Choline acetyltransferase in transected nerves, denervated muscles and Schwann cells of the frog: correlation of biochemical electron microscopical and electrophysiological observations.

The activity of choline acetyltransferase was increased in the peripheral nerve stumps of transected sciatic nerves of frogs (Rana esculenta) 5–21 days after the transection but had fallen markedly by the beginning of the second month. The increase of choline acetyltransferase activity began at a time when large myelinated fibres were still unaffected by degeneration according to both ultrastructural and electrophysiological criteria, whereas the small fibres were undergoing degeneration. It preceded the appearance of the miniature end-plate potentials attributable to the Schwann cells in the denervated sartorius muscle, which occurred 22–25 days after nerve section. The steep fall of choline acetyltransferase activity in the degenerating nerve coincided with the final stages of destruction of the large myelinated axons. One-fifth of control choline acetyltransferase activity was found in the degenerated nerve even 3 months after nerve section, when all axonal remnants were completely resorbed and the peripheral nerve stump was almost exclusively composed of proliferated Schwann cells and of the Büngner bands, formed by their processes. The activity of choline acetyltransferase in denervated sartorius muscle diminished, starting from the fifth day after nerve section. It did not fall below 60% of control level even 3 months after denervation, but it could not be excluded that in the muscle some of the synthesis of acetylcholine observed during assays of choline acetyltransferase activity was due to non-specific acetylcholine-synthesizing activity of carnitine acetyltransferase.The observations strongly suggest that choline acetyltransferase is present in the Schwann cells of degenerated frog nerves and that the enzyme is newly synthesized, rather than taken up from degenerating axons. The synthesis of choline acetyltransferase in degenerating nerves is not restricted to the Schwann cells in direct contact with the muscle fibres; apparently, it depends on the relationship between the Schwann cells and axons, and not solely (if at all) on the relationship between the Schwann cells and muscle fibres.