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Abstract
Previous studies have demonstrated age-related decreases in the transmitters glycine
and glutamate in the cochlear nucleus (CN) of the Fischer-344 (F344) rat, along with
declining levels of binding for glycine receptors. The purpose of this study was to
evaluate structural correlates to the transmitter and receptor losses that accompany
aging in the anteroventral CN (AVCN). Thin sections were obtained from the middle-frequency
area of the right AVCNs from five 3-month-, four 19-month-, and five 28-month-old
F344 rats. Montages were constructed from electron micrographs taken of several sites
in each AVCN section. The presynaptic terminals were classified by vesicle type and
postsynaptic target, and their perimeters and synaptic lengths were traced using morphometry
software. The calibers of all dendritic profiles were also measured, and cell counts
were performed on semi-thin sections. The data were compared among the three age groups
using analysis of variance followed by Tukey's Honestly Significant Difference for
pairwise comparisons. There were significant age-related decreases in the size of
terminals contacting small-caliber (<2 microm) dendrites. Dendrites of this size comprised
the largest percentage of dendrites in the AVCN. On these targets, round and pleomorphic-vesicle
terminals were reduced in volume by nearly 44% and 24%, respectively, in 28-month
olds when compared to the 3-month olds. On the other hand, the densities and numbers
of synaptic terminals and dendritic profiles did not differ among age groups, and
no neuronal losses were evident in the older animals. Also, there were no detectable
changes in synaptic area among groups. The decrease in terminal size may be related
to age-associated reductions in neurotransmitter levels previously described in the
F344 CN. The observations presented here contrast with those previously described
in the inferior colliculus (IC), in which there were significant age-related losses
of synaptic terminals and dendrites, but no change in the size of synaptic terminals.
The lack of synaptic and dendritic losses suggests that the structural connectivity
of the rat AVCN remains relatively intact during aging, which is interesting in light
of the synaptic and dendritic changes evident in the IC, a major target of its projections.