AMPA and NMDA Receptors in Hippocampus of Rats with Fluoride-Induced Cognitive Decline

This experimental study was performed to evaluate the alterations in the expression of a few subunits composing glutamate AMPA (a-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) and NMDA (N-methyl-D-aspartate) receptors in the hippocampal cells of Wistar rats in response to long-term fluoride (F^-) exposure. The animals were given water with background 0.4 (control), 5, 20, and 50 ppm F^- (as NaF) for 12 months. The cognitive capacities of rats were examined by novel object recognition (NOR), Y-maze test, and Morris water maze tests. RT-qPCR and Western blotting techniques were used to evaluate the expression of different AMPA and NMDA subunits at transcriptional and translational levels, respectively. Long-term F^- poisoning disturbed the formation of hippocampus-dependent working spatial and long-term non-spatial memory. The expression of <i>Gria1</i>, <i>Gria2</i>, and <i>Gria3</i> genes encoding different subunits of AMPA receptors were comparable in hippocampi of control and F^--exposed animals, although the levels of both <i>Grin2a</i> and <i>Grin2b</i> mRNA increased. Long-term F^- intake enhanced the ratio of phospho-GluA1/total-GluA1 proteins in subcellular fraction enriched with cytosolic proteins, while decreased content of GluA2 but elevated level of GluA3 were observed in subcellular fraction enriched with membrane proteins. Such changes were accompanied by increased phosphorylation of GluN2A and GluN2B subunits, higher ratios of GluN2A/GluN1 and GluN2B/GluN1 proteins in the cytosol, and GluN2A/GluN2B ratio in membranes. These changes indicate the predominance of Ca²⁺-permeable AMPARs in membranes and a shift between different NMDARs subunits in hippocampal cells of F^--exposed rats, which is typical for neurodegeneration and can at least partially underly the observed disturbances in cognitive capacities of animals.