Neurotransmitter release and its facilitation in crayfish. III. Amplitude of facilitation and inhibition of entry of calcium into the terminal by magnesium.

Excitatory postsynaptic currents (EPSCs) were recorded extracellularly from synaptic spots on crayfish opener muscles. Release of transmitter was determined by counting the average number of quanta which appear after a stimulus. When [Mg]0 was increased from 2.5 to 12.5 mM, release was inhibited. Quantitatively the effect of [Mg]0 could be described by a competitive inhibition of the entry (not of the release) of Ca2+ after an impulse, with apparent dissociation constants KMg between 1.4 and 18 mM [Mg]0, assuming saturation kinetics for entry of Ca2+ and release. At constant [Ca]0, twin pulse facilitation (Fs) for short intervals (about 10 ms) increased when [Mg]0 was raised from low values, reached a maximum at a certain [Mg]0(F) and unexpectedly decreased again at higher [Mg]0. At higher [Ca]0, [Mg]0(F) shifted to higher values. This maximum of facilitation is predicted qualitatively by our theoretical model. However, the amplitude of facilitation was larger than predicted theoretically, and the [Mg]0(F) were smaller than predicted. The theoretical possibilities to correct these discrepancies within the framework of 'residual calcium' based facilitation and saturation kinetics of entry and release were analyzed, but all were in conflict with experimental findings. It is concluded that an essential element is missing in the present theory of facilitation.