Melanopsin in the circadian timing system

J Mol Neurosci. 2003;21(1):73-89. doi: 10.1385/JMN:21:1:73.


In mammals, circadian rhythms are generated by a light-entrainable oscillator located in the hypothalamic suprachiasmatic nucleus (SCN). Light signals reach the SCN via a dedicated retinal pathway, the retinohypothalamic tract (RHT). One question that continues to elude scientists is whether the circadian system has its own dedicated photoreceptor or photoreceptors. It is well established that conventional photoreceptors, rods and cones, are not required for circadian photoreception, suggesting that the inner retinal layer might contribute to circadian photoreception. Melanopsin, a novel photo pigment expressed in retinal ganglion cells (RGCs), has been proposed recently as a candidate circadian photoreceptor. Melanopsin-containing RGCs are intrinsically photosensitive, form part of the RHT, and contain neurotransmitters known to play a critical role in the circadian response to light. Furthermore, melanopsin-containing RGCs do not depend on inputs from rods and cones to transmit light signals to the SCN. However, based on a review of the available information about melanopsin and on new data from our laboratory, we propose that melanopsin, in itself, is not necessary for circadian photoreception. In fact, it appears that of the known photoreceptor systems, none, in and of itself, is necessary for circadian photoreception. Instead, it appears that within the photoreceptive systems there is some degree of redundancy, each contributing in some way to photic entrainment.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Animals, Newborn
  • Antibodies, Monoclonal / pharmacology
  • Circadian Rhythm / drug effects
  • Circadian Rhythm / physiology*
  • Denervation
  • Eye Enucleation
  • Immunohistochemistry
  • Immunotoxins / pharmacology
  • Light Signal Transduction / drug effects
  • Light Signal Transduction / physiology
  • Male
  • N-Glycosyl Hydrolases
  • Neurotoxins / pharmacology
  • Photic Stimulation / methods
  • Rats
  • Rats, Wistar
  • Receptor, Nerve Growth Factor / drug effects
  • Receptor, Nerve Growth Factor / metabolism
  • Retina / cytology
  • Retina / drug effects
  • Retina / metabolism*
  • Retinal Ganglion Cells / drug effects
  • Ribosome Inactivating Proteins, Type 1
  • Rod Opsins / metabolism*
  • Saporins
  • Sensory Deprivation
  • Sodium Glutamate
  • Suprachiasmatic Nucleus / cytology
  • Suprachiasmatic Nucleus / drug effects
  • Suprachiasmatic Nucleus / metabolism*
  • Visual Pathways / cytology
  • Visual Pathways / drug effects
  • Visual Pathways / metabolism


  • 192 IgG-saporin
  • Antibodies, Monoclonal
  • Immunotoxins
  • Neurotoxins
  • Receptor, Nerve Growth Factor
  • Ribosome Inactivating Proteins, Type 1
  • Rod Opsins
  • melanopsin
  • N-Glycosyl Hydrolases
  • Saporins
  • Sodium Glutamate