Multiple interaction sites of galnon trigger its biological effects

Co-released norepinephrine and galanin act on different timescales to promote stress-induced anxiety-like behavior

Both the noradrenergic and galaninergic systems have been implicated in stress-related neuropsychiatric disorders, and these two neuromodulators are co-released from the stress-responsive locus coeruleus (LC); however, the individual contributions of LC-derived norepinephrine (NE) and galanin to behavioral stress responses are unclear. Here we aimed to disentangle the functional roles of co-released NE and galanin in stress-induced behavior. We used foot shock, optogenetics, and behavioral pharmacology in wild-type (WT) mice and mice lacking either NE (Dbh^-/-) or galanin (Gal^cKO-Dbh) specifically in noradrenergic neurons to isolate the roles of these co-transmitters in regulating anxiety-like behavior in the elevated zero maze (EZM) either immediately or 24 h following stress. Foot shock and optogenetic LC stimulation produced immediate anxiety-like behavior in WT mice, and the effects of foot shock persisted for 24 h. NE-deficient mice were resistant to the anxiogenic effects of acute stress and optogenetic LC stimulation, while mice lacking noradrenergic-derived galanin displayed typical increases in anxiety-like behavior. However, when tested 24 h after foot shock, both Dbh^-/- and Gal^cKO-Dbh mice lacked normal expression of anxiety-like behavior. Pharmacological rescue of NE, but not galanin, in knockout mice during EZM testing was anxiogenic. In contrast, restoring galanin, but not NE, signaling during foot shock normalized stress-induced anxiety-like behavior 24 h later. These results indicate that NE and noradrenergic-derived galanin play complementary, but distinguishable roles in behavioral responses to stress. NE is required for the expression of acute stress-induced anxiety, while noradrenergic-derived galanin mediates the development of more persistent responses following a stressor.

Opioid-galanin receptor heteromers mediate the dopaminergic effects of opioids

Identifying non-addictive opioid medications is a high priority in medical sciences, but μ-opioid receptors mediate both the analgesic and addictive effects of opioids. We found a significant pharmacodynamic difference between morphine and methadone that is determined entirely by heteromerization of μ-opioid receptors with galanin Gal1 receptors, rendering a profound decrease in the potency of methadone. This was explained by methadone's weaker proficiency to activate the dopaminergic system as compared to morphine and predicted a dissociation of therapeutic versus euphoric effects of methadone, which was corroborated by a significantly lower incidence of self-report of "high" in methadone-maintained patients. These results suggest that μ-opioid-Gal1 receptor heteromers mediate the dopaminergic effects of opioids that may lead to a lower addictive liability of opioids with selective low potency for the μ-opioid-Gal1 receptor heteromer, exemplified by methadone.

Preclinical evaluation of intravenous NAX 810-2, a novel GalR2-preferring analog, for anticonvulsant efficacy and pharmacokinetics

OBJECTIVE

Potential clinical utility of galanin or peptidic analogs has been hindered by poor metabolic stability, lack of brain penetration, and hyperglycemia due to galanin receptor subtype 1 (GalR1) activation. NAX 810-2, a galanin receptor subtype 2 (GalR2)-preferring galanin analog, possesses 15-fold greater affinity for GalR2 over GalR1 and protects against seizures in the mouse 6 Hz, corneal kindling, and Frings audiogenic seizure models. The purpose of these studies was to further evaluate the preclinical efficacy and pharmacokinetics of NAX 810-2 in mice.

METHODS

NAX 810-2 was administered by intravenous (i.v.; tail vein, bolus) injection to fully kindled (corneal kindling assay) or naive CF-1 mice (6 Hz assay and pharmacokinetic studies). Plasma NAX 810-2 levels were determined from trunk blood samples. NAX 810-2 was also added to human plasma at various concentrations for determination of plasma protein binding.

RESULTS

In the mouse corneal kindling model, NAX 810-2 dose-dependently blocked seizures following intravenous administration (median effective dose [ED₅₀ ], 0.5 mg/kg). In the mouse 6 Hz (32 mA) seizure model, it was demonstrated that NAX 810-2 dose-dependently blocked seizures following bolus administration (0.375-1.5 mg/kg, i.v.; ED₅₀ , 0.7 mg/kg), with a time-to-peak effect of 0.5 h posttreatment. Motor impairment was observed at 1.5 mg/kg, i.v., whereas one-half of this dose, 0.75 mg/kg, i.v., was maximally effective in the 6 Hz test. Plasma levels of NAX 810-2 show linear pharmacokinetics following intravenous administration and a half-life of 1.2 h. Functional agonist activity studies demonstrate that NAX 810-2 effectively activates GalR2 at therapeutic concentrations.

SIGNIFICANCE

These studies further suggest the potential utility of NAX 810-2 as a novel therapy for epilepsy.

In vitro toxicity of the galanin receptor 3 antagonist SNAP 37889

Galanin and its receptors (GAL1, GAL2, GAL3) modulate a range of neuronal, immune and vascular activities. In vivo administration of SNAP 37889 (1-phenyl-3-[[3-(trifluoromethyl)phenyl]imino]-1H-indol-2-one), a potent small non-peptidergic antagonist of GAL3, was reported to reduce anxiety- and depression-related behavior, ethanol consumption, and antagonizes the effect of galanin on plasma extravasation in rodent models. Accordingly, SNAP 37889 has been proposed as a potential therapeutic agent to treat anxiety and depression disorders. Therefore, we evaluated the toxicity of SNAP 37889 to different cell types. Our experiments revealed that SNAP 37889 (≥10μM) induced apoptosis in epithelial (HMCB) and microglial (BV-2) cell lines expressing endogenous GAL3, in peripheral blood mononuclear cells and promyelocytic leukemia cells (HL-60) expressing GAL2, and in a neuronal cell line (SH-SY5Y) lacking galanin receptor expression altogether. In conclusion, SNAP 37889 is toxic to a variety of cell types independent of GAL3 expression. We caution that the clinical use of SNAP 37889 at doses that might be used to treat anxiety- or depression- related diseases could have unexpected non-galanin receptor-mediated toxicity, especially on immune cells.

Regulation of neurological and neuropsychiatric phenotypes by locus coeruleus-derived galanin

Decades of research confirm that noradrenergic locus coeruleus (LC) neurons are essential for arousal, attention, motivation, and stress responses. While most studies on LC transmission focused unsurprisingly on norepinephrine (NE), adrenergic signaling cannot account for all the consequences of LC activation. Galanin coexists with NE in the vast majority of LC neurons, yet the precise function of this neuropeptide has proved to be surprisingly elusive given our solid understanding of the LC system. To elucidate the contribution of galanin to LC physiology, here we briefly summarize the nature of stimuli that drive LC activity from a neuroanatomical perspective. We go on to describe the LC pathways in which galanin most likely exerts its effects on behavior, with a focus on addiction, depression, epilepsy, stress, and Alzheimer׳s disease. We propose a model in which LC-derived galanin has two distinct functions: as a neuromodulator, primarily acting via the galanin 1 receptor (GAL1), and as a trophic factor, primarily acting via galanin receptor 2 (GAL2). Finally, we discuss how the recent advances in neuropeptide detection, optogenetics and chemical genetics, and galanin receptor pharmacology can be harnessed to identify the roles of LC-derived galanin definitively. This article is part of a Special Issue entitled SI: Noradrenergic System.

Galanin regulates blood glucose level in the zebrafish: a morphological and functional study

The present study has demonstrated the galaninergic innervation of the endocrine pancreas including sources of the galaninergic nerve fibers, and the influence of galanin receptor agonists on blood glucose level in the zebrafish. For the first time, a very abundant galaninergic innervation of the endocrine pancreas during development is shown, from the second day post-fertilization to adulthood. The fibers originated from ganglia consisting of galanin-IR, non-adrenergic (non-sensory) neurons located rostrally to the pancreatic tissue. The ganglia were found on the dorsal side of the initial part of the anterior intestinal segment, close to the intestinal branch of the vagus nerve. The galanin-IR neurons did not show immunoreactivity for applied antibodies against tyrosine hydroxylase, choline acetyltransferase, and vesicular acetylcholine transporter. Intraperitoneal injections of galanin analog NAX 5055 resulted in a statistically significant increase in the blood glucose level. Injections of another galanin receptor agonist, galnon, also caused a rise in blood glucose level; however, it was not statistically significant. The present findings suggest that, like in mammals, in the zebrafish galanin is involved in the regulation of blood glucose level. However, further studies are needed to elucidate the exact mechanism of the galanin action.

The galanin receptor agonist, galnon, attenuates cocaine-induced reinstatement and dopamine overflow in the frontal cortex

Relapse represents one of the most significant problems in the long-term treatment of drug addiction. Cocaine blocks plasma membrane monoamine transporters and increases dopamine (DA) overflow in the brain, and DA is critical for the motivational and primary reinforcing effects of the drug as well as cocaine-primed reinstatement of cocaine seeking in rats, a model of relapse. Thus, modulators of the DA system may be effective for the treatment of cocaine dependence. The endogenous neuropeptide galanin inhibits DA transmission, and both galanin and the synthetic galanin receptor agonist, galnon, interfere with some rewarding properties of cocaine. The purpose of this study was to further assess the effects of galnon on cocaine-induced behaviors and neurochemistry in rats. We found that galnon attenuated cocaine-induced motor activity, reinstatement and DA overflow in the frontal cortex at a dose that did not reduce baseline motor activity, stable self-administration of cocaine, baseline extracellular DA levels or cocaine-induced DA overflow in the nucleus accumbens (NAc). Similar to cocaine, galnon had no effect on stable food self-administration but reduced food-primed reinstatement. These results indicate that galnon can diminish cocaine-induced hyperactivity and relapse-like behavior, possibly in part by modulating DA transmission in the frontal cortex.

Physiology, signaling, and pharmacology of galanin peptides and receptors: three decades of emerging diversity

Galanin was first identified 30 years ago as a "classic neuropeptide," with actions primarily as a modulator of neurotransmission in the brain and peripheral nervous system. Other structurally-related peptides-galanin-like peptide and alarin-with diverse biologic actions in brain and other tissues have since been identified, although, unlike galanin, their cognate receptors are currently unknown. Over the last two decades, in addition to many neuronal actions, a number of nonneuronal actions of galanin and other galanin family peptides have been described. These include actions associated with neural stem cells, nonneuronal cells in the brain such as glia, endocrine functions, effects on metabolism, energy homeostasis, and paracrine effects in bone. Substantial new data also indicate an emerging role for galanin in innate immunity, inflammation, and cancer. Galanin has been shown to regulate its numerous physiologic and pathophysiological processes through interactions with three G protein-coupled receptors, GAL1, GAL2, and GAL3, and signaling via multiple transduction pathways, including inhibition of cAMP/PKA (GAL1, GAL3) and stimulation of phospholipase C (GAL2). In this review, we emphasize the importance of novel galanin receptor-specific agonists and antagonists. Also, other approaches, including new transgenic mouse lines (such as a recently characterized GAL3 knockout mouse) represent, in combination with viral-based techniques, critical tools required to better evaluate galanin system physiology. These in turn will help identify potential targets of the galanin/galanin-receptor systems in a diverse range of human diseases, including pain, mood disorders, epilepsy, neurodegenerative conditions, diabetes, and cancer.

Regulation of glucose and insulin release following acute and repeated treatment with the synthetic galanin analog NAX-5055

The neuropeptide galanin is widely expressed in both the central and peripheral nervous systems. However there is limited understanding of how individual galanin receptor (GalR1, 2, and 3) subtypes mediate the physiological activity of galanin in vivo. To address this issue we utilized NAX-5055, a systemically available, metabolically stable galanin analog. NAX-5055 displays a preference for GalR1 receptors and possesses potent anticonvulsant activity in vivo, suggesting that NAX-5055 engages central galanin receptors. To determine if NAX-5055 also modulates the activity of peripheral galanin receptors, we evaluated the effect of NAX-5055 on blood glucose and insulin levels in mice. Acute and repeated (once daily for four days) systemic administration of NAX-5055 (4 mg/kg) significantly increased blood glucose levels compared to vehicle treated mice. However, a hyperglycemic response was not observed following systemic administration of NAX-805-1, a scrambled analog of NAX-5055, with critical receptor binding residues, Trp(2) and Tyr(9), reversed. These results suggest that chemical modifications independent of the galanin backbone of NAX-5055 are not responsible for the hyperglycemic response. The effect of NAX-5055 on glucose homeostasis was further evaluated with a glucose tolerance test (GTT). Mice administered either acute or repeated (once daily for four days) injections of NAX-5055 (4 mg/kg) displayed impaired glucose handling and reduced insulin response to an acute glucose (1g/kg) challenge. Here we have shown that systemic administration of a centrally active GalR1-preferring galanin analog produces acute hyperglycemia and an inhibition of insulin release in vivo and that these effects are not attenuated with repeated administration. NAX-5055 thus provides a new pharmacological tool to further the understanding of function of both central and peripheral GalR1 receptors in vivo.

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.

Regulators of synaptic transmission: roles in the pathogenesis and treatment of epilepsy

Synaptic transmission is the communication between a presynaptic and a postsynaptic neuron, and the subsequent processing of the signal. These processes are complex and highly regulated, reflecting their importance in normal brain functioning and homeostasis. Sustaining synaptic transmission depends on the continuing cycle of synaptic vesicle formation, release, and endocytosis, which requires proteins such as dynamin, syndapin, synapsin, and synaptic vesicle protein 2A. Synaptic transmission is regulated by diverse mechanisms, including presynaptic modulators of synaptic vesicle formation and release, postsynaptic receptors and signaling, and modulators of neurotransmission. Neurotransmitters released presynaptically can bind to their postsynaptic receptors, the inhibitory γ-aminobutyric acid (GABA)ergic receptors or the excitatory glutamate receptors. Once released, glutamate activates a variety of postsynaptic receptors including α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), N-methyl-D-aspartate (NMDA), kainate, and metabotropic receptors. The activation of the receptors triggers downstream signaling cascades generating a vast array of effects, which can be modulated by a numerous auxiliary regulatory subunits. Moreover, different neuropeptides such as neuropeptide Y, brain-derived neurotrophic factor (BDNF), somatostatin, ghrelin, and galanin, act as regulators of diverse synaptic functions and along with the classic neurotransmitters. Abnormalities in the regulation of synaptic transmission play a critical role in the pathogenesis of numerous brain diseases, including epilepsy. This review focuses on the different mechanisms involved in the regulation of synaptic transmission, which may play a role in the pathogenesis of epilepsy: the presynaptic modulators of synaptic vesicle formation and release, postsynaptic receptors, and modulators of neurotransmission, including the mechanism by which drugs can modulate the frequency and severity of epileptic seizures.

Galanin receptors and ligands

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

Anticonvulsant neuropeptides as drug leads for neurological diseases

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

Behavioral effects of neuropeptides in rodent models of depression and anxiety

In recent years, studies have advocated neuropeptide systems as modulators for the behavioral states found in mood disorders such as depression and anxiety disorders. Neuropeptides have been tested in traditional animal models and screening procedures that have been validated by known antidepressants and anxiolytics. However, it has become clear that although these tests are very useful, neuropeptides have distinct behavioral effects and dose-dependent characteristics, and therefore, use of these tests with neuropeptides must be done with an understanding of their unique characteristics. This review will focus on the behavioral actions of neuropeptides and their synthetic analogs, particularly in studies utilizing various preclinical tests of depression and anxiety. Specifically, the following neuropeptide systems will be reviewed: corticotropin-releasing factor (CRF), urocortin (Ucn), teneurin C-terminal associated peptide (TCAP), neuropeptide Y (NPY), arginine vasopressin (AVP), oxytocin, the Tyr-MIF-1 family, cholecystokinin (CCK), galanin, and substance P. These neuropeptide systems each have a unique role in the regulation of stress-like behavior, and therefore provide intriguing therapeutic targets for mood disorder treatment.

Galanin, galanin receptor subtypes and depression-like behaviour

The pathophysiology of depression remains unclear, but involves disturbances in brain monoaminergic transmission. Current antidepressant drugs, which act by enhancing this type of neurotransmission, have limited therapeutic efficacy in a number of patients, and also cause serious side-effects, which limits their compliance. Increasing evidence suggests that neuropeptides, including galanin, can be of relevance in mood disorders. Galanin is co-expressed with and modulates noradrenaline and serotonin transmission, both implicated in depression. Pharmacological and genetic studies suggest a role for galanin in depression-like behaviour in rodents, involving specific receptor subtypes. Thus, stimulation of GalR1 and/or GalR3 receptors results in depression-like phenotype, while activation of the GalR2 receptor reduces depression-like behaviour in the rat. These findings suggest that galanin receptor subtypes may represent novel targets for the development of antidepressant drugs.

Galanin protects against behavioral and neurochemical correlates of opiate reward

The mechanisms underlying responses to drugs of abuse have been widely investigated; however, less is known about pathways normally protective against the development of drug reinforcement. These pathways are also important since they may regulate individual differences in vulnerability to addiction. The neuropeptide galanin and its binding sites are expressed in brain areas important for drug reward. Previous studies have shown that centrally infused galanin attenuates morphine place preference and peripheral injection of galnon, a galanin agonist, decreases opiate withdrawal signs. The current studies in galanin knockout (GKO) mice examined the hypothesis that galanin is an endogenous negative regulator of opiate reward and identified downstream signaling pathways regulated by galanin. We show that GKO mice demonstrate increased locomotor activation following morphine administration, which is inhibited by acute administration of galnon. GKO mice also show enhanced morphine place preference, supporting the idea that galanin normally antagonizes opiate reward. In addition, morphine-induced ERK1/2 phosphorylation was increased in the VTA of both wild-type and GKO mice, but only the GKO mice showed increases in ERK1/2 and CREB phosphorylation in the amygdala or nucleus accumbens. Furthermore, a single systemic injection of galnon in GKO mice was sufficient to reverse some of the biochemical changes brought about by morphine administration. These data suggest that galanin normally attenuates behavioral and neurochemical effects of opiates; thus, galanin agonists may represent a new class of therapeutic targets for opiate addiction.

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.

Galanin receptor antagonists : a potential novel pharmacological treatment for mood disorders

The pathophysiology of mood disorders involves several genetic and social predisposing factors, as well as a dysregulated response to chronic stress. Accumulated evidence during the last two decades has implicated disturbances in brain serotonin and/or noradrenaline (norepinephrine) neurotransmission in the aetiology of depression. In fact, current pharmacological treatment for mood disorders is based on the use of drugs that act mainly by enhancing brain serotonin and noradrenaline neurotransmission by blockade of the active reuptake mechanism for these neurotransmitters. However, current antidepressant drugs have a delayed onset of therapeutic action, and a substantial number of patients do not respond adequately to them. In addition, these drugs have a number of adverse effects that limit patient compliance. In view of this, there is an intense search to identify novel (receptor) targets for antidepressant therapy. Recent studies have indicated that several neuropeptides and their receptors are potential candidates for the development of novel antidepressant treatment. In this context, galanin is of particular interest, since it is co-localised with serotonin in the dorsal raphe nucleus and with noradrenaline in the locus coeruleus, nuclei known to play a major role in affective disorders and in the action of antidepressant drugs. The actions of galanin are mediated by three receptor subtypes (GAL1, GAL2 and GAL3), which are coupled to different intracellular effector systems. Studies in rats have shown that galanin administered intracerebroventricularly is a potent inhibitor of mesencephalic serotonergic neurotransmission, as indicated by a long-lasting reduction in the release of serotonin in the hippocampus. This inhibitory effect is related to activation of the galanin receptors located on the dorsal raphe neurons. Moreover, intracerebroventricular galanin alters the gene expression of serotonin 5-HT1A autoreceptors in the dorsal raphe and also changes their functional activity. In addition, galanin produces a functional blockade of postsynaptic 5-HT1A receptor-mediated responses. Both pharmacological and genetic studies suggest a role for galanin in depression-like behaviour in rodent models. Transgenic mice overexpressing galanin under the control of the platelet-derived growth factor-beta promoter display increased immobility in the forced swim test. Intracerebroventricular administration of galanin in the rat increases depression-like behaviour, and this is fully blocked by the nonselective peptide galanin receptor antagonist M35. Importantly, M35 alone administered intracerebroventricularly produces an antidepressant-like effect. Recently, newly developed receptor-specific nonpeptidergic galanin GAL3 receptor antagonists (SNAP-37889 and SNAP-398299), which cross the blood-brain barrier after systemic administration, have shown antidepressant-like activity in several animal models. On the other hand, stimulation of the GAL2 receptor at the raphe level by local application of the GAL2 receptor agonist galanin (2-11) has been shown to increase serotonin levels in the hippocampus and dorsal raphe. These results indicate an important (mainly inhibitory) role of galanin as a regulator of brain serotonin and 5-HT1A receptor-mediated transmission, which may be of potential importance for understanding mood disorders and for the development of antidepressant drugs. Taken together, the present evidence suggests that antidepressant efficacy may be associated with compounds acting as antagonists at the GAL3 and/or possibly GAL1 receptors, and/or agonists at the GAL2 receptor.

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 as a modulator of anxiety and depression and a therapeutic target for affective disease

Galanin is a 29 amino-acid (30 in humans) neuropeptide with a close functional relationship with neurotransmitter systems implicated in the pathophysiology and treatment of depression and anxiety disorders. In rodent models of depression-related behavior, treatment with galanin or compounds with agonist actions at galanin receptors has been shown to affect depression-related behaviors and the behavioral and neurochemical effects of antidepressants. Treatment with clinically efficacious antidepressants alters galanin and galanin receptor gene expression in rodents. Rodent anxiety-like behaviors appear to be modulated by galanin in a complex manner, with studies showing either increases, decreases and no effects of galanin treatments and galanin mutations on anxiety-like behavior in various tasks. One concept to emerge from this literature is that galanin recruitment during extreme behavioral and physiological provocations such as stress and opiate withdrawal may serve to attenuate negative emotional states caused by noradrenergic hyperactivation. The specific galanin receptor subtypes mediating the anxiety- and depression-related effects of galanin remains to be determined, with evidence supporting a possible contribution of GalR1, GalR2 and GalR3. While our understanding of the role of galanin as a modulator of emotion remains at an early stage, recent progress in this rapidly evolving field raise possibility of that galanin may represent a target for the development of novel antidepressant and anxiolytic drug treatments.