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

The Nerve Growth Factor Metabolic Pathway Dysregulation as Cause of Alzheimer's Cholinergic Atrophy

The cause of the loss of basal forebrain cholinergic neurons (BFCNs) and their terminal synapses in the cerebral cortex and hippocampus in Alzheimer's disease (AD) has provoked a decades-long controversy. The cholinergic phenotype of this neuronal system, involved in numerous cognitive mechanisms, is tightly dependent on the target-derived nerve growth factor (NGF). Consequently, the loss of BFCNs cholinergic phenotype in AD was initially suspected to be due to an NGF trophic failure. However, in AD there is a normal NGF synthesis and abundance of the NGF precursor (proNGF), therefore the NGF trophic failure hypothesis for the atrophy of BCNs was abandoned. In this review, we discuss the history of NGF-dependency of BFCNs and the atrophy of these neurons in Alzheimer's disease (AD). Further to it, we propose that trophic factor failure explains the BFCNs atrophy in AD. We discuss evidence of the occurrence of a brain NGF metabolic pathway, the dysregulation of which, in AD explains the severe deficiency of NGF trophic support for the maintenance of BFCNs cholinergic phenotype. Finally, we revise recent evidence that the NGF metabolic dysregulation in AD pathology starts at preclinical stages. We also propose that the alteration of NGF metabolism-related markers in body fluids might assist in the AD preclinical diagnosis.

The Brain NGF Metabolic Pathway in Health and in Alzheimer's Pathology

Emerging research has re-emphasized the role of the cortical cholinergic system in the symptomology and progression of Alzheimer's disease (AD). Basal forebrain (BF) cholinergic nuclei depend on target-derived NGF for survival during development and for the maintenance of a classical cholinergic phenotype during adulthood. In AD, BF cholinergic neurons lose their cholinergic phenotype and function, suggesting an impairment in NGF-mediated trophic support. We propose that alterations to the enzymatic pathway that controls the maturation of proNGF to mature NGF and the latter's ulterior degradation underlie this pathological process. Indeed, the NGF metabolic pathway has been demonstrated to be impaired in AD and other amyloid pathologies, and pharmacological manipulation of NGF metabolism has consequences in vivo for both levels of proNGF/NGF and the phenotype of BF cholinergic neurons. The NGF pathway may also have potential as a biomarker of cognitive decline in AD, as its changes can predict future cognitive decline in patients with Down syndrome as they develop preclinical Alzheimer's pathology. New evidence suggests that the cholinergic system, and by extension NGF, may have a greater role in the progression of AD than previously realized, as changes to the BF precede and predict changes to the entorhinal cortex, as anticholinergic drugs increase odds of developing AD, and as the use of donepezil can reduce rates of hippocampal and cortical thinning. These findings suggest that new, more sophisticated cholinergic therapies should be capable of preserving the basal forebrain thus having profound positive effects as treatments for AD.

Role of Nerve Growth Factor in Plasticity of Forebrain Cholinergic Neurons

Neuronal plastic rearrangements during the development and functioning of neurons are largely regulated by trophic factors, including nerve growth factor (NGF). NGF is also involved in the pathogenesis of Alzheimer's disease. In the brain, NGF is produced in structures innervated by basal forebrain cholinergic neurons and retrogradely transported along the axons to the bodies of cholinergic neurons. NGF is essential for normal development and functioning of the basal forebrain; it affects formation of the dendritic tree and modulates the activities of choline acetyltransferase and acetylcholinesterase in basal forebrain neurons. The trophic effect of NGF is mediated through its interactions with TrkA and p75 receptors. Experimental and clinical studies have shown that brain levels of NGF are altered in various pathologies. However, the therapeutic use of NGF is limited by its poor ability to penetrate the blood-brain barrier, adverse side effects that are due to the pleiotropic action of this factor, and the possibility of immune response to NGF. For this reason, the development of gene therapy methods for treating NGF deficit-associated pathologies is of particular interest. Another approach is creation of low molecular weight NGF mimetics that would interact with the corresponding receptors and display high biological activity but be free of the unfavorable effects of NGF.

Nerve growth factor promoter activity revealed in mice expressing enhanced green fluorescent protein

Nerve growth factor (NGF) and its precursor proNGF are perhaps the best described growth factors of the mammalian nervous system. There remains, however, a paucity of information regarding the precise cellular sites of proNGF/NGF synthesis. Here we report the generation of transgenic mice in which the NGF promoter controls the ectopic synthesis of enhanced green fluorescent protein (EGFP). These transgenic mice provide an unprecedented resolution of both neural cells (e.g., neocortical and hippocampal neurons) and non-neural cells (e.g., renal interstitial cells and thymic reticular cells) that display NGF promoter activity from postnatal development to adulthood. Moreover, the transgene is inducible by injury. At 2 days after sciatic nerve ligation, a robust population of EGFP-positive cells is seen in the proximal nerve stump. These transgenic mice offer novel insights into the cellular sites of NGF promoter activity and can be used as models for investigating the regulation of proNGF/NGF expression after injury.

Apoptosis in the rat basal forebrain during development and following lesions of connections

Evidence suggests that neurotrophins are essential for the survival and phenotypic maintenance of cholinergic basal forebrain (BF) neurons. We evaluated the pattern of programmed cell death in the BF of the rat during development and after ablations of the cerebral cortex, a major target area and source of neurotrophins for BF neurons. We identified dying cells using the TUNEL (terminal deoxynucleotidyl-transferase-mediated dUTP-biotin nick end labelling) method and confirmed their apoptotic morphology with electron microscopy. Moreover, we demonstrated the expression of the apoptotic marker active caspase-3 in cells with features of apoptosis. TUNEL(+) cells were present in the developing BF during the first two postnatal weeks. Their frequency peaked at postnatal day (P)1 and at P5. TUNEL used in conjunction with immunofluorescence for neuronal nuclear protein (NeuN) showed that, at both peak stages, the majority of apoptotic cells were neurons. Extensive lesions of the cerebral cortex at different ages (P0, P7 and P14) did not induce significant changes in the frequency of apoptotic BF neurons. However, they resulted in alterations in the morphological phenotype of choline acetyltransferase (ChAT)-immunoreactive neurons in the BF, and a reduction in their number which was inversely proportional to the age at which the lesions were performed. We suggest that: (i) apoptosis is temporally coordinated with the morphological and neurochemical differentiation of BF neurons and the establishment of connections with their target areas; and (ii) cortical ablations do not affect the survival of BF neurons, but they influence the phenotype of cholinergic BF neurons.

Altered neuronal responses and regulation of neurotrophic proteins in the medial septum following fimbria-fornix transection in CNTF- and leukaemia inhibitory factor-deficient mice

Degeneration of axotomized GABAergic septohippocampal neurones has been shown to be enhanced in ciliary neurotrophic factor (CNTF)-deficient mice following fimbria-fornix transection (FFT), indicating a neuroprotective function of endogenous CNTF. Paradoxically, however, the cholinergic population of septohippocampal neurones was more resistant to axotomy in these mutants. As leukaemia inhibitory factor (LIF) has been identified as a potential neuroprotective factor for the cholinergic medial septum (MS) neurones, FFT-induced responses were compared in CNTF(-/-), LIF(-/-) and CNTF/LIF double knockout mice. In CNTF(-/-) mice, FFT-induced cholinergic degeneration was confirmed to be attenuated as compared with wildtype mice. The expression of both LIF and LIF receptor beta was increased in the MS providing a possible explanation for the enhanced neuronal resistance to FFT in these animals. However, ablation of the LIF gene also produced paradoxical effects; following FFT in LIF(-/-) mice no loss of GABAergic or cholinergic MS neurones was detectable during the first postlesional week, suggesting that other efficient neuroprotective mechanisms are activated in these animals. In fact, enhanced activation of astrocytes, a source of neurotrophic proteins, was indicated by increased up-regulation of glial fibrillary acidic protein and vimentin expression. In addition, mRNA levels for neurotrophin signalling components (e.g. nerve growth factor, p75(NTR)) were differentially regulated. The positive effect on axotomized cholinergic neurones seen in CNTF(-/-) and LIF(-/-) mice as well as the increased up-regulation of astrogliose markers was abolished in CNTF/LIF double knockout animals. Our results indicate that endogenous CNTF and LIF are involved in the regulation of neuronal survival following central nervous system lesion and are integrated into a network of neurotrophic signals that mutually influence their expression and function.

Polysialic acid limits choline acetyltransferase activity induced by brain-derived neurotrophic factor

Choline acetyltransferase (ChAT), the enzyme synthesizing acetylcholine, is known to be activated by brain derived neurotrophic factor (BDNF). We found that the specific removal of the carbohydrate polysialic acid (PSA) significantly increased BDNF-induced ChAT-activity in embryonic septal neurons. Using a p75 neurotrophin receptor (p75(NTR)) function-blocking antibody and K252a, a-pan tropomyosin related kinase (Trk) inhibitor, we demonstrate that BDNF-induced ChAT activity requires the stimulation of p75(NTR) and TrkB. PSA removal drastically increased radioactive iodinated ([(125)I])BDNF's maximal binding capacity (Bmax), derived from concentrations of [(125)I]BDNF ranging from 1 pM to 3.2 nM. In the presence of unlabeled nerve growth factor to prevent the binding of [(125)I]BDNF to p75(NTR) sites, the impact of PSA removal on the binding capacity of [(125)I]BDNF was greatly reduced. In conclusion, PSA limits BDNF-induced ChAT activity and BDNF-receptor interactions. BDNF-induced ChAT activity is TrkB and p75(NTR) dependent, and upon PSA removal the additional binding of BDNF to its receptors, especially p75(NTR), likely contributes to the maximal ChAT activity observed. In vivo, the ontogenetic loss of PSA in the postnatal period may allow more interactions between BDNF and its receptors to increase ChAT activity and assure the proper development of the cholinergic septal 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.

Susceptibility to seizure-induced injury and acquired microencephaly following intraventricular injection of saporin-conjugated 192 IgG in developing rat brain

To study the role of neurotrophin-responsive neurons in brain growth and developmental resistance to seizure-induced injury, we infused saporin-conjugated 192-IgG (192 IgG-saporin), a monoclonal antibody directed at the P75 neurotrophin receptors (p75(NTR)), into the ventricles of postnatal day 8 (P8) rat pups. 7-10 days after immunotoxin treatment, loss of p75(NTR) immunoreactivity was associated with depletion of basal forebrain cholinergic projection to the neocortex and hippocampus. Kainic acid (KA)-induced seizures on P15 resulted in hippocampal neuronal injury in the majority of toxin-treated animals (13/16), but only rarely in saline-injected controls (2/25) (P < 0.001). In addition, widespread cerebral atrophy and a significant decrease in brain weight with preserved body weight were observed. Volumetric analysis of the hippocampal hilar region revealed a 2-fold reduction in perikaryal size and a 1.7-fold increase in cell packing density after 192 IgG-saporin injection. These observations indicate that neurotrophin-responsive neurons including basal forebrain magnocellular cholinergic neurons may be critical for normal brain growth and play a protective role in preventing excitotoxic neuronal injury during development.

Adulthood nicotine treatment alleviates behavioural impairments in rats neonatally treated with quinpirole: possible roles of acetylcholine function and neurotrophic factor expression

Increases in dopamine D(2) receptor sensitivity are known to be common in drug abuse and neurological disorders. Past data from this laboratory have shown that long-term increases in D(2) sensitivity can be produced by quinpirole treatment (a D(2)/D(3) agonist) during early development. The present investigation was designed to test the hypothesis that nicotine administration in adulthood would reduce both cognitive and skilled reaching impairments produced by increases in D(2) sensitivity. Female Sprague-Dawley rats were treated with quinpirole (1 mg/kg) or saline from postnatal day 1 (PD 1) to PD 21. Beginning in adulthood (PD 61), rats were treated with nicotine (0.3 mg/kg free base) or saline twice daily for 14 consecutive days before behavioural testing commenced. Animals neonatally treated with quinpirole demonstrated performance deficits on the Morris water task and a skilled reaching task compared to controls. Deficits on both tasks were completely alleviated by adulthood nicotine treatment. Animals neonatally treated with quinpirole demonstrated a significant 36% decrease of ChAT in the hippocampus compared to saline controls that was partially eliminated by nicotine. Additionally, neonatal quinpirole produced a significant decrease in hippocampal NGF content compared to controls, however, nicotine failed to alleviate this decrease in NGF. The results of this investigation demonstrate that long-term increases in dopamine D(2) receptor sensitivity produce significant decreases in hippocampal cholinergic and NGF expression that may result in cognitive impairment. Nicotine alleviates both cognitive and skilled reaching impairments caused by increases in D(2) sensitivity, but the mechanism through which nicotine is acting is currently unknown.

NGF induces appearance of adult-like response to spatial novelty in 18-day male mice

We investigated the effects of Nerve Growth Factor (NGF) administration on the maturation of reactivity to spatial and non-spatial novelty in developing mice. CD-1 mice of both sexes received intracerebral administration of NGF on postnatal day (pnd) 15, and their response to object displacement (spatial novelty) and object substitution (object novelty) were assessed in a spatial open-field with four objects on pnd 18 or 28. On pnd 18, NGF induced only in males precocious appearance of spatial novelty discrimination, while increasing choline acetyltransferase activity in neocortex and hippocampus of both sexes. The behavioral and neurochemical effects disappeared by pnd 28. NGF triggers adult-like responding to spatial novelty in developing mice and such effect is gender-specific.

Neurotrophins induce death of hippocampal neurons via the p75 receptor

Nerve growth factor (NGF) and related neurotrophins influence neuronal survival and differentiation via interactions with the trk family of receptors. Recent studies have demonstrated that neurotrophins may also induce cell death via the p75 receptor. The importance and generality of neurotrophin-induced death in the brain have not been defined but may play a critical role during development and in disease-associated neuronal death. Here we demonstrate for the first time that all four members of the neurotrophin family directly elicit the death of hippocampal neurons via the p75 receptor. The hippocampus is a complex structure with many different neuronal subpopulations, and signals that influence neuronal death during development may have a critical impact on the mature function of this structure. In these studies we show that each neurotrophin causes the death of hippocampal neurons expressing p75 but lacking the cognate trk receptor. Neurotrophin-induced neuronal death is mediated by activation of Jun kinase. These studies demonstrate that neurotrophins can regulate death as well as survival of CNS neurons.

Role of nitric oxide in the ethylcholine aziridinium model of delayed apoptotic neurodegeneration in vivo and in vitro

The involvement of nitric oxide in neurodegenerative processes still remains incompletely characterized. Although nitric oxide has been reported to be an important mediator in neuronal degeneration in different models of cell death involving NMDA-receptor activation, increasing evidence for protective mechanisms has been obtained. In this study the role of nitric oxide was investigated in a model of NMDA-independent, delayed apoptotic cell death, induced by the neurotoxin ethylcholine aziridinium ethylcholine aziridinium both in vivo and in vitro. For the in vivo evaluation rats received bilateral intracerebroventricular injections of ethylcholine aziridinium (2nmol/ventricle) or vehicle. In the hippocampus a transient decrease in nitric oxide synthase activity occurred, reaching its lowest levels three days after ethylcholine aziridinium treatment (51.7+/-9.8% of controls). The decrease coincided with the maximal reduction in choline acetyltransferase activity as marker for the extent of cholinergic lesion. The effect of pharmacological inhibition of nitric oxide synthase was tested by application of various nitric oxide synthase inhibitors with different selectivity for the nitric oxide synthase-isoforms. Unspecific nitric oxide synthase inhibition resulted in a significant potentiation of the loss of choline acetyltransferase activity in the hippocampus measured seven days after ethylcholine aziridinium application, whereas the specific inhibition of neuronal or inducible nitric oxide synthase was ineffective. These pharmacological data are suggestive for a neuroprotective role of nitric oxide generated by endothelial nitric oxide synthase. In vitro experiments were performed using serum-free primary neuronal cell cultures from hippocampus, cortex and septum of E15-17 Wistar rat embryos. Ethylcholine aziridinium-application in a range of 5-80microM resulted in delayed apoptotic neurodegeneration with a maximum after three days as confirmed by morphological criteria, life-death assays and DNA laddering. Nitric oxide synthase activity in harvested cells decreased in a dose- and time-dependent manner. Nitric oxide production as determined by measurement of the accumulated metabolite nitrite in the medium was equally low in controls and in ethylcholine aziridinium treated cells (range 0.77-1.86microM nitrite). An expression of inducible nitric oxide synthase messenger RNA could not be detected by semiquantitative RT-PCR 13h after ethylcholine aziridinium application. The present data indicate that in a model of delayed apoptotic neurodegeneration as induced by ethylcholine aziridinium neuronal cell death in vitro and in vivo is independent of the cytotoxic potential of nitric oxide. This is confirmed by a decrease in nitric oxide synthase activity, absence of nitric oxide production and absence of inducible nitric oxide synthase expression. In contrast, evidence for a neuroprotective role of nitric oxide was obtained in vivo as indicated by the exaggeration of the cholinergic lesion after unspecific nitric oxide synthase inhibition by N-nitro-L-arginine methylester.

Is there a metabolic basis for dietary supplementation?

To be efficacious, dietary supplements must either provide a nutrient that is normally undersupplied to cells or exert a pharmacologic effect on cellular processes. In the first case, optimal function is achieved when a nutrient required by the organism reaches a specific concentration within the cell. A supplement has benefit only when the normal intake of a bioavailable form of a nutrient is lower than the amount that would provide maximum benefit as judged from all biological perspectives. Metabolic, environmental, and genetic factors can make individual nutrient requirements differ from the estimated needs calculated from population-based data. For example, under certain circumstances intracellular antioxidants may be depleted and a dietary supplement might restore optimal antioxidant protection. In the second case, the dietary supplement contains a constituent that is normally not required by the cell, but this substance is capable of altering normal cell function. For example, herbal preparations may contain ephedrine (a drug), which might alter heart rate so that the amount of blood pumped by the heart is enhanced. An understanding of how the variation in nutrient requirements comes about and of the pharmacologic actions of nutrient supplements can help to identify which individuals are most likely to benefit from dietary supplements.

Postnatal intracerebroventricular administrations of NGF alter spatial memory in adulthood

The present work assessed the effects of intracerebroventricular injections (2x5 mg/2.5 ml) of recombined human nerve growth factor (rhNGF) at postnatal days 2 and 3 upon the development of spatial learning capacities in rats. The treated rats were trained at the age of 22 days to escape onto an invisible platform at a fixed position in space in a Morris navigation task. For half of the subjects, the training position was also cued, a procedure aimed at facilitating escape and reducing attention to the distant spatial cues. At the age of 2 months all the rats were retrained in the same task. Treatment effects were found in both immature and adult rats. The injection of NGF induced a slight alteration of the immature rats' performance. In contrast, a marked impairment of spatial abilities was shown in the 2-month-old rats. The most consistent effects were a significant increase in the escape latency and a decrease bias towards the training platform area during probe trials. The reduction of spatial memory was particularly marked if the subjects had been trained in a cued condition. Taken together, these experiments reveal that an acute pharmacological treatment that leads to transient modifications during early development might induce a behavioural change long after treatment. Thus, the development and the maintenance of an accurate spatial representation are tightly related to the development of brain structures that could be altered by precocious NGF administrations.

Nerve growth factor expression and release in allergic inflammatory disease of the upper airways

It is well known that allergic airways disease is characterized by inflammation and hyperresponsiveness, but the link between these two conditions has not been elucidated. We have previously shown that in allergic rhinitis, hyperresponsiveness is attributable to increased neural reactivity. We thus hypothesized that nerve growth factor (NGF), which is expressed by inflammatory cells and effects changes that lead to increased neural responsiveness, could be a pivotal mediator in this disease. Using reverse transcription-polymerase chain reaction (RT-PCR), Western immunoblotting, and ELISA to evaluate NGF expression and release, we found that subjects with allergic rhinitis have significantly decreased NGF mRNA in superficial nasal scrapings and significantly higher baseline concentrations of NGF protein in nasal lavage fluids, compared with control subjects. Nasal provocation with allergen significantly increased NGF protein in nasal lavage fluids of subjects with allergic rhinitis, but not of control subjects. The concentrations of NGF protein in nasal lavage fluids were not affected by provocation with the vehicle for allergen or with histamine. These data provide the first evidence of a steady state of dysregulation in mucosal NGF expression and release in allergic rhinitis, and support a role of this neurotrophin in the pathophysiology of allergic inflammatory disease of the human airways.

Enhanced visuospatial memory following intracerebroventricular administration of nerve growth factor

The present work assessed the effects of intracerebroventricular injections of rh recombined human nerve growth factor (rh NGF) (5 micrograms/2.5 microl) at postnatal days 12 and 13 upon the development of spatial learning capacities. The treated rats were trained at the age of 22 days to escape onto an invisible platform at a fixed position in space in a Morris navigation task. For half of the subjects, the training position was also cued, a procedure aimed at facilitating escape and at reducing attention to the distant spatial cues. Later, at the age of 6 months, all the rats were trained in a radial-arm maze task. Treatment effects were found in both immature and adult rats. The injection of NGF improved the performance in the Morris navigation task in both training conditions. There was a significant reduction in the escape latency and an increased bias toward the training platform quadrant during probe trials. The most consistent effect was the precocious development of an adult-like spatial memory. In the radial-arm maze, the NGF-treated rats made significantly fewer reentries than vehicle rats and this effect was particularly marked in the treated female rats. Taken together, these experiments reveal that the development and the maintenance of an accurate spatial representation are tightly related to the development of brain structures facilitated by the action of NGF. Moreover, these experiments demonstrate that an acute pharmacological treatment that leads to a transient modification in the choline acetyltransferase activity can induce a behavioral change long after the treatment.

Increased density of M1 receptors in the hippocampus of juvenile rats chronically deprived of NGF

Binding studies were used to assess the changes in affinity and/or number of M1 muscarinic receptors in hippocampi from juvenile rats chronically deprived of NGF. NGF deprivation was obtained by implanting into right ventricle at postnatal day 2 (P2) hybrydoma cells secreting high levels of monoclonal antibodies against NGF (alphaD11). Parenteral myeloma cells (P3U) were used as controls. Competition experiments were used to characterise the [3H]-PNZ binding sites in membrane preparations of hippocampi from rats sacrificed at P15. [3H]-PNZ bound M1 receptors both in P3U and alphaD11 group as shown by displacing potency order of antagonists: TLZ=4-DAMP>PNZ>p-F-HHSiD>MTC. The deprivation of NGF for two weeks significantly increased the number of M1 receptors without changing the Ki values of antagonists with exception of methoctramine which showed an increase in affinity in alphaD11 group. Similar changes in binding parameters were already observed after the first week of anti-NGF treatment. In contrast, a treatment for a week with implant at postnatal day 15 failed to produce any changes in M1 binding parameters. These results provide further physiological evidence for developmentally regulated modulatory role of NGF in the cholinergic function in the hippocampus.

Cholinergic function in the hippocampus of juvenile rats chronically deprived of NGF

Intracellular and extracellular recordings were used to assess the cholinergic function in hippocampal slices from juvenile rats chronically deprived of NGF. NGF was neutralised by implanting into the lateral ventricle of postnatal (P) day 2 rats, alphaD11 hybridoma cells (secreting monoclonal antibodies specific for NGF). Parental myeloma cells (P3U) were used as controls. At P15-P18, slow cholinergic EPSPs could be elicited in cells from both alphaD11- and P3U-treated rats. However, slices from alphaD11-implanted rats exhibited a 50% reduction in acetylcholine release following stimulation of cholinergic fibres. This effect was associated to a significant increase in the sensitivity of pyramidal cells to carbachol, as suggested by the shift to the left of the dose/response curve. This may reflect a compensatory mechanism for the reduced efficacy of cholinergic innervation in NGF-deprived rats. In both alphaD11- and P3U-treated rats, carbachol was able to induce a similar concentration-dependent depression of the field EPSPs, evoked by Schaffer collateral stimulation, suggesting that presynaptic muscarinic receptors were not altered. In rats implanted with alphaD11 cells at P15 and sacrificed at P21-P24, no changes in the sensitivity to carbachol were found. At this developmental stage, no differences in acetylcholine release were observed between P3U- and alphaD11-treated animals. These results provide physiological evidence for a regulatory role of NGF in the cholinergic function of the hippocampus during postnatal development.

Postnatal development of muscarinic autoreceptors modulating acetylcholine release in the septohippocampal cholinergic system. I. Axon terminal region: hippocampus

We studied the postnatal development of the release of acetylcholine (ACh) and of presynaptic, release-inhibiting muscarinic autoreceptors in the rat hippocampus. To this end, hippocampal slices (350 microns thick) from rats of various postnatal ages (postnatal day 3 [P3] to P16) were preincubated with [3H]choline and stimulated twice (S1, S2: 360 pulses, 2 ms, 3 Hz, 60 mA) during superfusion with physiological buffer containing hemicholinium-3 (10 microM). In parallel, the activities of hemicholinium-sensitive high-affinity choline uptake (HACU, in synaptosomes) and of choline acetyltransferase (ChAT, in crude homogenates) were determined as markers for the cholinergic ingrowth. In hippocampal slices preincubated with [3H]choline, the electrically evoked overflow of 3H at S1 increased from 0.11 (P3) to 0.81% of tissue 3H (P16), the latter value being still much lower than that of hippocampal slices from adult rats (2.89% of tissue 3H). Already at P3 the evoked overflow of 3H was Ca(2+)-dependent and sensitive to tetrodotoxin, indicating an action potential-evoked exocytotic mechanism of ACh release. The muscarinic agonist oxotremorine (1 microM) significantly inhibited the evoked ACh release in hippocampal slices with increasing effectivity from P4 to P16; no significant effect was detectable at P3. The ACh esterase inhibitor physostigmine and the muscarinic antagonist atropine (1 microM, each) exhibited significant inhibitory and facilitatory effects, respectively, only at P15-16. The specific activities of both hippocampal HACU (pmoles/mg protein/min) and ChAT (nmoles/mg protein/min) continuously increased from P3 to P16. It is concluded (1) that cholinergic nerve terminals arriving at the hippocampal formation during postnatal ingrowth are already endowed with the apparatus for action potential-induced, Ca(2+)-sensitive (exocytotic) ACh release; (2) that, in contrast, the expression of presynaptic muscarinic autoreceptors on these cholinergic axon terminals is delayed; and (3) that autoinhibition due to endogenous ACh develops even later, probably when the density of presynaptic terminals in the hippocampus and hence, the concentration of released ACh has reached a suprathreshold value.

Hypertrophy of basal forebrain neurons and enhanced visuospatial memory in perinatally choline-supplemented rats

The effects of choline supplementation during two time-frames of early development on radial-arm maze performance and the morphology of basal forebrain neurons immunoreactive for the P75 neurotrophin receptor (NTR) in male and female Sprague-Dawley rats were examined. In the first experiment, rats were supplemented with choline chloride from conception until weaning. At 80 days of age, subjects were trained once a day on a 12-arm radial maze for 30 days. Compared to control littermates, supplemented rats made fewer working and reference memory errors; however, the memory enhancing effects of choline supplementation were greater in males than females. A morphometric analysis of NTR-immunoreactive cell bodies at three levels through the medial septum/diagonal band (MS/DBv) of these rats revealed that perinatal choline supplementation caused the somata of cells in the MS/DBv to be larger by 8-15%. In a second experiment, choline supplementation was restricted to embryonic days 12-17. A developmental profile of NTR immunoreactive cell bodies in the MS/DBv of 0-, 8-, 16-, 30- and 90-day old male and female rats again revealed that cell bodies were larger in choline-supplemented rats than controls. As in the behavioral studies, the effect of choline supplementation was greater in male than female rats. These data are consistent with the hypothesis that supplementation with choline chloride during early development leads to an increase in the size of cell bodies of NTR-immunoreactive cells in the basal forebrain and that this change may contribute to long-term improvement in spatial memory.

NGF delays rather than prevents the cholinergic terminal damage and delayed neuronal death in the hippocampus after ischemia

Cerebral ischemia induces damage of cholinergic terminals in the hippocampus, which preceded the delayed neuronal death (DND) of the CA1 pyramidal cells. We investigated the effects of nerve growth factor (NGF) on the cholinergic terminal damage after ischemia. Continuous NGF infusion (0.5 microg/7 days) into the lateral ventricle before and after 5 min ischemia prevented a decrease in choline acetyltransferase (ChAT)-immunoreactivity and disturbance of acetylcholine (ACh) release on the 4th day after ischemia, but not on day 7, i.e., NGF infusion caused delay in the progress of the cholinergic terminal damage. These findings show that the cholinergic terminal damage may result from deficiency of endogenous NGF in an ischemic brain. In addition, we investigated whether NGF would prevent the DND after ischemia. NGF infusion also caused delay in the progress of the DND until day 14. Our results suggested that the neuroprotective effect of NGF on the DND may be secondarily yielded by maintenance of communication between cholinergic terminal and the target CA1 cell, and that prevention of cholinergic terminal damage may be useful for the treatment of cerebrovascular disease.

Developmental changes in the NGF content in the brain of young, growing, low-birth-weight rats

The NGF content in each region of the brain of four-week-old rats was ranked in the decreasing order of cerebral cortex, hippocampus, cerebellum, midbrain/diencephalon, and pons/medulla oblongata, and the NGF concentration, in the decreasing order of hippocampus, cerebral cortex, cerebellum, midbrain/diencephalon, and pons/medulla oblongata in both AFD and SFD groups. The NGF content and concentration in the cerebral cortex were about the same value at each age between those in the AFD and SFD groups. Those in the hippocampus were a little higher in the SFD group than in the AFD group at the ages of three and four weeks, unlike those in the other regions, where the values for the cerebellum, midbrain/diencephalon and pons/medulla oblongata tended to be somewhat higher in the AFD group than in the SFD group. The NGF concentrations in the hippocampus and cerebral cortex increased with growth: the concentration in the hippocampus at four weeks of age was about 4-fold of that at one week in the AFD group and about 5.7-fold of that at one week in the SFD group; and likewise the concentration in the cerebral cortex at four weeks of age was about 5.3-fold in the AFD group and about 7-fold in the SFD group. The NGF concentrations in the cerebellum decreased, and those in midbrain/diencephalon and pons/medulla oblongata hardly changed with growth in either AFD or SFD group. From these results NGF may have stronger implications for the neuronal growth in the hippocampus compared with those in the lower brain regions of the SFD rats.

c-Jun expression in adult rat dorsal root ganglion neurons: differential response after central or peripheral axotomy

The response of the mature central nervous system (CNS) to injury differs significantly from the response of the peripheral nervous system (PNS). Axotomized PNS neurons generally regenerate following injury, while CNS neurons do not. The mechanisms that are responsible for these differences are not completely known, but both intrinsic neuronal and extrinsic environmental influences are likely to contribute to regenerative success or failure. One intrinsic factor that may contribute to successful axonal regeneration is the induction of specific genes in the injured neurons. In the present study, we have evaluated the hypothesis that expression of the immediate early gene c-jun is involved in a successful regenerative response. We have compared c-Jun expression in dorsal root ganglion (DRG) neurons following central or peripheral axotomy. We prepared animals that received either a sciatic nerve (peripheral) lesion or a dorsal rhizotomy in combination with spinal cord hemisection (central lesion). In a third group of animals, several dorsal roots were placed into the hemisection site along with a fetal spinal cord transplant. This intervention has been demonstrated to promote regrowth of severed axons and provides a model to examine DRG neurons during regenerative growth after central lesion. Our results indicated that c-Jun was upregulated substantially in DRG neurons following a peripheral axotomy, but following a central axotomy, only 18% of the neurons expressed c-Jun. Following dorsal rhizotomy and transplantation, however, c-Jun expression was upregulated dramatically; under those experimental conditions, 63% of the DRG neurons were c-Jun-positive. These data indicate that c-Jun expression may be related to successful regenerative growth following both PNS and CNS lesions.

Developmental expression of the NGF receptor p140trk in the septohippocampal system of the rat: a quantitative analysis

An RNAse protection assay was used to identify p140trk mRNA in the developing rat septohippocampal system. In both the septum and hippocampus, levels of p140trk mRNA were low at birth and increased thereafter. Levels of transcripts were found to be much higher in the septum than in the hippocampus, whereas another brain region, the hypothalamus, showed levels of expression intermediate between these two structures. Only one isoform of the p140trk receptor was found to be expressed in the rat central nervous system (CNS) during development. This isoform corresponds to the one preferentially expressed in neural tissues in the adult animal. These data show that expression of the high affinity nerve growth factor (NGF) receptor is developmentally regulated during postnatal brain development and suggest that it might mediate NGF effects on developing central cholinergic systems.

Absence of p75NTR causes increased basal forebrain cholinergic neuron size, choline acetyltransferase activity, and target innervation

Emerging evidence suggests that the p75 neurotrophin receptor (p75NTR) mediates cell death; however, it is not known whether p75NTR negatively regulates other neuronal phenotypes. We found that mice null for p75NTR displayed highly significant increases in the size of basal forebrain cholinergic neurons, including those that are TrkA-positive. Cholinergic hippocampal target innervation also was increased significantly. Activity of the cholinergic neurotransmitter synthetic enzyme choline acetyltransferase (ChAT) was increased in both the medial septum and hippocampus. Upregulation of these cholinergic features was not associated with increased basal forebrain or hippocampal target NGF levels. In contrast, striatal cholinergic neurons, which do not express p75NTR, showed no difference in neuronal number, size, or ChAT activity between wild-type and p75NTR null mutant mice. These findings indicate that p75NTR negatively regulates cholinergic neuronal phenotype of the basal forebrain cholinergic neurons, including cell size, target innervation, and neurotransmitter synthesis.

Recovery of ChAT immunoreactivity in axotomized rat cholinergic septal neurons despite reduced NGF receptor expression

Previous studies have suggested that target-derived nerve growth factor (NGF) is essential for the survival of cholinergic basal forebrain neurons. Thus, axotomy of septohippocampal neurons in adult rats resulting in the withdrawal of target-derived NGF caused a dramatic loss of choline acetyltransferase (ChAT)-immunoreactive neurons in the medial septum-diagonal band complex. We have recently shown that this loss of immunolabelled neurons does not indicate cell death, since many septohippocampal cholinergic neurons recover their immunoreactivity for ChAT after a long survival time despite disconnection from target-derived neurotrophins. One possibility would be that these surviving ChAT-immunoreactive neurons have gained access to other, probably local, NGF sources. Here we provide evidence that the recovery of ChAT immunoreactivity after axotomy is not accompanied by a similar recovery of NGF receptor expression in these neurons. In situ hybridization for p75NTR mRNA and trkA mRNA 6 months after bilateral fimbria-fornix transection revealed a substantial loss of labelled cells. In addition, there was a persisting loss of p75NTR-immunoreactive and NGF-immunoreactive medial septal neurons. Cholinergic neurons in controls did not express NGF mRNA, but were heavily immunostained for NGF protein due to receptor-mediated uptake. These data suggest that at least some cholinergic septohippocampal neurons re-express ChAT either independently of NGF or with a reduced need for NGF.

Moderate lesion of the rat cholinergic septohippocampal pathway increases hippocampal nerve growth factor synthesis: evidence for long-term compensatory changes?

Moderate lesions of the septohippocampal pathway by intraventricular infusions of ethylcholine aziridinium (AF64A) induced a dose-dependent decrease of hippocampal choline acetyltransferase (ChAT) activity, which partially recovered between 1 and 5 weeks after treatment. The cholinergic deficit was associated with an increase in nerve growth factor (NGF) mRNA only within the hippocampal dentate gyrus and hilus by maximally 51% and 111% 3 and 7 weeks after AF64A treatment, respectively, whereas no changes in brain-derived neurotrophic factor- and neurotrophin-3 mRNA were observed. The content of NGF protein transiently increased in the ventral part of the hippocampus 3 weeks after AF64A infusion but returned to control levels at 5 weeks. At that time, however, NGF content as well as ChAT activity were significantly increased in the septum, suggesting an increased utilization of NGF by the remaining cholinergic neurons. Thus, the present data provide correlative evidence for a critical role of endogenous NGF in neuroregeneration and plasticity of the cholinergic basal forebrain in case of incipient damage.

Development of cholinergic and GABAergic neurons in the rat medial septum: effect of target removal in early postnatal development

During normal development of the nervous system, the target fields influence the survival and differentiation of projection neurons, but the factors regulating this interaction remain obscure. In the present study, we have raised the question whether the target region is essential for the postnatal development and maintenance of two different types of central projection neurons, cholinergic and GABAergic septohippocampal cells. In early postnatal rats (P5, P10), the hippocampus was eliminated by unilateral intrahippocampal injections of the excitotoxin N-methyl-D-aspartate. After a long survival time (at P70), we have immunostained serial sections of the septal region with antibodies against choline acetyltransferase (ChAT), the acetylcholine-synthesizing enzyme, or the calcium-binding protein parvalbumin (PARV) which is known to be contained in GABAergic septohippocampal neurons. In the medial septum ipsilateral to the lesioned side, about 60% of ChAT-immunoreactive neurons and 62% of PARV-immunoreactive neurons were found in adulthood even after complete elimination of the hippocampus. Some immunoreactive cells appeared heavily shrunken, but electron microscopic analysis revealed ultrastructural characteristics typical for medial septal neurons obtained from controls. Our results indicate that target elimination during development affected both types of projection cells, although only the cholinergic cells are known to be responsive to target-derived factors.

Glucocorticoids and the hippocampus. Developmental interactions facilitating the expression of behavioral inhibition

When threatened, the rapid induction of fear and anxiety responses is adaptive. This article summarizes the current knowledge of the neurobiological development of behavioral inhibition, a prominent response occurring in fear and anxiety-provoking situations. In the rat, behavioral inhibition as exemplified by freezing first appears near the end of the second postnatal week. This emergence of freezing coincides with the developmental period marked by the rapid increase in plasma concentrations of glucocorticoids. Studies show that removal of glucocorticoids at this time severely impairs the age-dependent appearance of freezing. This behavioral impairment produced by adrenalectomy, however, is prevented by exogenous glucocorticoid administration. The effectiveness of glucocorticoids in facilitating the development of freezing appears to be caused by its actions in the hippocampus. In particular, glucocorticoids appear to play a vital role in the postnatal cellular development of the hippocampal dentate gyrus. Doses of glucocorticoids shown to reverse the behavioral inhibitory deficits occurring after adrenalectomy are ineffective when hippocampal dentate granule neurons are destroyed by neurotoxins. Notably, site-specific administration of glucocorticoids to the dorsal hippocampus is successful in promoting the occurrence of freezing in the adrenalectomized rat pup. It is hypothesized that glucocorticoids exert their behavioral inhibitory effects by influencing the development of the septohippocampal cholinergic system. Support for this hypothesis is derived from work demonstrating the importance of glucocorticoids on nerve growth factor systems that play a critical role in septohippocampal cholinergic survival.

Nerve growth factor and the neurotrophic factor hypothesis

The discovery of nerve growth factor (NGF) over 40 years ago led to the formulation of the "Neurotrophic Factor Hypothesis". This hypothesis states that developing neurons compete with each other for a limited supply of a neurotrophic factor (NTF) provided by the target tissue. Successful competitors survive; unsuccessful ones die. Subsequent research on NTFs has shown that NTF expression and actions are considerably more complex and diverse than initially predicted. Even for NGF, different regulatory patterns are seen for different neuronal populations. As would be predicted by the "Neurotrophic Factor Hypothesis", NGF levels critically regulate basal forebrain cholinergic neuron size and neurochemical differentiation. In contrast, the level of trkA, the NGF receptor, regulates these properties in caudate-putamen cholinergic neurons. Understanding NTF regulation and actions on neurons has led to their use in clinical trials of human neurological diseases. NTFs may emerge as important therapies to prevent neuronal dysfunction and death.

Development of cholinergic and GABAergic neurons in the rat medial septum: different onset of choline acetyltransferase and glutamate decarboxylase mRNA expression

In the present study, we have investigated the developmental expression of the transmitter-synthesizing enzymes choline acetyltransferase (ChAT) and glutamate decarboxylase (GAD) in rat medial septal neurons by using in situ hybridization histochemistry. In addition, we have employed immunostaining for ChAT and the calcium-binding protein parvalbumin, known to be contained in septohippocampal GABAergic neurons. A large number of GAD67 mRNA-expressing neurons were already observed in the septal complex on embryonic day (E) 17, the earliest time point studied. During later developmental stages, there was mainly an increase in the intensity of labeling. Neurons expressing ChAT mRNA were first recognized at E 20, and their number slowly increased during postnatal development of the septal region. The adult pattern of ChAT mRNA-expressing neurons was observed around postnatal day (P) 16. By using a monoclonal ChAT antibody, the first immunoreactive cells were not seen before P 8. Similarly, the first weakly parvalbumin-immunoreactive neurons were seen in the septal complex by the end of the 1st postnatal week. These results indicate that in situ hybridization histochemistry may be an adequate method to monitor the different development of transmitter biosynthesis in cholinergic and GABAergic septal neurons. Moreover, the late onset of ChAT mRNA expression would be compatible with a role of target-derived factors for the differentiation of the cholinergic phenotype.

Systemic administration of anti-NGF antibodies to neonatal mice impairs 24-h retention of an inhibitory avoidance task while increasing ChAT immunoreactivity in the medial septum

Neonatal mice received subcutaneous injections of either antibody against murine NGF raised in goat (3 mg, injection volume 50 microliters) or preimmune serum on postnatal days 2, 4, 6, 8, 10, and 12. They were tested on postnatal days 15-16 or 20-21 for learning and 24-h retention of a passive avoidance step-through task. Immunostaining for choline acetyltransferase (ChAT) was measured in two cholinergic forebrain areas (septum and caudate-putamen) on postnatal day 16 or 21. Locomotor activity and exploratory behavior in an open-field test were also assessed on day 17 or 22, following a single administration of either scopolamine (2 mg/kg) or saline solution. While anti-NGF treatment did not affect acquisition on day 15, impairment in retention was evident on day 16. On days 20-21, no effects were found either on acquisition or on retention capabilities. Analysis of ChAT immunostaining revealed a significant increase of ChAT-immunopositive cells in the medial septal area in 16-day-old but not in 21-day-old mice. Behavior in the open-field test and age-typical response to scopolamine were not altered by anti-NGF at either of the two ages considered. These data support the view that immunological neutralization of endogenous NGF specifically affects the maturation of retention capabilities in altricial rodents, and confirm the involvement of forebrain cholinergic mechanisms in early memory processes.

Neurotrophins and the primate central nervous system: a minireview

The central nervous system (CNS) of primates is more complex than the CNS of other mammals. Details of the development and aging of the primate CNS have recently been revealed by various neurobiological techniques. It has become clear that the primate CNS has unique characteristics, for example, the capacity for the overproduction and elimination of fibers and synapses. Some differences have also been found in the distribution of and changes with development in levels of various neuroactive substances. Recent discoveries of a variety of neurotrophins in the mammalian CNS have led to research on the neurobiology of these molecules in the primate CNS. The distribution of and changes with development in levels of nerve growth factor (NGF) in the primate CNS are closely correlated with the cholinergic system of the basal forebrain. The administration of NGF into the monkey brain prevents the degeneration of the cholinergic neurons of the basal forebrain after axotomy, a result that suggests that neurotrophins might be very valuable agents for the future treatment of neurological diseases, such as Alzheimer's and Parkinson's diseases.

Neonatal exposure to anti-nerve growth factor antibodies affects exploratory behavior of developing mice in the hole board

The aim of this study was to assess in developing mice whether the neutralization of endogenous NGF following ICV administration of anti-NGF antibodies (50 micrograms/2 microliters) on postnatal days 3, 6, 9, and 12 affected locomotor activity, exploratory behavior, and response to the cholinergic blocker scopolamine. In Experiments 1 and 2 activity and age-typical scopolamine effects were evaluated on PND 13 or 17 in an automated apparatus. No significant main effect of anti-NGF treatment was found at either age. On day 13 scopolamine (0.2, 1, or 2 mg/kg) decreased locomotion in both anti-NGF and control animals. In Experiment 3, locomotion and exploratory behavior were analyzed in an open field arena or in a hole board apparatus on PND 16. No significant effects of anti-NGF treatment on general motor activity and investigation of a novel object in the open field was found, though anti-NGF animals tended to be less active than controls. In the hole board anti-NGF pups showed a different pattern of head dipping behavior from controls, exploring mainly the holes located in the periphery of the apparatus.

Role of nerve growth factor in the expression of trkA mRNA in cultured embryonic rat basal forebrain cholinergic neurons

Using a quantitative reverse transcription-polymerase chain reaction (RT-PCR), we studied the regulation of trkA mRNA expression in serum-free, cultured basal forebrain neurons from 17-day fetal rats. Besides increasing choline acetyltransferase (ChAT) activities, nerve growth factor (NGF) strikingly induced trkA gene expression in a time- and NGF concentration-dependent manner. Therefore, NGF might play a critical role in trkA gene expression during the development of basal forebrain cholinergic neurons. Furthermore, to investigate whether this up-regulation is connected with the trophic effects on basal forebrain cholinergic neurons, we examined the effects of some other neurotrophic agents (BDNF, NT-3, bFGF, CNTF, and 40 mM KCI) upon ChAT activity and trkA gene expression. Some neurotrophic factors increased ChAT activities to the same degree as NGF, whereas they did not stimulate trkA mRNA expression so potently. NT-3 plus K252b promotes the tyrosine phosphorylation of TrkA in PC12 cells and increases ChAT activity in cultured basal forebrain cholinergic neurons like NGF (Knusel et al., J Neurochem 59: 715-722, 1992). We found that NT-3 plus K252b upregulated the level of trkA mRNA. These results suggested that the expression of trkA mRNA is regulated directly by its specific ligand NGF, rather than neurotrophic effects upon basal forebrain cholinergic neurons and that the up-regulation is connected to a molecular event initiated by the binding of NGF to the TrkA receptor.

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.

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.

Changes in the response of cultured septal cholinergic neurons to nerve growth factor exposure and deprivation during the first postnatal month

The effects of nerve growth factor (NGF) and a blocking anti-NGF antibody were studied in cultures plated from postnatal day 1-28 (P1-P28) rat septum and maintained 3 weeks in vitro. 7S NGF (100 ng/ml = 0.75 nM) increased choline acetyltransferase (ChAT) activity in P7-P21 cultures. The largest increase was measured in P7-P14 cultures, where NGF addition produced ChAT activities 5-12 times higher than those measured in cultures grown in anti-NGF antibody. NGF also increased the number of acetylcholinesterase (AChE)-positive neurons in P7-P14 cultures. To determine whether this increase was due to enhanced survival of cholinergic neurons or simply to enhanced AChE expression, we examined cultures to which NGF was added only after an initial 1-2-week exposure to anti-NGF antibody. This delayed addition of NGF also increased ChAT activity and the number of AChE-positive neurons, indicating that cholinergic neurons survived the initial exposure to anti-NGF antibody. Thus even during a period when ChAT activity was most sensitive to NGF, postnatal septal cholinergic neurons did not require NGF for survival in vitro.

Localization and regulation of low affinity nerve growth factor receptor expression in the rat olfactory system during development and regeneration

Nerve growth factor (NGF), a classic neurotrophic factor, promotes neuronal survival, maintenance, regeneration and differentiation in the peripheral nervous system and parts of the central nervous system. NGF activity is mediated by cell surface bound receptors including the low affinity NGF receptor (LNGFr) which is expressed by some peripheral and central neurons and is present on peripheral nerve Schwann cells during development and regeneration. The olfactory system is a useful model for the study of the role of LNGFr in neuronal development and regeneration. The growth of olfactory axons into the brain begins in the embryo and continues through the first few postnatal weeks. In mature animals there is persistent turnover and generation of olfactory receptor neurons (ORNs) and continuous growth of new axons into the olfactory bulb. These new axons grow along the preexisting olfactory pathway. In the mature olfactory system, LNGFr has been observed in the glomerular layer of the olfactory bulb, the target of ORNs. However, neither the cellular localization nor the developmental expression of LNGFr has been characterized. Here, we tested the hypothesis that LNGFr expression is developmentally regulated in the olfactory nerve and is reinduced following injury to the mature olfactory nerve. LNGFr-immunoreactivity (IR) was first observed in the olfactory mucosa at embryonic day (E)13 and in the olfactory nerve at E14. LNGFr-IR increased in the nerve during embryonic development, began to decrease at around postnatal day (P)5 and was scarcely detectable in normal adults. The staining pattern suggests that LNGFr is located on the olfactory nerve Schwann cells. Streaks of LNGFr-IR were present in the adult olfactory nerve. We reasoned that these streaks might represent transient reexpression of LNGFr associated with normal olfactory neuron turnover and replacement. Consistent with this hypothesis, LNGFr was robustly reexpressed in the adult olfactory nerve following lesion of the olfactory epithelium. Starting late in development (E21) and in the adult, LNGFr-IR was also observed on fibers in deep layers of the olfactory bulb. LNGFr-IR was also observed in neurons of the nucleus of the diagonal band (NDB) in the basal forebrain. NDB is the sole source of cholinergic afferents of the olfactory bulb. Thus, we tested the hypothesis that LNGFr in the deep layers of the olfactory bulb is located on NDB axons by making lesions of NDB. Following the lesion, LNGFr-IR disappeared in the deep layers of the olfactory bulb but remained in the glomerular layer.(ABSTRACT TRUNCATED AT 400 WORDS)

Interleukin-6 protects cultured rat hippocampal neurons against glutamate-induced cell death

We examined the effect of interleukin-6 (human recombinant) on glutamate-induced neuronal death of cultured 20-day fetal rat hippocampal neurons. After 7 days in culture, the hippocampal neurons were markedly degenerated by the addition of L-glutamate and also N-methyl-D-aspartate. The neuronal death was prevented by the addition of MK801, a potent N-methyl-D-aspartate antagonist. Interleukin-6 at the concentration of 50 ng/ml has a significant preventive effect on the glutamate-induced neuronal death. Basic fibroblast growth factor at the concentration of 100 ng/ml gave also significant protective effect on hippocampal neurons, but nerve growth factor was ineffective in preventing the toxicity. It has been postulated that glutamate plays an important role in the pathogenesis of neuronal death such as ischemia and the various neurological diseases. Interleukin-6 might have somewhat physiological or pathological role in these events.

Nerve growth factor induces trkA mRNA expression in cultured basal forebrain cholinergic neurons from 17-day fetal rats

We first examined the basal forebrain tissues for developmental changes in expression of the high-affinity NGF receptor (trkA) gene. Our reverse transcriptase polymerase chain reaction (RT-PCR) analysis showed that trkA mRNA is present in those tissues of postnatal rats, but not in those of fetal rats. Then we determined the effect of NGF on trkA gene expression in serum-free cultures of basal forebrain neurons from 17-day fetal rats. NGF was found to induce trkA mRNA 36 h after the addition of NGF, while no trkA mRNA was detected in the absence of NGF.

Fibroblast growth factor-5 promotes differentiation of cultured rat septal cholinergic and raphe serotonergic neurons: comparison with the effects of neurotrophins

Fibroblast growth factor-5 (FGF-5) is a member of the fibroblast growth factor gene family, which has a signal sequence characteristic of secretory proteins. FGF-5 mRNA has previously been shown to be present in the adult mouse brain. Here we demonstrate that recombinant FGF-5 has neurotrophic activity on cultured rat septal cholinergic and raphe serotonergic neurons. The effect of FGF-5 on serotonin uptake was stronger than that evoked with either brain-derived neurotrophic factor or neurotrophin-3. FGF-5 also increased the choline acetyltransferase activity of cultured rat septal cholinergic neurons, the effect being additive to that of nerve growth factor. In situ hybridization experiments and immunohistochemistry using a specific anti-FGF-5 antibody demonstrated that FGF-5 is expressed in rat hippocampal neurons. Like nerve growth factor mRNA, the levels of FGF-5 mRNA in the rat hippocampus increased substantially during early postnatal development. In addition, injection of the muscarinic receptor agonist pilocarpine elevated FGF-5 mRNA. The presence of the secretory FGF-5 in the rat hippocampus, a target field of septal cholinergic and raphe serotonergic neurons, suggests that FGF-5 acts as a trophic factor for these neurons also in vivo.

Chronic 1,25-dihydroxyvitamin D3-mediated induction of nerve growth factor mRNA and protein in L929 fibroblasts and in adult rat brain

We have proposed that elevating levels of nerve growth factor (NGF) in the CNS is a rational strategy for treating certain neurodegenerative disorders. The present studies were conducted to determine: (1) if pharmacologically induced levels of NGF could be sustained for an extended time, and (2) if correlations exist between increases in NGF mRNA and NGF protein in L929 cells and in vivo. Short-term treatment of L929 cells with 1,25-dihydroxyvitamin D3 resulted in a two-fold increase in both NGF mRNA and NGF protein. These increases were sustained for up to 48 h with continuous exposure to 1,25-dihydroxyvitamin D3. In rats, 1,25-dihydroxyvitamin D3 (2.5 nmol; i.c.v.) induced NGF mRNA transiently, with peak two-fold increases observed 4 h post-injection. In contrast to L929 cells, 1,25-dihydroxyvitamin D3 did not elicit an increase in NGF protein after a single administration in vivo. However, consistent with long-term exposure in L929 cells, chronic 6 day infusion of 1,25-dihydroxyvitamin D3 resulted in induction of both NGF mRNA and NGF protein in the brain. These results indicate that 1,25-dihydroxyvitamin D3-mediated NGF induction in cultured L929 cells may predict of NGF induction in vivo, suggesting that L929 cells may have utility in studying underlying mechanisms of NGF induction by 1,25-dihydroxyvitamin D3. On the basis of NGF's ability to increase cholinergic function in animal models of cholinergic degeneration, these results are supportive of a role for NGF inducers as potential drugs for neurodegenerative disorders.

Trophic factors during normal brain aging and after functional damage

There is an increasing body of information concerning the physiological role of several target-derived neurotrophic proteins that are structurally and functionally related to the classical neurotrophic molecule NGF and which resemble a genetic family called neurotrophins. However apart from NGF, there is little knowledge about the pathophysiological role of these neurotrophins concerning aging or dementia. To our present knowledge, decreased NGF production does not seem to play a causal role in age-related cognitive impairment which is usually associated with neurodegenerative processes in the cholinergic basal forebrain system. However, there are several experimentally found indications that NGF might be of importance in the stimulation of compensatory changes and repair mechanisms. Moreover, recent findings suggest that disturbances in cerebral glucose metabolism may play an important role in cognitive disabilities during normal aging and also in dementia disorders such as Alzheimer's disease. Intracerebroventricular (ICV) injection of streptozotocin (STZ) has been reported to decrease cerebral glucose utilization and energy metabolism and to impair passive avoidance learning in adult rats. One week after ICV STZ treatment, NGF content was significantly decreased in the septal region, where NGF-responsive cell bodies are known to be located and where NGF exerts its neurotrophic action after retrograde transport from NGF-producing targets. In contrast, NGF levels were increased within 3 weeks after ICV STZ treatment by about the same magnitude as has been observed for aged learning-impaired rats in the target regions for the basal forebrain cholinergic neurons.(ABSTRACT TRUNCATED AT 250 WORDS)