Axotomy-induced changes of the molecular forms of acetylcholinesterase in the facial nucleus of the rat and guinea pig were investigated. Evidence is presented that facial motoneurons of the guinea pig are capable of synthesizing considerable amounts of 16S acetylcholinesterase, and furthermore that acetylcholinesterase isoenzymes show species differences in their response to axon transection. Three isoenzymes could be separated by velocity sedimentation, which correspond to G1 (4S), G4 (10S) and A12 (16S) acetylcholinesterase. After axotomy, G4 activity was decreased in both species by 40% 2-3 weeks after nerve transection. In the rat, G1 was even further depressed, whereas in guinea pig facial nucleus G1 showed only a slight change. A12 displayed a clear species difference: in the rat, it was decreased to 60% of control 5 days after axotomy. In guinea pig, however, A12 increased dramatically to values of 400-500% of the unoperated control, and maintained elevated levels even 120 days after operation. This result does not agree with the decrease of transmitter metabolism in regenerating nerves and provides support to the hypothesis that acetylcholinesterase in regenerating nerves may have functions different from transmitter hydrolysis.
