousal. Thus, sex differences within their underlying circuitry may explain sex biases in disease prevalence. This review describes clinical studies that identify sex differences within the activity of these circuits, as well as preclinical studies that demonstrate cellular and molecular sex differences in stress responses systems. These studies reveal sex differences from the molecular to the systems level that increase endocrine, emotional, and arousal responses to stress in females. Exploring these sex differences is critical because this research can reveal the neurobiological underpinnings of vulnerability to stress-related psychiatric disorders and guide the development of novel pharmacotherapies.

Adult neuroplasticity: more than 40 years of research

Within the last four decades, our view of the mature vertebrate brain has changed significantly. Today it is generally accepted that the adult brain is far from being fixed. A number of factors such as stress, adrenal and gonadal hormones, neurotransmitters, growth factors, certain drugs, environmental stimulation, learning, and aging change neuronal structures and functions. The processes that these factors may induce are morphological alterations in brain areas, changes in neuron morphology, network alterations including changes in neuronal connectivity, the generation of new neurons (neurogenesis), and neurobiochemical changes. Here we review several aspects of neuroplasticity and discuss the functional implications of the neuroplastic capacities of the adult and differentiated brain with reference to the history of their discovery.

Effect of acute and chronic tianeptine on the action of classical antiepileptics in the mouse maximal electroshock model

BACKGROUND

The aim of the study was to analyze the influence of acute and chronic treatment with tianeptine, an antidepressant selectively accelerating presynaptic serotonin reuptake, on the protective activity of classical antiepileptic drugs in the maximal electroshock test in mice.

METHODS

Electroconvulsions were produced by means of an alternating current (50 Hz, 25 mA, 0.2 s) delivered via ear-clip electrodes. Motor impairment and long-term memory deficits in animals were quantified in the chimney test and in the passive-avoidance task, respectively. Brain concentrations of antiepileptic drugs were measured by fluorescence polarization immunoassay.

RESULTS

Acute and chronic treatment with tianeptine (25-50 mg/kg) did not affect the electroconvulsive threshold. Furthermore, tianeptine applied in both acute and chronic protocols enhanced the anticonvulsant action of valproate and carbamazepine, but not that of phenytoin. Neither acute nor chronic tianeptine changed the brain concentrations of valproate, carbamazepine or phenytoin. On the other hand, both single and chronic administration of tianeptine diminished the brain concentration of phenobarbital. In spite of this pharmacokinetic interaction, the antidepressant enhanced the antielectroshock action of phenobarbital. In terms of adverse effects, acute/chronic tianeptine (50 mg/kg) and its combinations with classic antiepileptic drugs did not impair motor performance or long-term memory in mice.

CONCLUSION

The obtained results justify the conclusion that tianeptine may be beneficial in the treatment of depressive disorders in the course of epilepsy.

Long-term adaptive changes induced by serotonergic antidepressant drugs

The development of conventional antidepressants has been largely based on the hypothesis of monoaminergic dysfunctions and focuses particularly on the serotonin 5-hydroxytryptamine (5-HT) system. Hence, various classes of antidepressant treatments enhance 5-HT neurotransmission with a time course consistent with their delayed therapeutic effect. This delayed onset appears to be associated with the gradual development of specific adaptive changes of functional 5-HT receptors. However, recent theories suggest that major depressive disorders may be associated with impairments of functional plasticity and cellular flexibility. This review discusses several physiological mechanisms by which 5-HT function and hippocampal neuroplasticity are regulated. Knowledge of these long-term adaptations will increase not only our understanding of pathological processes underlying affective disorders, but could also lead to the development of new strategies to treat these devastating illnesses.

Adult hippocampal neurogenesis in depression: behavioral implications and regulation by the stress system

Adult hippocampal neurogenesis, the birth of new neurons in the dentate gyrus of the adult brain, can be regulated by stress and antidepressant treatment, and has consistently been implicated in the behavioral neurobiology of stress-related disorders, especially depression and anxiety. A reciprocal relationship between hippocampal neurogenesis and the hypothalamus-pituitary-adrenal (HPA) axis has recently been suggested, which may play a crucial role in the development and in the resolution of depressive symptoms. This chapter will review some of the existing evidence for stress- and antidepressant-induced changes in adult hippocampal neurogenesis, and critically evaluate the behavioral effects of these changes for depression and anxiety. The potential role of neurogenesis as a neurobiological mechanism for sustained remission from depressive symptoms will be discussed, integrating existing data from clinical studies, animal work, and cellular models. The effect of glucocorticoid hormones and the glucocorticoid receptor (GR) will thereby be evaluated as a central mechanism by which stress and antidepressant may exert their opposing effects on neurogenesis, and ultimately, on mood and behavior.

5-HTT deficiency affects neuroplasticity and increases stress sensitivity resulting in altered spatial learning performance in the Morris water maze but not in the Barnes maze

The purpose of this study was to evaluate whether spatial hippocampus-dependent learning is affected by the serotonergic system and stress. Therefore, 5-HTT knockout (-/-), heterozygous (+/-) and wildtype (+/+) mice were subjected to the Barnes maze (BM) and the Morris water maze (WM), the latter being discussed as more aversive. Additionally, immediate early gene (IEG) expression, hippocampal adult neurogenesis (aN), and blood plasma corticosterone were analyzed.

While the performance of 5-HTT-/- mice in the BM was undistinguishable from both other genotypes, they performed worse in the WM. However, in the course of the repeated WM trials 5-HTT-/- mice advanced to wildtype level. The experience of a single trial of either the WM or the BM resulted in increased plasma corticosterone levels in all genotypes. After several trials 5-HTT-/- mice exhibited higher corticosterone concentrations compared with both other genotypes in both tests. Corticosterone levels were highest in 5-HTT-/- mice tested in the WM indicating greater aversiveness of the WM and a greater stress sensitivity of 5-HTT deficient mice.

Quantitative immunohistochemistry in the hippocampus revealed increased cell counts positive for the IEG products cFos and Arc as well as for proliferation marker Ki67 and immature neuron marker NeuroD in 5-HTT-/- mice compared to 5-HTT+/+ mice, irrespective of the test. Most differences were found in the suprapyramidal blade of the dentate gyrus of the septal hippocampus. Ki67-immunohistochemistry revealed a genotype x environment interaction with 5-HTT genotype differences in naïve controls and WM experience exclusively yielding more Ki67-positive cells in 5-HTT+/+ mice. Moreover, in 5-HTT-/- mice we demonstrate that learning performance correlates with the extent of aN.

Overall, higher baseline IEG expression and increased an in the hippocampus of 5-HTT-/- mice together with increased stress sensitivity may constitute the neurobiological correlate of raised alertness, possibly impeding optimal learning performance in the more stressful WM.

Effects of different doses of doxepin on passive avoidance learning in rats

BACKGROUND

Studies have shown that Doxepin has anti-inflammatory effects and reduces oxidative stress. Due to the fact that other tricyclic antidepressants have been shown to have neuroprotective effects, this study aimed to investigate the effects of different doses of doxepin on passive avoidance learning in rats.

MATERIALS AND METHODS

Old male Wistar rats were used in this study. Doxepin was administered intraperitoneally (1, 5 and 10 mg/kg) for 21 days. Passive avoidance learning test was used for evaluation of learning and memory. Rats received foot electrical shock on fifteen day, and step through latencies were evaluated one week after the electrical shock in retention phase.

RESULTS

Administration of Doxepin considerably increased the step through latencies in the rats that received the doses of 1 and 5 mg/kg (P < 0.05). However, in the dose of 10 mg/kg, there wasn't any significant change comparing to control group.

CONCLUSION

These results indicate that Doxepin has desirable effects on cognitive functions in low doses. Therefore, Doxepin can be considered as memory enhancers that understanding the underling mechanisms need further investigation.

Differential effectiveness of tianeptine, clonidine and amitriptyline in blocking traumatic memory expression, anxiety and hypertension in an animal model of PTSD

Individuals exposed to life-threatening trauma are at risk for developing post-traumatic stress disorder (PTSD), a debilitating condition that involves persistent anxiety, intrusive memories and several physiological disturbances. Current pharmacotherapies for PTSD manage only a subset of these symptoms and typically have adverse side effects which limit their overall effectiveness. We evaluated the effectiveness of three different pharmacological agents to ameliorate a broad range of PTSD-like symptoms in our established predator-based animal model of PTSD. Adult male Sprague-Dawley rats were given 1-h cat exposures on two occasions that were separated by 10 days, in conjunction with chronic social instability. Beginning 24 h after the first cat exposure, rats received daily injections of amitriptyline, clonidine, tianeptine or vehicle. Three weeks after the second cat exposure, all rats underwent a battery of behavioral and physiological tests. The vehicle-treated, psychosocially stressed rats demonstrated a robust fear memory for the two cat exposures, as well as increased anxiety expressed on the elevated plus maze, an exaggerated startle response, elevated heart rate and blood pressure, reduced growth rate and increased adrenal gland weight, relative to the vehicle-treated, non-stressed (control) rats. Neither amitriptyline nor clonidine was effective at blocking the entire cluster of stress-induced sequelae, and each agent produced adverse side effects in control subjects. Only the antidepressant tianeptine completely blocked the effects of psychosocial stress on all of the physiological and behavioral measures that were examined. These findings illustrate the differential effectiveness of these three treatments to block components of PTSD-like symptoms in rats, and in particular, reveal the profile of tianeptine as the most effective of all three agents.

The action of antidepressants on the glutamate system: regulation of glutamate release and glutamate receptors

Recent compelling evidence has suggested that the glutamate system is a primary mediator of psychiatric pathology and also a target for rapid-acting antidepressants. Clinical research in mood and anxiety disorders has shown alterations in levels, clearance, and metabolism of glutamate and consistent volumetric changes in brain areas where glutamate neurons predominate. In parallel, preclinical studies with rodent stress and depression models have found dendritic remodeling and synaptic spines reduction in corresponding areas, suggesting these as major factors in psychopathology. Enhancement of glutamate release/transmission, in turn induced by stress/glucocorticoids, seems crucial for structural/functional changes. Understanding mechanisms of maladaptive plasticity may allow identification of new targets for drugs and therapies. Interestingly, traditional monoaminergic-based antidepressants have been repeatedly shown to interfere with glutamate system function, starting with modulation of N-methyl-D-aspartate (NMDA) receptors. Subsequently, it has been shown that antidepressants reduce glutamate release and synaptic transmission; in particular, it was found antidepressants prevent the acute stress-induced enhancement of glutamate release. Additional studies have shown that antidepressants may partly reverse the maladaptive changes in synapses/circuitry in stress and depression models. Finally, a number of studies over the years have shown that these drugs regulate glutamate receptors, reducing the function of NMDA receptors, potentiating the function of α-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid receptors, and, more recently, exerting variable effects on different subtypes of metabotropic glutamate receptors. The development of NMDA receptor antagonists has opened new avenues for glutamatergic, rapid acting, antidepressants, while additional targets in the glutamate synapse await development of new compounds for better, faster antidepressant action.

Synaptic regulation of affective behaviors; role of BDNF

Brain derived neurotrophic factor (BDNF), a neurotrophin essential for nervous system development and synaptic plasticity, has been found to have a significant influence on affective behaviors. The notion that an impairment in BDNF signaling might be involved in affective disorders is originated primarily from the opposing effects of antidepressants and stress on BDNF signaling. Antidepressants enhance BDNF signaling and synaptic plasticity. On the other hand, negative environmental factors such as severe stress suppress BDNF signaling, impair synaptic activity and increase susceptibility to affective disorders. Postmortem studies provided strong support for decreased BDNF signaling in depressive disorders. Remarkably, studies in humans with a single nucleotide polymorphism in the BDNF gene, the BDNF Val66Met which affects regulated release of BDNF, showed profound deficits in hippocampal and prefrontal cortical (PFC) plasticity and cognitive behaviors. BDNF regulates synaptic mechanisms responsible for various cognitive processes including attenuation of aversive memories, a key process in the regulation of affective behaviors. The unique role of BDNF in cognitive and affective behaviors suggests that cognitive deficits due to altered BDNF signaling might underlie affective disorders. Understanding how BDNF modulates synapses in neural circuits relevant to affective behaviors, particularly the medial prefrontal cortical (mPFC)-hippocampus-amygdala pathway, and its interaction with development, sex, and environmental risk factors might shed light on potential therapeutic targets for affective disorders. This article is part of the Special Issue entitled 'BDNF Regulation of Synaptic Structure, Function, and Plasticity'.

The growth factors cascade and the dendrito-/synapto-genesis versus cell survival in adult hippocampal neurogenesis: the chicken or the egg

The decision between cellular survival and death is governed by a balance between proapoptotic versus antiapoptotic signaling cascades. Growth factors are key actors, playing two main roles both at developmental and adult stages: a supporting antiapoptotic role through diverse actions converging in the mitochondria, and a promoter role of cell maturation and plasticity through dendritogenesis and synaptogenesis, especially relevant for the adult hippocampal neurogenesis, a case of development during adulthood. Here, both parallel roles mutually feed forward each other (the success in avoiding apoptosis lets the cell to grow and differentiate, which in turn lets the cell to reach new targets and form new synapses accessing new sources of growth factors to support cell survival) in a circular cause and consequence, or a "the chicken or the egg" dilemma. While identifying the first case of this dilemma makes no sense, one possible outcome might have biological relevance: the decision between survival and death in the adult hippocampal neurogenesis is mainly concentrated at a specific time window, and recent data suggest some divergences between the survival and the maturational promoter effect of growth factors. This review summarizes these evidences suggesting how growth factors might contribute to the live-or-die decision of adult-born immature granule neurons through influencing the maturation of the young neuron by means of its connectivity into a mature functional circuit.

Atrophy of pyramidal neurons and increased stress-induced glutamate levels in CA3 following chronic suppression of adult neurogenesis

Following their birth in the adult hippocampal dentate gyrus, newborn progenitor cells migrate into the granule cell layer where they differentiate, mature, and functionally integrate into existing circuitry. The hypothesis that adult hippocampal neurogenesis is physiologically important has gained traction, but the precise role of newborn neurons in hippocampal function remains unclear. We investigated whether loss of new neurons impacts dendrite morphology and glutamate levels in area CA3 of the hippocampus by utilizing a human GFAP promoter-driven thymidine kinase genetic mouse model to conditionally suppress adult neurogenesis. We found that chronic ablation of new neurons induces remodeling in CA3 pyramidal cells and increases stress-induced release of the neurotransmitter glutamate. The ability of persistent impairment of adult neurogenesis to influence hippocampal dendrite morphology and excitatory amino acid neurotransmission has important implications for elucidating newborn neuron function, and in particular, understanding the role of these cells in stress-related excitoxicity.