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D.-Q. Zhang, P. J. Sollars, G. E. Pickard, D. G. McMahon; Signaling by Ganglion-Cell photoreceptors to Dopaminergic Amacrine Cells Requires the Photopigment Melanopsin and AMPA-Type Glutamate Receptors. Invest. Ophthalmol. Vis. Sci. 2010;51(13):1206.
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© ARVO (1962-2015); The Authors (2016-present)
Our previous study strongly suggested that intrinsically photosensitive retinal ganglion cells (ipRGCs) drive a subclass of dopaminergic amacrine (DA) cells, providing a novel intra-retinal pathway that allows intrinsic melanopsin-mediated photopic information to flow from the inner retina to the outer retina through the dopaminergic system (Zhang et al., 2008). Here, we sought to determine whether melanopsin gene knockout abolishes sustained photosensitive responses of DA neurons thought to be driven by ipRGCs. The mechanisms which mediate signal transmission from ipRGCs to DA neurons were also determined.
Both loose patch and whole-cell patch-clamp recordings were performed on tyrosine hydroxylase (TH) driven red fluorescent protein (RFP) labeled DA neurons in flat mount retinas of wild-type and melanopsin knockout (Mel-KO) mice (Hattar et al., 2003).
Twenty-five light responsive DA neurons were recorded in Mel-KO retinas. All of them exhibited transient kinetic and terminated at light offset. 50 µM L-AP4, an ON channel blocker, applied to 13 DA neurons in Mel-KO retinas and it completely abolished all cells’ light-induced responses, suggesting that DA neurons in Mel-KO mice are excited purely through ON bipolar cells. Second, we determined whether the photopic information from ipRGCs could pass through ON bipolar cells via electrical synapses to excite DA neurons, because both DA neurons and ipRGCs have contacts with a subpopulation of ON bipolar cells in the OFF layer of the inner plexiform layer (Hoshi et al., 2009; Dumitrescu et al., 2009). Six ipRGC driven DA neurons in wild-type retinas were tested in the presence of 100 µM meclofenamic acid, a gap junction blocker, and light responses from all of them remained unchanged. Finally, we examined the glutamate receptor mediated postsynaptic mechanism of ipRGCs drive to DA neurons. 200 µM GYKI-52446, an AMPA receptor antagonist, completely eliminated ipRGC driven responses in five DA neurons tested.
Our results show that signaling by ganglion-cell photoreceptors to dopaminergic amacrine cells requires the photopigment melanopsin, and that ipRGCs drive DA neurons via activation of AMPA-type glutamate receptors. Future studies will determine physiological function of ipRGC driven activity of DA neurons upon light stimuli.
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