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  • 1
    Electronic Resource
    Electronic Resource
    Characterization of Serotonin N-Acetyltransferase in the Lateral Eye of the Green Frog Rana perezi: Protective Action of EGTA (1992)
    Oxford, UK : Blackwell Publishing Ltd
    Journal of neurochemistry 58 (1992), S. 0 
    Details
    ISSN: 1471-4159
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: Abstract: The kinetics of seRotonin N-acetyltransferase (NAT) from the lateral eye of Rana perezi have been characterized. NAT from ocular tissue reached maximal activity at a phosphate buffer concentration of 250 mM and a pH of 6.5. Reaction linearity was highly conserved within the homogenate fraction range tested (0.033-0.33). The time course of ocular NAT reaction showed a high linearity at 25 and 35°C. Km and Vmax estimations for acetyl-CoA at a 10 mM tryptamine concentration were 63.3 μM and 4.42 nmol/h per eye, respectively. Regardless of the acceptor amine (tryptamine or serotonin), the Km was not affected by the acetyl-CoA concentration (50 or 250 μM), whereas the Vmax was significantly increased at a 250 μM acetyl-CoA concentration. Ocular NAT showed a higher affinity for serotonin (Km= 20.7 μM) than for tryptamine (Km= 48-60 μM); Vmax however, was similar for both substrates. Acetyl-CoA does not protect ocular NAT; in contrast, the use of EGTA (〈inlineGraphic alt="geqslant R: gt-or-equal, slanted" extraInfo="nonStandardEntity" href="urn:x-wiley:00223042:JNC587:ges" location="ges.gif"/〉4 mM) in the assay is essential to protect the enzyme because NAT in ocular crude homogenate shows rapid inactivation. This result suggests that intracellular calcium levels are involved in the NAT inactivation mechanisms in frog ocular tissue.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1471-4159.1992.tb09759.x
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  • 2
    Electronic Resource
    Electronic Resource
    Regulation of Tryptophan Hydroxylase Activity in Xenopus laevis Photoreceptor Cells by Cyclic AMP (2000)
    Oxford UK : Blackwell Science Ltd.
    Journal of neurochemistry 74 (2000), S. 0 
    Details
    ISSN: 1471-4159
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: Abstract: 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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1046/j.1471-4159.2000.0741961.x
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  • 3
    Electronic Resource
    Electronic Resource
    Seasonal changes in haematology and metabolic resources in the tench (2003)
    Oxford, UK : Blackwell Science Ltd
    Journal of fish biology 62 (2003), S. 0 
    Details
    ISSN: 1095-8649
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Notes: Significant variations in the number of white and red blood cells, haematocrit and haemoglobin were found throughout the year in sexually mature male and female tench Tinca tinca. In general, the lowest values were observed during autumn–winter and the highest during summer, with males exhibiting higher values than females. Plasma glucose, cholesterol and triglycerides were lower during the winter than during the summer–autumn seasons in both sexes. Gonado-somatic and hepato-somatic indices were inversely correlated in female tench throughout the year. Seasonal patterns in liver metabolic resources were very similar for both sexes. For males and females, liver glycogen and proteins increased during the autumn, whereas the liver stored lipid during spring. Dorsal muscle mainly deposited glycogen, whereas lipid was mainly stored in the ventral muscle. Relations between seasonal changes in environmental factors, such as feeding and temperature are discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1046/j.1095-8649.2003.00066.x
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  • 4
    Electronic Resource
    Electronic Resource
    Serotonin N-acetyltransferase activity as a target for temperature in the regulation of melatonin production by frog retina (1994)
    Springer
    Pflügers Archiv 429 (1994), S. 153-159 
    Details
    ISSN: 1432-2013
    Keywords: Serotonin N-acetyltransferase ; Ocular melatonin production ; Eyecup culture ; Temperature ; Frog photoreceptors
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract The adaptive mechanisms of serotonin N-acetyltransferase (NAT) activity in the regulation of melatonin synthesis in frog retina in the face of chronic and acute temperature changes have been investigated. We performed thermal acclimation experiments to test different environmental temperatures at two seasons of the year (summer and winter), followed by the setup of an eyecup culture system to investigate the acute effects of temperature on NAT activity and melatonin production daily rhythms. Low temperature induced a significant increase in NAT activity, independent of both the time of the photocycle (midday or midnight) and the season of the year (winter or summer). Acute cold-induced stimulation of NAT activity may be associated with lower decreases in the enzyme synthesis rate, rather than decreases in the degradation rate. In contrast, acclimation to warm temperature (25° C) stimulated ocular melatonin production. Nocturnal melatonin production in eyecups cultured at 25° C was significantly higher than in eyecups cultured at 5° C. We suggest that this discrepancy in thermal regulation of melatonin synthesis can be justified by a seasonal variation in serotonin content within the photoreceptor cells, which determines the thermal response of melatonin production through changes in NAT kinetics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00374306
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  • 5
    Electronic Resource
    Electronic Resource
    Effect of constant and fluctuating temperature on daily melatonin production by eyecups from Rana perezi (1997)
    Springer
    Journal of comparative physiology 167 (1997), S. 221-228 
    Details
    ISSN: 1432-136X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Medicine
    Notes: Abstract We analysed the effect of daily temperature cycles in relation to constant temperature on day/night melatonin synthesis in frog eyecups in culture. Eyecups were cultured for 24 h under 12L:12D photoperiod and two thermal regimes, constant temperature (25, 15 and 5 °C) and thermoperiod (WL/CD, thermophase coinciding with photophase and cryophase coinciding with scotophase; and CL/WD, cryophase coinciding with photophase and thermophase coinciding with scotophase). A negative correlation between ocular serotonin N-acetyltransferase activity and culture temperature for both diurnal and nocturnal activities has been observed. This effect of increased ocular activity at low temperature is more pronounced than the well-known stimulatory effect of darkness, and it does not depend on the photoperiod phase. The lack of interactions between the phase of photoperiod and culture temperature indicates that the effects of both factors are independent. Nighttime temperature is the key factor in determining the amplitude of the melatonin rhythm in the Rana perezi retina. However, daytime temperature can not counteract the inhibitory effect of light on ocular melatonin synthesis.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s003600050068
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