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

A New Augmentation Strategy against Depression Combining SSRIs and the N-terminal Fragment of Galanin (1-15)

Depression is one of the most disabling mental disorders, with the second highest social burden; its prevalence has grown by more than 27% in recent years, affecting 246 million in 2021. Despite the wide range of antidepressants available, more than 50% of patients show treatment-resistant depression. In this review, we summarized the progress in developing a new augmentation strategy based on combining the N-terminal fragment of Galanin (1-15) and SSRI-type antidepressants in animal models.

Deciphering the role of siRNA in anxiety and depression

Anxiety and depression are central nervous system illnesses that are among the most prevalent medical concerns of the twenty-first century. Patients with this condition and their families bear psychological, financial, and societal hardship. There are currently restrictions when utilizing the conventional course of treatment. RNA interference is expected to become an essential approach in anxiety and depression due to its potent and targeted gene silencing. Silencing of genes by post-transcriptional modification is the mechanism of action of small interfering RNA (siRNA). The suppression of genes linked to disease is typically accomplished by siRNA molecules in an efficient and targeted manner. Unfavourable immune responses, off-target effects, naked siRNA instability, nuclease vulnerability, and the requirement to create an appropriate delivery method are some of the challenges facing the clinical application of siRNA. This review focuses on the use of siRNA in the treatment of anxiety and depression.

Galanin receptor 1 expressing neurons in hippocampal-prefrontal circuitry modulate goal directed attention and impulse control

While amino acid neurotransmitters are the main chemical messengers in the brain, they are co-expressed with neuropeptides which are increasingly recognized as modulators of cognitive pathways. For example, the neuropeptide galanin has been implicated in a wide range of pathological conditions in which frontal and temporal structures are compromised. In a recent study in rats, we discovered that direct pharmacological stimulation of galanin receptor type 1 (GalR1) in the ventral prefrontal cortex (vPFC) and ventral hippocampus (vHC) led to opposing effects on attention and impulse control behavior. In the present study, we investigate how subtypes of neurons expressing GalR1 in these two areas differentially contribute to these behaviors. We first establish that GalR1 is predominantly expressed in glutamatergic neurons in both the vPFC and HC. We develop a novel viral approach to gain genetic access to GalR1-expressing neurons and demonstrate that optogenetic excitation of GalR1 expressing neurons in the vPFC, but not vHC, selectively disrupts attention in a complex behavioral task. Finally, using fiber photometry, we measure the bulk calcium dynamics in GalR1-expressing neurons during the same task to demonstrate opposing activity in vPFC and vHC. These results are consistent with our previous work demonstrating differential behavioral effects induced by GalR1 activating in vPFC and vHC. These results indicate the distinct neuromodulatory and behavioral contributions of galanin mediated by subclasses of neurons in the hippocampal and prefrontal circuitry.

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.

Decreased medial prefrontal cortex activity related to impaired novel object preference task performance following GALR2 and Y1R agonists intranasal infusion

Different brain regions' interactions have been implicated in relevant neurological diseases, such as major depressive disorder (MDD), anxiety disorders, age-dependent cognitive decline, Alzheimer's disease (AD) and addiction. We aim to explore the role of the medial prefrontal cortex (mPFC) in the Neuropeptide Y (NPY) and Galanin (GAL) interaction since we have demonstrated specific NPY and GAL interactions in brain areas related to these brain diseases. We performed GALR2 and Y1R agonists intranasal infusion and analyzed the mPFC activation through c-Fos expression. To assess the associated cellular mechanism we studied the formation of Y1R-GALR2 heteroreceptor complexes with in situ proximity ligation assay (PLA) and the expression of the brain-derived neurotrophic factor (BDNF). Moreover, the functional outcome of the NPY and GAL interaction on the mPFC was evaluated in the novel object preference task. We demonstrated that the intranasal administration of both agonists decrease the medial prefrontal cortex activation as shown with the c-Fos expression. These effects were mediated by the decreased formation of Y1R-GALR2 heteroreceptor complexes without affecting the BDNF expression. The functional outcome of this interaction was related to an impaired performance on the novel object preference task. Our data may suggest the translational development of new heterobivalent agonist pharmacophores acting on Y1R-GALR2 heterocomplexes in the medial prefrontal cortex for the novel therapy on neurodegenerative and psychiatric diseases. DATA SHARING AND DATA ACCESSIBILITY: The data that support the findings of this study are openly available in Institutional repository of the University of Malaga (RIUMA) and from the corresponding author upon reasonable request.

Dysfunctional Heteroreceptor Complexes as Novel Targets for the Treatment of Major Depressive and Anxiety Disorders

Among mental diseases, major depressive disorder (MDD) and anxiety deserve a special place due to their high prevalence and their negative impact both on society and patients suffering from these disorders. Consequently, the development of novel strategies designed to treat them quickly and efficiently, without or at least having limited side effects, is considered a highly important goal. Growing evidence indicates that emerging properties are developed on recognition, trafficking, and signaling of G-protein coupled receptors (GPCRs) upon their heteromerization with other types of GPCRs, receptor tyrosine kinases, and ionotropic receptors such as N-methyl-D-aspartate (NMDA) receptors. Therefore, to develop new treatments for MDD and anxiety, it will be important to identify the most vulnerable heteroreceptor complexes involved in MDD and anxiety. This review focuses on how GPCRs, especially serotonin, dopamine, galanin, and opioid heteroreceptor complexes, modulate synaptic and volume transmission in the limbic networks of the brain. We attempt to provide information showing how these emerging concepts can contribute to finding new ways to treat both MDD and anxiety disorders.

Is Galanin a Promising Therapeutic Resource for Neural and Nonneural Diseases?

BACKGROUND

Galanin (GAL) constitutes a family of neuropeptides composed of four peptides: (i) galanin (GAL), (ii) galanin-message associated peptide (GAMP), (iii) galanin-like peptide (GALP), and (iv) alarin. GAL contains 29/30 amino acids, and its biological action occurs through the interactions with its various receptors (GALR1, GALR2, and GALR3). The neuropeptide GAL regulates several physiological and pathophysiological functions in the central nervous system, the peripheral nervous system, and the peripheral organs. GAL is secreted mainly by oligodendrocytes, astrocytes, and the gastrointestinal tract, and its effect depends on the interaction with its different receptors. These receptors are expressed mainly in the central, peripheral nervous systems and the intestines.

OBJECTIVE

The present review evaluates the role of GAL family in inflammatory diseases. An overview is given of the signaling and pharmacological effects due to the interaction between GAL and GALR in different cell types. The potential use of GAL as a therapeutic resource is critically discussed.

CONCLUSION

GAL is suggested to have an anti-inflammatory function in some situations and a proinflammatory function in others. The literature on GAL is controversial and currently not conclusive. This could be due to the complexity of the metabolic network signaling induced by the interactions between GAL and GALR. In the next future, GAL might be a promising therapeutic resource for several diseases, but its practical use for disease control is presently not advisable.

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 in an Agnathan: Precursor Identification and Localisation of Expression in the Brain of the Sea Lamprey Petromyzon marinus

Galanin is a neuropeptide that is widely expressed in the mammalian brain, where it regulates many physiological processes, including feeding and nociception. Galanin has been characterized extensively in jawed vertebrates (gnathostomes), but little is known about the galanin system in the most ancient extant vertebrate class, the jawless vertebrates or agnathans. Here, we identified and cloned a cDNA encoding the sea lamprey (Petromyzon marinus) galanin precursor (PmGalP). Sequence analysis revealed that PmGalP gives rise to two neuropeptides that are similar to gnathostome galanins and galanin message-associated peptides. Using mRNA in situ hybridization, the distribution of PmGalP-expressing neurons was mapped in the brain of larval and adult sea lampreys. This revealed PmGalP-expressing neurons in the septum, preoptic region, striatum, hypothalamus, prethalamus, and displaced cells in lateral areas of the telencephalon and diencephalon. In adults, the laterally migrated PmGalP-expressing neurons are observed in an area that extends from the ventral pallium to the lateral hypothalamus and prethalamus. The striatal and laterally migrated PmGalP-expressing cells of the telencephalon were not observed in larvae. Comparison with studies on jawed vertebrates reveals that the presence of septal and hypothalamic galanin-expressing neuronal populations is highly conserved in vertebrates. However, compared to mammals, there is a more restricted pattern of expression of the galanin transcript in the brain of lampreys. This work provides important new information on the early evolution of the galanin system in vertebrates and provides a genetic and neuroanatomical basis for functional analyses of the galanin system in lampreys.

Role of the galanin N-terminal fragment (1-15) in anhedonia: Involvement of the dopaminergic mesolimbic system

BACKGROUND

Anhedonia is a core feature of depressive disorders. The galanin N-terminal fragment (1-15) plays a role in mood regulation since it induces depression and anxiogenic-like effects in rats. In this study, we analysed galanin N-terminal fragment (1-15) actions in anhedonic-like behaviours in rats using operant and non-operant tests and the areas involved with these effects.

METHODS

Galanin N-terminal fragment (1-15) effects were analysed in saccharin self-administration, sucrose preference, novelty-suppressed feeding and female urine sniffing tests. The areas involved in galanin N-terminal fragment (1-15)-mediated effects were studied with positron emission tomography for in vivo imaging, and we analysed the ventral tegmental area and nucleus accumbens. Galanin N-terminal fragment (1-15) had effects on the mRNA expression of the dopamine transporters Dat and Vmat2; the C-Fos gene; the dopamine receptors D1, D2, D3, D5; and the galanin receptors 1 and 2.

RESULTS

Galanin N-terminal fragment (1-15) at a concentration of 3 nmol induced a strong anhedonia-like phenotype in all tests. The involvement of galanin receptor 2 was demonstrated with the galanin receptor 2 antagonist M871 (3 nmol). The 18F-fluorodeoxyglucose positron emission tomography images indicated the action of galanin N-terminal fragment (1-15) over several nuclei of the limbic system. Galanin N-terminal fragment (1-15)-mediated effects also involved changes in the expression of Dat, Vmat2, D3 and galanin receptors in the ventral tegmental area as well as the expression of C-Fos, D1, D2 and D3 and TH immunoreactivity in the nucleus accumbens.

CONCLUSIONS

Our results indicated that galanin N-terminal fragment (1-15) exerts strong anhedonic-like effects and that this effect was accompanied by changes in the dopaminergic mesolimbic system. These results may provide a basis for the development of novel therapeutic strategies using galanin N-terminal fragment (1-15) analogues for the treatment of depression and reward-related diseases.

Central administration of galanin N-terminal fragment 1-15 decreases the voluntary alcohol intake in rats

Alcohol consumption is considered a major risk factor for disease and mortality worldwide. In the absence of effective treatments in alcohol use disorders, it is important to find new biological targets that could modulate alcohol consumption. We tested the role of the N-terminal galanin fragment (1-15) [GAL(1-15)] in voluntary ethanol consumption in rats using the two-bottle choice paradigm as well as compare the effects of GAL(1-15) with the whole molecule of GAL. We describe for the first time that GAL(1-15), via central mechanisms, induces a strong reduction in preference and ethanol consumption in rats. These effects were significantly different than GAL. GAL receptor (GALR) 2 was involved in these effects, because the specific GALR2 antagonist M871 blocked GAL(1-15) mediated actions in preference and ethanol intake. Importantly, the mechanism of this action involves changes in GALR expression and also in immediate-early gene C-Fos and receptors-internalization-related gene Rab5 in the striatum. The relevance of the striatum as a target for GAL(1-15) was supported by the effect of GAL(1-15) on the locomotor activity of rats after ethanol administration. These results may give the basis for the development of novel therapeutics strategies using GAL(1-15) analogues for the treatment of alcohol use disorders in humans.

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

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

A Novel Integrative Mechanism in Anxiolytic Behavior Induced by Galanin 2/Neuropeptide Y Y1 Receptor Interactions on Medial Paracapsular Intercalated Amygdala in Rats

Anxiety is evoked by a threatening situation and display adaptive or defensive behaviors, found similarly in animals and humans. Neuropeptide Y (NPY) Y1 receptor (NPYY1R) and Galanin (GAL) receptor 2 (GALR2) interact in several regions of the limbic system, including the amygdala. In a previous study, GALR2 enhanced NPYY1R mediated anxiolytic actions on spatiotemporal parameters in the open field and elevated plus maze, involving the formation of GALR2/NPYY1R heteroreceptor complexes in the amygdala. Moreover, the inclusion of complementary ethological parameters provides a more comprehensive profile on the anxiolytic effects of a treatment. The purpose of the current study is to evaluate the anxiolytic effects and circuit activity modifications caused by coactivation of GALR2 and NPYY1R. Ethological measurements were performed in the open field, the elevated plus-maze and the light-dark box, together with immediate early gene expression analysis within the amygdala-hypothalamus-periaqueductal gray (PAG) axis, as well as in situ proximity ligation assay (PLA) to demonstrate the formation of GALR2/NPYY1R heteroreceptor complexes. GALR2 and NPYY1R coactivation resulted in anxiolytic behaviors such as increased rearing and head-dipping, reduced stretch attend postures and freezing compared to single agonist or aCSF injection. Neuronal activity indicated by cFos expression was decreased in the dorsolateral paracapsular intercalated (ITCp-dl) subregion of the amygdala, ventromedial hypothalamic (VMH) nucleus and ventrolateral part of the periaqueductal gray (vlPAG), while increased in the perifornical nucleus of the hypothalamus (PFX) following coactivation of GALR2 and NPYY1R. Moreover, an increased density of GALR2/NPYY1R heteroreceptor complexes was explicitly observed in ITCp-dl, following GALR2 and NPYY1R coactivation. Besides, knockdown of GALR2 was found to reduce the density of complexes in ITCp-dl. Taken together, these results open up the possibility that the increased anxiolytic activity demonstrated upon coactivation of NPYY1R and GALR2 receptor was related to actions on the ITCp-dl. GALR2-NPYY1R heteroreceptor complexes may inhibit neuronal activity, by also modifying the neuronal networks of the hypothalamus and the PAG. These results indicate that GALR2/NPYY1R interactions in medial paracapsular intercalated amygdala can provide a novel integrative mechanism in anxiolytic behavior and the basis for the development of heterobivalent agonist drugs targeting GALR2/NPYY1R heteromers, especially in the ITCp-dl of the amygdala for the treatment of anxiety.

The neuropeptides Galanin and Galanin(1-15) in depression-like behaviours

Galanin is a 29 amino acid neuropeptide widely distributed in neurons within the central nervous system. Galanin exerts its biological activities through three different G protein-receptors and participates in a number of functions, including mood regulation. Not only Galanin but also Galanin N-terminal fragments like Galanin(1-15) are active at the central level. In this work, we review the latest findings in studies on Galanin and Galanin(1-15) in depression-related behaviours. Our focus is on animal models for depression, and we pay some attention to research data obtained in human studies. Since Serotonin (5-HT), especially through 5-HT1A, and Galanin receptors interact at both pre-and postsynaptic level, the development of drugs targeting potential GAL₁-GAL₂-5-HT1A heteroreceptor complexes linked to the raphe-hippocampal 5-HT neurons may represent new treatment strategies in depression.

Galanin microinjection into the dorsal periaqueductal gray matter produces paradigm-dependent anxiolytic effects

Galanin is a peptide that is present in the central nervous system in mammals, including rodents and humans. The actions of galanin are mediated by three types of metabotropic receptors: GAL1, GAL2, and GAL3. GAL1 and GAL3 increase K(+) efflux, and GAL2 increases intracellular Ca(2+) levels. The distribution of galanin and its receptors suggests its involvement in fear and/or anxiety. The periaqueductal gray matter (PAG) is a key mediator of defensive behaviors that is both targeted by galaninergic projections and supplied with GAL1 receptors and, less markedly, GAL2 receptors. We examined the effects of galanin microinjections in the dorsal PAG (dPAG) on the performance of rats in different models of anxiety. Male Wistar rats (n=7-12) were implanted with guide cannulae in the dPAG. They received microinjections of either galanin (0.3, 1.0, and 3.0 nmol) or vehicle and were tested in the Vogel conflict test (VCT), elevated plus maze (EPM), and elevated T-maze (ETM). Rats that were tested in the ETM were further evaluated for exploratory activity in the open field test (OFT). Galanin microinjections had no effects on anxiety-like behavior in the EPM or VCT or exploratory activity in the EPM or OFT. In the ETM, however, microinjections of 3 nmol galanin impaired learned anxiety (i.e., avoidance of the open arms) without changing unconditioned fear (i.e., escape from the open arms). The present data suggest that galanin transmission in the dPAG inhibits the acquisition of anxiety-like responses in the ETM.

A role for galanin N-terminal fragment (1-15) in anxiety- and depression-related behaviors in rats

BACKGROUND

Galanin (GAL) plays a role in mood regulation. In this study we analyzed the action of the active N-terminal fragment [GAL(1-15)] in anxiety- and depression-related behavioral tests in rats.

METHODS

The effect of GAL(1-15) was analyzed in the forced swimming test, tail suspension test, open field test, and light/dark test. The proximity of GAL1 and GAL2 receptors was examined with the proximity ligation assay (PLA). We tested the GAL receptors involved in GAL(1-15) effects with the GAL2 receptor antagonist M871 and with an in vivo model of siRNA GAL2 receptor knockdown or siRNA GAL1 receptor knockdown rats. The effects of GAL(1-15) were also studied in the cell line RN33B.

RESULTS

GAL(1-15) induced strong depression-like and anxiogenic-like effects in all the tests. These effects were stronger than the ones induced by GAL. The involvement of the GAL2 receptor was demonstrated with M871 and with the siRNA GAL2 receptor knockdown rats. The PLA indicated the possible existence of GAL1 and GAL2 heteroreceptor complexes in the dorsal hippocampus and especially in the dorsal raphe nucleus. In the siRNA GAL1 receptor knockdown rats the behavioral actions of GAL(1-15) disappeared, and in the siRNA GAL2 receptor knockdown rats the reductions of the behavioral actions of GAL(1-15) was linked to a disappearance of PLA. In the cell line RN33B, GAL(1-15) decreased 5-HT immunoreactivity more strongly than GAL.

CONCLUSIONS

Our results indicate that GAL(1-15) exerts strong depression-related and anxiogenic-like effects and may give the basis for the development of drugs targeting GAL1 and GAL2 heteroreceptor complexes in the raphe-limbic system for the treatment of depression and anxiety.

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.

A role of supraspinal galanin in behavioural hyperalgesia in the rat

INTRODUCTION

In chronic pain disorders, galanin (GAL) is able to either facilitate or inhibit nociception in the spinal cord but the contribution of supraspinal galanin to pain signalling is mostly unknown. The dorsomedial nucleus of the hypothalamus (DMH) is rich in galanin receptors (GALR) and is involved in behavioural hyperalgesia. In this study, we evaluated the contribution of supraspinal GAL to behavioural hyperalgesia in experimental monoarthritis.

METHODS

In Wistar-Han males with a four week kaolin/carrageenan-induced monoarthritis (ARTH), paw-withdrawal latency (PWL) was assessed before and after DMH administration of exogenous GAL, a non-specific GALR antagonist (M40), a specific GALR1 agonist (M617) and a specific GALR2 antagonist (M871). Additionally, the analysis of c-Fos expression after GAL injection in the DMH was used to investigate the potential involvement of brainstem pain control centres. Finally, electrophysiological recordings were performed to evaluate whether pronociceptive On- or antinociceptive Off-like cells in the rostral ventromedial medulla (RVM) relay the effect of GAL.

RESULTS

Exogenous GAL in the DMH decreased PWL in ARTH and SHAM animals, an effect that was mimicked by a GALR1 agonist (M617). In SHAM animals, an unselective GALR antagonist (M40) increased PWL, while a GALR2 antagonist (M871) decreased PWL. M40 or M871 failed to influence PWL in ARTH animals. Exogenous GAL increased c-Fos expression in the RVM and dorsal raphe nucleus (DRN), with effects being more prominent in SHAM than ARTH animals. Exogenous GAL failed to influence activity of RVM On- or Off-like cells of SHAM and ARTH animals.

CONCLUSIONS

Overall, exogenous GAL in the DMH had a pronociceptive effect that is mediated by GALR1 in healthy and arthritic animals and is associated with alterations of c-Fos expression in RVM and DRN that are serotonergic brainstem nuclei known to be involved in the regulation of pain.

Galanin activated Gi/o-proteins in human and rat central nervous systems

The neuropeptide galanin (GAL) is involved in the control of hormone secretion, nociception, feeding behavior, attention, learning and memory. The anatomical localization of galanin receptors in the brain has been described using autoradiography and immunohistochemistry, but both techniques are limited by the availability of specific radioligands or antibodies. Functional autoradiography provides an alternative method by combining anatomical resolution and information of the activity mediated by G-protein coupled receptors. The present study analyzes the functional GAL receptors coupled to Gi/o-proteins in human and rat brain nuclei using [(35)S]GTPγS autoradiography. The results show the anatomical distribution of Gi/o-proteins activated by GAL receptors that trigger intracellular signaling mechanisms. The activity mediated by GAL receptors in human and rat brain showed a good correlation of the net stimulation in areas such as spinal cord, periaqueductal gray, putamen, CA3 layers of hippocampus, substantia nigra and diverse thalamic nuclei. The functional GAL receptors coupled to Gi/o-proteins showed a similar pattern for both species in most of the areas analyzed, but some discrete nuclei showed differences in the activity mediated by GAL, such as the ventroposteromedial thalamic nucleus, or areas that regulate learning and memory processes in the hippocampus. Taken into consideration the present results, the rat could be used as an experimental model for the study of the physiological role of GAL-mediated neurotransmission and the modulation of GAL receptors activity in the human CNS.

On the existence and function of galanin receptor heteromers in the central nervous system

Galanin receptor (GalR) subtypes 1-3 linked to central galanin neurons may form heteromers with each other and other types of G protein-coupled receptors in the central nervous system (CNS). These heteromers may be one molecular mechanism for galanin peptides and their N-terminal fragments (gal 1-15) to modulate the function of different types of glia-neuronal networks in the CNS, especially the emotional and the cardiovascular networks. GalR-5-HT1A heteromers likely exist with antagonistic GalR-5-HT1A receptor-receptor interactions in the ascending midbrain raphe 5-HT neuron systems and their target regions. They represent a novel target for antidepressant drugs. Evidence is given for the existence of GalR1-5-HT1A heteromers in cellular models with trans-inhibition of the protomer signaling. A GalR1-GalR2 heteromer is proposed to be a galanin N-terminal fragment preferring receptor (1-15) in the CNS. Furthermore, a GalR1-GalR2-5-HT1A heterotrimer is postulated to explain why only galanin (1-15) but not galanin (1-29) can antagonistically modulate the 5-HT1A receptors in the dorsal hippocampus rich in gal fragment binding sites. The results underline a putative role of different types of GalR-5-HT1A heteroreceptor complexes in depression. GalR antagonists may also have therapeutic actions in depression by blocking the antagonistic GalR-NPYY1 receptor interactions in putative GalR-NPYY1 receptor heteromers in the CNS resulting in increases in NPYY1 transmission and antidepressant effects. In contrast the galanin fragment receptor (a postulated GalR1-GalR2 heteromer) appears to be linked to the NPYY2 receptor enhancing the affinity of the NPYY2 binding sites in a putative GalR1-GalR2-NPYY2 heterotrimer. Finally, putative GalR-α2-adrenoreceptor heteromers with antagonistic receptor-receptor interactions may be a widespread mechanism in the CNS for integration of galanin and noradrenaline signals also of likely relevance for depression.

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.

Neurochemical modulation of central cardiovascular control: the integrative role of galanin

Galanin (GAL) is a peptide involved in multiple functions, including central cardiovascular control. In this review, the role of GAL and its fragments in the modulation of cardiovascular neuronal networks in the nucleus of the solitary tract is presented, including its interaction with the classical neurotransmitters and other neuropeptides involved in cardiovascular responses in this nucleus. First, we describe the cardiovascular responses of GAL and the pathway involved in these responses. Then we summarize findings obtained in our laboratory on how GAL, through its receptors, interacts with two other neuropeptides--Neuropeptide Y and Angiotensin II and their receptors--as they have particularly conspicuous cardiovascular effects. All these results strengthen the role of GAL in central cardiovascular control and indicate the existence of interactions among GAL receptor subtypes and alpha2-adrenergic receptors, AT1, and Y1 receptor subtypes. These interactions are crucial for understanding the integrative mechanisms responsible for the organization of the cardiovascular responses from the NTS.

Effects of galanin on monoaminergic systems and HPA axis: Potential mechanisms underlying the effects of galanin on addiction- and stress-related behaviors

Like a number of neuropeptides, galanin can alter neural activity in brain areas that are important for both stress-related behaviors and responses to drugs of abuse. Accordingly, drugs that target galanin receptors can alter behavioral responses to drugs of abuse and can modulate stress-related behaviors. Stress and drug-related behaviors are interrelated: stress can promote drug-seeking, and drug exposure and withdrawal can increase activity in brain circuits involved in the stress response. We review here what is known about the ability of galanin and galanin receptors to alter neuronal activity, and we discuss potential mechanisms that may underlie the effects of galanin on behaviors involved in responses to stress and addictive drugs. Understanding the mechanisms underlying galanin's effects on neuronal function in brain regions related to stress and addiction may be useful in developing novel therapeutics for the treatment of stress- and addiction-related disorders.

Collaborative depression care, screening, diagnosis and specificity of depression treatments in the primary care setting

The identification, referral and specific treatment of midlife patients in primary care who are distressed by mood, anxiety, sleep and stress-related symptoms, with or without clinically confirmed menopausal symptoms, are confounded by many structural issues in the delivery of women's healthcare. Diagnosis, care delivery, affordability of treatment, time commitment for treatment, treatment specificity for a particular patient's symptoms and patient receptiveness to diagnosis and treatment all play roles in the successful amelioration of symptoms in this patient population. The value of screening for depression in primary care, the limitations of commonly used screening instruments relative to culture and ethnicity, and which clinical care systems make best use of diagnostic screening programs will be discussed in the context of the midlife woman. The Sequenced Treatment Alternatives to Relieve Depression (STAR*D) program illustrates the relatively high rate of unremitted patients, regardless of clinical setting, who are receiving antidepressants. Nonmedication treatment approaches, referred to in the literature as 'nonsomatic treatments', for depression, anxiety and stress, include different forms of cognitive-behavioral therapy, interpersonal therapy, structured daily activities, mindfulness therapies, relaxation treatment protocols and exercise. The specificity of these treatments, their mechanisms of action, the motivation and time commitment required of patients, and the availability of trained practitioners to deliver them are reviewed. Midlife women with menopausal symptoms and depression/anxiety comorbidity represent a challenging patient population for whom an individualized treatment plan is often necessary. Treatment for depression comorbid with distressing menopausal symptoms would be facilitated by the implementation of a collaborative care program for depression in the primary care setting.

Distribution of galanin receptor-2 immunoreactive neurones in the ovine hypothalamus: no evidence for involvement in the control of gonadotrophin-releasing hormone secretion

Galanin is a small neuropeptide that mediates its effects via three receptor isoforms: galanin receptor-1, galanin receptor-2 and galanin receptor-3 (Gal-R1, Gal-R2 and Gal-R3). Galanin is thought to be an important intermediate in signalling in the hypothalamic-pituitary-gonadal axis and has been widely detected in the ovine hypothalamus. The expression of galanin and Gal-R1 has been reported to fluctuate during the reproductive cycle. Although the distribution of Gal-R1 has been determined in the ovine hypothalamus, the distribution of Gal-R2 was hitherto unknown. Using immunohistological and immunofluorescence techniques, we have mapped the distribution of Gal-R2 in the ovine hypothalamus, collected during the follicular phase of the oestrous cycle and examined colocalisation of Gal-R2 with oestrogen receptor alpha (ERalpha) and gonadotrophin-releasing hormone (GnRH). Gal-R2 was expressed in several regions of the hypothalamus (supraoptic nucleus, paraventricular nucleus, ventromedial nucleus, arcuate nucleus) but not as widely expressed as Gal-R1. Areas of Gal-R2 expression overlapped with those reported for Gal-R1. We observed that, in certain defined regions of the hypothalamus, up to 50% of neurones that express Gal-R2 also express ERalpha. No neurones coexpressed Gal-R2 and GnRH. Thus, we conclude that, in follicular phase animals, this receptor plays little or no role in direct intermediary signal transmission in GnRH-mediated control of the reproductive cycle.

Comparative immunohistochemical study of the dopaminergic systems in two inbred mouse strains (C57BL/6J and DBA/2J)

This study investigated possible neurochemical differences in the brain of two inbred mouse strains, C57BL/6J (C57) and DBA/2J (DBA) that in behavioral, memorization and learning tasks under normal and experimental conditions perform differently or often in an opposite manner. The immunohistochemical study, designed to investigate the dopaminergic system, identified many differences within the midbrain A10 area and less marked differences in areas A9 and A8. The number of dopamine transporter (DAT), vesicular monoamine transporter of type 2 (VMT) and tyrosine hydroxylase (TH) immunoreactive cell bodies was significantly higher in the midbrain of DBA mice than in C57 mice (on average +21.5%, P<0.001 in A10: +9.4% in A9, P<0.05: and +5.9% in A8, P<0.1). The distribution patterns of nerve fibres immunoreactive for same antisera also differed significantly in the two strains, especially at prelimbic, infralimbic and anterior cingulate cortical levels. In C57 mice these fibres were scanty whereas in DBA mice they were well represented. In the nucleus accumbens, also the territorial distribution of DAT immunoreactive nerve fibres differed in the two strains. In the midbrain, the galanin immunoreactive axons were more densely distributed in DBA than in C57 mice whereas neurotensin immunoreactive axons were more densely distributed in C57 than in DBA. These distinct immunohistochemical patterns could help to explain why performance differs in the two mouse strains.

Region specific galanin receptor/neuropeptide Y Y1 receptor interactions in the tel- and diencephalon of the rat. Relevance for food consumption

The aim of this work was to determine the interactions between NPY and GAL receptor (GALR) subtypes in the hypothalamus and the amygdala using quantitative receptor autoradiography to analyze the binding characteristics of NPY-Y1 and Y2 receptor subtypes in the presence and absence of GAL. Food intake in satiated animals was evaluated after intraventricular co-injections of GAL and NPY-Y1 or Y2 agonists. The expression of c-Fos IR in both regions was also investigated. GAL decreases NPY-Y1 agonist binding in the arcuate nucleus by about 15% (p<0.01), but increases NPY-Y1 agonist binding in amygdala (18%) (p<0.01). These effects were blocked with the GAL antagonist M35. Y2-agonist binding was not modified by GAL. GAL blocked the food intake induced by the Y1 agonist (p<0.01). Co-injections of Y1 agonist and GAL also reduced the c-Fos expression induced by the Y1 agonist in the arcuate nucleus and the dorsomedial hypothalamic nucleus but increased c-Fos expression in amygdala. These results indicate the existence of antagonistic interactions between GALR and NPY-Y1 receptors in the hypothalamus and their functional relevance for food intake. In contrast, a facilitatory interaction between GALR and Y1 receptors exists in the amygdala which may be of relevance for fear related behaviour.

Galanin type 1 receptor knockout mice show altered responses to high-fat diet and glucose challenge

Galanin, a brain and pancreatic peptide with three receptor subtypes (GALR1, GALR2, and GALR3), is hypothesized to participate in energy homeostasis and glucoregulation. Hypothalamic galanin expression is induced by dietary fat, and intra-hypothalamic galanin administration has orexigenic/anabolic properties. Systemic galanin infusion alters glucoregulation in non-human species, partly through direct actions on pancreatic islets. However, the physiologic significance of endogenous galanin-GALR signaling is unclear. The present studies tested the hypotheses that GALR1 deficiency alters food intake and feed efficiency following switches to high-fat diet and that GALR1 deficiency alters whole-body glucose homeostasis. Adult, male GALR1 knockout (-/-), heterozygote (+/-), and C57BL/6J control (+/+) mice were studied. GALR1 deficiency impaired adaptation to a 3-day high-fat diet challenge, leading to increased food intake, feed efficiency and weight gain. However, during the following 2 weeks, GALR1 knockout mice decreased intake, consuming less daily energy than while maintained on low-fat diet and also than heterozygote littermates. Chow-maintained GALR1 knockout mice showed relative hyperglycemia in fed and d-glucose (i.p. 1.5 g/kg)-challenged states. GALR1 knockout mice showed normal food intake, feed efficiency and weight accrual on low-fat diets, normal fasted glucose levels, and normal glucose sensitivity to porcine insulin (i.p. 1 IU/kg) in vivo. The results support the hypotheses that galanin-GALR1 systems help adapt food intake and metabolism to changes in dietary fat and modulate glucose disposition in mice.

Relaxin-3 in GABA projection neurons of nucleus incertus suggests widespread influence on forebrain circuits via G-protein-coupled receptor-135 in the rat

Relaxin-3 (RLX3) is a newly identified member of the relaxin/insulin peptide family that is highly conserved across a range of species from fish to mammals and is highly expressed in rat, mouse and human brain. Extensive pharmacological studies have demonstrated that RLX3 is a high affinity, selective ligand for G-protein-coupled receptor-135 (GPCR135, now classified as relaxin family peptide-3 receptor; RXFP3). In ongoing studies to understand the physiological functions of RLX3, the distribution of RLX3-containing neuronal elements in rat brain was determined by immunohistochemistry, using an affinity-purified polyclonal antiserum raised against a conserved segment of the RLX3 C-peptide (AS-R3(85-101)). Consistent with the distribution of RLX3 mRNA, neurons containing RLX3-like immunoreactivity (LI) were observed in the pontine nucleus incertus and the majority of these cells, which are known to express corticotropin-releasing factor receptor-1, were shown to express glutamic acid decarboxylase-65-immunoreactivity, suggesting a GABA phenotype. Nerve fibers and terminals containing RLX3-LI were observed adjacent to cells in the nucleus incertus and in various forebrain regions known to receive afferents from the nucleus incertus, including cortex, septum, hippocampus, thalamus, hypothalamus and midbrain. Regions that contained highest densities of RLX3-positive fibers included the medial septum, lateral preoptic area, lateral hypothalamus/medial forebrain bundle and ventral hippocampus; and additional fibers were observed in olfactory bulb and olfactory and frontal/cingulate cortices, bed nucleus of the stria terminalis, dorsal endopiriform, intergeniculate, and supramammillary nuclei, and the periaqueductal gray and dorsal raphe. The RLX3-positive network overlapped the regional distribution of GPCR135 mRNA and specific binding sites for an [125I]-GPCR135-selective, chimeric peptide. These anatomical findings further support the proposition that RLX3 is the endogenous ligand for GPCR135 in rat brain and provide evidence for broad modulatory activity of RLX3 in behavioral activation relating to autonomic and neuroendocrine control of metabolism and reproduction and higher-order processes such as stress and cognition.

Galanin expressed in the excitatory fibers attenuates synaptic strength and generalized seizures in the piriform cortex of mice

The neuropeptide galanin is considered to be an endogenous antiepileptic agent, presumably acting via inhibition of glutamate release. Previously, we have demonstrated that in mice ectopically overexpressing galanin in cortical and hippocampal neurons, particularly in granule cells and their axons, the mossy fibers, hippocampal kindling epileptogenesis is suppressed and is associated with attenuated frequency facilitation in mossy fiber-CA3 cell synapses. We hypothesized that changes in synaptic transmission might occur also in other excitatory synapses of the galanin overexpressing (GalOE) mouse, contributing to seizure suppression. Lateral olfactory tract (LOT) synapses, formed by axons of olfactory bulb (OB) mitral cells and targeting piriform cortex (PC) pyramidal cells, ectopically express galanin in GalOE mice. Using whole-cell patch-clamp recordings, we found that excitatory synaptic responses recorded in PC pyramidal cells during high frequency stimulation of the LOT were attenuated in GalOE mice as compared to wild-type controls. This effect was mimicked by bath application of galanin or its agonist galnon to wild-type slices, supporting the notion of ectopic galanin action. Since the high frequency activation induced in vitro resembles epileptic seizures in vivo, we asked whether the observed synaptic inhibition would result in altered epileptogenesis when animals were kindled via the same synapses. In male GalOE mice, we found that the latency to convulsions was prolonged, and once animals had experienced the first stage 5 seizure, generalized seizures were less sustainable. These data indicate that the PC is a possible target for epilepsy treatment by ectopically overexpressing galanin to modulate seizure activity.

A clinical genetic method to identify mechanisms by which pain causes depression and anxiety

Background

Pain patients are often depressed and anxious, and benefit less from psychotropic drugs than pain-free patients. We hypothesize that this partial resistance is due to the unique neurochemical contribution to mood by afferent pain projections through the spino-parabrachial-hypothalamic-amygdalar systems and their projections to other mood-mediating systems. New psychotropic drugs for pain patients might target molecules in such brain systems. We propose a method to prioritize molecular targets by studying polymorphic genes in cohorts of patients undergoing surgical procedures associated with a variable pain relief response. We seek molecules that show a significant statistical interaction between (1) the amount of surgical pain relief, and (2) the alleles of the gene, on depression and anxiety during the first postoperative year.

Results

We collected DNA from 280 patients with sciatica due to a lumbar disc herniation, 162 treated surgically and 118 non-surgically, who had been followed for 10 years in the Maine Lumbar Spine Study, a large, prospective, observational study. In patients whose pain was reduced >25% by surgery, symptoms of depression and anxiety, assessed with the SF-36 Mental Health Scale, improved briskly at the first postoperative measurement. In patients with little or no surgical pain reduction, mood scores stayed about the same on average. There was large inter-individual variability at each level of residual pain. Polymorphisms in three pre-specified pain-mood candidate genes, catechol-O-methyl transferase (COMT), serotonin transporter, and brain-derived neurotrophic factor (BDNF) were not associated with late postoperative mood or with a pain-gene interaction on mood. Although the sample size did not provide enough power to persuasively search through a larger number of genes, an exploratory survey of 25 other genes provides illustrations of pain-gene interactions on postoperative mood – the mu opioid receptor for short-term effects of acute sciatica on mood, and the galanin-2 receptor for effects of unrelieved post-discectomy pain on mood one year after surgery.

Conclusion

Genomic analysis of longitudinal studies of pain, depression, and anxiety in patients undergoing pain-relieving surgery may help to identify molecules through which pain alters mood. Detection of alleles with modest-sized effects will require larger cohorts.

A Short Estrogen-responsive N-terminal Galanin Homologue Found in Rat Brain and Gut with Antiserum Raised Against Rat Galanin(1-16)

Galanin-like peptide (GALP) is currently the only known galanin(1-29) homologue. However, three different galanin receptors, of which GalR3 exhibits comparatively low affinity for galanin(1-29), and molecular heterogeneity of immunoreactive galanin are arguments for presence of other endogenous galanin homologues. Since antibodies recognize three-dimensional structures of 3-5 amino acids in a peptide, we raised antibodies in rabbits against galanin(1-16) conjugated to bovine serum albumin, looking for the presence of endogenous N-terminal galanin homologues in rat tissues. The antiserum selected had 7,830 times higher avidity for galanin(1-16) compared to galanin(1-29). A single immunoreactive component with a Stokes radius of about 8 amino acids was found. Immunohistochemistry strongly suggested that this immunoreactivity is localised in the same neurons as galanin(1-29). Furthermore, its concentration was increased in response to estrogen treatment in the same brain regions as galanin(1-29), although not as rapidly. The present results indicate the presence of a novel endogenous N-terminal galanin homologue.

Association of galanin haplotypes with alcoholism and anxiety in two ethnically distinct populations

The neuropeptide galanin (GAL) is widely expressed in the central nervous system. Animal studies have implicated GAL in alcohol abuse and anxiety: chronic ethanol intake increases hypothalamic GAL mRNA; high levels of stress increase GAL release in the central amygdala. The coding sequence of the galanin gene, GAL, is highly conserved and a functional polymorphism has not yet been found. The aim of our study was, for the first time, to identify GAL haplotypes and investigate associations with alcoholism and anxiety. Seven single-nucleotide polymorphisms (SNPs) spanning GAL were genotyped in 65 controls from five populations: US and Finnish Caucasians, African Americans, Plains and Southwestern Indians. A single haplotype block with little evidence of historical recombination was observed for each population. Four tag SNPs were then genotyped in DSM-III-R lifetime alcoholics and nonalcoholics from two population isolates: 514 Finnish Caucasian men and 331 Plains Indian men and women. Tridimensional Personality Questionnaire harm avoidance (HA) scores, a dimensional measure of anxiety, were obtained. There was a haplotype association with alcoholism in both the Finnish (P=0.001) and Plains Indian (P=0.004) men. The SNPs were also significantly associated. Alcoholics were divided into high and low HA groups (>or= and Collapse

Key Words

• single-nucleotide polymorphism
• linkage disequilibrium
• haplotype
• galanin
• alcoholism
• anxiety

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MESH Headings

• Alcoholism/genetics
• Anxiety/genetics
• Base Sequence
• Black People/genetics
• DNA Primers
• Ethnicity/genetics
• Female
• Finland
• Galanin/genetics
• Genotype
• Harm Reduction
• Humans
• Indians, North American/genetics
• Male
• Surveys and Questionnaires
• United States
• White People/genetics
• Black or African American

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Grants

• Z99 AA999999 Intramural NIH HHS

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Affiliation(s)

I Belfer Pain and Neurosensory Mechanisms Branch, National Institute of Dental and Craniofacial Research, Bethesda, MD 20892, USA.
H Hipp
C McKnight
C Evans
B Buzas
A Bollettino
B Albaugh
M Virkkunen
Q Yuan
M B Max
D Goldman
M A Enoch

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Distribution of galanin receptor 1-immunoreactive neurons in the ovine hypothalamus: colocalization with GnRH

Galanin is implicated in numerous physiological functions, including reproduction. Where and how galanin acts in the brain is poorly understood, but recent evidence suggests that it is predominantly through the GAL-R1 receptor. Using an antibody raised against the third intracellular loop of rat GAL-R1, a region that is highly conserved among species, our first objective was to determine the distribution of cells expressing immunoreactive GAL-R1 in the hypothalamus of the sheep. GAL-R1-immunoreactive cells were spread widely in the ovine diencephalon and overlapped with the known distribution of GnRH neurons. Galanin has been shown to enhance GnRH secretion, but it is not known whether this effect is transduced at the level of the GnRH neuron or is indirect. Thus, our second objective was to establish if GnRH neurons throughout the hypothalamus expressed GAL-R1 receptors and, if so, whether GAL-R1 expression in GnRH neurons was influenced by season, gender and/or stage of the estrous cycle. In rams and ewes during the non-breeding season, only a tenth of the GnRH neurons expressed immunocytochemically detectable GAL-R1 receptors. In contrast, a fifth of the GnRH neurons expressed immunocytochemically detectable GAL-R1 in the luteal phase, whereas only a twentieth expressed GAL-R1 in the follicular phase. These data suggest that galanin may affect a subpopulation of GnRH neurons through the GAL-R1 receptor and that this affect may be modulated by steroids.

Administration of the galanin antagonist M40 into lateral septum attenuates shock probe defensive burying behavior in rats

Galanin (GAL) has been implicated in modulating anxiety, although a precise role remains unclear. Previous studies revealed anxiolytic effects, anxiogenic effects, or no effect, depending on the test, brain region, route of drug administration and context. We have shown previously that microinjection of the GAL antagonist M40 into central amygdala blocked an anxiolytic response to acute stress on the elevated plus maze when rats were pretreated with yohimbine, suggesting an anxiolytic effect of GAL. By contrast, we also showed that microinjection of M40 into the lateral bed nucleus of the stria terminalis attenuated anxiety-like behavioral responses to stress on the plus maze and social interaction tests, implying an anxiogenic effect for GAL. The behavioral response to stress on both these tests is a reduction of an ongoing behavior (open-arm exploration or social interaction, respectively). To better understand the anxiety-modulating role of GAL, it is also important to ascertain its effect on a response that represents an activation rather than suppression of behavior. Thus, in this study, we investigated an active behavioral response to acute stress in rats, the shock-probe defensive burying response. Bilateral microinjections of M40 into lateral septum (LS), a region important to this response and innervated by GAL, dose-dependently decreased burying without affecting immobility. No change was seen in hindpaw withdrawal latency on a thermosensitivity assay, suggesting that the reduction in burying behavior was not attributable to changes in cutaneous pain sensitivity. These results indicate that in LS, GAL facilitates the active anxiety-like behavioral response on the defensive burying test, similar to its facilitatory effect on anxiety-like stress-induced suppression of behavior in the lateral bed nucleus. These results highlight the fact that, rather than a unified system-like role in modulating anxiety, the effects of GAL can be either facilitating or attenuating, and are region-specific, context-specific and response-specific.

G protein-coupled galanin receptor distribution in the rat central nervous system

The action of galanin in the central nervous system is mediated by at least three galanin receptor subtypes (GalR1, GalR2 and GalR3) which belong to the family of G protein-coupled receptors. GalR1 and GalR2 are coupled to G(i/o) proteins, although the latter may also be coupled to G(q/11) proteins. The aim of the present study was to identify the anatomical distribution and quantify the density of GalRs coupled to G proteins. The galanin (10(-6) M) stimulated guanosine 5'-(gamma-[35S] thio)triphosphate binding assay was used in tissue sections from the rat brain. Maximal percentages of stimulation over basal levels were found in the anterior olfactory nucleus and in the lateral olfactory tract nucleus ( approximately 54%). High levels of stimulation were recorded in diverse hypothalamic nuclei (16-28%), in the amygdala (central amygdaloid nucleus, 40%), in the spinal trigeminal tract (23%) and in layers 1-2 of the spinal cord (26%). Moderate binding stimulation (5-13%) was observed in thalamus, substantia nigra pars compacta, parabrachial nucleus, locus coeruleus and dorsal raphe nucleus. The lowest stimulation induced by galanin was recorded in diverse areas of the cortex, striatum, hippocampus and substantia nigra pars reticulata. The results show an anatomical distribution similar to that described for GalR1. However, in diverse brain areas, in which a high density of these receptors has previously been reported, only a moderate coupling to G proteins was found. These findings would suggest that the efficacy of galanin to induce an effective coupling of its receptors to G proteins could be different depending on the brain area.

Estrogen induces a rapid increase in galanin levels in female rat hippocampal formation − possibly a nongenomic/indirect effect

Administration of 17beta-estradiol to ovariectomized rats increased the concentrations of galanin-like immunoreactivity (LI) in the hippocampal formation by 215% (P < 0.001) within 1 h. An increase of 125% (P < 0.05) was observed in the same brain region in the proestrous phase of a normal estrous cycle. Tamoxifen did not block the 17beta-estradiol-induced increase in the concentration of galanin-LI but resulted in a 62% decrease in the hypothalamus within 1 h. In vivo microdialysis in the dorsal hippocampal formation showed a decrease of extracellular galanin-LI (P < 0.001) 1-2 h after treatment with 17beta-estradiol, indicating a decreased release of galanin. For comparison, we studied the concentrations of neuropeptide Y, which were not influenced significantly in any of the regions studied. Taken together our results suggest that 17beta-estradiol inhibits galanin release, presumably from noradrenergic nerve terminals, and primarily via a nongenomic/indirect action, not necessarily involving the classical nuclear receptors ER-alpha or ER-beta. These rapid estrogen-induced changes in galanin release could influence transmitter signalling and plasticity in the hippocampal formation.

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.

Distribution of galanin-like immunoreactivity in the brain of the Siberian sturgeon (Acipenser baeri)

Galanin is a 29-amino acid peptide widely distributed in the central nervous system of vertebrates. The organization of galaninergic systems is well known in teleosts, the most advanced actinopterygians, but no data are available on primitive bony fish. To extend the evolutionary analysis of galaninergic systems we studied the distribution of galanin-like immunoreactive (GAL-ir) cells and fibers in the sturgeon brain, since chondrosteans are among the most primitive extant actinopterygians. Double-immunolabeling experiments were performed to compare the distribution of galanin with that of neurophysin, tyrosine hydroxylase, and serotonin. Numerous GAL-ir cells of cerebrospinal fluid-contacting (CSF-C) type were found in the ventral telencephalon, preoptic area, and in the tuberal and caudal hypothalamus. The distribution of GAL-ir elements in the sturgeon brain shows many similarities to that observed in other vertebrates, but also important differences, such as the abundance of GAL-ir CSF-C cells, which appear to be a primitive characteristic. GAL-ir neurons observed in the sturgeon telencephalic hemispheres perhaps represent the basic organization of common ancestors of bony fishes and tetrapods. In the preoptic-hypophyseal system, GAL-ir cells appeared to be related not only with neurophysin-expressing neurons (in the tuberal hypothalamus) but also with serotoninergic and catecholamines-synthesizing neurons (in preoptic and tuberal nuclei). Numerous GAL-ir fibers were observed in the median eminence and neural lobe of the hypophysis, indicating that galanin may play a role in the modulation of hypophyseal secretion. GAL-ir neurons were absent from the sturgeon brainstem, suggesting that their presence in other vertebrates could represent an evolutionary recent acquisition.

[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.