Circadian Norepinephrine Rhythm Regulates Outflow Facility and IOP in Rat Eyes

Purpose: This study sought to investigate the role of norepinephrine (NE) in circadian rhythms of rat IOP and conventional outflow facility (C). Methods: Sympathetic nerves and vascular structures in anterior segment sections of 10 Brown-Norway rat eyes were immunolabelled for tyrosine hydroxylase and CD31. The aqueous NE concentration, IOP, and C were respectively measured by ELISA, tonometry, and constant-pressure perfusion in eight rats during subjective day (SD) in light and dark and subjective night (SN) before and after tetrodotoxin instillation. Measurement relationships were evaluated using regression and mediation analyses. Distal drainage vessel (DV) sizes were quantified for six rats from optical coherence tomography images acquired during SD and SN. Results: Tyrosine hydroxylase-positive fibers were observed in the iris, ciliary body, and along DVs but not in the trabecular meshwork region. The aqueous NE concentration was higher during SN (2.0 ± 0.7 ng/mL) than SD in light and dark (0.4 ± 0.1 and 0.5 ± 0.2 ng/mL; P < 0.001). It decreased to daytime levels after nighttime tetrodotoxin instillation (0.5 ± 0.2 ng/mL; P = 0.98), indicating neural clock control. IOP behaved similarly, while C was lower during SN (13.0 ± 1.5 nL/min/mm Hg) than during SD (26.0 ± 4.0 nL/min/mm Hg; P < 0.01). Covariations in NE and IOP were partially mediated by C (14%). The cross-sectional DV area was smaller during SN (2171 µm2 [lower quartile, upper quartile, 1089, 3230 µm2]) than SD (2634 µm2 [lower quartile, upper quartile, 1556, 4190 µm2]; P < 0.005). Conclusions: The rat IOP rhythm is driven by clock sympathetic activity that elevates the aqueous NE level, constricts DVs, and decreases C at night. NE alters IOP partly through effects on C and mostly through other unspecified effectors.