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  • 1
    Electronic Resource
    Electronic Resource
    Solubilization and Biochemical Characterization of the Melatonin Deacetylase from Xenopus laevis Retina (1993)
    Oxford, UK : Blackwell Publishing Ltd
    Journal of neurochemistry 60 (1993), S. 0 
    Details
    ISSN: 1471-4159
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: Abstract: Melatonin deacetylase, an enzyme activity recently discovered in the Xenopus laevis retina, regulates local melatonin levels. The deacetylase occurs in retina, retinal pigment epithelium, and skin, all sites of melatonin action, and is widely distributed among vertebrates. We have solubilized the enzyme from Xenopus retina and pigment epithelium using nonionic detergents, and have developed a specific enzyme assay. We have characterized the enzyme and now report that the deacetylase is relatively specific for melatonin and is inhibited by the melatonin precursor N-acetylserotonin and the product of the deacetylase, 5-methoxytryptamine. Inhibition of deacetylase activity by eserine (physostigmine) suggests a relationship between deacetylase and cholinesterase activities. However, among a variety of cholinesterase inhibitors tested, only eserine inhibits the deacetylase. Furthermore, eserine is much less potent as an inhibitor of the deacetylase than the cholinesterases, and purified cholinesterases failed to deacetylate melatonin. We also show that melatonin deacetylase and aryl acylamidase (an enzyme related to cholinesterases) activities are differentially extractable from Xenopus ocular tissues, and that they exhibit different pH optima and inhibition profiles. Our results provide an initial characterization of the Xenopus retinal melatonin deacetylase, and indicate that deacetylase activity is distinct from cholinesterase and aryl acylamidase activities.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1471-4159.1993.tb03246.x
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  • 2
    Electronic Resource
    Electronic Resource
    Cyclic AMP Stimulates Serotonin N-Acetyltransferase Activity in Xenopus Retina In Vitro (1986)
    Oxford, UK : Blackwell Publishing Ltd
    Journal of neurochemistry 46 (1986), S. 0 
    Details
    ISSN: 1471-4159
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: Abstract: The possible involvement of cyclic AMP in the regulation of retinal serotonin N-acetyltransferase (NAT) activity was investigated using eye cups of Xenopus laevis cultured in a defined medium. Addition of dibutyrylcyclic AMP (dbcAMP) increased retinal NAT activity in eye cups cultured in light. Addition of adenosine or 5′-AMP under identical conditions was without effect. 3-Isobutylmethylxanthine (IBMX) increased both retinal cyclic AMP levels and NAT activity in light-exposed eye cups. Forskolin also increased the concentration of cyclic AMP and the activity of NAT, and the effect of forskolin on both of these parameters was synergistically enhanced by IBMX. The effects of forskolin and of dbcAMP did not require the addition of calcium to the medium; thus, Ca2+-dependent synaptic transmission does not appear to be required for the response to these drugs. Incubation conditions that activate cyclic AMP-dependent protein kinase in retinal homogenates had no effect on NAT activity, suggesting that direct phosphorylation of NAT was probably not involved in the response to elevating cyclic AMP in situ. The effect of dbcAMP was blocked by protein synthesis inhibitors. These results suggest that cyclic AMP increases retinal NAT activity by a mechanism that involves protein synthesis, and support a role for cyclic AMP in the nocturnal increase of NAT activity in darkness.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1471-4159.1986.tb12921.x
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  • 3
    Electronic Resource
    Electronic Resource
    Involvement of Calcium in the Regulation of Serotonin N-Acetyltransferase in Retina (1986)
    Oxford, UK : Blackwell Publishing Ltd
    Journal of neurochemistry 46 (1986), S. 0 
    Details
    ISSN: 1471-4159
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: Abstract: The possible involvement of calcium in the regulation of retinal serotonin N-acetyltransferase (NAT) activity was investigated using eye cups of Xenopus laevis cultured in defined medium. Omitting CaCl2 from the culture medium completely inhibited the dark-dependent increase of NAT activity at night. Approximately 10−4-10−3M free Ca2+ was found to be required for the maximal increase of NAT activity in the dark. Other divalent cations—Ba2+, Sr2+, and Mn2+—did not substitute for Ca2+. Antagonists of voltage-sensitive calcium channels, including nifedipine, methoxyverapamil (D600), Co2+, and Mg2+, were found to be effective inhibitors of the dark-dependent increase of retinal NAT activity. Trifluoperazine also decreased retinal NAT activity. These studies indicate that the increase of retinal NAT activity in the dark is mediated by a specific Ca2+-dependent process and that Ca2+ influx through voltage-sensitive calcium channels is involved.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1471-4159.1986.tb12928.x
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  • 4
    Electronic Resource
    Electronic Resource
    Tryptophan Hydroxylase Expression Is Regulated by a Circadian Clock in Xenopus laevis Retina (1994)
    Oxford, UK : Blackwell Publishing Ltd
    Journal of neurochemistry 62 (1994), S. 0 
    Details
    ISSN: 1471-4159
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: Abstract: A circadian clock has been localized to the photoreceptor layer in the Xenopus laevis retina. 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 synthesis in Xenopus laevis retina is tryptophan hydroxylase. A cDNA clone coding for Xenopus tryptophan hydroxylase was isolated, characterized, and used as a probe for analysis of tryptophan hydroxylase mRNA expression. Northern blot analyses of total retinal RNA show that the tryptophan hydroxylase message levels are low in the day and higher at night. The expression of tryptophan hydroxylase mRNA is under circadian control because rhythmic changes are also seen in constant darkness, with elevated levels during the subjective night. Nuclear run-on analysis during the first subjective day in constant darkness revealed that transcription initiation is low early in the day and increases throughout the day. Our observations suggest that the circadian clock modulates tryptophan hydroxylase gene expression. An understanding of how the circadian clock controls tryptophan hydroxylase expression may lead to a clearer understanding of the melatonin biosynthetic pathway, and possibly the clock itself.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1046/j.1471-4159.1994.62062420.x
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  • 5
    Electronic Resource
    Electronic Resource
    Circadian Regulation of Melatonin in the Retina of Xenopus laevis: Limitation by Serotonin Availability (1990)
    Oxford, UK : Blackwell Publishing Ltd
    Journal of neurochemistry 54 (1990), S. 0 
    Details
    ISSN: 1471-4159
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: Treatments expected to increase retinal serotonin levels were found to stimulate melatonin production by cultured eyecups from Xenopus laevis. The monoamine oxidase inhibitor pargyline (100 μM) caused a sixfold increase in melatonin release, and the serotonin precursor 5-hydroxy-L-tryptophan (100 μM) caused a 70-fold increase. Both acted synergistically with eserine, an inhibitor of melatonin deacetylation in the retina. The effect of 5-hydroxytryptophan was dose dependent, with effects increasing from 1 to 100 μM. Increasing the tryptophan level in the culture medium had no effect on melatonin release. These results indicate that the rate-limiting step in retinal melatonin synthesis is 5-hydroxylation of tryptophan. Melatonin released from individual eyecups in superfusion culture in constant darkness with and without added 5-hydroxy-L-tryptophan was monitored over a 5-day period. Control eyecups released low levels of melatonin, with circadian rhythmicity persisting for 1–3 days. With 5-hydroxy-L-tryptophan added, melatonin levels were elevated 10–20-fold at all times, and rhythmicity was apparent for as long as five cycles. This provides a model system for studies of the circadian clock in the eye.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1471-4159.1990.tb01932.x
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  • 6
    Electronic Resource
    Electronic Resource
    Regulation of photoreceptor Per1 and Per2 by light, dopamine and a circadian clock (2004)
    Oxford, UK : Blackwell Science Ltd
    European journal of neuroscience 20 (2004), S. 0 
    Details
    ISSN: 1460-9568
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: In the Xenopus laevis retina, a principal model for retinal circadian organization, photoreceptors have all the properties of circadian oscillators. However, rhythmic oscillations of Per1 gene expression in the inner retina (but not photoreceptors) have been reported in mice with the suggestion that mice and frogs have a different retinal circadian organization. Although it is known that two period genes (xPer1 and xPer2) exhibit different temporal patterns of expression in the Xenopus retina, and that one (xPer2) is directly responsive to light and dopamine, it is not known whether this reflects the properties of period genes within photoreceptor oscillators or among distinct retinal cell populations. We addressed this by determining the cellular site of light and dopamine regulated xPer2 expression, and the diurnal expression of both xPer1 and xPer2 using in situ hybridization. Our data show that both xPer1 and xPer2 are expressed in most cell types in the retina, including inner nuclear neurons and ganglion cells. However, light and quinpirole, a dopamine agonist, increase xPer2 levels specifically in photoreceptors, and the effect of quinpirole, but not light, is blocked by pCPT-cAMP. Furthermore, antiphasic diurnal expression of xPer1 and xPer2 also occurs in photoreceptors. Our analysis does not provide insight into the near constitutive expression of period genes in the inner retina, but supports a model in which light- and dopamine regulated-xPer2 and rhythmic xPer1 play critical roles in entrainment and circadian oscillations within photoreceptors.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1460-9568.2004.03479.x
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  • 7
    Electronic Resource
    Electronic Resource
    The “ON”-bipolar agonist, L-2-amino-4-phosphonobutyrate, blocks light-evoked cone contraction in xenopus eye cups (1992)
    Springer
    Neurochemical research 17 (1992), S. 75-80 
    Details
    ISSN: 1573-6903
    Keywords: Photoreceptor ; retinomotor movement ; dopamine ; “ON” bipolars ; L-2-amino-4-phosphonobutyrate
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract Rhythmic photoreceptor metabolism in relationship to light-dark cycles is now thought be regulated through a retinal feed-back mechanism with dopamine serving as a principal signal initiating light-evoked events. In order to test the hypothesis that depolarizing “ON”-bipolar neurons participate in the retinal signalling pathway, we determined the effects of L-2-amino-4-phosphonobutyrate (L-APB) on light-evoked cone contraction in eye cups fromXenopus laevis. L-APB blocked the response stereospecifically when applied over a broad concentration range. The high specificity of L-APB in retina suggests that sign-inverting bipolar neurons which depolarize in light are in the signalling pathway. One possibility is that this pathway conveys signals that regulate dopamine release.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00966867
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  • 8
    Electronic Resource
    Electronic Resource
    Circadian clock in Xenopus eye controlling retinal serotonin N -acetyltransferase (1983)
    [s.l.] : Nature Publishing Group
    Nature 305 (1983), S. 133-135 
    Details
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] Postmetamorphic, juvenile Xenopus laevis (Nasco, Fort Atkinson, Wisconsin) were maintained in our laboratory for a minimum of 1 month on a 12-h light, 12-h dark (LD 12:12) schedule at 23 ± 1 C before use in experiments. We have previously shown that the retina of this species exhibits a ...
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1038/305133a0
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  • 9
    Electronic Resource
    Electronic Resource
    Rhythmic regulation of retinal melatonin: Metabolic pathways, neurochemical mechanisms, and the ocular circadian clock (1991)
    Springer
    Cellular and molecular neurobiology 11 (1991), S. 529-560 
    Details
    ISSN: 1573-6830
    Keywords: Melatonin ; retina ; circadian rhythm ; serotoninN-acetyltransferase ; tryptophan hydroxylase ; melatonin deacetylase ; D2 dopamine receptor ; cyclic AMP
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Summary 1. Current knowledge of the mechanisms of circadian and photic regulation of retinal melatonin in vertebrates is reviewed, with a focus on recent progress and unanswered questions. 2. Retinal melatonin synthesis is elevated at night, as a result of acute suppression by light and rhythmic regulation by a circadian oscillator, or clock, which has been localized to the eye in some species. 3. The development of suitablein vitro retinal preparations, particularly the eyecup from the African clawed frog,Xenopus laevis, has enabled identification of neural, cellular, and molecular mechanisms of retinal melatonin regulation. 4. Recent findings indicate that retinal melatonin levels can be regulated at multiple points in indoleamine metabolic pathways, including synthesis and availability of the precursor serotonin, activity of the enzyme serotoninN-acetyltransferase, and a novel pathway for degradation of melatonin within the retina. 5. Retinal dopamine appears to act through D2 receptors as a signal for light in this system, both in the acute suppression of melatonin synthesis and in the entrainment of the ocular circadian oscillator. 6. A recently developedin vitro system that enables high-resolution measurement of retinal circadian rhythmicity for mechanistic analysis of the circadian oscillator is described, along with preliminary results that suggest its potential for elucidating general circadian mechanisms. 7. A model describing hypothesized interactions among circadian, neurochemical, and cellular mechanisms in regulation of retinal melatonin is presented.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00734814
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  • 10
    Electronic Resource
    Electronic Resource
    Immunocytochemical localization of opsin in rod photoreceptors during periods of rapid disc assembly (1995)
    Springer
    Journal of neurocytology 24 (1995), S. 371-388 
    Details
    ISSN: 1573-7381
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Summary Transport of opsin from photoreceptor inner to outer segments has been assumed to occur via the connecting cilium, the only permanent structural connection between these two regions. However, in prior work, little or no immunoreactive opsin has been detected in the cilium, despite the high rate of transport of this protein. This suggests that immune epitopes are masked during passage through the cilium or that opsin is transported via an extra-ciliary route. In this study, we stained the photoreceptors ofXenopus laevis with well-characterized monoclonal antibodies directed at the N-terminal, C-terminal, and 5–6 loop regions of bovine opsin. This was done on isolated retinas incubatedin vitro under conditions that support rapid disc assembly, to insure that opsin transport to forming discs was occurring at the time of fixation. Five MAbs that gave robust staining ofXenopus rod inner segment/rod outer segment preparations with the light microscope were utilized for electron microscopic studies on LR White embedded or cryo-ultrathin sections. Four of these stained outer segment discs and inner segment vesicles and plasma membrane. However, no significant staining of the connecting cilium was found. Furthermore, freeze-fractured mouse photoreceptors prepared by the ‘fracture-label’ technique showed extensive labelling of membrane compartments but lacked staining of the connecting cilium. Isolated retinas incubated under conditions that support robust rod disc synthesis contained many finger-like and vesicular projections of the apical inner segment plasma membrane and inner segment vesicles extending into them. Rod outer segment nascent discs usually made close contact with the inner segment. Both the vesicular profiles associated with the inner segment plasma membrane and the basal discs extending to the inner segment were heavily stained with all four anti-opsin antibodies. This suggests an alternate route for bulk transport of opsin to newly forming discs that involves direct transfer from apical inner segment plasma membrane to nascent discs.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01189064
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