title: Melanopsin is expressed in PACAP-containing retinal ganglion cells of the human retinohypothalamic tract.
abstract: OBJECTIVE
  The putative circadian photoreceptor melanopsin is found in rodents in a subpopulation of intrinsic light-sensitive retinal ganglion cells (RGCs) constituting the retinohypothalamic tract (RHT). The study was conducted to determine whether melanopsin is expressed in the human retina and costored with the neuropeptide pituitary adenylate cyclase activating polypeptide (PACAP), a marker for the RHT, projecting to the suprachiasmatic nucleus (SCN). Furthermore, whether melanopsin expression is conserved in retinas of blind patients with severe retinal degeneration was investigated.
  
  
  METHODS
  In situ hybridization and immunohistochemistry was used to demonstrate melanopsin synthesis in human eyes of 17 donors and two postmortem hypothalami containing the SCN. The coexistence of melanopsin and PACAP in elements of the retinohypothalamic tract was studied by dual-labeling immunocytochemistry.
  
  
  RESULTS
  Melanopsin expression was found in a subpopulation of RGCs located in the ganglion cell layer and displaced in the inner nuclear cell layer. Melanopsin-containing cells comprised approximately 0. 8% of all RGCs, with a distinct morphology characterized by two to four dendritic processes constituting a panretinal network. Melanopsin immunoreactivity was primary present at perikaryal boundaries and neuronal processes and to some extent also in the cytoplasm. PACAP and melanopsin were colocalized in the RGCs and PACAP-containing nerve fibers, seemingly innervating the retinorecipient part of the SCN. Melanopsin-expressing RGCs were conserved in retinas of blind patients with severe degeneration of the outer and/or inner layers.
  
  
  CONCLUSIONS
  Given the expression of melanopsin in PACAP-containing RGCs of the human RHT, this photoreceptor is a likely first base in the chain of events leading to photoentrainment of both normal and blind people.
authors.affiliation: Department of Clinical Biochemistry, Bispebjerg Hospital, University of Copenhagen, Denmark. j.hannibal@inet.uni2.dk
authors.forename: Jens
authors.initials: J
authors.lastname: Hannibal
authors.forename: Peter
authors.initials: P
authors.lastname: Hindersson
authors.forename: Jens
authors.initials: J
authors.lastname: Ostergaard
authors.forename: Birgitte
authors.initials: B
authors.lastname: Georg
authors.forename: Steffen
authors.initials: S
authors.lastname: Heegaard
authors.forename: Philip Just
authors.initials: PJ
authors.lastname: Larsen
authors.forename: Jan
authors.initials: J
authors.lastname: Fahrenkrug
mesh_terms: D000368:Aged; D000369:Aged, 80 and over; D000818:Animals; D001766:Blindness / Q000209:etiology / Q000378:metabolism; D003001:Cloning, Molecular; D005260:Female; D006801:Humans; D007124:Immunoenzyme Techniques; D017403:In Situ Hybridization; D008297:Male; D018613:Microscopy, Confocal; D009479:Neuropeptides / Q000378:metabolism*; D051219:Pituitary Adenylate Cyclase-Activating Polypeptide; D011817:Rabbits; D012162:Retinal Degeneration / Q000150:complications / Q000378:metabolism*; D012165:Retinal Ganglion Cells / Q000378:metabolism*; D017299:Rod Opsins / Q000378:metabolism*; D013493:Suprachiasmatic Nucleus / Q000378:metabolism*; D000099210:Melanopsin
chemical_list: D009479:Neuropeptides; D051219:Pituitary Adenylate Cyclase-Activating Polypeptide; D017299:Rod Opsins; D000099210:Melanopsin; C495401:ADCYAP1 protein, human