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

Abstract

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 6ulgaris. 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, h-linolenic acid, were found to reduce the duration of the feeding response in a 10 -100 mM 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 H. 6ulgaris, 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 k-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.Abbreviations. AA= arachidonic acid; BPB = 4-bromophenacyl bromide; cGMP=guanosine 3%:5%-cyclic monophosphate; CNS = central nervous system; DMSO = dimethyl sulfoxide; GABA =k-aminobutyric acid; GTP = guanosine 5%-triphosphate; (11-R)-HETE= 11-(R)-hydroxy-eicosa-5Z,8Z,12E,14Z-tetraenoic acid; (9-R)-HOTrE =9-(R)-hydroxy-octadeca-10E,12Z,15Z-trienoic acid; (11-R)-HPETE= 11-(R)-hydroperoxy-eicosa-5Z,8Z,12E,14Z-tetraenoic acid; HPLC= high-pressure liquid chromatography; (9-R)-HPOTrE =9-(R)-hydroperoxy-octadeca-10E,12Z,15Z-trienoic acid; h-LA= h-linolenic acid; OOPC= oleyloxyethylphosphorylcholine; PLA 2 = phospholipase A 2 ; PUFAs = polyunsaturated fatty acids; TBPS= t-butylbicyclophosphorothionate.The nerve net of the freshwater polyp Hydra (Cnidaria, Hydrozoa) is currently regarded as one of the most primitive nervous systems to have evolved [1]. Neurons are dispersed in the epithelia and are interconnected by neurites into a functional network, or to non-neuronal effectors by uni-and bidirectional synapses (see ref. 2 for a concise review). The nervous net spreads homogeneously over the entire body except in the head region, where the fibres are condensed to form a circular ring, possibly representing a rudimentary cephalization of the system [3]. This seemingly simple, two-dimensional nervous syste...

Alpha‐Linolenic Acid, but Not Palmitic Acid, Negatively Impacts Survival, Asexual Reproductive Rate, and Clonal Offspring Size in Hydra oligactis

“…Arachidonic acid appeared to be the dominant PUFA in hydra, irrespective of diet supplementation with PAM and ALA, and this corresponds with the results for other cnidarians, including marine Octacoralia (Berge & Barnathan, ). This fatty acid has also been reported to play an important role in the feeding behavior of hydra (Pierobon, De Petrocellis, Minei, & Di Marzo, ).…”

Hormonal signaling in cnidarians: do we understand the pathways well enough to know whether they are being disrupted?

“…While their role in cnidarian physiology has not been fully elucidated, LW-amides have been hypothesized to function in tentacle formation, metamorphosis and larval physiology (McFadden and Spencer, 1991;Brumwell and Martin, 1996). RF-amides affect cnidarian muscle contraction, feeding responses and exfoliation of gamete follicles (McFarlane and Grimmelikhuijzen, 1991;Pierobon et al, 1997b;Tremblay et al, 2004). The receptors for these peptides have not been identified, but genes encoding preprohormones have been sequenced from several species (Leviev et al, 1997;Darmer et al, 1998).…”

“…Considerable progress has been made in identifying bioactive molecules, particularly peptides, but in most cases their receptors have not been identified and their precise physiological function is unknown. Other classes of signaling molecules such as fatty acids and eicosanoids may also act as cnidarian hormones (Coll et al, 1994;Coll et al, 1995;Pierobon et al, 1997b;Pierobon et al, 1997a), but more research is needed to identify the functions of these compounds and their mechanisms of action. Iodinated organic molecules, such as thyroxine, have been proposed to hormonally regulate strobilation and metamorphosis in the scyphozoan jellyfish Aurelia aurita (Spangenberg, 1971).…”