The GABAA and NMDA-glutamate receptors as targets of toxicity

BABOT Z.; GALOFRÉ M.; GARCÍA D.A.; HERRERA N.; IRAOLA S.; SABATER D.; SUÑOL C.

Aranjuez, España

Congreso; XXVIII Reunión del Grupo Español de Neurotransmisión; 2007

Grupo Español de Neurotransmisión

Validated alternative test methods are required for safety toxicology of drugs and chemicals. Studies have demonstrated that there is ~70% correlation between in vitro cytotoxic concentrations and in vivo LD50 or human lethal concentrations. Therefore ++alerts and correctors must be defined that could account for the left 30% failure in prediction. Neurotoxic events may underlie acute human toxicity that may not be predicted by using an in vitro test that exclusively relies on cell death. Acute human toxicity correlating to adverse neuronal function is mainly a result of over-excitation or depression of the central or peripheral nervous system. The major molecular mechanisms involved include GABAergic, glutamatergic and cholinergic neurotransmission. It is expected that neurotoxic chemicals will produce their effect by different mechanisms. The evaluation of these mechanisms will allow establishing a hierarchy in the role of the different assays in their capacity of predicting the potential neurotoxicity of the compounds. In addition, long-term toxicity, either after permanent exposure or as a consequence to long-term effects after acute exposure, may underlie compensatory mechanisms known as synaptic scaling. Using primary neuronal cultures from cortical and from cerebellar granule neurons we have determined the effect of a set of compounds (including pharmaceuticals, pesticides, and other industrial chemicals) on GABAA and glutamate-NMDA receptors, after a short exposure period (15 minutes). The GABAA receptor was the neural target most sensitive for the analyzed compounds. Atropine, carbamazepine, lindane, dieldrin, chlorpyrifos, methadone and pentachlorophenol inhibited the GABAA receptor (determined by measuring the uptake of 36Cl- induced by GABA) and they have been reported to produce convulsions in humans after severe intoxication. When examining the effects of chlorinated and organophosphorus pesticides after prolonged periods of exposure (2 – 6 days in vitro), the activity of the NMDA receptor ([Ca2+]i using Fluo 3 and [3H]MK801 binding in intact cells) was found to be significantly reduced. It is interesting to note that none of the latest compounds inhibited the activity of the NMDA receptor, after a short exposure. Further examinations based on the effects of the organochlorine pesticide dieldrin (a persistent organic pollutant, POP) revealed that the total amount of the NR1 subunit of the NMDA receptor was not altered, as determined by immunoblotting. However, the immunolabelling of the NR1 subunit was reduced in the cell membrane, suggesting internalization of the NMDA receptor. Thus, the chronic reduction of GABAA receptor function induced by dieldrin decreases the number of functional NMDA receptors, which may be attributable to a mechanism of synaptic scaling. These effects could underlie neural mechanisms involved in cognitive impairment produced by low exposure to dieldrin.