Age-related synaptic changes in the anteroventral cochlear nucleus of Fischer-344 rats.

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.

Hsp70 in the inferior colliculus of Fischer-344 rats: effects of age and acoustic stress.

Heat shock proteins 72 and 73 (hsp72 and hsp73) were studied in the inferior colliculus (IC) of Fischer-344 rats to determine if their levels are altered during normal aging and following exposure to intense acoustic noise. Three age groups of rats (3, 18, and 25 months) were exposed to ambient sound (control) or broad-band noise at 108 dB sound pressure level (0.0004 dyn/cm2) for 30 min. Western blotting procedures were used to measure hsp72 and hsp73 in ICs and cerebella (positive control). Immunohistochemistry was performed using 3-month olds to study the localization patterns of hsp72 and hsp73 in both structures. The IC and cerebellum exhibited immunolabeling over neuronal somata and proximal dendrites. Ambient levels of hsp72 in supernatants from aged rats were reduced 56.5%+/-7.8% in the IC relative to 3-month olds. This decrease may render the IC more susceptible to stress-related damage. An increase in constitutive hsp73 (350.7%+/-70.4%) was observed in IC pellet fractions from animals exposed to the 108-dB noise when compared to the ambient-noise controls, suggestive of a lipoprotective role for hsp73. This elevation was consistent across age groups. No noise-induced changes in hsp72 were detectable in the IC, indicating that loud sounds may not be an appropriate stimulus for hsp72 induction in this structure.