Down-regulation of norepinephrine transporters on PC12 cells by transporter inhibitors

To investigate the regulation of norepinephrine transporters (NETs) in vitro, we measured the binding of the NET-selective ligand [H-3]nisoxeline in homogenates of PC12 cells after exposure of intact cells to the NET inhibitor desipramine (DMI). A 3-day exposure of PC12 cells to DMI robustly reduced the B-max but not the K-D, of [H-3] nisoxetine binding to NETs. Reduction of the binding of [H-3]nisoxetine was dependent on both the concentration of DMI and the time of exposure to DMI. Reduction of [H-3]nisoxetine binding to NETs produced by a I-day exposure to DMI reverted to preexposure levels 48 h after cessation of DMI exposure. Similar down-regulation of NETs was found when PC12 cells were exposed to another NET-selective drug, nisoxetine, which is structurally unrelated to DMI. In contrast, exposure of cells to the serotonin transporter-selective drug citalopram, or the NET substrate norepinephrine, had no effects on the binding of [H-3]nisoxetine to NETs. The down-regulation of NETs was paralleled by a DMI-induced reduction in the uptake of [H-3] norepinephrine in intact PC12 cells. It can be inferred from these data that inhibitors of the NET can down-regulate NETs directly, and do so in the absence of changes in the synaptic concentration of norepinephrine.