Cyanotriphenylborate: Subtype-specific blocker of glycine receptor chloride channels

The inhibitory glycine receptor is a ligand-gated ion-channel protein existing in different homo- and heterooligomeric isoforms. Here we show that the chloride channel of the recombinant α1-subunit homooligomeric glycine receptor is efficiently blocked by cyanotriphenylborate (CTB) with a concentration effecting 50% inhibition (IC₅₀) of 1.3 μM in the presence of 50 μM glycine. The antagonistic effect of CTB is noncompetitive, use dependent, and more pronounced at positive membrane potentials, suggesting open-channel block. In contrast to α1-subunit receptors, α2-subunit homooligomers are resistant to CTB (IC₅₀ >> 20 μM). By exchanging the channel-lining transmembrane segment M2 of the α1 polypeptide by that of the α2 polypeptide, we could transfer this resistance to α1 channels, indicating that a single glycine residue at position 254 of the α1 subunit is critical for CTB sensitivity. The blocker did not affect the cation- selective channel of the nicotinic acetylcholine receptor. Thus, CTB may prove useful as a tool to probe the subunit structure of native glycine receptors in mammalian neurons.