Date of Award
Doctor of Philosophy (PhD)
Learning and other forms of plasticity result from changes in transmission at existing synapses or the construction or elimination of synapses. Synapses occur at the juxtaposition of boutons, with their postsynaptic partners, dendrites and cell bodies. It has been assumed that connections in the primary visual cortex (V1) become static after the critical period. Recent studies show, however, that dendritic spines appear and disappear during adulthood in the normal brain. Our first objective was then to determine whether axonal branches and boutons also undergo morphological changes. To do so, we performed longitudinal studies of virally labeled neurons and their processes. An adeno-associated virus bearing the gene for enhanced green fluorescent protein (AAV.EGFP) provided long-term labeling of axons and their boutons in adult Macaque V1. To image the neurons in vivo, a custom-designed two-photon microscope and viewing chamber provided repeated imaging of selected locations. We examined the same EGFP-labeled axonal arbors at several time points over periods of weeks in the adult normal cortex. We found that axons are dynamic entities, in which a subset of boutons appeared and disappeared overtime, and that though axonal length and branching was largely stable, a small subset of terminals underwent elongation, retraction or appeared de novo. These results suggest an ongoing process of synaptogenesis and synapse elimination in adult V1. To further investigate structural plasticity in the adult V1, we studied the cortical reorganization that accompanies retinal lesions. Removal of visual input cause axonal sprouting of long-range horizontal connections from pyramidal cells in layer 2/3. Our in vivo approach allowed us to determine the dynamics of the process of sprouting. Immediately following retinal lesions, there was a remarkable rise in axonal density. In the following weeks, the massive increase in axon collaterals was accompanied by a comparable rate of axonal elimination. Also, boutons increased their rate of appearance and elimination beyond the rates seen in normal cortex. These data indicate that the initial sprouting of axons followed by the subsequent refinement, may account for the dynamics of receptive field changes observed during the course of topographic reorganization of visual cortex.
Yamahachi, Homare, "Circuit Dynamics of Adult Visual Cortex" (2009). Student Theses and Dissertations. 254.