Alpha-bungarotoxin binding to the myotome and choline acetyltransferase activity in the rabbit embryo

We have previously found incomplete sarcomeres and acetylcholinesterase activity in the myoblasts of the myotome of the rabbit at day 13 of gestation. We now report that an acetylcholine (ACh)-synthesizing enzyme and the nicotinic receptor are present at this stage as well. A study of the myotome using [125I]alpha-bungarotoxin shows that the mononucleated myoblasts have alpha-bungarotoxin binding sites before they migrate away to form multinucleated myotubes. Choline acetylcholinesterase activity and/or a different ACh-synthesizing enzyme are found at early stages of development, even before the spinal nerve has formed. An ACh-synthesizing enzyme is present in the notochord, a neural tube-dorsal root ganglion preparation, as well as in rows of myotomes separated from the latter preparation. Assays of isolated myotomes with very little adherent mesenchyme indicate that the enzyme is located either within the myotome or in its immediate vicinity. Cholinergic components, therefore, are associated with the mononucleated myoblasts of the myotome before they fuse to form myotubes and before they receive their permanent innervation.