Investigating the Effect of Kappa Opioid Receptor Agonists on Serotonin Transporter Function in HEK-293 Cells

The serotonin (5-HT) transporter (SERT) and the kappa opioid receptor (KOPr) are important brain proteins. Currently, it is known that KOPr modulates 5-HT concentrations but the biochemical mechanisms responsible remain enigmatic. The 5-HT and KOPr relationship is thought to involve SERT because both SERT and KOPr co-localise in the dorsal raphe nucleus (DRN), a brain region involved in drug addiction, affective disorders, and stress homeostasis. Thus, elucidating the KOPr and SERT relationship will clarify biomedical targets for these disorders. To investigate this relationship the effect of KOPr activation on SERT function was examined in HEK-293 cells co-expressing myc-rKOPr and eGFP-tagged hSERT. We found the KOPr agonists U50,488H, Salvinorin A (Sal A) and DS-3-240 had no acute (5 min) effect on SERT function as measured by rotating disc electrode voltammetry (RDEV) incubated with 1’-diethyl-2,2’-cyanine iodide (D-22; 1.5 mM) and by confocal microscopy. Interestingly, SERT function significantly decreased with chronic (30 min) exposure to U50,488H (**p<0.01) and Sal A (**p<0.01) as measured with a novel high-throughput assay; this decrease was also attenuated with 5 min pre-treatment of the SERT inhibitor, fluoxitene (***p<0.001). Furthermore, this novel high-throughput assay also replicated our laboratories recent finding that chronic (30 min) exposure to U50,488H (**p<0.01) and Sal A (**p<0.01) significantly increase dopamine transporter (DAT) function in HEK-293 cells co-expressing myc-rKOPr and eYFP-tagged DAT. Collectively, these findings suggest in this cell model that chronic (30 min) KOPr activation by U50,488H and Sal A decreases SERT function.