Expression of nerve growth factor receptor immunoreactivity in the rat choroid plexus.

The presence and localization of nerve growth factor receptors (NGFr) in the choroid plexus of the adult rat has been investigated immunohistochemically using an anti-rat NGFr monoclonal antibody (192-IgG). A moderate to strong immunoreaction was observed in the epithelial cells of the choroid plexus, whereas the choroidal blood vessels and connective tissue remained unlabelled. Moreover, no sex-differences were encountered in the NGFr immunoreaction intensity and Bouin fixative was more effective than 10% formaldehyde evidenciating the NGFr immunostain. Occasionally, ependymal cells displaying NGFr immunoreactivity were observed. Present data demonstrate that the choroid plexus of the rat contain NGFr, probably low-affinity NGFr, and suggest an involvement of NGF in the regulation of cerebrospinal fluid secretion, but the importance of these findings, if any, must be investigated in future studies.     

Lateral diffusion of nerve growth factor receptor: modulation by ligand-binding and cell-associated factors.

We compared the properties in human melanoma cell line A875 and rat pheochromocytoma cell line PC12 of nerve growth factor receptor (NGFr). We also analyzed NGFr and a truncated NGFR lacking the cytoplasmic domain, which were transiently expressed in COS cells. The full-length NGFR expressed in COS cells bound nerve growth factor (NGF) with positive cooperativity, but A875 NGFr and truncated NGFr in COS cells did not display positive cooperativity. The anti-human NGFr monoclonal antibody NGFR5 was characterized and found not to compete with NGF for binding to NGFr. Fabs were prepared from NGFR5 and 192, an anti-rat NGFR monoclonal antibody that was previously shown not to compete with NGF for binding. Fluorescein-labeled Fabs were used to measure the distribution and lateral diffusion of the NGFr. NGFr expressed on COS and A875 cells are diffusely distributed, but NGFr on the surface of PC12 cells appeared, for some cells, to be patched. In A875 cells, 51% of the NGFr was free to diffuse with diffusion coefficient (D) approximately 7 X 10(-10) cm2/s. In COS cells, 43% diffused with D approximately 5 X 10(-10) cm2/s. There was no significant difference in diffusibility between the full-length NGFr and the truncated NGFr. We compared NGFr diffusion on PC12 cells in suspension or adherent to collagen-coated coverslips. For suspension cells, we obtained 32% recovery with D approximately 2.5 X 10(-9) cm2/s. On adherent cells, we obtained 17% recovery with 6 X 10(-9) cm2/s. Binding of NGF enhanced lateral diffusion of NGFr in A875 cells and in PC12 cells in suspension but did not alter lateral diffusion of NGFr in COS cells or in adherent PC12 cells. NGF had no effect on the diffusing fraction or the distribution of NGFR for any cell line.     

Nerve growth factor receptor (NGFR) immunoreactivity in skeletal muscles of the rat.

The peroxidase-antiperoxidase (PAP) method, and a specific monoclonal antibody (192-IgG) were used to determine the localization of nerve growth factor receptor (NGFr) in the skeletal muscles of the adult rats. The rectus femoris and the gastrocnemius (medialis and lateralis) muscles were analyzed. Occurrence of NGFr immunoreactivity was observed in: 1) a subpopulation of myelinated nerve fibers within muscle nerve trunks; 2) the vascular adventitia and nerve-like profiles around the blood vessels; 3) the outer capsule and the surface of the intrafusal muscle fibers of muscle spindles. Conversely, images, suggesting the presence of NGFr on muscle fibers or in motor end-plates, were not found. Our results suggest the presence of NGF-binding sites in sensory and sympathetic nerve fibers, and/or their target tissues localized on the skeletal muscles of the rat, whereas the motor nerve fibers lack of NGFr. The dependence of sympathetic neurons, proprioceptive primary sensory neurons, and motoneurons innervating the mammalian muscles upon NGF or other neurotrophic factors is discussed.     

beta-Nerve growth factor (beta NGF) receptors on glial cells. Cell-cell interaction between neurones and Schwann cells in cultures of chick sensory ganglia.

Receptors for beta-nerve growth factor (beta NGF), so far regarded as specific cell surface markers of certain peripheral neurones, were found to be expressed on cultured non-neuronal cells of chick embryo dorsal root ganglia (drg) (Kd beta NGF = 2 X 10(-9) M). Autoradiography revealed that binding of [125I] beta NGF was restricted to a subpopulation of the non-neuronal drg cells. Cultured embryonic skin fibroblasts, liver cells, gut cells, muscle fibroblasts, myoblasts, and myotubes, as well as macrophages and the cell lines 3T3, 3T3SV40, BHK, BHK Py, PCC3 and ND1, did not express receptors for beta NGF. Non-neuronal drg cells obtained by a procedure designed for the preparation of pure Schwann cells, as well as RN6 Schwannoma cells, were beta NGF receptor positive. The beta NGF receptor-positive non-neuronal drg cells displayed behaviour typical of Schwann cells in their interaction with drg neurones in single cell, as well as explant cultures. Three stages of neurone-Schwann cell interaction were discernible: (1) association--neurites preferentially grew over beta NGF receptor-positive non-neuronal cells; (2) cell division/alignment--beta NGF receptor-positive non-neuronal cells were induced to proliferate and aligned and elongated along neurites; (3) ensheathment--the outline of beta NGF receptor-positive non-neuronal cells and neurites merged. In drg cell cultures prepared from embryonic stages E6-E10, 25-40% of the non-neuronal cells were beta NGF receptor-positive. Later in development, from E12 onward, less than or equal to 1% of the cultured non-neuronal cells expressed beta NGF receptors.     

High-affinity NGF receptor in the rat spinal cord during acute and chronic phases of experimental autoimmune encephalomyelitis: a possible functional significance

The biological effects of Nerve Growth Factor (NGF) are primarily mediated via its high affinity receptor-TrkA. In the present study, we examined the effect of experimental autoimmune encephalomyelitis (EAE) upon the expression of TrkA in neuronal and non-neuronal cells of the spinal cord of Lewis rats during the acute (14 days postimmunization) and chronic (12 months postimmunization) phases of the disease. In the normal spinal cord, both of mature and aged rats, we found TrkA immunoreaction (TrkA-IR) in the motoneurons of the Rexed lamina IX and in both oligo- and astroglia cells. In the acute phase of the disease, we found a reduction of TrkA immunoreactivity in motoneurons and its up-regulation in oligodendroglia, mainly in the white matter. We also confirmed our previous findings concerning the up-regulation of TrkA-IR in astroglia. Both neuronal and non-neuronal changes of TrkA immunoreactivity had a transient character: they were not seen in the chronic phase of the disease. Our results suggest that both neuronal and glial TrkA expression changes depend on inflammation. Moreover, our data indicate that, during the acute phase of EAE, the glial cells become more receptive to NGF, pointing to glia as an important target for pharmacological manipulations, particularly for exogenously administered NGF.     

NGF induction of NGF receptor gene expression and cholinergic neuronal hypertrophy within the basal forebrain of the adult rat

Chronic infusion of nerve growth factor (NGF) into the forebrain of the adult rat produced increases in NGF receptor (NGF-R) mRNA hybridization, NGF-R immunoreactivity, choline acetyltransferase (ChAT) mRNA hybridization, and neuronal hypertrophy, when compared with vehicle infusion or noninfused rat brain. In situ hybridization showed NGF induction of NGF-R gene expression, documented by increases in the number of NGF-R mRNA-positive cells within the medial septum, diagonal band, and nucleus basalis magnocellularis. NGF also produced hypertrophy of ChAT mRNA-positive neurons. These results suggest that NGF produces cholinergic neuronal hypertrophy through induction of NGF-R gene expression within the basal forebrain.     

<Emphasis Type="Italic">In vitro</Emphasis> human ependymoblastoma cells differentiate after exposure to nerve growth factor

Nerve Growth Factor (NGF) has a prominent action on immature crest-derived nerve cells and on differentiation and survival of neurons in central and peripheral nervous system. NGF is produced by a variety of neuronal and non-neuronal cells, including neoplastic cells. Its role in tumor cells is largely unknown and controversial. The aim of the present study was to investigate the effect of NGF on brain neoplastic cells using primary cultures from ependymoblastoma (EP) tissue. Human EP tissues were cultured to obtain in vitro cells and their structural, biochemical, and molecular responses to NGF were investigated. The results showed that under basal conditions, human EP cells are characterized by low presence of high-affinity NGF-receptors. Time-course and dose-response studies revealed that EP cells undergo differentiation after exposure to NGF. Our findings showed that in human EP cells, NGF exerts a marked action on differentiation rather than proliferation.     

Synthesis and secretion of nerve growth factor by mouse astroglial cells in culture

Astroglial cells cultured from the mouse brain have been found to synthesize and secrete a material(s) with nerve growth factor-like immunoreactivity (NGF-LI) into their culture medium. A material(s) with NGF-LI showed identical properties to those of beta NGF purified from the mouse submaxillary gland in immunoreactivity, molecular weight, isoelectric point, and neurite outgrowth stimulatory activity. These results indicate that astroglial cells cultured from mouse brain are able to synthesize and secrete beta NGF in culture.     

Laminin and fibronectin in normal and malignant neuroectodermal cells

Laminin and fibronectin, the major noncollagenous matrix glycoproteins, were studied in connection with normal brain cells and neuroectodermal cell lines. Laminin, a Mr 900,000 dalton matrix glycoprotein and an essential component of basement membranes, was found to be produced by cultured cells of several malignant cell lines of neuroectodermal origin. In cultured mouse C1300 neuroblastoma line cells laminin was localized, by immunoelectron microscopy, to the rough endoplasmic reticulum and, to sites of cell-to-cell and cell-to-substratum adhesion. Further experiments on the intracellular transport of this glycoprotein in C1300 cells confirmed that laminin is, at least partially, transported through the Golgi pathway. These results favor a role for laminin in attachment and cellular interactions of malignant neuronal cells. Laminin was also found in connection with neurons and glial cells from mammalian brain. In primary cultures from developing rat brain the vast majority of non-neuronal cells (80%) expressed immunoreactivity for the glial fibrillary acidic protein, a cytoskeletal protein specific for astrocytes. During the first week in culture all the glial fibrillary acidic protein-positive cells, with the exception of mature-looking star-shaped astrocytes, exhibited immunoreactivity for laminin. The intracellular laminin disappeared gradually after a few weeks in culture, but an extensive laminin matrix persisted and seemed to be localized on the upper surface of the non-neuronal cells. The neurofilament-positive neurons were negative for laminin. Pretreatment of the cultures with the ionophore monensin, caused accumulation of laminin-immunoreactivity within the Golgi region, which confirmed that laminin is, indeed, produced by cultured astrocytes and secreted through the Golgi complex. No fibronectin immunoreactivity was found in the majority of glial cells. However, under culture conditions where fibronectin was omitted from the culture medium there was, in the primary cultures, a minor population of glial fibrillary acidic protein-positive flat glial cells that exhibited intracytoplasmic immunofluorescence for fibronectin. In the presence of fibronectin in culture medium no fibronectin-positive glial cells could be detected. It thus appears that laminin, and to a minor extent fibronectin, are proteins that normal glial cells are capable of producing under specific conditions. Laminin and fibronectin were localized in adult rat brain in capillary and meningeal structures.(ABSTRACT TRUNCATED AT 400 WORDS)     

Effects of NGF and glucocorticoid on NGF receptor immunolabeling of cultured rhesus adrenal chromaffin cells

Nerve growth factor (NGF) promotes the outgrowth of neurites from cultured adrenal chromaffin cells from adult rhesus monkeys, but little is known about the distribution, at the cellular level, of the NGF receptors (NGFR) responsible for this response. We examined changes in immunostaining for NGFR in chromaffin cells cultured for 4 weeks in the presence or absence of NGF, with or without dexamethasone (DEX), which inhibits neuritic outgrowth from these cells. Purified cultures of adrenal chromaffin cells from adult rhesus monkeys were grown for up to 9 weeks in NGF, DEX, NGF plus DEX, or control medium. Cells were immunolabeled with three different monoclonal antibodies directed against different epitopes of the human NGFR. Although the distribution of immunolabeling was not uniform from cell to cell, the overall intensity of NGFR immunolabeling varied dramatically between different growth conditions. Of greatest interest, DEX-treated cells stained the most intensely at all time points, while the intensity of immunolabeling was much fainter in NGF-treated cells and decreased with time in culture. In contrast to the intensity of labeling, the proportion of chromaffin cells with immunoreactivity increased with time in all treatment groups. Thus, GCs do not appear to antagonize the effects of NGF merely by decreasing the total number of immunoreactive NGFR on the surface of these cells.     

Expression of the high-affinity choline transporter CHT1 in rat and human arteries.

The arterial vascular wall contains a non-neuronal intrinsic cholinergic system. The rate-limiting step in acetylcholine (ACh) synthesis is choline uptake. A high-affinity choline transporter, CHT1, has recently been cloned from neural tissue and has been identified in epithelial cholinergic cells. Here we investigated its presence in rat and human arteries and in primary cell cultures of rat vascular cells (endothelial cells, smooth muscle cells, fibroblasts). CHT1-mRNA was detected in the arterial wall and in all isolated cell types by RT-PCR using five different CHT1-specific primer pairs. Antisera raised against amino acids 29-40 of the rat sequence labeled a single band (50 kD) in Western blots of rat aorta, and an additional higher molecular weight band appeared in the hippocampus. Immunohistochemistry demonstrated CHT1 immunoreactivity in endothelial and smooth muscle cells in situ and in all cultured cell types. A high-affinity [3H]-choline uptake mechanism sharing characteristics with neuronal high-affinity choline uptake, i.e., sensitivity to hemicholinium-3 and dependence on sodium, was demonstrated in rat thoracic aortic segments by microimager autoradiography. Expression of the high-affinity choline transporter CHT1 is a novel component of the intrinsic non-neuronal cholinergic system of the arterial vascular wall, predominantly in the intimal and medial layers.     

Immunohistochemical distribution of NGF, BDNF, NT-3, and NT-4 in adult rhesus monkey brains.

Immunohistochemical distribution and cellular localization of neurotrophins was investigated in adult monkey brains using antisera against nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), and neurotrophin-4 (NT-4). Western blot analysis showed that each antibody specifically recognized appropriate bands of approximately 14.7 kDa, 14.2 kDa, 13.6 kDa, and 14.5 kDa, for NGF, BDNF, NT-3, and NT-4, respectively. These positions coincided with the molecular masses of the neurotrophins studied. Furthermore, sections exposed to primary antiserum preadsorbed with full-length NGF, BDNF, NT-3, and NT-4 exhibited no detectable immunoreactivity, demonstrating specificities of the antibodies against the tissues prepared from rhesus monkeys. The study provided a systematic report on the distribution of NGF, BDNF, NT-3, and NT-4 in the monkey brain. Varying intensity of immunostaining was observed in the somata and processes of a wide variety of neurons and glial cells in the cerebrum, cerebellum, hippocampus, and other regions of the brain. Neurons in some regions such as the cerebral cortex and the hippocampus, which stained for neurotrophins, also expressed neurotrophic factor mRNA. In some other brain regions, there was discrepancy of protein distribution and mRNA expression reported previously, indicating a retrograde or anterograde action mode of neurotrophins. Results of this study provide a morphological basis for the elucidation of the roles of NGF, BDNF, NT-3, and NT-4 in adult primate brains.     

Immunohistochemical localization of nerve growth factor (NGF) and NGF receptor (NGF-R) in the developing first molar tooth of the rat

Nerve growth factor (NGF) is a well established target-derived trophic factor supporting sympathetic and sensory innervation in the peripheral tissues as well as cholinergic innervation in the brain. Despite its name, NGF may have broader biological functions early in development in a wide range of non-neuronal differentiating cells. The many effects of NGF are directly dependent on initial binding of NGF to specific plasma membrane receptors on target cells. Here we use immunohistochemical methods to show that NGF and its receptor (NGF-R) are localized in a variety of embryonic epithelial and mesenchymal cells in the rat developing molar tooth. Dental cells known to play important roles in morphogenesis and inductive tissue interactions show NGF-like reactivity. Thus, labelling is seen in epithelial preameloblasts and mesenchymal odontoblasts. We also show a transient expression of NGF-R in restricted parts of the dental epithelium (inner dental epithelium) and dental mesenchyme differentiating cells (post-mitotic, polarizing odontoblasts). The expression patterns of NGF are different to those of NGF-R during embryogenesis and this is illustrated in detail in the developing tooth. The histochemical findings reported here support the notion that NGF may have multiple roles during morphogenetic and cytodifferentiation events in the tooth.     

Testicular Expression of NGF,TrkA and p75 During Seasonal Spermatogenesis of the Wild Ground Squirrel (Citellus Dauricus Brandt)

The nerve growth factor (NGF) not only has an essential effect on the nervous system, but also plays an important role in a variety of non-neuronal systems, such as the reproductive system. The aim of this study was to compare the quality and quantity in expression of NGF and its receptors (TrkA and p75) in testes of the wild ground squirrel during the breeding and nonbreeding seasons. Immunolocalization for NGF was detected mainly in Leydig cells and Sertoli cells in testes of the breeding and nonbreeding seasons. The immunoreactivity of TrkA was highest in the elongated spermatids, whereas p75 in spermatogonia and spermatocytes in testes of the breeding season. In the nonbreeding season testes, TrkA showed positive immunostainings in Leydig cells, spermatogonia and primary spermatocytes, while p75 showed positive signals in spermatogonia and primary spermatocytes. Consistent with the immunohistochemical results, the mean mRNA and protein level of NGF and TrkA were higher in the testes of the breeding season than in non-breeding season, and then decreased to a relatively low level in the nonbreeding season. In addition, the concentration of plasma gonadotropins and testosterone were assayed by radioimmunoassay (RIA), and the results showed a significant difference between the breeding and nonbreeding seasons with higher concentrations in breeding season. In conclusion, these results of this study provide the first evidence on the potential involvement of NGF and its receptor, TrkA and p75 in the seasonal spermatogenesis and testicular function change of the wild ground squirrel.Key words: NGF, p75, seasonal spermatogenesis, TrkA, wild ground squirrel     

Glycogen phosphorylase activity and immunoreactivity during pre- and postnatal development of rat brain

Catalytic activity and immunoreactivity of glycogen phosphorylase were studied in pre- and postnatal rat brain. The catalytic activity was assayed in brain homogenates; immunoreactivity was investigated by immunoblot analysis using a monoclonal anti-bovine brain glycogen phosphorylase antibody. The cellular localization and intensity of immunoreactivity were analysed on paraffin-embedded sections utilizing the same monoclonal antibody. The catalytic activity increased 10-fold from embryonic day 16 to adult; immunoreactivity became detectable on embryonic day 16 and increased in intensity as the enzyme activity rose to adult values. The first cellular elements to be stained immunohistochemically were ependymal cells lining the ventricles, ependymal cells of the choroid plexus, meningeal cells and a selected population of neurons in the brain stem. The immunoreactivity of plexus cells and meningeal cells was reduced or absent in the adult rat brain. The earliest appearance of glycogen phosphorylase immunoreactivity in astroglial cells was seen at postnatal day 9 in the hippocampus. The staining pattern of the adult brain was reached at day 22 post partum. The developmental changes in glycogen deposition and in glycogen phophorylase activity and immunoreactivity may indicate a variable physiological role of glycogen metabolism for different cell types in the pre- and postnatal periods.Dedicated to Professor Helmut Leonhardt on the occasion of his 75th birthday     

Epithelial rests of Malassez express immunoreactivity of TrkA and its distribution is regulated by sensory nerve innervation.

The periodontal ligament is the connective tissue that fills the space between the tooth and its bony socket. It is abundantly innervated by the sensory and sympathetic nerves. We first investigated the immunoreactivity of TrkA, which is a high-affinity receptor of nerve growth factor (NGF), in the periodontal ligament of rats. Immunoreactivity was observed at the epithelial cells in the cervical and furcation regions of the molars. These epithelial cells, which gather together to form clusters or networks, are known as the epithelial rests of Malassez. Immunoreactivity was not observed in other non-neuronal cells, such as osteoblasts, fibroblasts, odontoblasts, cementoblasts, endothelial cells, and/or osteoclasts. On the basis of these findings, we investigated the possible involvement of sensory nerve innervation in the immunoreactivity of the epithelial cells. Denervation of the inferior alveolar nerve resulted in a marked decrease in the distribution area and size of the clusters of immunoreactive cells compared with those of sham-operated rats. These findings suggest that sensory nerve innervation may have a regulatory role in maintenance of the epithelial rests of Malassez expressing TrkA in the periodontal ligament.     

Nerve growth factor receptor-like immunoreactivity in primary and permanent canine tooth pulps of the cat

Summary The distribution of nerve growth factor receptor (NGF receptor)-like immunoreactivity in pulps of developing primary and mature permanent cat canine teeth was examined, by use of a monoclonal antibody against NGF receptor detected by fluorescence immunohistochemistry and pre-embedding immunocytochemical light- and electron microscopy. Both primary and permanent pulps contained a vast number of NGF receptor-like immunoreactive nerves. Immunolabelling appeared to be localized both to axons and Schwann cells. In addition, many blood vessel walls in immature primary tooth pulps showed NGF receptor-like immunoreactivity, in contrast to permanent pulps where blood vessels rarely were NGF receptor-immunoreactive. Double-labelling immunofluorescence experiments revealed that in the permanent pulp a majority of the NGF receptor-positive nerves also showed calcitonin gene-related peptide (CGRP)-like immunoreactivity, and many showed substance P-like immunoreactivity. However, nerve fibers with neuropeptide Y-like immunoreactivity lacked NGF receptor-like immunoreactivity. In developing primary tooth pulps fewer NGF receptor-positive nerves were CGRP-like immunoreactive or substance P-like immunoreactive, as compared to the permanent pulp. Neuropeptide Y-like immunoreactive nerve fibers were not detected in the primary tooth pulp. The results suggest a role for nerve growth factor in both developing and mature sensory nerves of the tooth pulp.     

Quantitative analysis of nerve growth factor in the amniotic fluid during chick embryonic development

Nerve growth factor (NGF) and most neurotrophic factors support the proliferation and survival of particular types of neurons. Besidesthe pivotal role of NGF in the development of neuronal cells, it also has important functions on non-neuronal cells. The amnion surrounds the embryo, providing an aqueous environment for the embryo. A wide range of proteins has been identified in human amniotic fluid (AF). In this study, total protein concentration (TPC) and NGF level in AF samples from chick embryos were measured using a Bio-Rad protein assay, enzyme linked immunosorbent assay (ELISA) and Western blot. TPC increased from days E10 to day E18. There was a rapid increase in AF TPC on day E15 when compared to day E16. No significant changes in NGF levels have been seen from day E10 to day E14. There was a rapid increase in NGF content on days E15 and E16, and thereafter the levels decreased from day E16 to day E18. Since, NGF is important in brain development and changes in AF NGF levels have been seen in some CNS malformations, changes in the TPC and NGF levels in AF during chick embryonic development may be correlated with cerebral cortical development. It is also concluded that NGF is a constant component of the AF during chick embryogenesis.     

Widespread immunoreactivity for neuronal nuclei in cultured human and rodent astrocytes

The monoclonal antibody (mAb) neuronal nuclei (NeuN) labels the nuclei of mature neurons in vivo in vertebrates. NeuN has also been used to define post-mitotic neurons or differentiating neuronal precursors in vitro . In this study, we demonstrate that the NeuN mAb labels the nuclei of astrocytes cultured from fetal and adult human, newborn rat, and embryonic mouse brain tissue. A non-neuronal fibroblast cell line (3T3) also displayed NeuN immunoreactivity. We confirmed that NeuN labels neurons but not astrocytes in sections of P10 rat brain. Western blot analysis of NeuN immunoreactive species revealed a distribution of bands in nucleus-enriched fractions derived from the different cell lines that was similar, but not identical to adult rat brain homogenates. We then examined the hypothesis that the glial fibrillary acidic protein/NeuN-double positive population of cells might correspond to neuronal precursors. Although the NeuN-positive astrocytes were proliferating, no evidence of neurogenesis was detected. Furthermore, expression of additional neuronal precursor markers was not detected. Our results indicate that primary astrocytes derived from mouse, rat, and human brain express NeuN. Our findings are consistent with NeuN being a selective marker of neurons in vivo , but indicate that studies utilizing NeuN-immunoreactivity as a definitive marker of post-mitotic neurons in vitro should be interpreted with caution.     

Presence of a nerve growth factor-like immunoreactivity in auditory receptors during postnatal development in the rat

The presence of nerve growth factor (NGF) was investigated in the rat cochlea from birth to the adult stage, using immunohistochemical techniques NGF-like protein could be detected in the organ of Corti from birth up to day 8 and located within the hair cells, above the nuclei. No NGF-like immunoreactivity could be detected in the spiral ganglion. These results suggest that NGF may have a neurotropic action in the developing rat cochlea.