Glutamatergic modulators: the future of treating mood disorders?

Effects of venlafaxine and escitalopram treatments on NMDA receptors in the rat depression model

Depression may relate to neurocognitive impairment that results from alteration of N-methyl-D: -aspartate receptor (NMDAR) levels. Venlafaxine and escitalopram are two drugs commonly used to treat depression. The drugs may affect expression of NMDARs, which mediate learning and memory formation. The aim of the study was to examine whether the effects of venlafaxine and escitalopram treatments are associated with NMDARs in a rat model of depression. Forty male Wistar albino rats were randomly divided into four groups (n = 10) as follows: control group, chronic mild stress group (CMS), venlafaxine (20 mg/kg body weight per day) + CMS, and escitalopram (10 mg/kg body weight per day) + CMS. After induction of depression, a decrease in the concentration of NR2B was observed; venlafaxine treatment prevented the reduction of NR2B expression. Escitalopram treatment did not effect the reduced levels of NR2B resulting from depression. There was no significant difference in NR2A concentration among groups. The present data support the notion that venlafaxine plays a role in maintaining NR2B receptor in experimental depression. It may be possible that treatment with escitalopram has no effect on NMDARs in experimental depression.

Effects of venlafaxine and escitalopram treatments on NMDA receptors in the rat depression model

Depression may relate to neurocognitive impairment that results from alteration of N-methyl-D: -aspartate receptor (NMDAR) levels. Venlafaxine and escitalopram are two drugs commonly used to treat depression. The drugs may affect expression of NMDARs, which mediate learning and memory formation. The aim of the study was to examine whether the effects of venlafaxine and escitalopram treatments are associated with NMDARs in a rat model of depression. Forty male Wistar albino rats were randomly divided into four groups (n = 10) as follows: control group, chronic mild stress group (CMS), venlafaxine (20 mg/kg body weight per day) + CMS, and escitalopram (10 mg/kg body weight per day) + CMS. After induction of depression, a decrease in the concentration of NR2B was observed; venlafaxine treatment prevented the reduction of NR2B expression. Escitalopram treatment did not effect the reduced levels of NR2B resulting from depression. There was no significant difference in NR2A concentration among groups. The present data support the notion that venlafaxine plays a role in maintaining NR2B receptor in experimental depression. It may be possible that treatment with escitalopram has no effect on NMDARs in experimental depression.

Role of the mTOR signaling pathway in the rapid antidepressant action of ketamine

Some patients with major depressive disorder remain resistant to antidepressant medication. A randomized, placebo-controlled, double-blind trial demonstrated that a single subanesthetic dose (0.5 mg/kg) of the N-methyl-D-aspartate receptor antagonist ketamine caused a rapid antidepressant effect within hours in treatment-resistant patients with major depressive disorder. However, the precise cellular mechanisms underlying ketamine's rapid antidepressant actions were unclear, although it is proposed that the α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor might be involved in these mechanisms. Recently, Li et al. reported the role of the mammalian target of rapamycin (mTOR) signaling pathway, a ubiquitous protein kinase involved in protein synthesis and synaptic plasticity, in ketamine's rapid antidepressant effects. Here, these findings are put into context and their significance is discussed.

Social isolation-induced aggression potentiates anxiety and depressive-like behavior in male mice subjected to unpredictable chronic mild stress

Background

Accumulating epidemiological evidence shows that life event stressors are major vulnerability factors for psychiatric diseases such as major depression. It is also well known that social isolation in male mice results in aggressive behavior. However, it is not known how social isolation-induced aggression affects anxiety and depressive-like behavior in isolated male mice subjected to unpredictable chronic mild stress (CMS), an animal model of depression.

Methodology/Principal Findings

C57/B6 male mice were divided into 3 groups; non-stressed controls, in Group I; isolated mice subjected to the CMS protocol in Group II and aggression by physical contact in socially isolated mice subjected to the CMS protocol in Group III. In the sucrose intake test, ingestion of a 1% sucrose solution by mice in Groups II and III was significantly lower than in Group I. Furthermore, intake of this solution in Group III mice was significantly lower than in Group II mice. In the open field test, mice in Group III, showed reduced locomotor activity and reduced entry and retention time in the central zone, compared to Groups I and II mice. Moreover, the distances moved in 1 hour by Group III mice did not differ between night and morning. In the light/black box test, Groups II and III animals spent significantly less time in the light box compared to Group I animals. In the tail suspension test (TST) and forced swimming test (FST), the immobility times of Group II and Group III mice were significantly longer than in Group I mice. In addition, immobility times in the FST were significantly longer in Group III than in Group II mice.

Conclusions/Significance

These findings show that social isolation-induced aggression could potentiate anxiety and depressive -like behaviors in isolated male mice subjected to CMS.

Cognitive dysfunction in depression: neurocircuitry and new therapeutic strategies

Major depressive disorder (MDD) is a disabling medical condition associated with significant morbidity, mortality and public health costs. However, neurocircuitry abnormalities underlying depression remain incompletely understood and consequently current treatment options are unfortunately limited in efficacy. Recent research has begun to focus specifically on cognitive aspects of depression and potential neurobiological correlates. Two fundamental types of cognitive dysfunction observed in MDD are cognitive biases, which include distorted information processing or attentional allocation toward negative stimuli, and cognitive deficits, which include impairments in attention, short-term memory and executive functioning. In this article, we present a selective review of current research findings in these domains and examine neuroimaging research that is beginning to characterize the neurocircuitry underlying these biases and deficits. We propose that deficient cognitive functioning, attention biases and the sustained negative affect characteristic of MDD can be understood as arising in part from dysfunctional prefrontal-subcortical circuitry and related disturbances in the cognitive control of emotion. Finally, we highlight potential new pharmacological and non-pharmacological therapeutic strategies for MDD based on an evolving mechanistic understanding of the disorder.

Effects of chronic treatment with fluoxetine on receptor-stimulated increase of [Ca2+]i in astrocytes mimic those of acute inhibition of TRPC1 channel activity

Primary cultures of mouse astrocytes were used to investigate effects by chronic treatment (3-21 days) with fluoxetine (0.5-10 μM) on capacitative Ca(2+) influx after treatment with the SERCA inhibitor thapsigargin and on receptor agonist-induced increases in free cytosolic Ca(2+) concentration [Ca(2+)](i), determined with Fura-2. The agonists were the 5-HT(2B) agonist fluoxetine, the α(2)-adrenergic agonist dexmedetomidine, and ryanodine receptor (RyR) and IP(3) receptor (IP(3)R) agonists. In untreated sister cultures each agonist distinctly increased [Ca(2+)](i), but in cultures treated for sufficient length of time or with sufficiently high doses of fluoxetine, acute administration of fluoxetine, dexmedetomidine, or RyR or IP(3)R agonists elicited reduced, in some cases abolished, effects. Capacitative Ca(2+) entry, meditated by TRPC1 channels, was sufficiently inhibited to cause a depletion of Ca(2+) stores, which could explain the reduced agonist effects. All effects of chronic fluoxetine administration could be replicated by TRPC1 channel antibody or siRNA. Since increases in astrocytic [Ca(2+)](i) regulate release of gliotransmitters, these effects may have profound effects on brain function. They may be important for therapeutic effects of all 5 conventional 'serotonin-specific reuptake inhibitors' (SSRIs), which at concentrations used therapeutically (∼1 μM) share other of fluoxetine's chronic effects (Zhang et al., Neuron Glia Biol. 16 (2010) 1-13).

Late-life depression and Alzheimer's disease: the glutamatergic system inside of this mirror relationship

Late-life depressive syndromes often arise in the context of predementia, dementia syndromes, and Alzheimer's disease (AD). Conversely, patients with a history of mood disorders are at higher risk of developing cognitive impairment. The high rate of co-occurrence of these two disorders is becoming a major health problem in older subjects for both their epidemiological impact and the negative outcomes in terms of disability and increased mortality. In this perspective, it is possible to speculate on the presence of a mirror relationship between depressive and cognitive disorders in late-life. Indeed, although a causal contribution of genetic, environmental, and social factors is widely recognized in these disorders, the neurobiological links still remain largely unknown. l-glutamic acid and γ-aminobutyric acid are the principal excitatory and inhibitory neurotransmitters in the central nervous system, respectively, and increasing evidence suggests that alterations in this neurotransmitter system may contribute to the neurobiology linking depression and cognitive impairment. In the present review article, we examined the neurobiological bases of the relationship between late-life depressive syndromes and AD, with a particular attention to glutamatergic pathway signalling like a bridge connecting these two conditions. In addition, attempts have been made to explain changes in glutamatergic pathway, depression in older age, and dementia through the analysis of signal transduction mechanisms associated with these disabling disorders.

Roles of glutamate signaling in preclinical and/or mechanistic models of depression

Accumulating evidence suggests that the glutamatergic system plays important roles in the pathophysiology and treatment of major depressive disorder (MDD). Abnormalities in the glutamatergic system are definitely observed in this disorder, and certain glutamatergic agents exhibit antidepressant effects in patients with MDD. In this review, we summarize the preclinical findings suggesting the involvement of glutamate signaling in the pathophysiology and treatment of MDD. Preclinical animal models for depression are often characterized by changes in molecules related to glutamatergic signaling. Some antidepressants exert their effects by affecting glutamatergic system components in animals. Animals with genetically modified glutamatergic function exhibit depression-like behaviors or anti-depressive behavior. In addition, several types of glutamatergic agents have shown antidepressant-like effects in preclinical models for depression. Many types of glutamate receptors (NMDA, AMPA, and metabotropic glutamate receptors) or transporters appear to be involved in the etiology of depression or in the mechanisms of action of antidepressants. These functional proteins related to glutamate signal transduction are potential targets for a new generation of antidepressants with fast-onset effects, such as the NMDA antagonist ketamine.