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

Long-term enhancements in antidepressant efficacy and neurogenesis: Effects of intranasal co-administration of neuropeptide Y 1 receptor (NPY1R) and galanin receptor 2 (GALR2) agonists in the ventral hippocampus

This study evaluates the sustained antidepressant-like effects and neurogenic potential of a 3-day intranasal co-administration regimen of galanin receptor 2 (GALR2) agonist M1145 and neuropeptide Y Y1 receptor (NPY1R) agonist [Leu31, Pro34]NPY in the ventral hippocampus of adult rats, with outcomes analyzed 3 weeks post-treatment. Utilizing the forced swimming test (FST), we found that this co-administration significantly enhances antidepressant-like behaviors, an effect neutralized by the GALR2 antagonist M871, highlighting the synergistic potential of these neuropeptides in modulating mood-related behaviors. In situ proximity ligation assay (PLA) indicated a significant increase in GALR2/NPYY1R heteroreceptor complexes in the ventral hippocampal dentate gyrus, suggesting a molecular basis for the behavioral outcomes observed. Moreover, proliferating cell nuclear antigen (PCNA) immunolabeling revealed increased cell proliferation in the subgranular zone of the dentate gyrus, specifically in neuroblasts as evidenced by co-labeling with doublecortin (DCX), without affecting quiescent neural progenitors or astrocytes. The study also noted a significant uptick in the number of DCX-positive cells and alterations in dendritic morphology in the ventral hippocampus, indicative of enhanced neuronal differentiation and maturation. These morphological changes highlight the potential of these agonists to facilitate the functional integration of new neurons into existing neural circuits. By demonstrating the long-lasting effects of a brief, 3-day intranasal administration of GALR2 and NPY1R agonists, our findings contribute significantly to the understanding of neuropeptide-mediated neuroplasticity and herald novel therapeutic strategies for the treatment of depression and related mood disorders, emphasizing the therapeutic promise of targeting neurogenesis and neuronal maturation processes.

Enhanced neuronal survival and BDNF elevation via long-term co-activation of galanin 2 (GALR2) and neuropeptide Y1 receptors (NPY1R): potential therapeutic targets for major depressive disorder

BACKGROUND

Major Depressive Disorder (MDD) is a prevalent and debilitating condition, necessitating novel therapeutic strategies due to the limited efficacy and adverse effects of current treatments. We explored how galanin receptor 2 (GALR2) and Neuropeptide Y1 Receptor (NPYY1R) agonists, working together, can boost brain cell growth and increase antidepressant-like effects in rats. This suggests new ways to treat Major Depressive Disorder (MDD).

RESEARCH DESIGN AND METHODS

In a controlled laboratory setting, adult naive Sprague-Dawley rats were administered directly into the brain's ventricles, a method known as intracerebroventricular (ICV) administration, with GALR2 agonist (M1145), NPYY1R agonist, both, or in combination with a GALR2 antagonist (M871). Main outcome measures included long-term neuronal survival, differentiation, and behavioral.

RESULTS

Co-administration of M1145 and NPYY1R agonist significantly enhanced neuronal survival and maturation in the ventral dentate gyrus, with a notable increase in Brain-Derived Neurotrophic Factor (BDNF) expression. This neurogenic effect was associated with an antidepressant-like effect, an outcome partially reversed by M871.

CONCLUSIONS

GALR2 and NPYY1R agonists jointly promote hippocampal neurogenesis and exert antidepressant-like effects in rats without adverse outcomes, highlighting their therapeutic potential for MDD. The study's reliance on an animal model and intracerebroventricular delivery warrants further clinical exploration to confirm these promising results.

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.

Spexin as a neuroendocrine signal with emerging functions

Spexin (SPX), a novel peptide coevolved with the galanin/kisspeptin family, was first identified by bioinformatics prior to its protein purification/functional studies. Its mature peptide is highly conserved among different vertebrate classes. Based on the studies in mammals and fish models, SPX was found to be widely distributed at tissue level, secreted into systemic circulation, identified at notable levels in central nervous system and peripheral tissues, and has been confirmed/implicated in multiple functions in different tissues/organs, suggesting that SPX may serve as a neuroe​ndocrine signal with pleotropic functions. In this article, different isoforms of SPX and their binding with their cognate receptors GalR2 and GalR3, the biological functions of SPX reported in mammals including GI tract movement, energy balance and weight loss, fatty acid uptake, glucose homeostasis, nociception and cardiovascular/renal functions, as well as the recent findings in fish models regarding the role of SPX in reproduction and feeding control will be reviewed with interesting questions for future investigations.

Central galanin receptor 2 mediates galanin action to promote systemic glucose metabolism of type 2 diabetic rats

Although recent results of our and other studies have showed that galanin (GAL) is an antidiabetic and anti-inflammatory neuropeptide, the molecular mechanism how central GAL regulates energy homeostasis and insulin sensitivity is still not fully understood. The aim of this study was to investigate whether central type 2 of GAL receptors (GALR2) are involved in the regulation of systemic glucose metabolism and its underlying mechanisms. In the present study, type 2 diabetic rats were intracerebroventricularly (i.c.v.) given 100 nM/kg/d GALR2 agonist M1145 or GALR2 antagonist M871 in 5 μl artificial cerebrospinal fluid once a day for consecutive 21 days. Then insulin resistance indexes, inflammatory factor and many genes associated with the function of glucose metabolism were examined in peripheral tissues. The present findings showed that the intracerebroventricular injection of M1145 or M871 respectively increased or decreased glucose infusion rates in hyperinsulinemic euglycemic clamp tests, but attenuated or enhanced the plasma inflammatory factors and glucose concentration in type 2 diabetic rats. Moreover, administration of M1145 markedly increased PGC-1α and GLUT4 expression in skeletal muscles and adipocytes of type 2 diabetic rats. In conclusion, activation of central GALR2 promotes glucose metabolism and ameliorates insulin resistance mainly through the PGC-1α/GLUT4 pathways. The central GALR2 is crucial to whole-body insulin sensitivity and energy homeostasis.

Epilepsy-associated alterations in hippocampal excitability

The hippocampus exhibits a wide range of epilepsy-related abnormalities and is situated in the mesial temporal lobe, where limbic seizures begin. These abnormalities could affect membrane excitability and lead to overstimulation of neurons. Multiple overlapping processes refer to neural homeostatic responses develop in neurons that work together to restore neuronal firing rates to control levels. Nevertheless, homeostatic mechanisms are unable to restore normal neuronal excitability, and the epileptic hippocampus becomes hyperexcitable or hypoexcitable. Studies show that there is hyperexcitability even before starting recurrent spontaneous seizures, suggesting although hippocampal hyperexcitability may contribute to epileptogenesis, it alone is insufficient to produce epileptic seizures. This supports the concept that the hippocampus is not the only substrate for limbic seizure onset, and a broader hyperexcitable limbic structure may contribute to temporal lobe epilepsy (TLE) seizures. Nevertheless, seizures also occur in conditions where the hippocampus shows a hypoexcitable phenotype. Since TLE seizures most often originate in the hippocampus, it could therefore be assumed that both hippocampal hypoexcitability and hyperexcitability are undesirable states that make the epileptic hippocampal network less stable and may, under certain conditions, trigger seizures.

Short N-terminal galanin fragments are occurring naturally in vivo

The galanin family currently consists of four peptides, namely galanin, galanin-message associated peptide, galanin-like peptide and alarin. Unlike galanin that signals through three different G protein-coupled receptors; GAL₁, GAL₂, and GAL₃, binding at its N-terminal end, the cognate receptors for other members of the galanin family are currently unknown. Research using short N-terminal galanin fragments generated either by enzymatic cleavage or solid-phase synthesis has revealed differences in their receptor binding properties exerting numerous biological effects distinct from galanin(1-29) itself. Our studies on tissue extracts derived from rat small intestine and bovine gut using chromatographic techniques and sensitive galanin(1-16)-specific radioimmunoassay revealed the presence of immunoreactive compounds reacting with antiserum against galanin(1-16) distributed in distinct elution volumes. These results suggested a possible presence of short N-terminal galanin fragments also in vivo. Moreover, employing immunoaffinity chromatography and reverse-phase high performance liquid chromatography (HPLC) followed by mass spectrometry allowed specific enrichment of these immunoreactive compounds from rat tissues and identification of their molecular structure. Indeed, our study revealed presence of several distinct short N-terminal galanin sequences in rat tissue. To prove their receptor binding, four of the identified sequences were synthetized, namely, galanin(1-13), galanin(1-16), galanin(1-20), galanin(6-20), and tested on coronal rat brain sections competing with ¹²⁵I-labeled galanin(1-29). Our autoradiographs confirmed that galanin(1-13), galanin(1-16), and galanin(1-20) comprehensively displaced ¹²⁵I-galanin(1-29) but galanin(6-20) did not. Here we show, for the first time, that short N-terminal galanin fragments occur naturally in rat tissues and that similar or identical galanin sequences can be present also in tissues of other species.

BIOLOGICAL SIGNIFICANCE

This study is first to provide an evidence of the presence of short N-terminal galanin fragments in vivo in a biological system and provides further foundations for the previous studies using synthetized short N-terminal galanin fragments.

Central Administration of Galanin Receptor 1 Agonist Boosted Insulin Sensitivity in Adipose Cells of Diabetic Rats

Our previous studies testified the beneficial effect of central galanin on insulin sensitivity of type 2 diabetic rats. The aim of the study was further to investigate whether central M617, a galanin receptor 1 agonist, can benefit insulin sensitivity. The effects of intracerebroventricular administration of M617 on insulin sensitivity and insulin signaling were evaluated in adipose tissues of type 2 diabetic rats. The results showed that central injection of M617 significantly increased plasma adiponectin contents, glucose infusion rates in hyperinsulinemic-euglycemic clamp tests, GLUT4 mRNA expression levels, GLUT4 contents in plasma membranes, and total cell membranes of the adipose cells but reduced the plasma C-reactive protein concentration in nondiabetic and diabetic rats. The ratios of GLUT4 contents were higher in plasma membranes to total cell membranes in both nondiabetic and diabetic M617 groups than each control. In addition, the central administration of M617 enhanced the ratios of pAkt/Akt and pAS160/AS160, but not phosphorylative cAMP response element-binding protein (pCREB)/CREB in the adipose cells of nondiabetic and diabetic rats. These results suggest that excitation of central galanin receptor 1 facilitates insulin sensitivity via activation of the Akt/AS160 signaling pathway in the fat cells of type 2 diabetic rats.

Review: Hippocampal sclerosis in epilepsy: a neuropathology review

Hippocampal sclerosis (HS) is a common pathology encountered in mesial temporal lobe epilepsy (MTLE) as well as other epilepsy syndromes and in both surgical and post-mortem practice. The 2013 International League Against Epilepsy (ILAE) classification segregates HS into typical (type 1) and atypical (type 2 and 3) groups, based on the histological patterns of subfield neuronal loss and gliosis. In addition, granule cell reorganization and alterations of interneuronal populations, neuropeptide fibre networks and mossy fibre sprouting are distinctive features of HS associated with epilepsies; they can be useful diagnostic aids to discriminate from other causes of HS, as well as highlighting potential mechanisms of hippocampal epileptogenesis. The cause of HS remains elusive and may be multifactorial; the contribution of febrile seizures, genetic susceptibility, inflammatory and neurodevelopmental factors are discussed. Post-mortem based research in HS, as an addition to studies on surgical samples, has the added advantage of enabling the study of the wider network changes associated with HS, the long-term effects of epilepsy on the pathology and associated comorbidities. It is likely that HS is heterogeneous in aspects of its cause, epileptogenetic mechanisms, network alterations and response to medical and surgical treatments. Future neuropathological studies will contribute to better recognition and understanding of these clinical and patho-aetiological subtypes of HS.

Inhibitory control of the cough reflex by galanin receptors in the caudal nucleus tractus solitarii of the rabbit

The caudal nucleus tractus solitarii (NTS) is the main central station of cough-related afferents and a strategic site for the modulation of the cough reflex. The similarities between the characteristics of central processing of nociceptive and cough-related inputs led us to hypothesize that galanin, a neuropeptide implicated in the control of pain, could also be involved in the regulation of the cough reflex at the level of the NTS, where galanin receptors have been found. We investigated the effects of galanin and galnon, a nonpeptide agonist at galanin receptors, on cough responses to mechanical and chemical (citric acid) stimulation of the tracheobronchial tree. Drugs were microinjected (30–50 nl) into the caudal NTS of pentobarbital sodium-anesthetized, spontaneously breathing rabbits. Galnon antitussive effects on cough responses to the mechanical stimulation of the airway mucosa via a custom-built device were also investigated. Bilateral microinjections of 1 mM galanin markedly decreased cough number, peak abdominal activity, and increased cough-related total cycle duration. Bilateral microinjections of 1 mM galnon induced mild depressant effects on cough, whereas bilateral microinjections of 10 mM galnon caused marked antitussive effects consistent with those produced by galanin. Galnon effects were confirmed by using the cough-inducing device. The results indicate that galanin receptors play a role in the inhibitory control of the cough reflex at the level of the caudal NTS and provide hints for the development of novel antitussive strategies.

A Molecular Approach to Epilepsy Management: from Current Therapeutic Methods to Preconditioning Efforts

Epilepsy is the most common and chronic neurological disorder characterized by recurrent unprovoked seizures. The key aim in treating patients with epilepsy is the suppression of seizures. An understanding of focal changes that are involved in epileptogenesis may therefore provide novel approaches for optimal treatment of the seizure. Although the actual pathogenesis of epilepsy is still uncertain, recently growing lines of evidence declare that microglia and astrocyte activation, oxidative stress and reactive oxygen species (ROS) production, mitochondria dysfunction, and damage of blood-brain barrier (BBB) are involved in its pathogenesis. Impaired GABAergic function in the brain is probably the most accepted hypothesis regarding the pathogenesis of epilepsy. Clinical neuroimaging of patients and experimental modeling have demonstrated that seizures may induce neuronal apoptosis. Apoptosis signaling pathways are involved in the pathogenesis of several types of epilepsy such as temporal lobe epilepsy (TLE). The quality of life of patients is seriously affected by treatment-related problems and also by unpredictability of epileptic seizures. Moreover, the available antiepileptic drugs (AED) are not significantly effective to prevent epileptogenesis. Thus, novel therapies that are proficient to control seizure in people who are suffering from epilepsy are needed. The preconditioning method promises to serve as an alternative therapeutic approach because this strategy has demonstrated the capability to curtail epileptogenesis. For this reason, understanding of molecular mechanisms underlying brain tolerance induced by preconditioning is crucial to delineate new neuroprotective ways against seizure damage and epileptogenesis. In this review, we summarize the work to date on the pathogenesis of epilepsy and discuss recent therapeutic strategies in the treatment of epilepsy. We will highlight that novel therapy targeting such as preconditioning process holds great promise. In addition, we will also highlight the role of gene reprogramming and mitochondrial biogenesis in the preconditioning-mediated neuroprotective events.

Ghrelin, neuropeptide Y, and other feeding-regulatory peptides active in the hippocampus: role in learning and memory

The hippocampus is a brain region of primary importance for neurogenesis, which occurs during early developmental states as well as during adulthood. Increases in neuronal proliferation and in neuronal death with age have been associated with drastic changes in memory and learning. Numerous neurotransmitters are involved in these processes, and some neuropeptides that mediate neurogenesis also modulate feeding behavior. Concomitantly, feeding peptides, which act primarily in the hypothalamus, are also present in the hippocampus. This review aims to ascertain the role of several important feeding peptides in cognitive functions, either through their local synthesis in the hippocampus or through their actions via specific receptors in the hippocampus. A link between neurogenesis and the orexigenic or anorexigenic properties of feeding peptides is discussed.

Galanin receptors possibly modulate the obesity-induced change in pain threshold

Pain threshold may be up-regulated or down-regulated according to gender, age, race/ethnic and psychological state. Previous studies indicated that obesity may change pain threshold, both nociceptive and antinociceptive, which resulted from obesity-reduced variation of neuroendocrine. However there is a limited understanding of its molecular mechanism underlying this variation. A lot of evidence supports that galanin increases food intake and body weight to induce obesity in animals. This peptide may also modulate nociceptive susceptibility via central galanin receptor 1 and peripheral galanin receptor 2 in dorsal root ganglion. Whereas injury and obesity may up-regulate the galanin expression and stimulate its secretion to elevate the plasma levels of subjects. Pain may increase the risk of obesity through reduced physical activity. In this review, we highlighted the multiple bilateral interrelation between obesity and pain sensitivity, between galanin and obesity and between galanin and injure-induced pain. In view of the above, we reasoned that galanin receptors possibly participated in the modulation of the obesity-induced change in pain threshold, which need further direct evidence to support as yet. This review is helpful to explore the mechanism that galanin receptors regulate the obesity-induced change of pain sensitivity and to contribute to our understanding of the relation among galanin, obesity and pain threshold.

Exercise offers anxiolytic potential: a role for stress and brain noradrenergic-galaninergic mechanisms

Although physical activity reduces anxiety in humans, the neural basis for this response is unclear. Rodent models are essential to understand the mechanisms that underlie the benefits of exercise. However, it is controversial whether exercise exerts anxiolytic-like potential in rodents. Evidence is reviewed to evaluate the effects of wheel running, an experimental mode of exercise in rodents, on behavior in tests of anxiety and on norepinephrine and galanin systems in neural circuits that regulate stress. Stress is proposed to account for mixed behavioral findings in this literature. Indeed, running promotes an adaptive response to stress and alters anxiety-like behaviors in a manner dependent on stress. Running amplifies galanin expression in noradrenergic locus coeruleus (LC) and suppresses stress-induced activity of the LC and norepinephrine output in LC-target regions. Thus, enhanced galanin-mediated suppression of brain norepinephrine in runners is supported by current literature as a mechanism that may contribute to the stress-protective effects of exercise. These data support the use of rodents to study the emotional and neurobiological consequences of exercise.

Receptor subtype-dependent galanin actions on gamma-aminobutyric acidergic neurotransmission and ethanol responses in the central amygdala

The neuropeptide galanin and its three receptor subtypes (GalR1-3) are expressed in the central amygdala (CeA), a brain region involved in stress- and anxiety-related behaviors, as well as alcohol dependence. Galanin also has been suggested to play a role in alcohol intake and alcohol dependence. We examined the effects of galanin in CeA slices from wild-type and knockout (KO) mice deficient of GalR2 and both GalR1 and GalR2 receptors. Galanin had dual effects on gamma-aminobutyric acid (GABA)-ergic transmission, decreasing the amplitudes of pharmacologically isolated GABAergic inhibitory postsynaptic potentials (IPSPs) in over half of CeA neurons but augmenting IPSPs in the others. The increase in IPSP size was absent after superfusion of the GalR3 antagonist SNAP 37889, whereas the IPSP depression was absent in CeA neurons of GalR1 × GalR2 double KO and GalR2 KO mice. Paired-pulse facilitation studies showed weak or infrequent effects of galanin on GABA release. Thus, galanin may act postsynaptically through GalR3 to augment GABAergic transmission in some CeA neurons, whereas GalR2 receptors likely are involved in the depression of IPSPs. Co-superfusion of ethanol, which augments IPSPs presynaptically, together with galanin caused summated effects of ethanol and galanin in those CeA neurons showing galanin-augmented IPSPs, suggesting the two agents act via different mechanisms in this population. However, in neurons showing IPSP-diminishing galanin effects, galanin blunted the ethanol effects, suggesting a preemptive effect of galanin. These findings may increase understanding of the complex cellular mechanisms that underlie the anxiety-related behavioral effects of galanin and ethanol in CeA.

Central nervous system regulation of food intake and energy expenditure: role of galanin-mediated feeding behavior

Galanin is a neuropeptide widely expressed in the brain. It is implicated in energy expenditure, feeding, and the regulation of body weight. Numerous studies have revealed that galanin regulates food intake via galanin receptors, 5-HT(1A) receptor and adrenergic α-2 receptor. In this review, we summarize recent findings that reveal the essential role of galanin in increasing food intake as well as body weight and that identify the individual galanin receptor subtypes involved in the brain's modulation of food intake and energy expenditure, to provide a theoretical basis for further studies of different aspects of galanin action.

Current prospects and challenges for epilepsy gene therapy

This review addresses the state of gene therapy research for the treatment of epilepsy. Preclinical studies have demonstrated the anti-seizure efficacy of viral vector-based gene transfer through the use of a variety of strategies - from modulating classic neurotransmitter systems to targeting or overexpressing of neuropeptide receptors in seizure-specific brain regions. While these studies provide substantive proof of principle for viral vector gene therapy, future studies must address the challenges of vector immunity, cellular specificity and effective global delivery. As these issues are resolved, viral vector gene therapy should significantly impact the treatment of intractable epilepsy.

Galanin and epilepsy

Neuroanatomical localization and physiological properties of galanin suggest that the peptide may be involved in the regulation of seizures. Indeed, administration of galanin receptor agonists into brain areas pertinent to the initiation and propagation of epileptic activity attenuated seizure responses under conditions of animal models of epilepsy; pharmacological blocking of galanin receptors exerted proconvulsant effects. Functional deletion of both galanin and galanin type 1 receptor genes produced transgenic mice with either spontaneous seizure phenotype, or with enhanced susceptibility to seizure stimuli. At the same time, overexpression of galanin in seizure pathways, using both transgenic and virus vector transfection techniques, hindered the epileptic process. Galanin exerts anticonvulsant effects through both type 1 and type 2 receptors, with distinct downstream signaling cascades. Several synthetic agonists of galanin receptors with optimized bioavailability have been synthesized and inhibited experimental seizures upon systemic administration, thus opening an opportunity for the development of galanin-based antiepileptic drugs.

Regulation of the galanin system by repeated electroconvulsive seizures in mice

Even though induction of seizures by electroconvulsive stimulation (ECS) is a treatment widely used for major depression in humans, the working mechanism of ECS remains uncertain. The antiepileptic effect of ECS has been suggested to be involved in mediating the therapeutic effect of ECS. The neuropeptide galanin exerts antiepileptic and antidepressant-like effects and has also been implicated in the pathophysiology of depression. To explore a potential role of galanin in working mechanisms of ECS, the present study examined effects of repeated ECS on the galanin system using QRT-PCR, in situ hybridization, and [(125) I]galanin receptor binding. ECS was administered to adult mice daily for 14 days, and this paradigm was confirmed to exert antidepressant-like effect in the tail suspension test. Prominent increases in galanin gene expression were found in several brain regions involved in regulation of epileptic activity and depression, including the piriform cortex, hippocampal dentate gyrus, and amygdala. Likewise, GalR2 gene expression was up-regulated in both the central and the medial amygdala, whereas GalR1 gene expression showed a modest down-regulation in the medial amygdala. [(125) I]galanin receptor binding in the piriform cortex, hippocampus, and amygdala was found to be significantly down-regulated. These data show that the galanin system is regulated by repeated ECS in a number of brain regions implicated in seizure regulation and depression. These changes may play a role in the therapeutic effect of ECS.

Galanin: a potential role in mesolimbic dopamine-mediated instrumental behavior

The involvement of the neuropeptide galanin in the consumption of the primary "commodities" of food and water is well established. However, the present review describes anatomical and behavioral evidence that suggests that galanin may also modulate ascending mesolimbic dopamine function and thereby play an inhibitory role in the systems by which instrumental behavior is energized toward acquiring primary commodities. General anatomical frameworks for this interaction are presented and future studies that could evaluate it are discussed.

Feeding behaviour in galanin knockout mice supports a role of galanin in fat intake and preference

It has been widely suggested that saturated fat consumption has fuelled the current obesity epidemic. Macronutrient choices appear to be important not only as potential factors influencing obesity, but also independently as risk factors for diabetes, cardiovascular disease and cancer. The neuropeptide galanin has previously been implicated in the regulation of fat intake, although its precise role has been contested. The present study investigated mice with targeted knockout of the galanin gene (GKO). We demonstrate that, when only a high fat diet (HFD) was available, wild-type (WT) animals consumed significantly more energy than the GKO mice (89.85 +/- 4.57 kJ/day versus 76.84 +/- 3.55 kJ/day, P < 0.001, n = 17 versus 15). Consistent with this, WT animals gained more body weight when fed the HFD than GKO animals (3.48 +/- 0.44 g versus 2.02 +/- 0.62 g, P < 0.001, n = 17 versus 15). In a macronutrient choice scenario, WT mice ate almost three-fold more fat than GKO animals (0.63 +/- 0.02 g versus 0.23 +/- 0.01 g, P < 0.001, n = 18 versus 24). Chronic administration of galanin by mini-osmotic pumps into the lateral ventricle of GKO animals partially reversed the fat avoidance phenotype. Fat intake was significantly lower in the phosphate-buffered saline-treated GKO group compared to galanin-treated GKO animals (0.32 +/- 0.01 g versus 0.38 +/- 0.01 g, P < 0.005, n = 17 versus 17). These data are compatible with the hypothesis that galanin specifically regulates fat intake, and implies that an antagonist to one or more of the galanin receptor subtype(s) may be of use in the treatment of some forms of obesity.

Anxiolytic-like activity of the non-selective galanin receptor agonist, galnon

Galanin's influence on monoaminergic neurotransmission, together with its discrete CNS distribution in corticolimbic brain areas, points to a potential role for this neuropeptide in mediating anxiety- and depression-like responses. To evaluate this hypothesis, the non-selective galanin receptor agonist, galnon, was tested in multiple preclinical models of anxiolytic- and antidepressive-like activity. Acute administration of galnon (0.03-1mg/kg, i.p.) dose-dependently increased punished crossings in the four plate test, with magnitude similar to the effects of the endogenous ligand, galanin (0.1-1.0 microg, i.c.v.). Moreover, the effects of galnon and galanin were blocked by central administration of the non-selective galanin receptor antagonist, M35 (10 microg, i.c.v.). Interestingly, the benzodiazepine receptor antagonist, flumazenil (1mg/kg, i.p.), reversed galnon's effect in the four plate test, implicating GABAergic neurotransmission as a potential mechanism underlying this anxiolytic-like response. In the elevated zero maze, galnon (0.3-3.0mg/kg, i.p.) and galanin (0.03-0.3 microg, i.c.v.) increased the time spent in the open arms, while in the stress-induced hyperthermia model, galnon (0.3-30 mg/kg, i.p.) attenuated stress-induced changes in body temperature. Consistent with these anxiolytic-like effects, in vivo microdialysis showed that acute galnon (3mg/kg, i.p.) treatment preferentially elevated levels of GABA in the rat amygdala, a brain area linked to fear and anxiety behaviors. In contrast to the effects in anxiety models, neither galnon (1-5.6 mg/kg, i.p.) nor galanin (0.3-3.0 microg, i.c.v.) demonstrated antidepressant-like effects in the mouse tail suspension test. Galnon (1-10mg/kg, i.p.) also failed to reduce immobility time in the rat forced swim test. In vitro, galnon and galanin showed affinity for human galanin receptors expressed in Bowes melanoma cells (K(i)=5.5 microM and 0.2 nM, respectively). Galanin displayed high affinity and functional potency for membranes expressing rat GALR1 receptors (K(i)=0.85 nM; EC(50)=0.6 nM), while galnon (10 microM) failed to displace radiolabeled galanin or inhibit cAMP production in the same GALR1 cell line. Galnon (10 microM) showed affinity for NPY1, NK2, M5, and somatostatin receptors but no affinity for galanin receptors expressed in rat hippocampal membranes. Taken together, the present series of studies demonstrate novel effects of galnon in various preclinical models of anxiety and highlight the galaninergic system as a novel therapeutic target for the treatment of anxiety-related disorders. Moreover, these data indicate rodent GALR1 receptors do not mediate galnon's in vivo activity.

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.

Activation of the galanin receptor 2 (GalR2) protects the hippocampus from neuronal damage

Expression of the neuropeptide galanin is up-regulated in many brain regions following nerve injury and in the basal forebrain of patients with Alzheimer's disease. We have previously demonstrated that galanin modulates hippocampal neuronal survival, although it was unclear which receptor subtype(s) mediates this effect. Here we report that the protective role played by galanin in hippocampal cultures is abolished in animals carrying a loss-of-function mutation in the second galanin receptor subtype (GalR2-MUT). Exogenous galanin stimulates the phosphorylation of the serine/threonine kinase Akt and extracellular signal-regulated kinase (ERK) in wild-type (WT) cultures by 435 +/- 5% and 278 +/- 2%, respectively. The glutamate-induced activation of Akt was abolished in cultures from galanin knockout animals, and was markedly attenuated in GalR2-MUT animals, compared with WT controls. In contrast, similar levels of glutamate-induced ERK activation were observed in both loss-of-function mutants, but were further increased in galanin over-expressing animals. Using specific inhibitors of either ERK or Akt confirms that a GalR2-dependent modulation in the activation of the Akt and ERK signalling pathways contributes to the protective effects of galanin. These findings imply that the rise in endogenous galanin observed either after brain injury or in various disease states is an adaptive response that reduces apoptosis by the activation of GalR2, and hence Akt and ERK.

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.

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 and galanin-like peptide differentially modulate neuronal activities in rat arcuate nucleus neurons

Neuropeptides galanin and galanin-like peptide (GALP) share similar amino acid sequence and presumably interact with the same group of receptors, but they differentially regulate a variety of physiological and pathophysiological processes including metabolism and reproduction. Here we explored the neurophysiological basis of the in vivo differential effect between galanin and GALP by examining galanin and GALP modulation of neuronal activities of neurons in the arcuate nucleus (Arc), a brain region critically involved in energy homeostasis and reproductive function. We demonstrated that galanin and GALP inhibited excitatory and inhibitory postsynaptic currents in a similar way. In contrast, galanin and GALP differentially affected the intrinsic membrane property. In most recorded Arc neurons, galanin perfusion induced significant hyperpolarization of the resting membrane potential, which was not affected by GALP perfusion. In addition, galanin perfusion substantially suppressed the spontaneous spike firing in most Arc neurons, whereas in response to GALP perfusion, about half of the Arc neurons exhibited mild reduction in spontaneous spike firing and the other half showed enhancement. Furthermore, the Arc neurons that had been previously responsive to galanin perfusion no longer responded to galanin if co-applied with GALP, indicating that GALP can physiologically antagonize galanin effect. This differential effect appears to be mediated by G protein within the recorded cell, as the galanin effect on firing rate was abolished when the recorded cell was loaded with GDP-betaS, an agent that blocks G protein activity. Taken together, these differential effects of galanin and GALP may provide a neurophysiological mechanism through which galanin and GALP differentially regulate energy balance, reproductive function, and other physiological processes.

In vivo interaction between serotonin and galanin receptors types 1 and 2 in the dorsal raphe: implication for limbic seizures

The neuropeptide galanin suppresses seizure activity in the hippocampus by inhibiting glutamatergic neurotransmission. Galanin may also modulate limbic seizures through interaction with other neurotransmitters in neuronal populations that project to the hippocampus. We examined the role of galanin receptors types 1 and 2 in the dorsal raphe (DR) in the regulation of serotonergic transmission and limbic seizures. Infusion of a mixed agonist of galanin receptors types 1 and 2 [galanin (1-29)] into the DR augmented the severity of limbic seizures in both rats and wild-type mice and concurrently reduced serotonin concentration in the DR and hippocampus as measured by immunofluorescence or HPLC. In contrast, injection of the galanin receptor type 2 agonist galanin (2-11) mitigated the severity of seizures in both species and increased serotonin concentration in both areas. Injection of both galanin fragments into the DR of galanin receptor type 1 knockout mice exerted anticonvulsant effects. Both the proconvulsant activity of galanin (1-29) and seizure suppression by galanin (2-11) were abolished in serotonin-depleted animals. Our data indicate that, in the DR, galanin receptors types 1 and 2 modulate serotonergic transmission in a negative and a positive fashion, respectively, and that these effects translate into either facilitation or inhibition of limbic seizures.

Galanin and its receptors: introduction to the Third International Symposium, San Diego, California, USA, 21-22 October 2004

The Third Galanin Symposium presented many different and exciting results on galanin research reflecting a major progress since the previous symposium in 1998. A major impression was the many possible relationships of galaninergic mechanisms to important brain functions such as development, cognition and ageing as well as many aspects related to a wide spectrum of diseases, including Alzheimer's disease, anxiety/depression, addiction, obesity, pain and tumour growth. These studies were based on an extensive armament of methodologies including various strains of transgenic mice. Unfortunately, the pharmaceutical industry had only a minor participation. Nevertheless, exciting developments in the generation of agonists and antagonists are emerging, providing hope that we at the next symposium will be able to validitate many of the challenging hypotheses concerning galanin and disease with the help of pharmacological tools.