ESTIMATES FOR THE POOL SIZE OF RELEASABLE QUANTA AT A SINGLE CENTRAL SYNAPSE AND FOR THE TIME REQUIRED TO REFILL THE POOL

Local superfusion of limited dendritic areas with hypertonic or hyperk alemic solutions stimulates the release of quanta from a small populat ion of synapses made on rodent hippocampal neurons maintained in prima ry culture, and each quantal event can be detected in the postsynaptic neuron. With maintained stimulation, the initial release rate is abou t 20 quanta per sec per synapse, and this rate declines exponentially to a final low level. These observations can be interpreted as depleti on of available quanta and, with this interpretation, a bouton would c ontain one to two dozen quanta in its readily releasable pool. Tests w ith a second application of the solution that produces release reveal that the pool of readily releasable quanta is replenished with a time constant of about 10 sec (36 degrees C). The pool of quanta defined in this way may correspond to the population of vesicles docked at the b outon's active zone.