Melatonin does not increase IOP significantly in rabbits

New Zealand white rabbits have a circadian rhythm of intraocular pressure; pressure is higher during the dark than during the light. We explored the possibility that the dark phase increase of serum and/or ocular melatonin plays a role in regulating the rhythm of intraocular pressure. Exogenous melatonin was delivered by four routes: topical application (0.1 and 10 micrograms), intravenous injection (1 microgram), intravitreal injection (9 micrograms) and intra-arterial infusion (50 ng and 1 microgram). Melatonin delivered by these routes did not increase intraocular pressure. We also measured the concentration of melatonin in the aqueous after unilateral intra-arterial infusion to confirm that melatonin delivered by this route reached the eye. The concentration was 419 +/- 99 and 109 +/- 17 pg/ml (n = 8, p < 0.025) in the ipsilateral and contralateral eyes, respectively. The concentration of melatonin in the aqueous after intra-arterial infusion of saline was less than the sensitivity of the assay (< 16 pg/ml, n = 6). The results of these experiments indicate that melatonin does not increase IOP significantly under the experimental conditions employed in this study.