DECREASED INHIBITION TO LATERAL SUPERIOR OLIVE NEURONS IN YOUNG AND AGED SPRAGUE-DAWLEY RATS

Lateral superior olive (LSO) neurons in young and aged Sprague-Dawley rats have functional properties consistent with a limited contralatera l inhibition, which is markedly different from other animals. An unusu ally low proportion of LSO cells (36/113) exhibited contralateral inhi bition (and ipsilateral excitation, IE), while over 25% of LSO units e xhibited excitatory responses to contralateral stimuli. Inhibition of most IE LSO neurons was evident only when the contralateral intensity was greater than the ipsilateral intensity, resulting in a marked shif t in sensitivity to interaural intensity differences (IID). The firing rate of IE neurons was also affected more by a change in intensity of ipsilateral compared to contralateral stimuli. The shift in the IID s ensitivity and the relative decrease in effectiveness of contralateral ly driven inhibition in Sprague-Dawley rat LSO neurons could be due to decreased inhibitory inputs from the MNTB principal cells, increased contralateral excitatory effects and/or increased ipsilateral excitato ry effects, Age-related decreases in the numbers of MNTB neurons obser ved anatomically is not reflected in a change in LSO function. The Spr ague-Dawley rat may be a useful model for the effect of reduced inhibi tion in the superior olivary complex on auditory behavior.