A comparison of cocaine, GBR 12909, and phentermine self-administration by rhesus monkeys on a progressive-ratio schedule

Prospects for new drugs to treat binge-eating disorder: Insights from psychopathology and neuropharmacology

BACKGROUND

Binge-eating disorder (BED) is a common psychiatric condition with adverse psychological and metabolic consequences. Lisdexamfetamine (LDX) is the only approved BED drug treatment. New drugs to treat BED are urgently needed.

METHODS

A comprehensive review of published psychopathological, pharmacological and clinical findings.

RESULTS

The evidence supports the hypothesis that BED is an impulse control disorder with similarities to ADHD, including responsiveness to catecholaminergic drugs, for example LDX and dasotraline. The target product profile (TPP) of the ideal BED drug combines treating the psychopathological drivers of the disorder with an independent weight-loss effect. Drugs with proven efficacy in BED have a common pharmacology; they potentiate central noradrenergic and dopaminergic neurotransmission. Because of the overlap between pharmacotherapy in attention deficit hyperactivity disorder (ADHD) and BED, drug-candidates from diverse pharmacological classes, which have already failed in ADHD would also be predicted to fail if tested in BED. The failure in BED trials of drugs with diverse pharmacological mechanisms indicates many possible avenues for drug discovery can probably be discounted.

CONCLUSIONS

(1) The efficacy of drugs for BED is dependent on reducing its core psychopathologies of impulsivity, compulsivity and perseveration and by increasing cognitive control of eating. (2) The analysis revealed a large number of pharmacological mechanisms are unlikely to be productive in the search for effective new BED drugs. (3) The most promising areas for new treatments for BED are drugs, which augment noradrenergic and dopaminergic neurotransmission and/or those which are effective in ADHD.

Converging vulnerability factors for compulsive food and drug use

Highly palatable foods and substance of abuse have intersecting neurobiological, metabolic and behavioral effects relevant for understanding vulnerability to conditions related to food (e.g., obesity, binge eating disorder) and drug (e.g., substance use disorder) misuse. Here, we review data from animal models, clinical populations and epidemiological evidence in behavioral, genetic, pathophysiologic and therapeutic domains. Results suggest that consumption of highly palatable food and drugs of abuse both impact and conversely are regulated by metabolic hormones and metabolic status. Palatable foods high in fat and/or sugar can elicit adaptation in brain reward and withdrawal circuitry akin to substances of abuse. Intake of or withdrawal from palatable food can impact behavioral sensitivity to drugs of abuse and vice versa. A robust literature suggests common substrates and roles for negative reinforcement, negative affect, negative urgency, and impulse control deficits, with both highly palatable foods and substances of abuse. Candidate genetic risk loci shared by obesity and alcohol use disorders have been identified in molecules classically associated with both metabolic and motivational functions. Finally, certain drugs may have overlapping therapeutic potential to treat obesity, diabetes, binge-related eating disorders and substance use disorders. Taken together, data are consistent with the hypotheses that compulsive food and substance use share overlapping, interacting substrates at neurobiological and metabolic levels and that motivated behavior associated with feeding or substance use might constitute vulnerability factors for one another. This article is part of the special issue on 'Vulnerabilities to Substance Abuse'.

Changes in dopamine transporter binding in nucleus accumbens following chronic self-administration cocaine: heroin combinations

Concurrent use of cocaine and heroin (speedball) has been shown to exert synergistic effects on dopamine neurotransmission in the nucleus accumbens (NAc), as observed by significant increases in extracellular dopamine levels and compensatory elevations in the maximal reuptake rate of dopamine. The present studies were undertaken to determine whether chronic self-administration of cocaine, heroin or a combination of cocaine:heroin led to compensatory changes in the abundance and/or affinity of high- and low-affinity DAT binding sites. Saturation binding of the cocaine analog [(125) I] 3β-(4-iodophenyl)tropan-2β-carboxylic acid methyl ester ([(125) I]RTI-55) in rat NAc membranes resulted in binding curves that were best fit to two-site binding models, allowing calculation of dissociation constant (Kd ) and binding density (Bmax ) values corresponding to high- and low-affinity DAT binding sites. Scatchard analysis of the saturation binding curves clearly demonstrate the presence of high- and low- affinity binding sites in the NAc, with low-affinity sites comprising 85 to 94% of the binding sites. DAT binding analyses revealed that self-administration of cocaine and a cocaine:heroin combination increased the affinity of the low-affinity site for the cocaine congener RTI-55 compared to saline. These results indicate that the alterations observed following chronic speedball self-administration are likely due to the cocaine component alone; thus further studies are necessary to elaborate upon the synergistic effect of cocaine:heroin combinations on the dopamine system in the NAc.

Predicting abuse potential of stimulants and other dopaminergic drugs: overview and recommendations

Examination of a drug's abuse potential at multiple levels of analysis (molecular/cellular action, whole-organism behavior, epidemiological data) is an essential component to regulating controlled substances under the Controlled Substances Act (CSA). We reviewed studies that examined several central nervous system (CNS) stimulants, focusing on those with primarily dopaminergic actions, in drug self-administration, drug discrimination, and physical dependence. For drug self-administration and drug discrimination, we distinguished between experiments conducted with rats and nonhuman primates (NHP) to highlight the common and unique attributes of each model in the assessment of abuse potential. Our review of drug self-administration studies suggests that this procedure is important in predicting abuse potential of dopaminergic compounds, but there were many false positives. We recommended that tests to determine how reinforcing a drug is relative to a known drug of abuse may be more predictive of abuse potential than tests that yield a binary, yes-or-no classification. Several false positives also occurred with drug discrimination. With this procedure, we recommended that future research follow a standard decision-tree approach that may require examining the drug being tested for abuse potential as the training stimulus. This approach would also allow several known drugs of abuse to be tested for substitution, and this may reduce false positives. Finally, we reviewed evidence of physical dependence with stimulants and discussed the feasibility of modeling these phenomena in nonhuman animals in a rational and practical fashion. This article is part of the Special Issue entitled 'CNS Stimulants'.

Effects of chronic methylphenidate on cocaine self-administration under a progressive-ratio schedule of reinforcement in rhesus monkeys

It has been hypothesized that drugs that serve as substrates for dopamine (DA) and norepinephrine (NE) transporters may be more suitable medications for cocaine dependence than drugs that inhibit DA and NE uptake by binding to transporters. Previous studies have shown that the DA/NE releaser d-amphetamine can decrease cocaine self-administration in preclinical and clinical studies. The present study examined the effects of methylphenidate (MPD), a DA uptake inhibitor, for its ability to decrease cocaine self-administration under conditions designed to reflect clinically relevant regimens of cocaine exposure and pharmacotherapy. Each morning, rhesus monkeys pressed a lever to receive food pellets under a fixed-ratio 50 schedule of reinforcement; cocaine was self-administered under a progressive-ratio schedule of reinforcement in the evening. After cocaine (0.003-0.56 mg/kg per injection, i.v.) dose-response curves were determined, self-administration sessions were suspended and MPD (0.003-0.0056 mg/kg per hour, i.v.; or 1.0-9.0 mg/kg p.o., b.i.d.) was administered for several weeks. A cocaine self-administration session was conducted every 7 days. When a MPD dose was reached that either persistently decreased cocaine self-administration or produced disruptive effects, the cocaine dose-effect curve was re-determined. In most cases, MPD treatment either produced behaviorally disruptive effects or increased cocaine self-administration; it took several weeks for these effects to dissipate. These data are consistent with the largely negative results of clinical trials with MPD. In contrast to the positive effects with the monoamine releaser d-amphetamine under identical conditions, these results do not support use of monoamine uptake inhibitors like MPD as a medication for cocaine dependence.

Development of a simultaneous analytical method for selected anorectics, methamphetamine, MDMA, and their metabolites in hair using LC-MS/MS to prove anorectics abuse

Owing to the tight control of methamphetamine, it is presumed that phentermine, an amphetamine-type anorectic, has recently been considered a supplement for methamphetamine abusers in Korea. In addition, the abuse of other anorectics obtained by inappropriate means has become a social issue. Hair is a useful specimen to prove chronic drug use. Therefore, an analytical method for the simultaneous detection of phentermine, phendimetrazine, amfepramone, fenfluramine, mazindol, methamphetamine, and 3,4-methylenedioxymethamphetamine (MDMA), as well as their metabolites, which covers the major amphetamines and anorectic agents in Korea, in hair was established and validated using liquid chromatography-tandem mass spectrometry (LC-MS/MS). The drugs and their metabolites in hair were extracted using 1 % HCl in methanol and then filtered and analyzed by LC-MS/MS with electrospray ionization in positive mode. The validation results for selectivity, linearity, matrix effect, recovery, process efficiency, intra- and interassay precision and accuracy, and processed sample stability were satisfactory. The limits of detection ranged from 0.025 to 1 ng/10 mg hair and the limits of quantification were 0.25 ng/10 mg hair for every analyte except mazindol and phentermine, for which they were 10 ng/10 mg hair. The method was successfully applied for the segmental determination of selected anorectics, methamphetamine, MDMA, and their metabolites in hair from 39 drug suspects. Among the anorectics, phentermine and/or phendimetrazine were identified with or without methamphetamine in the hair samples. Closer supervision of the inappropriate use of anorectics is necessary. Also, hair analysis is useful for monitoring the abuse potential of unnoticed drugs.

Simultaneous measurement of extracellular dopamine and dopamine transporter occupancy by cocaine analogs in squirrel monkeys

Several classes of drugs bind to the dopamine transporter (DAT) with high affinity, but some are weaker positive reinforcers than cocaine, suggesting that affinity for and occupancy of the DAT is not the only determinant of a drug's reinforcing effectiveness. Other factors such as the rate of onset have been positively and strongly correlated with the reinforcing effects of DAT inhibitors in nonhuman primates. In the current studies, we examined the effects of acute systemic administration of cocaine and three cocaine analogs (RTI-150, RTI-177, and RTI-366) on binding to DAT in squirrel monkey brain using positron emission tomography (PET) neuroimaging. During the PET scan, we also measured drug effects on dopamine (DA) levels in the caudate using in vivo microdialysis. In general, our results suggest a lack of concordance between drug occupancy at DAT and changes in DA levels. These studies also indicate that acute cocaine administration decreases the availability of plasma membrane DAT for binding, even after cocaine is no longer blocking DA uptake as evidence by a return to basal DA levels.

Lower reinforcing strength of the phenyltropane cocaine analogs RTI-336 and RTI-177 compared to cocaine in nonhuman primates

Drugs that inhibit brain dopamine transporters (DAT) have been developed as potential agonist medications for cocaine abuse and dependence. Because the mechanism of action of such drugs is similar to cocaine, one concern regarding their use is the abuse potential of the medications themselves. The present study compared the reinforcing strength of cocaine (0.003-0.3mg/kg) and two 3-phenyltropane analogs of cocaine, RTI-336 (3beta-(4-chlorophenyl)-2beta-[3-(4'-methylphenyl)isoxazol-5-yl]tropane hydrochloride; 0.003-0.1mg/kg) and RTI-177 (3beta-(4-chlorophenyl)-2beta-[3-phenylisoxazol-5-yl]tropane hydrochloride; 0.003-0.1mg/kg), using a progressive-ratio (PR) schedule in rhesus monkeys (n=4). PR schedules of reinforcement are frequently used to measure reinforcing strength of drugs. Earlier research using limited-access conditions reported that cocaine was a stronger reinforcer than either RTI-336 or RTI-177. Because the 3-phenyltropanes have longer durations of action, one purpose of the present study was to examine reinforcing strength using longer experimental sessions. Under these conditions, cocaine functioned as a reinforcer in all monkeys, and RTI-336 and RTI-177 functioned as a reinforcer in three of four subjects. Consistent with their documented slower onset of neurochemical and pharmacological effects, RTI-336 and RTI-177 were weaker reinforcers, resulting in fewer injections than cocaine. On average, the potencies of the two RTI compounds were not different than that of cocaine. These results support the view that slow-onset DA-selective uptake inhibitors have lower abuse liability than cocaine. In addition, the present findings suggest that changes in PR session length can influence potency comparisons between drugs, but not measures of reinforcing strength.

Medication-related pharmacological manipulations of nicotine self-administration in the rat maintained on fixed- and progressive-ratio schedules of reinforcement

RATIONALE

The use of animal models to study existing medications for smoking cessation can elucidate the mechanism(s) of action of cessation agents and further validate the models for medication development.

OBJECTIVE

The objective of the study was to evaluate the response of nicotine self-administration (NSA) to pharmacological agents related to the smoking cessation medication bupropion and to nicotine dosing mimicking nicotine replacement on fixed-ratio (FR) and progressive-ratio (PR) schedules of reinforcement.

MATERIALS AND METHODS

NSA was maintained at a nicotine dose of 30 microg/kg/infusion i.v. in rats trained on FR5 and PR40% schedules. Pharmacological manipulations related to bupropion were examined by treating animals with a dopamine reuptake inhibitor [GBR 12909 (GBR)], a norepinephrine reuptake inhibitor [nisoxetine (NIS)], and a nicotinic antagonist [dihydro-beta-erythroidine (DHbetaE)]. The effect of nicotine replacement was examined on the PR schedule by chronic dosing with osmotic minipumps.

RESULTS

Significant treatment effects occurred with NIS and combinations of NIS-DHbetaE and with GBR on response rates. Chronic nicotine dosing reduced self-administration. The two schedules yielded different results with some treatments.

CONCLUSIONS

Noradrenergic-nicotinic cholinergic interactions and enhanced responding consequent to dopamine reuptake inhibition may be part of the complex behavioral pharmacology of bupropion-like compounds. Observation of differential results with the two schedules has implication for the use of self-administration techniques to elaborate the mechanisms of dependence as well as drug discovery.

Short-acting cocaine and long-acting GBR-12909 both elicit rapid dopamine uptake inhibition following intravenous delivery

The rewarding effects of cocaine have been reported to occur within seconds of administration. Extensive evidence suggests that these actions involve the ability of cocaine to inhibit the dopamine (DA) transporter. We recently showed that 1.5 mg/kg i.v. cocaine inhibits DA uptake within 5 s. Despite this evidence, there remains a lack of consensus regarding how quickly i.v. cocaine and other DA uptake inhibitors elicit DA uptake inhibition. The current studies sought to better characterize the onset of cocaine-induced DA uptake inhibition and to compare these effects to those obtained with the high-affinity, long-acting DA transporter inhibitor, GBR-12909 (1-(2-bis(4-fluorphenyl)-methoxy)-ethyl)-4-(3-phenyl-propyl)piperazine). Using in vivo fast scan cyclic voltammetry, we showed that i.v. cocaine (0.75, 1.5, and 3.0 mg/kg) significantly inhibited DA uptake in the nucleus accumbens of anesthetized rats within 5 s. DA uptake inhibition peaked at 30 s and returned to baseline levels in approximately 1 h. The effects of cocaine were dose-dependent, with the 3.0 mg/kg dose producing greater uptake inhibition at the early time points and exhibiting a longer latency to return to baseline. Further, the blood-brain barrier impermeant cocaine-methiodide had no effect on DA uptake or peak height, indicating that the generalized peripheral effects of cocaine do not contribute to the CNS alterations measured here. Finally, we show that GBR-12909 (0.75, 1.5, and 3.0 mg/kg) also significantly inhibited DA uptake within 5 s post-injection, although the peak effect and return to baseline were markedly delayed compared with cocaine, particularly at the highest dose. Combined, these observations indicate that the central effects of dopamine uptake inhibitors occur extremely rapidly following i.v. drug delivery.

Differential antagonism of cocaine self-administration and cocaine-induced disruptions of learning by haloperidol in rhesus monkeys

Six rhesus monkeys responding under a three-component multiple schedule were administered haloperidol to determine its effects on cocaine self-administration and on cocaine's disruptive effects on the repeated acquisition and performance of response chains. In the absence of haloperidol, 0.0032-0.032 mg/kg/infusion of cocaine increased response rate and the number of infusions in the self-administration component when compared to saline administration, whereas 0.1-0.32 mg/kg/infusion decreased response rate and the number of infusions. When compared to saline administration, the two lowest infusion doses of cocaine had little or no effect on responding in the acquisition and performance components; however, higher infusion doses of cocaine dose-dependently decreased response rate in these components. In addition, the higher doses of cocaine also increased the percentage of errors in the acquisition and performance components. Pretreatment with haloperidol (0.0032 or 0.01 mg/kg, i.m.) antagonized the effects of low doses of cocaine on the number of infusions in the self-administration component, whereas only the 0.01-mg/kg dose antagonized the effects of high doses of cocaine on the number of infusions. Neither dose of haloperidol antagonized the rate-decreasing effects of cocaine on responding in the acquisition and performance components significantly; the highest dose of haloperidol alone decreased rates of responding in each component. Antagonism of cocaine's error-increasing effects by haloperidol was only evident at one dose of cocaine (0.032 mg/kg/infusion), and was more complete in the performance components than in the acquisition components. Together, these data show the limited suitability of haloperidol for selectively antagonizing cocaine self-administration in the context of a multiple schedule involving transition behavior, and show the lack of uniform antagonism across operant behaviors.

Faster onset and dopamine transporter selectivity predict stimulant and reinforcing effects of cocaine analogs in squirrel monkeys

Although the behavioral-stimulant and reinforcing effects of cocaine and related psychomotor stimulants have been attributed to their actions at the dopamine transporter (DAT), the reinforcing effectiveness of these compounds varies. The properties that confer these differences are important considerations when developing agonist pharmacotherapies for the treatment of stimulant abuse. The present studies focused on the time course of action and pharmacological specificity of six 3-phenyltropane analogs of cocaine (RTI-112, RTI-126, RTI-150, RTI-171, RTI-177, and RTI-336) by observing their behavioral-stimulant, neurochemical, and reinforcing effects in squirrel monkeys. The faster-onset analogs (RTI-126, RTI-150, and RTI-336), and one of the slower-onset DAT selective analogs (RTI-177 and RTI-171) produced behavioral-stimulant effects, while the slower-onset nonselective analog RTI-112 did not. The time to the peak behavioral-stimulant effect and the peak caudate dopamine levels was strongly correlated, but the area under the curve of the time course of behavioral-stimulant effect and dopamine levels was not correlated. These results suggest that the rate of onset plays a more important role than duration of action in the stimulant effect of these analogs. In addition, the slower-onset nonselective analog (RTI-112) clearly did not exhibit any reinforcing effects while the faster-onset nonselective (RTI-126) and all the DAT-selective analogs showed robust reinforcing effects (RTI-150, and RTI-177) or showed trends towards reinforcing effects (RTI-336 and RTI-171). Hence, there was a general trend for compounds that had a faster onset and/or DAT selectivity to produce significant behavioral-stimulant and reinforcing effects.

Synthesis of dopamine transporter selective 3-diarylmethoxymethyl-8-arylalkyl-8-azabicyclo[3.2.1]octane derivatives

A series of diarylmethoxymethyltropane-GBR hybrid analogues with all three possible stereochemical orientations at C3 were synthesized and evaluated at dopamine and serotonin transporters. The 3alpha derivatives were found to be the most potent compounds with the 3alpha-di(4-fluorophenyl)methoxymethyl-8-(3-phenylpropyl)-8-azabicyclo[3.2.1]octane 15b (Ki = 5 nM) being the most potent compound of the series. The corresponding 3-di(4-fluorophenyl)-methoxymethyl-8-(3-phenylpropyl)-8-azabicyclo[3.2.1]oct-2-ene 12b (Ki = 12 nM) was slightly less potent than the 3alpha-analogue, while the 3beta-di(4-fluorophenyl)methoxymethyl-8-(3-phenylpropyl)-8-azabicyclo[3.2.1]octane 23b (Ki = 78 nM) exhibited only modest affinity for the dopamine transporter. Only the 3alpha-analogue 15b (SERT/DAT = 48) exhibited higher SERT/DAT selectivity than GBR 12909. These results indicate that the dopamine transporter can tolerate some variability in proximity of the benzhydryl ether to the basic nitrogen atom of the tropane without loss in potency. In addition, the structure-activity data for these tropane-GBR 12909 hybrid analogues support previous findings that the stereochemical and conformational effects imparted by unsaturation at C3 are important for dopamine transporter selectivity over the serotonin transporter.

Interactions of Cocaine with Dopamine Uptake Inhibitors or Dopamine Releasers in Rats Discriminating Cocaine

Several dopamine (DA) indirect agonists have been proposed as potential medications for treating cocaine abuse. The objective of the present study was to quantify the interactions among cocaine and DA uptake inhibitors or DA releasers to better understand how these drugs may be working when administered in combination. The DA uptake inhibitors GBR 12909 [1-{2-[bis-(4-fluorophenyl)methoxy]-ethyl}-4-(3-phenylpropyl)piperazine], WIN 35,428 [2beta-carbomethoxy-3beta-(4-fluorophenyl)tropane], methylphenidate, indatraline, nomifensine, and mazindol and DA releasers methamphetamine, d-amphetamine, methcathinone, cathinone, fencamfamine, and phentermine were examined alone and in combination with cocaine in rats trained to discriminate cocaine (10 mg/kg i.p.) from saline injections. All of the DA indirect agonists dose-dependently substituted for cocaine and shifted the cocaine dose-effect curve leftward. Isobolographic analysis indicated the interactions were generally additive, although both methamphetamine and d-amphetamine were quantitatively determined to be more potent than DA uptake inhibitors in shifting the cocaine dose-effect function to the left. The potential of d-amphetamine as an effective treatment for cocaine abuse and negative clinical results with dopamine uptake inhibitors suggest that differences in shifts in dose-effect curves should be further examined with emerging clinical data as a predictive index of potential treatments for cocaine abuse.

Pharmacological determinants of the reinforcing effects of psychostimulants: relation to agonist substitution treatment

Illicit use of psychostimulants, such as cocaine and methamphetamine, continues to pose a significant public health concern. On the basis of the relative success at treating opiate and tobacco users with agonist substitution treatments, this strategy has been pursued in the search for a pharmacotherapy for psychostimulant addiction. The reinforcing effects of drugs are central to their abuse liability; therefore, gaining a better understanding of the factors that determine the reinforcing effects of psychostimulants should inform the development of an effective treatment. Although the reinforcing effects of drugs are known to be multiply determined, the author's dissertation research focused on pharmacological factors. This review presents results from that research as well as findings reported in the extant literature, suggesting that the reinforcing effects of psychostimulant drugs are determined both by their pharmacodynamic and pharmacokinetic profiles. There is evidence to support the conclusion that affinity for dopamine transporters appears to be of critical importance, whereas serotonin transporters seem to serve a modulatory function. A more rapid rate of onset may enhance a drug's reinforcing effects, but a drug with a slow onset can still maintain self-administration. A drug's duration of action may only influence the rate but not the strength of responding that is maintained. Slow-onset, long-acting monoamine transporter ligands can be expected to have reinforcing effects and therefore abuse liability, which has implications for the use of these drugs as pharmacotherapies. Nonetheless, on the basis of promising preclinical and clinical findings, this appears to represent a viable treatment strategy.

The mesolimbic dopamine system: the final common pathway for the reinforcing effect of drugs of abuse?

In this review we will critically assess the hypothesis that the reinforcing effect of virtually all drugs of abuse is primarily dependent on activation of the mesolimbic dopamine system. The focus is on five classes of abused drugs: psychostimulants, opiates, ethanol, cannabinoids and nicotine. For each of these drug classes, the pharmacological and physiological mechanisms underlying the direct or indirect influence on mesolimbic dopamine transmission will be reviewed. Next, we evaluate behavioral pharmacological experiments that specifically assess the influence of activation of the mesolimbic dopamine system on drug reinforcement, with particular emphasis on animal experiments using drug self-administration paradigms. There is overwhelming evidence that all five classes of abused drugs increase dopamine transmission in limbic regions of the brain through interactions with a variety of transporters, ionotropic receptors and metabotropic receptors. Behavioral pharmacological experiments indicate that increased dopamine transmission is clearly both necessary and sufficient to promote psychostimulant reinforcement. For the other four classes of abused substances, self-administration experiments suggest that although increasing mesolimbic dopamine transmission plays an important role in the reinforcing effects of opiates, ethanol, cannabinoids and nicotine, there are also dopamine-independent processes that contribute significantly to the reinforcing effects of these compounds.

Molecular mechanisms underlying the rewarding effects of cocaine

The initially surprising observation that cocaine retains its rewarding effects in dopamine transporter (DAT) knockout (KO) mice led our laboratory to examine the effects of deletion of other monoaminergic genes on cocaine reward. Our initial approach to this problem was to combine DAT KO mice with serotonin transporter (SERT) KO mice to make combined DAT/SERT KO mice. The combination of these knockouts eliminates cocaine reward as assessed in the conditioned place preference (CPP) paradigm. We have also identified evidence that, in the absence of DAT, there is greater participation in cocaine reward by serotonin (SERT) and norepinephrine (NET) transporters. Both NET and SERT blockers (nisoxetine and fluoxetine) produced significant CPPs in DAT KO mice, but not in wild-type (WT) mice. The striking elimination of cocaine CPP in combined DAT/SERT KO mice contrasts with effects that we have identified in combined NET/SERT knockout mice, which display increases in cocaine reward, and with recent reports that suggest that DAT/NET combined KOs retain substantial cocaine CPP. Overall, these studies indicate important requirements for several monoaminergic system genes to fully explain cocaine reward, in particular those expressed by dopamine and serotonin systems.

Effects of dopamine indirect agonists and selective D1-like and D2-like agonists and antagonists on cocaine self-administration and food maintained responding in rats

A procedure is described for comprehensive evaluation of the effects of acute drug pretreatments on the reinforcing effects of cocaine using the rat self-administration assay in combination with a novel control assay of liquid-food maintained responding. In sessions comprised of five 20-min components, either complete dose-effect functions for cocaine self-administration or complete concentration-effect functions for liquid-food maintained responding were evaluated. The schedule of reinforcement (FR 5 TO 20-s), drug pretreatment doses and time intervals (0-30 min), and duration of sessions (108 min) were identical for cocaine- and food-reinforced test sessions. Whereas acute pretreatment with indirect dopamine agonists (D-amphetamine, GBR 12909) and D2-like agonists (7-OH-DPAT, quinelorane) produced dose-dependent leftward shifts in dose-effect functions for cocaine self-administration, D1-like agonists (SKF 82958, R-6-Br-APB) and dopamine antagonists (D1-like, SCH 39166; D2-like, eticlopride) shifted dose-effect functions for cocaine downward and rightward, respectively. Interestingly, with the indirect dopamine agonists but not the D2-like agonists, increased responding maintained by low cocaine doses was paralleled by increased responding maintained by low food concentrations. Moreover, three of the four direct agonists were moderately selective (< or =5-fold more potent) in decreasing cocaine self-administration relative to food maintained responding. When data were analyzed according to alterations in total cocaine intake, all of the agonists uniformly decreased total cocaine intake, whereas both antagonists increased total cocaine intake. Overall, this procedure was sensitive to leftward, downward and rightward shifts in cocaine dose-effect functions and should be useful for evaluating the nature of pharmacological interactions between novel compounds and self-administered cocaine, as well as the potential for altering cocaine self-administration selectively with candidate treatments for cocaine abuse and dependence.

Monoamine transporters and psychostimulant drugs

Most psychostimulants interact with monoamine transport proteins. This paper reviews work our laboratory has conducted to investigate the interaction of psychostimulants with monoamine transporters in order to advance our understanding of how these drugs affect the brain. We review two topics: (1) characterization of multiple binding sites for cocaine-like drugs and (2) an examination of the mechanisms of action of amphetamine-type anorectic agents. We conclude that the brain contains high abundance nonclassical binding sites for cocaine-like drugs that have micromolar affinity for cocaine and that none of the clinically available amphetamine-type appetite suppressants are equipotent substrates for dopamine transporter (DAT) and serotonin transporter (SERT) proteins. Future medications discovery efforts should focus on identifying new compounds which possess the equipotent substrate activity at DAT and SERT, but which lack the adverse effects of stimulants developed decades ago.

The reinforcing efficacy of psychostimulants in rhesus monkeys: the role of pharmacokinetics and pharmacodynamics

This study was undertaken to investigate pharmacological variables that influence the reinforcing efficacy of psychostimulants. Rhesus monkeys (n = 9) responded under a within-session, exponentially increasing, progressive ratio schedule of cocaine reinforcement. Doses of cocaine, methylphenidate (MP), cocaine analogs [(+/-)-2beta-propanoyl-3beta-(2-naphthyl)-tropane (WF-23), HD-23; (+/-)-2beta-propanoyl-3beta-(2-isopropenyl)tropane (WF-60), HD-60; and 2beta-propanoyl-3beta-(4-tolyl)-tropane (HD-11, WF-11), and 2beta-propanoyl-3beta-(4-tolyl)-tropane (HD-11, WF-11), PTT], and MP analogs [(alphaR,2R)-alpha-(2-naphthalenyl)-2-piperidineacetic acid methyl ester, HDMP-28; and (alphaR,2S)-alpha-(2-naphthalenyl)-2-pyrrolideneacetic acid methyl ester, HDMP-29] that varied in their pharmacokinetic and pharmacodynamic properties were substituted for cocaine. These drugs were chosen according to their selectivity for dopamine transporters (DAT) and 5-hydroxytryptamine (serotonin) transporters (5-HTT) as assessed in rodents and their duration of action. In addition, data pertaining to the rate of onset at DAT were collected for the cocaine analogs using an ex vivo binding assay in rodent tissue. Finally, the pharmacodynamic profile of select drugs was confirmed in primate brain tissue. All drugs had reinforcing effects except HDMP-29. The rank ordering of the peak breaking points (BPs) was cocaine = MP = HDMP-28 >or= HD-60 >or= PTT >or= HD-23 > HDMP-29. The time to peak DAT occupancy for the cocaine analogs was greater than 30 min. The potency to maintain peak BP was significantly correlated with DAT affinity. There was not a linear relationship between monoamine transporter affinity and reinforcing efficacy, but it appeared that in nonhuman primates there is a range of DAT affinity under which maximal responding is maintained. Interestingly, the 5-HTT-selective cocaine analog HD-60 functioned robustly as a reinforcer at several doses in all monkeys tested. These data question the dogma regarding the role of pharmacokinetic factors and the relative influence of DAT and 5-HTT in stimulant reinforcement.

Synthesis of dopamine transporter selective 3-[2-(diarylmethoxyethylidene)]-8-alkylaryl-8-azabicyclo[3.2.1]octanes

A series of 3-[2-(diarylmethoxyethylidene)]-8-alkylaryl-8-azabicyclo[3.2.1]octanes was synthesized and the binding affinities of the compounds were determined at the dopamine and serotonin transporters. The 8-phenylpropyl analogues 8a (K(i)=4.1 nM) and 8b (K(i)=3.7 nM) were the most potent compounds of the series with binding affinities 3 times greater than GBR-12909. In addition, 8a (SERT/DAT=327) was over 300-fold more selective for the dopamine transporter than the serotonin transporter.

Reinforcing strength of a novel dopamine transporter ligand: pharmacodynamic and pharmacokinetic mechanisms

Drugs that block dopamine uptake often function as positive reinforcers but can differ along the dimension of strength or effectiveness as a positive reinforcer. The present study was designed to examine pharmacological mechanisms that might contribute to differences in reinforcing strength between the piperidine-based cocaine analog (+)-methyl 4beta-(4-chlorophenyl)-1-methylpiperidine-3-alpha-carboxylate [(+)-CPCA] and cocaine. Drugs were made available to rhesus monkeys (n = 5) for i.v. self-administration under a progressive ratio schedule. Both compounds maintained responding with sigmoidal or biphasic dose-response functions (0.1-1.0 mg/kg/injection). (+)-CPCA was one-fourth as potent as cocaine and maintained fewer injections per session, at maximum. For in vitro binding in monkey brain tissue, (+)-CPCA was about one-half as potent as cocaine at the dopamine transporter (DAT), and the two compounds had similar affinities at the norepinephrine transporter. (+)-CPCA was less than 1/10 as potent as cocaine at the serotonin transporter. In ex vivo binding in rat striatum, occupancy of the DAT increased directly with dose to a maximum of approximately 80% for both compounds, and (+)-CPCA was about one-fourth as potent as cocaine. Ex vivo DAT occupancy was significantly higher for cocaine than (+)-CPCA at 2 min after injection but similar at other times. Thus, the primary differences between these compounds were in serotonin transporter affinity and the kinetics of DAT binding. These results suggest that (+)-CPCA is a weaker positive reinforcer than cocaine because it has a slower onset of action over the first few minutes after i.v. injection.

Synthesis and dopamine transporter binding affinities of 3alpha-benzyl-8-(diarylmethoxyethyl)-8-azabicyclo[3.2.1]octanes

A series of 3alpha-benzyl-8-(diarylmethoxyethyl)-8-azabicyclo[3.2.1]octanes was synthesized and the binding affinities of the compounds were determined at the dopamine transporter. The unsubstituted analogue 7b (K(i)=98nM) was the most potent compound of the series with binding affinity three-times greater than cocaine and only 5-fold less than GBR-12909. The structure-activity data for 7a-f suggests that these compounds may be binding at the dopamine transporter in a similar fashion to GBR 12909.

Appetite suppressants as agonist substitution therapies for stimulant dependence

Several lines of evidence support a dual-deficit model of stimulant withdrawal in which decreases in synaptic dopamine (DA) and serotonin (5-HT) contribute to withdrawal symptoms, drug craving, and relapse. According to the dual-deficit model, DA dysfunction during withdrawal underlies anhedonia and psychomotor disturbances, whereas 5-HT dysfunction gives rise to depressed mood, obsessive thoughts, and lack of impulse control. The model suggests that medications capable of normalizing stimulant-induced DA and 5-HT deficits should be effective treatment adjuncts. Furthermore, the model may explain why medications targeting only one neurotransmitter system (i.e., DA) have failed to treat cocaine dependence. Amphetamine-type appetite suppressants are logical choices for neurochemical normalization therapy of stimulant dependence, yet few clinical studies have tested anorectics in this regard. The chief purpose of the present work is to profile the activity of various anorectic agents at DA, 5-HT, and NE transporters, in order to identify possible medications for stimulant dependence. Compounds were tested in vitro for their ability to stimulate release and inhibit uptake of [(3)H]DA, [(3)H]NE, and [(3)H]5-HT. Selected compounds were tested in vivo for their ability to elevate extracellular levels of DA and 5-HT in rat nucleus accumbens. The results show that clinically available appetite suppressants display a wide range of activities at monoamine transporters. However, no single medication possesses equal potency at DA and 5-HT transporters, suggesting that none of the anorectics is ideally suited for treatment of stimulant addictions. Future efforts should focus on developing new medications that possess the desired therapeutic activity but lack the adverse effects associated with older amphetamine-type anorectics.