Detection of glutamate decarboxylase isoforms in rat inferior colliculus following acoustic exposure

The inferior colliculus is a central auditory structure which serves as a s ite for the integration of ascending and descending auditory information. C hanges in central auditory structures may occur with acoustic exposure, whi ch cannot be explained by alterations in cochlear function alone. Rats were exposed to a 10-kHz tone at 100 dB SPL for 9 h. Auditory brainstem respons e measures showed an initial 25-30-dB threshold shift across all tested fre quencies. By 30 days post-exposure, thresholds for clicks and most frequenc ies returned to near control levels; however, thresholds remained elevated at 10 and 20 kHz. Inner hair cell loss was confined to apical and basal end s of the cochlea, and did not exceed 20%. Inferior colliculus levels of the two isoforms of the GABA synthetic enzyme glutamate decarboxylase (65,000 and 67,000 mol. wt forms) were measured immediately post-exposure (0 h) and at two and 30 days post-exposure using quantitative immunocytochemical and western blotting techniques. Zero-hour measures revealed a significant inc rease in the level of glutamate decarboxylase (mol. wt 67,000) protein (118 %), as well as in the optical density (35%) of immunolabeled cells. By 30 d ays post-exposure, inferior colliculus protein levels of both glutamate dec arboxylase isoforms were significantly below unexposed controls (39% and 21 % for the 65,000 and 67,000 mol. wt forms, respectively). These studies describe increased markers for GABA immediately following aco ustic exposure, followed by a decline to below control levels from two to 3 0 days post-exposure. It remains to be determined whether noise trauma-indu ced changes in glutamate decarboxylase levels in the inferior colliculus re flect protective up-regulation in response to intense stimulation, followed by the establishment of new neurotransmitter equilibrium levels. (C) 1999 IBRO. Published by Elsevier Science Ltd.