PK2 mRNA rhythm in the SCN of melanopsin-deficient ( Opn4-/- ) mice and triple knockout mice ( Opn4-/- Gnat1-/- Cnga3-/- mice). (A) Temporal profiles of PK2 mRNA rhythm in wildtype (filled squares) and Opn4-/- mice (open triangles) under LD. Open and filled bars indicate light and dark periods, respectively. Each value represents the mean ± SEM of 3–4 animals. Two-way ANOVA indicated that there is no significant difference between genotypes. (B) Light pulse-induced PK2 mRNA expression in wildtype (shaded bars) and Opn4-/- mice (filled bars). PK2 mRNA was measured one and two hours after brief light pulse at ZT14. Each value represents the mean ± SEM of 6–8 animals. *p < 0.05, **p < 0.01, Student's t-test. (C) Light-pulse induced PK2 mRNA expression in triple knockout mice that lack melanopsin, rod and cone photoreceptive system (Opn4-/- Gnat1-/- Cnga3-/- mice). Dark controls received no light pulse. Each value represents the mean ± SEM of 3 animals. (D) PK2 mRNA expression in triple knockout mice at circadian time (CT) 3 and 15.