Background: Spinally delivered alpha 2-adrenoceptor agonists and N-methyl-D-aspartate antagonists each have been shown to have actions attenuating the hyperesthesia in rat models of nerve injury pain. Using a fixed-dose analysis and an isobolographic paradigm, the spinal interaction between the alpha 2-adrenoceptor agonist clonidine and the N-methyl-D-aspartate antagonist MK-801 is characterized in a rat model of nerve injury-induced tactile hyperesthesia.
Methods: Male Sprague Dawley rats were anesthetized with halothane, and the left L5 and L6 spinal nerves were ligated (Chung model). After 7-10 days' recovery, a Polyethylene tubing catheter was implanted into the lumbar intrathecal space. After recovery from catheter implantations (5-7 days), intrathecal dose-response curves were established for the antihyperesthesia effects of clonidine (3, 6, 10, and 20 micrograms) and MK-801 (1, 3, 10, and 20 micrograms) alone to obtain the ED50 for each agent. In separate studies, three doses of clonidine (1, 3, and 10 micrograms) were injected mixed with one dose of MK-801 (1 microgram) for fixed-dose analysis, and three doses of the two agents (2, 6, and 20 micrograms) were injected jointly in a fraction of the dose ratio 1:1 for isobolographic analysis. Thresholds for left hind paw withdrawal to von Frey hair application were assessed.
Results: Rats with nerve ligation showed a reliable tactile hyperesthesia (mechanical threshold 1-3 g vs. normal > 15 g). Intrathecal clonidine and MK-801 alone produced dose-dependent reductions of tactile hyperesthesia: ED50 9 micrograms and 10 micrograms, respectively. With the fixed-dose analysis, the log dose-response curves showed a left shift that considerably exceeds the theoretical curve made by a simple sum of the effects of clonidine alone and with MK-801 (1 microgram). With the isobolographic analysis, the combination ED50 was found to be statistically less than the theoretical additive dose combination. Intrathecal atipamezole, an alpha 2 antagonist, reversed the effects of clonidine and the clonidine/MK-801 mixture but not MK-801 alone. The side effect of clonidine was sedation and urination and that of MK-801 was motor weakness at doses above 10 micrograms. These effects were considerably less severe in rats after equiactive doses in the combination group.
Conclusions: The neuropathic pain is mediated by low-threshold mechanoreceptors, sympathetically dependent, and sensitive to both alpha 2 agonists and N-methyl-D-aspartate antagonists. Intrathecal clonidine may act to diminish sympathetic outflow, whereas MK-801 blocks the N-methyl-D-aspartate receptor that is associated with other spinal systems related to pain transmission mechanism. The two separate mechanisms may account for the powerful synergy observed in this study. Such combinations might be useful in neuropathic pain states to potentiate the antihyperpathic effects and to reduce the side effects of each agents.