Down-regulation of the rat serotonin transporter upon exposure to a selective serotonin reuptake inhibitor

The serotonin transporter (SERT) terminates serotonergic neurotransmission by rapid reuptake of 5-hydroxytryptamine (5-HT) into the nerve terminal or axonal varicosities. SERT represents the target of various antidepressants which inhibit 5-HT transport and are widely used for the pharmacotherapy of depression. Here, we have analyzed the function of SERT stably expressed in HEK 293 cells upon exposure to citalopram, a selective serotonin reuptake inhibitor (SSRI), with respect to 5-HT transport activity and protein expression as estimated by ligand binding experiments. Our results show that long-term exposure to an SSRI causes a down-regulation of transport activity as revealed by a reduction of the maximal transport rate, without affecting substrate affinity, accompanied by a decrease in ligand binding sites.