The rat retina has five types of ganglion-cell photoreceptors

Dec 3, 2014Experimental eye research

Five types of light-sensitive nerve cells in the rat retina

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Abstract

The rat possesses five types of intrinsically photosensitive retinal ganglion cells (ipRGCs), matching those found in mice.

All rat ipRGCs likely use melanopsin as their photopigment.
Rat ipRGCs display diverse spontaneous spike rates, with the M1 type spiking the least and M4 type the most, similar to mouse ipRGCs.
Both rat and mouse ipRGCs generate sluggish intrinsic photoresponses as well as fast, synaptically driven responses.
Rat M1 cells have more transient synaptic photoresponses compared to the more sustained responses of rat M2-M5 cells.
The difference in photoresponse characteristics between rat and mouse ipRGCs may explain variations in photoentrainment thresholds across species.
Rat ipRGCs exhibit three classifications of melanopsin-based spiking photoresponses, while only two types are found in mice.

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Intrinsically photosensitive retinal ganglion cells (ipRGCs) are inner retinal photoreceptors that mediate non-image-forming visual functions, e.g. pupillary constriction, regulation of pineal melatonin release, and circadian photoentrainment. Five types of ipRGCs were recently discovered in mouse, but whether they exist in other mammals remained unknown. We report that the rat also has five types of ipRGCs, whose morphologies match those of mouse ipRGCs; this is the first demonstration of all five cell types in a non-mouse species. Through immunostaining and λmax measurements, we showed that melanopsin is likely the photopigment of all rat ipRGCs. The various cell types exhibited diverse spontaneous spike rates, with the M1 type spiking the least and M4 spiking the most, just like we had observed for their mouse counterparts. Also similar to mouse, all ipRGCs in rat generated not only sluggish intrinsic photoresponses but also fast, synaptically driven ones. However, we noticed two significant differences between these species. First, whereas we learned previously that all mouse ipRGCs had equally sustained synaptic light responses, rat M1 cells' synaptic photoresponses were far more transient than those of M2-M5. Since M1 cells provide all input to the circadian clock, this rat-versus-mouse discrepancy could explain the difference in photoentrainment threshold between mouse and other species. Second, rat ipRGCs' melanopsin-based spiking photoresponses could be classified into three varieties, but only two were discerned for mouse ipRGCs. This correlation of spiking photoresponses with cell types will help researchers classify ipRGCs in multielectrode-array (MEA) spike recordings.

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The rat retina has five types of ganglion-cell photoreceptors

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