Analgesic effect of intrathecal ketamine in rats.

The analgesic effect of ketamine hydrochloride 200, 400, 800 and 1600 micrograms kg-1, administered intrathecally in rats, was evaluated using the tail-flick test. A short-lived analgesic effect was demonstrated with the larger doses and this could be abolished by pretreatment with naloxone hydrochloride 200 micrograms kg-1. Intrathecal morphine 100 micrograms kg-1 produced a more profound analgesic effect than intrathecal ketamine 1600 micrograms kg-1. There were three deaths shortly after intrathecal injection of ketamine. Histopathological examination of the spinal cords and nerve roots of these rats revealed the presence of vacuoles in the cells of the posterior root ganglion. Other rats sacrificed for histopathological examination showed no changes attributed to the injectate.