UC San Diego

Neocortical representations of sensory information are commonly arranged in topographic maps that can be modified and reorganized based on recent sensory experience. This map plasticity is thought to involve both anatomical and physiological changes of cortical circuits. In rat somatosensory cortex, it has been hypothesized that long- term synaptic depression (LTD) is a major mechanism for a common feature of cortical map plasticity, the reduction in cortical responsiveness to deprived sensory inputs. Recent work has shown that whisker deprivation weakens the layer 4 (L4) to L2/3 excitatory projection in columns deprived of their main sensory input, and that this weakening occludes subsequent attempts to induce LTD, suggesting that weakening represent, in part, LTD induced by sensory deprivation in vivo (Allen et al., 2003). The specific mechanisms that mediate this reduction in projection strength, and whether they share features with LTD, remain unclear. In chapter 2, we show that deprivation-induced weakening of L4 to L2/3 inputs represents synaptic weakening, and is mediated by a reduction in presynaptic, but not postsynaptic, efficacy, consistent with known mechanisms of LTD at this synapse. Chapter 3 focuses on the development and plasticity of L4 excitatory axonal arbors, which are the main columnar projection from L4 to L2/3. These arbors develop a high degree of columnar specificity from an initially less precise projection through targeted growth within columnar bounds. This process occurs identically regardless of sensory experience, indicating that deprivation-induced plasticity at this synapse does not involve large-scale axonal reorganization. Chapter 4 shows that whisker deprivation does not alter neuron density in L4, suggesting that whisker deprivation at these ages does not involve elimination of presynaptic neurons. In chapter 5, horizontal, trans-columnar axonal pathways are analyzed for their putative role in a second feature of map plasticity, the expansion of spared cortical representations in sensory maps