Conotoxin-sensitive and conotoxin-resistant Ca2+ currents in fish retinal ganglion cells

Using whole-cell patch-clamp methods, we tested whether omega-toxins from Conus block voltage-gated Ca2+ currents in teleost central neurons. The fractions omega-CTx-GVIA and omega-CTx-MVIIC, together with omega-toxins from Agelenopsis, the dihydropyridine BAY-K-8644, and voltage steps, produced effects indicating three types of Ca2+ current in dissociated goldfish retinal ganglion cells. One was activated by depolarization of most cells beyond -65 mV, primed at -95 mV but not at -45 mV, reduced by Ni2+, and unchanged by conotoxins, agatoxins, or BAY-K-8644. The second type constituted more than three-quarters of the total Ca2+ current in all cells, and at test potentials more positive than -30 mV, was reduced consistently by omega-CTx-GVIA, omega-CTx-MVIIC, and omega-Aga-IA, but not omega-Aga-IVA. The third Ca2+ current type was augmented by BAY-K-8644 at test potentials as negative as -45 mV, even in the presence of omega-CTx-GVIA. Replacement of extracellular Ca2+ by Ba2+ augmented current amplitude and slowed current decay. Conditioning depolarizations reduced Ca2+ current amplitude less than did omega-CTx-GVIA, and slowed current decay to imperceptible rates. These results provide the first description of conotoxin-sensitive, voltage-gated Ca2+ current recorded from teleost central neurons. Although most of the high-threshold Ca2+ current in these cells is blocked by omega-CTx-GVIA, it is also Ni(2+)-sensitive, and relatively resistant to omega-Aga-IIIA. The voltage sensitivities of low-and high-threshold Ca2+ current may suit current recruitment in situ after light-evoked hyperpolarizations end, and after light-evoked depolarizations begin, respectively.