Inhibition of large-conductance Ca(2+)-activated K(+) channels in hippocampal neurons by toosendanin.
The effect of toosendanin, a selective presynaptic blocker and effective antibotulismic agent, on large-conductance Ca(2+)-activated K(+) channels was studied in inside-out patches of pyramidal neurons freshly isolated from the hippocampal CA1 region of the rat. Toosendanin (1 x 10(-6)g/ml approximately 1 x 10(-4)g/ml) was found to inhibit large-conductance Ca(2+)-activated K(+) channels by reducing its open probability significantly in a concentration-dependent manner, although the effective concentration of toosendanin was lower in a symmetrical K(+) (150 mM) solution than under asymmetrical conditions (changing K(+) concentration in pipette solution to 5mM). The action was partially reversible by washing. By decreasing the slow open time constant, toosendanin shortened the open dwell time of large-conductance Ca(2+)-activated K(+) channels in a dose-dependent manner. A dose-dependent reduction of unitary current amplitude of the channel was detected after toosendanin perfusion. On elevating the intracellular free calcium concentration from 1 to 10 microM, a similar effect on large-conductance Ca(2+)-activated K(+) channels by toosendanin was also observed, but its efficacy was diminished. These results show that toosendanin inhibits large-conductance Ca(2+)-activated K(+) channels in hippocampal neurons by reducing the open probability and unitary current amplitude of the channel, and that Ca(2+) interferes with the effect. These data provide an explanation for toosendanin-induced facilitation of neurotransmitter release and the antibotulismic effect of the drug.
Wang
Z F
ZF
Key Laboratory of Neurobiology, Institute of Physiology, Shanghai Institutes for Biological Sciences, Chinese Academy of Sciences, 320 Yue-Yang Road, 200031, Shanghai, People's Republic of China.
Shi
Y L
YL
D000818:Animals; D002118:Calcium / Q000378:metabolism* / Q000494:pharmacology; D020013:Calcium Signaling / Q000187:drug effects* / Q000502:physiology; D002478:Cells, Cultured; D004305:Dose-Response Relationship, Drug; D004365:Drugs, Chinese Herbal / Q000494:pharmacology*; D006624:Hippocampus / Q000166:cytology / Q000187:drug effects* / Q000378:metabolism; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D009466:Neuromuscular Blocking Agents / Q000494:pharmacology*; D018408:Patch-Clamp Techniques; D015221:Potassium Channels / Q000187:drug effects* / Q000378:metabolism; D017966:Pyramidal Cells / Q000187:drug effects* / Q000378:metabolism; D051381:Rats; D017207:Rats, Sprague-Dawley; D013997:Time Factors; D014315:Triterpenes
D002118:Calcium; D004365:Drugs, Chinese Herbal; D009466:Neuromuscular Blocking Agents; D015221:Potassium Channels; C036454:toosendanin; D014315:Triterpenes