Decreased level of nerve growth factor (NGF) and its messenger RNA in the aged rat brain

Positive Allosteric Modulators of Trk Receptors for the Treatment of Alzheimer's Disease

Neurotrophins are important regulators of neuronal and non-neuronal functions. As such, the neurotrophins and their receptors, the tropomyosin receptor kinase (Trk) family of receptor tyrosine kinases, has attracted intense research interest and their role in multiple diseases including Alzheimer's disease has been described. Attempts to administer neurotrophins to patients have been reported, but the clinical trials have so far have been hampered by side effects or a lack of clear efficacy. Thus, much of the focus during recent years has been on identifying small molecules acting as agonists or positive allosteric modulators (PAMs) of Trk receptors. Two examples of successful discovery and development of PAMs are the TrkA-PAM E2511 and the pan-Trk PAM ACD856. E2511 has been reported to have disease-modifying effects in preclinical models, whereas ACD856 demonstrates both a symptomatic and a disease-modifying effect in preclinical models. Both molecules have reached the stage of clinical development and were reported to be safe and well tolerated in clinical phase 1 studies, albeit with different pharmacokinetic profiles. These two emerging small molecules are interesting examples of possible novel symptomatic and disease-modifying treatments that could complement the existing anti-amyloid monoclonal antibodies for the treatment of Alzheimer's disease. This review aims to present the concept of positive allosteric modulators of the Trk receptors as a novel future treatment option for Alzheimer's disease and other neurodegenerative and cognitive disorders, and the current preclinical and clinical data supporting this new concept. Preclinical data indicate dual mechanisms, not only as cognitive enhancers, but also a tentative neurorestorative function.

Multi-omics studies reveal ameliorating effects of physical exercise on neurodegenerative diseases

INTRODUCTION

Neurodegenerative diseases such as Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, and Huntington's disease, are heavy burdens to global health and economic development worldwide. Mounting evidence suggests that exercise, a type of non-invasive intervention, has a positive impact on the life quality of elderly with neurodegenerative diseases. X-omics are powerful tools for mapping global biochemical changes in disease and treatment.

METHOD

Three major databases were searched related to current studies in exercise intervention on neurodegenerative diseases using omics tools, including metabolomics, metagenomics, genomics, transcriptomics, and proteomics.

RESULT

We summarized the omics features and potential mechanisms associated with exercise and neurodegenerative diseases in the current studies. Three main mechanisms by which exercise affects neurodegenerative diseases were summed up, including adult neurogenesis, brain-derived neurotrophic factor (BDNF) signaling, and short-chain fatty acids (SCFAs) metabolism.

CONCLUSION

Overall, there is compelling evidence that exercise intervention is a feasible way of preventing the onset and alleviating the severity of neurodegenerative diseases. These studies highlight the importance of exercise as a complementary approach to the treatment and intervention of neurodegenerative diseases in addition to traditional treatments. More mechanisms on exercise interventions for neurodegenerative diseases, the specification of exercise prescriptions, and differentiated exercise programs should be explored so that they can actually be applied to the clinic.

Dracaena cochinchinensis stemwood extracts inhibit amyloid-β fibril formation and promote neuronal cell differentiation

Alzheimer’s disease (AD) is a progressive neurodegenerative disorder characterized by the deposition of amyloid plaques in the brain. The prevention of amyloid-β (Aβ)-induced neuronal toxicity is considered a major target for drug development for AD treatment. Dracaena cochinchinensis (Lour.) S.C. Chen, a Thai folk medicine named “Chan-Daeng,” is a member of the Asparagaceae family. The stemwood of D. cochinchinensis has been traditionally used for its antipyretic, pain relief, and anti-inflammatory effects. The aim of the present study was to determine the pharmacological activities of ethanol and water extracts of D. cochinchinensis stemwood in blocking the Aβ fibril formation, preventing Aβ-mediated cell toxicity, and promoting neuronal differentiation in cultured PC12 cells. The herbal extracts of D. cochinchinensis stemwood prevented the formation of Aβ fibrils and disassembled the aggregated Aβ in a dose-dependent manner. Additionally, they prevented Aβ fibril-mediated cell death. The synergy of the herbal extract with a low dose of the nerve growth factor showed an increase in the protein expression of neurofilaments, that is, NF68, NF160, and NF200. These findings suggest that the extracts of D. cochinchinensis stemwood may be used for AD treatment by targeting Aβ fibril formation and inducing neuron regeneration.

The Role of Neurotrophin Signaling in Age-Related Cognitive Decline and Cognitive Diseases

Neurotrophins are a family of secreted proteins expressed in the peripheral nervous system and the central nervous system that support neuronal survival, synaptic plasticity, and neurogenesis. Brain-derived neurotrophic factor (BDNF) and its high affinity receptor TrkB are highly expressed in the cortical and hippocampal areas and play an essential role in learning and memory. The decline of cognitive function with aging is a major risk factor for cognitive diseases such as Alzheimer’s disease. Therefore, an alteration of BDNF/TrkB signaling with aging and/or pathological conditions has been indicated as a potential mechanism of cognitive decline. In this review, we summarize the cellular function of neurotrophin signaling and review the current evidence indicating a pathological role of neurotrophin signaling, especially of BDNF/TrkB signaling, in the cognitive decline in aging and age-related cognitive diseases. We also review the therapeutic approach for cognitive decline by the upregulation of the endogenous BDNF/TrkB-system.

Relationship between Cognitive Dysfunction and Age-Related Variability in Oxidative Markers in Isolated Mitochondria of Alzheimer's Disease Transgenic Mouse Brains

Many neurodegenerative disorders, including Alzheimer's disease (AD), are strongly associated with the accumulation of oxidative damage. Transgenic animal models are commonly used to elucidate the pathogenic mechanism of AD. Beta amyloid (Aβ) and tau hyperphosphorylation are very famous hallmarks of AD and well-studied, but the relationship between mitochondrial dysfunction and the onset and progression of AD requires further elucidation. In this study we used transgenic mice (the strain name is 5xFAD) at three different ages (3, 6, and 20 months old) as an AD model. Cognitive impairment in AD mice occurred in an age-dependent manner. Aβ1-40 expression significantly increased in an age-dependent manner in all brain regions with or without AD, and Aβ1-42 expression in the hippocampus increased at a young age. In a Western blot analysis using isolated mitochondria from three brain regions (cerebral cortex, cerebellum, and hippocampus), NMNAT-3 expression in the hippocampi of aged AD mice was significantly lower than that of young AD mice. SOD-2 expression in the hippocampi of AD mice was lower than for the age-matched controls. However, 3-NT expression in the hippocampi of AD mice was higher than for the age-matched controls. NQO-1 expression in the cerebral cortex of AD mice was higher than for the age-matched controls at every age that we examined. However, hippocampal NQO-1 expression in 6-month-old AD mice was significantly lower than in 3-month-old AD mice. These results indicate that oxidative stress in the hippocampi of AD mice is high compared to other brain regions and may induce mitochondrial dysfunction via oxidative damage. Protection of mitochondria from oxidative damage may be important to maintain cognitive function.

Caesalpinia mimosoides Leaf Extract Promotes Neurite Outgrowth and Inhibits BACE1 Activity in Mutant APP-Overexpressing Neuronal Neuro2a Cells

Alzheimer’s disease (AD) is implicated in the imbalance of several proteins, including Amyloid-β (Aβ), amyloid precursor protein (APP), and BACE1. APP overexpression interferes with neurite outgrowth, while BACE1 plays a role in Aβ generation. Medicinal herbs with effects on neurite outgrowth stimulation and BACE1 inhibition may benefit AD. This study aimed to investigate the neurite outgrowth stimulatory effect, along with BACE1 inhibition of Caesalpinia mimosoides (CM), using wild-type (Neuro2a) and APP (Swedish mutant)-overexpressing (Neuro2a/APPSwe) neurons. The methanol extract of CM leaves stimulated neurite outgrowth in wild-type and APP-overexpressing cells. After exposure to the extract, the mRNA expression of the neurite outgrowth activation genes growth-associated protein-43 (GAP-43) and teneurin-4 (Ten-4) was increased in both Neuro2a and Neuro2a/APPSwe cells, while the mRNA expression of neurite outgrowth negative regulators Nogo receptor (NgR) and Lingo-1 was reduced. Additionally, the extract suppressed BACE1 activity in the APP-overexpressing neurons. Virtual screening demonstrated that quercetin-3′-glucuronide, quercetin-3-O-glucoside, clausarinol, and theogallin were possible inhibitors of BACE1. ADMET was analyzed to predict drug-likeness properties of CM-constituents. These results suggest that CM extract promotes neurite outgrowth and inhibits BACE1 activity in APP-overexpressing neurons. Thus, CM may serve as a source of drugs for AD treatment. Additional studies for full identification of bioactive constituents and to confirm the neuritogenesis in vivo are needed for translation into clinic of the present findings.

Peptides Derived from Growth Factors to Treat Alzheimer's Disease

Alzheimer's disease (AD) is a devastating neurodegenerative disease characterized by progressive neuron losses in memory-related brain structures. The classical features of AD are a dysregulation of the cholinergic system, the accumulation of amyloid plaques, and neurofibrillary tangles. Unfortunately, current treatments are unable to cure or even delay the progression of the disease. Therefore, new therapeutic strategies have emerged, such as the exogenous administration of neurotrophic factors (e.g., NGF and BDNF) that are deficient or dysregulated in AD. However, their low capacity to cross the blood-brain barrier and their exorbitant cost currently limit their use. To overcome these limitations, short peptides mimicking the binding receptor sites of these growth factors have been developed. Such peptides can target selective signaling pathways involved in neuron survival, differentiation, and/or maintenance. This review focuses on growth factors and their derived peptides as potential treatment for AD. It describes (1) the physiological functions of growth factors in the brain, their neuronal signaling pathways, and alteration in AD; (2) the strategies to develop peptides derived from growth factor and their capacity to mimic the role of native proteins; and (3) new advancements and potential in using these molecules as therapeutic treatments for AD, as well as their limitations.

Influence of aging on the peripheral nerve repair process using an artificial nerve conduit

The influence of aging on the induction of nerve regeneration in artificial nerve conduits has yet to be clarified. In the present study, artificial nerve conduit transplantation and histological analysis using the sciatic nerve of young and elderly mice were performed. Using 20 male C57BL/6 mice, an artificial nerve conduit was transplanted to the sciatic nerve at 8 weeks (Young group) or 70 weeks of age (Aged group), and the sciatic nerve was evaluated histologically at 1, 4 and 12 weeks after surgery. Using hematoxylin and eosin staining, the state of induction of nerve regeneration in the artificial nerve conduit was evaluated. Additionally, immunohistochemical staining was used to investigate an angiogenic marker [vascular endothelial growth factor A (VEGFA)], Schwann cell markers [sex determining region Y-box 10 (SOX10) and S100 calcium-binding protein β (S100β)] and a nerve damage marker [nerve growth factor (NGF)]. The results revealed that the induction of nerve regeneration was significantly higher in the Young group than in the Aged group. In addition, VEGFA and SOX10 expression at 1 week, SOX10 expression at 4 weeks and SOX10, S100β and NGF expression at 12 weeks in the proximal stump were significantly higher in the Young group than in the Aged group. At the center of the artificial nerve conduit, S100β and NGF expression at 4 weeks, and VEGFA, SOX10, S100β and NGF expression at 12 weeks were significantly higher in the Young group than in the Aged group. In the distal stump, no significant difference was noted in immunostaining at any week between the two groups. The present study suggested that the nerve regeneration-inducing functions decrease due to aging.

Neurite Outgrowth-Promoting Activity of Compounds in PC12 Cells from Sunflower Seeds

In the current super-aging society, the establishment of methods for prevention and treatment of Alzheimer’s disease (AD) is an urgent task. One of the causes of AD is thought to be a decrease in the revel of nerve growth factor (NGF) in the brain. Compounds showing NGF-mimicking activity and NGF-enhancing activity have been examined as possible agents for improving symptoms. In the present study, sunflower seed extract was found to have neurite outgrowth-promoting activity, which is an NGF-enhancing activity, in PC12 cells. To investigate neurite outgrowth-promoting compounds from sunflower seed extract, bioassay-guided purification was carried out. The purified active fraction was obtained by liquid-liquid partition followed by some column chromatographies. Proton nuclear magnetic resonance and gas chromatography-mass spectrometry analyses of the purified active fraction indicated that the fraction was a mixture of β-sitosterol, stigmasterol and campesterol, with β-sitosterol being the main component. Neurite outgrowth-promoting activities of β-sitosterol, stigmasterol, campesterol and cholesterol were evaluated in PC12 cells. β-Sitosterol and stigmasterol showed the strongest activity of the four sterol compounds (β-sitosterol ≈ stigmasterol > campesterol > cholesterol), and cholesterol did not show any activity. The results indicated that β-sitosterol was the major component responsible for the neurite outgrowth-promoting activity of sunflower seeds. Results of immunostaining also showed that promotion by β-sitosterol of neurite formation induced by NGF was accompanied by neurofilament expression. β-Sitosterol, which showed NGF-enhancing activity, might be a candidate ingredient in food for prevention of AD.

LncRNA SNHG14 aggravates invasion and migration as ceRNA via regulating miR-656-3p/SIRT5 pathway in hepatocellular carcinoma

Recurrence and adverse events after hepatocellular carcinoma (HCC) treatment occur frequently even treated with the most efficient therapy for HCC, liver transplantation. Therefore, better understanding of HCC progression is required to advance the therapeutic strategy of HCC. This study aims to explore the effect and mechanism of small nucleolar RNA host gene 14 (SNHG14) on HCC cell invasion and migration. SNHG14 and miR-656-3p expression in HCC tissues and cells were examined by qRT-PCR. After co-transfection with sh-SNHG14, miR-656-3p inhibitor, miR-656-3p mimic, si-SIRT5, pcDNA3.1-SIRT5 and corresponding negative controls, HepG2 and MHCC97H cell proliferation, invasion and migration were detected. Then the expression levels of SNHG14, miR-656-3p and SIRT5 were measured by qRT-PCR and Western blot. Luciferases reporter gene assay and RNA pull down identified the relation between SNHG14 and miR-656-3p and between miR-656-3p and SIRT5. SNHG14 was upregulated and miR-656-3p was downregulated in HCC cells. Inhibition of SNHG14 could inhibit HepG2 and MHCC97H cell proliferation, invasion and migration. Upregulation of miR-656-3p or knockdown of SIRT5 significantly suppressed the biological process of HepG2 and MHCC97H cells. SNHG14 directly acted on miR-656-3p and SIRT5 was a target gene of miR-656-3p. miR-656-3p inhibitor or pcDNA3.1-SIRT5 could reverse the inhibition of sh-SNHG14 on cell proliferation, invasion and migration of HCC cells. SNHG14 promotes HCC cell invasion and migration through regulating miR-656-3p/SIRT5 axis.

Toll-like receptor 4 deficiency ameliorates β2-microglobulin induced age-related cognition decline due to neuroinflammation in mice

Toll-like receptor 4 (TLR4) is a crucial receptor in neuroinflammation and apoptotic neuronal death, and increasing evidences indicated that β2-microglobulin (B2M) is thought to be a major contributor to age-related cognitive decline. In present study, we designed to investigate the effects of TLR4 on B2M-induced age-related cognitive decline. Wild-type (WT) C57BL/6, TLR4 knockout (TLR4 -KO) mice and hippocampal neurons from the two type mice were respectively divided into two groups: (1) Veh group; (2) B2M-treated group. The behavioral responses of mice were measured using Morris Water Maze. Hippocampal neurogenesis and neuronal damage, inflammatory response, apoptosis, synaptic proteins and neurotrophic factors, and TLR4/MyD88/NF-κB signaling pathway proteins were examined using molecular biological or histopathological methods. The results showed that WT mice received B2M in the DG exhibited age-related cognitive declines, increased TLR4 mRNA expression and high levels of interleukin-1β (IL-1β), tumor necrosis factor-alpha (TNF-α) and apoptotic neuronal death in the hippocampus, which were partially attenuated in TLR4-KO mice. Moreover, in absence of TLR4, B2M treatment improved hippocampus neurogenesis and increased synaptic related proteins. Our cell experiments further demonstrated that deletion of TLR4 could significantly increase synaptic related protein, decrease neuroinflammatory fators, inhibited apoptotic neuronal death, and regulated MyD88/NF-κB signal pathway after B2M treatment. In summary, our results support the TLR4 contributes to B2M-induced age-related cognitive decline due to neuroinflammation and apoptosis through TLR4/MyD88/NF-κB signaling pathway via a modulation of hippocampal neurogenesis and synaptic function. This may provide an important neuroprotective mechanism for improving age-related cognitive decline.

Hydrophilic Glycoproteins of an Edible Green Alga Capsosiphon fulvescens Prevent Aging-Induced Spatial Memory Impairment by Suppressing GSK-3β-Mediated ER Stress in Dorsal Hippocampus

Endoplasmic reticulum (ER) stress is involved in various neurodegenerative disorders. We previously found that Capsosiphon fulvescens (C. fulvescens) crude proteins enhance spatial memory by increasing the expression of brain-derived neurotrophic factor (BDNF) in rat dorsal hippocampus. The present study investigated whether the chronic oral administration of hydrophilic C. fulvescens glycoproteins (Cf-hGP) reduces aging-induced cognitive dysfunction by regulating ER stress in the dorsal hippocampus. The oral administration of Cf-hGP (15 mg/kg/day) for four weeks attenuated the aging-induced increase in ER stress response protein glucose-regulated protein 78 (GRP78) in the synaptosome of the dorsal hippocampus; this was attenuated by the function-blocking anti-BDNF antibody (1 μg/μL) and a matrix metallopeptidase 9 inhibitor 1 (5 μM). Aging-induced GRP78 expression was associated with glycogen synthase kinase-3 beta (GSK-3β) (Tyr216)-mediated c-Jun N-terminal kinase phosphorylation, which was downregulated upon Cf-hGP administration. The Cf-hGP-induced increase in GSK-3β (Ser9) phosphorylation was downregulated by inhibiting tyrosine receptor kinase B and extracellular signal-regulated kinase (ERK)1/2 with cyclotraxin-B (200 nM) and SL327 (10 μM), respectively. Cf-hGP administration or the inhibition of ER stress with salubrinal (1 mg/kg, i.p.) significantly decreased aging-induced spatial memory impairment. These findings suggest that the activation of the synaptosomal BDNF-ERK1/2 signaling in the dorsal hippocampus by Cf-hGP attenuates age-dependent ER stress-induced cognitive dysfunction.

Nerve Growth Factor Levels in Term Human Infants: Relationship to Prenatal Growth and Early Postnatal Feeding

BACKGROUND

Nerve growth factor (NGF) plays a key role in neuroprotection and developmental maturity. We assessed longitudinally the circulating concentrations of NGF in term healthy human newborns and infants as well as their association with prenatal growth and early postnatal feeding patterns.

METHODS

Circulating NGF and anthropometric measures (weight, length, body mass index, and ponderal index) were assessed longitudinally-at birth and at age 4 months-in 86 term infants born appropriate (AGA), small (SGA), or large for gestational age (LGA).

RESULTS

Cord blood NGF levels in SGA newborns were higher than those in AGA newborns (1.41 ± 0.2 pg/mL vs. 0.66 ± 0.1 pg/mL; p = 0.02) and not different from those in LGA neonates (0.79 ± 0.2 pg/mL). At age 4 months, SGA-breastfed infants showed the highest NGF concentrations (p = 0.02 and p = 0.01 vs. AGA and SGA-formula-fed infants, respectively), while LGA infants depicted a marginal increase. NGF levels in cord blood correlated negatively with the ponderal index at birth (r = -0.36; p = 0.0008).

CONCLUSIONS

Circulating NGF is related to both prenatal growth and early postnatal nutrition. The maintenance of increased NGF concentrations in SGA-breastfed infants at age 4 months might be a potential mechanism to counterbalance potential risks for developing cognitive and psychomotor disadvantages.

The effects of aging in the hippocampus and cognitive decline

Aging is a natural process that is associated with cognitive decline as well as functional and social impairments. One structure of particular interest when considering aging and cognitive decline is the hippocampus, a brain region known to play an important role in learning and memory consolidation as well as in affective behaviours and mood regulation, and where both functional and structural plasticity (e.g., neurogenesis) occur well into adulthood. Neurobiological alterations seen in the aging hippocampus including increased oxidative stress and neuroinflammation, altered intracellular signalling and gene expression, as well as reduced neurogenesis and synaptic plasticity, are thought to be associated with age-related cognitive decline. Non-invasive strategies such as caloric restriction, physical exercise, and environmental enrichment have been shown to counteract many of the age-induced alterations in hippocampal signalling, structure, and function. Thus, such approaches may have therapeutic value in counteracting the deleterious effects of aging and protecting the brain against age-associated neurodegenerative processes.

Role of Nerve Growth Factor in Plasticity of Forebrain Cholinergic Neurons

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

The involvement of BDNF, NGF and GDNF in aging and Alzheimer's disease

Aging is a normal physiological process accompanied by cognitive decline. This aging process has been the primary risk factor for development of aging-related diseases such as Alzheimer's disease (AD). Cognitive deficit is related to alterations of neurotrophic factors level such as brain-derived neurotrophic factor (BDNF), nerve growth factor (NGF) and glial cell-derived neurotrophic factor (GDNF). These strong relationship between aging and AD is important to investigate the time which they overlap, as well as, the pathophysiological mechanism in each event. Considering that aging and AD are related to cognitive impairment, here we discuss the involving these neurotrophic factors in the aging process and AD.

Glycation of the high affinity NGF-receptor and RAGE leads to reduced ligand affinity

AGEs are posttranslational modifications generated by irreversible non-enzymatic crosslinking reactions between sugars and proteins - a reaction referred to as glycation. Glycation, a feature of ageing, can lead to non-degradable and less functional proteins and enzymes and can additionally induce inflammation and further pathophysiological processes such as neurodegeneration. In this study we investigated the influence of glycation on the high affinity NGF-receptor TrkA and the AGE-receptor RAGE. We quantified the binding affinity of the TrkA-receptor and RAGE to their ligands by surface plasmon resonance (SPR) and compared these to the binding affinity after glycation. At the same time, we established a glycation procedure using SPR. We found that glycation of TrkA reduced the affinity to NGF by a factor of three, which could be shown to lead to a reduction of NGF-dependent neurite outgrowth in PC12 cells. Glycation of RAGE reduced binding affinity of AGEs by 10-fold.

Learning to remember: cognitive training-induced attenuation of age-related memory decline depends on sex and cognitive demand, and can transfer to untrained cognitive domains

Aging is associated with progressive changes in learning and memory. A potential approach to attenuate age-related cognitive decline is cognitive training. In this study, adult male and female rats were given either repeated exposure to a T-maze, or no exposure to any maze, and then tested on a final battery of cognitive tasks. Two groups of each sex were tested from 6 to 18 months old on the same T-maze; Group one received a version testing spatial reference memory, and Group two received only the procedural testing components with minimal cognitive demand. Groups three and four of each sex had no maze exposure until the final battery, and were comprised of aged or young rats, respectively. The final maze battery included the practiced T-maze plus two novel tasks, one with a similar, and one with a different, memory type to the practice task. Group five of each sex was not maze tested, serving as an aged control for the effects of maze testing on neurotrophin protein levels in cognitive brain regions. Results showed that adult intermittent cognitive training enhanced performance on the practice task when aged in both sexes, that cognitive training benefits transferred to novel tasks only in females, and that cognitive demand was necessary for these effects, since rats receiving only the procedural testing components showed no improvement on the final maze battery. Further, for both sexes, rats that showed faster learning when young demonstrated better memory when aged. Age-related increases in neurotrophin concentrations in several brain regions were revealed, which were related to performance on the training task only in females. This longitudinal study supports the tenet that cognitive training can help one remember later in life, with broader enhancements and associations with neurotrophins in females.

Increased micro-RNA 29b in the aged brain correlates with the reduction of insulin-like growth factor-1 and fractalkine ligand

Microglia develop an inflammatory phenotype during normal aging. The mechanism by which this occurs is not well understood, but might be related to impairments in several key immunoregulatory systems. Here we show that micro-RNA (miR)-29a and miR-29b, 2 immunoregulatory micro-RNAs, were increased in the brain of aged BALB/c mice compared with adults. Insulin-like growth factor-1 (IGF-1) and fractalkine ligand (CX3CL1) are negative modulators of microglial activation and were identified as targets of miR-29a and miR-29b using luciferase assay and primary microglia transfection. Indeed, higher expression of miR-29b in the brain of aged mice was associated with reduced messenger RNA (mRNA) levels of IGF-1 and CX3CL1. Parallel to these results in mice, miR-29a and miR-29b were also markedly increased in cortical brain tissue of older individuals (mean, 77 years) compared with middle-aged adults (mean, 45 years). Moreover, increased expression of miR-29b in human cortical tissue was negatively correlated with IGF-1 and CX3CL1 expression. Collectively, these data indicate that an age-associated increase in miR-29 corresponded with the reduction of 2 important regulators of microglia, IGF-1 and CX3CL1.

Age-dependent alterations in nerve growth factor (NGF)-related proteins, sortilin, and learning and memory in rats

The objective of this study was to evaluate the effects of aging on the performance of specific memory-related tasks in rats as well as to determine the levels of several nerve growth factor (NGF)-related proteins in relevant brain regions. The results indicated age-related impairments in spatial learning in a water maze task as well as deficits in recognition memory in a Spontaneous Novel Object Recognition task. In the prefrontal cortex and hippocampus, aged rats (compared to young controls) had elevated levels of the proneurotrophin, proNGF (+1.8-1.9 fold), p75(NTR) receptors (+1.6-1.8 fold) and sortilin (+1.8-2.1 fold), and decreased levels of mature NGF (-36 to 44%), and phospho-TrkA receptors (-45 to 49%). The results of this study support the argument that NGF signaling is altered in the aging brain, and that such alterations may contribute to an age-related decline in cognitive function. These results may also help to identify specific components of the NGF-signaling pathway that could serve as targets for novel drug discovery and development for age-related disorders of cognition (e.g., Alzheimer's disease).

Age-related changes in the sympathetic innervation of cerebral vessels and in carotid vascular responses to norepinephrine in the rat: in vitro and in vivo studies

We hypothesized that the density of sympathetic noradrenergic innervation of cerebral arteries and vasoconstrictor responses evoked in carotid circulation by norepinephrine (NE) increase with maturation and age. In rats of 4-5, 10-12, and 42-44 wk of age (juvenile, mature, middle aged), glyoxylic acid applied to stretch preparations showed the density of noradrenergic nerves in basilar and middle cerebral arteries was greater in mature than juvenile or middle-aged rats. In anesthetized rats, infusion of NE (2.5 mug/kg iv) increased mean arterial pressure (ABP) to approximately 180 mmHg in mature and middle-aged but to only approximately 150 mmHg in juveniles rats. Concomitantly, carotid blood flow (CBF) decreased in mature and middle-aged rats but remained constant in juveniles because carotid vascular conductance (CVC) decreased more in mature and middle-aged than juvenile rats. We also hypothesized that nitric oxide (NO) blunts cerebral vasoconstrictor responses to NE. Inhibition of NO synthase with l-NAME (10 mg/kg iv) induced similar increases in baseline ABP in each group, but larger decreases in CVC and CBF in mature and middle-aged than juvenile rats. Thereafter, the NE-evoked increase in ABP was similar in juvenile and mature but accentuated in middle-aged rats. Concomitantly, NE decreased CVC in juvenile and mature, but not middle-aged rats; in them, CBF increased. Thus, in juvenile rats, sparse noradrenergic innervation of cerebral arteries is associated with weak NE-evoked pressor responses and weak carotid vasoconstriction that allows autoregulation of CBF. Cerebral artery innervation density increases with maturation but lessens by middle age. Meanwhile, NE-evoked pressor responses and carotid vasoconstriction are stronger in mature and middle-aged rats, such that CBF falls despite the evoked increase in ABP. We propose that in juvenile and mature rats, NO does not modulate NE-evoked pressor responses, cerebral vasoconstriction, or CBF autoregulation, but by middle age, NO limits pressor responses and prevents breakthrough of CBF in the upper part of the autoregulatory range.

Long-term treadmill exposure protects against age-related neurodegenerative change in the rat hippocampus

The potential of exercise or environmental enrichment to prevent or reverse age-related cognitive decline in rats has been widely investigated. The data suggest that the efficacy of these interventions as neuroprotectants may depend upon the duration and nature of the protocols and age of onset. Investigations of the mechanisms underlying these neuroprotective strategies indicate a potential role for the neurotrophin family of proteins, including nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF). In this study, we have assessed the effects of 8 months of forced exercise, begun in middle-age, on the expression of long-term potentiation (LTP) and on spatial learning in the Morris water maze in aged Wistar rats. We also assessed these measures in a cage control group and in a group of rats exposed to the stationary treadmill for the same duration as the exercised rats. Our data confirm an age-related decline in expression of LTP and in spatial learning concomitant with decreased expression of NGF and BDNF mRNA in dentate gyrus (DG). The age-related impairments in both plasticity and growth factor expression were prevented in the long-term exercised group and, surprisingly, the treadmill control group. Given the extensive handling that the treadmill control group received and their regular exposure to an environment outside the home cage, this group can be considered to have experienced environmentally enriched conditions when compared with the cage control group. Significant correlations were observed between both learning and LTP and the expression of NGF and BDNF mRNA in the dentate gyrus. We conclude that decreased expression of NGF and BDNF in the dentate gyrus of aged rats is associated with impaired LTP and spatial learning. We suggest that the reversal of these age-related impairments by enrichment and exercise may be linked with prevention of the age-related decline in expression of these growth factors and, furthermore, that enrichment is as efficacious as exercise in preventing this age-related decline.

THIS ARTICLE HAS BEEN RETRACTED: Functional nerve growth factor and trkA autocrine/paracrine circuits in adult rat cortex are revealed by episodic ethanol exposure and withdrawal

The hypothesis tested is that cortical neurotrophins communicate through an inducible autocrine/paracrine mechanism. As ethanol (Et) can induce cortical nerve growth factor (NGF) expression, adult rats were challenged with Et on three consecutive days per week for 6 weeks. The focus of the study was layer V, the chief repository of receptor-expressing neuronal cell bodies. Brains were collected immediately after the sixth Et exposure or 72 h later [i.e., following withdrawal (WD)]. Double-label in situ hybridization-immunohistochemistry studies showed that many neuronal somata co-expressed NGF mRNA with NGF, trkA, or phosphorylated trk (p-trk), essential components of an inducible autocrine system. The frequencies of co-labeling were affected by neither Et nor WD. On the contrary, Et increased the number of NGF mRNA-expressing neurons and the amount of NGF mRNA expressed per cell. Et also increased total cortical concentration of NGF protein, the number of layer V neurons expressing trkA transcript, the amount of trkA mRNA expressed per neuron, and trkA phosphorylation. Following WD, the frequency of NGF-mRNA-expressing cells increased, although transcript and protein content fell. WD induced an increase in trkA mRNA and protein expression, however, p-trk expression was unaffected. Thus, Et treatment reveals that layer V has inducible autocrine/paracrine and anterograde neurotrophin systems. WD unveils the dynamism and recruitability of these systems.

Are neurodegenerative disorder and psychotic manifestations avoidable brain dysfunctions with adequate dietary omega-3?

The present mismatch between what our brain needs, and the modern diet neglects our marine heritage. Last century, the priority in nutrition and food production was to achieve a high protein diet and somatic growth and function. The dietary content of omega-3 (N-3) required by the brain was neglected although evidence for the essentiality of certain fatty acids was published in 1929 and specifically re-affirmed for omega 3 in the brain in the 1970s. Cognitive decline with age and neurodegenerative disorder with dementia are now rising. This review describes signs of N-3 deficit in Alzheimer and Parkinson Disease, where maximum change involves the primary sites: olfactory cortex and the hippocampus. The olfactory agnosia observed in schizophrenia supports an N-3 deficit as does a reduction of key ologodendrocyte- and myelin-related genes in this disorder and affective disorder, where a rise in dementia accords with a deficit of N-3 also in this disorder. N-3 normalizes cerebral excitability at all levels. That the two disorders are localized at the extremes of excitability, is supported by their opposing treatments: convulsant neuroleptics and anti-epileptic antidepressants. An adequate N-3 diet will probably prevent most psychotic episodes and prove that neurodegenerative disorder with dementia is also to a large extent not only preventable but avoidable.

Are neurodegenerative disorder and psychotic manifestations avoidable brain dysfunctions with adequate dietary omega-3?

The present mismatch between what our brain needs, and the modern diet neglects our marine heritage. Last century, the priority in nutrition and food production was to achieve a high protein diet and somatic growth and function. The dietary content of omega-3 (N-3) required by the brain was neglected although evidence for the essentiality of certain fatty acids was published in 1929 and specifically re-affirmed for omega 3 in the brain in the 1970s. Cognitive decline with age and neurodegenerative disorder with dementia are now rising. This review describes signs of N-3 deficit in Alzheimer and Parkinson Disease, where maximum change involves the primary sites: olfactory cortex and the hippocampus. The olfactory agnosia observed in schizophrenia supports an N-3 deficit as does a reduction of key ologodendrocyte- and myelin-related genes in this disorder and affective disorder, where a rise in dementia accords with a deficit of N-3 also in this disorder. N-3 normalizes cerebral excitability at all levels. That the two disorders are localized at the extremes of excitability, is supported by their opposing treatments: convulsant neuroleptics and anti-epileptic anti-depressants. An adequate N-3 diet will probably prevent most psychotic episodes and prove that neurodegenerative disorder with dementia is also to a large extent not only preventable but avoidable.

Nerve growth factor restores the expression of vasopressin and vasoactive intestinal polypeptide in the suprachiasmatic nucleus of aged rats

Aging leads to a decrease in the number of neurons expressing vasopressin (VP) and vasoactive intestinal polypeptide (VIP) in the suprachiasmatic nucleus (SCN) of the rat. Similar results were observed following prolonged alcohol consumption and withdrawal. In the latter circumstances, the administration of nerve growth factor (NGF) restored the synthesis and expression of those neuropeptides despite the absence of TrkA receptors in SCN neurons. Thus, we decided to test whether the administration of NGF would improve the expression of neuropeptides in the SCN of aged rats. For this purpose, NGF was delivered intraventricularly to aged rats over a period of 14 days. The somatic volume and the total number of VP- and VIP-immunostained SCN neurons were estimated by applying stereological methods. No age-related variations were found regarding the volume of the neuronal cell bodies. Yet, a striking reduction in the number of VP- and VIP-immunoreactive neurons was detected in aged animals and found to be completely retrieved by NGF. This finding shows that exogenous NGF administered to aged rats restores the neurochemical phenotype of the SCN. This might occur either through direct signaling of SCN neurons via p75NTR or through enhancement of the cholinergic input to the SCN.

Aging, peripheral nerve injury and nociception: effects of the antioxidant 16-desmethyltirilazad

Peripheral neuropathies increase with aging, and reactive oxygen species contribute to the symptomatology of neuropathic pain disorders. In this study, we examined age-related differences in the therapeutic efficacy of pre- or post-treatments of the amino-steroidal antioxidant 16-desmethyltirilazad in delaying the onset and/or limiting the duration of tactile-evoked allodynia following the induction of peripheral mononeuropathies in rats. Two different models of nerve injury were utilized to induce allodynia in young and aged rats: (1) the chronic constriction injury (CCI) model of Bennett and Xie [Bennett GJ, Xie Y-K. A peripheral mononeuropathy in rat that produces disorders of pain sensation like those seen in man. Pain 1988;33:87-107]; (2) the partial sciatic nerve ligation (PSNL) model of Seltzer et al. [Seltzer Z, Dubner R, Shir YA. Novel behavioral model of neuropathic pain disorders produced in rats by partial sciatic nerve injury. Pain 1990;43:205-18]. Calibrated von Frey filaments were used to examine changes in paw withdrawal threshold values. The results demonstrated that pre-treating young and aged rats with 16-desmethyltirilazad prior to the induction of peripheral mononeuropathies prevented the onset of neuropathic pain. However, once post-operative tactile allodynia was established, post-treatment therapy was ineffective at reversing the symptoms. These findings support the mediatory role of reactive oxygen species in neuropathic pain disorders, and suggest that the antiallodynic efficacy of antioxidant intervention is dependent on the time course of administration.

Downregulation of IL-4-induced signalling in hippocampus contributes to deficits in LTP in the aged rat

Ageing is characterized by deficits in learning and memory and by a deficit in long-term potentiation (LTP) in hippocampus. Several age-related changes, including dysfunction of calcium homeostatic mechanisms and upregulation of inflammatory processes are likely to contribute to these deficits. Here we exploited the fact that aged rats fall into a subgroup which fail to sustain LTP in perforant path granule cell synapses as a result of tetanic stimulation, and a subgroup which sustains LTP in a manner indistinguishable from young rats, in an effort to identify differential changes in the two subgroups. The age-related increase in IL-1beta concentration and IL-1beta-induced signalling was more profound in aged rats which failed to sustain LTP. We demonstrate that functional IL-4 receptors are expressed in rat hippocampus and that age is associated with a decrease in IL-4 concentration accompanied by a decrease in phosphorylation of JAK-1 and STAT-6. We propose that the imbalance between pro-inflammatory and anti-inflammatory cytokines in the aged brain significantly contributes to age-related deficits in synaptic function.

From superior adaptation and function to brain dysfunction--the neglect of epigenetic factors

With optimal pregnancy conditions (natural, enriched diet which includes fish) African (Digo) infants are 3-4 weeks ahead of European/American infants in sensorimotor terms at birth, and during the first year. Infants of semi-aquatic sea-gypsies swim before they walk, and have superior visual acuity compared with us. With adverse pregnancy behaviour (fear of fat, a trend to dieting), neglecting the need for brain fat to secure normal brain development and function, we run a risk of dysfunction--death. Sudden Infant Death Syndrome victims have depressed birth weight, lower levels of marine fat in brainstem than controls, and >80 suffer multiple hypoxic episodes prior to death. Depressed birth weight (more than 10% below mean) is seen in learning and behaviour disorders, and a trend towards weights of less than 3kg is increasing, which supports a rise in antenatal sub optimality. Given marine fat deficiency in pregnancy and infancy, neurons starved for fuel could delay myelination and maturation in the latest developed Frontal Lobes. The phylogenetic oldest Lateral Frontal Lobe System (feed-back mechanism etc.) derived from olfactory bulb-amygdala, which crosses in Anterior Commisure is probably spared, while the Medial Frontal Lobe System derived from Hippocampus-Cingulum and crosses in Corpus Callosum (delayed response task) is most likely affected. The rise in infantile autism (intact vision and hearing) with deficit in delayed response task only, could suggest a deficit in the Medial Frontal Lobe System. The human species is unique; 70% of total energy to the foetus goes to development of the brain, which mainly consists of marine fat. It undergoes pervasive regressive events, before birth, in infancy and at puberty. Minimal retraction of neuronal arborisation is advantageous. Attributable to adverse pregnancy childrearing practice, excessive retraction is likely prenatally and in infancy. Pubertal age affects the fundamental property of nervous tissue, excitability: excessive excitatory drive is seen in early, and a deficiency in late puberty. It is postulated that with adequate marine fat, there is probably no risk of psychopathology at the extremes, whereas a deficiency could lead to paroxysmal (subcortical) dysfunction in early puberty, and breakdown of cortical circuitry and cognitive dysfunctions in late puberty. The post-pubertal psychoses, schizophrenia and manic-depressive psychosis at the extremes of the pubertal age continuum, with contrasting excitability and biological treatment, are probably the result of continuous dietary deficiency, which has inactivated the expression of genes for myelin development and oligodendrocyte-related genes in their production of myelin. The beneficial effect of marine fat in both disorders, in other CNS disorders as well as in developmental dyslexia (DD) and ADHD among others, supports our usual diet is persistently deficient. We have neglected the similarity of our great brain to other mammals, and our marine heritage. Given the amount of marine fat needed to secure normal brain development and function is not known, nor the present dietary level, it seems unduly conjectural to postulate that a dietary deficiency in marine fat is causing brain dysfunction and death. However, all observations point in the same direction: our diet focusing on protein mainly, is deficient, the deficiency is most pronounced in maternal nutrition and in infancy.

Increased IL-1beta in cortex of aged rats is accompanied by downregulation of ERK and PI-3 kinase

Ageing is accompanied by a myriad of changes, which lead to deficits in synaptic function and recent studies have identified an increase in concentration of the proinflammatory cytokine, interleukin-1beta (IL-1beta), as a factor which significantly contributes to deterioration of cell function. Here, we consider that increased IL-1beta concentration and upregulation of IL-1beta-induced cell signalling cascades may be accompanied by downregulation of survival signals, perhaps as a consequence of decreased neurotrophins-associated signalling. The data indicate that increased IL-1beta concentration was coupled with downregulation of ERK and phosphoinositide-3 kinase (PI-3 kinase) in cortical tissue prepared from aged rats. These changes could not be attributed to decreased concentration of NGF or BDNF but the evidence suggested that they may be a consequence of an age-related change in the anti-inflammatory cytokine, IL-4. Significantly, treatment of aged rats with eicosapentaenoic acid reversed the age-related increases in IL-1beta and IL-1beta-induced signalling and also the age-related changes in IL-4, ERK and PI-3 kinase.

Long-term potentiation and memory

One of the most significant challenges in neuroscience is to identify the cellular and molecular processes that underlie learning and memory formation. The past decade has seen remarkable progress in understanding changes that accompany certain forms of acquisition and recall, particularly those forms which require activation of afferent pathways in the hippocampus. This progress can be attributed to a number of factors including well-characterized animal models, well-defined probes for analysis of cell signaling events and changes in gene transcription, and technology which has allowed gene knockout and overexpression in cells and animals. Of the several animal models used in identifying the changes which accompany plasticity in synaptic connections, long-term potentiation (LTP) has received most attention, and although it is not yet clear whether the changes that underlie maintenance of LTP also underlie memory consolidation, significant advances have been made in understanding cell signaling events that contribute to this form of synaptic plasticity. In this review, emphasis is focused on analysis of changes that occur after learning, especially spatial learning, and LTP and the value of assessing these changes in parallel is discussed. The effect of different stressors on spatial learning/memory and LTP is emphasized, and the review concludes with a brief analysis of the contribution of studies, in which transgenic animals were used, to the literature on memory/learning and LTP.

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.

Evidence for reduced accumulation of exogenous neurotrophin by aged sympathetic neurons

The present study investigated the potential for neurotrophin uptake by cerebrovascular axons and subsequent accumulation in the aged superior cervical ganglion (SCG) following a two week intracerebroventricular infusion of nerve growth factor (NGF). In the SCG from aged rats, NGF protein levels declined significantly compared with the SCG from young adult rats. Following NGF infusion, perivascular axons from both young adult and aged rats showed intense NGF immunostaining. In addition, significant increases in NGF protein were shown using enzyme-linked immunosorbent assay (ELISA) and in counts of NGF immunopositive cell bodies in the SCG when compared with age-matched controls. NGF accumulation in ganglia from aged rats, however, was significantly less when compared with ganglia from young adult rats. The results of the present study suggest that NGF protein is significantly reduced in aged ganglia with the neurons retaining some capacity to take up and transport exogenous neurotrophin. Even so, the potential for NGF accumulation is dramatically reduced in aged rats when compared with that of young adult rats. While previous results have shown robust NGF-induced neurotransmitter responses by sympathetic neurons from the aged animal, the present finding of reduced accumulation of NGF in aged sympathetic neurons suggests an age-related difference in the utilization or transport of NGF.

Environmental influences on brain neurotrophins in rats

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

Nerve Growth Factor Regulates Expression of the Nerve Growth Factor Receptor Gene in Adult Sensory Neurons

Sensory neurons of the adult rat dorsal root ganglion (DRG) can be maintained in culture in the absence of nerve growth factor (NGF). We have thus used dissociated cultures of these neurons to study effects of NGF on the regulation of expression of mRNA encoding the nerve growth factor receptor (NGF-R). In the absence of NGF, levels of NGF-R mRNA remained constant for 7 days in cultures of adult rat DRG neurons. In the presence of NGF, NGF-R mRNA levels rose two - three-fold after 2 days, reaching plateau levels (five - six-fold elevation) after 5 days. This NGF-induced up-regulation could be demonstrated even after prior NGF-deprivation for 3 - 4 days. NGF had no effect upon NGF-R mRNA levels in DRG non-neuronal cells. Epidermal growth factor (EGF), fibroblast growth factor (FGF) and ciliary neurotrophic factor (CNTF) were without effect on NGF-R mRNA levels, but 8-bromo-cAMP decreased NGF-R mRNA levels by 65% after 2 days. NGF also induced a rapid (30 min) rise in expression of c-fos in DRG neurons, but not in non-neuronal cells. Our results suggest that endogenous levels of NGF may regulate the expression of NGF-R in vivo.

Neuroprotection and aging of the cholinergic system: a role for the ergoline derivative nicergoline (Sermion)

The aging brain is characterized by selective neurochemical changes involving several neural populations. A deficit in the cholinergic system of the basal forebrain is thought to contribute to the development of cognitive symptoms of dementia. Attempts to prevent age-associated cholinergic vulnerability and deterioration therefore represent a crucial point for pharmacotherapy in the elderly. In this paper we provide evidence for the protective effect of nicergoline (Sermion) on the degeneration of cholinergic neurons induced by nerve growth factor deprivation. Nerve growth factor deprivation was induced by colchicine administration in rats 13 and 18 months old. Colchicine induces a rapid and substantial down-regulation of choline acetyltransferase messenger RNA level in the basal forebrain in untreated adult, middle-aged and old rats. Colchicine failed to cause these effects in old rats treated for 120 days with nicergoline 10 mg/kg/day, orally. Moreover, a concomitant increase of both nerve growth factor and brain-derived neurotrophic factor content was measured in the basal forebrain of old, nicergoline-treated rats. Additionally, the level of messenger RNA for the brain isoform of nitric oxide synthase in neurons of the basal forebrain was also increased in these animals. Based on the present findings, nicergoline proved to be an effective drug for preventing neuronal vulnerability due to experimentally induced nerve growth factor deprivation.

Aging and neuronal plasticity: lessons from a model

In spite of many well-documented examples of age-related reductions in neuronal plasticity, the causes of such changes remain largely unknown. One example of age-reduced plasticity involves an aberrant sprouting response of mature rat sympathetic neurons into the CNS (hippocampal formation). This phenomenon has proven to be useful for exploring the relative contribution of target aging (extrinsic influences) versus neuronal aging (intrinsic influences) to reduced sprouting. Aged sympathetic neurons mount a robust growth response when confronted with young target tissue or when exposed to exogenous trophic factor in vivo. In contrast, the aged target tissue (the hippocampal formation in this example) exhibits reduced receptivity for sympathetic sprouting. This change in the target does not appear to be due to alterations in baseline levels of trophic or substrate support for axonal growth. Rather, aging appears to dampen the consequences of target denervation so that the aged target elicits less sprouting. Age-related reductions in neuronal sprouting are speculated to reflect increasing commitment to information storage at the expense of neuronal plasticity.

The effects of growth hormone and IGF-1 deficiency on cerebrovascular and brain ageing

Research studies clearly indicate that age-related changes in cellular and tissue function are linked to decreases in the anabolic hormones, growth hormone and insulin-like growth factor (IGF)-1. Although there has been extensive research on the effects of these hormones on bone and muscle mass, their effect on cerebrovascular and brain ageing has received little attention. We have also observed that in response to moderate calorie restriction (a treatment that increases mean and maximal lifespan by 30-40%), age-related decreases in growth hormone secretion are ameliorated (despite a decline in plasma levels of IGF-1) suggesting that some of the effects of calorie restriction are mediated by modifying the regulation of the growth hormone/IGF-1 axis. Recently, we have observed that microvascular density on the surface of the brain decreases with age and that these vascular changes are ameliorated by moderate calorie restriction. Analysis of cerebral blood flow paralleled the changes in vasculature in both groups. Administration of growth hormone for 28 d was also found to increase microvascular density in aged animals and further analysis indicated that the cerebral vasculature is an important paracrine source of IGF-1 for the brain. In subsequent studies, administration of GHRH (to increase endogenous release of growth hormone) or direct administration of IGF-I was shown to reverse the age-related decline in spatial working and reference memory. Similarly, antagonism of IGF-1 action in the brains of young animals impaired both learning and reference memory. Investigation of the mechanisms of action of IGF-1 suggested that this hormone regulates age-related alterations in NMDA receptor subtypes (e.g. NMDAR2A and R2B). The beneficial role of growth hormone and IGF-1 in ameliorating vascular and brain ageing are counterbalanced by their well-recognised roles in age-related pathogenesis. Although research in this area is still evolving, our results suggest that decreases in growth hormone and IGF-1 with age have both beneficial and deleterious effects. Furthermore, part of the actions of moderate calorie restriction on tissue function and lifespan may be mediated through alterations in the growth hormone/IGF-1 axis.

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.

The effects of growth hormone and IGF-1 deficiency on cerebrovascular and brain ageing

Research studies clearly indicate that age-related changes in cellular and tissue function are linked to decreases in the anabolic hormones, growth hormone and insulin-like growth factor (IGF)-1. Although there has been extensive research on the effects of these hormones on bone and muscle mass, their effect on cerebrovascular and brain ageing has received little attention. We have also observed that in response to moderate calorie restriction (a treatment that increases mean and maximal lifespan by 30-40%), age-related decreases in growth hormone secretion are ameliorated (despite a decline in plasma levels of IGF-1) suggesting that some of the effects of calorie restriction are mediated by modifying the regulation of the growth hormone/IGF-1 axis. Recently, we have observed that microvascular density on the surface of the brain decreases with age and that these vascular changes are ameliorated by moderate calorie restriction. Analysis of cerebral blood flow paralleled the changes in vasculature in both groups. Administration of growth hormone for 28 d was also found to increase microvascular density in aged animals and further analysis indicated that the cerebral vasculature is an important paracrine source of IGF-1 for the brain. In subsequent studies, administration of GHRH (to increase endogenous release of growth hormone) or direct administration of IGF-I was shown to reverse the age-related decline in spatial working and reference memory. Similarly, antagonism of IGF-1 action in the brains of young animals impaired both learning and reference memory. Investigation of the mechanisms of action of IGF-1 suggested that this hormone regulates age-related alterations in NMDA receptor subtypes (e.g. NMDAR2A and R2B). The beneficial role of growth hormone and IGF-1 in ameliorating vascular and brain ageing are counterbalanced by their well-recognised roles in age-related pathogenesis. Although research in this area is still evolving, our results suggest that decreases in growth hormone and IGF-1 with age have both beneficial and deleterious effects. Furthermore, part of the actions of moderate calorie restriction on tissue function and lifespan may be mediated through alterations in the growth hormone/IGF-1 axis.

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.

Nerve growth factor brain concentration and stress: changes depend on type of stressor and age

In the relationship between the hippocampus and the hypothalamo-pituitary-adrenocortical axis, trophic and tropic actions of nerve growth factor are involved in parallel with those on the cholinergic nuclei of the basal forebrain. Here, we report the changes produced by stress activation of the hypothalamo-pituitary-adrenocortical axis on hippocampal and basal forebrain nerve growth factor concentrations in 3-month-old male Wistar rats. The stressors used were: restraint; cold exposure; foot-shock; and rotatory platform. Restraint stress tended to reduce nerve growth factor in the hippocampus and reduced it significantly in the basal forebrain. Nerve growth factor levels in the hippocampus were not modified by cold exposure. However, a single unrepeated exposure significantly increased nerve growth factor in the basal forebrain. Both acute and chronic foot-shock reduced nerve growth factor in the hippocampus, leaving the levels in the basal forebrain unmodified. Acute but not chronic rotatory platform reduced nerve growth factor in the hippocampus, while showing a tendency, more pronounced after chronic application, toward an increase in the basal forebrain. Since with aging both activity of the hypothalamus-pituitary-adrenal axis and nerve growth factor trophic and tropic functions change, we studied the effect of restraint and cold stress in the 24-month-old male rat. The variations in nerve growth factor concentrations in the basal forebrain following stress activation are no longer present in the aged rat. The picture that emerges is indicative of a complex relationship between stress and nerve growth factor which is influenced by the kind of stressor and by age. Lack of uniformity in the effects produced by different stressors might reside in different qualitative and/or quantitative degree of involvement of neurotransmitters and/or neurohormones for each of them.

Dietary n-3 fatty acid deficiency decreases nerve growth factor content in rat hippocampus

Dietary deprivation of alpha-linolenic acid (n-3) through two generations has been shown to lower performance in an operant-type brightness-discrimination learning test in rats. Here, we examined a possible correlation between nerve growth factor (NGF) content and n-3 fatty acid status in the brain. Female rats were fed a semipurified diet supplemented with safflower oil (n-3 fatty acid-deficient) and their offsprings were fed a diet supplemented with either 3% safflower oil (Saf group) or a mixture of 2.4% safflower oil plus 0.6% ethyl eicosapentaenoate (Saf+EPA group) after weaning. The brain docosahexaenoic acid (22:6n-3, DHA) content in the Saf group was less than half of that in the Per group fed a diet supplemented with 3% perilla oil (n-3 fatty acid-sufficient) throughout the duration of the experiment. The DHA level of the Saf+EPA group was restored to the level of the Per group. However, the NGF contents in the hippocampus of the Saf and Saf+EPA groups were half that of the Per group. In the piriform cortex, the NGF content tended to be higher in the Saf and Saf+EPA groups than in the Per group. These results indicate that dietary n-3 fatty acid deficiency and restoration affect NGF levels differently among different brain regions.

Repeated immunolesions display diminished stress response signal

Cholinergic basal forebrain neurons (CBFNs) retrogradely transport neurotrophins released in the hippocampus and cortex as part of a general response to injury in a process that is impaired in the aged rodent and can be spared by the exogenous addition of pharmacological doses of nerve growth factor (NGF). This observation suggests that components of stress response signal transduction pathways in the aged CNS can be exogenously activated. The extent and mechanism of the endogenous stimulation of NGF in response to injury can be mimicked via treatment with 192 IgG-saporin of rat CNS, an immunolesion model. Here we report on the use of a conditioning lesion paradigm to determine if repeated partial immunolesions have a conditioning effect on the immunolesion-induced increases in NGF protein or decreases in choline acetyltransferase (ChAT) and acetylcholinesterase (AChE) activity. We report that chronic repeated immunolesions, as used here, were not as effective as a one time equivalent immunolesion in terms of induced NGF protein increases or decreasing ChAT and AChE activity in the hippocampus and cortex. Thus, chronic lesions resulting in cholinergic impairment typical of the aged CNS may differ from acute toxic models as a result of desensitization due to a conditioning effect of chronic subthreshold lesioning events in the CNS.