NEUROENDOCRINE CONTROL OF OSMOTIC REGULATION IN THE FRESH-WATER SHRIMP MACROBRACHIUM-OLFERSII (WIEGMANN) (CRUSTACEA, DECAPODA) - FREE AMINO-ACID-CONCENTRATIONS IN THE HEMOLYMPH

The participation of neuroendocrine factors present within the central nervous system in the regulation of hemolymph free amino acid (FAA) c oncentrations was examined in the freshwater shrimp Macrobrachium olfe rsii. Test shrimps were injected intramuscularly with homogenates prep ared from the eyestalks (ES), ventral nerve cord (VNC), supraesophagea l (SEG), or thoracic ganglia (TG) of donor shrimps previously exposed for 6 hr to a high-salinity medium (HSM, 21 parts per thousand salinit y). After injection of the homogenate, the shrimps were maintained for up to 6 hr in either freshwater (FW) or HSM. Hemolymph was sampled by cardiac puncture and prepared for reverse phase HPLC, derivatizing th e FAA with phenylisothiocyanate. An FAA profile was determined and the [FAA]:[Cl-] ratios for the four FAA present in highest concentration (Gly, Arg, Ala, and Pro for ES and VNC experiments; Glu, Leu, Ala, and Val for SEG and TG experiments) were obtained. Nonparametric analyses revealed specific, notable effects resulting from homogenate injectio n, e.g., ES homogenate increased [Pro]/[Cl-] ratios in FW-exposed shri mps; SEG homogenate increased [Glu]/[Cl-] and [Val]/[Cl-] ratios in HS M-exposed shrimps; and TG homogenate increased [FAA]/[Cl-] ratios for Glu, Leu, Ala, and Val in HSM-exposed shrimps. Total FAA concentration s decreased after exposure of the shrimps to HSM but were increased by the injection of ES homogenate in FW-exposed shrimps and by TG homoge nate in HSM-exposed shrimps. The total [FAA]/[Cl-] ratio was also incr eased by TG homogenate in HSM-exposed animals. There were no clear eff ects on [Cl-] alone. These data are consistent with the notion that FA A titers in the hemolymph of M. olfersii reflect, anisosmotic extracel lular regulation and its modulation by endocrine factors which accumul ate in neurosecretory cells of the ES, SEG, and TG, particularly the l atter. The role of such modulation in the osmoregulatory process in vi vo is discussed. (C) 1995 Academic Press, Inc.