Attenuated pressor responses and Fos-like immunoreactivity to chronic Angiotensin infusion in rats with area postrema lesions

Neurally-mediated hypertension produced by chronic infusion of Angiotensin II (AII) has been shown to require an intact area postrema, the caudal brainstem circumventricular organ. The present study examined Fos-like immunoreactivity (FLI) in specific brain regions of rats with area postrema lesions (APX; n = 6) and in rats with sham-lesions (APS; n = 4) after 2 or 18 h of intravenous infusion of AII (50 ng/kg/min). After 2 h of AII infusion, arterial pressure was increased similarly in rats with APX and in rats with APS (142 mmHg vs. 151 mmHg, respectively) and FLI also was increased comparably between the groups. In contrast, after 18 h of AII infusion, the AII-induced increase in arterial pressure was blunted in rats with APX compared to that in rats with APS (126 mmHg vs. 153 mmHg, respectively). Additionally, after 18 h of AII infusion, FLI was selectively blunted in the rostral ventral lateral medulla (rVLM) of rats with APX (125% of the vehicle-treated control) compared to that observed in the rVLM of rats with APS (372% of the vehicle-treated control). The rVLM contains bulbospinal neurons and is associated with sympathoexcitation; therefore, these results suggest that the area postrema is necessary for the sympathetically-mediated component to the hypertension induced by chronic AII infusion in rats.