Dynamic regulation of BDNF and NT-3 expression during visual system development

Recent studies have proposed roles for neurotrophins in the formation and p lasticity of ocular dominance columns as well as in the regulation of dendr itic arborization in visual cortex of higher mammals. To assess potential r oles for neurotrophins in these processes, we have examined the development al expression of BDNF and NT-3 mRNA in the cat's visual system using in sit u hybridization. BDNF and NT-3 mRNAs are dynamically regulated in many CNS structures during embryonic and postnatal development, and both mRNAs under go striking developmental changes in laminar specificity and levels of expr ession within primary visual cortex during the critical period for ocular d ominance column formation. Within visual cortex, BDNF mRNA is found in neur ons in deep cortical layers (5 and 6) prior to eye opening, and in both dee p and superficial layers (2 and 3) shortly afterwards. Within layer 4, the target of thalamocortical axons, BDNF mRNA is low initially and rises to hi gh levels by the end of the critical period for ocular dominance column for mation. NT-3 mRNA is first detectable in small stellate neurons at the base of layer 4 (4c) after eye opening, and levels decrease near the end of the critical period. BDNF and NT-3 mRNAs can be detected in the lateral genicu late nucleus at birth, and levels peak during the critical period. In both structures, BDNF mRNA expression is maintained into adulthood, while NT-3 i s undetectable in the adult. The presence and dynamic regulation of these n eurotrophins in visual structures is consistent with suggested roles for bo th of these neurotrophins in axonal and dendritic remodeling known to accom pany the formation of ocular dominance columns. J. Comp. Neurol. 420:1-18, 2000. (C) 2000 Wiley-Liss, Inc.