In vivo development of cholinesterase at a neuromuscular junction in the absence of motor activity in Xenopus laevis.

Embryos of Xenopus laevis were selected prior to the onset of innervation and were raised for 2 days in the anaesthetic tricaine methanesulphonate (200 micrograms/ml). The gross development of these tricaine-reared animals appeared normal despite the absence of spontaneous motor activity and the lack of motor responses to prodding with a pin. Motor activity quickly appeared when the anaesthetic was withdrawn. Intracellular recording from the myotomes of intact, tricaine-maintained animals failed to reveal any spontaneous muscle action potentials. Synaptic potentials increased in frequency and amplitude upon withdrawing tricaine, but resting potentials remained unchanged. Cholinesterase activity, detected histochemically, was observed at the ends of the myotomes, the main site of innervation. The intensity of the histochemical reaction product at these sites appeared to be about as great in the myotomes of tricaine-reared animals as in control myotomes. Miniature end-plate currents (m.e.p.c.s), examined by focal external recording, declined with a time constant of 2.9 +/- 0.2 ms (mean +/- S.E. of mean) in the myotomes of tricaine-reared animals (stages 40-41). The time constants in the myotomes of control animals were 1.8 +/- 0.1 ms at stages 40-41 and 8.7 +/- 0.7 ms at stages 24-26 (shortly after the onset of innervation). The anticholinesterase neostigmine doubled m.e.p.c. time constants in the myotomes of tricaine-reared animals as well as in control myotomes at stages 40-41. It is concluded that motor activity is not required for the in vivo development of physiological levels of synaptic cholinesterase in Xenopus myotomal muscle.