Abstract
PurposeIntraocular pressure (IOP) is known to have a strong circadian rhythm, yet how light/dark cycles entrain this rhythm is unknown. The purpose of this study was to assess whether, like the retina, the mammalian ciliary body and IOP clocks have an intrinsic ability to entrain to light/dark cycles.MethodsIris-ciliary body complexes were obtained from period2:luciferase (PER2::LUC) mice and cultured to measure bioluminescence rhythmicity. Pairs of the iris-ciliary body complex were exposed to antiphasic 9:15 h light/dark cycle in vitro. After 4 days of exposure to light/dark cycles, bioluminescence was recorded to establish their circadian phases. In addition, pairs of the iris-ciliary body complex co-cultured with the retinas or corneas of wild-type mice were also investigated. The IOP circadian changes of free-running Opn4-/-;rd1/rd1 mice whose behavior was antiphasic to wild-type were measured by a rebound tonometry, and compared with wild-type mice. Opn3, Opn4, and Opn5 mRNA expression in the iris-ciliary body were analyzed using RT-PCR.ResultsThe iris/ciliary body complex expressed Opn3, Opn4, and Opn5 mRNA; however, unlike in retina and cornea, neither the iris-CB complex nor the co-cultured complex was directly entrained by light-dark cycle in vitro. The diurnal IOP change of Opn4-/-;rd1/rd1 mice showed an antiphasic pattern to wild-type mice and their rhythms followed the whole-animal behavioral rhythm.ConclusionsDespite expressing mRNA for several non-visual opsins, circadian rhythms of the iris-ciliary body complex of mice do not entrain directly to light-dark cycles ex vivo. Unlike retina, the iris/ciliary body clocks of blind mice remain synchronized to the organismal behavioral rhythm rather than local light-dark cycles. These results suggest that IOP rhythm entrainment is mediated by a systemic rather than local signal in mice.
Highlights
Intraocular pressure (IOP) in humans and mice has long been known to have a strong circadian rhythm [1,2,3]
The iris/ciliary body complex expressed Opn3, Opn4, and Opn5 mRNA; unlike in retina and cornea, neither the Iris-ciliary body (iris-CB) complex nor the co-cultured complex was directly entrained by light-dark cycle in vitro
These results suggest that IOP rhythm entrainment is mediated by a systemic rather than local signal in mice
Summary
Intraocular pressure (IOP) in humans and mice has long been known to have a strong circadian rhythm [1,2,3]. Maeda et al showed that mice lacking the core clock genes Cry and Cry lost their IOP rhythm [8], suggesting this rhythm depends on the core clock mechanism [9]. The diurnal rhythm of intraocular pressure is thought to derive at least in part from changes in ciliary body aqueous production in both animals [10] and humans [11,12]. Dalvin et al showed that the irisciliary body complex of mice have robust rhythmic core clock gene expression, which strongly correlated with IOP diurnal curve [13]
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