(A–F) Photomicrography showing representative cases of TH- and Fos-labelled cells located in(A, B) the pFRG, (C, D) the RLVM and (E, F) the cNTS after normoxia (21% O2, balanced with N2; N = 6) and after hypoxia (3 hr in 8% O2, balanced with N2; N = 6). (A’–F’) Higher magnification of pFRG, RVLM and cNTS showing neurons labeled by Fos and/or TH. (G–I) Number of Fos and/or TH-labeled cells in the pFRG, RVLM and cNTS. Cell count was obtained in coronary brain sections (five sections from the pFRG, six sections from the RVLM and three sections from the cNTS of 40 μm in thickness with 240 μm intervals between slices) from each rat. Abbreviations: cc, central canal; cNTS, commissural nucleus of the solitary tract; IO, inferior olive; pFRG, parafacial respiratory group; RVLM, rostral ventrolateral medulla; Sp5, spinal trigeminal tract; TH, tyrosine hydroxylase; VII, facial motor nucleus; XII, hypoglossal nucleus. *Different from normoxia. Scale bar in panel (F) = 100 μm, this scale bar also applies topanels (A–F), whereas the scale bar in panel (F’) = 20 μm applies to panels (A’-F’).