Arachidonic acid as an endogenous signal for the glutathione-induced feeding response in Hydra

Phospholipase A(2)-derived arachidonic acid (AA) and related metabolic products represent an important pathway involved in the regulation of growth and morphogenesis as well as in oxidative processes in cnidarian tissues. Here we present data on the participation of AA in the glutathione (GSH)-induced feeding response in Hydra vulgaris. Under conditions in which it produces the feeding response (which consists mainly of mouth opening followed by mouth closure), GSH dose-dependently induced the release of free arachidonic acid from live polyps. Phospholipase A(2) inhibitors blocked this release and enhanced the duration of GSH-induced Hydra mouth opening. Accordingly, AA and, to a smaller extent, alpha-linolenic acid, were found to reduce the duration of the feeding response in a 10-100 mu M concentration range, and this effect resulted mainly from earlier times of mouth closure. 11-(R)-hydroxy-eicosa-5Z,8Z,12E,14Z-tetraenoic acid, a lipoxygenase metabolite of AA in PI. vulgaris, reproduced the effects of its precursor fatty acid at lower concentrations. The kinetics of GSH-induced arachidonate release and GSH-induced feeding response were correlated, suggesting that AA might modulate the mechanisms controlling mouth closure. Finally, the role of AA in the facilitation of the mouth-closing phase of the feeding response was further supported by the finding that: (1) exogenous AA reversed the effect on the feeding response of gamma-aminobutyric acid, which normally delays the times of mouth closure, and (2) endogenous AA was also released in the absence of GSH from live polyps stimulated with antho-RFamide, a neuropeptide abundant in hydrozoans. We suggest that in Hydra AA may act as a second messenger following chemical stimulation.