Onset and offset of calcium current in response
to short, depolarizing voltage pulses were studied in giant
synaptic terminals of goldfish bipolar neurons. Tail-current
decays were extremely rapid (80 μs at −70 mV),
and exponentially slowed at more positive repolarization
potentials. The amplitude of tail current following voltage
pulses varied with the pulse amplitude, according to a
Bolzmann distribution with a V1/2
of −27.6 mV and a slope factor of 13.8 mV. When interpreted
with a Hodgkin-Huxley model, calcium-current onset was best
described by m2 kinetics and a time constant
(τm) of 0.62 ms at −10 mV. The
kinetics of calcium-current onset and offset are at least
two- to threefold faster than those described in other native
cells. Combined with other recent data, the results suggest
that the rapid kinetics may participate in a fast signaling
mode in the goldfish ON-center pathway.