RESPONSES OF YOUNG AND AGED FISCHER-344 RAT INFERIOR COLLICULUS NEURONS TO BINAURAL TONAL STIMULI

The inferior colliculus (IC) is one nucleus of the central auditory sy stem which displays age-related changes. Inputs to the IC use primaril y the amino acid neurotransmitters glutamate and gamma-aminobutryic ac id (GABA). Neurochemical and anatomical studies of the Fischer 344 (F3 44) rat IC have shown decreases in GABA and GABA receptor levels (see Caspary et al., 1995 for review). GABA neurotransmission affects binau ral response properties in the IC (Faingold et al., 1991a,b; Vater et al., 1992a; Park and Pollak, 1993, 1994). We hypothesized that aged F3 44 rats would show alterations in binaural IC neuronal response proper ties due to an imbalance in the relative levels of inhibition and exci tation. Extracellular recordings from 189 single units localized to th e IC of anesthetized aged (24 month) F344 rats were compared to those obtained from 221 IC units in young adult (3 month) animals. Quantitat ive analyses were performed to determine the distribution of ipsilater al and binaural rate/intensity functions (RIFs) in the central nucleus of the IC and external cortex of the IC units. The majority of IC uni ts in both young and aged F344 rats were not responsive to monaural ip silateral characteristic frequency tone bursts. Although there was som e shift in the distribution of binaural RIF shapes with age, it was no t statistically significant. The shift included a reduction in the per centage of units classified as E/I (excited by contralateral stimulati on/ipsilaterally inhibited during binaural stimulation), but an increa se with age in the percentage of units classified as E/f(excited by co ntralateral stimulation/further facilitated by the addition of low int ensity ipsilateral stimulation, but inhibited by higher intensity ipsi lateral stimulation). Despite the role of inhibitory neurotransmission in binaural processing in the IC, age-related neurochemical deficits in the IC do not appear to result in a major deficit in the processing of simple binaural stimuli in F344 rats.