Intrathecal Nalbuphine and Naloxone Produce Nociceptive and Motor Block in Rats.

INTRODUCTION

Naloxone blocks Na+ channels and produces cutaneous analgesia, but it is unknown whether naloxone or its analog, nalbuphine, produces spinal anesthesia. The purpose of this study was to assess the effects of intrathecal nalbuphine and naloxone on motor and nociceptive blockade.

METHODS

Sprague-Dawley rats received intrathecal injections of nalbuphine, naloxone, and the local anesthetic lidocaine (n = 6 for each group). Following intrathecal injection, motor function (extensor strength) and nociceptive responses (pinch test) were assessed. A combination of intrathecal nalbuphine and lidocaine at four fixed ratios was employed for isobolographic analysis.

RESULTS

Intrathecal injection of nalbuphine (1.6 mg) and naloxone (1.6 mg) produced motor and nociceptive blockade. The ED50 (50% effective dose) values of intrathecal nalbuphine for motor (1.50 [1.39-1.62] mg) and nociceptive (1.39 [1.28-1.50] mg) blockade were greater than those of lidocaine for motor (0.36 [0.32-0.40] mg) and nociceptive (0.31 [0.28-0.35] mg) blockade, respectively. At equianesthetic doses (ED25, ED50, and ED75), the duration of motor and nociceptive blockade was longer with intrathecal nalbuphine than with lidocaine (p<0.01). After an isobolographic analysis of the interaction between intrathecal nalbuphine and lidocaine, the experimental ED50 values did not differ significantly from the calculated additive ED50 values.

DISCUSSION: Intrathecal nalbuphine produces a dose-dependent spinal block that is weaker but longer- lasting than lidocaine, and its combination with lidocaine results in an additive spinal block effect, similar to two local anesthetics.

CONCLUSIONS

Intrathecal nalbuphine produces a dose-dependent spinal block. The spinal block induced by intrathecal nalbuphine is weaker than that of lidocaine but has a longer duration of action. The combined intrathecal injection of nalbuphine and lidocaine produces an additive spinal block effect, similar to the combination of two local anesthetics.