ABSTRACT
The presence and roles of NO are well demonstrated in the main invertebrate groups, show ing the widespread occurrence of this signaling molecule throughout the animal kingdom, from higher invertebrates down to porifera and even to prokaryotic cells. In invertebrates, experimental evidence suggests the presence of NOS isoforms other than those known for higher organisms. Noteworthy is the early appearance of NO during evolution and striking is the role played by the nitrergic pathway in the sensorial functions, from coelenterates up to mammals, mainly in olfactory-like systems.
