Brain-derived neurotrophic factor rapidly potentiates synaptic transmission through NMDA, but suppresses it through non-NMDA receptors in rat hippocampal neuron

The role of NMDA receptor in neurobiology and treatment of major depressive disorder: Evidence from translational research

There is accumulating evidence demonstrating that dysfunction of glutamatergic neurotransmission, particularly via N-methyl-d-aspartate (NMDA) receptors, is involved in the pathophysiology of major depressive disorder (MDD). Several studies have revealed an altered expression of NMDA receptor subtypes and impaired NMDA receptor-mediated intracellular signaling pathways in brain circuits of patients with MDD. Clinical studies have demonstrated that NMDA receptor antagonists, particularly ketamine, have rapid antidepressant effects in treatment-resistant depression, however, neurobiological mechanisms are not completely understood. Growing body of evidence suggest that signal transduction pathways involved in synaptic plasticity play critical role in molecular mechanisms underlying rapidly acting antidepressant properties of ketamine and other NMDAR antagonists in MDD. Discovering the molecular mechanisms underlying the unique antidepressant actions of ketamine will facilitate the development of novel fast acting antidepressants which lack undesirable effects of ketamine. This review provides a critical examination of the NMDA receptor involvement in the neurobiology of MDD including analyses of alterations in NMDA receptor subtypes and their interactive signaling cascades revealed by postmortem studies. Furthermore, to elucidate mechanisms underlying rapid-acting antidepressant properties of NMDA receptor antagonists we discussed their effects on the neuroplasticity, mostly based on signaling systems involved in synaptic plasticity of mood-related neurocircuitries.

BDNF acting in the hypothalamus induces acute pressor responses under permissive control of angiotensin II

Brain-derived neurotrophic factor (BDNF) expression increases in the paraventricular nucleus of the hypothalamus (PVN) during hypertensive stimuli including stress and hyperosmolarity, but its role in PVN cardiovascular regulatory mechanisms is unclear. Chronic BDNF overexpression in the PVN has been shown to elevate sympathetic tone and blood pressure in part by modulating central angiotensin (Ang) II mechanisms. However, the cardiovascular effects of short-term increases in PVN levels of BDNF and the mechanisms governing them are unknown. Therefore, we investigated whether acute BDNF microinjections into the PVN of conscious and anesthetized Sprague-Dawley rats induce blood pressure elevations and whether Ang II signaling is involved in these hypertensive responses. In conscious rats, unilateral BDNF (12.5ng) microinjections into the PVN increased mean arterial pressure (MAP) by 27±1mmHg (P<0.001 vs vehicle), which was significantly attenuated by intracerebroventricular infusion of the Ang II-type-1 receptor (AT1R) antagonist losartan and by ganglionic blockade with intravenous hexamethonium infusion. In anesthetized rats, unilateral PVN microinjection of BDNF increased MAP by 31±4mmHg (P<0.001 vs vehicle), which was prevented by PVN microinjection pretreatments with the high-affinity BDNF receptor TrkB antagonist ANA-12, losartan, the angiotensin converting enzyme inhibitor lisinopril, or by intravenous hexamethonium. Additional experiments in hypothalamic samples including the PVN revealed that BDNF-induced TrkB receptor phosphorylation was prevented by ANA-12 and losartan pretreatments. Collectively, these data indicate that BDNF acting within the PVN acutely raises blood pressure under permissive control of Ang II-AT1R mechanisms and therefore may play an important role in mediating acute pressor responses to hypertensive stimuli.

Stress, depression and Parkinson's disease

In this review, we focus on the relationship among Parkinson's disease (PD), stress and depression. Parkinson's disease patients have a high risk of developing depression, and it is possible that stress contributes to the development of both pathologies. Stress dysfunction may have a role in the etiology of preclinical non-motor symptoms of PD (such as depression) and, later in the course of the disease, may worsen motor symptoms. However, relatively few studies have examined stress or depression and the injured nigrostriatal system. This review discusses the effects of stress on neurodegeneration and depression, and their association with the symptoms and progression of PD.

The combined effects of exercise and foods in preventing neurological and cognitive disorders

OBJECTIVE

Exercise and select diets have important influences on health and plasticity of the nervous system, and the molecular mechanisms involved with these actions are starting to be elucidated. New evidence indicates that exercise, in combination with dietary factors, exerts its effects by affecting molecular events related to the management of energy metabolism and synaptic plasticity.

METHODS

Published studies in animals and humans describing the effects of exercise and diets in brain plasticity and cognitive abilities are discussed.

RESULTS

New evidence indicates that exercise and select diets exert their effects by affecting molecular events related to the management of energy metabolism and synaptic plasticity. An important instigator in the molecular machinery stimulated by exercise is brain-derived neurotrophic factor (BDNF), which acts at the interface of metabolism and plasticity.

CONCLUSIONS

Recent studies show that selected dietary factors share similar mechanisms with exercise, and in some cases they can complement the action of exercise. Therefore, exercise and dietary management appear as a non-invasive and effective strategy to counteract neurological and cognitive disorders.

EphA activation overrides the presynaptic actions of BDNF

The adult pattern of neural connectivity is shaped by repulsive and attractive factors, many of which are modulated by activity. Although much is known about the actions of these factors when studied in isolation, little is known about how they interact. To address this question, we examined the effects of sequential or coapplication of brain-derived neurotrophic factor (BDNF) and Fc-conjugated ephrin-A5 or EphA5 in cultured embryonic hippocampal neurons. BDNF promotes neurite outgrowth and synapse formation, and when applied acutely, it elicits an increase in ongoing synaptic activity. Members of the ephrin family of ligands and receptors can be repulsive and prevent formation of synaptic contacts. Acute exposure to either ephrin-A5-Fc or EphA5-Fc transiently enhanced synaptic activity when applied alone, but when applied prior to BDNF, they dramatically reduced the electrophysiological effects of the neurotrophin. Conversely, BDNF had no effect on subsequently applied ephrin-A5-Fc or EphA5-Fc. Consistent with this, ephrin-A5-Fc also prevented BDNF-induced activation of p42/44 MAPK. The effect of ephrin-A5-Fc appears to be presynaptic, as it prevented the BDNF-induced increase in spontaneous miniature postsynaptic current frequency, whereas EphA5-Fc did not. These results suggest that these factors can be categorized differently, with the contact-mediated activation of EphA receptors by ephrin-A5 overriding the diffusion-mediated effect of BDNF.

Circadian and developmental regulation of N-methyl-d-aspartate-receptor 1 mRNA splice variants and N-methyl-d-aspartate-receptor 3 subunit expression within the rat suprachiasmatic nucleus

The circadian rhythms of mammals are generated by the circadian clock located in the suprachiasmatic nucleus (SCN) of the hypothalamus. Its intrinsic period is entrained to a 24 h cycle by external cues, mainly by light. Light impinging on the SCN at night causes either advancing or delaying phase shifts of the circadian clock. N-methyl-d-aspartate receptors (NMDAR) are the main glutamate receptors mediating the effect of light on the molecular clockwork in the SCN. They are composed of multiple subunits, each with specific characteristics whose mutual interactions strongly determine properties of the receptor. In the brain, the distribution of NMDAR subunits depends on the region and developmental stage. Here, we report the circadian expression of the NMDAR1 subunit in the adult rat SCN and depict its splice variants that may constitute the functional receptor channel in the SCN. During ontogenesis, expression of two of the NMDAR1 subunit splice variants, as well as the NMDAR3A and 3B subunits, exhibits developmental loss around the time of eye opening. Moreover, we demonstrate the spatial and developmental characteristics of the expression of the truncated splice form of NMDAR1 subunit NR1-E in the brain. Our data suggest that specific properties of the NMDAR subunits we describe within the SCN likely influence the photic transduction pathways mediating the clock entrainment. Furthermore, the developmental changes in NMDAR composition may contribute to the gradual postnatal maturation of the entrainment pathways.

BDNF modulation of NMDA receptors is activity dependent

Brain-derived neurotrophic factor (BDNF), a potent modulator of synaptic transmission, is known to influence associative synaptic plasticity and refinement of neural connectivity. We now show that BDNF modulation of glutamate currents in hippocampal neurons exhibits the additional property of use dependence, a postsynaptic mechanism resulting in selective modulation of active channels. We demonstrate selectivity by varying the repetition rate of iontophoretically applied glutamate pulses during BDNF exposure. During relatively high-frequency glutamate pulses (0.1 Hz), BDNF application elicited a doubling of the glutamate current. During low-frequency pulses (0.0033 Hz), however, BDNF evoked a dramatically diminished response. This effect was apparently mediated by calcium because manipulations that prevented elevation of intracellular calcium largely eliminated the action of BDNF on glutamate currents. To confirm N-methyl-D-aspartate (NMDA) receptor involvement and assess spatial requirements, we made cell-attached single-channel recordings from somatic NMDA receptors. Inclusion of calcium in the pipette was sufficient to produce enhancement of channel activity by BDNF. Substitution of EGTA for calcium prevented BDNF effects. We conclude that BDNF modulation of postsynaptic NMDA receptors requires concurrent neuronal activity potentially conferring synaptic specificity on the neurotrophin's actions.

BDNF regulates the expression and traffic of NMDA receptors in cultured hippocampal neurons

The neurotrophin BDNF regulates the activity-dependent modifications of synaptic strength in the CNS. Physiological and biochemical evidences implicate the NMDA glutamate receptor as one of the targets for BDNF modulation. In the present study, we investigated the effect of BDNF on the expression and plasma membrane abundance of NMDA receptor subunits in cultured hippocampal neurons. Acute stimulation of hippocampal neurons with BDNF differentially upregulated the protein levels of the NR1, NR2A and NR2B NMDA receptor subunits, by a mechanism sensitive to transcription and translation inhibitors. Accordingly, BDNF also increased the mRNA levels for NR1, NR2A and NR2B subunits. The neurotrophin NT3 also upregulated the protein levels of NR2A and NR2B subunits, but was without effect on the NR1 subunit. The amount of NR1, NR2A and NR2B proteins associated with the plasma membrane of hippocampal neurons was differentially increased by BDNF stimulation for 30 min or 24 h. The rapid upregulation of plasma membrane-associated NMDA receptor subunits was correlated with an increase in NMDA receptor activity. The results indicate that BDNF increases the abundance of NMDA receptors and their delivery to the plasma membrane, thereby upregulating receptor activity in cultured hippocampal neurons.

Clinical relevance of the neurotrophins and their receptors

The neurotrophins are growth factors required by discrete neuronal cell types for survival and maintenance, with a broad range of activities in the central and peripheral nervous system in the developing and adult mammal. This review examines their role in diverse disease states, including Alzheimer's disease, depression, pain and asthma. In addition, the role of BDNF (brain-derived neurotrophic factor) in synaptic plasticity and memory formation is discussed. Unlike the other neurotrophins, BDNF is secreted in an activity-dependent manner that allows the highly controlled release required for synaptic regulation. Evidence is discussed which shows that sequestration of NGF (nerve growth factor) is able to reverse symptoms of inflammatory pain and asthma in animal models. Both pain and asthma show an underlying pathophysiology linked to increases in endogenous NGF and subsequent NGF-dependent increase in BDNF. Conversely, in Alzheimer's disease, there is a role for NGF in the treatment of the disease and a recent clinical trial has shown benefit from its exogenous application. In addition, reductions in BDNF, and changes in the processing and usage of NGF, are evident and it is possible that both NGF and BDNF play a part in the aetiology of the disease process. This highly selective choice of functions and disease states related to neurotrophin function, although in no way comprehensive, illustrates the importance of the neurotrophins in the brain, the peripheral nervous system and in non-neuronal tissues. Ways in which the neurotrophins, their receptors or agonists/antagonists may act therapeutically are discussed.

Revenge of the “Sit”: How lifestyle impacts neuronal and cognitive health through molecular systems that interface energy metabolism with neuronal plasticity

Exercise, a behavior that is inherently associated with energy metabolism, impacts the molecular systems important for synaptic plasticity and learning and memory. This implies that a close association must exist between these systems to ensure proper neuronal function. This review emphasizes the ability of exercise and other lifestyle implementations that modulate energy metabolism, such as diet, to impact brain function. Mechanisms believed to interface metabolism and cognition seem to play a critical role with the brain derived neurotrophic factor (BDNF) system. Behaviors concerned with activity and metabolism may have developed simultaneously and interdependently during evolution to determine the influence of exercise and diet on cognition. A look into our evolutionary past indicates that our genome remains unchanged from the times of our hunter-gatherer ancestors, whose active lifestyle predominated throughout almost 100% of humankind's existence. Consequently, the sedentary lifestyle and eating behaviors enabled by the comforts of technologic progress may be reaping "revenge" on the health of both our bodies and brains. In the 21st century we are confronted by the ever-increasing incidence of metabolic disorders in both the adult and child population. The ability of exercise and diet to impact systems that promote cell survival and plasticity may be applicable for combating the deleterious effects of disease and ageing on brain health and cognition.

Novel Alternative Splicing Predicts a Truncated Isoform of the NMDA Receptor Subunit 1 (NR1) in Embryonic Rat Brain

The expression of mesencephalic brain derived neurotrophic factor (BDNF) has been shown to be regulated by dopaminergic neuronal functioning and glutamate receptors (GluRs). In turn, BDNF participates in the regulation of mesencephalic GluRs' expression. In the present study we analyzed, using semi-quantitative RT-PCR, the effect of BDNF as well as of the GluRs agonists NMDA and trans-(+/-)-1-Amino-(1S,3R)-cyclopentane dicarboxylic acid (t-ACPD), on the expression levels of the NMDA GluR subunit 1 (NR1) mRNA, using rat cultured mesencephalic neurons. In the course of this study, a novel rat mRNA splice variant of NR1 was identified. This new NR1 mRNA isoform is characterized by the insertion of an 82 base pair intron containing an inframe stop codon, thus predicting the expression of a putative truncated protein of 465 amino acids. The RT-PCR and in situ hybridization reveals that the novel NR1 mRNA is expressed in various brain regions of the rat embryo, whereas no expression was detected in the adult rat brain. The modulation of the novel NR1 mRNA isoform by both BDNF and the metabotropic GluR agonist t-ACPD, suggests that the resulting putative NR1 truncated protein may be relevant in the regulatory network of glutamatergic neurotransmission in the developing central nervous system.

From modulator to mediator: rapid effects of BDNF on ion channels

Neurotrophins (NTs) are [?AUTHOR] a family of structurally related, secreted proteins that regulate the survival, differentiation and maintenance of function of different populations of peripheral and central neurons.1,2 Among these, BDNF (brain-derived neurotrophic factor) has drawn considerable interest because both its synthesis and secretion are increased by physiological levels of activity, indicating a unique role of this neurotrophin in coupling neuronal activity to structural and functional properties of neuronal circuits. In addition to its classical neurotrophic effects, which are evident within hours or days and which usually result from changes in cellular gene expression, BDNF exerts acute effects on synaptic transmission and is involved in the induction of long-term potentiation. Many of these rapid effects of BDNF are mediated by its modulation of ion channel properties following TrkB-mediated activation of intracellular second messenger cascades and protein phosphorylation. However, recent reports have shown that BDNF not only acts as a modulator of ion channels, but can also directly and rapidly gate a Na(+) channel, thereby assigning BDNF the properties of a classical excitatory transmitter. Thus, BDNF, in addition its role as a potent neuromodulator, emerges as an excitatory transmitter-like substance which acutely controls resting membrane potential, neuronal excitability, synaptic transmission and participates in the induction of synaptic plasticity.

A novel mechanism for the facilitation of theta-induced long-term potentiation by brain-derived neurotrophic factor

Brain-derived neurotrophic factor (BDNF) contributes to the induction of long-term potentiation (LTP) by theta-pattern stimulation, but the specific processes underlying this effect are not known. Experiments described here, using BDNF concentrations that have minor effects on baseline responses, show that the neurotrophin both reduces the threshold for LTP induction and elevates the ceiling on maximal potentiation. The enhanced LTP proved to be as stable and resistant to reversal as that recorded under control conditions. BDNF markedly increased the facilitation of burst responses that occurs within a theta train. This suggests that the neurotrophin acts on long-lasting events that (1) are set in motion by the first burst in a train and (2) regulate the amplitude of subsequent bursts. Whole-cell recordings established that BDNF causes a rapid reduction in the size of the long-lasting afterhyperpolarization (AHP) that follows individual theta bursts. Apamin, an antagonist of type 2 small-conductance Ca2+-activated potassium (SK2) channels, also reduced hippocampal AHPs and closely reproduced the effects of BDNF on theta-burst responses and LTP. The latter results were replicated with a newly introduced, highly selective inhibitor of SK2 channels. Immunoblot analyses indicated that BDNF increases SK2 serine phosphorylation in hippocampal slices. These findings point to the conclusion that BDNF-driven protein kinase cascades serve to depress the SK2 component, and possibly other constituents, of the AHP. It is likely that this mechanism, acting with other factors, promotes the formation and increases the magnitude of LTP.

Alterations in IGF-I, BDNF and NT-3 levels following experimental brain trauma and the effect of IGF-I administration

The effects of a unilateral, penetrating brain trauma on IGF-I, BDNF and NT-3 were studied immunocytochemically in the rat. BDNF and NT-3 were decreased in the peritraumatic area, but increased in the adjacent region, 4 and 12 h post-injury. One week following the trauma, BDNF remained low in the peritraumatic area, but was restored to normal levels in the adjacent, while no effect of injury on NT-3 levels was detected in either area. Injury resulted in an increase in IGF-I levels in the peritraumatic area, which was most pronounced 1 week following the trauma, indicating that IGF-I could participate in endogenous repair processes. We thus administered IGF-I immediately following the trauma and investigated its effects on injury-induced changes in neurotrophin levels. Administration of IGF-I partially reversed the injury-induced decrease in BDNF and NT-3 in the peritraumatic area observed 4 and 12 h post-injury, while at the same time-points, it completely cancelled the effects of injury in the adjacent region. One week after the trauma, BDNF levels were dramatically increased in both the peritraumatic and adjacent area, reaching levels even higher than those of the sham-operated animals, following IGF-I administration. Our results showing that IGF-I not only counteracts injury-induced changes in neurotrophins, but can also further increase their levels, indicate that this growth factor could mediate repair and/or protective processes, following brain trauma.

Protective effect of green tea polyphenol EGCG against neuronal damage and brain edema after unilateral cerebral ischemia in gerbils

Previous studies have demonstrated that a green tea polyphenol, (-)-epigallocatechine gallate (EGCG), has a potent free radical scavenging and antioxidant effect. Glutamate leads to excitotoxicity and oxidative stress, which are important pathophysiologic responses to cerebral ischemia resulting in brain edema and neuronal damage. We investigated the effect of EGCG on excitotoxic neuronal damage in a culture system and the effect on brain edema formation and lesion after unilateral cerebral ischemia in gerbils. In vitro, excitotoxicity was induced by 24-hr incubation with N-methyl-D-aspartate (NMDA; 10 microM), AMPA (10 microM), or kainate (20 microM). EGCG (5 microM) was added to the culture media alone or with excitotoxins. We examined malondialdehyde (MDA) level and neuronal viability to evaluate the effect of EGCG. In vivo, unilateral cerebral ischemia was induced by occlusion of the right common carotid artery for 30, 60, or 90 min and followed by reperfusion of 24 hr. Brain edema, MDA, and infarction were examined to evaluate the protective effect of EGCG. EGCG (25 or 50 mg/kg, intraperitoneally) was administered twice, at 30 min before and immediately after ischemia. EGCG reduced excitotoxin-induced MDA production and neuronal damage in the culture system. In the in vivo study, treatment of gerbils with the lower EGCG dose failed to show neuroprotective effects; however, the higher EGCG dose attenuated the increase in MDA level caused by cerebral ischemia. EGCG also reduced the formation of postischemic brain edema and infarct volume. These results demonstrate EGCG may have future possibilities as a neuroprotective agent against excitotoxicity-related neurologic disorders such as brain ischemia.

BDNF enhancement of postsynaptic NMDA receptors is blocked by ethanol

The neurotrophin brain-derived neurotrophic factor (BDNF) modulates several distinct aspects of synaptic transmission. Physiological and biochemical evidence implicates the NMDA glutamate receptor as one of the targets for BDNF modulation. In the present studies, murine brain slices containing hippocampus and neocortex were used to study the effects of BDNF on excitatory neurotransmission. Acute exposure to BDNF rapidly and reversibly enhanced the magnitude of NMDA-mediated, but not AMPA receptor-mediated, synaptic currents, specifically enhancing the activity of NMDA receptors containing the NR2B subunit. This effect of BDNF was dependent on activation of trkB neurotrophin receptors because similar effects were not seen with the related neurotrophins NT-3 or NGF. Furthermore, activation of trkB receptors in the postsynaptic neuron was required, as BDNF-induced potentiation was blocked by postsynaptic injection of a trk tyrosine kinase inhibitor. Interestingly, the effect of BDNF was also completely blocked by pretreatment with ethanol, even at concentrations of ethanol that had minimal direct effects on NMDA-mediated responses. These results provide a potential mechanism for the proposed role for BDNF in activity-dependent synaptic plasticity and, potentially, learning and memory processes.

Early maternal deprivation reduces the expression of BDNF and NMDA receptor subunits in rat hippocampus

It is well accepted that events that interfere with the normal program of neuronal differentiation and brain maturation may be relevant for the etiology of psychiatric disorders, setting the stage for synaptic disorganization that becomes functional later in life. In order to investigate molecular determinants for these events, we examined the modulation of the neurotrophin brain-derived neurotrophic factor (BDNF) and the glutamate NMDA receptor following 24 h maternal separation (MD) on postnatal day 9. We found that in adulthood the expression of BDNF as well as of NR-2A and NR-2B, two NMDA receptor forming subunits, were significantly reduced in the hippocampus of MD rats whereas, among other structures, a slight reduction of NR-2A and 2B was detected only in prefrontal cortex. These changes were not observed acutely, nor in pre-weaning animals. Furthermore we found that in MD rats the modulation of hippocampal BDNF in response to an acute stress was altered, indicating a persistent functional impairment in its regulation, which may subserve a specific role for coping with challenging situations. We propose that adverse events taking place during brain maturation can modulate the expression of molecular players of cellular plasticity within selected brain regions, thus contributing to permanent alterations in brain function, which might ultimately lead to an increased vulnerability for psychiatric diseases.

Interplay between brain-derived neurotrophic factor and signal transduction modulators in the regulation of the effects of exercise on synaptic-plasticity

This study was designed to identify molecular mechanisms by which exercise affects synaptic-plasticity in the hippocampus, a brain area whose function, learning and memory, depends on this capability. We have focused on the central role that brain-derived neurotrophic factor (BDNF) may play in mediating the effects of exercise on synaptic-plasticity. In fact, this impact of exercise is exemplified by our finding that BDNF regulates the mRNA levels of two end products important for neural function, i.e. cAMP-response-element binding (CREB) protein and synapsin I. CREB and synapsin I have the ability to modify neuronal function by regulating gene-transcription and affecting synaptic transmission, respectively. Furthermore, we show that BDNF is capable of concurrently increasing the mRNA levels of both itself and its tyrosine kinaseB (TrkB) receptor, suggesting that exercise may employ a feedback loop to augment the effects of BDNF on synaptic-plasticity. The use of a novel microbead injection method in our blocking experiments and Taqman reverse transcription polymerase reaction (RT-PCR) for RNA quantification, have enabled us to evaluate the contribution of different pathways to the exercise-induced increases in the mRNA levels of BDNF, TrkB, CREB, and synapsin I. We found that although BDNF mediates exercise-induced hippocampal plasticity, additional molecules, i.e. the N-methyl-D-aspartate receptor, calcium/calmodulin protein kinase II and the mitogen-activated protein kinase cascade, modulate its effects. Since these molecules have a well-described association to BDNF action, our results illustrate a basic mechanism through which exercise may promote synaptic-plasticity in the adult brain.

EGCG attenuates AMPA-induced intracellular calcium increase in hippocampal neurons

We have investigated the protective effect of (-)-epigallocatechin gallate (EGCG) on alpha-amino-3-hydroxy-5-methyl-4-isoxazolo propionate (AMPA)-induced toxicity in cultured rat hippocampal neurons. Treatment of 24 h AMPA (10 microM) reduced the neuronal viability in both survival neuron counting and MTT reduction assay compared with control, with increase in cellular concentrations of hydrogen peroxide and malondialdehyde. These responses to AMPA were significantly blocked by co-treatments with EGCG (10 microM), which effect was very similar to the protective ability of a known antioxidant catalase (2000 U/ml). AMPA (50 microM) elicited the increase in intracellular calcium concentration ([Ca(2+)]i) on which EGCG significantly attenuated both peak amplitude and sustained nature of that [Ca(2+)]i increase in a dose-dependent manner. These data suggest that EGCG has a neuroprotective effect against AMPA through inhibition of AMPA-induced [Ca(2+)]i increase and consequent attenuation of reactive oxygen species production and lipid peroxidation as an antioxidant and a radical scavenger.

From acquisition to consolidation: on the role of brain-derived neurotrophic factor signaling in hippocampal-dependent learning

One of the most rigorously investigated problems in modern neuroscience is to decipher the mechanisms by which experience-induced changes in the central nervous system are translated into behavioral acquisition, consolidation, retention, and subsequent recall of information. Brain-derived neurotrophic factor (BDNF) has recently emerged as one of the most potent molecular mediators of not only central synaptic plasticity, but also behavioral interactions between an organism and its environment. Recent experimental evidence indicates that BDNF modulates synaptic transmission and plasticity by acting across different spatial and temporal domains. BDNF signaling evokes both short- and long-term periods of enhanced synaptic physiology in both pre- and postsynaptic compartments of central synapses. Specifically, BDNF/TrkB signaling converges on the MAP kinase pathway to enhance excitatory synaptic transmission in vivo, as well as hippocampal-dependent learning in behaving animals. Emerging concepts of the intracellular signaling cascades involved in synaptic plasticity induced through environmental interactions resulting in behavioral learning further support the contention that BDNF/TrkB signaling plays a fundamental role in mediating enduring changes in central synaptic structure and function. Here we review recent literature showing the involvement of BDNF/TrkB signaling in hippocampal-dependent learning paradigms, as well as in the types of cellular plasticity proposed to underlie learning and memory.

Brain-derived neurotrophic factor delays hippocampal kindling in the rat

Epileptic seizures increase the expression of brain-derived neurotrophic factor in the hippocampus. Since this neurotrophin exerts modulatory effects on neuronal excitability in this structure, it may play an important role in hippocampal epileptogenesis. This question was addressed by studying the effects of chronic infusions of recombinant brain-derived neurotrophic factor and brain-derived neurotrophic factor antisense in the hippocampus during the first seven days of hippocampal kindling. Infusion with brain-derived neurotrophic factor (6-24 microg/day) significantly delayed the progression of standard hippocampal kindling and strongly suppressed seizures induced by rapid hippocampal kindling. These suppressive effects were dose dependent, long lasting, not secondary to neuronal toxicity and specific to this neurotrophin, as nerve growth factor accelerated hippocampal kindling progression. They also appeared to be specific to the hippocampus, as infusion of brain-derived neurotrophic factor (48 microg/day) in the amygdala only resulted in a slight and transient delay of amygdala kindling. Conversely to the protective effects of exogenous brain-derived neurotrophic factor, chronic hippocampal infusion of antisense oligodeoxynucleotides (12 nmol/day), resulting in reduced expression of endogenous brain-derived neurotrophic factor in the hippocampus, aggravated seizures during hippocampal kindling. Taken together, our results lead us to suggest that the seizure-induced increase in brain-derived neurotrophic factor expression in the hippocampus may constitute an endogenous regulatory mechanism able to restrain hippocampal epileptogenesis.

Brain-derived neurotrophic factor increases activity of NR2B-containing N-methyl-D-aspartate receptors in excised patches from hippocampal neurons

Growth factors, including members of the neurotrophin gene family, play a central role in the regulation of neuronal survival and differentiation during development. In addition to these relatively long-term actions of neurotrophins, recent studies have shown that these factors also rapidly modulate synaptic transmission. Brain-derived neurotrophic factor (BDNF), in particular, regulates both pre- and postsynaptic aspects of hippocampal synaptic transmission. The postsynaptic effects include an increase in glutamate responsiveness, mediated by the N-methyl-D-aspartate (NMDA) glutamate receptor subtype. It is not clear, however, where BDNF-trkB signal transduction is initiated, because trkB receptors are located in both pre- and postsynaptic membranes. In the present study, we used excised membrane patches from cultured hippocampal neurons to determine whether BDNF directly modulates postsynaptic NMDA receptor activity. The results indicate that acute exposure to BDNF increases NMDA single channel open probability via postsynaptic trkB receptors and that this effect is dependent on the presence of the NR2B subunit of the NMDA receptor.

Brain-derived neurotrophic factor differentially regulates excitatory and inhibitory synaptic transmission in hippocampal cultures

Brain-derived neurotrophic factor (BDNF) has been postulated to be a key signaling molecule in regulating synaptic strength and overall circuit activity. In this context, we have found that BDNF dramatically increases the frequency of spontaneously initiated action potentials in hippocampal neurons in dissociated culture. Using analysis of unitary synaptic transmission and immunocytochemical methods, we determined that chronic treatment with BDNF potentiates both excitatory and inhibitory transmission, but that it does so via different mechanisms. BDNF strengthens excitation primarily by augmenting the amplitude of AMPA receptor-mediated miniature EPSCs (mEPSCs) but enhances inhibition by increasing the frequency of mIPSC and increasing the size of GABAergic synaptic terminals. In contrast to observations in other systems, BDNF-mediated increases in AMPA-receptor mediated mEPSC amplitudes did not require activity, because blocking action potentials with tetrodotoxin for the entire duration of BDNF treatment had no effect on the magnitude of this enhancement. These forms of synaptic regulations appear to be a selective action of BDNF because intrinsic excitability, synapse number, and neuronal survival are not affected in these cultures. Thus, although BDNF induces a net increase in overall circuit activity, this results from potentiation of both excitatory and inhibitory synaptic drive through distinct and selective physiological mechanisms.

Brain-derived neurotrophic factor acutely inhibits AMPA-mediated currents in developing sensory relay neurons

Brain-derived neurotrophic factor (BDNF) is expressed by many primary sensory neurons that no longer require neurotrophins for survival, indicating that BDNF may be used as a signaling molecule by the afferents themselves. Because many primary afferents also express glutamate, we investigated the possibility that BDNF modulates glutamatergic AMPA responses of newborn second-order sensory relay neurons. Perforated-patch, voltage-clamp recordings were made from dissociated neurons of the brainstem nucleus tractus solitarius (nTS), a region that receives massive primary afferent input from BDNF-containing neurons in the nodose and petrosal cranial sensory ganglia. Electrophysiological analysis was combined in some experiments with anterograde labeling of primary afferent terminals to specifically analyze responses of identified second-order neurons. Our data demonstrate that BDNF strongly inhibits AMPA-mediated currents in a large subset of nTS cells. Specifically, AMPA responses were either completely abolished or markedly inhibited by BDNF in 73% of postnatal day (P0) cells and in 82% of identified P5 second-order sensory relay neurons. This effect of BDNF is mimicked by NT-4, but not NGF, and blocked by the Trk tyrosine kinase inhibitor K252a, consistent with a requirement for TrkB receptor activation. Moreover, analysis of TrkB expression in culture revealed a close correlation between the percentage of nTS neurons in which BDNF inhibits AMPA currents and the percentage of neurons that exhibit TrkB immunoreactivity. These data document a previously undefined mechanism of acute modulation of AMPA responses by BDNF and indicate that BDNF may regulate glutamatergic transmission at primary afferent synapses.

Identification of two persistently activated neurotrophin-regulated pathways in rat hippocampus

Brain-derived neurotrophic factor contributes profoundly to modulate activity-dependent synaptic plasticity in adult brain areas such as the hippocampus, but the mechanisms underlying this important role still remain unclear. Recently, we have shown that two serine/threonine kinases, calcium/calmodulin-dependent protein kinase-2 and casein kinase-2, are capable of mediating brain-derived neurotrophic factor responses in adult rat hippocampus. In the present study, using hippocampal slices from adult rat, we show that phospholipase C-regulated calcium signals couple the brain-derived neurotrophic factor receptor to two distinct pathways: a pathway in which calcium/calmodulin-dependent protein kinase-2 stimulates a signalling module involving the p38 subfamily of mitogen-activated protein kinases and its downstream target, usually named mitogen-activated protein kinase-activated protein kinase-2; and a pathway in which the extracellular signal-regulated kinase subfamily of mitogen-activated protein kinases activates casein kinase-2. Our results suggest that: (i) extracellular signal-regulated kinase is activated by B-Raf in response to a calcium-sensitive adenylate cyclase; and (ii) extracellular signal-regulated kinase activates casein kinase-2 via a protein phosphatase(s) that may be of the PP1 and/or PP2A type. Interestingly, we also show that neurotrophin-induced activation of the two signalling cascades promotes a sustained activation of mitogen-activated protein kinase-activated protein kinase-2 and casein kinase-2 in slices. Considering the ability of these two kinases to be persistently activated, and that most of the protein kinases which lie in these pathways are believed to be important for multiple events underlying neuronal plasticity, it is suggested that the mechanisms described here might contribute both to rapid synaptic changes through local effects and to long-lasting synaptic responses through new gene transcription in the hippocampus.

Casein kinase 2 as a potentially important enzyme in the nervous system

Protein kinase CK2 is a ubiquitous and pleiotropic seryl/threonyl protein kinase which is highly conserved in evolution indicating a vital cellular role for this kinase. The holoenzyme is generally composed of two catalytic (alpha and/or alpha') and two regulatory (beta) subunits, but the free alpha/alpha' subunits are catalytically active by themselves and can be present in cells under some circumstances. Special attention has been devoted to phosphorylation status and structure of these enzymic molecules, however, their regulation and roles remain intriguing. Until recently, CK2 was believed to represent a kinase especially required for cell cycle progression in non-neural cells. At present, with respect to recent findings, four essential features suggest potentially important roles for this enzyme in specific neural functions: (1) CK2 is much more abundant in brain than in any other tissue; (2) there appear to be a myriad of substrates for CK2 in both synaptic and nuclear compartments that have clear implications in development, neuritogenesis, synaptic transmission, synaptic plasticity, information storage and survival; (3) CK2 seems to be associated with mechanisms underlying long-term potentiation in hippocampus; and (4) neurotrophins stimulate activity of CK2 in hippocampus. In addition, some data are suggestive that CK2 might play a role in processes underlying progressive disorders due to Alzheimer's disease, ischemia, chronic alcohol exposure or immunodeficiency virus HIV. The present review focuses mainly on the latest data concerning the regulatory mechanisms and the possible neurophysiological functions of this enzyme.

Differential effects of BDNF, ADNF9, and TNFalpha on levels of NMDA receptor subunits, calcium homeostasis, and neuronal vulnerability to excitotoxicity

Calcium influx through N-methyl-d-aspartate (NMDA) receptors can result in neuronal apoptosis or necrosis and may play a pivotal role in neuronal death in many different neurodegenerative diseases. In the present study we employed primary neuronal cultures and three different excitoprotective factors, brain-derived neurotrophic factor (BDNF), activity-dependent neurotrophic factor (ADNF9), and tumor necrosis factor alpha (TNFalpha), to elucidate the mechanisms whereby trophic factors modify the excitotoxic process. Neurons pretreated with BDNF exhibited increased levels of the NMDA receptor subunits NR1 and NR2A, which was associated with increased calcium responses to NMDA and vulnerability to excitotoxic necrosis and reduced vulnerability to apoptosis. ADNF9 and TNFalpha suppressed calcium responses to glutamate and protected neurons against both excitotoxic necrosis and apoptosis, but had no effect on levels of NMDA receptor subunits. Inhibition of phosphorylation and DNA binding of NF-kappaB, by H7 and kappaB decoy DNA, respectively, suggest that the excitotoxic-modulating actions of BDNF are mediated by kinases, while those of ADNF9 and TNFalpha are mediated by both kinases and the transcription factor NF-kappaB. Our data show that, whereas BDNF increases neuronal responses to glutamate while ADNF9 and TNFalpha decrease the same, all three protect against excitotoxic apoptosis.

Neurotrophin-mediated potentiation of neuronal injury

The neurotrophins are a diverse family of peptides which activate specific tyrosine kinase-linked receptors. Over the past five decades, since the pioneering work of Levi-Montalcini and colleagues, the critical role that neurotrophins play in shaping the developing nervous system has become increasingly established. These molecules, which include the nerve growth factor (NGF)-related peptides, NGF, brain-derived neurotrophic factor (BDNF), NT-4/5 and NT-3, promote differentiation and survival in the developing nervous system, and to a lesser extent in the adult nervous system. As survival-promoting molecules, neurotrophins have been studied as potential neuroprotective agents, and have shown beneficial effects in many model systems. However, a surprising "dark side" to neurotrophin behavior has emerged from some of these studies implying that, under certain pathological conditions, neurotrophins may exacerbate, rather than alleviate, injury. How neurotrophins cause these deleterious consequences is a question which is only beginning to be answered, but initial work supports altered free radical handling or modification of glutamate receptor expression as possible mechanisms underlying these effects. This review will focus on evidence suggesting that neurotrophins may enhance injury under certain circumstances and on the mechanisms behind these injury-promoting aspects.

Classical and novel directions in neurotrophin transport and research: anterograde transport of brain-derived neurotrophic factor by sensory neurons

After the discovery of nerve growth factor, a classic model of neurotrophin action was developed. In this model, nerve endings compete for limited quantities of neurotrophic factors produced in neuronal target tissues. Neurotrophins are bound with high-affinity receptors expressed on the neuronal membrane and then endocytosed and retrogradely transported back to the cell body of responsive neurons. This classic model of target derived trophic support has been utilized to explain a wide range of trophic actions including effects on neuronal survival, terminal branching, and protein expression. However, a number of recent findings in the field of neurotrophin research cannot be explained using the classic model. In the peripheral nervous system (PNS), sensory neurons have been shown to contain mRNA for a member of the neurotrophin family, brain-derived neurotrophic factor (BDNF). Sensory neurons do not receive synaptic input so neurotrophin production by these cells does not fit into the classic target derived model. In contrast to target derived trophic support, BDNF produced by sensory neurons provides local autocrine and paracrine neurotrophic support in vitro. Furthermore, in vivo, sensory neurons transport BDNF in the anterograde direction away from the cell body, and opposite to the retrograde direction utilized in the classic model. Thus, out of necessity, a new direction for neurotrophin research has developed to study the production and anterograde transport of neurotrophins. The importance of this new mode of neurotrophin action in the PNS is indicated by results that implicate it in the response to pain, inflammation, and nerve injury.

Blockade of NR2B-Containing NMDA Receptors Prevents BDNF Enhancement of Glutamatergic Transmission in Hippocampal Neurons

Application of brain-derived neurotrophic factor (BDNF) to hippocampal neurons has profound effects on glutamatergic synaptic transmission. Both pre- and postsynaptic actions have been identified that depend on the age and type of preparation. To understand the nature of this diversity, we have begun to examine the mechanisms of BDNF action in cultured dissociated embryonic hippocampal neurons. Whole-cell patch-clamp recording during iontophoretic application of glutamate revealed that BDNF doubled the amplitude of induced inward current. Coexposure to BDNF and the NMDA receptor antagonist AP-5 markedly reduced, but did not entirely prevent, the increase in current. Coexposure to BDNF and ifenprodil, an NR2B subunit antagonist, reproduced the response observed with AP-5, suggesting BDNF primarily enhanced activity of NR2B-containing NMDA receptors with a lesser effect on non-NMDA receptors. Protein kinase involvement was confirmed with the broad spectrum inhibitor staurosporine, which prevented the response to BDNF. PKCI19-31 and H-89, selective antagonists of PKC and PKA, had no effect on the response to BDNF, whereas autocamtide-2-related inhibitory peptide, an antagonist of CaM kinase II, reduced response magnitude by 60%. These results demonstrate the predominant role of a specific NMDA receptor subtype in BDNF modulation of hippocampal synaptic transmission.