title: Serotonin (5-HT) transporter ligands affect plasma 5-HT in rats.
abstract: Dual dopamine (DA)/serotonin (5-HT)-releasing agents are promising candidate medications for stimulant addiction and other disorders. However, certain 5-HT transporter (SERT) substrates are associated with development of idiopathic pulmonary arterial hypertension (IPAH) and valvular heart disease (VHD). According to the "5-HT hypothesis," SERT substrates increase the risk for developing IPAH and VHD by increasing plasma 5-HT. To test this hypothesis directly, we determined the effects of acute and chronic fenfluramine, and other SERT ligands, on plasma 5-HT in male rats. For acute treatments, rats received i.v. vehicle or test drug (0.3 and 1.0 mg/kg), and serial blood samples were withdrawn. For chronic treatments, vehicle or test drug was infused via osmotic minipump (3 and 10 mg/kg/d) for 2 weeks. On the last day of infusion, rats received i.v. fenfluramine challenge (1 mg/kg), and serial blood samples were withdrawn. Plasma 5-HT was measured using ex vivo microdialysis in whole-blood samples. Baseline plasma 5-HT was <1.0 nM. Acute injection of fenfluramine or other SERT substrates caused large (up to 24-fold) dose-dependent increases in plasma 5-HT. Chronic fenfluramine at 3 and 10 mg/kg/d produced 1.7- and 3.5-fold increases in baseline plasma 5-HT, while chronic fluoxetine had no effect. Chronic infusions of fenfluramine or fluoxetine diminished the ability of acute fenfluramine to elevate dialysate 5-HT, and both drugs markedly reduced whole-blood 5-HT. Acute fenfluramine increases plasma 5-HT to concentrations that are below the micromolar levels necessary to produce adverse cardiovascular effects. Chronic fenfluramine and fluoxetine have minimal effects on plasma 5-HT, suggesting that the increased risk for IPAH associated with fenfluramine does not depend upon elevations in plasma 5-HT.
authors.affiliation: Clinical Psychopharmacology Section, Intramural Research Program, National Institute on Drug Abuse, National Institutes of Health, DHHS, Baltimore, Maryland, USA. rrothman@mail.nih.gov
authors.forename: Richard B
authors.initials: RB
authors.lastname: Rothman
authors.forename: Dorota
authors.initials: D
authors.lastname: Zolkowska
authors.forename: Michael H
authors.initials: MH
authors.lastname: Baumann
mesh_terms: D000818:Animals; D005277:Fenfluramine / Q000097:blood / Q000494:pharmacology; D005473:Fluoxetine / Q000097:blood / Q000494:pharmacology; D006801:Humans; D008024:Ligands; D008297:Male; D017551:Microdialysis; D051381:Rats; D017207:Rats, Sprague-Dawley; D012701:Serotonin / Q000097:blood*; D050486:Serotonin Plasma Membrane Transport Proteins / Q000378:metabolism*; D017367:Selective Serotonin Reuptake Inhibitors / Q000097:blood / Q000494:pharmacology
chemical_list: D005277:Fenfluramine; D005473:Fluoxetine; D008024:Ligands; D012701:Serotonin; D050486:Serotonin Plasma Membrane Transport Proteins; D017367:Selective Serotonin Reuptake Inhibitors