Endogenous hormonal changes and circadian elevation of intraocular pressure

A unique circadian rhythm of intraocular pressure (IOP) occurs in laboratory rabbits housed in a light-dark environment. The IOP is low in the light and high in the dark. In 32 rabbits studied, the IOP was 18.7 +/- 0.3 mm Hg (mean +/- the standard error of the mean) at 2 hr before the onset of dark and 24.9 +/- 0.6 mm Hg at 2 hr into the dark. The changes in endogenous hormones corresponding to this elevation of IOP were studied. Concentrations of norepinephrine (NE), epinephrine (Epi), corticosterone, and melatonin in the plasma and NE and melatonin in the aqueous humor were determined. Changes in plasma catecholamines were variable and statistically insignificant. A decrease of plasma corticosterone and an increase of plasma melatonin in the dark were observed. Norepinephrine and melatonin levels in the aqueous humor increased in the dark. In ten rabbits with unilateral transection of the cervical sympathetic trunk, the elevation of IOP was absent only in the decentralized eye. The changes in plasma NE, Epi, and melatonin levels from the light to the dark phase were insignificant. The plasma corticosterone level decreased. In the dark phase, aqueous humor NE level in the decentralized eye was significantly lower than that in the intact eye. Melatonin levels in these two eyes were indifferent. In 22 rabbits housed in a constant dark environment, aqueous humor NE concentration increased corresponding to the circadian elevation of IOP. The authors conclude that the activation of ocular sympathetic nerves is directly related to the circadian elevation of IOP.