Effects of anoxia, hyperoxia, and salinity on neurons in the leech Nephelopsis obscura (Erpobdellidae): RNA redistribution by fluorescence histochemistry

The histochemical distribution of cytoplasmic RNA in ganglion cells of the freshwater leech Nephelopsis obscura has been studied using the fluorochrome acridine orange as a marker of nucleic acids. Two series of experiments, employing 50 adult animals, involved changes in oxygen tension in the water and changes in salinity. Normal leech neurons exhibit finely granular orange fluorescence uniformly distributed throughout the cytoplasm, with a perinuclear ring of especially strong fluorescence. After exposure to anoxic (0% O2), hypoxic (20% O2), or hyperoxic (200% O2) conditions at 20 degrees C for 1-15 days, the orange cytoplasmic fluorescence is no longer uniformly distributed; the redistribution is generally toward the periphery, leaving the perinuclear zone without RNA fluorescence, but irregular zones of cytoplasm devoid of RNA also occur not as a gradient. Leeches exposed to salinity of, or greater than, 2.5 ppt for 15 days exhibit similar changes. These alterations are confirmed by electron microscopy. Seasonal fluctuations in oxygen tension and salinity of lake water affect the distribution and abundance of organisms. The acridine orange method provides one measure of stress to the nervous system in freshwater invertebrates that might be applicable to ecological studies as well as to metabolic studies of individual animals.