Potentiation of morphine analgesia by subanesthetic doses of pentobarbital.

Pentobarbital pretreatment reportedly either inhibits, enhances or has no effect on morphine analgesia. The effect of subanesthetic doses of sodium pentobarbital (8-12 mg kg-1, SC) delivered via a delivery system on analgesia of morphine (5 mg kg-1, SC or 1 mg kg-1, IV) acutely administered 45 min after the sodium pentobarbital pellet implantation was assessed using the warm water (55 degrees C)-induced tail-withdrawal reflex in male Wistar rats. Significant potentiation of morphine analgesia was observed in sodium pentobarbital as compared to the placebo-pelleted animals. Pharmacokinetic or dispositional factors were not involved in this potentiation, which was possibly due to the activation of the descending inhibitory control pathways of nociceptive spinal tail-withdrawal reflex by a combined interaction of two drugs at spinal and supraspinal sites of action, that mediate opiate antinociception.