ENDOGENOUS NEUROTROPHINS REGULATE NEUROMUSCULAR TRANSMISSION IN RAT DIAPHRAGM MUSCLE

Abstract

In a previous study, we found that the neurotrophins BDNF and NT-4 are expressed at phrenic motor neurons and that TrkB receptors are present at diaphragm muscle (DIAm) neuromuscular junctions. Furthermore, we found that exogenous BDNF and NT-4 treatment improves neuromuscular transmission during repeated stimulation. In the present study, we hypothesized that endogenously released neurotrophins play an important role in regulating neuromuscular transmission during repeated activation. To test this hypothesis, we determined the effects of TrkB-Fc (a soluble fusion protein containing the neurotrophin-binding domain of the neurotrophin receptor TrkB) on neuromuscular transmission at the rat DIAm. The DIAm together with the phrenic nerve was dissected and mounted at optimal length in a tissue bath for isometric force measurements induced by nerve stimulation. The muscle segments were preincubated with TrkB-Fc (750 ng/ml) for 30 min prior to force measurements. The extent of neuromuscular transmission failure (NMTF) was evaluated during repetitive nerve stimulation (40 Hz in 330 ms duration trains repeated each s) by superimposing direct muscle stimulation every 15 s during a 2-min period. The difference between force generated by nerve vs. direct muscle stimulation provided an index of NMTF. Muscle fatigue was also assessed during repetitive muscle stimulation in a separate muscle segment. Compared to controls (CTL), NMTF was ~20% greater in the TrkB-Fc treated group (P<0.05). In contrast, the extent of muscle fatigue was comparable between CTL and TrkB-Fc treated groups. These results support the hypothesis that endogenously released neurotrophins improve neuromuscular transmission in the rat DIAm. Supported by NIH grants AR51173 and HL37680.