Prefrontal dopamine efflux during exposure to drug-associated contextual cues in rats with prior repeated methamphetamine

Effects of chronic methamphetamine exposure on rewarding behavior and neurodegeneration markers in adult mice

Recreational and medical use of stimulants among young adults have gained popularity in the United States over the last decade and their use may increase vulnerability to brain biochemical changes and addictive behaviors. The long-term effects of chronic stimulant exposure in later adulthood have not been fully elucidated.Our study investigated whether chronic exposure to methamphetamine (METH), at a dose designed to emulate human therapeutic dosing for ADHD, would promote biochemical alterations and affect sensitivity to the rewarding effects of subsequent METH dosing.Groups of 3.5-month-old male and female C57BL/6J mice were administered non-contingent intraperitoneal injections of either saline or METH (1.4 mg/kg) twice a day for 1 month (5 days/week). METH (0.5 mg/kg)-induced conditioned place preference (CPP) was tested in mice to determine the effects of previous METH exposure on reward-related behavior. Mice were randomly assigned to Experiment I (males and females) or Experiment II (females only) in which CPP testing was respectively performed either 0.5 or 5 months after the end of METH injections, at ~5 or 10 months old respectively. The midbrain and striatum, regions involved in reward circuit, were assessed for markers associated with neurotoxicity, dopaminergic function, neuroinflammation and epigenetic changes after behavioral testing.Previous exposure to chronic METH did not have significant short-term effects on CPP response but led to a decreased CPP response in 10-month-old females. Previous exposure to METH induced some short-term changes to biochemical markers measured in a brain region and sex-dependent manner, while long-term changes were only observed with GFAP and KDM5C.In conclusion, our data suggest sex- and post-exposure duration-dependent outcomes and warrant further exploration of the long-term neurobehavioral consequences of psychostimulant use in both sexes.

Cardiolipin and OPA1 Team up for Methamphetamine-Induced Locomotor Activity by Promoting Neuronal Mitochondrial Fusion in the Nucleus Accumbens of Mice

Mitochondria are highly dynamic organelles with coordinated cycles of fission and fusion occurring continuously to satisfy the energy demands in the complex architecture of neurons. How mitochondria contribute to addicted drug-induced adaptable mitochondrial networks and neuroplasticity remains largely unknown. Through liquid chromatography-mass spectrometry-based lipidomics, we first analyzed the alteration of the mitochondrial lipidome of three mouse brain areas in methamphetamine (METH)-induced locomotor activity and conditioned place preference. The results showed that METH remodeled the mitochondrial lipidome of the hippocampus, nucleus accumbens (NAc), and striatum in both models. Notably, mitochondrial hallmark lipid cardiolipin (CL) was specifically increased in the NAc in METH-induced hyperlocomotor activity, which was accompanied by an elongated giant mitochondrial morphology. Moreover, METH significantly boosted mitochondrial respiration and ATP generation as well as the copy number of mitochondrial genome DNA in the NAc. By screening the expressions of mitochondrial dynamin-related proteins, we found that repeated METH significantly upregulated the expression of long-form optic atrophy type 1 (L-OPA1) and enhanced the interaction of L-OPA1 with CL, which may promote mitochondrial fusion in the NAc. On the contrary, neuronal OPA1 depletion in the NAc not only recovered the dysregulated mitochondrial morphology and synaptic vesicle distribution induced by METH but also attenuated the psychomotor effect of METH. Collectively, upregulated CL and OPA1 cooperate to mediate METH-induced adaptation of neuronal mitochondrial dynamics in the NAc, which correlates with the psychomotor effect of METH. These findings propose a potential therapeutic approach for METH addiction by inhibiting neuronal mitochondrial fusion.

Developmental treatments with Δ9- tetrahydrocannabinol and the MAGL inhibitor JZL184 persistently alter adult cocaine conditioning in contrasting ways

Using a songbird, zebra finches, as a developmental drug abuse model we found previously that cannabinoid agonists administered during the sensorimotor period of vocal learning (50-75 days of age) persistently alter song patterns and cocaine responsiveness in adulthood. However, these effects were not produced in adults exposed to similar treatment regimens. Currently, we have used the MAGL inhibitor, JZL184, to test whether enhanced endocannabinoid signaling may similarly alter cocaine responsiveness. We found that, as expected and consistent with prior results, repeated developmental (but not adult) treatments with Δ⁹-tetrahydrocannabinol (THC, 3 mg/kg QD IM) resulted in increased time spent in cocaine-paired chambers. Unexpectedly and in contrast, repeated developmental JZL184 (4 mg/kg QD IM) treatments decreased time spent in cocaine-conditioned chambers. That is, young finches repeatedly treated with JZL184 avoided cocaine-paired chambers later in adulthood, while similar development treatments with THC had the opposite effect. To begin to identify brain regions that may underly this differential responsiveness we used c-Fos expression as a marker of neuronal activity. Differences in c-Fos expression patterns following placement of cocaine-conditioned finches into vehicle- vs. cocaine-paired chambers suggest distinct involvement of circuits through striatal and amygdaloid regions in respective effects of THC and JZL184. Results demonstrate that, like exogenous cannabinoid exposure, inhibition of MAGL activity during late post-natal development persistently alters behavior in adulthood. Contrasting effects of THC vs. MAGL inhibition with JZL184 suggests the latter alters development of brain regions to favor promotion of aversive rather than appetitive cocaine responsiveness later in adulthood.

REM sleep deprivation before extinction or reinstatement alters methamphetamine reward memory via D1-like dopamine receptors

We aimed to determine whether REM sleep deprivation (RSD) affects extinction and reinstatement of methamphetamine (METH) reward memory in male rats and also to evaluate the possible role of dopamine D1-like and D2-like dopamine (DA) receptors in these processes. Male rats were trained to acquire METH-induced place preference (2 mg/kg, i.p.). METH reward memory was then reinstated following a 10-day extinction period. The animals underwent a 72-hour sleep deprivation episode by multiple platforms method (in separate groups), either before the extraction or before the reinstatement of METH reward memory. The animals received SCH 23390 (0.01 or 0.05 mg/kg, i.p.) or sulpiride (20 or 60 mg/kg, i.p.) as antagonists of D1-like and D2-like DA receptors, respectively, either immediately following each daily extinction session or before the reinstatement of METH-seeking behavior. The RSD episode postponed extinction and facilitated reinstatement of METH reward memory. Administration of SCH 23390, but not sulpiride, facilitated METH extinction and decreased reinstatement of the extinguished METH-seeking behavior. Moreover, locomotor activity was not affected by METH and/or the RSD paradigm. The results would seem to suggest that the D1-like, but not the D2-like, DA receptors may be involved in the extinction and reinstatement of the extinguished METH reward memory in RSD animals. Nonetheless, more investigations are needed to elucidate the exact mechanisms involved.

Dopamine and relapse to drug seeking

The actions of dopamine are essential to relapse to drug seeking but we still lack a precise understanding of how dopamine achieves these effects. Here we review recent advances from animal models in understanding how dopamine controls relapse to drug seeking. These advances have been enabled by important developments in understanding the basic neurochemical, molecular, anatomical, physiological and functional properties of the major dopamine pathways in the mammalian brain. The literature shows that although different forms of relapse to seeking different drugs of abuse each depend on dopamine, there are distinct dopamine mechanisms for relapse. Different circuit-level mechanisms, different populations of dopamine neurons and different activity profiles within these dopamine neurons, are important for driving different forms of relapse. This diversity highlights the need to better understand when, where and how dopamine contributes to relapse behaviours.

Significance of protein kinase C in the neuropsychotoxicity induced by methamphetamine-like psychostimulants

The abuse of methamphetamine (MA), an amphetamine (AMPH)-type stimulant, has been demonstrated to be associated with various neuropsychotoxicity, including memory impairment, psychiatric morbidity, and dopaminergic toxicity. Compelling evidence from preclinical studies has indicated that protein kinase C (PKC), a large family of serine/threonine protein kinases, plays an important role in MA-induced neuropsychotoxicity. PKC-mediated N-terminal phosphorylation of dopamine transporter has been identified as one of the prerequisites for MA-induced synaptic dopamine release. Consistently, it has been shown that PKC is involved in MA (or AMPH)-induced memory impairment and mania-like behaviors as well as MA drug dependence. Direct or indirect regulation of factors related to neuronal plasticity seemed to be critical for these actions of PKC. In addition, PKC-mediated mitochondrial dysfunction, oxidative stress or impaired antioxidant defense system has been suggested to play a role in psychiatric and cognitive disturbance induced by MA (or AMPH). In MA-induced dopaminergic toxicity, particularly PKCδ has been shown to trigger oxidative stress, mitochondrial dysfunction, pro-apoptotic changes, and neuroinflammation. Importantly, PKCδ may be a key mediator in the positive feedback loop composed of these detrimental events to potentiate MA-induced dopaminergic toxicity. This review outlines the role of PKC and its individual isozymes in MA-induced neuropsychotoxicity. Better understanding on the molecular mechanism of PKCs might provide a great insight for the development of potential therapeutic or preventive candidates for MA (or AMPH)-associated neuropsychotoxicity.

Genetic factors involved in risk for methamphetamine intake and sensitization

Lines of mice were created by selective breeding for the purpose of identifying genetic mechanisms that influence the magnitude of the selected trait and to explore genetic correlations for additional traits thought to be influenced by shared mechanisms. DNA samples from high and low methamphetamine-drinking (MADR) and high and low methamphetamine-sensitization lines were used for quantitative trait locus (QTL) mapping. Significant additive genetic correlations between the two traits indicated a common genetic influence, and a QTL on chromosome X was detected for both traits, suggesting one source of this commonality. For MADR mice, a QTL on chromosome 10 accounted for more than 50 % of the genetic variance in that trait. Microarray gene expression analyses were performed for three brain regions for methamphetamine-naïve MADR line mice: nucleus accumbens, prefrontal cortex, and ventral midbrain. Many of the genes that were differentially expressed between the high and low MADR lines were shared in common across the three brain regions. A gene network highly enriched in transcription factor genes was identified as being relevant to genetically determined differences in methamphetamine intake. When the mu opioid receptor gene (Oprm1), located on chromosome 10 in the QTL region, was added to this top-ranked transcription factor network, it became a hub in the network. These data are consistent with previously published findings of opioid response and intake differences between the MADR lines and suggest that Oprm1, or a gene that impacts activity of the opioid system, plays a role in genetically determined differences in methamphetamine intake.

Neural substrates of amphetamine-induced behavioral sensitization: unconditioned (zero context) and conditioned (switch versus same context) components in c-fos overexpression

The neural substrates of the unconditioned and conditioned components of amphetamine (AMPH)-induced behavioral sensitization remain unknown. The present study examines the brain activation of rats in response to an AMPH challenge with augmented locomotion in groups receiving chronic AMPH under chloral hydrate anesthetization (i.e., the 'zero context') or when tested in the 'same context' as a chronic treatment, or when tested in a 'different context'. The neural activations of the three groups reveal fairly consistent patterns: (a) The substantia nigra is activated in the same context condition and the pure AMPH effect (i.e., the zero context with the unconditioned component), but not in the switch context condition. (b) The ventral pallidum showed Fos expression in the switch context and the same context, but not in the zero context condition. (c) The other nuclei, including the medial prefrontal cortex, nucleus accumbens, caudate putamen, medial thalamus, hippocampus, amygdala, and ventral tegmental area, are activated in all contextual conditions and the pure AMPH effect (the zero context). The context exerts definable effects on the mesocorticolimbic dopamine system on AMPH-induced behavioral sensitization. (d) The ventral pallidum and the substantia nigra activations dissociate the unconditioned component from the conditioned component in behavioral sensitization. Further studies are needed to determine how these two nuclei mediate the effect in terms of primary and conditioned rewards.

Baclofen facilitates the extinction of methamphetamine-induced conditioned place preference in rats

The powerful, long-lasting association between the rewarding effects of a drug and contextual cues associated with drug administration can be studied using conditioned place preference (CPP). The GABA(B) receptor agonist baclofen facilitates the extinction of morphine-induced CPP in mice. The current study extended this work by determining if baclofen could enhance the extinction of methamphetamine (Meth) CPP. CPP was established using a six-day conditioning protocol wherein Meth-pairings were alternated with saline-pairings. Rats were subsequently administered baclofen (2 mg/kg i.p. or vehicle) immediately after each daily forced extinction session, which consisted of a saline injection immediately prior to being placed into the previously Meth- or saline-paired chamber. One extinction training cycle, consisted of six once-daily forced extinction sessions, mimicking the alternating procedure established during conditioning, followed by a test for preference (Ext test). CPP persisted for at least four extinction cycles in vehicle-treated rats. In contrast, CPP was inhibited following a single extinction training cycle. These data indicate that Meth-induced CPP was resistant to extinction, but extinction training was rendered effective when the training was combined with baclofen. These findings converge with the prior demonstration of baclofen facilitating the extinction of morphine-induced CPP indicating that GABA(B) receptor actions are independent of the primary (unconditioned) stimulus (i.e., the opiate or the stimulant) and likely reflect mechanisms engaged by extinction learning processes per se. Thus, baclofen administered in conjunction with extinction training may be of value for addiction therapy regardless of the class of drug being abused.

Context-dependent effects of a single administration of mirtazapine on the expression of methamphetamine-induced conditioned place preference

Re-exposure to cues repeatedly associated with methamphetamine (Meth) can trigger Meth-seeking and relapse in the abstinent abuser. Weakening the conditioned Meth-associated memory during cue re-exposure may provide a means for relapse-reduction pharmacotherapy. Accordingly, we sought to determine if the atypical antidepressant mirtazapine disrupted the persistence of Meth-induced conditioned place preference (CPP) when administered in conjunction with re-exposure to contextual conditioning cues, and if this effect was altered by Meth being present during cue re-exposure. First, we evaluated the effect of mirtazapine on the maintenance of Meth-induced CPP during re-exposure to either the saline- or Meth-paired chamber 12 days after conditioning. Meth-conditioned rats subsequently administered mirtazapine expressed CPP independent of re-exposure to the saline- or Meth-paired chamber; but the magnitude of CPP was significantly less for mirtazapine-treated rats re-exposed to the Meth-paired chamber. Next, we evaluated the effect of mirtazapine on a "reinforced re-exposure" to the Meth-paired context. Administration of mirtazapine vehicle and Meth, prior to re-exposure to the Meth-paired chamber did not disrupt the ability of rats to demonstrate CPP 15 days after conditioning; however, CPP was disrupted when rats were administered mirtazapine and Meth prior to re-exposure to the Meth-paired chamber. These results indicate that the capacity of mirtazapine to diminish Meth-induced CPP is promoted if mirtazapine treatment is coupled with Meth administration in the Meth-associated context and thus appears to be the consequence of disrupting processes necessary to reconsolidate CPP following activation of drug-associated memories.

Effects of sensitization on the detection of an instrumental contingency

While prior exposure to drugs of abuse permanently changes many behaviors, the underlying psychological mechanisms are relatively obscure. Here, the effects of sensitization on the detection of an action-outcome relationship were assessed, using a particularly stringent contingency degradation procedure. Rats were trained to leverpress until the probability of reinforcement for a response on one lever, or alternative reinforcement for a response on a second lever was reduced to 0.05 per second. Sensitization was then carried out (1mg/kg d-amphetamine/day for 7 days). Then, one reinforcer was also made available for a lack of response on either lever (p=0.05/s), maintaining its contiguity with the original response but eliminating its contingent relationship. Sensitized animals were more active, particularly early in the contingency degradation phase, but reduced responding directed at the degraded action-outcome contingency at a similar rate as controls. However, controls also reduced responding directed at the nondegraded contingency until very late in training, while sensitized animals maintained nondegraded responding at baseline levels. It was suggested that the relatively specific response shown by sensitized animals may reflect either improved action-outcome utilization or discrimination of relevant task features.

MDMA ("ecstasy") abuse as an example of dopamine neuroplasticity

A number of reviews have focused on the short- and long-term effects of MDMA and, in particular, on the persistent deficits in serotonin neurotransmission that accompany some exposure regimens. The mechanisms underlying the serotonin deficits and their relevance to various behavioral and cognitive consequences of MDMA use are still being debated. It has become clear, however, that some individuals develop compulsive and uncontrolled drug-taking that is consistent with abuse. For other drugs of abuse, this transition has been attributed to neuroadaptations in central dopamine mechanisms that occur as a function of repeated drug exposure. A question remains as to whether similar neuroadaptations occur as a function of exposure to MDMA and the impact of serotonin neurotoxicity in the transition from use to abuse. This review focuses specifically on this issue by first providing an overview of human studies and then reviewing the animal literature with specific emphasis on paradigms that measure subjective effects of drugs and self-administration as indices of abuse liability. It is suggested that serotonin deficits resulting from repeated exposure to MDMA self-administration lead to a sensitized dopaminergic response to the drug and that this sensitized response renders MDMA comparable to other drugs of abuse.

Enhancing the salience of dullness: behavioral and pharmacological strategies to facilitate extinction of drug-cue associations in adolescent rats

Extinction of drug-seeking is an integral part of addiction treatment, and can profoundly reverse or ameliorate the harmful consequences of drug use. These consequences may be the most deleterious during adolescence. The studies presented here build from recent evidence that adolescent rats are more resistant to extinction training than adults, and therefore may require unique treatment strategies. We used unbiased place-conditioning in male rats to show that passive, un-explicit extinction pairings resulted in delayed extinction in 40-day-old adolescents relative to 80-day-old adults. However, explicit-pairing of a previously cocaine-associated context with the absence of drug produces extinction in adolescents as rapidly as in adults. These data suggest that successful extinction of drug-paired associations in adolescents may be facilitated by stronger acquisition of a new (extinction) memory. Drug-paired associations are largely controlled by the prelimbic prefrontal cortex (plPFC) and its influence on the nucleus accumbens (NAc). This pathway mediates the motivational salience attributed to incoming stimuli through the D1 dopamine receptor. D1 receptors on plPFC outputs to the accumbens are transiently overproduced during adolescence. Since D1 receptors are selectively responsive to potent stimuli, we hypothesized that the adolescent plPFC hinders competition between potent drug-paired associations and the subtler, drug-free information necessary for extinction. To harness this unique profile of the adolescent plPFC, we aimed to increase the salience of unrewarded extinction memories by activating plPFC D1 receptors during extinction training. In a second study, extinction of drug-cue associations was facilitated in adolescents by elevating dopamine and norepinephrine in the PFC during extinction training with atomoxetine. In a third study, direct microinjection of the D1 receptor agonist SKF38393 mimicked this effect, also facilitating extinction in adolescent subjects. Furthermore, pharmacological intervention attenuated subsequent drug-primed reinstatement of cocaine-conditioned preferences. We establish a potential direction for distinct strategies to treat this vulnerable population.

Extinction of drug- and withdrawal-paired cues in animal models: relevance to the treatment of addiction

Conditioned drug craving and withdrawal elicited by cues paired with drug use or acute withdrawal are among the many factors contributing to compulsive drug taking. Understanding how to stop these cues from having these effects is a major goal of addiction research. Extinction is a form of learning in which associations between cues and the events they predict are weakened by exposure to the cues in the absence of those events. Evidence from animal models suggests that conditioned responses to drug cues can be extinguished, although the degree to which this occurs in humans is controversial. Investigations into the neurobiological substrates of extinction of conditioned drug craving and withdrawal may facilitate the successful use of drug cue extinction within clinical contexts. While this work is still in the early stages, there are indications that extinction of drug- and withdrawal-paired cues shares neural mechanisms with extinction of conditioned fear. Using the fear extinction literature as a template, it is possible to organize the observations on drug cue extinction into a cohesive framework.

Maternal separation fails to render animals more susceptible to methamphetamine-induced conditioned place preference

The maternal separation (MS) paradigm is an animal model that has been successfully used to study the long term effects of child abuse and neglect. Experiments showed that animals subjected to trauma and stress early in life display behavioural, endocrinological and growth factor abnormalities at a later stage in life, results that mirrored clinical conditions. It is apparent that adverse events early in life may affect the development and maturation of the brain negatively. The purpose of the present study was to investigate whether the abnormal brain development occurring in separated animals would also enhance the development of a preference for psychostimulant drug usage. Rats were subjected to maternal deprivation and further exposed to methamphetamine-induced conditioned place preference (CPP) which primarily measures drug reward (ventral striatum) learning and memory. Apomorphine-induced locomotor activity was also assessed to investigate the effects of methamphetamine on the dorsal (primarily locomotor activity) striatal dopaminergic system. We found that four consecutive injections of methamphetamine resulted in CPP behaviour 24 h after the 4th injection. A further four injections yielded similar CPP results and this effect lasted for at least 7 days until the third CPP assessment. These animals also had decreased ACTH and corticosterone secretions, but the prolactin levels were increased. Prior exposure to maternal separation did not have any effect on the CPP test. The ACTH and corticosterone secretions were also similarly reduced. However maternal separation decreased the release of prolactin and this reduction was not evident in the separated group that received methamphetamine. There was no significant difference in the apomorphine-induced locomotor activity of normally reared animals whether they received methamphetamine or saline. Interestingly there was a significant difference in locomotor activity between the two groups of animals that were subjected to maternal deprivation. The separated animals that received methamphetamine displayed markedly reduced locomotor activity upon apomorphine administration when compared to those that were treated with saline. Taken together, we conclude that maternal deprivation differentially influences dorsal and ventral striatal regions implicating dopaminergic mechanisms.

Behavioural and dopaminergic alterations induced by a low dose of WIN 55,212-2 in a conditioned place preference procedure

AIMS

This study investigated the role of the cannabinoid CB1 receptor agonist, WIN 55,212-2, on motor activity. Subsequently, the effects of a low, stimulatory dose of WIN 55,212-2 and cocaine, as a positive control, were evaluated using a conditioned place preference (CPP) procedure. Upon completion of CPP, in rats that had been treated with WIN 55,212-2, dopaminergic status and spontaneous and d-amphetamine-induced motor activity were assessed.

MAIN METHODS

Sprague-Dawley rats were evaluated for habituated motor activity following WIN 55,212-2 (0, 0.1, 0.3, 1 mg/kg, i.p.) administration. A stimulatory dose of WIN 55,212-2 (0.1 mg/kg, i.p.) and cocaine (20 mg/kg, i.p.) was selected to assess CPP behaviour. Upon completion of CPP, in one group, tissue levels of dopamine and its metabolites were measured in distinct brain regions (dorsal striatum, nucleus accumbens, prefrontal cortex, amygdala, hippocampus) using High Performance Liquid Chromatography with electrochemical detection. In another group, spontaneous and D-amphetamine-induced motor activity was evaluated in an open-field apparatus.

KEY FINDINGS

The lowest dose of WIN 55,212-2 increased motor activity but did not produce CPP. As expected, cocaine induced clear CPP. Dopaminergic status was increased in a region-specific way and motor activity was enhanced following a challenge of D-amphetamine in rats that had been administered with WIN 55,212-2 during conditioning.

SIGNIFICANCE

A stimulatory effect of WIN 55,212-2 on motor activity was not accompanied by place preference. Upon completion of the CPP procedure, this dose was found to induce region-specific hyperdopaminergia along with a greater sensitivity to a subsequent challenge dose of D-amphetamine.

Accumbal dopamine and serotonin activity throughout acquisition and expression of place conditioning: correlative relationships with preference and aversion

The ability of addictive drugs to induce adaptations in mesolimbic dopamine (DA) activity offers an attractive neurobiological explanation for enhanced incentive motivation toward drug-associated stimuli in addiction. However, direct evidence supporting this is sparse. By tracking neurochemical activity within the mouse nucleus accumbens via microdialysis during repeated pairing of morphine with environmental stimuli, we reveal a predictive relationship between enhanced DA responses to morphine and subsequent preference towards a morphine-paired stimulus. A similar relationship for serotonin (5-HT) was observed, suggesting that these neuromodulatory systems work in concert. During expression of preference towards a morphine-paired stimulus, extracellular DA was not enhanced but was negatively associated with this behavior on a subject-by-subject basis. In contrast, avoidance of an aversively-paired stimulus (the opiate antagonist naloxone) was associated with enhanced extracellular DA levels, and also the balance between DA and 5-HT responses. These findings reveal a tangible predictive relationship between drug-induced neural adaptations and conditioned behavior, and emphasize that DA activity is not generalized to all subcomponents of behavior conditioned by addictive drugs. They further provide evidence for an active role of DA-5-HT interactions in the expression of learned behavior.

Cue-elicited drug craving represses ERK activation in mice prefrontal association cortex

Morphine addiction is characterized by compulsive drug-taking behavior and high rates of relapse that reflect reward-controlled learning, consolidation and reconsolidation of drug cues. Extracellular signal-regulated protein kinase (ERK) is one of the cellular molecules that have been highly implicated in the synaptic plasticity processes of learning and memory in cocaine addiction. However, the roles of ERK in the morphine-paired conditioned place preference (CPP) are not clear. In the present study, we found that compared to the morphine-unpaired and saline-paired and saline-unpaired groups, morphine-paired mice showed depressed ERK2 activity in the Frontal Association Cortex (FrA), whereas ERK1 activity was not changed in the same region. In the Accumbens Nucleus (Acb) and Caudate Putamen (CPu) that are associated with cocaine addiction, the activities of ERK1 and ERK2 among four groups showed no difference. These results suggest that the FrA plays an important role in morphine craving and that ERK2 is involved in eliciting the environment-related morphine craving, which is totally different from those induced by morphine itself.

Microdialysis and the neurochemistry of addiction

Drug addiction is a process beginning with the initial exposure to a drug of abuse, and leading, in some individuals, to chronic habitual use, and high rates of relapse. Microdialysis allows researchers to monitor the neurochemical changes that occur in the brain after the initial exposure to a drug, and the neurochemical changes that occur with repeated exposure. These changes in the brain are often referred to as drug-induced neuroplasticity, and the aim of this article is to review studies that have utilized microdialysis to increase our understanding of the neuroplasticity that occurs in the process of addiction. We will review how several neurotransmitter systems, including glutamate, GABA, the monoamines, and others, are altered after chronic drug exposure, and how microdialysis can be used to determine if putative treatments for addiction can reverse the drug-induced neuroplasticity in these systems. We will also briefly discuss our recent research using a known change in GABA neurotransmission that occurs during reinstatement of drug-seeking to screen for possible novel treatments to prevent relapse. Overall, microdialysis in combination with other behavioral and pharmacological techniques has greatly increased our understanding of addiction-related neuroplasticity, and provides a means for discovering new ways to prevent these changes and treat addiction.