Euphorigenic doses of cocaine reduce [123I]beta-CIT SPECT measures of dopamine transporter availability in human cocaine addicts

Dopamine Transporter and Reward Anticipation in a Dimensional Perspective: A Multimodal Brain Imaging Study

Dopamine function and reward processing are highly interrelated and involve common brain regions afferent to the nucleus accumbens, within the mesolimbic pathway. Although dopamine function and reward system neural activity are impaired in most psychiatric disorders, it is unknown whether alterations in the dopamine system underlie variations in reward processing across a continuum encompassing health and these disorders. We explored the relationship between dopamine function and neural activity during reward anticipation in 27 participants including healthy volunteers and psychiatric patients with schizophrenia, depression, or cocaine addiction, using functional magnetic resonance imaging (fMRI) and positron emission tomography (PET) multimodal imaging with a voxel-based statistical approach. Dopamine transporter (DAT) availability was assessed with PET and [11C]PE2I as a marker of presynaptic dopamine function, and reward-related neural response was assessed using fMRI with a modified Monetary Incentive Delay task. Across all the participants, DAT availability in the midbrain correlated positively with the neural response to anticipation of reward in the nucleus accumbens. Moreover, this relationship was conserved in each clinical subgroup, despite the heterogeneity of mental illnesses examined. For the first time, a direct link between DAT availability and reward anticipation was detected within the mesolimbic pathway in healthy and psychiatric participants, and suggests that dopaminergic dysfunction is a common mechanism underlying the alterations of reward processing observed in patients across diagnostic categories. The findings support the use of a dimensional approach in psychiatry, as promoted by the Research Domain Criteria project to identify neurobiological signatures of core dysfunctions underling mental illnesses.

Is Mania the Hypertension of the Mood? Discussion of A Hypothesis

Beyond both being biphasic/bidirectional disorders (hypo)mania and essential hypertension share a surprising number of similarities and an overlap between their genetics, biological background, underlying personality and temperamental factors, precipitating factors, comorbidity and response to treatment, indicating a possibly partially shared biological background. Based on theoretical knowledge, similarities related to characteristics, manifestation and course, and the results of pharmacological studies related to the effects and side effects of pharmacotherapies used in the treatment of these two distinct disorders, the authors outline a hypothesis discussing the similar origins of these two phenomena and thus mania being the hypertension of mood in memory of Athanasios Koukopoulos, one of the greatest researchers and theoreticists of mania of all time.

The role of impulsivity in psychostimulant- and stress-induced dopamine release: Review of human imaging studies

Drug addiction is a debilitating disorder and its pivotal problem is the high relapse rate. To solve this problem, the aim is to prevent people from becoming addicted in the first place. One of the key questions that is still unanswered is why some people become addicted to drugs and others, who take drugs regularly, do not. In recent years extensive research has been done to untangle the many factors involved in this disorder. Here, we review some of the factors that are related to dopamine, i.e., impulsivity and stress (hormones), and aim to integrate this into a neurobiological model. Based on this, we draw two conclusions: (1) in order to understand the transition from recreational drug use to addiction, we need to focus more on these recreational users; and (2) research should be aimed at finding therapies that can restore inhibitory control/frontal functioning and improve stress resiliency in addicts.

The catecholaminergic-cholinergic balance hypothesis of bipolar disorder revisited

Bipolar disorder is a unique illness characterized by fluctuations between mood states of depression and mania. Originally, an adrenergic-cholinergic balance hypothesis was postulated to underlie these different affective states. In this review, we update this hypothesis with recent findings from human and animal studies, suggesting that a catecholaminergic-cholinergic hypothesis may be more relevant. Evidence from neuroimaging studies, neuropharmacological interventions, and genetic associations support the notion that increased cholinergic functioning underlies depression, whereas increased activations of the catecholamines (dopamine and norepinephrine) underlie mania. Elevated functional acetylcholine during depression may affect both muscarinic and nicotinic acetylcholine receptors in a compensatory fashion. Increased functional dopamine and norepinephrine during mania on the other hand may affect receptor expression and functioning of dopamine reuptake transporters. Despite increasing evidence supporting this hypothesis, a relationship between these two neurotransmitter systems that could explain cycling between states of depression and mania is missing. Future studies should focus on the influence of environmental stimuli and genetic susceptibilities that may affect the catecholaminergic-cholinergic balance underlying cycling between the affective states. Overall, observations from recent studies add important data to this revised balance theory of bipolar disorder, renewing interest in this field of research.

Sertraline delays relapse in recently abstinent cocaine-dependent patients with depressive symptoms

AIMS

Whether the selective serotonin re-uptake inhibitor sertraline at 200 mg/day delays relapse in recently abstinent cocaine-dependent individuals.

DESIGN

The study involved a 12-week, double-blind, placebo-controlled clinical trial with 2-week residential stay followed by 10-week out-patient participation.

SETTING

Veterans Affairs residential unit and out-patient treatment research program.

PARTICIPANTS

Cocaine-dependent volunteers (n = 86) with depressive symptoms (Hamilton score > 15), but otherwise no major psychiatric or medical disorder or contraindication to sertraline.

MEASUREMENTS

Participants were housed on a drug-free residential unit (weeks 1-2) and randomized to receive sertraline or placebo. Participants then participated on an out-patient basis during weeks 3-12 while continuing to receive study medication. Patients participated in a day substance abuse/day treatment program during weeks 1-3 and underwent weekly cognitive behavioral therapy during weeks 4-12. The primary outcome measure was thrice-weekly urine results and the secondary measure was Hamilton Depression scores.

FINDINGS

Pre-hoc analyses were performed on those who participated beyond week 2. Generally, no group differences in retention or baseline characteristics occurred. Sertraline patients showed a trend towards longer time before their first cocaine-positive urine ('lapse', χ(2) = 3.67, P = 0.056), went significantly longer before having two consecutive urine samples positive for cocaine ('relapse', χ(2) = 4.03, P = 0.04) and showed significantly more days to lapse (26.1 ± 16.7 versus 13.2 ± 10.5; Z = 2.89, P = 0.004) and relapse (21.3 ± 10.8 versus 32.3 ± 14.9; Z = 2.25, P = 0.02). Depression scores decreased over time (F = 43.43, P < 0.0001), but did not differ between groups (F = 0.09, P = 0.77).

CONCLUSIONS

Sertraline delays time to relapse relative to placebo in cocaine-dependent patients who initially achieve at least 2 weeks of abstinence.

Striatal dopamine transporter availability in unmedicated bipolar disorder

OBJECTIVES

Dopamine transmission abnormalities have been implicated in the etiology of bipolar disorder (BPD). However, there is a paucity of receptor imaging studies in BPD, and little information is available about the dopamine system in BPD. Reuptake of synaptic dopamine by the dopamine transporter (DAT) is the principal mechanism regulating dopamine neurotransmission, and is often used as a marker for presynaptic dopamine function. This positron emission tomography (PET) study investigated whether DAT availability differed between BPD and healthy control subjects.

METHODS

A total of 11 unmedicated BPD patients in either the euthymic or depressed phase and 13 closely matched healthy subjects underwent PET imaging with the DAT-selective radiotracer [(11) C]CFT and a structural magnetic resonance imaging (MRI) scan. Striatal binding potential (BP(ND) ) was estimated using the multilinear reference tissue model. Region of interest and analyses were conducted to test for differences in [(11) C]CFT BP(ND) between groups.

RESULTS

Unmedicated BPD subjects had significantly lower DAT availability relative to healthy controls in bilateral dorsal caudate.

CONCLUSIONS

The results of this study support the hypothesis that there are abnormalities in the dopaminergic system in BPD, and suggest that DAT availability may be related to the neuropathology of BPD. Future studies are needed to determine if DAT availability cycles with disease phase.

Pharmacological aspects of the combined use of 3,4-methylenedioxymethamphetamine (MDMA, ecstasy) and gamma-hydroxybutyric acid (GHB): a review of the literature

Epidemiological studies show that the use of club drugs is on the rise. Furthermore, the last few decades have seen a rise in patterns of polydrug use. One of the combinations frequently used is ecstasy (MDMA) with gammahydroxybutyrate (GHB). For effective prevention it is important to be aware of this phenomenon and of the pharmacology of these drugs. The effects of the combination extend to different neurotransmitter systems, including serotonin, dopamine and noradrenaline. Studies investigating the effects of combinations of psychoactive substances are limited. In this review we describe the subjective effects of the MDMA/GHB combination. Furthermore, we review the individual actions of MDMA on serotonin, dopamine and noradrenaline systems. In addition, actions of GHB on these systems are discussed as a possible pharmacological basis for the interaction of both drugs. It is postulated that GHB attenuates the unpleasant or dysphoric effects of MDMA by its effect on the central dopaminergic system.

Addiction and substance abuse in anesthesiology

Despite substantial advances in our understanding of addiction and the technology and therapeutic approaches used to fight this disease, addiction still remains a major issue in the anesthesia workplace, and outcomes have not appreciably changed. Although alcoholism and other forms of impairment, such as addiction to other substances and mental illness, impact anesthesiologists at rates similar to those in other professions, as recently as 2005, the drug of choice for anesthesiologists entering treatment was still an opioid. There exists a considerable association between chemical dependence and other psychopathology, and successful treatment for addiction is less likely when comorbid psychopathology is not treated. Individuals under evaluation or treatment for substance abuse should have an evaluation with subsequent management of comorbid psychiatric conditions. Participation in self-help groups is still considered a vital component in the therapy of the impaired physician, along with regular monitoring if the anesthesiologist wishes to attempt reentry into clinical practice.

An Open-Label Pilot Study of Risperidone in the Treatment of Methamphetamine Dependence

Psychopharmacological treatments for methamphetamine (MA) dependence have questionable efficacy. Open-label risperidone was evaluated in veterans seeking MA dependence treatment. Participants (N = 11) received four weeks of risperidone. They provided weekly self-reports of substance use, urine drug screens, and adverse effects. Neuropsychological assessments and psychiatric symptomatology (Brief Symptom Inventory; BSI) were measured at baseline and follow-up. The eight completers had an average risperidone dose of 3.6 mg/day and decreased days of MA use during the trial from a mean of 13.0 (SD = 6.5) in the 30 days prior to starting risperidone to a mean of 0.125 (SD = 0.4; t = 5.7, p = .001), When measured over time, fine motor function (Grooved Peg Board Dominant Hand) was the only neuropsychological domain to improve significantly. No other domain changed significantly from baseline to follow-up among study completers. BSI data were converted to demographically corrected T-scores utilizing appropriate normative data (mean = 50, SD = 10). BSI somatization T-scores declined from a mean of 59.0 (SD = 8.4) to 51.8 (SD = 8.3; t = 2.7, p <.05), and positive symptom distress declined from a mean of 52.8 (SD =8.0) to 41.7 (SD = 8.6; t= 3.0, p <.05). Risperidone was well tolerated and associated with decreased MA use.

Applications of Clinical Dopamine Imaging

Recent technologic advances make it increasingly possible to image neurotransmitter systems in living human brain, The dopamine system has been most intensively studied owing to its involvement in several brain disorders such as Parkinson's disease and Huntington's disease, as well as psychiatric disorders such as schizophrenia, depression, and compulsive behavioral disorders of multiple types. A variety of aspects of dopamine receptor density, function, and dopaminergic terminal status can now be assessed using the minimally invasive neuroimaging techniques of positron emission tomography and single-photon emission computed tomography. Although these techniques are currently used most often in the context of research, clinical applications are rapidly emerging.

A paradigm to investigate the self-regulation of cocaine administration in humans

INTRODUCTION

Current laboratory paradigms of human cocaine administration generally dictate the timing of drug access in ways that may limit assessing aspects of cocaine-taking behavior. Patient-controlled analgesia (PCA) methods, which allow individuals less restricted access to narcotic (i.e., opiate) analgesics, have proven safe and clinically effective for self-regulated treatment of pain. The current study assessed the feasibility, safety, and validity of a model of ad libitum cocaine self-administration, in which participants self-selected the timing of cocaine infusions, using PCA techniques.

METHODS

Eight nontreatment seeking, otherwise medically healthy, experienced cocaine users participated in a double-blind, placebo-controlled, escalating-dose regimen of intravenous cocaine (0, 8, 16, and 32 mg per 70 kg) on 4 test days, during which time participants had 2 h of access to cocaine via manual presses of a corded PCA pump button under a fixed ratio 1: time-out 5-min schedule.

RESULTS

Procedures were well-tolerated by participants, and no significant adverse events were noted. Measures of cocaine self-administration (e.g., number of responses and interinfusion intervals) indicated a significant main effect of cocaine dose, consistent with predicted dose-response relationships (i.e., decreasing responses and increasing interinfusion intervals with increasing injection dose). Participants appeared to regulate their cocaine intake in a carefully controlled manner, using considerably less cocaine (about half) that permitted by pump loading, PCA parameters, and session duration.

CONCLUSIONS

Data from this study support the validity of our PCA paradigm. Moreover, results suggest the apparent feasibility and safety of allowing experienced users to self-select the timing of cocaine infusions to intervals as short as 5 min. Such procedures may enhance our ability to identify effective pharmacological treatments for cocaine addiction.

Implications of chronic methamphetamine use: a literature review

Methamphetamine (MA) abuse is increasing to epidemic proportions, both nationally and globally. Chronic MA use has been linked to significant impairments in different arenas of neuropsychological function. To better understand this issue, a computerized literature search (PubMed, 1964-2004) was used to collect research studies examining the neurobiological and neuropsychiatric consequences of chronic MA use. Availability of MA has markedly increased in the United States due to recent technological improvements in both mass production and clandestine synthesis, leading to significant public health, legal, and environmental problems. MA intoxication has been associated with significant psychiatric and medical comorbidity. Research in animal models and human subjects reveals complicated mechanisms of neurotoxicity by which chronic MA use affects catecholamine neurotransmission. This pathology may underlie the characteristic cognitive deficits that plague chronic MA users, who experience impairments in memory and learning, psychomotor speed, and information processing. These impairments have the potential to compromise, in turn, the ability of MA abusers to engage in, and benefit from, psychosocially based chemical-dependency treatment. Development of pharmacological interventions to improve these cognitive impairments in this population may significantly improve the degree to which they may be able to participate in treatment. Atypical antipsychotics may have some promise in this regard.

Evidence for alterations in α2-adrenergic receptor sensitivity in rats exposed to repeated cocaine administration

It is well established that cocaine stimulates monoamine transmission by blocking reuptake of norepinephrine (NE), dopamine and serotonin into nerve cells, yet few investigations have addressed the effects of chronic cocaine on NE function. In the present study, we examined the effects of repeated cocaine injections on neuroendocrine responses evoked by the alpha2-adrenergic receptor agonist, clonidine. Previous findings show that clonidine increases pituitary growth hormone (GH) secretion by a central mechanism involving postsynaptic alpha2-adrenergic receptors. Male rats previously fitted with indwelling jugular catheters received two daily injections of cocaine (15 mg/kg, i.p.) or saline for 7 days. At 42 h and 8 days after treatment, rats were challenged with clonidine (25 microg/kg, i.v.) or saline, and serial blood samples were withdrawn. Plasma GH and corticosterone levels were measured by radioimmunoassay. Prior cocaine exposure did not affect basal levels of either hormone. However, cocaine-pretreated rats displayed a significant reduction in clonidine-evoked GH secretion at 42 h, and this blunted response was still apparent 8 days later. Corticosterone responses produced by clonidine were similar regardless of pretreatment. The present data suggest that withdrawal from repeated cocaine injections may be accompanied by desensitization of postsynaptic alpha2-adrenoreceptors coupled to GH secretion. Since human patients with depression often exhibit blunted GH responses to clonidine, our findings provide evidence that cocaine withdrawal might produce depressive-like symptoms via dysregulation of NE mechanisms.

Neuroimaging in drug abuse

Neuroimaging techniques, including positron emission tomography (PET), are ideally suited for studies of addiction. These minimally invasive modalities yield information about acute and long-term drug-induced structural and functional changes in the brain over time. Changes can be observed in the brains of human and animal subjects during drug self-administration. Neuroimaging with PET allows precise quantification and visualization of the drug and its rates of movement in the body. In addition, imaging reveals recovery of function and reappearance of neuronal markers in abstinent drug users. Evidence that suggests that PET may have use in identifying individuals predisposed to become addicted is emerging. Finally, candidate pharmacotherapies for drug addiction can be critically evaluated. These unique assets clearly point to the use of these strategies for addiction studies.

Biological markers of cocaine addiction: implications for medications development

The search for effective medications for cocaine addiction has been elusive. The failure to find such medications so far could be due to poor understanding of the underlying biology both in the premorbid condition and following the disease state of chronic cocaine use. Population heterogeneity could be a major factor in response to medications. In an attempt to highlight the issue of biomarkers we reviewed physiological, neuroendocrine and neuroimaging studies to identify specific biological changes/markers that could be used to characterize subgroups among chronic cocaine users. Merging the biology within medications studies of cocaine abusers could prove useful for targeting specific pharmacological agents to subgroups of patients, prediction of response to medication and relapse to use.

Cocaine abusers have an overexpression of alpha-synuclein in dopamine neurons

Alpha-synuclein is a presynaptic protein that has been implicated as a possible causative agent in the pathogenesis of Parkinson's disease. The native protein is a major component of nigral Lewy bodies in Parkinson's disease, and full-length alpha-synuclein accumulates in Lewy neurites. Here we present evidence that alpha-synuclein levels are elevated in midbrain dopamine (DA) neurons of chronic cocaine abusers. Western blot and immunoautoradiographic studies were conducted on postmortem neuropathological specimens from cocaine users and age-matched drug-free control subjects. The results demonstrated that alpha-synuclein levels in the DA cell groups of the substantia nigra/ventral tegmental complex were elevated threefold in chronic cocaine users compared with normal age-matched subjects. The increased protein levels in chronic cocaine users were accompanied by changes in the expression of alpha-synuclein mRNA in the substantia nigra and ventral tegmental area. Although alpha-synuclein expression is prominent in the hippocampus, there was no increase in protein expression in this brain region. The levels of beta-synuclein, a possible negative regulator of alpha-synuclein, also were not affected by cocaine exposure. Alpha-synuclein protein levels were increased in the ventral tegmental area, but not the substantia nigra, in victims of excited cocaine delirium who experienced paranoia, marked agitation, and hyperthermia before death. The overexpression of alpha-synuclein may occur as a protective response to changes in DA turnover and increased oxidative stress resulting from cocaine abuse. However, the accumulation of alpha-synuclein protein with long-term cocaine abuse may put addicts at increased risk for developing the motor abnormalities of Parkinson's disease.

Radiopharmaceuticals for single-photon emission computed tomography brain imaging

In the past 10 years, significant progress on the development of new brain-imaging agents for single-photon emission computed tomography has been made. Most of the new radiopharmaceuticals are designed to bind specific neurotransmitter receptor or transporter sites in the central nervous system. Most of the site-specific brain radiopharmaceuticals are labeled with (123)I. Results from imaging of benzodiazepine (gamma-aminobutyric acid) receptors by [(123)I]iomazenil are useful in identifying epileptic seizure foci and changes of this receptor in psychiatric disorders. Imaging of dopamine D2/D3 receptors ([(123)I]iodobenzamide and [(123)I]epidepride) and transporters [(123)I]CIT (2-beta-carboxymethoxy-3-beta(4-iodophenyl)tropane) and [(123)I]FP-beta-CIT (N-propyl-2-beta-carboxymethoxy-3-beta(4-iodophenyl)-nortropane has proven to be a simple but powerful tool for differential diagnosis of Parkinson's and other neurodegenerative diseases. A (99m)Tc-labeled agent, [(99m)Tc]TRODAT (technetium, 2-[[2-[[[3-(4-chlorophenyl)-8-methyl-8-azabicyclo [3,2,1]oct-2-yl]methyl](2-mercaptoethyl)amino]ethyl]amino] ethanethiolato(3-)]oxo-[1R-(exo-exo)]-), for imaging dopamine transporters in the brain has been successfully applied in the diagnosis of Parkinson's disease. Despite the fact that (123)I radiopharmaceuticals have been widely used in Japan and in Europe, clinical application of (123)I-labeled brain radiopharmaceuticals in the United States is limited because of the difficulties in supplying such agents. Development of (99m)Tc agents will likely extend the application of site-specific brain radiopharmaceuticals for routine applications in aiding the diagnosis and monitoring treatments of various neurologic and psychiatric disorders.

Pharmacokinetic imaging: a noninvasive method for determining drug distribution and action

Advances in positron emission tomography (PET), single photon emission computed tomography (SPECT) and magnetic resonance spectroscopy (MRS), and the ability to label a wide variety of compounds for in vivo use in humans, have created a new technology for making precise physiological and pharmacological measurements. Due to the noninvasive nature of these approaches, repetitive and/or continuous measurements have become possible. Thus far, these techniques have been primarily used for one-time assessments of individuals. However, experience suggests that a major use of this technology will be in the evaluation of new drug therapies. Already, these techniques have been used to measure precisely and noninvasively the pharmacokinetics of a variety of antimicrobial, antineoplastic and CNS agents. In the case of CNS drugs, imaging techniques (particularly PET) have been used to define the classes of neuroreceptors with which the drug interacts. The physiological, pharmacological and biochemical measurements that can be performed noninvasively using modern imaging techniques can greatly facilitate the evaluation of new therapies. These measurements are most likely to be useful during drug development in preclinical studies and in phase I/II human studies. Preclinically, new drugs can be precisely compared with standard therapies, or a series of analogues can be screened for further development on the basis of performance in animal models. In Phase I/II, imaging measurements can be combined with classical pharmacokinetic data to establish optimal administration schedules, evaluate the utility of interventions in specific clinical situations, and aid in the design of Phase III trials.

Dopamine transport function is elevated in cocaine users

Dopaminergic transmission has been suggested to be a primary mechanism mediating reinforcement, withdrawal and craving associated with psychostimulant addiction. Pyscho-stimulants attenuate dopamine transporter (DAT) clearance efficiency, resulting in a net increase in synaptic dopamine levels. Re-uptake rate is determined by the number of functional DAT molecules at the membrane surface. Previous in vivo imaging studies in humans and in vitro studies in post-mortem human brain have demonstrated that chronic cocaine abuse results in a neuroadaptive increase in DAT-binding site density in the limbic striatum. Whether this increase in DAT availability represents an increase in the functional activity of the transporter is unknown. Here, we present evidence that DAT function is elevated by chronic cocaine abuse. The effect of increasing post-mortem interval on the functional viability of synaptosomes was modeled in the baboon brain. Baboon brains sampled under conditions similar to human brain autopsies yielded synaptosomal preparations that were viable up to 24 h post-mortem. Dopamine (DA) uptake was elevated twofold in the ventral striatum from cocaine users as compared to age-matched drug-free control subjects. The levels of [3H]DA uptake were not elevated in victims of excited cocaine delirium, who experienced paranoia and marked agitation prior to death. In keeping with the increase in DAT function, [3H]WIN 35,428 binding was increased in the cocaine users, but not in the victims of excited delirium. These results demonstrate that DA uptake function assayed in cryopreserved human brain synaptosomes is a suitable approach for testing hypotheses of the mechanisms underlying human brain disorders and for studying the actions of addictive drugs in man.

[(11)]Cocaine: PET studies of cocaine pharmacokinetics, dopamine transporter availability and dopamine transporter occupancy

Cocaine was initially labeled with carbon-11 in order to track the distribution and pharmacokinetics of this powerful stimulant and drug of abuse in the human brain and body. It was soon discovered that [(11)C]cocaine was not only useful for measuring cocaine pharmacokinetics and its relationship to behavior but that it is also a sensitive radiotracer for dopamine transporter (DAT) availability. Measures of DAT availability were facilitated by the development of a graphical analysis method (Logan Plot) for reversible systems which streamlined kinetic analysis. This expanded the applications of [(11)C]cocaine to studies of DAT availability in the human brain and allowed the first comparative measures of the degree of DAT occupancy by cocaine and another stimulant drug methylphenidate. This article will summarize preclinical and clinical research with [(11)C]cocaine.

Effects of cocaine prior to and during bupropion maintenance in cocaine-abusing volunteers

The effects of cocaine were examined prior to and during bupropion maintenance in nonopioid-dependent cocaine abusers. Prior to bupropion maintenance, subjects underwent an experimental session during which repeated cocaine doses (0, 50, 100 mg/70 kg) were administered intranasally. Then subjects were maintained on bupropion (150 and 300 mg per day) and underwent experimental sessions as before. Cocaine, regardless of bupropion, produced dose-related increases in several stimulant-like self-reports, performance and cardiovascular measures. Bupropion decreased POMS ratings of friendliness and vigor, regardless of cocaine dose. Bupropion enhanced and attenuated cocaine-induced increases in ratings on the LSD and BG subscales of the ARCI, respectively. These results suggest that bupropion does not alter the acute subjective or cardiovascular effects of cocaine in a robust manner.

Imaging studies on the role of dopamine in cocaine reinforcement and addiction in humans

We summarize our studies with positron emission tomography investigating the role of dopamine (DA) in the reinforcing effects of cocaine and methylphenidate in humans and its involvement in cocaine addiction. These studies have shown that the rate at which cocaine and methylphenidate enter the brain and block the dopamine transporters (DAT) is the variable associated with the 'high', rather than the presence per se of the drug in the brain. Our studies also show that, while the level of DAT blockade is important in predicting the intensity of the 'high' induced by these drugs (DAT blockade > 50% is required for these drugs to induce a 'high'), the rate at which DAT are blocked determines whether the 'high' is perceived or not. Thus, oral methylphenidate, which leads to slow DAT blockade, does not induce a 'high', even at doses which block DAT more than 60%. In cocaine abusers, we have shown significant reductions in DA D2 receptors that are associated with decreased metabolism in cingulate gyrus and in orbitofrontal cortex. We suggest that this is one of the mechanisms by which DA disruption leads to compulsive drug administration in cocaine addiction. Cocaine abusers also show significant decreases in DA release, which coupled with the reduction in D2 receptors may result in decreased activation of reward circuits by physiological reinforcers and may perpetuate cocaine use as a means to compensate for this deficit. Thus, strategies to enhance DA brain function in ways that mimic physiological DA activity may be of help in overcoming cocaine addiction.

Structural and functional neuroimaging findings in substance-related disorders

Intoxication with alcohol results in depressed global glucose metabolism that continues into the stages of withdrawal and abstinence. The decrease in metabolism, however, is not equal across the brain, with certain regions more affected than others. Such a pattern of disturbance suggests that the effect of alcohol on the brain cannot simply be a nonspecific depressant effect secondary to decreased blood flow or glucose transport into the cells but may be related to the dysfunction of the various neurotransmitter systems. Different authors have suggested the dysfunction to be related to the GABAergic, cholinergic, and dopaminergic systems. Long-term alcoholism is associated with atrophy of several brain regions. The frontal lobes and limbic structures seem to be most vulnerable. The data are encouraging with regard to the normalization in brain metabolism and in size of vulnerable brain regions with continued abstinence. In addition to findings of improvement in cognitive functioning and many health parameters, these findings arm clinicians with further data on the benefits of abstinence in the struggle to aid patients in maintaining their sobriety. Several areas remain to be addressed. In particular, clinicians are in need of data, neuroimaging and otherwise, that serve as prognostic indicators, thus allowing patients at higher risk for relapse to be identified and provided with more intensive treatment. A similar need exists for indicators of diagnostic heterogeneity that would guide the development of more highly tailored treatment regimens for identified subgroups of patients. Currently, we have rudimentary knowledge of the gender differences of the effects of alcohol and cocaine on the brain.

Depression and stimulant dependence: neurobiology and pharmacotherapy

Depressive disorder rates in stimulant-dependent individuals are substantially higher than community rates. Further, depressive symptoms are considered a major component of stimulant withdrawal. The comorbidity of these disorders may reflect shared neurochemical alterations in the function of serotonin, dopamine, and peptide systems, such as corticotropin releasing factor (CRF) and neuropeptide Y (NPY). These alterations are observed in patients, and in animal models of depression and stimulant dependence, particularly in limbic brain structures. This shared neurobiology does not seem to result from significant shared heritability or genetic linkage; stimulants may induce changes in neurobiology that are similar to those found in depression, and these changes might provide a therapeutic target. Stimulant-dependent patients with a depressive disorder may be a specific subpopulation for antidepressant trials, and they might reduce their stimulant abuse when treated with antidepressants. Nevertheless, concomitant dependence on alcohol or opioids may influence this response, and antidepressants appear to be more effective for depression in combined stimulant and opioid dependence than in combined stimulant and alcohol dependence.

Assessment of dynamic neurotransmitter changes with bolus or infusion delivery of neuroreceptor ligands

To describe the effect of endogenous dopamine on [11C]raclopride binding, we previously extended the conventional receptor ligand model to include dynamic changes in neurotransmitter concentration. Here, we apply the extended model in simulations of neurotransmitter competition studies using either bolus or bolus-plus-infusion (B/I) tracer delivery. The purpose of this study was (1) to develop an interpretation of the measured change in tracer binding in terms of underlying neurotransmitter changes, and (2) to determine tracer characteristics that maximize sensitivity to neurotransmitter release. A wide range of kinetic parameters was tested based on existing reversible positron emission tomography tracers. In simulations of bolus studies, the percent reduction in distribution volume (deltaV) caused by a neurotransmitter pulse was calculated. For B/I simulations, equilibrium was assumed, and the maximum percent reduction in tissue concentration (deltaC) after neurotransmitter release was calculated. Both deltaV and deltaC were strongly correlated with the integral of the neurotransmitter pulse. The values of deltaV and deltaC were highly dependent on the kinetic properties of the tracer in tissue, and deltaV could be characterized in terms of the tissue free tracer concentration. The value of deltaV was typically maximized for binding potentials of approximately 3 to 10, with deltaC being maximized at binding potentials of approximately 1 to 2. Both measures increased with faster tissue-to-blood clearance of tracer and lower nonspecific binding. These simulations provide a guideline for interpreting the results of neurotransmitter release studies and for selecting radiotracers and experimental design.

Measuring dopamine transporter occupancy by cocaine in vivo: radiotracer considerations

Several recent neuroimaging studies in humans and in monkeys using different radiotracers have reported widely differing values of dopamine transporter (DAT) occupancy by doses of cocaine which are perceived as reinforcing by humans. Here we tested the hypothesis that the measurement of DAT occupancies by drugs with fast pharmacokinetics such as cocaine requires a radioligand with similar kinetics in order to effectively compete with the drug. We measured DAT occupancy by four different doses of cocaine (1.0, 0.5, 0.25, and 0.1 mg/kg) using [11C]d-threo-methylphenidate (a radiotracer which binds rapidly to the DAT in vivo) and compared them to estimates reported previously using [11C]cocaine in the same two baboons and with the same four doses of cocaine [Volkow et al. (1996b) Synapse 24:399-402). Cocaine reduced [11C]d-threo-methylphenidate binding in striatum in a dose-dependent manner, and values were significantly correlated with those obtained previously with [11C]cocaine (r = 0.9, F = 37, P < 0.001). The ED50s (50% occupancy of DAT by cocaine) were 0.27 and 0.17 mg/kg for [11C]d-threo-methylphenidate and [11C]cocaine, respectively. This is significantly lower than values obtained with labeled beta-CIT and other similar radiotracers with a slow uptake and clearance (ED50s: 3-7 mg/kg). We conclude that in vivo measurements of DAT occupancy by rapidly clearing drugs like cocaine requires the use of radiotracers having similar kinetics to the drug itself.

Concentration and occupancy of dopamine transporters in cocaine abusers with [11C]cocaine and PET

The concentration (Bmax) of the dopamine transporter (DAT) and the maximum and effective occupancies by cocaine doses of 0.1 mg/kg or 0.05 mg/kg were measured in the striatum of cocaine abusers (n = 12) by using [11C]cocaine as a radiotracer for the DAT and positron emission tomography (PET). Two methods based on a three-compartment model with one binding site (the nonlinear least squares (NLSQ) and the Farde pseudoequilibrium method) were used to estimate Bmax. Effective occupancies and maximum occupancies were calculated from the distribution volume ratios (DVR) and a three-compartment model, respectively. The NLSQ and Farde methods gave similar values of Bmax (average, 650 +/- 350 pmol/ml and 776 +/- 400 pmol/ml, respectively), but the individual estimates of Bmax were found to be very sensitive to small variations in other model parameters and were not correlated with the parameter Bmax/Kd (r = .07). The average maximum (and effective) occupancies were found to be 67% (50%) and 52% (39%) for the 0.1-mg/kg and the 0.05-mg/kg studies, respectively. The ED50 based on the effective occupancy corresponds to 0.1 mg/kg, which is significantly smaller than the ED50 of 3 mg/kg calculated from studies in which [123]beta-CIT is displaced by cocaine. The effect on the Bmax estimate of two binding sites with different Kd's is also considered by simulation. We conclude (1) that the lack of robustness in the Bmax estimate limits the usefulness of any one subject's Bmax and suggests that the combination parameter Bmax/Kd (or the DVR), which has been used extensively, is a more stable measure of free receptor/transporter concentration. The average Bmax may, however, provide an estimate of the expected concentration in humans. (2) The DVR can be used as a measure of DAT occupancy without applying an explicit model.

Molecular mechanisms of opiate and cocaine addiction

Chronic administration of opiates or cocaine has been shown to alter the activity or expression of diverse types of cellular proteins in specific target neurons within the central nervous system. Prominent examples include signaling proteins, such as receptors, G proteins, second-messenger synthetic enzymes, and protein kinases. It is now increasingly possible to relate particular molecular adaptations to specific behavioral actions of drugs of abuse in animal models of addiction. In addition, recent work has focused on a role for transcription factors, and the associated alterations in gene expression, in mediating part of this long-lasting, drug-induced molecular and behavioral plasticity.

Pharmacology and regulation of the neuronal dopamine transporter

The dopamine transporter, a member of the family of Na+,Cl(-)-dependent transporters, mediates uptake of dopamine into dopaminergic neurons by an electrogenic, Na(+)- and Cl(-)-transport-coupled mechanism. Dopamine and blockers of uptake such as cocaine probably bind to both shared and separate domains on the transporter, which can be influenced dramatically by the presence of cations. Regulation of the dopamine transporter occurs both by chronic occupancy with blocker and by acute effects of D2 dopamine receptors or second messengers such as diacylglycerol (protein kinase C) and arachidonic acid. The dopamine transporter is involved in the uptake of toxins generating Parkinson's disease; it is also an important target for psychostimulant drugs, ligands for in vivo imaging and medications used for neurologic diseases involving changes in the dopamine system.

Model for estimating dopamine transporter occupancy and subsequent increases in synaptic dopamine using positron emission tomography and carbon-11-labeled cocaine

Although increases in dopamine secondary to the inhibition of the dopamine transporter appear to underlie the reinforcing properties of cocaine, there is presently no model that relates the elevation of synaptic dopamine to the transporter occupancy by cocaine. We propose such a model based on positron emission tomographic (PET) measurements of the brain concentration of cocaine and the assumption of rapid equilibrium between free cocaine and cocaine bound to the dopamine transporter. A euphorigenic dose of cocaine (about 40 mg) is predicted to occupy 80-90% of the transporters, while a perceptible dose (about 5 mg) occupies about 40% of the transporters. If reuptake of dopamine is reduced in proportion to the fraction of transporters occupied by cocaine, our model indicates that synaptic dopamine rises supra-linearly with occupancy, so that 5 and 40 mg doses of cocaine give about 2- and 10-fold increases, respectively. A consequence is that a given dose of cocaine produces a similar degree of elevation of dopamine regardless of the prior level of occupation of the transporters by cocaine. This prediction is supported by recent PET/neuropsychological studies in our laboratory where dopamine transporter occupancy was measured after giving methylphenidate intravenously to volunteers; similarly intense "highs" were reported whether the initial occupancy was zero or 75-85%. It could also explain why attempts to block the psychostimulant-induced "high" by pretreating subjects with drugs that block the dopamine transporter have been unsuccessful, and why the use of methylphenidate to treat cocaine addicts led to increased cocaine consumption.