AMPA and NMDA receptor localization and recruitment in a central circuit of the mammalian retina
Date
2024
Authors
Chundekkad, Pavitra
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Abstract
Retinal direction-selective ganglion cells (DSGC) have the ability to encode direction of moving objects over a wide range of contrasts. In part, the DSGC response is shaped by graded excitatory information mediated by glutamatergic bipolar cells, via specialized ribbon synapses. Glutamate signals to DSGCs are known to be mediated via two types of receptors: AMPA and NMDA receptors (AMPARs & NMDARs, respectively). The main aim of my thesis was to investigate how AMPARs and NMDARs are distributed at the sub-cellular and synaptic levels, in an effort to understand how DSGCs encode responses over a range of conditions.
My research employed patch-clamp electrophysiology and pharmacological techniques to examine the voltage-dependence and contrast-dependence of glutamate input to DSGCs arising from presynaptic bipolar cells. First, I observed that the proportion of AMPARs and NMDARs across the DSGC dendritic arbor appeared constant when probed with small light spots. Interestingly, however, NMDARs and AMPARs were recruited over distinct contrast ranges. NMDARs primarily mediated responses to low contrast stimuli, while AMPARs were recruited at higher contrast levels. This indicated that AMPARs and NMDARs do not necessarily reside within the same bipolar cell synapses.
I also observed that NMDARs measured in pharmacological isolation could encode a wide range of stimulus contrast. Thus, the graded glutamate signals from bipolar cells appear to be effectively detected by NMDARs present on DSGCs. This was somewhat surprising because NMDARs are generally prone to saturation owing to their high affinity for glutamate. Furthermore, pharmacological investigation revealed that GluN2A and GluN2B subtypes of NMDARs contribute equally to the light-evoked responses, over the entire range of contrasts. Interestingly, currents triggered by spontaneous vesicle release from bipolar cells, which indicate the involvement of receptors directly opposite the release site, were almost entirely inhibited by the GluN2A antagonist. These findings point to a distinct localization of GluN2A and GluN2B in synaptic and extrasynaptic regions, respectively.
My results suggest that NMDARs encode a large range of stimulus contrasts and are able to evade saturation by employing distinct bipolar cell types as well as incorporating a combination of GluN2A and GluN2B receptors positioned at specific distances from the vesicle release site. Together, my work provides novel insights into how AMPARs and NMDARs contribute to the ability of DSGCs to respond to diverse stimuli.