Location of the area postrema pressor pathway in the dog brain stem

Electrical stimulation of the dog's area postrema (AP) induces a response that mimics the pressor response produced by intravertebral infusion of low-dose angiotensin II, which causes an increase in mean arterial pressure associated with transient tachycardia and increased peripheral resistance. The present study investigated in morphine-chloralose anesthetized dogs whether: 1) the characteristics of the AP pressor response are influenced by the presence of carotid sinus afferents; 2) structures rostral to the medulla influence the AP pressor response; and 3) the pressor pathway is initiated by neurons within the AP. Since bilateral cervical sinovagal denervation, which potentiated the phenylephrine pressor response, did not affect the pressor response to AP stimulation, the data provide evidence for an inhibitory influence exerted upon the central baroreflex mechanism by the AP pressor mechanism. The unaltered AP pressor response after midcollicular transection suggests that the efferent pathway is contained within the brain stem caudal to the pons. Finally, the elimination of the pressor response following kainic acid microinjection into the AP provides evidence that the AP pressor mechanism is initiated by neurons within the AP, rather than by fibers of passage from other pressor centers. These results suggest that the AP produces its facilitation of central sympathetic vasomotor outflow via a pathway contained within the medulla.