Neurochemical characterisation of sensory receptors in airway smooth muscle: comparison with pulmonary neuroepithelial bodies

Abstract

Descriptions of morphologically well-defined sensory airway receptors are sparse, in contrast to the multiplicity of airway receptors that have been identified electrophysiologically. The present study aimed at further determining the location, morphology and neurochemical coding of subepithelial receptor-like structures that have been sporadically reported in the wall of large diameter airways. The results were compared with those obtained for pulmonary neuroepithelial bodies (NEBs), which are complex intraepithelial sensory airway receptors. Multiple immunocytochemical staining showed branching laminar subepithelial receptor-like endings, which were found to intercalate in the smooth muscle layer of intrapulmonary conducting airways in rats. Because of the consistent intimate association with the airway smooth muscle, the laminar terminals will further be referred to as 'smooth muscle-associated airway receptors (SMARs)'. SMARs were characterised by their Na(+)/K(+)-ATPase alpha3, vesicular glutamate transporter 1 (VGLUT1) and VGLUT2-immunoreactivity, expression of the ATP receptor P2X(3), and the presence of calcium-binding proteins. Nerve fibres giving rise to SMARs were shown to be myelinated and to have a vagal origin. Interestingly, the neurochemical coding and receptor-like appearance of SMARs appeared to be almost identical to at least part of the complex vagal sensory terminals in NEBs. Intraepithelial nerve endings in pulmonary NEBs were indeed also shown to originate from myelinated vagal afferent nerve fibres, and to express Na(+)/K(+)-ATPase alpha3, VGLUT1, VGLUT2, P2X(3) and calcium-binding proteins. Since several of the latter proteins have been reported as markers for mechanoreceptor terminals in other organs, both SMARs and the vagal nodose nerve terminals in NEBs seem good candidates to represent the morphological counterparts of at least subsets of the extensive population of physiologically characterised myelinated vagal airway mechanoreceptors. The observation that SMARs and NEBs are regularly found in each other's immediate neighbourhood, and the very similar characteristics of their nerve terminals, point out that the interpretation of electrophysiological data based on 'local' stimuli should be made with great caution.

Expression of mechanogated two-pore domain potassium channels in mouse lungs: special reference to mechanosensory airway receptors

“…-ATPase a3) were used. These markers have been shown to label SMARs in rat airways (Brouns et al 2006b), and vagal (mechano)sensory nerve fibres associated with mouse and rat NEBs (Brouns et al 2006a(Brouns et al , 2009a, and in the present study additionally revealed subepithelial nerve terminals in mouse airways. To check the specific location of subepithelial nerve terminals visualised with VGLUT1, P2X 3 , Na ?…”

“…Using immunostaining for the above-mentioned molecules resulted in the identification of SMARs a few years ago. Morphologically, SMARs appear as branching laminar subepithelial receptor-like nerve endings that were found to intercalate in the smooth muscle layer of intrapulmonary conducting airways (Brouns et al 2006b;De Proost et al 2007). In rats, nerve fibres giving rise to SMARs were shown to be myelinated and to have a vagal origin (Brouns et al 2006b).…”

“…Morphologically, SMARs appear as branching laminar subepithelial receptor-like nerve endings that were found to intercalate in the smooth muscle layer of intrapulmonary conducting airways (Brouns et al 2006b;De Proost et al 2007). In rats, nerve fibres giving rise to SMARs were shown to be myelinated and to have a vagal origin (Brouns et al 2006b). The combined observation that rat SMAR fibres are myelinated, disappear after vagal denervation, and do not show characteristics of the motor innervation of airway smooth muscle strongly suggests that SMARs are the counterparts of at least a subgroup of the physiologically characterised myelinated vagal airway receptors (Adriaensen et al 2006;Brouns et al 2006b).…”

Pulmonary expression of voltage-gated calcium channels: special reference to sensory airway receptors

“…Since the expression of VGLUTs should be regarded as determining for the capacity of nerve endings to store and release glutamate as a neurotransmitter (Takamori et al 2000), depolarisation of SMAR terminals will likely cause the release of glutamate. Glutamate may consequently serve a local peripheral eVector function (Raab and Neuhuber 2003;Bewick et al 2005), for example inXuencing the activity of neighbouring smooth muscle cells, or may regulate the excitability and/or transduction of the sensory nerve Wbres (Brouns et al 2004(Brouns et al , 2006b.…”

Morphologic Characterization of Nerves in Whole-Mount Airway Biopsies

“…Yet, prominent discoveries of immunochemically distinct receptor end organs, such as the unmistakable guinea pig cough receptor (10) and the murine smooth muscle-associated airway receptor and neuroepithelial body (NEB) innervation (11), have been conspicuously absent in descriptions of human airway innervation to date (12). The sparsity of such vagal afferent mechanoreceptors in comparison with the easily identified vagal and spinal C fiber counterparts and lack of appropriate antibodies might explain this phenomenon.…”