Neurotrophic control of 16S acetylcholinesterase at the vertebrate neuromuscular junction

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Abstract
Endplate 16S acetylcholinesterase (16S-AChE) from rat anterior gracilis muscle was assessed, 6 hr to 10 days after denervation, by velocity sedimentation analysis on linear sucrose gradients. The innervating obturator nerve was transected either close (1-2 mm, short stump) or far (35-40 mm, long stump) from the muscle. In both instances, the activity of 16S-AChE gradually decreased and reached approximately the same level (10%-20% of control) by 6 days after denervation. However, enzymatic decay started considerably earlier in short stump (12-24 hr) as compared to long stump (4-5 days) preparations, i.e., the time of onset of 16S-AChE loss depended on the length of nerve that remained attached to the muscle. Whether this result extended to other AChE molecular forms (10S, 4S) in muscle endplates could not be determined because, in contrast to 16S-AChE, these forms were also detected in red blood cells (4S) and plasma (10S). Only small amounts of 16S-AChE were found in intact obturator nerves (1/100 of that in gracilis endplate regions). Thus a faster depletion of enzyme from shorter nerve stumps after axotomy could not entirely account for the substantial effect of nerve stump length on 16S-AChE. Since muscle contraction ceases immediately following nerve transection, regardless of nerve stump length, the results can be ascribed to the lack of some neural influence other than nerve-evoked muscle activity. The present findings are consistent with the view that maintenance of 16S-AChE at neuromuscular junctions primarily depends on regulatory substances which are conveyed by axonal transport and released from nerve terminals.