Functional interactions between native Gs-coupled 5-HT receptors in HEK-293 cells and the heterologously expressed serotonin transporter.

In HEK-293 cells, serotonin (5-hydroxytryptamine, 5-HT) was found to induce cAMP production showing pharmacological characteristics consistent with the 5-HT(7) receptor. The presence of 5-HT(7) (and 5-HT(6)) receptor mRNA was confirmed by RT-PCR. Stable HEK-293 cell lines expressing either wild-type or haemagglutinin (HA)-tagged human 5-HT transporter (SERT) were selected and SERT function was confirmed using [3H]5-HT transport. The presence of SERT caused a 10-fold reduction in the potency of 5-HT-induced cAMP production compared to control cells. Downstream signalling by 5-HT(6/7) receptors could be detected as 5-HT-induced protein kinase A activation and phosphorylation of MAP kinase and CREB using phospho-specific antibodies. SERT inhibitors reversed the reduction in potency of 5-HT-induced cAMP production caused by the presence of SERT, resulting in a concentration-dependent left shift in EC(50) values but also a progressive decrease in the maximal response. Thus, when antidepressants were used to block SERT activity, 5-HT receptor signalling was effectively clamped within a mid-range.