Specific retrograde transport of nerve growth factor (NGF) from neocortex to nucleus basalis in the rat

The role of cholinergic projections from the nucleus basalis in memory

The behavioral effects of lesions of the nucleus basalis magnocellularis (NBM) are reviewed, focusing on the anatomical extent of the lesion, the involvement of neurotransmitter systems and the alterations in memory processes. Most behavioral deficits after NBM lesions can be attributed to damage to the NBM itself, although during spontaneous or pharmacologically induced recovery, other brain structures might play a role. The neurochemical deficit underlying the behavioral impairments is most likely the decrease in cholinergic functioning, since, for example, enhancement of cholinergic functioning is sufficient for behavioral improvement. However, since the lesions are not specific for cholinergic neurons, the extent to which noncholinergic damage causes behavioral deficits is still unclear. Finally, lesions of the NBM impair memory, but affect also other behavioral processes, such as discrimination and habituation. A common process underlying these various impairments could be that of insufficiently focused processing of stimuli.

Acetyl-L-carnitine enhances the response of PC12 cells to nerve growth factor

We have demonstrated that treatment of rat pheochromocytoma (PC12) cells with acetyl-L-carnitine (ALCAR) stimulates the synthesis of nerve growth factor receptors (NGFR). ALCAR has also been reported to prevent some age-related impairments of the central nervous system (CNS). In particular, ALCAR reduces the loss of NGFR in the hippocampus and basal forebrain of aged rodents. On these bases, a study on the effect of NGF on the PC12 cells was carried out to ascertain whether ALCAR induction of NGFR resulted in an enhancement of NGF action. Treatment of PC12 cells for 6 days with ALCAR (10 mM) stimulated [125I]NGF PC12 cell uptake, consistent with increased NGFR levels. Also, neurite outgrowth elicited in PC12 cells by NGF (100 ng/ml) was greatly augmented by ALCAR pretreatment. When PC12 cells were treated with 10 mM ALCAR and then exposed to NGF (1 ng/ml), an NGF concentration that is insufficient to elicit neurite outgrowth under these conditions, there was an ALCAR effect on neurite outgrowth. The concentration of NGF necessary for survival of serum-deprived PC12 cells was 100-fold lower for ALCAR-treated cells as compared to controls. The minimal effective dose of ALCAR here was between 0.1 and 0.5 mM. This is similar to the reported minimal concentration of ALCAR that stimulates the synthesis of NGFR in these cells. The data here presented indicate that one mechanism by which ALCAR rescues aged neurons may be by increasing their responsiveness to neuronotrophic factors in the CNS.

Nerve growth factor mRNA-containing cells are distributed within regions of cholinergic neurons in the rat basal forebrain

It has been proposed that nerve growth factor (NGF) provides critical trophic support for the cholinergic neurons of the basal forebrain and that it becomes available to these neurons by retrograde transport from distant forebrain targets. However, neurochemical studies have detected low levels of NGF mRNA within basal forebrain areas of normal and experimental animals, thus suggesting that some NGF synthesis may actually occur within the region of the responsive cholinergic cells. In the present study with in situ hybridization and immunohistochemical techniques, the distribution of cells containing NGF mRNA within basal forebrain was compared with the distribution of cholinergic perikarya. The localization o NGF mRNA was examined by using a 35S-labeled RNA probe complementary to rat preproNGF mRNA and emulsion autoradiography. Hybridization of the NGF cRNA labeled a large number of cells within the anterior olfactory nucleus and the piriform cortex as well as neurons in a continuous zone spanning the lateral aspects of both the horizontal limb of the diagonal band of Broca and the magnocellular preoptic nucleus. In the latter regions, large autoradiographic grain clusters labeled relatively large Nissl-pale nuclei; it did not appear that glial cells were autoradiographically labeled. Comparison of adjacent tissue sections processed for in situ hybridization to NGF mRNA and immunohistochemical localization of choline acetyltransferase (ChAT) demonstrated overlapping fields of cRNA-labeled neurons and ChAT immunoreactive perikarya in both the horizontal limb of the diagonal band and magnocellular preoptic regions. However, no hybridization of the cRNA probe was observed in other principal cholinergic regions including the medial septum, the vertical limb of the diagonal band, or the nucleus basalis of Meynert.(ABSTRACT TRUNCATED AT 250 WORDS)

Molecular cloning of PC3, a putatively secreted protein whose mRNA is induced by nerve growth factor and depolarization

PC3 is an immediate early gene induced by nerve growth factor in PC12 cells, a cell line derived from a tumor of the adrenal medulla that undergoes neuronal differentiation in the presence of nerve growth factor. This induction is independent of new protein synthesis as it can occur in the presence of cycloheximide. PC3 is also induced with similar kinetics, but at lower levels, by membrane depolarization (both in vivo and in vitro) and epidermal growth factor. It is induced at much lower levels by fibroblast growth factor and interleukin 6. In vivo it is found expressed in tissues, such as brain at embryonic day 13.5, placenta, amnion, and spleen, which are proliferating and/or differentiating. The deduced protein sequence from the cDNA indicates the presence of a signal peptide, suggesting that PC3 is secreted.

Target cell specificity of synaptic connections in the hippocampus

A major question of neurobiological research is how precise connections between neurons are formed and maintained. In the hippocampus, afferent fiber systems are known to terminate in a laminated fashion. Previous studies have indicated that this lamination is largely due to spatiotemporal constraints during ontogenetic development. In this commentary, recent fine structural studies on the target cell specificity of the various hippocampal afferents are discussed. It becomes obvious that some afferent fibers establish synapses with all available target cells, whereas other afferents are restricted to distinct types of neurons. A high degree of neuronal specificity is found in the hippocampal and dentate axo-axonic cells, which are restricted not only to specific types of target cells (pyramidal neurons and granule cells, respectively) but also to distinct portions of the target cell's membrane (the axon initial segment). Altogether, these data indicate that there are different levels of target cell specificity in the hippocampus. It is suggested that specific molecular interactions between pre- and postsynaptic elements, in addition to spatial and temporal factors, play a role in the formation and stabilization of the various synaptic connections of the hippocampal formation.

Evidence that biological activity of NGF is mediated through a novel subclass of high affinity receptors

Trophic factors, such as NGF, regulate survival and differentiation of many classes of neurons by binding specific receptors. Two types of NGF receptors have been identified, which bind NGF with low and high affinity. The latter mediates the major biological actions of NGF. To determine the relationship between these two receptor types, polyclonal antibodies to the low affinity receptor have been prepared and used in ligand-binding, ligand-cross-linking, and biological assays. These antibodies eliminated binding of NGF to low affinity receptors and to one class of high affinity receptors, but did not prevent binding to a second class of high affinity receptors. The same antibodies did not inhibit NGF-stimulated neuronal survival or neurite outgrowth. Thus, a biologically important class of high affinity NGF receptors is antigenically distinct from the low affinity receptor and may be encoded by a novel gene.

Expression of NGF receptor and NGF receptor mRNA in the developing and adult rat retina

Nerve growth factor (NGF) has been recently found to rescue axotomized retinal ganglion cells (RGCs) of the adult rat from degeneration. Because the trophic effect of NGF involves a receptor-coupling event, the characterization and cellular localization of the NGF receptor (NGFR) in the retina are essential to understanding the possible specific action of NGF in this district of the central nervous system. We report here that the NGFR mRNA is expressed in fetal, neonatal, and adult rat retina. Using monoclonal antibody 192-IgG to immunoprecipitate and immunohistochemically identify NGFR, we also found that the NGFR from the retina has a molecular weight identical to that of the NGFR from PC12 cells. The NGFR is localized on RGCs and Müller cells. Finally, following ligation of the optic nerve, NGFR-immunopositive material was found to accumulate both distal and proximal to the site of ligation, suggesting that RGC axons anterogradely and retrogradely transport the NGFR. These data raise the possibility that NGF may play a specific role in rat RGCs.

Immunohistochemical detection of a monoclonal antibody directed against the NGF receptor in basal forebrain neurons following intraventricular injection

It has been shown by autoradiography that, following intraventricular administration, a monoclonal antibody directed against the rat nerve growth factor (NGF) receptor is specifically accumulated bilaterally by numerous cholinergic neurons of the basal forebrain. This is consistent with the evidence that cholinergic basal forebrain neurons have NGF receptors and respond to NGF under a variety of experimental conditions. The present study demonstrates that the immunohistochemical detection of unmodified monoclonal antibody in cholinergic forebrain neurons following transport from CSF is feasible, although injection of larger amounts of the antibody is required to obtain an image equivalent to the one obtained with the autoradiographic method. The location of the immunohistochemical product clearly indicates that the antibody has been internalized, probably in an endosomal compartment.

Two distinct monoclonal antibodies raised against mouse beta nerve growth factor. Generation of bi-specific anti-nerve growth factor anti-horseradish peroxidase antibodies for use in a homogeneous enzyme immunoassay

Two hybridomas producing monoclonal antibodies against mouse beta nerve growth factor (NGF) were obtained from the fusion of hyperimmune splenocytes from rats immunized with polymerized beta-NGF and Sp2/0.Ag mouse myeloma cells. The monoclonal antibodies coded IgG 24 and 30 produced and secreted by the hybrid cells are both of the IgG2a subclass. Both monoclonal antibodies are capable of recognizing native NGF coated on microassay plates as well as the denatured factor on Western blots. However, only IgG 30 has been found to block NGF-induced process outgrowth from the rat pheochromocytoma cell line (PC12) as well as NGF-induced increase in choline acetyltransferase activity in rat primary septal cell cultures. In addition, only IgG 30 was able to detect immunocytochemically NGF-immunoreactive sites in fixed tissue. And, finally, IgG 24 could not compete for IgG 30 binding to immobilized native NGF. Consequently, it appears that these antibodies are recognizing different epitopes on the NGF molecule. Neither monoclonal antibody displayed any crossreactivity with serum albumin, aprotinin, epidermal growth factor or insulin. A hybrid-hybridoma producing bi-specific anti-NGF anti-horseradish peroxidase (HRP) monoclonal antibodies was generated from the fusion of an azaguanine resistant anti-HRP hybridoma, coded RAP2.Ag and the anti-NGF IgG 30 hybridoma treated with emetine. The potential merits of using these bi-specific antibodies in combination with their mono-specific anti-NGF parent in a homogeneous sandwich immunoassay for the quantitation of NGF are discussed.

Receptor-mediated transport of human recombinant nerve growth factor from olfactory bulb to forebrain cholinergic nuclei

Receptors for nerve growth factor are present in the olfactory bulb and in cholinergic nuclei that send projections to the olfactory bulb. The retrograde transport of 125I-labeled recombinant human nerve growth factor (rhNGF) was demonstrated in the rat 18 h following an injection of [125I]rhNGF into the left olfactory bulb. In each of six animals, [125I]rhNGF label was observed in the ipsilateral horizontal limb of the diagonal band and, in four of the 6 animals, in the vertical limb of the diagonal band. Label was not observed in any other brain region except within the injected olfactory bulb. The transport of label to the diagonal band was blocked by the injection of 170-fold greater concentration of unlabeled rhNGF. Emulsion autoradiography of hematoxylin/eosin counterstained sections revealed silver grains clustered over numerous cell profiles that resembled neurons. In contrast, cerebellar injections of [125I]rhNGF, with or without unlabeled rhNGF, did not label diagonal band neurons, nor the lateral vestibular or red nuclei, from which originate the primary cholinergic afferents to cerebellum. The receptor-dependent transport of NGF from olfactory bulb to forebrain cholinergic nuclei suggests that this projection, unlike pontomesencephalic cholinergic pathways, may be responsive to endogenous NGF or exogenously administered rhNGF.

Production of transferable neuronotrophic factor(s) by human midgut carcinoid tumour cells; studies using cultures of rat fetal cholinergic neurons

A co-culture system was established between human midgut carcinoid tumour cells and rat fetal cholinergic neurons. In monocultures in serum-free media, only tumour cells survived, while neurons deteriorated. In serum-free co-cultures, neurons displayed outgrowth of neuritic processes. Neurons of neuronal serum-free monocultures thrived if supplemented with conditioned media from tumour cell cultures grown serum-free. This indicates that tumour cells produce transferable growth factor(s) with potent neuronotrophic actions. Immunocytochemical studies indicate that this growth factor resembles nerve growth factor immunologically, since tumour cells were strongly immunoreactive after incubation with a rabbit anti-nerve growth factor antiserum, and furthermore expressed immunoreactive nerve growth factor receptors.

Thyroid hormone causes sexually distinct neurochemical and morphological alterations in rat septal-diagonal band neurons

Sex differences were investigated in cholinergic neurons of the septal-diagonal band region of adult rats subjected to neonatal treatment with 3,3',5-triiodo-L-thyronine (T3). Neonatal hyperthyroidism resulted in a 44% increase in specific activity of choline acetyltransferase (ChAT; EC 2.3.1.6) in adult male rat septal-diagonal band region, whereas no change in ChAT activity could be detected in either dorsal or ventral hippocampus. An increase in muscarinic cholinergic receptors, as measured by [3H]quinuclidinyl benzilate [( 3H]QNB) binding, was discovered in both septum-diagonal band and dorsal hippocampus of the T3-treated male rats. Immunohistochemistry in the septal-diagonal band region indicated a more intense staining in the neonatally T3-treated adult male rats than in controls, with larger and more abundant ChAT-positive and nerve growth factor receptor (NGF-R)-positive varicosities. ChAT immunocytochemistry showed a substantial decrease in cell body area in the medial septum and in the vertical limb of the diagonal band of T3-treated male rats, while cell density increased twofold. Female littermates subjected to the same treatment showed no changes in any of the biochemical or immunohistochemical cholinergic markers. Only in the medial septum was morphology significantly altered in the female T3-treated rats in that ChAT-positive cell body area increased. These results indicate a marked sexual variation in the septal-diagonal band region with respect to the sensitivity of postnatally developing cholinergic neurons to the actions of excess thyroid hormone.

Nerve growth factor and the neostriatum

1. The present review summarizes evidence describing the expression, immunoreactivity, binding, transport, development, aging, and functions of NGF in the mammalian neostriatum. 2. Neostriatal NGF binding sites and intrinsic cholinergic neurons are co-localized, increase at a similar rate during ontogeny, and are lost to an equal extent following age- or injury-induced loss of neostriatal neurons. 3. Exogenously administered NGF augments ChAT activity in the intact caudate-putamen, nucleus accumbens, and following mechanical or excitotoxin-induced cholinergic injury. NGF antibodies lower ChAT in the intact caudate-putamen. 4. Neostriatal cholinergic interneurons are lost in the aged rat but also in Alzheimer's disease, Parkinson's disease, supranuclear palsy, and Huntington's chorea. Future studies need to address the extent to which these losses result from an abbreviation of NGF production, binding, or transport and whether rhNGF administration may retard or reverse these cholinergic losses.

Identification of high- and low-affinity NGF receptors during development of the chicken central nervous system

In order to study regulation of the nerve growth factor (NGF) receptor during embryogenesis in chick brain, we have used affinity crosslinking of tissues with 125I-NGF. NGF interacts with high- and low-affinity receptors; high-affinity receptors are required for the majority of NGF's actions. Most measurements of receptor levels do not distinguish between high- and low-affinity forms of the receptor. We have used the lipophilic crosslinking agent HSAB to identify the high-affinity, functional receptor during development of the chicken central nervous system. A peak of expression during Embryonic Days 5-10 was detected in all regions of the chicken central nervous system, but, shortly after birth, only the cerebellar region displays significant levels of NGF receptor protein. The time course of expression confirms the dramatic regulation of the NGF receptor gene during defined embryonic periods. The detection of high-affinity NGF receptors in brain and neural retina provides strong evidence that NGF is involved in essential ontogenetic events in the development of the chicken central nervous system.

Regulation of beta-nerve growth factor expression by inflammatory mediators in hippocampal cultures

Substances which regulate expression of nerve growth factor (NGF) were examined in embryonic rat hippocampal cultures containing both neurons and glial cells. Both cell types expressed NGF mRNA when cultivated in vitro. Lipopolysaccharide, an activator of macrophages, elicited a significant increase in NGF mRNA. Interleukin-1 beta evoked a similar increase in NGF mRNA which was accompanied by a rise in NGF protein. The Il-1-induced increase was partially blocked by indomethacin, suggesting that prostaglandins might mediate this effect. Treatment of the cultures directly with prostaglandin E2 resulted in elevated levels of both NGF mRNA and protein. Thus, agents which promote inflammatory activity appear to increase NGF expression. Moreover, a suppressor of inflammation, dexamethasone, decreased NGF expression. Our observations indicate that a variety of immunomodulators regulate NGF expression in the hippocampus.

Human beta nerve growth factor obtained from a baculovirus expression system has potent in vitro and in vivo neurotrophic activity

A baculovirus expression vector, which contains the coding sequences for human prepro (beta) nerve growth factor under control of the viral polyhedrin promoter, was constructed. Upon infection of insect cells with the recombinant virus, mature human beta nerve growth factor (rhNGF) was released into the culture fluid. The mature rhNGF was biologically active since rat pheochromocytoma (PC12) and human neuroblastoma (SH-SY5Y) cells were induced to extend neurites upon treatment with this material. This activity was abolished by treating with antiserum prepared against mature mouse beta NGF (mNGF). When compared with mNGF, rhNGF more rapidly elicited the differentiation response in both PC12 and SH-SY5Y cells. In an in vivo assay of cholinergic cell survival, rhNGF was nearly as potent as mNGF in protecting cholinergic neurons from degeneration following a fimbria-fornix lesion. These results show that the baculovirus expression system provides quantities of biologically potent human beta NGF suitable for a comprehensive program of research to ascertain beta NGF's potential as a therapeutic agent for the treatment of Alzheimer's disease.

Purification and characterization of Fab fragments from anti-mouse NGF polyclonal antibodies

A functional role for Nerve Growth Factor (NGF) in the peripheral nervous system is well-documented, but a similar case for NGF in the central nervous system remains to be established. One approach to answering this question would be the availability of high-affinity monospecific Fab fragments obtained against NGF. In the present studies we describe the preparation and characterization of such Fab fragments from anti-mouse NGF polyclonal antibodies. Following their purification by the use of a NGF Sepharose-coupled affinity column, the Fab fragments were examined for biological competence in several ways. In vitro, the anti-Fab fragments blocked the neuronotrophic activity of NGF, as measured by the survival of chicken embryonic day 8 dorsal root ganglion neurons. In vivo, these Fab fragments, when administered systemically to neonatal rats, produced a decrease of noradrenaline levels in two sympathetically innervated organs, the heart and the spleen. These findings suggest that affinity purified Fab fragments of anti-NGF antibodies can be a useful tool for studying the physiological function of NGF in the nervous system.

Environmental influence on behaviour and nerve growth factor in the brain

The influence of the environment on the endogenous levels of nerve growth factor (NGF) in the cortex, hippocampus and septum was examined in adult (82 days old) and juvenile (51 days old) rats. Animals were reared/housed for 30 days in an enriched, standard or isolated environment prior to analysis. In addition, another group of rats were given behavioural tests (4 days) after differential rearing/housing before measurements of NGF. We found complex variations in the level of NGF both in juvenile and adult hippocampus after differential environmental rearing/housing. Rearing/housing in an enriched environment improved performance in the Morris maze and decreased spontaneous motor activity. Exposure to behavioural tests caused alterations in adult hippocampus and septum NGF levels. The results show that testing in a novel environment causes small but significant changes in the hippocampal and septal NGF levels depending upon the environmental history of the animal. In view of the purported involvement of the septohippocampal pathway and NGF in the pathophysiology of Alzheimer's disease, our finding suggests that lack of adequate environmental stimulation might be of importance in age-related behavioural and neurochemical deficits.

Immunoelectron microscopic evidence of nerve growth factor receptor metabolism and internalization in rat nucleus basalis neurons

Nerve growth factor receptor (NGFr) immunoreactive neurons of the adult rat nucleus basalis magnocellularis were examined by electron microscopy. Prominent NGFr immunoreactivity (IR) was consistently present along the perikaryal cell membrane and frequently in intracellular sites of protein synthesis and modification such as the rough endoplasmic reticulum and Golgi apparatus, respectively. Immunoperoxidase reaction product was also seen along the nuclear membrane. Membrane-bound aggregates of immunoreactive vesicles were scattered throughout the perikaryon, being more concentrated in the perinuclear region and in the proximal neurites. These may represent either aggregates of receptor-containing vesicles on their way to/from the cell membrane or secondary lysosomes where NGFr reaction product is degraded. Immunostained cytoplasmic vesicles which possessed an electron-dense coat and were adjacent to or contiguous with the plasmalemma probably represented internalized receptor. This ultrastructural study of the subcellular distribution of NGFr-IR in basal forebrain neurons therefore demonstrates sites of receptor metabolism and potential receptor-ligand interaction.

Hippocampal neurotrophic factors influence the perikaryal size of septal acetylcholinesterase-containing neurons in culture

The septal neurons were cultured under the following conditions: (1) treated with 7S nerve growth factor (NGF) (50 ng/ml); (2) grown with hippocampal cell-conditioned medium supernatant; (3) cocultivated with hippocampal cells; (4) cocultivated with cerebellar cells; (5) no treatment. Acetylcholinesterase histochemistry was used to identify cholinergic cells after pretreatment with diisopropylfluorophosphate. The mean values of the perikaryal major axis and minor axis at day 14 of culture were significantly larger in septal cells cocultivated with hippocampal cells than in septal cells grown under other conditions. NGF-treated septal cells showed a smaller, but significant, increase in the mean value of the major axis of these neurons.

Developmental study of nerve growth factor receptor mRNA expression in the postnatal rat cerebellum

Nerve growth factor receptor (NGFR) mRNA expression was examined in the developing rat cerebellum with in situ hybridization histochemistry. The probe used in this study was the 57-mer oligonucleotide complementary to the rat NGFR cDNA (bases 798-855). The specificity of the hybridization was determined with a 'sense' probe. From the age of postnatal day 0 (PND 0), hybridization signals were detected in the external granule cell layer, Purkinje cell layer, and meningeal cells of the cerebellum. Until PND 10, the external granular cell layer consistently expressed high levels of NGFR message, with the densest hybridization signal observed in the proliferative zone. NGFR mRNA level in this layer rapidly decreased at later ages and was barely detectable after PND 16. In the Purkinje cell layer beyond PND 4, the hybridization signal became more intense, reached a peak level around PND 10, and then gradually decreased. NGFR mRNA was detected until PND 30 in this layer. Meningeal cells expressed NGFR message from PND 0 to PND 8. These results suggest that NGF may be involved in cerebellar development as a mediator of morphogenesis and/or synaptogenesis.

Axotomized, adult basal forebrain neurons can innervate fetal frontal cortex grafts: a double fluorescent tracer study in the rat

The ability of axonal regeneration of identified adult basal forebrain (BFB) neurons was examined after homotopic grafting of fetal neocortical tissue to a lesion cavity in the frontal neocortex. Using a four step experimental procedure, adult rats first received an injection of the fluorescent dye Fluoro-Gold (FG) into the sensorimotor cortex in order to label those neurons with projections to the area by retrograde axonal transport. After one week the injection area was removed by aspiration, leaving a cavity in the neocortex. One week later a block of fetal (E14) frontal cortical tissue was placed in the cavity. The animals were then allowed to survive for 6 weeks before a second fluorescent tracer, Nuclear Yellow (NY), was injected into the transplant. The animals were sacrificed 24 h later and analyzed by fluorescence microscopy. Both single labeled, FG and NY containing neurons and double labeled neurons containing both tracers were found in the BFB. The results demonstrate that adult BFB neurons can reestablish cortical projections into fetal cortical grafts (double labeled neurons), and they suggest that other BFB neurons, not initially innervating the lesioned cortical area, have sprouted into the transplant (NY labeled neurons).

Effect of basic fibroblast growth factor on neurons cultured from various regions of postnatal rat brain

Neurons from various brain regions of postnatal (15 days after birth) and fetal (16 days gestation) rats were cultured in the presence of basic fibroblast growth factor (bFGF). bFGF increased the survival of neurons from postnatal septum, striatum, midbrain, and hippocampus. Fetal neurons derived from cerebral cortex, septum, striatum, midbrain, thalamus, and colliculus were far more dependent on bFGF for survival in comparison with postnatal neurons. In contrast, cerebellum neurons of postnatal and fetal rat brain did not respond to bFGF. The increase of postnatal and fetal neuronal survival with bFGF treatment (0.01-10 ng/ml) was dose-dependent and reached 2-4-fold and 5-10-fold more than the control, respectively. Fetal cortical neurons showed almost complete dependence on bFGF since almost all neurons died in control cultures. Nerve growth factor was slightly effective only on postnatal septal and striatal neurons, being ineffective on the other neurons tested. These results indicate that bFGF can function as a neurotrophic factor not only on fetal but also on postnatal neurons of the central nervous system, and that bFGF has great potential for application in vivo.

Changes of NGF level in mouse hypothalamus following intermale aggressive behaviour: biological and immunohistochemical evidence

Nerve growth factor (NGF) immunoreactivity was detected in the hypothalamus of adult male mice. NGF-immunoreactive cell bodies were examined through consecutive brain sections, and it was found that most of the NGF-positive cells were located in the dorsomedial and mediolateral portions of the hypothalamus. Tissue culture bioassays showed that hypothalamic extract elicits neurite outgrowth from both chick sensory ganglia and rat superior cervical ganglia neurons, and that these effects are inhibited by addition of NGF antibodies. Our results also showed that intermale aggressive behaviour induced by 6-8 weeks of social isolation induces an NGF increase in the hypothalamic area, which is not abolished by sialoadenectomy, suggesting that the increased brain NGF is locally synthesized, and does not come from salivary sources. Likewise, the level of NGF in the hypothalamus of adrenalectomized fighting mice increased, although to a much lesser extent, when compared to hypothalamic levels of sham-operated fighting mice. The present results and a recent report showing that aggressive behaviour causes an increase of mRNANGF in hypothalamic areas are discussed in relation to a possible functional role of NGF in these brain structures.

Detection of beta-nerve growth factor mRNA in the human fetal brain

Nerve growth factor (NGF) is a trophic molecule recently demonstrated to interact with different structures in the central nervous system. The expression of the beta-NGF mRNA from human fetal cortices at the 15-16th week of gestational age has been demonstrated and quantitated by polymerase chain reaction amplification of the specific cDNA. beta-NGF mRNA expression in the human brain coincides with the period of active differentiation and synaptogenesis, suggesting that the trophic agent plays a role in the cerebral development.

Cholinergic cell loss and cognitive impairments following intraventricular or intradentate injection of colchicine

Bilateral injections of colchicine (3.5 or 7.0 micrograms/0.5 microliters/site) into either the dentate gyrus or the lateral cerebroventricles (i.c.v.) of Sprague-Dawley rats produced specific behavioral, histopathological and neurochemical alterations. Colchicine, administered via either route, produced impairments in the performance of a radial-arm maze task which did not subside during 8 weeks of testing. Intradentate colchicine decreased (1) the thickness of both blades of the dentate granule cell layer, (2) the size of the overlying molecular layer, (3) hippocampal volume, and (4) the number of cholinergic neurons in the medial septum/vertical limb of the diagonal band (MS/VLDB). I.c.v. administration of colchicine did not alter any index of hippocampal morphology but did significantly decrease the number of cholinergic neurons in the MS/VLDB. An analysis of the time course of cholinotoxicity revealed that both intradentate and i.c.v. colchicine decreased choline acetyltransferase (ChAT) activity and high affinity choline uptake (HAChU) in the hippocampus at 1 and 3, but not 9, weeks following surgery. Furthermore, i.c.v. colchicine decreased ChAT activity in the septum at both 3 and 9 weeks following surgery. Neither route of administration altered ChAT or HAChU in the frontal cortex, olfactory bulb or striatum. The decreases in presynaptic cholinergic parameters were paralleled by a reduction in acetylcholinesterase staining in the hippocampus which appeared to recover within 9 weeks. These data suggest that intradentate colchicine produces either (i) transsynaptic degeneration of cholinergic neurons due to a loss of their target sites (granule cells in the dentate gyrus), (ii) a direct cholinotoxic effect, or (iii) a combination of these mechanisms. The i.c.v. injection of colchicine appears to exert a direct toxic effect on cholinergic neurons and/or nerve terminals that results in the death of these neurons. Colchicine may be a useful tool for investigating the behavioral and neurobiological properties of the septohippocampal cholinergic pathway and its response to injury.

The ontogeny of specific retrograde transport of nerve growth factor by motoneurons of the brainstem and spinal cord

Radiolabeled Nerve Growth Factor (NGF) was injected into either the mandibular process of the first visceral arch or the limb bud of chick embryos at Days 3.5-14 or Days 4-13 of incubation, respectively. Control embryos received injections of labeled cytochrome-C or labeled NGF plus an excess of unlabeled NGF. The tissues were then processed for autoradiography. The 125I-NGF was retrogradely transported by motoneurons of the trigeminal (V) motor nucleus on Days 3.5-8 of incubation, but not at later stages. Similar transport was seen in motoneurons of the spinal cord lateral motor column from Days 4-10 of incubation, but not at later stages. Sensory neurons of the V ganglion and of the dorsal root ganglia transported NGF at all injection ages. In no instance was the 125I-cytochrome-C transported by sensory or motor neurons. The injection of an excess of cold NGF along with labeled NGF resulted in no evidence of retrograde transport of the labeled NGF indicating that the transport was saturable. The time of transport by these brainstem and spinal cord motoneurons corresponds closely to the points during development at which they have been found to exhibit specific NGF binding. The present results, then, provide further evidence for a possible biological role for NGF during early developmental stages of these motoneuron populations.

The response of cultured trigeminal and spinal cord motoneurons to nerve growth factor

Dissociated neurons from the trigeminal (V) region of the metencephalic basal plate or the ventral spinal cord from chick embryos of Day 4 (V basal plate) or Day 5 (spinal cord) were cultured on a laminin substratum either in the presence of nerve growth factor (NGF) or in control medium. Assessment was made of neuronal survival, the amount of neurite elaborated, and the percentage of neurons initiating neurites. The presence of motoneurons was verified by retrograde labeling with the fluorescent dye diI. NGF was found to significantly increase the quantity of neuritic processes produced by the spinal cord dissociates at both 24 and 48 hr in vitro. The percentage of neurons initiating neuritic processes was significantly increased by NGF in the trigeminal population at 48 hr in vitro. Neuronal survival was not enhanced by NGF in either group. Both trigeminal and spinal cord neurons were also found to specifically bind 125I-NGF in culture. These results provide direct evidence for an influence of NGF on process formation of early embryonic motoneurons in culture.

Specific neuronal expression of human NGF receptors in the basal forebrain and cerebellum of transgenic mice

Transgenic mice carrying multiple copies of the human NGF receptor gene have been generated. Using a monoclonal antibody specific for the human receptor, we have detected specific expression in cholinergic neurons in the basal forebrain and Purkinje cells in the cerebellum during the postnatal period. Expression in the PNS was exemplified by immunostaining of sympathetic and sensory neurons during an early embryonic age. Transection of the sciatic nerve in transgenic animals resulted in induction of human NGF receptors, indicating that the inserted gene can be appropriately regulated. These transgenic mice will provide an opportunity to study the elements regulating the NGF receptor. Furthermore, the ability to obtain specific expression in transgenic mice will permit directed expression of heterologous genes in discrete cells important in the cholinergic septal-hippocampal pathway and the PNS.

Localization of nerve growth factor receptor mRNA in the rat basal forebrain with in situ hybridization histochemistry

1. In situ hybridization histochemistry was used to localize nerve growth factor receptor (NGFR) mRNA in the adult rat basal forebrain. 2. In emulsion-dipped sections 35S-labeled RNA antisense probes produced a high density of silver grains over cells located in the medial septum, vertical and horizontal limbs of the diagonal band of Broca, and nucleus basalis. 3. This distribution of NGFR mRNA overlaps with the distribution of NGFR protein localized using immunocytochemical techniques. 4. No hybridization signal was detected when sections were hybridized with a 35S-labeled RNA sense (control) probe. 5. We suggest that NGFRs are synthesized in these basal forebrain nuclei and transported to terminal areas where NGF is thought to be bound and internalized, an initial step in the many actions of this neurotrophic factor.

Nerve growth factor and neuronal cell death

The regulation of neuronal cell death by the neuronotrophic factor, nerve growth factor (NGF), has been described during neural development and following injury to the nervous system. Also, reduced NGF activity has been reported for the aged NGF-responsive neurons of the sympathetic nervous system and cholinergic regions of the central nervous system (CNS) in aged rodents and man. Although there is some knowledge of the molecular structure of the NGF and its receptor, less is known as to the mechanism of action of NGF. Here, a possible role for NGF in the regulation of oxidant--antioxidant balance is discussed as part of a molecular explanation for the known effects of NGF on neuronal survival during development, after injury, and in the aged CNS.

125I-beta-nerve growth factor binding is reduced in rat brain after stress exposure

In the central nervous system (CNS), the presence of nerve growth factor (NGF) and its receptor, NGFR, in cholinergic neurons has been demonstrated. In this study we report that, after exposure to stress, there was a reduction in total binding of NGF in the hippocampus and basal forebrain of 3.5-month-old rats without significant changes in the frontal cortex or cerebellum. Chronic treatment with acetyl-l-carnitine (ALCAR), that prevents some age-related impairments of CNS, for 1.5 months, decreased NGF binding in hippocampus and basal forebrain but abolished the stress-related reduction of NGF binding observed in the hippocampus of untreated rats.

Gene therapy in the CNS: intracerebral grafting of genetically modified cells

Grafting cells to the CNS has been suggested and applied as a potential approach to CNS therapy through the selective replacement of cells lost as a result of disease or damage. Independently, studies aimed at direct genetic therapy in model systems have recently begun to suggest conceptually new approaches to the treatment of several kinds of human genetic disease, especially those caused by single gene enzyme deficiencies. We suggest that a combination of these two approaches, namely the graftment into the CNS of genetically modified cells, may provide a new approach toward the restoration of some functions in the damaged or diseased CNS. We present evidence for the feasibility of this approach, including a description of some current techniques for mammalian cell gene transfer and CNS grafting, and several possible approaches to clinical applications. Specifically, we report that fibroblasts, genetically modified to secrete NGF by infection with a retroviral vector and implanted into the brains of rats with a surgical lesion of the fimbria-fornix, prevented the degeneration of cholinergic neurons that would die without treatment.

Intracerebral NGF infusion induces hyperinnervation of cerebral blood vessels

The use of intracerebral NGF (nerve growth factor) infusions as a therapeutic tool to prevent the degeneration of cholinergic neurons in humans suffering from Alzheimer's disease has recently been suggested. In the present study, intracerebroventricular infusion of nerve growth factor into the adult rat brain was found to induce axonal sprouting of mature, uninjured axons associated with the intradural segment of the internal carotid artery. Following NGF infusion, a three-fold increase in the total number of axons associated with the vessel wall was observed when compared with vehicle-infused animals. This vascular hyperinnervation might also occur in humans. Before NGF infusion therapy is initiated, more research is necessary concerning the specificity, mechanisms, and functional significance of the sprouting response observed in this study.

Nerve growth factor infusions combined with fetal hippocampal grafts enhance reconstruction of the lesioned septohippocampal projection

A combination of intracerebral grafting and intraventricular infusion of nerve growth factor was used to attempt to reconstruct the cholinergic component of the septohippocampal pathway following fimbria-fornix lesions in the rat. Four groups were tested: lesion only, lesion plus fetal hippocampal graft, lesion plus nerve growth factor, and lesion plus graft plus nerve growth factor. Choline acetyltransferase immunoreactivity, acetylcholinesterase fiber staining and behavior-dependent theta activity on electroencephalogram were used to assess the extent of pathway reconstruction. Nerve growth factor was infused for the first two weeks following the fimbria-fornix lesion, while electrophysiological measurements and histological analysis were conducted six to eight months later. The lesion plus graft plus nerve growth factor infusion group had long-term savings of choline acetyltransferase-immunoreactive cells as compared to the lesion only or lesion plus graft groups. In addition the lesion plus graft plus nerve growth factor infusion group had more extensive reinnervation of the hippocampus compared to all other groups. Behavioral-dependent theta activity on electroencephalogram was observed in some animals of both lesion plus graft and lesion plus graft plus nerve growth factor infusion groups, but not in other groups; however, unlike intact animals, the restored theta could be blocked completely by scopalamine. These results demonstrate that a combination of short-term intraventricular nerve growth factor infusion and fetal hippocampal grafts enhances reconstruction of the damaged septohippocampal circuit.

NGF-dependent sprouting and regeneration in the hippocampus

While a variety of sprouting and regenerative responses have been investigated in the hippocampus, the cellular and molecular events responsible for these plastic responses have not been determined. One transmitter system, the cholinergic system, shows several distinct responses to damage in the septohippocampal circuit. Present evidence strongly supports a role for nerve growth factor (NGF) in these responses. NGF is not only important for the survival of the adult cholinergic neurons, but can also induce regrowth of the damaged fibers given an appropriate substratum for growth. These reparative effects of NGF can manifest themselves in functional recovery in the aged rat and the young rat with fimbria-fornix lesions. Finally, a role for glia cells is proposed to clarify how NGF availability may be regulated during the degenerative and regenerative events. While all plasticity events certainly cannot be explained by the coincidence of NGF and the cholinergic system, their interaction may provide a template for other transmitter/trophic factor interactions.

Nerve growth factor in the primate central nervous system: regional distribution and ontogeny

An enzyme immunoassay for nerve growth factor was developed to determine the regional distribution and ontogenic change in the macaque (Macaca fascicularis) CNS. The standard curve of mouse nerve growth factor paralleled the dilution curves of extracts from the primate CNS at the adult and pre-natal stages. Furthermore, the nerve growth factor immunoreactive material comigrated with mouse nerve growth factor by means of carboxy methyl cellulose chromatography. These findings suggest that the immunoreactive material extracted from the primate CNS is mouse nerve growth factor-like molecules. At the adult stage, the highest level of nerve growth factor was in the hippocampus, with relatively high levels also in the hypothalamus, the cerebral cortex, the amygdala, the basal nucleus of Meynert, the septal nucleus, the cerebellum and the caudate nucleus. No detectable amounts were observed in the spinal cord, the substantia nigra or the dentate nucleus. In addition to the CNS, the pituitary gland contained about four times the level found in the hippocampus. At embryonic day 120, a high level of nerve growth factor already existed in the occipital cortex (80% of the level at the adult stage) and in the hippocampus (70% of the level at the adult stage). Between embryonic day 120 and the newborn stage in the occipital cortex and between embryonic day 120 and postnatal day 60 in the hippocampus, nerve growth factor levels increased about 1.7-fold, and after that, they gradually decreased until the adult stage was reached. In contrast, in the cerebellum, the level was quite high during the pre-natal period and declined to one-third at postnatal day 60. The developmental changes in nerve growth factor and choline acetyltransferase activity in the hippocampus were well correlated (r = 0.963) between embryonic day 120 and postnatal day 60. Our studies reveal that nerve growth factor is present in the primate CNS. The high level of nerve growth factor during embryonic stages and the good correlation with choline acetyltransferase activity suggest a physiological role for nerve growth factor in the development of the primate CNS.

Distribution of nerve growth factor receptor-like immunoreactivity in the adult rat central nervous system. Effect of colchicine and correlation with the cholinergic system--I. Forebrain

Nerve growth factor receptor, as recognized by the monoclonal antibody 192-IgG, was localized to multiple regions of the adult rat forebrain. Immunoreactive cell bodies and fibers were seen in both sensory and motor regions which are known to contain cholinergic and non-cholinergic neurons. Specifically, nerve growth factor receptor immunoreactivity was present in cells lining the olfactory ventricle, rostral portion of the lateral ventricle, in basal forebrain nuclei, caudate putamen, globus pallidus, zona incerta and hypothalamus. Immunoreactive cells which were situated subpially along the olfactory ventricle and anterior portions of the lateral ventricle, and in the arcuate nucleus resembled neuroglia but could not definitively identified at the light microscopic level. Animals pretreated with intracerebroventricular colchicine displayed significantly increased nerve growth factor receptor immunoreactivity in all previously positive neurons and particularly in the medial preoptic area and ventral premammillary nucleus of the hypothalamus. In such animals, receptor immunoreactivity also appeared in previously non-immunoreactive cells of the hippocampal CA3 region and polymorph layer of the dentate gyrus as well as in the mitral cell layer of the olfactory bulb. Nerve growth factor receptor-immunoreactive fibers and varicosities were seen in the olfactory bulb, piriform cortex, neocortex, amygdala, hippocampus, thalamus, olivary pretectal nucleus and hypothalamus. In most regions, such fiber-like immunoreactive structures likely represented axon terminals, although in some areas, neuroglial or extracellular localizations could not be excluded. In this context, diffuse, non-fibrillar receptor immunoreactivity occurred in the lateral habenular nucleus and medial terminal nucleus of the accessory optic tract. Furthermore, intense nerve growth factor receptor immunoreactivity occurred along certain regions of the pial surface on the ventral surface of the brain. The distribution of nerve growth factor receptor-immunoreactive cell bodies and fibers in multiple sensory and motor nuclei suggests wide-spread influences of nerve growth factor throughout the adult rat forebrain. There is a high degree of overlap with regions containing choline acetyltransferase immunoreactivity. However, significant disparities exist suggesting that certain nerve growth factor receptor-containing non-cholinergic neurons of the rat forebrain may also be affected by nerve growth factor.

NGF receptor gene expression is decreased in the nucleus basalis in Alzheimer's disease

Basal forebrain neuronal atrophy in Alzheimer's disease (AD) may be caused by a deficit in the NGF responsiveness of magnocellular cholinergic neurons which project to the cerebral cortex and hippocampal formation. We have used in situ hybridization to show that NGF-receptor (NGF-R) mRNA-positive neurons are lost within all divisions of the nucleus basalis of Meynert (Ch4 cell group) in AD patients as compared to normal aged controls. The posterior division of the nucleus basalis showed the largest decrease in NGF-R mRNA hybridization in the disease, with no overlap in neuronal number between AD cases and normal controls. Northern (RNA) blotting showed decreased levels of NGF-R mRNA in the nucleus basalis in the disease. No differences in the number of NGF-R mRNA-positive neurons between normal aged and AD patients were detected within the NGF-responsive cell groups of the medial septum (Ch1) and nucleus of the vertical limb of the diagonal band (Ch2). These results show that NGF-R gene expression is selectively reduced within basal forebrain neuronal populations which exhibit degenerative changes in AD.

Localization of nerve growth factor receptor messenger RNA and protein in the adult rat brain

We have used in situ hybridization and immunocytochemistry to map the cellular localization of NGF receptor (NGF-R) mRNA and protein in the adult rat brain. In addition to basal forebrain magnocellular neurons, NGF-R is widely expressed within the CNS, including neurons of the caudate/putamen, ventral premamillary nucleus, mesencephalic trigeminal nucleus, prepositus hypoglossal nucleus, raphe nucleus, nucleus ambiguous, and Purkinje cells of the cerebellum. Cells of the vestibulocochlear ganglion also contain NGF-R mRNA and protein. Ventricular subependymal cells and tanycytes are clearly stained by immunocytochemistry, yet only very weak hybridization is detectable in these cells. Also, greater amounts of NGF-R protein than of mRNA appear to be present in the glomeruli of the olfactory bulb, area postrema, and nucleus tractus solitarius. Areas that contain only NGF-R immunoreactive fibers and terminals can be distinguished from the cellular sites of NGF-R biosynthesis and include the suprachiasmatic nucleus, the principal olivary pretectal nucleus, the superior colliculus, the inferior olive, and the principal and spinal trigeminal nuclei. This study shows that NGF-R is widely expressed within individual neurons in different areas of the rat brain and identifies new potential CNS target sites of endogenous NGF.

Expression of NGF receptor in the rat forebrain detected with in situ hybridization and immunohistochemistry

The expression of nerve growth factor (NGF) receptor mRNA and NGF receptor protein was examined in the adult rat basal forebrain using in situ hybridization and immunohistochemical techniques. NGF receptor mRNA and protein were detected within cells in the medial septum, diagonal band of Broca, and nucleus basalis of Meynert. Controls showed that the hybridization signal was not due to nonspecific binding of the probe to heterologous RNAs or other molecules. As expected, the distribution of NGF receptor mRNA-containing cells correlated nicely with the distribution of NGF receptor immunoreactive cells in each of these areas. These data extend previous work which suggests that neurons in these areas express the NGF receptor mRNA and manufacture functional NGF receptors. NGF receptor immunoreactivity was also detected in the arcuate nucleus of the hypothalamus, in the leptomeninges at the base of the brain and overlying the tectum, and within ependymal regions along the lateral walls of the cerebral ventricles. A few weakly stained neurons in the lateral hypothalamus and ventrolateral striatum were also consistently observed. In contrast, NGF receptor mRNA was not detected within any meningial, ependymal, or hypothalamic tissues using in situ hybridization. A cross-linking/immunoprecipitation assay demonstrated normal, membrane-bound NGF receptors within extracts of dorsal superior colliculus, ventromedial hypothalamic, and overlying meningial tissues, proving that the staining observed in these areas was not a non-specific artifact associated with the immunohistochemistry. The lack of hybridization in these areas may reflect levels of NGF receptor mRNA which are too low to be detected by the in situ hybridization methods being used. Alternatively, the staining may represent innervation of these areas by afferents whose cell bodies are located elsewhere, and whose terminals contain the NGF receptor protein.

The olfactory system and Alzheimer's disease

Alzheimer's disease (AD) is considered to be the number one health problem and seems to be reaching epidemic proportion in the USA. The cause of AD is not known, a reliable animal model of the disease has not been found and appropriate treatment of this dementia is wanting. The present review focuses on the possibility that a virus or exogenous toxic materials may gain access to the CNS using the olfactory mucosa as a portal of entry. Anterograde and retrograde transport of the virus/zeolites to olfactory forebrain regions, which receive primary and secondary projections from the main olfactory bulb (MOB) and which, in turn, project centrifugal axons to the MOB, may initiate cell degeneration at such loci. Pathological changes may, thus, be initially confined to projecting and intrinsic neurons localized in cortical and subcortical olfactory structures; arguments are advanced which favor the view that excitotoxic phenomena could be mainly responsible for the overall degenerative picture. Neurotoxic activity may follow infection by the virus itself, be facilitated by loss of GABAergic terminals in olfactory cortex, develop following repeated episodes of physiological long term potentiation (which unmasks NMDA receptors) or be due to excessive release, faculty re-uptake or altered glutamate receptor sensitivity. Furthermore, a reduction in central inhibitory inputs to the MOB might then result in disinhibition of mitral/tufted neurons and enhance the excitotoxic phenomena in the MOB projecting field. Within this context, and in line with recent studies, it is believed that pathology begins at cortical (mainly olfactory) regions, basal forebrain neurons being secondarily affected due to retrograde degeneration. In addition, failure to produce a critical level of neurotrophic factors by a damaged MOB and olfactory cortex, could adversely affect survival of basal cholinergic neurons which innervate both regions. Support for these hypothesis is provided, first, by recent reports on pathological findings in AD brains which seem to involve preferentially the olfactory and entorhinal cortices, the olfactory amygdala and the hippocampus, all of which receive primary or secondary projections from the MOB; secondly, by the presence of severe olfactory deficits in the early stages of the disease, mainly of a discriminatory nature, which points to a malfunction of central olfactory structures.

Distribution of beta-nerve growth factor receptors in the human basal forebrain

The distribution of neurons expressing the receptor for beta-nerve growth factor has been examined immunohistochemically in serial coronal sections of basal forebrain from aged normal human subjects. Neurons expressing the receptor were observed in the nucleus of the diagonal band of Broca and in the anterior, the intermediate, and the posterior portions of the nucleus basalis of Meynert. Neurons could also be seen in the medial septal nucleus and embedded in myelinated fibre tracts such as those of the external capsule, cingulum, medullary laminae of the globus pallidus, ansa penduncularis, ansa lenticularis, and anterior commissure. In situ hybridization with a 35S cDNA probe to the human beta-nerve growth factor receptor confirms a neuronal location as the site of synthesis of beta-nerve growth factor receptors in the nucleus basalis of Meynert in a fifth brain. A high percentage of Nissl-stained hyperchromic magnocellular neurons expressed the receptor for beta-nerve growth factor, suggesting that most neurons in the human cholinergic magnocellular basal forebrain system express these receptors. Recent data suggest that beta-nerve growth factor functions as a neurotrophic factor in basal forebrain cholinergic neurons. In Alzheimer's disease there is known to be a reduction in cholinergic function and an apparent loss of neurons in the cholinergic nucleus basalis of Meynert. For this reason we have examined the distribution of receptors for beta-nerve growth factor in the normal human basal forebrain in order to form a basis for comparison to those with Alzheimer's disease.

Nerve growth factor mRNA and protein increase in hypothalamus in a mouse model of aggression

The effects of intermale aggressive behavior induced by social isolation on the level of nerve growth factor (NGF) mRNA and protein were investigated in central and peripheral mouse tissues. A large increase in NGF mRNA and protein was observed in hypothalamus, with no changes in cerebral cortex, hippocampus, and cerebellum. No change in NGF mRNA levels was found in heart, spleen, vas deferens, and submaxillary salivary gland. The cellular localization of NGF mRNA in the central nervous system was investigated by in situ hybridization. Numerous nerve cells were specifically labeled in preoptic and ventrolateral nuclei of the hypothalamus, as well as in the cornu ammonis region of the hippocampus and throughout all layers of the cerebral cortex, with the highest concentration in layer III. The present results firmly establish that nerve cells constitute the major source in NGF in the brain. They also open the way to understanding the regulation of NGF biosynthesis in the central nervous system.