Antinociceptive effect of the S(+)-enantiomer of ketamine on carrageenan hyperalgesia after intrathecal administration in rats

Ketamine exerts antinociceptive effects in many pain tests. We investigated the antinociceptive effect of intrathecally administered racemic ketamine and its S(+)- and R(-)-enantiomer on carrageenan-induced thermal hyperalgesia with a paw withdrawal test and acute pain (hot plate and tailflick) tests. Rats were prepared with a chronic lumbar intrathecal catheter to receive either saline or ketamine enantiomers in cumulative doses. None of the ketamines (10, 50, or 100 mu g) had any effect on the withdrawal latency of the contralateral, noninjected paw. In the injected paw, intrathecal saline did not alter carrageenan-induced thermal hyperalgesia, whereas intrathecally applied S(+)-, R(-)-, and racemic ketamine decreased thermal hyperalgesia. However, compared with saline, racemic ketamine had a higher efficacy than S(+)-ketamine, whereas R(-)-ketamine did not achieve statistical significance. Neither S(+)- nor R(-)-ketamine had a significant effect in the tailflick test (10, 100, or 500 mu g). In the hot plate test, only the largest dose of ketamine (500 mu g) caused a non-stereospecific, significant increase in hot plate latency; this dose caused supraspinal effects as well. The results demonstrate that the behavioral hyperalgesia associated with carrageenan-induced hindpaw inflammation in rats is attenuated by the intrathecal administration of racemic and S(+)-ketamine, but not R(-)-ketamine, which only displayed an insignificant trend toward a dose-response relationship. This finding warrants further studies to investigate a possible clinical advantage of preservative-free S(+)-ketamine over the currently used preservative containing racemic mixture. Implications: In rats, intrathecal S(+)-ketamine was effective for treating inflammatory pain. Although racemic ketamine has a greater efficacy, S(+)-ketamine is available as a preservative-free drug and might be of clinical interest for future neuraxial administration in different pain states.