Timolol reduces IOP in normal NZW rabbits during the dark only

The role of beta-adrenergic receptors in mediating the circadian rhythm of intraocular pressure was examined by determining the effect on intraocular pressure of timolol applied topically at different times during the circadian cycle to New Zealand White rabbits entrained to alternating 12 hr periods of light and dark. Timolol lowered intraocular pressure when applied during the dark but not during the light phase of the circadian cycle. Timolol did not lower intraocular pressure when applied during the dark to animals after cervical ganglionectomy. These results are consistent with the idea that beta-adrenergic receptors participate in mediating the circadian rhythm of intraocular pressure, and suggest that endogenous agonists released by adrenergic nerves in the eye during the dark stimulate ocular beta-adrenergic receptors to increase intraocular pressure in rabbits.