Morphology and topographical organization of the retrospleniocollicular connection: A pathway to relay contextual information from the environment tothe superior colliculus

The retrospleniocollicular connection is of interest because it constitutes one link between the limbic system, which is considered the anatomical sub strate of emotional experience, and the superior colliculus (SC), which med iates approach and avoidance behavior. The morphology, topography, and orig in of the retrospleniocollicular connections were studied by using anterogr ade [biotinylated dextranamine 10,000 (BDA)] and retrograde [Fluoro-Gold (F G)] tracers. After BDA injections involving retrosplenial granular and agra nular cortices, terminal fibers innervating all collicular layers except st ratum griseum superficiale were found throughout nearly the entire collicul i. Axons branched within restricted portions of the dorsoventral collicular axis with variable morphologies, suggesting functional heterogeneity. Term inal fields originating in anterior and posterior regions of the retrosplen ial cortex were preferentially distributed in laterodorsal and medioventral collicular regions, respectively, but there were also large, densely inner vated regions in which the terminal fields overlapped. FG injections in the SC confirmed the retrospleniocollicular topography and demonstrated that t his connection originated from layer V pyramidal cells of all retrosplenial areas. The distribution of retrospleniocollicular boutons was related to t hat of the AChE modules, which are associated with connections in the inter mediate layers of the SC. In lateral portions of the SC intermediate layers , most retrospleniocollicular boutons were found in medium AChE stained reg ions, whereas in medial portions, they terminated in AChE-poor domains. The present results demonstrate that the retrosplenial cortex is the origin of a broad and dense network of axonal branches that may modulate SC-mediated motor and physiological responses involved in emotional behavior. J. Comp. Neurol. 425:393-408, 2000. (C) 2000 Wiley-Liss, Inc.