In the mammalian retina, melanopsin is expressed by a subset of intrinsically photosensitive retinal ganglion cells (ipRGCs), which also receive synaptic input from rods and cones ( Dacey et al., 2005 Schmidt et al., 2008 Weng et al., 2013). As such, this study provides the first evidence for a population of melanopsin interneurons in the mammalian retina. In the CMZ, between 20% (pigmented) and 24% (albino) of the melanopsin positive cells are unlabeled by the tracer and we suggest that this may be because they fail to send an axon into the ON. Rather surprisingly, application of a retrograde tracer to the optic nerve (ON) labels all ipRGCs, except for a sub-population of the d-ipRGCs (14% in pigmented and 28% in albino, respectively) and melanopsin positive cells residing in the ciliary marginal zone (CMZ) of the retina. Tracing from both superior colliculli shows that 98% (pigmented) and 97% (albino) of the total ipRGCs, become retrogradely labeled, while double immunodetection of melanopsin and Brn3a confirms that few ipRGCs express this transcription factor in mice. Both strains also contain displaced ipRGCs (d-ipRGCs) in the inner nuclear layer (INL) that account for 14% of total ipRGCs in pigmented mice and 5% in albinos. In both strains, ipRGCs are located in the retinal periphery, in the areas of lower Brn3a +RGC density. Our data show that although pigmented (C57Bl/6) and albino (Swiss) mice have a similar total number of ipRGCs, their distribution is slightly different: while in pigmented mice ipRGCs are more abundant in the temporal retina, in albinos the ipRGCs are more abundant in superior retina. Here we have studied the population of intrinsically photosensitive retinal ganglion cells (ipRGCs) in adult pigmented and albino mice. 2Department of Ocular Biology and Therapeutics, UCL-Institute of Ophthalmology, London, UK.1Departamento de Oftalmología, Facultad de Medicina, Campus de Espinardo, Universidad de Murcia, e Instituto Murciano de Investigación Biosanitaria-Hospital Clínico Universitario Virgen de la Arrixaca (IMIB-ARRIXACA), Murcia, Spain. Valiente-Soriano 1†, Diego García-Ayuso 1†, Arturo Ortín-Martínez 1†, Manuel Jiménez-López 1, Caridad Galindo-Romero 1, Maria Paz Villegas-Pérez 1, Marta Agudo-Barriuso 1, Anthony A.
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