Differential rat brain expression of EGR proteins and of the transcriptional corepressor NAB in response to acute or chronic cocaine administration

Characteristics of MK-801-induced locomotor sensitization

Repeated administration of drugs of abuse leads to progressively greater behavioral responses; this phenomenon is referred to as behavioral sensitization. MK-801 blocks the N-methyl-d-aspartate (NMDA) receptor and elicits behavioral sensitization. Ketamine and phencyclidine, are also NMDA antagonists and have well-documented abuse potential. This study investigated the characteristics of MK-801-induced behavioral sensitization and found that it induced sensitization rapidly; only five consecutive treatments were required. The optimal dose for robust sensitization was also identified, which corresponded to the typical doses of abused NMDA antagonists (i.e., between the doses inducing antidepressant and anesthetic effects). Following MK-801-induced behavioral sensitization, changes were observed in the expression and/or phosphorylation of NMDA receptor subunits. While the expression of early growth response protein 1, which serves as a marker of neuronal activation, was affected by MK-801 sensitization, extracellular signal-regulated kinase phosphorylation was not associated with MK-801 treatment.

Inducible CRISPR Epigenome Systems Mimic Cocaine Induced Bidirectional Regulation of Nab2 and Egr3

Substance use disorder is a chronic disease and a leading cause of disability around the world. The NAc is a major brain hub mediating reward behavior. Studies demonstrate exposure to cocaine is associated with molecular and functional imbalance in NAc medium spiny neuron subtypes (MSNs), dopamine receptor 1 and 2 enriched D1-MSNs and D2-MSNs. We previously reported repeated cocaine exposure induced transcription factor early growth response 3 (Egr3) mRNA in NAc D1-MSNs, and reduced it in D2-MSNs. Here, we report our findings of repeated cocaine exposure in male mice inducing MSN subtype-specific bidirectional expression of the Egr3 corepressor NGFI-A-binding protein 2 (Nab2). Using CRISPR activation and interference (CRISPRa and CRISPRi) tools combined with Nab2 or Egr3-targeted sgRNAs, we mimicked these bidirectional changes in Neuro2a cells. Furthermore, we investigated D1-MSN- and D2-MSN-specific expressional changes of histone lysine demethylases Kdm1a, Kdm6a, and Kdm5c in NAc after repeated cocaine exposure in male mice. Since Kdm1a showed bidirectional expression patterns in D1-MSNs and D2-MSNs, like Egr3, we developed a light-inducible Opto-CRISPR-KDM1a system. We were able to downregulate Egr3 and Nab2 transcripts in Neuro2A cells and cause similar bidirectional expression changes we observed in D1-MSNs and D2-MSNs of mouse repeated cocaine exposure model. Contrastingly, our Opto-CRISPR-p300 activation system induced the Egr3 and Nab2 transcripts and caused opposite bidirectional transcription regulations. Our study sheds light on the expression patterns of Nab2 and Egr3 in specific NAc MSNs in cocaine action and uses CRISPR tools to further mimic these expression patterns.SIGNIFICANCE STATEMENT Substance use disorder is a major societal issue. The lack of medication to treat cocaine addiction desperately calls for a treatment development based on precise understanding of molecular mechanisms underlying cocaine addiction. In this study, we show that Egr3 and Nab2 are bidirectionally regulated in mouse NAc D1-MSNs and D2-MSNs after repeated exposure to cocaine. Furthermore, histone lysine demethylations enzymes with putative EGR3 binding sites showed bidirectional regulation in D1- and D2-MSNs after repeated exposure to cocaine. Using Cre- and light-inducible CRISPR tools, we show that we can mimic this bidirectional regulation of Egr3 and Nab2 in Neuro2a cells.

Cocaine induces paradigm-specific changes to the transcriptome within the ventral tegmental area

During the initial stages of drug use, cocaine-induced neuroadaptations within the ventral tegmental area (VTA) are critical for drug-associated cue learning and drug reinforcement processes. These neuroadaptations occur, in part, from alterations to the transcriptome. Although cocaine-induced transcriptional mechanisms within the VTA have been examined, various regimens and paradigms have been employed to examine candidate target genes. In order to identify key genes and biological processes regulating cocaine-induced processes, we employed genome-wide RNA-sequencing to analyze transcriptional profiles within the VTA from male mice that underwent one of four commonly used paradigms: acute home cage injections of cocaine, chronic home cage injections of cocaine, cocaine-conditioning, or intravenous-self administration of cocaine. We found that cocaine alters distinct sets of VTA genes within each exposure paradigm. Using behavioral measures from cocaine self-administering mice, we also found several genes whose expression patterns corelate with cocaine intake. In addition to overall gene expression levels, we identified several predicted upstream regulators of cocaine-induced transcription shared across all paradigms. Although distinct gene sets were altered across cocaine exposure paradigms, we found, from Gene Ontology (GO) term analysis, that biological processes important for energy regulation and synaptic plasticity were affected across all cocaine paradigms. Coexpression analysis also identified gene networks that are altered by cocaine. These data indicate that cocaine alters networks enriched with glial cell markers of the VTA that are involved in gene regulation and synaptic processes. Our analyses demonstrate that transcriptional changes within the VTA depend on the route, dose and context of cocaine exposure, and highlight several biological processes affected by cocaine. Overall, these findings provide a unique resource of gene expression data for future studies examining novel cocaine gene targets that regulate drug-associated behaviors.

Escalated Oxycodone Self-Administration and Punishment: Differential Expression of Opioid Receptors and Immediate Early Genes in the Rat Dorsal Striatum and Prefrontal Cortex

Opioid use disorder (OUD) is characterized by compulsive drug taking despite adverse life consequences. Here, we sought to identify neurobiological consequences associated with the behavioral effects of contingent footshocks administered after escalation of oxycodone self-administration. To reach these goals, Sprague-Dawley rats were trained to self-administer oxycodone for 4 weeks and were then exposed to contingent electric footshocks. This paradigm helped to dichotomize rats into two distinct behavioral phenotypes: (1) those that reduce lever pressing (shock-sensitive) and (2) others that continue lever pressing (shock-resistant) for oxycodone during contingent punishment. The rats were euthanized at 2-h after the last oxycodone plus footshock session. The dorsal striata and prefrontal cortices were dissected for use in western blot and RT-qPCR analyses. All oxycodone self-administration rats showed significant decreased expression of Mu and Kappa opioid receptor proteins only in the dorsal striatum. However, expression of Delta opioid receptor protein was decreased in both brain regions. RT-qPCR analyses documented significant decreases in the expression of c-fos, fosB, fra2, junB, egr1, and egr2 mRNAs in the dorsal striatum (but not in PFC) of the shock-sensitive rats. In the PFC, junD expression was reduced in both phenotypes. However, egr3 mRNA expression was increased in the PFC of only shock-resistant rats. These results reveal that, similar to psychostimulants and alcohol, footshocks can dichotomize rats that escalated their intake of oxycodone into two distinct behavioral phenotypes. These animals also show significant differences in the mRNA expression of immediate early genes, mainly, in the dorsal striatum. The increases in PFC egr3 expression in the shock-resistant rats suggest that Egr3 might be involved in the persistence of oxycodone-associated memory under aversive conditions. This punishment-driven model may help to identify neurobiological substrates of persistent oxycodone taking and abstinence in the presence of adverse consequences.

Hippocampal BDNF regulates a shift from flexible, goal-directed to habit memory system function following cocaine abstinence

The transition from recreational drug use to addiction involves pathological learning processes that support a persistent shift from flexible, goal-directed to habit behavioral control. Here, we examined the molecular mechanisms supporting altered function in hippocampal (HPC) and dorsolateral striatum (DLS) memory systems following abstinence from repeated cocaine. After 3 weeks of cocaine abstinence (experimenter- or self-administered), we tested new behavioral learning in male rats using a dual-solution maze task, which provides an unbiased approach to assess HPC- versus DLS-dependent learning strategies. Dorsal hippocampus (dHPC) and DLS brain tissues were collected after memory testing to identify transcriptional adaptations associated with cocaine-induced shifts in behavioral learning. Our results demonstrate that following prolonged cocaine abstinence rats show a bias toward the use of an inflexible, habit memory system (DLS) in lieu of a more flexible, easily updated memory system involving the HPC. This memory system bias was associated with upregulation and downregulation of brain-derived neurotrophic factor (BDNF) gene expression and transcriptionally permissive histone acetylation (acetylated histone H3, AcH3) in the DLS and dHPC, respectively. Using viral-mediated gene transfer, we overexpressed BDNF in the dHPC during cocaine abstinence and new maze learning. This manipulation restored HPC-dependent behavioral control. These findings provide a system-level understanding of altered plasticity and behavioral learning following cocaine abstinence and inform mechanisms mediating the organization of learning and memory more broadly.

Immediate Early Genes Anchor a Biological Pathway of Proteins Required for Memory Formation, Long-Term Depression and Risk for Schizophrenia

While the causes of myriad medical and infectious illnesses have been identified, the etiologies of neuropsychiatric illnesses remain elusive. This is due to two major obstacles. First, the risk for neuropsychiatric disorders, such as schizophrenia, is determined by both genetic and environmental factors. Second, numerous genes influence susceptibility for these illnesses. Genome-wide association studies have identified at least 108 genomic loci for schizophrenia, and more are expected to be published shortly. In addition, numerous biological processes contribute to the neuropathology underlying schizophrenia. These include immune dysfunction, synaptic and myelination deficits, vascular abnormalities, growth factor disruption, and N-methyl-D-aspartate receptor (NMDAR) hypofunction. However, the field of psychiatric genetics lacks a unifying model to explain how environment may interact with numerous genes to influence these various biological processes and cause schizophrenia. Here we describe a biological cascade of proteins that are activated in response to environmental stimuli such as stress, a schizophrenia risk factor. The central proteins in this pathway are critical mediators of memory formation and a particular form of hippocampal synaptic plasticity, long-term depression (LTD). Each of these proteins is also implicated in schizophrenia risk. In fact, the pathway includes four genes that map to the 108 loci associated with schizophrenia: GRIN2A, nuclear factor of activated T-cells (NFATc3), early growth response 1 (EGR1) and NGFI-A Binding Protein 2 (NAB2); each of which contains the "Index single nucleotide polymorphism (SNP)" (most SNP) at its respective locus. Environmental stimuli activate this biological pathway in neurons, resulting in induction of EGR immediate early genes: EGR1, EGR3 and NAB2. We hypothesize that dysfunction in any of the genes in this pathway disrupts the normal activation of Egrs in response to stress. This may result in insufficient electrophysiologic, immunologic, and neuroprotective, processes that these genes normally mediate. Continued adverse environmental experiences, over time, may thereby result in neuropathology that gives rise to the symptoms of schizophrenia. By combining multiple genes associated with schizophrenia susceptibility, in a functional cascade triggered by neuronal activity, the proposed biological pathway provides an explanation for both the polygenic and environmental influences that determine the complex etiology of this mental illness.

Beyond Neuronal Activity Markers: Select Immediate Early Genes in Striatal Neuron Subtypes Functionally Mediate Psychostimulant Addiction

Immediate early genes (IEGs) were traditionally used as markers of neuronal activity in striatum in response to stimuli including drugs of abuse such as psychostimulants. Early studies using these neuronal activity markers led to important insights in striatal neuron subtype responsiveness to psychostimulants. Such studies have helped identify striatum as a critical brain center for motivational, reinforcement and habitual behaviors in psychostimulant addiction. While the use of IEGs as neuronal activity markers in response to psychostimulants and other stimuli persists today, the functional role and implications of these IEGs has often been neglected. Nonetheless, there is a subset of research that investigates the functional role of IEGs in molecular, cellular and behavioral alterations by psychostimulants through striatal medium spiny neuron (MSN) subtypes, the two projection neuron subtypes in striatum. This review article will address and highlight the studies that provide a functional mechanism by which IEGs mediate psychostimulant molecular, cellular and behavioral plasticity through MSN subtypes. Insight into the functional role of IEGs in striatal MSN subtypes could provide improved understanding into addiction and neuropsychiatric diseases affecting striatum, such as affective disorders and compulsive disorders characterized by dysfunctional motivation and habitual behavior.

Inhibition of DNA methyltransferases regulates cocaine self-administration by rats: a genome-wide DNA methylation study

DNA methylation is a major epigenetic process which regulates the accessibility of genes to the transcriptional machinery. In the present study, we investigated whether modifying the global DNA methylation pattern in the brain would alter cocaine intake by rats, using the cocaine self-administration test. The data indicate that treatment of rats with the DNA methyltransferase inhibitors 5-aza-2'-deoxycytidine (dAZA) and zebularine enhanced the reinforcing properties of cocaine. To obtain some insights about the underlying neurobiological mechanisms, a genome-wide methylation analysis was undertaken in the prefrontal cortex of rats self-administering cocaine and treated with or without dAZA. The study identified nearly 189 000 differentially methylated regions (DMRs), about half of them were located inside gene bodies, while only 9% of DMRs were found in the promoter regions of genes. About 99% of methylation changes occurred outside CpG islands. Gene expression studies confirmed the inverse correlation usually observed between increased methylation and transcriptional activation when methylation occurs in the gene promoter. This inverse correlation was not observed when methylation took place inside gene bodies. Using the literature-based Ingenuity Pathway Analysis, we explored how the differentially methylated genes were related. The analysis showed that increase in cocaine intake by rats in response to DNA methyltransferase inhibitors underlies plasticity mechanisms which mainly concern axonal growth and synaptogenesis as well as spine remodeling. Together with the Akt/PI3K pathway, the Rho-GTPase family was found to be involved in the plasticity underlying the effect of dAZA on the observed behavioral changes.

Opposing role for Egr3 in nucleus accumbens cell subtypes in cocaine action

An imbalance in molecular signaling cascades and transcriptional regulation in nucleus accumbens (NAc) medium spiny neuron (MSN) subtypes, those enriched in dopamine D1 versus D2 receptors, is implicated in the behavioral responses to psychostimulants. To provide further insight into the molecular mechanisms occurring in MSN subtypes by cocaine, we examined the transcription factor early growth response 3 (Egr3). We evaluated Egr3 because it is a target of critical cocaine-mediated signaling pathways and because Egr3-binding sites are found on promoters of key cocaine-associated molecules. We first used a RiboTag approach to obtain ribosome-associated transcriptomes from each MSN subtype and found that repeated cocaine administration induced Egr3 ribosome-associated mRNA in NAc D1-MSNs while reducing Egr3 in D2-MSNs. Using Cre-inducible adeno-associated viruses combined with D1-Cre and D2-Cre mouse lines, we observed that Egr3 overexpression in D1-MSNs enhances rewarding and locomotor responses to cocaine, whereas overexpression in D2-MSNs blunts these behaviors. miRNA knock-down of Egr3 in MSN subtypes produced opposite behavioral responses from those observed with overexpression. Finally, we found that repeated cocaine administration altered Egr3 binding to promoters of genes that are important for cocaine-mediated cellular and behavioral plasticity. Genes with increased Egr3 binding to promoters, Camk2α, CREB, FosB, Nr4a2, and Sirt1, displayed increased mRNA in D1-MSNs and, in some cases, a reduction in D2-MSNs. Histone and the DNA methylation enzymes G9a and Dnmt3a displayed reduced Egr3 binding to their promoters and reduced mRNA in D1-MSNs. Our study provides novel insight into an opposing role of Egr3 in select NAc MSN subtypes in cocaine action.

Cocaine induces ubiquitination of Egr-1 in the rat dorsal striatum

Early growth response (Egr) is a member of the zinc finger family of transcription factors that reflects neuronal activity induced by various stimuli. Acute cocaine administration elicits rapid and transient induction of several immediate early genes in brain neurons. However, the mechanism regulating the degradation of the Egr-1 protein is not clearly understood. In this study, rats were injected with cocaine and the relationships among locomotor activity, Egr-1 protein level, phosphorylation of upstream kinase extracellular regulated kinase (ERK)1/2, Egr-1 mRNA expression, and ubiquitination of the Egr-1 protein were measured in the dorsal striatum and the frontal cortex. Locomotor activity reached a peak at about 15 min, and phosphorylation of ERK1/2 and Egr-1 mRNA level also increased at that time. However, the Egr-1 protein level decreased initially in the dorsal striatum, probably due to ubiquitination-mediated degradation. When locomotor activity decreased substantially at 30 min, the phosphorylation of ERKs and expression levels of Egr-1 mRNA and protein reached their peak levels and the protein level subsequently increased. These findings indicate that immediate early gene protein levels would not be a reliable indicator of increased regional activity in the brain. Thus, observations spanning multiple time periods or the examination of mRNA rather than protein would be recommended in these situations.

Repeated electroconvulsive seizure treatment in rats reduces inducibility of early growth response genes and hyperactivity in response to cocaine administration

Regulated expression of immediate early genes (IEGs) in the brain reflects neuronal activity in response to various stimuli and recruits specific gene programs involved in long-term neuronal modification and behavioral alterations. Repeated electroconvulsive seizure (ECS) treatment reduces the expression level of several IEGs, such as c-fos, which play important roles in psychostimulant-induced behavioral changes. In this study, we investigated the effects of repeated ECS treatment on the basal expression level of IEGs and its effects on cocaine-induced activation of IEGs and locomotor activity in rats. Repeated ECS treatment for 10days (E10×) reduced Egr1, Egr2, Egr3, and c-fos mRNA and protein levels in the rat frontal cortex at 24h after the last ECS treatment, and these changes were evident in the neuronal cells of the prefrontal cortex. In particular, downregulation of Egr1 and c-fos was evident until 5days after the last ECS treatment. Moreover, E10× pretreatment attenuated the cocaine-induced increase in Egr1, Egr2, and c-fos expression in the rat frontal cortex, whereas phosphorylation of ERK1/2, one of the representative upstream activators of these genes, increased significantly following cocaine treatment. Additionally, E10× pretreatment attenuated the increase in locomotor activity in response to a cocaine injection. In conclusion, repeated ECS treatment reduced the expression and inducibility of Egrs and c-fos, which could attenuate the response of the brain to psychostimulants.

Transcription Mapping of Embryonic Rat Brain Reveals EGR-1 Induction in SOX2 Neural Progenitor Cells

Neuronal expression of the early growth response-1 (EGR-1; NGFI-A/Zif268) transcription factor has been extensively studied in the adult mammalian brain and linked to aspects of mature physiological/behavioral function. In contrast, this factor has not been studied in detail in the embryonic brain. Here, we used a fluorescent protein-encoding Egr-1 transgene to map the cellular distribution of Egr-1 transcription in embryonic rat brain. We identified a novel, widely distributed population of GFP(+) cells, characterized as a precursor/stem cell phenotype by co-localization with SOX2/nestin/vimentin/S-100β and exclusion from other known cellular markers including DCX/BLBP/TBR2/NURR1. At both E18 and E20, these cells were located across the developing brain but concentrated in the subplate and intermediate zones. The transgene was also highly expressed in developing (NeuN(+)) striatal neurons. The authentic expression pattern that we observed for the rEgr-1 transgene sequence indicates that restriction to neuronal/precursor cells is largely driven by proximal 5(') sequence. Deletion of conserved Egr-1 silencer (neuron restrictive silencer factor) elements did not markedly alter transcriptional activity in transfected cells; this is consistent with a dominant role for positive factors in the control of cell-specific Egr-1 expression. Induction of Egr-1 in a population of SOX2(+) cells indicates a co-incidence of extrinsic (EGR-1) and cell-intrinsic (SOX2) cellular signals that may form a novel level of progenitor cell regulation. The wide distribution of EGR-1 signaling in SOX2(+) cells suggests an organizational role during late embryonic brain development.

The dendritically targeted protein Dendrin is induced by acute nicotine in cortical regions of adolescent rat brain

The response of the brain to addictive substances such as nicotine includes the rapid induction of genes that influence synaptic events. This response is different in adolescent brain, which continues to undergo synaptic remodeling in regions that include reward-associated corticolimbic areas. We report here that acute nicotine (0.4 mg/kg), but not cocaine or exposure to a novel environment, induces the expression of the dendritically targeted, corticolimbic mRNA Dendrin in specific regions of adolescent brain. Acute nicotine resulted in an increase in Dendrin mRNA levels in the adolescent prefrontal cortex that was not evident in adult animals. The induction in Dendrin mRNA was a rapid, short-lived transcriptional event that resulted in changes in Dendrin protein. For example, an increase in Dendrin protein levels following nicotine treatment paralleled enhanced Dendrin immunoreactivity in the dendrites of pyramidal neurons of somatosensory cortex. As Dendrin is an important component of cytoskeletal modifications at the synapse, these results suggest that nicotine influences unique plasticity-related changes in the adolescent forebrain that differ from the adult.

Histone deacetylase inhibitors decrease cocaine but not sucrose self-administration in rats

Regulation of gene expression is known to contribute to the long-term adaptations taking place in response to drugs of abuse. Recent studies highlighted the regulation of gene transcription in neurons by chromatin remodeling, a process in which posttranslational modifications of histones play a major role. To test the involvement of epigenetic regulation on drug-reinforcing properties, we submitted rats to the cocaine operant self-administration paradigm. Using the fixed ratio 1 schedule, we found that the histone deacetylase (HDAC) inhibitors trichostatin A and phenylbutyrate dose-dependently reduced cocaine self-administration. Under the progressive ratio schedule, both trichostatin A and depudecin significantly reduced the breaking point, indicating that HDAC inhibition attenuated the motivation of rats for cocaine. Conversely, HDAC inhibition did not decrease self-administration for the natural reinforcer sucrose. This observation was correlated with measurements of HDAC activity in the frontal cortex, which was inhibited in response to cocaine, but not to sucrose self-administration. Control experiments showed that the decrease in the motivation for the drug was not attributable to a general motivational dysfunction because trichostatin A had no adverse effect on locomotion during the habituation session or on cocaine-induced hyperlocomotion. It was not attributable to anhedonia because the inhibitor had no effect on the sucrose preference test. In contrast, trichostatin A completely blocked the cocaine-induced behavioral sensitization. Together, the data show that epigenetic regulation of gene transcription in adult brain is able to influence a motivated behavior and suggest that HDAC inhibition may counteract the neural sensitization leading to drug dependence.

Regulation of genes involved in dopamine transporter modulation by acute cocaine in rat striatum

It is well established that acute administration of psychostimulants alters dopamine transport. However, the exact mechanism of this modulation is still unknown. In this study we examined the mRNA levels of several proteins involved in the various proposed processes following cocaine administration. The expression levels of several immediate early genes were also studied. This was performed in rat striatum using real-time quantitative PCR. As expected, a marked increase of the immediate early genes Fos, Egr1 and Egr3 was observed. Egr2 was also found up-regulated. Among the different genes studied only Synaptotagmin4 in the SNARE family and Synphilin1 in the synaptic vesicles binding family were modulated by acute cocaine treatment. Interestingly, acute amphetamine treatment did not increase either Synaptotagmin4 and Synphilin1 mRNA levels, although increases in early genes expression were noted.

The MAPK pathway and Egr-1 mediate stress-related behavioral effects of glucocorticoids

Many of the behavioral consequences of stress are mediated by the activation of the glucocorticoid receptor by stress-induced high levels of glucocorticoid hormones. To explore the molecular mechanisms of these effects, we combined in vivo and in vitro approaches. We analyzed mice carrying a brain-specific mutation (GR(NesCre)) in the glucocorticoid receptor gene (GR, also called Nr3c1) and cell lines that either express endogenous glucocorticoid receptor or carry a constitutively active form of the receptor (DeltaGR) that can be transiently induced. In the hippocampus of the wild-type [corrected] mice after stress, as well as in the cell lines, activation of glucocorticoid receptors greatly increased the expression and enzymatic activity of proteins in the MAPK signaling pathway and led to an increase in the levels of both Egr-1 mRNA and protein. In parallel, inhibition of the MAPK pathway within the hippocampus abolished the increase in contextual fear conditioning induced by glucocorticoids. The present results provide a molecular mechanism for the stress-related effects of glucocorticoids on fear memories.

Activation of the cGMP pathway in dopaminergic structures reduces cocaine-induced EGR-1 expression and locomotor activity

Nitric oxide (NO) and the C-type natriuretic peptide (CNP) exert their action on brain via the cGMP signaling pathway. NO, by activating soluble guanylyl cyclase, and CNP, by stimulating membrane-bound guanylyl cyclase, cause intracellular increases of cGMP, activating cGMP-dependent protein kinases (PKGs). We show here that injection of CNP into the rat ventral tegmental area strongly reduced cocaine-induced egr-1 expression in the nucleus accumbens in a dose-dependent manner. The effect of CNP was reversed by the previous injection of a selective PKG inhibitor, KT5823. Activation of PKG by 8-bromo-cGMP reduced, like CNP, cocaine-induced gene transcription in dopaminergic structures. To confirm the involvement of PKG, this was overexpressed in either the mesencephalon or the caudate-putamen. Using the polyethyleneimine delivery system, an active protein was expressed by injecting a plasmid vector containing the human PKG-Ialpha cDNA. PKG was overexpressed in dopaminergic and GABAergic neurons when the plasmid was injected in the ventral tegmental area, whereas overexpression was observed in medium spiny GABAergic neurons and in both cholinergic and GABAergic interneurons when the PKG vector was injected into the caudate-putamen. Activation of the overexpressed PKG reduced cocaine-induced egr-1 expression in dopaminergic structures and affected behavior (i.e., locomotor activity). These effects were again reversed by previous injection of the selective PKG inhibitor. The current data suggest that NO and the neuropeptide CNP are potential regulators of cocaine-related effects on behavior.

Hippocampal gene expression is modulated by hypergravity

We used the cDNA microarray technique to monitor simultaneously possible changes induced by hypergravity in the expression level of thousands of hippocampal genes. We tested the mRNA level of about 5000 genes in the hippocampus of mice subjected to 1.09 g (1g) or to 1.85 g (2g) for five repeated 1-h daily rotations in a centrifuge (g = 9.81 m/s2). Data were compared with those obtained for mice kept stationary (C). The ratios 1g/C and 2g/C identified genes affected by rotation and rotation + hypergravity, respectively, whereas 2g/1g ratio identified those affected by hypergravity. We found that about 200 genes were affected by rotation and/or rotation + hypergravity. Almost all the genes affected by rotation + hypergravity were up-regulated, only five being down-regulated. The modulated genes code for proteins involved in a wide range of cellular functions (DNA/RNA metabolism, protein processing, intermediate metabolism, cytoskeleton and motility, cell cycle and apoptosis, signal transduction, neuronal structure/function), suggesting that rotation + hypergravity may affect several aspects of the hippocampal function in order to compensate for environmental changes. Six genes directly or indirectly involved in synaptic transmission and plasticity (proSAAS, neuroblastoma ras oncogene, ESTs moderately similar to thymosin beta-10, syndet, inhibin beta E and Ngfi-A binding protein 2) were found to be significantly modulated by hypergravity and unaffected or only slightly affected by rotation. The modulation by hypergravity of these genes suggests that this stimulus might induce plastic remodelling of the hippocampal circuits, possibly both at structural and functional level.

Importance of ERK activation in behavioral and biochemical effects induced by MDMA in mice

Little is known about the cellular effects induced by 3,4-methylenedioxymethamphetamine (MDMA, ecstasy), although changes in gene expression have been observed following treatments with other psychostimulants. Thus, the aim of this study was to investigate in mice, the relationships between the ras-dependent protein kinase ERK and MDMA-induced reinforcement using the conditioned place preference (CPP) and locomotor activity measurements. This was completed using real-time quantitative PCR method by a study of immediate early-genes (IEGs) transcription known to be involved in neuronal plasticity. A significant CPP was observed after repeated MDMA treatment in CD-1 mice at a dose of 9 mg kg-1 i.p. but not at 3 and 6 mg kg-1. This rewarding effect was abolished by the selective inhibitor of ERK activation, SL327 (50 mg kg-1; i.p.). Similar results were obtained on MDMA-induced locomotor activity, clearly suggesting a role of ERK pathway in these behavioral responses. Following acute i.p. injection, MDMA induced a strong c-fos transcription in brain structures, such as caudate putamen, nucleus accumbens and hippocampus, whereas egr-1 and egr-3 transcripts were only increased in the caudate putamen. MDMA-induced IEGs transcription was selectively suppressed by SL327 in the caudate putamen, suggesting a role for other signaling pathways in regulation of IEGs transcription in the other brain structures. In agreement with these results, MDMA-induced c-fos protein expression was blocked by SL327 in the caudate putamen. This study confirms and extends to mice the reported role of ERK pathway in the development of addiction-like properties of MDMA. This could facilitate studies about the molecular mechanism of this process by using mutant mice.