High-Speed Imaging Reveals Neurophysiological Links to Behavior in an Animal Model of Depression

Adult hippocampal neurogenesis: an actor in the antidepressant-like action

Depression and anxiety are psychiatric illnesses that are major burdens in society and affect as much as 7% of the world's population. The heterogeneous nature of depression suggests an involvement of multiple distinct brain regions including amygdala, prefrontal cortex and the hippocampus, which may be responsible for the diversity of the symptoms. Besides its critical role in learning and memory, the hippocampus is one of only two areas in mammalian brain where adult neurogenesis occurs. Of the current leading hypotheses of the pathophysiology and treatment of depression, the neurogenesis hypothesis of depression deserves particular attention because changes in neurogenesis are only seen after chronic, but not acute, antidepressant treatment. This review revisits the role of adult hippocampal neurogenesis in the pathophysiology of mood disorders, especially anxiety/depression, and also in the antidepressant-like responses, especially in stressed rodents.

Identification of a role for the ventral hippocampus in neuropeptide S-elicited anxiolysis

Neuropeptide S (NPS) increasingly emerges as a potential novel treatment option for anxiety diseases like panic and posttraumatic stress disorder. However, the neural underpinnings of its anxiolytic action are still not clearly understood. Recently, we reported that neurons of the ventral hippocampus (VH) take up intranasally administered fluorophore-conjugated NPS and, moreover, that application of NPS to mouse brain slices affects neurotransmission and plasticity at hippocampal CA3-CA1 synapses. Although these previous findings define the VH as a novel NPS target structure, they leave open whether this brain region is directly involved in NPS-mediated anxiolysis and how NPS impacts on neuronal activity propagation in the VH. Here, we fill this knowledge gap by demonstrating, first, that microinjections of NPS into the ventral CA1 region are sufficient to reduce anxiety-like behavior of C57BL/6N mice and, second, that NPS, via the NPS receptor, rapidly weakens evoked neuronal activity flow from the dentate gyrus to area CA1 in vitro. Additionally, we show that intranasally applied NPS alters neurotransmission and plasticity at CA3-CA1 synapses in the same way as NPS administered to hippocampal slices. Thus, our study provides, for the first time, strong experimental evidence for a direct involvement of the VH in NPS-induced anxiolysis and furthermore presents a novel mechanism of NPS action.

Towards translational rodent models of depression

Rodent models of depression have been developed in an effort to identify novel antidepressant compounds and to further our understanding of the pathophysiology of depression. Various rodent models of depression and antidepressant-like behaviour are currently used but, clearly, none of these current models fully recapitulate all features of depression. Moreover, these models have not resulted in the development of novel non-monoaminergic-based antidepressants with clinical efficacy. Thus, a refinement of the current models of depression is required. The present review outlines the most commonly used models of depression and antidepressant drug-like activity and suggests several factors that should be considered when refining these models.

Cardiac optogenetics

Optogenetics is an emerging technology for optical interrogation and control of biological function with high specificity and high spatiotemporal resolution. Mammalian cells and tissues can be sensitized to respond to light by a relatively simple and well-tolerated genetic modification using microbial opsins (light-gated ion channels and pumps). These can achieve fast and specific excitatory or inhibitory response, offering distinct advantages over traditional pharmacological or electrical means of perturbation. Since the first demonstrations of utility in mammalian cells (neurons) in 2005, optogenetics has spurred immense research activity and has inspired numerous applications for dissection of neural circuitry and understanding of brain function in health and disease, applications ranging from in vitro to work in behaving animals. Only recently (since 2010), the field has extended to cardiac applications with less than a dozen publications to date. In consideration of the early phase of work on cardiac optogenetics and the impact of the technique in understanding another excitable tissue, the brain, this review is largely a perspective of possibilities in the heart. It covers the basic principles of operation of light-sensitive ion channels and pumps, the available tools and ongoing efforts in optimizing them, overview of neuroscience use, as well as cardiac-specific questions of implementation and ideas for best use of this emerging technology in the heart.

Electroconvulsive seizure, but not imipramine, rapidly up-regulates pro-BDNF and t-PA, leading to mature BDNF production, in the rat hippocampus

Electroconvulsive therapy is the most effective treatment for antidepressant-resistant depression, although its mechanism has not been fully elucidated. Previous studies have demonstrated that electroconvulsive seizures (ECS) induce expression of brain-derived neurotrophic factor (BDNF) in the rat hippocampus. However, in contrast with mature BDNF (mBDNF) known to have antidepressant effects, its precursor (pro-BDNF) has harmful effects on neurons. We therefore hypothesized that efficient processing of pro-BDNF is a critical requirement for the antidepressant effects of ECS. We found that single administration of ECS rapidly increased not only hippocampal levels of pro-BDNF but also those of prohormone convertase 1 (PC1) and tissue-plasminogen activator (t-PA), which are proteases involved in intra- and extracellular pro-BDNF processing, respectively. Interestingly, pro-BDNF and t-PA levels were increased in hippocampal synaptosomes after single ECS, suggesting their transport to secretory sites. In rats receiving 10-d repeated ECS, accumulation of pro-BDNF and a resultant increase in mBDNF levels were observed. While t-PA levels increased and accumulated following repeated ECS, PC1 levels did not, suggesting that intracellular processing capacity is limited. Finally, chronic administration of imipramine significantly increased mBDNF levels, but not pro-BDNF and protease levels, indicating that the therapeutic mechanism of imipramine differs from that of ECS. Taken together, these results suggest that, while intra- and extracellular proteases are involved in pro-BDNF processing in single ECS, t-PA plays a dominant role following repeated ECS. Such efficient pro-BDNF processing as well as strong induction of BDNF expression may contribute to the antidepressant effects of ECS.

Evidence for reduced dentate gyrus and fimbria volume in bipolar II disorder

OBJECTIVES

  Dentate gyrus (DG)-dependent inhibition of the stress response might play an important role in mood disorders. During stress, hippocampal projections traversing the fimbria, a white matter bundle on the hippocampal surface, inhibit the hypothalamic-pituitary-adrenal (HPA) axis. The aim of the present study was to measure the volumes of the DG-cornu ammonis 4 (DG-CA4) and fimbria in patients with bipolar II disorder (BD-II) and healthy controls using a recently developed magnetic resonance imaging (MRI)-based technique.

METHODS

  Thirty-seven individuals with a DSM-IV diagnosis of BD-II and 42 healthy controls underwent 3-Tesla MRI. Hippocampal subfield volumes were estimated using a novel segmentation algorithm implemented in FreeSurfer.

RESULTS

  In patients with BD-II there was a significant reduction in the volume of the left [analysis of covariance (ANCOVA), F = 7.84, p = 0.006] and total (left + right) (F = 4.01, p = 0.047) DG-CA4 and left (F = 4.38, p = 0.040) and total (F = 4.15, p = 0.045) fimbria compared to healthy controls. Explorative analyses indicated a smaller left CA2-3 volume in subjects with BD-II compared to healthy controls, and a reduced left fimbria volume in unmedicated patients compared to medicated patients and controls.

CONCLUSIONS

  Our results provide evidence for the involvement of the DG and fimbria in BD-II. Longitudinal studies of the DG and fimbria with assessments of the HPA axis in BD-II are warranted.

Depression uncouples brain hate circuit

It is increasingly recognized that we need a better understanding of how mental disorders such as depression alter the brain's functional connections to improve both early diagnosis and therapy. A new holistic approach has been used to investigate functional connectivity changes in the brains of patients suffering from major depression using resting-state functional magnetic resonance imaging (fMRI) data. A canonical template of connectivity in 90 different brain regions was constructed from healthy control subjects and this identified a six-community structure with each network corresponding to a different functional system. This template was compared with functional networks derived from fMRI scans of both first-episode and longer-term, drug resistant, patients suffering from severe depression. The greatest change in both groups of depressed patients was uncoupling of the so-called 'hate circuit' involving the superior frontal gyrus, insula and putamen. Other major changes occurred in circuits related to risk and action responses, reward and emotion, attention and memory processing. A voxel-based morphometry analysis was also carried out but this revealed no evidence in the depressed patients for altered gray or white matter densities in the regions showing altered functional connectivity. This is the first evidence for the involvement of the 'hate circuit' in depression and suggests a potential reappraisal of the key neural circuitry involved. We have hypothesized that this may reflect reduced cognitive control over negative feelings toward both self and others.

Sustained remission from depressive-like behavior depends on hippocampal neurogenesis

Impairment of hippocampal neurogenesis has been associated with the expression of depressive-like symptoms and some studies have suggested neurogenesis as a critical factor in the normalization of behavior by antidepressant (AD) drugs. This study provides robust evidence that ongoing neurogenesis is essential for the maintenance of behavioral homeostasis and that its pharmacological arrest precipitates symptoms commonly found in depressed patients. Further, the incorporation of newly born neurons and astrocytes into the preexisting hippocampal neurocircuitry is shown to be necessary for the spontaneous recovery from the adverse effects of stress and for long-term benefits of AD treatments.

Novel insights into depression and antidepressants: a synergy between synaptogenesis and neurogenesis?

Major depressive disorder has been associated with manifold pathophysiological changes. These include metabolic abnormalities in discreet brain areas; modifications in the level of stress hormones, neurotransmitters, and neurotrophic factors; impaired spinogenesis and synaptogenesis in crucial brain areas, such as the prefrontal cortex and the hippocampus; and impaired neurogenesis in the hippocampus. Antidepressant therapy facilitates remission by reversing most of these disturbances, indicating that these dysfunctions may participate causally in depressive symptomatology. However, few attempts have been made to integrate these different pathophysiologies into one model. The present chapter endeavors (1) to review the extant literature in the field, with particular focus on the role of neurogenesis and synaptogenesis in depression; (2) and to suggest a possible interplay between these two processes, as well as, describe the ways by which improving both neurogenesis and synaptogenesis may enable effective recovery by acting on a larger neuronal network.

GluA1 and its PDZ-interaction: a role in experience-dependent behavioral plasticity in the forced swim test

Glutamate receptor dependent synaptic plasticity plays an important role in the pathophysiology of depression. Hippocampal samples from clinically depressed patients display reduced mRNA levels for GluA1, a major subunit of AMPA receptors. Moreover, activation and synaptic incorporation of GluA1-containing AMPA receptors are required for the antidepressant-like effects of NMDA receptor antagonists. These findings argue that GluA1-dependent synaptic plasticity might be critically involved in the expression of depression. Using an animal model of depression, we demonstrate that global or hippocampus-selective deletion of GluA1 impairs expression of experience-dependent behavioral despair. This impairment is mediated by the interaction of GluA1 with PDZ-binding domain proteins, as deletion of the C-terminal leucine alone is sufficient to replicate the behavioral phenotype. Our results provide evidence for a significant role of hippocampal GluA1-containing AMPA receptors and their PDZ-interaction in experience-dependent expression of behavioral despair and link mechanisms of hippocampal synaptic plasticity with behavioral expression of depression.

The neurobiology of depression and antidepressant action

We present a comprehensive overview of the neurobiology of unipolar major depression and antidepressant drug action, integrating data from affective neuroscience, neuro- and psychopharmacology, neuroendocrinology, neuroanatomy, and molecular biology. We suggest that the problem of depression comprises three sub-problems: first episodes in people with low vulnerability ('simple' depressions), which are strongly stress-dependent; an increase in vulnerability and autonomy from stress that develops over episodes of depression (kindling); and factors that confer vulnerability to a first episode (a depressive diathesis). We describe key processes in the onset of a 'simple' depression and show that kindling and depressive diatheses reproduce many of the neurobiological features of depression. We also review the neurobiological mechanisms of antidepressant drug action, and show that resistance to antidepressant treatment is associated with genetic and other factors that are largely similar to those implicated in vulnerability to depression. We discuss the implications of these conclusions for the understanding and treatment of depression, and make some strategic recommendations for future research.

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.

Role of leaky neuronal ryanodine receptors in stress-induced cognitive dysfunction

The type 2 ryanodine receptor/calcium release channel (RyR2), required for excitation-contraction coupling in the heart, is abundant in the brain. Chronic stress induces catecholamine biosynthesis and release, stimulating β-adrenergic receptors and activating cAMP signaling pathways in neurons. In a murine chronic restraint stress model, neuronal RyR2 were phosphorylated by protein kinase A (PKA), oxidized, and nitrosylated, resulting in depletion of the stabilizing subunit calstabin2 (FKBP12.6) from the channel complex and intracellular calcium leak. Stress-induced cognitive dysfunction, including deficits in learning and memory, and reduced long-term potentiation (LTP) at the hippocampal CA3-CA1 connection were rescued by oral administration of S107, a compound developed in our laboratory that stabilizes RyR2-calstabin2 interaction, or by genetic ablation of the RyR2 PKA phosphorylation site at serine 2808. Thus, neuronal RyR2 remodeling contributes to stress-induced cognitive dysfunction. Leaky RyR2 could be a therapeutic target for treatment of stress-induced cognitive dysfunction.

Transcriptional suppression of the neuronal PAS domain 4 (Npas4) gene by stress via the binding of agonist-bound glucocorticoid receptor to its promoter

Neuronal PAS domain 4 (NPAS4), a brain-specific helix-loop-helix transcription factor, has recently been shown to regulate the development of GABAergic inhibitory neurons. We previously reported that Npas4 mRNA expression levels were reduced in the hippocampus of mice exposed to social isolation or restraint stress, which was accompanied by impairment of memory, emotional behavior, and hippocampal neurogenesis. Therefore, the reduction of NPAS4 expression may play a role in stress-induced brain dysfunction. In this study, to investigate the transcriptional regulation of Npas4 by stress, we focused on the effect of glucocorticoids (GCs) upon Npas4 transcription. Corticosterone treatment reduced Npas4 expression in the frontal cortex and hippocampus, whereas adrenalectomy caused an increase in expression. GC receptor (GR) antagonist, mifepristone, inhibited the stress-induced reduction of Npas4 expression. Putative negative glucocorticoid response elements (GREs) were found -2000 to -1000 upstream of the Npas4 transcription initiation site. Npas4 promoter activity was increased by mifepristone or by mutation of the negative GRE sequences. A chromatin immunoprecipitation assay revealed that restraint stress increased the binding of GR to Npas4 promoter region in the hippocampus. These results suggest that transcription of Npas4 is down-regulated by stress via the binding of agonist-bound GR to its promoter.

Enhancement of dorsal hippocampal activity by knockdown of HCN1 channels leads to anxiolytic- and antidepressant-like behaviors

The hippocampus is an integral brain region for affective disorders. TRIP8b knockout mice lacking functional HCN channels as well as both HCN1 and HCN2 knockout mice have been shown to display antidepressant-like behaviors. The mechanisms or brain regions involved in these alterations in behavior, however, are not clear. We developed a lentiviral shRNA system to examine whether knockdown of HCN1 protein in the dorsal hippocampal CA1 region is sufficient to produce antidepressant-like effects. We found that knockdown of HCN1 channels increased cellular excitability and resulted in physiological changes consistent with a reduction of I(h). Rats infused with lentiviral shRNA-HCN1 in the dorsal hippocampal CA1 region displayed antidepressant- and anxiolytic-like behaviors associated with widespread enhancement of hippocampal activity and upregulation of BDNF-mTOR signaling pathways. Our results suggest that HCN1 protein could be a potential target for treatment of anxiety and depression disorders.

Neurosteroids, stress and depression: potential therapeutic opportunities

Neurosteroids are potent and effective neuromodulators that are synthesized from cholesterol in the brain. These agents and their synthetic derivatives influence the function of multiple signaling pathways including receptors for γ-aminobutyric acid (GABA) and glutamate, the major inhibitory and excitatory neurotransmitters in the central nervous system (CNS). Increasing evidence indicates that dysregulation of neurosteroid production plays a role in the pathophysiology of stress and stress-related psychiatric disorders, including mood and anxiety disorders. In this paper, we review the mechanisms of neurosteroid action in brain with an emphasis on those neurosteroids that potently modulate the function of GABA(A) receptors. We then discuss evidence indicating a role for GABA and neurosteroids in stress and depression, and focus on potential strategies that can be used to manipulate CNS neurosteroid synthesis and function for therapeutic purposes.

Are we getting closer to valid translational models for major depression?

Advances in characterizing the neuropathology and functional dysconnectivity of depression and promising trials with emerging circuit-targeted and fast-onset therapeutics are providing unprecedented opportunities to gain deeper insight into the neurobiology of this devastating and pervasive disorder. Because of practical and ethical limitations to dissecting these mechanisms in humans, continued progress will critically depend on our ability to emulate aspects of depressive symptomatology and treatment response in nonhuman organisms. Although various experimental models are currently available, they often draw skepticism from both clinicians and basic research scientists. We review recent progress and highlight some of the best leads to diversify and improve discovery end points for preclinical depression research.

Anxiety- rather than depression-like behavior is associated with adult neurogenesis in a female mouse model of higher trait anxiety- and comorbid depression-like behavior

Adult neurogenesis has been implicated in affective disorders and the action of antidepressants (ADs) although the functional significance of this association is still unclear. The use of animal models closely mimicking human comorbid affective and anxiety disorders seen in the majority of patients should provide relevant novel information. Here, we used a unique genetic mouse model displaying higher trait anxiety (HAB) and comorbid depression-like behavior. We demonstrate that HABs have a lower rate of hippocampal neurogenesis and impaired functional integration of newly born neurons as compared with their normal anxiety/depression-like behavior (NAB) controls. In HABs, chronic treatment with the AD fluoxetine alleviated their higher depression-like behavior and protected them from relapse for 3 but not 7 weeks after discontinuation of the treatment without affecting neurogenesis. Similar to what has been observed in depressed patients, fluoxetine treatment induced anxiogenic-like effects during the early treatment phase in NABs along with a reduction in neurogenesis. On the other hand, treatment with AD drugs with a particularly strong anxiolytic component, namely the neurokinin-1-receptor-antagonist L-822 429 or tianeptine, increased the reduced rate of neurogenesis in HABs up to NAB levels. In addition, challenge-induced hypoactivation of dentate gyrus (DG) neurons in HABs was normalized by all three drugs. Overall, these data suggest that AD-like effects in a psychopathological mouse model are commonly associated with modulation of DG hypoactivity but not neurogenesis, suggesting normalization of hippocampal hypoactivity as a neurobiological marker indicating successful remission. Finally, rather than to higher depression-related behavior, neurogenesis seems to be linked to pathological anxiety.

The (Eigen)value of diffusion tensor imaging to investigate depression after traumatic brain injury

BACKGROUND

Many people with a traumatic brain injury (TBI), even mild to moderate, will develop major depression (MD). Recent studies of patients with MD suggest reduced fractional anisotropy (FA) in dorsolateral prefrontal cortex (DLPFC), temporal lobe tracts, midline, and capsule regions. Some of these pathways have also been found to have reduced FA in patients with TBI. It is unknown whether the pathways implicated in MD after TBI are similar to those with MD without TBI. This study sought to investigate whether there were specific pathways unique to TBI patients who develop MD.

METHODS

A sample of TBI-MD subjects (N = 14), TBI-no-MD subjects (N = 12), MD-no-TBI (N = 26), and control subjects (no TBI or MD, N = 23), using a strict measurement protocol underwent psychiatric assessments and diffusion tensor brain Magnetic Resonance Imaging (MRI).

RESULTS

The findings of this study indicate that (1) TBI patients who develop MD have reduced axial diffusivity in DLPFC, corpus callosum (CC), and nucleus accumbens white matter tracts compared to TBI patients who do not develop MD and (2) MD patients without a history of TBI have reduced FA along the CC. We also found that more severe MD relates to altered radial diffusivity.

CONCLUSIONS

These findings suggest that compromise to specific white matter pathways, including both axonal and myelination aspects, after a mild TBI underlie the susceptibility of these patients developing MD.

Disconnection and reconnection: the morphological basis of (mal)adaptation to stress

Maladaptive responses to stress and the associated hypersecretion of glucocorticoids cause psychopathologies ranging from hyperemotional states and mood dysfunction to cognitive impairments. Research in both humans and animal models has begun to identify morphological correlates of these functional changes. These include dendritic and synaptic reorganization, glial remodeling, and altered cell fate in cortical and subcortical structures. The emerging view is that stress induces a 'disconnection syndrome' whereby the transmission and integration of information that are critical for orchestrating appropriate physiological and behavioral responses are perturbed. High-resolution spatiotemporal mapping of the complete neural circuitry and identification of the cellular processes impacted by stress will help to advance discovery of strategies to reduce or reverse the burden of stress-related neuropsychiatric disorders.

Entorhinal theta-frequency input to the dentate gyrus trisynaptically evokes hippocampal CA1 LTP

There exists substantial evidence that some forms of explicit learning in mammals require long-term potentiation (LTP) at hippocampal CA3-CA1 synapses. While CA1 LTP has been well characterized at the monosynaptic level, it still remains unclear how the afferent systems to the hippocampus can initiate formation of this neuroplastic phenomenon. Using voltage-sensitive dye imaging (VSDI) in a mouse brain slice preparation, we show that evoked entorhinal cortical (EC) theta-frequency input to the dentate gyrus highly effectively generates waves of neuronal activity which propagate through the entire trisynaptic circuit of the hippocampus (“HTC-Waves”). This flow of activity, which we also demonstrate in vivo, critically depends on frequency facilitation of mossy fiber to CA3 synaptic transmission. The HTC-Waves are rapidly boosted by the cognitive enhancer caffeine (5 μM) and the stress hormone corticosterone (100 nM). They precisely follow the rhythm of the EC input, involve high-frequency firing (>100 Hz) of CA3 pyramidal neurons, and induce NMDA receptor-dependent CA1 LTP within a few seconds. Our study provides the first experimental evidence that synchronous theta-rhythmical spiking of EC stellate cells, as occurring during EC theta oscillations, has the capacity to drive induction of CA1 LTP via the hippocampal trisynaptic pathway. Moreover, we present data pointing to a basic filter mechanism of the hippocampus regarding EC inputs and describe a methodology to reveal alterations in the “input–output relationship” of the hippocampal trisynaptic circuit.

Neurogenesis-independent antidepressant-like effects on behavior and stress axis response of a dual orexin receptor antagonist in a rodent model of depression

Growing evidence indicates that an increase of orexin (or hypocretin) signaling is involved in the pathophysiology of major depression, but little is known regarding the causal link between the orexinergic system and depressive-like states. Here we blocked orexin receptors in mice subjected to unpredictable chronic mild stress (UCMS) to investigate putative antidepressant-like effects of this treatment, as well as the underlying mechanisms. BALB/c mice were exposed to 9 weeks of UCMS and from the third week onward treated daily with fluoxetine (20 mg/kg per day, per os) or with the dual orexin receptor antagonist almorexant (100 mg/kg per day, per os). The effects of UCMS regimen and pharmacological treatments were assessed by physical measures and behavioral testing. The dexamethasone suppression test was performed to examine the integrity of the negative feedback of the hypothalamic-pituitary-adrenal (HPA) axis, and immunohistochemical markers were used to assess cell proliferation (Ki-67), immature newborn neurons (doublecortin), and mature newborn neurons (5-bromo-2'-deoxyuridine/NeuN) in the dorsal and ventral parts of the hippocampus. Our results show that 7 weeks of fluoxetine or almorexant treatments counteract the UCMS-induced physical and behavioral alterations. Both treatments prevented the HPA axis dysregulation caused by UCMS, but only fluoxetine reversed the UCMS-induced decrease of hippocampal cell proliferation and neurogenesis, while chronic almorexant treatment decreased cell proliferation and neurogenesis specifically in the ventral hippocampus. Taken together, this is the first evidence that pharmacological blockade of the orexinergic system induces a robust antidepressant-like effect and the restoration of stress-related HPA axis defect independently from a neurogenic action.

Optogenetics: a novel optical manipulation tool for medical investigation

Optogenetics is a new and rapidly evolving gene and neuroengineering technology that allows optical control of specific populations of neurons without affecting other neurons in the brain at high temporal and spatial resolution. By heterologous expression of the light-sensitive membrane proteins, cell type-specific depolarization or hyperpolarization can be optically induced on a millisecond time scale. Optogenetics has the higher selectivity and specificity compared to traditional electrophysiological techniques and pharmaceutical methods. It has been a novel promising tool for medical research. Because of easy handling, high temporal and spatial precision, optogenetics has been applied to many aspects of nervous system research, such as tactual neural circuit, visual neural circuit, auditory neural circuit and olfactory neural circuit, as well as research of some neurological diseases. The review highlights the recent advances of optogenetics in medical study.

Taking the Perspective that a Depressive State Reflects Inflammation: Implications for the Use of Antidepressants

This paper reviews both the evidence that supports the characterization of depression as an inflammatory disorder and the different biochemical mechanisms that have been postulated for the connection between inflammation and depression. This association offers credible explanation for the short term efficacy of antidepressants, which have short term anti-inflammatory effects. Evidence for those anti-inflammatory effects is discussed. Evidence of the contrary long-term effects of antidepressants, which increase rather than decrease inflammation, is also reviewed. It is argued that this increase in inflammation would predict an increase in chronicity among depressed patients that have been treated with antidepressants drugs, which has been noted in the literature. A brief discussion of alternatives for decreasing inflammation, some of which have demonstrated efficacy in ameliorating depression, is presented.

Effects of chronic stress on prefrontal cortex transcriptome in mice displaying different genetic backgrounds

There is increasing evidence that depression derives from the impact of environmental pressure on genetically susceptible individuals. We analyzed the effects of chronic mild stress (CMS) on prefrontal cortex transcriptome of two strains of mice bred for high (HA)and low (LA) swim stress-induced analgesia that differ in basal transcriptomic profiles and depression-like behaviors. We found that CMS affected 96 and 92 genes in HA and LA mice, respectively. Among genes with the same expression pattern in both strains after CMS, we observed robust upregulation of Ttr gene coding transthyretin involved in amyloidosis, seizures, stroke-like episodes, or dementia. Strain-specific HA transcriptome affected by CMS was associated with deregulation of genes involved in insulin secretion (Acvr1c, Nnat, and Pfkm), neuropeptide hormone activity (Nts and Trh), and dopamine receptor mediated signaling pathway (Clic6, Drd1a, and Ppp1r1b). LA transcriptome affected by CMS was associated with genes involved in behavioral response to stimulus (Fcer1g, Rasd2, S100a8, S100a9, Crhr1, Grm5, and Prkcc), immune effector processes (Fcer1g, Mpo, and Igh-VJ558), diacylglycerol binding (Rasgrp1, Dgke, Dgkg, and Prkcc), and long-term depression (Crhr1, Grm5, and Prkcc) and/or coding elements of dendrites (Crmp1, Cntnap4, and Prkcc) and myelin proteins (Gpm6a, Mal, and Mog). The results indicate significant contribution of genetic background to differences in stress response gene expression in the mouse prefrontal cortex.

[A depression model of social defeat etiology using tree shrews]

Depression is a common neuropsychiatric disorder, marked by depressed mood for at least two weeks. The World Health Organization predicts that depression will be the number one leading cause of disease and injury burden by 2030. Clinical treatment faces at least three serious obstacles. First, the disease mechanism is not fully understood and thus there are no effective ways to predict and prevent depression and no biological method of diagnosis. Second, available antidepressants are based on monoamine mechanisms that commonly have a long delay of action and possibly cause a higher risk of suicide. Third, no other antidepressant mechanisms are available, with fast action and few side effects. Unfortunately, several decades of research based on rodent models of depression have not been successful in resolving these problems, at least partially due to the huge differences in brain function between rodents and people. Tree shrews are the closest sister to primates, and brain functions in these species are closer to those of humans. In this review, we discuss a tree shrew model of depression with social defeat etiology and aspects of construct, face and predicted validity of an animal model. Although a tree shrew model of depression has long been ignored and not fully established, its similarities to those aspects of depression in humans may open a new avenue to address this human condition.

Leptin restores adult hippocampal neurogenesis in a chronic unpredictable stress model of depression and reverses glucocorticoid-induced inhibition of GSK-3β/β-catenin signaling

Stress and glucocorticoid stress hormones inhibit neurogenesis, whereas antidepressants increase neurogenesis and block stress-induced decrease in neurogenesis. Our previous studies have shown that leptin, an adipocyte-derived hormone with antidepressant-like properties, promotes baseline neurogenesis in the adult hippocampus. This study aimed to determine whether leptin is able to restore suppression of neurogenesis in a rat chronic unpredictable stress (CUS) model of depression. Chronic treatment with leptin reversed the CUS-induced reduction of hippocampal neurogenesis and depression-like behaviors. Leptin treatment elicited a delayed long-lasting antidepressant-like effect in the forced swim behavioral despair test, and this effect was blocked by ablation of neurogenesis with X-irradiation. The functional isoform of the leptin receptor, LepRb, and the glucocorticoid receptor (GR) were colocalized in hippocampal neural stem/progenitor cells in vivo and in vitro. Leptin treatment reversed the GR agonist dexamethasone (DEX)-induced reduction of proliferation of cultured neural stem/progenitor cells from adult hippocampus. Further mechanistic analysis revealed that leptin and DEX converged on glycogen synthase kinase-3β (GSK-3β) and β-catenin. While DEX decreased Ser9 phosphorylation and increased Tyr216 phosphorylation of GSK-3β, leptin increased Ser9 phosphorylation and attenuated the effects of DEX at both Ser9 and Tyr216 phosphorylation sites of GSK-3β. Moreover, leptin increased total level and nuclear translocation of β-catenin, a primary substrate of GSK-3β and a key regulator in controlling hippocampal neural progenitor cell proliferation, and reversed the inhibitory effects of DEX on β-catenin. Taken together, our results suggest that adult neurogenesis is involved in the delayed long-lasting antidepressant-like behavioral effects of leptin, and leptin treatment counteracts chronic stress and glucocorticoid-induced suppression of hippocampal neurogenesis via activating the GSK-3β/β-catenin signaling pathway.

Potential utility of optogenetics in the study of depression

Novel antidepressants are needed to enhance the health and quality of life of the hundreds of millions of depressed individuals worldwide who remain inadequately treated with today's approaches. In reality, no new class of antidepressant medication has been introduced in over 50 years. This insufficiency of current drug treatments is evident to those eager to pursue invasive experimental options like that of deep brain stimulation. Encouragingly, human brain imaging studies and animal work implicate strong relationships between depressive symptoms and patterns of brain activity, which are now open to more empirical assessments using optogenetics. Recent advances in optogenetics permit control over specific subtypes of neurons or their afferent or efferent projections and can greatly further our understanding of the neural mechanisms involved in depression and the mechanism of action of deep brain stimulation and perhaps chemical antidepressants. Here, we discuss how optogenetic tools are being used to answer a broad range of molecular, cellular, and circuit-level questions pertaining to depression that, up until now, have been resistant to other experimental approaches. The emergence of optogenetic technology, when combined with the best-validated animal models of depression, will dramatically increase knowledge about the basic neurobiology of depression, as well as facilitate the development of more effective antidepressant treatments.

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.

Neurofibromin modulates adult hippocampal neurogenesis and behavioral effects of antidepressants

Neurogenesis persists in the rodent dentate gyrus (DG) throughout adulthood but declines with age and stress. Neural progenitor cells (NPCs) residing in the subgranular zone of the DG are regulated by an array of growth factors and respond to the microenvironment, adjusting their proliferation level to determine the rate of neurogenesis. Here we report that genetic deletion of neurofibromin (Nf1), a tumor suppressor with RAS-GAP activity, in adult NPCs enhanced DG proliferation and increased generation of new neurons in mice. Nf1 loss-associated neurogenesis had the functional effect of enhancing behavioral responses to subchronic antidepressants and, over time, led to spontaneous antidepressive-like behaviors. Thus, our findings establish an important role for the Nf1-Ras pathway in regulating adult hippocampal neurogenesis, and demonstrate that activation of adult NPCs is sufficient to modulate depression- and anxiety-like behaviors.

Beneficial behavioural and neurogenic effects of agomelatine in a model of depression/anxiety

Agomelatine (S20098) is a novel antidepressant drug with melatonergic agonist and 5-HT2C receptor antagonist properties, displaying antidepressant/anxiolytic-like properties in animal models and in humans. In a depression/anxiety-like mouse model in which the response of the HPA axis is blunted, we investigated whether agomelatine could reverse behavioural deficits related to depression/anxiety compared to the classical selective serotonin reuptake inhibitor, fluoxetine. Adult mice were treated for 8 wk with either vehicle or corticosterone (35 μg/ml.d) via drinking water. During the final 4 wk, animals were treated with vehicle, agomelatine (10 or 40 mg/kg i.p.) or fluoxetine (18 mg/kg i.p.) and tested in several behavioural paradigms and also evaluated for home-cage activity. Our results showed that the depressive/anxiety-like phenotype induced by corticosterone treatment is reversed by either chronic agomelatine or fluoxetine treatment. Moreover, agomelatine increased the dark/light ratio of home-cage activity in vehicle-treated mice and reversed the alterations in this ratio induced by chronic corticosterone, suggesting a normalization of disturbed circadian rhythms. Finally, we investigated the effects of this new antidepressant on neurogenesis. Agomelatine reversed the decreased cell proliferation in the whole hippocampus in corticosterone-treated mice and increased maturation of newborn neurons in both vehicle- and corticosterone-treated mice. Overall, the present study suggests that agomelatine, with its distinct mechanism of action based on the synergy between the melatonergic agonist and 5-HT2C antagonist properties, provides a distinct antidepressant/anxiolytic spectrum including circadian rhythm normalization.

Neuropeptide S alters anxiety, but not depression-like behaviour in Flinders Sensitive Line rats: a genetic animal model of depression

Neuropeptide S (NPS) and its receptor (NPSR) have been implicated in the mediation of anxiolytic-like behaviour in rodents. However, little knowledge is available regarding the NPS system in depression-related behaviours, and whether NPS also exerts anxiolytic effects in an animal model of psychopathology. Therefore, the aim of this work was to characterize the effects of NPS on depression- and anxiety-related parameters, using male and female rats in a well-validated animal model of depression: the Flinders Sensitive Line (FSL), their controls, the Flinders Resistant Line (FRL), and Sprague-Dawley (SD) rats. We found that FSL showed greater immobility in the forced swim test (FST) than FRL, confirming their phenotype. However, NPS did not affect depression-related behaviour in any rat line. No significant differences in baseline anxiety levels between the FSL and FRL strains were observed, but FSL and FRL rats displayed less anxiety-like behaviour compared to SD rats. NPS decreased anxiety-like behaviour on the elevated plus-maze in all strains. The expression of the NPSR in the amygdala, periventricular hypothalamic nucleus, and hippocampus was equal in all male strains, although a trend towards reduced expression within the amygdala was observed in FSL rats compared to SD rats. In conclusion, NPS had a marked anxiolytic effect in FSL, FRL and SD rats, but did not modify the depression-related behaviour in any strain, in spite of the significant differences in innate level between the strains. These findings suggest that NPS specifically modifies anxiety behaviour but cannot overcome/reverse a genetically mediated depression phenotype.

Optogenetic investigation of neural circuits underlying brain disease in animal models

Optogenetic tools have provided a new way to establish causal relationships between brain activity and behaviour in health and disease. Although no animal model captures human disease precisely, behaviours that recapitulate disease symptoms may be elicited and modulated by optogenetic methods, including behaviours that are relevant to anxiety, fear, depression, addiction, autism and parkinsonism. The rapid proliferation of optogenetic reagents together with the swift advancement of strategies for implementation has created new opportunities for causal and precise dissection of the circuits underlying brain diseases in animal models.

Immuno-Golgi as a tool for analyzing neuronal 3D-dendritic structure in phenotypically characterized neurons

Characterization of neuronal dendritic structure in combination with the determination of specific neuronal phenotype or temporal generation is a challenging task. Here we present a novel method that combines bromodioxyuridine (BrdU) immunohistochemistry with Golgi-impregnation technique; with this simple non-invasive method, we are able to determine the tridimensional structure of dendritic arborization and spine shape of neurons born at a specific time in the hippocampus of adult animals. This analysis is relevant in physiological and pathological conditions in which altered neurogenesis is implicated, such as aging or emotional disorders.

Pulsed light irradiation improves behavioral outcome in a rat model of chronic mild stress

BACKGROUND AND OBJECTIVE

Transcranial laser therapy (TLT) has been used successfully for the treatment of stroke in animal models and clinical trials. These results support the hypothesis that TLT could be used to treat other central nervous system conditions, such as depression. Current therapy for depression emphasizes pharmaco-therapeutics. However, these interventions often cause unwanted side effects. Here, TLT as a treatment for depression was studied in a rat model of chronic mild stress (CMS).

STUDY DESIGN/MATERIAL AND METHODS

Wistar rats were randomized into four experimental groups (n = 8): (1) No-stress; (2) stress without treatment (Stress); (3) stress treated with an antidepressant (Drug); and (4) stress treated with TLT (TLT). The rats in the stress groups were exposed sequentially to a variety of mild stressors for 8 weeks. Rats were weighed weekly. After 5 weeks of stressing, the Drug group received a daily injection of fluoxetine (10 mg/kg), and the TLT group was irradiated transcranially 3 times a week (810 nm wavelength laser, 3 mm diameter probe, 350 mW peak power, 100 Hz with 20% duty cycle, 2-minute treatment time, 120 J/cm(2) average energy density on skin surface). After 3 weeks of treatment, a forced swimming test (FST) was performed and recorded for behavioral assessment. Animals were euthanized after 8 weeks of the study.

RESULTS

The No-stress group had significantly higher body weight than stress groups from week 5 (P < 0.05). No weight difference was found between the stress groups before treatment. However, the Drug group had significantly less body weight than both Stress and TLT groups after 2 weeks of treatment (P < 0.05). FST showed that the Stress group had significantly more immobility than the No-stress group (P < 0.05). Both Drug and TLT groups had significantly less immobility than the stress group (P < 0.05). There was no significant difference in immobility between both Drug and TLT groups (P = 0.62).

CONCLUSIONS

TLT was comparable to fluoxetine in improving the behavioral outcome after CMS. TLT did not cause weight loss, which is consistently seen in patients treated with fluoxetine. This study demonstrates that TLT has potential as an effective treatment for depression.

Noradrenaline increases neural precursor cells derived from adult rat dentate gyrus through β2 receptor

Several preclinical researches indicate that increased neurogenesis in the adult hippocampus might underlie the therapeutic effect of antidepressant treatment. Most antidepressant drugs have ability to increase serotonin (5-HT) and/or noradrenaline (NA) in brain, and chronic treatment with antidepressant drugs increases the number of proliferating neural precursor cells and neurogenesis in hippocampus. However, the direct effects of antidepressant drugs, 5-HT and NA on the neural precursor cells remain largely unknown. Neural precursor cells in adult hippocampus are divided into stem/progenitor cells of four types based on stages of neural development. We recently established a culture system of adult rat dentate gyrus-derived neural precursor cells (ADPs), which correspond to be type 2a early progenitor cells. Here the direct effects of antidepressant drugs of four types (fluoxetine, imipramine, reboxetine, and tranylcypromine) and two neurotransmitters (5-HT and NA) on the proliferation of ADPs were investigated. Neither antidepressant drugs of all types nor 5-HT increased the number of ADPs. On the other hand, NA increased the number and the DNA synthesis of ADPs. The effect of NA on ADP proliferation was antagonized by propranolol and timolol (β-adrenergic receptor (AR) antagonists), but not by phentolamine (α-AR antagonist), prazosin (α1-AR antagonist), or yohimbine (α2-AR antagonist). Moreover, it was antagonized by ICI 118, 551 (β2-AR selective antagonist) and salmeterol (β2-AR selective agonist) promoted ADP proliferation. These results suggest that NA might increase the proliferation of early progenitor cells in adult hippocampus via β2-AR directly, but antidepressant drugs and 5-HT do not.

NMDA receptors and metaplasticity: mechanisms and possible roles in neuropsychiatric disorders

N-methyl-D-aspartate receptors (NMDARs) are key components of neural signaling, playing roles in synaptic transmission and in the synaptic plasticity thought to underlie learning and memory. NMDAR activation can also have neurotoxic consequences contributing to several forms of neurodegeneration. Additionally, NMDARs can modulate neuronal function and regulate the ability of synapses to undergo synaptic plasticity. Evidence gathered over the past 20 years strongly supports the idea that untimely activation of NMDARs impairs the induction of long-term potentiation (LTP) by a form of metaplasticity. This metaplasticity can be triggered by multiple stimuli including physiological receptor activation, and metabolic and behavioral stressors. These latter findings raise the possibility that NMDARs contribute to cognitive dysfunction associated with neuropsychiatric disorders. This paper examines NMDAR metaplasticity and its potential role in cognition. Recent studies using NMDAR antagonists for therapeutic purposes also raise the possibility that metaplasticity may contribute to clinical effects of certain drugs.

Antidepressants recruit new neurons to improve stress response regulation

Recent research suggests an involvement of hippocampal neurogenesis in behavioral effects of antidepressants. However, the precise mechanisms through which newborn granule neurons might influence the antidepressant response remain elusive. Here, we demonstrate that unpredictable chronic mild stress in mice not only reduces hippocampal neurogenesis, but also dampens the relationship between hippocampus and the main stress hormone system, the hypothalamo-pituitary-adrenal (HPA) axis. Moreover, this relationship is restored by treatment with the antidepressant fluoxetine, in a neurogenesis-dependent manner. Specifically, chronic stress severely impairs HPA axis activity, the ability of hippocampus to modulate downstream brain areas involved in the stress response, the sensitivity of the hippocampal granule cell network to novelty/glucocorticoid effects and the hippocampus-dependent negative feedback of the HPA axis. Remarkably, we revealed that, although ablation of hippocampal neurogenesis alone does not impair HPA axis activity, the ability of fluoxetine to restore hippocampal regulation of the HPA axis under chronic stress conditions, occurs only in the presence of an intact neurogenic niche. These findings provide a mechanistic framework for understanding how adult-generated new neurons influence the response to antidepressants. We suggest that newly generated neurons may facilitate stress integration and that, during chronic stress or depression, enhancing neurogenesis enables a dysfunctional hippocampus to restore the central control on stress response systems, then allowing recovery.

Hippocampal telomerase is involved in the modulation of depressive behaviors

Telomere and telomerase alterations have been reported in mood disorders. However, the role of telomerase in depression remains unclear. Here we show that chronic mild stress (CMS) led to a significant decrease in telomerase reverse transcriptase (TERT) level and telomerase activity in the hippocampus. Treatment with antidepressant fluoxetine reversed the CMS-induced TERT and telomerase activity changes. Inhibiting telomerase by systemic administration (100 mg · kg(-1) · d(-1), i.p., for 14 d), intrahippocampal microinjection (0.7 μmol, 2 μl), or infusion (using an osmotic minipump, 0.134 μg/μl, 0.25 μl/h) of 3'-azido-deoxythymidine (AZT) resulted in depression-like behaviors and impaired hippocampal neurogenesis in mice. In contrast, overexpressing telomerase by intrahippocampal infusion of recombinant adenovirus vector expressing mouse TERT (Ad-mTERT-GFP) led to neurogenesis upregulation, produced antidepressant-like behaviors, and prevented the CMS-induced behavioral modifications. Disrupting neurogenesis in the dentate gyrus by X-irradiation (15 Gy) of a restricted region of mouse brain containing the hippocampus abolished the antidepressant-like effect of Ad-mTERT-GFP. Additionally, AZT had no effect on DNA polymerase activity and did not cause cell damage in vitro and in vivo. Microinjection of AZT into the subventricular zone of lateral ventricle (0.7 μmol, 2 μl) inhibited local neurogenesis but had no behavioral effect. These results suggest that hippocampal telomerase is involved in the modulation of depression-related behaviors, possibly by regulating adult neurogenesis.

Antidepressants stimulate hippocampal neurogenesis by inhibiting p21 expression in the subgranular zone of the hipppocampus

The relationships among hippocampal neurogenesis, depression and the mechanism of action of antidepressant drugs have generated a considerable amount of controversy. The cyclin-dependent kinase (Cdk) inhibitor p21^Cip1 (p21) plays a crucial role in restraining cellular proliferation and maintaining cellular quiescence. Using in vivo and in vitro approaches the present study shows that p21 is expressed in the subgranular zone of the dentate gyrus of the hippocampus in early neuronal progenitors and in immature neurons, but not in mature neurons or astroglia. In vitro, proliferation is higher in neuronal progenitor cells derived from p21-/- mice compared to cells derived from wild-type mice. Proliferation is increased in neuronal progenitor cells after suppression of p21 using lentivirus expressing short hairpin RNA against p21. In vivo, chronic treatment with the non-selective antidepressant imipramine as well as the norepinephrine-selective reuptake inhibitor desipramine or the serotonin-selective reuptake inhibitor fluoxetine all decrease p21 expression, and this was associated with increased neurogenesis. Chronic antidepressant treatment did not affect the expression of other Cdk inhibitors. Untreated p21-/- mice exhibit a higher degree of baseline neurogenesis and decreased immobility in the forced swim test. Although chronic imipramine treatment increased neurogenesis and reduced immobility in the forced swim test in wild-type mice, it reduced neurogenesis and increased immobility in p21-/- mice. These results demonstrate the unique role of p21 in the control of neurogenesis, and support the hypothesis that different classes of reuptake inhibitor-type antidepressant drugs all stimulate hippocampal neurogenesis by inhibiting p21 expression.

Subchronic peripheral neuregulin-1 increases ventral hippocampal neurogenesis and induces antidepressant-like effects

Background

Adult hippocampal neurogenesis has been implicated in the mechanism of antidepressant action, and neurotrophic factors can mediate the neurogenic changes underlying these effects. The neurotrophic factor neuregulin-1 (NRG1) is involved in many aspects of brain development, from cell fate determination to neuronal maturation. However, nothing is known about the influence of NRG1 on neurodevelopmental processes occurring in the mature hippocampus.

Methods

Adult male mice were given subcutaneous NRG1 or saline to assess dentate gyrus proliferation and neurogenesis, as well as cell fate determination. Mice also underwent behavioral testing. Expression of ErbB3 and ErbB4 NRG1 receptors in newborn dentate gyrus cells was assessed at various time points between birth and maturity. The phenotype of ErbB-expressing progenitor cells was also characterized with cell type-specific markers.

Results

The current study shows that subchronic peripheral NRG1β administration selectively increased cell proliferation (by 71%) and neurogenesis (by 50%) in the caudal dentate gyrus within the ventral hippocampus. This pro-proliferative effect did not alter neuronal fate, and may have been mediated by ErbB3 receptors, which were expressed by newborn dentate gyrus cells from cell division to maturity and colocalized with SOX2 in the subgranular zone. Furthermore, four weeks after cessation of subchronic treatment, animals displayed robust antidepressant-like behavior in the absence of changes in locomotor activity, whereas acute treatment did not produce antidepressant effects.

Conclusions

These results show that neuregulin-1β has pro-proliferative, neurogenic and antidepressant properties, further highlight the importance of peripheral neurotrophic factors in neurogenesis and mood, and support the role of hippocampal neurogenesis in mediating antidepressant effects.

Tumor suppressor down-regulated in renal cell carcinoma 1 (DRR1) is a stress-induced actin bundling factor that modulates synaptic efficacy and cognition

Stress has been identified as a major causal factor for many mental disorders. However, our knowledge about the chain of molecular and cellular events translating stress experience into altered behavior is still rather scant. Here, we have characterized a murine ortholog of the putative tumor suppressor gene DRR1 as a unique stress-induced protein in brain. It binds to actin, promotes bundling and stabilization of actin filaments, and impacts on actin-dependent neurite outgrowth. Endogenous DRR1 localizes to some, but not all, synapses, with preference for the presynaptic region. Hippocampal virus-mediated enhancement of DRR1 expression reduced spine density, diminished the probability of synaptic glutamate release, and altered cognitive performance. DRR1 emerges as a protein to link stress with actin dynamics, which in addition is able to act on synaptic function and cognition.