ROLE OF NITRIC OXIDE (NO)

Role of Nitric Oxide (No) in the Central and Peripheral Nervous Systems of Vertebrates

Role of Nitric Oxide (No) in the Central and Peripheral Nervous Systems of Vertebrates

In the central nervous system, nitric oxide is produced enzymatically in postsynaptic structures in response to activation of excitatory amino acid receptors. It then diffuses out to act on neighbouring cellular elements, probably presynaptic nerve endings and astrocyte (glial) processes. Being of such a small size NO is freely diffusible across membranes. That is it does not react with receptors but diffuses into adjacent cells. Because it is so liable NO cannot be stored by conventional means nor inactivated after synaptic release. Its local biosynthesis constitutes the only means for regulating NO levels, hence NOS is one of the most regulated enzymes in biology. Thus the compartmentalization of NOS appears to be crucial for its functionality by providing local NO levels. (Abman, 2009)

NO is a major signal transduction molecule in vertebrates and serves as a neurotransmitter in the CNS and the GI tract and may be involved in memory and learning. Unlike most other neurotransmitters that only transmit information from a presynaptic to a postsynaptic neuron, the small nitric oxide molecule can diffuse all over and can thereby act on several nearby neurons, even on those not connected by a synapse. It is thought that this process may be involved in memory through the maintenance of long-term potentiation, or long-lasting strengthening of the connection between two nerve cells. The physiological role of nNOS in mechanisms such as long term potentiation has been shown to involve retrograde transport (diffusion) of NO synthesized in post synaptic neurons across the synaptic cleft into synapses, where it stimulated guanyl cyclase.

Guanylyl cyclase is the enzyme that catalyzes the formation of the messenger cyclic GMP (cGMP) from GTP. It has been established that cGMP plays a role in the relaxation of smooth muscle, the inhibition of platelet aggregation and participates in signal transduction within the nervous system. Moreover, cGMP is involved in the regulation of the water and electrolyte balance as well as in the metabolism of the bone. cGMP is also involved in retinal phototransduction--that is the conversion of a light signal received by a nerve receptor, to an electrical signal transmitted to the brain. Since cGMP is activated by nitric oxide (NO) and peptide hormones and concentrations of both these arise during an ...