Impaired Circadian Photoentrainment in Opn5-Null Mice

The master circadian pacemaker in mammals resides in the hypothalamic suprachiasmatic nuclei (SCN) and is synchronized to ambient light/dark cycles (i.e., photoentrainment). Melanopsin (Opn4) and classical rod-cone photoreceptors are believed to provide all the photic input necessary for circadian photoentrainment. Although the UVA-sensitive photopigment Opn5 is known to be expressed in retinal ganglion cells, its physiological role remains unclear and a potential role for Opn5 in the photoentrainment of the master clock has not been addressed. Here we report impaired photoentrainment and phase shifting to UVA light in Opn5-null mice. However, triple-knockout mice lacking all known functional circadian photoreceptors (i.e., rods, cones, and melanopsin) failed to entrain to UVA-light/dark cycles, despite the presence of Opn5, demonstrating that Opn5 alone is not sufficient for photoentrainment of the SCN clock. Since Opn5 is involved in the regulation of the retinal circadian clock, disrupted retinal function may cause impaired circadian photoentrainment in Opn5-null mice.