The long-term effect of bilateral inferior colliculus ablation on auditory brainstem response in awake cats.

In this study auditory brainstem responses (ABRs) were recorded repeatedly in three awake cats before and after bilateral aspiration of the inferior colliculi (ICs). All surgical procedures were performed under anaesthesia with Nembutal. Postoperative recordings were obtained 1 week, 3 months, 8 months and 13 months after bilateral aspiration of the ICs in order to study the long-term changes in ABRs. The P5 amplitude was markedly reduced only at 1 week after aspiration. P1 and P2 amplitudes were affected slightly at 3 months and 1 year, but those of P3 and P4 decreased significantly. After perfusion of these cats, histological examination of the brainstem revealed complete ablation of both ICs, neuronal loss in the nucleus of the lateral lemniscus and the superior olivary complex and preservation of the cochlear nucleus. Our results suggest that retrograde degeneration of neuronal cells of the brainstem, which project to the IC from the nucleus of the lateral lemniscus and superior olivary complex except the cochlear nucleus, affect the peak amplitudes of the ABR over the long-term.