Ouabain-like immunoreactivity in the medulla oblongata of rats

An isomer of ouabain, the ouabain-like compound (OI,C), may participate in the regulation of body fluid volume and vascular tone. Forebrain regions, especially the hypothalamus, are reported to be sites of OLC action in the central nervous system. The medulla oblongata is another critical area involved in central cardiovascular regulation. We reported that the microinjection of either monoclonal antibody to ouabain T8B11 or Fab fragment of digoxin-specific antibody into the rostral ventrolateral medulla significantly decreased mean arterial pressure and renal sympathetic nerve activity in anesthetized normotensive rats (TERUYA et al.: J. Clin. Invest. 99: 2791-2798, 1997). Using T8B11, we examined the ouabain-like immunoreactivity in the medulla oblongata of normotensive rats. In periodate-lysine-paraformaldehyde fixed tissues, ouabain-like immunoreactive neurons were detected in the nuclei and regions in the medulla oblongata including the ventrolateral medulla, ventromedial medulla, nucleus ambiguus, caudal raphe nuclei, nucleus of solitary tract, and dorsal motor nucleus of the vagus. When an Fab fragment of digoxin-specific antibody was used as a first antibody, the digoxin-like immunoreactive neurons were distributed in almost the same pattern as those observed with the use of T8B11. In the brain fixed with the "three-step" procedure developed by YAMADA et al. (1987), which was used in a previous ouabain immunohistochemical study of the hypothalamus, ouabain-like immunoreactivity in the medulla oblongata was much weaker in intensity and less restricted in distribution than that in the hypothalamus. These findings suggest that ouabain-like immunoreactivities are present in the medulla oblongata with a manner of distribution different from that seen in the hypothalamus. Some ouabain-immunopositive nuclei and regions in the medulla oblongata, especially the rostral ventrolateral medulla, may be other OLC action sites.