Anionic Ligand-Gated Ion Channels: The Convulsive Site and Mechanism of Action

Dibas, Mohammed I.
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Dibas, Mohammed, Anionic Ligand-Gated Ion channels: The Convulsive Site And Mechanism of Action. Doctor of Philosophy (Biomedical Sciences), August 2001, pp153, 1 table, 24 illustrations, 76 titles. Picrotoxin, a CNS convulsant inhibits all anionic ligand gated ion channels. The mechanism and the binding site for picrotoxin and its related ligands are still undefined. The second transmembrane (TMII) domain of these ligand gated ion channels is found to play a key role in the mechanism of block by picrotoxin. It has been shown that the incorporation of a phenylalanine residue in place of threonine at position 6’ within the TMII domain of B2 subunit conferred high resistance toward picrotoxin in GABAA a3B2(T6’F)y2 receptors. Mediating their blocking effect through the PTX-site, PTZ, TBPS, and U-93631 lost their inhibitory effects due to the same mutation B2(T6’F). Interestingly, this mutation uncovered a low affinity, highly efficacious stimulatory site for PTZ. PTZ seems to mediate its stimulatory effect through a novel distinct site different from that for benzodiazepine. The effect of varying subunit configuration of GABAA receptors dramatically affected the ability of the mutation B2(T6’F) to abolish the inhibitory effect of picrotoxin. While picrotoxin failed to block the current induced by GABA in a3B2(T6’F)y2 receptors, picrotoxin partially blocked the current in a3B2(T6’F)y2 receptors. In B2(T6’F)y2 receptors, picrotoxin restores its full efficacy. When phenylalanine was incorporated at position 6’ in the a1 subunit, picrotoxin completely blocked the current induced by GABA in a1(T6’F)B2y2 receptors. The combined results showed that the ability of (T6’F) mutation to regulate the inhibitory mechanism of picrotoxin as dependent on the subunit configurations and at which subunit is mutated. In addition, picrotoxin is known to inhibit GABAA receptors in use-facilitated mechanism, while it inhibits the glycine receptor in a non-use facilitated fashion. The molecular determinant behind the use-facilitated mechanism was modulated by the nature of the amino acid at position 15’ within the second transmembrane domain. The mutation of serine 15’ to either glutamine or asparagine in the glycine a1 receptors converted picrotoxin from a non-use facilitated blocker to a use-facilitated one. The latter finding suggested that this residue might residue within the PTX binding site or play a key role in the transduction pathway for picrotoxin mechanism. The overall results further support the fact that TMII domain plays a key role in the picrotoxin mechanism.