Block of Ca channels in rat central neurons by the spider toxin omega- Aga-IIIA

Abstract

The effects of the spider toxin w-Aga-WA were studied on Ca channel currents in rat central neurons. In hippocampal CA1 pyramidal neurons, o-Aga-IIIA blocked-70% of the high-threshold Ca currents and had no effect on low-threshold T-type current. Occlusion experiments with blockers of L-, N-, and P-type Ca currents showed that w-Aga-IIIA abolished dihydropyridine-sensitive L-type current and blocked a substantial fraction of the w-conotoxin (CgTX)-sensitive N-type and w-Aga-IVA-sensitive P-type Ca currents. The highthreshold current remaining with saturating concentrations of nimodipine; CgTX, and w-Aga-IVA was also partially blocked by w-Aga-MA in a variety of central neurons. Block of P-type current by o-Aga-IIIA was investigated in more detail in cerebellar Purkinje neurons. Block was potent (&-0.5 nM), but incomplete and voltage dependent. Tail current activation curves showed that channel gating is shifted in the depolarizing direction by-7 mV. The instantaneous current-voltage curve for P-type current was also altered; the toxin reduced Ba-carried inward currents by-40% and had little effect on Cs-carried outward currents. The partial, voltage-dependent reduction of P-type Ca current can be accounted for by a combination of toxin effects on channel permeation and gating. [Key words: Ca channels, CA 7 pyramidal neurons, Purkinje neurons, ion channel block, spider toxin, w-Aga-I/IA] Neurons possess multiple types of Ca channels (

Modulation of CaV2.1 Channels by the Neuronal Calcium-Binding Protein Visinin-Like Protein-2

“…Ca V 2.1, ␤ 2a , and ␤ 1b are also expressed in these regions (Stea et al, 1994;Tanaka et al, 1995;Westenbroek et al, 1995;Sakurai et al, 1996). The Ca V 2.1/␤ 2a currents reported here are similar to slowly inactivating P/Q-type Ca 2ϩ currents recorded in hippocampal CA1 neurons (Hillyard et al, 1992;Mintz, 1994), cerebellar granule neurons (Randall and Tsien, 1995), and Purkinje neurons (Mintz et al, 1992a,b). Thus, VILIP-2 is present in neurons in which Ca V 2.1/␤ 2a is expressed and is therefore able to modulate these channels in vivo.…”

Anti-Ca 2+ channel antibody attenuates Ca 2+ currents and mimics cerebellar ataxia in vivo

“…Like D-III antibody, -Aga-IIIA caused only partial block of N-type channels even at maximal effective doses, but it rendered the remaining N-type current impervious to further blockade by GVIA (21,35), as if it had occupied part of the GVIA-binding site. In a further parallel, both D-III antibody and -Aga-IIIA caused only partial inhibition of P/Q-type channels at saturating concentrations but failed to prevent further reduction of current by -Aga-IVA, a gating modifier (21,35). Nothing is published about the structural basis of -Aga-IIIA binding, but one can be reasonably confident that D-III antibody interacts with the S5-S6 region of Domain-III, in close proximity to structural determinants of GVIA binding (37,38).…”

“…Given the presence of antibodies against both kinds of VGCCs in the cerebrospinal fluid of PCA patients (16,17), it seems likely that reductions in both N-and P/Q-type Ca 2ϩ entry may contribute to the observed motor behavior. Interestingly, the actions of the D-III antibody on N-and P/Q-type channels were reminiscent of the effects of the spider toxin -Aga-IIIA (35,36). Like D-III antibody, -Aga-IIIA caused only partial block of N-type channels even at maximal effective doses, but it rendered the remaining N-type current impervious to further blockade by GVIA (21,35), as if it had occupied part of the GVIA-binding site.…”

“…Interestingly, the actions of the D-III antibody on N-and P/Q-type channels were reminiscent of the effects of the spider toxin -Aga-IIIA (35,36). Like D-III antibody, -Aga-IIIA caused only partial block of N-type channels even at maximal effective doses, but it rendered the remaining N-type current impervious to further blockade by GVIA (21,35), as if it had occupied part of the GVIA-binding site. In a further parallel, both D-III antibody and -Aga-IIIA caused only partial inhibition of P/Q-type channels at saturating concentrations but failed to prevent further reduction of current by -Aga-IVA, a gating modifier (21,35).…”

The Spider Toxin ω-Aga IIIA Defines a High Affinity Site on Neuronal High Voltage-activated Calcium Channels

“…Two buffers were used, binding buffer or "recording buffer," the latter designed to mimic ionic conditions used for patch clamp recordings (29). Recording buffer contained 5 mM BaCl 2 , 160 mM tetraethylammonium chloride, and 10 mM HEPES, adjusted to pH 7.4 with tetraethylammonium-OH.…”