Neurotransmitter release and its facilitation in crayfish
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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 facilitationTF 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.TF 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,R1 andR2. Mg0 inhibits only theR1 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.
Key wordsSynaptic facilitation Removal of calcium from terminals Magnesium
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