Presynaptic compensation sustains neuromuscular transmission in the mouse diaphragm at a late stage of aging

The diaphragm is the primary respiratory muscle. Like other skeletal muscles, it is subject to age-related changes, but it may be more resistant to sarcopenia due to compensatory mechanisms operating at the neuromuscular junction (NMJ). Here, we examined key parameters of neuromuscular transmission in the diaphragm of 24- and 3-month-old mice. In aged NMJs, a decrease in the amplitude of evoked postsynaptic responses in combination with a much greater reduction in miniature end-plate amplitude occurred. This was accompanied by an increase in spontaneous exocytosis intensity, presynaptic action potential propagation velocity, the quantal content of end-plate potentials upon single stimuli, and the asynchronous component of the evoked release. Moreover, aged NMJs exhibited increased exocytosis of acetylcholine quanta during short-term stimulus trains at 10-70 Hz and especially prolonged intermittent nerve stimulation at 20 Hz. The facilitation of neurotransmitter release between bursts (a sign of synaptic vesicle mobilization) and the estimated size of the readily releasable pool both increased with age. This correlated with a decrease in vesicle clustering and immunolabeling of synapsin 1, a protein restraining vesicles in the nonactive pool. Monitoring the exocytotic release of FM1-43 dye from synaptic vesicles suggested an increase in vesicle recycling rate during 20-Hz activity in aged NMJ. Thus, enhanced neurotransmitter release, probably due to increases in readily releasable pool size and synaptic vesicle mobilization, may partially compensate for the decrease in the postsynaptic membrane response to a single acetylcholine quantum in aged diaphragm NMJs. Such adaptations are likely necessary to preserve diaphragmatic function with aging.