Quantitative Ultrastructural Analysis of Hippocampal Excitatory Synapses

Abstract

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...

Quantal Size Is Independent of the Release Probability at Hippocampal Excitatory Synapses

“…It has been shown that most hippocampal glutamatergic axon terminals have only one synaptic specialization and that a single specialization contains a large number of docked vesicles (Sorra and Harris, 1993;Schikorski and Stevens, 1997). Indeed, we also found only one presynaptic specialization at each axon terminal and a large number of small synaptic vesicles attached to the presynaptic specialization at identified synaptic contacts.…”

Calmodulin enhances ribbon replenishment and shapes filtering of synaptic transmission by cone photoreceptors

“…At hippocampal synapses, D = 0.0042 µm 2 /s (Shtrahman et al, 2005) and  = 38 nm (Harris and Sultan, 1995). Vesicle density averages 1,800-3,100 v/µm 3 (Harris and Stevens, 1989;Schikorski and Stevens, 1997), but only 15% are mobile (Shtrahman et al, 2005), yielding an effective range for  of 270-465 v/µm 3 . From these values, the formula predicts a maximum kinetics of vesicle resupply from the cytoplasm of  = 13-23 s. For comparison, replenishment at hippocampal synapses involves a fast process with  7 s and a slower process with  1 min; the slower process is thought to reflect the kinetics of resupply to the readily releasable pool (Stevens and Wesseling, 1998;Wesseling and Lo, 2002;Garcia-Perez and Wesseling, 2008).…”

Reactive astrocytosis and glial glutamate transporter clustering are early changes in a spinocerebellar ataxia type 1 transgenic mouse model

“…We performed two measures of the length of the axon-spine interfaces. The first one was limited to the active zone (AZ), composed of the synaptic cleft bordered by vesicles on the presynaptic axonal bouton and corresponding to the postsynaptic density (PSD) on the dendritic spine (Schikorski and Stevens, 1997). The second measure, including the active zone, was extended to the nonsynaptic interface, characterized by the thin extracellular space bordered by spine and bouton membranes without specialization.…”