Effect of endocrine manipulation on anterior pituitary galanin in the rat

Galanin and the neuroendocrine axes

Galanin has diverse physiological functions, including nociception, arousal/sleep regulation, cognition, and many aspects of neuroendocrine activities that are associated with feeding, energy metabolism, thermoregulation, osmotic and water balance, and reproduction. This review will provide a brief overview of galanin action in some major neuroendocrine processes. Most of the recent data are about the role of galanin in the central regulation of food intake and energy metabolism, and to some extent, in the regulation of reproduction. It seems that galanin plays a modulatory role rather than a regulatory one in the central and peripheral branches of the neuroendocrine systems. In the hypothalamus, it functions as a neurotransmitter/ neuromodulator. In the pituitary and the peripheral endocrine glands, it acts via its receptors in a paracrine/autocrine fashion. The development of new, selective, and potent antagonists of GALRs should keep advancing our knowledge not only in the physiology of galanin but also in its pathophysiology.

Localization and role of galanin in the thyroid gland of Podarcis sicula lizard (reptilia, lacertide)

Galanin (GAL) is a 29-amino acid residue neuropeptide, which was initially isolated from porcine intestine extracts and since then, widely found in a variety of vertebrate organs, in correlation with multiple neuro-hormonal actions exerted and so receiving a constantly growing attention. Moreover, although the studies undertaken so far suggest a local intrathyroidal peptidergic regulatory action, the exact role of GAL on thyroid gland remains to be established. The aim of this study was to determine in the lizard, Podarcis sicula, (1) the presence of GAL immunoreactivity in the thyroid gland and (2) the short- and long-term effects of in vivo GAL administration by intraperitoneal injection on thyroid gland physiology. First of all, the presence of GAL in the thyroid gland of P. sicula was demonstrated by immunohistochemical technique (avidin-biotin-peroxidase complex--ABC method). Second, the role of GAL in the control of thyroid gland activity was studied in vivo using light microscopy (LM) technique coupled to a specific radioimmunoassay for thyroid-stimulating hormone (TSH) and thyroid hormones (T(4) and T(3)). Prolonged GAL administration [(0.4 mg/100 g body wt)/day] increased T(4) and T(3) release, but decreased the plasma concentration of TSH. In addition, using LM clear signs of stimulation of the thyroid gland were observed. These findings suggest that systemic administration of GAL was able to stimulate the thyroid gland of the lizard both at morphological and physiological level.

Increased caloric intake on a fat-rich diet: role of ovarian steroids and galanin in the medial preoptic and paraventricular nuclei and anterior pituitary of female rats

Previous studies in male rats have demonstrated that the orexigenic peptide galanin (GAL), in neurones of the anterior parvocellular region of the paraventricular nucleus (aPVN) projecting to the median eminence (ME), is stimulated by consumption of a high-fat diet and may have a role in the hyperphagia induced by fat. In addition to confirming this relationship in female rats and distinguishing the aPVN-ME from other hypothalamic areas, the present study identified two additional extra-hypothalamic sites where GAL is stimulated by dietary fat in females but not males. These sites were the medial preoptic nucleus (MPN), located immediately rostral to the aPVN, and the anterior pituitary (AP). The involvement of ovarian steroids, oestradiol (E(2)) and progesterone (PROG), in this phenomenon was suggested by an observed increase in circulating levels of these hormones and GAL in MPN and AP with fat consumption and an attenuation of this effect on GAL in ovariectomised (OVX) rats. Furthermore, in the same four areas affected by dietary fat, levels of GAL mRNA and peptide immunoreactivity were stimulated by E(2) and further by PROG replacement in E(2)-primed OVX rats and were higher in females compared to males. Because both GAL and PROG stimulate feeding, their increase on a fat-rich diet may have functional consequences in females, possibly contributing to the increased caloric intake induced by dietary fat. This is supported by the findings that PROG administration in E(2)-primed OVX rats reverses the inhibitory effect of E(2) on total caloric intake while increasing voluntary fat ingestion, and that female rats with higher GAL exhibit increased preference for fat compared to males. Thus, ovarian steroids may function together with GAL in a neurocircuit, involving the MPN, aPVN, ME and AP, which coordinate feeding behaviour with reproductive function to promote consumption of a fat-rich diet at times of increased energy demand.

Galanin and galanin receptors in human cancers

The increasing interest in peptides and peptide receptors in cancer is based on the possibility of receptor targeting, because peptide receptors are often expressed in different human tumors. The neuropeptide galanin has also been suggested to be involved in the development of neuroendocrine tumors based on the development of estrogen-induced tumors in estrogen-sensitive rats. This study summarizes our current knowledge on the expression of galanin peptide and galanin receptors in different human neuroendocrine tumors. The expression of both, peptide and corresponding receptor, seems to be a common feature of human gliomas, pheochromocytomas, pituitary and neuroblastic tumors. The co-expression of galanin and its receptors supports a role for galanin in tumor cell pathology via autocrine/paracrine mechanisms.

Pituitary cell lines and their endocrine applications

The pituitary gland is an important component of the endocrine system, and together with the hypothalamus, exerts considerable influence over the functions of other endocrine glands. The hypothalamus either positively or negatively regulates hormonal productions in the pituitary through its release of various trophic hormones which act on specific cell types in the pituitary to secrete a variety of pituitary hormones that are important for growth and development, metabolism, reproductive and nervous system functions. The pituitary is divided into three sections-the anterior lobe which constitute the majority of the pituitary mass and is composed primarily of five hormone-producing cell types (thyrotropes, lactotropes, corticotropes, somatotropes and gonadotropes) each secreting thyrotropin, prolactin, ACTH, growth hormone and gonadotropins (FSH and LH) respectively. There is also a sixth cell type in the anterior lobe-the non-endocrine, agranular, folliculostellate cells. The intermediate lobe produces melanocyte-stimulating hormone and endorphins, whereas the posterior lobe secretes anti-diuretic hormone (vasopressin) and oxytocin. Representative cell lines of all the six cell types of the anterior pituitary have been established and have provided valuable information on genealogy of the various cell lineages, endocrine feedback control of hormone synthesis and secretions, intrapituitary interactions between the various cell types, as well as the role of specific transcription factors that determine each differentiated cell phenotype. In this review, we will discuss the morphology and function of the cell types that make up the anterior pituitary, and the characteristics of the various functional anterior pituitary cell systems that have been established to be representative of each anterior pituitary cell lineage.

Regulation of rat pituitary cocaine- and amphetamine-regulated transcript (CART) by CRH and glucocorticoids

Cocaine- and amphetamine-regulated transcript (CART) was originally isolated from rat brain, but CART is also synthesized and stored in the anterior pituitary. The localization of pituitary CART and factors regulating its synthesis are largely unknown. The regulation of pituitary CART synthesis and release in response to CRH and glucocorticoids was examined in vitro and in vivo. CART immunoreactivity (CART-IR) was released from anterior pituitary segments. This release was increased 15-fold in response to corticotropin-releasing hormone (CRH). Intraperitoneal administration of CRH to rats significantly increased plasma CART-IR. Furthermore, CART-IR content and plasma CART-IR were significantly increased in adrenalectomized rats, and anterior pituitary CART mRNA expression, CART-IR content, and plasma CART-IR were significantly decreased in corticosterone-treated rats. Plasma CART-IR showed a pattern of diurnal variation similar to that of ACTH and corticosterone, and plasma CART-IR was positively correlated with corticosterone. CART-IR was detectable in the medium of the corticotroph cell line AtT-20. Dual in situ hybridization for prepro-CART (ppCART) mRNA expression and immunocytochemistry for ACTH showed localization of ppCART mRNA to a subpopulation of ACTH-immunoreactive cells. These findings demonstrate that pituitary CART expression and release are regulated by CRH and the glucocorticoid environment and that pituitary CART is partly localized to corticotrophs.

In vitro effects of mammalian leptin, neuropeptide-Y, β-endorphin and galanin on transcript levels of thyrotropin β and common α subunit mRNAs in the pituitary of bighead carp (aristichthys nobilis)

Thyrotropin (thyroid stimulating hormone, TSH) is a member of the pituitary glycoprotein hormones, consisting of two dissimilar subunits, alpha and beta. The two subunits are produced by different genes and are regulated independently. We have previously cloned a TSHbeta cDNA from bighead carp pituitary and investigated its gene regulation. We report here the direct effects of mammalian TSH-releasing hormone (TRH), leptin, neuropeptide-Y (NPY), beta-endorphin and galanin on mRNA levels of both TSHbeta and alpha-subunits in the pituitary of bighead carp in vitro. The dispersed pituitary cells of bighead carp were incubated at 25 degrees C for 6 h with different doses of these factors. The relative mRNA levels of TSHbeta and alpha-subunits were estimated by traditional polymerase chain reaction (PCR) analysis and fluorescence real-time PCR analysis. The results revealed that mammalian TRH, leptin and beta-endorphin produced dose-dependent stimulatory effects on mRNA levels of both TSHbeta and alpha-subunits while thyroxine (T4) and mammalian galanin suppressed mRNA levels of both TSHbeta and alpha-subunits. NPY suppressed TSHbeta mRNA level, but stimulated alpha-subunit mRNA level. This study has demonstrated that mammalian TRH, leptin, NPY, beta-endorphin and galanin were active in modulating the steady-state mRNA levels of TSHbeta and alpha-subunits of bighead carp pituitary in vitro. The results suggest that endogenous TRH, leptin, NPY, beta-endorphin and galanin may modulate transcript levels of TSHbeta and alpha-subunits in pituitary of bighead carp.

Peptidergic innervation in pars distalis of the human anterior pituitary

It has been previously shown that the peptidergic nerve fibers are present in the anterior pituitary of monkeys, dogs and rats. In our study, which is reported here, thick nerve fiber bundles, large numbers of peptidergic nerve fibers and their varicosities, which are substance P (SP)-, calcitonin gene-related peptide (CGRP)- and galanin (GAL)-immunoreactive (ir), are found in the human pituitary stalk. All these peptidergic nerve fibers run along the pituitary stalk and enter the pars distalis, and some GAL-ir nerve fibers even reach the center of the human anterior pituitary as well as in parenchyma of adenohypophysis. The number of SP-ir nerve fibers is much more than that of other kinds of peptidergic nerve fibers. All these peptidergic nerve fibers are mainly located in the medial part of the gland and distributed in its dorsal-posterior region. A substantial amount of these peptidergic nerve fibers with numerous varicosities are found to be close to the glandular tissue in the pars distalis of the human anterior pituitary. Furthermore, same SP-ir and CGRP-ir cells have been demonstrated in the pars distalis of the anterior pituitary. More or less, these peptidergic nerve fibers came also from the meningeal shell and enter the parenchyma of the anterior pituitary. Whatever the function of peptidergic nerve fibers in the human anterior pituitary might be, the concept that the adenohypophysis is regulated only hormonally by way of the portal system could be challenged.

Gender differences in the expression of galanin and vasoactive intestinal peptide in oestrogen-induced prolactinomas of Fischer 344 rats

We have previously described a sexual dimorphism in oestrogen-induced anterior pituitary tumorigenesis in Fischer 344 rats, with female tumours averaging twice the size of those of males. Neonatal androgenization of female Fischer 344 rats with 100 micro g of testosterone propionate reverted that effect, causing a 'male-like' phenotype. The peptides galanin and vasoactive intestinal peptide (VIP) are possible mediators of oestrogen effects on the anterior pituitary, including hyperprolactinemia and lactotroph proliferation. To further extend our previous findings, we investigated the expression of galanin and VIP in the anterior pituitary of control and oestrogenized male, female and neonatally androgenized female Fischer 344 rats. At 3 months of age, rats were deprived of their gonads and divided into control and diethylstilbestrol (DES)-treated groups. In the anterior pituitary of control rats, galanin and VIP immunoreactive cells were absent. However, in DES-treated rats, pituitaries from normal ovariectomized females showed higher number of galanin and VIP positive cells than pituitaries from neonatally androgenized ovariectomized females and gonadectomized males. This pattern correlated with changes in anterior pituitary weight and serum prolactin. Our study suggests that sexual differences in oestrogen-induced pituitary tumorigenesis could be due to the differential expression of galanin and VIP. Furthermore, our data support the fact that neonatal exposure to androgens, as in normal males and androgenized females, may condition the response of the pituitary gland to oestrogens in adult life.

Neuropeptides in anterior pituitary development

Recent studies using biotechnological methods have achieved significant advances in our knowledge of molecular mechanisms underlying pituitary gland development and the differentiation of pituitary cytotypes. A large number of neuropeptides have been reported in the adult pituitary gland as well as in the central and peripheral nervous system. The early presence of neuropeptides during pituitary development is reviewed here. Neuromedin U (NmU), galanin and the polypeptide 7B2 have been localised to different endocrine cells of the gland. Their expression seems to be manifold even though it is temporally and spatially regulated. There is now firm immunocytochemical evidence that neuropeptides are present during morphogenesis of the pituitary and can be present simultaneously with all pituitary hormones.

Targeted disruption of galanin: new insights from knock-out studies

The neuropeptide galanin has a widespread but no means ubiquitous expression pattern in the nervous and endocrine systems. Profound changes in the levels and distribution of the peptide occur in a range of path-physiological situations including nerve injury or damage and alterations in the circulating levels of a number of hormones. There is now a substantial body of work to indicate that galanin plays an important biological role as a regulator of neurotransmitter and hormone release in the adult. The recent generation of mice carrying a loss-of-function mutation within the galanin gene has allowed us new insights into the physiological actions of galanin. In this manuscript we detail three sets of data relating to the major phenotypic effects thus far delineated, putting them in the context of existing published data. These studies demonstrate that galanin acts as a developmental and trophic factor to subsets of neurons in the nervous and neuroendocrine systems.

Local action of estrogen and thyroid hormone on vasoactive intestinal peptide (VIP) and galanin gene expression in the rat anterior pituitary

Neuropeptides act within the pituitary as autocrine or paracrine factors, modulating the synthesis and release of primary pituitary hormones, and possibly regulating cell proliferation and/or plasticity. Manipulation of the endocrine status of rats produces dramatic long-term changes in the pituitary expression of several peptides, including the neuropeptides galanin and vasoactive intestinal peptide (VIP). Whether or not these changes are caused indirectly by hypothalamic factors, or by hormone actions directly in the pituitary, has been only partially addressed. To determine if estrogen or thyroid hormone can act directly within the pituitary to regulate VIP and galanin gene expression, cultured female rat pituitary cells were treated with 10 nM 1,17 beta-estradiol (E2) or triiodothyronine (T(3)). E2 treatment for three days resulted in an approximate 5-fold and 7-fold increase in VIP and galanin mRNA, respectively. In contrast, T(3) treatment reduced the mRNA levels of these neuropeptides to approximately 40% and 30% of control values. A time course study indicated that the actions of estrogen on VIP and galanin mRNA, and of thyroid hormone on VIP mRNA were readily apparent after 24h. The rat pituitary tumor cell line RC-4B/C was found to express easily detectable levels of galanin but not VIP mRNA. Galanin gene expression in these cells was moderately increased by E2 and decreased by T(3). Transfection of a series of luciferase plasmids containing 5 kb to 131 bp of the bovine galanin promoter fused to luciferase revealed cell-type specific enhancer sequences located between -452 and -131 bp of the galanin gene transcription start site. However, transfected plasmids were minimally responsive to E2 and T(3) treatment. Overall the results suggest that E2 and T(3) exert significant local actions in the pituitary on VIP and galanin gene expression. The bovine galanin gene fragment used in these studies contains a potential pituitary cell-type specific enhancer, but appears to lack strong E2-and T(3)-responsive sequences.

Galanin immunoreactive innervations of the anterior pituitary in the monkey and the dog

As we know, the anterior pituitary is regulated by hypothalamic hormones via the portal system. However, our recent studies have demonstrated the presence of a substantial amount of substance P (SP)- and calcitonin gene-related peptide (CGRP)-containing nerve fibers in the anterior pituitary. In the present paper, the existence of a considerable amount of galanin (GAL)like immunoreactive (ir) nerve fibers with numerous of varicosities in the anterior pituitaries of the Macaca mulatta monkey and the dog were reported. In the monkey, GAL-ir nerve fibers with a large amount of varicosities were mainly located in the medial part of the gland, dominantly in its dorso-posterior regions. A great majority of varicosities were found to be closely related to the glandular tissue although some were located along the walls of blood sinus. GAL-ir nerve fibers were traced to enter the pars distalis of the anterior pituitary from the stalk as well as from the meningeal sheath covering the upper part of the anterior pituitary. Numerous GAL-ir cells presented in the pars distalis and the pars intermedia of the adenohypophysis. In the dog pituitary, GAL-ir nerve fibers were mainly located in the tail part and some in oral region. A majority of GAL-ir nerve fibers were at the periphery of the gland, especially in the medial planes, although some could be found deep in the gland. They appeared in individual fibers or in patches. Many GAL-ir nerve fibers and fiber fascicles existed in the median eminence and the sheath covering the tail and the oral part of the pituitary.

Advantages of in situ hybridisation over direct or indirect in situ reverse transcriptase-polymerase chain reaction for localisation of galanin mRNA expression in rat small intestine and pituitary

In situ hybridisation (ISH) and direct or indirect in situ reverse transcriptase-polymerase chain reaction (RT-PCR) were used to detect galanin mRNA in paraffin sections of rat intestine and pituitary. With conventional ISH, a subset of intestinal neuronal ganglion cells and anterior pituitary endocrine cells were labelled. Direct in situ RT-PCR also labelled some cells in pituitary but not in intestine. Negative controls were unlabelled, but sections with 3' primer alone for RT-PCR appeared positive. No signal was apparent using the indirect in situ RT-PCR method. Investigation of the specificity of solution phase RT-PCR using RNA extracts from pituitary or intestine revealed that additional PCR products were detected under some conditions. The sequences of these PCR products suggested that one was the result of mispriming and single primer PCR, which could also have occurred in situ. Alternative galanin primers gave only the predicted RT-PCR product in solution phase yet still gave artefacts in tissue sections using direct in situ RT-PCR. ISH with probes transcribed from the correct PCR product gave identical labelling to the original galanin riboprobe. In conclusion, direct in situ RT-PCR is unreliable and requires validation, while indirect in situ RT-PCR may fail even though sufficient target exists for detection with conventional sensitive riboprobe ISH.

Galanin transgenic mice display cognitive and neurochemical deficits characteristic of Alzheimer's disease

Galanin is a neuropeptide with multiple inhibitory actions on neurotransmission and memory. In Alzheimer's disease (AD), increased galanin-containing fibers hyperinnervate cholinergic neurons within the basal forebrain in association with a decline in cognition. We generated transgenic mice (GAL-tg) that overexpress galanin under the control of the dopamine beta-hydroxylase promoter to study the neurochemical and behavioral sequelae of a mouse model of galanin overexpression in AD. Overexpression of galanin was associated with a reduction in the number of identifiable neurons producing acetylcholine in the horizontal limb of the diagonal band. Behavioral phenotyping indicated that GAL-tgs displayed normal general health and sensory and motor abilities; however, GAL-tg mice showed selective performance deficits on the Morris spatial navigational task and the social transmission of food preference olfactory memory test. These results suggest that elevated expression of galanin contributes to the neurochemical and cognitive impairments characteristic of AD.

Progestin regulation of galanin and prolactin gene expression in oestrogen-induced pituitary tumours

Galanin is a peptide widely distributed in the hypothalamic-pituitary axis. In the female rat pituitary, galanin is mainly present in lactotrophs, where it regulates their secretion and proliferation. Galanin expression is increased in oestrogen-induced prolactinomas, and it has been proposed that oestrogen effects on lactotroph function and proliferation could be mediated by galanin. Previous studies from our laboratory demonstrated that the synthetic progestin levonorgestrel antagonizes pituitary tumorigenesis of rats given oestrogen, reducing the number of proliferating cells and increasing cell death by nonapoptotic mechanism(s). To elucidate the role of galanin in levonorgestrel effects on the tumours, we examined galanin and prolactin mRNA and peptide expression in prolactinomas of rats receiving the progestin. Levonorgestrel reduced the pituitary weight and serum prolactin concentrations in oestrogen-treated rats. Galanin mRNA expression (determined by in situ hybridization), and the number of galanin expressing cells (determined by immunocytochemistry) were also reduced by the progestin in tumour-bearing rats. However, neither prolactin mRNA content, nor the number of prolactin-expressing cells, were modified by levonorgestrel treatment of oestrogen-receiving rats. The present study suggests that levonorgestrel controls pituitary growth by diminishing galanin expression. In contrast, changes in serum prolactin concentration seem to be more related to the reduction in tumour size, since the reduction in galanin expression was not large enough to regulate prolactin mRNA expression or the percentage of lactotrophs.

Galanin receptor 1 gene expression is regulated by cyclic AMP through a CREB-dependent mechanism

The galanin receptor-1 (GalR1) protein belongs to a family of G protein-coupled receptors for the neuropeptide galanin (GalR1, GalR2 and GalR3) distributed throughout the central and peripheral nervous system. Activation of galanin receptors by their ligands results in increased feeding, impaired learning, enhanced opiate analgesia and decreased opiate place preference. We have shown that opiate withdrawal, which is known to increase levels of cAMP in the locus coeruleus (LC), results in an increase in the number of galanin binding sites and the level of GalR1 mRNA in the LC. We have isolated a 3.6-kb fragment 5' of the inititiation codon of the mouse GalR1 gene and generated a series of deletion mutations of this fragment driving expression of luciferase for use in transient transfection assays in PC12 and Cath.a cell lines. Treatment with forskolin, but not dideoxyforskolin, up-regulates GalR1 transcription, likely through elevation of cAMP levels. The region between - 1050 and - 700 base pairs upstream of exon one is necessary both for basal activity of the GalR1 promoter and for forskolin-mediated increases in transcription. The forskolin effect can be blocked by simultaneous mutation of a CRE-like site and a CRE/DRE-like site, but not mutation of either site alone. Gel shift and super-shift experiments demonstrate that the transcription factor CREB can bind to both sites and is likely to be responsible for the cAMP-mediated increase in GalR1 promoter activity. This study provides a molecular mechanism for the increased GalR1 expression in the LC seen following opiate withdrawal.

Prolactin: structure, function, and regulation of secretion

Prolactin is a protein hormone of the anterior pituitary gland that was originally named for its ability to promote lactation in response to the suckling stimulus of hungry young mammals. We now know that prolactin is not as simple as originally described. Indeed, chemically, prolactin appears in a multiplicity of posttranslational forms ranging from size variants to chemical modifications such as phosphorylation or glycosylation. It is not only synthesized in the pituitary gland, as originally described, but also within the central nervous system, the immune system, the uterus and its associated tissues of conception, and even the mammary gland itself. Moreover, its biological actions are not limited solely to reproduction because it has been shown to control a variety of behaviors and even play a role in homeostasis. Prolactin-releasing stimuli not only include the nursing stimulus, but light, audition, olfaction, and stress can serve a stimulatory role. Finally, although it is well known that dopamine of hypothalamic origin provides inhibitory control over the secretion of prolactin, other factors within the brain, pituitary gland, and peripheral organs have been shown to inhibit or stimulate prolactin secretion as well. It is the purpose of this review to provide a comprehensive survey of our current understanding of prolactin's function and its regulation and to expose some of the controversies still existing.

In vitro effects of oestradiol on galanin gene expression in rat anterior pituitary cells

While the pituitary galanin gene is highly responsive to oestrogen stimulation in vivo, in vitro effects of oestrogens on pituitary galanin gene expression have been less studied. We therefore examined the short-term effects of 17beta-oestradiol on galanin synthesis by dispersed rat anterior pituitary cell cultures and investigated the mechanisms by which oestrogens may modulate galanin gene expression. 17beta-oestradiol increased galanin mRNA expression in a dose-dependent manner, with a maximal increase observed at a concentration of 10-7 M. The 17beta-oestradiol (10-7 M)-induced increase in galanin mRNA expression varied from 3- to 20-fold (average 12-fold) depending upon the experiments and was also time-dependent, reaching significance after 6 h. A 1-h exposure of anterior pituitary cells to 17beta-oestradiol was sufficient to induce markedly galanin mRNA expression after 24 h (by 16-fold) and 48 h (by 25-fold). Tamoxifen administered simultaneously with or following 17beta-oestradiol treatment completely abolished the oestrogen-induced increase of galanin mRNA levels. Cycloheximide (10 microg/ml), a protein synthesis inhibitor, also blocked 17beta-oestradiol-induced galanin gene expression. Using transcription blockade by actinomycin D, we observed similar decreases of pituitary galanin mRNA concentrations, in the presence and absence of 17beta-oestradiol, implying no oestrogen effect on mRNA stability. We conclude that oestrogens stimulate rat pituitary galanin gene expression, mainly through a transcriptional mechanism, and that this effect requires persistent binding of the hormone to its nuclear receptor and newly synthesized protein intermediates.

Targeted overexpression of galanin in lactotrophs of transgenic mice induces hyperprolactinemia and pituitary hyperplasia

We generated transgenic mice that carry 4.6 kb of the mouse galanin gene fused to 2.5 kb of the rat PRL promoter. In all transgenic lines that carried and transmitted the transgene, there were significant increases in galanin messenger RNA and peptide levels in the anterior pituitary in both male and female transgenic mice, and the elevation of galanin was restricted to the anterior lobe. Furthermore, galanin release from pituitary cells in vitro of both male and female transgenic mice was dramatically increased compared with that in control mice. At 2-4 months of age, pituitary PRL contents in female transgenic mice were increased compared with those in normal controls. Moreover, PRL messenger RNA levels were increased in female transgenic mice. However, plasma levels of PRL in female transgenic mice were not significantly higher until 6 months of age. By 11 months of age, cell numbers in the anterior pituitary were increased in female, but not male, transgenic mice. The percentage of lactotrophs in female transgenic mice as well as PRL gene expression per cell were significantly higher. No differences were detected in PRL content, gene expression, or release between normal and transgenic male mice. Six weeks of estrogen treatment significantly increased anterior pituitary weights and PRL secretion in male transgenic mice compared with that in normal male mice. In addition, anterior pituitary weights and PRL secretion were decreased in female transgenic mice compared with controls 6 weeks after ovariectomy. We conclude that overexpression of galanin in lactotrophs stimulates PRL synthesis and secretion and acts as a growth factor resulting in the formation of pituitary hyperplasia and hyperprolactinemia. Furthermore, estrogen appears critical for these galanin-mediated events.

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.

Short-term glucocorticoid administration decreases both hypothalamic and pituitary galanin synthesis in the adult male rat

Galanin (GAL) is a peptide that has been implicated in the regulation of the growth axis. It is generally accepted that GAL can increase serum growth hormone (GH) levels, although the underlying mechanism for this increase is unknown. It is well known that long-term glucocorticoid treatment alters in vivo GH secretion, since there is a decrease in serum GH in response to stimuli. It has previously been shown in our laboratory that administration of GAL can overcome the effects of glucocorticoid administration on GH secretion. The aim of the present study was to determine the effects of long-term glucocorticoid administration on the regulation of hypothalamic and pituitary GAL mRNA levels. Adult male rats were treated for 72 hours with the synthetic glucocorticoid dexamethasone ([DEX] 40 microg/kg/d intraperitoneal injections). RNase protection assays were performed on both the hypothalamus and pituitary for the presence of GAL mRNA. As expected, DEX significantly decreased somatic growth, as evidenced by a decrease (50%) in the weight gain of glucocorticoid-treated versus control animals. It was also demonstrated that in both the hypothalamus and pituitary, glucocorticoid treatment reduced the level of GAL mRNA (to 11% and 6.5%, respectively) compared with the control condition. We conclude that the decrease in GAL mRNA may lead to a decrease in GAL secretion, which in turn may be involved in the glucocorticoid-induced inhibition of GH secretion.

Targeted disruption of the murine galanin gene

The 29 amino acid neuropeptide galanin is widely distributed in the nervous and endocrine systems; highest levels of galanin synthesis and storage occur within the hypothalamus in the median eminence, but it is also abundantly expressed in the basal forebrain, the peripheral nervous system, and gut. To further define the role played by galanin in the peripheral nervous and endocrine systems, a mouse strain carrying a loss-of-function germ-line mutation of the galanin locus, engineered by targeted mutagenesis in embryonic stem cells, has been generated. The mutation removes the first five exons containing the entire coding region for the galanin peptide. Germ-line transmission of the disrupted galanin locus has been obtained, and the mutation has been bred to homozygosity on the inbred 129O1aHsd background. Phenotypic analysis of mice lacking a functional galanin gene demonstrate that these animals are viable, grow normally, and can reproduce. A marked reduction in both the anterior pituitary prolactin content and in circulating plasma levels of the hormone is evident. Lactation is abolished along with abrogation of the proliferative response of the lactotroph to estrogen. The responses of sensory neurons to injury in the mutants are markedly impaired. Peripheral nerve regeneration is reduced with associated long-term functional deficits. There is a striking reduction in the development of chronic neuropathic pain. These two phenotypic changes may be explained, in part, by the observation that a subset of dorsal root ganglion neurons is lost in the mutant animals, implying a role for galanin as a trophic cell survival factor. These initial findings have important implications for our understanding and potential therapeutic treatment of (a) sensory nerve regeneration and neuropathic pain and (b) disordered pituitary proliferation and the development of prolactinoma.

Galanin: analysis of its coexpression in gonadotropin-releasing hormone and growth hormone-releasing hormone neurons

Galanin is coexpressed in a subset of gonadotropin-releasing hormone (GnRH) and growth hormone-releasing hormone (GHRH) neurons in the brain and has an important role in the neuroendocrine regulation of gonadotropin and growth hormone secretion. Our overall goal has been to understand the functional significance of galanin as a cotransmitter with GnRH and GHRH in the regulation of these important physiologic processes. To this end, we studied the regulation of galanin's expression in GnRH and GHRH neurons under a variety of physiologic and experimental conditions. Using double-label in situ hybridization and computerized image analysis, we observed that in GnRH neurons, galanin's expression is increased over the course of development in both sexes. Galanin achieves a higher basal expression in GnRH neurons in females, and it is sexually differentiated in the adult as a result of the differential exposure to testosterone during the neonatal critical period. Galanin is induced in GnRH neurons coincident with and subsequent to the proestrous luteinizing hormone surge (reflecting the combined action of estradiol and progesterone) acting indirectly on GnRH neurons through a synaptic relay. Galanin's expression in GnRH neurons is inhibited during lactation, when the neuroendocrine reproductive axis is relatively quiescent. In GHRH neurons, the expression of galanin is also induced over the course of development in both sexes. Galanin's expression in GHRH neurons in the adult is sexually differentiated, but in this case, its expression is higher in males than females, reflecting the stimulatory effect of testosterone on galanin in the male. Galanin's expression in GHRH neurons is induced by growth hormone (GH), whereas the absence of GH leads to a reduction of galanin mRNA in these same cells. On the basis of these observations, we conclude that galanin is an important target for regulation by many hormones, and we postulate that as a cotransmitter, galanin acts presynaptically to modulate the secretion of GnRH and GHRH, possibly by altering their pulsatile release patterns, which in turn influences the release of the gonadotropins and GH from the pituitary.

Galanin regulates prolactin release and lactotroph proliferation

The neuropeptide galanin is predominantly expressed by the lactotrophs (the prolactin secreting cell type) in the rodent anterior pituitary and in the median eminence and paraventricular nucleus of the hypothalamus. Prolactin and galanin colocalize in the same secretory granule, the expression of both proteins is extremely sensitive to the estrogen status of the animal. The administration of estradiol-17beta induces pituitary hyperplasia followed by adenoma formation and causes a 3,000-fold increase in the galanin mRNA content of the lactotroph. To further study the role of galanin in prolactin release and lactotroph growth we now report the generation of mice carrying a loss-of-function mutation of the endogenous galanin gene. There is no evidence of embryonic lethality and the mutant mice grow normally. The specific endocrine abnormalities identified to date, relate to the expression of prolactin. Pituitary prolactin message levels and protein content of adult female mutant mice are reduced by 30-40% compared with wild-type controls. Mutant females fail to lactate and pups die of starvation/dehydration unless fostered onto wild-type mothers. Prolactin secretion in mutant females is markedly reduced at 7 days postpartum compared with wild-type controls with an associated failure in mammary gland maturation. There is an almost complete abrogation of the proliferative response of the lactotroph to high doses of estrogen, with a failure to up-regulate prolactin release, STAT5 expression or to increase pituitary cell number. These data further support the hypothesis that galanin acts as a paracrine regulator of prolactin expression and as a growth factor to the lactotroph.

Effects of estrous cyclicity on the expression of the galanin receptor Gal-R1 in the rat preoptic area: a comparison with the male

Variations in the number of galanin receptor (Gal-R1)-expressing cells and levels of Gal-R1 messenger RNA (mRNA) were determined in the preoptic area in intact female rats throughout the phases of the estrous cycle and compared with those in the male. Female and male Wistar rats were fixed by perfusion with 4% paraformaldehyde. Cryostat sections were hybridized with a 35S-labeled antisense Gal-R1 riboprobe. The number of Gal-R1 mRNA-expressing cells was lower in the rostral preoptic area than in the medial preoptic area. During the estrous cycle, the highest number of Gal-R1 mRNA-expressing cells in the rostral preoptic region was detected at 0800 h on proestrus, whereas in the medial preoptic area, the maximum number was observed at 1800 h on estrus. Gal-R1 mRNA levels in individual cells were low during diestrus and increased at estrus in both areas. In the male, the number of mRNA-expressing cells and the hybridization signal were significantly lower than those in females during estrus. The results demonstrate that Gal-R1 gene expression in the preoptic area varies during the estrous cycle and is low in males. Short term treatment of ovariectomized rats with estradiol plus progesterone caused significantly decreased preoptic Gal-R1 mRNA levels compared with those after treatment with estrogen only. These observations suggest that in the preoptic area, expression of Gal-R1 is influenced by progesterone. The variation in Gal-R1 expression is likely to influence the extent to which galanin can influence the preoptic cells implicated in the control of neighboring GnRH cells.

Galanin-like immunoreactive nerve fibres in the anterior pituitary of the normal and adrenalectomized rat

Our previous studies have shown the presence of substantial amounts of substance P and calcitonin gene-related peptide-like immunoreactive nerve fibres in the anterior pituitary of the monkey, dog and rat. Furthermore, synaptic relationships have been demonstrated between these nerve fibres and the gland cells in the dog and rat. The substance P and calcitonin gene-related nerve fibres increase in number following adrenalectomy and ovariectomy, respectively. The present study was aimed to investigate the galanin-containing nerve fibres in the anterior pituitary of normal and adrenalectomized rats. The results showed only a small amount of galanin-like immunoreactive nerve fibres in the normal anterior pituitary, which were present among the gland cells as well as along the blood vessels. Following adrenalectomy, the number of galanin-like immunoreactive nerve fibres increased and ramification appeared more frequently. The results substantiate our hypothesis of a dual neural-humoral regulation of the mammalian anterior pituitary gland.

The relationship of galanin immunoreactive nerve fibers to glandular cells in the anterior pituitary in the monkey

The anterior pituitary is known to be regulated by hypothalamic hormones via the portal system. However, our recent studies have demonstrated the presence of a substantial amount of substance P (SP)-, calcitonin gene-related peptide (CGRP)- and galanin (GAL)- immunoreactive (ir) nerve fibers with numerous varicosities in the anterior pituitaries of the Macaca mulatta monkey. The present study investigated the relationship of the GAL-ir nerve fibers to the glandular cells. The M. mulatta monkeys were used and sections of the anterior pituitary were double immunostained. GAL-ir nerve fibers and/or varicosities were found in proximity to contact directly with corticotropes, somatotropes, lactotropes, gonadotropes and thyrotropes without any exception. These findings indicate that a direct neural factor may be involved in the regulation of adenohypophyseal secretion.

Regulation of anterior pituitary galanin gene expression by thyroid hormone

Thyroid hormone is required for basal and estrogen-induced expression of anterior pituitary galanin. Steady-state anterior pituitary galanin mRNA levels decreased 6-fold in hypothyroid rats after 3 weeks of treatment. Similarly, hypothyroidism resulted in a 2.6-fold decrease in estrogen induction of galanin gene expression. The effect of thyroid hormone on anterior pituitary galanin gene expression appears to be exerted, at least in part, at the pituitary itself. Transient expression assays in GH3 cells suggest the involvement of transcriptional mechanisms in the regulation of galanin gene expression by thyroid hormone. A region between -41 and -132 bp upstream of the transcriptional start site confers thyroid hormone responsiveness to the galanin gene. Gel-mobility shift assays show specific binding of 'SPI-like' proteins in GH3 nuclear extracts to this region of the galanin gene. This binding was greatly enhanced by thyroid hormone.

An estrogen receptor binding site within the human galanin gene

Regulation of galanin gene expression in the anterior pituitary (AP) is positively influenced by estrogen in rodents and undetermined in humans. The objective of this study was to investigate the mechanism behind estrogen induction of galanin by identifying any putative estrogen receptor (ER) binding sequences within the human galanin promoter that may function as estrogen response elements (ERE). Two regions, gERE1 and gERE2, were identified in the galanin 5'-flanking sequence with similarity to the full 13-base ERE consensus previously defined in the vitellogenin gene (vERE). Both sequences were tested in mobility shift assays for the ability to bind nuclear proteins isolated from rat AP tissue or MtTW-10 pituitary tumors. Only the distal sequence at -527 (gERE1) yielded an ERE-specific DNA/protein complex distinguished by mobility and cross-competition with vERE. The gel mobility pattern of the DNA/protein complex was comparable between the pituitary tissue and tumor extracts. However, DNA/protein affinity estimations demonstrated a greater affinity of pituitary proteins for gERE1 over the vERE sequence. Evidence that the human ER (hER) does recognize the gERE1 sequence in the human galanin gene was provided by electrophoretic mobility shift assays (EMSAs) with Sf9 extracts enriched in recombinant hER. In addition, antibodies specific for the hER recognized the gERE1/protein complex in supershift experiments. Enhancer activity by gERE1 was detected in transient transfections of the rat GH3 pituitary cell line, resulting in a 4-fold induction of expression driven by the heterologous thymidine kinase promoter in the presence of estrogen. Evidence for ER regulation of the gERE1 enhancer was demonstrated by: 1) inhibition of enhancement using the specific ER antagonist ICI 164,384; and 2) enhancement in HeLa cells that was dependent upon coexpression with hER. Enhancement by gERE1 was half the magnitude as that from the vERE element and may reflect a difference in affinity or composition of the ER complex between the two sequences. These data demonstrate the presence of a functional ERE sequence within the human galanin gene that could potentially function as a regulatory element for estrogen action in the AP.

Galanin positively modulates prolactin secretion in normal women

It is widely accepted that, in man, galanin, a neuropeptide, has a clear GH-releasing effect while its stimulatory influence on PRL secretion is matter of debate. To clarify this point, in 6 normal young women (23-35 yr) in their early follicular phase, we studied the effect of galanin (pGAL, 80 pmol/kg. min infused i.v. over 60 min) on both basal and arginine (ARG, 0.5 g/kg i.v. in 30 min), TRH (400 micrograms i.v. as a bolus at 0 min) or metoclopramide (MCP, 10 mg i.v. as a bolus at 0 min)-stimulated PRL secretion. GAL infusion failed to significantly increase basal PRL levels (peak vs baseline: 12.2 +/- 3.6 vs 8.7 +/- 1.2 micrograms/L) but counteracted the spontaneous PRL decrease observed during saline infusion (AUC: 1216.6 +/- 282.1 vs 672.0 +/- 94.5 micrograms.min/L; p < 0.05). GAL infusion clearly enhanced the PRL response to TRH (AUC: 5806.3 +/- 743.0 vs 3952.1 +/- 423.9 micrograms.min/L, p < 0.05) and ARG (AUC: 3676.8 +/- 382.6 vs 2638.9 +/- 287.0 micrograms.min/L, p < 0.05), respectively. On the other hand, GAL failed to modify the MCP-induced PRL response (AUC: 15409.5 +/- 2085.3 vs 14,787.9 +/- 2045.5 micrograms.min/L). The PRL response to MCP was higher than that to TRH (p < 0.01) which, in turn, was higher than that to ARG (p < 0.01). During GAL infusion, the PRL response to TRH or ARG remained lower (p < 0.01) than that after MCP administration. Thus, in conclusion, present data demonstrate that in normal women galanin enhances the PRL response to ARG and TRH but fails to modify that induced by dopamine receptor blockade with metoclopramide. Based on evidence that the inhibition of central dopaminergic activity inhibits the lactotrope responsiveness to dopaminergic antagonists or TRH, it is unlikely that galanin influences PRL secretion via inhibition of dopaminergic tone.

Galanin gene expression in radiothyroidectomy-induced thyrotroph adenomas

Galanin gene expression is markedly increased in the anterior pituitary glands of estrogen-treated rats (lactotroph hyperplasia) as well as human growth hormone-releasing hormone transgenic mice (somatotroph hyperplasia). The objective of this study was to examine galanin in a mouse model of thyrotroph adenoma formation. Male mice were radiothyroidectomized by use of iodine-131 (131I), and galanin peptide levels were assessed in the hypothalamic-pituitary axis. Immunoreactive galanin concentrations in the anterior pituitaries of 131I-treated mice were decreased 80% at 3, 6, 9, and 12 mo after radiothyroidectomy. Galanin peptide levels in the hypothalamus were decreased 20-25% at these times. Treatment with either estradiol or 3,3',5-triiodo-L-thyronine increased galanin peptide concentrations in the anterior pituitaries of 131I-treated mice, but neither treatment restored galanin concentrations. Galanin mRNA levels were decreased > 80% 1 yr after radiothyroidectomy. We conclude that, unlike animal models of lactotroph and somatotroph hyperplasia, galanin gene expression is suppressed throughout the development of thyrotroph adenomas, suggesting that galanin does not have a stimulatory role in the proliferation of thyrotrophs. Moreover, these data show that thyroid hormones are important positive regulators of galanin gene expression in the mouse and that estrogen may stimulate galanin gene expression in the absence of thyroid hormones.

Sex-steroid control of galanin in the rat hypothalamic-pituitary axis

To further investigate how sex steroids regulate galanin (GAL) in the rat pituitary and hypothalamus, we examined the effects of prepubertal gonadectomy (Gx) and long-term (9 weeks) replacement with estradiol (E2) or testosterone (T) on pituitary and hypothalamic GAL concentrations in Wistar rats (5-6/group). Sham-operated animals served as controls (CTR). Pituitary GAL concentration was markedly higher in random-cycling CTR-females than in CTR-males (1391 +/- 247 vs 39 +/- 5 pg/mg protein, P < 0.01) and decreased after Gx only in females (20 +/- 3 pg/mg protein, P < 0.01). E2 strongly increased pituitary GAL in Gx-females and Gx-males (4470 +/- 365 and 3853 +/- 347 pg/mg protein, P < 0.01), whereas T had no effect. Inversely, hypothalamic GAL was higher in CTR males than in CTR females (5.4 +/- 0.3 vs 4.0 +/- 0.5 ng/mg protein, P < 0.05), and decreased significantly after gonadectomy in males (3.7 +/- 0.2 ng/mg protein, P < 0.01). The only steroid treatment that significantly modified hypothalamic GAL in Gx animals was administration of E2 to females (5.7 +/- 0.4 ng/mg protein, P < 0.01 vs non-treated Gx). We also studied in hypophysectomized (Hx) rats (8/group) the effects of sex steroids on hypothalamic GAL concentration and distribution. The low hypothalamic GAL concentration observed in male and female Hx rats (1.0 +/- 0.1 ng/mg protein) was significantly increased by T in males and in females (respectively, by 40% and by 50%, P < 0.02) and by E2 in males (by 60%, P < 0.02).(ABSTRACT TRUNCATED AT 250 WORDS)

Galanin--10 years with a neuroendocrine peptide

Galanin is a 29/30 amino acids long neuropeptide which does not belong to any known peptide family. The N-terminal first 16 amino acids of the molecule are both necessary and sufficient for receptor recognition and receptor activation. The main pharmacophores of galanin in its central and pancreatic actions are Gly1, Trp2, Asn5 and Tyr9, respectively. The neuropeptide galanin has multiple effects in both the central and peripheral nervous systems. Centrally, galanin potently stimulates fat intake and impairs cognitive performance. Anoxic glutamate release in the hippocampus is inhibited by galanin and the noradrenergic tonus in the brain is influenced by a hyperpolarizing action of galanin in the locus coeruleus. In the spinal cord galanin inhibits spinal excitability and potentiates the analgesic effect of morphine. In the neuroendocrine system galanin acts in a stimulatory manner on the release of growth hormone and prolactin, and peripherally galanin inhibits glucose induced insulin release. Galanin also causes contraction of the jejunum. The galanin receptor is a Gi-protein-coupled, membrane-bound glycoprotein with an estimated molecular mass of 53 kDa. Several putative tissue specific galanin receptor subtypes have been proposed on a pharmacological basis. The distribution of galanin receptors and of galanin like immunoreactivity are overlapping in the CNS, both being high in areas such as the locus coeruleus, raphe nucleus and hypothalamus. Galanin receptor activation leads to a reduced intracellular Ca(2+)-concentration, either by direct action on voltage sensitive Ca(2+)-channels or indirectly via opening of K(+)-channels or via inhibition of adenylyl cyclase activity. The lowered intracellular Ca2+ level subsequently leads to a reduced PLC activity. Galanin also inhibits cGMP synthesis induced by depolarization. A number of synthetic high affinity galanin receptor antagonists of the peptide type were developed recently, which have enabled the elucidation of functional roles of endogenous galanin in several systems. Furthermore, putative subtypes of galanin receptors can be distinguished by the use of these new galanin receptor ligands.

Characterization of trout galanin and its distribution in trout brain and pituitary

Galanin was purified from an extract of the stomach of the rainbow trout, Oncorhynchus mykiss, and its primary structure was established as Gly-Trp-Thr-Leu-Asn-Ser- Ala-Gly-Tyr-Leu10-Leu-Gly-Pro-His-Gly-Ile-Asp-Gly-His-Arg20- Thr-Leu-Ser-Asp- Lys-His-Gly-Leu-Ala. Trout galanin shows six amino acid substitutions compared with pig galanin, but the N-terminal region (residues 1-14) has been fully conserved. The distribution of galanin-immunoreactive (GAL-IR) structures in the trout brain and pituitary was studied via immunohistochemistry. GAL-IR cell bodies were observed only in the caudal telencephalon, the preoptic region, and the mediobasal hypothalamus. GAL-IR fibers, however, are widely distributed throughout the brain, with a much lower density in the midbrain and posterior brain than in the tel- and diencephalon. Particularly dense innervation of the mediobasal hypothalamus, the ventral and supracommissuralis parts of the caudal telencephalon, and the region above and below the anterior commissure was observed. A heavy innervation of the pituitary was consistently detected. GAL-IR fibers were present in neurohypophyseal digitations of both the anterior and intermediate lobes with highest density in the region of the proximal pars distalis, where growth hormone and gonadotropic cells are located. Fibers were also seen in digitations of the rostral pars distalis, in particular between the prolactin follicles. The distribution of GAL-IR neurons in the central nervous system and pituitary of the trout suggests that the peptide may exercise an important role in the regulation of neuroendocrine functions, particularly those related to reproduction.

Activation of galanin pathways across puberty in the male rat: galanin gene expression in the bed nucleus of the stria terminalis and medial amygdala

Galanin and vasopressin are coexpressed in the bed nucleus of the stria terminalis and medial amygdala of the male rat. In adult males, the level of gene expression for both peptides in these regions is dependent on circulating levels of testosterone. We hypothesized that galanin messenger RNA levels would be enhanced in adult males compared with prepubertal males due to the rise in plasma testosterone levels. We used in situ hybridization and quantitative autoradiography to measure galanin messenger RNA in cells of the bed nucleus of the stria terminalis and medial amygdala of prepubertal and adult male rats. Our results show that significantly (P < or = 0.05) more galanin messenger RNA expressing neurons are detectable in the bed nucleus of the stria terminalis of adult compared with prepubertal male rats. In contrast, no differences were observed between the groups in the number of labeled neurons detected within the medial amygdala. However, the average labeling intensity was significantly enhanced in both the bed nucleus of the stria terminalis (P < or = 0.001) and medial amygdala (P < or = 0.001) of adult compared with prepubertal animals. The present findings are consistent with the hypothesis that gonadal hormones regulate galanin gene expression in some brain regions and suggest that the activation of the hypothalamic-pituitary-gonadal axis which occurs naturally with puberty is associated with activation of galanin pathways in the bed nucleus of the stria terminalis and medial amygdala.

Adrenal steroid receptors: interactions with brain neuropeptide systems in relation to nutrient intake and metabolism

The glucocorticoid, corticosterone (CORT), is believed to have an important function in modulating nutrient ingestion and metabolism. Recent evidence described in this review suggests that the effects of this adrenal hormone are mediated through two steroid receptor subtypes, the type I mineralocorticoid receptor and the type II glucocorticoid receptor. These receptors, which have different affinities for CORT, respond to different levels of circulating hormone. They mediate distinct effects of the steroid, which can be distinguished by the specific nutrient ingested and by the particular period of the circadian cycle. Under normal physiological conditions, the type I receptor is tonically activated, either by low basal levels of circulating CORT (0.5-2 microgram %) normally available across the circadian cycle or possibly by the mineralocorticoid aldosterone. This type I activation is required for the maintenance of fat ingestion and fat deposition that occurs during most meals of the feeding cycle. In contrast, the type II receptor is phasically activated by moderate levels of CORT (2-10 micrograms %) normally reached during the circadian peak. Activation of this receptor is required for the natural surge in carbohydrate ingestion and metabolism that is essential at the onset of the active feeding cycle when the body's glycogen stores are at their nadir, and gluconeogenesis is needed to maintain blood glucose levels. This receptor is also activated during periods of increased energy requirements, such as, after exercise and food restriction, when CORT levels rise further (> 10 micrograms %) and when its catabolic effects on fat and protein stores predominate to provide additional substrates for glucose homeostasis. These functions of CORT on fat and carbohydrate balance are mediated, in part, by type I and type II receptors located within the hypothalamic paraventricular nucleus, which is known to have key functions in controlling nutrient intake and metabolism, as well as circulating CORT levels. Moreover, the type II receptors within this nucleus, in addition to the arcuate nucleus, may interact positively with the peptide, neuropeptide Y, and the catecholamine, norepinephrine, both of which act to enhance natural carbohydrate feeding and CORT release at the onset of the natural feeding cycle. Thus, under normal conditions, endogenous CORT has a primary function in controlling nutrient ingestion and metabolism over the natural circadian cycle, through the coordinated action of the type I and type II steroid receptor systems. Through this action, CORT has impact on total caloric intake and body weight gain over the long term.(ABSTRACT TRUNCATED AT 400 WORDS)

Hypothalamic galanin-like immunoreactivity and its gene expression in relation to circulating corticosterone

The neuropeptide galanin (GAL), which exists in dense concentrations within the hypothalamus, has physiological actions which are neuroendocrine in nature. In light of evidence showing GAL to alter the release of the adrenal steroid, corticosterone (CORT), a possible effect of this steroid on GAL gene expression and peptide production in discrete hypothalamic and brainstem sites was investigated. Using radioimmunoassay and in situ hybridization techniques, this peptide was examined in rats that had received SHAM surgery, adrenalectomy (ADX) and ADX+CORT replacement. The results showed a clear, site-specific change in GAL in relation to circulating CORT. A loss of CORT after ADX caused a dramatic decline in GAL peptide and mRNA levels in the arcuate nucleus and peptide levels in the median eminence, with no change occurring in other hypothalamic areas. In the brainstem, a similar change was detected in the dorsal raphe nucleus but not the locus coeruleus. The GAL peptide and mRNA levels in these specific brain areas of ADX rats was restored by CORT replacement, which had no impact on GAL in other brain sites. These findings demonstrate that CORT's impact on brain GAL is highly site specific, possibly determined by local concentrations of steroid receptors.

Distribution and chromatographic analysis of galanin message-associated peptide (GMAP)-like immunoreactivity in the rat

Galanin message-associated peptide (GMAP) constitutes the C-terminal part of the precursor protein encoding also the biologically active neuropeptide galanin (GAL). We have raised antisera against a species-conserved portion of GMAP, and investigated the localization of GMAP-like immunoreactivity (-LI) in relation to that of GAL-LI in the rat central and peripheral nervous system using the indirect immunofluorescence technique. In the central nervous system, GMAP-immunoreactive (-IR) cell bodies were observed in the hypothalamus, while GMAP-IR nerve fibers were demonstrated in the septum, hypothalamus, pons and spinal cord. In the posterior pituitary and in the connecting infundibular stalk, weakly fluorescent GMAP-IR nerve fibers were observed. GMAP-IR nerve fibers were also observed throughout the gastrointestinal tract, i.e., from the stomach down to the colon, and in all layers, except in the epithelium, of the wall. In general, the staining of consecutive tissue sections suggested that GMAP-IR was co-distributed with that of GAL-IR. A sensitive radioimmunoassay (RIA) for characterization of GMAP-IR in the rat central and peripheral nervous system was also developed. Characterization of GMAP-LI in acid extracts of rat brain and small intestine, using reverse phase high pressure liquid chromatography (rpHPLC), revealed multiple GMAP-IR forms that co-eluted with a synthetic porcine GMAP(19-41)-amide fragment, or were less or more polar than this fragment. The corresponding chromatographic analysis of GAL-LI revealed only one major form corresponding to rat GAL. The immunohistochemical data indicate that a GMAP-like peptide(s) probably is axonally transported and may possibly have pre- and/or post-synaptic functions. The nature of the multiple GMAP-IR components remains to be investigated, but may tentatively represent differently processed and/or chemically modified forms of rat GMAP(1-60).

Bioactive peptides in anterior pituitary cells

The anterior pituitary (AP) has been shown to contain a wide variety of bioactive peptides: brain-gut peptides, growth factors, hypothalamic releasing factors, posterior lobe peptides, opioids, and various other peptides. The localization of most of these peptides was first established by immunocytochemical methods and some of the peptides were localized in identified cell types. Although intracellular localization of a peptide may be the consequence of internalization from the plasma compartment, there is evidence for local synthesis of most of these peptides in the AP based on the identification of their messenger-RNA (mRNA). In several cases the release of the peptide from the AP cell has been shown and regulation of synthesis, storage and release have also been described. Because the amount of most of the AP peptides is very low (except for POMC peptides and galanin), endocrine functions are not expected. There is more evidence for paracrine, autocrine, or intracrine roles in growth, differentiation, and regeneration, or in the control of hormone release. To demonstrate such functions, in vitro AP experiments have been designed to avoid the interference of hypothalamic or peripheral hormones. The strategy is first to show a direct effect of the peptide after adding it to the in vitro system and, secondly, to explore if the endogenous AP peptide has a similar action by using blockers of peptide receptors or antisera immunoneutralizing the peptide.

Immunocytochemical localization of progesterone receptors in galanin neurons in the guinea pig hypothalamus

A double-label immunocytochemical technique was used to determine whether progesterone receptor-containing neurons in the female guinea-pig hypothalamus also contained galanin. Adult ovariectomized guinea-pigs were primed by estradiol to induce progesterone receptors and injected intracerebroventricularly with colchicine to visualize galanin-immunopositive neurons. A small proportion of progesterone receptor-containing perikarya in the medial preoptic area and the mediobasal hypothalamus were bound to be immunoreactive for galanin. The medial preoptic, periventricular and arcuate nuclei showed the greatest concentration of double-labelled cells. Galanin varicosities appeared in close proximity to neurons with progesterone receptor-containing nuclei. These results provide neuroanatomical evidence that a subset of hypothalamic galanin-immunoreactive neurons is directly regulated by progesterone.

Galanin regulates basal and oestrogen-stimulated lactotroph function

Oestrogen, an essential physiological regulator of reproductive function, controls lactotroph proliferation and prolactin release. The neuropeptide galanin co-localizes to the lactotroph, but its physiological function is largely unknown. Pituitary galanin expression is extremely sensitive to the oestrogen status of the animal. A marked elevation occurs during pregnancy and lactation, and exogenous 17 beta-oestradiol can cause a 4,000-fold increase in messenger RNA levels. Here we report that galanin is secreted by a minority of lactotrophs and is essential for the regulation of basal and vasoactive-intestinal-polypeptide-stimulated prolactin release. Hyperoestrogenization increases the number of galanin-secreting cells and the resulting increase in basal prolactin release is completely abolished by treatment with galanin antiserum. Galanin is a potent lactotroph growth factor and galanin-immunoneutralization completely inhibits the previously reported mitogenic effects of oestrogen on the lactotroph. These findings represent direct evidence for paracrine regulation of lactotroph function and demonstrate that the effect of oestrogen on lactotroph proliferation and prolactin release are mediated by locally secreted galanin.

Modulation of galanin and neuromedin U-like immunoreactivity in rat corticotropes after alteration of endocrine status

The localization of galanin in rat lactotropes and human corticotropes is well established. Neuromedin U immunoreactivity is present in rat corticotropes but radioimmunoassay of thyroid-manipulated rat pituitaries has also linked it to the thyroid axis. We found galanin immunoreactivity in some rat corticotropes, so we have re-examined rat anterior pituitary galanin- and neuromedin U-like immunoreactivity by use of immunocytochemistry and electron microscopy in rats in the normal state and after estrogen administration or adrenalectomy. In normal rats galanin immunoreactivity was present in a few corticotropes and lactotropes, females showing more than males; neuromedin U-like immunoreactivity was present in some thyrotropes and most corticotropes, in both sexes. Where galanin, neuromedin U and ACTH immunoreactivities were colocalized in corticotropes they were present in the same granules. Estrogen administration caused an increase in number of galanin immunoreactive lactotropes, as previously shown. The proportion of neuromedin U-positive corticotropes was not affected. After adrenalectomy, only females showed a significant increase in the proportion of galanin-positive corticotropes. Neuromedin U immunoreactivity was significantly increased in both sexes, as previously shown. Thus, in rat, as in man, galanin can be present in corticotropes and its expression appears to be sex-related. This finding, and the demonstration of thyrotrope neuromedin U (only examined in normal females), provide correlation with previous experiments. The influence of endocrine status on the expression of these novel peptides underlines the inherent plasticity of pituitary endocrine cells.

Galanin receptors in the brain of a teleost: autoradiographic distribution of binding sites in the Atlantic salmon

The distribution of galanin (GAL) binding sites in the brain of the Atlantic salmon (Salmo salar) was investigated by means of radioligand binding in conjunction with autoradiography by using high-performance liquid chromatography (HPLC) characterized radio-iodinated porcine galanin ([125I]GAL). On slide-mounted sections of frozen salmon brain homogenate, [125I]GAL (4 nM) bound rapidly and reversibly to a single population of sites with a Kd of 1.0 +/- 0.08 nM (n = 3) and Bmax of 2.38 +/- 0.19 fmol/mg wet tissue. Specific [125I]GAL binding was found in cellular regions, in fiber tracts, and in neuropil areas throughout the brain, except for in the olfactory bulb, pineal organ, and cerebellum. Autoradiographic microdensitometric measurements revealed high total [125I]GAL binding in the ventral hypothalamus (inferior lobes; around 7-12 fmol/mg tissue), the dorsal spinal cord (between 6 and 12 fmol/mg tissue), sublayers of the optic tectum (around 8 fmol/mg), torus semicircularis (around 7 fmol/mg), and glomerular complex (around 6 fmol/mg). Intermediate densities of [125I]GAL binding (3-5 fmol/mg tissue) were found in the pituitary, telencephalon, dorsolateral thalamic nucleus, and raphe nuclei and in association with the forebrain bundles. Except for in the optic tectum, there is a good concordance of [125I]GAL binding sites and GAL-immunoreactive fiber projections in most brain areas of the salmon. The wide distribution of GAL binding sites provides further evidence that a GAL-like substance might be involved in a diversity of brain functions of teleosts. The topographic distribution of target sites in the hypothalamo-hypophyseal axis indicates that GAL-like substances may have both direct and indirect effect on pituitary functions while in extrahypothalamic areas, functional implications by GAL may include involvement in somatosensory, central gustatory, olfactory, and visual functions. This study provides evidence for the presence of a specific GAL receptor in the brain of the Atlantic salmon. Together the distribution of GAL binding and GAL-like molecules provide a covering delineation of the GAL neuronal system in the brain of the Atlantic salmon. Comparisons with mammals suggest that the GAL receptor molecule has been well preserved during evolution and that GAL-like substances may be present, and even possess similar functional properties, throughout the vertebrate phylogeny.

Immunocytochemical localization of a galanin-like peptidergic system in the brain of two urodele and two anuran species (Amphibia)

Galanin-like immunoreactivity was localized in the brain of Urodela (Ambystoma, Pleurodeles) and Anura (Bufo, Xenopus) by immunocytochemistry with anti-porcine galanin antiserum. In the four species, immunoreactive perikarya were observed in the telencephalon (striatum, amygdala), diencephalon preoptic area mainly along the anterodorsal wall of the preoptic recessus, suprachiasmatic nucleus, lateral hypothalamus, ventral and dorsal infundibular nuclei, paraventricular organ, and rhombencephalon (nucleus of the solitary tract). Galaninergic fibres extended in similar regions and in the medial septum, ventral telencephalon, ventral hypothalamus, median eminence, and various mesencephalic and rhombencephalic regions. Contacts with the cerebrospinal fluid cavity occurred along the preoptic recessus (Ambystoma) and the ventral infundibular wall (all species). Fibres were scarce in the neurohypophysis. The distal and intermediate lobes of the pituitary were virtually devoid of immunoreactivity. The galaninergic system appeared more developed in adult amphibia than in young animals, suggesting the stimulating influence of sex steroids on the expression of galanin as previously described in Anguilla. The extensive distribution of the galanin-like immunoreactive neurons in amphibian brains suggests that this peptide may act as a neuromodulatur and/or neurotransmitter.

Dopaminergic regulation of galanin gene expression in the rat anterior pituitary gland

Estrogen dramatically increases galanin mRNA and peptide levels in the rat anterior pituitary gland. We recently reported that galanin secretion in vitro from estrogen-exposed anterior pituitary cells is regulated by hypothalamic factors; dopamine and somatostatin inhibit galanin secretion, and thyrotrophin-releasing hormone stimulates galanin release. To determine whether galanin is regulated by a dopaminergic mechanism in vivo, we used ovariectomized Fischer 344 rats treated with 17ß-estradiol-containing or empty Silastic capsules. Rats were also administered bromocriptine, a dopamine receptor agonist, haloperidol, a dopamine receptor antagonist, or placebo for 2 weeks. Galanin peptide levels were measured in the anterior pituitary, neurointermediate lobe, medial basal hypothalamus, and plasma by radioimmunoassay. Plasma and pituitary prolactin levels were also determined. Bromocriptine decreased gaianin peptide levels in the anterior pituitary gland of ovariectomized rats by 30%, but had no effect on galanin in the neurointermediate lobe or medial basal hypothalamus. In contrast, haloperidol had no effect on galanin in the anterior pituitary or medial basal hypothalamus of ovariectomized rats, but decreased galanin peptide levels in the neurointermediate lobe. In the anterior pituitary gland of estrogen-treated rats, bromocriptine increased and haloperidol decreased both galanin and prolactin levels. Galanin mRNA levels were quantified in the anterior pituitary gland by solution hybridization. Bromocriptine increased galanin mRNA levels 3-fold in the anterior pituitary, whereas haloperidol had no effect. Galanin mRNA levels in the anterior pituitary were elevated 10-fold by estrogen. Bromocriptine reduced galanin mRNA levels in the pituitary by 50% in estrogen-treated rats, where again haloperidol had no effect. Estrogen increased plasma galanin levels 4-fold compared to ovariectomized rats and this effect was reduced 60% by bromocriptine and increased 20% by haloperidol. We conclude 1) galanin synthesis and release from the estrogen-exposed anterior pituitary gland is inhibited by a dopaminergic mechanism in vivo, 2) dopamine regulates galanin gene expression in the ovariectomized rat, 3) the changes in galanin peptide levels in the anterior pituitary of rats treated with estrogen and dopamine receptor ligands are primarily due to alterations in peptide secretion, and 4) galanin release from the neurointermediate lobe may also be regulated by a dopaminergic mechanism in vivo. These data, in conjunction with previous studies, provide evidence for the co-regulation of galanin and prolactin in estrogen-treated rats, and further discriminate between the regulation of galanin in the hypothalamus and pituitary gland.

Interactions of galanin and arginine on growth hormone, prolactin, and insulin secretion in man

Galanin (GAL), a 29 amino acid neuropeptide, is known to increase both basal and growth hormone-releasing hormone (GHRH)-induced growth hormone (GH) secretion while not significantly increasing prolactin (PRL) secretion in man. GAL is also endowed with an inhibiting effect on glucose-stimulated insulin release in animals, but not in man. We studied the effect of GAL (80 pmol/kg/min infused over 60 minutes) on the arginine- (ARG, 30 g infused over 30 minutes) stimulated GH, PRL, insulin, and C-peptide secretion in eight healthy volunteers (age, 20 to 30 years). GAL induced an increase of GH (GAL v saline, area under curve [AUC], mean +/- SEM: 316.5 +/- 73.9 v 93.2 +/- 20.9 micrograms/L/h, P less than .05), but failed to modify both PRL and insulin secretion. GAL enhanced the ARG-induced stimulation of both GH (1,634.1 +/- 293.1 v 566.9 +/- 144.0 micrograms/L/h, P less than .02) and PRL secretion (1,541.9 +/- 248.8 v 1,023.8 +/- 158.7 micrograms/L/h, P less than .02). On the contrary, GAL blunted the ARG-stimulated insulin (816.3 +/- 87.7 v 1,322.7 +/- 240.9 mU/L/h, P less than .05), as well as C-peptide secretion (105.1 +/- 9.8 v 132.8 +/- 17.3 micrograms/L/h, P less than .02). ARG administration induced a transient increase of glucose levels (P less than .01 v baseline) followed by a significant decrease (P less than .05 v baseline). This latter effect was prevented by the coadministration of GAL. In conclusion, these results show that in man GAL potentiates the GH response to ARG, suggesting that these drugs act at the hypothalamic level, at least in part, via different mechanisms.(ABSTRACT TRUNCATED AT 250 WORDS)

Galanin-like immunoreactivity is increased in the brain of estradiol- and methyltestosterone-treated eels

A galanin-like peptidergic system was demonstrated in the brain of Anguilla. A group of immunoreactive perikarya was located in the nucleus preopticus periventricularis close to the recessus preopticus. Galaninergic fibers occurred in various brain areas. Galanin identified in mammalian pituitary cells was undetectable in fish adenohypophysial cells. Estradiol increased the immunostaining of the rostral perikarya and brain fibers in both male and female European eels kept in fresh water and in female American eels in sea water. Methyltestosterone, an aromatizable androgen, increased galanin immunoreactivity in rostral perikarya and brain fibers of male European eels and female American eels. The cross-sectional area of these perikarya increased significantly after both treatments whereas cell bodies of the posteroventral hypothalamus were slightly affected. Dihydrotestosterone showed no clear effect. Fibers close to the corticotropes were sometime increased, but galanin synthesis was not induced in pituitary cells. In contrast, estradiol induced galanin synthesis in rat pituitary cells, but had a still controversed effect on hypothalamic galanin. A putative influence of galanin on the pituitary-gonadal axis is discussed as gonadal hormones diversely affect gonadotropes and gonosomatic indices in Anguilla.