The effects of nerve section on the non-quantal release of ACh from the motor nerve terminal.

ABSTRACT

The spontaneous release of acetylcholine (ACh) from motor nerve terminals is now thought to occur by two mechanisms (a) quantal release, giving rise to miniature endplate potentials; and (b) non-quantal release. In this study we have examined the effect of nerve section on spontaneous non-quantal ACh release, and have compared the time-course of cessation of non-quantal and quantal ACh release. Non-quantal ACh release, measured by an electrophysiological technique, declined 4 h after nerve section to approximately 50% of the control value. At 8-10 h it briefly rose again, then gradually declined to undetectable levels. Spontaneous quantal release (frequency of miniature endplate potentials) in the same muscle fibers remained close to control levels for 8 h after nerve section, and also increased prior to failure. Decline of non-quantal ACh release appears to be the earliest change in the nerve terminal following nerve transection; it may therefore be relevant in understanding the effects of denervation on the consequent changes in muscle properties.