N-acetyltransferase activity and melatonin level in the eyes of glaucomatous chickens

The possible involvement of a melatonergic mechanism in the control of intraocular pressure (IOP) and the genesis of light-induced avian glaucoma (LIAG) was studied by measuring N-acetyltransferase (NAT) activity and melatonin levels in the iris, ciliary body and retina-choroid during the course of LIAG development, and in normal subjects by day and night. NAT activity was found to be significantly higher than normal in preglaucomatous and glaucomatous chicken eyes at 8, 16, and 24 weeks of age. The increase in NAT activity corresponded well with the decrease in aqueous outflow facility and preceded the development of elevated IOP by 5-6 weeks. Melatonin levels in iris and ciliary body of normal chickens and rabbits were elevated by night relative to day, which corresponded to the diurnal change in IOP. The melatonin levels in glaucomatous chicken iris and ciliary body at 24 weeks were significantly elevated relative to controls. The results suggest that melatonergic mechanisms could be involved in the elevation of IOP in LIAG. No statistical differences in choline acetyltransferase activity, adrenergic transmitter levels or dopamine concentrations could be found in tissues of LIAG eyes vs control eyes, indicating that cholinergic, adrenergic and dopaminergic mechanisms are probably not involved in LIAG.