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NEUROPHARMACOLOGY

Evidence for the Involvement of Dopamine Transporters in Behavioral Stimulant Effects of Modafinil

Clinical Psychopharmacology Section, Intramural Research Program, National Institute on Drug Abuse, National Institutes of Health, Baltimore, Maryland (D.Z., R.B.R., J.S.P., M.H.B.); Department of Hygiene, Medical University of Lublin, Lublin, Poland (D.Z.); National Drug Dependence Centre, All India Institute of Medical Sciences, Ansari Nagar, New Dehli, India (R.J.); Department of Pharmacology, Medicinal Chemistry, and Psychiatry, University of North Carolina School of Medicine, Chapel Hill, North Carolina (B.L.R., V.S.); and Department of Medicinal Chemistry, The University of Kansas, Lawrence, Kansas (T.E.P.)

Modafinil is prescribed for numerous medical conditions, but the drug's mechanism of action is unclear. Here, we examined the interaction of modafinil with receptors and transporters in vitro and compared pharmacological effects of the drug with those produced by indirect dopamine (DA) agonists 1-[2-[bis(4-fluorophenyl)methoxy]ethyl]-4-(3-phenylpropyl)piperazine (GBR12909) and (+)-methamphetamine (METH). Modafinil was screened at various receptors and transporters using binding assays. Transporter-mediated uptake and release were examined in rat brain synaptosomes. Effects of modafinil on motor activity and neurochemistry were determined in rats undergoing in vivo microdialysis in nucleus accumbens. Of the receptors and transporters assayed, modafinil displayed measurable potency only at DA transporters (DAT), inhibiting [³H]DA uptake, with an IC₅₀ value of 4.0 µM. Accordingly, modafinil pretreatment (10 µM) antagonized METH-induced release of the DAT substrate [³H]1-methyl-4-phenylpyridinium. Intravenous modafinil (20 and 60 mg/kg) produced dose-dependent increases in motor activity and extracellular DA, without affecting serotonin (5-HT). Analogous results were observed for GBR12909 (1 and 3 mg/kg), whereas METH (0.3 and 1 mg/kg) increased DA and 5-HT. Locomotor effects of all drugs were positively correlated with dialysate DA (P < 0.001). Interestingly, modafinil pretreatment reduced METH-induced ambulation and DA release. Our data show that modafinil interacts with DAT sites in rat brain, a property shared with agonist medications under investigation for treating cocaine dependence. Nondopaminergic mechanisms may also contribute to the pharmacology of modafinil. Finally, the results suggest that modafinil should be tested as an adjunct for treating METH addiction.

Address correspondence to: Dr. Michael H. Baumann, Intramural Research Program, National Institute on Drug Abuse, National Institutes of Health, 333 Cassell Dr., Suite 4500, Baltimore, MD 21224. E-mail: mbaumann{at}mail.nih.gov