Electrophysiological and ultrastructural correlates of cryoinjury in sciatic nerve of the freeze-tolerant wood frog, Rana sylvatica

We investigated function and ultrastructure of sciatic nerves isolated from wood frogs (Rana sylvatica) endemic to the Northwest Territories, Canada, following freezing at -2.5 degrees C, -5.0 degrees C, or -7.5 degrees C. All frogs frozen at -2.5 degrees C, and most frogs (71%) frozen at -5.0 degrees C, recovered within 14 h after thawing began; however, frogs did not survive exposure to -7.5 degrees C. Sciatic nerves isolated from frogs frozen at -7.5 degrees C were refractory to electrical stimulation, whereas those obtained from frogs surviving exposure to -2.5 degrees C or -5.0 degrees C generally exhibited normal characteristics of compound action potentials. Frogs responded to freezing by mobilizing hepatic glycogen reserves to synthesize the cryoprotectant glucose, which increased 20-fold in the liver and 40-fold in the blood. Ultrastructural analyses of nerves harvested from frogs in each treatment group revealed that freezing at -2.5 degrees C or -5.0 degrees C had little or no effect on tissue and cellular organization, but that (lethal) exposure to -7.5 degrees C resulted in marked shrinkage of the axon, degeneration of mitochondria within the axoplasm, and extensive delamination of myelin sheaths of the surrounding Schwann cells.