2-DEOXYGLUCOSE UPTAKE PATTERNS IN RESPONSE TO PURE-TONE STIMULI IN THE AGED RAT INFERIOR COLLICULUS

The tonotopic map of the inferior colliculus (IC) of aged rats (25 mon ths old) was examined to determine whether age-related changes known t o occur in the cochlea are reflected in the 2-deoxyglucose (2-DG) upta ke pattern of the IC. Because aged animals have hearing losses, audito ry brainstem response thresholds were measured. Animals with threshold shifts of no greater than 30 dB relative to young animals were used. Animals were injected with radiolabeled 2-DG and stimulated with conti nuous pure tones presented at 70 dB above the behavioral thresholds fo r young animals at either 1, 4 or 32 kHz for one hour in a sound atten uated booth. The stimulus sound pressure levels were chosen to achieve comparable sensation levels between the young and aged animals. The t onotopic map of the IC in aged rats was different from that reported p reviously for young animals (Ryan et al., 1988), in that, the regions stimulated by 1 and 4 kHz were shifted towards the higher frequencies and the uptake areas were twice as broad for the aged animals as for t he young animals. The observed 2-DG uptake patterns are consistent wit h an activation pattern of a high intensity stimulus and a loss of res ponsive elements in the cochlear apex. Similar broad and shifted bands of activated tissue may contribute to difficulties in auditory percep tion in aged humans with increased thresholds and sound amplification;