Possible involvement of arachidonic acid and eicosanoids in metamorphic events in Hydractinia echinata (Coelenterata; Hydrozoa)

Cnidarian chemical neurotransmission, an updated overview

The ultrastructural, histochemical, immunocytochemical, biochemical, molecular, behavioral and physiological evidence for non-peptidergic and peptidergic chemical neurotransmission in the Anthozoa, Hydrozoa, Scyphozoa and Cubozoa is surveyed. With the possible exception of data for the catecholamines and peptides in some animals, the set of cumulative data - the evidence from all methodologies - is incomplete. Taken together, the evidence from all experimental approaches suggests that both classical fast (acetylcholine, glutamate, GABA, glycine) and slow (catecholamines and serotonin) transmitters, as well as neuropeptides, are involved in cnidarian neurotransmission. Ultrastructural evidence for peptidergic, serotonergic, and catecholaminergic synaptic localization is available, but the presence of clear and dense-cored synaptic vesicles also suggests both fast and slow classical transmission. Immunocytochemical studies, in general, reveal a continuous, non-localized distribution of neuropeptides, suggesting a neuromodulatory role for them. Immunocytochemical and biochemical studies indicate the presence of glutamate, GABA, serotonin, catecholamines (and/or their receptors), RFamides, nitric oxide and eicosanoids in cnidarian neurons and tissues. Gene sequences for peptidergic preprohormones have been reported; putative gene homologies to receptor proteins for vertebrate transmitters have been found in Hydra. Behavioral and physiological studies implicate classical transmitters, neuropeptides, eicosanoids and nitric oxide in the coordination of the neuroeffector systems.

Metamorphosis in the Cnidaria

The free-living stages of sedentary organisms are an adaptation that enables immobile species to exploit scattered or transient ecological niches. In the Cnidaria the task of prospecting for and identifying a congenial habitat is consigned to tiny planula larvae or larva-like buds, stages that actually transform into the sessile polyp. However, the sensory equipment of these larvae does not qualify them to locate an appropriate habitat from a distance. They therefore depend on a hierarchy of key stimuli indicative of an environment that is congenial to them; this is exemplified by genera of the Anthozoa (Nematostella, Acropora), Scyphozoa (Cassiopea), and Hydrozoa (Coryne, Proboscidactyla, Hydractinia). In many instances the final stimulus that triggers settlement and metamorphosis derives from substrate-borne bacteria or other biogenic cues which can be explored by mechanochemical sensory cells. Upon stimulation, the sensory cells release, or cause the release of, internal signals such as neuropeptides that can spread throughout the body, triggering decomposition of the larval tissue and acquisition of an adult cellular inventory. Progenitor cells may be preprogrammed to adopt their new tasks quickly. Gregarious settlement favours the exchange of alleles, but also can be a cause of civil war. A rare and spatially restricted substrate must be defended. Cnidarians are able to discriminate between isogeneic and allogeneic members of a community, and may use particular nematocysts to eliminate allogeneic competitors. Paradigms for most of the issues addressed are provided by the hydroid genus Hydractinia.

Discovery of an 11 (R)- and 12(S)-lipoxygenase activity in ovaries of the mussel Mytilus edulis

Eicosanoid biosynthesis was investigated in mussel gonads by incubation of tissue homogenates with radiolabeled arachidonic acid and analysis of the products by radio-high-performance liquid chromatography. No radiolabeled metabolites were formed in homogenates of testes, but two major metabolites were synthesized by ovarian preparations. The radiolabeled metabolites were analyzed by mass spectrometry and chiral chromatography and identified as 11 (R)-hydroxy-5,8,1 2,14-eicosatetraenoic acid and 12(S)-hydroxy-5,8,10,14-eicosatetraenoic acid. In addition, four other nonlabeled metabolites, formed from endogenous substrates, were detected in ovarian extracts. Their structures, determined by mass spectrometric analysis, were the corresponding 11- and 12-hydroxy analogs formed from eicosapentaenoic acid (11-HEPE and 12-HEPE) and 9-hydroxy-6,10,12,15-octadecatetraenoic acid (9-HOTE) and 13-hydroxy-6,9,11,15-octadecatetraenoic acid formecl from stearidonic acid. The biosynthesis of the 11 - and 12-hydroxy products was calcium dependent, localized to the 100,000 x g supernatant cell fraction, and was inhibited by nordihydroguaiaretic acid, but not inhibited by the prostaglandin synthase inhibitors aspirin and indomethacin, or the monoxygenase inhibitor proadifen. Together these data suggested that both the 11 (R)- and 12(S)-hydroxy products were formed from lipoxygenase-type enzymes. Incubation of homogenates of immature ovaries with eicosapentaenoic acid revealed the major product to be I2-HEPE, whereas in mature ovaries mainly 11-HEPE was formed. Extraction of spawned eggs with methanol revealed that predominantly 11-HEPE and 9-HOTE were formed from endogenous substrates. This study shows that female gonads of the mussel express an 11(R)- and 12(S)-lipoxygenase activity whose expression is dependent on differentiation of the ovary.