Inactivity produces increases in neurotransmitter release and synapse size

When hippocampal synapses in culture are pharmacologically silenced for sev eral days, synaptic strength increases. The structural correlate of this ch ange in strength is an increase in the size of the synapses, with all synap tic components-active zone, postsynaptic density, and bouton-becoming large r. Further, the number of docked vesicles and the total number of vesicles per synapse increases, although the number of docked vesicles per area of a ctive zone is unchanged. In parallel with these anatomical changes, the phy siologically measured size of the readily releasable pool (RRP) and the rel ease probability are increased. Ultrastructural analysis of individual syna pses in which the RRP was previously measured reveals that, within measurem ent error, the same number of vesicles are docked as are estimated to be in the RRP.