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LETTER TO THE EDITOR
The question asked by Dr. Yu, whether neuroepithelial bodies (NEBs) are SARs or not (7, 9), seems to us an oversimplification. NEBs are groups of neuroendocrine cells that interact with many different nerve terminal populations, several of which originate in vagal sensory ganglia (for review see Refs. 1, 2). In our opinion, NEBs cannot be equated to an electrophysiological entity that is purely defined by the characteristics of a nerve fiber. A more relevant question would, therefore be: What types of electrophysiologically characterized vagal airway afferents connect to pulmonary NEBs?
Because of the complexity of a major part of the airway receptor end organs, activity generated in vagal fibers (or the absence of it) upon application of local stimuli will not necessarily lead to a straightforward interpretation. As recently also recognized by other groups in the field (4, 6), the neurochemical characteristics will be essential to eventually define the actual targets of the many subpopulations of airway-related vagal afferents.
The personal work (9) that Dr. Yu refers to as the basis for his comments was discussed in our review (1). As far as the combination between classic electrophysiology and morphological characterization of vagal airway receptors is concerned, the experimental design presented by Yu et al. (9) is unique in the field. Most likely due to technical problems (full discussion, see Ref. 3, p. 365, paragraph 3), the data shown for rat pulmonary NEBs, unfortunately, are insufficient to draw the strong conclusions that were formulated by Yu et al. (9) and repeated in the present Letter to the Editor (7). Our own observations, with a multitude of markers in hundreds of rat lungs, in many respects argue against the ideas put forward by the authors. The last sentence of the RESULTS section of Yu et al. (9) mentions that "SAR structures" had been identified in 8 out of 15 "successfully dissected" receptor blocks only. It is not clear, however, which nerve terminals may have been responsible for the SAR activity in the other half of the blocks. Apparently, the terminals did not stain for Na+-K+-ATPase, or at least not via the methods used by the authors.
The idea of paracrine interactions of substances secreted by NEBs with surrounding tissues, such as airway smooth muscle, is well accepted. We, therefore, do not share the interpretation of Dr. Yu (7) that the gap between the NEB and smooth muscle-associated airway receptor [in Fig. 3 of our review (1)] would be too wide (30–40 µm, according to our measurements) to allow for an interaction. On the other hand, the close proximity of both receptors as such does not prove a functional link.
Dr. Yu's remark (7) about the possibility that myelinated axons that connect to NEBs may belong to a population of nociceptor-like vagal afferents (high-threshold A
fiber receptors) is certainly interesting. However, the work of Yu et al. (8) that is referred to used rabbits, and vagal pulmonary fibers with similar characteristics have so far not been reported for rat airways. Furthermore, the work contains no direct link between morphology and physiology, and the reported data reveal some technical problems. Injection of the fluorescent neuronal tracer Dil in the nodose ganglion appears to have brightly labeled NEB cells (Ref. 8, see Fig. 1). The latter seems to be an artifact, since DiI will not pass from one cell (afferent neuron) to another (NEB cell), unless membranes are damaged, which makes identification of the actual receptor terminals impossible.
Dr. Yu (7) also suggests that there would be a unique link between continuously firing (slowly adapting) neurons and the expression of the 
3-subunit of the Na+-K+-ATPase ion exchanger. Unfortunately, it is not that simple, since different neuronal populations may use various mechanisms to achieve the necessary fast ion exchange.
In conclusion, a major part of the airway-related myelinated vagal afferents selectively connect to NEBs and the neurochemical characteristics of their terminals are astonishingly similar to those of smooth muscle-associated airway receptors, the smooth muscle receptors that are believed to have SAR-like properties.
FOOTNOTES
Address for reprint requests and other correspondence: D. Adriaensen, Laboratory of Cell Biology and Histology, Dept. of Veterinary Sciences, Univ. of Antwerp, Groenenborgerlaan 171, BE-2020 Antwerp, Belgium (e-mail: dirk.adriaensen{at}ua.ac.be)
REFERENCES
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