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

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.