Toxic effects of fluoride (F) are now well known. If chronically absorbed, F induces dental and skeletal abnormalities, neurotoxical effects, especially when F levels are higher than 4 ppm and/or
when cells are at an early stage of their development. Moreover a spontaneous acute pain syndrome of lower extremities is frequently observed in humans even without radiographic abnormalities of bones. So, in order to test new experimental animal models of neuropathic pain, it was of interest to assess pain appearing when high doses of F are administered to rats.
High fluoridated drinking water (100, 150 or 200 ppm in deionized water) was individually given for 7 Weeks to adult female SpragueDawley rats (100-125g) (n =8). Control received deionized water. Behavioral changes, hyperalgesia and allodynia, were assessed using noxious thermal (radiant heat) and non-noxious mechanical (Von Frey filaments) stimuli, respectively, to the two hind paws. Plasma F levels ranged from 62.1 to 589.8 fLgfl according to the dose and upper levels were similar as that found in humans ingesting 5-10 ppm F in drinking water.
Thermal sensitivity thresholds were always significantly reduced compared to control animals from W + 2 to W + 7 (from -16.1% to -38%). A rapid and significant reduction of Von Frey withdrawal
thresholds was also observed after I and 4 weeks with rats exposed to 100 and 150 ppm F, respectively, (from -51.1% to -82.1%) whereas rats exposed to 200 ppm F exhibited a significant allodynia only atW+ 7 (–60.4%). We are currently evaluating the histological changes which could explain the effects of F on peripheral nerve conduction, i.e. tactile allodynia and thermal hyperalgesia, as these effects are mediated by myelinated and unmyelinated fibers, respectively.