Neurotransmitters in coelenterates

NMDA and GABA B receptors are involved in controlling nematocyst discharge in hydra

The role of chemical neurotransmission in nematocyst discharge was investigated by stimulating the cnidocils of nematocysts in ablated tentacles of Hydra vulgaris with a piezoelectrically-driven glass probe, in the presence of selected neurotransmitters. Acetylcholine, dopamine, epinephrine, glycine, and serotonin (10(-4), 10(-6), 10(-8) M) per se, did not alter stenotele and desmoneme discharge. gamma-Amino-butyric acid (GABA) significantly increased desmoneme discharge when the cnidocil of another desmoneme in the same or adjacent battery cell complex was stimulated without affecting the discharge rates of the directly stimulated desmonemes or stenoteles. Baclofen (GABA(B) agonist) mimicked the increase; its antagonist, phaclofen, counteracted it. GABA(A) agonists and antagonists did not alter discharge rates. Glutamate caused a dose-dependent increase in the discharge rate of directly stimulated stenoteles; distant stenotele and desmoneme discharge rates were unaffected. Kainate, AMPA, and NMDA, per se, did not alter discharge rates. Co-administration of NMDA and kainate mimicked glutamate's effects. AMPA plus NMDA increased discharge rates. DAP-5 (NMDA antagonist) and CNQX, (kainate/AMPA antagonist) counteracted the increase. The findings suggest that metabotropic GABA is involved in recruiting desmonemes by disinhibiting those previously inhibited, and that the NMDA/kainate-AMPA mechanism regulating Ca(++) entry in higher neuroeffector systems is an early-evolved process, which, in hydra, modulates nematocyst discharge.

Distribution of beta 2-like adrenergic receptors in the cnidarian Renilla koellikeri as revealed by autoradiography and in situ hybridization

Autoradiography and in situ hybridization were used to examine the histological distribution of the previously characterized beta 2-like adrenergic receptors involved in the bioluminescent activity of the sea pansy Renilla koellikeri. The use of [3H]-(+/-)CGP12177 as radioligand revealed autoradiographic labelling of the refringent granule-filled endoderm at the base of autozooid tentacles and autozooid columns, and in the corresponding endoderm of siphonozooid polyps, all areas where photocytes are concentrated. The presence of excess (10 microM) unlabelled (+/-)CGP12177 or atenolol in the incubation mixture substantially reduced total [3H]-(+/-)CGP12177 labelling. Under low stringency hybridization washing, human beta 2-adrenoceptor oligonucleotide probe signals were detected in granular cells located in those areas of polyp endoderm that were labelled by [3H]-(+/-)CGP12177. These cells were previously shown to be distinct from, but in close proximity to photocytes. No other cell or tissue type was labelled in polyps or throughout colonial tissues. The results suggest that a conserved form of beta 2-adrenergic receptors is present and synthesized in a unique type of endodermal cell indirectly involved in sea pansy bioluminescence control.

Arg-Phe-amide-like peptides in the primitive nervous systems of coelenterates

By using immunocytochemistry and radioimmunoassays, several substances resembling vertebrate or invertebrate neuropeptides have been found in the nervous systems of coelenterates. The most abundant neuropeptides were those related to the molluscan neuropeptide Phe-Met-Arg-Phe-amide (FMRFamide). Of antisera against different fragments of FMRFamide, those against RFamide were superior in recognizing the coelenterate peptide. Incubation of whole mounts with these RFamide antisera visualized the coelenterate nervous system in such a detail as has previously not been possible. By using a radioimmunoassay with a RFamide antiserum and [J-125]-YFMRFamide as tracer, the RFamide-like peptide from sea anemones was isolated. After cation-exchange chromatography, gelfiltration and HPLC, this peptide was obtained in a pure form.