Analysis of Drug Interaction between Intrathecal Clonidine and MK-801 in Peripheral Neuropathic Pain Rat Model

Background: Spinally delivered alpha2 -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 alpha2 -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 micro gram) and MK-801 (1, 3, 10, and 20 micro gram) alone to obtain the ED50 for each agent. In separate studies, three doses of clonidine (l, 3, and 10 micro gram) were injected mixed with one dose of MK-801 (1 micro gram) for fixed-dose analysis, and three doses of the two agents (2, 6, and 20 micro gram) 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 micro gram and 10 micro gram, 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 micro gram). With the isobolographic analysis, the combination ED50 was found to be statistically less than the theoretical additive dose combination. lntrathecal atipamezole, an alpha2 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 micro gram. These effects were considerably less severe in rats after equiactive doses in the combination group.