Fig. 2.  The differential effects of ondansetron (OND) on opioid-induced hyperalgesia (OIH) when administered via  intrathecal or intraplantar injection. Mice were used after chronic morphine treatment (see Methods). The assessment mechanical sensitivity at baseline was conducted before morphine treatment (baseline), after morphine treatment and after subsequent OND administration (arrow ). (A ) The effect of intrathecal injection of OND (1 μg) or saline in morphine-treated mice. (B ) The effect of intraplantar injection of OND (1 and 10 μg) or saline in morphine-treated mice. Data are presented as mean ± SEM; **P < 0.01; ***P < 0.001. Six mice were used in each group.