Further evaluation of the reinforcing effects of the novel cocaine analog 2beta-propanoyl-3beta-(4-tolyl)-tropane (PTT) in rhesus monkeys

Evaluation of the Reinforcing Effect of Quetiapine, Alone and in Combination with Cocaine, in Rhesus Monkeys

There are several case reports of nonmedicinal quetiapine abuse, yet there are very limited preclinical studies investigating quetiapine self-administration. The goal of this study was to investigate the reinforcing effects of quetiapine alone and in combination with intravenous cocaine in monkeys. In experiment 1, cocaine-experienced female monkeys (N = 4) responded under a fixed-ratio (FR) 30 schedule of food reinforcement (1.0-g banana-flavored pellets), and when responding was stable, quetiapine (0.003-0.1 mg/kg per injection) or saline was substituted for a minimum of five sessions; there was a return to food-maintained responding between doses. Next, monkeys were treated with quetiapine (25 mg, by mouth, twice a day) for approximately 30 days, and then the quetiapine self-administration dose-response curve was redetermined. In experiment 2, male monkeys (N = 6) self-administered cocaine under a concurrent FR schedule with food reinforcement (three food pellets) as the alternative to cocaine (0.003-0.3 mg/kg per injection) presentation. Once choice responding was stable, the effects of adding quetiapine (0.03 or 0.1 mg/kg per injection) to the cocaine solution were examined. In experiment 1, quetiapine did not function as a reinforcer, and chronic quetiapine treatment did not alter these effects. In experiment 2, cocaine choice increased in a dose-dependent fashion. The addition of quetiapine to cocaine resulted in increases in low-dose cocaine choice and number of cocaine injections in four monkeys, while not affecting high-dose cocaine preference. Thus, although quetiapine alone does not have abuse potential, there was evidence of enhancement of the reinforcing potency of cocaine. These results suggest that the use of quetiapine in cocaine-addicted patients should be monitored.

The selective dopamine uptake inhibitor, D-84, suppresses cocaine self-administration, but does not occasion cocaine-like levels of generalization

A successful replacement pharmacotherapy for treating cocaine dependency would likely reduce cocaine's abuse, support a low abuse liability, overlap cocaine's subjective effects, and have a long duration of action. Inhibitors with varying selectivity at the dopamine transporter (DAT) have approximated these properties. The objective of the present study was to characterize the behavioural effects of an extremely selective DAT inhibitor, (+) trans-4-(2-Benzhydryloxyethyl)-1-(4-fluorobenzyl) piperadin-3-ol (D-84), a 3-hydroxy substituted piperidine derivative of GBR-12935, for its cocaine-like discriminative stimulus effects, its effects on cocaine self-administration, and for its own self-administration. During cocaine discrimination tests, cocaine occasioned the 10 mg/kg cocaine training stimulus with an ED(50) value of 3.13 (1.54-6.34) mg/kg, and reduced response rates with an ED(50) value of 20.39 (7.24-57.44) mg/kg. D-84 incompletely generalized to the cocaine stimulus occasioning a maximal 76% cocaine-lever responding, while reducing response rates with lower potency than cocaine (ED(50)=30.94 (12.34-77.60) mg/kg). Pretreatment with D-84 (9.6-30.4 mg/kg) significantly (P<0.05) reduced cocaine intake at 17.1 mg/kg D-84 when cocaine was self-administered at 0.5 mg/kg/infusion, and at 30.4 mg/kg D-84 when cocaine was self-administered at 0.1, 0.5 .and 1.0 mg/kg/infusion. During self-administration tests with D-84 (0.1-1 mg/kg/infusion), numbers of infusions significantly exceeded vehicle levels at 0.3 mg/kg/infusion. These results show that D-84 pretreatment can decrease cocaine intake especially when high doses of cocaine are being self-administered. This observation, combined with its incomplete generalization to the cocaine discriminative stimulus and its reported long duration of action, provides a profile consistent with a potential replacement therapy for treating cocaine-abusing patients.

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.

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.

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.

Assessment of the relative reinforcing strength of cocaine in socially housed monkeys using a choice procedure

Position in the social hierarchy can influence brain dopamine function and cocaine reinforcement in nonhuman primates during early cocaine exposure. With prolonged exposure, however, initial differences in rates of cocaine self-administration between dominant and subordinate monkeys dissipate. The present studies used a choice procedure to assess the relative reinforcing strength of cocaine in group-housed male cynomolgus monkeys with extensive cocaine self-administration histories. Responding was maintained under a concurrent fixed-ratio 50 schedule of food and cocaine (0.003-0.1 mg/kg per injection) presentation. Responding on the cocaine-associated lever increased as a function of cocaine dose in all monkeys. Although response distribution was similar across social rank when saline or relatively low or high cocaine doses were the alternative to food, planned t tests indicated that cocaine choice was significantly greater in subordinate monkeys when choice was between an intermediate dose (0.01 mg/kg) and food. When a between-session progressive-ratio procedure was used to increase response requirements for the preferred reinforcer (either cocaine or food), choice of that reinforcer decreased in all monkeys. The average response requirement that produced a shift in response allocation from the cocaine-associated lever to the food-associated lever was higher in subordinates across cocaine doses, an effect that trended toward significance (p = 0.053). These data indicate that despite an extensive history of cocaine self-administration, most subordinate monkeys were more sensitive to the relative reinforcing strength of cocaine than dominant monkeys.

Effects of the dopamine reuptake inhibitor PTT on reinstatement and on food- and cocaine-maintained responding in rhesus monkeys

RATIONALE

High-affinity, slow-onset, long-acting dopamine transporter (DAT) inhibitors are being considered as potential agonist replacement therapies for cocaine addiction, and therefore the ability of these drugs to reinstate cocaine seeking and to selectively decrease cocaine-maintained responding should be assessed.

OBJECTIVES

The purpose of these experiments was to evaluate the effects of the active enantiomer of a high-affinity, slow-onset, long-acting DAT inhibitor, (-)2beta-propanoyl-3beta-(4-tolyl)-tropane (PTT), and cocaine on food- and cocaine-maintained responding and on extinguished responding previously maintained by cocaine in non-human primates using a within-subjects design.

METHODS

Rhesus monkeys (n=3) responded under a multiple fixed-ratio schedule of food (1 g) and drug reinforcement, and cocaine dose-response curves (saline, 0.003-0.3 mg/kg per injection) were determined. The effects of pretreatment with (-)PTT (0.001-0.056 mg/kg, i.v.) and cocaine (0.03-0.3 mg/kg, i.v.) were determined when the dose of cocaine that maintained peak response rates (0.03 mg/kg per injection) or saline was available.

RESULTS

(-)PTT and cocaine reduced cocaine intake; (-)PTT affected cocaine self-administration only at doses that also decreased food-maintained responding. (-)PTT and cocaine reinstated responding that was previously reinforced by cocaine at lower doses than were necessary to decrease cocaine-maintained responding. For all studies, PTT was at least 1.0 log-unit more potent than cocaine. Compared to cocaine, PTT had a longer duration of action in all behavioral measures.

CONCLUSIONS

These results suggest that PTT would decrease cocaine use, but only at doses that disrupted other behaviors. It appears that the potency of this class of drugs to reinstate cocaine-seeking is substantially greater than their potency at decreasing cocaine self-administration.

Dopamine transporter as target for drug development of cocaine dependence medications

Because much evidence implicates the dopamine transporter in the reinforcing effects of cocaine, development of potential medications for cocaine dependence has included the dopamine transporter as a target. The present overview covers progress in the drug development area regarding several classes of dopamine uptake inhibitors, with an emphasis on structure-activity relationships that enhance potency and selectivity at transporters for dopamine compared with those for serotonin or norepinephrine. The following categories of compounds are covered: tropane, benztropine, 1-[2-[bis(4-fluorophenyl)methoxy]ethyl]-4-(3-phenylpropyl)piperazine (GBR), methylphenidate, mazindol, and phencyclidine analogs. Activity at transporters as well as on behavior is highlighted.

Bitonic dose-response functions for reinforcing and self-reported effects of nitrous oxide in humans

The reinforcing and self-reported effects of nitrous oxide (10%, 30%, and 50% N(2)O in O(2)) were examined in 13 humans. During each of three sessions, subjects sampled one dose of N(2)O and 100% O(2) (placebo) for 10 min each, separated by 30-min recovery periods. The agents were identified by letter code, and later in the session, subjects chose nine times, once every 5 min, among N(2)O (e.g., "Agent A"), placebo (e.g., "Agent B"), or "neither" (also 100% O(2), identified as "drug-free air"). Self-reported and psychomotor effects were measured at various times. Dose-response functions varied across subjects and included bitonic, monotonic increasing, monotonic decreasing, U-shaped, and flat dose-response functions for reinforcing and/or self-reported effects. For subjects who showed bitonic reinforcing effects, the descending limb of the dose-response function could not be attributed to behavioral impairment. This study replicates previous studies showing dose-dependent effects of N(2)O, as well as between-subject variability in N(2)O effects. Bitonic dose-response functions for some subjects extend the generality of the phenomenon of bitonicity of drug effects to N(2)O effects in humans.

The reinforcing efficacy of the dopamine reuptake inhibitor 2beta-propanoyl-3beta-(4-tolyl)-tropane (PTT) as measured by a progressive-ratio schedule and a choice procedure in rhesus monkeys

The present series of experiments was undertaken to investigate the variables that influence the reinforcing efficacy of psychostimulants. The time of onset for dopamine transporter (DAT) occupancy of the long-acting, high-affinity DAT blocker 2beta-propanoyl-3beta-(4-tolyl)-tropane (PTT) was measured using an ex vivo binding assay in rodents and was determined to be significantly longer than for cocaine (30 min versus 2 min). To assess the reinforcing efficacy of PTT relative to cocaine, a discrete-trials drug-drug choice procedure (n = 3) and a progressive-ratio (PR) schedule (n = 4) were used in rhesus monkeys. Cocaine (0.003-0.56 mg/kg/injection) and PTT (0.003-0.03 mg/kg/injection) maintained responding greater than saline under the PR schedule. Maximal breaking points were significantly higher for cocaine compared with PTT. A separate group of monkeys prepared with double-lumen catheters was allowed to choose between cocaine (saline and 0.03-0.3 mg/kg/injection) and PTT (saline, and 0.01 and 0.03 mg/kg/injection). Under these conditions, PTT was not preferred over saline. When saline or 0.01 mg/kg/injection PTT was available as alternatives to cocaine, the highest dose of cocaine maintained greater than 80% choice. When 0.03 mg/kg/injection PTT was the alternative to cocaine, cocaine choice declined to approximately 50%, and total cocaine intake was decreased by ~70% at the highest cocaine dose. These results suggest that the reinforcing efficacy of PTT is less than cocaine in nonhuman primates. Data from studies with PTT indicate that slow-onset, long-acting DAT inhibitors can decrease cocaine self-administration while not functioning robustly as reinforcers, and support the further investigation of these drugs as treatment for cocaine addiction.

Self-administration of cocaine and the cocaine analog RTI-113: relationship to dopamine transporter occupancy determined by PET neuroimaging in rhesus monkeys

Dopaminergic mechanisms are thought to play a central role in the reinforcing effects of cocaine. The present study examined the reinforcing effects of 3beta-(4-chlorophenyl)tropane-2beta-carboxylic acid phenyl ester (RTI-113), a long-acting, selective, high-affinity dopamine uptake inhibitor. Additionally, the effects of RTI-113 pretreatment on cocaine self-administration were determined. Monkeys were trained to respond under a second-order schedule for intravenous cocaine administration (0.10 or 0.17 mg/kg/infusion). When responding was stable, cocaine (0.0030-1.0 mg/kg/infusion) and RTI-113 (0.010-0.30 mg/kg/infusion) were substituted for the cocaine training dose. Cocaine and RTI-113 were equipotent for their reinforcing effects. However, cocaine maintained higher response rates in two of the three monkeys tested. When administered as a pretreatment, RTI-113 (0.10-0.30 mg/kg) dose-dependently reduced responding maintained by two doses of cocaine. Drug effects on behavior were related to dopamine transporter (DAT) occupancy in monkey striatum during neuroimaging with positron emission tomography. DAT occupancy was determined by displacement of 8-(2-[(18)F]fluroethyl)2beta-carbomethoxy-3beta-(4-chlorophenyl)nortropane (FECNT). DAT occupancy was between 65-76% and 94-99% for doses of cocaine and RTI-113 that maintained maximum response rates, respectively. DAT occupancy did not differ markedly across RTI-113 pretreatment doses and ranged between 72-84%. The results suggest that the pharmacokinetic profile of RTI-113 (i.e., long-acting) may influence its ability to maintain self-administration, and therefore its abuse liability. Additionally, high DAT occupancy is required for RTI-113 to reduce cocaine-maintained responding.

Synthesis of 2beta-acyl-3beta-(substituted naphthyl)-8-azabicyclo[3.2.1]octanes and their binding affinities at dopamine and serotonin transport sites

A series of 3beta-naphthyltropane derivatives were synthesized and found to show high affinity at both the dopamine and serotonin transporter sites, leading to some of the most potent inhibitors known based on the tropane structure. Comparative molecular field analysis (CoMFA) models were developed for both dopamine and serotonin transporter binding data. These models provide insights into those factors that influence binding at the two transporters.

Synthesis and monoamine transporter affinity of 3beta-(4-(2-pyrrolyl)phenyl)-8-azabicycl

3Beta-(5-indolyl)-8-azabicyclo[3.2.1]octanes display potent binding affinity for both the dopamine and serotonin transporters, while certain 3beta-(4-(2-pyrrolyl)phenyl)-8-azabicyclo[3.2.1]octanes selectively bind to the serotonin transporter.