The role of the subfornical organ in angiotensin II-salt hypertension in the rat

Hypertension caused by chronic infusion of angiotensin II (Ang II) in experimental animals is dependent, in part, on increased activity of the sympathetic nervous system. This chronic sympathoexcitatory response is amplified by a high-salt diet, suggesting an interaction of circulating Ang II and dietary salt on sympathetic regulatory pathways in the brain. The present study tested the hypothesis that the subfornical organ (SFO), a forebrain circumventricular organ known to be activated by circulating Ang II, is crucial to the pathogenesis of hypertension induced by chronic Ang II administration in rats on a high-salt diet (Ang II-salt model). Rats were randomly selected to undergo either subfornical organ lesion (SFOx) or sham surgery (Sham) and then placed on a high-salt (2% NaCl) diet. One week later, rats were instrumented for radiotelemetric measurement of mean arterial pressure (MAP) and heart rate (HR) and placed in metabolic cages to measure sodium and water balance. Baseline MAP was slightly (but not statistically) lower in SFOx compared with Sham rats during the 5 day control period. During the subsequent 10 days of Ang II administration, MAP was statistically lower in SFOx rats. However, when MAP responses to Ang II were analysed by comparing the change from the 5 day baseline period, only on the fifth day of Ang II was MAP significantly different between groups. There were no differences between groups for water or sodium balance throughout the protocol. We conclude that, although the SFO is required for the complete expression of Ang II-salt hypertension in the rat, other brain sites are also involved.