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Assessment of ropinirole as a reinforcer in rhesus monkeys
Drug and Alcohol Dependence 125 (2012) 173–177
Contents lists available at SciVerse ScienceDirect
Drug and Alcohol Dependence journal homepage: www.elsevier.com/locate/drugalcdep
Short communication
Assessment of ropinirole as a reinforcer in rhesus monkeys Kevin B. Freeman a,∗ , David J. Heal b , Andrew C. McCreary c , William L. Woolverton a a
Division of Neurobiology and Behavior Research, Department of Psychiatry and Human Behavior, The University of Mississippi Medical Center, Jackson, MS 39216, USA RenaSci Ltd, BioCity, Nottingham NG1 1Gf, UK c Brains On-Line, de Mudden 16, 9741 AW Groningen, The Netherlands b
a r t i c l e
i n f o
Article history: Received 24 January 2012 Received in revised form 29 March 2012 Accepted 1 April 2012 Available online 28 April 2012 Keywords: Ropinirole Cocaine Monkey Self-administration Abuse liability
a b s t r a c t Background: Ropinirole, a D2 /D3 /5-HT1A agonist, is used for the treatment of Parkinson’s disease and restless leg syndrome, and is currently being evaluated as a treatment for cocaine dependence. However, there is little information available on ropinirole’s reinforcing effects. Methods: The current study tested ropinirole in monkeys (n = 7) trained to self administer cocaine on a fixed-ratio 25 (FR 25) schedule of reinforcement to determine if it would function as a reinforcer. In addition, a behavioral economics approach was used in four monkeys to compare the reinforcing effectiveness of ropinirole to cocaine. Results: Cocaine (0.01–0.3 mg/kg/injection) functioned as a reinforcer in all monkeys under the FR 25 schedule, and ropinirole (0.01–0.1 mg/kg/injection) functioned as a reinforcer in all but one. Furthermore, cocaine was a more effective reinforcer than ropinirole as indexed by demand functions. Conclusion: The current data indicate that ropinirole has reinforcing effects in monkeys, although its effectiveness as a reinforcer is relatively weak. Published by Elsevier Ireland Ltd.
1. Introduction Ropinirole, a D2 /D3 /5-HT1A receptor agonist, is currently being used or tested for the treatment of a variety of disorders including Parkinson’s disease (Kulisevsky and Pagonabarraga, 2010), bipolar depression (Perugi et al., 2001) and restless leg syndrome (Hansen et al., 2009). Recently, ropinirole has also been tested as a treatment for cocaine abuse and is reported to be effective in reducing the subjective “rush” of and craving for cocaine (Maremmani et al., 2011; Meini et al., 2011). According to the U.S. Product Label for ropinirole, animal studies and human clinical trials have not indicated that the drug has abuse potential. However, to our knowledge there are no publicly available data on the reinforcing effects of ropinirole. Compulsive D2 agonist use has been reported in Parkinson’s patients (Evans et al., 2005), but these compounds do not appear to be reinforcing in the general population. They do function as reinforcers in monkeys, however (Nader and Mach, 1996; Ranaldi et al., 2001; Sinnott et al., 1999; Woolverton et al., 1984). Ropinirole is a high affinity, high efficacy D2 /D3 agonist (64–100% and 94.2–96% intrinsic efficacy at D2 and D3 receptors, respectively; Al-Fulaji et al., 2007; Coldwell et al., 1999; Gardner and Strange, 1998; Tadori et al., 2011), and although it has not been reported to be abused
∗ Corresponding author. Tel.: +1 601 815 9203; fax: +1 601 984 5998. E-mail address: [email protected] (K.B. Freeman). 0376-8716/$ – see front matter. Published by Elsevier Ireland Ltd. http://dx.doi.org/10.1016/j.drugalcdep.2012.04.001
by humans, the fact that it is being tested as a pharmacotherapy for cocaine-dependence creates a need for information on its potential as a reinforcer. The current study had two aims: (1) to investigate ropinirole as a reinforcer in monkeys and (2) to extend previous findings that full D2 agonists are reinforcers in this species. Initially, rhesus monkeys were allowed to self-administer ropinirole and cocaine under a FR schedule of reinforcement. Next, the reinforcing effectiveness of ropinirole and cocaine were compared in a behavioral economics procedure. Given that D2 agonists functioned as reinforcers in previous reports, it was hypothesized that ropinirole would function as a reinforcer but would be less effective than cocaine. 2. Materials and methods All animal-use procedures were approved by the University of Mississippi Medical Center’s Animal Care and Use Committee and were in accordance with the National Research Council’s Guide for Care and Use of Laboratory Animals (1996). 2.1. Animals and apparatus The subjects were seven male rhesus monkeys (Macaca mulatta) weighing between 6.0 and 12.0 kg at the beginning of the study. All monkeys had histories of cocaine self-administration under conditions similar to those used here. In addition to cocaine, the most recent histories of drug self-administration were with nicotine (CK6R and R0697; Freeman and Woolverton, 2009), methamphetamine and pseudoephedrine (M1388; Freeman et al., 2010; R99028 and R0209, unpublished data), and alfentanil, remifentanil, and RTI-117 (R0463; Woolverton et al., 2008). Feeding and daily maintenance for the monkeys occurred as previously described (Freeman et al., 2010).
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Fig. 1. Self-administration of cocaine and ropinirole and their respective saline conditions (S). Each panel represents an individual monkey with the exception of the bottom right panel, which is an average of all monkeys. For individual monkeys, each data point represents the mean of three stable sessions for the indicated drug at the indicated dose. For the averaged graph, each data point represents the mean for all monkeys for the indicated drug at the indicated dose. * indicates that ropinirole was a reinforcer at the specified dose. The error bars represent standard deviation.
Monkeys were housed in the operant chambers as previously described (Freeman et al., 2010). Two response levers (PRL-001, BRS/LVE, Beltsville, MD, USA) were mounted on the inside of the door. Four jeweled stimulus lights, two red and two white, were mounted above each lever. Drug injections were delivered by a peristaltic infusion pump (Cole-Parmer, Chicago, IL, USA) over a 10 s period at a volume of approximately 1 ml. A Macintosh computer with custom interface and software controlled all events in an experimental session and recorded data. 2.2. Procedure 2.2.1. Experiment 1. Each monkey was surgically implanted with an intravenous catheter as previously described (Freeman et al., 2010). Experimental sessions were conducted daily, lasted for 2 h, and began at the same time each day. After the session started, signaled by the illumination of all white lever lights, monkeys were allowed to press the right lever to receive intravenous injections of a baseline dose of cocaine (either 0.03 or 0.1 mg/kg/injection, individually determined based on response rate). Injections were delivered on a FR 25 schedule of reinforcement. Upon completion of the response requirement, the white lights were turned off and the red lights were illuminated during an injection. Responses on the left lever were counted but
had no programmed consequences. There was no imposed inter-injection interval (i.e., no timeout). When responding was stable (less than 15% variation in number of injections per session for at least three consecutive sessions with no trends), 0.9% saline was substituted for cocaine until responding declined and stabilized at low levels (<10 injections/session). Responding for cocaine was re-established under baseline conditions and test sessions began. The cocaine dose–response function was established by varying the dose per injection of cocaine available in sessions. The level of saline self-administration (vehicle control) was established at the beginning of the study and between drugs. Doses for a drug were available for at least the same number of sessions required to complete the previous saline condition, and baseline cocaine self-administration was re-established between doses of each drug. Ropinirole was assessed in the same manner as cocaine. At least 3 doses of each drug were evaluated in each monkey. Doses of cocaine and ropinirole were tested in a counterbalanced order across subjects. With occasional exceptions, doses of one drug were tested before testing the next drug. 2.2.2. Experiment 2. After assessing reinforcing effects under the FR 25 schedule, the response requirement for drug injections was systematically increased in four
K.B. Freeman et al. / Drug and Alcohol Dependence 125 (2012) 173–177
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Fig. 2. Number of injections of cocaine and ropinirole self-administered per session as a function of response requirement. Each panel represents an individual monkey, and each point represents the mean of three stable sessions. Statistics for the demand analysis for each monkey are provided in table insets within each panel.
monkeys (AV22, CK6R, M1388, and R99028). Typically, responding maintained by a reinforcer decreases as response requirement increases, and the rate at which this decrease occurs provides a measure of the demand for a reinforcer. Research suggests that relative demand for drugs provides a measure of their relative reinforcing effectiveness (Hursh and Silberberg, 2008). The dose of cocaine and ropinirole that maintained maximum responding in Experiment 1 was tested in Experiment 2. Response requirement for an injection was increased by doubling (i.e., FR 25, 50, 100, etc.) in an ascending order in all subjects until responding decreased to levels maintained by saline under the FR 25 schedule in Experiment 1. Each response requirement was in effect for at least the same number of sessions that were required for responding to stabilize when saline was available at FR 25 and until responding was stable according to the same criteria used in Experiment 1. Between each response requirement manipulation, baseline responding was re-established with cocaine under a FR 25 schedule. 2.3. Data analysis In Experiment 1, a dose of a test drug was considered to be a positive reinforcer in a particular monkey if the mean number of injections for the last 3 sessions of a test period exceeded the mean value for saline injections established while testing that specific compound, and the ranges did not overlap. In Experiment 2, data were analyzed using a behavioral economic demand curve approach which relates the consumption of a reinforcer (the number of injections per session) to its price (the response requirement for one drug injection). Drug injections per session were plotted as a function of response requirement and the data were fit with a demand equation introduced by Hursh and Silberberg (2008): log Q = log Q0 +
k(e−˛Q0 C
− 1),
where Q is the experimentally determined measure of consumption at any particular FR, Q0 is the predicted absolute consumption at price 0 and specifies the highest level of demand, k is the range of the exponential demand curve in log units shared across all individuals and conditions, ˛ is the rate of decline in consumption as price is increased, and C is a measure of cost, expressed here as the FR value. The ˛ statistic, which describes the elasticity of the demand curve, provides a measure of a reinforcer’s effectiveness (see Hursh and Silberberg, 2008). In cases where the injections per session were a value of 0 (i.e., responses were not sufficient to earn a reinforcer), a value of 0.1 was entered because the log of 0 cannot be calculated (see Banks et al., 2011). To compare the reinforcing effectiveness of ropinirole to cocaine, ˛ values were log-transformed and averaged across monkeys for each drug
and compared using a paired t-test with the hypothesis that ˛ values for cocaine would be significantly lower than for ropinirole. Significance was set at p ≤ 0.05.
3. Results 3.1. Experiment 1 Cocaine functioned as a reinforcer in all monkeys. In the six monkeys that self-administered three doses of cocaine above saline levels, responses were monophasic with the rate of selfadministration decreasing as the unit dose of cocaine was increased (Fig. 1). The seventh monkey (R0463) had a biphasic function, with cocaine functioning as a reinforcer at 0.1 and 0.3 mg/kg/injection but not at 0.03 mg/kg/injection. Ropinirole functioned as a reinforcer at three doses in two monkeys (CK6R and R0209), at two doses in two monkeys (M1388 and R0463), and at one dose in two monkeys (AV22 and R99028). Ropinirole did not function as a reinforcer at any dose in monkey R0697.
3.2. Experiment 2 Injections for cocaine and ropinirole decreased with increases in response requirement (see Fig. 2). One monkey (AV22) decreased its intake of ropinirole to saline levels with the first increase in response requirement (i.e., at FR 50). As such, the demand function for ropinirole was fitted using two data points. Q0 , ˛, and R2 values for the demand functions in each monkey for each drug are displayed in Fig. 2. A paired t-test on log-transformed alpha values revealed that the alpha average for cocaine was significantly smaller than the value for ropinirole (p = 0.032), indicating that cocaine was a more effective reinforcer than ropinirole.
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4. Discussion The current data demonstrate that ropinirole is a positive reinforcer in monkeys experienced in cocaine self-administration, a result that is consistent with previous reports investigating the reinforcing effects of D2 agonists in this species (Nader and Mach, 1996; Ranaldi et al., 2001; Sinnott et al., 1999; Woolverton et al., 1984). When the monkeys were tested using a behavioral economics approach, demand for cocaine was significantly greater than for ropinirole, suggesting that ropinirole was a weaker reinforcer than cocaine. To our knowledge, the current report is the first to compare the reinforcing effectiveness of a D2 agonist to cocaine in monkeys. The fact that ropinirole was a relatively weak reinforcer may be a factor in why ropinirole and other D2 agonists are not reinforcing in humans. Additionally, dopamine agonists cause nausea and emesis in humans, and monkeys are less sensitive to these effects (Samardzic and Beleslin, 1989). As such, a difference in the balance of rewarding and aversive effects between humans and monkeys may affect drug acceptability differentially between these species (see Riley, 2011). Weed and Woolverton (1995) previously reported that the reinforcing effects of D1 agonists in monkeys varied directly with their intrinsic efficacies at the D1 receptor. When the current results with ropinirole are considered in the context of previous reports with other D2 agonists, a similar relationship is suggested for D2 agonists and reinforcing effect. Ropinirole, along with the D2 agonists apomorphine, N-propyl-apomorphine, piribedil, R(+)-3-PPP, and bromocriptine, reportedly possess between 58 and 100% intrinsic efficacy as D2 receptor agonists, and all function as reinforcers (Al-Fulaji et al., 2007; Coldwell et al., 1999; Gardner and Strange, 1998; Lahti et al., 1992). However, S(−)-3-PPP and terguride, D2 agonists that possess between 10 and 34% intrinsic efficacy at the D2 receptor, are not reinforcers (Gardner and Strange, 1998; Lahti et al., 1992). A similar profile is suggested for D3 activity with some of these compounds (Millan, 2010). Although these reports do not provide correlation information between receptor efficacy and reinforcing effect, they do suggest that there is a threshold for reinforcement functionality falling somewhere between 34 and 58% intrinsic agonist efficacy. Ropinirole is currently being tested as a potential treatment for cocaine dependence (Maremmani et al., 2011; Meini et al., 2011). While some pharmacotherapies for drug dependence do not function as reinforcers (e.g., disulfiram), others are intrinsically reinforcing in animals and humans and even have demonstrated abuse potential (e.g., buprenorphine, methadone; Johanson et al., 2012). D2 agonists, including ropinirole, are anomalous in that they are reinforcing in animals but not in humans. However, the decision to test a pharmacotherapy for drug dependence should be informed, in part, by empirically generated data on the reinforcing and subjective effects of the potential therapeutic. The current results, in addition to post-marketing surveillance, provide relevant information for the development of ropinirole as a therapeutic for cocaine dependence and possibly other uses. Role of funding source This research was funded by Abbott Healthcare Products BV to gain scientific information about the reinforcing potential of the D2 agonist, anti-Parkinsonian drug, ropinirole. Contributors Woolverton, Heal and McCreary designed the study. All authors participated in compiling and reviewing relevant literature. Freeman and Woolverton collected and organized the data. Freeman,
Woolverton and Heal participated in the data analysis. Freeman composed the first draft of the manuscript, and all authors made significant contributions to the development of the current draft. Conflict of interest Heal is an Executive Director and shareholder of RenaSci Ltd, a contract research provider. At the time of the study, McCreary was employed by Solvay Pharmaceuticals, which was later acquired by Abbott. Freeman and Woolverton have no conflicts of interest to disclose. Acknowledgments The authors wish to thank Mr. John Wikle for expert technical assistance and Mr. Richard Brammer, Head of Biostatistics at RenaSci, for performing statistical analyses. References Al-Fulaji, M.A., Ren, Y., Beinborn, M., Kopin, A.S., 2007. Identification of amino acid determinants of dopamine 2 receptor synthetic agonist function. J. Pharmacol. Exp. Ther. 321, 298–307. Banks, M.L., Roma, P.G., Folk, J.E., Rice, K.C., Negus, S., 2011. Effects of the delta-opioid agonist SNC80 on the abuse liability of methadone in rhesus monkeys: a behavioral economic analysis. Psychopharmacology (Berl) 216, 431–439. Coldwell, M.C., Boyfield, I., Brown, T., Hagan, J.J., Middlemiss, D.N., 1999. Comparison of the functional potencies of ropinirole and other dopamine receptor agonists at human D2(long) , D3 and D4 receptors expressed in Chinese hamster ovary cells. Br. J. Pharmacol. 127, 1696–1702. Evans, A.H., Lawrence, A.D., Pott, J., Appel, S., Lees, A.J., 2005. Factors influencing susceptibility to compulsive dopaminergic drug use in Parkinson disease. Neurology 65, 1570–1574. Freeman, K.B., Wang, Z., Woolverton, W.L., 2010. Self-administration of (+)methamphetamine and (+)-pseudoephedrine, alone and combined, by rhesus monkeys. Pharmacol. Biochem. Behav. 95, 198–202. Freeman, K.B., Woolverton, W.L., 2009. Self-administration of cocaine and nicotine by rhesus monkeys. Psychopharmacology (Berl) 207, 99–106. Gardner, B., Strange, P.G., 1998. Agonist action at D2(long) dopamine receptors: ligand binding and functional assays. Br. J. Pharmacol. 124, 978–984. Hansen, R.A., Song, L., Moore, C.G., Gilsenan, A.W., Kiim, M.M., Calloway, M.O., Murray, M.D., 2009. Effect of ropinirole on sleep outcomes in patients with restless legs syndrome: meta-analysis of pooled individual patient data from randomized controlled trials. Pharmacotherapy 29, 255–262. Hursh, S.R., Silberberg, A., 2008. Economic demand and essential value. Psychol. Rev. 115, 186–198. Johanson, C.E., Arfken, C.L., Menza, S., Schuster, C.R., 2012. Diversion and abuse of buprenorphine: findings from national surveys of treatment patients and physicians. Drug Alcohol Depend. 120, 1–3. Kulisevsky, J., Pagonabarraga, J., 2010. Tolerability and safety of ropinirole versus other dopamine agonists and levodopa in the treatment of Parkinson’s Disease: meta-analysis of randomized controlled trials. Drug Saf. 33, 147–161. Lahti, R.A., Figur, L.M., Piercey, M.F., Ruppel, P.L., Evans, D.L., 1992. Intrinsic activity determinations at the dopamine D2 guanine nucleotide-binding proteincoupled receptor: utilization of receptor state binding affinities. Mol. Pharmacol. 42, 432–438. Maremmani, A.G.I., Pacini, M., Rovai, L., Rugani, F., Dell’Osso, L., Maremmani, I., 2011. Can ropinirole modulate reinforcing subjective effects of cocaine in humans? Front. Psychiatry 2, 1–5. Meini, M., Moncini, M., Cecconi, D., Cellesi, L., Biasci, L., Simoni, G., Ameglio, M., Pellegrini, M., Forgione, R.N., Rucci, P., 2011. Aripiprazole and ropinirole treatment for cocaine dependence: evidence from a pilot study. Curr. Pharm. Des. 17, 1–8. Millan, M.J., 2010. From the cell to the clinic: a comparative review of the D2 /D3 receptor agonist and ␣2-adrenoceptor antagonist, piribedil, in the treatment of Parkinson’s disease. Pharmacol. Ther. 128, 2290–3273. Nader, M.A., Mach, R.H., 1996. Self-administration of the dopamine D2 agonist 7-OH-DPAT in rhesus monkeys is modified by prior cocaine exposure. Psychopharmacology (Berl) 125, 13–22. Perugi, G., Toni, C., Ruffolo, G., Frare, R., Akiskal, H., 2001. Adjunctive dopamine agonists in treatment-resistant bipolar II depression: an open case series. Pharmacopsychiatry 34, 137–141. Ranaldi, R., Wang, Z., Woolverton, W.L., 2001. Reinforcing effects of D2 dopamine receptor agonists and partial agonists in rhesus monkeys. Drug Alcohol Depend. 64, 209–217. Riley, A.L., 2011. The paradox of drug taking: the role of the aversive effects of drugs. Physiol. Behav. 103, 69–78.
K.B. Freeman et al. / Drug and Alcohol Dependence 125 (2012) 173–177 Samardzic, R., Beleslin, D.B., 1989. Neurochemical mechanisms in area postrema and emesis. Physiol. Pharmacol. Acta 25 (Suppl. 8), 133–153. Sinnott, R.S., Mach, R.H., Nader, M.A., 1999. Dopamine D2 /D3 receptors modulate cocaine’s reinforcing and discriminative stimulus effects in rhesus monkeys. Drug Alcohol Depend. 54, 91–110. Tadori, Y., Forbes, R.A., McQuade, R.D., Kikuchi, T., 2011. Functional potencies of dopamine agonists and antagonists at human dopamine D2 and D3 receptors. Eur. J. Pharmacol. 666, 43–52.
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Weed, M.R., Woolverton, W.L., 1995. The relationship between reinforcing effects and in vitro effects of D1 agonists in monkeys. J. Pharmacol. Exp. Ther. 275, 1367–1374. Woolverton, W.L., Goldberg, L.I., Ginos, J.Z., 1984. Intravenous self-administration of dopamine receptor agonists by rhesus monkeys. J. Pharmacol. Exp. Ther. 230, 678–683. Woolverton, W.L., Wang, Z., Vasterling, T., Tallarida, R., 2008. Self-administration of cocaine-remifentanil mixtures by monkeys: an isobolographic analysis. Psychopharmacology (Berl) 198, 387–394.
Contents lists available at SciVerse ScienceDirect
Drug and Alcohol Dependence journal homepage: www.elsevier.com/locate/drugalcdep
Short communication
Assessment of ropinirole as a reinforcer in rhesus monkeys Kevin B. Freeman a,∗ , David J. Heal b , Andrew C. McCreary c , William L. Woolverton a a
Division of Neurobiology and Behavior Research, Department of Psychiatry and Human Behavior, The University of Mississippi Medical Center, Jackson, MS 39216, USA RenaSci Ltd, BioCity, Nottingham NG1 1Gf, UK c Brains On-Line, de Mudden 16, 9741 AW Groningen, The Netherlands b
a r t i c l e
i n f o
Article history: Received 24 January 2012 Received in revised form 29 March 2012 Accepted 1 April 2012 Available online 28 April 2012 Keywords: Ropinirole Cocaine Monkey Self-administration Abuse liability
a b s t r a c t Background: Ropinirole, a D2 /D3 /5-HT1A agonist, is used for the treatment of Parkinson’s disease and restless leg syndrome, and is currently being evaluated as a treatment for cocaine dependence. However, there is little information available on ropinirole’s reinforcing effects. Methods: The current study tested ropinirole in monkeys (n = 7) trained to self administer cocaine on a fixed-ratio 25 (FR 25) schedule of reinforcement to determine if it would function as a reinforcer. In addition, a behavioral economics approach was used in four monkeys to compare the reinforcing effectiveness of ropinirole to cocaine. Results: Cocaine (0.01–0.3 mg/kg/injection) functioned as a reinforcer in all monkeys under the FR 25 schedule, and ropinirole (0.01–0.1 mg/kg/injection) functioned as a reinforcer in all but one. Furthermore, cocaine was a more effective reinforcer than ropinirole as indexed by demand functions. Conclusion: The current data indicate that ropinirole has reinforcing effects in monkeys, although its effectiveness as a reinforcer is relatively weak. Published by Elsevier Ireland Ltd.
1. Introduction Ropinirole, a D2 /D3 /5-HT1A receptor agonist, is currently being used or tested for the treatment of a variety of disorders including Parkinson’s disease (Kulisevsky and Pagonabarraga, 2010), bipolar depression (Perugi et al., 2001) and restless leg syndrome (Hansen et al., 2009). Recently, ropinirole has also been tested as a treatment for cocaine abuse and is reported to be effective in reducing the subjective “rush” of and craving for cocaine (Maremmani et al., 2011; Meini et al., 2011). According to the U.S. Product Label for ropinirole, animal studies and human clinical trials have not indicated that the drug has abuse potential. However, to our knowledge there are no publicly available data on the reinforcing effects of ropinirole. Compulsive D2 agonist use has been reported in Parkinson’s patients (Evans et al., 2005), but these compounds do not appear to be reinforcing in the general population. They do function as reinforcers in monkeys, however (Nader and Mach, 1996; Ranaldi et al., 2001; Sinnott et al., 1999; Woolverton et al., 1984). Ropinirole is a high affinity, high efficacy D2 /D3 agonist (64–100% and 94.2–96% intrinsic efficacy at D2 and D3 receptors, respectively; Al-Fulaji et al., 2007; Coldwell et al., 1999; Gardner and Strange, 1998; Tadori et al., 2011), and although it has not been reported to be abused
∗ Corresponding author. Tel.: +1 601 815 9203; fax: +1 601 984 5998. E-mail address: [email protected] (K.B. Freeman). 0376-8716/$ – see front matter. Published by Elsevier Ireland Ltd. http://dx.doi.org/10.1016/j.drugalcdep.2012.04.001
by humans, the fact that it is being tested as a pharmacotherapy for cocaine-dependence creates a need for information on its potential as a reinforcer. The current study had two aims: (1) to investigate ropinirole as a reinforcer in monkeys and (2) to extend previous findings that full D2 agonists are reinforcers in this species. Initially, rhesus monkeys were allowed to self-administer ropinirole and cocaine under a FR schedule of reinforcement. Next, the reinforcing effectiveness of ropinirole and cocaine were compared in a behavioral economics procedure. Given that D2 agonists functioned as reinforcers in previous reports, it was hypothesized that ropinirole would function as a reinforcer but would be less effective than cocaine. 2. Materials and methods All animal-use procedures were approved by the University of Mississippi Medical Center’s Animal Care and Use Committee and were in accordance with the National Research Council’s Guide for Care and Use of Laboratory Animals (1996). 2.1. Animals and apparatus The subjects were seven male rhesus monkeys (Macaca mulatta) weighing between 6.0 and 12.0 kg at the beginning of the study. All monkeys had histories of cocaine self-administration under conditions similar to those used here. In addition to cocaine, the most recent histories of drug self-administration were with nicotine (CK6R and R0697; Freeman and Woolverton, 2009), methamphetamine and pseudoephedrine (M1388; Freeman et al., 2010; R99028 and R0209, unpublished data), and alfentanil, remifentanil, and RTI-117 (R0463; Woolverton et al., 2008). Feeding and daily maintenance for the monkeys occurred as previously described (Freeman et al., 2010).
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Fig. 1. Self-administration of cocaine and ropinirole and their respective saline conditions (S). Each panel represents an individual monkey with the exception of the bottom right panel, which is an average of all monkeys. For individual monkeys, each data point represents the mean of three stable sessions for the indicated drug at the indicated dose. For the averaged graph, each data point represents the mean for all monkeys for the indicated drug at the indicated dose. * indicates that ropinirole was a reinforcer at the specified dose. The error bars represent standard deviation.
Monkeys were housed in the operant chambers as previously described (Freeman et al., 2010). Two response levers (PRL-001, BRS/LVE, Beltsville, MD, USA) were mounted on the inside of the door. Four jeweled stimulus lights, two red and two white, were mounted above each lever. Drug injections were delivered by a peristaltic infusion pump (Cole-Parmer, Chicago, IL, USA) over a 10 s period at a volume of approximately 1 ml. A Macintosh computer with custom interface and software controlled all events in an experimental session and recorded data. 2.2. Procedure 2.2.1. Experiment 1. Each monkey was surgically implanted with an intravenous catheter as previously described (Freeman et al., 2010). Experimental sessions were conducted daily, lasted for 2 h, and began at the same time each day. After the session started, signaled by the illumination of all white lever lights, monkeys were allowed to press the right lever to receive intravenous injections of a baseline dose of cocaine (either 0.03 or 0.1 mg/kg/injection, individually determined based on response rate). Injections were delivered on a FR 25 schedule of reinforcement. Upon completion of the response requirement, the white lights were turned off and the red lights were illuminated during an injection. Responses on the left lever were counted but
had no programmed consequences. There was no imposed inter-injection interval (i.e., no timeout). When responding was stable (less than 15% variation in number of injections per session for at least three consecutive sessions with no trends), 0.9% saline was substituted for cocaine until responding declined and stabilized at low levels (<10 injections/session). Responding for cocaine was re-established under baseline conditions and test sessions began. The cocaine dose–response function was established by varying the dose per injection of cocaine available in sessions. The level of saline self-administration (vehicle control) was established at the beginning of the study and between drugs. Doses for a drug were available for at least the same number of sessions required to complete the previous saline condition, and baseline cocaine self-administration was re-established between doses of each drug. Ropinirole was assessed in the same manner as cocaine. At least 3 doses of each drug were evaluated in each monkey. Doses of cocaine and ropinirole were tested in a counterbalanced order across subjects. With occasional exceptions, doses of one drug were tested before testing the next drug. 2.2.2. Experiment 2. After assessing reinforcing effects under the FR 25 schedule, the response requirement for drug injections was systematically increased in four
K.B. Freeman et al. / Drug and Alcohol Dependence 125 (2012) 173–177
175
Fig. 2. Number of injections of cocaine and ropinirole self-administered per session as a function of response requirement. Each panel represents an individual monkey, and each point represents the mean of three stable sessions. Statistics for the demand analysis for each monkey are provided in table insets within each panel.
monkeys (AV22, CK6R, M1388, and R99028). Typically, responding maintained by a reinforcer decreases as response requirement increases, and the rate at which this decrease occurs provides a measure of the demand for a reinforcer. Research suggests that relative demand for drugs provides a measure of their relative reinforcing effectiveness (Hursh and Silberberg, 2008). The dose of cocaine and ropinirole that maintained maximum responding in Experiment 1 was tested in Experiment 2. Response requirement for an injection was increased by doubling (i.e., FR 25, 50, 100, etc.) in an ascending order in all subjects until responding decreased to levels maintained by saline under the FR 25 schedule in Experiment 1. Each response requirement was in effect for at least the same number of sessions that were required for responding to stabilize when saline was available at FR 25 and until responding was stable according to the same criteria used in Experiment 1. Between each response requirement manipulation, baseline responding was re-established with cocaine under a FR 25 schedule. 2.3. Data analysis In Experiment 1, a dose of a test drug was considered to be a positive reinforcer in a particular monkey if the mean number of injections for the last 3 sessions of a test period exceeded the mean value for saline injections established while testing that specific compound, and the ranges did not overlap. In Experiment 2, data were analyzed using a behavioral economic demand curve approach which relates the consumption of a reinforcer (the number of injections per session) to its price (the response requirement for one drug injection). Drug injections per session were plotted as a function of response requirement and the data were fit with a demand equation introduced by Hursh and Silberberg (2008): log Q = log Q0 +
k(e−˛Q0 C
− 1),
where Q is the experimentally determined measure of consumption at any particular FR, Q0 is the predicted absolute consumption at price 0 and specifies the highest level of demand, k is the range of the exponential demand curve in log units shared across all individuals and conditions, ˛ is the rate of decline in consumption as price is increased, and C is a measure of cost, expressed here as the FR value. The ˛ statistic, which describes the elasticity of the demand curve, provides a measure of a reinforcer’s effectiveness (see Hursh and Silberberg, 2008). In cases where the injections per session were a value of 0 (i.e., responses were not sufficient to earn a reinforcer), a value of 0.1 was entered because the log of 0 cannot be calculated (see Banks et al., 2011). To compare the reinforcing effectiveness of ropinirole to cocaine, ˛ values were log-transformed and averaged across monkeys for each drug
and compared using a paired t-test with the hypothesis that ˛ values for cocaine would be significantly lower than for ropinirole. Significance was set at p ≤ 0.05.
3. Results 3.1. Experiment 1 Cocaine functioned as a reinforcer in all monkeys. In the six monkeys that self-administered three doses of cocaine above saline levels, responses were monophasic with the rate of selfadministration decreasing as the unit dose of cocaine was increased (Fig. 1). The seventh monkey (R0463) had a biphasic function, with cocaine functioning as a reinforcer at 0.1 and 0.3 mg/kg/injection but not at 0.03 mg/kg/injection. Ropinirole functioned as a reinforcer at three doses in two monkeys (CK6R and R0209), at two doses in two monkeys (M1388 and R0463), and at one dose in two monkeys (AV22 and R99028). Ropinirole did not function as a reinforcer at any dose in monkey R0697.
3.2. Experiment 2 Injections for cocaine and ropinirole decreased with increases in response requirement (see Fig. 2). One monkey (AV22) decreased its intake of ropinirole to saline levels with the first increase in response requirement (i.e., at FR 50). As such, the demand function for ropinirole was fitted using two data points. Q0 , ˛, and R2 values for the demand functions in each monkey for each drug are displayed in Fig. 2. A paired t-test on log-transformed alpha values revealed that the alpha average for cocaine was significantly smaller than the value for ropinirole (p = 0.032), indicating that cocaine was a more effective reinforcer than ropinirole.
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4. Discussion The current data demonstrate that ropinirole is a positive reinforcer in monkeys experienced in cocaine self-administration, a result that is consistent with previous reports investigating the reinforcing effects of D2 agonists in this species (Nader and Mach, 1996; Ranaldi et al., 2001; Sinnott et al., 1999; Woolverton et al., 1984). When the monkeys were tested using a behavioral economics approach, demand for cocaine was significantly greater than for ropinirole, suggesting that ropinirole was a weaker reinforcer than cocaine. To our knowledge, the current report is the first to compare the reinforcing effectiveness of a D2 agonist to cocaine in monkeys. The fact that ropinirole was a relatively weak reinforcer may be a factor in why ropinirole and other D2 agonists are not reinforcing in humans. Additionally, dopamine agonists cause nausea and emesis in humans, and monkeys are less sensitive to these effects (Samardzic and Beleslin, 1989). As such, a difference in the balance of rewarding and aversive effects between humans and monkeys may affect drug acceptability differentially between these species (see Riley, 2011). Weed and Woolverton (1995) previously reported that the reinforcing effects of D1 agonists in monkeys varied directly with their intrinsic efficacies at the D1 receptor. When the current results with ropinirole are considered in the context of previous reports with other D2 agonists, a similar relationship is suggested for D2 agonists and reinforcing effect. Ropinirole, along with the D2 agonists apomorphine, N-propyl-apomorphine, piribedil, R(+)-3-PPP, and bromocriptine, reportedly possess between 58 and 100% intrinsic efficacy as D2 receptor agonists, and all function as reinforcers (Al-Fulaji et al., 2007; Coldwell et al., 1999; Gardner and Strange, 1998; Lahti et al., 1992). However, S(−)-3-PPP and terguride, D2 agonists that possess between 10 and 34% intrinsic efficacy at the D2 receptor, are not reinforcers (Gardner and Strange, 1998; Lahti et al., 1992). A similar profile is suggested for D3 activity with some of these compounds (Millan, 2010). Although these reports do not provide correlation information between receptor efficacy and reinforcing effect, they do suggest that there is a threshold for reinforcement functionality falling somewhere between 34 and 58% intrinsic agonist efficacy. Ropinirole is currently being tested as a potential treatment for cocaine dependence (Maremmani et al., 2011; Meini et al., 2011). While some pharmacotherapies for drug dependence do not function as reinforcers (e.g., disulfiram), others are intrinsically reinforcing in animals and humans and even have demonstrated abuse potential (e.g., buprenorphine, methadone; Johanson et al., 2012). D2 agonists, including ropinirole, are anomalous in that they are reinforcing in animals but not in humans. However, the decision to test a pharmacotherapy for drug dependence should be informed, in part, by empirically generated data on the reinforcing and subjective effects of the potential therapeutic. The current results, in addition to post-marketing surveillance, provide relevant information for the development of ropinirole as a therapeutic for cocaine dependence and possibly other uses. Role of funding source This research was funded by Abbott Healthcare Products BV to gain scientific information about the reinforcing potential of the D2 agonist, anti-Parkinsonian drug, ropinirole. Contributors Woolverton, Heal and McCreary designed the study. All authors participated in compiling and reviewing relevant literature. Freeman and Woolverton collected and organized the data. Freeman,
Woolverton and Heal participated in the data analysis. Freeman composed the first draft of the manuscript, and all authors made significant contributions to the development of the current draft. Conflict of interest Heal is an Executive Director and shareholder of RenaSci Ltd, a contract research provider. At the time of the study, McCreary was employed by Solvay Pharmaceuticals, which was later acquired by Abbott. Freeman and Woolverton have no conflicts of interest to disclose. Acknowledgments The authors wish to thank Mr. John Wikle for expert technical assistance and Mr. Richard Brammer, Head of Biostatistics at RenaSci, for performing statistical analyses. References Al-Fulaji, M.A., Ren, Y., Beinborn, M., Kopin, A.S., 2007. Identification of amino acid determinants of dopamine 2 receptor synthetic agonist function. J. Pharmacol. Exp. Ther. 321, 298–307. Banks, M.L., Roma, P.G., Folk, J.E., Rice, K.C., Negus, S., 2011. Effects of the delta-opioid agonist SNC80 on the abuse liability of methadone in rhesus monkeys: a behavioral economic analysis. Psychopharmacology (Berl) 216, 431–439. Coldwell, M.C., Boyfield, I., Brown, T., Hagan, J.J., Middlemiss, D.N., 1999. Comparison of the functional potencies of ropinirole and other dopamine receptor agonists at human D2(long) , D3 and D4 receptors expressed in Chinese hamster ovary cells. Br. J. Pharmacol. 127, 1696–1702. Evans, A.H., Lawrence, A.D., Pott, J., Appel, S., Lees, A.J., 2005. Factors influencing susceptibility to compulsive dopaminergic drug use in Parkinson disease. Neurology 65, 1570–1574. Freeman, K.B., Wang, Z., Woolverton, W.L., 2010. Self-administration of (+)methamphetamine and (+)-pseudoephedrine, alone and combined, by rhesus monkeys. Pharmacol. Biochem. Behav. 95, 198–202. Freeman, K.B., Woolverton, W.L., 2009. Self-administration of cocaine and nicotine by rhesus monkeys. Psychopharmacology (Berl) 207, 99–106. Gardner, B., Strange, P.G., 1998. Agonist action at D2(long) dopamine receptors: ligand binding and functional assays. Br. J. Pharmacol. 124, 978–984. Hansen, R.A., Song, L., Moore, C.G., Gilsenan, A.W., Kiim, M.M., Calloway, M.O., Murray, M.D., 2009. Effect of ropinirole on sleep outcomes in patients with restless legs syndrome: meta-analysis of pooled individual patient data from randomized controlled trials. Pharmacotherapy 29, 255–262. Hursh, S.R., Silberberg, A., 2008. Economic demand and essential value. Psychol. Rev. 115, 186–198. Johanson, C.E., Arfken, C.L., Menza, S., Schuster, C.R., 2012. Diversion and abuse of buprenorphine: findings from national surveys of treatment patients and physicians. Drug Alcohol Depend. 120, 1–3. Kulisevsky, J., Pagonabarraga, J., 2010. Tolerability and safety of ropinirole versus other dopamine agonists and levodopa in the treatment of Parkinson’s Disease: meta-analysis of randomized controlled trials. Drug Saf. 33, 147–161. Lahti, R.A., Figur, L.M., Piercey, M.F., Ruppel, P.L., Evans, D.L., 1992. Intrinsic activity determinations at the dopamine D2 guanine nucleotide-binding proteincoupled receptor: utilization of receptor state binding affinities. Mol. Pharmacol. 42, 432–438. Maremmani, A.G.I., Pacini, M., Rovai, L., Rugani, F., Dell’Osso, L., Maremmani, I., 2011. Can ropinirole modulate reinforcing subjective effects of cocaine in humans? Front. Psychiatry 2, 1–5. Meini, M., Moncini, M., Cecconi, D., Cellesi, L., Biasci, L., Simoni, G., Ameglio, M., Pellegrini, M., Forgione, R.N., Rucci, P., 2011. Aripiprazole and ropinirole treatment for cocaine dependence: evidence from a pilot study. Curr. Pharm. Des. 17, 1–8. Millan, M.J., 2010. From the cell to the clinic: a comparative review of the D2 /D3 receptor agonist and ␣2-adrenoceptor antagonist, piribedil, in the treatment of Parkinson’s disease. Pharmacol. Ther. 128, 2290–3273. Nader, M.A., Mach, R.H., 1996. Self-administration of the dopamine D2 agonist 7-OH-DPAT in rhesus monkeys is modified by prior cocaine exposure. Psychopharmacology (Berl) 125, 13–22. Perugi, G., Toni, C., Ruffolo, G., Frare, R., Akiskal, H., 2001. Adjunctive dopamine agonists in treatment-resistant bipolar II depression: an open case series. Pharmacopsychiatry 34, 137–141. Ranaldi, R., Wang, Z., Woolverton, W.L., 2001. Reinforcing effects of D2 dopamine receptor agonists and partial agonists in rhesus monkeys. Drug Alcohol Depend. 64, 209–217. Riley, A.L., 2011. The paradox of drug taking: the role of the aversive effects of drugs. Physiol. Behav. 103, 69–78.
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