Anatomical arrangement of hypercapnia-activated cells in the superficial ventral medulla of rats

1 Department of Medicine, Keio University, Tsukigase Rehabilitation Center, Shizuoka-ken 410-3293; 4 Department of Physiology, School of Medicine, Teikyo University, Tokyo 173-8605, Japan; 2 Department of Cell and Molecular Physiology, and 3 Lineberger Cancer Center, University of North Carolina, Chapel Hill, North Carolina 27599 The anatomical structure of central respiratory chemoreceptors in the superficial ventral medulla of rats was studied by using hypercapnia-induced c- fos labeling to identify cells directly stimulated by extracellular pH or P CO 2 . The distribution of c- fos -positive cells was found to be predominantly perivascular to surface vessels. In the superficial ventral medullary midline, parapyramidal, and ventrolateral regions where c- fos -positive cells were concentrated, we found a common, characteristic, anatomical architecture. The medullary surface showed an indentation covered by a surface vessel, and the marginal glial layer was thickened. We classified c- fos -positive cells into two types. One (type I cell) was small, was located inside the marginal glial layer and close to the medullary surface, and surrounded fine vessels. The other (type II cell) was large and located dorsal to the marginal glial layer. c- fos Expression under synaptic blockade suggested that type I cells are intrinsically chemosensitive. The chemosensitivity of surface cells (possible type I cells) surrounding vessels was confirmed electrophysiologically in slice preparations. We suggest that this characteristic anatomical structure may be the central chemoreceptor. respiratory control; chemoreceptor; central chemosensitivity; carbon dioxide; synaptic blocker; c- fos