Apparent acetylcholine receptor channel conversion at individual rat soleus end-plates in vitro

Miniature end-plate currents (m.e.p.c.s.) were recorded extracellularly from individual fibres in neonatal rat soleus muscles for 2-24 h. In agreement with previous studies, the decay phases of m.e.p.c.s at many end-plates were doubly exponential with time constants of approximately 6 ms and approximately 1.5 ms at 21 degrees C. Earlier studies have shown that doubly exponential decays are due to the combined action of embryonic-type acetylcholine (ACh) receptors (AChRs) with long channel open times and adult-type AChRs with brief open times. When individual end-plates with doubly exponential m.e.p.c.s were studied for several hours or more, the relative size of the slow decay component frequently decreased with time. There was no evidence for a corresponding decrease in total m.e.p.c. amplitude. The time constants of the fast and slow components did not change. M.e.p.c. decays were stable at end-plates that were either very mature (small slow decay component) or very immature (small fast decay component). In these cases, the decay phases were virtually singly exponential and the time constant did not change. These data indicate that at end-plates with a mixture of adult-type and embryonic-type channels, the fraction of adult-type AChRs increases with time. This is similar to what occurs at end-plates developing in vivo. The results of ACh noise analysis experiments support this interpretation.