FOS-LIKE IMMUNOREACTIVITY IN THE MEDULLA AFTER ACUTE AND CHRONIC ANGIOTENSIN-II INFUSION

Acute and chronic angiotensin (Ang) II hypertension are reported to ha ve different mechanisms that involve differential contributions of the peripheral vasculature and the nervous system. Acute Ang II hypertens ion is mediated primarily by Ang acting at vascular smooth muscle, whe reas chronic Ang II hypertension appears to have a neural component. I n our experiments, the transition from a peripheral to a neural effect occurs over 10 hr of Ang II infusion in rats. To identify the role of the central nervous system in this transition, we measured Fos immuno reactivity, an indicator of neural activity, in the nucleus of the sol itary tract (NTS), caudal ventrolateral medulla (CVL) and rostral vent rolateral medulla (RVL) in normal, sinoaortic denervated (SAD) and sha m SAD rats after 2- or 18-hr Ang II infusion (50 ng/kg/min intravenous ly). Vehicle (5% dextrose) was infused in normal rats as control. Comp arable increases in arterial pressure were produced by 2- and 18-hr An g II infusion in all groups. Fos was increased in the NTS in sham SAD rats by 2- and 18-hr Ang II infusion (P < .05 vs. vehicle control). In the CVL, only 2-hr Ang II infusion was associated with increased Fos in normal and sham SAD rats (P < .05 vs. vehicle control) but not in S AD rats. In the RVL, 18-hr Ang II infusion elevated Fos in all groups (P < .05 vs. vehicle control). Activation of NTS during Ang II infusio n is baroreceptor mediated and independent of infusion duration. Acute Ang II infusion produced a baroreceptor-mediated activation of the CV L, a region associated with baroreflex sympathoinhibition. Chronic Ang II infusion produced a baroreceptor-independent activation of the RVL , a brain area associated with sympathoexcitation, suggesting a centra lly mediated increase in sympathetic outflow that may be associated wi th chronically infused Ang II.