Antagonism of responses to excitatory amino acids on rat cortical neurones by the spider toxin, argiotoxin636

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Abstract
1 Several low molecular weight spider toxins have recently been shown to block potently glutama-tergic neuromuscular transmission at the invertebrate neuromuscular junction. The aim of the present investigation was to evaluate the effects of one such toxin, argiotoxin636, on excitatory amino acid receptor-mediated responses in mammalian neurones. 2 Membrane currents were recorded from rat cortical neurones after 2–6 weeks in cell culture, by the whole-cell variant of the patch-clamp technique. N-methyl-D-aspartate (NMDA) and kainate were used as selective agonists for their respective receptor subtypes. α-Amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) was used as a selective agonist for the quisqualate receptor subtype. 3 Responses to these agonists were characterised with respect to their concentration and voltage-dependence. Argiotoxin636 (3–30 μm) was found to attenuate markedly responses to NMDA in an agonist- and voltage-dependent manner. Thus, argiotoxin636 progressively reduced successive responses to NMDA when membrane potentials were voltage clamped between −40 mV to −100 mV. The more negative the membrane potential the more rapid the development of the block of inward current. 4 The antagonism of NMDA-induced currents by argiotoxin636 could be reversed by clamping the membrane at positive potentials (+ 20 to + 60 mV) and reapplying NMDA. 5 Responses to AMPA and kainate were less affected by argiotoxin636, with an antagonist action only becoming evident at a concentration of 100 μm 6 These results suggest that argiotoxin636 is an open-channel blocker of the NMDA activated ion-channel in mammalian neurones. Furthermore, our results indicate at least a 30 fold selectivity for NMDA over the quisqualate- and kainate-activated ion-channels.