Structural mechanisms of bidirectional synaptic plasticity in the dentate gyrus of freely behaving rats

Abstract

This research thesis is part of a collaborative study between the laboratories of Kristen M. Harris and Wickliffe C. Abraham; its purpose: to elucidate the structural mechanisms responsible for the expression of memory within the brain. Bidirectional plasticity is a term which delineates the two principal states of cellular memory, and is characterized by persistent increases (long-term potentiation, LTP) and decreases (long-term depression. LTD) in synaptic transmission. In chapter one, we used chronically implanted electrodes to investigate parameters that may regulate bidirectional synaptic plasticity within the dentate gyrus of freely moving rats. The induction and persistence of bidirectional synaptic plasticity were compared across two different rat strains, two different phases of the circadian cycle, and three patterns of high-frequency stimulation. Results indicated that LTP was larger and more persistent in Long-Evans than Sprague-Dawley rats. Additional testing in Long-Evans rats revealed that short-term potentiation but not LTP was greater when experiments took place during dark rather than light cycles of the circadian rhythm. Input-output testing demonstrated that these effects were associated with significant differences in dentate granule cell excitability. Bidirectional synaptic plasticity was also strongly dependent on the pattern of afferent stimulation: conventional theta-burst stimulation induced negligible amounts of plasticity, while trains utilizing a 400 Hz pulse frequency yielded consistently robust effects. The results from chapter one were used to optimize the procedures used in subsequent experiments. In chapter two we used serial section transmission electron microscopy (ssTEM) to examine features of anatomical ultrastructure within the dentate gyrus. Results from chapter two served to illustrate that dendrite caliber, microtubule number and spine density were systematically distributed throughout the granule cell dendritic tree. Knowledge of these systematic distributions was used to inform the sampling scheme of subsequent experiments. In chapter three, bidirectional synaptic plasticity was induced within the dentate gyrus of freely behaving rats; animals were sacrificed 30 min after induction and prepared for ssTEM. Results indicated that LTP was associated with systematic increases in spine head volume and post-synaptic density (PSD) area within the middle molecular layer of the granule cell dendritic tree, and systematic decreases in the inner molecular layer. Interestingly, these structural changes appeared to balance between adjacent layers. This finding indicated that excitatory synaptic structures could be homeostatically coordinated within the dentate granule cell dendritic tree. The experimenter concludes by discussing the implications of these findings, and suggests directions for future investigation.