Galanin-like immunoreactivity in hippocampal afferents in the rat, with special reference to cholinergic and noradrenergic inputs

The effects of galanin on long-term synaptic plasticity in the CA1 area of rodent hippocampus

Learning and memory involve complex changes in neuronal excitability including long-lasting synaptic plasticity of glutamatergic synapses. The observation that the neuropeptide galanin affects performance in a number of behavioural models predicts that galanin should affect synaptic processes underlying learning. The present study in rat and mouse hippocampal slices now demonstrates that galanin inhibits long-term potentiation induced by both tetanic and theta-burst stimulation in both apical and basal dendrites of CA1 pyramidal neurones but does not affect long-term depression. This selective effect on long-term potentiation does not appear to be mediated through inhibition of N-methyl-D-aspartate or metabotropic glutamate receptor function, but likely resides downstream of receptor activation, possibly at the level of the kinase cascade that converts short-term into long-term potentiation. Our results indicate possible mechanisms by which the neuropeptide galanin may act at the molecular level to influence learning and memory in vertebrates.

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.

Galanin reduces PDBu-induced protein phosphorylation in rat ventral hippocampus

The effect of galanin (GAL) on basal and phorbol-12,13-dibutyrate (PDBu) induced protein phosphorylation in rat ventral hippocampal miniprisms was investigated. GAL (0.5, 1 and 2 microM) inhibited PDBu stimulation in a concentration-dependent manner without altering basal protein phosphorylation. This inhibitory effect was prevented by the GAL antagonist galantide. GAL did not affect either the activity of protein kinase C (PKC) from rat brain or basal phosphorylation in ventral hippocampal hippogenates, suggesting that it did not directly modulate PKC activity. Depolarization of miniprisms from ventral hippocampi by 18 mM K+ prevented the effect of GAL on PDBu-induced phosphorylation. The results indicate that GAL indirectly regulates neuronal protein phosphorylation by a GAL receptor-mediated action.

Galanin: an endogenous anticonvulsant?

Galanin is a neuroendocrine peptide involved in the regulation of feeding, pain, sexual behavior, learning, and memory. The recent discovery, that galanin antagonized excitatory glutamatergic neurotransmission in the hippocampus, provided a rationale for its possible antiepileptic effects. Here we summarize the data on the effects of galanin on seizure activity in several animal models of epilepsy. Pharmacological and molecular biological evidence suggest potent anticonvulsant effects of galanin. Exogenous administration of galanin receptor agonists attenuated seizures, whereas application of galanin receptor antagonists potentiated seizure expression. Genetically engineered mice, with either deletion or overexpression of galanin gene, showed altered resistance to seizures, which was in direct correlation with galanin gene expression. Possible mechanisms of the anticonvulsant action of galanin include its effects on synaptic potentiation in hippocampal circuits and inhibition of the release of the excitatory neurotransmitter glutamate from principal hippocampal neurons.

Galanin: neurobiologic mechanisms and therapeutic potential for Alzheimer's disease

The neuropeptide galanin (GAL) is widely distributed in the mammalian CNS. Several lines of evidence suggest that GAL may play a critical role in cognitive processes such as memory and attention through an inhibitory modulation of cholinergic basal forebrain activity. Furthermore, GAL fibers hyperinnervate remaining cholinergic basal forebrain neurons in Alzheimer's disease (AD). This suggests that GAL activity impacts cholinergic dysfunction in advanced AD. Pharmacological and in vitro autoradiographic studies indicate the presence of heterogeneous populations of GAL receptor (GALR) sites in the basal forebrain which bind GAL with both high and low affinity. Interestingly, we have recently observed that GALR binding sites increase in the anterior basal forebrain in late-stage AD. Three G protein-coupled GALRs have been identified to date that signal through a diverse array of effector pathways in vitro, including adenylyl cyclase inhibition and phospholipase C activation. The repertoire and distribution of GALR expression in the basal forebrain remains unknown, as does the nature of GAL and GALR plasticity in the AD basal forebrain. Recently, GAL knockout and overexpressing transgenic mice have been generated to facilitate our understanding of GAL activity in basal forebrain function. GAL knockout mice result in fewer cholinergic basal forebrain neurons and memory deficits. On the other hand, mice overexpressing GAL display hyperinnervation of basal forebrain and memory deficits. These data highlight the need to explore further the putative mechanisms by which GAL signaling might be beneficial or deleterious for cholinergic cell survival and activity within basal forebrain. This information will be critical to understanding whether pharmacological manipulation of GALRs would be effective for the amelioration of cognitive deficits in AD.

Suppressed kindling epileptogenesis in mice with ectopic overexpression of galanin

The neuropeptide galanin has been shown to suppress epileptic seizures. In cortical and hippocampal areas, galanin is normally mainly expressed in noradrenergic afferents. We have generated a mouse overexpressing galanin in neurons under the platelet-derived growth factor B promoter. RIA and HPLC analysis revealed up to 8-fold higher levels of galanin in transgenic as compared with wild-type mice. Ectopic galanin overexpression was detected especially in dentate granule cells and hippocampal and cortical pyramidal neurons. Galanin-overexpressing mice showed retardation of seizure generalization during hippocampal kindling, a model for human complex partial epilepsy. The high levels of galanin in mossy fibers found in the transgenic mice were further increased after seizures. Frequency facilitation of field excitatory postsynaptic potentials, a form of short-term synaptic plasticity assessed in hippocampal slices, was reduced in mossy fiber-CA3 cell synapses of galanin-overexpressing mice, indicating suppressed glutamate release. This effect was reversed by application of the putative galanin receptor antagonist M35. These data provide evidence that ectopically overexpressed galanin can be released and dampen the development of epilepsy by means of receptor-mediated action, at least partly by reducing glutamate release from mossy fibers.

Effects of galanin receptor agonists on locus coeruleus neurons

Galanin exerts an inhibitory effect on locus coeruleus (LC) neurons via a postsynaptic, as yet unidentified galanin receptor. Using an in vitro intracellular recording technique the effect of two galanin receptor agonists on LC neurons was investigated. Bath application of [Sar(1), D-Ala(12)]gal(1-16)-NH(2) (AR-M961), an agonist both at galanin R1 and R2 (GALR1, GALR2) receptors, evoked a reversible membrane hyperpolarization and inhibition of spike discharge in all LC neurons tested (n=42). The action of AR-M961 was blocked by tetraethylammonium chloride. Hyperpolarizing responses induced by AR-M961 were retained in the presence of tetrodotoxin and high Mg(2+)/low Ca(2+) media. The selective GALR2 agonist Gal(2-11)-NH(2) (AR-M1896) only caused inhibition of spike discharge and a slight hyperpolarization in 26 of 34 LC neurons tested, and was on a molar basis much weaker than AR-M961. These results suggest that it mainly is the GALR1 receptor that mediates hyperpolarization of LC neurons.

Galanin enhances noradrenaline-induced outward current on locus coeruleus noradrenergic neurons

Interactions between galanin and noradrenaline (NA) were studied in a locus coeruleus (LC) slice preparation using intracellular recording technique. Both galanin (10-100 nM) and NA induced an outward current. Idazoxan, an alpha2-adrenoceptor antagonist, blocked the NA- but not the galanin-induced outward current, indicating that different receptors mediate these responses. Moreover, the outward current induced by NA was enhanced and prolonged by preincubation with a low concentration of galanin (0.05-0.1 nM), which had no detectable effect on the membrane current by itself. This sensitizing effect may be of physiological importance and could be caused by galanin released from dendrites and soma of galanin/NA neurons and/or from galanin afferents. Thus, besides a direct action of galanin on locus coeruleus neurons, our results also indicate an indirect, modulating effect.

The role of galanin as a multi-functional neuropeptide in the nervous system

The neuropeptide galanin is expressed developmentally in the DRG and is rapidly up-regulated 120-fold after peripheral nerve section in the adult. The generation and study of galanin knockout mice has indicated that the peptide is critical to the development and function of specific subsets of neurons in the central and peripheral nervous system. These data have important implications for the understanding, and potential therapeutic treatment, of sensory neuropathies and a number of neurological diseases, including Alzheimer's disease and epilepsy.

Galanin regulates the postnatal survival of a subset of basal forebrain cholinergic neurons

The neuropeptide galanin colocalizes with choline acetyltransferase, the synthetic enzyme for acetylcholine, in a subset of cholinergic neurons in the basal forebrain of rodents. Chronic intracerebroventricular infusion of nerve growth factor induces a 3- to 4-fold increase in galanin gene expression in these neurons. Here we report the loss of a third of cholinergic neurons in the medial septum and vertical limb diagonal band of the basal forebrain of adult mice carrying a targeted loss-of-function mutation in the galanin gene. These deficits are associated with a 2-fold increase in the number of apoptotic cells in the forebrain at postnatal day seven. This loss is associated with marked age-dependent deficits in stimulated acetylcholine release, performance in the Morris water maze, and induction of long-term potentiation in the CA1 region of the hippocampus. These data provide unexpected evidence that galanin plays a trophic role to regulate the development and function of a subset of septohippocampal cholinergic neurons.

Modulation of hippocampal excitability and seizures by galanin

Previous studies have shown that the expression of the neuropeptide galanin in the hippocampus is altered by seizures and that exogenous administration of galanin into the hippocampus attenuates seizure severity. To address the role of endogenous galanin in modulation of hippocampal excitability and its possible role in seizure mechanisms, we studied two types of transgenic mice: mice with a targeted disruption of the galanin gene (GalKO) and mice that overexpress the galanin gene under a dopamine-beta-hydroxylase promoter (GalOE). GalKO mice showed increased propensity to develop status epilepticus after perforant path stimulation or systemic kainic acid, as well as greater severity of pentylenetetrazol-induced convulsions. By contrast, GalOE mice had increased resistance to seizure induction in all three models. Physiological tests of hippocampal excitability revealed enhanced perforant path-dentate gyrus long-term potentiation (LTP) in GalKO and reduced LTP in GalOE. GalKO showed increased duration of afterdischarge (AD) evoked from the dentate gyrus by perforant path simulation, whereas GalOE had increased threshold for AD induction. Depolarization-induced glutamate release from hippocampal slices was greater in GalKO and lower in GalOE, suggesting that alterations of physiological and seizure responses in galanin transgenic animals may be mediated through modulation of glutamate release. Our data provide further evidence that hippocampal galanin acts as an endogenous anticonvulsant and suggest that genetically induced changes in galanin expression modulate both hippocampal excitability and predisposition to epileptic seizures.

Internalization of intracerebrally administered porcine galanin (1-29) by a discrete nerve cell population in the hippocampus of the rat

In spite of numerous studies utilizing intraventricular administration of porcine galanin (1-29), little is known about the spread and cellular distribution of exogenous galanin following intraventricular administration. In this study a discrete nerve cell body population with their dendrites became strongly galanin immunoreactive (IR) in the dorsal hippocampus following intraventricular porcine galanin (1.5 nmol/rat). Time course experiments showed that after time intervals of 10 and 20 min, but not at 60 min, scattered small- to medium-sized galanin-IR nerve cell bodies and their dendrites were present in all layers of the dorsal and ventral hippocampus. In double-immunolabeling experiments most of these nerve cells were identified as putative GABA interneurons costoring NPY-IR or somatostatin-IR in some cases. Twenty minutes after intraventricular injection of artificial cerebrospinal fluid (aCSF), only endogenous punctate and coarse galanin-IR terminals were found, but no galanin-IR cell bodies. Intrahippocampal injection of fluorophore-labeled galanin resulted in the appearance of fluorescent nerve cell bodies with the same morphology and localization as in the above experiments. Coadministration of the putative galanin antagonist M35 (0.5 nmol) and galanin (1.5 nmol) resulted in a reduced number of galanin-IR nerve cell bodies in the hippocampus of half of the rats. These findings support the existence of a population of putative hippocampal GABA interneurons with the ability to internalize and concentrate galanin and/or its fragments present in the extracellular fluid, possibly mediated by galanin receptors.

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

The effects of the 29-amino acid neuropeptide galanin [GAL (1-29)], GAL(1-15), GAL(1-16), and the GAL subtype 2 receptor agonist D-tryptophan(2)-GAL(1-29) were studied in the dorsal hippocampus in vitro with intracellular recording techniques. GAL(1-15) induced, in the presence of tetrodotoxin, a dose-dependent hyperpolarization in hippocampal CA3 neurons. Most of the GAL(1-15)-sensitive neurons did not respond to GAL(1-29), GAL(1-16), or D-tryptophan(2)-GAL(1-29). These results indicate the presence of a distinct, yet-to-be cloned GAL(1-15)-selective receptor on CA3 neurons in the dorsal hippocampus.

Galanin and learning

A number of studies indicate that galanin (GAL) is a potent modulator of basal acetylcholine release in the rat forebrain e.g. in the cholinergic neurons of the septo-hippocampal projections. Thus, GAL perfused through the microdialysis probe decreased basal acetylcholine release in the ventral hippocampus, while it enhanced acetylcholine release in the dorsal hippocampus. This finding indicates that GAL may act via different mechanisms within the subsystems of the hippocampus. This hypothesis has received support from studies using the Morris swim maze, a learning task dependent on hippocampal mechanisms. GAL (3 nmol/rat) infused into the ventral hippocampus impaired spatial learning acquisition, while it tended to facilitate when injected into the dorsal hippocampus. However, the effects of GAL on acetylcholine release and on spatial learning, which are due to activation of GAL-receptors, appear to be indirectly mediated possibly via noradrenaline transmission. GAL is also a potent inhibitor of mesencephalic 5-HT neurotransmission in vivo. These findings are discussed in relation to the role of acetylcholine and serotonin in cognition.

Quantitative evidence for increase in galanin-immunoreactive terminals in the hippocampal formation following entorhinal cortex lesions in the adult rat

The projection from the entorhinal cortex to the dentate gyrus and hippocampus is severely affected in Alzheimer's disease and there is a depletion of cholinergic terminals but an upregulation of the neuropeptide galanin, which inhibits the release of acetylcholine. Evidence for changes to galanin-immunoreactive terminals in the hippocampal formation was therefore examined after unilateral entorhinal cortex lesions in the adult rat. An increase in the density of galanin-immunoreactive terminals on the lesioned side was evident in the stratum lacunosum moleculare of the hippocampus and the outer molecular layer of the dentate gyrus at 17 days post-lesion, and it increased gradually until the last time point examined, at 40 days post-lesion. Thus we demonstrate that there is an increase in galanin-immunoreactive terminals in the hippocampal formation following entorhinal cortex lesions.

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.

Regulation of galanin in memory pathways

Based on early immunocytochemical findings, galanin (GAL) was postulated to function as an inhibitory cotransmitter in rat cholinergic memory pathways. However, recent studies indicate that in the basal state GAL is not widely expressed by forebrain cholinergic neurons in rats. Inhibition of cholinergic transmission by cosecreted GAL may be enhanced under certain conditions, because GAL gene expression in the cholinergic basal forebrain is significantly increased prior to puberty and following nerve growth factor treatment. Other sources of GAL in rat septohippocampus that could interact with cholinergic pathways include noradrenergic neurons in the locus ceruleus and vasopressinergic neurons in the bed nucleus of the stria terminalis (BST) and medial amygdala (Me). GAL is extensively colocalized within these steroid-sensitive cell groups where its expression is upregulated by gonadal hormones. GAL, acting via the GALR1 receptor subtype, does not appear to directly regulate the activity of cholinergic neurons, but it may regulate the release of vasopressin and GAL into septohippocampus from BST/Me neurons.

Galanin expression within the basal forebrain in Alzheimer's disease. Comments on therapeutic potential

The inhibitory neuropeptide galanin has widespread distribution throughout the central nervous system. Studies indicate that galanin modulates cognition by regulating cholinergic basal forebrain (CBF) neuron function. The chemoanatomic organization of galanin within the mammalian CBF differs across species. In monkeys, all CBF neurons coexpress galanin, whereas in apes and humans galanin is found within a separate population of interneurons that are in close apposition to the CBF perikarya. Pharmacologic investigations revealed a low and high affinity galanin receptor within the basal forebrain in humans. In vitro autoradiographic investigations of the primate brain indicate that galanin receptors are concentrated within the anterior subfields of the CBF as well as bed nucleus of the stria terminalis, amygdala, and entorhinal cortex. Galaninergic fibers hyperinnervate remaining CBF neurons in Alzheimer's disease. Because galanin inhibits the release of acetylcholine in the hippocampus, it has been suggested that the overexpression of galanin in Alzheimer's disease may downregulate the production of acetylcholine within CBF perikarya, further exacerbating cholinergic cellular dysfunction in this disorder. These observations suggest that the development of a potent galanin antagonist would be a useful step towards the successful pharmacologic treatment of Alzheimer's disease.

Galanin modulation of seizures and seizure modulation of hippocampal galanin in animal models of status epilepticus

We examined the role of hippocampal galanin in an animal model of status epilepticus (SE). Control rats showed abundant galanin-immunoreactive (Gal-IR) fibers in the dentate hilus, whereas no Gal-IR neurons were observed. Three hours after the onset of self-sustaining SE (SSSE), induced either by intermittent stimulation of the perforant path for 30 min (PPS) or by injection of lithium and pilocarpine, Gal-IR fibers disappeared in the hilus and remained absent for up to 1 week afterward. Twelve hours after the induction of SE by PPS or 3 hr after pilocarpine administration, Gal-IR neurons appeared in the hilus; these neurons increased in number after 1 d and gradually declined 3 and 7 d later. Galanin concentration in the hippocampus, measured by ELISA, significantly decreased on the plateau of SSSE and increased 24 hr after PPS. Galanin (0.05 nmol) injected into the hilus prevented the induction of SSSE, and 0.5 nmol of galanin stopped established SSSE. These effects were attenuated by galanin receptor antagonists (M35 > M40 >/= M15). 2-Ala-galanin (5 nmol), a putative agonist of galanin type 2 receptors, prevented but was unable to stop SSSE. M35 facilitated the development of SSSE when given before PPS. We suggest that hippocampal galanin acts as an endogenous anticonvulsant via galanin receptors. SE-induced galanin depletion in the hippocampus may contribute to the maintenance of seizure activity, whereas the increase of galanin concentration and the appearance of galanin-immunoreactive neurons may favor the cessation of SSSE. The seizure-protecting action of galanin SSSE opens new perspectives in the treatment of SE.

Intra-septal injections of glucose and glibenclamide attenuate galanin-induced spontaneous alternation performance deficits in the rat

Injection of the neuroactive peptide galanin into the rat hippocampus and medial septal area impairs spatial memory and cholinergic system activity. Conversely, injection of glucose into these same brain regions enhances spatial memory and cholinergic system activity. Glucose and galanin may both modulate neuronal activity via opposing actions at ATP-sensitive K+ (K-ATP) channels. The experiments described in this report tested the ability of glucose and the direct K-ATP channel blocker glibenclamide to attenuate galanin-induced impairments in spontaneous alternation performance in the rat. Intra-septal injection of galanin (2.5 microgram), 30 min prior to plus-maze spontaneous alternation performance, significantly decreased alternation scores compared to those of rats receiving injections of vehicle solution. Co-injection of glucose (20 nmol) or the K-ATP channel blocker glibenclamide (5 nmol) attenuated the galanin-induced performance deficits. Glibenclamide produced an inverted-U dose-response curve in its interaction with galanin, with doses of 0.5 and 10 nmol having no effect on galanin-induced spontaneous alternation deficits. Drug treatments did not alter motor activity, as measured by overall number of arm entries during spontaneous alternation testing, relative to vehicle injected controls. These findings support the hypothesis that, in the septal region, galanin and glucose act via K-ATP channels to modulate neural function and behavior.

Galanin receptors in the hippocampus and entorhinal cortex of aged Fischer 344 male rats

Galanin (GAL) has been proposed to be an inhibitory modulator of cholinergic memory pathways because it acts within the hippocampus to inhibit the release and antagonize the postsynaptic actions of acetylcholine. Here we have used: 1) slice binding and quantitative autoradiography to assess the density and occupancy of GAL receptors; and 2) in situ hybridization histochemistry to assess expression of the GALR1 receptor subtype in the ventral hippocampus of 3-month-old and 21-month-old Fischer 344 male rats. We detected a small but significant (p < or = 0.0003) age-related reduction in 125I-GAL binding-site density in the ventral hippocampus and entorhinal cortex under standard binding conditions. Post-hoc analysis indicated that this reduction with age persisted in the CA1 radiatum and entorhinal cortex following GTP-induced desaturation to unmask pre-existent GAL receptors occupied by endogenous ligand. It was not associated with a significant change in peak GALR1 gene expression in the hippocampus. Because a portion of GAL receptors in this region have been postulated to function as presynaptic auto-receptors on cholinergic fiber terminals, the reduction in GAL binding sites with age may be a consequence of age-related alterations in GAL receptor expression by basal forebrain cholinergic neurons which project to the ventral hippocampus.

Distribution and kinetics of galanin infused into the ventral hippocampus of the rat: relationship to spatial learning

A recent study has shown that ventral hippocampal galanin plays a role in spatial learning and that it has an inhibitory effect on basal acetylcholine release [Ogren S. O. et al. (1996) Neuroscience 75, 1127-1140]. The present studies were designed to compare the in vivo tissue distribution and kinetics of infused galanin (porcine) with the temporal effect of galanin on spatial learning in the rat. Daily bilateral microinfusions of galanin (1.5 nmol/side for five days) via chronic cannulae placed in the ventral hippocampus produced a significant impairment of acquisition of the spatial task when infused 20 min, but not 5 or 60 min, before the daily training session. No overall impairment of memory retention (examined 24 h after the last training session) was observed in the galanin-treated rats. These results indicate that galanin given in the ventral hippocampus produces a time-dependent effect on acquisition. Using an antibody to porcine galanin and immunohistochemistry, galanin infused in the ventral hippocampus was found to be distributed mainly within the ventral part of the hippocampus and around the infusion site. The infused galanin was rapidly cleared from the extracellular space between 5 and 20 min after infusion. Five minutes after infusion of galanin, a number of cells in the ventral hippocampus, both within and outside the zone of extracellularly located galanin, showed a positive galanin-like immunoreactivity. These cells appear morphologically to be medium-sized neurons with a similar position as cells showing neuropeptide Y-like immunoreactivity. At 20 and 60 min after infusion of galanin, no cells with detectable levels of galanin-like immunoreactivity could be seen. These results indicate that the temporal kinetics and distribution of infused galanin are of major importance for its behavioural effect in the ventral hippocampus. The rapid clearance of the infused galanin and its internalization by neuronal endocytotic mechanisms may be important for its effect on cognition.

Galanin/GMAP- and NPY-like immunoreactivities in locus coeruleus and noradrenergic nerve terminals in the hippocampal formation and cortex with notes on the galanin-R1 and -R2 receptors

By using immunofluorescence methodology, extensive galanin (GAL) and GAL message-associated peptide (GMAP)-positive terminal networks were observed in the hippocampal formation. The majority of the GAL/GMAP fibers were dopamine beta-hydroxylase- (DBH) positive, that is, they were noradrenergic. This finding was established with GAL/GMAP-DBH double-staining and with 6-hydroxy-dopamine treatment, which totally abolished all fibers in which GAL/GMAP and DBH coexisted. Also, reserpine treatment caused a marked depletion of GAL. No evidence for GAL/GMAP coexistence with 5-hydroxytryptamine was obtained. In the ventral hippocampus, GAL/GMAP-, DBH-negative fibers were seen in the stratum oriens, the anterior stratum radiatum, along the granule cell layer and in the strata oriens and alveus. In the locus coeruleus (LC), around 80% of the GMAP-positive neurons contained neuropeptide tyrosine (NPY), and about 40% of the NPY-positive neurons expressed GMAP. GAL-R1 receptor mRNA was expressed in Barrington's nucleus (close to the LC), but was not detected in the hippocampal formation/dorsal cortical areas. GAL-R2 receptor mRNA was found in the granule cell layer in the dentate gyrus. The present results show that most, but not all, immunohistochemically detectable GAL/GMAP in the hippocampal formation/dorsal cortex is present in noradrenergic nerve terminals originating in the LC, which has a robust GAL/GMAP synthesis. The functional role of GAL may be related to noradrenaline, possibly by a presynaptic action. However, the presence of GAL in other systems and of GAL-R2 receptor mRNA in granule cells also indicates other targets.

Characterization and localization of galanin receptors in human entorhinal cortex

The neuropeptide galanin (GAL) has a widespread distribution throughout the human cortex. The entorhinal cortex (ENT) plays a crucial role in the transfer of cortico-cortical information related to memory and displays severe degeneration in Alzheimer's disease (AD). However, very little is known about the pharmacology of the GAL receptor (GALR) in normal human ENT. Therefore, we pharmacologically visualized their distribution and characterized GALRs using in vitro receptor autoradiography and radioligand binding assays. Autoradiograms revealed intense GALR labeling, mainly in the substantia innominata, hypothalamus, the bed nucleus of the stria terminalis and within layers 2 and 4 of the ENT. Kinetic experiments showed that saturation of GALR sites by [125I]GAL (human) (hGAL) occurred within 2 h and that this binding readily reversed in the presence of a GTP analog, but not in the presence of excess unlabeled hGAL. Analysis of [125I]hGAL binding data from saturation experiments gave KD values of 98.6+/-21.6 pM, Bmax values of 52.9+/-32.4 fmol/mg protein and identified a high and low affinity state of the GALR. The presence of 5'-guanylylimidodiphosphate (GppNHp) or NaCl reduced the agonist labeling of hGALR in ENT membranes.

Autoradiographic quantitation and anatomical mapping of GTP sensitive-galanin receptors in the guinea pig central nervous system

Galanin is a 29-amino acid peptide widely distributed in the mammalian central nervous system. Galanin receptors in the guinea pig brain were visualized using [125I]galanin by in vitro receptor quantitative autoradiography. Scatchard analysis of [125I]galanin binding to slide-mounted sections revealed saturable binding to a single class of high affinity receptors with a KD of approximately 1 nM. Specific [125I]galanin binding sites were detected in a large number of brain areas (concentration range: from non detectable to 99.32 fmol/mg of tissular proteins). The anatomical mapping revealed high densities essentially in the telencephalon (e.g. lateral septal nuclei, amygdala, hippocampal dentate gyrus) and the diencephalon (e.g. the anterodorsal and medial habenular thalamic nuclei, the paraventricular, dorsomedian and median mammillary hypothalamic nuclei, the posterior lobe of the pituitary). Addition of Mg2+ and GTP increased binding in some areas such as the zona incerta, the median eminence and the arcuate nucleus, and decreased it in other areas such as the amygdala, the hippocampus and the mammillary nuclei. This regional heterogeneity in the effect of Mg2+ and GTP can be interpreted as: (1) different rates of galanin receptor occupancy by endogenous peptide; (2) a differential coupling of GTP binding proteins to galanin receptors in the brain structures; and (3) a different nature of receptors. At any rate, this study provides evidence for a specific GTP-sensitive galanin receptor in guinea pig brain with an extensive distribution suggesting various physiological implications. Comparison with studies performed in several mammals shows that the overall distribution of galanin receptors is well preserved among species. These data suggest that galanin may possess similar functional properties in the different species tested so far. Nevertheless, very distinct differences were found in some areas like the cortex, the hippocampus and the pituitary.

Effects of ventral hippocampal galanin on spatial learning and on in vivo acetylcholine release in the rat

The neuropeptide galanin coexists in the medial septum and diagonal band of Broca with a population of acetylcholine neurons which project mainly to the ventral hippocampus. The present studies investigated the role of ventral hippocampal galanin in spatial learning in the male rat using a spatial learning task. In addition, the effects of galanin on cholinergic function were monitored by in vivo microdialysis and high-performance liquid chromatography. Bilateral microinjections of galanin (3 nmol/ rat) via chronic cannulae placed into the ventral hippocampus (i.v.h.) produced a slight but significant impairment of acquisition of the spatial task, while the 1 nmol dose of galanin facilitated acquisition. The 6 nmol dose of galanin failed to affect performance. A trend for an impairment of long-term memory retention (examined seven days after the last training session) was observed after 3 nmol of galanin, while the 1 nmol dose facilitated retention performance. Scopolamine (0.1 mg/kg, s.c.) caused a marked impairment of acquisition. Galanin (3 nmol/rat) given i.v.h. failed to modify the acquisition impairment caused by scopolamine (0.1 mg/kg, s.c.). These results suggest that galanin given i.v.h. produces a biphasic dose-dependent effects on spatial learning. In freely moving rats, galanin (3 nmol/10 microliters) given into the lateral ventricle (i.c.v.) did not affect basal acetylcholine release. In contrast, perfusion (100 min) with galanin (0.1 or 0.3 nmol/1.25 microliters/min) through the ventral hippocampal probe resulted in a reduction of basal acetycholine release which was dose-dependent and reversible. Galanin given i.c.v. (3 nmol/10 microliters) or through the probe (0.3 nmol/1.25 microliters/min) attenuated the increase in acetylcholine release evoked by the muscarinic antagonist scopolamine (0.1 mg/kg, s.c.; 0.001 nmol/1.25 microliters/min through the probe). The galanin plus scopolamine combinations produced a 50% lower increase in the extracellular acetylcholine concentrations than scopolamine alone. This suggests that the mechanism(s) behind scopolamine- and galanin-induced stimulation of acetylcholine differ. These results indicate that ventral hippocampal galanin plays a role in cognition and that it has a powerful and modulatory effect on cholinergic transmission. However, the effects of exogenous galanin on spatial learning cannot be directly related to changes in in vivo cholinergic transmission in the ventral hippocampus. These discrepancies may relate to effects on subtypes of galanin receptors with different functional coupling. In addition, other hippocampal neurotransmitter systems (e.g. noradrenergic neurons) important for cognitive functions may also be modulated by ventral hippocampal galanin.

Minireview. Galanin-acetylcholine interactions: relevance to memory and Alzheimer's disease

The neuropeptide, galanin, and its receptors are localized in the cholinergic basal forebrain and its projection areas in mammalian brain. Centrally administered galanin inhibits acetylcholine release in the rat ventral hippocampus, and produces deficits in learning and memory tasks. In Alzheimer's disease, galanin is overexpressed in terminals innervating the nucleus basalis of Meynert cell bodies. Selective galanin receptor antagonists provide a novel approach for increasing cholinergic function, as a potential adjunct to the clinical treatment of dementias.

Improvement by dynorphin A (1-13) of galanin-induced impairment of memory accompanied by blockade of reductions in acetylcholine release in rats

1. Human galanin (0.32 nmol per rat, i.c.v.), an endogenous neuropeptide, administered 30 min before acquisition or retention trials, significantly impaired the acquisition of learning and recall of memory in a step-through type passive avoidance performance. 2. The role of dynorphin A (1-13) in learning and memory is controversial. Dynorphin A (1-13) (0.5 nmol per rat, i.c.v.) administered 5 min before galanin injection, completely antagonized these impairments. 3. Galanin significantly decreased acetylcholine release in the hippocampus 40 to 120 min after injection as determined by in vivo brain microdialysis. This peptide also decreased acetylcholine release, albeit to a lesser extent, from the frontal cortex. 4. Dynorphin A (1-13) (0.5 nmol per rat, i.c.v.) 5 min before galanin injection, completely blocked the decrease in extracellular acetylcholine concentration induced by galanin. 5. These antagonistic effects of dynorphin A (1-13) were abolished by treatment with norbinaltorphimine (5.44 nmol per rat, i.c.v.), a selective kappa-opioid receptor antagonist, 5 min before dynorphin A (1-13). 6. Dynorphin A (1-13) (0.5 nmol) itself had no effect on learning and memory and on the acetylcholine concentration in the hippocampus or the frontal cortex in normal rats. 7. These results suggest that the neuropeptide dynorphin A (1-13) ameliorates the galanin-induced impairment of learning and memory accompanied by abolition of reductions in acetylcholine release via kappa-opioid receptors.

Galanin microinjected into the medial septum inhibits scopolamine-induced acetylcholine overflow in the rat ventral hippocampus

Galanin-like immunoreactive terminals hyperinnervate the basal forebrain cholinergic neurons in Alzheimer's disease. To investigate the hypothesis that galanin acts directly on basal forebrain cell bodies, in vivo microdialysis studies were conducted in awake rats which analyzed the actions of galanin on acetylcholine release. Microinjection of galanin into the cholinergic cell body region of the medial septum-diagonal band (MS-DBB) inhibited acetylcholine release in the ventral hippocampus. These results are consistent with an interpretation that galanin terminals synapsing on cholinergic cell bodies of the basal forebrain may serve to inhibit the release of acetylcholine in the terminal fields of the cholinergic neurons.

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 receptor binding sites in the temporal and occipital cortex are minimally affected in Alzheimer's disease

Galanin receptor binding sites were examined in the inferior temporal and medial occipital gyri of patients with Alzheimer's disease (AD) and matched control subjects using quantitative autoradiography. In the inferior temporal gyrus, galanin binding was reduced selectively in layers V-VI of the AD cases compared to controls, the magnitude of the reduction (45%) being similar to that of choline acetyltransferase activity (40%) in this region. In the medial occipital gyrus, galanin binding in the AD cases was not different from controls in any cortical layer despite a reduction in choline acetyltransferase activity. Galanin binding did not correlate with the densities of neuritic plaques in either temporal or occipital gyri. Thus, despite a significant cortical cholinergic deficit in AD, there is an anatomically selective reduction of cortical galanin receptor binding sites suggesting that the majority of galanin receptors are not located on cholinergic terminals in the human cerebral cortex.

Expression, control, and probable functional significance of the neuronal theta-rhythm

The data on theta-modulation of neuronal activity in the hippocampus and related structures, obtained by the author and her colleagues have been reviewed. Analysis of extracellularly recorded neuronal activity in alert rabbits, intact and after various brain lesions, in slices and transplants of the hippocampus and septum allow one to make the following conclusions. Integrity of the medial septal area (MS-DB) and its efferent connections are indispensable for theta-modulation of neuronal activity and EEG of the hippocampus. The expression of hippocampal theta depends on the proportion of the MS-DB cells involved in the rhythmic process, and its frequency in the whole theta-range, is determined by the corresponding frequencies of theta-burst in the MS-DB. The neurons of the MS-DB have the properties of endogenous rhythmic burst and regular single spike oscillators. Input signals ascending to the MS-DB from the pontomesencephalic reticular formation increase both the frequency of the MS-DB theta-bursts and the proportion of neurons involved in theta-activity; serotonergic midbrain raphe nuclei have the opposite effect on the MS-DB rhythmic activity and hippocampal EEG theta. Increase of endogenous acetylcholine (by physostigmine) also increases the proportion of the MS-DB neurons discharging in theta-bursts (both in intact and basally-undercut septum), but does not influence the theta-frequency. The primary effect of the MS-DB on hippocampal neurons (pyramidal and non-pyramidal) consists in GABAergic reset inhibition. Reset inhibition, after which theta-modulation follows in constant phase relation, is triggered also by sensory stimuli. About two-thirds of the hippocampal pyramidal neurons are tonically inhibited by sensory stimuli which evoke EEG theta, while others are excited, or do not change their activity. Anticholinergic drugs restrict the population of rhythmic neurons but do not completely suppress theta-bursts in the MS-DB and hippocampus. Under their action, EEG theta can be evoked (presumably through GABAergic MS-DB influences) by strong reticular or sensory stimuli with corresponding high frequency. However information processing in this condition is defective: expression of reset is increased, responses to electrical stimulation of the perforant path and to sensory stimuli are often augmented, habituation to sensory stimuli is absent and tonic responses are curtailed. On a background of continuous theta induced by increase of endogenous acetylcholine, reset is absent or reduced, responsiveness of the hippocampal neurons to electrical and sensory stimulation is strongly reduced.(ABSTRACT TRUNCATED AT 400 WORDS)

Galanin induces a hyperpolarization of norepinephrine-containing locus coeruleus neurons in the brainstem slice

Galanin applied in the bath or by micropipette directly on to locus coeruleus neurons in an in vitro slice preparation caused a hyperpolarization accompanied by a small decrease in membrane resistance. Immunohistochemical staining of intracellularly filled neurons indicated that the effect of galanin was exerted on norepinephrine neurons of the locus coeruleus. The galanin effect was variable in amplitude and duration and often showed desensitization, with subsequent applications producing a smaller response. When cells were exposed to tetrodotoxin or tetrodotoxin/low calcium media, the galanin response was still present. Under voltage clamp galanin application caused a net outward current that did not reverse in normal potassium concentrations; however, by increasing extracellular potassium concentrations the net outward current was reversed and the reversal potential shifted to a less negative potential. The response to galanin was identical when either KCl or KAc was used as the intracellular electrode solution. Tetraethylammonium chloride significantly reduced or abolished the response to galanin in most cells, although in a few cells the galanin response was not affected. Glibenclamide, a blocker of ATP-sensitive potassium channels, did not affect the galanin hyperpolarization. In addition, diazoxide had no effect on the membrane properties of locus coeruleus neurons. These results demonstrate that galanin exerts its inhibitory effect in the locus coeruleus via an increase in K+ conductance; however, not via the pancreatic type of ATP-sensitive K+ channels. Cryostat sections of the locus coeruleus incubated in 125I-labeled galanin revealed binding sites in the locus coeruleus at all levels. Sections of the locus coeruleus processed for ultrastructural immunocytochemistry showed galanin immunoreactivity in many neuronal somata and dendritic processes within the nucleus, confirming earlier evidence for the coexistence of galanin and noradrenaline in locus coeruleus neurons. Galanin-immunoreactive soma and dendrites in the locus coeruleus less frequently received galanin-immunoreactive synapses of axonal origin. These findings suggest that endogenous galanin in the locus coeruleus is mainly released from noradrenaline galanin somata and/or dendrites to act on autoreceptors or on receptors on adjacent neurons.

Connection matrix of the hippocampal formation: I. The dentate gyrus

The hippocampal formation presents a special opportunity for realistic neural modeling since its structure, connectivity, and physiology are better understood than that of other cortical components. A review of the quantitative neuroanatomy of the rodent dentate gyrus (DG) is presented in the context of the development of a computational model of its connectivity. The DG is a three-layered folded sheet of neural tissue. This sheet is represented as a rectangle, having a surface area of 37 mm2 and a septotemporal length of 12 mm. Points, representing cell somata, are distributed in the model rectangle in a roughly uniform fashion. Synaptic connectivity is generated by assigning each presynaptic cell a spatial zone representing its axonal arbor. For each postsynaptic cell, a list of potential presynaptic cells is compiled, based on which arbor zones the given postsynaptic cell falls within. An appropriate number of presynaptic inputs are then selected at random. The principal cells of the DG, the granule cells, are represented in the model, as are non-principal cells, including basket cells, chandelier cells, mossy cells, and GABAergic peptidergic polymorphic (GPP) cells. The neurons of layer II of the entorhinal cortex are included also. The DG receives its main extrinsic input from these cells via the perforant path. The basket cells, chandelier cells, and GPP cells receive perforant path and granule cell input and exert both feedforward and feedback inhibition onto the granule cells. Mossy cells receive converging input from granule cells and send their output back primarily to distant septotemporal levels, where they contact both granule cells and non-principal cells. To permit numerical simulations, the model must be scaled down while preserving its anatomical structure. A variety of methods for doing this exist. Hippocampal allometry provides valuable clues in this regard.

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.

The role of galanin in cholinergically-mediated memory processes

1. Galanin, a 29 amino-acid neuroactive peptide, has been shown to affect diverse processes throughout the nervous system and to coexist with several "classical" neurotransmitters, including norepinephrine, serotonin, and acetylcholine. 2. Galanin coexists with acetylcholine in neurons of the medial septum, diagonal band, and nucleus basalis of Meynert, cells which degenerate during the course of Alzheimer's disease. 3. In the ventral hippocampus, galanin inhibits the release of acetylcholine and inhibits carbachol stimulated phosphatidyl inositol hydrolysis. 4. Galanin impairs choice accuracy in learning and memory paradigms in rats, and is therefore hypothesized to be a contributory factor in the memory and cognitive disabilities found in Alzheimer's patients. 5. Newly developed galanin antagonists, by eliminating putative inhibitory effects of endogenous galanin on cholinergic function, may serve as useful therapies for memory disorders.

Localization of preprogalanin mRNA in the monkey hippocampal formation

The existence of neurons expressing preprogalanin mRNA in the monkey hippocampal formation was demonstrated using in situ hybridization of a radio-labelled cRNA probe transcribed from human preprogalanin cDNA. Specific hybridization occurred in neurons of the hilus of the dentate gyrus, fields CA1-3 in Ammon's horn, subiculum, presubiculum, parasubiculum and occasionally in neurons of the entorhinal cortex. These findings suggest that galanin is synthesized by neurons intrinsic to the monkey hippocampal formation.

Distribution of enkephalin and its relation to serotonin in cat and monkey spinal cord and brain stem

The distribution of enkephalin (ENK)-like immunoreactivity (LI) in spinal cord and medulla oblongata of cat and gray monkey (Macaca fascicularis) was studied by use of immunofluorescence and peroxidase antiperoxidase (PAP) techniques. Possible coexistence between ENK- and 5-hydroxytryptamine (5-HT)-LI was also analyzed with double labeling immunofluorescence. Furthermore, in situ hybridization was used to demonstrate cell bodies in the brain stem expressing mRNA encoding for ENK. ENK-immunoreactive (IR) axonal varicosities and fibers were demonstrated throughout the spinal cord gray matter, with the highest density in the superficial dorsal horn, the area around the central canal, the intermediolateral cell column, the sacral parasympathetic nucleus, and in Onuf's nucleus. In the monkey ventral horn, ENK-IR varicose fibers could in some cases be demonstrated in very close apposition to cell bodies. A low degree of co-localization between ENK- and 5-HT-LI was seen in the spinal cord of both species. Still, fibers containing both compounds could as a rule be demonstrated in every section studied. The highest degree of coexistence was encountered in the motor nucleus of the ventral horn. Six weeks after a low thoracic spinal cord transection a decreased staining for ENK-LI was demonstrated in the ventral horn motor nucleus, whereas other parts of the spinal cord appeared unaffected. In the brain stem of cats after colchicine treatment, ENK-LI was found in a majority of the 5-HT-IR cell bodies in the raphe nuclei (nucleus raphe magnus, pallidus and obscurus) and in the lateral reticular nucleus (rostroventrolateral reticular nucleus). In cat not pretreated with colchicine, a few weakly stained ENK-IR cell bodies could be found in the midline raphe nuclei and in the lateral reticular nucleus with the PAP technique. In the monkey brain stem without colchicine treatment, using the PAP technique, heavily stained ENK-IR cell bodies could be seen in the lateral reticular nucleus whereas, as in the cat, only a few, weakly stained ENK-IR cell bodies could be seen in the midline raphe nuclei. Using in situ hybridization technique, ENK mRNA expressing cells were demonstrated in the lateral reticular nucleus while no convincing mRNA signal could be found over cell bodies in the raphe nuclei. It is concluded that part of the ENKergic innervation of the cord in both species derives from supraspinal or suprasegmental levels.(ABSTRACT TRUNCATED AT 400 WORDS)

Galanin inhibits the potassium-evoked release of acetylcholine and the muscarinic receptor-mediated stimulation of phosphoinositide turnover in slices of monkey hippocampus

In the ventral hippocampus of Cynomologus monkey, galanin, a 29 amino acid long neuropeptide, reduced the potassium-evoked release of [3H]acetylcholine from slices preloaded with [3H]choline and diminished the carbachol-stimulated accumulation of [3H]inositol polyphosphates in hippocampal microprisms preincubated with myo-[2-3H]inositol. Using receptor autoradiography a strong, specific binding of iodinated galanin was observed in the molecular layer of the dentate gyrus. These may thus be the sites where galanin exerts its inhibitory effects on acetylcholine (ACh) release and phosphoinositide breakdown. These data provide evidence that galanin is a modulator of cholinergic function in septo-hippocampal neurons of primates.

Regulation of histamine release in rat hypothalamus and hippocampus by presynaptic galanin receptors

The effect of galanin, a peptide present in a subpopulation of histaminergic neurons emanating from the rat posterior hypothalamus, was investigated on K(+)-evoked [3H]histamine release in slices and synaptosomes from rat cerebral cortex, striatum, hippocampus and hypothalamus. Porcine galanin (0.3 microM) significantly inhibited histamine release induced by 25 mM K+ in slices from hypothalamus and hippocampus, but not from cerebral cortex and striatum, i.e., only in regions in which a colocalization of histamine and galanin has been described. The inhibitory effect of galanin was concentration dependent, with an EC50 value of 5.8 +/- 1.9 nM. The maximal inhibition was of 30-40% in hypothalamic and hippocampal slices depolarized with 25 mM K+. The galanin-induced inhibition observed in hypothalamic slices was not prevented in the presence of 0.6 microM tetrodotoxin and also occurred in hippocampal and hypothalamic synaptosomes, strongly suggesting the activation by galanin of presynaptic receptors located upon histaminergic nerve endings. The maximal inhibitory effect of galanin in slices or synaptosomes was lower than that previously reported for histamine acting at H3-autoreceptors, possibly suggesting that not all histaminergic axon terminals, even in the hypothalamus and hippocampus, are endowed with galanin receptors. It increased progressively in hypothalamic and hippocampal synaptosomes as the strength of the depolarizing stimulus was reduced. It is concluded that galanin modulates histamine release via presynaptic receptors, presumably autoreceptors located upon nerve terminals of a subpopulation of cerebral histaminergic neurons.

Pertussis toxin-sensitive G-protein mediates galanin's inhibition of scopolamine-evoked acetylcholine release in vivo and carbachol-stimulated phosphoinositide turnover in rat ventral hippocampus

The effects of intracerebroventricular (i.c.v.) injections of pertussis toxin were investigated on the inhibitory action of galanin on acetylcholine release and phosphoinositide breakdown stimulated by muscarinic agents in rat ventral hippocampus. Pertussis toxin (0.6 micrograms, i.c.v., 96 h) counteracted the in vitro inhibitory effect of galanin (3.1 nmol) on phosphoinositide breakdown stimulated by carbachol without altering the stimulatory action of the cholinergic agonist on signal transduction, in miniprisms from rat ventral hippocampus. Pertussis toxin also abolished the in vivo effect of galanin on scopolamine-stimulated acetylcholine release in vivo but did not affect basal acetylcholine release. The results indicate that pertussis toxin-sensitive G-protein(s) mediates the galanin receptor regulation of pre- and postsynaptic cholinergic functions in the ventral hippocampus.

Galanin reduces carbachol stimulation of phosphoinositide turnover in rat ventral hippocampus by lowering Ca2+ influx through voltage-sensitive Ca2+ channels

The 29-amino-acid peptide galanin (GAL) caused concentration-dependent inhibition of the accumulation of 3H-inositol phosphates (3H-InsPs) induced by the muscarinic agonist carbachol (CARB; 10(-3)-10(-5) M) in the presence of 5 mM lithium, specifically in tissue miniprisms from rat ventral hippocampus. The inhibitory effect of GAL involved the mono-, bis-, tris-, and tetrakisphosphates formed during activation for 2 min of phospholipase C by CARB (1 mM) in the absence of lithium. GAL (1 microM) did not affect alpha-adrenergic or serotonergic type 2 receptor-mediated phosphoinositide (PI) breakdown in the same tissue. GAL by itself neither acted on basal levels of 3H-InsPs nor affected muscarinic receptors in binding studies. Blockade of the T-, N-, and L-types of voltage-sensitive calcium channel (VSCC) with 200 microM Cd2+ reduced muscarinic receptor-mediated PI breakdown by 50% and abolished the inhibitory effect of GAL (1 microM). Reduction of the extracellular Ca2+ concentration from 1.3 mM to 0.49 microM abolished the GAL inhibition of CARB-stimulated PI hydrolysis. Ca2+ influx promoted by 18 mM K+ depolarization or by 1 microM Bay K 8644, a selective agonist of the L-type VSCC, prevented the inhibitory effect of GAL. Blockade of the L-type VSCC with nifedipine (1 microM) potentiated the inhibitory effects of GAL without affecting muscarinic stimulation of PI breakdown.(ABSTRACT TRUNCATED AT 250 WORDS)

The role of interactions between the cholinergic system and other neuromodulatory systems in learning and memory

Extensive evidence indicates that disruption of cholinergic function is characteristic of aging and Alzheimer's disease (AD), and experimental manipulation of the cholinergic system in laboratory animals suggests age-related cholinergic dysfunction may play an important role in cognitive deterioration associated with aging and AD. Recent research, however, suggests that cholinergic dysfunction does not provide a complete account of age-related cognitive deficits and that age-related changes in cholinergic function typically occur within the context of changes in several other neuromodulatory systems. Evidence reviewed in this paper suggests that interactions between the cholinergic system and several of these neurotransmitters and neuromodulators--including norepinephrine, dopamine, serotonin, GABA, opioid peptides, galanin, substance P, and angiotensin II--may be important in learning and memory. Thus, it is important to consider not only the independent contributions of age-related changes in neuromodulatory systems to cognitive decline, but also the contribution of interactions between these systems to the learning and memory deficits associated with aging and AD.

Effects of central nervous system lesions on the expression of galanin: a comparative in situ hybridization and immunohistochemical study

We have used in situ hybridization and immunohistochemistry to study the expression of galanin mRNA and galanin-like immunoreactivity after decortication and lesions of the ventral hippocampus. After decortication the levels of both galanin mRNA and galanin-like immunoreactivity were increased in the dorsal raphe nucleus. In addition, in decorticated rats, but not in controls, galanin mRNA could be seen in dorsal and ventral nuclei of the thalamus and in the remaining parts of the cortex. Increases in galanin mRNA and galanin-like immunoreactivity were also observed in the septum-vertical diagonal band after electrocoagulation lesions of the ventral hippocampus. In contrast, no changes were found after ibotenic acid lesions of the same hippocampal area. These results suggest that increases in the expression of galanin occur in certain neuron populations after direct lesion of their axons and/or terminal fields.

The effect of galanin on baseline and GHRH-induced growth hormone secretion in obese children

We have evaluated the effect of the administration of galanin (Gal), a newly identified hypothalamic peptide, on baseline and GHRH-induced GH rise in five obese children and in seven controls. The GH response to GHRH (hpGRF(1-29), 1 microgram/kg i.v.), and to Gal (15 micrograms/kg/h for 1 h), evaluated both as the maximum GH peak and as integrated area under the curve (AUC), was significantly lower in the obese children than in the controls. Simultaneous administration of Gal plus GHRH significantly increased the GH response to GHRH in all the obese subjects, so that their mean peak GH levels and AUC after Gal plus GHRH were similar to those of the control children after GHRH. Also, in control children Gal caused a significant augmentation of the GH response to GHRH. Mean peak GH levels and mean AUC after Gal plus GHRH were significantly higher in the controls than in the obese children given the same treatment. Our data indicate that obese children have a blunted GH response to Gal, which, however, is able to enhance the GH response to GHRH. This observation strengthens the view that the mechanism of action of Gal involves modulation of endogenous somatostatin (SRIH) release. In addition, similarity between the effects of Gal and pyridostigmine on baseline and GHRH-stimulated GH release in obese children may indicate the existence of a cholinergic link in the action of Gal.

Galanin immunoreactivity is increased in the nucleus basalis of Meynert in Alzheimer's disease

A depletion of large cholinergic neurons in the nucleus basalis of Meynert is a consistent finding in Alzheimer's disease (AD). The nucleus basalis of Meynert also contains interneurons and afferents that may modulate its functioning. In the present study we examined neurochemical markers for neuropeptides, amino acid neurotransmitters, and monoaminergic neurotransmitters in postmortem samples of the nucleus basalis in 16 control subjects and 30 patients with AD. There were no significant changes in glutamate, aspartate, taurine, gamma-aminobutyric acid (GABA), and catecholamines; however, concentrations of serotonin, 5-hydroxyindoleacetic acid, and 5-hydroxytryptophol were significantly reduced. Choline acetyltransferase activity was significantly reduced, consistent with previous reports. Galanin immunoreactivity was significantly increased twofold in the patients with AD, but there were no significant changes in substance P, somatostatin, or neuropeptide Y immunoreactivity. Since galanin inhibits acetylcholine release, and produces cognitive deficits in animals, increased galanin immunoreactivity in the nucleus basalis of Meynert in AD may contribute to the cognitive deficits that characterize the illness.