Oral aspartame and plasma phenylalanine: pharmacokinetic difference between rodents and man,and relevance to CNS effects of phenylalanine

Summary The ingestion of aspartame, a phenylalanine-containing dipeptide, raises plasma phenylalanine levels. These increments are much greater in humans than rats, because the rat hydroxylates phenylalanine five times faster than man. Accordingly, dose comparisons of aspartame (or phenylalanine) between humans and rats have usually been corrected by a factor of five. Recently, a correction factor of sixty has been proposed (Wurtman and Maher, 1987); the rationale is based on a novel calculation of competitive phenylalanine transport into brain. An analysis of the logic behind this postulation reveals there to beno basis for accepting the higher dose conversion of 60 between rat and man.     

Acute effects of aspartame on systolic blood pressure in spontaneously hypertensive rats

Summary Exogenous tyrosine lowers blood pressure in spontaneously hypertensive rats (SHR). The artificial sweetener aspartame also elevates blood and brain tyrosine levels in rats by being hydrolyzed to phenylalanine, which is then rapidly hydroxylated to tyrosine in the liver. Hence we tested the ability of aspartame; its hydrolytic products phenylalanine, aspartic acid and methanol; and of tyrosine itself to lower blood pressure in SHR. For one week prior to experimentation rats were acclimated to the indirect blood pressure measurement technique; on the day of an experiment they received I.P. injections (mg/kg) of aspartame (12.5–200), tyrosine (25–200) or phenylalanine (100–200), or of aspartic acid or methanol in the doses theoretically contained within 200 mg/kg aspartame. Animals receiving 50, 100 or 200 mg/kg of aspartame exhibited maximum falls in blood pressure of 17.3, 24.2 and 19.3 mmHg, respectively. All changes were significant, as determined by ANOVA and the Newman-Keuls test (p<0.05). Tyrosine or phenylalanine also lowered blood pressure, but aspartic acid or methanol produced no significant effects. Co-administration of aspartame with valine, a large neutral amino acid that competes with phenylalanine or tyrosine for brain uptake, attenuated aspartame's hypotensive effect. These observations suggest that the neurochemical changes produced by aspartame lead to predicted tyrosine-induced changes in blood pressure.     

Effects of serotonin depletion and dopamine depletion on bimodal divided attention

Abstract

Objectives: This study aimed to explore the effects of acute phenylalanine tyrosine depletion (APTD) and acute tryptophan depletion (ATD) on bimodal divided attention. A balanced amino acid mixture (BAL) served as control condition.

Methods: Fifty-three healthy adults (final analyzed sample was N?=?49, age: M?=?23.8 years) were randomly assigned to APTD, ATD or BAL in a double-blind, between-subject approach. Divided attention was assessed after 4?h. Blood samples were taken before and 6?h after challenge intake.

Results: Amino acid concentrations following challenge intake significantly decreased (all P?≤?0.01). There was a significant difference in the mean reaction time (RT) towards auditory stimuli, but not towards visual stimuli between the groups. Post-hoc comparison of mean RTs (auditory stimuli) showed a significant difference between ATD (RT?=?604.0?ms, SD?=?56.9?ms) and APTD (RT?=?556.4?ms, SD?=?54.2?ms; P?=?0.037), but no RT difference between ATD and BAL or APTD and BAL (RT?=?573.6?ms, SD?=?45.7?ms).

Conclusions: The results indicate a possible dissociation between the effects of a diminished brain 5-HT and DA synthesis on the performance in a bimodal divided attention task. The difference was exclusively observed within the RT towards auditory signals.     

Drinking behavior and the development of hypothalamic lesions from aspartame ingestion in water-restricted weanling mice

Summary Weanling mice that had been deprived overnight of water, or of water and diet, were offered a solution containing L-aspartyl-L-phenylalanine-methylester (APM) or monosodium L-aspartate monohydrate (MSA) as the sole source of drinking water, and/or a diet containing large amounts of APM and/or MSA for 30 min, and were killed subsequently. Only those animals exposed to concentrated (6.64% w/v) MSA solutions developed neuronal lesions in the hypothalamic arcuate nuclei. Those offered MSA in the diet only, or APM either in the diet and/or in solution, failed to develop such lesions.Apparently water-restricted weanling mice lose their ability to regulate subsequent drinking behavior, and consume hyperosmolar MSA or APM solutions whose osmolarity or sweetness would be aversive to humans. Such animals are unsuitable for the safety evaluation of APM.     

苯丙氨酸及其代谢物对P19细胞神经分化的影响

探讨苯丙氨酸及其代谢物对P19细胞分化的影响及机制。实验各组从P19细胞神经分化诱导期开始分别加入苯丙氨酸、苯乙酸和苯乳酸 ,观察神经分化期细胞形态 ,MTT法检测神经分化期细胞存活率和LDH漏出量的变化 ,并通过流式细胞仪检测诱导后细胞周期有无变化。结果显示 :苯丙氨酸及其代谢物可使P19神经元肿胀、崩解 ,神经突起纤细且突起数少 ,可降低P19细胞存活率 ,但对LDH漏出量并无影响 ,对诱导后的P19细胞周期也无影响。以上结果提示 ,苯丙氨酸及其代谢物可干扰P19细胞的神经诱导过程 ,母源性苯丙酮尿症中枢神经系统损伤可能与其在脑发育早期阶段神经诱导及分化过程中受到高苯丙氨酸及其代谢物影响有关。     

Effects of an oral glucose load on plasma neurotransmitters in humans: involvement of REM sleep?

Plasma noradrenaline (NA), adrenaline (Ad), dopamine (DA), platelet serotonin (p5HT), free serotonin (f5HT), glucose, heart rate (HR) and blood pressure (BP) were measured before and after an oral load of glucose (OGTT) in 100 normal humans. One sham-feeding test was performed in every subject 2-3 weeks before OGTT. Aside from glucose rise, significant increases in NA, p5HT and the NA/Ad ratio were registered. No significant changes were observed in Ad, DA, f5HT, HR and BP mean +/- SE values. Significant reductions in the NA/p5HT, Ad/p5HT and DA/p5HT ratios' mean values were registered at 90 and 180 min. Several significant correlations were found amongst plasma neurotransmitters. Very high positive correlations were obtained when NA, Ad and DA were plotted against the ratio of each one of them over p5HT; however, they (r = 0.99) decreased significantly at 90 and 180 min. Upon evaluation of these results we infer that quiescence of adrenal glands occurs during OGTT. Under such circumstances, plasma neurotransmitters are left under the control of a central bipolar system: noradrenergic-parasympathetic. All numerical data strongly suggest that the noradrenergic system predominates at 60, 120 and 210 min, whereas parasympathetic predominance occurs at 90 and 180 min. The fact that the latter is interfered by atropine reinforces this hypothesis. Analyses of correlations also suggest that DA and p5HT probably act as a buffer and modulate the excessive increase in NA plasma levels registered during OGTT.     

Regional transport of phenylalanine across the blood-brain barrier

In adult rats, using the single-pass brain uptake technique with a tritiated water standard, L-phenylalanine was shown to enter brain across the blood-brain barrier (BBB) by both a saturable and diffusionary process. A kinetic analysis of the data revealed Michaelis constant (K_a) of 0.42 mM and V _max of 35 nmoles g^?1 min^?1 The saturable component of L-phenylalanine transport was adversely affected by dinitrophenol or low sodium concentration in the intracarotid injection solution. Intravascular ouabain did not affect transport and only partial inhibition was observed with 1.5 mM L-leucine. It appears that there is no regional difference in the capacity of the BBB to more L-phenylalanine from blood to brain. This process is directly or indirectly coupled to sodium movement and is partially dependent on cellular energy.     

Effects of sugar and aspartame on aggression and activity in children

Habitual sugar consumption and behavior following challenge by sugar and aspartame were studied in 30 preschool boys. The 18 subjects whose parents considered them sugar reactive had more disruptive behavior problems at baseline than the other 12 subjects. Habitual sugar consumption correlated only with duration of aggression against property in alleged responders. Double-blind crossover challenges with aspartame, saccharin, sucrose, and glucose produced no significant effect on aggression or observers' ratings of behavior. Lower actometer counts followed the trials of aspartame, but the difference was not apparent to observers. It is unlikely that sugar and aspartame are clinically significant causes of disruptive behavior.     

IV glycine and oral D-cycloserine effects on plasma and CSF amino acids in healthy humans.

BACKGROUND: The amino acid glycine, modulates neurotransmission via actions at GLY-A receptor and GLY-B receptor. The latter are coagonist sites associated with N-Methyl-D-Aspartate (NMDA) glutamate receptors. The central bioavailability of peripherally administered glycine has not been adequately characterized in humans. METHODS: Healthy human subjects were administered either oral D-cycloserine (50 mg or placebo) and intravenous glycine (saline, 100 mg/kg or 200 mg/kg) in random order over 4 test days under double-blind conditions. Cerebrospinal fluid was collected by lumbar puncture performed on the first test day was analyzed to determine amino acid levels. The acoustic startle response was measured on the second test day. RESULTS: Intravenous glycine dose-dependently increased both serum and CSF glycine and serine levels. Neither glycine nor DCS produced any significant effects on behavior, cognition or the acoustic startle response. Neither IV glycine nor DCS were associated with any toxicity. CONCLUSIONS: Thus, peripheral glycine administration raised CSF glycine levels without producing any clear central nervous system effects. Glycine and D-cycloserine did not worsen cognitive test performance and did not induce behavioral symptoms on their own. The possibility that glycine and D-cycloserine enhanced cognitive test performance cannot be excluded given the psychometric limitations of the test battery.     

Effect of short- and long-term exposition to high phenylalanine blood levels on oxidative damage in phenylketonuric patients

Phenylketonuria is the most frequent disturbance of amino acid metabolism. Treatment for phenylketonuric patients consists of phenylalanine intake restriction. However, there are patients who do not adhere to treatment and/or are not submitted to neonatal screening. These individuals are more prone to develop brain damage due to long-lasting toxic effects of high levels of phenylalanine and/or its metabolites. Oxidative stress occurs in late-diagnosed phenylketonuric patients, probably contributing to the neurological damage in this disorder. In this work, we aimed to compare the influence of time exposition to high phenylalanine levels on oxidative stress parameters in phenylketonuric patients who did not adhere to protein restricted diet. We evaluated a large spectrum of oxidative stress parameters in plasma and erythrocytes from phenylketonuric patients with early and late diagnosis and of age-matched healthy controls. Erythrocyte glutathione peroxidase activity and glutathione levels, as well as plasma total antioxidant reactivity were significantly reduced in both groups of patients when compared to the control group. Furthermore, protein oxidative damage, measured by carbonyl formation and sulfhydryl oxidation, and lipid peroxidation, determined by malondialdehyde levels, were significantly increased only in patients exposed for a long time to high phenylalanine concentrations, compared to early diagnosed patients and controls. In conclusion, exposition to high phenylalanine concentrations for a short or long time results in a reduction of non-enzymatic and enzymatic antioxidant defenses, whereas protein and lipid oxidative damage only occurs in patients with late diagnosis.     

Effect of dietary phenylalanine on the plasma concentrations of phenylalanine, phenylethylamine and phenylacetic acid in healthy volunteers

1. Phenylethylamine has been proposed as a neuromodulator in several psychiatric and other brain disorders, and its concentration and that of its major metabolite, phenylacetic acid, in plasma may prove useful as state or trait markers in diagnosis, treatment or in the elucidation of biochemical mechanisms of these disorders. 2. The effect of dietary phenylalanine intake and changes in dietary phenylalanine intake on the plasma concentrations and changes in plasma concentrations, respectively, of phenylalanine, phenylethylamine and unconjugated and conjugated phenylacetic acid have been investigated. 3. Dietary phenylalanine affects the concentration of plasma phenylalanine on the following day, but has no effect on phenylethylamine or phenylacetic acid concentrations. Thus single measurements per subject of phenylethylamine or phenylacetic acid do not need to take dietary factors into account. 4. Changes in dietary phenylalanine (whether in absolute amount or in the proportion of phenylalanine in the diet) are significantly correlated with changes in unconjugated phenylacetic acid. Therefore, in longitudinal studies, dietary factors should be taken into account.     

Plasma tyrosine in normal humans: Effects of oral tyrosine and protein-containing meals

Summary To test the effects of tyrosine ingestion and concurrent food consumption on plasma tyrosine levels and on the plasma tyrosine ratio, we measured plasma neutral amino acid levels in 11 subjects who consumed a diet containing 113 g protein and who also took 100 mg/kg/day of L-tyrosine (in three equally divided doses) before meals. Plasma tyrosine levels rose significantly (p<0.025) during the day when subjects consumed the diet alone; they increased markedly after tyrosine ingestion (p<0.005). Tyrosine administration did not affect plasma concentrations of the other neutral amino acids that compete with tyrosine for entry into the brain. Thus, the plasma tyrosine ratio increased from 0.13 to 0.21 (p<0.001) on the day fed subjects received the tyrosine. These observations indicate that tyrosine administration might increase brain tyrosine levels and perhaps accelerate catecholamine synthesis in humans with diseases in which catecholamine synthesis or release is deficient.     

Recovery from depression after electroconvulsive therapy is accompanied by evidence of increased tetrahydrobiopterin-dependent hydroxylation

Serum phenylalanine and tyrosine levels were measured in 26 patients with severe depression before and after receiving electroconvulsive therapy. The phenylalanine:tyrosine [P:T] ratio declined significantly for those responding to treatment but not for nonresponders. These findings are discussed in relation to tetrahydrobiopterin, the essential cofactor for the formation of noradrenaline, dopamine and serotonin and the hydroxylation of phenylalanine to tyrosine.     

Pre-treatment effects of peripheral tumors on brain and behavior: Neuroinflammatory mechanisms in humans and rodents

Cancer patients suffer high levels of affective and cognitive disturbances, which have been attributed to diagnosis-related distress, impairment of quality of life, and side effects of primary treatment. An inflammatory microenvironment is also a feature of the vast majority of solid tumors. However, the ability of tumor-associated biological processes to affect the central nervous system (CNS) has only recently been explored in the context of symptoms of depression and cognitive disturbances. In this review, we summarize the burgeoning evidence from rodent cancer models that solid tumors alter neurobiological pathways and subsequent behavioral processes with relevance to affective and cognitive disturbances reported in human cancer populations. We consider, in parallel, the evidence from human clinical cancer research demonstrating that affective and cognitive disturbances are common in some malignancies prior to diagnosis and treatment. We further consider the underlying neurobiological pathways, including altered neuroinflammation, tryptophan metabolism, prostaglandin synthesis and associated neuroanatomical changes, that are most strongly implicated in the rodent literature and supported by analogous evidence from human cancer populations. We focus on the implications of these findings for behavioral researchers and clinicians, with particular emphasis on methodological issues and areas of future research.     

Suppressant effects of dexamethasone on the availability of plasma L-tryptophan and tyrosine in healthy controls and in depressed patients

Formation in the brain of serotonin from L-tryptophan (L-TRP) and noradrenaline from tyrosine are pathways related to the pathophysiology of major depression and to the regulation of the hypothalamic-pituitary-adrenal (HPA) axis. In the past, decrements in L-TRP availability and disorders in the HPA axis have repeatedly been observed in major depressed patients; both factors were shown to be inversely correlated. In order to investigate the relationships between glucocorticosteroid activity and the availability of L-TRP and tyrosine, the authors measured L-TRP, tyrosine, valine, leucine, isoleucine and phenylalanine in baseline conditions and after treatment with 1 mg dexamethasone in 16 healthy controls and in 50 depressed patients. The ratios between L-TRP and tyrosine and the sums of the amino acids known to compete with them during transport across the blood-brain barrier were computed as an index of (respectively) the serotonin and noradrenaline synthesis in the brain. We found significantly decreased plasma L-TRP and tyrosine levels after treatment with dexamethasone compared with basal levels. Accordingly, the plasma ratios between L-TRP and tyrosine and the sum of the competing amino acids were significantly reduced by dexamethasone administration. It was hypothesized that through these actions of dexamethasone on peripheral amino acids, the central noradrenaline and serotonin control over the HPA-axis could be altered.     

Brain large neutral amino acids and catecholamines in parenterally nourished preterm rabbits

Total parenteral nutrition (TPN) has been adapted as a standard for providing nutrition to ill term and preterm infants. The availability of tyrosine in amino acid preparations utilized for TPN is limited and may potentiate a tyrosine-deficient state. Phenlyalanine hydroxylase activity, responsible for catalyzing tyrosine synthesis, has been suggested to be decreased in fetal and neonatal animals. Parenterally nourished premature rabbits (n=16) and suckled rabbits (n=19) were studied in order to compare growth parameters and amino acids in the plasma and brain, as well as whole brain catecholamine concentrations. Influx velocities into the brain of amino acids were also determined in these two groups.The preterm rabbit's average birth weight (42.6±6.0) was less than that of term rabbits (56.7±8.7, P<0.005). Significantly lower concentrations of the catecholamine precursor tyrosine were found in both the plasma and brain of the parenterally nourished animals compared to the suckled animals. Tyrosine is reduced in the brain in TPN-supported animals reflecting both low tyrosine intake and increased plasma concentrations of large neutral amino acids that compete for uptake at the blood-brain barrier. However, no difference was observed between the two groups in their brain catecholamine concentrations.The seven-day parenterally nourished rabbit appears to be tyrosine-deficient but no evident effects on brain catecholamine concentrations were seen. The effects and impact of a tyrosine-deficient state might better be evaluated by regional evaluation of catecholaminergic areas of the brain or over a longer period of parenteral nutrition.     

Effect of aspartame on N-methyl-d-aspartate-sensitive l-[H]glutamate binding sites in rat brain synaptic membranes

Aspartame (l-aspartyl-l-phenylalanine methyl ester), an artificial low-calorie sweetener, was shown to dose-dependently inhibitl-[³H]glutamate binding to its N-methyl-d-aspartate-specific receptors.l-Aspartic acid, a major endogenous metabolite of aspartame, inhibited the binding more stronger than aspartame, while the other metabolites,l-phenylalanine and methanol, had no effect at the same concentration. Aspartame caused a significant change in the affinities ofl-[³H]glutamate binding without altering the V_max values of the binding, suggesting the inhibition is competitive. These in vitro findings suggested that aspartame may act directly on the N-methyl-d-aspartate-sensitive glutamate recognition sites in the brain synaptic membranes.     

Comparative anatomy of the locus coeruleus in humans and nonhuman primates

The locus coeruleus (LC) is a dense cluster of neurons that projects axons throughout the neuroaxis and is located in the rostral pontine tegmentum extending from the level of the inferior colliculus to the motor nucleus of the trigeminal nerve. LC neurons are lost in the course of several neurodegenerative disorders, including Alzheimer's and Parkinson's diseases. In this study we used Nissl staining and tyrosine hydroxylase (TH) immunoreactivity to compare the human LC with that of closely related primate species, including great and lesser apes, and macaque monkeys. TH catalyzes the initial and rate‐limiting step in catecholamine biosynthesis. The number of TH‐immunoreactive (TH‐ir) neurons was estimated in each species using stereologic methods. In the LC of humans the mean total number of TH‐ir neurons was significantly higher compared to the other primates. Because the total number of TH‐ir neurons in the LC was highly correlated with the species mean volume of the medulla oblongata, cerebellum, and neocortical gray matter, we conclude that much of the observed phylogenetic variation can be explained by anatomical scaling. Notably, the total number of LC neurons in humans was most closely predicted by the nonhuman allometric scaling relationship relative to medulla size, whereas the number of LC neurons in humans was considerably lower than predicted according to neocortex and cerebellum volume. J. Comp. Neurol. 518:963–971, 2010. © 2009 Wiley‐Liss, Inc.     

Early patterns of electrical activity in the developing cerebral cortex of humans and rodents

During prenatal and early postnatal development, the cerebral cortex exhibits synchronized oscillatory network activity that is believed to be essential for the generation of neuronal cortical circuits. The nature and functional role of these early activity patterns are of central interest in neuroscience. Much of the research is performed in rodents and in vitro, but how closely do these model systems relate to the human fetal brain? In this review, we compare observations in humans with in vivo and in vitro rodent data, focusing on particular oscillatory activity patterns that share many common features: delta brushes, spindle bursts and spindle-like oscillations. There is considerable evidence that the basic functional properties of immature cortical networks are conserved through mammalian evolution, making the neonatal rodent an excellent model for studying early cortical activity and associated plasticity during the developmental period corresponding to the human fetal stage. This review is part of the INMED/TINS special issue "Nature and nurture in brain development and neurological disorders", based on presentations at the annual INMED/TINS symposium (http://inmednet.com/).