Galanin inhibits performance on rodent memory tasks

Neuroanatomical characterization of the G protein-coupled receptor activity evoked by galanin-related ligands

Galanin neuropeptide is distributed throughout the mammalian nervous system modulating a plethora of diverse physiological functions, including nociception, cognition and neuroendocrine regulation. The regulation of the galaninergic system is an interesting approach for the treatment of different diseases associated to those systems. Nevertheless, the pharmacological selectivity and activities of some galanin receptor (GalR) ligands are still in discussion and seem to depend on the dose, the receptor subtype and the second messengers to which they are coupled at different brain areas. The activity of different GalR ligands on G_i/o proteins, was evaluated by the guanosine 5'-(γ-[³⁵S]thio)triphosphate ([³⁵S]GTPγS) autoradiography in vitro assay applied to rat brain tissue slices in the presence of galanin, M15, M35, M40, gal(2-11) or galnon. The enhancement of the [³⁵S]GTPγS binding induced by the chimerical peptides M15, M35 and M40 was similar to that produced by Gal in those brain areas showing the highest stimulations, such as dorsal part of the olfactory nucleus and ventral subiculum. In contrast to these peptides, using gal(2-11) no effect was measured on G_i/o protein coupling in areas of the rat brain with high GalR₁ density such as posterior hypothalamic nucleus and amygdala, indicating low selectivity for GalR₁ receptors. The effects evoked by the non-peptide ligand, galnon, were different from those induced by galanin, behaving as agonist or antagonist depending on the brain area, but the stimulations were always blocked by M35. Thus, the activity of most used GalR ligands on G_i/o protein mediated signalling is complex and depends on the brain area. More selective and potent GalR ligands are necessary to develop new treatments aimed to modulate the galaninergic system.

Minimal Increments of Hydrophobic Collapse within the N-Terminus of the Neuropeptide Galanin

The neuropeptide galanin has a 35-year history as an intriguing target in drug design owing to its implication as a potential anticonvulsant and neuronal trophic factor among many other therapeutically interesting functions including analgesia and mood alteration. In this study, we report the structural characterization of three synthetic fragments of the galanin N-terminus in buffered aqueous solution: hGal(2-12)KK, hGal(1-12)KK, and hGal(1-17)KK. High-field two-dimensional ¹H-¹H nuclear magnetic resonance (NMR) data were acquired for these fragments and used to derive distance restraints. We further utilized modified hydrogen bonding and dihedral restraints to reflect chemical shift patterns in the data, which revealed the signature of a weakly folded helix. Together, these sets of restraints were used to generate NMR structures of all three fragments, which depict a core of hydrophobic residues that cluster together regardless of the presence of a helical structure, and correspond to residues in the N-terminus of galanin that have been previously shown to be critical for binding its receptors. The helical structure only appears following the inclusion of Gly(1) in the sequence, and at longer sequence lengths, unlike many other peptides, the helix does not propagate. Rather, a few turns of poorly ordered helix appear to be a secondary consequence of clusters of hydrophobic sidechains that are conserved across all of the peptides in this study; the helices themselves appear ordered as a consequence of this clustering, and these clusters compare directly to those observed recently to make contacts between galanin and two of its receptor subtypes. Collapsed hydrophobic residues therefore organize and compose the functional core of human galanin and raise interesting questions about the nature of the conformational order in ligands that bind cell surface receptors.

Increased Serum Neuropeptide Galanin Level Is a Predictor of Cognitive Dysfunction in Patients with Hip Fracture

Background

Hip fracture is a common occurrence in elderly populations and is frequently followed by various levels of cognitive dysfunction, leading to adverse functional outcomes. Risk stratification of hip fracture patients to identify high-risk subsets can enable improved strategies to mitigate cognitive complications. The neuropeptide galanin has multiple neurological functions, and altered levels are documented in dementia-type and depression disorders. The present study investigated the association of serum neuropeptide galanin levels in hip fracture patients with the occurrence of cognitive dysfunction during the first week of admission.

Methods

276 hip fracture patients without preexisting delirium, cognitive impairment, or severe mental disorders were included in a cross-sectional study. Serum galanin levels were assessed by ELISA on the second day of admission. Routine clinical and laboratory variables were documented. MoCA was performed within 1 week, and those with a score < 26 were categorized with “cognitive decline.” Inferential statistics including multiple linear regression analysis were applied to determine the association of serum galanin level and cognitive status.

Results

141 patients were categorized with “cognitive decline,” and 135 patients were categorized as “cognitively normal.” Serum galanin was highly significantly increased in the “cognitive decline” group (34.2 ± 4.8, pg/ml) compared to the “cognitively normal” group (28.9 ± 3.7, pg/ml) and showed significant negative correlation with MoCA scores (r = −0.229, p = 0.016). Regression analysis showed serum galanin as the sole significant independent predictor of lower MoCA scores (β = 0.231, p = 0.035) while age, gender, blood pressure, cholesterol, and blood glucose levels had no significant association.

Conclusion

Higher serum galanin predicted the development of cognitive dysfunction and worse MoCA scores in a cohort of hip fracture patients without preexisting cognitive impairment or delirium at admission, thus warranting large-scale studies investigating galanin as a candidate biomarker to identify hip fracture patients at risk of cognitive decline.

Food-Derived Opioid Peptides in Human Health: A Review

World Health Organization data suggest that stress, depression, and anxiety have a noticeable prevalence and are becoming some of the most common causes of disability in the Western world. Stress-related disorders are considered to be a challenge for the healthcare system with their great economic and social impact. The knowledge on these conditions is not very clear among many people, as a high proportion of patients do not respond to the currently available medications for targeting the monoaminergic system. In addition, the use of clinical drugs is also associated with various side effects such as vomiting, dizziness, sedation, nausea, constipation, and many more, which prevents their effective use. Therefore, opioid peptides derived from food sources are becoming one of the safe and natural alternatives because of their production from natural sources such as animals and plant proteins. The requirement for screening and considering dietary proteins as a source of bioactive peptides is highlighted to understand their potential roles in stress-related disorders as a part of a diet or as a drug complementing therapeutic prescription. In this review, we discussed current knowledge on opioid endogenous and exogenous peptides concentrating on their production, purification, and related studies. To fully understand their potential in stress-related conditions, either as a drug or as a therapeutic part of a diet prescription, the need to screen more dietary proteins as a source of novel opioid peptides is emphasized.

An assembly of galanin-galanin receptor signaling network

The galanin receptor family of proteins is present throughout the central nervous system and endocrine system. It comprises of three subtypes-GalR1, GalR2, and GalR3; all of which are G-protein-coupled receptors. Galanin predominantly acts as an inhibitory, hyper-polarizing neuromodulator, which has several physiological as well as pathological functions. Galanin has a role in mediating food intake, memory, sexual behavior, nociception and is also associated with diseases such as Alzheimer's disease, epilepsy, diabetes mellitus, and chronic pain. However, the understanding of signaling mechanisms of the galanin family of neuropeptides is limited and an organized pathway map is not yet available. Therefore, a detailed literature mining of the publicly available articles pertaining to the galanin receptor was followed by manual curation of the reactions and their integration into a map. This resulted in the cataloging of molecular reactions involving 64 molecules into five categories such as molecular association, activation/inhibition, catalysis, transport, and gene regulation. For enabling easy access of biomedical researchers, the galanin-galanin receptor signaling pathway data was uploaded to WikiPathways ( https://www.wikipathways.org/index.php/Pathway:WP4970 ), a freely available database of biological pathways.

(Neuro) Peptides, Physical Activity, and Cognition

Regular physical activity (PA) improves cognitive functions, prevents brain atrophy, and delays the onset of cognitive decline, dementia, and Alzheimer’s disease. Presently, there are no specific recommendations for PA producing positive effects on brain health and little is known on its mediators. PA affects production and release of several peptides secreted from peripheral and central tissues, targeting receptors located in the central nervous system (CNS). This review will provide a summary of the current knowledge on the association between PA and cognition with a focus on the role of (neuro)peptides. For the review we define peptides as molecules with less than 100 amino acids and exclude myokines. Tachykinins, somatostatin, and opioid peptides were excluded from this review since they were not affected by PA. There is evidence suggesting that PA increases peripheral insulin growth factor 1 (IGF-1) levels and elevated serum IGF-1 levels are associated with improved cognitive performance. It is therefore likely that IGF-1 plays a role in PA induced improvement of cognition. Other neuropeptides such as neuropeptide Y (NPY), ghrelin, galanin, and vasoactive intestinal peptide (VIP) could mediate the beneficial effects of PA on cognition, but the current literature regarding these (neuro)peptides is limited.

Fronto-temporal galanin modulates impulse control

RATIONALE

The neuropeptide galanin has been implicated in a wide range of pathological conditions in which frontal and temporal structures are compromised. It works through three subtypes of G-protein-coupled receptors. One of these, the galanin receptor 1 (Gal-R1) subtype, is densely expressed in the ventral hippocampus (vHC) and ventral prefrontal cortex (vPFC); two brain structures that have similar actions on behavioral control. We hypothesize that Gal-R1 contributes to cognitive-control mechanisms that require hippocampal-prefrontal cortical circuitry.

OBJECTIVE

To examine the effect of local vHC and vPFC infusions of M617, a Gal-R1 agonist, on inhibitory mechanisms of response control.

METHODS

Different cohorts of rats were implanted with bilateral guide cannulae targeting the vPFC or the vHC. Following infusion of the Gal-R1 agonist, we examined the animals' behavior using a touchscreen version of the 5-choice reaction time task (5-choice task).

RESULTS

The Gal-R1 agonist produced opposing behaviors in the vPFC and vHC, leading to disruption of impulse control when infused in the vPFC but high impulse control when infused into the vHC. This contrast between areas was accentuated when we added variability to the timing of the stimulus, which led to long decision times and reduced accuracy in the vPFC group but a general improvement in performance accuracy in the vHC group.

CONCLUSIONS

These results provide the first evidence of a selective mechanism of Gal-R1-mediated modulation of impulse control in prefrontal-hippocampal circuitry.

Galanin and Cognition

Since the neuropeptide galanin’s discovery in 1983, information has accumulated that implicates it in a wide range of functions, including pain sensation, stress responses, appetite regulation, and learning and memory. This article reviews the evidence for specific functions of galanin in cognitive processes. Consistencies as well as gaps in the literature are organized around basic questions of methodology and theory. This review shows that although regularities are evident in the observed behavioral effects of galanin across several methods for measuring learning and memory, generalization from these findings is tempered with concerns about confounds and a restricted range of testing conditions. Furthermore, it is revealed that many noncognitive behavioral constructs that are relevant for assessing potential roles for galanin in cognition have not been thoroughly examined. The review concludes by laying out how future theory and experimental work can overcome these concerns and confidently define the nature of the association of galanin with particular cognitive constructs.

Hippocampal plasticity during the progression of Alzheimer's disease

Neuroplasticity involves molecular and structural changes in central nervous system (CNS) throughout life. The concept of neural organization allows for remodeling as a compensatory mechanism to the early pathobiology of Alzheimer's disease (AD) in an attempt to maintain brain function and cognition during the onset of dementia. The hippocampus, a crucial component of the medial temporal lobe memory circuit, is affected early in AD and displays synaptic and intraneuronal molecular remodeling against a pathological background of extracellular amyloid-beta (Aβ) deposition and intracellular neurofibrillary tangle (NFT) formation in the early stages of AD. Here we discuss human clinical pathological findings supporting the concept that the hippocampus is capable of neural plasticity during mild cognitive impairment (MCI), a prodromal stage of AD and early stage AD.

New Insights on Retrieval-Induced and Ongoing Memory Consolidation: Lessons from Arc

The mainstream view on the neurobiological mechanisms underlying memory formation states that memory traces reside on the network of cells activated during initial acquisition that becomes active again upon retrieval (reactivation). These activation and reactivation processes have been called "conjunctive trace." This process implies that singular molecular events must occur during acquisition, strengthening the connection between the implicated cells whose synchronous activity must underlie subsequent reactivations. The strongest experimental support for the conjunctive trace model comes from the study of immediate early genes such as c-fos, zif268, and activity-regulated cytoskeletal-associated protein. The expressions of these genes are reliably induced by behaviorally relevant neuronal activity and their products often play a central role in long-term memory formation. In this review, we propose that the peculiar characteristics of Arc protein, such as its optimal expression after ongoing experience or familiar behavior, together with its versatile and central functions in synaptic plasticity could explain how familiarization and recognition memories are stored and preserved in the mammalian brain.

Galanin receptors as a potential target for neurological disease

INTRODUCTION

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

AREAS COVERED

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

EXPERT OPINION

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

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

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

Intranasal Delivery of Proteins and Peptides in the Treatment of Neurodegenerative Diseases

The blood-brain barrier (BBB) is a major impediment to the therapeutic delivery of peptides and proteins to the brain. Intranasal delivery often provides a non-invasive means to bypass the BBB. Advantages of using intranasal delivery include minimizing exposure to peripheral organs and tissues, thus reducing systemic side effects. It also allows substances that typically have rapid degradation in the blood time to exert their effect. Intranasal delivery provides the ability to target proteins and peptides to specific regions of the brain when administered with substrates like cyclodextrins. In this review, we examined the use of intranasal delivery of various proteins and peptides that have implications in the treatment of neurodegenerative diseases, focusing especially on albumin, exendin/GLP-1, GALP, insulin, leptin, and PACAP. We have described their rationale for use, distribution in the brain after intranasal injection, how intranasal administration differed from other modes of delivery, and their use in clinical trials, if applicable. Intranasal delivery of drugs, peptides, and other proteins could be very useful in the future for the prevention or treatment of brain related diseases.

Ghrelin, neuropeptide Y, and other feeding-regulatory peptides active in the hippocampus: role in learning and memory

The hippocampus is a brain region of primary importance for neurogenesis, which occurs during early developmental states as well as during adulthood. Increases in neuronal proliferation and in neuronal death with age have been associated with drastic changes in memory and learning. Numerous neurotransmitters are involved in these processes, and some neuropeptides that mediate neurogenesis also modulate feeding behavior. Concomitantly, feeding peptides, which act primarily in the hypothalamus, are also present in the hippocampus. This review aims to ascertain the role of several important feeding peptides in cognitive functions, either through their local synthesis in the hippocampus or through their actions via specific receptors in the hippocampus. A link between neurogenesis and the orexigenic or anorexigenic properties of feeding peptides is discussed.

The effects of galanin on dorsal root ganglion neurons with high glucose treatment in vitro

The exposure of neurons to high glucose concentrations is considered a determinant of diabetic neuropathy. The extracellular high concentration of glucose can cause neuronal cellular damage. Galanin (Gal) not only plays a role in processing of sensory information but also participates in energy homeostasis and glucoregulation. However, the effects of Gal on dorsal root ganglion (DRG) neurons with high glucose are not clear. Using an in vitro model of high glucose-treated DRG neurons in culture, the effects of Gal on intracellular reactive oxygen species (ROS) expression, cell viability, apoptosis, expression of Gal and its receptors (GalR1 and GalR2) of DRG neurons were investigated. Neurons were dissociated from embryonic day 15 (E15) rat DRG and cultured for 48 h and then maintained in serum-free neurobasal medium containing high glucose (45 mmol/L) or normal glucose (25 mmol/L) for 24h. Mannitol (20 mmol/L) was also used to create a high osmotic pressure mimicking the high glucose condition. The results showed that high glucose caused a rapid increasing of intracellular ROS, decreases of cell viability, and upregulation of Gal and its mRNA. Exogenous Gal (1 μmol/L) inhibited the above effects caused by high glucose. Interestingly, high glucose caused downregulation of GalR1 and its mRNA and administration of exogenous Gal could further decrease their expression, whereas expression of GalR2 and its mRNA was not affected at different experimental conditions. The results of the present study indicate for the first time that Gal and its receptor system are involved in high glucose-induced DRG neuronal injury. The contribution of exogenous Gal on neuroprotection appears to be quite significant. These results provide rationale and experimental evidence for development and further studies of Gal on therapeutic strategy for improving diabetic neuropathy.

Galanin fiber hyperinnervation preserves neuroprotective gene expression in cholinergic basal forebrain neurons in Alzheimer's disease

Fibers containing galanin (GAL) hyperinnervate cholinergic basal forebrain (CBF) nucleus basalis neurons in late stage Alzheimer's disease (AD), yet the molecular consequences of this phenomenon are unknown. To determine whether GAL alters the expression of genes critical to CBF cell survival in AD, single cell microarray analysis was used to determine mRNA levels within nucleus basalis neurons lacking GAL innervation from subjects who died with a clinical diagnosis of no cognitive impairment (NCI) compared to nucleus basalis neurons from AD cases either lacking GAL hyperinnervation (AD/GAL-) or those displaying prominent GAL hyperinnervation (AD/GAL+). Levels of mRNAs encoding putatively neuroprotective proteins such as the GluR2 Ca(2)-impermeable glutamate receptor subunit, superoxide dismutase 2, and the GLUT2 glucose transporter were significantly decreased in AD/GAL- nucleus basalis neurons compared to NCI and AD/GAL+ neurons. By contrast, mRNAs encoding calpain catalytic and regulatory subunits, which may contribute to cell death in AD, were increased in AD/GAL- compared to NCI and AD/GAL+ neurons. Hence, GAL fiber hyperinnervation appears to preserve the expression of genes subserving multiple neuroprotective pathways suggesting that GAL overexpression regulates CBF neuron survival in AD.

Galanin, galanin receptors, and drug targets

Galanin, a neuropeptide widely expressed in the central and peripheral nervous systems and in the endocrine system, has been shown to regulate numerous physiological and pathological processes through interactions with three G-protein-coupled receptors, GalR1 through GalR3. Over the past decade, some of the receptor subtype-specific effects have been elucidated through pharmacological studies using subtype selective ligands, as well as through molecular approaches involving knockout animals. In this chapter, we summarize the current data which constitute the basis of targeting GalR1, GalR2, and GalR3 for the treatment of various human diseases and pathological conditions, including seizure, Alzheimer's disease, mood disorders, anxiety, alcohol intake in addiction, metabolic diseases, pain and solid tumors.

Neuroprotective role for galanin in Alzheimer's disease

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

Impairment of memory consolidation by galanin correlates with in vivo inhibition of both LTP and CREB phosphorylation

Changes in the state of CREB phosphorylation and in LTP in the hippocampus have been associated with learning and memory. Here we show that galanin, the neuropeptide released in the hippocampal formation from cholinergic and noradrenergic fibers, that has been shown to produce impairments in memory consolidation in the Morris water maze task inhibits both LTP and CREB phosphorylation in the rat hippocampus in vivo. While there are many transmitters regulating CREB phosphorylation none has been shown to suppress behaviorally-induced hippocampal CREB phosphorylation as potently as galanin. The in vivo inhibition of dentate gyrus-LTP and of CREB phosphorylation by the agonist occupancy of GalR1 and GalR2-type galanin receptors provides strong in vivo cellular and molecular correlates to galanin-induced learning deficits and designates galanin as a major regulator of the memory consolidation process.

Response of substances co-expressed in hypothalamic magnocellular neurons to osmotic challenges in normal and Brattleboro rats

The intention of this review is to emphasize the current knowledge about the extent and importance of the substances co-localized with magnocellular arginine vasopressin (AVP) and oxytocin (OXY) as potential candidates for the gradual clarification of their actual role in the regulation of hydromineral homeostasis. Maintenance of the body hydromineral balance depends on the coordinated action of principal biologically active compounds, AVP and OXY, synthesized in the hypothalamic supraoptic and paraventricular nuclei. However, on the regulation of water-salt balance, other substances, co-localized with the principal neuropetides, participate. These can be classified as (1) peptides co-localized with AVP or OXY with unambiguous osmotic function, including angiotensin II, apelin, corticotropin releasing hormone, and galanin and (2) peptides co-localized with AVP or OXY with an unknown role in osmotic regulation, including cholecystokinin, chromogranin/secretogranin, dynorphin, endothelin-1, enkephalin, ferritin protein, interleukin 6, kininogen, neurokinin B, neuropeptide Y, vasoactive intestinal peptide, pituitary adenylate cyclase-activating polypeptide, TAFA5 protein, thyrotropin releasing hormone, tyrosine hydroxylase, and urocortin. In this brief review, also the responses of these substances to different hyperosmotic and hypoosmotic challenges are pointed out. Based on the literature data published recently, the functional implication of the majority of co-localized substances is still better understood in non-osmotic than osmotic functional circuits. Brattleboro strain of rats that does not express functional vasopressin was also included in this review. These animals suffer from chronic hypernatremia and hyperosmolality, accompanied by sustained increase in OXY mRNA in PVN and SON and OXY levels in plasma. They represent an important model of animals with constantly sustained osmolality, which in the future, will be utilizable for revealing the physiological importance of biologically active substances co-expressed with AVP and OXY, involved in the regulation of plasma osmolality.

Galanin in Alzheimer's disease: neuroinhibitory or neuroprotective?

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

Galanin receptor subtype 2 (GalR2) null mutant mice display an anxiogenic-like phenotype specific to the elevated plus-maze

The neuropeptide galanin has been implicated in anxiety-related behaviors, cognition, analgesia, and feeding in rodents. Neuromodulatory actions of galanin are mediated by three G-protein coupled receptors, GalR1, GalR2, and GalR3. The present study investigates the role of the GalR2 receptor by evaluating behavioral phenotypes of mice with a targeted mutation in the GalR2 gene. A three-tiered behavioral phenotyping approach first examined control measures of general health, body weight, neurological reflexes, sensory abilities and motor function. Mice were then assessed on several tests for cognitive and anxiety-like behaviors. GalR2 null mutants and heterozygotes were not significantly different from wildtype littermates on two cognitive tests previously shown to be sensitive to galanin manipulation: acquisition of the Morris water maze spatial task, and trace cued and contextual fear conditioning, an emotional learning and memory task. Two independent cohorts of GalR2 null mutant mice demonstrated an anxiogenic-like phenotype in the elevated plus-maze. No genotype differences were detected on several other measures of anxiety-like behavior. The discovery of an anxiogenic phenotype specific to the elevated plus-maze, similar to findings in GalR1 null mutants, highlights the potential therapeutic efficacy of targeting GalR1 and GalR2 receptors in treating anxiety disorders.

Galanin attenuates beta-amyloid (Abeta) toxicity in rat cholinergic basal forebrain neurons

In brains of Alzheimer's disease (AD) patients, expression of the neuropeptide galanin is significantly upregulated and galanin-immunoreactive fibers hypertrophy and hyperinnervate cholinergic neurons of the basal forebrain. However, the role of galanin in AD, whether it is detrimental or neuroprotective, remains controversial. In this study, using primary cultured neurons from the rat basal forebrain, we show that pretreatment with galanin protects cholinergic neurons against beta-amyloid-induced apoptotic cell death as judged by visual observation, MTT assay, Live/dead cell assay, TUNEL and cleaved caspase-3 staining. These effects are mimicked by the galanin receptor 2 (GALR2) agonist, AR-M1896. Western blot analysis revealed Abeta-induced decrease in phospho-PKC and phospho-Akt levels was reversed by galanin. Galanin also attenuated cleavage of caspases-3 and -9 following exposure to Abeta. These findings support a neuroprotective role for galanin and may have implications for development of compounds based on this peptide to treat AD.

Ectopic galanin expression and normal galanin receptor 2 and galanin receptor 3 mRNA levels in the forebrain of galanin transgenic mice

The functional interactions of the neuropeptide galanin (GAL) occur through its binding to three G protein-coupled receptor subtypes: galanin receptor (GALR) 1, GALR2 and GALR3. Previously, we demonstrated that GALR1 mRNA expression was increased in the CA1 region of the hippocampus and discrete hypothalamic nuclei in galanin transgenic (GAL-tg) mice. This observation suggested a compensatory adjustment in cognate receptors in the face of chronic GAL exposure. To evaluate the molecular alterations to GALR2 and GALR3 in the forebrain of GAL overexpressing mice, we performed complementary quantitative, real-time PCR (qPCR), in situ hybridization, and immunohistochemistry in select forebrain regions of GAL-tg mice to characterize the neuronal distribution and magnitude of GAL mRNA and peptide expression and the consequences of genetically manipulating the neuropeptide GAL on the expression of GALR2 and GALR3 receptors. We found that GAL-tg mice displayed dramatic increases in GAL mRNA and peptide in the frontal cortex, posterior cortex, hippocampus, septal diagonal band complex, amygdala, piriform cortex, and olfactory bulb. Moreover, there was evidence for ectopic neuronal GAL expression in forebrain limbic regions that mediate cognitive and affective behaviors, including the piriform and entorhinal cortex and amygdala. Interestingly, regional qPCR analysis failed to reveal any changes in GALR2 or GALR3 expression in the GAL-tg mice, suggesting that, contrary to GALR1, these receptor genes are not under ligand-mediated regulatory control. The GAL-tg mouse model may provide a useful tool for the investigation of GAL ligand-receptor relationships and their role in normal cognitive and affective functions as well as in the onset of neurological disease.

Role of galanin receptor 1 and galanin receptor 2 activation in synaptic plasticity associated with 3',5'-cyclic AMP response element-binding protein phosphorylation in the dentate gyrus: studies with a galanin receptor 2 agonist and galanin receptor 1 knockout mice

The neuropeptide galanin was shown to impair cognitive performance and reduce hippocampal CA1 long-term potentiation (LTP) in rodents. However, the contribution of the two main galanin receptors; GalR1 and GalR2, present in the hippocampus to these effects is not known. In the present study, we determined the protein expression levels of GalR1 and GalR2 in the mouse dentate gyrus (DG) and used galanin (2-11), a recently introduced GalR2 agonist, and GalR1 knockout mice to examine the contribution of GalR1 and GalR2 to the modulation of LTP and 3',5'-cyclic AMP response element-binding protein (CREB)-dependent signaling cascades. In the DG, 57+/-5% of the galanin binding sites were GalR2, and the remaining population corresponded to GalR1. In hippocampal slices, galanin (2-11) fully blocked the induction of DG LTP, whereas galanin (1-29), a high affinity agonist for both GalR1 and GalR2, strongly but not fully attenuated the late phase of LTP by 80+/-1.5%. Application of galanin (1-29) or galanin (2-11) after LTP induction caused a transient reduction in the maintenance phase of LTP, with the larger effect displayed by superfusion of galanin (2-11). The induction and maintenance of DG LTP was not altered in the GalR1 knockout mice. Superfusion of galanin (1-29) or galanin (2-11) blocked the LTP induction to the same degree indicating a role for GalR2 in the induction phase of DG LTP. Furthermore, we analyzed the effects of GalR1 and/or GalR2 activation on DG LTP-induced CREB phosphorylation, associated with the late transcriptional effects of LTP. In the lateral part of the granule cell layer, high-frequency trains stimulation caused a significant increase in the level of CREB phosphorylation, which was significantly reduced by application of either galanin (1-29) or galanin (2-11), indicating that both GalR1 and/or GalR2 can mediate some of their effects on LTP through inhibition of CREB-related signaling cascades.

Galanin and perseveration

Galanin is a 29/30 amino acid neuropeptide that has been shown to impair learning and memory task performance and also have roles in somatosensation, stress responses, sexual behavior, and feeding regulation. However, little is known about galanin involvement in higher cognitive processes, especially executive processes. Perseveration is a classic sign of frontal cortex damage and failure of executive control. Galanin has been shown to disrupt the performance of maze delayed alternation tasks and the operant, spatial delayed nonmatch-to-position (DNMTP) working memory task, tests especially sensitive to perseverative responding. To better understand this potential involvement of galanin in executive control, the present study tested the hypothesis that galanin induces perseveration. The first experiment examined the effects of galanin (10, 20 microg i.c.v.) on the performance of a simple operant response alternation task in which stimuli were assigned to one of two spatially distinct locations to produce extended sequences of presentations to one location, separated by a 10-s intertrial interval. The second experiment looked at the effects of galanin (5, 20 microg i.c.v.) on the performance of non-delayed match-to-position and nonmatch-to-position conditional discrimination operant tasks in which a minimal 1.0 s time interval separated responses. Finally, the effects of galanin (10, 20 microg i.c.v.) on delayed match-to-position (DMTP) performance were examined to determine whether response alternation (i.e., nonmatching) was critical to observing a galanin-induced impairment in this task. Galanin reduced the rate of trial completion in all the tasks, but did not alter simple or conditional discrimination accuracy. Galanin (10 microg) impaired DMTP performance in a delay-independent manner. Together, these data suggest that galanin does not produce perseveration, but are consistent with a galanin-induced decrease in reinforcer strength.

Effect of galnon on induction of long-term potentiation in dentate gyrus of C57BL/6 mice

The impairment of cognitive performance by galanin administration in rodents indicates a possible modulating effect of this neuropeptide on long-term potentiation (LTP) induction in the hippocampal formation. Galnon is a non-peptide, systemically active galanin receptor agonist which has been tested in feeding, seizure and forced swim task in in vivo rodent experimental models. Similarly to galanin (1-29) (i.c.v.), galnon (i.p.) has exhibited anticonvulsant effects in rats. We have investigated the effect of galnon on the synaptic transmission and plasticity in hippocampal dentate gyrus (DG) of C57Bl/6 mice and compared the galnon effects to the effect of galanin (1-29) and galmic, a non-peptide galanin receptor agonist. Similarly to galanin (1-29) and galmic, superfusion of galnon did not alter the input-output responses in DG. Administration of galnon (1 microM) significantly attenuated the LTP induction by 85.5 +/- 1% by 51 min after high frequency trains stimulation. This result was very similar to the effect of galanin (1-29) and galmic, which caused an 80 +/- 1.5% and 94 +/- 2% reduction in the level of field potentiation, respectively. The PPF responses, however, were not altered due to galnon superfusion which is in contrast to the effect of galanin (1-29) or galmic. In summary, these data indicate that the systemically active, non-peptide galanin receptor agonist, galnon can exert similar effects to galanin (1-29) in attenuation of DG LTP in mice.

Induction of galanin receptor-1 (GalR1) expression in external granule cell layer of post-natal mouse cerebellum

Galanin is a modulator of fast transmission in adult brain and recent evidence suggests that it also acts as a trophic factor during neurogenesis and neural injury and repair. Previous studies in our laboratory have identified galanin mRNA in Purkinje cells of adult and developing rat (but not adult mouse) cerebellum; and galanin-binding sites in adult mouse (but not rat) cerebellum. The post-natal development of the cerebellum provides a unique and convenient model for the investigation of developmental processes and to learn more about putative cerebellar galanin systems, the current study examined the presence and distribution of galanin-like-immunoreactivity (- LI), [(125)I]-galanin binding sites and galanin receptor-1 (GalR1) mRNA in post-natal mouse cerebellum. Using autoradiography and in situ hybridization, [(125)I]-galanin binding sites and GalR1 mRNA were first detected on post-natal day 10 (P10) in the external germinal layer of all lobes and high levels were maintained until P14. Quantitative real-time PCR assays detected GalR1 mRNA in whole cerebellum across the post-natal period, with a strong induction and peak of expression at P10. Assessment of galanin levels in whole cerebellum by radioimmunoassay revealed relatively similar concentrations from P5 to P20 and in adult mice (80-170 pg/mg protein), with a significantly higher concentration (250 pg/mg, p < 0.01) detected at P3. These concentrations were some four- to six-fold lower than those in adult forebrain samples. Using immunohistochemistry, galanin-like-immuno-reactivity was observed in prominent fibrous elements within the white matter tracts of the cerebellum at P3-5 and in more punctate elements in the internal granule cell layer and associated with the Purkinje cell layer at P12 and P20. Increased levels of GalR1 mRNA and galanin binding (attributed to GalR1) in the external granule cell layer at P10-12/(14) coincide with granule cell migration from the external to the inner granule cell layer and together with demonstrated effects of other neuropeptide-receptor systems suggest a role for GalR1 signalling in regulating this or related developmental processes.

[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.

Impaired and spared cholinergic functions in the hippocampus after lesions of the medial septum/vertical limb of the diagonal band with 192 IgG-saporin

To lesion the cholinergic input to the hippocampus, rats received injections of 192 IgG-saporin into the medial septum/vertical limb of the diagonal band (MS/VDB). The lesions produced near-total loss of choline acetyltransferase (ChAT)-positive neurons in the MS/VDB. The loss was accompanied, however, by only partial decreases (to 40% of control levels) in acetylcholine (ACh) release in the hippocampus. Moreover, ACh release in the hippocampus increased when lesioned and control rats were tested on a spontaneous alternation task, indicating that there was significant residual cholinergic function in the hippocampus. The lesions were sufficient to impair spontaneous alternation scores. However, this impairment could be reversed by either systemic or intra-hippocampal injections of the indirect cholinergic agonist, physostigmine, providing additional evidence of residual and effective cholinergic functions in the hippocampus of lesioned rats. Moreover, systemic injections of physostigmine at doses that produced mild tremors in control rats led to more severe tremors in the lesioned rats, suggesting upregulation of cholinergic mechanisms after saporin lesions, likely in brain areas other than the hippocampus. Thus, these findings provide evidence for decreases in cholinergic input to the hippocampus accompanied by deficits on a spontaneous alternation tasks. The findings also provide evidence for considerable residual cholinergic input to the hippocampus after saporin lesions of the MS/VDB. Together, the results suggest that 192 IgG-saporin lesions of the MS/VDB, using methods often employed, do not fully remove septohippocampal cholinergic input to the hippocampus but are nonetheless sufficient to produce impairments on a task impaired by hippocampal lesions.

Central galanin administration blocks consolidation of spatial learning

Galanin is a neuropeptide that inhibits the evoked release of several neurotransmitters, inhibits the activation of intracellular second messengers, and produces deficits in a variety of rodent learning and memory tasks. To evaluate the actions of galanin on encoding, consolidation, and storage/retrieval, galanin was acutely administered to Sprague-Dawley rats at time points before and after training trials in the Morris water maze. Intraventricular administration of galanin up to 3h after subjects had completed daily training trials in the Morris water task impaired performance on the probe trial, indicating that galanin-blocked consolidation. Pretreatment with an adenylate cyclase activator, forskolin, prevented the deficits in distal cue learning produced by galanin. Di-deoxyforskolin, an inactive analog of forskolin, had no effect. These results provide the first evidence that galanin interferes with long-term memory consolidation processes. A potential mechanism by which galanin produces this impairment may involve the inhibition of adenylate cyclase activity, leading to inhibition of downstream molecular events that are necessary for consolidation of long-term memory.

Dysregulated hippocampal acetylcholine neurotransmission and impaired cognition in M2, M4 and M2/M4 muscarinic receptor knockout mice

Among the five different muscarinic receptors that have been cloned and characterized, M2 and M4 receptors are localized both post- and presynaptically and are believed to have a pronounced autoreceptor role. The functional importance of these receptors in the regulation of acetylcholine release in the hippocampus and in cognitive processes was investigated by using M2 and M4 receptor single knockout (KO) as well as M2/M4 receptor double KO mice. We found profound alterations in acetylcholine homeostasis in the hippocampus of both M2- and M4-KO mice as well as of the combined M2/M4-KOs, as assessed by in vivo microdialysis. Basal acetylcholine efflux in the hippocampus was significantly increased in M4-KO and was elevated further in M2/M4-KOs. The increase in hippocampal acetylcholine induced by local administration of scopolamine was markedly reduced in M2-KO and completely abolished in M2/M4-KOs. In M2-KO and much more in M2/M4-KOs, the increase in hippocampal acetylcholine triggered by exposure to a novel environment was more pronounced both in amplitude and duration, with a similar trend observed for M4-KOs. Dysregulation of cholinergic function in the hippocampus, as it could result from perturbed autoreceptor function, may be associated with cognitive deficits. Importantly, M2- and M2/M4-KO, but not M4-KO, animals showed an impaired performance in the passive avoidance test. Together these results suggest a crucial role for muscarinic M2 and M4 receptors in the tonic and phasic regulation of acetylcholine efflux in the hippocampus as well as in cognitive processes.

Distribution and regulation of galanin receptor 1 messenger RNA in the forebrain of wild type and galanin-transgenic mice

To learn more about molecular alterations in the brain that occur as a consequence of either the chronic excess or absence of peptide neurotransmitters, we examined the impact of genetically manipulating the neuropeptide galanin on the expression of one of its cognate receptors, galanin receptor 1. First, we examined the distribution of galanin receptor 1 messenger RNA in the mouse forebrain, and found it to be abundantly expressed in many brain regions, including in numerous hypothalamic and other forebrain regions associated with neuroendocrine function. The distribution of galanin receptor 1 messenger RNA in the mouse was similar to previous reports in the rat, with additional expression noted in the caudate putamen and in several midbrain regions. Next, using quantitative in situ hybridization, we measured cellular levels of galanin receptor 1 messenger RNA in the brains of mice that either overexpress galanin (galanin transgenic) or lack a functional galanin gene (galanin knockout). We report that relative to wild-type controls, the expression of galanin receptor 1 messenger RNA was increased in discrete areas of the brain in galanin-transgenic mice, but that depletion of galanin/noradrenergic innervation to the hypothalamus with the neurotoxin 6-hydroxydopamine did not alter levels of galanin receptor 1 messenger RNA. We also report that levels of galanin receptor 1 messenger RNA were not different between galanin-knockout and wild-type mice. These results suggest that compensatory adjustments in the expression of cognate receptors represent one mechanism by which the developing nervous system attempts to maintain homeostasis in response to overexpression of a peptidergic transmitter. However, the lack of significant changes in galanin receptor 1 messenger RNA in galanin-knockout mice suggests that developmentally programmed levels of receptor expression are maintained even in the complete absence of ligand.

Galanin peptide levels in hippocampus and cortex of galanin-overexpressing transgenic mice evaluated for cognitive performance

Galanin-overexpressing transgenic mice (GAL-tg) generated on a dopamine beta-hydroxylase promoter were previously shown to express high levels of galanin mRNA in the locus coeruleus, and to perform poorly on challenging cognitive tasks. The present study employed radioimmunoassay to quantitate the level of galanin peptide overexpression in two brain regions relevant to learning and memory, the hippocampus and cerebral cortex. Approximately 4-fold higher levels of galanin were detected in the hippocampus of GAL-tg as compared to WT. Approximately 10-fold higher levels of galanin were detected in the frontal cortex of GAL-tg as compared to WT. A second cohort of GAL-tg and WT again showed high levels of galanin overexpression in GAL-tg as compared to WT in both brain regions. Correlation analyses were conducted between galanin peptide concentrations and behavioral scores on four learning and memory tasks: the Morris water maze, social transmission of food preference, standard delay fear conditioning, and trace fear conditioning. While some significant correlations were detected, neither hippocampal nor cortical galanin levels in the two cohorts of GAL-tg consistently correlated with performance across these diverse cognitive tasks. Several interpretations of these findings are discussed, including the possibility that a threshold level of galanin overexpression is sufficient to impair performance on learning and memory tasks in mice.

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.

Dorsal and ventral hippocampal cholinergic systems modulate anxiety in the plus-maze and shock-probe tests

There is emerging evidence that increased acetylcholine levels in brain reduce anxiety. More specifically there is evidence that some of these anxiolytic effects of acetylcholine are modulated by the hippocampus. In the present study we examined the roles of the cholinergic systems in the dorsal and ventral hippocampus in two animal models of anxiety: the elevated plus-maze and the shock-probe burying tests. We found that microinfusions (10 microg/0.5 microl) of the acetylcholinesterase inhibitor physostigmine in either the dorsal or the ventral hippocampus increased rats' open arm exploration in the plus-maze test, and decreased burying behavior in the shock-probe test. Interestingly, infusions in the ventral, but not the dorsal hippocampus also increased the number of contacts rats made with the shock-probe. Overall, the results suggest that cholinergic stimulation in the dorsal and ventral hippocampus modulate anxiety, but that only the ventral hippocampal cholinergic system is involved in the passive avoidance of painful stimuli.

Deficits in trace cued fear conditioning in galanin-treated rats and galanin-overexpressing transgenic mice

Galanin inhibits the release of several neurotransmitters and produces performance deficits in a variety of spatial and aversive learning and memory tasks. The experiments in this study investigated the role galanin has in emotional learning and memory using a standard delay cued and contextual fear conditioning task. Rats were administered galanin into the lateral ventricles before training, and scored for freezing behavior in the same context and in a novel context with and without an auditory cue (CS) that had been paired previously with an aversive stimulus (US). Galanin-overexpressing transgenic mice were tested in an identical behavioral protocol. The galanin-administered rats and the transgenic mice were not significantly different from their respective controls on this task. A more challenging trace cued and contextual fear conditioning procedure was administered to separate groups of galanin-treated rats and galanin-overexpressing transgenic mice. Subjects were trained with the same CS and US, however, a 2.5-sec delay was inserted between CS offset and US onset. Following the trace conditioning, rats administered galanin and mice overexpressing galanin both exhibited significantly less freezing to the CS in the novel context as compared with their control groups. These results indicate that the observed disruption of cued fear conditioning was specific to the more difficult trace conditioning task. These findings are the first demonstration that galanin impairs performance on an emotional memory task and support the hypothesis that galanin-induced deficits are specific to more difficult cognitive tasks.

Functional continuum of regulatory peptides (RPs): vector model of RP-effects representation

During the past decades, bioactive (regulatory) peptides have been identified as the major players in the regulation of many important biological processes. Dozens of peptides have found their application as pharmaceutical agents, which further stimulated research in this field making it one of the most rapidly developing areas on the edge of biological science and medicine. However, the fast accumulation of enormous amounts of experimental data has revealed a great difficulty in their analysis and demanded the development of a systematic approach for generalization of the obtained information. We propose a new computer-based algorithm for studying biological activities of regulatory peptides and their groups based on their representation as vectors in n -dimensional functional space. Our method allows the rapid analysis of databases containing thousands of polyfunctional regulatory peptides with overlapping spectra of physiological activity. The described method permits to perform several types of correlations which, when applied to the large databases, could reveal new important information about the system of regulatory peptides. It can select the groups of peptides with similar physiological role (peptide constellations) and search for the optimal peptide combinations with predetermined spectrum of effects and minimal side effects for their further pharmacological application. It can also reveal the role of regulatory peptides in induction of chain physiological reactions.

Galanin overexpressing transgenic mice

Galanin overexpressing transgenic mice (GAL-tg) were generated on two different promoters. Both lines of GAL-tg displayed high levels of galanin in the hippocampus and reduced sensitivity to seizures, as compared to their respective wildtype littermate controls (WT). Performance deficits on learning and memory tasks, impaired long-term potentiation, reduced hippocampal excitability, lower evoked glutamate release, and reduced numbers of choline acetyltransferase immunoreactive neurons in the horizontal limb of the diagonal band were detected in GAL-tg as compared to WT. Changes in sensitivity to nociceptive stimuli were demonstrated in one line. GAL-tg represent a new model for investigating the biological actions of endogenous galanin, and for testing novel therapeutics based on galanin receptor ligands.

Galanin: neurobiologic mechanisms and therapeutic potential for Alzheimer's disease

The neuropeptide galanin (GAL) is widely distributed in the mammalian CNS. Several lines of evidence suggest that GAL may play a critical role in cognitive processes such as memory and attention through an inhibitory modulation of cholinergic basal forebrain activity. Furthermore, GAL fibers hyperinnervate remaining cholinergic basal forebrain neurons in Alzheimer's disease (AD). This suggests that GAL activity impacts cholinergic dysfunction in advanced AD. Pharmacological and in vitro autoradiographic studies indicate the presence of heterogeneous populations of GAL receptor (GALR) sites in the basal forebrain which bind GAL with both high and low affinity. Interestingly, we have recently observed that GALR binding sites increase in the anterior basal forebrain in late-stage AD. Three G protein-coupled GALRs have been identified to date that signal through a diverse array of effector pathways in vitro, including adenylyl cyclase inhibition and phospholipase C activation. The repertoire and distribution of GALR expression in the basal forebrain remains unknown, as does the nature of GAL and GALR plasticity in the AD basal forebrain. Recently, GAL knockout and overexpressing transgenic mice have been generated to facilitate our understanding of GAL activity in basal forebrain function. GAL knockout mice result in fewer cholinergic basal forebrain neurons and memory deficits. On the other hand, mice overexpressing GAL display hyperinnervation of basal forebrain and memory deficits. These data highlight the need to explore further the putative mechanisms by which GAL signaling might be beneficial or deleterious for cholinergic cell survival and activity within basal forebrain. This information will be critical to understanding whether pharmacological manipulation of GALRs would be effective for the amelioration of cognitive deficits in AD.

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.

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 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.

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.