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

Abstract The effects of purified spider toxin (argiotoxin 636 ) 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 channnel behaviour were broadly defined when argiotoxin 636 was present either in the patch pipette or in the muscle bath; the type of channel behaviour being dependent upon the concentration of argiotoxin 636 and/or duration of its application. Type I behaviour was characterized by reductions in channel open probability ( P o ) and channel event frequency ( f ), by an increase in mean channel closed time ( m c ) and either no change in mean channel open time ( m o ) or, infrequently, an increase in this parameter; Type II behaviour was characterized by further reductions in P o and f , an increase in m c and by a reduction in m o ; Type III behaviour was characterized by an apparent absence of channel openings. For example, with 10 −12 M argiotoxin 636 in the patch pipette Type I behaviour changed to Type II behaviour after ∼60s and from Type II behaviour to Type III behaviour after ∼120s. The results of this study are consistent with the idea that argiotoxin 636 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 m o seen in a few recordings during the stage of argiotoxin 636 antagonism raises the possibility that the toxin interacts allosterically with the glutamate binding sites on the excitatory glutamate receptor.