Age-related synaptic changes in the central nucleus of the inferior colliculus of Fischer-344 rats

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.