Ageing‐induced weakness of mouse NMJs is associated with reduced active zone density, synaptic event kinetics and presynaptic calcium entry

Ageing has been shown to affect both the structure and function of the neuromuscular junction (NMJ). In our previous study, we documented a biphasic change (first an increase followed by a decrease) in neurotransmission over the ageing time course at male mouse NMJs. Here, we explored several potential mechanisms behind the reduction in presynaptic neurotransmitter release in the later stages of ageing. We found that the active zone (AZ) density significantly decreased in NMJs at 30 months, compared to 4- and 26-month-old mice. Furthermore, the decreased end plate potential (EPP) amplitude in these 30-month-old mice was associated with a significantly longer rise and decay time, and, although miniature end plate potential (mEPPs) amplitude and frequency were unchanged, these events also had a longer rise and decay time. Thirty-month-old NMJs also showed a significant reduction in the vesicle replenishment rate (VRR). Additionally, 30-33-month-old NMJs had reduced presynaptic calcium entry following a single presynaptic action potential. Taken together, these transmitter release site changes may explain age-induced reductions in neuromuscular neurotransmission in aged mice and may also lead to the identification of novel therapeutic targets or potential biomarkers for future research in this area. KEY POINTS: Neuromuscular junctions (NMJs) from 30-month-old mice, in comparison with 4- and 26-month-old mice, showed altered neurotransmitter release properties; namely, decreased quantal content, decreased end plate potential (EPP) amplitude with prolonged rise and decay time, and prolonged miniature end plate potential (mEPP) rise and decay time. The density of transmitter release sites (active zones) was reduced in NMJs from 30-month-old mice compared to 4- and 26-month-old NMJs. NMJs from 30-month-old mice showed no change in short-term synaptic plasticity compared to 4-month-old mice, whereas NMJs from 26-month-old mice showed significant synaptic facilitation, and this was restricted to the weakest synapses in this age group. NMJs from 30-month-old mice showed no change in readily releasable pool (RRP), but a slower vesicle replenishment rate (VRR) compared to 4-month-old NMJs. NMJs from 30-33-month-old mice showed a reduction in presynaptic calcium entry following a single action potential stimulus.