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Until now, melanopsin (OPN4) - a specialized photopigment being responsive especially to blue light wavelengths - has not been found in the human brain at protein level outside the retina. More specifically, OPN4 has only been found in about 2% of retinal ganglion cells (i.e. in intrinsically photosensitive retinal ganglion cells), and in a subtype of retinal cone-cells. Given that Allen Institute for Brain Science has described a wide distribution of OPN4 mRNA in two human brains, we aimed to investigate whether OPN4 is present in the human brain also at protein level. Western blotting and immunohistochemistry, as well as immunoelectron microscopy, were used to analyse the existence and distribution of OPN4 protein in 18 investigated areas of the human brain in samples obtained in forensic autopsies from 10 male subjects (54 ± 3.5 years). OPN4 protein expression was found in all subjects, and, furthermore, in 5 out of 10 subjects in all investigated brain areas localized in membranous compartments and cytoplasmic vesicles of neurons. To our opinion, the wide distribution of OPN4 in central areas of the human brain evokes a question whether ambient light has important straight targets in the human brain outside the retinohypothalamic tract (RHT). Further studies are, however, needed to investigate the putative physiological phototransductive actions of inborn OPN4 protein outside the RHT in the human brain.

We investigated whether transcranial bright light (TBL) affects nocturnal melatonin and cortisol secretion in sham-controlled crossover trial. Young healthy adults were exposed in random order to 24 minutes of TBL or sham exposure via ear canals at 01:10 h. Saliva and urine samples were collected hourly between 21 h-03 h and 06 h-09 h. There were no significant differences in melatonin or cortisol concentrations between TBL and sham exposures at any sampling point indicating that TBL via ear canals does not suppress nocturnal melatonin secretion. Thus, non-visual effects of TBL are mediated via a pathway not involving melatonin suppression.