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 tasks in the IC. Recently, a number of neurochemical and immunocytochemical studies have suggested an age-related decline in GABAergic inhibition in the ICc, and possibly excitant-amino-acid-mediated excitation as well. The objective of this study was to compare quantitatively changes in the synaptic organization of the ICc among three age groups (3, 19, and 28 months) of Fischer-344 rats. Immunogold electron microscopic methods were used to determine if there were age-related changes in the density, distribution, or morphology 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+ and GABA- synaptic terminals (~30% and ~24%, respectively) and synapses (~33% and ~26%, respectively) in the ICc of 28-month-old rats relative to 3-month-olds. The numeric values, which were adjusted to consider changes in volume of the IC with age, depict similar effects, although the effect magnitude for the adjusted values was reduced by approximately 9%. For both types of synapses, the decreases did not differ significantly from each other. The reductions in synaptic numbers appeared to be related to a similar numeric decline in dendrites, in particular those with calibers of between 0.5 and 1.5 μm. The number and distribution of synaptic terminals on the remaining dendrites of GABA- neurons appeared not to undergo major age-related changes. GABA+ neurons, on the other hand, may have evolved patterns of synaptic and dendritic change during aging in which the distribution of synaptic terminals shifts to dendrites of larger caliber. In the 19-month group, the synaptic areas were elevated in terminals apposed to dendrites with calibers of 1.5 μm or less. However, this increase in synaptic size did not persist in the aged animals. No neuronal 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.