Effects of the novel D1 dopamine receptor agonist ABT-431 on cocaine self-administration and reinstatement

Rationale and Development of Tavapadon, a D1/D5-Selective Partial Dopamine Agonist for the Treatment of Parkinson's Disease

Currently, available therapeutics for the treatment of Parkinson's disease (PD) fail to provide sustained and predictable relief from motor symptoms without significant risk of adverse events (AEs). While dopaminergic agents, particularly levodopa, may initially provide strong motor control, this efficacy can vary with disease progression. Patients may suffer from motor fluctuations, including sudden and unpredictable drop-offs in efficacy. Dopamine agonists (DAs) are often prescribed during early-stage PD with the expectation they will delay the development of levodopa-associated complications, but currently available DAs are less effective than levodopa for the treatment of motor symptoms. Furthermore, both levodopa and DAs are associated with a significant risk of AEs, many of which can be linked to strong, repeated stimulation of D2/D3 dopamine receptors. Targeting D1/D5 dopamine receptors has been hypothesized to produce strong motor benefits with a reduced risk of D2/D3-related AEs, but the development of D1-selective agonists has been previously hindered by intolerable cardiovascular AEs and poor pharmacokinetic properties. There is therefore an unmet need in PD treatment for therapeutics that provide sustained and predictable efficacy, with strong relief from motor symptoms and reduced risk of AEs. Partial agonism at D1/D5 has shown promise for providing relief from motor symptoms, potentially without the AEs associated with D2/D3-selective DAs and full D1/D5-selective DAs. Tavapadon is a novel oral partial agonist that is highly selective at D1/D5 receptors and could meet these criteria. This review summarizes currently available evidence of tavapadon's therapeutic potential for the treatment of early through advanced PD.

Discovery of Selective Dopamine Receptor Ligands Derived from (-)-Stepholidine via C-3 Alkoxylation and C-3/C-9 Dialkoxylation

We evaluated C-3 alkoxylated and C-3/C-9 dialkoxylated (-)-stepholidine analogues to probe the tolerance at the C-3 and C-9 positions of the tetrahydroprotoberberine (THPB) template toward affinity for dopamine receptors. A C-9 ethoxyl substituent appears optimal for D1R affinity since high D1R affinities were observed for compounds that contain an ethyl group at C-9, with larger C-9 substituents tending to decrease D1R affinity. A number of novel ligands were identified, such as compounds 12a and 12b, with nanomolar affinities for D1R and no affinity for either D2R or D3R, with compound 12a being identified as a D1R antagonist for both G-protein- and β-arrestin-based signaling. Compound 23b was identified as the most potent and selective D3R ligand containing a THPB template to date and functions as an antagonist for both G-protein- and β-arrestin-based signaling. Molecular docking and molecular dynamics studies validated the D1R and D3R affinity and selectivity of 12a, 12b, and 23b.

A dopamine D1 agonist vs. methylphenidate in modulating prefrontal cortical working memory

Methylphenidate is used widely to treat symptoms of attention-deficit/hyperactivity disorder (ADHD), but like other stimulants has significant side effects. This study utilized a rodent model (spontaneously hypertensive rat) of spatial working memory (sWM) to compare the effects of methylphenidate with the novel dopamine D₁-like receptor agonist 2-methyldihydrexidine. Acute oral administration of methylphenidate (1.5 mg/kg) caused sWM improvement in half of the tested rats, but impairment in the others. Both improvement or impairment were eliminated by administration of the D₁ antagonist SCH39266 directly into the prefrontal cortex (PFC). Conversely, 2-methyldihydrexidine showed greater sWM improvement compared to methylphenidate without significant impairment in any subject. Its effects correlated negatively with vehicle-treated baseline performance (i.e., rats with lower baseline performance improved more than rats with higher baseline performance). These behavioral effects were associated with neural activities in the PFC. Single neuron firing rate was changed, leading to the alteration in neuronal preference to correct or error behavioral responses. Overall, 2-methyldihydrexidine was superior to methylphenidate in decreasing the neuronal preference, prospectively, in the animals whose behavior was improved. In contrast, methylphenidate, but not 2-methyldihydrexidine, significantly decreased neuronal preference, retrospectively, in those animals who had impaired performance. These results suggest that a D₁ agonist may be more effective than methylphenidate in regulating sWM-related behavior through neural modulation of the PFC, and thus may be superior to methylphenidate or other stimulants as ADHD pharmacotherapy. Significance Statement Methylphenidate is effective in ADHD by its indirect agonist stimulation of dopamine and/or adrenergic receptors, but the precise effects on specific targets are unclear. We compared methylphenidate to a dopamine D₁ receptor-selective agonist by investigating effects on working memory occurring via neural modulation in the prefrontal cortex. The data suggest that pharmacological treatment selectively targeting the dopamine D₁ may offer a superior approach to ADHD pharmacotherapy.

Identification of C10 nitrogen-containing aporphines with dopamine D1 versus D5 receptor selectivity

New aporphines containing C10 nitrogen substituents (viz. nitro, aniline or amide moieties), were synthesized and evaluated for affinity at human serotonin 5-HT_1A and 5-HT_2A receptors and at human dopamine D₁, D₂ and D₅ receptors. Two series of analogs were investigated: series A which contain a sole C10 nitrogen substituent on the tetracyclic aporphine core and series B which are 1,2,10-trisubstituted aporphines. Remarkably, compounds from both series lacked affinity for the D₅ receptor, thus attaining D₁ versus D₅ selectivity. Compound 20c was the most potent D₁ ligand identified. Docking studies at D₁ and D₅ receptors indicate that the binding mode of 20c at the D₁ receptor allows for stronger hydrophobic contacts, (primarily with Phe residues) as compared to the D₅ receptor, accounting for its D₁ versus D₅ selectivity. Considering the lack of affinity for the D₅ receptor (and low affinity at other receptors tested), compound 20c represents an interesting starting point for further structural diversification of aporphines as sub-type selective D₁ receptor tools.

Dopamine D1 and D3 receptor polypharmacology as a potential treatment approach for substance use disorder

In the search for efficacious pharmacotherapies to treat cocaine addiction much attention has been given to agents targeting dopamine D1 or D3 receptors because of the involvement of these receptors in drug-related behaviors. D1-like and D3 receptor partial agonists and antagonists have been shown to reduce drug reward, reinstatement of drug seeking and conditioned place preference in rodents and non-human primates. However, translation of these encouraging results to clinical settings has been limited due to a number of factors including toxicity, poor pharmacokinetic properties and extrapyramidal and sedative side effects. This review highlights the role of D1 and D3 receptors in drug reward and seeking, the discovery of D1-D3 heteromers and their potential as targets in the treatment of addiction.

Use of translational pharmacodynamic biomarkers in early-phase clinical studies for schizophrenia

Schizophrenia is a severe mental disorder characterized by cognitive deficits, and positive and negative symptoms. The development of effective pharmacological compounds for the treatment of schizophrenia has proven challenging and costly, with many compounds failing during clinical trials. Many failures occur due to disease heterogeneity and lack of predictive preclinical models and biomarkers that readily translate to humans during early characterization of novel antipsychotic compounds. Traditional early-phase trials consist of single- or multiple-dose designs aimed at determining the safety and tolerability of an investigational compound in healthy volunteers. However, by incorporating a translational approach employing methodologies derived from preclinical studies, such as EEG measures and imaging, into the traditional Phase I program, critical information regarding a compound's dose-response effects on pharmacodynamic biomarkers can be acquired. Furthermore, combined with the use of patients with stable schizophrenia in early-phase clinical trials, significant 'de-risking' and more confident 'go/no-go' decisions are possible.

Supra-normal stimulation of dopamine D1 receptors in the prelimbic cortex blocks behavioral expression of both aversive and rewarding associative memories through a cyclic-AMP-dependent signaling pathway

Dopamine (DA) receptor transmission through either D(1) or D(2)-like subtypes is involved critically in the processing of emotional information within the medial prefrontal cortex (mPFC). However the functional role of specific DA D(1)-like receptor transmission in the expression of emotionally salient associative memories (either aversive or rewarding) is not currently understood. Here we demonstrate that specific activation of DA D(1) receptors in the prelimbic (PLC) division of the mPFC causes a transient block in the behavioral expression of both aversive and rewarding associative memories. We report that intra-PLC microinfusions of a selective D(1) receptor agonist block the spontaneous expression of an associative olfactory fear memory, without altering the stability of the original memory trace. Furthermore, using an unbiased place conditioning procedure (CPP), intra-PLC D(1) receptor activation blocks the spontaneous expression of an associative morphine (5 mg/kg; i.p.) reward memory, while leaving morphine-primed memory expression intact. Interestingly, both intra-PLC D(1)-receptor mediated block of either fear-related or reward-related associative memories were dependent upon downstream cyclic-AMP (cAMP) signaling as both effects were rescued by co-administration of a cAMP signaling inhibitor. The blockade of both rewarding and aversive associative memories is mediated through a D(1)-specific signaling pathway, as neither forms of spontaneous memory expression were blocked by intra-PLC microinfusions of a D(2)-like receptor agonist. Our results demonstrate that the spontaneous expression of either rewarding or aversive emotionally salient memories shares a common, D(1)-receptor mediated substrate within the mPFC.

Dopamine D1 receptor modulation in nucleus accumbens lowers voluntary wheel running in rats bred to run high distances

Dopamine signaling in the nucleus accumbens (NAc) has been postulated to influence reward development towards drugs of abuse and exercise. Herein, we used generation 4-5 rats that were selectively bred to voluntary run high (HVR) versus low (LVR) distances in order to examine if dopamine-like 1 (D1) receptor modulation in the NAc differentially affects nightly voluntary wheel running between these lines. A subset of generation 5-6 HVR and LVR rats were also used to study the mRNA expression of key genes related to reward and addiction in the NAc (i.e., DRD1, DRD5, DRD2, Nr4a2, FosB, and BDNF). In a crossover fashion, a D1-like agonist SKF 82958 (2 μg per side) or D1-like full antagonist SCH 23390 (4 μg per side) was bilaterally injected into the NAc of HVR and LVR female Wistar rats prior to their high running nights. Notably, during hours 2-4 (between 2000 and 2300) of the dark cycle there was a significant decrement in running distances in the HVR rats treated with the D1 agonist (p=0.025) and antagonist (p=0.017) whereas the running distances in LVR rats were not affected. Interestingly, HVR and LVR rats possessed similar NAc concentrations of the studied mRNAs. These data suggest that: a) animals predisposed to run high distances on a nightly basis may quickly develop a rewarding response to exercise due to an optimal D1-like receptor signaling pathway in the NAc that can be perturbed by either activation or blocking, b) D1-like agonist or antagonist injections do not increase running distances in rats that are bred to run low nightly distances, and c) running differences between HVR and LVR animals are seemingly not due to the expression of the studied mRNAs. Given the societal prevalence of obesity and extraneous physical inactivity, future studies should be performed in order to further determine the culprit for the low running phenotype observed in LVR animals.

The Discovery of Novel Selective D1 Dopaminergic Agonists: A-68930, A-77636, A-86929, and ABT-413

The novel selective D1 dopaminergic full agonists A-68930, A-77636 were discovered by the synthesis of molecules to probe the bioactive conformation of the partial agonist SKF-38393, by the use of this information to add D1 affinity and selectivity to a screening hit, and by traditional medicinal chemistry exploration of structure-activity relationships. The subsequent design of A-86929 and ABT-413 capitalized on these results, recently disclosed agonists, and traditional medicinal chemistry.

Stimulation of dopamine D2/D3 but not D1 receptors in the central amygdala decreases cocaine-seeking behavior

Alterations in dopamine output within the various subnuclei of the amygdala have previously been implicated in cocaine reinforcement, as well as cocaine-seeking behavior. To elucidate the potential for increased stimulation of D1- and D2-like receptors (D1Rs and D2Rs, respectively) specifically in the central nucleus of the amygdala (CeA) to modulate cue- and cocaine-elicited reinstatement of cocaine-seeking behavior, we infused either the D1R agonist, SKF-38393 (0-4.0 microg/side) or the D2R agonist, 7-OH-DPAT (0-4.0 microg/side) into the CeA immediately prior to tests for cue and cocaine-primed reinstatement. We also examined the effects of 7-OH-DPAT on cocaine self-administration as a positive behavioral control. 7-OH-DPAT decreased cue-and cocaine-primed reinstatement, and reduced the number of cocaine infusions obtained during self-administration; SKF-38393 produced no discernable effects. The results suggest that enhanced stimulation of D2Rs, but not D1Rs, in the CeA is sufficient to inhibit expression of the incentive motivational effects of cocaine priming and cocaine-paired cues. Together with previous findings that D1R blockade attenuates reinstatement of cocaine-seeking behavior, the results suggest that D1R stimulation may be necessary, but not sufficient, to modulate the incentive motivational effects of cues and cocaine priming.

Cocaine-induced neuroadaptations in glutamate transmission: potential therapeutic targets for craving and addiction

A growing body of evidence indicates that repeated exposure to cocaine leads to profound changes in glutamate transmission in limbic nuclei, particularly the nucleus accumbens. This review focuses on preclinical studies of cocaine-induced behavioral plasticity, including behavioral sensitization, self-administration, and the reinstatement of cocaine seeking. Behavioral, pharmacological, neurochemical, electrophysiological, biochemical, and molecular biological changes associated with cocaine-induced plasticity in glutamate systems are reviewed. The ultimate goal of these lines of research is to identify novel targets for the development of therapies for cocaine craving and addiction. Therefore, we also outline the progress and prospects of glutamate modulators for the treatment of cocaine addiction.

Anti-relapse medications: preclinical models for drug addiction treatment

Addiction is a chronic relapsing brain disease and treatment of relapse to drug-seeking is considered the most challenging part of treating addictive disorders. Relapse can be modeled in laboratory animals using reinstatement paradigms, whereby behavioral responding for a drug is extinguished and then reinstated by different trigger factors, such as environmental cues or stress. In this review, we first describe currently used animal models of relapse, different relapse triggering factors, and the validity of this model to assess relapse in humans. We further summarize the growing body of pharmacological interventions that have shown some promise in treating relapse to psychostimulant addiction. Moreover, we present an overview on the drugs tested in cocaine or methamphetamine addicts and examine the overlap of existing preclinical and clinical data. Finally, based on recent advances in our understanding of the neurobiology of relapse and published preclinical data, we highlight the most promising areas for future anti-relapse medication development.

Dopamine D1 receptors in cocaine dependence measured with PET and the choice to self-administer cocaine

The goal of this study was to determine D(1) receptor availability in human cocaine-dependent (CD) subjects and matched healthy controls (HCs). In addition, the CD subjects performed cocaine self-administration sessions in order to explore the association between D(1) receptor availability and cocaine-seeking behavior. Twenty-five CD subjects (40+/-4 years, 19M/6 F) and 23 matched HCs (38+/-4 years, 19M/4F) were scanned with PET and the radiotracer [(11)C]NNC 112. During the cocaine self-administration sessions, CD volunteers were given the choice to self-administer cocaine (0, 6, and 12 mg) or to receive a monetary voucher worth $5. D(1) receptor availability was measured in the limbic, associative, and sensori-motor striatum in addition to cortical brain regions. No difference in D(1) receptor availability was seen between the two groups. A negative association was seen between D(1) receptor BP(ND) in the limbic striatum and the choice for the 6 mg dose of cocaine (r=-0.47, p=0.02, corrected for age). These results do not support the hypothesis that cocaine dependence is associated with a reduction in D(1) receptor availability in the striatum. However, within the CD subjects, low D(1) receptor availability in the ventral striatum was associated with the choice to self-administer cocaine, suggesting that low D(1) receptor availability may be associated with an increased risk of relapse in cocaine dependence.

From vice to virtue: insights from sensitization in the nonhuman primate

Repeated, intermittent administration of psychomotor stimulants, or D1 agonists in dopamine-deficient states, induces behavioral sensitization, characterized by an enhanced response to a subsequent acute low dose challenge, which may be manifested in form of altered behavior or cognitive function. Amphetamine sensitization in the nonhuman primate encompasses profound and enduring changes to similar neuronal and neurochemical substrates that occur in rodents. The process of sensitization in the monkey also results in a long-lasting depression in baseline behavioral responding, as well as emergence of hallucinatory-like behaviors reminiscent of human psychosis in response to an acute challenge. Nonhuman primates show a reduction in spine density and dendritic length in prefrontal neurons and a marked reduction in basal dopamine turnover in both prefrontal cortex and striatum. A major hallmark of amphetamine sensitization in both nonhuman primates and rodents is the manifestation of deficits in executive function and working memory which rely upon the integrity of prefrontal cortex and thereby, may yield significant insights into the cognitive dysfunction associated with addiction. Together with evidence from human and rodent studies, it can be concluded that repeated exposure to psychomotor stimulants can lead to a corruption of neuroadaptive systems in the brain by an extraordinary influence on synaptic plasticity, learning, and memory. Actively harnessing this same process by repeated, intermittent D1 agonist administration may be the key to improved working memory and decision making in addiction and other dopamine dysfunctional states, such as schizophrenia.

Differential ability of D1 and D2 dopamine receptor agonists to induce and modulate expression and reinstatement of cocaine place preference in rats

RATIONALE

D1-Like agonists are self-administered by drug-naive animals, whereas D2-like agonists reinstate cocaine-seeking behavior, but the rewarding and reinstating effects of D1- and D2-like agonists in pavlovian-based conditioned place preference are equivocal.

OBJECTIVE

To compare the ability of D1 and D2 agonists to produce conditioned place preference with their modulation of expression and reinstatement of an established cocaine place preference.

METHODS

Using an unbiased procedure, we measured the place preference induced by the D1 receptor agonist SKF 81297 and the D2/D3 receptor agonist quinpirole in drug-naive or cocaine-exposed rats. The rewarding effects of the D1 agonists SKF 82958, ABT-431, A-77636, and the D2/D3 receptor agonist 7-OH-DPAT were also tested. Additionally, we tested the ability of SKF 81297 and quinpirole to modulate expression and reinstatement of an established cocaine place preference.

RESULTS

The D1 receptor agonists SKF 81297, SKF 82958, and ABT-431 produced dose-dependent conditioned place preferences, whereas A-77636 produced only place aversion, and the D2/D3 agonists quinpirole and 7-OH-DPAT were without effect in drug naive rats. In cocaine-treated rats, SKF-81297-induced place preference was reduced, whereas quinpirole-induced place preference was revealed. Pretreatment using either D1 or D2/D3 agonists blocked expression of an established cocaine place preference, but only the D1 agonist SKF 81297 and cocaine dose-dependently reinstated an extinguished cocaine place preference, whereas the D2/D3 agonist quinpirole induced place aversion but failed to alter cocaine-induced reinstatement.

CONCLUSIONS

D1, but not D2/D3, agonists mediate rewarding effects and reinstatement of cocaine place preference, but the reinstating effects differ markedly from self-administration paradigms.

Insights into the Role of Dopamine Receptor Systems in Learning and Memory

It is well established that learning and memory are complex processes involving and recruiting different brain modulatory neurotransmitter systems. Considerable evidence points to the involvement of dopamine in various aspects of cognition, and interest has been focused on investigating the clinical relevance of dopamine systems to age-related cognitive decline and manifestations of cognitive impairment in schizophrenia, Alzheimer's disease, Parkinson's disease and other neurodegenerative diseases. In the past decade or so, in spite of the molecular cloning of the five dopamine receptor subtypes, their specific roles in brain function remained inconclusive due to the lack of completely selective ligands that could distinguish between the members of the D1-like and D2-like dopamine receptor families. One of the most important advances in the field of dopamine research has been the generation of mutant mouse models permitting evaluation of the dopaminergic system using gene targeting technologies. These mouse models represent an important approach to explore the functional roles of closely related receptor subtypes. In this review, we present and discuss evidence on the role of dopamine receptors in different aspects of learning and memory at the cellular, molecular and behavioral levels. We compare evidence using conventional pharmacological, lesion or electrophysiological studies with results from mice with targeted deletions of different subtypes of dopamine receptor genes. We particularly focus on dopamine D1 and D2 receptors in an effort to delineate their specific roles in various aspects of cognitive function. We provide strong evidence, from our own recent work as well as others, that dopamine is part of the network that plays a very important role in cognitive function, and that although multiple dopamine receptor subtypes contribute to different aspects of learning and memory, the D1 receptor seems to play a more prominent role in mediating plasticity and specific aspects of cognitive function, including spatial learning and memory processes, reversal learning, extinction learning, and incentive learning.

Anatomy and pharmacology of cocaine priming-induced reinstatement of drug seeking

Cocaine addiction in human addicts is characterized by a high rate of relapse following successful detoxification. Relapse to drug taking/seeking can be precipitated by several stimuli including, but not limited to, re-exposure to cocaine itself. In order to understand the mechanisms underlying cocaine craving, a substantial effort has been devoted to elucidating the anatomical and neurochemical bases underlying cocaine priming-induced reinstatement, an animal model of relapse. Here, we review evidence that changes in dopaminergic and glutamatergic transmission in limbic/basal ganglia circuits of interconnected nuclei including the medial prefrontal cortex, nucleus accumbens, ventral pallidum, amygdala, hippocampus, orbitofrontal cortex, neostriatum and thalamus underlie cocaine priming-induced reinstatement of cocaine seeking. Maladaptive changes in the processing of motivationally relevant stimuli by these circuits following cocaine self-administration result in drug craving and compulsive drug seeking upon re-exposure to cocaine.

Pharmacogenetic tools for the development of target-oriented cognitive-enhancing drugs

The identification of the anatomical and physiological substrates involved in the regulation of the dorsolateral prefrontal cortex function in humans provided the basis for the understanding of mechanisms involved in cognitive and executive function under normal as well as pathological conditions. In this context, substantial evidence indicates that alterations in monaminergic function in the dorsolateral prefrontal cortex significantly contributes to the cognitive impairments present in schizophrenia, attention deficit disorders, and other neuropsychiatric conditions. The development of a number of compounds that selectively increase extracellular dopamine (DA) concentrations in the dorsolateral prefrontal cortex but not in subcortical areas by either blocking its metabolism or reuptake, or increasing its release, or that directly activate postsynaptic DA-1 receptor mechanisms provided powerful pharmacotherapeutic tools to mitigate the cognitive deficits brought about by the dopaminergic alterations of the prefrontal cortex. More recently, the findings that polymorphisms of the catecholamine-O-methyl-transferase gene may also modify the effect of these drugs on the prefrontal cortex points toward a more specific genotype-based neuropsychopharmacology for the treatment of cognitive deficits in schizophrenia as well as in a number of other neuropsychiatric conditions. The ability of these compounds to increase DA load selectively in the frontal cortex and not on subcortical systems allows a targeted intervention without the stimulant-like effects observed with older drugs used to treat those conditions.

Stimulation of D1-like or D2 dopamine receptors in the shell, but not the core, of the nucleus accumbens reinstates cocaine-seeking behaviour in the rat

Although increases in dopamine transmission in the brain are clearly involved in the reinstatement of cocaine seeking, the role of nucleus accumbens dopamine in cocaine priming-induced reinstatement remains controversial. The goal of these experiments was to evaluate the relative contributions of D1-like and D2-like dopamine receptors in the nucleus accumbens core and shell in the reinstatement of cocaine-seeking behaviour. Initially, rats were trained to press a lever for cocaine (0.25 mg, i.v.) using a fixed-ratio 5 (FR5) schedule of reinforcement. Responding was then extinguished by substituting saline for cocaine. During the reinstatement phase, subtype-specific dopamine receptor agonists were microinjected into the nucleus accumbens core or medial shell in order to assess their ability to induce cocaine seeking. Administration of the D1/D5 dopamine receptor agonist SKF-81297 (1.0 microg) into the nucleus accumbens shell, but not core, reinstated drug-seeking behaviour. Similarly, microinjection of quinpirole (3.0 microg), a D2/D3 dopamine receptor agonist, into the nucleus accumbens shell and not core reinstated drug-seeking behaviour. In contrast, administration of the D3- or D4-preferring dopamine receptor agonists PD 128,907 (1.5 and 3.0 microg) and PD 168,077 (0.3 and 3.0 microg), respectively, did not promote reinstatement when administered into either the core or the shell. Taken together, these results indicate that activation of D1/D5 or D2 dopamine receptors, in the limbic shell subregion of the nucleus accumbens but not the basal ganglia-orientated accumbens core, promotes the reinstatement of cocaine-seeking behaviour.

Association between dopamine receptor D1 A-48G polymorphism and methamphetamine abuse

Several lines of evidence have suggested that substance abuse is mediated by the dopaminergic rewarding system, primarily through the activity of the dopamine receptor D1 (DRD1). The purpose of the present study was to evaluate the association of DRD1 A-48G polymorphism with methamphetamine (MAP) abusers and MAP-induced psychosis patients. A total of 363 MAP abusers and 425 healthy normal controls were enrolled. The structural Diagnostic Interview for Genetic Study was used to evaluate all MAP abusers. The MAP abusers were classified into psychosis (n = 135) and non-psychosis (n = 228) groups. A-48G polymorphism was determined by polymerase chain reaction-restriction fragment length polymorphism. The results show that male sex and a higher frequency of MAP abuse were the predisposing factors in the development of MAP psychosis. The DRD1 -48G allele frequency in the MAP psychosis group, non-psychosis group and the healthy normal controls was 0.14, 0.18 and 0.16, respectively. No association was found between DRD1 A-48G polymorphism and MAP abuse and MAP psychosis. However, the data provided additional evidence of ethnicity-related differences in the distribution of polymorphism in comparison to previous studies.

Effects of SCH-23390 infused into the amygdala or adjacent cortex and basal ganglia on cocaine seeking and self-administration in rats

Amygdala D1 receptors have been implicated in the motivating effects of cocaine-conditioned cues and cocaine itself, but the specific nucleus involved is unclear. Thus, we infused the D1 antagonist, SCH-23390, into the rostral basolateral amygdala (rBLA), caudal basolateral amygdala (cBLA), or central amygdala (CEA), and tested its effects on self-administration of cocaine, as well as reinstatement of extinguished cocaine-seeking behavior by cocaine-conditioned cues or cocaine itself. Two anatomical controls, the posterior regions of basal ganglia (BG) and somatosensory/insular cortices (CTX), were also examined. Cocaine self-administration was increased and cue and cocaine reinstatement were decreased by SCH-23390 infusion into every region when examined across the hour test session, with the exception that cBLA infusion did not alter cocaine reinstatement. In the first 20 min of the session, when SCH-23390 was more localized in the target sites, self-administration was increased by infusion into the CEA, cBLA, BG, and CTX, with lesser increases in the rBLA. Cocaine reinstatement was attenuated during the first 20 min only by infusion into the CEA, rBLA, and CTX. Cue reinstatement was not reliably observed in the first 20 min, but there was a trend for attenuation by infusion into the cBLA, and surprisingly, significant attenuations in the BG and CTX. The findings suggest that D1 receptors in subregions of the amygdala play differential roles in the reinforcing/motivational effects of cocaine, while the cue reinstatement effects are less clear. Further research is needed to examine the novel findings that neighboring regions of the BG and CTX may play a role in motivation for cocaine.

Effects of intra-nucleus accumbens shell administration of dopamine agonists and antagonists on cocaine-taking and cocaine-seeking behaviors in the rat

RATIONALE

Dopamine signaling in the nucleus accumbens (NAc) plays an important role in regulating drug-taking and drug-seeking behaviors, but the role of D(1)- and D(2)-like receptors in this regulation remains unclear.

OBJECTIVES

Our objective was to study the role of NAc D(1)- and D(2)-like receptors in the reinstatement of cocaine-seeking behavior and the regulation of stabilized cocaine intake in rats.

METHODS

Using a within-session reinstatement procedure, whereby animals self-administer cocaine (90 min) and extinguish responding (150 min) in a single session, we assessed the ability of NAc microinfusions of the D(1) agonist SKF 81297 and the D(2) agonist 7-OH-DPAT to reinstate extinguished cocaine seeking. The effects of the D(1) antagonist SCH 23390 and the D(2) antagonist eticlopride pretreatment on agonist- and cocaine-primed reinstatement were also measured. Similar agonist and antagonist treatments were tested for their ability to modulate stabilized cocaine and sucrose self-administration.

RESULTS

Intra-NAc infusions of either SKF 81297 (0.3-3.0 microg) or 7-OH-DPAT (1.0-10.0 microg) dose-dependently reinstated cocaine seeking with greater efficacy in the medial core than in the shell subregion and at doses that also stimulated locomotor behavior. Intra-NAc shell infusions of SCH 23390 (1.0 microg) and eticlopride (3.0-10.0 microg) blocked cocaine-primed reinstatement (2.0 mg/kg, i.v.) and indiscriminately blocked reinstatement induced by either intra-NAc D(1) or D(2) agonists. Doses of agonists that triggered reinstatement failed to alter stabilized cocaine intake, whereas doses of antagonists that blocked reinstatement increased cocaine intake in the shell.

CONCLUSIONS

Both D(1) and D(2) receptors in the NAc play a prominent, and perhaps cooperative, role in regulating cocaine-taking and cocaine-seeking behaviors.

Factors associated with the transition from abuse to dependence among substance abusers: implications for a measure of addictive liability

This study was conducted to test the validity of a measure that has potential to bridge research on the addictive liability of drugs and on individuals' liability to addiction, which to date have evolved in largely parallel arenas. The length of time between onset of abuse and dependence (LOTAD) has evolved from recent findings on transitions through levels of addiction; it was hypothesized that shorter LOTAD is indicative of greater addictive liability. Hypotheses were based on animal studies and human studies. Retrospective data from the DSM-IV Substance Use Disorders Work Group were reanalyzed using configural frequency analysis, survival curves, bivariate Kendall's tau associations, and linear regression. The sample consisted of participants recruited from community and clinical settings. The measure was the Composite International Diagnostic Interview-Substance Abuse Module (CIDI-SAM). The shortest LOTADs were observed for disorders related to use of cocaine and opiates, followed by cannabis and then alcohol regardless of the subsample that was analyzed. As hypothesized, females and early initiators of drug use had shorter LOTADs compared to men and other initiators of drug use; no consistent differences in LOTAD were observed between African-Americans and Caucasians. None of the LOTAD variance associated with differences between drugs could be accounted for by gender, early use of the drug, or ethnicity. Specific areas of research where LOTAD might be useful as well as how LOTAD might be improved are discussed.

Cocaine-induced alterations in dopamine receptor signaling: Implications for reinforcement and reinstatement

The transition from casual drug use to addiction, and the intense drug craving that accompanies it, has been postulated to result from neuroadaptations within the limbic system caused by repeated drug exposure. This review will examine the implications of cocaine-induced alterations in mesolimbic dopamine receptor signaling within the context of several widely used animal models of addiction. Extensive evidence indicates that dopaminergic mechanisms critically mediate behavioral sensitization to cocaine, cocaine-induced conditioned place preference, cocaine self-administration, and the drug prime-induced reinstatement of cocaine-seeking behavior. The propagation of the long-term neuronal changes associated with recurring cocaine use appears to occur at the level of postreceptor signal transduction. Repeated cocaine treatment causes an up-regulation of the 3',5'-cyclic adenosine monophosphate (cAMP)-signaling pathway within the nucleus accumbens, resulting in a dys-regulation of balanced D1/D2 dopamine-like receptor signaling. The intracellular events arising from enhanced D1-like postsynaptic signaling mediate both facilitatory and compensatory responses to the further reinforcing effects of cocaine.

The dopamine D1 receptor agonist and D2 receptor antagonist LEK-8829 attenuates reinstatement of cocaine-seeking in rats

Various dopaminergic drugs have been studied for their efficacy in the treatment of cocaine addiction. Pretreatment with either selective dopamine D1 receptor agonists or selective dopamine D2 receptor antagonists prevents reinstatement of cocaine-seeking in animal models of drug craving and relapse. We tested a novel ergoline derivative with combined D1 agonistic and D2 antagonistic effects, 9,10-didehydro- N-methyl- N-(2-propynyl)-6-methyl-8beta-aminomethylergoline bimaleate (LEK-8829), for its effects on cocaine-seeking in the intravenous cocaine self-administration model in rats. Pretreatment with systemic injections of LEK-8829 attenuated reinstatement of cocaine-seeking induced by cocaine priming injections and diminished cocaine intake in cocaine self-administration sessions. LEK-8829 itself did not induce reinstatement of cocaine-seeking and did not maintain intravenous self-administration. The results of our study indicate that LEK-8829 is a candidate medication for the treatment of cocaine craving in cocaine addiction.

Agents in Development for the Management of Cocaine Abuse

Cocaine abuse is a serious health problem in many areas of the world, yet there are no proven effective medications for the treatment of cocaine dependence. Preclinical studies suggest that the reinforcing effect of cocaine that promotes its abuse is mediated by blockade of the presynaptic dopamine transporter. This results in increased dopamine activity in the mesolimbic or meso-accumbens dopamine reward system of brain. Development of new medications to treat cocaine dependence has focused on manipulation of this dopamine system, either by direct action on dopamine binding sites (transporter or receptors) or indirectly by affecting other neurotransmitter systems that modulate the dopamine system. In principle, a medication could act via one of three mechanisms: (i) as a substitute for cocaine by producing similar dopamine effects; (ii) as a cocaine antagonist by blocking the binding of cocaine to the dopamine transporter; or (iii) as a modulator of cocaine effects by acting at other than the cocaine binding site. The US National Institute on Drug Abuse has a Clinical Research Efficacy Screening Trial (CREST) programme to rapidly screen existing medications. CREST identified four medications warranting phase II controlled clinical trials: cabergoline, reserpine, sertraline and tiagabine. In addition, disulfiram and selegiline (deprenyl) have been effective and well tolerated in phase II trials. However, selegiline was found ineffective in a recent phase III trial. Promising existing medications probably act via the first or third aforementioned mechanisms. Sustained-release formulations of stimulants such as methylphenidate and amfetamine (amphetamine) have shown promise in a stimulant substitution approach. Disulfiram and selegiline increase brain dopamine concentrations by inhibition of dopamine-catabolising enzymes (dopamine-beta-hydroxylase and monoamine oxidase B, respectively). Cabergoline is a direct dopamine receptor agonist, while reserpine depletes presynaptic stores of dopamine (as well as norepinephrine and serotonin). Sertraline, baclofen and vigabatrin indirectly reduce dopamine activity by increasing activity of neurotransmitters (serotonin and GABA) that inhibit dopamine activity. Promising new medications act via the second or third aforementioned mechanisms. Vanoxerine is a long-acting inhibitor of the dopamine transporter which blocks cocaine binding and reduces cocaine self-administration in animals. Two dopamine receptor ligands that reduce cocaine self-administration in animals are also undergoing phase I human safety trials. Adrogolide is a selective dopamine D(1) receptor agonist; BP 897 is a D(3) receptor partial agonist.A pharmacokinetic approach to treatment would block the entry of cocaine into the brain or enhance its catabolism so that less cocaine reached its site of action. This is being explored in animals using the natural cocaine-metabolising enzyme butyrylcholinesterase (or recombinant versions with enhanced capabilities), catalytic antibodies, and passive or active immunisation to produce anti-cocaine binding antibodies. A recent phase I trial of a "cocaine vaccine" found it to be well tolerated and producing detectable levels of anti-cocaine antibodies for up to 9 months after immunisation.

Manipulation of dopamine d1-like receptor activation in the rat medial prefrontal cortex alters stress- and cocaine-induced reinstatement of conditioned place preference behavior

These studies examined the ability of the dopamine D1-like agonist SKF 81297 and D1-like antagonist SCH 23390 in the medial prefrontal cortex to alter the reinstatement of cocaine-induced conditioned place preference behavior. Male Sprague-Dawley rats were fitted with bilateral cannulae over the medial prefrontal cortex and subsequently trained in a conditioned place preference task. Animals were trained in this task using four pairings of cocaine (12 mg/kg, i.p.). Conditioned place preference was demonstrated in all animals, and this behavior was then extinguished over a 5-10-day period before testing for reinstatement. Just prior to reinstatement by immobilization stress or a cocaine priming injection (5 mg/kg, i.p.), a microinjection of the D1-like receptor antagonist SCH 23390 (0.01, 0.1 or 1.0 microg/side), or the D1-like receptor agonist SKF 81297 (0.1, 0.3 or 1.0 microg/side) was given into the medial prefrontal cortex. SCH 23390 blocked both stress- and cocaine-induced reinstatement of conditioned place preference after the two higher doses were administered into the medial prefrontal cortex. The highest dose of SKF 81297 (1.0 microg/side) prevented immobilization stress- but not cocaine-induced reinstatement. The highest dose of these drugs given in the absence of stress or cocaine did not produce reinstatement. The results indicate that immobilization stress given within the place-preference chamber is capable of producing reinstatement of cocaine-seeking behavior. The microinjection studies suggest that D1-like receptor antagonism within the prefrontal cortex is sufficient to block reinstatement by stress and cocaine. Furthermore, the results from D1-like receptor activation in the medial prefrontal cortex point to utilization of different neural pathways for stress- and cocaine-induced reinstatement.

The reinstatement model and relapse prevention: a clinical perspective

OBJECTIVES

This commentary assesses the degree to which the reinstatement model is homologous to the human experience of relapse.

RESULTS

A review of the literature suggests that the relationship is less clear than is often assumed, largely due to a lack of prospective data on the precipitants and process of relapse (especially relapse to heroin or cocaine abuse). However, reinstatement does not need to resemble relapse to have immediate clinical value; predictive validity as a medication screen would be sufficient. Whether the model has predictive validity is unknown, because, to date, very few clinical trials have tested medications that are effective in the reinstatement model, and even fewer have used designs comparable to those of reinstatement experiments. A clinical trial comparable to a reinstatement experiment would enroll participants who are already abstinent, and its main outcome measure would be propensity to undergo a specific type of relapse (e.g., relapse induced by stress or cues).

CONCLUSIONS

Until clinical and preclinical work are more comparable, criticisms of the reinstatement model's presumed shortcomings are premature.

Selective antagonism at dopamine D3 receptors prevents nicotine-triggered relapse to nicotine-seeking behavior

Drugs of abuse, including, nicotine have been shown to enhance brain reward functions in the mesocortico-limbic dopamine (DA) system in general, and the nucleus accumbens in particular. The latter occupies a prominent position in the ventral striatum and expresses a high density of DA D(3) receptors. As such, the present study aimed at investigating the effect of the selective D(3) receptor antagonist SB-277011-A on both the stable maintenance of intravenous nicotine self-administration and nicotine-triggered relapse to nicotine-seeking behavior in the rat. SB-277011-A (3-10 mg/kg i.p.) significantly reduced reinstatement of nicotine-seeking behavior without affecting nicotine self-administration per se. These results suggest that DA D(3) receptors are involved in the reinstatement of nicotine-seeking behavior independently of any interaction with the primary reinforcing effects of nicotine itself. These findings point toward the potential use of selective DA D(3) receptor antagonists for the pharmacotherapeutic management of relapse to drug-seeking behaviors.

D1-receptor drugs and cocaine-seeking behavior: investigation of receptor mediation and behavioral disruption in rats

RATIONALE

Dopamine D1-receptor antagonists and agonists both attenuate cocaine-seeking behavior (i.e., operant responding in the absence of cocaine reinforcement) elicited by a cocaine prime or cocaine-paired stimuli. It remains unclear whether these effects are D1-receptor mediated.

OBJECTIVES

The present study tested whether a D1 antagonist (SCH-23390) would reverse the attenuating effects of a D1 agonist (SKF-81297) on cocaine-seeking behavior and whether behavioral disruption is involved in these effects.

METHODS

Rats trained to press a lever for cocaine reinforcement with light and tone cues paired with each infusion underwent daily extinction sessions during which responding had no scheduled consequences (i.e., neither cocaine nor the cocaine-paired stimulus complex was available). After responding diminished, the effects of the D1 antagonist on the dose-response functions of the D1 agonist for reinstatement of cocaine-seeking behavior by response-contingent cue presentations or cocaine priming were examined. A separate experiment assessed the effects of the agonist on the dose-response function of the antagonist for cue reinstatement. Stereotyped behavior and activity were also measured during each test session.

RESULTS

The attenuating effects of SKF-81297 on cocaine-seeking behavior during cocaine-primed reinstatement were reversed by co-administration of SCH-23390. However, no evidence for reversal of the attenuation during cue reinstatement was found even though agonist-induced stereotypy and antagonist-induced hypoactivity were reversed by co-administration of the two drugs during the same test session.

CONCLUSIONS

The findings suggest that the attenuating effects of D1-receptor drugs on cocaine-seeking behavior during cocaine reinstatement are mediated by dopamine D1 receptors; however, it remains unclear whether the effects of these drugs on cocaine-seeking behavior during cue reinstatement are D1-receptor mediated. Nevertheless, it is evident that the attenuation of cocaine-seeking behavior by these drugs is not simply due to behavioral disruption.

The reinstatement model of drug relapse: history, methodology and major findings

RATIONAL AND OBJECTIVES

The reinstatement model is currently used in many laboratories to investigate mechanisms underlying relapse to drug seeking. Here, we review briefly the history of the model and describe the different procedures that have been used to study the phenomenon of reinstatement of drug seeking. The results from studies using pharmacological and neuroanatomical techniques to determine the neuronal events that mediate reinstatement of heroin, cocaine and alcohol seeking by acute priming injections of drugs, drug-associated cues and environmental stressors are summarized. In addition, several issues are discussed, including (1) the concordance between the neuronal mechanisms involved in drug-induced reinstatement and those involved in drug reward and discrimination, (2) the role of drug withdrawal states and periods in reinstatement of drug seeking, (3) the role of neuronal adaptations induced by exposure to drugs in relapse, and (4) the degree to which the rat reinstatement model provides a suitable preclinical model of relapse to drug taking.

CONCLUSIONS

The data derived from studies using the reinstatement model suggest that the neuronal events that mediate drug-, cue- and stress-induced reinstatement of drug seeking are not identical, that the mechanisms underlying drug-induced reinstatement are to some degree different from those mediating drug discrimination or reward, and that the duration of the withdrawal period following cocaine and heroin self-administration has a profound effect on reinstatement induced by drug cues and stress. Finally, there appears to be a good correspondence between the events that induce reinstatement in laboratory animals and those that provoke relapse in humans.

Adrogolide HCl (ABT-431; DAS-431), a prodrug of the dopamine D1 receptor agonist, A-86929: preclinical pharmacology and clinical data

Adrogolide (ABT-431; DAS-431) is a chemically stable prodrug that is converted rapidly (<1 min) in plasma to A-86929, a full agonist at dopamine D1 receptors. In in vitro functional assays, A-86929 is over 400 times more selective for dopamine D1 than D2 receptors. In rats with a unilateral loss of striatal dopamine, A-86929 produces contralateral rotations that are inhibited by dopamine D1 but not by dopamine D2 receptor antagonists. Adrogolide improves behavioral disability and locomotor activity scores in MPTP-lesioned marmosets, a model of Parkinson's disease (PD), and shows no tolerance upon repeated dosing for 28 days. In PD patients, intravenous (i.v.) adrogolide has antiparkinson efficacy equivalent to that of L-DOPA with a tendency towards a reduced liability to induce dyskinesia. The adverse events associated with its use were of mild-to-moderate severity and included injection site reaction, asthenia, headache, nausea, vomiting, postural hypotension, vasodilitation, and dizziness. Adrogolide can also attenuate the ability of cocaine to induce cocaine-seeking behavior and does not itself induce cocaine-seeking behavior in a rodent model of cocaine craving and relapse. In human cocaine abusers, i.v. adrogolide reduces cocaine craving and other cocaine-induced subjective effects. The results of animal abuse liability studies indicate that adrogolide is unlikely to have abuse potential in man. Adrogolide has also been reported to reverse haloperidol-induced cognitive deficits in monkeys, suggesting that it may be an effective treatment for the cognitive dysfunction associated with aging and disease. Adrogolide undergoes a high hepatic "first-pass" metabolism in man after oral dosing and, as a result, has a low oral bioavailability (approximately 4%). This limitation may potentially be circumvented by oral inhalation formulations for intrapulmonary delivery that greatly increase the bioavailability of adrogolide. As the first full dopamine D1 receptor agonist to show efficacy in PD patients and to reduce the craving and subjective effects of cocaine in cocaine abusers, adrogolide represents an important tool in understanding the pharmacotherapeutic potential of dopamine D1 receptor agonists.