• Wei Ni, Claudia S Wilhelm, Michael Bader, Dennis L Murphy, Keith Lookingland, and Stephanie W Watts.
• Department of Pharmacology and Toxicology, Michigan State University, East Lansing, 48824-1317, USA. niwei@msu.edu
• J. Pharmacol. Exp. Ther. 2005 Sep 1;314(3):953-60.

Abstract(+)-Norfenfluramine, the major metabolite of fenfluramine, causes vasoconstriction dependence on the 5-hydroxytryptamine (5-HT)(2A) receptor in rat. (+)-Norfenfluramine was reported as a 5-hydroxytryptamine transporter (5-HTT) substrate and 5-HT releaser. Because the arterial 5-HTT exists and is functional in the rat, we hypothesized that (+)-norfenfluramine causes vasoconstriction by releasing 5-HT from vascular smooth muscle via 5-HTT. The released 5-HT, in turn, activates the 5-HT(2A) receptor. Isometric contractility experiments showed that (+)-norfenfluramine-induced mouse aortic contraction was reduced by the 5-HTT inhibitor fluoxetine (1 micriM) but not by fluvoxamine (1 microM). Tryptophan hydroxylase (TPH)-deficient (Tph1-/-) mice lack peripheral 5-HT. (+)-Norfenfluramine (10 nM-100 microM)-contracted aorta from wild-type and Tph1-/- mice with equivalent potency (-log EC(50) [M], wild type = 5.73 +/- 0.02, Tph1-/- = 5.62 +/- 0.09), and these contractions were inhibited by the 5-HT(2A) receptor antagonist ketanserin (3 nM) by a similar magnitude in aorta from wild-type and Tph1-/- mice (wild type = 19.4, Tph1-/- = 15.4-fold rightward shift versus control), as did fluoxetine (1 microM) (wild type = 22.4, Tph1-/- = 28.8-fold rightward shift versus control). To further test the role of 5-HTT in (+)-norfenfluramine-induced aortic contraction, the 5-HTT-targeted mutation mouse was used. (+)-Norfenfluramine induced similar aortic contraction in wild-type and 5-HTT-targeted mutation mice, and these contractions were inhibited by fluoxetine (1 microM). Thus, (+)-norfenfluramine vasoconstriction is not dependent on 5-HTT-mediated release of endogenous 5-HT but by activating membrane 5-HT(2A) receptors directly. Understanding of the mechanism by which (+)-norfenfluramine induces vasoconstriction is important to characterize and understand the function of the serotonergic system in peripheral arterial vasculature.

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