Prenatal Monosodium Glutamate (MSG) Treatment Given Through the Mother's Diet Causes Behavioral Deficits in Rat Offspring

Diet's Impact on Post-Traumatic Brain Injury Depression: Exploring Neurodegeneration, Chronic Blood-Brain Barrier Destruction, and Glutamate Neurotoxicity Mechanisms

Traumatic brain injury (TBI) has a profound impact on cognitive and mental functioning, leading to lifelong impairment and significantly diminishing the quality of life for affected individuals. A healthy blood-brain barrier (BBB) plays a crucial role in guarding the brain against elevated levels of blood glutamate, making its permeability a vital aspect of glutamate regulation within the brain. Studies have shown the efficacy of reducing excess glutamate in the brain as a treatment for post-TBI depression, anxiety, and aggression. The purpose of this article is to evaluate the involvement of dietary glutamate in the development of depression after TBI. We performed a literature search to examine the effects of diets abundant in glutamate, which are common in Asian populations, when compared to diets low in glutamate, which are prevalent in Europe and America. We specifically explored these effects in the context of chronic BBB damage after TBI, which may initiate neurodegeneration and subsequently have an impact on depression through the mechanism of chronic glutamate neurotoxicity. A glutamate-rich diet leads to increased blood glutamate levels when contrasted with a glutamate-poor diet. Within the context of chronic BBB disruption, elevated blood glutamate levels translate to heightened brain glutamate concentrations, thereby intensifying neurodegeneration due to glutamate neurotoxicity.

Dietary glutamate and the brain: In the footprints of a Jekyll and Hyde molecule

Glutamate is a crucial neurotransmitter of the mammalian central nervous system, a molecular component of our diet, and a popular food-additive. However, for decades, concerns have been raised about the issue of glutamate's safety as a food additive; especially, with regards to its ability (or otherwise) to cross the blood-brain barrier, cause excitotoxicity, or lead to neuron death. Results of animal studies following glutamate administration via different routes suggest that an array of effects can be observed. While some of the changes appear deleterious, some are not fully-understood, and the impact of others might even be beneficial. These observations suggest that with regards to the mammalian brain, exogenous glutamate might exert a double-sided effect, and in essence be a two-faced molecule whose effects may be dependent on several factors. This review draws from the research experiences of the authors and other researchers regarding the effects of exogenous glutamate on the brain of rodents. We also highlight the possible implications of such effects on the brain, in health and disease. Finally, we deduce that beyond the culinary effects of exogenous glutamate, there is the possibility of a beneficial role in the understanding and management of brain disorders.

Risk Assessment Paradigm for Glutamate

BACKGROUND

Re-evaluation of the use of glutamic acid and glutamate salts (referred to as glutamate hereafter) by the European Food Safety Authority (EFSA) proposed a group acceptable daily intake (ADI) of 30 mg/kg body weight (bw)/day.

SUMMARY

This ADI is below the normal dietary intake, while even intake of free glutamate by breast-fed babies can be above this ADI. In addition, the pre-natal developmental toxicity study selected by EFSA, has never been used by regulatory authorities worldwide for the safety assessment of glutamate despite it being available for nearly 40 years. Also, the EFSA ignored that toxicokinetic data provide support for eliminating the use of an uncertainty factor for interspecies differences in kinetics. Key Messages: A 3-generation reproductive toxicity study in mice that includes extensive brain histopathology, provides a better point of departure showing no effects up to the highest dose tested of 6,000 mg/kg bw/day. Furthermore, kinetic data support use of a compound-specific uncertainty factor of 25 instead of 100. Thus, an ADI of at least 240 mg/kg bw/day would be indicated. In fact, there is no compelling evidence to indicate that the previous ADI of "not specified" warrants any change.

Re-evaluation of glutamic acid (E 620), sodium glutamate (E 621), potassium glutamate (E 622), calcium glutamate (E 623), ammonium glutamate (E 624) and magnesium glutamate (E 625) as food additives

The EFSA Panel on Food Additives and Nutrient Sources added to Food (ANS) provides a scientific opinion re-evaluating the safety of glutamic acid-glutamates (E 620-625) when used as food additives. Glutamate is absorbed in the intestine and it is presystemically metabolised in the gut wall. No adverse effects were observed in the available short-term, subchronic, chronic, reproductive and developmental studies. The only effect observed was increased kidney weight and increased spleen weight; however, the increase in organ weight was not accompanied by adverse histopathological findings and, therefore, the increase in organ weight was not considered as an adverse effect. The Panel considered that glutamic acid-glutamates (E 620-625) did not raise concern with regards to genotoxicity. From a neurodevelopmental toxicity study, a no observed adverse effect level (NOAEL) of 3,200 mg monosodium glutamate/kg body weight (bw) per day could be identified. The Panel assessed the suitability of human data to be used for the derivation of a health-based guidance value. Although effects on humans were identified human data were not suitable due to the lack of dose-response data from which a dose without effect could be identified. Based on the NOAEL of 3,200 mg monosodium glutamate/kg bw per day from the neurodevelopmental toxicity study and applying the default uncertainty factor of 100, the Panel derived a group acceptable daily intake (ADI) of 30 mg/kg bw per day, expressed as glutamic acid, for glutamic acid and glutamates (E 620-625). The Panel noted that the exposure to glutamic acid and glutamates (E 620-625) exceeded not only the proposed ADI, but also doses associated with adverse effects in humans for some population groups.

Effect of trans-fat, fructose and monosodium glutamate feeding on feline weight gain, adiposity, insulin sensitivity, adipokine and lipid profile

The incidence of obesity and type 2 diabetes mellitus (T2DM) is increasing, and new experimental models are required to investigate the diverse aspects of these polygenic diseases, which are intimately linked in terms of aetiology. Feline T2DM has been shown to closely resemble human T2DM in terms of its clinical, pathological and physiological features. Our aim was to develop a feline model of diet-induced weight gain, adiposity and metabolic deregulation, and to examine correlates of weight and body fat change, insulin homeostasis, lipid profile, adipokines and clinical chemistry, in order to study associations which may shed light on the mechanism of diet-induced metabolic dysregulation. We used a combination of partially hydrogenated vegetable shortening and high-fructose corn syrup to generate a high-fat-high-fructose diet. The effects of this diet were compared with an isoenergetic standard chow, either in the presence or absence of 1.125 % dietary monosodium glutamate (MSG). Dual-energy X-ray absorptiometry body imaging and a glucose tolerance test were performed. The present results indicate that dietary MSG increased weight gain and adiposity, and reduced insulin sensitivity (P < 0.05), whereas high-fat-high-fructose feeding resulted in elevated cortisol and markers of liver dysfunction (P < 0.01). The combination of all three dietary constituents resulted in lower insulin levels and elevated serum β-hydroxybutyrate and cortisol (P < 0.05). This combination also resulted in a lower first-phase insulin release during glucose tolerance testing (P < 0.001). In conclusion, markers of insulin deregulation and metabolic dysfunction associated with adiposity and T2DM can be induced by dietary factors in a feline model.

Interactive effects of neonatal exposure to monosodium glutamate and aspartame on glucose homeostasis

BACKGROUND

Recent evidence suggests that the effects of certain food additives may be synergistic or additive. Aspartame (ASP) and Monosodium Glutamate (MSG) are ubiquitous food additives with a common moiety: both contain acidic amino acids which can act as neurotransmitters, interacting with NMDA receptors concentrated in areas of the Central Nervous System regulating energy expenditure and conservation. MSG has been shown to promote a neuroendocrine dysfunction when large quantities are administered to mammals during the neonatal period. ASP is a low-calorie dipeptide sweetener found in a wide variety of diet beverages and foods. However, recent reports suggest that ASP may promote weight gain and hyperglycemia in a zebrafish nutritional model.

METHODS

We investigated the effects of ASP, MSG or a combination of both on glucose and insulin homeostasis, weight change and adiposity, in C57BL/6 J mice chronically exposed to these food additives commencing in-utero, compared to an additive-free diet. Pearson correlation analysis was used to investigate the associations between body characteristics and variables in glucose and insulin homeostasis.

RESULTS

ASP alone (50 mg/Kgbw/day) caused an increase in fasting blood glucose of 1.6-fold, together with reduced insulin sensitivity during an Insulin Tolerance Test (ITT) P < 0.05. Conversely MSG alone decreased blood triglyceride and total cholesterol (T-CHOL) levels. The combination of MSG (120 mg/Kgbw/day) and ASP elevated body weight, and caused a further increase in fasting blood glucose of 2.3-fold compared to Controls (prediabetic levels); together with evidence of insulin resistance during the ITT (P < 0.05). T-CHOL levels were reduced in both ASP-containing diets in both genders. Further analysis showed a strong correlation between body weight at 6 weeks, and body weight and fasting blood glucose levels at 17 weeks, suggesting that early body weight may be a predictor of glucose homeostasis in later life.

CONCLUSIONS

Aspartame exposure may promote hyperglycemia and insulin intolerance. MSG may interact with aspartame to further impair glucose homeostasis. This is the first study to ascertain the hyperglycemic effects of chronic exposure to a combination of these commonly consumed food additives; however these observations are limited to a C57BL/6 J mouse model. Caution should be applied in extrapolating these findings to other species.

Effect of the oral administration of monosodium glutamate during pregnancy and breast-feeding in the offspring of pregnant Wistar rats

PURPOSE

Determine the effects of the MSG (monosodium glutamate) in the offspring of pregnant rats through the comparison of the weight, NAL (nasal-anal length) and IL (Index of Lee) at birth and with 21 days of life.

METHODS

Pregnant Wistar rats and their offspring were divided into 3 groups: GC, G10 and G20. Each of the groups received 0%, 10% and 20% of MSG, respectively from coupling until the end of the weaning period.

RESULTS

Neither weight nor NAL were different among the groups at birth. The group G20 at birth had an IL lower than the group GC (p<0,05) and with 21 days of life presented weight and NAL lower than the groups G10 and this lower than the GC (p<0,01). Otherwise the G20 at 21 days of life had the IL similar to the other two groups. The weight profit percentage from birth to the 21st day of life was lower in the G20 regarding the other two groups (p<0,01). The G20 had a NAL increase percentage from birth to the 21st day of life lower than the G10 and this lower than the GC (p<0,01).

CONCLUSIONS

MSG presented a dose-dependent relation in the variables weight and NAL. It caused a decrease in the growth pattern as well as in the weight gain pattern until the 21st day of life. The IL of the group 20% had an increased in relation to the control group after 3 weeks of follow up.

Glutamate oxidation by trophoblasts in vitro

Catabolism of uniformly and 1-14C-labeled glutamate was investigated in human placental cytotrophoblasts and syncytiotrophoblasts cultured on uncoated plastic or a fibrin matrix. Product-labeling experiments resulted in 14C incorporation into carbon dioxide and tricarboxylic acid cycle intermediates. 14C incorporation above background was not detected for the putative products, glutamine, amino acids, glutathione, and protein. Inhibitors of specific metabolic pathways were used to elucidate the routes of glutamate oxidation. Incorporation of 14C into carbon dioxide from [1-14C]glutamate was inhibited by the glutamate dehydrogenase inhibitor pyridine-2,6-dicarboxylic acid and aminotransferase inhibitor aminooxyacetic acid. Production of 14CO2 was higher for syncytiotrophoblast compared with cytotrophoblast and for cells on uncoated plastic compared with a fibrin matrix. Oxidation of glutamate was unaffected by added glutamine as high as 2 mM. The primary route of glutamate metabolism by placental trophoblast in vitro is oxidation to carbon dioxide utilizing both the transferase and deamination pathways.

Differences in open-field behavior and in learning tasks between two rat strains differing in their reactivity to stressors

The study characterizes differences between inbred Wistar-Kyoto (WKYs) and Brown-Norway (BNs) rats in open-field behavior, and in discriminative learning and acquisition of an avoidance learning task. Hyper-reactivity of WKYs to novelty was demonstrated in an open-field test. Discriminative learning and retention thereof was slower in WKYs, but as efficient as in BNs. Acquisition of avoidance learning was also slower in WKYs, but their maximal avoidance score was much higher (approximately 85%) than in BNs. Also, recall of avoidance learning was slower for WKYs. We conclude: (1) hyper-reactivity of WKYs to novelty is expressed by their exceptional immobility and excess defecation in the open-field and is paralleled by their known hyper-reactivity to stressful stimuli, and (2) no strain differences exist in the ability to learn a discriminative task, but both acquisition and recall of an avoidance task are slower in WKYs. This may imply that the degree of reactivity to stressful environmental stimuli may play an important role in the acquisition of learning.

Prenatal caffeine causes long lasting behavioral and neurochemical changes

The effects of prenatal exposure to caffeine were studied on later physical development, behavior and brain neurochemistry. Daily doses (150, 300 or 450 mg/L) of caffeine were given to rat dams during the last week of pregnancy. Prenatal caffeine exposure resulted in a number of behavioral and neurochemical changes in the offspring which were long lasting and dose related. The low dose (150 mg/L) of prenatal caffeine caused hyperactivity in an open-field. The high dose of caffeine caused learning disabilities in complex visual and auditory discrimination learning paradigms while simple motor learning or a spatial orientation task were not affected. Both male and female offspring showed some behavioral effects of caffeine exposure. The medium and high doses of caffeine resulted in weight gain that was observable as early as 35 days of age and increased progressively with age. This weight gain was associated with increased food intake. The neurochemical studies carried out at 2-3 months of age revealed an increase in choline uptake in hippocampus, mainly in the animals treated with the lower doses of caffeine and higher protein concentration (microgram/mg wet tissue) in the cortex or hippocampus of offspring exposed to the higher doses of caffeine. At 15 months of age, choline uptake in the frontal cortex was significantly reduced in the animals prenatally exposed to the 300 and 450 mg/L dose.

The effects of mild maternal stress during pregnancy on the behavior of rat pups

Mild maternal stress in the form of chronic daily subcutaneous injections of saline or the vehicle for diazepam to pregnant rats was shown to result in some long term, subtle but reliable, changes in the behavior of the offspring. The same vehicle given for the same period of time in the dam's drinking water, without injection had no effect on the development of later behavior of rat pups. Chronic prenatal injections of saline or vehicle for diazepam, used in many experiments as controls for the evaluation of drug effects were shown to have some long lasting behavioral effects in the offspring of the treated dams. The series of experiments reported here compared the offspring of saline or vehicle injected dams to those of uninjected dams on a variety of developmental measurements, an open field behaviour and on learning performance in a complex brightness discrimination maze.

Prenatal monosodium glutamate causes long-lasting cholinergic and adrenergic changes in various brain regions

Prenatal monosodium glutamate (MSG) given through the mother's diet was found previously to cause behavioral changes in the offspring, including learning disabilities. In the present study, neurochemical parameters were measured in the brains of prenatally exposed rats at various ages throughout development up to adulthood. At 15 days of age, choline uptake and choline acetyltransferase (ChAT) activity in the frontal cortex were significantly reduced (by 80 and 25%, respectively) in MSG-exposed animals, whereas the same cholinergic parameters in hippocampus were not changed. During later development, choline uptake gradually increased, until in adulthood it became significantly higher in MSG-exposed animals than in the controls. This enhancement was found in both males and females. Our previous study showed that only the male offspring were learning disabled. Choline uptake and ChAT activity were enhanced in the hippocampus of adult male animals. Norepinephrine (NE) uptake was reduced (by 25%) in the frontal cortex of males only. There was no change in NE uptake in the hypothalamus.

Differential vulnerability of male and female rats to the timing of various perinatal insults

The results of five experiments showed that exposure to diazepam, hypoxia and monosodium glutamate during the prenatal or early postnatal period of rapid brain development may result in different behavioral consequences depending on the timing of the exposure rather than the nature of the agent. Moreover, male and female offspring may be affected differently by the same agent at different periods of development. Prenatal insults of various kinds impair the later performance of males but not the females in a complex learning task, while postnatal insults seem to affect detrimentally this same behavior in both males and females. The effects of perinatal insults on maze learning and open field activity do not lend themselves to explanation by "feminization" or "masculinization" of behavior caused by interference with prenatal gonadal hormones.