Genetic animal models of alcohol and drug abuse

Voluntary ethanol consumption by mice: genome-wide analysis of quantitative trait loci and their interactions in a C57BL/6ByJ x 129P3/J F2 intercross

Consumption of ethanol solutions by rodents in two-bottle choice tests is a model to study human alcohol intake. Mice of the C57BL/6ByJ strain have higher ethanol preferences and intakes than do mice of the 129P3/J strain. F2 hybrids between these two strains were phenotyped using two-bottle tests involving a choice between water and either 3% or 10% ethanol. High ethanol preferences and intakes of the B6 mice were inherited as additive or dominant traits in the F2 generation. A genome screen using these F2 mice identified three significant linkages. Two loci, on distal chromosome 4 (Ap3q) and proximal chromosome 7 (Ap7q), strongly affected 10% ethanol intake and weakly affected 3% ethanol intake. A male-specific locus on proximal chromosome 8 (Ap8q) affected 3% ethanol preference, but not indexes of 10% ethanol consumption. In addition, six suggestive linkages (on chromosomes 2, 9, 12, 13, 17, and 18) affecting indexes of 3% and/or 10% ethanol consumption were detected. The loci with significant and suggestive linkages accounted for 35-44% of the genetic variation in ethanol consumption phenotypes. No additive-by-additive epistatic interactions were detected for the primary loci with significant and suggestive linkages. However, there were a few modifiers of the primary linkages and a number of interactions among unlinked loci. This demonstrates a significant role of the genetic background in the variation of ethanol consumption.

Developmental expression of the beta-amyloid precursor protein and heat-shock protein 70 in the cerebral hemisphere region of the rat brain

Alzheimer's disease (AD) is characterized by depositions of the amyloid beta protein (A beta) in the brain in the form of extracellular plaques and cerebrovascular amyloid. A beta (approximately 4 kDa) is derived from a family of large (approximately 110 kDa) beta-amyloid precursor proteins (APP), which are integral membrane glycoproteins. Although a connection between AD and alcoholism has recently been suggested, this relationship has not been explored at the molecular level. Our hypothesis is that APP has a role in brain development and that abnormal APP levels may be involved in dementia associated with AD and alcoholism. We compared the profile of total APP levels between ethanol naïve alcohol-preferring (P) and alcohol-nonpreferring (NP) rats. We also investigated the possibility that APP levels can be regulated in an age-dependent manner in young rats. We studied the distribution of two proteins in the cerebral hemisphere region of the rat brain at various developmental periods. Six groups composed of the following different ages of rats were used: 7, 14, 21, 36, 43, and 78 (postnatal) days. Cell extracts from different regions of the brain were subjected to Western immunoblotting using mAb22C11. Our results suggest that levels of high-molecular-weight APP bands were greater in brain extracts from 7-day-old P rats than in other samples tested, and that the distribution of APP levels was more uneven in brain extracts from different ages of P than from NP rats. These initial results suggest that APP may play an important role in the early development of the rat brain and the alcohol-preferring trait may influence APP processing in the developing brain.

Association study of novel human serotonin 5-HT(1B) polymorphisms with alcohol dependence in Taiwanese Han

BACKGROUND

Abnormal serotonergic pathways are implicated in numerous neuropsychiatric disorders, such as depression, anxiety, migraine, substance abuse, and alcoholism. The human serotonin receptor 1B, encoded by the HTR1B gene, is a presynaptic serotonin autoreceptor that plays a role in regulating serotonin synthesis and release. Because the linkage of antisocial alcoholism to the HTR1B gene was recently reported in two populations, it was of interest to identify genetic variants at the HTR1B locus and study their association with alcoholism in the Taiwanese Han population.

METHODS

We sequenced DNA from Taiwanese Han to screen for genetic variation in the coding, promoter, and partial 3' untranslated regions of the HTR1B locus of 158 alcohol-dependent cases with withdrawal symptoms and 149 control subjects, who either never drank or drank only occasionally and in low quantities.

RESULTS

Seven variants were identified. Positive associations were found between variant A-161T and alcohol dependence at both the allelic and genotypic level. In addition, an expression study showed that the A-161T variant affected reporter gene activity.

CONCLUSIONS

Our results support an association between HTR1B and alcohol dependence. The HTR1B A-161T polymorphism may be valuable both as a functional and as an anonymous genetic marker for HTR1B.

Genetic contributions to addiction

Even the most extreme environmentalists along the nature-nurture continuum in psychology now acknowledge that genes often contribute to individual differences in behavior. Behavioral traits are complex, reflecting the aggregate effects of many genes. These genetic effects are interactive, inter se and with the environments in which they are expressed. Human studies of addictive behaviors have clearly implicated both environmental and genetic influences. This review selects drug dependence as a paradigmatic addiction, and further, concentrates on the extensive literature with genetic animal models. Both traditional studies with inbred strains and selected lines and studies exploiting the new molecularly based technologies of the genomics era are discussed. Future directions for further contribution of animal models studies to our understanding of the brain dysregulations characteristic of addictions are identified.

Fermenting fruit and the historical ecology of ethanol ingestion: is alcoholism in modern humans an evolutionary hangover?

In the field of addiction research, the possibility of ancestral exposure to psychoactive compounds has generally been excluded. A paleobiological approach to the human diet, however, illustrates the potential utility of historical data in interpreting modern-day addictive behaviors. Low-level dietary exposure to ethanol via ingestion of fermenting fruit has probably characterized the predominantly frugivorous anthropoid lineage for about 40 million years. Potentially adaptive primate behaviors associated with the natural occurrence of ethanol include the olfactory use of ethanol plumes to localize fruit crops, the use of ethanol as an appetitive stimulant to facilitate rapid consumption of transient nutritional resources, and the physiological exploitation of the caloric benefits of ethanol. Such behavioral and energetic advantages probably pertain to all animal taxa that consume fermenting fruit, and may have been retained in modern humans in spite of considerable dietary diversification over the last several million years. In contemporary human environments, excessive consumption of ethanol would then represent maladaptive cooption of ancestrally advantageous behaviors given essentially ad libitum access to a compound otherwise found only within scarce nutritional substrates. Epidemiologically demonstrated health benefits of low-level alcohol consumption are consistent with an ancient and potentially adaptive exposure of primate frugivores to this most common of the psychoactive substances.

Harnessing the mouse to unravel the genetics of human disease

Complex traits, i.e. those with multiple genetic and environmental determinants, represent the greatest challenge for genetic analysis, largely due to the difficulty of isolating the effects of any one gene amid the noise of other genetic and environmental influences. Methods exist for detecting and mapping the Quantitative Trait Loci (QTLs) that influence complex traits. However, once mapped, gene identification commonly involves reduction of focus to single candidate genes or isolated chromosomal regions. To reach the next level in unraveling the genetics of human disease will require moving beyond the focus on one gene at a time, to explorations of pleiotropism, epistasis and environment-dependency of genetic effects. Genetic interactions and unique environmental features must be as carefully scrutinized as are single gene effects. No one genetic approach is likely to possess all the necessary features for comprehensive analysis of a complex disease. Rather, the entire arsenal of behavioral genomic and other approaches will be needed, such as random mutagenesis, QTL analyses, transgenic and knockout models, viral mediated gene transfer, pharmacological analyses, gene expression assays, antisense approaches and importantly, revitalization of classical genetic methods. In our view, classical breeding designs are currently underutilized, and will shorten the distance to the target of understanding the complex genetic and environmental interactions associated with disease. We assert that unique combinations of classical approaches with current behavioral and molecular genomic approaches will more rapidly advance the field.

Expression profiling identifies strain-specific changes associated with ethanol withdrawal in mice

Mice that exhibit characteristics of physical dependence following ethanol exposure serve as useful models of alcoholism in humans. The DBA/2J and C57BL/6J inbred strains differ in their behavioral response to ethanol withdrawal. Alterations in gene expression are believed to underlie neuroadaptation to ethanol dependence and tolerance. Therefore, the differences in ethanol withdrawal severity observed between the DBA/2J and C57BL/6J strains may be related to differential regulation of gene expression. We have used cDNA microarrays to determine the gene expression profile in the hippocampus of DBA/2J and C57BL/6J mice during withdrawal after chronic and acute ethanol exposure. Of the 7634 genes surveyed, approximately 2% were consistently differentially expressed by at least 1.4-fold in DBA/2J mice during chronic ethanol withdrawal. Less than 1% of the genes showed altered expression in C57BL/6J mice under the same conditions, or in DBA/2J mice during acute ethanol withdrawal. Strain- and treatment-specific patterns of altered expression were observed for multiple genes associated with the Janus kinase/signal transducers and activators of transcription and the mitogen activated protein kinase pathways. Genes associated with both pathways are regulated in DBA/2J mice during chronic ethanol withdrawal, and to a lesser extent during acute ethanol withdrawal. Only those genes associated with the mitogen-activated protein kinase (MAPK) pathway exhibited changes in expression in C57BL/6J mice during ethanol withdrawal. Furthermore, genes associated with retinoic acid-mediated signaling show differential expression exclusively in C57BL/6J mice. These findings represent significant differences in cellular adaptation to ethanol between the DBA/2J and C57BL/6J strains.

Novel and previously reported single-nucleotide polymorphisms in the human 5-HT(1B) receptor gene: no association with cocaine or alcohol abuse or dependence

Evidence from animal self-administration and human genetics studies suggests that the serotonin(1B) (5-HT(1B)) receptor may be involved in modulating responses to cocaine or alcohol. We hypothesize that polymorphisms, including single-nucleotide polymorphisms (SNPs), in the human 5-HT(1B) receptor gene, may be associated with individual differences in vulnerability to cocaine or alcohol abuse or dependence. A total of 210 subjects were studied, including individuals with a primary diagnosis (DSM-IV criteria) of cocaine abuse or dependence, alcohol abuse or dependence, and controls with no history of previous or current illicit drug or alcohol abuse or dependence. Genomic DNA samples were isolated from each individual. For 157 of the subjects, polymerase chain reaction (PCR) was used to amplify the entire coding region of the 5-HT(1B) receptor gene as well as parts of the 5' and 3' untranslated regions. PCR products were sequenced in forward and reverse directions on an automated sequencer. Amplified DNA from an additional 53 subjects was sequenced in the 5' untranslated region to gain additional data on the frequency of one identified SNP. Seven polymorphisms were identified: one novel SNP in the 5' untranslated region (UTR) of the gene (A-161T); one SNP not reported in any published scientific communication (but found to be recorded in GenBank) in the 3' UTR (A1180G); two novel dinucleotide deletions at positions - 184/- 183 and - 182/- 181; and three previously identified SNPs (T-261G, C129T, G861C). Data were stratified by ethnicity and pooled Relative Risk was calculated for combined alcohol abuse and dependence cases and controls, and also for combined cocaine abuse and dependence cases and controls. No significant differences between cases and controls were found.

An animal model for the study of the genetic bases of behaviour in men: the multiple marker strains (MMS)

Animal models are often used for preclinical research on the neurobiology of psychiatric disorders. Whereas many are employed to screen new therapeutic agents, few of them are used to study the genetic bases of psychiatric diseases, probably because of the complex genetic determinism underlying quantitative behavioral traits such as mood, personality or intelligence. The present article presents a short review introducing an analysis model using mice: the marker strains model. Using this model it is possible both to display genetic determinism data and to locate some of the chromosomal fragments involved in the regulation of anxiogenic processes. At present it cannot accurately determine the position of one or more genes, but it does provide a valuable means of 'scanning' the genome for an approximation. Through genetic analysis, using the model, an attempt will be made to identify autosomal fragments which may be involved in two behavioural traits: anxiety and chemical-induced seizures. In this paper, after reviewing theoretical aspects of looking for genes involved in behaviour, we will successively introduce studies in genetic topics in psychiatric human studies as well as appropriated behavioural animal studies. Then we will present a genetic model in mice which allows us to locate chromosomal fragments associated with a behavioural trait: multiple marker strains.

The inebri-actometer: a device for measuring the locomotor activity of Drosophila exposed to ethanol vapor

Drosophila melanogaster can be used as a model organism for probing the genetic basis for alcohol sensitivity. In this paper, we describe a new device, the inebri-actometer, which measures the locomotor activity of up to 128 individual flies simultaneously. The device consists of 128 pairs of emitter/detector photodiodes connected in series through a computer interlink. A single fly is placed in each of the 128 chambers and humidified air or air containing variable amounts of ethanol vapor is pumped through the chambers. When a fly blocks the infrared signal transmitted by an emitter photodiode, the computer records one movement for that fly. We present preliminary results showing the effect of ethanol on the activity of wild-type Oregon R Drosophila. Five preliminary runs with 95% ethanol vapor revealed that this concentration induces an approximately 3- to 4-fold increase in locomotor activity which peaks at about 5 min after the addition of ethanol vapor. This is followed by a gradual decrease in activity leading to a nearly total cessation of movement after 30 min. Statistically significant dose-related activity increases were obtained for ethanol concentrations of 8, 19, 50, and 100% of maximum, assessed in two replications at each dose. Unlike the complete suppression of locomotion seen in the last 10 min of the session at maximum ethanol exposure, the initial stimulation effect at the 19% concentration was maintained across the 30-min session.

Dark Agouti and Fischer 344 rats: differential behavioral responses to morphine and biochemical differences in the ventral tegmental area

We sought to identify behavioral and biochemical differences between Dark Agouti and Fischer 344 inbred rat strains to assess whether they could serve as a model of genetically determined differences in sensitivity to drugs of abuse. We compared the strains for the following traits: morphine-induced locomotor activity and sensitization; circadian variation in plasma levels of corticosterone, a hormone reported to affect sensitivity to drugs of abuse; and several biochemical parameters in the ventral tegmental area and nucleus accumbens, brain regions implicated in the locomotor activating and reinforcing actions of drugs of abuse. Fischer 344 rats exhibited greater initial locomotor responses to morphine but, unlike Dark Agouti rats, did not develop sensitization to a second morphine exposure. Fischer 344 rats displayed a marked rise in basal plasma corticosterone levels in the late light phase and early dark phase, whereas Dark Agouti rats showed no significant circadian variation in corticosterone levels. Relative to drug-naive Fischer 344 rats, drug-naive Dark Agouti rats showed higher levels of tyrosine hydroxylase and glial fibrillary acidic protein, and lower levels of neurofilament proteins, in the ventral tegmental area. In contrast, no strain differences were found in levels of tyrosine hydroxylase, specific G protein subunits or protein kinase A in the nucleus accumbens. Together, these results demonstrate that Dark Agouti rats and Fischer 344 rats exhibit differences in specific behavioral, endocrine and biochemical parameters related to sensitivity to drugs of abuse.

Withdrawal from alcohol in withdrawal seizure-prone and -resistant mice: evidence for enkephalin resistance

Methionine enkephalin (Met-enkephalin) functions as an endogenous anticonvulsant. Peptide transport system-1 (PTS-1) is an important regulator of Met-enkephalin levels in brain and transports the peptide from brain to blood. In outbred mice, alcohol dependence is associated with decreased PTS-1 activity and increased levels of Met-enkephalin. In contrast, alcohol withdrawal is associated with recovery of PTS-1 activity, decreased levels of Met-enkephalin, and seizures. In this study, we examined the PTS-1/Met-enkephalin system in two replicates of withdrawal seizure-resistant (WSR) and withdrawal seizure-prone (WSP) mouse lines. We measured levels of preproenkephalin (PPE) mRNA and Met-enkephalin peptide in brain and the activity of PTS-1 during alcohol-naive, -dependent, and -withdrawal states. In alcohol-naive animals, Met-enkephalin levels were higher in WSP than in WSR mice. In alcohol-withdrawal animals, Met-enkephalin levels remained elevated in WSP mice, whereas they increased in WSR mice. Peptide levels were unrelated to levels of PPE mRNA or activity of PTS-1. Factorial analysis showed that proneness to seizures was genetically linked to Met-enkephalin levels in alcohol-naive, -dependent, and -withdrawing mice but not to mRNA levels or PTS-1 activity. Overall, these results may be explained by resistance to enkephalin in WSP mice and suggest that the dysregulation of the PTS-1/Met-enkephalin system contributes to susceptibility to seizures in WSP mice.

Rat strain differences in responses of plasma prolactin and PRL mRNA expression after acute amphetamine treatment or restraint stress

1. The aim of this study was to compare the effects of acute amphetamine (AMPH) treatment and restraint stress on plasma level of prolactin (PRL) and PRL mRNA expression in the adenohypophysis in Sprague-Dawley and Lewis male rats, the latter known to have a deficient hypothalamo-pituitary-adrenal (HPA) axis. 2. Both restraint stress and AMPH treatment (i.p. in a dose of 8 mg/kg of b.w.) were applied 15 or 30 min before termination of the experiment. Plasma PRL and corticosterone (CORT) were determined by radioimmunoassay. PRL mRNA expression was estimated by a dot-blot hybridization. 3. Restraint stress and AMPH treatment induced a significant increase in the CORT plasma level, as an indicator of stress response. Compared to Sprague-Dawley rats, the magnitude of CORT increase after both stimuli was significantly lower in Lewis rats. 4. Although restraint stress significantly increased the PRL plasma levels in both rat strains, AMPH treatment reduced the PRL levels in both rat strains. However, the changes of PRL plasma levels had another pattern in Lewis rats than in Sprague-Dawley rats. Control plasma PRL levels were significantly higher in Lewis rats, and in this rat strain AMPH treatment for 30 min increased the PRL levels as compared to the values obtained after AMPH treatment for 15 min. 5. Expression of PRL mRNA in adenohypophysis by restraint stress and AMPH treatment had a similar pattern. After a 15-min lasting restraint stress, the expression of PRL mRNA was decreased insignificantly in both rat strains. AMPH treatment induced in Sprague-Dawley rats a significant decrease of PRL mRNA after a 15-min interval while after 30 min there was a significant increase. However, in Lewis rats AMPH failed to significantly change PRL mRNA. 6. The results from the present study indicate that the mechanisms mediating the effects of acute restraint stress and acute AMPH treatment differ in PRL response in Sprague-Dawley and Lewis male rat strains. Differences in the observed responses in Lewis rats could be related to the deficient activity of HPA axis in this rat strain.

The pharmacodynamics of anticonvulsant and subconvulsant effects of ethanol in CBA and C57BL/6 mice

A method of determination of minimal effective doses (MEDs) of bicuculline causing clonic-tonic convulsions (CTC) and tonic extension (TE) was used to investigate ethanol pharmacodynamics in C57BL/6 and CBA mice, differing in levels of alcohol predisposition. It is observed that ethanol produces a powerful anticonvulsant action antagonizing convulsant effects of bicuculline. On a long-term scale, the pharmacological action of alcohol had two phases in both strains of mice: anticonvulsant (in the interval 5 min to 4 h after ethanol administration) and subconvulsant (4-24 h after ethanol administration). C57BL/6 mice were characterized by a more rapid development of the anticonvulsant effect and its faster decay in comparison to CBA strain. A possibility of correct quantitative evaluation of data allows using the method of MED determination as an express model of an acute alcohol abstinence syndrome, as well as for screening of new antialcohol drugs.

Pharmacogenetics and the serotonin system: initial studies and future directions

Serotonin (5-hydroxytryptamine, 5-HT) appears to play a role in the pathophysiology of a range of neuropsychiatric disorders, and serotonergic agents are of central importance in neuropharmacology. Genes encoding various components of the 5-HT system are being studied as risk factors in depression, schizophrenia, obsessive-compulsive disorder, aggression, alcoholism, and autism. Recently, pharmacogenetic research has begun to examine possible genetic influences on therapeutic response to drugs affecting the serotonin system. Genes regulating the synthesis (TPH), storage (VMAT2), membrane uptake (HTT), and metabolism (MAOA) of 5-HT, as well as a number of 5-HT receptors (HTR1A, HTR1B, HTR2A, HTR2C, and HTR5A), have been studied and this initial research is reviewed here. After a brief introduction to serotonin neurobiology and a general discussion of appropriate genetic methodology, each of the major 5-HT-related genes and their encoded proteins are reviewed in turn. For each gene, relevant polymorphisms and research on functional variants are discussed; following brief reviews of the disorder or trait association and linkage studies, pharmacogenetic studies performed to date are covered. The critical and manifold roles of the serotonin system, the great abundance of targets within the system, the wide range of serotonergic agents-available and in development-and the promising preliminary results suggest that the serotonin system offers a particularly rich area for pharmacogenetic research.

Pharmacogenomics and addiction to opiates

The risk of initiating and maintaining the use of opiates up to the point of abuse and dependence is to a large degree genetically transmitted and is separate from genetic risk factors for addiction to other drugs of abuse. Pharmacogenetic studies have so far focused on obvious candidate genes that are expected to be involved either in the pharmacokinetics or in the pharmacodynamics of opioids in the mesolimbic reward system of the brain. The few findings of a positive allelic association rarely withstand replication in independent case-control or less stratification-prone family-based association samples. A pharmacogenomic approach in the best sense of the word, however, involves an unbiased, genome-wide, parallel search for risk genes and gene expression patterns. So far, only quantitative trait loci mapping studies of inbred rodent strains and differential expression studies using high-density DNA microarrays fulfill these requirements. The present state of pharmacogenomic and pharmacogenetic studies in animals and humans with respect to opiate addiction is reviewed in this paper.

Effect of genetic cross on the detection of quantitative trait loci and a novel approach to mapping QTLs

A genome-wide scan was conducted in two F(2) intercrosses, C57BL/6J (B6)xDBA/2J (D2) and BALB/cJ (C)xLP/J (LP), for three different phenotypes: basal locomotor activity, ethanol-induced locomotor activity, and haloperidol-induced catalepsy. For basal activity, significant quantitative trait loci (QTLs, LOD> or =4.3) were detected on chromosomes 9 and 19 for the CxLP intercross and chromosome 1 for the B6xD2 intercross. Significant QTLs for ethanol-induced activation were detected on chromosome 6 for the CxLP intercross, and on chromosomes 1 and 2 for the B6xD2 intercross. For haloperidol-induced catalepsy, significant QTLs were detected on chromosome 14 (two different QTLs) in the CxLP intercross, and chromosomes 1 and 9 in the B6xD2 intercross. These data illustrate the importance of the genetic cross for QTL detection. Finally, the data reported here, and elsewhere, are also used to demonstrate a novel approach to QTL detection and localization.

Effects of repeated morphine on cerebral dopamine release and metabolism in AA and ANA rats

Cerebral dopaminergic mechanisms were studied in the nucleus accumbens and caudate-putamen of alcohol-preferring AA (Alko Alcohol) and alcohol-avoiding ANA (Alko Non-Alcohol) rats after 4-day repeated morphine treatment. This treatment has been shown to enhance the locomotor activity stimulating effect of morphine in the AA but not in the ANA rats. Morphine (1 or 3 mg/kg) or saline was administered subcutaneously once daily and the extracellular concentrations of dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) were measured, in freely moving rats by in vivo microdialysis on days 1 and 4. Morphine increased accumbal DA, DOPAC and HVA similarly in rats of both lines, and no sensitization of DA release or metabolism was seen in rats of either line given morphine repeatedly. In the caudate-putamen, morphine increased DA, DOPAC and HVA significantly only in the AA rats. During repeated treatment, the morphine-induced elevation of DA metabolites, but not that of DA, was enhanced similarly in rats of both lines. These results suggest that the effects of acute morphine administration on nigrostriatal dopaminergic mechanisms are stronger in the AA than in the ANA rats, whereas the effects of morphine on mesolimbic dopaminergic systems do not differ. Furthermore, in rats of both lines, repeated morphine treatment enhanced the responses of the nigrostriatal dopaminergic systems similarly, but no enhancement occurred in the mesolimbic systems of rats of either line. These findings do not support the critical role of accumbal dopaminergic systems in morphine-induced behavioural sensitization.

DNA

The authors predict that in a few years, many areas of psychology will be awash in specific genes responsible for the widespread influence of genetics on behavior. As the focus shifts from finding genes (genomics) to understanding how genes affect behavior (behavioral genomics), it is important for the future of psychology as a science that pathways between genes and behavior be examined not only at the molecular biological level of cells or the neuroscience level of the brain but also at the psychological level of analysis. After a brief overview of quantitative genetic research, the authors describe how genes that influence complex traits like behavioral dimensions and disorders in human and nonhuman animals are being found. Finally, the authors discuss behavioral genomics and predict that DNA will revolutionize psychological research and treatment early in the 21st century.

Functional ethanol tolerance in Drosophila

In humans, repeated alcohol consumption leads to the development of tolerance, manifested as a reduced physiological and behavioral response to a particular dose of alcohol. Here we show that adult Drosophila develop tolerance to the sedating and motor-impairing effects of ethanol with kinetics of acquisition and dissipation that mimic those seen in mammals. Importantly, this tolerance is not caused by changes in ethanol absorption or metabolism. Rather, the development of tolerance requires the functional and structural integrity of specific central brain regions. Mutants unable to synthesize the catecholamine octopamine are also impaired in their ability to develop tolerance. Taken together, these data show that Drosophila is a suitable model system in which to study the molecular and neuroanatomical bases of ethanol tolerance.

Effects of cocaine-induced sensitization on ethanol drinking: sex and strain differences

Sensitization induced by repeated drug exposure has been proposed to increase 'wanting' the drug and to facilitate the transition from moderate to excessive drug intake. The present study examined the effects of cocaine-induced sensitization on ethanol-drinking behavior in male and female rats from different strains. In experiment 1, rats were pretreated with six injections of saline or cocaine (10 mg/kg, i.p.), spaced by 3-day intervals, and were subsequently allowed access to ethanol intake in an unrestricted free-choice procedure. In experiment 2, rats had acquired ethanol-drinking behavior and were exposed to the sensitizing treatment described previously or were left undisturbed. Subsequently, all animals again had access to ethanol. Whatever the sex and strain concerned, sensitized and control animals did not differ in either the acquisition or the maintenance of ethanol-drinking behavior, suggesting that cocaine-induced behavioral sensitization does not modify ethanol intake. The present results also confirm the sex- and strain-dependent character of alcohol intake and of the 'alcohol deprivation effect'.

Abolition and reversal of strain differences in behavioral responses to drugs of abuse after a brief experience

Inbred strains of mice are largely used to identify the genetic basis of normal and pathological behaviors. This report demonstrates that a moderate period of food shortage, an ecologically common experience, can reverse or abolish strain differences in behavioral responses to the abused psychostimulant amphetamine. The period of food shortage occurred when the animals were mature and was terminated before the administration of amphetamine. Strain differences in behavior appear highly dependent on environmental experiences. Consequently, to identify biological determinants of behavior, an integrated approach considering the interaction between environmental and genetic factors needs to be used.

Vertical shifts in self-administration dose-response functions predict a drug-vulnerable phenotype predisposed to addiction

The role of individual differences in the etiology of addiction is a very controversial issue. Neuroendocrine phenotypes that are able to predispose an individual to the development of drug intake have been identified previously. However, such information has been gathered by comparing individuals who differ in their sensitivity to low doses of the drug. Consequently, it remains unclear whether a phenotype predicting a higher sensitivity to low drug doses would be relevant in environmental conditions, such as the ones encountered by humans in which high drug doses are available. In this report, we studied dose-response, dose-intake, and ratio-intake functions for intravenous cocaine self-administration in the laboratory rat. We show that individual differences in drug self-administration originate from vertical shift in the dose-response function. Thus, no matter the dose, drug intake is very high in some "vulnerable" subjects and very low in other "resistant" ones. Vulnerable subjects, the upward shifted ones, would then have a higher chance to develop drug abuse also when high drug doses are available. In conclusion, these results provide a solid foundation for the existence of a drug-vulnerable phenotype relevant for the etiology of addiction.

Genetic studies of substance abuse

Genetic studies of substance abuse indicate that variation in the risk for the disorder in the population is contributed by differences in both individual genotypes and environment. Recent developments in genetics raise the possibility of disentangling the complex system of genotype-environment interaction that determines the development of the individual behavioral phenotype. This paper reviews the concepts, methods and results pertaining to genetic investigation of substance abuse.

An assessment of novelty-seeking behavior in alcohol-preferring and nonpreferring rats

This study examined novelty-seeking behavior in rat populations selectively bred for high and low alcohol-drinking behavior. In Experiment 1, and "odor-enhanced" novel environment produced greater behavioral activation in P compared to NP rats. In Experiment 2, the activity of high alcohol-drinking P and HAD rats was enhanced to a greater extent following the presentation of novel odors in a familiar arena, compared to the NP and LAD rats. The results suggest that, when measuring locomotor activity, alcohol-preferring rats are more reactive to novelty than their nonpreferring counterparts. Experiments 3 and 4, however, did not support the hypothesis that novelty seeking is associated with genetic vulnerability to high alcohol-drinking behavior. When measuring nose-poking behavior in response to novel odors and preference for a novel vs. a familiar chamber, behavior of the preferring lines did not differ from that of the nonpreferring lines, although P rats were more active in the place-preference paradigm. The overall results indicate that the relationship between novelty and alcohol drinking is only modestly associated, and is observed under specific conditions. Moreover, this study underscores the importance of using multiple measures when assessing complex behaviors such as novelty seeking.

Absence of cocaine-induced place conditioning in serotonin 1B receptor knock-out mice

A large body of evidence suggests that genetic factors may affect the reinforcing properties of drugs of abuse. This study investigated the involvement of the serotonin 1B (5-HT1B) receptor in modulating cocaine-induced place conditioning by comparing the response of 5-HT1B receptor gene knock-out mice with wild type 129/Sv-ter mice. The rewarding effects of various doses of cocaine (0, 2.5, 5, 10, 20, and 40 mg/kg) were examined in both strains. Results clearly show that 5-HT1B receptor knock-out mice failed to display a conditioned place preference for stimuli paired with cocaine while wild type mice exhibited a conditioned place preference for the compartment paired with cocaine (5 and 20 mg/kg). As other studies showed that 5-HT1B knock-out mice self-administer cocaine, these results suggest a dissociation between the psychologic state linked to self-administration and the one measured in conditioned place preference.

Effect of 5-HT(1B) receptor ligands on self-administration of ethanol in an operant procedure in rats

Recent evidence suggests that 5-HT(1B) receptor activation modifies ethanol's reinforcing, intoxicating and discriminative stimulus effects. The present study further explored the role played by 5-HT(1A/1B) receptors by examining their influence on oral ethanol self-administration. Male Wistar rats were trained on an FR 4 schedule to obtain a reinforcer of 0.1 12% w/v ethanol solution. Once responding was stable, the effect of the 5-HT(1A/1B) agonist RU24969 alone and in combination with the 5-HT(1B) antagonist GR127935 or the 5-HT(1A) antagonists (+) WAY100135 and (+) WAY100635 was assessed. The effect of RU24969 on ethanol's pharmacokinetic profile and on operant oral saline self-administration was also examined to assess if alterations in oral ethanol self-administration were due to nonspecific effects on level pressing. For comparison, we examined the effect of another 5-HT(1A/1B) agonist, CGS12066B, on oral ethanol self-administration. Both RU24969 (0.1 to 1 mg/kg) and CGS12066B (0.1 to 1 mg/kg) significantly suppressed oral ethanol self-administration. Administration of GR127935 (1 mg/kg), significantly reversed the effects elicited by RU24969, whereas neither WAY100635 (1 mg/kg) nor (+)WAY100135 (1 mg/kg) had any effect. The effects of lower doses of RU24969 on oral ethanol self-administration were selective as oral saline self-administration and blood ethanol levels were not altered by these doses. These data demonstrate that 5-HT(1B) receptor activation suppresses oral ethanol self-administration. These studies provide further evidence that 5-HT(1B) receptors play a modulatory role in ethanol's behavioral effects.

Mouse genetics/genomics: an effective approach for drug target discovery and validation

The mouse has become the premier mammalian system for the identification of the genetic basis of both mono- and oligogenic disorders, as well as the understanding of complex diseases with gene-gene and gene-environment interactions. The similarity between human and mouse genetic disease is sometimes striking, while in other cases the phenotypes are less similar. The ability to genetically map and then clone single gene disorders rapidly, and the emerging technologies that will allow the economical identification of the polygenes controlling quantitative traits further demonstrate the utility of the mouse as a model for gene discovery. Additionally, the ability to genetically manipulate the mouse through transgenesis and gene targeting allows for the testing of hypotheses regarding specific gene function and their role in disease. The utility of the mouse extends beyond being just a gene discovery tool to provide prevalidated targets. It can also be used for the development of animal models, and the testing of compounds in specifically constructed transgenic and knockout strains to further define the target and pathway of a therapeutic compound.

Evaluation of an allelic association of the serotonin 5-HT1B G681C polymorphism with antisocial alcoholism in the German population

Our study tested whether an association of the 861C allele of the serotonin 5-HT1B gene (HTR1B) with antisocial alcoholism exists in the German population. The HTR1B G861C polymorphism was genotyped in 588 subjects of German descent, comprising 250 non-alcoholic controls and 338 alcohol-dependent subjects, of whom 56 exhibited a dissocial personality disorder (DSPD). The Tridimensional Personality Questionnaire was assessed in 109 alcohol-dependent males to explore an effect of the 861C allele to risk-taking behaviour. Our results revealed no evidence for an association of the 861C allele with antisocial alcoholism (p > 0.63). There were no significant differences in the personality traits, novelty-seeking, harm avoidance and reward dependence between 46 male alcoholics carrying the 861C allele compared to those 63 alcoholics lacking it (p > 0.52). Our results do not provide evidence that the 861C allele contributes a substantial vulnerability effect to antisocial behavior in German alcohol-dependent subjects.

Genetics of alcohol withdrawal

Alcohol withdrawal is a clinically and etiologically heterogeneous syndrome caused by a complex interaction of environmental (e.g., amount of ethanol) and genetic factors. Multiple genes are considered to be involved in various components of the syndrome, each of them contributing only modestly to withdrawal vulnerability. Association studies using candidate genes of the dopamine, serotonin, gabaergic and opioidergic systems are reviewed and methodological limitations are discussed.

Transition to alcohol dependence: clinical and neurobiological considerations

The transition to alcohol dependence is supposed to occur during a critical period that begins with increased drinking and ends with a loss of control. This process may last about 3 to 4 years, and is modified by gender and accelerated by premorbid traits (e.g., novelty-seeking) and comorbid disorders (e.g., dissocial personality disorders according to ICD-10). Genetic disposition, environmental influences (e.g., stress), and sensitization by exposure are factors implicated in dependence that alter brain functions, some possibly in an irreversible way. Underlying neurobiological mechanisms that may have different time patterns are beginning to be characterized on a systemic, cellular, and molecular level. Repeated free choices of the rewarding compound seem to be necessary for the transition to dependence.

Genetic studies of autism: from the 1970s into the millennium

Reviewers in the 1960s and early 1970s were skeptical about any substantial role for genetic factors in the etiology of autism. A realization that the 2% rate of autism in siblings (as estimated at that time) was far above the general population base rate, and that this suggested a possible high genetic liability, led to the first small-scale twin study of autism. The replicated evidence from both twin and family studies undertaken in the 1970s and 1980s indicated both strong genetic influences and the likelihood that they applied to a phenotype that was much broader than the traditional diagnostic category of autism. Medical and chromosomal findings also indicated genetic heterogeneity. Advances in molecular genetics led to genome-wide scans of affected relative pair samples with a positive log of the odds to base 10 score for a location on chromosome 7. The major remaining research challenges and the likely clinical benefits that should derive from genetic research are considered in relation to both current knowledge and that anticipated to emerge from research over the next decade.

Background genotype modulates the effects of γ-PKC on the development of rapid tolerance to ethanol-induced hypothermia

The role of γ-PKC in initial sensitivity and in the development of rapid tolerance to the hypothermic effects of ethanol were investigated in γ-PKC null mutant mice. Effects of the single gene mutation were evaluated on three different genetic backgrounds. Null mutants from a C57BL/6J X 129/SvJ mixed genetic background failed to develop rapid tolerance after 4 days of i.p. ethanol injections. However, when the null mutation was introgressed onto a C57BL/6J background for six generations to create a congenic line, the expression of rapid tolerance unexpectedly reoccurred in the null mutant mice. Subsequent outcrossing of the γ-PKC null mutation to a C57BL/6J X 129/SvEvTac mixed background did not restore the no tolerance phenotype. These observations, taken together with similar results reported previously concerning the development of chronic tolerance to ethanol in these same genotypes, ¹ indicate that the gene coding for gamma-PKC has pleiotropic effects in the expression of both rapid and chronic tolerance to ethanol-induced hypothermia. However, the impact of γ-PKC is modulated by the background genotype. These results stress the necessity of understanding interactions with genetic background when interpreting the effects of single gene mutations on complex behavioral traits.

Cocaine reward and MPTP toxicity: alteration by regional variant dopamine transporter overexpression

Polygenic factors play important roles in animal models of substance abuse and susceptibility to dopaminergic neurodegeneration. Genetic factors are also likely to contribute to the etiology of human drug abuse disorders, and may alter human vulnerabilities to Parkinsonian neurodegeneration. The dopamine transporter (DAT; SLC6A3) is densely expressed by the dopaminergic midbrain neurons that play central roles in drug reward and is believed to be a primary site of action for cocaine reward. This transporter is necessary for the action of selective dopaminergic neurotoxins, and is uniquely expressed on neurons that are the primary targets of Parkinsonian neurodegeneration. To study possible influences of variant DAT expression on these processes, we have constructed transgenic mice (THDAT) in which tyrosine hydroxylase (TH) promoter sequences drive expression of a rat DAT cDNA variant, increase striatal DAT expression by 20-30%, and provide modest alterations in striatal levels of dopamine and its metabolites. THDAT mice habituate more rapidly to a novel environment than wildtype littermates. These animals display enhanced reward conferred by cocaine, as measured by conditioned place preference. However, locomotor responses to cocaine administration are similar to those of wildtype mice, except at high cocaine doses. THDAT mice display more than 50% greater losses of dopaminergic neurons following a course of MPTP treatment than do wildtype control mice. These results document a model for allelic variation at a gene locus that can exert significant effects in murine models of human substance abuse vulnerability and dopaminergic neurodegeneration.

Strain and sex differences in the locomotor response and behavioral sensitization to cocaine in hyperactive rats

Individual variability in the behavioral responsiveness to psychostimulant drugs is due, in part, to genetic factors. The present study investigated the effects of acute and repeated administrations of cocaine (0, 5, 10 or 20 mg/kg, i.p.) on locomotor activity in male and female rats from genetically distinct strains often used as a model of human childhood hyperactivity/attentional deficit disorder: Wistar Kyoto Hyperactive (WKHA) rats, Spontaneous Hypertensive rats (SHR) and their control Wistar Kyoto (WKY). The results, expressed as percent change in locomotor activity relative to respective control groups, showed that cocaine elicits a dose-dependent hyperactivity in all strains and revealed neither strain nor sex differences in acute sensitivity to moderate doses of the drug. Nevertheless, across repeated administrations, strain and sex differences appeared: WKHA rats displayed a moderate extent of sensitization to psychomotor stimulant effects of cocaine and female rats showed more robust sensitization than males, whatever the strain. These findings support the genotype-dependence in the development of behavioral sensitization to cocaine and confirm the robustness of the sexual dimorphism across different inbred rat strains. Interestingly, the present results demonstrate that sensitization to psychostimulant drugs occurs in genetically hyperactive strains as well as in their normoactive control strain.

Interrelationship of genetic and environmental influences on conduct disorder and alcohol and marijuana dependence symptoms

Data from the Vietnam Era Twin (VET) Registry were analyzed to explore the degree to which the same genetic and environmental factors contribute to childhood conduct disorder symptoms and to alcohol and marijuana dependence symptoms. Data on conduct disorder and alcohol and marijuana dependence were obtained from administration of the Diagnostic Interview Schedule to 1,856 monozygotic and 1,479 dizygotic male-male twin pair members of the VET Registry. Multivariate genetic models were compared to determine the genetic and environmental influences common and or specific to all three phenotypes. A full model that allowed for common genetic and environmental influences to all three phenotypes gave a good fit to the data, but the best fitting reduced model did not allow for a genetic influence on conduct disorder symptoms. Under the best fitting reduced model, genes explained 44.7% of the variance in risk for alcohol dependence symptoms. The genetic liability for symptoms of marijuana dependence was due to a 36.3% specific contribution and a 7.6% contribution from genes common with alcohol dependence symptoms. Family environmental contributions common to all three phenotypes explained 46.7%, 11.9%, and 21.3% of variance in risk for symptoms of conduct disorder, alcohol dependence, and marijuana dependence, respectively. Common family environmental factors contribute to risk of conduct disorder symptoms and alcohol and marijuana dependence symptoms. Common genetic influences contribute to risk of symptoms of alcohol dependence and marijuana dependence. While our findings suggest genes do not contribute to co-morbid conduct disorder symptoms, comparisons with other twin studies suggest that the role of genes in risk for conduct disorder remains uncertain.