Abstract
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 [EC50 (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.
Footnotes
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This study was supported by National Institute on Drug Abuse Grants DA-10352 (to W.L.W.) and DA-12970 (to B.E.B.). W.L.W. is the recipient of National Institute on Drug Abuse Grant K05-DA15343. This work was previously presented at the 2004 meeting of the College on Problems of Drug Dependence, 12–17 June 2004, San Juan, Puerto Rico.
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doi:10.1124/jpet.104.080101.
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ABBREVIATIONS: CNS, central nervous system; DA, dopamine; NE, norepinephrine; DAT, dopamine transporter; 5-HT, 5-hydroxytryptamine (serotonin); SERT, serotonin transporter; HD-60, (±)-2β-propanoyl-3β-(2-isopropenyl)-tropane; FR, fixed ratio; PR, progressive ratio; GBR12935, 1-[2-(diphenylmethoxy)ethyl]-4-(3-phenylpropyl)-piperazine; LH, limited hold; TO, timeout; ANOVA, analysis of variance; MPP+, 1-methyl-4-phenylpyridinium.
- Received November 4, 2004.
- Accepted January 25, 2005.
- The American Society for Pharmacology and Experimental Therapeutics
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