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T. M. Schmidt, P. Kofuji; Differential Influence of the on Pathway on the Light-Evoked and Resting Properties of Distinct Subpopulations of Intrinsically Photosensitive Retinal Ganglion Cells. Invest. Ophthalmol. Vis. Sci. 2010;51(13):5179.
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© ARVO (1962-2015); The Authors (2016-present)
Intrinsically photosensitive retinal ganglion cells (ipRGCs) express the photopigment melanopsin and comprise a distinct subset of ganglion cells in the mammalian retina. At least three morphological subtypes of ipRGC have now been described: those with monostratified dendritic arbors in either the outer (M1) or inner (M2) inner plexiform layer (IPL), and those with dendritic arbors bistratifying in both the inner and outer IPL. These morphological subtypes are also physiologically distinct, with M1 cells having a larger and more sensitive intrinsic light response, a more depolarized resting membrane potential, and a larger input resistance compared to M2 cells. Furthermore, research now indicates that the outer-stratifying M1 cells receive functional synapses from On cone bipolar cells and are thus influenced by the On pathway. However this question has not been examined for M2 cells nor have the functional consequences of this On channel influence been described. The goal of the current study was to determine the functional consequences of synaptic inputs from the On pathway to M1 and M2 ipRGCs.
We utilized a transgenic mouse line in which ipRGCs are labeled in vivo with EGFP to target ipRGCs for whole cell recording and dye-filling.
We observed light-evoked and basal synaptic inputs from the ON pathway to both M1 and M2 cells. M1 and M2 cells responded to saturating white light stimulus with large, fast depolarizations. Upon L-AP4 application, the latency for the light-evoked responses of M2 cells increased while for the M1 cells it was only slightly affected. The magnitude of light-evoked responses of M2 but not M1 cells also became significantly smaller in the presence of bath L-AP4. In the dark, L-AP4 application resulted in significant hyperpolarization of M2 cells but slight depolarization of M1 cells.
These data indicate that while both M1 and M2 cells receive functional synaptic inputs from the On pathway, these inputs are much more influential in shaping the light-evoked signaling of M2 cells. Furthermore, the On pathway differentially affects M1 and M2 cell resting properties in the dark.
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