Pressor effects of the injection of noradrenaline into different cerebroventricular spaces in unanesthetized rats.

Injection of noradrenaline (NA) into the lateral cerebral ventricle (i.c.v.) was reported to cause blood pressure increase in unanesthetized rats, blocked by i.v. injection of vasopressin antagonists. We report similar responses to NA injection into the III or IV ventricles, suggesting multiple sites of action for i.c.v. NA. These responses were blocked by i.v. pretreatment with vasopressin antagonist, suggesting a common mediation by vasopressin release into circulation. Selected ventricular spaces were occluded with Nivea cream plugs to identify ventricular areas responding to i.c.v. NA. III ventricle or aqueduct occlusions markedly reduced pressor responses to i.c.v. NA. Microinjection of NA into the periaqueductal gray matter (PAG) caused pressor responses that were similar to those of i.c.v. NA, reinforcing the idea of a site of action in the aqueduct. IV ventricle occlusion only partially blocked the response to i.c.v. NA. The results suggest at least two sites of action for i.c.v. NA in unanesthetized rats. A primary site located in the PAG and another on the IV ventricle wall.