Regionally specific and rapid increases in brain-derived neurotrophic factor messenger RNA in the adult rat brain following seizures induced by systemic administration of kainic acid

BDNF receptor TrkB as the mediator of the antidepressant drug action

Brain-derived neurotrophic factor (BDNF) signaling through its receptor TrkB has for a long time been recognized as a critical mediator of the antidepressant drug action, but BDNF signaling has been considered to be activated indirectly through the action of typical and rapid-acting antidepressants through monoamine transporters and glutamate NMDA receptors, respectively. However, recent findings demonstrate that both typical and the fast-acting antidepressants directly bind to TrkB and thereby allosterically potentiate BDNF signaling, suggesting that TrkB is the direct target for antidepressant drugs. Increased TrkB signaling particularly in the parvalbumin-expressing interneurons orchestrates iPlasticity, a state of juvenile-like enhanced plasticity in the adult brain. iPlasticity sensitizes neuronal networks to environmental influences, enabling rewiring of networks miswired by adverse experiences. These findings have dramatically changed the position of TrkB in the antidepressant effects and they propose a new end-to-end model of the antidepressant drug action. This model emphasizes the enabling role of antidepressant treatment and the active participation of the patient in the process of recovery from mood disorders.

Brain-Derived Neurotrophic Factor Inhibits the Function of Cation-Chloride Cotransporter in a Mouse Model of Viral Infection-Induced Epilepsy

Well over 100 different viruses can infect the brain and cause brain inflammation. In the developing world, brain inflammation is a leading cause for epilepsy and often refractory to established anti-seizure drugs. Epilepsy generally results from an imbalance in excitatory glutamatergic and inhibitory GABAergic neurotransmission. GABAergic inhibition is determined by the intracellular Cl⁻ concentration which is established through the opposing action of two cation chloride cotransporters namely NKCC1 and KCC2. Brain-derived neurotrophic factor (BDNF) signaling is known to regulate expression of KCC2. Hence we hypothesized that viral induced epilepsy may result from aberrant BDNF signaling. We tested this hypothesis using a mouse model of Theiler’s murine encephalomyelitis virus (TMEV) infection-induced epilepsy. We found that BDNF levels in the hippocampus from TMEV-infected mice with seizures was increased at the onset of acute seizures and continued to increase during the peak of acute seizure as well as in latent and chronic phases of epilepsy. During the acute phase of epilepsy, we found significant reduction in the expression of KCC2 in hippocampus, whereas the level of NKCC1 was unaltered. Importantly, inhibiting BDNF using scavenging bodies of BDNF in live brain slices from TMEV-infected mice with seizures normalized the level of KCC2 in hippocampus. Our results suggest that BDNF can directly decrease the relative expression of NKCC1 and KCC2 such as to favor accumulation of chloride intracellularly which in turn causes hyperexcitability by reversing GABA-mediated inhibition. Although our attempt to inhibit the BDNF signaling mediated through tyrosine kinase B–phospholipase Cγ1 (TrkB-PLCγ1) using a small peptide did not change the course of seizure development following TMEV infection, alternative strategies for controlling the BDNF signaling could be useful in preventing seizure generation and development of epilepsy in this model.

Selective Estrogen Receptor Modulation Increases Hippocampal Activity during Probabilistic Association Learning in Schizophrenia

People with schizophrenia show probabilistic association learning impairment in conjunction with abnormal neural activity. The selective estrogen receptor modulator (SERM) raloxifene preserves neural activity during memory in healthy older men and improves memory in schizophrenia. Here, we tested the extent to which raloxifene modifies neural activity during learning in schizophrenia. Nineteen people with schizophrenia participated in a twelve-week randomized, double-blind, placebo-controlled, cross-over adjunctive treatment trial of the SERM raloxifene administered orally at 120 mg daily to assess brain activity during probabilistic association learning using functional magnetic resonance imaging (fMRI). Raloxifene improved probabilistic association learning and significantly increased fMRI BOLD activity in the hippocampus and parahippocampal gyrus relative to placebo. A separate region of interest confirmatory analysis in 21 patients vs 36 healthy controls showed a positive association between parahippocampal neural activity and learning in patients, but no such relationship in the parahippocampal gyrus of healthy controls. Thus, selective estrogen receptor modulation by raloxifene concurrently increases activity in the parahippocampal gyrus and improves probabilistic association learning in schizophrenia. These results support a role for estrogen receptor modulation of mesial temporal lobe neural activity in the remediation of learning disabilities in both men and women with schizophrenia.

Rapid changes in expression of class I and IV histone deacetylases during epileptogenesis in mouse models of temporal lobe epilepsy

A prominent role of epigenetic mechanisms in manifestation of epilepsy has been proposed. Thus altered histone H3 and H4 acetylation has been demonstrated in experimental models of temporal lobe epilepsy (TLE). We now investigated changes in the expression of the class I and class IV histone deacetylases (HDAC) in two complementary mouse TLE models. Unilateral intrahippocampal injection of kainic acid (KA) induced a status epilepticus lasting 6 to 24h, development of spontaneous limbic seizures (2 to 3 days after KA injection) and chronic epilepsy, as revealed by telemetric recordings of the EEGs. Mice were killed at different intervals after KA injection and expression of HDAC mRNAs was investigated by in situ hybridization. We observed marked decreases in the expression of HDACs 1, 2 and 11 (by up to 75%) in the granule cell and pyramidal cell layers of the hippocampus during the acute status epilepticus (2 to 6h after KA injection). This was followed by increased expression of all class I HDAC mRNAs in all principal cell layers of the hippocampus after 12 to 48 h. In the chronic phase, 14 and 28 days after KA, only modest increases in the expression of HDAC1 mRNA were observed in granule and pyramidal cells. Immunohistochemistry using an antibody detecting HDAC2 revealed results consistent with the mRNA data and indicates also expression in glial cells on the injection side. Similar changes as seen in the KA model were observed after a pilocarpine-induced status epilepticus except that decreases in HDACs 2, 3 and 8 were also seen at the chronic 28 day interval. The prominent decreases in HDAC expression during status epilepticus are consistent with the previously demonstrated increased expression of numerous proteins and with the augmented acetylation of histone H4. It is suggested that respective putative gene products could facilitate proconvulsive as well as anticonvulsive mechanisms. The increased expression of all class I HDACs during the "silent phase", on the other hand, may be related to decreased histone acetylation, which could cause a decrease in expression of certain proteins, a mechanism that could also promote epileptogenesis. Thus, addressing HDAC expression may have a therapeutic potential in interfering with a status epilepticus and with the manifestation of TLE.

Peripheral BDNF: a candidate biomarker of healthy neural activity during learning is disrupted in schizophrenia

BACKGROUND

Brain-derived neurotrophic factor (BDNF) is an important regulator of synaptogenesis and synaptic plasticity underlying learning. However, a relationship between circulating BDNF levels and brain activity during learning has not been demonstrated in humans. Reduced brain BDNF levels are found in schizophrenia and functional neuroimaging studies of probabilistic association learning in schizophrenia have demonstrated reduced activity in a neural network that includes the prefrontal and parietal cortices and the caudate nucleus. We predicted that brain activity would correlate positively with peripheral BDNF levels during probabilistic association learning in healthy adults and that this relationship would be altered in schizophrenia.

METHOD

Twenty-five healthy adults and 17 people with schizophrenia or schizo-affective disorder performed a probabilistic association learning test during functional magnetic resonance imaging (fMRI). Plasma BDNF levels were measured by enzyme-linked immunosorbent assay (ELISA).

RESULTS

We found a positive correlation between circulating plasma BDNF levels and brain activity in the parietal cortex in healthy adults. There was no relationship between plasma BDNF levels and task-related activity in the prefrontal, parietal or caudate regions in schizophrenia. A direct comparison of these relationships between groups revealed a significant diagnostic difference.

CONCLUSIONS

This is the first study to show a relationship between peripheral BDNF levels and cortical activity during learning, suggesting that plasma BDNF levels may reflect learning-related brain activity in healthy humans. The lack of relationship between plasma BDNF and task-related brain activity in patients suggests that circulating blood BDNF may not be indicative of learning-dependent brain activity in schizophrenia.

BDNF local translation in viable synaptosomes: implication in spine maturation

The neurotrophic factor, BDNF, is encoded by two transcripts, one short and another long 3' untranslated region containing mRNA. Long BDNF mRNA was found to transport to the dendrites; however report about its translation or regulation of translation in the dendrite remains unknown. Using synaptosomes, to isolate from the nucleus and other subcellular fractions involved in translation, we demonstrate that depolarization by KCl or excitation by glutamate can induce translation of BDNF. Such translation at the synaptosomes was also observed for mRNAs of CaMKllα, Homer and Arc, which are known to travel to dendrite. This synaptosomal translation system is critically dependent on glucose concentration. Other than glucose, BDNF translation in synaptosome is dependent on its own receptor TrkB function as well as on the rise in intra-synaptosomal Ca(2+), both of which are elevated during to depolarization or excitation. As BDNF-TrkB signaling causes maturation of spines by inducing LTP, this study also investigated the possibility of induction of spine maturation signaling in the isolated synaptosomes. Increased phospho-cofilin and phospho-PAK is detected in KCl or glutamate treated synaptosomes compared to control by Western blotting, suggesting a possibility of induction of spine maturation signaling.

L-type CaV1.2 deletion in the cochlea but not in the brainstem reduces noise vulnerability: implication for CaV1.2-mediated control of cochlear BDNF expression

Voltage-gated L-type Ca²⁺ channels (L-VGCCs) like Ca_V1.2 are assumed to play a crucial role for controlling release of trophic peptides including brain-derived neurotrophic factor (BDNF). In the inner ear of the adult mouse, besides the well-described L-VGCC Ca_V1.3, Ca_V1.2 is also expressed. Due to lethality of constitutive Ca_V1.2 knock-out mice, the function of this ion channel as well as its putative relationship to BDNF in the auditory system is entirely elusive. We recently described that BDNF plays a differential role for inner hair cell (IHC) vesicles release in normal and traumatized condition. To elucidate a presumptive role of Ca_V1.2 during this process, two tissue-specific conditional mouse lines were generated. To distinguish the impact of Ca_V1.2 on the cochlea from that on feedback loops from higher auditory centers Ca_V1.2 was deleted, in one mouse line, under the Pax2 promoter (Ca_V1.2^Pax2) leading to a deletion in the spiral ganglion neurons, dorsal cochlear nucleus, and inferior colliculus. In the second mouse line, the Egr2 promoter was used for deleting Ca_V1.2 (Ca_V1.2^Egr2) in auditory brainstem nuclei. In both mouse lines, normal hearing threshold and equal number of IHC release sites were observed. We found a slight reduction of auditory brainstem response wave I amplitudes in the Ca_V1.2^Pax2 mice, but not in the Ca_V1.2^Egr2 mice. After noise exposure, Ca_V1.2^Pax2 mice had less-pronounced hearing loss that correlated with maintenance of ribbons in IHCs and less reduced activity in auditory nerve fibers, as well as in higher brain centers at supra-threshold sound stimulation. As reduced cochlear BDNF mRNA levels were found in Ca_V1.2^Pax2 mice, we suggest that a Ca_V1.2-dependent step may participate in triggering part of the beneficial and deteriorating effects of cochlear BDNF in intact systems and during noise exposure through a pathway that is independent of Ca_V1.2 function in efferent circuits.

A novel anticonvulsant modulates voltage-gated sodium channel inactivation and prevents kindling-induced seizures

Here, we explore the mechanism of action of isoxylitone (ISOX), a molecule discovered in the plant Delphinium denudatum, which has been shown to have anticonvulsant properties. Patch-clamp electrophysiology assayed the activity of ISOX on voltage-gated sodium channels (VGSCs) in both cultured neurons and brain slices isolated from controls and rats with experimental epilepsy(kindling model). Quantitative transcription polymerase chain reaction (qRT-PCR) (QPCR) assessed brain-derived neurotrophic factor (BDNF) mRNA expression in kindled rats, and kindled rats treated with ISOX. ISOX suppressed sodium current (I(Na)) showing an IC50 value of 185 nM in cultured neurons. ISOX significantly slowed the recovery from inactivation (ISOX τ = 18.7 ms; Control τ = 9.4 ms; p < 0.001). ISOX also enhanced the development of inactivation by shifting the Boltzmann curve to more hyperpolarized potentials by -11.2 mV (p < 0.05). In naive and electrically kindled cortical neurons, the IC50 for sodium current block was identical to that found in cultured neurons. ISOX prevented kindled stage 5 seizures and decreased the enhanced BDNF mRNA expression that is normally associated with kindling (p < 0.05). Overall, our data show that ISOX is a potent inhibitor of VGSCs that stabilizes steady-state inactivation while slowing recovery and enhancing inactivation development. Like many other sodium channel blocker anti-epileptic drugs, the suppression of BDNF mRNA expression that usually occurs with kindling is likely a secondary outcome that nevertheless would suppress epileptogenesis. These data show a new class of anti-seizure compound that inhibits sodium channel function and prevents the development of epileptic seizures.

Transient domoic acid excitotoxicity increases BDNF expression and activates both MEK- and PKA-dependent neurogenesis in organotypic hippocampal slices

Background

We have previously reported evidence of cell proliferation and increased neurogenesis in rat organotypic hippocampal slice cultures (OHSC) after a transient excitotoxic injury to the hippocampal CA1 area induced by low concentrations of the AMPA/kainate agonist domoic acid (DOM). An increased baseline rate of neurogenesis may contribute to recovery from DOM-induced mild injury but the intracellular mechanism(s) responsible for neuronal proliferation remain unclear. The current study investigated the key intracellular pathways responsible for DOM-induced neurogenesis in OHSC including the effects of transient excitotoxicity on the expression of brain-derived neurotrophic factor (BDNF), a well-known regulator of progenitor cell mitosis.

Results

Application of a low concentration of DOM (2 μM) for 24 h followed by recovery induced a significant and long lasting increase in BDNF protein levels expressed by both neurons and microglial cells. Furthermore, the mild DOM toxicity stimulated both PKA and MEK-dependent intracellular signaling cascades and induced a significant increase in BDNF- transcription factor CREB activation and BDNF-receptor TrkB expression. Coexposure to specific inhibitors of PKA and MEK phosphorylation resulted in a significant decrease in the neurogenic marker doublecortin.

Conclusions

Our results suggest that transient excitotoxic insult induced by DOM produces BDNF and CREB overexpression via MEK and PKA pathways and that both pathways mediate, at least in part, the increased neural proliferation resulting from mild excitotoxicity.

A high cholesterol diet given to apolipoprotein E-knockout mice has a differential effect on the various neurotrophin systems in the hippocampus

Apolipoprotein E (apoE) is one of the major transporters of cholesterol in the body and is essential for maintaining various neural functions in the brain. Given that hypercholesterolemia is a risk factor in Alzheimer's disease (AD), it has been suggested that altered cholesterol metabolism may be involved in the development of the pathogenesis, including neural degeneration, commonly observed in AD patients. Neurotrophic factors and their receptors, which are known to regulate various neural functions, are also known to be altered in various neurodegenerative diseases. We therefore hypothesized that cholesterol metabolism may itself influence the neurotrophin system within the brain. We decided to investigate this possibility by modulating the amount of dietary cholesterol given to apoE-knockout (apoE-KO) and wild-type (WT) mice, and examining the mRNA expression of various neurotrophin ligands and receptors in their hippocampal formations. Groups of eight-week-old apoE-KO and WT mice were fed a diet containing either "high" (HCD) or "normal" (ND) levels of cholesterol for a period of 12 weeks. We found that high dietary cholesterol intake elevated BDNF mRNA expression in both apoE-KO and WT mice and TrkB mRNA expression in apoE-KO animals. On the other hand, NGF and TrkA mRNA levels remained unchanged irrespective of both diet and mouse type. These findings indicate that altered cholesterol metabolism induced by HCD ingestion combined with apoE deficiency can elicit a differential response in the various neurotrophin ligand/receptor systems in the mouse hippocampus. Whether such changes can lead to neural degeneration, and the mechanisms that may be involved in this, awaits further research.

CXCL12-mediated guidance of migrating embryonic stem cell-derived neural progenitors transplanted into the hippocampus

Stem cell therapies for neurodegenerative disorders require accurate delivery of the transplanted cells to the sites of damage. Numerous studies have established that fluid injections to the hippocampus can induce lesions in the dentate gyrus (DG) that lead to cell death within the upper blade. Using a mouse model of temporal lobe epilepsy, we previously observed that embryonic stem cell-derived neural progenitors (ESNPs) survive and differentiate within the granule cell layer after stereotaxic delivery to the DG, replacing the endogenous cells of the upper blade. To investigate the mechanisms for ESNP migration and repair in the DG, we examined the role of the chemokine CXCL12 in mice subjected to kainic acid-induced seizures. We now show that ESNPs transplanted into the DG show extensive migration through the upper blade, along the septotemporal axis of the hippocampus. Seizures upregulate CXCL12 and infusion of the CXCR4 antagonist AMD3100 by osmotic minipump attenuated ESNP migration. We also demonstrate that seizures promote the differentiation of transplanted ESNPs toward neuronal rather than astrocyte fates. These findings suggest that ESNPs transplanted into the adult rodent hippocampus migrate in response to cytokine-mediated signals.

Seizure activity involved in the up-regulation of BDNF mRNA expression by activation of central mu opioid receptors

Chemical-induced seizures up-regulated brain-derived neurotrophic factor (BDNF) mRNA expression. Intracerebroventricular (i.c.v.) administration of endogenous opioids preferentially activating mu opioid receptor (MOR) could also increase BDNF mRNA expression. The aim of this study was to determine to what extent i.c.v. administration of synthetic MOR-selective agonists in rats can modulate both seizure activity and up-regulation of BDNF mRNA expression. Effects and potencies of i.c.v. administration of morphine and [D-Ala(2), N-Me-Phe(4), Gly(5)-ol]-enkephalin (DAMGO), were directly investigated by scoring behavioral seizures and measuring BDNF mRNA expression. In addition, effects of the opioid receptor antagonist naloxone and antiepileptic drugs, diazepam, phenobarbital, and valproate, on i.c.v. MOR agonist-induced behavioral seizures and up-regulation of BDNF mRNA expression were determined. A single i.c.v. administration of morphine (10-100 microg) or DAMGO (0.15-1.5 microg) dose-dependently elicited behavioral seizures and increased BDNF mRNA expression in the widespread brain regions. However, s.c. administration of MOR agonists neither produced behavioral seizures nor increased BDNF mRNA expression. Pretreatment with naloxone 1 mg/kg significantly reduced behavioral seizure scores and the up-regulation of BDNF mRNA expression elicited by i.c.v. morphine or DAMGO. Similarly, diazepam 10 mg/kg and phenobarbital 40 mg/kg significantly blocked i.c.v. MOR agonist-induced actions. Pretreatment with valproate 300 mg/kg only attenuated behavioral seizures, but it did not affect morphine-induced increase of BDNF mRNA expression. This study provides supporting evidence that seizure activity plays an important role in the up-regulation of BDNF mRNA expression elicited by central MOR activation and that decreased inhibitory action of GABAergic system through the modulation on GABA receptor synaptic function by central MOR activation is involved in its regulation of BDNF mRNA expression.

Latent stem and progenitor cells in the hippocampus are activated by neural excitation

The regulated production of neurons in the hippocampus throughout life underpins important brain functions such as learning and memory. Surprisingly, however, studies have so far failed to identify a resident hippocampal stem cell capable of providing the renewable source of these neurons. Here, we report that depolarizing levels of KCl produce a threefold increase in the number of neurospheres generated from the adult mouse hippocampus. Most interestingly, however, depolarizing levels of KCl led to the emergence of a small subpopulation of precursors (approximately eight per hippocampus) with the capacity to generate very large neurospheres (> 250 microm in diameter). Many of these contained cells that displayed the cardinal properties of stem cells: multipotentiality and self-renewal. In contrast, the same conditions led to the opposite effect in the other main neurogenic region of the brain, the subventricular zone, in which neurosphere numbers decreased by approximately 40% in response to depolarizing levels of KCl. Most importantly, we also show that the latent hippocampal progenitor population can be activated in vivo in response to prolonged neural activity found in status epilepticus. This work provides the first direct evidence of a latent precursor and stem cell population in the adult hippocampus, which is able to be activated by neural activity. Because the latent population is also demonstrated to reside in the aged animal, defining the precise mechanisms that underlie its activation may provide a means to combat the cognitive deficits associated with a decline in neurogenesis.

Exercising our brains: how physical activity impacts synaptic plasticity in the dentate gyrus

Exercise that engages the cardiovascular system has a myriad of effects on the body; however, we usually do not give much consideration to the benefits it may have for our minds. An increasing body of evidence suggests that exercise can have some remarkable effects on the brain. In this article, we will introduce how exercise can impact the capacity for neurons in the brain to communicate with one another. To properly convey this information, we will first briefly introduce the field of synaptic plasticity and then examine how the introduction of exercise to the experimental setting can actually alter the basic properties of synaptic plasticity in the brain. Next, we will examine some of the candidate physiological processes that might underlay these alterations. Finally, we will close by noting that, taken together, this data points toward our brains being dynamic systems that are in a continual state of flux and that physical exercise may help us to maximize the performance of both our body and our minds.

Status epilepticus induces region-specific changes in dendritic spines, dendritic length and TrkB protein content of rat brain cortex

Induction of status epilepticus (SE) with kainic acid results in a large reorganization of neuronal brain circuits, a phenomenon that has been studied primarily in the hippocampus. The neurotrophin BDNF, by acting through its receptor TrkB, has been implicated in such reorganization. In the present work we investigated, by Western blot and immunohistochemistry, whether regional changes of TrkB expression within the rat brain cortex are correlated with altered neuronal morphology and/or with apoptotic cell death. We found that the full-length TrkB protein decreased within the cortex when measured 24 h to 1 week after induction of SE. Analysis by immunohistochemistry revealed that TrkB staining diminished within layer V of the retrosplenial granular b (RSGb) and motor cortices, but not within the auditory cortex. In layer II/III, differential changes were also observed: TrkB decreased in the motor cortex, did not change within the RSGb but increased within the auditory cortex. Reduced TrkB was associated with dendritic atrophy and decreased spine density in pyramidal neurons within layer V of the RSGb. No correlation was observed between regional and cellular changes of TrkB protein and apoptosis, measured by the TdT-mediated dUTP nick end labeling (TUNEL) method. The global decrease of TrkB within the neocortex and the associated dendritic atrophy may counteract seizure propagation in the epileptic brain but may also underlie cognitive impairment after seizures.

Effects of Manganese Complexes of Curcumin and Diacetylcurcumin on Kainic Acid-Induced Neurotoxic Responses in the Rat Hippocampus

This study aimed to investigate the mechanism underlying the protective effects of manganese complexes of curcumin (Cp-Mn) and diacetylcurcumin (DiAc-Cp-Mn) on kainic acid (KA)-induced excitotoxicity in the rat hippocampus. Systemic injection of KA (10 mg/kg, i.p.) caused seizures and increased the expression of neurotoxic markers, immediate early genes [c-jun, cyclooxygenase 2 (COX-2), brain-derived neurotrophic factor (BDNF), and heat shock protein 70 (hsp70)] and a delayed response gene [inducible nitric oxide synthase (iNOS)], which were measured at 6 and 72 h after KA injection, respectively, in the hippocampus. Pretreatment with Cp-Mn (50 mg/kg, i.p.) and DiAc-Cp-Mn (50 mg/kg, i.p.) but not with curcumin (50 mg/kg, i.p.) delayed the onset of KA-induced seizure without affecting the seizure score. KA injection induced c-Fos immunoreactivity in DG, CA1, and CA3 hippocampal regions, the expression of which peaked at 6 h after injection. Cp-Mn and DiAc-Cp-Mn treatment significantly decreased c-Fos expression elicited by KA. Moreover, Cp-Mn and DiAc-Cp-Mn administration suppressed the KA-induced expression of c-jun, COX-2, BDNF, and iNOS mRNA, whereas curcumin attenuated only iNOS mRNA expression. No compounds tested had an effect on KA-induced hsp70 expression. It is therefore likely that in addition to radical scavenging and SOD-like activities, the suppression of potential neuronal injury marker expression by Cp-Mn and DiAc-Cp-Mn, contributes to the neuroprotective activities of these compounds, which are superior to those of curcumin, on KA-induced excitotoxicity in the hippocampus. These results suggest the beneficial effects of Cp-Mn, and DiAc-Cp-Mn on the treatment of excitotoxicity-induced neurodegenerative diseases.

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.

Novel agonist monoclonal antibodies activate TrkB receptors and demonstrate potent neurotrophic activities

Tyrosine kinase receptor B (TrkB) mediates neurotrophic effects of brain-derived neurotrophic factor (BDNF) to increase neuronal survival, differentiation, synaptic plasticity, and neurogenesis. The therapeutic potential of TrkB activation using BDNF has been demonstrated well in several preclinical models of CNS diseases, validating TrkB as a promising drug target. Therefore, we aimed to develop TrkB-specific receptor agonists by using a monoclonal antibody approach. After generation of hybridoma clones and assessment of their binding and functional activity, we identified five mouse monoclonal antibodies that show highly selective binding to TrkB and that induce robust activation of TrkB signaling. Epitope mapping studies using competition analysis showed that each of the monoclonal antibodies recognizes a unique binding site on TrkB, some of which are distinct from BDNF docking sites. These antibodies behave as true agonists based on their ability to both activate proximal and secondary signaling molecules downstream of TrkB receptors and promote neuronal survival and neurite outgrowth. The binding affinities and the functional efficacy of these antibodies are comparable to those of BDNF, whereas they do not bind to the p75 low-affinity neurotrophin receptor at all. Therefore, they could represent novel reagents to explore the pathophysiological roles of TrkB and its potential therapeutic utility in treating CNS disorders.

c-fos modulates brain-derived neurotrophic factor mRNA expression in mouse hippocampal CA3 and dentate gyrus neurons

Excess neuronal excitation by glutamate induces neuron cell death, which may contribute to the pathogenesis of acute brain injuries and neurodegenerative diseases. Our previous studies using a mouse with hippocampal c-fos gene deletion showed that c-fos regulates neuronal excitability and excitotoxicity. Moreover, a delayed induction of brain-derived neurotrophic factor (BDNF) protein expression in response to kainic acid (KA) treatment was found in c-fos mutant mice compared to wildtype controls, suggesting that c-fos is important in the temporal control of BDNF induction. To further investigate mechanisms of in vivo regulation of c-fos on BDNF expression, we studied the expression of BDNF mRNA and its colocalization with c-Fos protein in the hippocampal formation in the presence and absence of KA. By in situ hybridization, we observed that the c-fos mutant and wildtype mice exhibited similar basal expression of BDNF in the absence of KA. In contrast, the KA-induced BDNF mRNA levels were significantly different in wildtype and c-fos mutant mice in CA3 and dentate gyrus regions. Our findings indicate that c-fos regulates expression of BDNF in distinct neuron populations of the hippocampal formation in vivo.

CA3 axonal sprouting in kainate-induced chronic epilepsy

Latency between an early neurological insult and development of spontaneous recurrent seizures suggests aberrant chronological reorganization in patients with mesial temporal sclerosis associated epilepsy. Kainate-induced status similarly results in delayed development of spontaneous recurrent seizures. Mossy fiber sprouting by the dentate granule cells is a well-characterized manifestation of such temporal structural reorganization in both patients and animal models. However, alterations in other components of hippocampal circuitry have not been evaluated. We present results from studies using precise anterograde and retrograde tract tracing methodologies to evaluate the reorganization of outflow of the CA3 pyramidal cells. Although septotemporal relationships of the normal CA3 outflow tract through the Schaffer collaterals are well known, their aberrant reorganization following kainate-induced spontaneous recurrent seizures is not known. We provide the first definitive evidence of widespread CA3 structural reorganization in the form of sprouting of CA3 axons to widespread areas throughout the hippocampus and entorhinal cortex. This includes an apparent increase in the density of projection to areas that normally receive CA3 outflow such as CA1 and subiculum as well as novel projections beyond the confines of the hippocampus to the pre and parasubiculum and medial and lateral entorhinal cortex. We provide the first evidence of novel CA3 Schaffer collateral projection to the entorhinal cortex. The sprouting of CA3 outflow to widespread regions of the hippocampus and the entorhinal cortex may provide insight into how the injured hippocampus propagates unconventional impulse excitation to cortical fields which have a critical role in providing excitatory inputs into the hippocampus possibly setting up reverberating excitatory circuits as well as widespread connections throughout the cortical mantle. Sprouting-related mechanisms may also explain the latency associated with development of spontaneous recurrent seizures, the hallmark of temporal lobe epilepsy.

Bone morphogenetic proteins and neurotrophins provide complementary protection of septal cholinergic function during phosphatase inhibitor-induced stress

Cultures of embryonic rat septum were exposed for 24-48 h to 2-5 nm okadaic acid (OA), an inhibitor of pp1A and pp2A phosphatases. This stress killed approximately 75% of neurons. A neurotrophin (NT) combination (nerve growth factor and brain-derived neurotrophic factor, each 100 ng/mL) plus a bone morphogenetic protein (BMP6 or BMP7, 5 nm) reduced the death of both cholinergic and non-cholinergic neurons, and preserved choline acetyltransferase (ChAT) activity assayed 2-6 days post-stress. This NT + BMP combination preserved ChAT activity better than either NTs or BMPs alone, and was effective even if trophic factor addition was delayed until 12 h after stress onset. A general caspase inhibitor (qVD-OPH, 10 micro g/mL) also increased survival of stressed cholinergic neurons, but its protection of ChAT activity was shorter lived than that produced by the NT + BMP combination. Neither the NT + BMP combination nor the caspase inhibitor reduced the OA-induced increase in tau phosphorylation. These findings indicate that NTs and BMPs have synergistic protective effects against an OA stress, and suggest that at least some of these protective effects occur upstream of caspase activation.

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.

Differential regulation of hippocampal BDNF mRNA by typical and atypical antipsychotic administration

Apart from their differential propensities to block dopamine D2 and serotonin 5-HT2 receptors, the molecular mechanisms underlying the clinical efficacy of typical and atypical antipsychotics in schizophrenia are largely unknown. Given recent interest in the effects of antipsychotics on neurotrophic and other growth related factors, the effects of antipsychotics on brain-derived neurotrophic factor (BDNF), a neurotrophin crucial to the structural integrity of adult neurons, were investigated in male Wistar rats. Chronic (19 day) but not acute (45 min) antipsychotic administration significantly altered levels of hippocampal BDNF mRNA. In addition, whereas chronic treatment with the strong D2 receptor-blocker haloperidol significantly downregulated hippocampal BDNF mRNA, the selective 5-HT2 receptor-blocker ritanserin significantly upregulated CA1 hippocampal BDNF mRNA in comparison to controls. Since high doses of risperidone and clozapine produce potent inhibition of both 5-HT2 and D2 receptors, while lower doses produce significantly greater 5-HT2 vs. D2 receptor blockade, a dose-response study was employed to determine whether low doses of these atypical antipsychotics would also upregulate hippocampal BDNF mRNA in the absence of significant D2 receptor blockade. Whereas chronic haloperidol and high-dose risperidone significantly downregulated hippocampal BDNF mRNA, intermediate and lower doses of risperidone and clozapine were, unlike ritanserin, without effect when compared to controls. Thus, although the long-term downregulation of hippocampal BDNF mRNA may underlie the different clinical profiles of certain antipsychotics, this effect seems to be associated with antipsychotic doses that not only cause significant D2 receptor inhibition, but are usually associated with side effects rather than therapeutic efficacies.

Neural growth, neural damage and neurotrophins in the kindling model of epilepsy

Do seizures change the brain? Studies on the kindling model--a widely used animal model of epilepsy--suggest that they do. Dr. Racine, one of the pioneers in the kindling field, describes the basic phenomena of kindling, and discusses the possible roles of cell growth and cell death in this model.

Epilepsy as an example of neural plasticity

Epilepsy is a devastating disease affecting more than 1% of the population. Yet, if one considers the neurobiological substrates of this disease, what is revealed is an array of phenomenon that exemplify the remarkable capacity for the brain to change its basic structure and function, that is, neural plasticity. Some of these alterations are transient and merely impressive for their extent, or for their robust nature across animal models and human epilepsy. Others are notable for their persistence, often enduring for months or years. As an example, the dentate gyrus, and specifically the principal cell of the dentate gyrus, the granule cell, is highlighted. This area of the brain and this particular cell type, for reasons that are currently unclear, hold an uncanny capacity to change after seizures. For those interested in plasticity, it is suggested that perhaps the best examples for studying plasticity lie in the field of epilepsy.

Spontaneous limbic seizures after intrahippocampal infusion of brain-derived neurotrophic factor

The results of several studies have contributed to the hypothesis that BDNF promotes seizure activity, particularly in adult hippocampus. To test this hypothesis, BDNF, vehicle (phosphate-buffered saline, PBS), or albumin was infused directly into the hippocampus for 2 weeks using osmotic minipumps. Rats were examined behaviorally, electrophysiologically, and anatomically. An additional group was tested for sensitivity to the convulsant pilocarpine. Spontaneous behavioral seizures were observed in BDNF-infused rats (8/32; 25%) but not in controls (0/20; 0%). In a subset of six animals (three BDNF, three albumin), blind electrophysiological analysis of scalp recordings contralateral to the infused hippocampus demonstrated abnormalities in all BDNF rats; but not controls. Neuronal loss in BDNF-treated rats was not detected relative to PBS- or albumin-treated animals, but immunocytochemical markers showed a pattern of expression in BDNF-treated rats that was similar to rats with experimentally induced seizures. Thus, BDNF-infused rats had increased expression of NPY in hilar neurons of the dentate gyrus relative to control rats. NPY and BDNF expression was increased in the mossy fiber axons of dentate gyrus granule cells relative to controls. The increase in NPY and BDNF expression in BDNF-treated rats was bilateral and occurred throughout the septotemporal axis of the hippocampus. Mossy fiber sprouting occurred in five BDNF-treated rats but no controls. In another group of infused rats that was tested for seizure sensitivity to the convulsant pilocarpine, BDNF-infused rats had a shorter latency to status epilepticus than PBS-infused rats. In addition, the progression from normal behavior to severe seizures was faster in BDNF-treated rats. These data support the hypothesis that intrahippocampal BDNF infusion can facilitate, and potentially initiate, seizure activity in adult hippocampus.

Soman-induced seizures: limbic activity, oxidative stress and neuroprotective proteins

Soman, a potent acetylcholinesterase inhibitor, induces status epilepticus in rats followed by conspicuous neuropathology, most prominent in piriform cortex and the CA3 region of the hippocampus. Cholinergic seizures originate in striatal-nigral pathways and with fast-acting agents (soman) rapidly spread to limbic related areas and finally culminate in a full-blown status epilepticus. This leads to neurochemical changes, some of which may be neuroprotective whereas others may cause brain damage. Pretreatment with lithium sensitizes the brain to cholinergic seizures. Likewise, other agents that increase limbic hyperactivity may sensitize the brain to cholinergic agents. The hyperactivity associated with the seizure state leads to an increase in intracellular calcium, cellular edema and metal delocalization producing an oxidative stress. These changes induce the synthesis of stress-related proteins such as heat shock proteins, metallothioneins and heme oxygenases. We show that soman-induced seizures cause a depletion in tissue glutathione and an increase in tissue 'catalytic' iron, metallothioneins and heme oxygenase-1. The oxidative stress induces the synthesis of stress-related proteins, which are indicators of 'stress' and possibly provide neuroprotection. These findings suggest that delocalization of iron may catalyze Fenton-like reactions, causing progressive cellular damage via free radical products.

Long-lasting induction of brain-derived neurotrophic factor is restricted to resistant cell populations in an animal model of status epilepticus

We have recently characterized an animal model of status epilepticus induced by a single intraseptal injection of kainate. Under these conditions, there is a delayed expanding apoptotic hippocampal and amygdalar cell death. In order to further characterize this animal model, we have performed a detailed time-course analysis of the appearance of cell death, brain-derived neurotrophic factor messenger RNA expression and astroglial and microglial response in different brain areas related to the limbic system. We found a long-lasting delayed apoptotic cell death in the hippocampal formation, amygdala, medial thalamus, dorsal endopiriform nucleus and multiple cortical areas from two to 21 days post-injection. There was a spatiotemporal correlation between the appearance of cell death and induction of brain-derived neurotrophic factor messenger RNA expression in the areas studied, and interestingly this induction was found in non-degenerating cells. We conclude that our animal model of status epilepticus exhibits remarkable features of recurrent seizure activity and provides evidence for a neuroprotective role of brain-derived neurotrophic factor against seizure-induced apoptotic cell death.

Absence of hippocampal mossy fiber sprouting in transgenic mice overexpressing brain-derived neurotrophic factor

Excess neuronal activity upregulates the expression of two neurotrophins, nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) in adult hippocampus. Nerve growth factor has been shown to contribute the induction of aberrant hippocampal mossy fiber sprouting in the inner molecular layer of the dentate gyrus, however the role of prolonged brain-derived neurotrophic factor exposure is uncertain. We examined the distribution and plasticity of mossy fibers in transgenic mice with developmental overexpression of brain-derived neurotrophic factor. Despite 2--3-fold elevated BDNF levels in the hippocampus sufficient to increase the intensity of neuropeptide Y immunoreactivity in interneurons, no visible changes in mossy fiber Timm staining patterns were observed in the inner molecular layer of adult mutant hippocampus compared to wild-type mice. In addition, no changes of the mRNA expression of two growth-associated proteins, GAP-43 and SCG-10 were found. These data suggest that early and persistent elevations of brain-derived neurotrophic factor in granule cells are not sufficient to elicit this pattern of axonal plasticity in the hippocampus.

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.

The role of cytokines and growth factors in seizures and their sequelae

Although the neuropathological changes caused by severe or repeated seizures have been well characterized, many questions about the molecular mechanisms involved remain unanswered. Neuronal cell death, reactive gliosis, enhanced neurogenesis, and axonal sprouting are four of the best-studied sequelae of seizures. In vitro, each of these pathological processes can be substantially influenced by soluble protein factors, including neurotrophins, cytokines, and growth factors. Furthermore, many of these proteins and their receptors are expressed in the adult brain and are up-regulated in response to neuronal activity and injury. We review the evidence that these intercellular signaling proteins regulate seizure activity as well as subsequent pathology in vivo. As nerve growth factor and brain derived neurotrophic factor are the best-studied proteins of this class, we begin by discussing the evidence linking these neurotrophins to epilepsy and seizure. More than a dozen additional cytokines, growth factors, and neurotrophins that have been examined in the context of epilepsy models are then considered. We discuss the effect of seizure on expression of cytokines and growth factors, and explore the regulation of seizure development and aftermath by exogenous application or antagonist perturbation of these proteins. The experimental evidence supports a role for these factors in each aspect of seizure and pathology, and suggests potential targets for future therapeutics.

Brain-derived neurotrophic factor in the control human brain, and in Alzheimer's disease and Parkinson's disease

Brain-derived neurotrophic factor (BDNF) is a small dimeric protein, structurally related to nerve growth factor, which is abundantly and widely expressed in the adult mammalian brain. BDNF has been found to promote survival of all major neuronal types affected in Alzheimer's disease and Parkinson's disease, like hippocampal and neocortical neurons, cholinergic septal and basal forebrain neurons, and nigral dopaminergic neurons. In this article, we summarize recent work on the molecular and cellular biology of BDNF, including current ideas about its intracellular trafficking, regulated synthesis and release, and actions at the synaptic level, which have considerably expanded our conception of BDNF actions in the central nervous system. But our primary aim is to review the literature regarding BDNF distribution in the human brain, and the modifications of BDNF expression which occur in the brain of individuals with Alzheimer's disease and Parkinson's disease. Our knowledge concerning BDNF actions on the neuronal populations affected in these pathological states is also reviewed, with an aim at understanding its pathogenic and pathophysiological relevance.

Seizures and sensory stimulation result in different patterns of brain derived neurotrophic factor protein expression in the barrel cortex and hippocampus

Brain-derived neurotrophic factor (BDNF) is important for the development and trophic support of neurons, and may be involved in controlling axonal sprouting and synaptic plasticity. In order to investigate the activity-dependent regulation of the BDNF gene, BDNF expression was examined within the rat somatosensory cortex (SSC) and hippocampus following vibrissae stimulation, kainic acid induced seizure, and pentylenetetrazol (PTZ) induced seizure. The specific goals of this study were to determine the time course and magnitude of BDNF's activity-dependent expression, and to compare the expression patterns of three commonly used neuronal activation paradigms. Our results demonstrate three novel observations. First, the patterns of BDNF protein expression are dependent upon the neuronal stimulation model used. Both unilateral whisker stimulation (a model of experience dependent plasticity) and kainic acid induced seizure were able to increase the levels of BDNF protein within the SSC and hippocampus. In contrast, PTZ induced seizure did not increase BDNF protein levels in either tissue. Second, there is a dissociation between BDNF mRNA and protein levels following PTZ induced seizure. PTZ seizures resulted in strong increases of BDNF mRNA levels without corresponding increases of the protein. Finally, whisker stimulation resulted in an unexpected increase in BDNF mRNA and protein levels within the hippocampus. These results suggest specific types of neuronal activity can regulate gene expression differently. Furthermore, temporal and spatial differences between the expression of BDNF protein and mRNA levels suggest that the BDNF gene is regulated at the level of translation as well as transcription.

GABA(B) receptor antagonists elevate both mRNA and protein levels of the neurotrophins nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) but not neurotrophin-3 (NT-3) in brain and spinal cord of rats

In this study we show that single, physiologically-active and non-convulsive doses of the three GABA(B) receptor antagonists CGP 36742, CGP 56433A and CGP 56999A increase NGF and BDNF mRNA levels by 200-400% and protein levels by 200-250% in rat neocortex, hippocampus as well as spinal cord. In all areas examined the increase in NGF protein preceded that of BDNF. Peak levels of both neurotrophins are transient and occur between 24 and 72 h, depending on the region. In contrast, NT-3 protein concentrations in the neocortex and hippocampus were decreased significantly to 50% of control values within 48-96 h. The decrease in the spinal cord was less than 30% and did not reach significant levels. These data clearly demonstrate that GABA(B) receptor antagonists induce a specific neurotrophin expression in the central nervous system at physiologically relevant doses, as opposed to the extreme conditions of seizure paradigms. The results are in line with the concept that neuronal neurotrophin synthesis and release in brain are controlled by afferent nerve activity. GABA(B) receptor antagonists could therefore be a valuable new approach to selectively increase endogenous neurotrophin levels in the central nervous system.

Subplate neuron ablation alters neurotrophin expression and ocular dominance column formation

Ocular dominance column formation in visual cortex depends on both the presence of subplate neurons and the endogenous expression of neurotrophins. Here we show that deletion of subplate neurons, which supply glutamatergic inputs to visual cortex, leads to a paradoxical increase in brain-derived neurotrophic factor mRNA in the same region of visual cortex in which ocular dominance columns are absent. Subplate neuron ablation also increases glutamic acid decarboxylase-67 levels, indicating an alteration in cortical inhibition. These observations imply a role for this special class of neurons in modulating activity-dependent competition by regulating levels of neurotrophins and excitability within a developing cortical circuit.

Differential expression of brain-derived neurotrophic factor, neurotrophin-3, and neurotrophin-4/5 in the adult rat spinal cord: regulation by the glutamate receptor agonist kainic acid

Previous in vitro studies indicate that select members of the neurotrophin gene family, namely brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), and neurotrophin-4/5 (NT-4/5), contribute to survival and differentiation of spinal cord motoneurons. To investigate the potential roles of these factors in the adult spinal cord, we examined their cellular localization and regulation after systemic exposure to an excitotoxic stimulus, kainic acid (KA). Of the neurotrophins examined, NT-4/5 mRNA was most robustly expressed in the lumbosacral spinal cord of the normal adult rat, including expression by neurons throughout the gray matter, and in a subpopulation of white and gray matter glia. Both BDNF and NT-3 mRNAs were also densely expressed by alpha motoneurons of lamina IX, but were detected at lower levels elsewhere in the gray matter. NT-3 mRNA was additionally expressed by spinal cord glia, but was less widespread compared to NT-4/5. In response to systemic administration of KA, NT-4/5 and BDNF mRNAs were dramatically upregulated in a spatially and temporally restricted fashion, whereas levels of NT-3 mRNA were unchanged. These results provide strong in vivo evidence to support the idea that BDNF, NT-3, and in particular, NT-4/5, play a role in the normal function of the adult spinal cord. Furthermore, our results indicate that the actions of BDNF and NT-4/5 participate in the response of the cord to excitotoxic stimuli, and that those of NT-4/5 and NT-3 include both neurons and glia.

Impact of neonatal kainate treatment on hippocampal insulin-like growth factor receptors

The insulin-like growth factors-I and -II have neurotrophic properties and act through specific membrane receptors. High levels of binding sites for these growth factors are distributed discretely throughout the brain, being concentrated in the hippocampal formation. Functionally, the insulin-like growth factors, in addition to their growth-promoting actions, are considered to play important roles in normal cell functions, as well as in response to pharmacological or surgical manipulations. In adult rats, we have previously shown that systemic injection of kainate produces an overall decrease, in a time-dependent manner, in insulin-like growth factor-I and -II receptor binding sites in the hippocampus [Kar S. et al. (1997) Neuroscience 80, 1041-1055]. Given the evidence that insulin-like growth factors play a critical role during the early stages of brain development, the present study is a logical extension of this earlier report and established the effect of neonatal kainate injection on the developmental profile of insulin-like growth factor receptors. We have evaluated the time-course alteration of these receptors following systemic injection of kainate to newborn rats. After injection of a sublethal dose of kainate (5 mg/kg, i.p.) to postnatal one-day-old pups, [125I]insulin-like growth factor-I, [125I]insulin-like growth factor-II and [125I]insulin binding sites were studied at different postnatal days (7, 14, 21, 28 and 35) using receptor autoradiography. In the developing hippocampus, insulin-like growth factor-I and insulin binding sites are concentrated primarily in the dentate gyrus and the CA2/CA3 subfields, whereas insulin-like growth factor-II binding is discretely localized to the pyramidal layer and the granular layer of the dentate gyrus. Following kainate injection, we observed a slight increase in insulin-like growth factor-I binding sites in given hippocampal subfields starting at postnatal day 14, being significant at day 21. At later days, a progressive decrease was noted. This transient increase may represent an attempt for neuronal plasticity by up-regulating receptor levels. In contrast, insulin-like growth factor-II and insulin receptor binding sites are found to be decreased in various regions of the hippocampus in kainate-treated pups. Taken together, these results provide further evidence for the existence and differential alterations of insulin-like growth factor-I, insulin-like growth factor-II and insulin receptors in the developing rat hippocampus following kainate-induced lesion, suggesting possible involvement of these growth factors in brain plasticity.

Immunohistochemical evidence of seizure-induced activation of trk receptors in the mossy fiber pathway of adult rat hippocampus

Recent work suggests that limiting the activation of the trkB subtype of neurotrophin receptor inhibits epileptogenesis, but whether or where neurotrophin receptor activation occurs during epileptogenesis is unclear. Because the activation of trk receptors involves the phosphorylation of specific tyrosine residues, the availability of antibodies that selectively recognize the phosphorylated form of trk receptors permits a histochemical assessment of trk receptor activation. In this study the anatomy and time course of trk receptor activation during epileptogenesis were assessed with immunohistochemistry, using a phospho-specific trk antibody. In contrast to the low level of phosphotrk immunoreactivity constitutively expressed in the hippocampus of adult rats, a striking induction of phosphotrk immunoreactivity was evident in the distribution of the mossy fibers after partial kindling or kainate-induced seizures. The anatomic distribution, time course, and threshold for seizure-induced phosphotrk immunoreactivity correspond to the demonstrated pattern of regulation of BDNF expression by seizure activity. These results provide immunohistochemical evidence that trk receptors undergo activation during epileptogenesis and suggest that the mossy fiber pathway is particularly important in the pro-epileptogenic effects of the neurotrophins.

Immunohistochemical expression of trk receptor proteins in focal cortical dysplasia with intractable epilepsy

Focal cortical dysplasia (FCD), which is often associated with intractable epilepsy, is a form of abnormal structure of the cerebral cortex caused by a disorder of normal neocortical development. In such cerebral lesions obtained from four patients (two male, two female; average age 32.3 years at operation), the immunohistochemical expression of Trk receptors, which interact with neurotrophins and result in both growth and maturational changes in neuronal cells, was investigated in relation to the possible histogenesis of these lesions. In all cases, a derangement of the cortical laminar structure, dysplastic cytomegalic neurones, and large round balloon cells were the characteristic histological features. Immunohistochemically, the TrkA expression was localized in large dysplastic cytomegalic neurones, and TrkB expression was observed in large dysplastic and relatively small neuronal cells within the affected cortex. Although the exact roles of neurotrophins and their receptors in the pathogenesis of FCD remain uncertain, its development might be governed by such neurotrophic influences, and thus possibly prevent the death of abnormal neuronal cells. In addition, Trk receptors in FCDs may also play a role in establishing in the intrinsic epileptogenicity of FCDs.

Brain-derived neurotrophic factor immunoreactivity in the limbic system of rats after acute seizures and during spontaneous convulsions: temporal evolution of changes as compared to neuropeptide Y

Seizures increase the synthesis of brain-derived neurotrophic factor in forebrain areas, suggesting this neurotrophin has biological actions in epileptic tissue. The understanding of these actions requires information on the sites and extent of brain-derived neurotrophic factor production in areas involved in seizures onset and their spread. In this study, we investigated by immunocytochemistry the changes in brain-derived neurotrophic factor in the hippocampus, entorhinal and perirhinal cortices of rats at increasing times after acute seizures eventually leading to spontaneous convulsions. We also tested the hypothesis that seizure-induced changes in brain-derived neurotrophic factor induce later modifications in neuropeptide Y expression by comparing, in each instance, their immunoreactive patterns. As early as 100 min after seizure induction, brain-derived neurotrophic factor immunoreactivity increased in CA1 pyramidal and granule neurons and in cells of layers II-III of the entorhinal cortex. At later times, immunoreactivity progressively decreased in somata while increasing in fibres in the hippocampus, the subicular complex and in specific layers of the entorhinal and perirhinal cortices. Changes in neuropeptide Y immunoreactivity were superimposed upon and closely followed those of brain-derived neurotrophic factor. One week after seizure induction, brain-derived neurotrophic factor and neuropeptide Y immunoreactivities were similar to controls in 50% of rats. In rats experiencing spontaneous convulsions, brain-derived neurotrophic factor and neuropeptide Y immunoreactivity was strongly enhanced in fibres in the hippocampus/parahippocampal gyrus and in the temporal cortex. In the dentate gyrus, changes in immunoreactivity depended on sprouting of mossy fibres as assessed by growth-associated protein-43-immunoreactivity. These modifications were inhibited by repeated anticonvulsant treatment with phenobarbital. The dynamic and temporally-linked alterations in brain-derived neurotrophic factor and neuropeptide Y in brain regions critically involved in epileptogenesis suggest a functional link between these two substances in the regulation of network excitability.

Activity-Dependent Activation of TrkB Neurotrophin Receptors in the Adult CNS

In this paper we have investigated the hypothesis that neural activity causes rapid activation of TrkB neurotrophin receptors in the adult mammalian CNS. These studies demonstrate that kainic acid-induced seizures led to a rapid and transient activation of TrkB receptors in the cortex. Subcellular fractionation demonstrated that these activated Trk receptors were preferentially enriched in the synaptosomal membrane fraction that also contained postsynaptic glutamate receptors. The fast activation of synaptic TrkB receptors could be duplicated in isolated cortical synaptosomes with KCl, presumably as a consequence of depolarization-induced BDNF release. Importantly, TrkB activation was also observed following pharmacological activation of brain-stem noradrenergic neurons, which synthesize and anterogradely transport BDNF; treatment with yohimbine led to activation of cortical TrkB receptors within 30 min. Pharmacological blockade of the postsynaptic α1-adrenergic receptors with prazosin only partially inhibited this effect, suggesting that the TrkB activation was partially due to a direct effect on postsynaptic cortical neurons. Together, these data support the hypothesis that activity causes release of BDNF from presynaptic terminals, resulting in a rapid activation of postsynaptic TrkB receptors. This activity-dependent TrkB activation could play a major role in morphological growth and remodelling in both the developing and mature nervous systems.

Cellular localization of brain-derived neurotrophic factor-like immunoreactivity in adult monkey brain

Immunohistochemical localization of brain-derived neurotrophic factor (BDNF) in the monkey brain was investigated using a polyclonal anti-BDNF antibody produced in our laboratory. The antibody recognized a single band in monkey brain homogenates, and the estimated molecular weight was approximately 14 kDa, which corresponds well to the molecular weight of BDNF monomer. BDNF-like immunoreactivity was observed in the somata and processes of discrete neuronal subpopulations in the monkey brain. BDNF-positive neurons were widely distributed in various structures of the brain, including the cerebral cortex, hippocampus, basal forebrain, basal ganglia, diencephalon, brainstem and cerebellum. In addition, immunopositive glial cells were found in some brain regions. These data suggest that BDNF may exist widely in the monkey brain, and may be concerned with various types of neurons in the monkey central nervous system.

Selective inhibition of kindling development by intraventricular administration of TrkB receptor body

Recent work has shown that neurotrophin gene expression is increased after seizures evoked in the kindling model of epilepsy, but whether neurotrophins regulate kindling development is as yet unclear. In this study, we attempted to block selectively the activation of distinct neurotrophin receptors throughout kindling development in the rat via chronic intracerebroventricular administration of trk receptor bodies. The efficacy and selectivity of the trk receptor bodies were established by inhibition of neurotrophin-induced trk receptor phosphorylation in pheochromocytoma (PC12) cells and primary cultures of cortical neurons. The intracerebroventricular infusion of trkB receptor body (trkB-Fc) inhibited development of kindling in comparison with that seen with saline or human IgG controls, trkA-Fc, or trkC-Fc. These results imply that activation of trkB receptors contributes to the development of kindling, a form of activity-dependent behavioral plasticity in the adult mammalian brain.

Heparin-binding epidermal growth factor-like growth factor in hippocampus: modulation of expression by seizures and anti-excitotoxic action

The expression of heparin-binding epidermal growth factor-like growth factor (HB-EGF), an EGF receptor ligand, was investigated in rat forebrain under basal conditions and after kainate-induced excitotoxic seizures. In addition, a potential neuroprotective role for HB-EGF was assessed in hippocampal cultures. In situ hybridization analysis of HB-EGF mRNA in developing rat hippocampus revealed its expression in all principle cell layers of hippocampus from birth to postnatal day (P) 7, whereas from P14 through adulthood, expression decreased in the pyramidal cell layer versus the dentate gyrus granule cells. After kainate-induced excitotoxic seizures, levels of HB-EGF mRNA increased markedly in the hippocampus, as well as in several other cortical and limbic forebrain regions. In the hippocampus, HB-EGF mRNA expression increased within 3 hr after kainate treatment, continued to increase until 24 hr, and then decreased; increases occurred in the dentate gyrus granule cells, in the molecular layer of the dentate gyrus, and in and around hippocampal pyramidal CA3 and CA1 neurons. At 48 hr after kainate treatment, HB-EGF mRNA remained elevated in vulnerable brain regions of the hippocampus and amygdaloid complex. Western blot analysis revealed increased levels of HB-EGF protein in the hippocampus after kainate administration, with a peak at 24 hr. Pretreatment of embryonic hippocampal cell cultures with HB-EGF protected neurons against kainate toxicity. The kainate-induced elevation of [Ca2+]i in hippocampal neurons was not altered in cultures pretreated with HB-EGF, suggesting an excitoprotective mechanism different from that of previously characterized excitoprotective growth factors. Taken together, these results suggest that HB-EGF may function as an endogenous neuroprotective agent after seizure-induced neural activity/injury.