THERMAL HYPERALGESIA ACCELERATES AND MK-801 PREVENTS THE DEVELOPMENT OF TACHYPHYLAXIS TO RAT SCIATIC-NERVE BLOCKADE

Background: Tachyphylaxis to local anesthetics has been shown to be promoted by longer interanalgesic intervals between een injections. We hypothesized that thermal hyperalgesia also would accelerate the development of tachyphylaxis. The n-methyl-D-aspartate antagonist ((+)-5 methyl-10,11-dihydro- 5H-dibenzo (a,d) cyclohepten-5,10-imine, or dizocilpine) (MK-801) has been shown to prevent thermal hyperalgesia. We therefore also hypothesized that MK-801 would prevent tachyphylaxis. Methods: Catheters were surgically implanted in rats along the sciatic nerve. After recovery and conditioning to the testing paradigm, they received repeated injections of lidocaine or 2-chloroprocaine followed by motor block testing with or without hot-plate testing at 48, 52, or 56 degrees C. In other experiments, MK-801 or saline was administered by intraperitoneal injection before sciatic nerve local anesthetic injection and sensory and motor testing. Results: Rats receiving repeated lidocaine or 2-chloroprocaine injections, when repeatedly subjected to hot-plate testing at 56 degrees C, developed thermal hyperalgesia and tachyphylaxis to motor and sensory blockade. Rats receiving either no hotplate exposure or hot-plate exposure at 48 degrees C developed no tachyphylaxis or hyperalgesia. Rats tested at 52 degrees C developed milder hyperalgesia and developed tachyphylaxis more slowly than did rats tested at 56 degrees C. Control experiments excluded artifacts due to circadian rhythm, injection volume, and learning. Rats pretreated with MK-801 showed no tachyphylaxis over a series of three injections. Conclusions: Thermal hyperalgesia accelerates the development of tachyphylaxis to rat sciatic nerve blockade, and MK-801 prevents tachyphylaxis in this model. n-Methyl-D-aspartate receptor antagonists may have future clinical utility in increasing the duration of effectiveness of prolonged local anesthetic administration.