Is ocular melatonin regulated by the adrenergic system?

Abstract

There is evidence supporting a role for melatonin as an endogenous agent regulating intraocular pressure, but little is known about whether ocular levels of melatonin are regulated by the sympathetic nervous system, as they are in the rat pineal gland. The effects of topically applied L-timolol, norepinephrine, forskolin, domperidone and pilocarpine on levels of melatonin in regions of rabbit and chicken eyes were determined. None of these drugs altered levels of melatonin in the rabbit iris-ciliary body or in the chicken iris or ciliary body, suggesting that the actions of these drugs were not mediated through an action on melatonin. Topical forskolin did not alter N-acetyltransferase activities in rabbit iris-ciliary body. Chemical sympathectomy with 6-hydroxydopamine had no effect on nighttime levels of melatonin in rabbit iris-ciliary body or on daytime levels in chicken eyes. These results suggest that melatonin in the eyes does not appear to be under the solitary control of the adrenergic sympathetic nervous system. The data suggested a transmitter other than norepinephrine regulates N-acetyltransferase activities and melatonin levels.