Neurotrophin-3 into the insular cortex strengthens conditioned taste aversion memory

Memory consolidation is an essential process of long-term memory formation. Neurotrophins have been suggested as key regulators of activity dependent changes in the synaptic efficacy and morphology, which are considered the downstream mechanisms of memory consolidation. The neurotrophin 3 (NT-3), a member of the neurotrophin family, and its high affinity receptor TrkC, are widely expressed in the insular cortex (IC), a region with a critical role in the consolidation of the conditioned taste aversion (CTA) paradigm, in which an animal associates a novel taste with nausea. Nevertheless, the role of this neurotrophin in the cognitive processes that the IC mediates remains unexamined. To answer whether NT-3 is involved in memory consolidation at the IC, adult male Wistar rats were administered with NT-3 or NT-3 in combination with the Trk receptors inhibitor K252a into the IC, immediately after CTA acquisition under two different conditions: a strong-CTA (0.2 M lithium chloride i.p.) or a weak-CTA (0.1 M lithium chloride i.p.). Our results show that NT-3 strengthens the memory trace of CTA, transforming a weak conditioning into a strong one, in a Trk-dependent manner. The present evidence suggests that NT-3 has a key role in the consolidation process of an aversive memory in a neocortical region.

Repair effect of neurotrophic factor III (NT-3) on rats with spinal injury model and its mechanism

The present study aimed to study the repair effect of neurotrophic factor III (NT-3) on spinal injury model rats and its mechanism. Wistar rats with spinal injury were established by accelerated compression stroke after the operation and divided into control group, model group, and NT-3 intervention group. The motor function of rats in each group was evaluated at different postoperative time points (3, 7, 14 d). HE staining was used to detect the changes in tissue structure and morphology of the injured spinal column in each group. The changes of SOD, MDA and GSH in serum of rats were detected. The concentrations of inflammatory cytokines IL-1β, IL-6, IL-17 and TNF-α in serum were detected by enzyme-linked immunosorbent assay (ELISA). Western blot was used to detect the expression changes of anti-apoptotic protein (Bcl-2) and pro-apoptotic protein (Bax) in injured spinal tissue of rats in each group. Compared with model group, motor function score of NT-3 intervention group increased gradually, and had statistical significance at 7 and 14 days (5.29±1.62 vs 9.33±2.16, 5.92±1.44 vs 14.56±2.45, T =7.386, 9.294, P =0.004, 0.000). The levels of SOD and GSH in serum of NT-3 intervention group were significantly increased (t=9.117, 12.207, P=0.000, 0.000), while the level of MDA was significantly decreased (t=5.089, P=0.011). Serum levels of inflammatory cytokines IL-1β, IL-6, IL-17 and TNF-α in NT-3 intervention group were significantly decreased (T =6.157, 7.958, 6.339, 6.288, P=0.008, 0.005, 0.005, 0.007). In the NT-3 treatment group, Bax protein was significantly decreased (0.24±0.05 vs 0.89±0.12, T =8.579, P=0.001), and the relative expression of Bcl-2 protein was significantly increased (0.75±0.06 vs 0.13±0.05, T =9.367, P=0.001). Neurotrophic factor III can promote spinal injury repair in spinal injury model rats, and play a role by enhancing antioxidant stress ability, inhibiting inflammatory factors, promoting Bcl-2 and decreasing Bax expression.

Impact of Job Types on Plasma Neurotrophins Levels: A Preliminary Study in Airline Pilots, Construction Workers, and Fitness Instructors

BACKGROUND

Neurotrophins (NTs) encompass a group of closely associated proteins regulating various aspects of neuronal growth and survival. The potential association between work-related factors and the levels of circulating NTs has not been extensively examined. In this preliminary investigation, we evaluated plasma concentrations of brain-derived neurotrophic factor (BDNF), nerve growth factor (NGF), neurotrophin-3 (NT-3), and neurotrophin-4 (NT-4) in a cohort of healthy individuals from three distinct professional categories, each with unique work environments and lifestyle factors.

METHODS

The study involved 60 men from three professional fields: airline pilots, construction laborers, and fitness trainers (20 participants per category) recruited during routine occupational health appointments. Plasma levels of NTs were measured using commercially available immunoassays and compared in the three professional groups.

RESULTS

Among the professions studied, fitness instructors displayed the highest concentrations of BDNF and NGF, with airline pilots ranking second, and construction workers showing the lowest levels. Significantly decreased NT-3 levels were observed in airline pilots compared to fitness instructors and construction workers, but no differences were found between the latter two occupations. NT-4 levels were similar across all three occupational groups.

CONCLUSIONS

Our pilot results suggest that plasma concentrations of NTs, which are involved in various aspects of neuronal and cognitive functioning, may display significant differences among healthy individuals depending on their occupation. These observations warrant additional research to explore potential implications for the field of occupational medicine.

AAV Vector Mediated Delivery of NG2 Function Neutralizing Antibody and Neurotrophin NT-3 Improves Synaptic Transmission, Locomotion, and Urinary Tract Function after Spinal Cord Contusion Injury in Adult Rats

NG2 is a structurally unique transmembrane chondroitin sulfate proteoglycan (CSPG). Its role in damaged spinal cord is dual. NG2 is considered one of key inhibitory factors restricting axonal growth following spinal injury. Additionally, we have recently detected its novel function as a blocker of axonal conduction. Some studies, however, indicate the importance of NG2 presence in the formation of synaptic contacts. We hypothesized that the optimal treatment would be neutralization of inhibitory functions of NG2 without its physical removal. Acute intraspinal injections of anti-NG2 monoclonal antibodies reportedly prevented an acute block of axonal conduction by exogenous NG2. For prolonged delivery of NG2 function neutralizing antibody, we have developed a novel gene therapy: adeno-associated vector (AAV) construct expressing recombinant single-chain variable fragment anti-NG2 antibody (AAV-NG2Ab). We examined effects of AAV-NG2Ab alone or in combination with neurotrophin NT-3 in adult female rats with thoracic T10 contusion injuries. A battery of behavioral tests was used to evaluate locomotor function. In vivo single-cell electrophysiology was used to evaluate synaptic transmission. Lower urinary tract function was assessed during the survival period using metabolic chambers. Terminal cystometry, with acquisition of external urethral sphincter activity and bladder pressure, was used to evaluate bladder function. Both the AAV-NG2Ab and AAV-NG2Ab combined with AAV-NT3 treatment groups demonstrated significant improvements in transmission, locomotion, and bladder function compared with the control (AAV-GFP) group. These functional improvements associated with improved remyelination and plasticity of 5-HT fibers. The best results were observed in the group that received combinational AAV-NG2Ab+AAV-NT3 treatment.SIGNIFICANCE STATEMENT We recently demonstrated beneficial, but transient, effects of neutralization of the NG2 proteoglycan using monoclonal antibodies delivered intrathecally via osmotic mini-pumps after spinal cord injury. Currently, we have developed a novel gene therapy tool for prolonged and clinically relevant delivery of a recombinant single-chain variable fragment anti-NG2 antibody: AAV-rh10 serotype expressing scFv-NG2 (AAV-NG2Ab). Here, we examined effects of AAV-NG2Ab combined with transgene delivery of Neurotrophin-3 (AAV-NT3) in adult rats with thoracic contusion injuries. The AAV-NG2Ab and AAV-NG2Ab+AAV-NT3 treatment groups demonstrated significant improvements of locomotor function and lower urinary tract function. Beneficial effects of this novel gene therapy on locomotion and bladder function associated with improved transmission to motoneurons and plasticity of axons in damaged spinal cord.

The BDNF/TrkB Neurotrophin System in the Sensory Organs of Zebrafish

The brain-derived neurotrophic factor (BDNF) was discovered in the last century, and identified as a member of the neurotrophin family. BDNF shares approximately 50% of its amino acid with other neurotrophins such as NGF, NT-3 and NT-4/5, and its linear amino acid sequences in zebrafish (Danio rerio) and human are 91% identical. BDNF functions can be mediated by two categories of receptors: p75NTR and Trk. Intriguingly, BDNF receptors were highly conserved in the process of evolution, as were the other NTs’ receptors. In this review, we update current knowledge about the distribution and functions of the BDNF-TrkB system in the sensory organs of zebrafish. In fish, particularly in zebrafish, the distribution and functions of BDNF and TrkB in the brain have been widely studied. Both components of the system, associated or segregated, are also present outside the central nervous system, especially in sensory organs including the inner ear, lateral line system, retina, taste buds and olfactory epithelium.

Effect of lurasidone vs olanzapine on neurotrophic biomarkers in unmedicated schizophrenia: A randomized controlled trial

Neurotrophic factors like Brain-Derived Neurotrophic Factor (BDNF), Neurotrophin 3 (NT3) and Nerve Growth Factor (NGF), play a role in neuroplasticity and neurogenesis contributing to the pathogenesis of schizophrenia. The objective of the present study was to investigate and compare the effect of olanzapine and lurasidone on the change in serum neurotrophins in patients with schizophrenia. The present study was a randomized, open-label, active-controlled, parallel design clinical trial. After randomization baseline evaluations of serum BDNF, NGF, NT3, Positive and Negative Syndrome Scale (PANSS) scoring, Social and Occupational Functioning Assessment Scale (SOFAS) scoring of 101 unmedicated schizophrenia patients were done. Patients were reassessed after 6 weeks of monotherapy with olanzapine or lurasidone. Serum BDNF increased after treatment with both the drug groups but rise with olanzapine was found to be significantly higher (916.22; 95 %CI: 866.07 to 966.37; p < 0.001) in comparison to lurasidone. Increase in levels NGF and NT3 was also observed but there was no significant difference between the groups (NGF: 2.32; CI: 3.54 to -3.53; p = 0.57 and NT3: 0.99; CI: 2.11 to 0.14; p = 0.086). The difference in improvement in PANSS and SOFASS with both the drugs was not statistically significant. Both the drugs alleviate the symptoms of schizophrenia but olanzapine was better tolerated. Our findings suggest that increase in serum BDNF with olanzapine monotherapy is significantly higher than that with lurasidone but there is no significant difference in change in serum NGF and NT3. TRIAL REGISTRATION: ClinicalTrials.gov identifier: (NCT03304457).

The effect of Xinkeshu tablets on depression and anxiety symptoms in patients with coronary artery disease: Results from a double-blind, randomized, placebo-controlled study

A large proportion of patients with coronary artery disease (CAD) suffer from depression or anxiety symptoms and this is associated with increased mortality [1]. This double-blinded, randomized, placebo-controlled, clinical trial (ChiCTR-IPR-17010940) aimed to explore whether Xinkeshu tablets can reduce anxiety or depressive symptoms in CAD patients and how this is related to the concentration of plasma cytokines. Sixty patients with CAD anda Hospital Anxiety and Depression Scale (HADS-a/HADS-d) score of ≥8 were treated with Xinkeshu tablets or placebo for 12 weeks following percutaneous revascularization. Depressive/anxiety symptoms and the levels of 440 peripheral blood cytokines were evaluated at baseline and after 12 weeks treatment. Results showed significantly lower (P < 0.05) HADS-a/HADS-d and PHQ-9 scores in CAD patients treated with Xinkeshu tablets than in those who received placebo. These improvements were associated with changes in certain peripheral blood cytokines; most notably trappin-2, adiponectin, interleukin 1β (IL-1β), thrombopoietin, activated leukocyte cell adhesion molecule (ALCAM), neurotrophin-3 (NT-3), and transferrin. A significant correlation between anxiety/depression symptoms and trappin-2, NT-3, transferrin, and ALCAM (p < 0.05) were observed in an independent cohort of patients with CAD. These findings were in-keeping with the anti-depressive effects of Xinkeshu tablets. This trial demonstrates that Xinkeshu tablets can improve anxiety and depression symtoms effectively address in patients with coronary heart disease possibly through increasing the blood ratio of anti-inflammatory:pro-inflammatory cytokines.

Brain, blood, cerebrospinal fluid, and serum biomarkers in schizophrenia

Over the last decade, finding a reliable biomarker for the early detection of schizophrenia (Scz) has been a topic of interest. The main goal of the current review is to provide a comprehensive view of the brain, blood, cerebrospinal fluid (CSF), and serum biomarkers of Scz disease. Imaging studies have demonstrated that the volumes of the corpus callosum, thalamus, hippocampal formation, subiculum, parahippocampal gyrus, superior temporal gyrus, prefrontal and orbitofrontal cortices, and amygdala-hippocampal complex were reduced in patients diagnosed with Scz. It has been revealed that the levels of interleukin 1β (IL-1β), IL-6, IL-8, and TNF-α were increased in patients with Scz. Decreased mRNA levels of brain-derived neurotrophic factor (BDNF), tropomyosin receptor kinase B (TrkB), neurotrophin-3 (NT-3), nerve growth factor (NGF), and vascular endothelial growth factor (VEGF) genes have also been reported in Scz patients. Genes with known strong relationships with this disease include BDNF, catechol-O-methyltransferase (COMT), regulator of G-protein signaling 4 (RGS4), dystrobrevin-binding protein 1 (DTNBP1), neuregulin 1 (NRG1), Reelin (RELN), Selenium-binding protein 1 (SELENBP1), glutamic acid decarboxylase 67 (GAD 67), and disrupted in schizophrenia 1 (DISC1). The levels of dopamine, tyrosine hydroxylase (TH), serotonin or 5-hydroxytryptamine (5-HT) receptor 1A and B (5-HTR1A and 5-HTR1B), and 5-HT1B were significantly increased in Scz patients, while the levels of gamma-aminobutyric acid (GABA), 5-HT transporter (5-HTT), and 5-HT receptor 2A (5-HTR2A) were decreased. The increased levels of SELENBP1 and Glycogen synthase kinase 3 subunit α (GSK3α) genes in contrast with reduced levels of B-cell translocation gene 1 (BTG1), human leukocyte antigen DRB1 (HLA-DRB1), heterogeneous nuclear ribonucleoprotein A3 (HNRPA3), and serine/arginine-rich splicing factor 1 (SFRS1) genes have also been reported. This review covers various dysregulation of neurotransmitters and also highlights the strengths and weaknesses of studies attempting to identify candidate biomarkers.

Grafts of Fibroblasts Genetically Modified to Secrete Ngf, Bdnf, Nt-3, or Basic Fgf Elicit Differential Responses in the Adult Spinal Cord

Neuronal and axonal responses to neurotrophic factors in the developing spinal cord have been relatively well characterized, but little is known about adult spinal responses to neurotrophic factors. We genetically modified primary rat fibroblasts to produce either nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), or basic fibroblast growth factor (bFGF), then grafted these neurotrophic factor-secreting cells into the central gray matter of the spinal cord in adult rats. Spinal cord lesions were not made prior to grafting. From 2 wk to 6 mo later, sensory neurites of dorsal root origin extensively penetrated NGF-, NT-3-, and bFGF-producing grafts, whereas BDNF-secreting grafts elicited no growth responses. Putative noradrenergic neurites also penetrated NGF-secreting cell grafts. Local motor and corticospinal motor axons did not penetrate any of the neurotrophic factor-secreting grafts. These results indicate that unlesioned or minimally lesioned adult spinal cord sensory and putative noradrenergic populations retain significant neurotrophic factor responsiveness, whereas motor neurites are comparatively resistant even to those neurotrophic factors to which they exhibit survival dependence during development. Grafts of genetically modified cells can be a useful tool for characterizing neurotrophic factor responsiveness in the adult spinal cord and designing strategies to promote axonal regeneration after injury.

Functional Diversity of Neurotrophin Actions on the Oculomotor System

Neurotrophins play a principal role in neuronal survival and differentiation during development, but also in the maintenance of appropriate adult neuronal circuits and phenotypes. In the oculomotor system, we have demonstrated that neurotrophins are key regulators of developing and adult neuronal properties, but with peculiarities depending on each neurotrophin. For instance, the administration of NGF (nerve growth factor), BDNF (brain-derived neurotrophic factor) or NT-3 (neurotrophin-3) protects neonatal extraocular motoneurons from cell death after axotomy, but only NGF and BDNF prevent the downregulation in ChAT (choline acetyltransferase). In the adult, in vivo recordings of axotomized extraocular motoneurons have demonstrated that the delivery of NGF, BDNF or NT-3 recovers different components of the firing discharge activity of these cells, with some particularities in the case of NGF. All neurotrophins have also synaptotrophic activity, although to different degrees. Accordingly, neurotrophins can restore the axotomy-induced alterations acting selectively on different properties of the motoneuron. In this review, we summarize these evidences and discuss them in the context of other motor systems.

Expression of neurotrophins and their receptors in human CD34+ bone marrow cells

Bone marrow (BM) CD34+ cells have the ability to secrete growth factors, cytokines, and chemotactic factors. We sought to better characterize this population and to investigate whether human BM CD34+ cells express neurotrophins (NTs) and their relevant receptors. We also compared their expression levels with BM nucleated cells (NCs). BM CD34+ cells were evaluated with respect to the expression levels of neurotrophins using qRT-PCR, immunofluorescent staining, and Western blotting. Next, the expression of specific (TrkA, TrkB, TrkC) and non-specific (p75NTR) neurotrophin receptors was detected by qRT-PCR and immunofluorescent staining in BM CD34+ cells. Using qRT- PCR, we show that even in the absence of inducing factors, CD34+ cells spontaneously express neurotrophins such as NGF, BDNF, NT-3, and NT-4. In addition, the NT expression levels in BM CD34+ cells are considerably higher than in NCs. Furthermore, we confirmed intracellular NT expression in BM CD34+ cells at the protein level using immunofluorescent staining and Western blotting. Using qRT-PCR, we found that immunomagnetically separated BM CD34+ cells spontaneously express high-affinity neurotrophin receptors (TrkA, TrkB, and TrkC) and the low-affinity receptor p75NTR at higher levels than NCs. Immunomagnetic CD34+ cell separation enables for the rapid and gentle sorting of stem/progenitor cells (SPCs) to prepare specific cell types for use in research and clinical applications. Our study suggests that BM CD34+ cells have the potential to support trophic factors for neural tissue and could contribute towards the protection and regeneration of neural cells.

[Therapy for Charcot-Marie-Tooth Disease: From the Standpoint of Neurologists]

To date, there is no approved pharmacologic treatment for any form of Charcot-Marie-Tooth disease (CMT). However, some clinical or preclinical trials for CMT1A have been undertaken, for example Neurotrophin-3, PXT3003, and neuregulin-1. Gene therapy for CMT1X, CMT2F and Giant axonal neuropathy using animal model or culture cells have been reported with some interesting results. Stem cell research for example iPS cells derived from patients with CMT2A or CMT2E, is being conducted to clarify the mechanism of CMT and find therapeutic clues. The development of new surrogate markers for clinical trials is also needed. Additionally, steps should be taken to improve the quality of life of patients with CMT, including pain control and life style enhancement.

Improved neurological outcome by intramuscular injection of human amniotic fluid derived stem cells in a muscle denervation model

Purpose

The skeletal muscle develops various degrees of atrophy and metabolic dysfunction following nerve injury. Neurotrophic factors are essential for muscle regeneration. Human amniotic fluid derived stem cells (AFS) have the potential to secrete various neurotrophic factors necessary for nerve regeneration. In the present study, we assess the outcome of neurological function by intramuscular injection of AFS in a muscle denervation and nerve anastomosis model.

Materials and Methods

Seventy two Sprague-Dawley rats weighing 200–250 gm were enrolled in this study. Muscle denervation model was conducted by transverse resection of a sciatic nerve with the proximal end sutured into the gluteal muscle. The nerve anastomosis model was performed by transverse resection of the sciatic nerve followed by four stitches reconnection. These animals were allocated to three groups: control, electrical muscle stimulation, and AFS groups.

Results

NT-3 (Neurotrophin 3), BDNF (Brain derived neurotrophic factor), CNTF (Ciliary neurotrophic factor), and GDNF (Glia cell line derived neurotrophic factor) were highly expressed in AFS cells and supernatant of culture medium. Intra-muscular injection of AFS exerted significant expression of several neurotrophic factors over the distal end of nerve and denervated muscle. AFS caused high expression of Bcl-2 in denervated muscle with a reciprocal decrease of Bad and Bax. AFS preserved the muscle morphology with high expression of desmin and acetylcholine receptors. Up to two months, AFS produced significant improvement in electrophysiological study and neurological functions such as SFI (sciatic nerve function index) and Catwalk gait analysis. There was also significant preservation of the number of anterior horn cells and increased nerve myelination as well as muscle morphology.

Conclusion

Intramuscular injection of AFS can protect muscle apoptosis and likely does so through the secretion of various neurotrophic factors. This protection furthermore improves the nerve regeneration in a long term nerve anastomosis model.

Effects of human OEC-derived cell transplants in rodent spinal cord contusion injury

Numerous reports indicate that rodent olfactory ensheathing cells (OECs) assist in spinal cord repair and clinical trials have been undertaken using autologous transplantation of human olfactory ensheathing cells (hOECs) as a treatment for spinal cord injury. However, there are few studies investigating the efficacy of hOECs in animal models of spinal cord injury. In this study hOECs were derived from biopsies of human olfactory mucosa, purified by culture in a serum-free medium containing neurotrophin-3, genetically labelled with EGFP, and stored frozen. These hOEC-derived cells were thawed and transplanted into the spinal cord injury site 7 days after a moderate contusion injury of the spinal cord at thoracic level T10 in the athymic rat. Six weeks later the animals receiving the hOEC-derived transplants had greater functional improvement in their hindlimbs than controls, assessed using open field (BBB scale) and horizontal rung walking tests. Histological analysis demonstrated beneficial effects of hOEC-derived cell transplantation: reductions in the volume of the lesion and the cavities within the lesion. The transplanted cells were located at the periphery of the lesion where they integrated with GFAP-positive astrocytes resulting in a significant reduction of GFAP staining intensity adjacent to the lesion. Although their mechanism of action is unclear we conclude that hOEC-derived cell transplants improved functional recovery after transplantation into the contused spinal cord, probably by modulating inflammatory responses and reducing secondary damage to the cord.

Neurotrophic factors in peripheral neuropathies: therapeutic implications

Neurotrophic factors are proteins which promote the survival of specific neuronal populations. Many have other physiological effects on neurons such as inducing morphological differentiation, enhancing nerve regeneration, stimulating neurotransmitter expression, and otherwise altering the physiological characteristics of neurons. These properties suggest that neurotrophic factors are highly promising as potential therapeutic agents for neurological disease. Neurotrophic factors will most likely be applied to the peripheral nervous system initially, since there are fewer problems for large proteins to gain access to peripheral neurons. Many of the most intensively studied factors are active in the peripheral nervous system. These include the neurotrophins (nerve growth factor, brain derived neurotrophic factor, neurotrophin-3, neurotrophin-4/5), the insulin like growth factors, ciliary neurotrophic factor, and glial cell derived neurotrophic factor and its related proteins. The biology of these factors and their receptors in the peripheral nervous system is reviewed here. We also review data suggesting that abnormal availability of some factors may contribute towards the pathogenesis of certain types of peripheral neuropathy. Finally, the pre-clinical data suggesting that individual factors might be effective in treating neuropathy is reviewed, along with data relating to possible side effects of neurotrophic factor therapy. Several factors have already entered clinical trials with variable success. The data from these trials is reviewed as well.

Neurotrophin-3 increases neurite outgrowth and apoptosis in explants of the chicken neural plate

This study examined the effect of neurotrophin-3 (NT-3) on neurite outgrowth and apoptosis of chicken neural plate explants prior to neural tube formation. In situ hybridization revealed that mRNA for the full-length (catalytic) NT-3 receptor, TrkC, was present in, and limited to, the neural plate (including the neural folds) coincident with its formation. Neural plate explants were maintained in vitro on a collagen gel under serum-free conditions in the presence or absence of exogenous NT-3 and/or an antibody to NT-3. In the absence of exogenous NT-3, explants exhibited neurite outgrowth after several days in vitro; apoptotic cells were also seen after 2 days in vitro. This does not appear to be due to endogenous NT-3, since the total number of neurites or apoptotic cells was unchanged if explants were exposed to an NT-3 antibody for the entire culture period. In the presence of exogenous NT-3, neural plate explants exhibited a dose-dependent statistically significant increase in the total number of neurites as compared to explants maintained under control conditions, as well as a statistically significant increase in apoptosis. These NT-3 effects were blocked by an NT-3 antibody. In contrast, NT-3 had no effect on the length of neurites. These findings suggest that NT-3 may play a role during early neural development in vivo.

Neurotrophin-3 contributes to the initiation of behavioral sensitization to cocaine by activating the Ras/Mitogen-activated protein kinase signal transduction cascade

These experiments were designed to assess the role of neurotrophins and the Ras/mitogen-activated protein kinase (MAP) signal transduction cascade in behavioral sensitization to cocaine. The first experiments evaluated the effect of three daily intra-ventral tegmental area (VTA) microinjections of neurotrophin-3 (NT-3) or brain-derived neurotrophic factor (BDNF) on the behavioral-activating effects of a subsequent challenge injection of cocaine in rats. Results indicated that, although NT-3 did not influence behavior across the three microinjection days, animals displayed a sensitized behavioral response to the subsequent cocaine challenge injection. In contrast, BDNF microinjections resulted in a progressive increase in behavioral activity but did not influence the subsequent behavioral response to cocaine. A second series of experiments assessed the effect of inhibiting the MAP kinase signal transduction cascade on the initiation of behavioral sensitization to cocaine. The MAP kinase kinase inhibitor PD98059, or its vehicle, was microinjected into the VTA before three daily cocaine injections. Although PD98059 did not influence the acute behavioral response to cocaine, it blocked sensitization. Finally, the effects of acute and repeated cocaine injections on NT-3 and BDNF mRNA levels in the VTA, substantia nigra, and hippocampus were assessed. Results indicated that an acute cocaine injection resulted in a transient increase in NT-3 mRNA levels in the VTA. Collectively, these results suggest that NT-3 contributes to the initiation of behavioral sensitization to cocaine by activating the Ras/MAP kinase signal transduction system. The present data also indicate that BDNF itself produced a progressive augmentation in behavioral activation with repeated administration.

Extracellular-regulated kinases and phosphatidylinositol 3-kinase are involved in brain-derived neurotrophic factor-mediated survival and neuritogenesis of the neuroblastoma cell line SH-SY5Y

Retinoic acid (RA) induces the differentiation of many cell lines, including those derived from neuroblastoma. RA treatment of SH-SY5Y cells induces the appearance of functional Trk B and Trk C receptors. Acute stimulation of RA-predifferentiated SH-SY5Y cells with brain-derived neurotrophic factor (BDNF), neurotrophin 3 (NT-3), or neurotrophin 4/5 (NT-4/5), but not nerve growth factor (NGF), induces Trk autophosphorylation, followed by phosphorylation of Akt and the extracellular signal-regulated kinases (ERKs) 1 and 2. In addition, BDNF, NT-3, or NT-4/5, but not NGF, promotes cell survival and neurite outgrowth in serum-free medium. The mitogen-activated protein kinase and ERK kinase (MEK) inhibitor PD98059 blocks BDNF-induced neurite outgrowth and growth-associated protein-43 expression but has no effects on cell survival. On the other hand, the phosphatidylinositol 3-kinase inhibitor LY249002 reverses the survival response elicited by BDNF, leading to a cell death with morphological features of apoptosis.

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

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

Relative contribution of endogenous neurotrophins in hippocampal long-term potentiation

Recent evidence has shown that brain-derived neurotrophic factor (BDNF) is involved in hippocampal long-term potentiation (LTP). Because the reagents used in acute experiments react not only with BDNF but also with neurotrophin-4/5 (NT4/5) and neurotrophin-3 (NT3), we examined the involvement of these neurotrophins in LTP using two highly specific, function-blocking monoclonal antibodies against BDNF and NT3, as well as a TrkB-IgG fusion protein. Our results show that NT3 antibodies did not have any effects on LTP. However, both TrkB-IgG fusion proteins and BDNF antibody similarly reduced LTP, suggesting that only BDNF but no other ligands of the TrkB-receptor are likely to be involved in LTP induction. The reduction in LTP depended on the inducing stimuli and was only observed with theta-burst stimulation (TBS) but not with tetanic stimulation. We further observed that LTP was only reduced if BDNF was blocked before and during TBS stimulation, and BDNF antibodies did not affect early or late stages of LTP if they were applied 10, 30, or 60 min after TBS stimulation. These results point toward a specific and unique role of endogenous BDNF but not of other neurotrophins in the process of TBS-induced hippocampal LTP. Additionally, they suggest that endogenous BDNF is required for a limited time period only shortly before or around LTP induction but not during the whole process of LTP.

Identification of a zinc finger protein whose subcellular distribution is regulated by serum and nerve growth factor

A subclass of zinc finger proteins containing a unique protein motif called the positive regulatory (PR) domain has been described. The members include the PRDI-BF1/Blimp-1 protein, the Caenorhabditis elegans egl-43 and EVI1 gene products, and the retinoblastoma interacting protein RIZ. Here we describe a member of this family, SC-1, that exhibits several distinctive features. First, SC-1 interacts with the p75 neurotrophin receptor and is redistributed from the cytoplasm to the nucleus after nerve growth factor (NGF) treatment of transfected COS cells. The translocation of SC-1 to the nucleus was specific for p75, as NGF binding to the TrkA receptor did not lead to nuclear localization of SC-1. Thus, SC-1 provides a downstream transducer for the effects of NGF through the p75 neurotrophin receptor. Under normal growth conditions, SC-1 was found predominantly in the cytoplasm. On serum-starvation, SC-1 also translocated into the nucleus. A direct correlation between nuclear expression of SC-1 with the loss of BrdUrd incorporation was observed. These results imply that SC-1 may be involved in events associated with growth arrest.

Early BDNF, NT-3, and NT-4 signaling events

Much more is known about nerve growth factor (NGF) signaling than that initiated by brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), or NT-4. We sought to study early BDNF, NT-3, and NT-4 signaling events. Using TrkB-expressing cells, we found that BDNF and NT-4 individually induced tyrosine phosphorylation of TrkB in a dose-dependent fashion. At maximally effective concentrations, BDNF or NT-4 induced robust TrkB tyrosine phosphorylation at 5 min; this progressively declined at 15, 30, and 60 min. Using immunoprecipitation, PI3-kinase and tyrosine phosphorylated PLC-gamma1 and SHC were shown to be associated with tyrosine phosphorylated TrkB in response to both BDNF and NT-4. BDNF and NT-4 induced similar intensities of phosphorylation of TrkB and signaling intermediates at equivalent doses. NT-3 treatment of TrkC-expressing cells induced very similar patterns for induction of TrkC tyrosine phosphorylation and recruitment of signaling intermediates. BDNF, NT-3, and NT-4 caused rapid tyrosine phosphorylation of ERK and SNT. These data suggest that the earliest signaling events for BDNF, NT-3, and NT-4 are very similar to those for NGF.

Peripheral nerve graft and neurotrophic factors enhance neuronal survival and expression of nitric oxide synthase in Clarke's nucleus after hemisection of the spinal cord in adult rat

The present study examined the effects of peripheral nerve (PN) graft and neurotrophic factors on the expression of nitric oxide synthase (NOS) and the survival of Clarke's nucleus (CN) neurons at the first lumbar spinal segment (L1) 15 days after hemisection of the spinal cord at T11. Normal intact CN neurons did not express NOS. Forty-one percent of the ipsilateral CN neurons survived after hemisection at T11, and 48% of the surviving neurons expressed NOS. Transplantation of PN graft at the lesion site promoted the survival of CN neurons to 71% and increased the expression of NOS to 70%. Continuous infusion of brain-derived neurotrophic factor, ciliary neurotrophic factor, and neurotrophic-3, but not glial cell-derived neurotrophic factor, at the lesion site enhanced the survival of CN neurons to about 65%. Among the surviving neurons about 70% were NOS-positive. These results indicated that transplantation of autologous PN graft or continuous infusion of neurotrophic factors could enhance the survival of axotomized CN neurons. In addition, the survival-promoting function of the neurotrophic agents was coincided with the upregulation of the expression of NOS. However, whether the upregulation of NOS expression in injured CN neurons is related to the rescue function or is a side effect of the neurotrophic factors is not clear and needed further investigation.

Selective regulation of trkC expression by NT3 in the developing peripheral nervous system

We have studied the influence of neurotrophin-3 (NT3) on the expression of its receptor tyrosine kinase, trkC, in embryonic mice. The expression of trkC transcripts encoding full-length and kinase-deficient receptors was almost entirely restricted to neurons in the trigeminal ganglion and increased markedly throughout development. In NT3(+/-) embryos, the level of trkC mRNA in the trigeminal ganglion was much lower than that in wild-type embryos, although there was no significant reduction in the total number of neurons in the ganglion. This demonstrates that endogenous NT3 regulates trkC expression in trigeminal neurons independently of changes in population size. In NT3(-/-) embryos, the number of neurons in the trigeminal ganglion was much lower than in wild-type embryos, and there was a further reduction in the mean neuronal level of trkC mRNA. Direct regulation of trkC mRNA expression in cultured trigeminal neurons was also observed, although the finding that trkC mRNA levels were sustained better in explant cultures than in dissociated cultures irrespective of the presence of NT3 suggests that trkC mRNA expression is regulated by additional factors within the ganglion. In contrast to trigeminal neurons, the level of trkC mRNA was sustained at normal levels in neurons of the sympathetic chain of NT3(-/-) embryos and was not increased by NT3 in sympathetic neuron cultures. TrkC mRNA expression in developing cutaneous tissues was also unaffected by the NT3 null mutation. In summary, our findings provide the first clear evidence that the expression of a trk receptor, tyrosine kinase, is regulated by physiological levels of its ligand in vivo and show that regulation by NT3 is cell type-specific.

Development and specification of muscle sensory neurons

Although the response properties and synaptic projections of muscle sensory neurons have been studied extensively, relatively little is known about how these sensory neurons develop their unique phenotypes during embryonic life. The explosion of new information on neurotrophins, however, has revealed that neurotrophin 3 (NT3) is critically involved in several aspects of this development, including the initial differentiation, survival, and perhaps even the terminal arborizations of muscle sensory neurons within the spinal cord. The ETS family of transcription factors, recently shown to be expressed in these sensory neurons, may help specify their choice of synaptic targets in the central nervous system.

The p75 neurotrophin receptor influences NT-3 responsiveness of sympathetic neurons in vivo

To determine the role of the p75 neurotrophin receptor (p75NTR) in sympathetic neuron development, we crossed transgenic mice with mutations in p75NTR, nerve growth factor (NGF) and neurotrophin-3 (NT-3). Neuron number is normal in sympathetic ganglia of adult p75NTR-/- mice. Mice heterozygous for a NGF deletion (NGF+/-) have 50% fewer sympathetic neurons. In the absence of p75NTR (p75NTR-/- NGF+/-), however, neuron number is restored to wild-type levels. When NT-3 levels are reduced (p75NTR-/- NGF+/- NT3 +/-), neuron number decreases compared to p75NTR-/- NGF+/- NT3+/+. Thus, without p75NTR, NT3 substitutes for NGF, suggesting that p75 alters the neurotrophin specificity of TrkA in vivo.

Opposing roles for neurotrophin-3 in targeting and collateral formation of distinct sets of developing cortical neurons

Neurotrophin-3 and its receptor TrkC are expressed during the development of the mammalian cerebral cortex. To examine whether neurotrophin-3 might play a role in the elaboration of layer-specific cortical circuits, slices of layer 6 and layers 2/3 neurons were cultured in the presence of exogenously applied neurotrophin-3. Results indicate that neurotrophin-3 promotes axonal branching of layer 6 axons, which target neurotrophin-3-expressing layers in vivo, and that it inhibits branching of layers 2/3 axons, which avoid neurotrophin-3-expressing layers. Such opposing effects of neurotrophin-3 on axonal branching were also observed with embryonic cortical neurons, indicating that the response to neurotrophin-3 is specified at early developmental stages, prior to cell migration. In addition to its effects on fiber branching, axonal guidance assays also indicate that neurotrophin-3 is an attractive signal for layer 6 axons and a repellent guidance cue for layers 2/3 axons. Experiments with specific antibodies to neutralize neurotrophin-3 in cortical membranes revealed that endogenous levels of neurotrophin-3 are sufficient to regulate branching and targeting of cortical axons. These opposing effects of neurotrophin-3 on specific populations of axons demonstrate that it could serve as one of the signals for the elaboration of local cortical circuits.

Neurotrophin action on sensory neurons in adults: an extension of the neurotrophic hypothesis

This brief review explores the action of neurotrophins on sensory neurons in adults. Neutralization of neurotrophins in adults does not cause sensory neurons to die as it does in prenatal animals. Thus they are not required as survival factors in adults. However, neurotrophins continue to play important roles in the postnatal development of sensory neurons. They also exert strong effects on the anatomy and physiology of these fibers after axotomy in adults. Here we review of the effects of NT-3 on spindle afferent fibers and NGF on nociceptive afferents and consider possible extension of the neurotrophic hypothesis to adults.

Effects of BDNF and NT-3 on hair cell survival in guinea pig cochlea damaged by kanamycin treatment

The aim of this study was to determine whether neurotrophic factors such as brain derived neurotrophic factor (BDNF) and neurotrophin 3 (NT-3) would protect auditory hair cells from ototoxicity by aminoglycoside antibiotic. Twenty-seven Wistar guinea pigs were divided into three groups of nine animals each. BDNF and NT-3 (100 microg/ml) were delivered into the right scala tympani of guinea pig cochlea through a cannula-osmotic pump device. Artificial perilymph (AP) was used as control. Immediately after implantation of the device, each animal was given five successive doses of kanamycin (400 mg/kg). At 15, 30 and 60 days after infusion, surviving inner and outer hair cells were counted at each turn of every cochlea with a Philips 515 scanning electron microscope. Multiple comparison tests were carried out among the groups, using ANOVA and Dunnett T3/Tukey HSD. Protective effects of NT-3 on hair cells were observed at 30 and 60 days after kanamycin injection. BDNF had no protective effect on hair cells at 15 and 60 days, but some at 30 days. This study suggests that NT-3 and BDNF may protect against cochlear hair cell damage caused by kanamycin treatment. Possible mechanisms for the otoprotective effects were discussed. No single mechanism postulated can explain fully the results seen in this study. It is possible that the mechanisms act in concert to produce the observed effects, or there are as yet undiscovered mechanisms or secondary messengers responsible for the otoprotective effects.

Characterization of bone morphogenetic protein family members as neurotrophic factors for cultured sensory neurons

The bone morphogenetic proteins have been implicated in several inductive processes throughout vertebrate development including nervous system patterning. Recently, these proteins have also emerged as candidates for regulating survival of mesencephalic dopaminergic and sympathetic neurons. Interestingly, we have found that several bone morphogenetic proteins can be detected in developing embryonic day 14 rat dorsal root ganglia by means of reverse transcription-polymerase chain reaction and immunocytochemistry. To further elucidate their potential role during the period of ontogenetic neuron death, serum-free cultures of dorsal root sensory neurons from developing chick and rat embryos were treated with distinct bone morphogenetic proteins with or without simultaneous addition of other "established" neurotrophic factors. Our results show that bone morphogenetic proteins exert survival promoting effects on their own, and that they can positively modulate the effects of neurotrophins on sensory neurons. In particular, growth/differentiation factor-5, bone morphogenetic protein-2, -4, -7 and -12 significantly increased the survival promoting effects of neurotrophin-3 and nerve growth factor on cultured dorsal root ganglion neurons. These results fit well into the current concept that neurotrophic factors may act synergistically in ensuring neuronal survival. Moreover, these data suggest potential instructive interactions of bone morphogentic proteins and neurotrophins during sensory neuron development. Finally, the documented neurotrophic capacity of bone morphogenetic protein family members may have potential relevance for the treatment of peripheral neuropathies.

Brain-derived neurotrophic factor, neurotrophin-3 and neurotrophin-4/5 differentially regulate the phenotype and prevent degenerative changes in striatal projection neurons after excitotoxicity in vivo

To determine whether growth factors of the neurotrophin family are able to regulate the phenotype of striatal projection neurons, cell lines overexpressing brain-derived neurotrophic factor, neurotrophin-3 or neurotrophin-4/5 were intrastriatally grafted. Striatal projection neurons were examined for the regulation of their soma areas and for the expression of glutamate decarboxylase 67, preprotachykinin A, preproenkephalin and prodynorphin messenger RNAs by in situ hybridization. Brain-derived neurotrophic factor, neurotrophin-3 and neurotrophin-4/5 differentially regulated the soma area of projection neurons at different distances from the graft, but did not modify their messenger RNA levels. Neurotrophin-3 induced an increase in the soma area of preproenkephalin- and preprotachykinin A-positive neurons, brain-derived neurotrophic factor increased the soma area of only preprotachykinin A-positive neurons, while neurotrophin-4/5 did not produce any effect. Because atrophy and neuronal loss are hallmarks of Huntington's disease, we next examined whether neurotrophins prevent degenerative changes in a quinolinate model of Huntington's disease. Seven days after intrastriatal quinolinate injection, we observed a halo of cell loss around the injection sites, reduced soma area of glutamate decarboxylase 67-, preproenkephalin- and preprotachykinin A-positive neurons bordering the lesion, and a decrease in the messenger RNA levels of glutamate decarboxylase 67 and these neuropeptides. Grafting of cell lines expressing brain-derived neurotrophic factor, neurotrophin-3 or neurotrophin-4/5 reduced the size of the lesion for preproenkephalin-, preprotachykinin- and glutamate decarboxylase 67-, but not for prodynorphin-positive neurons. Moreover, the three neurotrophins prevented the atrophy of all projection neurons, and the lesion-induced decrease in preproenkephalin and preprotachykinin A messenger RNA levels. We conclude that neurotrophins differentially regulate the phenotype of striatal projection neurons and prevent degenerative changes. The higher efficiency of neurotrophin-3 suggests a potential therapeutic application of this molecule in neurological disorders affecting striatal projection neurons, such as Huntington's disease.

The staurosporine-like compound L-753,000 (NB-506) potentiates the neurotrophic effects of neurotrophin-3 by acting selectively at the TrkA receptor

K-252b, a member of the staurosporine family of protein kinase inhibitors, selectively potentiates the activation of the nerve growth factor receptor, TrkA, by a nonpreferred ligand, neurotrophin-3 (NT-3), in a variety of cell types. At higher (micromolar) concentrations of K-252b, an inhibitory effect occurs because of the inhibitory action of K-252b on the Trk kinase. By examining analogs of K-252b, we identified the compound L-753,000 (NB-506), which potentiates the action of NT-3 on TrkA but is devoid of the inhibitory action of K-252b. L-753,000 was effective at nanomolar concentrations in a Chinese hamster ovary cell line that expressed TrkA but was devoid of p75, the low-affinity neurotrophin receptor. L-753,000 also potentiated the activation of mitogen-activating protein kinase signaling (downstream from Trk activation) by NT-3 in this cell line. Although L-753,000, like K-252b, had a negligible effect in the absence of NT-3, the compound was found to potentiate NT-3-induced survival in both rat and chick primary cultures of dissociated dorsal root ganglia (DRG) and on neurite outgrowth of chick DRG explants. Unlike K-252b, which at micromolar concentrations inhibits the survival response of NT-3 in dissociated rat DRG, L-753,000 continued to potentiate the actions of NT-3 up to a concentration of 10 microM. Furthermore, the compound, unlike K-252b, did not inhibit an unrelated protein kinase, protein kinase C, at concentrations up to 10 microM. Because L-753, 000 selectively potentiates the NT-3-induced stimulation of TrkA without inhibiting Trks and other protein kinases, it represents a novel class of selective modifiers of neurotrophin actions.

Intraspinal delivery of neurotrophin-3 using neural stem cells genetically modified by recombinant retrovirus

Neural stem cells have been shown to participate in the repair of experimental CNS disorders. To examine their potential in spinal cord repair, we used retroviral vectors to genetically modify a clone of neural stem cells, C17, to overproduce neurotrophin-3 (NT-3). The cells were infected with a retrovirus construct containing the NT-3.IRES.lacZ/neo sequence and cloned by limiting dilution and selection for lacZ expression. We studied the characteristics of the modified neural stem cells in vitro and after transplantation into the intact spinal cord of immunosuppressed adult rats. Our results show that: (i) most of the genetically modified cells express both NT-3 and lacZ genes with a high coexpression ratio in vitro and after transplantation; and (ii) large numbers of the xenografted cells survive in the spinal cord of adult rats for at least 2 months, differentiate into neuronal and glial phenotypes, and migrate for long distances. We conclude that genetically modified neural stem cells, acting as a source of neurotrophic factors, have the potential to participate in spinal cord repair.

Distribution of BDNF, NT-3 and NT-4 in the developing auditory brainstem

This study describes the developmental expression of three neurotrophins, brain derived neurotrophic factor (BDNF), neurotrophin 3 (NT-3) and neurotrophin (NT-4) in the rat auditory brain-stem using immunohistochemistry. At postnatal day 0 (PND 0), neurotrophins expression was virtually absent from all auditory nuclei in the brainstem, even though some positive neurons were observed in the mesencephalic trigeminal nucleus at this age. However, BDNF, NT-3 and NT-4 positive neurons were observed in most brainstem auditory nuclei by PND 6. At the following stages, there was a general increase in the intensity of the neurotrophins immunoreactivity and BDNF labeling was particularly prominent in most cochlear nucleus neurons. A differential pattern of staining emerged in cochlear nucleus subdivisions, with more intense staining present in the ventral part. The superior olivary complex nuclei followed a similar pattern of BDNF staining compared to the cochlear nucleus. In the adult, BDNF heavily labeled most neurons of the superior olivary nuclei and moderately labeled neurons of the inferior colliculus (IC). NT-3 and NT-4 showed a similar pattern of staining in most auditory brainstem nuclei. The first staining was observed by PND 6 in some neuronal cell bodies. NT-3 and NT-4 immunoreactivity increased in the following stages and in the adult moderate labelings were observed in most neurons of the cochlear nucleus, the superior olivary nuclei and the IC. These results show that neurotrophins are expressed 1 week before the onset of hearing and the increase of their expressions correlate with the appearance of sound-evoked activity in the system. The temporal distribution of neurotrophins does not correlate with neuronal birth, axonal outgrowth or the formation of connection in the auditory structures, suggesting a role primarily in the maintenance and/ or modulation of postnatal and adult functions.

Neurotrophins differentially regulate the survival and morphological complexity of human CNS model neurons

To determine the effect of neurotrophins on the survival and morphological differentiation of CNS neurons, we examined NT2-N cells, which provide a unique culture model for terminally differentiated and polar human neurons. Here we report the development of conditions for the long-term culture of NT2-N cells in low density and in chemically defined medium. We show that NT2-N cells express rRNAs for TrkA, TrkB, and TrkC tyrosine kinase receptors and the low-affinity nerve growth factor receptor (p75NTR). All members of the nerve growth factor-related family of neurotrophic factors promote neuronal survival in long-term cultures with approximately 1 ng/ml for half-maximal survival. At high concentrations (>20 ng/ml), the neurotrophins reversed the survival-promoting effect as judged by MTT [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide] conversion. In contrast to the uniform effect of all neurotrophins on neuronal survival, brain-derived neurotrophic factor selectively induced an increased dendritic complexity. These results demonstrate that NT2-N cells provide a useful model to analyze the effect of neurotrophins on the survival and morphological differentiation of CNS neurons in vitro. In addition, the data indicate that neuronal survival and the development of morphological complexity are differentially regulated in a multireceptor context.

Gating of BDNF-induced synaptic potentiation by cAMP

Neurotrophins have been implicated in activity-dependent synaptic plasticity, but the underlying intracellular mechanisms remain largely unknown. Synaptic potentiation induced by brain-derived neurotrophic factor (BDNF), but not neurotrophin 3, was prevented by blockers of adenosine 3',5'-monophosphate (cAMP) signaling. Activators of cAMP signaling alone were ineffective in modifying synaptic efficacy but greatly enhanced the potentiation effect of BDNF. Blocking cAMP signaling abolished the facilitation of BDNF-induced potentiation by presynaptic activity. Thus synaptic actions of BDNF are gated by cAMP. Activity and other coincident signals that modulate cAMP concentrations may specify the action of secreted neurotrophins on developing nerve terminals.

Target-dependent regulation of acetylcholine secretion at developing motoneurons in Xenopus cell cultures

1. Myocyte-dependent regulation of acetylcholine (ACh) quantal secretion from developing motoneurons was studied in day-3 Xenopus nerve-muscle co-cultures. Spontaneous synaptic currents (SSCs) were measured in manipulated synapses by using whole-cell voltage-clamped myocytes. Changes in SSC amplitude were assumed to reflect changes in the ACh content of secreted quantal packets. Compared with natural synapses, motoneurons without any contact with a myocyte (naive neurons) released ACh in smaller quantal packets. 2. Bipolar cultured motoneurons, which were in contact with a myocyte with one axon branch (contact-end) but remained free at another axon branch (free-end), were further used to examine quantal ACh secretion. The ACh quantal size recorded at free-end terminals was similar to that of naive neurons and was smaller than that at the contact-end, indicating that myocyte contact exerts differential regulation on quantal secretion in the same neuron. 3. Some of the neurons that formed a natural synapse with a myocyte continued to grow forward and ACh quantal secretion from the free growth cone was examined. The ACh quantal size recorded at free growth cones was inversely proportional to the distance to the natural synapse, implying localized regulation of quantal secretion by the myocyte. 4. Chronic treatment of day-1 cultures with veratridine and d-tubocurarine, respectively, increased and decreased the neurotrophic action of myocytes when assayed on day 3. 5. Taken together, these findings suggest that the myocyte is an important postsynaptic target in the regulation of quantal secretion and that the trophic action is spatially restricted to the neighbourhood of the neuromuscular junction.

TrkB isoforms with distinct neurotrophin specificities are expressed in predominantly nonoverlapping populations of avian dorsal root ganglion neurons

Alternative splicing of the avian trkB receptor generates an extracellular deletion (ED) isoform missing 11 amino acids from the neurotrophin-binding domain of the full-length (FL) receptor. When expressed in fibroblasts, the ED isoform exhibited restricted neurotrophin specificity compared with that of the FL receptor. Brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), and neurotrophin-4 (NT-4) activated the FL receptor, as determined by tyrosine phosphorylation. However, only BDNF was capable of significant activation of the ED isoform, although to a reduced level. Because positively charged residues in NT-3 are important for binding to trkB, two negatively charged aspartate residues within the 11 amino acid motif of FL trkB were mutated to examine the role of electrostatic interactions on ligand binding. As found for the ED isoform, the FL mutated receptor displayed a similar loss of NT-3- and NT-4-mediated activation, in addition to a diminished responsiveness to BDNF. Because of these profound effects on ligand specificity, reverse transcription-PCR was used to understand the expression of the FL and ED receptor isoforms at the level of single neurons. The predominant expression pattern of either FL or ED isoforms in single embryonic DRG neurons establishes the existence of two subpopulations exhibiting differential responsiveness to trkB ligands, indicating that regulated splicing of the extracellular domain of trkB may serve as a mechanism to restrict neuronal responsiveness to the neurotrophins.

NT-3, like NGF, is required for survival of sympathetic neurons, but not their precursors

Superior cervical ganglia of postnatal mice with a targeted disruption of the gene for neurotrophin-3 have 50% fewer neurons than those of wild-type mice. In culture, neurotrophin-3 increases the survival of proliferating sympathetic precursors. Both precursor death (W. ElShamy et al., 1996, Development 122, 491-500) and, more recently, neuronal death (S. Wyatt et al., 1997, EMBO J. 16, 3115-3123) have been described in mice lacking NT-3. Consistent with the second report, we found that, in vivo, neurogenesis and precursor survival were unaffected by the absence of neurotrophin-3 but neuronal survival was compromised so that only 50% of the normal number of neurons survived to birth. At the time of neuron loss, neurotrophin-3 expression, assayed with a lacZ reporter, was detected in sympathetic target tissues and blood vessels, including those along which sympathetic axons grow, suggesting it may act as a retrograde neurotrophic factor, similar to nerve growth factor. To explore this possibility, we compared neuron loss in neurotrophin-3-deficient mice with that in nerve growth factor-deficient mice and found that neuronal losses occurred at approximately the same time in both mutants, but were less severe in mice lacking neurotrophin-3. Eliminating one or both neurotrophin-3 alleles in mice that lack nerve growth factor does not further reduce sympathetic neuron number in the superior cervical ganglion at E17.5 but does alter axon outgrowth and decrease salivary gland innervation. Taken together these results suggest that neurotrophin-3 is required for survival of some sympathetic neurons that also require nerve growth factor.

Signalling by the RET receptor tyrosine kinase and its role in the development of the mammalian enteric nervous system

RET is a member of the receptor tyrosine kinase (RTK) superfamily, which can transduce signalling by glial cell line-derived neurotrophic factor (GDNF) and neurturin (NTN) in cultured cells. In order to determine whether in addition to being sufficient, RET is also necessary for signalling by these growth factors, we studied the response to GDNF and NTN of primary neuronal cultures (peripheral sensory and central dopaminergic neurons) derived from wild-type and RET-deficient mice. Our experiments show that absence of a functional RET receptor abrogates the biological responses of neuronal cells to both GDNF and NTN. Despite the established role of the RET signal transduction pathway in the development of the mammalian enteric nervous system (ENS), very little is known regarding its cellular mechanism(s) of action. Here, we have studied the effects of GDNF and NTN on cultures of neural crest (NC)-derived cells isolated from the gut of rat embryos. Our findings suggest that GDNF and NTN promote the survival of enteric neurons as well as the survival, proliferation and differentiation of multipotential ENS progenitors present in the gut of E12.5-13.5 rat embryos. However, the effects of these growth factors are stage-specific, since similar ENS cultures established from later stage embryos (E14. 5–15.5), show markedly diminished response to GDNF and NTN. To examine whether the in vitro effects of RET activation reflect the in vivo function(s) of this receptor, the extent of programmed cell death was examined in the gut of wild-type and RET-deficient mouse embryos by TUNEL histochemistry. Our experiments show that a subpopulation of enteric NC undergoes apoptotic cell death specifically in the foregut of embryos lacking the RET receptor. We suggest that normal function of the RET RTK is required in vivo during early stages of ENS histogenesis for the survival of undifferentiated enteric NC and their derivatives.

Expression of neurotrophins in hippocampal interneurons immunoreactive for the neuropeptides somatostatin, neuropeptide-Y, vasoactive intestinal polypeptide and cholecystokinin

Using a double detection method, which combines in situ hybridization for the detection of neurotrophin messenger RNA with immunocytochemistry against the neuropeptides somatostatin, neuropeptide Y, vasoactive intestinal polypeptide and cholecystokinin, we have analysed the expression of the neurotrophins, nerve growth factor, brain-derived neurotrophic factor and neurotrophin-3, in distinct populations of neuropeptide-immunoreactive hippocampal interneurons. Nerve growth factor messenger RNA expression was found in subsets of the four subpopulations of neuropeptide-immunoreactive interneurons. The highest degree of co-localization was observed in the neuropeptide-Y-positive cells (up to 70%) and in somatostatin-immunoreactive cells (48%). Only small subsets of cholecystokinin- and vasoactive intestinal polypeptide-positive neurons (21% and 10%, respectively) displayed nerve growth factor hybridization signals. In contrast, expression of neurotrophin-3 messenger RNA was exclusively observed in 26% of neuropeptide-Y-immunoreactive cells. Brain-derived neurotrophic factor hybridization signals were never detected in the neuropeptide-positive hippocampal interneurons. Morphological analysis of neuropeptide-immunoreactive interneurons that express or lack nerve growth factor messenger RNA revealed that most perisomatic inhibitory neurons, such as large vasoactive intestinal polypeptide/ cholecystokinin-immunoreactive cells, showed positive nerve growth factor hybridization signals. In addition, some somatostatin/neuropeptide-Y-immunoreactive interneurons, which are responsible for dendritic inhibition of principal hippocampal neurons, expressed nerve growth factor messenger RNA. In contrast, interneurons specialized to innervate other GABAergic cells, such as small vasoactive intestinal polypeptide-positive cells, lacked nerve growth factor expression. All these data indicate that expression of neurotrophins is differentially regulated in functionally distinct classes of hippocampal interneurons immunoreactive for neuropeptides. We also analysed whether neuropeptide-immunoreactive interneurons expressing neurotrophins were targets of the GABAergic septohippocampal pathway. We used a triple detection method, combining anterograde tracing of this connection, with in situ hybridization for the detection of neurotrophin mRNA, and immunocytochemistry against neuropeptides. Our data showed that the four populations of hippocampal interneurons studied (somatostatin, neuropeptide-Y, vasoactive intestinal polypeptide and cholescystokinin) received GABAergic afferents from the septum. However, no preference for neuropeptide-immunoreactive cells expressing neurotrophins was observed, compared to neuropeptide-positive neurons lacking neurotrophin expression.

Endogenous neurotrophin-3 is retrogradely transported in the rat sciatic nerve

To address the active transport of neurotrophins, nerve growth factor, brain-derived neurotrophic factor, and neurotrophin-3 in the peripheral nerves, we examined the levels of proteins and messenger RNAs in the sciatic nerve of adult rats following transection, using enzyme immunoassays and reverse transcription polymerase chain reaction method, respectively. Neurotrophin-3 protein increased one day after transection only in the distal segment next to the transection site and returned to the original level two days later. This was considered to reflect accumulation of neurotrophin-3 transported from the periphery toward the neuronal cell bodies, because the neurotrophin-3 messenger RNA level was not changed in any sciatic segments during this experimental period. An increase in brain-derived neurotrophic factor protein was observed simultaneously in both the distal and proximal stumps three days after transection. Brain-derived neurotrophic factor messenger RNA was elevated in the same stumps two days after transection, suggesting that brain-derived neurotrophic factor was produced within the transected stumps. These observations demonstrate that neurotrophin-3, like nerve growth factor, is retrogradely transported in the sciatic nerve but that brain-derived neurotrophic factor is not. This suggests that neurotrophin-3 plays a role in the conveyance of trophic signals from target organs to neurons.

Hyperalgesia due to nerve damage: role of nerve growth factor

The hypothesis that nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) contribute to hyperalgesia resulting from nerve damage was tested in rats in which the sciatic nerve was partially transected on one side. Administration of antisera raised against NGF and BDNF relieved mechanical and thermal hyperalgesia in these animals. It has been suggested that NGF may elicit hyperalgesia by inducing mast cells to release algesic agents such as serotonin (5-HT). We found that degranulation of mast cells with compound 48/80 relieved mechanical and thermal hyperalgesia produced by nerve damage. We also found that local injection of the 5-HT2A and 5-HT3 receptor antagonists ketanserin and ICS 205-930 into the affected hind paw relieved mechanical hyperalgesia in a dose-dependent fashion. These findings support the idea that in this rat model of hyperalgesia due to peripheral nerve damage, NGF acts on mast cells to induce release of 5-HT, which sensitizes nociceptors. Hyperalgesia due to nerve injury and hyperalgesia due to inflammation may share some common features.

Radicicol potentiates neurotrophin-mediated neurite outgrowth and survival of cultured sensory neurons from chick embryo

Radicicol, an antifungal antibiotic with markedly low toxicity, is a potent inhibitor of the Src family of protein tyrosine kinases and causes morphological reversion of v-src-transformed fibroblasts. Recently, this antibiotic was also found to inhibit Raf kinase. In the present study, we found that nanomolar concentrations of radicicol (10 ng/ml) enhanced the survival and neurite outgrowth of neurons from embryonic chick dorsal root ganglia (DRGs) and sympathetic ganglia. It potentiated the trophic effects of nerve growth factor, brain-derived neurotrophic factor, and neurotrophin-3 on the cultured DRG neurons. This concentration of radicicol did not alter the tyrosine phosphorylation of Trk receptors or the activity of mitogen-activated protein (MAP) kinases. Wortmannin, an inhibitor of phosphatidylinositol 3-kinase (PI3-kinase), did not inhibit radicicol, excluding the involvement of PI3-kinase in the radicicol-dependent trophic actions. These results suggest that radicicol mediates neuronal growth presumably via a mechanism not involving the activation of Trk receptors, MAP kinase, or PI3-kinase.

Cutting edge: clonally restricted production of the neurotrophins brain-derived neurotrophic factor and neurotrophin-3 mRNA by human immune cells and Th1/Th2-polarized expression of their receptors

Neurotrophins, such as neurotrophin-3 (NT-3) and brain-derived neurotrophic factor (BDNF), are potent regulators of neuronal functions. Here we show that human immune cells also produce NT-3 mRNA, secrete BDNF, and express their specific receptors trkB and trkC. The truncated trkB receptor, usually expressed in sensory neurons of the central nervous system, was also constitutively expressed in unstimulated Th cells. Full-length trkB was detectable in stimulated PBMC, B cell lines, and Th1, but not in Th2 and Th0 cell clones. Clonally restricted expression was also observed for trkC, until now not detected on blood cells. The Th1 cytokine IL-2 stimulated production of trkB mRNA but not of trkC, whereas the Th2 cytokine IL-4 enhanced NT-3 but not BDNF mRNA expression. Microbial Ags, which influence the Th1/Th2 balance, could therefore modulate the neurotrophic system and thereby affect neuronal synaptic activity of the central nervous system.

Expression of p75(NTR), trkB and trkC in nonmanipulated and axotomized motoneurons of aged rats

Several lines of evidence indicate that adult neurons remain dependent on neurotrophins and that changes in tissue expression of neurotrophins and/or their receptors may play a role in senile neurodegeneration. We have studied the expression of p75NTR, trkB and trkC, respectively, in lumbar motoneurons of young adult (2-3 months) and aged (30 months) rats subjected to sciatic transection using in situ hybridization and immunohistochemistry. Nonmanipulated age-matched animals were processed in parallel. In nonmanipulated aged rats, high levels of p75NTR could be seen in a number of motoneurons (10-15%), while in young adult animals no p75NTR could be detected. Seven days following sciatic axotomy, a conspicuous ipsilateral upregulation p75NTR was observed in young adult rats. Also in aged rats there was a marked ipsilateral increase in number of p75NTR expressing neurons ( approximately 100%). In comparison to young adult rats, aged rats showed a decreased expression of both trkB (5/6 animals) and trkC (6/6 animals). Furthermore, in response to sciatic transection, 3 out of 5 aged rats did not show an increased expression of trkB. In aged rats, axotomy did not induce any significant change in trkC expression. In the young adult rats, we recorded a side-to-side effect with lower values ipsilaterally, however, it cannot be excluded that this difference was caused by an upregulation in the contralateral motoneurons. Oligonucleotide probes against BDNF and NT3 mRNA showed only very few faintly positive neurons in both age groups. Our results indicate that the pattern of regulatory changes of NT receptors in response to axotomy is different in aged and young adult rats. The lack of covariation between p75NTR and trkB and trkC regulation in aged rats indicates a changed role for p75NTR in senescent motoneurons.

Expression of Trk receptors in the developing mouse trigeminal ganglion: in vivo evidence for NT-3 activation of TrkA and TrkB in addition to TrkC

Animals lacking neurotrophin-3 (NT-3) are born with deficits in almost all sensory ganglia. Among these, the trigeminal ganglion is missing 70% of the normal number of neurons, a deficit which develops during the major period of neurogenesis between embryonic stages (E) 10.5 and E13.5. In order to identify the mechanisms for this deficit, we used antisera specific for TrkA, TrkB, and TrkC to characterize and compare the expression patterns of each Trk receptor in trigeminal ganglia of wild type and NT-3 mutants between E10.5 and E15.5. Strikingly, TrkA, TrkB, and TrkC proteins appear to be exclusively associated with neurons, not precursors. While some neurons show limited co-expression of Trk receptors at E11.5, by E13. 5 each neuron expresses only one Trk receptor. Neuronal birth dating and cell counts show that in wild-type animals all TrkB- and TrkC-expressing neurons are generated before E11.5, while the majority of TrkA-expressing neurons are generated between E11.5 and E13.5. In mice lacking NT-3, the initial formation of the ganglion, as assessed at E10.5, is similar to that in wild-type animals. At E11.5, however, the number of TrkC-expressing neurons is dramatically reduced and the number of TrkC-immunopositive apoptotic profiles is markedly elevated. By E13.5, TrkC-expressing neurons are virtually eliminated. At E11.5, compared to wild type, the number of TrkB-expressing neurons is also reduced and the number of TrkB immunoreactive apoptotic profiles is increased. TrkA neurons are also reduced in the NT-3 mutants, but the major deficit develops between E12.5 and E13.5 when elevated numbers of TrkA-immunoreactive apoptotic profiles are detected. Normal numbers of TrkA- and TrkB-expressing neurons are seen in a TrkC-deficient mutant. Therefore, our data provide evidence that NT-3 supports the survival of TrkA-, TrkB- and TrkC-expressing neurons in the trigeminal ganglion by activating directly each of these receptors in vivo.

Developing Schwann cells acquire the ability to survive without axons by establishing an autocrine circuit involving insulin-like growth factor, neurotrophin-3, and platelet-derived growth factor-BB

Although Schwann cell precursors from early embryonic nerves die in the absence of axonal signals, Schwann cells in older nerves can survive in the absence of axons in the distal stump of transected nerves. This is crucially important, because successful axonal regrowth in a damaged nerve depends on interactions with living Schwann cells in the denervated distal stump. Here we show that Schwann cells acquire the ability to survive without axons by establishing an autocrine survival loop. This mechanism is absent in precursors. We show that insulin-like growth factor, neurotrophin-3, and platelet-derived growth factor-BB are important components of this autocrine survival signal. The secretion of these factors by Schwann cells has significant implications for cellular communication in developing nerves, in view of their known ability to regulate survival and differentiation of other cells including neurons.

A possible mechanism of TPA-mediated downregulation of neurotrophin-3 gene expression in rat cultured vascular smooth muscle cells

We have previously reported that in cultured rat vascular smooth muscle cells (VSMCs), neurotrophin-3 (NT-3) gene expression was suppressed by TPA (12-O-tetradecanoyl phorbol-13-acetate), which induces an AP-1 transcription factor. In the present study, to clarify the mechanism for TPA-mediated downregulation of NT-3 gene expression, effects of cycloheximide and dexamethasone (Dex) on the TPA-mediated downregulation were examined in VSMCs. Pretreatment with cycloheximide, an inhibitor of protein synthesis, or simultaneous treatment with Dex, an inhibitor of AP-1, suppressed the TPA-mediated downregulation of NT-3 gene expression. Furthermore, co-transfection of c-fos and c-jun expression vectors into VSMCs resulted in decrease in the NT-3 gene expression. The present findings suggest that TPA-induced AP-1 de novo synthesis causes the downregulation of NT-3 gene expression in VSMCs.