Molecular biology and pharmacology of galanin receptors

Class A and C GPCR Dimers in Neurodegenerative Diseases

Neurodegenerative diseases affect over 30 million people worldwide with an ascending trend. Most individuals suffering from these irreversible brain damages belong to the elderly population, with onset between 50 and 60 years. Although the pathophysiology of such diseases is partially known, it remains unclear upon which point a disease turns degenerative. Moreover, current therapeutics can treat some of the symptoms but often have severe side effects and become less effective in long-term treatment. For many neurodegenerative diseases, the involvement of G proteincoupled receptors (GPCRs), which are key players of neuronal transmission and plasticity, has become clearer and holds great promise in elucidating their biological mechanism. With this review, we introduce and summarize class A and class C GPCRs, known to form heterodimers or oligomers to increase their signalling repertoire. Additionally, the examples discussed here were shown to display relevant alterations in brain signalling and had already been associated with the pathophysiology of certain neurodegenerative diseases. Lastly, we classified the heterodimers into two categories of crosstalk, positive or negative, for which there is known evidence.

Galanin suppresses visceral afferent responses to noxious mechanical and inflammatory stimuli

Galanin is a neuropeptide expressed by sensory neurones innervating the gastrointestinal (GI) tract. Galanin displays inhibitory effects on vagal afferent signaling within the upper GI tract, and the goal of this study was to determine the actions of galanin on colonic spinal afferent function. Specifically, we sought to evaluate the effect of galanin on lumbar splanchnic nerve (LSN) mechanosensitivity to noxious distending pressures and the development of hypersensitivity in the presence of inflammatory stimuli and colitis. Using ex vivo electrophysiological recordings we show that galanin produces a dose-dependent suppression of colonic LSN responses to mechanical stimuli and prevents the development of hypersensitivity to acutely administered inflammatory mediators. Using galanin receptor (GalR) agonists, we show that GalR1 activation, but not GalR2/3 activation, suppresses mechanosensitivity. The effect of galanin on colonic afferent activity was not observed in tissue from mice with dextran sodium sulfate-induced colitis. We conclude that galanin has a marked suppressive effect on colonic mechanosensitivity at noxious distending pressures and prevents the acute development of mechanical hypersensitivity to inflammatory mediators, an effect not seen in the inflamed colon. These actions highlight a potential role for galanin in the regulation of mechanical nociception in the bowel and the therapeutic potential of targeting galaninergic signaling to treat visceral hypersensitivity.

GAL3 receptor knockout mice exhibit an alcohol-preferring phenotype

Galanin is a neuropeptide which mediates its effects via three G-protein coupled receptors (GAL_1-3 ). Administration of a GAL₃ antagonist reduces alcohol self-administration in animal models while allelic variation in the GAL₃ gene has been associated with an increased risk of alcohol use disorders in diverse human populations. Based on the association of GAL₃ with alcoholism, we sought to characterize drug-seeking behavior in GAL₃ -deficient mice for the first time. In the two-bottle free choice paradigm, GAL₃ -KO mice consistently showed a significantly increased preference for ethanol over water when compared to wildtype littermates. Furthermore, male GAL₃ -KO mice displayed significantly increased responding for ethanol under operant conditions. These differences in alcohol seeking behavior in GAL₃ -KO mice did not result from altered ethanol metabolism. In contrast to ethanol, GAL₃ -KO mice exhibited similar preference for saccharin and sucrose over water, and a similar preference for a high fat diet over a low fat diet as wildtype littermates. No differences in cognitive and locomotor behaviors were observed in GAL₃ -KO mice to account for increased alcohol seeking behavior. Overall, these findings suggest genetic ablation of GAL₃ in mice increases alcohol consumption.

Galanin decreases spontaneous resting contractions and potentiates acetyl choline-induced contractions of goldfish gut

Galanin (GAL) is a 29 amino acid peptide, first identified from the porcine intestine and widely distributed within the brain and peripheral tissues. Among GAL biological functions, its role as a potent appetite-stimulating peptide is probably the most studied. With galanin's established role in the modulation of food intake in fish, this study aims to evaluate the effects of GAL on the intestinal motility of the goldfish, Carassius auratus, using an organ bath system. Our results found that application of GAL to the organ bath causes a significant concentration-dependent decrease in the amplitude of spontaneous contractions of goldfish gut. Preincubations of intestinal strips with acetylcholine (ACh) and GAL showed that GAL increases the force of ACh-induced contractions of the goldfish gut. These results provide the first evidence for a role of GAL in gut motility in goldfish. This also suggests a crosstalk between the effects of GAL and ACh in such functions, thus pointing to a putative joint role between the two molecules. These findings offer novel information that strengthens the role of the galaninergic system in fish feeding.

The neuropeptides Galanin and Galanin(1-15) in depression-like behaviours

Galanin is a 29 amino acid neuropeptide widely distributed in neurons within the central nervous system. Galanin exerts its biological activities through three different G protein-receptors and participates in a number of functions, including mood regulation. Not only Galanin but also Galanin N-terminal fragments like Galanin(1-15) are active at the central level. In this work, we review the latest findings in studies on Galanin and Galanin(1-15) in depression-related behaviours. Our focus is on animal models for depression, and we pay some attention to research data obtained in human studies. Since Serotonin (5-HT), especially through 5-HT1A, and Galanin receptors interact at both pre-and postsynaptic level, the development of drugs targeting potential GAL₁-GAL₂-5-HT1A heteroreceptor complexes linked to the raphe-hippocampal 5-HT neurons may represent new treatment strategies in depression.

Understanding the Role of GPCR Heteroreceptor Complexes in Modulating the Brain Networks in Health and Disease

The introduction of allosteric receptor-receptor interactions in G protein-coupled receptor (GPCR) heteroreceptor complexes of the central nervous system (CNS) gave a new dimension to brain integration and neuropsychopharmacology. The molecular basis of learning and memory was proposed to be based on the reorganization of the homo- and heteroreceptor complexes in the postjunctional membrane of synapses. Long-term memory may be created by the transformation of parts of the heteroreceptor complexes into unique transcription factors which can lead to the formation of specific adapter proteins. The observation of the GPCR heterodimer network (GPCR-HetNet) indicated that the allosteric receptor-receptor interactions dramatically increase GPCR diversity and biased recognition and signaling leading to enhanced specificity in signaling. Dysfunction of the GPCR heteroreceptor complexes can lead to brain disease. The findings of serotonin (5-HT) hetero and isoreceptor complexes in the brain over the last decade give new targets for drug development in major depression. Neuromodulation of neuronal networks in depression via 5-HT, galanin peptides and zinc involve a number of GPCR heteroreceptor complexes in the raphe-hippocampal system: GalR1-5-HT1A, GalR1-5-HT1A-GPR39, GalR1-GalR2, and putative GalR1-GalR2-5-HT1A heteroreceptor complexes. The 5-HT1A receptor protomer remains a receptor enhancing antidepressant actions through its participation in hetero- and homoreceptor complexes listed above in balance with each other. In depression, neuromodulation of neuronal networks in the raphe-hippocampal system and the cortical regions via 5-HT and fibroblast growth factor 2 involves either FGFR1-5-HT1A heteroreceptor complexes or the 5-HT isoreceptor complexes such as 5-HT1A-5-HT7 and 5-HT1A-5-HT2A. Neuromodulation of neuronal networks in cocaine use disorder via dopamine (DA) and adenosine signals involve A2AR-D2R and A2AR-D2R-Sigma1R heteroreceptor complexes in the dorsal and ventral striatum. The excitatory modulation by A2AR agonists of the ventral striato-pallidal GABA anti-reward system via targeting the A2AR-D2R and A2AR-D2R-Sigma1R heteroreceptor complex holds high promise as a new way to treat cocaine use disorders. Neuromodulation of neuronal networks in schizophrenia via DA, adenosine, glutamate, 5-HT and neurotensin peptides and oxytocin, involving A2AR-D2R, D2R-NMDAR, A2AR-D2R-mGluR5, D2R-5-HT2A and D2R-oxytocinR heteroreceptor complexes opens up a new world of D2R protomer targets in the listed heterocomplexes for treatment of positive, negative and cognitive symptoms of schizophrenia.

Taste perception, associated hormonal modulation, and nutrient intake

It is well known that taste perception influences food intake. After ingestion, gustatory receptors relay sensory signals to the brain, which segregates, evaluates, and distinguishes the stimuli, leading to the experience known as "flavor." It is well accepted that five taste qualities – sweet, salty, bitter, sour, and umami – can be perceived by animals. In this review, the anatomy and physiology of human taste buds, the hormonal modulation of taste function, the importance of genetic chemosensory variation, and the influence of gustatory functioning on macronutrient selection and eating behavior are discussed. Individual genotypic variation results in specific phenotypes of food preference and nutrient intake. Understanding the role of taste in food selection and ingestive behavior is important for expanding our understanding of the factors involved in body weight maintenance and the risk of chronic diseases including obesity, atherosclerosis, cancer, diabetes, liver disease, and hypertension.

Zinc Is Involved in Depression by Modulating G Protein-Coupled Receptor Heterodimerization

5-Hydroxytryptamine 1A receptor and galanin receptor 1 belong to the G protein-coupled receptors superfamily, and they have been described to heterodimerize triggering an anomalous physiological state that would underlie depression. Zinc supplementation has been widely reported to improve treatment against major depressive disorder. Our work has focused on the study and characterization of these receptors and its relationships with zinc both under purified conditions and in cell culture. To this aim, we have designed a strategy to purify the receptors in a conformationally active state. We have used receptors tagged with the monoclonal Rho-1D4 antibody and employed ligand-assisted purification in order to successfully purify both receptors in a properly folded and active state. The interaction between both purified receptors has been analyzed by surface plasmon resonance in order to determine the kinetics of dimerization. Zinc effect on heteromer has also been tested using the same methodology but exposing the 5-hydroxytryptamine 1A receptor to zinc before the binding experiment. These results, combined with Förster resonance energy transfer (FRET) measurements, in the absence and presence of zinc, suggest that this ion is capable of disrupting this interaction. Moreover, molecular modeling suggests that there is a coincidence between zinc-binding sites and heterodimerization interfaces for the serotonin receptor. Our results establish a rational explanation for the role of zinc in the molecular processes associated with receptor-receptor interactions and its relationship with depression, in agreement with previously reported evidence for the positive effects of zinc in depression treatment, and the involvement of our target dimer in the same disease.

Preferential activation by galanin 1-15 fragment of the GalR1 protomer of a GalR1-GalR2 heteroreceptor complex

The three cloned galanin receptors show a higher affinity for galanin than for galanin N-terminal fragments. Galanin fragment (1-15) binding sites were discovered in the rat Central Nervous System, especially in dorsal hippocampus, indicating a relevant role of galanin fragments in central galanin communication. The hypothesis was introduced that these N-terminal galanin fragment preferring sites are formed through the formation of GalR1-GalR2 heteromers which may play a significant role in mediating galanin fragment (1-15) signaling. In HEK293T cells evidence for the existence of GalR1-GalR2 heteroreceptor complexes were obtained with proximity ligation and BRET(2) assays. PLA positive blobs representing GalR1-GalR2 heteroreceptor complexes were also observed in the raphe-hippocampal system. In CRE luciferase reporter gene assays, galanin (1-15) was more potent than galanin (1-29) in inhibiting the forskolin-induced increase of luciferase activity in GalR1-GalR2 transfected cells. The inhibition of CREB by 50nM of galanin (1-15) and of galanin (1-29) was fully counteracted by the non-selective galanin antagonist M35 and the selective GalR2 antagonist M871. These results suggested that the orthosteric agonist binding site of GalR1 protomer may have an increased affinity for the galanin (1-15) vs galanin (1-29) which can lead to its demonstrated increase in potency to inhibit CREB vs galanin (1-29). In contrast, in NFAT reporter gene assays galanin (1-29) shows a higher efficacy than galanin (1-15) in increasing Gq/11 mediated signaling over the GalR2 of these heteroreceptor complexes. This disbalance in the signaling of the GalR1-GalR2 heteroreceptor complexes induced by galanin (1-15) may contribute to depression-like actions since GalR1 agonists produce such effects.

Diversity and Bias through Receptor-Receptor Interactions in GPCR Heteroreceptor Complexes. Focus on Examples from Dopamine D2 Receptor Heteromerization

Allosteric receptor-receptor interactions in GPCR heteromers appeared to introduce an intermolecular allosteric mechanism contributing to the diversity and bias in the protomers. Examples of dopamine D2R heteromerization are given to show how such allosteric mechanisms significantly change the receptor protomer repertoire leading to diversity and biased recognition and signaling. In 1980s and 1990s, it was shown that neurotensin (NT) through selective antagonistic NTR-D2 like receptor interactions increased the diversity of DA signaling by reducing D2R-mediated dopamine signaling over D1R-mediated dopamine signaling. Furthermore, D2R protomer appeared to bias the specificity of the NTR orthosteric binding site toward neuromedin N vs. NT in the heteroreceptor complex. Complex CCK2R-D1R-D2R interactions in possible heteroreceptor complexes were also demonstrated further increasing receptor diversity. In D2R-5-HT2AR heteroreceptor complexes, the hallucinogenic 5-HT2AR agonists LSD and DOI were recently found to exert a biased agonist action on the orthosteric site of the 5-HT2AR protomer leading to the development of an active conformational state different from the one produced by 5-HT. Furthermore, as recently demonstrated allosteric A2A-D2R receptor-receptor interaction brought about not only a reduced affinity of the D2R agonist binding site but also a biased modulation of the D2R protomer signaling in A2A-D2R heteroreceptor complexes. A conformational state of the D2R was induced, which moved away from Gi/o signaling and instead favored β-arrestin2-mediated signaling. These examples on allosteric receptor-receptor interactions obtained over several decades serve to illustrate the significant increase in diversity and biased recognition and signaling that develop through such mechanisms.

Neuropeptides in epilepsy

Neuropeptides play an important role in modulating seizures and epilepsy. Unlike neurotransmitters which operate on a millisecond time-scale, neuropeptides have longer half lives; this leads to modulation of neuronal and network activity over prolonged periods, so contributing to setting the seizure threshold. Most neuropeptides are stored in large dense vesicles and co-localize with inhibitory interneurons. They are released upon high frequency stimulation making them attractive targets for modulation of seizures, during which high frequency discharges occur. Numerous neuropeptides have been implicated in epilepsy; one, ACTH, is already used in clinical practice to suppress seizures. Here, we concentrate on neuropeptides that have a direct effect on seizures, and for which therapeutic interventions are being developed. We have thus reviewed the abundant reports that support a role for neuropeptide Y (NPY), galanin, ghrelin, somatostatin and dynorphin in suppressing seizures and epileptogenesis, and for tachykinins having pro-epileptic effects. Most in vitro and in vivo studies are performed in hippocampal tissue in which receptor expression is usually high, making translation to other brain areas less clear. We highlight recent therapeutic strategies to treat epilepsy with neuropeptides, which are based on viral vector technology, and outline how such interventions need to be refined in order to address human disease.

Interaction of somatostatin receptor-2 and neuropeptide Y receptor-1 in mice dorsal root ganglion neurons on the pinch-nerve injury model

The aim of this paper was to evaluate the interactions of Somatostatin receptor-2 (SST2) and Neuropeptide Y receptors-1 (Y1) by molecular, pharmacological and behavioral studies. Double-immunolabeling of SST2 and Y1 has shown the colocalization of these two receptors in the dorsal root ganglion (DRG) neurons. On the basis of the Pinch nerve injury model, the mechanical hyperalgesia and severely painful behavior (autotomy) were detected after the application of SST2 antibody (anti-SST2; 200μg/kg) on the pinch-injured nerve. The differential distribution of Y1 and up-regulation of PKC expression in DRGs were observed after anti-SST2 treatment. Our results indicated for the first time the interactions of SST2 and Y1 in DRGs, which have functional role in pain modulation and might give rise to explore possible novel therapeutic strategies against pain.

Peptide regulators of peripheral taste function

The peripheral sensory organ of the gustatory system, the taste bud, contains a heterogeneous collection of sensory cells. These taste cells can differ in the stimuli to which they respond and the receptors and other signaling molecules they employ to transduce and encode those stimuli. This molecular diversity extends to the expression of a varied repertoire of bioactive peptides that appear to play important functional roles in signaling taste information between the taste cells and afferent sensory nerves and/or in processing sensory signals within the taste bud itself. Here, we review studies that examine the expression of bioactive peptides in the taste bud and the impact of those peptides on taste functions. Many of these peptides produced in taste buds are known to affect appetite, satiety or metabolism through their actions in the brain, pancreas and other organs, suggesting a functional link between the gustatory system and the neural and endocrine systems that regulate feeding and nutrient utilization.

On the existence and function of galanin receptor heteromers in the central nervous system

Galanin receptor (GalR) subtypes 1-3 linked to central galanin neurons may form heteromers with each other and other types of G protein-coupled receptors in the central nervous system (CNS). These heteromers may be one molecular mechanism for galanin peptides and their N-terminal fragments (gal 1-15) to modulate the function of different types of glia-neuronal networks in the CNS, especially the emotional and the cardiovascular networks. GalR-5-HT1A heteromers likely exist with antagonistic GalR-5-HT1A receptor-receptor interactions in the ascending midbrain raphe 5-HT neuron systems and their target regions. They represent a novel target for antidepressant drugs. Evidence is given for the existence of GalR1-5-HT1A heteromers in cellular models with trans-inhibition of the protomer signaling. A GalR1-GalR2 heteromer is proposed to be a galanin N-terminal fragment preferring receptor (1-15) in the CNS. Furthermore, a GalR1-GalR2-5-HT1A heterotrimer is postulated to explain why only galanin (1-15) but not galanin (1-29) can antagonistically modulate the 5-HT1A receptors in the dorsal hippocampus rich in gal fragment binding sites. The results underline a putative role of different types of GalR-5-HT1A heteroreceptor complexes in depression. GalR antagonists may also have therapeutic actions in depression by blocking the antagonistic GalR-NPYY1 receptor interactions in putative GalR-NPYY1 receptor heteromers in the CNS resulting in increases in NPYY1 transmission and antidepressant effects. In contrast the galanin fragment receptor (a postulated GalR1-GalR2 heteromer) appears to be linked to the NPYY2 receptor enhancing the affinity of the NPYY2 binding sites in a putative GalR1-GalR2-NPYY2 heterotrimer. Finally, putative GalR-α2-adrenoreceptor heteromers with antagonistic receptor-receptor interactions may be a widespread mechanism in the CNS for integration of galanin and noradrenaline signals also of likely relevance for depression.

Galanin receptor 3--a potential target for acute pancreatitis therapy

BACKGROUND

Galanin participates in the pathogenesis of acute pancreatitis (AP). The galanin receptor (GALR) sub-types involved, however, are unclear. We aimed to determine GALRs messenger RNA (mRNA) expression in mouse pancreas, describe their localization, and ascertain if GALR2 and GALR3 are involved in AP.

METHODS

Galanin receptor expression in murine whole pancreas, acinar, and islet cells was quantified by polymerase chain reaction amplification of reverse-transcribed RNA for mRNA, Western blot analysis for protein and in situ hybridization for GALR localization. Isolated acinar cells were used to determine galanin's effect on amylase secretion. Acute pancreatitis was induced in mice by caerulein injections. Mice, with and without AP, were treated with the highly selective GALR2 antagonist M871, or the specific GALR3 antagonist SNAP-37889. Indices of AP were measured at 12 h.

KEY RESULTS

Murine pancreas expresses mRNA for GALRs. In islets the expression of all GALR are comparable, whereas in acinar cells GALR3 is predominantly expressed. Western blot analysis confirmed that the GALR proteins are expressed by acinar cells. In situ hybridization analysis confirmed that GALR3 mRNA is present in islet and acinar cells, while mRNA for GALR1 and 2 is confined to islets. Galanin did not influence basal and caerulein-stimulated amylase release from acinar cells. M871 treatment reduced some, whereas SNAP-37889 treatment reduced all indices of AP (by 40-80%).

CONCLUSIONS & INFERENCES

Galanin receptor mRNA and protein are expressed in mouse pancreas, with GALR3 mRNA predominating. GALR3 antagonism reduced the severity of AP whereas GALR2 antagonism was less effective. GALR3 is a potential target for treatment of AP.

Chronic increase of circulating galanin levels induces obesity and marked alterations in lipid metabolism similar to metabolic syndrome

OBJECTIVE

Galanin (GAL) has a role in the regulation of food intake by way of acting on the central nervous system in rodents. High serum GAL levels have been observed in obese human subjects, suggesting that peripheral GAL has a role in the regulation of energy balance and that elevated circulating GAL levels contribute to the development of obesity and obesity-associated metabolic impairments. Currently, it is not known how chronically increased levels of circulating GAL affect energy balance. The purpose of this study is to clarify the importance of chronically increased levels of circulating GAL on energy balance in a transgenic mouse model.

RESEARCH DESIGN AND METHODS

Male wild-type and homozygous galanin transgenic (GAL-Tg) mice were used to study the peripheral effects of a 10-fold increase in circulating GAL on food intake, body weight, lipid metabolism, hepatic steatosis, glucose homeostasis and energy expenditure.

RESULTS

In the absence of an orexigenic effect, GAL-Tg mice had increased body weight, visceral adiposity, total serum cholesterol, total serum triglycerides and hyperinsulinemia, as well as impaired glucose tolerance. Compared with wild-type mice, the obese phenotype observed in the GAL-Tg mice was attributed to decreased oxygen consumption and carbon dioxide production, and this effect was independent of any changes in food intake or horizontal activity. In this obese model, GAL contributed to the development of fatty liver disease, which was associated with impaired glucose tolerance, as well as a reduction in heat production and metabolic rate.

CONCLUSIONS

Chronically elevated GAL may regulate body weight, metabolic rate, and lipid and carbohydrate metabolism through a mechanism that is independent of feeding regulation. The obese phenotype in the GAL-Tg mice is related to the reduced energy expenditure and insulin resistance. These findings support the hypothesis that increased circulating GAL levels contribute to the development of metabolic syndrome.

Galanin influences on vasopressin and oxytocin release: in vitro studies

Galanin (Gal) acts in the central nervous system as the neuromodulator of the hypothalamo-neurohypophysial system function. Present investigations in vitro were undertaken to study the influence of Gal, added to the incubative media at the concentrations of 10(-10), 10(-9), 10(-8) or 10(-7) M, on AVP and OT release from isolated rat hypothalamus (Hth), neurohypophysis (NH) and hypothalamo-neurohypophysial system (Hth-NH). The present results showed that Gal at the concentrations of 10(-10), 10(-9) and 10(-8) M inhibited basal AVP secretion from the all incubated tissues as well as OT release from the NH and Hth-NH explant. On the contrary, 10(-10) M Gal was the reason of intensified basal hypothalamic OT secretion. The presence of Gal at the concentrations of 10(-10) and 10(-8) M in the incubative media enriched in potassium ions excess was the cause of diminished AVP release from the NH and from the Hth-NH explant, respectively. Any effect of Gal on AVP release from the Hth has been observed. All the concentrations of Gal did not exert any effect on OT release from the NH as well as Hth-NH explants. However, the K(+)-evoked OT release from the Hth was distinctly intensified under influence of 10(-10)M as well as 10(-8) M Gal. It may be concluded that: * Gal modifies AVP and OT release in vitro at every level of Hth-NH system. * Gal has been supposed to perform the role of central inhibitory neuromodulator for AVP release from the Hth-NH system. * Gal exerts inhibitory effect on OT release in vitro from NH as well intact Hth-NH system but stimulatory influence on OT secretion at the level of Hth.

Neurochemical modulation of central cardiovascular control: the integrative role of galanin

Galanin (GAL) is a peptide involved in multiple functions, including central cardiovascular control. In this review, the role of GAL and its fragments in the modulation of cardiovascular neuronal networks in the nucleus of the solitary tract is presented, including its interaction with the classical neurotransmitters and other neuropeptides involved in cardiovascular responses in this nucleus. First, we describe the cardiovascular responses of GAL and the pathway involved in these responses. Then we summarize findings obtained in our laboratory on how GAL, through its receptors, interacts with two other neuropeptides--Neuropeptide Y and Angiotensin II and their receptors--as they have particularly conspicuous cardiovascular effects. All these results strengthen the role of GAL in central cardiovascular control and indicate the existence of interactions among GAL receptor subtypes and alpha2-adrenergic receptors, AT1, and Y1 receptor subtypes. These interactions are crucial for understanding the integrative mechanisms responsible for the organization of the cardiovascular responses from the NTS.

Roles of hormones in taste signaling

Proper nutrition, avoidance of ingesting substances that are harmful to the whole organism, and maintenance of energy homeostasis are crucial for living organisms. Additionally, mammals possess a sophisticated system to control the types and content of food that we swallow. Gustation is a vital sensory skill for determining which food stuffs to ingest and which to avoid, and for maintaining metabolic homeostasis. It is becoming apparent that there is a strong link between metabolic control and flavor perception. Although the gustatory system critically influences food preference, food intake, and metabolic homeostasis, the mechanisms for modulating taste sensitivity by metabolic hormones are just now being explored. It is likely that hormones produced in the tongue influence the amounts and types of food that we eat: the hormones that we associate with appetite control, glucose homeostasis and satiety, such as glucagon-like peptide-1, cholecystokinin, and neuropeptide Y are also produced locally in taste buds. In this report, we will provide an overview of the peptidergic endocrine hormone factors that are present or are known to have effects within the gustatory system, and we will discuss their roles, where known, in taste signaling.

Depression and antidepressants: molecular and cellular aspects

Clinical depression is viewed as a physical and psychic disease process having a neuropathological basis, although a clear understanding of its ethiopathology is still missing. The observation that depressive symptoms are influenced by pharmacological manipulation of monoamines led to the hypothesis that depression results from reduced availability or functional deficiency of monoaminergic transmitters in some cerebral regions. However, there are limitations to current monoamine theories related to mood disorders. Recently, a growing body of experimental data has showed that other classes of endogenous compounds, such as neuropeptides and amino acids, may play a significant role in the pathophysiology of affective disorders. With the development of neuroscience, neuronal networks and intracellular pathways have been identified and characterized, describing the existence of the interaction between monoamines and receptors in turn able to modulate the expression of intracellular proteins and neurotrophic factors, suggesting that depression/antidepressants may be intermingled with neurogenesis/neurodegenerative processes.

Impairment of memory consolidation by galanin correlates with in vivo inhibition of both LTP and CREB phosphorylation

Changes in the state of CREB phosphorylation and in LTP in the hippocampus have been associated with learning and memory. Here we show that galanin, the neuropeptide released in the hippocampal formation from cholinergic and noradrenergic fibers, that has been shown to produce impairments in memory consolidation in the Morris water maze task inhibits both LTP and CREB phosphorylation in the rat hippocampus in vivo. While there are many transmitters regulating CREB phosphorylation none has been shown to suppress behaviorally-induced hippocampal CREB phosphorylation as potently as galanin. The in vivo inhibition of dentate gyrus-LTP and of CREB phosphorylation by the agonist occupancy of GalR1 and GalR2-type galanin receptors provides strong in vivo cellular and molecular correlates to galanin-induced learning deficits and designates galanin as a major regulator of the memory consolidation process.

Differential effect of galanin on proliferation of PC12 and B104 cells

The effect of galanin (GAL) on neural cell proliferation was studied using PC12 and B104 cells. Reverse transcription-PCR was used to determine the expression of GAL and GAL receptors and a 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) assay of cell viability was employed to detect the effects of GAL on cell proliferation. These studies revealed firstly that PC12 cells express mRNAs encoding all three GAL receptors (GalR1-3) but not GAL mRNA, whereas B104 cells express GAL, GalR2 and GalR3 mRNAs, but not GalR1 mRNA; and secondly that GAL inhibited the proliferation of PC12 cells, but in contrast significantly activated the proliferation of B104 cells. Moreover, these effects of GAL were blocked by M35, a nonselective, chimera peptide antagonist of GAL receptors. These data suggest that GAL can alter neural cell proliferation via GAL receptor activation, and that different GAL receptors and/or cellular complements of receptors produce different net effects via activation of different signaling pathways.

The neuropeptide galanin as an in vivo modulator of brain 5-HT1A receptors: Possible relevance for affective disorders

The neuropeptide galanin is widely distributed throughout the central nervous system with multiple and diverse biological functions mediated by different receptor subtypes. In the rat, galanin-like immunoreactivity is expressed in a population of 5-hydroxytryptamine (5-HT, serotonin) neurons in the dorsal raphe with extensive projections to the forebrain areas, e.g., hippocampus. This review summarizes results from experimental studies in rodents showing that in vivo galanin is a potent modulator of brain 5-HT transmission, and in particular 5-HT1A receptor-mediated functions. Galanin, given intracerebroventricular (i.c.v.), was demonstrated to have strong inhibitory interactions with 5-HT1A receptor functions, particularly in the dorsal raphe but also in the hippocampus. Since pre- and postsynaptic 5-HT1A receptors in the dorsal raphe and hippocampus are implicated in the action of antidepressant drugs and in depressive disorders, it is suggested that galanin receptors may be an important target for development of novel antidepressant drugs. This view is supported by a recent study in the rat showing that the galanin antagonist M35, given i.c.v., could block the depression-like behavior in the forced swim test induced by galanin, while M35 produced an antidepressant-like effect on its own.

Involvement of galanin receptors 1 and 2 in the modulation of mouse vagal afferent mechanosensitivity

It is established that the gut peptide galanin reduces neuronal excitability via galanin receptor subtypes GALR1 and GALR3 and increases excitability via subtype GALR2. We have previously shown that galanin potently reduces mechanosensitivity in the majority of gastro-oesophageal vagal afferents, and potentiates sensitivity in a minority. These actions may have implications for therapeutic inhibition of gut afferent signalling. Here we investigated which galanin receptors are likely to mediate these effects. We performed quantitative RT-PCR on RNA from vagal (nodose) sensory ganglia, which indicated that all three GALR subtypes were expressed at similar levels. The responses of mouse gastro-oesophageal vagal afferents to graded mechanical stimuli were investigated before and during application of galanin receptor ligands to their peripheral endings. Two types of vagal afferents were tested: tension receptors, which respond to circumferential tension, and mucosal receptors which respond only to mucosal stroking. Galanin induced potent inhibition of mechanosensitivity in both types of afferents. This effect was totally lost in mice with targeted deletion of Galr1. The GALR1/2 agonist AR-M961 caused inhibition of mechanosensitivity in Galr1+/+ mice, but this was reversed to potentiation in Galr1-/- mice, indicating a minor role for GALR2 in potentiation of vagal afferents. We observed no functional evidence of GALR3 involvement, despite its expression in nodose ganglia. The current study highlights the complex actions of galanin at different receptor subtypes exhibiting parallels with the function of galanin in other systems.

Alcoholism is associated withGALR3but not two other galanin receptor genes

The neuropeptide galanin is widely expressed in the periphery and the central nervous system and mediates diverse physiological processes and behaviors including alcohol abuse, depression and anxiety. Four genes encoding galanin and its receptors have been identified (GAL, GALR1, GALR2 and GALR3). Recently we found that GAL haplotypes were associated with alcoholism, raising the possibility that genetic variation in GALR1, GALR2 and GALR3 might also alter alcoholism risk. Tag single nucleotide polymorphisms (SNPs) were identified by genotyping SNP panels in controls from five populations. For the association study with alcoholism, six GALR1, four GALR2 and four GALR3 SNPs were genotyped in a large cohort of Finnish alcoholics and non-alcoholics. GALR3 showed a significant association with alcoholism that was driven by one SNP (rs3,091,367). Moreover, the combination of the GALR3 rs3,091,367 risk allele and GAL risk haplotypes led to a modestly increased odds ratio (OR) for alcoholism (2.4) as compared with the effect of either GAL (1.9) or GALR3 alone (1.4). Likewise, the combination of the GALR3 and GAL risk diplotypes led to an increased OR for alcoholism (4.6) as compared with the effect of either GAL (2.0) or GALR3 alone (1.6). There was no effect of GALR1 or GALR2 on alcoholism risk. This evidence suggests that GALR3 mediates the alcoholism-related actions of galanin.

Changes in brain cholinergic markers and spatial learning in old galanin-overexpressing mice

The cholinergic forebrain system is involved in learning and memory, and its age-dependent decline correlates with a decrease in cognitive performance. Since the neuropeptide galanin participates in cholinergic neuron regulation, we have studied 19- to 23-month-old male mice overexpressing galanin under the platelet-derived growth factor B promoter (GalOE) and wild-type (WT) littermates by monitoring behavioral, neurochemical and morphological/histochemical parameters. In the Morris water maze test, old transgenic animals showed a significant impairment in escape latency in the hidden platform test compared to age-matched WT animals. The morphological/histochemical studies revealed that cholinergic neurons in the basal forebrain display a slight, age- but not genotype-related, alteration in choline acetyltransferase- (ChAT) immunoreactivity. The neurochemical studies showed an age-related decline in ChAT activity in the cerebral cortex of all mice, whereas in the hippocampal formation this effect was seen in GalOE but not WT animals. Expression of BDNF mRNA in the hippocampal formation, as evaluated by RT-PCR, was reduced in old animals; no age- or genotype-induced variations in NGF mRNA expression were observed. These data suggest that galanin overexpression further accentuates the age-related decline of the cholinergic system activity in male mice, resulting in impairment of water maze performance in old animals.

Galanin receptor 1 is expressed in a subpopulation of glutamatergic interneurons in the dorsal horn of the rat spinal cord

The 29/30 amino acid neuropeptide galanin has been implicated in pain processing at the spinal level and local dorsal horn neurons expressing the Gal(1) receptor may play a critical role. In order to determine the transmitter identity of these neurons, we used immunohistochemistry and antibodies against the Gal(1) receptor and the three vesicular glutamate transporters (VGLUTs), as well as in situ hybridization, to explore a possible glutamatergic phenotype. Gal(1) protein, which could not be demonstrated in Gal(1) knockout mice, colocalized with VGLUT2 protein, but not with glutamate decarboxylase, in many nerve endings in lamina II. Moreover, Gal(1) and VGLUT2 transcripts were often found in the same cell bodies in laminae I-IV. Gal(1)-protein and galanin-peptide showed an overlapping distribution but were not colocalized. Gal(1) staining did not appear to be affected by dorsal rhizotomy. Taken together, these findings provide strong evidence that Gal(1) is a heteroreceptor expressed on excitatory glutamatergic dorsal horn interneurons. Activation of such Gal(1) receptors may thus decrease the inhibitory tone in the superficial dorsal horn, and possibly cause antinociception.

Galanin receptor expression in cultured human keratinocytes and in normal human skin

Galanin (GAL) is a biologically active neuropeptide that is widely distributed in the nervous system. GAL exerts diverse action via the GAL receptors (GALR1, GALR2, and GALR3), which belong in the superfamily of G-protein-coupled transmembrane receptors. In human skin, GAL-like immunoreactivity has been reported in free nerve endings and fibers of the dermis. The extraneuronal expression of GAL has also been demonstrated. Although the GALRs are essential for biological functions, the expressions of different GALR subtypes in cultured human keratinocytes have not yet been investigated. The aim of our study was to investigate the mRNA and protein expressions of the different GALRs in the HaCaT immortalized keratinocyte cell line and in cultured human keratinocytes. When reverse transcription (RT)-polymerase chain reaction (PCR) was used with different GALR-specific primers, only GALR2 mRNA was identified in cultured HaCaT cells and keratinocytes. Sequencing of the PCR products proved the presence of GALR2 mRNA in the keratinocytes. The presence of GALR2 protein was next investigated, using a polyclonal antibody against human GALR2. Both the HaCaT cells and the cultured keratinocytes displayed specific immunohistochemical staining, with higher intensity on the surface of the keratinocytes. Immunohistochemical investigations of normal human skin specimens revealed that GALR2 was expressed with high intensity in the basal layer of the epidermis and also around the hair follicles in the dermis. GAL treatment of the keratinocytes resulted in an increase in cytosolic Ca2+ concentration, suggesting that GALR2 is a functional receptor. Further studies are necessary to clarify the biological effects of GAL in the skin.

Galanin expressed in the excitatory fibers attenuates synaptic strength and generalized seizures in the piriform cortex of mice

The neuropeptide galanin is considered to be an endogenous antiepileptic agent, presumably acting via inhibition of glutamate release. Previously, we have demonstrated that in mice ectopically overexpressing galanin in cortical and hippocampal neurons, particularly in granule cells and their axons, the mossy fibers, hippocampal kindling epileptogenesis is suppressed and is associated with attenuated frequency facilitation in mossy fiber-CA3 cell synapses. We hypothesized that changes in synaptic transmission might occur also in other excitatory synapses of the galanin overexpressing (GalOE) mouse, contributing to seizure suppression. Lateral olfactory tract (LOT) synapses, formed by axons of olfactory bulb (OB) mitral cells and targeting piriform cortex (PC) pyramidal cells, ectopically express galanin in GalOE mice. Using whole-cell patch-clamp recordings, we found that excitatory synaptic responses recorded in PC pyramidal cells during high frequency stimulation of the LOT were attenuated in GalOE mice as compared to wild-type controls. This effect was mimicked by bath application of galanin or its agonist galnon to wild-type slices, supporting the notion of ectopic galanin action. Since the high frequency activation induced in vitro resembles epileptic seizures in vivo, we asked whether the observed synaptic inhibition would result in altered epileptogenesis when animals were kindled via the same synapses. In male GalOE mice, we found that the latency to convulsions was prolonged, and once animals had experienced the first stage 5 seizure, generalized seizures were less sustainable. These data indicate that the PC is a possible target for epilepsy treatment by ectopically overexpressing galanin to modulate seizure activity.

Galanin-neuropeptide Y (NPY) interactions in central cardiovascular control: involvement of the NPY Y1receptor subtype

The interactions between neuropeptide Y (NPY), specifically through NPY Y(1) and Y(2) receptor subtypes, and galanin [GAL(1-29)] have been analysed at the cardiovascular level. The cardiovascular effects of intracisternal coinjections of GAL(1-29) with NPY or NPY Y(1) or Y(2) agonists, as well as quantitative receptor autoradiography of the binding characteristics of NPY Y(1) and Y(2) receptor subtypes in the nucleus of the solitary tract (NTS), in the presence or absence of GAL(1-29), have been investigated. The effects of coinjections of GAL(1-29) and the NPY Y(1) agonist on the expression of c-FOS immunoreactivity in the NTS were also studied. The coinjection of NPY with GAL(1-29) induced a significant vasopressor and tachycardic action with a maximum 40% increase (P < 0.001). The coinjection of the NPY Y(1) agonist and GAL(1-29) induced a similar increase in mean arterial pressure and heart rate as did NPY plus GAL(1-29), actions that were not observed with the NPY Y(2) agonist plus GAL(1-29). GAL(1-29), 3 nm, significantly and substantially (by approximately 40%) decreased NPY Y(1) agonist binding in the NTS. This effect was significantly blocked (P < 0.01) in the presence of the specific galanin antagonist M35. The NPY Y(2) agonist binding was not modified in the presence of GAL(1-29). At the c-FOS level, the coinjection of NPY Y(1) and GAL(1-29) significantly reduced the c-FOS-immunoreactive response induced by either of the two peptides. The present findings suggest the existence of a modulatory antagonistic effect of GAL(1-29) mediated via galanin receptors on the NPY Y(1) receptor subtype and its signalling within the NTS.

Differential distribution and regulation of galanin receptors- 1 and -2 in the rat lumbar spinal cord

The expression of the galanin receptor-1 and -2 (Gal(1) and Gal(2)) messenger ribonucleic acids (mRNAs) was studied in the lower spinal cord of rat by means of in situ hybridization, using ribonucleic acid probes (riboprobes). Naïve rats as well as rats with unilateral axotomy of the sciatic nerve or unilateral inflammation of the hindpaw were analyzed. In naïve rats, numerous Gal(1) mRNA-positive (+) neurons were detected in lamina (L) I-III. In addition, several Gal(1) mRNA(+) neurons were seen in deeper layers, including the ventral horns, area X, and the lateral spinal nucleus. In contrast, few and comparatively weakly labeled Gal(2) mRNA(+) neurons were observed, mostly in the ventral horns and in area X, with fewer in the dorsal horn and in the sympathetic and parasympathetic intermediate lateral cell columns. Axotomy induced a strong increase in intensity and number of Gal(2) mRNA(+) motoneurons ipsilateral to the lesion. In contrast, nerve cut or hindpaw inflammation did not alter the expression of Gal(1) or Gal(2) in the dorsal horn. The present (and previous) results suggest that galanin, acting through Gal(1) and Gal(2) receptors, has a modulatory role on spinal excitability, not only via interneurons in superficial dorsal horn, but also on neurons in deep layers and area X, as well as on the sympathetic and parasympathetic outflow. Furthermore, the nerve injury-induced Gal(2) upregulation in motor neurons suggests a role for galanin in survival/regeneration mechanisms.

Galanin modulates neuronal and synaptic properties in the rat supraoptic nucleus in a use and state dependent manner

The magnocellular neurons of the hypothalamic supraoptic nucleus (SON) synthesize and secrete oxytocin (OXT) and vasopressin (AVP) from their dendrites. These peptides, and several other neurotransmitters, have been shown to modulate afferent glutamatergic neurotransmission in the SON. The neuropeptide, galanin (GAL) is also localized in SON magnocellular neurons and in afferent fibers in the nucleus. We show that GAL dose-dependently reduces evoked excitatory postsynaptic currents (eEPSCs), alters paired pulse ratio and decreases mEPSC frequency, but not amplitude or decay kinetics in both OXT and AVP neurons. GAL therefore modulates excitatory neurotransmission at a likely presynaptic receptor. Neither OXT/AVP, GABA(B) nor cannabinoid antagonists blocked this effect. A GAL2/3 agonist mimicked GAL's action while GAL1 antagonist did not block GAL's effect, suggesting that GAL2/3 receptors mediate the presynaptic effect. In nondehydrated rats GAL causes a small postsynaptic response, as assessed by input resistance measurements. When the rats were water deprived for 2 days the presynaptic response to GAL was unaltered; however, the postsynaptic decrease in input resistance and hyperpolarization was increased, an effect consistent with a previously described increase in GAL1 receptor expression in dehydration. A GAL1 receptor antagonist blocked the postsynaptic effects. Last, when a train of eEPSCs was elicited, GAL was found to inhibit the earlier events in a train but not the latter. This indicates that GAL may modulate a single synaptic event more effectively than trains of synaptic inputs, thereby acting as a high-pass filter.

Galanin-like peptide stimulates food intake via activation of neuropeptide Y neurons in the hypothalamic dorsomedial nucleus of the rat

Galanin-like peptide (GALP), a 29-amino-acid neuropeptide, is located in the hypothalamic arcuate nucleus (ARC), binds to galanin receptor subtype 2, and induces food intake upon intracerebroventricular (icv) injection in rats. However, neural mechanisms underlying its orexigenic action remain unclear. We aimed to identify the nuclei and neuron species that mediate the food intake in response to icv GALP injection. Intracerebroventricular injection of GALP, as powerfully as that of neuropeptide Y (NYP), increased food intake for the initial 2 h. GALP injected focally into the dorsomedial nucleus (DMN), but not the ARC, lateral hypothalamus, or paraventricular nucleus (PVN), stimulated food intake for 2 h after injection. In contrast, galanin injected into the DMN had no effect. DMN-lesion rats that received icv GALP injection showed attenuated feeding compared with control rats. Intracerebroventricular GALP injection increased c-Fos expression in NPY-containing neurons in the DMN, but not the ARC. GALP increased the cytosolic calcium concentration ([Ca(2+)](i)) in NPY-immunoreactive neurons isolated from the DMN, but not the ARC. Furthermore, both anti-NPY IgG and NPY antagonists, when preinjected, counteracted the feeding induced by GALP injection. These data show that icv GALP injection induces a potent short-term stimulation of food intake mainly via activation of NPY-containing neurons in the DMN.

Phenotyping of sensory and sympathetic ganglion neurons of a galanin-overexpressing mouse--possible implications for pain processing

The distribution of galanin was studied in the lumbar 5 dorsal root ganglia (DRGs) and spinal cord, superior cervical ganglia (SCGs), and skin of transgenic mice overexpressing galanin under the dopamine beta-hydroxylase (DBH) promoter (GalOE-DBH mice) and in wild type (WT) mice. The DRGs and spinal cord were analysed before and after a unilateral, complete transection (axotomy) of the sciatic nerve and after dorsal rhizotomy. Both galanin protein and transcript were studied by, respectively, immunohistochemistry and in situ hybridization. Increased galanin expression was observed in several small, medium-sized and large DRG neuron profiles (NPs) in the naïve transgenic mouse, frequently in neurons lacking calcitonin gene-related peptide (CGRP) and isolectin B4-binding. This lack of coexistence was particularly evident in the medium-sized/large NPs. In the dorsal horn of the spinal cord, no differences were detected between GalOE-DBH and WT mice, both displaying a strong galanin-positive neuropil in the superficial laminae of the dorsal horn, but the transgenic mice showed a more abundant galanin-positive innervation of the ventral horn. A 12-day dorsal rhizotomy, surprisingly, failed to alter the galanin staining patterns in the dorsal (and ventral) dorsal horn. Unilateral axotomy induced upregulation of galanin in DRG NPs of all sizes in both types of mouse. In the hindpaw skin, a profuse galanin-positive fiber plexus was observed in sweat glands and around blood vessels of the transgenic mice, being much more restricted in WT mice. Finally, GalOE mice exhibited a strong galanin-like immunoreactivity in most SCG NPs. The overexpression of the peptide in DRGs and SCGs was paralleled by increased mRNA levels. The present results show that overexpression of galanin under the control of the DBH promoter does not only occur, as expected in these mice, in noradrenline/adrenaline neurons but also in DRG neurons, particularly in large and medium-sized NPs. To what extent and how this overexpression pattern is related to the previously shown elevated pain threshold under normal and lesion conditions is discussed [Grass, S., Crawley, J.N., Xu, X.J., Wiesenfeld-Hallin, Z., 2003a. Reduced spinal cord sensitization to C-fibre stimulation in mice over-expressing galanin. Eur. J. Neurosci. 17, 1829-1832; Hygge-Blakeman, K., Brumovsky, P., Hao, J.X., Xu, X.J., Hökfelt, T., Crawley, J.N., Wiesenfeld-Hallin, Z., 2004. Galanin over-expression decreases the development of neuropathic pain-like behaviour in mice after partial sciatic nerve injury. Brain Res. 1025, 152-158].

Sensory neuronal phenotype in galanin receptor 2 knockout mice: focus on dorsal root ganglion neurone development and pain behaviour

Galanin is a 29-amino-acid peptide expressed in dorsal root ganglion (DRG) neurones and spinal dorsal horn neurones. It affects pain threshold and has developmental and trophic effects. Galanin acts at three G-protein-coupled receptors, galanin receptors (GalR1-3), each expressed in the DRGs as suggested by in situ hybridization and/or reverse transcriptase-polymerase chain reaction. The GalR2 knockout (-/-) mice permit studies on the contributions of this receptor subtype to the role of galanin at the spinal level. At 1 week after sciatic nerve transection (axotomy), there were 16-20% fewer neurones in intact and contralateral DRGs of -/- mice as compared with wild-type (WT) mice. In addition, a significant neurone loss (26% reduction) was found in the ipsilateral DRGs of WT mice, whereas no further neurone loss was seen in -/- mice. Expression of several peptides has been examined after axotomy, including galanin, neuropeptide Y and two of its receptors as well as substance P, and no significant differences were found between -/- and WT mice in either ipsi- or contralateral DRGs, respectively. After thermal injury and spinal nerve ligation, onset and duration of hyperalgesia in the injured paw were similar in GalR2-/- and WT animals. Recovery from spinal nerve ligation-caused allodynia had the same kinetics in -/- and WT animals. These data are in line with earlier observations from the peripheral and central nervous system, suggesting that galanin actions mediated by GalR2 subtype are of importance in neurodevelopment and neuroprotection.

Expression of galanin and the galanin receptor in rat taste buds

Galanin, a 29-amino-acid neuropeptide, was initially isolated from porcine intestine. It has a wide spread distribution in the central nervous system and is also present in the primary sensory neuron. Galanin has been suggested to be involved in numerous neuronal and endocrine functions as a neurotransmitter and neuromodulator. We examined the expression of galanin and galanin receptors by using a reverse transcription-polymerase chain reaction (RT-PCR), immunohistochemistry, and in situ hybridization. RT-PCR analysis showed that mRNA of galanin and GalR2 were detected in the taste bud-containing epithelium of the circumvallate papilla of rats. Immunohistochemical analyses detected galanin was detected in a subset of taste bud cells of the circumvallate papillae. Double-label studies showed that galanin colocalized with alpha-gustducin, NCAM, and PLCbeta2. Our results of double staining with galanin and taste cell markers indicate that galanin-expressing taste cells are type II and type III cells. Taken together with previous studies, these findings show that galanin may function as a taste bud neurotransmitter. Furthermore, GalR2 mRNA was expressed in some taste bud cells. This suggests that, galanin release may not only excite the peripheral afferent nerve fiber but also may act on neighboring taste receptor cells via the activation of GalR2.

Distribution of galanin receptor 1-immunoreactive neurons in the ovine hypothalamus: colocalization with GnRH

Galanin is implicated in numerous physiological functions, including reproduction. Where and how galanin acts in the brain is poorly understood, but recent evidence suggests that it is predominantly through the GAL-R1 receptor. Using an antibody raised against the third intracellular loop of rat GAL-R1, a region that is highly conserved among species, our first objective was to determine the distribution of cells expressing immunoreactive GAL-R1 in the hypothalamus of the sheep. GAL-R1-immunoreactive cells were spread widely in the ovine diencephalon and overlapped with the known distribution of GnRH neurons. Galanin has been shown to enhance GnRH secretion, but it is not known whether this effect is transduced at the level of the GnRH neuron or is indirect. Thus, our second objective was to establish if GnRH neurons throughout the hypothalamus expressed GAL-R1 receptors and, if so, whether GAL-R1 expression in GnRH neurons was influenced by season, gender and/or stage of the estrous cycle. In rams and ewes during the non-breeding season, only a tenth of the GnRH neurons expressed immunocytochemically detectable GAL-R1 receptors. In contrast, a fifth of the GnRH neurons expressed immunocytochemically detectable GAL-R1 in the luteal phase, whereas only a twentieth expressed GAL-R1 in the follicular phase. These data suggest that galanin may affect a subpopulation of GnRH neurons through the GAL-R1 receptor and that this affect may be modulated by steroids.

The galanin-R2 agonist AR-M1896 reduces glutamate toxicity in primary neural hippocampal cells

Galanin is a neuropeptide involved in a variety of biological functions, including having a strong anticonvulsant activity. To assess a possible role of galanin in modulation of glutamatergic synapses and excitotoxicity, we studied effects of a galanin receptor 2(3) agonist (AR-M1896) on several molecular events induced by glutamate administration in primary neural hippocampal cells. Exposure of cells, after 5 days in vitro, to glutamate 0.5 mM for 10 min caused morphological alterations, including disaggregation of beta-tubulin and MAP-2 cytoskeletal protein assembly, loss of neurites and cell shrinkage. When present in culture medium together with glutamate, 1 and 10 nM of AR-M1896 reduced these alterations. Moreover, AR-M1896 counteracted glutamate-induced c-fos mRNA and c-Fos protein up-regulation after 30-150 min, and 24 h, respectively. Massive nuclear alterations (Hoechst 33258 staining), observed 24 h after glutamate exposure, were also antagonized by AR-M1896 (0.1-100 nM) in a dose-dependent manner. These findings indicate that galanin, probably mainly through its type 2 receptor, interferes with events associated with glutamate toxicity.

Role of galanin receptor 1 and galanin receptor 2 activation in synaptic plasticity associated with 3',5'-cyclic AMP response element-binding protein phosphorylation in the dentate gyrus: studies with a galanin receptor 2 agonist and galanin receptor 1 knockout mice

The neuropeptide galanin was shown to impair cognitive performance and reduce hippocampal CA1 long-term potentiation (LTP) in rodents. However, the contribution of the two main galanin receptors; GalR1 and GalR2, present in the hippocampus to these effects is not known. In the present study, we determined the protein expression levels of GalR1 and GalR2 in the mouse dentate gyrus (DG) and used galanin (2-11), a recently introduced GalR2 agonist, and GalR1 knockout mice to examine the contribution of GalR1 and GalR2 to the modulation of LTP and 3',5'-cyclic AMP response element-binding protein (CREB)-dependent signaling cascades. In the DG, 57+/-5% of the galanin binding sites were GalR2, and the remaining population corresponded to GalR1. In hippocampal slices, galanin (2-11) fully blocked the induction of DG LTP, whereas galanin (1-29), a high affinity agonist for both GalR1 and GalR2, strongly but not fully attenuated the late phase of LTP by 80+/-1.5%. Application of galanin (1-29) or galanin (2-11) after LTP induction caused a transient reduction in the maintenance phase of LTP, with the larger effect displayed by superfusion of galanin (2-11). The induction and maintenance of DG LTP was not altered in the GalR1 knockout mice. Superfusion of galanin (1-29) or galanin (2-11) blocked the LTP induction to the same degree indicating a role for GalR2 in the induction phase of DG LTP. Furthermore, we analyzed the effects of GalR1 and/or GalR2 activation on DG LTP-induced CREB phosphorylation, associated with the late transcriptional effects of LTP. In the lateral part of the granule cell layer, high-frequency trains stimulation caused a significant increase in the level of CREB phosphorylation, which was significantly reduced by application of either galanin (1-29) or galanin (2-11), indicating that both GalR1 and/or GalR2 can mediate some of their effects on LTP through inhibition of CREB-related signaling cascades.

Phenotypic analysis of mice deficient in the type 2 galanin receptor (GALR2)

Galanin is a neuropeptide implicated in the regulation of feeding, reproduction, cognition, nociception, and seizure susceptibility. There are three known galanin receptor (GALR) subtypes (GALR1, GALR2, and GALR3), which bind to galanin with different affinities and have their own unique distributions, signaling mechanisms, and putative functions in the brain and peripheral nervous system. To gain further insight into the possible physiological significance of GALR2, we created mutant mice that were deficient in GALR2 and compared their phenotype to that of wild-type (WT) littermate or age-matched controls, with respect to basic motor and sensory function, feeding behavior, reproduction, mood, learning and memory, and seizure susceptibility. Phenotypic analysis revealed that animals bearing a deletion of GALR2 did not differ significantly from their WT controls in any of the measured variables. We conclude that either GALR2 plays no role in these physiological functions or through redundancy or compensation these mutant animals can adapt to the congenital absence of GALR2. It is also conceivable that GALR2 plays only a subtle role in some of these functions and that the impact of its loss could not be detected by the analytical procedures used here.

Galanin-like peptide and ghrelin increase cytosolic Ca2+ in neurons containing growth hormone-releasing hormone in the arcuate nucleus

Galanin-like peptide (GALP), discovered in the porcine hypothalamus, is expressed predominantly in the arcuate nucleus (ARC), a feeding-controlling center. Intracerebroventricular injection of GALP has been shown to stimulate food intake in the rats. However, the mechanisms underlying the orexigenic effect of GALP are unknown. The present study aimed to determine the target neurons of GALP in the ARC. We investigated the effects of GALP on cytosolic free Ca2+ concentration ([Ca2+]i) in the neurons isolated from the rat ARC, followed by neurochemical identification of these neurons by immunocytochemistry using antisera against growth hormone-releasing hormone (GHRH), neuropeptide Y (NPY) and proopiomelanocortin (POMC), the peptides localized in the ARC. GALP at 10(-10) M increased [Ca2+]i in 11% of single neurons of the ARC, while ghrelin, an orexigenic and GH-releasing peptide, at 10(-10) M increased [Ca2+]i in 35% of the ARC neurons. Some of these GALP- and/or ghrelin-responsive neurons were proved to contain GHRH. In contrast, NPY- and POMC-containing neurons did not respond to GALP. These results indicate that GALP directly targets GHRH neurons, but not NPY and POMC neurons, and that ghrelin directly targets GHRH neurons in the ARC. The former action may be involved in the orexigenic effect of GALP and the latter in the GH-releasing and/or orexigenic effects ghrelin.

Role of galanin and galanin(1-15) on central cardiovascular control

Galanin and the N-terminal fragment Galanin(1-15) are involved in central cardiovascular regulation. The present paper reviews the recent cardiovascular results obtained by intracisternal injections of Galanin and Galanin(1-15) showing that: (A) the Galanin antagonist M40 blocks the central cardiovascular responses induced by Galanin(1-15) but not those elicited by Galanin; (B) both Galanin and Galanin(1-15) induce the expression of c-Fos in cardiovascular nuclei of the medulla oblongata with different temporal and spatial profiles; (C) the cardiovascular action of Galanin(1-15), but not Galanin, is mediated by peripheral beta-receptor stimulation; (D) and it is demonstrated an antagonistic Galanin/alpha2-adrenoceptors interaction as well as a differential modulation of central cardiovascular responses of Angiotensin II by Galanin or Galanin(1-15). All these data strengthen the involvement of both Galanin molecules as neuromodulators on central cardiovascular regulation.