Neonatal exposure to monosodium glutamate disrupts place learning ability in adult rats

A systematic review of the neuropathology and memory decline induced by monosodium glutamate in the Alzheimer's disease-like animal model

This systematic review analyzes monosodium glutamate (MSG) in the Alzheimer's disease-like condition to enhance translational research. Our review seeks to understand how MSG affects the brain and causes degenerative disorders. Due to significant preclinical data linking glutamate toxicity to Alzheimer's disease and the lack of a comprehensive review or meta-analysis, we initiated a study on MSG's potential link. We searched PubMed, ScienceDirect, ProQuest, DOAJ, and Scopus for animal research and English language papers without time constraints. This study used the PRISMA-P framework and PICO technique to collect population, intervention or exposure, comparison, and result data. It was registered in PROSPERO as CRD42022371502. MSG affected mice's exploratory behaviors and short-term working memory. The brain, hippocampus, and cerebellar tissue demonstrated neuronal injury-related histological and histomorphometric changes. A total of 70% of MSG-treated mice had poor nesting behavior. The treated mice also had more hyperphosphorylated tau protein in their cortical and hippocampus neurons. Glutamate and glutamine levels in the brain increased with MSG, and dose-dependent mixed horizontal locomotor, grooming, and anxiety responses reduced. MSG treatment significantly decreased phospho-CREB protein levels, supporting the idea that neurons were harmed, despite the increased CREB mRNA expression. High MSG doses drastically lower brain tissue and serum serotonin levels. In conclusion, MSG showed AD-like pathology, neuronal atrophy, and short-term memory impairment. Further research with a longer time span and deeper behavioral characterization is needed. Systematic review registration: https://www.crd.york.ac.uk/prospero/, identifier [CRD42022371502].

Enhancement of hippocampal-dependent spatial memory by Ashwagandha (Withania somnifera) characterized by activation of NMDA receptors against monosodium glutamate-induced neurotoxicity in rats

BACKGROUND AND AIM

Monosodium glutamate (MSG) is used in food-additives, and the Food and Drug Administration has placed it under intense scrutiny following several reports that it causes glutamate neurotoxicity. Ashwagandha (ASH) roots are traditionally used for memory enhancement. This study aimed to evaluate the nootropic activity of ASH as well as its therapeutic anti-amnesic activity against MSG-induced hippocampal-dependent spatial memory impairment and hippocampal-NMDAR modulation.

METHOD

A total of 36 rats were divided equally into six groups (n = 6 in each group); the rats in the normal and negative groups were administered daily doses of normal saline and MSG (300 mg/kg), respectively, for 21 days. Two nootropic groups were administered ASH at 300 and 500 mg/kg o.p., respectively, for 21 days. Two other treatment groups were administered daily doses of MSG 300 mg/kg o.p. as well as 300 mg/kg and 500 mg/kg o.p. of ASH for 21 days. The rats' spatial memory was assessed for five days using the MWM. Additionally, NMDAR were measured quantitatively by immunohistochemistry.

RESULTS

We found that the rats in the nootropic groups showed significantly enhanced nootropic activity characterized by improved hippocampal-dependent spatial memory, as well as increases in the level of NMDAR in the Cornu Ammonis 1 region of their hippocampus. Moreover, we elucidated the therapeutic potential of ASH to protect against the depression of spatial memory caused by MSG-induced neurotoxicity.

CONCLUSION

Further, we elucidated a strong correlation between NMDAR-positive cells in the hippocampus and enhancement of spatial learning induced by long-term administration of ASH as well as a strong correlation between NMDAR positive cells in the hippocampus and depression of spatial learning induced by long-term administration of ASH and MSG.

The protective effect of thymoquinone or/and thymol against monosodium glutamate-induced attention-deficit/hyperactivity disorder (ADHD)-like behavior in rats: Modulation of Nrf2/HO-1, TLR4/NF-κB/NLRP3/caspase-1 and Wnt/β-Catenin signaling pathways in rat model

Both thymoquinone (TQ) and thymol (T) have been proved to possess a positive impact on human health. In this research, we aimed to investigate the effect of these compounds separately and together on the Attention-deficit/hyperactivity disorder (ADHD)-like behavior induced by monosodium glutamate (MSG) in rats. Forty male, Spargue Dawley rat pups (postnatal day 21), were randomly allocated into five groups: Normal saline (NS), MSG, MSG+TQ, MSG+T, and MSG+TQ+T. MSG (0.4 mg/kg/day), TQ (10 mg/kg/day) and T (30 mg/kg/day) were orally administered for 8 weeks. The behavioral tests proved that rats treated with TQ and/or T showed improved locomotor, attention and cognitive functions compared to the MSG group with more pronounced effect displayed with their combination. All treated groups showed improvement in MSG-induced aberrations in brain levels of GSH, IL-1β, TNF-α, GFAP, glutamate, calcium, dopamine, norepinephrine, Wnt3a, β-Catenin and BDNF. TQ and/or T treatment also enhanced the mRNA expression of Nrf2, HO-1 and Bcl2 while reducing the protein expression of TLR4, NFκB, NLRP3, caspase 1, Bax, AIF and GSK3β as compared to the MSG group. However, the combined therapy showed more significant effects in all measured parameters. All of these findings were further confirmed by the histopathological examinations. Current results concluded that the combined therapy of TQ and T had higher protective effects than their individual supplementations against MSG-induced ADHD-like behavior in rats.

Neuroprotective Effect of Morin Hydrate against Attention-Deficit/Hyperactivity Disorder (ADHD) Induced by MSG and/or Protein Malnutrition in Rat Pups: Effect on Oxidative/Monoamines/Inflammatory Balance and Apoptosis

Monosodium glutamate (MSG) is one of the most widely used food additives. However, it has been linked to protein malnutrition (PM) and various forms of toxicities such as metabolic disorders and neurotoxic effects. The current study is the first to explore the association between MSG, PM, and induced brain injury similar to attention-deficit/hyperactivity disorder (ADHD). Moreover, we determined the underlying mechanistic protective pathways of morin hydrate (MH)―a natural flavonoid with reported multiple therapeutic properties. PM was induced by feeding animals with a low protein diet and confirmed by low serum albumin measurement. Subsequently, rat pups were randomized into seven groups of 10 rats each. Group I, III, and VI were normally fed (NF) and groups II, IV, V, and VII were PM fed. Group I served as normal control NF while Group II served as PM control animals. Group III received NF + 0.4 g/kg MSG, Group IV: PM + 0.4 g/kg MSG, Group V: PM + 60 mg/kg MH, Group VI: NF + 0.4 kg/g MSG + 60 mg/kg MH and Group VII: PM + 0.4 kg/kg MSG + 60 mg/kg MH. At the end of the experimental period, animals were subjected to behavioral and biochemical tests. Our results showed that treatment of rats with a combination of MSG + PM-fed exhibited inferior outcomes as evidenced by deteriorated effects on behavioral, neurochemical, and histopathological analyses when compared to rats who had received MSG or PM alone. Interestingly, MH improved animals’ behavior, increased brain monoamines, brain-derived neuroprotective factor (BDNF), antioxidant status and protein expression of Nrf2/HO-1. This also was accompanied by a significant decrease in brain MDA, inflammatory markers (NF-kB, TNF-α and IL1β), and suppression of TLR4/NLRP3/caspase-1 axis. Taken together, MSG and/or PM are associated with neuronal dysfunction. Our findings suggest MH as a potential neuroprotective agent against brain insults via targeting Nrf2/HO-1 and hindering TLR4/NLRP3 inflammasome signaling pathways.

Effects of in utero exposure to monosodium glutamate on locomotion, anxiety, depression, memory and KCC2 expression in offspring

In pregnancy, there is a significant risk for developing embryos to be adversely affected by everyday chemicals such as food additives and environmental toxins. In recent times, several studies have documented the detrimental effect of exposure to such chemicals on the behaviour and neurodevelopment of the offspring. This study evaluated the influence of the food additive, monosodium glutamate (MSG), on behaviour and development in mice. Pregnant dams were exposed to MSG 2 or 4 g/kg or distilled water from gestation day 10-20. On delivery, postnatal day 1 (PN 1), 3 pups were sacrificed and whole brain samples assayed for KCC2 expression by western blot. The remaining pups were housed until PN 43 before commencing behavioural assessment. Their weights were measured at birth and at 3 days intervals until PN 42. The impact of prenatal exposure to MSG on baseline exploratory, anxiety and depression behaviours as well as spatial and working memory was assessed. In utero exposure to 4 g/kg MSG significantly reduced exploratory drive and increased depression-like behaviours but did not exert any significant impact on anxiety-like behaviours (p < 0.01). Additionally, there was a two-fold increase in KCC2 expression in both 2 and 4 g/kg MSG-exposed offspring. CONCLUSION: This study indicates that, in utero exposure to MSG increases the expression of KCC2 and causes significant effect on locomotion and depression-like behaviours but only marginally affects memory function.

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.

Cortical tau burden and behavioural dysfunctions in mice exposed to monosodium glutamate in early life

Although monosodium glutamate (MSG)-induced neurotoxicity has been recognized for decades, the potential similarities of the MSG model to Alzheimer’s disease (AD)-type neuropathology have only recently been investigated. MSG-treated mice were examined behaviourally and histologically in relation to some features of AD. Four-week old mice received 5 subcutaneous MSG (2 g/kg) injections on alternate days, or saline. At age 10–12 weeks, they were given a battery of behavioural tests for species-typical behaviours and working memory. The mice were killed at 12 weeks and the brains excised. Accumulation of hyperphosphorylated tau protein was assessed in cortical and hippocampal neurons by immunohistochemistry, and in cerebral cortical homogenates. A 78% increase in cortical concentrations of phosphorylated tau protein was observed in the MSG mice. Intracellular hyperphosphorylated tau immunostaining was observed diffusely in the cortex and hippocampus, together with cortical atrophic neurons, extensive vacuolation and dysmorphic neuropil suggestive of spongiform neurodegeneration. Nest-building was significantly impaired, and spontaneous T-maze alternation was reduced, suggesting defective short-term working memory. Subcutaneous MSG treatment also induced a 56% reduction in exploratory head dips in a holeboard (P = 0.009), and a non-significant tendency for decreased burrowing behaviour (P = 0.085). These effects occurred in the absence of MSG-induced obesity or gross locomotor deficits. The findings point to subcutaneous MSG administration in early life as a cause of tau pathology and compromised species-typical behaviour in rodents. Determining whether MSG can be useful in modelling AD requires further studies of longer duration and full behavioural characterization.

Neonatal Monosodium Glutamate Administration Disrupts Place Learning and Alters Hippocampal-Prefrontal Learning-Related Theta Activity in the Adult Rat

Neonatal treatment with monosodium glutamate causes profound deficits in place learning and memory in adult rats evaluated in the Morris maze. Theta activity has been related to hippocampal learning, and increased high-frequency theta activity occurs through efficient place learning training in the Morris maze. We wondered whether the place learning deficits observed in adult rats that had been neonatally treated with monosodium glutamate (MSG), were related to altered theta patterns in the hippocampus and prelimbic cortex, which were recorded during place learning training in the Morris maze. The MSG-treated group had a profound deficit in place learning ability, with a marginal reduction in escape latencies during the final days of training. Learning-related changes were observed in the relative power distribution in control and MSG-treated groups in the hippocampal EEG, but not in the prelimbic cortex. Increased prefrontal and reduced hippocampal absolute power that appeared principally during the final days of training, and reduced coherence between regions throughout the training (4-12 Hz), were observed in the MSG-treated rats, thereby suggesting a misfunction of the circuits rather than a hyperexcitable general state. In conclusion, neonatal administration of MSG, which caused a profound deficit in place learning at the adult age, also altered the theta pattern both in the hippocampus and prelimbic cortex.

Dietary composition modulates impact of food-added monosodium glutamate on behaviour, metabolic status and cerebral cortical morphology in mice

Effects of food-added monosodium glutamate (MSG) on neurobehaviour, serum biochemical parameters, malondialdehyde (MDA) levels, and changes in cerebral cortex, liver and kidney morphology were assessed in mice fed standard diet (SD) or high-fat diet (HFD). Animals were assigned to 8 groups [SD control, HFD control, and six groups fed MSG plus SD or HFD at 0.1, 0.2 and 0.4 g/kg of feed]. Animals were fed for 8 weeks, behavioural tests were conducted, and blood was taken for estimation of biochemical parameters and MDA level. Whole brain was homogenised for neurochemical assays, while the cerebrum, liver and kidneys were processed for histology. In groups fed MSG/SD, there was a decrease in weight gain, increase in food-intake, an increase in locomotion, a decrease in rearing/grooming, and a decrease in anxiety-response. Also observed were derangements in biochemical parameters, increased MDA, and alteration of renal morphology. Compared to HFD, MSG/HFD groups had reduction in weight gain, food-intake, grooming and anxiety-response, an increase in locomotion, and improved memory. Protection against biochemical derangements and HFD-induced organ injuries were also observed. In conclusion, the findings suggest that possible interactions that may occur between dietary constituents and MSG are determinants of the effects of food-added MSG in mice.

Altered behavior of adult obese rats by monosodium l-glutamate neonatal treatment is related to hypercorticosteronemia and activation of hypothalamic ERK1 and ERK2

OBJECTIVES

Obesity is a metabolic and hormonal disorder with serious social and psychological impacts. There is a close relationship among obesity, neuroendocrine homeostasis and behavioral patterns. However, few data are available in the literature regarding this subject. This study assessed behavior and memory of adult obese rats by monosodium l-glutamate (MSG) neonatal treatment or highly palatable dietary treatment.

METHODS

MSG obesity was induced by subcutaneous injections of MSG (4 mg/g) during the first 5 days of life (Ob-MSG); control group (C-MSG), received saline solution equimolar. Both groups were fed with commercial chow. To induce dietary obesity, 21-day-old rats were assigned to two experimental diets: highly palatable diet (Ob-Diet) and control diet (C-Diet) composed of commercial chow. Ninety-day-old animals were submitted to behavioral assessment by the open-field test and short- and long-term memory by the object recognition test. Biometric variables were obtained, the Lee index was calculated and mass of retroperitoneal and perigonadal fat pads was measured. Furthermore, an altered behavioral profile was investigated by quantification of plasmatic corticosterone, expression, and activity of hypothalamic extracellular signal-regulated kinase protein (ERK) 1 and 2.

RESULTS

Increased Lee index and fat pads were observed in Ob-MSG and Ob-Diet groups. Ob-MSG presented a higher level of anxiety and impaired long-term memory compared to C-MSG, while there was no difference between Ob-Diet and C-Diet. The Ob-MSG group presented a higher level of plasmatic corticosterone and increased phosphorylation of hypothalamic ERK1 and 2.

DISCUSSION

Both treatments induced obesity but only Ob-MSG showed altered behavioral parameters, which is related to increased concentration of corticosterone and hypothalamic ERK1 and 2 activation.

Excitotoxicity triggered by neonatal monosodium glutamate treatment and blood-brain barrier function

It is likely that monosodium glutamate (MSG) is the excitotoxin that has been most commonly employed to characterize the process of excitotoxicity and to improve understanding of the ways that this process is related to several pathological conditions of the central nervous system. Excitotoxicity triggered by neonatal MSG treatment produces a significant pathophysiological impact on adulthood, which could be due to modifications in the blood-brain barrier (BBB) permeability and vice versa. This mini-review analyzes this topic through brief descriptions about excitotoxicity, BBB structure and function, role of the BBB in the regulation of Glu extracellular levels, conditions that promote breakdown of the BBB, and modifications induced by neonatal MSG treatment that could alter the behavior of the BBB. In conclusion, additional studies to better characterize the effects of neonatal MSG treatment on excitatory amino acids transporters, ionic exchangers, and efflux transporters, as well as the role of the signaling pathways mediated by erythropoietin and vascular endothelial growth factor in the cellular elements of the BBB, should be performed to identify the mechanisms underlying the increase in neurovascular permeability associated with excitotoxicity observed in several diseases and studied using neonatal MSG treatment.

Cognitive and biochemical effects of monosodium glutamate and aspartame, administered individually and in combination in male albino mice

The present study was designed to investigate the in vivo effects of monosodium glutamate (MSG) and aspartame (ASM) individually and in combination on the cognitive behavior and biochemical parameters like neurotransmitters and oxidative stress indices in the brain tissue of mice. Forty male Swiss albino mice were randomly divided into four groups of ten each and were exposed to MSG and ASM through drinking water for one month. Group I was the control and was given normal tap water. Groups II and III received MSG (8 mg/kg) and ASM (32 mg/kg) respectively dissolved in tap water. Group IV received MSG and ASM together in the same doses. After the exposure period, the animals were subjected to cognitive behavioral tests in a shuttle box and a water maze. Thereafter, the animals were sacrificed and the neurotransmitters and oxidative stress indices were estimated in their forebrain tissue. Both MSG and ASM individually as well as in combination had significant disruptive effects on the cognitive responses, memory retention and learning capabilities of the mice in the order (MSG+ASM)>ASM>MSG. Furthermore, while MSG and ASM individually were unable to alter the brain neurotransmitters and the oxidative stress indices, their combination dose (MSG+ASM) decreased significantly the levels of neurotransmitters (dopamine and serotonin) and it also caused oxidative stress by increasing the lipid peroxides measured in the form of thiobarbituric acid-reactive substances (TBARS) and decreasing the level of total glutathione (GSH). Further studies are required to evaluate the synergistic effects of MSG and ASM on the neurotransmitters and oxidative stress indices and their involvement in cognitive dysfunctions.

Asiatic acid, a pentacyclic triterpene in Centella asiatica, attenuates glutamate-induced cognitive deficits in mice and apoptosis in SH-SY5Y cells

AIM

To investigate whether asiatic acid (AA), a pentacyclic triterpene in Centella asiatica, exerted neuroprotective effects in vitro and in vivo, and to determine the underlying mechanisms.

METHODS

Human neuroblastoma SH-SY5Y cells were used for in vitro study. Cell viability was determined with the MTT assay. Hoechst 33342 staining and flow cytometry were used to examine the apoptosis. The mitochondrial membrane potential (MMP) and reactive oxygen species (ROS) were measured using fluorescent dye. PGC-1α and Sirt1 levels were examined using Western blotting. Neonatal mice were given monosodium glutamate (2.5 mg/g) subcutaneously at the neck from postnatal day (PD) 7 to 13, and orally administered with AA on PD 14 daily for 30 d. The learning and memory of the mice were evaluated with the Morris water maze test. HE staining was used to analyze the pyramidal layer structure in the CA1 and CA3 regions.

RESULTS

Pretreatment of SH-SY5Y cells with AA (0.1-100 nmol/L) attenuated toxicity induced by 10 mmol/L glutamate in a concentration-dependent manner. AA 10 nmol/L significantly decreased apoptotic cell death and reduced reactive oxygen species (ROS), stabilized the mitochondrial membrane potential (MMP), and promoted the expression of PGC-1α and Sirt1. In the mice models, oral administration of AA (100 mg/kg) significantly attenuated cognitive deficits in the Morris water maze test, and restored lipid peroxidation and glutathione and the activity of SOD in the hippocampus and cortex to the control levels. AA (50 and 100 mg/kg) also attenuated neuronal damage of the pyramidal layer in the CA1 and CA3 regions.

CONCLUSION

AA attenuates glutamate-induced cognitive deficits of mice and protects SH-SY5Y cells against glutamate-induced apoptosis in vitro.

Role of p38 MAPK and pro-inflammatory cytokines expression in glutamate-induced neuronal death of neonatal rats

Pro-inflammatory cytokines TNF-alpha, IL-1beta and IL-6 rises significantly during neuronal damage and activate the signaling p38 MAPK pathway, which is involved in the apoptotic (AP) neuronal death. Systemic administration of glutamate as monosodium salt (MSG) to newborn animals induces neuronal death, however whether neurons die by AP or necrosis through MAPK p38 pathway activation it is unknown. In this study, TNF-alpha, IL-1beta and IL-6 expression levels, AP neuronal death and cellular type that produces TNF-alpha was also identified in the cerebral cortex (CC) and striatum (St) of rats at 8, 10, and 14 days of age after neonatal exposure to MSG. TNF-alpha production and AP neuronal death was significantly increased in the CC at PD8-10, and in the St in all ages studied by excitotoxicity effect induced with MSG. This effect was completely inhibited by SB203580 (p38 inhibitor) in both regions studied. TNF-alpha, IL-1beta and IL-6 RNAm increased after MSG administration, whereas SB203580 did not modify their expression. These data indicates that neuronal death induced by excitotoxicity appears to be mediated through p38 signaling pathway activated by TNF-alpha and their inhibition may have an important neuroprotective role as part of anti-inflammatory therapeutic strategy.

Calorie restriction alters physical performance but not cognition in two models of altered neuroendocrine signaling

A major neuroendocrinological effect of calorie restriction (CR) is induction of neuropeptide Y (NPY) in the arcuate nucleus (ARC). Aside from its appetite-stimulating effects, NPY is thought to be involved in the modulation of behavioral processes including anxiety and learning and memory. In the present study physical fitness, anxiety, and learning/memory-related tasks were assessed in mice lacking NPY or a functional ARC after dietary manipulation by CR. Physical fitness was improved by CR when measured by inclined screen and rotarod, and this diet effect was not affected by NPY or ARC status. As has been observed previously, the NPY knockout mice displayed heightened anxiety in an open field. This phenotype was not fully recapitulated in the ARC-lesioned model. CR affected neither total locomotor activity in the open field nor thigmotaxic behavior in these models. Neither NPY nor CR had a significant effect on Morris water maze performance; however, ARC-damaged mice were unable to learn the task, and this deficit was not corrected by CR. We conclude that despite established effects of CR on ARC signaling, our results suggest a mechanistic separation between the two where behavior is concerned.

Specific timing of taurine supplementation affects learning ability in mice

The effects of taurine supplementation on visual discrimination in mice were examined. Taurine, 2-aminoethane-sulphonic acid, found in high concentrations in the central nervous system of mammals and in human milk, has been shown to be essential for development. Male mice were divided into four groups according to taurine supplementation periods. 1) Lifelong: taurine (400 mg/kg/day) was dissolved in distilled water and provided as drinking water. In the prenatal period, taurine was given via the mother. After weaning mice were administered taurine in drinking water. 2) Pre-weaning: mice were exposed to taurine prior to weaning, 3) Post-weaning: mice were exposed to taurine after weaning. 4) CONTROL: no supplementation of taurine. It was shown that the Lifelong group required a longer period of time to acquire visual discrimination than the CONTROL group. Conversely, in the Post-weaning group, mice learned the task faster than CONTROLs. Visual discrimination learning time in the Pre-weaning group showed no significant difference compared with that in the CONTROL group. From these results, we suggest that the perinatal to early postnatal period is a "sensitive period" where taurine supplementation can result in retardation of learning in later life. At the same time, taurine supplementation after weaning improved visual discrimination learning. Thus, timing of taurine supplementation affected learning.

The expression and binding of kainate receptors is modified in different brain regions by glutamate neurotoxicity during postnatal rat development

Kainic acid receptor (KA-R) subunits are differentially expressed during brain development, and they modulate both neural growth and survival. High concentrations of glutamate in the brain can induce neuronal injury through these receptors, altering normal development. However, it is unclear whether KAR subunit expression itself is also modified by neonatal exposure to high glutamate. To analyze this, monosodium glutamate (4mg/g of body weight) was subcutaneously administered on postnatal days 1, 3, 5 and 7, and the expression of GluR5, GluR6, KA1 and KA2, as well as [(3)H]-kainic acid (KA-R) binding, was evaluated on postnatal days 14, 21, 30 and 60 in different regions of rat brain. As a result, high levels of GluR5 expression associated with strong [(3)H]-kainic acid binding were observed on postnatal days 30 and 60 in the cerebral cortex of rats exposed to glutamate. Similarly, the changes induced by glutamate administration in the expression of the KA1 and KA2 subunits were paralleled by those of [(3)H]-kainic acid binding in the striatum at postnatal days 21 and 30. In contrast, while KAR subunits were over expressed in the hippocampus, no changes were observed in [(3)H]-kainic acid binding in adult rats that had been exposed to glutamate. Therefore, glutamate modifies both the expression of kainic acid receptor subunits and kainic acid binding in a determined spatial and temporal manner, which may be indicative of a regional susceptibility to glutamate neurotoxicity.