NADPH-DIAPHORASE ACTIVITY IN THE NERVOUS-SYSTEM OF THE EMBRYONIC AND JUVENILE POND SNAIL, LYMNAEA-STAGNALIS

inamide-adenine-dinucleotide-phosphated-diaphorase (NADPH-d) histochem istry has been applied in the present study to determine the distribut ion of putative nitric oxide (nitric oxide synthase)-producing cells d uring embryonic and early postembryonic development in the pond snail, Lymnaea stagnalis L., with special reference to the nervous system. T he first NADPH-d-positive structures appear as early as 18% of develop ment (E18, trochophore stage) and correspond to the pair of protonephr idia. These structures later show disintegration, although after metam orphosis (E26=75%) staining of their individually spreading cells can be observed until hatching. Peripheral sensory neurons in the foot, ma ntle edge and lips, and their afferents projecting to the central nerv ous system reveal NADPH-d activity in the postmetamorphosis period (E2 5-E27=E60%-E80%) of embryogenesis. After hatching (P1-P3), a number of stained sensory cells appear in the pharynx and esophagus. Some NADPH -d positive neuronal perikarya occur in the pedal and pleural ganglia, and a few weakly stained cells in the cerebral and buccal ganglia of juvenile snails. At the same time, a continuous bundle of reactive fib ers is formed in the neuropil both through and through around the circ umesophageal ganglion ring. The localization of NADPH-d activity in th e developing nervous system of Lymnaea suggests that nitric oxide part icipates mainly in sensory processes. However, its role in specific in traganglionic integrative events cannot be excluded following embryoni c metamorphosis.