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

Longitudinal axis of the hippocampus: both septal and temporal poles of the hippocampus support water maze spatial learning depending on the training protocol

It has been suggested previously that 30% sparing of the hippocampus is enough to support spatial learning of a reference memory task in a water maze provided the spared tissue is located septally (Moser et al. 1995, Proc Natl Acad Sci USA 92:9697-9701). Therefore, the temporal hippocampus may not be involved in spatial memory. Place cells are also found in this part of the structure, and it has been suggested that these place cells have larger, less well-tuned place fields than are found in the septal hippocampus. We tested the possibility that the temporal hippocampus might be involved in spatial learning when the animals are required to distinguish between different contexts. Experiment 1 was a replication of the findings reported by Moser et al., using their protocol (8 trials/day, 6 days) and the groups with 20-40% hippocampus spared septally or temporally (volume assessed by quantitative volumetric techniques). In experiment 2, rats with also 20-40% sparing of the hippocampus either septally or temporally were trained in two water maze concurrently (four trials/day/water maze, 8 days). Rats with 20-40% hippocampus spared temporally were able to learn the two water maze tasks normally, and no difference was observed between rats with septal and temporal hippocampus spared across different measures of performance. In experiment 3, rats with 20-40% hippocampus spared septally or temporally were trained in one water maze as in experiment 1, but using a spaced training protocol similar to that of experiment 2 (four trials/day, 8 days). Rats with temporal hippocampus spared developed a preference for the training quadrant and acquired levels of performance indistinguishable from those of rats with septal hippocampus spared. The results suggest that the temporal hippocampus can support the learning of two, but also one, spatial water maze reference memory task, provided the training protocol is adequate.

CSF galanin and cognition after shunt surgery in normal pressure hydrocephalus

BACKGROUND

"Normal" pressure hydrocephalus (NPH) is associated with injury to neurotransmitter and neuropeptide systems that recovers after surgery. This could be linked to changes in galanin, a neuropeptide with inhibitory effects on basal forebrain cognitive function.

OBJECTIVE

To examine changes in CSF galanin concentrations in patients with normal pressure hydrocephalus undergoing shunt surgery, and to investigate the relation between these changes and cognitive functioning.

METHODS

Eight patients underwent surgery for idiopathic normal pressure hydrocephalus. Lumbar CSF galanin determinations, cognitive status, and clinical status were quantified before operation and six months after. Cognition was assessed by an extensive battery of tests measuring attention, memory, speed of mental processing, visuospatial function, and frontal lobe function.

RESULTS

CSF galanin concentration decreased after surgery. This reduction correlated with improved clinical and cognitive functioning, specifically with attention and visuomotor speed, visuoconstructive and frontal functioning, and clinical status according to the NPH scale, including the sphincter and cognitive components.

CONCLUSIONS

The cognitive and clinical improvement after shunt implantation correlated with CSF galanin levels, suggesting that the distribution or function of this agent involves cerebral structures that have some potential for recovery. In this study, galanin was related to several cognitive functions that may be associated with the fronto-subcortical deficits underlying cognitive dysfunction in normal pressure hydrocephalus.

Analysis of the role of the 5-HT1B receptor in spatial and aversive learning in the rat

The present study examined the role of the 5-HT1B receptor in learning and memory. The ability of the 5-HT1B receptor agonist anpirtoline and the selective 5-HT1B receptor antagonist NAS-181 to affect spatial learning in the water maze (WM) and aversive learning in the passive avoidance (PA) task were examined in the rat. Anpirtoline (0.1-1.0 mg/kg, s.c.) caused a dose-dependent impairment of learning and memory in both the WM and PA tasks. NAS-181 (1.0-10 mg/kg, s.c.) failed to alter performance of the WM task, but produced a dose-dependent (0.1-20 mg/kg) facilitation of PA retention. Furthermore, treatment with NAS-181 (10 mg/kg) fully blocked the impairment of the WM and PA performance caused by anpirtoline (1.0 mg/kg). In contrast, NAS-181 (3.0-10 mg/kg) did not attenuate the spatial learning deficit and the impairment of PA retention caused by scopolamine (0.1 mg/kg in WM task, 0.3 mg/kg in PA task, s.c.), a nonselective muscarinic antagonist. Moreover, a subthreshold dose of scopolamine (0.1 mg/kg) blocked the facilitation of PA retention induced by NAS-181 (1.0-10 mg/kg). In addition, the behavioral disturbances (eg thigmotaxic swimming and platform deflections) induced by anpirtoline and scopolamine were analyzed in the WM task and correlated with WM performance. These results indicate that: (1) 5-HT1B receptor stimulation and blockade result in opposite effects in two types of cognitive tasks in the rat, and that (2) the 5-HT1B antagonist NAS-181 can facilitate some aspects of cognitive function, most likely via an increase of cholinergic transmission. These results suggest that 5-HT1B receptor antagonists may have a potential in the treatment of cognitive deficits resulting from loss of cholinergic transmission.

Central galanin administration blocks consolidation of spatial learning

Galanin is a neuropeptide that inhibits the evoked release of several neurotransmitters, inhibits the activation of intracellular second messengers, and produces deficits in a variety of rodent learning and memory tasks. To evaluate the actions of galanin on encoding, consolidation, and storage/retrieval, galanin was acutely administered to Sprague-Dawley rats at time points before and after training trials in the Morris water maze. Intraventricular administration of galanin up to 3h after subjects had completed daily training trials in the Morris water task impaired performance on the probe trial, indicating that galanin-blocked consolidation. Pretreatment with an adenylate cyclase activator, forskolin, prevented the deficits in distal cue learning produced by galanin. Di-deoxyforskolin, an inactive analog of forskolin, had no effect. These results provide the first evidence that galanin interferes with long-term memory consolidation processes. A potential mechanism by which galanin produces this impairment may involve the inhibition of adenylate cyclase activity, leading to inhibition of downstream molecular events that are necessary for consolidation of long-term memory.

Selective 5-HT1A antagonists WAY 100635 and NAD-299 attenuate the impairment of passive avoidance caused by scopolamine in the rat

Systemic administration of the muscarinic-receptor antagonists atropine and scopolamine produces cognitive deficits in humans, nonhuman primates and rodents. In humans, these deficits resemble symptoms of dementia seen in Alzheimer's disease. The passive avoidance (PA) task has been one of the most frequently used animal models for studying cholinergic mechanisms in learning and memory. The present study examined the ability of two selective 5-HT(1A) receptor antagonists WAY 100635 and NAD-299 (robalzotan) and two acetylcholinesterase (AChE) inhibitors tacrine and donepezil to attenuate the impairment of PA retention caused by the nonselective muscarinic receptor antagonist scopolamine in the rat. Although demonstrating differences in their temporal kinetics, both WAY 100635 and NAD-299 attenuated the impairment of PA caused by scopolamine (0.3 mg/kg s.c.). Donepezil did not block the PA deficit caused by the 0.3 mg/kg dose of scopolamine, but it prevented the inhibitory effects of the 0.2 mg/kg dose of scopolamine. In contrast, tacrine was effective vs both the 0.2 and 0.3 mg/kg doses of scopolamine. These results indicate that (1). a functional 5-HT(1A) receptor antagonism can attenuate the anterograde amnesia produced by muscarinic-receptor blockade, and (2). the AChE inhibitors tacrine and donepezil differ in their ability to modify muscarinic-receptor-mediated function in vivo. These results suggest that 5-HT(1A) receptor antagonists may have a potential in the treatment of cognitive symptoms in psychopathologies characterized by reduced ACh transmission such as Alzheimer's disease.

Galanin regulates spatial memory but not visual recognition memory or synaptic plasticity in perirhinal cortex

It has previously been shown that the neuropeptide galanin plays a role in the age-dependent regulation of hippocampal synaptic plasticity and spatial memory. Here, we further extend these studies by demonstrating that galanin knockout mice also have deficits in an object-in-place spatial memory task. In contrast however, there is no deficit in single item object recognition memory, a memory that depends on perirhinal cortex. Furthermore, in perirhinal cortex slices there are no differences in activity-dependent long-term potentiation or depotentiation, nor in muscarinic receptor-dependent long-term depression between galanin knockout mice and wild-type litter-mates. Therefore, these results suggest that galanin has a differential role in hippocampal-dependent and perirhinal cortex-dependent memory.

Deficits in trace cued fear conditioning in galanin-treated rats and galanin-overexpressing transgenic mice

Galanin inhibits the release of several neurotransmitters and produces performance deficits in a variety of spatial and aversive learning and memory tasks. The experiments in this study investigated the role galanin has in emotional learning and memory using a standard delay cued and contextual fear conditioning task. Rats were administered galanin into the lateral ventricles before training, and scored for freezing behavior in the same context and in a novel context with and without an auditory cue (CS) that had been paired previously with an aversive stimulus (US). Galanin-overexpressing transgenic mice were tested in an identical behavioral protocol. The galanin-administered rats and the transgenic mice were not significantly different from their respective controls on this task. A more challenging trace cued and contextual fear conditioning procedure was administered to separate groups of galanin-treated rats and galanin-overexpressing transgenic mice. Subjects were trained with the same CS and US, however, a 2.5-sec delay was inserted between CS offset and US onset. Following the trace conditioning, rats administered galanin and mice overexpressing galanin both exhibited significantly less freezing to the CS in the novel context as compared with their control groups. These results indicate that the observed disruption of cued fear conditioning was specific to the more difficult trace conditioning task. These findings are the first demonstration that galanin impairs performance on an emotional memory task and support the hypothesis that galanin-induced deficits are specific to more difficult cognitive tasks.

Galanin overexpressing transgenic mice

Galanin overexpressing transgenic mice (GAL-tg) were generated on two different promoters. Both lines of GAL-tg displayed high levels of galanin in the hippocampus and reduced sensitivity to seizures, as compared to their respective wildtype littermate controls (WT). Performance deficits on learning and memory tasks, impaired long-term potentiation, reduced hippocampal excitability, lower evoked glutamate release, and reduced numbers of choline acetyltransferase immunoreactive neurons in the horizontal limb of the diagonal band were detected in GAL-tg as compared to WT. Changes in sensitivity to nociceptive stimuli were demonstrated in one line. GAL-tg represent a new model for investigating the biological actions of endogenous galanin, and for testing novel therapeutics based on galanin receptor ligands.

The action of orexin A on passive avoidance learning. Involvement of transmitters

The action of orexin A on one-way passive avoidance learning was studied in rats. Orexin A administered into the lateral brain ventricle in conscious rats facilitated learning, the consolidation of learning and also retrieval processes in a dose-dependent manner in a passive avoidance paradigm. The involvement of transmitters was studied by pretreating the animals with receptor antagonists, which had proved to be effective with other neuropeptides in attenuating or blocking the action of orexin A. The following receptor blockers were used: haloperidol, phenoxybenzamine, propranolol, atropine, bicuculline, naloxone and nitro-L-arginine, which can block nitric oxide synthase. In the doses used all of the receptor blockers attenuated, but none of them fully blocked the action of orexin A on the consolidation of passive avoidance learning. The results demonstrate that orexin A is able to facilitate learning, consolidation of learning and also retrieval processes in a passive avoidance paradigm. A number of transmitters could be involved in the action of consolidation, but none of them is absolutely essential.

Facilitation of dopamine-mediated locomotor activity in adult rats following cholinergic denervation

The dopamine hypothesis of schizophrenia postulates hyperactivity of dopaminergic neurotransmission in the mesolimbic system. However, the possible underlying causes for this dopaminergic overfunction are not well understood. Therefore, the main aim of this study was to examine the effect of central cholinergic denervation on dopamine-mediated functions. We also examined the effect of neonatal cholinergic denervation upon adult brain function. The immunotoxin 192 IgG-saporin causes severe lesions of the basal forebrain cholinergic system when infused into the lateral ventricles by targeting neurons expressing the p75 neurotrophin receptor. The toxin may also damage p75-expressing Purkinje neurons in the cerebellum. We have compared the behavioral effects of intracerebroventricular injections of 192 IgG-saporin to adult rats with that of injections to neonate rats. As expected, adult treated rats displayed an almost complete cholinergic denervation of forebrain corticohippocampal areas concomitant with a marked impairment in the Morris water maze. When tested as adults, neonatally treated animals had a less complete cholinergic denervation and showed lesser impairments in water maze behaviors. Interestingly, adult treated rats showed increased spontaneous horizontal activity and a remarkable increase in locomotor response to d-amphetamine as evidenced by increased horizontal and vertical activity. There were no marked changes of spontaneous or drug-induced locomotor activity in adult rats treated with 192 IgG-saporin as neonates. These results suggest that cholinergic denervation of the forebrain causes a marked enhancement of behavioral responses related to dopaminergic activity, probably mainly mediated presynaptically. However, it cannot be fully excluded that damage to noncholinergic systems, e.g., Purkinje cells, might contribute to the effects. The striking overreaction to dopaminergic stimuli, presumably caused by the cholinergic deficit, is discussed in relation to the suggested role of cholinergic malfunctioning in schizophrenia.

Microdialysis in freely moving mice: determination of acetylcholine, serotonin and noradrenaline release in galanin transgenic mice

In the present study, we describe micro-surgical methods for simultaneous implantation of a microdialysis probe and an intraventricular injection cannula via their respective guide cannulas into the mouse brain. Basal and stimulated release of acetylcholine (ACh), serotonin (5-HT) and noradrenaline (NA) was determined in the ventral hippocampus of freely moving mice. NA and 5-HT were determined in one run by a newly developed HPLC method based on precolumn derivatization with benzylamine and fluorescence detection. The mice with a loss-of-function mutation of the galanin gene (KO) and the mice that over-expressed galanin (OE) were studied. No significant differences in basal, potassium-stimulated or scopolamine-induced extracellular ACh levels were observed in 4-month-old wild-type (WT) and KO mice. In the aged, 10-month-old animals, the basal extracellular ACh levels were significantly reduced in both WT and KO groups. Galanin (1 nmol i.c.v.) caused a significant reduction of basal extracellular NA by about 40% in both WT and galanin OE mice, however, in the latter group the effect was delayed by almost 2 h. A 10-min forced swimming stress caused a higher increase in release of NA and 5-HT in the OE group than in the corresponding WT mice. Finally, venlafaxin (10 mg/kg i.p.) increased extracellular NA to 400% of the control values in the CBA mice, but only to 250% in the C57BL mice. It is concluded that galanin may play an important role in the cholinergic mechanisms underlying cognitive disorders. Furthermore, modulation by galanin and by behavioral activation, of NA and 5-HT neurotransmission in galanin over-expressing mice indicates its possible role in the aetiology of mood disorders.

Distribution of galaninergic immunoreactivity in the brain of the mouse

The distribution of galaninergic immunoreactive (-ir) profiles was studied in the brain of colchicine-pretreated and non-pretreated mice. Galanin (GAL)-ir neurons and fibers were observed throughout all encephalic vesicles. Telencephalic GAL-ir neurons were found in the olfactory bulb, cerebral cortex, lateral and medial septum, diagonal band of Broca, nucleus basalis of Meynert, bed nucleus of stria terminalis, amygdala, and hippocampus. The thalamus displayed GAL-ir neurons within the anterodorsal, paraventricular, central lateral, paracentral, and central medial nuclei. GAL-ir neurons were found in several regions of the hypothalamus. In the midbrain, GAL-ir neurons appeared in the pretectal olivary nucleus, oculomotor nucleus, the medial and lateral lemniscus, periaqueductal gray, and the interpeduncular nucleus. The pons contained GAL-ir neurons within the dorsal subcoeruleus, locus coeruleus, and dorsal raphe. In the medulla oblongata, GAL-ir neurons appear in the anterodorsal and dorsal cochlear nuclei, salivatory nucleus, A5 noradrenergic cells, gigantocellular nucleus, inferior olive, solitary tract nucleus, dorsal vagal motor and hypoglossal nuclei. Only GAL-ir fibers were seen in the lateral habenula nucleus, substantia nigra, parabrachial complex, cerebellum, spinal trigeminal tract, as well as the motor root of the trigeminal and facial nerves. GAL-ir was also observed in several circumventricular organs. The widespread distribution of galanin in the mouse brain suggests that this neuropeptide plays a role in the regulation of cognitive and homeostatic functions.

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

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

Neuropeptides in hippocampus and cortex in transgenic mice overexpressing V717F beta-amyloid precursor protein--initial observations

Immunohistochemistry was used to analyse 18- and 26-month-old transgenic mice overexpressing the human beta-amyloid precursor protein under the platelet-derived growth factor-beta promoter with regard to presence and distribution of neuropeptides. In addition, antisera/antibodies to tyrosine hydroxylase, acetylcholinesterase, amyloid peptide, glial fibrillary acidic protein and microglial marker OX42 were used. These mice have been reported to exhibit extensive amyloid plaques in the hippocampus and cortex [Masliah et al. (1996) J. Neurosci. 16, 5795-5811]. The most pronounced changes were related to neuropeptides, whereas differences between wild-type and transgenic mice were less prominent with regard to tyrosine hydroxylase and acetylcholinesterase. The main findings were of two types; (i) involvement of peptide-containing neurites in amyloid beta-peptide positive plaques, and (ii) more generalized changes in peptide levels in specific layers, neuron populations and/or subregions in the hippocampal formation and ventral cortices. In contrast, the parietal and auditory cortices were comparatively less affected. The peptide immunoreactivities most strongly involved, both in plaques and in the generalized changes, were galanin, neuropeptide Y, cholecystokinin and enkephalin. This study shows that there is considerable variation both with regard to plaque load and peptide expression even among homozygotes of the same age. The most pronounced changes, predominantly increased peptide levels, were observed in two 26-month-old homozygous mice, for example, galanin-, enkephalin- and cholecystokinin-like immunoreactivities in stratum lacunosum moleculare, and galanin, neuropeptide Y, enkephalin and dynorphin in mossy fibers. Many peptides also showed elevated levels in the ventral cortices. However, decreases were also observed. Thus, galanin-like immunoreactivity could not any longer be detected in the diffusely distributed (presumably noradrenergic) fiber network in all hippocampal and cortical layers, and dynorphin-like immunoreactivity was decreased in stratum moleculare, cholecystokinin-like immunoreactivity in mossy fibers and substance P-like immunoreactivity in fibers around granule cells. The significance of generalized peptide changes is at present unclear. For example, the increase in the mainly inhibitory peptides galanin, neuropeptide Y, enkephalin and dynorphin and the decrease in the mainly excitatory peptide cholecystokinin in mossy fibers (and of substance P fibers around granule cells) indicate a shift in balance towards inhibition of the input to the CA3 pyramidal cell layer. Moreover, it may be speculated that the increase in levels of some of the peptides represents a reaction to nerve injury with the aim to counteract, in different ways, the consequences of injury, for example by exerting trophic actions. Further studies will be needed to establish to what extent these changes are typical for Alzheimer mouse models in general or are associated with the V717F mutation and/or the platelet-derived growth factor-beta promoter.

Pharmacological evidence supporting a role for galanin in cognition and affect

1. Galanin is localized in brain pathways involved in both cognition and affect. 2. Galanin has inhibitory actions on a variety of memory tasks including the Morris water maze, delayed nonmatching to position, T-maze delayed alternation, starburst maze, passive avoidance, active avoidance, and spontaneous alternation. 3. Galanin may inhibit learning and memory by inhibiting neurotransmitter release and neuronal firing rate. 4. Two signal transduction mechanisms through which galanin exerts its inhibitory actions are the inhibition of phosphatidyl inositol hydrolysis and the inhibition of adenylate cyclase. 5. Galanin released during periods of burst firing from noradrenergic locus coeruleus terminals in the ventral tegmental area (VTA) may lead to symptoms of depression through inhibition of dopaminergic VTA neurons. 6. Intraventricular galanin has anxiolytic effects in a punished drinking test. Intra-amygdala galanin has anxiogenic effects in a punished drinking test.

Application of in vivo microdialysis to the study of cholinergic systems

The application of in vivo microdialysis to the study of acetylcholine (ACh) release has contributed greatly to our understanding of cholinergic brain systems. This article reviews standard experimental procedures for dialysis probe selection and implantation, perfusion parameters, neurochemical detection, and data analysis as they relate to microdialysis assessments of cholinergic function. Particular attention is focused on the unique methodological considerations that arise when in vivo microdialysis is dedicated expressly to the recovery and measurement of ACh as opposed to other neurotransmitters. Limitations of the microdialysis technique are discussed, as well as methodological adaptations that may prove useful in overcoming these limitations. This is followed by an overview of recent studies in which the application of in vivo microdialysis has been used to characterize the basic pharmacology and physiology of cholinergic neurons. Finally, the usefulness of the microdialysis approach for testing hypotheses regarding the cholinergic systems' involvement in cognitive processes is examined. It can be concluded that, in addition to being a versatile and practical method for studying the neurochemistry of cholinergic brain systems, in vivo microdialysis represents a valuable tool in our efforts to better comprehend ACh's underlying role in a variety of behavioral processes.

Endomorphin-2 but not Leu-enkephalin modulates spatial learning when microinjected in the CA3 region of the rat hippocampus

The recently discovered endogenous mu-selective opioid peptide, endomorphin-2, and the endogenous delta-selective opioid peptide, Leu-enkephalin, were tested for their ability to affect spatial learning in the Morris water task. It was found that microinjection of 10 nmol endomorphin-2 into the CA3 region of the rat hippocampus significantly impaired spatial learning. However, the two lower doses tested, 3.3 and 1 nmol, had no effect in this test. Leu-enkephalin did not have any effect on spatial learning at the two doses tested (10 and 3.3 nmol). Neither peptide had any effect on motor performance as measured by swim speed. The results indicate that mu-receptors in the CA3 region of the rat hippocampus are more relevant than delta-receptors for spatial learning.

Multiple output pathways of the basal forebrain: organization, chemical heterogeneity, and roles in vigilance

Studies over the last decade have shown that the basal forebrain (BF) consists of more than its cholinergic neurons. The BF also contains non-cholinergic neurons, including gamma-aminobutyric acid-ergic neurons which co-distribute and co-project with the cholinergic neurons. Both types of neuron project, in variable proportions, to the cerebral cortex, hippocampus, thalamus, amygdala, and olfactory bulb, whereas descending projections to the posterior hypothalamus and brainstem nuclei are predominantly non-cholinergic. Some of the cholinergic and non-cholinergic projection neurons contain neuropeptides such as galanin, nitric oxide synthase, and possibly glutamate. To understand better the function of the BF, the organization of the multiple ascending and descending projections of BF neurons is reviewed along with their neurochemical heterogeneity, and possible functions of individual pathways are discussed. It is proposed that BF neurons belong to multiple systems with distinct cognitive, motivational, emotional, motor, and regulatory functions, and that through these pathways, the BF plays a role in controlling both cognitive and non-cognitive aspects of vigilance.

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.

Galanin and spatial learning in the rat. Evidence for a differential role for galanin in subregions of the hippocampal formation

Anatomical, neurochemical and behavioural evidence support a role for galanin in hippocampally mediated functions such as spatial learning and memory. To obtain more precise information on this role, galanin (3 nmol/rat) was infused via bilateral chronic cannulae into different areas of the hippocampal formation which are characterized by different galanin receptor subtypes and also by different galanin innervation patterns. The effects of infused galanin on spatial learning were examined in the Morris swim maze. Infusions of galanin into both the dorsal and ventral dentate gyrus, which mainly contain GAL-R2 receptor mRNA and a high degree of galanin-noradrenaline coexistence, significantly retarded spatial acquisition without affecting swim speed or performance in the visible platform test. This spatial learning deficit was fully blocked by pretreatment with the non-selective galanin antagonist M35. Analysis of retention performance suggested that the major effect of intrahippocampal galanin is mediated via a specific disruption of acquisition mechanisms of importance for performance in the probe trial. Galanin infused into the ventral CA1 (a mainly GAL-R1 receptor mRNA expressing region) or into anterior, ventral CA3 regions did not produce any deficits in spatial learning compared to control animals. These results suggest that galanin mediates its action on spatial learning mainly through the GAL-R2 receptor subtype in areas where most of the galanin is present in noradrenergic terminals. A possible role for the GAL-R1 receptor subtype in cognition in the dorsal and ventral hippocampus remains to be defined. The results suggest a differential functional role for galanin and galanin receptor subtypes within subregions of the hippocampal formation.

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.

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.

Rat strain differences in response to galanin on the Morris water task

Galanin acts as an inhibitory modulator of cholinergic transmission in the septohippocampal pathway of the rat. Centrally administered galanin induces performance deficits on rodent learning and memory tasks, including delayed non-matching to position, T-maze delayed alternation, passive avoidance, starbust radial maze acquisition, and the Morris water task. The present study investigates differences in responsiveness to intraventricularly administered galanin across three strains of laboratory rat on acquisition of spatial learning in the Morris water task. Sprague-Dawley rats showed normal performance during training, but lack of selective quadrant search on the probe trial in response to galanin treatment. Long-Evans rats showed no effects of galanin on performance during training or probe trial. Wistar rats showed longer latencies to reach the hidden platform during training, and lack of selective quadrant search on the probe trial in response to galanin. Performance on the visible platform task and on locomotor activity in the open field was normal in rats treated with galanin. These results are consistent with an interpretation of strain differences in sensitivity to the inhibitory actions of galanin on learning and memory.

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.

Modulation of acetylcholine and serotonin transmission by galanin. Relationship to spatial and aversive learning

This paper presents evidence that galanin is a potent in vivo modulator of basal acetylcholine release in the rat brain with qualitatively and quantitatively differential effects in the dorsal and ventral hippocampus. Galanin perfused through the microdialysis probe decreased basal acetylcholine release in the ventral hippocampus, while it enhanced acetylcholine release in the dorsal hippocampus. Galanin (3 nmol/rat) infused into the ventral hippocampus impaired spatial learning acquisition, while it tended to facilitate acquisition when injected into the dorsal hippocampus. These effects appear to be related to activation of GAL-R1 (ventral hippocampus) and GAL-R2 (dorsal hippocampus) receptors, respectively. However, the effects of galanin on acetylcholine release and on spatial learning appear not to be directly related to cholinergic mechanisms, but they may also involve interactions with noradrenaline and/or glutamate transmission. Galanin administered into the lateral ventricle failed to affect acetylcholine release, while this route of administration produced a long-lasting reduction in 5-HT release in the ventral hippocampus, indicating that galanin is a potent inhibitor of mesencephalic 5-HT neurotransmission in vivo. Subsequent studies supported this hypothesis, showing that the effects on 5-HT release in vivo are most likely mediated by a galanin receptor in the dorsal raphe. The implications of these findings are discussed in relation to the role of acetylcholine in cognitive functions in the forebrain and the role of the raphe 5-HT neurons in affective disorders.

Galanin inhibits performance on rodent memory tasks

Central administration of galanin produces performance deficits on a variety of rodent learning and memory tasks. Galanin impairs acquisition and/or retention of the Morris water task, delayed nonmatching to position, T-maze delayed alternation, starburst radial maze, and passive avoidance in normal rats. A primary site of action is the ventral hippocampus, with an additional modulatory site in the medial septum-diagonal band. The behavioral actions of galanin at rat septohippocampal sites mediating cognitive processes are consistent with previous reports of inhibitory actions of galanin on acetylcholine release and cholinergically activated transduction at the M1 muscarinic receptor in rat hippocampus. The peptidergic galanin receptor antagonist M40 blocks the inhibitory actions of galanin on memory tasks. Treatment combinations of M40 with an M1 agonist, TZTP, improves performance on delayed nonmatching to position, in rats with 192IgG-saporin-induced cholinergic lesions of basal forebrain neurons. Nonpeptide, bioavailable, subtype-selective galanin receptor antagonists may provide tools to test the hypothesis that antagonism of endogenous galanin, which is overexpressed in the basal forebrain in Alzheimer's patients, can contribute to the alleviation of the cognitive deficits associated with Alzheimer's disease.

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.

Simultaneous determination of acetylcholine, choline and physostigmine in microdialysis samples from rat hippocampus by microbore liquid chromatography/electrochemistry on peroxidase redox polymer coated electrodes

Microbore column liquid chromatography with post-column immobilized enzyme reactor (IMER) and electrochemical detection on redox polymer coated electrodes was used for detection of acetylcholine (ACh) and choline (Ch) in microdialysis samples. The sensitivity of the coated electrodes, decreased gradually by about 10%/day, with highest reduction of 30% within the first 16 h of use. A number of choline derivatives were tested as possible internal standards, of those acetylethylhomocholine (AEHCh) and butyrylcholine (BCh) were found the most suitable candidates since they both provided high enzymatic conversion in the IMER. Physostigmine produced a negative peak, possibly reflecting oxidation of eseroline--a decarbamoylated product of reversible reaction of physostigmine with immobilized acetylcholine esterase. The probes, implanted in the ventral hippocampi of awake rats were perfused at a flow-rate of 1.25 microl/min with Ringer solution containing 10 microM physostigmine or with artificial cerebrospinal fluid only. The concentrations of ACh in 10-microl samples at basal conditions were between 0.9-2.5 nM, whereas in the presence of physostigmine the ACh levels raised to 41-48 nM. Physostigmine concentration was reduced to 8.8 microM, indicating its in vivo delivery of about 12%. The coefficients of variation were reduced from 7.4% for external standard method after every sixth sample to 5.8% and 5.9% for internal standardization with AEHCh and BCh, respectively. The latter method shortened the total analysis time by about 15%, thus being especially suitable for continuous long-lasting off-line or on-line monitoring. Additionally, other endogenous cholines such as butyrylcholine or synthetic choline derivatives could be detected by the present method.

Hippocampal dynorphin B injections impair spatial learning in rats: a kappa-opioid receptor-mediated effect

The hippocampus plays a central role in the acquisition and storage of information. Long-term potentiation in the mossy fibre pathway to the CA3 region in the hippocampus, an animal model of memory acquisition, is modulated by dynorphin peptides. This study investigated the possible role of hippocampal dynorphin in spatial learning. Male rats were trained in the Morris Water Task after microinjection with different doses of dynorphin B (1, 3.3 or 10 nmol/rat) or artificial cerebrospinal fluid (as control) into the CA3 region of the dorsal hippocampus. Dynorphin B was found to impair spatial learning at all tested doses. The synthetic kappa1-selective opiate receptor antagonist nor-binaltorphimine (2 nmol) also given into the hippocampus fully blocked the acquisition impairment caused by dynorphin B (10 nmol), while nor-binaltorphimine alone did not affect learning performance. These findings suggest that dynorphin peptides could play a modulatory role in hippocampal plasticity by acting on hippocampal kappa-receptors and thereby impair spatial learning.

Galaninergic signalling and adenylate cyclase

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

Intraventricular galanin modulates a 5-HT1A receptor-mediated behavioural response in the rat

The present studies have examined whether the neuropeptide galanin can modulate brain serotoninergic (5-HT) neurotransmission in vivo and, particularly, 5-HT1A receptor-mediated transmission. For that purpose, we studied the ability of galanin (given bilaterally into the lateral ventricle, i.c.v.) to modify the impairment of passive avoidance retention induced by the selective 5-HT1A agonist 8-hydroxy-2-(di-n-propyloamino)tetralin (8-OH-DPAT) when injected prior to training. This impairment appears to be mainly related to activation of 5-HT1A receptors in the CNS. Galanin dose-dependently (significant at 3.0 nmol/rat) attenuated the passive avoidance impairment (examined 24 h after training) induced by the 0.2 mg/kg dose of 8-OH-DPAT. This 8-OH-DPAT dose produced signs of the 5-HT syndrome indicating a postsynaptic 5-HT1A receptor activation. Furthermore, both the impairment of passive avoidance and the 5-HT syndrome were completely blocked by the 5-HT1A receptor antagonist WAY 100635 (0.1 mg/kg). Galanin (0.3 or 3.0 nmol) or WAY 100635 (0.1 mg/kg) failed by themselves to affect passive avoidance retention. 8-OH-DPAT given at a low dose 0.03 mg/kg, which presumably stimulates somatodendritic 5-HT1A autoreceptors in vivo, did not alter passive avoidance retention or induce any visually detectable signs of the 5-HT syndrome. Galanin (0.3 or 3.0 nmol) given i.c.v. in combination with the 0.03 mg/kg dose of 8-OH-DPAT, did not modify passive avoidance. The immunohistochemical study of the distribution of i.c.v. administered galanin (10 min after infusion) showed a strong diffuse labelling in the periventricular zone (100-200 microm) of the lateral ventricle. Furthermore, in the dorsal and ventral hippocampus galanin-immunoreactive nerve cells appeared both in the dentate gyrus and the CA1, CA2 and CA3 layers of the hippocampus. In the septum only endogenous fibres could be seen while in the caudal amygdala also galanin-immunoreactive nerve cells were visualized far away from the labelled periventricular zone. At the level of the dorsal raphe nucleus a thin periventricular zone of galanin immunoreactivity was seen but no labelling of cells. These results suggest that galanin can modulate postsynaptic 5-HT1A receptor transmission in vivo in discrete cell populations in forebrain regions such as the dorsal and ventral hippocampus and parts of the amygdala. The indication that galanin administered intracerebroventrically may be taken up in certain populations of nerve terminals in the periventricular zone for retrograde transport suggests that this peptide may also affect intracellular events.

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.

GALR1 galanin receptor mRNA is co-expressed by galanin neurons but not cholinergic neurons in the rat basal forebrain

The neuropeptide galanin (GAL) has been proposed to be an inhibitory modulator of cholinergic transmission in the hippocampus and may impair memory by directly affecting the activity of basal forebrain (BF) cholinergic neurons. Alternatively, GAL may act indirectly and modulate the activity of other neurotransmitter systems which, in turn, influence cholinergic transmission. We have used double in situ hybridization histochemistry to evaluate the co-expression of the GAL receptor subtype, GALR1, within cholinergic neurons in the medial septum/diagonal band of adult male rats. In alternate brain sections, we assessed the co-expression of GALR1 mRNA within another forebrain cell group implicated in memory functions, the neurons of the bed nucleus of the stria terminalis (BNST) and medial amygdala (AMe) which co-express vasopressin (VP) and GAL and project to septo-hippocampus. Despite the abundance of GALR1 mRNA-expressing neurons in the cholinergic BF, we found no evidence for the co-expression of this receptor subtype within cholinergic neurons in the medial septum/diagonal band. In contrast, we detected an extensive co-expression (95%) of GALR1 mRNA within extrahypothalamic VP/GAL neurons. These results do not support the idea that GAL, acting via the GALR1 receptor, directly impairs BF cholinergic neurons but suggest, instead, that non-cholinergic neurons in the BF may play a role in mediating the inhibitory actions of GAL on cholinergic function. However, our findings provide anatomical evidence that GAL could directly modulate the activity and/or secretion pattern of extrahypothalmic VP/GAL neurons into septo-hippocampal regions.

Differential effects of the neuropeptide galanin on striatal acetylcholine release in anaesthetized and awake rats

1. In the present study the mechanisms were examined by which the neuropeptide galanin modulates the extracellular concentrations of striatal acetylcholine (ACh) in enflurane anaesthetized and in freely moving male rats by use of in vivo microdialysis and high performance liquid chromatography. 2. The perfusion of galanin through the microdialysis probe (0.3 nmol microl(-1), flow rate: 2 microl min(-1)) caused a statistically significant increase in the basal striatal ACh levels in anaesthetized but a decrease in awake animals. No significant effect was revealed after a low dose (0.1 nmol microl(-1), flow rate: 2 microl min(-1)) of galanin perfusion. Both the stimulating and inhibitory effects of galanin on basal ACh release were reversible. 3. The muscarinic antagonist scopolamine (0.1 mg kg(-1), subcutaneously (s.c.)) caused a significant increase in ACh release in both anaesthetized and awake animals. 4. The combination of galanin plus scopolamine attenuated the stimulant effect on ACh release caused by scopolamine alone in awake animals. 5. The putative galanin receptor antagonist M35 at 0.3 nmol microl(-1) but not at 0.1 nmol microl(-1) caused a significant reduction (20%) in ACh release, supporting the view that M35 at higher concentrations behaves as a partial agonist at the galanin receptor. When M35 (0.1 nmol microl(-1)) was co-infused with galanin (0.3 nmol microl(-1)) the galanin-evoked decrease in ACh release was completely blocked. 6. Taken together, these results indicate that galanin affects basal ACh release via stimulation of galanin receptors within the striatum. The mechanism involved is dependent on the anaesthesia procedure which may act via enhancement of gamma-aminobutyric acidA (GABA(A)) mediated transmission within striatal and/or output neurones. In addition, anaesthesia may also decrease the activity of glutamatergic striatal afferents. The results with M35 indicate that the role of galanin perfused in striatum is permissive in the normal rat. Furthermore, galanin is a potent inhibitory modulator of basal ACh release also in the striatum, as recently was shown in the ventral hippocampus in awake animals.

Nociceptin/orphanin FQ microinjected into hippocampus impairs spatial learning in rats

The newly discovered peptide nociceptin/orphanin FQ has been found to increase reactivity to pain and to influence locomotor activity after intracerebroventricular administration. This study investigated the possible role of hippocampal nociceptin/orphanin FQ in spatial learning and in spontaneous locomotion. Male rats were trained in the Morris water task after microinjection of 10 nmol nociceptin/orphanin FQ or artificial cerebrospinal fluid (as control) into the CA3 region of the dorsal hippocampus. Nociceptin/orphanin FQ was found to severely impair spatial learning without interfering with swimming performance. Intrahippocampal injection of nociceptin/ orphanin FQ markedly decreased exploratory locomotor activity including vertical movements (rearing). The data suggest that nociceptin/orphanin FQ is a potent modulator of synaptic plasticity within the hippocampus.