Objective To investigate the effects of circadian rhythm changes on the expression of opsin photopigment melanopsin in the retinsa. Methods14-day old C57BL/6J mice were randomly divided into experimental group and control group. The experimental group was maintained in continuous light. The control group was maintained in a 12:12 h light:dark cycle. Fluorescence immunocytochemistry and RT-PCR assays were used to detect the expressions of melanopsin at 1 week and 8 weeks after the light exposure. ResultsThe results of fluorescence immunocytochemistry showed that opsin melanopsin mainly expressed in the retinal ganglion cell layer and little in the inner nuclear layer. The total amount of melanopsin cells in the experimental group were less than that in the control group 1 week later. RT-PCR indicated that the total amount of melanopsin mRNA in the experimental group was less than that in the control group (P<0.01). ConclusionsConstant lighting down-regulates the expression of melanopsin. Melanopsin-expression ganglion cells are photosensitive and may play an important role in the maintenance of normal circadian rhythm.