Indirect and direct depressive effects of morphine on activation of lamina V cell of the spinal dorsal horn induced by intra-arterial injection of bradykinin.

Small doses of morphine (0.3-2.0 mg/kg i.v.) inhibited, in the majority of intact rabbits, the unitary activities of the lumbar lamina V dorsal horn neurons induced by intra-arterial injection of bradykinin, a potent pain producing substance, but not in spinal rabbits. A large dose of morphine (5.0 mg/kg), however, inhibited the bradykinin-induced response even in spinal rabbits. These results suggest that morphine, in small doses, inhibits the sensory transmission of bradykinin-induced pain at the dorsal horn of the spinal cord, through its facilitatory action on the descending inhibitory mechanism; in large doses the level of dorsal horn is directly inhibited.