Inhibition of rat brain glutamate receptors by philanthotoxin.
The actions of philanthotoxin (PhTX) were studied on the function of glutamate receptors expressed in Xenopus oocytes injected with rat brain mRNA and on binding of radioligands to rat brain glutamate receptors. PhTX reversibly inhibited the oocyte responses to quisqualate, N-methyl-D-aspartate (NMDA) and kainate in a dose-dependent manner. The NMDA receptor was the most sensitive to PhTX action (10-fold more than the kainate receptor) and the least sensitive was the smooth current component of the quisqualate response. Recovery from PhTX block differed among the three amino acids. NMDA responses recovered completely within a few minutes whereas responses to kainate and quisqualate recovered more slowly. PhTX had no effect on equilibrium binding of [3H]glutamate to rat brain cortical membranes studied in buffer treated to eliminate microorganisms. Based on the drug specificity of this [3H]glutamate binding, it is suggested to be mostly to the NMDA receptor. Low concentrations of PhTX (1-10 microM) potentiated binding of [3H] MK-801, a specific noncompetitive inhibitor of the NMDA receptor. However, higher PhTX concentrations inhibited this binding with an IC50 of 20 microM, similar to its inhibition of the oocyte-expressed NMDA receptor. Inhibition of [3H]MK-801 binding by PhTX was noncompetitive. It is suggested that PhTX, like the more potent MK-801, binds to an allosteric site on the NMDA receptor and inhibits its function but its binding site is not identical with the MK-801 binding site.
Ragsdale
D
D
Department of Psychobiology, University of California, Irvine.
Gant
D B
DB
Anis
N A
NA
Eldefrawi
A T
AT
Eldefrawi
M E
ME
Konno
K
K
Miledi
R
R
D000818:Animals; D001224:Aspartic Acid / Q000037:antagonists & inhibitors; D001514:Bee Venoms / Q000494:pharmacology*; D001921:Brain / Q000187:drug effects*; D005971:Glutamates / Q000378:metabolism; D018698:Glutamic Acid; D066298:In Vitro Techniques; D007608:Kainic Acid / Q000037:antagonists & inhibitors; D008297:Male; D016202:N-Methylaspartate; D009865:Oocytes / Q000187:drug effects; D010069:Oxadiazoles / Q000037:antagonists & inhibitors; D016318:Quisqualic Acid; D051381:Rats; D011919:Rats, Inbred Strains; D018091:Receptors, AMPA; D017470:Receptors, Glutamate; D018092:Kainic Acid Receptors; D016194:Receptors, N-Methyl-D-Aspartate; D017981:Receptors, Neurotransmitter / Q000037:antagonists & inhibitors*; D014862:Wasp Venoms / Q000494:pharmacology*; D014981:Xenopus
D001224:Aspartic Acid; D001514:Bee Venoms; D005971:Glutamates; D018698:Glutamic Acid; D007608:Kainic Acid; D016202:N-Methylaspartate; D010069:Oxadiazoles; D016318:Quisqualic Acid; D018091:Receptors, AMPA; D017470:Receptors, Glutamate; D018092:Kainic Acid Receptors; D016194:Receptors, N-Methyl-D-Aspartate; D017981:Receptors, Neurotransmitter; D014862:Wasp Venoms