Response properties in young and old Fischer-344 rat lateral superior olive neurons: a quantitative approach

Significant age-related functional deficits may result from a selective loss of inhibitory processing in the lateral superior olivary (LSO) nucleus. To test this hypothesis, methods were developed for quantitative comparisons of the excitation evoked by ipsilateral acoustic stimuli and the inhibition evoked by contralateral acoustic stimuli in neurons recorded from this binaural structure. Data were obtained from 103 LSO neurons from 22 young adult (3-6 month) and 70 LSO neurons from 14 old (20-23 month) Fischer-344 (F-344) rats. Age-related increases in the thresholds of auditory brainstem responses as well as in the inhibitory and excitatory responses of LSO neurons were observed. Spike-discharge rates were analyzed using multiple regression analysis for interaural intensity data and by calculating correlation coefficients between excitatory and inhibitory isointensity curves. Most LSO principal cells exhibited ipsilateral excitation and contralateral inhibition of similar strength for similar stimuli. Inhibitory and excitatory response areas of these neurons were similar based on visual inspection and correlation coefficients. No statistically significant age-related changes were observed for (a) rate-level functions generated by ipsilateral or contralateral stimuli; (b) maximal discharge rate; (c) conduction latencies; or (d) measures of binaural function. However, a small percentage of LSO neurons both in young and old rats displayed "unmatched" inhibitory and excitatory response areas. Quantitative methods developed in this study are used to examine age-related changes in binaural function in the inferior colliculus of F-344 rats and in the LSO of the Sprague-Dawley rat. Although no asymmetrical aging changes were observed for the F-344 rat LSO, there appear to be significant differences between the aging auditory system in the F-344 and Sprague-Dawley rats. Age-related changes have been previously described for the F-344 in other brainstem auditory structures.