Activation of the β2-adrenoceptor by formoterol induces calcium-dependent exocytosis of synaptic vesicles at the neuromuscular junction in a sex-specific manner

Abstract

Increasing evidence suggests that the sympathetic nervous system profoundly interacts with skeletal muscle, influencing both muscle fiber function and composition. β₂-ARs, the predominant adrenergic receptor subtype in muscle fibers, have been shown to enhance protein synthesis, reduce protein degradation, facilitate muscle contraction and relaxation, and improve neuromuscular junction (NMJ) transmission upon activation. In this study, we investigated the effects of Formoterol, a highly selective β₂-adrenoceptors (β₂-AR) agonist, on the presynaptic terminal of motor neurons. We used myography, FM1-43 fluorescent dye assays, and transmission electron microscopy (TEM) to evaluate the NMJ following β₂-AR activation. We demonstrated that β₂-AR activation by Formoterol enhances muscle contractility and both spontaneous and evoked exocytosis of acetylcholine (ACh)-containing synaptic vesicles at the mouse diaphragm NMJ. Formoterol-induced morphological changes in diaphragmatic NMJs were consistent with increased exo-endocytic activity. Notably, Formoterol-evoked exocytosis displayed sexual dimorphism, with females showing a significantly milder response compared to males. In females, Formoterol-induced synaptic vesicles exocytosis was mediated solely by P/Q-type voltage-activated Ca²⁺ channels, whereas in males, it involved both P/Q-type channels, transient receptor potential channel of the vanilloid subtype (TRPV) 1 calcium channels, and an additional, yet unidentified, component. Orchiectomized males exhibited responses to Formoterol similar to the females, whereas ovariectomy did not modify female drug responses, indicating that male hormonal environment orchestrates the sex-differences herein described. These findings not only highlight the importance of sex-specific mechanisms but also reveal a novel effect of β₂-AR activation directly on presynaptic terminals by Formoterol, enhancing exocytosis at the NMJ and thereby increasing neuromuscular transmission.