Cocaine-induced psychomotor activity is associated with its ability to induce c-fos mRNA expression in the subthalamic nucleus: effects of dose and repeated treatment

Nucleus Accumbens as a Novel Target for Deep Brain Stimulation in the Treatment of Addiction: A Hypothesis on the Neurochemical and Morphological Basis

Addiction is a major cause of mortality and morbidity. Apart from psychotropic substances, alcohol and nicotine remain the common addictive materials responsible for the majority of deaths. Conventional conservative therapies are beneficial to certain populations, but the majority may require interventional treatments such as deep brain stimulation (DBS) in view of increasing mortality from drug abuse in recent years. We present a brief review on a novel neuromodulation target of the nucleus accumbens (NA) and its promising role in the management of addiction. The three stages of the addiction cycle are known to be mediated by dopaminergic pathways located in the mesolimbic dopamine system with connections to dorsal striatum, extended amygdala, cingulate gyrus, orbitofrontal cortex, prefrontal cortex, and ventral tegmental area. Recent advanced neuroimaging in humans and several animal studies demonstrated NA to be a vital anatomical area modulating this network. DBS of NA in animals reduced addictive behavior to alcohol, cocaine, and other narcotics significantly. The accidental observation that DBS of NA for psychiatric illnesses induced relief from addiction to alcohol and smoking has encouraged further research of late. Bilateral NA ablative surgery had shown nonrelapse in more than 50% of cases. Small series of patients have benefited so far from DBS of NA, but larger numbers are required to provide evidence-based treatment. The modulation of dopaminergic pathways through DBS of NA as a valid treatment for addiction is substantiated extensively by animal studies and also in a few clinical studies. However, this needs to be validated by a well-structured, multicenter controlled study in a large group of patients suffering from substance abuse.

Deep brain stimulation for the treatment of addiction: basic and clinical studies and potential mechanisms of action

RATIONALE

Deep brain stimulation (DBS) has achieved substantial success as a treatment for movement disorders such as Parkinson's disease. The therapeutic efficacy and relative lack of serious side effects resulted in the expansion of DBS into the treatment of many other diseases, including obsessive-compulsive disorder, Tourette's, and depression, among others. More recently, a limited number of basic and clinical studies indicated that DBS may also be useful in the treatment of various addictions.

OBJECTIVES

Here, we briefly summarize the history of DBS and review the basic and clinical studies focused on DBS and addiction. We also examine the potential mechanisms that may underlie the effects of DBS.

RESULTS AND CONCLUSIONS

The available data indicate that DBS is a promising therapeutic modality for the treatment of addiction. Thus far, the nucleus accumbens and subthalamic nucleus are the most promising sites for DBS, reversing aspects of addiction. The mechanisms underlying DBS are complex and likely vary from region to region. Emerging evidence indicates that DBS of the nucleus accumbens produces its effects, at least in part, by antidromic activation of cortico-accumbal afferents that stimulate inhibitory medial prefrontal cortex interneurons via recurrent collaterals.

Neurocircuitry for modeling drug effects

The identification and functional understanding of the neurocircuitry that mediates alcohol and drug effects that are relevant for the development of addictive behavior is a fundamental challenge in addiction research. Here we introduce an assumption-free construction of a neurocircuitry that mediates acute and chronic drug effects on neurotransmitter dynamics that is solely based on rodent neuroanatomy. Two types of data were considered for constructing the neurocircuitry: (1) information on the cytoarchitecture and neurochemical connectivity of each brain region of interest obtained from different neuroanatomical techniques; (2) information on the functional relevance of each region of interest with respect to alcohol and drug effects. We used mathematical data mining and hierarchical clustering methods to achieve the highest standards in the preprocessing of these data. Using this approach, a dynamical network of high molecular and spatial resolution containing 19 brain regions and seven neurotransmitter systems was obtained. Further graph theoretical analysis suggests that the neurocircuitry is connected and cannot be separated into further components. Our analysis also reveals the existence of a principal core subcircuit comprised of nine brain regions: the prefrontal cortex, insular cortex, nucleus accumbens, hypothalamus, amygdala, thalamus, substantia nigra, ventral tegmental area and raphe nuclei. Finally, by means of algebraic criteria for synchronizability of the neurocircuitry, the suitability for in silico modeling of acute and chronic drug effects is indicated. Indeed, we introduced as an example a dynamical system for modeling the effects of acute ethanol administration in rats and obtained an increase in dopamine release in the nucleus accumbens-a hallmark of drug reinforcement-to an extent similar to that seen in numerous microdialysis studies. We conclude that the present neurocircuitry provides a structural and dynamical framework for large-scale mathematical models and will help to predict chronic drug effects on brain function.

Selective serotonin reuptake inhibitor antidepressants potentiate methylphenidate (Ritalin)-induced gene regulation in the adolescent striatum

The psychostimulant methylphenidate (Ritalin) is used in conjunction with selective serotonin reuptake inhibitors (SSRIs) in the treatment of medical conditions such as attention-deficit hyperactivity disorder with anxiety/depression comorbidity and major depression. Co-exposure also occurs in patients on SSRIs who use psychostimulant 'cognitive enhancers'. Methylphenidate is a dopamine/norepinephrine reuptake inhibitor that produces altered gene expression in the forebrain; these effects partly mimic gene regulation by cocaine (dopamine/norepinephrine/serotonin reuptake inhibitor). We investigated whether the addition of SSRIs (fluoxetine or citalopram; 5 mg/kg) modified gene regulation by methylphenidate (2-5 mg/kg) in the striatum and cortex of adolescent rats. Our results show that SSRIs potentiate methylphenidate-induced expression of the transcription factor genes zif268 and c-fos in the striatum, rendering these molecular changes more cocaine-like. Present throughout most of the striatum, this potentiation was most robust in its sensorimotor parts. The methylphenidate + SSRI combination also enhanced behavioral stereotypies, consistent with dysfunction in sensorimotor striatal circuits. In so far as such gene regulation is implicated in psychostimulant addiction, our findings suggest that SSRIs may enhance the addiction potential of methylphenidate.

Fos after single and repeated self-administration of cocaine and saline in the rat: emphasis on the Basal forebrain and recalibration of expression

The effects of addictive psychostimulant drugs on the brain change over repeated administrations. We evaluated a large sample of brain structures, particularly ones comprising basal forebrain macrosystems, and determined in which the immediate-early gene product, Fos, is expressed following a single and repeated self-administrations of cocaine. The caudate-putamen and accumbens, comprising the basal ganglia input structures, and the hypothalamic supraoptic and paraventricular nuclei, lateral and medial habenula, mesopontine rostromedial tegmental nucleus and anterior cingulate cortex exhibited Fos expression enhanced by acute self-administration of cocaine (SAC), but desensitized after repeated administrations. Fos expression was mainly enhanced by acutely self-administered cocaine in basal ganglia output and intrinsic structures and the intermediate nucleus of lateral septum, medial division of the central amygdaloid nucleus and zona incerta, but, in contrast, was sensitized in these structures after repeated administrations. Acute and repeated SAC left Fos expression unaffected or marginally enhanced in most extended amygdala structures, of which nearly all, however, exhibited robustly increased Fos expression after repeated saline self-administration, occasionally to levels exceeding those elicited by cocaine. Thus, self-administered cocaine mainly elicits Fos expression, which persists or increases with repeated administrations in some structures, but declines in others. In addition, Fos expression is sensitized in most extended amygdala structures merely by the act of repeated self-administering. Similar spatiotemporal patterns of cocaine- or saline-elicited Fos expression characterize functionally related clusters of structures, such as, eg, basal ganglia input structures, basal ganglia output structures, extended amygdala and structures in the brainstem to which forebrain macrosystems project.

DeltaFosB induction in orbitofrontal cortex potentiates locomotor sensitization despite attenuating the cognitive dysfunction caused by cocaine

The effects of addictive drugs change with repeated use: many individuals become tolerant of their pleasurable effects but also more sensitive to negative sequelae (e.g., anxiety, paranoia, and drug craving). Understanding the mechanisms underlying such tolerance and sensitization may provide valuable insight into the basis of drug dependency and addiction. We have recently shown that chronic cocaine administration reduces the ability of an acute injection of cocaine to affect impulsivity in rats. However, animals become more impulsive during withdrawal from cocaine self-administration. We have also shown that chronic administration of cocaine increases expression of the transcription factor DeltaFosB in the orbitofrontal cortex (OFC). Mimicking this drug-induced elevation in OFC DeltaFosB through viral-mediated gene transfer mimics these behavioural changes: DeltaFosB over-expression in OFC induces tolerance to the effects of an acute cocaine challenge but sensitizes rats to the cognitive sequelae of withdrawal. Here we report novel data demonstrating that increasing DeltaFosB in the OFC also sensitizes animals to the locomotor-stimulant properties of cocaine. Analysis of nucleus accumbens tissue taken from rats over-expressing DeltaFosB in the OFC and treated chronically with saline or cocaine does not provide support for the hypothesis that increasing OFC DeltaFosB potentiates sensitization via the nucleus accumbens. These data suggest that both tolerance and sensitization to cocaine's many effects, although seemingly opposing processes, can be induced in parallel via the same biological mechanism within the same brain region, and that drug-induced changes in gene expression within the OFC play an important role in multiple aspects of addiction.

Decrease of D2 receptor binding but increase in D2-stimulated G-protein activation, dopamine transporter binding and behavioural sensitization in brains of mice treated with a chronic escalating dose 'binge' cocaine administration paradigm

Understanding the neurobiology of the transition from initial drug use to excessive drug use has been a challenge in drug addiction. We examined the effect of chronic 'binge' escalating dose cocaine administration, which mimics human compulsive drug use, on behavioural responses and the dopaminergic system of mice and compared it with a chronic steady dose (3 x 15 mg/kg/day) 'binge' cocaine administration paradigm. Male C57BL/6J mice were injected with saline or cocaine in an escalating dose paradigm for 14 days. Locomotor and stereotypy activity were measured and quantitative autoradiographic mapping of D(1) and D(2) receptors, dopamine transporters and D(2)-stimulated [(35)S]GTPgammaS binding was performed in the brains of mice treated with this escalating and steady dose paradigm. An initial sensitization to the locomotor effects of cocaine followed by a dose-dependent increase in the duration of the locomotor effect of cocaine was observed in the escalating but not the steady dose paradigm. Sensitization to the stereotypy effect of cocaine and an increase in cocaine-induced stereotypy score was observed from 3 x 20 to 3 x 25 mg/kg/day cocaine. There was a significant decrease in D(2) receptor density, but an increase in D(2)-stimulated G-protein activity and dopamine transporter density in the striatum of cocaine-treated mice, which was not observed in our steady dose paradigm. Our results document that chronic 'binge' escalating dose cocaine treatment triggers profound behavioural and neurochemical changes in the dopaminergic system, which might underlie the transition from drug use to compulsive drug use associated with addiction, which is a process of escalation.

Repeated amphetamine administration outside the home cage enhances drug-induced Fos expression in rat nucleus accumbens

Induction of the immediate early gene protein product Fos has been used extensively to assess neural activation in the striatum after repeated amphetamine administration to rats in their home cages. However, this technique has not been used to examine striatal activation after repeated administration outside the home cage, an environment where repeated drug administration produces more robust psychomotor sensitization. We determined the dose-response relationship for amphetamine-induced psychomotor activity and Fos expression in nucleus accumbens and caudate-putamen 1 week after repeated administration of amphetamine or saline in locomotor activity chambers. Repeated administration of amphetamine enhanced amphetamine-induced locomotor activity and stereotypy and Fos expression in nucleus accumbens, but not in caudate-putamen. In comparison, levels of Fos expression induced by 1mg/kg amphetamine were not altered in nucleus accumbens or caudate-putamen by repeated amphetamine administration in the home cage. Double-labeling of Fos protein and enkephalin mRNA indicates that Fos is expressed in approximately equal numbers of enkephalin-negative and enkephalin-positive neurons in nucleus accumbens and caudate-putamen following injections outside the home cage. Furthermore, repeated amphetamine administration increased drug-induced Fos expression in enkephalin-positive, but not enkephalin-negative, neurons in nucleus accumbens. We conclude that repeated amphetamine administration outside the home cage recruits the activation of enkephalin-containing nucleus accumbens neurons during sensitized amphetamine-induced psychomotor activity.

Modeling the role of environment in addiction

The aim of this review is to provide an overview of the main types of animal models used to investigate the modulatory role of environment on drug addiction. The environment can alter the responsiveness to addictive drugs in at least three major ways. First, adverse life experiences can make an individual more vulnerable to develop drug addiction or to relapse into drug seeking. Second, neutral environmental cues can acquire, through Pavlovian conditioning, the ability to trigger drug seeking even after long periods of abstinence. Third, the environment immediately surrounding drug taking can alter the behavioral, subjective, and rewarding effects of a given drug, thus influencing the propensity to use the same drug again. We have focused in particular on the results obtained using an animal model we have developed to study the latter type of drug-environment interaction.

Food-associated cues alter forebrain functional connectivity as assessed with immediate early gene and proenkephalin expression

Background

Cues predictive of food availability are powerful modulators of appetite as well as food-seeking and ingestive behaviors. The neurobiological underpinnings of these conditioned responses are not well understood. Monitoring regional immediate early gene expression is a method used to assess alterations in neuronal metabolism resulting from upstream intracellular and extracellular signaling. Furthermore, assessing the expression of multiple immediate early genes offers a window onto the possible sequelae of exposure to food cues, since the function of each gene differs. We used immediate early gene and proenkephalin expression as a means of assessing food cue-elicited regional activation and alterations in functional connectivity within the forebrain.

Results

Contextual cues associated with palatable food elicited conditioned motor activation and corticosterone release in rats. This motivational state was associated with increased transcription of the activity-regulated genes homer1a, arc, zif268, ngfi-b and c-fos in corticolimbic, thalamic and hypothalamic areas and of proenkephalin within striatal regions. Furthermore, the functional connectivity elicited by food cues, as assessed by an inter-regional multigene-expression correlation method, differed substantially from that elicited by neutral cues. Specifically, food cues increased cortical engagement of the striatum, and within the nucleus accumbens, shifted correlations away from the shell towards the core. Exposure to the food-associated context also induced correlated gene expression between corticostriatal networks and the basolateral amygdala, an area critical for learning and responding to the incentive value of sensory stimuli. This increased corticostriatal-amygdalar functional connectivity was absent in the control group exposed to innocuous cues.

Conclusion

The results implicate correlated activity between the cortex and the striatum, especially the nucleus accumbens core and the basolateral amygdala, in the generation of a conditioned motivated state that may promote excessive food intake. The upregulation of a number of genes in unique patterns within corticostriatal, thalamic, and hypothalamic networks suggests that food cues are capable of powerfully altering neuronal processing in areas mediating the integration of emotion, cognition, arousal, and the regulation of energy balance. As many of these genes play a role in plasticity, their upregulation within these circuits may also indicate the neuroanatomic and transcriptional correlates of extinction learning.

Motor-skill learning-associated gene regulation in the striatum: effects of cocaine

Psychostimulant-induced molecular changes in cortico-basal ganglia-cortical circuits play a critical role in addiction and dependence. These changes include alterations in gene regulation particularly in projection neurons of the sensorimotor striatum. We previously showed that cocaine-induced gene regulation in such neurons is dependent on the behavior performed during drug action. Rats trained on a running wheel under the influence of cocaine for 4 days subsequently displayed greater c-fos induction by cocaine than untrained controls. This effect was selective for the sensorimotor striatum, which is known to mediate forms of motor learning. In the present study, we investigated whether this enhanced cellular responsiveness was associated with learning of wheel running or with prolonged running (exercising), by assessing c-fos inducibility after 1, 2, or 8 days of training. Wheel training was performed after injection of cocaine (25 mg/kg) or vehicle, and c-fos induction by a cocaine challenge was measured 24 h later. Rats that trained under cocaine (but not vehicle) showed a greater c-fos response in the striatum compared to locked-wheel controls. This effect was present after the 1-day training, peaked after 2 days, and dissipated by 8 days of training. Similar effects were found for substance P, but not enkephalin, expression. These changes in striatal gene regulation paralleled improvement in wheel running, which was facilitated by cocaine. Thus, these training-induced molecular changes do not appear to represent exercising effects, but may reflect motor learning-associated neuronal changes altered by cocaine. Such cocaine effects may contribute to aberrant motor learning implicated in psychostimulant addiction.

Cocaine‐induced locomotor activity and Fos expression in nucleus accumbens are sensitized for 6 months after repeated cocaine administration outside the home cage

Induction of the immediate early gene protein product Fos has been used extensively to assess neural activation in the striatum after repeated cocaine administration to rats in their home cages but rarely after repeated administration outside the home cage, which produces more robust locomotor sensitization. In the present study, we found cocaine-induced Fos expression in nucleus accumbens, but not caudate-putamen, was enhanced 1 and 6 months after repeated drug administration in locomotor activity chambers. Double-labelling of Fos protein and enkephalin mRNA indicated that Fos expression in nucleus accumbens was enhanced in enkephalin-positive, but not enkephalin-negative, medium spiny neurons. In contrast, cocaine-induced Fos expression was absent altogether in nucleus accumbens and unaltered in caudate-putamen 1 month after repeated cocaine administration in the home cage. As cocaine-induced locomotor activity was also enhanced 1 and 6 months after repeated cocaine administration in locomotor activity chambers, we wanted to confirm that neuronal activity in nucleus accumbens mediates cocaine-induced locomotor activity using our particular treatment regimen. Bilateral infusions of the GABA agonists baclofen and muscimol (1 microg/side) into nucleus accumbens of sensitized rats blocked cocaine-induced Fos expression and locomotor activity. Thus, while neuronal activity in both D1- and D2-type neurons in nucleus accumbens can mediate acute cocaine-induced locomotor activity, the enhanced activation of enkephalinergic D2-type neurons suggests that these latter neurons mediate the enhancement of cocaine-induced locomotor activity for up to 6 months after repeated drug administration outside the home cage.

Subthalamic nucleus lesions enhance the psychomotor-activating, incentive motivational, and neurobiological effects of cocaine

The subthalamic nucleus (STN) is traditionally thought to be involved in motor control, and dysfunction of the STN is thought to contribute to movement disorders. Here, we show that the STN also plays an important role in motivational processes and the response to drugs of abuse. Specifically, bilateral STN lesions produced a dose-dependent increase in the psychomotor-activating effects of cocaine, the rate at which animals acquired cocaine self-administration, and the motivation for cocaine assessed using a progressive ratio schedule. Furthermore, bilateral STN lesions enhanced the ability of cocaine to induce gene expression in the nucleus accumbens and caudate-putamen, two structures known to be involved in mediating the psychomotor-activating and incentive motivational effects of drugs of abuse. These findings suggest that engagement of the STN serves to dampen the psychomotor-activating and incentive motivational effects of drugs of abuse. Thus, the STN may serve as a novel target for therapeutic interventions aimed at treating drug dependence.

Relationship of cocaine-induced c-Fos expression to behaviors and the role of serotonin 5-HT2A receptors in cocaine-induced c-Fos expression

Serotonin 5-HT2A receptor antagonists have been shown to attenuate the locomotor stimulant effects of cocaine in rats. The present study used the expression of c-Fos protein as a marker to identify brain areas through which 5-HT2A receptors may modulate cocaine-induced behaviors. Significant correlations were observed between cocaine-induced hyperactivity and c-Fos expression in the nucleus accumbens core (NAcC), caudate-putamen (CPu), and subthalamic nucleus. In a separate experiment, a low, behaviorally relevant dose of cocaine was found to increase c-Fos immunoreactivity in the medial CPu, NAcC, and nucleus accumbens shell (NAcSh). The selective 5-HT2A receptor antagonist M100907 significantly attenuated cocaine-induced c-Fos expression in the medial CPu and in the NAcSh. These data suggest that 5-HT2A receptors in the NAcSh and CPu or in afferents to these regions may contribute to genomic responses to cocaine in the brain as well as to cocaine-induced locomotor activity.

Regulation of cocaine-induced activator protein 1 transcription factors by the extracellular signal-regulated kinase pathway

Extracellular signal-regulated kinases and activator protein 1 transcription factor have been functionally linked to addiction. It has also been shown that extracellular signal-regulated kinase activation can regulate cocaine-induced expression of c-Fos and FosB, two possible components of activator protein 1. A direct link between extracellular signal-regulated kinases and activator protein 1 activation has, however, remained unexplored. In this study, we investigated the role of extracellular signal-regulated kinases in the regulation of DNA-binding activity and composition of activator protein 1 induced in the mouse caudate putamen by cocaine treatment. We have found that pre-treatment with SL327, a selective inhibitor the extracellular signal-regulated kinase pathway, has no influence on cocaine-induced DNA-binding activity of activator protein 1, when examined one hour after an acute cocaine treatment. This phenomenon results from simultaneous decrease of c-Fos protein level and increases in JunB and deltaFosB protein levels. SL327 pre-treatment, however, reduces the DNA-binding activity of the activator protein 1 complex induced six hours after an acute cocaine treatment as well as one hour after the last of the chronic cocaine injections, a phenomenon that results from the concomitant reduction of all cocaine-induced proteins (c-Fos, FosB, deltaFosB, JunB). In conclusion, we have found that extracellular signal-regulated kinase inhibition may not only interfere with cocaine-induced gene expression and activator protein 1 complex activation, but may also disturb the time-course of gene expression and composition of activator protein 1 complex. Our results support the notion that inhibitors of the extracellular signal-regulated kinase pathway could be valuable tools to obliterate cocaine-induced molecular changes and the development of addiction.

Neural and behavioral plasticity associated with the transition from controlled to escalated cocaine use

BACKGROUND

Rats given extended access to cocaine develop several symptoms of addiction, including a gradual escalation of drug intake, whereas rats given limited access do not. We asked here whether extended access to cocaine also produces drug-induced sensitization, a form of neurobehavioral plasticity implicated in addiction.

METHODS

Rats were given limited (1 hour/session) or extended access (6 hours/session) to self-administered cocaine. Following a period of abstinence, rats were selected at random for assessment of their psychomotor response to cocaine or drug-seeking during extinction or for anatomic studies.

RESULTS

When re-exposed to cocaine, rats allowed extended drug access showed greater drug-seeking behavior and were hypersensitive (sensitized) to the psychomotor activating effects of cocaine compared with rats given limited access. Extended access to cocaine was also associated with a greater increase in the density of dendritic spines on neurons specifically in the core of the nucleus accumbens (and not in the shell or medial or orbital frontal cortex).

CONCLUSIONS

The transition from stable to escalated cocaine use, a hallmark of addiction, is associated with especially robust behavioral sensitization and synaptic reorganization in the core of the nucleus accumbens.

Habituation to test procedure modulates the involvement of dopamine D2- but not D1-receptors in ethanol-induced locomotor stimulation in mice

RATIONALE

Novelty associated with behavioral testing has been shown to enhance psychostimulant- and morphine-induced locomotor stimulation. Evidence has demonstrated that novelty increases dopamine (DA) activity, and habituation to a novel environment reduces such activation. However, it is not clear whether novelty modulates ethanol-induced behavioral stimulation and whether DA plays a role in this effect.

OBJECTIVES

The present work sought to demonstrate a role of habituation to test procedure as a factor that could modulate the involvement of DA in ethanol-induced locomotor stimulation.

METHODS

Non-habituated (NH) and habituated (H) Swiss mice pretreated with DA D1- (SCH23390; 0-0.045 mg/kg) or D2-receptor (sulpiride; 0-50 mg/kg) antagonists were tested for ethanol (0-2.5 g/kg)-induced locomotor stimulation. Experiments with amphetamine (0-4 mg/kg), morphine (0-5 mg/kg) and caffeine (0-15 mg/kg)were designed to compare their results to those obtained with ethanol. The effect of the non-selective opioid receptor antagonist naltrexone (0-1.5 mg/kg) was also tested on ethanol-induced locomotor stimulation.

RESULTS

NH and H animals did not differ in their locomotor response to ethanol or caffeine; however, amphetamine- and morphine-induced stimulation was greater in NH than in H mice. SCH23390 only reduced ethanol-induced stimulation at doses that also reduced spontaneous activity in both NH and H mice. Sulpiride decreased ethanol-stimulated behavior only in the NH condition. Habituation did not modify the effect of sulpiride on amphetamine-, morphine- or caffeine-induced activation. Naltrexone (0-1.5 mg/kg) reduced ethanol-induced stimulation regardless of habituation.

CONCLUSIONS

The present data suggest that the participation of DA D2-receptors in ethanol-induced behavioral stimulation requires the presence of novelty. Results also support the involvement of neurotransmitter systems other than DA (i.e., endogenous opioid system) as important substrates mediating ethanol-induced locomotor activation.

Extracellular signal-regulated kinases (ERKs) modulate cocaine-induced gene expression in the mouse amygdala

It is known that acute cocaine administration activates the extracellular signal-regulated kinase (ERK) pathway in the striatum, and results in transcription and translation of immediate early genes (IEGs). In the present study we investigated a possible involvement of ERK in the regulation of IEG expression in the amygdala, another brain structure known to be related to an addicted state. The patterns of cocaine-induced c-Fos, JunB and Zif268 protein expression were investigated, using an immunohistochemical approach, within distinct nuclei of the amygdala, either in the presence or absence of a selective inhibitor of the ERK pathway, SL327. Although these IEGs were similarly activated in the various nuclei of the amygdala after acute administration of cocaine, they showed different patterns after chronic injections. They also showed selective sensitivities to ERK inhibition. In particular, whereas c-Fos and JunB expressions were augmented following chronic cocaine treatment, as compared with acute treatment, Zif268 expression was decreased by this chronic treatment. Additionally, chronic blocking of ERK activation affected cocaine-induced c-Fos and JunB but not Zif268 expression. Thus, the differential involvement of ERK in chronic vs. acute regulation of IEGs may account for its specific role in addiction-related behavioral alterations, such as sensitization and tolerance.

The neural consequences of repeated cocaine exposure revealed by functional MRI in awake rats

The use of functional magnetic resonance imaging (fMRI) in animal models of cocaine addiction is an invaluable tool for investigating the neuroadaptations that lead to this psychiatric disorder. We used blood-oxygen-level-dependent (BOLD) MRI in awake rats to identify the neuronal circuits affected by repeated cocaine administration. Rats were given an injection of cocaine (15 mg/kg, i.p.) or its vehicle for 7 days, abstained from injections for 1 week, and challenged with an intracerebroventricular cocaine injection during functional imaging. Acute cocaine produced robust positive BOLD responses across well-known monoamine-enriched brain regions, such as the prefrontal cortex, nucleus accumbens, dorsal striatum, sensory cortex, hippocampus, thalamus, and midbrain areas. However, repeated cocaine administration resulted in lower BOLD responses in the prefrontal cortex, agranular insular cortex, nucleus accumbens, ventral pallidum, and dorsomedial thalamus, among other brain regions. Reductions in BOLD intensity were not associated with variations in cerebrovascular reactivity between drug naive rats and those repeatedly exposed to cocaine. Therefore, the lower metabolic activation in response to cocaine could reflect a reduced neuronal and/or synaptic activity upon repeated administration.

The subthalamic nucleus exerts opposite control on cocaine and 'natural' rewards

A challenge in treating drug addicts is preventing their pathological motivation for the drug without impairing their general affective state toward natural reinforcers. Here we have shown that discrete lesions of the subthalamic nucleus greatly decreased the motivation of rats for cocaine while increasing it for food reward. The subthalamic nucleus, a key structure controlling basal ganglia outputs, is therefore able to oppositely modulate the effect of 'natural' rewards and drugs of abuse on behavior. Modulating the activity of the subthalamic nucleus might prove to be a new target for the treatment of cocaine addiction.

Rapid delivery of nicotine promotes behavioral sensitization and alters its neurobiological impact

BACKGROUND

Nicotine is highly addictive when it is inhaled from tobacco smoke, whereas nicotine replacement products, which usually deliver nicotine orally or transdermally, rarely lead to addiction. It has been proposed that this is due in part to differences in the rate of nicotine delivery to the brain under the two conditions. However, the mechanism by which rapid nicotine delivery facilitates the transition to addiction is not known. The ability of drugs to alter gene regulation and to produce sensitization has been implicated in addiction. We hypothesized, therefore, that varying the rate of nicotine administration may modulate its ability to elicit this form of plasticity.

METHODS

Animals were treated with repeated intravenous infusions of nicotine over 5, 25, or 100 sec, and their locomotor responses were monitored over treatment days.

RESULTS

We found that increasing the rate of intravenous nicotine infusion potentiated its ability to produce locomotor sensitization, and to induce c-fos and arc mRNA expression in mesocorticolimbic structures.

CONCLUSIONS

We suggest that rapid administration may increase vulnerability to addiction by altering the neurobiological impact of nicotine and promoting a form of neurobehavioral plasticity (i.e., sensitization) that can lead to pathological incentive motivation for drugs.

The induction of behavioural sensitization is associated with cocaine-induced structural plasticity in the core (but not shell) of the nucleus accumbens

Repeated exposure to cocaine increases the density of dendritic spines on medium spiny neurons in the nucleus accumbens (Acb) and pyramidal cells in the medial prefrontal cortex (mPFC). To determine if this is associated with the development of psychomotor sensitization, rats were given daily i.p. injections of 15 mg/kg of cocaine (or saline) for 8 days, either in their home cage (which failed to induce significant psychomotor sensitization) or in a distinct and relatively novel test cage (which induced robust psychomotor sensitization). Their brains were obtained 2 weeks after the last injection and processed for Golgi-Cox staining. In the Acb core (AcbC) cocaine treatment increased spine density only in the group that developed psychomotor sensitization (i.e. in the Novel but not Home group), and there was a significant positive correlation between the degree of psychomotor sensitization and spine density. In the Acb shell (AcbS) cocaine increased spine density to the same extent in both groups; i.e. independent of psychomotor sensitization. In the mPFC cocaine increased spine density in both groups, but to a significantly greater extent in the Novel group. Furthermore, when rats were treated at Home with a higher dose of cocaine (30 mg/kg), cocaine now induced psychomotor sensitization in this context, and also increased spine density in the AcbC. Thus, the context in which cocaine is experienced influences its ability to reorganize patterns of synaptic connectivity in the Acb and mPFC, and the induction of psychomotor sensitization is associated with structural plasticity in the AcbC and mPFC, but not the AcbS.

The rate of cocaine administration alters gene regulation and behavioral plasticity: implications for addiction

The rapid delivery of drugs of abuse to the brain is thought to promote addiction, but why this occurs is unknown. In the present study, we characterized the influence of rate of intravenous cocaine infusion (5-100 sec) on three effects thought to contribute to its addiction liability: its ability to block dopamine (DA) uptake, to activate immediate early gene expression, and to produce psychomotor sensitization. Rapid infusions potentiated the ability of cocaine to block DA reuptake, to induce c-fos and arc mRNA expression, especially in mesocorticolimbic regions, and to produce psychomotor sensitization. Thus, the rate at which cocaine is delivered influences both its neurobiological impact and its ability to induce a form of drug experience-dependent plasticity implicated in addiction. We propose that rapidly delivered cocaine may be more addictive, in part, because this more readily induces forms of neurobehavioral plasticity that lead to the compulsive pursuit of drugs.

Morphine-induced c-fos mRNA expression in striatofugal circuits: modulation by dose, environmental context, and drug history

Opiates and psychostimulants produce many shared behavioral and neurobiological adaptations, such as behavioral sensitization and the induction of immediate early genes in the caudate-putamen (CPu). Previous studies indicate that factors such as dose, the environmental context surrounding drug administration and drug history can influence both morphine- and psychostimulant-induced behavioral sensitization. In addition, these factors can modulate the ability of psychostimulants to engage striatofugal circuits in the CPu. The present study, therefore, sought to examine whether these factors have similar influences over the ability of morphine to engage cortico-striatofugal circuits. We report that, when given in the home cage, morphine produced a small, but significant increase in the number of c-fos+ striatonigral cells and c-fos+ cells in cingulate cortex, but had no effect on the number of c-fos+ striatopallidal cells. When given in a novel test environment, however, morphine dramatically increased the number of c-fos+ striatonigral cells in a dose-dependent fashion, and this effect was maintained following repeated treatment. Unexpectedly, morphine treatment in a novel environment produced a dose-dependent reduction in the number of c-fos+ striatopallidal cells and c-fos+ cells in cingulate cortex, relative to exposure to novelty alone-effects that were reversed by repeated morphine treatment. We suggest that alterations in c-fos expression patterns in striatofugal circuits following morphine administration may be involved in drug-experience-dependent plasticity.

Drug-induced neurobehavioral plasticity: the role of environmental context

Repeated administrations of addictive drugs produce long-lasting changes in brain and behavior. However, drug-induced neurobehavioral plasticity is not a mere function of the neuropharmacological actions of drugs, but the result of complex drug-environment interactions. In the present review we summarize results obtained in a series of studies using an animal model of drug-environment interaction, showing that environmental context and past drug history interact to modulate the effects of amphetamine, cocaine and morphine on behavior, gene expression and structural plasticity. These findings may help shed some light on the conditions necessary for addictive drugs to enduringly alter brain and behavior.