Centrally administered galanin blocks morphine place preference in the mouse

Opioid-galanin receptor heteromers differentiate the dopaminergic effects of morphine and methadone

As the opioid addiction crisis reaches epidemic levels, the identification of opioid analgesics that lack abuse potential may provide a path to safer treatment of chronic pain. Preclinical studies have demonstrated that galanin affects physical dependence and rewarding actions associated with morphine. In the brain and periphery, galanin and opioids signal through their respective GPCRs, GalR1-3 and the μ-opioid receptor (MOR). In this issue of the JCI, Cai and collaborators reveal that heteromers between GalR1 and MOR in the rat ventral tegmental area attenuate the potency of methadone, but not other opioids, in stimulating the dopamine release that produces euphoria. These studies help us understand why some synthetic opioids, such as methadone, do not trigger the release of dopamine in the mesolimbic system but still possess strong analgesic properties.

Alpha-PVP induces the rewarding effect via activating dopaminergic neuron

A synthetic cathinone, 1-phenyl-2-(1-pyrrolidinyl)-1-pentanone (α-PVP), was occasionally found in the "bath salt" type of designer drugs, as an active ingredient. It has been reported that drivers who consumed α-PVP were in an excited state and incapable of controlling their behavior, causing traffic accidents. Despite its acute excitatory effects, there is no information on the psychological dependency elicited by α-PVP use. The purpose of the present study was to clarify whether the reward pathway is activated with repeated doses of α-PVP in experimental animals. Treatment of male C57BL/6j mice with α-PVP (25 mg/kg, i.p.), once a day, for 3 days significantly increased the conditioned place preference scores. Therefore, repeated doses of α-PVP were shown to induce palatability in mice. α-PVP increases extracellular dopamine levels in the nucleus accumbens shell immediately after administration. The number of cells immunopositive for phosphorylated cAMP-regulatory element binding protein (CREB) was significantly increased in the α-PVP-treated mice in our study. These results indicate that the administration of α-PVP activates the phosphorylation of CREB in the nucleus accumbens shell. Our results suggest that α-PVP stimulates the reward pathway by increasing the extracellular dopamine levels and CREB phosphorylation in the nucleus accumbens shell, eventually causing positive reinforcement in mice.

Functional μ-Opioid-Galanin Receptor Heteromers in the Ventral Tegmental Area

The neuropeptide galanin has been shown to interact with the opioid system. More specifically, galanin counteracts the behavioral effects of the systemic administration of μ-opioid receptor (MOR) agonists. Yet the mechanism responsible for this galanin-opioid interaction has remained elusive. Using biophysical techniques in mammalian transfected cells, we found evidence for selective heteromerization of MOR and the galanin receptor subtype Gal1 (Gal1R). Also in transfected cells, a synthetic peptide selectively disrupted MOR-Gal1R heteromerization as well as specific interactions between MOR and Gal1R ligands: a negative cross talk, by which galanin counteracted MAPK activation induced by the endogenous MOR agonist endomorphin-1, and a cross-antagonism, by which a MOR antagonist counteracted MAPK activation induced by galanin. These specific interactions, which represented biochemical properties of the MOR-Gal1R heteromer, could then be identified in situ in slices of rat ventral tegmental area (VTA) with MAPK activation and two additional cell signaling pathways, AKT and CREB phosphorylation. Furthermore, in vivo microdialysis experiments showed that the disruptive peptide selectively counteracted the ability of galanin to block the dendritic dopamine release in the rat VTA induced by local infusion of endomorphin-1, demonstrating a key role of MOR-Gal1R heteromers localized in the VTA in the direct control of dopamine cell function and their ability to mediate antagonistic interactions between MOR and Gal1R ligands. The results also indicate that MOR-Gal1R heteromers should be viewed as targets for the treatment of opioid use disorders.

SIGNIFICANCE STATEMENT

The μ-opioid receptor (MOR) localized in the ventral tegmental area (VTA) plays a key role in the reinforcing and addictive properties of opioids. With parallel in vitro experiments in mammalian transfected cells and in situ and in vivo experiments in rat VTA, we demonstrate that a significant population of these MORs form functional heteromers with the galanin receptor subtype Gal1 (Gal1R), which modulate the activity of the VTA dopaminergic neurons. The MOR-Gal1R heteromer can explain previous results showing antagonistic galanin-opioid interactions and offers a new therapeutic target for the treatment of opioid use disorder.

Regulation of neurological and neuropsychiatric phenotypes by locus coeruleus-derived galanin

Decades of research confirm that noradrenergic locus coeruleus (LC) neurons are essential for arousal, attention, motivation, and stress responses. While most studies on LC transmission focused unsurprisingly on norepinephrine (NE), adrenergic signaling cannot account for all the consequences of LC activation. Galanin coexists with NE in the vast majority of LC neurons, yet the precise function of this neuropeptide has proved to be surprisingly elusive given our solid understanding of the LC system. To elucidate the contribution of galanin to LC physiology, here we briefly summarize the nature of stimuli that drive LC activity from a neuroanatomical perspective. We go on to describe the LC pathways in which galanin most likely exerts its effects on behavior, with a focus on addiction, depression, epilepsy, stress, and Alzheimer׳s disease. We propose a model in which LC-derived galanin has two distinct functions: as a neuromodulator, primarily acting via the galanin 1 receptor (GAL1), and as a trophic factor, primarily acting via galanin receptor 2 (GAL2). Finally, we discuss how the recent advances in neuropeptide detection, optogenetics and chemical genetics, and galanin receptor pharmacology can be harnessed to identify the roles of LC-derived galanin definitively. This article is part of a Special Issue entitled SI: Noradrenergic System.

The galanin receptor agonist, galnon, attenuates cocaine-induced reinstatement and dopamine overflow in the frontal cortex

Relapse represents one of the most significant problems in the long-term treatment of drug addiction. Cocaine blocks plasma membrane monoamine transporters and increases dopamine (DA) overflow in the brain, and DA is critical for the motivational and primary reinforcing effects of the drug as well as cocaine-primed reinstatement of cocaine seeking in rats, a model of relapse. Thus, modulators of the DA system may be effective for the treatment of cocaine dependence. The endogenous neuropeptide galanin inhibits DA transmission, and both galanin and the synthetic galanin receptor agonist, galnon, interfere with some rewarding properties of cocaine. The purpose of this study was to further assess the effects of galnon on cocaine-induced behaviors and neurochemistry in rats. We found that galnon attenuated cocaine-induced motor activity, reinstatement and DA overflow in the frontal cortex at a dose that did not reduce baseline motor activity, stable self-administration of cocaine, baseline extracellular DA levels or cocaine-induced DA overflow in the nucleus accumbens (NAc). Similar to cocaine, galnon had no effect on stable food self-administration but reduced food-primed reinstatement. These results indicate that galnon can diminish cocaine-induced hyperactivity and relapse-like behavior, possibly in part by modulating DA transmission in the frontal cortex.

Orquestic regulation of neurotransmitters on reward-seeking behavior

The ventral tegmental area is strongly associated with the reward system. Dopamine is released in areas such as the nucleus accumbens and prefrontal cortex as a result of rewarding experiences such as food, sex, and neutral stimuli that become associated with them. Electrical stimulation of the ventral tegmental area or its output pathways can itself serve as a potent reward. Different drugs that increase dopamine levels are intrinsically rewarding. Although the dopaminergic system represent the cornerstone of the reward system, other neurotransmitters such as endogenous opioids, glutamate, γ-Aminobutyric acid, acetylcholine, serotonin, adenosine, endocannabinoids, orexins, galanin and histamine all affect this mesolimbic dopaminergic system. Consequently, genetic variations of neurotransmission are thought influence reward processing that in turn may affect distinctive social behavior and susceptibility to addiction. Here, we discuss current evidence on the orquestic regulation of different neurotranmitters on reward-seeking behavior and its potential effect on drug addiction.

A high-fat diet or galanin in the PVN decreases phosphorylation of CREB in the nucleus accumbens

A high-fat diet (HFD) can increase hypothalamic galanin (GAL). GAL has recently been shown to inhibit opiate reward, which in turn, decreases cAMP response element-binding protein (CREB) in the nucleus accumbens (NAc). We hypothesized that injection of GAL into the paraventricular nucleus (PVN), or consumption of a HFD, would be associated with a decrease in NAc CREB. In Exp. 1, GAL in the PVN of naïve rats decreased phosphorylated-CREB (pCREB) which is the activated form of CREB, in the NAc compared to saline-injected controls. In Exp. 2, rats fed ad libitum HFD for 4 weeks had reduced NAc pCREB levels compared to rats with sporadic tastes of the HFD. Body weight, serum triglyceride and leptin levels were also raised in the chronic HFD-fed rats. These data suggest that PVN GAL or chronic intake of a HFD can decrease NAc pCREB. The implications of these findings may help to explain the lack of opiate-like withdrawal that has been reported in response to overeating a HFD, thereby providing a potential mechanism underlying behavioral differences seen with addiction-like overconsumption of different types of palatable foods.

Galanin-induced decreases in nucleus accumbens/striatum excitatory postsynaptic potentials and morphine conditioned place preference require both galanin receptor 1 and galanin receptor 2

The neuropeptide galanin has been shown to alter the rewarding properties of morphine. To identify potential cellular mechanisms that might be involved in the ability of galanin to modulate opiate reward, we measured excitatory postsynaptic potentials (EPSPs), using both field and whole-cell recordings from striatal brain slices extracted from wild-type mice and mice lacking specific galanin receptor (GalR) subtypes. We found that galanin decreased the amplitude of EPSPs in both the dorsal striatum and nucleus accumbens. We then performed recordings in slices from knockout mice lacking either the GalR1 or GalR2 gene, and found that the ability of galanin to decrease EPSP amplitude was absent from both mouse lines, suggesting that both receptor subtypes are required for this effect. In order to determine whether behavioral responses to opiates were dependent on the same receptor subtypes, we tested GalR1 and GalR2 knockout mice for morphine conditioned place preference (CPP). Morphine CPP was significantly attenuated in both GalR1 and GalR2 knockout mice. These data suggest that mesolimbic excitatory signaling is significantly modulated by galanin in a GalR1-dependent and GalR2-dependent manner, and that morphine CPP is dependent on the same receptor subtypes.

Pharmacogenetic association of the galanin receptor (GALR1) SNP rs2717162 with smoking cessation

Galanin modulates dopaminergic neurotransmission in the mesolimbic dopamine system, thereby influencing the rewarding effects of nicotine. Variants in the galanin receptor 1 (GALR1) gene have been associated with retrospective craving severity and heaviness of smoking in prior research. We investigated pharmacogenetic associations of the previously studied GALR1 polymorphism, rs2717162, in 1217 smokers of European ancestry who participated in one of three pharmacogenetic smoking cessation clinical trials and were treated with nicotine patch (n=623), nicotine nasal spray (n=189), bupropion (n=213), or placebo (n=192). The primary endpoint was abstinence (7-day point prevalence, biochemically confirmed) at the end of treatment. Cravings to smoke were assessed on the target quit day (TQD). The longitudinal regression model revealed a significant genotype by treatment interaction (P=0.03). There was a reduced odds of quitting success with the presence of at least one minor (C) allele in the bupropion-treated group (OR=0.43; 95% CI=0.22-0.77; P=0.005) but equivalent quit rates by genotype in the nicotine-replacement therapy groups. This genotype by treatment interaction was reproduced in a Cox regression model of time to relapse (P=0.04). In the bupropion trial, smokers carrying the C allele also reported more severe TQD cravings. Further research to identify functional variants in GALR1 and to replicate pharmacogenetic associations is warranted.

Galanin negatively modulates opiate withdrawal via galanin receptor 1

RATIONALE

The neuropeptide galanin has been shown to modulate opiate dependence and withdrawal. These effects could be mediated via activation of one or more of the three distinct G protein-coupled receptors, namely galanin receptors 1 (GalR1), 2 (GalR2), and 3 (GalR3).

OBJECTIVES

In this study, we used several transgenic mouse lines to further define the mechanisms underlying the role played by galanin and its receptors in the modulation of morphine dependence. First, transgenic mice expressing β-galactosidase under the control of the galanin promoter were used to assess the regulation of galanin expression in response to chronic morphine administration and withdrawal. Next, the behavioral responses to chronic morphine administration and withdrawal were tested in mice that over-express galanin, lack the GalR1 gene, or lack the GalR2 gene.

METHODS

Transgenic and matched wild-type mice were given increasing doses of morphine followed by precipitation of withdrawal by naloxone and behavioral responses to withdrawal were assessed.

RESULTS

Both morphine administration and withdrawal increased galanin gene transcription in the locus coeruleus (LC). Increasing galanin levels in the brain reduced signs of opiate withdrawal. Mice lacking GalR1 undergo more severe opiate withdrawal, whereas mice lacking GalR2 show no significant difference in withdrawal signs, compare with matched wild-type controls.

CONCLUSIONS

Opiate administration and withdrawal increase galanin expression in the LC. Galanin opposes the actions of morphine which leads to opiate dependence and withdrawal, an effect that is mediated via GalR1.

The neuropeptide galanin and variants in the GalR1 gene are associated with nicotine dependence

The neuropeptide galanin and its receptors are expressed in brain regions implicated in drug dependence. Indeed, several lines of evidence support a role for galanin in modulating the effects of drugs of abuse, including morphine, cocaine, amphetamine, and alcohol. Despite these findings, the role of galanin and its receptors in the effects of nicotine is largely underexplored. Here, using mouse models of nicotine reward and withdrawal, we show that there is a significant correlation between mecamylamine-precipitated nicotine withdrawal somatic signs and basal galanin or galanin receptor 1 (GALR1) expression in mesolimbocortical dopamine regions across the BXD battery of recombinant inbred mouse lines. The non-peptide galanin receptor agonist, galnon, also blocks nicotine rewarding effects and reverses mecamylamine-precipitated nicotine withdrawal signs in ICR mice. Additionally, we conducted a meta-analysis using smoking information from six European-American and African-American data sets. In support of our animal data, results from the association study show that variants in the GALR1 gene are associated with a protective effect in nicotine dependence (ND). Taken together, our data suggest that galanin has a protective role against progression to ND, and these effects may be mediated through GALR1.

The galanin receptor 1 gene associates with tobacco craving in smokers seeking cessation treatment

Craving for tobacco is a major challenge for people with nicotine dependence (ND) who try to quit smoking. Galanin (GAL) and its receptors (GALRs) can alter addiction-related behaviors and are therefore good candidates for modulators of behavioral parameters associated with smoking. We performed a genetic association study in 486 subjects (432 European American, EA) recruited for smoking cessation trials. Twenty-six candidate genes for ND-related phenotypes were selected based on the literature. Subjects were assessed using the Minnesota Withdrawal Scale (MWS), which included a specific item for craving, the Fagerström Scale of Nicotine Dependence (FTND), and other ND-related instruments. One single-nucleotide polymorphism (SNP) in GALR1, rs2717162, significantly associated with severity of craving in EA samples (p=6.48 × 10(-6)) and in the combined sample (p=9.23 × 10(-6)). Individuals with TT and TC genotypes had significantly higher craving scores than CC subjects. We also observed that SNPs in the CHRNA5 locus, rs16969968 and rs684513, which have been associated with ND-related phenotypes in previous studies, were nominally associated with FTND scores, although these results did not meet Bonferroni-adjusted criteria for experiment-wide significance. Our findings suggest that variation at GALR1 associates with differences in the severity of past craving for tobacco among smokers motivated to quit. Taken together with preclinical evidence, these results, if replicated, suggest that GAL and GALRs may be useful therapeutic targets for the pharmacological treatment of ND. Our results also confirm previously reported associations between variation at CHRNA5 and ND.

The lateral hypothalamus as integrator of metabolic and environmental needs: from electrical self-stimulation to opto-genetics

As one of the evolutionary oldest parts of the brain, the diencephalon evolved to harmonize changing environmental conditions with the internal state for survival of the individual and the species. The pioneering work of physiologists and psychologists around the middle of the last century clearly demonstrated that the hypothalamus is crucial for the display of motivated behaviors, culminating in the discovery of electrical self-stimulation behavior and providing the first neurological hint accounting for the concepts of reinforcement and reward. Here we review recent progress in understanding the role of the lateral hypothalamic area in the control of ingestive behavior and the regulation of energy balance. With its vast array of interoceptive and exteroceptive afferent inputs and its equally rich efferent connectivity, the lateral hypothalamic area is in an ideal position to integrate large amounts of information and orchestrate adaptive responses. Most important for energy homeostasis, it receives metabolic state information through both neural and humoral routes and can affect energy assimilation and energy expenditure through direct access to behavioral, autonomic, and endocrine effector pathways. The complex interplays of classical and peptide neurotransmitters such as orexin carrying out these integrative functions are just beginning to be understood. Exciting new techniques allowing selective stimulation or inhibition of specific neuronal phenotypes will greatly facilitate the functional mapping of both input and output pathways.

Locomotion and self-administration induced by cocaine in 129/OlaHsd mice lacking galanin

Previous studies have demonstrated that the galanin system modulates responses to drugs of abuse such as morphine. The current study examined whether genetic deletion of galanin could affect the locomotor and reinforcing effects of cocaine in mice. We analyzed spontaneous motor activity and cocaine-induced hyperactivity in wild-type (GAL-WT) and knockout mice lacking galanin (GAL-KO) maintained on the 129/OlaHsd background. Our results indicate that cocaine enhanced locomotion (defined as moving more than 5 cm) dose-dependently in GAL-WT and GAL-KO mice. However, general activity (total beam breaks) was increased by cocaine only in GAL-WT mice. An additional experiment indicated that galnon, a nonselective galanin receptor agonist, did not affect cocaine-induced hyperactivity. In a second set of experiments, mice of both genotypes were trained to self-administer cocaine under a fixed ratio schedule, tested with various doses of cocaine and under different schedules of reinforcement. This set of experiments showed that cocaine self-administration did not differ markedly between genotypes. However, while GAL-WT mice acquired cocaine self-administration, a median split analysis showed that mice could be divided into large and small drug takers, whereas all GAL-KO mice behaved as small drug takers. Our results indicate that wild-type and galanin knockout mice on a congenic 129/OlaHsd background are responsive to the locomotor effects of cocaine and can acquire intravenous cocaine self-administration. However, the phenotype observed in GAL-KO mice does not support a major role for galanin in cocaine-induced hyperlocomotion and self-administration.

Galanin and addiction

There has been increasing interest in the ability of neuropeptides involved in feeding to modulate circuits important for responses to drugs of abuse. A number of peptides with effects on hypothalamic function also modulate the mesolimbic dopamine system (ventral tegmental area and nucleus accumbens). Similarly, common stress-related pathways can modulate food intake, drug reward and symptoms of drug withdrawal. Galanin promotes food intake and the analgesic properties of opiates, thus it initially seemed possible that galanin might potentiate opiate reinforcement. Instead, galanin agonists decrease opiate reward, measured by conditioned place preference, and opiate withdrawal signs, whereas opiate reward and withdrawal are increased in knockout mice lacking galanin. This is consistent with studies showing that galanin decreases activity-evoked dopamine release in striatal slices and decreases the firing rate of noradrenergic neurons in locus coeruleus, areas involved in drug reward and withdrawal, respectively. These data suggest that polymorphisms in genes encoding galanin or galanin receptors might be associated with susceptibility to opiate abuse. Further, galanin receptors might be potential targets for development of novel treatments for addiction.

Hypothalamic injection of non-opioid peptides increases gene expression of the opioid enkephalin in hypothalamic and mesolimbic nuclei: Possible mechanism underlying their behavioral effects

The peptides galanin (GAL) and orexin (OX) share common features with the opioid enkephalin (ENK) in their relationship to ingestive behavior, stimulating consumption of a fat-rich diet and ethanol when injected into the hypothalamus. Since receptors for GAL and OX are dense in areas where ENK-expressing neurons are concentrated, these non-opioid peptides may exert their effects, in part, through the stimulation of endogenous ENK. This study was conducted to determine whether injection of GAL or OX affects the expression of ENK in hypothalamic and mesolimbic nuclei involved in consummatory behavior. Rats were injected with GAL (1 microg), OX-A (1 microg), or saline vehicle just dorsal to the hypothalamic paraventricular nucleus (PVN). They were sacrificed 1h later for analysis of ENK mRNA levels in the PVN, ventral tegmental area (VTA), central nucleus of the amygdala (CeA), and nucleus accumbens (NAc). Both GAL and OX had similar effects, significantly increasing ENK mRNA expression in each of these areas, except for the NAc. This enhanced ENK expression in the PVN, VTA and CeA was demonstrated with real-time quantitative polymerase chain reaction and confirmed in separate groups using radiolabeled and digoxigenin-labeled in situ hybridization. These findings demonstrate that the non-opioid peptides, GAL or OX, which have similar effects on consummatory behavior, are also similar in their effect on endogenous ENK. In light of published findings showing an opioid antagonist to block GAL- and OX-induced feeding, these results provide additional evidence that ENK is involved in mediating the common behavioral effects of these peptides.

Effects of galanin on monoaminergic systems and HPA axis: Potential mechanisms underlying the effects of galanin on addiction- and stress-related behaviors

Like a number of neuropeptides, galanin can alter neural activity in brain areas that are important for both stress-related behaviors and responses to drugs of abuse. Accordingly, drugs that target galanin receptors can alter behavioral responses to drugs of abuse and can modulate stress-related behaviors. Stress and drug-related behaviors are interrelated: stress can promote drug-seeking, and drug exposure and withdrawal can increase activity in brain circuits involved in the stress response. We review here what is known about the ability of galanin and galanin receptors to alter neuronal activity, and we discuss potential mechanisms that may underlie the effects of galanin on behaviors involved in responses to stress and addictive drugs. Understanding the mechanisms underlying galanin's effects on neuronal function in brain regions related to stress and addiction may be useful in developing novel therapeutics for the treatment of stress- and addiction-related disorders.

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.

Galanin protects against behavioral and neurochemical correlates of opiate reward

The mechanisms underlying responses to drugs of abuse have been widely investigated; however, less is known about pathways normally protective against the development of drug reinforcement. These pathways are also important since they may regulate individual differences in vulnerability to addiction. The neuropeptide galanin and its binding sites are expressed in brain areas important for drug reward. Previous studies have shown that centrally infused galanin attenuates morphine place preference and peripheral injection of galnon, a galanin agonist, decreases opiate withdrawal signs. The current studies in galanin knockout (GKO) mice examined the hypothesis that galanin is an endogenous negative regulator of opiate reward and identified downstream signaling pathways regulated by galanin. We show that GKO mice demonstrate increased locomotor activation following morphine administration, which is inhibited by acute administration of galnon. GKO mice also show enhanced morphine place preference, supporting the idea that galanin normally antagonizes opiate reward. In addition, morphine-induced ERK1/2 phosphorylation was increased in the VTA of both wild-type and GKO mice, but only the GKO mice showed increases in ERK1/2 and CREB phosphorylation in the amygdala or nucleus accumbens. Furthermore, a single systemic injection of galnon in GKO mice was sufficient to reverse some of the biochemical changes brought about by morphine administration. These data suggest that galanin normally attenuates behavioral and neurochemical effects of opiates; thus, galanin agonists may represent a new class of therapeutic targets for opiate addiction.

Galanin: a potential role in mesolimbic dopamine-mediated instrumental behavior

The involvement of the neuropeptide galanin in the consumption of the primary "commodities" of food and water is well established. However, the present review describes anatomical and behavioral evidence that suggests that galanin may also modulate ascending mesolimbic dopamine function and thereby play an inhibitory role in the systems by which instrumental behavior is energized toward acquiring primary commodities. General anatomical frameworks for this interaction are presented and future studies that could evaluate it are discussed.

Measuring reward with the conditioned place preference (CPP) paradigm: update of the last decade

Conditioned place preference (CPP) continues to be one of the most popular models to study the motivational effects of drugs and non-drug treatments in experimental animals. This is obvious from a steady year-to-year increase in the number of publications reporting the use this model. Since the compilation of the preceding review in 1998, more than 1000 new studies using place conditioning have been published, and the aim of the present review is to provide an overview of these recent publications. There are a number of trends and developments that are obvious in the literature of the last decade. First, as more and more knockout and transgenic animals become available, place conditioning is increasingly used to assess the motivational effects of drugs or non-drug rewards in genetically modified animals. Second, there is a still small but growing literature on the use of place conditioning to study the motivational aspects of pain, a field of pre-clinical research that has so far received little attention, because of the lack of appropriate animal models. Third, place conditioning continues to be widely used to study tolerance and sensitization to the rewarding effects of drugs induced by pre-treatment regimens. Fourth, extinction/reinstatement procedures in place conditioning are becoming increasingly popular. This interesting approach is thought to model certain aspects of relapse to addictive behavior and has previously almost exclusively been studied in drug self-administration paradigms. It has now also become established in the place conditioning literature and provides an additional and technically easy approach to this important phenomenon. The enormous number of studies to be covered in this review prevented in-depth discussion of many methodological, pharmacological or neurobiological aspects; to a large extent, the presentation of data had to be limited to a short and condensed summary of the most relevant findings.

Alternation of galanin in nociceptive modulation in the central nervous system of rats during morphine tolerance: A behavioral and immunohistochemical study

The present study was performed to investigate the alternation of galanin in nociceptive modulation and galanin-like immunoreactivity in the central nervous system of rats after morphine tolerance. The hindpaw withdrawal latencies to both thermal and mechanical stimulation increased significantly after intracerebroventricular injection of 3 nmol of galanin in opioid-naive rats. The antinociceptive effect induced by galanin was attenuated remarkably at the same dose in morphine-tolerant rats. Furthermore, an up-regulation of galanin-like immunoreactivity in the arcuate nucleus of hypothalamus of morphine-tolerant rat was observed by immunohistochemical methods, whereas no significant changes were detected in periaqueductal gray. The present study demonstrated that there are alternations in both galanin-induced antinociception and galanin-like immunoreactivity in the brain of rat after morphine tolerance. The results suggest an involvement of galanin in the central nervous system in morphine tolerance.

Galanin attenuates cyclic AMP regulatory element-binding protein (CREB) phosphorylation induced by chronic morphine and naloxone challenge in Cath.a cells and primary striatal cultures

Repeated morphine administration leads to molecular alterations of the neural circuitry in the locus coeruleus and nucleus accumbens. These changes include increased activity of several components of the cAMP signaling pathway that are thought to be associated with psychological and somatic signs of opiate withdrawal. The neuropeptide galanin has been shown to attenuate cAMP signaling in multiple cell types. The current study demonstrates that acute galanin treatment blocks the consequences of increased cAMP signaling following chronic opiate administration and withdrawal in Cath.a cells and primary cultures of striatal neurons as measured by phosphorylation of the transcription factor cAMP regulatory element-binding protein (CREB). In addition, galanin-mediated attenuation of CREB phosphorylation is independent of galanin-induced extracellular signal-regulated kinase (ERK) 1/2 phosphorylation in Cath.a cells. These data suggest that galanin receptors may serve as an additional potential therapeutic target for the treatment of opiate withdrawal.

GalR1, but not GalR2 or GalR3, levels are regulated by galanin signaling in the locus coeruleus through a cyclic AMP-dependent mechanism

The galanin receptors GalR1, GalR2 and GalR3 are widely expressed throughout the mouse brain and are enriched in catecholaminergic nuclei. Here, we show that GalR1 protein levels are regulated by neuronal activity and changes in cAMP levels. GalR1, but not GalR2 or GalR3, is specifically up-regulated in the LC-like Cath.a cell line in a cAMP-dependent manner. GalR1 protein and mRNA levels are also up-regulated in the LC of galanin knockout mice, whereas GalR2 and GalR3 are not. Lack of galanin-maintained cAMP tone in the galanin knockout mouse appears to result in a loss of negative feedback resulting in increased levels of CREB phosphorylation and increased GalR1 expression. These findings suggest that changes in levels of GalR1 may play an important role in modulating signaling events and neuroplasticity underlying physiological functions of the LC.

Galanin can attenuate opiate reinforcement and withdrawal

Galanin and its receptors are expressed in brain areas associated with opiate reinforcement and withdrawal. An emerging body of data suggests that galanin can attenuate the neurochemical, physiological and behavioral signs of opiate reinforcement and withdrawal. Experiments in transgenic mice overexpressing galanin and knockout mice lacking the peptide support a role for endogenous galanin in modulating the actions of opiates on brain regions associated with reinforcement and withdrawal. These studies suggest that galanin receptor agonists could be useful therapeutic agents to combat opiate addiction. Further, genetic variation in the genes encoding galanin and its receptors could be associated with altered susceptibility to opiate dependence.

Characterization of GalR1, GalR2, and GalR3 immunoreactivity in catecholaminergic nuclei of the mouse brain

The distribution of immunoreactivity for the three identified neuropeptide galanin receptors, GalR1, GalR2, and GalR3, was determined in areas of the mouse brain involved in drug addiction, including the ventral tegmental area (VTA), substantia nigra (SN), nucleus accumbens (NA), and locus coeruleus (LC). All three galanin receptors are found in the VTA, SN, NA, and LC; however, GalR1 protein is most highly represented in the VTA, NA, and SN, suggesting that GalR1 may play a predominant role in galanin-mediated regulation of dopamine neurotransmission. GalR1 and GalR3 protein levels are high in the LC, suggesting that these isoforms may be important for galanin-mediated regulation of noradrenergic transmission during opiate withdrawal. Although the distribution of GalR1, GalR2, and GalR3 largely recapitulates the pattern of galanin binding throughout the brain, some discrepancies exist, suggesting that another galanin receptor(s) may be present in some brain areas. Overall, GalR1, GalR2, and GalR3 are distributed widely throughout the brain, correlate with widespread galanin binding, and colocalize with tyrosine hydroxylase in catecholaminergic brain areas.

Intracerebroventricular administration of galanin decreases free water intake and operant water reinforcer efficacy in water-restricted rats

The 29/30 amino acid neuropeptide galanin coexists with vasopressin in the hypothalamus and has been shown to inhibit the actions of vasopressin and aldosterone, suggesting an inhibitory role for galanin in physiological water retention mechanisms and water seeking and water consumption behavior. Little work, however, has examined a role for galanin in water intake regulation. Furthermore, many experiments that have reported galanin-induced impairments in the performance of tasks thought to measure learning and memory have used water restriction routines and water reinforcers to maintain responding. Therefore, the present study examined the effects of intracerebroventricularly administered galanin (5.0-20.0 microg/5 microl) on free water consumption during a 10 min test session and a follow up open field exploration, an operant progressive ratio (PR) schedule, a test used to assess reinforcer strength, and an operant fixed time schedule (FT 20) in 23.5h water restricted rats. Finally, in an additional experiment that was designed to simulate the effects of a galanin-induced decrease in water reinforcer efficacy, the rats were allowed access to water prior to testing in an operant delayed non-matching to position (DNMTP) task. A galanin-induced decrease in water consumption was observed in both the free access test and the FT 20 at the 20 microg dose, but no significant galanin-induced alterations in open field behavior. A decrease in responses emitted and rewards received was observed on the PR schedule at the 5, 10, and 20 microg doses. Pre-session access to water significantly reduced the number of trials per session in the DNMTP but did not reduce accuracy. This study is the first to observe a galanin-induced reduction in water intake and reinforced operant behavior, and suggests that galanin may play a role in regulating water intake and reinforcement. However, the present data also suggest that DNMTP choice accuracy deficits observed previously cannot be attributed to a galanin-induced change in reinforcer efficacy.

[125I]-Galanin binding in brain of wildtype, and galanin- and GalR1-knockout mice: Strain and species differences in GalR1 density and distribution

Widespread production of knockout and transgenic mice has led to an increased use of mice as animal models for studies of normal- and patho-physiology. Hence, the precise mapping of central transmitter/peptide systems in the mouse has become essential for the interpretation of functional studies and for the correct correlation with findings obtained in the rat, primates and/or human. In this regard, the current study reports the autoradiographic localization of [(125)I]-galanin (GAL) binding sites in brain of the common C57BL/6J and 129OlaHsd mouse strains, as well as in GAL and galanin receptor-1 (GalR1) knockout (KO) mice. In C57BL/6J and 129OlaHsd mice, [(125)I]-GAL binding sites were detected throughout the brain, including moderate-high relative densities in the basal ganglia (caudate putamen, nucleus [n.] accumbens, olfactory tubercle, substantia nigra), limbic regions (septum, bed n. stria terminalis, ventral hippocampus, amygdala), cingulate, retrosplenial, entorhinal cortex, centro-lateral/medial thalamic n., preoptic/lateral hypothalamus, midbrain (superior colliculus, periaqueductal gray), pons/medulla oblongata (parabrachial, pontine reticular and solitary tract n.) and cerebellar cortex. [(125)I]-GAL binding levels were low or absent in main olfactory bulb, neocortex, ventrolateral/geniculate thalamic n., dorsal hippocampus, inferior colliculus and cranial motor n. In simultaneous determinations, relative [(125)I]-GAL binding site densities in brain were generally lower in C57BL/6J than in 129OlaHsd mice, while the density and distribution of central binding in the GAL-KO mouse was essentially identical to that in its background-129OlaHsd strain. In contrast, no specific [(125)I]-GAL binding was detected in any region of GalR1-KO mouse brain, revealing that under the experimental conditions used, the peptide ligand binding is predominantly (exclusively) to the GalR1 subtype. This evaluation of GAL receptor site distribution in mouse brain has revealed similarities and some differences with the equivalent system in rat and provides a valuable reference for future comparative studies of central GAL transmission.

Intracerebroventricularly administered galanin does not alter operant reaction time or differentially reinforced high rate schedule operant responding in rats

Galanin (Gal) is a 29/30 amino acid neuroendocrine peptide that impairs learning and memory processes, stimulates feeding, and modulates somatosensory, sex, and stress responses. Anatomical markers for Gal are found throughout the brain, including in the caudate-putamen and substantia nigra motor regions. Many of the behavioral tests that have been used to study the involvement of Gal in complex behavioral processes are motorically demanding, but no research has specifically investigated the involvement of Gal in response initiation or the maintenance of fine motor action. Therefore, the present study examined the effects of intraventricularly administered Gal on two highly sensitive operant tasks designed to detect alteration of these response properties. Response initiation was studied using a light-dark discrimination reaction time task that required a correct response within 2.5s of a spatially and temporally uncertain stimulus onset. The ability to perform high local rates of responding was studied using an operant differential reinforcement of high rate (DRH) of responding task. Gal (10-20 microg, i.c.v.) did not alter reaction time or inter-response time distributions in either task, though did substantially reduce the total number of responses and reinforcers obtained on the DRH schedule. These results are consistent with a Gal-induced reduction of reinforcer efficacy rather than Gal-disruption of response initiation or response patterning.

Dopamine responsiveness to drugs of abuse: A shell-core investigation in the nucleus accumbens of the mouse

The existence of subterritories within the nucleus accumbens has now been widely supported by histochemical, neurochemical, electrophysiological, as well as morphological and ultrastructural studies and suggest specific afferent and efferent systems involved in different behavioral aspects. Microdialysis studies in the rat have consistently shown that most drugs of abuse increase extracellular dopamine levels preferentially in the shell subregion of the nucleus accumbens. The study of the relative roles of NAc subregions may considerably help our understanding of the neurobiological basis of drug addiction. Accordingly, the aim of the present work was to extend the outcome of rat studies to the mouse species. Five major drugs of abuse were systemically and acutely administered to mice with a microdialysis probe implanted in either the shell or the core. A statistical comparison was performed on data transformed as percentage values of baseline dopamine vs. logarithmic values with baseline dopamine as a covariate. Results show a significant increase in dopamine levels in both the shell and core subregions following cocaine, amphetamine, nicotine, ethanol, and morphine treatments. A difference between shell and core after cocaine, nicotine, and morphine was evident when data were analyzed as percent values of baseline. However, such a shell-core dichotomy became no longer significant when ANOVA was applied on the statistically more appropriate logarithmic transformation of data with baseline as a covariate. The significant baseline differences among groups of mice (dopamine levels in the shell significantly lower compared with dopamine levels in the core) may have compromised, at least in part, the statistical procedure usually applied in microdialysis studies. These findings suggest that a careful evaluation of the data is required when subtle changes in extracellular levels of DA are measured.

The neuropeptide galanin modulates behavioral and neurochemical signs of opiate withdrawal

Much research has focused on pathways leading to opiate addiction. Pathways opposing addiction are more difficult to study but may be critical in developing interventions to combat drug dependence and withdrawal. Galanin decreases firing of locus coeruleus neurons, an effect hypothesized to decrease signs of opiate withdrawal. The current study addresses whether galanin affects morphine withdrawal signs by using a galanin agonist, galnon, that crosses the blood-brain barrier, and mice genetically engineered to under- or overexpress galanin peptide. Galnon significantly decreased morphine withdrawal signs in C57BL/6 mice. Further, knockout mice lacking galanin showed exacerbated morphine withdrawal signs, suggesting that endogenous galanin normally counteracts opiate withdrawal. Transgenic mice overexpressing galanin in noradrenergic neurons also showed decreased morphine withdrawal signs, suggesting a possible neuroanatomical locus for these effects of galanin. Both c-fos immunoreactivity, a marker of neuronal activity, and phosphorylation of tyrosine hydroxylase at Ser-40, a marker of cAMP levels, are decreased in the locus coeruleus by galnon treatment after morphine withdrawal, suggesting a possible molecular mechanism for the behavioral effects of galanin. These studies suggest that galanin normally acts to counteract opiate withdrawal and that small molecule galanin agonists could be effective in diminishing the physical signs of withdrawal.

Reduction of cocaine place preference in mice lacking the protein phosphatase 1 inhibitors DARPP 32 or Inhibitor 1

BACKGROUND

Modulation of protein phosphorylation by dopamine is thought to play an important role in drug reward. Protein phosphatase-1 (PP-1) is known to mediate some of the changes in neuronal signaling that occur following activation of the dopaminergic system.

METHODS

Two endogenous inhibitors of PP-1 are dopamine and cyclic 3', 5' adenosine monophosphate-regulated phosphoprotein (DARPP-32) and Inhibitor-1 (I-1). Knockout mice lacking one or both of these PP-1 inhibitors were tested for responses to cocaine using in vivo amperometry and conditioned place preference.

RESULTS

Presynaptic dopaminergic function appears to be unaffected by these mutations because stimulation-evoked changes in extracellular dopamine levels were unchanged between wild type mice and mice lacking one or both of these PP-1 inhibitors. In contrast, conditioned place preference to cocaine is reduced in mice lacking DARPP-32, I-1, or both phosphoproteins. This does not appear to be due to a learning deficit because mice lacking both DARPP-32 and I-1 show normal passive avoidance learning.

CONCLUSIONS

These data imply that increased PP-1 function as a result of deficits in DARPP-32 or I-1 is sufficient to decrease the rewarding properties of cocaine. Furthermore, the mechanism for this altered cocaine place preference does not involve alteration of dopamine release or reuptake.

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.

Effects of dopamine antagonists with different receptor blockade profiles on morphine-induced place preference in male mice

The effects of dopamine (DA) antagonists with different selectivity for the DA receptors (SCH 23390, 0.5, 0.25, 0.125 mg/kg; haloperidol, 0.2, 0.1 mg/kg; raclopride, 1.2, 0.6, 0.3 mg/kg; risperidone, 0.4, 0.2, 0.1 mg/kg; U-99194A maleate, 40, 20 mg/kg; clozapine, 2.5, 1.25, 0.625 mg/kg) on the acquisition of place conditioning and morphine-induced conditioned place preference (CPP) were explored in male mice. Morphine (40 mg/kg) produced CPP while SCH 23390, haloperidol and clozapine (highest dose) and risperidone (lowest dose) produced conditioned place aversion (CPA). Raclopride and U-99194A maleate did not produce CPP or CPA. Morphine-induced CPP was reversed by the administration of SCH 23390 and risperidone (all doses), haloperidol (highest dose) and raclopride and clozapine (intermediate and lowest doses). U-99194A maleate did not reverse morphine-induced CPP. These results suggest that the conditioned rewarding effects of morphine are mediated by the different subtypes of DA receptors.

Galanin receptor 1 gene expression is regulated by cyclic AMP through a CREB-dependent mechanism

The galanin receptor-1 (GalR1) protein belongs to a family of G protein-coupled receptors for the neuropeptide galanin (GalR1, GalR2 and GalR3) distributed throughout the central and peripheral nervous system. Activation of galanin receptors by their ligands results in increased feeding, impaired learning, enhanced opiate analgesia and decreased opiate place preference. We have shown that opiate withdrawal, which is known to increase levels of cAMP in the locus coeruleus (LC), results in an increase in the number of galanin binding sites and the level of GalR1 mRNA in the LC. We have isolated a 3.6-kb fragment 5' of the inititiation codon of the mouse GalR1 gene and generated a series of deletion mutations of this fragment driving expression of luciferase for use in transient transfection assays in PC12 and Cath.a cell lines. Treatment with forskolin, but not dideoxyforskolin, up-regulates GalR1 transcription, likely through elevation of cAMP levels. The region between - 1050 and - 700 base pairs upstream of exon one is necessary both for basal activity of the GalR1 promoter and for forskolin-mediated increases in transcription. The forskolin effect can be blocked by simultaneous mutation of a CRE-like site and a CRE/DRE-like site, but not mutation of either site alone. Gel shift and super-shift experiments demonstrate that the transcription factor CREB can bind to both sites and is likely to be responsible for the cAMP-mediated increase in GalR1 promoter activity. This study provides a molecular mechanism for the increased GalR1 expression in the LC seen following opiate withdrawal.

Endogenous opiates: 1999

This paper is the twenty-second installment of the annual review of research concerning the opiate system. It summarizes papers published during 1999 that studied the behavioral effects of the opiate peptides and antagonists, excluding the purely analgesic effects, although stress-induced analgesia is included. The specific topics covered this year include stress; tolerance and dependence; learning, memory, and reward; eating and drinking; alcohol and other drugs of abuse; sexual activity, pregnancy, and development; mental illness and mood; seizures and other neurologic disorders; electrical-related activity; general activity and locomotion; gastrointestinal, renal, and hepatic function; cardiovascular responses; respiration and thermoregulation; and immunologic responses.