Abstract
In addition to some other functions, melanopsin-expressing retinal ganglion cells (RGCs) constitute the principal mediators of the circadian photoentrainment, a process by which the suprachiasmatic nucleus (the central clock of mammals), adjusts daily to the external day/night cycle. In the present study these RGCs were immunohistochemically labelled using a specific polyclonal antiserum raised against mouse melanopsin. A daily oscillation in the number of immunostained cells was detected in mice kept under a light / dark (LD) cycle. One hour before the lights were on (i.e., the end of the night period) the highest number of immunopositive cells was detected while the lowest was seen 4 h later (i.e., within the first hours of the light period). This finding suggests that some of the melanopsin-expressing RGCs “turn on” and “off” during the day/night cycle. We have also detected that these daily variations already occur in the early postnatal development, when the rod/cone photoreceptor system is not yet functional. Two main melanopsin-expressing cell subpopulations could be found within the retina: M1 cells showed robust dendritic arborization within the OFF sublamina of the inner plexiform layer (IPL), whilst M2 cells had fine dendritic processes within the ON sublamina of the IPL. These two cell subpopulations also showed different daily oscillations throughout the LD cycle. In order to find out whether or not the melanopsin rhythm was endogenous, other mice were maintained in constant darkness for 6 days. Under these conditions, no defined rhythm was detected, which suggests that the daily oscillation detected either is light-dependent or is gradually lost under constant conditions. This is the first study to analyze immunohistochemically the daily oscillation of the number of melanopsin-expressing cells in the mouse retina.
Highlights
INTRODUCTIONCircadian rhythms are oscillations with a period of about 24 h
Circadian rhythms are oscillations with a period of about 24 h.Animals express these rhythms in their behavior and their physiology
Other organs and cells oscillate because they are governed by primary autonomous oscillators or as a response to environmental fluctuations
Summary
Circadian rhythms are oscillations with a period of about 24 h. The perception of the external day/night cycle, which is perhaps the most important function of the so-called non-imageforming visual system, is mediated by rod/cone photoreceptors and by intrinsically photosensitive retinal ganglion cells (ipRGCs) whose phototransduction is based on the photopigment melanopsin (Berson et al, 2002; Hattar et al, 2003). Sakamoto et al (2004, 2005) demonstrated that melanopsin mRNA shows a daily oscillating pattern, with a peak in the transitional phase from day to night, whilst the minimum was observed at the end of the night This rhythm appears to be influenced by rod/cone inputs, since melanopsin expression in rats with retinal degeneration, lacking rod and cone photoreceptors, is lower than in controls and arrhythmic (Sakamoto et al, 2004). In the present work we have analyzed for the first time the daily variation of the number of the mouse melanopsin-expressing ipRGCs and of their main subpopulations, by means of immunohistochemistry, as well as the effects of darkness on such variation
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