Nerve growth factor mRNA in brain: localization by in situ hybridization

Kainic acid-induced seizures stimulate increased expression of nerve growth factor mRNA in rat hippocampus

The influence of kainic acid (KA)-induced limbic seizure activity on the expression of mRNA for nerve growth factor (NGF) in adult rat brain was studied using in situ hybridization and S1 nuclease protection techniques with RNA probes complementary to murine and rat NGF mRNA. Within hippocampus, intracerebroventricular injection of 0.5 microgram KA caused a dramatic bilateral increase in hybridization of the 35S-labeled cRNA within stratum granulosum. This increase was first evident 1 h post-KA, appeared maximal at approximately 20-fold control levels at 2-3 h post-injection, and declined to control levels by 48 h post-injection. During the period of maximal hybridization, all but the deepest cells within stratum granulosum appeared to be autoradiographically labeled. Hybridization of the NGF cRNA probe was also increased within superficial layers of piriform and entorhinal cortex and, to much lesser extent, within scattered neurons of layers II and III of neocortex in KA-treated rats. In olfactory cortical areas, hybridization was maximally elevated 15.5-24.5 h after KA injection. In contrast to these effects, KA treatment did not consistently influence the density of hybridization, or number of neurons labeled, within the dentate gyrus hilus or the hippocampus proper (CA1-CA3). In agreement with the in situ hybridization results, S1 nuclease protection assay detected KA-induced increases in hybridization within pooled dentate gyrus/CA1 samples, but not hippocampal CA3 samples. These data support the conclusion that seizure activity stimulates a transient increase in NGF expression by select populations of forebrain neurons and indicates that experimental seizure paradigms might be further exploited for analyses of the mechanisms of NGF regulation and processing in the adult brain.

Effect of recombinant human nerve growth factor on presynaptic cholinergic function in rat hippocampal slices following partial septohippocampal lesions: measures of [3H]acetylcholine synthesis, [3H]acetylcholine release and choline acetyltransferase activity

To determine whether intraventricular administration of nerve growth factor alters presynaptic cholinergic function in the intact hippocampus or following partial lesions of the fimbria, we investigated the effects of recombinant human nerve growth factor treatment on [3H]acetylcholine synthesis and release by hippocampal slices following various treatment regimens. For chronic nerve growth factor treatment, 1 microgram of recombinant human nerve growth factor was injected intraventricularly every second day. Lesions reduced [3H]acetylcholine synthesis (by 48%) and spontaneous and evoked [3H]acetylcholine release by 35 and 61%, respectively. Chronic nerve growth factor treatment over three weeks elevated [3H]acetylcholine synthesis (by 39%) and spontaneous and evoked [3H]acetylcholine release by 27 and 64%, respectively, over values in lesioned hippocampi of animals treated with a control protein (cytochrome c). The nerve growth factor-induced enhancement of presynaptic cholinergic function persisted for three weeks following the termination of nerve growth factor administration. Furthermore, chronic (nine-week) treatment with nerve growth factor increased [3H]acetylcholine by 118% over values in lesioned hippocampi of animals treated with cytochrome c. These findings indicate that chronic treatment with recombinant human nerve growth factor increases the capacity of hippocampal cholinergic neurons surviving a partial fimbrial transection to synthesize, store and release acetylcholine. Application of recombinant human nerve growth factor during the initial weeks after lesioning was necessary to product significant elevations in acetylcholine synthesis, since chronic recombinant human nerve growth factor treatment after delays of three or more weeks were ineffective. Furthermore, chronic nerve growth factor treatment failed to stimulate acetylcholine synthesis and release in intact hippocampal cholinergic systems. Single intraventricular injections of recombinant human nerve growth factor at the time of lesioning resulted in a small decrease in acetylcholine synthesis which, however, was not accompanied by a change in the rate of evoked acetylcholine release from cholinergic neurons surviving the lesion. The study indicates that chronic or repeated administration of nerve growth factor during the onset of degenerative events is necessary for the stimulation of presynaptic cholinergic function in the hippocampus of adult rats with partial fimbrial transections.

Nerve growth factor potentiates the neurotoxicity of beta amyloid

The role of growth factors in the pathogenesis of Alzheimer disease is unknown. The beta-amyloid protein accumulates abnormally in the brain in Alzheimer disease and is neurotoxic to differentiated hippocampal neurons in culture. Nerve growth factor (NGF) increased the neurotoxic potency of a beta-amyloid polypeptide by a factor of approximately 100,000, which resulted in a reduction of the beta-amyloid neurotoxic EC50 from 0.1 microM to 1 pM. This potentiating effect of NGF was reversed by a monoclonal antibody against NGF and was not observed for a variety of other neurotrophic growth factors. Exposure of hippocampal neurons to very low concentrations of beta amyloid alone resulted in a marked induction of immunoreactive NGF receptors. Addition of NGF with beta amyloid resulted in the appearance of neurodegenerative changes in NGF receptor-positive neurons. The early and profound degeneration of hippocampal and basal forebrain cholinergic neurons that occurs in Alzheimer disease may result from a neurotoxic interaction of beta amyloid with NGF.

Activity dependent regulation of BDNF and NGF mRNAs in the rat hippocampus is mediated by non-NMDA glutamate receptors

The mRNAs of nerve growth factor (NGF) and brain derived neurotrophic factor (BDNF) exhibit a similar, though not identical, regional and cellular distribution in the rodent brain. In situ hybridization experiments have shown that BDNF, like NGF, is predominantly expressed by neurons. The neuronal localization of the mRNAs of these two neurotrophic molecules raised the question as to whether neuronal activity might be involved in the regulation of their synthesis. After we had demonstrated that depolarization with high potassium (50 mM) resulted in an increase in the levels of both BDNF and NGF mRNAs in cultures of hippocampal neurons, we investigated the effect of a large number of transmitter substances. Kainic acid, a glutamate receptor agonist, was by far the most effective in increasing BDNF and NGF mRNA levels in the neurons, but neither N-methyl-D-aspartic acid (NMDA) nor inhibitors of the NMDA glutamate receptors had any effect. However, the kainic acid mediated increase was blocked by antagonists of non-NMDA receptors. Kainic acid also elevated levels of BDNF and NGF mRNAs in rat hippocampus and cortex in vivo. These results suggest that the synthesis of these two neurotrophic factors in the brain is regulated by neuronal activity via non-NMDA glutamate receptors.

Regional and cellular codistribution of interleukin 1 beta and nerve growth factor mRNA in the adult rat brain: possible relationship to the regulation of nerve growth factor synthesis

We have found a regional distribution of IL 1 beta mRNA and IL 1 activity in the normal adult rat brain, which reveals at least partially a colocalization with nerve growth factor (NGF). The predominantly neuronal signal patterns were found over the granule cells of the dentate gyrus, the pyramidal cells of the hippocampus, the granule cells of the cerebellum, the granule and periglomerular cells of the olfactory bulb, and over dispersed cells of the ventromedial hypothalamus and of the frontal cortex. In these areas also the highest levels of IL 1 activity were observed. In the striatum and septum much lower levels of IL 1 beta mRNA and IL 1 activity (shown for the striatum), most likely synthesized by glial cells, could be determined. IL 1 beta-expressing cells were mainly found in brain regions that also synthesize NGF mRNA as shown by in situ hybridization. NGF mRNA could be demonstrated over pyramidal cells of the hippocampus, granule cells of the dentate gyrus, periglomerular cells of the olfactory bulb and over prefrontal cortex neurons. These data indicate that IL 1 beta, among other factors, might also play a regulatory role in the synthesis of NGF in the CNS, as has been demonstrated in the peripheral nervous system (Lindholm, D., R. Heumann, M. Meyer, and H. Thoenen. 1987. Nature (Lond.). 330:658-659).

Long-term administration of mouse nerve growth factor to adult rats with partial lesions of the cholinergic septohippocampal pathway

Nerve growth factor (NGF), a neurotrophic factor acting on cholinergic neurons of the basal forebrain, has been proposed as a treatment for Alzheimer's disease. Experimental support for its pharmacological use is derived from short-term studies showing that intraventricular administration of NGF during 2-4 weeks protects cholinergic cell bodies from lesion-induced degeneration, stimulates synthesis of choline acetyltransferase, and improves various behavioral impairments. To investigate the consequences of long-term NGF administration, we tested whether cholinergic cell bodies are protected from lesion-induced degeneration and whether cholinergic axons are stimulated to regrow into the denervated hippocampus following fimbrial transections. We found that intraventricular injections of NGF twice a week for 5 months to adult rats resulted in extended protection of cholinergic cell bodies from lesion-induced degeneration and did not produce obvious detrimental effects on the animals. NGF treatment mildly stimulated growth of cholinergic neurites within the 2-mm area directly adjacent to the fimbrial lesion but it failed to induce significant homotypic growth of cholinergic neurites into the deafferented hippocampus.

Beta-adrenergic receptor regulation, through cyclic AMP, of nerve growth factor expression in rat cortical and cerebellar astrocytes

1. Type 1 astrocytes prepared from 3-day rat cortex and cerebellum express the 1.3-kb nerve growth factor (NGF) mRNA and synthesize and release beta-NGF. 2. Isoproterenol (IP), a beta-adrenergic agonist, stimulates NGF mRNA content in cortical astrocytes; this increase is blocked by the beta-adrenergic antagonist propranolol but not the alpha-antagonist phenoxybenzamine. The EC50 for the effect of IP is 5 nM. 3. IP increases astrocyte cyclic AMP as does forskolin, which directly activates adenylate cyclase and also increases NGF mRNA content. Cerebellar astrocytes contain about one-third as much NGF mRNA, which can also be increased by forskolin and cyclic AMP. 4. These results suggest that CNS astrocytes can serve as a source of NGF and that the NGF gene is one of the class of cyclic AMP regulated genes.

Induction of nerve growth factor gene expression by 12-O-tetradecanoyl phorbol 13-acetate

Nerve injury leads to activation of fibroblasts, including stimulation of nerve growth factor (NGF) gene expression. Although interleukin-1 has been implicated as a mediator of NGF gene induction, the underlying mechanisms are not known. We investigated whether 12-O-tetradecanoyl phorbol 13-acetate (TPA), also a known stimulator of protein kinase C, regulates NGF gene expression. We show here that TPA stimulates NGF mRNA in mouse kidney and L929 fibroblasts but not in dispersed salivary cells. NGF mRNA stimulation in L929 cells is delayed by 2 h, is transient, and is followed by a parallel increase in NGF secretion. The induction of NGF mRNA is inhibited by cycloheximide, NGF mRNA levels decrease to similar values after 4 h of incubation with actinomycin D alone or in combination with TPA. These results indicate that the TPA response is cell specific and suggest that it is mediated at the transcriptional level via newly synthesized protein.

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.

Neuronal localization of the nerve growth factor precursor-like immunoreactivity in the rat brain

The distribution of the nerve growth factor precursor(proNGF)-like immunoreactivity was examined in the adult rat brain with affinity-purified antisera directed against synthetic peptides that reproduce sequences of the precursor protein. Immunoreactivity was localized in defined areas of the neocortex, hippocampal formation, thalamus/hypothalamus, striatum, olfactory bulb, pons and spinal cord, which are regions previously reported to contain NGF mRNA. Interestingly, immunoreactivity was also observed in the septum and diagonal band of Broca known to contain very low NGF mRNA levels. Using immunohistochemical methods combined with the retrograde transport of a protein gold complex we demonstrate that proNGF-like immunoreactivity is localized within neuronal cell bodies, in the cortex, hippocampus and septum. These results suggest that the immunological approach may not only identify NGF-synthesizing cells, but also cells which may accumulate proNGF or some cleavage products by an uptake mechanism.

Growth factors and growth factor receptors in the hippocampus. Role in plasticity and response to injury

Various growth factors are present in the hippocampal formation and appear responsible for the prominent plasticity of this brain area. Although hormone-like growth-promoting polypeptides are the best known, recent studies emphasize the importance in the growth response of molecules such as laminin proteoglycans, neurotransmitters and growth inhibitors. The progress and problems in the study of these substances are reviewed.

Regulation of Nerve Growth Factor (NGF) Synthesis in the Rat Central Nervous System: Comparison between the Effects of Interleukin-1 and Various Growth Factors in Astrocyte Cultures and in vivo

In order to obtain information on the physiological regulation of NGF-synthesis in the central nervous system (CNS) we investigated the effects of a series of growth factors (known to be present in the CNS) in cultures of purified rat astrocytes and compared these effects with those observed after intraventricular injection of the same molecules. After preliminary experiments had shown that 10% fetal calf serum (FCS) produced a marked increase in NGF-mRNA levels in astrocytes (but neither in microglia nor oligodendrocytes) as demonstrated by Northern blot analysis and in situ hybridization the experiments were performed at low (0.5%) FCS concentrations. Supramaximal concentrations of IL-1 and various growth factors caused a 5- to 7-fold increase in NGF-mRNA after 6 h. By 24 h the NGF-mRNA levels approached control values again, most probably due to inactivation of the added factors since after readdition after 24 h the response was about the same as the initial one. Norepinephrine and 8-bromo-cAMP did not change NGF-mRNA levels. The growth factor-mediated changes in NGF-mRNA levels in astrocyte cultures were not consistently reflected by the changes observed after intraventricular injection. IL-1 produced by far the largest increase in hippocampal NGF-mRNA after intraventricular injection. This large response to IL-1 could result from a positive feedback mechanism, since IL-1beta injection not only increases NGF-mRNA but also IL-1beta-mRNA in the hippocampus. The understanding of the physiological regulation of NGF synthesis in the CNS is the basis for a rational approach to its pharmacological modification. This, in turn, is an attractive alternative to the (long-term) infusion of NGF or the transplantation of NGF-secreting cells with the goal of providing trophic support to the cholinergic neurons of the basal forebrain nuclei. These neurons are consistently affected in the early stages of Alzheimer's disease, their impaired function being essentially responsible for the cognitive deficits.

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.

Secretion of nerve growth factor in cultures of glial cells and neurons derived from different regions of the mouse brain

The regional ability of central neurons and glial cells to produce nerve growth factor (NGF) was studied in vitro. NGF secretion was compared in cultures of perinatal astrocytes or embryonic neurons that were derived from various mouse brain structures. No regional differences were detected among cultures of post-natal day 2 glial cells of hippocampal, cortical, striatal, or mesencephalic origin. In all cases, levels of NGF released by the cells were very similar. They were closely correlated to the growth rate as shown by the fact that exponentially growing cells produced relatively more factor than did confluent cells, a finding in agreement with previous observations. Unlike growth-phase cells, primary astrocytes immediately plated at high cell density did not secrete any assayable factor before the 7th day of culture. Levels of NGF found during the following days remained low. In contrast, striking differences were observed among cultures of embryonic neurons. NGF was found in relatively large amounts in cultures of embryonic day 17 or 19 striatal neurons, whereas media conditioned by neurons from the mesencephalon, cortex, or septum contained much less factor. Amounts of NGF assayed in cultures of hippocampal neurons varied with the time of sampling of this brain structure. Levels of factor were significantly higher in media conditioned by embryonic day 19 neurons than in media of embryonic day 17 neurons. However, amounts of NGF found in supernatants of hippocampal neurons remained smaller than those present in cultures of striatal nerve cells. Altogether, the results suggest that, in addition to astrocytes, central neurons may also synthesize and secrete NGF in vitro and that this phenomenum is dependent on both the origin and the developmental stage of the neuronal population.

Function of neurotrophic factors in the adult and aging brain and their possible use in the treatment of neurodegenerative diseases

This review summarizes the current knowledge of characterized neurotrophic factors, including nerve growth factor (NGF) which serves as paradigmatic example when studying novel molecules. Special consideration is given to the function of neurotrophic factors in the adult and aging brain. Strategies are discussed for the eventual development of pharmacological applications of these molecules in the treatment of neurodegenerative diseases.

Limbic seizures increase neuronal production of messenger RNA for nerve growth factor

Nerve growth factor (NGF) produced by telencephalic neurons provides critical trophic support for cholinergic neurons of the basal forebrain. In situ hybridization and nuclease protection analyses demonstrate that limbic seizures dramatically increase the amount of messenger RNA for NGF in the neurons of the hippocampal dentate gyrus within 1 hour of seizure onset and in broadly distributed neocortical and olfactory forebrain neurons some hours later. The increased messenger RNA species is indistinguishable from messenger RNA for transcript B of the beta subunit of NGF from mouse submandibular gland. Thus, the expression of a known growth factor is affected by unusual physiological activity, suggesting one route through which trophic interactions between neurons in adult brain can be modified.

NGF receptor increase in the olfactory bulb of the rat after early odor deprivation

In the olfactory bulb of normal rats, nerve growth factor (NGF) receptor (NGFR) immunoreactivity was largely confined to the glomerular layer. Unilateral closure of the nostril at postnatal day 2 (P2) increased NGFR immunoreactivity in the sealed bulb at both 19 and 60 days after the operation. The increase in NGFR density, measured by autoradiographic immunohistochemistry, was most dramatic 60 days postocclusion. These findings suggest that a compensatory increase in NGFRs may play a role in the maintenance of bulbar function after the early loss of sensory stimulation.

Intraventricular NGF infusion in the mature rat brain enhances sympathetic innervation of cerebrovascular targets but fails to elicit sympathetic ingrowth

The ability of peripheral axons to regenerate long distances in the peripheral nervous system (PNS) is well documented; however, examples of axonal elongation within the adult mammalian central nervous system (CNS) are rare. One example of axonal growth in the mature brain is the sprouting of sympathetic axons into the hippocampal formation following disruption of the septohippocampal pathway. A current hypothesis is that elevated hippocampal NGF levels, secondary to loss of retrograde transport by septal neurons, elicits sympathetic ingrowth, In this study, we sought to determine whether elevation of hippocampal NGF activity without septal denervation is sufficient to elicit sympathetic sprouting. Forty-one female rats were infused for two weeks with NGF or cytochrome C in the right lateral ventricle through cannulae connected to an osmotic minipump. In some animals the brains were sectioned and stained for acetylcholinesterase (AChE) activity and norepinephrine histofluorescence; in others, CNS tissue was assayed for nerve growth factor (NGF) content with a two-site ELISA. A Farrand microspectrophotometer was used to measure the intensity of catecholamine fluorescence around the internal carotid artery. The average fluorescence intensity of the sympathetic innervation of the internal carotid artery in the NGF-injected animals was over twice that of vehicle-injected rats indicating that the infused NGF was both accessible to the sympathetic axons and biologically active. However, in none of the cases with elevated hippocampal NGF levels were sympathetic axons observed within the hippocampal formation or any other brain region. These results suggest that simple elevation of brain NGF, while perhaps necessary, is insufficient to permit the growth of sympathetic axons into the mature mammalian CNS.

Cell-specific and developmental regulation of a nerve growth factor-human growth hormone fusion gene in transgenic mice

We recently showed that a nerve growth factor-human growth hormone (NGF-hGH) fusion gene containing the promoter and 750 bp of 5' flanking region is transcriptionally active in the NGF-secreting L929 mouse fibroblast cell line. For the present experiments, we extended the 5' flank by 5 kb and constructed transgenic mice. These mice began to secrete hGH into saliva at puberty. hGH was detected immunocytochemically in the granulated convoluted tubular cells of the submandibular gland (SMG). SMG levels of hGH mRNA were 10-fold higher in adult males than in females. hGH mRNA was very abundant in SMG, moderately abundant in heart, brain, and kidney, rare in skin and adrenal gland, and undetectable in lung, liver, and spleen. Thus, the NGF-hGH gene reflects NGF gene expression. We conclude that basal NGF gene transcription is regulated by sequences in the cloned NGF gene fragment.

Sustained elevation in hippocampal NGF-like biological activity following medial septal lesions in the rat

Several laboratories have documented an increase in hippocampal nerve growth factor (NGF) levels, measured with biological or immunological assays, within 1-2 weeks following septal lesions or fimbria/fornix transections. In the present study we have determined the increase in NGF-like biological activity in medium conditioned by hippocampal slices at more prolonged times following medial septal lesion. In contrast to reports based on immunological assays, which demonstrate a transient increase in hippocampal NGF, elevated NGF-like biological activity was present in hippocampal slice-conditioned medium up to one year after a medial septal lesion.

Nerve growth factor increases in adult rat brain after hypoxic injury

Brain injury increases the synthesis of many growth and trophic factors. We have measured nerve growth factor (NGF) content in brain of adult rats with bilateral carotid artery occlusion exposed to 7% O2. Five days after hypoxia the NGF content was increased in neostriatum, hippocampus and cerebral neocortex, areas exhibiting focal neuronal degeneration and inflammation. The increase was not related to changes in choline acetyltransferase activity that was only reduced in heavily damaged cortex. The findings may indicate that the increase in NGF content was due to enhanced NGF synthesis by inflammatory cells including stimulated glia.

Production and characterization of biologically active recombinant human nerve growth factor

Nerve growth factor (NGF) is required for the differentiation and maintenance of sympathetic and sensory neurons. In animal models, NGF prevents the death of septal and basal forebrain cholinergic neurons deprived of endogenous NGF, suggesting that NGF may be of benefit in neurodegenerative diseases of humans. However, little is known about NGF in human brain, partly because a sensitive assay for hNGF has been lacking. As a first step toward developing the tools for the study of NGF in humans, recombinant human NGF (rhNGF) was produced by expressing exon 4 of the human NGF gene in COS cells. The expression vector is driven by the adenovirus major late promoter and contains an SV40 origin of replication. NGF was secreted by transiently transfected cells. Conditioned medium was assayed with an enzyme immunoassay (EIA) that utilizes a monoclonal antibody (clone 27/21) against mouse beta-NGF, and contained 15 ng/ml of rhNGF. The rhNGF migrated as a dimer of 26-29 Kd on a gel permeation chromatography column, and stimulated neurite outgrowth and neuropeptide Y mRNA levels in PC12 cells. With optimization, the described expression system is capable of providing sufficient hNGF for research and therapeutic purposes.

NGF in CNS: experimental data and clinical implications

The presence of beta-nerve growth factor (NGF) and its cell surface receptor (NGF-R) in the brain has been well established by a variety of experimental techniques in recent years. In particular, the molecular cloning of NGF and NGF-R as well as the development of sensitive two-site ELISA techniques for determining the levels of NGF and antibodies to NGF-R suitable for immunohistochemistry have led to rapid accumulation of data in this field from many laboratories. A main finding is the function of NGF in the cholinergic neurons of the basal forebrain, expressing NGF receptors and responding to the factor by increased activity of choline acetyltransferase, and the production of NGF in cortical areas and hippocampus comprising terminal areas for the cholinergic projections from the basal forebrain. In addition, findings suggest that additional neurons in the brain and spinal cord may utilize NGF, notably during development and possibly also after lesion of the adult CNS. Moreover, observations indicate that endogenous levels of NGF are lowered in the aged rat brain concomitant with losses of NGF-dependent neurons in the basal forebrain. The involvement of NGF in human neurodegenerative diseases is not established but the application of NGF to degenerating cholinergic neurons in Alzheimer patients may prove useful. A promising approach to achieve this goal is the production of biologically active, recombinant NGF.

Differential expression of the nerve growth factor receptor gene in multiple brain areas

Recent work has indicated that the trophic protein, nerve growth factor (NGF), is detectable in several brain regions, in addition to its well-known localization to the periphery. In addition, a number of cholinergic populations in the brain respond to NGF by increasing enzymes involved in acetylcholine metabolism. It is well recognized that responsiveness to NGF is dependent on expression of specific receptors; we have recently detected expression by the responsive rat basal forebrain/septal, cholinergic neurons, suggesting that NGF plays a physiologic role in the development of this brain pathway. To define a potential role for NGF in other rat brain regions, we isolated a rat receptor cDNA clone to use as a probe to detect receptor message by sensitive Sl nuclease protection experiments. Our studies indicate that the NGF receptor (NGF-R) gene is expressed by anatomically, functionally and biochemically diverse populations, widely distributed in the rat brain, and is not restricted to the basal forebrain/septal region. We detect NGF receptor message in frontal cortex, hippocampus, caudate, cerebellum and olfactory bulb. Moreover, developmental profiles of steady-state quantities varied differently for each area. Our observations support the contention that NGF regulates multiple brain systems, in addition to forebrain cholinergic pathways.

Electron microscopic immunocytochemical localization of nerve growth factor in developing mouse olfactory neurons

The immunocytochemical localization of nerve growth factor (NGF) in the embryonic mouse has been examined using correlative light and electron microscope procedures. In flat-embedded thick sections, primary sensory neurons of the olfactory pathway were clearly visible by their high level of NGF-like immunoreactivity. When ultrathin sections were examined under the electron microscope, the HRP reaction product present in the nasal epithelium was confined to the cytoplasm of the chemoreceptor cells, including the dendrites projecting into the nasal cavity. The axons projecting to the olfactory bulb and axon bundles within the bulb were also stained specifically for NGF. The reaction product did not appear to be associated with any particular subcellular structure. Neither the columnar supporting cells nor the basal precursor cells showed any evidence of immunoreactivity. No evidence was found for the presence of NGF in cells such as epithelial or glial cells within the immediate environment of the receptor neurons. These first subcellular localization studies indicate the presence of high levels of NGF in neurons not known to be sensitive to the trophic molecule.

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

A rat beta-nerve growth factor (NGF) genomic sequence encoding the entire 3' exon of preproNGF was cloned, and its nucleotide sequence was determined. Rat NGF shows very high homology with other known NGFs in both the prepropeptide and the 3' untranslated regions. The presumptive signal sequence, the cysteine residues important for tertiary structure, possible glycosylation sites, and dibasic amino acids required for proteolytic cleavage to mature NGF are conserved across species. Comparison of the hydrophobicity plots and amino acid sequences revealed an evolutionary divergent domain on the external surface of NGF, which may account for the poor immunologic crossreactivities of the various NGFs. In situ hybridization with a rat-specific oligodeoxynucleotide indicated high levels of NGF mRNA synthesis in both hippocampal granule and pyramidal cell layers. These results are consistent with one role for NGF in the CNS as a neuronally released, retrogradely transported neurotrophic factor for basal forebrain cholinergic neurons.

Localization of high-affinity and low-affinity nerve growth factor receptors in cultured rat basal forebrain

Previous work has indicated that nerve growth factor specifically and selectively increases choline acetyltransferase and acetylcholinesterase in organotypic cultures of rat basal forebrain-medial septal area. To determine whether these actions are potentially receptor-mediated, organotypic and dissociated basal forebrain-medial septal area cultures were examined. Two independent methods, [125I]nerve growth factor binding and immunocytochemistry with a monoclonal nerve growth factor receptor antibody (192-IgG), detected specific receptors. The nerve growth factor receptors were localized to two different cellular populations: flat, large, non-neuron-like cells, and small, round, process-bearing, neuron-like cells. Dissociation studies with [125I]nerve growth factor suggested that high-affinity receptors were localized to the neuron-like population, while only low-affinity receptors were localized to the non-neuron-like cells. We tentatively conclude that nerve growth factor may elicit cholinergic effects by directly binding to high-affinity receptors on neurons. To begin examining receptor regulation, cultures were exposed to exogenous, unlabeled nerve growth factor continuously for 10 days before binding studies were performed. Prior exposure to nerve growth factor did not alter binding characteristics of the receptor, using the present methods.

Expression of the beta-nerve growth factor gene in hippocampal neurons

In situ hybridization with complementary DNA probes for nerve growth factor (NGF) was used to identify cells containing NGF messenger RNA in rat and mouse brain. The most intense labeling occurred in hippocampus, where hybridizing neurons were found in the dentate gyrus and the pyramidal cell layer. The neuronal identity of NGF mRNA-containing cells was further assessed by a loss of NGF-hybridizing mRNA in hippocampal areas where neurons had been destroyed by kainic acid or colchicine. RNA blot analysis also revealed a considerable decrease in the level of NGF mRNA in rat dentate gyrus after a lesion was produced by colchicine. This lesion also caused a decrease in the level of Thy-1 mRNA and an increase in the level of glial fibrillary acidic protein mRNA. Neuronal death was thus associated with the disappearance of NGF mRNA. These results suggest a synthesis of NGF by neurons in the brain and imply that, in hippocampus, NGF influences NGF-sensitive neurons through neuron-to-neuron interactions.

Visualization of growth factor receptor sites in rat forebrain

It is now known that various growth factors may also act in the central nervous system. Among them, it has recently been shown that epidermal growth factor (EGF) and insulin-like growth factor I (IGF-I) may possess trophic effects in the mammalian brain. We report here on the respective autoradiographic distribution of [125I]EGF and [125I]IGF-I receptor binding sites in the rat brain, both during ontogeny and in adulthood. It appears that [125I]EGF sites are mostly found in the rat forebrain during brain development. On the other hand, [125I]IGF-I sites are more widely distributed both during ontogeny and in adulthood. These results reveal the plasticity of the expression of EGF and IGF-I receptor sites in the mammalian brain. This could be relevant for the respective role of these two growth factors in the development and maintenance of neuronal function.

NGF receptor immunoreactivity in rat brain: topographic distribution and response to entorhinal ablation

Nerve growth factor receptor (NGFR) immunoreactivity was distributed in neuronal cell bodies, axons and/or dendrites of cholinergic and non-cholinergic structures of the rat brain. The dentate gyrus of normal animals showed immunostained fibers mainly in the supragranular band, whereas following entorhinal ablation a greater density of immunostained fibers appeared in the outer molecular layer and a wider clear zone was apparent in the inner molecular layer. The NGFR-positive fibers followed the pattern of reorganization of septal fibers visualized with acetylcholinesterase staining. The results suggest that the NGF/NGFR may have a role in reorganization of septal fibers following entohinal lesion.

Immunohistochemical evidence for the distribution of nerve growth factor in the embryonic mouse

The distribution of nerve growth factor-like immunoreactivity has been examined in the embryonic mouse with special reference to the CNS. The intensity of the immunoreactive stain was found to be greatest on embryonic days 15 and 16. The antigen is widespread and present in high concentrations in both the PNS and CNS. Most intense staining was detected in cranial nerve tracts, hippocampus, developing white matter of the spinal cord and tegmentum. Lower intensities were found within diencephalic regions, spinal cord grey matter, medullary fibre tracts and cerebellum. These results support the increasing evidence suggesting that the trophic molecule nerve growth factor has an important role to play in the development of central as well as peripheral neurons.

Sympathetic sprouting in the central nervous system: a model for studies of axonal growth in the mature mammalian brain

Sympathetic fibers innervate many peripheral tissues but are normally confined to extracerebral structures within the cranial cavity, e.g. blood vessels. The invasion of the central nervous system by vascular sympathetic axons is a unique example of neuronal plasticity which provides new information concerning the regulation and mechanisms of neuronal sprouting in both the peripheral and central nervous systems. In this paper, the principal findings concerning the conditions under which such sprouting occurs, the mechanisms which may be involved, and the question of its possible function are reviewed. Of special interest is the fact that a nerve growth factor-like brain factor may be involved in this growth response. The principles gleaned from studies of this sprouting phenomenon may be applicable to other models of neuronal plasticity and may have clinical relevance.