Do alpha-adrenergic receptors participate in control of the circadian rhythm of IOP?

The alpha 2-adrenergic antagonists, yohimbine and rauwolscine, and the alpha 1-adrenergic antagonist, bunazosin, were used to explore the role of alpha-adrenergic receptors in the regulation of the circadian rhythms of intraocular pressure and aqueous flow in New Zealand white rabbits. Blockade of alpha 2-adrenergic receptors with yohimbine or rauwolscine produced small decreases in intraocular pressure during both light and dark phases. Rauwolscine had no effect on aqueous flow during the light or dark, but it increased the concentration of norepinephrine in the aqueous during both light and dark. These observations are difficult to reconcile with earlier suggestions that increased sympathetic input to the eye increases intraocular pressure and aqueous flow during the dark. The role of alpha 2-adrenergic receptors in the control of the circadian rhythm of intraocular pressure is unclear. Blockade of alpha 1-adrenergic receptors with bunazosin produced a dose-dependent reduction of IOP during the dark phase of the circadian cycle, a smaller reduction during the light phase, and no reduction during either light or dark in rabbits after superior cervical ganglionectomy or preganglionic section of the cervical sympathetic trunk (decentralization). Bunazosin decreased pupil diameter during the dark phase but had no effect on aqueous flow. Because it is unlikely that alpha 1-adrenergic blockade increased outflow facility or uveoscleral outflow, the mechanism for the role of alpha 1-adrenergic receptors in the control of the circadian rhythm of intraocular pressure in rabbits remains to be identified.