Neurotrophin-3 as an essential signal for the developing nervous system

Targeting the Multiple Complex Processes of Hypoxia-Ischemia to Achieve Neuroprotection

Hypoxic-ischemic encephalopathy (HIE) is a major cause of newborn brain damage stemming from a lack of oxygenated blood flow in the neonatal period. Twenty-five to fifty percent of asphyxiated infants who develop HIE die in the neonatal period, and about sixty percent of survivors develop long-term neurological disabilities. From the first minutes to months after the injury, a cascade of events occurs, leading to blood-brain barrier (BBB) opening, neuronal death and inflammation. To date, the only approach proposed in some cases is therapeutic hypothermia (TH). Unfortunately, TH is only partially protective and is not applicable to all neonates. This review synthesizes current knowledge on the basic molecular mechanisms of brain damage in hypoxia-ischemia (HI) and on the different therapeutic strategies in HI that have been used and explores a major limitation of unsuccessful therapeutic approaches.

Effect of selegiline as a monomine oxidase B inhibitor on the expression of neurotrophin mRNA levels in a contusion rat model of spinal cord injury

OBJECTIVE

Spinal cord injury (SCI) is followed by a cascade of events at the site of injury, including vascular ischemia, an increase in free radicals, inflammation, and neuronal death. In these individuals, protection of nerves and supporting cells, as well as prevention of neuronal damage, may improve recovery opportunities. Neurotrophins are a family of polypeptides that regulate nerve differentiation, growth, and survival. Selegiline is a selective monoamine oxidase B (MAO-B) inhibitor used to treat Parkinson's disease. Selegiline has been found to have neuroprotective properties and may be useful for the expression of neurotrophins. The aim of this study was to evaluate the expression levels of neurotrophin genes in spinal cord rats treated with selegiline.

METHODS

Rats were divided into four groups: injury (control), laminectomy, sham (injured rat received 1 ml saline intraperitoneally) and treatment (injured rat received 5 mg/kg selegiline intraperitoneally for 7 days; once a day). The BBB scale (Basso, Beattie and Bresnahan) was performed once a week for 4 weeks to assess motor function after a spinal cord injury. On day 28 after SCI, the rat was sacrificed and the spinal cord lesion removed. A real-time PCR approach was used to assess neurotrophin gene expression.

RESULTS

The results showed that administration of selegiline improves locomotor function and increases mRNA levels of BDNF, GDNF, NT-3, and NT-4.

CONCLUSION

In summary, the results of this study suggest that selegiline may be an appropriate treatment for spinal cord injuries.

Growth factors and molecular-driven plasticity in neurological systems

It has been almost 70 years since the discovery of nerve growth factor (NGF), a period of a dramatic evolution in our understanding of dynamic growth, regeneration, and rewiring of the nervous system. In 1953, the extraordinary finding that a protein found in mouse submandibular glands generated a halo of outgrowing axons has now redefined our concept of the nervous system connectome. Central and peripheral neurons and their axons or dendrites are no longer considered fixed or static "wiring." Exploiting this molecular-driven plasticity as a therapeutic approach has arrived in the clinic with a slate of new trials and ideas. Neural growth factors (GFs), soluble proteins that alter the behavior of neurons, have expanded in numbers and our understanding of the complexity of their signaling and interactions with other proteins has intensified. However, beyond these "extrinsic" determinants of neuron growth and function are the downstream pathways that impact neurons, ripe for translational development and potentially more important than individual growth factors that may trigger them. Persistent and ongoing nuances in clinical trial design in some of the most intractable and irreversible neurological conditions give hope for connecting new biological ideas with clinical benefits. This review is a targeted update on neural GFs, their signals, and new therapeutic ideas, selected from an expansive literature.

Innervation in organogenesis

Proper innervation of peripheral organs helps to maintain physiological homeostasis and elicit responses to external stimuli. Disruptions to normal function can result in pathophysiological consequences. The establishment of connections and communication between the central nervous system and the peripheral organs is accomplished through the peripheral nervous system. Neuronal connections with target tissues arise from ganglia partitioned throughout the body. Organ innervation is initiated during development with stimuli being conducted through several types of neurons including sympathetic, parasympathetic, and sensory. While the physiological modulation of mature organs by these nerves is largely understood, their role in mammalian development is only beginning to be uncovered. Interactions with cells in target tissues can affect the development and eventual function of several organs, highlighting their significance. This chapter will cover the origin of peripheral neurons, factors mediating organ innervation, and the composition and function of organ-specific nerves during development. This emerging field aims to identify the functional contribution of innervation to development which will inform future investigations of normal and abnormal mammalian organogenesis, as well as contribute to regenerative and organ replacement efforts where nerve-derived signals may have significant implications for the advancement of such studies.

The Impact of Intermittent Fasting on Brain-Derived Neurotrophic Factor, Neurotrophin 3, and Rat Behavior in a Rat Model of Type 2 Diabetes Mellitus

Type 2 diabetes mellitus (T2DM) is known to be associated with an increased risk of dementia, specifically Alzheimer’s disease and vascular dementia. Intermittent fasting (IF) has been proposed to produce neuroprotective effects through the activation of several signaling pathways. In this study, we investigated the effect of IF on rat behavior in type 2 diabetic rats. Forty male Wistar Kyoto rats were divided into four groups (n = 10 for each): the ad libitum (Ad) group, the intermittent fasting group (IF), the streptozotocin-induced diabetic 2 group (T2DM) fed a high-fat diet for 4 weeks followed by a single intraperitoneal (i.p.) injection of streptozotocin (STZ) 25 mg kg⁻¹, and the diabetic group with intermittent fasting (T2DM+IF). We evaluated the impact of 3 months of IF (16 h of food deprivation daily) on the levels of brain-derived neurotrophic factor (BDNF), neurotrophin 3 (NT3), serotonin, dopamine, and glutamate in the hippocampus, and rat behavior was assessed by the forced swim test and elevated plus maze. IF for 12 weeks significantly increased (p < 0.05) the levels of NT3 and BDNF in both control and T2DM rats. Additionally, it increased serotonin, dopamine, and glutamic acid in diabetic rats. Moreover, IF modulated glucose homeostasis parameters, with a significant decrease (p < 0.05) in insulin resistance and downregulation of serum corticosterone level. Interestingly, T2DM rats showed a significant increase in anxiety and depression behaviors, which were ameliorated by IF. These findings suggest that IF could produce a potentially protective effect by increasing the levels of BDNF and NT3 in both control and T2DM rats. IF could be considered as an additional therapy for depression, anxiety, and neurodegenerative diseases.

Neurotrophin-3 modulates synaptic transmission

Neurotrophin-3 (NT-3) belongs to a family of growth factors called neurotrophins whose actions are centered in the nervous system. NT-3 is structurally related to other neurotrophins like brain-derived neurotrophic factor. The expression of NT-3 starts with the onset of neurogenesis and continues throughout life. A wealth of information links NT-3 to the growth, differentiation, and survival of hippocampal cells as well as sympathetic and sensory neurons. These studies have described the distribution of NT-3 and its receptors throughout development and in the mature nervous system. Prior works has begun to cell-type specific impact of NT-3 as well as identify the signaling pathways involved. However, much less is known about how NT-3 regulates synaptic transmission. This chapter focuses role of NT-3 in the modulation of synaptic transmission.

Neural Stem Cells, Neural Progenitors, and Neurotrophic Factors

Neural stem cells (NSCs) have been proposed as a promising cellular source for the treatment of diseases in nervous systems. NSCs can self-renew and generate major cell types of the mammalian central nervous system throughout adulthood. NSCs exist not only in the embryo, but also in the adult brain neurogenic region: the subventricular zone (SVZ) of the lateral ventricle. Embryonic stem (ES) cells acquire NSC identity with a default mechanism. Under the regulations of leukemia inhibitory factor (LIF) and fibroblast growth factors, the NSCs then become neural progenitors. Neurotrophic and differentiation factors that regulate gene expression for controlling neural cell fate and function determine the differentiation of neural progenitors in the developing mammalian brain. For clinical application of NSCs in neurodegenerative disorders and damaged neurons, there are several critical problems that remain to be resolved: 1) how to obtain enough NSCs from reliable sources for autologous transplantation; 2) how to regulate neural plasticity of different adult stem cells; 3) how to control differentiation of NSCs in the adult nervous system. In order to understand the mechanisms that control NSC differentiation and behavior, we review the ontogeny of NSCs and other stem cell plasticity of neuronal differentiation. The role of NSCs and their regulation by neurotrophic factors in CNS development are also reviewed.

In vitro non-viral murine pro-neurotrophin 3 gene transfer into rat bone marrow stromal cells

Neurotrophin 3 (NT-3) is an important factor for promoting prenatal neural development, as well as regeneration, axogenesis and plasticity in postnatal life. Therapy with NT-3 was reported to improve the condition of patients suffering from degenerative diseases and traumatic injuries, however, the disadvantage of NT-3 protein delivery is its short half-life, thus our alternative approach is the use of NT-3 gene therapy. In this study, the bone marrow stromal cells (BMSCs) were isolated from adult rats, cultured for 4 passages and transfected with either pEGFP-N1 or a constructed vector containing murine proNT-3 (pSecTag2/HygroB-murine proNT-3) using Lipofectamine 2000 followed by Hygromycin B (200mg/kg). The transfection efficiency of the transiently transfected BMSCs was evaluated using the green fluorescence protein containing vector (pEGFP-N1). A quantitative evaluation of the NT-3 expression of mRNA using real time qRT-PCR shows that there was double fold increase in NT-3 gene expression compared with non-transfected BMSCs, also, the culture supernatant yielded double fold increase in NT-3 using ELISA technique, the data were supported by immunoblotting technique. This suggests that the use of this transfection technique can be useful for gene therapy in different neurological disorders with neurodegenerative or traumatic origins.

Neurotrophin Signaling and Stem Cells-Implications for Neurodegenerative Diseases and Stem Cell Therapy

Neurotrophins (NTs) are members of a neuronal growth factor protein family whose action is mediated by the tropomyosin receptor kinase (TRK) receptor family receptors and the p75 NT receptor (p75NTR), a member of the tumor necrosis factor (TNF) receptor family. Although NTs were first discovered in neurons, recent studies have suggested that NTs and their receptors are expressed in various types of stem cells mediating pivotal signaling events in stem cell biology. The concept of stem cell therapy has already attracted much attention as a potential strategy for the treatment of neurodegenerative diseases (NDs). Strikingly, NTs, proNTs, and their receptors are gaining interest as key regulators of stem cells differentiation, survival, self-renewal, plasticity, and migration. In this review, we elaborate the recent progress in understanding of NTs and their action on various stem cells. First, we provide current knowledge of NTs, proNTs, and their receptor isoforms and signaling pathways. Subsequently, we describe recent advances in the understanding of NT activities in various stem cells and their role in NDs, particularly Alzheimer's disease (AD) and Parkinson's disease (PD). Finally, we compile the implications of NTs and stem cells from a clinical perspective and discuss the challenges with regard to transplantation therapy for treatment of AD and PD.

Emerging roles of the neurotrophin receptor TrkC in synapse organization

Tropomyosin-receptor-kinase (Trk) receptors have been extensively studied for their roles in kinase-dependent signaling cascades in nervous system development. Synapse organization is coordinated by trans-synaptic interactions of various cell adhesion proteins, a representative example of which is the neurexin-neuroligin complex. Recently, a novel role for TrkC as a synapse organizing protein has been established. Post-synaptic TrkC binds to pre-synaptic type-IIa receptor-type protein tyrosine phosphatase sigma (PTPσ). TrkC-PTPσ specifically induces excitatory synapses in a kinase domain-independent manner. TrkC has distinct extracellular domains for PTPσ- and NT-3-binding and thus may bind both ligands simultaneously. Indeed, NT-3 enhances the TrkC-PTPσ interaction, thus facilitating synapse induction at the pre-synaptic side and increasing pre-synaptic vesicle recycling in a kinase-independent fashion. A crystal structure study has revealed the detailed structure of the TrkC-PTPσ complex as well as competitive modulation of TrkC-mediated synaptogenesis by heparan sulfate proteoglycans (HSPGs), which bind the same domain of TrkC as PTPσ. Thus, there is strong evidence supporting a role for the TrkC-PTPσ complex in mechanisms underlying the fine turning of neural connectivity. Furthermore, disruption of the TrkC-PTPσ complex may be the underlying cause of certain psychiatric disorders caused by mutations in the gene encoding TrkC (NTRK3), supporting its role in cognitive functions.

Degeneration of proprioceptive sensory nerve endings in mice harboring amyotrophic lateral sclerosis-causing mutations

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease that primarily targets the motor system. Although much is known about the effects of ALS on motor neurons and glial cells, little is known about its effect on proprioceptive sensory neurons. This study examines proprioceptive sensory neurons in mice harboring mutations associated with ALS, in SOD1(G93A) and TDP43(A315T) transgenic mice. In both transgenic lines, we found fewer proprioceptive sensory neurons containing fluorescently tagged cholera toxin in their soma five days after injecting this retrograde tracer into the tibialis anterior muscle. We asked whether this is due to neuronal loss or selective degeneration of peripheral nerve endings. We found no difference in the total number and size of proprioceptive sensory neuron soma between symptomatic SOD1(G93A) and control mice. However, analysis of proprioceptive nerve endings in muscles revealed early and significant alterations at Ia/II proprioceptive nerve endings in muscle spindles before the symptomatic phase of the disease. Although these changes occur alongside those at α-motor axons in SOD1(G93A) mice, Ia/II sensory nerve endings degenerate in the absence of obvious alterations in α-motor axons in TDP43(A315T) transgenic mice. We next asked whether proprioceptive nerve endings are similarly affected in the spinal cord and found that nerve endings terminating on α-motor neurons are affected during the symptomatic phase and after peripheral nerve endings begin to degenerate. Overall, we show that Ia/II proprioceptive sensory neurons are affected by ALS-causing mutations, with pathological changes starting at their peripheral nerve endings.

Smooth-muscle-specific expression of neurotrophin-3 in mouse embryonic and neonatal gastrointestinal tract

Vagal gastrointestinal (GI) afferents are essential for the regulation of eating, body weight, and digestion. However, their functional organization and the way that this develops are poorly understood. Neurotrophin-3 (NT-3) is crucial for the survival of vagal sensory neurons and is expressed in the developing GI tract, possibly contributing to their survival and to other aspects of vagal afferent development. The identification of the functions of this peripheral NT-3 thus requires a detailed understanding of the localization and timing of its expression in the developing GI tract. We have studied embryos and neonates expressing the lacZ reporter gene from the NT-3 locus and found that NT-3 is expressed predominantly in the smooth muscle of the outer GI wall of the stomach, intestines, and associated blood vessels and in the stomach lamina propria and esophageal epithelium. NT-3 expression has been detected in the mesenchyme of the GI wall by embryonic day 12.5 (E12.5) and becomes restricted to smooth muscle and lamina propria by E15.5, whereas its expression in blood vessels and esophageal epithelium is first observed at E15.5. Expression in most tissues is maintained at least until postnatal day 4. The lack of colocalization of beta-galactosidase and markers for myenteric ganglion cell types suggests that NT-3 is not expressed in these ganglia. Therefore, NT-3 expression in the GI tract is largely restricted to smooth muscle at ages when vagal axons grow into the GI tract, and when vagal mechanoreceptors form in smooth muscle, consistent with its role in these processes and in vagal sensory neuron survival.

A novel point variant in NTRK3, R645C, suggests a role of this gene in the pathogenesis of Hirschsprung disease

Hirschsprung disease (HSCR) is a developmental disorder characterized by the absence of ganglion cells in the myenteric and submucosal plexuses due to a defect in the migration process of neural crest neuroblasts. Manifestation of the disease has been linked to the dysfunction of two principal signalling pathways involved in the enteric nervous system (ENS) formation: the RET-GDNF and the EDN3-EDNRB receptor systems. However, the NTF3/NTRK3 signalling pathway plays an essential role in the development of the ENS suggesting a potential role for those genes in the pathogenesis of HSCR. We have sought to evaluate the candidature of the NTRK3 gene, which encodes the TrkC receptor, as a susceptibility gene for Hirschsprung disease. Using dHPLC technology we have screened the NTRK3 coding region in 143 Spanish HSCR patients. A total of four previously described polymorphisms and 12 novel sequence variants were detected. Of note, the novel R645C mutation was detected in 2 affected siblings of a HSCR family also carrying a RET splicing mutation. Using bioinformatics tools we observed that the presence of an additional cysteine residue might implicate structural alterations in the mutated protein. We propose haploinsufficiency as the most probable mechanism for the NTRK3 R645C mutation. NTRK3 and RET mutations in this family only appear together in the HSCR patients, suggesting that they per se are necessary but not sufficient to produce the phenotype. In addition, it is quite probable that the contribution of other still unidentified modifier genes, may be responsible for the different phenotypes (length of aganglionosis) in the two affected members.

Neurotrophin-3, but not nerve growth factor, promotes survival of human intestinal mast cells

Neurotrophins are potent regulators of neuronal cell survival and function. Nerve growth factor (NGF) was shown to reduce apoptosis in cord blood-derived mast cells. Here, we examined the effect of the neurotrophins NGF and neurotrophin (NT)-3 on survival and mediator release of human intestinal mast cells. Mast cells isolated from normal intestinal tissue were cultured in the presence of NGF, NT-3, or stem cell factor (SCF) alone or in the presence of SCF together with each neurotrophin. NGF or NT-3 alone did not promote mast cell survival. In contrast, mast cell recovery was increased twofold when mast cells were cultured with NT-3 in addition to SCF for 14 days compared with control. Mast cell recovery was further increased following a combined addition of NT-3, SCF and IL-4. NT-3 mediated mast cell growth was dependent on the primary receptor for NT-3 TrkC. NGF in combination with SCF or with SCF and IL-4 showed no effect on mast cell survival. Histamine release and histamine content per mast cell remained unchanged, whereas leukotriene C4 release decreased if mast cells were cultured with NGF or NT-3 in addition to SCF. In summary, NT-3 affects mature human mast cells by promoting mast cell survival, whereas NGF does not.

Regulation of neurotrophin-3 gene transcription by Sp3 and Sp4 in neurons

Neurotrophin-3 (NT-3), a neurotrophin member, plays crucial roles in neuronal development, function and plasticity. Previous studies have demonstrated that NT-3 gene transcription is driven by alternative promoters A and B, located upstream of exons 1A (EIA) and 1B (EIB), respectively. However, the transcription factors and DNA elements that drive NT-3 gene transcription remain to be identified. Here, we analysed the promoter region of the NT-3 gene and found that an NT-3 transcript containing EIB is predominantly expressed in cortical neurons which preferentially utilize promoter B, and two tandemly repeated GC-boxes, located between -100 and -60 base pairs within promoter B, are required for the transcription. Electrophoretic mobility shift and chromatin immunoprecipitation assays revealed that both specificity protein (Sp)3 and Sp4 were able to bind to the Sp1 binding sequences within the GC boxes. Expression of dominant-negative Sp3 and Sp4 small interfering RNA in cortical neurons reduced the activity of the NT-3 gene promoter. Over-expression of Sp1 family members, especially Sp4, resulted in an increase of the NT-3 gene promoter. These findings indicate that the NT-3 gene is a target gene for Sp4 that is abundantly expressed in the brain.

Postnatal cerebral cortical multipotent progenitors: regulatory mechanisms and potential role in the development of novel neural regenerative strategies

In the developing postnatal cerebral cortex, protracted generation of glia and neurons occurs and precise matching of local cell types is needed for the functional organization of regional microdomains characteristic of complex CNS tissues. Recent studies have suggested that multipotent progenitors play an important role in neural lineage elaboration during neurogenesis and gliogenesis after migration from paramedian generative zones. The presence of a separate reservoir of cerebral cortical multipotent cells under strict local environmental regulation would provide an appropriate mechanism for terminal developmental sculpting and for reconstitution of regional cellular pools after injury. We have isolated distinct pools of EGF- and bFGF-responsive multipotent progenitors from the postnatal mammalian cerebral cortex independent of the subventricular zone. These progenitor populations are under tight environmental regulation by specific hierarchies of cytokine subclasses that program the progressive elaboration of intermediate lineage-restricted progenitors and differentiated type I and II astrocytes, myelinating oligodendrocytes and neuronal subtypes that express specific neuromodulatory proteins. Neural lineage development from these cortical multipotent progenitors is a graded developmental process involving sequential induction of specific cytokine receptors, acquisition of factor responsiveness and complex lineage interdependence. The cortical multipotent progenitor pathways program the elaboration of neural lineage species with distinct cellular response properties when compared with analogous species derived from subventricular zone progenitors, indicating that the cortical multipotent cells contribute to the establishment of lineage diversity within the developing cortical cortex. In addition, the cortical multipotent cells generate dynamic intermediate progenitor pools that utilize temporally-coded environmental cues to alter neural fate decisions. These cumulative observations suggest that postnatal cerebral cortical multipotent cells represent a novel set of progenitor pathways necessary for normal mammalian cortical maturation, and may have important implications for our understanding of a wide variety of neuropathological conditions and for the development of more effective regenerative strategies to combat these pervasive neurological disorders.

Endothelins stimulate the expression of neurotrophin-3 in rat brain and rat cultured astrocytes

Endothelins play a role in the regulation of astrocytic functions in brain pathologies such as hyperplasia and neurotrophic factor production. The present study examined the effects of endothelins on production of neurotrophin-3, a member of the neurotrophin family of neurotrophic factors, in cultured astrocytes and rat brain. Quantitative reverse transcription-PCR analysis of mRNA copy numbers showed that cultured astrocytes expressed comparable numbers of neurotrophin-3 and neurotrophin-4/5 mRNA copies to nerve growth factor and brain-derived neurotrophic factor. Endothelin-1 (100 nM) and Ala1,3,11,15-endothelin-1 (an endothelinB receptor agonist, 100 nM) caused a transient increase in neurotrophin-3 mRNA levels, but not in neurotrophin-4/5 levels, in cultured astrocytes. The increases in mRNA levels were accompanied with that in extracellular release of neurotrophin-3. The effects of endothelin-1 on neurotrophin-3 mRNA levels were reduced by BQ788, an endothelinB receptor antagonist. I.c.v. administration of 500 pmol/day Ala1,3,11,15-endothelin-1 increased mRNA and peptide levels of neurotrophin-3 in rat caudate putamen and cerebrum. On the other hand, neurotrophin-3 production in hippocampus was not affected by Ala1,3,11,15-endothelin-1. Immunohistochemical examination of Ala1,3,11,15-endothelin-1-infused rats showed that neurotrophin-3 was mainly expressed in glial fibrillary acidic protein-positive astrocytes in caudate putamen and cerebrum. endothelin-induced increases in neurotrophin-3 expression in cultured astrocytes were inhibited by chelation of intracellular Ca2+ and PD98095 (an ERK inhibitor). These results suggest that endothelin is an extracellular signal that stimulates astrocytic neurotrophin-3 production in brain pathologies.

Embryology of the neural crest: its inductive role in the neurocutaneous syndromes

Neural crest cells are first recognized at the lateral margin of the neural placode shortly after gastrulation, although they are not committed to their diverse fates until later. After dorsal closure of the neural tube, neural crest cells separate and migrate throughout the embryo to form many structures of ectodermal origin (eg, dorsal root and autonomic ganglia, peripheral nerve sheaths) and mesodermal origin (eg, blood vessels, melanocytes, adipose tissue, membranous bone, connective tissue, most of the ocular globe). Terminal differentiation occurs after migration is complete. Three regions of the neural tube generate neural crest: rhombencephalon, mesencephalon, and prosencephalon, each with a different migratory pattern. The most important genes promoting neural crest differentiation and migration are those with a dorsalizing influence in the vertical axis of the neural tube (eg, PAX3, BMP4, ZIC2), some segmentation genes (eg, WNT1), genes that inhibit neural crest (eg, EGR2), and neural crest-specific differentiating genes (eg, SLUG, SOX10). In the neurocutaneous syndromes, diverse features result from abnormal neural crest differentiation, providing a more encompassing embryologic basis for these disorders than the traditional view that these syndromes are somehow related to skin and brain because both are ectodermal derivatives. Abnormal angiogenesis, areas of abnormal pigmentation that sometimes follow the lines of Blashko, nerve sheath proliferations, disorders of chromaffin tissue, lipomes and benign and malignant tumors are frequent features. Many defective genes in neurocutaneous syndromes have an additional function as tumor suppressors. Interactions between genes associated with these disorders and others essential to neural crest formation, migration, and differentiation, are a likely molecular genetic basis for these diseases. The craniofacial abnormalities associated with many cerebral malformations and cutaneous lesions in some neurocutaneous syndromes emphasize an important inductive role of the neural tube in the development of non-neural tissues, mediated through neural crest.

NT3 inhibits FGF2-induced neural progenitor cell proliferation via the PI3K/GSK3 pathway

Neurotrophin 3 (NT3), a member of the neurotrophin family, antagonizes the proliferative effect of fibroblast growth factor 2 (FGF2) on cortical precursors. However, the mechanism by which NT3 inhibits FGF2-induced neural progenitor (NP) cell proliferation is unclear. Here, using an FGF2-dependent rat neurosphere culture system, we found that NT3 inhibits both FGF2-induced neurosphere growth and bromodeoxyuridine (BrdU) incorporation in a dose-dependent manner. U0126, a mitogen-activated protein kinase kinase 1/2 (MEK1/2) inhibitor, and LY294002, a phosphatidylinositol 3-kinase (PI3K) inhibitor, both inhibited FGF2-induced BrdU incorporation, suggesting that the extracellular signal-regulated kinase1/2 (ERK1/2) and PI3K pathways are required for FGF2-induced NP cell proliferation. NT3 significantly inhibited FGF2-induced phosphorylation of Akt and glycogen synthase kinase 3beta (GSK3beta), a downstream kinase of Akt, whereas phosphorylation of ERK1/2 was unaffected. The inhibitory effect of NT3 on FGF2-induced NP cell proliferation was abolished by LY294002, and treatment with SB216763, a specific GSK3 inhibitor, antagonized the NT3 effect, rescuing both neurosphere growth and BrdU incorporation. Moreover, experiments with anti-NT3 antibody revealed that endogenous NT3 also plays a role in inhibiting FGF2-induced NP cell proliferation, and that anti-NT3 antibody enhanced phospho-Akt and phospho-GSK3beta levels in the presence of FGF2. These findings indicate that FGF2-induced NP cell proliferation is inhibited by NT3 via the PI3K/GSK3 pathway.

Low density of sympathetic nerve fibres and increased density of brain derived neurotrophic factor positive cells in RA synovium

OBJECTIVE

To investigate the correlation between density of nerve fibres and the presence of BDNF(+) cells.

METHODS

Densities of nerve fibres and BDNF(+) cells were detected by quantitative immunohistochemistry in fresh synovial tissue from 52 patients with RA, 59 with OA, and 26 controls (Co). BDNF was also detected by in situ hybridisation.

RESULTS

Sympathetic nerve fibre density was similar in Co and OA but markedly reduced in RA (p = 0.002), whereas density of substance P positive (SP(+)) sensory nerve fibres was lower in OA than in Co and RA (p = 0.002). The ratio of sympathetic/SP(+) sensory nerve fibre density was highest in OA and Co, followed by RA. The correlation between density of sympathetic nerve fibres and SP(+) sensory nerve fibres in OA (R = 0.425, p = 0.001) was strongly positive, had a positive trend in Co (R = 0.243, NS), but was negative in RA (R = -0.292, p = 0.040). In RA and OA tissue the density of BDNF(+) cells was high in sublining areas but markedly lower in Co (p = 0.001). BDNF(+) cell density correlated positively with the ratio of sympathetic/SP(+) sensory nerve fibre density in Co (R = 0.433, p = 0.045) and in OA (R = 0.613, p = 0.015), but not in RA (R = 0.101, NS). Immunohistochemical double staining demonstrated that some macrophages and fibroblasts were positive for BDNF.

CONCLUSIONS

The correlation of density of SP(+) sensory with sympathetic nerve fibres was positive in Co and OA but negative in RA. BDNF may have a stimulatory role on growth of sympathetic in relation to SP(+) sensory nerve fibres in Co and OA, but not in RA.

Neurotrophin-3 signaling maintains maturational homeostasis between neuronal populations in the olfactory epithelium

Neurons within the olfactory system undergo functional turnover throughout life. This process of cell death and compensatory neurogenesis requires feedback between neuronal populations of different developmental ages. We examined the role of NT-3 in this process. NT-3 was localized within both the olfactory bulb and olfactory epithelium. Mice null for NT-3 showed increased numbers of immature neurons, without change in the number of mature neurons. This was due to compensatory alterations in apoptosis of mature and immature neuronal populations. Using a primary olfactory neuronal culture, NT-3 was found to directly activate the PI3K/Akt pathway and indirectly activate the MAPK and PLC pathways. Activated PI3K/Akt promoted mature neuronal survival and induced the release of secondary factors, which activated the MAPK and PLC pathways to reduce neuronal precursor proliferation and inhibit neuronal maturation. These effects of NT-3 serve to maintain homeostasis between neuronal populations within the olfactory epithelium.

Neurotrophin-3 in the development of the enteric nervous system

To date, the only neurotrophin that has been shown to influence the development of the enteric nervous system (ENS) is neurotrophin-3 (NT-3). NT-3 plays an essential role in the development of both the neural-crest-derived peripheral nervous system and the central nervous system (i.e., Chalazonitis, 1996, Mol. Neurobiol., 12: 39-53; Sieber-Blum, 1999, Neurotrophins and the Neural Crest, CRC Press, Boca Raton). This review integrates data obtained from our laboratory and from our collaboration with other investigators that demonstrate a late-acting role for NT-3 in the development of enteric neurons in vitro and in vivo. Studies of the biological actions of NT-3 on enteric neuronal precursors in vitro demonstrate that NT-3 acts directly on the precursor cells and that it also acts in combination with other neurotrophic factors such as glial cell line-derived neurotrophic factor and a ciliary neurotrophic factor-like molecule, to promote the survival and differentiation of enteric neurons and glia. Importantly, bone morphogenetic protein-2 (BMP-2) and BMP-4, members of the transforming growth factor-beta (TGF-beta) superfamily, regulate the onset of action of NT-3 during fetal gut development. Analyzes performed on mice deficient in the genes encoding NT-3 or its transducing tyrosine kinase receptor, TrkC, and conversely on transgenic mice that overexpress NT-3 substantiate a physiological role for NT-3 in the development and maintenance of a subset of enteric neurons. There is loss of neurons in both the myenteric and submucosal plexuses of mice lacking NT-3/TrkC signaling and selective hyperplasia in the myenteric plexus of mice overexpressing NT-3. Analyzes performed on transgenic mice that overexpress noggin, a specific BMP-4 antagonist, show significant decreases in the density of TrkC-expressing neurons but significant increase in overall neuronal density of both plexuses. Conversely, overexpression of BMP-4 is sufficient to produce, an increase in the proportion of TrkC-expressing neurons in both plexuses. Overall, our data point to a regulatory role of BMP-4 in the responses of subsets of myenteric and submucosal neurons to NT-3. NT-3 is required for the differentiation, maintenance and proper physiological function of late-developing enteric neurons that are important for the control of gut peristalsis.

Reduction of NT-3 or TrkC results in fewer putative vagal mechanoreceptors in the mouse esophagus

Intraganglionic laminar endings (IGLEs) represent major vagal afferent structures throughout the gastrointestinal tract. Both morphological and functional data suggested a mechanosensory role. Elucidation of their functional significance in a particular organ would be facilitated by the availability of animal models with significantly altered numbers of IGLEs. The present study was aimed at searching for mouse strains fulfilling this criterion in the esophagus. Anterograde wheat germ agglutinin-horseradish peroxidase tracing (WGA-HRP) from nodose ganglion was used in order to label esophageal IGLEs in mice deficient for neurotrophin-3 (NT-3) or tyrosine kinase C-receptor (TrkC) and in control littermates. This approach was feasible only in heterozygous mutants which are viable. IGLEs were counted in tetramethylbenzidine (TMB) processed wholemounts using a standardised protocol. Quantification of myenteric neurons was done in cuprolinic blue-stained specimens. Nodose neuron counts were performed in cryostat sections stained with cresyl violet. Numbers of IGLEs in the esophagus were significantly reduced in both heterozygous NT-3 (NT-3+/-) and heterozygous TrkC (TrkC+/-) mutants (65% and 40% reduction, respectively). Numbers of nodose neurons were also significantly reduced in NT-3+/- mice (48% reduction), while their reduction in TrkC+/- mutants was insignificant (11% reduction). There was no reduction of myenteric neurons in the esophagus of either mutant strain. The numeric deficiency of IGLEs was unlikely to be secondary to reduction of myenteric neurons. Although only heterozygous mutants could be studied, these results suggest that esophageal IGLEs share neurotrophin dependence on NT-3/TrkC with spinal proprioceptors and some cutaneous mechanosensors. This concurs with their proposed function as vagal mechanosensors crucial for reflex peristalsis.

Changes in neurotrophin levels in umbilical cord blood from infants with different gestational ages and clinical conditions

Apoptotic neuronal loss may be responsible for altered brain development associated with prematurity and perinatal insults. Neurotrophins play crucial roles in protecting neurons from entering or progressing along an apoptotic pathway. The present study examined levels of neurotrophins in human umbilical cord blood from infants at different gestational ages and clinical conditions. We collected 60 samples of cord blood and categorized them accordingly into three gestational age groups: group A (24-28 wk), group B (29-35 wk), and group C (>/=36 wk). Neurotrophin levels were determined by using brain-derived neurotrophic factor (BDNF) and neurotrophin 3 (NT3) ELISA. Clinical data were obtained by medical chart analysis. The BDNF levels were 884 +/- 386, 1421 +/- 616, and 2190 +/- 356 pg/mL in group A, group B, and group C, respectively. Significant differences were found between groups A and B (p = 0.038), groups A and C (p = 0.0001), and groups B and C (p = 0.001). Infants with severe intraventricular hemorrhage had significantly lower cord blood BDNF levels (925 +/- 513 pg/mL) compared with their normal counterparts (1650 +/- 674 pg/mL; p = 0.021). NT3 levels did not show significant change either across gestational ages or with the presence of intraventricular hemorrhage. Cord blood levels of BDNF may reflect the degree of neural maturity in premature infants. Interestingly, when a complete course of antenatal steroids was given, BDNF and NT3 cord blood levels were higher than when no steroid was given. Increased neurotrophins levels may also mediate improved neurodevelopmental outcome in infants who received antenatal steroids.

Expression of heregulin and ErbB/Her receptors in adult chinchilla cochlear and vestibular sensory epithelium

Immunolabeling of heregulin, a growth factor that enhances cell proliferation in damaged utricles, and one of its binding receptors, ErbB-2, has been briefly described in the P3 rat cochlea and utricle [Zheng et al. (1999) J. Neurocytol. 28, 901-912]. However, little is known about the distribution of heregulin and its three binding receptors in adult animals. Here we describe the immunolabeling patterns for heregulin, ErbB-2, ErbB-3 and ErB-4 in the cochlea, spiral ganglion, utricle and saccule of the adult chinchilla using confocal microscopy. Heregulin immunolabeling was intense along the apical pole of Deiters cells and Hensen cells and along the membrane of supporting cells of the utricle and saccule; light immunolabeling was present in the outer layer of the spiral prominence and cytoplasm of spiral ganglion neurons. In the cochlea, intense to moderate ErbB-2 immunolabeling was evident in the cytoplasm of pillar cells, outer hair cells (OHCs), border cells, stria vascularis and spiral ligament; moderate ErbB-2 immunolabeling was present in the cytoplasm of the hair cell and supporting cell layers of the utricle and saccule. In the cochlea, light ErbB-3 immunolabeling was present in the inner hair cells, OHCs, marginal and intermediate cell layers of the stria vascularis and spiral ganglion neurons; moderate ErbB-3 immunolabeling was present in the cytoplasm of hair cells and supporting cells of the utricle and saccule. In the cochlea, utricle and saccule, ErbB-4 immunolabeling was intense in the nuclei and light to moderate in the cytoplasm and membrane of sensory cells and supporting cells. These results suggest that heregulin acting through ErbB receptors and various receptor complexes may play an important role in cell proliferation and survival in the cochlea and vestibular system.

Development of brain-derived neurotrophic factor and neurotrophin-3 immunoreactivity in the lower auditory brainstem of the postnatal gerbil

The localization of brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) in the gerbil auditory brainstem was studied during normal postnatal development. The principal objective of this paper was to compare the developmental distribution of BDNF and NT-3 proteins to the known developmental distribution of their cognate, high-affinity tyrosine kinase receptors. BDNF and NT-3 proteins were localized using standard immunohistochemistry. No specific immunoreactivity for BDNF or NT-3 was detected on the day of birth (P0) in any auditory structure, although fibers comprising the spinal tract of the Vth cranial nerve were well labelled with antibodies against BDNF. Diffuse immunoreactivity for both BDNF and NT-3 was first detected at P3 in the cochlear nucleus and in several second order auditory nuclei in the superior olivary complex. This diffuse immunoreactivity became clustered and restricted to neuronal cell bodies by P10. Immunoreactivity for both BDNF and NT-3 transiently disappeared in the lateral and medial superior olivary nuclei at P10. However, neurons in the medial nucleus of the trapezoid body remained immunopositive for both BDNF and NT-3. Fibers in the trapezoid body were labelled with BDNF immunoreactivity by P12. Between P12 and P15, the distribution of BDNF and NT-3 immunoreactivity in the cochlear nucleus and superior olivary complex became comparable to adult (P140) immunolabel. These results show that the normal developmental distribution of the neurotrophins BDNF and NT-3 in the lower auditory brainstem occurs during the first two postnatal weeks in parallel with the developmental expression of their cognate receptors, trkB and trkC.

Neurotrophin-3 is required for the survival-differentiation of subsets of developing enteric neurons

Neurotrophin-3 (NT-3) promotes enteric neuronal development in vitro; nevertheless, an enteric nervous system (ENS) is present in mice lacking NT-3 or TrkC. We thus analyzed the physiological significance of NT-3 in ENS development. Subsets of neurons developing in vitro in response to NT-3 became NT-3 dependent; NT-3 withdrawal led to apoptosis, selectively in TrkC-expressing neurons. Antibodies to NT-3, which blocked the developmental response of enteric crest-derived cells to exogenous NT-3, did not inhibit neuronal development in cultures of isolated crest-derived cells but did so in mixed cultures of crest- and non-neural crest-derived cells; therefore, the endogenous NT-3 that supports enteric neuronal development is probably obtained from noncrest-derived mesenchymal cells. In mature animals, retrograde transport of (125)I-NT-3, injected into the mucosa, labeled neurons in ganglia of the submucosal but not myenteric plexus; injections of (125)I-NT-3 into myenteric ganglia, the tertiary plexus, and muscle, labeled neurons in underlying submucosal and distant myenteric ganglia. The labeling pattern suggests that NT-3-dependent submucosal neurons may be intrinsic primary afferent and/or secretomotor, whereas NT-3-dependent myenteric neurons innervate other myenteric ganglia and/or the longitudinal muscle. Myenteric neurons were increased in number and size in transgenic mice that overexpress NT-3 directed to myenteric ganglia by the promoter for dopamine beta-hydroxylase. The numbers of neurons were regionally reduced in both plexuses in mice lacking NT-3 or TrkC. A neuropoietic cytokine (CNTF) interacted with NT-3 in vitro, and if applied sequentially, compensated for NT-3 withdrawal. These observations indicate that NT-3 is required for the normal development of the ENS.

Neurotrophin-3 sorts to the constitutive secretory pathway of hippocampal neurons and is diverted to the regulated secretory pathway by coexpression with brain-derived neurotrophic factor

Hippocampal neurons release nerve growth factor (NGF) through the constitutive secretory pathway, thus allowing the protein to be continuously available for promoting nerve cell survival. In contrast, hippocampal neurons use the regulated secretory pathway to process brain-derived neurotrophic factor (BDNF), which alters synaptic activity when released acutely from dense-core vesicles. Thus, understanding how neurons sort and deliver neurotrophins may provide clues to their functions in brain. In this study, we monitored the processing and delivery of neurotrophin-3 (NT-3). Pulse-chase studies, immunocytochemistry, and secretagogue-induced release experiments were performed on cultured hippocampal neurons and AtT-20 cells infected with vaccinia viruses encoding the NT-3 precursor (pro-NT-3). Results show that most newly synthesized NT-3 is released through the constitutive secretory pathway as a result of furin-mediated endoproteolytic cleavage of pro-NT-3 in the trans-Golgi network. Pro-NT-3 can also be diverted into the regulated secretory pathway when cells are treated with alpha1-PDX, a selective inhibitor of furin-like enzymes, or when pro-NT-3 expression is increased by transient transfection methods. In cells coinfected with viruses coding for pro-NT-3 and pro-BDNF, NT-3 is sorted into the regulated pathway, stored in secretory granules, and released in response to extracellular cues together with BDNF, apparently as a result of heterodimerization, as suggested by coimmunoprecipitation data. Taken together, these data show that sorting of the NT-3 precursor can occur in both the constitutive and regulated secretory pathways, which is consistent with NT-3 having both survival-promoting and synapse-altering functions.

Neurotrophin signaling via Trks and p75

This review focuses on recent advances in our understanding of receptor-mediated signaling by the neurotrophins NGF, BDNF, NT3, and NT4/5. Two distinct receptor types have been distinguished, Trks and p75. The Trks are receptor tyrosine kinases that utilize a complex set of substrates and adapter proteins to activate defined secondary signaling cascades required for neurotrophin-promoted neuronal differentiation, plasticity, and survival. A specialized aspect of Trk/neurotrophin action in neurons is the requirement for retrograde signaling from the distal periphery to the cell body. p75 is a universal receptor for neurotrophins that is a member of the TNF receptor/Fas/CD40 superfamily. p75 appears to modify Trk signaling when the two receptor types are coexpressed. When expressed in the absence of Trks, p75 mediates responses to neurotrophins including promotion of apoptotic death. The mechanisms of p75 receptor signaling remain to be fully understood.

Nerve growth factor (NGF) supports tooth morphogenesis in mouse first branchial arch explants

Posterior midbrain and anterior hindbrain neuroectoderm trans-differentiate into cranial neural crest cells (CNCC), emigrate from the neural folds, and become crest-derived ectomesenchyme within the mandibular and maxillary processes. To investigate the growth factor requirement specific for the initiation of tooth morphogenesis, we designed studies to test whether nerve growth factor (NGF) can support odontogenesis in a first branchial arch (FBA) explant culture system. FBA explants containing neural-fold tissues before CNCC emigration and the anlagen of the FBA were microdissected from embryonic day 8 (E8) mouse embryos, and cultured for 8 days in medium supplemented with 10% fetal calf serum only, or serum-containing medium further supplemented with either NGF or epidermal growth factor (EGF) at three different concentrations: 50, 100, or 200 ng/ml. Morphological, morphometric, and total protein analyses indicated that growth and development in all groups were comparable. Meckel's cartilage and tongue formation were also observed in all groups. However, odontogenesis was only detected in explants cultured in the presence of exogenous NGF. NGF-supplemented cultures were permissive for bud stage (50 ng/ml) as well as cap stage of tooth morphogenesis (100 and 200 ng/ml). Morphometric analyses of the volume of tooth organs showed a significant dose-dependent increase in tooth volume as the concentration of NGF increased. Whole-mount in situ hybridization and semiquantitative reverse transcription-polymerase chain reaction for Pax9, a molecular marker of dental mesenchyme, further supported and confirmed the morphological data of the specificity and dose dependency of NGF on odontogenesis. We conclude that (1) E8 FBA explants contain premigratory CNCC that are capable of emigration, proliferation, and differentiation in vitro; (2) serum-supplemented medium is permissive for CNCC differentiation into tongue myoblasts and chondrocytes in FBA explants; and (3) NGF controls CNCC cell fate specification and differentiation into tooth organs.

Repetitive firing deficits and reduced sodium current density in retinal ganglion cells developing in the absence of BDNF

Previous work by Cellerino et al. has shown that chronic absence of brain-derived neurotrophic factor (BDNF) resulted in hypomyelination of the optic nerve. Since myelination is influenced by neuronal activity, it is possible that a deficiency in BDNF during early development could alter the firing properties of retinal neurons. To test this hypothesis, patch-clamp recordings were performed in retinal whole mounts from BDNF-deficient (bdnf-/-), heterozygote (bdnf+/-) or wild-type control mice (bdnf+/+). Ganglion cell layer neurons (RGNs) were tested at different age [postnatal day (P)1-11] for their ability to encode graded depolarization with variable action potential frequency. At all developmental ages examined, RGNs exhibiting frequency coding were less frequently encountered in bdnf-/- than in bdnf+/+ mice. At P1, none of the RGNs from bdnf-/- mice displayed repetitive firing compared to 50% in bdnf+/+ mice, and by P7-11, only 50% of bdnf-/- RGNs exhibited repetitive firing compared to 100% in bdnf+/+ mice. Moreover, in bdnf-/- RGNs repetitive discharge was characterized by a reduced frequency increment per current change. Acquisition of repetitive firing was paralleled by a decrease in input resistance and a steep increase of sodium current density. In bdnf-/- mice, the onset of this increase occurred at later stages of development than in wild-type controls (bdnf-/-: P6-11; bdnf+/+: P2-6). The discharge pattern of P7-11 bdnf-/- RGNs resembled that of RGNs in neonatal wild-type mice and was mimicked by acute application of a Ca(2+) channel blocker. We conclude that BDNF plays an important role in the ontogeny of repetitive firing of RGNs.

Apoptosis in developing retinal tissue

The mechanisms of apoptosis are strongly dependent on cell-cell interactions typical of organized tissues. Experimental studies of apoptosis using a histotypical preparation of retinal explants are reported in the present article. We found that various characteristics of apoptosis are selectively associated with retinal cell death depending on cell type, stage of maturation, and means of induction of apoptosis. Among these were: (1) the requirements of protein synthesis; (2) the role of cAMP; (3) the expression of certain apoptosis-associated proteins; and (4) the sensitivity to excitotoxicity, modulation of protein phosphatases and calcium mobilization. Dividing cells undergo apoptosis in response to several inducers in specific phases of the cell cycle, and in distinct regions within their pathway of interkinetic nuclear migration. Recent post-mitotic cells are selectively sensitive to apoptosis induced by blockade of protein synthesis, while both proliferating and differentiated cells are more resistant. We also studied the association of several proteins, some of which play critical roles in the cell cycle, with both differentiation and apoptosis in the retinal tissue. Detection of cell cycle markers did not support the hypothesis that retinal cells re-enter the cell cycle on their pathway to apoptosis, although some proteins associated with cell proliferation re-appeared in degenerating cells. The transcription factors c-Jun, c-Fos and c-Myc were found associated with apoptosis in retinal cells, but their sub-cellular location in apoptotic bodies is not consistent with their canonical functions in the control of gene expression. The bifunctional redox factor/AP endonuclease Ref-1 and the transcription factor Max are associated with progressive cell differentiation, and both are down-regulated during cell death in the retina. The data suggest that Ref-1 and Max may normally function as negative modulators of retinal apoptosis. The results indicate that nuclear exclusion of transcription factors and other important control proteins is a hallmark of retinal apoptosis. Histotypical explants may be a choice preparation for the experimental analysis of the mechanisms of apoptosis, in the context both of cell-cell interactions and of the dynamic behavior of developing cells within the organized retinal tissue.

A neuronal subpopulation in the mammalian enteric nervous system expresses TrkA and TrkC neurotrophin receptor-like proteins

Increasing evidence suggests that, in addition to peripheral sensory and sympathetic neurons, the enteric neurons are also under the control of neurotrophins. Recently, neurotrophin receptors have been detected in the developing and adult mammalian enteric nervous system (ENS). Nevertheless, it remains to be established whether neurotrophin receptors are expressed in all enteric neurons and/or in glial cells and whether expression is a common feature in the enteric nervous system of all mammals or if interspecific differences exist. Rabbit polyclonal antibodies against Trk proteins (regarded as essential constituents of the high-affinity signal-transducing neurotrophin receptors) and p75 protein (considered as a low-affinity pan-neurotrophin receptor) were used to investigate the cell localization of these proteins in the ENS of adult man, horse, cow, sheep, pig, rabbit, and rat. Moreover, the percentage of neurons displaying immunoreactivity (IR) for each neurotrophin receptor protein was determined. TrkA-like IR and TrkC-like IR were observed in a neuronal subpopulation in both the myenteric and submucous plexuses, from esophagus to rectum in humans, and in the jejunum-ileum of the other species. Many neurons, and apparently all glial cells, in the human and rat enteric nervous system also displayed p75 IR. TrkB-like IR was found restricted to the glial cells of all species studied, with the exception of humans, in whom IR was mainly in glial cells and a small percentage of enteric neurons (about 5%). These findings indicate that the ENS of adult mammals express neuronal TrkA and TrkC, glial TrkB, and neuronal-glial p75, this pattern of distribution being similar in all examined species. Thus, influence of specific neurotrophins on their cognate receptors may be considered in the physiology and/or pathology of the adult ENS.

Neurotrophic factors are required by mature sympathetic neurons for survival, transmission and connectivity

1. Two neuronal growth factors, nerve growth factor (NGF) and neurotrophin 3 (NT3), have been studied for their action on the developing and mature sympathetic nervous system. 2. Antibodies to each factor have proved useful as reagents for the detection and quantification of NGF and NT3. They have also proved valuable in uncovering the functional roles of each factor by their ability to neutralize the endogenous molecules. 3. Nerve growth factor acts on postnatal neurons to control neurotransmission, connectivity and survival. Like NGF, NT3 is synthesized by effector tissues and is retrogradely transported by post-ganglionic neurons to prevent cell death. However, the two factors have been shown to have quite distinct functions in mature neurons, indicating the existence of different signalling pathways. This differential action extends to secondary influences on satellite glia. 4. Pathological consequences result from excessive growth factor synthesis leading, in the hypertensive rat, to hyperinnervation and elevated blood pressure. Satellite glial cell synthesis of the factors and their receptors following peripheral nerve damage appears to be responsible for the establishment of inappropriate neuronal connections between sympathetic nerve terminals and sensory somata. 5. It is concluded that these potent factors control, by both coincident and independent mechanisms, sympathetic neuronal function throughout the life of the animal.

Encapsulated sensory receptors within intraorbital skeletal muscles of a camel

BACKGROUND

The occurrence of encapsulated sensory receptors in extrinsic ocular muscles differs among and within orders of mammals. Beyond indications that neuromuscular and neurotendinous spindles are present in extraocular muscles of the family Camilidae, little is known of their distributional characteristics. In fact there appear to be no distribution maps for any animal that show both major types of encapsulated muscle receptor in a full set of intraorbital skeletal muscles.

METHOD

Serial histological sections of all skeletal muscles from one orbit of an adult, one-humped camel (Camelus dromedarius) were examined for encapsulated receptors.

RESULTS

Encapsulated receptors were apparent in all the intraorbital skeletal muscles. Muscle spindles outnumbered tendon organs in the fleshy part of each muscle. For all muscles, spindles were most abundant in the half of the muscle near the origin; levator palpebrae superioris had a more even distribution of spindles along its length than did extraocular muscles. These longitudinal patterns of distribution for muscle spindles related in a general way to the nerve entry zone. Tendon organs occurred anywhere along a muscle's length, but they tended to be more frequent on either side of the major concentration of muscle spindles. Both types of encapsulated receptors were generally located nearer the perimeter than the center of cross sections through muscles.

CONCLUSIONS

Encapsulated receptors of classic appearance are plentiful and have distinctive configurations within intraorbital skeletal muscles of the adult dromedary. When analyzed in conjunction with other studies, the present data give rise to testable explanations for the variability among genera in the number of encapsulated receptors in extraocular muscles.