Role of galanin in stimulation of pituitary luteinizing hormone secretion as revealed by a specific receptor antagonist, galantide

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.

Spexin and Galanin in Metabolic Functions and Social Behaviors With a Focus on Non-Mammalian Vertebrates

Spexin (SPX) and galanin (GAL) are two neuropeptides that are phylogenetically related and have descended from a common ancestral gene. Considerable attention has been given to these two multifunctional neuropeptides because they share GAL receptors 1,2, and 3. Since GAL and SPX-synthesizing neurons have been detected in several brain areas, therefore, it can be speculated that SPX and GAL are involved in various neurophysiological functions. Several studies have shown the functions of these two neuropeptides in energy regulation, reproduction, and response to stress. SPX acts as a satiety factor to suppress food intake, while GAL has the opposite effect as an orexigenic factor. There is evidence that SPX acts as an inhibitor of reproductive functions by suppressing gonadotropin release, while GAL modulates the activity of gonadotropin-releasing hormone (GnRH) neurons in the brain and gonadotropic cells in the pituitary. SPX and GAL are responsive to stress. Furthermore, SPX can act as an anxiolytic factor, while GAL exerts anti-depressant and pro-depressive effects depending on the receptor it binds. This review describes evidence supporting the central roles of SPX and GAL neuropeptides in energy balance, reproduction, stress, and social behaviors, with a particular focus on non-mammalian vertebrate systems.

The circulating alarin level was elevated in infertile women with poor ovarian reserve

OBJECTIVE

We aimed to reveal the association of serum alarin level with POR status of the infertile women in the present study.

METHODS

The eligibility criteria for this prospective cross-sectional study included a total of 92 infertile women attending the Hitit University Hospital, and all participant women were categorized into two main groups of ovarian reserve: (i) Poor ovarian reserve (POR) group (n = 40) based on ESHRE consensus and (ii) Control group (NOR) (n = 52).

RESULTS

The mean adjusted-ages and BMI values of the NOR and POR groups were statistically comparable (p = .057 and p = .600, respectively). The mean E2, FSH, and LH levels were elevated in the POR group (p < .001, for all). The mean AFC and AMH concentration were significantly reduced in the POR group (p < .001, for both). In addition, there was a significant increase in the serum alarin level in the POR group (p < .001). Pearson's analysis revealed that the mean BMI value of the POR group had a weak and negative correlation (r = 0.318, p = .046). Also, there was no correlation between serum alarin with E2 and FSH levels in both study groups. A weak and positive correlation was found between serum alarin and LH concentration only in the POR group (r = 0.318, p = .045). The mean AMH and AFC values were not significantly correlated with serum alarin levels.

CONCLUSION

The circulating alarin level was significantly elevated in infertile women with POR patterns. In addition, the alarin level was significantly correlated with the serum LH concentration in the POR pattern.

Galanin Activates G Protein Gated Inwardly Rectifying Potassium Channels and Suppresses Kisspeptin-10 Activation of GnRH Neurons

GnRH neurons are regulated by hypothalamic kisspeptin neurons. Recently, galanin was identified in a subpopulation of kisspeptin neurons. Although the literature thoroughly describes kisspeptin activation of GnRH neurons, little is known about the effects of galanin on GnRH neurons. This study investigated whether galanin could alter kisspeptin signaling to GnRH neurons. GnRH cells maintained in explants, known to display spontaneous calcium oscillations, and a long-lasting calcium response to kisspeptin-10 (kp-10), were used. First, transcripts for galanin receptors (GalRs) were examined. Only GalR1 was found in GnRH neurons. A series of experiments was then performed to determine the action of galanin on kp-10 activated GnRH neurons. Applied after kp-10 activation, galanin 1-16 (Gal1-16) rapidly suppressed kp-10 activation. Applied with kp-10, Gal1-16 prevented kp-10 activation until its removal. To determine the mechanism by which galanin inhibited kp-10 activation of GnRH neurons, Gal1-16 and galanin were applied to spontaneously active GnRH neurons. Both inhibited GnRH neuronal activity, independent of GnRH neuronal inputs. This inhibition was mimicked by a GalR1 agonist but not by GalR2 or GalR2/3 agonists. Although Gal1-16 inhibition relied on Gi/o signaling, it was independent of cAMP levels but sensitive to blockers of G protein-coupled inwardly rectifying potassium channels. A newly developed bioassay for GnRH detection showed Gal1-16 decreased the kp-10-evoked GnRH secretion below detection threshold. Together, this study shows that galanin is a potent regulator of GnRH neurons, possibly acting as a physiological break to kisspeptin excitation.

The regulative effect of galanin family members on link of energy metabolism and reproduction

It is essential for the species survival that an efficient coordination between energy storage and reproduction through endocrine regulation. The neuropeptide galanin, one of the endocrine hormones, can potently coordinate energy metabolism and the activities of hypothalamic-pituitary-gonadal reproductive axis to adjust synthesis and release of metabolic and reproductive hormones in animals and humans. However, few papers have summarized the regulative effect of the galanin family members on the link of energy storage and reproduction as yet. To address this issue, this review attempts to summarize the current information available about the regulative effect of galanin, galanin-like peptide and alarin on the metabolic and reproductive events, with special emphasis on the interactions between galanin and hypothalamic gonadotropin-releasing hormone, pituitary luteinizing hormone and ovarian hormones. This research line will further deepen our understanding of the physiological roles of the galanin family in regulating the link of energy metabolism and reproduction.

Novel galanin receptors in teleost fish: identification, expression and regulation by sex steroids

In fish, the onset of puberty, the transition from juvenile to sexually reproductive adult animals, is triggered by the activation of pituitary gonadotropin secretion and its timing is influenced by external and internal factors that include the growth/adiposity status of the animal. Kisspeptins have been implicated in the activation of puberty but peripheral signals coming from the immature gonad or associated to the metabolic/nutritional status are also thought to be involved. Therefore we hypothesize the importance of the galinergic system in the brain and testis of pre-pubertal male sea bass as a candidate to translate the signals leading to activation of testicular maturation. Here, the transcripts for four galanin receptors (GALR), named GALR1a, 1b, 2a and 2b, were isolated from European sea bass, Dicentrarchus labrax. Phylogenetic analysis confirmed the previously reported duplication of GALR1 in teleost fish, and unravelled the duplication of GALR2 in teleost fish and in some tetrapod species. Comparison with human showed that the key amino acids involved in ligand binding are present in the corresponding GALR1 and GALR2 orthologs. Transcripts for all four receptors are expressed in brain and testes of adult fish with GALR1a and GALR1b abundant in testes and hardly detected in ovaries. In order to investigate whether GALR1 dimorphic expression was dependent on steroid context we evaluated the effect of 11-ketotestosterone and 17β-estradiol treatments on the receptor expression in brain and testes of pre-pubertal males. Interestingly, steroid treatments had no effect on the expression of GALRs in the brain while in the testes, GALR1a and GALR1b were significantly up regulated by 11KT. Altogether, these results support a role for the galaninergic system, in particular the GALR1 paralog, in fish reproductive function.

Afferent neuronal control of type-I gonadotropin releasing hormone neurons in the human

Understanding the regulation of the human menstrual cycle represents an important ultimate challenge of reproductive neuroendocrine research. However, direct translation of information from laboratory animal experiments to the human is often complicated by strikingly different and unique reproductive strategies and central regulatory mechanisms that can be present in even closely related animal species. In all mammals studied so far, type-I gonadotropin releasing hormone (GnRH) synthesizing neurons form the final common output way from the hypothalamus in the neuroendocrine control of the adenohypophysis. Under various physiological and pathological conditions, hormonal and metabolic signals either regulate GnRH neurons directly or act on upstream neuronal circuitries to influence the pattern of pulsatile GnRH secretion into the hypophysial portal circulation. Neuronal afferents to GnRH cells convey important metabolic-, stress-, sex steroid-, lactational-, and circadian signals to the reproductive axis, among other effects. This article gives an overview of the available neuroanatomical literature that described the afferent regulation of human GnRH neurons by peptidergic, monoaminergic, and amino acidergic neuronal systems. Recent studies of human genetics provided evidence that central peptidergic signaling by kisspeptins and neurokinin B (NKB) play particularly important roles in puberty onset and later, in the sex steroid-dependent feedback regulation of GnRH neurons. This review article places special emphasis on the topographic distribution, sexual dimorphism, aging-dependent neuroanatomical changes, and plastic connectivity to GnRH neurons of the critically important human hypothalamic kisspeptin and NKB systems.

Intimate associations between the neuropeptide Y system and the galanin-immunoreactive neurons in the human diencephalon

Galanin and neuropeptide Y (NPY) are among the most abundant neuropeptides in the hypothalamus. The role of NPY and galanin in the regulation of the secretory activity of the anterior pituitary has been well established. In addition, the two peptides interact with a number of neurons synthesizing the releasing and inhibiting hormones and a large number of other neuropeptides. The aim of the present studies was to explore if, as in rodents, NPY innervates galanin-immunoreactive (IR) neurons in the human diencephalon. Due to the long post mortem period and subsequent lack of optimal preservation of the cell membranes in the brain, electron microscopy could not be employed to show the presence of NPY-IR synapses on galanin-IR neurons. Therefore, we used light microscopic double label immunocytochemistry and high magnification microscopy with oil immersion to identify putative juxtapositions between NPY and galanin. Our studies show that similarly to rats, numerous NPY-IR nerve terminals surrounded galanin-IR neurons in the human hypothalamus. Among the hypothalamic regions, the infundibulum (infundibular or arcuate nucleus) contained the largest number of galanin-IR neurons heavily surrounded with NPY-IR nerve terminals. These en passant-type intimate associations between NPY-IR and galanin-IR neuronal elements may be functional synapses and may provide the morphological basis for the NPY-mediated galanin release. Consequently, NPY-galanin communication may mediate effects of NPY on neuronal systems innervated by galanin, and therefore may play a pivotal role in the regulation of reproduction, growth, energy and metabolism.

Alarin stimulates food intake in male rats and LH secretion in castrated male rats

Alarin is a newly identified member of the galanin family of neuropeptides that includes galanin-like peptide (GALP) and galanin. Alarin was discovered as an alternate transcript of the GALP gene in neuroblastoma cells, and subsequently alarin mRNA was detected in the brain of rodents. GALP and galanin are important central regulators of both feeding and reproductive behavior. We hypothesized, that, as a member of the galanin family of peptides, alarin would also have central effects on feeding and reproduction. To test this hypothesis, we treated male rats with alarin intracerebroventricularly (i.c.v.) and measured its effects on food intake and energy homeostasis as well as sexual behavior and luteinizing hormone (LH) secretion. We observed that i.c.v. injection of 1.0 nmol alarin significantly increased food intake (p<0.01) and body weight (p<0.05). Alarin did not affect sexual behavior in male rats; however, alarin did significantly (p<0.01) increase LH levels in castrated, but not intact, male rats. Alarin immunoreactive cell bodies were detected within the locus coeruleus and locus subcoeruleus of the midbrain, which is a brainstem nucleus involved in coordinating many physiological activities, including food intake and reproduction. Lastly, alarin stimulated Fos induction in hypothalamic nuclei, such as the paraventricular nucleus and the nucleus of the tractus solitarious. Our studies demonstrate that alarin, like other members of the galanin family, is a neuromediator of food intake and body weight.

Foetal hypothalamic and pituitary expression of gonadotrophin-releasing hormone and galanin systems is disturbed by exposure to sewage sludge chemicals via maternal ingestion

Animals and humans are chronically exposed to endocrine disrupting chemicals (EDCs) that are ubiquitous in the environment. There are strong circumstantial links between environmental EDC exposure and both declining human/wildlife reproductive health and the increasing incidence of reproductive system abnormalities. The verification of such links, however, is difficult and requires animal models exposed to 'real life', environmentally relevant concentrations/mixtures of environmental contaminants (ECs), particularly in utero, when sensitivity to EC exposure is high. The present study aimed to determine whether the foetal sheep reproductive neuroendocrine axis, particularly gondotrophin-releasing hormone (GnRH) and galaninergic systems, were affected by maternal exposure to a complex mixture of chemicals, applied to pasture, in the form of sewage sludge. Sewage sludge contains high concentrations of a spectrum of EDCs and other pollutants, relative to environmental concentrations, but is frequently recycled to land as a fertiliser. We found that foetuses exposed to the EDC mixture in utero through their mothers had lower GnRH mRNA expression in the hypothalamus and lower GnRH receptor (GnRHR) and galanin receptor (GALR) mRNA expression in the hypothalamus and pituitary gland. Strikingly, this, treatment had no significant effect on maternal GnRH or GnRHR mRNA expression, although GALR mRNA expression within the maternal hypothalamus and pituitary gland was reduced. The present study clearly demonstrates that the developing foetal neuroendocrine axis is sensitive to real-world mixtures of environmental chemicals. Given the important role of GnRH and GnRHR in the regulation of reproductive function, its known role programming role in utero, and the role of galanin in the regulation of many physiological/neuroendocrine systems, in utero changes in the activity of these systems are likely to have long-term consequences in adulthood and represent a novel pathway through which EC mixtures could perturb normal reproductive function.

Differential effects of Th1, monocyte/macrophage and Th2 cytokine mixtures on early gene expression for molecules associated with metabolism, signaling and regulation in central nervous system mixed glial cell cultures

BACKGROUND

Cytokines secreted by immune cells and activated glia play central roles in both the pathogenesis of and protection from damage to the central nervous system (CNS) in multiple sclerosis (MS).

METHODS

We have used gene array analysis to identify the initial direct effects of cytokines on CNS glia by comparing changes in early gene expression in CNS glial cultures treated for 6 hours with cytokines typical of those secreted by Th1 and Th2 lymphocytes and monocyte/macrophages (M/M).

RESULTS

In two previous papers, we summarized effects of these cytokines on immune-related molecules, and on neural and glial related proteins, including neurotrophins, growth factors and structural proteins. In this paper, we present the effects of the cytokines on molecules involved in metabolism, signaling and regulatory mechanisms in CNS glia. Many of the changes in gene expression were similar to those seen in ischemic preconditioning and in early inflammatory lesions in experimental autoimmune encephalomyelitis (EAE), related to ion homeostasis, mitochondrial function, neurotransmission, vitamin D metabolism and a variety of transcription factors and signaling pathways. Among the most prominent changes, all three cytokine mixtures markedly downregulated the dopamine D3 receptor, while Th1 and Th2 cytokines downregulated neuropeptide Y receptor 5. An unexpected finding was the large number of changes related to lipid metabolism, including several suggesting a switch from diacylglycerol to phosphatidyl inositol mediated signaling pathways. Using QRT-PCR we validated the results for regulation of genes for iNOS, arginase and P glycoprotein/multi-drug resistance protein 1 (MDR1) seen at 6 hours with microarray.

CONCLUSION

Each of the three cytokine mixtures differentially regulated gene expression related to metabolism and signaling that may play roles in the pathogenesis of MS, most notably with regard to mitochondrial function and neurotransmitter signaling in glia.

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.

Estrogen and estrogen receptor-{beta} (ER{beta})-selective ligands induce galanin expression within gonadotropin hormone-releasing hormone-immunoreactive neurons in the female rat brain

Among the many factors that integrate the activity of the GnRH neuronal system, estrogens play the most important role. In females, estrogen, in addition to the negative feedback, also exhibits a positive feedback influence upon the activity and output of GnRH neurons to generate the preovulatory LH surge and ovulation. Until recently, the belief has been that the GnRH neurons do not contain estrogen receptors (ERs) and that the action of estrogen upon GnRH neurons is indirect involving several, estrogen-sensitive neurotransmitter and neuromodulator systems that trans-synaptically regulate the activity of the GnRH neurons. Based on our recent findings that GnRH neurons of the female rat coexpress galanin, that galanin is a potent GnRH-releasing peptide, and that ERbeta is present in GnRH neurons, we have evaluated the effect of 17beta-estradiol and two ERbeta-selective agonists (WAY-200070, WAY-166818) on the expression of galanin within GnRH neurons. By combining immunocytochemistry for GnRH and in situ hybridization histochemistry for galanin, we demonstrate that 17beta-estradiol (20 mug/kg, sc) stimulates galanin expression within GnRH-immunoreactive neurons in a time-dependent manner. A significant increase was observed 2 h after its administration to ovariectomized rats. However, a more robust expression required 3-d treatment regimen. Treatment with the beta-selective ligands resulted in similar observations, although no statistical analysis is available for the 2 hr survival. These observations strongly suggest that estrogen and the ERbeta-selective ligands stimulate galanin expression within GnRH neurons via ERbeta, although an indirect mechanism via interneurons still cannot be ruled out.

Immunoreactive galanin expression in ovine gonadotropin-releasing hormone neurones: no effects of gender or reproductive status

The neuropeptide, galanin, has been implicated to play a significant role in numerous physiological functions, including reproduction. Studies on several species have shown that galanin enhances gonadotropin-releasing hormone (GnRH)-induced luteinizing hormone secretion. In rodents, a subset of GnRH neurones expresses galanin in a sexually dimorphic manner and it has been suggested that this may underpin the differences in GnRH secretion observed between the sexes. However, there are few data available for other species. Previous studies in sheep have shown that the distribution of GnRH neurones overlaps with galanin cells. The primary objectives of our study were to determine whether GnRH and galanin coexist in the sheep brain and, importantly, if a sex difference is apparent in the colocalization of these two peptides. Using immunocytochemistry coupled to high temperature antigen retrieval, we found that all GnRH neurones in the ovine brain colocalize with galanin. There is also a distinct population of galanin neurones that do not secrete GnRH. In addition, the distribution of galanin-immunoreactive cells was similar to that previously reported for colchicine treated ewes and, in agreement with earlier studies, the number of GnRH neurones did not differ between rams and ewes or between ewes killed at different stages of the oestrous cycle. These results suggest that, in sheep, GnRH and galanin may be cosecreted but the functional significance of this coexpression and possible cosecretion remains to be elucidated.

Differential patterns of Fos induction in the hypothalamus of the rat following central injections of galanin-like peptide and galanin

Galanin and its newly discovered relative galanin-like peptide (GALP) are neuropeptides that are implicated in the neuroendocrine regulation of body weight and reproduction. GALP has been shown to bind in vitro to galanin receptor subtypes 1 and 2, but whether it has its own specific receptor(s) is unknown. We reasoned that if GALP acts through a receptor that is distinct from galanin receptors, then GALP should activate central pathways that are different from those activated by galanin. The purpose of this study was to determine whether galanin and GALP produce different patterns of neuronal activation within the hypothalamus. Quantitative analysis of Fos immunoreactivity showed that galanin induced a significantly greater number of Fos-positive nuclei in the paraventricular nucleus compared with GALP (P < 0.001); however, compared with galanin, GALP induced significantly more Fos-positive cells in the horizontal limb of the diagonal band of Broca, caudal preoptic area, arcuate nucleus, and median eminence (P < 0.05). These observations suggest that GALP and galanin act through different receptor-mediated pathways to exert their effects on the regulation of body weight and reproduction and identify target cells for GALP's specific actions in the hypothalamus, including the preoptic area, paraventricular and arcuate nuclei, and the median eminence.

Galanin enhancement of gonadotropin-releasing hormone-stimulated luteinizing hormone secretion in female rats is estrogen dependent

The hypothalamic peptide GnRH is the primary neuroendocrine signal regulating pituitary LH in females. The neuropeptide galanin is cosecreted with GnRH from hypothalamic neurons, and in vitro studies have demonstrated that galanin can act at the level of the pituitary to directly stimulate LH secretion and also augment GnRH-stimulated LH secretion. Several lines of evidence have suggested that the hypophysiotropic effects of galanin are important for the generation of preovulatory LH surges. To determine whether the pituitary actions of galanin are enhanced by the preovulatory steroidal milieu, LH responses to galanin administration (with or without GnRH) were examined in: 1) ovariectomized (OVX); 2) OVX, estrogen (E)-primed; and 3) OVX, E- and progesterone-treated female rats. Results from the study indicate that galanin enhances GnRH-stimulated LH secretion only in the presence of E (in OVX, E-primed, or E- and progesterone-treated rats). Galanin alone does not directly stimulate LH secretion under any of the steroid conditions examined. In the absence of gonadal steroids (OVX rats), galanin inhibits GnRH-stimulated LH secretion. These findings suggest that the primary pituitary effect of galanin is to modulate GnRH-stimulated LH secretion, and that the potentiating effects of galanin occur only in the presence of E.

Testosterone-dependent effects of galanin on pituitary luteinizing hormone secretion in male rats

Galanin is a 29-amino-acid peptide that colocalizes with GnRH in hypothalamic neurons. High concentrations of galanin are present in portal vessel blood of both male and female rats, and galanin receptors are present on gonadotropes in both sexes. Results from studies of female rats indicate that galanin acts at the level of the pituitary to directly stimulate LH secretion and also to enhance GnRH-stimulated LH secretion. The effects of galanin on pituitary LH secretion in male rats are relatively uncharacterized; thus, the present in vivo study was conducted 1). to examine the ability of galanin to affect basal or GnRH-stimulated LH secretion in male rats and 2). to determine whether the effects of galanin on LH secretion in male rats are testosterone-dependent. All three doses of galanin used (1, 5, and 10 micro g/pulse) significantly enhanced GnRH-stimulated LH secretion in intact male rats. Only the highest dose of galanin directly stimulated LH secretion (without GnRH coadministration) in intact males. Galanin did not directly stimulate LH secretion or enhance GnRH-stimulated LH secretion in castrated male rats. In fact, the highest dose of galanin inhibited GnRH-stimulated LH secretion in castrated males. Upon testosterone replacement, the ability of galanin to directly stimulate LH secretion and to enhance GnRH-stimulated LH secretion was restored in castrated males. These results suggest a role for galanin in the regulation of LH release in male rats and demonstrate that testosterone upregulates the ability of the pituitary to respond to the stimulatory effects of galanin.

Increased galanin synapses onto activated gonadotropin-releasing hormone neuronal cell bodies in normal female mice and in functional preoptic area grafts in hypogonadal mice

Galanin synaptic input onto gonadotropin-releasing hormone (GnRH) neuronal cell bodies was analysed in female mice using the presynaptic vesicle-specific protein, synaptophysin (Syn) as a marker. In the first experiment, forebrain sections from normal ovariectomized ovarian steroid-primed mice exhibiting a surge of luteinizing hormone were processed for immunohistochemical labelling for GnRH, synaptophysin, galanin and Fos. Two representative sections from each brain, one passing through the anterior septum (anterior section) and the other through the organum vasculosum lamina terminalis-preoptic area (posterior section), were analysed under the confocal microscope. None of the GnRH cells analysed in the anterior sections were Fos immunoreactive (IR) or received input from galanin-IR fibres. In contrast, the majority of GnRH cells in the posterior sections analysed were Fos-positive. The number of galanin synapses onto the Fos-positive GnRH cells was significantly higher than that in the Fos-negative cells in this area of the brain, even though the number of Syn-IR appositions was comparable to each other. Transplantation of preoptic area (POA) into the third cerebral ventricle of hypogonadal (HPG) mice corrects deficits in the reproductive system. In the second experiment, synaptic input to GnRH cells was compared between HPG/POA mice with (functional graft) or without (nonfunctional graft) gonadal development. The mean numbers of Syn-IR appositions and galanin synapses per GnRH cell and the proportion of GnRH cells with galanin input were significantly higher in the functional than in the nonfunctional grafts. The results suggest that galanin can act directly on the GnRH cell bodies and may have an important regulatory role on the GnRH system.

Anatomical markers of activity in neuroendocrine systems: are we all 'fos-ed out'?

It has now been nearly 15 years since the immediate early gene, c-fos, and its protein product, Fos, were introduced as tools for determining activity changes within neurones of the nervous system. In the ensuing years, this approach was applied to neuroendocrine study with success. With it have come advances in our understanding of which neuroendocrine neurones respond to various stimuli and how other central nervous system components interact with neuroendocrine neurones. Use of combined tract-tracing approaches, as well as double-labelling for Fos and transmitter markers, have added to characterization of neuroendocrine circuits. The delineation of the signal transduction cascades that induce Fos expression has led to establishment of the relationship between neurone firing and Fos expression. Importantly, we can now appreciate that Fos expression is often, but not always, associated with increased neuronal firing and vice versa. There are remaining gaps in our understanding of Fos in the nervous system. To date, knowledge of what Fos does after it is expressed is still limited. The transience of Fos expression after stimulation (especially if the stimulus is persistent) complicates design of experiments to assess the function of Fos and makes Fos of little value as a marker for long-term changes in neurone activity. In this regard, alternative approaches must be sought. Useful alternative approaches employed to date to monitor neuronal changes in activity include examination of (i) signal transduction intermediates (e.g. phosphorylated CREB); (ii) transcriptional/translational intermediates (e.g. heteronuclear RNA, messenger RNA (mRNA), prohormones); and (iii) receptor translocation. Another capitalizes on the fact that many neuroendocrine systems show striking stimulus-transcription coupling in the regulation of their transmitter or its synthetic enzymes. Together, as we move into the 21st Century, the use of multiple approach to study activity within neuroendocrine systems will further our understanding of these important systems.

Multimeric CREB-binding sites in the promoter regions of a family of G-protein-coupled receptors related to the vertebrate galanin and nociceptin/orphanin-FQ receptor families

Four related genes encoding a family of G-protein-coupled receptors (GPCRs) have been isolated from the mollusc Lymnaea stagnalis. The coding regions of this family of receptors share 97-99% sequence similarity at both the protein and nucleotide level, and they also share high sequence identity with vertebrate galanin and orphanin-FQ/nociceptin GPCR families. Analysis of the promoter regions reveals shared domains, some of which encode highly conserved repeating units. One 27-bp repeating unit, which encodes a c-AMP response element (CRE) and binds CREB protein, is repeated 14 times in one promoter. In situ hybridization showed expression of these receptors in identified neurons of several behaviourly important networks including those involved in feeding and ion and water regulation. These Lymnaea receptors are likely to represent members of a novel family of invertebrate neuropeptide receptors extensively regulated in response to intracellular signalling cascades.

Evidence suggesting that the potentiating action of neuropeptide Y on luteinizing hormone (LH)-releasing hormone-induced LH release remains unaltered in aged female rats

Several lines of evidence suggest that one of the mechanisms by which the hypothalamic neuropeptide Y plays an obligatory role in the preovulatory luteinizing hormone (LH) discharge in young rats is to potentiate the action of LH-releasing hormone (LHRH) on LH release at the level of the pituitary. This study examined whether an alteration in the potentiating action of neuropeptide Y on LHRH-induced LH release may contribute to the attenuation or absence of LH surges during female reproductive ageing. Young regularly cycling (2-3-month-old) and old constant oestrous (19-20-month-old) rats ovariectomized for 7 days were primed with oestradiol-17beta-filled Silastic capsules. Two days later, rats received s.c. progesterone at 09.00 h and then were injected i.p. with either saline or pentobarbital at 13.30 hours. Pentobarbital-treated rats received i.v. pulses of neuropeptide Y, LHRH, a combination of neuropeptide Y and LHRH, or saline, every 30 min from 14.00 to 18.00 h via a jugular cannula. Hourly blood samples were collected between 11.00 and 21.00 h. In old rats, the progesterone-induced LH surge was significantly attenuated and delayed as compared to that of young rats. Pentobarbital injection completely blocked the LH surge. Neuropeptide Y pulses alone had no significant effect on LH release. In contrast, LHRH pulses increased LH release in both age groups, although the response was significantly reduced in older rats. While combined pulses of neuropeptide Y and LHRH significantly increased LH release in both young and old rats as compared to that of LHRH alone, the potentiating action of neuropeptide Y on LHRH-induced LH release remained unchanged between the two age groups. These results, together with our recent demonstration of altered hypothalamic neuropeptide Y neuronal activity in middle-aged pro-oestrous rats, suggest that a deficit in neuropeptide Y secretion and action in the hypothalamus, rather than a decrease in pituitary responsiveness to neuropeptide Y, may partially be responsible for the absence of LH surges in old rats.

Two olfactory placode derived galanin subpopulations: luteinizing hormone-releasing hormone neurones and vomeronasal cells

In adult rodents, the peptide galanin is expressed in a subpopulation of hypothalamic luteinizing hormone-releasing hormone (LHRH) neurones in an activity-dependent manner. In this investigation, we examined whether galanin mRNA expression in mice was activated coincident with LHRH mRNA expression, as LHRH neurones differentiate from the olfactory placode. Using in situ hybridization, we show (i) that galanin mRNA is coexpressed in LHRH neurones prenatally, (ii) that there is a decrease in galanin mRNA expression relative to LHRH mRNA expression once LHRH mRNA positive/galanin mRNA positive neurones migrate out of the olfactory pit and into the nasal septum, and (iii) the presence of a novel population of galanin mRNA positive/LHRH mRNA negative expressing neurones in the olfactory pit/vomeronasal organ which do not migrate into the central nervous systenm (CNS). This study demonstrates that there are at least two populations of galanin mRNA expressing neurones arising from the olfactory placode; one that remains in nasal regions, is LHRH mRNA negative and whose function is unknown, and one which is coexpressed with LHRH. In addition, the temporal expression of galanin mRNA in LHRH cells indicates that initial activation and subsequent inactivation of galanin mRNA expression is independent of synaptic CNS connections.

Expression of galanin immunoreactivity in gonadotropin-releasing hormone neurons in mice: a confocal microscopic study

The expression of galanin immunoreactivity (galanin-IR) in gonadotropin-releasing hormone (GnRH) neurons was investigated in mice using double label immunohistochemistry combined with confocal laser scanning microscopy. A large proportion of GnRH cells in proestrous mice and very few GnRH cells in male mice exhibited galanin-IR. These results are consistent with earlier reports in rats. Unlike in rats, the proportion of GnRH cells coexpressing galanin in mice was high following ovariectomy (OVX) and the treatment of OVX mice with estrogen decreased the number of GnRH cells with galanin-IR. The GnRH system can be considered more active during proestrous and following OVX since the output of luteinizing hormone is elevated during these phases in females. Since the induction of galanin-IR in GnRH cells is more pronounced in OVX and proestrous mice, the expression of galanin-IR in GnRH cells in mice appears to be an activation-dependent phenomenon rather than a direct effect of estrogen. However, in OVX mice treated with steroids to induce an LH surge the number of GnRH cells with galanin-IR was not proportionately increased. The possible reasons for this discrepancy are also discussed.

Cloned human and rat galanin GALR3 receptors. Pharmacology and activation of G-protein inwardly rectifying K+ channels

The neuropeptide galanin has been implicated in the regulation of processes such as nociception, cognition, feeding behavior, and hormone secretion. Multiple galanin receptors are predicted to mediate its effects, but only two functionally coupled receptors have been reported. We now report the cloning of a third galanin receptor distinct from GALR1 and GALR2. The receptor, termed GALR3, was isolated from a rat hypothalamus cDNA library by both expression and homology cloning approaches. The rat GALR3 receptor cDNA can encode a protein of 370 amino acids with 35% and 52% identity to GALR1 and GALR2, respectively. Localization of mRNA by solution hybridization/RNase protection demonstrates that the GALR3 transcript is widely distributed, but expressed at low abundance, with the highest levels in the hypothalamus and pituitary. We also isolated the gene encoding the human homologue of GALR3. The human GALR3 receptor is 90% identical to rat GALR3 and contains 368 amino acids. Binding of porcine 125I-galanin to stably expressed rat and human GALR3 receptors is saturable (rat KD = 0.98 nM and human KD = 2.23 nM) and displaceable by galanin peptides and analogues in the following rank order: rat galanin, porcine galanin approximately M32, M35 approximately porcine galanin-(-7 to +29), galantide, human galanin > M40, galanin-(1-16) > [D-Trp2]galanin-(1-29), galanin-(3-29). This profile resembles that of the rat GALR1 and GALR2 receptors with the notable exception that human galanin, galanin-(1-16), and M40 show lower affinity at GALR3. In Xenopus oocytes, activation of rat and human GALR3 receptors co-expressed with potassium channel subunits GIRK1 and GIRK4 resulted in inward K+ currents characteristic of Gi/Go-coupled receptors. These data confirm the functional efficacy of GALR3 receptors and further suggest that GALR3 signaling pathways resemble those of GALR1 in that both can activate potassium channels linked to the regulation of neurotransmitter release.

Galanin receptor-mediated inhibition of glutamate release in the arcuate nucleus of the hypothalamus

It is thought that galanin, a 29 amino acid neuropeptide, is involved in various neuronal functions, including the regulation of food intake and hormone release. Consistent with this idea, galanin receptors have been demonstrated throughout the brain, with high levels being observed in the hypothalamus. However, little is known about the mechanisms by which galanin elicits its actions in the brain. Therefore, we studied the effects of galanin and its analogs on synaptic transmission using an in vitro slice preparation of rat hypothalamus. In arcuate nucleus neurons, application of galanin resulted in an inhibition of evoked glutamatergic EPSCs and a decrease in paired-pulse depression, indicating a presynaptic action. The fragments galanin 1-16 and 1-15 produced a robust depression of synaptic transmission, whereas the fragment 3-29 produced a lesser degree of depression. The chimeric peptides C7, M15, M32, and M40, which have been reported to antagonize some actions of galanin, all produced varying degrees of depression of evoked EPSCs. In a minority of cases, C7, M15, and M40 antagonized the actions of galanin. Analysis of mEPSCs in the presence of TTX and Cd2+, or after application of alpha-latrotoxin, indicated a site of action for galanin downstream of Ca2+ entry. Thus, our data suggest that galanin acts via several subtypes of presynaptic receptors to depress synaptic transmission in the rat arcuate nucleus.

Temporal patterns of gonadotropin-releasing hormone (GnRH), c-fos, and galanin gene expression in GnRH neurons relative to the luteinizing hormone surge in the rat

Gonadotropin-releasing hormone (GnRH) neurons increase their expression of Fos and galanin coincident with the luteinizing hormone (LH) surge in the female rat. To define the temporal relationships between the expression of these genes and the GnRH gene itself and to gain insight about the possible functional interactions of these processes, we compared levels of c-fos, galanin, and GnRH mRNA in GnRH neurons and plasma levels of LH in the rat, beginning 6 hr before and continuing for 24 hr after a sex steroid-induced LH surge. LH levels were increased significantly by 1600 hr. They increased twofold further by 1800 hr and then returned to baseline by 2400 hr. Using in situ hybridization, we determined that levels of c-fos mRNA in GnRH neurons were elevated significantly at 1600 hr only, whereas levels of galanin mRNA in GnRH neurons first increased twofold by 1800 hr, increased an additional twofold by 2400 hr, and remained elevated at all time points sampled thereafter. There were no significant changes in cellular levels of GnRH mRNA over the time points sampled. These results are consistent with the hypothesis that the induction of c-fos gene expression in GnRH neurons leads to an increase in galanin gene expression, and that the sustained increase in galanin mRNA levels in GnRH neurons reflects either the need to replenish galanin stores that are depleted at the time of the LH surge or the involvement of galanin with physiological events that occur on the day of estrus.

Steroid imprinting and modulation of sexual dimorphism in the luteinizing hormone-releasing hormone neuronal system

1. Sex differences in the control of gonadotropin secretion and reproductive functions are a distinct characteristic in all mammalian species, including humans. Ovulation and cyclicity are among the most distinct neuroendocrine markers of female brain differentiation, along with sex behavioral traits that are also evident in different species. 2. The luteinizing hormone-releasing hormone (LHRH) neuronal system is the prime regulator of neuroendocrine events leading to ovulation and hormonal changes during the menstrual cycle and, as such, is the potential site where many of these sex differences may be expressed or, at the very least, integrated. However, until recently, no significant differences were seen in LHRH neurons between male and female brains, including cell number, pattern of distribution, and expression of message or peptide (LHRH) levels. 3. Recently, we reported that galanin (GAL), a brain-gut peptide, is coexpressed in LHRH neurons and that this coexpression is sexually dimorphic. When GAL is used as a marker for this neuronal system, it is clear that estradiol as well as progesterone profoundly affects the message and expression of the peptide and that this regulation, at least in rodents, is neonatally predetermined by gonadal steroid imprinting. 4. Changes in GAL expression and message can also be seen at puberty, during pregnancy and lactation, and in aging, all situations that affect the function of the LHRH neuronal system. Using an immortalized LHRH neuronal cell line (GT1) we have recently observed that these neurons express estrogen receptor (ER) and GAL and that estradiol can increase the expression of GAL, indicating functional activation of the endogenous ER.

Galanin--a neuropeptide with inhibitory actions

1. Galanin is a 29 (in humans 30) amino acids long neuropeptide with mostly inhibitory, hyperpolarizing actions. 2. Differential structural requirements of truncated forms of galanin and differential agonist/antagonist behaviour of chimeric peptides, high affinity galanin receptor ligands suggest the presence of pharmacologically distinct galanin receptor subtypes. 3. The galanin receptor from human Bowes melanoma cell line--a member of G-protein coupled receptor superfamily--has been cloned. 4. Galanin acts via Gi/G(o) proteins inhibiting cAMP production, inositol phosphate turnover, opening K+ channels or closing Ca2+ channels.

Vasopressin and galanin mRNAs coexist in the nucleus of the horizontal diagonal band: a novel site of vasopressin gene expression

Vasopressin (VP) neurons have been identified in several brain regions where VP has been hypothesized to act as a neurotransmitter or neuromodulator. In many sites, VP is colocalized with the neuropeptide galanin (GAL). Here, using single in situ hybridization histochemistry, we have identified a novel group of neurons within the nucleus of the horizontal diagonal band of Broca (HDB) that express the VP gene and have assessed the distribution of these cells in adult male and female rats (90 days old, n = 7/group). VP mRNA-expressing neurons were scattered throughout the rostrocaudal extent of the HDB, and the number of VP neurons detected unilaterally ranged from 1 to 17 cells per 20 microns section. Using double in situ hybridization histochemistry on alternate sections, we have assessed the number of cells expressing VP and/or GAL mRNA in the diagonal band and have determined the extent of their colocalization. Approximately 50% of all VP-expressing neurons in the HDB coexpressed GAL mRNA, and 33% of GAL-expressing neurons in this region coexpressed VP mRNA. No sex differences were detected in the number of neurons expressing either VP or GAL mRNA or in the incidence of coexpression of VP and GAL mRNAs in this region. VP neurons in the HDB exhibited a low level of expression, and cellular VP mRNA content did not differ between male and female rats. However, sex differences were present in the bed nucleus of the stria terminalis (BNST) of these same rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Galanin neurons exhibit estrogen receptor immunoreactivity in the female rat mediobasal hypothalamus

Galanin has been shown to augment the hypothalamic luteinizing hormone releasing hormone and pituitary luteinizing hormone release and to play an important role in the feedback effects of ovarian steroids on pituitary hormone secretion. To further characterize estrogen effects on galanin, we tested for the existence of estrogen receptors in arcuate nucleus galanin-producing cells. Hypothalamic vibratome sections from colchicine-pretreated female rats were double immunolabeled for estrogen receptor and galanin. Neurons which exhibited immunoreactivity for either estrogen receptor or galanin were distributed throughout the hypothalamus; and a population of neurons, located predominantly in the mediobasal hypothalamus, displayed immunoreactivity for both galanin and estrogen receptor. These results raise the possibility that estrogen may act directly on galanin-producing arcuate nucleus neurons to regulate pituitary hormone secretion.