2 COMPONENTS OF CALCIUM CURRENTS IN THE SOMA OF PHOTORECEPTORS OF HERMISSENDA

1. The proposed mechanism of cellular plasticity underlying classical conditioning of Hermissenda involves Ca2+ influx through voltage-activ ated channels. This influx triggers several molecular cascades and lea ds to the phosphorylation of K+ channels in identified photoreceptors. We studied Ca2+ currents from isolated photoreceptors of Heimissenda with the whole cell patch-clamp technique. Two distinct Ca2+ currents were identified in isolated photoreceptors on the basis of differences in their voltage dependence, kinetics, and pharmacology. 2. One Ca2current was transient (I-Ca(t)), with a fast chant (similar to-5 ms), activated at -50 mV from a holding potential of -90 mV, and peaked at O mV. The second Ca2+ current, designated as sustained (I-Ca(s)), exhi bited a delayed time-to-peak, activated at -30 mV, and reached maximum at 30 mV. 3. Steady-state activation curves for both currents were ge nerated from normalized currents and fitted with the Boltzmann functio n; estimates of half-activation voltages for I-Ca(t) were -38.8 +/-6.7 mV (mean +/- SD; n = 9) and 3.2 +/- 8.2 mV for I-Ca(s) (n = 11)with m aximum slopes of 8.9 +/- 1.6 mV (n = 9) and 11.0 +/- 2.4 mV (n=11). 4. The inactivation of ICa555 was slow (time constants >3 s) whereas I-C a(t), inactivated rapidly(time constant of inactivation at various vol tages; 75-600 ms). 5. Ni2+ (0.8 mM), Gd3+ (0.5 mM), and amiloride (10 mu M) produced a reversible block of I-Ca(t) without affecting I-Ca(s) ,pi-conotoxin GVIA (10 nM) irreversibly blocked I-Ca(s) whereas nitren dipine (20 mu M) produced a reversible block. 6. I-Ca(t) may be respon sible for steady-state membrane potential oscillations. I-Ca(s) may co ntribute to the maintenance of the amplitude of the plateau phase of t he generator potential.