PROJECTION OF INDIVIDUAL AXONS FROM THE PRETECTUM TO THE DORSAL LATERAL GENICULATE COMPLEX IN THE CAT

The dorsal lateral geniculate nucleus of the thalamus transmits visual signals from the retina to the cortex. Within the lateral geniculate nucleus, the ascending visual signals are modified by the actions of a number of afferent pathways. One such projection originates in the pr etectum and appears to be active in association with oculomotor activi ty. Much remains unknown about the pretectal-geniculate projection. Ou r purpose was to examine for the first time individual axon arbors fro m the pretectum that project to the lateral geniculate nucleus, descri bing their topography and nuclear and laminar targets. We made injecti ons of the anterograde tracer Phaseolus vulgaris leucoagglutinin into the cat pretectum, targeting the nucleus of the optic tract. Serial 40 mu m coronal sections were processed by using immunohistochemistry to reveal labeled axons that were then serially reconstructed using ligh t microscopy. Pretectal-geniculate axons appeared morphologically hete rogeneous in terms of swelling size, branching patterns, and laminar t arget. Most axons innervated the geniculate A laminae. A separate, sma ller population innervated the C laminae. All axons exhibited substant ially greater spread medial-laterally than rostral-caudally in the lat eral geniculate nucleus, displaying a topographical organization for v isual field elevation, but not azimuth. Many pretectal axons that proj ected to the LGN also innervated adjacent structures, including the me dial interlaminar nucleus, the perigeniculate nucleus, and/or the pulv inar. These results indicate that the projection from the pretectum to the dorsal lateral geniculate nucleus is heterogeneous, is semitopogr aphical, and may coordinate neural activity in the lateral geniculate nucleus and in neighboring visual thalamic structures in association w ith oculomotor events. (C) 1996 Wiley-Liss, Inc.