Graded or all-or-nothing release of transmitter quanta by local depolarizations of nerve terminals on crayfish muscle?

In opener muscles of the first walking leg of 3 species of crayfish, quantal synaptic currents were recorded focally at synaptic spots by means of a macro-patch-clamp electrode. Proximal stimulation of the motor axons elicited excitatory postsynaptic currents (nEPSCs). In addition, current pulses through the recording electrode depolarizing the nerve terminal elicited similar synaptic release (pEPSCs). Artefact waveforms generated in the recording electrode after a pulse were compensated by a special circuit, allowing the pEPSC to be recorded from 0.3 to 1.5 ms after the pulse. In all terminals identified by recording nEPSCs, pEPSCs were also elicited, with a threshold pulse amplitude between -0.1 and -2 microA at 2 ms pulse duration. Most of the investigated terminals showed graded pEPSCs to rising amplitudes and durations of depolarizing pulses, and no effect of tetrodotoxin (TTX) on the pEPSCs. In these inexcitable terminals pEPSCs and nEPSCs showed mutual facilitation, with no signs of refractoriness for intervals as short at 3 ms. Some excitable terminals were found also: in these the amplitude of the pEPSC rose very steeply in an approximately all-or-nothing response on passing a threshold, while application of TTX reduced this response to one similar to that of inexcitable terminals. However, stimulation of such excitable terminals did not lead to antidromic conduction of action potentials into the main axon. In both inexcitable and excitable terminals, approximately the product of suprathreshold pulse amplitude and pulse duration determined the rate of release. The dependence of this response on pulse amplitude showed characteristic differences in proximal and distal synapses. The maximal double-logarithmic slope of this relation (sD) was 3.3 on the average in proximal synapses, while for distal synapses the average sD was 6.3. Further, in proximal synapses the nEPSC reached on average 86% of the maximum pEPSC, while the nEPSC in distal synapses amounted to only 5% of the maximum pEPSC. Therefore, the point of block of conduction in the terminal branch seems to lie father from the terminal in distal than in proximal synapses.