Age-dependent modulation of hippocampal long-term potentiation by antioxidant enzymes

Sex differences in neuroimmunoendocrine communication. Involvement on longevity

Endocrine, nervous, and immune systems work coordinately to maintain the global homeostasis of the organism. They show sex differences in their functions that, in turn, contribute to sex differences beyond reproductive function. Females display a better control of the energetic metabolism and improved neuroprotection and have more antioxidant defenses and a better inflammatory status than males, which is associated with a more robust immune response than that of males. These differences are present from the early stages of life, being more relevant in adulthood and influencing the aging trajectory in each sex and may contribute to the different life lifespan between sexes.

The relationship between cognitive impairment and superoxide dismutase activity in untreated first-episode patients with schizophrenia

Objectives: Cognitive decline is an essential characteristic of schizophrenia and may be due to the disturbance between reactive oxygen species generation and antioxidant capacity. The study aimed to explore the association between cognitive deficits and antioxidant defence parameters in untreated first-episode patients with schizophrenia.Methods: We determined important antioxidant enzymes, total superoxide dismutase (SOD) and manganese SOD (MnSOD), and their relationship with cognitive impairment in 168 untreated patients with first-episode schizophrenia and 168 age- and sex-matched healthy controls. The evaluation of psychopathological symptoms of all patients was based on the Positive and Negative Syndrome Scale (PANSS). We measured cognitive function by the Repeated Battery for the Assessment of Neuropsychological Status (RBANS) and activities of total SOD and MnSOD in all participants.Results: The results showed that untreated patients with first-episode schizophrenia had deficient cognitive functioning in four RBANS indices and total scores, except for the visuospatial/constructional index, as well as higher plasma total SOD activity compared with the control subjects. In addition, significant negative correlations were identified between MnSOD activity and attention index or RBANS total score in patients.Conclusions: Our results suggest that oxidative stress may be partly responsible for cognitive dysfunction in the early course of schizophrenia.

Neuronal Calcium Imaging, Excitability, and Plasticity Changes in the Aldh2-/- Mouse Model of Sporadic Alzheimer's Disease

Background:

Dysregulated signaling in neurons and astrocytes participates in pathophysiological alterations seen in the Alzheimer’s disease brain, including increases in amyloid-β, hyperphosphorylated tau, inflammation, calcium dysregulation, and oxidative stress. These are often noted prior to the development of behavioral, cognitive, and non-cognitive deficits. However, the extent to which these pathological changes function together or independently is unclear.

Objective:

Little is known about the temporal relationship between calcium dysregulation and oxidative stress, as some reports suggest that dysregulated calcium promotes increased formation of reactive oxygen species, while others support the opposite. Prior work has quantified several key outcome measures associated with oxidative stress in aldehyde dehydrogenase 2 knockout (Aldh2^–/–) mice, a non-transgenic model of sporadic Alzheimer’s disease.

Methods:

Here, we tested the hypothesis that early oxidative stress can promote calcium dysregulation across aging by measuring calcium-dependent processes using electrophysiological and imaging methods and focusing on the afterhyperpolarization (AHP), synaptic activation, somatic calcium, and long-term potentiation in the Aldh2^–/– mouse.

Results:

Our results show a significant age-related decrease in the AHP along with an increase in the slow AHP amplitude in Aldh2^–/– animals. Measures of synaptic excitability were unaltered, although significant reductions in long-term potentiation maintenance were noted in the Aldh2^–/– animals compared to wild-type.

Conclusion:

With so few changes in calcium and calcium-dependent processes in an animal model that shows significant increases in HNE adducts, Aβ, p-tau, and activated caspases across age, the current findings do not support a direct link between neuronal calcium dysregulation and uncontrolled oxidative stress.

Molecular Factors Mediating Neural Cell Plasticity Changes in Dementia Brain Diseases

Neural plasticity-the ability to alter a neuronal response to environmental stimuli-is an important factor in learning and memory. Short-term synaptic plasticity and long-term synaptic plasticity, including long-term potentiation and long-term depression, are the most-characterized models of learning and memory at the molecular and cellular level. These processes are often disrupted by neurodegeneration-induced dementias. Alzheimer's disease (AD) accounts for 50% of cases of dementia. Vascular dementia (VaD), Parkinson's disease dementia (PDD), dementia with Lewy bodies (DLB), and frontotemporal dementia (FTD) constitute much of the remaining cases. While vascular lesions are the principal cause of VaD, neurodegenerative processes have been established as etiological agents of many dementia diseases. Chief among such processes is the deposition of pathological protein aggregates in vivo including β-amyloid deposition in AD, the formation of neurofibrillary tangles in AD and FTD, and the accumulation of Lewy bodies composed of α-synuclein aggregates in DLB and PDD. The main symptoms of dementia are cognitive decline and memory and learning impairment. Nonetheless, accurate diagnoses of neurodegenerative diseases can be difficult due to overlapping clinical symptoms and the diverse locations of cortical lesions. Still, new neuroimaging and molecular biomarkers have improved clinicians' diagnostic capabilities in the context of dementia and may lead to the development of more effective treatments. Both genetic and environmental factors may lead to the aggregation of pathological proteins and altered levels of cytokines, such that can trigger the formation of proinflammatory immunological phenotypes. This cascade of pathological changes provides fertile ground for the development of neural plasticity disorders and dementias. Available pharmacotherapy and disease-modifying therapies currently in clinical trials may modulate synaptic plasticity to mitigate the effects neuropathological changes have on cognitive function, memory, and learning. In this article, we review the neural plasticity changes seen in common neurodegenerative diseases from pathophysiological and clinical points of view and highlight potential molecular targets of disease-modifying therapies.

The aging mouse brain: cognition, connectivity and calcium

Aging is a complex process that differentially impacts multiple cognitive, sensory, neuronal and molecular processes. Technological innovations now allow for parallel investigation of neuronal circuit function, structure and molecular composition in the brain of awake behaving adult mice. Thus, mice have become a critical tool to better understand how aging impacts the brain. However, a more granular systems-based approach, which considers the impact of age on key features relating to neural processing, is required. Here, we review evidence probing the impact of age on the mouse brain. We focus on a range of processes relating to neuronal function, including cognitive abilities, sensory systems, synaptic plasticity and calcium regulation. Across many systems, we find evidence for prominent age-related dysregulation even before 12 months of age, suggesting that emerging age-related alterations can manifest by late adulthood. However, we also find reports suggesting that some processes are remarkably resilient to aging. The evidence suggests that aging does not drive a parallel, linear dysregulation of all systems, but instead impacts some processes earlier, and more severely, than others. We propose that capturing the more fine-scale emerging features of age-related vulnerability and resilience may provide better opportunities for the rejuvenation of the aged brain.

Citrulline prevents age-related LTP decline in old rats

The prevalence of cognitive decline is increasing as the ageing population is considerably growing. Restricting this age-associated process has become a challenging public health issue. The age-related increase in oxidative stress plays a major role in cognitive decline, because of its harmful effect on functional plasticity of the brain, such as long-term potentiation (LTP). Here, we show that citrulline (Cit) has powerful antioxidant properties that can limit ex vivo oxidative stress-induced LTP impairment in the hippocampus. We also illustrate that a three-month Cit supplementation has a protective effect on LTP in aged rats in vivo. The identification of a Cit oxidation byproduct in vitro suggests that the antioxidant properties of Cit could result from its own oxidation. Cit supplementation may be a promising preventive nutritional approach to limit age-related cognitive decline.

TRPM2 confers susceptibility to social stress but is essential for behavioral flexibility

Transient receptor potential melastatin 2 (TRPM2) is a Ca²⁺-permeable, nonselective cation channel and a member of the TRP channel superfamily that acts as a sensor of intracellular redox states. TRPM2 is widely distributed in many tissues and highly expressed in the brain, but the physiological roles of TRPM2 in the central nervous system remain unclear. In this study, TRPM2-deficient mice were examined in a series of behavioral tests. TRPM2-deficient mice did not significantly differ from wild-type littermates in muscle strength, light/dark transition test, rotarod, elevated plus maze, social interaction, prepulse inhibition, Y-maze, forced swim test, cued and contextual fear conditioning, and tail suspension test. In the Barnes circular maze, TRPM2-deficient mice learned the fixed escape box position at similar extent to wild-type littermates, suggesting normal reference memory. However, performance of the first reversal trial and probe test were significantly impaired in TRPM2-deficient mice. In the T-maze delayed alternation task, TRPM2 deficiency significantly reduced choice accuracy. These results indicate that TRPM2-deficient mice shows behavioral inflexibility. Meanwhile, social avoidance induced by repeated social defeat stress was significantly attenuated in TRPM2-deficient mice, suggesting that TRPM2 deficiency confers stress resiliency. Our findings indicate that TRPM2 plays an essential role in maintaining behavioral flexibility but it increases susceptibility to stress.

Vitamin D, reactive oxygen species and calcium signalling in ageing and disease

Vitamin D is a hormone that maintains healthy cells. It functions by regulating the low resting levels of cell signalling components such as Ca(2+) and reactive oxygen species (ROS). Its role in maintaining phenotypic stability of these signalling pathways depends on the ability of vitamin D to control the expression of those components that act to reduce the levels of both Ca(2+) and ROS. This regulatory role of vitamin D is supported by both Klotho and Nrf2. A decline in the vitamin D/Klotho/Nrf2 regulatory network may enhance the ageing process, and this is well illustrated by the age-related decline in cognition in rats that can be reversed by administering vitamin D. A deficiency in vitamin D has also been linked to two of the major diseases in man: heart disease and Alzheimer's disease (AD). In cardiac cells, this deficiency alters the Ca(2+) transients to activate the gene transcriptional events leading to cardiac hypertrophy and the failing heart. In the case of AD, it is argued that vitamin D deficiency results in the Ca(2+) landscape that initiates amyloid formation, which then elevates the resting level of Ca(2+) to drive the memory loss that progresses to neuronal cell death and dementia.This article is part of the themed issue 'Evolution brings Ca(2+) and ATP together to control life and death'.

Vulnerability of white matter tracts and cognition to the SOD2 polymorphism: A preliminary study of antioxidant defense genes in brain aging

Oxidative stress is a key mechanism of the aging process that can cause damage to brain white matter and cognitive functions. Polymorphisms in the superoxide dismutase 2 (SOD2) and catalase (CAT) genes have been associated with abnormalities in antioxidant enzyme activity in the aging brain, suggesting a risk for enhanced oxidative damage to white matter and cognition among older individuals with these genetic variants. The present study compared differences in white matter microstructure and cognition among 96 older adults with and without genetic risk factors of SOD2 (rs4880) and CAT (rs1001179). Results revealed higher radial diffusivity in the anterior thalamic radiation among SOD2 CC genotypes compared to CT/TT genotypes. Further, the CC genotype moderated the relationship between the hippocampal cingulum and processing speed, though this did not survive multiple test correction. The CAT polymorphism was not associated with brain outcomes in this cohort. These results suggest that the CC genotype of SOD2 is an important genetic marker of suboptimal brain aging in healthy individuals.

Drebrin A regulates hippocampal LTP and hippocampus-dependent fear learning in adult mice

Structural plasticity of dendritic spines, which underlies higher brain functions including learning and memory, is dynamically regulated by the actin cytoskeleton and its associated proteins. Drebrin A is an F-actin-binding protein preferentially expressed in the brain and localized in the dendritic spines of mature neurons. Isoform conversion from drebrin E to drebrin A and accumulation of the latter in dendritic spines occurs during synapse maturation. We have previously demonstrated that drebrin A plays a pivotal role in spine morphogenesis and plasticity. However, it is unclear whether drebrin A plays a specific role in processes required for structural plasticity, and whether drebrin E can substitute in this role. To answer these questions, we analyzed mutant mice (named DAKO mice), in which isoform conversion from drebrin E to drebrin A is disrupted. In DAKO mouse brain, drebrin E continues to be expressed throughout life instead of drebrin A. Electrophysiological studies using hippocampal slices revealed that long-term potentiation of CA1 synapses was impaired in adult DAKO mice, but not in adolescents. In parallel with this age-dependent impairment, DAKO mice exhibited impaired hippocampus-dependent fear learning in an age-dependent manner; the impairment was evident in adult mice, but not in adolescents. In addition, histological investigation revealed that the spine length of the apical dendrite of CA1 pyramidal cells was significantly longer in adult DAKO mice than in wild-type mice. Our data indicate that the roles of drebrin E and drebrin A in brain function are different from each other, that the isoform conversion of drebrin is critical, and that drebrin A is indispensable for normal synaptic plasticity and hippocampus-dependent fear memory in the adult brain.

Catalpol preserves neural function and attenuates the pathology of Alzheimer's disease in mice

Alzheimer's disease (AD) is the most common form of dementia and there currently are no effective treatment strategies available. Catalpol is an iridoid glucoside, and large quantities can be isolated from the genus Rehmannia (Orobanchaceae). The present study assessed whether catalpol had any protective effects against Alzheimer's disease using a murine model. Reactive oxygen species (ROS)-associated enzymes as well as soluble Aβ40 and Aβ42 were detected using kits. Thioflavin‑S staining was performed to detect senile plaques and reverse-transcription quantitative polymerase chain reaction was used to assess iroquois homeobox protein 3 (IRX3) and obesity‑associated genes, while western blot analysis was used for β‑secretase 1 (BACE1), insulin‑degrading enzyme (IDE) and neprilysin (NEP) detection. The Morris water maze was used to detect the learning ability and spatial memory. The results revealed that catalpol was able to reduce the oxidative stress in the cerebral cortex by regulating the activities and concentration of ROS‑associated enzymes superoxide dismutase, glutathione peroxidase and catalase, however not malondialdehyde. Catalpol was also identified to be able to reduce the levels of soluble Aβ40 and Aβ42 in the cerebral cortex and thus inhibit the formation of senile plaques. These effects were observed to be regulated by IDE, however not by BACE1 or NEP. It is suggested that catalpol is not capable of directly regulating the expression of IRX3 and obesity‑associated genes. Subsequent to the treatment with catalpol, impairments in learning and memory were also observed to be relieved using the Morris water maze test. The results of the present study indicate that catalpol may be a potential drug for the treatment of neurodegenerative diseases such as AD.

Long term consequences on spatial learning-memory of low-calorie diet during adolescence in female rats; hippocampal and prefrontal cortex BDNF level, expression of NeuN and cell proliferation in dentate gyrus

Calorie restriction (CR) is argued to positively affect general health, longevity and normally occurring age-related reduction of cognition. Obesity during adolescence may adversely affect cognition in adulthood but, to date effects of CR have not been investigated. We hypothesized that feeding with as low as 15% low-calorie diet (LCD) during adolescence would increase hippocampal and prefrontal BDNF (Brain-derived neurotrophic factor) levels, proliferative cells and neuron numbers in dentate gyrus (DG), thus positively affecting spatial memory in adulthood. Spatial learning-memory function was improved in adult female Sprague-Dawley rats fed with LCD during adolescence. PCNA (Proliferating cell nuclear antigen-cell proliferation marker) expressing cells and NeuN (Neuronal nuclear antigen-neuron marker) expressing cells in hippocampus DG which are critically involved in memory were increased. Hippocampus and prefrontal cortex BDNF levels were increased while serum glucose levels and level of lipid peroxidation indicator malondialdehyde in serum and hippocampus were reduced. Our unique results suggest that improved cognition in adult rats with LCD feeding during adolescence may result from the increase of neurogenesis and BDNF. These findings reveal the importance of nutrition in adolescence for cognitive function in adulthood. Our results may be useful for further studies aiming to treat age-related cognitive impairments.

Oxidative stress and genetic markers of suboptimal antioxidant defense in the aging brain: a theoretical review

Normal aging involves a gradual breakdown of physiological processes that leads to a decline in cognitive functions and brain integrity, yet the onset and progression of decline are variable among older individuals. While many biological changes may contribute to this degree of variability, oxidative stress is a key mechanism of the aging process that can cause direct damage to cellular architecture within the brain. Oligodendrocytes are at a high risk for oxidative damage due to their role in myelin maintenance and production and limited repair mechanisms, suggesting that white matter may be particularly vulnerable to oxidative activity. Antioxidant defense enzymes within the brain, such as superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx), and glutathione-S-transferase (GST), are crucial for breaking down the harmful end products of oxidative phosphorylation. Previous studies have revealed that allele variations of polymorphisms that encode these antioxidants are associated with abnormalities in SOD, CAT, GPx, and GST activity in the central nervous system. This review will focus on the role of oxidative stress in the aging brain and the impact of decreased antioxidant defense on brain integrity and cognitive function. Directions for future research investigations of antioxidant defense genes will also be discussed.

Daily rhythms of catalase and glutathione peroxidase expression and activity are endogenously driven in the hippocampus and are modified by a vitamin A-free diet

OBJECTIVES

Alterations in enzymatic antioxidant defense systems lead to a deficit of cognitive functions and altered hippocampal synaptic plasticity. The objectives of this study were to investigate endogenous rhythms of catalase (CAT) and glutathione peroxidase (GPx) expression and activity, as well as CREB1 mRNA, in the rat hippocampus, and to evaluate to which extent the vitamin A deficiency could affect those temporal patterns.

METHODS

Rats from control and vitamin A-deficient (VAD) groups received a diet containing 4000 IU of vitamin A/kg diet, or the same diet devoid of vitamin A, respectively, during 3 months. Rats were maintained under 12-hour-dark conditions, during 10 days before the sacrifice. Circadian rhythms of CAT, GPx, RXRγ, and CREB1 mRNA levels were determined by reverse transcriptrase polymerase chain reaction in hippocampus samples isolated every 4 hours during a 24-hour period. CAT and GPx enzymatic activities were also determined by kinetic assays. Regulatory regions of clock and antioxidant enzymes genes were scanned for E-box, RXRE, and CRE sites.

RESULTS

E-box, RXRE, and CRE sites were found on regulatory regions of GPx and CAT genes, which display a circadian expression in the rat hippocampus. VAD phase shifted CAT, GPx, and RXRγ endogenous rhythms without affecting circadian expression of CREB1.

DISCUSSION

CAT and GPx expression and enzymatic activity are circadian in the rat hippocampus. The VAD affected the temporal patterns antioxidant genes expression, probably by altering circadian rhythms of its RXR receptors and clock factors; thus, it would impair the temporal orchestration of hippocampal daily cognitive performance.

Coenzyme Q(10) supplementation reverses age-related impairments in spatial learning and lowers protein oxidation

Coenzyme Q10 (CoQ) is widely available as a dietary supplement and remains under consideration as a treatment for age-associated neurodegenerative conditions. However, no studies have determined if supplementation, initiated relatively late in life, could have beneficial effects on mild functional impairments associated with normal brain aging. Accordingly, the current study assessed the effect of CoQ intake in older mice for which cognitive and psychomotor impairments were already evident. Separate groups of young (3.5 months) and relatively old mice (17.5 months) were fed a control diet or a diet supplemented with low (0.72 mg/g) or high (2.81 mg/g) concentrations of CoQ for 15 weeks. After 6 weeks, the mice were given tests for spatial learning (Morris water maze), spontaneous locomotor activity, motor coordination, and startle reflex. Age-related impairments in cognitive and psychomotor functions were evident in the 17.5-month-old mice fed the control diet, and the low-CoQ diet failed to affect any aspect of the impaired performance. However, in the Morris water maze test, old mice on the high-CoQ diet swam to the safe platform with greater efficiency than the mice on the control diet. The old mice supplemented with the high-CoQ diet did not show improvement when spatial performance was measured using probe trials and failed to show improvement in other tests of behavioral performance. Protein oxidative damage was decreased in the mitochondria from the heart, liver, and skeletal muscle of the high-CoQ-supplemented mice and, to some extent, in the brain mitochondria. Contrasting with the deleterious effect of long-term CoQ supplementation initiated during young adulthood previously published, this study suggests that CoQ improves spatial learning and attenuates oxidative damage when administered in relatively high doses and delayed until early senescence, after age-related declines have occurred. Thus, in individuals with age-associated symptoms of cognitive decline, high-CoQ intake may be beneficial.

Impairment of mitochondria in adult mouse brain overexpressing predominantly full-length, N-terminally acetylated human α-synuclein

While most forms of Parkinson's Disease (PD) are sporadic in nature, a small percentage of PD have genetic causes as first described for dominant, single base pair changes as well as duplication and triplication in the α-synuclein gene. The α-synuclein gene encodes a 140 amino acid residue protein that interacts with a variety of organelles including synaptic vesicles, lysosomes, endoplasmic reticulum/Golgi vesicles and, reported more recently, mitochondria. Here we examined the structural and functional interactions of human α-synuclein with brain mitochondria obtained from an early, pre-manifest mouse model for PD over-expressing human α-synuclein (ASOTg). The membrane potential in ASOTg brain mitochondria was decreased relative to wildtype (WT) mitochondria, while reactive oxygen species (ROS) were elevated in ASOTg brain mitochondria. No selective interaction of human α-synuclein with mitochondrial electron transport complexes cI-cV was detected. Monomeric human α-synuclein plus carboxyl terminally truncated forms were the predominant isoforms detected in ASOTg brain mitochondria by 2-dimensional PAGE (Native/SDS) and immunoblotting. Oligomers or fibrils were not detected with amyloid conformational antibodies. Mass spectrometry of human α-synuclein in both ASOTg brain mitochondria and homogenates from surgically resected human cortex demonstrated that the protein was full-length and postranslationally modified by N-terminal acetylation. Overall the study showed that accumulation of full-length, N-terminally acetylated human α-synuclein was sufficient to disrupt brain mitochondrial function in adult mice.

Synaptoneurosome micromethod for fractionation of mouse and human brain, and primary neuronal cultures

Brain and primary neuron fractions enriched in synaptic terminals are important tools for neuroscientists in biochemical, neuroanatomical and physiological studies. We describe an annotated updated micro-method for preparing synaptoneurosomes (SNs) enriched in presynaptic and postsynaptic elements. An easy to follow, step-by-step, protocol is provided for making SNs from small amounts of mammalian brain tissue. This includes novel applications for material obtained from human neurosurgical procedures and primary rat neuronal cultures. Our updated method for preparing SNs using smaller amounts of tissue provides a valuable new tool and expands the capabilities of neuroscientists.

Reversal of age-related oxidative stress prevents hippocampal synaptic plasticity deficits by protecting D-serine-dependent NMDA receptor activation

Oxidative stress (OS) resulting from an imbalance between antioxidant defenses and the intracellular accumulation of reactive oxygen species (ROS) contributes to age-related memory deficits. While impaired synaptic plasticity in neuronal networks is thought to underlie cognitive deficits during aging, whether this process is targeted by OS and what the mechanisms involved are still remain open questions. In this study, we investigated the age-related effects of the reducing agent N-acetyl-L-cysteine (L-NAC) on the activation of the N-methyl-D-aspartate receptor (NMDA-R) by its co-agonist D-serine, because alterations in this mechanism contribute greatly to synaptic plasticity deficits in aged animals. Long-term dietary supplementation with L-NAC prevented oxidative damage in the hippocampus of aged rats. Electrophysiological recordings in the CA1 of hippocampal slices indicated that NMDA-R-mediated synaptic potentials and theta-burst-induced long-term potentiation (LTP) were depressed in aged animals, deficits that could be reversed by exogenous D-serine. Chronic treatment with L-NAC, but not acute application of the reducing agent, restored potent D-serine-dependent NMDA-R activation and LTP induction in aged rats. In addition, it is also revealed that the age-related decrease in D-serine levels and in the expression of the synthesizing enzyme serine racemase, which underlies the decrease in NMDA-R activation by the amino acid, was rescued by long-term dietary treatment with L-NAC. Our results indicate that protecting redox status in aged animals could prevent injury to the cellular mechanisms underlying cognitive aging, in part by maintaining potent NMDA-R activation through the D-serine-dependent pathway.

Reactive oxygen species in the regulation of synaptic plasticity and memory

The brain is a metabolically active organ exhibiting high oxygen consumption and robust production of reactive oxygen species (ROS). The large amounts of ROS are kept in check by an elaborate network of antioxidants, which sometimes fail and lead to neuronal oxidative stress. Thus, ROS are typically categorized as neurotoxic molecules and typically exert their detrimental effects via oxidation of essential macromolecules such as enzymes and cytoskeletal proteins. Most importantly, excessive ROS are associated with decreased performance in cognitive function. However, at physiological concentrations, ROS are involved in functional changes necessary for synaptic plasticity and hence, for normal cognitive function. The fine line of role reversal of ROS from good molecules to bad molecules is far from being fully understood. This review focuses on identifying the multiple sources of ROS in the mammalian nervous system and on presenting evidence for the critical and essential role of ROS in synaptic plasticity and memory. The review also shows that the inability to restrain either age- or pathology-related increases in ROS levels leads to opposite, detrimental effects that are involved in impairments in synaptic plasticity and memory function.

Altered hippocampal synaptic physiology in aged parkin-deficient mice

We examined synaptic function in the hippocampus of aged mice deficient for the Parkinson's disease-linked protein, parkin. Surprisingly, heterozygous but not homozygous parkin-deficient mice exhibited impairments in basal excitatory synaptic strength. Similarly heterozygous mice exhibited broad deficits in paired-pulse facilitation, while homozygous parkin-deficient mice exhibited more restricted deficits. In contrast to the measurements of basal synaptic function, synaptic plasticity was not altered in aged heterozygous parkin-deficient mice, but was enhanced in aged homozygous parkin-deficient mice, due to an absence of age-related decline. These findings of differential synaptic phenotypes in heterozygous vs. homozygous parkin deficiency suggest compensatory responses to genetic abnormalities could play an important role during the development of pathology in response to parkin deficiency.

Prolonged intake of coenzyme Q10 impairs cognitive functions in mice

Coenzyme Q(10) (CoQ(10)) is widely consumed as a dietary supplement to enhance bioenergetic capacity and to ameliorate the debilitative effects of the aging process or certain pathological conditions. Our main purpose in this study was to determine whether CoQ(10) intake does indeed attenuate the age-associated losses in motor, sensory, and cognitive functions or decrease the rate of mortality in mice. Mice were fed a control nonpurified diet or that diet containing 0.68 mg/g (low dosage) or 2.6 mg/g (high dosage) CoQ(10), starting at 4 mo of age, and were tested for sensory, motor, and cognitive function at 7, 15, and 25 mo of age. Amounts of the ubiquinols CoQ(9)H(2) and CoQ(10)H(2) measured in a parallel study were augmented in the cerebral cortex but not in any other region of the brain. Intake of the low-CoQ(10) diet did not affect age-associated decrements in muscle strength, balance, coordinated running, or learning/memory, whereas intake at the higher amount increased spontaneous activity, worsened the age-related losses in acuity to auditory and shock stimuli, and impaired the spatial learning/memory of old mice. The CoQ(10) diets did not affect survivorship of mice through 25 mo of age. Our results suggest that prolonged intake of CoQ(10) in low amounts has no discernable impact on cognitive and motor functions whereas intake at higher amounts exacerbates cognitive and sensory impairments encountered in old mice. These findings do not support the notion that CoQ(10) is a fitness-enhancing or an "antiaging" substance under normal physiological conditions.

Alterations in corticostriatal synaptic plasticity in mice overexpressing human alpha-synuclein

Most forms of Parkinson's disease (PD) are sporadic in nature, but some have genetic causes as first described for the alpha-synuclein gene. The alpha-synuclein protein also accumulates as insoluble aggregates in Lewy bodies in sporadic PD as well as in most inherited forms of PD. The focus of the present study is the modulation of synaptic plasticity in the corticostriatal pathway of transgenic (Tg) mice that overexpress the human alpha-synuclein protein throughout the brain (ASOTg). Paired-pulse facilitation was detected in vitro by activation of corticostriatal afferents in ASOTg mice, consistent with a presynaptic effect of elevated human alpha-synuclein. However basal synaptic transmission was unchanged in ASOTg, suggesting that human alpha-synuclein could impact paired-pulse facilitation via a presynaptic mechanism not directly related to the probability of neurotransmitter release. Mice lacking alpha-synuclein or those expressing normal and A53T human alpha-synuclein in tyrosine hydroxylase-containing neurons showed, instead, paired-pulse depression. High-frequency stimulation induced a presynaptic form of long-term depression solely in ASOTg striatum. A presynaptic, N-methyl-d-aspartate receptor-independent form of chemical long-term potentiation induced by forskolin (FSK) was enhanced in ASOTg striatum, while FSK-induced cAMP levels were reduced in ASOTg synaptoneurosome fractions. Overall the results suggest that elevated human alpha-synuclein alters presynaptic plasticity in the corticostriatal pathway, possibly reflecting a reduction in glutamate at corticostriatal synapses by modulation of adenylyl cyclase signaling pathways. ASOTg mice may recapitulate an early stage in PD during which overexpressed alpha-synuclein dampens corticostriatal synaptic transmission and reduces movement.

Antiapoptotic and neurotrophic effects of antidepressants: a review of clinical and experimental studies

Recent studies have strengthened the role of the abnormalities in neurotrophic pathways in the pathophysiology of depression. It has been shown that the depletion of growth factors, particularly brain-derived neurotrophic factor, may result in depression-like behavior in animals and may induce cellular changes that are reminiscent of those observed in depressed patients. Some authors even suggested that increased neuronal cell loss may contribute to the pathogenesis of depression. Hence, appreciable interest has been focused on the trophic and antiapoptotic effects of antidepressant drugs. In this paper, we put emphasis on the contribution of hippocampal atrophy, increased cell death and alterations in trophic factors to the pathogenesis of depression and their relationship to the potential of antidepressants to reverse these changes by modulating trophic factor cascades and preventing apoptosis. First, evidences for increased hippocampal atrophy and cell death in depression are discussed, followed by a review of selected studies of special interest that concern antiapoptotic action of antidepressant drugs. Next, depression-related neurotrophic abnormalities and their reversal by antidepressants are depicted. Finally, relationships among neurotrophins, antiapoptotic proteins and antioxidant enzymes in the pathology and treatment of depression are pointed out.

Brief novelty exposure facilitates dentate gyrus LTP in aged rats

Aging is associated with a decreased capacity for dentate gyrus (DG) granule cell depolarization as well as reduced perforant path activation. Although it is well established that the maintenance of DG long-term potentiation (LTP) over days is impaired in aged, as compared to young animals, the threshold for inducing this LTP has never been investigated in aged, awake animals. In addition, although exposure to novelty prior to theta-burst stimulation (TBS) increases both the induction and longevity of DG LTP in adult rats, the effects of exposure to novelty on LTP in aged rats have never been investigated. Here, we report that although TBS delivered in the home cage induces robust and long-lasting DG LTP in young rats, TBS fails to induce DG LTP in aged rats. Interestingly, delivery of TBS to aged rats exploring novel environments induces robust and long-lasting LTP, with the induction, but not the longevity, of this LTP being similar in magnitude to that observed in young rats delivered TBS in the home cage. These results indicate that although TBS-induced DG LTP is impaired in aged, as compared to young rats, TBS during exploration of novel environments is sufficient to rescue age-related deficits in DG LTP. We discuss these observations in the context of previous findings suggesting that the facilitation of LTP by exposure to novel environments results as a consequence of reduced network inhibition in the DG and we suggest that, in spite of age-related changes in the DG, this capacity persists in aged rats and represents a nondietary and nonpharmacological way to facilitate DG LTP during aging.

Reverse of age-dependent memory impairment and mitochondrial DNA damage in microglia by an overexpression of human mitochondrial transcription factor a in mice

Mitochondrial DNA (mtDNA) is highly susceptible to injury induced by reactive oxygen species (ROS). During aging, mutations of mtDNA accumulate to induce dysfunction of the respiratory chain, resulting in the enhanced ROS production. Therefore, age-dependent memory impairment may result from oxidative stress derived from the respiratory chain. Mitochondrial transcription factor A (TFAM) is now known to have roles not only in the replication of mtDNA but also its maintenance. We herein report that an overexpression of TFAM in HeLa cells significantly inhibited rotenone-induced mitochondrial ROS generation and the subsequent NF-kappaB (nuclear factor-kappaB) nuclear translocation. Furthermore, TFAM transgenic (TG) mice exhibited a prominent amelioration of an age-dependent accumulation of lipid peroxidation products and a decline in the activities of complexes I and IV in the brain. In the aged TG mice, deficits of the motor learning memory, the working memory, and the hippocampal long-term potentiation (LTP) were also significantly improved. The expression level of interleukin-1beta (IL-1beta) and mtDNA damages, which were predominantly found in microglia, significantly decreased in the aged TG mice. The IL-1beta amount markedly increased in the brain of the TG mice after treatment with lipopolysaccharide (LPS), whereas its mean amount was significantly lower than that of the LPS-treated aged wild-type mice. At the same time, an increased mtDNA damage in microglia and an impaired hippocampal LTP were also observed in the LPS-treated aged TG mice. Together, an overexpression of TFAM is therefore considered to ameliorate age-dependent impairment of the brain functions through the prevention of oxidative stress and mitochondrial dysfunctions in microglia.

Adult-onset calorie restriction attenuates kainic acid excitotoxicity in the rat hippocampal slice

Lifelong calorie restriction is the only known intervention that has been shown to consistently increase life span and reduce the effects of aging on the brain. Given the difficulties of replicating lifelong calorie restriction within human populations, we have sought to assess the effects of short-term adult-onset calorie restriction upon acute excitotoxic insults in the rat hippocampus. Adult animals (approximately 6 months of age) underwent calorie restriction (alternate day feeding) for 7-10 weeks. Utilizing both electrophysiological and immunocytochemical techniques, we report that calorie restriction had no effect upon long-term potentiation (LTP), a measure of neuronal function. In control animals, application of kainic acid (20 microM) resulted in only 35% recovery of CA1 population spikes post-insult. However calorie-restricted animals showed significantly improved recovery after kainic acid treatment (64%). This data was supported by immunocytochemical studies which noted widespread loss of microtubule-associated protein (MAP 2) immunoreactivity in control slices following treatment with kainic acid; however MAP 2 staining was preserved in the CA1 and CA3 regions of calorie-restricted animals. Interestingly there was no significant difference in the recovery of population spikes between groups when slices were treated with N-methyl-d-aspartate (15 microM). We conclude that short-term adult-onset calorie restriction does not alter normal neuronal function and serves to protect the hippocampus from acute kainic acid excitotoxicity.

Calcium homeostasis and modulation of synaptic plasticity in the aged brain

The level of intracellular Ca2+ plays a central role in normal and pathological signaling within and between neurons. These processes involve a cascade of events for locally raising and lowering cytosolic Ca2+. As the mechanisms for age-related alteration in Ca2+ dysregulation have been illuminated, hypotheses concerning Ca2+ homeostasis and brain aging have been modified. The idea that senescence is due to pervasive cell loss associated with elevated resting Ca2+ has been replaced by concepts concerning changes in local Ca2+ levels associated with neural activity. This article reviews evidence for a shift in the sources of intracellular Ca2+ characterized by a diminished role for N-methyl-D-aspartate receptors and an increased role for intracellular stores and voltage-dependent Ca2+ channels. Physiological and biological models are outlined, which relate a shift in Ca2+ regulation with changes in cell excitability and synaptic plasticity, resulting in a functional lesion of the hippocampus.

Hippocampal long-term potentiation is reduced in mature compared to young male rats but not in female rats

Aging is associated with a decline in cognitive function which could be due to reduced synaptic plasticity. Hippocampal long-term potentiation (LTP) is an activity-dependent form of increased transmission efficacy at synapses that is considered the basis for some forms of learning and memory. We studied the N-methyl-d-aspartic acid (NMDA) receptor-dependent LTP in the CA1 region of hippocampus in young (2 months) and mature (8 months) male and female rats. We have found that in young male rats the tetanus increased the magnitude of excitatory post-synaptic potentials to 204+/-10% of basal while in mature male rats the magnitude of the LTP was significantly lower reaching only 153+/-11% of basal. This decrease did not occur in female rats. Similar changes occurred in the content of the NMDA receptor subunits NR1 and NR2A in hippocampus. The amount of both subunits was reduced significantly (15-16%) in hippocampus of 8-month-old compared with 2-month-old male rats. This decrease was not observed in female rats. Moreover, there is a significant correlation between the content of NR1 subunit and the magnitude of the potentiation. These data suggest that some of the neurobiological changes induced in hippocampus by aging are different in males and females.

N-truncated amyloid-beta oligomers induce learning impairment and neuronal apoptosis

N-terminal-truncated forms of amyloid-beta (A beta) peptide have been recently suggested to play a pivotal role early in Alzheimer's disease (AD). Among them, A beta 3(pE)-42 peptide, starting with pyroglutamyl at residue Glu-3, is considered as the predominant A beta species in AD plaques and pre-amyloid lesions. Its abundance is reported to be directly proportional to the severity of the clinical phenotype. The present study investigates the effects of soluble oligomeric A beta 3(pE)-42 after intracerebroventricular injection on mice learning ability and the molecular mechanisms of its in vitro neurotoxicity. Mice injected with soluble A beta 3(pE)-42 or A beta(l-42) displayed impaired spatial working memory and delayed memory acquisition in Y-maze and Morris water maze tests, while those injected with soluble A beta(42-1) showed no effect. These cognitive alterations were associated with free radical overproduction in the hippocampus and olfactory bulbs, but not in the cerebral cortex or cerebellum. In vitro, A beta 3(pE)-42 oligomers induced a redox-sensitive neuronal apoptosis involving caspase activation and an arachidonic acid-dependent pro-inflammatory pathway. These data suggest that A beta 3(pE)-42 could mediate the neurodegenerative process and subsequent cognitive alteration occurring in preclinical AD stages.