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 Ca(2+) concentrations (YC3.6). We observed concentration-dependent increases in cytosolic Ca(2+) concentrations after treatment with histamine (1–100 µM) and complete suppression of histamine-induced Ca(2+) mobilizations by H(1) histamine receptor (H(1)R) antagonist d-chlorpheniramine (d-CPA) in hRPE-YC cells. Consistently, real-time RT-PCR assays revealed that H(1)R showed the highest expression among the four subtypes (H(1)–H(4)) of histamine receptors in hRPE-YC cells. Stimulation of hRPE-YC cells with histamine transiently increased nuclear localization of phosphorylated Ca(2+)/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 H(1)R antagonist, ketotifen, blocked the histamine-induced phase shifts. Furthermore, an H(2) 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 H(1)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.