No news on the flatworm front! Nitric oxide synthase in parasitic and free-living flatworms

The free radical nitric oxide (NO) has emerged as a simple and unique signa lling molecule that can serve as neurotransmitter, paracrine substance or h ormone. NO is a gas, formed by various neuronal cells, both centrally and p eripherally. NO regulates cyclic GMP synthesis. The production of NO can be detected using the NADPH diaphorase (NADPH-d) histochemical stain for nitr ic oxide synthase (NOS). NOS was detected in two parasitic flatworms, Diphy llobothrium dendriticum and Hymenolepis diminuta, and two free-living flatw orms, Planaria torva and Girardia tigrina. The staining for NOS was very st rong in the nervous system of both parasitic worms. The main nerve cords, t he transverse ring commmissures, nerves in association with the musculature , especially the cirrus musculature and sensory nerve endings showed NADPH- d staining. The NADPH-d staining in the free-living flatworms was much weak er. Still NOS activity was found in the neuropile of the brain and in assoc iation with the pharynx musculature. The demonstration of NOS in flatworms, indicates that NO is an old signal molecule in evolutionary terms.