Antidepressant mechanisms: functional and molecular correlates of excitatory amino acid neurotransmission

Fluoxetine reverses the behavioral despair induced by neurogenic stress in mice: role of N-methyl-d-aspartate and opioid receptors

Opioid and N-methyl-d-aspartate (NMDA) receptors mediate different effects of fluoxetine. We investigated whether opioid and NMDA receptors are involved in the protective effect of fluoxetine against the behavioral despair induced by acute physical stress in male mice. We used the forced swimming test (FST), tail suspension test (TST), and open-field test (OFT) for behavioral evaluation. We used fluoxetine, naltrexone (opioid receptor antagonist), MK-801 (NMDA receptor antagonist), morphine (opioid receptor agonist), and NMDA (NMDA receptor agonist). Acute foot-shock stress (FSS) significantly induced behavioral despair (depressive-like) and anxiety-like behaviors in tests. Fluoxetine (5 mg/kg) reversed the depressant-like effect of FSS, but it did not alter the locomotion and anxiety-like behavior in animals. Acute administration of subeffective doses of naltrexone (0.3 mg/kg) or MK-801 (0.01 mg/kg) potentiated the antidepressant-like effect of fluoxetine, while subeffective doses of morphine (1 mg/kg) and NMDA (75 mg/kg) abolished this effect of fluoxetine. Also, co-administration of subeffective doses of naltrexone (0.05 mg/kg) and MK-801 (0.003 mg/kg) with fluoxetine (1 mg/kg) induced a significant decrease in the immobility time in FST and TST. Our results showed that opioid and NMDA receptors (alone or in combination) are involved in the antidepressant-like effect of fluoxetine against physical stress.

A pilot integrative genomics study of GABA and glutamate neurotransmitter systems in suicide, suicidal behavior, and major depressive disorder

Gamma-amino butyric acid (GABA) and glutamate are the major inhibitory and excitatory neurotransmitters in the mammalian central nervous system, respectively, and have been associated with suicidal behavior and major depressive disorder (MDD). We examined the relationship between genotype, brain transcriptome, and MDD/suicide for 24 genes involved in GABAergic and glutamatergic signaling. In part 1 of the study, 119 candidate SNPs in 24 genes (4 transporters, 4 enzymes, and 16 receptors) were tested for associations with MDD and suicidal behavior in 276 live participants (86 nonfatal suicide attempters with MDD and 190 non-attempters of whom 70% had MDD) and 209 postmortem cases (121 suicide deaths of whom 62% had MDD and 88 sudden death from other causes of whom 11% had MDD) using logistic regression adjusting for sex and age. In part 2, RNA-seq was used to assay isoform-level expression in dorsolateral prefrontal cortex of 59 postmortem samples (21 with MDD and suicide, 9 MDD without suicide, and 29 sudden death non-suicides and no psychiatric illness) using robust regression adjusting for sex, age, and RIN score. In part 3, SNPs with subthreshold (uncorrected) significance levels below 0.05 for an association with suicidal behavior and/or MDD in part 1 were tested for eQTL effects in prefrontal cortex using the Brain eQTL Almanac (www.braineac.org). No SNPs or transcripts were significant after adjustment for multiple comparisons. However, a protein coding transcript (ENST00000414552) of the GABA A receptor, gamma 2 (GABRG2) had lower brain expression postmortem in suicide (P = 0.01) and evidence for association with suicide death (P = 0.03) in a SNP that may be an eQTL in prefrontal cortex (rs424740, P = 0.02). These preliminary results implicate GABRG2 in suicide and warrant further investigation and replication in larger samples.

AMPA Receptor-mTOR Activation is Required for the Antidepressant-Like Effects of Sarcosine during the Forced Swim Test in Rats: Insertion of AMPA Receptor may Play a Role

Sarcosine, an endogenous amino acid, is a competitive inhibitor of the type I glycine transporter and an N-methyl-d-aspartate receptor (NMDAR) coagonist. Recently, we found that sarcosine, an NMDAR enhancer, can improve depression-related behaviors in rodents and humans. This result differs from previous studies, which have reported antidepressant effects of NMDAR antagonists. The mechanisms underlying the therapeutic response of sarcosine remain unknown. This study examines the role of mammalian target of rapamycin (mTOR) signaling and α-amino-3-hydroxy-5-methylisoxazole-4-propionate receptor (AMPAR) activation, which are involved in the antidepressant-like effects of several glutamatergic system modulators. The effects of sarcosine in a forced swim test (FST) and the expression levels of phosphorylated mTOR signaling proteins were examined in the absence or presence of mTOR and AMPAR inhibitors. In addition, the influence of sarcosine on AMPAR trafficking was determined by analyzing the phosphorylation of AMPAR subunit GluR1 at the PKA site (often considered an indicator for GluR1 membrane insertion in neurons). A single injection of sarcosine exhibited antidepressant-like effects in rats in the FST and rapidly activated the mTOR signaling pathway, which were significantly blocked by mTOR inhibitor rapamycin or the AMPAR inhibitor 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo(f)quinoxaline (NBQX) pretreatment. Moreover, NBQX pretreatment eliminated the ability of sarcosine to stimulate the phosphorylated mTOR signaling proteins. Furthermore, GluR1 phosphorylation at its PKA site was significantly increased after an acute in vivo sarcosine treatment. The results demonstrated that sarcosine exerts antidepressant-like effects by enhancing AMPAR–mTOR signaling pathway activity and facilitating AMPAR membrane insertion.

Highlights

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A single injection of sarcosine rapidly exerted antidepressant-like effects with a concomitant increase in the activation of the mammalian target of rapamycin mTOR signaling pathway.

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The antidepressant-like effects of sarcosine occur through the activated AMPAR–mTOR signaling pathway.

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Sarcosine could enhance AMPAR membrane insertion via an AMPAR throughput.

Cannabinoid receptor activation prevents the effects of chronic mild stress on emotional learning and LTP in a rat model of depression

Most psychiatric disorders are characterized by emotional memory or learning disturbances. Chronic mild stress (CMS) is a common animal model for stress-induced depression. Here we examined whether 3 days of treatment using the CB1/2 receptor agonist WIN55,212-2 could ameliorate the effects of CMS on emotional learning (ie, conditioned avoidance and extinction), long-term potentiation (LTP) in the hippocampal-accumbens pathway, and depression-like symptoms (ie, coping with stress behavior, anhedonia, and weight changes). We also examined whether the ameliorating effects of WIN55,212-2 on behavior and physiology after CMS are mediated by CB1 and glucocorticoid receptors (GRs). Rats were exposed to CMS or handled on days 1-21. The agonist WIN55,212-2 or vehicle were administered on days 19-21 (IP; 0.5 mg/kg) and behavioral and electrophysiological measures were taken on days 23 and 28. The CB1 receptor antagonist AM251 (IP; 0.3 mg/kg) or the GR antagonist RU-38486 (IP; 10 mg/kg) were co-administered with WIN55,212-2. Our results show that CMS significantly modified physiological and behavioral reactions, as observed by the impairment in avoidance extinction and LTP in the hippocampal-accumbens pathway, and the alterations in depression-like symptoms, such as coping with stress behavior, weight gain, and sucrose consumption. The most significant effect observed in this study was that 3 days of WIN55,212-2 administration prevented the CMS-induced alterations in emotional memory (ie, extinction) and plasticity. This effect was mediated by CB1 receptors as the CB1 receptor antagonist AM251 prevented the ameliorating effects of WIN55,212-2 on extinction and LTP. The GR antagonist RU-38486 also prevented the CMS-induced alterations in extinction and plasticity, and when co-administered with WIN55,212-2, the preventive effects after CMS were maintained. The findings suggest that enhancing cannabinoid signaling could represent a novel approach to the treatment of cognitive deficits that accompany stress-related depression.

Measurement of glycine in gray and white matter in the human brain in vivo by 1H MRS at 7.0 T

The concentration of glycine (Gly) was measured in gray matter (GM) and white matter (WM) in the human brain using single-voxel localized (1)H MRS at 7 T. A point-resolved spectroscopy sequence with echo time = 150 ms was used for measuring Gly levels in various regions of the frontal and occipital lobes in 11 healthy volunteers and one subject with a glioblastoma. The point-resolved spectroscopy spectra were analyzed with LCModel using basis functions generated from density matrix simulations that included the effects of volume localized radio-frequency and gradient pulses. The fraction of GM and white matter within the voxels was obtained from T(1)-weighted image segmentation. The metabolite concentrations within the voxels, estimated with respect to the GM + WM water concentrations, were fitted to a linear function of fractional GM content. The Gly concentrations in pure GM and white matter were estimated to be 1.1 and 0.1 mM, with 95% confidence intervals 1.0-1.2 and 0.0-0.2, respectively.

Mice lacking the β4 subunit of the nicotinic acetylcholine receptor show memory deficits, altered anxiety- and depression-like behavior, and diminished nicotine-induced analgesia

RATIONALE

The role of β4-containing nicotinic acetylcholine receptors (nAChRs) in cognition, anxiety, depression, and analgesia in the absence of nicotine is unclear.

METHODS

Wild-type (β4(+/+)) and knockout (β4(-/-)) mice for the nAChR β4 subunit were tested in behavioral tests assessing cognitive function, affective behaviors, and nociception.

RESULTS

There were no learning and memory deficits in β4(-/-) mice compared with β4(+/+) mice during the acquisition of the Barnes maze, contextual fear conditioning, and Y maze tasks. In the Barnes maze memory retention test, male β4(-/-) mice showed reduced use of the spatial search strategy, indicating small spatial memory deficits compared with β4(+/+) mice. In the cue-induced fear conditioning memory retention test, β4(-/-) mice exhibited reduced freezing time compared with β4(+/+) mice. Compared with β4(+/+) mice, β4(-/-) mice exhibited decreased anxiety-like behavior in the light-dark box. Depression-like behavior in β4(-/-) mice was decreased in the tail suspension test and increased in the forced swim test compared with β4(+/+) mice. β4(-/-) mice did not differ from β4(+/+) mice in basal nociception but were less sensitive to the antinociceptive effect of nicotine in 2 tests of acute thermal pain.

CONCLUSIONS

Lack of β4-containing nAChRs resulted in small deficits in hippocampus- and amygdala-dependent memory retention functions. β4-containing nAChRs are involved in anxiety- and depression-like behaviors and contribute to the analgesic effects of nicotine.

Increased stress-evoked nitric oxide signalling in the Flinders sensitive line (FSL) rat: a genetic animal model of depression

Stress engenders the precipitation and progression of affective disorders, while stress-related release of excitatory mediators is implicated in the degenerative pathology observed especially in the hippocampus of patients with severe depression. Nitric oxide (NO) release following stress-evoked N-methyl-d-aspartate (NMDA) receptor activation modulates neurotransmission, cellular memory and neuronal toxicity. We have investigated the Flinders rat (FSL/FRL), a genetic animal model of depression, regarding the response of the hippocampal nitrergic system following exposure to an escapable stress/inescapable stress (ES-IS) paradigm. Hippocampal tissue from naive FSL/FRL rats and those exposed to ES-IS were studied with respect to constitutive nitric oxide synthase (cNOS) activity and neuronal nitric oxide synthase (nNOS) protein levels, as well as transcript expression of upstream regulatory proteins in the NMDA-NO signalling pathway, including NMDAR1, nNOS, CAPON, PIN and PSD95. Within stress-naive animals, no differences in hippocampal cNOS activity and nNOS expression or PIN were evident in FSL and FRL rats, although transcripts for NMDAR1 and CAPON were increased in FSL rats. Within the group of ES-IS animals, we found an increase in total hippocampal cNOS activity, nNOS protein levels and mRNA expression in FSL vs. FRL rats, together with an increase in PSD95 transcripts, and a reduction in PIN. In conclusion, ES-IS enhanced hippocampal cNOS activity in FSL rats, but not FRL rats, confirming the NMDA-NO cascade as an important vulnerability factor in the depressive phenotype of the FSL rat.

Antidepressants are a rational complementary therapy for the treatment of Alzheimer's disease

There is a high prevalence rate (30-50%) of Alzheimer's disease (AD) and depression comorbidity. Depression can be a risk factor for the development of AD or it can be developed secondary to the neurodegenerative process. There are numerous documented diagnosis and treatment challenges for the patients who suffer comorbidity between these two diseases. Meta analysis studies have provided evidence for the safety and efficacy of antidepressants in treatment of depression in AD patients. Preclinical and clinical studies show the positive role of chronic administration of selective serotonin reuptake inhibitor (SSRI) antidepressants in hindering the progression of the AD and improving patient performance. A number of clinical studies suggest a beneficial role of combinatorial therapies that pair antidepressants with FDA approved AD drugs. Preclinical studies also demonstrate a favorable effect of natural antidepressants for AD patients. Based on the preclinical studies there are a number of plausible antidepressants effects that may modulate the progression of AD. These effects include an increase in neurogenesis, improvement in learning and memory, elevation in the levels of neurotrophic factors and pCREB and a reduction of amyloid peptide burden. Based on this preclinical and clinical evidence, antidepressants represent a rational complimentary strategy for the treatment of AD patients with depression comorbidity.

Global brain gene expression analysis links glutamatergic and GABAergic alterations to suicide and major depression

BACKGROUND

Most studies investigating the neurobiology of depression and suicide have focused on the serotonergic system. While it seems clear that serotonergic alterations play a role in the pathogenesis of these major public health problems, dysfunction in additional neurotransmitter systems and other molecular alterations may also be implicated. Microarray expression studies are excellent screening tools to generate hypotheses about additional molecular processes that may be at play. In this study we investigated brain regions that are known to be implicated in the neurobiology of suicide and major depression are likely to represent valid global molecular alterations.

METHODOLOGY/PRINCIPAL FINDINGS

We performed gene expression analysis using the HG-U133AB chipset in 17 cortical and subcortical brain regions from suicides with and without major depression and controls. Total mRNA for microarray analysis was obtained from 663 brain samples isolated from 39 male subjects, including 26 suicide cases and 13 controls diagnosed by means of psychological autopsies. Independent brain samples from 34 subjects and animal studies were used to control for the potential confounding effects of comorbidity with alcohol. Using a Gene Ontology analysis as our starting point, we identified molecular pathways that may be involved in depression and suicide, and performed follow-up analyses on these possible targets. Methodology included gene expression measures from microarrays, Gene Score Resampling for global ontological profiling, and semi-quantitative RT-PCR. We observed the highest number of suicide specific alterations in prefrontal cortical areas and hippocampus. Our results revealed alterations of synaptic neurotransmission and intracellular signaling. Among these, Glutamatergic (GLU) and GABAergic related genes were globally altered. Semi-quantitative RT-PCR results investigating expression of GLU and GABA receptor subunit genes were consistent with microarray data.

CONCLUSIONS/SIGNIFICANCE

The observed results represent the first overview of global expression changes in brains of suicide victims with and without major depression and suggest a global brain alteration of GLU and GABA receptor subunit genes in these conditions.

The association study of three FYN polymorphisms with prophylactic lithium response in bipolar patients

FYN belongs to the protein kinase family that phosphorylates NMDA receptor subunits, participating in the regulation of ion transmission and BDNF/TrkB signal transduction pathway. Lithium inhibits glutamatergic transmission via NMDA receptors, exerting neuroprotective effect against excitotoxicity. The aim of this study was to find possible association of three polymorphisms of FYN gene with prophylactic lithium response in the group of bipolar patients. We analyzed 101 bipolar patients treated with lithium carbonate for 5-27 years (mean 15 years). Twenty-four patients were identified as excellent lithium responders (ER), 51 patients as partial responders (PRs), and 26 patients were non-responders. Genotypes of the three analyzed polymorphisms were established by PCR-RFLP. Statistical analysis was done with Statistica. No significant differences in genotype distribution and allele frequencies were observed between T/G and A/G FYN polymorphisms and lithium response. We observed a trend toward association of TT genotype and T allele of T/C polymorphism with worse lithium response. The results of the study demonstrated only marginal association between FYN polymorphisms and prophylactic lithium response in bipolar patients. The results are discussed in light of our previous studies on FYN gene in bipolar illness and BDNF gene in lithium response.

Possible involvement of mu-opioid receptors in effect of lithium on inhibitory avoidance response in mice

In the present study, effects of intracerebroventricular (i.c.v.) injections of mu-opioid receptor agonist and antagonist on lithium state-dependency were investigated. For memory assessment, a one-trial step-down inhibitory avoidance task was used in adult male NMRI mice. Intraperitoneal (i.p.) administration of lithium (10 mg/kg) after training impaired memory when retrieval was tested 24 h later. The memory impairment was reversed by pretest administration of the same dose of lithium, suggesting state-dependency induced by lithium. In addition, i.c.v. administration of both lithium (2 and 4 microg/mouse, i.c.v.) and morphine (3 and 6 microg/mouse, i.c.v.) before the test reversed memory impairment induced by post-training lithium (10 mg/kg, i.p.). On the other hand, pretest administration of naloxone (1 and 2 mg/kg) which had no effects alone on inhibitory avoidance response, prevented the improving effects of both morphine (3 microg/mouse, i.c.v.) and lithium (2 microg/mouse, i.c.v.) on memory retrieval. The results suggest that the mu-opioid receptors in the central nervous system may be involved in the retrieval of lithium state-dependent learning.

Neurotransmission and bipolar disorder: a systematic family-based association study

Neurotransmission pathways/systems have been proposed to be involved in the pathophysiology and treatment of bipolar disorder for over 40 years. In order to test the hypothesis that common variants of genes in one or more of five neurotransmission systems confer risk for bipolar disorder, we analyzed 1,005 tag single nucleotide polymorphisms in 90 genes from dopaminergic, serotonergic, noradrenergic, GABAergic, and glutamatergic neurotransmitter systems in 101 trios and 203 quads from Caucasian bipolar families. Our sample has 80% power to detect ORs >or= 1.82 and >or=1.57 for minor allele frequencies of 0.1 and 0.5, respectively. Nominally significant allelic and haplotypic associations were found for genes from each neurotransmission system, with several reaching gene-wide significance (allelic: GRIA1, GRIN2D, and QDPR; haplotypic: GRIN2C, QDPR, and SLC6A3). However, none of these associations survived correction for multiple testing in an individual system, or in all systems considered together. Significant single nucleotide polymorphism associations were not found with sub-phenotypes (alcoholism, psychosis, substance abuse, and suicide attempts) or significant gene-gene interactions. These results suggest that, within the detectable odds ratios of this study, common variants of the selected genes in the five neurotransmission systems do not play major roles in influencing the risk for bipolar disorder or comorbid sub-phenotypes.

Is major depressive disorder a metabolic encephalopathy?

Metabolic encephalopathy is an acute disturbance in cellular metabolism in the brain evoked by conditions of hypoxia, hypoglycaemia, oxidative stress and/or inflammation. It usually develops acutely or subacutely and is reversible if the systemic disorder is treated. If left untreated, however, metabolic encephalopathy may result in secondary structural damage to the brain. Most encephalopathies are present with neuropsychiatric symptoms, one in particular being depression. However, mood disorders are often co-morbid with cardiovascular, liver, kidney and endocrine disorders, while increasing evidence concurs that depression involves inflammatory and neurodegenerative processes. This would suggest that metabolic disturbances resembling encephalopathy may underscore the basic neuropathology of depression at a far deeper level than currently realized. Viewing depression as a form of encephalopathy, and exploiting knowledge gleaned from our understanding of the neurochemistry and treatment of metabolic encephalopathy, may assist in our understanding of the neurobiology of depression, but also in realizing new ideas in the pharmacotherapy of mood disorders.

Direct inhibitory effect of fluoxetine on N-methyl-D-aspartate receptors in the central nervous system

BACKGROUND

Data accumulated in the last decade indicate that N-methyl-D-aspartate (NMDA) receptors might be involved in the pathophysiology of depression and the mechanism of action of antidepressants, although a direct inhibitory effect has been reported only in connection with tricyclic compounds, which interact with a wide range of receptors.

METHODS

Using whole-cell patch-clamp recording in rat cortical cell cultures, we investigated whether the selective serotonin reuptake inhibitor fluoxetine, which has a much better adverse effect profile, has a direct effect on NMDA receptors, and we compared its action to that of the tricyclic desipramine.

RESULTS

Both desipramine (concentration that causes 50% inhibition (IC(50)) = 3.13 microM) and fluoxetine (IC(50) = 10.51 microM) inhibited NMDA-evoked currents with similar efficacy in the clinically relevant low micromolar concentration range. However, in contrast to desipramine, the inhibition by fluoxetine was not voltage-dependent, and fluoxetine partially preserved its ability to associate with NMDA receptor in the presence of Mg(2+), suggesting different binding sites for the two drugs.

CONCLUSIONS

The fact that different classes of antidepressants were found to be low-affinity NMDA antagonists suggests that direct inhibition of NMDA receptors may contribute to the clinical effects of antidepressants.

Enhanced long-term synaptic depression in an animal model of depression

BACKGROUND

A growing body of evidence suggests a disturbance of brain plasticity in major depression. In contrast to hippocampal neurogenesis, much less is known about the role of synaptic plasticity. Long-term potentiation (LTP) and long-term depression (LTD) regulate the strength of synaptic transmission and the formation of new synapses in many neural networks. Therefore, we examined the modulation of synaptic plasticity in the chronic mild stress animal model of depression.

METHODS

Adult rats were exposed to mild and unpredictable stressors for 3 weeks. Thereafter, long-term synaptic plasticity was examined in the hippocampal CA1 region by whole-cell patch clamp measurements in brain slices. Neurogenesis was assessed by doublecortin immunostaining.

RESULTS

Exposure to chronic mild stress facilitated LTD and had no effect on LTP. Chronic application of the antidepressant fluvoxamine during the stress protocol prevented the facilitation of LTD and increased the extent of LTP induction. Neurogenesis in the dentate gyrus was impaired after chronic stress.

CONCLUSIONS

In addition to neurogenesis, long-term synaptic plasticity is an important and ubiquitous form of brain plasticity that is disturbed in an animal model of depression. Facilitated depression of synaptic transmission might impair function and structure of brain circuits involved in the pathophysiology of major depression. Antidepressants might counteract these alterations.

Steroid pregnenolone sulfate enhances NMDA-receptor-independent long-term potentiation at hippocampal CA1 synapses: Role for L-type calcium channels and sigma-receptors

Severe stress elevates plasma and CNS levels of endogenous neuroactive steroids that can contribute to the influence of stress on memory formation. Among the neuroactive steroids, pregnenolone sulfate (PREGS) reportedly strengthens memories and is readily available as a memory-enhancing supplement. PREGS actions on memory may reflect its ability to produce changes in memory-related neuronal circuits, such as long-term potentiation (LTP) of excitatory transmission in hippocampus. Here, we report a previously undiscovered pathway by which PREGS exposure promotes activity-dependent LTP of field excitatory postsynaptic potentials at CA1 synapses in hippocampal slices. Thus, application of PREGS, but not the phosphated conjugate of the steroid, selectively facilitates the induction of a slow-developing LTP in response to high-frequency (100 Hz) afferent stimulation, which is not induced in the absence of the steroid. The slow-developing LTP is independent of NMDA-receptor function (i.e., dAP5 insensitive) but dependent on functional L-type voltage-gated calcium channels (VGCC) and sigma-receptors. By contrast, PREGS at the highest concentration tested produces a depression in NMDA-receptor-dependent LTP, which is evident when sigma-receptor function is compromised by the presence of a sigma-receptor antagonist. We found that at early times during the induction phase of L-type VGCC-dependent LTP, PREGS via sigma-receptors transiently enhances presynaptic function. As well, during the maintenance phase of L-type VGCC-dependent LTP, PREGS promotes a further increase in presynaptic function downstream of LTP induction, as evidenced by a decrease in paired-pulse facilitation. The identification of complex regulatory actions of PREGS on LTP, involving sigma-receptors, L-type VGCCs, NMDA-receptors, and inhibitory circuits will aid future research endeavors aimed at understanding the precise mechanisms by which this stress-associated steroid may engage multiple LTP-signaling pathways that alter synaptic transmission at memory-related synapses.

Increased hippocampal nitric oxide synthase activity and stress responsiveness after imipramine discontinuation: role of 5HT 2A/C-receptors

Chronic depressive illness may cause shrinkage of the hippocampus with stress-induced release of glutamate and nitric oxide possibly causally linked to this pathology. Poor antidepressant compliance may contribute to this pathology as well as to long term morbidity. However, antidepressant withdrawal-associated symptoms in depressed patients often reflect hyperserotonergia. The effect of chronic imipramine (IMI; 15 mg/kg/d ip x 3wks) treatment and withdrawal on swim stress responsiveness was studied in Sprague-Dawley rats together with assay of hippocampal NO synthase (NOS) activity. The dependence of any biobehavioral changes following IMI withdrawal on 5HT(2A/C) receptor-mediated events was studied using the 5HT(2A/C) receptor antagonist, ritanserin (RIT; 4 mg/kg/day ip x 7 days), administered alone or during IMI withdrawal. IMI significantly inhibited the situational stress response to forced swimming while also significantly decreasing NOS activity. IMI withdrawal was associated with a significant increase in swim immobility together with a significant increase in NOS activity compared to both control and IMI-treated groups. RIT re-established the anti-immobility effects and reversed NOS hyper-function during IMI withdrawal, although alone it increased NOS activity. Antidepressant discontinuation therefore increases stress responsiveness together with disinhibition of hippocampal NOS through a mechanism involving 5HT(2A/C) receptor activation. The resulting increased nitrergic activity may have significant implications for depressive illness and its treatment.

Correlation between plasma levels of glutamate, alanine and serine with severity of depression

The goal of this study was to evaluate the utility of using plasma levels of amino acids as an indicator of the severity of depression. The samples were collected from 23 depressed patients receiving antidepressant medication, and were compared to 31 healthy subjects. The plasma levels of amino acids were determined using HPLC with fluorometric detection. The severity of depression was evaluated using the Hamilton Depression Rating Scale (HAM-D) scores. Plasma levels of glutamate, glutamine, glycine and taurine were significantly increased in the depressed patients compared to the controls. Statistical analysis indicated a positive correlation between glutamate and alanine levels and HAM-D scores and a negative correlation of L-serine with HAM-D scores. The results indicate that plasma level of glutamate, alanine and L-serine could reflect the severity of depression rather than glutamine, glycine and taurine.

Chronic lithium chloride administration attenuates brain NMDA receptor-initiated signaling via arachidonic acid in unanesthetized rats

It has been proposed that lithium is effective in bipolar disorder (BD) by inhibiting glutamatergic neurotransmission, particularly via N-methyl-D-aspartate receptors (NMDARs). To test this hypothesis and to see if the neurotransmission could involve the NMDAR-mediated activation of phospholipase A2 (PLA2), to release arachidonic acid (AA) from membrane phospholipid, we administered subconvulsant doses of NMDA to unanesthetized rats fed a chronic control or LiCl diet. We used quantitative autoradiography following the intravenous injection of radiolabeled AA to measure regional brain incorporation coefficients k* for AA, which reflect receptor-mediated activation of PLA2. In control diet rats, NMDA (25 and 50 mg/kg i.p.) compared with i.p. saline increased k* significantly in 49 and 67 regions, respectively, of the 83 brain regions examined. The regions affected were those with reported NMDARs, including the neocortex, hippocampus, caudate-putamen, thalamus, substantia nigra, and nucleus accumbens. The increases could be blocked by pretreatment with the specific noncompetitive NMDA antagonist MK-801 ((5R,10S)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine hydrogen maleate) (0.3 mg/kg i.p.), as well by a 6-week LiCl diet sufficient to produce plasma and brain lithium concentrations known to be effective in BD. MK-801 alone reduced baseline values for k* in many brain regions. The results show that it is possible to image NMDA signaling via PLA2 activation and AA release in vivo, and that chronic lithium blocks this signaling, consistent with its suggested mechanism of action in BD.

Differential effects of predator stress and the antidepressant tianeptine on physiological plasticity in the hippocampus and basolateral amygdala

Stress can profoundly affect memory and alter the functioning of the hippocampus and amygdala. Studies have also shown that the antidepressant tianeptine can block the effects of stress on hippocampal and amygdala morphology and synaptic plasticity. We examined the effects of acute predator stress and tianeptine on long-term potentiation (LTP; induced by 100 pulses in 1 s) and primed burst potentiation (PB; a low threshold form of LTP induced by only five physiologically patterned pulses) in CA1 and in the basolateral nucleus (BLA) of the amygdala in anesthetized rats. Predator stress blocked the induction of PB potentiation in CA1 and enhanced LTP in BLA. Tianeptine blocked the stress-induced suppression of PB potentiation in CA1 without affecting the stress-induced enhancement of LTP in BLA. In addition, tianeptine administered under non-stress conditions enhanced PB potentiation in the hippocampus and LTP in the amygdala. These findings support the hypothesis that acute stress impairs hippocampal functioning and enhances amygdaloid functioning. The work also provides insight into the actions of tianeptine with the finding that it enhanced electrophysiological measures of plasticity in the hippocampus and amygdala under stress, as well as non-stress, conditions.

Reward-aversion circuitry in analgesia and pain: implications for psychiatric disorders

Sensory and emotional systems normally interact in a manner that optimizes an organism's ability to survive using conscious and unconscious processing. Pain and analgesia are interpreted by the nervous system as aversive and rewarding processes that trigger specific behavioral responses. Under normal physiological conditions these processes are adaptive. However, under chronic pain conditions, functional alterations of the central nervous system frequently result in maladaptive behaviors. In this review, we examine: (a) the interactions between sensory and emotional systems involved in processing pain and analgesia in the physiological state; (b) the role of reward/aversion circuitry in pain and analgesia; and (c) the role of alterations in reward/aversion circuitry in the development of chronic pain and co-morbid psychiatric disorders. These underlying features have implications for understanding the neurobiology of functional illnesses such as depression and anxiety and for the development and evaluation of novel therapeutic interventions.

Recruitment of the Sonic hedgehog signalling cascade in electroconvulsive seizure-mediated regulation of adult rat hippocampal neurogenesis

Electroconvulsive seizure (ECS) induces structural remodelling in the adult mammalian brain, including an increase in adult hippocampal neurogenesis. The molecular mechanisms that underlie this increase in the proliferation of adult hippocampal progenitors are at present not well understood. We hypothesized that ECS may recruit the Sonic hedgehog (Shh) pathway to mediate its effects on adult hippocampal neurogenesis, as Shh is known to enhance the proliferation of neuronal progenitors and is expressed in the adult basal forebrain, a region that sends robust projections to the hippocampus. Here we demonstrate that the ECS-induced increase in proliferation of adult hippocampal progenitors was completely blocked in animals treated with cyclopamine, a pharmacological inhibitor of Shh signalling. Our results suggest that both acute and chronic ECS enhance Shh signalling in the adult hippocampus, as we observed a robust upregulation of Patched (Ptc) mRNA, a component of the Shh receptor complex and a downstream transcriptional target of Shh signalling. This increase was rapid and restricted to the dentate gyrus, where the adult hippocampal progenitors reside. In addition, both acute and chronic ECS decreased Smoothened (Smo) mRNA, the other component of the Shh receptor complex, selectively within the dentate gyrus. However, ECS did not appear to influence Shh expression within the basal forebrain, the site from which it has been suggested to be anterogradely transported to the hippocampus. Together, our findings demonstrate that ECS regulates the Shh signalling cascade and indicate that the Shh pathway may be an important mechanism through which ECS enhances adult hippocampal neurogenesis.

Dépression et neuroplasticité : implication des systèmes sérotoninergiques

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

AMPA receptor stimulation mediates the antidepressant-like effect of a group II metabotropic glutamate receptor antagonist

(1R,2R,3R,5R,6R)-2-amino-3-(3,4-dichlorobenzyloxy)-6-fluorobicyclo[3.1.0]hexane-2,6-dicarboxylic acid (MGS0039), a selective group II metabotropic glutamate receptor (mGluR) antagonist, exhibits antidepressant-like activities in rodent models. In the present studies, to clarify the involvement of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptor activation in exhibition of the antidepressant-like properties of MGS0039, we examined the effect of an AMPA receptor antagonist, 2,3-dihydroxy-6-nitro-7-sulfamoylbenzo(f)quinoxaline (NBQX), on the antidepressant-like effect of MGS0039 in the mouse tail suspension test. We also examined the effects of NBQX on increased serotonin release after treatment with MGS0039 in the rat medial prefrontal cortex (mPFC) using in vivo microdialysis evaluation. In the tail suspension test, MGS0039 (0.3-3 mg/kg, i.p.) treatment dose-dependently and significantly reduced immobility time. Pretreatment with NBQX (10 mg/kg, s.c.) significantly prevented the antidepressant-like effect of MGS0039 in the tail suspension test, while NBQX itself had no effect on immobility time. In the microdialysis evaluation, administration of MGS0039 (10 mg/kg, i.p.) significantly increased serotonin levels in mPFC in freely moving rats, while NBQX (1 mg/kg, i.p.) itself had no effect on serotonin release in this region. Pretreatment with NBQX significantly attenuated the increase in serotonin release by MGS0039. These findings suggest that stimulation of postsynaptic AMPA receptors plays a role in mediating the pharmacological effects of MGS0039.

Magnesium-deficient diet alters depression- and anxiety-related behavior in mice—influence of desipramine and Hypericum perforatum extract

A relation between magnesium (Mg) status and mood disorders has been suggested, but evidence remains inconsistent. Therefore, we examined in mice whether Mg-depletion would alter behavior evaluated in established animal models of depression and anxiety and whether these effects would be sensitive to antidepressants. Compared to control mice fed with normal diet, mice receiving a low Mg diet (10% of daily requirement) for several weeks displayed increased immobility time in the forced swim test, indicating enhanced depression-like behavior. In addition, the partial Mg-depletion increased anxiety-related behavior in the light/dark and open field test, while locomotor activity or motor coordination was not influenced. Chronic oral administration of desipramine (30 mg/kg/day), or Hypericum extract LI160 (Hyp, 380 mg/kg/day) prevented the "pro-depression-like" forced swim behavior in Mg-depleted mice. Furthermore, the increase in anxiety-related behavior of Mg-depleted mice was abolished in both the open field and light dark test by Hyp. Taken together, we report that Mg-depletion leads to enhanced depression- and anxiety-related behavior in mice, which was further validated by the reversibility of the behavioral changes by known antidepressant and anxiolytic substances. Further, the utility of Mg-depletion as a new screening model for clinically active antidepressant and anxiolytic drugs is suggested.

Glutamate as a therapeutic target in psychiatric disorders

Glutamate is the primary excitatory neurotransmitter in the mammalian brain. Glutamatergic neurotransmission may be modulated at multiple levels, only a minority of which are currently being exploited for pharmaceutical development. Ionotropic receptors for glutamate are divided into N-methyl-D-aspartate receptor (NMDAR) and AMPA receptor subtypes. NMDAR have been implicated in the pathophysiology of schizophrenia. The glycine modulatory site of the NMDAR is currently a favored therapeutic target, with several modulatory agents currently undergoing clinical development. Of these, the full agonists glycine and D-serine have both shown to induce significant, large effect size reductions in persistent negative and cognitive symptoms when added to traditional or newer atypical antipsychotics in double-blind, placebo-controlled clinical studies. Glycine (GLYT1) and small neutral amino-acid (SNAT) transporters, which regulate glycine levels, represent additional targets for drug development, and may represent a site of action of clozapine. Brain transporters for D-serine have recently been described. Metabotropic glutamate receptors are positively (Group I) or negatively (Groups II and III) coupled to glutamatergic neurotransmission. Metabotropic modulators are currently under preclinical development for neuropsychiatric conditions, including schizophrenia, depression and anxiety disorders. Other conditions for which glutamate modulators may prove effective include stroke, epilepsy, Alzheimer disease and PTSD.

Effects of antidepressant drug imipramine on gene expression in rat prefrontal cortex

We have investigated gene expression changes produced by acute and chronic daily treatment with a prototypical antidepressant, imipramine, using DNA microarrays. The analysis of similarities in gene expression patterns among functionally related genes revealed four expression profile cluster areas that showed a highly significant overrepresentation of several functional classes. Genes encoding for proteins involved in cAMP metabolism, postsynaptic membrane proteins, and proto-oncogenes were overrepresented in different cluster areas. Furthermore, we found that serine proteases as a group were similarly regulated by chronic antidepressant treatment. Our data suggest that cAMP metabolism, synaptic function, and protein processing by serine proteases may be important targets of antidepressant treatment and potential objects for antidepressant drug development.

Neurobiology of antidepressant withdrawal: implications for the longitudinal outcome of depression

Inappropriate discontinuation of drug treatment and noncompliance are a leading cause of long-term morbidity during treatment of depression. Increasing evidence supports an association between depressive illness and disturbances in brain glutamate activity, nitric oxide synthesis, and gamma-amino butyric acid. Animal models also confirm that suppression of glutamate N-methyl-D-aspartate receptor activity or inhibition of the nitric oxide-cyclic guanosine monophosphate pathway, as well as increasing brain levels of gamma-amino butyric acid, may be key elements in antidepressant action. Imaging studies demonstrate, for the most part, decreased hippocampal volume in patients with depression, which may worsen with recurrent depressive episodes. Preclinical models link this potentially neurodegenerative pathology to continued stress-evoked synaptic remodeling, driven primarily by the release of glucocorticoids, glutamate, and nitric oxide. These stress-induced structural changes can be reversed by antidepressant treatment. In patients with depression, antidepressant withdrawal after chronic administration is associated with a stress response as well as functional and neurochemical changes. Preclinical data also show that antidepressant withdrawal evokes a behavioral stress response that is associated with increased hippocampal N-methyl-D-aspartate receptor density, with both responses dependent on N-methyl-D-aspartate receptor activation. Drawing from both clinical and preclinical studies, this article proposes a preliminary molecular perspective and hypothesis on the neuronal implications of adherence to and discontinuation of antidepressant medication.

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

Adult hippocampal neurogenesis has been demonstrated in several species and is regulated by both environmental and pharmacological stimuli. The present study seeks to determine whether hippocampal proliferation and neurogenesis are altered in adult animals exposed to inescapable shock (IS) in the learned helplessness model of depression. We report that exposure to avoidance testing, regardless of pre-exposure to IS, decreases cell proliferation in the hippocampus, extending previous studies demonstrating downregulation of neurogenesis by exposure to acute stressors. In addition, when the analysis was conducted 9 days after exposure to IS we observed a significant decrease in cell proliferation compared to nonshocked animals. Administration of fluoxetine, a serotonin selective reuptake inhibitor, from days 2-8 blocked the downregulation of cell proliferation resulting from IS. Fluoxetine treatment also reversed the deficit in escape latency observed in animals exposed to IS. Finally, at the 9 day time point, there was no significant difference in blood levels of corticosterone between nonshocked and IS exposed animals, indicating that the decreased cell proliferation that is observed is not due to increased levels of this adrenal steroid. These findings demonstrate that exposure to IS, which results in a state of behavioral despair, decreases hippocampal cell proliferation and that this effect can be reversed by fluoxetine treatment.

Animal models of anxiety disorders

Animal models may be useful in investigating the fundamental mechanisms underlying psychiatric disorders, and may contribute to the development of new medications. A computerized literature search was used to collect studies on recently developed animal models for anxiety disorders. Particular cognitive-affective processes (eg, fear conditioning, control of stereotypic movements, social submissiveness, and trauma sensitization) may be particularly relevant to understanding specific anxiety disorders. Delineation of the phenomenology and psychobiology of these processes in animals leads to a range of useful models of these conditions. These models demonstrate varying degrees of face, construct, and predictive validity.

Antidepressant and anxiolytic-like effects in mice lacking the group III metabotropic glutamate receptor mGluR7

Glutamatergic neurotransmission has been strongly implicated in the pathophysiology of affective disorders, such as major depression and anxiety. Of all glutamate receptors, the role of group III metabotropic glutamate receptors (mGluR4, mGluR6, mGluR7, mGluR8) in such disorders is the least investigated because of the lack of specific pharmacological tools. To this end, we examined the behavioural profiles of mice with a targeted deletion of the gene for mGluR7 (mGluR7-/-) in animal models of depression and anxiety. mGluR7-/- mice were compared with wild-type (mGluR7+/+) littermates and showed substantially less behavioural immobility in both the forced swim test and the tail suspension test. Both behavioural paradigms are widely used to predict antidepressant-like activity. Further, mGluR7-/- mice displayed anxiolytic activity in four different behavioural tests, i.e. the light-dark box, the elevated plus maze, the staircase test, and the stress-induced hyperthermia test, while their cognitive performance was normal in the passive avoidance paradigm. Analysis of locomotor activity in a novel environment demonstrated that mGluR7-/- mice were slightly more active in the initial minutes following placement in the chamber only. Together, these data suggest that mGluR7 may play a pivotal role in mechanisms that regulate behavioural responses to aversive states. Therefore, drugs acting at mGluR7 may provide novel treatments for psychiatric disorders such as depression and anxiety.

Local, but not systemic, administration of serotonergic antidepressants decreases hippocampal nitric oxide synthase activity

Nitric oxide (NO) is an unconventional transmitter molecule in the nervous system, synthesized by nitric oxide synthase (NOS) following activation of the N-methyl-D-aspartate (NMDA) receptor. Several in vivo studies have demonstrated that NO modulates the extracellular levels of various neurotransmitters in the central nervous system, while serotonin (5-HT) re-uptake may be influenced by the NO pathway. Moreover, inhibitors of NOS exhibit antidepressant-like and anxiolytic-like properties in various animal models. Therefore, the aims of the present study were to clarify the involvement of distinct antidepressants acting on the serotonin re-uptake site in the regulation of the activity of hippocampal NOS in vitro, in vivo and ex vivo. We found that citalopram, paroxetine, imipramine and N(G)-nitro-L-arginine dose dependently decreased the hippocampal NOS activity in vitro. Moreover, local administration of citalopram, paroxetine, tianeptine, imipramine and N(G)-nitro-L-arginine significantly decreased the hippocampal NOS activity in vivo at a concentration significantly lower than in vitro. No effect on NOS activity following retrodialysis with 5-HT was observed. Acute (5 mg/kg, s.c.) and chronic (3 weeks, 20 mg/kg/24 h) systemic administration of citalopram did not influence NOS activity ex vivo. The effects on NOS represent a response to structurally dissimilar serotonergic antidepressants. However, since these data reflect effects on basal NOS activity, we believe that serotonergic antidepressants do not directly affect NOS at dosages used clinically, but the findings may reflect a secondary action of antidepressants on the glutamate NMDA receptor following their primary inhibitory action at the 5-HT transporter.