Behavioral and neurobiological consequences of prolonged glucocorticoid exposure in rats: relevance to depression

FKBP5 and specific microRNAs via glucocorticoid receptor in the basolateral amygdala involved in the susceptibility to depressive disorder in early adolescent stressed rats

Exposure to stressful events induces depressive-like symptoms and increases susceptibility to depression. However, the molecular mechanisms are not fully understood. Studies reported that FK506 binding protein51 (FKBP5), the co-chaperone protein of glucocorticoid receptors (GR), plays a crucial role. Further, miR-124a and miR-18a are involved in the regulation of FKBP5/GR function. However, few studies have referred to effects of early life stress on depressive-like behaviours, GR and FKBP5, as well as miR-124a and miR-18a in the basolateral amygdala (BLA) from adolescence to adulthood. This study aimed to examine the dynamic alternations of depressive-like behaviours, GR and FKBP5, as well as miR-124a and miR-18a expressions in the BLA of chronic unpredictable mild stress (CUMS) rats and dexamethasone administration rats during the adolescent period. Meanwhile, the GR antagonist, RU486, was used as a means of intervention. We found that CUMS and dexamethasone administration in the adolescent period induced permanent depressive-like behaviours and memory impairment, decreased GR expression, and increased FKBP5 and miR-124a expression in the BLA of both adolescent and adult rats. However, increased miR-18a expression in the BLA was found only in adolescent rats. Depressive-like behaviours were positively correlated with the level of miR-124a, whereas GR levels were negatively correlated with those in both adolescent and adult rats. Our results suggested FKBP5/GR and miR-124a in the BLA were associated with susceptibility to depressive disorder in the presence of stressful experiences in early life.

Dorsal raphé nucleus glucocorticoid receptors inhibit tph2 gene expression in male C57BL/6J mice

The serotonergic dorsal raphé nucleus (DRN) expresses glucocorticoid receptors (GR), and systemic glucocorticoids have been shown to regulate expression and activity of tryptophan hydroxylase isoform 2, the rate-limiting enzyme for serotonin synthesis in brain. We have used intra-DRN injection of pseudotyped adeno-associated virus AAV2/9 transducing either green fluorescent protein (GFP control) or Cre recombinase (DRN GR deletion) in floxed GR mice to determine if DRN GR directly regulate DRN mRNA levels of tryptophan hydroxylase 2 (tph2). In a separate set of similarly-treated floxed GR mice, we also measured limbic forebrain region concentrations of serotonin (5-hydroxytryptamine; 5-HT) and its major metabolite, 5-hydroxyindoleacetic acid (5-HIAA). DRN GR deletion increased tph2 mRNA levels in the dorsal, lateral wing, and caudal parts of the DRN without altering tissue concentrations of 5-HT, 5-HIAA, or the 5-HIAA/5-HT ratio in limbic forebrain regions. We conclude that DRN GR inhibit DRN tph2 gene expression in mice without marked effects on serotonin metabolism, at least under basal conditions at the circadian nadir. These data provide the first evidence of localized control of DRN tph2 mRNA expression by DRN GR in mice.

microRNA-124 targets glucocorticoid receptor and is involved in depression-like behaviors

Dysregulation of microRNA (miRNA) has been shown to be involved in early observations of depression. MicroRNA-124-3p (miR-124) is the most abundant microRNA in the brain. Previous studies have shown that miR-124 plays a major role in depression. Here we showed that miR-124 directly targeted glucocorticoid receptor (GR) in HEK 293 cells. In addition, inhibition of miR-124 by its antagomir (2nmol/every two days) could reverse the decrease of sucrose preference and the increase of immobility time in mice exposed to chronic corticosterone (CORT, 40mg/kg) injection. Moreover, these effects on behavioral improvement were coupled to the activation of brain-derived neurotrophic factor (BDNF), TrkB, ERK, and CREB, as well as the induction of synaptogenesis and neuronal proliferation. Altogether, our study suggests that miR-124 can be served as a biomarker for depression and a novel target for drug development, and demonstrates that inhibition of miR-124 may be a strategy for treating depression by activating BDNF-TrkB signaling pathway in the hippocampus.

Hyperactivity of Hypothalamic-Pituitary-Adrenal Axis Due to Dysfunction of the Hypothalamic Glucocorticoid Receptor in Sigma-1 Receptor Knockout Mice

Sigma-1 receptor knockout (σ₁R-KO) mice exhibit a depressive-like phenotype. Because σ₁R is highly expressed in the neuronal cells of hypothalamic paraventricular nuclei (PVN), this study investigated the influence of σ₁R deficiency on the regulation of the hypothalamic-pituitary-adrenocortical (HPA) axis. Here, we show that the levels of basal serum corticosterone (CORT), adrenocorticotropic hormone (ACTH) and corticotrophin releasing factor (CRF) as well as the level of CRF mRNA in PVN did not significantly differ between adult male σ₁R-KO mice and wild-type (WT) mice. Acute mild restraint stress (AMRS) induced a higher and more sustainable increase in activity of HPA axis and CRF expression in σ₁R-KO mice. Percentage of dexamethasone (Dex)-induced reduction in level of CORT was markedly attenuated in σ₁R^-/- mice. The levels of glucocorticoid receptor (GR) and protein kinase C (PKC) phosphorylation were reduced in the PVN of σ₁R-KO mice and σ₁R antagonist NE100-treated WT mice. The exposure to AMRS in σ₁R-KO mice induced a stronger phosphorylation of cAMP-response element binding protein (CREB) in PVN than that in WT mice. Intracerebroventricular (i.c.v.) injection of PKC activator PMA for 3 days in σ₁R-KO mice not only recovered the GR phosphorylation and the percentage of Dex-reduced CORT but also corrected the AMRS-induced hyperactivity of HPA axis and enhancement of CRF mRNA and CREB phosphorylation. Furthermore, the injection (i.c.v.) of PMA in σ₁R-KO mice corrected the prolongation of immobility time in forced swim test (FST) and tail suspension test (TST). These results indicate that σ₁R deficiency causes down-regulation of GR by reducing PKC phosphorylation, which attenuates GR-mediated feedback inhibition of HPA axis and facilitates the stress response of HPA axis leading to the production of depressive-like behaviors.

Diverse action of repeated corticosterone treatment on synaptic transmission, neuronal plasticity, and morphology in superficial and deep layers of the rat motor cortex

One of the adverse effects of prolonged stress in rats is impaired performance of skilled reaching and walking tasks. The mechanisms that lead to these abnormalities are incompletely understood. Therefore, we compared the effects of twice daily repeated corticosterone injections for 7 days on miniature excitatory postsynaptic currents (mEPSCs), as well as on synaptic plasticity and morphology of layers II/III and V pyramidal neurons of the primary motor cortex (M1) of male Wistar rats. Corticosterone treatment resulted in increased frequency, but not amplitude, of mEPSCs in layer II/III neurons accompanied by increased complexity of the apical part of their dendritic tree, with no changes in the density of dendritic spines. The frequency and amplitude of mEPSCs as well as the parameters characterizing the complexity of the dendritic tree were not changed in layer V cells; however, their dendritic spine density was increased. While corticosterone treatment resulted in an increase in the amplitude of field potentials evoked in intralaminar connections within layer II/III, it did not influence field responses in layer V intralaminar connections, as well as the extent of chemically induced layer V long-term potentiation (chemLTP) by the application of tetraethylammonium (TEA, 25 mM). However, chemLTP induction in layer II/III was impaired in slices prepared from corticosterone-treated animals. These data indicate that repeated 7-day administration of exogenous corticosterone induces structural and functional plasticity in the M1, which occurs mainly in layer II/III pyramidal neurons. These findings shed light on potential sites of action and mechanisms underlying stress-induced impairment of motor functions.

Molecular Mechanisms Underlying the Anti-depressant Effects of Resveratrol: a Review

Major depression is a public health problem, affecting 121 million people worldwide. Patients suffering from depression present high rates of morbidity, causing profound economic and social impacts. Furthermore, patients with depression present cognitive impairments, which could influence on treatment adherence and long-term outcomes. The pathophysiology of major depression is not completely understood yet but involves reduced levels of monoamine neurotransmitters, bioenergetics, and redox disturbances, as well as inflammation and neuronal loss. Treatment with anti-depressants provides a complete remission of symptoms in approximately 50% of patients with major depression. However, these drugs may cause side effects, as sedation and weight gain. In this context, there is increasing interest in studies focusing on the anti-depressant effects of natural compounds found in the diet. Resveratrol is a polyphenolic phytoalexin (3,4',5-trihydroxystilbene; C₁₄H₁₂O₃; MW 228.247 g/mol) and has been found in peanuts, berries, grapes, and wine and induces anti-oxidant, anti-inflammatory, and anti-apoptotic effects in several mammalian cell types. Resveratrol also elicits anti-depressant effects, as observed in experimental models using animals. Therefore, resveratrol may be viewed as a potential anti-depressant agent, as well as may serve as a model of molecule to be modified aiming to ameliorate depressive symptoms in humans. In the present review, we describe and discuss the anti-depressant effects of resveratrol focusing on the mechanism of action of this phytoalexin in different experimental models.

Preventive Effects of Ginseng Total Saponins on Chronic Corticosterone-Induced Impairment in Astrocyte Structural Plasticity and Hippocampal Atrophy

To further explore the underlying antidepressant mechanism of ginseng total saponins (GTS), this study observed the effects on hippocampal astrocyte structural plasticity and hippocampal volume in the corticosterone-induced mouse depression model. Corticosterone (20 mg/kg/day) was administered subcutaneously for 5 weeks, and GTS (12.5, 25, and 50 mg/kg/day; namely GTSL, GTSM, and GTSH) or fluoxetine (10 mg/kg/day) were given intragastrically during the last 3 weeks. On day 33 and day 34, depression-like behavior was observed via a forced swimming test and a tail suspension test, respectively. At 6 h after the last dose of corticosterone (day 35), all mice were sacrificed followed by serum corticosterone assays, stereological analysis of hippocampal glial fibrillary acidic protein-positive (GFAP⁺ ) astroctyes and hippocampal volume, and hippocampal glycogen tests. Results showed that all doses of GTS ameliorated depression-like behavior and the decrease in hippocampal glycogen without normalizing hypercortisolism. Moreover, GTSH and GTSM reversed the corticosterone-induced reduction in the total number of hippocampal GFAP⁺ astrocytes and hippocampal volume. Additionally, GTSH alleviated the diminished protrusion length and somal volume of GFAP⁺ astrocytes induced by corticosterone. These findings imply that the effects of GTS on corticosterone-induced depression-like behavior may be mediated partly through the protection to hippocampal astrocyte structural plasticity. Copyright © 2017 John Wiley & Sons, Ltd.

Differential Behavioral and Neurobiological Effects of Chronic Corticosterone Treatment in Adolescent and Adult Rats

Adolescence is a critical period with ongoing maturational processes in stress-sensitive systems. While adolescent individuals show heightened stress-induced hormonal responses compared to adults, it is unclear whether and how the behavioral and neurobiological consequences of chronic stress would differ between the two age groups. Here we address this issue by examining the effects of chronic exposure to the stress hormone, corticosterone (CORT), in both adolescent and adult animals. Male Sprague-Dawley (SD) rats were injected intraperitoneally with CORT (40 mg/kg) or vehicle for 21 days during adolescence (post-natal day (PND) 29–49) or adulthood (PND 71–91) and then subjected to behavioral testing or sacrifice for western blot analyses. Despite of similar physical and neuroendocrine effects in both age groups, chronic CORT treatment produced a series of behavioral and neurobiological effects with striking age differences. While CORT-treated adult animals exhibited decreased sucrose preference, increased anxiety levels and cognitive impairment, CORT-treated adolescent animals demonstrated increased sucrose preference, decreased anxiety levels, and increased sensorimotor gating functions. These differential behavioral alterations were accompanied by opposite changes in the two age groups in the expression levels of brain-derived neurotrophic factor (BDNF), the phosphorylation of the obligatory subunit of the NMDA receptor, GluN1, and PSD-95 in rat hippocampus. These results suggest that prolonged glucocorticoid exposure during adolescence produces different behavioral and neurobiological effects from those in adulthood, which may be due to the complex interaction between glucocorticoids and the ongoing neurodevelopmental processes during this period.

Translational relevance of rodent models of hypothalamic-pituitary-adrenal function and stressors in adolescence

Elevations in glucocorticoids that result from environmental stressors can have programming effects on brain structure and function when the exposure occurs during sensitive periods that involve heightened neural development. In recent years, adolescence has gained increasing attention as another sensitive period of development, a period in which pubertal transitions may increase the vulnerability to stressors. There are similarities in physical and behavioural development between humans and rats, and rats have been used effectively as an animal model of adolescence and the unique plasticity of this period of ontogeny. This review focuses on benefits and challenges of rats as a model for translational research on hypothalamic-pituitary-adrenal (HPA) function and stressors in adolescence, highlighting important parallels and contrasts between adolescent rats and humans, and we review the main stress procedures that are used in investigating HPA stress responses and their consequences in adolescence in rats. We conclude that a greater focus on timing of puberty as a factor in research in adolescent rats may increase the translational relevance of the findings.

The exercise-glucocorticoid paradox: How exercise is beneficial to cognition, mood, and the brain while increasing glucocorticoid levels

Exercise is known to have beneficial effects on cognition, mood, and the brain. However, exercise also activates the hypothalamic-pituitary-adrenal axis and increases levels of the glucocorticoid cortisol (CORT). CORT, also known as the "stress hormone," is considered a mediator between chronic stress and depression and to link various cognitive deficits. Here, we review the evidence that shows that while both chronic stress and exercise elevate basal CORT levels leading to increased secretion of CORT, the former is detrimental to cognition/memory, mood/stress coping, and brain plasticity, while the latter is beneficial. We propose three preliminary answers to the exercise-CORT paradox. Importantly, the elevated CORT, through glucocorticoid receptors, functions to elevate dopamine in the medial prefrontal cortex under chronic exercise but not chronic stress, and the medial prefrontal dopamine is essential for active coping. Future inquiries may provide further insights to promote our understanding of this paradox.

Antidepressant-like effects of Xiaochaihutang in a neuroendocrine mouse model of anxiety/depression

ETHNOPHARMACOLOGICAL RELEVANCE

Hyperactivity of hypothalamic-pituitary-adrenal (HPA) axis is often observed in the pathophysiology of depression. Antidepressant therapy can restore hippocampal neurogenesis to rescue the HPA axis regulation defects. Xiaochaihutang (XCHT), a famous Chinese herbal formula, has been used clinically in depressive disorders in China. Our previous studies have demonstrated XCHT improved depressive-like behaviors in chronic unpredictable mild stress rat, but the underlying mechanisms of XCHT on hippocampal neurogenesis and the HPA axis were still unclear.

MATERIALS AND METHODS

We used chronic corticosterone (CORT)-induced mouse model of anxiety/depression to investigate antidepressant-like effects of XCHT by several physical and behavioral testing, including body weight, coat state, open field test, elevated plus maze, tail suspension test and forced swimming test. The integrity of negative feedback function on HPA axis was assessed by the dexamethasone (DEX) suppression test. In addition, Ki-67 and doublecortin (DCX) were performed to assess hippocampal cell proliferation and neurogenesis by immunohistochemistry. Chemical profile of active constituents in brain after oral administration of XCHT was revealed by ultra performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS).

RESULTS

Our results showed that oral administration of XCHT (2.3, 7 and 21g/kg) for 30 days remarkably normalized chronic CORT-induced the slowness in weight gain, the deterioration in coat state, the escape behavior in open field test and elevated plus maze, and the increase of immobility time in tail suspension test and forced swimming test. Moreover, XCHT significantly reversed chronic CORT-induced the reduction of DEX-induced plasma corticosterone/c-Fos suppression and Ki-67/DCX positive cells. Finally, a total 13 potential active constituents in brain were identified by UPLC-MS/MS after oral administration of XCHT, including 10 prototype components and 3 metabolites.

CONCLUSIONS

Our findings showed that XCHT could remarkably alleviate chronic CORT-induced anxiety/depression-like behaviors, which were probably attribute to promoting hippocampal neurogenesis and remodeling the integrity of the negative feedback loop on HPA axis. The constituents identified in brain might contribute to understanding the therapeutic basis of XCHT on depression.

Effect of subchronic corticosterone administration on α2-adrenoceptor functionality in rat brain: an in vivo and in vitro study

RATIONALE

Noradrenergic system plays a critical role in the hypothalamic-pituitary-adrenal (HPA) axis regulation and the stress response. A dysregulated HPA axis may be indicative of an increased biological vulnerability for depression. In addition, a variety of studies have focused on specific alterations of α₂-adrenoceptors as a mechanism involved in the pathogenesis of mood disorders and antidepressant response.

OBJECTIVES

This study aimed to evaluate the effect of subchronic corticosterone administration on rat brain α₂-adrenoceptor functionality by in vitro [³⁵S]GTPγS binding stimulation assays and in vivo dual-probe microdialysis determination of extracellular noradrenaline concentrations.

RESULTS

Implantation of a time release corticosterone pellet during 14 days induced sustained changes in endocrine function. However, there were no differences in α₂-adrenoceptor agonist UK14304-induced stimulation of [³⁵S]GTPγS binding in prefrontal cortex (PFC) between corticosterone-treated and control rats. In the same way, the in vivo evaluation of α₂-adrenoceptor-mediated noradrenaline release responses to the α₂-adrenoceptor agonist clonidine local administration into the locus coeruleus (LC), and the PFC did not show differences between the groups.

CONCLUSIONS

The present results show that subchronic corticosterone administration does not induce changes on functionality of α₂-adrenoceptors neither in the LC nor in noradrenergic cortical terminal areas.

Prevalencia de depresión y factores asociados en hombres

El objetivo del presente estudio fue evaluar la prevalencia de depresión, detectar el riesgo suicida e identificar los factores sociodemográficos y personales asociados a este trastorno. La muestra no aleatorizada estuvo conformada por 1525 hombres colombianos con edades entre 18 y 83 años procedentes de 22 departamentos y de distintos niveles educativos. Para evaluar la depresión se usó el Cuestionario de Depresión para Hombres (Álvarez y Londoño, 2012); para evaluar la comorbilidad con ansiedad se usó la Escala de Ansiedad HADS (Zigmond y Snaith, 1983) y el IMAFE (Lara, 1991); y para recolectar información acerca de los factores personales y sociodemográficos se usó una ficha de registro. Se analizaron los datos para calcular la prevalencia de corte, el riesgo suicida, la comorbilidad a través del uso del paquete estadístico SPSS. Se concluye que la prevalencia real reportada y el riesgo suicida en la población estudiada son más altos que los detectados usando un instrumento no sensible al género.

Sources of variation in plasma corticosterone and dehydroepiandrosterone in the male northern cardinal (Cardinalis cardinalis): I. Seasonal patterns and effects of stress and adrenocorticotropic hormone

The secretion of steroids from the adrenal gland is a classic endocrine response to perturbations that can affect homeostasis. During an acute stress response, glucocorticoids (GC), such as corticosterone (CORT), prepare the metabolic physiology and cognitive abilities of an animal in a manner that promotes survival during changing conditions. Although GC functions during stress are well established, much less is understood concerning how adrenal androgens, namely dehydroepiandrosterone (DHEA) are influenced by stress. I conducted three field studies (one experimental and two descriptive) aimed at identifying how both CORT and DHEA secretion in free-living male northern cardinals (Cardinalis cardinalis), vary during acute stress; across different circulations (brachial vs. jugular); in response to ACTH challenge; and during the annual cycle. As predicted, restraint stress increased plasma CORT, but unexpectedly DHEA levels decreased, but the latter effect was only seen for blood sampled from the jugular vein, and not the brachial. The difference in DHEA between circulations may result from increased neural uptake of DHEA during stress. Injection with exogenous adrenocorticotropic hormone (ACTH) increased CORT concentrations, but failed to alter DHEA levels, thus suggesting ACTH is not a direct regulator of DHEA. Monthly field sampling revealed distinct seasonal patterns to both initial and restraint stress CORT and DHEA levels with distinct differences in the steroid milieu between breeding and non-breeding seasons. These data suggest that the CORT response to stress remains relatively consistent, but DHEA secretion is largely independent of the response by CORT. Although CORT functions have been well-studied in wild animals, little research exists for the role of DHEA and their variable relationship sets the stage for future experimental research addressing steroid stress responses.

Everolimus improves memory and learning while worsening depressive- and anxiety-like behavior in an animal model of depression

Everolimus (EVR) is an orally-administered rapamycin analog that selectively inhibits the mammalian target of rapamycin (mTOR) kinase (mainly mTORC1 and likely mTORC2) and the related signaling pathway. mTOR is a serine/threonine protein kinase regulating multiple important cellular functions; dysfunction of mTOR signaling has also been implicated in the pathophysiology of several neurological, neurodegenerative, developmental and cognitive disorders. EVR is widely used as an anti-neoplastic therapy and more recently in children with tuberous sclerosis complex (TSC). However, no clear correlation exists between EVR use and development of central side effects e.g. depression, anxiety or cognitive impairment. We studied the effects of a 3 weeks administration of EVR in mice chronically treated with betamethasone 21-phosphate disodium (BTM) as a model of depression and cognitive decline. EVR treatment had detrimental effects on depressive- and anxiety-like behavior while improving cognitive performance in both control (untreated) and BTM-treated mice. Such effects were accompanied by an increased hippocampal neurogenesis and synaptogenesis. Our results therefore might support the proposed pathological role of mTOR dysregulation in depressive disorders and confirm some previous data on the positive effects of mTOR inhibition in cognitive decline. We also show that EVR, possibly through mTOR inhibition, may be linked to the development of anxiety. The increased hippocampal neurogenesis by EVR might explain its ability to improve cognitive function or protect from cognitive decline. Our findings suggest some caution in the use of EVR, particularly in the developing brain; patients should be carefully monitored for their psychiatric/neurological profiles in any clinical situation where an mTOR inhibitor and in particular EVR is used e.g. cancer treatment, TSC or immunosuppression.

The role of medial prefrontal corticosterone and dopamine in the antidepressant-like effect of exercise

Despite the well-documented beneficial effect of exercise on stress coping and depression treatment, its underlying neurobiological mechanism remains unclear. This is further complicated by a 'side effect' of exercise: it increases basal glucocorticoid (CORT), the stress hormone, which has been shown to be a mediator linking stress to depressive disorders. Here we show that three weeks of voluntary wheel running reduced rats' immobility in the forced swim test (FST), an antidepressant-like effect. Monitoring extracellular fluids in the medial prefrontal cortex PFC (mPFC) using microdialysis we found that, wheel running was associated with higher baseline CORT, but lower FST-responsive CORT. Further, wheel running resulted in a higher dopamine (DA) both at baseline and following FST. Interestingly, the antidepressant-like effect of wheel running was completely abolished by intra-mPFC pre-microinjection of a D2R (haloperidol) but not D1R (SCH23390) antagonist, at a dose that does not affect normal rats' performance in the FST. It suggests that exercise exerts antidepressant-like effect through upregulated DA and in a D2R dependent way in the mPFC. Importantly, the antidepressant-like effect of wheel running was also abolished by intra-mPFC pre-microinjection of a GR antagonist (RU486). Finally, intra-mPFC pre-microinjection of RU486 also downregulated the originally elevated basal and FST-responsive DA in the mPFC of exercise rats. These results suggest a causal pathway linking CORT, GR, DA, and D2R, to the antidepressant-like effect of exercise. In conclusion, exercise achieves antidepressant-like effect through the CORT-GR-DA-D2R pathway and that the increased basal CORT by exercise itself may be beneficial rather than detrimental.

Hypothalamic-pituitary-adrenocortical axis dysfunction in epilepsy

Epilepsy is a common neurological disease, affecting 2.4million people in the US. Among the many different forms of the disease, temporal lobe epilepsy (TLE) is one of the most frequent in adults. Recent studies indicate the presence of a hyperactive hypothalamopituitary- adrenocortical (HPA) axis and elevated levels of glucocorticoids in TLE patients. Moreover, in these patients, stress is a commonly reported trigger of seizures, and stress-related psychopathologies, including depression and anxiety, are highly prevalent. Elevated glucocorticoids have been implicated in the development of stress-related psychopathologies. Similarly, excess glucocorticoids have been found to increase neuronal excitability, epileptiform activity and seizure susceptibility. Thus, patients with TLE may generate abnormal stress responses that both facilitate ictal discharges and increase vulnerability for the development of comorbid psychopathologies. Here, we will examine the evidence that the HPA axis is disrupted in TLE, consider potential mechanisms by which this might occur, and discuss the implications of HPA dysfunction for seizuretriggering and psychiatric comorbidities.

New Hippocampal Neurons Mature Rapidly in Response to Ketamine But Are Not Required for Its Acute Antidepressant Effects on Neophagia in Rats

Virtually all antidepressant agents increase the birth of granule neurons in the adult dentate gyrus in rodents, providing a key basis for the neurogenesis hypothesis of antidepressant action. The novel antidepressant ketamine, however, shows antidepressant activity in humans within hours, far too rapid for a mechanism involving neuronal birth. Ketamine could potentially act more rapidly by enhancing maturation of new neurons born weeks earlier. To test this possibility, we assessed the effects of S-ketamine (S-(+)-ketamine hydrochloride) injection on maturation, as well as birth and survival, of new dentate gyrus granule neurons in rats, using the immediate-early gene zif268, proliferating cell nuclear antigen, and BrdU, respectively. We show that S-ketamine has rapid effects on new neurons, increasing the proportion of functionally mature young granule neurons within 2 h. A single injection of S-ketamine also increased cell proliferation and functional maturation, and decreased depressive-like behavior, for at least 4 weeks in rats treated with long-term corticosterone administration (a depression model) and controls. However, the behavioral effects of S-ketamine on neophagia were unaffected by elimination of adult neurogenesis. Together, these results indicate that ketamine has surprisingly rapid and long-lasting effects on the recruitment of young neurons into hippocampal networks, but that ketamine has antidepressant-like effects that are independent of adult neurogenesis.

Reelin-Related Disturbances in Depression: Implications for Translational Studies

The finding that reelin expression is significantly decreased in mood and psychotic disorders, together with evidence that reelin can regulate key aspects of hippocampal plasticity in the adult brain, brought our research group and others to study the possible role of reelin in the pathogenesis of depression. This review describes recent progress on this topic using an animal model of depression that makes use of repeated corticosterone (CORT) injections. This methodology produces depression-like symptoms in both rats and mice that are reversed by antidepressant treatment. We have reported that CORT causes a decrease in the number of reelin-immunopositive cells in the dentate gyrus subgranular zone (SGZ), where adult hippocampal neurogenesis takes place; that down-regulation of the number of reelin-positive cells closely parallels the development of a depression-like phenotype during repeated CORT treatment; that reelin downregulation alters the co-expression of reelin with neuronal nitric oxide synthase (nNOS); that deficits in reelin might also create imbalances in glutamatergic and GABAergic circuits within the hippocampus and other limbic structures; and that co-treatment with antidepressant drugs prevents both reelin deficits and the development of a depression-like phenotype. We also observed alterations in the pattern of membrane protein clustering in peripheral lymphocytes in animals with low levels of reelin. Importantly, we found parallel changes in membrane protein clustering in depression patients, which differentiated two subpopulations of naïve depression patients that showed a different therapeutic response to antidepressant treatment. Here, we review these findings and develop the hypothesis that restoring reelin-related function could represent a novel approach for antidepressant therapies.

Apigenin reverses depression-like behavior induced by chronic corticosterone treatment in mice

Previous researches found that apigenin exerted antidepressant-like effects in rodents. However, it is unclear whether the neurotrophic system is involved in the antidepressant-like effects of apigenin. Our present study aimed to explore the neurotrophic related mechanism of apigenin in depressive-like mice induced by chronic corticosterone treatment. Mice were repeatedly injected with corticosterone (40 mg/kg) subcutaneously (s.c) once daily for consecutive 21 days. Apigenin (20 and 40 mg/kg) and fluoxetine (20 mg/kg) were administered 30 min prior to the corticosterone injection. The behavioral tests indicated that apigenin reversed the reduction of sucrose preference and the elevation of immobility time in mice induced by chronic corticosterone treatment. In addition, the increase in serum corticosterone levels and the decrease in hippocampal brain-derived neurotrophic factor (BDNF) levels in corticosterone-treated mice were also ameliorated by apigenin administration. Taken together, our findings intensively confirmed the antidepressant-like effects of apigenin and indicated that the antidepressant-like mechanism of apigenin was mediated, at least partly by up-regulation of BDNF levels in the hippocampus.

Evidence for protective effect of lipoic acid and desvenlafaxine on oxidative stress in a model depression in mice

Oxidative stress is implicated in the neurobiology of depression. Here we investigated oxidative alterations in brain areas of animals submitted to the model of depression induced by corticosterone (CORT) and the effects of the antioxidant compound alpha-lipoic acid (ALA) alone or associated with the antidepressant desvenlafaxine (DVS) in these alterations. Female mice received vehicle or CORT (20 mg/kg) during 14 days. From the 15th to 21st days different animals received further administrations of: vehicle, DVS (10 or 20 mg/kg), ALA (100 or 200 mg/kg), or the combinations of DVS10+ALA100, DVS20+ALA100, DVS10+ALA200, or DVS20+ALA200. Twenty-four hours after the last drug administration prefrontal cortex (PFC), hippocampus (HC) and striatum (ST) were dissected for the determination of the activity of superoxide dismutase (SOD), reduced glutathione (GSH) and lipid peroxidation (LP) levels. CORT significantly increased SOD activity in the PFC and HC, decreased GSH levels in the HC and increased LP in all brain areas studied when compared to saline-treated animals. Decrements of SOD activity were observed in all groups and brain areas studied when compared to controls and CORT. The hippocampal decrease in GSH was reversed by ALA100, DVS10+ALA100, DVS20+ALA100 and DVS20+ALA200. The same DVS+ALA combination groups presented increased levels of GSH in the PFC and ST. The greater GSH levels were observed in the PFC, HC and ST of DVS20+ALA200 mice. LP was reversed in the groups ALA200 (PFC), DVS10+ALA100, DVS20+ALA100 (PFC, HC and ST), and DVS20+ALA200 (PFC, HC). Our findings contribute to the previous preclinical evidences implicating ALA as a promising agent for augmentation therapy in depression.

Short-term repeated corticosterone administration enhances glutamatergic but not GABAergic transmission in the rat motor cortex

It has been demonstrated that stress impairs performance of skilled reaching and walking tasks in rats due to the action of glucocorticoids involved in the stress response. Skilled reaching and walking are controlled by the primary motor cortex (M1); however, it is not known whether stress-related impairments in skilled motor tasks are related to functional and/or structural alterations within the M1. We studied the effects of single and repeated injections of corticosterone (twice daily for 7 days) on spontaneous excitatory and inhibitory postsynaptic currents (sEPSCs and sIPSCs) recorded from layer II/III pyramidal neurons in ex vivo slices of the M1, prepared 2 days after the last administration of the hormone. We also measured the density of dendritic spines on pyramidal cells and the protein levels of selected subunits of AMPA, NMDA, and GABA_A receptors after repeated corticosterone administration. Repeatedly administered corticosterone induced an increase in the frequency but not in the amplitude of sEPSCs, while a single administration had no effect on the recorded excitatory currents. The frequency and amplitude of sIPSCs as well as the excitability of pyramidal cells were changed neither after single nor after repeated corticosterone administration. Treatment with corticosterone for 7 days did not modify the density of dendritic spines on pyramidal neurons. Corticosterone influenced neither the protein levels of GluA1, GluA2, GluN1, GluN2A, and GluN2B subunits of glutamate receptors nor those of α₁, β₂, and γ₂ subunits of the GABA_A receptor. The increase in sEPSCs frequency induced by repeated corticosterone administration faded out within 7 days. These data indicate that prolonged administration of exogenous corticosterone selectively and reversibly enhances glutamatergic, but not GABAergic transmission in the rat motor cortex. Our results suggest that corticosterone treatment results in an enhancement of spontaneous glutamate release from presynaptic terminals in the M1 and thereby uncovers a potential mechanism underlying stress-induced motor functions impairment.

Reversal of corticosterone-induced BDNF alterations by the natural antioxidant alpha-lipoic acid alone and combined with desvenlafaxine: Emphasis on the neurotrophic hypothesis of depression

Brain derived neurotrophic factor (BDNF) is linked to the pathophysiology of depression. We hypothesized that BDNF is one of the neurobiological pathways related to the augmentation effect of alpha-lipoic acid (ALA) when associated with antidepressants. Female mice were administered vehicle or CORT 20mg/kg during 14 days. From the 15th to 21st days the animals were divided in groups that were further administered: vehicle, desvenlafaxine (DVS) 10 or 20mg/kg, ALA 100 or 200mg/kg or the combinations of DVS10+ALA100, DVS20+ALA100, DVS10+ALA200 or DVS20+ALA200. ALA or DVS alone or in combination reversed CORT-induced increase in immobility time in the forced swimming test and decrease in sucrose preference, presenting, thus, an antidepressant-like effect. DVS10 alone reversed CORT-induced decrease in BDNF in the prefrontal cortex (PFC), hippocampus (HC) and striatum (ST). The same was observed in the HC and ST of ALA200 treated animals. The combination of DVS and ALA200 reversed CORT-induced alterations in BDNF and even, in some cases, increased the levels of this neurotrophin when compared to vehicle-treated animals in HC and ST. Taken together, these results suggest that the combination of the DVS+ALA may be valuable for treating conditions in which BDNF levels are decreased, such as depression.

Creatine, Similar to Ketamine, Counteracts Depressive-Like Behavior Induced by Corticosterone via PI3K/Akt/mTOR Pathway

Ketamine has emerged as a novel strategy to treat refractory depression, producing rapid remission, but elicits some side effects that limit its use. In an attempt to investigate a safer compound that may afford an antidepressant effect similar to ketamine, this study examined the effects of the ergogenic compound creatine in a model of depression, and the involvement of phosphatidylinositol-3-kinase/protein kinase B/mammalian target of rapamycin (PI3K/Akt/mTOR) pathway in its effect. In order to induce a depressive-like behavior, mice were administered with corticosterone (20 mg/kg, per os (p.o.)) for 21 days. This treatment increased immobility time in the tail suspension test (TST), an effect abolished by a single administration of creatine (10 mg/kg, p.o.) or ketamine (1 mg/kg, i.p.), but not by fluoxetine (10 mg/kg, p.o., conventional antidepressant). Treatment of mice with wortmannin (PI3K inhibitor, 0.1 μg/site, intracerebroventricular (i.c.v.)) or rapamycin (mTOR inhibitor, 0.2 nmol/site, i.c.v.) abolished the anti-immobility effect of creatine and ketamine. None of the treatments affected locomotor activity of mice. The immunocontents of p-mTOR, p-p70S6 kinase (p70S6K), and postsynaptic density-95 protein (PSD95) were increased by creatine and ketamine in corticosterone or vehicle-treated mice. Moreover, corticosterone-treated mice presented a decreased hippocampal brain-derived neurotrophic factor (BDNF) level, an effect abolished by creatine or ketamine. Altogether, the results indicate that creatine shares with ketamine the ability to acutely reverse the corticosterone-induced depressive-like behavior by a mechanism dependent on PI3K/AKT/mTOR pathway, and modulation of the synaptic protein PSD95 as well as BDNF in the hippocampus, indicating the relevance of targeting these proteins for the management of depressive disorders. Moreover, we suggest that creatine should be further investigated as a possible fast-acting antidepressant.

Chronic corticosterone-mediated dysregulation of microRNA network in prefrontal cortex of rats: relevance to depression pathophysiology

Stress plays a major role in inducing depression, which may arise from interplay between complex cascades of molecular and cellular events that influence gene expression leading to altered connectivity and neural plasticity. In recent years, microRNAs (miRNAs) have carved their own niche owing to their innate ability to induce disease phenotype by regulating expression of a large number of genes in a cohesive and coordinated manner. In this study, we examined whether miRNAs and associated gene networks have a role in chronic corticosterone (CORT; 50 mg  kg(-1) × 21 days)-mediated depression in rats. Rats given chronic CORT showed key behavioral features that resembled depression phenotype. Expression analysis revealed differential regulation of 26 miRNAs (19 upregulated, 7 downregulated) in prefrontal cortex of CORT-treated rats. Interaction between altered miRNAs and target genes showed dense interconnected molecular network, in which multiple genes were predicated to be targeted by the same miRNA. A majority of altered miRNAs showed binding sites for glucocorticoid receptor element, suggesting that there may be a common regulatory mechanism of miRNA regulation by CORT. Functional clustering of predicated target genes yielded disorders such as developmental, inflammatory and psychological that could be relevant to depression. Prediction analysis of the two most prominently affected miRNAs miR-124 and miR-218 resulted into target genes that have been shown to be associated with depression and stress-related disorders. Altogether, our study suggests miRNA-mediated novel mechanism by which chronic CORT may be involved in depression pathophysiology.

Stress effects on the hippocampus: a critical review

Uncontrollable stress has been recognized to influence the hippocampus at various levels of analysis. Behaviorally, human and animal studies have found that stress generally impairs various hippocampal-dependent memory tasks. Neurally, animal studies have revealed that stress alters ensuing synaptic plasticity and firing properties of hippocampal neurons. Structurally, human and animal studies have shown that stress changes neuronal morphology, suppresses neuronal proliferation, and reduces hippocampal volume. Since the inception of stress research nearly 80 years ago, much focus has been on the varying levels of hypothalamic-pituitary-adrenal (HPA) axis neuroendocrine hormones, namely glucocorticoids, as mediators of the myriad stress effects on the hippocampus and as contributing factors to stress-associated psychopathologies such as post-traumatic stress disorder (PTSD). However, reports of glucocorticoid-produced alterations in hippocampal functioning vary widely across studies. This review provides a brief history of stress research, examines how the glucocorticoid hypothesis emerged and guides contemporary stress research, and considers alternative approaches to understanding the mechanisms underlying stress effects on hippocampal functioning.

Imipramine protects against the deleterious effects of chronic corticosterone on depression-like behavior, hippocampal reelin expression, and neuronal maturation

We have hypothesized that a downregulation of reelin and deficient maturation of adult-born hippocampal neurons are important factors in the pathogenesis of depression. This hypothesis is based on previous work showing that depression-like behavior in rats treated with protracted corticosterone develops in concert with decreased dendritic complexity in newborn hippocampal granule neurons and decreased reelin expression in the proliferative subgranular zone of the dentate gyrus. In addition, heterozygous reeler mice with approximately 50% of normal brain levels of reelin are more vulnerable to the depressogenic effects of corticosterone than wild-type mice. The purpose of this experiment was to provide pharmacological validation for the link between reelin, neuronal maturation, and depression by examining whether the deleterious effects of corticosterone on these measures could be prevented by co-administration of the antidepressant imipramine. Rats received corticosterone injections, corticosterone injections plus either 10 or 15mg/kg imipramine injections, or vehicle injections for 21 consecutive days. They were then subjected to the forced swim test to assess depression-like behavior and sacrificed for immunohistochemical examination of immature neuron number and dendritic complexity and the presence of reelin+cells. We found that corticosterone increases depression-like behavior, decreases the number of reelin+cells in the subgranular zone, and decreases the number and complexity of immature neurons in the granule cell layer. All of these behavioral and cellular phenotypes were prevented by imipramine, providing further support for the idea that reelin is involved in the pathogenesis of depression.

Pathogenesis of depression: Insights from human and rodent studies

Major depressive disorder (MDD) will affect one out of every five people in their lifetime and is the leading cause of disability worldwide. Nevertheless, mechanisms associated with the pathogenesis of MDD have yet to be completely understood and current treatments remain ineffective in a large subset of patients. In this review, we summarize the most recent discoveries and insights for which parallel findings have been obtained in human depressed subjects and rodent models of mood disorders in order to examine the potential etiology of depression. These mechanisms range from synaptic plasticity mechanisms to epigenetics and the immune system where there is strong evidence to support a functional role in the development of specific depression symptomology. Ultimately we conclude by discussing how novel therapeutic strategies targeting central and peripheral processes might ultimately aid in the development of effective new treatments for MDD and related stress disorders.

Behavioral effects of glucocorticoids during the first exposures to the forced swim stress

RATIONALE

Glucocorticoids facilitate coping with stress, but their high levels have been also implicated in mood disorders. Due to this duality, the role of glucocorticoid signaling in the development of the first episodes of stress-induced depression remains unclear.

OBJECTIVES

To address this issue, effects of the glucocorticoid signal modulation on depressive-like behavior during pretest and test Porsolt swim sessions were examined.

METHODS

Metyrapone (MET; 150 mg/kg, i.p.) was injected 3 h before pretest to block stress-induced increase in corticosterone levels. Dexamethasone (DEX; 0.2 mg/kg, s.c.) was applied to MET-treated rats 1 h before both pretest and test sessions. In addition to behavior during these sessions, glucocorticoid receptor (GR) expression was analyzed by immunohistochemistry 2 h after the second swim.

RESULTS

In pretest, MET-treated rats exhibited increased latency to immobility and shortened immobility. DEX reversed the behavioral effects of MET in the pretest. In the test, animals from MET + DEX group unexpectedly exhibited an antidepressant-like behavior. Swim stress increased GR expression in the frontal cortex irrespective of the pharmacological treatment. A significant elevation in GR expression was found in the prefrontal cortex (PFC) of stressed MET + DEX-treated rats and in the PFC of unstressed rats 6 h after injection of DEX alone.

CONCLUSION

The data suggest that the increase in glucocorticoid levels under swim stress during pretest directly contributes to the development of the immobility response. Transition of DEX effect from prodepressant in the pretest to an antidepressant in the test was associated with the elevation in the PFC GR expression.

Stress-induced deficits in cognition and emotionality: a role of glutamate

Stress is associated with a number of neuropsychiatric disorders, many of which are characterized by altered cognition and emotionality. Rodent models of stress have shown parallel behavioral changes such as impaired working memory, cognitive flexibility and fear extinction. This coincides with morphological changes to pyramidal neurons in the prefrontal cortex, hippocampus and amygdala, key cortical regions mediating these behaviors. Increasing evidence suggests that alteration in the function of the glutamatergic system may contribute to the pathology seen in neuropsychiatric disorders. Stress can alter glutamate transmission in the prefrontal cortex, hippocampus and amygdala and altered glutamate transmission has been linked to neuronal morphological changes. More recently, genetic manipulations in rodent models have allowed for subunit-specific analysis of the role of AMPA and NMDA receptors as well as glutamate transporters in behaviors shown to be altered by stress. Together these data point to a role for glutamate in mediating the cognitive and emotional changes observed in neuropsychiatric disorders. Furthering our understanding of how stress affects glutamate receptors and related signaling pathways will ultimately contribute to the development of improved therapeutics for individuals suffering from neuropsychiatric disorders.

Long-term betamethasone 21-phosphate disodium treatment has distinct effects in CD1 and DBA/2 mice on animal behavior accompanied by opposite effects on neurogenesis

One of the most peculiar characteristics of the stress response is the pronounced inter-individual and inter-strain variability both in behavioral and neurochemical outcomes. Several studies confirm that rodents belonging to the same or different strain and/or gender, when exposed to a stressor, may show behavioral and cognitive differences. We compared the effects of long-term betamethasone 21-phosphate disodium (BTM), a widely clinically used corticosteroid, on animal behavior and neurogenesis in CD1 and DBA/2 mice. BTM treatment, in CD1 mice, increased body weight gain and anxiety parameters while having pro-depressant effects. Furthermore, BTM significantly reduced neurogenesis in the dentate gyrus of the hippocampus. Finally, BTM treatment induced a significant impairment in memory and learning performance in the Morris water maze. At odds, BTM administration, in DBA/2 mice, caused a significant reduction in the body weight while not modifying anxiety parameters. In addition, both an increased synaptogenesis and neurogenesis were found. Similarly to CD1 mice, also in DBA/2 mice, memory and learning were impaired. Our data confirm that long-term exposure to corticosteroids can generate or aggravate psychiatric/neurologic disorders such as depression, anxiety, memory and learning. Our study did not reveal significant differences between corticosterone and BTM treatment in CD1 mice. In contrast, BTM treatment in mice with an anxious phenotype (DBA/2 mice) revealed some contrasting results indicating that genetic factors can influence corticosteroids dependent effects. Finally, our data further underline the need for a re-evaluation of neurogenesis role; the increased neurogenesis observed in DBA/2 mice and behavioral effects might be distinguished phenomena.

Converging translational evidence for the involvement of the serotonin 2A receptor gene in major depressive disorder

An association between serotonin 2A receptor (5-HT2AR), encoded by HTR2A gene, and major depressive disorder (MDD) has been suggested. Here, we combined preclinical and ecological clinical approaches to explore the impact of impaired 5-HT2AR-mediated transmission on MDD or anxio-depressive-like phenotype in mice. Htr2a knock-out mice (Htr2a(-/-)) and wild-type mice were compared for the ability of chronic corticosterone to elicit some anxio-depressive-like phenotype in three behavioral paradigms (elevated plus maze, tail suspension test and splash test). Accordingly, two single nucleotide polymorphisms of the HTR2A gene (rs6314 ie His452Tyr and rs6313 ie 102C/T), which specific allelic variants may decrease 5-HT2AR-mediated transmission (as in Htr2a(-/-)mice), were studied in a sample of 485 Caucasian patients with MDD. In response to chronic corticosterone exposure, Htr2a(-/-) mice displayed more pronounced anxiodepressive-like phenotype than wild-type mice, as shown by a significant higher "emotionality score" (p<0.01). In patients, the C allele of rs6313 was more frequent in depressed patients (p=0.019) and was also associated with a more severe major depressive episode (p=0.03). This translational and ecological study involving constitutive Htr2a(-/-) knock-out mice and related SNPs in depressed patients suggests that a lower neurotransmission at the 5-HT2AR may favor the susceptibility and severity of MDE. It also suggests that specific allelic variants of the rs6313 and rs6314 may reduce 5-HT2AR-mediated transmission.

The depressogenic-like effect of acute and chronic treatment with dexamethasone and its influence on the activity of antidepressant drugs in the forced swim test in adult mice

There is a close relationship between chronic stress, glucocorticoids and depression. Psychiatric and cognitive symptoms resembling major depression have been observed in patients experiencing elevated glucocorticoid levels, and a high percentage of people suffering from depression have undergone a stressful event/events prior to the onset of this mental disorder. In our study, we investigated whether acute and chronic treatment of dexamethasone induces depression-like behavior in mice and if dexamethasone therapy influences the activity of antidepressant drugs with diverse modes of action. The antidepressant-like effect was assessed by the forced swim test in adult mice. The depressogenic-like activity of dexamethasone turned out to be dose-dependent: only the highest tested dose of the glucocorticoid (i.e., 64μg/kg) given as a single injection increased immobility time, whereas 16μg/kg/day of dexamethasone (but not 4μg/kg/day) administered repeatedly induced a significant alteration in animal behavior. These depressogenic doses of dexamethasone (i.e., 64μg/kg and 16μg/kg/day for an acute and repeated administration, respectively) diminished the antidepressant potential of the therapeutic doses of imipramine (10mg/kg), amitriptyline (10mg/kg), tianeptine (25mg/kg), mianserin (10mg/kg), citalopram (15mg/kg) and moclobemide (25mg/kg). Two main findings of our study should be particularly underlined: (1) both single and repeated administration of dexamethasone evoked a depression-like behavior of mice, (2) both single and repeated administration of dexamethasone were able to modify the activity of the antidepressant agents from various pharmacological groups, which may lead to a considerable reduction in the efficacy of pharmacotherapy prescribed for patients with mood disorders.

Age-associated epigenetic upregulation of the FKBP5 gene selectively impairs stress resiliency

Single nucleotide polymorphisms (SNPs) in the FK506 binding protein 5 (FKBP5) gene combine with traumatic events to increase risk for post-traumatic stress and major depressive disorders (PTSD and MDD). These SNPs increase FKBP51 protein expression through a mechanism involving demethylation of the gene and altered glucocorticoid signaling. Aged animals also display elevated FKBP51 levels, which contribute to impaired resiliency to depressive-like behaviors through impaired glucocorticoid signaling, a phenotype that is abrogated in FKBP5^−/− mice. But the age of onset and progressive stability of these phenotypes remain unknown. Moreover, it is unclear how FKBP5 deletion affects other glucocorticoid-dependent processes or if age-associated increases in FKBP51 expression are mediated through a similar epigenetic process caused by SNPs in the FKBP5 gene. Here, we show that FKBP51-mediated impairment in stress resiliency and glucocorticoid signaling occurs by 10 months of age and this increased over their lifespan. Surprisingly, despite these progressive changes in glucocorticoid responsiveness, FKBP5^−/− mice displayed normal longevity, glucose tolerance, blood composition and cytokine profiles across lifespan, phenotypes normally associated with glucocorticoid signaling. We also found that methylation of Fkbp5 decreased with age in mice, a process that likely explains the age-associated increases in FKBP51 levels. Thus, epigenetic upregulation of FKBP51 with age can selectively impair psychological stress-resiliency, but does not affect other glucocorticoid-mediated physiological processes. This makes FKBP51 a unique and attractive therapeutic target to treat PTSD and MDD. In addition, aged wild-type mice may be a useful model for investigating the mechanisms of FKBP5 SNPs associated with these disorders.

Forebrain glucocorticoid receptor gene deletion attenuates behavioral changes and antidepressant responsiveness during chronic stress

Stress is an important risk factor for mood disorders. Stress also stimulates the secretion of glucocorticoids, which have been found to influence mood. To determine the role of forebrain glucocorticoid receptors (GR) in behavioral responses to chronic stress, the present experiments compared behavioral effects of repeated social defeat in mice with forebrain GR deletion and in floxed GR littermate controls. Repeated defeat produced alterations in forced swim and tail suspension immobility in floxed GR mice that did not occur in mice with forebrain GR deletion. Defeat-induced changes in immobility in floxed GR mice were prevented by chronic antidepressant treatment, indicating that these behaviors were dysphoria-related. In contrast, although mice with forebrain GR deletion exhibited antidepressant-induced decreases in tail suspension immobility in the absence of stress, this response did not occur in mice with forebrain GR deletion after defeat. There were no marked differences in plasma corticosterone between genotypes, suggesting that behavioral differences depended on forebrain GR rather than on abnormal glucocorticoid secretion. Defeat-induced gene expression of the neuronal activity marker c-fos in the ventral hippocampus, paraventricular thalamus and lateral septum correlated with genotype-related differences in behavioral effects of defeat, whereas c-fos induction in the nucleus accumbens and central and basolateral amygdala correlated with genotype-related differences in behavioral responses to antidepressant treatment. The dependence of both negative (dysphoria-related) and positive (antidepressant-induced) behaviors on forebrain GR is consistent with the contradictory effects of glucocorticoids on mood, and implicates these or other forebrain regions in these effects.