Visual conditions affecting eye growth alter diurnal levels of vitreous DOPAC

In chicks, the diurnal patterns of retinal dopamine synthesis and release are associated with refractive development. To assess the within-day patterns of dopamine release, we assayed vitreal levels of DOPAC (3,4-dihydroxyphenylacetic acid) using high performance liquid chromatography with electrochemical detection, at 4-h intervals over 24 h in eyes with experimental manipulations that change ocular growth rates. Chicks were reared under a 12 h light/12 h dark cycle; experiments began at 12 days of age. Output was assessed by modelling using the robust variance structure of Generalized Estimating Equations. Continuous spectacle lensdefocus or form deprivation: One group experienced non-restricted visual input to both eyes and served as untreated "normal" controls. Three experimental cohorts underwent monocular visual alterations known to alter eye growth and refraction: wearing a diffuser, a negative lens or a positive lens. After one full day of device-wear, chicks were euthanized at 4-h intervals over 24 h (8 birds per time/condition). Brief hyperopic defocus: Chicks wore negative lenses for only 2 daily hours either in the morning (starting at ZT 0; n = 16) or mid-day (starting at ZT 4; n = 8) for 3 days. Vitreal DOPAC was assayed. In chicks with bilateral non-restricted vision, or with continuous defocus or form-deprivation, there was a diurnal variation in vitreal DOPAC levels for all eyes (p < 0.001 for each). In normal controls, DOPAC was highest during the daytime, lowest at night, and equivalent for both eyes. In experimental groups, regardless of whether experiencing a growth stimulatory input (diffuser; negative lens) or growth inhibitory input (positive lens), DOPAC levels were reduced compared both to fellow eyes and to those of normal controls (p < 0.001 for each). These diurnal variations in vitreous DOPAC levels under different visual conditions indicate a complexity for dopaminergic mechanisms in refractive development that requires further study.

Circadian clock genes of goldfish, Carassius auratus: cDNA cloning and rhythmic expression of period and cryptochrome transcripts in retina, liver, and gut

Clock genes are known to be the molecular core of biological clocks of vertebrates. They are expressed not only in those tissues considered central pacemakers, but also in peripheral tissues. In the present study, partial cDNAs for 6 of the principal clock genes (Period 1-3 and Cryptochrome 1-3) were cloned from a teleost fish, the goldfish (Carassius auratus ). These genes showed high homology (approximately 90%) with the respective cDNAs of zebrafish (Danio rerio), the only other teleost from which clock genes have been cloned. The daily expression pattern of each gene in retina, gut, and liver of goldfish was investigated using quantitative RT-PCR and cosinor analysis. All clock genes analyzed in the retina showed circadian rhythmicity; however, only Per 2-3 and Cry 2-3 were rhythmic in goldfish liver and gut. The amplitude and phase of the expression in liver and gut were different from those found in goldfish retina. Such differences suggest that other cues, such as feeding time, may contribute to the entrainment of oscillators in goldfish liver and gut. Our results support the use of goldfish as a teleost model to investigate the location and functioning of the circadian oscillators.

Light regulation of retinal dopamine that is independent of melanopsin phototransduction

Light-dependent release of dopamine (DA) in the retina is an important component of light-adaptation mechanisms. Melanopsin-containing inner retinal photoreceptors have been shown to make physical contacts with DA amacrine cells, and have been implicated in the regulation of the local retinal environment in both physiological and anatomical studies. Here we determined whether they contribute to photic regulation of DA in the retina as assayed by the ratio of DA with its primary metabolite, 3,4-dihydroxyphenylacetic acid (DOPAC), and by c-fos induction in tyrosine hydroxylase (TH)-labelled DA amacrine cells. Light treatment (approximately 0.7 log W/m(2) for 90 min) resulted in a substantial increase in DA release (as revealed by an increase in the DOPAC : DA ratio), as well as widespread induction of nuclear c-fos in DA amacrine cells in wild-type mice and in mice lacking melanopsin (Opn4(-/-)). Light-induced DA release was also retained in mice lacking rod phototransduction (Gnat1(-/-)), although the magnitude of this response was substantially reduced compared with wild-types, as was the incidence of light-dependent nuclear c-fos in DAergic amacrines. By contrast, the DAergic system of mice lacking both rods and cones (rd/rd cl) showed no detectable light response. Our data suggest that light regulation of DA, a pivotal retinal neuromodulator, originates primarily with rods and cones, and that melanopsin is neither necessary nor sufficient for this photoresponse.

Postnatal control of ocular growth: dopaminergic mechanisms

A vision-dependent feedback mechanism contributes to the regulation of postnatal eye growth and refraction; this mechanism is located at least in part in the retina. In chicks and rhesus monkeys, visual deprivation leads to ocular enlargement and a myopic refractive error, and it also reduces the retinal concentration of dopamine. In neonatal chicks, local application of the dopamine agonist apomorphine limits the excessive axial elongation that is associated with visual deprivation. Both D1 and D2 dopamine receptor mechanisms may participate. Remarkably, apomorphine is geometrically selective, not limiting the exaggerated equatorial growth that occurs during visual deprivation. Preliminary results in neonatal rhesus monkeys indicate that apomorphine eye drops also suppress exaggerated axial growth and myopic refractive error during visual deprivation; equatorial diameters were not measured. Our observations are consistent with a retinal hypothesis for emmetropization and suggest that retinal dopamine participates in a mechanism linking ocular growth control to vision. In addition, we conclude that axial and equatorial dimensions are independently regulated in the chick.

Molecular cloning, localization and circadian expression of chicken melanopsin (Opn4): differential regulation of expression in pineal and retinal cell types

The avian retina and pineal gland contain autonomous circadian oscillators and photo-entrainment pathways, but the photopigment(s) that mediate entrainment have not been definitively identified. Melanopsin (Opn4) is a novel opsin involved in entrainment of circadian rhythms in mammals. Here, we report the cDNA cloning of chicken melanopsin and show its expression in retina, brain and pineal gland. Like the melanopsins identified in amphibians and mammals, chicken melanopsin is more similar to the invertebrate retinaldehyde-based photopigments than the retinaldehyde-based photopigments typically found in vertebrates. In retina, melanopsin mRNA is expressed in cells of all retinal layers. In pineal gland, expression was strong throughout the parenchyma of the gland. In brain, expression was observed in a few discrete nuclei, including the lateral septal area and medial preoptic nucleus. The retina and pineal gland showed distinct diurnal expression patterns. In pineal gland, melanopsin mRNA levels were highest at night at Zeitgeber time (ZT) 16. In contrast, transcript levels in the whole retina reached their highest levels in the early morning (ZT 0-4). Further analysis of melanopsin mRNA expression in retinal layers isolated by laser capture microdissection revealed different patterns in different layers. There was diurnal expression in all retinal layers except the ganglion cell layer, where heavy expression was localized to a small number of cells. Expression of melanopsin mRNA peaked during the daytime in the retinal pigment epithelium and inner nuclear layer but, like in the pineal, at night in the photoreceptors. Localization and regulation of melanopsin mRNA in the retina and pineal gland is consistent with the hypothesis that this novel photopigment plays a role in photic regulation of circadian function in these tissues.

Spatial, temporal, and intensive determinants of dopamine release in the chick retina

The retinal dopaminergic system is a global regulator of retinal function. Apart from the fact that the rates of dopamine synthesis and release are increased by increasing illumination, the visual image parameters that influence dopaminergic function are mostly unknown. Roles for spatial and temporal frequency and image contrast are suggested by the effects of form-deprivation with a diffusing goggle. Form-deprivation reduces the rates of dopamine synthesis and release, and induces myopia, which is prevented by dopamine agonists. Our purpose here was to identify visual stimulus parameters that activate dopaminergic amacrine cells and elicit dopamine release. White Leghorn cockerels 4–7 days old were exposed to 2 h of form-deprivation, reduced light intensity, or stimuli of varied temporal or spatial frequency. Activation of dopaminergic neurons, labeled for tyrosine hydroxylase (TH), was assessed with immunocytochemistry for c-Fos, and dopamine release was measured by HPLC analysis of dopamine metabolite accumulation in the vitreous body. Form-deprivation did not reduce TH+ cell activation or vitreal dopamine metabolite accumulation any more than did neutral-density filters of approximately equal transmittance. TH+ cell activation and vitreal metabolite accumulation were not affected significantly by exposure to 2, 5, 10, 15, or 20 Hz stroboscopic stimulation on a dark background, or by sine-wave gratings of 0.089, 0.44, 0.89, 1.04, or 3.13 cycles/deg compared to a uniform gray target of equal mean luminance. These data indicate that the retinal dopaminergic system does not respond readily to short-term changes in visual stimulus parameters, other than light intensity, under the conditions of these experiments.

Regulation of cAMP by light and dopamine receptors is dysfunctional in photoreceptors of dystrophic retinal degeneration slow(rds) mice

cAMP levels in dark and light were studied in dystrophic retinal degeneration slow(rds) mice, which carry a mutation in the rds/peripherin gene. cAMP levels were measured in vivo, in freshly isolated retinas, and in vitro in the presence of glutamate, which confines light modulation to photoreceptors. Experiments were conducted on young animals, when significant numbers of viable photoreceptor cells are present. In vivo levels of cAMP are higher in illuminated rds/rds retinas than levels measured in normal BALB/c retinas. Light-evoked down-regulation of cAMP levels was observed in vitro in normal photoreceptors. These measurements were made in the presence of the cyclic nucleotide phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine; therefore, they reflect an inhibition of cAMP formation. In contrast, light had no effect on cAMP formation in photoreceptors of mutant mice, measured under identical conditions. Thus, elevated levels of cAMP in rds/rds retinas in vivo result from abnormalities in cAMP synthesis in the mutant photoreceptor cells. In addition to regulation by light, cAMP formation in photoreceptor cells is regulated by dopamine, acting through dopamine D4 receptors. A dopamine D2/D4 receptor agonist, quinpirole, reduced cAMP levels in dark-adapted normal retinas in vitro, but not in rds/rds retinas. Our data indicate that alterations in a signal transduction pathway that leads to inhibition of adenylyl cyclase might underlie the abnormalities in cAMP levels in mutant rds/rds retinas. Heterozygous rds/+ photoreceptors demonstrated a normal pattern of light-evoked and quinpirole-mediated down-regulation of cAMP. Thus, partial expression of the normal phenotype is sufficient to render normal characteristics of cAMP regulation to the photoreceptors of the heterozygous mouse. The data obtained in the present study might be relevant to the understanding of photoreceptor pathology of patients with peripherin/rds mutations.

The role of dopamine in the locomotor stimulant effects and tolerance to these effects of caffeine

Current evidence indicates that the acute locomotor stimulant effects of caffeine involve dopamine (DA) receptor activation; however, few studies have investigated the role of DA receptors in mediating the development of tolerance to caffeine. Therefore, the present study was designed to determine the degree to which DA receptors mediate the development of tolerance to the locomotor stimulant effects of caffeine. Caffeine was examined alone and in combination with haloperidol (HAL), GBR 12909, nisoxetine and fluoxetine. HAL dose-dependently and completely blocked the acute effects of caffeine on locomotor activity, and the highest dose of GBR 12909 enhanced the effects of caffeine. Neither nisoxetine nor fluoxetine altered the effects of caffeine. HAL was infused via osmotic pumps (0.1 mg/kg/day) during a 14-day regimen of chronic caffeine administered in a caffeinated drinking solution ( approximately 136 mg/kg/day). HAL did not block the development of tolerance to the locomotor stimulant effects of caffeine, but did impair the recovery from tolerance following withdrawal of caffeine. [3H]SCH 23390 (DA D(1)) binding sites were downregulated in the nucleus accumbens and striatum and were upregulated in the prefrontal cortex of caffeine-treated vs. control rats; however, the affinity of [3H]SCH 23390 for these binding sites was unaltered. There were no differences between the caffeine-treated and control rats in number or affinity of [3H]spiperone (DA D(2)) binding sites. These results suggest that, although HAL did not alter the development of tolerance to caffeine, changes in DA D(1) receptors could be one component of the mechanism underlying caffeine-induced tolerance.

Diurnal metabolism of dopamine in dystrophic retinas of homozygous and heterozygous retinal degeneration slow (rds) mice

Dopamine metabolism was studied in dystrophic retinal degeneration slow (rds) mice which carry a mutation in the rds/peripherin gene. RDS mutations in humans cause several forms of retinal degeneration. Dopamine synthesis and utilization were analyzed at various time points in the diurnal cycle in homozygous rds/rds retinas which lack photoreceptor outer segments and heterozygous rds/+ retinas which have short malformed outer segments. Homozygous retinas exhibited depressed dopamine synthesis and utilization while the heterozygous retina retained a considerable level of activity which was, nevertheless, significantly lower than that of normal retinas. By one year, heterozygous rds/+ retinas which had lost half of the photoreceptors still maintained significant levels of dopamine metabolism. Normal characteristics of dopamine metabolism such as a spike in dopamine utilization at light onset were observed in mutant retinas. However, light intensity-dependent changes in dopamine utilization were observed in normal but not rds/+ retinas. The findings of this study suggest that human patients with peripherin/rds mutations, or other mutations that result in abnormal outer segments that can still capture light, might maintain light-evoked dopamine metabolism and dopamine-dependent retinal functions during the progression of the disease, proportional to remaining levels of light capture capabilities. However, visual deficits due to reduced light-evoked dopamine metabolism and abnormal patterns of dopamine utilization could be expected in such diseased retinas.

Behavioral evaluation of hemiparkinsonian MPTP monkeys following dopamine pharmacological manipulation and adrenal co-graft transplantation

Bradykinesia and rigidity are the symptoms that most directly correlate with loss of striatal dopamine in Parkinson's disease. In the hemiparkinsonian (HP) monkey, this is represented by paucity of movement as measured by coli puterized movement analysis, diminished manual dexterity on clinical examination, and diminished performance on operant behavioral tasks. The present study used an MPTP-induced HP model in rhesus monkeys to evaluate the effectiveness of adrenal medullary and peripheral nerve co-grafts in diminishing parkinsonian symptoms. Unoperated controls (N = 4), surgical controls with caudate lesioning (N = 4), and caudate co-grafted (N = 4) HP monkeys demonstrated diminished movement in the home cage following MPTP. This behavior persisted in unoperated controls, but improved in both surgical control and co-grafted monkeys. Functional hand dexterity evaluations demonstrated similar impairment in all three groups but only surgical controls and co-grafted monkeys demonstrated improvement. In general, rotational behavior in response to apomorphine was consistent with recovery of function in surgical controls and co grafted monkeys, but marked between-subject variability precluded group statistical analyses. None of the monkeys could perform the operant task using the affected limb following MPTP. However, the performance of two co-grafted animals demonstrated partial recovery. L-dopa improved operant performance, demonstrating a dopaminergic component to the task. The results demonstrate recovery of behavioral function after surgical treatment, with adrenal co-grafted monkeys showing the greatest degree of improvement.

Diurnal metabolism of dopamine in the mouse retina

Dopamine is an important retinal neurotransmitter and neuromodulator that regulates key diurnal cellular and physiological functions. In the present study we carried out a comprehensive analysis of dopamine metabolism during the light phase of the diurnal cycle and evaluated the presence of diurnal and circadian rhythms of dopaminergic activity in the mouse retina. Steady-state levels of dopamine did not change significantly between the dark phase (night) and the light phase (day) of the diurnal cycle, nor did they change between early and late points in the day. Dopamine synthesis and utilization, however, revealed significant alterations between the night and day and between early and late time points in the day. A spike in synthesis and utilization was measured immediately after light onset at the end of the night. Subsequently, dopamine synthesis and utilization partially declined and remained stable throughout the remainder of the day at a level that was significantly higher than that at night. The burst of dopamine synthesis and utilization at the beginning of the day is entirely light evoked and not driven by a circadian clock. Similarly, there was no circadian rhythm in dopamine synthesis and utilization in mice kept in constant darkness. This daily pattern of dopaminergic activity may impact upon a variety of temporally regulated retinal events. Moreover, these data will provide a basis for evaluating the role of dopamine in retinal pathology in mouse models of retinal degeneration where mutations affect light perception.

Regulation of tryptophan hydroxylase activity in Xenopus laevis photoreceptor cells by cyclic AMP

The aim of this study was to investigate the role of cyclic AMP in the regulation of tryptophan hydroxylase activity localized in retinal photoreceptor cells of Xenopus laevis, where the enzyme plays a key role in circadian melatonin biosynthesis. In photoreceptor-enriched retinas that lack serotonergic neurons, tryptophan hydroxylase activity is markedly stimulated by treatments that increase intracellular levels of cyclic AMP or activate cyclic AMP-dependent protein kinase, including forskolin, phosphodiesterase inhibitors, and cyclic AMP analogues. In contrast, cyclic AMP has no effect on tryptophan hydroxylase mRNA abundance. Experiments using cycloheximide and actinomycin D demonstrate that cyclic AMP exerts its regulatory effect via posttranslational mechanisms mediated by cyclic AMP-dependent protein kinase. The effect of cyclic AMP is independent of the phase of the photoperiod, suggesting that the nucleotide is not a mediator of the circadian rhythm of tryptophan hydroxylase. Cyclic AMP accumulation is higher in darkness than in light, as is tryptophan hydroxylase activity. Furthermore, the stimulatory effect of forskolin and that of darkness are inhibited by H89, an inhibitor of cyclic AMP-dependent protein kinase. In conclusion, cyclic AMP may mediate the acute effects of light and darkness on tryptophan hydroxylase activity of retinal photoreceptor cells.

Serotonin N-acetyltransferase mRNA levels in photoreceptor-enriched chicken retinal cell cultures: elevation by cyclic AMP

Serotonin N-acetyltransferase (AA-NAT; arylalkylamine N-acetyltransferase; EC 2.3.1.87) is a key regulatory enzyme in the biosynthesis of melatonin. Previous studies have shown that the activity of this enzyme in the chicken retina is regulated by a cyclic AMP-dependent mechanism. In the present report, we investigated whether cyclic AMP can regulate the levels of AA-NAT mRNA in photoreceptor-enriched chick retinal cell cultures. AA-NAT mRNA levels were elevated by acute treatment with cyclic AMP protagonists, including forskolin; this response was blocked by H-89, a selective inhibitor of cyclic AMP-dependent protein kinase. Forskolin did not alter the rate of disappearance of AA-NAT mRNA in actinomycin D-treated cells, suggesting that cyclic AMP enhances transcription of the AA-NAT gene. Forskolin-induced elevation of AA-NAT mRNA levels was enhanced by cycloheximide, which decreased the degradation of the transcript in cells treated with actinomycin D. These studies indicate that the abundance of AA-NAT mRNA is regulated in part through a cyclic AMP-dependent mechanism.

Diurnal rhythms of tryptophan hydroxylase activity in Xenopus laevis retina: opposing phases in photoreceptors and inner retinal neurons

Tryptophan hydroxylase (TPH) is the first enzyme in the biosynthetic pathways of melatonin in photoreceptor cells and of serotonin in amacrine cells. To assess the regulation of TPH activity in photoreceptor cells, we pretreated retinas with kainic acid. The neurotoxin selectively killed inner retinal neurons while sparing photoreceptors. TPH activity in both control and kainate-treated retinas undergoes a day-night rhythm. The rhythms in both preparations fit sinusoidal functions. However, the rhythm in intact retinas peaks at midday while that in kainate-lesioned retinas does so at midnight. The daily rhythm of tryptophan hydroxylase activity in photoreceptors parallels that of melatonin release. Comparing the mean level of activity in rhythms of intact and lesioned retinas, we calculate that the TPH activity in photoreceptors represents 24% of the total activity. Therefore, the TPH activity measured in intact retinas reflects mainly the enzymatic activity in serotonergic neurons, masking that from photoreceptors. In contrast, the levels and diurnal variation of TPH mRNA did not differ in intact and kainate-lesioned retinas indicating that measurements of TPH mRNA content reflect primarily that in photoreceptor cells. Thus, TPH mRNA levels and enzyme activity are differentially regulated in amacrine neurons and photoreceptor cells. This differential regulation markedly impacts the patterns of daily rhythms observed in the intact retina.

Effects of muscarinic cholinergic receptor antagonists on postnatal eye growth of rhesus monkeys

PURPOSE

To study a potential role for muscarinic receptors in the inhibition of deprivation-induced excessive axial elongation and myopia in a monkey model.

METHODS

The right eyes of 20 newborn rhesus monkeys were occluded with a black contact lens. In seven monkeys each, either atropine or pirenzepine was topically applied daily to the occluded eyes. The nonoccluded fellow eyes and both the occluded and nonoccluded fellow eyes of another six monkeys were treated with vehicle solution.

RESULTS

After 33 to 39 weeks, in 5 monkeys of the vehicle group, occluded eyes were longer and the myopic shift significantly greater than in the nonoccluded fellow eyes. In six atropine-treated monkeys, axial length and reduction of the initial hyperopia of occluded and nonoccluded fellow eyes were not different statistically. The myopic shift of the occluded eyes was significantly smaller than in the vehicle-treated occluded eyes. In the pirenzepine-treated group, axial length of the occluded eyes was similar to the nonoccluded eyes of controls and the occluded eyes of atropine-treated monkeys. There was a trend of pirenzepine to reduce the myopic shift of the occluded eye. No effect of atropine or pirenzepine was noted on muscarinic receptor density in retina, brain, or heart, but a small increase was observed in iris + ciliary body.

CONCLUSIONS

The drug treatment results implicate muscarinic receptors in postnatal eye growth regulation. Because of interanimal differences our data do not indicate whether nonselective or selective muscarinic blockade is more effective in reducing deprivation-induced myopia.

Dopamine mediates circadian rhythms of rod-cone dominance in the Japanese quail retina

A circadian clock modulates the functional organization of the Japanese quail retina. Under conditions of constant darkness, rods dominate electroretinogram (ERG) b-wave responses at night, and cones dominate them during the day, yielding a circadian rhythm in retinal sensitivity and rod-cone dominance. The activity of tyrosine hydroxylase, the rate-limiting enzyme in dopamine synthesis, also exhibits a circadian rhythm in the retina with approximately threefold higher levels during the day than at night. The rhythm of tyrosine hydroxylase activity is opposite in phase to the circadian activity of tryptophan hydroxylase, the first enzyme in the melatonin biosynthetic pathway. We tested whether dopamine may be related to the physiological rhythms of the retina by examining the actions of pharmacological agents that effect dopamine receptors. We found that blocking dopamine D2 receptors in the retina during the day mimics the nighttime state by increasing the amplitude of the b-wave and shifting the retina to rod dominance. Conversely, activating D2 receptors at night mimics the daytime state by decreasing the amplitude of the b-wave and shifting the retina to cone dominance. A selective antagonist for D1 dopamine receptors has no effect on retinal sensitivity or rod-cone dominance. Reducing retinal dopamine partially abolishes rhythms in sensitivity and yields a rod-dominated retina regardless of the time of day. These results suggest that dopamine, under the control of a circadian oscillator, has a key role in modulating sensitivity and rod-cone dominance in the Japanese quail retina.

Intracellular Ca2+ concentrations in cultured chicken photoreceptor cells: sustained elevation in depolarized cells and the role of dihydropyridine-sensitive Ca2+ channels

PURPOSE

Retinal photoreceptor cells are tonically depolarized in darkness. Ca2+ influx in darkness plays a critical role in the regulation of neurotransmitter release and melatonin synthesis in these sensory cells. The purpose of the present study was to examine the dynamic changes of intracellular Ca2+ concentrations ([Ca2+]in ) in response to a tonic depolarizing stimulus and to determine the role of dihydropyridine-sensitive calcium channels in the response.

METHODS

Photoreceptor cells were prepared from embryonic chick retina and cultured for 6-12 days. Cells were depolarized by exposure to 35 mM extracellular K+. [Ca2+]in of individual photoreceptor cell bodies/synaptic terminals was determined by ratiometric fura-2 image analysis.

RESULTS

Chemical depolarization with 35 mM [K+]out greatly increased [Ca2+]in of inner segment/synaptic terminal regions of photoreceptors. The increase usually reached a plateau after the first few minutes of stimulation and was sustained for prolonged periods (>2 h) in the presence of high K+. When the extracellular K+ concentration was reduced, the [Ca2+]in rapidly returned to the basal level. Substitution of 1 mM CoCl2 for CaCl2 in the superfusion medium rapidly and reversibly reduced the [Ca2+]in of depolarized photoreceptor cells. Antagonists of L-type Ca2+ channels, nitrendipine and nifedipine, inhibited the K+-evoked increase of [Ca2+]in. Bay K 8644, a dihydropyridine Ca2+ channel agonist, potentiated the increase of [Ca2+]in elicited by high K+. In some cells, Bay K 8644 alone increased [Ca2+]in under basal conditions.

CONCLUSIONS

The increase of [Ca2+]in elicited by depolarization with 35 mM extracellular K+ is due to influx of calcium through the dihydropyridine-sensitive voltage-gated channels. Intracellular [Ca2+] remains elevated for extended periods of time during tonic depolarization. This sustained response requires continuous Ca2+ channel activity.

Elevation of melatonin in chicken retina by 5-hydroxytryptophan: differential light/dark responses

Melatonin is synthesized in the chicken retina under the influence of a circadian clock, which also regulates the expression of tryptophan hydroxylase (TPH) and serotonin N-acetyltransferase (AA-NAT). In order to examine the role of substrate supply in the rhythmic synthesis of melatonin in chicken retina, tryptophan and 5-hydroxytryptophan were administered day and night in light or darkness. When administered systemically at night in darkness, 5-hydroxytryptophan, but not tryptophan, dramatically stimulates melatonin levels in the chick retina in a dose-dependent manner. Intraocular administration of 5-hydroxytryptophan also increases melatonin levels locally, indicating a retinal site of action of the serotonin precursor. The effect of 5-hydroxytryptophan is much greater at night, when TPH and AA-NAT activities are high, than during the day, when the enzyme activities are low. Similarly, unexpected light exposure at night, which inactivates AA-NAT, significantly reduces the ability of 5-hydroxytryptophan to increase retinal melatonin levels. The results suggest that TPH, but not AA-NAT or other enzymes in the melatonin biosynthetic pathway, is saturated with substrate in situ. The rate of melatonin production appears to be a function of the concentration of serotonin, which is regulated by TPH, and by the level of activity of AA-NAT.

The melatonin antagonist luzindole protects retinal photoreceptors from light damage in the rat

PURPOSE

Systemic administration of melatonin can increase retinal light damage in the rat. The role of retinal melatonin receptors in modulating light-damage susceptibility was investigated by intravitreally injecting the melatonin receptor antagonist luzindole into rats.

METHODS

Nine Sprague-Dawley albino rats 8 to 9 weeks of age were kept in 50 lux cyclic light for at least 7 days before receiving an intravitreal injection of 1 microl 1 mM luzindole in one eye and 1 microl vehicle in the other eye. The injection was given just before the beginning of the normal 12-hour dark phase. At the end of this dark period, animals were exposed to constant light of 2500 lux for 48 hours. Animals were returned to dim cyclic light for 7 days, and dark-adapted electroretinograms (ERGs) were then recorded from the two eyes simultaneously. The eyes were processed for retinal morphology. Photoreceptor nuclei were counted in the outer nuclear layer (ONL), and the thickness of the ONL and that of the rod outer-segment plus inner-segment layer were measured at several points along sections through the vertical meridian. Two age-matched control rats were maintained in dim cyclic light but received no injections.

RESULTS

Luzindole-treated eyes had ERG b-wave thresholds of 2.7 +/- 0.5 (mean +/- SEM) log candela (cd)/m2 lower than the fellow eyes injected with vehicle (P < 0.001), and the maximum b-wave amplitude was 1.0 +/- 0.2 log microV greater in luzindole-treated eyes (P < 0.001). Thresholds of the scotopic threshold response were 0.5 +/- 0.1 log cd/m2 lower than those in vehicle-injected eyes (P < 0.05). Luzindole-treated eyes on average had twice as many photoreceptor cells remaining (P < 0.005). In some areas, several rows of photoreceptor nuclei and outer segments remained in the luzindole-treated eye, whereas the fellow control eye showed cells only occasionally and no outer segments.

CONCLUSIONS

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. These results implicate the intraocular melatonin-dopamine system in the regulation of light-damage susceptibility.

Circadian expression of tryptophan hydroxylase mRNA in the chicken retina

Many aspects of retinal physiology are controlled by a circadian clock located within the eye. This clock controls the rhythmic synthesis of melatonin, which results in elevated levels during the night and low levels during the day. The rate-limiting enzyme in melatonin biosynthesis in retina appears to be tryptophan hydroxylase (TPH)[G.M. Cahill and J.C. Besharse, Circadian regulation of melatonin in the retina of Xenopus laevis: Limitation by serotonin availability, J. Neurochem. 54 (1990) 716-719]. In this report, we found that TPH mRNA is strongly expressed in the photoreceptor layer and the vitread portion of the inner nuclear layer; the message is also expressed, but to a lesser extent, in the ganglion cell layer. The abundance of retinal TPH mRNA exhibits a circadian rhythm which persists in constant light or constant darkness. The phase of the rhythm can be reversed by reversing the light:dark cycle. In parallel experiments we found a similar pattern of expression in the chicken pineal gland. However, whereas a pulse of light at midnight suppressed retinal TPH mRNA by 25%, it did not alter pineal TPH mRNA, suggesting that there are tissue-specific differences in photic regulation of TPH mRNA. In retinas treated with kainic acid to destroy serotonin-containing amacrine and bipolar cells, a high amplitude rhythm of TPH mRNA was observed indicating that melatonin-synthesizing photoreceptors are the primary source of the rhythmic message. These observations provide the first evidence that chick retinal TPH mRNA is under control of a circadian clock.

Highly sensitive and specific assay of plasma melatonin using high-performance liquid chromatography with fluorescence detection preceded by solid-phase extraction

This paper describes a new, highly sensitive, rapid method for the determination of low levels of endogenous melatonin, the pineal and retinal hormone, in fish plasma samples. The combination of solid-phase extraction and high-performance liquid chromatography with fluorescence detection significantly improves the separation and increases the sensitivity of the assay. The proposed method may be a useful alternative to previously reported procedures. The assay should be applicable to plasma and tissue samples from other animals, including humans, with only minor modifications.

Cellular and molecular regulation of serotonin N-acetyltransferase activity in chicken retinal photoreceptors

Serotonin N-acetyltransferase (AA-NAT; arylalkylamine N-acetyltransferase; EC 2.3.1.87) is the penultimate enzyme in melatonin synthesis and large changes in the activity of this enzyme appear to regulate the rhythm in melatonin synthesis. Recent advances have made it possible to study the mRNA encoding chicken AA-NAT, which has only been detected in the retina and pineal gland. Within the retina, AA-NAT mRNA is expressed primarily in photoreceptors. The levels of chicken retinal AA-NAT mRNA and activity exhibit 24-hour rhythms with peaks at night. These rhythms appear to reflect circadian clock control of AA-NAT mRNA abundance and independent effects of light and darkness on both mRNA levels and enzyme activity. The effects of darkness and light may occur through alterations in cAMP-dependent protein phosphorylation, which increases AA-NAT activity in photoreceptor cell cultures. The cAMP-dependent increase of AA-NAT enzyme activity reflects, at least in part, increased mRNA levels and inhibition of enzyme inactivation by a posttranslational mechanism. This review discusses a hypothetical model for the cellular and molecular regulation of AA-NAT activity by circadian oscillators and light in chicken retinal photoreceptor cells.

Depolarization and activation of dihydropyridine-sensitive Ca2+ channels stimulate inositol phosphate accumulation in photoreceptor-enriched chick retinal cell cultures

Elevated concentrations of extracellular K+ increased inositol phosphate accumulation in primary cultures of chick retinal photoreceptors and multipolar neurons. K+-evoked stimulation of inositol phosphate accumulation was greater in photoreceptor-enriched cell cultures than in cultures where multipolar neurons were the predominant cell type. Destroying multipolar neurons, but not photoreceptors, with kainic acid and N-methyl-D-aspartate did not reduce the K+-evoked stimulation of inositol phosphate accumulation. Both of these observations indicate that the observed effects occur in photoreceptor cells. The K+-evoked stimulation of inositol phosphate accumulation was blocked by omitting Ca2+ from the incubation medium or by adding the dihydropyridine-sensitive Ca2+-channel antagonists, nitrendipine and nifedipine. Bay K 8644, a dihydropyridine agonist, stimulated inositol phosphate accumulation and enhanced the effect of K+. omega-Conotoxin GVIA, an inhibitor of N-type Ca2+ channels, had no significant effect on K+-stimulated inositol phosphate accumulation. Pretreatment with pertussis toxin neither blocked K+-evoked inositol phosphate accumulation nor altered the inhibitory effect of nifedipine. K+-evoked inositol phosphate accumulation appears to reflect activation of phosphatidylinositol-specific phospholipase C, as it is inhibited by U-73122. These results indicate that Ca2+ influx through voltage-gated, dihydropyridine-sensitive channels activates phospholipase C in photoreceptor inner segments and/or synaptic terminals.

Avian melatonin synthesis: photic and circadian regulation of serotonin N-acetyltransferase mRNA in the chicken pineal gland and retina

The circadian rhythms in melatonin production in the chicken pineal gland and retina reflect changes in the activity of serotonin N-acetyltransferase (arylalkylamine N-acetyltransferase; AA-NAT; EC 2.3.1.87). Here we determined that the chicken AA-NAT mRNA is detectable in follicular pineal cells and retinal photoreceptors and that it exhibits a circadian rhythm, with peak levels at night. AA-NAT mRNA was not detected in other tissues. The AA-NAT mRNA rhythm in the pineal gland and retina persists in constant darkness (DD) and constant lighting (LL). The amplitude of the pineal mRNA rhythm is not decreased in LL. Light appears to influence the phase of the clock driving the rhythm in pineal AA-NAT mRNA in two ways: The peak is delayed by approximately 6 h in LL, and it is advanced by > 4 h by a 6-h light pulse late in subjective night in DD. Nocturnal AA-NAT mRNA levels do not change during a 20-min exposure to light, whereas this treatment dramatically decreases AA-NAT activity. These observations suggest that the rhythmic changes in chicken pineal AA-NAT activity reflect, at least in part, clock-generated changes in mRNA levels. In contrast, changes in mRNA content are not involved in the rapid light-induced decrease in AA-NAT activity.

The melatonin rhythm-generating enzyme: molecular regulation of serotonin N-acetyltransferase in the pineal gland

A remarkably constant feature of vertebrate physiology is a daily rhythm of melatonin in the circulation, which serves as the hormonal signal of the daily light/dark cycle: melatonin levels are always elevated at night. The biochemical basis of this hormonal rhythm is one of the enzymes involved in melatonin synthesis in the pineal gland-the melatonin rhythm-generating enzyme-serotonin N-acetyltransferase (arylalkylamine N-acetyltransferase, AA-NAT, E.C. 2.3.1.87). In all vertebrates, enzyme activity is high at night. This reflects the influences of internal circadian clocks and of light. The dynamics of this enzyme are remarkable. The magnitude of the nocturnal increase in enzyme activity ranges from 7- to 150-fold on a species-to-species basis among vertebrates. In all cases the nocturnal levels of AA-NAT activity decrease very rapidly following exposure to light. A major advance in the study of the molecular basis of these changes was the cloning of cDNA encoding the enzyme. This has resulted in rapid progress in our understanding of the biology and structure of AA-NAT and how it is regulated. Several constant features of this enzyme have become apparent, including structural features, tissue distribution, and a close association of enzyme activity and protein. However, some remarkable differences among species in the molecular mechanisms involved in regulating the enzyme have been discovered. In sheep, AA-NAT mRNA levels show relatively little change over a 24-hour period and changes in AA-NAT activity are primarily regulated at the protein level. In the rat, AA-NAT is also regulated at a protein level; however, in addition, AA-NAT mRNA levels exhibit a 150-fold rhythm, which reflects cyclic AMP-dependent regulation of expression of the AA-NAT gene. In the chicken, cyclic AMP acts primarily at the protein level and a rhythm in AA-NAT mRNA is driven by a noncyclic AMP-dependent mechanism linked to the clock within the pineal gland. Finally, in the trout, AA-NAT mRNA levels show little change and activity is regulated by light acting directly on the pineal gland. The variety of mechanisms that have evolved among vertebrates to achieve the same goal-a rhythm in melatonin-underlines the important role melatonin plays as the hormonal signal of environmental lighting in vertebrates.

Melatonin biosynthesis in cultured chick retinal photoreceptor cells: calcium and cyclic AMP protect serotonin N-acetyltransferase from inactivation in cycloheximide-treated cells

The aim of the present study was to examine the roles of membrane depolarization, calcium influx, and cyclic AMP synthesis in regulating the stability and inactivation of serotonin N-acetyltransferase activity (NAT) in cultured chick photoreceptor cells. NAT activity was induced by pretreating cells for 6 h with 1 microM forskolin. Cycloheximide was subsequently added, and the rate of loss of enzyme activity (inactivation) was determined. After induction, in the presence of cycloheximide, NAT activity declined with a half-life of approximately 30 min. The rate of inactivation was greatly reduced when depolarizing concentrations of K+, forskolin, 8-bromoadenosine 3',5'-cyclic monophosphate, or 3-isobutyl-1-methylxanthine were added together with cycloheximide. The apparent increase in NAT stability caused by K+ was abolished by addition of EGTA or nifedipine and potentiated by Bay K 8644, indicating the involvement of Ca2+ influx through dihydropyridine-sensitive channels. MDL-12330A, and inhibitor of K(+)-stimulated cyclic AMP formation, blocked the effect of depolarizing concentrations of K+. This result suggests that the effect of Ca2+ influx on the stability of NAT is at least partially mediated by increased levels of cyclic AMP. Thus, depolarization-evoked Ca2+ influx and cyclic AMP formation have two roles in the regulation of NAT activity in chick photoreceptor cells. First, they stimulate the de novo synthesis of NAT or a regulatory protein required for NAT activity. Second, they increase the half-life of the enzyme, presumably by regulating the turnover of existing enzyme molecules.

Melatonin biosynthesis in photoreceptor-enriched chick retinal cell cultures: role of cyclic AMP in the K(+)-evoked, Ca(2+)-dependent induction of serotonin N-acetyltransferase activity

The roles of cyclic AMP and calcium in the regulation of serotonin N-acetyltransferase (NAT) activity were studied in low density monolayer cultures of chick retinal photoreceptors and neurons. Photoreceptor-enriched retinal cell cultures were prepared from embryonic day 6 retinas and cultured for 6 days. NAT activity in these cultures could be induced by treatment with cyclic AMP protagonists, 8Br-cyclic AMP, forskolin, and 3-isobutyl-1-methylxanthine (IBMX), or by treatment with depolarizing concentrations of extracellular K+. The stimulatory effect of K+, which involves Ca2+ influx through dihydropyridine-sensitive channels, was mediated at least in part by cyclic AMP, as indicated by the following observations. Depolarizing concentrations of K+ stimulated the formation of cyclic AMP, and the stimulatory effects of K+ on both cyclic AMP formation and on NAT activity were synergistically potentiated by the cyclic nucleotide phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX). MDL 12,330A, a putative adenylate cyclase inhibitor, inhibited K(+)-evoked cyclic AMP accumulation and induction of NAT activity over the identical concentration range. In contrast, MDL 12,300A failed to inhibit the induction of NAT elicited by 8Br-cyclic AMP. H-89, an inhibitor of cyclic AMP-dependent protein kinase, antagonized the induction of NAT activity by either forskolin or K+ with equal potency for both stimuli. These results suggest that cyclic AMP plays an essential role in the induction of NAT activity that occurs as a consequence of membrane depolarization. Cyclic AMP and Ca2+ may also interact at a step distal to adenylate cyclase.(ABSTRACT TRUNCATED AT 250 WORDS)

Stimulation of dopaminergic amacrine cells by stroboscopic illumination or fibroblast growth factor (bFGF, FGF-2) injections: possible roles in prevention of form-deprivation myopia in the chick

Form-deprivation myopia (FDM) in the chick is a popular model for studying the postnatal regulation of ocular growth. Using this model, we have shown previously that dopamine and FGF-2 can counteract the effects of form-deprivation, thereby producing emmetropia. In the present study, we tested the hypothesis that the emmetropizing effects of flickering light and intraocular injections of FGF-2 in the chick are mediated by the activity of dopaminergic retinal amacrine cells. We have assessed the rate of dopamine synthesis in the retina by measuring the accumulation of 3,4-dihydroxyphenylalanine (DOPA). We found that form-deprivation reduces the rate of dopamine synthesis in the light-adapted retina, and that the normal rate of dopamine synthesis in the light can be restored by stroboscopic illumination at frequencies around 10 Hz. By labeling cells immunocytochemically we have shown that the synthesis of c-fos, a putative transcriptional regulator of the tyrosine hydroxylase gene, is induced in dopaminergic amacrine cells by stroboscopic illumination at around 10 Hz. These observations are consistent with a critical role for dopaminergic amacrine cells in the regulation of ocular growth by intermittent illumination. We have found also that intraocular injections of FGF-2 cause emmetropization without altering levels of expression of c-fos, amounts of tyrosine hydroxylase, or rates of dopamine synthesis with respect to vehicle-injected controls. We conclude that FGF acts either in parallel to or downstream from the dopaminergic amacrine cells, rather than through them. We observed that intravitreal injection per se induces high levels of c-fos expression in both form-deprived and non-deprived retinas, and causes partial emmetropization in form-deprived eyes, while inhibiting dopamine synthesis in non-deprived retinas. It is likely, therefore, that injection stimulates the production and/or release of unknown factors whose diverse effects on ocular growth and dopamine metabolism are mediated by complex pathways. Taken together, our results are consistent with the view that the retinal circuitry that controls postnatal ocular growth in the chick involves multiple messengers and pathways.

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

Melatonin and 5-methoxytryptamine inhibited forskolin-stimulated cyclic AMP formation in cultured neural cells prepared from embryonic chick retina. Both methoxyindoles exhibited similar potency and efficacy, with EC50 values of 0.8 nM for melatonin and 7.2 nM for 5-methoxytryptamine. Inhibition of cyclic AMP formation by 5-methoxytryptamine or melatonin was prevented by pretreatment with pertussis toxin. Pretreatment of cultures with 5-methoxytryptamine for 24 h reduced the subsequent inhibitory cyclic AMP response to 5-methoxytryptamine but not that to 2-iodomelatonin. Putative melatonin receptors on cultured retinal cells were labeled with 2-[125I]iodomelatonin. Melatonin displaced specific 2-[125I]iodomelatonin with a Ki value (0.8 nM) similar to the EC50 for inhibition of cyclic AMP formation. In contrast, 5-methoxytryptamine only inhibited 2-[125I]iodomelatonin binding at very high concentrations (Ki = 650 nM). Pretreating cultured cells for 24 h with 2-iodomelatonin or melatonin, but not with 5-methoxytryptamine, reduced subsequent 2-[125I]iodomelatonin binding. Thus, 5-methoxytryptamine appears to inhibit forskolin-stimulated cyclic AMP formation at a site distinct from the 2-iodomelatonin binding site.

Functional interaction of melatonin receptors and D1 dopamine receptors in cultured chick retinal neurons

The possible interaction of melatonin receptors and D1 dopamine receptors was investigated in neural cells prepared from embryonic day 8 chick retinas and cultured for 6 d. Dopamine stimulated cAMP accumulation in cultured retinal cells. This effect of dopamine was antagonized by addition of dopamine receptor antagonists (haloperidol and SCH23390) or melatonin receptor agonists (melatonin, 2-iodomelatonin, and 6-chloromelatonin). The inhibition of dopamine-stimulated cAMP accumulation by melatonin was concentration dependent, with half-maximal inhibition at approximately 160 pM. Melatonin inhibited the effect of dopamine at all dopamine concentrations, suppressing the maximal response to the neurotransmitter by approximately 70%. Melatonin also inhibited the stimulation of cAMP accumulation by SKF 82958, a selective D1 dopamine receptor agonist. Pretreatment of cultures with pertussis toxin had no significant effect on dopamine-stimulated cAMP accumulation, but inhibited the response to melatonin. In contrast to its effect on cAMP accumulation, melatonin had no effect on dopamine-stimulated inositol phosphate accumulation. These results suggest that melatonin receptors are coupled to dopamine receptor-regulated adenylate cyclase via an inhibitory G protein, and demonstrate another mechanism, in addition to inhibition of dopamine release, through which melatonin can modulate dopaminergic neurotransmission.

Regulation of endogenous dopamine release in amphibian retina by melatonin: the role of GABA

In the retina of the African clawed frog (Xenopus laevis), endogenous dopamine release increases in light and decreases in darkness. Exogenous melatonin and several chemical analogs of melatonin suppressed light-evoked dopamine release from frog retina in a concentration-dependent manner. The rank order of potency for inhibition of light-evoked dopamine release was melatonin >> 5-methoxytryptamine > or = N-acetylserotonin > 5-methoxytryptophol >>> serotonin. Melatonin did not suppress dopamine release below levels seen in darkness. The putative melatonin receptor antagonist luzindole inhibited the effect of melatonin. Luzindole enhanced dopamine release in darkness but had little effect in light. These data suggest a role for endogenous melatonin in dark-induced suppression of retinal dopamine. Picrotoxin and bicuculline, GABA-A receptor antagonists, blocked melatonin-induced suppression of dopamine release. In the presence of melatonin, bicuculline was significantly less potent in stimulating dopamine release. These results suggest that melatonin enhances GABAergic inhibition of light-evoked dopamine release. This mechanism may underlie the light/dark difference in dopamine release in vertebrate retina.

Regulation of endogenous dopamine release in amphibian retina by gamma-aminobutyric acid and glycine

Endogenous dopamine release in the retina of the African clawed frog (Xenopus laevis) increases in light and decreases in darkness. The roles of the inhibitory amino acid transmitters gamma-aminobutyric acid (GABA) and glycine in regulating this light/dark difference in dopamine release were explored in the present study. Exogenous GABA, the GABA-A receptor agonist muscimol, the GABA-B receptor agonist baclofen, and the GABA-C receptor agonist cis-aminocrotonic acid (CACA) suppressed light-evoked dopamine overflow from eyecups. The effects of GABA-A and -B receptor agonists were selectively reversed by their respective receptor-specific antagonists, whereas the effect of CACA was reversed by the competitive GABA-A receptor antagonist bicuculline. The benzodiazepine diazepam enhanced the effect of muscimol on light-evoked dopamine release. Both GABA-A and -B receptor antagonists stimulated dopamine release in light or darkness. Bicuculline was more potent in light than in darkness. These data suggest that retinal dopaminergic neurons are inhibited by GABA-A and -B receptor activation in both light and darkness but that GABA-mediated inhibitory tone may be greater in darkness than in light. Exogenous glycine inhibited light-stimulated dopamine release in a concentration-dependent and strychnine-sensitive manner. However, strychnine alone did not increase dopamine release in light or darkness, nor did it augment bicuculline-stimulated release in darkness. Additionally, both strychnine and 7-chlorokynurenate, an antagonist of the strychnine-insensitive glycine-binding site of the N-methyl-D-aspartate subtype of glutamate receptor, suppressed light-evoked dopamine release. Thus, the role of endogenous glycine in the regulation of dopamine release remains unclear.

Melatonin receptor-mediated inhibition of cyclic AMP accumulation in chick retinal cell cultures

Melatonin receptors were characterized in cultured neurons and photoreceptors prepared from chick embryo retina. Cultured cells contained high-affinity 2-[125I]iodomelatonin binding sites (KD = 41.6 pM), similar to those in intact retina. The effects of melatonin and related indoles on cyclic AMP accumulation were examined. Melatonin (10(-7) M) had no effect on basal or K(+)-stimulated cyclic AMP accumulation, but inhibited forskolin-stimulated cyclic AMP accumulation by approximately 50%. Melatonin inhibited forskolin-stimulated cyclic AMP accumulation in the presence or absence of the cyclic nucleotide phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine, suggesting an effect on cyclic AMP synthesis rather than degradation. Half-maximal inhibition was observed at 5.9 x 10(-10) M melatonin. The relative order of potency among melatonin analogues was 2-iodomelatonin > melatonin approximately 6-chloromelatonin > or = 6-hydroxymelatonin > N-acetylserotonin approximately 5-methoxytryptophol > serotonin. The EC50 value for inhibition of cyclic AMP accumulation by 2-iodomelatonin (36.7 pM) was comparable to the KD value for binding of the radioligand, suggesting that the binding sites represent functional receptors. The inhibitory effect of melatonin was antagonized by the putative melatonin antagonists luzindole, N-acetyltryptamine, and N-(2,4-dinitrophenyl)-5-methoxytryptamine, with estimated KB values of 0.12, 0.17, and 1 microM, respectively. At a concentration of 10 microM, N-(2,4-dinitrophenyl)-5-methoxytryptamine significantly inhibited forskolin-stimulated cyclic AMP accumulation when added alone; at 30 microM, luzindole and N-acetyltryptamine also had significant inhibitory effects. The inhibitory effect of melatonin was blocked by pretreatment with pertussis toxin.(ABSTRACT TRUNCATED AT 250 WORDS)

Alterations in light-evoked dopamine metabolism in dystrophic retinas of mutant rds mice

In dystrophic retinas of rds mice, which are devoid of photoreceptor outer segments, high steady state levels of dopamine were found in dark and light periods. These levels were similar to those observed in normal, BALB/c mouse retinas. Major differences were determined, however, between dopamine turnover in normal and dystrophic retinas. While substantial light-evoked elevation of dopamine synthesis and utilization was observed in normal retinas, dopamine synthesis and metabolism in rds retinas was very low and response to light was depressed. Retinal dopamine metabolism was already depressed in 2 week old rds mice, prior to the onset of photoreceptor cell death, relative to that in age-matched BALB/c mice. At 1 month of age, robust light/dark differences in retinal dopamine metabolism were observed in BALB/c mice, while no significant effect of light was seen in rds mice. The limited ability of the dopaminergic system in rds retinas to respond to light may be due to the absence of normal outer segments. Interestingly, in old rds retinas, although most photoreceptor cells had degenerated, a small but significant light-evoked increase in dopamine metabolism was measured. The presence of relatively high steady state levels of dopamine in rds retinas, despite the reduced dopamine synthetic activity, is maintained by a compensatory reduction in dopamine utilization. Thus, although a considerable amount of dopamine is present in the rds retina, it might not be available to exert its biological functions.

Inhibition of endogenous dopamine release in amphibian retina by L-2-amino-4-phosphonobutyric acid (L-AP4) and trans-2-aminocyclopentane-1,3-dicarboxylate (ACPD)

The metabotropic glutamate receptor agonists 2-amino-4-phosphonobutyric acid (AP4) and trans-2-aminocyclopentane-1,3-dicarboxylate (ACPD) blocked light-stimulated dopamine release from Xenopus laevis retina. ACPD suppressed release in darkness but AP4 did not. AP4 blocked release stimulated in darkness by picrotoxin, a GABA-A receptor antagonist. The data suggest that regulation of dopamine release in Xenopus retina involves subpopulations of metabotropic glutamate receptors.

Photomechanical movements of cultured embryonic photoreceptors: regulation by exogenous neuromodulators and by a regulable source of endogenous dopamine

In the retina of nonmammalian vertebrates, light regulates photoreceptor morphology by causing rod photoreceptor elongation and cone photoreceptor contraction. The opposite photomechanical movements occur in the dark, and proceed with a circadian rhythm in many species in vivo. Using dissociated cultures of embryonic chick retina cells, we have recently demonstrated that photoreceptor cells that differentiate in vitro acquire the capacity of responding to light/dark cycles with photomechanical movements (Stenkamp and Adler, 1993). Here we report that the putative neuromodulators melatonin and dopamine can mimic the effects of darkness and light, respectively, on in vitro photomechanical movement. Pharmacological studies showed that dopamine appears to function by means of a D2-type receptor negatively coupled to adenylate cyclase. The effects of light on the cultured photoreceptors were inhibited by dopamine D2 receptor antagonists, and were attenuated by the dopaminergic neurotoxin 6-hydroxydopamine and by the dopamine synthesis inhibitor alpha-methyl-p-tyrosine. The possible existence of an endogenous source of dopamine in the cultures was also suggested by the presence of tyrosine hydroxylase-like immunoreactivity, and of an Na(+)-dependent mechanism for the accumulation of 3H-dopamine, which was predominantly associated with nonphotoreceptor cells. Additionally, 3H-dopamine release occurred in vitro through a Ca(2+)-dependent mechanism, as well as through reverse function of a nomifensine-sensitive dopamine transporter. Both of these putative release mechanisms appeared to be regulated by light and by melatonin, suggesting a mechanism whereby the putative dopaminergic cells may interact with other cells present in the cultures. These studies suggest that complex paracrine neuromodulatory mechanisms can differentiate in low-density embryonic cell culture, that dopaminergic activities exist in vitro, and that they are important for mediating photomechanical movements.

Regulation of melatonin and dopamine biosynthesis in chick retina: the role of GABA

Melatonin biosynthesis in chick retina occurs as a circadian rhythm. Biosynthesis of the neurohormone is highest at night in darkness, and is suppressed by light. The role of gamma-aminobutyric acid (GABA) in the nocturnal regulation of melatonin synthesis was examined. Systemic or intravitreal administration of muscimol, a GABA-A receptor agonist, to light-exposed chicks at the beginning of the dark phase of the light/dark cycle increased retinal melatonin levels and the activity of serotonin N-acetyltransferase (NAT), a key regulatory enzyme of the melatonin biosynthetic pathway. Baclofen, a GABA-B receptor agonist, also increased NAT activity of light-exposed retinas, but muscimol was approximately 40-fold more potent than baclofen. Effects of both muscimol and baclofen on NAT activity were inhibited by GABA-A antagonists, bicuculline and picrotoxin, and the effect of baclofen was unaffected by the GABA-B selective antagonist, CGP 35348. Thus, activation of GABA-A receptors appears to be associated with increased melatonin biosynthesis. The GABA-uptake inhibitor, nipecotic acid, and the GABA-transaminase inhibitor, aminooxyacetic acid, also increased NAT activity of light-exposed retinas. The high levels of NAT activity associated with exposure to darkness were unaffected by either muscimol or baclofen, but picrotoxin and bicuculline significantly inhibited retinal NAT activity in darkness. The rate of dopamine synthesis, estimated from in situ tyrosine hydroxylase activity, was higher in light-exposed retinas than in darkness. Muscimol inhibited dopamine synthesis in light, and picrotoxin stimulated dopamine synthesis in darkness. The stimulation of melatonin synthesis by muscimol in light-exposed retinas appears to be related to inhibition of retinal dopamine neurons.(ABSTRACT TRUNCATED AT 250 WORDS)

Melatonin synthesis and circadian tryptophan hydroxylase activity in chicken retina following destruction of serotonin immunoreactive amacrine and bipolar cells by kainic acid

The neurotoxic excitatory amino acid analog, kainic acid, was used to destroy serotonin-immunoreactive inner retinal neurons, bipolar cells and amacrine cells, in retinas of chickens. Tryptophan hydroxylase activity and melatonin content were examined in the kainic acid-lesioned retinas. Tryptophan hydroxylase activity was present in kainic acid-lesioned retinas and displayed a circadian rhythm. Nocturnal levels of activity in lesioned and control retinas were similar. Melatonin synthesis occurred in kainic acid-lesioned retinas in a diurnal cycle as in normal retinas. Dark-phase melatonin content of kainic acid-lesioned retinas exceeded that of controls. We conclude that most, if not all, circadian tryptophan hydroxylase activity and melatonin synthesis occurs in cells other than the cells that contain most of the serotonin in retina, serotonin-immunoreactive bipolar and amacrine cells.

Melatonin synthesis in chicken retina: effect of kainic acid-induced lesions on the diurnal rhythm and D2-dopamine receptor-mediated regulation of serotonin N-acetyltransferase activity

The effect of kainic acid (KA)-induced lesions of retinal neurons on regulation of serotonin N-acetyltransferase (NAT) activity in chicken retina was investigated. Although NAT activity was higher in KA-lesioned retinas than in controls, the pattern of diurnal variation of enzyme activity throughout 36 h of constant darkness was similar for both tissues. Quinpirole, a selective D2-dopamine receptor agonist, inhibited the nocturnal increase of NAT activity in both control and KA-treated retinas. Quinpirole was significantly more potent in KA-treated retinas than in controls; the ED50 value for quinpirole was 3 times lower in KA-treated retinas than in control tissues. KA treatment markedly reduced retinal levels of dopamine (DA) and 3,4-dihydroxyphenylacetic acid (DOPAC). We conclude that: (1) NAT activity in retina is localized primarily to KA-insensitive cells, presumably photoreceptors; (2) KA-sensitive inner retinal neurons are not essential to the maintenance of the circadian rhythm of NAT activity; and (3) KA-induced lesions of retinal cells result in supersensitivity of D2-dopamine receptors regulating NAT activity in a mechanism that involves adaptive changes following a decline in retinal dopamine neurotransmission.

Circadian rhythm of tryptophan hydroxylase activity in chicken retina

1. Retinal tryptophan hydroxylase activity in chickens (1-4 weeks old and embryos) was estimated by determination of levels of 5-hydroxytryptophan (5HTP) in retinas at defined intervals after inhibition of aromatic L-amino acid decarboxylase with m-hydroxybenzylhydrazine (NSD1015). 2. The relationship of tryptophan hydroxylase activity to photoperiod was explored. In chickens maintained on a 12-hr light: 12-hr dark cycle, a diurnal cycle in tryptophan hydroxylase activity was observed. Activity during middark phase was 4.4 times that seen in midlight phase. Cyclic changes in tryptophan hydroxylase activity persisted in constant darkness with a period of approximately 1 day, indicating regulation of the enzyme by a circadian oscillator. The phase of the tryptophan hydroxylase rhythm was found to be determined by the phase of the light/dark cycle. The relationship of the tryptophan hydroxylase rhythm to the light/dark cycle mirrors previously described rhythms of melatonin synthesis and serotonin N-acetyltransferase (NAT) activity in the retina. 3. Light exposure for 1 hr during dark phase suppressed NAT activity by 82%, while tryptophan hydroxylase activity was suppressed by only 30%. 4. Based on the differential responses of retinal NAT activity and tryptophan hydroxylase activity to acute light exposure during dark phase, it was predicted that exposure to light during dark phase would divert serotonin in the retina from melatonin biosynthesis to oxidation by MAO. In support of this, levels of 5-hydroxyindole acetic acid (5HIAA) in retina were found to be elevated approximately two-fold in chickens exposed to 30 min of light during dark phase. In pargyline-treated chickens, 2 hr of light exposure during dark phase was found to increase retinal serotonin levels by 64% over pargyline-treated controls. 5. Cyclic changes in tryptophan hydroxylase activity and NAT activity persisted for 2-3 days in constant light. Tryptophan hydroxylase activity at mid-night gradually decreased on successive days in constant light; on the first day of constant light, tryptophan hydroxylase activity at mid-night was 70% of activity seen during middark phase of the normal light/dark cycle and decreased further on subsequent days. In contrast, on each of 3 days of constant light, NAT activity at mid-night was approximately 15% of normal middark phase activity. 6. Cycloheximide completely inhibited the nocturnal increase in tryptophan hydroxylase activity when given immediately before light offset. The nocturnal increase in NAT activity was inhibited in a similar fashion. 7. Like the development of the NAT rhythm, cyclic changes of tryptophan hydroxylase activity in the retinas of chickens began on or immediately before the day of hatching. hatching.(ABSTRACT TRUNCATED AT 400 WORDS)

K(+)-evoked depolarization stimulates cyclic AMP accumulation in photoreceptor-enriched retinal cell cultures: role of calcium influx through dihydropyridine-sensitive calcium channels

The effect of membrane depolarization on cyclic AMP synthesis was studied in glia-free, low-density, monolayer cultures of chick retinal photoreceptors and neurons. In photoreceptor-enriched cultures prepared from embryonic day 6 retinas and cultured for 6 days, elevated K+ concentrations increased the intracellular concentration of cyclic AMP and stimulated the conversion of [3H]adenine to [3H]cyclic AMP. The K(+)-evoked increase of cyclic AMP accumulation was blocked by omitting CaCl2 from the incubation medium, indicating a requirement for extracellular Ca2+. Stimulation of cyclic AMP accumulation was also inhibited by nifedipine, methoxyverapamil, Cd2+, Co2+, and Mg2+, and was enhanced by the dihydropyridine Ca2+ channel agonist Bay K 8644. The enhancement of K(+)-evoked cyclic AMP accumulation by Bay K 8644 was antagonized by nifedipine. Thus, Ca2+ influx through dihydropyridine-sensitive channel is required for depolarization-evoked stimulation of cyclic AMP accumulation in photoreceptor-enriched cultures.

Effects of apomorphine, a dopamine receptor agonist, on ocular refraction and axial elongation in a primate model of myopia

The authors examined the effect of local administration of a dopamine receptor agonist on visual deprivation-induced excessive ocular growth and myopia. Eight rhesus monkeys were monocularly deprived of vision from birth with opaque contact lenses. Four of the monkeys received drops of 1% apomorphine HCl 2-3 times/day in the occluded eye; the four control monkeys received vehicle only. Axial lengths were determined by A-scan ultrasonography at birth and at 5-7 months of age. The authors assessed the axial elongation by comparing the postnatal growth in the axial dimension of the occluded eyes with the postnatal growth in nonoccluded eyes. In three of the four control monkeys, occlusion increased axial growth by an average of 1.3 mm. In contrast, they found that growth of the occluded and nonoccluded eyes of the apomorphine-treated monkeys was equivalent, except in one monkey whose nonoccluded eye did not develop normally and was anomalously small. At 6.5-9.5 months of age, three of four controls had myopic refractive errors (-3 to -7 diopters) in the occluded eyes; three of four of the apomorphine-treated monkeys had hyperopic refractive errors (+1-(+)3 diopters) in their occluded eyes. The occluded eye of the fourth monkey was only -0.5 diopters myopic. The findings suggest that apomorphine administration retards excessive axial elongation and the concomitant development of myopia associated with visual deprivation in primates.

Alpha-2 adrenergic activity of bromocriptine and quinpirole in chicken pineal gland. Effects on melatonin synthesis and [3H]rauwolscine binding

In the pineal gland and retina of chickens, serotonin N-acetyl-transferase (NAT) activity and melatonin content are modulated by different receptors, alpha-2 adrenergic receptors in pineal gland and D2-dopamine receptors in retina. The effect of two D2-dopamine receptor agonists, bromocriptine and quinpirole (LY 171555), on melatonin synthesis in these tissues was investigated. Systemic administrations of bromocriptine and quinpirole decreased nocturnal NAT activity and melatonin content of both pineal gland and retina. Bromocriptine was equipotent in the two tissues, whereas quinpirole was approximately 100-fold more potent in retina than in pineal gland. In pineal gland, the suppressive effects of bromocriptine and quinpirole on NAT activity were blocked by yohimbine, a selective alpha-2 adrenergic receptor antagonist, but not by spiperone, a D2-dopamine receptor antagonist. In contrast, bromocriptine- and quinpirole-induced decreases of the enzyme activity in retina were antagonized by spiperone, and not affected by yohimbine. The nocturnal increase of NAT activity of pineal glands in vitro was inhibited with an order of potency clonidine greater than bromocriptine greater than quinpirole. Additionally, bromocriptine and quinpirole displaced the specific binding of [3H]rauwolscine, an alpha-2 adrenergic receptor antagonist, to membranes from chicken pineal gland, with potencies comparable to those observed for inhibition of NAT activity in vitro. It is suggested that bromocriptine and quinpirole, in addition to their D2-dopaminergic activity, can stimulate alpha-2 adrenergic receptors in pineal gland of chicken.

Cyclic AMP-dependent induction of serotonin N-acetyltransferase activity in photoreceptor-enriched chick retinal cell cultures: characterization and inhibition by dopamine

The activity of serotonin N-acetyltransferase (NAT), a key regulatory enzyme in the melatonin biosynthetic pathway, was examined in low-density monolayer cultures of chick embryo retinal cells prepared with three levels of photoreceptor enrichment. In cultures prepared from embryonic day 8 retinas (E8), photoreceptors represented approximately 30% of the total cell population, whereas in those prepared from embryonic day 6 retinas (E6), approximately 70% of the cells were photoreceptors. In E8 retinas treated with kainic acid to destroy neurons (E8K), the relative content of photoreceptors was increased to approximately 50%. NAT activity was detectable in the cultures under all conditions studied, and was markedly increased by drugs that increase intracellular cyclic AMP levels and cyclic AMP-dependent protein kinase activity: 8-bromocyclic AMP, forskolin, and 3-isobutyl-1-methylxanthine (IBMX). Consistent with the hypothesis that NAT is localized in photoreceptors, the effects of the stimulatory treatments were significantly greater in E6 and E8K cultures than in E8 cultures. The stimulation of NAT activity in E6 cultures was inhibited by actinomycin D and cycloheximide, suggesting the involvement of RNA and protein synthesis. Dopamine inhibited the induction of NAT activity by forskolin and IBMX, but not that elicited by 8-bromocyclic AMP. The dopamine-mediated suppression of activity was significantly inhibited by pertussis toxin and by spiperone and sulpiride, both D2-dopamine receptor antagonists, but not by SCH 23390, a D1-dopamine receptor blocker, or antagonists of alpha-adrenergic, beta-adrenergic, or serotonergic receptors. Because the inhibitory effect of dopamine on E6 and E8K cultures was at least as great as that on E8 cultures, the results suggest that dopamine acts on D2-like receptors on photoreceptors. The receptors appear to be coupled to adenylate cyclase through an inhibitory GTP-binding protein and to mediate inhibition of cyclic AMP synthesis and consequent induction of NAT activity.

Development of melatonin synthesis in chicken retina: regulation of serotonin N-acetyltransferase activity by light, circadian oscillators, and cyclic AMP

In chicken retinas, melatonin levels and the activity of serotonin N-acetyltransferase (NAT), a key regulatory enzyme of melatonin biosynthesis, are expressed as circadian rhythms with peaks of levels and activity occurring at night. In the present study, NAT activity was examined in retinas of embryonic and posthatch chicks to assess the ontogenic development of regulation of the enzyme by light, circadian oscillators, and the second messenger cyclic AMP. During embryonic development, NAT activity was consistently detectable by embryonic day 6 (E6). Significant light-dark differences were first observed on E20, and increased to a maximum amplitude of sixfold by posthatch day 3 (PH3). Circadian rhythmicity of NAT activity appears to develop at or prior to hatching, as evidenced by day-night differences of activity in constant darkness observed in PH1 chicks that had been exposed to a light-dark cycle in ovo only. NAT activity is regulated by a cyclic AMP-dependent mechanism. Activity was significantly increased by incubating retinas with forskolin or dibutyryl cyclic AMP as early as E7, and seven- to ninefold increases were observed following treatment with these agents on E14. Thus, development of the cyclic AMP-dependent mechanism for increasing NAT activity significantly precedes that of rhythmicity, suggesting that the onset of rhythmicity may be related to the onset of photoreception or development of the circadian oscillator in chick retina.

Arylalkylamine (serotonin) N-acetyltransferase assay using high-performance liquid chromatography with fluorescence or electrochemical detection of N-acetyltryptamine

A sensitive, rapid, and economical method has been developed for determination of serotonin N-acetyltransferase activity from a variety of enzyme sources. The assay is based upon separation and detection of N-acetyltryptamine formed from tryptamine and acetyl coenzyme A, by means of high-performance liquid chromatography with either electrochemical or fluorometric detection. The limit of sensitivity with both detection methods is less than 20 pmol of N-acetyltryptamine formed per sample. A method for synthesis of N-acetyltryptamine, used as an external standard in the assay, is described.

Preliminary report on adrenal-brain transplantation for parkinsonism in man

The attempt to restore lost neurological function through grafting of catecholaminergic tissue into the striatum is a dramatic new experimental treatment approach for parkinsonism. We have grafted autologous adrenal medullary tissue into the right caudate nucleus in 12 patients with advanced Parkinson's disease (10 males, 2 females, mean age 53 years, mean Hoehn and Yahr stage 'off' medications 4.2) applying a transcortical technique similar to that of Madrazo as modified by Allen using right adrenalectomy performed via a flank approach. There have been no deaths, surgical complications, or serious postoperative morbidity. Modified Columbia Parkinson's disease ratings of patients off all dopaminergic agents for 72 h revealed 22% improvement (p less than 0.01) 3-6 months postoperatively (n = 10) and 22% improvement (p less than 0.01) 7-12 months later (n = 9). Postoperative dyskinesias necessitated reduction of L-dopa by 11% and of carbidopa by 29%. Analysis of lumbar cerebrospinal fluid biogenic amine metabolites following a 4-day 'drug holiday' revealed that homovanillic acid increased to 159% of preoperative levels (p less than 0.01) 3-6 months (n = 9) and to 197% of preoperative levels (p less than 0.02) 7-12 months postoperatively (n = 5). We have observed a mild to moderate improvement persisting up to 1 year with this therapeutic approach, but further study is needed.