Structure and pharmacological properties of a molluscan glutamate-gated cation channel and its likely role in feeding behavior

Ion channel profiling of the Lymnaea stagnalis ganglia via transcriptome analysis

BACKGROUND

The pond snail Lymnaea stagnalis (L. stagnalis) has been widely used as a model organism in neurobiology, ecotoxicology, and parasitology due to the relative simplicity of its central nervous system (CNS). However, its usefulness is restricted by a limited availability of transcriptome data. While sequence information for the L. stagnalis CNS transcripts has been obtained from EST libraries and a de novo RNA-seq assembly, the quality of these assemblies is limited by a combination of low coverage of EST libraries, the fragmented nature of de novo assemblies, and lack of reference genome.

RESULTS

In this study, taking advantage of the recent availability of a preliminary L. stagnalis genome, we generated an RNA-seq library from the adult L. stagnalis CNS, using a combination of genome-guided and de novo assembly programs to identify 17,832 protein-coding L. stagnalis transcripts. We combined our library with existing resources to produce a transcript set with greater sequence length, completeness, and diversity than previously available ones. Using our assembly and functional domain analysis, we profiled L. stagnalis CNS transcripts encoding ion channels and ionotropic receptors, which are key proteins for CNS function, and compared their sequences to other vertebrate and invertebrate model organisms. Interestingly, L. stagnalis transcripts encoding numerous putative Ca2+ channels showed the most sequence similarity to those of Mus musculus, Danio rerio, Xenopus tropicalis, Drosophila melanogaster, and Caenorhabditis elegans, suggesting that many calcium channel-related signaling pathways may be evolutionarily conserved.

CONCLUSIONS

Our study provides the most thorough characterization to date of the L. stagnalis transcriptome and provides insights into differences between vertebrates and invertebrates in CNS transcript diversity, according to function and protein class. Furthermore, this study provides a complete characterization of the ion channels of Lymnaea stagnalis, opening new avenues for future research on fundamental neurobiological processes in this model system.

Sigma receptors [σRs]: biology in normal and diseased states

This review compares the biological and physiological function of Sigma receptors [σRs] and their potential therapeutic roles. Sigma receptors are widespread in the central nervous system and across multiple peripheral tissues. σRs consist of sigma receptor one (σ₁R) and sigma receptor two (σ₂R) and are expressed in numerous regions of the brain. The sigma receptor was originally proposed as a subtype of opioid receptors and was suggested to contribute to the delusions and psychoses induced by benzomorphans such as SKF-10047 and pentazocine. Later studies confirmed that σRs are non-opioid receptors (not an µ opioid receptor) and play a more diverse role in intracellular signaling, apoptosis and metabolic regulation. σ₁Rs are intracellular receptors acting as chaperone proteins that modulate Ca²⁺ signaling through the IP₃ receptor. They dynamically translocate inside cells, hence are transmembrane proteins. The σ₁R receptor, at the mitochondrial-associated endoplasmic reticulum membrane, is responsible for mitochondrial metabolic regulation and promotes mitochondrial energy depletion and apoptosis. Studies have demonstrated that they play a role as a modulator of ion channels (K⁺ channels; N-methyl-d-aspartate receptors [NMDAR]; inositol 1,3,5 triphosphate receptors) and regulate lipid transport and metabolism, neuritogenesis, cellular differentiation and myelination in the brain. σ₁R modulation of Ca²⁺ release, modulation of cardiac myocyte contractility and may have links to G-proteins. It has been proposed that σ₁Rs are intracellular signal transduction amplifiers. This review of the literature examines the mechanism of action of the σRs, their interaction with neurotransmitters, pharmacology, location and adverse effects mediated through them.

pT305-CaMKII stabilizes a learning-induced increase in AMPA receptors for ongoing memory consolidation after classical conditioning

The role of CaMKII in learning-induced activation and trafficking of AMPA receptors (AMPARs) is well established. However, the link between the phosphorylation state of CaMKII and the agonist-triggered proteasomal degradation of AMPARs during memory consolidation remains unknown. Here we describe a novel CaMKII-dependent mechanism by which a learning-induced increase in AMPAR levels is stabilized for consolidation of associative long-term memory. Six hours after classical conditioning the levels of both autophosphorylated pT305-CaMKII and GluA1 type AMPAR subunits are significantly elevated in the ganglia containing the learning circuits of the snail Lymnaea stagnalis. CaMKIINtide treatment significantly reduces the learning-induced elevation of both pT305-CaMKII and GluA1 levels and impairs associative long-term memory. Inhibition of proteasomal activity offsets the deleterious effects of CaMKIINtide on both GluA1 levels and long-term memory. These findings suggest that increased levels of pT305-CaMKII play a role in AMPAR dependent memory consolidation by reducing proteasomal degradation of GluA1 receptor subunits.

Glutamate transporters in the central nervous system of a pond snail

Previous studies on glutamate (GLU) and its receptors in the pond snail Lymnaea stagnalis have suggested that GLU functions as a neurotransmitter in various behaviors, particularly for generation of feeding rhythm. The uptake mechanism of GLU is not yet known in Lymnaea. In the present study, we characterized the GLU transporters and examined their functions in the feeding circuits of the central nervous system (CNS) in Lymnaea. First, measurement of the accumulation of (3)H-labeled GLU revealed the presence of GLU transport systems in the Lymnaea CNS. The highest accumulation rate was observed in the buccal ganglia, supporting the involvement of GLU transport systems in feeding behavior. Second, we cloned two types of GLU transporters from the Lymnaea CNS, the excitatory amino acid transporter (LymEAAT) and the vesicular GLU transporter (LymVGLUT). When we compared their amino acid sequences with those of mammalian EAATs and VGLUTs, we found that the functional domains of both types are well conserved. Third, in situ hybridization revealed that the mRNAs of LymEAAT and LymVGLUT are localized in large populations of nerve cells, including the major feeding motoneurons in the buccal ganglia. Finally, we inhibited LymEAAT and found that changes in the firing patterns of the feeding motoneurons that have GLUergic input were similar to those obtained following stimulation with GLU. Our results confirmed the presence of GLU uptake systems in the Lymnaea CNS and showed that LymEAAT is required for proper rhythm generation, particularly for generation of the feeding rhythm.

GABA(A)- and AMPA-like receptors modulate the activity of an identified neuron within the central pattern generator of the pond snail Lymnaea stagnalis

To examine the neurochemistry underlying the firing of the RPeD1 neuron in the respiratory central pattern generator of the pond snail, Lymnaea stagnalis, we examined electrophysiologically and pharmacologically either "active" or "silent" preparations by intracellular recording and pharmacology. GABA inhibited electrical firing by hyperpolarizing RPeD1, while picrotoxin, an antagonist of GABA(A) receptors, excited silent cells and reversed GABA-induced inhibition. Action potential activity was terminated by 1 mM glutamate (Glu) while silent cells were depolarized by the GluR agonists, AMPA, and NMDA. Kainate exerted a complex triphasic effect on membrane potential. However, only bath application of AMPA desensitized the firing. These data indicate that GABA inhibits RPeD1 via activation of GABA(A) receptors, while Glu stimulates the neuron by activating AMPA-sensitive GluRs.

Pharmacology of ionotropic and metabotropic glutamate receptors on neurons involved in feeding behavior in the pond snail, Helisoma trivolvis

Glutamate is a key regulatory neurotransmitter in the triphasic central pattern generator controlling feeding behavior in the pond snail, Helisoma trivolvis. It excites phase two motor neurons while inhibiting those in phases one and three. However, the receptors that mediate this regulation are only partially characterized. The purpose of these experiments was to further characterize the glutamate receptors on three buccal neurons modulated by glutamate. Intracellular recordings from B5, B19 and B27 neurons were taken during the perfusion of isolated buccal ganglia with agonists that are selective for different vertebrate glutamate receptors. The firing rate of all three neurons was inhibited in a dose-dependent manner by glutamate, including that of B27, a phase 2 motor neuron known to be excited by glutamate in vivo. Quisqualate also reduced the firing rate in all three neurons, and(1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD), a relatively non-selective metabotropic glutamate receptor (mGluR) agonist,reduced the firing rate in B5 neurons, but not in B19 or B27 neurons. Agonists selective for vertebrate group I, II and III mGluRs did not affect the firing rate in any of the Helisoma buccal neurons tested, suggesting that mGluR agonist binding sites on these neurons do not closely resemble those on any vertebrate mGluR subtypes. An increase in frequency of action potentials was observed in all three cell types in the presence of 100 μmol l–1 kainate (KA), suggesting the presence of excitatory(S)-α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/KA-like receptors. However, electrotonic coupling between B19 and B27 neurons, and a lack of effect of KA on isolated B19 neurons suggest the excitatory effects of KA on this neuron are indirect. These findings suggest the presence of multiple glutamate receptor subtypes in molluscan neurons that do not always resemble vertebrate receptors pharmacologically.

Cellular and subcellular localization of kainic acid in the marine red alga Digenea simplex

Polyclonal antibodies specific for the excitatory amino acid, kainic acid (KA), were raised in rabbits. The antibody recognized KA but did not cross-react with other structurally related amino acids, including glutamate. We used this anti-KA antibody to localize KA immunohistochemically in the KA-producing red alga Digenea simplex. KA immunoreactivity was most dense in the fine cylindrical thallus, which covers the middle to upper part of the alga. The cortical cells, but not the inner layers of the main axis, and cells of the rhizoid were also stained with this antibody. The presence of KA in cells that cover the surface of the alga might reflect its role in chemical defense. At the subcellular level, KA immunoreactivity was most intense in the nucleus, pit plugs, and the electron-dense areas denoted as "granule bodies", which were found only in the pericentral cells of the thallus.

Early socio-emotional experience induces expression of the immediate-early gene Arc/arg3.1 (activity-regulated cytoskeleton-associated protein/activity-regulated gene) in learning-relevant brain regions of the newborn chick

We have cloned a full-length complementary DNA from the chicken (Gallus gallus domesticus), which encodes a polypeptide that exhibits approximately 75% identity to the product of the mammalian gene Arc (activity-regulated cytoskeleton-associated protein), also known as arg3.1 (activity-regulated gene). Since this gene is an immediate-early gene that has been suggested to play a role in synaptic plasticity and learning and memory processes, its expression has been analyzed in a juvenile form of learning, namely, filial imprinting. Our results demonstrate that Arc/arg3.1 mRNA is detectable in the newborn chick brain, and that at this early age the level of this transcript can be altered by brief sensory/emotional experience. After postnatal exposure to a novel 30-min auditory imprinting stimulus, Arc/arg3.1 mRNA was found to be significantly increased in two higher associative areas, the mesopallium intermediomediale (P = 0.002) and the nidopallium dorso-caudale (P = 0.031), compared with naïve controls. The transcript level was also significantly elevated after imprinting in Area L pallii (P=0.045), which is analogous to the mammalian auditory cortex. In addition, increases were seen in the medio-rostral nidopallium/mesopallium (P = 0.054), which is presumed to be the analog of the mammalian prefrontal cortex, and the hyperpallium intercalatum (P = 0.054), but these did not quite reach significance. We discuss these data in the light of those obtained in an earlier study, in the same paradigm, for the avian immediate-early gene, zenk (an acronym for zif-268, egr-1, ngfi-a and krox-24, which are different names for the orthologous mammalian gene). We conclude that, although both the Arc/arg3.1 and zenk genes are induced by auditory imprinting, they are significantly up-regulated in different learning-relevant brain regions. It is, therefore, evident that they must be activated by different mechanisms.

Characterization of a domoic acid binding site from Pacific razor clam

The Pacific razor clam, Siliqua patula, is known to retain domoic acid, a water-soluble glutamate receptor agonist produced by diatoms of the genus Pseudo-nitzschia. The mechanism by which razor clams tolerate high levels of the toxin, domoic acid, in their tissues while still retaining normal nerve function is unknown. In our study, a domoic acid binding site was solubilized from razor clam siphon using a combination of Triton X-100 and digitonin. In a Scatchard analysis using [3H]kainic acid, the partially-purified membrane showed two distinct receptor sites, a high affinity, low capacity site with a KD (mean +/- S.E.) of 28 +/- 9.4 nM and a maximal binding capacity of 12 +/- 3.8 pmol/mg protein and a low affinity, high capacity site with a mM affinity for radiolabeled kainic acid, the latter site which was lost upon solubilization. Competition experiments showed that the rank order potency for competitive ligands in displacing [3H]kainate binding from the membrane-bound receptors was quisqualate > ibotenate > iodowillardiine = AMPA = fluorowillardiine > domoate > kainate > L-glutamate. At high micromolar concentrations, NBQX, NMDA and ATPA showed little or no ability to displace [3H]kainate. In contrast, Scatchard analysis using [3H]glutamate showed linearity, indicating the presence of a single binding site with a KD and Bmax of 500 +/- 50 nM and 14 +/- 0.8 pmol/mg protein, respectively. These results suggest that razor clam siphon contains both a high and low affinity receptor site for kainic acid and may contain more than one subtype of glutamate receptor, thereby allowing the clam to function normally in a marine environment that often contains high concentrations of domoic acid.

Are chlorella viruses a rich source of ion channel genes?

Plaque-forming dsDNA (>330 kb) viruses that infect certain unicellular, eukaryotic chlorella-like green algae contain approximately 375 protein-encoding genes. These proteins include a 94 amino acid K+ channel protein, called Kcv, as well as two putative ligand-gated ion channels. The viruses also encode other proteins that could be involved in the assembly and/or function of ion channels, including protein kinases and a phosphatase, polyamine biosynthetic enzymes and histamine decarboxylase.

Cloning and characterization of an ionotropic glutamate receptor subunit expressed in the squid nervous system

In this paper we describe the cloning of a putative ionotropic glutamate receptor subunit, SqGluR, and its distribution in the nervous system of the squid. A full-length cDNA was assembled from a cDNA library of the stellate ganglion/giant fibre lobe complex of Loligo opalescens. The deduced amino acid sequence of the mature SqGluR displayed 44-46% amino acid identity with mammalian GluR1-GluR4 and 53% with Lym-eGluR1 from Lymnaea stagnalis. In situ hybridizations in adult squid confirmed that the SqGluR mRNA is abundant in giant fibre lobe neurons, in large, presumptive motor neurons of the stellate ganglion proper and in the supraoesophageal and optic lobes of the central nervous system. In newborn squid, SqGluR mRNA expression was detected throughout the nervous system but not elsewhere. A synthetic peptide corresponding to the last 15 amino acids of the SqGluR C-terminus was used to generate polyclonal antibodies, which were used for immunoblot analysis to demonstrate widespread expression in the squid central and peripheral nervous systems. Injection of the synthetic peptide into the postsynaptic side of the giant synapse inhibited synaptic transmission.

Beyond the role of glutamate as a neurotransmitter

Glutamate is the principal excitatory neurotransmitter of the central nervous system, but many studies have expanded its functional repertoire by showing that glutamate receptors are present in a variety of non-excitable cells. How does glutamate receptor activation modulate their activity? Do non-excitable cells release glutamate, and, if so, how? These questions remain enigmatic. Here, we review the current knowledge on glutamatergic signalling in non-neuronal cells, with a special emphasis on astrocytes.

Cloning and functional pharmacology of two corticotropin-releasing factor receptors from a teleost fish

Although it is well established that fish possess corticotropin-releasing factor (CRF) and a CRF-like peptide, urotensin I, comparatively little is known about the pharmacology of their cognate receptors. Here we report the isolation and functional expression of two complementary DNAs (cDNAs), from the chum salmon Oncorhynchus keta, which encode orthologues of the mammalian and amphibian CRF type 1 (CRF(1)) and type 2 (CRF(2)) receptors. Radioligand competition binding experiments have revealed that the salmon CRF(1) and CRF(2) receptors bind urotensin I with approximately 8-fold higher affinity than rat/human CRF. These two peptides together with two related CRF-like peptides, namely, sauvagine and urocortin, were also tested in cAMP assays; for cells expressing the salmon CRF(1) receptor, EC(50) values for the stimulation of cAMP production were between 4.5+/-1.8 and 15.3+/-3.1 nM. For the salmon CRF(2) receptor, the corresponding values were: rat/human CRF, 9.4+/-0.4 nM; urotensin I, 21.2+/-2.1 nM; sauvagine, 0.7+/-0.1 nM; and urocortin, 2.2+/-0.7 nM. We have also functionally coupled the O. keta CRF(1) receptor, in Xenopus laevis oocytes, to the endogenous Ca(2+)-activated chloride conductance by co-expression with the G-protein alpha subunit, G(alpha16). The EC(50) value for channel activation by rat/human CRF (11.2+/-2.6 nM) agrees well with that obtained in cAMP assays (15.3+/-3.1 nM). We conclude that although sauvagine is 13- and 30-fold more potent than rat/human CRF and urotensin I, respectively, in activating the salmon CRF(2) receptor, neither receptor appears able to discriminate between the native ligands CRF and urotensin I.

Glutamate receptors in peripheral tissues: current knowledge, future research, and implications for toxicology

We illustrate the specific cellular distribution of different subtypes of glutamate receptors (GluRs) in peripheral neural and non-neural tissues. Some of the noteworthy locations are the heart, kidney, lungs, ovary, testis and endocrine cells. In these tissues the GluRs may be important in mediating cardiorespiratory, endocrine and reproductive functions which include hormone regulation, heart rhythm, blood pressure, circulation and reproduction. Since excitotoxicity of excitatory amino acids (EAAs) in the CNS is intimately associated with the GluRs, the toxic effects may be more generalized than initially assumed. Currently there is not enough evidence to suggest the reassessment of the regulated safety levels for these products in food since little is known on how these receptors work in each of these organs. More research is required to assess the extent that these receptors participate in normal functions and/or in the development of diseases and how they mediate the toxic effects of EAAs. Non-neural GluRs may be involved in normal cellular functions such as excitability and cell to cell communication. This is supported by the wide distribution in plants and animals from invertebrates to primates. The important tasks for the future will be to clarify the multiple biological roles of the GluRs in neural and non-neural tissues and identify the conditions under in which these are up- or down-regulated. Then this could provide new therapeutic strategies to target GluRs outside the CNS.

Multimeric CREB-binding sites in the promoter regions of a family of G-protein-coupled receptors related to the vertebrate galanin and nociceptin/orphanin-FQ receptor families

Four related genes encoding a family of G-protein-coupled receptors (GPCRs) have been isolated from the mollusc Lymnaea stagnalis. The coding regions of this family of receptors share 97-99% sequence similarity at both the protein and nucleotide level, and they also share high sequence identity with vertebrate galanin and orphanin-FQ/nociceptin GPCR families. Analysis of the promoter regions reveals shared domains, some of which encode highly conserved repeating units. One 27-bp repeating unit, which encodes a c-AMP response element (CRE) and binds CREB protein, is repeated 14 times in one promoter. In situ hybridization showed expression of these receptors in identified neurons of several behaviourly important networks including those involved in feeding and ion and water regulation. These Lymnaea receptors are likely to represent members of a novel family of invertebrate neuropeptide receptors extensively regulated in response to intracellular signalling cascades.

The activation of glutamate receptors by kainic acid and domoic acid

The neurotoxins kainic acid and domoic acid are potent agonists at the kainate and alphaamino-5-methyl-3-hydroxyisoxazolone-4-propionate (AMPA) subclasses of ionotropic glutamate receptors. Although it is well established that AMPA receptors mediate fast excitatory synaptic transmission at most excitatory synapses in the central nervous system, the role of the high affinity kainate receptors in synaptic transmission and neurotoxicity is not entirely clear. Kainate and domoate differ from the natural transmitter, L-glutamate, in their mode of activation of glutamate receptors; glutamate elicits rapidly desensitizing responses while the two neurotoxins elicit non-desensitizing or slowly desensitizing responses at AMPA receptors and some kainate receptors. The inability to produce desensitizing currents and the high affinity for AMPA and kainate receptors are undoubtedly important factors in kainate and domoate-mediated neurotoxicity. Mutagenesis studies on cloned glutamate receptors have provided insight into the molecular mechanisms responsible for these unique properties of kainate and domoate.

Glutamate receptor 5/6/7-like and glutamate transporter-1-like immunoreactivity in the leech central nervous system

Previous physiological and pharmacological evidence has suggested a neurotransmitter role for the excitatory amino acid glutamate in the leech central nervous system (CNS). In the present study, we sought to localize glutamate receptor (GluR) subunits (GluR 5/6/7, GluR 2/3 and N-methyl-D-aspartate receptor 1 [NMDAR 1]) and a glutamate transporter subtype [GLT-1] within the leech CNS using mono- and polyclonal antibodies. In whole-mounted tissue, small cells of the outer capsule and putative microglia labeled with both GluR 5/6/7 and GluR 2/3 but not NMDAR 1 subunit antisera. In general, GluR 5/6/7-like immunofluorescence was both more intense and more widespread than GluR 2/3-like immunolabeling. Cryostat-sectioned tissue revealed extensive GluR 5/6/7-like immunoreactivity throughout the neuropil as well as labeling within a few neuronal somata. GLT-1-like immunoreactivity localized to the inner capsule, which is the interface between neuronal somata and the neuropil and is deeply invested by processes of neuropil glia. These results complement previous physiological and pharmacological findings indicating that the leech CNS possesses the cellular machinery to respond to glutamate and to transport glutamate from extracellular spaces. Together, they provide further evidence for glutamate's role as a neurotransmitter within the leech CNS.

Immunochemical localization of the metabotropic glutamate receptors in the rat heart

The localization of the glutamate receptor outside of the central nervous system is becoming more evident. These receptors have been implicated in brain function and pathology. It can also be envisioned that they play a vital role in the physiology of other organs and systems. We recently reported the presence of ionotropic glutamate receptors in the rat heart. These were distributed differentially in specific cardiac structures, including nerve terminals, ganglion cells, and the conducting system. In this study, we investigated the presence and localization of the metabotropic glutamate receptors (mGluRs) in the rat heart by immunohistochemistry. The experimental data show that the mGluR 1alpha, mGLuR 2/3, and mGluR 5 are present in the rat heart. Their preferential localization includes nerve terminals, ganglion cells, and elements of the conducting system. The mGluR 5 was the only receptor located in the intercalated disks of the cardiac muscle and in the endothelial lining of the blood vessels. This preferential localization to the different components of the conducting system and cardiac neural structures suggest that they play a role in the physiology of the heart.

Glutamate is the transmitter for N2v retraction phase interneurons of the Lymnaea feeding system

Electrophysiological and pharmacological methods were used to examine the role of glutamate in mediating the excitatory and inhibitory responses produced by the N2v rasp phase neurons on postsynaptic cells of the Lymnaea feeding network. The N2v --> B3 motor neuron excitatory synaptic response could be mimicked by focal or bath application of -glutamate at concentrations of >/=10(-3) M. Quisqualate and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) were potent agonists for the B3 excitatory glutamate receptor (10(-3) M), whereas kainate only produced very weak responses at the same concentration. This suggested that non-N-methyl--aspartate (NMDA), AMPA/quisqualate receptors were present on the B3 cell. The specific non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 10(-5) M) blocked 85% of the excitatory effects on the B3 cell produced by focal application of glutamate (10(-3) M), confirming the presence of non-NMDA receptors. CNQX also blocked the major part of the excitatory postsynaptic potentials on the B3 cell produced by spontaneous or current-evoked bursts of spikes in the N2v cell. As with focal application of glutamate, a small delayed component remained that was CNQX insensitive. This provided direct evidence that glutamate acting via receptors of the non-NMDA, AMPA/quisqualate type were responsible for mediating the main N2v --> B3 cell excitatory response. NMDA at 10(-2) M also excited the B3 cell, but the effects were much more variable in size and absent in one-third of the 25 B3 cells tested. NMDA effects on B3 cells were not enhanced by bath application of glycine at 10(-4) M or reduction of Mg2+ concentration in the saline to zero, suggesting the absence of typical NMDA receptors. The variability of the B3 cell responses to NMDA suggested these receptors were unlikely to be the main receptor type involved with N2v --> B3 excitation. Quisqualate and AMPA at 10(-3) M also mimicked N2v inhibitory effects on the B7 and B8 feeding motor neurons and the modulatory slow oscillator (SO) interneuron, providing further evidence for the role of AMPA/quisqualate receptors. Similar effects were seen with glutamate at the same concentration. However, CNQX could not block either glutamate or N2v inhibitory postsynaptic responses on the B7, B8, or SO cells, suggesting a different glutamate receptor subtype for inhibitory responses compared with those responsible for N2v --> B3 excitation. We conclude that glutamate is a strong candidate transmitter for the N2v cells and that AMPA/quisquate receptors of different subtypes are likely to be responsible for the excitatory and inhibitory postsynaptic responses.

Opioid receptors from a lower vertebrate (Catostomus commersoni): sequence, pharmacology, coupling to a G-protein-gated inward-rectifying potassium channel (GIRK1), and evolution

The molecular evolution of the opioid receptor family has been studied by isolating cDNAs that encode six distinct opioid receptor-like proteins from a lower vertebrate, the teleost fish Catostomus commersoni. One of these, which has been obtained in full-length form, encodes a 383-amino acid protein that exhibits greatest sequence similarity to mammalian mu-opioid receptors; the corresponding gene is expressed predominantly in brain and pituitary. Transfection of the teleost cDNA into HEK 293 cells resulted in the appearance of a receptor having high affinity for the mu-selective agonist [D-Ala2, MePhe4-Gly-ol5]enkephalin (DAMGO) (Kd = 0.63 +/- 0.15 nM) and for the nonselective antagonist naloxone (Kd = 3.1 +/- 1.3 nM). The receptor had negligible affinity for U50488 and [D-Pen2, D-Pen5]enkephalin (DPDPE), which are kappa- and delta-opioid receptor selective agonists, respectively. Stimulation of transfected cells with 1 microM DAMGO lowered forskolin-induced cAMP levels, an effect that could be reversed by naloxone. Experiments in Xenopus oocytes have demonstrated that the fish opioid receptor can, in an agonist-dependent fashion, activate a coexpressed mouse G-protein-gated inward-rectifying potassium channel (GIRK1). The identification of six distinct fish opioid receptor-like proteins suggests that additional mammalian opioid receptors remain to be identified at the molecular level. Furthermore, our data indicate that the mu-opioid receptor arose very early in evolution, perhaps before the appearance of vertebrates, and that the pharmacological and functional properties of this receptor have been conserved over a period of approximately 400 million years implying that it fulfills an important physiological role.

Cloning and functional expression of a Drosophila metabotropic glutamate receptor expressed in the embryonic CNS

The excitatory neurotransmitter glutamate plays important roles in the mammalian brain, ranging from synaptic plasticity to memory. To mediate these functions, glutamate activates two types of receptors: ligand-gated channels and metabotropic receptors coupled to G-proteins. Both families of glutamate receptors share no sequence homology and possess original structural features compared with other ligand-gated channels and G-protein-coupled receptors, respectively. Glutamate-gated receptor-channel subunits have already been characterized in invertebrates. Here we report the cloning and functional characterization of an invertebrate metabotropic glutamate receptor (DmGluRA) isolated from Drosophila melanogaster. This receptor displays 45 and 43% amino acid sequence identity with its mammalian homologs mGluR3 and mGluR2, respectively. Moreover, its pharmacology and transduction mechanisms are surprisingly similar to those of mGluR2 and mGluR3. DmGluRA is expressed in the CNS of the late embryo. These results indicate that the original structural features of both glutamate receptor types are conserved from insects to mammals and suggest that the functions of these receptors have been highly conserved during evolution.