Effects of carbamazepine on muscle spindle discharges

Spinal and alpha-chloralose anesthetized cats were prepared for single unit recording from fibers originating in deefferented spindles of the triceps surae muscles. Carbamazepine (CBZ), 200 and 300 mg/kg, suspended in 1% methylcellulose in saline was administered i.p. and plasma concentrations were determined by gas liquid chromatography. Stretch induced by a 500 g weight drop was used to evaluate spontaneous, static stretch, and poststretch spindle activity, while stretch induced by stimulation of the antagonist muscle group was used to evaluate a phasic stretch response. Preparations were monitored from 15 minutes before to 90 minutes after drug administration. CBZ produced a concentration-dependent depression of most aspects of spindle activity at concentrations ranging from approximately 9 to 45 microgram/ml. Activity during the poststretch period exhibited the greatest sensitivity to CBZ depression, spontaneous and static stretch activity exhibited intermediate sensitivity and phasic responses were relatively insensitive. No difference between Ia and II sensory endings were observed. Spindle depression was evident at concentrations which had no effect on axonal conduction velocity. The possible influence of this peripheral activity of CBZ is discussed with regard to neurotoxicity and use in the treatment of various neuropathological states.