The striatum plays a key role in motor learning. Striatal function depends strongly on dopaminergic neurotransmission, but little is known about neuroadaptions of the dopamine system during striatal learning. Using an established task that allows differentiation between acquisition and consolidation of motor learning, we here investigate D1 and D2-like receptor binding and transcriptional levels after initial and long-term training of mice. We found profound reduction in D1 binding within the dorsomedial striatum (DMS) after the first training session on the accelerated rotarod and a progressive reduction in D2-like binding within the dorsolateral striatum (DLS) after extended training. Given that similar phase- and region-specific striatal neuroadaptations have been found also during learning of complex procedural tasks including habit formation and automatic responding, the here observed neurochemical alterations are important for our understanding of neuropsychiatric disorders that show a dysbalance in the function of striatal circuits, such as in addictive behaviors.
dopamine receptors; receptor binding; rotarod; striatum; gene expression; neuroadaptation; learning
Alcoholism is a complex behavioral disorder in which interactions between stressful life events and heritable susceptibility factors contribute to the initiation and progression of disease. Neural substrates of these interactions remain largely unknown. Here, we examined the role of the nociceptin/orphanin FQ (N/OFQ) system, using an animal model in which genetic selection for high alcohol preference has led to co-segregation of elevated behavioral sensitivity to stress (msP rats).
msP and Wistar rats trained to self-administer alcohol received central injections of N/OFQ. In situ hybridization, and receptor binding assays were also performed to evaluate N/OFQ receptor (NOP) function in naïve msP and Wistar rats.
Intracerebroventricular (ICV) injection of N/OFQ significantly inhibited alcohol self-administration in msP but not in nonselected Wistar rats. NOP receptor mRNA expression and binding was upregulated across most brain regions in msP compared to Wistar rats. However, in msP rats [35S]GTPγS binding revealed a selective impairment of NOP receptor signaling in the central amygdala (CeA). Ethanol self-administration in msP rats was suppressed after N/OFQ microinjection into the CeA but not into the bed nucleus of the stria terminalis or the basolateral amygdala.
These findings indicate that dysregulation of N/OFQ-NOP receptor signaling in the CeA contributes to excessive alcohol intake in msP rats, and that this phenotype can be rescued by local administration of pharmacological doses of exogenous N/OFQ. Data are interpreted based of the anti-CRF actions of N/OFQ and the significance of the CRF system in promoting excessive alcohol drinking in msP rats.
Addiction, Nociceptin/Orphanin FQ; NOP receptors; Central Amygdala; Alcohol Preferring Rats; Alcoholism
A key deficit in alcohol dependence is disrupted prefrontal function leading to excessive alcohol seeking, but the molecular events underlying the emergence of addictive responses remain unknown. Here we show by convergent transcriptome analysis that the pyramidal neurons of the infralimbic cortex are particularly vulnerable for the long-term effects of chronic intermittent ethanol intoxication. These neurons exhibit a pronounced deficit in mGluR2. Also, alcohol dependent rats do not respond to mGluR2/3 agonist treatment with reducing extracellular glutamate levels in the nucleus accumbens. Together these data imply a loss of autoreceptor feedback control. Alcohol dependent rats show escalation of ethanol seeking, which was abolished by restoring mGluR2 expression in the infralimbic cortex via viral-mediated gene transfer. Human anterior cingulate cortex from alcoholic patients shows a significant reduction in mGluR2 transcripts compared to control subjects suggesting that mGluR2 loss in the rodent and human cortico-accumbal neurocircuitry may be a major consequence of alcohol dependence and a key pathophysiological mechanism mediating increased propensity to relapse. Normalization of mGluR2 function within this brain circuit may be of therapeutic value.
The rewarding effects of alcohol have been attributed to interactions between opioid and dopaminergic system within the mesolimbic reward pathway. We have previously shown that ablation of β-arrestin 2 (Arrb2), a crucial regulator of μ-opioid receptor function, attenuates alcohol-induced hyperlocomotion and c-fos activation in the nucleus accumbens.
Here, we further investigated the role of Arrb2 in modulating alcohol-induced dopamine (DA) release and conditioned place preference (CPP). We also assessed the functional importance of Arrb2 for μ-opioid receptor surface expression and signaling following an acute alcohol challenge.
Alcohol-evoked (0.375, 0.75 and 1.5 g/kg intraperitoneally, i.p.) DA release was measured by in vivo microdialysis in the shell of nucleus accumbens. Reward was assessed by the CPP paradigm. Receptor function was assessed by μ-receptor binding and [35S]GTP-γ-S autoradiography.
In Arrb2 knockout mice accumbal DA levels reach maximum response at a lower dose compared to wild-type (wt) animals. In line with these results, Arrb2 knockout mice display increased CPP for alcohol as compared to wt mice. Finally, Arrb2 mutant mice display increased μ-opioid receptor signaling in the ventral and dorsal striatum and amygdala in response to a low dose of alcohol, indicating impaired desensitization mechanisms in these mice.
Our results show that Arrb2 modulates the response to low doses of alcohol on various levels including μ-opioid receptor signaling, DA release, and reward. They also reveal a clear dissociation between the effects of Arrb2 on psychomotor and reward behaviors.
Arrestin; opioid; dopamine; alcohol; reward; nucleus accumbens
Studies in humans and animals suggest a role for NPY in the mediation of behavioral stress responses. Here, we examined whether the NPY promoter variant rs16147:T>C is functional for expression of NPY in a brain region relevant for behavioral control, anxiety and depression, the anterior cingulate cortex. In silico analysis of DNA structural profile changes produced by rs16147 variation suggests allelic differences in protein binding at the rs16147 site. This was confirmed by electrophoretic mobility shift assay, demonstrating that the rs16147 C-allele has strongly reduced affinity for a yet unknown factor compared to the T-allele. Analyzing 107 human post-mortem brain samples we show that allelic variation at rs16147 contributes to regulation of NPY mRNA and peptide levels in this region. Specifically, the C-allele leads to increased gene expression. In agreement with the molecular findings, rs16147:T>C is associated with anxiety and depressive symptoms in 314 young adults via a gene × environment interaction with early childhood adversity, replicating the recent finding of rs16147-C as a risk factor for stress related psychopathology. Our results show the importance of rs16147:T>C for regulation of NPY gene expression and brain function.
NPY; promoter variant; brain; gene expression; stress coping
Long-term changes in brain gene expression have been identified in alcohol dependence, but underlying mechanisms remain unknown. Here, we examined the potential role of microRNAs for persistent gene expression changes in the rat medial prefrontal cortex after a history of alcohol dependence. Two-bottle free-choice alcohol consumption increased following 7-week exposure to intermittent alcohol intoxication. A bioinformatic approach using microarray analysis, qPCR, bioinformatic analysis, and microRNA-mRNA integrative analysis identified expression patterns indicative of a disruption in synaptic processes and neuroplasticity. 41 rat-microRNAs and 165 mRNAs in the medial prefrontal cortex were significantly altered after chronic alcohol exposure. A subset of the microRNAs and mRNAs was confirmed by qPCR. Gene ontology categories of differential expression pointed to functional processes commonly associated with neurotransmission, neuroadaptation, and synaptic plasticity. microRNA-mRNA expression pairing identified 33 microRNAs putatively targeting 89 mRNAs suggesting transcriptional networks involved in axonal guidance and neurotransmitter signaling. Our results demonstrate a significant shift in microRNA expression patterns in the medial prefrontal cortex following a history of dependence. Due to their global regulation of multiple downstream target transcripts, microRNAs may play a pivotal role in the reorganization of synaptic connections and long term neuroadaptations in alcohol dependence. microRNA-mediated alterations of transcriptional networks may be involved in disrupted prefrontal control over alcohol-drinking observed in alcoholic patients.
alcohol; addiction; neuroadaptation; gene expression; microRNA; medial prefrontal cortex
The comparability of gene expression between blood and brain tissues is a central issue in neuropsychiatric research where the analysis of molecular mechanisms in the brain is of high importance for the understanding of the diseases and the discovery of biomarkers. However, the accessibility of brain tissue is limited. Therefore, knowledge about how easily accessible peripheral tissue, e. g. blood, is comparable to and reflects gene expression of brain regions will help to advance neuropsychiatric research.
Gene expression in the blood, hippocampus (HC) and prefrontal cortex (PFC) of genetically identical rats was compared using a genome-wide Affymetrix gene expression microarray covering 29,215 expressed genes. A total of 56.8% of 15,717 expressed genes were co-expressed in blood and at least one brain tissue, while 55.3% of all genes were co-expressed in all three tissues simultaneously. The overlapping genes included a set of genes of relevance to neuropsychiatric diseases, in particular bipolar disorder, schizophrenia and alcohol addiction. These genes included CLOCK, COMT, FAAH, NPY, NR3C1, NRGN, PBRM1, TCF4, and SYNE.
This study provides baseline data on absolute gene expression and differences between gene expression in the blood, HC and PFC brain tissue of genetically identical rats. The present data represents a valuable resource for future studies as it might be used for first information on gene expression levels of genes of interest in blood and brain under baseline conditions. Limitations of our study comprise possible contamination of brain tissue with blood and the non-detection of genes with very low expression levels. Genes that are more highly expressed in the brain than in the blood are of particular interest since changes in their expression, e.g. due to disease status, or treatment, are likely to be detected in an experiment. In contrast, genes with higher expression in the blood than in the brain are less informative since their higher baseline levels could superimpose variation in brain.
Gene expression; Brain; Blood; Comparison; Rat
The most common functional single nucleotide polymorphism of the human OPRM1 gene, A118G, has been shown to be associated with inter-individual differences in opioid analgesic requirements, particularly with morphine, in patients with acute postoperative pain. The purpose of the present study was to examine whether this polymorphism would modulate the morphine and fentanyl pharmacological profile of sensory neurons isolated from a humanized mouse model homozygous for either the 118A or 118G allele.
The coupling of wild-type and mutant mu opioid receptors to voltage-gated Ca2+ channels after exposure to either ligand was examined by employing the whole-cell variant of the patch-clamp technique in acutely dissociated trigeminal ganglion neurons. Morphine-mediated antinociception was measured in mice carrying either the 118AA or 118GG allele.
The biophysical parameters (cell size, current density, and peak current amplitude potential) measured from both groups of sensory neurons were not significantly different. In 118GG neurons, morphine was approximately 5-fold less potent and 26% less efficacious than that observed in 118AA neurons. On the other hand, the potency and efficacy of fentanyl were similar for both groups of neurons. Morphine-mediated analgesia in 118GG mice was significantly reduced compared to the 118AA mice.
This study provides evidence to suggest that the diminished clinical effect observed with morphine in 118G carriers results from an alteration of the receptor’s pharmacology in sensory neurons. Additionally, the impaired analgesic response with morphine may explain why carriers of this receptor variant have an increased susceptibility to become addicted to opioids.
Excessive alcohol use, a major cause of morbidity and mortality, is less well understood than other addictive disorders. Dopamine release in ventral striatum is a common element of drug reward, but alcohol has an unusually complex pharmacology, and humans vary greatly in their alcohol responses. This variation is related to genetic susceptibility for alcoholism, which contributes more than half of alcoholism risk. Here, we report that a functional OPRM1 A118G polymorphism is a major determinant of striatal dopamine responses to alcohol. Social drinkers recruited based on OPRM1 genotype were challenged in separate sessions with alcohol and placebo under pharmacokinetically controlled conditions, and examined for striatal dopamine release using positron emission tomography and [11C]-raclopride displacement. A striatal dopamine response to alcohol was restricted to carriers of the minor 118G allele. To directly establish the causal role of OPRM1 A118G variation, we generated two humanized mouse lines, carrying the respective human sequence variant. Brain microdialysis showed a four-fold greater peak dopamine response to an alcohol challenge in h/mOPRM1-118GG than in h/mOPRM1-118AA mice. OPRM1 A118G variation is a genetic determinant of dopamine responses to alcohol, a mechanism by which it likely modulates alcohol reward.
alcohol; dopamine; opioids; reward; polymorphism; positron emission tomography
Understanding the pathophysiology of addictive disorders is critical for development of new treatments. A major focus of addiction research has for a long time been on systems that mediate acute positively reinforcing effects of addictive drugs, most prominently the mesolimbic dopaminergic (DA) system and its connections. This research line has been successful in shedding light on the physiology of both natural and drug reward, but has not led to therapeutic breakthroughs. The role of classical reward systems is perhaps least clear in alcohol addiction. Here, recent work is summarized that points to some clinically important conclusions. First, important pharmacogenetic differences exist with regard to positively reinforcing effects of alcohol and the ability of this drug to activate classical reward pathways. This offers an opportunity for personalized treatment approaches in alcoholism. Second, brain stress and fear systems become pathologically activated in later stages of alcoholism and their activation is a major influence in escalation of alcohol intake, sensitization of stress responses, and susceptibility to relapse. These findings offer a new category of treatment mechanisms. Corticotrophin-releasing hormone (CRH) signaling through CRH1 receptors is a major candidate target in this category, but recent data indicate that antagonists for substance P (SP) neurokinin 1 (NK1) receptors may have a similar potential.
alcoholism; reward; opioids; pharmacogenetics; stress; anxiety; amygdala; corticotrophin-releasing hormone; substance P; neurokinin
This article represents the proceedings of a symposium at the 2004 International Society for Biomedical Research on Alcoholism in Mannheim, Germany, organized and co-chaired by Susan E. Bergeson and Wolfgang Sommer. The presentations and presenter were (1) Gene Expression in Brains of Alcohol-Preferring and Non-Preferring Rats, by Howard J. Edenberg (2) Candidate Treatment Targets for Alcoholism: Leads from Functional Genomics Approaches, by Wolfgang Sommer (3) Microarray Analysis of Acute and Chronic Alcohol Response in Brain, by Susan E. Bergeson (4) On the Integration of QTL and Gene Expression Analysis, by Robert J. Hitzemann (5) Microarray and Proteomic Analysis of the Human Alcoholic Brain, by Peter R. Dodd.
Several components in the Wnt pathway, including β-catenin and glycogen synthase kinase 3 beta, have been implied in AD pathogenesis. Here, mRNA brain levels from five-month-old tg-ArcSwe and nontransgenic mice were compared using Affymetrix microarray analysis. With surprisingly small overall changes, Wnt signaling was the most affected pathway with altered expression of nine genes in tg-ArcSwe mice. When analyzing mRNA levels of these genes in human brain, transcription factor 7-like 2 (TCF7L2) and v-myc myelocytomatosis viral oncogene homolog (MYC), were increased in Alzheimer's disease (AD) (P < .05). Furthermore, no clear differences in TCF7L2 and MYC mRNA were found in brains with frontotemporal lobar degeneration, suggesting that altered regulation of these Wnt-related genes could be specific to AD. Finally, mRNA levels of three neurogenesis markers were analyzed. Increased mRNA levels of dihydropyrimidinase-like 3 were observed in AD brain, suggesting that altered Wnt pathway regulation may signify synaptic rearrangement or neurogenesis.
Neuropeptide Y (NPY) is important to countering stress and is involved in neuroadaptations that drive escalated alcohol drinking following repeated alcohol exposure in rodents. In humans, haplotype-driven diminution in NPY expression is predictive of amygdala response and emotional reactivity to stress. Genetic variation that affects the NPY system could impact resilience to stress and to developing addiction with continued alcohol use.
To determine whether functional NPY variation influences CSF NPY, behavioral adaptation to stress, and alcohol consumption in a nonhuman primate model of early adversity (peer rearing).
We sequenced the rhesus macaque NPY locus (rhNPY) and performed in silico analysis to identify functional variants. We performed gel shift assays for a −1002 T>G using nuclear extract from testes, brain and hypothalamus. Levels of NPY in CSF were measured by RIA, and mRNA levels were assessed in amygdala using RT-PCR. During infancy, animals were exposed to repeated social separation stress, and tested for individual differences in alcohol consumption as young adults. Animals were genotyped for −1002 T>G, and the effects of this variant on mRNA expression, CSF NPY, behavior arousal during stress, and ethanol consumption were assessed by ANOVA.
The G allele altered binding of regulatory proteins in all nuclear extracts tested, and −1002 T>G resulted in lower levels of NPY expression in amygdala. Macaques exposed to adversity had lower CSF NPY and exhibited higher levels of arousal during stress, but only as a function of the G allele. We also found that stress-exposed G allele carriers consumed more alcohol and exhibited an escalation in intake over cycles of alcohol availability and deprivation.
Our results suggest a role for NPY promoter variation in the susceptibility to alcohol use disorders and point to NPY as a candidate for examining GxE interactions in humans.
The brain renin–angiotensin system (RAS) participates importantly in the regulation of endocrine, autonomic, and behavioral response to stress. Recent data indicate that central action of AT1 receptor antagonists can reduce anxiety symptoms in experimental animals. Furthermore, central inhibition of RAS activity decreases ethanol intake in an animal model of alcoholism. Pathological anxiety responses and the development of substance dependence are both critically mediated through corticotrophin-releasing hormone (CRH) systems, and the RAS is positioned to interact both with hypothalamic as well as extrahypothalamic CRH systems. The thesis of this paper is that the RAS is part of the neurochemical dysregulation underlying negative affective states, anxiety disorders, and ethanol dependence and that medications targeting the RAS should be considered to augment the treatment of these disorders.
Angiotensin receptor; ACE; Alcoholism; Anxiety; Gene expression
Mitogen-activated and extracellular regulated kinase (MEK) and extracellular signal-regulated protein kinase (ERK) pathways may underlie ethanol-induced neuroplasticity. Here, we used the MEK inhibitor UO126 to probe the role of MEK/ERK signaling for the cellular response to an acute ethanol challenge in rats with or without a history of ethanol dependence. Ethanol (1.5g/kg, i.p.) induced expression of the marker genes c-fos and egr-1 in brain regions associated both with rewarding and stressful ethanol actions. Under non-dependent conditions, alcohol-induced c-fos expression was generally not affected by MEK inhibition, with the exception of medial amygdala (MeA). In contrast, following a history of dependence, a markedly suppressed c-fos response to acute ethanol was found in medial prefrontal-/orbitofrontal cortex (OFC), nucleus accumbens shell (AcbSh) and paraventricular nucleus (PVN). The suppressed ethanol response in the OFC and AcbSh, key regions involved in ethanol preference and seeking, was restored by pre-treatment with UO126, demonstrating a recruitment of an ERK/MEK mediated inhibitory regulation in the post-dependent state. Conversely, in brain areas involved in stress responses (MeA, PVN), a MEK/ERK mediated cellular activation by acute ethanol was lost following a history of dependence.
These data reveal region-specific neuroadaptations encompassing the MEK/ERK pathway in ethanol dependence. Recruitment of MEK/ERK mediated suppression of the ethanol response in OFC and AcbSh may reflect devaluation of ethanol as a reinforcer, while loss of a MEK/ERK mediated response in MeA and PVN may reflect tolerance to its aversive actions. These two neuroadaptations could act in concert to facilitate progression into ethanol dependence.
Alcoholism; animal model; mitogen-activated protein kinase; immediate early genes; extended amygdala; in situ hybridization