Tonic endogenous opioid inhibition of visceral noxious information in rabbits.

BACKGROUND & AIMS: A tonic intrinsic spinal inhibitory system on spinal motor reflexes in rabbits has been shown earlier. The aim of this study was to examine the effects of different opioid antagonists against visceral noxious stimulation in awake rabbits.

METHODS

The opioid receptor antagonists examined were naloxone (nonselective), MR2266 (kappa), and naltrindole (delta). The effects on the visceromotor response thresholds induced by colorectal distention in rabbits were determined after intrathecal and intramuscular administration of the antagonists.

RESULTS

Intrathecal naloxone resulted in a dose-dependent decrease of visceromotor response thresholds. The selective antagonists MR2266 and naltrindole had no significant effects. In the presence of MR2266, intrathecal naloxone reduced thresholds to the same degree as when given alone. Analysis of the data from all rabbits showed a statistically significant reduction in visceromotor response thresholds after intrathecal naloxone compared with intramuscular administration.

CONCLUSIONS

In rabbits, tonic active intrinsic spinal and supraspinal endogenous opioids modulate visceral noxious information. This inhibition is exerted at the mu opioid receptor.