TrkB signaling contributes to transdiaphragmatic pressure generation in aged mice

Abstract

Ventilatory deficits are common in old age and may result from neuromuscular dysfunction. Signaling via the tropomyosin-related kinase receptor B (TrkB) regulates neuromuscular transmission and, in young mice, is important for the generation of transdiaphragmatic pressure (Pdi). Loss of TrkB signaling worsened neuromuscular transmission failure and reduced maximal Pdi, and these effects are similar to those observed in old age. Administration of TrkB agonists such as 7,8-dihydroxyflavone (7,8-DHF) improves neuromuscular transmission in young and old mice (18 mo; 75% survival). We hypothesized that TrkB signaling contributes to Pdi generation in old mice, particularly during maximal force behaviors. Old male and female TrkB^F616A mice, with a mutation that induces 1NMPP1-mediated TrkB kinase inhibition, were randomly assigned to systemic treatment with vehicle, 7,8-DHF, or 1NMPP1 1 h before experiments. Pdi was measured during eupneic breathing (room air), hypoxia-hypercapnia (10% O₂/5% CO₂), tracheal occlusion, spontaneous deep breaths ("sighs"), and bilateral phrenic nerve stimulation (Pdi_max). There were no differences in the Pdi amplitude across treatments during ventilatory behaviors (eupnea, hypoxia-hypercapnia, occlusion, or sigh). As expected, Pdi increased from eupnea and hypoxia-hypercapnia (∼7 cm H₂O) to occlusion and sighs (∼25 cm H₂O), with no differences across treatments. Pdi_max was ∼50 cm H₂O in the vehicle and 7,8-DHF groups and ∼40 cm H₂O in the 1NMPP1 group (F_8,74 = 2; P = 0.02). Our results indicate that TrkB signaling is necessary for generating maximal forces by the diaphragm muscle in old mice and are consistent with aging effects of TrkB signaling on neuromuscular transmission.NEW & NOTEWORTHY TrkB signaling is necessary for generating maximal forces by the diaphragm muscle. In 19- to 21-mo-old TrkB^F616A mice susceptible to 1NMPP1-induced inhibition of TrkB kinase activity, maximal Pdi generated by bilateral phrenic nerve stimulation was ∼20% lower after 1NMPP1 compared with vehicle-treated mice. Treatment with the TrkB agonist 7,8-dihydroxyflavone did not affect Pdi generation when compared with age-matched mice. Inhibition of TrkB kinase activity did not affect the forces generated during lower force behaviors in old age.

Keywords: aging; breathing; diaphragm muscle; neuromuscular transmission; neurotrophins.

Graphical abstract

Figure 1.

Representative transdiaphragmatic pressure (Pdi) tracings from a single, aged (19–21 mo old) TrkB^F616A mouse from each treatment group during eupnea (breathing room air), hypoxia-hypercapnia (10% O₂-5% CO₂), breathing against an occluded airway, spontaneous deep breaths (sighs), and maximal Pdi generated by bilateral phrenic nerve stimulation at 150 Hz (Pdi_max). TrkB^F616A mice were treated intraperitoneally with 5 µL of vehicle (0.3% DMSO), 7,8-dihydroxyflavone (7,8-DHF; 10 µM), or 1NMPP1 (10 mM). Note expected differences in Pdi across behaviors. Across treatment groups, Pdi during eupneic breathing of room air and hypoxia-hypercapnia, as well as during occlusion and sighs are comparable. Pdi_max, maximal Pdi generation.

Figure 2.

Summary results of transdiaphragmatic pressure (Pdi) generated during eupnea, hypoxia-hypercapnia (10% O₂-5% CO₂), tracheal occlusion, spontaneous deep breaths (sighs), and bilateral phrenic nerve stimulation at 150 Hz (Pdi_max) in TrkB^F616A mice treated with vehicle (n = 7), 7,8-dihydroxyflavone (7,8-DHF; n = 7) or 1NMPP1 (n = 8) for 1 h. Bars show means ± 95% confidence interval (CI). Unique symbols are matched across behaviors per animal in each treatment group. Data were analyzed using a mixed linear model with animal as a random effect (see methods). There was a significant effect on Pdi amplitude of behavior (F_4,74 = 164; P < 0.01) and treatment (F_2,20 = 6; P < 0.01), with a behavior × treatment interaction (F_8,74 = 2; P = 0.02). *Significantly different the other groups in post hoc Tukey–Kramer honestly significant difference (HSD) tests (P < 0.05). Note that the treatment effects are limited to Pdi_max across treatment groups and that there were no significant differences in any of the lower force, ventilatory behaviors. Unique symbols are matched across behaviors per animal in each treatment group.