Down-regulation of inhibition following unilateral deafening

Physiological and neurochemical experiments described here suggest that uni lateral deafening causes a reduction in inhibition in the adult gerbil infe rior colliculus (IC) contralateral to the deafened ear. Multiple-unit recor dings were made from single electrode penetrations in the IC prior to and d irectly after contralateral cochlear ablation. These recordings showed up t o 60% increases in the proportion of sampled loci at which neural activity excited by ipsilateral stimulation was observed after the ablation. Novel e xcitatory responses were evident within minutes of the ablation. Western bl otting for glutamic acid decarboxylase protein levels showed significant de creases in the IC contralateral to cochlear ablation, relative to those in the ipsilateral IC, at 24 h and 7 days survival after the ablation. Four ho ur or 1 year survival post-ablation did not produce significant contralater al/ipsilateral differences in relation to the control group. Taken together , these results suggest the presence of at least two, short-term mechanisms involved in the central response to cochlear removal, both of which appear to implicate a decreased inhibitory influence. One is a very rapid, stimul us-related, functional unmasking. The other is a more delayed reduction in the capacity of gamma-aminobutyric acid synthesis in the IC. (C) 2000 Elsev ier Science B.V. All rights reserved.