Temporal properties of direct current sensory block of the rat sciatic nerve using the C-fiber reflex

Direct current (DC) electrical block of peripheral sensory axons has potential for clinical applications in pain management. The C-fiber reflex (CFR), elicited via noxious stimulation of the foot, is suitable for investigating the activation of unmyelinated C-fiber nerves, the fiber class that is responsible for lingering pain sensations. In anesthetized rats, the CFR was elicited via electrical stimulation to the plantar surface of the hindpaw, and the resulting C-fiber-evoked electromyograms (EMG) were recorded from the ipsilateral biceps femoris muscle. A Carbon Separated Interface Nerve Electrode (C-SINE), was used to deliver DC block to arrest action potentials in the sciatic nerve. The efficacy of the block was observed as a reduction/abolition of the magnitude of the EMG in a time window corresponding to the latency of C-fibers activity. Complete cessation of nerve activity could be achieved instantaneously by applying DC at the “block threshold”. At amplitudes below the block threshold, complete block could be induced over a period of seconds to minutes, with lower currents being correlated with longer induction times. When block was applied for prolonged periods of time, block was sustained following the cessation of DC delivery. This “recovery period” was longer for longer durations of block application. The CFR is an established method to investigate pharmaceutical pain therapies, yet to date, has not been used to assess electrical block of sensory axons. Therefore, anatomical and electrophysiological methods were used to validate this method. DC nerve block shows promise for clinical pain management applications. Furthermore, the temporal properties described here could be used to reduce overall electrical current delivery and improve safety.