Spinal antinociceptive action of Na+-K+ pump inhibitor ouabain and its interaction with morphine and lidocaine in rats.

BACKGROUND The Na+,K+-adenosine triphosphatase is a ubiquitous enzyme system that maintains the ion gradient across the plasma membrane of a variety of cell types, including cells in the central nervous system. We investigated the antinociceptive effect of intrathecally administered ouabain and examined its potential interaction with spinal morphine and lidocaine. METHODS Using rats chronically implanted with lumbar intrathecal catheters, the ability of intrathecally administered ouabain, morphine, and lidocaine and of mixtures of ouabain-morphine and ouabain-lidocaine to alter tail-flick latency was examined. To characterize any interactions, isobolographic analysis was performed. The effects of pretreatment with intrathecally administered atropine or naloxone also were tested. RESULTS Intrathecally administered ouabain (0.1-5.0 microg), morphine (0.2-10.0 microg), and lidocaine (25-300 microg) given alone produced significant dose- and time-dependent antinociception, but systemic administration of ouabain did not produce such an effect. The median effective dose (ED50) values for intrathecally administered ouabain, morphine, and lidocaine were 2.3, 5.0, and 227.0 microg, respectively. Isobolographic analysis exhibited a synergistic interaction after the coadministration of ouabain and morphine. With ouabain and lidocaine, there was no such evidence of synergism. Intrathecally administered atropine, but not naloxone, completely blocked the antinociceptive effect of ouabain and attenuated its interaction with spinally administered morphine. CONCLUSIONS Intrathecally administered ouabain produces antinociception, at least in part, via an enhancement of cholinergic transmission in the spinal nociceptive processing system. The results of the interaction of ouabain with morphine and lidocaine suggest that modulation of Na+-,K+-electrochemical gradients and thus subsequent release of neurotransmitters in the spinal cord are likely to play important roles in the spinal antinociceptive effect of intrathecally administered ouabain.