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

Honey Bee Allatostatins Target Galanin/Somatostatin-Like Receptors and Modulate Learning: A Conserved Function?

Sequencing of the honeybee genome revealed many neuropeptides and putative neuropeptide receptors, yet functional characterization of these peptidic systems is scarce. In this study, we focus on allatostatins, which were first identified as inhibitors of juvenile hormone synthesis, but whose role in the adult honey bee (Apis mellifera) brain remains to be determined. We characterize the bee allatostatin system, represented by two families: allatostatin A (Apime-ASTA) and its receptor (Apime-ASTA-R); and C-type allatostatins (Apime-ASTC and Apime-ASTCC) and their common receptor (Apime-ASTC-R). Apime-ASTA-R and Apime-ASTC-R are the receptors in bees most closely related to vertebrate galanin and somatostatin receptors, respectively. We examine the functional properties of the two honeybee receptors and show that they are transcriptionally expressed in the adult brain, including in brain centers known to be important for learning and memory processes. Thus we investigated the effects of exogenously applied allatostatins on appetitive olfactory learning in the bee. Our results show that allatostatins modulate learning in this insect, and provide important insights into the evolution of somatostatin/allatostatin signaling.

Hypothalamic peptides controlling alcohol intake: differential effects on microstructure of drinking bouts

Different alcohol drinking patterns, involving either small and frequent drinking bouts or large and long-lasting bouts, are found to differentially affect the risk for developing alcohol-related diseases, suggesting that they have different underlying mechanisms. Such mechanisms may involve orexigenic peptides known to stimulate alcohol intake through their actions in the hypothalamic paraventricular nucleus (PVN). These include orexin (OX), which is expressed in the perifornical lateral hypothalamus, and galanin (GAL) and enkephalin (ENK), which are expressed within as well as outside the PVN. To investigate the possibility that these peptides affect different aspects of consumption, a microstructural analysis of ethanol drinking behavior was performed in male, Sprague-Dawley rats trained to drink 7% ethanol and implanted with guide shafts aimed at the PVN. While housed in specialized cages containing computerized intake monitors (BioDAQ Laboratory Intake Monitoring System, Research Diets Inc., New Brunswick, NJ) that measure bouts of ethanol drinking, these rats were given PVN injections of OX (0.9 nmol), GAL (1.0 nmol), or the ENK analog D-Ala2-met-enkephalinamide (DALA) (14.2 nmol), as compared to saline vehicle. Results revealed clear differences between the effects of these peptides. While all 3 stimulated ethanol intake, they had distinct effects on patterns of drinking, with OX increasing the number of drinking bouts, GAL increasing the size of the drinking bouts, and DALA increasing both the size and duration of the bouts. In contrast, these peptides had little impact on water or food intake. These results support the idea that different peptides can increase ethanol consumption by promoting distinct aspects of the ethanol drinking response. The stimulatory effect of OX on drinking frequency may be related to its neuronally stimulatory properties, while the stimulatory effect of GAL and ENK on bout size and duration may reflect a suppressive effect of these neuronally inhibitory peptides on the satiety-controlling PVN.

Cellular, molecular, and genetic substrates underlying the impact of nicotine on learning

Addiction is a chronic disorder marked by long-lasting maladaptive changes in behavior and in reward system function. However, the factors that contribute to the behavioral and biological changes that occur with addiction are complex and go beyond reward. Addiction involves changes in cognitive control and the development of disruptive drug-stimuli associations that can drive behavior. A reason for the strong influence drugs of abuse can exert on cognition may be the striking overlap between the neurobiological substrates of addiction and of learning and memory, especially areas involved in declarative memory. Declarative memories are critically involved in the formation of autobiographical memories, and the ability of drugs of abuse to alter these memories could be particularly detrimental. A key structure in this memory system is the hippocampus, which is critically involved in binding multimodal stimuli together to form complex long-term memories. While all drugs of abuse can alter hippocampal function, this review focuses on nicotine. Addiction to tobacco products is insidious, with the majority of smokers wanting to quit; yet the majority of those that attempt to quit fail. Nicotine addiction is associated with the presence of drug-context and drug-cue associations that trigger drug seeking behavior and altered cognition during periods of abstinence, which contributes to relapse. This suggests that understanding the effects of nicotine on learning and memory will advance understanding and potentially facilitate treating nicotine addiction. The following sections examine: (1) how the effects of nicotine on hippocampus-dependent learning change as nicotine administration transitions from acute to chronic and then to withdrawal from chronic treatment and the potential impact of these changes on addiction, (2) how nicotine usurps the cellular mechanisms of synaptic plasticity, (3) the physiological changes in the hippocampus that may contribute to nicotine withdrawal deficits in learning, and (4) the role of genetics and developmental stage (i.e., adolescence) in these effects.

Central nervous system regulation of food intake and energy expenditure: role of galanin-mediated feeding behavior

Galanin is a neuropeptide widely expressed in the brain. It is implicated in energy expenditure, feeding, and the regulation of body weight. Numerous studies have revealed that galanin regulates food intake via galanin receptors, 5-HT(1A) receptor and adrenergic α-2 receptor. In this review, we summarize recent findings that reveal the essential role of galanin in increasing food intake as well as body weight and that identify the individual galanin receptor subtypes involved in the brain's modulation of food intake and energy expenditure, to provide a theoretical basis for further studies of different aspects of galanin action.

Genome-wide association study of major depressive disorder: new results, meta-analysis, and lessons learned

Major depressive disorder (MDD) is a common complex disorder with a partly genetic etiology. We conducted a genome-wide association study of the MDD2000+ sample (2431 cases, 3673 screened controls and >1 M imputed single-nucleotide polymorphisms (SNPs)). No SNPs achieved genome-wide significance either in the MDD2000+ study, or in meta-analysis with two other studies totaling 5763 cases and 6901 controls. These results imply that common variants of intermediate or large effect do not have main effects in the genetic architecture of MDD. Suggestive but notable results were (a) gene-based tests suggesting roles for adenylate cyclase 3 (ADCY3, 2p23.3) and galanin (GAL, 11q13.3); published functional evidence relates both of these to MDD and serotonergic signaling; (b) support for the bipolar disorder risk variant SNP rs1006737 in CACNA1C (P=0.020, odds ratio=1.10); and (c) lack of support for rs2251219, a SNP identified in a meta-analysis of affective disorder studies (P=0.51). We estimate that sample sizes 1.8- to 2.4-fold greater are needed for association studies of MDD compared with those for schizophrenia to detect variants that explain the same proportion of total variance in liability. Larger study cohorts characterized for genetic and environmental risk factors accumulated prospectively are likely to be needed to dissect more fully the etiology of MDD.

Dopamine-galanin receptor heteromers modulate cholinergic neurotransmission in the rat ventral hippocampus

Previous studies have shown that dopamine and galanin modulate cholinergic transmission in the hippocampus, but little is known about the mechanisms involved and their possible interactions. By using resonance energy transfer techniques in transfected mammalian cells, we demonstrated the existence of heteromers between the dopamine D(1)-like receptors (D(1) and D(5)) and galanin Gal(1), but not Gal(2) receptors. Within the D(1)-Gal(1) and D(5)-Gal(1) receptor heteromers, dopamine receptor activation potentiated and dopamine receptor blockade counteracted MAPK activation induced by stimulation of Gal(1) receptors, whereas Gal(1) receptor activation or blockade did not modify D(1)-like receptor-mediated MAPK activation. Ability of a D(1)-like receptor antagonist to block galanin-induced MAPK activation (cross-antagonism) was used as a "biochemical fingerprint" of D(1)-like-Gal(1) receptor heteromers, allowing their identification in the rat ventral hippocampus. The functional role of D(1)-like-Gal receptor heteromers was demonstrated in synaptosomes from rat ventral hippocampus, where galanin facilitated acetylcholine release, but only with costimulation of D(1)-like receptors. Electrophysiological experiments in rat ventral hippocampal slices showed that these receptor interactions modulate hippocampal synaptic transmission. Thus, a D(1)-like receptor agonist that was ineffective when administered alone turned an inhibitory effect of galanin into an excitatory effect, an interaction that required cholinergic neurotransmission. Altogether, our results strongly suggest that D(1)-like-Gal(1) receptor heteromers act as processors that integrate signals of two different neurotransmitters, dopamine and galanin, to modulate hippocampal cholinergic neurotransmission.

Galanin receptor 2 overexpressing mice display an antidepressive-like phenotype: possible involvement of the subiculum

The behavioral phenotype of a transgenic mouse overexpressing a galanin receptor 2 (GalR2)-enhanced, green fluorescent protein (EGFP)-construct under the platelet-derived growth factor-B promoter, and of controls, was assessed in various behavioral tests, such as the Porsolt forced swim test, as well as the open field, elevated plus maze and passive avoidance tests. In addition, the distribution of GalR2-EGFP expressing cell bodies and processes was studied in the brain of these mice using histochemical methods. Three age groups of the transgenic mice demonstrated decreased levels of immobility in the forced swim test, indicative of antidepressive-like behavior and/or increased stress resistance. Anxiety-like behaviors, measured in two different tests, did not differ between the GalR2-overexpressing and the wild-type mice, nor did motor activity levels, emotional learning or memory behaviors. High levels of GalR2 mRNA and protein expression were observed in the presubiculum, subiculum, cingulate cortex, retrosplenial granular and agranular cortices, subregions of prefrontal cortex, and the olfactory bulb, regions which are directly or indirectly implicated in depression-like behavior. These results may contribute to the understanding of the pathophysiology of major depressive disorder and the role of GalR2 in the regulation of mood, and suggest a potential therapeutic effect by targeting the GalR2 for treatment of depressive disorders.

Galanin and consummatory behavior: special relationship with dietary fat, alcohol and circulating lipids

Galanin (GAL) plays an integral role in consummatory behavior. In particular, hypothalamic GAL has a positive, reciprocal relationship with dietary fat and alcohol. In this relationship, GAL increases the consumption of fat or alcohol which, in turn, stimulates the expression of GAL, ultimately leading to overconsumption. Through actions in the amygdala, this relationship may become especially important in stress-induced food or drug intake. These effects of GAL in promoting overconsumption may involve various neurotransmitters, with GAL facilitating intake by stimulating norepinephrine and dopamine and reducing satiety by decreasing serotonin and acetylcholine. In addition, GAL in the hypothalamus stimulates the opioid, enkephalin, throughout the brain, which also promotes overconsumption. The relationship between GAL, fat, and alcohol may involve triglycerides, circulating lipids that are released by fat or alcohol and that correlate positively with hypothalamic GAL expression. In females, levels of endogenous GAL also fluctuate across the reproductive cycle, driven by a rise in the ovarian steroids, estrogen, and progesterone. They peak during the proestrous phase and also at puberty, simultaneous to a sharp increase in preference for fat to meet energy demands. Prenatal exposure to a high-fat diet also enhances hypothalamic expression of GAL into adulthood because of an increase in neurogenesis and proliferation of GAL-expressing neurons in this region. This organizational change may reflect the role of GAL in neuronal development, including neurite growth in adulthood, cell survival in aging, and cell stability in the disease state. By responding positively to fat and alcohol and guiding further neuronal development, GAL potentiates a long-term propensity to overconsume fat and alcohol.

Subarachnoid transplantation of immortalized galanin-overexpressing astrocytes attenuates chronic neuropathic pain

Treatment of chronic neuropathic pain resulted from peripheral nerve injury is one of the most difficult problems in modern clinical practice. The use of cell lines as biologic "minipumps" to chronically deliver anti-nociceptive molecules into the pain-processing centers of spinal cord is a newly developing technique for the treatment of pain. Moreover, spinal administration of exogenous galanin (GAL) is a useful target for the treatment of chronic pain after nerve injury. Because of better histocompatibility, lower immunogenicity and reproducibility, immortalized astrocytes (IAST) have been served as a promising cellular vehicle to deliver therapeutic molecules into CNS. In this study, the rat IAST was transfected with rat preprogalanin cDNA and the galanin-synthesizing and secreting cell line, IAST/GAL, was isolated. After cells were transplanted into the subarachnoid space of rats with chronic neuropathic pain induced by spared nerve injury (SNI) of sciatic nerve, their analgesic potential was evaluated by behavioral tests. The results showed that IAST/GAL transfected with preprogalanin gene could express and secrete significantly higher level of GAL protein in vitro and in vivo as compared with control cells. In addition, the pain-related behaviors, thermal hyperalgesia and mechanical allodynia were significantly alleviated during the 1-7 weeks after grafts of IAST/GAL cells, which could be reversed by galanin receptor antagonist M35 temporarily. Taken together, these data suggest that subarachnoid transplant of immortalized galanin-overexpressing astrocytes near the pain-processing centers was able to reverse the development of chronic neuropathic pain, which offers an adjunct approach to currently used therapies for the pain management.

Neuropeptides in learning and memory processes with focus on galanin

Neuropeptides represent by far the most common signalling molecules in the central nervous system. They are involved in a wide range of physiological functions and can act as neurotransmitters, neuromodulators or hormones in the central nervous system and in the periphery. Accumulating evidence during the past 40 years has implicated a number of neuropeptides in various cognitive functions including learning and memory. A major focus has been on the possibility that neuropeptides, by coexisting with classical neurotransmitters, can modulate classical transmitter function of importance for cognition. It has become increasingly clear that most transmitter systems in the brain can release a cocktail of signalling molecules including classical transmitters and several neuropeptides. However, the neuropeptides seem to come into action mainly under conditions of severe stress or aversive events, which have linked their action also to regulation of affective components of behaviour. This paper summarises some of the results of three neuropeptides, which can impact on hippocampal cognition by intrinsic (dynorphins, nociceptin) or extrinsic (galanin) modulation. The results obtained with these neuropeptides in rodent studies indicate that they are important for various aspects of hippocampal learning and memory as well as hippocampal plasticity. Recent studies in humans have also shown that dysregulation of these neuropeptides may be of importance for both neurodegenerative and neuropsychiatric disorders associated with cognitive impairments. It is concluded that compounds acting on neuropeptide receptor subtypes will represent novel targets for a number of disorders, which involve cognitive deficiencies.

Impairment of memory consolidation by galanin correlates with in vivo inhibition of both LTP and CREB phosphorylation

Changes in the state of CREB phosphorylation and in LTP in the hippocampus have been associated with learning and memory. Here we show that galanin, the neuropeptide released in the hippocampal formation from cholinergic and noradrenergic fibers, that has been shown to produce impairments in memory consolidation in the Morris water maze task inhibits both LTP and CREB phosphorylation in the rat hippocampus in vivo. While there are many transmitters regulating CREB phosphorylation none has been shown to suppress behaviorally-induced hippocampal CREB phosphorylation as potently as galanin. The in vivo inhibition of dentate gyrus-LTP and of CREB phosphorylation by the agonist occupancy of GalR1 and GalR2-type galanin receptors provides strong in vivo cellular and molecular correlates to galanin-induced learning deficits and designates galanin as a major regulator of the memory consolidation process.

Effects of galanin on cocaine-mediated conditioned place preference and ERK signaling in mice

RATIONALE

The neuropeptide galanin and its receptors are expressed in brain regions implicated in the rewarding effects of natural stimuli and drugs of abuse. Galanin has been shown to attenuate neurochemical, physiological, and behavioral signs of opiate and amphetamine reinforcement.

OBJECTIVE

In the current study, we present evidence that galanin modulates neurochemical and behavioral correlates of cocaine response.

METHODS

Mice lacking the neuropeptide galanin (Gal -/-) and wild-type (Gal +/+) controls were used to analyze the effects of galanin in an unbiased conditioned place preference paradigm. We then examined cocaine-induced activation of extracellular signal-regulated kinase (ERK) activity as a marker of intracellular signaling in the mesolimbic dopaminergic pathway induced by acute cocaine administration

RESULTS

Gal -/- mice showed significantly greater conditioned place preference at a threshold dose of cocaine (3 mg/kg) than Gal +/+ mice, and this was reversed by administration of the galanin receptor agonist galnon. Consistent with the results of behavioral experiments, there was a significant increase in ERK activation in the ventral tegmental area (VTA) and nucleus accumbens (NAc) of Gal -/- mice but not Gal +/+ mice following acute, systemic cocaine injection at the threshold dose. In the NAc, but not VTA, this effect was reversed by administration of galnon.

CONCLUSIONS

These data, coupled with previous studies on the effects of morphine and amphetamine, demonstrate that galanin normally attenuates drug reinforcement, potentially via modulation of the mesolimbic dopamine system.

Serotonergic regulation of galanin-induced selective macronutrient intake in self-selecting rats

We investigated macronutrient intake after intraventricular injection of galanin (GAL, 3 nmol/5 microl/rat) and/or serotonin (5-HT, 50 nmol/5 microl/rat) in self-selecting fasted rats with preferences for either carbohydrates or fats. GAL injection significantly increased carbohydrate and total intake in all rats irrespective of macronutrient preference, whereas 5-HT alone did not affect macronutrient intake. The GAL-induced increase in total intake decreased to the level of saline controls when GAL was coinjected with 5-HT. The ratio of kilocalories of carbohydrates, fats and proteins (macronutrient energy ratio) after injections of GAL and/or 5-HT was similar to the saline control. In carbohydrate-preferring rats, GAL increased carbohydrate, protein and fat intake as well as total intake. Coinjection of GAL and 5-HT tended to decrease carbohydrate intake, but increase protein and fat intake. The macronutrient energy ratio after injection of GAL did not change, but the carbohydrate energy ratio decreased after 5-HT was injected, with or without GAL. In contrast, in fat-preferring rats, GAL significantly increased carbohydrate intake. Injection of 5-HT with or without GAL did not change total macronutrient intake. The macronutrient energy ratio did not change after GAL injection with or without 5-HT. These differences suggest that macronutrient preferences should be considered in any macronutrient intake study, and that serotonergic neurons play a part in the regulation of GAL-induced macronutrient intake.

Postendocytotic traffic of the galanin R1 receptor: a lysosomal signal motif on the cytoplasmic terminus

The neuropeptide galanin R1 receptor (GalR1) was tagged at its C terminus with EGFP (GalR1-EGFP) to study receptor localization and trafficking. In PC12 and HEK293 cells, functional GalR1-EGFP was expressed on the plasma membrane and internalized into cytoplasmic vesicles after galanin stimulation. The internalization was blocked by 0.4 M sucrose and by silencing of clathrin with siRNA methodology. Internalized GalR1-EGFP and LysoTracker, a lysosomal marker, overlapped in intracellular vesicles after prolonged galanin stimulation. This colocalization was strongly reduced after site-directed mutagenesis of the motif YXXØ on the C terminus of GalR1 (where Ø is a bulky hydrophobic residue and X any amino acid). Taken together, these data suggest that GalR1 is internalized via the clathrin-dependent, endocytic pathway and then, to a large extent, delivered to lysosomes for degradation through the lysosome-targeting signal YXXØ.

The neuropeptide galanin as an in vivo modulator of brain 5-HT1A receptors: Possible relevance for affective disorders

The neuropeptide galanin is widely distributed throughout the central nervous system with multiple and diverse biological functions mediated by different receptor subtypes. In the rat, galanin-like immunoreactivity is expressed in a population of 5-hydroxytryptamine (5-HT, serotonin) neurons in the dorsal raphe with extensive projections to the forebrain areas, e.g., hippocampus. This review summarizes results from experimental studies in rodents showing that in vivo galanin is a potent modulator of brain 5-HT transmission, and in particular 5-HT1A receptor-mediated functions. Galanin, given intracerebroventricular (i.c.v.), was demonstrated to have strong inhibitory interactions with 5-HT1A receptor functions, particularly in the dorsal raphe but also in the hippocampus. Since pre- and postsynaptic 5-HT1A receptors in the dorsal raphe and hippocampus are implicated in the action of antidepressant drugs and in depressive disorders, it is suggested that galanin receptors may be an important target for development of novel antidepressant drugs. This view is supported by a recent study in the rat showing that the galanin antagonist M35, given i.c.v., could block the depression-like behavior in the forced swim test induced by galanin, while M35 produced an antidepressant-like effect on its own.

[Characteristics of galanin and vasoactive intestinal peptide immunoreactivity in the rat amygdala complex]

INTRODUCTION

Morphological features and morphometric parameters of galanin (GAL) and vasoactive intestinal peptide (VIP) immunoreactive neurons and neuronal fibres were studied in all nuclei of adult male rat amygdala.

MATERIAL AND METHODS

After perfusion and fixation, rat brains were immunohistochemically stained with antibodies against GAL and VIP and then visualized by avidin-biotin-peroxidase complex.

RESULTS AND DISCUSSION

The greatest number of galanin-immunoreactive neurons were identified in the medial part of the central nucleus and in the dorsal part of the medial nucleus. In the first case, most neurons were bipolar (37%), and in the second, they were ovoid (45%). GAL-immunoreactive fibers were identified in the medial nucleus, "bed nucleus" of the accessory olfactory tract, fiontal cortical nucleus, amygdalo-hippocampal area and basolateral nucleus. VIP-immunoreactive neurons were diffusely distributed in more nuclei than the previous, mostly in the lateral, basolateral, and basomedial nucleus. They were mostly ovoid (40%). VIP-immunoreactive fibers were observed in the lateral part oJ'the central nucleus, while long and radially oriented fibers were present in the frontal and dorsal cortical nucleus.

CONCLUSION

By distribution analysis of GAL and VIP immunoreactive neurons and fibers, and according to literature data, it can be assumed that the medial part of the central nucleus receives VIP fibers from other parts of the amygdaloid body, and then sends GAL fibers to the medial nucleus.

Galanin enhances and a galanin antagonist attenuates depression-like behaviour in the rat

The effect of intracerebroventricular infusion of galanin and/or the galanin antagonist M35 was studied in the forced swim test. Animals were pre-exposed to water for 15 min 24 h prior to test. Immobility and climbing were assessed during the second, 5 min exposure to water. Rats receiving a single infusion of galanin (3 nmol) displayed a significant increase of immobility. This effect was blocked by co-administration of M35 (1 nmol). M35 alone (1 nmol) produced a significant decrease of immobility. The results further support the hypothesis that galanin may play a role in mood disorders, and that galanin antagonists may represent new candidates for antidepressant treatment.

Changes in brain cholinergic markers and spatial learning in old galanin-overexpressing mice

The cholinergic forebrain system is involved in learning and memory, and its age-dependent decline correlates with a decrease in cognitive performance. Since the neuropeptide galanin participates in cholinergic neuron regulation, we have studied 19- to 23-month-old male mice overexpressing galanin under the platelet-derived growth factor B promoter (GalOE) and wild-type (WT) littermates by monitoring behavioral, neurochemical and morphological/histochemical parameters. In the Morris water maze test, old transgenic animals showed a significant impairment in escape latency in the hidden platform test compared to age-matched WT animals. The morphological/histochemical studies revealed that cholinergic neurons in the basal forebrain display a slight, age- but not genotype-related, alteration in choline acetyltransferase- (ChAT) immunoreactivity. The neurochemical studies showed an age-related decline in ChAT activity in the cerebral cortex of all mice, whereas in the hippocampal formation this effect was seen in GalOE but not WT animals. Expression of BDNF mRNA in the hippocampal formation, as evaluated by RT-PCR, was reduced in old animals; no age- or genotype-induced variations in NGF mRNA expression were observed. These data suggest that galanin overexpression further accentuates the age-related decline of the cholinergic system activity in male mice, resulting in impairment of water maze performance in old animals.

Galanin receptor antagonists : a potential novel pharmacological treatment for mood disorders

The pathophysiology of mood disorders involves several genetic and social predisposing factors, as well as a dysregulated response to chronic stress. Accumulated evidence during the last two decades has implicated disturbances in brain serotonin and/or noradrenaline (norepinephrine) neurotransmission in the aetiology of depression. In fact, current pharmacological treatment for mood disorders is based on the use of drugs that act mainly by enhancing brain serotonin and noradrenaline neurotransmission by blockade of the active reuptake mechanism for these neurotransmitters. However, current antidepressant drugs have a delayed onset of therapeutic action, and a substantial number of patients do not respond adequately to them. In addition, these drugs have a number of adverse effects that limit patient compliance. In view of this, there is an intense search to identify novel (receptor) targets for antidepressant therapy. Recent studies have indicated that several neuropeptides and their receptors are potential candidates for the development of novel antidepressant treatment. In this context, galanin is of particular interest, since it is co-localised with serotonin in the dorsal raphe nucleus and with noradrenaline in the locus coeruleus, nuclei known to play a major role in affective disorders and in the action of antidepressant drugs. The actions of galanin are mediated by three receptor subtypes (GAL1, GAL2 and GAL3), which are coupled to different intracellular effector systems. Studies in rats have shown that galanin administered intracerebroventricularly is a potent inhibitor of mesencephalic serotonergic neurotransmission, as indicated by a long-lasting reduction in the release of serotonin in the hippocampus. This inhibitory effect is related to activation of the galanin receptors located on the dorsal raphe neurons. Moreover, intracerebroventricular galanin alters the gene expression of serotonin 5-HT1A autoreceptors in the dorsal raphe and also changes their functional activity. In addition, galanin produces a functional blockade of postsynaptic 5-HT1A receptor-mediated responses. Both pharmacological and genetic studies suggest a role for galanin in depression-like behaviour in rodent models. Transgenic mice overexpressing galanin under the control of the platelet-derived growth factor-beta promoter display increased immobility in the forced swim test. Intracerebroventricular administration of galanin in the rat increases depression-like behaviour, and this is fully blocked by the nonselective peptide galanin receptor antagonist M35. Importantly, M35 alone administered intracerebroventricularly produces an antidepressant-like effect. Recently, newly developed receptor-specific nonpeptidergic galanin GAL3 receptor antagonists (SNAP-37889 and SNAP-398299), which cross the blood-brain barrier after systemic administration, have shown antidepressant-like activity in several animal models. On the other hand, stimulation of the GAL2 receptor at the raphe level by local application of the GAL2 receptor agonist galanin (2-11) has been shown to increase serotonin levels in the hippocampus and dorsal raphe. These results indicate an important (mainly inhibitory) role of galanin as a regulator of brain serotonin and 5-HT1A receptor-mediated transmission, which may be of potential importance for understanding mood disorders and for the development of antidepressant drugs. Taken together, the present evidence suggests that antidepressant efficacy may be associated with compounds acting as antagonists at the GAL3 and/or possibly GAL1 receptors, and/or agonists at the GAL2 receptor.

Galanin expressed in the excitatory fibers attenuates synaptic strength and generalized seizures in the piriform cortex of mice

The neuropeptide galanin is considered to be an endogenous antiepileptic agent, presumably acting via inhibition of glutamate release. Previously, we have demonstrated that in mice ectopically overexpressing galanin in cortical and hippocampal neurons, particularly in granule cells and their axons, the mossy fibers, hippocampal kindling epileptogenesis is suppressed and is associated with attenuated frequency facilitation in mossy fiber-CA3 cell synapses. We hypothesized that changes in synaptic transmission might occur also in other excitatory synapses of the galanin overexpressing (GalOE) mouse, contributing to seizure suppression. Lateral olfactory tract (LOT) synapses, formed by axons of olfactory bulb (OB) mitral cells and targeting piriform cortex (PC) pyramidal cells, ectopically express galanin in GalOE mice. Using whole-cell patch-clamp recordings, we found that excitatory synaptic responses recorded in PC pyramidal cells during high frequency stimulation of the LOT were attenuated in GalOE mice as compared to wild-type controls. This effect was mimicked by bath application of galanin or its agonist galnon to wild-type slices, supporting the notion of ectopic galanin action. Since the high frequency activation induced in vitro resembles epileptic seizures in vivo, we asked whether the observed synaptic inhibition would result in altered epileptogenesis when animals were kindled via the same synapses. In male GalOE mice, we found that the latency to convulsions was prolonged, and once animals had experienced the first stage 5 seizure, generalized seizures were less sustainable. These data indicate that the PC is a possible target for epilepsy treatment by ectopically overexpressing galanin to modulate seizure activity.

Differential distribution and regulation of galanin receptors- 1 and -2 in the rat lumbar spinal cord

The expression of the galanin receptor-1 and -2 (Gal(1) and Gal(2)) messenger ribonucleic acids (mRNAs) was studied in the lower spinal cord of rat by means of in situ hybridization, using ribonucleic acid probes (riboprobes). Naïve rats as well as rats with unilateral axotomy of the sciatic nerve or unilateral inflammation of the hindpaw were analyzed. In naïve rats, numerous Gal(1) mRNA-positive (+) neurons were detected in lamina (L) I-III. In addition, several Gal(1) mRNA(+) neurons were seen in deeper layers, including the ventral horns, area X, and the lateral spinal nucleus. In contrast, few and comparatively weakly labeled Gal(2) mRNA(+) neurons were observed, mostly in the ventral horns and in area X, with fewer in the dorsal horn and in the sympathetic and parasympathetic intermediate lateral cell columns. Axotomy induced a strong increase in intensity and number of Gal(2) mRNA(+) motoneurons ipsilateral to the lesion. In contrast, nerve cut or hindpaw inflammation did not alter the expression of Gal(1) or Gal(2) in the dorsal horn. The present (and previous) results suggest that galanin, acting through Gal(1) and Gal(2) receptors, has a modulatory role on spinal excitability, not only via interneurons in superficial dorsal horn, but also on neurons in deep layers and area X, as well as on the sympathetic and parasympathetic outflow. Furthermore, the nerve injury-induced Gal(2) upregulation in motor neurons suggests a role for galanin in survival/regeneration mechanisms.

Galanin and perseveration

Galanin is a 29/30 amino acid neuropeptide that has been shown to impair learning and memory task performance and also have roles in somatosensation, stress responses, sexual behavior, and feeding regulation. However, little is known about galanin involvement in higher cognitive processes, especially executive processes. Perseveration is a classic sign of frontal cortex damage and failure of executive control. Galanin has been shown to disrupt the performance of maze delayed alternation tasks and the operant, spatial delayed nonmatch-to-position (DNMTP) working memory task, tests especially sensitive to perseverative responding. To better understand this potential involvement of galanin in executive control, the present study tested the hypothesis that galanin induces perseveration. The first experiment examined the effects of galanin (10, 20 microg i.c.v.) on the performance of a simple operant response alternation task in which stimuli were assigned to one of two spatially distinct locations to produce extended sequences of presentations to one location, separated by a 10-s intertrial interval. The second experiment looked at the effects of galanin (5, 20 microg i.c.v.) on the performance of non-delayed match-to-position and nonmatch-to-position conditional discrimination operant tasks in which a minimal 1.0 s time interval separated responses. Finally, the effects of galanin (10, 20 microg i.c.v.) on delayed match-to-position (DMTP) performance were examined to determine whether response alternation (i.e., nonmatching) was critical to observing a galanin-induced impairment in this task. Galanin reduced the rate of trial completion in all the tasks, but did not alter simple or conditional discrimination accuracy. Galanin (10 microg) impaired DMTP performance in a delay-independent manner. Together, these data suggest that galanin does not produce perseveration, but are consistent with a galanin-induced decrease in reinforcer strength.

Medial septal galanin and acetylcholine: influence on hippocampal acetylcholine and spatial learning

Neurochemical and behavioral studies in the rat have provided evidence for the view that galanin impairs learning via an inhibitory modulation of cholinergic neurons in the septohippocampal projection, believed to be important for learning and memory. To test this hypothesis, galanin was microinjected via a unilateral chronic cannula located in MS/dBB of rats. Infusion of galanin in the MS/dBB, which contains a high number of 125I-galanin binding sites, did not impair spatial acquisition or memory. On the contrary, spatial acquisition tended to be facilitated by 1 and 3 nmoles of galanin, while the 0.3 nmol dose had no effect. Intraseptal injections of scopolamine (10 microg/rat), a non-specific muscarinic antagonist, also failed to alter learning performance. In contrast, co-injections of galanin (3 nmol) and scopolamine (10 microg) resulted in a marked impairment of spatial acquisition. The effect of intraseptal galanin on basal acetylcholine release in the ventral hippocampus was examined by in vivo microdialysis and high-performance liquid chromatography. Both galanin (3 nmol/rat) and scopolamine (10 microg/rat) infused into the MS/dBB increased basal acetylcholine release in the ventral hippocampus. The combined injections of galanin and scopolamine resulted in an excessive increase in acetylcholine release. These results indicate, that galanin activates septohippocampal cholinergic neurons, suggesting that septal galanin may have a facilitatory role in spatial learning. Moreover, the level of muscarinic activity within the septal area appears to be critical for the effects of galanin on cognitive functions, since the combination of galanin and scopolamine produced a marked impairment in spatial learning, despite a marked increase in hippocampal acetylcholine release. In summary, a limited range of cholinergic muscarinic transmission may contribute to optimal hippocampal function, a finding that has important implications for therapeutic approaches in the treatment of disorders of memory function.

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

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

Molecular endocrinology and physiology of the aging central nervous system

Aging is associated with a progressive decline in physical and cognitive functions. The impact of age-dependent endocrine changes regulated by the central nervous system on the dynamics of neuronal behavior, neurodegeneration, cognition, biological rhythms, sexual behavior, and metabolism are reviewed. We also briefly review how functional deficits associated with increases in glucocorticoids and cytokines and declining production of sex steroids, GH, and IGF are likely exacerbated by age-dependent molecular misreading and alterations in components of signal transduction pathways and transcription factors.

A role for galanin in antidepressant actions with a focus on the dorsal raphe nucleus

Selective serotonin reuptake inhibitors, such as fluoxetine (FLX), are the most commonly used drugs in the treatment of major depression. However, there is a limited understanding of their molecular mechanism of action. Although the acute effect of selective serotonin reuptake inhibitors in elevating synaptic serotonin concentrations is well known, the clinical amelioration of depressive symptoms requires 14-21 days of treatment, suggesting that numerous other rearrangements of function in the CNS must take place. In the present study, we demonstrated that 14 days of FLX treatment up-regulated galanin mRNA levels by 100% and GalR2-binding sites by 50%, in the rat dorsal raphe nucleus, where galanin coexists with serotonin. Furthermore, a galanin receptor antagonist, M40, attenuated the antidepressant-like effect of FLX in the forced swim test, a rodent preclinical screen commonly used to evaluate antidepressant-like efficacy. Direct activation of galanin receptors by a galanin receptor agonist, galnon, was found to produce an antidepressant-like effect in the same task. Two other antidepressant treatments also affected the galaninergic system in the monoaminergic nuclei: Electroconvulsive shock elevated galanin mRNA levels in dorsal raphe nucleus, whereas sleep deprivation increased galanin mRNA levels in the locus coeruleus, further underlining the connection between activation of the galaninergic system and antidepressant action of various clinically proven treatments.

Distribution of galanin and galanin transcript in the brain of a galanin-overexpressing transgenic mouse

The distribution of galanin mRNA-expressing cells and galanin-immunoreactive (IR) cell bodies and processes was studied in the brain of mice overexpressing galanin under the PDGF-B promoter (GalOE mice) and of wild type (WT) mice, both in colchicine-treated and non-treated animals. In this abstract, we only describe the results in GalOE mouse. A widespread ectopic expression of galanin (both mRNA and peptide) was found, that is a situation when neither transcript nor peptide could be seen in WT mice, not even after colchicine treatment. However, in some regions, such as claustrum, basolateral amygdala, thalamus, CA1 pyramidal cells, and Purkinje cells only galanin mRNA could be detected. In the forebrain galanin was seen in the mitral cells of the olfactory bulb, throughout the cortex, in the basolateral amygdaloid nucleus, claustrum, granular and pyramidal cell layers of the hippocampus, subiculum and presubiculum. In the thalamus, the anterodorsal, mediodorsal, intermediodorsal and mediodorsal lateral nuclei, the reuniens and reticular nuclei showed ectopic expression of galanin. Within the hypothalamus, neurons of the suprachiasmatic nucleus contained galanin. In the mesencephalon, the geniculate nucleus, nucleus ruber, the mesencephalic trigeminal and reticulotegmental nuclei ectopically expressed galanin. In the cerebellum, galanin was observed in the Purkinje cells and in the lateral and interposed cerebellar nuclei. In the pons, sensory and motor nuclei of the trigeminal nerve, the laterodorsal and dorsal tegmental nuclei, the pontine, reticulotegmental and gigantocellular reticular nuclei expressed galanin. Within the medulla oblongata, labeled cells were detected in the facial, ambiguus, prepositus, lateral paragigantocellular and lateral reticular nuclei, and spinal trigeminal nucleus. High densities of galanin-IR fibers were found in the axonal terminals of the lateral olfactory tract, the hippocampal and presumably the cerebellar mossy fibers system, in several thalamic and hypothalamic regions and the lower brain stem. Possible functional consequences of galanin overexpression are discussed.

Galanin in pituitary adenomas

Tumor galanin content was measured in extracts from human pituitary adenomas using a specific RIA method for monitoring human galanin. Twenty-two out of twenty-four tumors contained galanin with notably high levels in corticotroph adenomas, varying levels in clinically inactive tumors, and low levels in GH secreting adenomas. Tumor galanin and ACTH contents were closely correlated in all tumors. In four young patients with microadenomas and highly active Mb Cushing tumor galanin was inversely related to tumor volume. The molecular form of tumor galanin, studied with reverse-phase HPLC, was homogeneous with the majority of tumor galanin coeluting with standard human galanin. In the tumors analysed with in situ hybridization there was a good correlation between galanin peptide levels and galanin mRNA expression. In some tumors galanin mRNA and POMC levels coexisted, in others they were essentially in different cell populations. Levels of plasma galanin-LI were not related to tumor galanin concentration, and galanin levels were in the same range in sinus petrosus close to the pituitary venous drainage as in peripheral blood. Corticotrophin releasing hormone injections in two patients caused ACTH, but no detectable galanin release into sinus petrosus. Our results demonstrate that corticotroph, but not GH adenomas, express high levels of galanin, in addition to ACTH, and that in some tumors both polypeptides are synthesised in the same cell population. However, galanin levels in plasma were not influenced by the tumor galanin content.

Enhanced hippocampal noradrenaline and serotonin release in galanin-overexpressing mice after repeated forced swimming test

Basal and forced swimming (FS) stress-induced release of noradrenaline (NA) and serotonin (5-HT) were determined by in vivo microdialysis in the ventral hippocampus of mice overexpressing galanin under the platelet-derived growth factor B promoter (GalOE/P) or under the dopamine beta-hydroxylase promoter (GalOE/D) (only NA). WT mice served as controls. Intraventricular infusion of galanin significantly reduced basal extracellular NA in WT mice and in GalOE/P mice (albeit less so). Microdialysis sampling during a 10-min FS showed that NA and 5-HT release were elevated to 213% and 156%, respectively, in the GalOE/P group, whereas in the WT group the increases were only 127% and 119%, respectively. The second (repeated) 10-min FS (RFS) caused a marked enhancement of NA and 5-HT release in the GalOE/P mice to 344% and 275%, respectively. However, the RFS caused only a 192% increase of extracellular NA levels in the GalOE/D mice. Pretreatment with the putative peptidergic galanin receptor antagonist M35 almost completely blocked the elevation of NA and 5-HT levels in the GalOE/P after RFS. These results suggest that the NA and 5-HT hippocampal afferents in GalOE/P mice are hypersensitive to both conditioned and unconditioned stressful stimuli, such as FS, and that this effect is mediated by galanin receptors. The present findings support a role of galanin in the regulation of release of NA and 5-HT, two neurotransmitters involved in mood control.

Neuropeptide alterations in the hippocampal formation and cortex of transgenic mice overexpressing β-amyloid precursor protein (APP) with the Swedish double mutation (APP23)

The role of neuropeptides and the significance of peptidergic mechanisms in neurodegenerative diseases are still unclear. In the periphery, nerve injury results in dramatic changes in the expression of neuropeptides. An important question regards to what extent similar changes occur, and similar mechanisms operate, after lesions and/or degeneration in the brain. The purpose of this work is, therefore, to study neuropeptides with regard to their presence and distribution in the APP23 mouse (HuAPP(751) K670M/N671L under the murine Thy-1 promoter), a model for Alzheimer's disease, or cerebral amyloidosis, using the immunohistochemical technique. In addition, tyrosine hydroxylase and acetylcholinesterase were analyzed. This study shows marked neuropeptide changes in the hippocampal formation and the ventral cortex, whereas the dorsolateral neocortex was less affected. There was a considerable variation with regard to peptide expression among animals of the same age which was related to the variation in Abeta deposition. Dystrophic and varicose fibers containing galanin, neuropeptide Y, enkephalin, and especially cholecystokinin were commonly seen in close proximity to amyloid plaques. In addition, generalized changes were observed, such as increases of enkephalin and neuropeptide Y in stratum lacunosum moleculare and of neuropeptide Y, enkephalin, and dynorphin in mossy fibers. In contrast, cholecystokinin was decreased in mossy fibers. Comparatively small differences were observed between wild-type and transgenic mice with regard to tyrosine hydroxylase (noradrenergic but also dopaminergic fibers) and acetylcholine esterase (mainly cholinergic fibers). The increase of neuropeptides in dystrophic fibers in this model may represent a response to nerve injury caused by the amyloid accumulation and may reflect attempts to counteract degeneration by initiating protective and/or regenerative processes.

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.

Plasma galanin response to head-up tilt in normal subjects and patients with recurrent vasovagal syncope

Neurohumoral factors may contribute to cardiovascular changes associated with vasovagal syncope (VVS). Galanin (GAL) is a neuropeptide, widely distributed in the central and peripheral nervous systems, that interacts with both sympathetic and vagal systems as well as with neurotransmitters, such as serotonin. We investigated the changes in plasma GAL and catecholamine levels during head-up tilt (HUT) test in patients with recurrent VVS. Twenty-two patients (11 women, aged 33.1 +/- 4.2 years) with a history of VVS and 10 healthy subjects (5 women, aged 38.0 +/- 5.8 years) underwent HUT test (60 degrees, 45 minutes). GAL and catecholamine plasma levels were measured in the supine position, during HUT and, in patients with positive response, at presyncope, syncope, and after recovery of consciousness. Thirteen patients developed syncope during HUT, whereas no healthy subjects had a positive response. In healthy subjects, GAL did not change during HUT. By contrast, in patients with a history of VVS and a negative response to tilting (no syncope), GAL significantly (P <.001) increased in response to tilting (supine, 10.2 +/- 0.6 pmol/L; tilting, 18.1 +/- 1.1 pmol/L at 45 minutes) and correlated positively with the increases in blood pressure (BP) and heart rate (HR). In patients with a positive response, GAL did not change either before the loss of consciousness or during syncope. In patients with a positive response, norepinephrine (NE) significantly (P <.001) increased during tilting and then remained practically unchanged during syncope, whereas epinephrine (E) significantly (P <.001) increased during tilting and then showed further significant increases at presyncope and syncope. In conclusion, this study shows that circulating GAL levels progressively increase in correlation with the cardiovascular parameters during a negative HUT in patients with a history of VVS, whereas they remain unchanged in healthy subjects. Moreover, in the patients with tilting-induced syncope GAL does not change either before or during loss of consciousness. These data suggest a role for endogenous GAL in the adaptive responses to acute orthostatic stress preventing syncope in susceptible individuals.

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.

Neuropeptides in neuropathic and inflammatory pain with special emphasis on cholecystokinin and galanin

Neuropeptides present in primary afferents and the dorsal horn of the spinal cord have an important role in the mediation of nociceptive input under normal conditions. Under pathological conditions, such as chronic inflammation or following peripheral nerve injury, the production of peptides and peptide receptors is dramatically altered, leading to a number of functional consequences. In this review, the role of two neuropeptides that undergo such altered expression under pathological conditions, cholecystokinin (CKK) and galanin, is reviewed.

Distribution of galanin messenger RNA-expressing cells in murine brain and their regulation by leptin in regions of the hypothalamus

Galanin is widely distributed throughout the mammalian brain and has been implicated in the regulation of food intake, metabolism and reproduction-functions that are also thought to be under the control of leptin. To investigate the possible role of galanin in mediating the physiological effects of leptin in the mouse, we had three experimental objectives: first, to map the distribution of galanin messenger RNA-expressing cells in the brain of the mouse; second, to assess the effects of leptin on galanin gene expression in areas of the brain thought to be involved in the regulation of body weight and reproduction; and third, to determine whether galanin neurons in these regions express leptin receptor messenger RNA. We found the pattern of galanin messenger RNA expression in the mouse brain to be similar, but not identical, to that in the rat. Leptin treatment (2microg/g for six days) significantly reduced cellular levels of galanin messenger RNA in the hypothalamic periventricular nucleus of leptin-deficient obese (ob/ob) mice (P<0.01) by approximately 30%; however, leptin did not appear to influence the expression of galanin in the arcuate or dorsomedial nucleus of the hypothalamus. Galanin-producing neurons in the arcuate, dorsomedial and periventricular nuclei did not appear to express leptin receptor messenger RNA (P>0.05). These results demonstrate that galanin distribution patterns in the mouse brain are comparable to other species and, yet, possess unique features. In addition, galanin-expressing neurons in the hypothalamic periventricular nucleus are targets for regulation by leptin; however, the effect of leptin on galanin gene expression is likely to be mediated indirectly, perhaps through either proopiomelanocortin- or neuropeptide Y-expressing cells in the hypothalamus.

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.

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

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

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.

Effect of intrathecal galanin and its putative antagonist M35 on pain behavior in a neuropathic pain model

There is currently some debate over a possible role of galanin in pain processing. It was recently reported that the levels of galanin in dorsal root ganglia (DRGs) seem related to development of allodynia after unilateral sciatic nerve constriction injury. In our present study, we aimed at characterizing the effect of exogenous and endogenous galanin on pain behavior in allodynic and non-allodynic rats in which the levels of galanin in DRG neurons are low and high, respectively [28]. The results show that in allodynic rats, the mechanical threshold increases dose-dependently after intrathecal (i.t.) injection of galanin, while no significant changes were observed in groups treated with the putative galanin antagonist M35 or saline. In non-allodynic rats i.t. injection of M35 induced a significant mechanical allodynic state, which did not occur after injection of galanin, bradykinin, the bradykinin fragment(2-9) or saline. The results suggest that in the present experimental paradigm exogenous galanin has an anti-allodynic effect in the allodynic rats, and that endogenous galanin has a tonic inhibitory effect in the non-allodynic group.

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 and spinal nociceptive mechanisms: recent advances and therapeutic implications

Galanin is a peptide consisting of 29 or 30 (in humans) amino acids that is present in sensory and spinal dorsal horn neurons. Endogenous galanin may have an important modulatory function on nociceptive input at the spinal level. In addition, exogenously administered galanin exerts complex effects on spinal nociceptive transmission, where inhibitory action appears to predominate. Peripheral nerve injury and inflammation, conditions associated with chronic pain, upregulate the synthesis of galanin in sensory neurons and spinal cord neurons, respectively. Hence, the sensory effect of galanin may be increased under these conditions, raising the possibility that modulation of the activity of the galanin system may produce antinociception.

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

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