| Synaptic plasticity is widely regarded as a neuronal correlate of the whole-brain phenomena of learning and memory. Both Hebbian and homeostatic mechanisms of plasticity are thought to be required for maintenance and storage of information. Synaptic scaling is one important form of homeostatic plasticity which helps to maintain a basal level of neuronal activity through regulation of the number of AMPA receptors (AMPARs) at the synapse. Current models suggest that various subunits of the AMPAR play distinct roles in the movement of AMPARs and in the their maintenance at the synapse, depending on the activity history of that synapse and of the neuron in general. GluR1 subunits, for example, are thought to regulate insertion of AMPARs during LTP, while GluR2 is involved in constitutive receptor cycling and LTD. The molecular mechanisms of synaptic scaling, however, are largely undetermined. In this thesis I have attempted to further the understanding of the molecular pathways involved in synaptic scaling.; I have used RNA interference (RNAi) in primary cortical cultures to dissect the molecular bases for scaling, by targeting GluR2 mRNA for degradation. 24 hours post-transfection, GluR2 expression was greatly reduced as measured by immunostaining and increased AMPAR rectification. However, despite the loss of GluR2, knockdown neurons maintained synaptic puncta and synapse function at levels similar to control. GluR2 knockdown completely blocked the synaptic scaling of AMPARs induced by chronic TTX treatment. The intensity of GluR1 at synaptic sites and mEPSC amplitude were maintained at levels comparable to non-TTX treated controls. Chemical induction of LTP in knockdown neurons increased mEPSC amplitude similar to controls, indicating that GluR2 is not required for this mechanism of AMPAR insertion. Finally, overexpression of an RNAi-insensitive GluR2 subunit was able to partially rescue scaling in knockdown neurons. These data suggest a novel role for GluR2 in the molecular machinery of synaptic scaling. |
