This paper reviews comparative and evolutionary aspects of nitric oxide (NO) signaling
in major systematic groups such as prokaryotes, plants, fungi and invertebrate animals.
It appears that NO-mediated signaling can be as old as cellular organization itself.
Both non-enzymatic and enzymatic (in addition to NOS) synthetic pathways can contribute
to NO formation in living systems. The evolutionary roots of this means of gaseous
signaling can be traced back to the role of NO in non-immune defensive mechanisms
and the role of NO in control of gene expression, chemical ecology and, perhaps, symbiotic
interactions in the ancient prokaryotic world. These functions of NO can be preserved
in practically all modern taxons and be widely expressed in the nervous system. However,
it is hypothesized that neuronal NO signaling is a relatively new evolutionary invention
and it is likely to have happened several times during animal evolution. Although
a comparative analysis of neuronal NO signaling is still in its early stages, the
hypothesis is proposed that in many invertebrate lineages one of the primary neuronal
functions of NO was regulation of feeding patterns, chemosensory processing and neurodevelopment.
