Binding and agonist/antagonist actions of M35, galanin(1-13)-bradykinin(2-9)amide chimeric peptide, in Rin m 5F insulinoma cells

Gut AstA mediates sleep deprivation-induced energy wasting in Drosophila

Severe sleep deprivation (SD) has been highly associated with systemic energy wasting, such as lipid loss and glycogen depletion. Despite immune dysregulation and neurotoxicity observed in SD animals, whether and how the gut-secreted hormones participate in SD-induced disruption of energy homeostasis remains largely unknown. Using Drosophila as a conserved model organism, we characterize that production of intestinal Allatostatin A (AstA), a major gut-peptide hormone, is robustly increased in adult flies bearing severe SD. Interestingly, the removal of AstA production in the gut using specific drivers significantly improves lipid loss and glycogen depletion in SD flies without affecting sleep homeostasis. We reveal the molecular mechanisms whereby gut AstA promotes the release of an adipokinetic hormone (Akh), an insulin counter-regulatory hormone functionally equivalent to mammalian glucagon, to mobilize systemic energy reserves by remotely targeting its receptor AstA-R2 in Akh-producing cells. Similar regulation of glucagon secretion and energy wasting by AstA/galanin is also observed in SD mice. Further, integrating single-cell RNA sequencing and genetic validation, we uncover that severe SD results in ROS accumulation in the gut to augment AstA production via TrpA1. Altogether, our results demonstrate the essential roles of the gut-peptide hormone AstA in mediating SD-associated energy wasting.

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.

Galanin receptors and ligands

The neuropeptide galanin was first discovered 30 years ago. Today, the galanin family consists of galanin, galanin-like peptide (GALP), galanin-message associated peptide (GMAP), and alarin and this family has been shown to be involved in a wide variety of biological and pathological functions. The effect is mediated through three GPCR subtypes, GalR1-3. The limited number of specific ligands to the galanin receptor subtypes has hindered the understanding of the individual effects of each receptor subtype. This review aims to summarize the current data of the importance of the galanin receptor subtypes and receptor subtype specific agonists and antagonists and their involvement in different biological and pathological functions.

Anticonvulsant neuropeptides as drug leads for neurological diseases

Anticonvulsant neuropeptides are best known for their ability to suppress seizures and modulate pain pathways. Galanin, neuropeptide Y, somatostatin, neurotensin, dynorphin, among others, have been validated as potential first-in-class anti-epileptic or/and analgesic compounds in animal models of epilepsy and pain, but their therapeutic potential extends to other neurological indications, including neurodegenerative and psychatric disorders. Disease-modifying properties of neuropeptides make them even more attractive templates for developing new-generation neurotherapeutics. Arguably, efforts to transform this class of neuropeptides into drugs have been limited compared to those for other bioactive peptides. Key challenges in developing neuropeptide-based anticonvulsants are: to engineer optimal receptor-subtype selectivity, to improve metabolic stability and to enhance their bioavailability, including penetration across the blood–brain barrier (BBB). Here, we summarize advances toward developing systemically active and CNS-penetrant neuropeptide analogs. Two main objectives of this review are: (1) to provide an overview of structural and pharmacological properties for selected anticonvulsant neuropeptides and their analogs and (2) to encourage broader efforts to convert these endogenous natural products into drug leads for pain, epilepsy and other neurological diseases.

A novel GalR2-specific peptide agonist

The galanin peptide family and its three receptors have with compelling evidence been implicated in several high-order physiological disorders. The co-localization with other neuromodulators and the distinct up-regulation during and after pathological disturbances has drawn attention to this neuropeptide family. In the current study we present data on receptor binding and functional response for a novel galanin receptor type 2 (GalR2) selective chimeric peptide, M1145 [(RG)(2)-N-galanin(2-13)-VL-(P)(3)-(AL)(2)-A-amide]. The M1145 peptide shows more than 90-fold higher affinity for GalR2 over GalR1 and a 76-fold higher affinity over GalR3. Furthermore, the peptide yields an agonistic effect in vitro, seen as an increase in inositol phosphate (IP) accumulation, both in the absence or the presence of galanin. The peptide design with a N-terminal extension of galanin(2-13), prevails new insights in the assembly of novel subtype specific ligands for the galanin receptor family and opens new possibilities to apply the galanin system as a putative drug target.

The axotomy-induced neuropeptides galanin and pituitary adenylate cyclase-activating peptide promote axonal sprouting of primary afferent and cranial motor neurones

The neuropeptides galanin and pituitary adenylate cyclase-activating peptide (PACAP) are markedly up-regulated in response to peripheral nerve lesion. Both peptides are involved in neuronal differentiation and neurite outgrowth during development. In this study, we investigated the effects of galanin and PACAP on axonal elongation and sprouting by adult rat sensory neurones in vitro and facial motor neurones in vivo. Dissociated rat dorsal root ganglion neurones were plated on laminin substrate and analysed morphometrically. Both the mean axonal length and the number of branch points significantly increased in the presence of galanin or PACAP (2-5 microm). Effects on axonal collateralization were investigated in the rat facial nerve lesion model by direct application of the peptides to collagen-filled conduits entubulating the transected facial nerve stumps. Triple retrograde labelling of brainstem neurones confirmed that the peptides potently induce axonal sprouting of cranial motor neurones. The number of neurones regenerating into identified rami of the facial nerve increased up to fivefold. Biometrical analysis of whisking behaviour revealed that galanin and PACAP impaired the functional outcome when compared with vehicle-treated animals 8 weeks after surgery. In conclusion, although galanin and PACAP have been established as neurotrophic molecules with respect to axonal development and regeneration, their potential as treatments for peripheral nerve lesions appears limited because of the extensive stimulation of collateral axon branching. These branches are misrouted towards incorrect muscles and cause impairment in their coordinated activity.

Regulation of Kindling Epileptogenesis by Hippocampal Galanin Type 1 and Type 2 Receptors: The Effects of Subtype-Selective Agonists and the Role of G-Protein-Mediated Signaling

The search for antiepileptic drugs that are capable of blocking the progression of epilepsy (epileptogenesis) is an important problem of translational epilepsy research. The neuropeptide galanin effectively suppresses acute seizures. We examined the ability of hippocampal galanin receptor type 1 (GalR1) and type 2 (GalR2) to inhibit kindling epileptogenesis and studied signaling cascades that mediate their effects. Wistar rats received 24-h-long intrahippocampal infusion of a GalR1/2 agonist galanin(1-29), GalR1 agonist M617 [galanin(1-13)-Gln14-bradykinin(2-9)-amide], or GalR2 agonist galanin(2-11). The peptides were administered alone or combined with an inhibitor of Gi protein pertussis toxin (PTX), Gi-protein activated K+ channels (GIRK) inhibitor tertiapin Q (TPQ), G(q/11) protein inhibitor [D-Arg1,D-Trp(5,7,9),Leu11]-substance P (dSP), or an inhibitor of intracellular Ca2+ release dantrolene. Sixteen hours into drug delivery, the animals were subjected to rapid kindling-60 electrical trains administered to ventral hippocampus every 5 min. M617 delayed epileptogenesis, whereas galanin(1-29) and galanin(2-11) completely prevented the occurrence of full kindled seizures. TPQ abolished anticonvulsant effect of M617 but not of galanin(2-11). PTX blocked anticonvulsant effects of M617 and inversed the action of galanin(1-29) and galanin(2-11) to proconvulsant. dSP and dantrolene did not modify seizure suppression through GalR1 and GalR2, but eliminated the proconvulsant effect of PTX + galanin(1-29) and PTX + galanin(2-11) combinations. We conclude that hippocampal GalR1 exert their disease-modifying effect through the Gi-GIRK pathway. GalR2 is antiepileptogenic through the Gi mechanism independent of GIRK. A secondary proconvulsant pathway coupled to GalR2 involves G(q/11) and intracellular Ca2+. The data are important for understanding endogenous mechanisms regulating epileptogenesis and for the development of novel antiepileptogenic drugs.

Galanin (2-11) binds to GalR3 in transfected cell lines: limitations for pharmacological definition of receptor subtypes

The neuropeptide galanin regulates a variety of physiological and pathophysiological processes through three G protein coupled receptors, GalR1, GalR2, and GalR3. The studies on galanin receptor subtype specific effects have been hampered by the lack of high affinity subtype selective antagonist and/or agonist to any of these three galanin receptor subtypes. Since its recent introduction in 2003, galanin (2-11) has been widely used as a GalR2 selective agonist in several in vitro and in vivo studies. In the present paper, we demonstrate that galanin (2-11) binds to rat GalR3 receptors in transfected cell lines with a similar affinity as it binds to GalR2. As none of the available antagonists are galanin receptor subtype selective, as shown here for M35 and M40, more work is needed to confirm whether a galanin (2-11) effect is GalR2 mediated and there is an urgent need for high affinity galanin receptor subtype selective ligands. For now one needs to interpret the data obtained at lower galanin (2-11) concentrations as effects mediated by non-GalR1 type galanin receptors, i.e., GalR2 and/or GalR3.

Galanin receptor ligands

The three galanin receptor subtypes (GalR1-3) belong to the G protein-coupled receptor superfamily. The widespread distribution of galanin and its receptors in the CNS and PNS and the numerous physiological and pharmacological effects of galanin (for review, cf. Vrontakis, 2002) render the three galanin receptors attractive drug targets. The industrial efforts, however, have not yet resulted in a wealth of receptor subtype specific agonists or antagonists with high affinity and selectivity. The present paper summarizes the properties of the galanin ligands used at the end of 2004 in the ca. 2000 publications and complements their pharmacological characterization with new data.

The galanin antagonist M35 has intrinsic agonistic activity in the dorsal root ganglion

The chimeric peptide M35 (galanin(1-3)-bradykinin(2-9)amide) is a high-affinity galanin receptor ligand which acts as a galanin receptor antagonist in many experimental models such as the flexor reflex and chronic constriction injury in rat. However, more recently there have been conflicting reports that M35 may act as a galanin receptor agonist in certain systems. Here we demonstrate that in the absence of endogenous galanin M35 has an agonistic effect, significantly enhancing neurite outgrowth from cultured adult mouse dorsal root ganglion neurons, albeit at a lower potency than galanin peptide itself. However, in the presence of galanin its agonistic activity is masked and thus it appears to act as a galanin receptor antagonist.

Novel excitatory actions of galanin on rat cholinergic basal forebrain neurons: implications for its role in Alzheimer's disease

Galanin, a 29-amino-acid neuropeptide, is generally viewed as an inhibitory neuromodulator in a variety of central systems. Galanin expression is upregulated in the cholinergic basal forebrain nuclei in Alzheimer's disease (AD) and is postulated to play an important role in memory and cognitive function. In this study, application of galanin to acutely dissociated rat neurons from the basal forebrain nucleus diagonal band of Broca (DBB), caused a decrease in whole cell voltage-activated currents in a majority of cells. Galanin reduces a suite of potassium currents, including calcium-activated potassium (I(C)), the delayed rectifier (I(K)), and transient outward potassium (I(A)) conductances, but not calcium or sodium currents. Under current-clamp conditions, application of galanin evoked an increase in excitability and a loss of accommodation in cholinergic DBB neurons. Using single-cell RT-PCR technique, we determined that galanin actions were specific to cholinergic, but not GABAergic DBB neurons The notion that galanin plays a deleterious role in AD is based, in part, on galanin hyperinnervation of cholinergic cells in the basal forebrain of AD patients, its ability to depress acetylcholine release and its inhibitory actions at other CNS sites. However, our results suggest that by virtue of its excitatory actions on cholinergic neurons, galanin may in fact play a compensatory role by augmenting the release of acetylcholine from remaining cholinergic basal forebrain neurons. This action might serve to delay the progression of AD pathology linked to a reduction in central cholinergic tone.

Galanin/alpha2-adrenoceptor interactions in telencephalic and diencephalic regions of the rat

The aim of this study was to evaluate whether galanin could affect central alpha2-adrenoceptors in telencephalic and diencephalic regions in the rat using quantitative receptor autoradiography with the alpha2 agonist radioligand [3H]p-aminoclonidine. Galanin 1 nM significantly and substantially increased the Kd value of the [3H]p-aminoclonidine binding sites in the medial hypothalamus and amygdala by 86% (p < 0.01) and 73% (p < 0.05) respectively. The Bmax value was only significantly increased with 3 nM galanin in the amygdala and the medial hypothalamus (both p < 0.05). The antagonist M35 counteracted the increase of the Kd values of the alpha2-adrenoceptor agonist binding sites produced by galanin 1 nM in the amygdala and the medial hypothalamus (both p < 0.001). These findings suggest the existence of an antagonistic galanin/alpha2 adrenoceptor interaction in the medial hypothalamus and amygdala that may be of relevance for alpha2-adrenoceptor-regulated neuroendocrine functions and food intake.

Effect of intrathecal galanin and its putative antagonist M35 on pain behavior in a neuropathic pain model

There is currently some debate over a possible role of galanin in pain processing. It was recently reported that the levels of galanin in dorsal root ganglia (DRGs) seem related to development of allodynia after unilateral sciatic nerve constriction injury. In our present study, we aimed at characterizing the effect of exogenous and endogenous galanin on pain behavior in allodynic and non-allodynic rats in which the levels of galanin in DRG neurons are low and high, respectively [28]. The results show that in allodynic rats, the mechanical threshold increases dose-dependently after intrathecal (i.t.) injection of galanin, while no significant changes were observed in groups treated with the putative galanin antagonist M35 or saline. In non-allodynic rats i.t. injection of M35 induced a significant mechanical allodynic state, which did not occur after injection of galanin, bradykinin, the bradykinin fragment(2-9) or saline. The results suggest that in the present experimental paradigm exogenous galanin has an anti-allodynic effect in the allodynic rats, and that endogenous galanin has a tonic inhibitory effect in the non-allodynic group.

Galanin/alpha2-receptor interactions in central cardiovascular control

The modulation of the central cardiovascular effects of alpha(2)-adrenoceptor activation by galanin and its N-terminal fragment galanin-(1-15) has been evaluated by quantitative receptor autoradiography and cardiovascular analysis. Intracisternal coinjections of threshold doses of galanin and the selective and hypotensive alpha(2)-receptor agonist clonidine induced rapid and maintained vasopressor and tachycardic responses (p<0.001) instead of a hypotensive response, whereas the coinjections of threshold doses of the N-terminal galanin fragment (1-15) and clonidine did not elicit significant cardiovascular changes. Receptor autoradiographical experiments showed that galanin (1 nM) significantly increased the K(d) (p<0.01) and B(max) values (p<0.01) of [(3)H]p-Aminoclonidine binding sites in the nucleus tractus solitarii (NTS) compatible with a possible antagonistic interaction with the alpha(2)-adrenoceptors, and this effect was blocked by the presence of the specific galanin receptor antagonist M35. In addition, clonidine (30 nM) induced a 50% increase in the B(0) values of galanin based on competition experiments with [(125)I]-galanin binding in the NTS. These findings suggest the existence of an antagonistic effect of galanin, but not of galanin fragment (1-15), on the cardiovascular responses mediated by alpha(2)-receptors as well as a reciprocal facilitatory effect of alpha(2)-receptors on galanin binding. These mechanisms could be mediated by a reciprocal galanin-alpha(2) receptor interaction within the NTS.

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.

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.

Structural determinants for binding to CGRP receptors expressed by human SK-N-MC and Col 29 cells: studies with chimeric and other peptides

Structure-activity relationships for the binding of human alpha-calcitonin gene-related peptide 8-37 (halphaCGRP8-37) have been investigated at the CGRP receptors expressed by human SK-N-MC (neuroblastoma) and Col 29 (colonic epithelia) cells by radioligand binding assays and functional assays (halphaCGRP stimulation of adenylate cyclase). On SK-N-MC cells the potency order was halphaCGRP8-37 > halphaCGRP19-37 = AC187 > rat amylin8-37 > halpha[Tyr0]-CGRP28-37 (apparent pKBs of 7.49+/-0.25, 5.89+/-0.20, 6.18+/-0.19, 5.85+/-0.19 and 5.25+/-0.07). The SK-N-MC receptor appeared CGRP1-like. On Col 29 cells, only halphaCGRP8-37 of the above compounds was able to antagonize the actions of halphaCGRP (apparent pKB=6.48+/-0.28). Its receptor appeared CGRP2-like. halpha[Ala11,18]-CGRP8-37, where the amphipathic nature of the N-terminal alpha-helix has been reduced, bound to SK-N-MC cells a 100 fold less strongly than halphaCGRP8-37. On SK-N-MC cells, halphaCGRP8-18,28-37 (M433) and mastoparan-halphaCGRP28-37 (M432) had apparent pKBs of 6.64+/-0.16 and 6.42+/-0.26, suggesting that residues 19-27 play a minor role in binding. The physico-chemical properties of residues 8-18 may be more important than any specific side-chain interactions. M433 was almost as potent as halphaCGRP8-37 on Col 29 cells (apparent pKB=6.17+/-0.20). Other antagonists were inactive.

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.

Novel galanin receptor ligands

Galanin is a neuroendocrine peptide which is 29/30 amino acids in length and is recognised by G-protein-coupled central nervous system receptors via its N-terminus. We synthesised several galanin receptor ligands and fragments around C-terminal extensions of galanin(1-13) to yield chimeric peptides with C-terminals corresponding to bioactive peptides like bradykinin(2-9), mastoparan, neuropeptide Y(25-36) or substance P(5-11), respectively. We also synthesised short galanin analogs in which galanin(1-13) was C-terminally elongated with Lys14; different pharmacologically active small molecules were then attached to the epsilon-amino group of Lys14. Several cysteine-substituted linear and ring closed analogs of galanin(1-9) and galanin(1-16) were also synthesised. The equilibrium binding constants for these peptides at hypothalamic galanin receptors were determined and found in the subnanomolar to micromolar range. The large number of peptides and their binding affinities presented here permit structure-activity relationship analysis of peptide-type ligands to galanin receptors.

Cell penetration by transportan

Transportan is a 27 amino acid-long peptide containing 12 functional amino acids from the amino terminus of the neuropeptide galanin and mastoparan in the carboxyl terminus, connected via a lysine. Transportan is a cell-penetrating peptide as judged by indirect immunofluorescence using N epsilon13-biotinyl-transportan. The internalization of biotinyl-transportan is energy independent and takes place efficiently at 37 degrees, 4 degrees, and 0 degrees C. Cellular uptake of transportan is probably not mediated by endocytosis, since it cannot be blocked by treating the cells with phenylarsine oxide or hyperosmolar sucrose solution and is nonsaturable. The kinetics of internalization was studied with the aid of the 125I-labeled peptide. At 37 degrees C, the maximal intracellular concentration is reached in about 20 min. The internalized transportan is protected from trypsin. The cell-penetrating ability of transportan is not restricted by cell type, but seems to be a general feature of this peptide. In Bowes' melanoma cells, transportan first localizes in the outer membrane and cytoplasmatic membrane structures. This is followed by a redistribution into the nuclear membrane and uptake into the nuclei where transportan concentrates in distinct substructures, probably the nucleoli.

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.

Galanin inhibits continuous and phasic firing in rat hypothalamic magnocellular neurosecretory cells

The effects of galanin (GAL) on magnocellular neurosecretory cells (MNCs) were examined during microelectrode recordings from supraoptic neurons in superfused hypothalamic explants. Application of the full-length peptide (GAL1-29) or of the N-terminal fragment GAL1-16 produced reversible membrane hyperpolarization with an IC50 near 10 nM. These effects were associated with an increase of membrane conductance, with a reversal potential near -70 mV, and were not blocked by tetrodotoxin, indicating that the receptors mediating these effects are located postsynaptically. Hyperpolarizing responses were also observed in response to the GAL-like chimeric ligands M35 and M40, suggesting that these behave as partial agonists at galanin receptors. The reversal potential of the GAL-mediated effect was unaffected by reducing extracellular chloride or by intracellular chloride injection, indicating that the effects of galanin are not mediated by modulation of chloride conductances. In contrast, reducing the external concentration of potassium ions from 3 to 1 mM shifted the reversal potential of the responses to -85 mV, suggesting the involvement of a potassium conductance. When tested on spontaneously active MNCs, the hyperpolarizing effects of galanin were associated with a suppression of firing in both continuously active and phasically active neurons. Inhibition of phasic bursts was mediated both through the inhibitory effects of the hyperpolarization and through a GAL-mediated inhibition of the depolarizing afterpotential that is responsible for the production of individual bursts. These results suggest that galanin may be a potent endogenous modulator of firing pattern in hypothalamic neuroendocrine cells.

Differential effects of the neuropeptide galanin on striatal acetylcholine release in anaesthetized and awake rats

1. In the present study the mechanisms were examined by which the neuropeptide galanin modulates the extracellular concentrations of striatal acetylcholine (ACh) in enflurane anaesthetized and in freely moving male rats by use of in vivo microdialysis and high performance liquid chromatography. 2. The perfusion of galanin through the microdialysis probe (0.3 nmol microl(-1), flow rate: 2 microl min(-1)) caused a statistically significant increase in the basal striatal ACh levels in anaesthetized but a decrease in awake animals. No significant effect was revealed after a low dose (0.1 nmol microl(-1), flow rate: 2 microl min(-1)) of galanin perfusion. Both the stimulating and inhibitory effects of galanin on basal ACh release were reversible. 3. The muscarinic antagonist scopolamine (0.1 mg kg(-1), subcutaneously (s.c.)) caused a significant increase in ACh release in both anaesthetized and awake animals. 4. The combination of galanin plus scopolamine attenuated the stimulant effect on ACh release caused by scopolamine alone in awake animals. 5. The putative galanin receptor antagonist M35 at 0.3 nmol microl(-1) but not at 0.1 nmol microl(-1) caused a significant reduction (20%) in ACh release, supporting the view that M35 at higher concentrations behaves as a partial agonist at the galanin receptor. When M35 (0.1 nmol microl(-1)) was co-infused with galanin (0.3 nmol microl(-1)) the galanin-evoked decrease in ACh release was completely blocked. 6. Taken together, these results indicate that galanin affects basal ACh release via stimulation of galanin receptors within the striatum. The mechanism involved is dependent on the anaesthesia procedure which may act via enhancement of gamma-aminobutyric acidA (GABA(A)) mediated transmission within striatal and/or output neurones. In addition, anaesthesia may also decrease the activity of glutamatergic striatal afferents. The results with M35 indicate that the role of galanin perfused in striatum is permissive in the normal rat. Furthermore, galanin is a potent inhibitory modulator of basal ACh release also in the striatum, as recently was shown in the ventral hippocampus in awake animals.

Possible evidence for endogenous production of a novel galanin-like peptide

Galanin mRNA and peptide are not detectable in normal islets. We studied the effect of galanin antagonists on insulin secretion in the rat beta cell line, RIN5AH, and in perifused rat islets. In RIN cell membranes galanin and its antagonists showed high affinity for 125I-galanin binding sites [Kd: (galanin) 0.03+/-0.01; Ki for galanin antagonists: (C7) 0.12+/- 0.02, (M35) 0.21+/-0.04, and (M40) 0.22+/-0.03 nM, mean+/- SEM, n = 4]. Galanin (1 microM) inhibited glucose-induced insulin release in islets (control 21.2+/-1.5 vs. galanin 4.5+/-0.2 fmol/islet per min, P < 0.001, n = 6) and RIN5AH cells (control 0.26+/-0.01 vs. galanin 0.15+/-0.02 pmol/10(6) cells per h, P < 0.001, n = 9). In RIN5AH cells, all antagonists blocked the inhibitory effects of galanin and stimulated insulin release in the absence of galanin. C7 and M40 (1 microM) alone significantly stimulated glucose-induced insulin secretion. Purified porcine galanin antibody (GAb) enhanced glucose-induced insulin release from islets (control 100+/- 16.3% vs. GAb 806.1+/-10.4%, P < 0.001, n = 6), and RIN5AH cells (control 100+/-9.6% vs. GAb 149+/-6.8%, P < 0. 01, n = 6). Western blotting of dexamethasone-treated islet extracts using GAb showed a specific band of similar molecular weight to porcine galanin not detected using a rat specific galanin antibody. One possible explanation for these results is the presence of an endogenous galanin-like peptide.

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.