The Expression of Vesicular Glutamate Transporter 1 (VGLUT1) in the Rat Larynx and Implications for Laryngeal Proprioception

Abstract

ABSTRACTProprioception plays a crucial role in laryngeal function. Further, dysfunctional proprioception likely contributes to disorders such as laryngeal dystonia, dysphagia and vocal fold paresis. Despite this, the physiology of laryngeal proprioception is not well-understood. Controversy remains over whether canonical proprioceptive organs, like muscle spindles (MS) even exist in the intrinsic laryngeal muscles (ILM). Vesicular Glutamate Transporter 1 (VGLUT1) expression has been described in the sensory afferents of MS. This study’s primary aim is to determine whether the ILM contain MS using VGLUT1. This is a novel approach, as prior studies have relied on morphology and myosin composition to study this question. Secondarily, we describe the pattern of VGLUT1 expression in the rat larynx, Larynges of 62 Sprague-Dawley rats distributed across 5 age groups (P3, P8, P11, P14-15, and adult), were sectioned and immunostained for VGLUT1 and beta-tubulin III. Other markers (S46, GNAT3, PLCβ2, S100b, CGRP) were used to further characterize identified afferent innervation. Of 62 rats, MS were identified in the lateral thyroarytenoid muscles of just three P8 rats, and no golgi tendon organs (GTO) were seen. VGLUT1-positive intramuscular receptor-like entities were observed ILM, and VGLUT1-positive nerve endings were observed in the laryngeal mucosa, concentrated around the arytenoid cartilage. Employing VGLUT1 immunostaining, this study shows that rat intrinsic laryngeal muscles rarely express MS and do not express GTO. This leaves open the possibility that the larynx exhibits a unique proprioceptive apparatus. VGLUT1-positive intramuscular and mucosal structures provide candidates for an alternative system. Further defining the role of these sensory organs will increase our understanding of vocal fold function and ultimately lead to better treatment of vocal fold disorders.KEY POINTSDysfunctional laryngeal proprioception likely contributes to disorders such as laryngeal dystonia, dysphagia, and vocal fold paresis. Unlike proprioception of skeletal muscles, proprioception of the intrinsic laryngeal muscles is poorly understood.In the present study we demonstrate that canonical proprioceptive organs (muscles spindles and Golgi tendon organs) are rarely expressed in the rat larynx, by studying the expression pattern of VGLUT1.We also demonstrate the presence of other sensory innervation and structures which may contribute to an alternative proprioceptive circuitry, which requires further study.