Acute effects of aspartame on large neutral amino acids and monoamines in rat brain

Learning and memory deficits produced by aspartame are heritable via the paternal lineage

Environmental exposures produce heritable traits that can linger in the population for one or two generations. Millions of individuals consume substances such as artificial sweeteners daily that are declared safe by regulatory agencies without evaluation of their potential heritable effects. We show that consumption of aspartame, an FDA-approved artificial sweetener, daily for up to 16-weeks at doses equivalent to only 7-15% of the FDA recommended maximum daily intake value (equivalent to 2-4 small, 8 oz diet soda drinks per day) produces significant spatial learning and memory deficits in mice. Moreover, the cognitive deficits are transmitted to male and female descendants along the paternal lineage suggesting that aspartame's adverse cognitive effects are heritable, and that they are more pervasive than current estimates, which consider effects in the directly exposed individuals only. Traditionally, deleterious environmental exposures of pregnant and nursing women are viewed as risk factors for the health of future generations. Environmental exposures of men are not considered to pose similar risks. Our findings suggest that environmental exposures of men can produce adverse impact on cognitive function in future generations and demonstrate the need for considering heritable effects via the paternal lineage as part of the regulatory evaluations of artificial sweeteners.

Neurometabolic effects of sweetened solution intake during adolescence related to depressive-like phenotype in rats

OBJECTIVE

Exposure to artificial sweeteners, such as aspartame, during childhood and adolescence has been increasing in recent years. However, the safe use of aspartame has been questioned owing to its potentially harmful effects on the developing brain. The aim of this study was to test whether the chronic consumption of aspartame during adolescence leads to a depressive-like phenotype and to investigate the possible mechanisms underlying these behavioral changes.

METHODS

Adolescent male and female rats were given unlimited access to either water, solutions of aspartame, or sucrose in their home cages from postnatal day 21 to 55.

RESULTS

Forced swim test revealed that both chronic aspartame and sucrose intake induced depressive-like behaviord, which was more pronounced in males. Additionally, repeated aspartame intake was associated with increased cerebrospinal fluid (CSF) aspartate levels, decreased hippocampal neurogenesis, and reduced activation of the hippocampal leptin signaling pathways in males. In females, we observed a main effect of aspartame: reducing PI3K/AKT one of the brain-derived neurotrophic factor pathways; aspartame also increased CSF aspartate levels and decreased the immunocontent of the GluN2A subunit of the N-methyl-d-aspartic acid receptor.

CONCLUSION

The findings revealed that repeated aspartame intake during adolescence is associated with a depressive-like phenotype and changes in brain plasticity. Interestingly, males appear to be more vulnerable to the adverse neurometabolic effects of aspartame than females, demonstrating a sexually dimorphic response. The present results highlighted the importance of understanding the effects caused by the constant use of this artificial sweetener in sensitive periods of development and contribute to regulation of its safe use.

Astrogliosis and decreased neural viability as consequences of early consumption of aspartame and acesulfame potassium in male Wistar rats

Artificial sweeteners are mainly used as substitutes for sucrose derivates. In this study, we analyzed if the chronic consumption of aspartame or acesulfame potassium at an early age, produces histological alterations, astrogliosis and decreased neuronal viability, in hippocampus, prefrontal cortex, amygdala and hypothalamus of male Wistar rats. A histological analysis was performed on male Wistar rats that consumed aspartame or acesulfame potassium during 90 days, initiating the consumption of sweeteners immediately after weaning. The evaluation of neuronal morphology in different areas of the brain was performed with hematoxylin - eosin staining. To measure astrogliosis and neuronal viability, we used the immunohistochemical technique, with the glial fibrillary acidic protein immunomodulators (GFAP) and with neuronal-specific enolase (NSE). The consumption of aspartame or acesulfame potassium promoted morphological changes of neurons including increased pyknotic nuclei and vacuolization in all the brain areas studied. In hippocampus, prefrontal cortex, amygdala and hypothalamus, astrogliosis and reduction of neural viability were observed in sweeteners consumers in comparison with the control group. Chronic consumption of ASP and ACK from early stages of development and during long periods, may promote neural modifications, astrogliosis and decrease neuronal viability in prefrontal cortex, amygdala, hippocampus, and hypothalamus.

Alterations in behaviour, cerebral cortical morphology and cerebral oxidative stress markers following aspartame ingestion

OBJECTIVE

The study evaluated changes in open field behaviours, cerebral cortical histomorphology and biochemical markers of oxidative stress following repeated administration of aspartame in mice.

METHODOLOGY

Adult mice were assigned into five groups of twelve each. Vehicle (distilled water), or aspartame (20, 40, 80 and 160mg/kg body weight) were administered orally for 28days. Horizontal locomotion, rearing and grooming were assessed after the first and last dose of aspartame. Sections of the cerebral cortex were processed and stained for general histology, and also examined for neuritic plaques using the Bielschwosky's protocol. Glial fibrillary acidic protein (GFAP) and neuron specific enolase (NSE) immunoreactivity were assessed using appropriate antibodies. Aspartate and antioxidant levels were also assayed from cerebral cortex homogenates. Data obtained were analysed using descriptive and inferential statistics.

RESULTS

Body weight and food consumption decreased significantly with aspartame consumption. Locomotion, rearing and grooming increased significantly after first dose, and with repeated administration of aspartame. Histological changes consistent with neuronal damage were seen at 40, 80 and 160mg/kg. Neuritic plaque formation was not evident; while GFAP-reactive astrocytes and NSE-reactive neurons increased at 40 and 80mg/kg but decreased at 160mg/kg. Superoxide dismutase and nitric oxide increased with increasing doses of aspartame, while aspartate levels showed no significant difference.

CONCLUSION

The study showed morphological alterations consistent with neuronal injury and biochemical changes of oxidative stress. These data therefore supports the need for caution in the indiscriminate use of aspartame as a non-nutritive sweetener.

Acute effect of aspartame-induced oxidative stress in Wistar albino rat brain

The present study was carried out to investigate the acute effect of aspartame on oxidative stress in the Wistar albino rat brain. We sought to investigate whether acute administration of aspartame (75 mg/kg) could release methanol and induce oxidative stress in the rat brain 24 hours after administration. To mimic human methanol metabolism, methotrexate treated rats were used to study aspartame effects. Wistar strain male albino rats were administered with aspartame orally as a single dose and studied along with controls and methotrexate treated controls. Blood methanol and formate level were estimated after 24 hours and rats were sacrificed and free radical changes were observed in discrete regions by assessing the scavenging enzymes, reduce dglutathione (GSH), lipid peroxidation and protein thiol levels. There was a significant increase in lipid peroxidation levels, superoxide dismutase activity (SOD), glutathione peroxidase levels (GPx), and catalase activity (CAT) with a significant decrease in GSH and protein thiol. Aspartame exposure resulted in detectable methanol even after 24 hours. Methanol and its metabolites may be responsible for the generation of oxidative stress in brain regions. The observed alteration in aspartame fed animals may be due to its metabolite methanol and elevated formate. The elevated free radicals due to methanol induced oxidative stress.

Prooxidative effects of aspartame on antioxidant defense status in erythrocytes of rats

Since aspartame (L-aspartyl-L-phenylalanine methyl ester, ASP) is one of the most widely used artificial sweeteners, the aim of the present study was to investigate its effects on serum glucose and lipid levels as well as its effects on oxidative/antioxidative status in erythrocytes of rats. The experiment included two groups of animals: the control group was administered with water only, while the experimental group was orally administered with ASP (40 mg/kg b.w.) daily, for a period of six weeks. When compared with the control group, the group administrated with ASP indicated higher values of serum glucose, cholesterol and triglycerides. Significantly increased concentrations of superoxide anion (O2 .-), hydrogen peroxide (H2O2), peroxynitrite (?N??-) and lipid peroxides (LPO) were recorded in the erythrocytes of ASP treated group in comparison to the control group. In the course of chronic ASP administration, the following was observed: the concentration of reduced glutathione (GSH) and the activity of catalase (CAT) increased. Thus, these findings suggest that long-term consumption of ASP leads to hyperglycemia and hyperlipidemia, as well as to oxidative stress in erythrocytes.

The protective effect of N-acetylcysteine on oxidative stress in the brain caused by the long-term intake of aspartame by rats

Long-term intake of aspartame at the acceptable daily dose causes oxidative stress in rodent brain mainly due to the dysregulation of glutathione (GSH) homeostasis. N-Acetylcysteine provides the cysteine that is required for the production of GSH, being effective in treating disorders associated with oxidative stress. We investigated the effects of N-acetylcysteine treatment (150 mg kg(-1), i.p.) on oxidative stress biomarkers in rat brain after chronic aspartame administration by gavage (40 mg kg(-1)). N-Acetylcysteine led to a reduction in the thiobarbituric acid reactive substances, lipid hydroperoxides, and carbonyl protein levels, which were increased due to aspartame administration. N-Acetylcysteine also resulted in an elevation of superoxide dismutase, glutathione peroxidase, glutathione reductase activities, as well as non-protein thiols, and total reactive antioxidant potential levels, which were decreased after aspartame exposure. However, N-acetylcysteine was unable to reduce serum glucose levels, which were increased as a result of aspartame administration. Furthermore, catalase and glutathione S-transferase, whose activities were reduced due to aspartame treatment, remained decreased even after N-acetylcysteine exposure. In conclusion, N-acetylcysteine treatment may exert a protective effect against the oxidative damage in the brain, which was caused by the long-term consumption of the acceptable daily dose of aspartame by rats.

Chronic Effect of Aspartame on Ionic Homeostasis and Monoamine Neurotransmitters in the Rat Brain

Aspartame is one of the most widely used artificial sweeteners globally. Data concerning acute neurotoxicity of aspartame is controversial, and knowledge on its chronic effect is limited. In the current study, we investigated the chronic effects of aspartame on ionic homeostasis and regional monoamine neurotransmitter concentrations in the brain. Our results showed that aspartame at high dose caused a disturbance in ionic homeostasis and induced apoptosis in the brain. We also investigated the effects of aspartame on brain regional monoamine synthesis, and the results revealed that there was a significant decrease of dopamine in corpus striatum and cerebral cortex and of serotonin in corpus striatum. Moreover, aspartame treatment significantly alters the tyrosine hydroxylase activity and amino acids levels in the brain. Our data suggest that chronic use of aspartame may affect electrolyte homeostasis and monoamine neurotransmitter synthesis dose dependently, and this might have a possible effect on cognitive functions.

Aspartame: a safety evaluation based on current use levels, regulations, and toxicological and epidemiological studies

Aspartame is a methyl ester of a dipeptide used as a synthetic nonnutritive sweetener in over 90 countries worldwide in over 6000 products. The purpose of this investigation was to review the scientific literature on the absorption and metabolism, the current consumption levels worldwide, the toxicology, and recent epidemiological studies on aspartame. Current use levels of aspartame, even by high users in special subgroups, remains well below the U.S. Food and Drug Administration and European Food Safety Authority established acceptable daily intake levels of 50 and 40 mg/kg bw/day, respectively. Consumption of large doses of aspartame in a single bolus dose will have an effect on some biochemical parameters, including plasma amino acid levels and brain neurotransmitter levels. The rise in plasma levels of phenylalanine and aspartic acid following administration of aspartame at doses less than or equal to 50 mg/kg bw do not exceed those observed postprandially. Acute, subacute and chronic toxicity studies with aspartame, and its decomposition products, conducted in mice, rats, hamsters and dogs have consistently found no adverse effect of aspartame with doses up to at least 4000 mg/kg bw/day. Critical review of all carcinogenicity studies conducted on aspartame found no credible evidence that aspartame is carcinogenic. The data from the extensive investigations into the possibility of neurotoxic effects of aspartame, in general, do not support the hypothesis that aspartame in the human diet will affect nervous system function, learning or behavior. Epidemiological studies on aspartame include several case-control studies and one well-conducted prospective epidemiological study with a large cohort, in which the consumption of aspartame was measured. The studies provide no evidence to support an association between aspartame and cancer in any tissue. The weight of existing evidence is that aspartame is safe at current levels of consumption as a nonnutritive sweetener.

Direct and indirect cellular effects of aspartame on the brain

The use of the artificial sweetener, aspartame, has long been contemplated and studied by various researchers, and people are concerned about its negative effects. Aspartame is composed of phenylalanine (50%), aspartic acid (40%) and methanol (10%). Phenylalanine plays an important role in neurotransmitter regulation, whereas aspartic acid is also thought to play a role as an excitatory neurotransmitter in the central nervous system. Glutamate, asparagines and glutamine are formed from their precursor, aspartic acid. Methanol, which forms 10% of the broken down product, is converted in the body to formate, which can either be excreted or can give rise to formaldehyde, diketopiperazine (a carcinogen) and a number of other highly toxic derivatives. Previously, it has been reported that consumption of aspartame could cause neurological and behavioural disturbances in sensitive individuals. Headaches, insomnia and seizures are also some of the neurological effects that have been encountered, and these may be accredited to changes in regional brain concentrations of catecholamines, which include norepinephrine, epinephrine and dopamine. The aim of this study was to discuss the direct and indirect cellular effects of aspartame on the brain, and we propose that excessive aspartame ingestion might be involved in the pathogenesis of certain mental disorders (DSM-IV-TR 2000) and also in compromised learning and emotional functioning.

Sustained improvement of motor function in hemiparkinsonian rats chronically treated with low doses of caffeine or trihexyphenidyl

The effects of chronic oral treatment with low doses of caffeine (1-3 mg/kg) and trihexyphenidyl (0.1-0.2 mg/kg) were tested on hemiparkinsonian rats, which received the following treatments in a counterbalanced order: vehicle, caffeine, trihexyphenidyl, and caffeine plus trihexyphenidyl. Three preclinical models were used: the stepping test, the cylinder test, and the staircase test. Compared to pre-lesion values, the forepaw contralateral to the dopamine-denervated side showed impaired stepping, fewer wall contacts in the cylinder test, and fewer pellets retrieved in the staircase test. In the stepping test both doses of caffeine produced a complete recovery of motor function (100%), whereas the effect of trihexyphenidyl was less intense (77-80%). In this same test the maximal effect of drugs did not develop tolerance during 2-3 weeks, and was completely reversible after drug cessation. In the cylinder test only the wall contacts performed simultaneously with both forepaws were significantly increased by caffeine (3 mg/kg) and trihexyphenidyl (0.2 mg/kg), and this effect was also reversible. In the staircase test none of the treatments improved food pellet retrieval with the contralateral forepaw. Altogether, these results show that chronic treatment with caffeine, at doses similar to daily human consumption, produces a sustained improvement in the use of the contralateral forelimb in unilaterally 6-hydroxydopamine denervated rats, without the development of tolerance. Although the combined administration of caffeine plus trihexyphenidyl showed no synergism in these models, the results suggest that low doses of caffeine (1-3 mg/kg/day) could be of therapeutic value for the reversal of motor symptoms in parkinsonian patients.

Aspartame: review of safety

Over 20 years have elapsed since aspartame was approved by regulatory agencies as a sweetener and flavor enhancer. The safety of aspartame and its metabolic constituents was established through extensive toxicology studies in laboratory animals, using much greater doses than people could possibly consume. Its safety was further confirmed through studies in several human subpopulations, including healthy infants, children, adolescents, and adults; obese individuals; diabetics; lactating women; and individuals heterozygous (PKUH) for the genetic disease phenylketonuria (PKU) who have a decreased ability to metabolize the essential amino acid, phenylalanine. Several scientific issues continued to be raised after approval, largely as a concern for theoretical toxicity from its metabolic components--the amino acids, aspartate and phenylalanine, and methanol--even though dietary exposure to these components is much greater than from aspartame. Nonetheless, additional research, including evaluations of possible associations between aspartame and headaches, seizures, behavior, cognition, and mood as well as allergic-type reactions and use by potentially sensitive subpopulations, has continued after approval. These findings are reviewed here. The safety testing of aspartame has gone well beyond that required to evaluate the safety of a food additive. When all the research on aspartame, including evaluations in both the premarketing and postmarketing periods, is examined as a whole, it is clear that aspartame is safe, and there are no unresolved questions regarding its safety under conditions of intended use.

Effects of long-term ingestion of aspartame on hypothalamic neuropeptide Y, plasma leptin and body weight gain and composition

The aim of this study was to determine the effects of the chronic ingestion of aspartame (ASP) on brain neuropeptide Y (NPY) concentrations, plasma hormones, food intake and body fat. Two groups of male Long-Evans rats, fed on a control (C) well-balanced diet, had to drink either a 0.1% ASP solution or water for a period of 14 weeks starting at weaning. Food intake and body weight were weekly recorded. At the end of the experiment, fat pads were sampled, leptin and insulin were measured in the plasma and NPY in several microdissected brain areas. Substituting ASP for water led to lower body weight (-8%; P<.004) and lower fat depot weight (-20%; P<.01) with no differences in energy intake or plasma insulin concentrations. Plasma leptin was significantly reduced by 34% (P<.05). Leptin concentrations were well-correlated with final body weight (r=.47; P<.025) and fat pad mass (r=.53; P<.01). NPY concentrations were 23% lower (P<.03) in the arcuate nucleus of ASP rats with no differences in other brain areas. The beneficial effects on body composition could be related to the decreased effects of NPY on lipid and energy metabolism, independently of insulin. The reasons for the NPY decrease (regulatory or toxicological) are not obvious. The constitutive amino acids of the ASP molecule might participate in the NPY regulation.

Effects of two diketopiperazines, cyclo (His-Pro) and cyclo (Asp-Phe), on striatal dopamine: a microdialysis study

The effects of two diketopiperazines, Cyclo (His-Pro) (CHP) and Cyclo (Asp-Phe) (CAP), on striatal extracellular levels of dopamine (DA), dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), and 5-hydroxyindoleacetic acid (5-HIAA) were examined using in vivo microdialysis in anaesthetized rats. Treatment with neither CHP (0.1-10 mg/kg IP and 0.3 mg/kg i.v.) nor CAP (0.1-10 mg/kg IP and 10 mg/kg PO) significantly changed the efflux of DA, DOPAC, HVA, or 5-HIAA when compared to the effects of treatment with saline. Our results suggest that systemic administration of CHP or CAP alone does not modify striatal dopaminergic neurotransmission. The previous findings of enhanced DA release by systemic administration of thyrotropin releasing hormone (TRH) are probably not explained by formation of CHP from TRH.

Effect of tyrosine on the potentiation by aspartame and phenylalanine of metrazol-induced convulsions in rats

Male rats were treated by oral intubation with tyrosine (Tyr), at doses of 0.5 and 1.0 g/kg body weight, alone or together with 1 g aspartame (APM)/kg body weight, or an equivalent dose of phenylalanine (Phe; 0.5 g/kg body weight); the effects on seizures induced by an effective dose of metrazol (ED50) were observed. Tyr (0.5 g/kg body weight) had a protective effect against the Phe-potentiation of metrazol-induced clonic-tonic convulsions. At the same dose Tyr had no effect on the seizure-promoting activity of APM, but at 1 g/kg it reduced the proconvulsant potential of the sweetener. Analysis of the brain and plasma amino acid concentrations indicated that the Tyr to Phe ratio tended to be enhanced in Tyr-Phe treated rats compared with those treated with Phe alone. This ratio remained essentially constant in the brain of APM-treated rats, compared with those treated with APM plus 1 g Tyr/kg body weight, whereas an increase in this ratio in the plasma was observed. These results confirm that Tyr antagonizes the proconvulsant effect of Phe and APM and they further suggest that no simple relationship exists between the relative brain concentrations of the two amino acids and the response to metrazol convulsions.

Metabolism of aspartame and its L-phenylalanine methyl ester decomposition product by the porcine gut

The intestinal metabolism of aspartame (N-L-alpha-aspartyl-L-phenylalanine methyl ester; APM) and its L-phenylalanine methyl ester (PME) decomposition product was evaluated in six young pigs. Equimolar doses (2.5 mmol/kg body weight) of APM, PME, and L-phenylalanine (PHE) administered to the proximal jejunum produced similar increases in portal blood PHE concentrations. Methanol, nondetectable in portal blood after PHE ingestion, increased markedly after administration of either APM or PME. Portal blood aspartate concentrations were unchanged after PME and PHE administration, but increased significantly after APM administration. Increases in portal blood PHE concentrations were significantly greater than were increases in aspartate concentrations following APM administration. Neither APM, PME, nor aspartyl-phenylalanine (AspPhe) were detected in portal or vena caval blood following administration of any test compound. Steady-state perfusion of the small intestine with APM showed a net intraluminal appearance rate of AspPhe at 36% of the disappearance rate of APM. During steady-state PME perfusion, PHE had a significantly greater net appearance rate than during APM perfusion. Methanol appearance rates were slightly, but not significantly, higher during PME than during APM perfusions. The data suggest that (1) APM is hydrolyzed to AspPhe and both APM and PME are hydrolyzed to their constituent amino acids and and methanol prior to entering the portal circulation; (2) AspPhe is an important intraluminal intermediate in aspartame metabolism; and (3) aspartate is rapidly metabolized by the enterocyte.

Interspecies and interstrain studies on the increased susceptibility to metrazol-induced convulsions in animals given aspartame

The ability of aspartame (APM) to increase the susceptibility to metrazol-induced convulsions was studied in two strains of mice (CD1 and DBA/2J) and in guinea-pigs. Rats were included as known positive controls. Plasma and brain levels of phenylalanine (Phe) and tyrosine (Tyr) were measured in CD1 mice and guinea-pigs at various intervals after a dose of 1 g APM/kg body weight (administered orally to mice and ip to guinea-pigs). In mice, peak levels of Phe and Tyr were observed in plasma after 30 min and in brain after 60 min. In guinea-pigs peak plasma levels of Phe and Tyr occurred 30 min after treatment. Phe was at a maximum in guinea-pig brain after 30 min, while Tyr levels reached a peak at 120 min. In further experiments Phe and Tyr levels were measured 1 hr after APM doses of 0.5, 0.75 or 1 g/kg. In CD1 mice, plasma Phe and Tyr levels were increased significantly only at the highest dose, whereas in brain, Tyr concentrations were significantly increased by 0.75 or 1 g APM/kg and Phe was significantly increased by all three doses. In the guinea-pig, plasma Phe and Tyr were increased significantly only by 1 g APM/kg and in brain this dose significantly raised only the Phe levels. Monoamine and metabolite levels were determined in the brain striata of CD1 and DBA/2J mice 1 hr after the oral administration of 1 or 2 g APM/kg body weight; no differences from control values were found in either strain. The studies of potentiation of metrazol-induced convulsions showed that APM, at doses of up to 2 g/kg body weight, had no such effect in mice or guinea-pigs. In contrast, as expected, the potentiation was significant in the rat at 1 g/kg.

Effect of precursors on the synthesis of catecholamines and on neurotransmission in the superior cervical ganglion of the rat

Male Sprague-Dawley rats (325-350 g) were anesthetized with urethane (1.5 g/kg i.p.) and treated with physiological saline, Aspartame (APM; 552 mumols/kg), or tyrosine (Tyr; 552 mumols/kg). Ganglionic transmission and the synthesis of dopamine (DA) and norepinephrine (NE) were measured in the superior cervical ganglion (SCG) following electrical stimulation of the cervical sympathetic trunk (CST). When the CST was stimulated with single pulses, neither APM nor Tyr affected the synthesis of NE or DA. However, in response to low- (5 Hz, 20 s) and high- (20 Hz, 20 s) frequency pulses, the metabolism of DA was increased (p less than 0.05), but to the same extent after saline, APM, or Tyr. In rats stimulated with similar low- and high-frequency pulses, the synthesis of NE was increased significantly (p less than 0.05) after Tyr, but not after APM or saline. In saline-treated controls, ganglionic transmission was not changed in response to single pulses, or low- or high-frequency stimulation. However, after treatment with APM, ganglionic transmission was depressed significantly (p less than 0.01) in response to high-frequency stimulation (single: 0.46 +/- 0.09 mV; low: 0.39 +/- 0.07 mV; high: 0.27 +/- 0.07 mV). After treatment with Tyr, ganglionic transmission was depressed significantly (p less than 0.05) in response to both low- and high-frequency stimulation (single: 0.44 +/- 0.04 mV; low: 0.22 +/- 0.12 mV; high: 0.26 +/- 0.07 mV). In the nonstimulated SCG, L-3,4-dihydroxy-phenylalanine (25 mg/kg) caused a rapid, significant (p less than 0.01) increase in the synthesis and metabolism of DA, but not of NE.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of aspartame on N-methyl-D-aspartate-sensitive L-[3H]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 inhibit L-[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 of L-[3H]glutamate binding without altering the Vmax 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.

Plasma and brain kinetics of large neutral amino acids and of striatum monoamines in rats given aspartame

Two doses (250 and 1000 mg/kg body weight) of aspartame were administered orally to male rats, and plasma and brain phenylalanine and tyrosine kinetic profiles were studied. In both plasma and brain the maximum increase in phenylalanine and tyrosine levels was reached 60 min after treatment. The changes in brain levels of phenylalanine or tyrosine 0, 60, 120 or 180 min after treatment with 1000 mg AMP/kg were directly correlated with the ratio of the plasma concentration of phenylalanine or tyrosine to the overall plasma concentration of the other large neutral amino acids. The time course of monoamine and metabolite concentrations, in the corpora striatum of the brain, was studied after an oral dose of 500 mg phenylalanine/kg. No significant modifications of monoamine levels were found at any of the times studied, up to 5 hr after dosing.

Acute effects of aspartame on concentrations of brain amines and their metabolites in selected brain regions of Fischer 344 and Sprague-Dawley rats

This study is the first in a series to define a rodent model to document the effects of amino acid-modulating compounds on central neurotransmitter function. A time-response curve for a single dose of orally intubated aspartame was determined in male Fischer 344 and Sprague-Dawley rats. Regional brain concentrations of norepinephrine (NE), dopamine (DA), serotonin (5-HT) and their metabolites were analyzed in the hypothalamus, cerebellum, pons/medulla, hippocampus, striatum, cortex, and midbrain/thalamus at 30, 60, 120, or 240 min after oral aspartame (1000 mg/kg) administration. Without consideration for time and group variables, levels of most compounds were higher in the brain regions of Fischer than Sprague-Dawley rats. Aspartame in Fischer 344 or Sprague-Dawley rats had no significant effect on levels of the catecholamines or indoleamines at any of the time points monitored following its acute administration. From the results of this study, large oral loads of aspartame do not appear to lead to regional alterations in brain biogenic amine levels.

Aspartame fails to facilitate pentylenetetrazol-induced convulsions in CD-1 mice

Concentrations of plasma amino acids and brain monoamines as well as pentylenetetrazol-induced seizures were monitored in CD-1 mice treated with aspartame in acute oral doses from 0 to 2500 mg/kg. One hour after administration aspartame produced increases in plasma concentrations of phenylalanine and tyrosine and modest reductions in concentrations of brain serotonin and 5-hydroxyindole acetic acid. However, these effects of the sweetener had no influence on the convulsive dose fifty (CD50) of pentylenetetrazol. Moreover, aspartame failed to alter the percentage of mice exhibiting seizures when exposed to an approximate CD50 of pentylenetetrazol. Finally, aspartame had no effect on brain norepinephrine or dopamine concentrations. In sharp contrast to previously reported studies, these observations suggest that aspartame, given in heroic doses, does not alter the propensity to seizure activity in CD-1 mice. We conclude that changes in plasma amino acids and brain serotonin produced by large oral bolus doses of aspartame are insufficient to result in functional deficits which might have the capacity to facilitate pentylenetetrazol-induced seizures.

Failure of aspartame to affect seizure susceptibility in kindled rats

The effect of aspartame administered by gavage to rats on amygdala and hippocampal kindled seizures was assessed. Despite the administration of a wide range of doses (25-2000 mg/kg) no evidence for an effect of aspartame on afterdischarge threshold or seizure strength was obtained when testing was done at a time when serum and brain levels of neutral amino acids are known to be significantly elevated as a result of this treatment. There is controversy whether dietary aspartame (N-L-aspartyl-L-phenylalanine 1-methyl ester), a food additive sweetner, can lead to seizures in susceptible humans and in laboratory animals. A proseizure effect of high consumption of aspartame has been alleged (Wurtman, 1985; Walton, 1986) and denied (Gaull, 1985). Recent studies using mice have yielded mixed results. Thus, Kim and Kim (1986) and Pinto and Maher (1988) observed potentiating effects of high loads of aspartame on chemically induced seizures, but Nevins, Arnolde and Haigler (1986) observed no effect on chemical and ECS seizures. We used the electrical kindling model of epilepsy to assess whether aspartame can alter seizure threshold or strength in rats. The kindled response is highly repeatable and stable and has been shown to be sensitive to a large variety of pharmacological treatments (Racine, 1978) and to dietary manipulation (McCann, Cain and Philbrick, 1983).

Pharmacological effects of phenylalanine on seizure susceptibility: an overview

The effects of excessive doses of phenylalanine on seizure susceptibility were examined in animal models in the past, primarily because of their relevance to phenylketonuria. It was thought that such effects might involve brain monoaminergic mechanisms. Recently, this issue has been pursued with a renewed interest but for a different reason. The dipeptide sweetener, aspartame, contains a phenylalanine residue. In the last three years, a number of studies involving as many as nine animal models of seizures have reexamined the effects of phenylalanine (and aspartame) on seizure thresholds. Data from these studies are in general agreement that aspartame at dosage levels below 1,000 mg/kg, or phenylalanine at equimolar doses, is without an effect on seizure susceptibility in animals. When the dosage level of aspartame reaches 1,000 mg/kg, the findings between various laboratories and from different animal models of seizures are inconsistent, showing either no effect or a proconvulsant effect. The Acceptable Daily Intake of aspartame in humans set by the Food and Drug Administration is 50 mg/kg/day. Thus, the data from the excessive bolus doses in rodents do not appear to be relevant to human use. This article provides a detailed review of the data from both early and recent studies and points out the methodological problems apparent at such high doses.

Behavioral assessment of the toxicity of aspartame

Six experiments with rats assessed the toxicity of aspartame with behavioral measures. The first three experiments used a conditioned taste aversion procedure since taste aversions are typically observed after a taste is followed by a toxin. Thirty min after thirsty rats drank a sweet solution they were intraperitoneally injected (Experiment 1) or intragastrically intubated (Experiment 2) with saline or 176, 352, or 704 mg/kg of aspartame. Relative to rats given saline, rats injected with 704 and 352 mg/kg aspartame showed strong and mild aversions, respectively. Rats injected with 176 mg/kg of aspartame or intubated with any dose of aspartame did not show taste aversions. In Experiment 3, rats voluntarily consumed an aspartame solution sweetened with saccharin for 7 hr each day. Consumption of the taste paired with aspartame was not reduced. When 352 mg/kg aspartame was injected (Experiment 4), but not when intubated (Experiment 5), 5 min prior to access to a running wheel, running was reduced. Wheel running was not affected by the voluntary consumption of aspartame (Experiment 6). The route of administration effect (intraperitoneal vs. intragastric) on behavior corresponded with the amino acid levels in blood plasma (Experiment 7). Aspartate, phenylalanine, tyrosine and glutamate levels increased more after the injection, than the intubation, of aspartame (176 mg/kg). Overall, the results suggest that aspartame may have adverse effects when intraperitoneally injected but not when the route of administration is oral.

Effect of aspartame on seizures in various models of experimental epilepsy

We investigated in rats whether aspartame intake affected the susceptibility to seizures induced chemically (metrazol, quinolinic acid) or electrically (electroshock). Aspartame (0.75-1.0 g/kg), given orally as a single bolus to 16-hr fasted animals 60 min before metrazol, significantly increased the number of animals showing clonic-tonic seizures. At 1.0 g/kg the ED50 for clonic-tonic convulsions was lowered by 23%. A similar increase in seizure susceptibility was observed with 0.25-0.5 g/kg of the aspartame's metabolite phenylalanine. When aspartame was administered to fasted rats in three divided doses (0.33 g/kg) over 120 min or to fed animals after a meal, or overnight with the diet, no significant changes in the incidence of animals showing seizures was observed. One gram per kilogram aspartame and 0.5 g/kg phenylalanine did not modify the CC50 (mA) for tonic hindlimb extension induced by electroshock and the electroencephalographic seizures caused by intrahippocampal injection of 120 nmol quinolinic acid. Plasma and brain levels of phenylalanine and tyrosine significantly raised after both 1 g/kg aspartame as a single bolus (plasma: Phe 285%, Tyr 288%; brain: Phe 146%, Tyr 192%; above controls) or in three divided doses (plasma: Phe 207%, Tyr 315%; brain Phe 103%, Tyr 211%; above controls) and 0.5 g/kg phenylalanine (plasma: Phe 339%, Tyr 410%; brain: Phe 219%, Tyr 192%; above controls), but the ratio Phe/Tyr was not modified. Our data indicate that aspartame cannot be regarded as a general proconvulsant agent. The mechanisms of potentiation of seizures induced by metrazol after the administration of the sweetner in a single rapid intake will be discussed.

Effects of repeated doses of aspartame on serotonin and its metabolite in various regions of the mouse brain

Following a finding that single doses (approximating to average intakes and to potential 'over-use') of aspartame administered orally to mice caused significant increases in norepinephrine and dopamine concentrations in various brain regions, the effect of repeated exposure to aspartame was studied. Male CD-1 mice were given a daily oral dose of 0, 13, 133 or 650 mg/kg for 30 days and 1 day after the last dose the animals were decapitated and their brain regions were quickly isolated. Analyses of the different regions for catecholamine and indoleamine neurotransmitters and their major metabolites indicated that the increases in adrenergic chemicals observed shortly after a single exposure were not apparent after repeated dosing. In contrast, concentrations of serotonin and its metabolite, 5-hydroxyindoleacetic acid, were decreased in several regions. An increased supply of phenylalanine may be responsible for a decrease in tryptophan uptake by the brain tissue or for a depression in tryptophan conversion to serotonin.

Sugar and sugar substitutes. Comparisons and indications

Public confusion and concern about the use of sugar and sugar substitutes are widespread. Physicians must be prepared to answer patients' inquiries about these substances. Some population groups should avoid certain sugar substitutes. In particular, pregnant women and young children should avoid saccharin, and phenylketonuric homozygous persons should avoid aspartame. In a varied, balanced diet, the use of aspartame and saccharin is one safe way for the general population to enjoy sweet foods with fewer calories and less cariogenic potential. Sugar substitutes may be helpful in dietary compliance for overweight and diabetic patients.

Plasma amino acid concentrations in normal adults administered aspartame in capsules or solution: lack of bioequivalence

Some clinical studies require administration of test compounds in capsules to assure that the compound cannot be distinguished from a placebo. This raises the question of whether the pharmacokinetic responses produced by capsule administration are similar to values obtained when test compounds are ingested in solution. To test this, plasma phenylalanine and aspartate concentrations were compared in ten normal subjects ingesting 3 g aspartame in solution and in capsules in a balanced Latin square design. Peak plasma phenylalanine levels were significantly higher (191 +/- 65.4 v 117 +/- 39.5 mumol/L, mean +/- SD) and were reached significantly earlier (32 +/- 15 v 123 +/- 74 minutes) when aspartame was administered in solution than when it was administered in capsules. The area under the four-hour plasma phenylalanine concentration-time curve was significantly higher (15,340 +/- 4,820 v 8,465 +/- 3,356 mumol/L X min) when aspartame was ingested in solution. Administration in solution also produced a significantly higher ratio of plasma phenylalanine concentration to the sum of the plasma concentrations of the other large neutral amino acids (0.36 +/- 0.12 v 0.23 +/- 0.06). Similarly, peak plasma aspartate concentrations were significantly higher 26.2 +/- 16.3 v 10.4 +/- 5.0 mumol/L) and were reached significantly earlier (30 +/- 14 v 106 +/- 61.3 min) when aspartame was administered in solution. The data indicate different plasma phenylalanine and aspartate pharmacokinetics between solution and capsule administration of aspartame.

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

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 less than 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.

Neurobiochemical alterations induced by the artificial sweetener aspartame (NutraSweet)

The dipeptide aspartame (NutraSweet) is a newly approved and widely used artificial sweetener in foods and beverages. Consumption of aspartame (ASM) has been reported to be responsible for neurologic and behavioral disturbances in sensitive individuals. Unfasted male CD-1 mice were dosed orally with 13, 130, or 650 mg/kg ASM in corn oil, while control animals received corn oil alone. Three hours after dosing, the animals were killed, and the concentrations of the catecholamines norepinephrine (NE) and dopamine (DA), catecholamine metabolites 3-methoxy-4-hydroxymandelic acid (VMA), homovanillic acid (HVA), and dihydroxyphenylacetic acid (DOPAC), the indoleamine serotonin (5-HT), and its metabolite 5-hydroxyindoleacetic acid (5-HIAA) were determined by electrochemical high-performance liquid chromatography in six brain regions. ASM exerted its primary effect on adrenergic neurotransmitters in various brain regions. In the hypothalamus, the region richest in NE, increases in NE concentrations of 12, 49, and 47% were found in the low, medium, and high dose groups, respectively, relative to control. Significant increases of NE in the medulla oblongata and corpus striatum were also observed. Increases of the catecholamine DA and catecholamine metabolites VMA, HVA, and DOPAC were seen in various regions. The indoleamine serotonin and its metabolite 5-HIAA were unaffected by ASM treatment. These findings are consistent with ASM-induced increases in the brain catecholamine precursor amino acids phenylalanine and tyrosine, as reported earlier. Such observed alterations in brain neurotransmitter concentrations may be responsible for the reported clinical and behavioral effects associated with ASM ingestion.

Effect of mealing on plasma and brain amino acid, and brain monoamine in rats after oral aspartame

Aspartame (APM; L-aspartyl-L-phenylalanine methyl ester) was investigated for its ability to alter brain amino acids and monoamines in overnight fasted rats allowed to consume commercial diets for 60 minutes. In addition, the effects of mealing on the changes in plasma and brain amino acids and brain monoamines induced by glucose and/or insulin, and known pharmacologically active compounds, were studied. The consumption of the commercial chow largely prevented changes in blood glucose and amino acids, and brain amino acids and the monoamines dopamine, norepinephrine and serotonin that might be expected to occur following glucose with or without insulin. Feeding failed to prevent changes in the above parameters when 5-hydroxy-tryptophan, p-chlorophenylalanine and reserpine were administered. The oral administration of up to 250 mg/kg BW APM with water or glucose followed by free feeding failed to alter brain monoamines. These studies demonstrate the potent ability of food to normalize biochemical parameters in blood and brain that otherwise might occur, and clearly show the lack of effect on brain monoamine levels of abuse doses of APM when administered with food.

Dietary aspartame with protein on plasma and brain amino acids, brain monoamines and behavior in rats

Aspartame (APM; L-aspartyl-L-phenylalanine methyl ester), was investigated for its ability to alter levels of the large neutral amino acids and monoamines in overnight fasted rats allowed to consume meals with or without protein for two hours. Additionally, the possible long term behavioral consequences of APM in 25% casein diets with or without 10% sucrose were determined. Acute APM ingestion increased both plasma and brain phenylalanine and tyrosine levels, but brain tryptophan levels were not altered regardless of dietary protein. Brain norepinephrine and dopamine levels were unaltered by any of the diet while serotonin levels were slightly increased when a protein-free diet was consumed. But APM and/or protein ingestion minimized this increase of brain serotonin levels as much as controls. Chronic APM ingestion failed to influence diurnal feeding patterns, meal size distributions, or diurnal patterns of spontaneous motor activity. The chronic ingestion of abuse doses of APM produced no significant chemical changes in brain capable of altering behavioral parameters believed to be controlled by monoamines in rats.