Melatonin in the eye: Implications for glaucoma

Experimental Eye Research 84 (2007) 1021e1030 www.elsevier.com/locate/yexer

Review

Melatonin in the eye: Implications for glaucoma Per O. Lundmark a, S.R. Pandi-Perumal b,*, V. Srinivasan c, D.P. Cardinali d, R.E. Rosenstein e a

Department of Optometry and Vision Sciences, Buskerud University College, Kongsberg, 3601 Ko, Norway Comprehensive Center for Sleep Medicine, Department of Pulmonary, Critical Care and Sleep Medicine, Mount Sinai School of Medicine, 1178-5th avenue, 6th floor, NY 10029, USA c Department of Physiology, School of Medical Sciences, University Sains Malaysia 16150, Kubang kerian, Kelantan, Malaysia d Departamento de Fisiologı´a, Facultad de Medicina, Universidad de Buenos Aires, 1121 Buenos Aires, Argentina e Departamento de Bioquimica Humana, Facultad de Medicina, Universidad de Buenos Aires, 1121 Buenos Aires, Argentina b

Received 13 September 2006; accepted in revised form 30 October 2006 Available online 14 December 2006

Abstract Melatonin synthesis occurs in the retina of most animals as well as in humans. Circadian oscillators that control retinal melatonin synthesis have been identified in the eyes of different animal species. The presence of melatonin receptors is demonstrable by immunocytochemical studies of ocular tissues. These receptors may have different functional roles in different parts of the eye. In view that melatonin is a potent antioxidant molecule, it can be effective in scavenging free radicals that are generated in ocular tissues. By this mechanism melatonin could protect the ocular tissues against disorders like glaucoma, age-related macular degeneration, retinopathy of prematurity, photo-keratitis and cataracts. Although an increased intraocular pressure is an important risk factor in glaucoma, other concomitant phenomena like increased glutamate levels, altered nitric oxide metabolism and increased free radical generation seem to play a significant role in its pathogenesis. Data are discussed indicating that melatonin, being an efficient antioxidant with antinitridergic properties, has a promising role in the treatment and management of glaucoma. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: melatonin; glaucoma; intraocular pressure; antioxidant properties; glutamate excitotoxicity; free radicals

1. Introduction Melatonin is a ubiquitous natural substance widely distributed in nature, both in plants and animals. It is probably one of the first regulatory compounds that appeared in living organisms (Claustrat et al., 2005; Pandi-Perumal et al., 2006). In all mammals including humans, circulating melatonin derives primarily from the pineal gland. Local synthesis of melatonin also occurs in several peripheral organs (Kvetnoy, 2002) such as bone marrow, gut, gastrointestinal tract, lymphocytes and various parts of the eye including the retina (Cardinali and Rosner, 1971a,b; Tosini and Menaker, 1998), ciliary body (Martin et al., 1992) and lacrimal gland (Mhatre et al., 1988).

* Corresponding author. Tel.: þ1 212 241 5098; fax: þ1 212 241 4828. E-mail address: [email protected] (S.R. Pandi-Perumal). 0014-4835/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.exer.2006.10.018

In the eye, locally synthesized melatonin may regulate retinomotor movements (Pierce and Besharse, 1985), rod outer segment disc shedding (Wiechmann and Rada, 2003), dopamine synthesis and release (Doyle et al., 2002) and intraocular pressure (IOP) (Wiechmann and Wirsig-Wiechmann, 2001). Moreover, melatonin can be an effective antioxidant in the retina, acting as a direct and indirect free radical scavenger. In this sense, melatonin was shown to protect photoreceptor outer segment membranes from free radical attack induced by light (Marchiafava and Longoni, 1999; Siu et al., 1999, 2006). The reduction in antioxidant defenses has been suggested as one of the causes for early stage glaucoma (Bunin et al., 1992; Moreno et al., 2004). Selective death of retinal ganglion cells (RGCs) that occurs in glaucoma leads to optic neuropathy (Osborne et al., 1999). Intraocular hypertension and vascular insufficiency in the optic nerve are suggested as main risk factors for glaucoma

1022

P.O. Lundmark et al. / Experimental Eye Research 84 (2007) 1021e1030

(Ritch, 2000). Although the current management of glaucoma is mainly directed at the IOP control, the use of neuroprotective agents represents a new avenue for effective therapy in glaucoma (Chew and Ritch, 1997; Osborne et al., 1999; Weinreb and Levin, 1999). In the last decade melatonin has emerged as a promising neuroprotective agent in experimental animal models of various neurological and neurodegenerative disorders (Reiter, 1998; Srinivasan et al., 2005, 2006). In the present review, we will discuss the evidence supporting the use of melatonin as a useful therapeutic strategy in the management of glaucoma.

2. Melatonin biosynthesis in the eye In the eye, melatonin is synthesized through the same pathway as that described in the pineal gland. Tryptophan is taken up from the blood and is converted to serotonin. Serotonin is then converted into N-acetyl serotonin by the enzyme arylamine N-acetyl transferase (AA-NAT). N-acetyl serotonin is converted to melatonin by the enzyme hydroxyindoleO-methyl transferase (HIOMT). The existence of melatonin biosynthetic pathway in the mammalian retina was initially supported by the discovery of retinal HIOMT activity (Cardinali and Rosner, 1971b) and by the finding that labeled serotonin is converted into melatonin in the rat retina (Cardinali and Rosner, 1971a). The presence of HIOMT in the retina (both protein and mRNA) has been confirmed in other animals (Bernard et al., 1999; Liu et al., 2004). The gene encoding HIOMT is selectively expressed in retinal photoreceptors. In a recent study, the promoter of HIOMT gene Otx2 was localized in the retina and the pineal gland (Dinet et al., 2006). The findings are significant in view of the fact that the Otx2 protein is present at ‘‘the right place and at right time’’ to play a role in the onset of HIOMT gene expression in retinal photoreceptors and pineal gland of chickens (Dinet et al., 2006). Although available data strongly support that photoreceptors synthesize melatonin independently from the rest of the retina, recent evidence indicates that melatonin could also be synthesized by chick RGCs (Garbarino-Pico et al., 2004). As in the pineal gland, retinal melatonin content significantly changes during the 24-h cycle (Hamm and Menaker, 1980; Zawilska and Nowak, 1989). The enzyme AA-NAT exhibits a circadian rhythm with peak concentration occurring at night (Niki et al., 1998; Iuvone et al., 2002). It has been suggested that retinal melatonin synthesis is related to cyclic events that normally occur in the retina (Wiechmann, 1986). The existence of a diurnal rhythm of melatonin in the retina of rats suggests its involvement in the regulation of the diurnal rhythm of eye pigmentation in vertebrates (Pang et al., 1980).

3. Regulation of melatonin biosynthesis in the eye Melatonin biosynthesis by the pineal gland is regulated by the light/dark cycle (Moore, 1997). Specialized melanopsin-

containing neurons that respond to light has been detected in the eye (Brainard et al., 2001a,b; Foster and Hankins, 2002) and this is the neural source of light input to the suprachiasmatic nuclei (SCN). This unique subset of intrinsically photosensitive retinal ganglion cells express melanopsin, the primary circadian photopigment in rodents and primates. Action spectra of melatonin suppression by light have shown that light in the 446e477 nm range, distinct from the visual system’s peak sensitivity, is optimal for stimulating the human circadian system (Brainard et al., 2001a,b; Foster and Hankins, 2002). The finding that isolated photoreceptor cells rhythmically secrete melatonin suggests that photoreceptors contain an endogenous ‘‘clock’’ that regulates melatonin biosynthesis (Cahill and Besharse, 1993). This has been confirmed in the mammalian retina; photoreceptors, either rods or cones, contain circadian oscillators (Tosini and Menaker, 1996, 1998). The genes that have been recognized as components of the core oscillator in the SCN are also present in the retina. The clock genes Cry 1 and Cry 2 are expressed in inner retinal neurons and ganglion cell layer (Bailey et al., 2002; Haque et al., 2002). Other genes, like Per 1 and Per 2 were identified in the inner retina (Namihira et al., 2001), in the inner nuclear layer, and in few ganglion cells of mouse and human retina (Witkovsky et al., 2003; Thompson et al., 2004). Experiments with the rd (rodless) mouse have shown that melatonin synthesis is not abolished by the complete loss of photoreceptors but its circadian expression disappears (Tosini and Menaker, 1998; Tosini, 2000). This finding suggests that rods are necessary for the rhythmic synthesis of melatonin. Retinal melatonin levels are regulated by the interaction between the circadian clock and the photic environment. Retinal melatonin levels rise rapidly during darkness and decrease after exposure to light (Fukuhara et al., 2001). The depolarization of photoreceptors that takes place during darkness induces AA-NAT activity by a Ca2þ- and cAMP-dependent mechanism (Ivanova and Michael, 2003). Depolarization of the photoreceptor membrane opens dihydropyridine sensitive voltage gated Ca2þ channels resulting in a sustained increase of intracellular Ca2þ concentration in the inner segments of photoreceptors which in turn stimulates cAMP formation through activation of a calmodulin-dependent adenyl cyclase (Gan et al., 1995; Uchida and Iuvone, 1999). This increased formation of cAMP induces AA-NAT gene transcription and increases AA-NAT activity, thus causing the increased production of melatonin (Alonso-Gomez and Iuvone, 1995; Greve et al., 1999). A circadian clock that gates melatonin synthesis and that involves transcription factors has recently been proposed (Fukuhara et al., 2004). The gating is effected through E box-mediated transcriptional activation of the AC1 gene. This regulates melatonin synthesis through the expression of type 1 adenyl cyclase and the synthesis of cAMP in photoreceptors (Fukuhara et al., 2004). The gating of cAMP signaling presumably plays a key role as input and output components of the central circadian axis, i.e., the retina, the SCN and the pineal gland.

P.O. Lundmark et al. / Experimental Eye Research 84 (2007) 1021e1030

4. Melatonin receptors in the eye Since many functions of melatonin can be attributed to melatonin receptor subtypes (Dubocovich et al., 2000), the study of the distribution of melatonin receptors in the eye is relevant to the discussion. Table 1 summarizes available evidence on melatonin receptor localization in the retina of several species. Immunocytochemical analysis of ocular tissues obtained from various animals like chickens, rats, and humans has shown that melatonin receptors (MT1, MT2) are distributed in the cornea, choroid, sclera, photoreceptors, RGCs and retinal blood vessels (Fujieda et al., 1999; Scher et al., 2002, 2003; Savaskan et al., 2002; Wiechmann and Rada, 2003; Rada and Wiechmann, 2006). MT1 receptors have also been identified in the corneal epithelium, stroma, sclera, and endothelium of Xenopus eyes (Wiechmann and Rada, 2003). Collectively, the

1023

expression of melatonin receptors in ocular areas suggests that melatonin plays a role in the differential regulation of the growth and remodeling of the fibrous and cartilaginous scleral layers that affect eye size and refraction (Wiechmann and Rada, 2003). Additionally, the existence of melatonin receptors in the iris and ciliary processes has led to the proposal that they are involved in aqueous humor secretion and the circadian rhythm of intraocular pressure (IOP) (Osborne and Chidlow, 1994). Three types of melatonin receptors, namely Mel1a (MT1), Mel1b (MT2) and Mel1c are localized in the retina. Distinct diurnal rhythms of receptor protein expression occur in the retina-pigmentary epithelium (RPE)-choroid with peak levels of MT1 and MT2 receptors during the night, and peak levels of Mel1c receptors during the day (Rada and Wiechmann, 2006). The possible involvement of melatonin receptors in

Table 1 Melatonin receptors in the eye and their possible functional significance Type of receptor

Animal species

2-[125I]-iodomelatonin binding sites 2-[125I]-iodomelatonin binding sites 2-[125I]-iodomelatonin binding sites Melatonin receptors

Chicken Retinal membranes and rabbits Rana Inner plexiform layer of frog retina pipiens Rabbit Iris and ciliary processes

2-[125I]-iodomelatonin binding sites 2-[125I]-iodomelatonin binding sites Mel1b (MT2) receptor

Lizard

Inner plexiform layer of retina

Rabbit

Iris-ciliary body

Chicken

Retinas

Humans

Retina

Rabbit

Retina

Xenopus

Retina and retinal pigment epithelium

Mel1a (MT1) and Mel1b (MT2) receptors Mel1a (MT1), Mel1b (MT2) and Mel1c receptors Mel1c receptors

Xenopus

MT1 receptor

Humans

MT1 receptor

Humans

Mel1a (MT1) and Mel1c receptors MT1 receptor

Chicken

MT1 receptor

Mel1a (MT1) and Mel1c receptors Mel1c receptors MT1 receptor Mel1a (MT1), Mel1b (MT2) and Mel1c receptors

Location of the receptor

Humans Monkeys and humans Xenopus

transgenic Xenopus Rana perezi Chicken

Probable function

Reference

Inhibition of Ca2þ dependent release of [3H]-dopamine

Maintaining intraocular pressure

(Dubocovich and Takahashi, 1987) (Wiechmann and WirsigWiechmann, 1991) (Osborne, 1994)

Stimulates cyclic AMP, regulates intraocular pressure Modulation of dopaminergic neurotransmission

(Wiechmann and WirsigWiechmann, 1994) (Osborne and Chidlow, 1994) (Iuvone et al., 1995)

Inhibits Ca2þ dependent release of dopamine and other light dependent functions Inhibition of dopamine release

(Reppert et al., 1995) (Dubocovich et al., 1997).

Phagocytosis of photoreceptor outer segment and (Wiechmann et al., 1999; intracellular migration of pigment granules Wiechmann and Smith, 2001) Nonpigmented epithelia Rate of aqueous secretion (Wiechmann and Wirsigof ciliary body Wiechmann, 2001) Retina, inner segments of photoreceptors, Human vision (Scher et al., 2002) inner nuclear layer, ganglion cell layer Retina, photoreceptor cells, Modulation of dopamine release; maintenance of (Savaskan et al., 2002) ganglion cells, amacrine cells photoreceptor cells; mediates melatonin’s and retinal blood vessels action on blood vessels Retina, inner segments of photoreceptors, Modulation of retinal physiology (Natesan and Cassone, retinal ganglion cell layer 2002) Inner segments of rods and cones, Involved in physiological functions of the eye (Meyer et al., 2002) retinal ganglion cells AII amacrine cells Retinal light adaptation to lower light intensities (Scher et al., 2003)

Sclera, corneal epithelium

Neural retina

Differential regulation of cellular functions in sclera; circadian rhythms in corneal repair and ocular elongation Mediates responsiveness of rod photoreceptors to light Visual functions

(Isorna et al., 2004)

Cornea, choroid, sclera, and retina

Regulation of the diurnal rhythm of ocular growth

(Rada and Wiechmann, 2006)

Rod photoreceptors

(Wiechmann and Rada, 2003) (Wiechmann et al., 2003)

1024

P.O. Lundmark et al. / Experimental Eye Research 84 (2007) 1021e1030

the regulation of the diurnal rhythm of ocular growth was postulated (Rada and Wiechmann, 2006). The presence of MT2 receptors in the apical microvillar cell membrane but not on the basement membrane of the RPE strongly suggests that melatonin is involved in photoreceptor outer segment disk shedding and phagocytosis (Wiechmann and Rada, 2003). In the human retina MT2 receptors are highly expressed (Reppert et al., 1995) as compared to a relatively lower expression of MT1 in photoreceptors and other retinal cells (Scher et al., 2002; Meyer et al., 2002). MT2 receptor sites are also expressed in the sclera, lens, RPE and neural retina of Xenopus eye (Wiechmann et al., 2004). MT1 receptors are present in RGCs and in amacrine cells of the inner and outer plexiform layers (Fujieda et al., 1999, 2000). Scher et al. (2003) identified MT1 receptors in AII amacrine cells, which are critical neurons in the rod pathway of mammalian retina. A study of the melatonin receptor subtype present in RGCs and amacrine cells of Alzheimer’s patients revealed that MT1 receptors were up-regulated (Savaskan et al., 2002) presumably as a response to the decreased nocturnal circulating melatonin levels seen in this disease (Srinivasan et al., 2006). MT1 receptors have also been identified in ocular vessels of the human retina and in the adventitia of retinal vessels suggesting an indirect action of melatonin on vascular smooth muscle (Savaskan et al., 2002). Although initially considered as a receptor, the so called ‘‘MT3 receptor’’ has been purified and identified as the quinone reductase 2 enzyme (Nosjean et al., 2000). This enzyme is expressed in liver, kidney, brain, lung, intestine and spleen of hamster, mouse and monkey (Nosjean et al., 2001) and it has also been shown to be involved in IOP regulation (Pintor et al., 2001). Topical application of the melatonin agonist 5-methoxycarbonylamino-N-acetyltryptamine (5-MCA-NAT) reduced intraocular pressure in glaucomatous monkey eyes presumably by interaction with the quinone reductase 2 enzyme (Serle et al., 2004). 5. Ocular functions of melatonin Systemic administration of melatonin resulted in significant changes in anterior chamber and vitreous chamber depth, a finding compatible with the hypothesis that melatonin plays a role in the ocular growth and development (Rada and Wiechmann, 2006). Retinal melatonin may also act as a neuromodulator that mediates dark adaptive regulation of retinomotor movements (Pierce and Besharse, 1985). The expression of MT1 receptors in most dopaminergic amacrine cells of the human retina strongly implicates melatonin in the modulation of retinal dopaminergic function (Scher et al., 2003). Both dopamine and melatonin are important regulators of retinal rhythmicity and these two substances are mutually inhibitory, acting as chemical analogs of day and night, respectively (Doyle et al., 2002). The evidence for this has been obtained in the retina of C3Hþ/þ mice lacking melatonin, in which there is also a lack of circadian rhythmicity of dopamine content; in these animals the administration of melatonin in a cyclical manner re-established normal retinal dopamine

rhythm (Doyle et al., 2002). Indeed, the inhibition of dopamine release by melatonin has also been demonstrated in specific areas of the CNS like the hypothalamus, hippocampus, striatum and medullaepons (Zisapel, 2001). The action of melatonin in influencing the rod pathway at the level of horizontal and amacrine cells may play a role in retinal adaptation to low light intensities (Scher et al., 2002). Due to the strong correlation between circulating melatonin levels and electroretinographic (ERG) responses in humans their hypothesis that melatonin is associated with diurnal variations of ERG was put forth (Rufiange et al., 2002). Free radical generation is a major contributing factor for various eye diseases and the possibility that melatonin can act as an important antioxidant in the eye is supported by a number of studies (Lundmark et al., in press; Siu et al., 2006). The increased oxidative stress as a consequence of aging is one of the reasons for age-related macular degeneration (AMD), in which there is progressive degeneration of photoreceptors and their underlying retinal pigment epithelium in the macular area of the retina (Cai et al., 2000; Liang and Godley, 2003). AMD is a leading cause of severe visual loss in people over 50 years of age and accounts for approximately 50% of all cases of registered blindness (Klein and Klein, 1982). Photoreceptor death leads to the vision loss in AMD patients (Green and Key, 1977). Environmental factors like exposure to sunlight, intense illumination or cigarette smoking have been suggested as factors responsible for AMD through free radical generation (Hammond et al., 1996; Cruickshanks et al., 2001). The most important pathologic change that threatens visual acuity in AMD is subretinal neovascularization, which can lead to hemorrhage, retinal edema, exudates or detachment of the RPE. All forms of subretinal neovascularization (i.e., extrafoveal, juxtafoveal, subfoveal) imply a great potential risk. Within 3 years, about 44% of patients lost six lines or more of their vision (Avery et al., 1996). Melatonin has been demonstrated to be effective in protecting the photoreceptor outer segment membranes from free radicals generated by light (Siu et al., 1999). Prolonged exposure to melatonin (0.1e200 mM) for 3 days has been shown to protect the RPE cells from H2O2-induced cell death. As melatonin was effective only after prolonged administration, the stimulation of antioxidant enzymes like glutathione peroxidase, glutathione reductase or superoxide dismutase was suggested as a possible mechanism for melatonin-induced reduction of oxidative stress in the eye (Liang et al., 2004). In a caseecontrol study with a follow-up of 6e24 months, 100 patients with AMD were diagnosed and 3 mg melatonin was given orally each night at bedtime for at least 3 months (Yi et al., 2005). Both dry and wet forms of AMD were included in the study. In 55 patients followed longer than 6 months, the visual acuity was kept stable, a result that is better than the otherwise estimated natural course (Avery et al., 1996). Only four patients showed more retinal bleeding and three retinal exudates. The majority had reduced pathologic macular changes indicating that the daily use of 3 mg melatonin delays macular degeneration (Yi et al., 2005).

P.O. Lundmark et al. / Experimental Eye Research 84 (2007) 1021e1030

6. Oxidative stress in the eye Exposure of eye to intense illumination from focal light (Dorey et al., 1990; Cruickshanks et al., 1993), the presence of high oxygen tension in the eye (Alder and Cringle, 1985) and free radicals generated from phagocytosis and lipid peroxidation as an age-related process (Cai et al., 2000; Militante and Lombardini, 2004), all contribute to increased oxidative stress in RPE. Exposure of outer segments of rod photoreceptor cells to bright light of 485 nm wavelength induces oxidative stress followed by irreversible damage (Marchiafava and Longoni, 1999). Current evidence favors an important role of oxidative stress in RPE apoptosis (Cai et al., 2000; Liang and Godley, 2003; Liang et al., 2004). The constant shedding of the outer segments coupled with the synthesis of new membranes results in the accumulation of polyunsaturated fatty acids. Peroxidation of these lipids results in significant damage to RPE cells triggering apoptosis (Cai et al., 2000). The retina is endowed with a high mitochondrial population in the inner segments of photoreceptors. These mitochondria are involved in the production of ATP coupled to oxidative phosphorylation, through respiratory complexes of the electron transport system which uses molecular oxygen. In addition, the leakage of some electrons causes the formation of free radicals like the superoxide anion radical (O2 ), the hydroxyl radical (OH) and other reactive oxygen species (Nohl et al., 2005; Hardeland et al., 2006). Free radicals attack mitochondria causing impaired function and reduced ATP formation. The opening of mitochondrial transition pores and subsequent release of cytochrome c activate apoptosis leading to cellular loss in the retina (Acun˜a-Castroviejo et al., 2002). Oxidative stress is suggested as the main cause for retinopathy seen in preterm infants since this situation is associated with low levels of reduced glutathione (Papp et al., 1999) and vitamin E (Chan et al., 1999). Corneal epithelial cell exposure to UV-B radiation causes free radical release resulting in keratitis. Melatonin protected corneal epithelial cells from oxidative damage caused by exposure to UV radiation (Ciuffi et al., 2003). This finding assumes significance as it clearly shows the antioxidant effects of melatonin in the eye. 

7. Melatonin as an antioxidant Tan et al. (1993) by using spin trapping and electron resonance spectroscopy, demonstrated that melatonin has the capacity to directly scavenge the highly reactive hydroxyl radicals. Since then, a number of reports have shown that melatonin acts as a free radical scavenger and an efficient antioxidant (Hardeland et al., 1995; Reiter et al., 1997; Reiter, 1998; Pandi-Perumal et al., 2006). Not only melatonin but also several of its metabolites generated during its free radical scavenging action also act as antioxidants (Hardeland, 2005). The kynurenic pathway of melatonin metabolism includes a series of radical scavengers with the possible sequence: melatonin / cyclic 3-hydroxymelatonin / N1-acetyl-N2formyl-5-methoxykynuramine (AFMK) / N1-acetyl-5-methoxykynuramine (AMK). In the metabolic step from melatonin to

1025

AFMK up to four free radicals can be consumed (Tan et al., 2003; Guenther et al., 2005; Hardeland, 2005). Because of this pathway, melatonin’s efficacy as an antioxidant is greatly increased. Melatonin has been shown to scavenge free radicals generated in the mitochondria, to reduce electron leakage from the respiratory complexes and to improve ATP synthesis (Leon et al., 2004). Moreover, melatonin also maintains reduced glutathione levels within the mitochondria thereby enhancing the antioxidative potential. By the actions in free radical scavenging mechanism, increasing the antioxidative defense systems and improving the electron transport chain activities at the mitochondrial level, melatonin is able to protect the ocular tissues from free radical induced damage (Siu et al., 2006). It must be noted, however, that there have been at least two reports (Bubenik and Purtill, 1980; Wiechmann and O’Steen, 1992) indicating that melatonin administration may cause light-induced damage to the retinal photoreceptors. Additionally, rat eyes pretreated with the melatonin receptor competitive antagonist luzindole before the dark phase preceding constant light exposure were substantially protected from light damage to the retinal photoreceptors (Sugawara et al., 1998). 8. Melatonin and IOP Melatonin is detectable in aqueous humor with circadian rhythmicity, with peak levels occurring during the dark period, as in the plasma (Yu et al., 1990; Liu and Dacus, 1991). Since the circadian rhythm of aqueous humor secretion likely contributes to the circadian rhythm in IOP (Smith and Gregory, 1989), a modulatory role of melatonin on IOP rhythm was put forth (Wiechmann and Wirsig-Wiechmann, 2001). In nocturnal animals, the IOP rhythm is low during the light period and high at darkness (Frampton et al., 1987; Samples et al., 1988; Smith and Gregory, 1989; Liu and Dacus, 1991; Moore et al., 1996), whereas studies in humans revealed a diurnal rhythmicity of IOP with highest level occurring during daytime (Kitazawa and Horie, 1975; Samples et al., 1988). In view of this relation, melatonin has been suggested to influence IOP rhythm (Osborne and Chidlow, 1994; Pintor et al., 2001). 9. Glaucoma: role of elevated IOP and glutamate excitotoxicity Glaucoma is one of the leading causes of blindness worldwide, characterized by specific visual field defects due to the loss of RGCs and damage of the optic nerve head. The result is a patchy loss of vision generally in a peripheral to central manner. It is estimated that half of those affected may not be aware of their condition because symptoms may not occur during the early stages of the disease. When vision loss appears, considerable permanent damage has already occurred. Medications and surgery can help to slow the progression of some forms of glaucoma, but at present, there is no cure for the disease. Although an increased IOP is probably the most important risk factor in primary open-angle glaucoma, several concomitant factors, like elevation of glutamate levels, disorganized

P.O. Lundmark et al. / Experimental Eye Research 84 (2007) 1021e1030

1026

nitric oxide (NO) metabolism and oxidative damage, among others, could significantly contribute to the neurodegeneration (for a review, see Kaushik et al., 2003). Although therapy that prevents the death of RGCs should be the main goal of treatment, current management of glaucoma is mainly directed at the IOP control. Glutamate is present in RGCs and acts as an important neurotransmitter in the retina (Sucher et al., 1991). Excess glutamate release stimulates the N-methyl-D-aspartate (NMDA) receptor resulting in increased intracellular calcium levels (Sucher et al., 1991; Siliprandi et al., 1992). This leads to excess free radical accumulation, lipid peroxidation, mitochondrial dysregulation and excess activation of nitric oxide synthase (NOS) and catabolic enzymes, thus resulting in cellular death especially of RGCs (Dreyer et al., 1994). High glutamate levels occur in the vitreous chamber of patients with glaucoma and also in monkeys with experimentally-induced glaucoma (Dreyer et al., 1994). Moreover, there exists a significant alteration of the retinal glutamate/glutamine cycle activity in rats exposed to experimentally elevated IOP, which supports a significant increase in glutamate synaptic levels in retinas from hypertensive eyes (Moreno et al., 2005b) (Fig. 1). It is this glutamatergic injury that has been advocated as the main contributing factor for the death of RGCs in glaucoma (Moreno et al., 2005b). In fact, changes in glutamate recycling preceded functional and histological alterations induced by ocular hypertension (Moreno et al., 2005a). In support of this concept, evidence has been obtained from the studies undertaken in patients with glaucoma indicating the existence of high glutamate levels in their vitreous humor (Dreyer et al., 1996). Intracameral injection of hyaluronic acid (HA) has been postulated as a model of ocular hypertension in rats (Benozzi et al., 2002) since it results in highly consistent intraocular hypertension with preservation of daily variations in IOP

(Moreno et al., 2005a). To assess the functional state of the retinas from HA-treated eyes, scotopic ERGs were performed. Treatment of rats with HA for 6 or 10 weeks resulted in the reduction of scotopic ERG a and b waves. Examination of axons of HA-treated eyes, revealed that spacing between bundles of axons increased by an apparent development of glial profiles, glial nuclei, and axon loss. In addition, individual axons also exhibited distension and distortion. There was also a significant loss of medium and large diameter axons in HA-treated eyes (Moreno et al., 2005b). These results indicate that the chronic administration of HA to rats induces both functional and histological alterations consistent with some features of chronic open-angle glaucoma. NO is believed to play a significant role in experimental glaucoma (Neufeld, 2004). Elevation of IOP increases NO synthesis in the retina causing 30% loss of RGCs (Siu et al., 2002). Excessive release of NO by coupling with O2  will generate the highly reactive peroxynitrite anion (ONOO), therefore increasing oxidative stress (Siu et al., 2006). It has been reported that the inducible form of NOS (NOS-2) occurs in the optic nerve head astrocytes from human glaucomatous eyes (Neufeld et al., 1997) and in rat eyes with chronic elevation of IOP (Shareef et al., 1999). Based on this, aminoguanidine, an inhibitor of NOS-2, was applied for preventing the glaucomatous cupping (Shareef et al., 1999). Indeed, rats that did not receive aminoguanidine exhibited pallor and cupping while animals treated with aminoguanidine for 6 weeks appeared normal. In the untreated eyes there was loss of 36% of RGCs, whereas in treated animals the loss was less than 10% (Shareef et al., 1999). Experimental evidence indicates that melatonin is a potent inhibitor of the retinal nitridergic pathway, because it decreases retinal NOS activity, L-arginine uptake and the effect of L-arginine on cGMP accumulation (Saenz et al., 2002). Therefore, melatonin could be a useful neuroprotective agent in glaucoma. 

Müllercell

Presynaptic neuron Glutamine

Glutamine

Glutaminase

Glutamine Glutamine synthetase

Glutamate Glutamate

Glutamate

L-arginine Postsynaptic neuron

NOS

Fig. 1. Schematic representation of the retinal glutamate/glutamine cycle, the retinal nitridergic pathway, and their modulation by melatonin. As shown, melatonin increases glutamate uptake, glutamate release, glutamine synthetase activity, and glutamine uptake, whereas it decreases glutaminase activity. In addition, melatonin decreases L-arginine uptake and retinal NOS activity. On the other hand, ocular hypertension decreases glutamate uptake and glutamine synthetase activity, whereas it increases glutamine uptake and release, glutaminase and NOS activities. Positive effects of melatonin are noted as whereas indicates negative modulations.

P.O. Lundmark et al. / Experimental Eye Research 84 (2007) 1021e1030

10. Neuroprotective drugs in the treatment of glaucoma According to Osborne et al. (1999) neuroprotectors will be more beneficial to patients in which neurons die slowly as in glaucoma, than in a disease in which the death of a given set of neurons is more rapid (Osborne et al., 1999). Many compounds such as betaxolol, brimonidine, calcium channel blockers, antioxidants such as vitamin E, coenzyme Q or Ginkgo biloba extract have all been tried in animals and have been shown to offer protection for retina against free radical damage and lipid peroxidation (Ritch, 2000). Calcium channel blockers neutralize glutamate-NMDA-induced intracellular Ca2þ influx. Netland et al. (1993) demonstrated a decrease in glaucoma progression in open-angle glaucoma patients treated with Ca2þ channel blockers. The administration of the NMDA antagonist memantine effectively blocks the excitotoxic response of RGCs both in culture and in vivo conditions (Vorwerk et al., 1996) and is undergoing testing in placebo controlled randomized muticenter trial in the US (Kaushik et al., 2003).

11. Conclusions Melatonin has been demonstrated as a very good neuroprotective agent in various experimental models and is being used in the treatment of neurodegenerative diseases like Alzheimer’s disease and parkinsonism, where it has been shown to improve the clinical condition of the patients (Reiter, 1998; Srinivasan et al., 2005, 2006). As discussed above, melatonin acts as an efficient retinal antioxidant as well as a potent inhibitor of the nitridergic pathway. Melatonin also reduces the NOinduced retinal oxidative damage both in vivo and in vitro (Siu et al., 2006). Based on these findings it is proposed that melatonin could be a promising resource in the management of glaucoma, because it exhibits both antioxidant and antinitridergic properties and may be helpful in increasing the retinal glutamate clearance (Moreno et al., 2005b). In addition, both diurnal and nocturnal melatonin levels were significantly reduced in retinas from hypertensive eyes (Moreno et al., 2004). As melatonin influences the ciliary epithelium and regulates IOP, it can be more useful in the treatment of ocular diseases that involve an increase in IOP (Osborne and Chidlow, 1994; Pintor et al., 2001). Since the glaucomatous monkey eye was introduced as the presumably ideal animal model for open-angle glaucoma, several drugs like isopropyl unprostone have been tested in this model (Lee et al., 1987; Wang et al., 1990a,b; Serle et al., 1998). Recently the effect of topical application of 5-MCA-NAT, a putative melatonin ‘‘MT3’’ receptor agonist (see above) on IOP in glaucomatous monkey’s eye was evaluated (Serle et al., 2004). The topical application of 5-MCA-NAT substantially reduced IOP in glaucomatous monkeys by 10% on day 1 and by 19% on day 5 of treatment. The ocular hypotensive effect lasted for at least 18 h. There was enhanced efficacy with the use of 5 MCA-NAT when compared to the oral

1027

administration of melatonin (Samples et al., 1988; Viggiano et al., 1994) in patients or with topical application of melatonin in monkey’s eyes (Serle et al., 2004). In summary, available evidence supports that impairment of glutamate neurotoxicity, decrease in NO levels, manipulation of intracellular redox status using antioxidants, and adequate IOP control, or preferably the combination of all these treatments, may be a therapeutic strategy to prevent glaucomatous cell death. Melatonin could be a promissory resource in the management of glaucoma since, by itself, it exhibits antioxidant and antinitridergic properties, increases retinal glutamate clearance, and may regulate IOP. Further studies are needed to substantiate this hypothesis, mainly in view of the observation that under certain circumstances melatonin may increase retinal light damage in the rat (Bubenik and Purtill, 1980; Wiechmann and O’Steen, 1992). References Acun˜a-Castroviejo, D., Escames, G., Carazo, A., Leon, J., Khaldy, H., Reiter, R.J., 2002. Melatonin, mitochondrial homeostasis and mitochondrial-related diseases. Curr. Top. Med. Chem. 2, 133e151. Alder, V.A., Cringle, S.J., 1985. The effect of the retinal circulation on vitreal oxygen tension. Curr. Eye Res. 4, 121e129. Alonso-Gomez, A.L., Iuvone, P.M., 1995. Melatonin biosynthesis in cultured chick retinal photoreceptor cells: calcium and cyclic AMP protect serotonin N-acetyltransferase from inactivation in cycloheximide-treated cells. J. Neurochem. 65, 1054e1060. Avery, R.L., Fekrat, S., Hawkins, B.S., Bressler, N.M., 1996. Natural history of subfoveal subretinal hemorrhage in age-related macular degeneration. Retina 16, 183e189. Bailey, M.J., Chong, N.W., Xiong, J., Cassone, V.M., 2002. Chicken’s Cry2: molecular analysis of an avian cryptochrome in retinal and pineal photoreceptors. FEBS Lett. 513, 169e174. Benozzi, J., Nahum, L.P., Campanelli, J.L., Rosenstein, R.E., 2002. Effect of hyaluronic acid on intraocular pressure in rats. Investig. Ophthalmol. Vis. Sci. 43, 2196e2200. Bernard, M., Guerlotte, J., Greve, P., Grechez-Cassiau, A., Iuvone, M.P., Zatz, M., Chong, N.W., Klein, D.C., Voisin, P., 1999. Melatonin synthesis pathway: circadian regulation of the genes encoding the key enzymes in the chicken pineal gland and retina. Reprod. Nutr. Dev. 39, 325e334. Brainard, G.C., Hanifin, J.P., Greeson, J.M., Byrne, B., Glickman, G., Gerner, E., Rollag, M.D., 2001a. Action spectrum for melatonin regulation in humans: evidence for a novel circadian photoreceptor. J. Neurosci. 21, 6405e6412. Brainard, G.C., Hanifin, J.P., Rollag, M.D., Greeson, J., Byrne, B., Glickman, G., Gerner, E., Sanford, B., 2001b. Human melatonin regulation is not mediated by the three cone photopic visual system. J. Clin. Endocrinol. Metab. 86, 433e436. Bubenik, G.A., Purtill, R.A., 1980. The role of melatonin and dopamine in retinal physiology. Can. J. Physiol. Pharmacol. 58, 1457e1462. Bunin, A.I., Filina, A.A., Erichev, V.P., 1992. A glutathione deficiency in open-angle glaucoma and the approaches to its correction. Vestn. Oftalmol. 108, 13e15. Cahill, G.M., Besharse, J.C., 1993. Circadian clock functions localized in Xenopus retinal photoreceptors. Neuron 10, 573e577. Cai, J., Nelson, K.C., Wu, M., Sternberg Jr., P., Jones, D.P., 2000. Oxidative damage and protection of the RPE. Prog. Retin. Eye Res. 19, 205e221. Cardinali, D.P., Rosner, J.M., 1971a. Metabolism of serotonin by the rat retina ‘‘in vitro’’. J. Neurochem. 18, 1769e1770. Cardinali, D.P., Rosner, J.M., 1971b. Retinal localization of the hydroxyindoleO-methyl transferase (HIOMT) in the rat. Endocrinology 89, 301e303. Chan, D.K., Lim, M.S., Choo, S.H., Tan, I.K., 1999. Vitamin E status of infants at birth. J. Perinatal. Med. 27, 395e398.

1028

P.O. Lundmark et al. / Experimental Eye Research 84 (2007) 1021e1030

Chew, S.J., Ritch, R., 1997. Neuroprotection: the next breakthrough in glaucoma? Proceedings of the third annual optic nerve rescue and restoration think tank. J. Glaucoma 6, 263e266. Ciuffi, M., Pisanello, M., Pagliai, G., Raimondi, L., Franchi-Micheli, S., Cantore, M., Mazzetti, L., Failli, P., 2003. Antioxidant protection in cultured corneal cells and whole corneas submitted to UV-B exposure. J. Photochem. Photobiol. B Biol. 71, 59e68. Claustrat, B., Brun, J., Chazot, G., 2005. The basic physiology and pathophysiology of melatonin. Sleep Med. Rev. 9, 11e24. Cruickshanks, K.J., Klein, R., Klein, B.E., 1993. Sunlight and age-related macular degeneration. The Beaver Dam eye study. Arch. Ophthalmol. 111, 514e518. Cruickshanks, K.J., Klein, R., Klein, B.E., Nondahl, D.M., 2001. Sunlight and the 5-year incidence of early age-related maculopathy: the beaver dam eye study. Arch. Ophthalmol. 119, 246e250. Dinet, V., Girard-Naud, N., Voisin, P., Bernard, M., 2006. Melatoninergic differentiation of retinal photoreceptors: activation of the chicken hydroxyindole-O-methyltransferase promoter requires a homeodomain-binding element that interacts with Otx2. Exp. Eye Res. 83, 276e290. Dorey, C.K., Delori, F.C., Akeo, K., 1990. Growth of cultured RPE and endothelial cells is inhibited by blue light but not green or red light. Curr. Eye Res. 9, 549e559. Doyle, S.E., Grace, M.S., McIvor, W., Menaker, M., 2002. Circadian rhythms of dopamine in mouse retina: the role of melatonin. Vis. Neurosci. 19, 593e601. Dreyer, E.B., Pan, Z.H., Storm, S., Lipton, S.A., 1994. Greater sensitivity of larger retinal ganglion cells to NMDA-mediated cell death. Neuroreport 5, 629e631. Dreyer, E.B., Zurakowski, D., Schumer, R.A., Podos, S.M., Lipton, S.A., 1996. Elevated glutamate levels in the vitreous body of humans and monkeys with glaucoma. Arch. Ophthalmol. 114, 299e305. Dubocovich, M.L., Cardinali, D.P., Delagrange, P., Krause, D.N., Strosberg, D., Sugden, D., Yocca, F.D., 2000. Melatonin receptors. In: IUPHAR (Ed.), The IUPHAR Compendium of Receptor Characterization and Classification, second ed. IUPHAR Media, London, pp. 271e277. Dubocovich, M.L., Masana, M.I., Iacob, S., Sauri, D.M., 1997. Melatonin receptor antagonists that differentiate between the human Mel1a and Mel1b recombinant subtypes are used to assess the pharmacological profile of the rabbit retina ML1 presynaptic heteroreceptor. Naunyn Schmiedeberg’s Arch. Pharmacol. 355, 365e375. Dubocovich, M.L., Takahashi, J.S., 1987. Use of 2-[125I] iodomelatonin to characterize melatonin binding sites in chicken retina. Proc. Natl. Acad. Sci. U.S.A. 84, 3916e3920. Foster, R.G., Hankins, M.W., 2002. Non-rod, non-cone photoreception in the vertebrates. Prog. Retin. Eye Res. 21, 507e527. Frampton, P., Da Rin, D., Brown, B., 1987. Diurnal variation of intraocular pressure and the overriding effects of sleep. Am. J. Optom. Physiol. Opt. 64, 54e61. Fujieda, H., Hamadanizadeh, S.A., Wankiewicz, E., Pang, S.F., Brown, G.M., 1999. Expression of mt1 melatonin receptor in rat retina: evidence for multiple cell targets for melatonin. Neuroscience 93, 793e799. Fujieda, H., Scher, J., Hamadanizadeh, S.A., Wankiewicz, E., Pang, S.F., Brown, G.M., 2000. Dopaminergic and GABAergic amacrine cells are direct targets of melatonin: immunocytochemical study of mt1 melatonin receptor in guinea pig retina. Vis. Neurosci. 17, 63e70. Fukuhara, C., Dirden, J.C., Tosini, G., 2001. Photic regulation of melatonin in rat retina and the role of proteasomal proteolysis. Neuroreport 12, 3833e3837. Fukuhara, C., Liu, C., Ivanova, T.N., Chan, G.C., Storm, D.R., Iuvone, P.M., Tosini, G., 2004. Gating of the cAMP signaling cascade and melatonin synthesis by the circadian clock in mammalian retina. J. Neurosci. 24, 1803e1811. Gan, J., Alonso-Gomez, A.L., Avendano, G., Johnson, B., Iuvone, P.M., 1995. Melatonin biosynthesis in photoreceptor-enriched chick retinal cell cultures: role of cyclic AMP in the Kþ-evoked, Ca2þ-dependent induction of serotonin N-acetyltransferase activity. Neurochem. Int. 27, 147e155. Garbarino-Pico, E., Carpentieri, A.R., Contin, M.A., Sarmiento, M.I., Brocco, M.A., Panzetta, P., Rosenstein, R.E., Caputto, B.L.,

Guido, M.E., 2004. Retinal ganglion cells are autonomous circadian oscillators synthesizing N-acetylserotonin during the day. J. Biol. Chem. 279, 51172e51181. Green, W.R., Key III, S.N., 1977. Senile macular degeneration: a histopathologic study. Trans. Am. Ophthalmol. Soc. 75, 180e254. Greve, P., Alonso-Gomez, A., Bernard, M., Ma, M., Haque, R., Klein, D.C., Iuvone, P.M., 1999. Serotonin N-acetyltransferase mRNA levels in photoreceptor-enriched chicken retinal cell cultures: elevation by cyclic AMP. J. Neurochem. 73, 1894e1900. Guenther, A.L., Schmidt, S.I., Laatsch, H., Fotso, S., Ness, H., Ressmeyer, A.R., Poeggeler, B., Hardeland, R., 2005. Reactions of the melatonin metabolite AMK (N-acetyl-5-methoxykynuramine) with reactive nitrogen species: formation of novel compounds, 3-acetamidomethyl-6-methoxycinnolinone and 3-nitro-AMK. J. Pineal Res. 39, 251e260. Hamm, H.E., Menaker, M., 1980. Retinal rhythms in chicks: circadian variation in melatonin and serotonin N-acetyltransferase activity. Proc. Natl. Acad. Sci. U.S.A. 77, 4998e5002. Hammond Jr., B.R., Wooten, B.R., Snodderly, D.M., 1996. Cigarette smoking and retinal carotenoids: implications for age-related macular degeneration. Vis. Res. 36, 3003e3009. Haque, R., Chaurasia, S.S., Wessel III, J.H., Iuvone, P.M., 2002. Dual regulation of cryptochrome 1 mRNA expression in chicken retina by light and circadian oscillators. Neuroreport 13, 2247e2251. Hardeland, R., 2005. Antioxidative protection by melatonin: multiplicity of mechanisms from radical detoxification to radical avoidance. Endocrine 27, 119e130. Hardeland, R., Balzer, I., Poeggeler, B., Fuhrberg, B., Uria, H., Behrmann, G., Wolf, R., Meyer, T.J., Reiter, R.J., 1995. On the primary functions of melatonin in evolution: mediation of photoperiodic signals in a unicell, photooxidation, and scavenging of free radicals. J. Pineal Res. 18, 104e111. Hardeland, R., Pandi-Perumal, S.R., Cardinali, D.P., 2006. Melatonin. Int. J. Biochem. Cell Biol. 38, 313e316. Isorna, E., Guijarro, A., Jesus, D.M., Alonso-Bedate, M., AlonsoGomez, A.L., 2004. Characterization of melatonin binding sites in the brain and retina of the frog Rana perezi. Gen. Comp. Endocrinol. 135, 259e267. Iuvone, P.M., Brown, A.D., Haque, R., Weller, J., Zawilska, J.B., Chaurasia, S.S., Ma, M., Klein, D.C., 2002. Retinal melatonin production: role of proteasomal proteolysis in circadian and photic control of arylalkylamine N-acetyltransferase. Investig. Ophthalmol. Vis. Sci. 43, 564e572. Iuvone, P.M., Gan, J., Alonso-Gomez, A.L., 1995. 5-Methoxytryptamine inhibits cyclic AMP accumulation in cultured retinal neurons through activation of a pertussis toxin-sensitive site distinct from the 2[125I]iodomelatonin binding site. J. Neurochem. 64, 1892e1895. Ivanova, T.N., Michael, I.P., 2003. Melatonin synthesis in retina: circadian regulation of arylalkylamine N-acetyltransferase activity in cultured photoreceptor cells of embryonic chicken retina. Brain Res. 973, 56e63. Kaushik, S., Pandav, S.S., Ram, J., 2003. Neuroprotection in glaucoma. J. Postgrad. Med. 49, 90e95. Kitazawa, Y., Horie, T., 1975. Diurnal variation of intraocular pressure in primary open-angle glaucoma. Am. J. Ophthalmol. 79, 557e566. Klein, B.E., Klein, R., 1982. Cataracts and macular degeneration in older Americans. Arch. Ophthalmol. 100, 571e573. Kvetnoy, I., 2002. Extrapineal melatonin in pathology: new perspectives for diagnosis, prognosis and treatment of illness. Neuroendocrinol. Lett. 23 (Suppl. 1), 92e96. Lee, P.Y., Podos, S.M., Serle, J.B., Camras, C.B., Severin, C.H., 1987. Intraocular pressure effects of multiple doses of drugs applied to glaucomatous monkey eyes. Arch. Ophthalmol. 105, 249e252. Leon, J., Acun˜a-Castroviejo, D., Sainz, R.M., Mayo, J.C., Tan, D.X., Reiter, R.J., 2004. Melatonin and mitochondrial function. Life Sci. 75, 765e790. Liang, F.Q., Godley, B.F., 2003. Oxidative stress-induced mitochondrial DNA damage in human retinal pigment epithelial cells: a possible mechanism for RPE aging and age-related macular degeneration. Exp. Eye Res. 76, 397e403.

P.O. Lundmark et al. / Experimental Eye Research 84 (2007) 1021e1030 Liang, F.Q., Green, L., Wang, C., Alssadi, R., Godley, B.F., 2004. Melatonin protects human retinal pigment epithelial (RPE) cells against oxidative stress. Exp. Eye Res. 78, 1069e1075. Liu, C., Fukuhara, C., Wessel III, J.H., Iuvone, P.M., Tosini, G., 2004. Localization of Aa-nat mRNA in the rat retina by fluorescence in situ hybridization and laser capture microdissection. Cell Tissue Res. 315, 197e201. Liu, J.H., Dacus, A.C., 1991. Endogenous hormonal changes and circadian elevation of intraocular pressure. Investig. Ophthalmol. Vis. Sci. 32, 496e500. Lundmark, P.O., Pandi-Perumal, S.R., Srinivasan, V., Cardinali, D.P. Role of melatonin in the eye and ocular dysfunctions. Vis. Neurosci., in press. Marchiafava, P.L., Longoni, B., 1999. Melatonin as an antioxidant in retinal photoreceptors. J. Pineal Res. 26, 184e189. Martin, X.D., Malina, H.Z., Brennan, M.C., Hendrickson, P.H., Lichter, P.R., 1992. The ciliary body e the third organ found to synthesize indoleamines in humans. Eur. J. Ophthalmol. 2, 67e72. Meyer, P., Pache, M., Loeffler, K.U., Brydon, L., Jockers, R., Flammer, J., Wirz-Justice, A., Savaskan, E., 2002. Melatonin MT-1-receptor immunoreactivity in the human eye. Br. J. Ophthalmol. 86, 1053e1057. Mhatre, M.C., van Jaarsveld, A.S., Reiter, R.J., 1988. Melatonin in the lacrimal gland: first demonstration and experimental manipulation. Biochem. Biophys. Res. Commun. 153, 1186e1192. Militante, J., Lombardini, J.B., 2004. Age-related retinal degeneration in animal models of aging: possible involvement of taurine deficiency and oxidative stress. Neurochem. Res. 29, 151e160. Moore, C.G., Johnson, E.C., Morrison, J.C., 1996. Circadian rhythm of intraocular pressure in the rat. Curr. Eye Res. 15, 185e191. Moore, R.Y., 1997. Circadian rhythms: basic neurobiology and clinical applications. Annu. Rev. Med. 48, 253e266. Moreno, M.C., Campanelli, J., Sande, P., Sanez, D.A., Keller Sarmiento, M.I., Rosenstein, R.E., 2004. Retinal oxidative stress induced by high intraocular pressure. Free Radic. Biol. Med. 37, 803e812. Moreno, M.C., Marcos, H.J., Oscar, C.J., Sande, P.H., Campanelli, J., Jaliffa, C.O., Benozzi, J., Rosenstein, R.E., 2005a. A new experimental model of glaucoma in rats through intracameral injections of hyaluronic acid. Exp. Eye Res. 81, 71e80. Moreno, M.C., Sande, P., Marcos, H.A., de Zavalia, N., Keller Sarmiento, M.I., Rosenstein, R.E., 2005b. Effect of glaucoma on the retinal glutamate/glutamine cycle activity. FASEB J. 19, 1161e1162. Namihira, M., Honma, S., Abe, H., Masubuchi, S., Ikeda, M., Honmaca, K., 2001. Circadian pattern, light responsiveness and localization of rPer1 and rPer2 gene expression in the rat retina. Neuroreport 12, 471e475. Natesan, A.K., Cassone, V.M., 2002. Melatonin receptor mRNA localization and rhythmicity in the retina of the domestic chick, Gallus domesticus. Vis. Neurosci. 19, 265e274. Netland, P.A., Chaturvedi, N., Dreyer, E.B., 1993. Calcium channel blockers in the management of low-tension and open-angle glaucoma. Am. J. Ophthalmol. 115, 608e613. Neufeld, A.H., 2004. Pharmacologic neuroprotection with an inhibitor of nitric oxide synthase for the treatment of glaucoma. Brain Res. Bull. 62, 455e459. Neufeld, A.H., Hernandez, M.R., Gonzalez, M., 1997. Nitric oxide synthase in the human glaucomatous optic nerve head. Arch. Ophthalmol. 115, 497e503. Niki, T., Hamada, T., Ohtomi, M., Sakamoto, K., Suzuki, S., Kako, K., Hosoya, Y., Horikawa, K., Ishida, N., 1998. The localization of the site of arylalkylamine N-acetyltransferase circadian expression in the photoreceptor cells of mammalian retina. Biochem. Biophys. Res. Commun. 248, 115e120. Nohl, H., Gille, L., Staniek, K., 2005. Intracellular generation of reactive oxygen species by mitochondria. Biochem. Pharmacol. 69, 719e723. Nosjean, O., Ferro, M., Coge, F., Beauverger, P., Henlin, J.M., Lefoulon, F., Fauchere, J.L., Delagrange, P., Canet, E., Boutin, J.A., 2000. Identification of the melatonin-binding site MT3 as the quinone reductase 2. J. Biol. Chem. 275, 31311e31317. Nosjean, O., Nicolas, J.P., Klupsch, F., Delagrange, P., Canet, E., Boutin, J.A., 2001. Comparative pharmacological studies of melatonin receptors: MT1, MT2 and MT3/QR2. Tissue distribution of MT3/QR2. Biochem. Pharmacol. 61, 1369e1379.

1029

Osborne, N.N., 1994. Serotonin and melatonin in the iris/ciliary processes and their involvement in intraocular pressure. Acta Neurobiol. Exp. (Wars) 54 (Suppl.), 57e64. Osborne, N.N., Chidlow, G., 1994. The presence of functional melatonin receptors in the iris-ciliary processes of the rabbit eye. Exp. Eye Res. 59, 3e9. Osborne, N.N., Chidlow, G., Nash, M.S., Wood, J.P., 1999. The potential of neuroprotection in glaucoma treatment. Curr. Opin. Ophthalmol. 10, 82e92. Pandi-Perumal, S.R., Srinivasan, V., Maestroni, G.J.M., Cardinali, D.P., Poeggeler, B., Hardeland, R., 2006. Melatonin: nature’s most versatile biological signal? FEBS J. 273, 2813e2838. Pang, S.F., Yu, H.S., Suen, H.C., Brown, G.M., 1980. Melatonin in the retina of rats: a diurnal rhythm. J. Endocrinol. 87, 89e93. Papp, A., Nemeth, I., Karg, E., Papp, E., 1999. Glutathione status in retinopathy of prematurity. Free Radic. Biol. Med. 27, 738e743. Pierce, M.E., Besharse, J.C., 1985. Circadian regulation of retinomotor movements. I. Interaction of melatonin and dopamine in the control of cone length. J. Gen. Physiol. 86, 671e689. Pintor, J., Martin, L., Pelaez, T., Hoyle, C.H., Peral, A., 2001. Involvement of melatonin MT3 receptors in the regulation of intraocular pressure in rabbits. Eur. J. Pharmacol. 416, 251e254. Rada, J.A., Wiechmann, A.F., 2006. Melatonin receptors in chick ocular tissues: implications for a role of melatonin in ocular growth regulation. Investig. Ophthalmol. Vis. Sci. 47, 25e33. Reiter, R.J., 1998. Oxidative damage in the central nervous system: protection by melatonin. Prog. Neurobiol. 56, 359e384. Reiter, R.J., Carneiro, R.C., Oh, C.S., 1997. Melatonin in relation to cellular antioxidative defense mechanisms. Horm. Metab. Res. 29, 363e372. Reppert, S.M., Godson, C., Mahle, C.D., Weaver, D.R., Slaugenhaupt, S.A., Gusella, J.F., 1995. Molecular characterization of a second melatonin receptor expressed in human retina and brain: the Mel1b melatonin receptor. Proc. Natl. Acad. Sci. U.S.A. 92, 8734e8738. Ritch, R., 2000. Neuroprotection: is it already applicable to glaucoma therapy? Curr. Opin. Ophthalmol. 11, 78e84. Rufiange, M., Dumont, M., Lachapelle, P., 2002. Correlating retinal function with melatonin secretion in subjects with an early or late circadian phase. Investig. Ophthalmol. Vis. Sci. 43, 2491e2499. Saenz, D.A., Turjanski, A.G., Sacca, G.B., Marti, M., Doctorovich, F., Sarmiento, M.I., Estrin, D.A., Rosenstein, R.E., 2002. Physiological concentrations of melatonin inhibit the nitridergic pathway in the Syrian hamster retina. J. Pineal Res. 33, 31e36. Samples, J.R., Krause, G., Lewy, A.J., 1988. Effect of melatonin on intraocular pressure. Curr. Eye Res. 7, 649e653. Savaskan, E., Wirz-Justice, A., Olivieri, G., Pache, M., Krauchi, K., Brydon, L., Jockers, R., Muller-Spahn, F., Meyer, P., 2002. Distribution of melatonin MT1 receptor immunoreactivity in human retina. J. Histochem. Cytochem. 50, 519e526. Scher, J., Wankiewicz, E., Brown, G.M., Fujieda, H., 2002. MT1 melatonin receptor in the human retina: expression and localization. Investig. Ophthalmol. Vis. Sci. 43, 889e897. Scher, J., Wankiewicz, E., Brown, G.M., Fujieda, H., 2003. AII amacrine cells express the MT1 melatonin receptor in human and macaque retina. Exp. Eye Res. 77, 375e382. Serle, J.B., Podos, S.M., Kitazawa, Y., Wang, R.F., 1998. A comparative study of latanoprost (Xalatan) and isopropyl unoprostone (Rescula) in normal and glaucomatous monkey eyes. Jpn. J. Ophthalmol. 42, 95e100. Serle, J.B., Wang, R.F., Peterson, W.M., Plourde, R., Yerxa, B.R., 2004. Effect of 5-MCA-NAT, a putative melatonin MT3 receptor agonist, on intraocular pressure in glaucomatous monkey eyes. J. Glaucoma 13, 385e388. Shareef, S., Sawada, A., Neufeld, A.H., 1999. Isoforms of nitric oxide synthase in the optic nerves of rat eyes with chronic moderately elevated intraocular pressure. Investig. Ophthalmol. Vis. Sci. 40, 2884e2891. Siliprandi, R., Canella, R., Carmignoto, G., Schiavo, N., Zanellato, A., Zanoni, R., Vantini, G., 1992. N-methyl-D-aspartate-induced neurotoxicity in the adult rat retina. Vis. Neurosci. 8, 567e573. Siu, A.W., Leung, M.C., To, C.H., Siu, F.K., Ji, J.Z., So, K.F., 2002. Total retinal nitric oxide production is increased in intraocular pressure-elevated rats. Exp. Eye Res. 75, 401e406.

1030

P.O. Lundmark et al. / Experimental Eye Research 84 (2007) 1021e1030

Siu, A.W., Maldonado, M., Sanchez-Hidalgo, M., Tan, D.X., Reiter, R.J., 2006. Protective effects of melatonin in experimental free radical-related ocular diseases. J. Pineal Res. 40, 101e109. Siu, A.W., Reiter, R.J., To, C.H., 1999. Pineal indoleamines and vitamin E reduce nitric oxide-induced lipid peroxidation in rat retinal homogenates. J. Pineal Res. 27, 122e128. Smith, S.D., Gregory, D.S., 1989. A circadian rhythm of aqueous flow underlies the circadian rhythm of IOP in NZW rabbits. Investig. Ophthalmol. Vis. Sci. 30, 775e778. Srinivasan, V., Pandi-Perumal, S.R., Maestroni, G.J., Esquifino, A.I., Hardeland, R., Cardinali, D.P., 2005. Role of melatonin in neurodegenerative diseases. Neurotoxicol. Res. 7, 293e318. Srinivasan, V., Pandi-Perumal, S.R., Poeggeler, B., Hardeland, R., 2006. Melatonin in Alzheimer’s disease and other neurodegenerative disorders. Behav. Brain Funct. 2, 15. Sucher, N.J., Lei, S.Z., Lipton, S.A., 1991. Calcium channel antagonists attenuate NMDA receptor-mediated neurotoxicity of retinal ganglion cells in culture. Brain Res. 551, 297e302. Sugawara, T., Sieving, P.A., Iuvone, P.M., Bush, R.A., 1998. The melatonin antagonist luzindole protects retinal photoreceptors from light damage in the rat. Investig. Ophthalmol. Vis. Sci. 39, 2458e2465. Tan, D.X., Chen, B., Poeggeler, B., Manchester, L.C., Reiter, R.J., 1993. Melatonin: a potent, endogenous hydroxyl radical scavenger. Endocr. J. 1, 57e60. Tan, D.X., Hardeland, R., Manchester, L.C., Poeggeler, B., Lopez-Burillo, S., Mayo, J.C., Sainz, R.M., Reiter, R.J., 2003. Mechanistic and comparative studies of melatonin and classic antioxidants in terms of their interactions with the ABTS cation radical. J. Pineal Res. 34, 249e259. Thompson, C.L., Selby, C.P., Partch, C.L., Plante, D.T., Thresher, R.J., Araujo, F., Sancar, A., 2004. Further evidence for the role of cryptochromes in retinohypothalamic photoreception/phototransduction. Brain Res. Mol. Brain Res. 122, 158e166. Tosini, G., 2000. Melatonin circadian rhythm in the retina of mammals. Chronobiol. Int. 17, 599e612. Tosini, G., Menaker, M., 1996. Circadian rhythms in cultured mammalian retina. Science 272, 419e421. Tosini, G., Menaker, M., 1998. The clock in the mouse retina: melatonin synthesis and photoreceptor degeneration. Brain Res. 789, 221e228. Uchida, K., Iuvone, P.M., 1999. Intracellular Ca2þ concentrations in cultured chicken photoreceptor cells: sustained elevation in depolarized cells and the role of dihydropyridine-sensitive Ca2þ channels. Mol. Vis. 5, 1. Viggiano, S.R., Koskela, T.K., Klee, G.G., Samples, J.R., Arnce, R., Brubaker, R.F., 1994. The effect of melatonin on aqueous humor flow in humans during the day. Ophthalmology 101, 326e331. Vorwerk, C.K., Lipton, S.A., Zurakowski, D., Hyman, B.T., Sabel, B.A., Dreyer, E.B., 1996. Chronic low-dose glutamate is toxic to retinal ganglion cells. Toxicity blocked by memantine. Investig. Ophthalmol. Vis. Sci. 37, 1618e1624.

Wang, R.F., Serle, J.B., Podos, S.M., Sugrue, M.F., 1990a. The ocular hypotensive effect of the topical carbonic anhydrase inhibitor L-671,152 in glaucomatous monkeys. Arch. Ophthalmol. 108, 511e513. Wang, R.F., Serle, J.B., Podos, S.M., Surgrue, M.F., 1990b. The effect of MK-927, a topical carbonic anhydrase inhibitor, on IOP in glaucomatous monkeys. Curr. Eye Res. 9, 163e168. Weinreb, R.N., Levin, L.A., 1999. Is neuroprotection a viable therapy for glaucoma? Arch. Ophthalmol. 117, 1540e1544. Wiechmann, A.F., 1986. Melatonin: parallels in pineal gland and retina. Exp. Eye Res. 42, 507e527. Wiechmann, A.F., Campbell, L.D., Defoe, D.M., 1999. Melatonin receptor RNA expression in Xenopus retina. Brain Res. Mol. Brain Res. 63, 297e303. Wiechmann, A.F., O’Steen, W.K., 1992. Melatonin increases photoreceptor susceptibility to light-induced damage. Investig. Ophthalmol. Vis. Sci. 33, 1894e1902. Wiechmann, A.F., Rada, J.A., 2003. Melatonin receptor expression in the cornea and sclera. Exp. Eye Res. 77, 219e225. Wiechmann, A.F., Smith, A.R., 2001. Melatonin receptor RNA is expressed in photoreceptors and displays a diurnal rhythm in Xenopus retina. Brain Res. Mol. Brain Res. 91, 104e111. Wiechmann, A.F., Udin, S.B., Summers Rada, J.A., 2004. Localization of Mel1b melatonin receptor-like immunoreactivity in ocular tissues of Xenopus laevis. Exp. Eye Res. 79, 585e594. Wiechmann, A.F., Vrieze, M.J., Dighe, R., Hu, Y., 2003. Direct modulation of rod photoreceptor responsiveness through a Mel1c melatonin receptor in transgenic Xenopus laevis retina. Investig. Ophthalmol. Vis. Sci. 44, 4522e4531. Wiechmann, A.F., Wirsig-Wiechmann, C.R., 1991. Localization and quantification of high-affinity melatonin binding sites in Rana pipiens retina. J. Pineal Res. 10, 174e179. Wiechmann, A.F., Wirsig-Wiechmann, C.R., 1994. Melatonin receptor distribution in the brain and retina of a lizard, Anolis carolinensis. Brain Behav. Evol. 43, 26e33. Wiechmann, A.F., Wirsig-Wiechmann, C.R., 2001. Multiple cell targets for melatonin action in Xenopus laevis retina: distribution of melatonin receptor immunoreactivity. Vis. Neurosci. 18, 695e702. Witkovsky, P., Veisenberger, E., LeSauter, J., Yan, L., Johnson, M., Zhang, D.Q., McMahon, D., Silver, R., 2003. Cellular location and circadian rhythm of expression of the biological clock gene Period 1 in the mouse retina. J. Neurosci. 23, 7670e7676. Yi, C., Pan, X., Yan, H., Guo, M., Pierpaoli, W., 2005. Effects of melatonin in age-related macular degeneration. Ann. N.Y. Acad. Sci. 1057, 384e392. Yu, H.S., Yee, R.W., Howes, K.A., Reiter, R.J., 1990. Diurnal rhythms of immunoreactive melatonin in the aqueous humor and serum of male pigmented rabbits. Neurosci. Lett. 116, 309e314. Zawilska, J., Nowak, J.Z., 1989. Melatonin in the retina of vertebrates. Acta Physiol. Pol. 40 (Suppl. 33), 28e96. Zisapel, N., 2001. Melatonin-dopamine interactions: from basic neurochemistry to a clinical setting. Cell. Mol. Neurobiol. 21, 605e616.