GalR2-positive allosteric modulator exhibits anticonvulsant effects in animal models

Insights into Potential Targets for Therapeutic Intervention in Epilepsy

Epilepsy is a chronic brain disease that affects approximately 65 million people worldwide. However, despite the continuous development of antiepileptic drugs, over 30% patients with epilepsy progress to drug-resistant epilepsy. For this reason, it is a high priority objective in preclinical research to find novel therapeutic targets and to develop effective drugs that prevent or reverse the molecular mechanisms underlying epilepsy progression. Among these potential therapeutic targets, we highlight currently available information involving signaling pathways (Wnt/β-catenin, Mammalian Target of Rapamycin (mTOR) signaling and zinc signaling), enzymes (carbonic anhydrase), proteins (erythropoietin, copine 6 and complement system), channels (Transient Receptor Potential Vanilloid Type 1 (TRPV1) channel) and receptors (galanin and melatonin receptors). All of them have demonstrated a certain degree of efficacy not only in controlling seizures but also in displaying neuroprotective activity and in modifying the progression of epilepsy. Although some research with these specific targets has been done in relation with epilepsy, they have not been fully explored as potential therapeutic targets that could help address the unsolved issue of drug-resistant epilepsy and develop new antiseizure therapies for the treatment of epilepsy.

Neuropeptide and Small Transmitter Coexistence: Fundamental Studies and Relevance to Mental Illness

Neuropeptides are auxiliary messenger molecules that always co-exist in nerve cells with one or more small molecule (classic) neurotransmitters. Neuropeptides act both as transmitters and trophic factors, and play a role particularly when the nervous system is challenged, as by injury, pain or stress. Here neuropeptides and coexistence in mammals are reviewed, but with special focus on the 29/30 amino acid galanin and its three receptors GalR1, -R2 and -R3. In particular, galanin's role as a co-transmitter in both rodent and human noradrenergic locus coeruleus (LC) neurons is addressed. Extensive experimental animal data strongly suggest a role for the galanin system in depression-like behavior. The translational potential of these results was tested by studying the galanin system in postmortem human brains, first in normal brains, and then in a comparison of five regions of brains obtained from depressed people who committed suicide, and from matched controls. The distribution of galanin and the four galanin system transcripts in the normal human brain was determined, and selective and parallel changes in levels of transcripts and DNA methylation for galanin and its three receptors were assessed in depressed patients who committed suicide: upregulation of transcripts, e.g., for galanin and GalR3 in LC, paralleled by a decrease in DNA methylation, suggesting involvement of epigenetic mechanisms. It is hypothesized that, when exposed to severe stress, the noradrenergic LC neurons fire in bursts and release galanin from their soma/dendrites. Galanin then acts on somato-dendritic, inhibitory galanin autoreceptors, opening potassium channels and inhibiting firing. The purpose of these autoreceptors is to act as a 'brake' to prevent overexcitation, a brake that is also part of resilience to stress that protects against depression. Depression then arises when the inhibition is too strong and long lasting - a maladaption, allostatic load, leading to depletion of NA levels in the forebrain. It is suggested that disinhibition by a galanin antagonist may have antidepressant activity by restoring forebrain NA levels. A role of galanin in depression is also supported by a recent candidate gene study, showing that variants in genes for galanin and its three receptors confer increased risk of depression and anxiety in people who experienced childhood adversity or recent negative life events. In summary, galanin, a neuropeptide coexisting in LC neurons, may participate in the mechanism underlying resilience against a serious and common disorder, MDD. Existing and further results may lead to an increased understanding of how this illness develops, which in turn could provide a basis for its treatment.

The galanin and galanin receptor subtypes, its regulatory role in the biological and pathological functions

The multitalented neuropeptide galanin was first discovered 30 years ago but initially no biologic activity was found. Further research studies discovered the presence of galanin in the brain and some peripheral tissues, and galanin was identified as a modulator of neurotransmission in the central and peripheral nervous system. Over the last decade there were performed very intensive studies of the neuronal actions and also of nonneuronal actions of galanin. Other galanin family peptides have been described, namely galanin, galanin-like peptide, galanin-message associated peptide and alarin. The effect of these peptides is mediated through three galanin receptors subtypes, GalR1, GalR2 and GalR3 belonging to G protein coupled receptors, and signaling via multiple transduction pathways, including inhibition of cyclic AMP/protein kinase A (GalR1, GalR3) and stimulation of phospholipase C (GalR2). This also explains why one specific molecule of galanin can be responsible for different roles in different tissues. The present review summarizes the information currently available on the relationship between the galaninergic system and known pathological states. The research of novel galanin receptor specific agonists and antagonists is also very promising for its future role in pharmacological treatment. The galaninergic system is important target for current and future biomedical research.

Pharmacological stimulation of GAL1R but not GAL2R attenuates kainic acid-induced neuronal cell death in the rat hippocampus

The neuropeptide galanin is widely distributed in the central and peripheral nervous systems and part of a bigger family of bioactive peptides. Galanin exerts its biological activity through three G-protein coupled receptor subtypes, GAL1-3R. Throughout the last 20years, data has accumulated that galanin can have a neuroprotective effect presumably mediated through the activation of GAL1R and GAL2R. In order to test the pharmaceutical potential of galanin receptor subtype selective ligands to inhibit excitotoxic cell death, the GAL1R selective ligand M617 and the GAL2R selective ligand M1145 were compared to the novel GAL1/2R ligand M1154, in their ability to reduce the excitotoxic effects of intracerebroventricular injected kainate acid in rats. The peptide ligands were evaluated in vitro for their binding preference in a competitive (125)I-galanin receptor subtype binding assay, and G-protein signaling was evaluated using both classical signaling and a label-free real-time technique. Even though there was no significant difference in the time course or severity of the kainic acid induced epileptic behavior in vivo, administration of either M617 or M1154 before kainic acid administration significantly attenuated the neuronal cell death in the hippocampus. Our results indicate the potential therapeutic value of agonists selective for GAL1R in the prevention of neuronal cell death.

Homology modeling, docking, and molecular dynamics simulation of the receptor GALR2 and its interactions with galanin and a positive allosteric modulator

Galanin receptor type 2 (GALR2) is a class A G-protein-coupled receptor (GPCR), and it has been reported that orthosteric ligands and positive allosteric modulators (PAMs) of GALR2 could potentially be used to treat epilepsy. So far, the X-ray structure of this receptor has not been resolved, and knowledge of the 3D structure of GALR2 may prove informative in attempts to design novel ligands and to explore the mechanism for the allosteric modulation of this receptor. In this study, homology modeling was used to obtain several GALR2 models using known templates. ProSA-web Z-scores and Ramachandran plots as well as pre-screening against a test dataset of known compounds were all utilized to select the best model of GALR2. Molecular dockings of galanin (a peptide) and a nonpeptide ligand were carried out to choose the (GALR2 model)-galanin complex that showed the closest agreement with the corresponding experimental data. Finally, a 50-ns MD simulation was performed to study the interactions between the GALR2 model and the synthetic and endogenous ligands. The results from docking and MD simulation showed that, besides the reported residues, Tyr160(4.60), Ile105(3.32), Ala274(7.35), and Tyr163(ECL2) also appear to play important roles in the binding of galanin. The potential allosteric binding pockets in the GALR2 model were then investigated via MD simulation. The results indicated that the mechanism for the allosteric modulation caused by PAMs is the binding of the PAM at pocket III, which is formed by galanin, ECL2, TM2, TM3, and ECL1; this results in the disruption of the Na(+)-binding site and/or the Na(+) ion pathway, leading to GALR2 agonism.

Galanin receptors as a potential target for neurological disease

INTRODUCTION

Galanin is a 29/30 amino acid long neuropeptide that is widely expressed in the brains of many mammals. Galanin exerts its biological activities through three different G protein-coupled receptors, GalR1, GalR2 and GalR3. The widespread distribution of galanin and its receptors in the CNS and the various physiological and pharmacological effects of galanin make the galanin receptors attractive drug targets.

AREAS COVERED

This review provides an overview of the role of galanin and its receptors in the CNS, the involvement of the galaninergic system in various neurological diseases and the development of new galanin receptor-specific ligands.

EXPERT OPINION

Recent advances and novel approaches in migrating the directions of subtype-selective ligand development and chemical modifications of the peptide backbone highlight the importance of the galanin neurochemical system as a potential target for drug development.

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.

Galanin modulates human and murine neutrophil activation in vitro

AIMS

Polymorphonuclear neutrophils are key players in innate immunity. The innate immune system needs to be tightly controlled to ensure proper activation but also no overactivation. Galanin has been shown to regulate inflammatory reactions, and therefore, we aimed to elucidate the expression of galanin and its three receptors (GAL1 -GAL3 ) in polymorphonuclear neutrophils and to evaluate whether galanin exerts direct or indirect effects on human and murine polymorphonuclear neutrophils.

METHODS

Human peripheral polymorphonuclear neutrophils were isolated from fresh blood of healthy donors, and murine polymorphonuclear neutrophils were isolated from bone marrow of C57BL/6N mice. Gene expression was evaluated by qRT-PCR. As a marker for polymorphonuclear neutrophil activation, CD11b integrin surface expression was measured by FACS analysis. Furthermore, a label-free technology measuring ligand-induced dynamic mass redistribution was used to evaluate the response of polymorphonuclear neutrophils to galanin.

RESULTS

GAL2 receptor expression was found in both human and murine polymorphonuclear neutrophils, galanin and GAL3 receptor were exclusively expressed in murine bone marrow polymorphonuclear neutrophils, and GAL1 receptor was not detectable in polymorphonuclear neutrophils of either species. Galanin treatment was not able to induce CD11b integrin surface expression or dynamic mass redistribution in human polymorphonuclear neutrophils and murine bone marrow polymorphonuclear neutrophils. However, galanin treatment significantly enhanced the response of polymorphonuclear neutrophils of both species to interleukin-8.

CONCLUSION

Galanin can be regarded as an immunomodulatory peptide as it can sensitize polymorphonuclear neutrophils towards pro-inflammatory cytokines in humans and mice.

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.

Efficacy and tolerability of the galanin analog NAX 5055 in the multiple-hit rat model of symptomatic infantile spasms

Infantile spasms are seizures manifesting in infantile epileptic encephalopathies that are associated with poor epilepsy and cognitive outcomes. The current therapies are not always effective or are associated with serious side effects. Early cessation of spasms has been proposed to improve long-term outcomes. To identify new therapies for infantile spasms with rapid suppression of spasms, we are using the multiple-hit rat model of infantile spasms, which is a model of refractory infantile spasms. Here, we are testing the efficacy and tolerability of a single dose of the galanin receptor 1 preferring analog, NAX 5055, in the multiple-hit model of spasms. To induce the model, postnatal day 3 (PN3) male Sprague-Dawley rats underwent right intracerebral infusions of doxorubicin and lipopolysaccharide; p-chlorophenylalanine was then injected intraperitoneally (i.p.) at PN5. After the onset of spasms at PN4, 11-14 rats/group were injected i.p. with either NAX 5055 (0.5, 1, 2, or 4mg/kg) or vehicle. Video monitoring for spasms included a 1h pre-injection period, followed by 5h of recording post-injection, and two 2h sessions on PN5. The study was conducted in a randomized, blinded manner. Neurodevelopmental reflexes were assessed daily as well as at 2h after injection. Respiratory function, heart rate, pulse distension, oximetry and blood glucose were measured 4h after injection. The relative expression of GalR1 and GalR2 mRNA over β-actin in the cerebral cortex and hippocampus was determined with real time reverse transcription polymerase chain reaction. There was no acute effect of NAX 5055 on spasm frequency after the single dose of NAX 5055 (n=11-13 rats/group, following exclusions). Neurodevelopmental reflexes, vital signs, blood glucose measured 4h post-injection, and survival were not affected. A reduction in pulse and breath distention of unclear clinical significance was observed with the 7mg/kg NAX 5055 dose. GalR1 mRNA was present in the cerebral cortex and hippocampus of PN4 and adult rats. The hippocampal - but not the cortical - GalR1 mRNA expression was significantly lower in PN4 pups than in adults. GalR1 mRNA was also at least 20 times less abundant in the PN4 cortex than GalR2 mRNA. In conclusion, a single dose of NAX 5055 has no acute efficacy on spasms or toxicity in the multiple hit rat model of medically refractory infantile spasms. Our findings cannot exclude the possibility that repetitive NAX 5055 administration may show efficacy on spasms. The higher expression of GalR2 in the PN4 cortex suggests that GalR2-preferring analogs may be of interest to test for efficacy on spasms.

Positive allosteric modulators to peptide GPCRs: a promising class of drugs

The task of finding selective and stable peptide receptor agonists with low molecular weight, desirable pharmacokinetic properties and penetrable to the blood-brain barrier has proven too difficult for many highly coveted drug targets, including receptors for endothelin, vasoactive intestinal peptide and galanin. These receptors and ligand-gated ion channels activated by structurally simple agonists such as glutamate, glycine and GABA present such a narrow chemical space that the design of subtype-selective molecules capable of distinguishing a dozen of glutamate and GABA receptor subtypes and possessing desirable pharmacokinetic properties has also been problematic. In contrast, the pharmaceutical industry demonstrates a remarkable success in developing 1,4-benzodiazepines, positive allosteric modulators (PMAs) of the GABAA receptor. They were synthesized over 50 years ago and discovered to have anxiolytic potential through an in vivo assay. As exemplified by Librium, Valium and Dormicum, these allosteric ligands of the receptor became the world's first blockbuster drugs. Through molecular manipulation over the past 2 decades, including mutations and knockouts of the endogenous ligands or their receptors, and by in-depth physiological and pharmacological studies, more peptide and glutamate receptors have become well-validated drug targets for which an agonist is sought. In such cases, the pursuit for PAMs has also intensified, and a working paradigm to identify drug candidates that are designed as PAMs has emerged. This review, which focuses on the general principles of finding PAMs of peptide receptors in the 21st century, describes the workflow and some of its resulting compounds such as PAMs of galanin receptor 2 that act as potent anticonvulsant agents.

Neuropeptides and neuropeptide receptors: drug targets, and peptide and non-peptide ligands: a tribute to Prof. Dieter Seebach

The number of neuropeptides and their corresponding receptors has increased steadily over the last fourty years: initially, peptides were isolated from gut or brain (e.g., Substance P, somatostatin), then by targeted mining in specific regions (e.g., cortistatin, orexin in the brain), or by deorphanization of G-protein-coupled receptors (GPCRs; orexin, ghrelin receptors) and through the completion the Human Genome Project. Neuropeptides (and their receptors) have regionally restricted distributions in the central and peripheral nervous system. The neuropeptide signaling is somewhat more distinct spatially than signaling with classical, low-molecular-weight neurotransmitters that are more widely expressed, and, therefore, one assumes that drugs acting at neuropeptide receptors may have more selective pharmacological actions with possibly fewer side effects than drugs acting on glutamatergic, GABAergic, monoaminergic, or cholinergic systems. Neuropeptide receptors, which may have a few or multiple subtypes and splice variants, belong almost exclusively to the GPCR family also known as seven-transmembrane receptors (7TM), a favorite class of drug targets in the pharmaceutical industry. Most neuropeptides are co-stored and co-released with classic neurotransmitters, albeit often only at higher frequencies of stimulation or at bursting activity, thus restricting the neuropeptide signaling to specific circumstances, another reason to assume that neuropeptide drug mimics may have less side effects. Neuropeptides possess a wide spectrum of functions from neurohormone, neurotransmitter to growth factor, but also as key inflammatory mediators. Neuropeptides become 'active' when the nervous system is challenged, e.g., by stress, injury, drug abuse, or neuropsychiatric disorders with genetic, epigenetic, and/or environmental components. The unsuspected number of true neuropeptides and their cognate receptors provides opportunities to identify novel targets for the treatment of both central and peripheral nervous system disorders. Both, receptor subtype-selective antagonists and agonists are being developed, as illustrated by the success of somatostatin agonists, angiotensin, and endothelin antagonists, and the expected clinical applications of NK-1/2/3 (substance P) receptor antagonists, CRF, vasopressin, NPY, neurotensin, orexin antagonists, or neuropeptide receptor modulators; such ligands have efficacy in preclinical or clinical models of pain and neuropsychiatric diseases, such as migraine, chronic/neuropathic pain, anxiety, sleep disorders, depression, and schizophrenia. In addition, both positive and negative allosteric modulators have been described with interesting in vivo activities (e.g., at galanin receptors). The field has become more complex now that an increasing number of heteromeric neuropeptide receptors are described, e.g., ghrelin receptors with 5-HT(2C) or dopamine D(1), D(2) receptors. At long last, structure-based drug discovery can now be envisaged with confidence, since crystal or solution structure of GPCRs and GPCR-ligand complexes, including peptide receptors, are published almost on a monthly basis. Finally, although most compounds acting at peptide receptors are still peptidomimetics, the last decade has seen the emergence of low-molecular-weight nonpeptide ligands (e.g., for orexin, ghrelin, or neurokinin receptors), and surprising progress has been made with β- and γ-peptides as very stable and potent mimetics of, e.g., somatostatin (SRIF), where the native SRIF has a half-life limited to 2-3 min. This last point will be illustrated more specifically, as we have had a long-standing collaboration with Prof. D. Seebach to whom this review is dedicated at the occasion of his 75th birthday.

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.

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.

Synthesis and biological evaluation of novel pyrimidine derivatives as sub-micromolar affinity ligands of GalR2

GalR1 and GalR2 represent unique pharmacological targets for treatment of seizures and epilepsy. A novel series of 2,4,6-triaminopyrimidine derivatives were synthesized and found to have sub-micromolar affinity for GalR2. Optimization of a series of 2,4,6-triaminopyrimidines led to the discovery of several analogs with IC50 values ranging from 0.3 to 1 μM.

Rap1 mediates galanin receptor 2-induced proliferation and survival in squamous cell carcinoma

Previously we showed that galanin, a neuropeptide, is secreted by human squamous cell carcinoma of the head and neck (SCCHN) in which it exhibits an autocrine mitogenic effect. We also showed that rap1, a ras-like signaling protein, is a critical mediator of SCCHN progression. Given the emerging importance of the galanin cascade in regulating proliferation and survival, we investigated the effect of GAL on SCCHN progression via induction of galanin receptor 2 (GALR2)-mediated rap1 activation. Studies were performed in multiple SCCHN cell lines by inducing endogenous GALR2, by stably overexpressing GALR2 and by downregulating endogenous GALR2 with siGALR2. Cell proliferation and survival, mediated by the ERK and AKT signaling cascades, respectively, were evaluated by functional and immunoblot analysis. The role of rap1 in GALR2-mediated proliferation and survival was evaluated by modulating expression. Finally, the effect of GALR2 on tumor growth was determined. GALR2 stimulated proliferation and survival via ERK and AKT activation, respectively. Knockdown or inactivation of rap1 inhibited GALR2-induced, AKT and ERK-mediated survival and proliferation. Overexpression of GALR2 promoted tumor growth in vivo. GALR2 promotes proliferation and survival in vitro, and promotes tumor growth in vivo, consistent with an oncogenic role for GALR2 in SCCHN.

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.

Endogenous galanin protects mouse hippocampal neurons against amyloid toxicity in vitro via activation of galanin receptor-2

Expression of the neuropeptide galanin is known to be upregulated in the brain of patients with Alzheimer's disease (AD). We and others have shown that galanin plays a neuroprotective role in a number of excitotoxic injury paradigms, mediated by activation of the second galanin receptor subtype (GAL2). In the present study, we investigated whether galanin/GAL2 plays a similar protective role against amyloid-β(Aβ) toxicity. Here we report that galanin or the GAL2/3-specific peptide agonist Gal2-11, both equally protect primary dispersed mouse wildtype (WT) neonatal hippocampal neurons from 250 nM Aβ1-42 toxicity in a dose dependent manner. The amount of Aβ1-42 induced cell death was significantly greater in mice with loss-of-function mutations in galanin (Gal-KO) or GAL2 (GAL2-MUT) compared to strain-matched WT controls. Conversely, cell death was significantly reduced in galanin over-expressing (Gal-OE) transgenic mice compared to strain-matched WT controls. Exogenous galanin or Gal2-11 rescued the deficits in the Gal-KO but not the GAL2-MUT cultures, confirming that the protective effects of endogenous or exogenous galanin are mediated by activation of GAL2. Despite the high levels of endogenous galanin in the Gal-OE cultures, the addition of exogenous 100 nM or 50 nM galanin or 100 nM Gal2-11 further significantly reduced cell death, implying that GAL2-mediated neuroprotection is not at maximum in the Gal-OE mice. These data further support the hypothesis that galanin over-expression in AD is a neuroprotective response and imply that the development of a drug-like GAL2 agonist might reduce the progression of symptoms in patients with AD.