Connectivity of the tectal zones coding for upward and downward oblique eye movements in goldfish

Deep layers of the goldfish tectum code movements in a topographically orde red motor map. This work studies the relationship between tectal sites (cod ing eye movements with different vertical directions) and the distributions of boutons (left by their projections), within rostral mesencephalic struc tures and rhombencephalic reticular formations. These regions have been inv olved in the generation of the vertical and horizontal components of eye mo vement, respectively, as suggested by the Cartesian hypothesis of de-codifi cation of tectal signal. With this aim, discrete injections of biotinylated dextran amine (BDA) and Fluoro-Ruby (FR) were made into functionally ident ified tectal sites, coding oblique eye movements with similar amplitude of the horizontal component but opposite upward and downward vertical directio ns, and the distribution of synaptic endings was determined. The main findi ngs of the present work were as follows: 1) within the tectal descending tr act, axons were organized according to the location of injected sites withi n the tectum; 2) BDA and FR boutons were distributed in separate clusters w ithin the medial longitudinal fasciculus and oculomotor nuclei, as well as in the nearby mesencephalic reticular formation; and 3) the regions contain ing both types of bouton overlapped moderately within the mesencephalic ret icular formation at the isthmus level. Overlapping was more extended at the different levels of the rhombencephalic reticular formation, although a sh ift in the distribution of both types of bouton was always observed. These results suggest that, within the vertical generator, the endings were separ ated to contact the different neuronal population that codes the upward and downward components of movements. In contrast, in the horizontal generator , tectal endings more likely converge on the same neuronal population to co de the horizontal component of movements, irrespective of whether the obliq ue movements were directed upward or downward. (C) 2000 Wiley-Liss, Inc.