TrkB signaling modulates neuromuscular transmission in a frequency-dependent manner in the mouse diaphragm muscle

Signaling via tropomyosin-related kinase receptor B (TrkB) plays an important role in synaptic function and plasticity at the neuromuscular junction. Multiple conditions show selective effects on the most forceful and fatigable motor units that require high-frequency activation and are necessary for maximal force generation. The goal of the present study is to investigate the effects of inhibiting TrkB kinase activity on neuromuscular transmission in mouse diaphragm muscle during repeated stimulation at frequencies that reflect the recruitment of different motor units (10 vs. 75 Hz). TrkB^F616A mice, which possess a mutation that renders TrkB kinase activity susceptible to rapid inhibition by 1NMPP1, were used at 6-8 mo old (n = 12; 6 females). Neuromuscular transmission failure (NMTF) was estimated in diaphragm-phrenic nerve preparations following 1 h treatment with 1NMPP1 or vehicle by assessing the difference in forces evoked by 2 min of repetitive nerve and muscle stimulation at either 10 or 75 Hz. There was no effect of 1NMPP1 on muscle contractile properties, but there was a frequency-dependent effect of 1NMPP1 treatment on the extent of NMTF. At 10-Hz stimulation, 1NMPP1 decreased the extent of NMTF compared with vehicle by 35%. At 75 Hz, 1NMPP1 increased the extent of NMTF compared with vehicle by 28%. The greater negative effect of TrkB kinase inhibition during higher-frequency stimulation suggests reliance on TrkB signaling for sustained activation predominantly at higher force, more fatigable motor units, which are responsible for the high forces necessary for airway clearance (i.e., coughing and sneezing).NEW & NOTEWORTHY This study found that inhibiting TrkB kinase activity decreases the extent of neuromuscular transmission failure during repetitive stimulation at low frequency while increasing failure with high-frequency stimulation. The frequency-dependent effect of 1NMPP1 treatment suggests that the role of TrkB signaling in maintaining neuromuscular transmission during repetitive stimulation may differ between motor unit types (slow-twitch, fatigue-resistant vs. fast-twitch, fatigable) and likely reflect effects at the presynaptic terminal without major effects on the muscle itself.