Critical role for GALR1 galanin receptor in galanin regulation of neuroendocrine function and seizure activity

[125I]-Galanin binding in brain of wildtype, and galanin- and GalR1-knockout mice: Strain and species differences in GalR1 density and distribution

Widespread production of knockout and transgenic mice has led to an increased use of mice as animal models for studies of normal- and patho-physiology. Hence, the precise mapping of central transmitter/peptide systems in the mouse has become essential for the interpretation of functional studies and for the correct correlation with findings obtained in the rat, primates and/or human. In this regard, the current study reports the autoradiographic localization of [(125)I]-galanin (GAL) binding sites in brain of the common C57BL/6J and 129OlaHsd mouse strains, as well as in GAL and galanin receptor-1 (GalR1) knockout (KO) mice. In C57BL/6J and 129OlaHsd mice, [(125)I]-GAL binding sites were detected throughout the brain, including moderate-high relative densities in the basal ganglia (caudate putamen, nucleus [n.] accumbens, olfactory tubercle, substantia nigra), limbic regions (septum, bed n. stria terminalis, ventral hippocampus, amygdala), cingulate, retrosplenial, entorhinal cortex, centro-lateral/medial thalamic n., preoptic/lateral hypothalamus, midbrain (superior colliculus, periaqueductal gray), pons/medulla oblongata (parabrachial, pontine reticular and solitary tract n.) and cerebellar cortex. [(125)I]-GAL binding levels were low or absent in main olfactory bulb, neocortex, ventrolateral/geniculate thalamic n., dorsal hippocampus, inferior colliculus and cranial motor n. In simultaneous determinations, relative [(125)I]-GAL binding site densities in brain were generally lower in C57BL/6J than in 129OlaHsd mice, while the density and distribution of central binding in the GAL-KO mouse was essentially identical to that in its background-129OlaHsd strain. In contrast, no specific [(125)I]-GAL binding was detected in any region of GalR1-KO mouse brain, revealing that under the experimental conditions used, the peptide ligand binding is predominantly (exclusively) to the GalR1 subtype. This evaluation of GAL receptor site distribution in mouse brain has revealed similarities and some differences with the equivalent system in rat and provides a valuable reference for future comparative studies of central GAL transmission.

Patterns of seizures, hippocampal injury and neurogenesis in three models of status epilepticus in galanin receptor type 1 (GalR1) knockout mice

The neuropeptide galanin exhibits anticonvulsant effects in experimental epilepsy. Two galanin receptor subtypes, GalR1 and GalR2, are present in the brain. We examined the role of GalR1 in seizures by studying the susceptibility of GalR1 knockout (KO) mice to status epilepticus (SE) and accompanying neuronal injury. SE was induced in GalR1 KO and wild type (WT) mice by Li-pilocarpine, 60 min electrical perforant path stimulation (PPS), or systemic kainic acid (KA). Seizures were analyzed using Harmonie software. Cell injury was examined by FluoroJade B- and terminal deoxynucleotidyl transferase-mediated uridine triphosphate nick end labeling; neurogenesis was studied using bromodeoxyuridine labeling. Compared with WT littermates, GalR1 KO showed more severe seizures, more profound injury to the CA1 pyramidal cell layer, as well as injury to hilar interneurons and dentate granule cells upon Li-pilocarpine administration. PPS led to more severe seizures in KO, as compared with WT mice. No difference in the extent of neuronal degeneration was observed between the mice of two genotypes in CA1 pyramidal cell layer; however, in contrast to WT, GalR1 KO developed mild injury to hilar interneurons on the side of PPS. KA-induced seizures did not differ between GalR1 KO and WT animals, and led to no injury to the hippocampus in either of experimental group. No differences were found between KO and WT mice in both basal and seizure-induced neuronal progenitor proliferation in all seizure types. Li-pilocarpine led to more extensive glia proliferation in GalR1 KO than in WT, and in both mouse types in two other SE models. In conclusion, GalR1 mediate galanin protection from seizures and seizure-induced hippocampal injury in Li-pilocarpine and PPS models of limbic SE, but not under conditions of KA-induced seizures. The results justify the development and use of GalR1 agonists in the treatment of certain forms of epilepsy.

Distribution of galanin and galanin transcript in the brain of a galanin-overexpressing transgenic mouse

The distribution of galanin mRNA-expressing cells and galanin-immunoreactive (IR) cell bodies and processes was studied in the brain of mice overexpressing galanin under the PDGF-B promoter (GalOE mice) and of wild type (WT) mice, both in colchicine-treated and non-treated animals. In this abstract, we only describe the results in GalOE mouse. A widespread ectopic expression of galanin (both mRNA and peptide) was found, that is a situation when neither transcript nor peptide could be seen in WT mice, not even after colchicine treatment. However, in some regions, such as claustrum, basolateral amygdala, thalamus, CA1 pyramidal cells, and Purkinje cells only galanin mRNA could be detected. In the forebrain galanin was seen in the mitral cells of the olfactory bulb, throughout the cortex, in the basolateral amygdaloid nucleus, claustrum, granular and pyramidal cell layers of the hippocampus, subiculum and presubiculum. In the thalamus, the anterodorsal, mediodorsal, intermediodorsal and mediodorsal lateral nuclei, the reuniens and reticular nuclei showed ectopic expression of galanin. Within the hypothalamus, neurons of the suprachiasmatic nucleus contained galanin. In the mesencephalon, the geniculate nucleus, nucleus ruber, the mesencephalic trigeminal and reticulotegmental nuclei ectopically expressed galanin. In the cerebellum, galanin was observed in the Purkinje cells and in the lateral and interposed cerebellar nuclei. In the pons, sensory and motor nuclei of the trigeminal nerve, the laterodorsal and dorsal tegmental nuclei, the pontine, reticulotegmental and gigantocellular reticular nuclei expressed galanin. Within the medulla oblongata, labeled cells were detected in the facial, ambiguus, prepositus, lateral paragigantocellular and lateral reticular nuclei, and spinal trigeminal nucleus. High densities of galanin-IR fibers were found in the axonal terminals of the lateral olfactory tract, the hippocampal and presumably the cerebellar mossy fibers system, in several thalamic and hypothalamic regions and the lower brain stem. Possible functional consequences of galanin overexpression are discussed.

Mutant G-protein-coupled receptors as a cause of human diseases

G-protein-coupled receptors (GPCR) are involved in directly and indirectly controlling an extraordinary variety of physiological functions. Their key roles in cellular communication have made them the target for more than 60% of all currently prescribed drugs. Mutations in GPCR can cause acquired and inherited diseases such as retinitis pigmentosa (RP), hypo- and hyperthyroidism, nephrogenic diabetes insipidus, several fertility disorders, and even carcinomas. To date, over 600 inactivating and almost 100 activating mutations in GPCR have been identified which are responsible for more than 30 different human diseases. The number of human disorders is expected to increase given the fact that over 160 GPCR have been targeted in mice. Herein, we summarize the current knowledge relevant to understanding the molecular basis of GPCR function, with primary emphasis on the mechanisms underlying GPCR malfunction responsible for different human diseases.

Galanin and galanin receptors in epilepsy

The shift in the balance between the inhibition and the excitation in favor of the latter is a major mechanism of the evolvement of epileptic seizures. On the neurotransmitter level two major players contribute to such misbalance: an inhibitory transmitter gamma-aminobutyric acid, and an excitatory amino acid glutamate. Neuropeptides are powerful modulators of classical neurotransmitters, and thus represent an intriguing tool for restoring the balance between the inhibition and the excitation, through either blocking or activating peptide receptors depending on whether a peptide is pro- or anticonvulsant. Galanin, a 29-amino acid residues neuropeptide which inhibits glutamate release in the hippocampus, is a likely member of the anticonvulsant peptide family. During the past decade growing evidence has been suggesting that galanin is in fact a powerful inhibitor of seizure activity. This review summarizes the state of research of galanin in epilepsy, beginning with the first simple experiments which showed that central injection of galanin agonists inhibited seizures, and that seizures themselves affected galanin signaling in the hippocampus; exploring the impact of active manipulation with the expression of galanin and galanin receptors on seizures, using transgenic animals, antisense and peptide-expressing vector approaches; and concluding with the recent advances in pharmacology, which led to the synthesis of non-peptide galanin receptor agonists with anticonvulsant properties. We also address recently established functions of galanin in seizure-associated neuronal degeneration and neuronal plasticity.

Galanin type 2 receptors regulate neuronal survival, susceptibility to seizures and seizure-induced neurogenesis in the dentate gyrus

The neuropeptide galanin has been implicated in inhibiting seizures and protecting hippocampal neurons from excitotoxic injury. In the hippocampus galanin acts through two receptor subtypes, GalR1, expressed in CA1, and GalR2, abundant in dentate gyrus. We developed an approach to induce and to study selective semichronic knockdown of GalR2 in the rat hippocampus. A 50% reduction of GalR2 binding was achieved by continuous infusion of complementary peptide nucleic acid antisense oligonucleotide into the dentate gyrus. This resulted in an increase in the severity of self-sustaining status epilepticus induced by electrical stimulation of the perforant path, induced mild neuronal injury in the dentate hilus, augmented seizure-induced hilar injury and inhibited seizure-induced neurogenesis in the subgranular zone of the dentate gyrus. Our data suggest that in the dentate gyrus, galanin, acting through GalR2, inhibits seizures, promotes viability of hilar interneurons and stimulates seizure-induced neurogenesis. These results are important for understanding the role of galanin and galanin receptor subtypes in the hippocampus both under normal conditions and in excitotoxic injury.

Galmic, a nonpeptide galanin receptor agonist, affects behaviors in seizure, pain, and forced-swim tests

The pharmacological exploitation of the galanin receptors as drug targets for treatment of epilepsy, depression, and pain has been hampered by the lack of workable compounds for medicinal chemists from random screening of large chemical libraries. The present work uses the tripeptidomimetic galnon and displays its presumed pharmacophores on a rigid molecular scaffold. The scaffold is related to marine natural products and presents three functional groups near one another in space, in a manner reminiscent of a protein surface. An active compound, Galmic, was identified from a small synthetic library and tested in vitro and in vivo for its affinity and efficacy at galanin receptors. Galmic has micromolar affinity for GalR1 receptors (Ki = 34.2 microM) and virtually no affinity for GalR2 receptors. In vitro, Galmic, like galanin, suppresses long-term potentiation in the dentate gyrus; it blocks status epilepticus when injected intrahippocampally or administered i.p. Galmic applied i.p. shows antidepressant-like effects in the forced-swim test, and it is a potent inhibitor of flinching behavior in the inflammatory pain model induced by formalin injection. These data further implicate brain and spinal cord galanin receptors as drug targets and provide an example of a systemically active compound based on a scaffold that mimics protein surfaces.

Obesity and endocrine dysfunction in mice with deletions of both neuropeptide Y and galanin

Neuropeptide Y (NPY) and galanin have both been implicated in the regulation of body weight, yet mice bearing deletions of either of these molecules have unremarkable metabolic phenotypes. To investigate whether galanin and NPY might compensate for one another, we produced mutants lacking both neuropeptides (GAL(-/-)/NPY(-/-)). We found that male GAL(-/-)/NPY(-/-) mice ate significantly more and were much heavier (30%) than wild-type (WT) controls. GAL(-/-)/NPY(-/-) mice responded to a high-fat diet by gaining more weight than WT mice gain, and they were unable to regulate their weight normally after a change in diet. GAL(-/-)/NPY(-/-) mice had elevated levels of leptin, insulin, and glucose, and they lost more weight than WT mice during chronic leptin treatment. Galanin mRNA was increased in the hypothalamus of NPY(-/-) mice, providing evidence of compensatory regulation in single mutants. The disruption of energy balance observed in GAL(-/-)/NPY(-/-) double knockouts is not found in the phenotype of single knockouts of either molecule. The unexpected obesity phenotype may result from the dysregulation of the leptin and insulin systems that normally keep body weight within the homeostatic range.

Interneuron Diversity series: Interneuronal neuropeptides--endogenous regulators of neuronal excitability

Interneurons are often classified according to neuropeptide content. However, it is becoming increasingly clear that neuropeptides are more than convenient neurochemical markers and can act as important modulators of neuronal activity. Recent advances in understanding neuropeptide release and physiological actions suggest that the interneuronal system of neuropeptides is crucial for maintaining appropriate brain function under normal and pathophysiological conditions. In particular, interneuronal neuropeptides appear to play roles in cognition and as endogenous anti-epileptic agents. This article describes current understanding of the conditions under which neuropeptides are released from interneurons, their specific effects on neuronal excitability and synaptic transmission, and the consequences of their loss of function.

Learning and memory performance in mice lacking the GAL-R1 subtype of galanin receptor

The neuropeptide galanin induces performance deficits in a wide range of cognitive tasks in rodents. Three G-protein-coupled galanin receptor subtypes, designated GAL-R1, GAL-R2 and GAL-R3, have been cloned. The present study examined the role of GAL-R1 in cognition by testing mice with a null mutation in Galr1 on several different types of learning and memory tasks. Assessments of general health, neurological reflexes, sensory abilities and motor functions were conducted as control measures. Mutant mice were unimpaired in social transmission of food preference and the Morris water maze. In tests of fear conditioning, mutant mice were unimpaired in a delay version of cued fear conditioning. However, mice homozygous for the null mutation were impaired in a trace version of cued fear conditioning. Mutant mice were unimpaired in contextual fear conditioning, whether training was by the delay or trace protocol. General health, neurological reflexes, sensory abilities and motor functions did not differ across genotypes, indicating that the trace fear conditioning deficit was not an artifact of procedural disabilities. The findings of normal performance on several cognitive tasks and a selective deficit in trace cued fear conditioning in homozygous GAL-R1 mutant mice are discussed in terms of hypothesized roles of the GAL-R1 subtype. The generally normal phenotype of GAL-R1 null mutants supports the use of this line for identification of the receptor subtypes that mediate the cognitive deficits produced by exogenous galanin.

Genetic control of sensitivity to hippocampal cell death induced by kainic acid: A quantitative trait loci analysis

Host genetic factors are likely to contribute to differences in individual susceptibility to seizure-induced excitotoxic neuronal damage. Similarly, inbred strains of mice differ in their susceptibility to the kainic acid (KA) model of seizure-induced cell death, but the genes responsible for the differences are not known. Here, we define the inheritance patterns of susceptibility to KA-induced neurodegeneration in the hippocampus by assessing 331 back-cross (N2) progeny of two inbred mouse strains, C57BL/6 and FVB/N, previously shown to display resistance and sensitivity to KA-induced cell death, respectively. Results of phenotypic analysis suggest that the difference in susceptibility between these two strains is conferred by a single dominant gene. Therefore, we used an N2 back-cross between the inbred C57BL/6 and FVB/N strains for a genome-wide search for quantitative trait loci (QTLs), which are chromosomal sites containing genes influencing the magnitude of susceptibility. Genome-wide interval mapping in N2 progeny identified a locus on distal chromosome (Chr) 18 with a peak LOD score of 4.9 localized between D18Mit186 and D18Mit4 as having the strongest and most significant effect in this model. QTLs of minor effect were detected on Chr 15 (D15Mit174-D15Mit156) and Chr 4 (D4Mit264-D4Mit91), with peak LOD scores of 3.02 and 2.46, respectively. The three significant QTLs (Chrs 4, 15, 18) together account for nearly 25% of the trait variance for both genders combined. Reduced KA-induced cell death susceptibility was observed in a congenic strain in which the highly susceptible FVB/N strain carried putative resistance alleles from the C57BL/6 strain on Chr 18.

Galanin and neuropeptide Y reduce cholinergic transmission in the heart of the anaesthetised mouse

(1) This study investigated the effects of galanin (GAL) on inhibition of cholinergic (vagal) activity in the mouse heart using control galanin knockout (GAL-KO) and GAL-1R receptor knockout (GAL-1R-KO) mice. (2) In pentobarbitone anaesthetised mice, supramaximal stimulation every 30 s of the vagus nerve innervating the heart, increased pulse interval (PI) by approximately 50 ms or decreased heart rate by approximately 100 beats min-1. This response was attenuated by intravenous administration of GAL (dose ranged from 0.8 to 13 nmol kg-1) in a dose-dependent manner. (3) In GAL-KO mice, the magnitude of inhibition of the increase in PI (DeltaPI) following a bolus dose of GAL was not different from the DeltaPI in control mice, and neuropeptide Y (NPY), previously shown to attenuate vagal inhibitory activity in mice, evoked a comparative inhibition of DeltaPI in GAL-KO mice. (4) In GAL-1R-KO mice, an intravenous, bolus injection of GAL had no inhibitory effect on vagal activity. (5) In control mice, stimulation of the sympathetic nerve at 25 V, 10 Hz for 2 min in the presence of propranolol evoked a long-lasting attenuation of DeltaPI. The inhibitory effect on DeltaPI was reduced in the presence of the NPY Y2 antagonist, BIIE0246. (6) In GAL-1R-KO mice, stimulation of the sympathetic nerve in the presence of propranolol evoked an attenuation of DeltaPI not significantly different from the response in control mice in the presence of BIIE0246. Following administration of BIIE0246 in GAL-1R-KO mice, the inhibition of DeltaPI that followed stimulation of the sympathetic nerve was abolished. (7) These findings support the view that the nerve terminals of parasympathetic neurons in the mouse heart possess both GAL-1R and NPY Y2 receptors which, when activated, reduce acetylcholine release.

Neuropeptides: opportunities for drug discovery

The role of peptides as signalling molecules in the nervous system has been studied for more than 30 years. Neuropeptides and their G-protein-coupled receptors are widely distributed throughout the body and they commonly occur with, and are complementary to, classic neurotransmitters. The functions of neuropeptides range from neurotransmitter to growth factor. They are present in glial cells, are hormones in the endocrine system, and are messengers in the immune system. Much evidence indicates that neuropeptides are of particular importance when the nervous system is challenged (eg, by stress, injury, or drug abuse). These features and the large number of neuropeptides and neuropeptide receptors provide many opportunities for the discovery of new drug targets for the treatment of nervous-system disorders. In fact, receptor-subtype-selective antagonists and agonists have been developed, and recently a substance P receptor (neurokinin 1) antagonist has been shown to have clinical efficacy in the treatment of major depression and chemotherapy-induced emesis. Several other neuropeptide receptor ligands are in clinical trials for various indications.

Altered hippocampal expression of neuropeptides in seizure-prone GALR1 knockout mice

PURPOSE

Mice carrying a deletion of the GALR1 galanin receptor have recently showed spontaneous seizure phenotype with 25% penetrance. To better understand the role of neuropeptides, which are known to undergo complex plasticity changes with development of epileptic seizures, we characterized their expression in the hippocampal formation in GALR1- knockout (-KO) mice with or without seizures and in wild-type (WT) mice.

METHODS

Immunohistochemistry and in situ hybridization were used to study expression of galanin, neuropeptide Y (NPY), substance P, enkephalin, dynorphin, and cholecystokinin (CCK).

RESULTS

In GALR1-KO mice that had been displaying seizures, a strong upregulation of galanin immunoreactivity (ir) and messenger RNA (mRNA) was found in the polymorph layer of the dentate gyrus; galanin-ir also appeared in a dense fiber network in the supragranular layer. A strong upregulation of enkephalin was found in the granule cells/mossy fibers, whereas dynorphin mRNA levels were modestly decreased. NPY was strongly expressed in the granule cells/mossy fibers, and an increase of NPY mRNA levels in the polymorph cells was paralleled by an increase of NPY-ir in the molecular layer. An upregulation of substance P-ir was confined to the fibers in the granule and molecular layers, whereas substance P mRNA was increased in the cells of the polymorph layer. Both CCK-ir and mRNA were strongly downregulated in the granule cell/mossy fiber system, but CCK-ir appeared increased in the supragranular and molecular layers. No changes in neuropeptide-ir were found in GALR1-KO mice not displaying seizures.

CONCLUSIONS

Complex changes in neuropeptide expression in some principal hippocampal neurons and interneurons appear as a characteristic feature of the spontaneous-seizure phenotype in GALR1-KO mice. However, to what extent causal relations exist between this "epilepsia peptidergic profile" and development of seizures requires further clarification.

Flexor reflex excitability in mice lacking galanin receptor galanin-R1

This study was conducted to examine the excitability of the nociceptive flexor reflex and its sensitization by repetitive stimulation of C-fibers in anesthetized mice that lack the galanin-R1 receptor. Repetitive stimulation of C-fibers induced a gradual increase in reflex magnitude during the stimulation (wind-up), and a subsequent increase in spinal reflex excitability (central sensitization). This occurred in GAL-R1 -/-, GAL-R1 +/-, and +/+ wild-type controls, with no significant differences observed between genotypes. Intrathecal administration of galanin markedly blocked the sensitization following the repetitive stimulation in all three groups. No differences between wild-type or galanin-R1 receptor knockout mice were seen. These results confirm previous studies in rats, showing that intrathecal galanin reduces the central sensitization following wind-up. The present data indicate that this effect is probably mediated by receptors other than GAL-R1.

Galanin GAL-R1 receptor null mutant mice display increased anxiety-like behavior specific to the elevated plus-maze

The neuropeptide galanin coexists with norepinephrine and serotonin in neural systems mediating emotion. Previous findings suggested that galanin modulates anxiety-related behaviors in rodents. Three galanin receptor subtypes have been cloned; however, understanding their functions has been limited by the lack of galanin receptor subtype-selective ligands. To study the role of the galanin GAL-R1 receptor subtype in mediating anxiety-related behavior, we generated mice with a null mutation in the Galr1 gene. GAL-R1 -/- are viable and show no abnormalities in health, neurological reflexes, motoric functions, or sensory abilities. On a battery of tests for anxiety-like behavior, GAL-R1 -/- showed increased anxiety-like behavior on the elevated plus-maze test. Anxiety-related behaviors on the light/dark exploration, emergence, and open field tests were normal in GAL-R1 -/-. This test-specific anxiety-like phenotype was confirmed in a second, independent cohort of GAL-R1 null mutant mice and +/+ controls. Principal components factor analysis of behavioral scores from 279 mice suggested that anxiety-like behavior on the elevated plus-maze was qualitatively distinct from behavior on other tests in the battery. In addition, exposure to the elevated plus-maze produced a significantly greater neuroendocrine response than exposure to the light/dark exploration test, as analyzed in normal C57BL/6J mice. These behavioral findings in the first galanin receptor null mutant mouse are consistent with the hypothesis that galanin exerts anxiolytic actions via the GAL-R1 receptor under conditions of relatively high stress.