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A CRITICAL PERIOD FOR VESTIBULAR DEVELOPMENT IN ZEBRAFISH (DANIO RERIO).   S.J. Moorman, R. Cordova, and S.A. Davies. Dept of Anatomy, Case Western Res Univ, Cleveland, OH. 

A critical period is defined as the briefest period of time during development when stimulus deprivation results in long lasting or permanent sensory deficits. Analogies to the visual and auditory systems suggested the existence of a critical period during vestibular development. We have used a bioreactor designed by NASA to simulate microgravity for cells in culture to determine a critical period of vestibular development in zebrafish. Zebrafish eggs were collected within 3 hours of having been laid and fertilized. Experiment #1; eggs were placed in the bioreactor at 3, 24, 30, 36, 48, or 72 hours post-fertilization (hpf) and maintained in the bioreactor until 96 hpf. Experiment #2; eggs were placed in the bioreactor immediately after they were collected and maintained in the bioreactor until 24, 36, 48, 60, 66, 72, or 96 hpf. Beginning at 96 hpf, all larvae had their vestibulo-ocular reflexes (VOR) evaluated once each day for 5 days. Only larvae that hatched from eggs that were placed in the bioreactor prior to 30 hpf in the experiment #1 or removed from the bioreactor later than 66 hpf in experiment #2 had VOR deficits that persisted for at least 5 days. These data suggest a critical period for vestibular development in the zebrafish that begins prior to 30 hpf and ends after 66 hpf. To confirm this, zebrafish eggs were placed in the bioreactor at 24 hpf and removed after 66 hpf. VORs were evaluated in these larvae once each day for 5 days beginning at 96 hpf. These larvae had VOR deficits that persisted for at least 5 days. It should be noted that none of the larvae in the first experiment of in this last experiment had any obvious or significant changes in morphology of the inner ear. Overall, these data support the idea that there is a critical period for functional maturation of the zebrafish vestibular system. The developmental period identified includes the time frame during which the primary afferent neurons are born, innervate their central and peripheral targets and remodel their central projections. 

(Supported by NASA: NAG2-1356.)