Abstract
Aging disrupts multiple homeostatic processes, including autophagy, a cellular process for the recycling and degradation of defective cytoplasmic structures. Acute treatment with the autophagy inhibitor chloroquine blunts the maximal forces generated by the diaphragm muscle, but the mechanisms underlying neuromuscular dysfunction in old age remain poorly understood. We hypothesized that chloroquine treatment increases the presynaptic retention of the styryl dye FM 4-64 following high-frequency nerve stimulation, consistent with the accumulation of unprocessed bulk endosomes. Diaphragm-phrenic nerve preparations from 24-month-old male and female C57BL/6 × 129 J mice were incubated with FM 4-64 (5 µM) and either chloroquine (50 µM) or vehicle during 80 Hz phrenic nerve stimulation. Acute chloroquine treatment significantly decreased FM 4-64 intensity at diaphragm neuromuscular junctions following 80 Hz phrenic nerve stimulation, consistent with disrupted synaptic vesicle recycling. A similar reduction was evident in regions with the greatest FM 4-64 fluorescence intensity, which most likely surround synaptic vesicle release sites. In the absence of nerve stimulation, chloroquine treatment significantly increased FM 4-64 intensity at diaphragm neuromuscular junctions. These findings highlight the importance of autophagy in regulating presynaptic vesicle retrieval (including vesicle recycling and endosomal processing) and support the role of autophagy impairments in age-related neuromuscular dysfunction.