title: Wiring the retinal circuits activated by light during early development.
abstract: BACKGROUND
  Light information is sorted by neuronal circuits to generate image-forming (IF) (interpretation and tracking of visual objects and patterns) and non-image-forming (NIF) tasks. Among the NIF tasks, photic entrainment of circadian rhythms, the pupillary light reflex, and sleep are all associated with physiological responses, mediated mainly by a small group of melanopsin-expressing retinal ganglion cells (mRGCs). Using Xenopus laevis as a model system, and analyzing the c-fos expression induced by light as a surrogate marker of neural activity, we aimed to establish the developmental time at which the cells participating in both systems come on-line in the retina.
  
  
  RESULTS
  We found that the peripheral retina contains 80% of the two melanopsin-expressing cell types we identified in Xenopus: melanopsin-expressing horizontal cells (mHCs; opn4m+/opn4x+/Prox1+) and mRGCs (2.7% of the total RGCs; opn4m+/opn4x+/Pax6+/Isl1), in a ratio of 6:1. Only mRGCs induced c-fos expression in response to light. Dopaminergic (tyrosine hydroxylase-positive; TH+) amacrine cells (ACs) may be part of the melanopsin-mediated circuit, as shown by preferential c-fos induction by blue light. In the central retina, two cell types in the inner nuclear layer (INL) showed light-mediated induction of c-fos expression [(On-bipolar cells (Otx2+/Isl1+), and a sub-population of ACs (Pax6-/Isl1-)], as well as two RGC sub-populations (Isl1+/Pax6+ and Isl1+/Pax6-). Melanopsin and opsin expression turned on a day before the point at which c-fos expression could first be activated by light (Stage 37/38), in cells of both the classic vision circuit, and those that participate in the retinal component of the NIF circuit. Key to the classic vision circuit is that the component cells engage from the beginning as functional 'unit circuits' of two to three cells in the INL for every RGC, with subsequent growth of the vision circuit occurring by the wiring in of more units.
  
  
  CONCLUSIONS
  We identified melanopsin-expressing cells and specific cell types in the INL and the RGC layer which induce c-fos expression in response to light, and we determined the developmental time when they become active. We suggest an initial formulation of retinal circuits corresponding to the classic vision pathway and melanopsin-mediated circuits to which they may contribute.
authors: Gabriel E
authors: GE
authors: Bertolesi
authors: Carrie L
authors: CL
authors: Hehr
authors: Department of Cell Biology and Anatomy, Hotchkiss Brain Institute, University of Calgary, 3330 Hospital Dr, NW, Health Sciences Building, Room 2164, Calgary AB T2N4N1, Canada. smcfarla@ucalgary.ca.
authors: Sarah
authors: S
authors: McFarlane
mesh_terms: D025042:Amacrine Cells / Q000502:physiology; D000818:Animals; D008027:Light; D010775:Photic Stimulation; D016760:Proto-Oncogene Proteins c-fos / Q000378:metabolism; D012160:Retina / Q000166:cytology / Q000196:embryology* / Q000378:metabolism / Q000528:radiation effects; D051245:Retinal Bipolar Cells / Q000502:physiology; D012165:Retinal Ganglion Cells / Q000502:physiology; D055351:Retinal Neurons / Q000166:cytology / Q000378:metabolism / Q000502:physiology*; D017299:Rod Opsins / Q000378:metabolism; D014446:Tyrosine 3-Monooxygenase / Q000378:metabolism; D014982:Xenopus laevis; D000099210:Melanopsin
chemical_list: D016760:Proto-Oncogene Proteins c-fos; D017299:Rod Opsins; D014446:Tyrosine 3-Monooxygenase; D000099210:Melanopsin