Inhibition of acetylcholinesterase accelerates axon terminal withdrawal at the developing rat neuromuscular junction

Abstract

In developing skeletal muscles, the rate at which superfluous innervation is lost from the endplates depends on the general level of neuromuscular activity. Whether it is activity of the presynaptic or postsynaptic structures (or both) that is critical is not well established. In this work, we transitorily inhibited the AChE of soleus muscle in postnatal rats, in order to increase postsynaptic activity, without directly altering activity of the nerve terminals. We then followed the time course of disappearance of axon terminals from the endplates of treated and normal muscles, using electron-microscope techniques. Three hours after inhibition of AChE, the muscle fibres exhibited local supercontracture and ultrastructural damage in the region of the endplate, consistent with local elevation of Ca²⁺ levels. At the same time, small electron-opaque vesicles, apparently of muscular origin, appeared in the synaptic cleft. The nerve terminals, however, were entirely normal in number and appearance. One day after treatment, endplates of esteraseinhibited muscles showed accelerated loss of nerve terminals, compared to endplates of normally developing muscles. No further loss of nerve terminals occurred, once AChE activity returned at the endplate. These results suggest that the rate at which superfluous nerve terminals retract from the developing neuromuscular junction is regulated by the level of activation of the muscle. It seems likely that activity of postsynaptic sites may similarly regulate changes in innervation patterns, in other developing or adapting neuro-neuronal or neuro-effector systems.