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 neuromuscular junction (nmj) under a variety of experimental conditions. Our experiments were designed both to test the hypothesis that elevated [Ca2+]i is necessary and sufficient for the induction of a form of synaptic enhancement that persists for several seconds after tetanic stimulation--augmentation--and to determine the quantitative relationship between 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 approximately 7 sec at 20 degrees C with < 200 microM fura-2 in terminals), [Ca2+]i was elevated 700 nM or less above rest and an essentially 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 component of synaptic enhancement corresponding to augmentation. The slope of the relationship relating delta [Ca2+]i to augmentation was not changed. The time course of augmentation and recovery of [Ca2+]i remained correlated as the temperature of the preparation was changed from about 10 degrees C to 20 degrees C, but the quantitative relationship of enhancement to [Ca2+]i was increased more than two- to threefold. During moderate frequency trains of action potentials, a slowly developing component of the total synaptic enhancement was approximately linearly related to residual [Ca2+]i measured with fura-2. The quantitative relationship between [Ca2+]i and this component of synaptic enhancement during trains was the same as that during synaptic augmentation after trains.(ABSTRACT TRUNCATED AT 250 WORDS)
Delaney
K R
KR
Biological Computation Research Department, AT&T Bell Laboratories, Murray Hill, New Jersey 07974.
Tank
D W
DW
D000200:Action Potentials / Q000187:drug effects / Q000502:physiology; D000818:Animals; D003400:Astacoidea; D002118:Calcium / Q000032:analysis / Q000378:metabolism*; D005470:Fluorometry; D016257:Fura-2; D007089:Image Enhancement / Q000379:methods; D008856:Microscopy, Fluorescence / Q000379:methods; D008985:Monensin / Q000494:pharmacology; D018482:Muscle, Skeletal / Q000502:physiology; D009469:Neuromuscular Junction / Q000502:physiology; D010042:Ouabain / Q000494:pharmacology; D017729:Presynaptic Terminals / Q000737:chemistry / Q000187:drug effects / Q000378:metabolism*; D012016:Reference Values; D012044:Regression Analysis; D000254:Sodium-Potassium-Exchanging ATPase / Q000187:drug effects; D013696:Temperature
D010042:Ouabain; D008985:Monensin; D000254:Sodium-Potassium-Exchanging ATPase; D002118:Calcium; D016257:Fura-2