ω-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 ω-agatoxin-TK, a toxin of the venom of Agelenopsis aperta, which is 10 times more concentrated than the P/Q type Ca2+ channel blocker, ω-agatoxin-IVA in the venom, inhibits the high K+ depolarisation-induced rise in internal Ca2+ (Cai, as determined with fura-2) dose dependently in cerebral (striatal and hippocampal) isolated nerve endings, with calculated IC50's of about 60 nM. The maximal inhibition exerted by ω-agatoxin-TK in striatal synaptosomes (61 ± 11%) is 10% larger than in hippocampal synaptosomes, suggesting a larger population of ω-agatoxin-TK-sensitive Ca2+ channels in striatal than in hippocampal nerve endings. The N-type Ca2+ channel blocker, ω-conotoxin-GVIA (1 μM), inhibits part of the ω-agatoxin-TK-insensitive rise in Cai induced by high K+. In contrast to the inhibition exerted by ω-agatoxin-TK on the Cai response to high K+, ω-agatoxin-TK failed to inhibit the tetrodotoxin-sensitive elevations in Cai and in internal Na+ (Nai, as determined with SBFI) induced by veratridine, indicating that the Ca2+ influx activated by veratridine does not involve ω-agatoxin-TK-sensitive channels. High K+ does not increase Nai. In [3H]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 [3H]Glu induced by high K+ is absolutely dependent on the entrance of external Ca2+. This exocytotic release of [3H]Glu attained in the absence of a chemical Na+ gradient is inhibited with the same potency and efficacy by ω-agatoxin-TK and by ω-agatoxin-IVA, which is known to differ from ω-agatoxin-TK in its amino terminal moiety. These results indicate that ω-agatoxin-TK represents a good pharmacological tool to study P/Q type Ca2+ channel-mediated responses in cerebral nerve endings.