Single channel studies of non-competitive antagonism of a quisqualate-sensitive glutamate receptor by argiotoxin636--a fraction isolated from orb-web spider venom.

The effects of purified spider toxin (argiotoxin636) on single glutamate-activated channels in voltage-clamped locust muscle fibres have been examined using a megaohm seal, patch-clamp technique. Four experimental protocols were employed in which the composition of the patch pipette and bathing solutions were varied. Three types of channel behaviour were broadly defined when argiotoxin636 was present either in the patch pipette or in the muscle bath; the type of channel behaviour being dependent upon the concentration of argiotoxin636 and/or the duration of its application. Type I behaviour was characterized by reductions in channel open probability (Po) and channel event frequency (f), by an increase in mean channel closed time (mc) and either no change in mean channel open time (mo) or, infrequently, an increase in this parameter; Type II behaviour was characterized by apparent absence of channel openings. For example, with 10(-12) M argiotoxin636 in the patch pipette Type I behaviour changed to Type II behaviour after approximately 60 s and from Type II behaviour to Type III behaviour after approximately 120 s. The results of this study are consistent with the idea that argiotoxin636 blocks the cation-selective channel gated by excitatory glutamate receptors in insect muscle at the level of the open channel although there remains the possibility that it is also either a closed channel blocker and/or a competitive antagonist. The increase in mo seen in a few recordings during the initial stage of argiotoxin636 antagonism raises the possibility that the toxin interacts allosterically with the glutamate binding sites on the excitatory glutamate receptor.