Galanin: an unassuming neuropeptide moves to center stage in reproduction

Organization of the galaninergic neuronal system in the brain of the gecko Hemidactylus frenatus

Galanin (GAL) is a 29 amino acid peptide present in the central nervous system (CNS) as well as peripheral tissues in vertebrates. However, the brain distribution pattern of GAL is understudied in reptiles. The aim of this study was to determine the organization of galaninergic neuronal system in the brain of the gecko Hemidactylus frenatus, a tropical and sub-tropical lizard, using rabbit anti-galanin antibody. In the telencephalon, GAL-ir perikarya and fibres were found in the lateral septal nucleus, but only GAL-ir fibres were observed in the striatum, nucleus accumbens, anterior commissure, nucleus centralis amygdalae, dorsal and medial septal nuclei, nucleus of the diagonal band of Broca and in the optic chiasma. In the preoptic region, a cluster of GAL-ir cells and fibres was observed in the periventricular preoptic area and lateral preoptic area. GAL-ir perikarya and fibres were observed in hypothalamic areas such as the supraoptic nucleus, suprachiasmatic nucleus, paraventricular nucleus, periventricular nucleus of the hypothalamus, infundibular recess nucleus and in the median eminence, whereas GAL-ir fibres were present in the pars distalis of the pituitary gland. In the thalamus, GAL-ir fibres were observed in the dorsomedial, dorsolateral, and medial thalamic nuclei. GAL-ir fibres were also detected in mesencephalic areas such as the optic tectum, torus semicircularis, ventral tegmental area and substantia nigra, brain stem as well as the spinal cord. The organization of GAL-ir cells and fibres throughout the gecko brain suggests several neuroendocrine, neuromodulatory and behavioural functions for GAL in lizards. The study provides new insights into the evolutionarily conserved nature of GAL peptide in squamate reptiles and forms a valuable basis for future comparative studies.

Galanin expression varies with parental care and social status in a wild cooperatively breeding fish

As many busy parents will attest, caring for young often comes at the expense of having time to feed and care for oneself. Galanin is a neuropeptide that regulates food intake and modulates parental care; however, the relative importance of galanin in the regulation of feeding versus caring by parents has never been evaluated before under naturalistic settings. Here, we assessed how expression of the galanin system varied in two brain regions, the hypothalamus (which regulates feeding) and the preoptic area (which modulates social behaviours including care) in a wild cichlid fish, Neolamprologus pulcher. Females with young had higher hypothalamic expression of galanin receptor 1a, and the highest expression of galanin and galanin receptor 1a was observed in females that foraged the least. However, expression of five other feeding-related neuropeptides did not change while females were caring for young suggesting that changes in the hypothalamic galanin system may not have been directly related to changes in food intake. The preoptic galanin system was unaffected by the presence of young, but preoptic galanin expression was higher in dominant females (which are aggressive, regularly reproduce and care for young) compared to subordinate females (which are submissive, rarely reproduce but often help care for young). Additionally, preoptic galanin expression was higher in fish that performed more territory defense. Overall, our results indicate that galanin has brain-region-specific roles in modulating both parental care and social status in wild animals.

Neuropeptide galanin and its effects on metabolic and reproductive disturbances in female rats with estradiol valerate (EV) - Induced polycystic ovary syndrome (PCOS)

A functional role of the neuropeptide galanin, executed through the three G-protein coupled receptor subtypes GAL₁₋₃, has been demonstrated in many biological systems and under pathological circumstances. Galanin is involved in many central and peripheral actions, in particular associated with endocrine mechanisms such as anterior pituitary hormone regulation, reproduction, glucose metabolism and also inflammation. The role of galanin in the pathology of the polycystic ovary syndrome (PCOS) and possible therapeutic effects are unknown. However, based on the well known neuroendocrine changes in PCOS patients, it may be assumed that galanin via effects on gonadotropin-releasing hormone (GnRH) secretory neurons could play a significant role in the development of PCOS. The aim of this study was to examine possible therapeutic effects of galanin on hormonal, metabolic and molecular parameters in PCOS. Accordingly, intraperitoneal injection of galanin in a dose- dependent manner in female PCOS rats induced a significant reduction in inflammatory markers (TNF-α, IL-6), an increase in FSH and a decrease in LH, insulin and testosterone (using ELISA kit) compared to the PCOS groups. Moreover, data from real-time quantitative PCR showed significantly ameliorated changes in the mRNA levels of the steroidogenic acute regulatory protein (StAR) and aromatase cytochrome P450 (CYP19). Taken together, galanin has neuroendocrine, anti- and pro-inflammatory and metabolic effects, and we therefore suggest that treatment with this peptide could represent new therapeutic approach for managing hormonal and metabolic disturbances in the PCOS disease.

Effects of cycle stage on regionalised galanin, galanin receptors 1-3, GNRH and GNRH receptor mRNA expression in the ovine hypothalamus

The neurotransmitter galanin has been implicated in the steroidogenic regulation of reproduction based on work mainly conducted in rodents. This study investigated the temporal changes in the expression of galanin and its three receptor isoforms and GNRH and GNRHR mRNA in specific hypothalamic nuclei known to be involved in the regulation of reproductive cyclicity, namely the medial pre-optic area (mPOA), the rostral mPOA/organum vasculosum of the lamina terminalis, the paraventricular nucleus and the arcuate nucleus using an ovine model. Following synchronisation of their oestrous cycles, tissues were collected from ewes at five time points: the early follicular, mid follicular (MF) and late follicular phases and the early luteal and mid luteal phases. The results indicated significant differences in regional expression of most of the genes studied, with galanin mRNA expression being highest during the MF phase at the start of the GNRH/LH surge and the expression of the three galanin receptor (GalR) isoforms and GNRH and its receptor highest during the luteal phase. These findings are consistent with a role for galanin in the positive feedback effects of oestradiol (E(2)) on GNRH secretion and a role for progesterone induced changes in the pattern of expression of GalRs in the regulation of the timing of E(2)'s positive feedback through increased sensitivity of galanin-sensitive systems to secreted galanin.

Expression of mRNA for galanin, galanin-like peptide and galanin receptors 1–3 in the ovine hypothalamus and pituitary gland: effects of age and gender

The neurotransmitters/neuromodulators galanin (GAL) and galanin-like peptide (GALP) are known to operate through three G protein-coupled receptors, GALR1, GALR2 and GALR3. The aim of this study was to investigate changes in expression of mRNA for galanin, GALP and GALR1–3 in the hypothalamus and pituitary gland, of male and female sheep, to determine how expression changed in association with growth and the attainment of reproductive competence. Tissue samples from the hypothalami and pituitary glands were analysed from late foetal and pre-pubertal lambs and adult sheep. Although mRNA for galanin and GALR1-3 was present in both tissues, at all ages and in both genders, quantification of GALP mRNA was not possible due to its low levels of expression. mRNA expression for both galanin and its receptors was seen to change significantly in both tissues as a function of age. Specifically, hypothalamic galanin mRNA expression increased with age in the male, but decreased with age in the female pituitary gland. mRNA expression for all receptors increased between foetal and pre-pubertal age groups and decreased significantly between pre-pubertal and adult animals. The results indicate that the expression of mRNA for galanin and its receptors changes dynamically with age and those significant differences exist with regard to tissue type and gender. These changes suggest that galaninergic neuroendocrine systems could be involved in the regulation of ovine growth and or the development of reproductive competence. The roles played by these systems in the sheep, however, may differ from other species, in particular the neuroendocrine link between nutrition and reproduction and GALR1's role in pituitary signalling.

Profiling neurotransmitter receptor expression in mouse gonadotropin-releasing hormone neurons using green fluorescent protein-promoter transgenics and microarrays

The definition of neurotransmitter receptors expressed by individual neuronal phenotypes is essential for our understanding of integrated neural regulation. We report here a single-neuron strategy using green fluorescent protein (GFP)-promoter transgenic mice and oligonucleotide microarrays that has enabled us to provide a qualitative profile of the neurotransmitter receptors expressed by the gonadotropin- releasing hormone (GnRH) neurons, critical for the neural regulation of fertility. Acute brain slices were prepared from adult female GnRH-GFP transgenic mice and single GnRH neurons identified and patched. The contents of GnRH neurons underwent reverse transcription and cDNA amplification using the switch mechanism at the 5' end of RNA templates system, and hybridization to mouse gene oligonucleotide arrays. Fifty different neurotransmitter receptor subunit mRNAs were detected in GnRH neurons. Many of the classical amino acid and aminergic receptors were present in addition to 14 distinct, and in most cases novel, neuropeptidergic receptor signaling families. Four of the latter were selected for functional validation with gramicidin-perforated patch-clamp electrophysiology. Galanin, GnRH and neuromedin B were all found to exert direct depolarizing actions upon GnRH neurons whereas somatostatin induced a potent hyperpolarizing response. These studies demonstrate a relatively straightforward approach for transcriptome profiling of specific neuronal phenotypes. The stimulatory actions of GnRH and galanin upon GnRH neurons found here indicate that positive ultrashort feedback loops exist among the GnRH neuronal population.

Galanin-like peptide as a link in the integration of metabolism and reproduction

The arcuate nucleus is a hypothalamic center that couples energetics and reproduction. Peptide-releasing neurons in the arcuate nucleus receive and process humoral signals from the periphery and relay this information to other nuclei in the hypothalamus and preoptic area. Galanin-like peptide (GALP) is expressed in the arcuate nucleus, and GALP-containing neurons are targets for the action of leptin. GALP-containing neurons are closely apposed to gonadotropin-releasing hormone (GnRH) neurons in the preoptic area, and CNS injections of GALP stimulate GnRH-mediated secretion of luteinizing hormone. These observations indicate that GALP is a molecular signal that couples circulating indices of metabolism to the neuroendocrine reproductive system and, thus, regulates reproductive activity as a function of the energy state. In this article, we describe the involvement of GALP in metabolism and reproduction, and in the coupling between these two processes.

Distribution of galanin messenger RNA-expressing cells in murine brain and their regulation by leptin in regions of the hypothalamus

Galanin is widely distributed throughout the mammalian brain and has been implicated in the regulation of food intake, metabolism and reproduction-functions that are also thought to be under the control of leptin. To investigate the possible role of galanin in mediating the physiological effects of leptin in the mouse, we had three experimental objectives: first, to map the distribution of galanin messenger RNA-expressing cells in the brain of the mouse; second, to assess the effects of leptin on galanin gene expression in areas of the brain thought to be involved in the regulation of body weight and reproduction; and third, to determine whether galanin neurons in these regions express leptin receptor messenger RNA. We found the pattern of galanin messenger RNA expression in the mouse brain to be similar, but not identical, to that in the rat. Leptin treatment (2microg/g for six days) significantly reduced cellular levels of galanin messenger RNA in the hypothalamic periventricular nucleus of leptin-deficient obese (ob/ob) mice (P<0.01) by approximately 30%; however, leptin did not appear to influence the expression of galanin in the arcuate or dorsomedial nucleus of the hypothalamus. Galanin-producing neurons in the arcuate, dorsomedial and periventricular nuclei did not appear to express leptin receptor messenger RNA (P>0.05). These results demonstrate that galanin distribution patterns in the mouse brain are comparable to other species and, yet, possess unique features. In addition, galanin-expressing neurons in the hypothalamic periventricular nucleus are targets for regulation by leptin; however, the effect of leptin on galanin gene expression is likely to be mediated indirectly, perhaps through either proopiomelanocortin- or neuropeptide Y-expressing cells in the hypothalamus.

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.

Distribution of galanin immunoreactivity in the sheep diencephalon

Although the physiological role of galanin has been demonstrated in several endocrine regulations in sheep, the anatomical characteristics of this neuronal system has never been studied. The distribution of galanin-containing neurones was described by immunohistochemistry using galanin antiserum in the diencephalon of adult ewes, both ovariectomized or treated with colchicine. Galanin-immunoreactivity was found throughout the diencephalon. In the ovariectomized ewes, galanin-immunoreactive neurones were mainly observed in the medial preoptic area and the infundibular nucleus. The highest density of immunoreactive fibres was found in the external layer of the median eminence. Numerous galanin-immunoreactive fibres were also observed in the preoptic area, the mediobasal hypothalamus, the periphery of the supraoptic and the paraventricular nuclei. With colchicine treatment, the number of labelled neurones increased, and additional galanin-immunoreactive perikarya were observed in the bed nucleus of the stria terminalis, the lateral septum, the supraoptic, the paraventricular and the periventricular nuclei and the paraventricular nucleus of the thalamus. In the caudal part of the diencephalon, the density of labelled neurones was lower in both groups of animals than in other species studied. Regardless of treatment, labelling was not seen in the suprachiasmatic nucleus and only rarely in the ventromedial nucleus. These results describe, for the first time, the distribution of galanin-immunoreactive neurones in the sheep diencephalon. Compared to other species studied, distribution in the sheep diencephalon has several distinct differences. In ovariectomized animals, the medial preoptic area presents more labelled neurones in sheep than in monkeys, whereas in the supraoptic nucleus the density of labelled neurones is lower in sheep than in humans or opossums. After colchicine treatment only very few differences were observed between sheep and rats, but in contrast to other species, the suprachiasmatic nucleus of the sheep does not contain labelled neurones.

Galanin and galanin receptors

The development of a strain of galanin knockout mice has provided confirmation of a neuroendocrine role for galanin, as well as supporting results of previous physiological investigations indicating a role for galanin in analgesia and neuropathic pain, and potentially in neuronal growth and regeneration processes. Whether elevation of galanin expression in neurodegenerative disorders such as Alzheimer's disease represents a survival response or exacerbates functional deficit in afflicted individuals remains to be determined. More detailed analysis of the phenotype of the galanin knockout mouse should provide insights into the physiological role of galanin in memory and learning processes, as well as in hypothalamic function and other aspects of neuroendocrine regulation. Biochemical and molecular cloning efforts have demonstrated that the multiplicity of actions of galanin is matched by complexity in the distribution and regulation of galanin and its receptors. A focus on characterisation of galanin receptors has resulted in the molecular cloning of three receptor subtypes to date. The distribution and functional properties of these receptors have not yet been fully elucidated, currently precluding assignment of discrete functions of galanin to any one receptor subtype. It is not currently possible to reconcile available pharmacological data using analogs of galanin and chimeric peptides in functional assay systems with the pharmacological properties of cloned receptor subtypes. This highlights the value of further knockout approaches targeting galanin receptor subtypes, but also raises the possibility of the existence of additional receptor subtypes that have yet to be cloned, or that receptor activity may be modulated by regulatory molecules that remain to be identified. The development of receptor subtype-specific compounds remains a high priority to advance work in this area. The ability to selectively modulate the many different actions of galanin, through a clearer understanding of receptor structure-function relationships and neuronal distribution, promises to provide important insights into the molecular and cellular basis of galanin action in normal physiology, and may provide lead compounds with therapeutic application in the prevention and treatment of a range of disorders.

Galanin stimulates steroidogenesis in rat Leydig cells

The present study was designed to evaluate whether galanin could play a role in the regulation of testicular steroidogenesis. To this purpose, using purified rat Leydig cells, we examined the effects of galanin on basal and hCG- or LHRH-induced testosterone production and the interference of a specific galanin receptor antagonist, galantide, on galanin activity. Moreover, since it has been shown that galanin-induced stimulation of LHRH secretion appears to involve the release of prostaglandin E2 (PGE2) as intracellular mediator, we evaluated also the effect of galanin on Leydig cells PGE2 output and the interference of indomethacin, a cycloxygenase blocker, on its activity. Furthermore, the effect of nordihydroguaiaretic acid (NDGA), a lipoxygenase inhibitor, was also examined. Data obtained indicate that galanin amplified testosterone response to hCG or LHRH whilst galantide prevented its potentiating activity. Moreover, galanin stimulated PGE2 output though this fatty acid is not involved in galanin activity on Leydig cells as indomethacin failed to affect its amplification of testosterone production. The possible involvement of leukotrienes should also be excluded as NDGA did not modify galanin action. In summary, the present study indicates that galanin potentiates acute gonadotropin or LHRH steroidogenic action on Leydig cells and that this activity is specific and receptor-mediated as it is prevented by a specific receptor antagonist.

Mechanisms of galanin-induced contraction in the rat myometrium

A neuropeptide, galanin, regulates the reproductive process and directly induces myometrial contraction. The aim of this study was to determine the mechanism of galanin-induced myometrial contraction. For this purpose, we simultaneously measured intracellular Ca2+ concentration ([Ca2+]i) and tension using fura-PE3-fluorometry and the rat longitudinal myometrium. The effect of galanin on the Ca2+ sensitivity of the contractile apparatus was examined in beta-escin permeabilized strips. The expression of galanin and the galanin receptors mRNAs in the rat myometrium were determined by reverse transcription polymerase chain reaction (RT-PCR). Galanin (10-300 nM) induced phasic contraction with or without oscillation in the pregnant rat myometrium in a concentration-dependent manner. The maximal response was obtained at 100 nM. There was no significant difference either in the maximal responses or EC50 values for galanin-induced myometrial contractions among myometriums from non-pregnant and pregnant (day 4, day 11, day 20, day 22) rats. In the day 20 and 22 pregnant myometriums, assigning the levels of [Ca2+]i and tension at 40 mM K+-depolarization to be 100%, galanin increased the [Ca2+]i and tension to 126.9+/-2.9% and 116.3+/-2.7%, respectively. Diltiazem (10 microM) inhibited the galanin-induced elevation of [Ca2+]i and tension to 71.9+/-2.4% and 16.2+/-0.7%, respectively. Ni2+, by itself, decreased the basal [Ca2+]i to -50.2+/-3.9% without affecting resting tension. After Ni2+ treatment, galanin-induced increases in [Ca2+]i and tension were -19.6+/-3.4% and 0.9+/-0.1%, respectively. In myometrium treated with diltiazem, no oscillation in [Ca2+]i and tension was observed. In Ca2+-free solution with 0.1 mM EGTA, galanin increased [Ca2+]i from -40.2+/-2.7% to -18.0+/-2.6% and induced transient contraction (3.6+/-0.8%). In beta-escin permeabilized myometrium, galanin enhanced the contraction induced by 0.3 microM Ca2+ in the presence of GTP. In the presence of GDPbetaS (1 mM) instead of GTP, galanin failed to increase the Ca2+ sensitivity of the contractile apparatus. RT-PCR revealed that galanin mRNA was hardly expressed in the non-pregnant rat myometrium and increased to reach a maximal level at mid pregnancy (day 11), but decreased to the same level as in the non-pregnant myometrium at term (day 22). Type 2 galanin receptor (GALR2) mRNA was found to be expressed in the rat myometrium whereas type 1 galanin receptor (GALR1) mRNA expression was not detected. In conclusion, galanin induces contraction of the rat myometrium by increasing [Ca2+]i as well as by increasing Ca2+ sensitivity of the contractile apparatus. Galanin-induced increases in [Ca2+]i are caused by both intracellular Ca2+ release and Ca2+ influx from extracellular space. The responsiveness of the rat myometrium to galanin does not change during pregnancy. The galanin mRNA is expressed in the rat myometrium and it is upregulated during mid-pregnancy. Rat myometrium expresses GALR2 but not GALR1 mRNA. By changing mRNA expression in the myometrium during pregnancy, galanin may act as a paracrine or autocrine mediator in the regulation of myometrial contractility.

Expression of functional estrogen receptors and galanin messenger ribonucleic acid in immortalized luteinizing hormone-releasing hormone neurons: estrogenic control of galanin gene expression

The activity of estradiol on the LHRH neuronal network is crucial in the regulation of reproduction. In vivo, estradiol induces galanin (GAL) gene expression in LHRH neurons and GAL/LHRH colocalization is sexually dimorphic and neonatally determined by steroid exposure. The effects of estradiol on LHRH neurons, however, are considered to be indirect because estrogen receptors (ER) have not been detected in LHRH neurons in vivo. Using immortalized mouse LHRH neurons (GT1-7 cells), we demonstrated by RT-PCR and Southern blotting that GT1-7 cells express ER messenger RNA (mRNA). Sequencing of the amplification products indicated that GT1-7 ER is of the alpha-subtype (ER alpha). Additionally, estrogen receptors in GT1-7 cells were characterized by competitive radioligand receptor binding and IC50 values for 17beta-estradiol and ICI-182,780 were found to be 0.24 and 4.1 nM, respectively. The ability of endogenous GT1-7 cell ER to regulate transcription was determined in transient transfection studies using a construct that consisted of a luciferase reporter gene that is driven by tandem estrogen response elements (ERE) and a minimal herpes simplex virus thymidine kinase promoter. 17Beta-estradiol was found to enhance luciferase activity by 2.5-fold at physiological concentrations with an ED50 value of 47 pM. This induction was completely inhibited by ICI-182,780 which had an IC50 value of 4.8 nM. Raloxifene, tamoxifen, 4-hydroxytamoxifen, and droloxifene also fully blocked estrogen-mediated luciferase induction with IC50 values of 58.4, 89.2, 33.2, and 49.8 nM, respectively. In addition, GAL mRNA was detected and identified by RT-PCR followed by Southern blotting using a rat GAL complementary DNA (cDNA) probe. The ability of 17beta-estradiol to modulate expression of the endogenous GAL gene in immortalized LHRH neurons was also determined. Quantitative RT-PCR demonstrated that physiological concentrations of estrogen increase GAL gene expression by 2-fold with an ED50 value of 23 pM. ICI-182,780, raloxifene, and droloxifene completely blocked this induction. In summary, our data demonstrate the presence of ER alpha and GAL mRNA in GT1-7 cells. The ER in GT1-7 cells is biologically active because 17beta-estradiol enhances both endogenous GAL gene expression and an ERE-driven reporter gene. These results suggest that estrogenic control of GAL gene expression in immortalized LHRH neurons may be transduced by ER. Thus, hypothalamic-derived LHRH neurons appear to have the capacity to be directly regulated by estrogen.

Cloning and expression of the human galanin receptor GalR2

Galanin is a peptide hormone which modulates a wide variety of physiological processes, including secretion, muscle contraction, cognitive function, the reproductive axis, and feeding. Two galanin receptor subtypes, GalR1 and GalR2, have been cloned; however, for GalR2 only the rat sequence has been reported in the literature. Our cloning of human GalR2 reveals its amino acid sequence to be 85% identical to rat GalR2 and 39% identical to human GalR1. Binding of [125I]galanin to the human GalR2 receptor transiently expressed in COS-7 cells was saturable (Kd = 0.24 nM +/- 0.06 nM) with a receptor density of 383 +/- 66 fmol/mg protein. Human galanin(1-30) bound with high affinity to the human GalR2 receptor, with a Ki value of 0.86 +/- 0.12 nM. With the identification of a second galanin receptor subtype, the specific functions of human galanin receptor subtypes can now begin to be addressed.