PREMETAMORPHIC AND POSTMETAMORPHIC ORGANIZATION OF THE VESTIBULAR NUCLEAR-COMPLEX IN THE TURBOT EXAMINED BY RETROGRADE TRACER SUBSTANCES

During metamorphosis of flatfish larvae, eye migration leads to a 90 d egrees misalignment of the visual and vestibular frames of reference. In order to maintain vestibular eye stabilization, the vestibulo-ocula r (V-O) pathways have to be radically reorganized. Here, we have exami ned the vestibular projections in turbot larvae and juveniles by means of conventional neurohistological techniques using horseradish peroxi dase and fluorescent dextranamines as tracers. We have found that the vestibular projections to the rostral eye motor nuclei consist of five densely clustered groups of neurons projecting to the rostral eye mot or nuclei, some through the ipsilateral, others through the contralate ral medial longitudinal fascicle (MLF). In addition, there are three g roups of vestibule-spinal neurons. The most prominent of these gives r ise to the ipsilateral vestibule-spinal tract. The other two project c ontralaterally, one descending in the MLF, the other more laterally in the anterior funiculus of the spinal cord. These subnuclei of the ves tibular complex are easily identifiable in larvae before metamorphosis , as well as in juvenile turbots. The number of projection neurons in each of the subnuclei is approximately doubled over the period of meta morphosis. Applying different tracers to rostrally and caudally projec ting pathways, we found no double-labeled neurons, indicating that the V-O and vestibule-spinal groups are distinct entities. However, by ap plying the two tracers ipsi- and contralaterally in the terminal field s in the rostral eye motor nuclei after metamorphosis, we found many d ouble-labeled neurons in all the V-O subgroups. In contrast, we found only a small fraction of double-labeled vestibular neurons when the sa me strategy was applied to larval preparations. We conclude that 1) th e basic organization of the vestibular nuclei of the turbot is similar to that of other teleosts, in larvae as well as juveniles; 2) there i s a substantial increase in projection neurons over the period of meta morphosis in all the subgroups of the vestibular nuclear complex; and 3) many more of the V-O neurons project bilaterally to the rostral eye motor nuclei in juvenile than in larval turbots. (C) 1996 Wiley-Liss, Inc.