An Interaction between Brain-Derived Neurotrophic Factor and Stress-Related Glucocorticoids in the Pathophysiology of Alzheimer's Disease.

Both the brain-derived neurotrophic factor (BDNF) and glucocorticoids (GCs) play multiple roles in various aspects of neurons, including cell survival and synaptic function. BDNF and its receptor TrkB are extensively expressed in neurons of the central nervous system (CNS), and the contribution of the BDNF/TrkB system to neuronal function is evident; thus, its downregulation has been considered to be involved in the pathogenesis of Alzheimer's disease (AD). GCs, stress-related molecules, and glucocorticoid receptors (GRs) are also considered to be associated with AD in addition to mental disorders such as depression. Importantly, a growing body of evidence suggests a close relationship between BDNF/TrkB-mediated signaling and the GCs/GR system in the CNS. Here, we introduce the current studies on the interaction between the neurotrophic system and stress in CNS neurons and discuss their involvement in the pathophysiology of AD.

The Role of Neurotrophin Signaling in Age-Related Cognitive Decline and Cognitive Diseases.

Neurotrophins are a family of secreted proteins expressed in the peripheral nervous system and the central nervous system that support neuronal survival, synaptic plasticity, and neurogenesis. Brain-derived neurotrophic factor (BDNF) and its high affinity receptor TrkB are highly expressed in the cortical and hippocampal areas and play an essential role in learning and memory. The decline of cognitive function with aging is a major risk factor for cognitive diseases such as Alzheimer's disease. Therefore, an alteration of BDNF/TrkB signaling with aging and/or pathological conditions has been indicated as a potential mechanism of cognitive decline. In this review, we summarize the cellular function of neurotrophin signaling and review the current evidence indicating a pathological role of neurotrophin signaling, especially of BDNF/TrkB signaling, in the cognitive decline in aging and age-related cognitive diseases. We also review the therapeutic approach for cognitive decline by the upregulation of the endogenous BDNF/TrkB-system.

An iPSC-based neural model of sialidosis uncovers glycolytic impairment-causing presynaptic dysfunction and deregulation of Ca2+ dynamics.

Sialidosis is a neuropathic lysosomal storage disease caused by a deficiency in the NEU1 gene-encoding lysosomal neuraminidase and characterized by abnormal accumulation of undigested sialyl-oligoconjugates in systemic organs including brain. Although patients exhibit neurological symptoms, the underlying neuropathological mechanism remains unclear. Here, we generated induced pluripotent stem cells (iPSCs) from skin fibroblasts with sialidosis and induced the differentiation into neural progenitor cells (NPCs) and neurons. Sialidosis NPCs and neurons mimicked the disease-like phenotypes including reduced neuraminidase activity, accumulation of sialyl-oligoconjugates and lysosomal expansions. Functional analysis also revealed that sialidosis neurons displayed two distinct abnormalities, defective exocytotic glutamate release and augmented α-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate receptor (AMPAR)-mediated Ca2+ influx. These abnormalities were restored by overexpression of the wild-type NEU1 gene, demonstrating causative role of neuraminidase deficiency in functional impairments of disease neurons. Comprehensive proteomics analysis revealed the significant reduction of SNARE proteins and glycolytic enzymes in synaptosomal fraction, with downregulation of ATP production. Bypassing the glycolysis by treatment of pyruvate, which is final metabolite of glycolysis pathway, improved both the synaptsomal ATP production and the exocytotic function. We also found that upregulation of AMPAR and L-type voltage dependent Ca2+ channel (VDCC) subunits in disease neurons, with the restoration of AMPAR-mediated Ca2+ over-load by treatment of antagonists for the AMPAR and L-type VDCC. Our present study provides new insights into both the neuronal pathophysiology and potential therapeutic strategy for sialidosis.

Brain-Derived Neurotrophic Factor Signaling in the Pathophysiology of Alzheimer's Disease: Beneficial Effects of Flavonoids for Neuroprotection.

The function of the brain-derived neurotrophic factor (BDNF) via activation through its high-affinity receptor Tropomyosin receptor kinase B (TrkB) has a pivotal role in cell differentiation, cell survival, synaptic plasticity, and both embryonic and adult neurogenesis in central nervous system neurons. A number of studies have demonstrated the possible involvement of altered expression and action of the BDNF/TrkB signaling in the pathogenesis of neurodegenerative diseases, including Alzheimer's disease (AD). In this review, we introduce an essential role of the BDNF and its downstream signaling in neural function. We also review the current evidence on the deregulated the BDNF signaling in the pathophysiology of AD at gene, mRNA, and protein levels. Further, we discuss a potential usefulness of small compounds, including flavonoids, which can stimulate BDNF-related signaling as a BDNF-targeting therapy.

Actions of Brain-Derived Neurotrophin Factor in the Neurogenesis and Neuronal Function, and Its Involvement in the Pathophysiology of Brain Diseases.

It is well known that brain-derived neurotrophic factor, BDNF, has an important role in a variety of neuronal aspects, such as differentiation, maturation, and synaptic function in the central nervous system (CNS). BDNF stimulates mitogen-activated protein kinase/extracellular signal-regulated kinase (MAPK/ERK), phosphoinositide-3kinase (PI3K), and phospholipase C (PLC)-gamma pathways via activation of tropomyosin receptor kinase B (TrkB), a high affinity receptor for BDNF. Evidence has shown significant contributions of these signaling pathways in neurogenesis and synaptic plasticity in in vivo and in vitro experiments. Importantly, it has been demonstrated that dysfunction of the BDNF/TrkB system is involved in the onset of brain diseases, including neurodegenerative and psychiatric disorders. In this review, we discuss actions of BDNF and related signaling molecules on CNS neurons, and their contributions to the pathophysiology of brain diseases.

Actions of Brain-Derived Neurotrophic Factor and Glucocorticoid Stress in Neurogenesis.

Altered neurogenesis is suggested to be involved in the onset of brain diseases, including mental disorders and neurodegenerative diseases. Neurotrophic factors are well known for their positive effects on the proliferation/differentiation of both embryonic and adult neural stem/progenitor cells (NSCs/NPCs). Especially, brain-derived neurotrophic factor (BDNF) has been extensively investigated because of its roles in the differentiation/maturation of NSCs/NPCs. On the other hand, recent evidence indicates a negative impact of the stress hormone glucocorticoids (GCs) on the cell fate of NSCs/NPCs, which is also related to the pathophysiology of brain diseases, such as depression and autism spectrum disorder. Furthermore, studies including ours have demonstrated functional interactions between neurotrophic factors and GCs in neural events, including neurogenesis. In this review, we show and discuss relationships among the behaviors of NSCs/NPCs, BDNF, and GCs.

Basic fibroblast growth factor induces miR-134 upregulation in astrocyte for cell maturation.

Evidence suggests that neuronal microRNAs (miRs) contribute to synaptic plasticity, although a role of glial miRs have been unknown. Growth factors including brain-derived neurotrophic factor (BDNF) regulate neuronal functions via upregulation of miRs, while possible influences on expression/function of glial miRs have not been fully understood. Here, we report that basic fibroblast growth factor (bFGF) increased miR-134 expression in astrocyte. The miR-134 was upregulated through stimulating extracellular signal-regulated kinase and phosphatidylinositol 3-kinase signaling, because inhibitors for each signaling blocked the miR-134 induction by bFGF. We also found upregulation of glial fibrillary acidic protein (astrocyte marker) and decreased extracellular concentration of glutamate after miR-134 overexpression and bFGF application, suggesting that astroglial cell maturation is enhanced by bFGF through induction of miR-134.

Teneurin-4 promotes cellular protrusion formation and neurite outgrowth through focal adhesion kinase signaling.

Teneurin-4 (Ten-4), a transmembrane protein, is highly expressed in the central nervous system; however, its cellular and molecular function in neuronal differentiation remains unknown. In this study, we aimed to elucidate the function of Ten-4 in neurite outgrowth. Ten-4 expression was induced during neurite outgrowth of the neuroblastoma cell line Neuro-2a. Ten-4 protein was localized at the neurite growth cones. Knockdown of Ten-4 expression in Neuro-2a cells decreased the formation of the filopodia-like protrusions and the length of individual neurites. Conversely, overexpression of Ten-4 promoted filopodia-like protrusion formation. In addition, knockdown and overexpression of Ten-4 reduced and elevated the activation of focal adhesion kinase (FAK) and Rho-family small GTPases, Cdc42 and Rac1, key molecules for the membranous protrusion formation downstream of FAK, respectively. Inhibition of the activation of FAK and neural Wiskott-Aldrich syndrome protein (N-WASP), which is a downstream regulator of FAK and Cdc42, blocked protrusion formation by Ten-4 overexpression. Further, Ten-4 colocalized with phosphorylated FAK in the filopodia-like protrusion regions. Together, our findings show that Ten-4 is a novel positive regulator of cellular protrusion formation and neurite outgrowth through the FAK signaling pathway.

Phencyclidine rapidly decreases neuronal mRNA of brain-derived neurotrophic factor.

Downregulation of brain-derived neurotrophic factor (BDNF), a member of neurotrophin family, has been implicated in psychiatric diseases including schizophrenia. However, detailed mechanisms of its reduction in patients with schizophrenia remain unclear. Here, using cultured cortical neurons, we monitored BDNF mRNA levels following acute application of phencyclidine [PCP; an N-methyl-d-aspartate (NMDA) receptor blocker], which is known to produce schizophrenia-like symptoms. We found that PCP rapidly caused a reduction in total amount of BDNF transcripts without effect on cell viability, while mRNA levels of nerve growth factor was intact. Actinomycin-D (ActD), an RNA synthesis inhibitor, decreased total BDNF mRNA levels similar to PCP, and coapplication of ActD with PCP did not show further reduction in BDNF mRNA compared with solo application of each drug. Among BDNF exons I, IV, and VI, the exon IV, which is positively regulated by neuronal activity, was highly sensitive to PCP. Furthermore, PCP inactivated cAMP response element-binding protein (CREB; a regulator of transcriptional activity of exon IV). The inactivation of CREB was also achieved by an inhibitor for Ca(2+) /calmodulin kinase II (CaMKII), although coapplication with PCP induced no further inhibition on the CREB activity. It is possible that PCP decreases BDNF transcription via blocking the NMDA receptor/CaMKII/CREB signaling.

Impairments in brain-derived neurotrophic factor-induced glutamate release in cultured cortical neurons derived from rats with intrauterine growth retardation: possible involvement of suppression of TrkB/phospholipase C-γ activation.

Low birth weight due to intrauterine growth retardation (IUGR) is suggested to be a risk factor for various psychiatric disorders such as schizophrenia. It has been reported that developmental cortical dysfunction and neurocognitive deficits are observed in individuals with IUGR, however, the underlying molecular mechanisms have yet to be elucidated. Brain-derived neurotrophic factor (BDNF) and its receptor TrkB are associated with schizophrenia and play a role in cortical development. We previously demonstrated that BDNF induced glutamate release through activation of the TrkB/phospholipase C-γ (PLC-γ) pathway in developing cultured cortical neurons, and that, using a rat model for IUGR caused by maternal administration of thromboxane A2, cortical levels of TrkB were significantly reduced in IUGR rats at birth. These studies prompted us to hypothesize that TrkB reduction in IUGR cortex led to impairment of BDNF-dependent glutamatergic neurotransmission. In the present study, we found that BDNF-induced glutamate release was strongly impaired in cultured IUGR cortical neurons where TrkB reduction was maintained. Impairment of BDNF-induced glutamate release in IUGR neurons was ameliorated by transfection of human TrkB (hTrkB). Although BDNF-stimulated phosphorylation of TrkB and of PLC-γ was decreased in IUGR neurons, the hTrkB transfection recovered the deficits in their phosphorylation. These results suggest that TrkB reduction causes impairment of BDNF-stimulated glutamatergic function via suppression of TrkB/PLC-γ activation in IUGR cortical neurons. Our findings provide molecular insights into how IUGR links to downregulation of BDNF function in the cortex, which might be involved in the development of IUGR-related diseases such as schizophrenia.

Phencyclidine-induced decrease of synaptic connectivity via inhibition of BDNF secretion in cultured cortical neurons.

Repeated administration of phencyclidine (PCP), a noncompetitive N-methyl-D-aspartate (NMDA) receptor blocker, produces schizophrenia-like behaviors in humans and rodents. Although impairment of synaptic function has been implicated in the effect of PCP, the molecular mechanisms have not yet been elucidated. Considering that brain-derived neurotrophic factor (BDNF) plays an important role in synaptic plasticity, we examined whether exposure to PCP leads to impaired BDNF function in cultured cortical neurons. We found that PCP caused a transient increase in the level of intracellular BDNF within 3 h. Despite the increased intracellular amount of BDNF, activation of Trk receptors and downstream signaling cascades, including MAPK/ERK1/2 and PI3K/Akt pathways, were decreased. The number of synaptic sites and expression of synaptic proteins were decreased 48 h after PCP application without any impact on cell viability. Both electrophysiological and biochemical analyses revealed that PCP diminished glutamatergic neurotransmission. Furthermore, we found that the secretion of BDNF from cortical neurons was suppressed by PCP. We also confirmed that PCP-caused downregulation of Trk signalings and synaptic proteins were restored by exogenous BDNF application. It is possible that impaired secretion of BDNF and subsequent decreases in Trk signaling are responsible for the loss of synaptic connections caused by PCP.

Large-scale animal model study uncovers altered brain pH and lactate levels as a transdiagnostic endophenotype of neuropsychiatric disorders involving cognitive impairment.

Increased levels of lactate, an end-product of glycolysis, have been proposed as a potential surrogate marker for metabolic changes during neuronal excitation. These changes in lactate levels can result in decreased brain pH, which has been implicated in patients with various neuropsychiatric disorders. We previously demonstrated that such alterations are commonly observed in five mouse models of schizophrenia, bipolar disorder, and autism, suggesting a shared endophenotype among these disorders rather than mere artifacts due to medications or agonal state. However, there is still limited research on this phenomenon in animal models, leaving its generality across other disease animal models uncertain. Moreover, the association between changes in brain lactate levels and specific behavioral abnormalities remains unclear. To address these gaps, the International Brain pH Project Consortium investigated brain pH and lactate levels in 109 strains/conditions of 2294 animals with genetic and other experimental manipulations relevant to neuropsychiatric disorders. Systematic analysis revealed that decreased brain pH and increased lactate levels were common features observed in multiple models of depression, epilepsy, Alzheimer's disease, and some additional schizophrenia models. While certain autism models also exhibited decreased pH and increased lactate levels, others showed the opposite pattern, potentially reflecting subpopulations within the autism spectrum. Furthermore, utilizing large-scale behavioral test battery, a multivariate cross-validated prediction analysis demonstrated that poor working memory performance was predominantly associated with increased brain lactate levels. Importantly, this association was confirmed in an independent cohort of animal models. Collectively, these findings suggest that altered brain pH and lactate levels, which could be attributed to dysregulated excitation/inhibition balance, may serve as transdiagnostic endophenotypes of debilitating neuropsychiatric disorders characterized by cognitive impairment, irrespective of their beneficial or detrimental nature.

Neurotrophins and Other Growth Factors in the Pathogenesis of Alzheimer's Disease.

The involvement of the changed expression/function of neurotrophic factors in the pathogenesis of neurodegenerative diseases, including Alzheimer's disease (AD), has been suggested. AD is one of the age-related dementias, and is characterized by cognitive impairment with decreased memory function. Developing evidence demonstrates that decreased cell survival, synaptic dysfunction, and reduced neurogenesis are involved in the pathogenesis of AD. On the other hand, it is well known that neurotrophic factors, especially brain-derived neurotrophic factor (BDNF) and its high-affinity receptor TrkB, have multiple roles in the central nervous system (CNS), including neuronal maintenance, synaptic plasticity, and neurogenesis, which are closely linked to learning and memory function. Thus, many investigations regarding therapeutic approaches to AD, and/or the screening of novel drug candidates for its treatment, focus on upregulation of the BDNF/TrkB system. Furthermore, current studies also demonstrate that GDNF, IGF1, and bFGF, which play roles in neuroprotection, are associated with AD. In this review, we introduce data demonstrating close relationships between the pathogenesis of AD, neurotrophic factors, and drug candidates, including natural compounds that upregulate the BDNF-mediated neurotrophic system.

Involvement of brain-derived neurotrophic factor signaling in the pathogenesis of stress-related brain diseases.

Neurotrophins including brain-derived neurotrophic factor, BDNF, have critical roles in neuronal differentiation, cell survival, and synaptic function in the peripheral and central nervous system. It is well known that a variety of intracellular signaling stimulated by TrkB, a high-affinity receptor for BDNF, is involved in the physiological and pathological neuronal aspects via affecting cell viability, synaptic function, neurogenesis, and cognitive function. As expected, an alteration of the BDNF/TrkB system is suspected to be one of the molecular mechanisms underlying cognitive decline in cognitive diseases and mental disorders. Recent evidence has also highlighted a possible link between the alteration of TrkB signaling and chronic stress. Furthermore, it has been demonstrated that downregulation of the BDNF/TrkB system and chronic stress have a role in the pathogenesis of Alzheimer's disease (AD) and mental disorders. In this review, we introduce current evidence showing a close relationship between the BDNF/TrkB system and the development of cognition impairment in stress-related disorders, and the possible contribution of the upregulation of the BDNF/TrkB system in a therapeutic approach against these brain diseases.

Enhanced social reward response and anxiety-like behavior with downregulation of nucleus accumbens glucocorticoid receptor in BALB/c mice.

Social anhedonia is a psychological state with difficulty in experiencing pleasure from social interactions and is observed in various diseases, such as depressive disorders. Although the relationships between social reward responses and anxiety- and depression-like behaviors have remained unclear, a social reward conditioned place preference (SCPP) test can be used to analyze the rewarding nature of social interactions. To elucidate these relationships, we used 5-week-old male mice of AKR, BALB/c, and C57BL/6J strains and conducted behavioral tests in the following order: elevated plus-maze test (EPM), open field test (OFT), SCPP, saccharin preference test (SPT), and passive avoidance test. The nucleus accumbens of these mice were collected 24 hr after these behavioral tests and were used for western blotting to determine the levels of receptors for brain-derived neurotrophic factors and glucocorticoids. BALB/c mice displayed the highest levels of anxiety-like behavior in EPM and OFT as well as physical anhedonia-like behaviors in SPT. They also showed increased responses to social rewards and huddling behaviors in SCPP, with downregulated glucocorticoid receptor (GR). Regression analysis results revealed positive influences of anxiety- and physical anhedonia-like behaviors and expressions of GR on social reward responses. Collectively, temperament associated with anxiety and physical anhedonia may affect social reward responses, which possibly is influenced by the expression of GR that can modify these psychological traits.

Glucocorticoid receptor interaction with TrkB promotes BDNF-triggered PLC-gamma signaling for glutamate release via a glutamate transporter.

An increase in glucocorticoid levels and down-regulation of BDNF (brain-derived neurotrophic factor) are supposed to be involved in the pathophysiology of depressive disorders. However, possible crosstalk between glucocorticoid- and BDNF-mediated neuronal functions in the CNS has not been elucidated. Here, we examined whether chronic glucocorticoid exposure influences BDNF-triggered intracellular signaling for glutamate release via a glutamate transporter. We found that chronic exposure to dexamethasone (DEX, a synthetic glucocorticoid) suppressed BDNF-induced glutamate release via weakening the activation of the PLC-gamma (phospholipase C-gamma)/Ca(2+) system in cultured cortical neurons. We demonstrated that the GR (glucocorticoid receptor) interacts with receptor tyrosine kinase for BDNF (TrkB). Following DEX treatment, TrkB-GR interaction was reduced due to the decline in GR expression. Corticosterone, a natural glucocorticoid, also reduced TrkB-GR interaction, BDNF-stimulated PLC-gamma, and BDNF-triggered glutamate release. Interestingly, BDNF-dependent binding of PLC-gamma to TrkB was diminished by DEX. SiRNA transfection to induce a decrease in endogenous GR mimicked the inhibitory action of DEX. Conversely, DEX-inhibited BDNF-activated PLC-gamma signaling for glutamate release was recovered by GR overexpression. We propose that TrkB-GR interaction plays a critical role in the BDNF-stimulated PLC-gamma pathway, which is required for glutamate release, and the decrease in TrkB-GR interaction caused by chronic exposure to glucocorticoids results in the suppression of BDNF-mediated neurotransmitter release via a glutamate transporter.

[The hypothalamic-pituitary-adrenal axis and depressive disorder: recent progress].

Depression is a stress-induced disorder and there is compelling evidence for the involvement of hypothalamic-pituitary-adrenal (HPA) axis abnormalities in the disease. Chronic hyperactivity of the HPA axis and resultant excessive glucocorticoid (hypercortisolism) may be causal to depression. We demonstrated that the dexamethasone (DEX)/CRH test is a sensitive state-dependent marker to monitor HPA axis abnormalities. Restoration from HPA axis abnormalities occurs with clinical responses to treatment. Brain-derived neurotrophic factor (BDNF) has also been implicated in depression. We found that glucocorticoid (DEX) suppresses BDNF-induced dendrite outgrowth and synaptic formation via blocking the MAPK pathway in early-developing cultured hippocampal neurons. Furthermore, we demonstrated that glucocorticoid receptor (GR) and TrkB (a specific receptor of BDNF) interact and that DEX acutely suppresses BDNF-induced glutamate release by affecting the PLC-gamma pathway in cultured cortical neurons, indicating a mechanism underlying the effect of excessive glucocorticoid on BDNF function and resultant damage in cortical neurons. In a macroscopic view using magnetic resonance imaging (MRI), we found that individuals with hypercortisolism detected by the DEX/CRH test demonstrated volume loss in gray matter and reduced neural network assessed with diffusion tensor imaging in several brain regions. Finally, we observed that individuals with hypocortisolism detected by the DEX/CRH test tend to present more distress symptoms, maladaptive coping styles, and schizotypal personality traits than their counterparts, which points to the important role of hypocortisolism as well as hypercortisolism in depression spectrum disorders.

Rapid and Simplified Induction of Neural Stem/Progenitor Cells (NSCs/NPCs) and Neurons from Human Induced Pluripotent Stem Cells (hiPSCs).

Human induced pluripotent stem cells (iPSCs) and their progeny displaying tissue-specific characteristics have paved the way for regenerative medicine and research in various fields such as the elucidation of the pathological mechanism of diseases and the discovery of drug candidates. iPSC-derived neurons are particularly valuable as it is difficult to analyze neural cells obtained from the central nervous system in humans. For neuronal induction with iPSCs, one of the commonly used approaches is the isolation and expansion of neural rosettes, following the formation of embryonic bodies (EBs). However, this process is laborious, inefficient, and requires further purification of the cells. To overcome these limitations, we have developed an efficient neural induction method that allows for the generation of neural stem/progenitor cells (NSCs/NPCs) from iPSCs within 7 days and of functional mature neurons. Our method yields a PAX6-positive homogeneous cell population, a cortical NSCs/NPCs, and the resultant NSCs/NPCs can be cryopreserved, expanded, and differentiated into functional mature neurons. Moreover, our protocol will be less expensive than other methods since the protocol requires fewer neural supplements during neural induction. This article also presents the FM1-43 imaging assay, which is useful for the presynaptic assessment of the iPSCs-derived human neurons. This protocol provides a quick and simplified way to generate NSCs/NPCs and neurons, enabling researchers to establish in vitro cellular models to study brain disease pathology.

Small noncoding vault RNA modulates synapse formation by amplifying MAPK signaling.

The small noncoding vault RNA (vtRNA) is a component of the vault complex, a ribonucleoprotein complex found in most eukaryotes. Emerging evidence suggests that vtRNAs may be involved in the regulation of a variety of cellular functions when unassociated with the vault complex. Here, we demonstrate a novel role for vtRNA in synaptogenesis. Using an in vitro synapse formation model, we show that murine vtRNA (mvtRNA) promotes synapse formation by modulating the MAPK signaling pathway. mvtRNA is transported to the distal region of neurites as part of the vault complex. Interestingly, mvtRNA is released from the vault complex in the neurite by a mitotic kinase Aurora-A-dependent phosphorylation of MVP, a major protein component of the vault complex. mvtRNA binds to and activates MEK1 and thereby enhances MEK1-mediated ERK activation in neurites. These results suggest the existence of a regulatory mechanism of the MAPK signaling pathway by vtRNAs as a new molecular basis for synapse formation.

Interface between hypothalamic-pituitary-adrenal axis and brain-derived neurotrophic factor in depression.

Although the pathophysiology of depressive disorder remains elusive, two hypothetical frameworks seem to be promising: the involvement of hypothalamic pituitary-adrenal (HPA) axis abnormalities and brain-derived neurotrophic factor (BDNF) in the pathogenesis and in the mechanism of action of antidepressant treatments. In this review, we focused on research based on these two frameworks in relation to depression and related conditions and tried to formulate an integrated theory of the disorder. Hormonal challenge tests, such as the dexamethasone/corticotropin-releasing hormone test, have revealed elevated HPA activity (hypercortisolism) in at least a portion of patients with depression, although growing evidence has suggested that abnormally low HPA axis (hypocortisolism) has also been implicated in a variety of stress-related conditions. Several lines of evidence from postmortem studies, animal studies, blood levels, and genetic studies have suggested that BDNF is involved in the pathogenesis of depression and in the mechanism of action of biological treatments for depression. Considerable evidence has suggested that stress reduces the expression of BDNF and that antidepressant treatments increase it. Moreover, the glucocorticoid receptor interacts with the specific receptor of BDNF, TrkB, and excessive glucocorticoid interferes with BDNF signaling. Altered BDNF function is involved in the structural changes and possibly impaired neurogenesis in the brain of depressed patients. Based on these findings, an integrated schema of the pathological and recovery processes of depression is illustrated.