Depression of nociceptive sympathetic reflexes by the intrathecal administration of midazolam.

The effect of the intrathecal administration of midazolam 0.5-1.0 mg in 1-2 ml of physiological saline solution, has been observed on responses evoked in renal sympathetic nerves by supramaximal electrical stimulation of radial and tibial nerves. In artificially ventilated dogs anaesthetized with alpha-chloralose, the intrathecal administration of midazolam caused a marked depression of reflexes evoked from the tibial nerve but had no effect on either spontaneous sympathetic activity or reflexes evoked by radial nerve stimulation. Neither the small amount (1-2 microliters) of benzyl alcohol, present as a preservative (administered intrathecally), nor midazolam 1 mg kg-1 i.v. caused any significant depression of the evoked somato--sympathetic reflexes. The effects of intrathecal midazolam were reversed by the benzodiazepine antagonists Ro 15-1788 1 mg kg-1 i.v. and Ro 15-3505 1-2 mg kg-1 i.v. but not by naloxone 2 mg i.v. It is suggested that the antinociceptive effect of locally applied midazolam could be the result of a non-opioid GABA-mediated system which may have implications in the management of pain.