Diurnal Variations in Cyclic AMP and Melatonin Content of Golden Hamster Retina

Abstract: The diurnal variations and photic regulation of cyclic AMP and melatonin content in golden hamster retina were studied. Both parameters showed significant diurnal variations with maximal values at night. Light exposure during the night inhibited retinal cyclic AMP and melatonin levels, whereas exposure to darkness during the day significantly increased cyclic AMP and melatonin content. Incubation with melatonin of retinas excised at different intervals indicated that the methoxyindole inhibited cyclic AMP accumulation in a time-dependent manner. The inhibitory effect of melatonin at 2400 h and at noon showed a threshold concentration of 1 and 10 pM, respectively. At 0400 h melatonin did not affect cyclic AMP accumulation. The results indicate a diurnal variability of retinal cyclic AMP and melatonin content in hamsters, mainly influenced by a photic stimulus. Cyclic AMP could be a putative second messenger for melatonin action in golden hamster retina.     

Pineal and serum melatonin at midday and midnight following pinealectomy or castration in male rats.

Melatonin has been extractkd by chloroform from rat serum and quantified by the Rana pipiens tadpole bioassay. Like pineal melatonin, serum melatonin was high at mid-dark and low at mid-light. This finding suggests a diurnal rhythm of serum melatonin in the rat. Serum samples obtained at mid-dark had melatonin levels ranging from 0.02 to 0.05 ng per ml of serum. When rats were pinealectomized, melatonin activity in the serum was abolished. Thus, rat serum melatonin may originate primarily from the pineal. There was no significant change in pineal melatonin content nor in serum melatonin concentrations 7 or 12 days following orchidectomy.     

Melatonin and its generating system in vertebrate retina: circadian rhythm, effect of environmental lighting and interaction with dopamine

Retinas of rats, rabbits, chicks and carp possess enzymes, i.e. serotonin N-acetyltransferase (NAT) and hydroxyindole-O-methyltransferase (HIOMT), which convert serotonin (5-HT) to melatonin, NAT activity and melatonin levels, but not HIOMT activity, show distinct circadian rhythms, with peak values occurring during the dark (night) phase of the 12 h light-dark cycle. Exposure of the animals to light at night inhibited the night-stimulated NAT activity. Treatment of rats and rabbits with the dopaminergic agonist, apomorphine, inhibited the retinal NAT activity. Dopamine levels in the rabbit retina showed diurnal variations, with higher contents seen during the light phase of both the 12 h light-dark cycle with lights on between 06:00–18:00, and that with reversed periods of illumination (lights on between 18:00–06:00). Melatonin potently inhibited the electrically-evoked calcium-dependent release of [³H]dopamine from pieces of retina from both albino and pigmented rabbits. Our results indicate that the light-regulated melatonin-generating system does operate in the vertebrate retina. The present data, together with other findings, suggest that in the retina there is an antagonistic interplay between melatonin and dopamine. Thus, melatonin inhibits dopamine synthesis in, and release from, the retinal dopaminergic cells, whilst dopamine inhibits the night (dark)-stimulated melatonin formation by decreasing NAT activity. Since light increases metabolic activity of the retinal dopaminergic cells (it enhances the amine synthesis, levels and release), it seems likely that the retinal dopamine plays a role of a “light” messenger in the inhibition of melatonin synthesis. It is suggested that an interplay between melatonin and dopamine in the retina is responsible for regulation of those retinal events which follow circadian rhythmicity, and/or are dependent on light-dark conditions.     

Negative correlation of age and the levels of pineal melatonin, pineal N-acetylserotonin, and serum melatonin in male rats

Pineal concentrations of N-acetylserotonin and melatonin and serum levels of melatonin were studied in 3-wk-old (prepubertal), 8-wk-old (adult), and 17-mo-old (senile) male rats. They were adapted to a photoperiod of 12 h light/12 h darkness for a minimum of 1 wk and killed at mid-light and mid-dark. Melatonin and N-acetylserotonin were determined by radioimmunoassay. The concentrations of pineal N-acetylserotonin and melatonin were high in the dark period and low in the light period. Statistical analysis indicated that pineal N-acetylserotonin and melatonin levels per 100 gm body weight declined with age. Similarly, serum melatonin demonstrated diurnal changes in all the age groups studied. In addition, there was a significant reduction in the levels of serum melatonin with age. The parallel patterns of decrease in pineal and serum melatonin levels with age suggest a decline in pineal secretion of melatonin in the older animals.     

MELATONIN CIRCADIAN RHYTHM IN THE RETINA OF MAMMALS

Melatonin has been traditionally considered to be derived principally from the pineal gland. However, several investigations have now demonstrated that melatonin synthesis occurs also in the retina (and in other organs as well) of several vertebrate classes, including mammals. As in the pineal, melatonin synthesis in the retina is elevated at night and reduced during the day. Since melatonin receptors are present in the retina and retinal melatonin does not contribute to the circulating levels, retinal melatonin probably acts locally as a neuromodulator. Melatonin synthesis in the retinas of mammals is under control of a circadian oscillator located within the retina itself, and circadian rhythms in melatonin synthesis and/or release have been described for several species of rodents. These rhythms are present in vivo, persist in vitro, are entrained by light, and are temperature compensated. The recent cloning of the gene responsible for the synthesis of the enzyme arylalkylamine N-acetyltransferase (the only enzyme unique to the melatonin synthetic pathway) will facilitate localizing the cellular site of melatonin synthesis in the retina and investigating the molecular mechanism responsible for the generation of retinal melatonin rhythmicity. Melatonin has been implicated in many retinal functions, and the levels of melatonin and dopamine appear to regulate several aspects of retinal physiology that relate to light and dark adaptation. In conclusion, it seems that retinal melatonin is involved in several functions, but its precise role is yet to be understood. (Chronobiology International, 17(5), 599–612, 2000)     

Melatonin binding sites in senegal sole: day/night changes in density and location in different regions of the brain

We localized melatonin binding sites in different brain regions (optic tectum, telencephalon, cerebellum, hypothalamus, olfactory bulbs, and medulla oblongata) of Senegal sole, a species of aquaculture interest, and checked day/night changes in density (B(max)) at mid-light (ZT06) and mid-dark (ZT18). Plasma melatonin was measured using a radioimmunoassay, while binding assays were performed using 2-[(125)I]iodomelatonin as a radioligand. Plasma melatonin concentrations were significantly lower at mid-light (189.5+/-46 pg/ml) than mid-dark (455.5+/-163 pg/ml). Values of B(max) were statistically significantly higher in the optic tectum (5.6+/-0.6 and 12.3+/-1 fmol/mg prot, at mid-light and mid-dark, respectively) and in the cerebellum (7.7+/-1.1 and 10.6+/-1.3 fmol/mg prot, at mid-light and mid-dark, respectively). Significant day/night differences were only observed in these two tissues. These results show for the first time the distribution of melatonin binding sites within the brain of a flatfish species and their lack of down-regulation.     

Inferior retinal light exposure is more effective than superior retinal exposure in suppressing melatonin in humans

Illumination of different areas of the human retina elicits differences in acute light-induced suppression of melatonin. The aim of this study was to compare changes in plasma melatonin levels when light exposures of equal illuminance and equal photon dose were administered to superior, inferior, and full retinal fields. Nine healthy subjects participated in the study. Plexiglass eye shields were modified to permit selective exposure of the superior and inferior halves of the retinas of each subject. The Humphrey Visual Field Analyzer was used both to confirm intact full visual fields and to quantify exposure of upper and lower visual fields. On study nights, eyes were dilated, and subjects were exposed to patternless white light for 90 min between 0200 and 0330 under five conditions: (1) full retinal exposure at 200 lux, (2) full retinal exposure at 100 lux, (3) inferior retinal exposure at 200 lux, (4) superior retinal exposure at 200 lux, and (5) a dark-exposed control. Plasma melatonin levels were determined by radioimmunoassay. ANOVA demonstrated a significant effect of exposure condition (F = 5.91, p < 0.005). Post hoc Fisher PLSD tests showed significant (p < 0.05) melatonin suppression of both full retinal exposures as well as the inferior retinal exposure; however, superior retinal exposure was significantly less effective in suppressing melatonin. Furthermore, suppression with superior retinal exposure was not significantly different from that of the dark control condition. The results indicate that the inferior retina contributes more to the light-induced suppression of melatonin than the superior retina at the photon dosages tested in this study. Findings suggest a greater sensitivity or denser distribution of photoreceptors in the inferior retina are involved in light detection for the retinohypothalamic tract of humans.     

Melatonin in octopus (<Emphasis Type="Italic">Octopus vulgaris)</Emphasis>: tissue distribution,daily changes and relation with serotonin and its acid metabolite

Information regarding melatonin production in molluscs is very limited. In this study the presence and daily fluctuations of melatonin levels were investigated in hemolymph, retina and nervous system-related structures in the cephalopod Octopus vulgaris. Adult animals were maintained in captivity under natural photoperiod and killed at different times in a regular daily cycle. Levels of melatonin, serotonin (5-HT) and its acid metabolite (5-hydroxyindole acetic acid, 5-HIAA) in the hemolymph, retina, optic lobe, and cerebral ganglion were assayed by HPLC. Melatonin content fluctuated rhythmically in the retina and hemolymph, peaking at night. In the retina, but not in the other neural tissues, the rhythm was opposite to that of 5-HT, which displayed basal levels at night. Also, 5-HIAA levels in the retina were higher during the night, supporting that rhythmic melatonin production could be linked to diurnal changes in 5-HT degradation. The high levels of melatonin found in the retina point to it as the major source of melatonin in octopus; in addition, a large variation of melatonin content was found in the optic lobe with maximal values at night. All these data suggest that melatonin might play a role in the transduction of the light–dark cycle information for adjustment of rhythmic physiological events in cephalopods.     

Melatonin circadian rhythm in the retina of mammals

Melatonin has been traditionally considered to be derived principally from the pineal gland. However, several investigations have now demonstrated that melatonin synthesis occurs also in the retina (and in other organs as well) of several vertebrate classes, including mammals. As in the pineal, melatonin synthesis in the retina is elevated at night and reduced during the day. Since melatonin receptors are present in the retina and retinal melatonin does not contribute to the circulating levels, retinal melatonin probably acts locally as a neuromodulator. Melatonin synthesis in the retinas of mammals is under control of a circadian oscillator located within the retina itself, and circadian rhythms in melatonin synthesis and/or release have been described for several species of rodents. These rhythms are present in vivo, persist in vitro, are entrained by light, and are temperature compensated. The recent cloning of the gene responsible for the synthesis of the enzyme arylalkylamine N-acetyltransferase (the only enzyme unique to the melatonin synthetic pathway) will facilitate localizing the cellular site of melatonin synthesis in the retina and investigating the molecular mechanism responsible for the generation of retinal melatonin rhythmicity. Melatonin has been implicated in many retinal functions, and the levels of melatonin and dopamine appear to regulate several aspects of retinal physiology that relate to light and dark adaptation. In conclusion, it seems that retinal melatonin is involved in several functions, but its precise role is yet to be understood.     

Daily oscillation in melatonin synthesis in the Turkey pineal gland and retina: diurnal and circadian rhythms

The aim of the present study was to examine arylalkylamine N-acetyltransferase (AANAT) activity and melatonin content in the pineal gland and retina as well as the melatonin concentration in plasma of the turkey (Meleagris gallopavo), an avian species in which several physiological processes, including reproduction, are controlled by day length. In order to investigate whether the analyzed parameters display diurnal or circadian rhythmicity, we measured these variables in tissues isolated at regular time intervals from birds kept either under a regular light-dark (LD) cycle or under constant darkness (DD). The pineal gland and retina of the turkey rhythmically produced melatonin. In birds kept under a daily LD cycle, melatonin levels in the pineal gland and retina were high during the dark phase and low during the light phase. Rhythmic oscillations in melatonin, with high night-time concentrations, were also found in the plasma. The pineal and retinal melatonin rhythms mirrored oscillations in the activity of AANAT, the penultimate enzyme in the melatonin biosynthetic pathway. Rhythmic oscillations in AANAT activity in the turkey pineal gland and retina were circadian in nature, as they persisted under conditions of constant darkness (DD). Transferring birds from LD into DD, however, resulted in a potent decline in the amplitude of the AANAT rhythm from the first day of DD. On the sixth day of DD, pineal AANAT activity was still markedly higher during the subjective dark than during the subjective light phase; whereas, AANAT activity in the retina did not exhibit significant oscillations. The results indicate that melatonin rhythmicity in the turkey pineal gland and retina is regulated both by light and the endogenous circadian clock. The findings suggest that environmental light may be of primary importance in the maintenance of the high-amplitude melatonin rhythms in the turkey.     

Melatonin Increases Serotonin N-Acetyltransferase Activity and Decreases Dopamine Synthesis in Light-Exposed Chick Retina: In Vivo Evidence Supporting Melatonin-Dopamine Interaction in Retina

The administration of melatonin, either peripherally (0.01-10 mg/kg) or intraocularly (0.001-10 mumol/eye), to light-exposed chicks dose-dependently increased serotonin N-acetyltransferase (NAT) activity in retina but not in pineal gland. The effect of melatonin was slightly but significantly reduced by luzindole (2-benzyl-N-acetyltryptamine), and not affected by two other purported melatonin antagonists, N-acetyltryptamine and N-(2,4-dinitrophenyl)-5-methoxytryptamine (ML-23). The elevation of the enzyme activity induced by melatonin was substantially stronger than that evoked by 5-hydroxytryptamine, N-acetyl-5-hydroxytryptamine, or 5-methoxytryptamine. The melatonin-evoked rise in the retinal NAT activity was counteracted by two dopamine D2 receptor agonists, quinpirole and apomorphine, and prevented by the dopamine D2 receptor blocker spiroperidol, and by an inhibitor of dopamine synthesis, alpha-methyl-p-tyrosine. Melatonin (0.1-10 mg/kg i.p.) dose-dependently decreased the levels of dopamine and 3,4-dihydroxyphenylacetic acid (DOPAC), as well as the DOPAC/dopamine ratio, in chick retina but not in forebrain. The results obtained (1) indicate that melatonin in vivo potently inhibits dopamine synthesis selectively in retina, and (2) suggest that the increase in retinal NAT activity evoked by melatonin in light-exposed chicks is an indirect action of the compound, and results from the disinhibition of the NAT induction process from the dopaminergic (inhibitory) signal. The results provide in vivo evidence supporting the idea (derived on the basis of in vitro findings) that a mutually antagonistic interaction between melatonin and dopamine operates in retinas of living animals.     

Effects of melatonin and 6-methoxy-tetrahydro-beta-carboline in light induced retinal damage: a computerized morphometric method

The effects of melatonin and a related 5-methoxy-indole, 6-methoxy-1,2,3,4-tetrahydro-beta-carboline (6-MeO-THBC) were investigated in rats on the development of retinal degeneration in presence of high intensity illumination (HII). A morphometric method is used in which the degree of degeneration was evaluated by a computer-coupled graphical analyzer. Instead of measuring individual thicknesses of different retinal layers at various loci we measured large areas of retinal light microscopic sections. Thus the influence of sporadic artefactual and other fluctuations in the thickness of various layers of the retina can be essentially reduced. Continuous light produced significant degeneration of the retina and the degree of degeneration was further increased by both studied compounds and even more by 6-MeO-THBC. The role of melatonin and 6-MeO-THBC in retinal physiology is discussed.     

Effect of GABA on melatonin content in golden hamster retina

The effect of GABA on melatonin content in vitro was studied in the golden hamster retina. GABA significantly increased melatonin levels in a dose-dependent manner, its effect being reversed by a GABA(A) receptor antagonist, bicuculline, but not by saclofen, a GABA(B) antagonist. Moreover, an equimolar concentration of muscimol, a GABA(A) receptor agonist, significantly increased retinal melatonin content, whereas baclofen, a GABA(B) receptor agonist, was ineffective. The darkness-induced increase in melatonin content in vitro was inhibited by bicuculline, whereas saclofen was ineffective. Retinal GABA turnover rate was significantly higher at midnight than at midday. GABA significantly decreased cyclic AMP and increased cyclic GMP accumulation in the golden hamster retina. The effect of GABA on both nucleotide levels was reversed by bicuculline, but baclofen had no effect. Cyclic GMP analogues (i.e., 8-bromoguanosine 3',5'-cyclic monophosphate and 2'-O-dibutyrylguanosine 3',5'-cyclic monophosphate) significantly increased retinal melatonin content in vitro. Taken together, these results support the hypothesis that GABA may be important for the "dark message" in the hamster retina.     

Gicerin,a cell adhesion molecule,participates in the histogenesis of retina

Gicerin is a novel cell adhesion molecule that belongs to the immunoglobulin superfamily. Gicerin protein adheres to neurite outgrowth factor (NOF), an extracellular matrix protein in the laminin family, and also exhibits homophilic adhesion. Heterophilic adhesion of gicerin to NOF is thought to play an active role in neurite outgrowth of developing retinal cells in vitro. In this study, we examined the adhesion activity of gicerin during the retinal development of Japanese quail using an antibody directed against gicerin, to elucidate the biological importance of gicerin in retinal histogenesis. Immunohistochemical and Western blot analysis showed that gicerin was highly expressed in the developing retina but suppressed in the mature retina. The aggregation of neural retinal cells from 5-day embryonic quail retina was significantly inhibited when incubated with a polyclonal antibody to gicerin, suggesting that gicerin protein participates in the adhesion of neural retinal cells of the developing retina. Furthermore, histogenesis of retina both in the organ cultures and in ovo embryos was severely disrupted by incubation with a gicerin antibody. These findings provide evidence that gicerin plays an important role in retinal histogenesis. © 1997 John Wiley & Sons, Inc. J Neurobiol 33: 769–780, 1997     

Daily Oscillation in Melatonin Synthesis in The Turkey Pineal Gland and Retina: Diurnal and Circadian Rhythms

The aim of the present study was to examine arylalkylamine N‐acetyltransferase (AANAT) activity and melatonin content in the pineal gland and retina as well as the melatonin concentration in plasma of the turkey (Meleagris gallopavo), an avian species in which several physiological processes, including reproduction, are controlled by day length. In order to investigate whether the analyzed parameters display diurnal or circadian rhythmicity, we measured these variables in tissues isolated at regular time intervals from birds kept either under a regular light‐dark (LD) cycle or under constant darkness (DD). The pineal gland and retina of the turkey rhythmically produced melatonin. In birds kept under a daily LD cycle, melatonin levels in the pineal gland and retina were high during the dark phase and low during the light phase. Rhythmic oscillations in melatonin, with high night‐time concentrations, were also found in the plasma. The pineal and retinal melatonin rhythms mirrored oscillations in the activity of AANAT, the penultimate enzyme in the melatonin biosynthetic pathway. Rhythmic oscillations in AANAT activity in the turkey pineal gland and retina were circadian in nature, as they persisted under conditions of constant darkness (DD). Transferring birds from LD into DD, however, resulted in a potent decline in the amplitude of the AANAT rhythm from the first day of DD. On the sixth day of DD, pineal AANAT activity was still markedly higher during the subjective dark than during the subjective light phase; whereas, AANAT activity in the retina did not exhibit significant oscillations. The results indicate that melatonin rhythmicity in the turkey pineal gland and retina is regulated both by light and the endogenous circadian clock. The findings suggest that environmental light may be of primary importance in the maintenance of the high‐amplitude melatonin rhythms in the turkey.     

Vitamin A deficiency reduces the responsiveness of pineal gland to light in Japanese quail (Coturnix japonica)

Synthesis of melatonin in pineal gland is under the control of light environment. The recent finding of the presence of rhodopsin-like photopigment (pinopsin) and retinal in the avian pinealocytes has led to a hypothesis that vitamin A is involved in photoresponses of the pineal gland. We have thus analyzed the effect of vitamin A deficiency on the regulatory system of melatonin synthesis in the pineal gland of Japanese quail. Depletion of vitamin A from Japanese quails was attained by feeding them with a vitamin A-free diet supplemented with retinoic acid. In the vitamin A-deficient birds, diurnal rhythm in melatonin production persisted such that the phase of the wave was similar to that seen in the control birds. However, the amplitude of the nighttime surge of pineal melatonin was damped by vitamin A deficiency. When the control birds were briefly exposed to light at night, pineal melatonin dropped to the daytime level. In contrast, only slight decrease was observed in the vitamin A-deficient quails. The light responsiveness was restored after feeding the vitamin A-deficient quails with the control diet for 1 week. These results indicate that vitamin A plays essential roles in maintaining sufficient responsiveness of the avian pineal gland to photic input.     

Melatonin binding sites in the brain of European sea bass (Dicentrarchus labrax)

Characteristics, day-night changes, guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS) modulation, and localization of melatonin binding sites in the brain of a marine teleost, European sea bass Dicentrarchus labrax, were studied by radioreceptor assay using 2-[(125)I]iodomelatonin as a radioligand. The specific binding to the sea bass brain membranes was rapid, stable, saturable and reversible. The radioligand binds to a single class of receptor site with the affinity (Kd) of 9.3 +/-0.6 pM and total binding capacity (Bmax) of 39.08 +/-0.86 fmol/mg protein (mean+/-SEM, n=4) at mid-light under light-dark (LD) cycles of 12:12. Day-night changes were observed neither in the Kd nor in the Bmax under LD 12:12. Treatment with GTPgammaS significantly increased the Kd and decreased the Bmax both at mid-light and mid-dark. The binding sites were highly specific for 2-phenylmelatonin, 2-iodomelatonin, melatonin, and 6-chloromelatonin. Distribution of melatonin binding sites in the sea bass brain was uneven: The Bmax was determined to be highest in mesencephalic optic tectum-tegmentum and hypothalamus, intermediate in telencephalon, cerebellum-vestibulolateral lobe and medulla oblongata-spinal cord, and lowest in olfactory bulbs with the Kd in the low picomolar range. These results indicate that melatonin released from the pineal organ and/or retina plays neuromodulatory roles in the sea bass brain via G protein-coupled melatonin receptors.     

Pertussis toxin blocks melatonin-induced inhibition of forskolin-stimulated adenylate cyclase activity in the chick brain.

The high-affinity guanine nucleotide-sensitive receptor sites for melatonin in the mammalian hypothalamus and pars tuberalis mediate inhibition of adenylate cyclase (AC) activity. Therefore, we have examined whether similar sites in the chick brain and retina also modulate AC activity. Melatonin did not alter basal or forskolin-stimulated AC activity in whole forebrain or retinal homogenates. In contrast, melatonin significantly inhibited forskolin-stimulated AC activity in forebrain synaptosomal membranes and partially purified retinal membranes in a concentration-dependent manner. Maximal inhibition (approximately 25-30%) of stimulated AC activity was observed at 10-100nM melatonin, while the concentrations (EC50's) which caused half-maximal effects were 22 +/- 6 pM and 30 +/- 5 pM in the brain and retina respectively. Pretreatment of forebrain slices with pertussis toxin abolished the inhibitory effect of melatonin on stimulated AC activity. These data provide the first evidence that melatonin suppresses AC activity in the chick CNS via a pertussis toxin-sensitive G-protein.     

Effects of water salinity on melatonin levels in plasma and peripheral tissues and on melatonin binding sites in European sea bass (Dicentrarchus labrax)

Sea bass is an euryhaline fish that lives in a wide range of salinities and migrates seasonally from lagoons to the open sea. However, to date, the influence of water salinity on sea bass melatonin levels has not been reported. Here, we evaluated the differences in plasma and tissue melatonin contents and melatonin binding sites in sea bass under four different salinity levels: seawater (36‰), isotonic water (15‰), brackish water (4‰) and freshwater (0‰). The melatonin content was evaluated in plasma, whole brain, gills, intestine and kidney, while melatonin binding sites were analyzed in different brain regions and in the neural retina. Plasma melatonin levels at mid-dark varied, the lowest value occurring in seawater (102 pg/mL), and the highest in freshwater (151 pg/mL). In gills and intestine, however, the highest melatonin values were found in the seawater group (209 and 627 pg/g tissue, respectively). Melatonin binding sites in the brain also varied with salinity, with the highest density observed at the lower salinities in the optic tectum, cerebellum and hypothalamus (30.3, 13.0, and 8.0 fmol/mg protein, respectively). Melatonin binding sites in the retina showed a similar pattern, with the highest values being observed in freshwater. Taken together, these results reveal that salinity influences melatonin production and modifies the density of binding sites, which suggests that this hormone could play a role in timing seasonal events in sea bass, including those linked to fish migration between waters of different salinities for reproduction and spawning.     

Retinal oxidative stress induced by high intraocular pressure

Glaucoma is an optic neuropathy in which retinal ganglion cells die probably through an apoptotic process. Apoptosis is known to involve free radicals in several systems including the retina. In this context, the aim of the present work was to analyze retinal oxidative damage in rats with glaucoma induced by the chronic injection of hyaluronic acid in the eye anterior chamber. The results showed a significant decrease in total retinal superoxide dismutase and catalase activities after 6 and 3 weeks of treatment with hyaluronic acid, respectively. Also, although GPX activity increased after 10 weeks of ocular hypertension, GSH levels significantly decreased at 6 weeks of treatment with hyaluronic acid. Moreover, retinal lipid peroxidation significantly increased in a time-of-hypertension-dependent manner. On the other hand, a significant decrease in both diurnal and nocturnal retinal melatonin content was detected at 3, 6, or 10 weeks of treatment with hyaluronic acid. The present results suggest that retinal oxidative stress may be involved in glaucomatous cell death. Thus, manipulation of intracellular redox status using antioxidants may be a new therapeutic tool to prevent glaucomatous neurodegeneration.