title: Direct noncompetitive inhibition of 5-HT(3) receptor-mediated responses by forskolin and steroids.
abstract: 5-HT(3) receptors cloned from NCB-20 cells were expressed in Xenopus oocytes, and the effects of forskolin and steroids on the function of the receptors were investigated using the two-electrode voltage-clamp technique. Forskolin, 17-beta-estradiol, and progesterone inhibited the currents activated by 1 microM 5-HT in a reversible and concentration-dependent manner, with IC(50) values of 12, 33, and 89 microM, respectively. The inhibitory effects of forskolin and 17-beta-estradiol were independent of the membrane potential. Forskolin and 17-beta-estradiol significantly reduced the maximal amplitude of the 5-HT concentration-response curve (E(max)) without significantly affecting the EC(50), indicating that these compounds act as noncompetitive inhibitors of the 5-HT(3) receptor. The cAMP analogue, 8-Br-cAMP (0.2 mM), and the protein kinase A activator, Sp-cAMP (0.1 mM), did not affect the amplitude of 5-HT(3) receptor-mediated currents. The membrane-permeable protein kinase A inhibitor Rp-cAMP (0.1 mM) and the estrogen-receptor antagonist tamoxifen (1 microM) did not affect the inhibition of 5-HT-activated current. In addition, 5-HT(3) receptor-mediated currents were inhibited by both 1,9-dideoxy forskolin (30 microM), which does not activate adenylyl cyclase, and wForskolin (30 microM), a charged hydrophilic analogue of forskolin that is membrane impermeable. These results indicate that both forskolin and 17-beta-estradiol inhibit the function of the 5-HT(3) receptor in a noncompetitive manner and that this inhibition is independent of cAMP levels.
authors.affiliation: National Institute on Drug Abuse, Cellular Neurobiology Section, National Insititues of Health, 5500 Nathan Shock Drive, Baltimore, MD 21224, USA. moz@intra.nida.nih.gov
authors.forename: Murat
authors.initials: M
authors.lastname: Oz
authors.forename: Li
authors.initials: L
authors.lastname: Zhang
authors.forename: Charles E
authors.initials: CE
authors.lastname: Spivak
mesh_terms: D015124:8-Bromo Cyclic Adenosine Monophosphate / Q000494:pharmacology; D000262:Adenylyl Cyclases / Q000187:drug effects / Q000378:metabolism; D000818:Animals; D002478:Cells, Cultured; D005576:Colforsin / Q000494:pharmacology*; D006224:Cricetinae; D017868:Cyclic AMP-Dependent Protein Kinases / Q000187:drug effects / Q000378:metabolism; D004305:Dose-Response Relationship, Drug; D020536:Enzyme Activators / Q000494:pharmacology; D004791:Enzyme Inhibitors / Q000494:pharmacology; D004958:Estradiol / Q000494:pharmacology*; D004965:Estrogen Antagonists / Q000494:pharmacology; D005260:Female; D008564:Membrane Potentials / Q000187:drug effects* / Q000502:physiology; D051379:Mice; D008845:Microinjections; D009865:Oocytes / Q000166:cytology / Q000187:drug effects / Q000378:metabolism; D018408:Patch-Clamp Techniques; D011374:Progesterone / Q000494:pharmacology*; D011985:Receptors, Serotonin / Q000187:drug effects* / Q000235:genetics / Q000378:metabolism; D044406:Receptors, Serotonin, 5-HT3; D012701:Serotonin / Q000494:pharmacology; D013629:Tamoxifen / Q000494:pharmacology; D014162:Transfection; D014982:Xenopus laevis
chemical_list: D020536:Enzyme Activators; D004791:Enzyme Inhibitors; D004965:Estrogen Antagonists; D011985:Receptors, Serotonin; D044406:Receptors, Serotonin, 5-HT3; D013629:Tamoxifen; D005576:Colforsin; D015124:8-Bromo Cyclic Adenosine Monophosphate; D012701:Serotonin; D011374:Progesterone; D004958:Estradiol; D017868:Cyclic AMP-Dependent Protein Kinases; D000262:Adenylyl Cyclases