ROLE OF PERIAQUEDUCTAL GRAY IN THE PRESSOR-RESPONSE PRODUCED BY CENTRAL INJECTIONS OF ANGIOTENSIN-II

The present studies were undertaken to determine the role of rostral p eriaqueductal gray (PAG) in mediating the pressor effect produced by i ntracerebroventricular (icv) injection of angiotensin II (ANG II, 200 ng). Two functionally and anatomically distinct sites were identified in rostral PAG: a dorsomedial site involved in the hemodynamic respons es produced by electrical stimulation of the anteroventral third ventr icle (AV3V) region and a ventromedial site required for the pressor re sponse elicited by icv administration of ANG II. In Saffan-anesthetize d rats, injection of lidocaine (LIDO, 4%) in dorsomedial PAG, but not in ventromedial PAG, significantly attenuated the decrease in hindquar ter resistance (HQR) produced by electrical stimulation of the AV3V re gion, and the poststimulatory increase in mean arterial pressure (MAP) and HQR. The injection of LIDO in ventromedial PAG had no effect on t he hemodynamic responses produced by electrical stimulation of the AV3 V region in anesthetized rats but significantly attenuated the pressor response produced by icv administration of ANG II in conscious rats. The hypothesis that these two sites receive separate projections was a ddressed by microinjecting two retrogradely transported fluorescent dy es, Fluoro-Gold and Fast Blue. The anatomic findings suggest that sepa ration of the pathways activated by electrical and chemical stimulatio n of the AV3V region occurs at the level of rostral PAG.