Director-General Invited Seminar

Nitric oxide was identified as a biological molecule just over 25 years ago, and its presence and potential importance in the nervous system was immediately noted. With the cloning of NO synthase and the physiological NO receptor, soluble guanylyl cyclase, a variety of methods quickly led to a rather complete picture of where NO is produced and acts in the body. In particular we developed the NADPH-diaphorase histochemical method, which allows for a very simple and elegant detection of the NO syntase enzyme activity. NO was quickly recognized as a major intercellular messenger in the central and peripheral nervous systems. It is distinct from other neurotransmitters many key ways. NO is not stored in synaptic vesicles nor released in a quantal fashion. Instead NO production is regulated by intracellular calcium in cells expressing NO synthase. Rather than acting discretely in a temporal and spatial way at synapses, NO quickly spreads from the source of production to affect an entire volume of brain tissue. The details regarding the subcellular localization of NO synthase and the identity of its molecular binding partners require further clarification. Although the hypothesis that calcium influx via activation of NMDA receptors is a key trigger for NO production has proven very popular and led to suggested roles for NO in synaptic plasticity, there is little direct evidence to support this notion. Instead, studies from the peripheral nervous system indicate a key role for voltage-sensitive calcium channels in regulating NO synthase activity. In the peripheral nervous system NO acts to regulate various organ systems on a timescale of minutes, rather than milliseconds. A similar mechanism may also be important in central neurons where NO production appears to vary with behavioural state. It remains an important task to identify the precise sources of calcium regulating NO production in specific NO neurons. Since cGMP production appears to mediate the physiological signaling by NO, understanding the specific roles of cGMP-dependent ion channels, protein kinases and phosphodiesterases in mediating NO action is key. Our recent work points to cGMP-stimulated phosphodiesterase as a key mediator of NO action. This NO-regulated enzyme exists in multipe forms which can be targeted to particular sites within neurons to regulate cAMP levels. The cGMP-regulated enzymes may provide useful targets for pharmacological manipulation of the NO system at specific sites.