Neuropeptide Y Y5 receptor antagonism attenuates cocaine-induced effects in mice

Neuropeptides Modulate Feeding via the Dopamine Reward Pathway

Dopamine (DA) is a catecholamine neurotransmitter widely distributed in the central nervous system. It participates in various physiological functions, such as feeding, anxiety, fear, sleeping and arousal. The regulation of feeding is exceptionally complex, involving energy homeostasis and reward motivation. The reward system comprises the ventral tegmental area (VTA), nucleus accumbens (NAc), hypothalamus, and limbic system. This paper illustrates the detailed mechanisms of eight typical orexigenic and anorexic neuropeptides that regulate food intake through the reward system. According to recent literature, neuropeptides released from the hypothalamus and other brain regions regulate reward feeding predominantly through dopaminergic neurons projecting from the VTA to the NAc. In addition, their effect on the dopaminergic system is mediated by the prefrontal cortex, paraventricular thalamus, laterodorsal tegmental area, amygdala, and complex neural circuits. Research on neuropeptides involved in reward feeding can help identify more targets to treat diseases with metabolic disorders, such as obesity.

Actions of feeding-related peptides on the mesolimbic dopamine system in regulation of natural and drug rewards

The mesolimbic dopamine system is the primary neural circuit mediating motivation, reinforcement, and reward-related behavior. The activity of this system and multiple behaviors controlled by it are affected by changes in feeding and body weight, such as fasting, food restriction, or the development of obesity. Multiple different peptides and hormones that have been implicated in the control of feeding and body weight interact with the mesolimbic dopamine system to regulate many different dopamine-dependent, reward-related behaviors. In this review, we summarize the effects of a selected set of feeding-related peptides and hormones acting within the ventral tegmental area and nucleus accumbens to alter feeding, as well as food, drug, and social reward.

Disruption of the PDZ domain-binding motif of the dopamine transporter uniquely alters nanoscale distribution, dopamine homeostasis, and reward motivation

The dopamine (DA) transporter (DAT) is part of a presynaptic multiprotein network involving interactions with scaffold proteins via its C-terminal PDZ domain-binding sequence. Using a mouse model expressing DAT with mutated PDZ-binding sequence (DAT-AAA), we previously demonstrated the importance of this binding sequence for striatal expression of DAT. Here, we show by application of direct stochastic reconstruction microscopy not only that the striatal level of transporter is reduced in DAT-AAA mice but also that the nanoscale distribution of this transporter is altered with a higher propensity of DAT-AAA to localize to irregular nanodomains in dopaminergic terminals. In parallel, we observe mesostriatal DA adaptations and changes in DA-related behaviors distinct from those seen in other genetic DAT mouse models. DA levels in the striatum are reduced to ∼45% of that of WT, accompanied by elevated DA turnover. Nonetheless, fast-scan cyclic voltammetry recordings on striatal slices reveal a larger amplitude and prolonged clearance rate of evoked DA release in DAT-AAA mice compared with WT mice. Autoradiography and radioligand binding show reduced DA D2 receptor levels, whereas immunohistochemistry and autoradiography show unchanged DA D1 receptor levels. In behavioral experiments, we observe enhanced self-administration of liquid food under both a fixed ratio of one and progressive ratio schedule of reinforcement but a reduction compared with WT when using cocaine as reinforcer. In summary, our data demonstrate how disruption of PDZ domain interactions causes changes in DAT expression and its nanoscopic distribution that in turn alter DA clearance dynamics and related behaviors.

The Role of Neuropeptide Y in the Nucleus Accumbens

Neuropeptide Y (NPY), an abundant peptide in the central nervous system, is expressed in neurons of various regions throughout the brain. The physiological and behavioral effects of NPY are mainly mediated through Y1, Y2, and Y5 receptor subtypes, which are expressed in regions regulating food intake, fear and anxiety, learning and memory, depression, and posttraumatic stress. In particular, the nucleus accumbens (NAc) has one of the highest NPY concentrations in the brain. In this review, we summarize the role of NPY in the NAc. NPY is expressed principally in medium-sized aspiny neurons, and numerous NPY immunoreactive fibers are observed in the NAc. Alterations in NPY expression under certain conditions through intra-NAc injections of NPY or receptor agonists/antagonists revealed NPY to be involved in the characteristic functions of the NAc, such as alcohol intake and drug addiction. In addition, control of mesolimbic dopaminergic release via NPY receptors may take part in these functions. NPY in the NAc also participates in fat intake and emotional behavior. Accumbal NPY neurons and fibers may exert physiological and pathophysiological actions partly through neuroendocrine mechanisms and the autonomic nervous system.

Roles of Neuropeptide Y in Neurodegenerative and Neuroimmune Diseases

Neuropeptide Y (NPY) is a neurotransmitter or neuromodulator that mainly exists in the nervous system. It plays a neuroprotective role in organisms and widely participates in the regulation of various physiological processes in vivo. Studies in both humans and animal models have been revealed that NPY levels are altered in some neurodegenerative and neuroimmune disorders. NPY plays various roles in these diseases, such as exerting a neuroprotective effect, increasing trophic support, decreasing excitotoxicity, regulating calcium homeostasis, and attenuating neuroinflammation. In this review, we will focus on the roles of NPY in the pathological mechanisms of neurodegenerative and neuroimmune diseases, highlighting NPY as a potential therapeutic target in these diseases.

Glucagon-Like Peptide-1 (GLP-1) and 5-Hydroxytryptamine 2c (5-HT2c) Receptor Agonists in the Ventral Tegmental Area (VTA) Inhibit Ghrelin-Stimulated Appetitive Reward

Current literature indicates that the orexigenic peptide ghrelin increases appetitive motivation via signaling in the mesolimbic reward system. Another gastric peptide, glucagon-like peptide-1 (GLP-1), and the neurotransmitter 5-hydroxytryptamine (5-HT), are both known to suppress operant responding for food by acting on key mesolimbic nuclei, including the ventral tegmental area (VTA). In order to investigate the interaction effects of ghrelin, GLP-1, and 5-HT within the VTA, we measured operant responding for sucrose pellets after the administration of ghrelin, the GLP-1 receptor agonist exendin-4 (Ex-4), and the 5-HT_2c receptor agonist Ro60-0175 in male Sprague-Dawley rats. Following training on a progressive ratio 3 (PR3) schedule, animals were first injected with ghrelin into the VTA at doses of 3 to 300 pmol. In subsequent testing, separate rats were administered intraperitoneal (IP) Ex-4 (0.1–1.0 µg/kg) or VTA Ex-4 (0.01–0.1 µg) paired with 300 pmol ghrelin. In a final group of rats, the 5-HT_2c agonist Ro60-0175 was injected IP (0.25–1.0 mg/kg) or into the VTA (1.5–3.0 µg), and under both conditions paired with 300 pmol ghrelin delivered into the VTA. Our results indicated that ghrelin administration increased operant responding for food reward and that this effect was attenuated by IP and VTA Ex-4 pretreatment as well as pre-administration of IP or VTA Ro60-0175. These data provide compelling evidence that mesolimbic GLP-1 and serotonergic circuitry interact with the ghrelinergic system to suppress ghrelin’s effects on the mediation of food reinforcement.

PICK1-Deficient Mice Exhibit Impaired Response to Cocaine and Dysregulated Dopamine Homeostasis

Protein interacting with C-kinase 1 (PICK1) is a widely expressed scaffold protein known to interact via its PSD-95/discs-large/ZO-1 (PDZ)-domain with several membrane proteins including the dopamine (DA) transporter (DAT), the primary target for cocaine's reinforcing actions. Here, we establish the importance of PICK1 for behavioral effects observed after both acute and repeated administration of cocaine. In PICK1 knock-out (KO) mice, the acute locomotor response to a single injection of cocaine was markedly attenuated. Moreover, in support of a role for PICK1 in neuroadaptive changes induced by cocaine, we observed diminished cocaine intake in a self-administration paradigm. Reduced behavioral effects of cocaine were not associated with decreased striatal DAT distribution and most likely not caused by the ∼30% reduction in synaptosomal DA uptake observed in PICK1 KO mice. The PICK1 KO mice demonstrated preserved behavioral responses to DA receptor agonists supporting intact downstream DA receptor signaling. Unexpectedly, we found a prominent increase in striatal DA content and levels of striatal tyrosine hydroxylase (TH) in PICK1 KO mice. Chronoamperometric recordings showed enhanced DA release in PICK1 KO mice, consistent with increased striatal DA pools. Viral-mediated knock-down (KD) of PICK1 in cultured dopaminergic neurons increased TH expression, supporting a direct cellular effect of PICK1. In summary, in addition to demonstrating a key role of PICK1 in mediating behavioral effects of cocaine, our data reveal a so far unappreciated role of PICK1 in DA homeostasis that possibly involves negative regulation of striatal TH levels.

The Role of Neuropeptide Y (NPY) in Alcohol and Drug Abuse Disorders

Neuropeptide Y (NPY) is a neuromodulator that is widely expressed throughout the central nervous system (CNS) and which is cosecreted with classic neurotransmitters including GABA and glutamate. There is a long history of research implicating a role for NPY in modulating neurobiological responses to alcohol (ethanol) as well as other drugs of abuse. Both ethanol exposure and withdrawal from chronic ethanol have been shown to produce changes in NPY and NPY receptor protein levels and mRNA expression in the CNS. Importantly, manipulations of NPY Y1 and Y2 receptor signaling have been shown to alter ethanol consumption and self-administration in a brain region-specific manner, with Y1 receptor activation and Y2 receptor blockade in regions of the extended amygdala promoting robust reductions of ethanol intake. Similar observations have been made in studies examining neurobiological responses to nicotine, psychostimulants, and opioids. When taken together with observations of potential genetic linkage between the NPY system and the human alcohol abuse disorders, NPY represents a promising target for treating problematic alcohol and drug use, and in protecting individuals from relapse during abstinence.

Effects of drugs of abuse on the central neuropeptide Y system

Neuropeptide Y (NPY), which is widely expressed in the central nervous system is involved in several neuropathologies including addiction. Here we comprehensively and systematically review alterations on the central NPY system induced by several drugs. We report on the effects of psychostimulants [cocaine, amphetamine, methamphetamine, 3,4-methylenedioxymethamphetamine (MDMA) and nicotine], ethanol, and opioids on NPY protein levels and expression of different NPY receptors. Overall, expression and function of NPY and its receptors are changed under conditions of drug exposure, thus affecting several physiologic behaviors, such as feeding, stress and anxiety. Drugs of abuse differentially affect the components of the NPY system. For example methamphetamine and nicotine lead to a consistent increase in NPY mRNA and protein levels in different brain sites whereas ethanol and opioids decrease NPY mRNA and protein expression. Drug-induced alterations on the different NPY receptors show more complex regulation pattern. Manipulation of the NPY system can have opposing effects on reinforcing and addictive properties of drugs of abuse. NPY can produce pro-addictive effects (nicotine and heroin), but can also exert inhibitory effects on addictive behavior (AMPH, ethanol). Furthermore, NPY can act as a neuroprotective agent in chronically methamphetamine and MDMA-treated rodents. In conclusion, manipulation of the NPY system seems to be a potential target to counteract neural alterations, addiction-related behaviors and cognitive deficits induced by these drugs.

Glucagon-like peptide 1 receptor activation regulates cocaine actions and dopamine homeostasis in the lateral septum by decreasing arachidonic acid levels

Agonism of the glucagon-like peptide 1 (GLP-1) receptor (GLP-1R) has been effective at treating aspects of addictive behavior for a number of abused substances, including cocaine. However, the molecular mechanisms and brain circuits underlying the therapeutic effects of GLP-1R signaling on cocaine actions remain elusive. Recent evidence has revealed that endogenous signaling at the GLP-1R within the forebrain lateral septum (LS) acts to reduce cocaine-induced locomotion and cocaine conditioned place preference, both considered dopamine (DA)-associated behaviors. DA terminals project from the ventral tegmental area to the LS and express the DA transporter (DAT). Cocaine acts by altering DA bioavailability by targeting the DAT. Therefore, GLP-1R signaling might exert effects on DAT to account for its regulation of cocaine-induced behaviors. We show that the GLP-1R is highly expressed within the LS. GLP-1, in LS slices, significantly enhances DAT surface expression and DAT function. Exenatide (Ex-4), a long-lasting synthetic analog of GLP-1 abolished cocaine-induced elevation of DA. Interestingly, acute administration of Ex-4 reduces septal expression of the retrograde messenger 2-arachidonylglycerol (2-AG), as well as a product of its presynaptic degradation, arachidonic acid (AA). Notably, AA reduces septal DAT function pointing to AA as a novel regulator of central DA homeostasis. We further show that AA oxidation product γ-ketoaldehyde (γ-KA) forms adducts with the DAT and reduces DAT plasma membrane expression and function. These results support a mechanism in which postsynaptic septal GLP-1R activation regulates 2-AG levels to alter presynaptic DA homeostasis and cocaine actions through AA.

A novel dopamine transporter transgenic mouse line for identification and purification of midbrain dopaminergic neurons reveals midbrain heterogeneity

Midbrain dopaminergic (DAergic) neurons are a heterogeneous cell group, composed of functionally distinct cell populations projecting to the basal ganglia, prefrontal cortex and limbic system. Despite their functional significance, the midbrain population of DAergic neurons is sparse, constituting only 20 000-30 000 neurons in mice, and development of novel tools to identify these cells is warranted. Here, a bacterial artificial chromosome mouse line [Dat1-enhanced green fluorescent protein (eGFP)] from the Gene Expression Nervous System Atlas (GENSAT) that expresses eGFP under control of the dopamine transporter (DAT) promoter was characterized. Confocal microscopy analysis of brain sections showed strong eGFP signal reporter in midbrain regions and striatal terminals that co-localized with the DAergic markers DAT and tyrosine hydroxylase (TH). Thorough quantification of co-localization of the eGFP reporter signal with DAT and TH in the ventral midbrain showed that a vast majority of eGFP-expressing neurons are DAergic. Importantly, expression profiles also revealed DAergic heterogeneity when comparing substantia nigra and ventral tegmental area. Dat1-eGFP mice showed neither change in synaptosomal DA uptake nor altered levels of DAT and TH in both striatum and midbrain. No behavioural difference between Dat1-eGFP and wild-type was found, suggesting that the strain is not aberrant. Finally, cell populations highly enriched in DAergic neurons can be obtained from postnatal mice by fluorescence-activated cell sorting and the sorted neurons can be cultured in vitro. The current investigation demonstrates that eGFP expression in this mouse line is selective for DAergic neurons, suggesting that the Dat1-eGFP mouse strain constitutes a promising tool for delineating new aspects of DA biology.

Dopaminergic Regulation of Striatal Interneurons in Reward and Addiction: Focus on Alcohol

Corticobasal ganglia networks coursing through the striatum are key structures for reward-guided behaviors. The ventral striatum (nucleus accumbens (nAc)) and its reciprocal connection with the ventral tegmental area (VTA) represent a primary component of the reward system, but reward-guided learning also involves the dorsal striatum and dopaminergic inputs from the substantia nigra. The majority of neurons in the striatum (>90%) are GABAergic medium spiny neurons (MSNs), but both the input to and the output from these neurons are dynamically controlled by striatal interneurons. Dopamine is a key neurotransmitter in reward and reward-guided learning, and the physiological activity of GABAergic and cholinergic interneurons is regulated by dopaminergic transmission in a complex manner. Here we review the role of striatal interneurons in modulating striatal output during drug reward, with special emphasis on alcohol.

The glucagon-like peptide 1 (GLP-1) receptor agonist exendin-4 reduces cocaine self-administration in mice

Glucagon-like peptide 1 (GLP-1) analogues are used for the treatment of type 2 diabetes. The ability of the GLP-1 system to decrease food intake in rodents has been well described and parallels results from clinical trials. GLP-1 receptors are expressed in the brain, including within the ventral tegmental area (VTA) and the nucleus accumbens (NAc). Dopaminergic neurons in the VTA project to the NAc, and these neurons play a pivotal role in the rewarding effects of drugs of abuse. Based on the anatomical distribution of GLP-1 receptors in the brain and the well-established effects of GLP-1 on food reward, we decided to investigate the effect of the GLP-1 analogue exendin-4 on cocaine- and dopamine D1-receptor agonist-induced hyperlocomotion, on acute and chronic cocaine self-administration, on cocaine-induced striatal dopamine release in mice and on cocaine-induced c-fos activation. Here, we report that GLP-1 receptor stimulation reduces acute and chronic cocaine self-administration and attenuates cocaine-induced hyperlocomotion. In addition, we show that peripheral administration of exendin-4 reduces cocaine-induced elevation of striatal dopamine levels and striatal c-fos expression implicating central GLP-1 receptors in these responses. The present results demonstrate that the GLP-1 system modulates cocaine's effects on behavior and dopamine homeostasis, indicating that the GLP-1 receptor may be a novel target for the pharmacological treatment of drug addiction.

Neurotransmitter and psychostimulant recognition by the dopamine transporter

Na(+)/Cl(-)-coupled biogenic amine transporters are the primary targets of therapeutic and abused drugs, ranging from antidepressants to the psychostimulants cocaine and amphetamines, and to their cognate substrates. Here we determine X-ray crystal structures of the Drosophila melanogaster dopamine transporter (dDAT) bound to its substrate dopamine, a substrate analogue 3,4-dichlorophenethylamine, the psychostimulants d-amphetamine and methamphetamine, or to cocaine and cocaine analogues. All ligands bind to the central binding site, located approximately halfway across the membrane bilayer, in close proximity to bound sodium and chloride ions. The central binding site recognizes three chemically distinct classes of ligands via conformational changes that accommodate varying sizes and shapes, thus illustrating molecular principles that distinguish substrates from inhibitors in biogenic amine transporters.

The dark side of emotion: the addiction perspective

Emotions are "feeling" states and classic physiological emotive responses that are interpreted based on the history of the organism and the context. Motivation is a persistent state that leads to organized activity. Both are intervening variables and intimately related and have neural representations in the brain. The present thesis is that drugs of abuse elicit powerful emotions that can be interwoven conceptually into this framework. Such emotions range from pronounced euphoria to a devastating negative emotional state that in the extreme can create a break with homeostasis and thus an allostatic hedonic state that has been considered key to the etiology and maintenance of the pathophysiology of addiction. Drug addiction can be defined as a three-stage cycle-binge/intoxication, withdrawal/negative affect, and preoccupation/anticipation-that involves allostatic changes in the brain reward and stress systems. Two primary sources of reinforcement, positive and negative reinforcement, have been hypothesized to play a role in this allostatic process. The negative emotional state that drives negative reinforcement is hypothesized to derive from dysregulation of key neurochemical elements involved in the brain incentive salience and stress systems. Specific neurochemical elements in these structures include not only decreases in incentive salience system function in the ventral striatum (within-system opponent processes) but also recruitment of the brain stress systems mediated by corticotropin-releasing factor (CRF), dynorphin-κ opioid systems, and norepinephrine, vasopressin, hypocretin, and substance P in the extended amygdala (between-system opponent processes). Neuropeptide Y, a powerful anti-stress neurotransmitter, has a profile of action on compulsive-like responding for drugs similar to a CRF1 receptor antagonist. Other stress buffers include nociceptin and endocannabinoids, which may also work through interactions with the extended amygdala. The thesis argued here is that the brain has specific neurochemical neurocircuitry coded by the hedonic extremes of pleasant and unpleasant emotions that have been identified through the study of opponent processes in the domain of addiction. These neurochemical systems need to be considered in the context of the framework that emotions involve the specific brain regions now identified to differentially interpreting emotive physiological expression.

Enhanced inhibitory control by neuropeptide Y Y5 receptor blockade in rats

RATIONALE

The neuropeptide Y (NPY) system acts in synergy with the classic neurotransmitters to regulate a large variety of functions including autonomic, affective, and cognitive processes. Research on the effects of NPY in the central nervous system has focused on food intake control and affective processes, but growing evidence of NPY involvement in attention-deficit/hyperactivity disorder (ADHD) and other psychiatric conditions motivated the present study.

OBJECTIVES

We tested the effects of the novel and highly selective NPY Y5 receptor antagonist Lu AE00654 on impulsivity and the underlying cortico-striatal circuitry in rats to further explore the possible involvement of the NPY system in pathologies characterized by inattention and impulsive behavior.

RESULTS

A low dose of Lu AE00654 (0.03 mg/kg) selectively facilitated response inhibition as measured by the stop-signal task, whereas no effects were found at higher doses (0.3 and 3 mg/kg). Systemic administration of Lu AE00654 also enhanced the inhibitory influence of the dorsal frontal cortex on neurons in the caudate-putamen, this fronto-striatal circuitry being implicated in the executive control of behavior. Finally, by locally injecting a Y5 agonist, we observed reciprocal activation between dorsal frontal cortex and caudate-putamen neurons. Importantly, the effects of the Y5 agonist were attenuated by pretreatment with Lu AE00654, confirming the presence of Y5 binding sites modulating functional interactions within frontal-subcortical circuits.

CONCLUSIONS

These results suggest that the NPY system modulates inhibitory neurotransmission in brain areas important for impulse control, and may be relevant for the treatment of pathologies such as ADHD and drug abuse.

Neuropeptide y gates a stress-induced, long-lasting plasticity in the sympathetic nervous system

Acute stress evokes the fight-or-flight reflex, which via release of the catecholamine hormones affects the function of every major organ. Although the reflex is transient, it has lasting consequences that produce an exaggerated response when stress is reexperienced. How this change is encoded is not known. We investigated whether the reflex affects the adrenal component of the sympathetic nervous system, a major branch of the stress response. Mice were briefly exposed to the cold-water forced swim test (FST) which evoked an increase in circulating catecholamines. Although this hormonal response was transient, the FST led to a long-lasting increase in the catecholamine secretory capacity measured amperometrically from chromaffin cells and in the expression of tyrosine hydroxylase. A variety of approaches indicate that these changes are regulated postsynaptically by neuropeptide Y (NPY), an adrenal cotransmitter. Using immunohistochemistry, RT-PCR, and NPY(GFP) BAC mice, we find that NPY is synthesized by all chromaffin cells. Stress failed to increase secretory capacity in NPY knock-out mice. Genetic or pharmacological interference with NPY and Y1 (but not Y2 or Y5) receptor signaling attenuated the stress-induced change in tyrosine hydroxylase expression. These results indicate that, under basal conditions, adrenal signaling is tonically inhibited by NPY, but stress overrides this autocrine negative feedback loop. Because acute stress leads to a lasting increase in secretory capacity in vivo but does not alter sympathetic tone, these postsynaptic changes appear to be an adaptive response. We conclude that the sympathetic limb of the stress response exhibits an activity-dependent form of long-lasting plasticity.

Addiction is a Reward Deficit and Stress Surfeit Disorder

Drug addiction can be defined by a three-stage cycle - binge/intoxication, withdrawal/negative affect, and preoccupation/anticipation - that involves allostatic changes in the brain reward and stress systems. Two primary sources of reinforcement, positive and negative reinforcement, have been hypothesized to play a role in this allostatic process. The negative emotional state that drives negative reinforcement is hypothesized to derive from dysregulation of key neurochemical elements involved in the brain reward and stress systems. Specific neurochemical elements in these structures include not only decreases in reward system function (within-system opponent processes) but also recruitment of the brain stress systems mediated by corticotropin-releasing factor (CRF) and dynorphin-κ opioid systems in the ventral striatum, extended amygdala, and frontal cortex (both between-system opponent processes). CRF antagonists block anxiety-like responses associated with withdrawal, block increases in reward thresholds produced by withdrawal from drugs of abuse, and block compulsive-like drug taking during extended access. Excessive drug taking also engages the activation of CRF in the medial prefrontal cortex, paralleled by deficits in executive function that may facilitate the transition to compulsive-like responding. Neuropeptide Y, a powerful anti-stress neurotransmitter, has a profile of action on compulsive-like responding for ethanol similar to a CRF1 antagonist. Blockade of the κ opioid system can also block dysphoric-like effects associated with withdrawal from drugs of abuse and block the development of compulsive-like responding during extended access to drugs of abuse, suggesting another powerful brain stress system that contributes to compulsive drug seeking. The loss of reward function and recruitment of brain systems provide a powerful neurochemical basis that drives the compulsivity of addiction.

The role of melanocortins and Neuropeptide Y in food reward

The Neuropeptide Y and the melanocortin peptides are two well-described hypothalamic feeding peptides regulating energy balance. Predominantly expressed within the arcuate nucleus, these neurons project to different brain areas and modulate various aspects of feeding. Hedonic feeding, where one overindulges in palatable food consumption beyond one's nutritional necessities, is one such aspect regulated by NPY/melanocortin signaling. Research suggests that NPY/melanocortin regulate hedonic aspects of feeding through its projections to the brain reward circuitry (ventral tegmental area, lateral hypothalamus, nucleus accumbens etc.), however, exact target areas have not yet been identified. The current work explores literature to provide a mechanistic explanation for the effects of these peptides on food reward.

A C-terminal PDZ domain-binding sequence is required for striatal distribution of the dopamine transporter

The dopamine transporter (DAT) mediates reuptake of dopamine from the synaptic cleft. The cellular mechanisms controlling DAT levels in striatal nerve terminals remain poorly understood. DAT contains a C-terminal PDZ (PSD-95/Discs-large/ZO-1) domain binding sequence believed to bind synaptic scaffolding proteins, but its functional significance is uncertain. Here we demonstrate that two different DAT knock-in mice with disrupted PDZ-binding motifs (DAT-AAA and DAT+Ala) are characterized by dramatic loss of DAT expression in the striatum, causing hyperlocomotion and attenuated response to amphetamine. In cultured dopaminergic neurons and striatal slices from DAT-AAA mice, we find markedly reduced DAT surface levels and evidence for enhanced constitutive internalization. In DAT-AAA neurons, but not in wild type neurons, surface levels are rescued in part by expression of a dominant-negative dynamin mutation (K44A). Our findings suggest that PDZ domain interactions are critical for synaptic distribution of DAT in vivo and thereby for proper maintenance of dopamine homeostasis.

Mood and memory-associated behaviors in neuropeptide Y5 knockout mice

Recent data led to suggest that in addition to Y1 and Y2 subtypes, Y5 receptors may be involved in mood-related behaviors (Morales-Medina et al., 2010). In the present study, using a battery of behavioral tests to assess anxiety and depression-like paradigms, as well as memory function, we evaluated the potential behavioral changes induced in mice devoid of Y5 receptors. Those paradigms were assessed using the open field (OF), elevated plus maze (EPM), forced swim test (FST), social interaction test (SI), object recognition test (ORT) and Morris water maze (MWM) in Y5 knockout (KO) mice and wild type (WT) animals. In the tests associated to anxiety related behaviors (OF, EPM and SI), no difference for locomotion and time spent in the lateral area of open field were observed between Y5 KO and WT mice. Similar results were observed for time and number of entries in open arms in EPM. Additionally, in SI test, Y5 KO mice spent same amount of time and number of entries in the stranger chamber as compared to WT animals. In the FST, as compared to WT mice, Y5 KO mice had similar immobility time on day 1. No memory dysfunction was observed in the MWM and ORT in Y5 KO mice, as compared to WT. Altogether these data suggest that under basal conditions Y5 KO and WT mice display similar mood behaviors and memory functions. However, as compared to WT, Y5 KO mice display increased grooming and rearing in the OF, lower ratio entries in open arms in the EPM and increased immobility time on the second day of the FST.

Neuropeptide Y Y5 receptor antagonism causes faster extinction and attenuates reinstatement in cocaine-induced place preference

Several studies have suggested a role for neuropeptide Y (NPY) in addiction to drugs of abuse, including cocaine. Recently, our group showed a role for the NPY Y5 receptor in the modulation of acute reinforcing effects of cocaine using self-administration and hyperlocomotion paradigms. In the present study, we further explored potential anti-addiction-related effects of Y5 antagonism in another murine model of cocaine addiction-related behavior: conditioned place-preference (CPP). Using this model, it was tested whether blockade or deficiency of the NPY Y5 receptor could influence the induction, extinction or reinstatement of a conditioned cocaine response. We found that the Y5 antagonist L-152,804 causes faster extinction and reduced reinstatement of cocaine-induced CPP but did not reduce the ability of cocaine to induce CPP. Similarly, Y5-KO mice displayed faster extinction, and reinstatement of cocaine-induced CPP was absent. The development of CPP for cocaine was similar between Y5-KO and WT mice. Taken together, the present data show that Y5 antagonism attenuates relapse to cocaine addiction-related behavior. Prevention of relapse is considered to be of pivotal importance for the development of an effective treatment against cocaine addiction and therefore Y5 receptors could be a potential future therapeutic target in cocaine addiction.

Characterization of Highper, an ENU-induced mouse mutant with abnormal psychostimulant and stress responses

RATIONALE

Chemical mutagenesis in the mouse is a forward genetics approach that introduces random mutations into the genome, thereby providing an opportunity to annotate gene function and characterize phenotypes that have not been previously linked to a given gene.

OBJECTIVES

We report on the behavioral characterization of Highper, an N-ethyl-N-nitrosourea (ENU)-induced mutant mouse line.

METHODS

Highper and B6 control mice were assessed for locomotor activity in the open field and home cage environments. Basal and acute restraint stress-induced corticosterone levels were measured. Mice were tested for locomotor response to cocaine (5, 20, 30, and 45 mg/kg), methylphenidate (30 mg/kg), and ethanol (0.75, 1.25, and 1.75 g/kg). The rewarding and reinforcing effects of cocaine were assessed using conditioned place preference and self-administration paradigms.

RESULTS

Highper mice are hyperactive during behavioral tests but show normal home cage locomotor behavior. Highper mice also exhibit a twofold increase in locomotor response to cocaine, methylphenidate, and ethanol and prolonged activation of the hypothalamic-pituitary-adrenal axis in response to acute stress. Highper mice are more sensitive to the rewarding and reinforcing effects of cocaine, although place preference in Highper mice appears to be significantly influenced by the environment in which the drug is administered.

CONCLUSIONS

Altogether, our findings indicate that Highper mice may provide important insights into the genetic, molecular, and biological mechanisms underlying stress and the drug reward pathway.

Multifaces of neuropeptide Y in the brain--neuroprotection, neurogenesis and neuroinflammation

Neuropeptide Y (NPY) has been implicated in the modulation of important features of neuronal physiology, including calcium homeostasis, neurotransmitter release and excitability. Moreover, NPY has been involved as an important modulator of hippocampal and thalamic circuits, receiving particular attention as an endogenous antiepileptic peptide and as a potential master regulator of feeding behavior. NPY not only inhibits excessive glutamate release (decreasing circuitry hyperexcitability) but also protects neurons from excitotoxic cell death. Furthermore, NPY has been involved in the modulation of the dynamics of dentate gyrus and subventricular zone neural stem cell niches. In both regions, NPY is part of the chemical resource of the neurogenic niche and acts through NPY Y1 receptors to promote neuronal differentiation. Interestingly, NPY is also considered a neuroimmune messenger. In this review, we highlight recent evidences concerning paracrine/autocrine actions of NPY involved in neuroprotection, neurogenesis and neuroinflammation. In summary, the three faces of NPY, discussed in the present review, may contribute to better understand the dynamics and cell fate decision in the brain parenchyma and in restricted areas of neurogenic niches, in health and disease.

An allosteric enhancer of M₄ muscarinic acetylcholine receptor function inhibits behavioral and neurochemical effects of cocaine

RATIONALE

The mesostriatal dopamine system plays a key role in mediating the reinforcing effects of psychostimulant drugs like cocaine. The muscarinic M₄ acetylcholine receptor subtype is centrally involved in the regulation of dopamine release in striatal areas. Consequently, striatal M₄ receptors could be a novel target for modulating psychostimulant effects of cocaine.

OBJECTIVES

For the first time, we here addressed this issue by investigating the effects of a novel selective positive allosteric modulator of M₄ receptors, VU0152100, on cocaine-induced behavioral and neurochemical effects in mice.

METHODS

To investigate the effect of VU0152100 on the acute reinforcing effects of cocaine, we use an acute cocaine self-administration model. We used in vivo microdialysis to investigate whether the effects of VU0152100 in the behavioral studies were mediated via effects on dopaminergic neurotransmission. In addition, the effect of VU0152100 on cocaine-induced hyperactivity and rotarod performance was evaluated.

RESULTS

We found that VU0152100 caused a prominent reduction in cocaine self-administration, cocaine-induced hyperlocomotion, and cocaine-induced striatal dopamine increase, without affecting motor performance. Consistent with these effects of VU0152100 being mediated via M₄ receptors, its inhibitory effects on cocaine-induced increases in striatal dopamine were abolished in M₄ receptor knockout mice. Furthermore, selective deletion of the M₄ receptor gene in dopamine D₁ receptor-expressing neurons resulted in a partial reduction of the VU0152100 effect, indicating that VU0152100 partly regulates dopaminergic neurotransmission via M₄ receptors co-localized with D₁ receptors.

CONCLUSIONS

These results show that positive allosteric modulators of the M₄ receptor deserve attention as agents in the future treatment of cocaine abuse.

Mice lacking neuropeptide Y show increased sensitivity to cocaine

There is increasing data implicating neuropeptide Y (NPY) in the neurobiology of addiction. This study explored the possible role of NPY in cocaine-induced behavior using NPY knockout mice. The transgenic mice showed a hypersensitive response to cocaine in three animal models of cocaine addiction. Whether this is due to an observed compensatory increase in striatal dopamine transporter binding or an anxiogenic phenotype of the transgenic mice remains to be determined.