Differential gene expression profiling of short and long term denervated muscle

Skeletal muscle function and viability are dependent upon intact innervation. Peripheral nerve injury and muscle denervation cause muscle atrophy. Time to re-innervation is one of the most important determinants of functional outcome. While short-term denervation can result in nearly fully reversible changes in muscle mass, prolonged denervation leads to irreversible muscle impairment from profound atrophy, myocyte death and fibrosis. We performed transcriptional profiling to identify genes that were altered in expression in short-term (1 month) and long-term (3 month) denervated muscle and validated the microarray data by RT-PCR and Western blotting. Genes controlling cell death, metabolism, proteolysis, stress responses and protein synthesis/translation were altered in expression in the denervated muscle. A differential pattern of expression of genes encoding cell cycle regulators and extracellular matrix components was identified that correlated with the development of irreversible post-denervation changes. Genes encoding mediators of protein degradation were differentially expressed between 1 and 3 month denervated muscle suggesting different signaling networks are recruited over time to induce and maintain muscle atrophy. Understanding of the timing and type of pathological processes that are triggered by denervation may allow the design of interventions that delay or protect muscle from loss of nerve function.

Precursor form of brain-derived neurotrophic factor and mature brain-derived neurotrophic factor are decreased in the pre-clinical stages of Alzheimer's disease

Brain-derived neurotrophic factor (BDNF) is critical for the function and survival of neurons that degenerate in the late stage of Alzheimer's disease (AD). There are two forms of BDNF, the BDNF precursor (proBDNF) and mature BDNF, in human brain. Previous studies have shown that BDNF mRNA and protein, including proBDNF, are dramatically decreased in end-stage AD brain. To determine whether this BDNF decrease is an early or late event during the progression of cognitive decline, we used western blotting to measure the relative amounts of BDNF proteins in the parietal cortex of subjects clinically classified with no cognitive impairment (NCI), mild cognitive impairment (MCI) or mild to moderate AD. We found that the amount of proBDNF decreased 21 and 30% in MCI and AD groups, respectively, as compared with NCI, consistent with our previous results of a 40% decrease in end-stage AD. Mature BDNF was reduced 34 and 62% in MCI and AD groups, respectively. Thus, the decrease in mature BDNF and proBDNF precedes the decline in choline acetyltransferase activity which occurs later in AD. Both proBDNF and mature BDNF levels were positively correlated with cognitive measures such as the Global Cognitive Score and the Mini Mental State Examination score. These results demonstrate that the reduction of both forms of BDNF occurs early in the course of AD and correlates with loss of cognitive function, suggesting that proBDNF and BDNF play a role in synaptic loss and cellular dysfunction underlying cognitive impairment in AD.

Differential actions of nerve growth factor receptors TrkA and p75NTR in a rat model of epileptogenesis

Kindling, an experimental model of epileptogenesis, and activation-induced synaptic reorganization are modulated by nerve growth factor (NGF), but whether NGF acts via its high-affinity receptor TrkA and/or the common neurotrophin receptor p75NTR is unknown. We previously demonstrated, and confirmed in this study, that inhibition of NGF binding to both TrkA and p75NTR inhibited kindling and decreased kindling-induced mossy fiber sprouting. We now report specific inhibition of TrkA.NGF binding, but not p75NTR.NGF binding, retarded perforant path kindling progression. However, mossy fiber sprouting was inhibited by either selective TrkA.NGF or p75NTR.NGF antagonists. Our results suggest that TrkA, but not p75NTR, plays a role in kindling, while both receptors modulate kindling-induced mossy fiber sprouting. This implicates different mechanisms of neurotrophin action on kindling (mediated by TrkA) and neuronal sprouting (mediated by both TrkA and p75NTR) and suggests that sprouting involves kindling-independent neurotrophin action via p75NTR.

Precursor form of brain-derived neurotrophic factor and mature brain-derived neurotrophic factor are decreased in the pre-clinical stages of Alzheimer's disease

Brain-derived neurotrophic factor (BDNF) is critical for the function and survival of neurons that degenerate in the late stage of Alzheimer's disease (AD). There are two forms of BDNF, the BDNF precursor (proBDNF) and mature BDNF, in human brain. Previous studies have shown that BDNF mRNA and protein, including proBDNF, are dramatically decreased in end-stage AD brain. To determine whether this BDNF decrease is an early or late event during the progression of cognitive decline, we used western blotting to measure the relative amounts of BDNF proteins in the parietal cortex of subjects clinically classified with no cognitive impairment (NCI), mild cognitive impairment (MCI) or mild to moderate AD. We found that the amount of proBDNF decreased 21 and 30% in MCI and AD groups, respectively, as compared with NCI, consistent with our previous results of a 40% decrease in end-stage AD. Mature BDNF was reduced 34 and 62% in MCI and AD groups, respectively. Thus, the decrease in mature BDNF and proBDNF precedes the decline in choline acetyltransferase activity which occurs later in AD. Both proBDNF and mature BDNF levels were positively correlated with cognitive measures such as the Global Cognitive Score and the Mini Mental State Examination score. These results demonstrate that the reduction of both forms of BDNF occurs early in the course of AD and correlates with loss of cognitive function, suggesting that proBDNF and BDNF play a role in synaptic loss and cellular dysfunction underlying cognitive impairment in AD.

Time-dependent effect of kainate-induced seizures on glutamate receptor GluR5, GluR6, and GluR7 mRNA and Protein Expression in rat hippocampus

PURPOSE

Glutamate receptor 6 is strongly implicated in human refractory epilepsy and in kainic acid (KA)-induced status epilepticus (SE). In vitro pharmacologic studies with newer antiepileptic drugs (AEDs) are increasingly indicating the role of glutamate receptor 5 (GluR5) in epilepsy. Glutamate receptor 7 (GluR7) has been the least investigated in the context of epilepsy. We studied the messenger ribonucleic acid (mRNA) and protein expression of GluR5, GluR6, and GluR7 in rat hippocampus 72 h, 90 days, and 180 days after KA-induced SE.

METHODS

SE was induced by injecting KA intraperitoneally (i.p.) into adult rats. The hippocampi were isolated 72 h, 90 days, and 180 days after SE. Reverse transcription-polymerase chain reaction (RT-PCR) was performed for mRNA expression. Western blots determined the protein expression.

RESULTS

A significant increase was noted in GluR5 expression in KA-treated animals compared with controls at 72 h and 180 days, with no significant difference at the intervening 90-day point. Protein levels for GluR5 increased at 72 h and remained elevated until 180 days. GluR7 mRNA showed a significant decrease at 90 days after seizures. Neither the mRNA expression nor the protein levels of GluR6 differed from controls at any of the times after SE.

CONCLUSIONS

KA-induced SE leads to an upregulation of GluR5 mRNA and protein and a downregulation of GluR7 mRNA in rat hippocampus, with no change in GluR6 mRNA or protein expression.

Contribution of the distal nerve sheath to nerve and muscle preservation following denervation and sensory protection

The goal of this study was to determine the contribution of the distal nerve sheath to sensory protection. Following tibial nerve transection, rats were assigned to one of the following groups: (1) saphenous-to-tibial nerve neurorrhaphy; (2) saphenous-to-gastrocnemius neurotization; (3) unprotected controls (tibial nerve transection); or (4) immediate common peroneal-to-tibial nerve neurorrhaphy. After a 6-month denervation period and motor reinnervation, ultrastructural, histologic, and morphometric analyses were performed on the distal tibial nerve and gastrocnemius muscle cross-sections. Sensory axons neurotized to muscle maintain existing muscle integrity, as demonstrated by less fibrosis, collagenization, and fat deposition, more than unprotected muscle, and preserve the distribution pattern of fast twitch fibers. However, neurorrhaphy of the sensory nerve to the distal tibial nerve (involving the distal nerve sheath) improves existing endoneurial sheath structure, demonstrated by reduced collagen, and enhances regeneration, shown by improved axon-to-Schwann cell coupling and increased axon area. The authors conclude that sensory protection of muscle does not require the distal nerve sheath, but that preservation of the distal sheath may contribute to enhanced nerve regeneration.

NGF, BDNF, NT-3, and GDNF mRNA expression in rat skeletal muscle following denervation and sensory protection

Poor muscle and nerve functional recovery after nerve damage is a serious clinical problem, particularly if there is prolonged delay before nerve-muscle contact is reestablished. Our previous studies showed that sensory nerve cross-anastomosis (sensory protection) provides support to the denervated muscle. In the present study, we analyzed neurotrophic factor mRNA expression by RT-PCR in denervated rat gastrocnemius muscle receiving sensory protection with the saphenous nerve, compared to normal innervated muscle, to denervated muscle, and to denervated muscle repaired immediately with the peroneal (motor) nerve, after periods of 3 days to 3 months. No significant differences in mRNA levels of beta-actin, nerve growth factor, brain-derived neurotrophic factor or neurotrophin-3 were found between the sensory protection treatment and the denervated or the motor repair groups. However, sensory protection resulted in levels of muscle glial cell line-derived neurotrophic factor mRNA expression that were lower than in denervated muscle and higher than in muscle given immediate motor repair. These results demonstrate that glial cell line-derived neurotrophic factor mRNA is elevated following denervation but is partially down-regulated by sensory protection. Our study suggests that sensory protection provides a modified trophic environment by modulating neurotrophic factor synthesis in muscle.

The effects of brain-derived neurotrophic factor (BDNF) administration on kindling induction, Trk expression and seizure-related morphological changes

Brain-derived neurotrophic factor (BDNF) is a member of the neurotrophin family that mediates synaptic plasticity and excitability in the CNS. Recent evidence has shown that increased BDNF levels can lead to hyperexcitability and epileptiform activities, while suppression of BDNF function in transgenic mice or by antagonist administration retards the development of seizures. However, several groups, including our own, have reported that increasing BDNF levels by continuous intrahippocampal infusion inhibits epileptogenesis. It is possible that the continuous administration of BDNF produces a down-regulation of its high-affinity TrkB receptor, leading to a decrease of neuronal responsiveness to BDNF. If so, then animals should respond differently to bolus injections of BDNF, which presumably do not alter Trk expression, compared with continuous infusion. To test this hypothesis, we compared the effects of intrahippocampal BDNF continuous infusion and bolus injections on kindling induction. We showed that continuous infusion of BDNF inhibited the development of behavioral seizures and decreased the level of phosphorylated Trks or TrkB receptors. In contrast, multiple bolus microinjections of BDNF accelerated kindling development and did not affect the level of phosphorylated Trks or TrkB receptors. Our results indicate that different administration protocols yield opposite effects of BDNF on neuronal excitability, epileptogenesis and Trk expression. Unlike nerve growth factor and neurotrophin-3, which affect mossy fiber sprouting, we found that BDNF administration had no effect on the mossy fiber system in naive or kindled rats. Such results suggest that the effects of BDNF on epileptogenesis are not modulated by its effect on sprouting, but rather by its effects on excitability.

Kindling and status epilepticus models of epilepsy: rewiring the brain

This review focuses on the remodeling of brain circuitry associated with epilepsy, particularly in excitatory glutamate and inhibitory GABA systems, including alterations in synaptic efficacy, growth of new connections, and loss of existing connections. From recent studies on the kindling and status epilepticus models, which have been used most extensively to investigate temporal lobe epilepsy, it is now clear that the brain reorganizes itself in response to excess neural activation, such as seizure activity. The contributing factors to this reorganization include activation of glutamate receptors, second messengers, immediate early genes, transcription factors, neurotrophic factors, axon guidance molecules, protein synthesis, neurogenesis, and synaptogenesis. Some of the resulting changes may, in turn, contribute to the permanent alterations in seizure susceptibility. There is increasing evidence that neurogenesis and synaptogenesis can appear not only in the mossy fiber pathway in the hippocampus but also in other limbic structures. Neuronal loss, induced by prolonged seizure activity, may also contribute to circuit restructuring, particularly in the status epilepticus model. However, it is unlikely that any one structure, plastic system, neurotrophin, or downstream effector pathway is uniquely critical for epileptogenesis. The sensitivity of neural systems to the modulation of inhibition makes a disinhibition hypothesis compelling for both the triggering stage of the epileptic response and the long-term changes that promote the epileptic state. Loss of selective types of interneurons, alteration of GABA receptor configuration, and/or decrease in dendritic inhibition could contribute to the development of spontaneous seizures.

Increased proNGF levels in subjects with mild cognitive impairment and mild Alzheimer disease

Nerve growth factor (NGF) is critical for the regulation, differentiation, and survival of basal forebrain cholinergic neurons that degenerate in the late stage of Alzheimer disease (AD). The precursor of NGF (proNGF) is the predominant form of NGF in brain and is increased in end stage AD. To determine whether this increase in proNGF is an early or late change during the progression of cognitive decline, we used Western blotting to measure the relative amounts of proNGF protein in the parietal cortex from subjects clinically classified with no cognitive impairment (NCI; n = 20), mild cognitive impairment (MCI; n = 20), or mild to moderate AD (n = 19). We found that proNGF increased during the prodromal stage of AD. The amount of proNGF protein was 1.4-fold greater in the MCI group as compared to NCI, and was 1.6-fold greater in mild-moderate AD as compared to NCI, similar to our previous findings of a 2-fold increase in end stage AD. There was a negative correlation between proNGF levels and Mini Mental Status Examination (MMSE) score, demonstrating that the accumulation of proNGF is correlated with loss of cognitive function. These findings demonstrate that proNGF levels increase during the preclinical stage of AD and may reflect an early biological marker for the onset of AD.

Strain differences affect the induction of status epilepticus and seizure-induced morphological changes

Genetic deficits have been discovered in human epilepsy, which lead to alteration of the balance between excitation and inhibition, and ultimately result in seizures. Rodents show similar genetic determinants of seizure induction. To test whether seizure-prone phenotypes exhibit increased seizure-related morphological changes, we compared two standard rat strains (Long-Evans hooded and Wistar) and two specially bred strains following status epilepticus. The special strains, namely the kindling-prone (FAST) and kindling-resistant (SLOW) strains, were selectively bred based on their amygdala kindling rate. Although the Wistar and Long-Evans hooded strains experienced similar amounts of seizure activity, Wistar rats showed greater mossy fiber sprouting and hilar neuronal loss than Long-Evans hooded rats. The mossy fiber system was affected differently in FAST and SLOW rats. FAST animals showed more mossy fiber granules in the naïve state, but were more resistant to seizure-induced mossy fiber sprouting than SLOW rats. These properties of the FAST strain are consistent with those observed in juvenile animals, further supporting the hypothesis that the FAST strain shares circuit properties similar to those seen in immature animals. Furthermore, the extent of mossy fiber sprouting was not well correlated with sensitivity to status epilepticus, but was positively correlated with the frequency of spontaneous recurrent seizures in the FAST rats only, suggesting a possible role for axonal sprouting in the development of spontaneous seizures in these animals. We conclude that genetic factors clearly affect seizure development and related morphological changes in both standard laboratory strains and the selectively bred seizure-prone and seizure-resistant strains.

The nerve growth factor precursor proNGF exhibits neurotrophic activity but is less active than mature nerve growth factor

Nerve growth factor (NGF) promotes neuronal survival and differentiation and stimulates neurite outgrowth. NGF is synthesized as a precursor, proNGF, which undergoes post-translational processing to generate mature beta-NGF. It has been assumed that, in vivo, NGF is largely processed into the mature form and that mature NGF accounts for the biological activity. However, we recently showed that proNGF is abundant in CNS tissues whereas mature NGF is undetectable, suggesting that proNGF has biological functions beyond its role as a precursor. To determine whether proNGF exhibits biological activity, we mutagenized the precursor-processing site and expressed unprocessed, cleavage-resistant proNGF protein in insect cells. Survival and neurite outgrowth assays on murine superior cervical ganglion neurons and PC12 cells indicated that proNGF exhibits neurotrophic activity similar to mature 2.5S NGF, but is approximately fivefold less active. ProNGF binds to the high-affinity receptor, TrkA, as determined by cross-linking to PC12 cells, and is also slightly less active than mature NGF in promoting phosphorylation of TrkA and its downstream signaling effectors, Erk1/2, in PC12 and NIH3T3-TrkA cells. These data, coupled with our previous report that proNGF is the major form of NGF in the CNS, suggest that proNGF could be responsible for much of the biological activity normally attributed to mature NGF in vivo.

EphA/ephrin-A interactions regulate epileptogenesis and activity-dependent axonal sprouting in adult rats

The Eph family of tyrosine kinase receptors and their ligands, ephrins, are distributed in gradients and serve as molecular guidance cues for axonal patterning during neuronal development. Most of these molecules are also expressed in mature brain. Thus, we examine here the potential roles of such molecules in plasticity and activity-dependent mossy fiber sprouting of adult CNS. We show that the ligand ephrin-A3 and the receptor EphA5 are expressed in complementary gradients in the adult rat mossy fiber system. Using the kindling model, we demonstrate that exogenous immunoadhesins that affect the interaction of endogenous EphA receptors and ephrin-A ligands modulate the development of kindling, one type of long-term plasticity, in mature rat brain. These immunoadhesins, combined with epileptogenic stimulations, alter both the extent and the pattern of collateral axonal sprouting in the mossy fiber pathway. Our results suggest that EphA receptors and ephrin-A ligands modify neuronal plasticity and may serve as spatial cues that modulate the development and pattern of activation-dependent axonal growth in adult CNS.

ProNGF: a neurotrophic or an apoptotic molecule?

Nerve growth factor (NGF) acts on various classes of central and peripheral neurons to promote cell survival, stimulate neurite outgrowth and modulate differentiation. NGF is synthesized as a precursor, proNGF, which undergoes processing to generate mature NGF. It has been assumed, based on studies in the mouse submandibular gland, that NGF in vivo is largely mature NGF, and that mature NGF accounts for the molecule's biological activity. However, recently we have shown that proNGF is abundant in central nervous system tissues whereas mature NGF is undetectable, suggesting that proNGF may have a function distinct from its role as a precursor. A recent report that proNGF has apoptotic activity contrasts with other data demonstrating that proNGF has neurotrophic activity. This chapter will review the structure and processing of NGF and what is known about the biological activity of proNGF. Possible reasons for the discrepancies in recent reports are discussed.

A ligand of the p65/p95 receptor suppresses perforant path kindling, kindling-induced mossy fiber sprouting, and hilar area changes in adult rats

Kindling, an animal model of epilepsy, results in an increased volume of the hilus of the dentate gyrus and sprouting of the mossy fiber pathway in the hippocampus. Our previous studies have revealed that chronic infusion of neurotrophins can regulate not only seizure development, but also these kindling-induced structural changes. Kindling, in turn, can alter the expression of neurotrophins and their receptors. We previously showed that intraventricular administration of a synthetic peptide that interferes with nerve growth factor stability and thus its binding to TrkA and p75(NTR) receptors suppressed kindling and sprouting. However, the precise involvement of TrkA, p75(NTR), and downstream signaling effectors of neurotrophins on kindling, sprouting and hilar changes are unknown. One of these downstream effectors is Ras. In the present study, we find that intraventricular infusion of the synthetic peptide Reo3Y, which binds to p65/p95 receptors and causes a rapid inactivation of Ras protein, impairs development of perforant path kindling, reduces the growth in afterdischarge duration, blocks kindling-induced mossy fiber sprouting in area CA3 of hippocampus and in inner molecular layer of the dentate gyrus, and prevents kindling-induced increases in hilar area. These results are consistent with a mediation of neurotrophin effects on kindling, hilar area, and axonal sprouting via Trk receptors, and suggest important roles for Ras in kindling and in kindling-induced structural changes.

Pro-brain-derived neurotrophic factor is decreased in parietal cortex in Alzheimer's disease

Brain-derived neurotrophic factor (BDNF) promotes the function and survival of the major neuronal types affected in Alzheimer disease, such as hippocampal, cortical and basal forebrain cholinergic neurons. We and others have demonstrated a reduction in BDNF mRNA expression in Alzheimer's disease hippocampus and cortex, which may help to explain the selective vulnerability of these neurons. Several studies have also shown decreased BDNF protein in Alzheimer's disease. BDNF protein is synthesized as a precursor, proBDNF, which is cleaved to the mature 14-kDa form. We demonstrate here that BDNF exists as a mixture of proBDNF and mature BDNF in all regions tested of human brain. Using Western blotting, we observe a 40% reduction in proBDNF levels in Alzheimer's disease parietal cortex compared to controls. Thus, decreased BDNF protein measured by ELISA and immunohistochemistry likely represents a mixture of the two BDNF forms, and previously reported decreases in BDNF protein may be due, at least in part, to a significant reduction in proBDNF levels. Although the biological activity of proBDNF is unknown, reduced proBDNF may have functional consequences for the selective neuronal degeneration in Alzheimer's disease brain.

Continuous infusion of neurotrophin-3 triggers sprouting, decreases the levels of TrkA and TrkC, and inhibits epileptogenesis and activity-dependent axonal growth in adult rats

Neurotrophin-3 (NT-3), a member of the neurotrophin family of neurotrophic factors, is important for cell survival, axonal growth and neuronal plasticity. Epileptiform activation can regulate the expression of neurotrophins, and increases or decreases in neurotrophins can affect both epileptogenesis and seizure-related axonal growth. Interestingly, the expression of nerve growth factor and brain-derived neurotrophic factor is rapidly up-regulated following seizures, while NT-3 mRNA remains unchanged or undergoes a delayed down-regulation, suggesting that NT-3 might have a different function in epileptogenesis. In the present study, we demonstrate that continuous intraventricular infusion of NT-3 in the absence of kindling triggers mossy fiber sprouting in the inner molecular layer of the dentate gyrus and the stratum oriens of the CA3 region. Furthermore, despite this NT-3-related sprouting effect, continuous infusion of NT-3 retards the development of behavioral seizures and inhibits kindling-induced mossy fiber sprouting in the inner molecular layer of the dentate gyrus. We also show that prolonged infusion of NT-3 leads to a decrease in kindling-induced Trk phosphorylation and a down-regulation of the high-affinity Trk receptors, TrkA and TrkC, suggesting an involvement of both cholinergic nerve growth factor receptors and hippocampal NT-3 receptors in these effects. Our results demonstrate an important inhibitory role for NT-3 in seizure development and seizure-related synaptic reorganization.

Activity-dependent changes in synaptophysin immunoreactivity in hippocampus, piriform cortex, and entorhinal cortex of the rat

Synaptophysin, an integral membrane glycoprotein of synaptic vesicles, has been widely used to investigate synaptogenesis in both animal models and human patients. Kindling is an experimental model of complex partial seizures with secondary generalization, and a useful model for studying activation-induced neural growth in adult systems. Many studies using Timm staining have shown that kindling promotes sprouting in the mossy fiber pathway of the dentate gyrus. In the present study, we used synaptophysin immunohistochemistry to demonstrate activation-induced neural sprouting in non-mossy fiber cortical pathways in the adult rat. We found a significant kindling-induced increase in synaptophysin immunoreactivity in the stratum radiatum of CA1 and stratum lucidum/radiatum of CA3, the hilus, the inner molecular layer of the dentate gyrus, and layer II/III of the piriform cortex, but no significant change in layer II/III of the entorhinal cortex, 4 weeks after the last kindling stimulation. We also found that synaptophysin immunoreactivity was lowest in CA3 near the hilus and increased with increasing distance from the hilus, a reverse pattern to that seen with Timm stains in stratum oriens following kindling. Furthermore, synaptophysin immunoreactivity was lowest in dorsal and greatest in ventral sections of both CA3 and dentate gyrus in both kindled and non-kindled animals. This demonstrates that different populations of sprouting axons are labeled by these two techniques, and suggests that activation-induced sprouting extends well beyond the hippocampal mossy fiber system.

Neurotrophic factors and Alzheimer's disease: are we focusing on the wrong molecule?

Brain derived neurotrophic factor (BDNF) promotes cholinergic neuron function and survival. In Alzheimer's disease, BDNF mRNA and protein are decreased in basal forebrain cholinergic neuron target tissues such as cortex and hippocampus. Using RT-PCR, we demonstrate that BDNF is synthesized in basal forebrain, supplying cholinergic neurons with a local as well as a target-derived source of this factor. BDNF mRNA levels are decreased 50% in nucleus basalis of Alzheimer disease patients compared to controls. Thus, not only do the basal forebrain cholinergic neurons have a reduced supply of target-derived BDNF, but also of local BDNF. We also show by Western blotting that human CNS tissue contains both proBDNF and mature BDNF protein. Moreover, we demonstrate a significant (2.25-fold) deficit in proBDNF protein in Alzheimer's disease parietal cortex compared to controls. Thus, reduced BDNF mRNA and protein levels in Alzheimer's disease suggests that BDNF administration may be an effective therapeutic strategy for this disorder.

Glial cell line-derived neurotrophic factor modulates kindling and activation-induced sprouting in hippocampus of adult rats

Kindling, a phenomenon in which repeated electrical stimulation of certain forebrain structures leads to an increase in the evoked epileptogenic response, is widely used to investigate the mechanisms of epilepsy. Kindling also results in sprouting of the dentate gyrus mossy fiber pathway and triggers astrocyte hypertrophy and increased volume of the hilus of the dentate gyrus. Our previous studies showed that infusion of the neurotrophin nerve growth factor accelerated the behavioral progression of amygdala kindling and affected kindling-induced structural changes in the brain, whereas intrahilar infusion of another neurotrophin, brain-derived neurotrophic factor, delayed amygdala kindling-induced seizure development and reduced the growth in afterdischarge duration, but had little effect on kindling-induced structural changes. In this paper, we report the effects of infusion of glial cell line-derived neurotrophic factor, a neurotrophic factor of the TGF-beta superfamily having similar central nervous system neuronal targets as brain-derived neurotrophic factor. We show that continuous intraventricular infusion of glial cell line-derived neurotrophic factor inhibits the behavioral progression of perforant path kindling-induced seizures without affecting afterdischarge duration. In addition, we demonstrate that intraventricular administration of glial cell line-derived neurotrophic factor prevents kindling-induced increases in hilar area and blocks mossy fiber sprouting in the CA3 region of the hippocampus. Glial cell line-derived neurotrophic factor did not have a statistically significant effect on the mossy fiber density in the inner molecular layer. Our results raise the possibility that glial cell line-derived neurotrophic factor plays a role in kindling and activation-induced neural growth via mechanisms distinct from those of the neurotrophins.

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.

Differential expression of nerve growth factor transcripts in glia and neurons and their regulation by transforming growth factor-beta1

Nerve growth factor (NGF) influences neuronal development, function, and response to injury. Using reverse transcriptase polymerase chain reaction, we find that mouse and rat cortex and spinal cord, and both neurons and glia in culture, express NGF mRNA. In the mouse, NGF is regulated by at least two promoters that govern synthesis of four different transcripts, A through D, that are all expressed in the mouse tissues and cells examined. In contrast, rat NGF expression varies with tissue and with cell type: transcript C is expressed strongly in brain but weakly in spinal cord, and transcript D is undetectable in rat central nervous system (CNS). In addition to species- and tissue-specific expression, NGF transcripts also exhibit cell type-specific expression: transcripts B, C and D are expressed in rat astrocytes but poorly or not at all in rat neurons, identifying glia as an important source of NGF in rat. NGF increases sharply after injury. TGF-beta1, which also increases immediately after injury, induces NGF mRNA and protein in rat and mouse glia but not in neurons. Furthermore, transcripts A, B and D, but not C, are upregulated by TGF-beta1 in mouse glia, whereas in rat glia, the major responsive transcript is C. Thus, there may be multiple TGF-beta1-responsive elements in the NGF promoters located upstream of exons 1 and 3 that may differ between mouse and rat. Moreover, NGF transcripts are differentially expressed in a species-, cell type-, and inducer-specific manner. These results have implications for the use of mice versus rats as models for the study of NGF regulation following CNS injury.

A new brain-derived neurotrophic factor transcript and decrease in brain-derived neurotrophic factor transcripts 1, 2 and 3 in Alzheimer's disease parietal cortex

Brain-derived neurotrophic factor (BDNF) supports hippocampal, cortical and basal forebrain cholinergic neurons, which lose function in Alzheimer's disease. In Alzheimer's tissues such as hippocampus and parietal cortex, brain- derived neurotrophic factor mRNA is decreased three- to four-fold compared with controls. However, the molecular mechanism of the down-regulation of BDNF in Alzheimer's disease is unknown. The human brain-derived neurotrophic factor gene has multiple promoters governing six non-coding upstream exons that are spliced to one downstream coding exon, leading to six different transcripts. Here we report an alternate human splice variant within exon 4I for a total of seven transcripts. Previous brain-derived neurotrophic factor mRNA measurements in Alzheimer's disease tissue were done using the downstream coding exon present in all transcripts. Using RT-PCR primers specific for each upstream exon, we observe a significant decrease in three human brain-derived neurotrophic factor mRNA transcripts in Alzheimer's disease samples compared with controls. Transcripts 1 and 3 each exhibit a two-fold decrease, and transcript 2 shows a five-fold decrease. There are no significant differences between control and Alzheimer's disease samples for the other transcripts, including the new splice variant. In rat, both transcripts 1 and 3 are regulated through the transcription factor cAMP response element binding protein, whose phosphorylation is decreased in the Alzheimer's disease brain. This could lead to specific down-regulation of the brain-derived neurotrophic factor transcripts shown here.