Membrane potential was recorded intracellularly near presynaptic
terminals of the excitor axon of the crayfish opener neuromuscular junction (NMJ),
while transmitter release was recorded postsynaptically. This study focused on the
effects of a presynaptic calcium-activated potassium conductance, gK(Ca), on the
transmitter release evoked by single and paired depolarizing current pulses.
Blocking gK(Ca) by adding tetraethylammonium ion (TEA; 5-20 mM) to a solution
containing tetrodotoxin and aminopyridines caused the relation between presynaptic
potential and transmitter release to steepen and shift to less depolarized
potentials. When two depolarizing current pulses were applied at 20-ms intervals
with gK(Ca) not blocked, the presynaptic voltage change to the second (test) pulse was
inversely related to the amplitude of the first (conditioning) pulse. This effect of the
conditioning prepulse on the response to the test pulse was eliminated by 20 mM
TEA and by solutions containing 0 mM Ca2+/1 mM EGTA, suggesting that the
reduction in the amplitude of the test pulse was due to activation of gK(Ca) by calcium
remaining from the conditioning pulse. In the absence of TEA, facilitation of
transmitter release evoked by a test pulse increased as the conditioning pulse grew
from -40 to -20 mV, but then decreased with further increase in the conditioning
depolarization. A similar nonmonotonic relationship between facilitation and the
amplitude of the conditioning depolarization was reported in previous studies using
extracellular recording, and interpreted as supporting an additional voltagedependent
step in the activation of transmitter release. We suggest that this result
was due instead to activation of a gK(Ca) by the conditioning depolarization, since
facilitation of transmitter release increased monotonically with the amplitude of the
conditioning depolarization, and the early time course of the decay of facilitation
was prolonged when gK(Ca) was blocked. The different time courses for decay of the
presynaptic potential (20 ms) and facilitation (> 50 ms) suggest either that residual free calcium does not account for facilitation at the crayfish NMJ or that the
transmitter release mechanism has a markedly higher affinity or stoichiometry for
internal free calcium than does g K(Ca). Finally, our data suggest that the calcium
channels responsible for transmitter release at the crayfish NMJ are not of the L, N,
or T type.This work was partially supported by NIAAA grant AA0776 to G. D. Bittner.Neuroscienc
