The galanin antagonist, M-15, inhibits growth hormone release in rats

Neuroendocrine Regulation of Growth Hormone Secretion

This article reviews the main findings that emerged in the intervening years since the previous volume on hormonal control of growth in the section on the endocrine system of the Handbook of Physiology concerning the intra- and extrahypothalamic neuronal networks connecting growth hormone releasing hormone (GHRH) and somatostatin hypophysiotropic neurons and the integration between regulators of food intake/metabolism and GH release. Among these findings, the discovery of ghrelin still raises many unanswered questions. One important event was the application of deconvolution analysis to the pulsatile patterns of GH secretion in different mammalian species, including Man, according to gender, hormonal environment and ageing. Concerning this last phenomenon, a great body of evidence now supports the role of an attenuation of the GHRH/GH/Insulin-like growth factor-1 (IGF-1) axis in the control of mammalian aging.

Stimulation of porcine pituitary luteinizing hormone release by galanin: putative auto/paracrine regulation

It has repeatedly been suggested that galanin acts within the anterior pituitary (AP) in an auto/paracrine manner to modulate luteinizing hormone (LH) release. Except for one recent report in the rat, evidence for this notion is absent. The purpose of this study was to investigate in the pig the effects of galanin on LH and growth hormone (GH) release and to evaluate putative local effects using various AP culture systems (monolayer, perifusion, reaggregates). Independent of age galanin dose dependently (0.05, 0.2, 1 microM) stimulated basal but not gonadotropin-releasing hormone (GnRH; > or =0.01 nM)-induced LH release. Neither basal nor GH-releasing hormone (GHRH)-stimulated GH release was affected at any age. Of 4 galanin receptor antagonists (0.2, 1 microM) tested C7 proved to have agonistic effects, whereas M40 and M15 (galantide) were ineffective in blocking galanin (0.2 microM)-induced LH secretion or affecting basal or GnRH-induced LH release. M32 [galanin (1-13) NPY (25-36) amide] inhibited (p < or = 0.05) GnRH-induced LH release at doses of > or =2 microM, an effect which could be totally compensated by 1 microM galanin. However, the neuropeptide (NPY) antagonist BIBP 3226 (1 microM) partially overcame the effect of M32 (M32 is known to also bind to NPY receptors and NPY is inhibitory in the pig). In further studies using APs from preovulatory gilts a specific well-characterized galanin antiserum diluted 1:20 or 1:50 attenuated GnRH-induced LH release (p < or = 0.05). However, an NPY antiserum (also affinity purified and at the same dilution) used as control unexpectedly inhibited GnRH (and galanin)-induced LH release as well, thus suggesting that attenuation of GnRH-induced LH release by galanin antiserum might be at least partly nonspecific. Furthermore 96-hour exposure of AP reaggregates to two types of porcine preprogalanin antisense oligodeoxynucleotides neither affected basal nor GnRH-induced LH release. In line with the failure to unequivocally prove paracrine effects of galanin, concentrations of galanin in AP cultures and AP culture medium were very low (< or =2 pg galanin/10(5) AP cells). In conclusion the present study provides some evidence to ascribe a hypophysiotropic role to galanin in regulating LH but not GH secretion in the pig. The study also points to the critical role of appropriate controls when trying to prove auto/paracrine control mechanisms within the anterior pituitary. Our findings do not provide convincing evidence to support the notion that intrapituitary galanin is involved in the fine tuning of LH secretion, at least in the preovulatory pig.

The differential effects of galanin-(1-30) and -(3-30) on anterior pituitary hormone secretion in vivo in humans

Intravenous injection of galanin increases plasma growth hormone (GH) and prolactin (PRL) concentrations. In the rat, the effects of galanin on GH appear to be mediated via the hypothalamic galanin receptor GAL-R(1), at which galanin-(3-29) is inactive. In contrast, the effect of galanin on PRL is mediated via the pituitary-specific galanin receptor GAL-R(W), at which galanin-(3-29) is fully active. We investigated the effects of an intravenous infusion of human galanin (hGAL)-(1-30) and -(3-30) on anterior pituitary hormone levels in healthy females. Subjects were infused with saline, hGAL-(1-30) (80 pmol. kg(-1). min(-1)), and hGAL-(3-30) (600 pmol. kg(-1). min(-1)) and with boluses of gonadotropin-releasing hormone, thyrotropin-releasing hormone, and growth hormone-releasing hormone (GHRH). Both hGAL-(1-30) and -(3-30) potentiated the rise in GHRH-stimulated GH levels [area under the curve (AUC), saline, 2,810 +/- 500 vs. hGAL-(1-30), 4,660 +/- 737, P < 0.01; vs. hGAL-(3-30), 6, 870 +/- 1,550 ng. min. ml(-1), P < 0.01]. In contrast to hGAL-(1-30), hGAL-(3-30) had no effect on basal GH levels (AUC, saline, -110 +/- 88 vs. hGAL 1-30, 960 +/- 280, P < 0.002; vs. hGAL-(3-30), 110 +/- 54 ng. min. ml(-1), P = not significant). These data suggest that the effects of galanin on basal and stimulated GH release are mediated via different receptor subtypes and that the human equivalent of GAL-R(W) may exist.

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.

In vivo studies of the cerebral glutamate receptor/NO/cGMP pathway

Overwhelming evidence indicates that the glutamate/nitric oxide (NO) synthase/soluble guanylyl cyclase system is of primary importance in a variety of physiological and pathological processes of the brain. Most of our knowledge on this neurochemical pathway derives from in vitro and ex vivo studies but the recent improvement of microdialysis techniques combined with extremely sensitive measurements of the amplified end-product cyclic GMP (cGMP) has given new impulses to the investigation of this cascade of events, its modulation by neurotransmitters and its functional relevance, in a living brain. The first reports, appeared in the early 90's, have demonstrated that microdialysis monitoring of cGMP in the extracellular environment of the cerebellum and hippocampus exactly reflects what is expected to occur at the intracellular level; thus, in vivo extracellular cGMP is sensitive to NO-synthase and soluble guanylyl cyclase inhibitors, can be increased by NO-donors or phosphodiesterase blockers and is modulated by glutamate receptor stimulation in a NO-dependent fashion. Since then, other microdialysis studies have been reported showing that the brain NO synthase/guanylyl cyclase pathway is mainly controlled by NMDA, AMPA and metabotropic glutamate receptors but can be also influenced by other transmitters (GABA, acetylcholine, neuropeptides) through polysynaptic circuits interacting with the glutamatergic system. The available data indicate that this technique, applied to freely-moving animals and combined with behavioural tests, could be useful to get a better insight into the functional roles played by NO and cGMP in physiological and pathological situations such as learning, memory formation, epilepsy, cerebral ischemia and neurodegenerative diseases.

Galanin stimulates the N-methyl-D-aspartate receptor/nitric oxide/cyclic GMP pathway in vivo in the rat ventral hippocampus

We investigated whether the neuropeptide galanin affects the nitric oxide synthase/cyclic GMP pathway in rat hippocampus by measuring in vivo the extracellular cyclic GMP levels during microdialysis. Galanin (2.5 and 3.5 nmol; i.c.v.) dose-dependently raised the extracellular levels of cyclic GMP in the ventral but not the dorsal hippocampus. The effect of 3.5 nmol galanin was blocked by local application of tetrodotoxin and inhibited by the high-affinity galanin antagonist M40 (galanin-[1-12]-Pro3-[Ala-Leu]2-Ala amide). The non-competitive N-methyl-D-aspartate receptor antagonist dizocilpine maleate (30 microM infused into the ventral hippocampus or 0.2 mg/kg, i.p.) and the competitive one, 3-([R]-carboxypiperazin-4-yl)-propyl-phosphonic acid (50 microM infused), but not local perfusion of the AMPA antagonist 6-nitro-7-sulphamoylbenzo(f)quinoxaline-2,3-dione (15 microM) abolished the galanin-evoked cyclic GMP response in the hippocampus. Inhibitors of nitric oxide synthase, L-Arg(NO2)-OMe.HCl and 7-nitroindazole monosodium salt, applied locally, blocked the galanin-induced increase in hippocampal extracellular cyclic GMP. This increase was also prevented by local application of 1H-(1,2,4)oxadiazolo(4,3a) quinoxalin-1-one, a selective inhibitor of soluble guanylyl cyclase. The galanin receptors mediating the rise in cyclic GMP reside outside the hippocampus, as galanin (0.35-3 nmol) locally applied had no effect. The results provide in vivo evidence that galanin stimulates the N-methyl-D-aspartate receptor/nitric oxide synthase/cyclic GMP pathway in the ventral hippocampus, which may be of importance in memory processes.

Galaninergic signalling and adenylate cyclase

Galanin is a hyperpolarizing, inhibitory neurotransmitter; its recognition by seven transmembrane spanning G-protein-coupled receptors leads to a change in accumulation of cAMP (3'5'-cyclic AMP). Different subtypes of galanin receptor and G-proteins could be manifested in the mode of inhibitory action of galanin receptor on the production of cAMP by adenylate cyclase. Galanin analogues, acting at the adenylate cyclase level as subtype-specific galanin antagonists, can selectively block the inhibitory effect of endogenous galanin and thereby have potential as therapeutic agents for several endocrine, neuroendocrine and neuronal disorders. In this review, the latest results in the field of interplay between galanin-initiated signal transduction and the cAMP pathway are summarized.

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.

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.

CNS effects of peptides: a cross-listing of peptides and their central actions published in the journal Peptides, 1986-1993

The centrally mediated effects of peptides as published in the journal Peptides from 1986 to 1993 are tabulated in two ways. In one table, the peptides are listed alphabetically. In another table, the effects are arranged alphabetically. Most of the effects observed after administration of peptides are grouped, wherever possible, into categories such as cardiovascular and gastrointestinal. The species used in most cases has been rats; where other animals were used, the species is noted. The route of administration of peptides and source of information also are included in the tables, with a complete listing provided at the end. Many peptides have been shown to exert a large number of centrally mediated effects.

Functional interactions of galanin and acetylcholine: relevance to memory and Alzheimer's disease

Galanin, a 29-amino acid neuropeptide, is the only peptide known to coexist with acetylcholine in the basal forebrain neurons which degenerate early in the progression of Alzheimer's disease. Biochemical and neurophysiological studies demonstrated inhibitory actions of galanin on cholinergic functions. Behavioral investigations found that intracerebrally administered galanin produces deficits on spatial learning and memory tasks in rats. Taken together, the current literature suggests that galanin acts as an inhibitory modulator of acetylcholine in this coexistence. Particularly in the case of Alzheimer's disease, where cholinergic activity is severely compromised, the negative actions of galanin may be particularly deleterious. Recently developed galanin antagonists may provide a novel therapeutic approach toward enhancing memory processes in Alzheimer's disease, by removing the putative inhibitory actions of endogenous galanin on the remaining basal forebrain cholinergic neurons.