Antinociceptive effect of intrathecal butorphanol through opioid receptors: Compared to morphine

This study compared the antinociceptive effect of intrathecal (i.t.) butorphanol, a potent mixed agonist/antagonist opioid analgesic, to i.t. morphine as measured by the tail-flick test. Pretreatment with i.t. naloxone (10.4 nmol/10μl/rat) or saline (10μl/rat) 10 min before a single i.t. injection of butorphanol 26, 52, or 104 nmol/10μl/rat or morphine 26 nmol/10μl/rat was performed in 6-8 male Sprague-Dawley rats per group. In butorphanol groups, the antinociception showed a dose-dependent manner as evidenced by increases in tail-flick latencies. The duration and the peak of antinociceptive effect of i.t. butorphanol, with either the same or higher doses, were less than those of i.t. morphine. Pretreatment with naloxone significantly attenuated the peak effects and overall antinociceptive responses of morphine more than those of butorphanol. These results demonstrate that i.t. butorphanol elicits spinal analgesic effect against thermal stimuli through opioid receptors and is less potent than i.t. morphine.