Acute application of NGF increases the firing rate of aged rat basal forebrain neurons

A Review of Techniques for Biodelivery of Nerve Growth Factor (NGF) to the Brain in Relation to Alzheimer's Disease

Age-dependent progressive neurodegeneration and associated cognitive dysfunction represent a serious concern worldwide. Currently, dementia accounts for the fifth highest cause of death, among which Alzheimer's disease (AD) represents more than 60% of the cases. AD is associated with progressive cognitive dysfunction which affects daily life of the affected individual and associated family. The cognitive dysfunctions are at least partially due to the degeneration of a specific set of neurons (cholinergic neurons) whose cell bodies are situated in the basal forebrain region (basal forebrain cholinergic neurons, BFCNs) but innervate wide areas of the brain. It has been explicitly shown that the delivery of the neurotrophic protein nerve growth factor (NGF) can rescue BFCNs and restore cognitive dysfunction, making NGF interesting as a potential therapeutic substance for AD. Unfortunately, NGF cannot pass through the blood-brain barrier (BBB) and thus peripheral administration of NGF protein is not viable therapeutically. NGF must be delivered in a way which will allow its brain penetration and availability to the BFCNs to modulate BFCN activity and viability. Over the past few decades, various methodologies have been developed to deliver NGF to the brain tissue. In this chapter, NGF delivery methods are discussed in the context of AD.

The Effects of Nerve Growth Factor on Nicotinic Synaptic Transmission in Mouse Airway Parasympathetic Neurons

In autonomic ganglia, acetylcholine (ACh) is released from preganglionic nerve terminals and binds to nicotinic ACh receptors (nAChRs) on postganglionic neurons, resulting in a brief, short-lived synaptic potential (fast excitatory postsynaptic potential [fEPSP]). Although nerve growth factor (NGF) is known to affect sensory and sympathetic nerves, especially during development, little is known regarding its effect on parasympathetic nerves, especially on adult neurons. Elevated levels of NGF and NGF-mediated neural plasticity may have a role in airway diseases, such as asthma and chronic obstructive pulmonary disease. In this study, we characterize the composition and response of nAChRs in parasympathetic neurons located in lower airways of mice, and note the effects of NGF on fEPSPs and on nicotinic currents. Based on immunohistochemical staining, nAChRs are made up of α-3 and β-4 subunits; in addition, tropomyosin-related kinase A, the receptor for NGF, is also expressed by the neurons. Vagus nerve evoked fEPSPs and inward currents evoked by a nicotinic receptor agonist (1,1-dimethyl-4-phenylpiperazinium) were increased by NGF. NGF also affected the action potential after hyperpolarization. These studies were done in mice, which are routinely used to study airway diseases, such as asthma, where the allergen-induced contraction of airway smooth muscle has a well-defined parasympathetic cholinergic component.

Effects of a saturated fat and high cholesterol diet on memory and hippocampal morphology in the middle-aged rat

Diets rich in cholesterol and/or saturated fats have been shown to be detrimental to cognitive performance. Therefore, we fed a cholesterol (2%) and saturated fat (hydrogenated coconut oil, Sat Fat 10%) diet to 16-month old rats for 8 weeks to explore the effects on the working memory performance of middle-aged rats. Lipid profiles revealed elevated plasma triglycerides, total cholesterol, HDL, and LDL for the Sat-Fat group as compared to an iso-caloric control diet (12% soybean oil). Weight gain and food consumption were similar in both groups. Sat-Fat treated rats committed more working memory errors in the water radial arm maze, especially at higher memory loads. Cholesterol, amyloid-beta peptide of 40 (Abeta40) or 42 (Abeta42) residues, and nerve growth factor in cortical regions was unaffected, but hippocampal Map-2 staining was reduced in rats fed a Sat-Fat diet, indicating a loss of dendritic integrity. Map-2 reduction correlated with memory errors. Microglial activation, indicating inflammation and/or gliosis, was also observed in the hippocampus of Sat-Fat fed rats. These data suggest that saturated fat, hydrogenated fat and cholesterol can profoundly impair memory and hippocampal morphology.

Galantamine effects on memory, spatial cue utilization, and neurotrophic factors in aged female rats

Galantamine is an acetylcholine esterase inhibitor that has been approved for use in Alzheimer's disease. However, even though clinical studies indicate efficacy in attenuating some of the symptoms associated with the disease, there are a paucity of studies evaluating the effects of galantamine administration on cognitive performance and brain parameters in aged rats. Further, because all previous animal studies using galantamine have been performed in male rats, there is no information on how females respond to galantamine treatment. Therefore, we studied the effects of 0.3, 0.6, and 1.2 mg/kg/day galantamine in 20-month-old female rats in terms of performance on the working and reference memory water radial arm maze task. Galantamine did not influence maze performance. Furthermore, a probe trial procedure to determine extra-maze cue utilization while solving the water radial arm maze established that aged female rats utilized extramaze cues, and that they did not rely on a nonspatial chaining strategy to locate hidden platforms. Galantamine treatment had no effect on use of extramaze cues or chaining. In addition, there were no significant changes in neurotrophin levels in the frontal cortex, entorhinal cortex, hippocampus, or basal forebrain after galantamine administration. Therefore, the data reported here suggest that aged animals do utilize spatial strategies for solving a working memory task, but galantamine has no appreciable effects on this task, at least not at the doses tested.

Effects of NGF and BDNF on baseline glutamate and dopamine release in the hippocampal formation of the adult rat

It has been shown using in vitro techniques that BDNF and NGF evoke neurotransmitter release in the hippocampus but this phenomenon has not been demonstrated in vivo to date. We therefore performed in vivo microdialysis in urethane-anesthetized Fischer 344 rats. The microdialysis probe was implanted stereotaxically into the CA1 area of the hippocampus. Three hours after the implantation of the probe, glutamate (Glu) and dopamine (DA) levels had reached a stable baseline. Four baseline samples were collected every 15 min at a flow rate of 1 microL/min. The growth factors were delivered (1 microL/10 min) using a microinjector attached to the microdialysis probe. We found that BDNF and NGF, when administered into the hippocampus, evoked dopamine and glutamate release in a dose-dependent fashion. NGF produced a biphasic response in the release of Glu, and a uniphasic response in the release of DA, both of which were calcium dependent. The neurotransmitter release induced by NGF was blocked by tetrodotoxin, indicating neuronal origin of this response. The BDNF induced release of DA and Glu was decreased in low calcium conditions, indicating that it is at least partially calcium dependent. Furthermore, BDNF-induced neurotransmitter release was partially blocked by pre-treatment with K252a, an antagonist for tyrosine kinase receptors, indicating that BDNF is acting through Trk receptors to induce neurotransmitter release. These results demonstrate a close relationship between the growth factors BDNF and NGF and the neurotransmitters DA and Glu in the hippocampus of intact animals.

Age-dependent loss of NGF signaling in the rat basal forebrain is due to disrupted MAPK activation

The loss of nerve growth factor (NGF) and its high affinity receptor TrkA has been implicated in the loss of cholinergic tone and function in Alzheimer's disease (AD) and normal aging. We employed an animal model of aging, the aged rat, which also exhibits memory loss and NGF alterations. Basal forebrain TrkA levels increased after injection of NGF in the hippocampus within 1h in young rats, but this response was diminished in aged animals as determined by Western blot analysis. Further, NGF activated MAPK pathways without changing total ERK levels and the activation of these pathways was also diminished in aged animals. The exogenous NGF injection did not appear to activate the PI-3K pathway or alter total levels of Akt significantly. These data shed light on mechanisms of NGF signaling in the CNS, and alterations in this signaling cascade associated with age and memory loss. These findings might lead to development of novel treatment therapies for the memory loss associated with AD and other age-associated neurodegenerative diseases.

ERK-mediated NGF signaling in the rat septo-hippocampal pathway diminishes with age

RATIONALE

Degeneration of basal forebrain cholinergic neurons (BFCNs) plays an important role in aging and Alzheimer's disease (AD) pathology. This degeneration may be a result of disrupted nerve growth factor (NGF) signaling. Aged rats have memory deficits, BFCN degeneration, and disrupted NGF signaling.

OBJECTIVE

In this study we identify a rapid NGF signaling pathway in BFCNs and the second messenger system associated with that signaling. We also identify age-dependent alterations in this signaling pathway.

MATERIALS AND METHODS

After cognitive assessment using the Morris water maze, rats were given an intra-hippocampal NGF injection. Basal forebrain immunohistochemical analysis, confocal microscopy, and inhibitor studies were performed.

RESULTS

An increase in immunoreactivity for the NGF receptor TrkA was found in cell bodies of BFCNs 15 min and 1 h post-NGF injection. Immunohistochemistry studies with phospho-ERK and phospho-AKT antibodies showed that this rapid signaling occurred through MAP kinase, but not PI-3 kinase pathways. MAPK inhibitor studies attenuated the NGF-induced effects. Both TrkA and phospho-ERK (extracellular signal-regulated kinase) immunoreactivities were diminished in aged rats and phospho-ERK immunoreactivity-correlated with aged rat performance in the Morris water maze.

CONCLUSIONS

Rapid NGF signaling likely occurs in the rat CNS through the MAPK signaling pathway. This rapid signaling pathway is diminished in aged rats compared to young ones and may contribute to memory deficits observed in aged rats. As cholinergic degeneration coupled with altered levels of NGF and TrkA receptors are also seen in human aging and AD, ERK-related dysfunction may be relevant in human conditions as well.

Nerve growth factor in treatment and pathogenesis of Alzheimer's disease

The etiology of Alzheimer's disease (AD) is still unknown. In addition, this terrible neurodegenerative disease will increase exponentially over the next two decades due to longer lifespan and an aging "baby-boomer" generation. All treatments currently approved for AD have moderate efficacy in slowing the rate of cognitive decline in patients, and no efficacy in halting progression of the disease. Hence, there is an urgent need for new drug targets and delivery methods to slow or reverse the progression of AD. One molecule that has received much attention in its potential therapeutic role in AD is nerve growth factor (NGF). This review will demonstrate data from humans and animals which promote NGF as a potential therapeutic target by (1) outlining the hypothesis behind using NGF for the treatment of AD, (2) reviewing both the normal and AD altered signaling pathways and effects of NGF in the central nervous system (CNS), and (3) examining the results of NGF treatment obtained from animal models of AD and AD patients.

Chronic nicotine improves working and reference memory performance and reduces hippocampal NGF in aged female rats

The cholinergic system is involved in cognition and several forms of dementia, including Alzheimer's disease, and nicotine administration has been shown to improve cognitive performance in both humans and rodents. While experiments with humans have shown that nicotine improves the ability to handle an increasing working memory load, little work has been done in animal models evaluating nicotine effects on performance as working memory load increases. In this report, we demonstrate that in aged rats nicotine improved the ability to handle an increasing working memory load as well as enhanced performance on the reference memory component of the water radial arm maze task. The dose required to exert these effects (0.3mg/kg/day) was much lower than doses shown to be effective in young rats and appears to be a lower maintenance dose than is seen in light to moderate smokers. In addition, our study reports a nicotine-induced reduction in nerve growth factor (NGF) protein levels in the hippocampus of the aged rat. The effects of nicotine on hippocampal NGF levels are discussed as a potential mechanism of nicotine-induced improvements in working and reference memory.

Behavioral and neurobiological markers of Alzheimer's disease in Ts65Dn mice: effects of estrogen

Individuals with Down's syndrome (DS) develop neuropathological features similar to Alzheimer's disease (AD) early in life, including dementia, accumulation of beta-amyloid, and irregular phosphorylation of tau proteins. Ts65Dn mice, an animal model of DS, provide a unique method to investigate the mechanisms related to AD-like symptoms in DS and possible therapeutic interventions. Ts65Dn mice undergo a decline in cholinergic phenotype and cognitive deterioration beginning at 6-8 months of age. In middle-aged female Ts65Dn mice, estrogen supplementation alleviated these cholinergic and cognitive impairments. The current study investigated AD-like markers and the effects of estrogen in male Ts65Dn mice. Estrogen treatment prior to behavioral testing did not improve cognitive deficits in 6-month-old male Ts65Dn mice, but decreased total and phosphorylated (pS199) tau in the entorhinal cortex compared to normosomic animals. Hippocampal beta-amyloid(1-42) levels were increased in Ts65Dn animals, regardless of estrogen treatment. These findings further support Ts65Dn mice as a model for specific AD-like symptoms, and demonstrate that estrogen treatment of this type does not improve the cognitive ability of male Ts65Dn mice.

Stroke, dementia, and drug delivery

Stroke and dementia represent a major health burden for elderly subjects as they are associated with significant morbidity and mortality. The rates of stroke and dementia are progressively increasing due to the ageing population in most westernized countries. Therefore, both these conditions represent a major therapeutic target. However, the therapeutic options available for the management of stroke and dementia remain largely unsatisfactory, the main reason being the difficulty in transferring the results obtained in animal and in vitro studies to the clinical setting. This review focuses on the recent advances in pathophysiology and treatment of these conditions and future directions for research. Moreover, the technique of functional magnetic resonance imaging is discussed in detail as a tool to assess the effects of therapeutic agents on the central nervous system and monitor the progression of diseases. Finally, an overview of the issue of drug delivery into the central nervous system is presented.

Regional alterations in amyloid precursor protein and nerve growth factor across age in a mouse model of Down's syndrome

Individuals with Down's syndrome (DS) develop the pathological hallmarks of Alzheimer's (AD) disease at an early age, subsequently followed by memory decline and dementia. We have utilized an animal model for DS, mice with segmental trisomy of chromosome 16 (Ts65Dn), to study biological events linked to memory loss. Previous studies demonstrated a cognitive decline and loss of cholinergic markers after 6-8 months of age. In the current study, we found increased levels of amyloid precursor protein (APP) in the striatum by 6-8 months of age, and in the hippocampus and parietal cortex by 13-16 months of age in Ts65Dn but not in normosomic mice. Additionally, Ts65Dn mice exhibited alterations in nerve growth factor (NGF) levels in the basal forebrain and hippocampus. Ts65Dn mice demonstrated a significant decline in NGF levels in the basal forebrain with age, as well as a reduction in hippocampal NGF by 13-16 months of age. These findings demonstrate that elevated APP and decreased NGF levels in limbic areas correlate with the progressive memory decline and cholinergic degeneration seen in middle-aged trisomic mice.

Exogenous NGF restores endogenous NGF distribution in the brain of the cognitively impaired aged rat

Alzheimer's disease and normal aging may impair retrograde transport of nerve growth factor (NGF) from cortical areas to basal forebrain cholinergic neurons. We demonstrate a relationship between performance in a spatial reference memory task and NGF distribution in the aged rat brain. In addition, exogenous NGF restored endogenous NGF distribution in cognitively impaired aged rats. These data suggest that NGF administration restores utilization of endogenous growth factor in the brain of cognitively impaired aged rats.

Frontal cortex BDNF levels correlate with working memory in an animal model of Down syndrome

Individuals with Down syndrome (DS) develop most neuropathological hallmarks of Alzheimer's disease early in life, including loss of cholinergic markers in the basal forebrain. Ts65Dn mice, an animal model of DS, perform poorly on tasks requiring spatial memory and also exhibit basal forebrain pathology beginning around 6 months of age. We evaluated memory as well as brain-derived neurotrophic factor (BDNF) and nerve growth factor (NGF) protein levels in basal forebrain, frontal cortex, hippocampus, and striatum in Ts65Dn mice at the age when cholinergic degeneration is first observed, and compared values to normosomic controls. Six-month-old Ts65Dn mice exhibited impairments in working and reference memory as assessed on a water radial-arm maze. The working memory deficit was related to the inability of Ts65Dn mice to successfully sustain performance as the working memory load increased. Coupled with cognitive performance deficiencies, Ts65Dn mice also exhibited lower frontal cortex BDNF protein levels than controls. Further, BDNF levels were negatively correlated with working memory errors during the latter portion of testing in Ts65Dn mice, thereby suggesting that lower BDNF protein levels in the frontal cortex may be associated with the observed working memory impairment.

Estrogen restores cognition and cholinergic phenotype in an animal model of Down syndrome

Estrogen maintains normal function of basal forebrain (BF) cholinergic neurons and estrogen replacement therapy (ERT) has therefore been proposed as a therapy for Alzheimer's disease (AD). We provide evidence to support this hypothesis in an animal model of Down syndrome (DS), a chromosome 16 segmental trisomy (Ts65Dn) mouse. These mice develop cholinergic degeneration similar to young adults with DS and AD patients. ERT has not been tested in women with DS, even though they are more likely than normosomic women to develop early menopause and AD. Female Ts65Dn and normosomic mice (11-15 months) received a subcutaneous estrogen pellet or a sham operation. After 60 days, estrogen treatment improved learning of a T-maze task and normalized behavior in the Ts65Dn mice in reversal learning of the task, a measure of cognitive flexibility. Stereological evaluation of choline acetyltransferase (ChAT) immunopositive BF neurons showed that estrogen increased cell size and total number of cholinergic neurons in the medial septum of Ts65Dn mice. In addition, estrogen increased NGF protein levels in the BF of trisomic mice. These findings support the emerging hypothesis that estrogen may play a protective role during neurodegeneration and cognitive decline, particularly in cholinergic BF neuronal systems underlying cognition. The findings also indicate that estrogen may act, at least partially, via endogenous growth factors. Collectively, the data suggest that ERT may be a viable therapeutic approach for women with DS coupled with dementia.

Amelioration of cholinergic neurons dysfunction in aged rats depends on the continuous supply of NGF

The present study was designed to examine whether NGF-induced improvement in morphology of senile basal forebrain cholinergic neurons persist after discontinuation of NGF treatment. Trophic effect of continuous intraventricular infusion of NGF was tested in the 4- and 28 months old male Wistar rats immediately after cessation of NGF and 3 or 6 weeks after termination of treatment. Immunohistochemical procedure for ChAT, TrkA, and p75(NTR) receptor has been applied to identify cholinergic cells in the basal forebrain structures. Using the quantitative image analyzer, morphometric and densitometric parameters of cholinergic cells were measured. In untreated 28-month-old rats a reduction in the number, size and intensity of staining of cholinergic neurons was observed in all basal forebrain structures. NGF significantly improved morphological parameters of ChAT- and TrkA-positive cells in aged rats. In 28-month-old rats tested within 3 and 6 weeks after discontinuation of infusion a renewed progressive deterioration of cholinergic phenotype of basal forebrain neurons was observed when compared with the NGF-treated immediately tested group. The parallel staining for p75(NTR) revealed normal morphology of the basal forebrain neurons, despite of the age of rats or the NGF treatment. Analysis of Nissl stained sections also showed that 28-month-old rats did not display significant losses of neurons in the basal forebrain when compared with the young animals. These findings demonstrate that senile impairment of cholinergic neurons is induced by a loss of cholinergic phenotype rather than an acute degeneration of cell bodies. NGF may be beneficial in enhancing cholinergic neurochemical parameters, but the protective effects seem to be dependent on the continuous supply of NGF.

Plasticity of aged perivascular axons following exogenous NGF: analysis of catecholamines

The present study investigated the atrophy of aged perivascular sympathetic axons and the response of these cerebrovascular neurons to the neurotrophin nerve growth factor (NGF). Using high performance liquid chromatography coupled with electrochemical detection (HPLC-ECD) to quantify catecholamines and immunohistochemical methods to quantify the density of TH immunoreactive fibers, we found a significant decrease in norepinephrine (NE) and TH in aged sympathetic axons. However, following in vivo administration of exogenous neurotrophin, aged neurons exhibited a robust response to NGF that was similar to the young adult, suggesting little decline in the capability of aged neurons to utilize exogenous neurotrophin. These results suggest that the age-related atrophy of aged sympathetic axons may result primarily from reduced availability of target-derived neurotrophin rather than from intrinsic alterations of neuronal function.

Neuroprotective peptide drug delivery and development: potential new therapeutics

Alzheimer's disease and related neurodegenerative disorders are prevalent among the elderly and might be considered as the plague of the 21st century. It is thus imperative to find cures for these conditions. The use of nerve growth factor proteins as neuroprotective therapeutics is limited by their hindered mobility through the blood-brain barrier. Peptides provide an attractive alternative. However, do peptide derivatives retain the activity of the entire protein? Are they stable? Would peptides cross the blood-brain barrier and what are the potential side effects? Examples are put forth to strengthen our opinion that peptides are important candidates for future drug development.

Nerve growth factor rapidly induces prolonged acetylcholine release from cultured basal forebrain neurons: differentiation between neuromodulatory and neurotrophic influences

Long-term exposure to nerve growth factor (NGF) is well established to have neurotrophic effects on basal forebrain cholinergic neurons, but its potential actions as a fast-acting neuromodulator are not as well understood. We report that NGF (0.1-100 ng/ml) rapidly (<60 min) and robustly enhanced constitutive acetylcholine (ACh) release (148-384% of control) from basal forebrain cultures without immediate persistent increases in choline acetyltransferase activity. More ACh was released in response to NGF when exposure was coupled with a higher depolarization level, suggesting activity dependence. In a long-term potentiation-like manner, brief NGF exposure (10 ng/ml; 60 min) induced robust and prolonged increases in ACh release, a capacity that was shared with the other neurotrophins. K252a (10-100 nm), BAPTA-AM (25 microm), and Cd(2+) (200 microm) prevented NGF enhancement of ACh release, suggesting the involvement of TrkA receptors, Ca(2+), and voltage-gated Ca(2+) channels, respectively. Forskolin (10 microm), a cAMP generator, enhanced constitutive ACh release but did not interact synergistically with NGF. Tetrodotoxin (1 microm) and cycloheximide (2 microm) did not prevent NGF-induced ACh release, indicative of action at the level of the cholinergic nerve terminal and that new protein synthesis is not required for this neurotransmitter-like effect, respectively. In contrast, after a 24 hr NGF treatment, distinct protein synthesis-dependent and independent effects on choline acetyltransferase activity and ACh release were observed. These results indicate that neuromodulator/neurotransmitter-like (protein synthesis-independent) and neurotrophic (translation-dependent) actions likely make distinct contributions to the enhancement of cholinergic activity by NGF.

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.

Long-term environmental enrichment leads to regional increases in neurotrophin levels in rat brain

A number of studies have demonstrated that both morphological and biochemical indices in the brain undergo alterations in response to environmental influences. In previous work we have shown that rats raised in an enriched environmental condition (EC) perform better on a spatial memory task than rats raised in isolated conditions (IC). We have also found that EC rats have a higher density of immunoreactivity than IC rats for both low and high affinity nerve growth factor (NGF) receptors in the basal forebrain. In order to determine if these alterations were coupled with altered levels of neurotrophins in other brain regions as well, we measured neurotrophin levels in rats that were raised in EC or IC conditions. Rats were placed in the different environments at 2 months of age and 12 months later brain regions were dissected and analyzed for NGF, brain-derived neurotrophic factor (BDNF), and neurotrophin-3 (NT-3) levels using Promega ELISA kits. We found that NGF and BDNF levels were increased in the cerebral cortex, hippocampal formation, basal forebrain, and hindbrain in EC animals compared to age-matched IC animals. NT-3 was found to be increased in the basal forebrain and cerebral cortex of EC animals as well. These findings demonstrate significant alterations in NGF, BDNF, and NT-3 protein levels in several brain regions as a result of an enriched versus an isolated environment and thus provide a possible biochemical basis for behavioral and morphological alterations that have been found to occur with a shifting environmental stimulus.