Identifying specific RGC types may shed light on their idiosyncratic responses to neuroprotection.

Abstract

Retinal ganglion cells (RGCs) are located in the innermost layer of the retina and are the only retinal output neurons, conveying light information to the main retinorecipient target regions of the brain responsible for the image- and non-image-forming visual functions. There are well over twenty RGC types, each with its own dendritic morphology and physiological characteristics, target territories and visual functions. The study of the responses of RGCs to various injury-induced or inherited retinal degenerations requires the specific identification of these neuron types. RGCs share their anatomical location in the RGC layer with the displaced amacrine cells, a population as numerous as the population of RGCs itself, with overlapping soma sizes. Consequently, classical morphological methods do not allow distinction between these two classes of retinal neurons. Up to now, RGCs have been identified mainly with retrograde tracers applied to their principal retinorecipient target nuclei in the brain, the superior colliculi (SCi), or applied to their main axonal output, the optic nerve (ON). These methods, although the most efficient to label the great majority of RGCs, do not distinguish between different RGC types. Retrogradely transported tracers applied to the ON result in massive retrograde labelling of RGCs, while tracer application to the SCi labels most RGCs projecting to these nuclei as well as to neighbouring areas. Indeed, injection into the optic tract or tracer application over the SCi may result in spurious labelling of neurons located in neighbouring areas (Vidal-Sanz et al., 2002; Valiente-Soriano et al., 2014).