From three-dimensional reconstructions of CA1 excitatory synapses in the rodent hippocampus and in culture, we have estimated statistical distributions of active zone and postsynaptic density (PSD) sizes (average area ϳ0.04 m 2 ), the number of active zones per bouton (usually one), the number of docked vesicles per active zone (ϳ10), and the total number of vesicles per bouton (ϳ200), and we have determined relationships between these quantities, all of which vary from synapse to synapse but are highly correlated. These measurements have been related to synaptic physiology. In particular, we propose that the distribution of active zone areas can account for the distribution of synaptic release probabilities and that each active zone constitutes a release site as identified in the standard quantal theory attributable to Katz (1969).Key words: synaptic vesicle; active zone; release; statistical distribution; hippocampus; release probabilitySince the pioneering work of Katz and his collaborators on synaptic f unction (summarized in Katz, 1969) and that of the early electron microscopists on synaptic structure (Palay and Palade, 1955;L use, 1956;Wyckoff and Young, 1956), a consistent goal of neurobiologists has been to identif y the structural basis for the entities identified in Katz's theory of synaptic transmission (Katz, 1969). The synaptic vesicle is generally accepted as corresponding to Katz's quantum, although still without definitive evidence. The number of release sites [N s in Katz's (1969) scheme] associated with an axon has been identified with the total number of releasable vesicles, with the number of boutons, and with the number of active zones (Zucker, 1973;Jack et al., 1981;Korn et al., 1981;Neale et al., 1983;Redman and Walmsley, 1983;Walmsley et al., 1985;Pun et al., 1986;Propst and Ko, 1987;Redman, 1990;Walmsley, 1991;Pierce and Mendell, 1993;Pierce and Lewin, 1994). A possible anatomical counterpart of Katz's release probability p has been subject to less speculation, but several authors have noted that p might be related to synaptic size (see Pierce and Lewin, 1994).With the development of optical techniques to study synaptic transmission and the extension of these techniques to hippocampal neurons in culture (Ryan et al., 1993;Ryan and Smith, 1995;Ryan et al., 1996), many synaptic properties can now be investigated in culture at the level of single central synapses. Although individual hippocampal synapses differ greatly from one another with respect to any one of these properties, a variety of the properties are highly correlated with the release probability of the synapse (Ryan et al., 1996;Murthy et al., 1997). To relate results from physiological investigations of synapse populations to synaptic structure, one must know the statistical distribution of morphological characteristics. Current statistical information about hippocampal synapses comes from studies by Harris and Stevens (1989), Harris et al. (1992), Sorra and Harris (1993), and Harris and Sultan (1995), but this work has focus...

When hippocampal synapses in culture are pharmacologically silenced for several days, synaptic strength increases. The structural correlate of this change in strength is an increase in the size of the synapses, with all synaptic components--active zone, postsynaptic density, and bouton--becoming larger. Further, the number of docked vesicles and the total number of vesicles per synapse increases, although the number of docked vesicles per area of active zone is unchanged. In parallel with these anatomical changes, the physiologically measured size of the readily releasable pool (RRP) and the release probability are increased. Ultrastructural analysis of individual synapses in which the RRP was previously measured reveals that, within measurement error, the same number of vesicles are docked as are estimated to be in the RRP.

By combining photoconversion of FM1-43-stained vesicles and electron microscopy of hippocampal synapses, we find evidence that the population of morphologically docked synaptic vesicles corresponds to the release-ready neurotransmitter quanta. Furthermore, those synaptic vesicles that are participating in cycles of exo- and endocytosis tend to be closer to the active zone than vesicles that are being held in reserve.