Responses of subfornical organ neurons projecting to the hypothalamic paraventricular nucleus to hemorrhage.

The activity of subfornical organ (SFO) neurons that were antidromically identified by electrical stimulation of the rat hypothalamic paraventricular nucleus (PVN) was tested for a response to microiontophoretic application of angiotensin II (ANG II) or hemorrhage (10 ml/kg b.w.t.). Microiontophoretically (MIPh) applied ANG II caused an increased excitability in 24 out of 28 neurons tested and the excitation was blocked by MIPh-applied saralasin (Sar), a specific ANG II antagonist. Of these neurons that responded to ANG II, 14 displayed an increase in neuronal firing in response to hemorrhage, while 10 were unresponsive. The excitatory response to hemorrhage in 5 out of 14 neurons tested was prevented by MIPh-applied Sar, whereas the response of the remaining neurons was not affected. These results show that part of SFO neurons projecting to the PVN may receive neural inputs from the peripheral baroreceptors, and suggest that the inputs may be partially attributable to the involvement of central angiotensinergic circuits.