Regionally specific regulation of ERK MAP kinase in a model of antidepressant-sensitive chronic depression

Bidirectional crosstalk between stress-induced gastric ulcer and depression under chronic stress

Stress contributes to a variety of diseases and disorders such as depression and peptic ulcer. The present study aimed to investigate the correlation between stress ulcer and depression in pathogenesis and treatment by using chronic stress depression (CSD), chronic psychological stress ulcer (CPSU) and water immersion restrain stress models in rats. Our data showed that the ulcer index of the animals after CSD exposure was significantly higher than that of controls. Depression-like behaviors were observed in rat after CPSU exposure. Fluoxetine hydrochloride significantly reduced the ulcer index of rats exposed to CPSU stress, while ranitidine inhibited depression-like behavior of the animals in CSD group. The ulcer index of rats administered with mifepristone after CPSU stress was markedly reduced compared to CPSU group, although there was no significant difference in the depression-like behavior between mifepristone-treated CSD group and naive controls. We also found that the rats exposed to CPSU or CSD stress displayed a lower level of corticosterone than naive controls, however, the acute stress (AS) group showed an opposite result. Additionally, in order to study the relevance of H(2) receptors and depression, we treated the CSD group with cimetidine and famotidine respectively. The data showed that cimetidine inhibited depression-like behavior in CSD rats, and famotidine had no impact on depression. Overall our data suggested that the hypothalamic-pituitary-adrenal (HPA) axis dysfunction may be the key role in triggering depression and stress ulcer. Acid-suppressing drugs and antidepressants could be used for treatment of depression and stress ulcer respectively. The occurrence of depression might be inhibited by blocking the central H(2) receptors.

Somatic drugs for psychiatric diseases: aspirin or simvastatin for depression?

The evolution in the understanding of the neurobiology of most prevalent mental disorders such as major depressive disorder (MDD), bipolar disorder or schizophrenia has not gone hand in hand with the synthesis and clinical use of new drugs that would represent a therapeutic revolution such as that brought about by selective serotonin reuptake inhibitors (SSRIs) or atypical antipsychotics. Although scientists are still a long way from understanding its true aetiology, the neurobiological concept of depression has evolved from receptor regulation disorder, to a neurodegenerative disorder with a hippocampal volume decrease with the controversial reduction in neurotrophins such as BDNF, to current hypotheses that consider depression to be an inflammatory and neuroprogressive process. As regards antidepressants, although researchers are still far from knowing their true mechanism of action, they have gone from monoaminergic hypotheses, in which serotonin was the main protagonist, to emphasising the anti-inflammatory action of some of these drugs, or the participation of p11 protein in their mechanism of action.In the same way, according to the inflammatory hypothesis of depression, it has been proposed that some NSAIDS such as aspirin or drugs like simvastatin that have an anti-inflammatory action could be useful in some depressive patients. Despite the fact that there may be some data to support their clinical use, common sense and the evidence advise us to use already tested protocols and wait for the future to undertake new therapeutic strategies.

Action control is mediated by prefrontal BDNF and glucocorticoid receptor binding

Stressor exposure biases decision-making strategies from those based on the relationship between actions and their consequences to others restricted by stimulus-response associations. Chronic stressor exposure also desensitizes glucocorticoid receptors (GR) and diminishes motivation to acquire food reinforcement, although causal relationships are largely not established. We show that a history of chronic exposure to the GR ligand corticosterone or acute posttraining GR blockade with RU38486 makes rodents less able to perform actions based on their consequences. Thus, optimal GR binding is necessary for the consolidation of new response-outcome learning. In contrast, medial prefrontal (but not striatal) BDNF can account for stress-related amotivation, in that selective medial prefrontal cortical Bdnf knockdown decreases break-point ratios in a progressive-ratio task. Knockdown also increases vulnerability to RU38486. Despite the role of BDNF in dendritic spine reorganization, deep-layer spine remodeling does not obviously parallel progressive-ratio response patterns, but treatment with the Na(+)-channel inhibitor riluzole reverses corticosteroid-induced motivational deficits and restores prefrontal BDNF expression after corticosterone. We argue that when prefrontal neurotrophin systems are compromised, and GR-mediated hypothalamic-pituitary-adrenal axis feedback is desensitized (as in the case of chronic stress hormone exposure), amotivation and inflexible maladaptive response strategies that contribute to stress-related mood disorders result.

Ethanol extracts from Hemerocallis citrina attenuate the decreases of brain-derived neurotrophic factor, TrkB levels in rat induced by corticosterone administration

ETHNOPHARMACOLOGICAL RELEVANCE

Hemerocallis citrina, a traditional herbal medicine, has been used for the improvement of behavioral and emotional status in Eastern-Asia countries.

AIM OF THE STUDY

Our previous studies have demonstrated that the ethanol extracts of H. citrina flowers (HCE) reversed the behavioral alterations and monoamine neurotransmitter dysfunctions in stressed mice. However, the relation of its antidepressant-like action with neurotrophic molecular expressions remains unknown.

MATERIALS AND METHODS

To clarify this, we explored the effect of HCE (32.5, 65, 130mg/kg, p.o.) on the behavior, brain-derived neurotrophic factor (BDNF) and its receptor (TrkB) in depression-like rats induced by exogenous administration of the stress hormone corticosterone (40mg/kg, s.c.).

RESULTS

It was observed that repeated administration of corticosterone induced an elevation on the serum corticosterone levels, which caused the abnormalities observed in the sucrose preference test and forced swimming test (FST). Administration of HCE (65 and 130mg/kg) reversed the changes above and up-regulated the BDNF and TrkB receptor protein expressions in the brain region of frontal cortex and hippocampus.

CONCLUSION

These findings confirm that HCE produce an antidepressant-like effect in corticosterone-induced depression-like model of rats and this effect is at least partly mediated by BDNF-TrkB signaling in the frontal cortex and hippocampus.

Antidepressant-like effects of ginsenoside Rg1 are due to activation of the BDNF signalling pathway and neurogenesis in the hippocampus

BACKGROUND AND PURPOSE Ginsenoside Rg1 (Rg1) is one of the major bioactive ingredients of Panax ginseng with little toxicity and has been shown to have neuroprotective effects. In this study, we investigated the antidepressant-like effect of Rg1 in models of depression in mice. EXPERIMENTAL APPROACH The effects of Rg1 were assessed in the forced swimming test (FST) and tail suspension test (TST) in mice. Rg1 was also investigated in the chronic mild stress (CMS) mouse model of depression with imipramine as the positive control. Changes in hippocampal neurogenesis and spine density, the brain-derived neurotrophic factor (BDNF) signalling pathway, and serum corticosterone level after chronic stress and Rg1 treatment were then investigated. The tryptophan hydroxylase inhibitor and the tyrosine kinase B inhibitor were also used to explore the antidepressive mechanisms of Rg1. KEY RESULTS Ginsenoside Rg1 exhibited antidepressant-like activity in the FST and TST in mice without affecting locomotor activity. It was also effective in the CMS model of depression. Furthermore, Rg1 up-regulated the BDNF signalling pathway in the hippocampus and down-regulated serum corticosterone level during the CMS procedure. In addition, Rg1 was able to reverse the decrease in dendritic spine density and hippocampal neurogenesis caused by CMS. However, Rg1 had no discernable effect on the monoaminergic system. CONCLUSIONS AND IMPLICATIONS Our results provide the first evidence that Rg1 has antidepressant activity via activation of the BDNF signalling pathway and up-regulation of hippocampal neurogenesis.

Oxytocin: a therapeutic target for mental disorders

We review here that oxytocin (OT) is released into blood and within distinct brain regions in response to stressful and social stimuli, and has been shown to have an antidepressant-like effect in animal studies. Clinical reports suggest OT to be a promising drug for psychiatric diseases such as depression, anxiety disorders, schizophrenia, and autism. OT may also have therapeutic potential in the treatment of major depressive disorders, even though OT administered into blood does not readily cross the blood-brain barrier. Physiological functions such as sexual activity and mating induce the release of OT in the central nervous system. A drug for the treatment of sexual dysfunction, sildenafil, enhances the electrically evoked release of OT from the posterior pituitary. This drug has antidepressant-like effects through activation of an OT signaling pathway. These results suggest that sildenafil may have promise as a potential antidepressant.

Anatomic, hematologic, and biochemical features of C57BL/6NCrl mice maintained on chronic oral corticosterone

Metabolic syndrome is a condition that typically includes central obesity, insulin resistance, glucose intolerance, dyslipidemia, and hypertension. Disruption of the hypothalamic-pituitary-adrenal axis, a regulator of corticosterone secretion, occurs in some cases of metabolic syndrome and obesity, and Cushing hypercortisolemia is associated with obesity and metabolic disorders. We therefore assessed anatomic and clinical pathology in C57BL/6NCrl mice to evaluate the effects of chronic corticosterone in the drinking water at doses of 25, 50, and 100 μg/mL for 25 d. Treated mice developed obesity, glucose intolerance, electrolyte aberrations, and dyslipidemia that were dose-dependent and most severe in the 100-mu;g/mL treatment group. To evaluate return to normal function, additional C57BL/6NCrl mice received corticosterone-free water for 2 wk after the 25-d treatment period. According to results of gross examination, mice appeared to recover within days of exogenous corticosterone withdrawal; however, adrenal gland vacuolation and protein, lipid, and electrolyte abnormalities persisted. Together, these findings support chronic corticosterone exposure through the drinking water as a potentially useful, noninvasive method to induce some features of metabolic syndrome.

Metabotropic glutamate 2/3 receptor antagonists improve behavioral and prefrontal dopaminergic alterations in the chronic corticosterone-induced depression model in mice

Metabotropic glutamate 2/3 (mGlu2/3) receptor antagonists have an antidepressant-like effect, but the exact mechanism still remains unclear. This study examined the effects of mGlu2/3 receptor antagonists in chronic corticosterone-treated mice which could be used as an animal model of depression. In the forced swim test, the mGlu2/3 receptor antagonists MGS0039 (1.0 mg/kg, i.p.) and LY341495 (0.3 mg/kg, i.p) significantly reduced the increased immobility time of mice pretreated with corticosterone (20 mg/kg, s.c.) for 21 days, while desipramine (30 mg/kg, i.p.) and fluoxetine (30 mg/kg, i.p.) did not. The antidepressant-like effect of LY341495 was not blocked by the α-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) receptor antagonist NBQX (10 mg/kg, i.p.). Systemic administration of LY341495 did not affect basal release of glutamate, dopamine or serotonin in the prefrontal cortex of the control or chronic corticosterone-treated mice. Chronic corticosterone markedly enhanced high K(+)-induced release of dopamine, but not serotonin or glutamate, in the prefrontal cortex. This neurochemical change was blocked by systemic administration of MGS0039 and LY341495, but not desipramine or fluoxetine. These results suggest that chronic corticosterone-treated mice could be used as an animal model of treatment-resistant depression. This study also suggests that the prefrontal dopaminergic system is involved in the antidepressant-like effect of mGlu2/3 receptor antagonists in the chronic corticosterone-induced depression model.

Glucocorticoid regulation of brain-derived neurotrophic factor: relevance to hippocampal structural and functional plasticity

Glucocorticoids serve as key stress response hormones that facilitate stress coping. However, sustained glucocorticoid exposure is associated with adverse consequences on the brain, in particular within the hippocampus. Chronic glucocorticoid exposure evokes neuronal cell damage and dendritic atrophy, reduces hippocampal neurogenesis and impairs synaptic plasticity. Glucocorticoids also alter expression and signaling of the neurotrophin, brain-derived neurotrophic factor (BDNF). Since BDNF is known to promote neuroplasticity, enhance cell survival, increase hippocampal neurogenesis and cellular excitability, it has been hypothesized that specific adverse effects of glucocorticoids may be mediated by attenuating BDNF expression and signaling. The purpose of this review is to summarize the current state of literature examining the influence of glucocorticoids on BDNF, and to address whether specific effects of glucocorticoids arise through perturbation of BDNF signaling. We integrate evidence of glucocorticoid regulation of BDNF at multiple levels, spanning from the well-documented glucocorticoid-induced changes in BDNF mRNA to studies examining alterations in BDNF receptor-mediated signaling. Further, we delineate potential lines of future investigation to address hitherto unexplored aspects of the influence of glucocorticoids on BDNF. Finally, we discuss the current understanding of the contribution of BDNF to the modulation of structural and functional plasticity by glucocorticoids, in particular in the context of the hippocampus. Understanding the mechanistic crosstalk between glucocorticoids and BDNF holds promise for the identification of potential therapeutic targets for disorders associated with the dysfunction of stress hormone pathways.

Organic cation transporter 2 controls brain norepinephrine and serotonin clearance and antidepressant response

High-affinity transporters for norepinephrine (NE) and serotonin (5-HT), which ensure neurotransmitter clearance at the synapse, are the principal targets of widely used antidepressant drugs. Antidepressants targeting these high-affinity transporters, however, do not provide positive treatment outcomes for all patients. Other monoamine transport systems, with lower affinity, have been detected in the brain, but their role is largely unknown. Here we report that OCT2, a member of the polyspecific organic cation transporter (OCT) family, is expressed notably in the limbic system and implicated in anxiety and depression-related behaviors in the mouse. Genetic deletion of OCT2 in mice produced a significant reduction in brain tissue concentrations of NE and 5-HT and in ex vivo uptake of both these neurotransmitters in the presence of the dual 5-HT-NE transport blocker, venlafaxine. In vivo clearance of NE and 5-HT evaluated using microiontophoretic electrophysiology was diminished in the hippocampus of OCT2(-/-) mice in the presence of venlafaxine, thereby affecting postsynaptic neuronal activity. OCT2(-/-) mice displayed an altered sensitivity to acute treatments with NE- and/or 5-HT-selective transport blockers in the forced-swim test. Moreover, the mutant mice were insensitive to long-term venlafaxine treatment in a more realistic, corticosterone-induced, chronic depression model. Our findings identify OCT2 as an important postsynaptic determinant of aminergic tonus and mood-related behaviors and a potential pharmacological target for mood disorders therapy.

5-HT2 ligands in the treatment of anxiety and depression

INTRODUCTION

One third of depressed patients do not respond adequately to conventional antidepressants including the selective serotonin reuptake inhibitors (SSRIs). Therefore, multi-target drugs or augmentation strategies have been developed for the management of SSRIs-resistant patients. In this context, the 5-HT(2) receptor subtypes represent promising targets but their precise roles have yet to be determined.

AREAS COVERED

The aim of this review is to shed some light on the preclinical evidence supporting the use of 5-HT(2A) and/or 5-HT(2C) receptor antagonists such as antipsychotics, as potential effective adjuncts in SSRIs-resistant depression. This review synthesizes the current literature about the behavioral, electrophysiological and neurochemical effects of 5-HT(2) receptors ligands on the monoaminergic systems but also on adult hippocampal neurogenesis.

EXPERT OPINION

Although studies support the hypothesis that the inactivation of 5-HT(2A) and/or 5-HT(2C) receptors might be of interest to reinforce different facets of the therapeutic activity of SSRIs, this pharmacological strategy remains debatable notably because of the lack of chronic data in relevant animal models. Conversely, emerging evidence suggests that the activation of 5-HT(2B) receptor is required for antidepressant-like activity, opening the way to new therapeutic approaches. However, the potential risks related to the enhancement of monoaminergic neurotransmissions could represent a major concern.

Allopregnanolone regulates neurogenesis and depressive/anxiety-like behaviour in a social isolation rodent model of chronic stress

Chronic stress has been implicated as a causal factor in depression and anxiety, and is associated with neuroendocrine dysfunction and impaired hippocampal neurogenesis. The neurosteroid allopregnanolone (3α,5α-THP; ALLO) has been shown to be reduced in depressed patients. ALLO is "stress responsive" and plays a major role in regulating hypothalamic-pituitary-adrenal (HPA) axis function. We propose that reduced ALLO levels following chronic stress leads to HPA hyperactivity due to diminished ALLO regulation. This will result in increased glucocorticoid levels and reduced BDNF expression, leading to impaired hippocampal neurogenesis and the precipitation of depression/anxiety. To investigate this, chronic stress was induced using the social isolation model and depressive/anxiety-like behaviour assessed using the novelty-suppressed feeding test and forced-swim test. The social isolation model was associated with a significant reduction in endogenous ALLO levels and a depressive/anxiety-like behavioural profile. When exogenous ALLO was administered from the onset of isolation it prevented the development of depressive/anxiety-like behaviours and impairment of hippocampal neurogenesis. When treatment was initiated following six weeks of social isolation, behavioural profile was restored and deficits in BDNF and neurogenesis were not observed. Supporting our hypothesis we observed that socially isolated animals exhibited reduced HPA responsiveness, which was either prevented or normalised with ALLO treatment. Combined, these results indicate that administration of exogenous ALLO either during or following a period of chronic stress can prevent or normalise HPA dysfunction and impairment of hippocampal neurogenesis respectively, precluding the establishment of depressive/anxiety-like behaviours. ALLO may therefore provide a novel therapeutic target for the treatment of depression/anxiety.

Assessing behavioural effects of chronic HPA axis activation using conditional CRH-overexpressing mice

The corticotropin-releasing hormone (CRH) and its cognate receptors have been implicated in the pathophysiology of stress-related disorders. Hypersecretion of central CRH and elevated glucocorticoid levels, as a consequence of impaired feedback control, have been shown to accompany mood and anxiety disorders. However, a clear discrimination of direct effects of centrally hypersecreted CRH from those resulting from HPA axis activation has been difficult. Applying a conditional strategy, we have generated two conditional CRH-overexpressing mouse lines: CRH-COE ( Del ) mice overexpress CRH throughout the body, while CRH-COE ( APit ) mice selectively overexpress CRH in the anterior and intermediate lobe of the pituitary. Both mouse lines show increased basal plasma corticosterone levels and consequently develop signs of Cushing's syndrome. However, while mice ubiquitously overexpressing CRH exhibited increased anxiety-related behaviour, overexpression of CRH in the pituitary did not produce alterations in emotional behaviour. These results suggest that chronic hypercorticosteroidism alone is not sufficient to alter anxiety-related behaviour but rather that central CRH hyperdrive on its own or in combination with elevated glucocorticoids is responsible for the increase in anxiety-related behaviour. In conclusion, the generated mouse lines represent valuable animal models to study the consequences of chronic CRH overproduction and HPA axis activation.

Chronic corticosterone exposure during adolescence reduces impulsive action but increases impulsive choice and sensitivity to yohimbine in male Sprague-Dawley rats

Chronic stress during adolescence is associated with an increased risk for alcoholism and addictive disorders. Addiction is also associated with increased impulsivity, and stress during adolescence could alter cortical circuits responsible for response inhibition. Therefore, the present study determined the effect of chronic exposure to the stress hormone corticosterone (CORT) during adolescence on tests of impulsivity in adulthood and examined possible biochemical mechanisms. Male Sprague-Dawley rats were exposed to CORT by their drinking water during adolescence (post-natal day 30-50). The rats were then tested in adulthood to assess behavior on the 5-choice serial reaction time task (5CSRTT), stop-signal reaction time task (SSRTT), and the delay-discounting task, which differentially assess attention, impulsive action, and impulsive choice. Yohimbine-induced impulsivity on the 5CSRTT and biochemical analysis of the lateral orbital frontal cortex (lOFC) was also assessed owing to the ability of yohimbine to activate the hypothalamic-pituitary-adrenal axis and influence impulsivity. Adolescent CORT-treated rats were found to behave largely like controls on the 5CSRTT, but did show reduced premature responses when the intertrial interval was increased. Nevertheless, the CORT-treated rats tended to have more yohimbine-induced impulsive responses at low doses on this task, which was not found to be due to increased pCREB in the lOFC, but could be related to a higher expression/activity of the AMPA receptor subunit GluR1. Adolescent CORT-treated rats performed more accurately on the SSRTT, but showed greater impulsivity on the delay-discounting task, as indicated by steeper discounting functions. Therefore, adolescent CORT exposure reduced impulsive action but increased impulsive choice, indicating that chronic stress hormone exposure in adolescence can have long-term consequences on behavior.

Modulation of neuroplasticity pathways and antidepressant-like behavioural responses following the short-term (3 and 7 days) administration of the 5-HT₄ receptor agonist RS67333

It has been recently suggested that activation of 5-HT₄ receptors might exert antidepressant-like effects in rats after 3 d treatment, suggesting a new strategy for developing faster-acting antidepressants. We studied the effects of 3 d and 7 d treatment with the 5-HT₄ receptor partial agonist RS67333 (1.5 mg/kg.d) in behavioural tests of chronic efficacy and on neuroplastic-associated changes, such as adult hippocampal neurogenesis, expression of CREB, BDNF, β-catenin, AKT and 5-HT₄ receptor functionality. RS67333 treatment up-regulated hippocampal cell proliferation, β-catenin expression and pCREB/CREB ratio after 3 d treatment. This short-term treatment also reduced immobility time in the forced swim test (FST), together with a partial reversion of the anhedonic-like state (sucrose consumption after chronic corticosterone). Administration of RS67333 for 7 d resulted in a higher increase in the rate of hippocampal cell proliferation, a significant desensitization of 5-HT₄ receptor-coupled adenylate cyclase activity and a more marked increase in the expression of neuroplasticity-related proteins (BDNF, CREB, AKT): these changes reached the same magnitude as those observed after 3 wk administration of classical antidepressants. Consistently, a positive behavioural response in the novelty suppressed feeding (NSF) test and a complete reversion of the anhedonic-like state (sucrose consumption) were also observed after 7 d treatment. These results support the antidepressant-like profile of RS67333 with a shorter onset of action and suggest that this time period of administration (3-7 d) could be a good approximation to experimentally predict the onset of action of this promising strategy.

OX2R activation induces PKC-mediated ERK and CREB phosphorylation

Deficiencies in brain orexins and components of mitogen activated protein kinase (MAPK) signaling pathway have been reported in either human depression or animal model of depression. Brain administration of orexins affects behaviors toward improvement of depressive symptoms. However, the documentation of endogenous linkage between orexin receptor activation and MAPK signaling pathway remains to be insufficient. In this study, we report the effects of orexin 2 receptor (OX2R) activation on cell signaling in CHO cells over-expressing OX2R and in mouse hypothalamus cell line CLU172. Short-term extracellular signal-regulated kinase (ERK) phosphorylation and long-term cyclic adenosine monophosphate (cAMP) response element binding protein (CREB) phosphorylation were subsequently observed in CHO cells that over-express OX2R while 20 min of ERK phosphorylation was significantly detected in mouse adult hypothalamus neuron cell line CLU172. Orexin A, which can also activate OX2R, mediated ERK phosphorylation was as the same as orexin B in CHO cells. A MAPK inhibitor eliminated ERK phosphorylation but not CREB phosphorylation in CHO cells. Also, ERK and CREB phosphorylation was not mediated by protein kinase A (PKA) or calmodulin kinase (CaMK). However, inhibition of protein kinase C (PKC) by GF 109203X eliminated the phosphorylation of ERK and CREB in CHO cells. A significant decrease in ERK and CREB phosphorylation was observed with 1 μM GF 109203X pre-treatment indicating that the conventional and novel isoforms of PKC are responsible for CREB phosphorylation after OX2R activation. In contrast, ERK phosphorylation induced by orexin B in CLU172 cells cannot be inhibited by 1 μM of protein kinase C inhibitor. From above observation we conclude that OX2R activation by orexin B induces ERK and CREB phosphorylation and orexin A played the same role as orexin B. Several isoforms of PKC may be involved in prolonged CREB phosphorylation. Orexin B induced ERK phosphorylation in mouse hypothalamus neuron cells differs from CHO cell line and cannot be inhibited by PKC inhibitor GF 109203X. And hypothalamus neuron cells may use different downsteam pathway for orexin B induced ERK phosphorylation. This result supports findings that orexins might have anti-depressive roles.

Antidepressant-like effects of Δ⁹-tetrahydrocannabinol and rimonabant in the olfactory bulbectomised rat model of depression

The endocannabinoid signalling system is widely accepted to play a role in controlling the affective state. Plant cannabinoids are well known to have behavioural effects in animals and humans and the cannabinoid CB(1) receptor antagonist rimonabant has recently been shown to precipitate depression-like symptoms in clinical trial subjects. The aim of the present study was to investigate the behavioural and neurochemical effects of chronic administration of Δ⁹-tetrahydrocannabinol (THC) and rimonabant on intact and olfactory bulbectomised (OB) rats used as a model of depression. As expected, OB rats were hyperactive in the open field. Repeated THC (2 mg/kg, i.p. once every 48 h for 21 days) and rimonabant (5 mg/kg, i.p. once every 48 h for 21 days) reduced this hyperactivity, which is typical of clinically effective antidepressant drugs. In intact animals, chronic THC increased brain derived neurotrophic factor (BDNF) expression levels in the hippocampus and frontal cortex but rimonabant had no effect. Rimonabant increased the levels of phosphorylated extracellular signal regulated kinases (p-ERKs(1/2)) in the hippocampus and prefrontal cortex and THC also increased expression in frontal cortex. OB did not affect BDNF or p-ERK(1/2) expression in the hippocampus or frontal cortex and in, contrast to the intact animals, neither THC nor rimonabant altered expression in the OB rats. These findings indicate antidepressant-like behavioural properties of both THC and rimonabant in OB rats although additional studies are required to clarify the relationship between the chronic effects of cannabinoids in other pre-clinical models and in human depression.

Regulator of calmodulin signaling knockout mice display anxiety-like behavior and motivational deficits

Regulator of calmodulin (CaM) signaling (RCS), when phosphorylated by protein kinase A (PKA) on Ser55, binds to CaM and inhibits CaM-dependent signaling. RCS expression is high in the dorsal striatum, nucleus accumbens and amygdala, suggesting that the protein is involved in limbic-striatal function. To test this hypothesis, we examined RCS knockout (KO) mice in behavioral models dependent on these brain areas. Mice were tested for food-reinforced instrumental conditioning and responding under a progressive ratio (PR) schedule of reinforcement and in models of anxiety (elevated plus maze and open field). While RCS KO mice showed normal acquisition of a food-motivated instrumental response, they exhibited a lower breakpoint value when tested on responding under a PR schedule of reinforcement. RCS KO mice also displayed decreased exploration in both the open arms of an elevated plus maze and in the center region of an open field, suggesting an enhanced anxiety response. Biochemical studies revealed a reduction in the levels of dopamine and cAMP-regulated phosphoprotein (DARPP-32) in the striatum of RCS KO mice. DARPP-32 is important in reward-mediated behavior, suggestive of a possible role for DARPP-32 in mediating some of the effects of RCS. Together these results implicate a novel PKA-regulated phosphoprotein, RCS, in the etiology of motivational deficits and anxiety.

Arg kinase regulates prefrontal dendritic spine refinement and cocaine-induced plasticity

Adolescence is characterized by vulnerability to the development of neuropsychiatric disorders including drug addiction, as well as prefrontal cortical refinement that culminates in structural stability in adulthood. Neuronal refinement and stabilization are hypothesized to confer resilience to poor decision making and addictive-like behaviors, although intracellular mechanisms are largely unknown. We characterized layer V prefrontal dendritic spine development and refinement in adolescent wild-type mice and mice lacking the cytoskeletal regulatory protein Abl-related gene (Arg) kinase. Relative to hippocampal CA1 pyramidal neurons, which exhibited a nearly linear increase in spine density up to postnatal day 60 (P60), wild-type prefrontal spine density peaked at P31, and then declined by 18% by P56-P60. In contrast, dendritic spines in mice lacking Arg destabilized by P31, leading to a net loss in both structures. Destabilization corresponded temporally to the emergence of exaggerated psychomotor sensitivity to cocaine. Moreover, cocaine reduced dendritic spine density in wild-type orbitofrontal cortex and enlarged remaining spine heads, but arg(-/-) spines were unresponsive. Local application of Arg or actin polymerization inhibitors exaggerated cocaine sensitization, as did reduced gene dosage of the Arg substrate, p190RhoGAP. Genetic and pharmacological Arg inhibition also retarded instrumental reversal learning and potentiated responding for reward-related cues, providing evidence that Arg regulates both psychomotor sensitization and decision-making processes implicated in addiction. These findings also indicate that structural refinement in the adolescent orbitofrontal cortex mitigates psychostimulant sensitivity and support the emerging perspective that the structural response to cocaine may, at any age, have behaviorally protective consequences.

Antidepressant-like properties of oral riluzole and utility of incentive disengagement models of depression in mice

RATIONALE

The neuroprotective agent riluzole has antidepressant-like properties in humans, but its mechanisms of action are unclear. Despite the increasing utility of transgenic and knockout mice in addressing such issues, previous studies aimed at characterizing biochemical mechanisms have been conducted in rats.

OBJECTIVES

We sought to optimize an oral riluzole administration protocol with antidepressant-like consequences in C57BL/6 mice, a common background strain in genetically modified mice.

METHODS

Riluzole (6-60 μg/ml) was dissolved in tap water and replaced regular drinking water for up to 3 weeks; sensitivity to tail suspension, forced swimming, and the locomotor response to extinction training in a model of "incentive disengagement" were tested. Peripheral and central effects of long-term 60-μg/ml treatment were also evaluated.

RESULTS

Riluzole had dose-dependent antidepressant-like effects in the forced swim test, and like chronic fluoxetine, exerted antidepressant-like actions in an adaptation of the "incentive disengagement" model at the highest concentration tested. This 60-μg/ml concentration also restored hippocampal brain-derived neuroptrophic factor (BDNF) expression after chronic corticosteroid exposure and increased glutamate glial transporter 1 (GLT-1, or EAAT2) expression without significantly affecting baseline locomotor activity, thymus and adrenal gland weights, or blood serum corticosterone. The lowest 6-μg/ml concentration increased locomotor activity, potentially consistent with an anxiolytic-like effect.

CONCLUSIONS

Riluzole's therapeutic potential for treating mood disorders may involve GLT-1 and BDNF, and we suggest this protocol could be used to further characterize its precise long-term biochemical mechanisms of action in animal models of depression.

Antidepressant-like effect of sildenafil through oxytocin-dependent cyclic AMP response element-binding protein phosphorylation

Oxytocin (OT) levels in plasma increase during sexual response and are significantly lower in patients with depression. A drug for the treatment of sexual dysfunction, sildenafil, enhances the electrically evoked release of OT from the posterior pituitary. In this study, we showed that sildenafil had an antidepressant-like effect through activation of an OT signaling pathway. Application of sildenafil reduced depression-related behavior in male mice. The antidepressant-like effect was blocked by an OT receptor (OTR) antagonist and was absent in OTR knockout (KO) mice. Sildenafil increased the phosphorylation of cAMP response element-binding protein (CREB) in the hippocampus. The OTR antagonist inhibited sildenafil-induced CREB phosphorylation and sildenafil had no effect on CREB phosphorylation in OTR KO mice. These results suggest sildenafil to have an antidepressant-like effect through the activation of OT signaling and to be a promising drug for the treatment of depression.

IQGAP1 regulates NR2A signaling, spine density, and cognitive processes

General or brain-region-specific decreases in spine number or morphology accompany major neuropsychiatric disorders. It is unclear, however, whether changes in spine density are specific for an individual mental process or disorder and, if so, which molecules confer such specificity. Here we identify the scaffolding protein IQGAP1 as a key regulator of dendritic spine number with a specific role in cognitive but not emotional or motivational processes. We show that IQGAP1 is an important component of NMDAR multiprotein complexes and functionally interacts with the NR2A subunits and the extracellular signal-regulated kinase 1 (ERK1) and ERK2 signaling pathway. Mice lacking the IQGAP1 gene exhibited significantly lower levels of surface NR2A and impaired ERK activity compared to their wild-type littermates. Accordingly, primary hippocampal cultures of IQGAP1(-/-) neurons exhibited reduced surface expression of NR2A and disrupted ERK signaling in response to NR2A-dependent NMDAR stimulation. These molecular changes were accompanied by region-specific reductions of dendritic spine density in key brain areas involved in cognition, emotion, and motivation. IQGAP1 knock-outs exhibited marked long-term memory deficits accompanied by impaired hippocampal long-term potentiation (LTP) in a weak cellular learning model; in contrast, LTP was unaffected when induced with stronger stimulation paradigms. Anxiety- and depression-like behavior remained intact. On the basis of these findings, we propose that a dysfunctional IQGAP1 gene contributes to the cognitive deficits in brain disorders characterized by fewer dendritic spines.

Proteomic and metabolomic profiling of a trait anxiety mouse model implicate affected pathways

Depression and anxiety disorders affect a great number of people worldwide. Whereas singular factors have been associated with the pathogenesis of psychiatric disorders, growing evidence emphasizes the significance of dysfunctional neural circuits and signaling pathways. Hence, a systems biology approach is required to get a better understanding of psychiatric phenotypes such as depression and anxiety. Furthermore, the availability of biomarkers for these disorders is critical for improved diagnosis and monitoring treatment response. In the present study, a mouse model presenting with robust high versus low anxiety phenotypes was subjected to thorough molecular biomarker and pathway discovery analyses. Reference animals were metabolically labeled with the stable (15)N isotope allowing an accurate comparison of protein expression levels between the high anxiety-related behavior versus low anxiety-related behavior mouse lines using quantitative mass spectrometry. Plasma metabolomic analyses identified a number of small molecule biomarkers characteristic for the anxiety phenotype with particular focus on myo-inositol and glutamate as well as the intermediates involved in the tricarboxylic acid cycle. In silico analyses suggested pathways and subnetworks as relevant for the anxiety phenotype. Our data demonstrate that the high anxiety-related behavior and low anxiety-related behavior mouse model is a valuable tool for anxiety disorder drug discovery efforts.

Effects of maternal L-tryptophan depletion and corticosterone administration on neurobehavioral adjustments in mouse dams and their adolescent and adult daughters

Major depressive disorder (MDD), a pathology characterized by mood and neurovegetative disturbances, depends on a multi-factorial contribution of individual predisposition (e.g., diminished serotonergic transmission) and environmental factors (e.g., neonatal abuse or neglect). Despite its female-biased prevalence, MDD basic research has mainly focused on male rodents. Most of present models of depression are also devalued due to the fact that they typically address only one of the aforementioned pathogenetic factors. In this paper we first describe the basic principles behind mouse model development and evaluation and then articulate that current models of depression are intrinsically devalued due to poor construct and/or external validity. We then report a first attempt to overcome this limitation through the design of a mouse model in which the genetic and the environmental components of early risk factors for depression are mimicked together. Environmental stress is mimicked through the supplementation of corticosterone in the maternal drinking water while biological predisposition is mimicked through maternal access to an L-tryptophan (the serotonin precursor) deficient diet during the first week of lactation. CD1 dams and their offspring exposed to the L-tryptophan deficient diet (T) and to corticosterone (80mg/l; C) were compared to animal facility reared (AFR) subjects. T and C mice served as intermediate reference groups. Adolescent TC offspring, compared to AFR mice, showed decreased time spent floating in the forced-swim test and increased time spent in the open sectors of an elevated 0-maze. Adult TC offspring showed reduced preference for novelty, decreased breakpoints in the progressive ratio operant procedure and major alterations in central BDNF levels and altered HPA regulation. The route of administration and the possibility to control the independent variables predisposing to depressive-like symptoms disclose novel avenues towards the development of animal models with increased external and construct validity. Furthermore, the observation that, compared to adult subjects, adolescent mice display an opposite profile suggests that peri-pubertal developmental processes may interact with neonatal predispositions to calibrate the adult abnormal phenotype.

Intravenous injection of neural progenitor cells improved depression-like behavior after cerebral ischemia

Poststroke depression (PSD) occurs in approximately one-third of stroke survivors and is one of the serious sequelae of stroke. The onset of PSD causes delayed functional recovery by rehabilitation and also increases cognitive impairment. However, appropriate strategies for the therapy against ischemia-induced depression-like behaviors still remain to be developed. Such behaviors have been associated with a reduced level of brain-derived neurotrophic factor (BDNF). In addition, accumulating evidence indicates the ability of stem cells to improve cerebral ischemia-induced brain injuries. However, it remains to be clarified as to the effect of neural progenitor cells (NPCs) on PSD and the association between BDNF level and PSD. Using NPCs, we investigated the effect of intravenous injection of NPCs on PSD. We showed that injection of NPCs improved ischemia-induced depression-like behaviors in the forced-swimming test and sucrose preference test without having any effect on the viable area between vehicle- and NPC-injected ischemic rats. The injection of NPCs prevented the decrease in the level of BDNF in the ipsilateral hemisphere. The levels of phosphorylated CREB, ERK and Akt, which have been implicated in events downstream of BDNF signaling, were also decreased after cerebral ischemia. NPC injection inhibited these decreases in the phosphorylation of CREB and ERK, but not that of Akt. Our findings provide evidence that injection of NPCs may have therapeutic potential for the improvement of depression-like behaviors after cerebral ischemia and that these effects might be associated with restoring BDNF-ERK-CREB signaling.

Neurogenesis and affective disorders

The neurogenesis hypothesis of depression was originally formed upon the demonstration that stress impacts levels of adult neurogenesis in the hippocampus. Since then much work has established that newborn neurons in the dentate gyrus are required for mediating some of the beneficial effects of antidepressant treatment. Recent studies combining behavioral, molecular and electrophysiological approaches have attempted to make sense of the role young neurons play in modulating mood by demonstrating a potential role in regulating the circuitry in the brain that underlies depression. Here we discuss the work that led to the neurogenesis hypothesis of depression, and the subsequent studies that have sought to test this hypothesis. We also discuss different animal models of depression that have been used to test the role of neurogenesis in mediating the antidepressant response.

Molecular aspects involved in swimming exercise training reducing anhedonia in a rat model of depression

Patients suffering from depression frequently display hyperactivity of the hypothalamic-pituitary-adrenal axis (HPA) resulting in elevated cortisol levels. One main symptom of this condition is anhedonia. There is evidence that exercise training can be used as a rehabilitative intervention in the treatment of depressive disorders. In this scenario, the aim of the present study was to assess the effect of an aerobic exercise training protocol on the depressive-like behavior, anhedonia, induced by repeated dexamethasone administration. The study was carried out on adult male Wistar rats randomly divided into four groups: the "control group" (C), "exercise group" (E), "dexamethasone group" (D) and the "dexamethasone plus exercise group" (DE). The exercise training consisted of swimming (1 h/d, 5 d/wk) for 3 weeks, with an overload of 5% of the rat body weight. Every day rats were injected with either dexamethasone (D/DE) or saline solution (C/E). Proper positive controls, using fluoxetine, were run in parallel. Decreased blood corticosterone levels, reduced adrenal cholesterol synthesis and adrenal weight (HPA disruption), reduced preference for sucrose consumption and increased immobility time (depressive-like behavior), marked hippocampal DNA oxidation, increased IL-10 and total brain-derived neurotrophic factor (BDNF; pro-plus mature-forms) and a severe loss of body mass characterized the dexamethasone-treated animals. Besides increasing testosterone blood concentrations, the swim training protected depressive rats from the anhedonic state, following the same profile as fluoxetine, and also from the dexamethasone-induced impaired neurochemistry. The data indicate that physical exercise could be a useful tool in preventing and treating depressive disorders.

Chronic corticosterone administration from adolescence through early adulthood attenuates depression-like behaviors in mice

There is evidence that depression may have a different neural basis at different ages. Although chronic stress and elevated glucocorticoid levels have been demonstrated to lead to the emergence of mood disorders, it remains unclear how moderate elevation of glucocorticoid levels in young animals influences depression-like behaviors and brain functions. To address this issue, the present study examines how chronic corticosterone (CORT) administration during adolescence and early adulthood influences depression-like behaviors, hypothalamic-pituitary-adrenal (HPA) axis response and hippocampal cell proliferation. Male mice were chronically administrated with CORT drinking water (20mg/L) during adolescence. After two months of treatment, serum CORT levels were measured using enzyme immunoassay. Hippocampal glucocorticoid and mineralocorticoid receptors were characterized using Western blot. Tail suspension and forced swim tests were used to assess depression-related behaviors in mice. Immunohistochemistry was performed to measure bromodeoxyuridine (BrdU) incorporation in order to assess cell proliferation in the hippocampus. Our results suggest that chronic CORT administration induced a mild but not significant elevation in basal CORT levels and attenuated the physiological responses to stress. Chronic CORT administration also reduced expression of the hippocampal mineralocorticoid receptor and decreased immobility time in both the tail suspension test and the forced swim test. Moreover, chronic CORT administration increased the BrdU immunoreactivities in the hippocampus. Taken together, these findings suggest that chronic mild elevation by CORT administration during the adolescence and early adulthood attenuates depression-like behaviors.

Glutamate N-methyl-D-aspartate receptor antagonists rapidly reverse behavioral and synaptic deficits caused by chronic stress exposure

BACKGROUND

Despite widely reported clinical and preclinical studies of rapid antidepressant actions of glutamate N-methyl-D-aspartate (NMDA) receptor antagonists, there has been very little work examining the effects of these drugs in stress models of depression that require chronic administration of antidepressants or the molecular mechanisms that could account for the rapid responses.

METHODS

We used a rat 21-day chronic unpredictable stress (CUS) model to test the rapid actions of NMDA receptor antagonists on depressant-like behavior, neurochemistry, and spine density and synaptic function of prefrontal cortex neurons.

RESULTS

The results demonstrate that acute treatment with the noncompetitive NMDA channel blocker ketamine or the selective NMDA receptor 2B antagonist Ro 25-6981 rapidly ameliorates CUS-induced anhedonic and anxiogenic behaviors. We also found that CUS exposure decreases the expression levels of synaptic proteins and spine number and the frequency/amplitude of synaptic currents (excitatory postsynaptic currents) in layer V pyramidal neurons in the prefrontal cortex and that these deficits are rapidly reversed by ketamine. Blockade of the mammalian target of rapamycin protein synthesis cascade abolishes both the behavioral and biochemical effects of ketamine.

CONCLUSIONS

The results indicate that the structural and functional deficits resulting from long-term stress exposure, which could contribute to the pathophysiology of depression, are rapidly reversed by NMDA receptor antagonists in a mammalian target of rapamycin dependent manner.

A translational research framework for enhanced validity of mouse models of psychopathological states in depression

Depression presents as a disorder of feelings and thoughts that debilitate daily functioning and can be life threatening. Increased understanding of these specific emotional-cognitive pathological states and their underlying pathophysiologies and neuropathologies is fundamental to an increased understanding of the disorder and, therefore, to development of much-needed improved therapies. Despite this, there is a current lack of emphasis on development and application of translational (i.e. valid) neuropsychological measures in depression research. The appropriate strategy is neuropsychological research translated, bi-directionally, between epidemiological and clinical human research and in vivo - ex vivo preclinical research conducted, primarily, with mice. This paper presents a translational framework to stimulate and inform such research, in four inter-dependent sections. (1) A depression systems-model describes the pathway between human environment-gene (E-G) epidemiology, pathophysiology, psycho- and neuropathology, symptoms, and diagnosis. This model indicates that G→emotional-cognitive endophenotypes and E-G/endophenotype→emotional-cognitive state markers are central to experimental and translational depression research. (2) Human neuropsychological tests with (potential) translational value for the quantitative study of these endophenotypes and state markers are presented. (3) The analogous rodent behavioural tests are presented and their translational validity in terms of providing analogue emotional-cognitive endophenotypes and state markers are discussed. (4) The need for aetiological validity of mouse models in terms of G→endophenotypes and E-G→state markers is presented. We conclude that the informed application of the proposed neuropsychological translational framework will yield mouse models of high face, construct and aetiological validity with respect to emotional-cognitive dysfunction in depression. These models, together with the available technological tools, can then be studied to increase understanding of depression pathophysiology and neuropathology, leading to identification and validation of novel therapeutic targets and the development of effective, personalized antidepressant treatments.

Modeling treatment-resistant depression

Depression is a polygenic and highly complex psychiatric disorder that is currently a major burden on society. Depression is highly heterogeneous in presentation and frequently exhibits high comorbidity with other psychiatric and somatic disorders. Commonly used treatments, such as selective serotonin reuptake inhibitors (SSRIs), are not ideal since only a subset of patients achieve remission. In addition, the reason why some individuals respond to SSRIs while others don't are unknown. Here we begin to ask what the basis of treatment resistance is, and propose new strategies to model this phenomenon in animals. We focus specifically on animal models that offer the appropriate framework to study treatment resistance with face, construct and predictive validity.

Integrated behavioral z-scoring increases the sensitivity and reliability of behavioral phenotyping in mice: relevance to emotionality and sex

Defining anxiety- and depressive-like states in mice (emotionality) is best characterized by the use of complementary tests, leading sometimes to puzzling discrepancies and lack of correlation between similar paradigms. To address this issue, we hypothesized that integrating measures along the same behavioral dimensions in different tests would reduce the intrinsic variability of single tests and provide a robust characterization of the underlying "emotionality" of individual mouse, similarly as mood and related syndromes are defined in humans through various related symptoms over time. We describe the use of simple mathematical and integrative tools to help phenotype animals across related behavioral tests (syndrome diagnosis) and experiments (meta-analysis). We applied z-normalization across complementary measures of emotionality in different behavioral tests after unpredictable chronic mild stress (UCMS) or prolonged corticosterone exposure - two approaches to induce anxious-/depressive-like states in mice. Combining z-normalized test values, lowered the variance of emotionality measurement, enhanced the reliability of behavioral phenotyping, and increased analytical opportunities. Comparing integrated emotionality scores across studies revealed a robust sexual dimorphism in the vulnerability to develop high emotionality, manifested as higher UCMS-induced emotionality z-scores, but lower corticosterone-induced scores in females compared to males. Interestingly, the distribution of individual z-scores revealed a pattern of increased baseline emotionality in female mice, reminiscent of what is observed in humans. Together, we show that the z-scoring method yields robust measures of emotionality across complementary tests for individual mice and experimental groups, hence facilitating the comparison across studies and refining the translational applicability of these models.

Implications of the functional integration of adult-born hippocampal neurons in anxiety-depression disorders

Adult neurogenesis in the dentate gyrus of the hippocampus has gained considerable attention as a cellular substrate for both the pathophysiology and treatment of depression. Overall, the studies of adult hippocampal neurogenesis are still in their infancy because most of them explore only one stage of this process. Importantly, given the built-in homeostatic mechanisms that act at each stage during the progression from stem cells to mature neurons (proliferation, differentiation, maturation, survival), it is very difficult to extrapolate the efficiency of a drug on adult neurogenesis from analysis of one stage alone. Here, we review the most significant data on hippocampal neurogenesis, focusing on the importance of studying each stage of adult hippocampal neurogenesis and also on the importance of choosing the appropriate mouse strain to perform the experiment. Specifically, strains with a high number of basal proliferating cells in the dentate gyrus of the hippocampus should be used only under stressed conditions to detect the effects of antidepressants on adult neurogenesis. We also discuss how adult hippocampal neurogenesis could be involved in affective state disorders such as depression and anxiety. Finally, we reveal that the behavioral effects of fluoxetine are mediated through both neurogenesis-dependent and -independent actions.

Contributions of ERK signaling in the striatum to instrumental learning and performance

The striatum is critical for learning and decision making; however, the molecular mechanisms that govern striatum function are not fully understood. The extracellular signal regulated kinase (ERK) cascade is an important signaling pathway that underlies synaptic plasticity, cellular excitability, learning and arousal. This review focuses on the role of ERK signaling in striatum function. ERK is activated in the striatum by coordinated dopamine and glutamate receptor signaling, where it underlies corticostriatal synaptic plasticity and influences striatal cell excitability. ERK activation in the dorsal striatum is necessary for action-outcome learning and performance of goal-directed actions. In the ventral striatum, ERK is necessary for the motivating effects of reward-associated stimuli on instrumental performance. Dysregulation of ERK signaling in the striatum by repeated drug exposure contributes to the development of addictive behavior. These results highlight the importance of ERK signaling in the striatum as a critical substrate for learning and as a regulator of ongoing behavior. Furthermore, they suggest that ERK may be a suitable target for therapeutics to treat disorders of learning and decision making that arise from compromised striatum function.

Neurobiological alterations induced by exercise and their impact on depressive disorders [corrected]

BACKGROUND

The impact of physical activity on brain metabolic functions has been investigated in different studies and there is growing evidence that exercise can be used as a preventive and rehabilitative intervention in the treatment of depressive disorders. However, the exact neuronal mechanisms underlying the latter phenomenon have not been clearly elucidated. The present article summarises key results derived from studies that focussed on the neurobiological impact of exercise on brain metabolic functions associated with depressive disorders. Since major depressive disorder (MDD) is a life threatening disease it is of great significance to find reliable strategies to prevent or to cure this illness. Therefore, the aim of this paper is to review (1) the physiological relationship between physical activity and depressive disorders and (2) the potential neurobiological alterations induced by exercise that might lead to the relief of mental disorders like depression.

METHODS

We searched electronic databases for literature concerning the relationship between exercise and depression from 1963 until 2009.

RESULTS

The data suggests an association between physical inactivity and higher levels of depressive symptoms. Properly designed studies could show that exercise training can be as effective as antidepressive medications.

CONCLUSION

The exact mechanisms how exercise affects the brain are not fully understood and the literature lacks of well designed studies concerning the effects of exercise training on depressive disorders. But the observed antidepressant actions of exercise are strong enough that it already can be used as an alternative to current medications in the treatment of depressive disorders.

Behavioral animal models of depression

Depression is a chronic, recurring and potentially life-threatening illness that affects up to 20% of the population across the world. Despite its prevalence and considerable impact on human, little is known about its pathogenesis. One of the major reasons is the restricted availability of validated animal models due to the absence of consensus on the pathology and etiology of depression. Besides, some core symptoms such as depressed mood, feeling of worthlessness, and recurring thoughts of death or suicide, are impossible to be modeled on laboratory animals. Currently, the criteria for identifying animal models of depression rely on either of the 2 principles: actions of known antidepressants and responses to stress. This review mainly focuses on the most widely used animal models of depression, including learned helplessness, chronic mild stress, and social defeat paradigms. Also, the behavioral tests for screening antidepressants, such as forced swimming test and tail suspension test, are also discussed. The advantages and major drawbacks of each model are evaluated. In prospective, new techniques that will be beneficial for developing novel animal models or detecting depression are discussed.

Peripheral BDNF produces antidepressant-like effects in cellular and behavioral models

Recent clinical studies demonstrate that serum levels of brain-derived neurotrophic factor (BDNF) are significantly decreased in patients with major depressive disorder (MDD) and that antidepressant treatments reverse this effect, indicating that serum BDNF is a biomarker of MDD. These findings raise the possibility that serum BDNF may also have effects on neuronal activity and behavior, but the functional significance of altered serum BDNF is unknown. To address this issue, we determined the influence of peripheral BDNF administration on depression- and anxiety-like behavior, including the forced swim test (FST), chronic unpredictable stress (CUS)/anhedonia, novelty-induced hypophagia (NIH) test, and elevated-plus maze (EPM). Furthermore, we examined adult hippocampal neurogenesis as well as hippocampal and striatal expression of BDNF, extracellular signal-regulated kinase (ERK) and cAMP response element-binding protein (CREB), in order to determine whether peripherally administered BDNF produces antidepressant-like cellular responses in the brain. Peripheral BDNF administration increased mobility in the FST, attenuated the effects of CUS on sucrose consumption, decreased latency in the NIH test, and increased time spent in the open arms of an EPM. Moreover, adult hippocampal neurogenesis was increased after chronic, peripheral BDNF administration. We also found that BDNF levels as well as expression of pCREB and pERK were elevated in the hippocampus of adult mice receiving peripheral BDNF. Taken together, these results indicate that peripheral/serum BDNF may not only represent a biomarker of MDD, but also have functional consequences on molecular signaling substrates, neurogenesis, and behavior.

Linking molecules to mood: new insight into the biology of depression

Major depressive disorder is a heritable psychiatric syndrome that appears to be associated with subtle cellular and molecular alterations in a complex neural network. The affected brain regions display dynamic neuroplastic adaptations to endocrine and immunologic stimuli arising from within and outside the CNS. Depression's clinical and etiological heterogeneity adds a third level of complexity, implicating different pathophysiological mechanisms in different patients with the same DSM diagnosis. Current pharmacological antidepressant treatments improve depressive symptoms through complex mechanisms that are themselves incompletely understood. This review summarizes the current knowledge of the neurobiology of depression by combining insights from human clinical studies and molecular explanations from animal models. The authors provide recommendations for future research, with a focus on translating today's discoveries into improved diagnostic tests and treatments.

Melatonin treatment mimics the antidepressant action in chronic corticosterone-treated mice

Melatonin, involved in circadian cycle, provides protective effects on neuronal cells and acts as antidepressant by restoration of corticosterone levels. A mouse model of anxiety/depressive-like behavior, induced by chronic corticosterone treatment, has been used to evaluate behavior and adult hippocampal neurogenesis in mice and their possible modulation under melatonin. With this aim, CD1 mice were subjected to 7 wk of corticosterone administration, and then behavioral tests as novelty-suppressed feeding, open field and a forced swim test were performed. Cell proliferation in hippocampal dentate gyrus (DG) was investigated by 5-bromo-2'-deoxyuridine and doublecortin immunohistochemistry techniques, and stereological procedure was used to quantify labeled cells. Golgi-impregnated method was used to evaluate the changes of dendritic spines in DG neurons. A new therapeutic approach with antidepressant-like substances (3 wk) such as melatonin (8 mg/kg) was employed to possibly modulate neural development in the adult hippocampus and the behavioral changes. The depressive-like state caused by chronic corticosterone treatment was reversed by exogenous administration of melatonin; the proliferation of progenitor cells in mice hippocampus was significantly reduced under chronic corticosterone treatment (cort- 83.7 +/- 20.3 versus cort+ 60.5 +/- 18.2; P < 0.05), whereas long-term treatment with melatonin prevented the corticosterone-induced reduction in hippocampal cell proliferation (cort- 60.5 +/- 18.2 versus mel 133.4 +/- 26.9; P < 0.05). Corticosterone-treated mice exhibited a reduced spine density, which was ameliorated by melatonin administration. These findings suggest a strong correspondence between behavior and neurogenesis, strengthening the hypothesis that neurogenesis contributes to the effects of melatonin as an antidepressant.