Physiology of the aged Fischer 344 rat inferior colliculus: responses to contralateral monaural stimuli.

1. Presbycusis, age-related hearing loss, is an ever increasing problem in our aging society. It involves changes in both the peripheral and central portions of the auditory system. The inferior colliculus (IC) has been shown to display age-related changes including decreased gamma-aminobutryic acid (GABA) levels, decreased glutamate decarboxylase levels, and decreased binding by GABAB receptors, as well as rearrangement of axon terminals in aging Fischer 344 (F344) rats. Age-related physiological changes have also been noted in the ICs of C57 and CBA mice. 2. Given the age-related alterations in the inhibitory neurotransmitter system, we hypothesized that aged F344 rats would show alterations in the physiological response properties of their IC neurons due to an imbalance in the relative levels of inhibition and excitation affecting the neuronal firing. 3. In vivo extracellular single-unit recordings were made from 297 IC neurons in ketamine/xylazine-anesthetized aged (24 mo) F344 rats. Locations of recorded units were determined from the electrode track marks and horseradish peroxidase marks. Results were compared with those obtained from young adult (3 mo) animals. 4. Average threshold increased from 25.4 dB SPL in young rats to 56.1 dB SPL in aged rats. 5. Although there was a reduction in the percentage of units recorded at either extreme of the frequency range in aged animals, the characteristic frequency (CF) range and mean did not differ between the two groups. 6. For the IC as a whole, no differences were noted in spontaneous activity, first spike latency, dynamic range, percentage of units with nonmonotonic contralateral CF tone rate/intensity functions (RIFs), or percentage of units sensitive to change in CF tone presentation rate. 7. In aged rats, a higher percentage of units was poorly responsive to auditory stimulation. 8. In the aged rat, there was a 12% reduction in the maximum discharge rate, a 12% increase in the percentage of units classified as onset in their temporal response pattern, and an 18% increase in the breadth of the isointensity functions at 30 dB above threshold. 9. Age-related changes in the central nucleus of the IC (CIC) frequently differed from those observed in the external cortex of the IC (ECIC). The percentage of units classified as having nonmonotonic contralateral tone RIFs decreased with age in the CIC but increased with age in the ECIC, and the percentage of units classified as onset in their temporal response pattern increased with age in the CIC but did not change with age in the ECIC. 10. The results of this study support the hypothesis that there is an age-related shift to higher intensities in the working range of most CIC units along with a small, selective deficit in inhibitory processing. When considered in conjunction with the mouse aging studies conducted by other researchers and with the results of a similar study of single units in the visual system (lateral geniculate nucleus) of young and aged rhesus monkeys, these results suggest that compensatory mechanisms are highly active in sensory systems as animals age. Despite deficits that lead to reduced input to the IC and neurochemical changes affecting neurotransmitter levels, IC neurons in aged rats are able to respond to most simple auditory stimuli in a fashion quite similar to that observed in young rats.