Distribution and ultrastructure of tachykinin-like immunoreactivity in thefrog (Rana esculenta) spinal cord, notably, the dorsal horn

Tachykinins are involved in pain transmission at the spinal level. In frog, at least four tachykinins [TK] have been isolated from the brain, but thei r organization in the dorsal born of the spinal cord is still poorly known. We have reexamined TK distribution by immunocytochemistry using an antibod y recognizing the sequence common to all tachykinins in the spinal cord and dorsal root ganglia of the green frog Rana esculenta. A dense tachykinin-l ike immunoreactivity (TK-LI) was observed in the dorsolateral fasciculus or Lissauer's tract running ventromedial to the entry of the dorsal root and in numerous small and medium-sized dorsal root ganglion cells showing a pri mary afferent origin for part of TK-LI of the dorsal horn. The observation of numerous cell bodies in the dorsal horn, in addition, suggested a local or propriospinal origin. One group of cells was localized at the entrance o f the Lissauer's tract TK-LI fibers into the dorsal horn, and another group was localized in the upper dorsal horn, a region with a low density of TK- LI fibers. It was suggested that the latter group may correspond to neuroki nin B. Electron microscopic examination of the Lissauer's tract showed nume rous immunoreactive axons, some located at the center of glomerular-like ar rangements, suggesting that the information brought by these fibers may be transmitted and most probably modulated before their entry in the dorsal ho rn. In conclusion, the functional organization of tachykinins in the frog s pinal cord seems to be similar to that of mammals, albeit with a different morphological organization. J. Comp. Neurol. 433:183-192, 2001. (C) 2001 Wi ley-Liss, Inc.