Lack of effect of intrathecally administered N-methyl-D-aspartate receptor antagonists in a rat model for postoperative pain

Evidence from experiments by others indicates an important role for excitatory amino acids activating spinal n-methyl-d-aspartate (NMDA) receptors in models of persistent pain. The purpose of this study was to examine the effect of intrathecal (+)-5-methyl-10,11-dihydro-5H-dibenzo(a,d)cyclohepten-5,10-imine (MK-801), a noncompetitive NMDA receptor antagonist, 2-amino-5-phosphonovaleric acid (AP5), a competitive NMDA receptor antagonist, and N-G-nitro-L-arginine methyl ester (L-NAME), a nitric oxide synthase inhibitor, on pain behaviors in a rat model of postoperative pain. Rats with intrathecal catheters were anesthetized and underwent a plantar incision. Withdrawal threshold to punctate stimulation applied adjacent to the wound, response frequency to application of a nonpunctate stimulus applied directly to the wound, and nonevoked pain behaviors were measured before and after intrathecal administration of MK-801 or AP5. The effect of intrathecal L-NAME on mechanical hyperalgesia was also examined. Mechanical hyperalgesia increased and was persistent after plantar incision and was not decreased by intrathecal administration of 4, 14, or 40 nmol MK-801 or 10 nmol AP5. Only the greatest dose of AP5, 30 nmol, caused a small decrease in punctate and nonpunctate hyperalgesia. Intrathecal L-NAME had no effect. Neither intrathecal MK-801 nor intrathecal AP5 affected nonevoked pain behaviors. The greatest doses caused motor deficits. Unlike intrathecal and systemic morphine, intrathecal NMDA receptor antagonists did not modify pain behaviors in this rat model of postoperative pain. These data suggest that NMDA receptors do not play an important role in the maintenance of postoperative pain behaviors and that NMDA receptor antagonists, administered spinally by themselves during the postoperative period, will not be useful for the treatment of postoperative pain in humans.