Periodic Reporting for period 2 - NovelNMDA (Novel NMDA receptor signaling in cortical synaptic depression)
Période du rapport: 2020-08-01 au 2022-01-31
Neurons communicate primarily through fast chemical synapses, where an active presynaptic neuron releases neurotransmitter that is detected by a postsynaptic cell to regenerate electrical signals. In the mammalian brain, glutamate is the principle excitatory neurotransmitter. The NMDA receptor (NMDAR), a glutamate-gated ion channel, is essential for many forms of synaptic plasticity, including long-term depression. This synaptic plasticity is essential for experience-dependent changes in neuronal connectivity, and is thought to underlie the formation of receptive field properties over development. Understanding how these plastic properties of synapses arise can help us understand how the developmental refinement of cortical representations of sensory information occurs, and whether it can be manipulated.
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.
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.
This ERC starting grant provided an opportunity to establish my laboratory at the European Neuroscience Institute Göttingen, and acquire the necessary equipment to complete this research project. We have begun work on all objectives, however, our main focus has been on establishing the experimental paradigm relating to long-term depression at cortical l4-l2/3 synapses, and establishing imaging techniques to assay single synapse function before and after plasticity induction.
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.
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.
During the work towards this project, we noticed a short-term plasticity phenomenon arising from both facilitation and depression processes. This was beyond our expectations, as the cortical layer 4 to 2/3 synapse has been traditionally thought of as a high release probability, depressing synapse. We further found that the short-term dynamics were due to seemingly independent release properties which arose from single synaptic sites. This synaptic property may be an important function for sensory cortical processing, acting as high-fidelity transmitter as well as a high-pass filter. This finding also feeds back into our long-term plasticity experimental work, as these short-term dynamic properties may be differentially altered by long-term plasticity mechanisms.