Substance P afferent terminals innervate vagal preganglionic neurons projecting to the trachea of the ferret

Abstract

Airway disorders, such as asthma and chronic obstructive bronchitis, are, in part, due to abnormalities in the nervous control of the airways. However, the ultrastructural circuitry and neurochemical anatomy of afferents modulating the output of airway-related vagal preganglionic neurons (VPNs) in the nucleus ambiguus are poorly understood. We have examined the potential role of substance P (SP) immunoreactive afferents in the regulation of anatomically identified airway VPNs. Cholera toxin b-subunit conjugated to horseradish peroxidase was used as a retrograde cell body tracer to identify the central VPNs innervating the extra-thoracic trachea. Immunocytochemistry was employed to identify SP afferents. The external formation of the nucleus ambiguus was examined by electron microscopy using a simultaneous double labeling method. Cell bodies of tracheal VPNs were 31.7±1.18×23.0±1.3 μm (means±S.E.M.) in size, contained abundant endoplasmic reticulum, had a round nucleus with a prominent nucleolus, no satellite body and displayed somatic and dendritic spines. Somato-somatic appositions, somato-dendritic appositions without intervening glial processes and dendritic “bundling” commonly seen in esophageal motoneurons were not observed. The ultrastructural morphology of tracheal VPNs were also clearly distinguishable from pharyngeal and laryngeal motoneurons in other divisions of the nucleus ambiguus which lack somatic spines. These data are consistent with the hypothesis that differences in the ultrastructure and synaptology of the different divisions of the nucleus ambiguus may be associated with specific physiological functions. The mean size (±S.E.M.) of SP nerve terminals was 1.57±0.06×0.79±0.03 μm. SP terminals formed 17.5% of the axo-dendritic and 15.9% of the axo-somatic synapses which were observed upon retrogradely labeled tracheal VPNs. Synaptic contacts observed were both symmetric and asymmetric. These synaptic interactions define, in part, the neurochemical anatomy of neuronal circuits modulating vagal preganglionic control of tracheal functions.