THE CONCEPT OF CALCIUM-CONCENTRATION MICRODOMAINS IN SYNAPTIC TRANSMISSION

Ever since the initial measurements of presynaptic calcium currents it has been evident that calcium triggers transmitter release quite rapi dly. Several models indicate, as did the initial voltage clamp measure ments, that the calcium concentration triggering such release could be very high at the entry site and that this concentration should be ver y short lasting. In order to determine this time course, calcium entry was studied at the squid giant synapse by imaging light emission from n-aequorin-J, intracellularly injected into the presynaptic terminal. The imaging utilized a video system capable of acquiring 4000 frames per sec. The results indicate that the calcium entry, triggered by act ion potentials, reaches a peak within 200 mu sec and has an overall du ration of close to 800 mu sec, closely matching the duration of the pr esynaptic calcium current determined by voltage clamp results under si milar conditions.