Cortistatin-expressing interneurons require TrkB signaling to suppress neural hyper-excitability.

Signaling of brain-derived neurotrophic factor (BDNF) via tropomyosin receptor kinase B (TrkB) plays a critical role in the maturation of cortical inhibition and controls expression of inhibitory interneuron markers, including the neuropeptide cortistatin (CST). CST is expressed exclusively in a subset of cortical and hippocampal GABAergic interneurons, where it has anticonvulsant effects and controls sleep slow-wave activity (SWA). We hypothesized that CST-expressing interneurons play a critical role in regulating excitatory/inhibitory balance, and that BDNF, signaling through TrkB receptors on CST-expressing interneurons, is required for this function. Ablation of CST-expressing cells caused generalized seizures and premature death during early postnatal development, demonstrating a critical role for these cells in providing inhibition. Mice in which TrkB was selectively deleted from CST-expressing interneurons were hyperactive, slept less and developed spontaneous seizures. Frequencies of spontaneous excitatory post-synaptic currents (sEPSCs) on CST-expressing interneurons were attenuated in these mice. These data suggest that BDNF, signaling through TrkB receptors on CST-expressing cells, promotes excitatory drive onto these cells. Loss of excitatory drive onto CST-expressing cells that lack TrkB receptors may contribute to observed hyperexcitability and epileptogenesis.

Methylene Blue (Tetramethylthionine Chloride) Influences the Mobility of Adult Neural Stem Cells: A Potentially Novel Therapeutic Mechanism of a Therapeutic Approach in the Treatment of Alzheimer's Disease.

An interest in neurogenesis in the adult human brain as a relevant and targetable process has emerged as a potential treatment option for Alzheimer's disease and other neurodegenerative conditions. The aim of this study was to investigate the effects of tetramethylthionine chloride (methylene blue, MB) on properties of adult murine neural stem cells. Based on recent clinical studies, MB has increasingly been discussed as a potential treatment for Alzheimer's disease. While no differences in the proliferative capacity were identified, a general potential of MB in modulating the migratory capacity of adult neural stem cells was indicated in a cell mobility assay. To our knowledge, this is the first time that MB could be associated with neural mobility. The results of this study add insight to the spectrum of features of MB within the central nervous system and may be helpful for understanding the molecular mechanisms underlying a potential therapeutic effect of MB.

Bdnf mRNA splice variants differentially impact CA1 and CA3 dendrite complexity and spine morphology in the hippocampus.

Brain-derived neurotrophic factor (BDNF) is an activity-dependent neurotrophin critical for neuronal plasticity in the hippocampus. BDNF is encoded by multiple transcripts with alternative 5' untranslated regions (5'UTRS) that display activity-induced targeting to distinct subcellular compartments. While individual Bdnf 5'UTR transcripts influence dendrite morphology in cultured hippocampal neurons, it is unknown whether Bdnf splice variants impact dendrite arborization in functional classes of neurons in the intact hippocampus. Moreover, the contribution of Bdnf 5'UTR splice variants to dendritic spine density and shape has not been explored. We analyzed the structure of CA1 and CA3 dendrite arbors in transgenic mice lacking BDNF production from exon (Ex) 1, 2, 4, or 6 splice variants (Bdnf-e1, -e2, -e4, and -e6-/- mice) and found that loss of BDNF from individual Bdnf mRNA variants differentially impacts the complexity of apical and basal arbors in vivo. Consistent with the subcellular localization studies, Bdnf Ex2 and Ex6 transcripts significantly contributed to dendrite morphology in both CA1 and CA3 neurons. While Bdnf-e2-/- mice showed increased branching proximal to the soma in CA1 and CA3 apical arbors, Bdnf-e6-/- mice showed decreased apical and basal dendrite complexity. Analysis of spine morphology on Bdnf-e6-/- CA1 dendrites revealed changes in the percentage of differently sized spines on apical, but not basal, branches. These results provide further evidence that Bdnf splice variants generate a spatial code that mediates the local actions of BDNF in distinct dendritic compartments on structural and functional plasticity.

Functional Role of BDNF Production from Unique Promoters in Aggression and Serotonin Signaling.

Brain-derived neurotrophic factor (BDNF) regulates diverse biological functions ranging from neuronal survival and differentiation during development to synaptic plasticity and cognitive behavior in the adult. BDNF disruption in both rodents and humans is associated with neurobehavioral alterations and psychiatric disorders. A unique feature of Bdnf transcription is regulation by nine individual promoters, which drive expression of variants that encode an identical protein. It is hypothesized that this unique genomic structure may provide flexibility that allows different factors to regulate BDNF signaling in distinct cell types and circuits. This has led to the suggestion that isoforms may regulate specific BDNF-dependent functions; however, little scientific support for this idea exists. We generated four novel mutant mouse lines in which BDNF production from one of the four major promoters (I, II, IV, or VI) is selectively disrupted (Bdnf-e1, -e2, -e4, and -e6 mice) and used a comprehensive comparator approach to determine whether different Bdnf transcripts are associated with specific BDNF-dependent molecular, cellular, and behavioral phenotypes. Bdnf-e1 and -e2 mutant males displayed heightened aggression accompanied by convergent expression changes in specific genes associated with serotonin signaling. In contrast, BDNF-e4 and -e6 mutants were not aggressive but displayed impairments associated with GABAergic gene expression. Moreover, quantifications of BDNF protein in the hypothalamus, prefrontal cortex, and hippocampus revealed that individual Bdnf transcripts make differential, region-specific contributions to total BDNF levels. The results highlight the biological significance of alternative Bdnf transcripts and provide evidence that individual isoforms serve distinct molecular and behavioral functions.

Antidepressant-like Effects of Electroconvulsive Seizures Require Adult Neurogenesis in a Neuroendocrine Model of Depression.

BACKGROUND: Neurogenesis continues throughout life in the hippocampal dentate gyrus. Chronic treatment with monoaminergic antidepressant drugs stimulates hippocampal neurogenesis, and new neurons are required for some antidepressant-like behaviors. Electroconvulsive seizures (ECS), a laboratory model of electroconvulsive therapy (ECT), robustly stimulate hippocampal neurogenesis. HYPOTHESIS: ECS requires newborn neurons to improve behavioral deficits in a mouse neuroendocrine model of depression. METHODS: We utilized immunohistochemistry for doublecortin (DCX), a marker of migrating neuroblasts, to assess the impact of Sham or ECS treatments (1 treatment per day, 7 treatments over 15 days) on hippocampal neurogenesis in animals receiving 6 weeks of either vehicle or chronic corticosterone (CORT) treatment in the drinking water. We conducted tests of anxiety- and depressive-like behavior to investigate the ability of ECS to reverse CORT-induced behavioral deficits. We also determined whether adult neurons are required for the effects of ECS. For these studies we utilized a pharmacogenetic model (hGFAPtk) to conditionally ablate adult born neurons. We then evaluated behavioral indices of depression after Sham or ECS treatments in CORT-treated wild-type animals and CORT-treated animals lacking neurogenesis. RESULTS: ECS is able to rescue CORT-induced behavioral deficits in indices of anxiety- and depressive-like behavior. ECS increases both the number and dendritic complexity of adult-born migrating neuroblasts. The ability of ECS to promote antidepressant-like behavior is blocked in mice lacking adult neurogenesis. CONCLUSION: ECS ameliorates a number of anxiety- and depressive-like behaviors caused by chronic exposure to CORT. ECS requires intact hippocampal neurogenesis for its efficacy in these behavioral indices.

Atrophy of pyramidal neurons and increased stress-induced glutamate levels in CA3 following chronic suppression of adult neurogenesis.

Following their birth in the adult hippocampal dentate gyrus, newborn progenitor cells migrate into the granule cell layer where they differentiate, mature, and functionally integrate into existing circuitry. The hypothesis that adult hippocampal neurogenesis is physiologically important has gained traction, but the precise role of newborn neurons in hippocampal function remains unclear. We investigated whether loss of new neurons impacts dendrite morphology and glutamate levels in area CA3 of the hippocampus by utilizing a human GFAP promoter-driven thymidine kinase genetic mouse model to conditionally suppress adult neurogenesis. We found that chronic ablation of new neurons induces remodeling in CA3 pyramidal cells and increases stress-induced release of the neurotransmitter glutamate. The ability of persistent impairment of adult neurogenesis to influence hippocampal dendrite morphology and excitatory amino acid neurotransmission has important implications for elucidating newborn neuron function, and in particular, understanding the role of these cells in stress-related excitoxicity.

Role of activity-dependent BDNF expression in hippocampal-prefrontal cortical regulation of behavioral perseverance.

Activity-dependent gene transcription, including that of the brain-derived neurotrophic factor (Bdnf) gene, has been implicated in various cognitive functions. We previously demonstrated that mutant mice with selective disruption of activity-dependent BDNF expression (BDNF-KIV mice) exhibit deficits in GABA-mediated inhibition in the prefrontal cortex (PFC). Here, we show that disruption of activity-dependent BDNF expression impairs BDNF-dependent late-phase long-term potentiation (L-LTP) in CA1, a site of hippocampal output to the PFC. Interestingly, early-phase LTP and conventional L-LTP induced by strong tetanic stimulation were completely normal in BDNF-KIV mice. In parallel, attenuation of activity-dependent BDNF expression significantly impairs spatial memory reversal and contextual memory extinction, two executive functions that require intact hippocampal-PFC circuitry. In contrast, spatial and contextual memory per se were not affected. Thus, activity-dependent BDNF expression in the hippocampus and PFC may contribute to cognitive and behavioral flexibility. These results suggest distinct roles for different forms of L-LTP and provide a link between activity-dependent BDNF expression and behavioral perseverance, a hallmark of several psychiatric disorders.

Glucocorticoids orchestrate divergent effects on mood through adult neurogenesis.

Both social defeat stress and environmental enrichment stimulate adrenal glucocorticoid secretion, but they have opposing effects on hippocampal neurogenesis and mood. Hypothalamic-pituitary-adrenal axis dysregulation and decreased neurogenesis are consequences of social defeat. These outcomes are correlated with depressive states, but a causal role in the etiology of depression remains elusive. The antidepressant actions of environmental enrichment are neurogenesis-dependent, but the contribution of enrichment-elevated glucocorticoids is unexplored. Importantly, for both social defeat and environmental enrichment, how glucocorticoids interact with neurogenesis to alter mood is unknown. Here, we investigate causal roles of glucocorticoids and neurogenesis in induction of depressive-like behavior and its amelioration by environmental enrichment in mice. By blocking neurogenesis and surgically clamping adrenal hormone secretions, we showed that neurogenesis, via hypothalamic-pituitary-adrenal axis interactions, is directly involved in precipitating the depressive phenotype after social defeat. Mice adrenalectomized before social defeat showed enhanced behavioral resiliency and increased survival of adult-born hippocampal neurons compared with sham-operated defeated mice. However, mice lacking hippocampal neurogenesis did not show protective effects of adrenalectomy. Moreover, glucocorticoids secreted during environmental enrichment promoted neurogenesis and were required for restoration of normal behavior after social defeat. The data demonstrate that glucocorticoid-dependent declines in neurogenesis drive changes in mood after social defeat and that glucocorticoids secreted during enrichment promote neurogenesis and restore normal behavior after defeat. These data provide new evidence for direct involvement of neurogenesis in the etiology of depression, suggesting that treatments promoting neurogenesis can enhance stress resilience.

Cholinergic impact on neuroplasticity drives muscarinic M1 receptor mediated differentiation into neurons.

OBJECTIVES: Increasing evidence indicates that canonical neurotransmitters act as regulatory signals during neuroplasticity. Here, we report that muscarinic cholinergic neurotransmission stimulates differentiation of adult neural stem cells in vitro. METHODS: Adult neural stem cells (ANSC) dissociated from the adult mouse hippocampus were expanded in culture with basic fibroblast growth factor (BFGF) and epidermal growth factor (EGF). RESULTS: Carbachol (CCh), an analog of acetylcholine (ACh) significantly enhanced de novo differentiation into neurons on bFGF- and EGF-deprived stem cells as shown by the percentage of TUJ1 positive cells. By contrast, pirenzepine (PIR), a muscarinic M1 receptor antagonist, reduced the generation of neurons. CONCLUSION: Activation of cholinergic signaling drives the de novo differentiation of uncommitted stem cells into neurons. These effects appear to be predominantly mediated via the muscarinic M1 receptor subtype.

The complex role of the serotonin transporter in adult neurogenesis and neuroplasticity. A critical review.

OBJECTIVES: Studies on the serotonin transporter (SERT) with regard to neurogenesis and neuroplastic effects on the adult brain are scarce. This is intriguing since neurogenesis is believed to play a decisive role in modulating the effect of selective serotonin reuptake inhibitors (SSRI), which are targeting SERT. METHODS: Therefore, we reviewed the current scientific literature about the influence of serotonin on neurogenesis with particular emphasis on SERT in various settings, both in vivo and in vitro. RESULTS: Experiments using SERT KO (knock-out) animal models showed that SERT does not directly or indirectly influence neurogenesis in vitro, whereas compensatory mechanism seem to participate in vivo. CONCLUSION: At least with regard to adult neural stem cells, the impact of serotonin (5-HT) on neuroplasticity and neurogenesis is not due to SERT-mediated effcts. Instead, serotonergic fine-tuning may be exerted by a number of other different mechanisms including endogenous production of 5-HT in adult neural stem cells, uptake of 5-HT into adult neural stem cells by other monoamine transporters, and actions of the 5-HT1A receptors present on these cells.

Acetylcholinesterase inhibition ameliorates deficits in motivational drive.

BACKGROUND: Apathy is frequently observed in numerous neurological disorders, including Alzheimer's and Parkinson's, as well as neuropsychiatric disorders including schizophrenia. Apathy is defined as a lack of motivation characterized by diminished goal-oriented behavior and self-initiated activity. This study evaluated a chronic restraint stress (CRS) protocol in modeling apathetic behavior, and determined whether administration of an anticholinesterase had utility in attenuating CRS-induced phenotypes. METHODS: We assessed behavior as well as regional neuronal activity patterns using FosB immunohistochemistry after exposure to CRS for 6 h/d for a minimum of 21 d. Based on our FosB findings and recent clinical trials, we administered an anticholinesterase to evaluate attenuation of CRS-induced phenotypes. RESULTS: CRS resulted in behaviors that reflect motivational loss and diminished emotional responsiveness. CRS-exposed mice showed differences in FosB accumulation, including changes in the cholinergic basal forebrain system. Facilitating cholinergic signaling ameliorated CRS-induced deficits in initiation and motivational drive and rescued immediate early gene activation in the medial septum and nucleus accumbens. CONCLUSIONS: Some CRS protocols may be useful for studying deficits in motivation and apathetic behavior. Amelioration of CRS-induced behaviors with an anticholinesterase supports a role for the cholinergic system in remediation of deficits in motivational drive.

Estrogen effects on the forced swim test differ in two outbred rat strains.

Changes in reproductive hormones, such as estrogen, play a role in mood regulation. The present study examined strain differences (Long-Evans vs. Wistar-Hannover) in the behavioral and biochemical effects of estrogen manipulation. Adult ovariectomized female rats were treated with estradiol, vehicle, or withdrawn from estradiol. The two strains demonstrated differential behavioral responses to short-term estradiol administration in the forced swim test; estradiol induced an antidepressant-like effect in Long-Evans rats but not in Wistar rats. Conversely, withdrawal from estradiol resulted in a depressive-like state in the Wistar rats but not in the Long-Evans rats. Western blot analyses found no differences in estrogen receptors α and ß within the hippocampus or the frontal cortex, two brain areas strongly implicated in affective disorders. These data demonstrate the importance of strain as a variable when interpreting behavioral effects of estrogen.

Mood-stabilizing drugs: mechanisms of action.

Mood-stabilizing drugs are the most widely prescribed pharmacological treatments for bipolar disorder, a disease characterized by recurrent episodes of mania and depression. Despite extensive clinical utilization, significant questions concerning their mechanisms of action remain. In recent years, a diverse set of molecular and cellular targets of these drugs has been identified. Based on these findings, downstream effects on neural and synaptic plasticity within key circuits have been proposed. Here, we discuss recent data, identify current challenges impeding progress and define areas for future investigation. Further understanding of the primary targets and downstream levels of convergence of mood-stabilizing drugs will guide development of novel therapeutic strategies and help translate discoveries into more effective treatments with less burdensome adverse-effect profiles.

Roles of p75(NTR), long-term depression, and cholinergic transmission in anxiety and acute stress coping.

BACKGROUND: Stress is causally associated with anxiety. Although the underlying cellular mechanisms are not well understood, the basal forebrain cholinergic neurons have been implicated in stress response. p75(NTR) is a panneurotrophin receptor expressed almost exclusively in basal forebrain cholinergic neurons in adult brain. This study investigated whether and how p75(NTR), via regulation of the cholinergic system and hippocampal synaptic plasticity, influences stress-related behaviors. METHODS: We used a combination of slice electrophysiology, behavioral analyses, pharmacology, in vivo microdialysis, and neuronal activity mapping to assess the role of p75(NTR) in mood and stress-related behaviors and its underlying cellular and molecular mechanisms. RESULTS: We show that acute stress enables hippocampal long-term depression (LTD) in adult wild-type mice but not in mice lacking p75(NTR). The p75(NTR) mutant mice also exhibit two distinct behavioral impairments: baseline anxiety-like behavior and a deficit in coping with and recovering from stressful situations. Blockade of stress-enabled LTD with a GluA2-derived peptide impaired stress recovery without affecting baseline anxiety. Pharmacological manipulations of cholinergic transmission mimicked the p75(NTR) perturbation in both baseline anxiety and responses to acute stress. Finally, we show evidence of misregulated cholinergic signaling in animals with p75(NTR) deletion. CONCLUSIONS: Our results suggest that loss of p75(NTR) leads to changes in hippocampal cholinergic signaling, which may be involved in regulation of stress-enabled hippocampal LTD and in modulating behaviors related to stress and anxiety.

TrkB as a potential synaptic and behavioral tag.

Late-phase long-term potentiation (L-LTP), a cellular model for long-term memory (LTM), requires de novo protein synthesis. An attractive hypothesis for synapse specificity of long-term memory is "synaptic tagging": synaptic activity generates a tag, which "captures" the PRPs (plasticity-related proteins) derived outside of synapses. Here we provide evidence that TrkB, the receptor of BDNF (brain-derived neurotrophic factor), may serve as a "synaptic tag." TrkB is transiently activated by weak theta-burst stimulation (TBS) that induces only early-phase LTP (E-LTP). This TrkB activation is independent of protein synthesis, and confined to stimulated synapses. Induction of L-LTP by strong stimulation in one synaptic pathway converts weak TBS-induced E-LTP to L-LTP in a second, independent pathway. Transient inhibition of TrkB around the time of weak TBS to the second pathway diminished L-LTP in that pathway without affecting the first one. Behaviorally, weak training, which induces short-term memory (STM) but not LTM, can be consolidated into LTM by exposing animals to novel but not familiar environment 1 h before training. Inhibition of TrkB during STM training blocked such consolidation. These results suggest TrkB as a potential tag for synapse-specific expression of L-LTP and LTM.

Activity-dependent brain-derived neurotrophic factor expression regulates cortistatin-interneurons and sleep behavior.

BACKGROUND: Sleep homeostasis is characterized by a positive correlation between sleep length and intensity with the duration of the prior waking period. A causal role for brain-derived neurotrophic factor (BDNF) in sleep homeostasis has been suggested, but the underlying mechanisms remain unclear. Cortistatin, a neuropeptide expressed primarily in a subset of cortical GABAergic interneurons, is another molecule implicated in sleep homeostasis. RESULTS: We confirmed that sleep deprivation leads to an increase in cortical cortistatin mRNA expression. Disruption of activity-dependent BDNF expression in a genetically modified mouse line impairs both baseline levels of cortistatin mRNA as well as its levels following sleep deprivation. Disruption of activity-dependent BDNF also leads to a decrease in sleep time during the active (dark) phase. CONCLUSION: Our studies suggest that regulation of cortistatin-expressing interneurons by activity-dependent BDNF expression may contribute to regulation of sleep behavior.