Light and electron microscopic evidence for projections from the thalamic nucleus rotundus to targets in the basal ganglia, the dorsal ventricular ridge, and the amygdaloid complex in a lizard

To elucidate the organization and evolution of the tectorotundotelencephali c pathways in birds and reptiles, we reinvestigated at both light and elect ron microscopic levels the efferent projections of nucleus rotundus in a li zard, using the sensitive tracer biotinylated dextran amine. Our results in dicate that nucleus rotundus projects to targets in the basal ganglia (late ral parts of striatum and olfactory tubercle and possibly the globus pallid us), the anterior dorsal ventricular ridge (ADVR), and the amygdaloid compl ex (the central and possibly lateral amygdaloid nuclei). In these targets, the rotundal axon terminals establish asymmetric, presumably excitatory syn aptic contacts, usually with dendrites of local cells. In the ADVR, the rot undal projection terminates in two separate radial regions showing distinct , cytoarchitecture: 1) a dorsolateral region that extends radially from the dorsolateral ADVR ventricular surface to the ventral part of the lateral c ortex and 2) the lateral part of a ventromedial region that extends radiall y from the dorsomedial and medial ADVR ventricle to a superficial area inte rposed between the dorsolateral ADVR and the striatum. These two ADVR regio ns have different connections with the thalamus and telencephalon, which su ggests that they may be involved in different degrees of integration. Our s tudy also suggests that the rotundal projection to the ventromedial ADVR fi eld of lizards may be comparable to the rotundoectostriatal/periectostriata l projection of birds. The connections and pathways involving nucleus rotun dus suggest that this nucleus conveys visual information which may play a r ole in visuomotor, emotional, and visceral functions. J. Comp. Neurol. 424: 216-232, 2000. (C) 2000 Wiley-Liss, Inc.