Antihyperalgesic effects of intrathecally administered magnesium sulfate in rats

Intrathecal administration of MgSO4 is reported to cause paralysis. However , the characteristic sensory disturbances have not been thoroughly investig ated. We examined the effect of intrathecally administered MgSO4 on the noc iceptive threshold, using three different nociceptive measures, formalin te st, hot plate test and paw pressure rest in rats. The:dose of MgSO4 was 30, 100 or 300 mu g. In acute nociceptive tests, intrathecal MgSO4 did not cau se any significant changes in the pain threshold. However, phase 2 of the f ormalin test was suppressed dose-dependently. It is known that spinal NMDA receptors are involved in the changes seen during the second (tonic) phase of the formalin test and in vitro studies showed that Mg2+ can cause voltag e-dependent blockade of NMDA receptor channel in the neurons of spinal dors al hem. Thus, the suppressive effect of intrathecally administered MgSO4 on the tonic inflammation-evoked behavior is mediated by the spinal NMDA rece ptors. Our results suggest that intrathecal administration of MgSO4 may be therapeutically beneficial for patients with tonic pain involving the spina l NMDA receptors. (C) 2000 International Association for the Study of Pain. Published by Elsevier Science B.V.