PHYSIOLOGY OF THE YOUNG-ADULT FISCHER-344 RAT INFERIOR COLLICULUS - RESPONSES TO CONTRALATERAL MONAURAL STIMULI

This study was designed to establish the young adult (3 month) Fischer 344 (F344) rat as a model of inferior colliculus (IC) physiology, pro viding a baseline for analysis of changes in single unit responses as the animals age and for the study of noise induced hearing loss. The r esponse properties of units localized to the central nucleus of the IC (CIC) and those localized to the external cortex of the IC (ECIC) wer e compared in order to better characterize differences between these t wo subnuclei in the processing of simple auditory stimuli. In vivo ext racellular single unit recordings were made from IC neurons in ketamin e/xylazine anesthetized young adult F344 rats. When a unit was electri cally isolated, the spontaneous activity level, characteristic frequen cy (CF) and CF threshold were determined. Rate/intensity functions (RI Fs) in response to contralateral CF;tones and to contralateral noise b ursts were obtained as were tone isointensity functions. The recording site was marked by ejecting horseradish peroxidase (HRP) from an elec trode. Locations of recorded units were determined from electrode trac k marks and HRP marks in serial brain sections. Recordings were made f rom 320 neurons in the IC; 176 were localized to the CIC and 87 to the ECIC. Thirteen percent of the units in each subdivision were found to be poorly responsive to auditory stimulation (clicks, tones or noise) , and spontaneous activity was generally low. Characteristic frequenci es representative of the full rat audiogram were found in each subdivi sion with the mean threshold significantly higher in the ECIC (28.7 dB SPL) than in the CIC (22.3 dB SPL). The mean maximum discharge rate t o CF tone bursts was near 24 spikes/s in each subdivision. Dynamic ran ge tended to be higher in the ECIC (28.3 dB) than in the CIC (23.2 dB) , reflecting the lower percentage of nonmonotonic units found in the E CIC. Most units responded more robustly with a slower tone presentatio n rate, displayed lower levels of discharge to noise bursts than to to ne bursts, and had differently shaped tone and noise RIFs. Most units were classified as onset responders to CF tone bursts in both subdivis ions, with the percentage of onset responders higher in the ECIC (68.9 %) than in the CIC (57.8%). First spike latency did not differ signifi cantly between the subdivisions, but tended to be shorter in the CIC. The breadth of the excitatory receptive fields did not differ signific antly between subdivisions, although the mean was slightly larger in t he ECIC. These results are generally consistent with the results of CI C studies from other species, establishing the F344 rat as a model of CIC physiology. Differences between CIC and ECIC units included a high er percentage of nonmonotonic RIFs and lower percentage of onset tempo ral response patterns in the CIC than in the ECIC. Some properties whi ch have been previously used as hallmarks for differentiation between CIC and ECIC units, namely broader tuning and longer first spike laten cies in the ECIC, did not reach statistical significance in this study . These may reflect species differences and/or the highly variable and largely overlapping sets of responses evident in the large sample siz e used in this study.