Abstract
Despite the importance of 5-hydroxytryptamine (5-HT)2C (serotonin) receptors in the control of depressive states, actions of antidepressants at these receptors remain poorly characterized. This issue was addressed both in human embryonic kidney (HEK)-293 cells coexpressing unedited human 5-HT2CINI receptors and Gαq protein and in cultured mouse cortical neurons. Indicative of constitutive activity, the inverse agonist SB206,553 decreased basal inositol phosphate (IP) production in HEK-293 cells. The tetracyclic antidepressants mirtazapine and mianserin likewise suppressed basal IP formation. Conversely, the tricyclics amitriptyline and clomipramine, the m-chlorophenylpiperazine derivatives trazodone and nefazodone, and the 5-HT reuptake inhibitors fluoxetine and citalopram were inactive alone, although they blocked 5-HT-induced IP production. Inverse agonist actions of 5-methyl-1-(3-pyridylcarbamoyl)-1,2,3,5-tetrahydropyrrolo[2,3-f]indole (SB206,553) and mirtazapine were abolished by the neutral antagonist 6-chloro-5-methyl-1-[6-(2-methylpyridin-3-yloxy)pyridin-3-ylcarbamoyl]indoline (SB242,084), which was inactive alone. As assessed by confocal microscopy and enzyme-linked immunosorbent assay, prolonged treatment of HEK-293 cells with SB206,553, mirtazapine, or mianserin, but not the other antidepressants, enhanced cell surface expression of 5-HT2C receptors: 5-HT-induced IP production was also increased, and both these actions were blocked by SB242,084. Cortical neurons were shown by reverse transcription-polymerase chain reaction to predominantly express constitutively active 5-HT2C receptor isoforms. Prolonged pretreatment with SB206,553 or mirtazapine triggered an otherwise absent 5-HT-induced elevation in cytosolic Ca2+ concentrations. SB242,084, which was inactive alone, abolished these effects of SB206,553 and mirtazapine. In conclusion, the tetracyclic antidepressants mirtazapine and mianserin, but not other clinically established antidepressants, suppress constitutive activity at recombinant and native 5-HT2C receptors. The clinical significance of inverse agonist versus neutral antagonist properties both during and after drug administration will be of interest to elucidate.
Footnotes
-
P.M. was supported by grants from Centre National de la Recherche Scientifique, Institut National de la Santé et de la Recherche Médicale, le Ministère Français de la Recherche (contract ACI-JC 5075 and ANR Neurosciences-2005) and la Fondation pour la Recherche Médicale (Equipe FRM-2005). B.C. was a recipient of a Convention Industrielle de Formation par la Recherche fellowship from Servier Pharmaceuticals.
-
ABBREVIATIONS: 5-HT, 5-hydroxytryptamine (serotonin); SB206,553, 5-methyl-1-(3-pyridylcarbamoyl)-1,2,3,5-tetrahydropyrrolo[2,3-f]indole; SB242,084, 6-chloro-5-methyl-1-[6-(2-methylpyridin-3-yloxy)pyridin-3-ylcarbamoyl]indoline; PLC, phospholipase C; NA, noradrenalin; mCPP, m-chlorophenylpiperazine; SSRI, selective serotonin reuptake inhibitor; h, human; HEK, human embryonic kidney; IP, inositol phosphate; ELISA, enzyme-linked immunosorbent assay; PBS, phosphate-buffered saline; BSA, bovine serum albumin; ANOVA, analysis of variance; NE, norepinephrine; RTR-PCR, reverse transcription-polymerase chain reaction; SSCP, single-strand conformational polymorphism; PCR, polymerase chain reaction; bp, base pair(s); CHO, Chinese hamster ovary; Ro-60,0175, 2(S)-1-(6-chloro-5-fluoro-1H-indol-1-yl)-2-propanamine fumarate.
- Received September 5, 2007.
- Accepted December 13, 2007.
- The American Society for Pharmacology and Experimental Therapeutics
MolPharm articles become freely available 12 months after publication, and remain freely available for 5 years.Non-open access articles that fall outside this five year window are available only to institutional subscribers and current ASPET members, or through the article purchase feature at the bottom of the page.
|