Antagonists of NMDA glutamate receptors have been shown to alleviate neuropathic pain in rats and humans. However, NMDA antagonists can cause significant side effects ranging from behavioral disturbances to injury of neurons in the posterior cingulate/retrosplenial (PC/RS) cortex. We have found that alpha-2 adrenergic agonists prevent the PC/RS neurotoxic side effects of NMDA antagonists. In the present study of adult female rats subjected to sciatic nerve ligation (Bennett neuropathic pain model) and tested for paw withdrawal latency (PWL) following a thermal stimulus, we evaluated the ability of the NMDA antagonist, MK-801, to alleviate neuropathic pain either by itself or when administered together with the alpha-2 adrenergic agonist, clonidine. We found that MK-801, at a dose (0.05 mg/kg s.c.) that is known to cause mild hyperactivity but is subthreshold for producing PC/RS neurotoxic changes, relieved the neuropathic pain state associated with sciatic nerve ligation. However, the relief at this dose was very transient, and no neuropathic pain-relieving effect was observed at a lower dose (0. 025 mg/kg s.c.) of MK-801. Clonidine, at a dose (0.05 mg/kg s.c.) that prevents the cerebrocortical neurotoxic effects of MK-801, decreased sensitivity to the thermal stimulus equally under all conditions (ligated, sham ligated, unoperated), but did not specifically relieve neuropathic pain in the ligated limb. Combining this dose of clonidine with an ineffective dose (0.025 mg/kg s.c.) of MK-801 provided specific, complete and long lasting (up to 4 h) relief from neuropathic pain. Rats receiving this drug combination did not display hyperactivity or any other behavioral disturbance typically associated with MK-801 treatment, nor show neurotoxic changes in cerebrocortical neurons. In separate experiments on normal unoperated rats, we found that clonidine (0.05 mg/kg s.c.) counteracted the hyperactivity induced by MK-801 (0.05 mg/kg s.c.) and returned activity levels to a normal range. These findings signify that clonidine, which does not specifically relieve neuropathic pain, can potentiate the neuropathic pain-relieving action of MK-801, while also protecting against neurotoxicity and hyperactivity side effects of MK-801. The potentiation is of a sufficient magnitude that it permits cutting the MK-801 dose requirement in half, thereby achieving prolonged neuropathic pain relief while doubling the margin of safety against any type of side effect that might be mediated by blockade of NMDA receptors.
Copyright 1998 Elsevier Science B.V.