Stimulatory Effect Of Light Research Articles

Plant-specific environmental and developmental signals regulate the mismatch repair protein MSH6 in Arabidopsis thaliana

The DNA mismatch repair (MMR) is a postreplicative system that guarantees genomic stability by correcting mispaired and unpaired nucleotides. In eukaryotic nuclei, MMR is initiated by the binding of heterodimeric MutS homologue (MSH) complexes to the DNA error or lesion. Among these proteins, MSH2-MSH6 is the most abundant heterodimer. Even though the MMR mechanism and proteins are highly conserved throughout evolution, physiological differences between species can lead to different regulatory features. Here, we investigated how light, sugar, and/or hormones modulate Arabidopsis thaliana MSH6 expression pattern. We first characterized the promoter region of MSH6. Phylogenetic shadowing revealed three highly conserved regions. These regions were analyzed by the generation of deletion constructs of the MSH6 full-length promoter fused to the β-glucuronidase (GUS) gene. Combined, our in silico and genetic analyses revealed that a 121-bp promoter fragment was necessary for MSH6 expression and contained potential cis-acting elements involved in light- and hormone-responsive gene expression. Accordingly, light exposure or sugar treatment of four-day old A. thaliana seedlings triggered an upregulation of MSH6 in shoot and root apical meristems. Appropriately, MSH6 was also induced by the stem cell inducer WUSCHEL. Further, the stimulatory effect of light was dependent on the presence of phyA. In addition, treatment of seedlings with auxin or cytokinin also caused an upregulation of MSH6 under darkness. Consistent with auxin signals, MSH6 expression was suppressed in the GATA23 RNAi line compared with the wild type. Our results provide evidence that endogenous factors and environmental signals controlling plant growth and development regulate the MSH6 protein in A. thaliana.

Seed germination ecology of meadow knapweed (Centaurea × moncktonii) populations in New York State, USA

AbstractThe introduced meadow knapweed (Centaurea × moncktonii C.E. Britton), a hybrid of black (Centaurea nigra L.) and brown (Centaurea jacea L.) knapweeds, is increasingly common in pastures, meadows, and waste areas across many U.S. states, including New York. We evaluated the effects of temperature, light, seed stratification, scarification, and population on percent germination in four experiments over 2 yr. Percent germination ranged from 3% to 100% across treatment combinations. Higher temperatures (30:20, 25:15, and sometimes 20:10 C day:night regimes compared with 15:5 C) promoted germination, especially when combined with the stimulatory effect of light (14:10 h L:D compared with continuous darkness). Under the three lowest temperature treatments, light increased percent germination by 15% to 86%. Cold-wet seed stratification also increased germination rates, especially at lower germination temperatures, but was not a prerequisite for germination. Scarification did not increase percent germination. Differences between C. × moncktonii populations were generally less significant than differences between temperature, light, and stratification treatments. Taken together, these results indicate that C. × moncktonii is capable of germinating under a broad range of environments, which may have facilitated this species’ range expansion in recent decades. However, C. × moncktonii also shows evidence of germination polymorphism: some seeds will germinate under suboptimal conditions, while others may remain dormant until the abiotic environment improves. Subtle differences in dormancy mechanisms and their relative frequencies may affect phenological traits like the timing of seedling emergence and ultimately shape the sizes and ranges of C. × moncktonii populations.

Loss of dopamine disrupts circadian rhythms in a mouse model of Parkinson's disease

Although a wide range of physiological functions regulated by dopamine (DA) display circadian variations, the role of DA in the generation and/or modulation of these rhythms is unknown. In Parkinson's disease (PD) patients, in addition to the classical motor symptoms, disturbances of the pattern of daily rest/wake cycles are common non-motor symptoms. We investigated daily and circadian rhythms of rest/activity behaviors in a transgenic MitoPark mouse model with selective inactivation of mitochondrial transcription factor A (Tfam) resulting in a slow and progressive degeneration of DA neurons in midbrain structures. Correlated with this, MitoPark mice show a gradual reduction in locomotor activity beginning at about 20weeks of age. In a light–dark cycle, MitoPark mice exhibit a daily pattern of rest/activity rhythms that shows an age-dependent decline in both the amplitude and the stability of the rhythm, coupled with an increased fragmentation of day/night activities. When the circadian system is challenged by exposure to constant darkness or constant light conditions, control littermates retain a robust free-running circadian locomotor rhythm, whereas in MitoPark mice, locomotor rhythms are severely disturbed or completely abolished. Re-exposure to a light/dark cycle completely restores daily locomotor rhythms. MitoPark mice and control littermates express similar masking behaviors under a 1h light/1h dark regime, suggesting that the maintenance of a daily pattern of rest/activity in arrhythmic MitoPark mice can be attributed to the acute inhibitory and stimulatory effects of light and darkness. These results imply that, in addition to the classical motor abnormalities observed in PD, the loss of the midbrain DA neurons leads to impairments of the circadian control of rest/activity rhythms.

An updated view on the role of dopamine in myopia

A large body of data is available to support the hypothesis that dopamine (DA) is one of the retinal neurotransmitters involved in the signaling cascade that controls eye growth by vision. Initially, reduced retinal DA levels were observed in eyes deprived of sharp vision by either diffusers (“deprivation myopia”, DM) or negative lenses (“lens induced myopia”, LIM). Simulating high retinal DA levels by intravitreal application of a DA agonist can suppress the development of both DM and LIM. Also more recent studies using knock-out mouse models of DA receptors support the idea of an association between decreased DA levels and DM. There seem to be differences in the magnitude of the effects of DA on DM and LIM, with larger changes in DM but the degrees of image degradation by both treatments need to be matched to support this conclusion. Although a number of studies have shown that the inhibitory effects of dopamine agonists on DM and LIM are mediated through stimulation of the D2-receptor, there is also recent evidence that the balance of D2- and D1-receptor activation is important. Inhibition of D2-receptors can also slow the development of spontaneous myopia in albino guinea pigs. Retinal DA content displays a distinct endogenous diurnal, and partially circadian rhythm. In addition, retinal DA is regulated by a number of visual stimuli like retinal illuminance, spatial frequency content of the image, temporal contrast and, in chicks, by the light input from the pineal organ. A close interaction was found between muscarinergic and dopaminergic systems, and between nitric oxide and dopaminergic pathways, and there is evidence for crosstalk between the different pathways, perhaps multiple binding of the ligands to different receptors. It was shown that DA agonists interact with the immediate early signaling molecule ZENK which triggers the first steps in eye growth regulation. However, since long treatment periods were often needed to induce significant changes in retinal dopamine synthesis and release, the role of dopamine in the early steps is unclear. The wide spatial distribution of dopaminergic amacrine cells in the retina and the observation that changes in dopamine levels can be locally induced by local retinal deprivation is in line with the assumption that dopaminergic mechanisms control both central and peripheral eye growth. The protective effect of outdoor activity on myopia development in children seems to be partly mediated by the stimulatory effect of light on retinal dopamine production and release. However, the dose–response function linking light exposure to dopamine and to the suppression of myopia is not known and requires further studies.

Antipsychotic-induced D2-receptor upregulation — differential effect of continuous versus transient D2-receptor blockade

A large body of data is available to support the hypothesis that dopamine (DA) is one of the retinal neurotransmitters involved in the signaling cascade that controls eye growth by vision. Initially, reduced retinal DA levels were observed in eyes deprived of sharp vision by either diffusers (“deprivation myopia”, DM) or negative lenses (“lens induced myopia”, LIM). Simulating high retinal DA levels by intravitreal application of a DA agonist can suppress the development of both DM and LIM. Also more recent studies using knock-out mouse models of DA receptors support the idea of an association between decreased DA levels and DM. There seem to be differences in the magnitude of the effects of DA on DM and LIM, with larger changes in DM but the degrees of image degradation by both treatments need to be matched to support this conclusion. Although a number of studies have shown that the inhibitory effects of dopamine agonists on DM and LIM are mediated through stimulation of the D2-receptor, there is also recent evidence that the balance of D2- and D1-receptor activation is important. Inhibition of D2-receptors can also slow the development of spontaneous myopia in albino guinea pigs. Retinal DA content displays a distinct endogenous diurnal, and partially circadian rhythm. In addition, retinal DA is regulated by a number of visual stimuli like retinal illuminance, spatial frequency content of the image, temporal contrast and, in chicks, by the light input from the pineal organ. A close interaction was found between muscarinergic and dopaminergic systems, and between nitric oxide and dopaminergic pathways, and there is evidence for crosstalk between the different pathways, perhaps multiple binding of the ligands to different receptors. It was shown that DA agonists interact with the immediate early signaling molecule ZENK which triggers the first steps in eye growth regulation. However, since long treatment periods were often needed to induce significant changes in retinal dopamine synthesis and release, the role of dopamine in the early steps is unclear. The wide spatial distribution of dopaminergic amacrine cells in the retina and the observation that changes in dopamine levels can be locally induced by local retinal deprivation is in line with the assumption that dopaminergic mechanisms control both central and peripheral eye growth. The protective effect of outdoor activity on myopia development in children seems to be partly mediated by the stimulatory effect of light on retinal dopamine production and release. However, the dose–response function linking light exposure to dopamine and to the suppression of myopia is not known and requires further studies.

Light Irradiation of Mouse Spermatozoa: Stimulation of In Vitro Fertilization and Calcium Signals

Irradiation of mouse spermatozoa by 630 nm He-Ne laser was found to enhance the intracellular calcium levels and fertilizing potential of these cells. The effect of light on calcium transport and on fertilization rate was abrogated in the absence of Ca2+ during the irradiation time, indicating that the effect of light is Ca2+ dependent. The stimulatory effect of light on Ca2+ uptake was abolished in the presence of a voltage-dependent Ca(2+)-channel inhibitor nifedipine, indicating the involvement of a plasma membrane voltage-dependent Ca2+ channel. Furthermore, the stimulatory effect of light was completely inhibited by the mitochondrial uncoupler FCCP, indicating that laser irradiation might affect the mitochondrial Ca2+ transport mechanisms. A causal association between laser irradiation, reactive oxygen species (ROS) generation and sperm function was indicated by studies with ROS scavengers, superoxide dismutase (SOD) and catalase, and exogenous hydrogen peroxide. The SOD treatment, which enhanced H2O2 production, resulted in increased Ca2+ uptake and enhanced fertilization rate. On the other hand, catalase, which decomposes H2O2, impaired the light-induced stimulation in Ca2+ uptake and the fertilization rate. Taken together, the data suggest that H2O2 might be involved in the irradiation effects, and indeed laser irradiation enhances the production of H2O2 by spermatozoa. These results indicate that the effect of 630 nm He-Ne laser irradiation is mediated through the generation of H2O2 by the spermatozoa and that this effect plays a significant role in the augmentation of the sperm cells' capability to fertilize metaphase II-arrested eggs in vitro.

Light Irradiation of Mouse Spermatozoa: Stimulation of In Vitro Fertilization and Calcium Signals

Irradiation of mouse spermatozoa by 630 nm He-Ne laser was found to enhance the intracellular calcium levels and fertilizing potential of these cells. The effect of light on calcium transport and on fertilization rate was abrogated in the absence of Ca2+during the irradiation time, indicating that the effect of light is Ca2+dependent. The stimulatory effect of light on Ca2+uptake was abolished in the presence of a voltage-dependent Ca2+-channel inhibitor nifedipine, indicating the involvement of a plasma membrane voltage-dependent Ca2+channel. Furthermore, the stimulatory effect of light was completely inhibited by the mitochondrial uncoupler FCCP, indicating that laser irradiation might affect the mitochondrial Ca2+transport mechanisms. A causal association between laser irradiation, reactive oxygen species (ROS) generation and sperm function was indicated by studies with ROS scavengers, superoxide dismutase (SOD) and catalase, and exogenous hydrogen peroxide. The SOD treatment, which enhanced H2O2 production, resulted in increased Ca2+uptake and enhanced fertilization rate. On the other hand, catalase, which decomposes H2O2, impaired the light-induced stimulation in Ca2+uptake and the fertilization rate. Taken together, the data suggest that H2O2 might be involved in the irradiation effects, and indeed laser irradiation enhances the production of H2O2, by spermatozoa. These results indicate that the effect of 630 nm He-Ne laser irradiation is mediated through the generation of H2O2 by the spermatozoa and that this effect plays a significant role in the augmentation of the sperm cells' capability to fertilize metaphase H-arrested eggs in vitro.

Circadian regulation of hydroxyindole-O-methyltransferase mRNA in the chicken pineal gland in vivo and in vitro.

The production of the pineal hormone melatonin displays circadian variations with high levels at night. The last enzyme involved in melatonin biosynthesis is hydroxyindole-O-methyltransferase (HIOMT, EC 2.1.1.4). The expression of the mRNA encoding chicken HIOMT was investigated in vivo and in vitro throughout the light/dark cycle, in constant darkness and with light interruption of the dark phase. The stability of HIOMT mRNA was also examined. A day/night rhythm of HIOMT mRNA levels, with a peak at the midlight phase, was observed in vivo as well as in vitro. Constant darkness did not abolish this rhythm in vivo. One cycle of the HIOMT mRNA rhythm could be observed in constant darkness in vitro. In addition, a stimulatory effect of light on HIOMT mRNA levels during the dark phase could be observed in vivo as well as in vitro. HIOMT mRNA stability was not affected by light or dark conditions, as demonstrated by chase experiments with actinomycin D. The results indicate that the daily changes in HIOMT mRNA concentration reflect transcriptional regulation by circadian oscillators and photosensory mechanisms that are endogenous to the pineal gland.

N2-Fixation by Freshly Isolated Nostoc from Coralloid Roots of the Cycad Macrozamia riedlei (Fisch. ex Gaud.) Gardn.

Nitrogenase (EC 1.7.99.2) activity (acetylene reduction) and nitrogen fixation ((15)N(2) fixation) were measured in cyanobacteria freshly isolated from the coralloid roots of Macrozamia riedlei (Fisch. ex Gaud.) Gardn. Light and gas phase oxygen concentration had marked interactive effects on activity, with higher (up to 100-fold) rates of acetylene reduction and (15)N(2) fixation in light. The relationship between ethylene formation and N(2)-fixation varied in the freshly isolated cyanobacteria from 4 to 7 nanomoles of C(2)H(4) per nanomole (15)N(2). Intact coralloid roots, incubated in darkness and ambient air, showed a value of 4.3. Maximum rates of nitrogenase activity occurred at about 0.6% O(2) in light, while in darkness there was a broad optimum around 5 to 8% O(2). Inhibition of nitrogenase, in light, by pO(2) above 0.6% was irreversible. Measurements of light-dependent O(2) evolution and (14)CO(2) fixation indicated negligible photosynthetic electron transport involving photosystem II and, on the basis of inhibitor studies, the stimulatory effect of light was attributed to cyclic photophos-phorylation. Nitrogenase activity of free-living culture of an isolate from Macrozamia (Nostoc PCC 73102) was only slightly inhibited by O(2) levels above 6% O(2) and the inhibition was reversible. These cells showed rates of light-dependent O(2) evolution and (14)CO(2) fixation which were 100- to 200-fold higher than those by the freshly isolated symbiont. Furthermore, nitrogenase activity was dependent on both photosynthetic electron transport and photophosphorylation. These data indicate that cyanobacteria within cycad coralloid roots are differentiated specifically for symbiotic functioning in a microaerobic environment. Specializations include a high heterocyst frequency, enhanced permeability to O(2), and a direct dependence on the cycad for substrates to support nitrogenase activity.

Circadian and Photoperiodic Effects of Brief Light Pulses in Male Djungarian Hamsters1

The effects of brief light pulses (1-60 min in duration) on the circadian rhythm of locomotor activity and/or the neuroendocrine-gonadal axis was investigated in male Djungarian hamsters. Exposure of hamsters free-running in constant darkness to a single 1-h pulse of light induced phase-dependent phase shifts in the rhythm of locomotor activity. The general shape of the "phase-response curve" was similar to that observed in other animals; phase-delays and phase-advances were induced by light pulses delivered in the early and late subjective night, respectively, while light pulses during the subjective day induced little or no phase-shift in the activity rhythm. Animals exposed for 7 days to 1-min of light during the night in animals otherwise exposed to 6L:18D resulted in increased levels of serum FSH and testicular weight. Daily exposure to two 1-h or two 10-min pulses of light (but not two 1-min pulses) for 10 days resulted in stable entrainment of the activity rhythm as well as testicular weight gains and serum FSH increases. When two 10-min pulses of light were presented 8 and 16 h apart, some animals showed a short-day entrainment pattern (i.e., locomotor activity confined to the long period of darkness) while other animals showed a long-day entrainment pattern (i.e., locomotor activity confined to the short period of darkness). Importantly, the stimulatory effects of light on neuroendocrine-gonadal activity were clearly dependent on the phase-relationship between the light pulses and the circadian rhythm of locomotor activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Digitoxin Biosynthesis in Isolated Mesophyll Cells and Cultured Cells of <italic>Digitalis</italic>

In the laminae of Digitalis, most of the digitoxin present is found in the mesophyll. A new method for determining the amount of digitoxin biosynthesis using a digitoxin antibody was devised to estimate this activity in isolated mesophyll cells and cultured cells. Isolated mesophyll cells showed significant activity, which suggests that the site of biosynthesis and the accumulation of cardenolides in a lamina of Digitalis is mainly in the mesophyll cells. Of five liquid cultures of D. purpurea; green shoot-forming cultures, white shoot-forming cultures, root-forming cultures, undifferentiated green cells and undifferentiated white cells, the green shoot-forming cultures had the highest activity. The white shoot-forming cultures had about one-third the activity of the green shoot-forming cultures, and the other three cultures had very low activity. No stimulatory effect of light was found during the 48-h incubation.

Light stimulates glycerate uptake by spinach chloroplasts

Uptake of glycerate into the stroma of isolated spinach chloroplasts has been studied by silicone oil filtering centrifugation. In the dark, glycerate uptake was slow but it was increased more than five-fold by illumination of the chloroplasts. The stimulatory effect of light was reversed by uncoupling agents. By chromatography of chloroplast extracts it was demonstrated that the concentration of glycerate in the chloroplast stroma exceeded that in the surrounding medium. Glycerate uptake was dependent on temperature and pH and showed saturation kinetics. A number of weak acids inhibited glycerate uptake. It is concluded that glycerate uptake in chloroplasts is mediated by a carrier which is stimulated by illumination of the chloroplasts.

Hormones are no causal links in phytochrome-mediated adventitious root formation in mustard seedlings (Sinapis alba L.)

The question of whether or not hormones are causal links in the realization of phytochrome control during photomorphogenesis was investigated using the phytochrome-dependent formation of adventitious roots in hypocotyl cuttings excised from mustard seedlings as a test system. Histological examination of regenerating "rest" seedlings revealed that phytochrome (operationally, continuous far-red light) mediates the de novo formation of root primordia in the pericycle region of the hypocotyl near the cutting surface withing 12-24 h after excision.Auxin (IAA), gibberellin (GA3), Cytokinin (kinetin), abscisic acid (ABA), and ethylene had no promotive effect on primordium formation in dark-grown or far-red irradiated rest seedlings. Depending on concentration, the application of these hormones was either ineffective or inhibitory in the rooting response. It is concluded that phytochrome does not operate through changes of hormone (auxin, gibberellin, cytokinin, ABA, ethylene) levels.While externally applied ethylene had no specific effect on primordium formation, the number of primordia produced in darkness could be increased to the far-red light level by removing the endogenously formed ethylene. Since the stimulatory effect of light could not be related to a lower ethylene level, it is concluded that ethylene interferes with primordium formation by modulating the susceptibility of this process to phytochrome control. This ethylene effect takes place in a concentration range below the range that can be manipulated by external application of the hormone.

ASPARTATE‐DEPENDENT ALANINE PRODUCTION BY LEAF DISCS OF AMARANTHUS PANICULATUS, AN ASPARTATE UTILIZING NAD‐MALIC ENZYME TYPE C4 PLANT

SUMMARYIlluminated leaf discs of Amaranthus paniculatus formed alanine rapidly after infiltration with aspartate. a‐oxoglutarate and glutamate accelerated aspartate‐dependent alanine production. Alanine was also formed by A. paniculatus leaf discs infiltrated with oxalacetate; this production was stimulated only by glutamate. Our results confirm that alanine can be formed rapidly from aspartate in C4 plants. Aspartate‐dependent alanine production was stimulated greatly by light. The stimulatory effect of light could almost be replaced by ATP in presence of sodium ions. These findings are discussed in relation to a possible light‐dependent active transport of amino acids into the bundle sheath, mediated by Na+‐dependent ATPase activity.

Light-induced changes of enzyme activities in parsley cell suspension cultures: Effects of inhibitors of RNA and protein synthesis

Light-induced increases in the activities of phenylalanine ammonia-lyase, p-coumarate:CoA ligase, and chalcone-flavanone isomerase in cell suspension cultures of Petroselinum hortense were similarly affected by several inhibitors of transcription and translation. In particular, strong inhibition by low doses of actinomycin D or cycloheximide indicated that both RNA and cytoplasmic protein synthesis were involved in the stimulatory effects of light. The increases in enzyme activities were preceded by a lag period of about 2.5 hr. When actinomycin D or cycloheximide was added at different times after the onset of light, characteristic differences in the inhibitory effects on the increases in phenylalanine ammonia-lyase activity were observed. The results suggested that RNA synthesis commenced immediately upon illumination, while an enhanced rate of synthesis of specific protein(s) did not begin before the end of the lag period. By comparison, the activities of some enzymes not involved in phenylpropanoid metabolism and not affected by light were also not affected by actinomycin D or cycloheximide at the concentrations used. This seems to exclude the possibility of nonspecific effects of these inhibitors on enzyme activities under the conditions of experimentation.

GERMINATION OF THE LIGHT‐SENSITIVE SEEDS OF OCIMUM AMERICANUM LINN.

SummaryA brief light exposure is essential for the germination of dark imbibed seeds of Ocimum americanum Linn. Seeds with testa broken at the radicular end were independent of light requirement for germination. Red and far‐red light were found to affect germination in a fashion typical of other photomorphogenic reactions mediated through a phytochrome system. The stimulatory effect of light was not constant if the imbibed seeds were maintained in darkness for long periods. Imbibed seeds entered a secondary dormancy (skotodormancy) under prolonged darkness. This was relieved by cold treatment. The ecological implications of skotodormancy are discussed.

Light Regulation in Turkey Management: 2. Female Reproductive Performance

CONSIDERABLE information has been published recently about the stimulatory effect of light on turkey reproductive activity. The publications of Margolf et al. (1947), Asmundson et al. (1946), Asmundson and Moses (1950), Harper and Parker (1957, 1961), Marr et al. (1956), Ogasowara et al. (1959), McCartney et al. (1956), Eaton et al. (1960), Garland et al. (1961), Leighton (1960) and Leighton and Shoffner (1961a, b) attest to the interest in photoperiod response. Light intensity, length of day, and alteration of day-length have been shown to be important in photo-stimulation. The female turkey is also dark-dependent, being relatively insensitive to stimulation unless previously exposed to a short-day period.Maximizing egg production by light management has potential in several ways, such as onset of early lay and increased persistency during the normal season, as well as encouraging off-season production. The provision of a suitable environment for early and extended lay may be of …

The stimulatory effect of light on nucleic acid synthesis in the mould Blastocladiella emersonii.

SUMMARY: The fungus Blastocladiella emersonii possesses neither chlorophyll nor coloured carotenoids; yet, visible light induces increased nuclear multiplication, increased synthesis of DNA relative to total nucleic acid, and detectable changes in the soluble phosphorus pool of the fungus. The stimulatory effect is detectable through ontogeny, from the first few hours after germination to the end of the generation time of c. 15 to 25 hr. (depending on temperature). Light could be replaced by providing the plants with thymidine or thymine, but not with the corresponding RNA base, uracil. The DNA of B. emersonii contained the four principal bases usually associated with this nucleic acid. Finally, the interrelationships between DNA and RNA, and the effects thereon of nucleic acid analogues and inhibitors, were also studied. In particular, it was found that the DNA per cell doubled just before the first mitosis, while the total nucleic acid concentration did not change. The results of the latter experiments were interpreted in terms of the transformations which occur in the RNA-rich nuclear cap in the spores of the fungus during germination.

STIMULATORY AND INHIBITORY EFFECT OF LIGHT ON THE NITRATE ASSIMILATION BY CHLORELLA

1. Using Chlorella ellipsoidea as material it was demonstrated that the process of assimilation of nitrate or nitrite was affected by light in quite different ways according as the cells had been placed under dark-adapted or light-adapted condition. The cells cultured with nitrate as the nitrogen source accumulated a large amount of nitrite on long continued illumination, and the nitrite thus accumulated rapidly disappeared when light was turned off (inhibitory effect of light). When the cells were kept in the dark for a sufficient length of time, re-accumulation of nitrite took place. However, this nitrite decreased markedly when the cells (dark-adapted cell) were illuminated (stimulatory effect of light). The rate of formation of amino acids decreased when nitrite was accumulated, and increased when the level of nitrite dropped, indicating that the change of nitrite level was closely related to the amino acid and follow-up protein syntheses.2. The addition of keto acids to the light-adapted cells decreased the level of nitrite, while practically no effect of the addition was observed with the dark-adapted cells. This observation together with the fact that the amino acids content of light-adapted cells increased markedly when the cells were kept in the dark for a short period of time (30 minutes) led us to the inference that in the light-adapted cells keto acids form the limiting factor for the nitrate assimilation and that the keto acids are accumulated in the dark conceivably owing to the enhanced operation of the Krebs cycle in the darkness.3. Various experimental results were explained by assuming two factors controlling the process of amino acids synthesis from nitrate. One factor is a photogenic reducing agent(s), R, which is required for the reduction of nitrate and the intermediates between nitrate and amino acids, and the other is keto acids, which react with hydroxylamine or ammonia to form amino acids. The light-adapted cells contain a large amount of R but only a small amount of keto acids, while dark-adapted cells contain a small amount of R but a large amount of keto acids. Along the line of this reasoning, it was postulated that the assimilation of nitrate or nitrite by algal cells would proceed most efficiently when the cells are subject to an intermittent illumination of appropriate intervals.

Researches on the texture of iron

A large body of data is available to support the hypothesis that dopamine (DA) is one of the retinal neurotransmitters involved in the signaling cascade that controls eye growth by vision. Initially, reduced retinal DA levels were observed in eyes deprived of sharp vision by either diffusers (“deprivation myopia”, DM) or negative lenses (“lens induced myopia”, LIM). Simulating high retinal DA levels by intravitreal application of a DA agonist can suppress the development of both DM and LIM. Also more recent studies using knock-out mouse models of DA receptors support the idea of an association between decreased DA levels and DM. There seem to be differences in the magnitude of the effects of DA on DM and LIM, with larger changes in DM but the degrees of image degradation by both treatments need to be matched to support this conclusion. Although a number of studies have shown that the inhibitory effects of dopamine agonists on DM and LIM are mediated through stimulation of the D2-receptor, there is also recent evidence that the balance of D2- and D1-receptor activation is important. Inhibition of D2-receptors can also slow the development of spontaneous myopia in albino guinea pigs. Retinal DA content displays a distinct endogenous diurnal, and partially circadian rhythm. In addition, retinal DA is regulated by a number of visual stimuli like retinal illuminance, spatial frequency content of the image, temporal contrast and, in chicks, by the light input from the pineal organ. A close interaction was found between muscarinergic and dopaminergic systems, and between nitric oxide and dopaminergic pathways, and there is evidence for crosstalk between the different pathways, perhaps multiple binding of the ligands to different receptors. It was shown that DA agonists interact with the immediate early signaling molecule ZENK which triggers the first steps in eye growth regulation. However, since long treatment periods were often needed to induce significant changes in retinal dopamine synthesis and release, the role of dopamine in the early steps is unclear. The wide spatial distribution of dopaminergic amacrine cells in the retina and the observation that changes in dopamine levels can be locally induced by local retinal deprivation is in line with the assumption that dopaminergic mechanisms control both central and peripheral eye growth. The protective effect of outdoor activity on myopia development in children seems to be partly mediated by the stimulatory effect of light on retinal dopamine production and release. However, the dose–response function linking light exposure to dopamine and to the suppression of myopia is not known and requires further studies.