Neural control of respiratory musculature in the mdx mouse model of Duchenne muscular dystrophy

Abstract

Duchenne muscular dystrophy (DMD) is an inherited fatal neuromuscular disease. Despite deficits in respiratory muscle performance being well recognised, there is a paucity of knowledge in respect of the neural control of breathing in dystrophinopathies. We sought to examine neural control of the diaphragm and upper airway musculature in dystrophin deficient mdx mice.Wild‐type (WT) and mdx mice were studied to examine diaphragm and genioglossus (GG) EMG activity using fine concentric needle electrodes in urethane (1.7mg/kg) anaesthetized spontaneously breathing mice. Basal EMG and responsiveness to chemostimulation were determined. In separate studies, phrenic and hypoglossal (XII) motor output were assessed in anaesthetized, vagotomized, paralyzed and mechanically ventilated WT and mdx mice.Diaphragm and upper airway (sternohyoid) muscle form and function were assessed. Monoamine concentrations, density of glial cells, and pro‐inflammatory gene expression were determined in the spinal cord (C3–C5) and brainstem. Diaphragm and GG EMG activities were depressed in mdx compared to WT during baseline and hypoxic‐hypercapnic challenge. Phrenic and XII nerve recordings revealed enhanced neural drive to diaphragm and GG in mdx mice during hypoxic‐hypercapnic challenge. Excitatory monoamine concentrations were elevated in mdx spinal cord (noradrenaline and serotonin) and brainstem (serotonin). Putative phrenic and hypoglossal motor nuclei in mdx mice had unaltered expression of microglia and astrocytes compared to WT. NFkB and GFAP mRNA expression were increased in mdx brainstem compared to WT, while GFAP mRNA was reduced in mdx spinal cord.Our study revealed a potentiated neural drive in phrenic and XII motor pathways in mdx mice during chemoactivation, suggesting plasticity in motor pathways facilitating breathing. Depressed EMG activity despite potentiated neural outputs, suggests compromised neuromuscular transmission, which we have also observed in respiratory EMG recordings during protracted airway obstruction (Burns et al., 2019, J Physiol). Studies of the fundamental control of breathing in models of DMD should provide a comprehensive vista of the disease, with the potential to identify novel therapeutic targets.