Neuronal localization and evoked release of norepinephrine in the cnidarianRenilla koellikeri

Catecholamine Involvement in the Bioluminescence Control of Two Species of Anthozoans

Bioluminescence, the ability of living organisms to emit visible light, is an important ecological feature for many marine species. To fulfil the ecological role (defence, offence, or communication), bioluminescence needs to be finely controlled. While many benthic anthozoans are luminous, the physiological control of light emission has only been investigated in the sea pansy, Renilla koellikeri. Through pharmacological investigations, a nervous catecholaminergic bioluminescence control was demonstrated for the common sea pen, Pennatula phosphorea, and the tall sea pen, Funiculina quadrangularis. Results highlight the involvement of adrenaline as the main neuroeffector triggering clusters of luminescent flashes. While noradrenaline and octopamine elicit flashes in P. phosphorea, these two biogenic amines do not trigger significant light production in F. quadrangularis. All these neurotransmitters act on both the endodermal photocytes located at the base and crown of autozooids and specific chambers of water-pumping siphonozooids. Combined with previous data on R. koellikeri, our results suggest that a catecholaminergic control mechanisms of bioluminescence may be conserved in Anthozoans.

Neuroactive compounds induce larval settlement in the scleractinian coral Leptastrea purpurea

Settlement of pelagic coral larvae is commonly induced by chemical cues that originate from biofilms and coralline algae. These natural settlement cues initiate signal pathways leading to attachment and metamorphosis of the coral larva. In order to investigate the settlement process and its natural inducers, it is necessary to gain a better understanding of these signal pathways. At present, the pathways and neurotransmitters involved in this signal transduction are still widely unknown. In this study, we exposed larvae of the brooding coral Leptastrea purpurea to five neuroactive compounds known to be present in cnidarians, and K⁺ Ions. All compounds were applied at different dilutions and settlement behavior of the larvae was documented over 48 h. Dopamine, glutamic acid and epinephrine significantly induced settlement in the coral larvae. The highest observed metamorphosis response was 54% in 10^-5 M dopamine. Serotonin, L-DOPA and K⁺ ions did not have an influence on settlement behavior in our experiments. Exposing larvae to settlement-inducing neurotransmitters and thus bypassing the initial induction could be utilized in coral aquaculture. The active neurotransmitters should be used to further study the settlement process in L. purpurea in greater detail. Their role and relevance should also be assessed for other coral species as they may represent or reveal a universal inducer for coral settlement.

Chemical transmission in the sea anemone Nematostella vectensis: A genomic perspective

The sequencing of the starlet sea anemone (Nematostella vectensis) genome provides opportunities to investigate the function and evolution of genes associated with chemical neurotransmission and hormonal signaling. This is of particular interest because sea anemones are anthozoans, the phylogenetically basal cnidarians least changed from the common ancestors of cnidarians and bilaterian animals, and because cnidarians are considered the most basal metazoans possessing a nervous system. This analysis of the genome has yielded 20 orthologues of enzymes and nicotinic receptors associated with cholinergic function, an even larger number of genes encoding enzymes, receptors and transporters for glutamatergic (28) and GABAergic (34) transmission, and two orthologues of purinergic receptors. Numerous genes encoding enzymes (14), receptors (60) and transporters (5) for aminergic transmission were identified, along with four adenosine-like receptors and one nitric oxide synthase. Diverse neuropeptide and hormone families are also represented, mostly with genes encoding prepropeptides and receptors related to varying closeness to RFamide (17) and tachykinin (14), but also galanin (8), gonadotropin-releasing hormones and vasopressin/oxytocin (5), melanocortins (11), insulin-like peptides (5), glycoprotein hormones (7), and uniquely cnidarian peptide families (44). Surprisingly, no muscarinic acetylcholine receptors were identified and a large number of melatonin-related, but not serotonin, orthologues were found. Phylogenetic tree construction and inspection of multiple sequence alignments reveal how evolutionarily and functionally distant chemical transmitter-related proteins are from those of higher metazoans.

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.

Antho-RFamide-containing neurons in the primitive nervous system of the anthozoan Renilla koellikeri

The neuropeptide Antho-RFamide is extremely abundant in Renilla koellikeri (sea pansy), a representative of the cnidarians (octocorallians) considered to be closest to the stem ancestors of metazoans with nervous systems. Therefore, a knowledge of the distribution of Antho-RFamide-containing neurons in this species would contribute to our understanding of the early evolution of nervous systems. Using antisera raised against RFamide and FMRFamide, we detected immunostaining in numerous neurons throughout the nervous system of the sea pansy. The antisera revealed ectodermal nerve-nets on the upper and lower sides of the colony and on the oral side of tentacles, in the oral disk, and in the pharynx of feeding polyps. Neurons were immunostained also in the mesogleal nerve-net of feeding polyps and in the through-conducting mesogleal nerve-net of the colonial mass. Varying densities of stained neurons were observed in the different compartments of the endoderm: muscular walls of the feeding and water circulation polyps, mesenteric filaments and their derived follicles containing either ovocytes or spermatophores, in the endodermal channels connecting the different compartments of the colony, and in circular muscle of the peduncle. The distribution of immunostained neurons suggests that they play important roles in feeding, reproduction, neuromuscular transmission, and in neuro-neuronal transmission coordinating the different parts of the colony.

Monoamine release by neurons of a primitive nervous system: an amperometric study

We measured monoamine release from dissociated neurons of the sea pansy Renilla koellikeri, a representative of the most evolutionarily ancient animals with nervous systems, by real-time monitoring of exocytosis using the amperometric method with carbon-fiber microelectrodes. Depolarization-induced, as well as spontaneously active, neurons exhibited calcium-dependent exocytotic events at both the soma and the terminal bulb of neuritic processes. All spontaneously active neurons exhibited a bursting activity pattern in which amplitudes of exocytotic events appeared to be distributed in a quantal-like fashion. Fast Fourier transform analysis of bursting activity in 20 such neurons revealed burst harmonics with a major frequency of 8 Hz and a dominant rate of 95 Hz for individual exocytotic events within bursts. The results suggest that exocytotic transmitter release is as ancient as neurons and that endogenously bursting neurons in the sea pansy are as complex as those of higher animals. In addition, the observation that both soma and neuritic terminals of the same neuron can release transmitter suggests that local release sites in these cnidarian neurons are not critical for nerve net function.

Evidence for gonadotropin-releasing hormone-like peptides in a cnidarian nervous system

There is increasing evidence that peptides of the gonadotropin-releasing hormone (GnRH) family, long considered a vertebrate preserve, are also present in invertebrate (molluscan) nervous systems. The possibility was examined that GnRHs are present and bioactive in cnidarians, considered to be representatives of the most primitive animals possessing a nervous system. Immunoreactive GnRH was detected in endodermal neurons of two anthozoans, the sea pansy Renilla koellikeri and the sea anemone Nematostella vectensis. In the sea pansy, immunoreactivity was detected throughout the autozooid polyps, including gamete-producing endoderm. High-performance liquid chromatography and radioimmunoassays of extracts from whole sea pansy colonies yielded two elution peaks exhibiting GnRH immunoreactivity with antisera raised against shark or mammalian GnRH. Vertebrate GnRHs as well as the two sea pansy GnRH-like factors inhibited the amplitude and frequency of peristaltic contractions in the sea pansy, and these actions were blocked by the LHRH analog [D-pGlu(1),D-Phe(2),D-Trp(3,6)]-LHRH. These results suggest that the GnRH family of neuropeptides is more widespread in metazoans than previously thought. Although our physiological data are preliminary, they point to a role for GnRHs as inhibitory modulators of neuromuscular transmission in the sea pansy.