Changes in aqueous norepinephrine and cyclic adenosine monophosphate during the circadian cycle in rabbits

The concentrations of catecholamines and cyclic adenosine monophosphate (AMP) in the aqueous of rabbits entrained to 12-hr light:12-hr dark were measured at six times during the circadian cycle. Aqueous norepinephrine was higher in the dark than in the light; the minimum concentration at 1.5 hr after lights on, and maximum concentration at 6 hr after lights off, were 1.21 +/- 0.17 and 4.60 +/- 0.79 ng/ml, respectively. The concentrations of epinephrine and dopamine were less than the sensitivity of the assay (0.1 and 0.4 ng/ml, respectively) at all times. Aqueous cyclic AMP was also higher in the dark than in the light; the minimum concentration at 6 hr after lights on, and maximum concentration at 10.5 hr after lights off, were 11.5 +/- 1.2 and 29.4 +/- 2.7 pmol/ml, respectively. Superior cervical ganglionectomy reduced aqueous norepinephrine during light and dark to less than the sensitivity of the assay (0.1 ng/ml); preganglionic section of the cervical sympathetic trunk (decentralization) did not reduce aqueous norepinephrine as much as ganglionectomy but markedly reduced its concentration during the light and dark. Superior cervical ganglionectomy or pretreatment with 0.1% timolol virtually eliminated, and decentralization markedly reduced, the dark-phase increase of aqueous cyclic AMP. Most of the daily changes in norepinephrine and cyclic AMP persisted in animals housed in constant dark; this is consistent with the fraction of both changes which persisted in constant dark being circadian. These results support the idea that increased adrenergic activity during the dark increases the rate of aqueous flow and intraocular pressure in rabbits.