A systems medicine research approach for studying alcohol addiction

Convergent evidence from alcohol-dependent humans and rats for a hyperdopaminergic state in protracted abstinence

A major hypothesis in addiction research is that alcohol induces neuroadaptations in the mesolimbic dopamine (DA) system and that these neuroadaptations represent a key neurochemical event in compulsive drug use and relapse. Whether these neuroadaptations lead to a hypo- or hyperdopaminergic state during abstinence is a long-standing, unresolved debate among addiction researchers. The answer is of critical importance for understanding the neurobiological mechanism of addictive behavior. Here we set out to study systematically the neuroadaptive changes in the DA system during the addiction cycle in alcohol-dependent patients and rats. In postmortem brain samples from human alcoholics we found a strong down-regulation of the D1 receptor- and DA transporter (DAT)-binding sites, but D2-like receptor binding was unaffected. To gain insight into the time course of these neuroadaptations, we compared the human data with that from alcohol-dependent rats at several time points during abstinence. We found a dynamic regulation of D1 and DAT during 3 wk of abstinence. After the third week the rat data mirrored our human data. This time point was characterized by elevated extracellular DA levels, lack of synaptic response to D1 stimulation, and augmented motor activity. Further functional evidence is given by a genetic rat model for hyperdopaminergia that resembles a phenocopy of alcohol-dependent rats during protracted abstinence. In summary, we provide a new dynamic model of abstinence-related changes in the striatal DA system; in this model a hyperdopaminergic state during protracted abstinence is associated with vulnerability for relapse.

Activation of Melatonin Receptors Reduces Relapse-Like Alcohol Consumption

Melatonin is an endogenous synchronizer of biological rhythms and a modulator of physiological functions and behaviors of all mammals. Reduced levels of melatonin and a delay of its nocturnal peak concentration have been found in alcohol-dependent patients and rats. Here we investigated whether the melatonergic system is a novel target to treat alcohol addiction. Male Wistar rats were subjected to long-term voluntary alcohol consumption with repeated abstinence phases. Circadian drinking rhythmicity and patterns were registered with high temporal resolution by a drinkometer system and analyzed by Fourier analysis. We examined potential antirelapse effect of the novel antidepressant drug agomelatine. Given that agomelatine is a potent MT1 and MT2 receptor agonist and a 5-HT2C antagonist we also tested the effects of melatonin itself and the 5-HT2C antagonist SB242084. All drugs reduced relapse-like drinking. Agomelatine and melatonin administered at the end of the light phase led to very similar changes on all measures of the post-abstinence drinking behavior, suggesting that effects of agomelatine on relapse-like behavior are mostly driven by its melatonergic activity. Both drugs caused a clear phase advance in the diurnal drinking pattern when compared with the control vehicle-treated group and a reduced frequency of approaches to alcohol bottles. Melatonin given at the onset of the light phase had no effect on the circadian phase and very small effects on alcohol consumption. We conclude that targeting the melatonergic system in alcohol-dependent individuals can induce a circadian phase advance, which may restore normal sleep architecture and reduce relapse behavior.

Quantification of alcohol drinking patterns in mice

The use of mice in alcohol research provides an excellent model system for a better understanding of the genetics and neurobiology of alcohol addiction. Almost 60 years ago, alcohol researchers began to test strains of mice for alcohol preference and intake. In particular, various voluntary alcohol drinking paradigms in the home cage were developed. In mouse models of voluntary oral alcohol consumption, animals have concurrent access to water and either one or several concentrated alcohol solutions in their home cages. Although these models have high face validity, many experimental conditions require a more precise monitoring of alcohol consumption in mice in order to capture the role of specific strains or genes, or any other manipulation on alcohol drinking behavior. Therefore, we have developed a fully automated, highly precise monitoring system for alcohol drinking in mice in the home cage. This system is now commercially available. We show that this drinkometer system allows for detecting differences in drinking behavior (i) in transgenic mice, (ii) following alcohol deprivation, and (iii) following stress applications that are usually not detected by classical home-cage drinking paradigms. In conclusion, our drinkometer system allows disturbance-free and high resolution monitoring of alcohol drinking behavior. In particular, micro-drinking and circadian drinking patterns can be monitored in genetically modified and inbred strains of mice after environmental and pharmacological manipulation, and therefore this system represents an improvement in measuring behavioral features that are of relevance for the development of alcohol use disorders.

The effects of single nucleotide polymorphisms in glutamatergic neurotransmission genes on neural response to alcohol cues and craving

The aim of the current study was to determine genotype effects of four single nucleotide polymorphisms (SNPs) in the genes of the N-Methyl-d-aspartate receptor (GRIN1, GRIN2A, GRIN2C) and kainate receptor (GRIK1), which have been previously associated with alcoholism, on behavior, neural cue-reactivity and drinking outcome. Eighty-six abstinent alcohol dependent patients were recruited from an in-patient setting. Neuropsychological tests, genotyping and functional magnetic resonance imaging (fMRI) were used to study genotype effects. GRIN2C risk allele carriers displayed increased alcohol cue-induced activation in the anterior cingulate cortex (ACC) and dorsolateral prefrontal cortex (dlPFC). Neural activation in the ACC positively correlated with craving for alcohol (r = 0.201, P = 0.032), whereas activation in the dlPFC showed a negative association (r = -0.215, P = 0.023). In addition, dlPFC activation predicted time to first relapse (HR = 2.701, 95%CI 1.244-5.864, P = 0.012). GRIK1 risk allele carriers showed increased cue-induced activation in the medial prefrontal (PFC) and orbitofrontal cortex (OFC) and in the lateral PFC and OFC. Activation in both clusters positively correlated with alcohol craving (rmedOFC, medPFC  = 0.403, P = 0.001, rlatOFC, latPFC  = 0.282, P = 0.008), and activation in the cluster that encompassed the medial OFC predicted time to first relapse (HR = 1.911, 95%CI 1.030-3.545, P = 0.040). Findings indicate that SNPs in the GRIN2C and GRIK1 genes are associated with altered cue-induced brain activation that is related to craving for alcohol and relapse risk.

Predicting the future relapse of alcohol-dependent patients from structural and functional brain images

In alcohol dependence, individual prediction of treatment outcome based on neuroimaging endophenotypes can help to tailor individual therapeutic offers to patients depending on their relapse risk. We built a prediction model for prospective relapse of alcohol-dependent patients that combines structural and functional brain images derived from an experiment in which 46 subjects were exposed to alcohol-related cues. The patient group had been subdivided post hoc regarding relapse behavior defined as a consumption of more than 60 g alcohol for male or more than 40 g alcohol for female patients on one occasion during the 3-month assessment period (16 abstainers and 30 relapsers). Naïve Bayes, support vector machines and learning vector quantization were used to infer prediction models for relapse based on the mean and maximum values of gray matter volume and brain responses on alcohol-related cues within a priori defined regions of interest. Model performance was estimated by leave-one-out cross-validation. Learning vector quantization yielded the model with the highest balanced accuracy (79.4 percent, p < 0.0001; 90 percent sensitivity, 68.8 percent specificity). The most informative individual predictors were functional brain activation features in the right and left ventral tegmental areas and the right ventral striatum, as well as gray matter volume features in left orbitofrontal cortex and right medial prefrontal cortex. In contrast, the best pure clinical model reached only chance-level accuracy (61.3 percent). Our results indicate that an individual prediction of future relapse from imaging measurement outperforms prediction from clinical measurements. The approach may help to target specific interventions at different risk groups.

Losing Control: Excessive Alcohol Seeking after Selective Inactivation of Cue-Responsive Neurons in the Infralimbic Cortex

Loss of control over drinking is a key deficit in alcoholism causally associated with malfunction of the medial prefrontal cortex (mPFC), but underlying molecular and cellular mechanisms remain unclear. Cue-induced reinstatement of alcohol seeking activates a subset of mPFC neurons in rats, identified by their common expression of the activity marker cFos and comprised of both principal and interneurons. Here, we used cFos-lacZ and pCAG-lacZ transgenic rats for activity-dependent or nonselective inactivation of neurons, respectively, which by their lacZ encoded β-galactosidase activity convert the inactive prodrug Daun02 into the neurotoxin daunorubicin. We report that activity-dependent ablation of a neuronal ensemble in the infralimbic but not the prelimbic subregion induced excessive alcohol seeking. The targeted neuronal ensemble was specific for the cue-induced response because stress-induced reinstatement was not affected in these animals. Importantly, nonselective inactivation of infralimbic neurons, using pCAG-lacZ rats, was without functional consequence on the cue-induced reinstatement task. Thus, inhibitory control over alcohol seeking is exerted by distinct functional ensembles within the infralimbic cortex rather than by a general inhibitory tone of this region on the behavioral output. This indicates a high level of functional compartmentation within the rat mPFC whereat many functional ensembles could coexist and interact within the same subregion.

SIGNIFICANCE STATEMENT

Hebb's (1949) idea of memories as being represented in local neuronal networks is supported by identification of transiently stable activity patterns within subgroups of neurons. However, it is difficult to link individual networks to specific memory tasks, for example a learned behavior. By a novel approach of activity-dependent ablation, here we identify a specific neuronal ensemble located in the infralimbic subregion of the medial prefrontal cortex that controls a seeking response for alcohol in rats. Our data demonstrate that functional output depends on specific neuronal ensembles within a given brain region rather than on the global activity of that region, which raises important questions about the interpretation of numerous earlier experiments using site-directed silencing or stimulation for elucidating brain function.

Long-lasting effect of NMDA receptor antagonist memantine on ethanol-cue association and relapse

It is well known that the glutamatergic system plays a crucial role in alcohol addiction and especially in relapse-like behaviour. However, results of clinical studies on compounds that influence the activity of the glutamatergic system have been disappointing so far. The aim of our study was to establish treatment conditions under which the N-methyl-d-aspartate receptor (NMDAR) antagonist memantine may produce more reliable treatment effect with respect to alcohol relapse-like behaviour. For this purpose, male Wistar rats were trained to associate several discrete stimuli with ethanol delivery. Thereafter, half of the animals received a brief memory reactivation session followed by two administrations of 20 mg/kg of memantine, while the other half received the same treatment without memory reactivation. Afterwards, a cue-induced ethanol-seeking behaviour test was performed followed by repeated extinction sessions and a reacquisition test. Our data show that administration of memantine reduced responding on the ethanol-associated lever in a cue-induced ethanol-seeking test. This reduction did not depend on whether or not a memory reactivation session was introduced prior to memantine administration. Following extinction, however, reacquisition of ethanol self-administration was only impaired in the group where memantine was given after a short memory reactivation session, showing that this schedule of drug administration produced a long-lasting disruption of the association between the conditioned stimuli and the delivery of ethanol. In conclusion, we show that memantine disrupted the drug-cue association, which consequently interfered with relapse-like behaviour supporting the possibility that memantine is a treatment option for alcoholism. Our data supports the possibility that memantine is a treatment option for alcoholism. However, the effectiveness of this drug seems to lie in its ability to disrupt conditioned behaviours and should be given in conjunction with exposure to conditioned drug stimuli.

Advances in nicotine research in Addiction Biology

The aim of Addiction Biology is to advance our understanding of the action of drugs of abuse and addictive processes via the publication of high-impact clinical and pre-clinical findings resulting from behavioral, molecular, genetic, biochemical, neurobiological and pharmacological research. As of 2013, Addiction Biology is ranked number 1 in the category of Substance Abuse journals (SCI). Occasionally, Addiction Biology likes to highlight via review important findings focused on a particular topic and recently published in the journal. The current review summarizes a number of key publications from Addiction Biology that have contributed to the current knowledge of nicotine research, comprising a wide spectrum of approaches, both clinical and pre-clinical, at the cellular, molecular, systems and behavioral levels. A number of findings from human studies have identified, using imaging techniques, alterations in common brain circuits, as well as morphological and network activity changes, associated with tobacco use. Furthermore, both clinical and pre-clinical studies have characterized a number of mechanistic targets critical to understanding the effects of nicotine and tobacco addiction. Together, these findings will undoubtedly drive future studies examining the dramatic impact of tobacco use and the development of treatments to counter nicotine dependence.

Increased mesolimbic cue-reactivity in carriers of the mu-opioid-receptor gene OPRM1 A118G polymorphism predicts drinking outcome: a functional imaging study in alcohol dependent subjects

The endogenous opioid system is involved in the pathophysiology of alcohol-use disorders. Genetic variants of the opioid system alter neural and behavioral responses to alcohol. In particular, a single nucleotide polymorphism rs1799971 (A118G) in the mu-opioid receptor gene (OPRM1) is suggested to modulate alcohol-related phenotypes and neural response in the mesocorticolimbic dopaminergic system. Little is known about the clinical implications of these changes. The current study investigated the relationship of genotype effects on subjective and neural responses to alcohol cues and relapse in a sample of abstinent alcohol-dependent patients. Functional magnetic resonance imaging (fMRI) was used to investigate alcohol cue-reactivity and drinking outcome of 81 abstinent alcohol-dependent patients. G-allele carriers displayed increased fMRI cue-reactivity in the left dorsal striatum and bilateral insulae. Neural responses to alcohol cues in these brain regions correlated positively with subjective craving for alcohol and positive expectations of alcohol׳s effects. Moreover, alcohol cue-reactivity in the left dorsal striatum predicted time to first severe relapse. Current results show that alcohol-dependent G-allele carriers׳ increased cue-reactivity is associated with an increased relapse risk. This suggests that genotype effects on cue-reactivity might link the OPRM1 A118G risk allele with an increased relapse risk that was reported in earlier studies. From a clinical perspective, risk-allele carriers might benefit from treatments, such as neuro-feedback or extinction-based therapy that are suggested to reduce mesolimbic reactivity.

A common functional allele of the Nogo receptor gene, reticulon 4 receptor (RTN4R), is associated with sporadic amyotrophic lateral sclerosis in a French population

Amyotrophic lateral sclerosis is sporadic (SALS) in 90% of cases and has complex environmental and genetic influences. Nogo protein inhibits neurite outgrowth and is overexpressed in muscle in ALS. Our aims were to study the reticulon 4 receptor gene RTN4R which encodes Nogo 1 receptor (NgR1) in SALS, to test if the variants were associated with variable expression of the gene and whether NgR1 protein expression was modified in a transgenic mouse model of ALS. We genotyped three single nucleotide polymorphisms (SNPs; rs701421, rs701427, and rs1567871) of the RTN4R gene in 364 SALS French patients and 430 controls. We examined expression of RTN4R mRNA by quantitative PCR in control post mortem human brain tissue. We determined the expression of NgR1 protein in spinal motor neurons from a SOD1 G86R ALS mouse model. We observed significant associations between SALS and RTN4R alleles. Messenger RNA expression from RTN4R in human cortical brain tissue correlated significantly with the genotypes of rs701427. NgR1 protein expression was reduced in Nogo A positive motor neurons from diseased transgenic animals. In conclusion, these observations suggest that a functional RTN4R gene variant is associated with SALS. This variant may act in concert with other genetic variants or environmental influences.

Frequency of alcohol consumption in humans; the role of metabotropic glutamate receptors and downstream signaling pathways

Rodent models implicate metabotropic glutamate receptors (mGluRs) and downstream signaling pathways in addictive behaviors through metaplasticity. One way mGluRs can influence synaptic plasticity is by regulating the local translation of AMPA receptor trafficking proteins via eukaryotic elongation factor 2 (eEF2). However, genetic variation in this pathway has not been examined with human alcohol use phenotypes. Among a sample of adults living in Detroit, Michigan (Detroit Neighborhood Health Study; n = 788; 83% African American), 206 genetic variants across the mGluR-eEF2-AMPAR pathway (including GRM1, GRM5, HOMER1, HOMER2, EEF2K, MTOR, EIF4E, EEF2, CAMK2A, ARC, GRIA1 and GRIA4) were found to predict number of drinking days per month (corrected P-value < 0.01) when considered as a set (set-based linear regression conducted in PLINK). In addition, a CpG site located in the 3'-untranslated region on the north shore of EEF2 (cg12255298) was hypermethylated in those who drank more frequently (P < 0.05). Importantly, the association between several genetic variants within the mGluR-eEF2-AMPAR pathway and alcohol use behavior (i.e., consumption and alcohol-related problems) replicated in the Grady Trauma Project (GTP), an independent sample of adults living in Atlanta, Georgia (n = 1034; 95% African American), including individual variants in GRM1, GRM5, EEF2, MTOR, GRIA1, GRIA4 and HOMER2 (P < 0.05). Gene-based analyses conducted in the GTP indicated that GRM1 (empirical P < 0.05) and EEF2 (empirical P < 0.01) withstood multiple test corrections and predicted increased alcohol consumption and related problems. In conclusion, insights from rodent studies enabled the identification of novel human alcohol candidate genes within the mGluR-eEF2-AMPAR pathway.

Clock genes × stress × reward interactions in alcohol and substance use disorders

Adverse life events and highly stressful environments have deleterious consequences for mental health. Those environmental factors can potentiate alcohol and drug abuse in vulnerable individuals carrying specific genetic risk factors, hence producing the final risk for alcohol- and substance-use disorders development. The nature of these genes remains to be fully determined, but studies indicate their direct or indirect relation to the stress hypothalamo-pituitary-adrenal (HPA) axis and/or reward systems. Over the past decade, clock genes have been revealed to be key-players in influencing acute and chronic alcohol/drug effects. In parallel, the influence of chronic stress and stressful life events in promoting alcohol and substance use and abuse has been demonstrated. Furthermore, the reciprocal interaction of clock genes with various HPA-axis components, as well as the evidence for an implication of clock genes in stress-induced alcohol abuse, have led to the idea that clock genes, and Period genes in particular, may represent key genetic factors to consider when examining gene × environment interaction in the etiology of addiction. The aim of the present review is to summarize findings linking clock genes, stress, and alcohol and substance abuse, and to propose potential underlying neurobiological mechanisms.

A pharmacogenetic determinant of mu-opioid receptor antagonist effects on alcohol reward and consumption: evidence from humanized mice

BACKGROUND

It has been proposed that therapeutic responses to naltrexone in alcoholism are moderated by variation at the mu-opioid receptor gene locus (OPRM1). This remains controversial because human results vary and no prospectively genotyped studies have been reported. We generated humanized mice carrying the respective human OPRM1 A118G alleles. Here, we used this model system to examine the role of OPRM1 A118G variation for opioid antagonist effects on alcohol responses.

METHODS

Effects of naltrexone on alcohol reward were examined using intracranial self-stimulation. Effects of naltrexone or nalmefene on alcohol intake were examined in continuous access home cage two-bottle free-choice drinking and operant alcohol self-administration paradigms.

RESULTS

Alcohol lowered brain stimulation reward thresholds in 118GG mice in a manner characteristic of rewarding drugs, and this effect was blocked by naltrexone. Brain stimulation reward thresholds were unchanged by alcohol or naltrexone in 118AA mice. In the home cage, increased alcohol intake emerged in 118GG mice with increasing alcohol concentrations and was 33% higher at 17% alcohol. At this concentration, naltrexone selectively suppressed alcohol intake in 118GG animals to a level virtually identical to that of 118AA mice. No effect of naltrexone was found in the latter group. Similarly, both naltrexone and nalmefene were more effective in suppressing operant alcohol self-administration in 118GG mice.

CONCLUSIONS

In a model that allows close experimental control, OPRM1 A118G variation robustly moderates effects of opioid antagonism on alcohol reward and consumption. These findings strongly support a personalized medicine approach to alcoholism treatment that takes into account OPRM1 genotype.

Bipolar disorder comorbid with alcohol use disorder: focus on neurocognitive correlates

Bipolar disorder (BD) and alcohol use disorders (AUDs) are usually comorbid, and both have been associated with significant neurocognitive impairment. Patients with the BD-AUD comorbidity (dual diagnosis) may have more severe neurocognitive deficits than those with a single diagnosis, but there is paucity of research in this area. To explore this hypothesis more thoroughly, we carried out a systematic literature review through January 2015. Eight studies have examined the effect of AUDs on the neurocognitive functioning of BD patients. Most studies found that BD patients with current or past history of comorbid AUDs show more severe impairments, especially in verbal memory and executive cognition, than their non-dual counterparts. Greater neurocognitive dysfunction is another facet of this severe comorbid presentation. Implications for clinical practice and research are discussed. Specifically, the application of holistic approaches, such as clinical staging and systems biology, may open new avenues of discoveries related to the BD-AUD comorbidity.

The neurometabolic fingerprint of excessive alcohol drinking

'Omics' techniques are widely used to identify novel mechanisms underlying brain function and pathology. Here we applied a novel metabolomics approach to further ascertain the role of frontostriatal brain regions for the expression of addiction-like behaviors in rat models of alcoholism. Rats were made alcohol dependent via chronic intermittent alcohol vapor exposure. Following a 3-week abstinence period, rats had continuous access to alcohol in a two-bottle, free-choice paradigm for 7 weeks. Nontargeted flow injection time-of-flight mass spectrometry was used to assess global metabolic profiles of two cortical (prelimbic and infralimbic) and two striatal (accumbens core and shell) brain regions. Alcohol consumption produces pronounced global effects on neurometabolomic profiles leading to a clear separation of metabolic phenotypes between treatment groups, particularly. Further comparisons of regional tissue levels of various metabolites, most notably dopamine and Met-enkephalin, allow the extrapolation of alcohol consumption history. Finally, a high-drinking metabolic fingerprint was identified indicating a distinct alteration of central energy metabolism in the accumbens shell of excessively drinking rats that could indicate a so far unrecognized pathophysiological mechanism in alcohol addiction. In conclusion, global metabolic profiling from distinct brain regions by mass spectrometry identifies profiles reflective of an animal's drinking history and provides a versatile tool to further investigate pathophysiological mechanisms in alcohol dependence.

XRCC5 as a risk gene for alcohol dependence: evidence from a genome-wide gene-set-based analysis and follow-up studies in Drosophila and humans

Genetic factors have as large role as environmental factors in the etiology of alcohol dependence (AD). Although genome-wide association studies (GWAS) enable systematic searches for loci not hitherto implicated in the etiology of AD, many true findings may be missed owing to correction for multiple testing. The aim of the present study was to circumvent this limitation by searching for biological system-level differences, and then following up these findings in humans and animals. Gene-set-based analysis of GWAS data from 1333 cases and 2168 controls identified 19 significantly associated gene-sets, of which 5 could be replicated in an independent sample. Clustered in these gene-sets were novel and previously identified susceptibility genes. The most frequently present gene, ie in 6 out of 19 gene-sets, was X-ray repair complementing defective repair in Chinese hamster cells 5 (XRCC5). Previous human and animal studies have implicated XRCC5 in alcohol sensitivity. This phenotype is inversely correlated with the development of AD, presumably as more alcohol is required to achieve the desired effects. In the present study, the functional role of XRCC5 in AD was further validated in animals and humans. Drosophila mutants with reduced function of Ku80-the homolog of mammalian XRCC5-due to RNAi silencing showed reduced sensitivity to ethanol. In humans with free access to intravenous ethanol self-administration in the laboratory, the maximum achieved blood alcohol concentration was influenced in an allele-dose-dependent manner by genetic variation in XRCC5. In conclusion, our convergent approach identified new candidates and generated independent evidence for the involvement of XRCC5 in alcohol dependence.

Dopamine systems adaptation during acquisition and consolidation of a skill

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.

Analytical approximations of the firing rate of an adaptive exponential integrate-and-fire neuron in the presence of synaptic noise

Computational models offer a unique tool for understanding the network-dynamical mechanisms which mediate between physiological and biophysical properties, and behavioral function. A traditional challenge in computational neuroscience is, however, that simple neuronal models which can be studied analytically fail to reproduce the diversity of electrophysiological behaviors seen in real neurons, while detailed neuronal models which do reproduce such diversity are intractable analytically and computationally expensive. A number of intermediate models have been proposed whose aim is to capture the diversity of firing behaviors and spike times of real neurons while entailing the simplest possible mathematical description. One such model is the exponential integrate-and-fire neuron with spike rate adaptation (aEIF) which consists of two differential equations for the membrane potential (V) and an adaptation current (w). Despite its simplicity, it can reproduce a wide variety of physiologically observed spiking patterns, can be fit to physiological recordings quantitatively, and, once done so, is able to predict spike times on traces not used for model fitting. Here we compute the steady-state firing rate of aEIF in the presence of Gaussian synaptic noise, using two approaches. The first approach is based on the 2-dimensional Fokker-Planck equation that describes the (V,w)-probability distribution, which is solved using an expansion in the ratio between the time constants of the two variables. The second is based on the firing rate of the EIF model, which is averaged over the distribution of the w variable. These analytically derived closed-form expressions were tested on simulations from a large variety of model cells quantitatively fitted to in vitro electrophysiological recordings from pyramidal cells and interneurons. Theoretical predictions closely agreed with the firing rate of the simulated cells fed with in-vivo-like synaptic noise.

CREB activity in dopamine D1 receptor expressing neurons regulates cocaine-induced behavioral effects

It is suggested that striatal cAMP responsive element binding protein (CREB) regulates sensitivity to psychostimulants. To test the cell-specificity of this hypothesis we examined the effects of a dominant-negative CREB protein variant expressed in dopamine receptor D1 (D1R) neurons on cocaine-induced behaviors. A transgenic mouse strain was generated by pronuclear injection of a BAC-derived transgene harboring the A-CREB sequence under the control of the D1R gene promoter. Compared to wild-type, drug-naïve mutants showed moderate alterations in gene expression, especially a reduction in basal levels of activity-regulated transcripts such as Arc and Egr2. The behavioral responses to cocaine were elevated in mutant mice. Locomotor activity after acute treatment, psychomotor sensitization after intermittent drug injections and the conditioned locomotion after saline treatment were increased compared to wild-type littermates. Transgenic mice had significantly higher cocaine conditioned place preference, displayed normal extinction of the conditioned preference, but showed an augmented cocaine-seeking response following priming-induced reinstatement. This enhanced cocaine-seeking response was associated with increased levels of activity-regulated transcripts and prodynorphin. The primary reinforcing effects of cocaine were not altered in the mutant mice as they did not differ from wild-type in cocaine self-administration under a fixed ratio schedule at the training dose. Collectively, our data indicate that expression of a dominant-negative CREB variant exclusively in neurons expressing D1R is sufficient to recapitulate the previously reported behavioral phenotypes associated with virally expressed dominant-negative CREB.

The alcohol deprivation effect model for studying relapse behavior: a comparison between rats and mice

Understanding the psychological mechanisms and underlying neurobiology of relapse behavior is essential for improving the treatment of addiction. Because the neurobiology of relapse behavior cannot be well studied in patients, we must rely on appropriate animal models. The alcohol deprivation effect (ADE) is a phenomenon in laboratory animals that models a relapse-like drinking situation, providing excellent face and predictive validity. In rodents, relapse-like behavior is largely influenced by the genetic make-up of an animal. It is not clear which other factors are responsible for variability of this behavior, but there seems to be no correlation between levels of baseline alcohol intake and the occurrence, duration, and robustness of the ADE. Rats that undergo long-term alcohol drinking for several months with repeated deprivation phases develop a compulsive drinking behavior during a relapse situation, characterized by insensitivity to taste adulteration with quinine, a loss of circadian drinking patterns during relapse-like drinking, and a shift toward drinking highly concentrated alcohol solutions to rapidly increase blood alcohol concentrations and achieve intoxication. Some mouse strains also exhibit an ADE, but this is usually of shorter duration than in rats. However, compulsive drinking in mice during a relapse situation has yet to be demonstrated. We extend our review section with original data showing that during long-term alcohol consumption, mice show a decline in alcohol intake, and the ADE fades with repeated deprivation phases. Furthermore, anti-relapse compounds that produce reliable effects on the ADE in rats produce paradoxical effects in mice. We conclude that the rat provides a better model system to study alcohol relapse and putative anti-relapse compounds.

Stress and alcohol interactions: animal studies and clinical significance

Alcohol is frequently consumed for stress relief, but the individual determinants and the temporal course of stress-induced alcohol use are not well understood. Preclinical studies may help shed light on these factors. We synthesize here the findings from numerous rodent studies of stress and alcohol interactions. Stress-induced alcohol consumption is age-dependent, has a high genetic load, and results from an interaction of the stress and reward systems. Specifically, glucocorticoids, acting within the nucleus accumbens (NAc), are important mediators of this stress-induced alcohol intake. In addition, increased activation of the corticotropin-releasing hormone (CRH) system within the extended amygdala appears to mediate stress-induced relapse. Finally, these preclinical studies have helped to identify several attractive targets for novel treatments of alcohol abuse and addiction.

Convergent functional genomics in addiction research - a translational approach to study candidate genes and gene networks

Convergent functional genomics (CFG) is a translational methodology that integrates in a Bayesian fashion multiple lines of evidence from studies in human and animal models to get a better understanding of the genetics of a disease or pathological behavior. Here the integration of data sets that derive from forward genetics in animals and genetic association studies including genome wide association studies (GWAS) in humans is described for addictive behavior. The aim of forward genetics in animals and association studies in humans is to identify mutations (e.g. SNPs) that produce a certain phenotype; i.e. "from phenotype to genotype". Most powerful in terms of forward genetics is combined quantitative trait loci (QTL) analysis and gene expression profiling in recombinant inbreed rodent lines or genetically selected animals for a specific phenotype, e.g. high vs. low drug consumption. By Bayesian scoring genomic information from forward genetics in animals is then combined with human GWAS data on a similar addiction-relevant phenotype. This integrative approach generates a robust candidate gene list that has to be functionally validated by means of reverse genetics in animals; i.e. "from genotype to phenotype". It is proposed that studying addiction relevant phenotypes and endophenotypes by this CFG approach will allow a better determination of the genetics of addictive behavior.