Cell proliferation in adult hippocampus is decreased by inescapable stress: reversal by fluoxetine treatment

Decreased Hippocampal Neurogenesis Following Olfactory Bulbectomy is Reversed by Repeated Citalopram Administration

1. Whereas much progress has been made in the treatment of depression, the exact pathogenetic mechanisms of the disorder are still poorly understood. It has been proposed that one possible mechanism could be a decrease in adult hippocampal neurogenesis. 2. The olfactory bulbectomy (OB) in rats is widely accepted as an animal model of depression. In the present study, we investigated whether hippocampal neurogenesis is affected by an OB, and whether chronic citalopram, a serotonin selective reuptake inhibitor, counteracts OB-induced impairment of neurogenesis. 3. Our study shows that OB decreases proliferation of the neuronal precursors in the dentate gyrus and retards their differentiation into mature granule neurons. In OB rats, repeated administration of citalopram restores reduced proliferative activity and enhances the differentiation of precursors into mature calbindin-positive neurons. 4. The obtained data demonstrate that a citalopram-induced increase in neurogenesis in OB rats could be one possible mechanism by which antidepressants alleviate OB-induced depressive-like behavior.

Hippocampal neurogenesis: regulation by stress and antidepressants

Accumulating evidence implicates hippocampal neurogenesis in the pathophysiology of depression. Psychosocial stress reduces neurogenesis in rodents, whereas chronic treatment with antidepressants increases neurogenesis and blocks the effects of stress. The effects of stress and antidepressant treatment on hippocampal neurogenesis parallel behavioral changes in animal models. Moreover, ablating hippocampal neurogenesis renders antidepressants inactive in behavioral paradigms used to model antidepressant response and anxiety-like behavior in mice. In humans, monoamine-modulating antidepressants demonstrate clinical efficacy in treating depression and anxiety, which are often precipitated by psychosocial stress. This review examines the mounting evidence that stress and antidepressant treatment regulate neurogenesis in animals. Special attention is paid to the cellular and molecular mechanisms by which this regulation takes place. An analysis of current animal models used to study response to stress and antidepressants indicates the importance of modeling chronic treatment, which reflects both changes in neurogenesis and clinical response. Exploring responses of hippocampal neurogenesis to experimental challenges in appropriate animal models should delineate the role of adult-born neurons in hippocampal physiology. Focusing on neurogenic response to experimental paradigms of stress and antidepressant treatment is particularly interesting for understanding the pathophysiology of major depressive disorder.

A neurotrophic model for stress-related mood disorders

There is a growing body of evidence demonstrating that stress decreases the expression of brain-derived neurotrophic factor (BDNF) in limbic structures that control mood and that antidepressant treatment reverses or blocks the effects of stress. Decreased levels of BDNF, as well as other neurotrophic factors, could contribute to the atrophy of certain limbic structures, including the hippocampus and prefrontal cortex that has been observed in depressed subjects. Conversely, the neurotrophic actions of antidepressants could reverse neuronal atrophy and cell loss and thereby contribute to the therapeutic actions of these treatments. This review provides a critical examination of the neurotrophic hypothesis of depression that has evolved from this work, including analysis of preclinical cellular (adult neurogenesis) and behavioral models of depression and antidepressant actions, as well as clinical neuroimaging and postmortem studies. Although there are some limitations, the results of these studies are consistent with the hypothesis that decreased expression of BDNF and possibly other growth factors contributes to depression and that upregulation of BDNF plays a role in the actions of antidepressant treatment.

Agomelatine, a new antidepressant, induces regional changes in hippocampal neurogenesis

BACKGROUND

Antidepressant treatments increase neural plasticity and adult neurogenesis, especially in the hippocampus. Here, we determined the effects of agomelatine (S-20098), a new antidepressant, on various phases of neurogenesis in the dentate gyrus of adult rat.

METHODS

Animals were injected with agomelatine for different time periods. Immunostaining for bromodeoxyuridine, neuron specific nuclear protein, and glial fibrillary acid protein, as well as for the highly polysialylated form of neuronal cell adhesion molecule and doublecortin, was used to detect changes in cell proliferation, neurogenesis, and survival. Cell death was estimated by terminal deoxynucleotidyl transferase-mediated biotinylated deoxyuridine triphosphate nick end labeling and cresyl violet staining.

RESULTS

Chronic (3 weeks) but not acute (4 hours) or subchronic (1 week) administration of agomelatine increased cell proliferation and neurogenesis in the ventral dentate gyrus, a region notably implicated in response to emotion, which is consistent with the antidepressant-anxiolytic properties of the drug. Extending agomelatine treatment over several weeks, however, increases survival of newly formed neurons in the entire dentate gyrus. Finally, agomelatine treatment does not affect mature granule cells.

CONCLUSIONS

This study shows that an antidepressant can affect differentially various stages of neurogenesis in the dorsal and ventral hippocampus. Altogether, these changes lead to a pronounced augmentation in the total number of new granule cells.

Cell proliferation is influenced by bulbectomy and normalized by imipramine treatment in a region-specific manner

Growing evidence indicates that alterations of neuroplasticity may contribute to the pathophysiology of depression. In contrast, various antidepressants increase adult hippocampal neurogenesis and block the effects of stress. These findings result in the 'neurogenesis hypothesis of depression'. The present study seeks to determine out whether cell proliferation is altered in the hippocampus, subventricular zone (SVZ), and basolateral amygdala of adult rats exposed to bilateral olfactory bulbectomy, another established model of depression and, if so, how imipramine effects bulbectomy-induced changes of cell genesis. Bulbectomy results in a significant reduction of cell proliferation in the hippocampus and SVZ, an effect that is normalized by subchronic doses of imipramine. Moreover, an increase in cell genesis in the basolateral amygdala, which is not affected by imipramine, is demonstrated. TUNEL staining indicates an enhanced apoptosis after bulbectomy in the SVZ that cannot be reduced by imipramine. Cell death rates in the hippocampus and amygdala are not affected by bulbectomy. The opposing effects of bulbectomy and imipramine treatment in the hippocampus and amygdala demonstrate that these structures of the limbic system, both integrated in emotional processing, react quite differently with regard to neuroplasticity. Further to this, we discuss a possible link between the pathogenesis of depression and changed neuronal plasticity in the SVZ.

Fluoxetine targets early progenitor cells in the adult brain

Chronic treatment with antidepressants increases neurogenesis in the adult hippocampus. This increase in the production of new neurons may be required for the behavioral effects of antidepressants. However, it is not known which class of cells within the neuronal differentiation cascade is targeted by the drugs. We have generated a reporter mouse line, which allows identification and classification of early neuronal progenitors. It also allows accurate quantitation of changes induced by neurogenic agents in these distinct subclasses of neuronal precursors. We use this line to demonstrate that the selective serotonin reuptake inhibitor antidepressant fluoxetine does not affect division of stem-like cells in the dentate gyrus but increases symmetric divisions of an early progenitor cell class. We further demonstrate that these cells are the sole class of neuronal progenitors targeted by fluoxetine in the adult brain and suggest that the fluoxetine-induced increase in new neurons arises as a result of the expansion of this cell class. This finding defines a cellular target for antidepressant drug therapies.

Meditation states and traits: EEG, ERP, and neuroimaging studies

Neuroelectric and imaging studies of meditation are reviewed. Electroencephalographic measures indicate an overall slowing subsequent to meditation, with theta and alpha activation related to proficiency of practice. Sensory evoked potential assessment of concentrative meditation yields amplitude and latency changes for some components and practices. Cognitive event-related potential evaluation of meditation implies that practice changes attentional allocation. Neuroimaging studies indicate increased regional cerebral blood flow measures during meditation. Taken together, meditation appears to reflect changes in anterior cingulate cortex and dorsolateral prefrontal areas. Neurophysiological meditative state and trait effects are variable but are beginning to demonstrate consistent outcomes for research and clinical applications. Psychological and clinical effects of meditation are summarized, integrated, and discussed with respect to neuroimaging data.

Multi-target strategies for the improved treatment of depressive states: Conceptual foundations and neuronal substrates, drug discovery and therapeutic application

Major depression is a debilitating and recurrent disorder with a substantial lifetime risk and a high social cost. Depressed patients generally display co-morbid symptoms, and depression frequently accompanies other serious disorders. Currently available drugs display limited efficacy and a pronounced delay to onset of action, and all provoke distressing side effects. Cloning of the human genome has fuelled expectations that symptomatic treatment may soon become more rapid and effective, and that depressive states may ultimately be "prevented" or "cured". In pursuing these objectives, in particular for genome-derived, non-monoaminergic targets, "specificity" of drug actions is often emphasized. That is, priority is afforded to agents that interact exclusively with a single site hypothesized as critically involved in the pathogenesis and/or control of depression. Certain highly selective drugs may prove effective, and they remain indispensable in the experimental (and clinical) evaluation of the significance of novel mechanisms. However, by analogy to other multifactorial disorders, "multi-target" agents may be better adapted to the improved treatment of depressive states. Support for this contention is garnered from a broad palette of observations, ranging from mechanisms of action of adjunctive drug combinations and electroconvulsive therapy to "network theory" analysis of the etiology and management of depressive states. The review also outlines opportunities to be exploited, and challenges to be addressed, in the discovery and characterization of drugs recognizing multiple targets. Finally, a diversity of multi-target strategies is proposed for the more efficacious and rapid control of core and co-morbid symptoms of depression, together with improved tolerance relative to currently available agents.

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Modulation of adult hippocampal neurogenesis by thyroid hormones: implications in depressive-like behavior

Hormonal imbalances are involved in many of the age-related pathologies, as neurodegenerative and psychiatric diseases. Specifically, thyroid state alterations in the adult are related to psychological changes and mood disorders as depression. The dentate gyrus of the hippocampal formation undergoes neurogenesis in adult mammals including humans. Recent evidence suggests that depressive disorders and their treatment are tightly related to the number of newly born neurons in the dentate gyrus. We have studied the effect of thyroid hormones (TH) on hippocampal neurogenesis in adult rats in vivo. A short period of adult-onset hypothyroidism impaired normal neurogenesis in the subgranular zone of the dentate gyrus with a 30% reduction in the number of proliferating cells. Hypothyroidism also reduced the number of newborn neuroblasts and immature neurons (doublecortin (DCX) immunopositive cells) which had a severely hypoplastic dendritic arborization. To correlate these changes with hippocampal function, we subjected the rats to the forced swimming and novel object recognition tests. Hypothyroid rats showed normal memory in object recognition, but displayed abnormal behavior in the forced swimming test, indicating a depressive-like disorder. Chronic treatment of hypothyroid rats with TH not only normalized the abnormal behavior but also restored the number of proliferative and DCX-positive cells, and induced growth of their dendritic trees. Therefore, hypothyroidism induced a reversible depressive-like disorder, which correlated to changes in neurogenesis. Our results indicate that TH are essential for adult hippocampal neurogenesis and suggest that mood disorders related to adult-onset hypothyroidism in humans could be due, in part, to impaired neurogenesis.

Inhibition of neuronal nitric oxide synthase in the rat hippocampus induces antidepressant-like effects

RATIONALE

Systemic inhibition of neuronal nitric oxide synthase (nNOS) induces antidepressant-like effects in rodents. The mechanisms and brain regions mediating this effect are still unknown. The hippocampus is a brain region proposed to mediate adaptation to stress and antidepressant behavioral effects. Therefore, it could be involved in the antidepressant effects of NOS inhibitors.

OBJECTIVES

To test the hypothesis that nNOS inhibition in the dorsal hippocampus will induce antidepressant-like effects in the forced swimming test (FST) in rats.

METHODS

Rats implanted with cannulas aimed at the dorsal hippocampus were submitted to 15 min of forced swimming (pretest). Immediately before or after pretest they received bilateral microinjections of the nNOS inhibitor 7-nitroindazole (7-NI; 50, 100, or 200 nmol/0.5 microl) or vehicle, alone or combined with L-arginine. Additional groups received SIN-1 (125 or 250 nmol/0.5 microl), a NO donor, either before or after the pretest. Twenty-four hours later, immobility time was registered for 5 min in the FST.

RESULTS

7-NI (100 nmol) significantly decreased immobility time when administered either before or after pretest. Pretreatment with L-arginine (100 nmol/0.5 microl) prevented these effects but produced no significant effects per se. SIN-1 did not induce any significant effect.

CONCLUSION

These data suggest that the reduction of NO levels within the hippocampus can induce antidepressant-like effects; thus implicating endogenous hippocampal NO in the neurobiology of stress and depression.

Serotonin 5-HT2C receptors as a target for the treatment of depressive and anxious states: focus on novel therapeutic strategies

Serotonin (5-HT)2C receptors play an important role in the modulation of monoaminergic transmission, mood, motor behaviour, appetite and endocrine secretion, and alterations in their functional status have been detected in anxiodepressive states. Further, 5-HT2C sites are involved in the actions of several classes of antidepressant. At the onset of treatment, indirect activation of 5-HT2C receptors participates in the anxiogenic effects of selective 5-HT reuptake inhibitors (SSRIs) as well as their inhibition of sleep, sexual behaviour and appetite. Conversely, progressive down-regulation of 5-HT2C receptors parallels the gradual onset of clinical efficacy of SSRIs. Other antidepressants, such as nefazodone or mirtazapine, act as direct antagonists of 5-HT2C receptors. These observations underpin interest in 5-HT2C receptor blockade as a strategy for treating depressive and anxious states. This notion is supported by findings that 5-HT2C receptor antagonists stimulate dopaminergic and adrenergic pathways, exert antidepressant and anxiolytic actions in behavioural paradigms, and favour sleep and sexual function. In addition to selective antagonists, novel strategies for exploitation of 5-HT2C receptors embrace inverse agonists, allosteric modulators, ligands of homo/heterodimers, modulators of interactions with 'postsynaptic proteins', dual melatonin agonists/5-HT2C receptor antagonists and mixed 5-HT2C/alpha2-adrenergic antagonists. Intriguingly, there is evidence that stimulation of regionally discrete populations of 5-HT2C receptors is effective in certain behavioural models of antidepressant activity, and promotes neurogenesis in the hippocampus. This article explains how these ostensibly paradoxical actions of 5-HT2C antagonists and agonists can be reconciled and discusses both established and innovative strategies for the exploitation of 5-HT2C receptors in the improved management of depressed and anxious states.

Hippocampal upregulation of the cyclooxygenase-2 gene following neonatal clomipramine treatment (a model of depression)

Although a putative role has been attributed to inflammation in the pathogenesis of depressive disorders, the relationship of prostaglandins, known mediators of inflammation, and depression has not been elucidated. Clomipramine is an antidepressive drug with a pro-depressive paradoxical effect in adult rats when administrated neonatally. Using this effect as a model of depression, we investigated the differential expression of the cyclooxygenase (COX-2) gene in rat brains. Rats injected neonatally with clomipramine showed depressive-like symptoms in adulthood, as well as decreased levels of the brain-derived neurotrophic factor (BDNF) and a quantitative differential expression of the COX-2 gene (Real Time PCR) and protein (immunohistochemistry) in the hippocampus. As evidenced, the relationship between a key enzyme in the prostaglandin synthesis and biological and behavioral depression-like changes opens an interesting line of investigation regarding the molecular bases of depression and its potential treatment through immunomodulatory drugs.

Innovative approaches for the development of antidepressant drugs: current and future strategies

Depression is a highly debilitating disorder that has been estimated to affect up to 21% of the world population. Despite the advances in the treatment of depression with selective serotonin reuptake inhibitors (SSRIs) and serotonin and norepinephrine reuptake inhibitors (SNRIs), there continue to be many unmet clinical needs with respect to both efficacy and side effects. These needs range from efficacy in treatment resistant patients, to improved onset, to reductions in side effects such as emesis or sexual dysfunction. To address these needs, there are numerous combination therapies and novel targets that have been identified that may demonstrate improvements in one or more areas. There is tremendous diversity in the types of targets and approaches being taken. At one end of a spectrum is combination therapies that maintain the benefits associated with SSRIs but attempt to either improve efficacy or reduce side effects by adding additional mechanisms (5-HT1A, 5-HT1B, 5-HT1D, 5-HT2C, alpha-2A). At the other end of the spectrum are more novel targets, such as neurotrophins (BDNF, IGF), based on recent findings that antidepressants induce neurogenesis. In between, there are many approaches that range from directly targeting serotonin receptors (5-HT2C, 5-HT6) to targeting the multiplicity of potential mechanisms associated with excitatory (glutamate, NMDA, mGluR2, mGluR5) or inhibitory amino acid systems (GABA) or peptidergic systems (neurokinin 1, corticotropin-releasing factor 1, melanin-concentrating hormone 1, V1b). The present review addresses the most exciting approaches and reviews the localization, neurochemical and behavioral data that provide the supporting rationale for each of these targets or target combinations.

Unique patterns of FOS, phospho-CREB and BrdU immunoreactivity in the female rat brain following chronic stress and citalopram treatment

Affective disorders are common psychiatric illnesses characterized by marked gender-related prevalence. Recent evidence links chronic stress and dysregulation of neurotrophin signaling with the development of depression, while novel theories suggest that antidepressants may act by promoting intracellular adaptations linked to neuroplasticity. Although selective serotonin reuptake inhibitors (SSRIs) efficaciously improve a variety of dysfunctions in males, their neuroendocrine effects and intracellular signaling patterns in females are not well determined. Here we show that chronic footshock stress (21 days) promotes HPA axis hyperactivity (as seen by the increased FOS-ir in the paraventricular hypothalamic nucleus (PVN), plasma corticosterone and adrenal hypertrophy), reduces hippocampal BrdU immunoreactivity and suppresses cortical-limbic CREB phosphorylation in female rats. Long-term citalopram treatment, in contrast, attenuates stress-induced elevation of corticosterone levels and adrenal hypertrophy, although it does not reverse footshock-mediated induction of FOS-ir in the PVN, inhibition of CREB phosphorylation and reduction of hippocampal BrdU-labeling. Moreover, citalopram administration was also associated with significant hypophagic effects and inhibition of CREB phosphorylation. These data suggest that, in female rats, normalization of chronic stress-induced HPA axis abnormalities may represent an initial phase of citalopram-mediated therapeutic actions and despite this SSRI's apparent lack of effects on neuroplasticity, we cannot exclude the possibility that some neurochemical adaptations occur in a later stage which may require more than 3 weeks of treatment to manifest.

Dopamine specifically inhibits forebrain neural stem cell proliferation, suggesting a novel effect of antipsychotic drugs

Neurogenesis has been implicated in antidepressant drug action and animal models of depression, suggesting that proliferating cells play a role in psychiatric disorders. Similar studies using antipsychotic drugs have yielded conflicting results, perhaps because of the lack of focus on specific cell types. We examine the effect of haloperidol on neural stem cells (NSCs), the ultimate precursors for adult cell genesis. We show that haloperidol increases NSC numbers, resulting in more progenitors and more new neurons and glia in the adult rat brain. The increase in NSCs by haloperidol is dependent on central dopamine D2 receptors, and these receptors are expressed by NSCs. D2 receptor stimulation in vitro inhibits NSC proliferation, which is reversed by haloperidol. Thus, haloperidol increases adult mammalian brain proliferation by antagonizing dopamine at D2 receptors on NSCs. These findings demonstrate a direct link between neural activity and NSC proliferation and implicate cell genesis in antipsychotic drug effects.

'One night' sleep deprivation stimulates hippocampal neurogenesis

Neurogenesis in the adult hippocampus can be up- or downregulated in response to a variety of physiological and pathological conditions. Among these, dysregulation of hippocampal neurogenesis has been recently implicated in the pathogenesis of depression. In addition, in animal models of depression, a variety of antidepressant treatments reverse that condition by increasing neurogenesis. As one night sleep deprivation is known to improve mood in depressed patients for at least 1 day, we investigated whether a comparable treatment may affect hippocampal neurogenesis in adult rats. Accordingly, rats were sleep-deprived by gentle handling for 12 h during their physiological period of rest, and were injected with bromodeoxyuridine 4 h and 2 h before the end of sleep deprivation. They were then perfused immediately thereafter, or after 15 days and 30 days. We found that 12 h sleep deprivation significantly increased cell proliferation and the total number of surviving cells in the hippocampal dentate gyrus soon after sleep deprivation, as well as 15 days and 30 days later, in comparison to control rats allowed to sleep. No changes were instead found in the subventricular zone of the lateral ventricles, indicating that 12 h sleep deprivation selectively triggers neurogenic signals to the hippocampus. The present data include acute sleep deprivation among the conditions which upregulate hippocampal neurogenesis and raise the possibility that such response could be implicated in the beneficial effects elicited in depressed patients by one night sleep deprivation. Thus, the findings could contribute to the understanding of the intriguing relationship between depression and neurogenesis in the adult brain.

Selective serotonin depletion does not regulate hippocampal neurogenesis in the adult rat brain: differential effects of p-chlorophenylalanine and 5,7-dihydroxytryptamine

Serotonin is suggested to regulate adult hippocampal neurogenesis, and previous studies with serotonin depletion reported either a decrease or no change in adult hippocampal progenitor proliferation. We have addressed the effects of serotonin depletion on distinct aspects of adult hippocampal neurogenesis, namely the proliferation, survival and terminal differentiation of hippocampal progenitors. We used the serotonin synthesis inhibitor p-chlorophenylalanine (PCPA) or the serotonergic neurotoxin 5,7-dihydroxytryptamine (5,7-DHT) to deplete serotonin levels. 5,7-DHT selectively decreased hippocampal serotonin levels, while PCPA resulted in a significant decline in both serotonin and norepinephrine levels. We observed a robust decline in the proliferation and survival of adult hippocampal progenitors following PCPA treatment. This was supported by a decrease in the number of doublecortin-positive cells in the neurogenic niche in the hippocampus. In striking contrast, 5,7-DHT did not alter the proliferation or survival of adult hippocampal progenitors and did not alter the number of doublecortin-positive cells. The terminal differentiation of adult hippocampal progenitors was not altered by either PCPA or 5,7-DHT treatment. An acute increase in serotonin levels also did not influence adult hippocampal progenitor proliferation. These results suggest that selective serotonin depletion or an acute induction in serotonin levels does not regulate adult hippocampal neurogenesis, whereas treatment with PCPA that induces a decline in both serotonin and norepinephrine levels results in a significant decrease in adult hippocampal neurogenesis. Our results highlight the need for future studies to examine the role of other monoamines in both the effects of stress and antidepressants on adult hippocampal neurogenesis.

Corticosterone-induced inhibition of gliogenesis in rat hippocampus is counteracted by electroconvulsive seizures

BACKGROUND

Volumetric changes and glial pathology have been reported in the central nervous system (CNS) of patients with depressive disorder, an illness often associated with elevated glucocorticoid levels. Glucocorticoids reduce gliogenesis in the adult rat CNS. Electroconvulsive seizure (ECS)-treatment, an animal model for the antidepressant treatment electroconvulsive therapy, can enhance proliferation of glial cells. This study examined glial cell proliferation in response to ECS in rats whose glucocorticoid levels were elevated to mimic the conditions seen in depression.

METHODS

Rats were injected daily for seven days with either corticosterone or vehicle. ECS- or sham- treatment was given once daily during the first five days. Proliferating cells in the hippocampus were labeled with bromodeoxyuridine and analyzed for co-labeling with the glial cell markers NG2, Ox42, S-100beta and Rip.

RESULTS

ECS counteracted the glucocorticoid-induced inhibition of NG2+, Ox42+ and Rip+ cell proliferation, and the gliogenesis rate was restored to baseline levels. Volumetric changes in rats treated with ECS were detected.

CONCLUSIONS

Our results show that ECS-treatment affects the proliferation of glial cells even in the presence of elevated levels of glucocorticoids. This result adds to an increasing number of studies suggesting that antidepressant treatment can counteract degenerative processes associated with major depression.

Is there a role for the endocannabinoid system in the etiology and treatment of melancholic depression?

With advances in basic and clinical neuroscience, many gaps have appeared in the traditional monoamine theory of depression that have led to reformulation of the hypotheses concerning the neurobiology of depression. The more recent hypotheses suggest that melancholic depression is characterized by central glucocorticoid resistance that results in hypercortisolemia, which in turn leads to down-regulation of neurotrophins and subsequent neurodegeneration. Examining the neurobiology of depression from this perspective suggests that the endocannabinoid system may play a role in the etiology of melancholic depression. Specifically, pharmacological and genetic blockade of the cannabinoid CB1 receptor induces a phenotypic state that is analogous to melancholic depression, including symptoms such as reduced food intake, heightened anxiety, increased arousal and wakefulness, deficits in extinction of aversive memories and supersensitivity to stress. These similarities between melancholic depression and an endocannabinoid deficiency become more interesting in light of recent findings that endocannabinoid activity is down-regulated by chronic stress and possibly increased by some antidepressant regimens. We propose that an endocannabinoid deficiency may underlie some of the symptoms of melancholic depression, and that enhancement of this system may ultimately be a novel form of pharmacotherapy for treatment-resistant depression.

Synergetic effects of quetiapine and venlafaxine in preventing the chronic restraint stress-induced decrease in cell proliferation and BDNF expression in rat hippocampus

Clinical studies show better response rates of patients with depression and schizophrenia to combinations of atypical antipsychotics and antidepressants, compared to responses to either type of drugs alone. Animal studies demonstrate that some antipsychotics and antidepressants increase neurogenesis and BDNF expression in the hippocampus, which is reduced in volume in patients with depression or schizophrenia. We hypothesized that the better therapeutic effects of combined treatment seen in schizophrenia and depression patients are related to the additive or synergistic effects of combined treatment on hippocampal neurogenesis and BDNF expression. To test this hypothesis, we investigated the effects of chronic administration of quetiapine, venlafaxine, and their combination, on hippocampal cell proliferation and BDNF expression in rats, when subjected to chronic restraint stress (CRS) during the last 2 weeks of a 3-week drug administration period. We found (1) CRS decreased hippocampal cell proliferation and BDNF expression; (2) chronic administration of quetiapine or venlafaxine dose-dependently prevented these decreases in hippocampal cell proliferation and BDNF expression caused by CRS (6 h/day for 14 days); (3) the combination of lower doses of quetiapine (5 mg/kg) and venlafaxine (2.5 mg/kg) increased hippocampal cell proliferation and prevented BDNF decrease in stressed rats, whereas each of the drugs exerted mild or no effects; (4) individual higher doses of quetiapine (10 mg/kg) or venlafaxine (5 mg/kg) exerted effects comparable to those produced by their combination. These results support our hypothesis and can lead to future studies to develop new therapeutic approaches for treatment-resistant depression and the negative symptoms of schizophrenia.

Alcohol and adult neurogenesis: Roles in neurodegeneration and recovery in chronic alcoholism

The concept of "structural plasticity" has emerged as a potential mechanism in neurodegenerative and psychiatric diseases such as drug abuse, depression, and dementia. Chronic alcoholism is a progressive neurodegenerative disease while the person continues to abuse alcohol, though clinical and imaging studies show that some recovery may occur with abstinence. The neural plasticity observed in chronic alcoholism coupled with conflicting reports on alcohol-induced hippocampal neuropathology make this disease ripe for reconsideration in terms of the phenomenon of adult neurogenesis. This review describes opposing neurogenic processes that occur with alcohol intoxication and abstinence following alcohol dependence and how these opposing events relate to neurodegeneration and recovery from chronic alcoholism.

Stress and adult neurogenesis

Stress hormones have potent growth-inhibiting effects on a variety of peripheral tissues. Consistent with this general function, stress has been shown to inhibit cell proliferation and, ultimately, neurogenesis in the hippocampus. This effect appears to be common across mammalian species, life stages, and most types of stressors. Although some evidence points to a role for glucocorticoids in mediating this effect, contradictory data exist. This review considers the growing literature on this subject with specific emphasis on paradoxical findings and the role of glucocorticoids in modulating adult neurogenesis.

Hippocampal neurogenesis: Opposing effects of stress and antidepressant treatment

The hippocampus is one of several limbic brain structures implicated in the pathophysiology and treatment of mood disorders. Preclinical and clinical studies demonstrate that stress and depression lead to reductions of the total volume of this structure and atrophy and loss of neurons in the adult hippocampus. One of the cellular mechanisms that could account for alterations of hippocampal structure as well as function is the regulation of adult neurogenesis. Stress exerts a profound effect on neurogenesis, leading to a rapid and prolonged decrease in the rate of cell proliferation in the adult hippocampus. In contrast, chronic antidepressant treatment up-regulates hippocampal neurogenesis, and could thereby block or reverse the atrophy and damage caused by stress. Recent studies also demonstrate that neurogenesis is required for the actions of antidepressants in behavioral models of depression. This review discusses the literature that has lead to a neurogenic hypothesis of depression and antidepressant action, as well as the molecular and cellular mechanisms that underlie the regulation of adult neurogenesis by stress and antidepressant treatment.

Impaired fear memory and decreased hippocampal neurogenesis following olfactory bulbectomy in rats

It has been proposed that a decrease in adult hippocampal neurogenesis provides a biological and cellular basis for major depression. The olfactory bulbectomy (OB) in rats is widely accepted as an animal model of depression. In the present study, we investigated the effect of OB on memory formation in the memory tasks related to the hippocampal function and adult hippocampal neurogenesis. OB induced a behavioural syndrome, which was characterized by an increased activity in the open-field test and impairment in passive avoidance behaviour and contextual fear conditioning. The behavioural changes, following OB, were accompanied by a decrease in the number of proliferating cells in the dentate gyrus. Furthermore, the differentiation of the newly born cells, into mature calbindin-positive neurons, was also retarded. Stereological analysis revealed a decrease in the total granule neuron numbers within the granule cell layer of the dentate gyrus, without a significant decrease in volume of the dentate gyrus. Although a relationship between altered neurogenesis and behavioural syndrome, induced by OB, is not established yet, our results suggest that decreased neurogenesis might at least partly contribute for behavioural deficits following OB.

Electroconvulsive seizures induce angiogenesis in adult rat hippocampus

BACKGROUND

Electroconvulsive seizure (ECS)-treatment, a model for electroconvulsive therapy (ECT) has been shown to induce proliferation of endothelial cells in the dentate gyrus (DG) of adult rats. Here we quantified the net angiogenic response after chronic ECS-treatment in the molecular layer (ML) of the dentate gyrus. Patients undergoing ECT are routinely oxygenated to prevent hypoxia, a known inducer of angiogenesis. Therefore we also examined the effect of oxygenation on ECS-induced proliferation of endothelial cells.

METHODS

Total endothelial cell numbers and vessel length were estimated utilizing design based stereological analysis methods. Endothelial cell proliferation in the DG after ECS with or without oxygenation was assessed using bromodeoxyuridine.

RESULTS

The total number of endothelial cells and total vessel length was increased. Oxygenation did not abolish the ECS-induced proliferation of endothelial cells in the DG.

CONCLUSIONS

ECS-treatment induces a dramatic increase in endothelial cell proliferation leading to a 30% increase in the total number of endothelial cells. The increase in cell number resulted in a 16% increase in vessel length. These findings raise the possibility that similar vascular growth is induced by clinically administered ECT.

Age-dependent susceptibility of adult hippocampal cell proliferation to chronic psychosocial stress

Stress inhibits neurogenesis in the adult dentate gyrus. It is not known, however, whether age has any influence on this process. We subjected adult male tree shrews to 5 weeks of psychosocial stress, after which dentate cytogenesis was determined using BrdU immunohistochemistry. We found that older animals were significantly more vulnerable to the adverse effect of stress on dentate cell proliferation.

Cognitive disorders and neurogenesis deficits in Huntington's disease mice are rescued by fluoxetine

Huntington's disease (HD) is a neurodegenerative disorder caused by an expanded CAG trinucleotide repeat encoding an extended polyglutamine tract in the huntingtin protein. Affected individuals display progressive motor, cognitive and psychiatric symptoms (including depression), leading to terminal decline. Given that transgenic HD mice have decreased hippocampal cell proliferation and that a deficit in neurogenesis has been postulated as an underlying cause of depression, we hypothesized that decreased hippocampal neurogenesis contributes to depressive symptoms and cognitive decline in HD. Fluoxetine, a serotonin-reuptake inhibitor commonly prescribed for the treatment of depression, is known to increase neurogenesis in the dentate gyrus of wild-type mouse hippocampus. Here we show that hippocampal-dependent cognitive and depressive-like behavioural symptoms occur in HD mice, and that the administration of fluoxetine produces a marked improvement in these deficits. Furthermore, fluoxetine was found to rescue deficits of neurogenesis and volume loss in the dentate gyrus of HD mice.

The pathogenesis of clinical depression: stressor- and cytokine-induced alterations of neuroplasticity

Stressful events promote neurochemical changes that may be involved in the provocation of depressive disorder. In addition to neuroendocrine substrates (e.g. corticotropin releasing hormone, and corticoids) and central neurotransmitters (serotonin and GABA), alterations of neuronal plasticity or even neuronal survival may play a role in depression. Indeed, depression and chronic stressor exposure typically reduce levels of growth factors, including brain-derived neurotrophic factor and anti-apoptotic factors (e.g. bcl-2), as well as impair processes of neuronal branching and neurogenesis. Although such effects may result from elevated corticoids, they may also stem from activation of the inflammatory immune system, particularly the immune signaling cytokines. In fact, several proinflammatory cytokines, such as interleukin-1, tumor necrosis factor-alpha and interferon-gamma, influence neuronal functioning through processes involving apoptosis, excitotoxicity, oxidative stress and metabolic derangement. Support for the involvement of cytokines in depression comes from studies showing their elevation in severe depressive illness and following stressor exposure, and that cytokine immunotherapy (e.g. interferon-alpha) elicited depressive symptoms that were amenable to antidepressant treatment. It is suggested that stressors and cytokines share a common ability to impair neuronal plasticity and at the same time altering neurotransmission, ultimately contributing to depression. Thus, depressive illness may be considered a disorder of neuroplasticity as well as one of neurochemical imbalances, and cytokines may act as mediators of both aspects of this illness.

Dépression et neuroplasticité : implication des systèmes sérotoninergiques

Neuroplasticity contributes to both normal and pathological brain function. A recent hypothesis links depression to lack of adaptive responses to stress or other aversive stimuli, and effects of antidepressant treatments on adult neurogenesis are more and more extensively studied because of the structural changes involved in the pathophysiology of depression. Indeed, neuronal remodelling in hippocampal formation is associated with chronic stress and is reversed by antidepressant treatments in animals. Decrease in hippocampal volume has also been associated to cognitive deficits in patients with major depression. Interestingly, serotonergic (5-HT) systems play a major role both as antidepressants and by increasing hippocampal neurogenesis through various receptor subtypes. Recently, we have also demonstrated that agomelatine, a new antidepressant drug having serotonergic and melatonergic properties, can increase proliferation and survival of newly formed hippocampal cells. Although the mechanisms underlying such effects are still unknown, these data reinforce the view that changes in hippocampal neurogenesis might belong to the cellular correlates of mood disorders.

The role of 5-HT1A receptors in the proliferation and survival of progenitor cells in the dentate gyrus of the adult hippocampus and their regulation by corticoids

These experiments explore the role of 5-HT1A receptors in the regulation of cell proliferation in the dentate gyrus of the intact and adrenalectomized adult rat. Depleting 5-HT with p-chlorophenylalanine (300 mg/kg initially followed by 100 mg/kg/day) or stimulating 5-HT1A receptors with 8-OH-DPAT (1 mg/kg or 2 mg/kg, s.c. injections twice daily) for 14 days had no effect on cell proliferation as measured by Ki-67 or BrdU (5-bromo-3-deoxyuridine) immunocytochemistry in the dentate gyrus. However, combined treatment with p-chlorophenylalanine followed by 8-OH-DPAT significantly increased cell proliferation compared with p-chlorophenylalanine alone. Micro-injection of the 5-HT neurotoxin 5,7-dihydroxytryptamine into the fimbria-fornix (3.0 microg/side) and the cingulate bundle (1.8 microg/side) depleted hippocampal 5-HT locally but did not change cell proliferation 3 weeks after the surgery. However, 8-OH-DPAT (1 mg/kg, twice daily) stimulated cell proliferation in the dentate gyrus of hippocampal 5-HT-depleted rats compared with controls. These results suggest that 5-HT(1A) modulates cell proliferation in the hippocampus by a direct post-synaptic effect. Previous studies demonstrate that adrenalectomy increases hippocampal 5-HT1A receptor expression and binding, and thus we investigated whether the effect of adrenalectomy on cell proliferation and survival was dependent on the activity of the 5-HT1A receptors. In contrast to the null effect following twice-daily s.c. injection, 8-OH-DPAT (2.0 mg/kg/day) delivered by s.c. osmotic pumps increased proliferation in intact rats. The 5-HT1A antagonist WAY-100635 (1.5 mg/kg/day also delivered by osmotic pump) by itself did not alter cell proliferation, confirming that reduced serotonin activity does not change proliferation, but blocked the effect of 8-OH-DPAT. However, WAY-100635 could not block the stimulating action of adrenalectomy cell proliferation. 5-HT1A mRNA expression was not altered in the hippocampus by adrenalectomy. Thus, the effect of adrenalectomy on cell proliferation and survival is not 5-HT1A dependent, despite the interaction between 5-HT1A and corticosterone.

Decreased proliferation in the adult rat hippocampus after exposure to the Morris water maze and its reversal by fluoxetine

Granular cell proliferation in the adult hippocampus decreases during aging and after chronic stress, while it can be increased by physical activity or treatment with the antidepressant fluoxetine. We investigated whether the physical and cognitive stimulation accompanied by stress in the commonly used Morris water maze affects the rate of proliferation and whether the induced changes can be influenced by antidepressant treatment with fluoxetine. Proliferating cells in the dentate gyrus were labeled by three injections of BrdU during the 24h preceding sacrifice. Early differentiation to neuronal progeny was studied by immunohistochemical staining for doublecortin (DCX), a microtubule binding protein expressed in newborn neurons. Acquisition learning in the water maze for 15 days caused a significant decrease in granular cell proliferation in the granular cell layer of the hippocampus. The decrease in the number of BrdU- and DCX-positive cells was reversed to control levels by the use of fluoxetine during the water maze training. Fluoxetine treatment alone increased the number of BrdU-positive cells, but did not increase the number of DCX-positive cells. We conclude that the exposure of adult male rats to water maze acquisition trials is a stressful experience that significantly suppresses the production of new granular cells and that this stressful effect can be blocked by the concomitant administration of the antidepressant fluoxetine.

Voltage-Sensitive and Ligand-Gated Channels in Differentiating Neural Stem-Like Cells Derived from the Nonhematopoietic Fraction of Human Umbilical Cord Blood

Fetal cells with the characteristics of neural stem cells (NSCs) can be derived from the nonhematopoietic fraction of human umbilical cord blood (HUCB), expanded as a nonimmortalized cell line (HUCB-NSC), and further differentiated into neuron-like cells (HUCB-NSCD); however, the functional and neuronal properties of these cells are poorly understood. To address this issue, we used whole-cell patch-clamp recordings, gene microarrays, and immunocytochemistry to identify voltage-gated channels and ligand-gated receptors on HUCB-NSCs and HUCB-NSCDs. Gene microarray analysis identified genes for voltage-dependent potassium and sodium channels and the neurotransmitter receptors acetylcholine (ACh), gamma-aminobutyric acid (GABA), glutamate, glycine, 5-hydroxytryptamine (5-HT), and dopamine (DA). Several of these genes (GABA-A, glycine and glutamate receptors, voltage-gated potassium channels, and voltage-gated sodium type XII alpha channels) were not expressed in the HUCB mono-nuclear fraction (HUCB-MC), which served as a starting cell population for HUCB-NSC. HUCB-NSCD acquired neuronal phenotypes and displayed an inward rectifying potassium current (Kir) and an outward rectifying potassium current (I(K+)). Kir was present on most HUCB-NSCs and HUCB-NSCDs, whereas I(K+) was present only on HUCB-NSCDs. Many HUCB-NSCDs were immunopositive for glutamate, glycine, nicotinic ACh, DA, 5-HT, and GABA receptors. Kainic acid (KA), a non-N-methyl-D-asparate (NMDA) glutamate-receptor agonist, induced an inward current in some HUCB-NSCDs. KA, glycine, DA, ACh, GABA, and 5-HT partially blocked Kir through their respective receptors. These results suggest that HUCB-NSCs differentiate toward neuron-like cells, with functional voltage- and ligand-gated channels identified in other neuronal systems.

Thyroid hormone regulates hippocampal neurogenesis in the adult rat brain

We have examined the influence of thyroid hormone on adult hippocampal neurogenesis, which encompasses the proliferation, survival and differentiation of dentate granule cell progenitors. Using bromodeoxyuridine (BrdU), we demonstrate that adult-onset hypothyroidism significantly decreases hippocampal neurogenesis. This decline is predominantly the consequence of a significant decrease in the survival and neuronal differentiation of BrdU-positive cells. Both the decreased survival and neuronal differentiation of hippocampal progenitors could be rescued by restored euthyroid status. Adult-onset hyperthyroidism did not influence hippocampal neurogenesis, suggesting that the effects of thyroid hormone may be optimally permissive at euthyroid levels. Our in vivo and in vitro results revealed that adult hippocampal progenitors express thyroid receptor isoforms. The in vitro studies demonstrate that adult hippocampal progenitors exhibit enhanced proliferation, survival and glial differentiation in response to thyroid hormone. These results support a role for thyroid hormone in the regulation of adult hippocampal neurogenesis and raise the possibility that altered neurogenesis may contribute to the cognitive and behavioral deficits associated with adult-onset hypothyroidism.

Nelumbinis Semen reverses a decrease in hippocampal 5-HT release induced by chronic mild stress in rats

Depression is associated with a dysfunctional serotonin system. Recently, several lines of evidence have suggested that a very important evoking factor in depression may be a serotonin deficit in the hippocampus. This study assessed the antidepression effects of Nelumbinis Semen (NS) through increasing serotonin concentrations under normal conditions and reversing a decrease in serotonin concentrations in rat hippo-campus with depression-like symptoms induced by chronic mild stress (CMS). Using an in-vivo microdialysis technique, the serotonin-enhancing effect of NS on rat hippocampus was investigated and its effects compared with those of two well-known antidepressants, Hypericum perforatum (St John's wort) and fluoxetine (Prozac). Rats were divided into five groups: saline-treated normal, without CMS; saline-treated stress control; NS-, St John's wort- and fluoxetine-treated rats under CMS for 8 weeks or no stress treatment. NS and fluoxetine significantly increased serotonin in normal conditions and reversed a CMS-induced decrease in serotonin release in the hippocampus (P<0.05 compared with normal group or control group under CMS). These results suggest that NS increases the serotonin levels normally decreased in depression, resulting in an enhancement of central serotonergic transmission and possible therapeutic action in depression. It is suggested that NS may present an antidepressant effect through enhancement of serotonin.

The role of monoamines in the actions of established and "novel" antidepressant agents: a critical review

Monoaminergic pathways are highly responsive to aversive stimuli and play a crucial role in the control of affect, cognition, endocrine secretion, chronobiotic rhythms, appetite, and motor function, all of which are profoundly disrupted in depressive states. Accordingly, a perturbation of monoaminergic transmission is implicated in the aetiology of depressive disorders, and all clinically available antidepressants increase corticolimbic availability of monoamines. However, their limited efficacy, delayed onset of action, and undesirable side effects underlie ongoing efforts to identify improved therapeutic agents. Sequencing the human genome has raised the hope not only of better symptomatic control of depression, but even of the prevention or cure of depressive states. In the pursuit of these goals, there is currently a tendency to focus on selective ligands of "novel" nonmonoaminergic targets. However, certain classes of novel agent (such as neurokinin(1) receptor antagonists) indirectly modulate the activity of monoaminergic networks. Others may act "downstream" of them, converging onto common cellular substrates controlling gene expression, synaptic plasticity, and neurogenesis. Further, by analogy to the broad-based actions of currently employed drugs, multitarget agents may be better adapted than selective agents to the management of depression-a complex disorder with hereditary, developmental, and environmental origins. It is, thus, important to continue the creative exploration of clinically validated and innovative monoaminergic strategies within a multitarget framework. In this light, drugs combining monoaminergic and nonmonoaminergic mechanisms of action may be of particular interest. The present article provides a critical overview of monoaminergic strategies for the treatment of depressive states, both established and under development, and discusses interactions of novel "nonmonoaminergic" antidepressants with monoaminergic mechanisms.

Neurobiology of mood, anxiety, and emotions as revealed by studies of a unique antidepressant: tianeptine

Recent studies have provided evidence that structural remodeling of certain brain regions is a feature of depressive illness, and the postulated underlying mechanisms contribute to the idea that there is more to antidepressant actions that can be explained exclusively by a monoaminergic hypothesis. This review summarizes recent neurobiological studies on the antidepressant, tianeptine (S-1574, [3-chloro-6-methyl-5,5-dioxo-6,11-dihydro-(c,f)-dibenzo-(1,2-thiazepine)-11-yl) amino]-7 heptanoic acid, sodium salt), a compound with structural similarities to the tricyclic antidepressant agents, the efficacy and good tolerance of which have been clearly established. These studies have revealed that the neurobiological properties of tianeptine involve the dynamic interplay between numerous neurotransmitter systems, as well as a critical role of structural and functional plasticity in the brain regions that permit the full expression of emotional learning. Although the story is far from complete, the schema underlying the effect of tianeptine on central plasticity is the most thoroughly studied of any antidepressants. Effects of tianeptine on neuronal excitability, neuroprotection, anxiety, and memory have also been found. Together with clinical data on the efficacy of tianeptine as an antidepressant, these actions offer insights into how compounds like tianeptine may be useful in the treatment of neurobiological features of depressive disorders.

Mood-stabilizing drugs: are their neuroprotective aspects clinically relevant?

The possibility that there may be subtypes of bipolar disorder and the slow progress in understanding the therapeutic mechanism for approved mood-stabilizing drugs make the challenges of intelligent drug design seem daunting. Nonetheless, the numerous shortcomings in current pharmaco-therapy underscore the need to develop novel therapies. There are significant problems with currently approved mood-stabilizing drugs: 1. Up to 40% of patients fail to respond to monotherapy with either lithium or valproic acid. 2. Common use of polypharmacotherapy increases the side effects associated with treatment. 3. Treatment must continue for weeks to months for therapeutic effects to be greater than placebo. 4. Up to 60% of patients will discontinue therapy, which is somewhat attributable to unwanted side effects. Thus, it is critical that new medications without these problems be developed for bipolar disorder. The hypothesis that mood-stabilizing drugs are neuroprotective is an important first step in new drug development. To determine if the clinical efficacy of mood-stabilizing drugs is dependent on the neuroprotective or neurogenic properties of these medications, greater strides need to be made in relating findings from cell culture and animal models to human imaging and pathology. Mounting evidence supports the neuroprotective and neurogenic properties of lithium and valproic acid ina variety of cell-culture models. It is important for clinical, biochemical, and in vitro differences between these medications to be examined, not ignored,because these differences may reveal critical distinctions between the neural mechanisms of these drugs. Continuation of the in vitro work will aid in the understanding of the mechanism by which these drugs are neuroprotective,but such studies do not advance the understanding of whether these effects are critical for the clinical efficacy of these medications. In attempting to understand the in vivo effects of these medications, a variety of evidence supports the neuroprotective and neurogenic aspects of lithium and valproic acid in healthy rodents and animal models of gross brain insult. More work needs to be done to assess whether these effects occur in animal models for bipolar disorder. The proof of principle for supporting the claim that the neuroprotective or neurogenic properties are important clinically will come from longitudinal clinical studies that compare brain morphology and function before and during treatment. If enough evidence supports the hypothesis that the neuroprotective and neurogenic properties of mood-stabilizing drugs are important for their clinical efficacy, new medications that are more efficacious and have fewer side effects will be designed based on this discovery.

Effects of active shock avoidance learning on hippocampal neurogenesis and plasma levels of corticosterone

Hippocampal granule neurons that are newly formed during adulthood might be involved in learning and memory processes. Experimental data suggest that only hippocampus-dependent learning tasks stimulate neurogenesis. To further address this issue, the effects of active shock avoidance (ASA) learning on hippocampal progenitor proliferation and survival of newly formed cells were investigated. ASA training, although considered as hippocampus-independent, is known to induce several neurobiological alterations in the hippocampus. Adult Wistar rats were trained in a shuttle box using a 1-day or 4-day paradigm and brains were analyzed for the mitotic marker Ki-67. Effects on survival of newly generated cells were examined by immunocytochemistry for 5-bromo-2-deoxyuridine (BrdU), which was injected 1 week before the training. Neither proliferation nor survival was affected by the ASA learning task. Because elevated glucocorticoid levels have a negative impact on hippocampal neurogenesis, blood samples were taken throughout the 4-day training paradigm. Both trained animals and control rats that were only placed in the shuttle box without receiving foot shocks showed a similar rise in corticosterone, enabling us to exclusively investigate the effects of ASA learning on neurogenesis without differential interference of stress between groups. On the other hand, the finding that ASA induced elevations in plasma corticosterone, but did not influence proliferation or survival of newly formed cells, indicates that this type of stress does not affect neurogenesis. The present study shows that, in line with the existing data on other hippocampus-independent learning tasks, ASA training has no effect on hippocampal neurogenesis.

Chronic treatment with milnacipran reverses the impairment of synaptic plasticity induced by conditioned fear stress

RATIONALE

Recent studies have focused on neural plasticity at the cellular and molecular levels in the etiology and treatment of stress-related disorders; however, there are no reports concerning modulation of synaptic plasticity in the hippocampus underlying therapeutic effects of antidepressants and/or anxiolytics.

OBJECTIVES

To elucidate the functional interaction between the stress-induced alteration of synaptic plasticity and therapeutic effects, we examined the anxiolytic mechanism(s) of milnacipran, focusing on modulation of long-term potentiation (LTP) in the hippocampal CA1 field.

METHODS

Rats that received footshock stimulation five times (intensity, 0.5 mA; duration, 2 s; shock interval, 30 s) for 5 days were treated with milnacipran (30 mg kg(-1), p.o.) or vehicle for 14 days. On the 15th day, rats were subjected to conditioned fear stress (CFS) to evaluate freezing behavior. Separate from the behavioral study, electrophysiological approach was performed to evaluate the synaptic efficacy under anesthesia.

RESULTS

Exposure to CFS suppressed LTP in the CA1 field. Chronic treatment with milnacipran (30 mg kg(-1), i.p. after 30 mg kg(-1) day(-1), p.o. x14 days), but not acute treatment (30 mg kg(-1), i.p. after vehicle 5 ml kg(-1) day(-1), p.o. x14 days), reduced freezing behavior and reversed the impairment of LTP induced by CFS.

CONCLUSION

The present data suggest that a correspondence exists between fear-related behavior and synaptic plasticity in the hippocampus. In other words, anxiolytic mechanism(s) of chronic treatment with milnacipran may be explained by reversal effects on the psychological stress-induced impairment of synaptic plasticity.

Acute diclofenac treatment attenuates lipopolysaccharide-induced alterations to basic reward behavior and HPA axis activation in rats

RATIONALE

Non-steroidal anti-inflammatory drugs (NSAIDs) counteract stress hormone and pro-inflammatory cytokine activation, and are being considered as therapeutics for Alzheimer's and Parkinson's disease, and multiple sclerosis. Previous data from our laboratory revealed that repeated treatment with the NSAID diclofenac attenuated lipopolysaccharide (LPS)-induced alterations to reward behavior, implicating a role for NSAIDs in alleviating depressive-like behavior.

OBJECTIVES

To extend these findings, we sought to determine whether acute treatment with diclofenac would attenuate LPS-induced alterations to basic reward behavior, as well as neuroendocrine and neuroimmune function.

METHODS

Male, Wistar rats (n=8-9/grp) pressed a lever for sucrose pellet reward and after establishing a steady baseline were exposed to an injection of saline (1 ml/kg, SC) or diclofenac (2.5 mg/kg, SC) 30 min prior to a second injection of saline or LPS (20 microg/kg, IP).

RESULTS

In saline pre-treated rats, LPS significantly reduced rate of sucrose pellet self-administration and total reinforcers obtained, suggestive of an anhedonia response. In addition, LPS increased corticosterone release, increased plasma intereleukin (IL)-1beta release, increased IL-1beta and IL-6 mRNA in hippocampus, increased corticotropin releasing hormone (CRH) mRNA in pituitary, and decreased CRH-1 mRNA in pituitary. Importantly, the behavioral and neuroendocrine effects, but not neuroimmune effects, produced by LPS were significantly attenuated in rats pre-treated with diclofenac.

CONCLUSIONS

These new data provide a comprehensive assessment of the acute effects of diclofenac on LPS exposure in rats and confirm a role for NSAIDs in attenuating endotoxin-induced anhedonia. Of particular importance, the data reveal that the observed effects are mediated via the hypothalamic pituitary adrenal axis at the level of the pituitary or above.

Repeated stress and structural plasticity in the brain

Although adrenal steroid receptors are distributed widely throughout the central nervous system, specific limbic and cortical regions targeted by stress hormones play a key role in integrating behavioral and physiological responses during stress and adaptation to subsequent stressors. When the stressor is of a sufficient magnitude or prolonged, it may result in abnormal changes in brain plasticity that, paradoxically, may impair the ability of the brain to appropriately regulate and respond to subsequent stressors. Here we review how repeated stress produces alterations in brain plasticity in animal models, and discuss its relevance to behavioral changes associated with these regions. Interestingly, prolonged stress produces opposing effects on structural plasticity, notably dendritic atrophy and excitatory synapse loss in the hippocampus and prefrontal cortex, and growth of dendrites and spines in the amygdala. The granule cells of the dentate gyrus are also significantly affected through a decrease in the rate neurogenesis following prolonged stress. How functional impairments in these brain regions play a role in stress-related mental illnesses is discussed in this context. Finally, we discuss the cumulative impact of stress-induced structural plasticity in aging.

Adult neurogenesis: from precursors to network and physiology

The discovery that the adult mammalian brain creates new neurons from pools of stemlike cells was a breakthrough in neuroscience. Interestingly, this particular new form of structural brain plasticity seems specific to discrete brain regions, and most investigations concern the subventricular zone (SVZ) and the dentate gyrus (DG) of the hippocampal formation (HF). Overall, two main lines of research have emerged over the last two decades: the first aims to understand the fundamental biological properties of neural stemlike cells (and their progeny) and the integration of the newly born neurons into preexisting networks, while the second focuses on understanding its relevance in brain functioning, which has been more extensively approached in the DG. Here, we propose an overview of the current knowledge on adult neurogenesis and its functional relevance for the adult brain. We first present an analysis of the methodological issues that have hampered progress in this field and describe the main neurogenic sites with their specificities. We will see that despite considerable progress, the levels of anatomic and functional integration of the newly born neurons within the host circuitry have yet to be elucidated. Then the intracellular mechanisms controlling neuronal fate are presented briefly, along with the extrinsic factors that regulate adult neurogenesis. We will see that a growing list of epigenetic factors that display a specificity of action depending on the neurogenic site under consideration has been identified. Finally, we review the progress accomplished in implicating neurogenesis in hippocampal functioning under physiological conditions and in the development of hippocampal-related pathologies such as epilepsy, mood disorders, and addiction. This constitutes a necessary step in promoting the development of therapeutic strategies.

A molecular cell biology of lithium

Lithium (Li(+)), a mood stabilizer, has profound effects on cultured neurons, offering an opportunity to investigate its cellular biological effects. Here we consider the effect of Li(+) and other psychotropic drugs on growth cone morphology and chemotaxis. Li(+) inhibits GSK-3 (glycogen synthase kinase-3) at a therapeutically relevant concentration. Treated cells show a number of features that arise due to GSK-3 inhibition, such as altered microtubule dynamics, axonal branching and loss of semaphorin 3A-mediated growth cone collapse. Li(+) also causes growth cones to spread; however, a similar effect is seen with two other mood stabilizers, valproic acid and carbamazepine, but without changes in microtubules or axon branching. This common effect of mood stabilizers is mediated by changes in inositol phosphate signalling, not GSK-3 activity. Given the presence of neurogenesis in the adult brain, we speculate that changes in growth cone behaviour could also occur during treatment of mental disorders.

Effect of chronic intermittent restraint stress on hippocampal expression of marker proteins for synaptic plasticity and progenitor cell proliferation in rats

Chronic restraint stress may change hippocampal mRNA levels of markers for synaptic plasticity such as synaptophysin, growth-associated protein 43 (GAP-43), and brain-derived neurotrophic factor (BDNF). In order to examine the relation between that stressor and those biochemical markers on protein level as well as the Ki-67 protein, a marker of progenitor cell proliferation, we subjected rats to chronic intermittent restraint stress for 6 h per day for 14 days excluding the weekends. This stress intensity caused a significant increase in adrenal gland weight and decrease in body weight gain. However, we did not find significant alteration of protein expression levels for synaptophysin, GAP-43, and BDNF by using Western blot analysis. Unlike these findings, the hippocampal protein expression of Ki-67 was significantly reduced by using both Western blot and immunohistochemical analyses. This reduction of Ki-67 expression in chronically stressed rats was correlated with increased adrenal gland weight and decreased body weight gain. All marker proteins used did not show any changes of hippocampal expression level after a single restraint stress session of 3 h. In conclusion, chronic intermittent restraint stress caused changes in the physiological stress response in rats, and a decrease of hippocampal progenitor cells using the Ki-67 protein as marker which indicates a suppression of adult neurogenesis. The results might contribute to understand the relationship between stress and cellular neurobiology of depression, since chronic antidepressant treatment have been shown to increase adult neurogenesis in the rat hippocampus.

Stress in early life inhibits neurogenesis in adulthood

Both structure and function of the hippocampus are altered by stress: by increasing levels of corticosteroids, stress causes atrophy of CA3 pyramidal cell dendrites, inhibits adult neurogenesis in the dentate gyrus, and impairs hippocampus-dependent learning. A recent study shows that adverse experience limited to early life, specifically removal of rat pups from their mother for three hours each day, decreases production of new granule neurons in adulthood through a corticosteroid-dependent mechanism. This finding suggests that stress in early life could permanently impair hippocampus-dependent learning and memory and increase susceptibility to depression by inhibiting adult neurogenesis in the hippocampus.

5-HT1A receptors, gene repression, and depression: guilt by association

The serotonin system is implicated in major depression and suicide and is negatively regulated by somatodendritic 5-HT1A autoreceptors. Desensitization of 5-HT1A autoreceptors is implicated in the 2- to 3-week latency for antidepressant treatments. Alterations in 5-HT1A receptor levels are reported in depression and suicide, and gene knockout of the 5-HT1A receptor results in an anxiety phenotype, suggesting that abnormal transcriptional regulation of this receptor gene may underlie these disorders. The 5-HT1A receptor gene is negatively regulated in neurons by repressors including REST/NRSF, Freud-1, NUDR/Deaf-1, and Hes5. The association with major depression, suicide, and panic disorder of a new functional 5-HT1A polymorphism at C(-1019)G that selectively blocks repression of the 5-HT1A autoreceptor by NUDR further suggests a causative role for altered regulation of this receptor in predisposition to mental illness. The authors review evidence that altered transcription of the 5-HT1A receptor can affect the serotonin system and limbic and cortical areas, leading to predisposition to depression.