Evidence for a physiological role of nerve growth factor in the central nervous system of neonatal rats

Forebrain cholinergic neurons have been shown to respond in vivo to administration of nerve growth factor (NGF) with a prominent and selective increase of choline acetyltransferase (ChAT) activity. This has suggested that NGF can act as a trophic factor for these neurons. To test this hypothesis directly, anti-NGF antibodies (and their Fab fragments) were intracerebroventricularly injected into neonatal rats to neutralize endogenously occurring NGF. The anti-NGF antibody administration produced a decrease of ChAT activity in the hippocampus, septal area, cortex, and striatum of rat pups. This finding was substantiated by a concomitant decrease of immunopositive staining for ChAT in the septal area. These effects indicate that the occurrence of endogenous NGF in the CNS is physiologically relevant for regulating the function of forebrain cholinergic neurons.     

Hippocampal Membranes Contain a Neurotrophic Activity That Stimulates Cholinergic Properties of Fetal Rat Septal Neurons Cultured Under Serum-Free Conditions

Primary cultures of fetal rat septal neurons were used to identify a membrane-associated cholinergic neurotrophic activity. Under serum-free culture conditions, approximately 98% of the septal cells are neurons, and approximately 6% of the neurons are cholinergic as determined immunocytochemically. Crude membranes prepared from rat hippocampal homogenates stimulate choline acetyltransferase (ChAT) activity in treated septal neurons. The membrane-associated trophic activity is apparent at lower protein concentrations than activity present in the soluble fraction and is unevenly distributed in various brain regions; it is highest in hippocampus and striatum and negligible in cerebellum. Membrane trophic activity is developmentally regulated, is heat and trypsin sensitive, and increases the rate of expression of ChAT in septal neurons. Upon gel filtration chromatography of a high-salt membrane extract, trophic activity elutes as a broad peak in the 500 kilodalton (kD) molecular mass range. Stimulation of septal neuronal ChAT activity by either crude membranes or partially purified preparations is not inhibited by antibodies against nerve growth factor (NGF), and its maximal activity is additive to maximally active doses of NGF. The results indicate that hippocampal membranes contain cholinergic neurotrophic activity which may be important for the development of septal cholinergic neurons.     

Expression of neuronal-NOS in developing basal forebrain cholinergic neurons: Regulation by NGF

Nerve growth factor (NGF) acts through the receptor tyrosine kinase trkA to serve as a trophic factor for cholinergic neurons in the medial septal nucleus and vertical limb of the diagonal band. We have previously shown that the neuronal isoform of nitric oxide synthase (NOS) is selectively expressed in a large fraction of trkA-expressing cholinergic neurons in these brain regions in the adult rat, and that NGF induces the expression of neuronal-NOS in these cells. Herein, we show that: 1) neuronal-NOS is also localized to these neurons in the developing septum; 2) the expression of neuronal-NOS is regulated in the developing medial septal nucleus and vertical limb of the diagonal band; 3) neuronal-NOS regulation parallels that for other markers of basal forebrain cholinergic neuron differentiation, such as cholineacetyltransferase; and 4) NGF infusion in the postnatal period induces robust increases in neuronal-NOS mRNA and in NOS activity in the basal forebrain. Taken together with earlier findings, our results suggest that neuronal-NOS has a role in the differentiation and mature function of septal cholinergic neurons. Through enhancing neuronal-NOS synthesis, endogenous NGF is likely to regulate NO functions in vivo. Special issue dedicated to Dr. Hans Thoenen.     

Intracellular Storage and Evoked Release of Acetylcholine from Postnatal Rat Basal Forebrain Cholinergic Neurons in Culture with Nerve Growth Factor

Cholinergic neurons from the septum area, the vertical limb of the diagonal band of Broca, and the nucleus basalis of Meynert of postnatal 13-day-old rats were cultured with or without nerve growth factor (NGF) conditions. Total choline acetyltransferase (ChAT) activities, acetylcholine (ACh) contents, and survival numbers of cholinergic neurons in culture from each of three distinct regions were increased by NGF treatment, but little difference was found in cellular ChAT activities and ACh contents obtained in cultures with or without NGF. The result shows that NGF promotes the survival of cholinergic neurons from 13-day-old rats. Furthermore, the release of ACh from cultured neurons was investigated. The cells cultured with NGF showed a larger increase of the high K+-evoked ACh release than those cultured without NGF. However, NGF had no effect on spontaneous release. This suggests that NGF could regenerate and sustain the stimulation-evoked release mechanisms of ACh in cultured cholinergic neurons from postnatal rats.     

Overexpression of tumour necrosis factor alpha in the brain of transgenic mice differentially alters nerve growth factor levels and choline acetyltransferase activity

Tumour necrosis factor alpha (TNF-alpha) is a pleiotrophic cytokine synthesized primarily by macrophages and monocytes, which exerts a variety of biological activities during inflammatory responses, immune reactions, and wound healing. Within the central nervous system (CNS), the basal levels of TNF-alpha are almost undetectable, but increase after neurological insults. Using transgenic mice expressing high levels of TNF-alpha in the CNS, we investigated the effect of this cytokine on the levels of brain nerve growth factor (NGF), a neurotrophin playing a crucial role in the development, maintenance and regeneration of basal forebrain cholinergic neurons. The immunoenzymatic assay and in situ hybridization revealed that the constitutive expression of NGF decreased in the hippocampus, increased in the hypothalamus, while remained unchanged in the cortex. Moreover, septal cholinergic neurons which receive trophic support from NGF produced in the hippocampus display loss of choline acetyltransferase immunoreactivity, suggesting that the reduced availability of NGF may influence negatively the synthesis of brain cholinergic neurons. These observations indicate that the basal level of brain NGF can be influenced negatively or positively by local expression of TNF-alpha and that this cytokine, through dose-dependent regulation of NGF synthesis and release, may be involved in neurodegenerative events associated with aging.     

PACAP and NGF cooperatively enhance choline acetyltransferase activity in postnatal basal forebrain neurons by complementary induction of its different mRNA species

Both nerve growth factor (NGF) and pituitary adenylate cyclase activating polypeptide (PACAP) have neurotrophic effects on basal forebrain cholinergic neurons. They promote differentiation, maturation, and survival of these cholinergic neurons in vivo and in vitro. Here we report on the cooperative effects of NGF and PACAP on postnatal, but not embryonic, cholinergic neurons cultured from rat basal forebrain. Combined treatment with NGF, brain-derived neurotrophic factor (BDNF), neurotrophin-4 (NT-4), and PACAP induced an additive increase in choline acetyltransferase (ChAT) activity. There were no cooperative effects on the number of cholinergic neurons, suggesting that ChAT mRNA expression had been induced in each cholinergic neuron. Further analysis revealed that NGF and PACAP led to complementary induction of different ChAT mRNA species, thus enhancing total ChAT mRNA expression. These results explain the cooperative neurotrophic action of NGF and PACAP on postnatal cholinergic neurons.     

Effect of Nerve Growth Factor in Ethylcholine Mustard Aziridinium (AF64A)-Treated Rats: Sensitization of Cholinergic Enzyme Activity in the Septohippocampal Pathway

Abstract: In this study, we examined the effects of nerve growth factor (NGF) administration on cholinergic enzyme activity in both normal and ethylcholine mustard aziridinium (AF64A)-treated rats. Choline acetyltransferase (ChAT) and acetylcholinesterase activity were measured in the hippocampus and septum of rats chronically administered NGF (0.36–2.85 µg/day) into the lateral ventricle for 14 days. In both normal and AF64A-treated rats, NGF increased cholinergic enzyme activity in a dose-dependent manner. Furthermore, although NGF increased ChAT activity in normal rats by 147%, it had a greater effect in AF64A-treated rats, increasing ChAT activity as much as 273%. NGF increased acetylcholinesterase activity in normal rats by only 125% but produced a 221% increase in this activity in AF64A-treated rats. These data indicate that AF64A produces an increased sensitivity to NGF in cholinergic neurons.     

Developmental changes of nerve growth factor and its mRNA in the rat hippocampus: Comparison with choline acetyltransferase

Previous experiments have demonstrated that in the septo-hippocampal system choline acetyltransferase (ChAT) is induced by nerve growth factor (NGF) (Gnahn et al. (1983) Dev. Brain Res. 9, 45-52) and that hippocampal NGF and mRNANGF levels are correlated with the density of cholinergic innervation (Korsching et al. (1985) EMBO J. 4, 1389-1393). In the present investigation we have compared the developmental changes of ChAT, NGF, and mRNANGF levels in this system. During the postnatal development of the hippocampus the time courses of NGF and ChAT were well correlated including the most rapid increase between P12 and P14. This increase in hippocampal NGF was preceded by a corresponding increase in mRNANGF. The developmental changes in hippocampal NGF levels were also closely reflected by corresponding changes in the septum. This, together with previous observations (Korsching et al., 1985) that the adult septum, in spite of relatively high NGF levels, does not contain measurable quantities of mRNANGF, suggests that the NGF levels in the septum are determined by the quantity of NGF transported retrogradely from the field of innervation rather than by local synthesis. During the prenatal period hippocampal NGF levels were relatively high, whereas the mRNANGF was below the level of detection. Since the ingrowth of septal fibers, and with that also the removal of NGF by retrograde transport, begins around birth, the relatively high prenatal NGF levels probably result from an accumulation produced by a small copy number of mRNANGF prior to the removal of NGF by retrograde axonal transport. It is concluded that the correlation of the developmental changes in NGF and mRNANGF with the ChAT activity in the hippocampus further supports the concept of a physiological role of NGF in the central nervous system.     

Levels of nerve growth factor and its mRNA in the central nervous system of the rat correlate with cholinergic innervation.

The levels of nerve growth factor (NGF) and its mRNA in the rat central nervous system were determined by two-site enzyme immunoassay and quantitative Northern blots, respectively. Relatively high NGF levels (0.4-1.4 ng NGF/g wet weight) were found both in the regions innervated by the magnocellular cholinergic neurons of the basal forebrain (hippocampus, olfactory bulb, neocortex) and in the regions containing the cell bodies of these neurons (septum, nucleus of the diagonal band of Broca, nucleus basalis of Meynert). Comparatively low, but significant NGF levels (0.07-0.21 ng NGF/g wet weight) were found in various other brain regions. mRNANGF was found in the hippocampus and cortex but not in the septum. This suggests that magnocellular cholinergic neurons of the basal forebrain are supplied with NGF via retrograde axonal transport from their fields of innervation. These results, taken together with those of previous studies showing that these neurons are responsive to NGF, support the concept that NGF acts as trophic factor for magnocellular cholinergic neurons.     

Maturation and maintenance of cholinergic medial septum neurons require glucocorticoid receptor signaling

Glucocorticoids have been shown to influence trophic processes in the nervous system. In particular, they seem to be important for the development of cholinergic neurons in various brain regions. Here, we applied a genetic approach to investigate the role of the glucocorticoid receptor (GR) on the maturation and maintenance of cholinergic medial septal neurons between P15 and one year of age by using a mouse model carrying a CNS-specific conditional inactivation of the GR gene (GRNesCre). The number of choline acetyltransferase and p75NTR immuno-positive neurons in the medial septum (MS) was analyzed by stereology in controls versus mutants. In addition, cholinergic fiber density, acetylcholine release and cholinergic key enzyme activity of these neurons were determined in the hippocampus. We found that in GRNesCre animals the number of medial septal cholinergic neurons was significantly reduced during development. In addition, cholinergic cell number further decreased with aging in these mutants. The functional GR gene is therefore required for the proper maturation and maintenance of medial septal cholinergic neurons. However, the loss of cholinergic neurons in the medial septum is not accompanied by a loss of functional cholinergic parameters of these neurons in their target region, the hippocampus. This pinpoints to plasticity of the septo-hippocampal system, that seems to compensate for the septal cell loss by sprouting of the remaining neurons.     

Ciliary neurotrophic factor prevents neuronal degeneration and promotes low affinity NGF receptor expression in the adult rat CNS.

Recombinant human ciliary neurotrophic factor (CNTF) was infused for 2 weeks into the lateral ventricle of fimbria-fornix transected adult rats, and its effects were compared with those of purified mouse nerve growth factor (NGF). We provide evidence that CNTF can prevent degeneration and atrophy of almost all injured medial septum neurons (whereas NGF protects only the cholinergic ones). CNTF is also involved in up-regulation of immunostainable low affinity NGF receptor (LNGFR) in cholinergic medial septum and neostriatal neurons and in a population of lateral septum neurons. In contrast to NGF, CNTF did not stimulate choline acetyltransferase in the lesioned septum and normal neostriatum (pointing to different mechanisms for the regulation of choline acetyltransferase and LNGFR), cause hypertrophy of septal or neostriatal cholinergic neurons, or cause sprouting of LNGFR-positive (cholinergic) septal fibers.     

NGF receptor reexpression and NGF-mediated cholinergic neuronal hypertrophy in the damaged adult neostriatum

Adult cholinergic interneurons of the neostriatum are not immunoreactive for monoclonal antibody to NGF receptor, whereas the developing neostriatum is immunoreactive for this same antibody. Chronic NGF infusion into the adult neostriatum resulted in reexpression of the NGF receptor such that many cholinergic interneurons became immunoreactive for NGF receptor. NGF infusion dramatically increased the size and choline acetyltransferase immunoreactivity of these same cholinergic neurons. Additionally, in situ hybridization demonstrated an increase in the number of cells expressing NGF receptor mRNA in the NGF-infused striatum. These findings indicate that central cholinergic neurons which lose their NGF receptors during postnatal development will resume their NGF responsiveness when the tissue is damaged. Such a damage-induced mechanism may act to enhance the action of trophic factors, including NGF, released at the site of injury and enhance the responsiveness of damaged CNS neurons to exogenously administered trophic factors.     

Developmental regulation of nerve growth factor and its receptor in the rat caudate-putamen

In prior studies, nerve growth factor (NGF) administration induced a robust, selective increase in the neurochemical differentiation of caudate-putamen cholinergic neurons. In this study, expression of NGF and its receptor was examined to determine whether endogenous NGF might serve as a neurotrophic factor for these neurons. The temporal pattern of NGF gene expression and the levels of NGF mRNA and protein were distinct from those found in other brain regions. NGF and high-affinity NGF binding were present during cholinergic neurochemical differentiation and persisted into adult-hood. An increase in NGF binding during the third postnatal week was correlated with increasing choline acetyltransferase activity. The data are consistent with a role for endogenous NGF in the development and, possibly, the maintenance of caudate-putamen cholinergic neurons.     

Regulation of cholinergic gene expression by nerve growth factor depends on the phosphatidylinositol-3'-kinase pathway

Nerve growth factor (NGF) exerts anti-apoptotic, trophic and differentiating actions on sympathetic neurons and cholinergic cells of the basal forebrain and activates the expression of genes regulating the synthesis and storage of the neurotransmitter acetylcholine (ACh). We have been studying the intracellular signaling pathways involved in this process. Although, in the rat pheochromocytoma cell line PC12, NGF strongly activates the mitogen-activated protein kinase (MAPK) pathway, prolonged inhibition of MAPK kinase (MEK) activity by PD98059 or U0126 did not affect the ability of NGF to up-regulate choline acetyltransferase (ChAT) or to increase intracellular ACh levels. In contrast, the treatment with the phosphatidylinositol 3'-kinase (PI3K) inhibitor LY294002, but not with its inactive analogue LY303511, completely abolished the NGF-induced production of ACh. Inhibition of PI3K also eliminated the NGF effect on the intracellular ACh level in primary cultures of septal neurons from E18 mouse embryos. Blocking the PI3K pathway prevented the activation of cholinergic gene expression, as demonstrated in RT/PCR assays and in transient transfections of PC12 cells with cholinergic locus promoter-luciferase reporter constructs. These results indicate that the PI3K pathway, but not the MEK/MAPK pathway, is the mediator of NGF-induced cholinergic differentiation.     

Cholinergic fiber growth in co-cultures of CNS tissue.

In co-cultures prepared from the septum and the hippocampus, cholinergic fibers originating in the septal slices grew into the neighboring hippocampal tissue and established functional cholinergic connections with pyramidal cells. To get further insight into the mechanisms governing cholinergic fiber growth, we have added TTX to the growth medium (2 x 10(-7) M) to block propagated electrical activity. Under these conditions, considerably fewer cholinergic cells appeared to survive. A few cholinergic fibers still invaded hippocampal target tissue, but their number was markedly reduced compared with control cultures. Simultaneous application of NGF together with TTX, however, not only increased enzyme levels and enhanced survival of cholinergic neurons, but also led to hippocampal ingrowth in virtually all septo-hippocampal co-cultures. These data, therefore, suggest, that in the absence of spiking activity, cholinergic fibers are capable of growing into a co-cultured target tissue. To test the specificity of growth of septal cholinergic fibers, we have co-cultured septal slices with slices of various brain areas which in situ lack a major cholinergic innervation, in particular the cerebellum. In the vast majority of such co-cultures, cholinergic fibers remained restricted within the septal slices, without innervating cerebellar tissue. This failure might in part be related to the lack of trophic factors released by the target tissue. We have, therefore, grown septo-cerebellar cultures in the presence and absence of NGF. Following application of 100 ng/ml NGF during the entire growth of the cultures, numerous AChE-positive fibers originating in the septal slices invaded the co-cultured cerebellar slices.(ABSTRACT TRUNCATED AT 250 WORDS)     

Involvement of Nerve Growth Factor and Its Receptor in the Regulation of the Cholinergic Function in Aged Rats

The role of nerve growth factor (NGF) and its receptor (NGFR) in the regulation of cholinergic activity has been studied during the aging process. NGFRs were quantified in cortical membranes using a radioactive binding assay. NGF levels and choline acetyltransferase (ChAT) activity were determined in cortex, hippocampus, neostriatum, and septum. These assays were performed in both adult (6-month-old) and aged (36-month-old) rats. High- and low-affinity 125I-NGF binding sites were present in cortex of adult and aged rats. Furthermore, we observed a decrease in number and affinity of both NGFRs in aged rats. ChAT activity in these rats was lower (approximately 30%) than in adult rats in all the brain regions examined. NGF levels were not modified in cortex and hippocampus and were decreased in neostriatum (55%) and septum (35%). In conclusion, our results suggest that, during the aging process, the cholinergic impairment is related to a decrease in NGF levels in neostriatum but not in cortex and hippocampus. The reduction in level of NGF protein in septum could be due to a decrease in number of high-affinity 125I-NGF binding sites.     

Nerve Growth Factor Increases the Intracellular Content of Acetylcholine in Cultured Septal Neurons from Developing Rats

The effects of nerve growth factor (NGF) on the intracellular content of acetylcholine (ACh) in cultured septal neurons from developing rats have been examined. The content of ACh could be measured by using HPLC and electrochemical detection (HPLC-ECD), coupled with an immobilized enzyme column. This method of determination is very simple and rapid, and is highly sensitive. The content of ACh and the activity of choline acetyltransferase (ChAT) in cultured postnatal day 1 (P1) septal neurons grown on an astroglial "feeder" layer was increased during the period of cultivation by the addition of NGF. The activities of ChAT and the content of ACh increased in a dose-dependent manner in direct relationship to the different amounts of NGF employed. These effects of NGF, i.e., elevating the intracellular content of ACh, accompanied by an increase in activity of ChAT, also were confirmed in the P1 septal organotypic cultures. Additionally, embryonic day 17 (E17) septal neurons in a serum-free medium displayed a similar responsiveness to NGF with respect to the elevation in the content of ACh and the increase in activity of ChAT. These results suggest that intracellular levels of ACh are likely to be regulated by NGF in a fashion similar to that of the activity levels of the biosynthetic enzyme.     

Correlative Regulation of Nerve Growth Factor Level and Choline Acetyltransferase Activity by Thyroxine in Particular Regions of Infant Rat Brain

Abstract: Effects of thyroxine (T₄) on nerve growth factor (NGF) level and choline acetyltransferase (ChAT) activity of rat brains were investigated. Repetitive intraperitoneal administration of T₄ caused increases in both NGF level and ChAT activity in the frontal cortex, septum, hippocampus, and striatum and decreases in the cerebellum in 2-day-old rats. Only ChAT activity was elevated in the olfactory bulb, and the NGF level remained unchanged there. No changes were observed in the midbrain and pons/medulla. Furthermore, T₄ was effective on the post-natal rats only up to day 11. These results suggest that T₄ plays a role in the developmental regulation of NGF level and ChAT activity in rat brain in a region- and/or stage-specific manner. That (1) changes in NGF level and ChAT activity occurred in regions nearly identical to those that contained NGF-responding neurons, and (2) the change in NGF level in the hippocampus and frontal cortex was followed by the change of ChAT activity after a single injection of T₄ suggest that the effects of T₄ on cholinergic differentiation are, at least in part, mediated via NGF, which itself is quantitatively regulated by T₄.     

Differential Effects of Insulin on Choline Acetyltransferase and Glutamic Acid Decarboxylase Activities in Neuron-Rich Striatal Cultures

We studied the effects of insulin, nerve growth factor (NGF), and tetrodotoxin (TTX) on cellular metabolism and the activity of glutamic acid decarboxylase (GAD) and choline acetyltransferase (ChAT) in neuron-rich cultures prepared from embryonic day 15 rat striatum. Insulin (5 micrograms/ml) increased glucose utilization, protein synthesis, and GAD activity in cultures plated over a range of cell densities (2,800-8,400 cells/mm2). TTX reduced GAD activity; NGF had no effect on GAD activity. Insulin treatment reversibly reduced ChAT activity in cultures plated at densities of greater than 4,000 cells/mm2, and the extent of this reduction increased with increasing cell density. The number of acetylcholinesterase-positive neurons was not reduced by insulin, suggesting that insulin acts by down-regulating ChAT rather than by killing cholinergic neurons. Insulin-like growth factor-1 (IGF-1) reduced ChAT activity at concentrations 10-fold lower than insulin, suggesting that insulin's effect on ChAT may involve the IGF-1 receptor. NGF increased ChAT activity; TTX had no effect on ChAT activity. These results suggest that striatal cholinergic and GABAergic neurons are subject to differential trophic control.     

Nerve growth factor expression in the developing hippocampus isolated in vitro.

Nerve growth factor (NGF) is synthesized in the hippocampus and neocortex and provides trophic support for afferent cholinergic neurons of the basal forebrain. To determine the capacity of the developing hippocampus to express NGF in the absence of NGF-responsive afferents, embryonic hippocampal cells isolated prior to septal innervation were studied in reaggregating cell culture. The expression of NGF protein in vitro was qualitatively and quantitatively similar to that observed in situ. The expression of NGF mRNA exhibited an initial increase in vitro but then plateaued and was maintained at a steady level. This latter finding was in contrast to the steady rise in NGF mRNA levels observed in situ. These data suggest that (i) intrinsic hippocampal interactions regulate the onset of NGF expression, but that (ii) additional extrinsic developmental signals may be required for proper regulation of hippocampal NGF expression during ontogeny.