Synaptic vesicle pools at diaphragm neuromuscular junctions vary with motoneuron soma, not axon terminal, inactivity

Both spinal hemisection (SH) at C₂ and tetrodotoxin (TTX) phrenic nerve blockade result in diaphragm muscle paralysis and inactivity of the phrenic axon terminals. However, phrenic motoneuron somata are inactive with SH but remain active with TTX phrenic nerve blockade. Neuromuscular transmission failure with repeated activation decreases following SH and increases following TTX phrenic nerve blockade, suggesting that matching (or mismatching) of somal and synaptic inactivities of phrenic motoneurons differentially regulates synaptic vesicle pools at diaphragm neuromuscular junctions. At individual type-identified rat diaphragm presynaptic terminals, the size of the releasable pool of synaptic vesicles was analyzed by fluorescence confocal microscopy of N-(3-triethylammoniumpropyl)-4-(6-(4-(diethylamino)phenyl)hexatrienyl) pyridinium dibromide (FM4-64) uptake and synaptic vesicle density at active zones was determined using transmission electron microscopy. After 14 days of SH and TTX-induced diaphragm muscle inactivity, neuromuscular junction size was not different at type I or IIa fibers, but increased at type IIx and/or IIb fibers (by 51% in SH and 35% in TTX) compared with control. With SH, synaptic vesicle pool size and density increased at presynaptic terminals innervating type I or IIa fibers (17 and 63%, respectively; P<0.001) and type IIx and/or IIb fibers (41 and 31%, respectively; P<0.001) when compared with controls. Following TTX, synaptic vesicle pool size and density decreased by 64 and 17%, respectively, at presynaptic terminals innervating type I or IIa fibers, and by 50 and 36%, respectively, at type IIx and/or IIb fibers (P<0.001, for all comparisons). Thus, matching motoneuron soma and axon terminal inactivity (SH) increases the size and density of releasable synaptic vesicle pools at adult rat diaphragm neuromuscular junctions. Mismatching motoneuron soma and axon terminal inactivities (TTX) results in converse presynaptic adaptations. Inactivity-induced neuromuscular plasticity reflects specific adaptations in the size and density of synaptic vesicle pools that depend on motoneuron soma rather than axon terminal (or muscle fiber) inactivity.