Indirect and direct suppressive actions of morphine on dorsal horn neurons in rabbits.

The analgesic mechanism of morphine on the spinal nociceptive transmission was compared in rabbits with the intact and cold-blocked states of the spinal cord. The degree of the suppressive effect of morphine (2 mg/kg) on the bradykinin-induced activity was significantly greater in the intact than in the cold-blocked states. Morphine (4 mg/kg) suppressed the nociceptive responses to similar levels in both states. These results suggest that in a small dose, the indirect suppressive action is more important than the direct action. In a larger dose, the suppressive action is probably exerted primarily by the direct spinal action.