Long-term exposure to dieldrin prevents glutamate toxicity by reducing NMDA receptor and mGLUR5 functionality in primary cultures of cortical neurons.

Abstract

Trabajo presentado en el 12th Biennial Meeting of the International Neurotoxicology Association, celebrado en Jerusalén (Israel) del 07 al 12 de julio de 2009.[Purpose]: Dieldrin is a previously used pesticide that accumulates in the adipose tissue and the brain of mammals. Despite it is known that an acute exposure to dieldrin evokes convulsions, due to its antagonism on the GABAA receptor, little is known about the effects of a chronic exposure to this pollutant. We have previously reported that long-term exposure to dieldrin reduces the number of functional NMDA receptors in cerebellar granule cells. In the present work we use primary cultures of cortical neurons, mainly composed of GABAergic neurons, to support these observations in a different neuronal population and to further deep into the mechanisms involved in the toxic action of this pesticide. [Methods]: Cell viability: MTT and LDH assays. Functionality of the glutamate receptors (GluR): by measuring the [Ca2+]i with the fluorescence probe Fluo-3AM. Levels and localization of GluR: Western Blot and immunocytochemistry. [Results]: We have observed that the long-term (2 and 6DIV) exposure to a subcitotoxic concentration (60nM) of dieldrin prevents the excitotoxicity and reduces the [Ca2+]i increase caused by glutamate. However only after 6 DIV of treatment, dieldrin was able to reduce NMDAR and type-I metabotropic GluR functionalities. The NMDAR immunolabelling revealed that it undergoes internalization after 6 DIV of exposure to dieldrin. Whereas mGLUR1 expression remained unaffected, mGLUR5 was downregulated after the longer treatment. In contrast no changes were observed neither on receptor functionality nor on the receptor levels after 2 DIV of exposure to dieldrin. However double immunostaining for NMDA and mGLUR5 showed that these receptors loosed colocalization on the cell membrane in cells treated with dieldrin either for 2 and 6 DIV. [Conclusions]: We confirmed that the permanent blockade of the GABAA receptor by this persistent pesticide triggers adaptative neuronal changes consisting on the reduction of the glutamatergic transmission. Moreover it seems that the presence of mGLUR5 plays crucial role on the glutamate excitotoxicity in cortical neurons. Altough these results could attribute protective effects to this pollutant in a excitotoxic model, it seems properly that, through interfering with the glutamatergic transmission, dieldrin might impair memory and learning and could also alter behaviour.Supported by FIS PI 06121