Autoradiographic distribution of 125I-galanin binding sites in the rat central nervous system

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

Galanin, acting at the GalR1-3 subtypes of galanin receptors, is involved in the regulation of cognition, mood, feeding, seizure activity and pain. The understanding of galanin's effects in molecular and cellular terms has been hampered by the lack of receptor subtype selective ligands and antibodies. Previous in situ hybridization data showed that GalR1 and GalR2 receptors are abundant in the rat brain, while the distribution of GalR3 is contradictory and most studies demonstrated a low expression levels in the rat brain. The distribution of galanin receptor subtypes at protein level is unknown. In the present study, we report the regional distribution of the galanin receptors: GalR1 and non-GalR1 receptors, using a recently synthesized high affinity GalR2/3 selective ligand, galanin (2-11), and galanin (1-29), as competitors, in saturating (125)I-galanin membrane binding assay. We show that paraventricular nucleus (PVN) express predominantly GalR1, whereas areas like the dorsal raphe nucleus (DRN), hippocampus and amygdala express both the GalR1 and non-GalR1 receptors. We speculate that the GalR2/3 binding sites detected by galanin (2-11) binding in our study probably represent mostly GalR2 receptors. In addition, we show regionally specific and subtype specific regulation of galanin receptors. Status epilepticus (SE), known to deplete galanin from axonal projections of locus coeruleus and septum/diagonal band neurons in the hippocampus and to induce galanin expression in a subset of hippocampal cells, down regulates GalR2 receptor mRNA and proteins by 30% without altering the GalR1 receptors.

Galanin receptor ligands

The three galanin receptor subtypes (GalR1-3) belong to the G protein-coupled receptor superfamily. The widespread distribution of galanin and its receptors in the CNS and PNS and the numerous physiological and pharmacological effects of galanin (for review, cf. Vrontakis, 2002) render the three galanin receptors attractive drug targets. The industrial efforts, however, have not yet resulted in a wealth of receptor subtype specific agonists or antagonists with high affinity and selectivity. The present paper summarizes the properties of the galanin ligands used at the end of 2004 in the ca. 2000 publications and complements their pharmacological characterization with new data.

Age-related impairments of synaptic plasticity in the lateral perforant path input to the dentate gyrus of galanin overexpressing mice

In the present study, electrophysiological recordings were made from hippocampal slices obtained from mice overexpressing galanin under the promoter for the platelet-derived growth factor-B (GalOE mice). In these mice, a particularly strong galanin expression is seen in the granule cell layer/mossy fibers. Paired-pulse facilitation (PPF) of excitatory postsynaptic field potentials (fEPSPs) at the lateral perforant path (LPP)-dentate gyrus synapses was elicited in the dentate gyrus after stimulation with different interpulse intervals. Slices from young adult wild-type (WT) animals showed significant PPF of the 2nd EPSP evoked with paired-pulse stimuli, while PPF was reduced in slices from young adult GalOE mice, as well as aged WT mice, but were not observed at all in slices from aged GalOE animals. Application of the putative galanin antagonist M35 increased PPF in slices from aged WT mice as well as from adult and aged GalOE mice, but had no effect in slices taken from young adult WT mice. These data indicate that galanin is involved in hippocampal synaptic plasticity, in particular in age-related reduction of synaptic plasticity in the LPP input to the dentate gyrus. Galaninergic mechanisms may therefore represent therapeutic targets for treatment of age-related memory deficits and Alzheimer's disease.

Constitutive and ligand-induced internalization of EGFP-tagged galanin R2 and Rl receptors in PC12 cells

In the present experiments trafficking of the galanin R1-(GALR1) and, in particular, the galanin R2 receptor (GALR2) was studied after fusion with enhanced green fluorescent protein (GALR1-EGFP and GALR2-EGFP) and transfection into PC12 cells. Both fusion proteins were predominantly localized on the plasma membrane and internalized in a dose dependent manner after incubation with galanin. Preincubation with M35 or M40 did not prevent galanin-induced internalization of GALR1-EGFP or GALR2-EGFP. However, AR-M1896, a selective GALR2 agonist, caused GALR2, but not GALR1 internalization. Hyperosmotic sucrose inhibited internalization of GALR2-EGFP. After co-incubation with galanin, GALR2-EGFP was co-localized with internalized Texas Red transferrin, a marker of the clathrin endocytic pathway. Experiments with protein synthesis inhibition and Texas Red transferrin suggest that GALR2 is constitutively internalized. Studies in progress will show if this is the case also for GALR1.

Behavioral and neurochemical studies on brain aging in galanin overexpressing mice

To study possible involvement of galanin in brain aging quality, we have investigated behavioral, neurochemical and morphological parameters in aged mice overexpressing galanin under the platelet-derived growth factor B promoter (GalOE mice) compared to wild-type littermates (WT mice). The behavioral analysis in the forced swim test showed that old GalOE animals spent more time in immobility compared to WT. In the activity cage test, galanin overexpression counteracted the age-induced decrease in exploratory behavior. The neurochemical analysis showed a 30% decrease in noradrenaline overflow in the cerebral cortex of WT old mice that was not present in age-matched GalOE mice. Our results indicate that overexpression of galanin can influence several behavioral and neurochemical parameters in old mice.

Inhibitory effects of different galanin compounds and fragments on osmotically and histamine-induced enhanced vasopressin secretion in rats

The effects of rat, porcine and human galanin, and the human 1-16 and human 16-30 terminal galanin fragments on vasopressin secretion were studied in rat. The plasma vasopressin level was determined by radioimmunoassay (RIA). There were no changes in the basal vasopressin secretion after galanin administration. A significant increase in vasopressin concentration was detected following 2.5% NaCl or histamine administration. I.c.v. injected rat, porcine or human galanin or the 1-16 N-terminal galanin fragment prevented the plasma vasopressin level enhancement. Following the i.v. administration of rat galanin or the i.c.v. injected 16-30 C-terminal galanin fragment, the vasopressin concentration did not return to the normal level. Administration of the galanin antagonist galantid (M15) i.c.v. before the rat galanin i.c.v. injection prevented the inhibitory effect on the increased plasma vasopressin level following 2.5% NaCl solution or histamine administration. The results indicate that there is no significant difference in the inhibitory effect of rat, porcine or human galanin or the 1-16 galanin fragment on the enhanced plasma vasopressin secretion induced by hyperosmosis or histamine administration. Our findings suggest that galanin, as a peptide modulator, is physiologically involved in the regulation of vasopressin release following different forms of stimulation: an osmotic response or histamine administration.

Electrophysiological studies on galanin effects in brain--progress during the last six years

The effects of galanin and galanin fragments have been studied on neurons in various brain regions of rodents using electrophysiological techniques. Here, we mainly review reports published during the last six years, that is after the second galanin symposium in 1998. These papers deal with locus coeruleus (LC), the hippocampal formation (HF), hypothalamus, the nucleus of the diagonal band of Broca (DBB) and the dorsal vagal complex (DVC). In most cases galanin has an inhibitory effect by increasing a potassium conductance or reducing a calcium conductance. In LC, beside a direct inhibitory effect, galanin exerts an indirect effect enhancing the noradrenaline-induced hyperpolarization. In the HF, galanin (1-15), but not galanin (1-29), induces hyperpolarization in CA3 pyramidal neurons. Inhibitory effects of galanin on several forms of synaptic plasticity including long-term potentiation, frequency facilitation and paired-pulse facilitation have also been demonstrated in normal and transgenic animals. In the hypothalamic arcuate nucleus galanin has a presynaptic action inhibiting glutamate release, as well as a postsynaptic effect via the galanin R1 receptor. In the DVC, galanin inhibits dorsal vagal motor neurons projecting to the stomach by activation of a postsynaptic galanin receptor. However, excitatory effects of galanin have also been reported in several regions, such as the DBB nucleus, where galanin increases excitability by decreasing a K+ conductance. Taken together, electrophysiological studies have further supported the role of galanin as a neurotransmitter/neuromodulator in the brain.

Intracerebroventricular administration of galanin decreases free water intake and operant water reinforcer efficacy in water-restricted rats

The 29/30 amino acid neuropeptide galanin coexists with vasopressin in the hypothalamus and has been shown to inhibit the actions of vasopressin and aldosterone, suggesting an inhibitory role for galanin in physiological water retention mechanisms and water seeking and water consumption behavior. Little work, however, has examined a role for galanin in water intake regulation. Furthermore, many experiments that have reported galanin-induced impairments in the performance of tasks thought to measure learning and memory have used water restriction routines and water reinforcers to maintain responding. Therefore, the present study examined the effects of intracerebroventricularly administered galanin (5.0-20.0 microg/5 microl) on free water consumption during a 10 min test session and a follow up open field exploration, an operant progressive ratio (PR) schedule, a test used to assess reinforcer strength, and an operant fixed time schedule (FT 20) in 23.5h water restricted rats. Finally, in an additional experiment that was designed to simulate the effects of a galanin-induced decrease in water reinforcer efficacy, the rats were allowed access to water prior to testing in an operant delayed non-matching to position (DNMTP) task. A galanin-induced decrease in water consumption was observed in both the free access test and the FT 20 at the 20 microg dose, but no significant galanin-induced alterations in open field behavior. A decrease in responses emitted and rewards received was observed on the PR schedule at the 5, 10, and 20 microg doses. Pre-session access to water significantly reduced the number of trials per session in the DNMTP but did not reduce accuracy. This study is the first to observe a galanin-induced reduction in water intake and reinforced operant behavior, and suggests that galanin may play a role in regulating water intake and reinforcement. However, the present data also suggest that DNMTP choice accuracy deficits observed previously cannot be attributed to a galanin-induced change in reinforcer efficacy.

Behavioural characterisation of young adult transgenic mice overexpressing galanin under the PDGF-B promoter

The behavioural phenotype of transgenic mice (3- to 5-months old) overexpressing galanin (GalOE) under the platelet-derived growth factor B (PDGF-B) promoter was evaluated in a battery of tests, including open field, locomotor cages, light-dark exploration test, elevated plus-maze and the Porsolt forced swim test. Learning and memory were assessed in the passive avoidance and the Morris water maze tasks. No difference between genotypes was found in exploratory activity in the open field. GalOE mice showed a slight increase in spontaneous locomotor activity assessed in the locomotor cages, but the amphetamine-induced increase in locomotor activity was somewhat lower in GalOE mice. Anxiety-like behaviour in the three different tests including open field, light-dark exploration and elevated plus-maze did not differ between genotypes. In the Porsolt forced swim test, GalOE mice displayed an increased time of immobility, indicative of increased learned helplessness possibly reflecting increased stress-susceptibility and/or depression-like behaviour. GalOE mice showed normal learning and memory retention in the passive avoidance and the Morris water maze tasks. These data support the hypothesis that galanin may have a role in functions related to mood states including affective disorders.

[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.

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.

Intracerebroventricularly administered galanin does not alter operant reaction time or differentially reinforced high rate schedule operant responding in rats

Galanin (Gal) is a 29/30 amino acid neuroendocrine peptide that impairs learning and memory processes, stimulates feeding, and modulates somatosensory, sex, and stress responses. Anatomical markers for Gal are found throughout the brain, including in the caudate-putamen and substantia nigra motor regions. Many of the behavioral tests that have been used to study the involvement of Gal in complex behavioral processes are motorically demanding, but no research has specifically investigated the involvement of Gal in response initiation or the maintenance of fine motor action. Therefore, the present study examined the effects of intraventricularly administered Gal on two highly sensitive operant tasks designed to detect alteration of these response properties. Response initiation was studied using a light-dark discrimination reaction time task that required a correct response within 2.5s of a spatially and temporally uncertain stimulus onset. The ability to perform high local rates of responding was studied using an operant differential reinforcement of high rate (DRH) of responding task. Gal (10-20 microg, i.c.v.) did not alter reaction time or inter-response time distributions in either task, though did substantially reduce the total number of responses and reinforcers obtained on the DRH schedule. These results are consistent with a Gal-induced reduction of reinforcer efficacy rather than Gal-disruption of response initiation or response patterning.

Galanin in the brain: chemoarchitectonics and brain cartography--a historical review

We present a review of galanin in the brain from a historical perspective of the development of "chemoarchitectonics" and "brain cartography" accomplished in the Histopharmacology Section at the National Institutes of Health. It was the mapping of potential brain neuroregulators that served as a springboard of ideas from which behavioral studies emanate. The integration of the known localization of neurotransmitter/neuromodulatory nerves ("chemoarchitectonic maps") and receptor binding sites with biochemical data derived from brain micropunches coupled with behavioral analysis at the level of discrete brain allows one to define the anatomical circuits which support behavioral changes and which ultimately will improve our understanding of mental disorders.

Analysis of the contribution of galanin receptors 1 and 2 to the central actions of galanin-like peptide

Galanin-like peptide (GALP) shares partial sequence identity with galanin and exhibits agonistic activity at two of the galanin receptor subtypes (GALR1 and GALR2) in vitro. The goal of these experiments was to determine whether galanin receptors mediate the effects of central GALP administration on food intake, body weight, and luteinizing hormone (LH) secretion in the mouse. We first evaluated the effects of intracerebroventricular injections of GALP or its vehicle alone in GALR1 knockout mice, GALR2 knockout mice, and their respective wild-type controls. GALP reduced food intake and body weight after 24 h to a similar degree in wild-type, GALR1 knockout, and GALR2 knockout mice. The wild-type, GALR1 knockout, and GALR2 knockout mice also exhibited significant increases in serum levels of LH following the GALP injections. To help delineate the biologically active moiety of the GALP molecule, we injected wild-type mice with shorter fragments of the full-length GALP peptide. Neither GALP((1-21)) (the fragment containing the galanin-homologous sequence) nor GALP((22-60)) (the C-terminal portion of the GALP molecule lacking sequence identity with galanin) had any discernable effect on food intake, body weight or circulating LH. These observations demonstrate that neither GALR1 nor GALR2 are essential for mediating the effects of GALP on feeding, body weight or LH secretion. Furthermore, the galanin-homologous region of the GALP molecule is not sufficient to mimic the effects of full-length GALP. Together, these findings argue against the hypothesis that GALP signals solely through galanin receptors in vivoand suggest the existence of a yet-to-be-identified GALP-specific receptor.

Expression and plasticity of galanin systems in cortical neurons, oligodendrocyte progenitors and proliferative zones in normal brain and after spreading depression

Neocortex contains very few galanin neurons but receives a moderate galanin innervation from various subcortical loci. Recent data suggest that galanin helps regulate the tonic neuronal excitability of hippocampus and probably cerebral cortex but relatively little is known about the anatomy and functional regulation of cortical galanin systems. Therefore, we examined, in the rat, the effect of the intense but benign stimulus, cortical spreading depression (CSD), on the expression of galanin and galanin receptors (GalR1 and GalR2) in the neocortex and associated regions, revealing complex, multicellular responses. Thus, following acute, unilateral KCl-induced CSD, a delayed and transient induction (onset after 48 h, lasting approximately 24 h) of galanin mRNA and peptide production occurred across the ipsilateral cerebral cortex in activated oligodendrocyte progenitor cells (OPCs), identified by specific NG2 proteoglycan immunostaining. An increase in GalR1 mRNA, immunoreactivity and receptor binding occurred in neurons within layers II and V of neocortex and in piriform cortex at 7-28 days after CSD, associated with a long-lasting depletion of galanin-positive nerve fibres in these regions. In contrast, GalR2 mRNA expression was largely unaltered after CSD. Additional novel findings in normal, adult brain were the detection of galanin mRNA and immunoreactivity in OPCs within the medial corpus callosum and in immature progenitor cells in the subventricular zone and rostral migratory stream. GalR1 and GalR2 mRNA was also present in these latter regions. These findings and the complex modulation of galanin and galanin receptors in multiple cell types (neurons/OPCs) following acute cortical activation/depression further demonstrate the potential plasticity of neuronal and non-neuronal galanin systems under physiological and pathological conditions and strongly suggest additional functions for this pleiotropic peptide in mammalian brain.

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.

Differential galanin receptor-1 and galanin expression by 5-HT neurons in dorsal raphé nucleus of rat and mouse: evidence for species-dependent modulation of serotonin transmission

Galanin and galanin receptors are widely expressed by neurons in rat brain that either synthesize/release and/or are responsive to, classical transmitters such as gamma-aminobutyric acid, acetylcholine, noradrenaline, histamine, dopamine and serotonin (5-hydroxytryptamine, 5-HT). The dorsal raphé nucleus (DRN) contains approximately 50% of the 5-HT neurons in the rat brain and a high percentage of these cells coexpress galanin and are responsive to exogenous galanin in vitro. However, the precise identity of the galanin receptor(s) present on these 5-HT neurons has not been previously established. Thus, the current study used a polyclonal antibody for the galanin receptor-1 (GalR1) to examine the possible expression of this receptor within the DRN of the rat and for comparative purposes also in the mouse. In the rat, intense GalR1-immunoreactivity (IR) was detected in a substantial population of 5-HT-immunoreactive neurons in the DRN, with prominent receptor immunostaining associated with soma and proximal dendrites. GalR1-IR was also observed in many cells within the adjacent median raphé nucleus. In mouse DRN, neurons exhibited similar levels and distribution of 5-HT-IR to that in the rat, but GalR1-IR was undetectable. Consistent with this, galanin and GalR1 mRNA were also undetectable in mouse DRN by in situ hybridization histochemistry, despite the detection of GalR1 mRNA (and GalR1-IR) in adjacent cells in the periaqueductal grey and other midbrain areas. 5-HT neuron activity in the DRN is primarily regulated via 5-HT1A autoreceptors, via inhibition of adenylate cyclase and activation of inward-rectifying K+ channels. Notably, the GalR1 receptor subtype signals via identical mechanisms and our findings establish that galanin modulates 5-HT neuron activity in the DRN of the rat via GalR1 (auto)receptors. However, these studies also identify important species differences in the relationship between midbrain galanin and 5-HT systems, which should prompt further investigations in relation to comparative human neurochemistry and which have implications for studies of animal models of relevant neurological conditions such as stress, anxiety and depression.

Inhibitory effect of galanin on dopamine-induced enhanced vasopressin secretion in rat neurohypophyseal tissue cultures

The effect of galanin (GAL) on vasopressin (VP) secretion was studied in 13-14-day cultures of isolated rat neurohypophyseal (NH) tissue. The VP content of the supernatant was determined by radioimmunoassay (RIA) after a 1- or 2-h incubation. A significantly decreased content of VP was detected following the administration of 10(-6)-10(-9) M doses of GAL. Dopamine (DA) and the DA-active drugs apomorphine (APM) and Pro-Lys-Gly (PLG) (10(-6) M in each medium) increased the VP level of NH tissue cultures. This VP concentration elevation could be blocked by the administration of GAL together with DA, APM or PLG. The DA-blocking effect of GAL was prevented by previous treatment with the GAL receptor antagonist galantid (M15). The results indicate that VP release is directly influenced by the GAL-ergic system. The GAL-ergic control of VP secretion from NH tissue in rats can occur independently of the hypothalamus, at the level of the posterior pituitary.

Restricted distribution of galanin receptor 3 (GalR3) mRNA in the adult rat central nervous system

Recent molecular cloning studies have established the existence of a third rat galanin receptor subtype, GalR3, however its precise distribution in the mammalian central nervous system (CNS) is not well established. In the present study, we examined the regional and cellular distribution of GalR3 mRNA in the CNS of the rat by in situ hybridization. Our findings indicate that GALR3 mRNA expression in the rat brain is discrete and highly restricted, concentrated mainly in the preoptic/hypothalamic area. Within the hypothalamus, GalR3 expression was confined to the paraventricular, ventromedial and dorsomedial hypothalamic nuclei. In addition to these hypothalamic nuclei, GalR3 mRNA-expressing cells were observed in the medial septum/diagonal band of Broca complex, the bed nucleus of the stria terminalis, the medial amygdaloid nucleus, the periaqueductal gray, the lateral parabrachial nucleus, the dorsal raphe nucleus, the locus coeruleus, the medial medullary reticular formation and in one of the circumventricular organs, the subfornical organ. In the spinal cord, a faint but specific ISH signal was observed over the laminae I-II with a few moderately labeled cells distributed in laminae V and X. The neuroanatomical distribution of GalR3 suggests it might be involved in mediating documented effects of galanin on food intake, fluid homeostasis, cardiovascular function and nociception.

Galanin inhibits tyrosine hydroxylase expression in midbrain dopaminergic neurons

Galanin (GAL) inhibits midbrain dopamine (DA) activity in several experimental paradigms, yet the mechanism underlying this inhibition is unclear. We examined the effects of GAL on the expression of tyrosine hydroxylase (TH) in primary cultures of rat embryonic (E14) ventral mesencephalon (VM). One micromolar GAL had no effect on the number of TH-immunoreactive (ir) neurons in VM cultures. However, 1 micro m GAL reduced an approximately 100% increase in TH-ir neurons in 1 mm dibutyryl cAMP (dbcAMP)-treated cultures by approximately 50%. TH-ir neuron number in dbcAMP-treated VM cultures was dose-responsive to GAL and the GAL receptor antagonist M40 blocked GAL effects. Semi-quantitative RT-PCR and quantitative immunoblotting experiments revealed that GAL had no effect on TH mRNA levels in VM cultures but reduced TH protein. VM cultures expressed GALR1, GALR2, and GALR3 receptor mRNA. However, dbcAMP treatment resulted in a specific approximately 200% increase in GALR1 mRNA. GALR1 activity is linked to a pertussis toxin (PTX)-sensitive opening of G protein-gated K+ channels (GIRKs). GAL reduction of TH-ir neuron number in dbcAMP + GAL-treated cultures was sensitive to both PTX and tertiapin, a GIRK inhibitor. GAL inhibition of midbrain DA activity may involve a GALR1- mediated reduction of TH in midbrain dopaminergic neurons.

On the Role of Galanin, Substance P and Other Neuropeptides in Primary Sensory Neurons of the Rat: Studies on Spinal Reflex Excitability and Peripheral Axotomy

The interaction of intrathecally (i.t.) applied galanin (GAL) with substance P (SP), calcitonin gene-related peptide (CGRP), vasoactive intestinal polypeptide (VIP), somatostatin (SOM) and C-fibre conditioning stimulation (CS) with regard to their effects on the spinal nociceptive flexor reflex was studied in decerebrate, spinalized, unanaesthetized rats with intact or sectioned sciatic nerves. SP, CGRP, VIP and SOM applied onto the surface of lumbar spinal cord or a brief CS train (1 Hz, 20 s) to the sural nerve facilitated the flexor reflex for several minutes in animals with intact or sectioned nerves. Pretreatment with GAL, which by itself had a biphasic effect on the flexor reflex in a dose-dependent manner, antagonized the reflex facilitation induced by sural CS before and after sciatic nerve section. SP-induced facilitation of the flexor reflex was antagonized by GAL in rats with intact sciatic nerves, but not after nerve section. In contrast, VIP-induced reflex facilitation was antagonized by GAL only after sectioning of the sciatic nerve. GAL was effective in antagonizing the facilitatory effect of CGRP under both situations, but had no effect on SOM-induced facilitation. A parallel immunohistochemical study revealed that after sciatic nerve section GAL-like immunoreactivity (LI) and VIP-LI are increased in the dorsal root ganglia and that these two peptides coexist in many cells. The present results indicate that GAL antagonizes the excitatory effect of some neuropeptides which exist in the spinal cord. This antagonism could explain the inhibitory effect of GAL on C-fibre CS-induced facilitation of the flexor reflex, which is presumably due to the release of some of these neuropeptides from the terminals of primary afferents. Furthermore, the interaction between GAL and other neuropeptides is altered by sciatic nerve section, paralleling changes in the levels of these neuropeptides in primary afferents and their pattern of coexistence after nerve section. It is proposed that SP and CGRP are important mediators of the spinal flexor reflex in intact rats. However, after axotomy VIP may replace SP in this capacity, paralleling the decrease in SP and marked increase in VIP levels. In general the study provides further support for involvement of peptides in sensory function.

Distribution of Galanin mRNA Containing Cells and Galanin Receptor Binding Sites in Human and Rat Hypothalamus

The distribution of cells containing galanin mRNA and that of galanin receptor binding sites were investigated using in situ hybridization histochemistry and receptor autoradiography in male rat hypothalamus and in postmortem hypothalamic tissues from control human brains. Oligonucleotide probes labelled with 32P were used for hybridization experiments. The specificity of the hybridization signal was ascertained using several probes, competition assays and Northern blot analysis. High levels of hybridization were found in the paraventricular, supraoptic and arcuate nuclei of rat and human hypothalamus. Human intermediate nuclei and scattered cells of the posterior perifornical nucleus also contained galanin mRNA. Galanin mRNA was also found in the dorsomedial nucleus of the rat. The distribution of galanin receptor sites was investigated by receptor autoradiography using 125I-labelled porcine galanin. The specificity of the binding was assessed by competition with different neuropeptides. While galanin blocked the binding at nanomolar concentrations, the other neuropeptides examined were ineffective at 10-7 M concentrations. The highest densities of galanin binding sites were seen in the preoptic area, ventromedial and lateral nuclei, of rat and human hypothalamus. In contrast, very low densities of binding sites were observed in the paraventricular, supraoptic and arcuate nuclei. Our results show that the distribution of neurons expressing galanin is complementary to that of galanin receptors in the rat and human hypothalamus. This suggests that receptors for galanin are not located on the cell bodies of galaninergic neurons, but are probably presynaptic on or postsynaptic to the processes of these cells.

Galanin receptor over-expression within the amygdala in early Alzheimer's disease: an in vitro autoradiographic analysis

Galanin receptor (GALR) expression is increased in various areas of the limbic system in end stage Alzheimer's disease (AD). The amygdaloid complex is a key component of the limbic circuit, is involved in homeostatic and cognitive functions, is impacted in AD and contains the peptide and receptor for galanin. Although GALR expression occurs in the amygdala in end stage AD, it remains to be determined whether a plasticity response occurs early or late in the disease. Therefore, we analyzed the distribution and associated changes in GALR binding in the amygdala during the progression of AD using an in vitro receptor autoradiographic method. Human galanin ([125I]hGAL) receptor binding was performed on brain sections from early and late AD cases, as well as normal age-matched control subjects. In aged controls, densities of [125I]hGAL binding sites were found in the central and the corticomedial nuclei. Relative to controls, possible/early AD cases displayed significantly greater numbers of [125I]hGAL binding sites in the central nucleus and cortico-amygdaloid transition area. In contrast, we found a decrease in the number of binding sites for [125I]hGAL in late as compared to early AD cases. The over-expression of GALRs in subfields of the amygdaloid early in AD suggests that galaninergic systems play a key role in limbic related behavioral changes during the disease process.

Galanin/GALP and galanin receptors: role in central control of feeding, body weight/obesity and reproduction?

Scientific and commercial pharmacological interest in the role of galanin and galanin receptors in the regulation of food intake, energy balance, and obesity has waned recently, following initial enthusiasm during the 1980-1990s. It has been replaced by efforts to understand the role of newly discovered peptide systems such as the hypocretin/orexins, melanocortins and cocaine- and amphetamine-regulated transcript (CART) and their relationship to the important hormones, leptin and insulin. Thus, while numerous studies have revealed the ability of galanin to stimulate food intake via actions at sites within the hypothalamus, and shown reliable changes in hypothalamic galanin synthesis in response to food ingestion; findings including the lack of a 'body weight/obesity' phenotype in galanin transgenic mouse strains and a lack of agonists/antagonists for galanin receptor subtypes have probably served to reduce enthusiasm. However, as more is learnt about the general and galanin-related neurochemistry of brain pathways involved in feeding, metabolism and body weight control, the potential importance of galanin systems is again in focus. Studies of the newly discovered galanin family peptide, 'galanin-like peptide' (GALP), highlight the likely role of galanin peptides and receptors in the physiological coupling of body weight, adiposity and reproductive function. GALP is produced by a discrete population of neurons within the basomedial arcuate nucleus (and median eminence) that send projections to the anterior paraventricular nucleus and that make close contacts with leutinizing hormone-releasing hormone (LHRH) neurons in basal forebrain. Furthermore, GALP neurons express leptin receptors and respond to leptin treatment by increasing their expression of GALP mRNA. Centrally administered GALP activates LHRH-immunoreactive neurons and increases plasma LH levels. These findings suggest a direct stimulatory action of endogenous GALP on gonadotropin secretion via actions within the hypothalamus/basal forebrain, with leptin actions linking this system to body adipose levels.

Expression of galanin receptor-1 (GALR1) in the rat trigeminal ganglia and molar teeth

The expression of galanin receptor-1 (GALR1) was investigated in the rat trigeminal ganglion by using immunocytochemistry and in situ hybridization. In addition, the regional distribution of GALR1-immunoreactive pulpal nerves and their ultrastructure were examined in the molar teeth. In the trigeminal ganglion, the immunoreactivity for GALR1 was recognizable in about 30% of the total number of neurons. Most of the cell bodies were small to medium in size. Analysis of serially cut sections alternately stained with GALR1 and galanin antisera demonstrated that some GALR1-positive cells displayed immunoreactivity for galanin. In situ hybridization analysis, expression of GALR1 mRNA was detected in trigeminal ganglion cells. The cell size distribution was similar to that of GALR1-immunoreactive cells. In the dental pulp, a small number of nerve fibers displayed immunoreactivity for GALR1. The labeled fibers formed terminal arbors in the coronal pulp around and within the odontoblast cell layer, but never penetrated into the predentin and dentin. Ultrastructurally, GALR1 immunoreactivity in the dental pulp was confined to the axoplasm of unmyelinated nerve fibers. The present study provided new evidence that unmyelinated primary afferents innervating dental pulp possessed galanin receptor, and suggests the existence of nociceptive primary afferents functioning as autocrine cells.

Galanin: an endogenous anticonvulsant?

Galanin is a neuroendocrine peptide involved in the regulation of feeding, pain, sexual behavior, learning, and memory. The recent discovery, that galanin antagonized excitatory glutamatergic neurotransmission in the hippocampus, provided a rationale for its possible antiepileptic effects. Here we summarize the data on the effects of galanin on seizure activity in several animal models of epilepsy. Pharmacological and molecular biological evidence suggest potent anticonvulsant effects of galanin. Exogenous administration of galanin receptor agonists attenuated seizures, whereas application of galanin receptor antagonists potentiated seizure expression. Genetically engineered mice, with either deletion or overexpression of galanin gene, showed altered resistance to seizures, which was in direct correlation with galanin gene expression. Possible mechanisms of the anticonvulsant action of galanin include its effects on synaptic potentiation in hippocampal circuits and inhibition of the release of the excitatory neurotransmitter glutamate from principal hippocampal neurons.

Food deprivation decreases blood galanin-like peptide and its rapid entry into the brain

Galanin-like peptide (GALP) was recently isolated from the hypothalamus, where its expression is influenced by leptin and food deprivation. Since leptin crosses the blood-brain barrier (BBB) by a saturable transport system that is downregulated by fasting, we examined the effect of leptin and fasting on the entry of GALP into mouse brain. Multiple-time regression analysis showed that the basal influx of 125I-GALP from blood was rapid (K(i) = 9.49 +/- 0.72 x 10(-4) ml/g x min). This influx was not affected by leptin but was significantly decreased by food deprivation for 24 or 48 h, accompanied by decreased immunoreactive plasma GALP at 48 h, but not at 24 h. By contrast, pretreatment of mice fasted for 24 h with glucose resulted in a significant increase in the blood-to-brain influx of GALP that was not accompanied by increased immunoreactive plasma GALP. HPLC showed that most of the GALP crossed the BBB in an intact form, and capillary depletion studies showed that more than 93% of the GALP crossing entered the parenchyma of the brain rather than being bound to the endothelial cells of the capillaries composing the BBB or being reversibly associated with the vasculature. Efflux of 125I-GALP occurred at the rate of the normal reabsorption of CSF, and the octanol-buffer partition coefficient showed insufficient lipophilicity to explain the fast rate of influx. When 125I-GALP was perfused in blood-free buffer, the self-inhibition characteristic of a saturable transport system was evident even though capillary gel electrophoresis showed GALP aggregating as a trimer. Capillary zone electrophoresis showed protein binding of GALP in serum, perhaps facilitating its interactions at the BBB. In particular, these studies show for the first time (1) that immunoreactive GALP is present in blood where (2) its concentrations are reduced by food deprivation, and (3) that there is a rapid blood-to-brain influx of intact GALP (4) which is decreased by fasting and (5) increased by pretreatment with glucose.

Axon terminals containing tyrosine hydroxylase- and dopamine-beta-hydroxylase immunoreactivity form synapses with galanin immunoreactive neurons in the lateral division of the bed nucleus of the stria terminalis in the rat

Catecholaminergic projections from brainstem sources to the bed nucleus of the stria terminalis play a central role in the neurochemically mediated modulation/regulation of stress response. The lateral division of the bed nucleus of the stria terminalis (BSTL) exhibits several galanin immunoreactive (ir) neurons that are also central in the modulatory control of acute stress responses. The distribution of galaninergic nervous structures overlaps with that of the dopaminergic and noradrenergic axon terminals in the BSTL. Since both monoamines and galanin regulate/modulate the central regulatory pathways of endocrine, behavioral and physiological responses during stress, the aim of this study was to demonstrate synaptic interaction between galanin-ir nervous structures and fiber terminals immunopositive for dopamine or noradrenaline in the BSTL, thereby providing morphological data to understand better the significance of catecholamine-galanin interactions in brain areas responding to stressful stimuli. Double-labeling immunohistochemistry applied both at light and electron microscopic levels made it possible to demonstrate synaptic interactions between galanin-ir nervous structures and axon terminals immunopositive for either dopamine or noradrenaline. The dopaminergic fiber terminals innervated galanin-ir cells and dendrites in the laterodorsal division of the bed nucleus of the stria terminalis (BST), whereas the noradrenergic axons contacted galaninergic neurons and dendrites in the lateroventral BST. In this study, interactions between monoamines and galanin-ir structures were demonstrated in the BSTL which can be central in the modulatory control of the major stress regulatory pathway of the limbic-hypothalamo-pituitary-adrenal axis.

Central galanin and N-terminal galanin fragment induce c-Fos immunoreactivity in the medulla oblongata of the anesthetized rat

This immunohistochemical study analyzed the c-Fos expression (c-Fos-ir) induced by galanin injections. Galanin and N-terminal galanin fragment (1-15) induced a significant increase of c-Fos expression (c-ir) within the medulla oblongata 90 min and 6 h. after intracisternal injections. This expression has been studied mainly in the nucleus of the solitary tract and in the ventrolateral medulla showing different temporal profiles for both peptides. The presence of c-Fos-ir in TH-positive cells was analyzed in all the groups. These results may be relevant to understand the role of galanin in several functions including central cardiovascular control.

Distribution of galaninergic immunoreactivity in the brain of the mouse

The distribution of galaninergic immunoreactive (-ir) profiles was studied in the brain of colchicine-pretreated and non-pretreated mice. Galanin (GAL)-ir neurons and fibers were observed throughout all encephalic vesicles. Telencephalic GAL-ir neurons were found in the olfactory bulb, cerebral cortex, lateral and medial septum, diagonal band of Broca, nucleus basalis of Meynert, bed nucleus of stria terminalis, amygdala, and hippocampus. The thalamus displayed GAL-ir neurons within the anterodorsal, paraventricular, central lateral, paracentral, and central medial nuclei. GAL-ir neurons were found in several regions of the hypothalamus. In the midbrain, GAL-ir neurons appeared in the pretectal olivary nucleus, oculomotor nucleus, the medial and lateral lemniscus, periaqueductal gray, and the interpeduncular nucleus. The pons contained GAL-ir neurons within the dorsal subcoeruleus, locus coeruleus, and dorsal raphe. In the medulla oblongata, GAL-ir neurons appear in the anterodorsal and dorsal cochlear nuclei, salivatory nucleus, A5 noradrenergic cells, gigantocellular nucleus, inferior olive, solitary tract nucleus, dorsal vagal motor and hypoglossal nuclei. Only GAL-ir fibers were seen in the lateral habenula nucleus, substantia nigra, parabrachial complex, cerebellum, spinal trigeminal tract, as well as the motor root of the trigeminal and facial nerves. GAL-ir was also observed in several circumventricular organs. The widespread distribution of galanin in the mouse brain suggests that this neuropeptide plays a role in the regulation of cognitive and homeostatic functions.

Regulation by osmotic stimuli of galanin-R1 receptor expression in magnocellular neurones of the paraventricular and supraoptic nuclei of the rat

Neurones of the supraoptic nucleus (SON) and the magnocellular and parvocellular divisions of the paraventricular nucleus (PVN) express galanin and [125I]galanin binding sites. Although the precise role(s) of galanin in these different cell populations is still unknown, it has been shown to regulate the electrophysiological, neurochemical and secretory activity of magnocellular neurones. In light of the well-described effects of hyperosmotic stimuli, such as salt-loading on magnocellular neurone activity and galanin synthesis and release, and the recent identification of multiple galanin receptors in brain, this study assessed the possible regulation of galanin receptor subtype expression in the PVN/SON of salt-loaded, dehydrated and food-deprived rats. Gal-R1 mRNA was abundant in the SON (and magnocellular PVN) of control rats and levels were increased in these same cells after 4 days of salt-loading (2% NaCl solution as drinking water) or water deprivation. The density of specific [125I]galanin(1-29) binding and the intensity of Gal-R1-like immunostaining were also increased in the characteristically enlarged, magnocellular neurones of the PVN and SON after these treatments. Gal-R2 mRNA was detected in the parvocellular PVN, but levels were not altered by the hyperosmotic stimuli. In contrast, food deprivation (4 days), which has been shown to reduce levels of several neurochemical markers in magnocellular neurones, produced a significant reduction in Gal-R1 (and galanin) mRNA levels in the SON, but no consistent change in neurone size, [125I]galanin binding levels, or Gal-R1 immunostaining. Along with previous findings from this and other laboratories, these data suggest that the expression of galanin and Gal-R1 receptors is regulated in parallel with functional and morphological changes in hypothalamic magnocellular neurones. Furthermore, Gal-R1 immunoreactivity was primarily detected in somatodendritic areas and thus galanin may influence the activity of these cells, particularly vasopressin synthesis/release, via autocrine or paracrine activation of Gal-R1 receptors, especially during long-lasting stimulation.

Distribution, regulation and role of hypothalamic galanin systems: renewed interest in a pleiotropic peptide family

1. Galanin peptide and galanin receptor-binding sites are known to be widely distributed within the central nervous system, particularly in the hypothalamus in the preoptic area, the paraventricular (PVN) and supraoptic (SON) nuclei and the arcuate nucleus/median eminence. 2. The present brief review focuses on some recent studies of the regional and cellular localization of mRNA encoding galanin and two galanin receptor subtypes (GalR1 and GalR2) in the hypothalamus, regulation of galanin and/or galanin receptor expression in various nuclei by physiological stimuli, electrophysiological effects of galanin on hypothalamic neurons and the isolation and cloning of galanin-like peptide (GALP), a putative endogenous ligand for GalR2. 3. In situ hybridization studies in rat brain have demonstrated an abundance of GalR1 mRNA in SON, magnocellular (m) and parvocellular (p) PVN and dorsomedial, ventromedial and arcuate nuclei. In contrast, GalR2 mRNA is enriched in pPVN, but not mPVN, and is not detected in SON. In addition, GalR2 mRNA is present in the dorsomedial nucleus and is enriched in the arcuate nucleus compared with GalR1 transcripts, with numerous labelled cells in all subdivisions. 4. Neurons of the SON and PVN contain vasopressin and/or oxytocin, along with several other peptides, and the production and release of these hormones and peptides are modulated by various physiological stimuli. In relation to galanin systems, GalR1 and galanin expression is increased in magnocellular neurons by salt loading and is downregulated by lactation, consistent with an increased inhibition by galanin of vasopressin release following osmotic stimulation and a decreased inhibition of oxytocin release during lactation. 5. Powerful inhibitory effects of galanin on the electrical (and secretory) activity of magnocellular neurons and complex presynaptic actions of galanin on the synaptic release of glutamate in the arcuate nucleus in vitro suggest an active role for multiple galanin receptor subtypes in the regulation of these hypothalamic systems in vivo. 6. The recent isolation of a peptide from porcine hypothalamus (GALP-1-60) that is structurally related to galanin and appears to be selective for GalR2 over GalR1 and the subsequent cloning of GALP cDNA from pig, rat and humans should allow studies to help reveal the physiological role played by galanin receptor subtypes (especially GalR2) and their multiple ligands in the hypothalamus and other brain areas.

Effects of centrally administered galanin (1-16) on galanin expression in the rat hypothalamus

In the rat hypothalamic magnocellular neurons, galanin coexists with vasopressin and might be involved in hydro-osmotic regulation. In the present study, we investigated the ability of galanin to also regulate the osmotically stimulated expression of galanin itself in hypothalamic magnocellular neurons. Ten minutes after galanin injection, galanin mRNA rate decreased in salt-loaded rats whereas the level of galanin immunoreactivity increased. Both effects were suppressed by the injection of a galanin antagonist together with galanin. Moreover, electron microscope studies demonstrated synaptic contacts between galanin-containing fibers and magnocellular neurons. Galanin may exert inhibitory roles in the regulation of magnocellular neurons. However, galanin and vasopressin expression displayed differences upon galanin injection. Possible mechanisms underlying these discrepancies are further discussed.

Acute and chronic restraint stress: effects on [125I]-galanin binding in normotensive and hypertensive rat brain

The neuropeptide galanin (GAL) has been implicated in the neural response to a number of stressors including restraint; however, the effect of restraint stress on GAL receptor density in the central nervous system (CNS) has not been investigated. Normotensive (Wistar-Kyoto; WKY) and hypertensive (spontaneously hypertensive; SHR) rats were subjected to a daily 60-min restraint stress paradigm for 0 (control), 1, 3, 5 or 10 consecutive days, and the density of [125I]-GAL binding sites following exposure to restraint was compared between strains using quantitative autoradiography. Significant differences in basal (no stress) levels of GAL receptor density between WKY and SHR were detected in regions such as the central nucleus of the amygdala (Ce) and ventromedial hypothalamus (VMH) (P<0.05). In WKY, restraint stress (1 day) induced significant decreases in GAL receptor density in forebrain regions such as the Ce (-41%) and medial nucleus of the amygdala (-41%) (P<0.05). Chronic restraint (10 days) did not induce significant decreases in these nuclei in WKY, indicating that forebrain neurons containing GAL receptors in WKY possessed a functional ability to adapt to repeated restraint. In addition, restraint stress induced significant decreases in GAL receptor density in SHR in regions such as the lateral parabrachial nucleus (-43%; 5 days of restraint) and hypoglossal nucleus ( approximately -18% for entire restraint period) (P<0.05). In conclusion, restraint stress resulted in region- and strain-specific alterations in GAL receptor density, some of which may contribute to the altered stress response previously observed in hypertensive rats. The results clearly support the hypothesis that neuropeptides such as GAL are an integral component of the neural response to psychological stress, although the functional significance of the changes in GAL receptor density described in this study awaits elucidation.

Galanin-R1 and -R2 receptor mRNA expression during the development of rat brain suggests differential subtype involvement in synaptic transmission and plasticity

The present study employed 35S-labelled oligonucleotides and in situ hybridization to examine the distribution in the developing rat brain of mRNA encoding two galanin receptor subtypes, i.e. Gal-R1 and Gal-R2. Gal-R1 and/or Gal-R2 mRNA was detected at embryonic day (E) 20 and from postnatal day (P) 0-70. Gal-R1 mRNA was highly expressed in olfactory regions, ventral hippocampal CA fields, dorsomedial thalamic areas and many hypothalamic nuclei at all ages studied. In adult brain, Gal-R2 mRNA was most abundant in the dentate gyrus, anterior and posterior hypothalamus, raphe and spinal trigeminal nuclei, and in the dorsal motor nucleus of the vagus. At P0-P7, Gal-R2 mRNA was more widely distributed and abundant than at other ages, with highest levels of expression detected throughout the neocortex and thalamus. Thus, Gal-R2 transcripts had a more restricted distribution than Gal-R1 and were differentially abundant at different ages, while the distribution and relative abundance of Gal-R1 mRNA did not alter substantially during postnatal development. In general, Gal-R1 and -R2 mRNAs were localized in regions previously shown to contain [125I]-galanin binding sites and galanin-positive terminals in adult brain. Galanin-immunostaining was assessed in postnatal brain to determine whether peptide innervation correlated with observed transient receptor expression, but was not particularly enriched in Gal-R2 mRNA-positive areas of P4 or P7 brain. These results, together with earlier findings [e.g. Burazin, T. C. D. & Gundlach, A. L. (1998) J. Neurochem., 71, 879-882], suggest that Gal-R1 receptors have a broad role in normal synaptic transmission, while Gal-R2 receptors, in addition to a similar role in particular pathways, may be involved in processes prominent during the establishment and maturation of synaptic connections in developing brain and during neural damage and repair in the mature nervous system.

The galanin receptor antagonist M40 blocks the central cardiovascular actions of the galanin N-terminal fragment (1-15)

It has been shown that galanin plays a role in central cardiovascular regulation. Galanin administered centrally induces an increase of heart rate and a weak vasodepressor response, whereas the N-terminal galanin fragment (1-15) elicits vasopressor effects and tachycardia. Furthermore, it has been shown that galanin-(1-15), but not galanin-(1-29), decreases the baroreceptor reflex sensitivity. Since these data demonstrate that both galanin and its N-terminal fragment (1-15) exert a different modulation on central cardiovascular control, the aim of this work has been to study if the specific galanin receptor antagonist Galanin-(1-12)-Pro-(Ala-Leu)(2)-Ala]-amide (M40) could modulate their cardiovascular actions. Urethane anaesthetized rats were injected intracisternally and the changes in mean arterial pressure and heart rate were monitored. Two doses of M40 alone have been tested for their cardiovascular effects. With the dose of 1.0 nmol, a significant tachycardia was observed (P<0.001), but 0.1 nmol was ineffective. This suggests a possible agonistic effect for the higher doses of M40. The galanin receptor antagonist M40 at the dose of 0.1 nmol failed to modify the weak vasodepressor effects and tachycardia induced by 3.0 nmol of galanin-(1-29). However, the same dose completely blocked the vasopressor and tachycardic responses elicited by 3.0 nmol of galanin-(1-15). These data show that M40 differentially counteracts the central cardiovascular responses of the galanin fragment and give a functional support for the existence of galanin receptor subtypes within the brainstem. Therefore, the present findings can be explained on the basis that the cardiovascular actions of galanin-(1-29) could be mediated by one type of galanin receptor, whereas a galanin receptor subtype that recognizes N-terminal fragments of galanin may mediate the actions of galanin-(1-15).

Galanin and spatial learning in the rat. Evidence for a differential role for galanin in subregions of the hippocampal formation

Anatomical, neurochemical and behavioural evidence support a role for galanin in hippocampally mediated functions such as spatial learning and memory. To obtain more precise information on this role, galanin (3 nmol/rat) was infused via bilateral chronic cannulae into different areas of the hippocampal formation which are characterized by different galanin receptor subtypes and also by different galanin innervation patterns. The effects of infused galanin on spatial learning were examined in the Morris swim maze. Infusions of galanin into both the dorsal and ventral dentate gyrus, which mainly contain GAL-R2 receptor mRNA and a high degree of galanin-noradrenaline coexistence, significantly retarded spatial acquisition without affecting swim speed or performance in the visible platform test. This spatial learning deficit was fully blocked by pretreatment with the non-selective galanin antagonist M35. Analysis of retention performance suggested that the major effect of intrahippocampal galanin is mediated via a specific disruption of acquisition mechanisms of importance for performance in the probe trial. Galanin infused into the ventral CA1 (a mainly GAL-R1 receptor mRNA expressing region) or into anterior, ventral CA3 regions did not produce any deficits in spatial learning compared to control animals. These results suggest that galanin mediates its action on spatial learning mainly through the GAL-R2 receptor subtype in areas where most of the galanin is present in noradrenergic terminals. A possible role for the GAL-R1 receptor subtype in cognition in the dorsal and ventral hippocampus remains to be defined. The results suggest a differential functional role for galanin and galanin receptor subtypes within subregions of the hippocampal formation.

Galanin-(1-16) modulates 5-HT1A receptors in the ventral limbic cortex of the rat

The aim of the present study was to evaluate whether galanin-(1-16) of the rat and porcine type and rat galanin-(1-29) can modulate the 5-HT1A receptors, using [3H]8-OH-DPAT as a radioligand, in membrane preparations from the ventral limbic cortex of the rat. Galanin-(1-16) produced a concentration dependent increase in the Kd value of [3H]8-OH-DPAT binding sites with a maximal effect of approximately 61% at 30 nM without changing the Bmax values. The galanin antagonist M35 blocked these effects. Rat galanin produced the same pattern of response but was less potent and effective. These results indicate the existence of a galanin receptor subtype in the ventral limbic cortex mainly recognizing N-terminal galanin fragments and capable of more strongly modulating 5-HT1A receptors than cloned galanin receptors.

Internalization of intracerebrally administered porcine galanin (1-29) by a discrete nerve cell population in the hippocampus of the rat

In spite of numerous studies utilizing intraventricular administration of porcine galanin (1-29), little is known about the spread and cellular distribution of exogenous galanin following intraventricular administration. In this study a discrete nerve cell body population with their dendrites became strongly galanin immunoreactive (IR) in the dorsal hippocampus following intraventricular porcine galanin (1.5 nmol/rat). Time course experiments showed that after time intervals of 10 and 20 min, but not at 60 min, scattered small- to medium-sized galanin-IR nerve cell bodies and their dendrites were present in all layers of the dorsal and ventral hippocampus. In double-immunolabeling experiments most of these nerve cells were identified as putative GABA interneurons costoring NPY-IR or somatostatin-IR in some cases. Twenty minutes after intraventricular injection of artificial cerebrospinal fluid (aCSF), only endogenous punctate and coarse galanin-IR terminals were found, but no galanin-IR cell bodies. Intrahippocampal injection of fluorophore-labeled galanin resulted in the appearance of fluorescent nerve cell bodies with the same morphology and localization as in the above experiments. Coadministration of the putative galanin antagonist M35 (0.5 nmol) and galanin (1.5 nmol) resulted in a reduced number of galanin-IR nerve cell bodies in the hippocampus of half of the rats. These findings support the existence of a population of putative hippocampal GABA interneurons with the ability to internalize and concentrate galanin and/or its fragments present in the extracellular fluid, possibly mediated by galanin receptors.

Electrophysiological evidence for a hyperpolarizing, galanin (1-15)-selective receptor on hippocampal CA3 pyramidal neurons

The effects of the 29-amino acid neuropeptide galanin [GAL (1-29)], GAL(1-15), GAL(1-16), and the GAL subtype 2 receptor agonist D-tryptophan(2)-GAL(1-29) were studied in the dorsal hippocampus in vitro with intracellular recording techniques. GAL(1-15) induced, in the presence of tetrodotoxin, a dose-dependent hyperpolarization in hippocampal CA3 neurons. Most of the GAL(1-15)-sensitive neurons did not respond to GAL(1-29), GAL(1-16), or D-tryptophan(2)-GAL(1-29). These results indicate the presence of a distinct, yet-to-be cloned GAL(1-15)-selective receptor on CA3 neurons in the dorsal hippocampus.

Galanin and learning

A number of studies indicate that galanin (GAL) is a potent modulator of basal acetylcholine release in the rat forebrain e.g. in the cholinergic neurons of the septo-hippocampal projections. Thus, GAL perfused through the microdialysis probe decreased basal acetylcholine release in the ventral hippocampus, while it enhanced acetylcholine release in the dorsal hippocampus. This finding indicates that GAL may act via different mechanisms within the subsystems of the hippocampus. This hypothesis has received support from studies using the Morris swim maze, a learning task dependent on hippocampal mechanisms. GAL (3 nmol/rat) infused into the ventral hippocampus impaired spatial learning acquisition, while it tended to facilitate when injected into the dorsal hippocampus. However, the effects of GAL on acetylcholine release and on spatial learning, which are due to activation of GAL-receptors, appear to be indirectly mediated possibly via noradrenaline transmission. GAL is also a potent inhibitor of mesencephalic 5-HT neurotransmission in vivo. These findings are discussed in relation to the role of acetylcholine and serotonin in cognition.

Galanin-R1 receptor mRNA expression in the hypothalamus of the Brattelboro rat

Using in situ hybridization the regulation of mRNA encoding the galanin receptor R1 was investigated in the mutant Brattelboro (diabetes insipidus) rat. We here report an increase of the galanin receptor R1 mRNA levels in the hypothalamic supraoptic and paraventricular nuclei of the mutant strains. The increase seemed to be confined to magnocellular neurons, since no changes were detected in galanin receptor R1 mRNA levels in the extra-hypothalamic nucleus of the olfactory tract. The results confirm that osmotic stimulation induces up-regulation of galanin receptor R1 mRNA levels. This may increase the sensitivity to galanin peptide, the endogenous ligand for this receptor.

Ultrastructural localization of increased neuropeptide immunoreactivity in the axons and cells of the gracile nucleus following chronic constriction injury of the sciatic nerve

Neuropeptide plasticity in the gracile nucleus is thought to play a role in the development of neuropathic pain following nerve injury. Two weeks after chronic constriction injury of adult rat sciatic nerve, galanin, neuropeptide Y and calcitonin gene-related peptide immunoreactivities were increased in fibers and cells in the gracile nucleus ipsilateral to injury. At the electron microscopic level, this increased neuropeptide immunoreactivity was localized in myelinated axons, boutons, dendrites, neurons and glial cells. Galanin-, neuropeptide Y- and calcitonin gene-related peptide-immunoreactive boutons were frequently presynaptic to dendrites of both immunoreactive and non-immunoreactive neurons. However, no neuropeptide Y, galanin and calcitonin gene-related peptide messenger RNA was detected in the injured side gracile nuclei by in situ hybridization. These results show that partial nerve injury to the sciatic nerve induces increases in the content of galanin, neuropeptide Y and calcitonin gene-related peptide immunoreactivities in synaptic terminals within the gracile nucleus, which suggests that there may be increased release of these neuropeptides following sensory or spontaneous stimulation of large-diameter primary afferents following partial nerve injury, perhaps one mechanism involved in neuropathic pain. We also show an apparent transfer of these neuropeptides to the cells of the gracile nucleus, both neurons and glial cells, an intriguing phenomenon of unknown functional significance.