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 presynapt ic terminals of squid. Brief trains of presynaptic action potentials w ere evoked to open Ca2+ channels within the giant presynaptic terminal s and elevate presynaptic Ca2+ concentration. 2. Electrical stimulatio n produced pronounced rises in presynaptic Ca2+ concentration. These r ises were much larger in the terminal region than in the adjacent axon al 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. D uring stimulation, Ca2+ concentration was highest in the compartment o f the presynaptic terminal closest to the postsynaptic neuron. This su ggests that the Ca2+ channels are localized to this region of the pres ynaptic terminal. 4. Following the end of action potential trains, the rises in Ca2+ concentration became uniform across the width of the te rminal. The redistribution of Ca2+ presumably is due to diffusion of C a2+ throughout the presynaptic cytoplasm. Stimulus-evoked rises in Ca2 + declined slowly over several tens of seconds. 5. Histological examin ation of a giant presynaptic terminal used for imaging experiments rev ealed that the spatial compartments where stimulus-induced rises in Ca 2+ 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 C a2+ channels selectively at active zones and that they do so to enhanc e the magnitude of Ca2+ signals in the vicinity of the active zone. 6. Longitudinal gradients in Ca2+ concentration also occur within presyn aptic terminals and can be quantitatively accounted for by gradients i n surface/volume ratio and density of active zones along the length of the presynaptic terminal.