The present studies were undertaken to determine the role of rostral periaqueductal gray (PAG) in mediating the pressor effect produced by intracerebroventricular (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 responses produced by electrical stimulation of the anteroventral third ventricle (AV3V) region and a ventromedial site required for the pressor response elicited by icv administration of ANG II. In Saffan-anesthetized rats, injection of lidocaine (LIDO, 4%) in dorsomedial PAG, but not in ventromedial PAG, significantly attenuated the decrease in hindquarter resistance (HQR) produced by electrical stimulation of the AV3V region, and the poststimulatory increase in mean arterial pressure (MAP) and HQR. The injection of LIDO in ventromedial PAG had no effect on the hemodynamic responses produced by electrical stimulation of the AV3V 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 addressed by microinjecting two retrogradely transported fluorescent dyes, Fluoro-Gold and Fast Blue. The anatomic findings suggest that separation of the pathways activated by electrical and chemical stimulation of the AV3V region occurs at the level of rostral PAG.
Effect of Dorsal and Ventral Hippocampal Lesions on Contextual Fear Conditioning and Unconditioned Defensive Behavior Induced by Electrical Stimulation of the Dorsal Periaqueductal Gray