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

Distribution of Brain-Derived Neurotrophic Factor in the Brain of the Small-Spotted Catshark Scyliorhinus canicula, and Evolution of Neurotrophins in Basal Vertebrates

Neurotrophins (NTFs) are structurally related neurotrophic factors essential for differentiation, survival, neurite outgrowth, and the plasticity of neurons. Abnormalities associated with neurotrophin-signaling (NTF-signaling) were associated with neuropathies, neurodegenerative disorders, and age-associated cognitive decline. Among the neurotrophins, brain-derived neurotrophic factor (BDNF) has the highest expression and is expressed in mammals by specific cells throughout the brain, with particularly high expression in the hippocampus and cerebral cortex. Whole genome sequencing efforts showed that NTF signaling evolved before the evolution of Vertebrates; thus, the shared ancestor of Protostomes, Cyclostomes, and Deuterostomes must have possessed a single ortholog of neurotrophins. After the first round of whole genome duplication that occurred in the last common ancestor of Vertebrates, the presence of two neurotrophins in Agnatha was hypothesized, while the monophyletic group of cartilaginous fishes, or Chondrichthyans, was situated immediately after the second whole genome duplication round that occurred in the last common ancestor of Gnathostomes. Chondrichthyans represent the outgroup of all other living jawed vertebrates (Gnathostomes) and the sister group of Osteichthyans (comprehensive of Actinopterygians and Sarcopterygians). We were able to first identify the second neurotrophin in Agnatha. Secondly, we expanded our analysis to include the Chondrichthyans, with their strategic phylogenetic position as the most basal extant Gnathostome taxon. Results from the phylogenetic analysis confirmed the presence of four neurotrophins in the Chondrichthyans, namely the orthologs of the four mammalian neurotrophins BDNF, NGF, NT-3, and NT-4. We then proceeded to study the expression of BDNF in the adult brain of the Chondrichthyan Scyliorhinus canicula. Our results showed that BDNF is highly expressed in the S. canicula brain and that its expression is highest in the Telencephalon, while the Mesencephalic and Diencephalic areas showed expression of BDNF in isolated and well-defined cell groups. NGF was expressed at much lower levels that could be detected by PCR but not by in situ hybridization. Our results warrant further investigations in Chondrichthyans to characterize the putative ancestral function of neurotrophins in Vertebrates.

Environmental influences on brain neurotrophins in rats

Environmental factors can have profound influences on the brain. Enriching environments with physical, social and sensory stimuli are now established to be beneficial to brain development and ageing. A multitude of responses from cellular and molecular mechanisms to macroscopic changes in neural morphology and neurogenesis have been considered in the context for evidences that environmental inputs can regulate brain plasticity in the rat at all stages of life. Data from our laboratory have revealed that enriched environment increased nerve growth factor (NGF) gene expression and protein levels in the hippocampus, and this may contribute to events underlying environmentally induced neural plasticity. Because neurotrophic factors are essential for neural development and survival, they are likely to be involved in the cerebral consequences modified by enriched experiences.

Differential Regulation of Nerve Growth Factor (NGF) Synthesis in Neurons and Astrocytes by Glucocorticoid Hormones

Glucocorticoid hormones are important regulators of brain development and ageing. Here we show that dexamethasone, a synthetic glucocorticoid, differentially affects the expression of nerve growth factor (NGF) in cultured neurons and astrocytes. Dexamethasone increased the levels of NGF mRNA in cultured hippocampal neurons in a time- and concentration-dependent manner, whereas it down-regulated the NGF mRNA levels in astrocytes. However, dexamethasone had no effect on the mRNA levels of brain-derived neurotrophic factor in the hippocampal neurons. Aldosterone, a mineralocorticoid, in higher concentrations also up-regulated NGF mRNA levels in the hippocampal neurons. Dexamethasone increased the levels of NGF mRNA in the rat hippocampus in vivo, but not to the same extent as observed with kainic acid, a glutamate receptor agonist. There is no apparent diurnal rhythm in the hippocampal NGF protein levels corresponding to circadian variations in the levels of glucocorticoid hormones in serum. The increase in NGF mRNA in the hippocampus in vivo following dexamethasone treatments may reflect the physiological response of hippocampal neurons to high glucocorticoid levels reached under conditions of stress.

Changes in brain nerve growth factor levels and nerve growth factor receptors in rats exposed to environmental enrichment for one year

This study examined the effects of long-term differential rearing on levels of brain nerve growth factor, its receptors, and their relationships to cognitive function. Adult rats (two months old) were placed into either enriched or standard housing conditions where they remained for 12 months. Animals from the enriched condition group had significantly higher levels of nerve growth factor in hippocampus, visual and entorhinal cortices compared with animals housed in isolated condition. Immunohistochemical analysis of brain tissue from the medial septal area revealed higher staining intensity and fibre density with both the low-affinity and the high-affinity nerve growth factor receptors. Enriched rats performed better than isolated rats in acquisition of spatial learning and had lower locomotion scores in the open field. These results provide further evidence that experimental stimulation results in increased production of trophic factors and structural reorganization in specific brain regions known to be involved in cognitive function.

Regulation of the hypothalamic-pituitary-adrenal axis by cytokines: actions and mechanisms of action

Glucocorticoids are hormone products of the adrenal gland, which have long been recognized to have a profound impact on immunologic processes. The communication between immune and neuroendocrine systems is, however, bidirectional. The endocrine and immune systems share a common "chemical language," with both systems possessing ligands and receptors of "classical" hormones and immunoregulatory mediators. Studies in the early to mid 1980s demonstrated that monocyte-derived or recombinant interleukin-1 (IL-1) causes secretion of hormones of the hypothalamic-pituitary-adrenal (HPA) axis, establishing that immunoregulators, known as cytokines, play a pivotal role in this bidirectional communication between the immune and neuroendocrine systems. The subsequent 10-15 years have witnessed demonstrations that numerous members of several cytokine families increase the secretory activity of the HPA axis. Because this neuroendocrine action of cytokines is mediated primarily at the level of the central nervous system, studies investigating the mechanisms of HPA activation produced by cytokines take on a more broad significance, with findings relevant to the more fundamental question of how cytokines signal the brain. This article reviews published findings that have documented which cytokines have been shown to influence hormone secretion from the HPA axis, determined under what physiological/pathophysiological circumstances endogenous cytokines regulate HPA axis activity, established the possible sites of cytokine action on HPA axis hormone secretion, and identified the potential neuroanatomic and pharmacological mechanisms by which cytokines signal the neuroendocrine hypothalamus.

Stimulation of beta2-adrenoceptors inhibits apoptosis in rat brain after transient forebrain ischemia

We have previously demonstrated that the neuroprotective effect of the beta2-adrenoceptor agonist clenbuterol in vitro and in vivo was most likely mediated by an increased nerve growth factor (NGF) expression. In the present study, we examined whether clenbuterol was capable of inhibiting apoptosis caused by ischemia. Transient forebrain ischemia was performed in male Wistar rats (300 to 350 g) by clamping both common carotid arteries and reducing the blood pressure to 40 mm Hg for 10 minutes. Clenbuterol (0.1, 0.5, and 1.0 mg/kg intraperitoneally) was administered 3 hours before ischemia or immediately after ischemia. The brains were removed for histologic evaluation 7 days after ischemia. The time course of DNA fragmentation was determined 1, 2, 3 and 4 days after ischemia. Staining with terminal deoxynucleotidyl transferase (TdT)-mediated dUTP nick end-labeling (TUNEL) was used for further analysis of DNA fragments in situ 3 days after ischemia. The NGF protein was assayed by enzyme-linked immunosorbent assay. Ten-minute forebrain ischemia damaged 80% to 90% of the neurons in the hippocampal CA1 region evaluated 7 days after ischemia. Pretreatment with clenbuterol (0.5 and 1.0 mg/kg) reduced the neuronal damage by 18.1% (P < 0.01) and 13.1% (P < 0.05), respectively. The neuroprotective effect also was found when clenbuterol (0.5 mg/kg) was administered immediately after ischemia (P < 0.05). The DNA laddering appeared in striatum 1 day and in hippocampus 2 days after ischemia and peaked on the third day in both regions. The DNA laddering was nearly abolished in the hippocampus and partially blocked in striatum and cortex by 0.5 mg/kg clenbuterol. These results were confirmed by TUNEL staining. Clenbuterol (0.5 mg/kg intraperitoneally) elevated the NGF protein level by 33% (P < 0.05) in the hippocampus and 41% (P < 0.05) in the cortex 6 hours after ischemia. Three days after ischemia, the NGF levels in these regions were no longer different between the clenbuterol-treated and control groups. This study clearly demonstrates that clenbuterol possesses a neuroprotective activity and a marked capacity to inhibit DNA degradation after global ischemia. The results suggest that clenbuterol increases NGF expression during the first hours after global ischemia and thereby protects neurons against apoptotic damage.

Environmental enrichment results in higher levels of nerve growth factor mRNA in the rat visual cortex and hippocampus

Evidence for structural modifications in the brain following environmental changes have been provided during the last decades. The most pronounced alterations following environmental manipulations have been found in the visual cortex. These plastic changes are supposed to reflect reorganization of neuronal connections involved in postnatal development and adult adjustments of connections involved in sensori-perceptual processing and learning. Potential candidates to mediate these changes are neurotrophins. Nerve growth factor (NGF) has been associated with cognitive functions and shown to improve the performance of aged rats in spatial learning and memory task. In the central nervous system, NGF is of importance for development and maintenance of cholinergic neurons and atrophy of cholinergic neurons is strongly correlated with learning and memory impairments. Exposure to enriched environmental conditions improves learning and problem-solving ability and results in plastic changes in the brain. This study examined the effect of environmental enrichment on expression of NGF mRNA in the rat visual cortex and hippocampus. Rats housed in groups in a stimulus-rich environment for 30 days had significantly higher levels of NGF mRNA than rats housed individually in single cages without stimulus-enrichment. We have recently presented results showing higher levels of neurotrophin-3 (NT-3) mRNA and improved spatial learning following environmental enrichment, and suggest that an interplay involving the neurotrophins NGF and NT-3 may be mediating experience-induced structural changes.

Localization of nerve growth factor, trkA and P75 immunoreactivity in the hippocampal formation and basal forebrain of adult rats

In the immunohistochemical staining of nerve growth factor, it has been reported that fixation-dependent lability of nerve growth factor hampers its localization. In the present study, we used two different polyclonal antibodies to immunostain nerve growth factor in rat brain tissue. We found that in paraformaldehyde-fixed (immersion- or perfusion-fixed) brains, nerve growth factor-like immunoreactivity was located primarily in the cytoplasmic membrane and fiber tract of hippocampal neurons and was sparse in cortical neurons. When fresh frozen brain sections were fixed in paraformaldehyde solution, nerve-growth factor-like immunoreactivity was distributed evenly in the cell body. However, when fresh frozen brain sections were fixed in acetone, immunoreactivity to nerve growth factor was present as discrete or confluent dense particles in the cell body, especially in the nuclear region. Also, when paraformaldehyde-perfusion-fixed brain sections were heat treated in salt solution before immunostaining, nerve growth factor-like immunoreactivity could be retrieved in the cytoplasmic and nuclear regions. The hippocampal formation, cerebral cortex and basal forebrain expressed nerve growth factor-like immunoreactivity. Double immunostaining in fresh frozen brains showed that the low-affinity nerve growth factor receptor (p75) co-expressed with nerve growth factor and trkA proto-oncogene in basal forebrain neurons. Our study shows that formaldehyde fixation can mask nerve growth factor antigen, and special treatment, such as heating, is needed to retrieve nerve growth factor antigen to permit immunohistochemical detection. For immunohistochemical study of nerve growth factor in rat brain tissue, successful immunostaining can be obtained by using fresh frozen brains to prevent the masking effect of fixatives or by using paraformaldehyde-fixed brains with heat treatment. It is likely that nerve growth factor is synthesized and accumulated mainly in the cell body but not in the fiber tracts, which is similar to the distribution of its messenger RNA. The co-existence of p75 with nerve growth factor and trkA in basal forebrain neurons suggests the role of low- and high-affinity receptors in regulating the trophic effect of nerve growth factor.

FGF-2 induces nerve growth factor expression in cultured rat hippocampal neurons

Basic fibroblast growth factor (FGF-2) is expressed in the hippocampus and has been demonstrated to promote neurotrophic effects on hippocampal neurons in vitro. We show that these neurons, even at the embryonic stage, express the mRNAs encoding the FGF receptors, bek and flg. We have characterized the effects of FGF-2 on the expression of nerve growth factor (NGF) using the reverse transcription-coupled polymerase chain reaction, in situ hybridization and immunocytochemistry. In hippocampal neurons grown in the absence of serum, FGF-2 exposure induces an important elevation of NGF mRNA expression followed by a marked increase in NGF immunoreactivity. Combining in situ hybridization with an NGF probe and microtubule-associated protein-2 (MAP2) immunocytochemistry we show that the induction of NGF mRNA by FGF-2 is localized in MAP2-immunoreactive neurons. These results suggest roles for FGF-2 in the development of hippocampal neurons and in the maintenance of connections in the central nervous system, particularly the septo-hippocampal pathway, via the regulation of an important neurotrophin.

Seizure-induced differential expression of messenger RNAs for neurotrophins and their receptors in genetically fast and slow kindling rats

Levels of messenger RNAs for brain-derived neurotrophic factor, nerve growth factor and neurotrophin-3, and their high-affinity receptors, TrkB and TrkC, were analysed in the brains of genetically fast and slow kindling rats using in situ hybridization. Basal expression of neurotrophins and Trk messenger RNAs in the hippocampal formation, amygdala, frontoparietal and piriform cortices did not differ between the two strains. At 2 h after the third generalized grade 5 seizure, induced by kindling stimulations in the amygdala, increased expression of brain-derived neurotrophic factor messenger RNA was detected in the dentate gyrus granule cell layer, amygdala, frontoparietal and piriform cortices of the fast kindlers. Similar seizure-evoked increases of brain-derived neurotrophic factor messenger RNA levels were also observed in the amygdala and piriform cortex of slow kindlers. However, in these animals, brain-derived neurotrophic factor messenger RNA expression was not significantly altered by the seizures in the dentate gyrus granule cell layer and frontoparietal cortex. Furthermore, the seizure-induced increase of nerve growth factor, TrkB and TrkC messenger RNAs and decrease of neurotrophin-3 messenger RNA levels in the dentate gyrus granule cell layer was only observed in fast, but not in slow, kindlers. The neurotrophins are believed to regulate synaptic plasticity and efficacy and to facilitate long-term potentiation and kindling epileptogenesis. The present data suggest that the slow and fast kindling rates in the two strains studied here might partly be due to differences in seizure-evoked neurotrophin and Trk synthesis.

Clenbuterol protects mouse cerebral cortex and rat hippocampus from ischemic damage and attenuates glutamate neurotoxicity in cultured hippocampal neurons by induction of NGF

It has been shown previously that clenbuterol, a beta 2-adrenergic receptor agonist, enhances NGF synthesis in adult rat brain. Since NGF is able to protect neurons against damage, we tried to find out whether clenbuterol can rescue cultured hippocampal neurons from excitotoxic damage by induction of NGF. The neuroprotective activity of clenbuterol on neurons in the vulnerable CA1 subfield of the hippocampus was tested in a rat model of transient forebrain ischemia. Additionally, in the mouse model of focal cerebral ischemia the ability of clenbuterol to reduce the infarct size was examined. Exposure of mixed neuronal/glial hippocampal cultures to clenbuterol (1 to 100 microM) enhanced significantly the content of NGF measured in the culture medium by two-site ELISA. The excitotoxic injury was induced in the same type of cells after 14 days in vitro by exposure to 1 mM L-glutamate for 1 h in serum-free medium. NGF itself (0.15 to 100 ng/ml) added to the growth medium 4 h before until 18 h after induction of injury (the point of glutamate-toxicity measurement), protected hippocampal neurons from excitotoxic damage. Clenbuterol (1 to 100 microM) provided similar neuroprotection as NGF under the same experimental conditions. The neuroprotective activity of clenbuterol (100 microM) against glutamate-induced damage in hippocampal cultures was blocked by anti-NGF monoclonal antibodies (0.5 microgram/ml) added to the medium during the clenbuterol exposure, demonstrating that the neuronal rescue is mediated by NGF. Propranolol, a beta-adrenergic receptor antagonist (10 microM) added 20 min before and kept in the medium during exposure of the cultures to clenbuterol (1 microM) reversed the neuroprotective activity, suggesting that the induction of NGF and neuroprotection caused by clenbuterol are mediated via beta-adrenergic receptor activation. The capacity of clenbuterol to protect hippocampal neurons was also demonstrated in vivo in a rat model of transient forebrain ischemia. Clenbuterol (4 x 1 mg/kg) administered intraperitoneally increased the number of viable neurons in CA1 subfield of the rat hippocampus. Furthermore, clenbuterol (0.3 and 1 mg/kg, i.p. and 1 mg/kg, s.c.) reduced significantly the infarct area on the mouse brain surface after occlusion of the middle cerebral artery. The present data demonstrate that clenbuterol induces NGF synthesis in cultured hippocampal cells and protects hippocampal neurons from excitotoxic damage. The neuroprotective activity of clenbuterol is also demonstrated in vivo in two rodent models of cerebral ischemia. The results offer strong evidence that the neuroprotective activity of clenbuterol is caused by activation of beta-adrenergic receptors and the subsequent increased expression of NGF.

Age-related decrease of nerve growth factor-like immunoreactivity in the basal forebrain of senescence-accelerated mice

The senescence-accelerated mouse P10 (SAMP10) is a murine model of accelerated senescence characterized by the deterioration of learning and memory with advancing age. In the present study, we examined the distribution of nerve growth factor (NGF) immunohistochemically in SAMP10 mice and its control strain, SAMR1. In both strains, NGF-like immunoreactivity (NGF-IR) was observed in neurons throughout the entire forebrain and in glial cells in a particular location. In aged SAMP10 mice, each layer of the cerebral cortex retained its NGF-IR, although the thickness of the cortical mantle was markedly decreased in comparison with younger animals. There was an age-related decline in NGF-IR in the substantia innominata of SAMP10 mice at the age of 10 months, when compared to 2-month-old SAMP10. These results indicate age-related decrease of NGF in the basal forebrain in SAMP10 mice.

Degeneration of rat cholinergic basal forebrain neurons and reactive changes in nerve growth factor expression after chronic neurotoxic injury--II. Reactive expression of the nerve growth factor gene in astrocytes

Long-term consumption of ethanol both in human and rodent induces a process of chronic degeneration of cholinergic basal forebrain neurons which results in a cholinergic deafferentation of the cortical mantle. We have used quantitative northern blot analysis and in situ hybridization to demonstrate that these degenerative events in rat evoke an increase in the expression of the nerve growth factor gene in a number of brain areas, including the cholinergic basal forebrain nuclei and their cortical target regions. By combining non-radioactive in situ hybridization and immunohistochemistry activated astrocytes were identified as the major source of altered nerve growth factor gene expression. This increased nerve growth factor expression is paralleled by a dendritic remodelling of basal forebrain neurons, while the expression of choline acetyltransferase in surviving neurons remains the same. This failure of nerve growth factor to rescue the expression of choline acetyltransferase differs from the effects of exogenously administered nerve growth factor in acutely lesioned systems. The results indicate that under certain conditions of chronic neurodegeneration, the utilization of nerve growth factor might be impaired, which could be due to a defective nerve growth factor signalling mechanism.

The effect of mercury vapour on cholinergic neurons in the fetal brain: studies on the expression of nerve growth factor and its low- and high-affinity receptors

The effects of mercury vapour on the production of nerve growth factor during development have been examined. Pregnant rats were exposed to two different concentrations of mercury vapour during either embryonic days E6-E11 (early) or E13-E18 (late) in pregnancy, increasing the postnatal concentration of mercury in the brain from 1 ng/g tissue to 4 ng/g tissue (low-dose group) or 11 ng/g (high-dose group). The effect of this exposure in offspring was determined by looking at the NGF concentration at postnatal days 21 and 60 and comparing these levels to age-matched controls from sham-treated mothers. Changes in the expression of mRNA encoding NGF, the low- and high-affinity receptors for NGF (p75 and p140 trk, respectively) and choline acetyltransferase (ChAT) were also determined. When rats were exposed to high levels of mercury vapour during early embryonic development there was a significant (62%) increase in hippocampal NGF levels at P21 accompanied by a 50% decrease of NGF in the basal forebrain. The expression of NGF mRNA was found to be unaltered in the dentate gyrus. The expression of p75 mRNA was significantly decreased to 39% of control levels in the diagonal band of Broca (DB) and to approximately 50% in the medial septal nucleus (MS) whereas no alterations in the level of trk mRNA expression were detectable in the basal forebrain. ChAT mRNA was slightly decreased in the DB and MS, significantly in the striatum. These findings suggest that low levels of prenatal mercury vapour exposure can alter the levels of the NGF and its receptors, indicating neuronal damage and disturbed trophic regulations during development.

The stimulation of thalamic neurite outgrowth by cortex-derived growth factors in vitro: the influence of cortical age and activity

Recent in vitro experiments have provided useful insights into the development of connections between the thalamus and the cortex. While most of these previous studies focused on neurite guidance and target recognition, our experiments used a serum-free culture system to examine the possible roles of unidentified diffusible cortex-derived growth factors. We demonstrated that occipital cortical explants release diffusible growth factors that enhance neurite outgrowth from explants of the posterior thalamus (the region around the developing lateral geniculate nucleus). The amount of thalamic outgrowth was dependent on the age of the cocultured cortical slices. Our results suggest that there is an overall increase in the release of cortex-derived growth factors during the first three postnatal weeks in mice; this parallels known postnatal increases in the production of several identified growth factors. We found evidence for two peaks in the release of cortex-derived growth factors during the general upward trend, the first at around postnatal day 6 (shortly after thalamocortical innervation of layer 4) and a second between postnatal days 14 and 18 (just after eye-opening). The increased release of cortex-derived growth factors was not found when cortical slices were from mice that had been dark-reared from birth, suggesting that neural activity may be important for enhancing release. Other regions of the central nervous system, including the cerebellum and medulla, were also capable of stimulating some thalamic outgrowth; neither additional explants of the thalamus nor hepatic explants enhanced outgrowth. Fibroblast growth factor is one substance that is distributed preferentially among those tissues that were stimulatory in our experiments. Its level of transcription is known to increase in the brain during the first three postnatal weeks and to be influenced by neural activity. At low doses, fibroblast growth factor greatly increased outgrowth from isolated posterior thalamic explants. Nerve growth factor, another candidate molecule, was less effective. Overall, our results complement the in vivo observations of others on the synthesis of identified growth factors in the cortex and the factors that influence their production. They suggest that growth factors may influence thalamic neurons, and indicate that fibroblast growth factor, and possibly nerve growth factor, are two candidates for molecules mediating the in vitro effects.

Nerve growth factor released by transgenic astrocytes enhances the function of adrenal chromaffin cell grafts in a rat model of Parkinson's disease

Previous studies have demonstrated that astrocytes genetically modified to express recombinant nerve growth factor (NGF) support the survival and neuronal transdifferentiation of intrastriatal adrenal chromaffin cell grafts at 2 weeks post-transplantation [15]. The present study was performed to determine whether these effects would be maintained at longer times post-transplantation and, if so, whether the co-grafts would reduce rotational behavior in the unilateral 6-hydroxydopamine-lesioned rat. In the present study, we have demonstrated that primary type I rat astrocytes infected with a replication-defective retrovirus conferring expression of a mouse beta-NGF cDNA sequence secrete NGF at a rate that is approximately 40-fold higher than that of controls (i.e., 8.0 vs. 0.2 pg NGF/h/10(5) cells, respectively). The genetically modified astrocytes were also found to express recombinant NGF following intrastriatal transplantation, as indicated by a 23% increase in striatal NGF content compared with controls, measured at 4 weeks post-transplantation. When NGF-producing astrocytes and adrenal chromaffin cells were co-grafted into the dopamine-denervated striatum of the unilateral 6-hydroxydopamine-lesioned rat, the chromaffin cells displayed extensive neurite outgrowth and a 5-12-fold increase in survival compared to controls at 10 weeks post-grafting. These effects were paralleled by a 60% reduction of apomorphine-induced rotational behavior, suggesting a partial normalization of striatal function. These results suggest that genetically modified astrocytes promote the prolonged survival and function of adrenal chromaffin cell grafts in a rat model of Parkinson's disease.

1,25-dihydroxyvitamin D3 regulates the synthesis of nerve growth factor in primary cultures of glial cells

The effect of 1,25-dihydroxyvitamin D3 (1,25-(OH)2 D3) on nerve growth factor (NGF) synthesis was investigated in primary cultures of astrocytes prepared from brain of neonatal rats. 1,25-(OH)2 D3 elicited a dose-dependent increase of NGF mRNA with a maximal effect at 10(-7) M, which persisted for at least 48 h. Northern blot analysis revealed an expression of the vitamin D3 receptor (VDR) gene in primary glial cells. Treatment of cells with 1,25-(OH)2 D3 led to an increase in the VDR mRNA levels. Similar results were obtained in C6 glioma cells. Exposure of primary glial cells to 10(-8) M 1,25-(OH)2 D3 caused only a 2-fold increase of the levels of cell-secreted NGF after 3 days of treatment. However, a 5-fold increase was observed three days after a second addition of vitamin D3. Likewise, a pretreatment with lower doses of hormone such as 10(-10) M or 10(-9) M enhanced the responsiveness of the cells to a 24 h treatment with 10(-8) M hormone. It appears, therefore, that the duration of the treatment influences the level of synthesis of NGF, possibly as a consequence of the increase of the VDR gene expression. The specificity of 1,25-(OH)2 D3 is supported by the fact that a concentration of 10(-7) M of an another vitamin D3 metabolite, 24,25-(OH)2 D3, had no effect on NGF synthesis. Several lines of evidence indicate that astrocytes constitute the major cell type responsive to 1,25-(OH)2 D3 in primary cultures of glial cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Nerve growth factor and Alzheimer's disease

Nerve growth factor (NGF) is a well-characterized protein that exerts pharmacological effects on a group of cholinergic neurons known to atrophy in Alzheimer's disease (AD). Considerable evidence from animal studies suggests that NGF may be useful in reversing, halting, or at least slowing the progression of AD-related cholinergic basal forebrain atrophy, perhaps even attenuating the cognitive deficit associated with the disorder. However, many questions remain concerning the role of NGF in AD. Levels of the low-affinity receptor for NGF appear to be at least stable in AD basal forebrain, and the recent finding of AD-related increases in cortical NGF brings into question whether endogenous NGF levels are related to the observed cholinergic atrophy and whether additional NGF will be useful in treating this disorder. Evidence regarding the localization of NGF within the central nervous system and its presumed role in maintaining basal forebrain cholinergic neurons is summarized, followed by a synopsis of the relevant aspects of AD neuropathology. The available data regarding levels of NGF and its receptor in the AD brain, as well as potential roles for NGF in the pathogenesis and treatment of AD, are also reviewed. NGF and its low affinity receptor are abundantly present within the AD brain, although this does not rule out an NGF-related mechanism in the degeneration of basal forebrain neurons, nor does it eliminate the possibility that exogenous NGF may be successfully used to treat AD. Further studies of the degree and distribution of NGF within the human brain in normal aging and in AD, and of the possible relationship between target NGF levels and the status of basal forebrain neurons in vivo, are necessary before engaging in clinical trials.

Neurotrophic factor mRNA expression in dentate gyrus is increased following angular bundle transection

In the central nervous system, the highest levels of nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) mRNA are found in the hippocampal formation. In the present study, we report that a unilateral transection of the angular bundle, which relays cortical information via the entorhinal cortex to the hippocampal formation, increases NGF and BDNF mRNA in the ipsilateral dentate gyrus. Within 4 hours following transection, the hybridization signal for NGF and BDNF mRNA increases in stratum granulosum 3- and 5-fold, respectively, compared to control levels. This lesion-induced increase of both mRNA returns to control levels within 24 hours and is maintained for at least 5 days. The induction is not prevented by pretreatment with AP-5, CNQX, or cholinergic denervation due to transection of the fimbria-fornix. Finally, the induction of neurotrophin mRNA is preceded by an increase in c-fos mRNA. These results provide evidence that transection of the cortical input to the hippocampal formation upregulates NGF and BDNF mRNA selectively in stratum granulosum. We suggest that the increased expression of NGF and BDNF mRNA may be an early step in the synaptic rearrangement of neurotrophin responsive cholinergic afferents observed following damage to the entorhinal cortex.

Cellular localization of nerve growth factor-like immunoreactivity in adult rat brain: quantitative and immunohistochemical study

To elucidate the role and the mechanism of action of nerve growth factor in the adult central nervous system, we investigated the localization of nerve growth factor-like immunoreactivity in adult rat brain, both quantitatively and immunohistochemically, using polyclonal anti-nerve growth factor immunoglobulin G. We raised rabbit polyclonal anti-mouse nerve growth factor antibody with an extremely high titer as 10(-9) determined by an enzyme immunoassay. The affinity-purified anti-nerve growth factor immunoglobulin G specifically recognized nerve growth factor with no cross-reaction to recombinant brain-derived neurotrophic factor and neurotrophin-3 evaluated by an enzyme immunoassay. We quantified nerve growth factor content in each layer of the adult rat cerebral cortex and in each small piece (0.225 mg wet weight tissue) of the diencephalon, brainstem and cerebellum with a highly sensitive two-site enzyme immunoassay. Nerve growth factor content was unevenly distributed in the cerebral cortex (dense in layers II/III and V/VI and sparse in layers I and IV). Moderate to high levels of nerve growth factor were registered in the habenular nuclei, zona incerta, ventral tegmental area, substantia nigra, locus coeruleus, ventral cochlear nucleus, trapezoid body, lateral vestibular nucleus, cerebellar nuclei and paraflocculus. Immunohistochemically, the nerve growth factor-like immunoreactivity was found in the cell bodies, dendrites and axons of adult rat central neurons, not only in the cerebral cortex, hippocampus and basal forebrain, but also in the diencephalon, brainstem and cerebellum. The population of neurons with nerve growth factor-like immunoreactivity was limited, but unexpectedly widespread, and the density of these cells correlated well with the content determined by an enzyme immunoassay in the present and a previous study [Nishio T. et al. (1992) Expl Neurol. 116, 76-84]. The monoamine neurons, including dopaminergic, noradrenergic and serotonergic neurons, showed intense nerve growth factor-like immunoreactivity, indicating that the central monoaminergic neuronal system may also be involved in the nerve growth factor trophic system. To visualize nerve growth factor transported in the axons and to enhance the immunostaining in the nerve growth factor-producing cells, we injected colchicine, a potent inhibitor of microtubule polymerization and a blocker of axoplasmic transport, into the lateral ventricle of adult Wistar rat brain. Colchicine treatment enhanced the intensities of nerve growth factor-like immunoreactivity in the axons and cell bodies, especially in the axon hillocks and the proximal axons of the nerve growth factor-producing neurons. This observation may suggest the existence of an orthograde axonal transport system for nerve growth factor in the central neurons.(ABSTRACT TRUNCATED AT 400 WORDS)

Hippocampal sympathetic ingrowth and cholinergic denervation alter hippocampal muscarinic cholinergic receptors

Cholinergic denervation of the hippocampus by medial septal (MS) lesions results in the ingrowth of peripheral sympathetic fibers, originating from the superior cervical ganglia, into the hippocampus. To determine the effect of hippocampal sympathetic ingrowth (HSI) [3H]-QNB (L-quinuclidinyl [benzilic-4,4(n)] binding was assessed in the dorsal and ventral hippocampus four weeks after MS lesions. In dorsal hippocampus, HSI was found to significantly increase the number (Bmax) of [3H]-QNB binding sites and to normalize the decrease in affinity found in animals with MS lesions plus ganglionectomy (i.e., no ingrowth). In ventral hippocampus, HSI was found to normalize the increased number of binding sites and decreased affinity found in animals with MS lesions without ingrowth. No effect on either Kd or Bmax was found in animals that had undergone ganglionectomy with sham MS lesions. These results suggest that HSI can induce changes in hippocampal muscarinic cholinergic receptors.

Cell-type-specific expression and regulation of a c-fos-NGF fusion gene in neurons and astrocytes of transgenic mice

A mouse line transgenic for nerve growth factor (NGF) was developed using the mouse prepro-NGF cDNA inserted within a plasmid containing the proximal region (-10 to -550 bp) of the c-fos promoter and the transcription termination and polyadenylation signals of the rabbit beta-globin gene. No significant modification of gross behavior or central nervous system anatomy was detected in adult animals as assessed by immunohistochemistry and in situ hybridization for NGF and choline acetyltransferase. The expression of the transgene and the possible regulation of its expression by agents acting on the promoter were investigated in vitro. Despite the presence of an additional pool of NGF mRNA specific to the transgene, basal levels of NGF in the supernatant of transgenic astrocytes were similar to normal ones. On the other hand, transgenic neurons spontaneously synthesized and released levels of NGF two to three times higher than normal neurons, while mRNA levels were barely detectable by conventional Northern blotting. The tissue-specificity of NGF expression was respected, with higher levels in hippocampal than neocortical neurons. Increases of NGF mRNA by agents acting on the promoter could be observed in normal and transgenic astrocytes only after inhibition of the protein synthesis by cycloheximide, suggesting a similar rapid turnover of normal and transgenic transcripts. Cyclic AMP agonists specifically increased the secretion of NGF protein by transgenic astrocytes and neurons, while activators of the protein kinase C had a similar effect on transgenic and normal cells. Differences between amounts of NGF secreted by neurons and astrocytes with regards to their respective content in mRNA suggest that transgenic transcripts are subject to normal cell- and tissue-specific post-transcriptional regulations. Agents acting on the c-fos promoter through the protein kinase C or cyclic AMP routes differentially increased the secretion of NGF by transgenic astrocytes or neurons, supporting this hypothesis.

Altered gene expression in cerebral ischemia

BACKGROUND

Using the techniques of molecular biology, recent experimental studies have shown that cerebral ischemia induces a variety of changes in gene expression in the brain.

SUMMARY OF REVIEW

During the early postischemic stages, protein synthesis in the brain is generally suppressed, but specific genes are expressed and their corresponding proteins may be synthesized, such as immediate-early gene products (c-fos, c-jun, and zinc finger gene), heat-shock proteins, and amyloid precursor protein. The ability of neurons to induce such stress responses, which depends on both the severity of ischemia and the intrinsic nature of the neuronal populations, may be directly associated with neuronal death and survival after ischemia. Nerve growth factor and fibroblast growth factor are also induced after ischemia and may be related to repair processes, in which a role of glial cells is suggested. Postischemic events that may be associated with the altered gene expression include (1) induction of tolerance to ischemia after pretreatment with sublethal ischemia, (2) slow, progressive neuronal changes and the development of neuronal plasticity after ischemia, and (3) delayed neuronal changes in remote areas outside the cerebral ischemic focus.

CONCLUSIONS

Because a variety of harmful stresses, including ischemia, elicit the same stress response and because this response is induced when total protein synthesis in the brain is nearly completely suppressed, this response may be vital to cell survival and repair. A successful induction of this response may induce resistance and survival of neurons after ischemia. However, failure or abortion of the response and persistent stresses may lead to neuronal death and possibly long-term changes and degeneration.

Female circulating sex hormones and hippocampal sympathetic ingrowth

Following cholinergic denervation of the hippocampal formation, via medial septal (MS) lesions, sympathetic fibers, originating from the superior cervical ganglia, growth into the hippocampus. Previous studies have demonstrated a sexually dimorphic effect of this neuronal rearrangement on recovery of a spatial-learning task, with this rearrangement being detrimental in male but protective in female rats. Circulating male sex hormones were found to interact with this effect in male animals. In this study we assessed the role of circulating female sex hormones on the behavioral and biochemical effects of hippocampal sympathetic ingrowth (HSI). For the behavioral studies female rats underwent either sham ovariectomy (sham OVARX) or OVARX and were taught a standard radial-8-arm maze task. Following attainment of criterion, animals underwent one of three surgical procedures: sham surgery; MS lesions+sham ganglionectomy (MS); HSI group; MS lesions+ganglionectomy (MSGx). As in our previous study, animals with HSI (i.e. MS group) were found to recover learning faster (in fact, these animals did not differ from controls) than animals with MS lesions without HSI. Gonadal status did not affect this behavioral recovery. For the biochemical studies hippocampal norepinephrine (NE) and choline acetyltransferase (ChAT) were measured in animals sham OVARX and OVARX, 8-12 weeks after the neurosurgical procedure. MS lesions (i.e. MSGx; MS) were found to reduce ChAT activity, regardless of circulating sex hormones. In controls NE levels were similar between OVARX and sham OVARX. NE levels were markedly elevated in the OVARX MS group compared to all other groups including sham OVARX. In the MSGx groups, NE levels were reduced compared to controls, while comparisons between these groups revealed a significant reduction in NE levels in the OVARX MSGx group compared to sham OVARX MSGx group. These studies suggest that female circulating sex hormones interact with brain injury in a very complex manner. However, this interaction does not appear to mediate the changes in behavior observed after HSI.

Long-term protective effects of human recombinant nerve growth factor and monosialoganglioside GM1 treatment on primate nucleus basalis cholinergic neurons after neocortical infarction

Neocortical infarction induces biochemical and morphological retrograde degenerative changes in cholinergic neurons of the rat nucleus basalis magnocellularis [Sofroniew et al. (1983) Brain Res. 289, 370-374]. In the present study, this lesion model has been reproduced in the non-human primate (Cercopithecus aethiops) to investigate whether degenerative changes affecting the cortex surrounding the lesioned area and the ipsilateral basal forebrain are prevented by the early administration of recombinant human nerve growth factor alone or in combination with the monosialoganglioside GM1. Six months after surgery and treatment, the monkeys were processed either for biochemistry (choline acetyltransferase assay) or immunocytochemistry. In lesioned vehicle-treated animals, choline acetyltransferase activity significantly decreased by 28% in the cortex surrounding the injured area and by 31% in the ipsilateral nucleus basalis of Meynert when compared with values of sham-operated monkeys. These biochemical changes were fully prevented with the administration of nerve growth factor alone or in combination with the monosialoganglioside GM1. The morphometrical analysis revealed a significant shrinkage of cholinergic neurons (61 +/- 1.4% of sham-operated cell size) and loss of neuritic processes (59 +/- 10% of sham-operated values) within the intermediate nucleus basalis region of lesioned vehicle-treated animals. Although a protection of the cholinergic cell bodies within the nucleus basalis was found with both treatments, a significant recovery of the neuritic processes (84 +/- 7.2% of sham-operated values) was assessed only in the double-treated monkeys. These results indicate that the early administration of nerve growth factor alone or in combination with the monosialoganglioside GM1 induces a long-term protective effect on the nucleus basalis cholinergic neurons in cortical injured non-human primates.

Effect of chronic ethanol on the septohippocampal system: a role for neurotrophic factors?

The mechanisms by which chronic ethanol exposure produces neuronal damage have not been established. Potentially ethanol may reduce normal neurotrophic influences necessary for neuronal survival, growth, and function. We hypothesized that chronic ethanol exposure might produce a decrease in the synthesis, availability, upregulation, delivery, and/or the biological activity of normally occurring neurotrophic factors, or may alter the capacity of target neurons to respond to these factors. The available evidence leading to this hypothesis and supporting data from our laboratory are discussed.

Arachidonic acid lipoxygenation may mediate interleukin-1 stimulation of nerve growth factor secretion in astroglial cultures

Interleukin-1 beta (IL-1) stimulates by about fivefold NGF secretion from rat neonatal cortical astrocytes in primary culture. We investigated the possible intracellular second messenger mechanisms involved in the IL-1 induced NGF secretion. Basal NGF secretion did not require extracellular Ca2+, whereas Ca2+ was necessary for the maximal NGF secretion stimulated by IL-1 (10 units/ml). The protein kinase C activator TPA stimulated by sixfold NGF secretion, but in this case, TPA acted synergistically with IL-1 to increase NGF secretion. Treatment of cells with the phospholipase A2 inhibitor mepacrine (30 microM) inhibited basal (by 50%) and IL-1 stimulated (by 80%) NGF secretion. Indomethacin, a cyclooxygenase inhibitor, produced a slight increase in basal NGF secretion at low concentrations, while PGE2 (10 microM) inhibited basal and IL-1 stimulated NGF secretion. In contrast, treatment of cells with nordihydroguaiaretic acid (NDGA), a lipoxygenase inhibitor, blocked in a concentration-dependent manner (IC50 = 10 microM) IL-1 stimulation of NGF secretion. The leukotriene LTB4 increased basal NGF secretion and this effect was not additive with IL-1 when both agents were added at saturating concentrations, indicating a common mechanism of action for these two agents. Thus, one possible mechanism by which IL-1 stimulates NGF secretion from astrocytes is by activation of the phospholipase A2-lipoxygenase pathway.

Lesion-induced expression of low-affinity nerve growth factor receptor-immunoreactive protein in Purkinje cells of the adult rat

Normal adult cerebellar Purkinje cells in the rat rarely express low-affinity nerve growth factor receptor immunoreactivity. However, intense anti-low-affinity nerve growth factor receptor immunostaining was observed as early as one day after a lesion of the cerebellar cortex. Low-affinity nerve growth factor receptor immunoreactivity was confined to a selected group of Purkinje cells, the number of which reached a maximum at three days postlesion, and, in some neurons, persisted up to 10 days after damage. The intensity of Purkinje cell immunolabeling decayed abruptly with distance from the lesion site. Reactive Purkinje cells exhibited deposition of immunoreaction product in the cell soma, dendrites and axons. Characteristically, most Purkinje cell axons exhibiting intense low-affinity nerve growth factor receptor immunoreactivity had beaded, varicose morphology. Varicose fibres with the appearance of recurrent collaterals of Purkinje cell axons were also low-affinity nerve growth factor receptor-positive. Our results indicate that adult rat Purkinje cells increase low-affinity nerve growth factor receptor-immunoreactive protein in response to injury, suggesting that, in the cerebellum, low-affinity nerve growth factor receptor or low-affinity nerve growth factor receptor-like molecules may be involved in regulating neuronal plasticity during adulthood.

Cortical neurons inhibit basal and interleukin-1-stimulated astroglial cell secretion of nerve growth factor

Primary cultures of neonatal rat cortical neurons and astrocytes synthesize and secrete nerve growth factor (NGF). Co-culturing neurons with astrocytes decreased NGF secretion in the co-cultures. The inhibition of co-culture NGF secretion was partially reversible upon selectively decreasing the number of neurons by glutamate treatment. Interleukin-1 beta (IL-1) stimulated NGF secretion from astrocytes, and the magnitude of this secretion was decreased in the co-cultures. Thus, co-culture with neurons decreases astroglial cell secretion of NGF and down-regulates astroglial responsiveness to IL-1.

Reduction of NGF protein level in rat dorsal hippocampus following administration of kainic acid

Intraperitoneal administration of kainic acid (KA) into adult rats caused a profound increase in nerve growth factor (NGF) mRNA and a significant reduction of NGF protein level in the hippocampus. Diazepam pretreatment suppressed both. The reduction of NGF level was apparent in the dorsal hippocampus at 2 h after KA administration, but a marked elevation of NGF protein was observed in the ventral hippocampus at 4 h. These results suggest that non-N-methyl-D-aspartate (NMDA) receptor agonists negatively influence NGF synthesis or stimulate NGF protein degradation in the dorsal hippocampus involving the CA1 sector.

Nerve growth factor precursors in the rat thyroid and hippocampus

A nerve growth factor (NGF) precursor form of about 24 kDa was identified in homogenates of rat thyroid and hippocampus by immunoprecipitation using three sera raised against a synthetic peptide that reproduces the sequence -71 to -46 of the proNGF molecule. Besides this species, a 31 kDa protein, as well as a cleavage product of 12 kDa were also immunoprecipitated in both tissues by one of these sera.

Detailed distribution of nerve growth factor in rat brain determined by a highly sensitive enzyme immunoassay

We modified a previously reported enzyme immunoassay method to make it more sensitive for quantification of nerve growth factor (NGF), and succeeded in measuring the NGF content in as small as 2 mg (wet weight) of rat brain tissue. Rat brain was cut into about 600 pieces of the same size, and the NGF content in each piece was determined by this method. The findings were as follows: (i) In the cerebral cortex, NGF contents were unevenly distributed, ranging from less than 0.1 to 1.8 ng/g wet wt. The level was highest in the caudal parietal and rostral occipital cortices and lowest in the lateral parietal cortex. (ii) Areas comprising the limbic system such as the cingulate gyrus, pyriform cortex, amygdala, anterior and medial thalamus, hippocampus, septum, and diagonal band of Broca contained high levels of NGF. (iii) In the brain stem and cerebellum, the levels were low; however, a relatively high level was registered in the cerebellar nuclei, lateral vestibular nucleus, ventral cochlear nucleus, superior olive, and pontine reticular nuclei. These findings, taken together with previously published information, suggest that the neurons in the anterior and medial thalamus, pontine reticular nuclei, superior olive, ventral cochlear nucleus, and cerebellar Purkinje cells may be additional populations of NGF-responsive neurons in the rat brain.

Subcellular localization of insulin-like growth factor I (IGF-I) in Purkinje cells of the adult rat: an immunocytochemical study

Insulin-like growth factor I (IFG-I) was localized by light and electron microscopic immunocytochemistry in the adult rat cerebellum. Neurons showing the same cytological characteristics and pattern of synaptic input as Purkinje cells were IGF-I immunoreactive. In these neurons, intense electron-dense reaction product was consistently located in the rough endoplasmic reticulum and, occasionally, in multivesicular bodies and associated with the nuclear envelope. Thus, the results of this study suggest synthesis IGF-I in Purkinje cells and furthermore provide anatomical support for IGF-I participation in paracrine or autocrine regulatory systems in the brain.

Cooperative regulation of nerve growth factor synthesis and secretion in fibroblasts and astrocytes by fibroblast growth factor and other cytokines

Acidic fibroblast growth factor (aFGF) enhances nerve growth factor (NGF) synthesis by astrocytes obtained from various brain regions. NGF secretion by fibrous-shaped astrocytes transformed by dibutyryl-cAMP (db-cAMP) pretreatment was less than that by untreated astrocytes. However, aFGF also enhanced NGF secretion by fibrous-shaped astrocytes. The effects of various kinds of intracellular signaling modulators on NGF synthesis were examined. None of the following second messenger effectors had an effect on NGF synthesis: protein kinase C (PKC) agonist (phorbol myristate acetate (PMA)) or antagonist (sphingosine (SP)). LiCl, and ionomycin (Iono). Further, increases of intracellular cAMP by forskolin (FK) or db-cAMP have no significant effect on NGF synthesis in astrocytes under a standard culture condition. However, NGF synthesis by astrocytes in the presence of aFGF was significantly enhanced by db-cAMP, but not by FK or sodium butyrate. These results indicate that an excessive amount of cAMP enhances the effect of aFGF on NGF synthesis in astrocytes. NGF synthesis in astrocytes was not affected by treatment with anti-aFGF or anti-bFGF neutralizing antibodies, indicating that FGFs are not involved in the autocrine regulation of NGF synthesis in astrocytes. Transforming growth factor-beta 1 (TGF-beta 1), which inhibits some effects of FGFs, increased NGF synthesis in concert with aFGF. Furthermore, the highest NGF synthesis was observed when astrocytes were stimulated by all of the following cytokines: aFGF, interleukin-1 beta (IL-1 beta), tumor necrosis factor-alpha (TNF-alpha) and TGF-beta 1. The mechanism regulating NGF synthesis in fibroblasts obtained from prenatal rat skin was also investigated. Acidic FGF, basic FGF (bFGF), epidermal growth factor (EGF), platelet-derived growth factor (PDGF), transforming growth factor-alpha (TGF-alpha), TGF-beta 1, IL-1 beta, and TNF-alpha were found to be regulators of NGF synthesis in skin fibroblasts. Among these cytokines, aFGF is the most potent regulator of NGF synthesis in fibroblasts. NGF synthesis by skin fibroblasts, either in the presence or absence of aFGF, was not modified by any of the following: FK, PMA, SP, LiCl, and Iono. However, db-cAMP significantly enhanced NGF synthesis in both conditions. Sodium butyrate enhanced NGF synthesis in the presence of aFGF, but not in the absence of aFGF. These results suggest that an excessive amount of cAMP and butyrate moiety regulate NGF synthesis in skin fibroblasts in different ways.(ABSTRACT TRUNCATED AT 400 WORDS)

Molecular mechanisms for generation of neural diversity and specificity: roles of polypeptide factors in development of postmitotic neurons

Development of postmitotic neurons is influenced by two groups of polypeptide factors. Neurotrophic factors promote neuronal survival both in vivo and in vitro. Neuronal differentiation factors influence transmitter phenotypes without affecting neuronal survival. The list of neurotrophic factors is increasing partly because certain growth factors and cytokines have been shown to possess neurotrophic activities and also because new neurotrophic factors including new members of the nerve growth factor (NGF) family have been identified at the molecular level. In vitro assays using recombinant neurotrophic factors and distributions of their mRNAs and proteins have indicated that members of a neurotrophic gene family may play sequential and complementary roles during development and in the adult nervous system. Most of the receptors for neurotrophic factors contain tyrosine kinase domains, suggesting the importance of tyrosine phosphorylation and subsequent signal transduction for their effects. Molecules such as LIF (leukemia inhibitory factor) and CNTF (ciliary neurotrophic factor) have been identified as neuronal differentiation factors in vitro. At the moment, however, it remains to be determined whether or not the receptors for a group of neuronal differentiation factors constitute a gene family or contain domains of kinase or phosphatase activity. Synergetic combinations of neurotrophic and neuronal differentiation factors as well as their receptors may contribute to the generation of neural specificity and diversity.

Interplay between glutamate and gamma-aminobutyric acid transmitter systems in the physiological regulation of brain-derived neurotrophic factor and nerve growth factor synthesis in hippocampal neurons

In the central nervous system brain-derived neurotrophic factor (BDNF) and nerve growth factor (NGF) are predominantly located in neurons. Here we demonstrate that the balance between the activity of the glutamatergic and gamma-aminobutyric acid (GABA)ergic systems controls the physiological levels of BDNF and NGF mRNAs in hippocampal neurons in vitro and in vivo. The blockade of the glutamate receptors and/or stimulation of the GABAergic system reduces BDNF and NGF mRNAs in hippocampus and NGF protein in hippocampus and septum. The reduction of NGF in the septum reflects the diminished availability of NGF in the projection field of NGF-dependent septal cholinergic neurons. These neurons do not synthesize NGF themselves but accumulate it by retrograde axonal transport. The refined and rapid regulation of BDNF and NGF synthesis by the glutamate and GABA transmitter systems suggests that BDNF and NGF might be involved in activity-dependent synaptic plasticity.

Hippocampal damage and kainic acid injection induce a rapid increase in mRNA for BDNF and NGF in the rat brain

In situ hybridization and Northern blots were used to study expression of mRNAs for members of the nerve growth factor family in the rat brain following an excitatory stimulus. One hour after a unilateral needle insertion or saline injection into the dorsal hippocampus, the level of brain-derived neurotrophic factor (BDNF) mRNA increased markedly in granular neurons of the dentate gyrus and in the piriform cortex ipsilateral to the injection. The same treatment also increased the level of NGF mRNA in granular neurons of the ipsilateral dentate gyrus. The rapid increase in BDNF and NGF mRNA after a needle insertion or injection of saline was transient and preceded by an increase in c-fos mRNA in the same brain regions. In contrast to a needle insertion per se or a saline injection, 7 h after a unilateral injection of kainic acid into the dorsal hippocampus, the level of BDNF mRNA was dramatically increased in the ipsilateral hippocampus, as well as in the ipsilateral frontoparietal, piriform and perihinal cortex, the amygdaloid complex, claustrum, and ventromedial hypothalamus. A less pronounced increase was also seen in these brain areas on the contralateral side. Northern blots revealed that the level of BDNF mRNA increased 5- and 40-fold in the contra- and ipsilateral hippocampus, respectively, compared to sham-operated control animals. In contrast to BDNF and NGF, the level of hippocampus-derived neurotrophic factor/neurotrohin-3 (HDNF/NT-3) mRNA was not altered by either needle insertion or injection of saline or kainic acid.(ABSTRACT TRUNCATED AT 250 WORDS)

Structural and functional identification of regulatory regions and cis elements surrounding the nerve growth factor gene promoter

The transcriptional mechanisms which contribute to the regulation of nerve growth factor (NGF) production are still largely unknown. We previously expressed the NGF promoter region in transgenic mice to localize cis regulatory elements to within 5 kb of the promoter. To further map these elements, and to begin to study the corresponding transacting factors, we here assayed the effects of 5' deletions and point mutations and examined the binding of nuclear factors to the NGF promoter region using L929 cell fibroblasts. Sequential deletions delineated regions upstream from the promoter which stimulated and inhibited transcription. DNAse-1 footprinting experiments identified four upstream segments, designated F2, F4, F6 and F8, which bound L929 cell nuclear proteins. F2 and F4 mapped to stimulatory and F6 and F8 to inhibitory regions. Competition experiments using a heptanucleotide present in both F2 and F4 segments suggested that they may be bound by related factors. Gel shift assays showed that the F8 binding proteins are less abundant in L929 cells than in NIH 3T3 fibroblasts and B16 melanoma cells. In addition to the upstream segments, a downstream AP-1 consensus sequence bound L929 nuclear proteins. Mutation of the AP-1 consensus sequence eliminated binding of nuclear proteins and reduced transcriptional activity. Our results indicate that transcriptional activator as well as suppressor regions surround the NGF gene promoter. The regulation of NGF production is likely to involve cis elements within these regions and transacting factors that bind to them.

Regulation of NGF gene expression in CNS glia by cell-cell contact

Nerve growth factor (NGF) gene expression in central nervous system (CNS) glia appears to be associated with active glial growth. To study the underlying molecular mechanisms, we examined the effects of a number of growth-related factors on NGF mRNA expression in glial cultures. Our results suggest that glial membrane interaction, as a consequence of growth, actively inhibits NGF gene expression in CNS glia.

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)

Chronic treatment with scopolamine and physostigmine changes nerve growth factor (NGF) receptor density and NGF content in rat brain

Nerve growth factor (NGF) and NGF receptors were measured in cortex and hippocampus of rats treated with drugs affecting cholinergic neurotransmission. High (Kd = 0.045 nM) and low (Kd = 21 nM) affinity 125I-NGF binding sites were present in both cortical and hippocampal membranes with hippocampus containing higher numbers of both sites than cortex. Chronic treatment of rats with the muscarinic receptor antagonist scopolamine (5 mg/kg, twice daily) decreased the density of high- and low-affinity sites by 50-90% in cortical and hippocampal membranes. These changes were seen after 7 days, but not 3 days, of scopolamine treatment. Chronic infusion of physostigmine (1 mg/kg/day) using minipumps increased the number of high- and low-affinity sites in cortex 3- and 6-fold, respectively. The changes in receptor-binding parameters induced by physostigmine were transient as they were evident after 3 days of treatment, but returned to control levels after 7 days. NGF content in cortex and hippocampus was reduced by about 50% following 7, but not 3, days of chronic physostigmine infusion. In contrast, scopolamine treatment failed to change NGF levels in the cholinergic neuronal target regions but it decreased NGF content in the septal area. The content of NGF mRNA in the cortex measured by Northern blot analysis failed to change following either scopolamine or physostigmine treatment. The results suggest that the levels of NGF and NGF receptors in the target regions of cholinergic neurons are regulated by the extent of cholinergic neurotransmitter activity.

Transplantation of a temperature-sensitive, nerve growth factor-secreting, neuroblastoma cell line into adult rats with fimbria-fornix lesions rescues cholinergic septal neurons

The HT4 cell line was derived from infection of a mouse neuroblastoma cell line with a retrovirus that encoded the temperature-sensitive (ts) mutant of SV40 large T antigen. At nonpermissive temperature, HT4 cells differentiated with neuronal morphology, expressed neuronal antigens, synthesized nerve growth factor (NGF) mRNA, and secreted biologically active NGF in vitro. We sought to establish whether transplanted HT4 cells expressed class I major histocompatibility complex (MHC) antigens, a partial requirement for recognition by cytotoxic T lymphocytes (CTL), and thus be susceptible to xenograft rejection. Differentiated HT4 cells expressed marginally detectable levels of class I MHC antigens, but demonstrated higher levels of class I MHC expression after treatment with interferon-gamma. However, HT4 cells were resistant to direct lysis by perforin, the pore-forming protein of CTLs, and thus may have potential use in xenograft experiments. To address whether HT4 cells secrete NGF in vivo, HT4 cells were transplanted into adults rats with unilateral fimbria-fornix transections. A ts cell line derived from P4 cerebellum, BT1, that does not differentiate with neuronal phenotype or synthesize NGF in vitro, was transplanted as a control. Six weeks posttransplant. HT4 cells had integrated into host CNS without forming tumors. In BT1 transplants, the number of medial septal acetylcholinesterase (AChE)-positive cells was reduced to 26-39% of the contralateral control side, depending on the rostrocaudal level. In HT4 transplants, the number of cholinergic septal neurons was 58-78% of the contralateral side. This percentage was significantly (P less than 0.005) greater than that seen with BT1 transplants, indicating that transplanted HT4 cells secrete NGF in vivo and rescue cholinergic septal neurons following fimbria-fornix transection.

Promotion of central cholinergic and dopaminergic neuron differentiation by brain-derived neurotrophic factor but not neurotrophin 3

Recombinant human brain-derived neurotrophic factor (rhBDNF) and neurotrophin 3 (rhNT-3), two recently cloned molecules closely related to nerve growth factor (NGF), were produced from human cDNA expressed in human embryonic kidney cells. The recombinant proteins were tested in cultures of dissociated fetal rat brain cells containing basal forebrain cholinergic neurons. rhBDNF stimulated the differentiation of the cholinergic neurons, similar to NGF, which is well established as a neurotrophic factor for these cells. However, rhBDNF was particularly effective during the first few days in vitro, whereas the stimulatory action of rhNGF was more pronounced later in the development of the cultures. This finding indicates the existence of different time periods of responsiveness of the cholinergic neurons to BDNF and NGF. To assess the selectivity of the effect of rhBDNF on cholinergic neurons, its actions were tested in cultures of ventral mesencephalon containing dopaminergic cells. In contrast to NGF, which does not affect central dopaminergic neurons, rhBDNF increased dopamine uptake activity. The findings suggest that BDNF stimulates survival or differentiation of other cells besides the cholinergic neurons.

Fibroblast growth factors stimulate nerve growth factor synthesis and secretion by astrocytes

Nerve growth factor (NGF) is produced and secreted by astrocytes and fibroblasts, but not by microglia, in a primary non-neuronal cell culture derived from newborn rat brains under a standard culture condition. NGF secretion by astrocytes was highest just after passage and then gradually decreased. There is no significant difference in NGF secretion by astrocytes from five sites of origin tested: cerebral cortex, striatum, hippocampus, septum, and cerebellum. Acidic and basic fibroblast growth factors (aFGF and bFGF) significantly increased NGF secretion by astrocytes. The effect of aFGF was greater than that of bFGF, and the effect of both FGFs was not additive at the maximum concentration. The peak of NGF secretion stimulated by aFGF occurred 3-12 h after the addition of aFGF. On the other hand, the dramatic increase in cell numbers was observed 12-48 h after stimulation, and the morphological change became significant 24 h after aFGF stimulation. NGF synthesized by astrocytes is rapidly secreted into the culture medium and aFGF enhances NGF secretion from the transcription level, because cycloheximide and actinomycin-D completely inhibited NGF secretion by astrocytes in the presence or absence of aFGF. Epidermal growth factor (EGF), interleukin-1 beta (IL-1 beta), and tumor necrosis factor-alpha (TNF-alpha) also increased NGF secretion by astrocytes to a certain extent. NGF secretion by astrocytes in the presence of a maximum dose of aFGF was enhanced by the addition of IL-1 beta or TNF-alpha, but not EGF. However, platelet-derived growth factor, interleukin-3, and interleukin-6 had no significant effects. FGFs also enhanced NGF secretion by fibroblasts derived from meninges, but not by microglia.

The genes encoding nerve growth factor and its receptor are expressed in the developing female rat hypothalamus

Nerve growth factor (NGF), its messenger RNA (mRNA) and its receptor protein and mRNA are found in several brain regions. Little attention has been given to the possibility that the hypothalamus, which controls the endocrine system, may produce NGF and/or express NGF receptors. This would indicate the existence in this brain area of neuronal populations which are either target for NGF-responsive cells or sensitive to NGF, respectively. Blot hybridization of polyadenylated (poly(A)+) RNA to a mouse NGF cRNA probe revealed the presence of NGF mRNA in both the suprachiasmatic region (henceforth, called preoptic area [POA]) and medial basal hypothalamus (MBH) of developing female rats. The mRNA was already detectable on fetal day 18 and reached maximal levels around postnatal day 3 (MBH) and 12 (POA), declining thereafter. This pattern was temporally different than that of areas known to produce NGF, i.e., the neocortex (Cc) and hippocampus (Hc). In agreement with previous reports, NGF mRNA levels in these areas were negligible before birth, became maximal between the second and third week of postnatal life and decreased moderately thereafter. The concentration of NGF protein, measured by a two-site enzyme immunoassay, was 3 times higher in the POA than in the MBH at an infantile age (day 12), increasing 2-fold in the POA of juvenile animals (day 28) but not in the MBH. This developmental pattern was similar to that seen in the Hc, though the NGF concentrations were significantly lower in the POA.(ABSTRACT TRUNCATED AT 250 WORDS)