Environmental modulation of amphetamine-induced c-fos expression in D1 versus D2 striatal neurons

Effect of Combined Methamphetamine and Oxycodone Use on the Synaptic Proteome in an In Vitro Model of Polysubstance Use

Polysubstance use (PSU) generally involves the simultaneous use of an opioid along with a stimulant. In recent years, this problem has escalated into a nationwide epidemic. Understanding the mechanisms and effects underlying the interaction between these drugs is essential for the development of treatments for those suffering from addiction. Currently, the effect of PSU on synapses-critical points of contact between neurons-remains poorly understood. Using an in vitro model of primary neurons, we examined the combined effects of the psychostimulant methamphetamine (METH) and the prescription opioid oxycodone (oxy) on the synaptic proteome using quantitative mass-spectrometry-based proteomics. A further ClueGO analysis and Ingenuity Pathway Analysis (IPA) indicated the dysregulation of several molecular functions, biological processes, and pathways associated with neural plasticity and structural development. We identified one key synaptic protein, Striatin-1, which plays a vital role in many of these processes and functions, to be downregulated following METH+oxy treatment. This downregulation of Striatin-1 was further validated by Western blot. Overall, the present study indicates several damaging effects of the combined use of METH and oxy on neural function and warrants further detailed investigation into mechanisms contributing to synaptic dysfunction.

The Medial Prefrontal Cortex, Nucleus Accumbens, Basolateral Amygdala, and Hippocampus Regulate the Amelioration of Environmental Enrichment and Cue in Fear Behavior in the Animal Model of PTSD

A growing body of evidence showed that environmental enrichment (EE) ameliorated footshock-induced fear behavior of posttraumatic stress disorder (PTSD). However, no research comprehensively tested the effect of EE, cue, and the combination of EE and cue in footshock-induced fear behavior of PTSD symptoms. The present study addressed this issue and examined whether the medial prefrontal cortex (mPFC, including the cingulate cortex 1 (Cg1), prelimbic cortex (PrL), and infralimbic cortex (IL)), the nucleus accumbens (NAc), the basolateral amygdala (BLA), and the hippocampus (e.g., CA1, CA3, and dentate gyrus (DG)) regulated the amelioration of the EE, cue, or the combination of EE and cue. The results showed that EE or cue could reduce fear behavior. The combination of EE and cue revealed a stronger decrease in fear behavior. The cue stimulus may play an occasion setting or a conditioned stimulus to modulate the reduction in fear behavior induced by footshock. Regarding the reduction of the EE in fear behavior, the Cg1 and IL of the mPFC and the NAc upregulated the c-Fos expression; however, the BLA downregulated the c-Fos expression. The mPFC (i.e., the Cg1, PrL, and IL) and the hippocampus (i.e., the CA1, CA3, and DG) downregulated the c-Fos expression in the suppression of the cue in fear behavior. The interaction of EE and cue in reduction of fear behavior occurred in the Cg1 and NAc for the c-Fos expression. The data of c-Fos mRNA were similar to the findings of the c-Fos protein expression. These findings related to the EE and cue modulations in fear behavior may develop a novel nonpharmacological treatment in PTSD.

Drug-activated cells: From immediate early genes to neuronal ensembles in addiction

Beyond their rapid rewarding effects, drugs of abuse can durably alter an individual's response to their environment as illustrated by the compulsive drug seeking and risk of relapse triggered by drug-associated stimuli. The persistence of these associations even long after cessation of drug use demonstrates the enduring mark left by drugs on brain reward circuits. However, within these circuits, neuronal populations are differently affected by drug exposure and growing evidence indicates that relatively small subsets of neurons might be involved in the encoding and expression of drug-mediated associations. The identification of sparse neuronal populations recruited in response to drug exposure has benefited greatly from the study of immediate early genes (IEGs) whose induction is critical in initiating plasticity programs in recently activated neurons. In particular, the development of technologies to manipulate IEG-expressing cells has been fundamental to implicate broadly distributed neuronal ensembles coincidently activated by either drugs or drug-associated stimuli and to then causally establish their involvement in drug responses. In this review, we summarize the literature regarding IEG regulation in different learning paradigms and addiction models to highlight their role as a marker of activity and plasticity. As the exploration of neuronal ensembles in addiction improves our understanding of drug-associated memory encoding, it also raises several questions regarding the cellular and molecular characteristics of these discrete neuronal populations as they become incorporated in drug-associated neuronal ensembles. We review recent efforts towards this goal and discuss how they will offer a more comprehensive understanding of addiction pathophysiology.

One Is Not Enough: Understanding and Modeling Polysubstance Use

Substance use disorder (SUD) is a chronic, relapsing disease with a highly multifaceted pathology that includes (but is not limited to) sensitivity to drug-associated cues, negative affect, and motivation to maintain drug consumption. SUDs are highly prevalent, with 35 million people meeting criteria for SUD. While drug use and addiction are highly studied, most investigations of SUDs examine drug use in isolation, rather than in the more prevalent context of comorbid substance histories. Indeed, 11.3% of individuals diagnosed with a SUD have concurrent alcohol and illicit drug use disorders. Furthermore, having a SUD with one substance increases susceptibility to developing dependence on additional substances. For example, the increased risk of developing heroin dependence is twofold for alcohol misusers, threefold for cannabis users, 15-fold for cocaine users, and 40-fold for prescription misusers. Given the prevalence and risk associated with polysubstance use and current public health crises, examining these disorders through the lens of co-use is essential for translatability and improved treatment efficacy. The escalating economic and social costs and continued rise in drug use has spurred interest in developing preclinical models that effectively model this phenomenon. Here, we review the current state of the field in understanding the behavioral and neural circuitry in the context of co-use with common pairings of alcohol, nicotine, cannabis, and other addictive substances. Moreover, we outline key considerations when developing polysubstance models, including challenges to developing preclinical models to provide insights and improve treatment outcomes.

Non-pharmacological factors that determine drug use and addiction

Based on their pharmacological properties, psychoactive drugs are supposed to take control of the natural reward system to finally drive compulsory drug seeking and consumption. However, psychoactive drugs are not used in an arbitrary way as pure pharmacological reinforcement would suggest, but rather in a highly specific manner depending on non-pharmacological factors. While pharmacological effects of psychoactive drugs are well studied, neurobiological mechanisms of non-pharmacological factors are less well understood. Here we review the emerging neurobiological mechanisms beyond pharmacological reinforcement which determine drug effects and use frequency. Important progress was made on the understanding of how the character of an environment and social stress determine drug self-administration. This is expanded by new evidence on how behavioral alternatives and opportunities for drug instrumentalization generate different patterns of drug choice. Emerging evidence suggests that the neurobiology of non-pharmacological factors strongly determines pharmacological and behavioral drug action and may, thus, give rise for an expanded system's approach of psychoactive drug use and addiction.

Opposite environmental gating of the experienced utility ('liking') and decision utility ('wanting') of heroin versus cocaine in animals and humans: implications for computational neuroscience

BACKGROUND

In this paper, we reviewed translational studies concerned with environmental influences on the rewarding effects of heroin versus cocaine in rats and humans with substance use disorder. These studies show that both experienced utility ('liking') and decision utility ('wanting') of heroin and cocaine shift in opposite directions as a function of the setting in which these drugs were used. Briefly, rats and humans prefer using heroin at home but cocaine outside the home. These findings appear to challenge prevailing theories of drug reward, which focus on the notion of shared substrate of action for drug of abuse, and in particular on their shared ability to facilitate dopaminergic transmission.

AIMS

Thus, in the second part of the paper, we verified whether our findings could be accounted for by available computational models of reward. To account for our findings, a model must include a component that could mediate the substance-specific influence of setting on drug reward RESULTS: It appears of the extant models that none is fully compatible with the results of our studies.

CONCLUSIONS

We hope that this paper will serve as stimulus to design computational models more attuned to the complex mechanisms responsible for the rewarding effects of drugs in real-world contexts.

Heroin versus cocaine: opposite choice as a function of context but not of drug history in the rat

RATIONALE

Previous studies have shown that rats trained to self-administer heroin and cocaine exhibit opposite preferences, as a function of setting, when tested in a choice paradigm. Rats tested at home prefer heroin to cocaine, whereas rats tested outside the home prefer cocaine to heroin. Here, we investigated whether drug history would influence subsequent drug preference in distinct settings. Based on a theoretical model of drug-setting interaction, we predicted that regardless of drug history rats would prefer heroin at home and cocaine outside the home.

METHODS

Rats with double-lumen catheters were first trained to self-administer either heroin (25 μg/kg) or cocaine (400 μg/kg) for 12 consecutive sessions. Twenty-six rats were housed in the self-administration chambers (thus, they were tested at home), whereas 30 rats lived in distinct home cages and were transferred to self-administration chambers only for the self-administration session (thus, they were tested outside the home). The rats were then allowed to choose repeatedly between heroin and cocaine within the same session for seven sessions.

RESULTS

Regardless of the training drug, the rats tested outside the home preferred cocaine to heroin, whereas the rats tested at home preferred heroin to cocaine. There was no correlation between drug preference and drug intake during the training phase.

CONCLUSION

Drug preferences were powerfully influenced by the setting but, quite surprisingly, not by drug history. This suggests that, under certain conditions, associative learning processes and drug-induced neuroplastic adaptations play a minor role in shaping individual preferences for one drug or the other.

Identifying the In Vivo Cellular Correlates of Antipsychotic Drugs

GPCRs such as 5-HT_2A and D2 are implicated in the therapeutic and the side effects of antipsychotics. However, the pattern of brain activity that leads to the behavioral effects of antipsychotics is poorly understood. To address this question, we used the transgenic ‘FosTRAP’ mice (Mus musculus), where a fluorescent reporter marks the cells responsive to the stimulus of interest. Here, the stimulus was an administration of various antipsychotic drugs. In case of typical antipsychotics such as Haloperidol, the c-fos active cells were predominantly found in the striatum, whereas in case of the atypical antipsychotics (Clozapine and Olanzapine), c-fos-induced cells were more numerous in the cortical regions, e.g., orbital cortex, piriform cortex. Curiously, we also observed ependymal cells to be a novel cellular target of atypical antipsychotics. 5-HT_2A is considered to be a major target for atypical antipsychotics. Therefore, we bred ‘FosTRAP’ mice with 5-HT_2A knock-out (KO) mice and tested their response to the prototype of atypical antipsychotics, Clozapine. Interestingly, the absence of 5-HT_2A did not significantly affect the number of c-fos-induced cells in the cortical regions. However, the ependymal cells showed a dramatically reduced response to Clozapine in the absence of 5-HT_2A. In summary, the TRAP system has allowed us to identify various region-specific activity induced by antipsychotics and novel cellular targets of the antipsychotics. These results serve as a “proof of principle” study that can be extended to explore the biochemical and physiological changes brought about by antipsychotics and specifically identify antipsychotic-responsive cells in the live tissue.

The Affective and Neural Correlates of Heroin versus Cocaine Use in Addiction Are Influenced by Environmental Setting But in Opposite Directions

Previous studies have shown that individuals with heroin and cocaine addiction prefer to use these drugs in distinct settings: mostly at home in the case of heroin and mostly outside the home in the case of cocaine. Here we investigated whether the context would modulate the affective and neural responses to these drugs in a similar way. First, we used a novel emotional task to assess the affective state produced by heroin or cocaine in different settings, based on the recollections of male and female drug users. Then we used fMRI to monitor neural activity during drug imagery (re-creating the setting of drug use) in male drug users. Consistent with our working hypothesis, the majority of participants reported a shift in the affective valence of heroin from mostly pleasant at home to mostly unpleasant outside the home (p < 0.0001). The opposite shift was observed for cocaine; that is, most participants who found cocaine pleasant outside the home found it unpleasant when taken at home (p < 0.0014). Furthermore, we found a double dissociation, as a function of drug and setting imagery, in BOLD signal changes in the left PFC and caudate, and bilaterally in the cerebellum (all p values <0.01), suggesting that the fronto-striatal-cerebellar network is implicated in the contextualization of drug-induced affect. In summary, we report that the same setting can influence in opposite directions the affective and neural response to psychostimulants versus opiates in humans, adding to growing evidence of distinct substrates for the rewarding effects of these two drug classes.SIGNIFICANCE STATEMENT The rewarding effects of addictive drugs are often thought to depend on shared substrates. Yet, environmental influences can unmask striking differences between psychostimulants and opiates. Here we used emotional tasks and fMRI to explore the influence of setting on the response to heroin versus cocaine in individuals with addiction. Simply moving from one setting to another significantly decreased heroin pleasure but increased cocaine pleasure, and vice versa. Similar double dissociation was observed in the activity of the fronto-striatal-cerebellar network. These findings suggest that the effects of opiates and psychostimulants depend on dissociable psychological and neural substrates and that therapeutic approaches to addiction should take into account the peculiarities of different drug classes and the settings of drug use.

Parvalbumin Interneurons of the Mouse Nucleus Accumbens are Required For Amphetamine-Induced Locomotor Sensitization and Conditioned Place Preference

To determine the requirement for parvalbumin (PV) expressing GABAergic interneurons of the nucleus accumbens (NAc) in the behavioral adaptations induced by amphetamine (AMPH), we blocked synaptic vesicle release from these neurons using Cre-inducible viral expression of the tetanus toxin light chain in male and female PV-Cre mice. Silencing PV+ interneurons of the NAc selectively inhibited the expression of locomotor sensitization following repeated injections of AMPH and blocked AMPH-induced conditioned place preference (CPP). AMPH induced significantly more expression of the activity-dependent gene Fos in both D1 and D2 dopamine receptor-expressing medium spiny neurons (MSNs) of the NAc of PV+ interneuron silenced mice, suggesting a function for PV+ interneuron-mediated MSN inhibition in the expression of AMPH-induced locomotor sensitization and CPP. These data show a requirement for PV+ interneurons of the NAc in behavioral responses to AMPH, and they raise the possibility that modulation of PV+ interneuron function may alter the development or expression of psychostimulant-induced behavioral adaptations.

Stress-Induced Reduction of Dorsal Striatal D2 Dopamine Receptors Prevents Retention of a Newly Acquired Adaptive Coping Strategy

The inability to learn an adaptive coping strategy in a novel stressful condition leads to dysfunctional stress coping, a marker of mental disturbances. This study tested the involvement of dorsal striatal dopamine receptors in the dysfunctional coping with the Forced Swim test fostered by a previous experience of reduced food availability. Adult male mice were submitted to a temporary (12 days) reduction of food availability [food-restricted (FR)] or continuously free-fed (FF). Different groups of FF and FR mice were used to evaluate: (1) dorsal striatal mRNA levels of the two isoforms of the dopamine D2 receptor (D2S, D2L). (2) Forced Swim-induced c-fos expression in the dorsal striatum; (3) acquisition and 24 h retention of passive coping with Forced Swim. Additional groups of FF mice were tested for 24 h retention of passive coping acquired during a first experience with Forced Swim immediately followed by intra-striatal infusion of vehicle or two doses of the dopamine D2/D3 receptors antagonist sulpiride or the D1/D5 receptors antagonist SCH23390. Previous restricted feeding selectively reduced mRNA levels of both D2 isoforms and abolished Forced Swim-induced c-fos expression in the left Dorsolateral Striatum and selectively prevented 24 h retention of the coping strategy acquired in a first experience of Forced Swim. Finally, temporary blockade of left Dorsolateral Striatum D2/D3 receptors immediately following the first Forced Swim experience selectively reproduced the behavioral effect of restricted feeding in FF mice. In conclusion, the present results demonstrate that mice previously exposed to a temporary reduction of food availability show low striatal D2 receptors, a known marker of addiction-associated aberrant neuroplasticity, as well as liability to relapse into maladaptive stress coping strategies. Moreover, they offer strong support to a causal relationship between reduction of D2 receptors in the left Dorsolateral Striatum and impaired consolidation of newly acquired adaptive coping.

Role of Central Amygdala Neuronal Ensembles in Incubation of Nicotine Craving

The craving response to smoking-associated cues in humans or to intravenous nicotine-associated cues in adult rats progressively increases or incubates after withdrawal. Here, we further characterized incubation of nicotine craving in the rat model by determining whether this incubation is observed after adolescent-onset nicotine self-administration. We also used the neuronal activity marker Fos and the Daun02 chemogenetic inactivation procedure to identify cue-activated neuronal ensembles that mediate incubation of nicotine craving. We trained adolescent and adult male rats to self-administer nicotine (2 h/d for 12 d) and assessed cue-induced nicotine seeking in extinction tests (1 h) after 1, 7, 14, or 28 withdrawal days. In both adult and adolescent rats, nicotine seeking in the relapse tests followed an inverted U-shaped curve, with maximal responding on withdrawal day 14. Independent of the withdrawal day, nicotine seeking in the relapse tests was higher in adult than in adolescent rats. Analysis of Fos expression in different brain areas of adolescent and adult rats on withdrawal days 1 and 14 showed time-dependent increases in the number of Fos-positive neurons in central and basolateral amygdala, orbitofrontal cortex, ventral and dorsal medial prefrontal cortex, and nucleus accumbens core and shell. In adult Fos-lacZ transgenic rats, selective inactivation of nicotine-cue-activated Fos neurons in central amygdala, but not orbitofrontal cortex, decreased "incubated" nicotine seeking on withdrawal day 14. Our results demonstrate that incubation of nicotine craving occurs after adolescent-onset nicotine self-administration and that neuronal ensembles in central amygdala play a critical role in this incubation.

SIGNIFICANCE STATEMENT

The craving response to smoking-associated cues in humans or to intravenous nicotine-associated cues in adult rats progressively increases or incubates after withdrawal. It is currently unknown whether incubation of craving also occurs after adolescent-onset nicotine self-administration. The brain areas that mediate such incubation are also unknown. Here, we used a rat model of incubation of drug craving, the neuronal activity marker Fos, and the Daun02 chemogenetic inactivation method to demonstrate that incubation of nicotine craving is also observed after adolescent-onset nicotine self-administration and that neuronal ensembles in the central nucleus of the amygdala play a critical role in this incubation in adult rats.

The neuroendocrine response to stress under the effect of drugs: Negative synergy between amphetamine and stressors

There have been numerous studies into the interaction between stress and addictive drugs, yet few have specifically addressed how the organism responds to stress when under the influence of psychostimulants. Thus, we studied the effects of different acute stressors (immobilization, interleukin-1β and forced swimming) in young adult male rats simultaneously exposed to amphetamine (AMPH, 4 mg/kg SC), evaluating classic biological markers. AMPH administration itself augmented the plasma hypothalamic-pituitary-adrenal (HPA) hormones, adrenocorticotropin (ACTH) and corticosterone, without affecting plasma glucose levels. By contrast, this drug dampened the peripheral HPA axis, as well as the response of glucose to the three stressors. We also found that AMPH administration completely blocked the forced swim-induced expression of the corticotropin-releasing hormone (hnCRH) and it partially reduced c-fos expression in the paraventricular nucleus of the hypothalamus (PVN). Indeed, this negative synergy in the forced swim test could even be observed with a lower dose of AMPH (1mg/kg, SC), a dose that is usually received in self-administration experiments. In conclusion, when rats that receive AMPH are subjected to stress, a negative synergy occurs that dampens the prototypic peripheral physiological response to stress and activation of the PVN.

Incubation of methamphetamine craving is associated with selective increases in expression of Bdnf and trkb, glutamate receptors, and epigenetic enzymes in cue-activated fos-expressing dorsal striatal neurons

Cue-induced methamphetamine seeking progressively increases after withdrawal (incubation of methamphetamine craving), but the underlying mechanisms are largely unknown. We determined whether this incubation is associated with alterations in candidate genes in dorsal striatum (DS), a brain area implicated in cue- and context-induced drug relapse. We first measured mRNA expression of 24 candidate genes in whole DS extracts after short (2 d) or prolonged (1 month) withdrawal in rats following extended-access methamphetamine or saline (control condition) self-administration (9 h/d, 10 d). We found minimal changes. Next, using fluorescence-activated cell sorting, we compared gene expression in Fos-positive dorsal striatal neurons, which were activated during "incubated" cue-induced drug-seeking tests after prolonged withdrawal, with nonactivated Fos-negative neurons. We found significant increases in mRNA expression of immediate early genes (Arc, Egr1), Bdnf and its receptor (Trkb), glutamate receptor subunits (Gria1, Gria3, Grm1), and epigenetic enzymes (Hdac3, Hdac4, Hdac5, GLP, Dnmt3a, Kdm1a) in the Fos-positive neurons only. Using RNAscope to determine striatal subregion and cell-type specificity of the activated neurons, we measured colabeling of Fos with Drd1 and Drd2 in three DS subregions. Fos expression was neither subregion nor cell-type specific (52.5 and 39.2% of Fos expression colabeled with Drd1 and Drd2, respectively). Finally, we found that DS injections of SCH23390 (C17H18ClNO), a D1-family receptor antagonist known to block cue-induced Fos induction, decreased incubated cue-induced methamphetamine seeking after prolonged withdrawal. Results demonstrate a critical role of DS in incubation of methamphetamine craving and that this incubation is associated with selective gene-expression alterations in cue-activated D1- and D2-expressing DS neurons.

Effects of differential rearing on amphetamine-induced c-fos expression in rats

BACKGROUND

Rearing rats in environmental enrichment alters psychostimulant-induced locomotor activity as well as neurotransmitter expression. Exposure to novelty and psychostimulants induces c-fos expression in neurons in the mesolimbic dopamine (DA) pathway. Here we investigated changes in the expression of the immediate early gene c-fos in the mesolimbic DA pathway of enriched, isolated, or socially reared rats due to the neurobiological changes that result from rearing conditions and influence drug taking behavior.

METHODS

Rats were reared in either enriched (EC), isolated (IC), or social (SC) conditions for 30 days, after which they received an acute amphetamine or saline injection (1.0 mg/kg) and locomotor activity was measured. Following immunohistochemical staining c-fos positive neurons were quantified in the NAcc, mPFC, and amygdala.

RESULTS

Greater locomotor activity was observed in differentially reared rats treated with amphetamine compared to saline, as well as in SC compared to EC and IC rats. Rats reared in an IC context demonstrated greater c-fos expression than EC rats in the NAcc when treated with amphetamine, and EC saline rats demonstrated greater c-fos expression in the cingulate and prelimbic cortices compared to SC saline rats. Additionally, IC amphetamine rats displayed greater c-fos expression in the NAcc compared to IC saline rats, while EC saline rats displayed greater c-fos expression in the prelimbic cortex compared to EC amphetamine rats.

CONCLUSIONS

These results suggest regional specificity of psychostimulant-induced c-fos expression in the prelimbic/NAcc pathway that is altered in differential rearing, and influences initial c-fos activation following psychostimulant exposure.

Using c-fos to study neuronal ensembles in corticostriatal circuitry of addiction

Learned associations between drugs and environment play an important role in addiction and are thought to be encoded within specific patterns of sparsely distributed neurons called neuronal ensembles. This hypothesis is supported by correlational data from in vivo electrophysiology and cellular imaging studies in relapse models in rodents. In particular, cellular imaging with the immediate early gene c-fos and its protein product Fos has been used to identify sparsely distributed neurons that were strongly activated during conditioned drug behaviors such as drug self-administration and context- and cue-induced reinstatement of drug seeking. Here we review how Fos and the c-fos promoter have been employed to demonstrate causal roles for Fos-expressing neuronal ensembles in prefrontal cortex and nucleus accumbens in conditioned drug behaviors. This work has allowed identification of unique molecular and electrophysiological alterations within Fos-expressing neuronal ensembles that may contribute to the development and expression of learned associations in addiction.

Selective expression of a dominant-negative type Iα PKA regulatory subunit in striatal medium spiny neurons impairs gene expression and leads to reduced feeding and locomotor activity

Striatal medium spiny neurons (MSNs) mediate many of the physiological effects of dopamine, including the regulation of feeding and motor behaviors. Dopaminergic inputs from the midbrain modulate MSN excitability through pathways that involve cAMP and protein kinase A (PKA), but the physiological role of specific PKA isoforms in MSN neurons remains poorly understood. One of the major PKA regulatory (R) subunit isoforms expressed in MSNs is RIIβ, which localizes the PKA holoenzyme primarily to dendrites by interaction with AKAP5 and other scaffolding proteins. However, RI (RIα and RIβ) subunits are also expressed in MSNs and the RI holoenzyme has a weaker affinity for most scaffolding proteins and tends to localize in the cell body. We generated mice with selective expression of a dominant-negative RI subunit (RIαB) in striatal MSNs and show that this dominant-negative RIαB localizes to the cytoplasm and specifically inhibits type I PKA activity in the striatum. These mice are normal at birth; however, soon after weaning they exhibit growth retardation and the adult mice are hypophagic, lean, and resistant to high-fat diet-induced hyperphagia and obesity. The RIαB-expressing mice also exhibit decreased locomotor activity and decreased dopamine-regulated CREB phosphorylation and c-fos gene expression in the striatum. Our results demonstrate a critical role for cytoplasmic RI-PKA holoenzyme in gene regulation and the overall physiological function of MSNs.

Substance-specific environmental influences on drug use and drug preference in animals and humans

Epidemiological, clinical, and preclinical evidence indicate that the setting of drug use can exert a powerful modulatory influence on drug reward and that this influence is substance-specific. When heroin and cocaine co-abusers, for example, report on the circumstances of drug use, they indicate distinct settings for the two drugs: heroin being used preferentially at home and cocaine being used preferentially outside the home. Similar results were obtained in laboratory rats. These findings will be interpreted in the light of a novel model of drug reward, based on the emotional appraisal of central and peripheral drug effects as a function of environmental context. I argue here that drug addiction research has not paid sufficient attention to the substance-specific aspects of drug abuse and this may have contributed to the present dearth of effective treatments. Pharmacological and cognitive-behavioral therapy, for example, should be tailored so as to allow the addict to anticipate, and cope with, the risks associated, in a substance-specific manner, to the different settings of drug use.

Addiction-related gene regulation: risks of exposure to cognitive enhancers vs. other psychostimulants

The psychostimulants methylphenidate (Ritalin, Concerta), amphetamine (Adderall), and modafinil (Provigil) are widely used in the treatment of medical conditions such as attention-deficit hyperactivity disorder and narcolepsy and, increasingly, as "cognitive enhancers" by healthy people. The long-term neuronal effects of these drugs, however, are poorly understood. A substantial amount of research over the past two decades has investigated the effects of psychostimulants such as cocaine and amphetamines on gene regulation in the brain because these molecular changes are considered critical for psychostimulant addiction. This work has determined in some detail the neurochemical and cellular mechanisms that mediate psychostimulant-induced gene regulation and has also identified the neuronal systems altered by these drugs. Among the most affected brain systems are corticostriatal circuits, which are part of cortico-basal ganglia-cortical loops that mediate motivated behavior. The neurotransmitters critical for such gene regulation are dopamine in interaction with glutamate, while other neurotransmitters (e.g., serotonin) play modulatory roles. This review presents (1) an overview of the main findings on cocaine- and amphetamine-induced gene regulation in corticostriatal circuits in an effort to provide a cellular framework for (2) an assessment of the molecular changes produced by methylphenidate, medical amphetamine (Adderall), and modafinil. The findings lead to the conclusion that protracted exposure to these cognitive enhancers can induce gene regulation effects in corticostriatal circuits that are qualitatively similar to those of cocaine and other amphetamines. These neuronal changes may contribute to the addiction liability of the psychostimulant cognitive enhancers.

Mechanisms of psychostimulant-induced structural plasticity

Psychostimulants robustly induce alterations in neuronal structural plasticity throughout brain reward circuits. However, despite our extensive understanding of how these circuits modulate motivated behavior, it is still unclear whether structural plasticity within these regions drives pathological behavioral responses in addiction. Although these structural changes have been subjected to an exhaustive phenomenological characterization, we still have a limited understanding of the molecular mechanisms regulating their induction and the functional relevance of such changes in mediating addiction-like behavior. Here we have highlighted the known molecular pathways and intracellular signaling cascades that regulate psychostimulant-induced changes in neuronal morphology and synaptic restructuring, and we discuss them in the larger context of addiction behavior.

Common liability to addiction and "gateway hypothesis": theoretical, empirical and evolutionary perspective

BACKGROUND

Two competing concepts address the development of involvement with psychoactive substances: the "gateway hypothesis" (GH) and common liability to addiction (CLA).

METHOD

The literature on theoretical foundations and empirical findings related to both concepts is reviewed.

RESULTS

The data suggest that drug use initiation sequencing, the core GH element, is variable and opportunistic rather than uniform and developmentally deterministic. The association between risks for use of different substances, if any, can be more readily explained by common underpinnings than by specific staging. In contrast, the CLA concept is grounded in genetic theory and supported by data identifying common sources of variation in the risk for specific addictions. This commonality has identifiable neurobiological substrate and plausible evolutionary explanations.

CONCLUSIONS

Whereas the "gateway" hypothesis does not specify mechanistic connections between "stages", and does not extend to the risks for addictions, the concept of common liability to addictions incorporates sequencing of drug use initiation as well as extends to related addictions and their severity, provides a parsimonious explanation of substance use and addiction co-occurrence, and establishes a theoretical and empirical foundation to research in etiology, quantitative risk and severity measurement, as well as targeted non-drug-specific prevention and early intervention.

Repeated methamphetamine administration differentially alters fos expression in caudate-putamen patch and matrix compartments and nucleus accumbens

Background

The repeated administration of psychostimulant drugs produces a persistent and long-lasting increase (“sensitization”) in their psychomotor effects, which is thought to be due to changes in the neural circuitry that mediate these behaviors. One index of neuronal activation used to identify brain regions altered by repeated exposure to drugs involves their ability to induce immediate early genes, such as c-fos. Numerous reports have demonstrated that past drug experience alters the ability of drugs to induce c-fos in the striatum, but very few have examined Fos protein expression in the two major compartments in the striatum—the so-called patch/striosome and matrix.

Methodology/Principal Findings

In the present study, we used immunohistochemistry to investigate the effects of pretreatment with methamphetamine on the ability of a subsequent methamphetamine challenge to induce Fos protein expression in the patch and matrix compartments of the dorsolateral and dorsomedial caudate-putamen and in the ventral striatum (nucleus accumbens). Animals pretreated with methamphetamine developed robust psychomotor sensitization. A methamphetamine challenge increased the number of Fos-positive cells in all areas of the dorsal and ventral striatum. However, methamphetamine challenge induced Fos expression in more cells in the patch than in the matrix compartment in the dorsolateral and dorsomedial caudate-putamen. Furthermore, past experience with methamphetamine increased the number of methamphetamine-induced Fos positive cells in the patch compartment of the dorsal caudate putamen, but not in the matrix or in the core or shell of the nucleus accumbens.

Conclusions/Significance

These data suggest that drug-induced alterations in the patch compartment of the dorsal caudate-putamen may preferentially contribute to some of the enduring changes in brain activity and behavior produced by repeated treatment with methamphetamine.

Opiate versus psychostimulant addiction: the differences do matter

The publication of the psychomotor stimulant theory of addiction in 1987 and the finding that addictive drugs increase dopamine concentrations in the rat mesolimbic system in 1988 have led to a predominance of psychobiological theories that consider addiction to opiates and addiction to psychostimulants as essentially identical phenomena. Indeed, current theories of addiction - hedonic allostasis, incentive sensitization, aberrant learning and frontostriatal dysfunction - all argue for a unitary account of drug addiction. This view is challenged by behavioural, cognitive and neurobiological findings in laboratory animals and humans. Here, we argue that opiate addiction and psychostimulant addiction are behaviourally and neurobiologically distinct and that the differences have important implications for addiction treatment, addiction theories and future research.

Basal Ganglia disorders associated with imbalances in the striatal striosome and matrix compartments

The striatum is composed principally of GABAergic, medium spiny striatal projection neurons (MSNs) that can be categorized based on their gene expression, electrophysiological profiles, and input–output circuits. Major subdivisions of MSN populations include (1) those in ventromedial and dorsolateral striatal regions, (2) those giving rise to the direct and indirect pathways, and (3) those that lie in the striosome and matrix compartments. The first two classificatory schemes have enabled advances in understanding of how basal ganglia circuits contribute to disease. However, despite the large number of molecules that are differentially expressed in the striosomes or the extra-striosomal matrix, and the evidence that these compartments have different input–output connections, our understanding of how this compartmentalization contributes to striatal function is still not clear. A broad view is that the matrix contains the direct and indirect pathway MSNs that form parts of sensorimotor and associative circuits, whereas striosomes contain MSNs that receive input from parts of limbic cortex and project directly or indirectly to the dopamine-containing neurons of the substantia nigra, pars compacta. Striosomes are widely distributed within the striatum and are thought to exert global, as well as local, influences on striatal processing by exchanging information with the surrounding matrix, including through interneurons that send processes into both compartments. It has been suggested that striosomes exert and maintain limbic control over behaviors driven by surrounding sensorimotor and associative parts of the striatal matrix. Consistent with this possibility, imbalances between striosome and matrix functions have been reported in relation to neurological disorders, including Huntington’s disease, L-DOPA-induced dyskinesias, dystonia, and drug addiction. Here, we consider how signaling imbalances between the striosomes and matrix might relate to symptomatology in these disorders.

The striatal balancing act in drug addiction: distinct roles of direct and indirect pathway medium spiny neurons

The striatum plays a key role in mediating the acute and chronic effects of addictive drugs, with drugs of abuse causing long-lasting molecular and cellular alterations in both dorsal striatum and nucleus accumbens (ventral striatum). Despite the wealth of research on the biological actions of abused drugs in striatum, until recently, the distinct roles of the striatum’s two major subtypes of medium spiny neurons (MSNs) in drug addiction remained elusive. Recent advances in cell-type-specific technologies, including fluorescent reporter mice, transgenic, or knockout mice, and viral-mediated gene transfer, have advanced the field toward a more comprehensive understanding of the two MSN subtypes in the long-term actions of drugs of abuse. Here we review progress in defining the distinct molecular and functional contributions of the two MSN subtypes in mediating addiction.

FACS identifies unique cocaine-induced gene regulation in selectively activated adult striatal neurons

Numerous studies with the neural activity marker Fos indicate that cocaine activates only a small proportion of sparsely distributed striatal neurons. Until now, efficient methods were not available to assess neuroadaptations induced specifically within these activated neurons. We used fluorescence-activated cell sorting (FACS) to purify striatal neurons activated during cocaine-induced locomotion in naive and cocaine-sensitized cfos-lacZ transgenic rats. Activated neurons were labeled with an antibody against β-galactosidase, the protein product of the lacZ gene. Cocaine induced a unique gene expression profile selectively in the small proportion of activated neurons that was not observed in the nonactivated majority of neurons. These genes included altered levels of the immediate early genes arc, fosB, and nr4a3, as well as genes involved in p38 MAPK signaling and cell-type specificity. We propose that this FACS method can be used to study molecular neuroadaptations in specific neurons encoding the behavioral effects of abused drugs and other learned behaviors.

Continuous, but not intermittent, antipsychotic drug delivery intensifies the pursuit of reward cues

Chronic exposure to antipsychotic medications can persistently change brain dopamine systems. Most studies on the functional significance of these neural changes have focused on motor behavior and few have addressed how long-term antipsychotic treatment might influence dopamine-mediated reward function. We asked, therefore, whether a clinically relevant antipsychotic treatment regimen would alter the incentive motivational properties of a reward cue. We assessed the ability of a Pavlovian-conditioned stimulus to function as a conditioned reward, as well as to elicit approach behavior in rats treated with haloperidol, either continuously (achieved via subcutaneous osmotic minipump) or intermittently (achieved via daily subcutaneous injections). Continuous, but not intermittent, treatment enhanced the ability of amphetamine to potentiate the conditioned reinforcing effects of a cue associated with water. This effect was not related to differences in the ability to attribute predictive value to a conditioned stimulus (as measured by conditioned approach behavior), but was potentially linked to the development of behavioral supersensitivity to amphetamine and to augmented amphetamine-induced immediate early-gene expression (c-fos and Nur77) in dorsal striatopallidal and striatonigral cells. By enhancing the ability of reward cues to control behavior and by intensifying dopamine-mediated striatopallidal and striatonigral cell activity, standard (ie, continuous) antipsychotic treatment regimens might exacerbate drug-seeking and drug-taking behavior in schizophrenia. Achieving regular but transiently high antipsychotic levels in the brain (as modeled in the intermittent condition) might be a viable option to prevent these changes. This possibility should be explored in the clinic.

Role of dopamine D1 receptors in the activation of nucleus accumbens extracellular signal-regulated kinase (ERK) by cocaine-paired contextual cues

Exposure to drug-paired cues can trigger addicts to relapse into drug seeking. Although the molecular mechanisms underlying cue-elicited cocaine seeking are incompletely understood, the protein kinase extracellular signal-regulated kinase (ERK) is known to have an important role. Psychostimulants and their associated cues can activate ERK in medium spiny neurons of the nucleus accumbens core (AcbC). These medium spiny neurons can be classified according to their projections (to ventral pallidum and/or substantia nigra) and by their mRNA expression. The present experiments were designed to determine which distinct set of AcbC projection neurons expresses phosphorylated ERK (pERK) in response to cocaine-paired contextual cues. Combined use of the retrograde label Flurogold with immunohistochemical staining of pERK was used to show that the AcbC pERK accompanying preference for cocaine-paired contexts occurs in both the accumbens (Acb)-nigral and Acb-pallidal projections. The gene expression characteristics of the neurons expressing pERK in response to cocaine-paired cues was further investigated using combined in situ hybridization and immunocytochemistry to show that AcbC pERK+ cells correspond to D1, but not preproenkephalin, mRNA+ cells. Furthermore, intra-AcbC infusion of the D1-antagonist SCH23390 attenuated cue-induced AcbC pERK expression. In aggregate, these results indicate that (i) the D1-expressing AcbC neurons evidence long-term plasticity related to drug-cue memories and (ii) local dopamine D1 receptors are necessary for the expression of cocaine-paired cue-induced pERK in these AcbC neurons.

Transient neuronal inhibition reveals opposing roles of indirect and direct pathways in sensitization

The dorsal striatum plays an important role in the development of drug addiction; however, a precise understanding of the roles of striatopallidal (indirect) and striatonigral (direct) pathway neurons in regulating behaviors remains elusive. Using a novel approach that relies on the viral-mediated expression of an engineered GPCR (hM₄D), we demonstrated that activation of hM₄D receptors with clozapine-N-oxide (CNO) potently reduced striatal neuron excitability. When hM₄D receptors were selectively expressed in either direct or indirect pathway neurons in rats, CNO did not change acute locomotor responses to amphetamine but altered behavioral plasticity associated with repeated drug treatment. Specifically, transiently disrupting striatopallidal neuronal activity facilitated behavioral sensitization whereas decreasing excitability of striatonigral neurons impaired its persistence. These findings suggest that acute drug effects can be parsed from the behavioral adaptations associated with repeated drug exposure and highlight the utility of this approach for deconstructing neuronal pathway contributions to behaviors such as sensitization.

Exposure to cocaine dynamically regulates the intrinsic membrane excitability of nucleus accumbens neurons

Drug-induced malfunction of nucleus accumbens (NAc) neurons underlies a key pathophysiology of drug addiction. Drug-induced changes in intrinsic membrane excitability of NAc neurons are thought to be critical for producing behavioral alterations. Previous studies demonstrate that, after short-term (2 d) or long-term (21 d) withdrawal from noncontingent cocaine injection, the intrinsic membrane excitability of NAc shell (NAcSh) neurons is decreased, and decreased membrane excitability of NAcSh neurons increases the acute locomotor response to cocaine. However, animals exhibit distinct cellular and behavioral alterations at different stages of cocaine exposure, suggesting that the decreased membrane excitability of NAc neurons may not be a persistent change. Here, we demonstrate that the membrane excitability of NAcSh neurons is differentially regulated depending on whether cocaine is administered contingently or noncontingently. Specifically, the membrane excitability of NAcSh medium spiny neurons (MSNs) was decreased at 2 d after withdrawal from either 5 d intraperitoneal injections (15 mg/kg) or cocaine self-administration (SA). At 21 d of withdrawal, the membrane excitability of NAcSh MSNs, which remained low in intraperitoneally pretreated rats, returned to a normal level in SA-pretreated rats. Furthermore, after a reexposure to cocaine after long-term withdrawal, the membrane excitability of NAcSh MSNs instantly returned to a normal level in intraperitoneally pretreated rats. Conversely, in SA-pretreated rats, the reexposure elevated the membrane excitability of NAcSh MSMs beyond the normal level. These results suggest that the dynamic alterations in membrane excitability of NAcSh MSNs, together with the dynamic changes in synaptic input, contribute differentially to the behavioral consequences of contingent and noncontingent cocaine administration.

Patterns of neural activity associated with differential acute locomotor stimulation to cocaine and methamphetamine in adolescent versus adult male C57BL/6J mice

Adolescence is a time period when major changes occur in the brain with long-term consequences for behavior. One ramification is altered responses to drugs of abuse, but the specific brain mechanisms and implications for mental health are poorly understood. Here, we used a mouse model in which adolescents display dramatically reduced sensitivity to the acute locomotor stimulating effects of cocaine and methamphetamine. The goal was to identify key brain regions or circuits involved in the differential behavior. Male adolescent (postnatal day (PN), 30-35) and young adult (PN, 69-74) C57BL/6J mice were administered an i.p. injection of cocaine (0, 15, 30 mg/kg) or methamphetamine (0, 2, 4 mg/kg) and euthanized 90 min later. Locomotor activity was monitored continuously in the home cage by video tracking. Immunohistochemical detection of Fos protein was used to quantify neuronal activation in 16 different brain regions. As expected, adolescents were less sensitive to the locomotor stimulating effects of cocaine and methamphetamine as indicated by a rightward shift in the dose response relationship. After a saline injection, adolescents showed similar levels of Fos as adults in all regions except the dorsal caudate (CPuD) and lateral caudate (CPuL) where levels were lower in adolescents. Cocaine and methamphetamine dose dependently increased Fos in all brain regions sampled in both adolescents and adults, but Fos levels were similar in both age groups for a majority of regions and doses. Locomotor activity was correlated with Fos in several brain areas within adolescent and adult groups, and adolescents had a significantly greater induction of Fos for a given amount of locomotor activity in key brain regions including the caudate where they showed reduced Fos under baseline conditions. Future research will identify the molecular and cellular events that are responsible for the differential psychostimulant-induced patterns of brain activation and behavior observed in adolescent versus adult mice.

Positron emission tomography measures of endogenous opioid neurotransmission and impulsiveness traits in humans

CONTEXT

The endogenous opioid system and opioid mu receptors (mu-receptors) are known to interface environmental events, positive (eg, relevant emotional stimuli) and negative (eg, stressors), with pertinent behavioral responses and to regulate motivated behavior.

OBJECTIVE

To examine the degree to which trait impulsiveness (the tendency to act on cravings and urges rather than to delay gratification) is predicted by baseline mu-receptor availability or the response of this system to a standardized, experientially matched stressor.

DESIGN, SETTING, AND PATIENTS

Nineteen young healthy male volunteers completed a personality questionnaire (NEO Personality Inventory, Revised) and underwent positron emission tomography scans with the mu-receptor-selective radiotracer carfentanil labeled with carbon 11. Measures of receptor concentrations were obtained at rest and during receipt of an experimentally maintained pain stressor of matched intensity between subjects.

MAIN OUTCOME MEASURES

Baseline receptor levels and stress-induced activation of mu-opioid system neurotransmission compared between subjects scoring above and below the population median on the NEO Personality Inventory, Revised, impulsiveness subscale and the orthogonal dimension (deliberation) expected to interact with it.

RESULTS

High impulsiveness and low deliberation scores were associated with significantly higher regional mu-receptor concentrations and greater stress-induced endogenous opioid system activation. Effects were obtained in the prefrontal and orbitofrontal cortices, anterior cingulate, thalamus, nucleus accumbens, and basolateral amygdala-all regions involved in motivated behavior and the effects of drugs of abuse. Availability of the mu-receptor and the magnitude of stress-induced endogenous opioid activation in these regions accounted for 17% to 49% of the variance in these personality traits.

CONCLUSIONS

Individual differences in the function of the endogenous mu-receptor system predict personality traits that confer vulnerability to or resiliency against risky behaviors such as the predisposition to develop substance use disorders. These personality traits are also implicated in psychopathological states (eg, personality disorders) in which variations in the function of this neurotransmitter system also may play a role.

Long-lasting dysregulation of gene expression in corticostriatal circuits after repeated cocaine treatment in adult rats: effects on zif 268 and homer 1a

Human imaging studies show that psychostimulants such as cocaine produce functional changes in several areas of cortex and striatum. These may reflect neuronal changes related to addiction. We employed gene markers (zif 268 and homer 1a) that offer a high anatomical resolution to map cocaine-induced changes in 22 cortical areas and 23 functionally related striatal sectors, in order to determine the corticostriatal circuits altered by repeated cocaine exposure (25 mg/kg, 5 days). Effects were investigated 1 day and 21 days after repeated treatment to assess their longevity. Repeated cocaine treatment increased basal expression of zif 268 predominantly in sensorimotor areas of the cortex. This effect endured for 3 weeks in some areas. These changes were accompanied by attenuated gene induction by a cocaine challenge. In the insular cortex, the cocaine challenge produced a decrease in zif 268 expression after the 21-day, but not 1-day, withdrawal period. In the striatum, cocaine also affected mostly sensorimotor sectors. Repeated cocaine resulted in blunted inducibility of both zif 268 and homer 1a, changes that were still very robust 3 weeks later. Thus, our findings demonstrate that cocaine produces robust and long-lasting changes in gene regulation predominantly in sensorimotor corticostriatal circuits. These neuronal changes were associated with behavioral stereotypies, which are thought to reflect dysfunction in sensorimotor corticostriatal circuits. Future studies will have to elucidate the role of such neuronal changes in psychostimulant addiction.

Drug context differently regulates cocaine versus heroin self-administration and cocaine- versus heroin-induced Fos mRNA expression in the rat

RATIONALE

We have previously reported that cocaine self-administration is facilitated in male rats not residing in the test chambers (Non Resident rats) relative to rats living in the test chambers at all times (Resident rats). Surprisingly, the opposite was found for heroin.

MATERIALS AND METHODS

We predicted that, when given access to both cocaine and heroin on alternate days, Non Resident rats would take more cocaine relative to heroin than Resident rats. Heroin (25.0 microg/kg) and cocaine (400 microg/kg), were made alternately available for 14 self-administration sessions, on a fixed ratio (FR) schedule that was progressively increased from FR1 to FR5. Next, some rats underwent a progressive-ratio procedure for heroin and cocaine. The other rats continued to alternate heroin and cocaine self-administration for 12 additional sessions, during which the FR schedule was progressively increased from FR10 to FR100. The second aim of the study was to investigate Fos mRNA expression in Resident and Non Resident rats treated with non-contingent intravenous infusion of "self-administration doses" of heroin (25.0 microg/kg) and cocaine (400 microg/kg).

RESULTS

We found that: (1) drug-taking context differentially modulates intravenous cocaine versus heroin self-administration; (2) very low doses of cocaine and heroin are sufficient to induce Fos mRNA expression in the posterior caudate; (3) drug-administration context differentially modulates cocaine- versus heroin-induced Fos mRNA expression.

CONCLUSIONS

Our study indicates that the context of drug taking can play a powerful role in modulating cocaine versus heroin intake in the laboratory rat.

Striatal dopamine and glutamate receptors modulate methamphetamine-induced cortical Fos expression

Methamphetamine (mAMPH) is a psychostimulant drug that increases extracellular levels of monoamines throughout the brain. It has previously been observed that a single injection of mAMPH increases immediate early gene (IEG) expression in both the striatum and cerebral cortex. Moreover, this effect is modulated by dopamine and glutamate receptors since systemic administration of dopamine or glutamate antagonists has been found to alter mAMPH-induced striatal and cortical IEG expression. However, because dopamine and glutamate receptors are found in extra-striatal as well as striatal brain regions, studies employing systemic injection of dopamine or glutamate antagonists fail to localize the effects of mAMPH-induced activation. In the present experiments, the roles of striatal dopamine and glutamate receptors in mAMPH-induced gene expression in the striatum and cerebral cortex were examined. The nuclear expression of Fos, the protein product of the IEG c-fos, was quantified in both the striatum and the cortex of animals receiving intrastriatal dopamine or glutamate antagonist administration. Intrastriatal infusion of dopamine (D1 or D2) or glutamate [N-methyl-D-aspartic acid (NMDA) or alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)] antagonists affected not only mAMPH-induced striatal, but also cortical, Fos expression. Overall, the effects of the antagonists occurred dose-dependently, in both the infused and non-infused hemispheres, with greater influences occurring in the infused hemisphere. Finally, unilateral intrastriatal infusion of dopamine or glutamate antagonists changed the behavior of the rats from characteristic mAMPH-induced stereotypy to rotation ipsilateral to the infusion. These results demonstrate that mAMPH's actions on striatal dopamine and glutamate receptors modulate the widespread cortical activation induced by mAMPH. It is hypothesized that dopamine release from nigrostriatal terminals modulates activity within striatal efferent pathways, thereby disinhibiting thalamo-cortical circuits. By extension, these results suggest processes through which repeated exposure to mAMPH might influence cortical function in mAMPH abusers.

Neurobiological mechanisms of placebo responses

Expectations, positive or negative, are modulating factors influencing behavior. They are also thought to underlie placebo effects, potentially impacting perceptions and biological processes. We used sustained pain as a model to determine the neural mechanisms underlying placebo-induced analgesia and affective changes in healthy humans. Subjects were informed that they could receive either an active agent or an inactive compound, similar to routine clinical trials. Using PET and the mu-opioid selective radiotracer [(11)C]carfentanil we demonstrate placebo-induced activation of opioid neurotransmission in a number of brain regions. These include the rostral anterior cingulate, orbitofrontal and dorsolateral prefrontal cortex, anterior and posterior insula, nucleus accumbens, amygdala, thalamus, hypothalamus, and periaqueductal grey. Some of these regions overlap with those involved in pain and affective regulation but also motivated behavior. The activation of endogenous opioid neurotransmission was further associated with reductions in pain report and negative affective state. Additional studies with the radiotracer [(11)C]raclopride, studies labeling dopamine D2/3 receptors, also demonstrate the activation of nucleus accumbens dopamine during placebo administration under expectation of analgesia. Both dopamine and opioid neurotransmission were related to expectations of analgesia and deviations from those initial expectations. When the activity of the nucleus accumbens was probed with fMRI using a monetary reward expectation paradigm, its activation was correlated with both dopamine, opioid responses to placebo in this region and the formation of placebo analgesia. These data confirm that specific neural circuits and neurotransmitter systems respond to the expectation of benefit during placebo administration, inducing measurable physiological changes.

Expression of c-fos mRNA in the basal ganglia associated with contingent tolerance to amphetamine-induced hypophagia

Tolerance to the hypophagic effect of psychostimulants is contingent on having access to food while intoxicated. Rats given chronic injections of such drugs with access to food learn to suppress stereotyped movements, which interfere with feeding. In contrast, controls given the drug after food access do not learn to suppress stereotypy and, therefore, do not become tolerant. To determine the role of the basal ganglia in this phenomenon, we used in situ hybridization to measure the expression of c-fos mRNA, a marker for neural activation, in the brains of tolerant and nontolerant rats. Rats given chronic amphetamine injections prior to food access learned to suppress stereotyped movements, whereas yoked controls given the drug after feeding did not. Following an acute injection of amphetamine, both of these groups had higher levels of c-fos mRNA than saline-treated controls throughout the striatum, in the nucleus accumbens core, the ventral pallidum and layers V-VI of the motor cortex. In contrast, tolerant rats, which had learned to suppress stereotypy, had higher levels of c-fos mRNA than both amphetamine- and saline-treated controls in the entopeduncular nucleus, globus pallidus, subthalamic nucleus, pedunculopontine nucleus, nucleus accumbens shell, olfactory tubercle, somatosensory cortex, and layers II-IV of motor cortex. These data suggest that the learned suppression of amphetamine-induced stereotypy involves the activation of dorsal striatal pathways previously implicated in response selection as well as the ventral striatum, long implicated in appetitive motivation and reinforcement.

Opposing patterns of signaling activation in dopamine D1 and D2 receptor-expressing striatal neurons in response to cocaine and haloperidol

Psychostimulants and other drugs of abuse activate extracellular signal-regulated kinase (ERK) in the striatum, through combined stimulation of dopamine D(1) receptors (D1Rs) and glutamate NMDA receptors. Antipsychotic drugs activate similar signaling proteins in the striatum by blocking dopamine D(2) receptors (D2Rs). However, the neurons in which these pathways are activated by psychotropic drugs are not precisely identified. We used transgenic mice, in which enhanced green fluorescent protein (EGFP) expression was driven by D1R promoter (drd1a-EGFP) or D2R promoter (drd2-EGFP). We confirmed the expression of drd1a-EGFP in striatonigral and drd2-EGFP in striatopallidal neurons. Drd2-EGFP was also expressed in cholinergic interneurons, whereas no expression of either promoter was detected in GABAergic interneurons. Acute cocaine treatment increased phosphorylation of ERK and its direct or indirect nuclear targets, mitogen- and stress-activated kinase-1 (MSK1) and histone H3, exclusively in D1R-expressing output neurons in the dorsal striatum and nucleus accumbens. Cocaine-induced expression of c-Fos and Zif268 predominated in D1R-expressing neurons but was also observed in D2R-expressing neurons. One week after repeated cocaine administration, cocaine-induced signaling responses were decreased, with the exception of enhanced ERK phosphorylation in dorsal striatum. The responses remained confined to D1R neurons. In contrast, acute haloperidol injection activated phosphorylation of ERK, MSK1, and H3 only in D2R neurons and induced c-fos and zif268 predominantly in these neurons. Our results demonstrate that cocaine and haloperidol specifically activate signaling pathways in two completely segregated populations of striatal output neurons, providing direct evidence for the selective mechanisms by which these drugs exert their long-term effects.

Modulation by the dorsal, but not the ventral, hippocampus of the expression of behavioural sensitization to amphetamine

Although the dorsal hippocampus (DH) and the ventral hippocampus (VH) densely innervate the nucleus accumbens, which mediates the expression of behavioural sensitization, the respective and specific contribution of DH and VH in the expression of behavioural sensitization to amphetamine has not been investigated. In the present study, we investigated how lidocaine infused in DH or VH modulated behavioural locomotor sensitization induced by repeated administration of systemic amphetamine. Rats, well habituated to their environmental conditions and experimental protocol, were given repeated administration of systemic amphetamine. Once behavioural sensitization was developed, rats were challenged with amphetamine and infused with saline (controls) or lidocaine into DH or VH. We found that reversible inhibition by lidocaine of DH, but not VH, blocks the expression of behavioural sensitization to amphetamine. Control animals injected with saline solution do express behavioural sensitization. Our results bring new insights on the role of the hippocampus complex in the expression of behavioural sensitization, indicating that, in individuals well habituated to the drug-associated context, DH but not VH would play a key role. The results provide experimental evidence for clinical studies in human addicts that have demonstrated that exposure to environmental stimuli associated with drug-taking behaviour elicits craving and can promote relapse, and further suggest that in drug abusers, once addiction has occurred, the contextual and spatial conditions that are associated with drug consumption may play a critical role in the maintenance of drug abuse.

Brain regional Fos expression elicited by the activation of mu- but not delta-opioid receptors of the ventral tegmental area: evidence for an implication of the ventral thalamus in opiate reward

Both mu-opioid receptors (MORs) and delta-opioid receptors (DORs) are expressed in the ventral tegmental area (VTA) and are thought to be involved in the addictive properties of opiates. However, their respective contributions to opiate reward remain unclear. We used intracranial self-administration (ICSA) to study the rewarding effects of morphine microinjections into the VTA of male and female MOR-/- and DOR-/- mice. In brains of mice tested for intra-VTA morphine self-administration, we analyzed regional Fos protein expression to investigate the neural circuitry underlying this behavior. Male and female WT and DOR-/- mice exhibited similar self-administration performances, whereas knockout of the MOR gene abolished intra-VTA morphine self-administration at all doses tested. Naloxone (4 mg/kg) disrupted this behavior in WT and DOR mutants, without triggering physical signs of withdrawal. Morphine ICSA was associated with an increase in Fos within the nucleus accumbens, striatum, limbic cortices, amygdala, hippocampus, the lateral mammillary nucleus (LM), and the ventral posteromedial thalamus (VPM). This latter structure was found to express high levels of Fos exclusively in self-administering WT and DOR-/- mice. Abolition of morphine reward in MOR-/- mice was associated with a decrease in Fos-positive neurons in the mesocorticolimbic dopamine system, amygdala, hippocampus (CA1), LM, and a complete absence within the VPM. We conclude that (i) VTA MORs, but not DORs, are critical for morphine reward and (ii) the role of VTA-thalamic projections in opiate reward deserves to be further explored.

Opposite environmental regulation of heroin and amphetamine self-administration in the rat

RATIONALE

The circumstances of drug taking are thought to play a role in drug abuse but the evidence of it is anecdotal. Previous studies have shown that the intravenous self-administration of cocaine is facilitated in rats non-residing in the test chambers relative to rats that live in the test chambers at all times. We investigated here whether environmental context could exert its modulatory influence on heroin and amphetamine self-administration as well.

MATERIALS AND METHODS

Independent groups of rats were given the possibility to self-administer different doses of heroin or amphetamine (12.5, 25.0, or 50.0 microg/kg). Some animals were housed in the self-administration chambers (resident groups) whereas other rats were transported to the self-administration chambers only for the test sessions (non-resident groups).

RESULTS

Amphetamine-reinforcing effects were more pronounced in non-resident rats than in resident rats, as previously reported for cocaine. Quite unexpectedly, the opposite was found for heroin. Because of this surprising dissociation, some of the rats trained to self-administer amphetamine were later given the opportunity to self-administer heroin. Also in this case, resident rats took more heroin than non-resident rats.

CONCLUSIONS

These findings suggest an unforeseen dissociation between opioid and psychostimulant reward and demonstrate that even in the laboratory rat some contexts are associated with the propensity to self-administer more opioid than psychostimulant drugs and vice versa, thus indicating that drug taking is influenced not only by economical or cultural factors but also can be modulated at a much more basic level by the setting in which drugs are experienced.

Expression of amphetamine sensitization is associated with recruitment of a reactive neuronal population in the nucleus accumbens core

RATIONALE

Repeated exposure to psychostimulant drugs causes a long-lasting increase in the psychomotor and reinforcing effects of these drugs and an array of neuroadaptations. One such alteration is a hypersensitivity of striatal activity such that a low dose of amphetamine in sensitized animals produces dorsal striatal activation patterns similar to acute treatment with a high dose of amphetamine.

OBJECTIVES

To extend previous findings of striatal hypersensitivity with behavioral observations and with cellular activity in the nucleus accumbens and prefrontal cortex in sensitized animals.

MATERIALS AND METHODS

Rats treated acutely with 0, 1, 2.5, or 5 mg/kg i.p. amphetamine and sensitized rats challenged with 1 mg/kg i.p. amphetamine were scored for stereotypy, rearing, and grooming, and locomotor activity recorded. c-fos positive nuclei were quantified in the nucleus accumbens and prefrontal cortex after expression of sensitization with 1 mg/kg i.p. amphetamine.

RESULTS

Intense stereotypy was seen in animals treated acutely with 5 mg/kg amphetamine, but not in the sensitized group treated with 1 mg/kg amphetamine. The c-fos response to amphetamine in the accumbens core was augmented in amphetamine-pretreated animals with a shift in the distribution of optical density, while no effect of sensitization was seen in the nucleus accumbens shell or prefrontal cortex.

CONCLUSIONS

A lack of stereotypy in the sensitized group indicates a dissociation of behavioral responses to amphetamine and striatal immediate-early gene activation patterns. The increase in c-fos positive nuclei and shift in the distribution of optical density observed in the nucleus accumbens core suggests recruitment of a new population of neurons during expression of sensitization.

Modulation by group I mGLU receptor activation and group III mGLU receptor blockade of locomotor responses induced by D1-like and D2-like receptor agonists in the nucleus accumbens

Evidence for functional motor interactions between group I and group III metabotropic glutamatergic (mGlu) receptors and dopamine neurotransmission is now clearly established [David, H.N., Abraini, J.H., 2001a. The group I metabotropic glutamate receptor antagonist S-4-CPG modulates the locomotor response produced by the activation of D1-like, but not D2-like, dopamine receptors in the rat nucleus accumbens. Eur. J. Neurosci. 15, 2157-2164, David, H.N., Abraini, J.H., 2002. Group III metabotropic glutamate receptors and D1-like and D2-like dopamine receptors interact in the rat nucleus accumbens to influence locomotor activity. Eur. J. Neurosci. 15, 869-875]. Nevertheless, whether or not and how, activation of group I and blockade of group III mGlu receptors modulate the motor responses induced by the activation of dopaminergic receptors in the NAcc still remains unknown. Answering this question needs to be assessed since functional interactions between neurotransmitters in the NAcc are well known to depend upon the level of activation of glutamatergic and/or dopaminergic receptors and because the effects of glutamatergic receptor agonists and antagonists on dopaminergic receptor-mediated locomotor responses are not always reciprocal as shown in previous studies. Our results show that activation of group I mGlu receptors by DHPG in the NAcc potentiated the locomotor response induced by intra-NAcc activation of D1-like receptors and blocked those induced by D2-like presynaptic or postsynaptic receptors. Alternatively, blockade of group III mGlu receptors by MPPG in the NAcc potentiated the locomotor responses mediated by D1-like receptors and by D2-like postsynaptic receptors and inhibited that induced by D2-like presynaptic receptors. These results compiled with previous data demonstrate that group I mGlu receptors and group III mGlu receptors can modulate the locomotor responses produced by D1-like and/or D2-like receptor agonists in a complex phasic and tonic fashion.

Hippocampal modulation of locomotor activity induced by focal activation of postsynaptic dopamine receptors in the core of the nucleus accumbens

The locomotor effects of intra-NAcc injection of dopamine receptor agonists following discrete lesion or inhibition of the DH or the VH have been poorly investigated using only the indirect dopamine receptor agonist amphetamine. In the present study, we investigated how lidocaine in the DH or the VH modulated hyperlocomotion induced by focal injection into the NAcc core of the selective D1-like receptor agonist, SKF 38393, or coinjection of SKF 38393, and the selective D2-like receptor agonist, LY 171555; the latter pharmacological condition being required for the full expression of the postsynaptic effects of D2-like receptor agonists, and recognized to produce a locomotor response mainly mediated by D2-like postsynaptic receptors. Rats were given the D1-like receptor agonist SKF 38393 alone or in combination with the D2-like receptor agonist LY 171555 into the NAcc core, and lidocaine into the DH or the VH. Then, locomotor activity was recorded. Focal injection into the NAcc core of SKF 38393 alone or in combination with LY 171555 resulted in an increase of locomotor activity. Administration of lidocaine into the DH further potentiated the increase in locomotor activity induced by activation of D1-like receptors or co-activation of D1-like and D2-like receptors in the NAcc core. Administration of lidocaine into the VH also potentiated the increase in locomotor activity induced by D1-like receptor activation, but decreased that produced by co-activation of D1-like and D2-like receptors in the NAcc core. Taken together, these results suggest that under lidocaine-free conditions the DH may exert a tonic inhibitory modulation on hyperlocomotion mediated by D1-like and D2-like postsynaptic receptors in the NAcc core, while the VH may exert a tonic inhibitory on hyperlocomotion mediated by D1-like receptors and a tonic facilitatory control on hyperlocomotion mediated by D2-like postsynaptic receptors.

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.

Modulatory effect of environmental context and drug history on heroin-induced psychomotor activity and fos protein expression in the rat brain

The goal of the present study was to investigate the role of environmental context and drug history in modulating the effects of heroin on locomotor activity and Fos protein expression in the neocortex and striatal complex of the rat. It was found that (1) repeated i.p. administrations of a relatively low dose of heroin (1 mg/kg, i.p.) induced psychomotor sensitization only when the treatment was administered in a relatively 'novel' environment (ie, a unique test environment distinct from the home cage) but not when the same treatment was administered in the home cage; (2) environmental novelty facilitated heroin-induced Fos expression in the caudate, particularly in its most caudal regions; (3) environmental context also modulated heroin-induced Fos expression in the nucleus accumbens and in the neocortex; (4) repeated exposures to heroin dramatically altered its effects on Fos expression in the caudate and in the neocortex; and (5) Fos protein levels in the postero-dorsal caudate, in the shell of the nucleus accumbens, and in the barrel field cortex predicted most of the variance in heroin-induced activity scores, as shown by multiple regression analysis. The present report demonstrates that environment and drug history powerfully interact in shaping the neurobehavioral response to heroin, as previously shown for amphetamine and cocaine. Thus, a full understanding of the mechanisms responsible for the neurobehavioral adaptations produced by addictive drugs will also require taking into due consideration the environment in which drugs are experienced.

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.

Dysregulation of gene induction in corticostriatal circuits after repeated methylphenidate treatment in adolescent rats: differential effects on zif 268 and homer 1a

Psychostimulants and other dopamine agonists produce molecular changes in neurons of cortico-basal ganglia-cortical circuits, and such neuronal changes are implicated in behavioural disorders. Methylphenidate, a psychostimulant that causes dopamine overflow (among other effects), alters gene regulation in neurons of the striatum. The present study compared the effects of acute and repeated methylphenidate treatment on cortical and striatal gene regulation in adolescent rats. Changes in the expression of the immediate-early genes zif 268 and homer 1a were mapped in 23 striatal sectors and 22 cortical areas that provide input to these striatal sectors, in order to determine whether specific corticostriatal circuits were affected by these treatments. Acute administration of methylphenidate (5 mg/kg, i.p.) produced modest zif 268 induction in cortical areas. These cortical zif 268 responses were correlated in magnitude with zif 268 induction in functionally related striatal sectors. In contrast, after repeated methylphenidate treatment (10 mg/kg, 7 days), cortical and striatal gene induction were dissociated. In these animals, the methylphenidate challenge (5 mg/kg) produced significantly greater gene induction (zif 268 and homer 1a) in the cortex. This enhanced response was widespread but regionally selective, as it occurred predominantly in premotor, motor and somatosensory cortical areas. At the same time, striatal gene induction was partly suppressed (zif 268) or unchanged (homer 1a). Thus, repeated methylphenidate treatment disrupted the normally coordinated gene activation patterns in cortical and striatal nodes of corticostriatal circuits. This drug-induced dissociation in cortical and striatal functioning was associated with enhanced levels of behavioural stereotypies, suggesting disrupted motor switching function.