The novel high-affinity antagonist, galantide, blocks the galanin-mediated inhibition of glucose-induced insulin secretion

Galanin and Cognition

Since the neuropeptide galanin’s discovery in 1983, information has accumulated that implicates it in a wide range of functions, including pain sensation, stress responses, appetite regulation, and learning and memory. This article reviews the evidence for specific functions of galanin in cognitive processes. Consistencies as well as gaps in the literature are organized around basic questions of methodology and theory. This review shows that although regularities are evident in the observed behavioral effects of galanin across several methods for measuring learning and memory, generalization from these findings is tempered with concerns about confounds and a restricted range of testing conditions. Furthermore, it is revealed that many noncognitive behavioral constructs that are relevant for assessing potential roles for galanin in cognition have not been thoroughly examined. The review concludes by laying out how future theory and experimental work can overcome these concerns and confidently define the nature of the association of galanin with particular cognitive constructs.

Octreotide negates the benefit of galantide when used in the treatment of caerulein-induced acute pancreatitis in mice

BACKGROUND

We have previously shown that galantide, a non-specific galanin receptor antagonist, ameliorates acute pancreatitis (AP) induced in mice. Octreotide, a somatostatin analogue, has been used in the treatment of AP with inconsistent outcomes. This study set out to compare the efficacy of a combined treatment of galantide and octreotide with the efficacy of each agent individually in experimental AP.

METHODS

Acute pancreatitis was induced in mice with 7-hourly caerulein injections. Galantide and/or octreotide were co-administered with each caerulein injection commencing with the first injection. Control animals received galantide, octreotide or saline alone. Pancreata were harvested for histological examination and estimation of myeloperoxidase (MPO) activity. Plasma amylase and lipase activities were measured.

RESULTS

Galantide significantly reduced AP-induced hyperenzymaemia by 39-45%. Octreotide alone, or in combination with galantide, did not significantly alter AP-induced hyperenzymaemia. Plasma enzyme activity in the control groups was comparable with pre-treatment activity. Galantide and octreotide administered individually reduced MPO activity by 79% and 50%, respectively; however their combination was without effect. Galantide, octreotide and their combination significantly reduced the percentage of abnormal acinar cells by 28-45%.

CONCLUSIONS

Treatment with galantide alone ameliorated most of the indices of AP studied, whereas treatment with octreotide reduced pancreatic MPO activity and acinar cell damage. Combining the two peptides appears to negate their individual benefits, which suggests an interaction in their mechanism of action.

The efficacy of combining feG and galantide in mild caerulein-induced acute pancreatitis in mice

We have previously shown that galantide ameliorates mild acute pancreatitis (AP), and the salivary tripeptide analogue, feG, ameliorates severe AP in mice. In this study, we compared the efficacy of combining galantide and feG with that of the individual agents in treating mild AP induced in mice with 7-hourly caerulein injections. Galantide was co-administered with each caerulein injection commencing with the first injection. feG was co-administered with the first injection of caerulein as a single intraperitoneal injection. Combination of the agents was also administered. Control animals received galantide, feG, or saline alone. Pancreata were harvested for histological examination and estimation of myeloperoxidase (MPO) activity. Plasma enzyme activities were measured. Galantide significantly reduced AP-induced hyperenzymemia by 41-49%. The combination of galantide and feG significantly reduced AP-induced hyperenzymemia by 39-40%, whereas feG alone was without effect. Plasma enzyme activity in the control groups was comparable with pre-treatment activity. Galantide, feG, and their combination significantly reduced MPO activity by 83, 44 and 74% respectively, and % abnormal acinar cells by 32, 29 and 36% respectively. This study demonstrates for the first time the beneficial effect of feG in mild caerulein-induced AP. Moreover the data indicate that the hyperenzymemia in mild caerulein-induced AP at 12h possibly reflect a larger secretory component as compared to enzyme release due to neutrophil-mediated acinar cell damage. The effects of the treatment with both peptides indicate a possible role for galantide in modulating neutrophil chemotaxis/activation and supports the hypothesis that galantide may influence neurogenic inflammation in AP.

The combination of neurokinin-1 and galanin receptor antagonists ameliorates caerulein-induced acute pancreatitis in mice

Both galanin and substance P have been separately implicated in the pathogenesis of acute pancreatitis. We compared the efficacy of the combination of the galanin antagonist galantide and the neurokinin-1 receptor antagonist L703,606 with that of either alone in the treatment of acute pancreatitis. Acute pancreatitis was induced in mice with 7-hourly caerulein injections. Galantide was co-administered with each caerulein injection commencing with the first injection (prophylactic) or 2h after the first injection (therapeutic). L703,606 was administered either 30 min before (prophylactic), or 2h after the first caerulein injection (therapeutic). Combination of the two agents was also administered. Control groups received galantide, L703,606, or saline, without caerulein. Pancreata were harvested for histological examination and estimation of myeloperoxidase activity. Plasma amylase activity was measured. Prophylactic and therapeutic administration of galantide reduced the hyperamylasemia by 37% and 30% respectively whereas only prophylactic L703,606 reduced hyperamylasemia (by 34%). Prophylactic administration of the combined antagonists reduced the hyperamylasemia by 44%. In contrast, therapeutic administration of the combination significantly increased plasma amylase levels by 27%. The plasma amylase activity in the control groups was similar to basal levels. Prophylactic and therapeutic administration of either antagonist or the combination significantly reduced myeloperoxidase activity. Galantide and L703,606 individually, and in combination, significantly reduced the acute pancreatitis-induced necrosis score. The administration of the combined antagonists does not offer any further benefit as compared to galantide alone. An interaction between neurokinin-1 and galanin receptors may occur to modulate amylase secretion.

Galanin potentiates supramaximal caerulein-stimulated pancreatic amylase secretion via its action on somatostatin secretion

Galanin inhibits pancreatic amylase secretion from mouse lobules induced by physiological concentrations of caerulein via an insulin-dependent mechanism. We aimed to determine the effect and elucidate the mechanism of action of exogenous galanin on pancreatic amylase secretion induced by supramaximal concentrations of caerulein. Amylase secretion from isolated murine pancreatic lobules was measured. Lobules were coincubated with galanin (10(-12)-10(-7) M) and caerulein (10(-7) M). Lobules were preincubated with atropine (10(-5) M), tetrodotoxin (10(-5) M), diazoxide (10(-7) M), or the galanin antagonist galantide (10(-12)-10(-7) M) for 30 min followed by incubation with caerulein alone, or combined with galanin (10(-12) M). Lobules were also coincubated with combinations of galanin (10(-12) M), caerulein, octreotide (10(-12)-10(-7) M) or cyclo-(7-aminoheptanoyl-Phe-D-Trp-Lys-Thr[BZL]), a somatostatin receptor antagonist (10(-9) M). Amylase secretion was expressed as percent of total lobular amylase. Caerulein stimulated amylase secretion to 124% of control. Diazoxide pretreatment abolished the caerulein-stimulated amylase secretion, whereas atropine or tetrodotoxin caused a partial inhibition. Galanin (10(-12)-10(-7) M) potentiated caerulein-stimulated amylase secretion to 160% of control. Preincubation with a combination of atropine and diazoxide abolished the potentiating effect of galanin, indicating muscarinic receptor and insulin mediation. Preincubation with galantide abolished the galanin effect, implying galanin receptor involvement. Coincubation with caerulein, galanin, and octreotide significantly reduced the potentiating effect galanin. However, coincubation with the somatostatin receptor antagonist, alone or in combination with galanin, significantly increased caerulein-stimulated amylase secretion to a level comparable to the galanin potentiation. Taken together, these data suggest that, at supramaximal caerulein concentrations, galanin acts via its receptors to further increase caerulein-stimulated amylase secretion by inhibiting the caerulein-induced release of somatostatin.

Galanin modulates vagally induced contractions in the mouse oesophagus

Nitrergic myenteric neurons co-innervating motor endplates were previously shown to inhibit vagally induced contractions of striated muscle in the rodent oesophagus. Immunohistochemical demonstration of putative co-transmitters, e.g. galanin, in enteric neurons prompted us to study a possible role of galanin in modulating vagally mediated contractions in an in vitro vagus nerve-oesophagus preparation of the mouse. Galanin (1-16) (1-100 nmol L(-1)), in the presence of the peptidase inhibitor, phenanthroline monohydrate, inhibited vagally induced contractions in a concentration-dependent manner (control: 100%; galanin 1 nmol L(-1): 95.6 +/- 1.6%; galanin 10 nmol L(-1): 57.3 +/- 6.5%; galanin 100 nmol L(-1): 31.2 +/- 8.1%, n = 5). The non-selective galanin receptor antagonist, galantide (100 nmol L(-1)), blocked the inhibitory effect of galanin (10 nmol L(-1)) while the selective non-galanin receptor 1 and galanin receptor 3 antagonists, M871 (1 micromol L(-1)) and SNAP37889 (100 nmol L(-1)), respectively, and the nitric oxide synthase inhibitor, NG-nitro-l-arginine methyl ester (L-NAME) (200 micromol L(-1)), failed to affect this galanin-induced response. Simultaneous application of galantide (100 nmol L(-1)) and L-NAME (200 micromol L(-1)) significantly reduced the inhibitory effect of capsaicin (30 mumol L(-1)) on vagally induced contractions when compared with its effect in the presence of L-NAME alone or in combination with the selective galanin receptor 2 or 3 antagonists. An inhibitory effect of piperine on vagally induced contractions was reduced neither by galantide nor by L-NAME. Immunohistochemistry revealed galanin immunoreactive myenteric neurons and nerve fibres intermingling with cholinergic vagal terminals at motor endplates. These data suggest that galanin from co-innervating enteric neurons co-operates with nitric oxide in modulating vagally induced contractions in the mouse oesophagus.

Galanin type 1 receptor knockout mice show altered responses to high-fat diet and glucose challenge

Galanin, a brain and pancreatic peptide with three receptor subtypes (GALR1, GALR2, and GALR3), is hypothesized to participate in energy homeostasis and glucoregulation. Hypothalamic galanin expression is induced by dietary fat, and intra-hypothalamic galanin administration has orexigenic/anabolic properties. Systemic galanin infusion alters glucoregulation in non-human species, partly through direct actions on pancreatic islets. However, the physiologic significance of endogenous galanin-GALR signaling is unclear. The present studies tested the hypotheses that GALR1 deficiency alters food intake and feed efficiency following switches to high-fat diet and that GALR1 deficiency alters whole-body glucose homeostasis. Adult, male GALR1 knockout (-/-), heterozygote (+/-), and C57BL/6J control (+/+) mice were studied. GALR1 deficiency impaired adaptation to a 3-day high-fat diet challenge, leading to increased food intake, feed efficiency and weight gain. However, during the following 2 weeks, GALR1 knockout mice decreased intake, consuming less daily energy than while maintained on low-fat diet and also than heterozygote littermates. Chow-maintained GALR1 knockout mice showed relative hyperglycemia in fed and d-glucose (i.p. 1.5 g/kg)-challenged states. GALR1 knockout mice showed normal food intake, feed efficiency and weight accrual on low-fat diets, normal fasted glucose levels, and normal glucose sensitivity to porcine insulin (i.p. 1 IU/kg) in vivo. The results support the hypotheses that galanin-GALR1 systems help adapt food intake and metabolism to changes in dietary fat and modulate glucose disposition in mice.

Inhibitory effects of different galanin compounds and fragments on osmotically and histamine-induced enhanced vasopressin secretion in rats

The effects of rat, porcine and human galanin, and the human 1-16 and human 16-30 terminal galanin fragments on vasopressin secretion were studied in rat. The plasma vasopressin level was determined by radioimmunoassay (RIA). There were no changes in the basal vasopressin secretion after galanin administration. A significant increase in vasopressin concentration was detected following 2.5% NaCl or histamine administration. I.c.v. injected rat, porcine or human galanin or the 1-16 N-terminal galanin fragment prevented the plasma vasopressin level enhancement. Following the i.v. administration of rat galanin or the i.c.v. injected 16-30 C-terminal galanin fragment, the vasopressin concentration did not return to the normal level. Administration of the galanin antagonist galantid (M15) i.c.v. before the rat galanin i.c.v. injection prevented the inhibitory effect on the increased plasma vasopressin level following 2.5% NaCl solution or histamine administration. The results indicate that there is no significant difference in the inhibitory effect of rat, porcine or human galanin or the 1-16 galanin fragment on the enhanced plasma vasopressin secretion induced by hyperosmosis or histamine administration. Our findings suggest that galanin, as a peptide modulator, is physiologically involved in the regulation of vasopressin release following different forms of stimulation: an osmotic response or histamine administration.

Human galanin expresses amphipathic properties that modulate its vasoreactivity in vivo

The purpose of this study was to determine whether human galanin, a pleiotropic 30-amino acid neuropeptide, expresses amphipathic properties in vitro and, if so, whether these properties modulate its vasoactive effects in the intact peripheral microcirculation. We found that human galanin aggregates in an aqueous solution and forms micelles with a critical micellar concentration (CMC) of 0.4 microM. In addition, the peptide interacted with model membrane as indicated by long and significant increase of the surface pressure of the biomimetic monolayer membrane in vitro. Interactions of human galanin with sterically stabilized phospholipid micelles (SMM) were not associated with a significant change in peptide conformation. Using intravital microscopy, we found that suffusion of human galanin alone elicited significant concentration-dependent vasoconstriction in the intact hamster cheek pouch. This response was amplified when human galanin in SSM was suffused onto the cheek pouch. The effects of human galanin alone and in SSM were mediated by galanin receptors because galantide, a galanin receptor antagonist, abrogated galanin-induced vasoconstriction. Collectively, these data show that human galanin expresses amphipathic properties in the presence of phospholipids which in turn amplifies its vasoactive effects in the intact peripheral microcirculation.

Inhibitory effect of galanin on dopamine-induced enhanced vasopressin secretion in rat neurohypophyseal tissue cultures

The effect of galanin (GAL) on vasopressin (VP) secretion was studied in 13-14-day cultures of isolated rat neurohypophyseal (NH) tissue. The VP content of the supernatant was determined by radioimmunoassay (RIA) after a 1- or 2-h incubation. A significantly decreased content of VP was detected following the administration of 10(-6)-10(-9) M doses of GAL. Dopamine (DA) and the DA-active drugs apomorphine (APM) and Pro-Lys-Gly (PLG) (10(-6) M in each medium) increased the VP level of NH tissue cultures. This VP concentration elevation could be blocked by the administration of GAL together with DA, APM or PLG. The DA-blocking effect of GAL was prevented by previous treatment with the GAL receptor antagonist galantid (M15). The results indicate that VP release is directly influenced by the GAL-ergic system. The GAL-ergic control of VP secretion from NH tissue in rats can occur independently of the hypothalamus, at the level of the posterior pituitary.

Galanin modulates the activity of proopiomelanocortin neurons in the isolated mediobasal hypothalamus of the male rat

It has become apparent that galanin as well as proopiomelanocortin-derived peptides, such as beta-endorphin, play an important role in the hypothalamic circuitry that regulates neuroendocrine functions and appetite behavior. We have recently shown that GalR1 and GalR2 galanin receptor mRNAs are expressed in proopiomelanocortin neurons of the arcuate nucleus, suggesting a direct modulatory action of galanin on the proopiomelanocortin neuronal system. In the present study, we investigated the effect of galanin on beta-endorphin release and proopiomelanocortin mRNA expression from male rat mediobasal hypothalamic fragments incubated ex vivo. Galanin induced a decrease of spontaneous beta-endorphin release within the first 30-60 min of incubation and this effect was blocked by the galanin receptor antagonist galantide. Co-incubation of galanin with FK-506 (tacrolimus), a calcineurin inhibitor, suppressed the inhibitory effect of galanin on beta-endorphin release, suggesting that calcineurin is involved in the galanin-evoked decrease in beta-endorphin release. Measurement of beta-endorphin levels in the tissues at the end of the incubation period (120 min) revealed that galanin caused a two-fold increase of beta-endorphin peptide concentration in the mediobasal hypothalamic tissues. Concurrently, galanin induced an increase in the mean density of silver grains overlying proopiomelanocortin neurons after 60 min of incubation, an effect antagonized by galantide. Finally, reverse transcription-polymerase chain reaction analysis revealed that the mRNAs for the three galanin receptor subtypes (i.e. GalR1, GalR2, and GalR3) were expressed in the incubated mediobasal hypothalamic fragments. Taken as a whole, our results indicate that galanin plays a modulatory role on proopiomelanocortin neurons and this interrelation contributes to the elucidation of the neural circuitry that controls, among others, gonadotropin-releasing hormone function.

Galanin receptor antagonists decrease fat preference in Brattleboro rat

The Brattleboro rat eats spontaneously 46% of its diet per day in fat when given a choice of carbohydrate, protein and fat. An overexpression of galanin (GAL) has been also observed in the hypothalamic paraventricular nuclei (PVN). This associative correlation has led to a hypothesis of a functional relation between central galanin expression and the preference for a lipid diet. In the present experiments, the effects of two GAL receptor antagonists, C7 and galantide, on fat consumption and central overexpression of GAL were investigated. Both antagonists were injected into either the cerebral ventricles or directly above the PVN, and the diet consumption followed for the subsequent 24h. C7 decreased significantly fat consumption when injected into the ventricles or directly above the PVN. In contrast, galantide must be injected above the PVN to show the same effect. However, the two antagonists did not modify GAL mRNA expression in the PVN when they were injected 2h before sacrifice. These experiments confirm a functional link between the preferential consumption of fat and hypothalamic Galanin; different subtypes of the GAL receptor are probably involved, since both Galanin antagonists were differently efficient in decreasing spontaneous fat selection of the Brattleboro rat.

Functional and immunocytochemical evidence that galanin is a physiological regulator of human jejunal motility

The neuropeptide galanin has species-dependent effects on intestinal motility. It has a contractile effect on rat jejunal muscle while it relaxes guinea-pig ileum by inhibiting cholinergic transmission. Its effect on human gut motility has been unknown. Extensive work led to the discovery of selective galanin analogues such as M15 [galanin(1-12)-Pro-substance-P(5-11)], M35 [galanin(1-12)-Pro-bradykinin(2-9)-amide] that competitively inhibit various actions of galanin in the central nervous system. The present study was designed to examine the effect of galanin, M15 and M35 on longitudinal jejunal smooth muscle strips isolated from humans and rats, and to localize galanin-immunoreactivity in human jejunum. Galanin and ACh were equally effective in stimulating contractions of the isolated jejunal muscle: sigmoid curve fitting showed that maximal contractile response to galanin and ACh were 25.7+/-11.1 mN and 23.7+/-9.7 in humans, while 8.0+/-0.6 and 8.1+/-0.3 mN in rats, respectively. These effects of galanin were not inhibited by either atropine (5 x 10(-6) M) or tetrodotoxin (3 x 10(-6) M). The potency of galanin inducing the contractile actions were similar in humans and rats. Interestingly, neither M15 nor M35 (up to 10(-7) M) were able to inhibit the responses of the smooth muscle to galanin. However, both putative galanin receptor antagonists showed agonist effects in our experimental models. In accordance with the functional studies, both the longitudinal and the circular muscle layers were abundant in nerve fibers and varicosities showing galanin immunoreactivity. Our data suggest that galanin is a potent physiological regulator of jejunal contractions in humans. Its action on the jejunum, however, is mediated by galanin receptors that are different from those located in the central nervous system.

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.

Antisecretory effects of galanin and its putative antagonists M15, M35 and C7 in the rat stomach

The neuropeptide galanin has been reported to have a wide range of biological actions both in the central nervous system and in the gastrointestinal tract. Recent works led to the discovery of selective galanin receptor antagonists including M15 (galanin(1-12)-Pro-substanceP(5-11)-amide), M35 (galanin(1-12)-Pro-bradykinin(2-9)-amide) and C7 (galanin(1-12)-Pro-spantide-amide). These antagonists were shown to competitively inhibit actions of galanin in the central nervous system. The present study was designed to investigate the effect of galanin, M15, M35 and C7 on gastric acid secretion and gastric emptying. Pentagastrin-stimulated gastric acid secretion was inhibited by galanin (0.1-9 nmol x kg(-1) x h(-1), i.v.) in a dose-dependent manner (ID50 = 1.8 +/- 0.3 nmol x kg(-1) x h(-1)). When 9 nmol x kg(-1) x h(-1) galanin infusion was given, inhibition became almost complete. M15, M35 and C7 (1-9 nmol x kg(-1) x h(-1)) did not modify responses of the stomach to galanin, but acted as agonists of galanin on acid secretion. Neither galanin nor its putative antagonists affected the emptying of non-caloric liquids from the stomach. In conclusion, galanin may play an antisecretory role in the regulation of gastric acid secretion but not in the control of gastric emptying of liquids in rats. Its antisecretory action on the stomach is mediated by galanin receptors that are distinct from those in the central nervous system.

In vivo studies of the cerebral glutamate receptor/NO/cGMP pathway

Overwhelming evidence indicates that the glutamate/nitric oxide (NO) synthase/soluble guanylyl cyclase system is of primary importance in a variety of physiological and pathological processes of the brain. Most of our knowledge on this neurochemical pathway derives from in vitro and ex vivo studies but the recent improvement of microdialysis techniques combined with extremely sensitive measurements of the amplified end-product cyclic GMP (cGMP) has given new impulses to the investigation of this cascade of events, its modulation by neurotransmitters and its functional relevance, in a living brain. The first reports, appeared in the early 90's, have demonstrated that microdialysis monitoring of cGMP in the extracellular environment of the cerebellum and hippocampus exactly reflects what is expected to occur at the intracellular level; thus, in vivo extracellular cGMP is sensitive to NO-synthase and soluble guanylyl cyclase inhibitors, can be increased by NO-donors or phosphodiesterase blockers and is modulated by glutamate receptor stimulation in a NO-dependent fashion. Since then, other microdialysis studies have been reported showing that the brain NO synthase/guanylyl cyclase pathway is mainly controlled by NMDA, AMPA and metabotropic glutamate receptors but can be also influenced by other transmitters (GABA, acetylcholine, neuropeptides) through polysynaptic circuits interacting with the glutamatergic system. The available data indicate that this technique, applied to freely-moving animals and combined with behavioural tests, could be useful to get a better insight into the functional roles played by NO and cGMP in physiological and pathological situations such as learning, memory formation, epilepsy, cerebral ischemia and neurodegenerative diseases.

Suggestive evidence for inhibitory effects of galanin on mesolimbic dopaminergic neurotransmission

The objective was to examine effects of galaninrat on forebrain monoamine synthesis and on spontaneous locomotor activity in the rat. The rate of monoamine synthesis was estimated by measuring the accumulation of l-DOPA and 5-HTP, following inhibition of cerebral aromatic l-amino acid decarboxylase by means of NSD-1015 (100 mg kg-1 i.p.), after i.c.v. or intracerebral administration of galanin in adult male Wistar rats. Spontaneous locomotor activity was observed in an automated open-field arena ( approximately 0.5 m2). The i.c.v. administration of galanin (0.5-5.0 nmol bilaterally) produced a dose-dependent, statistically significant, increase in DOPA accumulation throughout the neostriatum, and in the olfactory bulb, indicating an increase in the rate of DA synthesis. No increase was observed in brain areas where noradrenaline is the predominant catecholamine, such as the neocortex or the ventral hippocampus. In addition, there was a tendency for an increase in 5-HTP accumulation in the dorso-lateral neostriatum and in the accumbens. The same i.c.v. administration of galanin produced a dose-dependent, and statistically significant, decrease in spontaneous locomotor activity. The effect on forebrain DA synthesis could also be produced by local bilateral application of galanin (2x1 nmol) into the ventral tegmental area, but not the nucleus accumbens (2x2 nmol). There were no effects on forebrain DOPA or 5-HTP accumulation by the local application of galanin into the locus coeruleus, or into the dorsal raphe nucleus. It is concluded that the neuropeptide galanin modulates forebrain dopaminergic neurotransmission. The effect appears to be mediated at the somato-dendritic level of the meso-neostriatal pathway, and could perhaps be utilized to normalize perturbations ascribed to dysfunction in this neuronal pathway, such as schizophrenia.

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.

Galanin stimulates the N-methyl-D-aspartate receptor/nitric oxide/cyclic GMP pathway in vivo in the rat ventral hippocampus

We investigated whether the neuropeptide galanin affects the nitric oxide synthase/cyclic GMP pathway in rat hippocampus by measuring in vivo the extracellular cyclic GMP levels during microdialysis. Galanin (2.5 and 3.5 nmol; i.c.v.) dose-dependently raised the extracellular levels of cyclic GMP in the ventral but not the dorsal hippocampus. The effect of 3.5 nmol galanin was blocked by local application of tetrodotoxin and inhibited by the high-affinity galanin antagonist M40 (galanin-[1-12]-Pro3-[Ala-Leu]2-Ala amide). The non-competitive N-methyl-D-aspartate receptor antagonist dizocilpine maleate (30 microM infused into the ventral hippocampus or 0.2 mg/kg, i.p.) and the competitive one, 3-([R]-carboxypiperazin-4-yl)-propyl-phosphonic acid (50 microM infused), but not local perfusion of the AMPA antagonist 6-nitro-7-sulphamoylbenzo(f)quinoxaline-2,3-dione (15 microM) abolished the galanin-evoked cyclic GMP response in the hippocampus. Inhibitors of nitric oxide synthase, L-Arg(NO2)-OMe.HCl and 7-nitroindazole monosodium salt, applied locally, blocked the galanin-induced increase in hippocampal extracellular cyclic GMP. This increase was also prevented by local application of 1H-(1,2,4)oxadiazolo(4,3a) quinoxalin-1-one, a selective inhibitor of soluble guanylyl cyclase. The galanin receptors mediating the rise in cyclic GMP reside outside the hippocampus, as galanin (0.35-3 nmol) locally applied had no effect. The results provide in vivo evidence that galanin stimulates the N-methyl-D-aspartate receptor/nitric oxide synthase/cyclic GMP pathway in the ventral hippocampus, which may be of importance in memory processes.

Galanin inhibition of enterochromaffin-like cell function

BACKGROUND & AIMS

Galanin, a 29-amino acid neuropeptide found in the gastric mucosa, inhibits basal and pentagastrin-stimulated acid secretion. Its cellular target is unknown. The aim of this study was to determine whether galanin inhibits Ca2+ signaling and histamine release in enterochromaffin-like (ECL) cells.

METHODS

Isolated rat ECL cells were purified to 85% homogeneity by a combination of elutriation, density gradient centrifugation, and 48-hour culture. Intracellular calcium concentration ([Ca2+]i) was determined using video imaging with Fura-2 in a 37 degreesC superfusion chamber. Histamine was measured by radioimmunoassay.

RESULTS

Reverse-transciption polymerase chain reaction of the ECL cell RNA showed a galanin type I receptor subtype. Galanin inhibited gastrin, Bay K8644, and K+ depolarization-induced calcium mobilization and entry as well as reduced basal calcium levels. Pretreatment with pertussis toxin decreased the effect of galanin. Galanin inhibited basal and gastrin-stimulated histamine release by approximately 60% with a median effective concentration of 1.10(-10) mol/L. The inhibitory actions of galanin on histamine release and Ca2+ influx could be reduced by a galanin antagonist, galantide.

CONCLUSIONS

Galanin's inhibition of acid secretion can be explained in part by inhibition of calcium signaling and histamine release from the ECL cells due to activation of a Gi,o protein-coupled receptor.

Galanin receptor-mediated inhibition of glutamate release in the arcuate nucleus of the hypothalamus

It is thought that galanin, a 29 amino acid neuropeptide, is involved in various neuronal functions, including the regulation of food intake and hormone release. Consistent with this idea, galanin receptors have been demonstrated throughout the brain, with high levels being observed in the hypothalamus. However, little is known about the mechanisms by which galanin elicits its actions in the brain. Therefore, we studied the effects of galanin and its analogs on synaptic transmission using an in vitro slice preparation of rat hypothalamus. In arcuate nucleus neurons, application of galanin resulted in an inhibition of evoked glutamatergic EPSCs and a decrease in paired-pulse depression, indicating a presynaptic action. The fragments galanin 1-16 and 1-15 produced a robust depression of synaptic transmission, whereas the fragment 3-29 produced a lesser degree of depression. The chimeric peptides C7, M15, M32, and M40, which have been reported to antagonize some actions of galanin, all produced varying degrees of depression of evoked EPSCs. In a minority of cases, C7, M15, and M40 antagonized the actions of galanin. Analysis of mEPSCs in the presence of TTX and Cd2+, or after application of alpha-latrotoxin, indicated a site of action for galanin downstream of Ca2+ entry. Thus, our data suggest that galanin acts via several subtypes of presynaptic receptors to depress synaptic transmission in the rat arcuate nucleus.

Inhibition of gastric acid secretion by galanin in rats. Relation to endogenous histamine release

Inhibitory effect of galanin on basal and secretagogs-stimulated gastric acid secretion was investigated in urethane-anesthetized rats. A rat stomach was mounted in an ex-vivo chamber, perfused with saline, and either gastric acid or alkaline secretion was determined by titrating the perfusate. Gastric mucosal blood flow (GMBF) was measured by a laser Doppler flowmeter. Intravenous infusion of galanin dose-dependently inhibited the increase of acid secretion induced by pentagastrin and carbachol but not by histamine, without any influence on the GMBF response. Galanin also reduced basal acid secretion while increasing GMBF, but did not evoke any change in basal gastric alkaline secretion. M15, which is a galanin receptor antagonist in the central nervous system but acts as a full agonist in the gastrointestinal smooth muscle, also suppressed pentagastrin-induced acid secretion, similar to galanin. In addition, pentagastrin increased the release of histamine into the gastric lumen, and this response was significantly inhibited by galanin. These results suggest that the inhibitory effect of galanin on acid secretion is mediated by suppression of endogenous histamine release from enterochromaffin-like cells and that the process may be related to the activation of the same subtype of galanin receptors as in the central nervous system and pancreatic beta-cells.

Pharmacological characterization of the contractile effects of galanin (1-29)-NH2, galantide and galanin (1-14)-(alpha-aminobutyric acid8)scyliorhinin-I in the rat gastric fundus

Porcine galanin (1-29)-NH2, galantide (M15) and galanin (1-14)-(alpha-aminobutyric acid8)-scyliorhinin-I used in concentrations of 300, 1,000 and 3,000 nM respectively caused contractions of rat fundus strips. The contractile responses to galanin(1-29)-NH2 were not modified by atropine (10 microM), guanethidine (10 microM), naloxone (1 microM), a mixture of propranolol (10 microM) and phentolamine (10 microM), indomethacin (10 microM), a mixture of mepyramine (10 microM) and cimetidine (10 microM), saralasin (10 microM), and spantide (100 microM). The effects of M15 and galanin(1-14)-(alpha-aminobutyric acid8)-scyliorhinin-I were significantly decreased by atropine for 36 and 18% and by spantide for 37 and 26% respectively. Indomethacin inhibited the muscle response to M15 without influence on the galanin (1-14)-(alpha-aminobutyric acid8)-scyliorhinin-I-induced action. These results support findings that galanin (1-29)-NH2 contracts rat gastric fundus strips by stimulating specific receptors localized on the surface of smooth muscle cells. M15 and galanin(1-14)-(alpha-aminobutyric acid8)-scyliorhinin-I seem to contract smooth muscles not only by acting at galanin receptors, but by interacting with muscarinic or tachykinin receptors or modulating the release of acetylcholine and substance P. Diltiazem (EC50 825 nM), dantrolene (EC50 30.2 microM) and the phospholipase C inhibitors U-73122 (EC50 549 microM) and U-73343 (EC50 751 microM) lowered the contraction to galanin(1-29)-NH2 in a concentration-dependent manner. These observations imply that though the extracellular Ca2+ influx plays a major role in the action of galanin(1-29)-NH2, the release of Ca2+ ions from the intracellular stores contributes to the response of smooth muscles of galanin(1-29) NH2. Norepinephrine (30, 60, 100 and 300 nM) concentration-dependently reduced the Emax to galanin (1-29)-NH2 and reduced the slopes of the concentration-contraction curves, without a notable change in EC50. Pertussis toxin pre-treatment (10 and 30 mg/kg intravenous [i.v.]), 120 h before the experiment, notably increased the maximal response of the rat gastric fundus to galanin(1-29)-NH2, without a significant change in the properties of the concentration-contraction curves (EC50, slopes). The observations may suggest that pertussis toxin-sensitive GTP-binding proteins are involved in the modulation of the excitatory effects of galanin(1-29)-NH2 in the rat gastric fundus.

Effects of galanin and the galanin receptor antagonist galantide on plasma catecholamine levels during a psychosocial stress stimulus in rats

The aim of this study was to investigate the role played by the neuropeptide galanin (GAL) in the regulation of sympathoadrenal function. We evaluated the effects of rat GAL (rGAL) and of the putative GAL receptor antagonist galantide (GLT) on epinephrine (E) and norepinephrine (NE) plasma levels in conscious freely moving male rats, during a psychosocial stress condition. Four groups of male rats were challenged by a stress stimulus, obtained by exposing the animals to a resident conspecific fighter (intruder model), following an intravenous injection with (1) rGAL + saline (SAL), (2) GLT + SAL, (3) rGAL + GLT, or (4) SAL + SAL. Plasma levels of both E and NE were also measured in an additional group of male rats not exposed to any stressor stimulus. The results (mean+/-SEM) showed that rats exposed to the stressor stimulus (intruder rats) exhibited a significant increase above baseline in circulating levels of both E (peak values of 834.13+/-115.13 pmol/l vs. basal values of 309.31+/-32.93 pmol/l; p < 0.01) and NE (peak values of 5,299.03+/-450.62 pmol/l vs. basal values of 2,798.24+/-311.56 pmol/l; p < 0.01) in comparison to control, nonstressed rats. The comparison of the areas under the curve response (AUC) among treatments in the intruder rats revealed that rGAL + SAL injections resulted in a further increase in E levels when compared to SAL + SAL treatment (AUC values: 8.26+/-0.64 vs. 25.38+/-5.52 nmol/ 1/20 min in SAL + SAL vs. rGAL + SAL treatment, respectively; p < 0.02). No significant changes in stress-induced E plasma levels were found following GLT + SAL treatment in comparison to SAL + SAL injections. When the intruder rats were submitted to rGAL + GLT injections, the increments in E levels were found to be higher than those observed following SAL + SAL treatment (AUC values: 8.26+/-0.64 vs. 36.00+/-13.76 nmol/ 1/20 min in SAL + SAL vs. rGAL + GLT treatment, p < 0.03); however, the values were not significantly different from those observed in rGAL + SAL-injected rats. No significant changes in stress-induced NE levels were found in either treatment group when compared to SAL + SAL-injected intruder rats. The results of this study demonstrate that rGAL administration leads to an increase in the E response to the stress stimulus without any effect on NE response. Galantide does not affect either the physiological stress-induced elevation of plasma catecholamines or the effects of rGAL on E plasma levels in response to stress. Therefore, GLT does not appear to behave as a GAL receptor antagonist in the regulation of sympathoadrenal function in rats.

Galanin stimulates glucocorticoid secretion in rats through a receptor-dependent activation of the adenylate cyclase/protein kinase A-dependent signaling pathway

Galanin, a 29-amino acid peptide widely distributed in the central and peripheral nervous systems, was found to induce a concentration-dependent increase in corticosterone secretion and cyclic-AMP release by dispersed rat inner adrenocortical cells (maximal effective concentration, 10(-7) M). The effect of 10(-7) M galanin was blocked by 10(-6) M galantide, a specific antagonist of galanin receptors. Galanin (10(-7) M) also enhanced corticosterone and cyclic-AMP responses of dispersed cells to submaximal but not maximal (10(-9) M) effective concentrations of ACTH, and again this effect was reversed by galantide. The ACTH-receptor antagonist corticotropin-inhibiting peptide (10(-6) M) blocked corticosterone response of dispersed cells to 10(-9) M ACTH but not to 10(-7) M galanin; conversely, the specific protein kinase A inhibitor H-89 (10(-5) M) annulled the secretory response to both ACTH and galanin. In light of these findings, we conclude that galanin stimulates adrenal glucocorticoid secretion in rats, acting through specific receptors, coupled, like those of ACTH, with the adenylate cyclase/protein kinase A-dependent signaling pathway.

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 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.

Effect of three galanin antagonists on the pressor response to galanin in the Cane toad, Bufo marinus

Galanin is a neuropeptide that causes a marked pressor response in several non-mammalian vertebrate species, and some marsupials. In this study, the effect of three galanin antagonists were tested on the pressor response to an intravenous dose (6.3 nmol/kg) of porcine galanin in anaesthetised Cane toads, Bufo marinus. Antagonists were injected at either 20 or 50 times the molar dose (x MD) of galanin. The antagonist, C7 (Galanin 1-13-spantide) reduced the pressor effect of galanin by 32.2 +/- 6.0% when delivered at 20 x MD (n = 4) and by 42.9 +/- 15.7% when delivered at 50 x MD (n = 4) of galanin, the response recovering within 30 min. A second antagonist, M32a (Galanin 1-13-NPY 24-36) had no effect on the pressor response to galanin at 20 x MD (n = 4), but significantly reduced the pressor effect by 54.8 +/- 6.4% at 50 x MD (n = 5), which also recovered within 30 min. Administration of a third antagonist, galantide or M15 (Galanin 1-13-Substance P5-11), resulted in a profound drop in blood pressure, and did not affect the response to galanin at either dose. In conclusion, C7 and M32a are effective, short-term antagonists of the blood pressure effects of galanin in the toad.

Actions of galanin and some of its analogues on rat isolated gastric fundus

The study was undertaken to characterize the effects of porcine galanin (pGal) and some of its analogues on rat gastric fundus muscle strips. pGal, galantide (M15) and pGal(1-14)-[Abu8]SCY-I evoked reproducible concentration-dependent contractions in concentrations of 1-300, 3-1,000 and 100-3,000 nM, respectively, with EC50 values of 13, 70 and 187 nM. Hill's coefficient for pGal is 1.03, indicating an interaction of one pGal molecule with one receptor, fulfilling criteria of classical receptor theory. For M15 and pGal(1-14)-[Abu8]SCY-I, Hill's coefficients are significantly different from 1, namely 0.73 and 1.56, so that one drug molecule may not interact with one receptor. The stimulatory effects of pGal were not modified by dibenamine 10 microM or glybenclamide 1 or 10 microM. Diltiazem 0.1, 1 and 10 microM, papaverine 0.1, 10 microM or dibutyryl cAMP (dib cAMP) 100 and 300 microM, blocked the contraction to pGal in a concentration-dependent manner, indicating an important role for the influx of extracellular calcium ions and regulation by cAMP the pGal-evoked contraction. Diltiazem, dibutyryl cAMP and papaverine were not competitive antagonists of pGal in the stomach smooth muscle.

Human galanin modulates human colonic motility in vitro. Characterization of structural requirements

BACKGROUND

Human galanin (hGal) is a 30-residue non-amidated gut-brain peptide that shows considerable sequence divergence compared with galanin (Gal) forms of other species. Conflicting results have been reported with regard to the structural requirements for its modulatory action on gut motility.

METHODS

We investigated the effect of human and rat Gal and substituted analogues of Gal on the contractility of longitudinal muscle strips of the human colon in vitro.

RESULTS

Both hGal and rGal contracted the preparations in a concentration-dependent and tetrodotoxin-resistant manner without difference in sensitivity. The NH2-terminally truncated peptides hGal (3-30) and rGal (3-29) were inactive, whereas the NH2-terminal fragments, hGal (1-21) and rGal (1-18), remained fully responsive. Single amino acid substitutions at NH2-terminal positions showed divergent results: substitution of Trp2 reduced significantly potency and efficacy, whereas substitutions at positions 1, 3, 4, or 5 did not markedly modify the bioactivity of Gal. Galantide, a high-affinity Gal antagonist in the central nervous system, is a full agonist in human colonic smooth muscle.

CONCLUSION

The COOH-terminal part of Gal contributes mainly the receptor-binding affinity of the peptide, whereas the NH2-terminal region is essential for biologic activity.

Galanin modulates sphincter of Oddi function in the Australian brush-tailed possum

The neuropeptide galantin (GAL) is found in neurons in the biliary tract of several species. We mapped the distribution of GAL-like immunoreactive nerve (GAL-LI) fibers in the sphincter of Oddi of the Australian brush-tailed possum by immunohistochemistry. The pharmacological effects of GAL in vitro and in vivo were studied by measuring sphincter of Oddi muscle strip contractility and transsphincteric flow, respectively. Muscle layers, and ganglionated and perivascular plexuses, contained GAL-LI nerve fibers. Exogenous GAL caused a concentration-dependent (10(9)-10(-6)M) increase in the spontaneous longitudinal but not circular muscle contractions. At 10(-6) M GAL, contractile activity was elevated two- to fourfold. This response was tetrodotoxin insensitive but competitively inhibited by galantide (10(-8)-10(-7) M). In vivo, intra-arterial bolus injections of GAL (1001000 ng/kg), decreased transsphincteric flow, with a maximum reduction to 80.2 +/- 6.8% of control. In conclusion, GAL appears to selectively stimulate longitudinally oriented sphincter of Oddi smooth muscle via a direct mechanism, which results in a modest reduction in transsphincteric flow.

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.

cDNA sequence, ligand biding, and regulation of galanin/GMAP in mouse brain

The cDNA encoding the 29 amino acid-long neuropeptide galanin and its flanking peptide galanin message associated peptide (GMAP), has been cloned and sequenced from mouse hypothalamic cDNA. The primary sequence of mouse galanin is GWTLNSAGYLLGPHAIDNHRSFSDKHGLT, followed by an amidation signal GKR. There are now 12 galanin sequences known: human, porcine, dog, rat, bovine, chicken, sheep, alligator, bowfin, dogfish, trout and mouse. The N-terminal 14 amino acids are identical in all of these species and the whole primary sequence of mouse galanin is identical to that of rate galanin. The mouse C-terminal flanking peptide, the GMAP, which is encoded on the same mRNA as galanin, shows a high degree of homology with all other known GMAP sequences but is not identical to any of them and it is more charged than the other GMAP sequences. Synthetic mouse galanin was found to displace [125I]mono-iodo-Tyr26 galanin (porcine) from receptors in the mouse hypothalamic membranes with high affinity (KD = 0.9 nM). Estrogen treatment of mice (0.1 mg/kg i.p.) for 6 h, which elevates the rat pituitary galanin mRNA levels, does not affect the galanin mRNA levels in mouse hypothalamus and pituitary. Neither does a subchronic glucocorticoid treatment (dexamethasone, 0.5 mg/kg i.p. for 7 days) affect these mRNA levels.

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

The chimeric peptide M35 [galanin(1-13)-bradykinin(2-9) amide] is a high-affinity galanin receptor ligand acting as a galanin receptor antagonist in the rat spinal cord, rat hippocampus and isolated mouse pancreatic islets. We have radiolabelled M35 and performed equilibrium binding studies with [125I]M35 on the rat pancreatic beta-cell line Rin m 5F, whereby we show the existence of high-affinity binding site (KD = 0.9 +/- 0.1 nM) with a Bmax of 72 +/- 3 fmol/mg protein. Galanin displaces [125I]M35 with the same affinity (KD = 1 nM) as it displaces [125I]galanin. Displacement of [125I]galanin by M35 from Rin m 5F cell membranes shows the presence of two binding sites for M35 with KD-values of 0.3 +/- 0.1 nM and 0.52 +/- 0.03 microM, respectively. The GTP- and pertussis toxin-sensitivity of M35 binding to Rin m 5F membranes shows that binding of [125I]M35 is almost completely abolished by the presence of GTP or after pertussis toxin treatment of the cells, indicating an agonist-like binding of M35 to the galanin receptors. M35 has a dual effect on the galanin mediated inhibition of forskolin stimulated cyclic AMP production in Rin m 5F cells: at low concentrations M35 antagonises the effect of galanin, whereas at concentrations above 10 nM M35 acts as a galanin receptor agonist. These agonist-like effects of galanin and M35 are not additive, thus the mixed agonist/antagonist properties arise from the chimeric nature of M35[galanin(1-13)-bradykinin(2-9)amide] acting solely at galanin receptors.

Effects of GLP-1 and 2,5-anhydro-D-mannitol on insulin secretion and plasma glucose in mice

The truncated glucagon-like peptide-1 (GLP-1(7-36)amide or GLP-1) stimulates insulin secretion, enhances glucose elimination and is of potential interest in diabetes treatment. We studied the hypoglycemic action of GLP-1 in normal mice when given alone or together with the fructose analogue, 2,5-anhydro-D-mannitol (2,5-AM), which inhibits glycogenolysis and gluconeogenesis. GLP-1 (32 nmol/kg iv) lowered plasma glucose levels after 25 min to 4.6 +/- 0.2 mmol/l compared with 7.3 +/- 0.4 mmol/l in controls (P < 0.001). Also 2,5-AM (0.5 mumol/kg iv) reduced plasma glucose levels, to 5.6 +/- 0.3 mmol/l (P < 0.01). When given together, the glucose lowering action of GLP-1 and 2,5-AM was additive, since the 25 min glucose level was 2.8 +/- 0.2 mmol/l. At 5 min after injection, GLP-1 had increased plasma insulin levels to 693 +/- 68 pmol/l compared with 342 +/- 42 pmol/l in controls (P < 0.01). 2,5-AM abolished this increase. Furthermore, GLP-1 (32 nmol/kg) did not affect the glycogen content, neither in the liver nor in the gastrocnemic muscle in samples taken at 30 min after injection. Moreover, in isolated islets incubated at 3.3 and 8.3 mmol/l glucose, 2,5-AM at 75 mmol/l inhibited glucose-stimulated insulin secretion (P < 0.05) showing that 2,5-AM inhibits insulin secretion both in vivo and in vitro. We conclude that GLP-1 may reduce plasma glucose levels also to levels below the basal levels under normal conditions, and that an insulin- and liver-independent action of the peptide contributes to its hypoglycemic action in normal animals.

Galanin contracts and relaxes guinea pig and canine intestinal smooth muscle cells through distinct receptors

BACKGROUND/AIMS

Galanin induces a contraction or a relaxation of digestive smooth muscle. Receptors mediating these effects have not been pharmacologically characterized. The aim of the study was to evaluate properties of two specific galanin antagonists M15 and M35 on galanin effects on muscle cells.

METHODS

Isolated muscle cells were obtained separately from circular and longitudinal layers of guinea pig and dog ileums. Contraction was expressed as percentage decrease in cell length from control.

RESULTS

Galanin induced a contraction of cells from guinea pig circular layer (50% effective concentration [EC50], 80 pmol/L) and dog longitudinal layer (EC50, 100 pmol/L). The antagonists inhibited galanin-induced contraction. The most potent was M15 (50% inhibitory concentration [IC50], 80 pmol/L in guinea pig; 90 pmol/L in dog) which was > M35 (IC50, 4 nmol/L in guinea pig; 1 nmol/L in dog). In dog circular layer, galanin inhibited cholecystokinin-induced contraction by relaxing the cells (EC50, 3 pmol/L). The antagonists inhibited this relaxation. The most potent was M35 (IC50, 60 pmol/L) which was > M15 (IC50, 900 pmol/L).

CONCLUSIONS

Galanin antagonists M15 and M35 inhibit the contraction and the relaxation induced by galanin with different potency, suggesting the presence of distinct galanin receptors in gastrointestinal tract that each mediates a specific effect.

N-terminally elongated fragments of galanin(1-16) inhibit insulin secretion from isolated mouse islets

The neuropeptide galanin inhibits insulin secretion and has been suggested to be an adrenergic co-transmitter in the endocrine pancreas. Recently, N-terminally elongated forms of galanin have been identified in both porcine brain and adrenals. Whether these elongated peptides show galanin-like biological effects is not known. We therefore synthesized two N-terminally elongated fragments of galanin(1-16), which contains the active site of galanin. The synthesized peptides were galanin(-9-16) and galanin(-7-16), which correspond to amino acids 24-61 and 26-61 in the galanin precursor molecule. Both these peptides were found to potently inhibit glucose-(11.1 mM)-stimulated insulin secretion from isolated mouse islets of Langerhans in all concentrations studied (1-1000 nM) (P < 0.0001). The potency of the peptides was not different from that of synthetic rat galanin. Thus, at 100 nM, insulin secretion was inhibited by galanin(-7-16) by 83 +/- 7% and by galanin(-9-16) by 71 +/- 17% and by rat galanin by 93 +/- 4% (not statistically different). Furthermore, the galanin receptor antagonist, M35 (10 nM), prevented the inhibitory action of the two N-terminally galanin fragments. This study thus shows that N-terminally elongated galanin-fragments as entire galanin inhibits insulin and thus indicates that the effect of galanin on insulin secretion is not dependent on a free amino-terminus.

Comparison of the solution structures of the chimeric peptides galanin(1-12)-Ala-neuropeptide Y(25-36)amide and galanin(1-12)-Pro-neuropeptide Y(25-36)amide

In a previous study [Arvidsson, K., Land, T., Langel, U., Bartfai, T. & Ehrenberg, A. (1993) Biochemistry 32, 7787-7798], the 25-amino-acid-residue chimeric peptide M32, galanin(1-12)-Pro-neuropeptide Y(25-36)amide, was found to bind very strongly to galanin receptors and also to have considerable affinity to neuropeptide Y receptors. The solution structure of M32 in 30% (by vol.) 1,1,1,3,3,3-hexafluoro-2-propanol, was determined from structural restraints obtained by two-dimensional 1H-NMR. Another peptide, M88, with Ala instead of Pro in position 13, was shown to bind with tenfold lower affinity to galanin receptors, i.e. with nearly the same affinity as galanin itself. The binding to neuropeptide Y receptors was altered in the opposite sense. The peptide M88 has now been examined by two-dimensional 1H-NMR under the same conditions as applied for M32. Using NMR-derived restraints, we have calculated structures of M88 by means of the programs CALIBA, HABAS and DIANA, and refined them by restrained energy minimisation and restrained molecular dynamics. The use of CALIBA and HABAS meant that the restraints were less restrictively formulated, than in the previous work. Hence, structures of M32 were recalculated in the same way as for M88, also including some additional restraints, possible to identify by comparison of the NMR spectra of the two peptides. The new structures of M32 confirm an alpha-helix starting at Pro13, but now continuing until Gln23, instead of ending at Ile20. Conversely, no secondary structure is now expressed in the N-terminal section. It is concluded that in the case of peptides the details of the calculated structures depend on how the restraints are calibrated. In M88, an alpha-helix is found over a long section, approximately Ser6-Gln23. Possible correlations between the binding affinities to receptors and the solution structures of the peptides M32 and M88 are discussed. This is done with particular reference to the length and stability of the alpha-helix and the flexibility of the N-terminal section and its bending direction versus the helix.

Functional effects of a family of galanin antagonists on the cardiovascular system in anaesthetised cats

Previous studies have shown that injection of galanin (GAL: 6.2 nmol/kg) causes prolonged inhibition of cardiac vagal action in anaesthetised cats. Stimulation of the cardiac sympathetic nerve (16 Hz for 5 min) also produces inhibition of cardiac vagal action, an effect which has been proposed to be due to the release of endogenous GAL from sympathetic nerves. In a previous study we tested galantide (M15) and in this study we compared galantide with two other GAL antagonists for their GAL antagonist activity in our experimental model. Each of these incorporate the N-terminal fragment GAL 1-13 and a C-terminal portion of another bioactive peptide and all are C-terminally amidated. GAL 1-13 Substance P 5-11 amide (galantide: M15: 62 nmol/kg and 156 nmol/kg), GAL 1-13 Spantide amide (C7: 156 nmol/kg) and GAL 1-13 NPY 24-36 amide (M32a: 62 nmol/kg) all significantly reduced the cardiac vagal inhibitory effect of exogenous GAL and also reduced the effect of sympathetic stimulation on subsequent cardiac vagal slowing, giving strong support to our hypothesis that GAL is involved in this phenomenon. No antagonist reduced the depressor effect of GAL. This study demonstrates the GAL antagonist properties of these agents on autonomic neuroeffector functions making them useful tools in elucidating further functions of endogenous GAL.