CONOTOXIN-SENSITIVE AND CONOTOXIN-RESISTANT CA2-CELLS( CURRENTS IN FISH RETINAL GANGLION)

Using whole-cell patch-clamp methods, rye tested whether omega-toxins from Conus block voltage-gated Ca2+ currents in teleost central neuron s. The fractions omega-CTx-GVIA and omega-CTx-MVIIC, together with ome ga-toxins from Agelenopsis, the dihydropyridine BAY-K-8644, and voltag e steps, produced effects indicating three types of Ca2+ current in di ssociated goldfish retinal ganglion cells, One was activated by depola rization 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 tota l 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 w as augmented by BAY-K-8644 at test potentials as negative as -45 mV, e ven in the presence of omega-CTx-GVIA. Replacement of extracellular Ca 2+ by Ba2+ augmented current amplitude and slowed current decay, Condi tioning depolarizations reduced Ca2+ current amplitude less than did o mega-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 mos t of the high-threshold Ca2+ current in these cells is blocked by omeg a-CTx-GVIA, it is also Ni2+-sensitive, and relatively resistant to ome ga-Aga-IIIA, The voltage sensitivities of low- and high-threshold Ca2 current may suit current recruitment in situ after light-evoked hyper polarizations end, and after light-evoked depolarizations begin, respe ctively. (C) 1996 John Wiley & Sons, Inc.