Early Postnatal Expression of L1 by Retinal Fibers in the Optic Tract and Synaptic Targets of the Syrian Hamster ALVIN W. LYCKMAN, 1,2 KENNETH L. MOYA, 3 ANNAMARIA CONFALONI, 2,4 AND SONAL JHAVERI 1 * 1 Department of Brain and Cognitive Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 2 INSERM U334, Service Hospitalier Fre ´de ´ric Joliot, CEA/DSV/DRM, 91401 Orsay, France 3 CEA-CNRS URA 2210, Service Hospitalier Fre ´de ´ric Joliot, DSV/DRM, 91401 Orsay, France 4 Laboratorio de Metabolismo e Biochimica Patologica, Istituto Superiore di Sanita `, 00161 Rome, Italy ABSTRACT Previous immunohistochemical studies in mouse, rat, and chick have reported that the expression of the glycoprotein and cell adhesion molecule L1, a member of the immunoglob- ulin superfamily, shows regulation during development of retina and optic nerve. To extend our understanding of the role of L1 in developing neural circuitry, we have examined L1 expression in the optic tract and thalamic and midbrain synaptic targets of retinal fibers in the early postnatal Syrian hamster, a well-characterized developmental model of the primary visual projection. Metabolic labeling studies reveal that a synaptically targeted, sulfated, and glycosylated form of L1 undergoes rapid axonal transport from the retina. Retinofugal transport of L1 decreases commensurate with the decline in immunoreactivity of retinal fibers in the visual pathway. Retinal ganglion cell axons show intense L1 immunoreactivity as they navigate in highly fasciculated bundles in the optic tract overlying the lateral geniculate body and in the superior colliculus. We found no evidence of L1 immunoreactivity on retinal axon collaterals as they defasciculate from the optic tract and branch into target neuropils. L1 immunoreactivity wanes in optic tract as axon terminal arbors are elaborated in the lateral geniculate body and superior colliculus and as myelination in the visual pathway commences. This pattern of L1 expression suggests that, in the early postnatal period, L1 may support fasciculation of retinal fibers, maintaining them within the optic tract, and that subsequent down-regulation of L1 may facilitate their terminal arborization and myelination. J. Comp. Neurol. 423:40 –51, 2000. © 2000 Wiley-Liss, Inc. Indexing terms: development; visual system; retinotectal; fasciculation; growth cone L1, a membrane-spanning glycoprotein of the immuno- globulin superfamily, is implicated in mediating cell– cell interactions necessary for axonal guidance (Cohen et al., 1998), neuronal migration (Asou et al., 1992; Tsuru et al., 1997), myelination (Wood et al., 1990), and modulation of activity-dependent synaptic plasticity (Lu ¨ thi et al., 1994). L1 is likely to function in basic cellular mechanisms such as cell adhesion, cell– cell recognition, and signal trans- duction (Hortsch, 1996). Embryonic and neonatal regula- tion of L1 in the developing retina and optic nerve has been studied in mouse and rat (Bartsch et al., 1989; Hankin and Lagenaur, 1994; Mi et al., 1998), but L1 expression has not been examined distal to the optic chi- asm nor in synaptic targets of retinal fibers. Much of our understanding of visual pathway development in rodents arises from anatomical studies using the Syrian hamster, a species whose birth is coincident with or precedes key events in visual pathway formation (Frost and Schneider, 1979; Moya et al., 1988; So et al., 1990; Jhaveri et al., 1991, 1996). In the hamster, retinal ganglion cell axons *Correspondence to: Dr. Sonal Jhaveri, Department of Brain and Cog- nitive Sciences, Massachusetts Institute of Technology, E25-642a, Cam- bridge, MA 02139. E-mail: sonal@mit.edu Received 17 June 1999; Revised 9 March 2000; Accepted 10 March 1999 THE JOURNAL OF COMPARATIVE NEUROLOGY 423:40 –51 (2000) © 2000 WILEY-LISS, INC.