Changes in basal and cocaine-evoked extracellular dopamine uptake and release in the rat nucleus accumbens during early abstinence from cocaine: quantitative determination under transient conditions

The influence of dopamine autoreceptors on temperament and addiction risk

As a major regulator of dopamine (DA), DA autoreceptors (DAARs) exert substantial influence over DA-mediated behaviors. This paper reviews the physiological and behavioral impact of DAARs. Individual differences in DAAR functioning influences temperamental traits such as novelty responsivity and impulsivity, both of which are associated with vulnerability to addictive behavior in animal models and a broad array of externalizing behaviors in humans. DAARs additionally impact the response to psychostimulants and other drugs of abuse. Human PET studies of D2-like receptors in the midbrain provide evidence for parallels to the animal literature. These data lead to the proposal that weak DAAR regulation is a risk factor for addiction and externalizing problems. The review highlights the potential to build translational models of the functional role of DAARs in behavior. It also draws attention to key limitations in the current literature that would need to be addressed to further advance a weak DAAR regulation model of addiction and externalizing risk.

Cocaine increases dopaminergic connectivity in the nucleus accumbens

The development of addictive behavior is associated with functional and structural plasticity in the mesocorticolimbic pathway. Increased connectivity upon cocaine administration has been inferred from increases in dendritic spine density, but without observations of presynaptic elements. Recently, we established a method that enables analyses of both dendritic spines and glutamatergic boutons and presented evidence that cocaine induces changes in striatal connectivity. As the pharmacological and behavioral effects of cocaine directly implicate dopaminergic neurons and their afferents, a remaining question is whether dopaminergic striatal innervations also undergo structural plasticity. To address this issue, we generated transgenic mice in which the fluorophore tdTomato is expressed under the promoter of the dopamine transporter gene. In these mice, specific labeling of dopaminergic boutons was observed in the striatum. Of note, the accordance of our results for control mice with previous electron microscopy studies confirms that our method can be used to decipher the spatial organization of boutons in relation to dendritic elements. Following repeated cocaine administration that led to behavioral locomotor sensitization, an increased density of dopaminergic boutons was observed 1 day later in the nucleus accumbens shell specifically, and not in other striatal regions. Combined labeling of dopaminergic boutons and striatal dendrites showed that cocaine significantly increased the percentage of dendritic spines associated with a dopaminergic bouton. Our results show that chronic cocaine administration induces structural plasticity of dopaminergic boutons that could participate in dopamine-dependent neuronal adaptations in the striatum.

Brain microdialysis and its applications in experimental neurochemistry

Abstract Microdialysis is one of the most powerful neurochemistry techniques, which allows the monitoring of changes in the extracellular content of endogenous and exogenous substances in the brain of living animals. The strength as well as wide applicability of this experimental approach are based on the bulk theory of brain neurotransmission. This methodological review introduces basic principles of chemical neurotransmission and emphasizes the difference in neurotransmission types.Clear understanding of their significance and degree of engagement in regulation of physiological processes is an ultimate prerequisite not only for choosing an appropriate method of monitoring for interneuronal communication via chemical messengers but also for accurate data interpretation. The focus on the processes of synthesis/metabolism, receptor interaction/neuronal signaling or the behavioral relevance of neurochemical events sculpts the experiment design. Brain microdialysis is an important method for examining changes in the content of any substances, irrespective of their origin, in living animals. This article compares contemporary approaches and techniques that are used for monitoring neurotransmission (including in vivo brain microdialysis, voltammetric methods, etc). We highlight practical aspects of microdialysis experiments in particular to those researchers who are seeking to increase the repertoire of their experimental techniques with brain microdialysis.

Target-dependent expression of the netrin-1 receptor, UNC5C, in projection neurons of the ventral tegmental area

We have shown previously that the netrin-1 receptor, unc-5 homologue C (UNC5C), is expressed by ventral tegmental area (VTA) dopamine (DA) neurons of rodents, but only from adolescence onwards (Manitt et al., 2010; Auger et al., 2013). The goal of this study was to characterize the expression of UNC5C by these neurons. Specifically, we assessed whether UNC5C expression is selective to DA neurons that project to the medial prefrontal cortex (mPFC), which undergo significant maturation during the adolescent period. To this end, we injected fluorescent retrograde tracer beads into the mPFC, nucleus accumbens (NAcc) core, or NAcc lateral shell of adult male wild-type C57Bl/6J mice and processed their brains for tyrosine hydroxylase (TH) and UNC5C immunofluorescence 2-3weeks later. VTA neurons with any combination of these immunolabels were visualized and counted using optical fractionator stereology. Our analysis revealed two main findings: (1) there are no differences in the proportions of UNC5C-positive DA neurons projecting to the mPFC, NAcc core, or NAcc lateral shell, and (2) the proportion of non-DA UNC5C-positive neurons targeting the mPFC is greater than the proportions of non-DA UNC5C-positive neurons targeting the NAcc core or lateral shell. These findings show that, contrary to our hypothesis, DA neurons projecting to the mPFC do not express UNC5C selectively. However, UNC5C expression by non-DA VTA neurons is predominantly found in those projecting to the mPFC and, as such, may play a role in their function.

Advances in studying phasic dopamine signaling in brain reward mechanisms

The last sixty years of research has provided extraordinary advances of our knowledge of the reward system. Since its discovery as a neurotransmitter by Carlsson and colleagues (1), dopamine (DA) has emerged as an important mediator of reward processing. As a result, a number of electrochemical techniques have been developed to measure DA in the brain. Together, these techniques have begun to elucidate the complex roles of tonic and phasic DA signaling in reward processing and addiction. In this review, we will first provide a guide for the most commonly used electrochemical methods for DA detection and describe their utility in furthering our knowledge about DA's role in reward and addiction. Second, we will review the value of common in vitro and in vivo preparations and describe their ability to address different types of questions. Last, we will review recent data that has provided new mechanistic insight of in vivo phasic DA signaling and its role in reward processing and reward-mediated behavior.

Hyperactivity induced by the dopamine D2/D3 receptor agonist quinpirole is attenuated by inhibitors of endocannabinoid degradation in mice

The present study was designed to investigate the effect of pharmacological inhibition of endocannabinoid degradation on behavioural actions of the dopamine D2/D3 receptor agonist quinpirole in male C57Bl/6J mice. In addition, we studied the effects of endocannabinoid degradation inhibition on both cocaine-induced psychomotor activation and behavioural sensitization. We analysed the effects of inhibition of the two main endocannabinoid degradation enzymes: fatty acid amide hydrolase (FAAH), using inhibitor URB597 (1 mg/kg); monoacylglycerol lipase (MAGL), using inhibitor URB602 (10 mg/kg). Administration of quinpirole (1 mg/kg) caused a temporal biphasic response characterized by a first phase of immobility (0-50 min), followed by enhanced locomotion (next 70 min) that was associated with the introduction of stereotyped behaviours (stereotyped jumping and rearing). Pretreatment with both endocannabinoid degradation inhibitors did not affect the hypoactivity actions of quinpirole. However, this pretreatment resulted in a marked decrease in quinpirole-induced locomotion and stereotyped behaviours. Administration of FAAH or MAGL inhibitors did not attenuate the acute effects of cocaine. Furthermore, these inhibitors did not impair the acquisition of cocaine-induced behavioural sensitization or the expression of cocaine-induced conditioned locomotion. Only MAGL inhibition attenuated the expression of an already acquired cocaine-induced behavioural sensitization. These results suggest that pharmacological inhibition of endocannabinoid degradation might exert a negative feedback on D2/D3 receptor-mediated hyperactivity. This finding might be relevant for therapeutic approaches for either psychomotor disorders (dyskinesia, corea) or disorganized behaviours associated with dopamine-mediated hyperactivity.

Presynaptic dopamine modulation by stimulant self-administration

The mesolimbic dopamine system is an essential participant in the initiation and modulation of various forms of goal-directed behavior, including drug reinforcement and addiction processes. Dopamine neurotransmission is increased by acute administration of all drugs of abuse, including the stimulants cocaine and amphetamine. Chronic exposure to these drugs via voluntary self-administration provides a model of stimulant abuse that is useful in evaluating potential behavioral and neurochemical adaptations that occur during addiction. This review describes commonly used methodologies to measure dopamine and baseline parameters of presynaptic dopamine regulation, including exocytotic release and reuptake through the dopamine transporter in the nucleus accumbens core, as well as dramatic adaptations in dopamine neurotransmission and drug sensitivity that occur with acute non-contingent and chronic, contingent self-administration of cocaine and amphetamine.

Chronic escalating cocaine exposure, abstinence/withdrawal, and chronic re-exposure: effects on striatal dopamine and opioid systems in C57BL/6J mice

Cocaine addiction is a chronic relapsing disease with periods of chronic escalating self-exposure, separated by periods of abstinence/withdrawal of varying duration. Few studies compare such cycles in preclinical models. This study models an "addiction-like cycle" in mice to determine neurochemical/molecular alterations that underlie the chronic, relapsing nature of this disease. Groups of male C57BL/6J mice received acute cocaine exposure (14-day saline/14-day withdrawal/13-day saline + 1-day cocaine), chronic cocaine exposure (14 day cocaine) or chronic re-exposure (14-day cocaine/14-day withdrawal/14-day cocaine). Escalating-dose binge cocaine (15-30 mg/kg/injection × 3/day, i.p. at hourly intervals) or saline (14-day saline) was administered, modeling initial exposure. In "re-exposure" groups, after a 14-day injection-free period (modeling abstinence/withdrawal), mice that had received cocaine were re-injected with 14-day escalating-dose binge cocaine, whereas controls received saline. Microdialysis was conducted on the 14th day of exposure or re-exposure to determine striatal dopamine content. Messenger RNA levels of preprodynorphin (Pdyn), dopamine D1 (Drd1) and D2 (Drd2) in the caudate putamen were determined by real-time PCR. Basal striatal dopamine levels were lower in mice after 14-day escalating exposure or re-exposure than in those in the acute cocaine group and controls. Pdyn mRNA levels were higher in the cocaine groups than in controls. Long-term adaptation was observed across the stages of this addiction-like cycle, in that the effects of cocaine on dopamine levels were increased after re-exposure compared to exposure. Changes in striatal dopaminergic responses across chronic escalating cocaine exposure and re-exposure are a central feature of the neurobiology of relapsing addictive states.

AMPA receptor plasticity in the nucleus accumbens after repeated exposure to cocaine

This review focuses on cocaine-induced postsynaptic plasticity in the nucleus accumbens (NAc) involving changes in AMPA receptor (AMPAR) transmission. First, fundamental properties of AMPAR in the NAc are reviewed. Then, we provide a detailed and critical analysis of literature demonstrating alterations in AMPAR transmission in association with behavioral sensitization to cocaine and cocaine self-administration. We conclude that cocaine exposure leads to changes in AMPAR transmission that depend on many factors including whether exposure is contingent or non-contingent, the duration of withdrawal, and whether extinction training has occurred. The relationship between changes in AMPAR transmission and responding to cocaine or cocaine-paired cues can also be affected by these variables. However, after prolonged withdrawal in the absence of extinction training, our findings and others lead us to propose that AMPAR transmission is enhanced, resulting in stronger responding to drug-paired cues. Finally, many results indicate that the state of synaptic transmission in the NAc after cocaine exposure is associated with impairment of AMPAR-dependent plasticity. This may contribute to a broad range of addiction-related behavioral changes.

Dopamine signaling in the nucleus accumbens of animals self-administering drugs of abuse

Abuse of psychoactive substances can lead to drug addiction. In animals, addiction is best modeled by drug self-administration paradigms. It has been proposed that the crucial common denominator for the development of drug addiction is the ability of drugs of abuse to increase extracellular concentrations of dopamine in the nucleus accumbens (NAcc). Studies using in vivo microdialysis and chronoamperometry in the behaving animal have demonstrated that drugs of abuse increase tonic dopamine concentrations in the NAcc. However, it is known that dopamine neurons respond to reward-related stimuli on a subsecond timescale. Thus, it is necessary to collect neurochemical information with this level of temporal resolution, as achieved with in vivo fast-scan cyclic voltammetry (FSCV), to fully understand the role of phasic dopamine release in normal behavior and drug addiction. We review studies that investigated the effects of drugs of abuse on NAcc dopamine levels in freely moving animals using in vivo microdialysis, chronoamperometry, and FSCV. After a brief introduction of dopamine signal transduction and anatomy and a section on current theories on the role of dopamine in natural goal-directed behavior, a discussion of techniques for the in vivo assessment of extracellular dopamine in behaving animals is presented. Then, we review studies using these techniques to investigate changes in phasic and tonic dopamine signaling in the NAcc during (1) response-dependent and -independent administration of abused drugs, (2) the presentation of drug-conditioned stimuli and operant behavior in self-administration paradigms, (3) drug withdrawal, and (4) cue-induced reinstatement of drug seeking. These results are then integrated with current ideas on the role of dopamine in addiction with an emphasis on a model illustrating phasic and tonic NAcc dopamine signaling during different stages of drug addiction. This model predicts that phasic dopamine release in response to drug-related stimuli will be enhanced over stimuli associated with natural reinforcers, which may result in aberrant goal-directed behaviors contributing to drug addiction.

Ventral and dorsal striatal dopamine efflux and behavior in rats with simple vs. co-morbid histories of cocaine sensitization and neonatal ventral hippocampal lesions

RATIONAL

Exposing animal models of mental illness to addictive drugs provides an approach to understanding the neural etiology of dual diagnosis disorders. Previous studies have shown that neonatal ventral hippocampal lesions (NVHL) in rats produce features of both schizophrenia and addiction vulnerability.

OBJECTIVE

This study investigated ventral and dorsal striatal dopamine (DA) efflux in NVHL rats combined with behavioral sensitization to cocaine.

METHODS

Adult NVHL vs. SHAM-operated rats underwent a 5-day injection series of cocaine (15 mg/kg/day) vs. saline. One week later, rats were cannulated in nucleus accumbens SHELL, CORE, or caudate-putamen. Another week later, in vivo microdialysis sampled DA during locomotor testing in which a single cocaine injection (15 mg/kg) was delivered.

RESULTS

NVHLs and cocaine history significantly increased behavioral activation approximately 2-fold over SHAM-saline history rats. DA efflux curves corresponded time dependently with the cocaine injection and locomotor curves and varied significantly by striatal region: Baseline DA levels increased 5-fold while cocaine-stimulated DA efflux decreased by half across a ventral to dorsal striatal gradient. However, NVHLs, prior cocaine history, and individual differences in behavior were not underpinned by differential DA efflux overall or within any striatal region.

CONCLUSION

Differences in ventral/dorsal striatal DA efflux are not present in and are not required for producing differential levels of acute cocaine-induced behavioral activation in NVHLs with and without a behaviorally sensitizing cocaine history. These findings suggest other neurotransmitter systems, and alterations in striatal network function post-synaptic to DA transmission are more important to understanding the interactive effects of addictive drugs and mental illness.

Altered dopamine transporter function and phosphorylation following chronic cocaine self-administration and extinction in rats

Cocaine binds with the dopamine transporter (DAT), an effect that has been extensively implicated in its reinforcing effects. However, persisting adaptations in DAT regulation after cocaine self-administration have not been extensively investigated. Here, we determined the changes in molecular mechanisms of DAT regulation in the caudate-putamen (CPu) and nucleus accumbens (NAcc) of rats with a history of cocaine self-administration, followed by 3weeks of withdrawal under extinction conditions (i.e., no cocaine available). DA uptake was significantly higher in the CPu of cocaine-experienced animals as compared to saline-yoked controls. DAT V(max) was elevated in the CPu without changes in apparent affinity for DA. In spite of elevated CPu DAT activity, total and surface DAT density and DAT-PP2Ac (protein phosphatase 2A catalytic subunit) interaction remained unaltered, although p-Ser- DAT phosphorylation was elevated. In contrast to the CPu, there were no differences between cocaine and saline rats in the levels of DA uptake, DAT V(max) and K(m) values, total and surface DAT, p-Ser-DAT phosphorylation, or DAT-PP2Ac interactions in the NAcc. These results show that chronic cocaine self-administration leads to lasting, regionally specific alterations in striatal DA uptake and DAT-Ser phosphorylation. Such changes may be related to habitual patterns of cocaine-seeking observed during relapse.

Dopamine transporter down-regulation following repeated cocaine: implications for 3,4-methylenedioxymethamphetamine-induced acute effects and long-term neurotoxicity in mice

BACKGROUND AND PURPOSE

3,4-Methylenedioxymethamphetamine (MDMA) and cocaine are two widely abused psychostimulant drugs targeting the dopamine transporter (DAT). DAT availability regulates dopamine neurotransmission and uptake of MDMA-derived neurotoxic metabolites. We aimed to determine the effect of cocaine pre-exposure on the acute and long-term effects of MDMA in mice.

EXPERIMENTAL APPROACH

Mice received a course of cocaine (20 mg*kg(-1), x2 for 3 days) followed by MDMA (20 mg*kg(-1), x2, 3 h apart). Locomotor activity, extracellular dopamine levels and dopaminergic neurotoxicity were determined. Furthermore, following the course of cocaine, DAT density in striatal plasma membrane and endosome fractions was measured.

KEY RESULTS

Four days after the course of cocaine, challenge with MDMA attenuated the MDMA-induced striatal dopaminergic neurotoxicity. Co-administration of the protein kinase C (PKC) inhibitor NPC 15437 prevented cocaine protection. At the same time, after the course of cocaine, DAT density was reduced in the plasma membrane and increased in the endosome fraction, and this effect was prevented by NPC 15437. The course of cocaine potentiated the MDMA-induced increase in extracellular dopamine and locomotor activity, following challenge 4 days later, compared with those pretreated with saline.

CONCLUSIONS AND IMPLICATIONS

Repeated cocaine treatment followed by withdrawal protected against MDMA-induced dopaminergic neurotoxicity by internalizing DAT via a mechanism which may involve PKC. Furthermore, repeated cocaine followed by withdrawal induced behavioural and neurochemical sensitization to MDMA, measures which could be indicative of increased rewarding effects of MDMA.

Regulation of dynamin 2 and G protein-coupled receptor kinase 2 in rat nucleus accumbens during acute and repeated cocaine administration

Exposure to cocaine causes many neuroadaptations including alterations in several neurotransmitter receptors and transporters. This study investigated potential mechanisms of cocaine-induced receptor and transporter regulation by measuring levels of two proteins involved in receptor and transporter trafficking, dynamin 2 and G protein-coupled receptor kinase 2 (GRK2). Male Fischer rats received three daily injections of cocaine, 15 mg/kg, in a binge-pattern (at 1 h intervals) for 1, 3, or 14 days. Brain regions of interest were collected 30 min after the last injection and proteins measured by Western blot. Acute binge-pattern cocaine administration produced a significant increase in both dynamin 2- and GRK2-immunoreactivity (227% and 358% of control) in the nucleus accumbens and GKR2 (150% of control) in the caudate putamen. Tolerance to this effect occurred, as levels of both proteins returned to baseline after 3 days of cocaine. In contrast, dynamin 2 and GRK2 were significantly decreased in the nucleus accumbens after chronic cocaine. This pattern of regulation was unique to the nucleus accumbens and not seen in the frontal cortex or substantia nigra. Pretreatment with either the dopamine (DA) D1 receptor antagonist SCH 23390 or D2 receptor antagonist eticlopride prior to acute cocaine blocked the upregulation of dynamin 2 and GRK2 in the nucleus accumbens. However, only eticlopride was effective in attenuating the decrease in these proteins following chronic cocaine exposure. These results demonstrate that two proteins involved in receptor and transporter trafficking are selectively regulated in the nucleus accumbens following acute versus chronic cocaine exposure, and dopamine receptor activation is required for this regulation.

Lower level of endogenous dopamine in patients with cocaine dependence: findings from PET imaging of D(2)/D(3) receptors following acute dopamine depletion

OBJECTIVE

Previous positron emission tomography (PET) imaging studies have demonstrated that cocaine dependence is associated with a decrease in dopamine type 2 and 3 (D(2)/D(3)) receptor binding in cocaine-dependent individuals relative to healthy comparison subjects. However, given the nature of PET imaging, it is possible that the measured decrease in radiotracer binding results from an increase in baseline dopamine levels. The purpose of this study was to measure D(2)/D(3) receptors following acute dopamine depletion in cocaine-dependent volunteers relative to healthy comparison subjects.

METHOD

Cocaine-dependent volunteers (N=15) and healthy matched comparison subjects (N=15) were scanned using PET, with the dopamine receptor radiotracer [(11)C]raclopride, at baseline and again following acute depletion of endogenous dopamine via alpha-methyl-para-tyrosine (AMPT) administration. Changes in radiotracer binding were measured in the subdivisions of the striatum (caudate, putamen, and ventral striatum) in addition to the striatum as a whole.

RESULTS

Findings revealed that cocaine-dependent volunteers exhibited lower levels of endogenous dopamine relative to comparison subjects, which was measured as an increase in [(11)C]raclopride binding following AMPT administration. The increase in [(11)C]raclopride binding in the striatum was 11.1% (SD=4.4%) in healthy comparison subjects and 5.7% (SD=5.9%) in cocaine-dependent volunteers. Similar differences were seen in the subdivisions of the striatum.

CONCLUSIONS

The decrease in striatal D(2)/D(3 )receptors associated with cocaine dependence cannot be attributed to higher levels of endogenous dopamine.

Striatal dopamine responses to intranasal cocaine self-administration in humans

BACKGROUND

The effect of self-administered cocaine on extracellular dopamine (DA) levels has not been measured in humans.

METHODS

Ten nondependent cocaine users underwent positron emission tomography [11C]raclopride scans following intranasal self-administration of cocaine hydrochloride (1.0 mg/kg) and placebo powder.

RESULTS

Compared with placebo, intranasal cocaine self-administration decreased [11C]raclopride binding values in the ventral limbic striatum and putamen. Individual differences in the magnitude of the [11C]raclopride response in the ventral striatum were predicted by lifetime histories of stimulant drug use.

CONCLUSIONS

The results suggest that 1) intranasal cocaine self-administration increases synaptic DA levels in human striatum and 2) prior use of stimulant drugs on the street is associated with progressively greater cocaine-induced DA responses. These dopaminergic effects might influence susceptibility to drug-seeking behavior and the progression to substance abuse.

Cocaine abuse and sensitization of striatal dopamine transmission: a critical review of the preclinical and clinical imaging literature

Much effort has been devoted in the preclinical addiction literature to understanding the phenomenon of sensitization, an enhanced dopaminergic response in the nucleus accumbens that occurs after repeated exposure to psychostimulant drugs. Although sensitization has been reported in preclinical studies, studies of sensitization in humans measuring behavioral and physiological responses have been mixed and inconclusive. However, imaging studies with positron emission tomography (PET) and single photon emission computed tomography (SPECT) using a stimulant challenge to induce dopamine (DA) release provide a unique opportunity to probe DA transmission in cocaine dependent human subjects. In contrast to the basic science literature that predicted sensitization, three independent cohorts have shown a blunted DA response, or the opposite of sensitization, in human cocaine dependent subjects. This article reviews the methodological differences between the preclinical and clinical PET studies that have investigated DA sensitization in order to address the discrepancy between the human and animal literature.

The neurocircuitry of addiction: an overview

Drug addiction presents as a chronic relapsing disorder characterized by persistent drug-seeking and drug-taking behaviours. Given the significant detrimental effects of this disease both socially and economically, a considerable amount of research has been dedicated to understanding a number of issues in addiction, including behavioural and neuropharmacological factors that contribute to the development, loss of control and persistence of compulsive addictive behaviours. In this review, we will give a broad overview of various theories of addiction, animal models of addiction and relapse, drugs of abuse, and the neurobiology of drug dependence and relapse. Although drugs of abuse possess diverse neuropharmacological profiles, activation of the mesocorticolimbic system, particularly the ventral tegmental area, nucleus accumbens, amygdala and prefrontal cortex via dopaminergic and glutamatergic pathways, constitutes a common pathway by which various drugs of abuse mediate their acute reinforcing effects. However, long-term neuroadaptations in this circuitry likely underlie the transition to drug dependence and cycles of relapse. As further elucidated in more comprehensive reviews of various subtopics on addiction in later sections of this special issue, it is anticipated that continued basic neuroscience research will aid in the development of effective therapeutic interventions for the long-term treatment of drug-dependent individuals.

Dysregulation of dopamine transporter trafficking and function after abstinence from cocaine self-administration in rats: evidence for differential regulation in caudate putamen and nucleus accumbens

The profound alterations produced by cocaine on dopamine (DA) neurotransmission raise the possibility that dopamine transporter (DAT)-expressing neurons may modify DA transport in response to repeated cocaine exposure to maintain the appropriate efficiency of DA clearance. In this study, we determined the changes in molecular mechanisms of DAT regulation in rats with a history of repeated cocaine self-administration followed by 3 weeks of abstinence. Using ex vivo caudate putamen (CPu) and nucleus accumbens (NAcc) synaptosomal preparations, we found that DA uptake was significantly higher in the CPu and NAcc of cocaine-experienced animals compared with yoked saline animals. Surface distribution, p-Ser phosphorylation, and protein phosphatase 2A catalytic subunit (PP2Ac) interaction of DAT were all altered in the CPu. Maximal velocity (V(max)) values were elevated both in the CPu and NAcc of cocaine-experienced rats compared with saline controls. Although there was no change in the apparent affinity for DA in the CPu, increased DA affinity was evident in the NAcc. Consistent with elevated DAT activity in cocaine-experienced animals, a higher level of surface DAT, DAT-PP2Ac association, and decreased serine phosphorylation of DAT were observed in the CPu, but not in the NAcc. These results, for the first time, suggest that chronic cocaine self-administration followed by abstinence leads to persisting alterations in normal DAT trafficking and catalytic regulatory cascades in the CPu and NAcc in a brain region-specific manner.

Repeated electrical stimulation of reward-related brain regions affects cocaine but not "natural" reinforcement

Drug addiction is associated with long-lasting neuronal adaptations including alterations in dopamine and glutamate receptors in the brain reward system. Treatment strategies for cocaine addiction and especially the prevention of craving and relapse are limited, and their effectiveness is still questionable. We hypothesized that repeated stimulation of the brain reward system can induce localized neuronal adaptations that may either potentiate or reduce addictive behaviors. The present study was designed to test how repeated interference with the brain reward system using localized electrical stimulation of the medial forebrain bundle at the lateral hypothalamus (LH) or the prefrontal cortex (PFC) affects cocaine addiction-associated behaviors and some of the neuronal adaptations induced by repeated exposure to cocaine. Repeated high-frequency stimulation in either site influenced cocaine, but not sucrose reward-related behaviors. Stimulation of the LH reduced cue-induced seeking behavior, whereas stimulation of the PFC reduced both cocaine-seeking behavior and the motivation for its consumption. The behavioral findings were accompanied by glutamate receptor subtype alterations in the nucleus accumbens and the ventral tegmental area, both key structures of the reward system. It is therefore suggested that repeated electrical stimulation of the PFC can become a novel strategy for treating addiction.

Conditioned and sensitized responses to stimulant drugs in humans

In animal models considerable evidence suggests that increased motivation to seek and ingest drugs of abuse are related to conditioned and sensitized activations of the mesolimbic dopamine (DA) system. Direct evidence for these phenomena in humans, though, is sparse. However, recent studies support the following. First, the acute administration of drugs of abuse across pharmacological classes increases extracellular DA levels within the human ventral striatum. Second, individual differences in the magnitude of this response correlate with rewarding effects of the drugs and the personality trait of novelty seeking. Third, transiently diminishing DA transmission in humans decreases drug craving, the propensity to preferentially respond to reward-paired stimuli, and the ability to sustain responding for future drug reward. Finally, very recent studies suggest that repeated exposure to stimulant drugs, either on the street or in the laboratory, can lead to conditioned and sensitized behavioral responses and DA release. In contrast to these findings, though, in individuals with a long history of substance abuse, drug-induced DA release is decreased. This diminished DA release could reflect two different phenomena. First, it is possible that drug withdrawal related decrements in DA cell function persist longer than previously suspected. Second, drug-paired stimuli may gain marked conditioned control over the release of DA and the expression of sensitization leading to reduced DA release when drug-related cues are absent. Based on these observations a two-factor hypothesis of the role of DA in drug abuse is proposed. In the presence of drug cues, conditioned and sensitized DA release would occur leading to focused drug-seeking behavior. In comparison, in the absence of drug-related stimuli DA function would be reduced, diminishing the ability of individuals to sustain goal-directed behavior and long-term objectives. This conditioned control of the expression of sensitized DA release could aggravate susceptibility to relapse, narrow the range of interests and perturb decision-making, accounting for a wide range of addiction related phenomena.

Microdialysis and the neurochemistry of addiction

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

Altered basal and stimulated accumbens dopamine release in obese OLETF rats as a function of age and diabetic status

The Otsuka Long-Evans Tokushima Fatty (OLETF) rat lacking the CCK-1 receptor is hyperphagic, prefers palatable and high-calorie meals, and gradually develops obesity and type 2 diabetes. To determine dopamine levels in this strain, we used in vivo quantitative (no net flux) microdialysis at three different ages representing nondiabetic (8 wk), prediabetic (18 wk), and diabetic (56 wk) stages in OLETF and age-matched lean Long-Evans Tokushima Otsuka (LETO) controls. Results showed significantly elevated basal dopamine levels in the caudomedial nucleus accumbens of OLETF rats compared with LETO at younger ages (8 wk: 20.10 +/- 5.61 nM vs. 15.85 +/- 5.63 nM; 18 wk: 7.37 +/- 3.71 nM vs. 4.75 +/- 1.25 nM, means +/- SD). In contrast, at 56 wk of age, a profound decline in extracellular dopamine concentrations was seen in both strains with a tendency for a greater effect in OLETF rats (1.78 +/- 0.40 nM vs. 2.39 +/- 0.42 nM). Further, extracellular fraction, an index for reuptake, was higher in 56-wk-old OLETF compared with LETO (0.648 +/- 0.049 vs. 0.526 +/- 0.057). Potassium-stimulated dopamine efflux revealed an increased capacity of vesicular pool in OLETF rats compared with LETO across all age groups with an accentuated strain difference at 56 wk. These findings demonstrate altered striatal dopamine functions (i.e., increased stimulated release and uptake) in obese OLETF rat. This could be due to the lack of functional CCK-1 receptors, or metabolic and hormonal factors associated with the development of obesity and insulin resistance, or both.

Accumbal neurons that are activated during cocaine self-administration are spared from inhibitory effects of repeated cocaine self-administration

Hypoactivity of the accumbens is induced by repeated cocaine exposure and is hypothesized to play a role in cocaine addiction. However, it is difficult to understand how a general hypoactivity of the accumbens, which facilitates multiple types of motivated behaviors, could contribute to the selective increase in drug-directed behavior that defines addiction. Electrophysiological recordings, made during sessions in which rats self-administer cocaine, show that most accumbal neurons that encode events related to drug-directed behavior achieve and maintain higher firing rates during the period of cocaine exposure (Task-Activated neurons) than do other accumbal neurons (Task-Non-Activated neurons). We have hypothesized that this difference in activity makes the neurons that facilitate drug-directed behavior less susceptible than other neurons to the chronic inhibitory effects of cocaine. A sparing of neurons that facilitate drug-directed behavior from chronic hypoactivity might lead to a relative increase in the transmission of neuronal signals that facilitate drug-directed behavior through accumbal circuits and thereby contribute to changes in behavior that characterize addiction (ie differential inhibition hypothesis). A prediction of the hypothesis is that neurons that are activated in relation to task events during cocaine self-administration sessions will show less of a decrease in firing across repeated self-administration sessions than will other neurons. To test this prediction, rats were exposed to 30 daily (6 h/day) cocaine self-administration sessions. Chronic extracellular recordings of single accumbal neurons were made during the second to third session and the 30th session. Between-session comparisons showed that decreases in firing were exhibited by Task-Non-Activated, but not by Task-Activated, neurons. During the day 30 session, the magnitude of the difference in firing rate between the two groups of neurons was positively related to the propensity of animals to seek and take cocaine. The findings of the present study are consistent with a basic prediction of the differential inhibition hypothesis and may be relevant to understanding cocaine addiction.

Chronic ethanol exposure during adolescence increases basal dopamine in the nucleus accumbens septi during adulthood

BACKGROUND

In humans, adolescent exposure to alcohol is associated with the onset of adult alcohol dependency and suggests that early use potentiates vulnerability to addiction. The aim of the present study was to address whether chronic ethanol exposure during adolescence would alter nucleus accumbens septi (NAcc) dopamine (DA) levels in the adult brain.

METHODS

Rats were injected daily from postnatal day (PND) 30 to 50 with either 0.75 g/kg/i.p. ethanol or saline followed by an ethanol-abstinent period from PND 51 to 65. Changes in extracellular DA levels in the anterior NAcc shell were measured via the no net flux (NNF) paradigm.

RESULTS

Extracellular DA levels were greater in rats chronically treated with ethanol during adolescence (6.5 nM DA) in comparison with saline-exposed controls (3.6 nM DA). There were no differences in extraction fraction (E(d)), an indirect measure of DA reuptake, between ethanol-treated (87%) and nontreated (68%) rats.

CONCLUSIONS

Together these findings suggest that changes in extracellular DA may be an underlying physiological mechanism in adolescent vulnerability to the rewarding properties of ethanol.

Adolescents differ from adults in cocaine conditioned place preference and cocaine-induced dopamine in the nucleus accumbens septi

In humans, adolescent exposure to illicit drugs predicts the onset of adult drug abuse and suggests that early drug use potentiates vulnerability to drug addiction. Cocaine conditioned place preferences were measured in early adolescent [postnatal day (PND) 35], late adolescent (PND 45) and young adult (PND 60) rats by injecting either 0, 5 or 20 mg/kg cocaine and conditioning them to environmental cues. Cocaine preferences were found for all ages at the high dose. PND 35s were the only age group to have a preference at the low dose. To address whether age-related differences in cocaine place preferences were related to differences in the mesolimbic dopaminergic system, we measured extracellular dopamine levels in the nucleus accumbens septi of PND 35, PND 45 and PND 60 rats via quantitative microdialysis under transient conditions. Rats were injected daily with either 5 mg/kg/ip or saline for 4 days and surgically implanted with a microdialysis probe aimed at the nucleus accumbens. Rats were perfused with either 0, 1, 10 or 40 nM dopamine and the extracellular dopamine concentration was measured. Our results show that adolescents differ from adults in basal dopamine. All cocaine treated rats, regardless of age, showed a significant increase in dopamine over baseline in response to a cocaine challenge. Additionally, there were age-related differences in the extraction fraction (E(d)), an indirect measure of dopamine reuptake. Together these findings suggest ontogenetic differences in extracellular dopamine and dopamine reuptake and that these differences may provide an explanation for adolescent vulnerability to addiction.

Paradoxical effects of prodynorphin gene deletion on basal and cocaine-evoked dopaminergic neurotransmission in the nucleus accumbens

Quantitative and conventional microdialysis were used to investigate the effects of constitutive deletion of the prodynorphin gene on basal dopamine (DA) dynamics in the nucleus accumbens (NAc) and the responsiveness of DA neurons to an acute cocaine challenge. Saline- and cocaine-evoked locomotor activity were also assessed. Quantitative microdialysis revealed that basal extracellular DA levels were decreased, while the DA extraction fraction, an indirect measure of DA uptake, was unchanged in dynorphin (DYN) knockout (KO) mice. The ability of cocaine to increase NAc DA levels was reduced in KO. Similarly, cocaine-evoked locomotor activity was decreased in KO. The selective kappa opioid receptor agonist U-69593 decreased NAc dialysate DA levels in wildtype mice and this effect was enhanced in KO. Administration of the selective kappa opioid receptor (KOPr) antagonist nor-binaltorphimine to KO mice attenuated the decrease in cocaine-induced DA levels. However, it was ineffective in altering the decreased locomotor response to cocaine. These studies demonstrate that constitutive deletion of prodynorphin is associated with a reduction of extracellular NAc DA levels and a decreased responsiveness to acute cocaine. Data regarding the effects of U-69593 and nor-binaltorphimine in KO suggest that the kappa opioid receptor is up-regulated as a consequence of prodynorphin gene deletion and that this adaptation underlies the decrease in basal DA dynamics and cocaine-evoked DA levels observed in DYN KO mice. These findings suggest that the phenotype of DYN KO mice is not solely due to loss of endogenous opioid peptide but also reflects developmental compensations that occur at the level of the opioid receptor.

Reduced dopamine terminal function and insensitivity to cocaine following cocaine binge self-administration and deprivation

Despite large numbers of studies describing neuroadaptations caused by chronic cocaine exposure, there remains considerable uncertainty as to whether alterations in dopamine (DA) neurotransmission are responsible for progression into an addicted state. High-intake, 24-h access cocaine self-administration (SA, 10 days) followed by an extended (7 days), but not 1 day deprivation period produces an increased motivation to SA cocaine as measured by a progressive ratio protocol. Following binge cocaine SA and deprivation, the status of DA terminals in the nucleus accumbens (NAc) was investigated using microdialysis in freely moving rats and voltammetry in brain slices. At 1 and 7 days following binge cocaine SA, baseline extracellular DA concentrations in the NAc core were decreased by 40 and 55% of control levels, in the 1 and 7 day deprivation groups, respectively. Acute cocaine (1.5 mg/kg, i.v.) administration increased extracellular DA (350%) in the NAc core of naïve animals but failed to significantly increase DA at 1 or 7 days following binge cocaine SA. The shell of the NAc showed a similar lack of effect of cocaine. Analysis of DA terminals in brain slices showed that cocaine was markedly less effective in inhibiting DA uptake at 1 and 7 days of cocaine deprivation (max effect 40% of control). Electrically stimulated DA release was decreased at 1 day and further decreased at 7 days of deprivation (67 and 49% of control, respectively). The rate of DA uptake was increased (150% of control) following binge SA, irrespective of deprivation period. Finally, presynaptic autoreceptors were subsensitive at both time points, as measured by the ability of quinpirole, a D2-like DA receptor agonist, to inhibit DA release. Thus, the NAc was hypodopaminergic and DA terminals were less sensitive to cocaine following binge cocaine SA and deprivation.

Preferentially impaired neurotransmitter release sites not their discreteness compromise the validity of microdialysis zero-net-flux method

Intracerebral microdialysis is a popular technique for studying neurochemistry and neural circuits in various brain regions. Recent studies called into question the validity of the microdialysis zero-net-flux (ZNF) method by suggesting that this method significantly underestimates the basal level of extracellular dopamine as a result of the discreteness of dopamine release sites as well as the preferential damage to dopamine release over uptake. To identify which factor is most important in undermining the microdialysis ZNF measurements and the extent of underestimation, two mathematical models were developed to explore the influences of the discrete nature and the probe-induced impairment in the neurotransmitter release. The two models differ in their characterizations of the transmitter release as spatially discrete and homogeneous, respectively. Simulations using physiologically reasonable parameters for striatal dopamine systems indicate that the preferential release site damage surrounding the implanted probe is the most important determinant to the underestimation of the microdialysis ZNF concentration. Under normal physiological conditions, the discreteness of neurotransmitter release sites is of minor importance, except when neuronal degeneration occurs. It is concluded that homogeneous models can adequately describe microdialysis operating processes as long as the corresponding tissue damage parameters in such models are appropriately incorporated.

Long-lasting alterations of the mesolimbic dopamine system after periadolescent ethanol drinking by alcohol-preferring rats

BACKGROUND

This study tested the hypothesis that ethanol consumption by alcohol-preferring (P) rats during the periadolescent period causes persistent alterations in the mesolimbic dopamine (DA) system. After ethanol drinking during periadolescence, P rats were examined for alterations in basal locomotor activity, changes in extracellular DA levels and extraction fraction in the nucleus accumbens (NAc) by using no-net-flux (NNF) microdialysis, and changes in the response of the mesolimbic DA system to ethanol.

METHODS

Male P rat pups were given 24-hr free-choice access to 15% (v/v) ethanol from postnatal day (PD) 30 through PD 60. On PD 70, rats were assessed for locomotor activity. On PD 70 to 80, rats were implanted with bilateral guide cannulas aimed above the NAc. After at least 5 days, microdialysis probes were inserted bilaterally; on the following day, NNF microdialysis experiments were conducted. On the day after the NNF experiment, conventional microdialysis experiments were conducted to measure extracellular levels of DA in response to intraperitoneal injection of saline or ethanol 2.5 g/kg.

RESULTS

Compared with the ethanol-naive group, ethanol drinking by P rats during periadolescence did not alter basal locomotor activity, nor did it alter the basal extracellular concentration of DA. There was, however, a significant increase in the extraction fraction of DA of ethanol-drinking animals relative to the controls (57.4 +/- 2.7% and 45.8 +/- 2.3%, respectively). Additionally, compared with controls, P rats with exposure to ethanol during the periadolescent period showed a prolonged increase in the extracellular levels of DA after a challenge dose of ethanol.

CONCLUSIONS

The results of the microdialysis experiments suggest that periadolescent ethanol drinking by P rats increases basal DA neurotransmission (as indicated by higher DA clearance while maintaining the same extracellular DA concentrations) and prolongs the response of DA neurotransmission to ethanol.

Repeated cocaine injections have no influence on tyrosine hydroxylase activity in the rat nucleus accumbens core or shell

Numerous reports have demonstrated augmented cocaine-evoked release of dopamine in the nucleus accumbens of rats pre-treated with cocaine. However, the extent to which repeated cocaine injections affect basal levels of dopamine is unclear. There have been reports of increases, decreases, or no change in basal levels of extracellular accumbal dopamine resulting from repeated psychostimulant administration. The present study assessed the activity of tyrosine hydroxylase, the rate-limiting enzyme in dopamine synthesis, in the nucleus accumbens following either acute or repeated cocaine administration. The in vivo microdialysis technique was used to measure accumulation of the dopamine precursor DOPA following intra-accumbal administration of the DOPA decarboxylase inhibitor NSD 1015 through the microdialysis probe. This method provides an estimate of tyrosine hydroxylase activity within the nucleus accumbens. Results indicate that neither acute nor repeated cocaine administration produced any change in DOPA accumulation in either the nucleus accumbens shell or core. These data indicate that dopamine synthesis is not altered by cocaine administration.

Implications of protein kinase C in the nucleus accumbens in the development of sensitization to methamphetamine in rats

Repeated treatment with methamphetamine leads to an enhancement in the methamphetamine-induced dopamine release and its related behaviors. This phenomenon is called sensitization or reverse tolerance. Protein kinase C (PKC) controls numerous signaling cascades by virtue of its ability to phosphorylate target proteins that include other kinases. The purpose of study was then to investigate the implication of PKC in the development of sensitization to the rewarding effect and to the extracellular dopamine release induced by methamphetamine in rats. The conditioned place preference paradigm and in vivo microdialysis assay were performed in the present study. An intra-nucleus accumbens injection of a selective PKC inhibitor chelerythrine chloride abolished the enhancement of the methamphetamine-induced place preference following repeated treatment with methamphetamine. Furthermore, intra-nucleus accumbens injection of chelerythrine chloride blocked the development of sensitization to dopamine release and to the decrease in the major dopamine metabolites, 3'4-dihydroxyphenylacetic acid and homovanillic acid, in the nucleus accumbens induced by repeated methamphetamine treatment. Under these conditions, the immunoreactivity of the cytosolic phosphorylated conventional- or classic-type PKC in the limbic forebrain region including the nucleus accumbens was slightly, but significantly increased in methamphetamine-sensitized rats. The present data provide evidence for the implication of PKC in the nucleus accumbens in the development of sensitization to the methamphetamine-induced rewarding effect, dopamine release and inhibition of dopamine metabolism/re-uptake in rats.

Neuroadaptive changes in the mesoaccumbens dopamine system after chronic nicotine self-administration: A microdialysis study

There is little evidence to date to indicate if mesoaccumbens dopamine function at the neurochemical level is altered during early abstinence from chronic i.v. nicotine self-administration. Thus, a quantitative microdialysis (no-net-flux) approach was used to measure basal extracellular concentrations and extraction fractions of dopamine in the nucleus accumbens (ACB) of rats that self-administered nicotine i.v. for 25 days, as well as in rats serving as yoked comparison groups (yoked nicotine and yoked saline). After 24-48 h of the final self-administration session, there was a significant reduction in basal extracellular dopamine levels in the ACB of the self-administration group compared with the yoked saline group (1.35+/-0.15 nM versus 3.70+/-0.28 nM). The basal extracellular dopamine levels in the yoked nicotine group (1.46+/-0.20 nM) were not significantly different compared with the nicotine self-administration group. The in vivo extraction fraction of dopamine, an indirect measure of dopamine uptake, was significantly increased in the nicotine self-administration (86%) and yoked nicotine (91%) groups compared with the yoked saline group (77%). In addition, a marked reduction in the elevation of extracellular dopamine levels in the ACB occurred after a nicotine challenge as measured by conventional microdialysis in the self-administration (112% of basal) and yoked nicotine (121% of basal) groups as compared with a yoked saline (154% of basal) group. The reduced basal ACB dopamine levels in the nicotine groups during early abstinence appears to be due to increased clearance, suggesting increased dopamine uptake is occurring as a result of the chronic nicotine treatment. The reduced elevation of extracellular dopamine levels in the ACB upon nicotine challenge suggests a functional desensitization or downregulation phenomenon involving acetylcholine receptors (nicotinic nAChRs). Overall, these results provide clear evidence for a neuroadaptive change that alters dopamine transmission in the ACB during abstinence from chronic i.v. nicotine exposure.

Enhanced rewarding properties of morphine, but not cocaine, in beta(arrestin)-2 knock-out mice

The reinforcing and psychomotor effects of morphine involve opiate stimulation of the dopaminergic system via activation of mu-opioid receptors (muOR). Both mu-opioid and dopamine receptors are members of the G-protein-coupled receptor (GPCR) family of proteins. GPCRs are known to undergo desensitization involving phosphorylation of the receptor and the subsequent binding of beta(arrestins), which prevents further receptor-G-protein coupling. Mice lacking beta(arrestin)-2 (beta(arr2)) display enhanced sensitivity to morphine in tests of pain perception attributable to impaired desensitization of muOR. However, whether abrogating muOR desensitization affects the reinforcing and psychomotor properties of morphine has remained unexplored. In the present study, we examined this question by assessing the effects of morphine and cocaine on locomotor activity, behavioral sensitization, conditioned place preference, and striatal dopamine release in beta(arr2) knock-out (beta(arr2)-KO) mice and their wild-type (WT) controls. Cocaine treatment resulted in very similar neurochemical and behavioral responses between the genotypes. However, in the beta(arr2)-KO mice, morphine induced more pronounced increases in striatal extracellular dopamine than in WT mice. Moreover, the rewarding properties of morphine in the conditioned place preference test were greater in the beta(arr2)-KO mice when compared with the WT mice. Thus, beta(arr2) appears to play a more important role in the dopaminergic effects mediated by morphine than those induced by cocaine.

Basal and morphine-evoked dopaminergic neurotransmission in the nucleus accumbens of MOR- and DOR-knockout mice

Conventional and no net flux microdialysis were used to quantify basal and morphine-induced extracellular dopamine (DA) levels and the basal extraction fraction, which provides an estimate of the rate of DA uptake, in the nucleus accumbens (NAc) of wild-type mice and those with a constitutive deletion of mu (MOR)- or delta (DOR)-opioid receptors. Locomotor activity was assessed in these same animals. No difference between genotypes in basal dialysate DA levels was seen. No net flux studies revealed significant decreases in the DA extraction fraction in both MOR- and DOR-knockout mice, indicating decreased basal DA uptake in both genotypes. Extracellular DA, however, was unchanged. Because extracellular neurotransmitter levels are determined by the dynamics of both release and uptake, these findings provide suggestive evidence that basal DA release is decreased in mutant mice. Systemic administration of morphine significantly increased locomotor activity and dialysate DA levels in wild-type mice. MOR-knockout mice failed to exhibit a behavioural response to morphine. The ability of morphine to increase DA levels, however, was reduced but not prevented. No alteration in the effects of morphine was observed in DOR-knockout mice. These data provide genetic evidence for the existence of tonically active MOR and DOR systems that modulate basal DA neurotransmission in the NAc. Furthermore, they demonstrate that in contrast to the locomotor-activating effects of morphine, a small component of morphine-evoked DA release occurs independently of MOR activation.

Glutamatergic Agents for Cocaine Dependence

Effective medications for cocaine dependence are needed to improve outcome in this chronic, relapsing disorder. Medications affecting glutamate function are reasonable candidates for investigation, given the involvement of glutamate circuits in reward-related brain regions and evidence of cocaine-induced glutamatergic dysregulation. In addition, it is increasingly apparent that glutamatergic mechanisms underlie several clinical aspects of cocaine dependence, including euphoria, withdrawal, craving, and hedonic dysfunction. Even denial, traditionally viewed as purely psychological, may result, in part, from dysfunctional glutamate-rich cortical regions. We review the involvement of glutamate in reward-related circuits, the acute and chronic effects of cocaine on these pathways, and glutamatergic mechanisms that contribute to the neurobiology of cocaine dependence. We also present preliminary data from our research of modafinil, a glutamate-enhancing agent with promise in the treatment of cocaine-addicted individuals.

Rapid regulation of dopamine transporter function by substrates, blockers and presynaptic receptor ligands

The extracellular actions of dopamine are terminated primarily through its binding to dopamine transporters and translocation back into dopamine neurons. The transporter thereby serves as an optimal target to regulate dopamine neurotransmission. Although acute pharmacological blockade of dopamine transporters is known to reversibly inhibit transporter function by preventing the binding of its endogenous substrate dopamine, it recently has become clear that dopamine transporter substrates, such as amphetamines, and blockers, such as cocaine, also have the ability to rapidly and persistently regulate transporter function after their direct pharmacological effect has subsided. Presynaptic receptor ligands can also regulate dopamine transporter function. This has been investigated most extensively for dopamine D2 receptors, but there is also evidence for regulation by gamma-aminobutyric acid (GABA) GABAB receptors, metabotropic glutamate, nicotinic acetylcholine, serotonin, sigma2- and kappa-opioid receptors. The focus of this review is the rapid, typically reversible, regulation of dopamine transporter velocity by substrates, blockers and presynaptic receptor ligands. The research discussed here suggests that a common mechanism through which these different classes of compounds regulate transporter activity is by altering the cell surface expression of dopamine transporters.

Morphine acutely regulates opioid receptor trafficking selectively in dendrites of nucleus accumbens neurons

Morphine stimulates the internalization of mu-opioid receptors (MORs) in transfected cell models to a lesser degree than opioid peptides and other analgesic drugs, such as methadone, and previous studies have reported that morphine does not produce a detectable redistribution of MORs in neural tissue after either acute or chronic administration. Nevertheless, morphine produces profound physiological effects, raising the question of whether receptor trafficking plays any role in the in vivo actions of morphine. We investigated the effects of opiate drugs on recombinant and native opioid receptors in the nucleus accumbens, which plays an important role in mediating the behavioral effects of opiate drugs. Morphine and methadone differed in their effects on the internalization of epitope-tagged MORs in cell bodies, introduced by viral gene transfer and imaged by fluorescence microscopy. A mutation of the cytoplasmic tail that confers morphine-induced internalization in cultured cells had a similar effect on receptor trafficking in nucleus accumbens cell bodies. Surprisingly, in contrast to its failure to affect MOR distribution detectably in cell bodies, acute morphine administration produced a pronounced change in MOR distribution visualized in the processes of the same neurons. A similar effect of acute morphine administration was observed for endogenously expressed MORs by immunoelectron microscopy; the acute administration of morphine increased the density of MORs associated with internal membrane structures specifically in dendrites. These results provide the first evidence that morphine regulates the distribution of MORs in neuronal processes, suggesting that "compartment-selective" membrane trafficking represents a previously unanticipated type of opioid receptor regulation contributing to the in vivo effects of opiate drugs on a physiologically relevant population of CNS neurons.

Enhanced responsiveness to novelty and cocaine is associated with decreased basal dopamine uptake and release in the nucleus accumbens: quantitative microdialysis in rats under transient conditions

Male rats were screened for their response to a novel environment and designated as high responders (HRs) or low responders (LRs). They then received daily injections of saline or cocaine (20 mg/kg, i.p.). Basal and cocaine-evoked extracellular dopamine (DA(ext)) levels as well as basal DA uptake rate and cocaine-evoked inhibition of uptake in the nucleus accumbens were determined on abstinence day 3 using quantitative microdialysis under transient conditions. The kinetics of uptake, dopamine transporter (DAT) expression, and [(3)H](-)-2-beta-carbomethoxy-3-beta-(4-fluorophenyl)tropane 1,5-naphthalenedisulfonate ([(3)H]WIN35428) binding were also examined. The locomotor activating effects of cocaine and the magnitude of behavioral sensitization were greater in HRs. Saline-treated HRs had lower basal uptake than LRs. DA uptake after cocaine challenge was also lower in these animals. Although basal DA(ext) did not differ, cocaine-evoked DA(ext) was greater in HRs. The K(m) and V(max) of DA uptake were higher in naive HRs than LRs, as were the K(d) and B(max) of [(3)H]WIN35428 binding. DAT protein expression did not differ. Previous cocaine exposure decreased basal DA uptake. It increased cocaine-evoked DA(ext) and decreased the cocaine-induced inhibition of uptake, especially in HRs, indicating greater DA release during cocaine challenge in this phenotype. We hypothesize that lower basal uptake in HRs results from a decrease in DAT binding affinity that is compensated for, in part, by an increased number of plasma membrane binding sites. Basal uptake, but not DA(ext), was lower in HRs, indicating lower basal DA release in HRs. The finding that cocaine-evoked DA(ext) is higher in naive and cocaine-exposed HRs suggests that the greater responsiveness of DA neurons in HRs may underlie the enhanced behavioral responses that characterize this phenotype.

Individual differences in cocaine-induced locomotor sensitization in low and high cocaine locomotor-responding rats are associated with differential inhibition of dopamine clearance in nucleus accumbens

Behavioral sensitization to cocaine reflects neuroadaptive changes that intensify drug effects. However, repeated cocaine administration does not induce behavioral sensitization in all male Sprague-Dawley rats. Because cocaine inhibits the dopamine (DA) transporter (DAT), we investigated whether altered DAT function contributes to these individual differences. Freely moving rats had electrochemical microelectrode/microcannulae assemblies chronically implanted in the nucleus accumbens so that exogenous DA clearance signals were recorded simultaneous with behavior. The peak DA signal amplitude (A(max)) and efficiency of clearance (k) were used as indices of in vivo DAT function. Low and high cocaine responders (LCRs and HCRs, respectively) were identified based on their locomotor responsiveness to an initial injection of cocaine (10 mg/kg i.p.). Consistent with DAT inhibition, cocaine elevated A(max) and reduced k in HCRs, but not in LCRs. The same dose of cocaine was administered for six additional days and after a 7-day withdrawal. Baseline behavioral and dopamine clearance indices were unaltered by repeated cocaine or after withdrawal. Only LCRs expressed cocaine-induced sensitized locomotor activation, and this was accompanied by cocaine-induced elevations in A(max) and reductions in k. These sensitized responses to cocaine persisted in LCRs after withdrawal. In contrast, neither locomotor nor electrochemical responses were altered by repeated saline administration or a saline challenge after repeated cocaine administration, suggesting that conditioning did not significantly contribute. Our results suggest that increased DAT inhibition by cocaine is associated with locomotor sensitization and that DAT serves as a common substrate for mediating both the initial and sensitized locomotor responsiveness to cocaine.

Behavioural sensitization and enhanced dopamine response in the nucleus accumbens after intravenous cocaine self-administration in mice

The behavioural effects of cocaine are enhanced in animals with a prior history of repeated cocaine administration. This phenomenon, referred to as sensitization, is also associated with an increase in cocaine-evoked extracellular dopamine levels in the nucleus accumbens. Behavioural and neurochemical sensitization has been demonstrated in rats with a prior history of cocaine self-administration and in those that had received experimenter-administered cocaine. Although it is clear that the repeated non-contingent administration also results in behavioural sensitization in the mouse, the issue of whether behavioural and neurochemical sensitization also occur in this species following intravenous cocaine self-administration has not been assessed. The present study used the technique of in vivo microdialysis in conjunction with operant self-administration to characterize cocaine-evoked locomotor activity and dopamine levels in the nucleus accumbens in mice with a prior history of intravenous cocaine self-administration or those that had received yoked infusions of cocaine. Mice that had received contingent or non-contingent infusions of cocaine exhibited an enhanced behavioural response to cocaine and increased cocaine-evoked dopamine levels in the nucleus accumbens. There was no difference between groups in the magnitude of this effect. Prior exposure to cocaine did not modify baseline dopamine levels in the nucleus accumbens. These data demonstrate that mice with previous cocaine self-administration experience show an enhanced behavioural and dopamine response to cocaine in the nucleus accumbens. Furthermore, control over cocaine infusion does not significantly alter the magnitude of the sensitized behavioural and presynaptic dopamine responses observed in response to a challenge dose of cocaine.