EXN-4 activates medullary catecholamine neurons projecting to the spinal cord. (a–c) Double-label immunohistochemistry demonstrates that neurons that innervate sympathetic preganglionic neurons (brown cytoplasm) also contain i.c.v. EXN-4–induced Fos-IR (black nuclei) in (a) the PVH, (b) the Arc/RCA, and (c) the LHA of the hypothalamus. (d–f) Higher-magnification view of the boxed areas in a–c, respectively. (g–i) Double-label immunohistochemistry demonstrates that TH-immunoreactive neurons (brown cytoplasm) also contain i.c.v. EXN-4–induced Fos-IR (black nuclei) in (g) the NTS, (h) the LC, and (i) the CVLM. (j–l) Higher-magnification view of the boxed areas in g–i, respectively. (m–p) Triple-label immunohistochemistry reveals that many medullary catecholamine neurons (red cytoplasm in m and o) that project to the IML in the spinal cord (green neurons in n and p) also contain i.c.v. EXN-4–induced Fos-IR (black nuclei) in the RVLM (m and n) and the A5 (o and p). Scale bar = 100 μm in a–c and g–i, 25 μm in d–f and j–l, and 50 μm in m–p.