Phasic alterations in dopamine and serotonin release in striatum and prefrontal cortex in response to cocaine predictive cues in behaving rhesus macaques

Advancements in Brain Research: The In Vivo/In Vitro Electrochemical Detection of Neurochemicals

Neurochemicals, crucial for nervous system function, influence vital bodily processes and their fluctuations are linked to neurodegenerative diseases and mental health conditions. Monitoring these compounds is pivotal, yet the intricate nature of the central nervous system poses challenges. Researchers have devised methods, notably electrochemical sensing with micro-nanoscale electrodes, offering high-resolution monitoring despite low concentrations and rapid changes. Implantable sensors enable precise detection in brain tissues with minimal damage, while microdialysis-coupled platforms allow in vivo sampling and subsequent in vitro analysis, addressing the selectivity issues seen in other methods. While lacking temporal resolution, techniques like HPLC and CE complement electrochemical sensing's selectivity, particularly for structurally similar neurochemicals. This review covers essential neurochemicals and explores miniaturized electrochemical sensors for brain analysis, emphasizing microdialysis integration. It discusses the pros and cons of these techniques, forecasting electrochemical sensing's future in neuroscience research. Overall, this comprehensive review outlines the evolution, strengths, and potential applications of electrochemical sensing in the study of neurochemicals, offering insights into future advancements in the field.

Differential roles of medial prefrontal subregions in the regulation of drug seeking

The prefrontal cortex plays an important role in shaping cognition and behavior. Many studies have shown that medial prefrontal cortex (mPFC) plays a key role in seeking, extinction, and reinstatement of cocaine seeking in rodent models of relapse. Subregions of mPFC appear to play distinct roles in these behaviors, such that the prelimbic cortex (PL) is proposed to drive cocaine seeking and the infralimbic cortex (IL) is proposed to suppress cocaine seeking after extinction. This dichotomy of mPFC function may be a general attribute, as similar dorsal-ventral distinctions exist for expression vs. extinction of fear conditioning. However, other results indicate that the role of mPFC neurons in reward processing is more complex than a simple PL-seek vs. IL-extinguish dichotomy. Both PL and IL have been shown to drive and inhibit drug seeking (and other types of behaviors) depending on a range of factors including the behavioral context, the drug-history of the animal, and the type of drug investigated. This heterogeneity of findings may reflect multiple subcircuits within each of these PFC areas supporting unique functions. It may also reflect the fact that the mPFC plays a multifaceted role in shaping cognition and behavior, including those overlapping with cocaine seeking and extinction. Here we discuss research leading to the hypothesis that dorsal and ventral mPFC differentially control drug seeking and extinction. We also present recent results calling the absolute nature of a PL vs. IL dichotomy into question. Finally, we consider alternate functions for mPFC that correspond less to response execution and inhibition and instead incorporate the complex cognitive behavior for which the mPFC is broadly appreciated.

Imaging dopamine transmission in the frontal cortex: a simultaneous microdialysis and [11C]FLB 457 PET study

In a recent human positron emission tomography (PET) study we demonstrated the ability to detect amphetamine-induced dopamine (DA) release in the prefrontal cortex as a reduction in the binding of the DA D(2/3) radioligand [(11)C]FLB 457. A key requirement for validating this paradigm for use in clinical studies is demonstrating that the changes in [(11)C]FLB 457 binding observed with PET following amphetamine are related to changes in dialysate DA concentration as measured with microdialysis. Microdialysis and PET experiments were performed to compare, in five rhesus monkeys, amphetamine-induced DA release and [(11)C]FLB 457 displacement in the frontal cortex after three doses of amphetamine (0.3 mg kg(-1), 0.5 mg kg(-1) and 1.0 mg kg(-1)). Amphetamine led to a significant dose-dependent increase in dialysate (0.3 mg kg(-1): 999±287%; 0.5 mg kg(-1): 1320±432%; 1.0 mg kg(-1): 2355±1026%) as measured with microdialysis and decrease in [(11)C]FLB 457 binding potential (BP(ND), 0.3 mg kg(-1): -6±6%; 0.5 mg kg(-1): -16±4%; 1.0 mg kg(-1): -24±2%) as measured with PET. The relationship between amphetamine-induced peak ΔDA and Δ[(11)C]FLB 457 BP(ND) in the frontal cortex was linear. The results of this study clearly demonstrate that the magnitude of dialysate DA release is correlated with the magnitude of the reduction in [(11)C]FLB 457 BP(ND) in the frontal cortex. The use of the [(11)C]FLB 457-amphetamine imaging paradigm in humans should allow for characterization of prefrontal cortical DA release in neuropsychiatric disorders such as schizophrenia and addiction.

Serotonin at the nexus of impulsivity and cue reactivity in cocaine addiction

Cocaine abuse and addiction remain great challenges on the public health agendas in the U.S. and the world. Increasingly sophisticated perspectives on addiction to cocaine and other drugs of abuse have evolved with concerted research efforts over the last 30 years. Relapse remains a particularly powerful clinical problem as, even upon termination of drug use and initiation of abstinence, the recidivism rates can be very high. The cycling course of cocaine intake, abstinence and relapse is tied to a multitude of behavioral and cognitive processes including impulsivity (a predisposition toward rapid unplanned reactions to stimuli without regard to the negative consequences), and cocaine cue reactivity (responsivity to cocaine-associated stimuli) cited as two key phenotypes that contribute to relapse vulnerability even years into recovery. Preclinical studies suggest that serotonin (5-hydroxytryptamine; 5-HT) neurotransmission in key neural circuits may contribute to these interlocked phenotypes well as the altered neurobiological states evoked by cocaine that precipitate relapse events. As such, 5-HT is an important target in the quest to understand the neurobiology of relapse-predictive phenotypes, to successfully treat this complex disorder and improve diagnostic and prognostic capabilities. This review emphasizes the role of 5-HT and its receptor proteins in key addiction phenotypes and the implications of current findings to the future of therapeutics in addiction. This article is part of a Special Issue entitled 'NIDA 40th Anniversary Issue'.

Functional status of the serotonin 5-HT2C receptor (5-HT2CR) drives interlocked phenotypes that precipitate relapse-like behaviors in cocaine dependence

Relapse vulnerability in cocaine dependence is rooted in genetic and environmental determinants, and propelled by both impulsivity and the responsivity to cocaine-linked cues ('cue reactivity'). The serotonin (5-hydroxytryptamine, 5-HT) 5-HT2C receptor (5-HT2CR) within the medial prefrontal cortex (mPFC) is uniquely poised to serve as a strategic nexus to mechanistically control these behaviors. The 5-HT2CR functional capacity is regulated by a number of factors including availability of active membrane receptor pools, the composition of the 5-HT2CR macromolecular protein complex, and editing of the 5-HT2CR pre-mRNA. The one-choice serial reaction time (1-CSRT) task was used to identify impulsive action phenotypes in an outbred rat population before cocaine self-administration and assessment of cue reactivity in the form of lever presses reinforced by the cocaine-associated discrete cue complex during forced abstinence. The 1-CSRT task reliably and reproducibly identified high impulsive (HI) and low impulsive (LI) action phenotypes; HI action predicted high cue reactivity. Lower cortical 5-HT2CR membrane protein levels concomitant with higher levels of 5-HT2CR:postsynaptic density 95 complex distinguished HI rats from LI rats. The frequency of edited 5-HT2CR mRNA variants was elevated with the prediction that the protein population in HI rats favors those isoforms linked to reduced signaling capacity. Genetic loss of the mPFC 5-HT2CR induced aggregate impulsive action/cue reactivity, suggesting that depressed cortical 5-HT2CR tone confers vulnerability to these interlocked behaviors. Thus, impulsive action and cue reactivity appear to neuromechanistically overlap in rodents, with the 5-HT2CR functional status acting as a neural rheostat to regulate, in part, the intersection between these vulnerability behaviors.

Corticotropin-releasing factor receptor type-2 is involved in the cocaine-primed reinstatement of cocaine conditioned place preference in rats

Here we explored the in vivo role of brain corticotropin-releasing factor receptor type-2 (CRFR2) in cocaine-primed reinstatement of drug seeking. Conditioned place preference (CPP) procedure was used to assess the acquisition, extinction and reinstatement of cocaine-seeking behavior in rats. First, expressions of CRFR2 were shown to be affected in a brain region-specific manner within cocaine-induced CPP and cocaine-extinct CPP models. Bilateral blockade of CRFR2 in the dorsal portion of the medial prefrontal cortex (mPFC), or hippocampus (HP) was partially inhibited, but in the dorsal striatum (DS) did not affect, the cocaine-primed reinstatement of cocaine CPP.

Differential serotonin transport is linked to the rh5-HTTLPR in peripheral blood cells

The human serotonin transporter (SERT) gene possesses a 43-base pair (bp) insertion-deletion promoter polymorphism, the h5-HTTLPR. Genotype at this locus correlates with variation in anxiety-related personality traits and risk for major depressive disorder in many studies. Yet, the complex effects of the h5-HTTLPR, in combination with closely associated single-nucleotide polymorphisms (SNPs), continue to be debated. Moreover, although SERT is of high clinical significance, transporter function in vivo remains difficult to assess. Rhesus express a promoter polymorphism related to the h5-HTTLPR. The rh5-HTTLPR has been linked to differences in stress-related behavior and cognitive flexibility, although allelic variations in serotonin uptake have not been investigated. We studied the serotonin system as it relates to the 5-HTTLPR in rhesus peripheral blood cells. Sequencing of the rh5-HTTLPR revealed a 23-bp insertion, which is somewhat longer than originally reported. Consistent with previous reports, no SNPs in the rh5-HTTLPR and surrounding genomic regions were detected in the individuals studied. Reductions in serotonin uptake rates, cell surface SERT binding, and 5-hydroxyindoleacetic acid/serotonin ratios, but not SERT mRNA levels, were associated with the rh5-HTTLPR short allele. Thus, serotonin uptake rates are differentiable with respect to the 5-HTTLPR in an easily accessible native peripheral tissue. In light of these findings, we foresee that primary blood cells, in combination with high sensitivity functional measurements enabled by chronoamperometry, will be important for investigating alterations in serotonin uptake associated with genetic variability and antidepressant responsiveness in humans.

The neural basis of drug stimulus processing and craving: an activation likelihood estimation meta-analysis

BACKGROUND

The capacity of drug cues to elicit drug-seeking behavior is believed to play a fundamental role in drug dependence; yet the neurofunctional basis of human drug cue-reactivity is not fully understood. We performed a meta-analysis to identify brain regions that are consistently activated by presentation of drug cues. Studies involving treatment-seeking and nontreatment-seeking substance users were contrasted to determine whether there were consistent differences in the neural response to drug cues between these populations. Finally, to assess the neural basis of craving, consistency across studies in brain regions that show correlated activation with craving was assessed.

METHODS

Appropriate studies, assessing the effect of drug-related cues or manipulations of drug craving in drug-user populations across the whole brain, were obtained via the PubMed database and literature search. Activation likelihood estimation, a method of quantitative meta-analysis that estimates convergence across experiments by modeling the spatial uncertainty of neuroimaging data, was used to identify consistent regions of activation.

RESULTS

Cue-related activation was observed in the ventral striatum (across both subgroups), amygdala (in the treatment-seeking subgroup and overall), and orbitofrontal cortex (in the nontreatment-seeking subgroup and overall) but not insula cortex. Although a different pattern of frontal and temporal lobe activation between the subgroups was observed, these differences were not significant. Finally, right amygdala and left middle frontal gyrus activity were positively associated with craving.

CONCLUSIONS

These results substantiate the key neural substrates underlying reactivity to drug cues and drug craving.

Involvement of 5-HT2A receptors in MDMA reinforcement and cue-induced reinstatement of MDMA-seeking behaviour

The serotonergic system appears crucial for (±)-3,4-methylenedioxymethamphetamine (MDMA) reinforcing properties. Current evidence indicates that serotonin 5-HT2A receptors (5-HT2ARs) modulate mesolimbic dopamine (DA) activity and several behavioural responses related to the addictive properties of psychostimulants. This study evaluated the role of 5-HT2ARs in MDMA-induced reinforcement and hyperlocomotion, and the reinstatement of MDMA-seeking behaviour. Basal and MDMA-stimulated extracellular levels of DA in the nucleus accumbens (NAc) and serotonin and noradrenaline in the prefrontal cortex were also assessed. Self-administration of MDMA was blunted in 5-HT2AR knockout (KO) mice compared to wild-type (WT) littermates at both doses tested (0.125 and 0.25 mg/kg per infusion). Horizontal locomotion was increased by MDMA (10 and 20 mg/kg i.p.) to a higher extent in KO than in WT mice. DA outflow in the NAc was lower in KO compared to WT mice under basal conditions and after MDMA (20 mg/kg) challenge. In WT mice, MDMA (5 and 10 mg/kg i.p.) priming did not reinstate MDMA-seeking behaviour, while cue-induced reinstatement was prominent. This cue-induced reinstatement was blocked by administration of the selective 5-HT2AR antagonist, SR46349B (eplivanserin) at a dose of 0.5 mg/kg, but not at 0.25 mg/kg. Our results indicate that 5-HT2ARs are crucial for MDMA-induced reinforcement and cue-induced reinstatement of MDMA-seeking behaviour. These effects are probably due to the modulation of mesolimbic dopaminergic activity.

Extended characterisation of the serotonin 2A (5-HT2A) receptor-selective PET radiotracer 11C-MDL100907 in humans: quantitative analysis, test-retest reproducibility, and vulnerability to endogenous 5-HT tone

INTRODUCTION

Scanning properties and analytic methodology of the 5-HT2A receptor-selective positron emission tomography (PET) tracer 11C-MDL100907 have been partially characterised in previous reports. We present an extended characterisation in healthy human subjects.

METHODS

64 11C-MDL100907 PET scans with metabolite-corrected arterial input function were performed in 39 healthy adults (18-55 years). 12 subjects were scanned twice (duration 150 min) to provide data on plasma analysis, model order estimation, and stability and test-retest characteristics of outcome measures. All other scans were 90 min duration. 3 subjects completed scanning at baseline and following 5-HT2A receptor antagonist medication (risperidone or ciproheptadine) to provide definitive data on the suitability of the cerebellum as reference region. 10 subjects were scanned under reduced 5-HT and control conditions using rapid tryptophan depletion to investigate vulnerability to competition with endogenous 5-HT. 13 subjects were scanned as controls in clinical protocols. Pooled data were used to analyse the relationship between tracer injected mass and receptor occupancy, and age-related decline in 5-HT2A receptors.

RESULTS

Optimum analytic method was a 2-tissue compartment model with arterial input function. However, basis function implementation of SRTM may be suitable for measuring between-group differences non-invasively and warrants further investigation. Scan duration of 90 min achieved stable outcome measures in all cortical regions except orbitofrontal which required 120 min. Binding potential (BPP and BPND) test-retest variability was very good (7-11%) in neocortical regions other than orbitofrontal, and moderately good (14-20%) in orbitofrontal cortex and medial temporal lobe. Saturation occupancy of 5-HT2A receptors by risperidone validates the use of the cerebellum as a region devoid of specific binding for the purposes of PET. We advocate a mass limit of 4.6 μg to remain below 5% receptor occupancy. 11C-MDL100907 specific binding is not vulnerable to competition with endogenous 5-HT in humans. Paradoxical decreases in BPND were found in right prefrontal cortex following reduced 5-HT, possibly representing receptor internalisation. Mean age-related decline in brain 5-HT2A receptors was 14.0±5.0% per decade, and higher in prefrontal regions.

CONCLUSIONS

Our data confirm and extend support for 11C-MDL100907 as a PET tracer with very favourable properties for quantifying 5-HT2A receptors in the human brain.

Cortical and sub-cortical effects in primate models of cocaine use: implications for addiction and the increased risk of psychiatric illness

Drug abuse is a serious risk factor for the incidence and severity of multiple psychiatric illnesses. Understanding the neurobiological consequences of repeated exposure to abused drugs can help to inform how those risks are manifested in terms of specific neurochemical mechanisms and brain networks. This review examines selective studies in non-human primates that employed a cocaine self-administration model. Neurochemical consequences of chronic exposure appear to differ from observations in rodent studies. Whereas chronic intermittent exposure in the rodent is usually associated with a dose-dependent increase in dopaminergic response to a cocaine challenge, in the rhesus monkey, high cumulative exposure was not observed to cause a sensitized dopamine response. These non-human primate observations are concordant with clinical findings in human users. The results of cue exposure studies on dopaminergic transmission are also reviewed. Direct microdialysis measurements indicate that there is not a sustained increase in dopamine associated with cocaine-linked cues. As an alternative to striatal dopaminergic mechanisms mediating cue effects, single unit studies in prefrontal cortex during self-administration in monkeys suggests the orbitofrontal and anterior cingulate cortex are strongly engaged by cocaine cues. Based on the strong clinical imaging literature on cortical and cognitive dysfunction associated with addiction, it is proposed that the strong engagement of cortical systems during repeated cocaine reinforcement results in maladaptive changes that contribute to the risks of drug use for exacerbation of other psychiatric disorders.

Blockade of the serotonin 5-HT2A receptor suppresses cue-evoked reinstatement of cocaine-seeking behavior in a rat self-administration model

The serotonin 5-HT2A receptor (5-HT-sub(2A)R) may play a role in reinstatement of drug-seeking. This study investigated the ability of a selective 5-HT-sub(2A)R antagonist to suppress reinstatement evoked by exposure to cues conditioned to cocaine self-administration. Cocaine self-administration (0.75 mg/kg/0.1 mL/6 s infusion; FR 4) was trained in naïve, free-fed rats to allow interpretation of results independent from changes related to food deprivation stress. Pretreatment with the selective 5-HT-sub(2A)R antagonist M100907 (volinanserin) failed to reduce rates of operant responding for cocaine infusions. On the other hand, M100907 (0.001-0.8 mg/kg ip) significantly suppressed the cue-induced reinstatement of cocaine-seeking behavior following extinction; effective M100907 doses did not alter operant responding for cues previously associated with sucrose self-administration. Importantly, a greater magnitude of active lever presses on the initial extinction session (high extinction responders) predicted the maximal susceptibility to M100907-induced suppression of cue-evoked reinstatement. The findings indicate that blockade of the 5-HT-sub(2A)R attenuates the incentive-motivational effects of cocaine-paired cues, particularly in high extinction responders, and suggests that M100907 may afford a therapeutic advance in suppression of cue-evoked craving and/or relapse.

Serotonergic mechanisms in addiction-related memories

Drug-associated memories are a hallmark of addiction and a contributing factor in the continued use and relapse to drugs of abuse. Repeated association of drugs of abuse with conditioned stimuli leads to long-lasting behavioral responses that reflect reward-controlled learning and participate in the establishment of addiction. A greater understanding of the mechanisms underlying the formation and retrieval of drug-associated memories may shed light on potential therapeutic approaches to effectively intervene with drug use-associated memory. There is evidence to support the involvement of serotonin (5-HT) neurotransmission in learning and memory formation through the families of the 5-HT(1) receptor (5-HT(1)R) and 5-HT(2)R which have also been shown to play a modulatory role in the behavioral effects induced by many psychostimulants. While there is a paucity of studies examining the effects of selective 5-HT(1A)R ligands, the available dataset suggests that 5-HT(1B)R agonists may inhibit retrieval of cocaine-associated memories. The 5-HT(2A)R and 5-HT(2C)R appear to be integral in the strong conditioned associations made between cocaine and environmental cues with 5-HT(2A)R antagonists and 5-HT(2C)R agonists possessing potency in blocking retrieval of cocaine-associated memories following cocaine self-administration procedures. The complex anatomical connectivity between 5-HT neurons and other neuronal phenotypes in limbic-corticostriatal brain structures, the heterogeneity of 5-HT receptors (5-HT(X)R) and the conflicting results of behavioral experiments which employ non-specific 5-HT(X)R ligands contribute to the complexity of interpreting the involvement of 5-HT systems in addictive-related memory processes. This review briefly traces the history of 5-HT involvement in retrieval of drug-cue associations and future targets of serotonergic manipulation that may reduce the impact that drug cues have on addictive behavior and relapse.

Dopamine detection with fast-scan cyclic voltammetry used with analog background subtraction

Fast-scan cyclic voltammetry has been used in a variety of applications and has been shown to be especially useful to monitor chemical fluctuations of neurotransmitters such as dopamine within the mammalian brain. A major limitation of this procedure, however, is the large amplitude of the background current relative to the currents for the solution species of interest. Furthermore, the background tends to drift, and this drift limits the use of digital background subtraction techniques to intervals less than 90 s before distortion of dopamine signals occurs. To minimize the impact of the background, a procedure termed analog background subtraction is reported here. The background is recorded, and its inverse is played back to the current transducer during data acquisition so that it cancels the background in subsequent scans. Background drift still occurs and is recorded, but its magnitude is small compared to the original background. This approach has two advantages. First it allows the use of higher gains in the current transducer, minimizing quantization noise. Second, because the background amplitude is greatly reduced, principal component regression could be used to separate the contributions from drift, dopamine, and pH when appropriate calibrations were performed. We demonstrate the use of this approach with several applications. First, transient dopamine fluctuations were monitored for 15 min in a flowing injection apparatus. Second, evoked release of dopamine was monitored for a similar period in the brain of an anesthetized rat. Third, dopamine was monitored in the brain of freely moving rats over a 30 min interval. By analyzing the fluctuations in each resolved component, we were able to show that cocaine causes significant fluctuations in dopamine concentration in the brain while those for the background and pH remain unchanged from their predrug value.

Time-resolved microdialysis for in vivo neurochemical measurements and other applications

Monitoring changes in chemical concentrations over time in complex environments is typically performed using sensors and spectroscopic techniques. Another approach is to couple sampling methods, such as microdialysis, with chromatographic, electrophoretic, or enzymatic assays. Recent advances of such coupling have enabled improvements in temporal resolution, multianalyte capability, and automation. In a sampling and analysis method, the temporal resolution is set by the mass sensitivity of the analytical method, analysis time, and zone dispersion during sampling. Coupling methods with high speed and mass sensitivity to microdialysis sampling help to reduce some of these contributions to yield methods with temporal resolution of seconds. These advances have been primarily used in monitoring neurotransmitters in vivo. This review covers the problems associated with chemical monitoring in the brain, recent advances in using microdialysis for time-resolved in vivo measurements, sample applications, and other potential applications of the technology such as determining reaction kinetics and process monitoring.

Monitoring dopamine in vivo by microdialysis sampling and on-line CE-laser-induced fluorescence

Microdialysis sampling was coupled on-line to micellar electrokinetic chromatography (MEKC) to monitor extracellular dopamine concentration in the brains of rats. Microdialysis probes were perfused at 0.3 microL/min and the dialysate mixed on-line with 6 mM naphthalene-2,3-dicarboxaldehye and 10 mM potassium cyanide pumped at 0.12 microL/min each into a reaction capillary. The reaction mixture was delivered into a flow-gated interface and separated at 90-s intervals. The MEKC separation buffer consisted of 30 mM phosphate, 6.5 mM SDS, and 2 mM HP-beta-CD at pH 7.4, and the electric field was 850 V/cm applied across a 14-cm separation distance. Analytes were detected by laser-induced fluorescence excited using the 413-nm line of a 14-mW diode-pumped laser. The detection limit for dopamine was 2 nM when sampling by dialysis. The basal dopamine concentration in dialysates collected from the striatum of anesthetized rats was 18 +/- 3 nM (n = 12). The identity of the putative dopamine peak was confirmed by showing that dopamine uptake inhibitors increased the peak and dopamine synthesis inhibitors eliminated the peak. The utility of this method for behavioral studies was demonstrated by correlating dopamine concentrations in vivo and with psychomotor behavior in freely moving rats following the intravenous administration of cocaine. Over 60 additional peaks were detected in the electropherograms, suggesting the potential for monitoring many other substances in vivo by this method.

Cocaine sensitization and dopamine mediation of cue effects in rodents, monkeys, and humans: areas of agreement, disagreement, and implications for addiction

BACKGROUND

Sensitization of mesocorticolimbic dopamine projections has been a valuable model of neurobiological adaptation to chronic exposure to cocaine and other psychostimulants.

DISCUSSIONS

In addition to providing an explanation of exaggerated responses to drugs that might explain their increased ability to serve as reinforcers, sensitization has also been incorporated into influential theories of how drug associated cues can acquire increased salience and incentive motivation. However, almost all of the work exploring behavioral and neurochemical sensitization has been conducted in rodents. Importantly, the relatively small amount of work conducted in human and nonhuman primates differs from the rodent work in some important regards. This review will examine areas of convergence and divergence between the rodent and primate literature on sensitization and the ability of drug associated environmental cues to elicit dopamine release. The implications of this comparison for expanding addiction research beyond dopaminergic mechanisms in the striatum/nucleus accumbens will be considered.

Differential impact of pavlovian drug conditioned stimuli on in vivo dopamine transmission in the rat accumbens shell and core and in the prefrontal cortex

RATIONALE

Conditioned stimuli (CSs) by pavlovian association with reinforcing drugs (US) are thought to play an important role in the acquisition, maintenance and relapse of drug dependence.

OBJECTIVE

The aim of this study was to investigate by microdialysis the impact of pavlovian drug CSs on behaviour and on basal and drug-stimulated dopamine (DA) in three terminal DA areas: nucleus accumbens shell, core and prefrontal cortex (PFCX).

METHODS

Conditioned rats were trained once a day for 3 days by presentation of Fonzies filled box (FFB, CS) for 10 min followed by administration of morphine (1 mg/kg), nicotine (0.4 mg/kg) or saline, respectively. Pseudo-conditioned rats were presented with the FFB 10 h after drug or saline administration. Rats were implanted with microdialysis probes in the shell, core and PFCX. The effect of stimuli conditioned with morphine and nicotine on DA and on DA response to drugs was studied.

RESULTS

Drug CSs elicited incentive reactions and released DA in the shell and PFCX but not in the core. Pre-exposure to morphine CS potentiated DA release to morphine challenge in the shell but not in the core and PFCX. This effect was related to the challenge dose of morphine and was stimulus-specific since a food CS did not potentiate the shell DA response to morphine. Pre-exposure to nicotine CS potentiated DA release in the shell and PFCX.

CONCLUSION

The results show that drug CSs stimulate DA release in the shell and medial PFCX and specifically potentiate the primary stimulant drug effects on DA transmission.

Dopaminergic responses to self-administered cocaine in Rhesus monkeys do not sensitize following high cumulative intake

Sensitization of mesolimbic dopamine (DA) systems by administration of psychostimulants has been observed repeatedly in rodents. This phenomenon has been incorporated into theories of neurobiological adaptation underlying addiction, and is believed to be a mechanism whereby drug-associated cues acquire the ability to control behaviour via a conditioned release of DA. However, we have previously demonstrated in nonhuman primates that drug cues that cause cocaine seeking do not promote a conditioned increase in DA release of sufficient endurance to be measured in 2-min samples. In addition, imaging studies in humans and nonhuman primates that have been chronically exposed to psychostimulants have not demonstrated an increase in DA release upon psychostimulant challenge. Here we report that following 32 weeks of self-administration by rhesus monkeys, no increase over time in the DA response to self-administered cocaine was observed in any striatal subregion or individual animal. These results are consistent with clinical imaging studies showing a lack of DA sensitization, and might provide a mechanism to explain our previous observation that the rodent and primate differ in neurochemical response to drug-associated cues.

Progression of cellular adaptations in medial prefrontal and orbitofrontal cortex in response to repeated amphetamine

Recent theories on addiction implicate adaptive changes in prefrontal cortex (PFC) neurons in reinforcing and psychotomimetic properties of psychostimulants, yet little is known about how neuronal responses to these drugs change over time. Here we describe electrophysiological evidence for a progressive and sustained change in the response of PFC neurons to amphetamine during repeated exposure. In spontaneously behaving rats and in rats engaged in an instrumental responding task, we followed the activity of medial PFC (mPFC) and orbitofrontal cortex (OFC) neurons during daily exposure to amphetamine and after a post-withdrawal challenge. Repeated amphetamine increased the number of responsive neurons and the magnitude of responses and modified spontaneous burst patterns. These changes were apparent after a few exposures to amphetamine, were amplified after withdrawal, and were region specific in that repeated amphetamine increasingly produced inhibitory responses in mPFC and excitatory responses in OFC. In behaviorally engaged animals, the gradual enhancement in mPFC inhibition and OFC overactivation correlated with a progressive impairment of instrumental responding. Furthermore, these changes were evident predominately in neurons that displayed phasic responses during task-related events. These rapid-onset and sustained cellular adaptations suggest that even limited exposure to psychostimulants may reduce the influence of mPFC neurons on behavior while at the same time exaggerating information encoded by OFC neurons.

Serotonin2C receptors (5-HT2C R) control expression of cocaine-induced conditioned hyperactivity

Environmental cues can become classically conditioned to cocaine exposure and are known to contribute to drug craving and relapse in addicts. The 5-HT2C receptor (5-HT2C R) has been shown to control the behavioral effects of acute cocaine administration and, in the present study, we investigated the role of this receptor in the expression of cocaine-induced conditioned hyperactivity. Rats received repeated pairings of a distinct test environment with either saline or cocaine (15 mg/kg, i.p.) for 7 days. In a drug-free test 2 days after the last pairing, expression of conditioned hyperactivity was seen in the rats previously exposed to cocaine in the test environment. The 5-HT2C R agonist MK 212 (0.0625-0.5 mg/kg, i.p., 5 min before test) significantly decreased, while the 5-HT2C R antagonist SB 242084 (0.5-1 mg/kg, i.p. 30 min prior to test) enhanced, expression of cocaine-induced conditioned hyperactivity. The effective doses of MK 212 and SB 242084 did not alter basal activity on the test session. These results suggest that the 5-HT2C R controls expression of cocaine-induced conditioned hyperactivity and suggest that such ligands may be useful in preventing relapse and promoting abstinence in cocaine-dependent individuals.

Differential involvement of the prelimbic cortex and striatum in conditioned heroin and sucrose seeking following long-term extinction

Relapse to drug taking is triggered by stimuli previously associated with consumption of drugs of misuse (cues) and involves brain systems controlling motivated behaviour towards natural reinforcers. In this study, we aimed to identify and compare neuronal pathways in corticostriatal systems that control conditioned heroin or natural reward (sucrose) seeking. To that end, rats were trained to self-administer heroin or sucrose in association with an identical compound cue. After more than 3 weeks of abstinence during extinction training, cue exposure robustly reinstated heroin and sucrose seeking, but induced distinct and even opposing changes in the expression of the neuronal activation marker zif268 in the prelimbic cortex and striatal complex, respectively. Because in the prelimbic area zif268 expression was enhanced during cue-induced heroin seeking but unaffected during sucrose seeking, a pharmacological intervention was aimed at this prefrontal region. Injection of a GABA agonist mixture within the prelimbic area enhanced conditioned heroin seeking, but had no effect on conditioned sucrose seeking. Our findings suggest a differential role of the prelimbic area and the striatum in the persistence of heroin vs. sucrose seeking following long-term extinction.