Can repeated exposure to morphine change the spinal analgesic effects of lidocaine in rats?*

BACKGROUND Chronic opium exposure leads to altered response to opioid compounds. The aim of this study was to assess the behavioral effects of opium tolerance on the analgesic effects of intrathecal lidocaine in rats. METHODS Twenty-four adult male Sprague Dawley rats with intrathecal (IT) catheters were divided into 3 groups of 8. The first group was morphine tolerant and received IT lidocaine (ML). Rats in the second group were not morphine tolerant and received IT lidocaine (L), while the third group consisted of not morphine tolerant rats that received IT placebo. Tail flick test was done and maximal possible antinociceptive effects (MPAE) were compared using analysis of variance (ANOVA). RESULTS While percent of MPAE significantly increased in the L group, it had a significant reduction in the ML group (P < 0.001). CONCLUSIONS After intrathecal lidocaine administration, a hyperalgesic response was seen in morphine tolerant rats and an analgesic response was seen in the lidocaine group.