Microarray studies of psychostimulant-induced changes in gene expression

Stress, dysregulation of drug reward pathways, and the transition to drug dependence

This review provides a neuroadaptive perspective regarding the role of the hormonal and brain stress systems in drug addiction with a focus on the changes that occur during the transition from limited access to drugs to long-term compulsive use of drugs. A dramatic escalation in drug intake with extended access to drug self-administration is characterized by a dysregulation of brain reward pathways. Hormonal studies using an experimenter-administered cocaine binge model and an escalation self-administration model have revealed large increases in ACTH and corticosterone in rats during an acute binge with attenuation during the chronic binge stage and a reactivation of the hypothalamic-pituitary-adrenal axis during acute withdrawal. The activation of the hypothalamic-pituitary-adrenal axis with cocaine appears to depend on feed-forward activation of the mesolimbic dopamine system. At the same time, escalation in drug intake with either extended access or dependence-induction produces an activation of the brain stress system's corticotropin-releasing factor outside of the hypothalamus in the extended amygdala, which is particularly evident during acute withdrawal. A model of the role of different levels of hormonal/brain stress activation in addiction is presented that has heuristic value for understanding individual vulnerability to drug dependence and novel treatments for the disorder.

Long-lasting up-regulation of orexin receptor type 2 protein levels in the rat nucleus accumbens after chronic cocaine administration

Hypothalamic orexin (hypocretin) neurons project to the key structures of the limbic system and orexin receptors, both orexin receptor type 1 (OXR1) and type 2 (OXR2), are expressed in most limbic regions. Emerging evidence suggests that orexin is among important neurotransmitters that regulate addictive properties of drugs of abuse. In this study, we examined the effect of psychostimulant cocaine on orexin receptor protein abundance in the rat limbic system in vivo. Intermittent administration of cocaine (20 mg/kg, i.p., once daily for 5 days) caused a typical behavioral sensitization response to a challenge cocaine injection at a 14-day withdrawal period. Repeated cocaine administration at the same withdrawal time also increased OXR2 protein levels in the nucleus accumbens while repeated cocaine had no effect on OXR1 and orexin neuropeptide (both orexin-A and orexin-B) levels in this region. In contrast to the nucleus accumbens, OXR2 levels in the frontal cortex, the ventral tegmental area, the hippocampus, and the dorsal striatum (caudate putamen) were not altered by cocaine. Remarkably, the up-regulated OXR2 levels in the nucleus accumbens showed a long-lasting nature as it persisted up to 60 days after the discontinuation of repeated cocaine treatments. In contrast to chronic cocaine administration, an acute cocaine injection was insufficient to modify levels of any orexin receptor and peptide. Our data identify the up-regulation of OXR2 in the nucleus accumbens as an enduring molecular event that is correlated well with behavioral plasticity in response to chronic psychostimulant administration. This OXR2 up-regulation may reflect a key adaptation of limbic orexinergic transmission to chronic drug exposure and may thus be critical for the expression of motor plasticity.

Microarrays

Microarrays are revolutionizing genetics by making it possible to genotype hundreds of thousands of DNA markers and to assess the expression (RNA transcripts) of all of the genes in the genome. Microarrays are slides the size of a postage stamp that contain millions of DNA sequences to which single-stranded DNA or RNA can hybridize. This miniaturization requires little DNA or RNA and makes the method fast and inexpensive; multiple assays of each target make the method highly accurate. DNA microarrays with hundreds of thousands of DNA markers have made it possible to conduct systematic scans of the entire genome to identify genetic associations with complex disorders or dimensions likely to be influenced by many genes of small effect size. RNA microarrays can provide snapshots of gene expression across all of the genes in the genome at any time in any tissue, which has far-reaching applications such as structural and functional 'genetic neuroimaging' and providing a biological basis for understanding environmental influence.

Quantitative peptidomics in mice: effect of cocaine treatment

We recently developed a quantitative peptidomics method using stable isotopic labels and mass spectrometry to both quantify and identify a large number of peptides. To test this approach and screen for peptides regulated by cocaine administration, 32 Cpefat/fat mice and 16 wild-type mice were treated twice daily for 5 d either with saline or 10 mg/kg cocaine. Peptides were extracted from striatum, hypothalamus, hippocampus, and prefrontal cortex, and extracts from groups of eight mice were labeled with the N-hydroxysuccinimide ester of trimethylammonium butyrate containing either nine deuterium or nine hydrogen atoms. Pools of heavy- and light-labeled peptides were combined, purified on an anhydrotrypsin affinity column, and analyzed on a reversephase column coupled to an electrospray ionization quadrapole time-of-flight mass spectrometer. Changes in peptide levels upon cocaine treatment were determined from the relative peak intensities of the cocaine versus saline peaks, and peptides were identified from collision-induced dissociation spectra. Ten peptides were found to increase or decrease in each of two separate analyses from distinct groups of mice. Peptides found to increase corresponded to fragments of proenkephalin, prothyrotropin-releasing hormone, provasopressin, proSAAS, secretogranin II, chromogranin B, and peptidyl-glycine-alpha-amidating mono-oxygenase in the hypothalamus. The same peptidyl-glycine-alpha-amidating mono-oxygenase peptide decreased in the prefrontal cortex, along with striatal neurokinin B and two unidentified peptides. Thirty other peptides were not substantially affected by cocaine treatment in both replicates. Taken together, the quantitative peptidomics approach provides an efficient method to screen for changes in a large number of peptides.

Cytosolic proteomic alterations in the nucleus accumbens of cocaine overdose victims

Chronic cocaine use in humans and animal models is known to lead to pronounced alterations in neuronal function in the nucleus accumbens (NAc), a brain region associated with drug reinforcement. Two-dimensional gel electrophoresis was used to compare protein alterations in the NAc between cocaine overdose (COD) victims (n=10) and controls (n=10). Following image normalization, spots with significantly differential image intensities (P<0.05) were identified, excised, trypsin digested and analyzed by matrix-assisted laser desorption ionization-time of flight-time of flight. A total of 1407 spots were found to be present in a minimum of five subjects per group and the intensity of 18 spots was found to be differentially abundant between the groups, leading to positive identification of 15 proteins by peptide mass fingerprinting (PMF). Of an additional 37 protein spots that were constitutively expressed, 32 proteins were positively identified by PMF. Increased proteins in COD included beta-tubulin, liprin-alpha3 and neuronal enolase, whereas decreased proteins included parvalbumin, ATP synthase beta-chain and peroxiredoxin 2. The present data provide a preliminary protein profile of COD, suggesting the involvement of novel proteins and pathways in the expression of this complex disease. Additional studies are warranted to further characterize alterations in the differentially regulated proteins. Understanding the coordinated involvement of multiple proteins in cocaine abuse provides insight into the molecular basis of the disease and offers new targets for pharmacotherapeutic intervention for drug abuse-related disorders.

Differential regulation of immediate-early gene expression in the prefrontal cortex of rats with a high vs low behavioral response to methamphetamine

Methamphetamine (METH) administration mimics many of the symptoms of mania and can produce psychosis after chronic use. Both rodents and man display interindividual variation in response to METH. The molecular mechanisms underlying these differences might be relevant to both stimulant addiction and endogenous psychosis. We treated 50 Sprague-Dawley rats acutely with METH (4.0 mg/kg) and 10 control rats with saline, and measured their behavior for 3 h after drug administration. Animals were divided into high responders (HR) (top 20%) and low responders (LR) (lowest 20%) based on their stereotypy response. They were killed 24 h after injection. Total RNA was extracted from the prefrontal cortex (PFC) and the expression of approximately 30 000 transcripts were analyzed using Affymetrix 230 2.0 GeneChips. Real-time reverse transcription-polymerase chain reaction was used to validate the expression of a select group of genes. Forty-three genes exhibited significant differences in expression in HR vs LR 24 h after METH treatment including a group of immediate-early genes (IEGs) (eg, c-fos, junB, NGFI-B, serum-regulated glucocorticoid kinase). These IEG expression differences were accompanied by the significant downregulation of many of these genes compared to saline in the HR but not LR, suggesting a differential responsiveness of signal transduction pathways in these two groups of rats. In addition, the expression of other transcription factors in the PFC was significantly different in HR compared to LR. These gene expression changes may contribute to individual differences in responsiveness to stimulants and the development of mania and psychosis.

Involvement of the AMPA receptor GluR-C subunit in alcohol-seeking behavior and relapse

Craving and relapse are core symptoms of drug addiction and alcoholism. It is suggested that, after chronic drug consumption, long-lasting neuroplastic changes within the glutamatergic system are important determinants of addictive behavior. Here, we show that the AMPA type glutamate receptor plays a crucial role in alcohol craving and relapse. We observed, in two animal models of alcohol craving and relapse, that the AMPA antagonist GYKI 52466 [1-(4-aminophenyl)-4-methyl-7, 8-methylenedioxy-5H-2, 3-benzodiazepine] dose-dependently reduced cue-induced reinstatement of alcohol-seeking behavior and the alcohol deprivation effect. The involvement of the AMPA receptor in these phenomena was further studied using mice deficient for the GluR-C AMPA subunit [GluR-C knock-out (KO)]. GluR-C KOs displayed a blunted, cue-induced reinstatement response and alcohol deprivation effect, when compared with wild-type controls; however, no differences between genotypes could be observed regarding ethanol self-administration under operant or home cage drinking conditions. These results imply a role for GluR-C in alcohol relapse, although this phenotype could also be attributable to a reduction in the total number of AMPA receptors in specific brain areas. In conclusion, AMPA receptors seem to be involved in the neuroplastic changes underlying alcohol seeking behavior and relapse. Thus, AMPA receptors represent a novel therapeutic target in preventing relapse.

Bioinformatic analysis of neuropeptide and receptor expression profiles during midgut metamorphosis in Drosophila melanogaster

Neuropeptides are important messenger molecules in invertebrates, serving as neuromodulators in the nervous system and as regulatory hormones released into the circulation. Understanding the function of neuropeptides will require the integration of genetic, biochemical, physiological and behavioral information. The advent of DNA microarrays and bioinformatic databases provides a wealth of data describing the expression profiles of thousands of genes during biological processes. One such array catalogs the developmental patterns of gene expression during the metamorphic transformation of the Drosophila midgut. We have mined the data from this experiment to explore changes of expression in genes coding for known neuropeptides, peptide hormones, and their receptors during the metamorphosis of the midgut. We found small but significant changes in the expression of the peptides diuretic hormone, FGLa-type allatostatins, myoinhibiting peptide, ecdysis-triggering hormone, drosokinin and the burs subunit of bursicon, as well as the receptors DAR-2, NPFR1, ALCR-2, Lkr and DH-R. Just as advances have been made in understanding the molecular basis of invertebrate neuropeptide action by analysis of genome projects, data mining of gene expression databases can help to integrate molecular, biochemical and physiological knowledge of biological processes.

Antipsychotic pathway genes with expression altered in opposite direction by antipsychotics and amphetamine

To develop a new strategy for identifying possible psychotic- or antipsychotic-related pathway genes, rats were treated with clinical doses of haloperidol and clozapine for 4 days, and the altered expression of genes was compared with the genes altered in expression after amphetamine sensitization. The objective was to identify genes with expression altered in the same direction by haloperidol and clozapine but in the opposite direction in the amphetamine-sensitized rat striatum. These criteria were met by 21 genes, consisting of 15 genes upregulated by amphetamine, and 6 genes downregulated by amphetamine. Of the 21 genes, 15 are not presently identified, and only 3 genes (cathepsin K, GRK6, and a gene with accession number AI177589) are located in chromosome regions known to be associated with schizophrenia.

Expression profiling methods used in drug abuse research

A variety of analytical methodologies to investigate gene expression patterns in cells or tissues have been developed. For screening purposes, a large number of target mRNAs have to be interrogated simultaneously. These requirements have been met more or less comprehensively by Differential Display (DD) RT-PCR, Suppression Subtractive Hybridization (SSH), Serial Analysis of Gene Expression (SAGE), and DNA chips. The ultimate goal to cover any gene transcript potentially expressed by a given cell is on the way to be achieved by microbead arrays and by Affymetrix gene chips. Once targets of interest are identified, techniques employing low degrees of multiplexing, such as RNAse protection assays or some bead-based techniques (Luminex) eventually provide extremely fast results on the diagnostic level. With the aid of powerful computer programs, expression profiling technologies have opened intriguing new insights into the complex world of gene regulation. These new techniques have also been applied in drug abuse research recently and some examples of such approaches are described.