Control of extracranial and hindlimb blood flow by the midbrain periaqueductal grey of the cat.

Microinjections of D,L homocysteic acid (DLH, 40 nmoles) made in the lateral PAG of the unanesthetized and paralyzed decerebrate cat evoked distinct patterns of extracranial and hindlimb blood flow. One pattern, evoked from the pretentorial part of the lateral PAG (A3.3-A2.5), consisted of an extracranial vasodilation associated with an iliac vasoconstriction. Another pattern, evoked from the subtentorial part of the lateral PAG (P0.2-P0.9), consisted of an extracranial vasoconstriction associated with an iliac vasodilation. Both patterns were associated with increased arterial blood pressure. These results indicate (i) that the PAG contains neurons regulating head and hindlimb vasculature, and (ii) that these neurons are viscerotopically organized, in the sense that different representations of the head and hindlimb are found at different rostrocaudal levels. The significance of the results is discussed in the context of our previous findings that different kinds of defense reactions are evoked from these same pretentorial and subtentorial PAG regions.