Histamine Regulates Molecular Clock Oscillations in Human Retinal Pigment Epithelial Cells via H 1 Receptors.

Vertebrate eyes are known to contain circadian clocks, but their regulatory mechanisms remain largely unknown. To address this, we used a cell line from human retinal pigment epithelium (hRPE-YC) with stable coexpression of reporters for molecular clock oscillations ( Bmal1-luciferase ) and intracellular Ca2+ concentrations ( YC3.6 ). We observed concentration-dependent increases in cytosolic Ca2+ concentrations after treatment with histamine (1-100 µM) and complete suppression of histamine-induced Ca2+ mobilizations by H1 histamine receptor (H1 R) antagonist d -chlorpheniramine ( d -CPA) in hRPE-YC cells. Consistently, real-time RT-PCR assays revealed that H1 R showed the highest expression among the four subtypes (H1 -H4 ) of histamine receptors in hRPE-YC cells. Stimulation of hRPE-YC cells with histamine transiently increased nuclear localization of phosphorylated Ca2+ /cAMP-response element-binding protein that regulates clock gene transcriptions. Administration of histamine also shifted the Bmal1-luciferase rhythms with a type-1 phase-response curve, similar to previous results with carbachol stimulations. Treatment of hRPE-YC cells with d -CPA or with more specific H1 R antagonist, ketotifen, blocked the histamine-induced phase shifts. Furthermore, an H2 histamine receptor agonist, amthamine, had little effect on the Bmal1-luciferase rhythms. Although the function of the in vivo histaminergic system within the eye remains obscure, the present results suggest histaminergic control of the molecular clock via H1 R in retinal pigment epithelial cells. Also, since d -CPA and ketotifen have been widely used (e.g., to treat allergy and inflammation) in our daily life and thus raise a possible cause for circadian rhythm disorders by improper use of antihistamines.