Abstract

The study was designed to evaluate an influence of excessive fluoride (F^–) intake on cognitive capacities of adult rats and on proteins of memory-related calpain signaling in hippocampus. Control animals were given water with natural F^– content of 0.4 ppm; rats from other groups consumed the same water supplemented with 5, 20, and 50 ppm F^– (as NaF) for 12 months. The efficiency of learning and memory formation was evaluated by novel object recognition (NOR) and Morris water maze tests. The expression of enzymes of calpain-1 and calpain-2 signaling in hippocampus was detected by Western blotting. Excessive F^– consumption had moderate impact on short-term memory, but impaired spatial learning and long-term memory of animals. Intoxication of rats with 5-50 ppm F^– led to stimulation of calpain-1 in hippocampal cells and its translocation from cytosol to membranes, accompanied by activation of GTPase RhoA. Exposure to 20-50 ppm F^– resulted in proteolytic cleavage of phosphatase PHLPP1 and increased expression of phospho-ERK1/2 kinase with insignificant decline of total ERK1/2 activity. In contrast, F^– did not change the expression of calpain-2 and its substrates-phosphatase PTEN and kinase mTOR. However, F^– intake led to downregulation of cAMP-response element binding protein (CREB) and brain-derived neurotrophic factor (BDNF). Thus, altered expression of calpain-1 and its downstream effectors at a background of stable activity of calpain-2 indicates overstimulation of signaling pathways of early LTP phase and disrupted link between early and late LTP phases, most probably due to altered activity of transcriptional and neurotrophic factors.

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