Neurotrophic control of channel properties at neuromuscular synapses of rat muscle

Abstract

Ectopic neuromuscular synapses formed with the fibular nerve was implanted into the proximal part of rat soleus muscle and the soleus nerve was cut. The gating properties of acetylcholine (ACh) receptor channels in the newly formed ectopic and in the denervated original end-plates were examined at various stages of ectopic synapse formation. At ectopic end-plates the apparent mean open time of ACh receptor channels changing during synaptic development. Channels in immature ectopic end-plates, examined 1 wk after cutting the soleus nerve, have apparent mean open times of .apprx. 4 ms (at -70 mV, 22.degree. C), similar to those of the extrasynaptic ACh receptor channels of completely denervated fibers. The channel gating of mature ectopic end-plates, examined 3-7 wk after nerve section, is characterized by a mean open time of .apprx. 1 ms and resembles that found in normal end-plates of adult fibers. The conversion of end-plate channel gating occurs during the 2nd and 3rd wk of synapse formation. During this period, 2 discrete classes of channels with different gating behavior are present in the ectopic end-plate. Examination of ectopic end-plates in the EM reveals that junctional folds begin to appear in the subsynaptic membrane during the period of channel conversion. At the denervated original end-plates of ectopically innervated fibers the apparent mean open time of ACh receptor channels remains similar to that of the normally innervated end-plates. Original end-plates retain the normal synaptic class of channel for at least 42 days after denervation. At this time, most of the ACh receptors present originally in the membrane were replaced by newly inserted receptors. At former end-plates of completely denervated fibers ACh activates 2 classes of channels, even when most of the ACh receptors originally present in the end-plate were replaced by new receptors. During synapse formation a neurally controlled conversion of ACh receptor channels occurs about 2-3 wk after establishment of the nerve muscle contact. End-plate channel properties are independent of neural influences. These observations are consistent with a mechanism of channel conversion whereby the nerve modifies not the ACh receptor channel itself, but another constituent of the end-plate membrane which determines the gating properties of end-plate channels.