Impact of melatonin receptors on pCREB and clock-gene protein levels in the murine retina.

In several mammalian species, the retina is capable of synthesizing melatonin and contains an autonomous circadian clock that relies on interlocking transcriptional/translational feedback loops involving several clock genes, such as Per1 and Cry2. Our previous investigations have shown remarkable differences in retinae of melatonin-deficient (C57BL) and melatonin-proficient (C3H) mice with regard to the protein levels of PER1, CRY2, and phosphorylated (p) cyclic AMP response element binding protein (CREB). To elucidate the melatonin receptor type possibly responsible for these differences, we have performed immunocytochemical analyses for PER1, CRY2, and pCREB in retinae of melatonin-proficient wild type (WT) mice and mice with targeted deletions of the MT1 receptor (MelaaBB) or the MT1 and MT2 receptors (Melaabb) at four different time points. Immunoreactions for PER1, CRY2 and pCREB were localized to the nuclei of cells in the inner nuclear layer (INL) and ganglion cell layer (GC) of all strains. Surprisingly, in MelaaBB and Melaabb the day/night rhythm of pCREB, PER1, and CRY2 levels was not abolished, but the maxima and minima of PER1 were 180 degrees out of phase as compared to the WT. These data suggest that MT1 and MT2 melatonin receptors are not necessary to maintain rhythmic changes in clock-gene protein levels in the murine retina, but, as shown for PER1, appear to be involved in internal synchronization.