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Yong-Mei Zhong, Shi-Jun Weng, Wei Zhou, Xiong-Li Yang; Orexin-A potentiates intrinsic light responses of intrinsically photosensitive retinal ganglion cells in mouse retina. Invest. Ophthalmol. Vis. Sci. 2019;60(9):5258.
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© ARVO (1962-2015); The Authors (2016-present)
The neuropeptide orexin-A is widely expressed in mammalian retina and orexin receptor 1 (OX1R) is localized to intrinsically photosensitive retinal ganglion cells (ipRGCs) that express the photopigment melanopsin and respond to light directly. However, the role of orexin-A in visual function is still largely unknown. Here we sought to investigate whether orexin-A modulates melanopsin-driven (intrinsic) light responses of ipRGC M2 subtype.
We used BAC transgenic mice in which ipRGCs were labeled with td-Tomato. Cell-attached and/or whole-cell patch-clamp recording techniques were used to record the light response and membrane potential of genetically-labeled ipRGCs in flat-mount mouse retinas. To record intrinsic light responses of ipRGCs, L-AP4, DNQX, D-AP5 were included in Ames’ medium to block rod/cone signaling.
Exogenous orexin-A (500 nM) significantly increased intrinsic light-evoked spiking rates of M2 cells, and the effect was abolished by the OX1R antagonist SB334867, but not by the OX2R antagonist TCS OX229. In contrast, SB334867 significantly decreased the intrinsic light-evoked firing rates of M2 cells, indicating that endogenous orexins also potentiate the intrinsic light responses. In complete darkness with intact synapses, orexin-A directly depolarized M2 cells and increased their spontaneous spiking rates through activation of OX1Rs. After applying L-AP4, D-AP5 and DNQX to block glutamatergic inputs to ipRGCs, along with TTX and carbenoxolone to block Na+-mediated action potentials and gap junction transmission, respectively, we found that the amplitudes of orexin-A-induced depolarization of M2 cells were not altered, suggesting that the orexin effect is independent on presynaptic inputs. Moreover, the K+ channel blocker BaCl2 or the nonselective cation channel blocker FFA partly blocked the orexin-A-induced depolarization, and co-application of BaCl2 and FFA totally eliminated the orexin-A effect on M2 cells. However, blockage of Ca2+ channel or Na+-Ca2+ exchanger did not change the depolarizing amplitudes of M2 cells.
Orexin-A directly depolarizes M2 cells through activation of OX1Rs. Dual ionic mechanisms including activation of the nonselective cation channels and block of an inward rectifier K+ channels mediate the effects of orexin-A on M2 cells.
This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.
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