Excitatory neuromuscular transmission in crayfish: Calcium dependence is unaffected by picrotoxin

Abstract

Picrotoxin, 1 X 10(-5)M to 1.6 X 10(-3)M, had little or no effect on the amplitude of intracellularly recorded excitatory junctional potentials (EJPs) at extracellular calcium concentrations [Ca2+]0 ranging from 0.5 to 15 mM. The slope of the log EJP vs. log[Ca2+]0 relationship was approximately 1 with or without picrotoxin. The reduction EJP amplitude resulting from the addition of 5 X 10(-5)M GABA was largely reversed by 10(-5)M picrotoxin.

Post-tetanic decay of evoked and spontaneous transmitter release and a residual-calcium model of synaptic facilitation at crayfish neuromuscular junctions.

“…In regard to the second assumption implicit in our model, it has often been reported that the relation between external calcium concentration and transmitter release is linear in crayfish (Bracho and Orkand, 1970;Ortiz and Bracho, 1972;Zucker, 1974;Staggs et al ., 1980), although it has been pointed 36 9 out that saturation of calcium entry could mask a nonlinearity between active calcium and transmitter release (Parnas and Segel, 1981) . Recently, more careful measurements of the relation between transmitter release and external calcium indicate that the relation is nonlinear (Dudel, 1981) and that it is in fact consistent with a fourthor higher-power dependence of transmitter release on intracellular active calcium (Parnas et al ., 1982) .…”

Transmitter release in crayfish neuromuscular junction: Comments on the article by Staggs et al. (1980)

“…This finding was interpreted to contradict the observation of Parnas, Rahamimoff, and Sarne (1975) that picrotoxin, by removing a tonic inhibition, leads to an increase of EPSPs. It is important to note that the latter results were obtained at the neuromuscular junction of a crab (Ocypode), while the findings of Staggs et al (1980) were obtained at crayfish neuromuscular junctions and thus are not seen to "contradict," especially since their conditions were unfavorable for the observation of tonic inhibition. Staggs et al (1980) also emphasized and gave evidence that in crayfish there is a linear dependence of the amplitude of the EPSP on [Ca],, as reported earlier (Bracho and Orkand, 1970;Ortiz and Bracho, 1972;Zucker, 1974).…”

“…It is important to note that the latter results were obtained at the neuromuscular junction of a crab (Ocypode), while the findings of Staggs et al (1980) were obtained at crayfish neuromuscular junctions and thus are not seen to "contradict," especially since their conditions were unfavorable for the observation of tonic inhibition. Staggs et al (1980) also emphasized and gave evidence that in crayfish there is a linear dependence of the amplitude of the EPSP on [Ca],, as reported earlier (Bracho and Orkand, 1970;Ortiz and Bracho, 1972;Zucker, 1974). Since in vertebrate endplates this dependence is very nonlinear (d log EPSP)I(d log [Calo) = 4 (Dodge and Rahamimoff, 1967), and since this nonlinearity is a prerequisite for explaining the observed facilitation by the residual calcium mechanism, this apparent and "suspect" (Atwood, 1977) deviation of EPSP behavior in crayfish should not be left without comment.…”

“…This agrees with the findings of Nickell and Boyarski (1980), an extensive study of synaptic facilitation (Parnas et al, submitted), and on quantum content (Dudel, 1981) at different [Calo in the crayfish neuromuscular junction. Therefore, there is no reason to assume that the [Ca]o dependence of synaptic transmission, and more importantly, the mechanism of facilitation are basically different in crayfish and vertebrates.The consistent failure of Staggs et al (1980) to observe nonlinearity may be explained by insufficient access of the low-[Calo solution to the synapses. We have found that careful removal of obstructing fine tissue layers, rapid super-…”

“…The consistent failure of Staggs et al (1980) to observe nonlinearity may be explained by insufficient access of the low-[Calo solution to the synapses. We have found that careful removal of obstructing fine tissue layers, rapid super-…”