Antinociceptive actions of morphine and buprenorphine given intrathecally in the conscious rat.

1 The antinociceptive effects of morphine and buprenorphine given intrathecally and subcutaneously have been compared in the conscious rat. 2 In the paw pressure test, when given subcutaneously buprenorphine 0.001-0.1 mg/kg s.c., was approximately 100 times more potent than morphine 0.1-3 mg/kg s.c., but in the hot plate test, buprenorphine 0.03-3.0 mg/kg s.c., produced a bell-shaped dose-response curve of low maximum effect and was about equipotent with morphine 0.03-3 mg/kg s.c. 3 When given intrathecally buprenorphine 10 micrograms and morphine, 10-60 micrograms, were approximately equipotent in both paw pressure and hot plate tests. Furthermore, morphine produced these effects at 1/25th of the minimum effective parenteral dose while the dose of buprenorphine exceeded the parenteral dose. 4 It is concluded that the predominant site of the analgesic action of buprenorphine is supraspinal. The significance of these findings in relation to the role of spinal opiate receptors is discussed.