■ OBJECTIVES: According to recent experimental studies of brain ischemia, neuronal cell death due to severe ischemia may be relieved by a preceding minor ischemia. This phenomenon is called ischemic tolerance, and is extensively studied in the brain and the heart. At present,there is no report that mentions this phenomenon in the cochlea. The aim of the present study is to reveal if such mechanism works in the cochlea using an animal model of transient cochlear ischemia.

■ METHODS: Adult male (60-80g) Mongolian gerbil was used in this study, since it lacks the posterior communicating arteries and the cochlea is nourished solely by the vertebral arteries. Cochlear ischemia was introduced in animals by occluding the bilateral vertebral arteries for 2-min as a preliminary minor ischemia or 15-min as a severe ischemia. Hearing was assessed sequentially by recording auditory brainstem responses (ABR). Our animal model features that the damage of the cochlea was severer at the basal turn, and that the inner hair cells (IHCs) were more fragile than the outer hair cells. In histological study, therefore, loss of IHC was counted in specimens stained with rhodamine-phalloidin and Hoechst 33342. The animals were divided into two groups; 1) single ischemia group (n=8, 16 ears), that was loaded with 15-min ischemia, and 2) double ischemia group (n=6, 12 ears), that was treated with 2-min ischemia 2 days before loading of 15-min ischemia. The results were compared between the two groups.