In vitro radioligand binding studies were carried out in rat brain membranes to assess the affinity of various reuptake inhibitors for the serotonin (5-hydroxytryptamine, 5-HT) and the norepinephrine transporters using the selective ligands [3H]cyanoimipramine and [3H]nisoxetine, respectively. The selective 5-HT reuptake inhibitors paroxetine, indalpine and fluvoxamine displayed a high affinity for the 5-HT transporter, whereas the norepinephrine reuptake inhibitor desipramine had a high affinity for the norepinephrine transporter. Duloxetine, a dual 5-HT and norepinephrine reuptake inhibitor, displayed a high affinity for both the 5-HT and the norepinephrine transporters. Interestingly, venlafaxine, a dual 5-HT and norepinephrine reuptake inhibitor, displayed only a moderate affinity for the 5-HT transporter (Ki = 74 nM) and a very low affinity for the norepinephrine transporter (Ki = 1.26 microM). The relatively low affinities of venlafaxine contrast with its potent in vivo 5-HT and norepinephrine reuptake blocking properties. These results raise the possibility that the in vivo effects on the 5-HT and norepinephrine reuptake observed with venlafaxine may not be mediated solely by its binding to the [3H]cyanoimipramine and [3H]nisoxetine binding sites.
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