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Toxityping rat brain calcium channels with omega-toxins from spider and cone snail venoms
M E Adams1, R A Myers, J S Imperial
1Department of Entomology, University of California, Riverside 92521.
Abstract:
Different types of voltage-sensitive Ca2+ channels in the brain can be defined by specific ligands: L-type Ca2+ channels are uniquely sensitive to dihydropyridines, and N-type Ca2+ channels are selectively blocked by the Conus peptide omega-CTX-GVIA. Cloning data have revealed additional calcium channel types in mammalian brain for which selective ligands would be desirable. We describe binding experiments involving three newer ligands that block dihydropyridine- and omega-CTX-GVIA-resistant Ca channels: omega-Aga-IIIA and omega-Aga-IVA from venom of the spider Agelenopsis aperta and omega-CTX-MVIIC from Conus magus. [125I]omega-Aga-IVA binds with high affinity (IC50 = approximately 50 nM) to receptors in rat brain which may correspond to P-like calcium channels. A second high-affinity site (IC50 = approximately 1 nM) is defined by [125I]omega-CTX-MVIIC, proposed here to be on an "O"-type calcium channel. [125I]omega-Aga-IIIA targets homologous binding sites present on multiple Ca channel types. The IIIA sites overlap with the binding sites for [125I]omega-CTX-GVIA and [125I]omega-CTX-MVIIC. The IIIA sites do not overlap with the site defined by omega-Aga-IVA. Thus toxin ligands may be highly specific for a particular Ca channel (i.e., GVIA for the N-type channel) or exhibit broader specificity (i.e., omega-Aga-IIIA, which appears to bind L-, N-, P-, and O-type Ca2+ channels).
Insights
New spider and cone snail toxins selectively target brain calcium channels. These ligands, including omega-Aga-IVA and omega-CTX-MVIIC, help identify novel P-like and O-type calcium channels.
Area of Science:
- Neuroscience
- Pharmacology
- Biochemistry
Background:
- Voltage-sensitive calcium channels (CaV) are crucial for neuronal function.
- Existing ligands like dihydropyridines and omega-CTX-GVIA identify L-type and N-type CaV channels, respectively.
- New CaV subtypes in the mammalian brain necessitate novel selective ligands for characterization.
Purpose of the Study:
- To identify and characterize novel ligands for dihydropyridine- and omega-CTX-GVIA-resistant CaV channels.
- To explore the binding specificities of new toxins from spider and cone snail venoms.
- To potentially define new CaV channel subtypes in the mammalian brain.
Main Methods:
- Radioligand binding assays using [125I]-labeled toxins: omega-Aga-IVA, omega-Aga-IIIA, and omega-CTX-MVIIC.
- Experiments conducted on rat brain tissue.
- Analysis of binding affinities (IC50 values) and cross-reactivity between ligands and CaV channel subtypes.
Main Results:
- [125I]omega-Aga-IVA exhibited high-affinity binding to putative P-like CaV channels (IC50 ≈ 50 nM).
- [125I]omega-CTX-MVIIC identified a high-affinity binding site on proposed O-type CaV channels (IC50 ≈ 1 nM).
- [125I]omega-Aga-IIIA demonstrated broad specificity, binding to multiple CaV channel types, including L-, N-, P-, and O-types.
Conclusions:
- Novel spider and cone snail toxins serve as valuable pharmacological tools for CaV channel research.
- omega-Aga-IVA and omega-CTX-MVIIC represent selective ligands for P-like and O-type CaV channels, respectively.
- Toxin ligands display varying specificities, from highly selective (omega-CTX-GVIA) to broadly reactive (omega-Aga-IIIA), aiding in CaV channel classification.