Reflex sympathetic activity after intravenous administration of midazolam in anesthetized cats.

BACKGROUND

Although intrathecal midazolam has been reported to produce antinociceptive effects mediated by gamma-aminobutyric acid type A-benzodiazepine receptor complexes in the spinal cord, the effects of systemic midazolam on nociception remain unclear. We performed this study to examine the effects of IV-administered midazolam on somatosympathetic Adelta and C reflex discharges in brain-intact cats and decerebrate cats (with transection at midbrain level).

METHODS

Somatosympathetic Adelta and C reflexes were elicited in the inferior cardiac sympathetic nerve by electrical stimulation of myelinated (Adelta) and unmyelinated (C) afferent fibers of the superficial peroneal nerve in 28 mature cats. After control somatosympathetic reflex responses were obtained, midazolam was administered IV to four groups of randomly allocated cats as follows: brain-intact cats at a dose of 0.03 mg/kg, brain-intact cats at a dose of 0.1 mg/kg, brain-intact cats at a dose of 0.5 mg/kg, and decerebrate cats at a dose of 0.1 mg/kg.

RESULTS

C reflex discharges were significantly augmented at the dose of 0.03 mg/kg and significantly depressed at the dose of 0.1 and 0.5 mg/kg in brain-intact cats. C reflex discharges were also significantly depressed at the dose of 0.1 mg/kg in decerebrate cats.

CONCLUSIONS

We have demonstrated that IV midazolam produces dose-related effects on somatosympathetic reflex discharges. The clinical implication of these findings is that the effect of midazolam on nociception depends on its dosage. It also appears that the infra-midbrain region plays a major role in mediating the depressive effects of midazolam on somatosympathetic C reflex discharges.