A post-natal decrease in acetylcholine channel open time at rat end-plates.

1. The mean channel open time (tau) of ACh receptors in adult developing rat skeletal muscle was estimated by analysis of ACh current fluctuations. 2. At adult end-plates, tau was 1.4 msec at 21 degrees C. Time constants of miniature end-plate current decays (taus) were equally brief. 3. On the other hand, tau and taus were long (4.5 msec, 21 degrees C) at neonatal rat end-plates. These estimates are comparable to values obtained at extrajunctional sites on embryonic rat myotubes grown in culture and on adult denervated muscle fibres. 4. Both tau and taus decreased to values characteristic of adult end-plates during the first 2 weeks after birth. 5. Complex ACh power spectra were obtained at most end-plates during the transition period and at some, miniature synaptic current decays were doubly exponential. End-plates in transition might contain a mixture of slow, neonatal channels and fast, adult channels.