Thermal block of mammalian unmyelinated C fibers by local cooling to 15-25°C after a brief heating at 45°C

Abstract

The purpose of this study was to examine the changes in cold block of unmyelinated C fibers in the tibial nerve by preconditioning with heating and to develop a safe method for thermal block of C-fiber conduction. In seven cats under a-chloralose anesthesia, C-fiber-evoked potentials elicited by electri-cal stimulation were recorded on the tibial nerve during block of axonal conduction induced by exposing a small segment (9 mm) of the nerve to cooling (from 35 degrees C to <= 5 degrees C) or heating (45 degrees C). Before heating, partial, reproducible, and reversible cold block was first detected at a threshold cold block temperature of 15 degrees C and complete cold block occurred at a temperature of <= 5 degrees C. After the nerve was heated at 45 degrees C for 5-35 min, the threshold cold block temperature significantly (P < 0.05) increased from 15 degrees C to 25 degrees C and the complete cold block temperature significantly (P < 0.05) increased from <= 5 degrees C to 15 degrees C on average. The increased cold block tempera-tures persisted for the duration of the experiments (30-100 min) while the amplitude of the C-fiber-evoked potential measured at 35 degrees C recovered significantly (P < 0.05) to similar to 80% of control. This study discovered a novel thermal method to block mammalian C fibers at an elevated temperature (15-25 degrees C), providing the opportunity to develop a thermal nerve block technology to suppress chronic pain of periph-eral origin. The interaction between heating and cooling effects on C-fiber conduction indicates a possible interaction between different temperature-sensitive channels known to be present in the mammalian C fibers. NEW & NOTEWORTHY Our study discovered that the temperature range for producing a partial to complete cold block of mammalian C-fiber axons can be increased from 5-15 degrees C to 15-25 degrees C on average after a preheating at 45 degrees C. This discovery raises many basic scientific questions about the influence of temperature on nerve conduction and block. It also raises the possibility of developing a novel implantable nerve block device to treat many chronic diseases including chronic pain.