Elsevier

Regulatory Peptides

Volume 71, Issue 1, 23 July 1997, Pages 29-36
Regulatory Peptides

Abnormal afferent nerve endings in the soft palatal mucosa of sleep apnoics and habitual snorers

https://doi.org/10.1016/S0167-0115(97)01016-1Get rights and content

Abstract

Habitual snoring precedes obstructive sleep apnea (OSA), but the pathophysiological mechanisms behind progression are still unclear. The patency of upper airways depends on a reflexogen mechanism reacting on negative intrapharyngeal pressure at inspiration, probably mediated by mucosal receptors, i.e., via afferent nerve endings. Such nerves contain a specific nerve protein, protein-gene product 9.5 (PGP 9.5) and in some cases substance P (SP) and calcitonin gene-related peptide (CGRP). Biopsies of the soft palatal mucosa were obtained from non-smoking men ten OSA patients, 11 habitual snorers and 11 non-snoring controls. The specimens were immunohistochemically analyzed for PGP 9.5, SP and CGRP. As compared to controls, an increased number of PGP-, SP- and CGRP-immunoreactive nerves were demonstrated in the mucosa in 9/10 OSA patients and 4/11 snorers, in addition to varicose nerve endings in the papillae and epithelium. Using double staining methodology, it could be shown that SP- and CGRP-like immunoreactivities (LIs) often coexisted in these fibres, as did CGRP- and PGP 9.5-LIs. The increased density in sensory nerve terminals are interpreted to indicate an afferent nerve lesion. Our results support the hypothesis of a progressive neurogenic lesion as a contributory factor to the collapse of upper airways during sleep in OSA patients.

Introduction

Obstructive sleep apnea (OSA) is characterized by a collapse of the upper airways (UA) during inspiration and sleep. The subocclusive stage of habitual snoring usually precedes the development of OSA [1], but the pathophysiological mechanisms underlying progression are not known. The patency of UA depends on a reflexogen mechanism reacting on the negative intrapharyngeal pressure at inspiration [2]. It has been suggested that intra- and/or sub-mucosal receptors mediate this reflex via afferent and efferent nerve pathways to dilating muscles [2]. It is our working hypothesis that the mechanical trauma of snoring causes a disturbance of these nerves, and that this lesion is a contributory factor to the pharyngeal collapse seen in OSA patients.

Previously, histological changes indicative of a denervation process of the efferent pathways to the palatopharyngeus muscle were demonstrated in OSA patients [3]. Furthermore, a focal degeneration of myelinated nerve fibres was shown in the uvula of severe apnoics [4], and signs of an afferent nerve lesion with impaired temperature thresholds have been found in the soft palatal mucosa of OSA patients [5]. Some of the afferent nerve endings, i.e., polymodal nociceptors are responsible for propagating mechanical, chemical and thermal stimuli as well as causing vascular reactions after stimulation [6]. The vascular reaction has been shown to be caused by a release of calcitonin gene-related peptide (CGRP) and substance P (SP) [7]from these endings. CGRP and SP have previously been demonstrated by immunohistochemistry in the human soft palatal mucosa [8]. In a recent study by Friberg et al. (unpublished observations), abnormal vascular reactions were seen after afferent nerve stimulation of the soft palatal mucosa in habitual snorers and sleep apnoics when compared to controls, indirectly indicating an afferent nerve lesion.

The aim of this study was to investigate whether or not there are any further signs of an afferent neuropathy in the soft palatal mucosa of patients with habitual snoring and OSA, compared to non-snoring controls. By using immunohistochemistry, the content of CGRP, SP and protein-gene peptide 9.5 (PGP), a specific nerve protein was semiquantitatively estimated in the mucosa of these subjects.

Section snippets

Subjects

Altogether 32 non-smoking men undergoing pharyngeal surgery were included. In 11 non-snoring controls the surgery was because of chronic tonsillitis (except one with a pharyngeal tumour) and in ten OSA patients and 11 habitual snorers the surgery was uvulopalatopharyngoplasty. OSA patients or snorers with gross abnormality of the upper airways, e.g., severe tonsillar hyperplasia, or with previous treatment for their snoring/OSA, e.g., continues positive airway pressure (CPAP) and/or dental

Control specimens

Nerve endings staining for PGP 9.5 (Fig. 1a–c), CGRP (Fig. 1d,e) and SP were found in the submucosa – especially around blood vessels and in the lamina propria. The SP- and CGRP-immunoreactive (IR) nerves were also PGP 9.5-positive. However, there were more nerves staining for PGP 9.5. Also, the PGP 9.5-like immunoreactivity (LI) was much stronger than the CGRP and SP signals. The majority of control specimens exhibited immunoreactivities as in Fig. 1a,d. However, in 2 of 11 control specimens (

Discussion

The present study demonstrates that in snorers and OSA patients there is an increased number of varicose nerve endings in the epithelium of the mucosa as visualized with immunohistochemistry and antibodies to three molecules, PGP 9.5, CGRP and SP. Whereas PGP 9.5 is a general marker for nerve fibres, the two neuropeptides CGRP [19]and SP [20]are in the skin and mucous membranes generally assumed to be present mainly in sensory nerve fibres of the C and A-delta type [21]. The localization and

Acknowledgements

This study was supported by the Karolinska Institute, the Swedish MRC (04X-2887). The authors thank associate Professor Eva Svanborg, Dept. of Neurophysiology, Karolinska Hospital, for valuable advises and for performing the sleep recordings and Ms Katarina Åman, Dept. of Neuroscience, Karolinska Institute, for skilful technical assistance.

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