Block and allosteric modulation of GABAergic currents by oenanthotoxin in rat cultured hippocampal neurons.
OBJECTIVE
Oenanthotoxin (OETX), a polyacetylenic alcohol from plants of the genus Oenanthe, has recently been identified as potent inhibitor of GABA-evoked currents. However, the effects of OETX on the inhibitory postsynaptic currents (IPSCs), as well as the pharmacological mechanism(s) underlying its effects on GABA(A) receptors, remain unknown. The purpose of this study was to elucidate the mechanism underlying the inhibition of GABAergic currents by OETX.


METHODS
Effects of OETX on GABAergic currents were studied using the patch clamp technique on rat cultured hippocampal neurons. Miniature IPSCs (mIPSCs) were recorded in the whole-cell configuration, while the current responses were elicited by ultrafast GABA applications onto the excised patches.


RESULTS
OETX potently inhibited both mIPSCs and current responses, but its effect was much stronger on synaptic currents. Analysis of the effects of OETX on mIPSCs and evoked currents disclosed a complex mechanism: allosteric modulation of both GABA(A) receptor binding and gating properties and a non-competitive, probably open channel block mechanism. In particular, OETX reduced the binding rate and nearly abolished receptor desensitization. A combination of rapid clearance of synaptic GABA and OETX-induced slowing of binding kinetics is proposed to underlie the potent action of OETX on mIPSCs.


CONCLUSIONS
OETX shows a complex blocking mechanism of GABA(A) receptors, and the impact of this toxin is more potent on mIPSCs than on currents evoked by exogenous GABA. Such effects on GABAergic currents are compatible with the convulsions and epileptic-like activity reported for OETX.
Wyrembek
Paulina
P
Department of Biophysics, Wrocław Medical University, Chałubińskiego, Wrocław, Poland.
Lebida
Katarzyna
K
Mercik
Katarzyna
K
Szczuraszek
Katarzyna
K
Szczot
Marcin
M
Pollastro
Federica
F
Appendino
Giovanni
G
Mozrzymas
Jerzy W
JW
D000494:Allosteric Regulation / Q000187:drug effects; D000818:Animals; D002478:Cells, Cultured; D053281:Enediynes / Q000494:pharmacology*; D005233:Fatty Alcohols / Q000494:pharmacology*; D018756:GABA Antagonists / Q000494:pharmacology*; D058787:GABA-A Receptor Antagonists; D006624:Hippocampus / Q000187:drug effects / Q000378:metabolism; D053444:Inhibitory Postsynaptic Potentials / Q000187:drug effects*; D055369:Miniature Postsynaptic Potentials / Q000187:drug effects; D009474:Neurons / Q000187:drug effects / Q000378:metabolism; D031050:Oenanthe / Q000737:chemistry; D018408:Patch-Clamp Techniques; D011485:Protein Binding; D051381:Rats; D017208:Rats, Wistar; D011963:Receptors, GABA-A / Q000378:metabolism
D053281:Enediynes; D005233:Fatty Alcohols; D018756:GABA Antagonists; D058787:GABA-A Receptor Antagonists; D011963:Receptors, GABA-A; C010719:enanthotoxin