5-HTTLPR genotype and gender, but not chronic fluoxetine administration, are associated with cortical TREK1 protein expression in rhesus macaques.

TREK1 is a widely expressed background potassium channel. Similar to mice treated with selective serotonin reuptake inhibitors (SSRIs), TREK1 knockout mice are resistant to depression-like behavior and have elevated serotonin levels leading to speculation that TREK1 inhibition may contribute to the therapeutic effects of SSRIs. This study examined how chronic fluoxetine administration and a common functional polymorphism in the serotonin-transporter-linked promoter region (5-HTTLPR) influence cortical TREK1 expression in 24 rhesus monkeys. The short rh5-HTTLPR allele as well as female gender were associated with reduced cortical TREK1 protein expression but chronic SSRI administration had no effect. These results suggest that serotonin may influence TREK1, but that chronic SSRI treatment does not result in long lasting changes in cortical TREK1 protein expression. TREK1 gender differences may be related to gender differences in serotonin and require further research.

Self-administration of cocaine-pentobarbital mixtures by rhesus monkeys.

A number of experiments have evaluated self-administration of the combination of a stimulant and an opioid. Less is known about the combination of a stimulant and a CNS depressant. The present experiment was designed to examine self-administration of the mixture of cocaine and pentobarbital (PB). Rhesus monkeys (n=4) prepared with i.v. catheters were allowed to self-administer cocaine or saline under a progressive-ratio schedule. When responding was stable, doses of cocaine and PB, alone or in combination, were made available in test sessions. Cocaine functioned as a positive reinforcer in a dose-related manner in all monkeys. PB functioned as a relatively weaker reinforcer in one of four monkeys. Self-administration of intermediate doses of cocaine (0.025-0.1mg/kg per injection) was decreased when mixed with PB (0.05-0.2mg/kg per injection); full maximum responding was re-established when cocaine dose was increased. The magnitude of the shift to the right in the cocaine dose-response function was directly related to PB dose. When PB was given as an i.v. pretreatment there was no effect on cocaine self-administration up to a sedative dose of PB (5.6 mg/kg), suggesting that responding was not non-specifically suppressed by PB. Thus, simultaneous self-administration of PB diminished the potency but not the strength of cocaine as a reinforcer, potentially encouraging self-administration of larger doses of cocaine.

Self-administration of cocaine-remifentanil mixtures by monkeys: an isobolographic analysis.

RATIONALE: Abuse of mixtures of stimulants and opioids ("speedball") is common. Although this combination has been studied in the laboratory, conclusions about the nature of the cocaine/opioid interaction have been mixed. OBJECTIVES: The objectives of the present experiment were to allow monkeys to self-administer mixtures of cocaine and the mu opioid agonist remifentanil and to quantify the interaction using the isobolographic approach. Our hypothesis was that the drugs would be super-additive in their reinforcing effects. MATERIALS AND METHODS: Rhesus monkeys (n = 5) prepared with i.v. catheters were allowed to self-administer cocaine or saline under a progressive-ratio schedule. When responding was stable, doses of cocaine or remifentanil were made available in test sessions. Next, mixtures of doses of the drugs were tested over a range of doses in 1:1, 1:2, and 2:1 ratios of their ED(50)s. Results were analyzed using isobolographic techniques. RESULTS: Both drugs alone and all drug mixtures functioned as positive reinforcers in a dose-related manner. Cocaine maintained more responding at maximum than did remifentanil, i.e., was a stronger reinforcer. The experimentally determined equi-effective dose for the 1:1 and 1:2 cocaine/remifentanil mixtures tended toward super-additivity, but the difference from additivity did not achieve statistical significance. The 2:1 mixture was super-additive. Maximum responding maintained by the mixtures was higher than that maintained by remifentanil but not different from cocaine. CONCLUSIONS: Combinations of cocaine and remifentanil can be additive or super-additive as positive reinforcers, depending on proportions of each. Interactions between stimulants and opioids may contribute to the abuse of these mixtures.

Self-administration of drug mixtures by monkeys: combining drugs with comparable mechanisms of action.

RATIONALE: Abuse of drug mixtures is common. Drug interactions that are super-additive in terms of reinforcing effects may contribute to this phenomenon. Although quantitative methods for assessing drug interactions have been developed, they have not been widely applied to the analysis of reinforcing effects. OBJECTIVES: The present experiment was designed to study self-administration of mixtures of drugs with comparable pharmacological mechanisms of action. Our hypothesis was that the drugs would be dose-additive. MATERIALS AND METHODS: Rhesus monkeys prepared with i.v. catheters were allowed to self-administer cocaine or saline under a progressive-ratio schedule in baseline sessions. When responding was stable, two mu opioid agonists, alfentanil and remifentanil, were tested alone in one group (n = 5). Two dopamine (DA) uptake blockers, cocaine and RTI-117 were tested in the other group (n = 6). Next, mixtures of doses of the two opioids or the two DA uptake blockers were tested in approximate 1:1, 1:2, and 2:1 ratios of their ED50s. Results were analyzed using isobolographic techniques. RESULTS: All drugs alone and drug mixtures functioned as positive reinforcers in a dose-related manner. There was no difference between experimentally determined ED50 values and predicted additive ED50 values for any mixture. Maximum responding maintained by mixtures, a measure of reinforcing strength, did not differ from that for single drugs. CONCLUSIONS: Mixtures of various proportions of two drugs with comparable mechanisms of action were additive, i.e., they did not interact. This result will serve as the basis for comparison to studies of mixtures of drugs with various mechanisms of action.

Relationship between the serotonergic activity and reinforcing effects of a series of amphetamine analogs.

It has been reported that among drugs with mixed actions on central nervous system monoamine systems, increased serotonergic activity is associated with decreased potency as a reinforcer. The present experiment was designed to examine this relationship for amphetamine analogs that varied in serotonin releasing potency and to evaluate whether serotonergic actions can affect reinforcing efficacy. Compounds PAL 313 and 314 are para- and meta-methylamphetamine, respectively. PAL 303 and 353 are para- and meta-fluoroamphetamine, respectively. All compounds had similar potencies as in vitro releasers of dopamine (DA) and norepinephrine (NE) but differed in potency for 5-hydroxytryptamine (serotonin) (5-HT) release [EC(50) (nanomolar) PAL 313 = 53.4; PAL 314 = 218; PAL 303 = 939; PAL 353 = 1937]. When made available to rhesus monkeys (Macaca mulatta)(n = 4) for self-administration under a fixed-ratio 25 schedule, all were positive reinforcers with biphasic dose-response functions (0.003-1.0 mg/kg) and were equipotent. PAL 313 was self-administered at a lower rate than the other compounds, which were indistinguishable. Under a progressive-ratio schedule (n = 5), all drugs were positive reinforcers. Dose-response functions increased to a maximum or were biphasic (0.01-1.0 mg/kg), and drugs were equipotent. At maximum, PAL 313 maintained less responding than other PAL drugs, which maintained similar maxima. Thus, all compounds were positive reinforcers under both schedules, consistent with their potent DA actions. Responding was lower when 5-HT potency was higher and comparable with DA and NE potency. The results suggest that the mechanism for this effect involves a decrease in reinforcing potency and efficacy among monoamine releasing agents when 5-HT releasing potency is increased relative to DA.

Reinforcing and discriminative stimulus effects of 1-benzylpiperazine and trifluoromethylphenylpiperazine in rhesus monkeys.

1-Benzylpiperazine (BZP) and 1-(3-trifluoromethylphenyl)piperazine (TFMPP) are two designer drugs that are often sold in combination tablets via the internet. The discriminative stimulus properties and reinforcing effects of these compounds have not previously been assessed in laboratory primates. In this regard, the reinforcing effects of BZP and TFMPP (alone, and in combination) were assessed via intravenous self-administration in rhesus monkeys previously trained to self-administer cocaine, while the discriminative stimulus effects of these compounds were determined in rhesus monkeys trained to discriminate amphetamine (AMPH) from saline. BZP was an effective reinforcer in self-administration tests, and appeared to induce long-lasting direct effects on behavior following sessions where BZP intakes were large. Additionally, BZP occasioned AMPH-appropriate responding in a dose-dependent manner, and produced full generalization in all monkeys tested. In contrast, TFMPP was not self-administered by any subjects and occasioned essentially no AMPH-appropriate responding at any dose tested. Non-contingent TFMPP administration had direct effects on behavior and abolished subsequent cocaine-maintained responding. Similarly, self-administration of various ratios of BZP:TFMPP combinations engendered less responding than did BZP alone. The present results suggest that BZP has abuse liability of the amphetamine type, but that such effects are not shared by TFMPP.

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.

Reinforcing and discriminative-stimulus effects of ephedrine isomers in rhesus monkeys.

Ephedrine is a sympathomimetic drug that is currently found in many over-the-counter preparations. This compound exists as four isomers which, in addition to a racemic mixture, were evaluated for their positive reinforcing effects and for their similarity to (+)-amphetamine as a discriminative stimulus. Rhesus monkeys (N=3) with intravenous cocaine (0.1 mg/kg/inj) or saline as a consequence for lever pressing were shown to self-administer all of the ephedrine compounds (range tested: 0.03-3.0 mg/kg/inj), with the exception of (-)-pseudoephedrine, when each drug/dose was substituted for cocaine or saline during test sessions. However, the (-)-pseudoephedrine isomer was evaluated within a limited dose range due to solubility limitations. Systematically increasing the number of responses required for an injection indicated that these isomers were not as effective as reinforcers as was cocaine. Rhesus monkeys (N=3) trained to discriminate intragastric 1.0 mg/kg (+)-amphetamine from saline were given substitution tests with the ephedrine isomers and the racemic mixture. When given intragastrically, at least one dose of all the ephedrine isomers substituted for the (+)-amphetamine discriminative stimulus in at least one of the subjects tested. However, (+)-amphetamine-like effects were not systematically related to dose. When the discriminative-stimulus effects of (-)-ephedrine were also compared with those of (+)-amphetamine across three different routes of administration, full, dose-related, (+)-amphetamine-like responding was observed with both the intramuscular and intravenous routes. Taken together, these results suggest that the ephedrines have psychomotor stimulant-like abuse potential, lower than that of cocaine. Parenteral administration may enhance psychomotor-stimulant-like effects.

Role of the dopamine transporter and the sodium channel in the cocaine-like discriminative stimulus effects of local anesthetics in rats.

RATIONALE: Local anesthetics bind to the dopamine transporter (DAT), inhibit dopamine (DA) uptake and have been reported to have cocaine-like discriminative stimulus effects. The hypothesis of the present study was that affinity at the DAT and potency as a DA uptake blocker determines potency as a cocaine-like discriminative stimulus among local anesthetics, and maximum DA uptake inhibition determines maximum cocaine-like discriminative stimulus effects. OBJECTIVES: Cocaine-like discriminative potency was compared to DAT affinity and DA uptake inhibition potency, and maximum cocaine-like discriminative stimulus effects were compared to maximum DA uptake inhibition for procaine, chloroprocaine, dimethocaine, tetracaine and lidocaine. METHODS: Discriminative stimulus effects were determined in two groups of rats using 10 mg/kg and 3.0 mg/kg cocaine training doses. DAT affinity and DA uptake inhibition effects were determined in vitro in rat caudate nucleus tissue. Additionally, sodium channel affinity was determined in rat frontal cortex tissue. RESULTS: In the 10 mg/kg group, none of the local anesthetics fully substituted for cocaine and all decreased response rate. Rate decreasing potencies were positively correlated with sodium channel affinities. In the low training dose group, all the local anesthetics except tetracaine substituted fully for cocaine. Discriminative potencies were positively correlated with sodium channel affinities. Maximum DA uptake inhibition did not adequately predict maximum discriminative stimulus effects. CONCLUSIONS: Cocaine-like discriminative stimulus effects of local anesthetics were more prominent at a low than at a high training dose of cocaine. Sodium channels seem to have a direct influence on discriminative effects at low cocaine doses, whereas they have an indirect influence on discriminative effects at high cocaine doses by decreasing response rates.

Reinforcing effects of D2 dopamine receptor agonists and partial agonists in rhesus monkeys.

Dopamine (DA) receptors play a role in the reinforcing effects of psychomotor stimulants and other drugs. Both D1 and D2 DA receptor agonists have been reported to function as positive reinforcers in maintaining self-administration in non-human subjects. The purpose of the present study was to evaluate, in monkeys, the reinforcing effects of DA D2 receptor agonists that vary in their efficacy as D2 agonists. Rhesus monkeys were prepared with venous catheters and lever pressing was maintained by i.v. cocaine (n=5, 0.03 mg/kg/inj) in daily baseline sessions (2 h/day, fixed ratio 25). Various doses of cocaine or D2 agonists were then made available for at least four to seven sessions, and until responding was stable. At least one dose of the higher-efficacy D2 agonists, R(-)-propylnorapomorphine (NPA) (n=4, 0.001-0.01 mg/kg/inj), R(-)-apomorphine (APO) (n=4, 0.003-0.1 mg/kg/inj) and R(+)-3-(3-hydroxyphenyl)-N-propylpiperidine HCl [R(+)-3-PPP] (n=4, 0.03-0.3 mg/kg/inj), functioned as a positive reinforcer in all the monkeys tested. In contrast, no dose of the lower-efficacy D2 agonists, R(+)-terguride (n=4, 0.001-0.3 mg/kg/inj) and S(-)-3-(3-hydroxyphenyl)-N-propylpiperidine HCl [S(-)-3-PPP] (n=4, 0.001-0.3 mg/kg/inj), maintained self-administration. In in vitro binding studies with monkey brain tissue NPA and terguride had high affinities for the D2 receptor, while APO had intermediate affinity, and the 3-PPPs had low affinity. Among the compounds that were reinforcers potency as a reinforcer was directly related to D2 affinity in three of the four monkeys, consistent with D2 receptor involvement in the reinforcing effect of these compounds. The data suggest that the efficacy at D2 receptors is directly related to the reinforcing effect.

Further studies of the reinforcing effects of benztropine analogs in rhesus monkeys.

RATIONALE: Several halogenated analogs of benztropine (BZT) have previously been characterized as potent DA uptake inhibitors with behavioral profiles that indicate diminished psychomotor stimulant effects relative to cocaine. In a previous study using a fixed-ratio 10 schedule, two chloro-analogs (3'-Cl-BZT and 4'-Cl-BZT) maintained i.v. self-administration in monkeys but appeared to be weak positive reinforcers. OBJECTIVES: The present experiments were designed to test the hypothesis that 3'-Cl-BZT and 4'-Cl-BZT are relatively weak reinforcers by evaluating reinforcing effects under increased response requirements. To examine further the effect of this halogen substitution on self-administration, 3',4"-diCl-BZT was also evaluated for reinforcing effects. METHODS: Four rhesus monkeys self-administered cocaine (0.03 mg/kg per injection, i.v.) under a fixed-ratio 25 (FR25) schedule until stable responding was established. Saline, various doses of cocaine (0.003-0.2 mg/kg per injection), the BZT analogs (0.012-0.2 mg/kg per injection), GBR 12909 (0.012-0.2 mg/kg per injection), and compounds with known reinforcing effects (d-amphetamine, morphine, pentobarbital, ketamine) were then made available for self-administration. Various doses (0.01-0.3 mg/kg per injection) of the compounds that maintained self-administration under the FR schedule were then substituted for cocaine (0.1 mg/kg per injection) under progressive-ratio (PR) schedules. RESULTS: Reinforcing effects were evident under the FR schedule for 3'-Cl-BZT, 4'-Cl-BZT, GBR 12909, and the control compounds, but not by 3',4"-diCl-BZT. Results with the PR suggested that the rank order of these compounds for their effectiveness as reinforcers was cocaine > GBR 12909 > 3'-Cl-BZT = 4'-Cl-BZT >> 3',4"-diCl-BZT. CONCLUSIONS: This study confirms and extends previous results suggesting that compounds with high DAT affinity can have strong, moderate, weak, or no effectiveness as reinforcers. The mechanisms that may underlie this variation in reinforcing effectiveness of these DAT ligands remain to be established.

Dopamine transporter binding without cocaine-like behavioral effects: synthesis and evaluation of benztropine analogs alone and in combination with cocaine in rodents.

RATIONALE: Previous SAR studies demonstrated that small halogen substitutions on the diphenylether system of benztropine (BZT), such as a para-Cl group, retained high affinity at the cocaine binding site on the dopamine transporter. Despite this high affinity, the compounds generally had behavioral effects different from those of cocaine. However, compounds with meta-Cl substitutions had effects more similar to those of cocaine. OBJECTIVES: A series of phenyl-ring analogs of benztropine (BZT) substituted with 3'-, 4'-, 3',4"- and 4',4"-position Cl-groups were synthesized and their pharmacology was evaluated in order to assess more fully the contributions to pharmacological activity of substituents in these positions. METHODS: Compounds were synthesized and their pharmacological activity was assessed by examining radioligand binding and behavioral techniques. RESULTS: All of the compounds displaced [3H]WIN 35,428 binding with affinities ranging from 20 to 32.5 nM. Affinities at norepinephrine ([3H]nisoxetine) and serotonin ([3H]citalopram) transporters, respectively, ranged from 259 to 5120 and 451 to 2980 nM. Each of the compounds also inhibited [3H]pirenzepine binding to muscarinic M1 receptors, with affinities ranging from 0.98 to 47.9 nM. Cocaine and the BZT analogs produced dose-related increases in locomotor activity in mice. However, maximal effects of the BZT analogs were uniformly less than those produced by cocaine, and were obtained 2-3 h after injection compared to the relatively rapid onset (within 30 min) of cocaine effects. In rats trained to discriminate i.p. saline from 29 mumol/kg cocaine (10 mg/kg), cocaine produced a dose-related increase in responding on the cocaine lever, reaching 100% at the training dose; however, none of the BZT analogs fully substituted for cocaine, with maximum cocaine responding from 20 to 69%. Despite their reduced efficacy compared to cocaine in cocaine discrimination, none of the analogs antagonized the effects of cocaine. As has been reported previously for 4'-Cl-BZT, the cocaine discriminative-stimulus effects were shifted left-ward by co-administration of the present BZT analogs. CONCLUSIONS: The present results indicate that although the BZT analogs bind with relatively high affinity and selectivity at the dopamine transporter, their behavioral profile is distinct from that of cocaine. The present results suggest that analogs of BZT may be useful as treatments for cocaine abuse in situations in which an agonist treatment is indicated. These compounds possess features such as reduced efficacy compared to cocaine and a long duration of action that may render them particularly useful leads for the development of therapeutics for cocaine abusers.

Behavioral effects of flunitrazepam: reinforcing and discriminative stimulus effects in rhesus monkeys and prevention of withdrawal signs in pentobarbital-dependent rats.

Flunitrazepam was evaluated in several procedures that have been used extensively to study the behavioral effects and abuse potential of positive GABA(A) modulators. One group of monkeys (n=3) responded to receive injections of methohexital or saline (i.v.) while other groups (n=2-4/group) discriminated vehicle from either pentobarbital or triazolam. Other monkeys (n=2) received diazepam daily and discriminated flumazenil from vehicle. Finally, the ability of flunitrazepam to prevent the emergence of withdrawal signs in pentobarbital-treated rats was evaluated. Flunitrazepam maintained i.v. self-administration that was, on average, less than that maintained by methohexital and greater than that maintained by saline. In drug discrimination studies, flunitrazepam substituted for pentobarbital and for triazolam and failed to substitute for flumazenil. In rats (n=3-6/group), signs of withdrawal were not evident when flunitrazepam treatment replaced pentobarbital treatment; withdrawal signs emerged when either pentobarbital or flunitrazepam treatment was terminated. Taken together with data from previous studies, these data suggest that the abuse liability of flunitrazepam is comparable to that of other benzodiazepines.

Discriminative stimulus effects of panadiplon (U-78875), a partial agonist at the benzodiazepine site, in pentobarbital-trained rhesus monkeys.

The pentobarbital-like discriminative stimulus effects of the benzodiazepine receptor partial agonist panadiplon were assessed in rhesus monkeys trained to discriminate pentobarbital (10 mg/kg) from saline. In test sessions, pentobarbital and the benzodiazepine full agonist triazolam engendered near 100% drug-appropriate responding in all monkeys, whereas panadiplon engendered near 100% drug-appropriate responding in two of four monkeys. Panadiplon pretreatment resulted in leftward shifts in the pentobarbital dose-response function but predominantly rightward shifts of the triazolam dose-response function. These results are consistent with findings that benzodiazepine partial agonists have pentobarbital-like discriminative stimulus effects in some subjects only, and further suggest that partial agonists enhance the effects of pentobarbital but antagonize the effects of a benzodiazepine full agonist.

Concurrent variable-interval drug self-administration and the generalized matching law: a drug-class comparison.

It has previously been shown that self-administration of cocaine under concurrent variable-interval schedules is well described by the generalized matching law. That is, choice between two cocaine-maintained options was apportioned in accordance with relative frequency of reinforcement. However, the generality of this conclusion to drugs of other pharmacological classes has not been determined. In the present study, four male rhesus monkeys (Macaca mulatta) lever pressed under various pairs of concurrent variable-interval schedules with drug injection as the maintaining event. An opioid (alfentanil, 0.001 or 0.004 mg/kg/injection), a barbiturate (methohexital, 0.25 or 0.5 mg/kg/injection), and a psychomotor stimulant (cocaine, 0.05 mg/kg/injection) were selected as representatives of major classes of abused drugs and because of their relatively short duration of action. As has been found for cocaine, choice was well accounted for by the generalized matching law. There were no systematic differences in matching-law parameters across drugs and/or doses. As in earlier studies with drug and nondrug reinforcers, undermatching was a consistent finding. Therefore, the conclusion that relative reinforcement frequency is a crucial determinant of choice, as proposed by the generalized matching law, can be extended to behavior maintained by drugs from a variety of pharmacological classes.

3'- and 4'-chloro-substituted analogs of benztropine: intravenous self-administration and in vitro radioligand binding studies in rhesus monkeys.

RATIONALE: The reinforcing effects of many psychomotor stimulants have been related to increased dopaminergic neurotransmission. Drugs that block dopamine (DA) uptake have generally been found to function as positive reinforcers. Benztropine (BZT) and several of its halogenated analogs have previously been characterized as potent DA-uptake inhibitors with behavioral profiles that indicate diminished psychomotor stimulant effects relative to cocaine. OBJECTIVES: The present experiments were designed to examine, in rhesus monkeys, the reinforcing effects of the DA-uptake inhibitor BZT and two chloro-analogs 3'-Cl-BZT and 4'-Cl-BZT, and to compare self-administration and binding profiles. METHODS: Four rhesus monkeys self-administered cocaine i.v. under a fixed-ratio 10 (FR10) schedule until stable responding was established. Saline, and various doses of cocaine, BZT, and the BZT analogs were then made available for self-administration. Binding of these compounds to monoaminergic and cholinergic sites in monkey brain were determined using standard radioligand binding techniques. RESULTS: Self-administration was maintained by both 3'-Cl-BZT and 4'-Cl-BZT, but not by BZT. Results suggested that 3'-Cl-BZT and 4'-Cl-BZT were weak positive reinforcers. BZT and analogs bound DA transporters (DAT) with affinities higher than that of cocaine and had affinity for muscarinic binding sites. CONCLUSIONS: Surprisingly, high affinity at DATs was associated with weak or no reinforcing effects. The mechanism(s) that may underlie this dissociation between DAT actions and reinforcing effects remains to be established. These data support the proposal that a lead for the discovery of a pharmacotherapeutic agent for cocaine abuse may come from this group of compounds.

Reinforcing and discriminative stimulus effects of RTI 111, a 3-phenyltropane analog, in rhesus monkeys: interaction with methamphetamine.

RATIONALE: The neuronal actions of methamphetamine (MA) include an increase in extracellular levels of monoamines, presumably via reverse transport involving the monoamine transporters. This action is thought to play an important role in the effects of MA. Therefore, in the present experiment, it was hypothesized that a monoamine uptake blocker would block behavioral effects of MA related to its abuse. OBJECTIVE: RTI 111, a newly synthesized 3-phenyltropane analog with high affinity for the dopamine, norepinephrine, and serotonin transporters, was evaluated alone and in combination with MA for its ability to block the reinforcing and discriminative stimulus effects of MA in rhesus monkeys. METHODS: RTI 111 (0.0003-0.03 mg/kg, i.v.) was made available to four rhesus monkeys for self-administration under a fixed-ratio 25 (FR 25) schedule of reinforcement. RTI 111 (0.01-0.1 mg/kg, i.m.) was also administered as a pretreatment (15 min prior) to four monkeys self-administering MA (0.0-0.3 mg/kg per injection, i.v.) on a progressive-ratio schedule of reinforcement. MA (0.01-1.0 mg/kg, i.m.), RTI 111 (0.001-0.1 mg/kg, i.m.), or the combination of MA and RTI 111 were administered to four monkeys trained to discriminate (+)-amphetamine (AMPH; 1.0 or 1.7 mg/kg, intragastric) from saline. RESULTS: When RTI 111 was made available for self-administration under an FR 25 schedule it functioned as a positive reinforcer in all four monkeys tested. When RTI 111 was given as a pretreatment to monkeys self-administering MA under a progressive-ratio schedule, the MA dose-response function shifted to the left and down. When RTI 111 or MA were given to monkeys trained to discriminate AMPH from saline, full AMPH-like responding was observed for both drugs. Given in combination, RTI 111 shifted the MA dose-response function to the left. CONCLUSIONS: These data suggest that RTI 111 is behaviorally similar to traditional psychomotor stimulants that act at the DA transporter and that it increases, rather than blocks, the behavioral potency of MA.

On the relationship between the dopamine transporter and the reinforcing effects of local anesthetics in rhesus monkeys: practical and theoretical concerns.

RATIONALE: Drugs that are self-administered appear to vary in their potency and effectiveness as positive reinforcers. Understanding mechanisms that determine relative effectiveness of drugs as reinforcers will enhance our understanding of drug abuse. OBJECTIVES: The hypothesis of the present study was that differences among dopamine transporter (DAT) ligands in potency and effectiveness as a positive reinforcers were related to potency and effectiveness as DA uptake inhibitors. Accordingly, self-administration of a group of local anesthetics that are DAT ligands was compared to their effects as DA uptake blockers in vitro in brain tissue. METHODS: Rhesus monkeys were allowed to self-administer cocaine and other local anesthetics i.v. under a progressive-ratio schedule. The same compounds were compared in standard in vitro DA uptake assays using monkey caudate tissue. RESULTS: The rank order of both potency and effectiveness as reinforcers was cocaine > dimethocaine > procaine > chloroprocaine. Tetracaine did not maintain self-administration. For inhibiting DA uptake, the potency order was cocaine > dimethocaine > tetracaine > procaine > chloro-procaine. At maximum, these compounds were equally effective in blocking DA uptake. Lidocaine did not inhibit DA uptake. CONCLUSIONS: The potency of local anesthetics as positive reinforcers is likely related to their potency as DA uptake inhibitors. Variation in their effectiveness as positive reinforcers was not a function of differences in effectiveness as DA uptake blockers, but may be related to relative potency over the concentrations that are achieved in vivo. Effects at sodium channels may limit the reinforcing effects of local anesthetics.

Discriminative stimulus effects of ethyl-beta-carboline-3-carboxylate at two training doses in rats.

RATIONALE AND OBJECTIVES: The role of benzodiazepine (BZ) receptor mechanisms in modulating the stimulus effects of the BZ partial inverse agonist ethyl-beta-carboline-3-carboxylate (beta-CCE) are not well understood. The purpose of the present experiments was to assess the role of BZ and non-BZ receptor stimulation in the discriminative stimulus effects of beta-CCE in rats. METHODS: Adult male rats were trained to discriminate either a relatively high dose (10 mg/kg, n = 8) or a relatively low dose (5.0 mg/kg, n = 7) of beta-CCE from saline under a fixed-ratio 10 schedule of food presentation. RESULTS: Under the high-dose training condition, beta-CCE engendered an increase in responding on the drug-paired lever up to 100% drug-lever responding, with no decrease in response rate. Diazepam, pentobarbital, flumazenil, (+)-amphetamine, and morphine did not share stimulus effects with 10 mg/kg beta-CCE up to doses that suppressed rate of responding. The BZ full inverse agonist dimethoxy-4-ethyl-beta-carboline-3-carboxylate also did not engender > or = 80% beta-CCE-lever responding up to doses that suppressed response rate and produced seizures in some animals. The BZ partial inverse agonists Ro 15-4513 and sarmazenil fully reproduced the stimulus effects of beta-CCE. Flumazenil antagonized the effects of beta-CCE with an in vivo apparent pA2 value of 6.1 (slope = -0.86). Under the low-dose condition, beta-CCE engendered an increase in drug-lever responding, with no changes in response rate. In contrast to the high-dose condition, diazepam, pentobarbital, and (+)-amphetamine engendered high levels of beta-CCE-lever responding (up to 77, 96, and 75%, respectively), whereas flumazenil and morphine did not engender full beta-CCE-lever responding. CONCLUSIONS: These results indicated that the stimulus effects of the high dose of beta-CCE appeared consistent with mediation by the drug's partial inverse agonist effects at BZ receptors. The discriminative stimulus effects of beta-CCE at the lower training dose, however, appeared to be relatively non-specific.

Reinforcing and discriminative stimulus effects of the neuroactive steroids pregnanolone and Co 8-7071 in rhesus monkeys.

RATIONALE AND OBJECTIVES: The present study was designed to assess possible abuse-related effects of the endogenous neuroactive steroid pregnanolone (3alpha-hydroxy-5beta-pregnan-20-one) and the orally bioavailable, water-soluble neuroactive steroid pro-drug Co 8-7071 (3alpha,21-dihydroxy-3beta-trifluoromethyl-5beta-pregnan-20- one, 21-hemisuccinate). METHODS: Four rhesus monkeys were prepared with chronic intravenous (i.v.) catheters and trained to press a lever under a ten-response fixed-ratio (FR) schedule of methohexital injection (0.1 mg/kg per injection). Three rhesus monkeys were trained to discriminate intragastric infusions of pentobarbital (10 mg/kg) from saline infusions under a FR5 schedule of stimulus-shock termination. RESULTS: At least two doses of pregnanolone (0.003-0.1 mg/kg per injection) maintained injections per session above saline levels in the four monkeys tested, whereas Co 8-7071 (0.01-1.0 mg/kg per injection) maintained injections per session above saline levels in two of four monkeys at relatively low levels of injections per session. In rhesus monkeys trained to discriminate pentobarbital, i.v. pregnanolone injections (0.1-1.7 mg/kg, 5-min presession) dose-dependently reproduced the discriminative stimulus effects of pentobarbital in all monkeys tested. Intravenous administration of Co 8-7071 (1-10 mg/kg, 5-min presession) resulted in a dose-dependent increase to >80% pentobarbital-appropriate responding in two of three monkeys tested. Following intragastric infusions of Co 8-7071 (1.0-30 mg/kg), > or =80% pentobarbital-appropriate responding occurred in one out of three monkeys at 10 mg/kg when administered 60 min before the session. When administered 120 min before the session, however, 10-30 mg/kg Co 8-7071 reproduced the discriminative stimulus effects of pentobarbital in each of the three monkeys tested. CONCLUSIONS: These data demonstrate barbiturate-like abuse-related effects that differed between two pregnane steroids. Whereas pregnanolone functioned as a reinforcer, suggesting that this compound has abuse potential, Co 8-7071 did not, despite having pentobarbital-like discriminative effects.