Rh. Helfert et al., Age-related synaptic changes in the central nucleus of the inferior colliculus of Fischer-344 rats, J COMP NEUR, 406(3), 1999, pp. 285-298
The central nucleus of the inferior colliculus (ICc) is a major processing
center for the ascending auditory pathways. Gamma-aminobutyric acid (GABA)
and excitant amino acids (EAAs) are essential for coding many auditory task
s in the IC. Recently, a number of neurochemical and immunocytochemical stu
dies have suggested an age-related decline in GABAergic inhibition in the I
Cc, and possible excitant-amino-acid-mediated excitation as well. The objec
tive of this study was to compare quantitatively changes in the synaptic or
ganization of the ICc among three age groups (3, 19, and 28 months) of Fisc
her-344 rats. Immunogold electron microscopic methods were used to determin
e if there were age-related changes in the density, distribution, or morpho
logy of GABA-immunoreactive (+) and GABA-immunonegative (-) synapses in the
ICc. The data suggest similar losses of excitatory and inhibitory synapses
in the ICc. There were significant reductions in the densities of GABA+ an
d GABA- synaptic terminals (similar to 30% and similar to 24%, respectively
) and synapses (similar to 33% and similar to 26%, respectively) in the ICc
of 28-month-old rats relative to S-month-olds. The numeric values, which w
ere adjusted to consider changes in volume of the IC with age, depict simil
ar effects, although the effect magnitude for the adjusted values was reduc
ed by approximately 9%. For both types of synapses, the decreases did not d
iffer significantly from each other. The reductions in synaptic numbers app
eared to be related to a similar numeric decline in dendrites, in particula
r those with calibers of between 0.5 and 1.5 mu m. The number and distribut
ion of synaptic terminals on the remaining dendrites of GABA- neurons appea
red not to undergo major age-related changes. GABA+ neurons, on the other h
and, may have evolved patterns of synaptic and dendritic change during agin
g in which the distribution of synaptic terminals shifts to dendrites of la
rger caliber. In the Is-month group, the synaptic areas were elevated in te
rminals apposed to dendrites with calibers of 1.5 Fun or less. However, thi
s increase in synaptic size did not persist in the aged animals. No neurona
l losses were detectable among the three age groups. Thus, the decrease in
GABA and EAAs identified in the IC by previous studies may be attributable
to synaptic and dendritic declines, rather than cell loss. (C) 1999 Wiley-L
iss, Inc.