Chemoreception in Hydra vulgaris (attenuata): initial characterization of two distinct binding sites for L-glutamic acid

Linalool acts as a fast and reversible anesthetic in Hydra

The ability to make transgenic Hydra lines has allowed for quantitative in vivo studies of Hydra regeneration and physiology. These studies commonly include excision, grafting and transplantation experiments along with high-resolution imaging of live animals, which can be challenging due to the animal’s response to touch and light stimuli. While various anesthetics have been used in Hydra studies, they tend to be toxic over the course of a few hours or their long-term effects on animal health are unknown. Here, we show that the monoterpenoid alcohol linalool is a useful anesthetic for Hydra. Linalool is easy to use, non-toxic, fast acting, and reversible. It has no detectable long-term effects on cell viability or cell proliferation. We demonstrate that the same animal can be immobilized in linalool multiple times at intervals of several hours for repeated imaging over 2–3 days. This uniquely allows for in vivo imaging of dynamic processes such as head regeneration. We directly compare linalool to currently used anesthetics and show its superior performance. Linalool will be a useful tool for tissue manipulation and imaging in Hydra research in both research and teaching contexts.

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.

Hydra's feeding response: Effect of GABAB ligands on GSH-induced electrical activity in the hypostome of H. vulgaris

The feeding response in the Cnidarian, hydra, consists of mouth opening, tentacle writhing, and the cessation of pacemaker-controlled tentacle and body contractions. The behavior can be induced by reduced glutathione (GSH), contained in body fluids that leak from prey impaled by hydra's cnidocysts. Mouth, tentacle, and body-contraction behavior is carried out by hydra's ectodermal and endodermal epitheliomuscular cells. Here, we present the first evidence of GSH-induced electrical activity in the hypostome and its modification by GABA and GABA_B ligands. The 'heads' of hydra were ablated and the tentacles removed. Suction electrodes, positioned on the mouth, recorded electrical activity produced by GSH, contained either within the electrode, or in the surrounding bath, the mouth being shielded. Recorded impulses were characterized, according to size and temporal pattern, as small, medium and (large) pacemaker impulses. GSH applied in the bath caused a frequency increase of small and medium impulses and a decrease in pacemaker bursts. The changes in frequencies of medium and pacemaker bursts, though not obviously affected by GABA, were counteracted by blocking GABA_B inhibition with phaclofen. Only the highest concentration of GSH applied at the mouth potentially decreased pacemaker frequency and potentially increased medium impulses, without affecting small impulses. GABA caused a significant increase in small and medium impulses relative to GSH which was counteracted by baclofen and/or baclofen plus phaclofen. The results indicate that considerable GSH-receptor circuitry is located in hypostomal tissue proximal to hydra's mouth, and substantiate GABA and GABA_B inhibition within the neuroeffector network of the feeding response.

Molecular characterization of larval development from fertilization to metamorphosis in a reef-building coral

BACKGROUND

Molecular mechanisms underlying coral larval competence, the ability of larvae to respond to settlement cues, determine their dispersal potential and are potential targets of natural selection. Here, we profiled competence, fluorescence and genome-wide gene expression in embryos and larvae of the reef-building coral Acropora millepora daily throughout 12 days post-fertilization.

RESULTS

Gene expression associated with competence was positively correlated with transcriptomic response to the natural settlement cue, confirming that mature coral larvae are "primed" for settlement. Rise of competence through development was accompanied by up-regulation of sensory and signal transduction genes such as ion channels, genes involved in neuropeptide signaling, and G-protein coupled receptor (GPCRs). A drug screen targeting components of GPCR signaling pathways confirmed a role in larval settlement behavior and metamorphosis.

CONCLUSIONS

These results gives insight into the molecular complexity underlying these transitions and reveals receptors and pathways that, if altered by changing environments, could affect dispersal capabilities of reef-building corals. In addition, this dataset provides a toolkit for asking broad questions about sensory capacity in multicellular animals and the evolution of development.

Glutamate signalling: A multifaceted modulator of oligodendrocyte lineage cells in health and disease

Myelin is essential for the mammalian brain to function efficiently. Whilst many factors have been associated with regulating the differentiation of oligodendroglia and myelination, glutamate signalling might be particularly important for learning-dependent myelination. The majority of myelinated projection neurons are glutamatergic. Oligodendrocyte precursor cells receive glutamatergic synaptic inputs from unmyelinated axons and oligodendrocyte lineage cells express glutamate receptors which enable them to monitor and respond to changes in neuronal activity. Yet, what role glutamate plays for oligodendroglia is not fully understood. Here, we review glutamate signalling and its effects on oligodendrocyte lineage cells, and myelination in health and disease. Furthermore, we discuss whether glutamate signalling between neurons and oligodendroglia might lay the foundation to activity-dependent white matter plasticity. This article is part of the Special Issue entitled 'Oligodendrocytes in Health and Disease'.

Regional modulation of the response to glutathione in Hydra vulgaris

In the presence of prey, or upon exposure to reduced glutathione (GSH), Hydra polyps open a mouth to ingest the captured prey and close it after feeding; at rest the mouth is not evident. In previous papers we have shown that GABA, glycine and NMDA modulate the mechanisms of mouth closure through ligand-gated-ion-channel receptors that are similar to their mammalian analogues in terms of biochemical and pharmacological properties. In order to study the regional distribution of these receptors, we have applied the GSH assay to polyps amputated at different levels of the body column. The response to 1-10 µmol l(-1) GSH of polyps lacking either peduncle and foot or the entire body columns (heads) was not different from control, whole animals. In the presence of GABA or muscimol, duration of the response was significantly decreased in heads; the decrease was suppressed by the GABA antagonists gabazine and bicuculline. By contrast, in animals lacking peduncle and foot, duration of the response did not vary upon GABA administration. Conversely, in the presence of glycine, duration of the response in heads preparations was similar to control, whereas in footless polyps, it was significantly reduced. The decrease was mimicked by the glycine agonists taurine and β-alanine, and counteracted by strychnine. These results suggest a regional distribution of receptors to GABA and glycine in the neuromuscular circuitry modulating the feeding behaviour.

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.

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.

Measurement of glutamate and aspartate in Planaria

INTRODUCTION

The major excitatory neurotransmitters in the mammalian central nervous system are glutamate and aspartate. We developed a rapid and efficient method for the extraction and measurement of these amino acids in Planaria--a valuable model for mammalian processes because of their simple, centralized nervous system and similar neurotransmitter systems.

METHOD

The method utilized buffer extraction (perchloric acid containing 0.025% of L-cystine and Na2EDTA), simple derivatization, high-pressure liquid chromatography (HPLC), and fluorescence detection.

RESULTS

The mean+/-S.E.M. amounts of glutamate and aspartate were 322.6+/-43.6 and 188.6+/-27.6 pmol/mg-planarian, respectively.

DISCUSSION

The method provides the ability to investigate changes in glutamate and aspartate in response to drug administration or withdrawal.

Nitrogen regulation of root branching

BACKGROUND

Many plant species can modify their root architecture to enable them to forage for heterogeneously distributed nutrients in the soil. The foraging response normally involves increased proliferation of lateral roots within nutrient-rich soil patches, but much remains to be understood about the signalling mechanisms that enable roots to sense variations in the external concentrations of different mineral nutrients and to modify their patterns of growth and development accordingly.

SCOPE

In this review we consider different aspects of the way in which the nitrogen supply can modify root branching, focusing on Arabidopsis thaliana. Our current understanding of the mechanism of nitrate stimulation of lateral root growth and the role of the ANR1 gene are summarized. In addition, evidence supporting the possible role of auxin in regulating the systemic inhibition of early lateral root development by high rates of nitrate supply is presented. Finally, we examine recent evidence that an amino acid, L-glutamate, can act as an external signal to elicit complex changes in root growth and development.

CONCLUSIONS

It is clear that plants have evolved sophisticated pathways for sensing and responding to changes in different components of the external nitrogen supply as well as their own internal nitrogen status. We speculate on the possibility that the effects elicited by external L-glutamate represent a novel form of foraging response that could potentially enhance a plant's ability to compete with its neighbours and micro-organisms for localized sources of organic nitrogen.

Evolving visual pigments: hints from the opsin-based proteins in a phylogenetically old "eyeless" invertebrate

Visual pigments are photosensitive receptor proteins that trigger the transduction process producing the visual excitation once they have absorbed photons. In spite of the molecular and morpho-functional complexity that has characterized the development of animal eyes and eyeless photoreceptive systems, opsin-based protein family appears ubiquous along metazoan visual systems. Moreover, in addition to classic rhodopsin photoreceptors, all Metazoa have supplementary non-visual photosensitive structures, mainly located in the central nervous system, that sense light without forming an image and that rather regulate the organism's temporal physiology. The investigation of novel non-visual photopigments exerting extraretinal photoreception is a challenging field in vision research. Here we propose the cnidarian Hydra as a useful tool of investigation for molecular and functional differences between these pigment families. Hydra is the first metazoan owning a nervous system and it is an eyeless invertebrate showing only an extraocular photoreception, as it has no recognized visual or photosensitive structures. In this paper we provide an overview of the molecular and functional features of the opsin-based protein subfamilies and preliminary evidences in a phylogenetically old species of both image-forming and non-visual opsins. Then we give new insights on the molecular biology of Hydra photoreception and on the evolutionary pathways of visual pigments.

Pacemaker activity in hydra is modulated by glycine receptor ligands

In the mammalian central nervous system, the neurotransmitter, glycine, acts both on an inhibitory, strychnine-sensitive receptor (GlyR) and an excitatory, strychnine-insensitive site at the NMDA receptor. Here we present electrophysiological evidence that the strychnine-sensitive glycine agonists, glycine and taurine, and the antagonist, strychnine, affect the endodermal rhythmic potential (RP) system and that the ectodermal contraction burst (CB) pacemaker system is modulated by glycine and strychnine in hydra. The RP and CB pacemaker systems are responsible for the respective elongation and contraction of hydra's body column. Activity of the CB system, quantified by the rate of contraction bursts (CBs), the number of pulses per contraction burst (P/CB), and the duration of bursts, was decreased by glycine. Glycine, coadministered with the strychnine-insensitive glycine site blocker, indole-2-carboxylic acid (I2CA), decreased RPs but not CBs or P/CB. The effect was mimicked by taurine. Strychnine increased the duration of RP production, and decreased CB duration. The effect of glycine with I2CA was counteracted by strychnine. The results support the idea that a vertebrate-like GlyR may be involved in modulating activity of the endodermal RP system and suggest that a glycine site on an NMDA receptor may be involved in the CB system.

Putative NMDA receptors in Hydra: a biochemical and functional study

The feeding behaviour of the freshwater polyp Hydra vulgaris (Cnidaria, Hydrozoa) is modulated by a number of molecules acting as neurotransmitters in other nervous systems. Here we present biochemical and functional evidence of the occurrence of putative NMDA receptors in Hydra tissues. Saturation experiments showed the presence of one population of binding sites with nanomolar affinity and low capacity for [3H]MK-801. Before equilibrium, [3H]MK-801 binding was increased by the agonists glutamate and glycine as well as by reduced glutathione (GSH). In vivo the glutamate receptor agonist NMDA markedly decreased the duration of the response to GSH. This effect was linearly related to ligand doses in the nanomolar concentration range and was counteracted by either the NMDAR-specific antagonist D-AP5 or by the d-serine antagonist DCKA. When NMDA concentration was increased to 10 or 100 microm, duration of the response to GSH was no longer affected unless the lectin concanavalin A, which prevents receptor desensitization in other systems, was added to the test medium. Simultaneous administration of ineffective doses of NMDA and strychnine, glycine or d-serine, an agonist at the glycine binding site of the NMDA receptor in vertebrate CNS, resulted in a strong reduction of response duration. Both D-AP5 and DCKA suppressed this effect. These results, together with the decrease in response duration produced by d-serine, support the hypothesis that NMDA-like glutamate receptors may occur in Hydra tissues where they are involved in modulation of the response to GSH with opposite actions to those of GABA and glycine.

Neuronal evolution: analysis of regulatory genes in a first-evolved nervous system, the hydra nervous system

Cnidarians represent the first animal phylum with an organized nervous system and a complex active behavior. The hydra nervous system is formed of sensory-motoneurons, ganglia neurons and mechanoreceptor cells named nematocytes, which all differentiate from a common stem cell. The neurons are organized as a nerve net and a subset of neurons participate in a more complex structure, the nerve ring that was identified in most cnidarian species at the base of the tentacles. In order to better understand the genetic control of this neuronal network, we analysed the expression of evolutionarily conserved regulatory genes in the hydra nervous system. The Prd-class homeogene prdl-b and the nuclear orphan receptor hyCOUP-TF are expressed at strong levels in proliferating nematoblasts, a lineage where they were found repressed during patterning and morphogenesis, and at low levels in distinct subsets of neurons. Interestingly, Prd-class homeobox and COUP-TF genes are also expressed during neurogenesis in bilaterians, suggesting that mechanoreceptor and neuronal cells derive from a common ancestral cell. Moreover, the Prd-class homeobox gene prdl-a, the Antp-class homeobox gene msh, and the thrombospondin-related gene TSP1, which are expressed in distinct subset of neurons in the adult polyp, are also expressed during early budding and/or head regeneration. These data strengthen the fact that two distinct regulations, one for neurogenesis and another for patterning, already apply to these regulatory genes, a feature also identified in bilaterian related genes.

GABA and glutamate receptors are involved in modulating pacemaker activity in hydra

The effects of gamma-amino butyric acid (GABA) and glutamate, their ionotropic agonists and antagonists on hydra's ectodermal and endodermal pacemaker systems were studied. GABA decreased ectodermal body contraction bursts (CBs) and the number of pulses in a burst (P/CB) and endodermal rhythmic potentials (RPs); tentacle pulses (TPs) were not affected. The GABA(A) agonist, muscimol, and the benzodiazepine receptor agonist, diazepam, mimicked the effects of GABA on the endodermal system. The GABA(A) antagonist bicuculline counteracted GABA's effects. Low concentrations of glutamate increased CBs and RPs. Higher concentrations required concanavalin A (Con A) to produce the same effect on CBs and P/CB. TPs were increased by high concentrations of glutamate and kainate. The ionotropic glutamate agonist, alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) also required Con A to increase CBs and RPs. The effects of AMPA were antagonized by 6-nitro-7-sulfamoylbenzo[f]quinoxaline-2,3-dione (NBQX), which, per se, decreased CBs. The results indicate that GABA and glutamate, acting on their ionotropic receptors, modify the impulses of hydra's pacemaker systems. On the whole GABA decreased the outputs of both ectodermal and endodermal impulse generating systems, while glutamate increased them.

Thiol-induced discharge of acontial nematocytes

The discharge of nematocytes, the stinging cells of Coelenterata, is a poorly understood phenomenon. In particular, little is known about the chemical stimuli that trigger the discharge. In this paper, we show that thiols are able to initiate the nematocyst discharge in isolated nematocytes. Among the thiols tested, reduced glutathione and cysteine were found to be the most effective. The effect of glutathione was likely two-fold: it formed mixed disulfides with membrane thiols, as shown by the ability of the mercapto-blocking reagent iodoacetamide to abolish its action; and it bound to the membrane through the glutamate moiety, as demonstrated by competitive experiments with free glutamate. Glutathione triggered the discharge at concentrations higher than those sufficient to activate the feeding response of Coelenterates. However, our results demonstrate for the first time that the modification of membrane thiols by selective agents may be a key event in the discharge of nematocytes.

Putative glycine receptors in Hydra: a biochemical and behavioural study

Glycine acts as an inhibitory transmitter in the lower brain stem and spinal cord of vertebrate species, while very few data are yet available to support a similar role in invertebrate nervous systems. Here we report the identification and characterization of glycine receptors in the freshwater polyp Hydra vulgaris (Cnidaria, Hydrozoa) by biochemical and behavioural studies. Saturation experiments revealed the occurrence of one population of binding sites of nanomolar affinity (KD = 33 nm) and low capacity (Bmax = 79 fmol/mg protein) for [(3)H]strychnine. The addition of glycine or taurine (0.1 microm-1 mm) produced a dose-dependent inhibition of [(3)H]strychnine binding. Beta-alanine (0.1-1 mm) did not significantly affect [(3)H]strychnine binding. The pharmacological properties of these receptors compare with those of vertebrate glycine receptors. Stimulation of Hydra polyps by reduced glutathione resulted in a significant increase in the duration of mouth opening in the presence of glycine, taurine or beta-alanine. The enhancement of the response was related both to amino acid (10-100 microm) and to glutathione concentration (1-10 microm). The effects of glycine or its agonists were suppressed by strychnine (1-10 microm). D-serine, a glycine agonist at the vertebrate NMDA receptor, produced opposite effects to those of glycine. The effects of d-serine were suppressed by 5,7-dichlorokynurenic acid but not by strychnine. In vitro, [(3)H]strychnine binding was not displaced by d-serine. These results indicate a dual action of glycine in Hydra tissues. The hypothesis that NMDA receptors may also be present in this elementary nervous system is proposed.

The spawning pheromone cysteine-glutathione disulfide ('nereithione') arouses a multicomponent nuptial behavior and electrophysiological activity in Nereis succinea males

The pheromone nereithione (cysteine-glutathione disulfide), which is released by swimming females of the polychaete Nereis succinea to activate spawning behavior of N. succinea males, has recently been identified and synthesized. Nereithione activates sperm release at less than 10(-6) M, one to two orders of magnitude less than oxidized glutathione or any other glutathione derivative tested. The glutathione fragment gamma-glu-cys inhibited sperm release. Nereithione aroused three components of the male nuptial behavior: circling, sperm release, and accelerated swimming. Electrophysiological activity elicited by nereithione near the sperm release site consisted of initial large spikes, cyclic bursting activity, and small spikes lasting up to a minute and was dose dependent, rapid, reversible, and repeatable. This preparation is an excellent model system for characterizing the receptors and functions of a marine pheromone.

Modulation of gamma-aminobutyric acid (GABA) receptors and the feeding response by neurosteroids in Hydra vulgaris

Gamma-Aminobutyric acid (GABA) receptors are present in membrane preparations from Hydra vulgaris, one of the most primitive organisms with a nervous system. These receptors are sensitive to muscimol and benzodiazepines and appear to be important in the regulation of the feeding response. The effects of neurosteroids, general anaesthetics, and GABA antagonists on GABA(A) receptors in membranes prepared from Hydra and on the feeding response have now been investigated. The neurosteroids tetrahydroprogesterone and tetrahydrodeoxycorticosterone increased [3H]GABA binding to hydra membranes with nanomolar potency (EC50, 141+/-11 and 623+/-36 nM, respectively) and high efficacy (maximal increase 79+/-6.5 and 62+/-4%, respectively), whereas the 3beta-hydroxy epimer of tetrahydroprogesterone was ineffective. The benzodiazepine receptor ligands diazepam (100 microM), clonazepam (100 microM) and abecarnil (30 microM) enhanced [3H]GABA binding to Hydra membranes by 22, 20 and 24%, respectively; effects abolished by the specific benzodiazepine antagonist flumazenil (100 microM). On the contrary, the peripheral benzodiazepine receptor ligand 4'chlorodiazepam failed to affect [3H]GABA binding to Hydra membranes. The general anaesthetics propofol and alphaxalone similarly increased (+38% and +30% respectively) [3H]GABA binding. Moreover, [3H]GABA binding to Hydra membranes was completely inhibited by the GABA(A) receptor antagonist SR 95531, whereas bicuculline was without effect. The modulation of GABA(A) receptors in vitro by these various drugs correlated with their effects on the glutathione-induced feeding response in the living animals. Tetrahydroprogesterone and tetrahydrodeoxy-corticosterone (1 to 10 microM) prolonged, in a dose-dependent manner, the duration of mouth opening induced by 10 microM glutathione, with maximal effects of +33 and +29%, respectively, apparent at 10 microM neurosteroid. Alphaxalone (10 microM) similarly increased (+33%) the effect of glutathione. The effects of steroids on the feeding response were inhibited by SR 95531 in a dose-dependent manner; t-butylbyclophosphorothyonate (1 microM), a specific Cl- channel blocker, which per se, like picrotoxin but not bicuculline, shortened the duration of the response, also counteracted the steroids effects at 1 microM. These results suggest that the modulation of GABA(A) receptors by steroids is an ancient characteristic of the animal kingdom and that the pharmacological properties of these receptors have been highly conserved through evolution.

Two novel types of L-glutamate receptors with affinities for NMDA and L-cysteine in the olfactory organ of the Caribbean spiny lobster Panulirus argus

A subset of olfactory receptor neurons of the Caribbean spiny lobster Panulirus argus possesses receptors for L-glutamate that can mediate both excitatory and inhibitory responses (P.C. Daniel, M.F. Burgess, C.D. Derby, Responses of olfactory receptor neurons in the spiny lobster to binary mixtures are predictable using a non-competitive model that incorporates excitatory and inhibitory transduction pathways, J. Comp. Physiol. A 178 (1992) 523-536). In this study, we have used biochemical and electrophysiological techniques to understand the role of these receptors in olfactory transduction, and to compare these olfactory glutamate receptors with peripheral and central L-glutamate receptors in other animals. Using a radioligand-binding assay with a membrane-rich preparation from the dendrites of olfactory receptor neurons, we have identified two types of binding sites for L-glutamate. Both sites showed rapid, reversible, and saturable association with radiolabeled L-glutamate, and their Kd values (1 nM and 3 microM) are effective in physiological studies of glutamate-sensitive olfactory neurons, suggesting these binding sites are receptors involved in olfactory transduction. Both sites were completely inhibited by high concentrations of NMDA and L-cysteine, and only partially inhibited by other L-glutamate analogs and odorants. Electrophysiological recordings from L-glutamate-best olfactory receptor neurons showed that NMDA and L-cysteine are both partial agonists and antagonists of glutamate receptors. Together, these results suggest the olfactory L-glutamate receptors of spiny lobsters are novel types of L-glutamate receptors that are functionally important in mediating olfactory responses.

Nitric oxide involvement in Hydra vulgaris very primitive olfactory-like system

Hydra feeding response is a very primitive olfactory-like behavior present in a multicellular organism. We investigated the role of nitric oxide (NO) in the induction and control of hydra feeding response. Under basal conditions, hydra specimens produce detectable amounts of nitrite (NO2-), the breakdown product of NO. When hydra were incubated with reduced glutathione (GSH), the typical activator of feeding response, an increase of basal NO production was observed. This effect was inhibited by glutamic or alpha-aminoadipic acids, two GSH antagonists, which block GSH-induced feeding response, and by the NO synthase (NOS) inhibitor L-NAME. Moreover, we found that hydra possess a calcium-dependent (but calmodulin-independent) NOS isoform. By using exogenous NO donors and NOS inhibitors, we demonstrated that NO stimulus can participate both in triggering tentacular movements and in recruiting neighbor tentacles during hydra feeding response. By using dbt2-cGMP, an analog to cGMP, we observed that the NO effect was independent of cGMP pathway. Our results strongly implicate NO involvement in hydra very primitive feeding behavior, thus confirming its preservation throughout evolution.

Affinity purification of Hydra glutathione binding proteins

The association of glutathione (GSH) with putative external chemoreceptors elicits feeding behavior in Hydra. In the present study, solubilized membrane proteins were chromatographed on an affinity column of immobilized GSH in order to isolate GSH-binding proteins that may represent the Hydra GSH chemoreceptor. The most abundant of the affinity-purified proteins was a triplet of peptides ranging in molecular weight from 24.5-26 kDa. Antiserum generated against the 24.5-26 kDa triplet peptides inhibited GSH-stimulated feeding behavior by 47%, implicating a role for one or more of these peptides in Hydra chemoreception.

Stereoselective detection of amino acids by lobster olfactory receptor neurons

1. Biochemical and electrophysiological assays were used to test the hypothesis that the olfactory system of the Caribbean spiny lobster, Panulirus argus, contains populations of chemosensory receptors that are differentially sensitive to the L- and D-stereoisomers of the amino acid alanine. 2. Independent binding sites for L-alanine (dissociation constant (KD) of 6.6 microM and maximum binding (Bmax) of 16.8 fmole/microgram protein) and for D-alanine (KD of 21.6 microM and Bmax of 17.8 fmole/microgram protein) were characterized biochemically. The interaction of ligand with each binding site is rapid, reversible and saturable with respect to both time and concentration. 3. Based on a difference of at least 20% in the relative sensitivity of an olfactory receptor cell to alanine enantiomers, 44% and 34% of the 77 neurons tested were classified as L-alanine and D-alanine sensitive, respectively. The relative sensitivity to alanine enantiomers was independent of the concentration tested. Stereoselective receptors are likely for 17 of 20 other amino acids tested. 4. The congruence of biochemical and electrophysiological results leads to the conclusion that the lobster's responses to D- and L-alanine are mediated by receptors specific for each stereoisomer and that the receptors are differentially distributed among receptor cells.

Suspected chemoreceptors in coelenterates and ctenophores

Chemoreceptors in coelenterates and ctenophores have not been identified with certainty. Among prospective chemoreceptive cells are the sensory nerve cells, the cnidocyst-bearing cnidocytes, and the epitheliomuscular cells that are likely to be involved in feeding or aggression. Both behaviors are mediated by coordinated chemical and mechanical reception. This is reflected in the close apposition of putative chemo- and mechanoreceptors. Among the structures that have been designated as likely chemo- and/or mechanoreceptors are stereocilia, kinocilia, and/or microvilli which are universally present on all the putative chemoreceptor complexes, while gland cells and mucous secretions are prevalent. Evidence that the actin-containing stereocilia are chemically modulated mechanoreceptors is presented for several forms.

Glutathione-binding proteins identified by monoclonal antibodies which depress the behavioral response evoked by glutathione in Hydra

Hydra shows at least 5 components of the behavioral response (R1-R5) which are evoked at different concentrations of S-methylglutathione (GSM). We have prepared several monoclonal antibodies (mAbs), each of which depressed specific responses and visualized specific structures. In this paper, we analyzed molecules participating in the signaling pathway of R5 response with use of three mAbs (J245, J5 and J5/1), all of which depressed the response. A behavioral analysis of the response in the presence of mixtures of these mAbs suggested that J245 and J5 both acted on a component of the receptor-effector system which was not affected by J5/1. Correspondingly, an immunoblotting analysis showed two 220 kDa proteins which reacted with both J245 and J5 but not with J5/1. ELISA analyses also showed that the J245 antigens formed only a portion of the J5 antigens. A labeled major peak was found in the 220 kDa protein fraction by gel permeation HPLC after immunoprecipitation of the J245 antigen photolabeled with [35S]-S-(p-azidophenacyl)-glutathione. Competition of the photolabeling by the ligands GSM and L-glutamate (a competitive inhibitor of the R5 response) indicated dissociation constants for their binding to the protein of 55 and 90 microM, respectively. These values were consistent with those expected from behavioral experiments. The 220 kDa proteins therefore appear to be candidates for the receptor molecules mediating R5.

Chemoreception in hydra: specific binding of glutathione to a membrane fraction

Specific binding of reduced [35S]glutathione (GSH) was measured using a crude membrane fraction of Hydra vulgaris (attenuata). The specific binding shows both rapid displaceable and nondisplaceable components. Rapid displaceable binding accounted for 20% of the total specific binding. Data from saturation binding experiments indicates half maximal total specific binding occurs at 2 microM GSH which is similar to reported EC50 values from behavioral experiments. Calcium is required for displaceable binding of GSH to the putative receptor. Oxidized glutathione (GSSG), an antagonist of the GSH-activated feeding response, and S-methylglutathione (GSM), an agonist of the feeding response, inhibit the binding of radiolabeled GSH to the putative receptor. Glutamate, a putative competitive antagonist of the GSH-activated feeding response in hydra, does not inhibit the specific binding of radiolabeled GSH to the receptor and must therefore block the feeding response by a mechanism other than competitive inhibition.

Biochemical characterization of independent olfactory receptor sites for 5'-AMP and taurine in the spiny lobster

To understand the initial events in chemosensory transduction (i.e. binding of odorants to olfactory receptor sites), we have utilized a radioligand-receptor binding assay on a tissue preparation that is enriched in dendritic membrane from olfactory receptor cells in the spiny lobster Panulirus argus. Radioligands used were tritiated adenosine 5'-monophosphate (AMP) and taurine, which are two of the most excitatory compounds for spiny lobsters. Our results indicate the existence of two independent types of binding sites--a taurine binding site and an AMP binding site. For both the taurine and AMP binding sites, odorant binding is rapid, reversible, and saturable. KD values for the taurine and AMP binding sites are 2.3 and 2.0 microM, respectively, and Bmax values are 159 and 3.2 fmol/micrograms protein, respectively. The fact that the specificity, affinity, and independence of these two binding sites as defined in these biochemical studies are in agreement with those from electrophysiological studies suggests that these binding sites are olfactory receptor sites involved in sensory transduction.