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

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. 24 adult male Sprague Dawley rats divided into 3 groups (8 in each); all with an intrathecal catheter. The first group was morphine tolerated; then received IT lidocaine (ML); the 2nd group not morphine tolerated; received IT lidocaine (L); the 3rd group, not morphine tolerated, received IT placebo. Tail flick test was done and maximal possible antinociceptive effect (MPAE) was compared. A significant increase in the L group and decrease in the ML group for MPAE results (p value for ANOVA=0.000). After intrathecal lidocaine administration a hyperalgesic response is seen in the morphine tolerated rats and an analgesic response is seen in the lidocaine group.