Daily Profile in Melanopsin Transcripts Depends on Seasonal Lighting Conditions in the Rat Retina

Abstract

The retinal photopigment melanopsin (Opn4) mediates photoentrainment of the circadian system. In the present study, seasonal regulation of the melanopsin gene was investigated in comparison with the arylalkylamine N-acetyltransferase (AA-NAT) gene as an indicator of retinal pacemaker output. For this purpose, the daily profiles in the amount of melanopsin mRNA and AA-NAT mRNA were monitored under 8 : 16 h light/dark, 12 : 12 h light/dark and 16 : 8 h light/dark photoperiods using real-time polymerase chain reaction analysis. We found that, under all of the lighting regimes, melanopsin and AA-NAT expression oscillated with a peak around dark onset and the middle of the dark phase, respectively. The lighting regime influenced both genes, but in an opposing manner. Under long photoperiods, the duration of peak expression was prolonged for melanopsin, whereas it was shortened for AA-NAT. Under constant darkness, the rhythm of mRNA was abolished for melanopsin, but persisted for AA-NAT whereas, under constant light, the rhythm of mRNA was abolished for both genes. Our findings suggest that, in contrast to the AA-NAT gene, the daily and photoperiod-dependent regulation of the melanopsin gene does not rely on a circadian oscillator but is directly illumination-dependent.

Melanopsin Gene Variations Interact With Season to Predict Sleep Onset and Chronotype

“…Because melanopsin may be differentially expressed as a function of acute light exposure and photoperiod in rats (Hannibal, 2006; Mathes et al, 2007), it is possible that sequence variations affect melanopsin expression levels. Yet, other explanations for our results could involve the role of melanopsin in acute alerting effects of light.…”

Circadian organization of the mammalian retina: From gene regulation to physiology and diseases

“…These ganglion cell photoreceptors exhibit daily rhythms in the expression of the melanopsin pigment gene that are influenced by light and dopamine (Sakamoto et al 2004; Sakamoto et al 2005; Mathes et al 2007), suggesting that they may be clock neurons generating endogenous circadian rhythms. Indeed, there are daily rhythms in the responsiveness of ipRGCs to light (Weng et al 2009), and in the amplitude of the pupillary light response driven by these cells (Zele et al 2011), indicating circadian modulation in the function of these retinal ganglion cells.…”

Intrinsically Photosensitive Retinal Ganglion Cells

“…Under a variety of light-dark cycles, melanopsin mRNA rises at the onset of light and reaches a peak near the transition to darkness (76, 123, 170). As a consequence, melanopsin protein is higher during the day (76).…”

“…This diurnal variation in melanopsin, in addition to being dictated by light, is also under control of the circadian clock (but see Refs. 123, 170). A higher level of melanopsin expression would be expected to improve photon-capture and thus the sensitivity of ipRGCs (39), although only modest diurnal variation in the excitability of ipRGCs has been reported (223).…”