Short Term Memory May Be the Depletion of the Readily Releasable Pool of Presynaptic Neurotransmitter Vesicles

Figure 10. Average time course of depletion of the RRP for the 16 synapses, each
normalized to its initial RRP size (12). NEED MORE OF THESE TO GET THE SPREAD

Endocytosis, the reversal of the RRP depletion, is a possible candidate for the tagging
decay. Figure 11 displays the endocytosis of synaptic vesicles from a variety of paired pulse
experiments on hippocampal mouse synapses and one on rat calyx of Held synapse together
with the memory decay curve from above. Experiments not included on this graph which do not
fit straight lines were those on chromaffin (14) and mossy-fiber-CA3 synapses (15). All other data
sets investigated show a straight line (logarithmic decay - please see Appendix I for a further
discussion) and the slope is remarkably similar to the memory decay (0.11-0.14/sec for
endocytosis experiments versus 0.11/sec for the memory curve). There is a spread in the
experimental offset though this spread does not seem to overlap with the memory curve. The
experimental curves are each averages over several synapses and it could be that the memory
curve is determined by the synapses with the slowest response perhaps explaining the
discrepancy in offset (see difference between a collection of fast and slow cells from (16) in
Figure 12) or that human synapses have a different offset than those of mice (the author is not
aware of data on short term memory decay in mice or on endocytosis in human synapses).

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