The human galanin receptor: ligand-binding and functional characteristics in the Bowes melanoma cell line

Distribution and differential regulation of galanin receptor subtypes in rat brain: effects of seizure activity

Galanin, acting at the GalR1-3 subtypes of galanin receptors, is involved in the regulation of cognition, mood, feeding, seizure activity and pain. The understanding of galanin's effects in molecular and cellular terms has been hampered by the lack of receptor subtype selective ligands and antibodies. Previous in situ hybridization data showed that GalR1 and GalR2 receptors are abundant in the rat brain, while the distribution of GalR3 is contradictory and most studies demonstrated a low expression levels in the rat brain. The distribution of galanin receptor subtypes at protein level is unknown. In the present study, we report the regional distribution of the galanin receptors: GalR1 and non-GalR1 receptors, using a recently synthesized high affinity GalR2/3 selective ligand, galanin (2-11), and galanin (1-29), as competitors, in saturating (125)I-galanin membrane binding assay. We show that paraventricular nucleus (PVN) express predominantly GalR1, whereas areas like the dorsal raphe nucleus (DRN), hippocampus and amygdala express both the GalR1 and non-GalR1 receptors. We speculate that the GalR2/3 binding sites detected by galanin (2-11) binding in our study probably represent mostly GalR2 receptors. In addition, we show regionally specific and subtype specific regulation of galanin receptors. Status epilepticus (SE), known to deplete galanin from axonal projections of locus coeruleus and septum/diagonal band neurons in the hippocampus and to induce galanin expression in a subset of hippocampal cells, down regulates GalR2 receptor mRNA and proteins by 30% without altering the GalR1 receptors.

A role for galanin in antidepressant actions with a focus on the dorsal raphe nucleus

Selective serotonin reuptake inhibitors, such as fluoxetine (FLX), are the most commonly used drugs in the treatment of major depression. However, there is a limited understanding of their molecular mechanism of action. Although the acute effect of selective serotonin reuptake inhibitors in elevating synaptic serotonin concentrations is well known, the clinical amelioration of depressive symptoms requires 14-21 days of treatment, suggesting that numerous other rearrangements of function in the CNS must take place. In the present study, we demonstrated that 14 days of FLX treatment up-regulated galanin mRNA levels by 100% and GalR2-binding sites by 50%, in the rat dorsal raphe nucleus, where galanin coexists with serotonin. Furthermore, a galanin receptor antagonist, M40, attenuated the antidepressant-like effect of FLX in the forced swim test, a rodent preclinical screen commonly used to evaluate antidepressant-like efficacy. Direct activation of galanin receptors by a galanin receptor agonist, galnon, was found to produce an antidepressant-like effect in the same task. Two other antidepressant treatments also affected the galaninergic system in the monoaminergic nuclei: Electroconvulsive shock elevated galanin mRNA levels in dorsal raphe nucleus, whereas sleep deprivation increased galanin mRNA levels in the locus coeruleus, further underlining the connection between activation of the galaninergic system and antidepressant action of various clinically proven treatments.

[125I]-Galanin binding in brain of wildtype, and galanin- and GalR1-knockout mice: Strain and species differences in GalR1 density and distribution

Widespread production of knockout and transgenic mice has led to an increased use of mice as animal models for studies of normal- and patho-physiology. Hence, the precise mapping of central transmitter/peptide systems in the mouse has become essential for the interpretation of functional studies and for the correct correlation with findings obtained in the rat, primates and/or human. In this regard, the current study reports the autoradiographic localization of [(125)I]-galanin (GAL) binding sites in brain of the common C57BL/6J and 129OlaHsd mouse strains, as well as in GAL and galanin receptor-1 (GalR1) knockout (KO) mice. In C57BL/6J and 129OlaHsd mice, [(125)I]-GAL binding sites were detected throughout the brain, including moderate-high relative densities in the basal ganglia (caudate putamen, nucleus [n.] accumbens, olfactory tubercle, substantia nigra), limbic regions (septum, bed n. stria terminalis, ventral hippocampus, amygdala), cingulate, retrosplenial, entorhinal cortex, centro-lateral/medial thalamic n., preoptic/lateral hypothalamus, midbrain (superior colliculus, periaqueductal gray), pons/medulla oblongata (parabrachial, pontine reticular and solitary tract n.) and cerebellar cortex. [(125)I]-GAL binding levels were low or absent in main olfactory bulb, neocortex, ventrolateral/geniculate thalamic n., dorsal hippocampus, inferior colliculus and cranial motor n. In simultaneous determinations, relative [(125)I]-GAL binding site densities in brain were generally lower in C57BL/6J than in 129OlaHsd mice, while the density and distribution of central binding in the GAL-KO mouse was essentially identical to that in its background-129OlaHsd strain. In contrast, no specific [(125)I]-GAL binding was detected in any region of GalR1-KO mouse brain, revealing that under the experimental conditions used, the peptide ligand binding is predominantly (exclusively) to the GalR1 subtype. This evaluation of GAL receptor site distribution in mouse brain has revealed similarities and some differences with the equivalent system in rat and provides a valuable reference for future comparative studies of central GAL transmission.

Galanin type 2 receptors regulate neuronal survival, susceptibility to seizures and seizure-induced neurogenesis in the dentate gyrus

The neuropeptide galanin has been implicated in inhibiting seizures and protecting hippocampal neurons from excitotoxic injury. In the hippocampus galanin acts through two receptor subtypes, GalR1, expressed in CA1, and GalR2, abundant in dentate gyrus. We developed an approach to induce and to study selective semichronic knockdown of GalR2 in the rat hippocampus. A 50% reduction of GalR2 binding was achieved by continuous infusion of complementary peptide nucleic acid antisense oligonucleotide into the dentate gyrus. This resulted in an increase in the severity of self-sustaining status epilepticus induced by electrical stimulation of the perforant path, induced mild neuronal injury in the dentate hilus, augmented seizure-induced hilar injury and inhibited seizure-induced neurogenesis in the subgranular zone of the dentate gyrus. Our data suggest that in the dentate gyrus, galanin, acting through GalR2, inhibits seizures, promotes viability of hilar interneurons and stimulates seizure-induced neurogenesis. These results are important for understanding the role of galanin and galanin receptor subtypes in the hippocampus both under normal conditions and in excitotoxic injury.

Galmic, a nonpeptide galanin receptor agonist, affects behaviors in seizure, pain, and forced-swim tests

The pharmacological exploitation of the galanin receptors as drug targets for treatment of epilepsy, depression, and pain has been hampered by the lack of workable compounds for medicinal chemists from random screening of large chemical libraries. The present work uses the tripeptidomimetic galnon and displays its presumed pharmacophores on a rigid molecular scaffold. The scaffold is related to marine natural products and presents three functional groups near one another in space, in a manner reminiscent of a protein surface. An active compound, Galmic, was identified from a small synthetic library and tested in vitro and in vivo for its affinity and efficacy at galanin receptors. Galmic has micromolar affinity for GalR1 receptors (Ki = 34.2 microM) and virtually no affinity for GalR2 receptors. In vitro, Galmic, like galanin, suppresses long-term potentiation in the dentate gyrus; it blocks status epilepticus when injected intrahippocampally or administered i.p. Galmic applied i.p. shows antidepressant-like effects in the forced-swim test, and it is a potent inhibitor of flinching behavior in the inflammatory pain model induced by formalin injection. These data further implicate brain and spinal cord galanin receptors as drug targets and provide an example of a systemically active compound based on a scaffold that mimics protein surfaces.

Galanin contributes to the excess colonic fluid secretion observed in dextran sulfate sodium murine colitis

Galanin is present in enteric nerves lining the gastrointestinal (GI) tract where it is normally involved in regulating intestinal motility by binding to the galanin-1 receptor (Gal1R) subtype expressed by smooth muscle cells. In contrast, although epithelial cells lining the colon do not normally express Gal1R, this protein is up-regulated by the inflammation-associated transcription factor NF-kappaB. We previously showed that the murine colitis induced by dextran sulfate sodium (DSS) was associated with increased Gal1R expression as well as by increased colonic fluid secretion. Although Gal1R up-regulation by colonic epithelial cells results in increased intestinal Cl- secretion, the relative contributions of galanin to this excess colonic fluid secretion could not be determined. We therefore created a mouse genetically incapable of synthesizing Gal1R (GAL1R-/- mice). We herein demonstrate that both wild-type and GAL1R-/- mice developed identical histologic lesions in response to DSS. This was characterized by a marked inflammatory infiltrate, activation of NF-kappaB in both enterocytes and enteric nerves, and a threefold increase in neuronal galanin. Colonic fluid secretion, while increased, was approximately half that in GAL1R-/- mice as compared with their wild-type littermates. Overall, then, these findings strongly suggest that approximately half of the increase in colonic fluid secretion in DSS colitis is due to up-regulation of the Gal1R.

Actions of galanin on neurotransmission in the submucous plexus of guinea pig small intestine

Electrophysiologic recording methods were used to study the actions of galanin on synaptic transmission in the submucous plexus of guinea pig ileum. Exposure to galanin resulted in concentration-dependent suppression of slow noradrenergic inhibitory postsynaptic potentials and fast nicotinic excitatory postsynaptic potentials in the majority of neurons. Failure of galanin to suppress nicotinic depolarizing responses to micropressure pulses of acetylcholine and failure to suppress hyperpolarizing responses to micropressure pulses of norepinephrine suggested that galanin acted at presynaptic inhibitory receptors to suppress release of acetylcholine and norepinephrine. Galanin suppressed slow excitatory postsynaptic potentials in eight of eight neurons with AH (after-hyperpolarization) type electrical behavior and in none of 26 neurons with S (synaptic) type electrical behavior. Suppression of excitatory neurotransmission in AH neurons was always associated with membrane hyperpolarization. Excitatory responses caused by experimentally applied substance P were also inhibited by galanin. Galanin-(1-16) and galanin-like peptide mimicked the inhibitory actions of galanin on neurotransmission. The selective galanin GAL2 receptor agonist [D-Trp(2)]galanin was inactive. The chimeric peptides, galanin-(1-13)-spantide I, galantide, galanin-(1-13)-neuropeptide Y(25-36) amide, galanin-(1-13)-bradykinin-(2-9)amide and galanin-(1-13)-Pro-Pro-Ala-Leu-Ala-Leu-Ala amide all produced varying degrees of suppression of the synaptic potentials. The evidence suggests that the galanin GAL1 receptor, but not the galanin GAL2 receptor, mediated the presynaptic and postsynaptic inhibitory actions of galanin.

Galanin and galanin receptors in human gliomas

Galanin-like immunoreactivity (GAL-LI) and specific GAL binding sites have been shown to be widely distributed in the central nervous system (CNS) and in CNS tumors. GAL and its receptors have also been shown to be present in glial cells, but to date it is still unknown whether human gliomas produce GAL and express GAL receptors. In this study 20 brain tumors consisting of 15 glioblastomas, 4 meningiomas and 1 gliosarcoma were investigated for the presence of GAL-LI and GAL receptors. Immunofluorescence analysis revealed a dense network of GAL-LI positive cellular processes and cell bodies in 18 of the 20 tumors. In contrast, in vitro (125)I-labeled GAL receptor autoradiography showed substantial GAL binding in only 6 glioblastoma tissues. Reverse transcription-PCR analysis detected mRNA of all three known galanin receptors in the tumor tissues, with most tumors expressing multiple receptor subtypes. Pharmacological analysis of tumor membrane homogenates with GAL and the specific GAL receptor GalR2 agonist, AR-M1896, revealed that the GAL receptor GalR1 is most likely the receptor responsible for the observed GAL binding in the glioblastomas. No correlation could be found between GAL-LI, the level of GAL binding and proliferative activity as determined by immunostaining with the cell proliferation marker Ki-67.

Galanin inhibits tyrosine hydroxylase expression in midbrain dopaminergic neurons

Galanin (GAL) inhibits midbrain dopamine (DA) activity in several experimental paradigms, yet the mechanism underlying this inhibition is unclear. We examined the effects of GAL on the expression of tyrosine hydroxylase (TH) in primary cultures of rat embryonic (E14) ventral mesencephalon (VM). One micromolar GAL had no effect on the number of TH-immunoreactive (ir) neurons in VM cultures. However, 1 micro m GAL reduced an approximately 100% increase in TH-ir neurons in 1 mm dibutyryl cAMP (dbcAMP)-treated cultures by approximately 50%. TH-ir neuron number in dbcAMP-treated VM cultures was dose-responsive to GAL and the GAL receptor antagonist M40 blocked GAL effects. Semi-quantitative RT-PCR and quantitative immunoblotting experiments revealed that GAL had no effect on TH mRNA levels in VM cultures but reduced TH protein. VM cultures expressed GALR1, GALR2, and GALR3 receptor mRNA. However, dbcAMP treatment resulted in a specific approximately 200% increase in GALR1 mRNA. GALR1 activity is linked to a pertussis toxin (PTX)-sensitive opening of G protein-gated K+ channels (GIRKs). GAL reduction of TH-ir neuron number in dbcAMP + GAL-treated cultures was sensitive to both PTX and tertiapin, a GIRK inhibitor. GAL inhibition of midbrain DA activity may involve a GALR1- mediated reduction of TH in midbrain dopaminergic neurons.

Characterisation of a new chimeric ligand for galanin receptors: galanin(1-13)-[D-Trp(32)]-neuropeptide Y(25-36)amide

In this work, we studied a novel chimeric peptide, M242, galanin(1-13)-[D-Trp(32)]-neuropeptide Y(25-36)amide, and examined its properties in comparison with its parent peptide, M32, galanin(1-13)-neuropeptide Y(25-36)amide, a previously known high-affinity ligand for galanin receptors, and galanin itself. Binding assays performed in Bowes cells known to express human galanin receptor type 1 (hGalR1) and in Chinese hamster ovary cells overexpressing human galanin receptor type 2 (hGalR2) revealed that all three ligands had comparable affinities: at hGalR1<1 nM and at hGalR2<10 nM. However, in rat hippocampal membranes M242 had a 24-fold lower affinity than galanin (9.4 vs. 0.4 nM) and 134-fold lower affinity than M32 (9.4 vs. 0.07 nM). In the same tissue, we also examined the effects of these peptides on adenylate cyclase activity. M32 showed a weak antagonistic behaviour but M242 acted as a potent biphasic regulator of adenylate cyclase. In conclusion, we present and characterise a new peptide M242, which could be a useful tool in studies of galaninergic signalling.

Biphasic response to human galanin of extracellular acidification in human Bowes melanoma cells

The metabolic response of galanin GAL1 receptor subtype, endogenously expressed in human Bowes melanoma (HBM) cells, was investigated. Cytosensor microphysiometry was used to determine the extracellular acidification rate. A biphasic response, consisting of a rapid increase in the extracellular acidification rate followed by a decrease below the basal level, was observed after perfusion with human galanin. The magnitude and the rate of onset of both phases were dependent on the galanin concentration. The increase in the extracellular acidification rate (maximum of 25% of basal level; -log(EC(50))=7.23+/-0.14) was transient, whereas the following decrease (maximum of 40% of basal level; -log(EC(50))=7.77+/-0.23) was sustained. The EC(50) values for the increase and decrease were in a similar range. After consecutive galanin administration, the magnitude of the response was the same as for the unexposed cells, indicating the absence of galanin receptor desensitization or internalization in HBM cells. Responses were blocked by pretreatment with pertussis toxin and phorbol-12-myristate-13-acetate (PMA), indicating a G-protein/protein kinase C signalling pathway. Our microphysiometry results show a biphasic response of the extracellular acidification rate mediated by the galanin receptor expressed in HBM cells which has not been described previously for any other endogenously expressed neuropeptide receptor.

Binding of chimeric NPY/galanin peptides M32 and M242 to cloned neuropeptide Y receptor subtypes Y1, Y2, Y4, and Y5

Ligand binding to neuropeptide Y (NPY) receptors Y1, Y2, Y4, and Y5 from guinea-pig was investigated using the two NPY-galanin hybrids M32 (galanin1-13-NPY25-36-amide) and M242 ([D-Trp(32)]M32). The affinity of M32 for Y1, Y2, and Y4 receptors was 13, 4, and 30nM, respectively, similar to that of NPY18-36 and NPY22-36 but 40-fold to 300-fold lower than the affinity of intact porcine NPY. M242 bound to the Y1, Y2, and Y4 receptors with 9-fold to 20-fold lower affinity than did M32. The affinities of M32 and M242 for Y5 were 400 and 800 nM, respectively. Thus, M32 seems to gain affinity relative to both of its constituent peptide portions although the NPY25-36 part may be sufficient for NPY-receptor recognition, especially at the Y2 receptor. This suggests that the galanin portion of M32 influences and/or stabilizes the conformation of the NPY portion, similar to the effect seen for the NPY portion of M32 in binding to galanin receptors.

Development of a fluorescent ligand-binding assay using the AcroWell filter plate

One of the most powerful tools for receptor research and drug discovery is the use of receptor-ligand affinity screening of combinatorial libraries. Early work involved the use of radioactive ligands to identify a binding event; however, there are numerous limitations involved in the use of radioactivity for high throughput screening. These limitations have led to the creation of highly sensitive, nonradioactive alternatives to investigate receptor-ligand interactions. Pall Gelman Laboratory has introduced the AcroWell, a patented low-fluorescent-background membrane and sealing process together with a filter plate design that is compatible with robotic systems. Taken together, these allow the AcroWell 96-well filter plate to detect trace quantities of lanthanide-labeled ligands for cell-, bead-, or membrane-based assays using time-resolved fluorescence. Using europium-labeled galanin, we have demonstrated that saturation binding experiments can be performed with low-background fluorescence and signal-to-noise ratios that rival traditional radioisotopic techniques while maintaining biological integrity of the receptor-ligand interaction. In addition, the ability to discriminate between active and inactive compounds in a mock galanin screen is demonstrated with low well-to-well variability, allowing reliable determination of positive hits even for low-affinity interactions.

Amyloid peptide Abeta(1-42) binds selectively and with picomolar affinity to alpha7 nicotinic acetylcholine receptors

We have recently reported evidence that a very high affinity interaction between the beta-amyloid peptide Abeta(1-42) and the alpha7 nicotinic acetylcholine receptor (alpha7nAChR) may be a precipitating event in the formation of amyloid plaques in Alzheimer's disease. In the present study, the kinetics for the binding of Abeta(1-42) to alpha7nAChR and alpha4beta2nAChR were determined using the subtype-selective nicotinic receptor ligands [(3)H]methyllycaconitine and [(3)H]cytisine. Synaptic membranes prepared from rat and guinea pig cerebral cortex and hippocampus were used as the source of receptors. Abeta(1-42) bound to the alpha7nAChR with exceptionally high affinity, as indicated by K(i) values of 4.1 and 5.0 pM for rat and guinea pig receptors, respectively. When compared with the alpha7nAChR, the affinity of Abeta(1-42) for the alpha4beta2nAChR was approximately 5,000-fold lower, as indicated by corresponding K(i) values of 30 and 23nM. The results of this study support the concept that an exceptionally high affinity interaction between Abeta(1-42) and alpha7nAChR could serve as a precipitating factor in the formation of amyloid plaques and thereby contribute to the selective degeneration of cholinergic neurons that originate in the basal forebrain and project to the cortex and hippocampus.

2,3-Dihydro-dithiin and -dithiepine-1,1,4,4-tetroxides: small molecule non-peptide antagonists of the human galanin hGAL-1 receptor

The neuropeptide galanin modulates several physiological functions such as cognition, learning, feeding behavior, and depression, probably via the galanin 1 receptor (GAL-R1). Using an HTS assay based on 125I-human galanin binding to the human galanin-1 receptor (hGAL-R1), we discovered a series of 1,4-dithiin and dithiipine-1,1,4,4-tetroxides that exhibited binding affinity IC50's to hGAL-R1 ranging from 190 to 2700 nM. Two of the dithiepin analogues, 7 and 23, behaved pharmacologically as hGAL-R1 antagonists in secondary assays involving adenylate cyclase activity and GTP binding to G-proteins. Analogues 7 and 23 were also active in functional assays involving galanin, reversing the inhibitory effect of galanin on acetylcholine (ACh) release in rat brain hippocampal slices and electrically-stimulated guinea pig ileum twitch.

Dextran sulfate sodium-induced murine colitis activates NF-kappaB and increases galanin-1 receptor expression

Galanin is widely distributed in enteric nerve terminals and acts to modulate intestinal motility by altering smooth muscle contraction. This ligand causes Cl(-) secretion when colonic epithelial cells express the galanin-1 receptor (Gal1-R) subtype. Because Gal1-R expression by colonic epithelia is upregulated by the transcription factor nuclear factor-kappaB (NF-kappaB), increasingly appreciated as critical in the pathophysiology of inflammatory bowel disease, we wondered whether the diarrhea associated with this condition could be due to NF-kappaB-mediated increases in Gal1-R expression. To test this hypothesis, we provided oral dextran sulfate sodium (DSS) to C57BL/6J mice. Although Gal1-R are not normally expressed by epithelial cells lining the mouse colon, DSS treatment resulted in increased NF-kappaB activation and Gal1-R expression. Whereas galanin had no effect on murine colonic tissues studied ex vivo, it progressively increased short-circuit current and colonic fluid secretion in DSS-treated mice. Concomitant parenteral administration of the NF-kappaB inhibitor dexamethasone attenuated the activation of this transcription factor by DSS, inhibiting the increase in Gal1-R expression. Although Gal1-R-specific antagonists do not exist, intracolonic administration of commercially available galanin antibody diminished the DSS-induced increase in colonic fluid accumulation. Overall, these data demonstrate that a significant component of the excessive fluid secretion observed in DSS-treated mice is due to increased Gal1-R expression.

Galanin receptor subtypes

The neuropeptide galanin, which is widely expressed in brain and peripheral tissues, exerts a broad range of physiological effects. Pharmacological studies using peptide analogues have led to speculation about multiple galanin receptor subtypes. Since 1994, a total of three G-protein-coupled receptor (GPCR) subtypes for galanin have been cloned (GAL1, gal2 and gal3). Potent, selective antagonists are yet to be found for any of the cloned receptors. Major challenges in this field include linking the receptor clones with each of the known physiological actions of galanin and evaluating the evidence for additional galanin receptor subtypes.

beta-Amyloid(1-42) binds to alpha7 nicotinic acetylcholine receptor with high affinity. Implications for Alzheimer's disease pathology

Alzheimer's disease pathology is characterized by the presence of neuritic plaques and the loss of cholinergic neurons in the brain. The underlying mechanisms leading to these events are unclear, but the 42-amino acid beta-amyloid peptide (Abeta(1-42)) is involved. Immunohistochemical studies on human sporadic Alzheimer's disease brains demonstrate that Abeta(1-42) and a neuronal pentameric cation channel, the alpha7 nicotinic acetylcholine receptor (alpha7nAChR), are both present in neuritic plaques and co-localize in individual cortical neurons. Using human brain tissues and cells that overexpress either alpha7nAChR or amyloid precursor protein as the starting material, Abeta(1-42) and alpha7nAChR can be co-immunoprecipitated by the respective specific antibodies, suggesting that they are tightly associated. The formation of the alpha7nAChR.Abeta(1-42) complex can be efficiently suppressed by Abeta(12-28), implying that this Abeta sequence region contains the binding epitope. Receptor binding experiments show that Abeta(1-42) and alpha7nAChR bind with high affinity, and this interaction can be inhibited by alpha7nAChR ligands. Human neuroblastoma cells overexpressing alpha7nAChR are readily killed by Abeta(1-42), whereas alpha7nAChR agonists such as nicotine and epibatidine offered protection. Because Abeta(1-42) inhibits alpha7nAChR-dependent calcium activation and acetylcholine release, two processes critically involved in memory and cognitive functions, and the distribution of alpha7nAChR correlates with neuritic plaques in Alzheimer's disease brains, we propose that interaction of the alpha7nAChR and Abeta(1-42) is a pivotal mechanism involved in the pathophysiology of Alzheimer's disease.

Galanin and galanin receptors

The development of a strain of galanin knockout mice has provided confirmation of a neuroendocrine role for galanin, as well as supporting results of previous physiological investigations indicating a role for galanin in analgesia and neuropathic pain, and potentially in neuronal growth and regeneration processes. Whether elevation of galanin expression in neurodegenerative disorders such as Alzheimer's disease represents a survival response or exacerbates functional deficit in afflicted individuals remains to be determined. More detailed analysis of the phenotype of the galanin knockout mouse should provide insights into the physiological role of galanin in memory and learning processes, as well as in hypothalamic function and other aspects of neuroendocrine regulation. Biochemical and molecular cloning efforts have demonstrated that the multiplicity of actions of galanin is matched by complexity in the distribution and regulation of galanin and its receptors. A focus on characterisation of galanin receptors has resulted in the molecular cloning of three receptor subtypes to date. The distribution and functional properties of these receptors have not yet been fully elucidated, currently precluding assignment of discrete functions of galanin to any one receptor subtype. It is not currently possible to reconcile available pharmacological data using analogs of galanin and chimeric peptides in functional assay systems with the pharmacological properties of cloned receptor subtypes. This highlights the value of further knockout approaches targeting galanin receptor subtypes, but also raises the possibility of the existence of additional receptor subtypes that have yet to be cloned, or that receptor activity may be modulated by regulatory molecules that remain to be identified. The development of receptor subtype-specific compounds remains a high priority to advance work in this area. The ability to selectively modulate the many different actions of galanin, through a clearer understanding of receptor structure-function relationships and neuronal distribution, promises to provide important insights into the molecular and cellular basis of galanin action in normal physiology, and may provide lead compounds with therapeutic application in the prevention and treatment of a range of disorders.

Determination of ligand binding affinities for endogenous seven-transmembrane receptors using fluorometric microvolume assay technology

We have developed a fluorescence-based mix and read method for the quantitative determination of receptor-ligand binding interactions. This method was used to determine IC(50) values for peptide ligands of two endogenous seven-transmembrane receptors that are expressed in cultured human cancer cells. Substance P, neurokinin A, and galanin were labeled with Cy5 and were shown to retain their native binding affinities. The cell-associated fluorescence was quantified using a fluorometric microvolume assay technology (FMAT) scanner that was designed to perform high-throughput screening assays in multiwell plates with no wash steps. The binding of fluorescently labeled substance P and neurokinin A was tested on the human astrocytoma cell line UC11 that expresses endogenous NK(1) receptor. Galanin binding was measured on endogenous galanin type 1 receptors in the Bowes neuroblastoma cell line. IC(50) values were determined for substance P, neurokinin A, and galanin and were found to correspond well with reported values from radioligand binding determinations. To demonstrate FMAT as instrumentation for high-throughput screening, it was utilized to successfully identify individual wells in a 96-well plate in which Cy5-substance P binding in UC11 cells was competed with unlabeled substance P. In addition, we developed a two-color multiplex assay in which cells individually expressing neuropeptide Y and substance P receptors were mixed in the same well. In this assay, the fluorescent ligands substance P and neuropeptide Y bound only to their respective cell types and binding was specifically competed. Therefore, two different seven-transmembrane receptor targets can be tested in one screen to minimize reagent consumption and increase throughput.

Human colonic epithelial cells express galanin-1 receptors, which when activated cause Cl- secretion

Galanin is a peptide hormone widely expressed in the central nervous system and gastrointestinal (GI) tract. Within the GI tract galanin is present in enteric nerve terminals where it is known to modulate intestinal motility by altering smooth muscle contraction. Recent studies also show that galanin can alter intestinal short-circuit current (Isc) but with differing results observed in rats, rabbits, guinea pigs, and pigs. In contrast, nothing is known about the ability of galanin to alter ion transport in human intestinal epithelial tissues. By RT-PCR, we determined that these tissues express only the galanin-1 receptor (Gal1-R) subtype. To evaluate Gal1-R pharmacology and physiology, we studied T84 cells. Gal1-R expressed by these cells bound galanin rapidly (half time 1-2 min) and with high affinity (inhibitor constant 0.7 +/- 0.2 nM). T84 cells were then studied in a modified Ussing chamber and alterations in Isc, a measure of all ion movement across the tissue, were determined. Maximal increases in Isc were observed in a concentration-dependent manner around 2 min after stimulation with peptide, with 1 microM galanin causing Isc to rise more than eightfold and return to baseline occurring within 10 min. The increase in galanin-induced Isc was shown by 125I efflux studies to be due to Cl- secretion, which occurred independently of alterations in cAMP and phospholipase C. Rather, Cl- secretion is mediated via a Ca2+-dependent, pertussis toxin-sensitive mechanism. These data suggest that galanin released by enteric nerves may act as a secretagogue in the human colon by activating Gal1-R.

Molecular biology and pharmacology of galanin receptors

Galanin was first isolated 15 years ago. Diversity of galanin receptors has been suspected from the study of native tissues and functional responses to galanin and galanin-like peptides in vitro and in vivo. The recent application of molecular biologic techniques to clone galanin receptors has extended this diversity. So far, three galanin receptor subtypes, GALR1, GALR2, and GALR3, have been cloned from both human and rat. Their molecular structure, pharmacologic profiles, tissue distribution, and signal transduction properties have been partially elucidated.

Molecular characterization, pharmacological properties and chromosomal localization of the human GALR2 galanin receptor

The neuropeptide galanin mediates a diverse spectrum of biological activities by interacting with specific G protein-coupled receptors. We have used homology genomic library screening and polymerase chain reaction (PCR) techniques to isolate both genomic and cDNA clones encoding the human homolog of the recently cloned rat GALR2 galanin receptor. By fluorescence in situ hybridization, the gene encoding human GALR2 (GALNR2) has been localized to chromosome 17q25.3. The two coding exons of the human GALNR2 gene, interrupted by an intron positioned at the end of transmembrane domain III, encode a 387 amino acid G protein-coupled receptor with 87% overall amino acid identity with rat GALR2. In HEK-293 cells stably expressing human GALR2, binding of [125I]porcine galanin is saturable and can be displaced by galanin, amino-terminal galanin fragments and chimeric galanin peptides but not by carboxy-terminal galanin fragments. In HEK-293 cells, human GALR2 couples both to Galphaq/11 to stimulate phospholipase C and increase intracellular calcium levels and to Galphai/o to inhibit forskolin-stimulated intracellular cAMP accumulation. A wide tissue distribution is observed by reverse transcriptase (RT)-PCR analysis, with human GALR2 mRNA being detected in many areas of the human central nervous system as well as in peripheral tissues.

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.

Cloning and characterization of the human galanin GALR2 receptor

We present the molecular cloning and characterization of the human galanin receptor, hGALR2. hGALR2 shares 85%, 39%, and 57% amino acid identities to rGALR2, hGALR1, and hGALR3, respectively. hGALR2, along with rGALR2, can be distinguished from the other cloned galanin receptors by a tolerance for both N-terminal extension and C-terminal deletion of galanin, as well as by a primary signaling mechanism involving phosphatidyl inositol hydrolysis and calcium mobilization. By RT-PCR, GALR2 mRNA was abundant in human hippocampus, hypothalamus, heart, kidney, liver, and small intestine. A weak GALR2 mRNA signal was detected in human retina, and no signal was detected in cerebral cortex, lung, spleen, stomach, or pituitary.

Pituitary adenylate cyclase-activating peptide (PACAP) and PACAP type 1 receptor expression in regenerating adult mouse and rat superior cervical ganglia in vitro

Pituitary adenylate cyclase-activating polypeptide (PACAP), a regulatory peptide belonging to the vasoactive intestinal peptide (VIP) family, is widely distributed in the central and peripheral nervous system. Recent studies have shown that PACAP expression is upregulated in sensory neurons in response to axonal injury. Here we report that PACAP and PACAP type 1 receptors are located in rat and mouse superior cervical ganglia (SCG). PACAP-immunoreactivity (-IR) was demonstrated in preganglionic fibers, whereas only occasional PACAP-IR cell bodies could be observed. In situ hybridization histochemistry using 35S-labeled deoxyribonucleotide probes confirmed that PACAP mRNA was present only in occasional cell bodies. In contrast, PACAP type 1 receptor mRNA was expressed in virtually all cell bodies within the ganglia. After removal and culturing of the SCG for 24 h, there was a marked increase in PACAP mRNA, whilst PACAP type 1 receptor mRNA expression appeared to be downregulated in most nerve cell bodies except for a few scattered neurons displaying a strong upregulation. The total specific binding of PACAP to isolated SCG membranes as assayed by [125I]PACAP-27 binding showed an increase in SCG cultured for 48 h. PACAP-27 neither affected axonal outgrowth from the cultured SCG nor the survival of cells within the SCG. We conclude that PACAP and PACAP receptors are rapidly upregulated in sympathetic ganglia in response to axonal injury and that PACAP may play a role during nerve regeneration.

Cloning, pharmacological characterization and distribution of a novel galanin receptor

The neuropeptide galanin mediates a diverse spectrum of biological activities by interacting with specific G-protein-coupled receptors. Through expression cloning, human and rat GALR1 receptor cDNA clones have previously been isolated and characterized. In this study, we have used homology screening to isolate a rat brain cDNA clone encoding a second galanin receptor subtype, the GALR2 receptor. The isolated cDNA encodes a 372-amino-acid G-protein-coupled receptor that shares 38% overall amino-acid identity with the rat GALR1 receptor. The pharmacological profile of the rat GALR2 receptor is similar to that of the rat GALR1 receptor. The rat GALR2 receptor binds galanin, N-terminal galanin fragments, and the putative galanin receptor antagonists galantide, C7, M35 and M40 with high affinity but it does not bind C-terminal galanin fragments. Galanin increases intracellular inositol phosphate levels in HEK 293 cells expressing the rat GALR2 receptor via a pertussis toxin-insensitive G-protein. The rat GALR2 receptor mRNA is highly expressed in several brain regions, including hypothalamus and hippocampus as well as the anterior pituitary, with lower levels of expression detected in amygdala, and regions of cortex. It is also highly expressed in the GH3 pituitary cell line and in gut tissues, and to a lower extent in spleen, lung, skeletal muscle, heart, kidney, liver and testis. These results suggest that GALR2 receptor mediates galanin's regulation of pituitary hormone secretion and possibly food intake.

Mutagenesis and ligand modification studies on galanin binding to its GTP-binding-protein-coupled receptor GalR1

In this study, a large number of receptor mutants were generated and several N-terminally modified galanin analogues synthesized to refine the previously proposed binding site model for galanin to its GTP-binding-protein-coupled receptor GalR1. In addition to ligand-binding studies, the functionality of mutant receptors was evaluated by assessing their ability to mediate galaninergic inhibition of isoproterenol-stimulated adenylyl cyclase activity. The His264Ala and Phe282Ala receptor mutants, although deficient in binding in the concentration range of galanin used, remain functional albeit 20-fold less efficient than the wild-type receptor in mediating inhibition of stimulated cAMP production by galanin. The His267Ala mutant is, apart from being deficient in galanin binding, also severely impaired in functional coupling. While His264 and Phe282 seem to be important in forming the binding pocket for galanin, His267 might play a role in forming or stabilizing the active conformation of the GalR1 receptor rather than directly participating in the formation of the binding pocket for galanin. N-terminal carboxylic acid analogues of galanin have low affinity to wild-type GalR1, but substantially increased affinity to the Glu271Lys receptor mutant. This, together with the finding that an alanine substitution of Phe115 in TM III results in a tenfold decrease in affinity for galanin, suggests that the N-terminus of galanin interacts with Phe115. In contrast to the Phe282Ala mutation in TM VII, a conservative mutation of Phe282 to tyrosine did not alter the affinity for galanin. Thus, the interaction between Tyr9 of galanin and Phe282 is likely to be of an aromatic-aromatic nature.

Pharmacological characterization and tissue distribution of the human and rat GALR1 receptors

The diverse biological functions of galanin are mediated via membrane bound high-affinity receptors. In order to identify and characterize potential galanin receptor subtypes, we have examined the specific 125I-galanin binding to the CHP-212 human neuroblastoma cell line. The galanin receptors expressed in CHP-212 cells, like GALR1 have high affinity for galanin (Kd = 0.07 nM) and the potency for inhibition of 125I-galanin binding by galanin peptides parallels that of hGALR1 expressed in a stable CHO cell line. We confirmed that GALR1 is expressed in these cells by RT-PCR. We further determined the tissue expression patterns of hGALR1 which is expressed in a variety of human tissues at a very low level, with the highest levels seen in heart, small intestine and prostate. A species of approximately 70 kDa is recognized by antisera specific for hGALR1 by Western blot analysis and should allow future measurements of receptor protein expression.

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.

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.

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.

Galantide distinguishes putative subtypes of galanin receptors in mudpuppy parasympathetic neurons

The effect of the chimeric ligand galantide on the galanin-induced activation of membrane K+ conductance and inhibition of voltage-dependent Ca2+ conductance has been studied using voltage-clamped dissociated mudpuppy parasympathetic neurons. Galantide did not activate the K+ conductance but produced a concentration-dependent antagonism (IC50 = 4 nM) of the galanin-induced increase in K+ conductance. Galantide acted like galanin and inhibited the voltage-dependent Ba2+ current (IBa). The inhibition of IBa also was concentration dependent (IC50 = 16 nM) and the maximum inhibition produced by galantide was approximately 40%. We also demonstrate that the galanin-(1-16) fragment increased the membrane K+ conductance and decreased IBa, suggesting that the NH2 portion of the galanin molecule is sufficient to mediate both actions. One interpretation of these observations is that different galanin receptors mediate the different effects of galanin on the mudpuppy parasympathetic neurons.

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.

Differential regulation of adenylate cyclase activity in rat ventral and dorsal hippocampus by rat galanin

Rat galanin inhibits basal as well as forskolin-stimulated adenylate cyclase activity in rat ventral and dorsal hippocampus. The inhibition of adenylate cyclase activity, both basal and forskolin-stimulated, is characterised by IC50 values being 250-fold lower in ventral hippocampus (IC50 = 1.1 nM) compared to the dorsal hippocampus (IC50 = 270 nM). The maximal inhibition of basal and forskolin-stimulated adenylate cyclase activity in both ventral and dorsal hippocampus in the presence of 10 microM rat galanin is 34-45%. The analysis of the binding data obtained with 125I-labelled Tyr26-porcine galanin as a tracer reveals similar binding constants for rat galanin in both ventral and dorsal hippocampus with 4.8-fold higher concentration of galanin receptors in the ventral hippocampus. Putative galanin receptor subtype differences between the ventral and dorsal hippocampus have been noted by Hedlund et al. (Eur. J. Pharmacol., 224 (1992) 203-205). This study yields further confirmation for the existence of different galanin receptor subtypes or for differential coupling of galanin receptors to the adenylate cyclase in the dorsal versus ventral hippohampus.

Cloning, pharmacological characterization, and anatomical distribution of a rat cDNA encoding for a galanin receptor

We have cloned and expressed a rat cDNA, designated GALR1-rat, that encodes a galanin receptor based on homology, pharmacology, and anatomical criteria. This cDNA was isolated from a rat brain cDNA library. The nucleotide sequence of the cloned receptor revealed an open reading frame encoding a 346-amino-acid protein, showing 90.8% identity with the previously cloned human galanin receptor. Membranes prepared from COS cells transiently expressing GALR1-rat specifically bind 125I-galanin with high affinity (Kd = 0.12 +/- 0.01 nM). Rat, porcine, and human galanin were able to displace 125I-galanin with nanomolar Ki (0.08 +/- 0.03, 0.10 +/- 0.01, and 0.14 +/- 0.03 nM, respectively), whereas the Ki values for the porcine galanin fragments galanin-(1-16), galanin-(2-29), and galanin-(3-29) were 0.95 +/- 0.21 nM, 7.14 +/- 0.51 nM, and > 1 microM, respectively. The rank order potency of these ligands is consistent with that reported for the native galanin receptor. The distribution of the mRNA corresponding to the galanin receptor encoded by GALR1-rat was determined by in situ hybridization to rat brain sections. High levels of galanin receptor mRNA were detected in the ventral hippocampal formation, thalamic, amygdala, and medulla oblongata nuclei, and in the dorsal horn of the spinal cord.