MEDULLARY NEURONS ACTIVATED BY ANGIOTENSIN-II IN THE CONSCIOUS RABBIT

Previous studies have shown that angiotensin II (Ang II) can activate cardiovascular neurons within the medulla oblongata via an action on s pecific receptors. The purpose of this study was to determine the dist ribution of neurons within the medulla activated by infusion of Ang II into the fourth ventricle of conscious rabbits, using the expression of Fos, the protein product of the immediate early gene c-fos as a mar ker of neuronal activation. Experiments were done in both intact and b arodenervated animals. In comparison with a control group infused with Ringer's solution alone, in both intact and barodenervated animals, f ourth ventricular infusion of Ang II (4 to 8 pmol/min) induced a signi ficant increase in the number of Fos-positive neurons in the nucleus o f the solitary tract and in the rostral, intermediate, and caudal part s of the ventrolateral medulla. Double-labeling for Fos and tyrosine h ydroxylase immunoreactivity showed that 50% to 75% of Fos-positive cel ls in the rostral, intermediate, and caudal ventrolateral medulla and 30% to 40% of Fos-positive cells in the nucleus of the solitary tract were also positive for tyrosine hydroxylase in both intact and baroden ervated animals. The distribution of Fos-positive neurons corresponded very closely to the location of Ang II receptor binding sites as prev iously determined in the rabbit. The results indicate that medullary n eurons activated by Ang II are located in discrete regions within the nucleus of the solitary tract and ventrolateral medulla and include, i n all of these regions, both catecholamine and noncatecholamine neuron s.