Anterograde transport of endogenous brain-derived neurotrophic factor in hippocampal mossy fibers

Intervention of Brain-Derived Neurotrophic Factor and Other Neurotrophins in Adult Neurogenesis

Cell survival during adult neurogenesis and the modulation of each step, namely, proliferation, lineage differentiation, migration, maturation, and functional integration of the newborn cells into the existing circuitry, is regulated by intrinsic and extrinsic factors. Transduction of extracellular niche signals triggers the activation of intracellular mechanisms that regulate adult neurogenesis by affecting gene expression. While the intrinsic factors include transcription factors and epigenetic regulators, the extrinsic factors are molecular signals that are present in the neurogenic niche microenvironment. These include morphogens, growth factors, neurotransmitters, and signaling molecules secreted as soluble factors or associated to the extracellular matrix. Among these molecular mechanisms are neurotrophins and neurotrophin receptors which have been implicated in the regulation of adult neurogenesis at different levels, with brain-derived neurotrophic factor (BDNF) being the most studied neurotrophin. In this chapter, we review the current knowledge about the role of neurotrophins in the regulation of adult neurogenesis in both the subventricular zone (SVZ) and the hippocampal subgranular zone (SGZ).

Lack of antidepressant effects of burst-suppressing isoflurane anesthesia in adult male Wistar outbred rats subjected to chronic mild stress

Post-ictal emergence of slow wave EEG (electroencephalogram) activity and burst-suppression has been associated with the therapeutic effects of the electroconvulsive therapy (ECT), indicating that mere “cerebral silence” may elicit antidepressant actions. Indeed, brief exposures to burst-suppressing anesthesia has been reported to elicit antidepressant effects in a subset of patients, and produce behavioral and molecular alterations, such as increased expression of brain-derived neurotrophic factor (BDNF), connected with antidepressant responses in rodents. Here, we have further tested the cerebral silence hypothesis by determining whether repeated exposures to isoflurane anesthesia reduce depressive-like symptoms or influence BDNF expression in male Wistar outbred rats (Crl:WI(Han)) subjected to chronic mild stress (CMS), a model which is responsive to repeated electroconvulsive shocks (ECS, a model of ECT). Stress-susceptible, stress-resilient, and unstressed rats were exposed to 5 doses of isoflurane over a 15-day time period, with administrations occurring every third day. Isoflurane dosing is known to reliably produce rapid EEG burst-suppression (4% induction, 2% maintenance; 15 min). Antidepressant and anxiolytic effects of isoflurane were assessed after the first, third, and fifth drug exposure by measuring sucrose consumption, as well as performance on the open field and the elevated plus maze tasks. Tissue samples from the medial prefrontal cortex and hippocampus were collected, and levels of BDNF (brain-derived neurotrophic factor) protein were assessed. We find that isoflurane anesthesia had no impact on the behavior of stress-resilient or anhedonic rats in selected tests; findings which were consistent—perhaps inherently related—with unchanged levels of BDNF.

Modeling the neuron as a nanocommunication system to identify spatiotemporal molecular events in neurodegenerative disease

AIM

In tauopathies such as Alzheimer's disease (AD), molecular changes spanning multiple subcellular compartments of the neuron contribute to neurodegeneration and altered axonal signaling. Computational modeling of end-to-end linked events benefit mechanistic analysis and can be informative to understand disease progression and accelerate development of effective therapies. In the calcium-amyloid beta model of AD, calcium ions that are an important regulator of neuronal function undergo dysregulated homeostasis that disrupts cargo loading for neurotrophic signaling along axonal microtubules (MTs). The aim of the present study was to develop a computational model of the neuron using a layered architecture simulation that enables us to evaluate the functionalities of several interlinked components in the calcium-amyloid beta model.

METHODS

The elevation of intracellular calcium levels is modeled upon binding of amyloid beta oligomers (AβOs) to calcium channels or as a result of membrane insertion of oligomeric Aβ1-42 to form pores/channels. The resulting subsequent Ca2+ disruption of dense core vesicle (DCV)-kinesin cargo loading and transport of brain-derived neurotrophic factor (BDNF) on axonal MTs are then evaluated. Our model applies published experimental data on calcium channel manipulation of DCV-BDNF and incorporates organizational complexity of the axon as bundled polar and discontinuous MTs. The interoperability simulation is based on the Institute of Electrical and Electronics Engineers standard association P1906.1 framework for nanoscale and molecular communication.

RESULTS

Our analysis provides new spatiotemporal insights into the end-to-end signaling events linking calcium dysregulation and BDNF transport and by simulation compares the relative impact of dysregulation of calcium levels by AβO-channel interactions, oligomeric Aβ1-42 pores/channel formation, and release of calcium by internal stores. The flexible platform of our model allows continued expansion of molecular details including mechanistic and morphological parameters of axonal cytoskeleton networks as they become available to test disease and treatment predictions.

CONCLUSION

The present model will benefit future drug studies on calcium homeostasis and dysregulation linked to measurable neural functional outcomes. The algorithms used can also link to other multiscale multi-cellular modeling platforms to fill in molecular gaps that we believe will assist in broadening and refining multiscale computational maps of neurodegeneration.

Dysregulation of neurotrophin signaling in the pathogenesis of Alzheimer disease and of Alzheimer disease in Down syndrome

Neurotrophic factors, including the members of the neurotrophin family, play important roles in the development and maintenance of the nervous system. Trophic factor signals must be transmitted over long distances from axons and dendrites to the cell bodies of neurons. A mode of signaling well suited to the challenge of robust long distance signaling is the signaling endosome. We review the biology of signaling endosomes and the "signaling endosome hypothesis". Evidence for disruption of signaling endosome function in disorders of the nervous system is also reviewed. Changes in endosome structure in Alzheimer disease (AD) and Down syndrome (DS) are present early in these disorders. Data for the APP products responsible are reviewed and the consequent changes in signaling from endosomes discussed. We conclude by pointing to the need for additional studies to explore the biology of signaling endosomes in normal neurons and to elucidate their role in the pathogenesis of neurodegeneration.

The Contribution of Spatial and Temporal Molecular Networks in the Induction of Long-term Memory and Its Underlying Synaptic Plasticity

The ability to form long-lasting memories is critical to survival and thus is highly conserved across the animal kingdom. By virtue of its complexity, this same ability is vulnerable to disruption by a wide variety of neuronal traumas and pathologies. To identify effective therapies with which to treat memory disorders, it is critical to have a clear understanding of the cellular and molecular mechanisms which subserve normal learning and memory. A significant challenge to achieving this level of understanding is posed by the wide range of distinct temporal and spatial profiles of molecular signaling induced by learning-related stimuli. In this review we propose that a useful framework within which to address this challenge is to view the molecular foundation of long-lasting plasticity as composed of unique spatial and temporal molecular networks that mediate signaling both within neurons (such as via kinase signaling) as well as between neurons (such as via growth factor signaling). We propose that evaluating how cells integrate and interpret these concurrent and interacting molecular networks has the potential to significantly advance our understanding of the mechanisms underlying learning and memory formation.

BDNF-TRKB signaling system of the dorsal periaqueductal gray matter is implicated in the panicolytic-like effect of antidepressant drugs

A wealth of evidence implicates the BDNF-TRKB system in the therapeutic effects of antidepressant drugs (ADs) on mood disorders. However, little is known about the involvement of this system in the panicolytic property also exerted by these compounds. In the present study we evaluated the participation of the BDNF-TRKB system of the dorsal periaqueductal gray matter (DPAG), a core structure involved in the pathophysiology of panic disorder, in AD-induced panicolytic-like effects in rats. The results showed that short- (3 days) or long-term (21 days) systemic treatment with the tricyclic ADs imipramine, clomipramine or desipramine increased BDNF levels in the DPAG. Only longterm treatment with the selective serotonin reuptake inhibitor fluoxetine was able to increase BDNF levels in this structure. After 21-day treatment, fluoxetine and the three tricyclic ADs used also increased BDNF concentration in the hippocampus, a key area implicated in their mood-related actions. Neither in the DPAG nor hippocampus did long-term treatment with the standard anxiolytics diazepam, clonazepam or buspirone affect BDNF levels. Imipramine, both after short and long-term administration, and fluoxetine under the latter regimen, raised the levels of phosphorylated TRKB in the DPAG. Short-term treatment with imipramine or BDNF microinjection inhibited escape expression in rats exposed to the elevated T maze, considered as a panicolytic-like effect. This anti-escape effect was attenuated by the intra-DPAG administration of the TRK receptor antagonist k252a. Altogether, our data suggests that facilitation of the BDNF-TRKB system in the DPAG is implicated in the panicolytic effect of ADs.

Activation of postsynaptic 5-HT1A receptors improve stress adaptation

RATIONALE

Serotonin-1A (5-HT1A) receptors modulate the stress response and have been implicated in the etiology and treatment of depression and anxiety disorders. A reduction in postsynaptic 5-HT1A receptor function in limbic areas has consistently been observed following exposure to chronic stress.

OBJECTIVES

To investigate the hypothesis that increased activation of 5-HT1A receptors in rats having reduced 5-HT function may improve stress adaptation and the behavioral sequelae commonly associated with chronic stress.

METHODS

One hundred forty-four Sprague-Dawley rats received injections of para-chlorophenylalanine to partially deplete 5-HT then were given daily systemic pretreatment with the 5-HT1A receptor agonist, 8-hydroxy-2- (di-n-propylamino) tetralin (8-OH-DPAT), the antagonist, WAY 100635, or vehicle prior to either restraint stress (6 h/day for 10 daily sessions) or control conditions. Anxiety- and depressive-like behaviors were then assessed using the open field and sucrose preference tests. Protein level of hippocampal glucocorticoid receptors (GR) and mineralocorticoid receptors was detected by immunohistochemistry and brain-derived neurotrophic factor (BDNF) was determined by in situ hybridization.

RESULTS

8-OH-DPAT pretreatment prior to stress exposure attenuated later stress-induced anxiety- and depression-like behaviors and increased GR and BDNF mRNA expression in the hippocampus relative to vehicle- and WAY 100635-pretreated, stressed animals.

CONCLUSION

The stress-related impairments associated with 5-HT deficiency can be improved by 8-OH-DPAT pretreatment prior to stress exposure and are associated with an augmentation of GR-like immunoreactivity and BDNF mRNA expression in the hippocampus. It suggested that selective activation of 5-HT1A receptors may be a potential treatment strategy for stress-related disorders such as anxiety and depression.

NGF, BDNF, NT3, and NT4

The discovery of nerve growth factor (NGF) was a seminal event in history of research in developmental neurobiology. The further discovery that NGF was just one of a family of structurally similar growth factors, neurotrophins, provided important insights into the way nerve cells communicate, during development of the nervous system, and in neuroplasticity, memory, and learning in the adult nervous system. Four neurotrophins, NGF, brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT3), and neurotrophin-4 (NT4), regulate a wide variety of neural functions, acting upon p75NTR, TrkA, TrkB, and TrkC receptors.

Pre- and postsynaptic twists in BDNF secretion and action in synaptic plasticity

Overwhelming evidence collected since the early 1990's strongly supports the notion that BDNF is among the key regulators of synaptic plasticity in many areas of the mammalian central nervous system. Still, due to the extremely low expression levels of endogenous BDNF in most brain areas, surprisingly little data i) pinpointing pre- and postsynaptic release sites, ii) unraveling the time course of release, and iii) elucidating the physiological levels of synaptic activity driving this secretion are available. Likewise, our knowledge regarding pre- and postsynaptic effects of endogenous BDNF at the single cell level in mediating long-term potentiation still is sparse. Thus, our review will discuss the data currently available regarding synaptic BDNF secretion in response to physiologically relevant levels of activity, and will discuss how endogenously secreted BDNF affects synaptic plasticity, giving a special focus on spike timing-dependent types of LTP and on mossy fiber LTP. We will attempt to open up perspectives how the remaining challenging questions regarding synaptic BDNF release and action might be addressed by future experiments. This article is part of the Special Issue entitled 'BDNF Regulation of Synaptic Structure, Function, and Plasticity'.

Physical exercise and antidepressants enhance BDNF targeting in hippocampal CA3 dendrites: further evidence of a spatial code for BDNF splice variants

Brain-derived neurotrophic factor (BDNF) is encoded by multiple BDNF transcripts, whose function is unclear. We recently showed that a subset of BDNF transcripts can traffic into distal dendrites in response to electrical activity, while others are segregated into the somatoproximal domains. Physical exercise and antidepressant treatments exert their beneficial effects through upregulation of BDNF, which is required to support survival and differentiation of newborn dentate gyrus (DG) neurons. While these DG processes are required for the antidepressant effect, a role for CA1 in antidepressant action has been excluded, and the effect on CA3 neurons remains unclear. Here, we show for the first time that physical exercise and antidepressants induce local increase of BDNF in CA3. Voluntary physical exercise for 28 consecutive days, or 2-week treatment with 10 mg/kg per day fluoxetine or reboxetine, produced a global increase of BDNF mRNA and protein in the neuronal somata of the whole hippocampus and a specific increase of BDNF in dendrites of CA3 neurons. This increase was accounted for by BDNF exon 6 variant. In cultured hippocampal neurons, application of serotonin or norepinephrine (10-50 μM) induced increase in synaptic transmission and targeting of BDNF mRNA in dendrites. The increased expression of BDNF in CA3 dendrites following antidepressants or exercise further supports the neurotrophin hypothesis of antidepressants action and confirms that the differential subcellular localization of BDNF mRNA splice variants provides a spatial code for a selective expression of BDNF in specific subcellular districts. This selective expression may be exploited to design more specific antidepressants.

Impact of the NGF maturation and degradation pathway on the cortical cholinergic system phenotype

Cortical cholinergic atrophy plays a significant role in the cognitive loss seen with aging and in Alzheimer's disease (AD), but the mechanisms leading to it remain unresolved. Nerve growth factor (NGF) is the neurotrophin responsible for the phenotypic maintenance of basal forebrain cholinergic neurons in the mature and fully differentiated CNS. In consequence, its implication in cholinergic atrophy has been suspected; however, no mechanistic explanation has been provided. We have previously shown that the precursor of NGF (proNGF) is cleaved extracellularly by plasmin to form mature NGF (mNGF) and that mNGF is degraded by matrix metalloproteinase 9. Using cognitive-behavioral tests, Western blotting, and confocal and electron microscopy, this study demonstrates that a pharmacologically induced chronic failure in extracellular NGF maturation leads to a reduction in mNGF levels, proNGF accumulation, cholinergic degeneration, and cognitive impairment in rats. It also shows that inhibiting NGF degradation increases endogenous levels of the mature neurotrophin and increases the density of cortical cholinergic boutons. Together, the data point to a mechanism explaining cholinergic loss in neurodegenerative conditions such as AD and provide a potential therapeutic target for the protection or restoration of this CNS transmitter system in aging and AD.

The influence of naturalistic experience on plasticity markers in somatosensory cortex and hippocampus: effects of whisker use

We have previously demonstrated that exposure of adult rat to a type of enriched environment, known as 'naturalistic habitat' (NH), induces extensive functional plasticity in the whiskers' representations within the primary somatosensory cortex. Here we have investigated the molecular basis for such functional plasticity involved in this model. Based on the role of BDNF on synaptic plasticity and neuronal growth, the focus of this study is on BDNF and its downstream effectors CREB, synapsin I, and GAP-43. In particular, we determined the effects of natural whisker use during 2, 7 or 28 days exposure to a NH on barrel cortex and hippocampus, as compared to standard cage controls. Naturalistic whisker use resulted in increased levels of mRNAs and proteins for BDNF and its downstream effectors. Level changes for these markers were already detected after 2 days in the naturalistic habitat and grew larger over longer exposures (7 and 28 days). The cerebral cortex was found to be sensitive to the naturalistic habitat exposure at all time points, and more sensitive than the hippocampus to the trimming of the whiskers. Trimming of the whiskers decreased the level of most of the markers under study, suggesting that whiskers exert a tonic influence on plasticity markers that can be further enhanced by naturalistic use. These results implicate BDNF and its downstream effectors in the plasticity induced by the naturalistic habitat. The critical action of experience on molecular substrates of plasticity seems to provide molecular basis for the design of experienced-based rehabilitative strategies to enhance brain function.

Altering BDNF expression by genetics and/or environment: impact for emotional and depression-like behaviour in laboratory mice

According to the "neurotrophin hypothesis", brain-derived neurotrophic factor (BDNF) is an important candidate gene in depression. Moreover, environmental stress is known to represent a risk factor in the pathophysiology and treatment of this disease. To elucidate, whether changes of BDNF availability signify cause or consequence of depressive-like alterations, it is essential to look for endophenotypes under distinct genetic conditions (e.g. altered BDNF expression). Furthermore it is crucial to examine environment-driven BDNF regulation and its effect on depressive-linked features. Consequently, gene × environment studies investigating prospective genetic mouse models of depression in different environmental contexts become increasingly important. The present review summarizes recent findings in BDNF-mutant mice, which have been controversially discussed as models of depression and anxiety. It furthermore illustrates the potential of environment to serve as naturalistic stressor with the potential to modulate the phenotype in wildtype and mutant mice. Moreover, environment may exert protective effects by regulating BDNF levels as attributed to "environmental enrichment". The effect of this beneficial condition will also be discussed with regard to probable "curative/therapeutic" approaches.

BDNF signaling in the formation, maturation and plasticity of glutamatergic and GABAergic synapses

In the past 15 years numerous reports provided strong evidence that brain-derived neurotrophic factor (BDNF) is one of the most important modulators of glutamatergic and GABAergic synapses. Remarkable progress regarding localization, kinetics, and molecular mechanisms of BDNF secretion has been achieved, and a large number of studies provided evidence that continuous extracellular supply of BDNF is important for the proper formation and functional maturation of glutamatergic and GABAergic synapses. BDNF can play a permissive role in shaping synaptic networks, making them more susceptible for the occurrence of plastic changes. In addition, BDNF appears to be also an instructive factor for activity-dependent long-term synaptic plasticity. BDNF release just in response to synaptic stimulation might be a molecular trigger to convert high-frequency synaptic activity into long-term synaptic memories. This review attempts to summarize the current knowledge in synaptic secretion and synaptic action of BDNF, including both permissive and instructive effects of BDNF in synaptic plasticity.

Mechanisms, locations, and kinetics of synaptic BDNF secretion: an update

Brain-derived neurotrophic factor (BDNF) and other members of the protein family of neurotrophins have been implicated in a multitude of processes that are important for neuronal development and synaptic plasticity in the rodent central nervous system. In comparison to the wealth of information available with respect to the biological functions of neurotrophins, our knowledge regarding the processes that govern synaptic secretion of neurotrophins is scarce. Using live cell imaging of GFP-tagged neurotrophins in primary neurons, immunocytochemical detection of endogenous BDNF in fixed cells, and by blocking the action of endogenously released BDNF by means of TrkB receptor bodies in living neurons, several studies in recent years have allowed to better understand the time course and the mechanisms of synaptic secretion of neurotrophins. This review will summarize the current knowledge regarding the intracellular processing of proneurotrophins, the targeting of neurotrophin vesicles to axons and dendrites, and the mechanisms of activity-dependent secretion of BDNF at synapses. Since these processes are known to be cell type dependent, special emphasis is given to observations gained from experiments in primary neurons.

Targeting the hippocampal mossy fiber synapse for the treatment of psychiatric disorders

It is widely known that new neurons are continuously generated in the dentate gyrus of the hippocampus in the adult mammalian brain. This neurogenesis has been implicated in depression and antidepressant treatments. Recent evidence also suggests that the dentate gyrus is involved in the neuropathology and pathophysiology of schizophrenia and other related psychiatric disorders. Especially, abnormal neuronal development in the dentate gyrus may be a plausible risk factor for the diseases. The synapse made by the mossy fiber, the output fiber of the dentate gyrus, plays a critical role in regulating neuronal activity in its target CA3 area. The mossy fiber synapse is characterized by remarkable activity-dependent short-term synaptic plasticity that is established during the postnatal development and is supposed to be central to the functional role of the mossy fiber. Any defects, including developmental abnormalities, in the dentate gyrus and drugs acting on the dentate gyrus can modulate the mossy fiber-CA3 synaptic transmission, which may eventually affect hippocampal functions. In this paper, I review recent evidence for involvement of the dentate gyrus and mossy fiber synapse in psychiatric disorders and discuss potential importance of drugs targeting the mossy fiber synapse either directly or indirectly in the therapeutic treatments of psychiatric disorders.

Electroconvulsive therapy and its different indications

In spite of recent developments in the pharmacotherapy of depressive disorders, the delay until clinical improvement can be achieved, and the considerable rate of nonresponse and nonremission, are major problems which remain unresolved. Electroconvulsive therapy (ECT) is a nonpharmacologic biological treatment which has been proven to be a highly effective treatment option, predominantly for depression, but also for schizophrenia and other indications. Though there is a lack of controlled investigations on long-term treatments, ECT can also be used for relapse prevention during maintenance therapies. The safety and tolerability of electroconvulsive treatment have been enhanced by the use of modified stimulation techniques and by progress in modern anesthesia. Thus, today a safe treatment can also be offered to patients with higher somatic risks. ECT still represents an important option, especially in the therapy of treatment-resistant psychiatric disorders after medication treatment failures. Earlier consideration of ECT may reduce the rate of chronic and difficult-to-treat psychiatric disorders.

Is it time to reassess the BDNF hypothesis of depression?

The brain-derived neurotrophic factor (BDNF) hypothesis of depression postulates that a loss of BDNF is directly involved in the pathophysiology of depression, and that its restoration may underlie the therapeutic efficacy of antidepressant treatment. While this theory has received considerable experimental support, an increasing number of studies have generated evidence which is not only inconsistent, but also directly contradicts the hypothesis. This article provides a critical review of the clinical and preclinical studies which have been responsible for this controversy, outlining pharmacological, behavioural and genetic evidence which demonstrates the contrasting role of BDNF in regulating mood and antidepressant effects throughout the brain. I will also review key studies, both human and animal, which have investigated the association of a BDNF single-nucleotide polymorphism (Val66Met) with depression pathogenesis, and detail the number of inconsistencies which also afflict this novel area of BDNF research. The article will conclude by discussing why now is a critical time to reassess the original BDNF hypothesis of depression, and look towards the formation of new models that can provide a more valid account of the complex relationships between growth factors, mood disorders and their treatment.

Laser capture microdissection and microarray analysis of dividing neural progenitor cells from the adult rat hippocampus

Neural progenitor cells reside in the hippocampus of adult rodents and humans and generate granule neurons throughout life. Knowledge about the molecular processes regulating these neurogenic cells is fragmentary. In order to identify genes with a role in the proliferation of adult neural progenitor cells, a protocol was elaborated to enable the staining and isolation of such cells under RNA-preserving conditions with a combination of immunohistochemistry and laser capture microdissection. We increased proliferation of neural progenitor cells by electroconvulsive treatment, one of the most effective antidepressant treatments, and isolated Ki-67-positive cells using this new protocol. RNA amplification via in vitro transcription and subsequent microarray analysis revealed over 100 genes that were differentially expressed in neural progenitor cells due to electroconvulsive treatment compared to untreated control animals. Some of these genes have already been implicated in the functioning of neural progenitor cells or have been induced by electroconvulsive treatment; these include brain-derived neurotrophic factor (Bdnf), PDZ-binding kinase (Pbk) and abnormal spindle-like microcephaly-associated (Aspm). In addition, genes were identified for which no role in the proliferation of neurogenic progenitors has been described so far, such as enhancer of zeste homolog 2 (Ezh2).

Electroconvulsive seizures stimulate glial proliferation and reduce expression of Sprouty2 within the prefrontal cortex of rats

BACKGROUND

Reductions in cell number are found within the medial prefrontal cortex (PFC) in major depression and bipolar disorder, conditions for which electroconvulsive therapy (ECT) is a highly effective treatment. We investigated whether electroconvulsive seizure (ECS) in rats stimulates cellular proliferation in the PFC immediately and four weeks after the treatments. In parallel, we examined if ECS also alters the expression of Sprouty2 (SPRY2), an inhibitor of cell proliferation.

METHODS

Sprague-Dawley rats received 10 days of ECS treatments and bromodeoxyuridine (BrdU) injections. After a four week survival period, we estimated the density and number of BrdU-, proliferating cell nuclear antigen (PCNA)-, and SPRY2-immunoreactive cells in the medial (infralimbic) PFC (ILPFC). We also determined the percentage of BrdU-labeled cells that were immunoreactive for markers specific to oligodendrocytes, astrocytes, endothelial cells and neurons.

RESULTS

ECS dramatically enhanced the proliferation of new cells in the infralimbic PFC, and this effect persisted four weeks following the treatments. The percentage of new cells expressing oligodendrocyte precursor cell markers increased slightly following ECS. In contrast, ECS dramatically reduced the number of cells expressing SPRY2.

CONCLUSIONS

ECS stimulates long-lasting increases in glial proliferation within the ILPFC. ECS also decreases SPRY2 expression in the same region, an effect that might contribute to increased glial proliferation.

Brain-derived neurotrophic factor and its receptor tropomyosin-related kinase B in the mechanism of action of antidepressant therapies

The focus of this review is to critically examine and review the literature on the role of brain-derived neurotrophic factor (BDNF) and its primary receptor, tropomyosin-related kinase B (TrkB), in the actions of pharmacologically diverse antidepressant treatments for depression. This will include a review of the studies on the regulation of BDNF and TrkB by different types of antidepressant drug treatments and animal in models of depression, as well as altered levels of BDNF and TrkB in the blood and postmortem brain of patients with depression. Results from clinical and basic studies have demonstrated that stress and depression decrease BDNF expression and neurogenesis and antidepressant treatment reverses or blocks these effects, leading to the neurotrophic hypothesis of depression. Clinical studies demonstrate an association between BDNF levels and several disorders, including depression, epilepsy, bipolar disorder, Parkinson's and Alzheimer's diseases. Physical activity and diet exert neurotrophic effects and positively modulate BDNF levels. A common single nucleotide polymorphism (SNP) in the BDNF gene, a methionine substitution for valine, is associated with alterations in brain anatomy and memory, but what role it has in clinical disorders is unclear. Findings suggest that early childhood events and adult stress produce neurodegenerative alterations in the brain that can eventually cause breakdown of information processing in the neuronal networks regulating mood. Antidepressant treatments elevate activity-dependent neuronal plasticity by activating BDNF, thereby gradually restoring network function and ultimately mood.

Costorage of BDNF and neuropeptides within individual dense-core vesicles in central and peripheral neurons

Some central and peripheral neurons synthesize brain-derived neurotrophic factor (BDNF), and, after anterograde transport, release it at synapses. By immunocytochemistry, we examined, in rat and mouse, the subcellular localization of BDNF and BDNF/peptide coexistence, under normal conditions or after intrathecal infusion of nerve growth factor. In dorsal root ganglion neurons and afferent terminals, and in the parabrachial projection to amygdala, we show that BDNF is costored in individual dense-core vesicles (DCVs) with the neuropeptides calcitonin gene-related peptide (CGRP) and substance P. At both locations, nerve endings costoring all three peptides were fairly rare. Remarkably however, costorage occurred in a stoichiometric ratio of 0.7 BDNF:1 CGRP:1 substance P, and DCVs contained 31 (spinal cord) -36 (amygdala) times the amount of BDNF detected in agranular vesicles. This is the first direct demonstration in peripheral and central neurons from two different mammals, that a growth factor is selectively packaged together with neuropeptide transmitters within individual DCVs. It provides structural bases for differential release upon stimulation, and has important implications for understanding BDNF transmitter function.

Mossy fiber synaptic transmission: communication from the dentate gyrus to area CA3

Communication between the dentate gyrus (DG) and area CA3 of the hippocampus proper is transmitted via axons of granule cells--the mossy fiber (MF) pathway. In this review we discuss and compare the properties of transmitter release from the MFs onto pyramidal neurons and interneurons. An examination of the anatomical connectivity from DG to CA3 reveals a surprising interplay between excitation and inhibition for this circuit. In this respect it is particularly relevant that the major targets of the MFs are interneurons and that the consequence of MF input into CA3 may be inhibitory or excitatory, conditionally dependent on the frequency of input and modulatory regulation. This is further complicated by the properties of transmitter release from the MFs where a large number of co-localized transmitters, including GABAergic inhibitory transmitter release, and the effects of presynaptic modulation finely tune transmitter release. A picture emerges that extends beyond the hypothesis that the MFs are simply "detonators" of CA3 pyramidal neurons; the properties of synaptic information flow from the DG have more subtle and complex influences on the CA3 network.

Neurotrophins in the dentate gyrus

Since the discovery of nerve growth factor (NGF) in the 1950s and brain-derived neurotrophic factor (BDNF) in the 1980s, a great deal of evidence has mounted for the roles of neurotrophins (NGF; BDNF; neurotrophin-3, NT-3; and neurotrophin-4/5, NT-4/5) in development, physiology, and pathology. BDNF in particular has important roles in neural development and cell survival, as well as appearing essential to molecular mechanisms of synaptic plasticity and larger scale structural rearrangements of axons and dendrites. Basic activity-related changes in the central nervous system (CNS) are thought to depend on BDNF modulation of synaptic transmission. Pathologic levels of BDNF-dependent synaptic plasticity may contribute to conditions such as epilepsy and chronic pain sensitization, whereas application of the trophic properties of BDNF may lead to novel therapeutic options in neurodegenerative diseases and perhaps even in neuropsychiatric disorders. In this chapter, I review neurotrophin structure, signal transduction mechanisms, localization and regulation within the nervous system, and various potential roles in disease. Modulation of neurotrophin action holds significant potential for novel therapies for a variety of neurological and psychiatric disorders.

A neurotrophic model for stress-related mood disorders

There is a growing body of evidence demonstrating that stress decreases the expression of brain-derived neurotrophic factor (BDNF) in limbic structures that control mood and that antidepressant treatment reverses or blocks the effects of stress. Decreased levels of BDNF, as well as other neurotrophic factors, could contribute to the atrophy of certain limbic structures, including the hippocampus and prefrontal cortex that has been observed in depressed subjects. Conversely, the neurotrophic actions of antidepressants could reverse neuronal atrophy and cell loss and thereby contribute to the therapeutic actions of these treatments. This review provides a critical examination of the neurotrophic hypothesis of depression that has evolved from this work, including analysis of preclinical cellular (adult neurogenesis) and behavioral models of depression and antidepressant actions, as well as clinical neuroimaging and postmortem studies. Although there are some limitations, the results of these studies are consistent with the hypothesis that decreased expression of BDNF and possibly other growth factors contributes to depression and that upregulation of BDNF plays a role in the actions of antidepressant treatment.

What is the biological significance of BDNF mRNA targeting in the dendrites? Clues from epilepsy and cortical development

The neurotrophin brain-derived neurotrophic factor (BDNF) is a regulatory factor of several, partially contrasting, aspects of the biology of neural cells, including survival, growth, differentiation, and cell death. Regulation of the local availability of BDNF at distinct subcellular domains such as the cell soma, dendrites, axons, and spines appears to be the key to conferring spatial and temporal specificity of the different effects elicited by this neurotrophin. This article reviews recent findings in the context of epileptogenesis and visual cortex maturation that showed that different BDNF messenger RNA (mRNA) transcripts are localized at different subcellular locations in hippocampal and cortical neurons. It also reviews findings demonstrating that strong depolarizing stimuli, both in vitro and in vivo, elicit accumulation of BDNF mRNA and protein in the distal dendrites through a signaling pathway involving the activation of the N-methyl-D-aspartate and tyrosine kinase B receptors and an intracellular increase in Ca2+ concentration. Finally, this article proposes that the regulation of the delivery of BDNF mRNA and protein to the different subcellular domains--particularly the dendritic compartment--may represent a fundamental aspect of the processes of cellular and synaptic morphological rearrangements underlying epileptogenesis and postnatal development of the visual cortex.

A role for AMPA receptors in mood disorders

Major antidepressant agents increase synaptic levels of monoamines. Although the monoamine hypothesis of depression remains a cornerstone of our understanding of the pathophysiology of depression, emerging data has suggested that the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor subtype of glutamate receptor may also play a pivotal role in depression. Positive allosteric modulators of AMPA receptors increase brain levels of brain-derived neurotrophic factor (BDNF) that impacts the viability and generation of neurons in key brain structures. AMPA receptor potentiators are active in rodent models predictive of antidepressant efficacy. The mechanisms by which AMPA receptor potentiators produce these biological effects, however, are uncertain. Current evidence points to an antidepressant mechanism that is independent of monoaminergic facilitation that is driven by neurogenesis, a process facilitated by increased BDNF expression. However, alternative hypotheses need to be considered given uncertainties in the relationship between BDNF increases and the effects of conventional antidepressant medications. Electrophysiological and protein conformational data indicate that structural variants of AMPA receptor potentiators can differentially modulate AMPA receptor-mediated currents, although the manner in which this impacts antidepressant efficacy is yet to be understood. Conventional antidepressants such as fluoxetine positively modulate AMPA receptors. This potentiation is engendered by specific phosphorylation pathways activated through the dopamine- and cAMP-regulated phosphoprotein of Mr 32,000 (DARPP-32). Other novel compounds with antidepressant-like effects in rodents may also produce their in vivo effects through potentiation of AMPA receptors. Thus, AMPA receptor potentiation might be a general mechanism through which the clinical outcome of antidepressant efficacy is achieved.

Repeated electroconvulsive stimuli increase brain-derived neurotrophic factor in ACTH-treated rats

Electroconvulsive therapy is considered to be an effective treatment for severe depression. We have already shown that the antidepressant-like effects of tricyclic antidepressants in the rat forced swim test are blocked by repeated treatment with adrenocorticotropic hormone (ACTH). In the present study, we investigated the effect of repeated electroconvulsive stimuli on the forced swim test and on brain-derived neurotrophic factor (BDNF) protein levels in ACTH-treated rats. Electroconvulsive stimuli (50 mA, 0.2 s) was administered 30 min after ACTH treatment (100 microg/rat, s.c.) once daily for 14 days. In both saline and ACTH-treated rats, repeated electroconvulsive stimuli for 6 or 14 days decreased the immobility time in the forced swim test and increased the BDNF protein levels in the hippocampus. However, repeated imipramine administration (10 mg/kg, i.p. for 14 days) had no effect on the hippocampus BDNF protein levels in ACTH-treated rats. These results suggest that electroconvulsive stimuli has decreasing effects of immobility time in the forced swim test in the tricyclic antidepressant-resistant depressive model of rats induced by repeated ACTH treatment, and that increased BDNF may be involved in this phenomenon.

Antidepressant action: to the nucleus and beyond

After decades of effort, the field of depression research is far from understanding how antidepressant drugs mediate their clinical effects. The time lag of 2-6 weeks of therapy that is necessary to obtain antidepressant efficacy indicates a requirement for long-term regulation of molecules activated by drug treatment. The focus of antidepressant research has thus expanded from examining acute monoamine-mediated mechanisms to include long-term transcriptional regulators such as cAMP response element-binding protein (CREB) and trophic factors such as brain-derived nerve growth factor and insulin-like growth factor. In addition, the recent discovery of antidepressant-induced neurogenesis provides another avenue by which antidepressants might exert their effects. Current efforts are aimed at understanding how CREB and trophic factor signaling pathways are coupled to neurogenic effects and how alterations in behavioral, molecular and cellular endpoints are related to the alleviation of the symptoms of depression.

Plasma concentrations of neuroactive steroids before and after electroconvulsive therapy in major depression

There is evidence that both cerebrospinal fluid (CSF) and plasma concentrations of 3alpha-reduced neuroactive steroids are decreased in major depressive disorder. Successful antidepressant pharmacotherapy, for example, with selective serotonin reuptake inhibitors (SSRIs), over several weeks is accompanied by an increase in CSF and plasma concentrations of these neuroactive steroids. However, no such increase has been observed during nonpharmacological treatments such as partial sleep deprivation or repetitive transcranial magnetic stimulation. In order to investigate whether concentration changes in neuroactive steroids are an important component of clinically effective antidepressant treatment, we examined plasma concentrations of the neuroactive steroids 3alpha,5alpha-tetrahydroprogesterone, 3alpha,5beta-tetrahydroprogesterone, 3beta,5alpha-tetrahydroprogesterone, and their precursors progesterone, 5alpha-dihydroprogesterone, and 5beta-dihydroprogesterone in 31 pharmacotherapy-resistant depressed in-patients before and after unilateral electroconvulsive therapy (ECT) as a monotherapy over 4 weeks. Samples were quantified for neuroactive steroids by means of a highly sensitive and specific combined gas chromatography/mass spectrometry analysis. In all, 51.6% of the patients were treatment responders. There was no influence of ECT on the plasma concentrations of any neuroactive steroid studied. Moreover, neuroactive steroid levels did not differ between treatment responders and nonresponders. Our study shows that changes in neuroactive steroid plasma levels are not a mandatory factor for successful antidepressant treatment by ECT. Thus, the previously observed changes in plasma concentrations of neuroactive steroids following treatment with antidepressants such as SSRIs more likely reflect distinct pharmacological properties of these compounds rather than clinical improvement.

Cellular mechanisms underlying the effects of an early experience on cognitive abilities and affective states

In the present study we investigated the effects of neonatal handling, an animal model of early experience, on spatial learning and memory, on hippocampal glucocorticoid (GR), mineralocorticoid (MR) and type 1A serotonin (5-HT1A) receptors, as well as brain derived neurotrophic factor (BDNF), and on circulating leptin levels, of male rats.

METHOD

Spatial learning and memory following an acute restraint stress (30 min) were assessed in the Morris water maze. Hippocampal GR, MR and BDNF levels were determined immunocytochemically. 5-HT1A receptors were quantified by in vitro binding autoradiography. Circulating leptin levels, following a chronic forced swimming stress, were measured by radioimmunoassay (RIA). Data were statistically analyzed by analysis of variance (ANOVA).

RESULTS

Neonatal handling increased the ability of male rats for spatial learning and memory. It also resulted in increased GR/MR ratio, BDNF and 5-HT1A receptor levels in the hippocampus. Furthermore, leptin levels, body weight and food consumption during chronic forced swimming stress were reduced as a result of handling.

CONCLUSION

Neonatal handling is shown to have a beneficial effect in the males, improving their cognitive abilities. This effect on behavior could be mediated by the handling-induced increase in hippocampal GR/MR ratio and BDNF levels. The handling-induced changes in BDNF and 5-HT1A receptors could underlie the previously documented effect of handling in preventing "depression". Furthermore, handling is shown to prevent other maladaptive states such as stress-induced hyperphagia, obesity and resistance to leptin.

Fluoxetine-induced change in rat brain expression of brain-derived neurotrophic factor varies depending on length of treatment

Recent studies indicate that brain-derived neurotrophic factor (BDNF) may be implicated in the clinical action of antidepressant drugs. Repeated (2-3 weeks) administration of antidepressant drugs increases BDNF gene expression. The onset of this response as well as concomitant effects on the corresponding BDNF protein is however, unclear. The present study investigated the effects of acute and chronic administration of the selective serotonin reuptake inhibitor, fluoxetine (10mg/kg p.o.), upon regional rat brain levels of BDNF mRNA and protein expression. To improve the clinical significance of the study, fluoxetine was administered orally and mRNA and protein levels were determined ex vivo using the techniques of in situ hybridisation histochemistry and immunocytochemistry respectively. Direct measurement of BDNF protein was also carried out using enzyme-linked immunosorbent assay (ELISA). Four days of once daily oral administration of fluoxetine induced decreases in BDNF mRNA (hippocampus, medial habenular and paraventricular thalamic nuclei). Whilst 7 days of treatment showed a non-significant increase in BDNF mRNA, there were marked and region-specific increases following 14 days of treatment. BDNF protein levels remained unaltered until 21 days of fluoxetine treatment, when the numbers of BDNF immunoreactive cells were increased, reaching significance in the pyramidal cell layer of CA1 and CA3 regions of Ammon's horn (CA1 and CA3) but not in the other sub-regions of the hippocampus. Indicative of the highly regional change within the hippocampus, the ELISA method failed to demonstrate significant up-regulation at 21 days, measuring levels of BDNF protein in the whole hippocampus. In contrast to the detected time dependent and biphasic response of the BDNF gene, activity-regulated, cytoskeletal-associated protein (Arc) mRNA showed a gradual increase during the 14-day course of treatment. The results presented here show that BDNF is expressed differentially depending on length of fluoxetine administration, which could contribute in explaining the slow onset of antidepressant activity observed with selective serotonin reuptake inhibitors.

Mutant mouse models of depression: Candidate genes and current mouse lines

Depression is a multifactorial and multigenetic disease. At present, three main theories try to conceptualize its molecular and biochemical mechanisms, namely the monoamine-, the hypothalamus-pituitary-adrenal- (HPA-) system- and the neurotrophin-hypotheses. One way to explore, validate or falsify these hypotheses is to alter the expression of genes that are involved in these systems and study their respective role in animal behavior and neuroendocrinological parameters. Following an introduction in which we briefly describe each hypothesis, we review here the different mouse lines generated to study the respective molecular pathways. Among the many mutant lines generated, only a few can be regarded as genetic depression models or as models of predisposition for a depressive syndrome after stress exposure. However, this is likely to reflect the human situation where depressive syndromes are complex, can vary to a great extent with respect to their symptomatology, and may be influenced by a variety of environmental factors. Mice with mutations of candidate genes showing depression-like features on behavioral or neurochemical levels may help to define a complex molecular framework underlying depressive syndromes. Because it is conceivable that manipulation of one single genetic function may be necessary but not sufficient to cause complex behavioral alterations, strategies for improving genetic modeling of depression-like syndromes in animals possibly require a simultaneous targeted dysregulation of several genes involved in the pathogenesis of depression. This approach would correspond to the new concept of 'endophenotypes' in human depression research trying to identify behavioral traits which are thought to be encoded by a limited set of genes.

Neurotrophin expression in the hippocampus and cerebellum is affected by chronic placental insufficiency in the late gestational ovine fetus

Our aim was to determine the effects of chronic placental insufficiency (CPI) during late gestation on the expression of neurotrophic factors and their receptors in the hippocampus and cerebellum in the near-term fetus. Structural alterations were also assessed in these brain regions. CPI was induced in eight fetal sheep by umbilicoplacental embolization (UPE) from 120 to 140 days of gestation (term approximately 147d) such that fetal arterial O2 saturation (SaO2) was maintained at approximately 50% of pre-UPE values. Five non-UPE fetuses served as controls. UPE resulted in fetal hypoxemia, hypoglycaemia, and growth restriction. In hippocampi from UPE fetuses, there were reductions in the optical density (OD) of the immunoreactivity (IR) of brain-derived neurotrophic factor (BDNF) protein within the mossy fibre collaterals of the polymorphic layer and in stratum lucidum (p<0.05); there was no consistent effect on tyrosine-related kinase (Trk) B receptor or neurotrophin-3 (NT-3) expression. Within the cerebellum, there was an increase in BDNF-IR (p<0.05) in the molecular layer; however, Trk B-IR and NT-3-IR were unaltered. There were no significant alterations to the structural parameters measured in the hippocampus. We conclude that CPI in late gestation affects the expression of BDNF in the fetal hippocampus and cerebellum, but these changes do not have a well-defined relationship to structural outcome.

A role for glia in the action of electroconvulsive therapy

In this article, we consider possible mechanisms of action for electroconvulsive therapy (ECT) based on evidence regarding cellular changes in affective and psychotic illnesses. Postmortem investigations of major depression and schizophrenia have revealed abnormalities in the number of neuronal and glial cells. Such cellular changes could indicate a perturbed balance between neurogenesis and neuronal death in the adult brain. Psychotropic drugs and ECT have been shown to stimulate neurogenesis, giving rise to the hypothesis that this generation of new cells mediates some of their therapeutic effect. A possible trophic effect on glial cells has not been examined. Since glial cells are essential for proper neuronal function, treatments that alter glial function would have significant effects on brain function. We suggest that the effectiveness of ECT is, in part, related to its effect on glial cells. This testable hypothesis may advance our understanding and treatment of psychiatric disorders.

Anterograde transport of neurotrophic factors: possible therapeutic implications

The actions of neurotrophic factors are classically thought to be mediated by their retrograde transport from target tissues to the cell bodies. There is now evidence that specific trophic factors are trafficked anterogradely along peripheral and central axons and released to postsynaptic cells. This review focuses on recent experiments that demonstrate the involvement of the anterograde transfer of neurotrophic factors in various physiological processes, including the regulation of developmental neuronal death, the modulation of synaptic transmission, and the control of axonal and dendritic architecture. The authors also discuss whether anterograde transport of exogenous trophic factors can be exploited to protect damaged postsynaptic neurons and spare their function. This issue has clear implications for possible therapeutic applications of neurotrophic factors.

Ontogeny of seizure-induced increases in BDNF immunoreactivity and TrkB receptor activation in rat hippocampus

The present work tested the hypothesis that the anatomic and developmental patterns of status epilepticus-induced increases of brain-derived neurotrophic factor (BDNF) protein coincided with status epilepticus-induced increases of phospho-Trk immunoreactivity, a measure of TrkB receptor activation, in rat hippocampus. In P22 rats, robust increases of phospho-Trk immunoreactivity were detected in the mossy fiber pathway of the hippocampus one day following kainate-induced status epilepticus. Conversely, no change in phospho-Trk immunoreactivity was detected in P8 or P14 rats. In P17 rats, intermediate levels of increased phospho-Trk immunoreactivity were detected, again in the mossy fiber pathway. Like phospho-Trk immunoreactivity, marked increases of BDNF immunoreactivity were detected in the mossy fiber pathway of P22 but not P14 rats. Dissociations were found in P17 rats following status epilepticus in that striking increases of BDNF, but not phospho-Trk immunoreactivity were detected. Immunoprecipitation and Western blot analyses of hippocampal extracts after status epilepticus showed increased phospho-TrkB, but not TrkB immunoreactivity in P22 rats, thereby confirming and extending the immunohistochemical findings. While most of the findings support the hypothesis, important dissociations among individual animals at P17 were identified. Together the findings are consistent with the proposal that status epilepticus-induced increase of BDNF content in the mossy fibers is necessary, but not sufficient, to effect activation of TrkB, as revealed by phospho-Trk immunoreactivity. Furthermore, these results provide the first characterization of seizure-induced increases in BDNF protein and TrkB receptor activation in developing animals.

Role of neurotrophic factors in the etiology and treatment of mood disorders

Basic research in rodents has demonstrated that exposure to stress decreases levels of brain-derived neurotrophic factor (BDNF) in brain regions associated with depression. In contrast, antidepressant treatment produces the opposite effect and blocks the effects of stress on BDNF. BDNF upregulation and possibly other neurotrophic/growth factors could reverse or block the atrophy and cell loss that has been observed in rodent stress models and in depressed patients. The morphological alterations observed in depressed patients could result from decreased size or number of glia and/or neurons and may include regulation of adult neurogenesis. This article reviews the primary work leading to a neurotrophic hypothesis of depression and antidepressant action and the cellular mechanisms and signal transduction pathways that underlie these effects.

Role of neurotrophic factors in the etiology and treatment of mood disorders

Basic research in rodents has demonstrated that exposure to stress decreases levels of brain-derived neurotrophic factor (BDNF) in brain regions associated with depression. In contrast, antidepressant treatment produces the opposite effect and blocks the effects of stress on BDNF. BDNF upregulation and possibly other neurotrophic/growth factors could reverse or block the atrophy and cell loss that has been observed in rodent stress models and in depressed patients. The morphological alterations observed in depressed patients could result from decreased size or number of glia and/or neurons and may include regulation of adult neurogenesis. This article reviews the primary work leading to a neurotrophic hypothesis of depression and antidepressant action and the cellular mechanisms and signal transduction pathways that underlie these effects.

What is the importance of multivesicular bodies in retrograde axonal transport in vivo?

Neurons with long axons have a unique problem in generating signaling cascades that are able to reach the nucleus after receptor activation by neurotrophins at the nerve terminal. The straightforward concept of receptor binding and local generation of 2nd second messenger cascades is too simplistic. In this review we will outline a mechanism that would enable the complex signals generated at the nerve terminal to be conveyed intact to the cell body. There are three different sites in the neuron where 2nd messenger proteins can interact with the signaling complex and be activated. Signaling cascades are initiated both at the nerve terminal and at the cell body when 2nd messengers are recruited to the plasma membrane by activated receptors. After receptor-mediated endocytosis, 2nd messenger molecules continue to be recruited to the internalized vesicle; however, the mix of proteins differs in the nerve terminal and in the cell body. At the nerve terminal the activated pathways result in the formation of the neurotrophin signaling endosome, which includes molecules to be retrogradely transported to the cell body. When the retrograde neurotrophin signaling endosome reaches the cell body, it can recruit additional 2nd messenger molecules to finally generate the unique signal derived from the nerve terminal. We propose that the multivesicular body observed in vivo functions as an endosome carrier vehicle or retrosome. This retrosome enables the mix of signaling molecules recruited at the terminal to be transported intact to the cell body. This will allow the cell body to receive a snapshot of the events occurring at the nerve terminal at the time the retrosome is formed.

Persistent regional increases in brain-derived neurotrophic factor in the flurothyl model of epileptogenesis are dependent upon the kindling status of the animal

Brain-derived neurotrophic factor (BDNF) appears to be both regulated by and a regulator of epileptogenesis. In the flurothyl (HFE) model of kindling mice exposed to successive flurothyl trials over 8 days express a rapid, long-lasting reduction in generalized seizure threshold and a more slowly evolving change in seizure phenotype in response to subsequent flurothyl exposure. The BDNF genotype of particular mouse strains appears to influence the epileptogenic progression in this model. Thus, we hypothesized that BDNF signaling pathways are altered by flurothyl-induced seizures. Following HFE kindling, fully kindled (eight seizures) adult male C57BI/6J mice had significantly elevated whole brain BDNF levels through at least 28 days after their final seizure. Mice that received only four HFE seizures (not kindled) had elevated BDNF levels, but only at 1 day post-seizure (DPSz), while BDNF levels were not significantly altered in mice receiving just one HFE seizure at any time point studied. Regional expression patterns of BDNF in the hippocampus, hypothalamus, and frontal cortex were also elevated by one DPSz and returned to control values by 14 DPSz in mice that received four HFE seizures. No changes were seen in the cerebellum, striatum, or piriform cortex. In contrast, fully kindled mice had significantly elevated BDNF levels within the hippocampus, hypothalamus, neocortex, and striatum that remained elevated through at least 14 DPSz, while levels were unchanged in the cerebellum and piriform cortex. Regional results were confirmed using anti-BDNF immunohistochemistry (IHC). Despite changes in BDNF levels following HFE kindling, we were unable to demonstrate alterations either in full-length tyrosine kinase receptor B (TrkB) expression (Western blot and IHC) or in truncated TrkB (IHC) expression levels. Together, these data suggest a model of a positive feedback loop involving seizure activity and seizure number and persistently modified BDNF signaling pathways that influences seizure phenotypes within the HFE kindling paradigm. Thus, long-term elevations in BDNF may be responsible in part for epileptogenic processes and the development of human refractory epilepsies.

Spike Train Timing-Dependent Associative Modification of Hippocampal CA3 Recurrent Synapses by Mossy Fibers

In the CA3 region of the hippocampus, extensive recurrent associational/commissural (A/C) connections made by pyramidal cells may function as a network for associative memory storage and recall. We here report that long-term potentiation (LTP) at the A/C synapses can be induced by association of brief spike trains in mossy fibers (MFs) from the dentate gyrus and A/C fibers. This LTP not only required substantial overlap between spike trains in MFs and A/C fibers, but also depended on the temporal order of these spike trains in a manner not predicted by the well-known rule of spike timing-dependent plasticity and requiring activation of type 1 metabotropic glutamate receptors. Importantly, spike trains in a putative single MF input provided effective postsynaptic activity for the induction of LTP at A/C synapses. Thus, the timing of spike trains in individual MFs may code information that is crucial for the associative modification of CA3 recurrent synapses.

The Role of BDNF in Epilepsy and Other Diseases of the Mature Nervous System

The neurotrophin brain-derived neurotrophic factor (BDNF) is ubiquitous in the central nervous system (CNS) throughout life. In addition to trophic effects on target neurons, BDNF appears to be part of a general mechanism for activity-dependent modification of synapses in the developing and adult nervous system. Thus, diseases of abnormal trophic support (such as neurodegenerative diseases) and diseases of abnormal excitability (such as epilepsy and central pain sensitization) can be related in some cases to abnormal BDNF signaling. For example, various studies have shown that BDNF is upregulated in areas implicated in epileptogenesis, and interference with BDNF signal transduction inhibits the development of the epileptic state. Further study of the cellular and molecular mechanisms by which BDNF influences cell survival and excitability will likely provide novel concepts and targets for the treatment of diverse CNS diseases.

Expression of BDNF mRNA in substantia nigra is dependent on target integrity and independent of neuronal activation

We have analyzed the regulation of brain-derived neurotrophic factor (BDNF) mRNA expression in the nigrostriatal system following neurotoxin ablation of striatal targets by means of kainate (KA) or quinolinic acid (QA) injections. Loss of nigral target cells in the striatum was accompanied by significant induction of BDNF mRNA levels in the ipsilateral substantia nigra (SN) at 12 and 24 h post lesion. Dual tyrosine hydroxylase (TH) and BDNF mRNA in situ hybridization (ISH) confirmed the dopaminergic nature of the BDNF mRNA expressing cells. Analysis of neuronal activity in terms of cFos mRNA expression demonstrated intense induction of this marker in the ipsilateral SN pars reticulata (SNPR), but not in SN pars compacta. Dual glutamic acid decarboxylase (GAD) and cFos mRNA ISH confirmed this view. Colchicine injections into the medial forebrain bundle to specifically disrupt neuronal trafficking between SN and striatum induced BDNF mRNA levels in the ipsilateral SNPC, thus demonstrating that nigral expression of BDNF mRNA is dependent of striatal target tissue. In addition, we found significant elevations of BDNF in the subthalamic nucleus following striatal excitotoxic lesion, which may bring novel roles of BDNF in the basal ganglia complex.