SECONDARY VESTIBULOOCULOMOTOR PROJECTIONS IN LARVAL SEA LAMPREY - ANTERIOR OCTAVOMOTOR NUCLEUS

The ventral octavolateral area of lampreys contains three nuclei: the anterior, intermediate and posterior octavomotor nuclei, formed of lar ge neurons that are contacted by thick primary vestibular fibres. We u sed horseradish peroxidase (HRP) or fluorescein-dextran-amine (FDA) la belling to study the projections of the anterior octavomotor nucleus ( AON) in the larval sea lamprey, Petromyzon marinus. The tracers were i njected either in the AON, the oculomotor nucleus or the rostralmost s pinal cord. HRP injection in the AON labelled thick axons that coursed to the basal mesencephalic tegmentum, where most decussate and projec t to the oculomotor nucleus and the third Muller cell. Electron micros copy confirmed that AON axons contact with the contralateral third Mul ler cell and with oculomotor neurons. Some AON axons run in the mesenc ephalic tegmentum and the ventral diencephalon. An AON axon was observ ed to run close to the axon of the contralateral third Muller cell, es tablishing what appeared to be en passant contacts. HRP injection in t he AON also revealed commissural fibres projecting to the contralatera l octavolateral area. HRP or FDA injections in the oculomotor nucleus labelled both large and small neurons of the AON, mostly contralateral to the injection site, as well as of cells in the intermediate octavo motor nucleus, mainly ipsilateral. HRP injection in the AON or in the rostral spinal cord did not reveal any projections from the AON to the spinal cord. Our results indicate that the pattern of octavo-oculomot or connections in the lamprey is different from that observed in other vertebrates. (C) 1996 Wiley-Liss, Inc.