Changes in the properties of synaptic channels opened by acetylcholine in denervated frog muscle

Abstract

Acetylcholine (ACh)-activated channels in end-plates of frog sartorius muscle were studied at various times after denervation. Mean open times of the synaptic membrane channels were derived from the time constant of decay of miniature end-plate currents (tau MEPC) evoked by ACh quanta released from Schwann cells, which replace the motor nerve terminals after these degenerate. Membrane current noise, elicited by iontophoretic application of ACh to voltage-clamped end-plates, was also used to determine mean open time (tau noise) and conductance of the ion channels. About 1 week after denervation, soon after Schwann cell MEPCs appeared, they had a tau similar to that of the neural MEPC in innervated end-plates. However, 5-6 weeks after denervation tau MEPC was increased by a factor of about 5. Circa 4 weeks after denervation, cholinesterase activity of the denervated muscle decreased to 76% of that in the contralateral, innervated muscle, and even 4 months after the operation it was still 64%. Thus, it is unlikely that a change in acetylcholinesterase activity is the main factor responsible for the increase in tau of Schwann cell MEPC. About 1 week after denervation tau noise was close to that in innervated end-plates (about 2 ms). Twelve to 24 days after denervation the average channel open time was 4.5 +/- 1.0 ms, with some end-plates still showing normal 'fast' channels. However, in muscles denervated for 47-113 days the open time was 12.9 +/- 1.9 ms. In the early and intermediate periods, ACh-induced noise spectra with two components were obtained from many end-plates, indicating the simultaneous activation of two different types of channels. At some end-plates during the early and intermediate periods after denervation, but not after about 5 weeks, neostigmine caused the appearance of a component, which was as fast as that of normal end-plate channels. In other experiments small doses of alpha-bungarotoxin were applied in order to predominantly block extra-junctional receptors. In the early period of denervation, when two components were present in the noise spectra, alpha-bungarotoxin eliminated the slow component leaving channels as fast as in innervated end-plates. After prolonged denervation, a component with tau of about 5.5 ms was occasionally disclosed by application of alpha-bungarotoxin. tau noise and tau MEPC from the same end-plate closely agreed. Our results indicate that at frog end-plates the open time of the majority of the synaptic channels opened by ACh becomes longer with increasing time after denervation.