A QUANTITATIVE MEASUREMENT OF THE DEPENDENCE OF SHORT-TERM SYNAPTIC ENHANCEMENT ON PRESYNAPTIC RESIDUAL CALCIUM

We simultaneously measured presynaptic free calcium ion concentration ([Ca2+](i)) and synaptic strength at the crayfish claw opener neuromus cular junction (nmj) under a variety of experimental conditions. Our e xperiments were designed both to test the hypothesis that elevated [Ca 2+](i), is necessary and sufficient for the induction of a form of syn aptic enhancement that persists for several seconds after tetanic stim ulation-augmentation-and to determine the quantitative relationship be tween elevated [Ca2+](i) and this enhancement. Action potential trains increased [Ca2+](i) and enhanced transmission. During the decay phase of synaptic enhancement known as augmentation (time constant of decay approximate to 7 sec at 20 degrees C with < 200 mu M fura-2 in termin als), [Ca2+](i) was elevated 700 nM or less above rest and an essentia lly linear relationship between [Ca2+](i) and enhancement was observed . Introduction of exogenous Ca2+ buffers into the presynaptic terminal slowed the buildup and recovery kinetics of both [Ca2+](i) and the co mponent of synaptic enhancement corresponding to augmentation. The slo pe of the relationship relating Delta[Ca2+](i) to augmentation was not changed. The time course of augmentation and recovery of [Ca2+](i) re mained correlated as the temperature of the preparation was changed fr om about 10 degrees C to 20 degrees C, but the quantitative relationsh ip of enhancement to [Ca2+](i) was increased more than two- to threefo ld. During moderate frequency trains of action potentials, a slowly de veloping component of the total synaptic enhancement was approximately linearly related to residual [Ca2+](i) measured with fura-2. The quan titative relationship between [Ca2+](i) and this component of synaptic enhancement during trains was the same as that during synaptic augmen tation after trains. Bath application of the calcium ionophore A23187 increased [Ca2+](i) and resulted in synaptic enhancement. The quantita tive relationship between increased [Ca2+](i) and synaptic enhancement was similar to that seen after augmentation-inducing action potential trains. Blocking Na+/K+ pump with ouabain also increased [Ca2+](i), p ossibly through reversal or slowing of Na+/Ca2+ exchange. With increas ed [Ca2+](i) of 300 nM or less above basal, the relationship between e nhancement and [Ca2+](i) produced by ouabain application was comparabl e to that seen during synaptic augmentation after a train. Our results in conjunction with previous work (Delaney et al., 1989) support the hypothesis that elevated [Ca2+](i) of about 1 mu M or less is a necess ary and sufficient component for the production of augmentation and a longer-lasting form of activity-dependent synaptic enhancement, postte tanic potentiation (ptp). We conclude that the differing time constant s of augmentation and ptp largely reflect multicomponent recovery kine tics of [Ca2+](i) that occur following the stimulus conditions that pr oduce these two forms of synaptic enhancement. Our experiments also su ggest that shorter-lasting facilitation of release, termed F1 and F2 f acilitation, which decays within a second of the offset of stimulation , is not significantly activated by the increases in [Ca2+](i) that ar e sufficient to produce augmentation and ptp. Evidence is presented th at at higher [Ca2+](i) other facilitatory processes (possibly correspo nding to F1 or F2 facilitation) contribute to the enhancement of both spontaneous and action potential-evoked release.