Brain Amino Acid Concentrations Research Articles

The increasing importance of LNAA supplementation in phenylketonuria at higher plasma phenylalanine concentrations

BackgroundLarge neutral amino acid (LNAA) treatment has been suggested as alternative to the burdensome severe phenylalanine-restricted diet. While its working mechanisms and optimal composition have recently been further elucidated, the question whether LNAA treatment requires the natural protein-restricted diet, has still remained. ObjectiveFirstly, to determine whether an additional liberalized natural protein-restricted diet could further improve brain amino acid and monoamine concentrations in phenylketonuria mice on LNAA treatment. Secondly, to compare the effect between LNAA treatment (without natural protein) restriction and different levels of a phenylalanine-restricted diet (without LNAA treatment) on brain amino acid and monoamine concentrations in phenylketonuria mice. DesignBTBR Pah-enu2 mice were divided into two experimental groups that received LNAA treatment with either an unrestricted or semi phenylalanine-restricted diet. Control groups included Pah-enu2 mice on the AIN-93 M diet, a severe or semi phenylalanine-restricted diet without LNAA treatment, and wild-type mice receiving the AIN-93 M diet. After ten weeks, brain and plasma samples were collected to measure amino acid profiles and brain monoaminergic neurotransmitter concentrations. ResultsAdding a semi phenylalanine-restricted diet to LNAA treatment resulted in lower plasma phenylalanine but comparable brain amino acid and monoamine concentrations as compared to LNAA treatment (without phenylalanine restriction). LNAA treatment (without phenylalanine restriction) resulted in comparable brain monoamine but higher brain phenylalanine concentrations compared to the severe phenylalanine-restricted diet, and significantly higher brain monoamine but comparable phenylalanine concentrations as compared to the semi phenylalanine-restricted diet. ConclusionsPresent results in PKU mice suggest that LNAA treatment in PKU patients does not need the phenylalanine-restricted diet. In PKU mice, LNAA treatment (without phenylalanine restriction) was comparable to a severe phenylalanine-restricted diet with respect to brain monoamine concentrations, notwithstanding the higher plasma and brain phenylalanine concentrations, and resulted in comparable brain phenylalanine concentrations as on a semi phenylalanine-restricted diet.

Multiplexed analysis of amino acids in mice brain microdialysis samples using isobaric labeling and liquid chromatography-high resolution tandem mass spectrometry

We developed a new multiplexed reversed phase liquid chromatography-high resolution tandem mass spectrometric (LC-MS/MS) method. The method is based on isobaric labeling with a tandem mass tag (TMT10-plex) and stable isotope-labeled internal standards, and was used to analyze amino acids in mouse brain microdialysis samples. The TMT10-plex labeling of amino acids allowed analysis of ten samples in one LC-MS/MS run, significantly increasing the sample throughput. The method provides good chromatographic performance (peak half-width between 0.04–0.12 min), allowing separation of all TMT-labeled amino acids with acceptable resolution and high sensitivity (limits of detection typically around 10 nM). The use of stable isotope-labeled internal standards, together with TMT10-plex labeling, ensured good repeatability (relative standard deviation ≤ 12.1 %) and linearity (correlation coefficient > 0.994), indicating good quantitative performance of the multiplexed method. The method was applied to study the effect of d-amphetamine microdialysis perfusion on amino acid concentrations in the mouse brain. All amino acids were reliably detected and quantified, indicating that the method is sensitive enough to detect low concentrations of amino acids in brain microdialysis samples.

Intracerebroventricular injection of L-arginine and D-arginine induces different effects under an acute stressful condition

Central administration of L-arginine was reported to attenuate stress responses in neonatal chicks. The present study aimed to elucidate the differential effects of centrally administered L-arginine and its enantiomer, D-arginine, on the stress response in chicks and the associated mechanisms. Intracerebroventricular injection of L-arginine attenuated acute isolation stress by inducing sleep-like behavior, while central administration of D-arginine potentiated the stress response, reducing the time spent standing motionless with eyes open and increasing distress vocalizations compared to the control. The brain concentrations of amino acids and monoamines following L- and D-arginine administration during stress were also determined. L-Arginine significantly increased the mesencephalic L-glutamine concentration. D-Arginine administration did not affect the levels of L-arginine or other amino acids in the examined brain regions. 3,4-Dihydroxyphenylacetic acid (DOPAC) level and dopamine (DA) metabolic rate (DOPAC/DA) were significantly higher in the diencephalon in the D-arginine group compared to the L-arginine group, while the mesencephalic DA level was significantly lower in the D-arginine group compared to the control. In vitro experiment using the brain slice culture demonstrated that extracellular perfusion of D-arginine significantly elevated the mRNA expression level of monoamine oxidase B, the major enzyme involved in DA metabolism, in the locus coeruleus region of the brainstem. In conclusion, in neonatal chicks, central administration of D-arginine exerted a stimulant effect on the stress response, in contrast to the stress-attenuating effects of L-arginine, partly through an effect on brain dopaminergic metabolism and not through competition with the L-stereoisomer.

Maternal Immune Activation Alters Placental and Fetal Brain Amino Acid Transport in Rats

ProblemMaternal immune activation (MIA) during pregnancy is an environmental risk factor for neurodevelopmental disorders such as autism and schizophrenia. However, the exact mechanism through which this occurs is unknown. Due to the important roles that amino acids play in neurodevelopment, altered transport of amino acids across the placenta and throughout the developing fetal brain could play a role in neurodevelopmental disorders. Our objectives were to determine whether virally mediated MIA alters amino acid transporter expression in the placenta and fetal brain, and to explore the underlying mechanism(s) associated with these changes.MethodsPregnant rats received poly(I:C) on gestational day 14 and placental and fetal tissues were collected 6, 24, and 48 hours later. Amino acid transporter expression in the placenta and fetal brain was measured using qPCR and immunoblotting. Free amino acid concentrations in the fetal brain were quantified using HPLC. To explore a possible mechanism, the expression and phosphorylation of components of the placental mTOR and AMPK signaling pathways were investigated using immunoblotting.ResultsPoly(I:C) induced the mRNA expression of multiple amino acid transporters in the placenta and fetal brain over all three timepoints. Conversely, poly(I:C) significantly decreased protein expression of ASCT1 and EAAT2 in the placenta, and SNAT5, EAAT1, and GLYT1 in the fetal brain. Changes in transporter expression were accompanied by functional changes in the concentrations of numerous amino acids in the fetal brain. We observed activation of AMPK and inhibition of mTOR signaling in the placentas of poly(I:C)‐treated dams.ConclusionsMIA induced by poly(I:C) significantly alters placental and fetal brain amino acid transporter expression as well as amino acid concentrations in the fetal brain. Due to the importance of amino acids in neurodevelopment, changes in amino acid transporters could contribute to the development of a neurodevelopmental disorder. AMPK and mTOR have been previously shown to regulate amino acid transporter expression. Since poly(I:C) altered the activation of placental and AMPK and mTOR, these could provide novel treatment targets to alleviate MIA‐induced changes in amino acid transport and investigate the implications of these changes in neurodevelopment.Support or Funding InformationThis work was supported by an operating grant from the Canadian Institutes of Health Research (CIHR) awarded to Dr. Piquette‐Miller [MOP 13346]. Eliza McColl was a recipient of a CIHR Canadian Graduate Scholarship (Master’s) and an Ontario Graduate Scholarship, and is currently funded by a CIHR Banting and Best Canadian Graduate Scholarship (Doctoral) [GSD‐164238].

Poly(I:C) alters placental and fetal brain amino acid transport in a rat model of maternal immune activation.

Maternal immune activation (MIA) during pregnancy is associated with increased chances of neurodevelopmental disorders including schizophrenia and autism spectrum disorder (ASD). However, the exact mechanism through which MIA contributes to altered neurodevelopment is unknown. Due to the important role that amino acids play in neurodevelopment, alteredamino acid transport could play a role in neurodevelopmental disorders. Indeed, altered plasma concentrations of multiple amino acids have been reported in individuals with ASD or schizophrenia. Therefore,our objective was to determine whether virally mediated MIA induces changes in amino acid transporters in the placenta and fetal brain. Pregnant rats were administered poly(I:C) on gestational day 14, and placental and fetal tissues were collected 6, 24, and 48hours later. Amino acid transporter expression was measured in the placenta and fetal brain using qPCR, Western blotting, and Simple Western. Free amino acid concentrations in the fetal brain were quantified using HPLC. Poly(I:C) increased mRNA expression of several amino acid transporters in the placenta and fetal brain over these timepoints. Conversely, poly(I:C) imposed significant decreases in the protein expression of ASCT1 and EAAT2 in placenta and expression of SNAT5, EAAT1, and GLYT1 in fetal brain. Functional consequences of altered transporter expression were demonstrated through widespread changes in the concentrations of free amino acids in the fetal brains. Together, these results represent novel findings with the poly(I:C) MIA model and contribute to the understanding of how MIA during pregnancy potentially leads to neurodevelopmental disorders.

L-Leucine acts as a potential agent in reducing body temperature at hatching and affords thermotolerance in broiler chicks

Thermal manipulation (TM) of incubation temperature causes metabolic alterations and contributes to improving thermotolerance in chicks post hatching. However, there has been no report on amino acid metabolism during TM and the part it plays in thermotolerance. In this study, we therefore first analyzed free amino acid concentrations in the embryonic brain and liver during TM (38.6°C, 6h/d during embryonic day (ED) 10 to ED 18). It was found that leucine (Leu), phenylalanine and lysine were significantly decreased in the embryonic brain and liver. We then chose l-Leu and other branched-chain amino acids (l-isoleucine (L-Ile) and l-valine (l-Val)) for in ovo injection on ED 7 to reveal their roles in thermoregulation, growth, food intake and thermotolerance in chicks. It was found that in ovo injection of l-Leu, but not of l-Ileu or l-Val, caused a significant decline in body temperature at hatching and increased food intake and body weight gain in broiler chicks. Interestingly, in ovo injection of l-Leu resulted in the acquisition of thermotolerance under high ambient temperature (35±1°C for 180min) in comparison with the control thermoneutral temperature (28±1°C for 180min). These results indicate that the free amino acid concentrations during embryogenesis were altered by TM. l-Leu administration in eggs caused a reduction in body temperature at hatching, and afforded thermotolerance in heat-exposed young chicks, further suggesting that l-Leu may be one of the key metabolic factors involved in controlling body temperature in embryos, as well as in producing thermotolerance after hatching.

L-Ornithine is a potential acute satiety signal in the brain of neonatal chicks

Recently, we observed that neonatal chicks exhibit feeding behavior characterized by frequent food intake and short resting intervals, with changes detected in the brain amino acid and monoamine concentrations. In this study, we aimed to clarify further the relationship between the appetite of neonatal chicks and brain amino acid metabolism. In Experiment 1, changes were investigated in free amino acids in the brain under conditions of regulated appetite induced by fasting and subsequent short-term re-feeding. Chicks (5days old) were distributed into four treatment groups — namely, fasting for 3h, and fasting for 3h followed by re-feeding for 10, 20 or 30min. Brain samples were collected after treatment to analyze free amino acid concentrations. Amino adipic acid and proline in all brain parts as well as arginine and ornithine in all brain parts – except mesencephalic arginine and cerebellar ornithine – were increased in a time-dependent manner following re-feeding. In Experiment 2, we further examined the effect of exogenous administration of some amino acids altered in association with feeding behavior in Experiment 1. We chose l-arginine and its functional metabolite, l-ornithine, to analyze their effects on food intake in chicks. Intracerebroventricular injection (2μmol) of l-ornithine, but not l-arginine, significantly inhibited food intake in neonatal chicks. In Experiment 3, we found that central injection of l-ornithine (2, 4, and 6μmol) dose-dependently suppressed food intake in chicks. These results suggested that l-ornithine may have an important role in the control of food intake as an acute satiety signal in the neonatal chick brain.

Large Neutral Amino Acid Supplementation Exerts Its Effect through Three Synergistic Mechanisms: Proof of Principle in Phenylketonuria Mice.

BackgroundPhenylketonuria (PKU) was the first disorder in which severe neurocognitive dysfunction could be prevented by dietary treatment. However, despite this effect, neuropsychological outcome in PKU still remains suboptimal and the phenylalanine-restricted diet is very demanding. To improve neuropsychological outcome and relieve the dietary restrictions for PKU patients, supplementation of large neutral amino acids (LNAA) is suggested as alternative treatment strategy that might correct all brain biochemical disturbances caused by high blood phenylalanine, and thereby improve neurocognitive functioning.ObjectiveAs a proof-of-principle, this study aimed to investigate all hypothesized biochemical treatment objectives of LNAA supplementation (normalizing brain phenylalanine, non-phenylalanine LNAA, and monoaminergic neurotransmitter concentrations) in PKU mice.MethodsC57Bl/6 Pah-enu2 (PKU) mice and wild-type mice received a LNAA supplemented diet, an isonitrogenic/isocaloric high-protein control diet, or normal chow. After six weeks of dietary treatment, blood and brain amino acid and monoaminergic neurotransmitter concentrations were assessed.ResultsIn PKU mice, the investigated LNAA supplementation regimen significantly reduced blood and brain phenylalanine concentrations by 33% and 26%, respectively, compared to normal chow (p<0.01), while alleviating brain deficiencies of some but not all supplemented LNAA. Moreover, LNAA supplementation in PKU mice significantly increased brain serotonin and norepinephrine concentrations from 35% to 71% and from 57% to 86% of wild-type concentrations (p<0.01), respectively, but not brain dopamine concentrations (p = 0.307).ConclusionsThis study shows that LNAA supplementation without dietary phenylalanine restriction in PKU mice improves brain biochemistry through all three hypothesized biochemical mechanisms. Thereby, these data provide proof-of-concept for LNAA supplementation as a valuable alternative dietary treatment strategy in PKU. Based on these results, LNAA treatment should be further optimized for clinical application with regard to the composition and dose of the LNAA supplement, taking into account all three working mechanisms of LNAA treatment.

Changes in free amino acid and monoamine concentrations in the chick brain associated with feeding behavior.

Domesticated chicks are precocial and therefore have relatively well-developed feeding behavior. The role of hypothalamic neuropeptides in food-intake regulation in chicks has been reported for decades. However, we hypothesized that nutrients and their metabolites in the brain may be involved in food intake in chicks because these animals exhibit a very frequent feeding pattern. Therefore, the purpose of this study was to examine the feeding behavior of chicks as well as the associated changes in free amino acid and monoamine concentrations in the chick brain. The feeding behavior of chicks was recorded continuously for 6 h. The next day, brain and blood samples were collected when the chicks either attempted to have food (hungry group) or turned food down (satiated group), in order to analyze the concentrations of the free amino acids and monoamines. We confirmed that the feeding behavior of neonatal chicks was characterized by short resting periods between very brief times spent on food intake. Several free amino acids in the mesencephalon were significantly lower in the satiated group than in the hungry group, while l-histidine and l-glutamine were significantly higher. Notably, there was no change in the free amino acid concentrations in other brain regions or plasma. As for monoamines, serotonin and norepinephrine were significantly lower in the mesencephalon of the hungry group compared with the satiated group, but 5 hydroxyindolacetic acid (5-HIAA) was higher. In addition, serotonin and norepinephrine levels were significantly higher in the brain stem of the hungry chicks compared with the satiated group, but levels of 5-HIAA and homovanillic acid were lower. Levels of both dopamine and its metabolite, 3,4-dihydroxyphenylacetic acid, were significantly higher in the diencephalon and telencephalon of the chicks in the hungry group. In conclusion, the changes in the free amino acids and monoamines in the brain may have some role in the feeding behavior of neonatal chicks.

Paracetamol impairs the profile of amino acids in the rat brain

In our experiment we investigated the effect of subcutaneous administration of paracetamol on the levels of amino acids in the brain structures. Male Wistar rats received for eight weeks paracetamol at two doses: 10mg/kg b.w. (group P10, n=9) and 50mg/kg b.w. per day s.c. (group P50, n=9).The regional brain concentrations of amino acids were determined in the prefrontal cortex, hippocampus, hypothalamus and striatum of control (Con, n=9) and paracetamol-treated groups using HPLC. Evaluation of the biochemical results indicated considerable decrease of the content of amino acids in the striatum (glutamine, glutamic acid, taurine, alanine, aspartic acid) and hypothalamus (glycine) between groups treated with paracetamol compared to the control. In the prefrontal cortex paracetamol increased the level of γ-aminobutyric acid (GABA). The present study demonstrated significant effect of the long term paracetamol treatment on the level of amino acids in the striatum, prefrontal cortex and hypothalamus of rats.

The impact of chronic imipramine treatment on amino acid concentrations in the hippocampus of mice

The relationship between antidepressants and monoamine concentrations in the brain has been well investigated, but few studies have investigated the relationship between antidepressants and amino acid concentrations in the brain. The purpose of the present study was therefore to investigate the effect of the chronic antidepressant imipramine on amino acid and monoamine concentrations in the mouse brain and plasma. Chronic imipramine treatment decreased the concentration of 5-hydroxyindoleaceticacid/5-hydroxytryptamine in the cerebral cortex and increased that of norepinephrine (NE) in the hippocampus. Since these changes were conspicuous effects of the antidepressant, we concluded that imipramine acts on the central nervous system. No change in amino acid concentrations in plasma was induced by chronic imipramine treatment, but several changes were confirmed in the cerebral cortex, the hypothalamus and the hippocampus. Chronic imipramine treatment caused increases in l-methionine, l-tyrosine, and l-lysine in the cerebral cortex, and an increase in l-aspartate in the hypothalamus. Contrary to this, the concentrations of l-aspartate, l-serine, l-asparagine, glycine, l-glutamine, gamma-aminobutyric acid, l-threonine, l-arginine, l-proline, l-valine, and l-methionine in the hippocampus were decreased by chronic imipramine treatment. The present results demonstrate that the metabolism of several amino acids in the brain, but not of those in plasma, was altered by chronic imipramine treatment. The findings in the present study may help to further elucidate the relationship between amino acids and the effects and side effects of antidepressants.

Oral administration of Excitin-1 (β-alanyl-L-leucine) alters behavior and brain monoamine and amino acid concentrations in rats

We previously demonstrated that β-alanyl-branched chain amino acids have excitatory effects. Therefore, we named β-alanyl-L-leucine, β-alanyl-L-isoleucine and β-alanyl-L-valine as Excitin-1, -2, and -3 , respectively. Since there is little known about the effects of Excitins, we clarified whether oral administration of Excitin-1 affects behavior in rats, alters the monoamine and amino acid levels in the central nervous system, whether Excitin-1 is incorporated into the brain, and how long it remains in the blood. Excitin-1 increased motor behavior, increasing the distance of path and number of rearings in the open field. Excitin-1 influenced some monoamine and amino acid levels in the cerebral cortex and hypothalamus. Following oral administration, Excitin-1 was detected in the cerebral cortex, hypothalamus, hippocampus and olfactory bulb. In the plasma, Excitin-1 and its metabolites β-alanine and L-leucine were recorded. The present study demonstrated that Excitin-1 was incorporated in the brain and promoted behavioral changes in rats.

Forced swimming and imipramine modify plasma and brain amino acid concentrations in mice

The relationships between monoamine metabolism and forced swimming or antidepressants have been well studied, however information is lacking regarding amino acid metabolism under these conditions. Therefore, the aim of the present study was to investigate the effects of forced swimming and imipramine on amino acid concentrations in plasma, the cerebral cortex and the hypothalamus in mice. Forced swimming caused cerebral cortex concentrations of l-glutamine, l-alanine, and taurine to be increased, while imipramine treatment caused decreased concentrations of l-glutamate, l-alanine, l-tyrosine, l-methionine, and l-ornithine. In the hypothalamus, forced swimming decreased the concentration of l-serine while imipramine treatment caused increased concentration of β-alanine. Forced swimming caused increased plasma concentration of taurine, while concentrations of l-serine, l-asparagine, l-glutamine and β-alanine were decreased. Imipramine treatment caused increased plasma concentration of all amino acid, except for l-aspartate and taurine. In conclusion, forced swimming and imipramine treatment modify central and peripheral amino acid metabolism. These results may aid in the identification of amino acids that have antidepressant-like effects, or may help to refine the dosages of antidepressant drugs.

Nicotine increases microdialysate brain amino acid concentrations and induces conditioned place preference

The action of nicotine on the nicotinic receptor-mediated release of inhibitory and excitatory acids in the nucleus accumbens, NAC, of freely moving rats was studied in order to clarify their effects' on reinforcing behavior as estimated by conditioned place preference (CPP). Using the technique of microdialysis, intraperitoneal (i.p.) injections of nicotine (0.15-0.3-0.6 mg/kg), significantly increased aspartate, glutamate, arginine, taurine, and alanine microdialysate content in the nucleus accumbens. The same doses of nicotine were able to elicit a reinforcing effect in a CPP paradigm which was probably associated with the increased brain levels of excitatory acids triggering additional dopamine release in the mesolimbic system.

Modulation of absence seizures by branched-chain amino acids: correlation with brain amino acid concentrations.

The occurrence of absence seizures might be due to a disturbance of the balance between excitatory and inhibitory neurotransmissions in the thalamo-cortical loop. In this study, we explored the consequences of buffering the glutamate content of brain cells on the occurrence and duration of seizures in Genetic Absence Epilepsy Rats from Strasbourg (GAERS), a genetic model of generalized non-convulsive epilepsy. Branched-chain amino acids (BCAAs) and alpha-ketoisocaproate (alpha-KIC), the ketoacid of leucine were repeatedly shown to have a critical role in brain glutamate metabolism. Thus, GAERS were injected by intraperitoneal (i.p.) or intracerebroventricular (i.c.v.) route with these compounds, then the effects on seizures were evaluated on the electroencephalographic recording. We also measured the concentration of amino acids in thalamus and cortex after an i.p. injection of leucine or alpha-KIC. Intracerebroventricular injections of leucine or alpha-KIC did not influence the occurrence of seizures, possibly because the substances reached only the cortex. BCAAs and alpha-KIC, injected intraperitoneally, increased the number of seizures whereas they had only a slight effect on their duration. Leucine and alpha-KIC decreased the concentration of glutamate in thalamus and cortex without affecting GABA concentrations. Thus, BCAAs and alpha-KIC, by decreasing the effects of glutamatergic neurotransmission could facilitate those of GABAergic neurotransmission, which is known to increase the occurrence of seizures in GAERS.

Reduction of large neutral amino acid levels in plasma and brain of hyperleucinemic rats

Neurological dysfunction is common in patients with maple syrup urine disease (MSUD). However, the mechanisms underlying the neuropathology of this disorder are poorly known. In the present study we investigated the effect of acute hyperleucinemia on plasma and brain concentrations of amino acids. Fifteen-day-old rats were injected subcutaneously with 6 μmol l-leucine per gram body weight. Controls received saline in the same volumes. The animals were sacrificed 30–120 min after injection, blood was collected and their brain rapidly removed and homogenized. The amino acid concentrations were determined by HPLC using orthophtaldialdehyde for derivatization and fluorescence for detection. The results showed significant reductions of the large neutral amino acids (LNAA) l-phenylalanine, l-tyrosine, l-isoleucine, l-valine and l-methionine, as well as l-alanine, l-serine and l-histidine in plasma and of l-phenylalanine, l-isoleucine, l-valine and l-methionine in brain, as compared to controls. In vitro experiments using brain slices to study the influence of leucine on amino acid transport and protein synthesis were also carried out. l-Leucine strongly inhibited [ 14C]- l-phenylalanine transport into brain, as well as the incorporation of the [ 14C]-amino acid mixture, [ 14C]- l-phenylalanine and [ 14C]- l-lysine into the brain proteins. Although additional studies are necessary to evaluate the importance of these effects for MSUD, considering previous findings of reduced levels of LNAA in plasma and CSF of MSUD patients during crises, it may be speculated that a decrease of essential amino acids in brain may lead to reduction of protein and neurotransmiter synthesis in this disorder.

Uremic Anorexia: A Consequence of Persistently High Brain Serotonin Levels? The Tryptophan/Serotonin Disorder Hypothesis

Anorexia is a frequent part of uremic syndrome, contributing to malnutrition in dialysis patients. Many factors have been suggested as responsible for uremic anorexia. In this paper we formulate a new hypothesis to explain the appetite disorders in dialysis patients: "the tryptophan/serotonin disorder hypothesis." We review current knowledge of normal hunger-satiety cycle control and the disorders described in uremic patients. There are four phases in food intake regulation: (1) the gastric phase, during which food induces satiety through gastric distention and satiety peptide release; (2) the post absorptive phase, during which circulating compounds, including glucose and amino acids, cause satiety by hepatic receptors via the vagus nerve; (3) the hepatic phase, during which adenosine triphosphate (ATP) concentration is the main stimulus inducing hunger or satiety, with cytokines inhibiting ATP production; and (4) the central phase, during which appetite is regulated through peripheral (circulating plasma substances and neurotransmitters) and brain stimuli. Brain serotonin is the final target for peripheral mechanisms controlling appetite. High brain serotonin levels and a lower serotonin/dopamine ratio cause anorexia. Plasma and brain amino acid concentrations are recognized factors involved in neurotransmitter synthesis and appetite control. Tryptophan is the substrate of serotonin synthesis. High plasma levels of anorectics such as tryptophan (plasma and brain), cholecystokinin, tumor necrosis factor alpha, interleukin-1, and leptin, and deficiencies of nitric oxide and neuropeptide Y have been described in uremia; all increase intracerebral serotonin. We suggest that brain serotonin hyperproduction due to a uremic-dependent excess of tryptophan may be the final common pathway involved in the genesis of uremic anorexia. Various methods of ameliorating anorexia by decreasing the central effects of serotonin are proposed.

Consequences of renal mass reduction on amino acid and biogenic amine levels in nephrectomized mice.

Amino acid and biogenic amine changes were investigated in nephrectomized mice ten days postsurgery. Uremic mice exhibited changes in amino acid concentrations in plasma, urine and brain. Particularly plasma methionine, citrulline and arginine levels were significantly enhanced in nephrectomized mice compared to controls whereas serine was decreased. Urinary excretion of methionine, citrulline and alanine was higher in nephrectomized mice compared to controls whereas many amino acids were increased in brain of nephrectomized mice. Brain and urinary amino acid changes were more pronounced in the 75% than in the 50% nephrectomized mice. Brain norepinephrine and dopamine and its metabolites 3,4-dihydroxyphenylacetic acid and homovanillic acid were significantly increased whereas serotonin was decreased comparing the 75% nephrectomized mice to the sham-operated mice. This study demonstrates that at very early stages of renal insufficiency, specific amino acid and biogenic amine changes occur in plasma, urine and brain. These alterations might depend qualitatively and quantitatively on the degree of functional renal mass reduction.

Small Changes in Essential Amino Acid Concentrations Alter Diet Selection in Amino Acid–Deficient Rats

The rat's sensitivity to changes in the dietary limiting amino acid concentration (LAA) was examined on the basis of dietary selection. Rats were adapted to purified low protein basal (Basal) diets in which threonine (Thr) was the LAA (0.188–0.212% wt/wt of diet). In Experiment 1, rats made a clear selection for their adaptation diet over a diet containing 0.012% less threonine after 2–3 d of choice. Rats made no clear dietary selection when given a choice between their adaptation diet and a diet containing 0.012% more threonine. Experiment 2 was conducted to examine the rat's sensitivity to small decreases in the LAA concentration. Rats adapted to a 0.200% Thr-Basal diet clearly responded to decreases as small as 0.009% in the concentration of threonine and selected against the more deficient diet when given a choice between it and the 0.200% Thr-Basal adaptation diet. Because plasma and brain amino acid concentrations are important for detection of other amino acid deficiencies, these variables were measured to determine whether they were affected by such small changes in dietary amino acid concentration. In Experiments 3 and 4, rats were adapted to the 0.200% Thr-Basal diet and then fed 0.188, 0.200 or 0.212% Thr-Basal diets for 6 h, or 0.188 and 0.212% Thr-Basal for 54 h. Amino acid concentrations in plasma, prepiriform cortex and anterior cingulate cortex were not significantly different among treatments. Norepinephrine concentration in the prepiriform cortex was not affected by dietary treatment. We conclude that small decreases in LAA concentration can cause selection against the more deficient diet, but that detection of such deficiencies does not require significant changes in plasma and brain amino acid concentrations.

1,25-dihydroxyvitamin D3 regulates γ-glutamyl transpeptidase activity in rat brain

γ-Glutamyl transpeptidase (γ-GT), primarily described as a kidney enzyme, is also expressed in several cell types of the central nervous system (CNS). It is involved in the glutathione cycle and in cysteine transport. Here we report that the specific activity of this enzyme is transiently increases in the rat brain, following a treatment with 1,25-dihydroxyvitamin D 3 (1,25-D 3 ), the active form of vitamin D. In vitro experiments showed that this positive regulatory effect does not affect endothelial cells of the brain microvessels, but does affect pericytes and parenchymal astrocytes. Changes in the specific activity of γ-GT were not correlated with any important modification of brain amino acid concentrations. Since γ-GT is thought to participate in the scavenging of reactive oxygen species, these data suggest that 1,25-D 3 could be an effector controlling detoxification processes in the brain.