Responses of raphe nucleus projecting subfornical organ neurons to angiotensin II in rats

The subfornical organ (SFO) is an important central site of action of circulating angiotensin II (ANG II). Although neuroanatomical tracing studies have identified the efferent pathways from the SFO to the midbrain raphe nucleus (RN), the functional role of the pathways is unknown. The present study was carried out to examine the responses of SFO neurons projecting to the dorsal RN (DRN) to microiontophoretic application or intracarotid injection of ANG II in male rats under urethane anesthesia. Twenty-three neurons in the SFO were antidromically identified by electrical stimulation of the midbrain DRN. Of these identified units, 13 were excited by ANG II applied iontophoretically, while 10 were unresponsive. ANG II-induced excitation was prevented by the ANG II antagonist saralasin (Sar) applied iontophoretically. The activity of seven out of 10 units that displayed this excitation to iontophoretically applied ANG II was also enhanced by intracarotid injection of ANG II. These results suggest that SFO neurons projecting to the DRN may monitor the circulating level of ANG II and carry the information to the DRN.