Electrophysiological evidence for a hyperpolarizing, galanin (1-15)-selective receptor on hippocampal CA3 pyramidal neurons

Cortical spreading depolarization is a potential target for rat brain excitability modulation by Galanin

The inhibitory neuropeptide Galanin (Gal) has been shown to mediate anticonvulsion and neuroprotection. Here we investigated whether Gal affects cortical spreading depolarization (CSD). CSD is considered the pathophysiological neuronal mechanism of migraine aura, and a neuronal mechanism aggravating brain damage upon afflictions of the brain. Immunohistochemistry localized Gal and the Gal receptors 1-3 (GalR1-3) in native rat cortex and evaluated microglial morphology after exposure to Gal. In anesthetized rats, Gal was applied alone and together with the GalR antagonists M40, M871, or SNAP 37889 locally to the exposed cortex. The spontaneous electrocorticogram and CSDs evoked by remote KCl pressure microinjection were measured. In rat cortex, Gal was present in all neurons of all cortical layers, but not in astrocytes, microglia and vessels. GalR2 and GalR3 were expressed throughout all neurons, whereas GalR1 was preponderantly located at neurons in layers IV and V, but only in about half of the neurons. In susceptible rats, topical application of Gal on cortex decreased CSD amplitude, slowed CSD propagation velocity, and increased the threshold for KCl to ignite CSD. In some rats, washout of previously applied Gal induced periods of epileptiform patterns in the electrocorticogram. Blockade of GalR2 by M871 robustly prevented all Gal effects on CSD, whereas blockade of GalR1 or GalR3 was less effective. Although microglia did not express GalRs, topical application of Gal changed microglial morphology indicating microglial activation. This effect of Gal on microglia was prevented by blocking neuronal GalR2. In conclusion, Gal has the potential to ameliorate CSD thus reducing pathophysiological neuronal events caused by or associated with CSD.

Beneficial effects of galanin system on diabetic peripheral neuropathic pain and its complications

Diabetic peripheral neuropathic pain (DPNP) is a distal spontaneous pain, caused by lesion of sensory neurons and accompanied by depression and anxiety frequently, which reduce life quality of patients and increase society expenditure. To date, antidepressants, serotonin-noradrenaline reuptake inhibitors and anticonvulsants are addressed as first-line therapy to DPNP, alone or jointly. It is urgently necessary to develop novel agents to treat DPNP and its complications. Evidences indicate that neuropeptide galanin can regulate multiple physiologic and pathophysiological processes. Pain, depression and anxiety may upregulate galanin expression. In return, galanin can modulate depression, anxiety, pain threshold and pain behaviors. This article provides a new insight into regulative effects of galanin and its subtype receptors on antidepressant, antianxiety and against DPNP. Through activating GALR1, galanin reinforces depression-like and anxiogenic-like behaviors, but exerts antinociceptive roles. While via activating GALR2, galanin is referred to as anti-depressive and anti-anxiotropic compounds, and at low and high concentration facilitates and inhibits nociceptor activity, respectively. The mechanism of the galanin roles is relative to increase in K⁺ currents and decrease in Ca²⁺ currents, as well as neurotrophic and neuroprotective roles. These data are helpful to develop novel drugs to treat DPNP and its complications.

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

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

Short N-terminal galanin fragments are occurring naturally in vivo

The galanin family currently consists of four peptides, namely galanin, galanin-message associated peptide, galanin-like peptide and alarin. Unlike galanin that signals through three different G protein-coupled receptors; GAL₁, GAL₂, and GAL₃, binding at its N-terminal end, the cognate receptors for other members of the galanin family are currently unknown. Research using short N-terminal galanin fragments generated either by enzymatic cleavage or solid-phase synthesis has revealed differences in their receptor binding properties exerting numerous biological effects distinct from galanin(1-29) itself. Our studies on tissue extracts derived from rat small intestine and bovine gut using chromatographic techniques and sensitive galanin(1-16)-specific radioimmunoassay revealed the presence of immunoreactive compounds reacting with antiserum against galanin(1-16) distributed in distinct elution volumes. These results suggested a possible presence of short N-terminal galanin fragments also in vivo. Moreover, employing immunoaffinity chromatography and reverse-phase high performance liquid chromatography (HPLC) followed by mass spectrometry allowed specific enrichment of these immunoreactive compounds from rat tissues and identification of their molecular structure. Indeed, our study revealed presence of several distinct short N-terminal galanin sequences in rat tissue. To prove their receptor binding, four of the identified sequences were synthetized, namely, galanin(1-13), galanin(1-16), galanin(1-20), galanin(6-20), and tested on coronal rat brain sections competing with ¹²⁵I-labeled galanin(1-29). Our autoradiographs confirmed that galanin(1-13), galanin(1-16), and galanin(1-20) comprehensively displaced ¹²⁵I-galanin(1-29) but galanin(6-20) did not. Here we show, for the first time, that short N-terminal galanin fragments occur naturally in rat tissues and that similar or identical galanin sequences can be present also in tissues of other species.

BIOLOGICAL SIGNIFICANCE

This study is first to provide an evidence of the presence of short N-terminal galanin fragments in vivo in a biological system and provides further foundations for the previous studies using synthetized short N-terminal galanin fragments.

Alterations in the neuropeptide galanin system in major depressive disorder involve levels of transcripts, methylation, and peptide

Major depressive disorder (MDD) is a substantial burden to patients, families, and society, but many patients cannot be treated adequately. Rodent experiments suggest that the neuropeptide galanin (GAL) and its three G protein-coupled receptors, GAL_1-3, are involved in mood regulation. To explore the translational potential of these results, we assessed the transcript levels (by quantitative PCR), DNA methylation status (by bisulfite pyrosequencing), and GAL peptide by RIA of the GAL system in postmortem brains from depressed persons who had committed suicide and controls. Transcripts for all four members were detected and showed marked regional variations, GAL and galanin receptor 1 (GALR1) being most abundant. Striking increases in GAL and GALR3 mRNA levels, especially in the noradrenergic locus coeruleus and the dorsal raphe nucleus, in parallel with decreased DNA methylation, were found in both male and female suicide subjects as compared with controls. In contrast, GAL and GALR3 transcript levels were decreased, GALR1 was increased, and DNA methylation was increased in the dorsolateral prefrontal cortex of male suicide subjects, however, there were no changes in the anterior cingulate cortex. Thus, GAL and its receptor GALR3 are differentially methylated and expressed in brains of MDD subjects in a region- and sex-specific manner. Such an epigenetic modification in GALR3, a hyperpolarizing receptor, might contribute to the dysregulation of noradrenergic and serotonergic neurons implicated in the pathogenesis of MDD. Thus, one may speculate that a GAL₃ antagonist could have antidepressant properties by disinhibiting the firing of these neurons, resulting in increased release of noradrenaline and serotonin in forebrain areas involved in mood regulation.

Depression-like behavior in rat: Involvement of galanin receptor subtype 1 in the ventral periaqueductal gray

The neuropeptide galanin coexists in rat brain with serotonin in the dorsal raphe nucleus and with noradrenaline in the locus coeruleus (LC), and it has been suggested to be involved in depression. We studied rats exposed to chronic mild stress (CMS), a rodent model of depression. As expected, these rats showed several endophenotypes relevant to depression-like behavior compared with controls. All these endophenotypes were normalized after administration of a selective serotonin reuptake inhibitor. The transcripts for galanin and two of its receptors, galanin receptor 1 (GALR1) and GALR2, were analyzed with quantitative real-time PCR using laser capture microdissection in the following brain regions: the hippocampal formation, LC, and ventral periaqueductal gray (vPAG). Only Galr1 mRNA levels were significantly increased, and only in the latter region. After knocking down Galr1 in the vPAG with an siRNA technique, all parameters of the depressive behavioral phenotype were similar to controls. Thus, the depression-like behavior in rats exposed to CMS is likely related to an elevated expression of Galr1 in the vPAG, suggesting that a GALR1 antagonist could have antidepressant effects.

Effects of 17β-estradiol on galanin(1-29)- and galanin(1-16)-like immunoreactivities

There are reasons to believe that the galanin neuropeptide family could include more than the two hitherto known members (galanin(1-29) and galanin-like peptide), such as the existence of at least three galanin receptors and the fact that synthetic short-chain homologues have effects and binding sites that are distinct from those of galanin(1-29). The current study uses a radioimmunoassay based on a polyclonal rabbit antiserum raised against galanin(1-16) to study the concentrations of galanin(1-16) like immunoreactivity (LI) in the various parts of the brain and gut of ovariectomized female rats, and investigates the effects of different concentrations of estradiol on these concentrations in relation to galanin(1-29)-LI. Galanin(1-29) concentrations were increased by 17β-estradiol administration in almost all examined tissues whereas galanin(1-16)-LI was increased by 17β-estradiol treatment in most of the gut, but only in the pituitary of the brain. Furthermore, the relation between galanin(1-29)-LI and galanin(1-16)-LI varied substantially from tissue to tissue. The main hypothesis, that galanin(1-16)-LI would be affected by 17β-estradiol in brain and/or gut, was confirmed in addition to the secondary hypothesis, stating that the pattern of galanin(1-16)-LI changes would differ from that of galanin(1-29). The study indicates that galanin(1-16)-LI is estrogen-responsive but that its concentrations are regulated differently from that of galanin(1-29). This is strongly indicative of a biological relevance of this potentially new member of the galanin neuropeptide family.

Galanin systems and ischemia: peptide and receptor plasticity in neurons and oligodendroglial precursors

Cerebral cortex contains few if any galanin neurons, but receives galanin-ergic inputs from subcortical areas. Apart from our earlier study on the response to cortical spreading depression, little is known about the presence and function of galanin in normal or injured cortex and to gain more insight into its possible roles, we investigated the temporal effects of focal ischemia on the expression of galanin and galanin receptors (GalRs). Focal ischemia induced in the rat by unilateral middle cerebral artery occlusion increased galanin and GalR1 mRNAs in penumbral/undamaged areas on the first and second day post-ischemia, while increased GalR2 mRNA was observed in the same regions only on the second day. Immunohistochemical studies revealed galanin immunoreactive neurons in the frontal/ cingulate cortex and abundant galanin-immunoreactivity in nerve axons/fibres within the penumbral areas, between the third and the seventh day after ischemia. Galanin mRNA and immunoreactivity was also increased in a population of small cells thought to be NG2-positive oligodendrocyte precursors. Up-regulation of galanin and GalRs in various cell populations following severe ischemic injury further demonstrates the marked plasticity of galanin and GalR1/2 expression after brain injury, and together with data reported elsewhere in this volume, suggests a functional role for galanin signalling in such pathophysiological conditions.

Selective stimulation of GalR1 and GalR2 in rat substantia gelatinosa reveals a cellular basis for the anti- and pro-nociceptive actions of galanin

Galanin modulates spinal nociceptive processing by interacting with two receptors, GalR1 and GalR2. The underlying neurophysiological mechanisms were examined by whole-cell recording from identified neurons in the substantia gelatinosa of young adult rats. GalR1 was activated with a 'cocktail' containing the GalR1/2 agonist, AR-M 961 (0.5 microM), in the presence of the GalR2 antagonist, M871 (1.0-2.5 microM). GalR2 was activated with the selective agonist, AR-M 1896 (0.5-1.0 microM). Application of the 'GalR1 agonist cocktail' often activated an inwardly-rectifying conductance in delay firing (excitatory) and tonically firing (inhibitory) neurons. This conductance was not activated by AR-M 1896 which instead decreased or increased an outwardly-rectifying conductance at voltages positive to -70 mV. Despite this variability in its actions on current-voltage relationships, AR-M 1896 very consistently decreased membrane excitability, as measured by cumulative action potential latency in response to a depolarizing current ramp. This strong GalR2-mediated effect was seen in neurons where membrane conductance was decreased, and where membrane excitability might be predicted to increase. GalR2 was also located presynaptically, as AR-M 1896 increased the interevent interval of spontaneous EPSCs in both delay and tonic cells. By contrast, the 'GalR1 agonist cocktail' had little effect on spontaneous EPSCs, suggesting that presynaptic terminals do not express GalR1. These diverse actions of GalR1 and GalR2 activation on both inhibitory and excitatory neurons are discussed in relation to the known spinal antinociceptive and pro-nociceptive actions of galanin, to the possible association of GalR1 with the inhibitory G-protein, G(i/o) and to report that GalR2 activation suppresses Ca2+ channel currents.

Neuropeptide gene therapy for epilepsy: viral vectors, stem cells and neurogenesis

Gene therapy for epilepsy is a relatively novel concept compared with previous approaches, which have relied on primary embryonic cells to deliver gene products of interest into localized brain regions. In vivo and ex vivo gene transfer offer promising, but yet insufficiently explored, possibilities to inhibit seizures, either by genetically modifying postmitotic neurons of the brain using viral vectors, or by transplanting genetically modified and in vitro tested cell lines, particularly stem cell lines, to produce and release gene products of interest. In this regard, neuropeptides are discussed as emerging candidates for such gene therapy approaches. Selective modification of newly generated neurons in the dentate gyrus by retroviral vector-based gene delivery opens novel possibilities in gene therapy for epilepsy. However, the limited number of new neurons targeted remains a main obstacle. Despite its early stage, gene therapy for epilepsy might not be a remote prospect for clinical trials, particularly in patients with intractable temporal lobe epilepsy. Ex vivo gene transfer using encapsulated genetically modified cells could be of particular value for such initial trials.

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

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

The galanin-R2 agonist AR-M1896 reduces glutamate toxicity in primary neural hippocampal cells

Galanin is a neuropeptide involved in a variety of biological functions, including having a strong anticonvulsant activity. To assess a possible role of galanin in modulation of glutamatergic synapses and excitotoxicity, we studied effects of a galanin receptor 2(3) agonist (AR-M1896) on several molecular events induced by glutamate administration in primary neural hippocampal cells. Exposure of cells, after 5 days in vitro, to glutamate 0.5 mM for 10 min caused morphological alterations, including disaggregation of beta-tubulin and MAP-2 cytoskeletal protein assembly, loss of neurites and cell shrinkage. When present in culture medium together with glutamate, 1 and 10 nM of AR-M1896 reduced these alterations. Moreover, AR-M1896 counteracted glutamate-induced c-fos mRNA and c-Fos protein up-regulation after 30-150 min, and 24 h, respectively. Massive nuclear alterations (Hoechst 33258 staining), observed 24 h after glutamate exposure, were also antagonized by AR-M1896 (0.1-100 nM) in a dose-dependent manner. These findings indicate that galanin, probably mainly through its type 2 receptor, interferes with events associated with glutamate toxicity.

Role of galanin and galanin(1-15) on central cardiovascular control

Galanin and the N-terminal fragment Galanin(1-15) are involved in central cardiovascular regulation. The present paper reviews the recent cardiovascular results obtained by intracisternal injections of Galanin and Galanin(1-15) showing that: (A) the Galanin antagonist M40 blocks the central cardiovascular responses induced by Galanin(1-15) but not those elicited by Galanin; (B) both Galanin and Galanin(1-15) induce the expression of c-Fos in cardiovascular nuclei of the medulla oblongata with different temporal and spatial profiles; (C) the cardiovascular action of Galanin(1-15), but not Galanin, is mediated by peripheral beta-receptor stimulation; (D) and it is demonstrated an antagonistic Galanin/alpha2-adrenoceptors interaction as well as a differential modulation of central cardiovascular responses of Angiotensin II by Galanin or Galanin(1-15). All these data strengthen the involvement of both Galanin molecules as neuromodulators on central cardiovascular regulation.

Age-related impairments of synaptic plasticity in the lateral perforant path input to the dentate gyrus of galanin overexpressing mice

In the present study, electrophysiological recordings were made from hippocampal slices obtained from mice overexpressing galanin under the promoter for the platelet-derived growth factor-B (GalOE mice). In these mice, a particularly strong galanin expression is seen in the granule cell layer/mossy fibers. Paired-pulse facilitation (PPF) of excitatory postsynaptic field potentials (fEPSPs) at the lateral perforant path (LPP)-dentate gyrus synapses was elicited in the dentate gyrus after stimulation with different interpulse intervals. Slices from young adult wild-type (WT) animals showed significant PPF of the 2nd EPSP evoked with paired-pulse stimuli, while PPF was reduced in slices from young adult GalOE mice, as well as aged WT mice, but were not observed at all in slices from aged GalOE animals. Application of the putative galanin antagonist M35 increased PPF in slices from aged WT mice as well as from adult and aged GalOE mice, but had no effect in slices taken from young adult WT mice. These data indicate that galanin is involved in hippocampal synaptic plasticity, in particular in age-related reduction of synaptic plasticity in the LPP input to the dentate gyrus. Galaninergic mechanisms may therefore represent therapeutic targets for treatment of age-related memory deficits and Alzheimer's disease.

Electrophysiological studies on galanin effects in brain--progress during the last six years

The effects of galanin and galanin fragments have been studied on neurons in various brain regions of rodents using electrophysiological techniques. Here, we mainly review reports published during the last six years, that is after the second galanin symposium in 1998. These papers deal with locus coeruleus (LC), the hippocampal formation (HF), hypothalamus, the nucleus of the diagonal band of Broca (DBB) and the dorsal vagal complex (DVC). In most cases galanin has an inhibitory effect by increasing a potassium conductance or reducing a calcium conductance. In LC, beside a direct inhibitory effect, galanin exerts an indirect effect enhancing the noradrenaline-induced hyperpolarization. In the HF, galanin (1-15), but not galanin (1-29), induces hyperpolarization in CA3 pyramidal neurons. Inhibitory effects of galanin on several forms of synaptic plasticity including long-term potentiation, frequency facilitation and paired-pulse facilitation have also been demonstrated in normal and transgenic animals. In the hypothalamic arcuate nucleus galanin has a presynaptic action inhibiting glutamate release, as well as a postsynaptic effect via the galanin R1 receptor. In the DVC, galanin inhibits dorsal vagal motor neurons projecting to the stomach by activation of a postsynaptic galanin receptor. However, excitatory effects of galanin have also been reported in several regions, such as the DBB nucleus, where galanin increases excitability by decreasing a K+ conductance. Taken together, electrophysiological studies have further supported the role of galanin as a neurotransmitter/neuromodulator in the brain.

Galanin and galanin receptors in epilepsy

The shift in the balance between the inhibition and the excitation in favor of the latter is a major mechanism of the evolvement of epileptic seizures. On the neurotransmitter level two major players contribute to such misbalance: an inhibitory transmitter gamma-aminobutyric acid, and an excitatory amino acid glutamate. Neuropeptides are powerful modulators of classical neurotransmitters, and thus represent an intriguing tool for restoring the balance between the inhibition and the excitation, through either blocking or activating peptide receptors depending on whether a peptide is pro- or anticonvulsant. Galanin, a 29-amino acid residues neuropeptide which inhibits glutamate release in the hippocampus, is a likely member of the anticonvulsant peptide family. During the past decade growing evidence has been suggesting that galanin is in fact a powerful inhibitor of seizure activity. This review summarizes the state of research of galanin in epilepsy, beginning with the first simple experiments which showed that central injection of galanin agonists inhibited seizures, and that seizures themselves affected galanin signaling in the hippocampus; exploring the impact of active manipulation with the expression of galanin and galanin receptors on seizures, using transgenic animals, antisense and peptide-expressing vector approaches; and concluding with the recent advances in pharmacology, which led to the synthesis of non-peptide galanin receptor agonists with anticonvulsant properties. We also address recently established functions of galanin in seizure-associated neuronal degeneration and neuronal plasticity.

Propranolol blocks the tachycardia induced by galanin (1-15) but not by galanin (1-29)

The efferent pathways involved in the tachycardia induced by intracisternal injections of the N-terminal galanin fragment (1-15) (GAL (1-15)) and galanin (GAL (1-29)) has been evaluated in rats pretreated with the cholinergic antagonist atropine or the beta-antagonist propranolol. The pretreatment with propranolol significantly blocked the tachycardic and vasopressor effect produced by intracisternal injection of GAL (1-15) (p<0.05), but the pretreatment with atropine did not modify these cardiovascular effects. However, the cardiovascular response elicited by GAL (1-29) is modified by the pretreatment with atropine (p<0.05) but not by propranolol. These findings demonstrate that the central cardiovascular action of GAL (1-15), but not GAL (1-29), is mediated by beta-receptor stimulation and this suggests the existence of a different pathway involved in the cardiovascular response produced by the N-terminal galanin fragment as compared with the parent molecule GAL (1-29).

Anticonvulsant activity of a nonpeptide galanin receptor agonist

Galanin is a neuropeptide with a wide variety of biological functions, including that of a strong endogenous anticonvulsant. No nonpeptide ligands, capable of activating galanin receptors, are available today. Based on known pharmacophores of galanin, a combinatorial library was designed, synthesized, and screened at the rat hippocampal galanin receptor. A low molecular weight galanin receptor agonist, 7-((9-fluorenylmethoxycarbonyl)cyclohexylalanyllysyl)amino-4-methylcoumarin (galnon) was found to displace (125)I-galanin with micromolar affinity at Bowes cellular and rat hippocampal membranes. Autoradiographic binding assay on rat spinal cord sections confirmed the ability of galnon to displace (125)I-galanin from its binding sites. Galnon inhibited adenylate cyclase activity, suggesting an agonist action at galanin receptors. When injected i.p. galnon reduced the severity and increased the latency of pentylenetetrazole-induced seizures in mice and reversed the proconvulsant effects of the galanin receptor antagonist M35, injected into a lateral ventricle. Intrahippocampal injection of galnon also shortened the duration of self-sustaining status epilepticus in rats, confirming its agonist properties in vivo. Pretreatment of rats with antisense peptide nucleic acid targeted to galanin receptor type 1 mRNA abolished the effect of galnon, suggesting mediation of its anticonvulsant properties through this receptor subtype. These findings introduce a systemically active nonpeptide galanin agonist anticonvulsant.

Novel excitatory actions of galanin on rat cholinergic basal forebrain neurons: implications for its role in Alzheimer's disease

Galanin, a 29-amino-acid neuropeptide, is generally viewed as an inhibitory neuromodulator in a variety of central systems. Galanin expression is upregulated in the cholinergic basal forebrain nuclei in Alzheimer's disease (AD) and is postulated to play an important role in memory and cognitive function. In this study, application of galanin to acutely dissociated rat neurons from the basal forebrain nucleus diagonal band of Broca (DBB), caused a decrease in whole cell voltage-activated currents in a majority of cells. Galanin reduces a suite of potassium currents, including calcium-activated potassium (I(C)), the delayed rectifier (I(K)), and transient outward potassium (I(A)) conductances, but not calcium or sodium currents. Under current-clamp conditions, application of galanin evoked an increase in excitability and a loss of accommodation in cholinergic DBB neurons. Using single-cell RT-PCR technique, we determined that galanin actions were specific to cholinergic, but not GABAergic DBB neurons The notion that galanin plays a deleterious role in AD is based, in part, on galanin hyperinnervation of cholinergic cells in the basal forebrain of AD patients, its ability to depress acetylcholine release and its inhibitory actions at other CNS sites. However, our results suggest that by virtue of its excitatory actions on cholinergic neurons, galanin may in fact play a compensatory role by augmenting the release of acetylcholine from remaining cholinergic basal forebrain neurons. This action might serve to delay the progression of AD pathology linked to a reduction in central cholinergic tone.

Prolonged effects of intraventricular galanin on a 5-hydroxytryptamine(1A) receptor mediated function in the rat

Galanin (3 nmol/rat), 2 h after its intracerebroventricular (i.c.v.) administration to male rats, attenuated the passive avoidance (PA) retention deficit induced by the 5-hydroxytryptamine (HT)(1A) receptor agonist 8-hydroxy-2-(di-N-propylamino)tetraline (8-OH-DPAT) (0.2 mg/kg) The reduction in the postjunctional 5-HT(1A) receptor-mediated response after i.c.v. galanin was not associated with changes in the mRNA levels and agonist binding properties of cortical limbic 5-HT(1A) receptors, believed to be the target receptors mediating the PA deficit caused by 8-OH-DPAT. These results suggest that acute increases of galanin transmission in vivo even after 2 h can counteract limbic 5-HT(1A) receptor-mediated responses of relevance for affective disorders without significantly affecting gene expression and binding characteristics of cortical limbic 5-HT(1A) receptors.

Distribution, regulation and role of hypothalamic galanin systems: renewed interest in a pleiotropic peptide family

1. Galanin peptide and galanin receptor-binding sites are known to be widely distributed within the central nervous system, particularly in the hypothalamus in the preoptic area, the paraventricular (PVN) and supraoptic (SON) nuclei and the arcuate nucleus/median eminence. 2. The present brief review focuses on some recent studies of the regional and cellular localization of mRNA encoding galanin and two galanin receptor subtypes (GalR1 and GalR2) in the hypothalamus, regulation of galanin and/or galanin receptor expression in various nuclei by physiological stimuli, electrophysiological effects of galanin on hypothalamic neurons and the isolation and cloning of galanin-like peptide (GALP), a putative endogenous ligand for GalR2. 3. In situ hybridization studies in rat brain have demonstrated an abundance of GalR1 mRNA in SON, magnocellular (m) and parvocellular (p) PVN and dorsomedial, ventromedial and arcuate nuclei. In contrast, GalR2 mRNA is enriched in pPVN, but not mPVN, and is not detected in SON. In addition, GalR2 mRNA is present in the dorsomedial nucleus and is enriched in the arcuate nucleus compared with GalR1 transcripts, with numerous labelled cells in all subdivisions. 4. Neurons of the SON and PVN contain vasopressin and/or oxytocin, along with several other peptides, and the production and release of these hormones and peptides are modulated by various physiological stimuli. In relation to galanin systems, GalR1 and galanin expression is increased in magnocellular neurons by salt loading and is downregulated by lactation, consistent with an increased inhibition by galanin of vasopressin release following osmotic stimulation and a decreased inhibition of oxytocin release during lactation. 5. Powerful inhibitory effects of galanin on the electrical (and secretory) activity of magnocellular neurons and complex presynaptic actions of galanin on the synaptic release of glutamate in the arcuate nucleus in vitro suggest an active role for multiple galanin receptor subtypes in the regulation of these hypothalamic systems in vivo. 6. The recent isolation of a peptide from porcine hypothalamus (GALP-1-60) that is structurally related to galanin and appears to be selective for GalR2 over GalR1 and the subsequent cloning of GALP cDNA from pig, rat and humans should allow studies to help reveal the physiological role played by galanin receptor subtypes (especially GalR2) and their multiple ligands in the hypothalamus and other brain areas.

Galanin-like peptide mRNA in neural lobe of rat pituitary. Increased expression after osmotic stimulation suggests a role for galanin-like peptide in neuron-glial interactions and/or neurosecretion

Galanin-like peptide (GALP) was recently identified in the porcine hypothalamus, pituitary gland and gut, and has reported selectivity for the GalR2, c.f. the GalR1 receptor. GALP cDNAs have been cloned from pig, rat and human, and GALP mRNA expression is restricted to the arcuate nucleus in normal rat brain. This study examined the regional and cellular distribution of GALP mRNA in the rat pituitary gland, and subsequently determined the effect of osmotic stimulation on GALP transcript levels. GALP mRNA was not detected in the anterior or intermediate lobes, but moderate levels of GALP mRNA were present in the neural (posterior) lobe, in presumed pituicytes, of normal male and female rats. Osmotic stimulation by dehydration or salt loading produced a time-dependent increase in GALP mRNA levels in the neural lobe. Thus, dehydration for 4 days increased GALP mRNA 40-fold, while salt loading for 4, 7 or 10 days increased GALP levels 14-, 21- and 25-fold, respectively (p < or = 0.001). Levels of vasopressin (VP) mRNA in the neural lobe were also increased by these treatments, consistent with previous reports. Galanin (GAL) and GalR2 receptor mRNAs were not detected in the neural lobe, under normal or osmotic stimulation conditions. In addition, GALP mRNA levels in the arcuate nucleus were not altered in dehydrated or salt-loaded rats; and GALP mRNA was not detected in magnocellular neurons of the supraoptic or paraventricular nucleus, despite the characteristic up-regulation of VP and GAL mRNA in these cells. In view of the established anatomy and function of VP/oxytocin neurons in the hypothalamo-neurohypophysial system and the role played by pituicytes in their regulation, the likely synthesis/release of GALP by these specialized astrocytes strongly suggests a role for this novel peptide in regulation of pituicyte morphology/function and/or neurohormone release.

Galanin receptor subtypes

The neuropeptide galanin, which is widely expressed in brain and peripheral tissues, exerts a broad range of physiological effects. Pharmacological studies using peptide analogues have led to speculation about multiple galanin receptor subtypes. Since 1994, a total of three G-protein-coupled receptor (GPCR) subtypes for galanin have been cloned (GAL1, gal2 and gal3). Potent, selective antagonists are yet to be found for any of the cloned receptors. Major challenges in this field include linking the receptor clones with each of the known physiological actions of galanin and evaluating the evidence for additional galanin receptor subtypes.