Effects of some centrally acting muscle relaxants on spinal root potentials: a comparative study

The effects of intravenously administered mephenesin, tolperisone, baclofen, diazepam and midazolam on reflex activity were studied in unanesthetized spinal cats. Monosynaptic, as well as polysynaptic ventral root reflexes, the dorsal root potential and the dorsal root reflex were recorded simultaneously from L6-S1 segments. An analogue integrating method was developed for quantitative monitoring and recording ipsilateral spinal root potentials evoked by stimulation of the tibial nerve. Mephenesin (12.5-50 mg/kg) caused a significant and dose-dependent reduction in the polysynaptic and the dorsal root reflexes, slightly decreased the dorsal root potential but minimally affected the monosynaptic ventral root reflex. Tolperisone (2.5-10 mg/kg) dose-dependently inhibited both ventral root reflexes and the dorsal root reflex. It slightly prolonged the dorsal root potential without affecting the amplitude. Baclofen (0.5 mg/kg) abolished the monosynaptic reflex, partially inhibited the polysynaptic reflex, while dorsal root responses were less attenuated. Both benzodiazepines exerted similar actions, both qualitatively as well as quantitatively: the polysynaptic reflex was partially reduced while the monosynaptic reflex was not modified by diazepam or midazolam. Dorsal root responses were enhanced and the half-time of decay of the dorsal root potential was prolonged. Different patterns of action of muscle relaxants studied here are discussed in terms of their possible mechanisms of action. Profound depressant effects of mephenesin and tolperisone on the dorsal root reflex are in contrast to the small effect of both drugs on the dorsal root potential and might reflect their inhibition of spike-generating mechanisms. For a yet unknown reason, various spinal pathways are affected differentially by baclofen. In spinal cats, the reduction by benzodiazepines of the polysynaptic reflex may be related to the potentiation of some unidentified GABA-ergic inhibitory processes. The use of water-soluble midazolam, as a model compound instead of diazepam, is suggested because the usual organic solvents for diazepam may affect its action.