Neonatal denervation inhibits the normal postnatal decrease in endplate channel open time.

The apparent mean channel open time (tau) of acetylcholine receptors (AChR) at skeletal muscle endplates decreases greater than 3-fold during development. In rat soleus muscles, the change occurs between postnatal days 8 and 18 as channels with long apparent open times (tau = 4.5 msec) disappear while channels with short apparent open times (tau = 1.5 msec) increase in number. We studied the role of innervation in this process by denervating neonatal soleus muscles prior to channel conversion. Tau at the denervated endplates was assayed at various times between days 8 and 18 by using fluctuation analysis. We found that early denervation blocked, or at least delayed, channel conversion. Unexpectedly, there was enhanced extrajunctional ACh sensitivity in the innervated muscles contralateral to the denervated ones. This observation allowed us to compare the apparent open times of junctional AChRs with those of extrajunctional AChRs 200 micron distant in the same innervated fibers. In developing muscles, tau at the extrajunctional sites decreased in parallel with tau at the endplates. Thus, neural regulation of AChR channel gating extends well beyond the endplate boundaries.