ß2-Adrenergic Regulation of the Neuromuscular Transmission and Its Lipid-Dependent Switch.

ABSTRACT

ß2-Adrenoceptors (ß2-ARs) are the most abundant subtype of adrenergic receptors in skeletal muscles. Their activation via a stabilization of postsynaptic architecture has beneficial effects in certain models of neuromuscular disorders. However, the ability of ß2-ARs to regulate neuromuscular transmission at the presynaptic level is poorly understood. Using electrophysiological recordings and fluorescent FM dyes, we found that ß2-AR activation with fenoterol enhanced an involvement of synaptic vesicles in exocytosis and neurotransmitter release during intense activity at the neuromuscular junctions of mouse diaphragm. This was accompanied by an improvement of contractile responses to phrenic nerve stimulation (but not direct stimulation of the muscle fibers) at moderate-to-high frequencies. ß2-ARs mainly reside in lipid microdomains enriched with cholesterol and sphingomyelin. The latter is hydrolyzed by sphingomyelinases, whose upregulation occurs in many conditions characterized by muscle atrophy and sympathetic nerve hyperactivity. Sphingomyelinase treatment reversed the effects of ß2-AR agonist on the neurotransmitter release and synaptic vesicle recruitment to the exocytosis during intense activity. Inhibition of Gi protein with pertussis toxin completely prevented the sphingomyelinase-mediated inversion in the ß2-AR agonist action. Note that lipid raft disrupting enzyme cholesterol oxidase had the same effect on ß2-AR agonist-mediated changes in neurotransmission as sphingomyelinase. Thus, ß2-AR agonist fenoterol augmented recruitment and release of synaptic vesicles during intense activity in the diaphragm neuromuscular junctions. Sphingomyelin hydrolysis inversed the effects of ß2-AR agonist on neurotransmission probably via switching to Gi protein-dependent signaling. This phenomenon may reflect a dependence of the ß2-AR signaling on lipid raft integrity in the neuromuscular junctions.