COMPARATIVE ASPECTS ON NITRIC-OXIDE IN BRAIN AND ITS ROLE AS A CEREBRAL VASODILATOR

Histological studies have detected nitric oxide (NO) synthase in the c entral nervous system of all vertebrates examined, from lampreys to ma mmals. However, there are still very few comparative physiological stu dies on the function of NO synthase in the brain of non-mammalian vert ebrates. So far, we know that acetylcholine can cause an NO-dependent increase in brain blood flow in turtles and some fish species (crucian carp and rainbow trout), whereas some other fishes appear to lack suc h a mechanism. Hypercapnia can induce NO-dependent cerebral vasodilati on in mammals, but such a mechanism appears to be lacking in the ectot hermic vertebrates examined. The number of species studied needs to be expanded before we can draw any firm conclusions about the origin of NO-dependent brain blood flow regulation: if it has evolved more than once or if it has been occasionally lost during evolution. We conclude that NO synthase may be present in all vertebrate brains but that its functions can vary, as judged from its role in cerebral blood flow re gulation. The diversity of functions that NO has proven to have within the mammalian brain is likely to be paralleled by the same degree of diversity of function between vertebrate groups. (C) 1997 Elsevier Sci ence Inc.