Abstract
Purpose :
The organization of synaptic inputs from bipolar cells (BCs) to starburst amacrine cells (SACs) is thought to play an important role in generating directional computation in SAC dendrites. However, the functional circuitry and spatiotemporal dynamics of BC transmission to SAC remain to be directly examined at the synaptic level and in a BC type-specific manner. In this study, we aimed to characterize synaptic transmission from individually identified cone bipolar cell (CBC) types to SAC dendrites using dual patch clamp and glutamate imaging in the wholemount mouse retina.
Methods :
Whole-cell patch recordings were made from pairs of morphologically and physiologically identified CBC types and SACs in wholemount retinas of ChAT-Cre/Ai162 or ChAT-Cre/Ai14 mice. Glutamate release from single CBC axonal terminals were analyzed under simultaneous patch clamp and two-photon imaging with iGluSnFR, which was expressed in the retina via intraocular injection of AAV2/2-CAG-SF-iGluSnFR.A184S.
Results :
CBC types were identified based on their axonal and dendritic morphology, as well as their intrinsic and light response physiology. Postsynaptic excitatory currents were detected in SACs in response to voltage stimulation of single BCs of various types located at various dendritic distances from SAC soma. The postsynaptic currents showed fast/transient and delayed/sustained components, with relative amplitudes that were functions of CBC type, SAC dendritic location, and gap junction coupling strength. Two-photon imaging detected glutamate release from axon terminals of individually voltage-clamped CBCs. Activation of different CBC types produced different spatiotemporal patterns of glutamate signals, ranging from transient glutamate “hot spots” to sustained glutamate “clouds”, depending on stimulation strength and duration, as well as gap-junction coupling strength.
Conclusions :
The results provided direct functional characterization of synaptic transmission from specific CBC types to SAC dendrites and revealed chemical and electrical synaptic organizations shaping spatiotemporal response properties of SAC dendrites.
This abstract was presented at the 2023 ARVO Annual Meeting, held in New Orleans, LA, April 23-27, 2023.