Neurotransmitter release and its facilitation in crayfish

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Abstract
Excitatory synaptic currents (EPSCs) were recorded extracellulary from synaptic spots on crayfish opener muscles. The decay of facilitation after a first pulse was measured by a second pulse given at increasing intervals; the duration of facilitationT F was the interval after which the second EPSC had 1.1 times the amplitude of the first one. Increasing [Mg]0 in the range from 0.5–12.5 mM at low [Ca]0 (1.7–4.5 mM) led to a monotonic prolongation of facilitation.T F showed a S-shaped dependence on [Mg]0, rising very steeply at 2–5 mM [Mg]0. At higher [Ca]0, and also at half normal [Na]0, an increase of [Mg]0 did not affect the decay of facilitation appreciably. As shown before, the decay of facilitation is due to two Cai-removal processes,R 1 andR 2. Mg0 inhibits only theR 1 process, which is also inhibited by high [Ca]0, is dependent on normal [Na]0 and has the characteristics of a Cai√Na0 exchange. As one possible mechanism, competition of Mg0 with Na0 at the extracellular loading site of the exchange is discussed quantitatively.

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