Interaction of intrathecal morphine with bupivacaine and lidocaine in the rat

The interaction in the rat between intrathecal morphine and local anesthetics (bupivacaine and lidocaine) on nociception (52.5 degrees C hot plate and paw pressure), motor function, and autonomic function (blood pressure [BP] and heart rate [HR]) was examined over a range of doses for both morphine and the local anesthetics. High doses of intrathecal bupivacaine (75 micrograms) or lidocaine (500 micrograms) produced motor block and hypotension (150 micrograms bupivacaine) lasting approximately 15 and 7 min, respectively, whereas low doses of intrathecal bupivacaine (25 micrograms) and lidocaine (100 micrograms) produced only a transient motor weakness lasting 2 min or less. Alone, neither agent altered the hot plate or paw pressure response at doses, or at times, where the agents had no effect upon motor function. In contrast, at the low dose of either local anesthetic, after the resolution of the transient motor weakness, these doses resulted in a significant leftward shift in the dose-response curves for intrathecal morphine on both the hot plate and paw pressure, as measured by the maximum observed peak effect and by the area under the time-effect curve. Thus, for example, the morphine ED50 (95% confidence intervals) for morphine/saline was 1.7 micrograms (0.7-1.9) on the hot plate and 1.1 micrograms (0.8-1.4) on the paw pressure versus for morphine/bupivacaine (25 micrograms): hot plate 0.25 micrograms (0.21-0.42) and paw pressure 0.28 micrograms (0.2-0.4). Intrathecal morphine was not observed to have any effect on the dose-dependent effects of intrathecal bupivacaine on motor or autonomic blockade. Comparable results were also observed with lidocaine (bupivacaine was found to have no significant effect on spinal cord morphine clearance). We conclude that low doses of intrathecal lidocaine and bupivacaine, which alone have no antinociceptive effect, at times when motor function was clearly unimpaired, are able to significantly augment the antinociceptive activity of intrathecal morphine on the hot plate and paw pressure tests. This prominent and selective potentiation appears to occur via a non-pharmacokinetic mechanism and probably reflects upon the interaction of low concentrations of local anesthetics with systems in the spinal dorsal horn that process acute high threshold afferent input.