Prefrontal cortex astroglia modulate anhedonia-like behavior.

Reductions of astroglia expressing glial fibrillary acidic protein (GFAP) are consistently found in the prefrontal cortex (PFC) of patients with depression and in rodent chronic stress models. Here, we examine the consequences of PFC GFAP+ cell depletion and cell activity enhancement on depressive-like behaviors in rodents. Using viral expression of diphtheria toxin receptor in PFC GFAP+ cells, which allows experimental depletion of these cells following diphtheria toxin administration, we demonstrated that PFC GFAP+ cell depletion induced anhedonia-like behavior within 2 days and lasting up to 8 days, but no anxiety-like deficits. Conversely, activating PFC GFAP+ cell activity for 3 weeks using designer receptor exclusively activated by designer drugs (DREADDs) reversed chronic restraint stress-induced anhedonia-like deficits, but not anxiety-like deficits. Our results highlight a critical role of cortical astroglia in the development of anhedonia and further support the idea of targeting astroglia for the treatment of depression.

Prefrontal Cortex Astroglia Modulate Anhedonia-like Behavior.

Reductions of astroglia expressing glial fibrillary acidic protein (GFAP) are consistently found in the prefrontal cortex (PFC) of patients with depression and in rodent chronic stress models. Here, we examine the consequences of PFC GFAP+ cell depletion and cell activity enhancement on depressive-like behaviors in rodents. Using viral expression of diphtheria toxin receptor in PFC GFAP+ cells, which allows experimental depletion of these cells following diphtheria toxin administration, we demonstrated that PFC GFAP+ cell depletion induced anhedonia-like behavior within 2 days and lasting up to 8 days, but no anxiety-like deficits. Conversely, activating PFC GFAP+ cell activity for 3 weeks using designer receptor exclusively activated by designer drugs (DREADDs) reversed chronic restraint stress-induced anhedonia-like deficits, but not anxiety-like deficits. Our results highlight a critical role of cortical astroglia in the development of anhedonia and further support the idea of targeting astroglia for the treatment of depression.

BDNF release and signaling are required for the antidepressant actions of GLYX-13.

Conventional antidepressant medications, which act on monoaminergic systems, display significant limitations, including a time lag of weeks to months and low rates of therapeutic efficacy. GLYX-13 is a novel glutamatergic compound that acts as an N-methyl-D-aspartate (NMDA) modulator with glycine-like partial agonist properties; like the NMDA receptor antagonist ketamine GLYX-13 produces rapid antidepressant actions in depressed patients and in preclinical rodent models. However, the mechanisms underlying the antidepressant actions of GLYX-13 have not been characterized. Here we use a combination of neutralizing antibody (nAb), mutant mouse and pharmacological approaches to test the role of brain-derived neurotrophic factor-tropomyosin-related kinase B (BDNF-TrkB) signaling in the actions of GLYX-13. The results demonstrate that the antidepressant effects of GLYX-13 are blocked by intra-medial prefrontal cortex (intra-mPFC) infusion of an anti-BDNF nAb or in mice with a knock-in of the BDNF Val66Met allele, which blocks the processing and activity-dependent release of BDNF. We also demonstrate that pharmacological inhibitors of BDNF-TrkB signaling or of L-type voltage-dependent Ca2+ channels (VDCCs) block the antidepressant behavioral actions of GLYX-13. Finally, we examined the role of the Rho GTPase proteins by injecting a selective inhibitor into the mPFC and found that activation of Rac1 but not RhoA is involved in the antidepressant effects of GLYX-13. Together, these findings indicate that enhanced release of BDNF through exocytosis caused by activation of VDCCs and subsequent TrkB-Rac1 signaling is required for the rapid and sustained antidepressant effects of GLYX-13.

Transiently increased glutamate cycling in rat PFC is associated with rapid onset of antidepressant-like effects.

Several drugs have recently been reported to induce rapid antidepressant effects in clinical trials and rodent models. Although the cellular mechanisms involved remain unclear, reports suggest that increased glutamate transmission contributes to these effects. Here, we demonstrate that the antidepressant-like efficacy of three unique drugs, with reported rapid onset antidepressant properties, is coupled with a rapid transient rise in glutamate cycling in the medial prefronal cortex (mPFC) of awake rats as measured by ex vivo 1H-[13C]-nuclear magnetic resonance spectroscopy. Rats were acutely pretreated by intraperitoneal injection with a single dose of ketamine (1, 3, 10, 30 and 80 mg kg-1), Ro 25-6981 (1, 3 and 10 mg kg-1), scopolamine (5, 25 and 100 µg kg-1) or vehicle (controls). At fixed times after drug injection, animals received an intravenous infusion of [1,6-13C2]glucose for 8 min to enrich the amino-acid pools of the brain with 13C, followed by rapid euthanasia. The mPFC was dissected, extracted with ethanol and metabolite 13C enrichments were measured. We found a clear dose-dependent effect of ketamine and Ro 25-6981 on behavior and the percentage of 13C enrichment of glutamate, glutamine and GABA (γ-aminobutyric acid). Further, we also found an effect of scopolamine on both cycling and behavior. These studies demonstrate that three pharmacologically distinct classes of drugs, clinically related through their reported rapid antidepressant actions, share the common ability to rapidly stimulate glutamate cycling at doses pertinent for their antidepressant-like efficacy. We conclude that increased cycling precedes the antidepressant action at behaviorally effective doses and suggest that the rapid change in cycling could be used to predict efficacy of novel agents or identify doses with antidepressant activity.

Remodeling of axo-spinous synapses in the pathophysiology and treatment of depression.

Dendritic spines provide a compartment for assembly and functional organization of synaptic machinery that plays a fundamental role in neuronal communication and neuroplasticity. Studies in humans as well as in animal models have demonstrated abnormal spine architecture in several psychiatric disorders, including depression and other stress-related illnesses. The negative impact of stress on the density and organization of spines is thought to contribute to the behavioral deficits caused by stress exposure. Moreover, there is now evidence that medication-induced recovery involves changes in synaptic plasticity and dendrite morphology, including increased expression of pre- and postsynaptic plasticity-related proteins, as well as the density and function of axo-spinous synapses. Here we review the evidence from brain imaging and postmortem studies demonstrating that depression is accompanied by structural and functional alterations of cortical and limbic brain regions, including the prefrontal cortex, hippocampus and amygdala. In addition, we present more direct evidence from basic research studies that exposure to stress alters spine morphology, function and plasticity and that antidepressants, particularly new rapid acting agents, reverse these effects. Elucidation of the signaling pathways and molecular mechanisms that control spine synapse assembly and plasticity will contribute to a better understanding of the pathophysiology of depression and development of novel, more effective therapeutic agents.

The roles of neurotrophic factor and Wnt signaling in depression.

Depression is a debilitating disease with a lifetime prevalence of ~16% in the American population. In addition to the monoamine hypothesis, altered expressions of neurotrophic factors, growth factors, and Wnt signaling are implicated in the pathophysiology and treatment of depression. This review focuses on intracellular signaling cascades that underlie depression and treatment response.

Glial pathology in an animal model of depression: reversal of stress-induced cellular, metabolic and behavioral deficits by the glutamate-modulating drug riluzole.

Growing evidence indicates that glia pathology and amino-acid neurotransmitter system abnormalities contribute to the pathophysiology and possibly the pathogenesis of major depressive disorder. This study investigates changes in glial function occurring in the rat prefrontal cortex (PFC) after chronic unpredictable stress (CUS), a rodent model of depression. Furthermore, we analyzed the effects of riluzole, a Food and Drug Administration-approved drug for the treatment of amyotrophic laterosclerosis, known to modulate glutamate release and facilate glutamate uptake, on CUS-induced glial dysfunction and depressive-like behaviors. We provide the first experimental evidence that chronic stress impairs cortical glial function. Animals exposed to CUS and showing behavioral deficits in sucrose preference and active avoidance exhibited significant decreases in 13C-acetate metabolism reflecting glial cell metabolism, and glial fibrillary associated protein (GFAP) mRNA expression in the PFC. The cellular, metabolic and behavioral alterations induced by CUS were reversed and/or blocked by chronic treatment with the glutamate-modulating drug riluzole. The beneficial effects of riluzole on CUS-induced anhedonia and helplessness demonstrate the antidepressant action of riluzole in rodents. Riluzole treatment also reversed CUS-induced reductions in glial metabolism and GFAP mRNA expression. Our results are consistent with recent open-label clinical trials showing the drug's effect in mood and anxiety disorders. This study provides further validation of hypothesis that glial dysfunction and disrupted amino-acid neurotransmission contribute to the pathophysiology of depression and that modulation of glutamate metabolism, uptake and/or release represent viable targets for antidepressant drug development.

Neurobiology and treatment of anxiety: signal transduction and neural plasticity.

The stress-dependence and chronic nature of anxiety disorders along with the anxiolytic effectiveness of antidepressant drugs suggests that neuronal plasticity may play a role in the pathophysiology of anxiety. Intracellular signaling pathways are known in many systems to be critical links in the cascades from surface signals to the molecular alterations that result in functional plasticity. Chronic antidepressant treatments can regulate intracellular signaling pathways and can induce molecular, cellular, and structural changes over time. These changes may be important to the anxiolytic effectiveness of these drugs. In addition, the signaling proteins implicated in the actions of chronic antidepressant action, such as cAMP response element binding protein (CREB), have also been implicated in conditioned fear and in anxiety. The cellular mechanisms underlying conditioned fear indicate roles for additional signaling pathways; however, less is known about such mechanisms in anxiety. The challenge to identify intracellular signaling pathways and related molecular and structural changes that are critical to the etiology and treatment of anxiety will further establish the importance of mechanisms of neuronal plasticity in functional outcome and improve treatment strategies.

Inducible, brain region-specific expression of a dominant negative mutant of c-Jun in transgenic mice decreases sensitivity to cocaine.

Administration of cocaine induces the Fos family of transcription factors in the striatum, including the nucleus accumbens (NAc), a brain region important for the rewarding effects of addictive drugs. Several Fos proteins are induced acutely by cocaine, with stable isoforms of DeltaFosB predominating after chronic drug administration. However, it has been difficult to study the functional consequences of these Fos responses in vivo. Fos proteins heterodimerize with members of the Jun family to form active AP-1 transcription factor complexes. In the present study, we took advantage of this property and generated transgenic mice, using the tetracycline gene regulation system, that support the inducible, brain region-specific expression of a dominant negative mutant form of c-Jun (Deltac-Jun), which can antagonize the actions of Fos proteins. Expression of Deltac-Jun in the striatum and certain other brain regions of adult mice decreases their development of cocaine-induced conditioned place preference, suggesting reduced sensitivity to the rewarding effects of cocaine. In contrast, Deltac-Jun expression had no effect on cocaine-induced locomotor activity or sensitization. However, expression of Deltac-Jun in adult mice blocked the ability of chronic cocaine administration to induce three known targets for AP-1 in the NAc: the AMPA glutamate receptor subunit GluR2, the cyclin-dependent protein kinase Cdk5, and the transcription factor nuclear factor-kappaB (NFkappaB), without affecting several other proteins examined for comparison. Taken together, these results provide further support for an important role of AP-1-mediated transcription in some of the behavioral and molecular mechanisms underlying cocaine addiction.

Localization of phosphorylated cAMP response element-binding protein in immature neurons of adult hippocampus.

Neurogenesis continues to occur in the adult hippocampus, although many of the newborn cells degenerate 1-2 weeks after birth. The number and survival of newborn cells are regulated by a variety of environmental stimuli, but very little is known about the intracellular signal transduction pathways that control adult neurogenesis. In the present study, we examine the expression of the phosphorylated cAMP response element-binding protein (pCREB) in immature neurons in adult hippocampus and the role of the cAMP cascade in the survival of new neurons. The results demonstrate that virtually all immature neurons, identified by triple immunohistochemistry for bromodeoxyuridine (BrdU) and polysialic acid-neural cell adhesion molecule (PSA-NCAM), are also positive for pCREB. In addition, upregulation of cAMP (via pharmacological inhibition of cAMP breakdown or by antidepressant treatment) increases the survival of BrdU-positive cells. A possible role for pCREB in the regulation of PSA-NCAM, a marker of immature neurons involved in neuronal remodeling and neurite outgrowth, is supported by cell culture studies demonstrating that the cAMP-CREB pathway regulates the expression of a rate-limiting enzyme responsible for the synthesis of PSA-NCAM. These findings indicate that the cAMP-CREB pathway regulates the survival, and possibly the differentiation and function, of newborn neurons.

[Molecular aspects of antidepressive therapy. Transsynaptic effects on signal transduction, gene expression and neuronal plasticity]. / Molekulare Aspekte antidepressiver Therapie. Transsynaptische Effekte auf Signaltransduktion, Genexpression und neuronale Plastizität.

Simple neurotransmitter theories cannot sufficiently explain the mode of action of antidepressant drugs. Molecular pharmacological studies demonstrate that antidepressive treatment initially modulates the neurotransmitter-receptor interaction, subsequently influences signal transduction cascades beyond the synapse and gene transcription mechanisms, and ultimately triggers the expression of specific target genes. Such genes often code for molecules which play an important role in the maintenance of neural and synaptic plasticity. Chronic (but not acute) treatment with antidepressants modulates, for example, the cAMP-second-messenger system and increases the expression of neurotrophic factors. Furthermore, antidepressants promote hippocampal neurogenesis. Stress, an important risk factor for psychiatric disorders, often induces opposite effects. A better understanding of the molecular and cellular effects of stress and therapy with psychotropic drugs will stimulate the development of innovative treatment strategies for which an optimised antidepressant efficacy with a simultaneously improved tolerance is expected.

Synaptic plasticity and mood disorders.

Recent studies demonstrate that the molecular elements known to regulate neuronal plasticity in models of learning and memory are also involved in the actions of drugs used for the treatment of depression and bipolar disorder. This includes up-regulation of transcription factors, such as the cAMP response element binding protein and neurotrophic factors, such as brain derived neurotrophic factor. These findings raise the possibility that regulation of neural plasticity in specific neuronal circuits is integrally involved in the therapeutic intervention of mood disorders. Atypical antipsychotic drugs, including clozapine and olanzapine, are also effective for the treatment of bipolar disorder, and are used as add-on medication for unipolar depression. The possibility that these atypical antipsychotic drugs also influence the molecular determinants of synaptic plasticity that are involved in the response to drugs used for the treatment of mood disorders, is discussed.

Pathophysiology of depression: the concept of synaptic plasticity.

Neuronal plasticity or remodeling is most often discussed with regard to cellular and behavioral models of learning and memory. However, neuronal plasticity is a fundamental process by which the brain acquires information and makes the appropriate adaptive responses in future-related settings. Dysfunction of these fundamental processes could thereby contribute to the pathophysiology of mood disorders, and recovery could occur by induction of the appropriate plasticity or remodeling. These possibilities are supported by preclinical and clinical studies demonstrating that there are structural alterations that occur in response to stress and in patients with mood disorders. Moreover, antidepressant treatment may oppose these effects by regulation of signal transduction and gene expression pathways linked to neuronal plasticity. These findings comprise a novel conceptual framework for future studies of the etiology of mood disorders and for the development of novel therapeutic interventions.

Regulation of adult neurogenesis by antidepressant treatment.

Demonstration of neurogenesis in adult brain represents a major advance in our understanding of the cellular mechanisms underlying neuronal remodeling and complex behavior. Recent studies from our laboratory and others demonstrate that chronic administration of an antidepressant, including either a 5-HT or norepinephrine selective reuptake inhibitor, up-regulates neurogenesis in adult rodent hippocampus. Up-regulation of neurogenesis could block or reverse the effects of stress on hippocampal neurons, which include down-regulation of neurogenesis, as well as atrophy. The possibility that the cAMP signal transduction cascade contributes to the regulation of neurogenesis by antidepressants is supported by previous studies and by recent work. Although additional studies must be conducted to determine the significance of adult neurogenesis in humans, these findings will stimulate new avenues of research to identify the cellular and molecular basis of stress-related mood disorders as well as the development of novel therapeutic strategies.

Depresssion--emerging insights from neurobiology.

An emerging hypothesis suggests that the pathogenesis and treatment of depression is likely to involve a plasticity of neuronal pathways. The inability of neuronal systems to exhibit appropriate, adaptive plasticity could contribute to the pathogenesis of depression. Antidepressant treatments may exert their therapeutic effects by stimulating appropriate adaptive changes in neuronal systems. Recent studies have demonstrated that chronic antidepressant administration up-regulates the cAMP signal transduction cascade resulting in an increased expression and function of the transcription factor CREB. Enhanced CREB expression leads to an up-regulation of specific target genes, including the neurotrophin BDNF. Chronic antidepressant treatments enhance BDNF expression within hippocampal and cortical neurons and can prevent the stress-induced decrease in BDNF expression. Stress has been shown to: (i) induce neuronal atrophy/death; and (ii) decrease neurogenesis of hippocampal neurons. Clinical studies indicate significant hippocampal damage in cases of major, recurrent depression. It is possible that antidepressant treatments through enhanced expression of growth and survival promoting factors like BDNF may prevent or reverse the atrophy and damage of hippocampal neurons. Indeed, studies have indicated that chronic antidepressant treatments enhance hippocampal neurogenesis, promote neuronal sprouting and prevent atrophy. The molecular mechanisms underlying the effects of antidepressant treatments including adaptations in the cAMP transduction cascade, CREB and BDNF gene expression, and structural neuronal plasticity are discussed.

Stress differentially regulates synaptophysin and synaptotagmin expression in hippocampus.

BACKGROUND: In view of the effects of stress on synaptic plasticity, the regulation of synaptophysin and synaptotagmin expression by immobilization was analyzed by in situ hybridization. METHODS: Rats were exposed to immobilization stress, which induced typical behavioral alterations, such as reduced locomotor activity after stress exposure. Determination of mRNA levels of the integral synaptic vesicle proteins was performed immediately after acute or chronic immobilization. RESULTS: The results demonstrate that stress exposure leads to reduced expression of synaptophysin but increased expression of synaptotagmin in the hippocampus. CONCLUSIONS: This rapid and differential regulation of synaptic vesicle proteins could be responsible for some of the morphological, biochemical, and behavioral changes observed after stress exposure. These changes may be relevant to such clinical disorders as psychoses, depression, and posttraumatic stress disorder that are sensitive to stress and involve changes in neural and synaptic plasticity.

Regulation of adult neurogenesis by psychotropic drugs and stress.

Proliferation and maturation of neurons has been demonstrated to occur at a significant rate in discrete regions of adult brain, including the hippocampus and subventricular zone. Moreover, adult neurogenesis is an extremely dynamic process that is regulated in both a positive and negative manner by neuronal activity and environmental factors. It has been suggested to play a role in several important neuronal functions, including learning, memory, and response to novelty. In addition, exposure to psychotropic drugs or stress regulates the rate of neurogenesis in adult brain, suggesting a possible role for neurogenesis in the pathophysiology and treatment of neurobiological illnesses such as depression, post-traumatic stress disorder, and drug abuse. As the mechanisms that control adult neurogenesis continue to be identified, the exciting prospect of developing pharmacological agents that specifically regulate the proliferation and maturation of neurons in the adult brain could be fulfilled.

Expression of the cAMP response element binding protein (CREB) in hippocampus produces an antidepressant effect.

BACKGROUND: Recent studies have demonstrated that chronic antidepressant treatment increases the expression of the cyclic amp (cAMP) response element binding protein (CREB) in rat hippocampus. The study presented here was conducted to determine if CREB is a relevant target that produces an antidepressant-like effect. METHODS: We employed the herpes simplex virus (HSV)-mediated gene transfer technique to overexpress CREB and determined its effect on the learned helplessness and forced swim tests, two established models used for pharmacological screening of antidepressant drugs. RESULTS: In the learned helplessness model, rats that received bilateral microinjection of HSV-CREB into the dentate gyrus showed significantly fewer escape failures in the subsequent conditioned avoidance test than those injected with control vector (HSV-LacZ). In contrast, microinjection of HSV-CREB in either the CA1 pyramidal cell layer of hippocampus or the prefrontal cortex did not produce an antidepressant response. In the forced swim test, CREB expression in the dentate gyrus resulted in a significantly shorter immobility time than those injected with HSV-LacZ. CONCLUSIONS: These results demonstrate that over-expression of CREB in hippocampus results in an antidepressant effect and suggest that CREB may serve as a potential molecular target for novel therapeutic agents.

ECS-Induced mossy fiber sprouting and BDNF expression are attenuated by ketamine pretreatment.

Recent evidence suggests hippocampal and possibly cortical atrophy is associated with major depression. Chronic electroconvulsive seizures (ECS) induce brain-derived neurotrophic factor (BDNF) expression and sprouting of the mossy fiber pathway in the hippocampus, effects that may be related to electroconvulsive therapy's (ECT) mechanism of action. The objective of this study was to investigate the role of NMDA (N-methyl-D-aspartate) receptor in mediating the ECS-induced mossy fiber sprouting and BDNF expression. Timm histochemistry and in situ hybridization methodologies were used to determine the effect of pretreatment with ketamine, an NMDA antagonist, on ECS-induced sprouting and BDNF expression. The results demonstrate the ability of ketamine pretreatment to attenuate ECS-induced sprouting in the dentate gyrus and BDNF expression in the medial prefrontal cortex and the dentate gyrus. In addition, we found a significant decrease in seizure duration with ketamine pretreatment. These data suggest that NMDA receptor activation contributes to both the regulation of neurotrophic factor expression and the morphological changes associated with seizure activity. However, other effects resulting from shortened seizure duration and seizure intensity cannot be excluded. These findings are of increasing interest, as they relate to the use of ECT in the treatment of depression, and the specific anesthetic agents that are used.

Regulation of GFRalpha-1 and GFRalpha-2 mRNAs in rat brain by electroconvulsive seizure.

The influence of both acute and chronic electroconvulsive seizure (ECS) or antidepressant drug treatments on expression of mRNAs encoding glial cell line-derived neurotrophic factor (GDNF) and its receptors, GFRalpha-1, GFRalpha-2, and c-Ret proto-oncogene (RET) in the rat hippocampus was examined by in situ hybridization. Two hours after acute ECS, levels of GFRalpha-1 mRNA in the dentate gyrus were significantly increased. This increase peaked to nearly 3-fold at 6 h after acute ECS and returned to basal levels 24 h after treatment. Chronic (once daily for 10 days) ECS significantly increased the expression of GFRalpha-1 mRNA nearly 5-fold after the last treatment. Levels of GFRalpha-2 mRNA in the dentate gyrus were also significantly increased by acute and chronic ECS, although this effect was less than that observed for GFRalpha-1. Maximum induction of GFRalpha-2 was 30% and 70% compared to sham in response to acute or chronic ECS, respectively. Levels of GDNF and RET mRNAs were not significantly changed following either acute or chronic ECS treatment at the time points examined. Chronic (14 days) administration of different classes of antidepressant drugs, including tranylcypromine, desipramine, or fluoxetine, did not significantly affect the GDNF, GFRalpha-1, GFRalpha-2, or RET mRNA levels in CA1, CA3, and dentate gyrus areas of hippocampus. The results demonstrate that acute ECS increases the expression of GFRalpha-1 and GFRalpha-2 and that these effects are enhanced by chronic ECS. The results also imply that regulation of the binding components of GDNF receptor complex may mediate the adaptive responses of the GDNF system to acute and chronic stimulation.