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.