Synaptic inputs to retinal ganglion cells that set the circadian clock

Jorge Alberto Perez-Leon, Erin J. Warren, Charles N. Allen, David W. Robinson, R. Lane Brown

Research output: Contribution to journalArticlepeer-review

83 Scopus citations


Melanopsin-containing retinal ganglion cells (RGCs) project to the suprachiasmatic nuclei (SCN) and mediate photoentrainment of the circadian system. Melanopsin is a novel retinal-based photopigment that renders these cells intrinsically photosensitive (ip). Although genetic ablation of melanopsin abolishes the intrinsic light response, it has a surprisingly minor effect on circadian photoentrainment. This and other non-visual responses to light are lost only when the melanopsin deficiency is coupled with mutations that disable classical rod and cone photoreceptors, suggesting that melanopsin-containing RGCs also receive rod- and cone-driven synaptic inputs. Using whole-cell patch-clamp recording, we demonstrate that light triggers synaptic currents in ipRGCs via activation of ionotropic glutamate and γ-aminobutyric acid (GABA) receptors. Miniature postsynaptic currents (mPSCs) were clearly observed in ipRGCs, although they were less robust and were seen less frequently than those seen in non-ip cells. Pharmacological treatments revealed that the majority of ipRGCs receive excitatory glutamatergic inputs that were blocked by DNQX and/or kynurenic acid, as well as inhibitory GABAergic inputs that were blocked by bicuculline. Other ipRGCs received either glutamatergic or GABAergic inputs nearly exclusively. Although strychnine (Strych)-sensitive mPSCs were evident on many non-ipRGCs, indicating the presence of glycinergic inputs, we saw no evidence of Strych-sensitive events in ipRGCs. Based on these results, it is clear that SCN-projecting RGCs can respond to light both via an intrinsic melanopsin-based signaling cascade and via a synaptic pathway driven by classical rod and/or cone photoreceptors. It remains to be determined how the ipRGCs integrate these temporally distinct inputs to generate the signals that mediate circadian photoentrainment and other non-visual responses to light.

Original languageEnglish (US)
Pages (from-to)1117-1123
Number of pages7
JournalEuropean Journal of Neuroscience
Issue number4
StatePublished - Aug 2006


  • Circadian rhythms
  • Intrinsic light response
  • Melanopsin
  • Photoentrainment
  • Photoreceptor
  • Postsynaptic current

ASJC Scopus subject areas

  • General Neuroscience


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