Three-Dimensional Reconstruction of a Calyx of Held and Its Postsynaptic Principal Neuron in the Medial Nucleus of the Trapezoid Body

Abstract

The three-dimensional morphology of the axosomatic synaptic structures between a calyx of Held and a principal neuron in the medial nucleus of the trapezoid body (MNTB) in the brainstem of young postnatal day 9 rats was reconstructed from serial ultrathin sections. In the apposition zone between the calyx and the principal neuron two types of membrane specializations were identified: synaptic contacts (SCs) with active zones (AZs) and their associated postsynaptic densities (PSDs) constituted approximately 35% (n = 554) of the specializations; the remaining 65% (n = 1010) were puncta adherentia (PA). Synaptic contacts comprised approximately 5% of the apposition area of presynaptic and postsynaptic membranes. A SC had an average area of 0.100 microm(2), and the nearest neighbors were separated, on average, by 0.59 microm. Approximately one-half of the synaptic vesicles in the calyx were clustered within a distance of 200 nm of the AZ membrane area, a cluster consisting of approximately 60 synaptic vesicles (n = 52 SCs). Approximately two synaptic vesicles per SC were "anatomically docked." Comparing the geometry of the synaptic structure with its previously studied functional properties, we find that during a single presynaptic action potential (AP) (1) approximately 35% of the AZs release a transmitter quantum, (2) the number of SCs and anatomically docked vesicles is comparable with the low estimates of the readily releasable pool (RRP) of quanta, and (3) the broad distribution of PSD areas [coefficient of variation (CV) = 0.9] is likely to contribute to the large variability of miniature EPSC peaks. The geometry of the reconstructed synapse suggests that each of the hundreds of SCs is likely to contribute independently to the size and rising phase of the EPSC during a single AP.

Synaptic vesicle recycling at the calyx of Held

“…Acta Pharmacologica Sinica npg capacitance, indicating exocytosis, agreed with the singlevesicle capacitance estimated from EM studies [22] ( Figure 4B). Approximately 20% of the time, capacitance flicker was observed.…”

“…In control conditions, most of the up-and down-steps are smaller than about 220 aF, which is a regular vesicle's maximal capacitance, based on findings from EM studies [22] . This suggests that most of the observed releases are from single vesicles.…”

Involvement of Actin Polymerization in Vesicle Recruitment at the Calyx of Held Synapse

“…Rather, an intact actin network may be important for guiding synaptic vesicles toward the plasma membrane, and other proteins may assist mobilization. The calyx of Held has ϳ600 active zones, and each active zone has three to five vesicles within 20 nm from the plasma membrane (Sätzler et al, 2002). The total number of such vesicles in the whole terminal (3000 vesicles) matches quite well with the size of physiologically identified releasable vesicles (Sakaba and Neher, 2001b,c).…”

Patch pipettes are more useful than initially thought: simultaneous pre- and postsynaptic recording from mammalian CNS synapses in vitro and in vivo

“…2a). Estimates of the number of release sites made anatomically ( [33], Fig. 2b) or functionally suggest ≈600 synaptic contacts [27].…”

“…5a). Typically, ∼600 synaptic contacts are established by a single connection [33]. Of these, about 200 AZs are active upon a sensory stimulus.…”