Inhibition of adenylate cyclase activity by galanin in rat insulinoma cells is mediated by the G-protein Gi3

The Galaninergic System: A Target for Cancer Treatment

Simple Summary

Peptidergic systems play an important role in cancer progression. The galaninergic system (the peptide galanin and its receptors: galanin 1, 2 and 3) is involved in tumorigenesis, the invasion and migration of tumor cells and angiogenesis and it has been correlated with tumor stage/subtypes, metastasis and recurrence rate in many types of cancer. Galanin exerts a dual action in tumor cells: a proliferative or an antiproliferative effect depending on the galanin receptor involved in these mechanisms. Galanin receptors could be used in certain tumors as therapeutic targets and diagnostic markers for treatment, prognosis and surgical outcome. This review shows the importance of the galaninergic system in the development of tumors and suggests future promising clinical antitumor applications using galanin agonists or antagonists.

Abstract

The aim of this review is to show the involvement of the galaninergic system in neuroendocrine (phaeochromocytomas, insulinomas, neuroblastic tumors, pituitary tumors, small-cell lung cancer) and non-neuroendocrine (gastric cancer, colorectal cancer, head and neck squamous cell carcinoma, glioma) tumors. The galaninergic system is involved in tumorigenesis, invasion/migration of tumor cells and angiogenesis, and this system has been correlated with tumor size/stage/subtypes, metastasis and recurrence rate. In the galaninergic system, epigenetic mechanisms have been related with carcinogenesis and recurrence rate. Galanin (GAL) exerts both proliferative and antiproliferative actions in tumor cells. GAL receptors (GALRs) mediate different signal transduction pathways and actions, depending on the particular G protein involved and the tumor cell type. In general, the activation of GAL₁R promoted an antiproliferative effect, whereas the activation of GAL₂R induced antiproliferative or proliferative actions. GALRs could be used in certain tumors as therapeutic targets and diagnostic markers for treatment, prognosis and surgical outcome. The current data show the importance of the galaninergic system in the development of certain tumors and suggest future potential clinical antitumor applications using GAL agonists or antagonists.

Go2 G protein mediates galanin inhibitory effects on insulin release from pancreatic β cells

The neuropeptide galanin regulates numerous physiological activities in the body, including feeding and metabolism, learning and memory, nociception and spinal reflexes, and anxiety and related behaviors. Modulation of blood glucose levels by suppressing insulin release was the first reported activity for galanin. This inhibition was mediated by one or more pertussis toxin-sensitive G proteins of the G(i/o) subfamily. However, the molecular identities of the specific G protein(s) and intracellular effectors have not been fully revealed. Recently, we demonstrated that mice lacking G(o)2, but not other members of the G(i/o) protein family, secrete more insulin than controls upon glucose challenge, indicating that G(o)2 is a major transducer for the inhibitory regulation of insulin secretion. In this study, we investigated galanin signaling mechanisms in β cells using cell biological and electrophysiological approaches. We found that islets lacking G(o)2, but not other G(i/o) proteins, lose the inhibitory effect of galanin on insulin release. Potentiation of ATP-sensitive potassium (K(ATP)) and inhibition of calcium currents by galanin were disrupted by anti-G(o)2α antibodies. Galanin actions on K(ATP) and calcium currents were completely lost in G(o)2(-/-) β cells. Furthermore, the hyperglycemic effect of galanin is also blunted in G(o)2(-/-) mice. Our results demonstrate that G(o)2 mediates the inhibition of insulin release by galanin by regulating both K(ATP) and Ca(2+) channels in mice. Our findings provide insight into galanin's action in glucose homeostasis. The results may also be relevant to the understanding of galanin signaling in other biological systems, especially the central nervous system.

26RFa, a novel orexigenic neuropeptide, inhibits insulin secretion in the rat pancreas

26RFa is a novel orexigenic neuropeptide identified as the endogenous ligand of the orphan G protein-coupled receptor GPR103. GPR103 shares sequence identity with the receptors for neuropeptide-Y and galanin, two peptides known to inhibit insulin secretion. We have investigated the effect of 26RFa on insulin and glucagon secretion in the perfused rat pancreas. 26RFa dose-dependently reduced glucose-induced insulin release, inhibited the insulin responses to both arginine and exendin-4 and did not affect glucagon output. The inhibitory effect of 26RFa on exendin-4-induced insulin secretion was not observed in pancreata from pertussis toxin-treated rats, thus suggesting that 26RFa may inhibit insulin secretion, at least in part, via a pertussis toxin-sensitive G(i) protein coupled to the adenylyl cyclase system.

Pertussis toxin-sensitive pathway inhibits glucose-stimulated Ca2+ signals of rat islet β-cells by affecting L-type Ca2+ channels and voltage-dependent K+ channels

A role of pertussis toxin (PTX)-sensitive pathway in regulation of glucose-stimulated Ca2+ signaling in rat islet beta-cells was investigated by using clonidine as a selective agonist to alpha2-adrenoceptors which link to the pathway. An elevation of extracellular glucose concentration from 5.5 to 22.2 mM (glucose stimulation) increased the levels of [Ca2+]i of beta-cells, and clonidine reversibly reduced the elevated levels of [Ca2+]i. This clonidine effect was antagonized by yohimbine, and abolished in beta-cells pre-treated with PTX. Clonidine showed little effect on membrane currents including those through ATP-sensitive K+ channels induced by voltage ramps from -90 to -50 mV. Clonidine showed little effect on the magnitude of whole-cell currents through L-type Ca2+ channels (ICa(L)), but increased the inactivation process of the currents. Clonidine increased the magnitude of the voltage-dependent K+ currents (IVK). These clonidine effects on ICa(L) and IVK were abolished in beta-cells treated with PTX or GDP-betaS. These results suggest that the PTX-sensitive pathway increases IVK activity and decreases ICa(L) activity of islet beta-cells, resulting in a decrease in the levels of [Ca2+]i elevated by depolarization-induced Ca2+ entry. This mechanism seems responsible at least in part for well-known inhibitory action of PTX-sensitive pathway on glucose-stimulated insulin secretion from islet beta-cells.

Masked excitatory action of noradrenaline on rat islet beta-cells via activation of phospholipase C

The effect of noradrenaline (NE) on rat islet beta-cells was examined. NE reduced insulin secretion from rat islets exposed to extracellular solutions containing glucose at 5.5 or 16.6 mM. In islets treated with pertussis toxin (PTX), however, NE increased insulin secretion. The NE-induced augmentation of insulin secretion was inhibited by prazosin. In intact islets, NE increased phospholipase C (PLC) activity, an effect that was prevented by treatment of islets with U-73122. NE elevated intracellular [Ca2+] ([Ca2+]i) in isolated beta-cells independently of PTX. Although this NE effect was inhibited by prazosin, phenylephrine did not mimic it. The [Ca2+]i response to NE was also prevented by the treatment of cells with U-73122. NE produced depolarization of beta-cells followed by nifedipine-sensitive action potentials. NE reduced the whole-cell membrane currents through ATP-sensitive K+ channels (KATP), responsible for the depolarization. This NE effect was prevented by treatment of beta-cells with U-73122 or BAPTA/AM. Although at least some of our results imply the presence of alpha1-adrenoceptors, beta-cells were not stained by a polyclonal IgG antibody recognizing all adrenergic alpha1-receptor subtypes so far identified. These results suggest that an interaction of NE with an unknown type of receptor activates rat islet beta-cells via a PLC-dependent signal pathway. This effect is, however, masked by the inhibitory action via a PTX-sensitive pathway also activated by NE.

G protein specificity: traffic direction required

This review focuses on the coupling specificity of the Galpha and Gbetagamma subunits of pertussis toxin (PTX)-sensitive G(i/o) proteins that mediate diverse signaling pathways, including regulation of ion channels and other effectors. Several lines of evidence indicate that specific combinations of G protein alpha, beta and gamma subunits are required for different receptors or receptor-effector networks, and that a higher degree of specificity for Galpha and Gbetagamma is observed in intact systems than reported in vitro. The structural determinants of receptor-G protein specificity remain incompletely understood, and involve receptor-G protein interaction domains, and perhaps other scaffolding processes. By identifying G protein specificity for individual receptor signaling pathways, ligands targeted to disrupt individual pathways of a given receptor could be developed.

The G-protein inhibitor, pertussis toxin, inhibits the secretion of brain-derived neurotrophic factor

Secretion of neurotrophins is critical for the delivery of neurotrophic support. Brain-derived neurotrophic factor is targeted to a regulated secretory pathway in neurons as well as the neurosecretory AtT-20 cells. Here, we show that pertussis toxin, which inactivates Gi and Go G proteins, inhibits up to 50% of the regulated release of brain derived neurotrophic factor by AtT-20 cells. To determine whether pertussis toxin-sensitive G proteins may regulate brain-derived neurotrophic factor release in vivo, the effect of intraocular pertussis toxin was assessed on the isthmo-optic nucleus in the developing chick visual system. The isthmo-optic nucleus projects axons from the midbrain to innervate retinal amacrine cells and depends on target-derived brain-derived neurotrophic factor between embryonic days 13 and 17 (E13-17). During this period approximately 50% of isthmo-optic neurons are eliminated by programmed cell death. Intraocular pertussis toxin administered at E13 increased cell death of isthmo-optic neurons by 42%, whereas injections at E19 had no effect. Co-injection of brain-derived neurotrophic factor with pertussis toxin rescued approximately 50% of isthmo-optic neurons from enhanced cell death, although overall retinal brain derived neurotrophic factor protein levels were unaffected by pertussis toxin. Retrograde transport of exogenous 125I-labeled brain derived neurotrophic factor from the retina to the midbrain was increased by co-administration of pertussis toxin, possibly owing to diminished competition from endogenously released brain-derived neurotrophic factors for the receptors that mediate retrograde axonal transport. These data suggest that the release of a major fraction of brain-derived neurotrophic factor in the secretory pathway in vitro and in vivo is regulated by the activity of pertussis toxin-sensitive G proteins.

Elucidating cell signaling mechanisms using antisense technology

Many diseases result from defects in cell signaling. Achieving an in-depth understanding of the complex mechanisms by which cells transduce extracellular signals into cellular responses in both normal and diseased systems is a crucial step in the discovery of more effective drugs to treat human diseases. Traditional approaches for studying cell signaling have some limitations. Antisense oligonucleotides represent a novel approach for studying signal transduction processes that offers significant advantages in terms of specificity and versatility. This article reviews the opportunities that antisense oligonucleotides offer for the study of signal transduction pathways and identification of inhibitors of these pathways for drug development.

Galphai-2 is required for carbachol-induced stress fiber formation in human airway smooth muscle cells

To determine which heterotrimeric G protein couples muscarinic receptors to stress fiber formation [measured by an increase in the filamentous (F)- to monomeric (G)-actin ratio] in human airway smooth muscle (ASM) cells, cultured human ASM cells expressing the M2 muscarinic receptor were grown to confluence. Cells were exposed for 6 days to 10 microM antisense oligonucleotides designed to specifically bind to the mRNA encoding Galphai-2, Galphai-3, or Gqalpha. A randomly scrambled oligonucleotide served as a control. F- to G-actin ratios were measured with dual-fluorescence labeling after 5 min of carbachol exposure, which is known to increase the F- to G-actin ratio. Cells in parallel wells were harvested for immunoblot analysis of G protein alpha-subunit expression. Oligonucleotide antisense treatment decreased protein expression of the respective G protein alpha-subunit. Antisense depletion of the Galphai-2 protein but not of Galphai-3 or Gqalpha protein blocked the carbachol-induced increase in the F- to G-actin ratio. These results show that the Galphai-2 protein couples muscarinic receptors to stress fiber formation in ASM.

Mechanisms of galanin-induced contraction in the rat myometrium

A neuropeptide, galanin, regulates the reproductive process and directly induces myometrial contraction. The aim of this study was to determine the mechanism of galanin-induced myometrial contraction. For this purpose, we simultaneously measured intracellular Ca2+ concentration ([Ca2+]i) and tension using fura-PE3-fluorometry and the rat longitudinal myometrium. The effect of galanin on the Ca2+ sensitivity of the contractile apparatus was examined in beta-escin permeabilized strips. The expression of galanin and the galanin receptors mRNAs in the rat myometrium were determined by reverse transcription polymerase chain reaction (RT-PCR). Galanin (10-300 nM) induced phasic contraction with or without oscillation in the pregnant rat myometrium in a concentration-dependent manner. The maximal response was obtained at 100 nM. There was no significant difference either in the maximal responses or EC50 values for galanin-induced myometrial contractions among myometriums from non-pregnant and pregnant (day 4, day 11, day 20, day 22) rats. In the day 20 and 22 pregnant myometriums, assigning the levels of [Ca2+]i and tension at 40 mM K+-depolarization to be 100%, galanin increased the [Ca2+]i and tension to 126.9+/-2.9% and 116.3+/-2.7%, respectively. Diltiazem (10 microM) inhibited the galanin-induced elevation of [Ca2+]i and tension to 71.9+/-2.4% and 16.2+/-0.7%, respectively. Ni2+, by itself, decreased the basal [Ca2+]i to -50.2+/-3.9% without affecting resting tension. After Ni2+ treatment, galanin-induced increases in [Ca2+]i and tension were -19.6+/-3.4% and 0.9+/-0.1%, respectively. In myometrium treated with diltiazem, no oscillation in [Ca2+]i and tension was observed. In Ca2+-free solution with 0.1 mM EGTA, galanin increased [Ca2+]i from -40.2+/-2.7% to -18.0+/-2.6% and induced transient contraction (3.6+/-0.8%). In beta-escin permeabilized myometrium, galanin enhanced the contraction induced by 0.3 microM Ca2+ in the presence of GTP. In the presence of GDPbetaS (1 mM) instead of GTP, galanin failed to increase the Ca2+ sensitivity of the contractile apparatus. RT-PCR revealed that galanin mRNA was hardly expressed in the non-pregnant rat myometrium and increased to reach a maximal level at mid pregnancy (day 11), but decreased to the same level as in the non-pregnant myometrium at term (day 22). Type 2 galanin receptor (GALR2) mRNA was found to be expressed in the rat myometrium whereas type 1 galanin receptor (GALR1) mRNA expression was not detected. In conclusion, galanin induces contraction of the rat myometrium by increasing [Ca2+]i as well as by increasing Ca2+ sensitivity of the contractile apparatus. Galanin-induced increases in [Ca2+]i are caused by both intracellular Ca2+ release and Ca2+ influx from extracellular space. The responsiveness of the rat myometrium to galanin does not change during pregnancy. The galanin mRNA is expressed in the rat myometrium and it is upregulated during mid-pregnancy. Rat myometrium expresses GALR2 but not GALR1 mRNA. By changing mRNA expression in the myometrium during pregnancy, galanin may act as a paracrine or autocrine mediator in the regulation of myometrial contractility.

Galaninergic signalling and adenylate cyclase

Galanin is a hyperpolarizing, inhibitory neurotransmitter; its recognition by seven transmembrane spanning G-protein-coupled receptors leads to a change in accumulation of cAMP (3'5'-cyclic AMP). Different subtypes of galanin receptor and G-proteins could be manifested in the mode of inhibitory action of galanin receptor on the production of cAMP by adenylate cyclase. Galanin analogues, acting at the adenylate cyclase level as subtype-specific galanin antagonists, can selectively block the inhibitory effect of endogenous galanin and thereby have potential as therapeutic agents for several endocrine, neuroendocrine and neuronal disorders. In this review, the latest results in the field of interplay between galanin-initiated signal transduction and the cAMP pathway are summarized.

The P2U purinoceptor obligatorily engages the heterotrimeric G protein G16 to mobilize intracellular Ca2+ in human erythroleukemia cells

To assess the role of G16, a trimeric G protein exclusively expressed in hematopoietic cells, Galpha16 antisense RNA was stably expressed in human erythroleukemia (HEL) cells. Western blot analysis showed that in transfected cell lines, the expression of endogenous Galpha16 protein was suppressed, but the expression of Galphaq/11, Galphai2, and Galphai3 remained unaffected. Suppression of Galpha16 in transfected HEL cells did not interfere with transient elevations of intracellular free Ca2+ concentrations induced by prostaglandin E1 (PGE1), platelet-activating factor, or thrombin. In parental HEL cells, UTP and ATP mobilized Ca2+ from intracellular stores with half-maximum effective concentrations of 3. 6 +/- 0.7 and 4.7 +/- 1.6 microM, respectively, apparently by stimulating P2U purinoceptors. By contrast, Ca2+ mobilization by UTP or ATP was completely abrogated in Galpha16-suppressed cells, indicating specific coupling of G16 to P2U purinoceptors. Pertussis toxin inhibited the effect of UTP in parental HEL cells by 57.6 +/- 4.9%. These data indicate that signaling by the P2U purinoceptor obligatorily requires G16 but may be modulated further by activation of Gi. Priming of HEL cells with UTP or ATP prior to stimulation with PGE1 markedly enhanced the PGE1-induced intracellular Ca2+ release. This indirect, potentiating effect of UTP and ATP was not impaired in Galpha16-suppressed cells but was inhibited by pertussis toxin, indicating that functional P2U purinoceptors are present on these cells and that the potentiating effect primarily depends on Gi. The data demonstrate (i) that Galpha16 antisense RNA selectively inhibits endogenous Galpha16 protein expression in HEL cells; (ii) that stimulation of endogenous P2U (P2Y2) purinoceptors leads to the mobilization of intracellular Ca2+ by a mechanism that strictly depends on Galpha16; and (iii) that P2U purinoceptors in HEL cells can communicate with two distinct signaling pathways diverging at the G protein level.

Galanin receptors: involvement in feeding, pain, depression and Alzheimer's disease

Galanin, a neuroendocrine peptide with a multitude of functions, binds to and acts on specific G-protein coupled receptors. Only one galanin receptor subtype, GalRI, has been cloned so far, although pharmacological evidence suggests the presence of more than one galanin receptor subtype. These receptors mediate via different Gi/Go-proteins the inhibition of adenylyl cyclase, opening of K+-channels and closure of Ca2+-channels. Galanin inhibits secretion of insulin, acetylcholine, serotonin and noradrenaline, while it stimulates prolactin and growth hormone release. Determination of structural components of galanin receptors required for binding of the peptide ligand as carried out recently will facilitate the screening and design of molecules specifically acting on galaninergic systems with therapeutic potential in Alzheimer's disease, feeding disorders, pain and depression.

Pancreatic beta-cell receptors and G proteins coupled to adenylyl cyclase

Glucagon and tGLP-1 receptors can be either coexpressed or selectively expressed in beta-cell models. Our results indicate that both these peptides can regulate insulin secretion from beta-cells through their own specific receptors. The finding of a selective expression of G proteins in insulin and glucagon cells indicates a clear difference in their transduction pathways. A key role of the G alpha s family in beta-cell function is further supported by its conserved cell distribution between different species. In conclusion, one could postulate that in the human beta-cells, tGLP-1 and glucagon receptors could mediate their action through different G protein alpha-subunits of the G alpha s family.

Mechanisms of inhibition of insulin release

Several agonists including norepinephrine, somatostatin, galanin, and prostaglandins inhibit insulin release. The inhibition is sensitive to pertussis toxin, indicating the involvement of heterotrimeric Gi and/or Go proteins. Receptors for the different agonists have different selectivity for these G proteins. After G protein activation, the alpha- and beta gamma-subunits dissociate and interact with multiple targets to inhibit release. These include 1) the ATP-sensitive K+ channel and perhaps other K+ channels, 2) L-type voltage-dependent Ca2+ channels, 3) adenylyl cyclase, and 4) a "distal" site late in stimulus-secretion coupling. The latter effect, which may be exerted close to the final stage of exocytosis, is the most powerful of the individual inhibitory mechanisms. G protein action on the target molecules is determined by the individual G proteins activated and their specificity for the targets. The L-type Ca2+ channel is inhibited by G(o)-1. Adenylyl cyclase is inhibited by Gi-2 and Gi-3. The distal inhibition can be exerted by Gi-1, Gi-2, Gi-3, and G(o)-2. Thus there is both selectivity and promiscuity in G protein action in the beta-cell. These characteristics allow an inhibitory ligand to be effective at multiple targets and to act differentially from other inhibitory ligands.

Minireview. Galanin-acetylcholine interactions: relevance to memory and Alzheimer's disease

The neuropeptide, galanin, and its receptors are localized in the cholinergic basal forebrain and its projection areas in mammalian brain. Centrally administered galanin inhibits acetylcholine release in the rat ventral hippocampus, and produces deficits in learning and memory tasks. In Alzheimer's disease, galanin is overexpressed in terminals innervating the nucleus basalis of Meynert cell bodies. Selective galanin receptor antagonists provide a novel approach for increasing cholinergic function, as a potential adjunct to the clinical treatment of dementias.

G proteins in adipocytes and preadipocytes: characterization, subcellular distribution, and potential roles for Gi2 and/or Gi3 in the control of cell proliferation

Guanosine triphosphate (GTP)-binding protein subunits were studied by immunoblot analysis in particulate fractions from mature adipocytes, confluent preadipocytes, and in vitro-differentiated preadipocytes. Mature adipocytes express Gi alpha 1, Gi alpha 2, Gi alpha 3, Go alpha, Gq/11 alpha, G13 alpha and the long and short isoforms of Gs alpha, but no Gz alpha or G12 alpha. Confluent and differentiated preadipocytes differ in having a higher content of Gi alpha 3 and G13 alpha and expressing G12 alpha. In contrast, they lack Gi alpha 1, Go alpha, and the short from of Gs alpha. The G-protein alpha subunits Gi alpha 2, Gs alpha (long isoform), and Gq/11 alpha, and G-protein beta subunits were unchanged throughout the differentiation process. By immunoblot and indirect immunofluorescence studies on confluent preadipocytes, we showed that Gi alpha 2 is present in the endoplasmic reticulum and marginally in plasma membranes and nuclei. In contrast, antibodies to Gi alpha 3 stained the Golgi apparatus. The role of G proteins on preadipocyte proliferation was studied using Bordetella pertussis toxin. Exposure of growing cells to this toxin in the presence of fetal calf serum (FCS) decreased [3H]thymidine incorporation by 40% and induced a 40% increase in doubling time. This resulted in a 30% decrease in cell number per well after 48 h. These effects of B. pertussis toxin did not appear to be related to an increase in cyclic adenosine monophosphate (cAMP) concentration, because forskolin had the opposite effect on cell proliferation. Finally, B. pertussis toxin prevented serum-induced Raf1 association to the plasma membrane, possibly by disrupting FCS-induced G beta gamma effects on the Ras/Raf1 pathway. Since Go alpha and Gi alpha 1 subunits were absent in preadipocytes, we conclude that Gi2 and/or Gi3 proteins transduce some mitogenic signals of FCS through release of G beta gamma subunits. The subcellular distribution of Gi alpha 2 and Gi alpha 3 suggests that part of their functions result from interactions with components other than the plasma membrane.

G alpha i RNA antisense expression demonstrates the exclusive coupling of peptide YY receptors to G(i)2 proteins in renal proximal tubule cells

A clone PKSV-PCT Cl.10 referred to as Cl.10 was selected from the PKSV-PCT renal proximal tubule cell line which expressed peptide YY (PYY) receptors (Voisin, T., Bens, M., Cluzeaud, F., Vandewalle, A., and Laburthe, M. (1993) J. Biol. Chem. 268, 20547-20554). In order to identify G(i) protein(s) coupled to PYY receptors, antisense G alpha i protein RNAs were expressed in Cl.10 cells by transfecting the pcDNA3 vector into which were inserted 39 bases of the 5'-noncoding region of G alpha i2 or G alpha i3 used as specific antisense templates. A Cl.10/alpha i2-clone was selected which displayed a drastic decrease (> 90%) of the expression of G alpha i2 without changes of G alpha i3, G alpha s, and G beta subunits (G alpha i1 is not present in Cl.10 cells) as evidenced by Western blots. When compared to untransfected cells, this clone exhibited: (i) an increase in the dissociation constant of PYY receptors (5.3 versus 0.6 nM) identical to that observed in pertussis toxin-treated untransfected cells; (ii) an absence of inhibition of 125I-PYY binding by guanosine 5'-O-(thiotriphosphate) (GTP gamma S); and (iii) the failure of PYY to inhibit cAMP levels and to stimulate [methyl-3H]thymidine incorporation into DNA. A clone was also selected which exhibited a specific decrease (> 80%) of G alpha i3 as compared to untransfected cells. The sensitivity to GTP gamma S and the dissociation constant of PYY receptors as well as PYY-mediated inhibition of cAMP were identical to those observed in untransfected cells. These findings support an exclusive coupling of PYY receptors to G alpha i2.

Cloning and characterization of the rat GALR1 galanin receptor from Rin14B insulinoma cells

Galanin is a ubiquitous neuropeptide that regulates a wide array of physiological processes via interaction with specific G protein-coupled receptors. A rat galanin receptor cDNA was cloned from the Rin14B insulinoma cell line. The isolated cDNA encodes a 346 amino acid G protein-coupled receptor that is 92% identical to the recently reported human GALR1 galanin receptor. [125I]Galanin binds with high affinity to two receptor states in COS1 cell membranes containing the rat GALR1 receptor, consistent with coupling of the receptor to a G protein in these membranes. N-terminal galanin fragments and the putative galanin receptor antagonists galantide, C7, M35 and M40 bind with high affinity to the rat GALR1 receptor. In contrast, C-terminal galanin fragments do not bind to this receptor. Galanin inhibits basal and forskolin-stimulated cAMP formation in CHO cells expressing the rat GALR1 receptor via a pertussis toxin-sensitive G protein. The GALR1 receptor is expressed in rat spinal cord, small intestine, Rin14B insulinoma cells and several brain regions, particularly ventral hippocampus, amygdala, supraoptic nucleus, hypothalamus, thalamus, lateral parabrachial nucleus and locus coeruleus. Cloning of the rat GALR1 galanin receptor cDNA will permit many new experimental strategies to be applied to studies of the structure and function of galanin receptors.

Galanin--a neuropeptide with inhibitory actions

1. Galanin is a 29 (in humans 30) amino acids long neuropeptide with mostly inhibitory, hyperpolarizing actions. 2. Differential structural requirements of truncated forms of galanin and differential agonist/antagonist behaviour of chimeric peptides, high affinity galanin receptor ligands suggest the presence of pharmacologically distinct galanin receptor subtypes. 3. The galanin receptor from human Bowes melanoma cell line--a member of G-protein coupled receptor superfamily--has been cloned. 4. Galanin acts via Gi/G(o) proteins inhibiting cAMP production, inositol phosphate turnover, opening K+ channels or closing Ca2+ channels.

Direct control of a large conductance K(+)-selective channel by G-proteins in adrenal chromaffin granule membranes

We report here the presence of a Ca(2+)-independent K(+)-channel of large conductance in adrenal chromaffin cell secretory vesicle membranes which is controlled by inhibitory as well as stimulatory heterotrimeric GTP-binding proteins. Using antibodies against specific alpha subunits for immunoblot analysis, we were able to identify the presence of the inhibitory G(i)2 and G(i)3 subtypes, as well as the stimulatory G(o) and Gs subtypes, but not G(i)1 in adrenal chromaffin granules. Furthermore, functional analysis of the K(+)-channel incorporated into planar lipid bilayers showed that GDP beta S and GTP gamma S have opposite effects on channel activity inducing interconversions between a low and a high open-probability state. Consistent with these findings, the same antibodies antagonized the effects of the nonhydrolyzable analogues on the open probability of the K(+)-channel.