Intravenous Midazolam Suppresses Noxiously Evoked Activity of Spinal Wide Dynamic Range Neurons in Cats

The effects of intravenously (IV) administered midazolam on noxiously evoked activity of spinal wide dynamic range (WDR) neurons were investigated in decerebrate, spinal-cord-transected cats. Extracellular, single-unit recordings were measured during stimulation by pinching the receptive field on the hind paw and the effect of midazolam at doses of 0.25, 0.5, 1, 2, and 4 mg/kg were measured. Two series of experiments were performed to characterize the analgesic effects of midazolam. In the first, dose-response experiments (n = 59) demonstrated a dose-dependent suppression of the noxiously evoked activity of spinal WDR neurons after midazolam administration. This effect of midazolam was maximal at a dose of 1 mg/kg IV. The second series of experiments (n = 14) demonstrated that a benzodiazepine antagonist, flumazenil (n = 8), promptly reversed the effect of midazolam, while an opioid antagonist, naloxone (n = 6), had no effect on the effect of midazolam. The present study demonstrates that IV administered midazolam suppresses noxiously evoked activity of spinal WDR neurons that is reversible by a benzodiazepine antagonist. This is consistent with an analgesic action of midazolam. (Anesth Analg 1995;80:58-63)