A large second-order nonlinearity [chi((2)) 1 pm/V 0.2 chi((2)) (22) for LiNbO(3)] is induced in the near-surface ( 4 microm) region of commercial fused-silica optical flats by a temperature (250-325 degrees C) and electric-field (E ~ 5 x 10(4) V/cm) poling process. Once formed, the nonlinearity, which is roughly 10(3)-10(4) times larger than that found in fiber second-harmonic experiments, is extremely stable at room temperature and laboratory ambient. The nonlinearity can be cycled by repeated depoling (temperature only) and repoling (temperature and electric field) processes without history effects. Possible mechanisms, including nonlinear moieties and electric-field-induced second-order nonlinearities, are discussed.

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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).