Electrophysiological effects of BDNF and TrkB signaling at type-identified diaphragm neuromuscular junctions

Previous studies show that synaptic quantal release decreases during repetitive stimulation, i.e., synaptic depression. Neurotrophin brain-derived neurotrophic factor (BDNF) enhances neuromuscular transmission via activation of tropomyosin-related kinase receptor B (TrkB). We hypothesized that BDNF mitigates synaptic depression at the neuromuscular junction and that the effect is more pronounced at type IIx and/or IIb fibers compared to type I or IIa fibers given the more rapid reduction in docked synaptic vesicles with repetitive stimulation. Rat phrenic nerve-diaphragm muscle preparations were used to determine the effect of BDNF on synaptic quantal release during repetitive stimulation at 50 Hz. An ∼40% decline in quantal release was observed during each 330-ms duration train of nerve stimulation (intratrain synaptic depression), and this intratrain decline was observed across repetitive trains (20 trains at 1/s repeated every 5 min for 30 min for 6 sets). BDNF treatment significantly enhanced quantal release at all fiber types (P < 0.001). BDNF treatment did not change release probability within a stimulation set but enhanced synaptic vesicle replenishment between sets. In agreement, synaptic vesicle cycling (measured using FM4-64 fluorescence uptake) was increased following BDNF [or neurotrophin-4 (NT-4)] treatment (∼40%; P < 0.05). Conversely, inhibiting BDNF/TrkB signaling with the tyrosine kinase inhibitor K252a and TrkB-IgG (which quenches endogenous BDNF or NT-4) decreased FM4-64 uptake (∼34% across fiber types; P < 0.05). The effects of BDNF were generally similar across all fiber types. We conclude that BDNF/TrkB signaling acutely enhances presynaptic quantal release and thereby may serve to mitigate synaptic depression and maintain neuromuscular transmission during repetitive activation.NEW & NOTEWORTHY Neurotrophin brain-derived neurotrophic factor (BDNF) enhances neuromuscular transmission via activation of tropomyosin-related kinase receptor B (TrkB). Rat phrenic nerve-diaphragm muscle preparations were used to determine the rapid effect of BDNF on synaptic quantal release during repetitive stimulation. BDNF treatment significantly enhanced quantal release at all fiber types. BDNF increased synaptic vesicle cycling (measured using FM4-64 fluorescence uptake); conversely, inhibiting BDNF/TrkB signaling decreased FM4-64 uptake.