Reversible neural inactivation by cooling in anesthetized and freely behaving rats

The design and use of a miniaturized cryoprobe to allow reversible neural inactivation were investigated in the rat. Tissue temperature measurements around the cryoprobe in the cerebellum of anesthetized rats established that significant heat loss (lower than 20 degrees C) was localized within 1.5 mm of the cryoprobe tip. To physiologically test the effectiveness of the cryoprobe, two specific behaviors mediated by well defined neural structures were measured; electrically elicited hindleg flexion in ketamine-anesthetized rats, mediated by the pyramidal tract; and the acoustic startle reflex in freely behaving rats, mediated by the ventral nucleus of the lateral lemniscus. Cooling of the pyramidal tract at a tip temperature of -5 degrees C reversibly blocked electrically elicited hindleg flexions within about 21 s. Cooling of the ventral nucleus of the lateral lemniscus at 0 degrees C reversibly blocked the acoustic startle reflex. These effects were obtained when the cryoprobe tip was within 0.5 mm of the respective neural targets. The results suggest that cooling produces fully reversible neural blockade with a very rapid onset and offset. The advantages and limitations of cooling in the rat are discussed.