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

Fronto-temporal galanin modulates impulse control

RATIONALE

The neuropeptide galanin has been implicated in a wide range of pathological conditions in which frontal and temporal structures are compromised. It works through three subtypes of G-protein-coupled receptors. One of these, the galanin receptor 1 (Gal-R1) subtype, is densely expressed in the ventral hippocampus (vHC) and ventral prefrontal cortex (vPFC); two brain structures that have similar actions on behavioral control. We hypothesize that Gal-R1 contributes to cognitive-control mechanisms that require hippocampal-prefrontal cortical circuitry.

OBJECTIVE

To examine the effect of local vHC and vPFC infusions of M617, a Gal-R1 agonist, on inhibitory mechanisms of response control.

METHODS

Different cohorts of rats were implanted with bilateral guide cannulae targeting the vPFC or the vHC. Following infusion of the Gal-R1 agonist, we examined the animals' behavior using a touchscreen version of the 5-choice reaction time task (5-choice task).

RESULTS

The Gal-R1 agonist produced opposing behaviors in the vPFC and vHC, leading to disruption of impulse control when infused in the vPFC but high impulse control when infused into the vHC. This contrast between areas was accentuated when we added variability to the timing of the stimulus, which led to long decision times and reduced accuracy in the vPFC group but a general improvement in performance accuracy in the vHC group.

CONCLUSIONS

These results provide the first evidence of a selective mechanism of Gal-R1-mediated modulation of impulse control in prefrontal-hippocampal circuitry.

Galanin and Cognition

Since the neuropeptide galanin’s discovery in 1983, information has accumulated that implicates it in a wide range of functions, including pain sensation, stress responses, appetite regulation, and learning and memory. This article reviews the evidence for specific functions of galanin in cognitive processes. Consistencies as well as gaps in the literature are organized around basic questions of methodology and theory. This review shows that although regularities are evident in the observed behavioral effects of galanin across several methods for measuring learning and memory, generalization from these findings is tempered with concerns about confounds and a restricted range of testing conditions. Furthermore, it is revealed that many noncognitive behavioral constructs that are relevant for assessing potential roles for galanin in cognition have not been thoroughly examined. The review concludes by laying out how future theory and experimental work can overcome these concerns and confidently define the nature of the association of galanin with particular cognitive constructs.

Galanin impairs cognitive abilities in rodents: relevance to Alzheimer's disease

The neuropeptide galanin and its receptors are localized in brain pathways that mediate learning and memory. Central microinjection of galanin impairs performance of a variety of cognitive tasks in rats. Transgenic mice overexpressing galanin display deficits in some learning and memory tests. The inhibitory role of galanin in cognitive processes, taken together with the fact that overexpression of galanin occurs in Alzheimer's disease, suggests that galanin antagonists may offer a novel therapeutic approach to treat memory loss in patients suffering from Alzheimer's.

Galanin impairs cognitive abilities in rodents: relevance to Alzheimer's disease

The neuropeptide galanin and its receptors are localized in brain pathways mediating learning and memory. Central microinjection of galanin impairs performance of a variety of cognitive tasks in rats. Transgenic mice overexpressing galanin display deficits in some learning and memory tests. The inhibitory role of galanin in cognitive processes, taken together with the overexpression of galanin in Alzheimer's disease, suggests that galanin antagonists may offer a novel therapeutic approach to treat memory loss in Alzheimer's patients.

Performance of galanin transgenic mice in the 5-choice serial reaction time attentional task

The neuropeptide galanin impairs learning and memory in rodents. The mechanism underlying the cognitive effects of galanin may be related to inhibitory effects of galanin on cholinergic transmission. As cholinergic function is thought to modulate sustained attention, the present study examined whether galanin-overexpressing transgenic mice have impairments in sustained attention. Galanin transgenic (GAL-tg) mice and wild-type (WT) littermate controls were trained in a 5-choice serial reaction time task, modified to assess sustained attention. GAL-tg and WT mice performed similarly during acquisition with respect to accuracy, total omissions, and response speed. Attentional mechanisms were challenged by parametric changes including increased event rate, event asynchrony, or decreased stimulus duration. Singly, these challenges did not differentially affect performance between genotypes. Concurrent administration of these challenges, which represents an optimal test of sustained attention, also had similar effects on GAL-tg and WT mice. When stimulus discriminability was reduced by constant illumination of the house light, GAL-tg mice omitted more trials than WT mice, but other measures of performance did not differ by genotype. Moreover, intraventricular injection of galanin in WT mice did not affect sustained attention. These data indicate that previously reported learning and memory effects of galanin are not secondary to attentional dysfunction.

Patterns of seizures, hippocampal injury and neurogenesis in three models of status epilepticus in galanin receptor type 1 (GalR1) knockout mice

The neuropeptide galanin exhibits anticonvulsant effects in experimental epilepsy. Two galanin receptor subtypes, GalR1 and GalR2, are present in the brain. We examined the role of GalR1 in seizures by studying the susceptibility of GalR1 knockout (KO) mice to status epilepticus (SE) and accompanying neuronal injury. SE was induced in GalR1 KO and wild type (WT) mice by Li-pilocarpine, 60 min electrical perforant path stimulation (PPS), or systemic kainic acid (KA). Seizures were analyzed using Harmonie software. Cell injury was examined by FluoroJade B- and terminal deoxynucleotidyl transferase-mediated uridine triphosphate nick end labeling; neurogenesis was studied using bromodeoxyuridine labeling. Compared with WT littermates, GalR1 KO showed more severe seizures, more profound injury to the CA1 pyramidal cell layer, as well as injury to hilar interneurons and dentate granule cells upon Li-pilocarpine administration. PPS led to more severe seizures in KO, as compared with WT mice. No difference in the extent of neuronal degeneration was observed between the mice of two genotypes in CA1 pyramidal cell layer; however, in contrast to WT, GalR1 KO developed mild injury to hilar interneurons on the side of PPS. KA-induced seizures did not differ between GalR1 KO and WT animals, and led to no injury to the hippocampus in either of experimental group. No differences were found between KO and WT mice in both basal and seizure-induced neuronal progenitor proliferation in all seizure types. Li-pilocarpine led to more extensive glia proliferation in GalR1 KO than in WT, and in both mouse types in two other SE models. In conclusion, GalR1 mediate galanin protection from seizures and seizure-induced hippocampal injury in Li-pilocarpine and PPS models of limbic SE, but not under conditions of KA-induced seizures. The results justify the development and use of GalR1 agonists in the treatment of certain forms of epilepsy.

Galanin and galanin receptors in epilepsy

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

Selective reduction in ventral hippocampal acetylcholine release in awake galanin-treated rats and galanin-overexpressing transgenic mice

The neuropeptide galanin is an inhibitory modulator of hippocampal acetylcholine (ACh) release and cognitive functions. Anatomical evidence demonstrated some differences between the dorsal and ventral hippocampi notably in the expression of galanin receptor subtypes, and the neuronal population on which galanin-like immunoreactivity is expressed. This is suggestive of a differential role for this peptide in these two areas of the hippocampal formation. Using in vivo microdialysis, we investigated the role of galanin on ACh release in the dorsal and ventral hippocampi. Two models were studied: galanin-administered rats and transgenic mice over-expressing galanin (GAL-tg). In rats, galanin (2.0 and 10.0 microM) infused locally through the dialysis probe induced a significant decrease in ACh release in the ventral hippocampus, confirming previous findings, while no effect was seen in the dorsal hippocampus. Using the no net flux method, a significant reduction in ACh levels was noted only in the ventral hippocampus of GAL-tg compared to wild-type littermates. These results suggest that excess endogenous galanin can suppress basal ACh release, with anatomical specificity, to the ventral hippocampus. These results are of interest in the context of galanin receptor subtypes in the dorsal and ventral hippocampus, and the differential alterations of hippocampal subregions in neurological diseases such as Alzheimer's dementia.

GABAergic septohippocampal neurons are not necessary for spatial memory

The medial septum/vertical limb of the diagonal band of Broca (MSDB) provides a major input to the hippocampus and is important for spatial memory. Both cholinergic and GABAergic MSDB neurons project to the hippocampus, and nonselective lesions of the MSDB or transections of the septohippocampal pathway impair spatial memory. However, selective lesions of cholinergic MSDB neurons using 192-IgG saporin (SAP) do not impair or only mildly impair spatial memory. Previously, intraseptal kainic acid was found to reduce levels of glutamic acid decarboxylase, a marker of GABAergic neurons, but not to alter the levels of choline acetyltransferase, a marker of cholinergic neurons. The present study further characterized the effects of kainic acid on GABAergic MSDB neurons and examined the effects of intraseptal kainic acid on spatial memory. Saline, kainic acid, SAP, or the combination of kainic acid and SAP was administered into the MSDB of rats. Spatial memory was assessed in an eight-arm radial maze and a water maze. Kainic acid destroyed GABAergic septohippocampal neurons, but spared cholinergic neurons. SAP eliminated MSDB cholinergic neurons, sparing noncholinergic neurons. Coadministration of kainic acid and SAP destroyed GABAergic and cholinergic MSDB neurons. Acquisition of the radial maze task and performance on this task with 4-h delays were unimpaired by intraseptal kainic acid or SAP, but were impaired by coadministration of kainic acid and SAP. Acquisition of the water maze task was unaffected by intraseptal kainic acid, delayed slightly by SAP, and impaired severely by coadministration of kainic acid and SAP. These results provide evidence that kainic acid at appropriate concentrations effectively destroys GABAergic septohippocampal neurons, while sparing cholinergic MSDB neurons. Furthermore, lesions of the GABAergic septohippocampal neurons do not impair spatial memory. While lesions of cholinergic MSDB neurons may mildly impair spatial memory, the combined lesion of GABAergic and cholinergic septohippocampal neurons resulted in a memory impairment that was greater than that observed after a selective lesion to either population. Thus, damage of GABAergic or cholinergic MSDB neurons, which together comprise the majority of the septohippocampal pathway, cannot totally account for the spatial memory impairment that is observed after nonselective lesions of the MSDB.

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.

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.

Galanin antagonizes vasopressin-stimulated flank marking in male golden hamsters

Microinjection of vasopressin (VP) into the anterior hypothalamus (AH) of golden hamsters induces a rapid bout of flank marking, a stereotyped scent marking behavior used for olfactory communication. In rats, VP is colocalized with galanin (GAL) in several brain regions. GAL has been shown to antagonize the postsynaptic actions of other cosecreted neurotransmitters including acetylcholine and norepinephrine; however, the ability of GAL to modulate the postsynaptic actions of VP has not been assessed. Here, we report that coadministration of GAL can block VP-induced flank marking in golden hamsters in a dose dependent manner. These findings provide the first evidence in any species that GAL can antagonize the central actions of VP. Using slice binding and receptor autoradiography, we have identified GAL binding sites in the AH and two other regions implicated in flank marking behavior (the lateral septum and central grey). These findings raise the possibility that endogenous GAL may function as an inhibitory modulator of this stereotypic scent marking behavior.

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.

Coadministration of galanin antagonist M40 with a muscarinic M1 agonist improves delayed nonmatching to position choice accuracy in rats with cholinergic lesions

The neuropeptide galanin is overexpressed in the basal forebrain in Alzheimer's disease (AD). In rats, galanin inhibits evoked hippocampal acetylcholine release and impairs performance on several memory tasks, including delayed nonmatching to position (DNMTP). Galanin(1-13)-Pro2-(Ala-Leu)2-Ala-NH2 (M40), a peptidergic galanin receptor ligand, has been shown to block galanin-induced impairment on DNMTP in rats. M40 injected alone, however, does not improve DNMTP choice accuracy deficits in rats with selective cholinergic immunotoxic lesions of the basal forebrain. The present experiments used a strategy of combining M40 with an M1 cholinergic agonist in rats lesioned with the cholinergic immunotoxin 192IgG-saporin. Coadministration of intraventricular M40 with intraperitoneal 3-(3-S-n-pentyl-1,2,5-thiadiazol-4-yl)-1,2,5, 6-tetrahydro-1-methylpyridine (TZTP), an M1 agonist, improved choice accuracy significantly more than a threshold dose of TZTP alone. These results suggest that a galanin antagonist may enhance the efficacy of cholinergic treatments for the cognitive deficits of AD.

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.

Galparan induces in vivo acetylcholine release in the frontal cortex

The chimeric peptide galparan (galanin(1-13)-mastoparan) induced the in vivo release of acetylcholine in the frontal cortex of rats when injected intracerebroventricularly, i.c.v. The ACh-releasing effects of galparan are reversible, dose-dependent, and not exerted at galanin receptors or at sites where mastoparan acts. Pertussis toxin pretreatment (i.c.v.) of the rats for 96 h prior to injection of galparan or of mastoparan completely prevented the ACh-releasing effects of both galparan and mastoparan. It appears that galparan acts at a novel site in the release of ACh in the cerebral cortex in vivo.

Effects of galanin on 8-OH-DPAT induced decrease in body temperature and brain 5-hydroxytryptamine metabolism in the mouse

Central administration of galanin dose-dependently (minimum effective dose, M.E.D. = 1 nmol) blocked the hypothermia induced by the 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT, 0.5 mg/kg s.c.), in mice. This inhibitory effect was reversed by pretreatment with the galanin receptor antagonist galantide (0.3 nmol) and also by pretreatment with the ATP-sensitive potassium channel blockers glibenclamide (10 nmol) and gliquidone (10 nmol). The hypothermic response to 8-OH-DPAT was also blocked by the 5-HT1A receptor antagonist (N-(2,4(2-methoxyphenyl)-1-piperazinyl)ethyl-N-(2-pyridinyl)cyclohexane, (WAY 100,635, M.E.D. = 0.01 mg/kg s.c.), and the centrally acting muscarinic receptor antagonist scopolamine (M.E.D. = 10 mg/kg i.p.) but not the peripheral muscarinic receptor antagonist N-methylscopolamine. 8-OH-DPAT (0.5 mg/kg s.c.) also decreased cortical and hypothalamic 5-HT (5-hydroxytryptamine, serotonin) metabolism, an effect which was not blocked by pretreatment with galanin (0.3-3 nmol intracerebroventricular, i.c.v.). Neither did galanin (0.03-3 nmol/5 microliters i.c.v.) affect basal 5-HT metabolism in these brain regions. Furthermore, pretreatment in vitro of mouse cortical membranes with galanin (10 or 1000 nM) had no effect on 5-HT1A receptor affinity, Bmax or pharmacology determined using [3H]8-OH-DPAT. These results suggest that the inhibition of 8-OH-DPAT induced hypothermia by galanin is probably not mediated by an interaction with 5-HT1A receptors but more likely by blocking the indirect activation by 8-OH-DPAT of central cholinergic pathways involved in temperature regulation.

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.

Side effects of physostigmine as a pretreatment in guinea pigs

To prevent incapacitation following nerve agent intoxications, it is proposed to replace pyridostigmine by the centrally active carbamate physostigmine (PHY). Behavioral and neurophysiological effects of PHY were determined and whether these effects would be counteracted by scopolamine. In addition, we compared them with the effects of another reversible cholinesterase (ChE) inhibitor ethyl-p-nitrophenylphosphoramidate (PNF) At similar levels of blood AChE inhibition, PHY caused a larger shuttlebox performance decrement than PNF, which was antagonized by scopolamine (0.1 mg/kg). SCO enhanced the PHY-induced increase of the auditory startle response, whereas PNF, with or without scopolamine, had no effect. In the EEG, PHY led to a power increase at the theta 2-alpha 1 band, also found after PNF, and at the theta 1 band. SCO antagonized all EEG effects, but not the effects of PHY on visual evoked responses, in contrast to those of PNF. Based on the different effects of both inhibitors, it is suggested that at relevant doses several PHY-induced phenomena occur that are unrelated to AChE inhibition.

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.

Binding and agonist/antagonist actions of M35, galanin(1-13)-bradykinin(2-9)amide chimeric peptide, in Rin m 5F insulinoma cells

The chimeric peptide M35 [galanin(1-13)-bradykinin(2-9) amide] is a high-affinity galanin receptor ligand acting as a galanin receptor antagonist in the rat spinal cord, rat hippocampus and isolated mouse pancreatic islets. We have radiolabelled M35 and performed equilibrium binding studies with [125I]M35 on the rat pancreatic beta-cell line Rin m 5F, whereby we show the existence of high-affinity binding site (KD = 0.9 +/- 0.1 nM) with a Bmax of 72 +/- 3 fmol/mg protein. Galanin displaces [125I]M35 with the same affinity (KD = 1 nM) as it displaces [125I]galanin. Displacement of [125I]galanin by M35 from Rin m 5F cell membranes shows the presence of two binding sites for M35 with KD-values of 0.3 +/- 0.1 nM and 0.52 +/- 0.03 microM, respectively. The GTP- and pertussis toxin-sensitivity of M35 binding to Rin m 5F membranes shows that binding of [125I]M35 is almost completely abolished by the presence of GTP or after pertussis toxin treatment of the cells, indicating an agonist-like binding of M35 to the galanin receptors. M35 has a dual effect on the galanin mediated inhibition of forskolin stimulated cyclic AMP production in Rin m 5F cells: at low concentrations M35 antagonises the effect of galanin, whereas at concentrations above 10 nM M35 acts as a galanin receptor agonist. These agonist-like effects of galanin and M35 are not additive, thus the mixed agonist/antagonist properties arise from the chimeric nature of M35[galanin(1-13)-bradykinin(2-9)amide] acting solely at galanin receptors.

Galanin--10 years with a neuroendocrine peptide

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

Galanin-like immunoreactivity is increased in the postmortem cerebral cortex from patients with Alzheimer's disease

Galanin is a peptide that is associated with cholinergic neurons of the basal forebrain, and, thus, of interest for the neuropathology of Alzheimer's disease. In the present study, human galanin-like immunoreactivity was measured in postmortem human cerebral cortical tissues by using a homologous radioimmunoassay. In an initial study, six cerebral cortical regions were evaluated from nine elderly controls, 13 neuropathologically verified Alzheimer's disease patients, and 19 elderly schizophrenics. A significant 65% increase in galanin was found in frontal cortex Brodmann area 8 of Alzheimer's disease patients compared with controls. In contrast, cerebral cortical tissues from elderly schizophrenics were not different from those from elderly controls in any region. In a second study, 10 cerebral cortical regions were evaluated from 50 neuropathologically verified Alzheimer's disease patients and nine elderly controls. Concentrations of galanin were increased significantly 26-61% in six of 10 cerebral cortical regions examined (Brodmann areas F8, F44, T20, T21, T36, and P22). Purification of brain extracts by size-exclusion Sephadex G-50 chromatography revealed that human galanin-like immunoreactivity eluted in two peaks of different molecular weights. These studies reveal increased concentrations of galanin in the cerebral cortex of Alzheimer's disease, similar to previous findings in basal forebrain tissue. Because galanin inhibits cholinergic neurotransmission, these findings may have important implications in the understanding of Alzheimer's disease neuropathology and associated cognitive deficits.

The effect of neuropeptides on the release of neurotransmitter amino acids from rat striatum

Neuropeptide Y (NPY), peptide YY (PYY), and galanin are found throughout the central nervous system with appreciable levels occurring in the striatum. In this study we have investigated the effects of these peptides on the potassium-stimulated release of endogenous neurotransmitter amino acids from slices of rat striatum. The release of glutamate was significantly reduced by nanomolar concentrations of each peptide, whilst the release of aspartate, gamma-aminobutyric acid (GABA) and glycine was not affected. The reduction in release due to galanin (200 nM) was inhibited by glibenclamide (5 microM). These results support the view that NPY, PYY and galanin modulate neurotransmitter release possibly by a presynaptic action. The results with glibenclamide suggest that the action of galanin is mediated through an ATP-sensitive potassium channel.

Galanin-receptor ligand M40 peptide distinguishes between putative galanin-receptor subtypes

The galanin-receptor ligand M40 [galanin-(1-12)-Pro3-(Ala-Leu)2-Ala amide] binds with high affinity to [mono[125I]iodo-Tyr26]galanin-binding sites in hippocampal, hypothalamic, and spinal cord membranes and in membranes from Rin m5F rat insulinoma cells (IC50 = 3-15 nM). Receptor autoradiographic studies show that M40 (1 microM) displaces [mono[125I]iodo-Tyr26]galanin from binding sites in the hippocampus, hypothalamus, and spinal cord. In the brain, M40 acts as a potent galanin-receptor antagonist: M40, in doses comparable to that of galanin, antagonizes the stimulatory effects of galanin on feeding, and it blocks the galaninergic inhibition of the scopolamine-induced acetylcholine release in the ventral hippocampus in vivo. In contrast, M40 completely fails to antagonize both the galanin-mediated inhibition of the glucose-induced insulin release in isolated mouse pancreatic islets and the inhibitory effects of galanin on the forskolin-stimulated accumulation of 3',5'-cAMP in Rin m5F cells; instead M40 is a weak agonist at the galanin receptors in these two systems. M40 acts as a weak antagonist of galanin in the spinal flexor reflex model. These results suggest that at least two subtypes of the galanin receptor may exist. Hypothalamic and hippocampal galanin receptors represent a putative central galanin-receptor subtype (GL-1-receptor) that is blocked by M40. The pancreatic galanin receptor may represent another subtype (GL-2-receptor) that recognizes M40, but as a weak agonist. The galanin receptors in the spinal cord occupy an intermediate position between these two putative subtypes.

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.

Galanin and galanin antagonists: molecular and biochemical perspectives

The neuropeptide galanin potently inhibits insulin release, hippocampal acetylcholine release and firing of locus coeruleus cells, and stimulates feeding and release of growth hormone. Galanin regulates K+ channels, adenylyl cyclase and phospholipase C by acting at Gi/Go protein-coupled high-affinity receptors. Galanin receptor agonists such as the N-terminal fragment galanin1-16 act synergistically with morphine in the somatosensory system and have potential analgetic application. Galanin antagonists may be useful therapeutic agents in endocrinology, neurology and psychiatry. The enhancing effect of such agents on hippocampal cholinergic function would be useful in treatment of Alzheimer's disease. Recent synthesis of a series of high-affinity galanin antagonists, reviewed, along with galanin's actions, by Tamas Bartfai and colleagues, opens the possibility of examining the functions of endogenous galanin and test the pharmacological usefulness of antagonism of galanin function in the endocrine, somatosensory and central nervous systems.