NGF deprivation of adult rat brain results in cholinergic hypofunction and selective impairments in spatial learning

CNS Gene Therapy and a Nexus of Complexity: Systems and Biology at a Crossroads

Gene therapy is a potentially promising new treatment for neurodegenerative disorders such as Alzheimer's disease (AD), which has been difficult to treat with conventional therapeutics. Viral vector-mediated somatic gene therapy is a rapidly developing methodology for providing never before achieved capability to deliver specific genes to the CNS in a highly localized and controlled manner. With the advent and refinements of this technology one focus is directed to which genes are the most appropriate to select for specific disease indications. Nerve growth factor (NGF), a potent survival factor for critical cell populations that degenerate in AD, has been chosen already for clinical gene therapy trials in human AD patients. Much knowledge about the pathophysiological underpinnings of AD is still lacking to make clear which patients may benefit from a gene therapy approach. Moreover, a detailed understanding of sustained NGF action in the normal and diseased CNS needs to be resolved before conclusions can be drawn regarding the utility of NGF gene therapy. Systematic efforts to acquire this new knowledge should compel clinically and biologically sophisticated efforts to advance gene therapy for neurodegenerative diseases.

Growth factor signaling and memory formation: temporal and spatial integration of a molecular network

Growth factor (GF) signaling is critically important for developmental plasticity. It also plays a crucial role in adult plasticity, such as that required for memory formation. Although different GFs interact with receptors containing distinct types of kinase domains, they typically signal through converging intracellular cascades (e.g., Ras–MEK–MAPK) to mediate overlapping functional endpoints. Several GFs have been implicated in memory formation, but due to a high level of convergent signaling, the unique contributions of individual GFs as well as the interactions between GF signaling cascades during the induction of memory is not well known. In this review, we highlight the unique roles of specific GFs in dendritic plasticity, and discuss the spatial and temporal profiles of different GFs during memory formation. Collectively, the data suggest that the roles of GF signaling in long-lasting behavioral and structural plasticity may be best viewed as interactive components in a complex molecular network.

NGF is essential for hippocampal plasticity and learning

Nerve growth factor (NGF) is produced in the hippocampus throughout life and is retrogradely trafficked to septal cholinergic neurons, providing a potential mechanism for modulating cholinergic inputs and, thereby, hippocampal plasticity. To explore NGF modulation of hippocampal plasticity and function, NGF levels were augmented or blocked in intact adult rats, and subsequent in vivo effects on cholinergic neurons, hippocampal long-term potentiation (LTP), and learning were examined. NGF augmentation significantly enhanced cholinergic neuronal markers and facilitated induction of hippocampal LTP. Blockade of endogenous NGF significantly reduced hippocampal LTP and impaired retention of spatial memory. These findings reveal an essential role for NGF in regulating biological mechanisms related to plasticity and memory in the intact adult brain.

Sex differences in water maze performance and cortical neurotrophin levels of LHX7 null mutant mice

Mice lacking both alleles of the LIM-homeobox gene Lhx7 display dramatically reduced number of forebrain cholinergic neurons. Given the fact that sex differences are consistently observed in forebrain cholinergic function, in the present study we investigated whether the absence of LHX7 differentially affects water maze performance in the two sexes. Herein we demonstrate that LHX7 null mutants display a sex-dependent impairment in water maze, with females appearing more affected than males. Moreover, neurotrophin assessment revealed a compensatory increase of brain-derived neurotrophic factor and neurotrophin 3 in the neocortex of both male and female mutants and an increase of nerve growth factor levels only in the females. Nevertheless, the compensatory increase of cortical neurotrophin levels did not restore cognitive abilities of Lhx7 homozygous mutants. Finally, our analysis revealed that cortical neurotrophin levels correlate negatively with water maze proficiency, indicating that there is an optimal neurotrophin level for successful cognitive performance.

Beta/A4-Amyloid increases nerve growth factor production in rat primary hippocampal astrocyte cultures

Nerve growth factor (NGF), a member of the neurotrophin family, is an essential mediator of neuronal activity and synaptic plasticity of basal forebrain cholinergic neurons (BFCN). In processes of chronic degeneration of BFCN like in Alzheimer's disease (AD), characterized among others by amyloid containing plaques, NGF has been shown to improve cognitive decline and rescue BFCN but also to reduce survival of hippocampal neurons via p75 neurotrophin receptor (p75). Little is known about the mechanisms of NGF regulation in glial cells under pathological conditions in AD. This study investigates the influence of amyloid administration on the NGF protein secretion in rat primary hippocampal astrocytes. Astrocytes were stimulated with "aged" beta/A4-Amyloid (1-40), and NGF was measured in different fractions, such as supernatant, vesicles, and cytosol fraction. Treatment with amyloid at a final concentration of 10 microM for 72 h led to increased NGF protein levels up to 30-fold increase compared to unstimulated controls. This observation may be an endogenous neuroprotective mechanism possibly contributing to a delay of amyloid-dependent loss of cholinergic neurons or contribute to accelerated neuronal death by activation of p75 within Alzheimer pathology.

Translational considerations for CNS gene therapy

Gene transfer is being rigorously evaluated in the laboratory in the preparation for the development of clinical therapies. Many CNS diseases, which have proved more challenging to treat than peripheral disorders, are prime candidates for gene therapy. However, there are numerous considerations in the development of gene therapy, including delivery, maintenance of expression, transgene level regulation, toxicity of the viral vector system and safety of the gene product. The authors review these issues and discuss various approaches used in preclinical studies. Alzheimer's and Parkinson's disease are employed as models, in which much research has already been performed, to address disease-specific questions about gene therapy approaches.

The modular systems biology approach to investigate the control of apoptosis in Alzheimer's disease neurodegeneration

Apoptosis is a programmed cell death that plays a critical role during the development of the nervous system and in many chronic neurodegenerative diseases, including Alzheimer's disease (AD). This pathology, characterized by a progressive degeneration of cholinergic function resulting in a remarkable cognitive decline, is the most common form of dementia with high social and economic impact. Current therapies of AD are only symptomatic, therefore the need to elucidate the mechanisms underlying the onset and progression of the disease is surely needed in order to develop effective pharmacological therapies. Because of its pivotal role in neuronal cell death, apoptosis has been considered one of the most appealing therapeutic targets, however, due to the complexity of the molecular mechanisms involving the various triggering events and the many signaling cascades leading to cell death, a comprehensive understanding of this process is still lacking.

Modular systems biology is a very effective strategy in organizing information about complex biological processes and deriving modular and mathematical models that greatly simplify the identification of key steps of a given process.

This review aims at describing the main steps underlying the strategy of modular systems biology and briefly summarizes how this approach has been successfully applied for cell cycle studies. Moreover, after giving an overview of the many molecular mechanisms underlying apoptosis in AD, we present both a modular and a molecular model of neuronal apoptosis that suggest new insights on neuroprotection for this disease.

The effects of adulthood olanzapine treatment on cognitive performance and neurotrophic factor content in male and female rats neonatally treated with quinpirole

Male and female Sprague-Dawley rats were administered quinpirole (1 mg/kg, i.p.) or saline once daily from postnatal day (P)1 to P21. This drug treatment has been shown to produce long-term priming of the D2 receptor. Beginning on P62, rats were administered the atypical antipsychotic olanzapine (2.5 mg/kg) or saline twice daily (i.p.) for 28 days. One day after olanzapine treatment ceased, rats were tested on the place and match-to-place versions of the Morris water maze (MWM) for seven consecutive days. Dopamine D2 receptor priming was verified through a yawning behavioural test, a D2 receptor-mediated event, before olanzapine was administered as well as after olanzapine treatment and behavioural testing were complete. Results showed that neonatal quinpirole treatment induced D2 priming that was eliminated by olanzapine treatment. On the MWM place version, D2-primed rats demonstrated a significant impairment that was eliminated by olanzapine treatment, but olanzapine treatment to animals neonatally treated with saline produced a significant deficit on the place version of the MWM. There were no significant deficits on the match-to-place version. Brain tissue analyses revealed that neonatal quinpirole treatment produced a significant decrease in hippocampal NGF, BDNF and ChAT that was eliminated by olanzapine treatment. Neonatal quinpirole treatment produced a significant decrease in BDNF and ChAT in the frontal cortex that was unaffected by olanzapine treatment. These results show that olanzapine eliminates D2 receptor priming and cognitive impairment and also alleviates decreases in neurotrophins and acetylcholinergic markers produced by D2 priming in the hippocampus.

Area-specific effects of brain-derived neurotrophic factor (BDNF) genetic ablation on various neuronal subtypes of the mouse brain

The effects of brain-derived neurotrophic factor (BDNF) on the development of presynaptic terminals and of neuronal subtypes in various brain areas were studied in BDNF-knockout (BDNF-/-) mice at postnatal days 15-17. Western analysis revealed no changes in the overall amount of a variety of synaptic proteins in BDNF-/- mice as compared to wild type mice. In addition, the complex between the vesicular proteins, synaptophysin and synaptobrevin, as well as their respective homodimers were unaltered. Moreover, no changes in the density of neurons were found in, e.g., the CA3 region of the hippocampus and the nucleus nervi facialis of BDNF-/- mice. However, cholinergic cells were reduced by 20% in the medial septum of BDNF-/- mice associated with a decrease in the activity of choline acetyltransferase and protein levels of nerve growth factor in the hippocampus by 16% and 44%, respectively. In the striatum, however, the total number of cholinergic cells were comparable in both groups, although the activity of choline acetyltransferase was decreased by 46%. In GABAergic interneurons, the expression of neuropeptides in various brain areas was differentially affected by BDNF deletion as revealed by immunohistochemistry. In the hippocampus and cortex of BDNF-/- mice, the density of neuropeptide Y-, somatostatin-, and parvalbumin-immunoreactive cells was drastically reduced, whereas the density of calretinin-positive cells was increased. The extent of these changes in neuropeptide-containing cells varied among hippocampal subregions. In the striatum, only the density of parvalbumin-immunoreactive cells was decreased by approximately 45%. In conclusion, BDNF deficiency is accompanied by a differential dysregulation in the expression of neuropeptides and calcium-binding proteins in otherwise intact GABAergic and glutamatergic neurons in a region-specific manner.

Repeated nicotine exposure in rats: effects on memory function, cholinergic markers and nerve growth factor

A decrease in the number of nicotinic-acetylcholine receptors (nAChRs) in the brain is thought to contribute to the cognitive dysfunction associated with diseases as diverse as Alzheimer's disease and schizophrenia. Interestingly, nicotine and similar compounds have been shown to enhance memory function and increase the expression of nAChRs and therefore, could have a therapeutic role in the aforementioned diseases. Nicotine has also been shown to exert positive effects on certain neurotrophins such as nerve growth factor (NGF), and therefore could play a role beyond mere symptomatic therapy. However, to date, comprehensive studies of nicotine's effects on the expression of specific acetylcholine (ACh) receptor subtypes, key cholinergic proteins (that are regulated by NGF) such as choline acetyltransferase (ChAT) and the vesicular ACh transporter (VAChT) are lacking. Studies to further investigate the effects of nicotine on NGF especially its high- and low-affinity receptors are also needed. In the present study, male Wistar rats exposed a relatively low dosage of nicotine (0.35 mg/kg every 12 h) for 14 days demonstrated improved memory performance (assessed in two separate water maze testing methods) when compared with controls. Autoradiographic experiments indicated that nicotine increased [3H]-epibatidine, [125I]-alpha-bungarotoxin and [3H]-AFDX384, but not [3H]-pirenzepine binding sites in several learning- and memory-related brain areas. The expression of ChAT, VAChT, as well as tropomyosin-receptor kinase A (TrkA) NGF receptors and phospho-TrK receptors was increased by nicotine in the hippocampus. No changes were observed in the levels of the NGF peptide or low affinity p75 neurotrophin receptors (p75NTR), however. These results suggest that repeated exposure to nicotine results in positive effects on central cholinergic markers and memory function, which may be mediated via effects on high-affinity NGF receptors.

Mice lacking leukocyte common antigen-related (LAR) protein tyrosine phosphatase domains demonstrate spatial learning impairment in the two-trial water maze and hyperactivity in multiple behavioural tests

Leukocyte common antigen-related (LAR) protein is a cell adhesion molecule-like receptor-type protein tyrosine phosphatase. We previously reported that in LAR tyrosine phosphatase-deficient (LAR-Delta P) mice the number and size of basal forebrain cholinergic neurons as well as their innervation of the hippocampal area was reduced. With the hippocampus being implicated in behavioural activity aspects, including learning and memory processes, we assessed possible phenotypic consequences of LAR phosphatase deficiency using a battery of rodent behaviour tests. Motor function and co-ordination tests as well as spatial learning ability assays did not reveal any performance differences between wildtype and LAR-Delta P mice. A spatial learning impairment was found in the difficult variant of the Morris water maze. Exploration, nestbuilding and activity tests indicated that LAR-Delta P mice were more active than wildtype littermates. The observed hyperactivity in LAR-Delta P mice could not be explained by altered anxiety or curiosity levels, and was found to be persistent throughout the nocturnal period. In conclusion, behavioural testing of the LAR-Delta P mice revealed a spatial learning impairment and a significant increase in activity.

Rescue of NGF-deficient mice II: basal forebrain cholinergic projections require NGF for target innervation but not guidance

Basal forebrain cholinergic (BFC) neurons are an important substrate of cognitive function and are hypothesized to require the presence of nerve growth factor (NGF) for survival and target innervation. NGF-deficient mice develop BFC neurons that extend projections into telencephalic targets, but the mice perish before innervation is fully established. Rescue of NGF-deficient mice by transgenic expression of NGF under the keratin promoter yields viable mice with disrupted CNS expression of NGF. In the current study, rescued NGF-deficient mice contain normal numbers of septal cholinergic neurons yet reveal severe compromise of cholinergic innervation of both cortex and hippocampus. Surprisingly, intracerebroventricular infusion of NGF into juvenile mice can induce an essentially normal pattern of cholinergic innervation of the hippocampus. These results indicate that NGF is required for induction of proper innervation by BFC neurons, but that the cellular pattern of expression of this factor is not critical for specifying the distribution of axon terminals.

Chronic cannabis abuse raises nerve growth factor serum concentrations in drug-naive schizophrenic patients

Long-term cannabis abuse may increase the risk of schizophrenia. Nerve growth factor (NGF) is a pleiotropic neurotrophic protein that is implicated in development, protection and regeneration of NFG-sensitive neurones. We tested the hypothesis that damage to neuronal cells in schizophrenia is precipitated by the consumption of cannabis and other neurotoxic substances, resulting in raised NGF serum concentrations and a younger age for disease onset. The NGF serum levels of 109 consecutive drug-naive schizophrenic patients were measured and compared with those of healthy controls. The results were correlated with the long-term intake of cannabis and other illegal drugs. Mean (+/- SD) NGF serum levels of 61 control persons (33.1 +/- 31.0 pg/ml) and 76 schizophrenics who did not consume illegal drugs (26.3 +/- 19.5 pg/ml) did not differ significantly. Schizophrenic patients with regular cannabis intake (> 0.5 g on average per day for at least 2 years) had significantly raised NGF serum levels of 412.9 +/- 288.4 pg/ml (n = 21) compared to controls and schizophrenic patients not consuming cannabis (p < 0.001). In schizophrenic patients who abused not only cannabis, but also additional substances, NGF concentrations were as high as 2336.2 +/- 1711.4 pg/ml (n = 12). On average, heavy cannabis consumers suffered their first episode of schizophrenia 3.5 years (n = 21) earlier than schizophrenic patients who abstained from cannabis. These results indicate that cannabis is a possible risk factor for the development of schizophrenia. This might be reflected in the raised NGF-serum concentrations when both schizophrenia and long-term cannabis abuse prevail.

Chronic nicotine administration increases NGF-like immunoreactivity in frontoparietal cerebral cortex

Nicotine/nicotine agonists, which have been proposed as therapeutic agents for the treatment of Alzheimer's disease and other neurodegenerative disorders, produce a wide variety of effects on the nervous system. Some mechanisms involved remain poorly understood. In this work, immunohistochemical techniques were used to determine the effect of nicotine on nerve growth factor (NGF) in the frontoparietal (motor, somatosensory) brain cortex of the albino rat. Nicotine was chronically administered intraperitoneally using osmotic pumps (0.35 mg nicotine base/kg body weight/day for 14 days). An increase in the number and the immunoreaction intensity of NGF-like positive pyramidal and nonpyramidal neurons of these cortical areas was observed after treatment. Immunopositive astroglial cells were always seen in sections of treated animals but not in controls. The neuropil of control animals was, in general, devoid of reaction, but in treated animals, immunopositive prolongations were located randomly, some in close association with capillaries. At the electron microscopic level, these prolongations were demonstrated as belonging to neurons (dendrites and axons) and astroglial cells. Nicotinic activation of selected neurons and glial cells seems to trigger NGF/neurotrophic mechanisms, suggesting their use may be of benefit in prevention and treatment of neurodegenerative diseases.

Improvement of spatial learning and memory after adenovirus-mediated transfer of the nerve growth factor gene to aged rat brain

Adenovirus-mediated transfer of the nerve growth factor gene promotes significant recovery of age-related cholinergic neuronal deficits in aged rats, but the effects of such treatment on cognitive dysfunction remain unclear. Herein we report a beneficial effect of first-generation adenovirus-mediated nerve growth factor gene transfer (AdNGF) on the spatial learning and memory of aged rats. The NGF protein was detected by enzyme-linked immunosorbent assay in cerebrospinal fluid as early as 3 days after gene transfer and was expressed for at least 30 days. Escape latency in the Morris water maze hidden-platform test was significantly improved on day 8 postinoculation in memory-impaired rats treated with AdNGF as well as at later testing intervals. Ultimately, the escape latency values for the AdNGF group become indistinguishable from those for aged rats with normal learning capacity. Immunohistochemical analysis of septal cholinergic neurons for choline acetyltransferase (ChAT) showed significant increases in both the number and somal distribution of ChAT-positive cells after inoculation of memory-impaired rats with AdNGF. Improvement in memory performance was positively correlated with increases in both NGF concentration in cerebrospinal fluid (r = 0.73, p = 0.005) and the number of ChAT-staining cells (r = 0.77, p = 0.0022). We conclude that AdNGF can improve cognitive function in memory-impaired aged rats and, with refinements in vector-driven expression of the transgene, may prove suitable for use in humans.

Neuroplasticity in Alzheimer's disease

Ramon y Cajal proclaimed in 1928 that "once development was ended, the founts of growth and regeneration of the axons and dendrites dried up irrevocably. In the adult centers the nerve paths are something fixed, ended and immutable. Everything must die, nothing may be regenerated. It is for the science of the future to change, if possible, this harsh decree." (Ramon y Cajal, 1928). In large part, despite the extensive knowledge gained since then, the latter directive has not yet been achieved by 'modern' science. Although we know now that Ramon y Cajal's observation on CNS plasticity is largely true (for lower brain and primary cortical structures), there are mechanisms for recovery from CNS injury. These mechanisms, however, may contribute to the vulnerability to neurodegenerative disease. They may also be exploited therapeutically to help alleviate the suffering from neurodegenerative conditions.

Delayed NGF infusion fails to reverse axotomy-induced degeneration of basal forebrain cholinergic neurons in adult p75(LNTR)-deficient mice

The p75 low-affinity neurotrophin receptor (p75(LNTR)) appears to have various functions that include enhancing nerve growth factor (NGF)-mediated survival by increasing TrkA (high-affinity NGF receptor) efficiency, and mediating apoptosis by acting as a ligand-regulated pro-apoptotic receptor. Here, we investigated the role of p75(LNTR) for adult cholinergic basal forebrain neurons by comparing neuronal responses to injury in control and p75(LNTR)-deficient mice. In both types of mice, approximately 70% of the cholinergic neurons in the ipsilateral medial septum had lost their markers choline acetyltransferase and tyrosine kinase A by 28 days following unilateral transection of the dorsal septohippocampal pathway (fimbria fornix). A 7-day delayed infusion of NGF that started 28 days after the injury resulted in reversal of choline acetyltransferase expression and cell atrophy in control, but not in p75(LNTR)-deficient, mice. This lack of response to delayed NGF treatment in p75(LNTR)-deficient mice was most likely not due to cell death, as all of the septohippocampal neurons, labeled with Fluorogold before the lesion, were present at 28 days post-lesion, similar to control mice. p75(LNTR)-deficient cholinergic neurons can respond to NGF as they were protected by NGF infusions that started immediately after the injury. These observations, the fact that lesioned p75(LNTR)-deficient neurons atrophy faster, and that non-lesioned neurons hypertrophy in response to NGF in control but not in p75(LNTR)-deficient mice, suggest that p75(LNTR) is needed for tyrosine kinase A and NGF signaling efficiency.In conclusion, during adulthood p75(LNTR) appears to play a beneficial role in the response of cholinergic neurons to injury, consistent with the proposed role of p75(LNTR) in the enhancement of TrkA signaling and the transport of neurotrophins by these neurons.

Acute cholinergic rescue of synaptic plasticity in the neurodegenerating cortex of anti-nerve-growth-factor mice

Deficits in cholinergic systems innervating cerebral cortex are associated with cognitive impairment during senescence and in age-related neurodegenerative pathologies. However, little is known about the role of cholinergic pathways in modulating cortical plasticity. Basal forebrain cholinergic neurons are a major target for nerve-growth factor (NGF). In order to investigate the relationship between cholinergic innervation and cortical synaptic plasticity, we exploited a transgenic mouse model in which the activity of NGF in the adult nervous system is neutralized by the expression of blocking antibodies to NGF itself (anti-NGF mice) [Ruberti, F. et al. (2000). J. Neurosci. 20, 2589-2601]. In 6-month-old anti-NGF mice, we show that the reduction in cholinergic innervation of the cortex is associated with different forms of synaptic plasticity impairment. A local, acute increase in the availability of acetylcholine rescues these synaptic plasticity deficits, thus indicating that a cholinergic system mediates the impairment of cortical plasticity at this early stage of the neurodegenerative process triggered by NGF neutralization. Our results represent an important step in unveiling the pivotal role of cholinergic transmission in modulating adult cortical plasticity.

The precursor pro-nerve growth factor is the predominant form of nerve growth factor in brain and is increased in Alzheimer's disease

Nerve growth factor (NGF) is important for regulation, differentiation, and survival of peripheral and central nervous system neurons, including basal forebrain cholinergic neurons (BFCN) which degenerate in Alzheimer's disease (AD). Mature NGF protein is processed from a larger precursor, proNGF. We demonstrate that proNGF is the predominant form of NGF in mouse, rat, and human brain tissue, whereas little or no mature NGF is detected. Previous reports showed NGF protein, measured by ELISA, is increased in AD BFCN target regions such as hippocampus and cortex. Using Western blotting, we demonstrate a twofold increase in proNGF in AD parietal cortex compared to controls, indicating that it is this precursor form, proNGF, that accumulates in AD. This increase may reflect either a role for biologically active proNGF or posttranslational disturbances in NGF biosynthesis that decrease the processing of proNGF to mature NGF in AD.

Serotonergic lesion of median raphe nucleus alters nerve growth factor content and vulnerability of cholinergic septohippocampal neurons in rat

About 45% of the serotonergic raphe neurons are reported to express nerve growth factor (NGF) receptors. We therefore investigated whether selective serotonergic lesions of the median or dorsal raphe nuclei are associated with changes in NGF protein levels of the brain and whether the loss of serotonergic function alters the vulnerability of cholinergic septohippocampal neurons. In adult rats the hippocampal NGF content changed in a biphasic way after lesion of the median raphe nucleus by 5,7-dihydroxytryptamine (5,7-DHT), with a significant increase after 2-3 weeks of up to 35%, followed by a significant reduction of 22% below control levels after 7 weeks, and a return to control levels within the following 4 weeks. By contrast, the decrease in hippocampal serotonin and 5-hydroxyindoleacetic acid remained throughout the observation period of 11 weeks, being still reduced to 15 and 30% of the control levels, respectively. In the frontal cortex the partial loss of the serotonergic innervation projecting from the median raphe was associated 5 weeks after 5,7-DHT injection with an increase in NGF protein of 39.7+/-9.6% (P<0.05), which remained elevated up to 11 weeks. At 9 weeks after 5,7-DHT, the lesion of the septohippocampal cholinergic neurons induced by the cholinotoxin ethylcholine aziridinium (AF64A) was exaggerated (P<0.05) as compared to AF64A-treated rats with intact serotonergic innervation. The present data indicate that a serotonergic lesion of the median raphe nucleus results in biphasic changes of NGF protein content and in a delayed increase in the vulnerability of septohippocampal cholinergic neurons.

Elevation of nerve growth factor and antisense knockdown of TrkA receptor during contextual memory consolidation

We report here a series of experiments establishing a role for nerve growth factor and its high-affinity receptor TrkA in contextual memory consolidation. In all experiments, we trained rats in a novel chamber using tone and shock. Our first experiment revealed that endogenous nerve growth factor (NGF) increases in the hippocampus at a critical time during consolidation that occurs 1 week after training. NGF levels at other intervals (24 hr and 2 and 4 weeks after training) did not differ from those of naive control animals. In our second experiment, we blocked effects that NGF has at 1 week after training by infusing antisense TrkA phosphorothioate DNA oligonucleotide. Reduction of septohippocampal TrkA receptor expression selectively impaired memory consolidation for context but not for tone. Animals with antisense TrkA oligonucleotide infused into the medial septal area or CA1 of the hippocampus froze less when placed in the training chamber than did animals infused with inactive randomized oligonucleotide. At 4 weeks after training, antisense TrkA oligonucleotide had no effect on freezing. Third, we correlated levels of freezing with choline acetyltransferase (ChAT) and vesicular acetylcholine transporter (VAChT) immunohistochemistry. Antisense TrkA infused into CA1 of the hippocampus reduced cell body cross-sectional area for cholinergic cells in the medial septal area and decreased the density of hippocampal terminals labeled for ChAT and VAChT proteins. Cholinergic cell body measurements were significantly correlated with freezing. Taken together, these results indicate a role for nerve growth factor acting via the TrkA receptor on ChAT and VAChT proteins in contextual memory consolidation.

A decrease in size and number of basal forebrain cholinergic neurons is paralleled by diminished hippocampal cholinergic innervation in mice lacking leukocyte common antigen-related protein tyrosine phosphatase activity

The leukocyte common antigen-related (LAR) receptor, composed of an extracellular region with three immunoglobulin-like and eight fibronectin type III-like domains, and a cytoplasmic region containing two protein tyrosine phosphatase domains, is thought to play a role in axonal outgrowth and guidance during neural development. LAR mutant mice were generated completely lacking the two cytoplasmic protein tyrosine phosphatase domains, resulting in the loss of ability to bind intracellular associating proteins, but (may be) still containing the ability to perform extracellular functions. A reduction in size of basal forebrain cholinergic neurons and diminished hippocampal innervation reported for knockout mice that contain a leaky gene trap inserted into the 5' part of the LAR gene [Yeo T. T. et al. (1997) J. Neurosci. Res. 47, 348-360] warranted a computer-assisted quantitative image analysis throughout the basal forebrain and hippocampus of our LAR mutant mice. The total number, longest diameter and cell body area were calculated for the choline acetyltransferase-positive neurons in the medial septum and vertical diagonal band, and optical density measurements were performed to determine the extent of acetyl cholinesterase-positive fibre innervation of the different layers in the dentate gyrus. In LAR mutant mice, the number of cholinergic cells was significantly reduced (approximately 25%) in the vertical diagonal band. Also, the cross-sectional area of the cholinergic neurons in the medial septum and vertical diagonal band was reduced (5%). These findings were paralleled by a diminished cholinergic innervation of the supragranular (18%) and molecular (4%) layers of the dentate gyrus. Thus, LAR protein tyrosine phosphatase activity appears crucial for size, number and target projection of basal forebrain cholinergic neurons, further strengthening a role for LAR in CNS development.

Is there nicotinic modulation of nerve growth factor? Implications for cholinergic therapies in Alzheimer's disease

Studies on the neurobiology of nerve growth factor (NGF) reveal a diverse range of actions. Through alterations in gene expression, NGF is important in maintaining and regulating the phenotype of neurons that express the high-affinity receptor, trkA. Nerve growth factor also has a rapid action, revealed by its role in pain signaling in bladder and in skin. In the central nervous system (CNS), NGF has an intimate relationship with the cholinergic system. It promotes cholinergic neuron survival after experimental injury but also maintains and regulates the phenotype of uninjured cholinergic neurons. In addition to these effects mediated by gene expression, NGF has a rapid neurotransmitter-like action to regulate cholinergic neurotransmission and neuronal excitability. Consistent with its actions on the cholinergic system, NGF can enhance function in animals with cholinergic lesions and has been proposed to be useful in humans with Alzheimer's disease (AD); however, the problems of CNS delivery and of side effects (particularly pain) limit the clinical efficacy of NGF. Drug treatment strategies to enhance production of NGF in the CNS may be useful in the treatment of AD. Nicotine is one such agent, which, when administered directly to the hippocampus in rats, produces long-lasting elevation of NGF production.

Identification and expression of a novel gene in odour-taste associative learning in the terrestrial slug

Odour-taste associative learning in the terrestrial slug offers a useful model for long-term memory formation and retention. The genes which are expressed over 2 h after a learning event are thought to include those related to memory formation and/or consolidation. It is very important to examine what kinds of genes are expressed following associative learning. We identified a novel slug gene, the expression of which was regulated by associative learning and mostly restricted to the procerebrum (PC), a place that olfactory information is believed to be processed in slug. This gene encodes a 121 amino acid, 18 kDa secretory protein which we term LAPS18. Expression of the LAPS18 gene was induced in somata and the protein spread to neurites in the PC of slugs subjected to paired conditioning. Recombinant LAPS18 promoted the aggregation and movement of PC neurones in culture and they were blocked by the anti-LAPS18 antibody. Beads coupled with LAPS18 protein attached to PC neurones and the beads aggregated through PC cells but not by themselves, suggesting that LAPS18 may require a counterpart molecule for PC neurone aggregation. An increased expression and translocation of LAPS18 protein after paired conditioning may be needed for long-term memory formation and retention in the slug. Since genes homologous to LAPS18 genes in the land slug Limax are found from vertebrates including human, analysing the expression and function of LAPS18 may be important in understanding the molecular mechanism of memory formation and retention.

Gene-experience interaction alters the cholinergic septohippocampal pathway of mice

Spatial learning requires the septohippocampal pathway. The interaction of learning experience with gene products to modulate the function of a pathway may underlie use-dependent plasticity. The regulated release of nerve growth factor (NGF) from hippocampal cultures and hippocampus, as well as its actions on cholinergic septal neurons, suggest it as a candidate protein to interact with a learning experience. A method was used to evaluate NGF gene-experience interaction on the septohippocampal neural circuitry in mice. The method permits brain region-specific expression of a new gene by using a two-component approach: a virus vector directing expression of cre recombinase; and transgenic mice carrying genomic recombination substrates rendered transcriptionally inactive by a "floxed" stop cassette. Cre recombinase vector delivery into transgenic mouse hippocampus resulted in recombination in 30% of infected cells and the expression of a new gene in those cells. To examine the interaction of the NGF gene and experience, adult mice carrying a NGF transgene with a floxed stop cassette (NGFXAT) received a cre recombinase vector to produce localized unilateral hippocampal NGF gene expression, so-called "activated" mice. Activated and control nonactivated NGFXAT mice were subjected to different experiences: repeated spatial learning, repeated rote performance, or standard vivarium housing. Latency, the time to complete the learning task, declined in the repeated spatial learning groups. The measurement of interaction between NGF gene expression and experience on the septohippocampal circuitry was assessed by counting retrogradely labeled basal forebrain cholinergic neurons projecting to the hippocampal site of NGF gene activation. Comparison of all NGF activated groups revealed a graded effect of experience on the septohippocampal pathway, with the largest change occurring in activated mice provided with repeated learning experience. These data demonstrate that plasticity of the adult spatial learning circuitry can be robustly modulated by experience-dependent interactions with a specific hippocampal gene product.

Enhanced learning in mice parallels vector-mediated nerve growth factor expression in hippocampus

Spatial learning requires the integrity of the nerve growth factor (NGF)-responsive septohippocampal pathway. Loss of a single NGF allele at the mouse NGF locus (heterozygous null, ngf(+/-)) reduces septohippocampal NGF levels and NGF-regulated cholinergic neurotransmitter enzymes and results in spatial learning deficits in adult animals. A herpes simplex virus (HSV) amplicon vector was utilized to locally deliver NGF to the hippocampus of mice heterozygous and wild type (ngf(+/+)) at the NGF gene locus. NGF gene transfer produced transient increases in NGF protein levels and choline acetyltransferase activity in both ngf(+/-) and ngf(+/+) mice. However, spatial learning capability was improved only in ngf(+/-) mice. In aggregate, these findings suggest that amplicon-directed expression of NGF in subjects with baseline septohippocampal dysfunction can correct spatial learning deficits.

Peripheral PYY inhibits intracisternal TRH-induced gastric acid secretion by acting in the brain

The site of action of peripheral peptide YY (PYY)-induced inhibition of vagally stimulated gastric acid secretion was studied using immunoneutralization with PYY antibody in urethan-anesthetized rats. Gastric acid secretion (59+/-7 micromol/90 min) stimulated by intracisternal injection of the stable thyrotropin-releasing hormone (TRH) analog RX-77368 (14 pmol/rat) was dose-dependently inhibited by 52%, 69%, and 83% by intravenous infusion of 0.25, 0.5, and 1.0 nmol. kg(-1) x h(-1) PYY, respectively. PYY or PYY(3-36) (2.4 pmol/rat) injected intracisternally also inhibited the acid response to intracisternal RX-77368 by 73% and 80%, respectively. Intravenous pretreatment with PYY antibody (4.5 mg/rat), which shows a 35% cross-reaction with PYY(3-36) by RIA, completely prevented the inhibitory effect of intravenously infused PYY (1 nmol x kg(-1) x h(-1)). When injected intracisternally, the PYY antibody (280 microg/rat) reversed intracisternal PYY (2.4 pmol)- and intravenous PYY (1 nmol x kg(-1) x h(-1))-induced inhibition of acid response to intracisternal RX-77368 by 64% and 93.5%, respectively. These results provide supporting evidence that peripheral PYY inhibits central vagal stimulation of gastric acid secretion through an action in the brain.

Comparative effects of dopamine D(1) and D(2) receptor antagonists on nerve growth factor protein induction

We previously reported that following acute administration of haloperidol or (-)-sulpiride, both dopamine D(2)-receptor antagonists, to mice induced nerve growth factor (NGF) gene expression, mediated by the interaction of c-fos with the AP-1 binding site present in the first intron on the NGF gene. In contrast, the D(1)-receptor antagonist R-(-)-8-chloro-2,3,4, 5-tetrahydro-3,1-methyl-5-phenyl-11-3-benzyoepine-7-ol (SCH23390) did not induce NGF mRNA expression. We report here immunohistochemical and Western blot analyses showing that following injection of these drugs for 14 consecutive days, the amount of NGF protein increased gradually and was induced significantly in the hippocampus, piriform cortex, amygdala, dorsal striatum, and nucleus accumbens neurons. NGF enhances the release of acetylcholine from these regions. Cholinergic innervation in the striatum and nucleus accumbens neurons is believed to be related to late-onset extrapyramidal symptoms, while in the hippocampus and piriform cortex it is involved in enhancing cognition. Thus, our data suggest that haloperidol- and (-)-sulpiride-induced NGF expression may be associated with both beneficial and adverse effects.

Phenotypic knockout of nerve growth factor in adult transgenic mice reveals severe deficits in basal forebrain cholinergic neurons, cell death in the spleen, and skeletal muscle dystrophy

The disruption of the nerve growth factor (NGF) gene in transgenic mice leads to a lethal phenotype (Crowley et al., 1994) and hinders the study of NGF functions in the adult. In this study the phenotypic knockout of NGF in adult mice was achieved by expressing transgenic anti-NGF antibodies, under the control of the human cytomegalovirus promoter. In adult mice, antibody levels are 2000-fold higher than in newborns. Classical NGF targets, including sympathetic and sensory neurons, are severely affected. In the CNS, basal forebrain and hippocampal cholinergic neurons are not affected in the early postnatal period, whereas they are greatly reduced in the adult (55 and 62% reduction, respectively). Adult mice show a reduced ability in spatial learning behavioral tasks. Adult, but not neonatal, transgenic mice further show a new phenotype at the level of peripheral tissues, such as apoptosis in the spleen and dystrophy of skeletal muscles. The analysis of this novel comprehensive transgenic model settles the controversial issue regarding the NGF dependence of cholinergic neurons in adult animals and reveals new NGF functions in adult non-neuronal tissues. The results demonstrate that the decreased availability of NGF in the adult causes phenotypic effects via processes that are at least partially distinct from early developmental effects of NGF deprivation.

P75 nerve growth factor receptor is important for retrograde transport of neurotrophins in adult cholinergic basal forebrain neurons

The role of the p75 nerve growth factor receptor in the retrograde transport of neurotrophins in the adult CNS was investigated by comparing the transport of 125I-labeled neurotrophins by normal and p75 nerve growth factor receptor-deficient cholinergic septohippocampal neurons. In control mice, nerve growth factor was selectively transported from the hippocampal formation to the cholinergic neurons in the septum. Nerve growth factor labeling was found in three to four times as many septal cholinergic neuronal cell bodies than labeling for neurotrophin-3 or neurotrophin-4/5, and transported brain-derived neurotrophic factor was barely detectable. Cells were considered as labeled when the number of grains per cell exceeded five times background. In p75 nerve growth factor receptor-deficient mice, the number of cholinergic neurons labeled with each of the neurotrophins was reduced by 85-95%. Retrograde labeling of septohippocampal neurons with Fluorogold was not obviously reduced in p75 nerve growth factor receptor-deficient mice, suggesting that general transport mechanisms were not impaired. Despite the reduced neurotrophin transport, cholinergic neurons of p75 nerve growth factor receptor-deficient mice were larger than controls and had an apparently normal density of immunostaining for choline acetyltransferase. Since nerve growth factor is reportedly involved in size regulation and choline acetyltransferase expression, this raises the possibility that the retrograde transport itself is not essential for these events. Thus, p75 nerve growth factor receptor plays an important, although not exclusive, role in the transport of neurotrophins by cholinergic basal forebrain neurons, and retrograde transport of nerve growth factor may not be needed for regulating certain cellular processes.

Hippocampal neurotrophin and trk receptor mRNA levels are altered by local administration of nicotine, carbachol and pilocarpine

Cholinergic receptor agonists nicotine (nicotinic), carbachol (nicotinic/muscarinic) and pilocarpine (muscarinic) were administered into the hippocampus and mRNA levels of neurotrophins and their receptors determined using in situ hybridisation. Drug doses were carefully chosen to avoid the potentially confounding effects of seizure and cell death. Nicotine caused a long-lasting increase in nerve growth factor (NGF) mRNA in all subfields of the hippocampus. The increase was evident from 24 h up to 72 h after drug administration. This increase was dependent on excitatory amino acid neurotransmission as it was blocked by administration of an AMPA or NMDA receptor antagonist. In contrast, carbachol and pilocarpine produced a transient increase in NGF mRNA levels present 4-8 h after drug administration. Pilocarpine caused a transient increase in hippocampal brain-derived neurotrophic factor (BDNF) levels, with carbachol and nicotine showing the same trend. Nicotine and carbachol caused transient decreases in NT-3 mRNA levels in dentate gyrus and CA2 with pilocarpine showing a similar trend. Increases in mRNA encoding full-length trkB were seen 8 h after nicotine, with nicotine also causing elevations in a mRNA encoding a truncated isoform (trkB.T2). TrkC mRNA was not altered by any of the conditions used. The study suggests that muscarinic and nicotinic receptor activation in the hippocampus causes transient changes in all of the neurotrophins, but that NGF levels are selectively up-regulated by nicotinic receptor stimulation. The reciprocal interaction between NGF and ascending cholinergic systems may be a component of the cognitive enhancing effects of nicotine.

The effects of anti-nerve growth factor monoclonal antibodies on developing basal forebrain neurons are transient and reversible

In order to reassess the role of nerve growth factor (NGF) on rat basal forebrain cholinergic neurons (BFCNs) survival and/or phenotype maturation during the early postnatal life, we immunoneutralized NGF in vivo. Hybridoma cells producing the neutralizing anti-NGF monoclonal antibody alphaD11 were implanted in the lateral ventricle of the rat at different postnatal ages (P2, P8 and P15) and the effects on the number and the soma size of cholinacetyltransferase (ChAT) positive neurons were analysed 1, 2 or 3 weeks after the injection. A marked decrease in the number and in the soma size of BFCNs was observed implanting hybridoma cells at P2 and performing the analysis 1 week later. These effects are reversed 3 weeks after the implant of hybridoma cells at P2. At this time point, the levels of alphaD11 antibodies in the brain parenchyma are still in a vast molar excess over endogenous NGF. No effects on BFCNs were observed implanting alphaD11 cells at P15 while LGN neurons showed marked shrinkage. Our results demonstrate that the reduction in the number of ChAT-positive neurons during the first two postnatal weeks of anti-NGF treatment is not due to cell death. We conclude that NGF is not a survival factor for BFCNs, and that the influence of NGF on BFCNs cell maturation during the first 2 postnatal weeks is transient and reversible. Our results on tyrosine kinase (Trk) coexpression, suggest that NGF may cooperate with other factors in the cholinergic phenotype differentiation and maintenance after the second postnatal week.

Synaptic numbers across cortical laminae and cognitive performance of the rat during ageing

In this study, we have investigated the changes in the number of individual presynaptic boutons in the neocortex of rats and correlated them with cognitive performance. Brown Norway x Fischer 344 F1 hybrid rats, aged from one to 24 months, were used. Using synaptophysin as a marker for presynaptic boutons, we found that in the parietal II region of the neocortex an age-related decrease in the density of immunostained punctae representing presynaptic boutons occurred. Regression analysis showed that this decline in the number of presynaptic boutons correlates with ageing (r=0.495, P<0.05). Interestingly, we found that this age-related depletion of presynaptic boutons was more intense in the deeper cortical lamina, such as laminae V and VI (mean decrease of 18%), than in the superficial laminae (mean decrease of 8% in laminae I-IV). Using the Morris water maze test, we observed that young rats acquired the task at twice the speed of aged animals (48.9 +/- 9.0 s and 91.0 +/- 4.9 s for young and aged animals, respectively). Furthermore, at the end of the training period, the aged cohort still showed significantly higher escape latencies in the Morris water maze. The present findings support the concept that the decline in cognitive performances in ageing is related to the loss of synapses in the cerebral cortex.

Disruption of a single allele of the nerve growth factor gene results in atrophy of basal forebrain cholinergic neurons and memory deficits

Administration of nerve growth factor (NGF) to aged or lesioned animals has been shown to reverse the atrophy of basal forebrain cholinergic neurons and ameliorate behavioral deficits. To examine the importance of endogenous NGF in the survival of basal forebrain cholinergic cells and in spatial memory, mice bearing a disruption mutation in one allele of the NGF gene were studied. Heterozygous mutant mice (ngf+/-) have reduced levels of NGF mRNA and protein within the hippocampus and were found to display significant deficits in memory acquisition and retention in the Morris water maze. The behavioral deficits observed in NGF-deficient mice were accompanied by both shrinkage and loss of septal cells expressing cholinergic markers and by a decrease in cholinergic innervation of the hippocampus. Infusions of NGF into the lateral ventricle of adult ngf+/- mice abolished the deficits on the water maze task. Prolonged exposure to NGF may be required to induce cognitive effects, because reversal of the acquisition deficit was seen after long (5 weeks) but not short (3 d) infusion. Although NGF administration did not result in any improvement in the number of septal cells labeled for choline acetyltransferase, this treatment did effectively correct the deficits in both size of cholinergic neurons and density of cholinergic innervation of the hippocampus. These findings demonstrate the importance of endogenous NGF for survival and function of basal forebrain cholinergic neurons and reveal that partial depletion of this trophic factor is associated with measurable deficits in learning and memory.

Chronic intraventricular infusion with NGF improves LTP in old cognitively-impaired rats

Aged (21 months) cognitively-impaired male Sprague-Dawley rats received intraventricular infusion of nerve growth factor (NGF) or cytochrome C (Cit C) for 14 or 28 days using miniosmotic pumps and were evaluated either 1 week or 3 months after treatment. Groups of untreated young, aged-impaired and aged non-impaired rats were also evaluated. Under narcose recording and stimulating electrodes were stereotactically implanted in the dentate gyrus and the perforant path. The stimulation intensity was individually adjusted to obtain a half-maximal population spike (P) for test stimuli and a quarter-maximal for tetanization. The amplitude and latency of P and the slope (S) of the field EPSP were determined before and at 2, 5, 15, 30 and 60 min after tetanization at 400 Hz. Paired stimuli at 30 ms interval were also applied before and after tetanization. Aged, cognitively impaired rats showed an absent S potentiation and a delayed P potentiation, both in amplitude and latency, while non-impaired rats behaved like the young controls. Paired pulse inhibition showed no difference among groups before or after tetanization suggesting that the impaired potentiation is not due to an increased retroactive inhibition. NGF treatment ameliorates LTP deficits to levels equivalent to non-impaired rats, while Cit C controls showed no improvement. No differences appear among NGF treated groups, but evidence suggest that the animals evaluated 3 months after treatment developed a stronger potentiation.

Aging in the hippocampus: interrelated actions of neurotrophins and glucocorticoids

Over the past two decades, evidence has been accumulating that diffusible molecules, such as growth factors and steroids hormones, play an important part in neural senescence, particularly in the hippocampus. There is also evidence that these molecules do not act as independent signals, but show interrelated regulation and cooperative control over the aging process. Here, we review some of the changes that occur in the hippocampus with age, and the influence of two classes of signaling substances: glucocorticoids and neurotrophins. We also examine the interactions between these substances and how this could influence the aging process.

Learning impairment and cholinergic deafferentation after cortical nerve growth factor deprivation

Cholinergic basal forebrain (CBF ) neurons have been shown to respond in vivo to exogenous administration of NGF. Although neurotrophins and their receptors are widely expressed in the CNS, little data exist for the physiological significance of endogenous neurotrophin signaling in CNS neurons. To test directly whether cortically derived NGF is functionally required for the cholinergic functions mediated by the cerebral cortex, repeated injections of anti-NGF mAbs were locally applied into the insular cortex (IC) of rats. The biochemical results, using an in vivo microdialysis technique, showed a dramatic lack of extracellular release of acetylcholine after high potassium stimulation compared with controls. Furthermore, by using small injections of the neurotracer fluorogold, we found a corresponding disruption in the connectivity between the IC and the CBF. Behavioral experiments showed that the NGF antibodies applied into the IC produced a significant disruption on the acquisition of conditioned taste aversion and inhibitory avoidance learning. However, the same animals were able to recall the taste aversion when the conditioning trial was established before injections of NGF antibodies. Given these results, it seems that cortical cholinergic functions are actively dependent on locally derived NGF in the adult normal brain, and that the cholinergic activity from the CBF is not necessary for recalling aversive stimuli, but is necessary for the acquisition of aversively motivated conditionings.

Inhibition of tumor necrosis factor-alpha action within the CNS markedly reduces the plasma adrenocorticotropin response to peripheral local inflammation in rats

The present study tested the hypothesis that the cytokine tumor necrosis factor-alpha (TNF-alpha) is an important CNS mediator of the hypothalamo-pituitary-adrenal (HPA) axis response to local inflammation in the rat. Recombinant murine TNF-alpha administered directly into the cerebroventricles of normal rats produced a dose-dependent increase in plasma adrenocorticotropin (ACTH) concentration. Local inflammation induced by the intramuscular injection of turpentine (50 microl/100 gm body weight) also produced an increase in plasma ACTH, peaking at 160-200 pg/ml at 7.5 hr after injection (compared with 10-30 pg/ml in controls). Intracerebroventricular pretreatment with either 5 microl of anti-TNF-alpha antiserum or 1-50 microg of soluble TNF receptor construct (rhTNFR:Fc) reduced the peak of the ACTH response to local inflammation by 62-72%. In contrast, intravenous treatment with the same doses of anti-TNF-alpha or rhTNFR:Fc had no significant effect on the ACTH response to local inflammation. Although these data indicated an action of TNF-alpha specifically within the brain, no increase in brain TNF-alpha protein (measured by bioassay) or mRNA (assessed using either in situ hybridization histochemical or semi-quantitative RT-PCR procedures) was demonstrable during the onset or peak of HPA activation produced by local inflammation. Furthermore, increased passage of TNF-alpha from blood to brain seems unlikely, because inflammation did not affect plasma TNF-alpha biological activity. Collectively these data demonstrate that TNF-alpha action within the brain is critical to the elaboration of the HPA axis response to local inflammation in the rat, but they indicate that increases in cerebral TNF-alpha synthesis are not a necessary accompaniment.

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.

Long-term functional recovery from age-induced spatial memory impairments by nerve growth factor gene transfer to the rat basal forebrain

Nerve growth factor (NGF) stimulates functional recovery from cognitive impairments associated with aging, either when administered as a purified protein or by means of gene transfer to the basal forebrain. Because gene transfer procedures need to be tested in long-term experimental paradigms to assess their in vivo efficiency, we have used ex vivo experimental gene therapy to provide local delivery of NGF to the aged rat brain over a period of 2.5 months by transplanting immortalized central nervous system-derived neural stem cells genetically engineered to secrete NGF. By grafting them at two independent locations in the basal forebrain, medial septum and nucleus basalis magnocellularis, we show that functional recovery as assessed in the Morris water maze can be achieved by neurotrophic stimulation of any of these cholinergic cell groups. Moreover, the cholinergic neurons in the grafted regions showed a hypertrophic response resulting in a reversal of the age-associated atrophy seen in the learning-impaired aged control rats. Long-term expression of the transgene lead to an increased NGF tissue content (as determined by NGF-ELISA) in the transplanted regions up to at least 10 weeks after grafting. We conclude that the gene transfer procedure used here is efficient to provide the brain with a long-lasting local supply of exogenous NGF, induces long-term functional recovery of cognitive functions, and that independent trophic stimulation of the medial septum or nucleus basalis magnocellularis has similar consequences at the behavioral level.

Expression of messenger RNA coding for the nerve growth factor receptor trkA in the hippocampus of the adult rat

Nerve growth factor is a member of the neurotrophin gene family and acts as a neurotrophic factor on a variety of neuronal populations. Nerve growth factor biological action is mediated by binding to the transmembrane tyrosine kinase trkA, although the low affinity neurotrophin receptor p75 may also play a role. TrkA messenger RNA in the central nervous system is localized within a small number of specific neuronal populations, as opposed to the widespread expression of the other members of the trk family, trkB and trkC. In particular, cholinergic neurons of the basal forebrain, the prototype of nerve growth factor-sensitive neurons in the brain express trkA. Several lines of evidence indicate that other populations of central neurons, in particular hippocampal neurons, may be responsive to nerve growth factor as well. In fact, nerve growth factor rescues hippocampal neurons from ischemic cell death in vivo and increases neurotransmitter release from hippocampal neurons in culture. Moreover, nerve growth factor has been implicated in spatial learning, a process known to be dependent on the hippocampal formation. The following paper reports expression of trkA messenger RNA in the rat hippocampus by in situ hybridization and reverse transcription-polymerase chain reaction. This finding supports the notion of hippocampal neurons as an nerve growth factor-sensitive population.