The permeability of gamma-aminobutyric acid-gated chloride channels is described by the binding of a "cage" convulsant, t-butylbicyclophosphoro[35S]thionate.

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Abstract
The "cage" convulsant t-butylbicyclophosphoro[35S]thionate ([35S]TBPS) binds with high affinity to sites at or near a .gamma.-aminobutyric acid (GABA)-gated chloride channel according to current hypothesis. We now report that the potencies of a series of anions in enhancing [35S]TBPS binding correlated highly with their relative permeabilities through GABA-gated chloride channels. Furthermore, statistically significant correlations are obtained between the apparent affinity (Kd) of [35S]TBPS estimated in the presence of these anions and their relative permeabilities through GABA-gated chloride channels. The latter relationships obtained whether the Kd of [35S]TBPS as estimated in rat cerebral cortex was correlated with the relative permeabilities of these anions in either frog dorsal root ganglion cells or primary cultures of either spinal cord neurons. These findings strongly suggest that [35S]TBPS binds to GABA-gated chloride channels and that the apparent affinity of this radioligand is directly related to the permeability of these channels. Thus, radioreceptor techniques using [35S]TBPS may provide a simple means of describing permeability characteristics of GABA-gated chloride channels.