The spatial distribution of calcium signals in squid presynaptic terminals.
1. The fluorescent Ca2+ indicator dye, fura-2, was used to examine the spatial distribution of intracellular Ca2+ signals in giant presynaptic terminals of squid. Brief trains of presynaptic action potentials were evoked to open Ca2+ channels within the giant presynaptic terminals and elevate presynaptic Ca2+ concentration. 2. Electrical stimulation produced pronounced rises in presynaptic Ca2+ concentration. These rises were much larger in the terminal region than in the adjacent axonal region of the presynaptic neuron, suggesting that Ca2+ channels are most abundant in the terminal. 3. Stimulation also produced gradients in Ca2+ concentration across the width of the presynaptic terminal. During stimulation, Ca2+ concentration was highest in the compartment of the presynaptic terminal closest to the postsynaptic neuron. This suggests that the Ca2+ channels are localized to this region of the presynaptic terminal. 4. Following the end of action potential trains, the rises in Ca2+ concentration became uniform across the width of the terminal. The redistribution of Ca2+ presumably is due to diffusion of Ca2+ throughout the presynaptic cytoplasm. Stimulus-evoked rises in Ca2+ declined slowly over several tens of seconds. 5. Histological examination of a giant presynaptic terminal used for imaging experiments revealed that the spatial compartments where stimulus-induced rises in Ca2+ concentration were highest were also enriched in active zones, the presynaptic sites of transmitter secretion. The co-localization of Ca2+ transients and active zones strongly suggests that neurons cluster Ca2+ channels selectively at active zones and that they do so to enhance the magnitude of Ca2+ signals in the vicinity of the active zone. 6. Longitudinal gradients in Ca2+ concentration also occur within presynaptic terminals and can be quantitatively accounted for by gradients in surface/volume ratio and density of active zones along the length of the presynaptic terminal.
Smith
S J
SJ
Department of Molecular Physiology, Beckman Center, Stanford University Medical School, Palo Alto, CA 94305-5425.
Buchanan
J
J
Osses
L R
LR
Charlton
M P
MP
Augustine
G J
GJ
D000200:Action Potentials / Q000502:physiology; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D015220:Calcium Channels / Q000378:metabolism; D002451:Cell Compartmentation; D049832:Decapodiformes / Q000378:metabolism* / Q000648:ultrastructure; D004558:Electric Stimulation; D005071:Evoked Potentials / Q000502:physiology; D016257:Fura-2; D066298:In Vitro Techniques; D007424:Intracellular Fluid / Q000378:metabolism; D017136:Ion Transport / Q000502:physiology; D017729:Presynaptic Terminals / Q000378:metabolism*; D015398:Signal Transduction / Q000502:physiology
D015220:Calcium Channels; D002118:Calcium; D016257:Fura-2