The Degree of Analgesia Correlates to Spinal Morphine Concentration after Intrathecal Administration in Rats

Abstract

Antinociceptive effects of intrathecal injections of morphine and pethidine were evaluated in rats using the hot‐plate and tail‐flick tests. Both drugs elicited antinociceptive effects either it was evaluated with maximal response latency (sec.) or with area under the curve (AUC) (sec. × min.). The effects of pethidine were shortlived. The concentration of morphine in the spinal cord correlated to its antinociceptive effects when tested on hot plate. A minimal morphine concentration of 3 pmol/mg in the spinal cord was required to induce experimental analgesia in the rat.