Ageing alters behavioural function and brain arginine metabolism in male Sprague-Dawley rats

Biodistribution of Agmatine to Brain and Spinal Cord after Systemic Delivery

Agmatine, an endogenous polyamine, has been shown to reduce chronic pain behaviors in animal models and in patients. This reduction is due to inhibition of the GluN2B subunit of the N-methyl-D-aspartate receptor (NMDAR) in the central nervous system (CNS). The mechanism of action requires central activity, but the extent to which agmatine crosses biologic barriers such as the blood-brain barrier (BBB) and intestinal epithelium is incompletely understood. Determination of agmatine distribution is limited by analytical protocols with low sensitivity and/or inefficient preparation. This study validated a novel bioanalytical protocol using high-performance liquid chromatography tandem mass spectrometry (HPLC-MS/MS) for quantification of agmatine in rat biologic matrices. These protocols were then used to determine the plasma pharmacokinetics of agmatine and the extent of distribution to the CNS. Precision and accuracy of the protocol met US Food and Drug Administration (FDA) standards in surrogate matrix as well as in corrected concentrations in appropriate matrices. The protocol also adequately withstood stability and dilution conditions. Upon application of this protocol to pharmacokinetic study, intravenous agmatine showed a half-life in plasma ranging between 18.9 and 14.9 minutes. Oral administration led to a prolonged plasma half-life (74.4-117 minutes), suggesting flip-flop kinetics, with bioavailability determined to be 29%-35%. Intravenous administration led to a rapid increase in agmatine concentration in brain but a delayed distribution and lower concentrations in spinal cord. However, half-life of agmatine in both tissues is substantially longer than in plasma. These data suggest that agmatine adequately crosses biologic barriers in rat and that brain and spinal cord pharmacokinetics can be functionally distinct. SIGNIFICANCE STATEMENT: Agmatine has been shown to be an effective nonopioid therapy for chronic pain, a significantly unmet medical necessity. Here, using a novel bioanalytical protocol for quantification of agmatine, we present the plasma pharmacokinetics and the first report of agmatine oral bioavailability as well as variable pharmacokinetics across different central nervous system tissues. These data provide a distributional rationale for the pharmacological effects of agmatine as well as new evidence for kinetic differences between brain and spinal cord.

Neuroprotection by agmatine: Possible involvement of the gut microbiome?

Agmatine, an endogenous polyamine derived from L-arginine, elicits tremendous multimodal neuromodulant properties. Alterations in agmatinergic signalling are closely linked to the pathogeneses of several brain disorders. Importantly, exogenous agmatine has been shown to act as a potent neuroprotectant in varied pathologies, including brain ageing and associated comorbidities. The antioxidant, anxiolytic, analgesic, antidepressant and memory-enhancing activities of agmatine may derive from its ability to regulate several cellular pathways; including cell metabolism, survival and differentiation, nitric oxide signalling, protein translation, oxidative homeostasis and neurotransmitter signalling. This review briefly discusses mammalian metabolism of agmatine and then proceeds to summarize our current understanding of neuromodulation and neuroprotection mediated by agmatine. Further, the emerging exciting bidirectional links between agmatine and the resident gut microbiome and their implications for brain pathophysiology and ageing are also discussed.

KL1 Domain of Longevity Factor Klotho Mimics the Metabolome of Cognitive Stimulation and Enhances Cognition in Young and Aging Mice

Cognitive deficits are a major biomedical challenge-and engagement of the brain in stimulating tasks improves cognition in aged individuals (Wilson et al., 2002; Gates et al., 2011) and rodents (Aidil-Carvalho et al., 2017), through unknown mechanisms. Whether cognitive stimulation alters specific metabolic pathways in the brain is unknown. Understanding which metabolic processes are involved in cognitive stimulation is important because it could lead to pharmacologic intervention that promotes biological effects of a beneficial behavior, toward the goal of effective medical treatments for cognitive deficits. Here we show using male mice that cognitive stimulation induced metabolic remodeling of the mouse hippocampus, and that pharmacologic treatment with the longevity hormone α-klotho (KL), mediated by its KL1 domain, partially mimicked this alteration. The shared, metabolic signature shared between cognitive stimulation and treatment with KL or KL1 closely correlated with individual mouse cognitive performance, indicating a link between metabolite levels and learning and memory. Importantly, the treatment of mice with KL1, an endogenous circulating factor that more closely mimicked cognitive stimulation than KL, acutely increased synaptic plasticity, a substrate of cognition. KL1 also improved cognition, itself, in young mice and countered deficits in old mice. Our data show that treatments or interventions mimicking the hippocampal metabolome of cognitive stimulation can enhance brain functions. Further, we identify the specific domain by which klotho promotes brain functions, through KL1, a metabolic mimic of cognitive stimulation.SIGNIFICANCE STATEMENT Cognitive deficits are a major biomedical challenge without truly effective pharmacologic treatments. Engaging the brain through cognitive tasks benefits cognition. Mimicking the effects of such beneficial behaviors through pharmacological treatment represents a highly valuable medical approach to treating cognitive deficits. We demonstrate that brain engagement through cognitive stimulation induces metabolic remodeling of the hippocampus that was acutely recapitulated by the longevity factor klotho, mediated by its KL1 domain. Treatment with KL1, a close mimic of cognitive stimulation, enhanced cognition and countered cognitive aging. Our findings shed light on how cognition metabolically alters the brain and provide a plausible therapeutic intervention for mimicking these alterations that, in turn, improves cognition in the young and aging brain.

Effects of sex and estrous cycle on the brain and plasma arginine metabolic profile in rats

L-arginine is a versatile amino acid with a number of bioactive metabolites. Increasing evidence implicates altered arginine metabolism in the aging and neurodegenerative processes. The present study, for the first time, determined the effects of sex and estrous cycle on the brain and blood (plasma) arginine metabolic profile in naïve rats. Female rats displayed significantly lower levels of L-arginine in the frontal cortex and three sub-regions of the hippocampus when compared to male rats. Moreover, female rats had significantly higher levels of L-arginine and γ-aminobutyric acid, but lower levels of L-ornithine, agmatine and putrescine, in plasma relative to male rats. The observed sex difference in brain L-arginine appeared to be independent of the enzymes involved in its metabolism, de novo synthesis and blood-to-brain transport (cationic acid transporter 1 protein expression at least), as well as circulating L-arginine. While the estrous cycle did not affect L-arginine and its metabolites in the brain, there were estrous cycle phase-dependent changes in plasma L-arginine. These findings demonstrate the sex difference in brain L-arginine in the estrous cycle-independent manner. Since peripheral blood has been increasingly used to identify biomarkers of brain pathology, the influences of sex and estrous cycle on blood arginine metabolic profile need attention when experimental research involves female rodents.

Untargeted GC/TOFMS unravel metabolic profiles in cerebrospinal fluid of Chinese people living with HIV

Background

Metabolic syndrome becomes a focus of clinical cares to people living with HIV (PLHIV) globally. This study aimed to explore the metabolic profiles in cerebrospinal fluid (CSF) of Chinese people living with HIV (PLHIV).

Methods

Cerebrospinal fluid samples from PLHIV and healthy controls were collected from our hospital. Then, the metabolic profiles of CSFs were analyzed PLHIV with healthy individual as the normal controls using the untargeted GC/TOFMS. Following this, kyoto encyclopedia of genes and genomes annotation and pathway analysis were performed to further explore the underlying mechanism of these metabolic alterations in cognitive impairment of PLHIV.

Results

Both PCA analysis and OPLS‐DA had presented that most samples were localized in 95% CI and the gap between control and HIV could significantly separate from each other. Upon this quality control, a total of 82 known metabolites were identified in CSF between PLHIV and healthy controls. Clustering of these metabolites presented that these differentially expressed metabolites could markedly distinguish HIV from healthy controls. Further pathway analyses showed that TCA cycle (citric acid, fumaric acid, lactate, et al.), amino acid (arginine, proline, alanine, aspartate, glutamine, et al.), lipid (cholesterol, butyrate, et al.) metabolisms were significantly changed in CSF of PLHIV, which might affect the cognitive status of PLHIV via affecting neuron energy support, signaling transduction, and neuroinflammation.

Conclusion

Metabolic profiles were significantly altered in CSF and might play key roles in the etiology of cognitive impairment of PHLIV. Further explore the exact mechanism for these metabolic changes might be useful for cognitive impairment management of PHLIV.

Altered brain arginine metabolism with age in the APPswe/PSEN1dE9 mouse model of Alzheimer's disease

Amyloid-beta (Aβ) cleaved from amyloid precursor protein (APP) has been proposed to play a central and causative role in the aetiology of Alzheimer's disease (AD). APP_swe/PSEN1_dE9 (APP/PS1) transgenic mice display chronic Aβ accumulation and deposition in the brain. L-arginine is a semi-essential amino acid with a number of bioactive metabolites, and altered arginine metabolism has been implicated in the pathogenesis and/or the development of AD. This study systematically investigated how arginine metabolic profiles changed in the frontal cortex, hippocampus, parahippocampal region and cerebellum of male APP/PS1 mice at 4, 9 and 17 months of age relative to their sex- and age-matched wildtype controls. Immunohistochemistry demonstrated age-related Aβ deposition in the brain. High-performance liquid chromatography and mass spectrometry revealed age-related increases in glutamine, spermidine and spermine in APP/PS1 mice in a region-specific manner. Notably, genotype-related increases in spermine were found in the frontal cortex at the 9-month age point and in the frontal cortex, hippocampus and parahippocampal region at 17 months of age. Given the existing literature indicating the role of polyamines (spermine in particular) in modulating the aggregation and toxicity of Aβ oligomers, increased spermidine and spermine levels in APP/PS1 mice may be a neuroprotective mechanism to combat Aβ toxicity. Future research is required to better understand the functional significance of these changes.

Metabolism of Amino Acids in the Brain and Their Roles in Regulating Food Intake

Amino acids (AAs) and their metabolites play an important role in neurological health and function. They are not only the building blocks of protein but are also neurotransmitters. In the brain, glutamate and aspartate are the major excitatory neurotransmitters, whereas γ-aminobutyrate (GABA, a metabolite of glutamate) and glycine are the major inhibitory neurotransmitters. Nitric oxide (NO, a metabolite of arginine), H₂S (a metabolite of cysteine), serotonin (a metabolite of tryptophan) and histamine (a metabolite of histidine), as well as dopamine and norepinephrine (metabolites of tyrosine) are neurotransmitters to modulate synaptic plasticity, neuronal activity, learning, motor control, motivational behavior, emotion, and executive function. Concentrations of glutamine (a precursor of glutamate and aspartate), branched-chain AAs (precursors of glutamate, glutamine and aspartate), L-serine (a precursor of glycine and D-serine), methionine and phenylalanine in plasma are capable of affecting neurotransmission through the syntheses of glutamate, aspartate, and glycine, as well as the competitive transport of tryptophan and tyrosine across from the blood-brain barrier. Adequate consumption of AAs is crucial to maintain their concentrations and the production of neurotransmitters in the central nervous system. Thus, the content and balance of AAs in diets have a profound impact on food intake by animals. Knowledge of AA transport and metabolism in the brain is beneficial for improving the health and well-being of humans and animals.

Altered neurovascular coupling and brain arginine metabolism in endothelial nitric oxide synthase deficient mice

Nitric oxide (NO) produced by endothelial NO synthase (eNOS) is a key regulator of cerebral blood flow (CBF) dynamics. Mice with eNOS deficiency (eNOS^-/-) display age-related increases in amyloid beta in the brain and memory deficits, implicating eNOS dysfunction in the neuropathogenesis and/or development of Alzheimer's disease (AD). The present study systematically investigated behavioural, CBF and brain arginine metabolic profile changes in male and female wildtype (WT) and eNOS^-/- mice at 14 months of age. eNOS^-/- mice displayed altered behaviour in the Y-maze and open field tests. A real-time microcirculation imager revealed a significant sex difference in the basal CBF and significantly increased perfusion response to whisker stimulations in the Barrel cortex in both male and female eNOS^-/- mice relative to their sex-matched WT controls. The treatment of 7-nitroindazole blocked the increased perfusion response to whisker stimulations in eNOS^-/- mice. Neurochemically, the most intriguing changes were markedly reduced glutamine levels in both male and female eNOS^-/- mice in the frontal cortex, hippocampus, parahippocampal region and cerebellum. These findings demonstrate altered behavioural function, neurovascular coupling and brain arginine metabolism (glutamine in particular) under the condition of eNOS deficiency, which further supports the role of eNOS dysfunction in the AD neuropathogenesis.

Altered brain arginine metabolism in a mouse model of tauopathy

Tauopathies consist of intracellular accumulation of hyperphosphorylated and aggregated microtubule protein tau, which remains a histopathological feature of Alzheimer's disease (AD) and frontotemporal dementia. L-Arginine is a semi-essential amino acid with a number of bioactive molecules. Its downstream metabolites putrescine, spermidine, and spermine (polyamines) are critically involved in microtubule assembly and stabilization. Recent evidence implicates altered arginine metabolism in the pathogenesis of AD. Using high-performance liquid chromatographic and mass spectrometric assays, the present study systematically determined the tissue concentrations of L-arginine and its nine downstream metabolites in the frontal cortex, hippocampus, parahippocampal region, striatum, thalamus, and cerebellum in male PS19 mice-bearing human tau P301S mutation at 4, 8, and 12-14 months of age. As compared to their wild-type littermates, PS19 mice displayed early and/or prolonged increases in L-ornithine and altered polyamine levels with age. There were also genotype- and age-related changes in L-arginine, L-citrulline, glutamine, glutamate, and γ-aminobutyric acid in a region- and/or chemical-specific manner. The results demonstrate altered brain arginine metabolism in PS19 mice with the most striking changes in L-ornithine, polyamines, and glutamate, indicating a shift of L-arginine metabolism to favor the arginase-polyamine pathway. Given the role of polyamines in maintaining microtubule stability, the functional significance of these changes remains to be explored in future research.

Age-Dependent Impairment of Neurovascular and Neurometabolic Coupling in the Hippocampus

Neurovascular and neurometabolic coupling are critical and complex processes underlying brain function. Perturbations in the regulation of these processes are, likely, early dysfunctional alterations in pathological brain aging and age-related neurodegeneration. Evidences support the role of nitric oxide (•NO) as a key messenger both in neurovascular coupling, by signaling from neurons to blood vessels, and in neurometabolic coupling, by modulating O₂ utilization by mitochondria. In the present study, we investigated the functionality of neurovascular and neurometabolic coupling in connection to •NO signaling and in association to cognitive performance during aging. For this, we performed in vivo simultaneous measurements of •NO, O₂ and cerebral blood flow (CBF) in the hippocampus of F344 rats along chronological age in response to glutamatergic activation and in correlation with cognitive performance. Firstly, it is evidenced the temporal sequence of events upon glutamate stimulation of hippocampal dentate gyrus, encompassing the local and transitory increase of •NO followed by transitory local changes of CBF and pO₂. Specifically, the transient increase of •NO is followed by an increase of CBF and biphasic changes of the local pO₂. We observed that, although the glutamate-induced •NO dynamics were not significantly affected by aging, the correspondent hemodynamic was progressively diminished accompanying a decline in learning and memory. Noteworthy, in spite of a compromised blood supply, in aged rats we observed an increased ΔpO₂ associated to the hemodynamic response, suggestive of a decrease in the global metabolic rate of O₂. Furthermore, the impairment in the neurovascular coupling observed along aging in F344 rats was mimicked in young rats by promoting an unbalance in redox status toward oxidation via intracellular generation of superoxide radical. This observation strengthens the idea that oxidative stress may have a critical role in the neurovascular uncoupling underlying brain aging and dysfunction. Overall, data supports an impairment of neurovascular response in connection with cognition decline due to oxidative environment-dependent compromised •NO signaling from neurons to vessels during aging.

Altered plasma arginine metabolome precedes behavioural and brain arginine metabolomic profile changes in the APPswe/PS1ΔE9 mouse model of Alzheimer's disease

While amyloid-beta (Aβ) peptides play a central role in the development of Alzheimer's disease (AD), recent evidence also implicates altered metabolism of L-arginine in the pathogenesis of AD. The present study systematically investigated how behavioural function and the brain and plasma arginine metabolic profiles changed in a chronic Aβ accumulation model using male APPswe/PS1ΔE9 transgenic (Tg) mice at 7 and 13 months of age. As compared to their wild-type (WT) littermates, Tg mice displayed age-related deficits in spatial water maze tasks and alterations in brain arginine metabolism. Interestingly, the plasma arginine metabolic profile was markedly altered in 7-month Tg mice prior to major behavioural impairment. Receiver operating characteristic curve analysis revealed that plasma putrescine and spermine significantly differentiated between Tg and WT mice. These results demonstrate the parallel development of altered brain arginine metabolism and behavioural deficits in Tg mice. The altered plasma arginine metabolic profile that preceded the behavioural and brain profile changes suggests that there may be merit in an arginine-centric set of ante-mortem biomarkers for AD.

Effects of maternal immune activation on brain arginine metabolism of postnatal day 2 rat offspring

l-Arginine is a versatile semi-essential amino acid with a number of bioactive metabolites, and altered arginine metabolism has been implicated in the pathogenesis of schizophrenia. Earlier research has demonstrated that maternal immune activation (MIA; a risk factor for schizophrenia) alters arginine metabolism in the prefrontal cortex and hippocampus of the adult offspring. The present study investigated how MIA affected the levels of l-arginine and its downstream metabolites in the whole forebrain, frontal cortex, hippocampus and cerebellum of male and female rat offspring at the age of postnatal day 2. While no effects were evident in the forebrain, MIA significantly increased l-arginine, glutamate, putrescine, spermidine and spermine levels and the glutamate/GABA ratio, but decreased the glutamine/glutamate ratio, in the frontal cortex, hippocampus and/or cerebellum with no marked sex differences. Cluster analyses revealed that l-arginine and its main metabolites formed distinct groups, which changed as a function of MIA or sex in all four brain regions examined. These results demonstrate, for the first time, that MIA alters brain arginine metabolism in the rat offspring during early neonatal development, and further support the involvement of arginine metabolism in the pathogenesis of schizophrenia.

Effects of Aging and Tocotrienol-Rich Fraction Supplementation on Brain Arginine Metabolism in Rats

Accumulating evidence suggests that altered arginine metabolism is involved in the aging and neurodegenerative processes. This study sought to determine the effects of age and vitamin E supplementation in the form of tocotrienol-rich fraction (TRF) on brain arginine metabolism. Male Wistar rats at ages of 3 and 21 months were supplemented with TRF orally for 3 months prior to the dissection of tissue from five brain regions. The tissue concentrations of L-arginine and its nine downstream metabolites were quantified using high-performance liquid chromatography and liquid chromatography tandem mass spectrometry. We found age-related alterations in L-arginine metabolites in the chemical- and region-specific manners. Moreover, TRF supplementation reversed age-associated changes in arginine metabolites in the entorhinal cortex and cerebellum. Multiple regression analysis revealed a number of significant neurochemical-behavioral correlations, indicating the beneficial effects of TRF supplementation on memory and motor function.

Further studies of the effects of aging on arginine metabolites in the rat vestibular nucleus and cerebellum

Some studies have demonstrated that aging is associated with impaired vestibular reflexes, especially otolithic reflexes, resulting in postural instability. However, the neurochemical basis of these age-related changes is still poorly understood. The l-arginine metabolic system has been implicated in changes in the brain associated with aging. In the current study, we examined the levels of l-arginine and its metabolizing enzymes and downstream metabolites in the vestibular nucleus complex (VNC) and cerebellum (CE) of rats with and without behavioral testing which were young (4months old), middle-aged (12months old) or aged (24months old). We found that aging was associated with lower nitric oxide synthase activity in the CE of animals with testing and increased arginase in the VNC and CE of animals with testing. l-citrulline and l-ornithine were lower in the VNC of aged animals irrespective of testing, while l-arginine and l-citrulline were lower in the CE with and without testing, respectively. In the VNC and CE, aging was associated with lower levels of glutamate in the VNC, irrespective of testing. In the VNC it was associated with higher levels of agmatine and putrescine, irrespective of testing. In the CE, aging was associated with higher levels of putrescine in animals without testing and with higher levels of spermine in animals with testing, and spermidine, irrespective of testing. Multivariate analyses indicated significant predictive relationships between the different variables, and there were correlations between some of the neurochemical variables and behavioral measurements. Cluster analyses revealed that aging altered the relationships between l-arginine and its metabolites. The results of this study demonstrate that there are major changes occurring in l-arginine metabolism in the VNC and CE as a result of age, as well as behavioral activity.

Effects of a chronic l-arginine supplementation on the arginase pathway in aged rats

While ageing is frequently associated with l-arginine deficiency, clinical and experimental studies provided controversial data on the interest of a chronic l-arginine supplementation with beneficial, no or even deleterious effects. It was hypothesized that these discrepancies might relate to a deviation of l-arginine metabolism towards production of l-ornithine rather than nitric oxide as a result of age-induced increase in arginase activity. This study investigated the effect of ageing on arginase activity/expression in target tissues and determined whether l-arginine supplementation modulated the effect of ageing on arginase activity. Arginase activity and expression were measured in the heart, vessel, brain, lung, kidney and liver in young rats (3-months old) and aged Wistar rats (22-24-months-old) with or without l-arginine supplementation (2.25% in drinking water for 6weeks). Plasma levels of l-arginine and l-ornithine were quantified in order to calculate the plasma l-arginine/l-ornithine ratio, considered as a reflection of arginase activity. Cardiovascular parameters (blood pressure, heart rate) and aortic vascular reactivity were also studied. Ageing dramatically reduced plasma l-arginine and l-arginine/l-ornithine ratio, decreased liver and kidney arginase activities but did not change activities in other tissues. l-Arginine supplementation normalized plasma l-arginine and l-arginine/l-ornithine ratio, improved endothelial function and decreased systolic blood pressure. These effects were associated with decreased arginase activity in aorta along with no change in the other tissues except in the lung in which activity was increased. A strong mismatch was therefore observed between arginase activity and expression in analyzed tissues. The present study reveals that ageing selectively changes arginase activity in clearance tissues, but does not support a role of the arginase pathway in the potential deleterious effect of the l-arginine supplementation in aged patients. Moreover, our data argue against the use of the measurement of plasma l-arginine/l-ornithine ratio to estimate arginase activity in aged patients.

Aging rather than stress strongly influences amino acid metabolisms in the brain and genital organs of female mice

Aging and stress affect quality of life, and proper nourishment is one of means of preventing this effect. Today, there is a focus on the amount of protein consumed by elderly people; however, changes in the amino acid metabolism of individuals have not been fully considered. In addition, the difference between average life span and healthy life years is larger in females than it is in males. To prolong the healthy life years of females, in the present study we evaluated the influence of stress and aging on metabolism and emotional behavior by comparing young and middle-aged female mice. After 28 consecutive days of immobilization stress, behavioral tests were conducted and tissue sampling was performed. The results showed that the body weight of middle-aged mice was severely lowered by stress, but emotional behaviors were hardly influenced by either aging or stress. Aging influenced changes in amino acid metabolism in the brain and increased various amino acid levels in the uterus and ovary. In conclusion, we found that aged mice were more susceptible to stress in terms of body-weight reduction, and that amino acid metabolisms in the brain and genital organs were largely influenced by aging rather than by stress.

Effects of withdrawal from repeated phencyclidine administration on behavioural function and brain arginine metabolism in rats

Phencyclidine (PCP) induces behavioural changes in humans and laboratory animals that resemble positive and negative symptoms, and cognitive impairments in schizophrenia. It has been shown repeated treatment of PCP leading to persistent symptoms even after the drug discontinuation, and there is a growing body of evidence implicating altered arginine metabolism in the pathogenesis of schizophrenia. The present study investigated the effects of withdrawal from repeated daily injection of PCP (2mg/kg) for 12 consecutive days on animals'behavioural performance and arginine metabolism in the hippocampus and prefrontal cortex in male young adult rats. Repeated PCP treatment reduced spontaneous alternations in the Y-maze and exploratory and locomotor activities in the open field under the condition of a washout period of 24h, but not 4days. Interestingly, the PCP treated rats also displayed spatial working memory deficits when tested 8-10days after withdrawal from PCP and showed altered levels of arginase activities and eight out of ten l-arginine metabolites in neurochemical- and region-specific manner. Cluster analyses showed altered relationships among l-arginine and its three main metabolites as a function of withdrawal from repeated PCP treatment in a duration-specific manner. Multiple regression analysis revealed significant neurochemical-behavioural correlations. Collectively, the results suggest both the residual and long-term effects of withdrawal from repeated PCP treatment on behavioural function and brain arginine metabolism. These findings demonstrate, for the first time, the influence of the withdrawal duration on animals' behaviour and brain arginine metabolism.

Altered brain arginine metabolism in schizophrenia

Previous research implicates altered metabolism of l-arginine, a versatile amino acid with a number of bioactive metabolites, in the pathogenesis of schizophrenia. The present study, for we believe the first time, systematically compared the metabolic profile of l-arginine in the frontal cortex (Brodmann's area 8) obtained post-mortem from schizophrenic individuals and age- and gender-matched non-psychiatric controls (n=20 per group). The enzyme assays revealed no change in total nitric oxide synthase (NOS) activity, but significantly increased arginase activity in the schizophrenia group. Western blot showed reduced endothelial NOS protein expression and increased arginase II protein level in the disease group. High-performance liquid chromatography and liquid chromatography/mass spectrometric assays confirmed significantly reduced levels of γ-aminobutyric acid (GABA), but increased agmatine concentration and glutamate/GABA ratio in the schizophrenia cases. Regression analysis indicated positive correlations between arginase activity and the age of disease onset and between l-ornithine level and the duration of illness. Moreover, cluster analyses revealed that l-arginine and its main metabolites l-citrulline, l-ornithine and agmatine formed distinct groups, which were altered in the schizophrenia group. The present study provides further evidence of altered brain arginine metabolism in schizophrenia, which enhances our understanding of the pathogenesis of schizophrenia and may lead to the future development of novel preventions and/or therapeutics for the disease.

Region-specific changes in presynaptic agmatine and glutamate levels in the aged rat brain

During the normal aging process, the brain undergoes a range of biochemical and structural alterations, which may contribute to deterioration of sensory and cognitive functions. Age-related deficits are associated with altered efficacy of synaptic neurotransmission. Emerging evidence indicates that levels of agmatine, a putative neurotransmitter in the mammalian brain, are altered in a region-specific manner during the aging process. The gross tissue content of agmatine in the prefrontal cortex (PFC) of aged rat brains is decreased whereas levels in the temporal cortex (TE) are increased. However, it is not known whether these changes in gross tissue levels are also mirrored by changes in agmatine levels at synapses and thus could potentially contribute to altered synaptic function with age. In the present study, agmatine levels in presynaptic terminals in the PFC and TE regions (300 terminals/region) of young (3month; n=3) and aged (24month; n=3) brains of male Sprague-Dawley rats were compared using quantitative post-embedding immunogold electron-microscopy. Presynaptic levels of agmatine were significantly increased in the TE region (60%; p<0.001) of aged rats compared to young rats, however no significant differences were detected in synaptic levels in the PFC region. Double immunogold labeling indicated that agmatine and glutamate were co-localized in the same synaptic terminals, and quantitative analyses revealed significantly reduced glutamate levels in agmatine-immunopositive synaptic terminals in both regions in aged rats compared to young animals. This study, for the first time, demonstrates differential effects of aging on agmatine and glutamate in the presynaptic terminals of PFC and TE. Future research is required to understand the functional significance of these changes and the underlying mechanisms.

Excessive levels of nitric oxide in rat model of Parkinson's disease induced by rotenone

Systemic rotenone models of Parkinson’s disease (PD) are highly reproducible and may provide evidence on the pathogenesis of PD. In the present study, male Sprague-Dawley rats (1-year-old) were subcutaneously administered with rotenone (1.5 mg/kg/day) for six days and observed for the following three weeks. Compared with the control rats, a significant decrease was observed in the body weight and a marked increase was observed in the areas under the behavioral scoring curves in the rotenone-treated rats. Immunohistochemical staining revealed that the abundance of nigral tyrosine hydroxylase (TH)-positive neurons was markedly reduced following rotenone treatment. ELISA and neurochemical assays demonstrated a significant increase in the levels of nitric oxide (NO) and NO synthase, whereas a marked decrease was observed in the thiol levels in the brains of the rotenone-treated rats. Thus, subacute rotenone treatment was found to induce behavioral deficits and the loss of nigral TH-positive neurons which may be associated with the excessive levels of NO in the rat brains.

Age-associated changes of nitric oxide concentration dynamics in the central nervous system of Fisher 344 rats

The increase in life expectancy is accompanied by an increased risk of developing neurodegenerative disorders and age is the most relevant risk factor for the appearance of cognitive decline. While decreased neuronal count has been proposed to be a major contributing factor to the appearance of age-associated cognitive decline, it appears to be insufficient to fully account for the decay in mental function in aged individuals. Nitric oxide ((•)NO) is a ubiquitous signaling molecule in the mammalian central nervous system. Closely linked to the activation of glutamatergic transmission in several structures of the brain, neuron-derived (•)NO can act as a neuromodulator in synaptic plasticity but has also been linked to neuronal toxicity and degenerative processes. Many studies have proposed that changes in the glutamate-(•)NO signaling pathway may be implicated in age-dependent cognitive decline and that the exact effect of such changes may be region specific. Due to its peculiar physical-chemical properties, namely hydrophobicity, small size, and rapid diffusion properties, the rate and pattern of (•)NO concentration changes are critical determinants for the understanding of its bioactivity in the brain. Here we show a detailed study of how (•)NO concentration dynamics change in the different regions of the brain of Fisher 344 rats (F344) during aging. Using microelectrodes inserted into the living brain of anesthetized F344 rats, we show here that glutamate-induced (•)NO concentration dynamics decrease in the hippocampus, striatum, and cerebral cortex as animals age. performance in behavior testing of short-term and spatial memory, suggesting that the impairment in the glutamate:nNOS pathway represents a functional critical event in cognitive decline during aging.

Altered arginine metabolism in Alzheimer's disease brains

L-arginine is a semi-essential amino acid with a number of bioactive metabolites. Accumulating evidence suggests the implication of altered arginine metabolism in the pathogenesis of Alzheimer's disease (AD). The present study systematically compared the metabolic profile of L-arginine in the superior frontal gyrus, hippocampus, and cerebellum from AD (mean age 80 years) and normal (mean age 80 or 60 years) cases. The activity and protein expression of nitric oxide synthase and arginase were altered with AD and age in a region-specific manner. There were also AD- and age-related changes in the tissue concentrations of L-arginine and its downstream metabolites (L-citrulline, L-ornithine, agmatine, putrescine, spermidine, spermine, glutamate, γ-aminobutyric acid, and glutamine) in a metabolite- or region-specific manner. These findings demonstrate that arginine metabolism is dramatically altered in diverse regions of AD brains, thus meriting further investigation to understand its role in the pathogenesis and/or progression of the disease.

Valsartan ameliorates ageing-induced aorta degeneration via angiotensin II type 1 receptor-mediated ERK activity

Angiotensin II (Ang II) plays important roles in ageing-related disorders through its type 1 receptor (AT1 R). However, the role and underlying mechanisms of AT1R in ageing-related vascular degeneration are not well understood. In this study, 40 ageing rats were randomly divided into two groups: ageing group which received no treatment (ageing control), and valsartan group which took valsartan (selective AT1R blocker) daily for 6 months. 20 young rats were used as adult control. The aorta structure were analysed by histological staining and electron microscopy. Bcl-2/Bax expression in aorta was analysed by immunohistochemical staining, RT-PCR and Western blotting. The expressions of AT1 R, AT2 R and mitogen-activated protein kinases (MAPKs) were detected. Significant structural degeneration of aorta in the ageing rats was observed, and the degeneration was remarkably ameliorated by long-term administration of valsartan. With ageing, the expression of AT1R was elevated, the ratio of Bcl-2/Bax was decreased and meanwhile, an important subgroup of MAPKs, extracellular signal-regulated kinase (ERK) activity was elevated. However, these changes in ageing rats could be reversed to some extent by valsartan. In vitro experiments observed consistent results as in vivo study. Furthermore, ERK inhibitor could also acquire partial effects as valsartan without affecting AT1R expression. The results indicated that AT1R involved in the ageing-related degeneration of aorta and AT1R-mediated ERK activity was an important mechanism underlying the process.

Mass spectrometric quantification of L-arginine and its pathway related substances in biofluids: the road to maturity

The amino acid L-arginine together with its metabolites and related substances is in the center of many biologically important pathways, especially the urea cycle and the nitric oxide (NO) synthesis. Therefore, the concentrations of these substances in various biological fluids are of great interest as predictive markers for health and disease. Yet, they provide major analytical difficulties as they are very polar in nature and therefore not easily to be separated on standard reversed phase HPLC stationary phases. Furthermore, as endogenous substances, no analyte-free matrix is available, a fact that results in complicated calibration procedures. This review evaluates the analytical literature for the determination of L-arginine, symmetric dimethylarginine, asymmetric dimethylarginine, monomethylarginine, L-citrulline, L-ornithine, L-homoarginine, agmatine and dimethylguanidinovaleric acid in biological fluids. Papers are discussed, which were published since 2007 and describe methods applying capillary electrophoresis (CE), gas chromatography (GC), reversed phase HPLC or polar phase HPLC, coupled to mass spectrometric quantification. Nowadays, many carefully developed and validated methods for L-arginine and its related substances are available to the scientific community. The use of stable isotope labeled internal standards enables high precision and accuracy in mass spectrometry-based quantitative analysis.

Altered arginine metabolism in the hippocampus and prefrontal cortex of maternal immune activation rat offspring

Altered arginine metabolism has been implicated in the pathogenesis of schizophrenia. The present study measured the levels of L-arginine and its downstream metabolites in the sub-regions of the hippocampus, prefrontal cortex and cerebellum in adult rats that had been exposed to maternal immune activation (MIA; a risk factor for schizophrenia). MIA significantly increased L-arginine, L-ornithine and putrescine levels and decreased agmatine levels in the hippocampus and prefrontal cortex in a region-specific manner. Correlational analysis revealed a significant neurochemical-behavioural correlation. Cluster analyses showed that L-arginine and its main metabolites formed distinct groups, which changed as a function of MIA. These results demonstrate, for the first time, that MIA leads to altered arginine metabolism in the hippocampus and prefrontal cortex of the adult offspring.

Effects of prolonged agmatine treatment in aged male Sprague-Dawley rats

Increasing evidence suggests that altered arginine metabolism contributes to cognitive decline during ageing. Agmatine, decarboxylated arginine, has a variety of pharmacological effects, including the modulation of behavioural function. A recent study demonstrated the beneficial effects of short-term agmatine treatment in aged rats. The present study investigated how intraperitoneal administration of agmatine (40mg/kg, once daily) over 4-6weeks affected behavioural function and neurochemistry in aged Sprague-Dawley rats. Aged rats treated with saline displayed significantly reduced exploratory activity in the open field, impaired spatial learning and memory in the water maze and object recognition memory relative to young rats. Prolonged agmatine treatment improved animals' performance in the reversal test of the water maze and object recognition memory test, and significantly suppressed age-related elevation in nitric oxide synthase activity in the dentate gyrus of the hippocampus and prefrontal cortex. However, this prolonged supplementation was unable to improve exploratory activity and spatial reference learning and memory in aged rats. These findings further demonstrate that exogenous agmatine selectively improves behavioural function in aged rats.