In the adult leopard frog (Rana pibiens), optic fibers regenerating after surgical displacement of the optic nerve into the telencephalon regularly form a laminar, synaptic, terminal field in the primary olfactory cortex. Preliminary data suggest that the potential exists for the development of retinotopic order in this projection, although the olfactory cortex lacks topographic organization with respect to its normal afferents. This could signify the capacity of retinal fiber terminals to become self-assembled with minimal reference to target-based coordinate systems. The current project will continue its electrophysiological study of the ectopic projection in order to fully document the receptive field organization of its elements and the topographic characteristics of the map that develops over time. Anatomical tracing studies will be included in the research plan to assist in determining the global order of the projection. Retinal fiber terminals normally form glomerular synaptic complexes with beaded dendritic processes appended to the apical dendrites of tectal neurons. Elaborate dendritic appendages of similar form are not observed in Golgi preparations of normal olfactory cortex. Evidence suggests that specific components of the short-latency, visually evoked, electrical signals characteristically obtained from normal frog tectum reflect the electrical activity of oriented dendritic appendages within the glomerular complexes. Visual stimuli evoke signals with identical waveforms in the area of anomalous projection in olfactory cortex, but neither olfactory nor visual stimuli evoke these signals in the normal cortex. In order to determine whether retinal terminals induce the formation of appropriately oriented dendritic appendages in the olfactory cortex, the current project will include Golgi and EM studies of the cortical neurons innervated by a regenerating optic nerve. The degree of selectivity of the retinorecipient area of the olfactory cortex in supporting foreign afferent innervation will be examined by using the nerve graft method to conduct regenerating auditory and olfactory nerve fibers into this area. The questions under study are important in further understanding the biological basis of synaptic organization in the CNS. It is expected that these studies will have potential relevance for clinical approaches to the treatment of neurological trauma and degeneration diseases.
