Antihyperalgesic effects of intrathecally administered magnesium sulfate in rats

Intrathecal administration of MgSO(4) is reported to cause paralysis. However, the characteristic sensory disturbances have not been thoroughly investigated. We examined the effect of intrathecally administered MgSO(4) on the nociceptive threshold, using three different nociceptive measures, formalin test, hot plate test and paw pressure test in rats. The dose of MgSO(4) was 30, 100 or 300 microg. In acute nociceptive tests, intrathecal MgSO(4) did not cause any significant changes in the pain threshold. However, phase 2 of the formalin 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 Mg(2+) can cause voltage-dependent blockade of NMDA receptor channel in the neurons of spinal dorsal horn. Thus, the suppressive effect of intrathecally administered MgSO(4) on the tonic inflammation-evoked behavior is mediated by the spinal NMDA receptors. Our results suggest that intrathecal administration of MgSO(4) may be therapeutically beneficial for patients with tonic pain involving the spinal NMDA receptors.