Rat beta-nerve growth factor sequence and site of synthesis in the adult hippocampus

Neural progenitor cell implants in the lesioned medial longitudinal fascicle of adult cats regulate synaptic composition and firing properties of abducens internuclear neurons

Transplants of neural progenitor cells (NPCs) into the injured CNS have been proposed as a powerful tool for brain repair, but, to date, few studies on the physiological response of host neurons have been reported. Therefore, we explored the effects of NPC implants on the discharge characteristics and synaptology of axotomized abducens internuclear neurons, which mediate gaze conjugacy for horizontal eye movements. NPCs were isolated from the subventricular zone of neonatal cats and implanted at the site of transection in the medial longitudinal fascicle of adult cats. Abducens internuclear neurons of host animals showed a complete restoration of axotomy-induced alterations in eye position sensitivity, but eye velocity sensitivity was only partially regained. Analysis of the inhibitory and excitatory components of the discharge revealed a normal re-establishment of inhibitory inputs, but only partial re-establishment of excitatory inputs. Moreover, their inhibitory terminal coverage was similar to that in controls, indicating that there was ultimately no loss of inhibitory synaptic inputs. Somatic coverage by synaptophysin-positive contacts, however, showed intermediate values between control animals and animals that had undergone axotomy, likely due to partial loss of excitatory inputs. We also demonstrated that severed axons synaptically contacted NPCs, most of which were VEGF immunopositive, and that abducens internuclear neurons expressed the VEGF receptor Flk1. Together, our results suggest that VEGF neurotrophic support might underlie the increased inhibitory-to-excitatory balance observed in the postimplant cells. The noteworthy improvement of firing properties of injured neurons following NPC implants indicates that these cells might provide a promising therapeutic strategy after neuronal lesions.

Intermittent vs Continuous Administration of Nerve Growth Factor to Injured Medial Septal Cholinergic Neurons in Rat Basal Forebrain

Many medial septal neurons of the basal forebrain are dependent on nerve growth factor (NGF) from the hippocampus for survival and maintenance of a cholinergic phenotype. When deprived of their source of NGF by axotomy, medial septal neuronal cell bodies atrophy and lose their cholinergic markers. This is similar to what is observed in the basal forebrain during Alzheimer’s disease (AD). In the present study, medial septal neurons were axotomized in female rats by way of a fimbria/fornix lesion. For fourteen days following axotomy, varying NGF doses (1 – 250 μg/ml) were administered to the lateral cerebral ventricle with either mini-osmotic infusion or daily injection. The responsiveness of medial septal neurons was evaluated with choline acetyltransferase immunohistochemistry. Within the mini-osmotic pumps, NGF activity diminished greatly during the first five days of implantation, but increased dramatically in the CSF after five days of infusion. The responsiveness of medial septal neurons to NGF was dose dependent and the ED₅₀ for NGF injection was determined to be 14.08 μg/ml compared to 27.60 μg/ml for NGF infusion. Intermittent injections at varying intervals were evaluated with 30 μg/ml NGF over a fourteen-day time period (2, 3, 6, or 12 injections). No differences occurred in the number of choline acetyltransferase neurons from rats that received weekly injections to those that received daily injections of NGF. NGF administration has been suggested as a therapy for AD. The results of these studies continue to highlight the need for NGF stability within the delivery system and AD patient CSF, the choice of delivery system, frequency of administration, and the NGF dose for maintaining basal forebrain cholinergic neurons during AD.

Furostanol saponins from the rhizomes of Dioscorea japonica and their effects on NGF induction

The rhizome of Dioscorea japonica is a food and medicinal source known as 'San Yak' in Korea. Two new furostanol saponins, coreajaponins A (1) and B (2), together with 10 known compounds (3-12) were isolated from the rhizomes of D. japonica. Their structures were determined by spectroscopic methods, including 1D and 2D NMR techniques, HRMS, and chemical methods. Nerve growth factor (NGF), a crucial factor for neuronal survival and differentiation, can potentially improve neurodegenerative diseases and diabetic polyneuropathy. We evaluated the effects of isolates (1-12) on NGF induction in a C6 rat glioma cell line. Coreajaponin B (2) upregulated NGF content without significant cell toxicity, as did 6, 8, 9, and 11.

Intravesical antisense therapy for cystitis using TAT-peptide nucleic acid conjugates

The present study investigated the potential of intravesical instillation for localized reduction of NGF (nerve growth factor) expression in the urinary bladder. Overexpression of NGF has been linked to the pathogenesis of interstitial cystitis (IC). A minimum free energy algorithm was used to predict suitable regions in mRNA of rat betaNGF, which can be targeted for an antisense approach. The candidate antisense oligos were evaluated for their ability to reduce NGF expression in vitro by cotransfecting HEK293 cells with NGF cDNA. A single oligonucleotide ODN sequence was chosen for testing in an acute cystitis model in rat induced by cyclophosphamide. Overexpression of NGF is known to mediate inflammation of bladder in this model. For improved stability, antisense ODN was replaced with antisense peptide nucleic acid (PNA) and its penetration into bladder was facilitated by tethering TAT peptide sequence. Rat bladders were instilled with either antisense or its scrambled control prior to cystitis induction. Cystometrograms performed on rats under urethane anaesthesia exhibited bladder contraction frequency that was significantly decreased in the antisense treated rats than rats treated with the control. NGF immunoreactivity was also decreased in the urothelium of the antisense treated bladders. Our findings demonstrate the feasibility of using TAT-PNA conjugates for intravesical antisense therapy.

Cerebrospinal fluid nerve growth factor levels in patients with Alzheimer's disease

BACKGROUND

Alzheimer's disease (AD) is the most common cause of dementia in Western countries and in Japan. Numerous blood and cerebrospinal fluid (CSF) tests based on the disease pathology have been proposed for early detection of AD. By comparing the CSF proteome of AD patients and controls it might be possible to identify proteins that play a role in the disease process and thus study the pathogenesis of AD.

PATIENTS AND METHODS

Samples of CSF from normal (n=20) and AD patients (n=20) were collected by lumbar puncture. The total concentration of proteins in the CSF of normal subjects and AD patients was determined by Bio-Rad protein assay based on the Bradford dye binding procedure. The presence and level of NGF in the CSF of normal and AD patients was measured by enzyme-linked immunosorbent assay (ELISA), SDS-PAGE and western blot.

RESULTS

The total protein concentration of all samples was within the normal range (0.10-0.44 g/L). A western blot analysis using anti-NGF antibody showed the presence of NGF in human CSF. By ELISA, the level of NGF in the CSF of AD patients was higher than in the CSF of normal subjects (81.5A+/-15.03 pg/mL vs. 4.2A+/-1.92 pg/mL, P<0.0001).

CONCLUSION

We suggest that the NGF level in the CSF may provide additional information in the differential diagnosis of AD. We also conclude that NGF could be significantly involved in the pathophysiology of AD.

Cytoskeletal Transport in the Aging Brain: Focus on the Cholinergic System

There is now compelling evidence for the aging-related breakdown of cytoskeletal support in neurons. Similarly affected are the principal components of the intracellular microtubule system, the transport units involved in active shuttle of organelles and molecules in an antero- and retrograde manner, and the proteins stabilizing the cytoskeleton and providing trophic support. Here, we review the basic organization of the cytoskeleton, and describe its elements and their interactions. We then critically assess the role of these cytoskeletal proteins in physiological aging and aging-related malfunction. Our focus is on the microtubule-associated protein tau, for which comprehensive investigations suggest a critical role in neurodegenerative diseases, for instance tauopathies. These diseases frequently lead to cognitive decline and are often paralleled by reductions in cholinergic neurotransmission. We propose this reduction to be due to destabilization of the cytoskeleton and protein transport mechanisms in these neurons. Therefore, maintenance of the neuronal cytoskeleton during aging may prevent or delay neurodegeneration as well as cognitive decline during physiological aging.

Expression of nerve growth factor in cerebrospinal fluid of congenital hydrocephalic and normal children

Cerebrospinal fluid (CSF) is secreted by the choroids plexuses and has the potential to act as a signaling pathway for physiological control as it has been demonstrated to contain molecules such as interleukins, leukoterins, neuropeptides, growth transforming factor-beta (TGF-beta) and nerve growth factor (NGF), which are present at specific times during development. In this study, CSF from hydrocephalic and normal children were analysed using SDS-PAGE followed by silver staining. In order to obtain semi-quantitative estimates of the relative amounts of 26 kDa protein, an image analyzer was used to determine the intensities of the band in the respective lanes in silver-stained gels. Quantification of the silver-stained gels from repeated experiments showed that the amount of 26 kDa protein was clearly increases in the hydrocephalic CSF when compared with the normal CSF. A Western blot analysis using anti-NGF antibody as a probe confirmed the presence of NGF. Using enzyme-linked immunosorbent assay (ELISA), it was shown that the level of NGF in the hydrocephalic CSF is higher than in normal CSF. It is concluded that NGF is not only a constant component of human CSF but could also be significantly involved in the pathophysiology of hydrocephalus.

Synergistic effect of dexamethasone and beta-adrenergic receptor agonists on the nerve growth factor gene transcription

Activation of beta-adrenergic receptor (betaAR) increases the synthesis of nerve growth factor (NGF) in the brain and in C6-2B glioma cells. However, in the brain, the betaAR-mediated increase in NGF expression appears to require the presence of glucocorticoids, suggesting that NGF promoter may be sensitive to cAMP and glucocorticoid-dependent transcription factors. We tested this hypothesis by exposing C6-2B glioma cells to dexamethasone (DEX) in combination with agents that increase cAMP levels and examining the DNA binding activity of two cAMP-dependent transcription factors that regulate NGF expression: cAMP responsive element binding protein (CREB) and CCAAT/enhancer binding protein delta (C/EBPdelta). Electrophoretic mobility shift assays revealed that the beta(2)AR agonist clenbuterol (CLE) or high levels of cAMP elicited a time-dependent increase in C/EBPdelta binding activity as well as phosphorylated CREB (P-CREB). When DEX, which per se showed little effect on these transcription factors, was combined with CLE, dibutyryl cAMP or isoproterenol, enhanced induction of P-CREB and C/EBP binding activity as well as NGF mRNA was observed. Moreover, the increase in NGF mRNA in the presence of DEX was prolonged compared to that obtained by CLE or other cAMP inducing agents alone. In fact, NGF mRNA levels remained significantly elevated at least for 24 h. These studies suggest that the synergistic effect of DEX on the induction of NGF mRNA may include the ability of this glucocorticoid to potentiate the betaAR-mediated induction of transcription factors.

Contribution of degeneration of motor and sensory fibers to pain behavior and the changes in neurotrophic factors in rat dorsal root ganglion

To elucidate the role of the degeneration of motor and sensory fibers in neuropathic pain, we examined the pain-related behaviors and the changes of brain-derived neurotrophic factor (BDNF) in the L4/5 dorsal root ganglion (DRG) and the spinal cord after L5 ventral rhizotomy. L5 ventral rhizotomy, producing a selective lesion of motor fibers, produced thermal hyperalgesia and increased BDNF expression in tyrosine kinase A-containing small- and medium-sized neurons in the L5 DRG and their central terminations within the spinal cord, but not in the L4 DRG. Furthermore, L5 ventral rhizotomy up-regulated nerve growth factor (NGF) protein in small to medium diameter neurons in the L5 DRG and also in ED-1-positive cells in the L5 spinal nerve, suggesting that NGF synthesized in the degenerative fibers is transported to the L5 DRG and increases BDNF synthesis. On the other hand, L5 ganglionectomy, producing a selective lesion of sensory fibers, produced heat hypersensitivity and an increase in BDNF and NGF in the L4 DRG. These data indicate that degeneration of L5 sensory fibers distal to the DRG, but not motor fibers, might influence the neighboring L4 nerve fibers and induce neurotrophin changes in the L4 DRG. We suggest that these changes of neurotrophins in the intact primary afferents of neighboring nerves may be one of many complex mechanisms, which can explain the abnormal pain behaviors after nerve injury. The ventral rhizotomy and ganglionectomy models may be useful to investigate the pathophysiological mechanisms of neuropathic pain after Wallerian degeneration in motor or sensory or mixed nerve.

Glial cell line-derived neurotrophic factor (GDNF) from adult rat tooth serves a distinct population of large-sized trigeminal neurons

Glial cell line-derived neurotrophic factor (GDNF) mediates trophic effects for specific classes of sensory neurons. The adult tooth pulp is a well-defined target of sensory trigeminal innervation. Here we investigated potential roles of GDNF in the regulation of adult trigeminal neurons and the dental pulp nerve supply of the rat maxillary first molar. Western blot analysis and radioactive 35S-UTP in situ hybridization revealed that GDNF in the dental pulp and its mRNAs were localized with Ngf in the coronal pulp periphery, in particular in the highly innervated subodontoblast layer. Retrograde neuronal transport of iodinated GDNF and Fluorogold (FG) from the dental pulp indicated that GDNF was transported in about one third of all the trigeminal dental neurons. Of the GDNF-labelled neurons, nearly all (97%) were large-sized (> or =35 microm in diameter). Analysis of FG-labelled neurons revealed that, of the trigeminal neurons supporting the adult dental pulp, approximately 20% were small-sized, lacked isolectin B4 binding and did not transport GDNF. Of the large-sized dental trigeminal neurons approximately 40% transported GDNF. About 90% of the GDNF-accumulating neurons were negative for the high-temperature nociceptive marker VRL-1. Our results show that a subclass of large adult trigeminal neurons are potentially dependent on dental pulp-derived GDNF while small dental trigeminal neurons seems not to require GDNF. This suggests that GDNF may function as a neurotrophic factor for subsets of nerves in the tooth, which apparently mediate mechanosensitive stimuli. As in dorsal root ganglia both small- and large-sized neurons are known to be GDNF-dependent; these data provide molecular evidence that the sensory supply in the adult tooth differs, in some aspects, from the cutaneous sensory system.

Differential expression of brain-derived neurotrophic factor transcripts after pilocarpine-induced seizure-like activity is related to mode of Ca2+ entry

Activity-dependent brain-derived neurotrophic factor (BDNF) expression is Ca2+-dependent, yet little is known about the Ca2+ channel contributions that might direct selective expression of the multiple BDNF transcripts. Here, effects of pilocarpine-induced seizure activity on total BDNF expression and on the individual sensitivity of BDNF transcripts to glutamate receptor and Ca2+ channel blockers were evaluated using hippocampal slice cultures and in situ hybridization of transcript-specific cRNA probes directed against mRNAs for the four 5' exons (I-IV) of the BDNF gene. mRNAs for nerve growth factor (NGF) and tyrosine kinase B (trkB) also were studied. Pilocarpine (5 mM) induced a dose- and time-dependent increase in total BDNF (exon V) mRNA expression in the dentate granule cells and CA3-CA1 pyramidal cells with maximal effects at 6 and 24 h, respectively. Increases were blocked by co-treatment with the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid/kainate 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX: 25 microM) and the N-methyl-d-aspartic acid receptor antagonist 2-amino-5-phosphonovaleric acid (APV; 25 microM), whereas the L-type voltage sensitive Ca2+ channel blocker nifedipine (20 microM) was without detectable effect. Maximal NGF and trkB mRNA expression was induced by pilocarpine at 4 and 12 h, respectively. For the individual BDNF transcripts, APV blocked pilocarpine-induced increases in transcript II, whereas nifedipine blocked increases in transcripts I and III. Transcript IV levels were not altered by treatment. These results indicate that transcript II makes the greatest contribution to pilocarpine effects on total BDNF mRNA content in this model and provides evidence for regional and Ca2+ channel-specific differences in activity-dependent regulation of the different BDNF transcripts in hippocampus.

Neurotrophins regulate proliferation and survival of two microglial cell lines in vitro

Microglia are thought to play a key role in the development and regeneration of the central nervous system although the mechanisms regulating their presence and activity are not fully understood. Substantial evidence suggests that members of the neurotrophin family such as nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), and neurotrophin-3 and -4 (NT-3/4) have a dramatic effect on both neurons and perineuronal cells. This study employed two murine microglial lines, BV-2 and N9, to examine the action of these neurotrophins on the mitotic activity and survival of microglia in vitro. Neurotrophins were incorporated into the media at the time of plating and cell number and levels of mitochondrial dehydrogenase activity (MTT) were determined at various time points in vitro. NGF increased cell number and MTT levels of both cell lines in a dose-dependent manner. BV-2 was more sensitive to NGF than N9. Similar responses were elicited by BDNF, although the sensitivity of each cell line was different than that found for NGF. NT-3 and NT-4 had no effect on cell proliferation. However, NT-4 had an effect on the survival of BV-2 and N9 cells. The response of these cells to neurotrophins was blocked by K252a, a tyrosine kinase inhibitor, suggesting that actions of neurotrophins were mediated by high-affinity tyrosine kinase receptors (Trk). Immunolocalization studies revealed positive Trk (pan) reactivity in the above cell lines and in primary microglia, but an absence of the low-affinity p75 neurotrophin receptor. Western blot analysis supported the above observations. These studies suggest that in addition to their neurotrophic actions, NGF and BDNF may also regulate microglial dynamics, thereby influencing the surrounding milieu during neuronal regeneration.

Sympathetic neurons synthesize and secrete pro-nerve growth factor protein

Postmitotic sympathetic neuronal survival is dependent upon nerve growth factor (NGF) provided by peripheral targets, and this dependency serves as a central tenet of the neurotrophic hypothesis. In some other systems, NGF has been shown to play an autocrine role, although the pervasiveness and significance of this phenomenon within the nervous system remain unclear. We show here that rat sympathetic neurons synthesize and secrete NGF. NGF mRNA is expressed in nearly half of superior cervical ganglion sympathetic neurons at embryonic day 17, rising to over 90% in the early postnatal period, and declining in the adult. Neuronal immunoreactivity is reduced when retrograde transport is interrupted by axotomy, but persists in a subpopulation of neurons despite diminished mRNA expression, suggesting that intrinsic protein synthesis occurs. Cultured neonatal neurons express NGF mRNA, which is maintained even when they are undergoing apoptosis. To determine which NGF isoforms are secreted, we performed metabolic labeling and immunoprecipitation of NGF-immunoreactive proteins synthesized by cultured NGF-dependent and -independent neurons. Conditioned medium contained high molecular weight NGF precursor proteins, which varied depending upon the state of NGF dependence. Mature NGF was undetectable by these methods. High molecular weight NGF isoforms were also detected in ganglion homogenates, and persisted at diminished levels following axotomy. We conclude that sympathetic neurons express NGF mRNA, and synthesize and secrete pro-NGF protein. These findings suggest that a potential NGF-sympathetic neuron autocrine loop may exist in this prototypic target-dependent system, but that the secreted forms of this neurotrophin apparently do not support neuronal survival.

Nerve growth factor prevents cell death and induces hypertrophy of basal forebrain cholinergic neurons in rats withdrawn from prolonged ethanol intake

We have previously reported that the hippocampal cholinergic fiber network is severely damaged in animals withdrawn from ethanol, and that a remarkable recovery in fiber density occurs following hippocampal grafting, a finding that we suggested to be underpinned by the graft production of neurotrophic factors, which are known to be decreased after ethanol exposure. It is widely accepted that nerve growth factor (NGF) signals the neurons of the brain cholinergic system, including those of the medial septum/vertical limb of the diagonal band of Broca (MS/VDB) nuclei, from which the septohippocampal projection arises. Because neurons in these nuclei are vulnerable to ethanol consumption and withdrawal we thought of interest to investigate, in withdrawn rats previously submitted to a prolonged period of ethanol intake, the effects of intraventricular delivery of NGF upon the MS/VDB cholinergic neurons. Stereological methods were applied to estimate neuron numbers and neuronal volumes in choline acetyltransferase (ChAT)-immunostained and Nissl-stained material. We have found that in ethanol-fed rats there was a significant reduction in the total number of Nissl-stained and cholinergic neurons in the MS/VDB, and that the suppression of ethanol intake further decreased neuron numbers. In addition, the somatic size of ChAT-IR neurons was reduced by ethanol intake, and withdrawal further aggravated neuronal atrophy. NGF treatment prevented the withdrawal-associated loss, and induced hypertrophy, of cholinergic neurons. These findings show that exogenous NGF protects the phenotype and prevents the withdrawal-induced degeneration of cholinergic neurons in the MS/VDB. These effects might be due to the trophic action of NGF upon the basal forebrain cholinergic neurons, including the hippocampal fiber network that conveys this neurotrophin retrogradely to the MS/VDB, and/or upon their targets, that is, the hippocampal formation neurons.

Neurotrophic properties of olfactory ensheathing glia

Olfactory ensheathing cells (OEC) constitute a specialized population of glia that accompany primary olfactory axons and have been reported to facilitate axonal regeneration after spinal cord injury in vivo. In the present report we describe OEC neurotrophic factor expression and neurotrophic properties of OECs in vitro. Investigation of the rat olfactory system during development and adulthood by radioactive in situ hybridization revealed positive labeling in the olfactory nerve layer for the neurotrophic molecules S-100beta, CNTF, BMP-7/OP-1, and artemin, as well as for the neurotrophic factor receptors RET and TrkC. Ribonuclease protection assay of cultured OEC revealed expression of NGF, BDNF, GDNF, and CNTF mRNA, while NT3 and NT4 mRNA were not detectable. In vitro bioassays of neurotrophic activity involved coculturing of adult OEC with embryonic chick ganglia and demonstrated increased neurite outgrowth from sympathetic, ciliary, and Remak's ganglia. However, when culturing the ganglia with OEC-conditioned medium, neurite outgrowth was not stimulated to any detectable extent. Our results suggest that the neurotrophic properties of OEC may involve secretion of neurotrophic molecules but that cellular interactions are crucial.

The effects of repeated minimal electroconvulsive shock exposure on levels of mRNA encoding fibroblast growth factor-2 and nerve growth factor in limbic regions

Chronic, but not acute, exposure to minimal electroconvulsive shock (ECS) has been shown to decrease vulnerability to neuronal cell death, without itself causing neuronal damage. One potential mechanism for the neuroprotective effect of ECS is the increase in fibroblast growth factor-2 (FGF-2) which occurs after chronic, but not acute, ECS exposure. This raises the possibility that repeated seizures over a period of several days may alter the transcriptional regulation of FGF-2. To test this hypothesis, the present study compared the effect of acute (1 day) vs. chronic (7 days) ECS treatment on levels of mRNA for FGF-2 in rhinal and frontal cortices, hippocampus, and olfactory bulbs. In addition, mRNA for another prominent neurotrophic factor, nerve growth factor (NGF), was assayed concurrently. At 8 h after acute ECS, mRNA levels increased by 60% for FGF-2 and 136% for NGF in rhinal cortex, 32% for FGF-2 and 36% for NGF in frontal cortex, and by 13% for NGF in hippocampus. After 7 days of ECS treatment the respective increases were 72% and 80%, 53% and 38%, and 28%. No increases were observed in olfactory bulbs after either treatment regimen. The peak increases in FGF-2 mRNA were consistently greater after chronic treatment, but the differences from those seen acutely reached significance in frontal cortex only. However, the duration over which mRNA for FGF-2 was elevated did not differ between the acute and chronic ECS groups. NGF mRNA induction was neither enhanced nor prolonged as a result of chronic ECS as compared to acute ECS treatment. These results suggest that chronic ECS treatment may lead to an enhanced rate of transcription of message for FGF-2 but not for NGF, in selected brain regions. At the same time, the results indicate that chronic ECS treatment induces FGF-2 and NGF mRNA expression in a tissue-specific manner and that this induction is maintained over the 7-day treatment period. The sustained increases in mRNAs for these trophic factors may contribute to the neuroprotective actions of chronic ECS treatment.

The effects of chronic ethanol consumption on neurotrophins and their receptors in the rat hippocampus and basal forebrain

Damage to the basal forebrain frequently results in deficits in learning and memory. Mnenonic dysfunction also occurs following prolonged ethanol consumption in humans and in animal models of chronic ethanol intake, accompanied by specific abnormalities in synaptic transmission between the basal forebrain and hippocampus. The integrity of at least some of the reciprocal neuronal connections between these brain regions is influenced by target-derived neurotrophic factors. We used a semiquantitative reverse transcription polymerase chain reaction technique to measure the messenger RNA for neurotrophins BDNF and NGF, and for their receptors trkB, trkA, and the low affinity receptor, p75(NTR) in the hippocampus and basal forebrain of rats after 28 weeks of alcohol consumption without malnutrition. This chronic ethanol treatment (CET) resulted in a marked and selective reduction in basal forebrain trkA mRNA. Western blotting revealed a similar reduction of basal forebrain trkA protein. CET effects on basal forebrain trkA may reflect impaired NGF signaling that could compromise septohippocampal synaptic connections, cholinergic differentiation, and emergent functional abilities dependent on these properties.

Expression of neurotrophin messenger RNAs during rat urinary bladder development

The family of neurotrophins, encompassing nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3) and neurotrophin-4/5 (NT-4/5), is important in the regulation of neuronal development and function. We examined the expression of neurotrophin messenger RNAs (mRNAs) in the rat urinary bladder during pre- and postnatal development using competitive reverse transcription-polymerase chain reaction. The mRNA levels showed a biphasic pattern of expression; one peak was at prenatal ages (embryonic day (E)15-E18) and the other peak was at postnatal ages (postnatal day (P)14-P28). NT-4/5, BDNF and NGF mRNA levels were greatest at E15, E16 and E18, respectively. In contrast, NT-3 mRNA levels were significantly highest at P14. These data suggest that neurotrophins are involved in the mechanisms of bladder nerve growth for the prenatal period and reorganization of bladder reflex pathways during the second to the fourth postnatal week.

Nerve growth factor and nerve growth factor receptors in respiratory syncytial virus-infected lungs

Nerve growth factor (NGF) controls sensorineural development and responsiveness and modulates immunoinflammatory reactions. Respiratory syncytial virus (RSV) potentiates the proinflammatory effects of sensory nerves in rat airways by upregulating the substance P receptor, neurokinin 1 (NK(1)). We investigated whether the expression of NGF and its trkA and p75 receptors in the lungs is age dependent, whether it is upregulated during RSV infection, and whether it affects neurogenic inflammation. Pathogen-free rats were killed at 2 (weanling) to 12 (adult) wk of age; in addition, subgroups of rats were inoculated with RSV or virus-free medium. In pathogen-free rats, expression of NGF and its receptors in the lungs declined with age, but RSV doubled expression of NGF, trkA, and p75 in weanling and adult rats. Exogenous NGF upregulated NK(1) receptor expression in the lungs. Anti-NGF antibody inhibited NK(1) receptor upregulation and neurogenic inflammation in RSV-infected lungs. These data indicate that expression of NGF and its receptors in the lungs declines physiologically with age but is upregulated by RSV and is a major determinant of neurogenic inflammation.

Regulation of Neurotrophin mRNA Expression in the Rat Brain by Glucocorticoids

Northern blot analysis was used to examine the effects of glucocorticoids on neurotrophin mRNA expression in the rat cerebral cortex and hippocampus. The results show that 3 days after adrenalectomy the mRNA levels for nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) decreased significantly in both these regions. In adrenalectomized animals given dexamethasone replacement the mRNA levels for the three neurotrophins were restored to control levels. The effect of a single dose of dexamethasone (5 mg/kg) administered i.p. to intact animals on the expression of neurotrophins was also examined. NGF and NT-3 mRNAs showed a 2.5-fold and a 1.4-fold increase, respectively, during the first 4 h after the injection. The increase was followed by a decrease, with levels approximately 50% of control 24 and 48 h after the injection. In contrast, the level of BDNF mRNA did not change during the first 10 h after the injection, but decreased to 70% of control 48 h after the injection. These data indicate that glucocorticoids regulate neurotrophin mRNA expression both in the cortex and in the hippocampus, and suggest further that the known effects of glucocorticoids on neuronal survival in the brain could be due to changes in the levels of neurotrophins in the brain.

Differential Regulation of Nerve Growth Factor (NGF) Synthesis in Neurons and Astrocytes by Glucocorticoid Hormones

Glucocorticoid hormones are important regulators of brain development and ageing. Here we show that dexamethasone, a synthetic glucocorticoid, differentially affects the expression of nerve growth factor (NGF) in cultured neurons and astrocytes. Dexamethasone increased the levels of NGF mRNA in cultured hippocampal neurons in a time- and concentration-dependent manner, whereas it down-regulated the NGF mRNA levels in astrocytes. However, dexamethasone had no effect on the mRNA levels of brain-derived neurotrophic factor in the hippocampal neurons. Aldosterone, a mineralocorticoid, in higher concentrations also up-regulated NGF mRNA levels in the hippocampal neurons. Dexamethasone increased the levels of NGF mRNA in the rat hippocampus in vivo, but not to the same extent as observed with kainic acid, a glutamate receptor agonist. There is no apparent diurnal rhythm in the hippocampal NGF protein levels corresponding to circadian variations in the levels of glucocorticoid hormones in serum. The increase in NGF mRNA in the hippocampus in vivo following dexamethasone treatments may reflect the physiological response of hippocampal neurons to high glucocorticoid levels reached under conditions of stress.

Effect of ischemic preconditioning on the expression of putative neuroprotective genes in the rat brain

Previous studies have demonstrated that sublethal ischemic insults protect from subsequent ischemia in the intact brain. There are two windows for the induction of tolerance by ischemic preconditioning (IPC). One occurs within 1 h following IPC, and the other one develops from 1 to 3 days after IPC. The goal of this study was to determine whether IPC neuroprotection may be mediated by expression of known neuroprotective genes and to characterize the temporal and spatial expression patterns of these genes. IPC was produced by bilateral carotid artery occlusions and hypotension (50 mmHg) for 2 min. After various survival times, the expression of MAP-2, brain-derived neurotrophic factor (BDNF), c-jun, c-fos, nerve growth factor (NGF) and HSP70 was assessed by in situ hybridization of coronal brain sections with 35S labeled probes. BDNF, NGF, and c-jun were significantly upregulated in the hippocampus. c-fos was detected in the hippocampus, cortex and striatum. HSP70 mRNA was induced in the cortex, hippocampus, striatum, and thalamus. MAP-2 showed no change in expression, confirming previous studies that no cell death occurs following IPC. The increase in expression of these stress-related, neurotrophic and immediate early genes in response to a mild preconditioning insult may help mediate the protection of vulnerable neurons to subsequent lethal ischemic insults.

Streptozotocin-induced diabetes reduces retrograde axonal transport in the afferent and efferent vagus nerve

Diabetes-induced alterations in nerve function include reductions in the retrograde axonal transport of neurotrophins. A decreased axonal accumulation of endogenous nerve growth factor (NGF) and neurotrophin-3 (NT-3) in the vagus nerve of streptozotocin (STZ)-induced diabetic rats was previously shown. In the current study, no changes in the NGF and NT-3 protein or mRNA levels in the stomach or atrium, two vagally innervated organs, were noted after 16 or 24 weeks of diabetes. Moreover, the amounts of neurotrophin receptor (p75, TrkA, TrkC) mRNAs in the vagus nerve and vagal afferent nodose ganglion were not reduced in diabetic rats. These data suggest that neither diminished access to target-derived neurotrophins nor the loss of relevant neurotrophin receptors accounts for the diabetes-induced alteration in the retrograde axonal transport of neurotrophins. To assess whether diabetes causes a defect in axonal transport that may not be specific to neurotrophin transport, we studied the ability of a neuronal tracer (FluoroGold, FG) to be retrogradely transported by vagal neurons of control and diabetic rats. After vagal target tissue (stomach) injections of FG, the numbers of FG-labeled afferent and efferent vagal neurons were counted in the nodose ganglion and in the dorsal motor nucleus of the vagus, respectively. After 24 weeks of diabetes, FG was retrogradely transported to more than 50% fewer afferent and efferent vagal neurons in the STZ-diabetic compared to control rats. The diabetes-induced deficit in retrograde axonal transport of FG is likely to reflect alterations in basic axonal transport mechanisms in both the afferent and efferent vagus nerve that contribute to the previously observed reductions in neurotrophin transport.

Accumulation of nerve growth factor protein at both rostral and caudal stumps in the transected rat spinal cord

Changes in the nerve growth factor (NGF) content in the rat spinal cord during development or after traumatic spinal cord injury were examined by using a two-site enzyme immunoassay (EIA) system and an immunohistochemical technique. From embryonic day (E) 14 to postnatal day (P) 70, the spinal cord contained 200-300 pg NGF/g of wet tissue evenly in all regions tested. After complete spinal cord transection of P49 rats, the NGF level started to increase in the rostral and caudal stumps nearest to the injury site at 2 and 4 days, respectively. The NGF level of the caudal side returned to the original level by 2 weeks, but that of the rostral side remained high even 3 weeks, after the injury. At 4 days after the injury, NGF-like immunoreactivity in both stumps was predominantly localized in the axon-like structures of the white matter and in cells morphologically resembling immune cells. These observations suggest that the NGF was transported within the spinal tracts, and that NGF secreted from immune cells that had invaded into the injured spinal cord had accumulated around the transection site. Increased NGF at the injury site may be advantageous for injured neurons and involved in mechanisms directing to axonal regeneration of the injured spinal cord.

Antidepressant treatments induce the expression of basic fibroblast growth factor in cortical and hippocampal neurons

New experimental evidence suggests that the mechanism of action of antidepressants includes the induction of neurotrophic factor synthesis in selected brain areas. The present study is aimed at establishing whether prolonged antidepressant treatments increase the expression of basic fibroblast growth factor (FGF2), a polypeptide growth factor that has a broad neurotrophic activity in the adult central nervous system. Rats received a single dose or long-term (3 weeks) administration of desipramine (DMI), fluoxetine (FLU), and mianserin (MIA), then were sacrificed at 5 and 24 h after the last injection. RNase protection assay and Western blot analysis revealed that all antidepressant drugs elicited an anatomically specific increase in FGF2 mRNA and protein. The increase in FGF2 mRNA after a single injection was seen only at 5 h after the injection and was restricted to the entorhinal cortex, whereas the effect of the long-term treatments lasted up to 24 h and occurred in the entire cortex and hippocampus. Immunohistochemical analysis of FGF2 immunoreactivity was carried out to investigate which cell types responded to the antidepressant treatments. DMI and MIA increased FGF2 proteins predominantly in neurons of layer V throughout the cerebral cortex and in some neurofilament-positive cells of the hippocampus. FLU increased FGF2 immunoreactivity mainly in neurofilament-positive cells of the hippocampus. These findings may explain the therapeutic efficacy of antidepressants in affective disorders.

Cortico-hippocampal APP and NGF levels are dynamically altered by cholinergic muscarinic antagonist or M1 agonist treatment in normal mice

To determine whether altered cholinergic neurotransmission can modify the long-term secretion of amyloid precursor protein (APP), endogenous levels of APP and nerve growth factor (NGF), we administered a selective M1 muscarinic receptor agonist (RS86) or the muscarinic antagonist, atropine, for 7 days in vivo into young adult mice (C57BL/6j). The levels of NGF and total APP in the hippocampus, frontal cortex, striatum, parietal cortex and cerebrospinal fluid (CSF) were examined by ELISA and Western blot. We found that this repeated i.m. administration of M1 receptor agonist resulted in decreased total APP levels in the hippocampus, frontal cortex and parietal cortex, and increased secreted alpha-APPs levels in the CSF. M1 agonist treatment also resulted in decreased NGF levels in the hippocampus and CSF. These effects of the M1 muscarinic agonist could be blocked by atropine, which by itself elevated tissue levels of total APP. Interestingly, we found that the decrease of total APP in the hippocampus and striatum after M1 agonist treatment inversely correlated with the change in NGF levels. These data suggest that a sustained increased cholinergic, M1-mediated neurotransmission will enhance secretion of alpha-APPs in CSF and adaptively reduce the levels of total APP and NGF in the corticohippocampal regions of normal mice. The dynamic and adaptive regulation linking total APP and NGF levels in normal adult mice is relevant for understanding the pathophysiology of conditions with cholinergic and APP related pathologies, like Alzheimer's disease and Down's syndrome.

Maternal deprivation increases cell death in the infant rat brain

Prolonged separation from the mother can interfere with normal growth and development and is a significant risk factor for adult psychopathology. In rodents, separation of a pup from its mother increases the behavioral and endocrine responses to stress for the lifetime of the animal. Here we investigated whether maternal deprivation could affect brain development of infant rats via changes in the rate of cell death as measured by labeling the 3' end of DNA fragments using terminal transferase (ApopTag). At postnatal day 12 (P12), the number of cells undergoing cell death approximately doubled in the cerebral cortex, cerebellar cortex and in several white matter tracts following 24 h of maternal deprivation. Deprivation strongly increased the number of ApopTag-labeled cells at P6 but not at P20. Stroking the infant rats only partially reversed the effects of maternal deprivation. Increased cell death in white matter tracts correlated with an induction of nerve growth factor which has been previously associated with oligodendrocyte cell death. Cell birth was either unchanged or decreased in response to deprivation. These results indicate that maternal deprivation can alter normal brain development by increasing cell death of neurons and glia, and provides a potential mechanism by which early environmental stressors may influence subsequent behavior.

Nerve growth factor plays a divergent role in mediating growth of rat C6 glioma cells via binding to the p75 neurotrophin receptor

Dysregulation of proliferation, differentiation and cell death play a major role in glial tumors, and there is evidence for regulatory mechanisms involving nerve growth factor (NGF) and its receptors in various CNS-derived tumor cell lines. The aim of our study was to observe the effect of exogenous recombinant NGF on C6 rat glioma growth, to characterize the role of endogenous NGF and the p75 neurotrophin receptor (p75) and to rule out whether p75 is necessary to mediate the effect of exogenous NGF. Recombinant exogenous NGF (1-100 ng/ml) was applied under different serum conditions (0%, 1%, 5%) and knockdown of endogenous NGF and p75 was achieved by lipid-mediated antisense oligonucleotide treatment. In presence of serum, NGF had a positive whereas in absence of serum NGF produced a negative effect on C6 cell number. A knockdown of NGF or p75 increased cell numbers and enhanced BrdU incorporation. In p75-knocked down cells NGF did not enhance C6 glioma growth in presence of serum. We conclude that (1) exogenous recombinant NGF enhances C6 glioma growth under serum conditions but decreases cell number in absence of serum, that (2) the effect of exogenous NGF is mediated by p75 alone or by heterodimers containing p75 and that (3) either basal levels of endogenous NGF or basal levels of p75 receptor moderate C6 glioma growth and represent an autoregulatory potential of C6 glioma cells.

Loss of cardiac sympathetic neurotransmitters in heart failure and NE infusion is associated with reduced NGF

Sympathetic neurotransmitters are diminished in cardiac efferent nerve endings in congestive heart failure (CHF). Similar changes occur after exogenous norepinephrine (NE) infusion. Since NE reduces nerve growth factor (NGF) in cultured cardiomyocytes, we proposed to determine whether the loss of noradrenergic transmitters in the failing heart is caused by the NE-mediated reduction of NGF or its neurotrophic receptor tyrosine kinase A (TrKA). Dogs were assigned to receive either rapid ventricular pacing (225 beats/min) or NE infusion (0.5 microg/kg/min) for 8 wk. Control animals received either cardiac pacing of 100 beats/min or saline infusion. We measured NGF and TrKA proteins by Western blot and immunocytochemistry and measured NGF and TrKA mRNAs by reverse transcription polymerase chain reaction, neuronal catecholaminergic histofluorescence, tyrosine hydroxylase-immunostained profiles, and plasma NE. Rapid ventricular pacing produced CHF with increased plasma NE, decreased myocardial NGF protein (0.61 +/- 0.07 vs. 1.04 +/- 0.04, P < 0.05), TrKA protein (0.75 +/- 0.08 vs. 0.98 +/- 0.06, P < 0.05), NGF and TrKA mRNAs and reduced catecholaminergic histofluorescence (197 +/- 23 vs. 485 +/- 43, P < 0.05), and tyrosine hydroxylase profiles (360 +/- 51 vs. 773 +/- 36, P < 0.05). Decreases in tissue NGF and TrKA protein were also noted by immunocytochemistry. Similar changes occurred in NE-treated animals. Tissue NGF and TrKA levels correlated closely with the noradrenergic transmitter profiles. We conclude that cardiac NGF and TrKA are reduced by rapid ventricular pacing and NE infusion, and that these changes correlate with decreases of cardiac catecholaminergic and tyrosine hydroxylase profiles. Findings indicate that decrease of cardiac sympathetic transmitters in heart failure is associated with NE-mediated reduction of NGF and TrKA.

Nerve growth factor signaling of p75 induces differentiation and ceramide-mediated apoptosis in Schwann cells cultured from degenerating nerves

In peripheral nerve regeneration or remyelination, immature Schwann cells expressing p75(NTR) play cardinal roles in the support and regeneration of axons (Griffin JW, Hoffman PN. Peripheral Neuropathy 361-376, 1993). Only one of four to six Schwann cells participate in remyelination of damaged or regenerating axons. The rest of the cells, or supernumerary Schwann cells, show severe atrophy and gradually decrease in number, reestablishing a 1:1 axon-Schwann cell relationship (Said G, Duckett S. Acta Neuropathol (Berl) 53:173-179, 1981). Recent reports demonstrated that severely atrophied supernumerary Schwann cells are eliminated by apoptosis during axonal regeneration or remyelination (Hirata H, Hibasami H. Apoptosis 3:353-360, 1998; Berciano MT, Calle E. Acta Neuropathol (Berl) 95:269-279, 1998). The mechanism to induce selective death of supernumerary Schwann cells without causing any damage to axon-associated Schwann cells or axons remains to be determined. In this article, we report that p75(NTR), the low-affinity receptor for all members of neurotrophins, signals both cell differentiation and apoptosis through intracellular ceramide elevation. The final response is dependent on the intracellular ceramide level and Schwann cells modulate their response by changing expression level of p75(NTR). This effect was selective for nerve growth factor (NGF). Taken together, the present study suggests that NGF contributes both to phenotypic regulation and to elimination of the dedifferentiated Schwann cells, while supporting survival or regeneration of certain types of axons during peripheral nerve repair or regeneration.

Neuropathology of mice carrying mutant APP(swe) and/or PS1(M146L) transgenes: alterations in the p75(NTR) cholinergic basal forebrain septohippocampal pathway

Cholinergic basal forebrain (CBF) projection systems are defective in late Alzheimer's disease (AD). We examined the brains of 12-month-old singly and doubly transgenic mice overexpressing mutant amyloid precursor protein (APP(swe)) and/or presenilin-1 (PS1(M146L)) to investigate the effects of these AD-related genes on plaque and tangle pathology, astrocytic expression, and the CBF projection system. Two types of beta-amyloid (Abeta)-immunoreactive (ir) plaques were observed: type 1 were darkly stained oval and elongated deposits of Abeta, and type 2 were diffuse plaques containing amyloid fibrils. APP(swe) and PS1(M146L) mouse brains contained some type 1 plaques, while the doubly transgenic (APP(swe)/PS1(M146L)) mice displayed a greater abundance of types 1 and 2 plaques. Sections immunostained for the p75 NGF receptor (p75(NTR)) revealed circular patches scattered throughout the cortex and hippocampus of the APP(swe)/PS1(M146L) mice that contained Abeta, were innervated by p75(NTR)-ir neurites, but displayed virtually no immunopositive neurons. Tau pathology was not seen in any transgenic genotype, although a massive glial response occurred in the APP(swe)/PS1(M146L) mice associated with amyloid plaques. Stereology revealed a significant increase in p75(NTR)-ir medial septal neurons in the APP(swe) and PS1(M146L) singly transgenic mice compared to the APP(swe)/PS1(M146L) mice. No differences in size or optical density of p75(NTR)-ir neurons were observed in these three mutants. p75(NTR)-ir fibers in hippocampus and cortex were more pronounced in the APP(swe) and PS1(M146L) mice, while the APP(swe)/PS1(M146L) mice showed the least p75(NTR)-ir fiber staining. These findings suggest a neurotrophic role for mutant APP and PS1 upon cholinergic hippocampal projection neurons at 12 months of age.

Brain-derived neurotrophic factor increases in the uninjured dorsal root ganglion neurons in selective spinal nerve ligation model

Nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) are two major members of the neurotrophin family. Using immunohistochemistry and in situ hybridization histochemistry, we examined the effect of L5 spinal nerve ligation (SPNL), a neuropathic pain model, on the expression of BDNF in the uninjured L4 dorsal root ganglion (DRG). After L5 SPNL, both immunoreactivity for BDNF and the hybridization intensity for BDNF mRNA increased mainly in the small- and medium-sized neurons. The percentage of BDNF mRNA-expressing neurons increased in the ipsilateral L4 DRG compared with the contralateral DRG from the third to 28th day after ligation. A significantly greater number of BDNF-immunoreactive neurons were observed in the ipsilateral L4 DRG than contralateral side 14 d after ligation. To test the contribution of BDNF to the thermal hyperalgesia produced in this model, we intrathecally injected anti-BDNF antibody at third day after ligation. This treatment clearly attenuated thermal hyperalgesia for a few hours. Almost all BDNF mRNA-expressing neurons coexpressed trkA, a high-affinity NGF receptor, mRNA. The percentage of BDNF mRNA-expressing cells of trkA cells significantly increased in the ipsilateral L4 DRG 14 d after ligation. Furthermore, we examined the contribution of NGF on this phenotypic change using ELISA, Northern blot analysis, and anti-NGF antibody. NGF content in the ipsilateral L4 DRG linearly increased and reached a statistical significant level 14 d after L5 SPNL. Moreover, at this time point, the increase in NGF mRNA was observed in the ipsilateral L5 DRG and sciatic nerve, but not in the ipsilateral L4 DRG or L4 spinal nerve. Local application of anti-NGF antibody to the L4 spinal nerve beside the L5 spinal nerve-ligation site prevented the development of thermal hyperalgesia for 5 d after ligation. Our data suggest that BDNF, which increased in the uninjured L4 DRG neurons, acts as a sensory neuromodulator in the dorsal horn and contributes to thermal hyperalgesia in this neuropathic pain model. The contribution of locally synthesized NGF to thermal hyperalgesia was also demonstrated. These dynamic alterations in the expression and content of BDNF and NGF in the uninjured DRG neurons might be involved in the pathomechanisms of neuropathic pain.

Arrival of afferents and the differentiation of target neurons: studies of developing cholinergic projections to the dentate gyrus

This study examined the relationship between the development of cholinergic axons originating from the septum and a group of their target cells, the granule cells of the dentate gyrus of the rat. Acetylcholinesterase histochemistry was used to identify septal cholinergic afferents to the dentate gyrus; parallel studies used anterograde movement of a carbocyanine dye to label the septal projections. Septal cholinergic axons are present in the molecular layer of the internal blade of the dentate gyrus shortly after birth, but these axons do not reach the external blade until several days later. Results demonstrate that acetylcholinesterase positive septal axons grow into the external blade of the dentate gyrus only after the recently generated granule cells have coalesced to form a clearly defined layer. Results from studies using in situ hybridization techniques demonstrate that dentate gyrus granule cells express messenger RNAs for brain derived neurotrophic factor and for neurotrophic factor 3 shortly after formation of the granule cell layer. Ingrowth of septal cholinergic axons follows two days after the formation of the external blade of the dentate gyrus and the expression of neurotrophin messenger RNAs by the dentate granule cells. These data support the hypothesis that target cell development is a prerequisite for attracting the ingrowth of septal afferent axons.

Streptozotocin-induced diabetes causes metabolic changes and alterations in neurotrophin content and retrograde transport in the cervical vagus nerve

Abnormal availability of neurotrophins, such as nerve growth factor (NGF), has been implicated in diabetic somatosensory polyneuropathy. However, the involvement of neurotrophins in diabetic neuropathy of autonomic nerves, particularly the vagus nerve which plays a critical role in visceral afferent and in autonomic motor functions, is unknown. To assess the effects of hyperglycemia on the neurotrophin content and transport in this system, cervical vagus nerves of streptozotocin (STZ)-induced diabetic rats were studied at 8, 16, and 24 weeks after the induction of diabetes. Elevations in vagus nerve hexose (glucose and fructose) and polyol levels (sorbitol), and their normalization with insulin treatment, verified that the STZ treatment resulted in hyperglycemia-induced metabolic abnormalities in the nerve. Neurotrophin (NGF and neurotrophin-3; NT-3) content and axonal transport were assessed in the cervical vagus nerves from nondiabetic control rats, STZ-induced diabetic rats, and diabetic rats treated with insulin. The NGF, but not the NT-3, content of intact vagus nerves from diabetic rats was increased at 8 and 16 weeks (but not at 24 weeks). Using a double-ligation model to assess the transport of endogenous neurotrophins, the retrograde transport of both NGF and NT-3 was found to be significantly reduced in the cervical vagus nerve at later stages of diabetes (16 and 24 weeks). Anterograde transport of NGF or NT-3 was not apparent in the vagus nerve of diabetic or control rats. These data suggest that an increase in vagus nerve NGF is an early, but transient, response to the diabetic hyperglycemia and that a subsequent reduction in neuronal access to NGF and NT-3 secondary to decreased retrograde axonal transport may play a role in diabetes-induced damage to the vagus nerve.

Neurotrophic factors and receptors in the immature and adult spinal cord after mechanical injury or kainic acid

Delivery of neurotrophic factors to the injured spinal cord has been shown to stimulate neuronal survival and regeneration. This indicates that a lack of sufficient trophic support is one factor contributing to the absence of spontaneous regeneration in the mammalian spinal cord. Regulation of the expression of neurotrophic factors and receptors after spinal cord injury has not been studied in detail. We investigated levels of mRNA-encoding neurotrophins, glial cell line-derived neurotrophic factor (GDNF) family members and related receptors, ciliary neurotrophic factor (CNTF), and c-fos in normal and injured spinal cord. Injuries in adult rats included weight-drop, transection, and excitotoxic kainic acid delivery; in newborn rats, partial transection was performed. The regulation of expression patterns in the adult spinal cord was compared with that in the PNS and the neonate spinal cord. After mechanical injury of the adult rat spinal cord, upregulations of NGF and GDNF mRNA occurred in meningeal cells adjacent to the lesion. BDNF and p75 mRNA increased in neurons, GDNF mRNA increased in astrocytes close to the lesion, and GFRalpha-1 and truncated TrkB mRNA increased in astrocytes of degenerating white matter. The relatively limited upregulation of neurotrophic factors in the spinal cord contrasted with the response of affected nerve roots, in which marked increases of NGF and GDNF mRNA levels were observed in Schwann cells. The difference between the ability of the PNS and CNS to provide trophic support correlates with their different abilities to regenerate. Kainic acid delivery led to only weak upregulations of BDNF and CNTF mRNA. Compared with several brain regions, the overall response of the spinal cord tissue to kainic acid was weak. The relative sparseness of upregulations of endogenous neurotrophic factors after injury strengthens the hypothesis that lack of regeneration in the spinal cord is attributable at least partly to lack of trophic support.

Expression of c-fos, heat shock protein 70, neurotrophins, and cyclooxygenase-2 mRNA in response to focal cerebral ischemia/reperfusion in rats and their modification by magnesium sulfate

The marginal area surrounding a region of ischemic brain tissue, designated as the penumbra, is of interest as a potential area for the rescue of neurons from cell death. Despite its clinical importance, relatively little is known about the molecular events leading to changes in brain cells in the penumbra following ischemia. In the first part of this study, we used in situ hybridization to investigate the temporal and spatial expression of c-fos, heat shock protein 70 (HSP70), neurotrophins and inducible cyclooxygenase-2 (COX-2) in the rat brain following a 2-h occlusion of the middle cerebral artery (MCA) with reperfusion. In the penumbra and surrounding cortex, upregulation of c-fos, brain-derived neurotrophic factor (BDNF), and COX-2 mRNAs was observed, while expression of HSP70 mRNA was restricted to the penumbra. This spatial discrepancy of mRNA expression suggests that different mechanisms are involved in the regulation of c-fos/BDNF/COX-2 and HSP70 expression. Intravenous infusion of magnesium sulfate (25 mg/kg) decreased both the infarct volume and upregulation of these mRNAs, suggesting its therapeutic potential.

Axotomy alters neurotrophin and neurotrophin receptor mRNAs in the vagus nerve and nodose ganglion of the rat

Neurotrophins and neurotrophin receptors play an important role in survival and growth of injured peripheral nerves. To study the injury-mediated neurotrophic response in autonomic nerves, we investigated changes in mRNA expression of neurotrophins and their receptors in the transected vagus nerve and nodose ganglion. Studies using in situ hybridization histochemistry showed that axotomy of the cervical vagus nerve resulted in increased expression of mRNAs for nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), and neurotrophin-3 (NT-3), and for TrkA, TrkB, and TrkC receptors in non-neuronal cells at both the proximal and distal segments of the transected cervical vagus nerve. Moreover, NGF protein was increased in the distal end, and NT-3 protein was increased in both the proximal and the distal ends of the transected nerve 3 days after axotomy. No change of p75(NTR) mRNA was detected in the transected vagus nerve. The induction of each neurotrophin and Trk receptor mRNA was apparent within 1 day after the axotomy and was sustained at least 14 days. By 45 days after the axotomy, a time when axonal reconnection with target tissue is made (integrity of the nerve-target connection was confirmed by the retrograde transport of FluoroGold from the stomach to vagal cell bodies), the levels of neurotrophin and Trk mRNAs in the vagus nerve declined to pre-axotomy levels. TrkA, TrkC, and p75(NTR) mRNA-containing vagal sensory neurons in the nodose ganglion were reduced in number after cervical vagotomy. Neurotrophin-mRNA-containing neurons were not found in the nodose ganglia from either intact or vagotomized rats. The axotomy-induced up-regulation of neurotrophins and Trk receptors mainly in the non-neuronal cells at or near the site of transection suggests that neurotrophins are involved in the survival and regeneration process of the vagus nerve after injury.

Nerve growth factor expression in parasympathetic neurons: regulation by sympathetic innervation

Interactions between sympathetic and parasympathetic nerves are important in regulating visceral target function. Sympathetic nerves are closely apposed to, and form functional synapses with, parasympathetic axons in many effector organs. The molecular mechanisms responsible for these structural and functional interactions are unknown. We explored the possibility that Nerve Growth Factor (NGF) synthesis by parasympathetic neurons provides a mechanism by which sympathetic-parasympathetic interactions are established. Parasympathetic pterygopalatine ganglia NGF-gene expression was examined by in situ hybridization and protein content assessed by immunohistochemistry. Under control conditions, NGF mRNA was present in approximately 60% and NGF protein was in 40% of pterygopalatine parasympathetic neurons. Peripheral parasympathetic axons identified by vesicular acetylcholine transporter-immunoreactivity also displayed NGF immunoreactivity. To determine if sympathetic innervation regulates parasympathetic NGF expression, the ipsilateral superior cervical ganglion was excised. Thirty days postsympathectomy, the numbers of NGF mRNA-positive neurons were decreased to 38% and NGF immunoreactive neurons to 15%. This reduction was due to a loss of sympathetic nerve impulse activity, as similar reductions were achieved when superior cervical ganglia were deprived of preganglionic afferent input for 40 days. These findings provide evidence that normally NGF is synthesized by parasympathetic neurons and transported anterogradely to fibre terminals, where it may be available to sympathetic axons. Parasympathetic NGF expression, in turn, is augmented by impulse activity within (and presumably transmitter release from) sympathetic axons. It is suggested that parasympathetic NGF synthesis and its modulation by sympathetic innervation provides a molecular basis for establishment and maintenance of autonomic axo-axonal synaptic interactions.

Spatiotemporal expression of BDNF in the hippocampus induced by the continuous intracerebroventricular infusion of beta-amyloid in rats

The beta-amyloid protein (Abeta) is the major component of senile plaques found in the brain in Alzheimer's disease (AD). Its neurotoxic properties in vivo, however, are not well defined. Since the expression of neurotrophin genes is considered an important component of the intrinsic neuroprotective response to insults, we analyzed the gene expression of neurotrophins in the brains of rats which received a continuous infusion of Abeta-(1-42) into the cerebroventricle. Northern blot analysis revealed a significant increase in brain-derived neurotrophic factor (BDNF) expression in the hippocampus but no change in the cerebral cortices. The alteration peaked on days 3-7 and returned to the basal level on day 14 after the start of Abeta-(1-42) infusion. No significant changes in nerve growth factor or neurotrophin-3 mRNA expression were observed. The infusion of Abeta-(1-40) and (25-35) also triggered the expression of BDNF mRNA, whereas neither Abeta-(40-1) nor (1-16) had any effect. In situ hybridization histochemistry revealed that the expression mainly occurred in the hilus and granular layer of the dentate gyrus and to a lesser extent in the pyramidal neurons of the CA1 region. These results demonstrate that the continuous intracerebroventricular infusion of Abeta induces selective and spatiotemporal expression of BDNF mRNA in the hippocampus.

Nerve growth factor but not neurotrophin-3 is synthesized by hippocampal GABAergic neurons that project to the medial septum

Conventional uptake of neurotrophins takes place at axon terminals via specific receptors, and is followed by retrograde transport. Recent studies demonstrated that, with the exception of nerve growth factor, other neurotrophins may be delivered anterogradely to the region containing the receptor expressing neurons. In this study we used a triple labeling method that combines retrograde tract tracing, in situ hybridization and immunocytochemistry to examine whether non-principal cells projecting from the hippocampus to the septum synthesize nerve growth factor. Our results show that, on average, 59% of the horseradish peroxidase-labeled hippocamposeptal nonpyramidal neurons also display nerve growth factor messenger RNA hybridization signal. The ratio was slightly higher in the CA1 stratum oriens and the hilus of the dentate gyrus (64 and 62%, respectively) compared to stratum oriens of the CA3 region (58%). In addition, we demonstrated that many nerve growth factor-positive septally projecting neurons also contain the calcium-binding protein calbindin D-28K, whereas nerve growth factor-negative projecting cells mostly lack this neurochemical marker. In contrast to nerve growth factor, neurotrophin-3 has never been found in hippocamposeptal cells. Hippocamposeptal GABAergic cells are reciprocally connected with the medial septum, thus they are in a key position to regulate nerve growth factor release as a function of the activity level in the septohippocampal system. Furthermore, our results raise the intriguing possibility that nerve growth factor may be transported also in an anterograde manner. Regardless of the direction of transport, the presence of nerve growth factor in hippocamposeptal cells suggests that long distance fast synaptic mechanisms and slow neurotrophin action are coupled in these neurons.

Induction of postnatal schwann cell death by the low-affinity neurotrophin receptor in vitro and after axotomy

Schwann cells express the low-affinity neurotrophin receptor (p75), but no role for either the neurotrophins or their cognate receptors in Schwann cell development has been established. We have found that Schwann cells isolated from postnatal day 1 (P1) or P2 mice that were p75-deficient exhibited potentiated survival compared to wild-type cells after growth factor and serum withdrawal. There was, however, no disparity in the survival of p75-deficient and wild-type Schwann cells isolated at embryonic day 15, suggesting that the death-inducing effects of p75 are developmentally regulated. A comparable degree of cell death was also observed in the sciatic nerves of both wild-type and p75-deficient mice at P1. However, 24 hr after axotomy, there was a 13-fold increase in the percentage of apoptotic nuclei in the distal nerve stumps of the transected sciatic nerves of neonatal wild-type but not p75-deficient mice. The expression of both the p75 and nerve growth factor (NGF) genes was upregulated after axotomy in neonatal wild-type nerves. Collectively, these results suggest that NGF-mediated activation of p75 is likely to be an important mediator of Schwann cell apoptosis in the context of peripheral nerve injury.

Regulation of NGF-family ligands and receptors in adulthood and senescence: correlation to degenerative and regenerative changes in cutaneous innervation

During development, a highly differential neurotrophin dependency is reported for various types of nerve endings in the whisker follicle. To what extent these dependencies extend and play a role in adulthood is largely unresolved. We show here, using in situ hybridization and immunohistochemistry that the expression of neurotrophins and trk/p75 receptors persists in adulthood. As suggested by their expression profiles, many classes of cutaneous nerve endings disclose similar ligand-receptor dependencies in adult animals as during development, while other populations appear to switch their dependency. Furthermore, our data suggest that sensory endings that have a high turnover due to mechanical wear and tear, e. g. Merkel cell-neurite complexes at the level of ring sinus show a more complex ligand-receptor expression phenotype than do endings with a less vulnerable location, e.g. the Merkel cell-neurite complexes at the rete ridge collar. Thus, neurotrophin-3 (NT3)/trkA signalling is suggested to be important for a continuous terminal plasticity of Merkel cell-neurite complexes at the level of ring sinus in adulthood. Evidence supporting a role for neurotrophin signalling in maintaining the adult cutaneous innervation also comes from the close correlation between altered ligand-receptor expression(s) and axonal/terminal aberrations in senescence. Thus, an ageing-related decrease in target neurotrophin expression, notably NT3 and NT4, results in a site-specific loss of sensory terminals concomitant with an aberrant growth of regenerating/sprouting axons into new target fields. Ageing of the cutaneous innervation, manifested in degenerative and regenerative events, seems strongly associated with changes in neurotrophic interactions between sensory neurons and target tissues.

Maternal care, hippocampal synaptogenesis and cognitive development in rats

We report that variations in maternal care in the rat promote hippocampal synaptogenesis and spatial learning and memory through systems known to mediate experience-dependent neural development. Thus, the offspring of mothers that show high levels of pup licking and grooming and arched-back nursing showed increased expression of NMDA receptor subunit and brain-derived neurotrophic factor (BDNF) mRNA, increased cholinergic innervation of the hippocampus and enhanced spatial learning and memory. A cross-fostering study provided evidence for a direct relationship between maternal behavior and hippocampal development, although not all neonates were equally sensitive to variations in maternal care.

Expression of lysophosphatidic acid receptor in rat astrocytes: mitogenic effect and expression of neurotrophic genes

Lysophosphatidic acid (LPA) is a phospholipid mediator with a variety of biological activities. It remains unknown, however, which cells in the brain express the LPA receptor. The present study was undertaken to identify cells in the rat brain expressing functional LPA receptors, and to explore biological roles of LPA in these cells. We found that the LPA receptor was most dominantly expressed in rat astrocytes, determined by LPA-induced Ca2+ imaging, and by Northern blot analyses. LPA induced a mitogenic response and expression of immediate early genes in astrocytes, through pertussis-toxin sensitive G-protein(s). LPA also stimulated the expression of various cytokine genes, including nerve growth factor, interleukin (IL)-1beta, IL-3 and IL-6. Thus, astrocytes are the major target of LPA in the brain. We propose that LPA may play important roles in neuronal development, gliosis and wound-healing process in the brain.

Commercial recombinant human beta-nerve growth factor and adult rat dorsal root ganglia contain an identical molecular species of nerve growth factor prohormone

Examination of commercial recombinant human beta-nerve growth factor (rh-beta-NGF) preparations with polyclonal antibodies specific to 13-kDa NGF and pro-NGF-specific domains revealed the presence of high-molecular-mass immunoreactive proteins, including a 60-kDa NGF prohormone. On incubation with a mixture of N- and O-specific glycosidases, the 60-kDa NGF pro-hormone generated a 32-kDa protein corresponding to the molecular size of NGF precursor predicted by the cloned human NGF cDNA. Highly sensitive chemiluminescence immunoblot analysis of adult rat dorsal root ganglia, spinal cord, and colon tissues with NGF- and pro-NGF domain-specific antibodies also revealed the presence of high-molecular-mass proteins, including the 60-kDa NGF prohormone. Based on the presence of the 60-kDa NGF prohormone in dorsal root ganglia and its efferent tissues, we suggest that proteolytically unprocessed, glycosylated NGF prohormone may mediate interactions between neurons and the tissues they innervate.

Dexamethasone induces TrkA and p75NTR immunoreactivity in the cerebral cortex and hippocampus

Nerve growth factor (NGF) plays a crucial role in synaptic plasticity during brain development and adulthood by activating a dual receptor system composed of TrkA and p75 (p75NTR) receptors. Exogenous NGF modulates the expression of both receptors. Little is known about the ability of endogenous NGF to regulate the expression of these receptors in basal forebrain cholinergic terminals. The ability of glucocorticoids to increase NGF expression in the hippocampus prompted us to investigate whether the synthetic glucocorticoid dexamethasone (DEX) increases TrkA and p75NTR expression in NGF-target cholinergic neurons in developing rats. We first examined the effect of DEX on NGF mRNA by in situ hybridization. DEX given systemically (0.5 mg/kg, sc) for 1 week to 7-day-old rats elicited an increase in NGF mRNA levels in the dentate gyrus of the hippocampus and superficial layers II and III of the cerebral cortex. Immunohistochemical analysis of p75NTR and TrkA levels revealed a dramatic increase in p75NTR immunoreactivity (IR) in both basal forebrain and hippocampus and TrkA IR in the hippocampus. Interestingly, in DEX-treated rats more axonal terminals were immunopositive for p75NTR in the hippocampus and cortex, suggesting an increase in p75NTR IR in cell bodies as well as in terminals. Our data indicate that the endogenously produced NGF elicits biological changes similar to those of the exogenously delivered NGF. We suggest that glucocorticoids might regulate and coordinate cholinergic neuronal maturation by increasing the biosynthesis of NGF.

Positive modulation of AMPA receptors increases neurotrophin expression by hippocampal and cortical neurons

This study investigated whether positive modulators of AMPA-type glutamate receptors influence neurotrophin expression by forebrain neurons. Treatments with the ampakine CX614 markedly and reversibly increased brain-derived neurotrophic factor (BDNF) mRNA and protein levels in cultured rat entorhinal/hippocampal slices. Acute effects of CX614 were dose dependent over the range in which the drug increased synchronous neuronal discharges; threshold concentrations for acute responses had large effects on mRNA content when applied for 3 d. Comparable results were obtained with a second, structurally distinct ampakine CX546. Ampakine-induced upregulation was broadly suppressed by AMPA, but not NMDA, receptor antagonists and by reducing transmitter release. Antagonism of L-type voltage-sensitive calcium channels blocked induction in entorhinal cortex but not hippocampus. Prolonged infusions of suprathreshold ampakine concentrations produced peak BDNF mRNA levels at 12 hr and a return to baseline levels by 48 hr. In contrast, BDNF protein remained elevated throughout a 48 hr incubation with the drug. Nerve growth factor mRNA levels also were increased by ampakines but with a much more rapid return to control levels during chronic administration. Finally, intraperitoneal injections of CX546 increased hippocampal BDNF mRNA levels in aged rats and middle-aged mice. The present results provide evidence of regional differences in mechanisms via which activity regulates neurotrophin expression. Moreover, these data establish that changes in synaptic potency produce sufficient network level physiological effects for inducing neurotrophin genes, indicate that the response becomes refractory during prolonged ampakine exposure, and raise the possibility of using positive AMPA modulators to regulate neurotrophin levels in aged brain.