Synergy between the antinociceptive effects of intrathecal clonidine and systemic morphine in the rat.

These experiments tested the hypothesis that intrathecal alpha 2-adrenergic antinociception could be potentiated by the concurrent administration of systemic morphine. Thirty-four male rats, implanted with chronic indwelling intrathecal catheters, received a subcutaneous injection of either morphine sulfate or an equal volume of saline, followed by an intrathecal injection of clonidine HCl or an equal volume of vehicle. Antinociception was assessed using the tail-flick test. Tail-flick latencies following subcutaneous morphine plus intrathecal vehicle, or subcutaneous saline plus intrathecal clonidine were not significantly different from baseline. However, the combination of subcutaneous morphine plus intrathecal clonidine produced a significant antinociceptive effect. Such potentiation may prove to be a useful clinical strategy to help maximize analgesia, minimize side effects and attenuate the development of tolerance.