Non‐quantal acetylcholine release in the mouse diaphragm after phrenic nerve crush and during recovery

The progressive decline and recovery of spontaneous quantal acetylcholine (ACh) release (miniature endplate potentials, MEPPs) and the H‐effect were measured in the mouse diaphragm after nerve crush and during regeneration. The H‐effect is the hyperpolarization of the muscle fibre membrane produced by the addition of (+)tubocurarine, which indicates non‐quantal ACh release. One hour after nerve crush the H‐effect had declined to 50% of control values and 4 h later the H‐effect disappeared completely. There were no substantial changes in the MEPP frequency and amplitude during the first 4 h after denervation. MEPP frequency then increased, but after 6 h of denervation it decreased and after 16 h no MEPPs were found in any of the muscle fibres. The times of onset of these denervation changes in the proximal, central and distal parts of diaphragm were similar. During reinnervation, the H‐effect was detectable in all muscle parts 3 days before the reappearance of MEPPs. The H‐effect developed first on day 8 in the proximal endplates and then, with a delay of 3 and 6 days, in the central and distal areas, respectively. During axonal regrowth the non‐quantal release was restored before detectable quantal release. Non‐quantal release is the first indication of the ability of the nerve terminal to release ACh in the process of reinnervation.