OCTAVOLATERAL NEURONS PROJECTING TO THE MIDDLE AND POSTERIOR RHOMBENCEPHALIC RETICULAR NUCLEI OF LARVAL LAMPREY - A RETROGRADE HORSERADISH-PEROXIDASE LABELING STUDY

The octavolateral area of lampreys, which receives primary fibers from the octaval and lateral line nerves, is involved in the premotor orga nization of body movements through secondary projections to the reticu lar formation. Here, the typology of neurons of the three octavolatera l nuclei (ventral, medial, and dorsal) that putatively project to the middle and posterior rhombencephalic reticular nuclei were studied by retrograde transport of horseradish peroxidase (HRP) applied to these reticular nuclei. Several types of neurons were labeled in the ventral nucleus, both ipsilateral and contralateral to the site of HRP applic ation. Some of these neurons showed a rather simple morphology (octavo motor neurons, monopolar cells), but most had more- or less-branched d endrites that were associated with one, or several, fields of terminal fibers in the octavolateral area. Unlike those of the ventral nucleus , labeled neurons of the medial nucleus were homogeneous in appearance (mostly pear-shaped). The dorsal nucleus was scarcely developed in la rvae, as judged from the very simple and small labeled cells. The pres ence of terminal or ''en-passant'' boutons of secondary octavolateral fibers in the reticular area and the commissural nature of these fiber s were also investigated by means of application of HRP or indocarbocy anine dye to the octavolateral nuclei. In addition, neurons of other a lar plate nuclei that were labeled by the HRP application to the retic ular nuclei (trigeminal descending root nucleus and solitary nucleus) were also characterized. The functional significance of these results is discussed. (C) 1997 Wiley-Liss, Inc.