Carbofuran causes neuronal vulnerability to glutamate by decreasing GluA2 protein levels in rat primary cortical neurons.

Glutamate receptor 2 (GluA2/GluR2) is one of the four subunits of α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor (AMPAR); an increase in GluA2-lacking AMPARs contributes to neuronal vulnerability to excitotoxicity because of the receptor's high Ca2+ permeability. Carbofuran is a carbamate pesticide used in agricultural areas to increase crop productivity. Due to its broad-spectrum action, carbofuran has also been used as an insecticide, nematicide, and acaricide. In this study, we investigated the effect of carbofuran on GluA2 protein expression. The 9-day treatment of rat primary cortical neurons with 1 µM and 10 µM carbofuran decreased GluA2 protein expression, but not that of GluA1, GluA3, or GluA4 (i.e., other AMPAR subunits). Decreased GluA2 protein expression was also observed on the cell surface membrane of 10 µM carbofuran-treated neurons, and these neurons showed an increase in 25 µM glutamate-triggered Ca2+ influx. Treatment with 50 µM glutamate, which did not affect the viability of control neurons, significantly decreased the viability of 10 µM carbofuran-treated neurons, and this effect was abolished by pre-treatment with 300 µM 1-naphthylacetylspermine, an antagonist of GluA2-lacking AMPAR. At a concentration of 100 µM, but not 1 or 10 µM, carbofuran significantly decreased acetylcholine esterase activity, a well-known target of this chemical. These results suggest that carbofuran decreases GluA2 protein expression and increases neuronal vulnerability to glutamate toxicity at concentrations that do not affect acetylcholine esterase activity.

Prenatal Exposure to Tributyltin Decreases GluR2 Expression in the Mouse Brain.

Tributyltin (TBT), a common environmental contaminant, is widely used as an antifouling agent in paint. We previously reported that exposure of primary cortical neurons to TBT in vitro decreased the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor subunit glutamate receptor 2 (GluR2) expression and subsequently increased neuronal vulnerability to glutamate. Therefore, to identify whether GluR2 expression also decreases after TBT exposure in vivo, we evaluated the changes in GluR2 expression in the mouse brain after prenatal or postnatal exposure to 10 and 25 ppm TBT through pellet diets. Although the mean feed intake and body weight did not decrease in TBT-exposed mice compared with that in control mice, GluR2 expression in the cerebral cortex and hippocampus decreased after TBT exposure during the prenatal period. These results indicate that a decrease in neuronal GluR2 may be involved in TBT-induced neurotoxicity, especially during the fetal period.

Methoxychlor and fenvalerate induce neuronal death by reducing GluR2 expression.

GluR2, an α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptor subunit, plays important roles in neuronal survival. We previously showed that exposure of cultured rat cortical neurons to several chemicals decreases GluR2 protein expression, leading to neuronal toxicity. Methoxychlor, the bis-p-methoxy derivative of dichlorodiphenyltrichloroethane, and fenvalerate, a synthetic pyrethroid chemical, have been used commercially as agricultural pesticides in several countries. In this study, we investigated the effects of long-term methoxychlor and fenvalerate exposure on neuronal glutamate receptors. Treatment of cultured rat cortical neurons with 1 or 10 µM methoxychlor and fenvalerate for 9 days selectively decreased GluR2 protein expression; the expression of other AMPA receptor subunits GluR1, GluR3, and GluR4 did not change under the same conditions. Importantly, the decreases in GluR2 protein expression were also observed on the cell surface membrane where AMPA receptors typically function. In addition, both chemicals decreased neuronal viability, which was blocked by pretreatment with 1-naphtylacetylspermine, an antagonist of GluR2-lacking AMPA receptors, and MK-801, an N-methyl-d-aspartate (NMDA) receptor antagonist. These results suggest that long-term exposure to methoxychlor and fenvalerate decreases GluR2 protein expression, leading to neuronal death via overactivation of GluR2-lacking AMPA and NMDA receptors.

Protein extracts from cultured cells contain nonspecific serum albumin.

Serum is an important component of cell culture media. The present study demonstrates contamination of intracellular protein extract by bovine serum albumin from the culture media and illustrates how this contamination can cause the misinterpretation of western blot results. Preliminary experiments can prevent the misinterpretation of some experimental results, and optimization of the washing process may enable specific protein detection.

Development of a simple measurement method for GluR2 protein expression as an index of neuronal vulnerability.

In vitro estimating strategies for potential neurotoxicity are required to screen multiple substances. In a previous study, we showed that exposure to low-concentrations of some chemicals, such as organotin, decreased the expression of GluR2 protein, which is a subunit of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)-type glutamate receptors, and led to neuronal vulnerability. This result suggested that GluR2 decreases as an index of neuronal cell sensitivity and vulnerability to various toxic insults. Accordingly, we developed a versatile method that is a large scale determination of GluR2 protein expression in the presence of environmental chemicals by means of AlphaLISA technology. Various analytical conditions were optimized, and then GluR2 protein amount was measured by the method using AlphaLISA. The GluR2 amounts were strongly correlated with that of measured by western blotting, which is currently used to determine GluR2 expression. An ideal standard curve could be written with the authentic GluR2 protein from 0 ng to 100 ng. Subsequently, twenty environmental chemicals were screened and nitenpyram was identified as a chemical which lead to decrease in GluR2 protein expression. This assay may provide a tool for detecting neurotoxic chemicals according to decreases in GluR2 protein expression.