Neurometer measurement of current stimulus threshold in rats

We examined the current stimulus threshold in rats with the Neurometer, a d evice used clinically for measuring perception and pain thresholds. Althoug h many studies have indicated the usefulness of this device in the quantifi cation of nerve dysfunction in patients, we have found no published reports on the use of the Neurometer in animals. Transcutaneous nerve stimuli of t he three sine-wave pulses produced by the Neurometer (at 2000, 250, and 5 H z) were applied to plantar surface of rats. The intensity of each stimulati on at which rats vocalized or were hardly startled was defined as the curre nt stimulus threshold. With repeated stimulation, the thresholds were almos t constant. Repeated topical application to the area around the stimulating electrode of a high concentration of capsaicin, which acts on small-diamet er fibers, increased the thresholds at 250 and 5 Hz, but did not affect the 2000-Hz threshold. Intravenous morphine (2-5 mg/kg) increased all three th resholds, whereas intrathecal morphine (20 or 80 mug) increased only the 5- Hz threshold. Intravenous injection of a minor tranquilizer, diazepam, at 1 mg/kg raised the thresholds at 2000 and 250 Hz, but did not affect the 5-H z threshold. Higher dose of diazepam increased all three thresholds. These results suggest that the Neurometer makes possible selective examination of subsets of nerve fibers that differ in diameter not only in humans but als o in animals. The present study in rats, in which we established a method o f measurement, may provide helpful suggestions for the interpretation of da ta in humans.