Omega-agatoxin-TK is a useful tool to study P-type Ca2+ channel-mediated changes in internal Ca2+ and glutamate release in depolarised brain nerve terminals.
The present study shows that omega-agatoxin-TK, a toxin of the venom of Agelenopsis aperta, which is 10 times more concentrated than the P/Q type Ca(2+) channel blocker, omega-agatoxin-IVA in the venom, inhibits the high K(+) depolarisation-induced rise in internal Ca(2+) (Ca(i), as determined with fura-2) dose dependently in cerebral (striatal and hippocampal) isolated nerve endings, with calculated IC(50)'s of about 60nM. The maximal inhibition exerted by omega-agatoxin-TK in striatal synaptosomes (61 +/- 11%) is 10% larger than in hippocampal synaptosomes, suggesting a larger population of omega-agatoxin-TK-sensitive Ca(2+) channels in striatal than in hippocampal nerve endings. The N-type Ca(2+) channel blocker, omega-conotoxin-GVIA (1muM), inhibits part of the omega-agatoxin-TK-insensitive rise in Ca(i) induced by high K(+). In contrast to the inhibition exerted by omega-agatoxin-TK on the Ca(i) response to high K(+), omega-agatoxin-TK failed to inhibit the tetrodotoxin-sensitive elevations in Ca(i) and in internal Na(+) (Na(i), as determined with SBFI) induced by veratridine, indicating that the Ca(2+) influx activated by veratridine does not involve omega-agatoxin-TK-sensitive channels. High K(+) does not increase Na(i). In [(3)H]Glu preloaded hippocampal synaptosomes super-fused with low Na(+) Krebs Ringer HEPES (a condition that guarantees the elimination of neurotransmitter transporters-mediated release), the release of [(3)H]Glu induced by high K(+) is absolutely dependent on the entrance of external Ca(2+). This exocytotic release of [(3)H]Glu attained in the absence of a chemical Na(+) gradient is inhibited with the same potency and efficacy by omega-agatoxin-TK and by omega-agatoxin-IVA, which is known to differ from omega-agatoxin-TK in its amino terminal moiety. These results indicate that omega-agatoxin-TK represents a good pharmacological tool to study P/Q type Ca(2+) channel-mediated responses in cerebral nerve endings.
Sitges
María
M
Departmento de Biología Celular y Fisiología, Instituto de Investigaciones Biomédicas, Apartado Postal 70228,Ciudad Universitaria 04510, México, DF. sitgas@biomedicas.unam.mx
Galindo
Carlos Alberto
CA
D060848:Agatoxins; D000818:Animals; D001921:Brain / Q000166:cytology* / Q000187:drug effects; D001923:Brain Chemistry / Q000187:drug effects; D002021:Buffers; D002118:Calcium / Q000378:metabolism*; D002121:Calcium Channel Blockers / Q000494:pharmacology*; D020865:Calcium Channels, P-Type / Q000187:drug effects* / Q000378:metabolism*; D018698:Glutamic Acid / Q000378:metabolism*; D006624:Hippocampus / Q000187:drug effects / Q000378:metabolism; D008297:Male; D017072:Neostriatum / Q000187:drug effects / Q000378:metabolism; D009411:Nerve Endings / Q000187:drug effects / Q000378:metabolism*; D011188:Potassium / Q000494:pharmacology; D051381:Rats; D017208:Rats, Wistar; D015222:Sodium Channels / Q000187:drug effects / Q000378:metabolism; D013111:Spider Venoms / Q000494:pharmacology*; D013574:Synaptosomes / Q000187:drug effects / Q000378:metabolism; D020866:omega-Conotoxin GVIA / Q000494:pharmacology
D060848:Agatoxins; D002021:Buffers; D002121:Calcium Channel Blockers; D020865:Calcium Channels, P-Type; D015222:Sodium Channels; D013111:Spider Venoms; C083855:omega-agatoxin-Aa4b; D018698:Glutamic Acid; D020866:omega-Conotoxin GVIA; D011188:Potassium; D002118:Calcium