(+)-Norfenfluramine-Induced Arterial Contraction Is Not Dependent on Endogenous 5-Hydroxytryptamine or 5-Hydroxytryptamine Transporter

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 μM) but not by fluvoxamine (1 μM). Tryptophan hydroxylase (TPH)-deficient (Tph1–/–) mice lack peripheral 5-HT. (+)-Norfenfluramine (10 nM–100 μM)-contracted aorta from wild-type and Tph1–/– mice with equivalent potency (–log EC₅₀ [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 μM) (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 μM). 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.