Galanin receptor-1 knockout mice exhibit spontaneous epilepsy, abnormal EEGs and altered inhibition in the hippocampus

Galanin diminishes cortical spreading depolarization across rodents - A candidate for treatment?

Galanin (Gal) is a neuropeptide with the potential to ameliorate cortical spreading depolarization (CSD), an electrophysiological phenomenon occurring after brain injury or in migraine aura. Gal is expressed in all cortical neurons both in rat and in mouse cortices. Here we investigated whether the effect of Gal on CSD previously described in the rat is conserved in the mouse cortex. In rats, the topical application of Gal to the cortex for 1 h did not induce any change in CSD amplitudes, propagation velocity, or threshold of elicitation. Rather, topical application of Gal for 3 h was necessary to obtain a significant decrease in these CSD parameters and to develop a remarkable increase in the KCl threshold to elicit a CSD in rat cortex. In contrast, the topical application of Gal on cortical surface for 1 h in mice was sufficient to significantly attenuate CSD amplitudes and increase threshold. A thinner cortex, a faster diffusion or different affinity/expression of receptors for Gal are possible reasons to explain this difference in the time course between rats and mice. Our data are relevant to postulate Gal as a potential target for inhibition of CSD under pathological situations such as stroke or ischemia. SIGNIFICANCE STATEMENT: The neuropeptide Galanin (Gal) is expressed in all neurons throughout the cerebral cortex, both in rats and mice, and is able to reduce or even inhibit Cortical Spreading Depolarization, thus, Gal has the potential to control neuronal excitability that may identify Gal as a target in drug development against CSD.

Cortical spreading depolarization is a potential target for rat brain excitability modulation by Galanin

The inhibitory neuropeptide Galanin (Gal) has been shown to mediate anticonvulsion and neuroprotection. Here we investigated whether Gal affects cortical spreading depolarization (CSD). CSD is considered the pathophysiological neuronal mechanism of migraine aura, and a neuronal mechanism aggravating brain damage upon afflictions of the brain. Immunohistochemistry localized Gal and the Gal receptors 1-3 (GalR1-3) in native rat cortex and evaluated microglial morphology after exposure to Gal. In anesthetized rats, Gal was applied alone and together with the GalR antagonists M40, M871, or SNAP 37889 locally to the exposed cortex. The spontaneous electrocorticogram and CSDs evoked by remote KCl pressure microinjection were measured. In rat cortex, Gal was present in all neurons of all cortical layers, but not in astrocytes, microglia and vessels. GalR2 and GalR3 were expressed throughout all neurons, whereas GalR1 was preponderantly located at neurons in layers IV and V, but only in about half of the neurons. In susceptible rats, topical application of Gal on cortex decreased CSD amplitude, slowed CSD propagation velocity, and increased the threshold for KCl to ignite CSD. In some rats, washout of previously applied Gal induced periods of epileptiform patterns in the electrocorticogram. Blockade of GalR2 by M871 robustly prevented all Gal effects on CSD, whereas blockade of GalR1 or GalR3 was less effective. Although microglia did not express GalRs, topical application of Gal changed microglial morphology indicating microglial activation. This effect of Gal on microglia was prevented by blocking neuronal GalR2. In conclusion, Gal has the potential to ameliorate CSD thus reducing pathophysiological neuronal events caused by or associated with CSD.

Modelling epilepsy in the mouse: challenges and solutions

In most mouse models of disease, the outward manifestation of a disorder can be measured easily, can be assessed with a trivial test such as hind limb clasping, or can even be observed simply by comparing the gross morphological characteristics of mutant and wild-type littermates. But what if we are trying to model a disorder with a phenotype that appears only sporadically and briefly, like epileptic seizures? The purpose of this Review is to highlight the challenges of modelling epilepsy, in which the most obvious manifestation of the disorder, seizures, occurs only intermittently, possibly very rarely and often at times when the mice are not under direct observation. Over time, researchers have developed a number of ways in which to overcome these challenges, each with their own advantages and disadvantages. In this Review, we describe the genetics of epilepsy and the ways in which genetically altered mouse models have been used. We also discuss the use of induced models in which seizures are brought about by artificial stimulation to the brain of wild-type animals, and conclude with the ways these different approaches could be used to develop a wider range of anti-seizure medications that could benefit larger patient populations.

Summary: This Review discusses the challenges of modelling epilepsy in mice, a condition in which the outward manifestation of the disorder appears only sporadically, and reviews possible solutions encompassing both genetic and induced 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.

Hippocampal Transcriptomic Profiles: Subfield Vulnerability to Age and Cognitive Impairment

The current study employed next-generation RNA sequencing to examine gene expression differences related to brain aging, cognitive decline, and hippocampal subfields. Young and aged rats were trained on a spatial episodic memory task. Hippocampal regions CA1, CA3, and the dentate gyrus were isolated. Poly-A mRNA was examined using two different sequencing platforms, Illumina, and Ion Proton. The Illumina platform was used to generate seed lists of genes that were statistically differentially expressed across regions, ages, or in association with cognitive function. The gene lists were then retested using the data from the Ion Proton platform. The results indicate hippocampal subfield differences in gene expression and point to regional differences in vulnerability to aging. Aging was associated with increased expression of immune response-related genes, particularly in the dentate gyrus. For the memory task, impaired performance of aged animals was linked to the regulation of Ca²⁺ and synaptic function in region CA1. Finally, we provide a transcriptomic characterization of the three subfields regardless of age or cognitive status, highlighting and confirming a correspondence between cytoarchitectural boundaries and molecular profiling.

The novel anticonvulsant neuropeptide and galanin analogue, NAX-5055, does not alter energy and amino acid metabolism in cultured brain cells

A large body of evidence suggests that the neuropeptide galanin plays an important role in seizure control. In line with this, it was demonstrated that the galanin analogue, NAX-5055, exerts a potent anticonvulsant activity in animal seizure models. We recently found that the NAX-5055-mediated anticonvulsant action involves modulation of both excitatory and inhibitory neurotransmission. Since homeostasis of neurotransmitters and cerebral energy metabolism are intimately linked, it was investigated whether the effects of NAX-5055 on neurotransmission involve changes in energy metabolism and in particular glucose- and amino acid metabolism. With this aim, cultured neurons from mouse brain were incubated with [U-¹³ C]glucose in absence or presence of NAX-5055. Since effects of NAX-5055 on neurotransmission were detected during repetitive stimulation, we tested potential metabolic effects while mimicking repetitive bursts of neurotransmitter release as occurring in the intact brain. The metabolic pathways were mapped using gas-chromatography coupled to mass-spectrometry. We found that NAX-5055 does not modify glucose metabolism in glutamatergic and GABAergic neurons. Furthermore, the effect of NAX-5055 on astrocyte-neuron metabolic interactions was investigated by incubating co-cultures of astrocytes and either glutamatergic or GABAergic neurons with [U-¹³ C]glucose or the glial-selective substrate [1,2-¹³ C]acetate, with or without NAX-5055. In the presence of NAX-5055, no changes in the metabolic landscape were traced. The findings suggest that the anticonvulsant action of NAX-5055 and the accompanying changes in neurotransmission do not involve alterations in energy and amino acid metabolism. Hence, NAX-5055 appears to be an anti-seizure drug candidate displaying no unwanted side effects concerning brain energy and amino acid homeostasis. © 2017 Wiley Periodicals, Inc.

The anticonvulsant action of the galanin receptor agonist NAX-5055 involves modulation of both excitatory- and inhibitory neurotransmission

The endogenous neuropeptide galanin is ubiquitously expressed throughout the mammalian brain. Through the galanin receptors GalR1-3, galanin has been demonstrated to modulate both glutamatergic and GABAergic neurotransmission, and this appears to be important in epilepsy and seizure activity. Accordingly, galanin analogues are likely to provide a new approach to seizure management. However, since peptides are generally poor candidates for therapeutic agents due to their poor metabolic stability and low brain bioavailability, a search for alternative strategies for the development of galanin-based anti-convulsant drugs was prompted. Based on this, a rationally designed GalR1 preferring galanin analogue, NAX-5055, was synthesized. This compound demonstrates anti-convulsant actions in several animal models of epilepsy. However, the alterations at the cellular level leading to this anti-convulsant action of NAX-5055 are not known. Here we investigate the action of NAX-5055 at the cellular level by determining its effects on excitatory and inhibitory neurotransmission, i.e. vesicular release of glutamate and GABA, respectively, in cerebellar, neocortical and hippocampal preparations. In addition, its effects on cell viability and neurotransmitter transporter capacity were examined to evaluate potential cell toxicity mediated by NAX-5055. It was found that vesicular release of glutamate was reduced concentration-dependently by NAX-5055 in the range from 0.1 to 1000 nM. Moreover, exposure to 1 μM NAX-5055 led to a reduction in the extracellular level of glutamate and an elevation of the extracellular level of GABA. Altogether these findings may at least partly explain the anti-convulsant effect of NAX-5055 observed in vivo.

MicroRNA profiles in hippocampal granule cells and plasma of rats with pilocarpine-induced epilepsy--comparison with human epileptic samples

The identification of biomarkers of the transformation of normal to epileptic tissue would help to stratify patients at risk of epilepsy following brain injury, and inform new treatment strategies. MicroRNAs (miRNAs) are an attractive option in this direction. In this study, miRNA microarrays were performed on laser-microdissected hippocampal granule cell layer (GCL) and on plasma, at different time points in the development of pilocarpine-induced epilepsy in the rat: latency, first spontaneous seizure and chronic epileptic phase. Sixty-three miRNAs were differentially expressed in the GCL when considering all time points. Three main clusters were identified that separated the control and chronic phase groups from the latency group and from the first spontaneous seizure group. MiRNAs from rats in the chronic phase were compared to those obtained from the laser-microdissected GCL of epileptic patients, identifying several miRNAs (miR-21-5p, miR-23a-5p, miR-146a-5p and miR-181c-5p) that were up-regulated in both human and rat epileptic tissue. Analysis of plasma samples revealed different levels between control and pilocarpine-treated animals for 27 miRNAs. Two main clusters were identified that segregated controls from all other groups. Those miRNAs that are altered in plasma before the first spontaneous seizure, like miR-9a-3p, may be proposed as putative biomarkers of epileptogenesis.

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.

Neuropeptides as targets for the development of anticonvulsant drugs

Epilepsy is a common neurological disorder characterized by recurrent seizures. These seizures are due to abnormal excessive and synchronous neuronal activity in the brain caused by a disruption of the delicate balance between excitation and inhibition. Neuropeptides can contribute to such misbalance by modulating the effect of classical excitatory and inhibitory neurotransmitters. In this review, we discuss 21 different neuropeptides that have been linked to seizure disorders. These neuropeptides show an aberrant expression and/or release in animal seizure models and/or epilepsy patients. Many of these endogenous peptides, like adrenocorticotropic hormone, angiotensin, cholecystokinin, cortistatin, dynorphin, galanin, ghrelin, neuropeptide Y, neurotensin, somatostatin, and thyrotropin-releasing hormone, are able to suppress seizures in the brain. Other neuropeptides, such as arginine-vasopressine peptide, corticotropin-releasing hormone, enkephalin, β-endorphin, pituitary adenylate cyclase-activating polypeptide, and tachykinins have proconvulsive properties. For oxytocin and melanin-concentrating hormone both pro- and anticonvulsive effects have been reported, and this seems to be dose or time dependent. All these neuropeptides and their receptors are interesting targets for the development of new antiepileptic drugs. Other neuropeptides such as nesfatin-1 and vasoactive intestinal peptide have been less studied in this field; however, as nesfatin-1 levels change over the course of epilepsy, this can be considered as an interesting marker to diagnose patients who have suffered a recent epileptic seizure.

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.

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.

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.

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.

Galanin and consummatory behavior: special relationship with dietary fat, alcohol and circulating lipids

Galanin (GAL) plays an integral role in consummatory behavior. In particular, hypothalamic GAL has a positive, reciprocal relationship with dietary fat and alcohol. In this relationship, GAL increases the consumption of fat or alcohol which, in turn, stimulates the expression of GAL, ultimately leading to overconsumption. Through actions in the amygdala, this relationship may become especially important in stress-induced food or drug intake. These effects of GAL in promoting overconsumption may involve various neurotransmitters, with GAL facilitating intake by stimulating norepinephrine and dopamine and reducing satiety by decreasing serotonin and acetylcholine. In addition, GAL in the hypothalamus stimulates the opioid, enkephalin, throughout the brain, which also promotes overconsumption. The relationship between GAL, fat, and alcohol may involve triglycerides, circulating lipids that are released by fat or alcohol and that correlate positively with hypothalamic GAL expression. In females, levels of endogenous GAL also fluctuate across the reproductive cycle, driven by a rise in the ovarian steroids, estrogen, and progesterone. They peak during the proestrous phase and also at puberty, simultaneous to a sharp increase in preference for fat to meet energy demands. Prenatal exposure to a high-fat diet also enhances hypothalamic expression of GAL into adulthood because of an increase in neurogenesis and proliferation of GAL-expressing neurons in this region. This organizational change may reflect the role of GAL in neuronal development, including neurite growth in adulthood, cell survival in aging, and cell stability in the disease state. By responding positively to fat and alcohol and guiding further neuronal development, GAL potentiates a long-term propensity to overconsume fat and alcohol.

Neuroprotective role for galanin in Alzheimer's disease

Galanin (GAL) and GAL receptors (GALR) are overexpressed in degenerating brain regions associated with cognitive decline in Alzheimer's disease (AD). The functional consequences of GAL plasticity in AD are unclear. GAL inhibits cholinergic transmission in the hippocampus and impairs spatial memory in rodent models, suggesting that GAL overexpression exacerbates cognitive impairment in AD. By contrast, gene expression profiling of individual cholinergic basal forebrain (CBF) neurons aspirated from AD tissue revealed that GAL hyperinnervation positively regulates mRNAs that promote CBF neuronal function and survival. GAL also exerts neuroprotective effects in rodent models of neurotoxicity. These data support the growing concept that GAL overexpression preserves CBF neuron function, which may in turn delay the onset of symptoms of AD. Further elucidation of GAL activity in selectively vulnerable brain regions will help gauge the therapeutic potential of GALR ligands in the treatment of AD.

Variation in Galr1 expression determines susceptibility to exocitotoxin-induced cell death in mice

Inbred strains of mice differ in their susceptibility to excitotoxin-induced cell death, but the genetic basis of individual variation in differential susceptibility is unknown. Previously, we identified a highly significant quantitative trait locus (QTL) on chromosome 18 that influenced susceptibility to kainic acid-induced cell death (Sicd1). Comparison of susceptibility to seizure-induced cell death between reciprocal congenic lines for Sicd1 and parental background mice indicates that genes influencing this trait were captured in both strains. Two positional gene candidates, Galr1 and Mbp, map to 55 cM, where the Sicd1 QTL had been previously mapped. Thus, this study was undertaken to determine if Galr1 and/or Mbp could be considered as candidate genes. Genomic sequence comparison of these two functional candidate genes from the C57BL/6J (resistant at Sicd1) and the FVB/NJ (susceptible at Sicd1) strains showed no single-nucleotide polymorphisms. However, expression studies confirmed that Galr1 shows significant differential expression in the congenic and parental inbred strains. Galr1 expression was downregulated in the hippocampus of C57BL/6J mice and FVB.B6-Sicd1 congenic mice when compared with FVB/NJ or B6.FVB-Sicd1 congenic mice. A survey of Galr1 expression among other inbred strains showed a significant effect such that 'susceptible' strains showed a reduction in Galr1 expression as compared with 'resistant' strains. In contrast, no differences in Mbp expression were observed. In summary, these results suggest that differential expression of Galr1 may contribute to the differences in susceptibility to seizure-induced cell death between cell death-resistant and cell death-susceptible strains.

Galanin in Alzheimer's disease: neuroinhibitory or neuroprotective?

Galanin (GAL) and GAL receptors (GALRs) are overexpressed in degenerating brain regions associated with cognitive decline in Alzheimer's disease (AD). The functional consequences of GAL plasticity in AD are unclear. GAL inhibits cholinergic transmission in the hippocampus and impairs spatial memory in rodent models, suggesting GAL overexpression exacerbates cognitive impairment in AD. By contrast, gene expression profiling of individual cholinergic basal forebrain (CBF) neurons aspirated from AD tissue revealed that GAL hyperinnervation positively regulates mRNAs that promote CBF neuronal function and survival. GAL also exerts neuroprotective effects in rodent models of neurotoxicity. These data support the growing concept that GAL overexpression preserves CBF neuron function which in turn may slow the onset of AD symptoms. Further elucidation of GAL activity in selectively vulnerable brain regions will help gauge the therapeutic potential of GALR ligands for the treatment of AD.

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

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

Neuropeptide gene therapy for epilepsy: viral vectors, stem cells and neurogenesis

Gene therapy for epilepsy is a relatively novel concept compared with previous approaches, which have relied on primary embryonic cells to deliver gene products of interest into localized brain regions. In vivo and ex vivo gene transfer offer promising, but yet insufficiently explored, possibilities to inhibit seizures, either by genetically modifying postmitotic neurons of the brain using viral vectors, or by transplanting genetically modified and in vitro tested cell lines, particularly stem cell lines, to produce and release gene products of interest. In this regard, neuropeptides are discussed as emerging candidates for such gene therapy approaches. Selective modification of newly generated neurons in the dentate gyrus by retroviral vector-based gene delivery opens novel possibilities in gene therapy for epilepsy. However, the limited number of new neurons targeted remains a main obstacle. Despite its early stage, gene therapy for epilepsy might not be a remote prospect for clinical trials, particularly in patients with intractable temporal lobe epilepsy. Ex vivo gene transfer using encapsulated genetically modified cells could be of particular value for such initial trials.

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.