Melatonergic involvement in diurnal changes of intraocular pressure in rabbit eyes

Endogenous biochemical regulation of diurnal changes in intraocular pressure was investigated in rabbits. Various biochemical parameters in eye tissues, particularly the iris and ciliary body, were studied at peak (21:00 hr) and trough (09:00 hr) points of IOP. No statistical difference in choline acetyltransferase activity, adrenergic transmitter levels and dopamine concentration could be detected at these points. On the other hand, serotonin N-acetyltransferase activity was significantly higher at 21:00 hr (2.84 +/- 0.14 nmoles/mg protein/hr) than at 09:00 hr (2.18 +/- 0.16 nmoles/mg protein/hr) indicating that melatonin might be involved in the diurnal changes in IOP. Intracameral injections of various agents into rabbit eyes revealed that melatonin but not serotonin nor N-acetylserotonin raised IOP markedly, indicating that melatonin but not its precursors is involved in IOP regulation. Topical application of melatonin did not affect the IOP presumably because it does not cross the cornea effectively.