Involvement of mitogen-stimulated p70-S6 kinase in the development of sensitization to the methamphetamine-induced rewarding effect in rats

The PI3-Kinase p110β Isoform Controls Severity of Cocaine-Induced Sequelae and Alters the Striatal Transcriptome

BACKGROUND

The PI3-kinase (PI3K) complex is a well-validated target for mitigating cocaine-elicited sequelae, but pan-PI3K inhibitors are not viable long-term treatment options. The PI3K complex is composed of p110 catalytic and regulatory subunits, which can be individually manipulated for therapeutic purposes. However, this possibility has largely not been explored in behavioral contexts.

METHODS

Here, we inhibited PI3K p110β in the medial prefrontal cortex (mPFC) of cocaine-exposed mice. Behavioral models for studying relapse, sensitization, and decision-making biases were paired with protein quantification, RNA sequencing, and cell type-specific chemogenetic manipulation and RNA quantification to determine whether and how inhibiting PI3K p110β confers resilience to cocaine.

RESULTS

Viral-mediated PI3K p110β silencing reduced cue-induced reinstatement of cocaine seeking by half, blocked locomotor sensitization, and restored mPFC synaptic marker content after exposure to cocaine. Cocaine blocked the ability of mice to select actions based on their consequences, and p110β inhibition restored this ability. Silencing dopamine D₂ receptor-expressing excitatory mPFC neurons mimicked cocaine, impairing goal-seeking behavior, and again, p110β inhibition restored goal-oriented action. We verified the presence of p110β in mPFC neurons projecting to the dorsal striatum and orbitofrontal cortex and found that inhibiting p110β in the mPFC altered the expression of functionally defined gene clusters within the dorsal striatum and not orbitofrontal cortex.

CONCLUSIONS

Subunit-selective PI3K silencing potently mitigates drug seeking, sensitization, and decision-making biases after exposure to cocaine. We suggest that inhibiting PI3K p110β provides neuroprotection against cocaine by triggering coordinated corticostriatal adaptations.

The role of the mTOR pathway in models of drug-induced reward and the behavioural constituents of addiction

BACKGROUND

Exposure to drugs of abuse induces neuroadaptations in critical nodes of the so-called reward systems that are thought to mediate the transition from controlled drug use to the compulsive drug-seeking that characterizes addictive disorders. These neural adaptations are likely to require protein synthesis, which is regulated, among others, by the mechanistic target of the rapamycin kinase (mTOR) signalling cascade.

METHODS

We have performed a narrative review of the literature available in PubMed about the involvement of the mTOR pathway in drug-reward and addiction-related phenomena.

AIMS

The aim of this study was to review the underlying architecture of this complex intracellular network and to discuss the alterations of its components that are evident after exposure to drugs of abuse. The aim was also to delineate the effects that manipulations of the mTOR network have on models of drug reward and on paradigms that recapitulate some of the psychological components of addiction.

RESULTS

There is evidence for the involvement of the mTOR pathway in the acute and rewarding effects of drugs of abuse, especially psychostimulants. However, the data regarding opiates are scarce. There is a need to use sophisticated animal models of addiction to ascertain the real role of the mTOR pathway in this pathology and not just in drug-mediated reward. The involvement of this pathway in behavioural addictions and impulsivity should also be studied in detail in the future.

CONCLUSIONS

Although there is a plethora of data about the modulation of mTOR by drugs of abuse, the involvement of this signalling pathway in addictive disorders requires further research.

Protein Translation and Psychiatric Disorders

Although historically research has focused on transcription as the central governor of protein expression, protein translation is now increasingly being recognized as a major factor for determining protein levels within cells. The central nervous system relies on efficient updating of the protein landscape. Thus, coordinated regulation of mRNA localization, initiation, or termination of translation is essential for proper brain function. In particular, dendritic protein synthesis plays a key role in synaptic plasticity underlying learning and memory as well as cognitive processes. Increasing evidence suggests that impaired mRNA translation is a common feature found in numerous psychiatric disorders. In this review, we describe how malfunction of translation contributes to development of psychiatric diseases, including schizophrenia, major depression, bipolar disorder, and addiction.

Morphine self-administration alters the expression of translational machinery genes in the amygdala of male Lewis rats

BACKGROUND

Addiction is a chronic disorder with a high risk of relapse. The neural mechanisms mediating addictions require protein synthesis, which could be relevant for the development of more effective treatments. The mTOR signaling pathway regulates protein synthesis processes that have recently been linked to the development of drug addiction.

AIMS

To assess the effects of morphine self-administration and its subsequent extinction on the expression of several genes that act in this pathway, and on the levels of specific phosphoproteins (Akt, Gsk3α/β, mTOR, PDK1 and p70 S6 kinase) in the amygdala, nucleus accumbens, and the prefrontal cortex.

METHODS

Male Lewis rats underwent morphine self-administration (1 mg/kg) for 19 days. They subsequently were submitted to extinction training for 15 days. Rats were killed either after self-administration or extinction, their brains extracted, and gene expression or phosphoprotein levels were assessed.

RESULTS

We found an increase in Raptor and Eif4ebp2 expression in the amygdala of rats that self-administered morphine, even after extinction. The expression of Insr in the amygdala of control animals decreased over time while the opposite effect was seen in the rats that self-administered morphine.

CONCLUSIONS

Our results suggest that morphine self-administration affects the gene expression of some elements of the translational machinery in the amygdala.

Quantitative Systems Pharmacological Analysis of Drugs of Abuse Reveals the Pleiotropy of Their Targets and the Effector Role of mTORC1

Existing treatments against drug addiction are often ineffective due to the complexity of the networks of protein-drug and protein-protein interactions (PPIs) that mediate the development of drug addiction and related neurobiological disorders. There is an urgent need for understanding the molecular mechanisms that underlie drug addiction toward designing novel preventive or therapeutic strategies. The rapidly accumulating data on addictive drugs and their targets as well as advances in machine learning methods and computing technology now present an opportunity to systematically mine existing data and draw inferences on potential new strategies. To this aim, we carried out a comprehensive analysis of cellular pathways implicated in a diverse set of 50 drugs of abuse using quantitative systems pharmacology methods. The analysis of the drug/ligand-target interactions compiled in DrugBank and STITCH databases revealed 142 known and 48 newly predicted targets, which have been further analyzed to identify the KEGG pathways enriched at different stages of drug addiction cycle, as well as those implicated in cell signaling and regulation events associated with drug abuse. Apart from synaptic neurotransmission pathways detected as upstream signaling modules that “sense” the early effects of drugs of abuse, pathways involved in neuroplasticity are distinguished as determinants of neuronal morphological changes. Notably, many signaling pathways converge on important targets such as mTORC1. The latter emerges as a universal effector of the persistent restructuring of neurons in response to continued use of drugs of abuse.

mTOR signaling in the nucleus accumbens mediates behavioral sensitization to methamphetamine

Chronic psychostimulant treatment in rodents readily produces behavioral sensitization, which reflects altered brain function in response to repeated drug exposure. Numerous morphological and biochemical investigations implicate altered neural plasticity in striatal medium spiny neurons (MSNs) as an essential component in behavioral sensitization. The mammalian target of the rapamycin (mTOR) signaling pathway, a key regulator of synaptic neuroplasticity, in the ventral striatum of methamphetamine (METH) -sensitized mice was investigated to determine if a link exists with the development of METH sensitization. Behaviorally, METH-sensitized mice possessed increased levels of phosphorylated mTOR/S2448 and its down-stream regulator p70S6K and pS6 in the ventral striatum. Systemic treatment with rapamycin, a specific mTOR inhibitor, coincident with a daily METH injection suppressed the induction of METH sensitization and reduced the number of dendritic spines in the shell and core of the nucleus accumbens. The infusion of lentivirus-expressing mTOR-shRNA into the shell region of the nucleus accumbens inhibited the induction of behavioral sensitization to METH, which was comparable to the effect of rapamycin. These results suggest that mTORC1-mediated signaling in the nucleus accumbens mediates the development of behavioral sensitization to METH.

Effects of ketamine administration on mTOR and reticulum stress signaling pathways in the brain after the infusion of rapamycin into prefrontal cortex

Recent studies show that activation of the mTOR signaling pathway is required for the rapid antidepressant actions of glutamate N-methyl-D-aspartate (NMDA) receptor antagonists. A relationship between mTOR kinase and the endoplasmic reticulum (ER) stress pathway, also known as the unfolded protein response (UPR) has been shown. We evaluate the effects of ketamine administration on the mTOR signaling pathway and proteins of UPR in the prefrontal cortex (PFC), hippocampus, amygdala and nucleus accumbens, after the inhibiton of mTOR signaling in the PFC. Male adult Wistar rats received pharmacological mTOR inhibitor, rapamycin (0.2 nmol), or vehicle into the PFC and then a single dose of ketamine (15 mg/kg, i.p.). The immunocontent of mTOR, eukaryotic initiation factor 4E-binding protein 1 (4E-BP1), eukaryotic elongation factor 2 kinase (eEF2K) homologous protein (CHOP), PKR-like ER kinase (PERK) and inositol-requiring enzyme 1 (IRE1) - alpha were determined in the brain. The mTOR levels were reduced in the rapamycin group treated with saline and ketamine in the PFC; p4EBP1 levels were reduced in the rapamycin group treated with ketamine in the PFC and nucleus accumbens; the levels of peEF2K were increased in the PFC in the vehicle group treated with ketamine and reduced in the rapamycin group treated with ketamine. The PERK and IRE1-alpha levels were decreased in the PFC in the rapamycin group treated with ketamine. Our results suggest that mTOR signaling inhibition by rapamycin could be involved, at least in part, with the mechanism of action of ketamine; and the ketamine antidepressant on ER stress pathway could be also mediated by mTOR signaling pathway in certain brain structures.

Essential role of D1R in the regulation of mTOR complex1 signaling induced by cocaine

The mammalian target of rapamycin (mTOR) is a serine/threonine kinase that is involved in neuronal adaptions that underlie cocaine-induced sensitization and reward. mTOR exists in two functionally distinct multi-component complexes known as mTORC1 and mTORC2. In this study, we show that increased mTORC1 activity induced by cocaine is mediated by the dopamine D1 receptor (D1R). Specifically, cocaine treatment increased the phosphorylation on residues Thr2446 and Ser2481 but not on Ser2448 in the nucleus accumbens (NAc) and that this increase in phosphorylated mTOR levels was also apparent when complexed with its binding partner Raptor. Furthermore, the increase in phosphorylated mTOR levels, as well as phosphorylated 4E-BP1 and S6K, downstream targets of mTORC1 were blocked with SCH23390 treatment. Similar results were also observed in the dopamine-transporter knockout mice as the increase in phosphorylated mTOR Thr2446 and Ser2481 was blocked by SCH23390 but not with raclopride. To further validate D1R role in mTORC1 signaling, decrease in phosphorylated mTOR levels were observed in D1R knockout mice, whereas administration of SKF81297 elevated phosphorylated mTOR in the NAc. Lastly deletion of mTOR or Raptor in D1R expressing neurons reduced cocaine-induced locomotor activity. Together, our data supports a mechanism whereby mTORC1 signaling is activated by cocaine administration through the stimulation of D1R.

mTORC1 inhibition in the nucleus accumbens 'protects' against the expression of drug seeking and 'relapse' and is associated with reductions in GluA1 AMPAR and CAMKIIα levels

The mechanistic target of rapamycin complex 1 (mTORC1) is necessary for synaptic plasticity, as it is critically involved in the translation of synaptic transmission-related proteins, such as Ca(2+)/Calmodulin-dependent kinase II alpha (CAMKIIα) and AMPA receptor subunits (GluAs). Although recent studies have implicated mTORC1 signaling in drug-motivated behavior, the ineffectiveness of rapamycin, an mTORC1 inhibitor, in suppressing cocaine self-administration has raised questions regarding the specific role of mTORC1 in drug-related behaviors. Here, we examined mTORC1's role in three drug-related behaviors: cocaine taking, withdrawal, and reinstatement of cocaine seeking, by measuring indices of mTORC1 activity and assessing the effect of intra-cerebroventricular rapamycin on these behaviors in rats. We found that withdrawal from cocaine self-administration increased indices of mTORC1 activity in the nucleus accumbens (NAC). Intra-cerebroventricular rapamycin attenuated progressive ratio (PR) break points and reduced phospho-p70 ribosomal S6 kinase, GluA1 AMPAR, and CAMKIIα levels in the NAC shell (NACsh) and core (NACc). In a subsequent study, we treated rats with intra-NACsh infusions of rapamycin (2.5 μg/side/day for 5 days) during cocaine self-administration and then tracked the expression of addiction-relevant behaviors through to withdrawal and extinction. Rapamycin reduced drug seeking in signaled non-drug-available periods, PR responding, and cue-induced reinstatement, with these effects linked to reduced mTORC1 activity, total CAMKIIα, and GluA1 AMPAR levels in the NACsh. Together, these data highlight a role for mTORC1 in the neural processes that control the expression and maintenance of drug reward, including protracted relapse vulnerability. These effects appear to involve a role for mTORC1 in the regulation of GluA1 AMPARs and CAMKIIα in the NACsh.

mTOR complex 1: a key player in neuroadaptations induced by drugs of abuse

The mammalian (or mechanistic) target of rapamycin (mTOR) complex 1 (mTORC1) is a serine and threonine kinase that regulates cell growth, survival, and proliferation. mTORC1 is a master controller of the translation of a subset of mRNAs. In the central nervous system mTORC1 plays a crucial role in mechanisms underlying learning and memory by controlling synaptic protein synthesis. Here, we review recent evidence suggesting that the mTORC1 signaling pathway promotes neuroadaptations following exposure to a diverse group of drugs of abuse including stimulants, cannabinoids, opiates, and alcohol. We further describe potential molecular mechanisms by which drug-induced mTORC1 activation may alter brain functions. Finally, we propose that mTORC1 is a focal point shared by drugs of abuse to mediate drug-related behaviors such as reward seeking and excessive drug intake, and offer future directions to decipher the contribution of the kinase to mechanisms underlying addiction. Recent studies suggesting that exposure to diverse classes of drugs of abuse as well as exposure to drug-associated memories lead to mTORC1 kinase activation in the limbic system. In turn, mTORC1 controls the onset and the maintenance of pathological neuroadaptions that underlie several features of drug addiction such as drug seeking and relapse. Therefore, we propose that targeting mTORC1 and its effectors is a promising strategy to treat drug disorders.

Interleukin-18 augments growth ability of primary human melanocytes by PTEN inactivation through the AKT/NF-κB pathway

Normal human skin relies on melanocytes to provide photoprotection and thermoregulation by producing melanin. The growth and behavior of melanocytes are controlled by many factors. Interleukin-18 (IL-18) is expressed in both immune and non-immune cells and participates in the adjustment of multitude cellular functions. Nonetheless, the regulative roles of IL-18 in melanogenesis and growth of melanocytes have not been explored. The present study was conducted to investigate the effects of IL-18 on melanocytes and elucidate the underlying mechanisms. We proved that IL-18 increased the tyrosinase activity and melanin content in normal human foreskin-derived epidermal melanocytes (NHEM). Treatment with IL-18 (20 ng/ml) enhanced the expression of c-Kit, microphtalmia-associated transcription factor (MITF) and its downstream tyrosinase-related protein 1 (TRP-1), and TRP-2. In addition, IL-18 induced NHEM migration at concentration of 20 ng/ml. These results indicated a promotive action of IL-18 on melanogenesis in NHEM. Our data revealed that IL-18 stimulated ERK1/2 and NF-κB activation, improved p-Akt, p70 S6K and anti-apoptotic Bcl-2 levels, and deactivated phosphatase and tensin homolog deleted on chromosome 10 (PTEN) in NHEM. Besides, IL-18 increased level of PTEN phosphorylation to protect NHEM from damage induced by H(2)O(2). These results in vitro showed the accommodation of IL-18 in melanocytes growth. Therefore, we suggested an important regulating action of IL-18 to melanogenesis and cell growth ability of skin melanocytes.

Rapamycin attenuates the expression of cocaine-induced place preference and behavioral sensitization

The mammalian target of rapamycin (mTOR) is a serine-threonine kinase that controls global protein synthesis, in part, by modulating translation initiation, a rate-limiting step for many mRNAs. Previous studies implicate mTOR in regulating stimulant-induced sensitization and antidepressive-like behavior in rodents, as well as drug craving in abstinent heroin addicts. To determine if signaling downstream of mTOR is affected by repeated cocaine administration in reward-associated brain regions, and if inhibition of mTOR alters cocaine-induced behavioral plasticity, C57BL/6J mice received four intraperitoneal (i.p.) injections of 15 mg/kg cocaine and levels of phosphorylated P70S6 kinase and ribosomal S6 protein-two translational regulators directly downstream of mTOR-were analyzed by immunoblotting across several brain regions. Cocaine place preference and locomotor sensitization were elicited by four pairings of cocaine with a distinct environment and the effects of mTOR inhibition were assessed by pre-treating the mice with 10 mg/kg rapamycin, 1 hour prior to: (1) each saline/cocaine conditioning session; (2) a post-conditioning test; or (3) a test for locomotor sensitization conducted at 3 weeks withdrawal. While systemic pre-treatment with 10 mg/kg rapamycin during conditioning failed to alter the development of a cocaine place preference or locomotor sensitization, pre-treatment prior to the post-conditioning test attenuated the expression of the place preference. Additionally, rapamycin pre-treatment prior to a cocaine challenge 3 weeks post-conditioning blocked the expression of the sensitized locomotor response. These findings suggest a role for mTOR activity, and perhaps translational control, in the expression of cocaine-induced place preference and locomotor sensitization.

An emerging role for the Mammalian target of rapamycin in "pathological" protein translation: relevance to cocaine addiction

Complex neuroadaptations within key nodes of the brain's "reward circuitry" are thought to underpin long-term vulnerability to relapse. A more comprehensive understanding of the molecular and cellular signaling events that subserve relapse vulnerability may lead to pharmacological treatments that could improve treatment outcomes for psychostimulant-addicted individuals. Recent advances in this regard include findings that drug-induced perturbations to neurotrophin, metabotropic glutamate receptor, and dopamine receptor signaling pathways perpetuate plasticity impairments at excitatory glutamatergic synapses on ventral tegmental area and nucleus accumbens neurons. In the context of addiction, much previous work, in terms of downstream effectors to these receptor systems, has centered on the extracellular-regulated MAP kinase signaling pathway. The purpose of the present review is to highlight the evidence of an emerging role for another downstream effector of these addiction-relevant receptor systems - the mammalian target of rapamycin complex 1 (mTORC1). mTORC1 functions to regulate synaptic protein translation and is a potential critical link in our understanding of the neurobiological processes that drive addiction and relapse behavior. The precise cellular and molecular changes that are regulated by mTORC1 and contribute to relapse vulnerability are only just coming to light. Therefore, we aim to highlight evidence that mTORC1 signaling may be dysregulated by drug exposure and that these changes may contribute to aberrant translation of synaptic proteins that appear critical to increased relapse vulnerability, including AMPARs. The importance of understanding the role of this signaling pathway in the development of addiction vulnerability is underscored by the fact that the mTORC1 inhibitor rapamycin reduces drug-seeking in pre-clinical models and preliminary evidence indicating that rapamycin suppresses drug craving in humans.

Down-regulated striatal gene expression for synaptic plasticity-associated proteins in addiction and relapse vulnerable animals

Reducing the likelihood of relapse represents one of the greatest obstacles in the successful treatment of cocaine addiction. Dysregulation of the synaptic plasticity processes long-term potentiation (LTP) and long-term depression (LTD) is thought to be associated with protracted relapse risk. To improve our understanding of the molecular mechanisms contributing to relapse vulnerability we trained rats (n=52) to self-administer cocaine and phenotyped animals as relapse-vulnerable or relapse-resilient using procedures adapted from Deroche-Gamonet et al. (Science 2004, 305, 1014-1017). Gene expression analysis, targeted at synaptic plasticity-related genes, revealed significant transcript down-regulation in the ventral and dorsal striatum of relapse-vulnerable animals compared to relapse-resilient controls. This included reduced expression of genes encoding proteins implicated in the dendritic translation of synaptic plasticity-related transcripts, the dynamic regulation and trafficking of ionotropic glutamate receptors important for LTP and LTD, along with neuronal surface receptors that initiate downstream signalling pathways associated with synaptic plasticity. Together, our data are consistent with recent reports of an inability to evoke LTD in the striatum of addiction-vulnerable rats. To our knowledge, this is the first study to demonstrate down-regulated synaptic plasticity-associated gene expression not only in the ventral striatum, where the majority of addiction-related synaptic plasticity studies have been conducted, but also in the dorsal striatum of animals categorized as relapse-vulnerable. As these neural correlates were elucidated using an approach incorporating individual behavioural differences, they potentially provide more relevant insight into addiction and assist the development of novel pharmacotherapies to treat relapse.

Inhibition of the mammalian target of rapamycin pathway by rapamycin blocks cocaine-induced locomotor sensitization

Repeated cocaine exposure induces locomotor sensitization, which is mediated by adaptive changes in synaptic transmission in the mesolimbic dopamine pathway. The molecular mechanisms underlying this adaptation remain poorly understood. One pathway that may play a role is the mammalian target of rapamycin (mTOR) which is implicated in synaptic plasticity. In the present study, we found that cocaine exposure stimulates mTOR activity in rat brain. Furthermore, inhibition of mTOR by rapamycin blocked the induction as well as the expression of cocaine-induced locomotor sensitization in rats. These data elucidate a novel mechanism by which the mTOR pathway mediates cocaine-induced behavioral changes and could suggest a new interventional strategy for drug abuse.

Photic regulation of the mTOR signaling pathway in the suprachiasmatic circadian clock

Here we analyzed the light-responsiveness of the mammalian target of rapamycin (mTOR) cascade, a key regulator of inducible translation, in the suprachiasmatic nuclei (SCN), the locus of the master circadian clock. Brief light exposure during the subjective night, but not during the subjective day, triggered rapid phosphorylation (a marker of catalytic activity) of the mTOR translation effectors p70 S6K, ribosomal S6 protein (S6) and 4E-BP1. In the absence of photic stimulation, marked S6 and 4E-BP1 phosphorylation was detected, indicating tonic mTOR activity in the SCN. Light stimulated the colocalized activation of p70 S6K and extracellular signal-regulated protein kinase (ERK), and pharmacological disruption of ERK signaling abolished light-induced mTOR activity, revealing that the MAPK cascade is an essential intermediate that couples light to mTOR. Together these data identify a light-responsive mTOR cascade in the SCN, and thus, raise the possibility that inducible translation contributes to the clock entrainment process.

The role of organic cation transporter-3 in methamphetamine disposition and its behavioral response in rats

Organic cation transporter-3 (OCT3) is expressed in several tissues including the brain. We have previously demonstrated that rats with behavioral sensitization to methamphetamine (METH) increased the brain penetration of METH with decreased expression of OCT3 in brain. Considering the earlier in vitro studies demonstrating that 1) OCT3 could transport dopamine (DA) and 2) the specific transport via OCT3 could be inhibited by METH, these results suggest that decreased OCT3 might decrease the efflux of METH and/or DA from brain, subsequently causing the development of behavioral sensitization. Thus, in the present study, behavioral task related to DA and pharmacokinetic experiment were performed using rats treated with antisense against OCT3 (OCT3-AS) since no specific ligands for OCT3 are still available. The continuous infusion of OCT3-AS into the third ventricle significantly decreased the expression of OCT3 in choroid plexus (CP) epithelial cells. Both METH-induced hyperlocomotion and METH-induced extracellular DA levels in nucleus accumbens and prefrontal cortex were significantly increased in OCT3-AS-treated rats. Moreover, the concentrations of METH were significantly increased in cerebrospinal fluid as well as extracellular areas at the nucleus accumbens in OCT3-AS-treated rats. These results suggested that decreased OCT3 elevated the concentration of METH and/or DA in brain, subsequently enhancing dopaminergic neuronal transmission and increasing METH-induced hyperlocomotion. In summary, OCT3 at the CP could regulate the effect of METH by controlling the levels of METH and/or DA in brain. Thus, these results suggest that OCT3 may be a new molecular target to treat METH-related disorders such as drug abuse and schizophrenia.

Measuring reward with the conditioned place preference (CPP) paradigm: update of the last decade

Conditioned place preference (CPP) continues to be one of the most popular models to study the motivational effects of drugs and non-drug treatments in experimental animals. This is obvious from a steady year-to-year increase in the number of publications reporting the use this model. Since the compilation of the preceding review in 1998, more than 1000 new studies using place conditioning have been published, and the aim of the present review is to provide an overview of these recent publications. There are a number of trends and developments that are obvious in the literature of the last decade. First, as more and more knockout and transgenic animals become available, place conditioning is increasingly used to assess the motivational effects of drugs or non-drug rewards in genetically modified animals. Second, there is a still small but growing literature on the use of place conditioning to study the motivational aspects of pain, a field of pre-clinical research that has so far received little attention, because of the lack of appropriate animal models. Third, place conditioning continues to be widely used to study tolerance and sensitization to the rewarding effects of drugs induced by pre-treatment regimens. Fourth, extinction/reinstatement procedures in place conditioning are becoming increasingly popular. This interesting approach is thought to model certain aspects of relapse to addictive behavior and has previously almost exclusively been studied in drug self-administration paradigms. It has now also become established in the place conditioning literature and provides an additional and technically easy approach to this important phenomenon. The enormous number of studies to be covered in this review prevented in-depth discussion of many methodological, pharmacological or neurobiological aspects; to a large extent, the presentation of data had to be limited to a short and condensed summary of the most relevant findings.

Gene Expression Profiling of Rewarding Effect in Methamphetamine Treated Bax-deficient Mouse

Methamphetamine is an illicit drug that is often abused and can cause neuropsychiatric and neurotoxic damage. Repeated administration of psychostimulants such as methamphetamine induces a behavioral sensitization. According to a previous study, Bax was involved in neurotoxicity by methamphetamine, but the function of Bax in rewarding effect has not yet been elucidated. Therefore, we have studied the function of Bax in a rewarding effect model. In the present study, we treated chronic methamphetamine exposure in a Bax-deficient mouse model and examined behavioral change using a conditioned place preference (CPP) test. The CPP score in Bax knockout mice was decreased compared to that of wild-type mice. Therefore, we screened for Bax-related genes that are involved in rewarding effect using microarray technology. In order to confirm microarray data, we applied the RT-PCR method to observe relative changes of Bcl2, a pro-apoptotic family gene. As a result, using our experiment microarray, we selected genes that were associated with Bax in microarray data, and eventually selected the Tgfbr2 gene. Expression of the Tgfbr2 gene was decreased by methamphetamine in Bax knockout mice, and the gene was overexpressed in Bax wild-type mice. Additionally, we confirmed that Creb, FosB, and c-Fos were related to rewarding effect and Bax using immunohistochemistry.