Endogenous angiotensin II in the paraventricular nucleus regulates arterial pressure during hypotension in rat, a single-unit study.

The hypothalamic paraventricular nucleus (PVN) controls cardiovascular regulation through vasopressin and sympathetic system. The PVN contains angiotensin II (AngII) and AngII receptors. We have already shown that microinjection of AngII into PVN produced a pressor response concomitant with an increase in firing rate of some PVN neurons. This study was performed to find if PVN AngII plays a regulatory function during hypotension. Hypovolemic-hypotension was induced and the possible role of the PVN AngII in returning arterial pressure toward normal was assessed by monitoring cardiovascular response and single-unit activity of the PVN neurons. Hemorrhage augmented the pressor, tachycardic and single-unit responses to AngII. After-hemorrhage injection of PD123319, an AT2 antagonist, into PVN resulted in a significant decrease in firing rate of some neurons, indicating that AngII was released into the PVN due to hemorrhage. Using single-unit recording, we found that PVN receives electrical signals from baroreceptors and from circulating AngII through circumventricular organs. In addition, by producing hemorrhagic-hypotension and bilateral blockade of AT2 receptors of the PVN, we found that AngII regulates arterial pressure toward normal during hypotension. So for the first time, it was verified that brain renin-angiotensin system is also a major regulatory system of the cardiovascular system.