Ultrastructural basis of synaptic transmission between endbulbs of Held and bushy cells in the rat cochlear nucleus

• Published in: The Journal of physiology Vol. 539; no. 3; pp. 713 - 723
• Main Authors: Nicol, Madeleine J., Walmsley, Bruce
• Format: Journal Article
• Language: English
• Published: Oxford, UK The Physiological Society 15-03-2002; Blackwell Publishing Ltd; Blackwell Science Inc
• Subjects: Animals; Cochlear Nucleus - cytology; Cochlear Nucleus - physiology; Cochlear Nucleus - ultrastructure; Microscopy, Electron; Nerve Endings - physiology; Nerve Endings - ultrastructure; Original; Rats; Rats, Wistar; Synapses - ultrastructure; Synaptic Transmission - physiology
• ISSN: 0022-3751; 1469-7793
• DOI: 10.1113/jphysiol.2001.012972

Auditory nerve fibres make large excitatory synaptic contacts, the endbulbs of Held, with bushy cells in the anteroventral
cochlear nucleus (AVCN). We have used serial-section electron microscopy to reconstruct seven endbulbs of Held in contact
with three different AVCN bushy cells from a 25-day-old rat, as a basis for interpreting our previous physiological results
at this connection. Four endbulbs of Held contacting the same bushy cell were completely reconstructed. The number of separate
synaptic specializations within these endbulbs varied from 85 to 217, with a mean of 155. Detailed measurements were obtained
from high magnification segments of four endbulbs contacting three different bushy cells. Large variability was found in the
size of synaptic specializations within an individual endbulb. The size of postsynaptic densities (PSDs) varied between endbulbs
(mean PSD area 0.03, 0.07, 0.07 and 0.18 μm 2 ; n = 4 endbulbs). The number of morphologically docked vesicles at individual specializations within the same endbulb varied
considerably (between 1 and 102). The mean number of morphologically docked vesicles per specialization differed between endbulbs
(mean numbers of docked vesicles per specialization = 2.1, 3.7, 5.3, 14.8; n = 4 endbulbs). Despite these large differences, the density of docked vesicles per square micron of PSD was similar between
endbulbs (54, 80, 81, 83 docked vesicles per μm 2 ; n = 4 endbulbs). Within an endbulb, a linear relationship was found between the number of docked vesicles and PSD area, and
between PSD area and the number of undocked vesicles within 150 nm of the active zone. The ratio of undocked vesicles (< 150
nm) to docked vesicles ranged from 2 to 5 in different endbulbs ( n = 4 endbulbs). These structural observations are discussed in relation to the functional properties of synaptic transmission
between endbulbs of Held and bushy cells in the AVCN.