Effects of Agelenopsis aperta toxins on the N-methyl-D-aspartate receptor: polyamine-like and high-affinity antagonist actions.
The effects of synthetic alpha-agatoxins, corresponding to toxins found in the venom of the spider Agelenopsis aperta, on the N-methyl-D-aspartate (NMDA) receptor complex have been investigated by using ligand-binding assays with [3H]MK-801 [(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine] and electrophysiological studies of NMDA receptors expressed in Xenopus oocytes after injection of rat brain RNA. The alpha-agatoxins Agel-489 and Agel-505 (3-30 microM) enhanced the binding of [3H]MK-801 to NMDA receptors on membranes prepared from rat brain. The stimulatory effect of Agel-505 was attenuated by the polyamine antagonists diethylenetriamine and N-(3-aminopropyl)-1,10-diaminodecane. Higher concentrations of Agel-489 and Agel-505 inhibited the binding of [3H]MK-801. A related toxin, Argiotoxin-636, had inhibitory but not stimulatory effects on the binding of [3H]MK-801. In Xenopus oocytes voltage-clamped at -70 mV, Agel-505 inhibited responses to NMDA with an IC50 of 13 nM. This effect of Agel-505 occurred at concentrations approximately 10,000-fold lower than those that cause inhibition of [3H]MK-801 binding. Antagonism of NMDA-induced currents by Agel-505 in Xenopus oocytes was strongly voltage-dependent and may represent an open-channel blocking effect of the toxin, possibly due to an interaction at the Mg++ binding site. Although alpha-agatoxins can interact at the positive allosteric polyamine site on the NMDA receptor, stimulatory effects produced by this interaction may be masked in functional assays due to a separate action of the toxins as high-affinity, noncompetitive antagonists of the receptor.
Williams
K
K
Department of Pharmacology, University of Pennsylvania School of Medicine, Philadelphia.
D000818:Animals; D001923:Brain Chemistry; D016291:Dizocilpine Maleate / Q000378:metabolism; D004594:Electrophysiology; D005260:Female; D009865:Oocytes / Q000502:physiology; D011073:Polyamines / Q000378:metabolism / Q000494:pharmacology*; D012313:RNA / Q000494:pharmacology; D051381:Rats; D016194:Receptors, N-Methyl-D-Aspartate / Q000037:antagonists & inhibitors* / Q000378:metabolism; D013111:Spider Venoms / Q000378:metabolism / Q000494:pharmacology*; D013268:Stimulation, Chemical; D014118:Toxins, Biological / Q000378:metabolism / Q000494:pharmacology*; D014316:Tritium; D014981:Xenopus
D011073:Polyamines; D016194:Receptors, N-Methyl-D-Aspartate; D013111:Spider Venoms; D014118:Toxins, Biological; D014316:Tritium; D012313:RNA; D016291:Dizocilpine Maleate