(A) HEK 293 cells were transfected with a plasmid expressing Sm5HTR-FLAG or empty plasmid (mock control) and probed with anti-FLAG antibody, followed by a FITC-conjugated secondary antibody. Typical in situ immunofluorescence results show expression of FLAG-tagged receptor in the test cells. (B) Schematic of the key signaling mechanisms of serotonin-activated GPCRs. Receptors can signal through changes in intracellular cAMP or Ca2+, depending on which G protein (Gs, Gi/o, Gq) is activated. Preliminary experiments showed that serotonin activation of Sm5HTR in HEK 293 cells elevated cAMP but had no effect on cytosolic Ca2+ (data not shown). AC, adenylate cyclase; PLCβ, phospholipase C-β; IP3, inositol trisphosphate; ER, endoplasmic reticulum. (C) Sm5HTR activation causes an increase in intracellular cAMP. HEK 293 cells expressing Sm5HTR were treated with biogenic amines, known serotonergic agonists and antagonists, each at 10−4 M. Antagonists were tested in the presence of 10−4 M serotonin. After incubation, the cells were lysed and the lysates were assayed for cAMP. The data were normalized relative to control cells transfected with empty plasmid (mock control). Substances tested included: serotonin (5HT), histamine (HA), acetylcholine (ACh), dopamine (DA), octopamine (OA), tyramine (TA), adrenaline (A), metanephrine (MTN), tryptamine (Trp), o-methyl-serotonin (methyl-5HT), buspirone (BUS), 8-Hydroxy-DPAT (DPAT), mianserin (MIAN), chlorpromazine (CHP), cyproheptadine (CPH). a significantly different from untreated at p<0.01; b significantly different from serotonin –induced cAMP level at p<0.01. Experiments were repeated with variable concentrations of serotonin (D) and o-methyl-serotonin (E) to obtain dose-response curves. Data were baseline-subtracted and normalized relative to the maximum agonist-induced response. Each data point is the mean and SEM of at least three independent experiments performed in duplicates or triplicates.