Abstract
Intrinsically photosensitive retinal ganglion cells (ipRGCs) are involved in circadian and other non-image forming visual responses. An open question is whether the activity of these neurons may also be under the regulation mediated by the neurohormone melatonin. In the present work, by double-staining immunohistochemical technique, we studied the expression of MT1 and MT2, two known subtypes of mammalian melatonin receptors, in rat ipRGCs. A single subset of retinal ganglion cells labeled by the specific antibody against melanopsin exhibited the morphology typical of M1-type ipRGCs. Immunoreactivity for both MT1 and MT2 receptors was clearly seen in the cytoplasm of all labeled ipRGCs, indicating that these two receptors were co-expressed in each of these neurons. Furthermore, labeling for both the receptors were found in neonatal M1 cells as early as the day of birth. It is therefore highly plausible that retinal melatonin may directly modulate the activity of ipRGCs, thus regulating non-image forming visual functions.
Highlights
Photosensitive retinal ganglion cells, a unique population of mammalian retinal ganglion cells (RGCs), express the novel photopigment melanopsin and signal light directly [1,2,3,4]
The morphological features of the labeled cells, as shown in both the flat-mount and the section, were typical of the M1-type Intrinsically photosensitive retinal ganglion cells (ipRGCs), which is characterized by dendritic varicosities, as well as sparsely branched dendritic arbors monostratifying at the outmost of the inner plexiform layer (IPL)
Immunoreactivity for MT2 is seen in RGCs of human and rat [25,30]
Summary
Photosensitive retinal ganglion cells (ipRGCs), a unique population of mammalian retinal ganglion cells (RGCs), express the novel photopigment melanopsin and signal light directly [1,2,3,4]. These cells send their axons to hypothalamic suprachiasmatic nucleus (SCN), a site of the master biological pacemaker, and other non-image forming (NIF) visual centers, mediating a wide variety of physiological processes, such as photoentrainment of circadian rhythms, pupillary light reflex and nocturnal suppression of pineal melatonin secretion, etc. IpRGC-controlled human post-illumination pupil responses are shown to exhibit circadian
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