Expression of growth-associated proteins (GAPs) in injured sensory axons of frogs acclimatized to 15 °C and 25 °C.

Labelled proteins conveyed by fast axonal transport into the sensory axons of frog sciatic nerve following axotomy have been studied by 2D gel electrophoresis. Previous work showed that in frogs acclimatized to 25 °C a cell body reaction occurs, along with regeneration of axons to their targets. In contrast, frogs acclimatized to 15 °C showed no cell body reaction and though regeneration began, it stalled after approximately 35 days. We found that axotomy at 25 °C was followed by an increase in transport of specific labelled proteins corresponding to growth-associated proteins (GAPs) identified in other regenerating systems. Surprisingly, axotomy at 15 °C also induced a similar increase, though with a slower onset, so that the highest levels of expression of GAPs occurred during the time when the axons had stalled. We conclude that sensory neurons in 15 °C frogs do detect that their axons have been injured, as shown by their ability to increase GAP synthesis. Slow and limited axonal growth is possible during a period when GAP synthesis is low compared to levels in rapidly regenerating nerves, but even when the ability to produce GAPs increases, this alone is not sufficient to sustain regeneration in the absence of other components of the cell body reaction to injury.