title: Rhynchophylline and isorhynchophylline inhibit NMDA receptors expressed in Xenopus oocytes.
abstract: Rhynchophylline and isorhynchophylline are major tetracyclic oxindole alkaloid components of Uncaira species, which have been long used as medicinal plants. In this study, the effects of rhynchophylline and isorhynchophylline on the ionotropic and metabotropic glutamate receptor-mediated current responses were examined using Xenopus oocytes injected with total RNA prepared from rat cortices or cerebelli. Rhynchophylline and isorhynchophylline (1-100 microM) per se failed to induce membrane current, but these alkaloids reversibly reduced N-methyl-D-aspartate (NMDA)-induced current in a concentration-dependent but voltage-independent manner. The IC(50) values of rhynchophylline and isorhynchophylline were 43.2 and 48.3 microM, respectively. Substitution of Ba(2+) for Ca(2+) in the recording medium did not alter the extent of rhynchophylline- and isorhynchophylline-induced suppression of NMDA currents. In contrast, neither alkaloid had an effect on the currents mediated by ionotropic kainic acid-type and (+/-)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-type glutamate receptors or by the metabotropic glutamate receptor(1 and 5) (mGlu(1/5)). Rhynchophylline and isorhynchophylline (30 microM) significantly reduced the maximal current responses evoked by NMDA and glycine (a co-agonist of NMDA receptor), but had no effect on the EC(50) values and Hill coefficients of NMDA and glycine for inducing currents. These alkaloids showed no interaction with the polyamine binding site, the Zn(2+) site, proton site or redox modulatory site on the NMDA receptor. These results suggest that rhynchophylline and isorhynchophylline act as noncompetitive antagonists of the NMDA receptor and that this property may contribute to the neuroprotective and anticonvulsant activity of the Uncaira species plant extracts.
authors.affiliation: Department of Pharmacology, Institute of Natural Medicine, Toyama Medical and Pharmaceutical University, 2630 Sugitani, Toyama 930-0194, Japan.
authors.forename: Tai-Hyun
authors.initials: TH
authors.lastname: Kang
authors.forename: Yukihisa
authors.initials: Y
authors.lastname: Murakami
authors.forename: Kinzo
authors.initials: K
authors.lastname: Matsumoto
authors.forename: Hiromitsu
authors.initials: H
authors.lastname: Takayama
authors.forename: Mariko
authors.initials: M
authors.lastname: Kitajima
authors.forename: Norio
authors.initials: N
authors.lastname: Aimi
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Watanabe
mesh_terms: D000470:Alkaloids / Q000302:isolation & purification / Q000494:pharmacology*; D000818:Animals; D026121:Indole Alkaloids; D008297:Male; D008564:Membrane Potentials / Q000187:drug effects; D009865:Oocytes / Q000187:drug effects; D000078183:Oxindoles; D012313:RNA / Q000494:pharmacology; D051381:Rats; D017208:Rats, Wistar; D016194:Receptors, N-Methyl-D-Aspartate / Q000187:drug effects*; D013237:Stereoisomerism; D014981:Xenopus
chemical_list: D000470:Alkaloids; D026121:Indole Alkaloids; D000078183:Oxindoles; D016194:Receptors, N-Methyl-D-Aspartate; C052714:rhyncophylline; D012313:RNA