Effects of ifenprodil on the N-methyl-D-aspartate receptor ionophore complex in rat brain.
The effects of a cerebral anti-ischemic drug ifenprodil on the receptor ionophore complex of an N-methyl-D-aspartate (NMDA)-sensitive subclass of central excitatory amino acid receptors were examined using [3H](+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10- imine (MK-801) binding in rat brain synaptic membrane preparations as a biochemical measure. The binding in membrane preparations not extensively washed was markedly inhibited not only by competitive NMDA antagonists such as (+/-)-3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic, D-2-amino-5-phosphonovaleric and D-2-amino-7-phosphonoheptanoic acids, but also by competitive antagonists at the strychnine-insensitive glycine (Gly) site including 7-chlorokynurenic acid and 6,7-dichloroquinoxaline-2,3-dione. Among several proposed ligands for alpha-adrenergic receptors tested, ifenprodil most potently inhibited the binding in these membrane preparations due to a decrease in the density of the binding sites without significantly affecting the affinity. Ifenprodil also inhibited the binding of [3H]N-[1-(2-thienyl)cyclohexyl]piperidine as well as of [3H]MK-801 to open NMDA channels in a concentration-dependent manner at concentrations above 10 nM in membrane preparations extensively washed but not treated by a detergent, with a Hill coefficient of less than unity. Further treatment of extensively washed membrane preparations with a low concentration of Triton X-100 resulted in an almost complete abolition of [3H]MK-801 binding, and the binding was restored to the level found in membrane preparations not extensively washed following the addition of both L-glutamic acid (Glu) and Gly. Ifenprodil was effective in inhibiting [3H]MK-801 binding via reducing both initial association and dissociation rates in Triton-treated membrane preparations, irrespective of the presence of Glu and Gly added. The binding in Triton-treated membrane preparations was additionally potentiated by the polyamine spermidine in a concentration-dependent manner at concentrations above 10 microM in the presence of both Glu and Gly at maximally effective concentrations. Ifenprodil invariably diminished the abilities of these three stimulants to potentiate [3H]MK-801 binding at concentrations over 1 microM in a manner that the maximal responses each were reduced. These results suggest that ifenprodil does not interfere with the NMDA receptor complex as a specific isosteric antagonist at the polyamine domain in contrast to the prevailing view.
Ogita
K
K
Department of Pharmacology, Setsunan University, Osaka, Japan.
Ohkawara
A
A
Suzuki
T
T
Ohgaki
T
T
Uchida
S
S
Meguri
H
H
Yoneda
Y
Y
D000317:Adrenergic alpha-Antagonists / Q000494:pharmacology*; D000818:Animals; D001667:Binding, Competitive; D001921:Brain / Q000378:metabolism*; D016291:Dizocilpine Maleate / Q000378:metabolism*; D005971:Glutamates / Q000378:metabolism; D018698:Glutamic Acid; D005998:Glycine / Q000378:metabolism; D007700:Kinetics; D008297:Male; D010880:Piperidines / Q000494:pharmacology*; D011869:Radioligand Assay; D051381:Rats; D017208:Rats, Wistar; D016194:Receptors, N-Methyl-D-Aspartate / Q000187:drug effects* / Q000378:metabolism; D013570:Synaptic Membranes / Q000378:metabolism*; D014316:Tritium
D000317:Adrenergic alpha-Antagonists; D005971:Glutamates; D010880:Piperidines; D016194:Receptors, N-Methyl-D-Aspartate; D014316:Tritium; D018698:Glutamic Acid; D016291:Dizocilpine Maleate; C010739:ifenprodil; D005998:Glycine