Periodic Reporting for period 2 - NovelNMDA (Novel NMDA receptor signaling in cortical synaptic depression)
Okres sprawozdawczy: 2020-08-01 do 2022-01-31
The functional relevance of NMDARs in long-term plasticity has been attributed to the flux of calcium through the NMDAR ion channel. However, recent work has shown that NMDARs may signal through a non-ionic mechanism in long-term depression. Understanding the functional role of NMDARs in synaptic plasticity is the ultimate goal of this research project.
To achieve this goal, we first aim to understand how long-term depression changes the synapses between cortical layer 4 and layer 2/3 neurons, and how NMDAR activity triggers those changes. We are applying functional imaging using 2-photon laser scanning microscopy, coupled with electrophysiological measurements to observe how single, identified synapses function and change after plasticity induction. As we develop approaches to study the activity of NMDARs in this cortical synapse, we are in parallel testing whether NMDARs in other synapse types behave in a similar manner, or whether there are unique properties in these synapses that are important for cortical experience dependent plasticity.
Objectives 1 and 2 focus on the outcome of long-term depression, induced by spike-timing dependent pairing (Objective 1), and other forms of plasticity induction (Objective 2). Both objective share similar approaches, and so far, have shown similar results. We first used electrophysiological methods to test for the locus of plasticity expression, which has produced results constant with presynaptic alterations after both spike-timing dependent plasticity (Objective 1) and low-frequency stimulation induced long-term depression (Objective 2). In addition, both forms of synaptic plasticity requires NMDAR signaling, but not ion flux through the NMDAR ion channel. Consistent with this, optical measurements of synaptic function also show presynaptic alteration after plasticity induction.
Objective 3 is designed to test NMDAR subunit identity and function at single synapses between cortical layer 4 and 2/3 neurons. using the 2-photon imaging system acquired through this grant, we have been able to image single synaptic sites and can measure the contribution of NMDARs to the postsynaptic calcium influx. We are currently working toward identifying NMDAR subunit composition in individual synapses.
Objective 4 is aimed at understanding the potential role of non-ionic NMDAR signaling at other synapse types. We have tested plasticity induction in hippocampal Schaffer collateral to CA1 synapses, and whether NMDAR pharmacology can bias plasticity towards long-term depression. This has produced mixed results that require further experiments.