OUABAIN-LIKE IMMUNOREACTIVITY IN THE MEDULLA-OBLONGATA OF RATS

An isomer of ouabain, the ouabain-like compound (OLC), may participate in the regulation of body fluid volume and vascular tone. Forebrain r egions, especially the hypothalamus, are reported to be sites of OLC a ction in the central nervous system. The medulla oblongata is another critical area involved in central cardiovascular regulation, We report ed that the microinjection of either monoclonal antibody to ouabain T8 B11 or Fab fragment of digoxin-specific antibody into the rostral vent rolateral medulla significantly decreased mean arterial pressure and r enal sympathetic nerve activity in anesthetized normotensive rats (TER UYA et al,: J, Clin, Invest, 99: 2791-2798, 1997), Using T8B11, we exa mined the ouabain-like immunoreactivity in the medulla oblongata of no rmotensive rats. In periodate-lysine-paraformaldehyde fixed tissues, o uabain-like immunoreactive neurons were detected in the nuclei and reg ions in the medulla oblongata including the ventrolateral medulla, ven tromedial medulla, nucleus ambiguus, caudal raphe nuclei, nucleus of s olitary tract, and dorsal motor nucleus of the vagus, When an Fab frag ment of digoxin-specific antibody was used as a first antibody, the di goxin-like immunoreactive neurons were distributed in almost the same pattern as those observed with the use of T8B11, In the brain fixed wi th the ''three-step'' procedure developed by YAMADA et al, (1987), whi ch was used in a previous ouabain immunohistochemical study of the hyp othalamus, 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 immunor eactivities are present in the medulla oblongata with a manner of dist ribution different from that seen in the hypothalamus, Some ouabain-im munopositive nuclei and regions in the medulla oblongata, especially t he rostral ventrolateral medulla, may be other OLC action sites.