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Biomed Pharmacother. Author manuscript; available in PMC 2008 October 31.
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Published in final edited form as:
Biomed Pharmacother. 2005 October ; 59(Suppl 1): S209–S212.
Chronomics reveal and quantify circadian rhythmic melatonin in
duodenum of rats
K. Stebelovaa, M. Zemana, G. Cornélissenb, G. Bubenikc, R. Jozsad, R. Hardelande, B.
Poeggelere, G. Huethere, A. Olahd, G. Nagyf, V. Csernusd, J. Kazsakig, W. Panh, K. Otsukai,
E. E. Bakkenj, and F. Halbergb,*
a Comenius University, Bratislava, Slovakia
b University of Minnesota, Minneapolis, MN, USA
c University of Guelph, Guelph, Ont., Canada
d Department of anatomy (MTA-TK1) University Pecs, Pecs, Hungary
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e Georg-August University, GOttingen, Germany
f Semmelweis University of Medicine, Budapest, Hungary
g University of Szeged, Hungary
h Pennington Biomedical Research Center, Baton Rouge, LA, USA
i Tokyo Women’s Medical University, Medical Center East, Tokyo, Japan
j North Hawaii Community Hospital Inc., Kamuela, HI, USA
Abstract
A circadian rhythm is documented in duodenal melatonin in rats, peaking 16.8 hours after light onset.
This component is more readily detected after logl0-transformation of the data. It differs between
male and female rats, females having a larger circadian amplitude and an earlier acrophase. The
circadian rhythm in duodenal melatonin is also found to lead that of pineal melatonin. The results
are qualified by the presence at the start of mapping of the second extremum of a double magnetic
storm.
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Keywords
Chronomics; Duodenum; Rats; Circadian; Melatonin
1. Aim
To examine in rats, as a widely used model, whether the mammalian gut is characterized by a
variation for which the zero circadian amplitude assumption can be rejected. If by this objective
time-microscopic cosinor approach, a rhythm can be validated, we can also examine the timing
of a duodenal rhythm in the light of circadian acrophases in other tissues obtained
concomitantly. Data from the same rats on other tissues were also analyzed to assess their time
structure by chronomics, in international cooperative studies along the scale of the day and the
week.
*Corresponding author. E-mail address: halbe00l @ umn.edu (F. Halberg).
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2. Background
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As a rule, in health [1–5] in many species, a prominent time-macroscopic human, murine and
avian circadian melatonin rhythm characterizes not only the circulation but also the pineal.
Melatonin is found to peak during the daily dark span, with but few exceptions in humans [6]
and other organisms [7–9], when time-microscopy is used. The variation is obvious to the
naked eye and quantifiable with its parameters by time- microscopy [7]. Chronomics, the study
of chronomes (time structures) in data of the stroke-prone spontaneously hypertensive
Okamoto rat, has revealed phase differences among circadian melatonin rhythms in the
hypothalamus and the pituitary vs. that in the pineal [8]. This study was designed to explore
any circadian variation of melatonin in the duodenum, in the context of that in other different
tissues of the same group of rats, sampled around the clock for 7 days, even though each rat
could only be sampled once. Duodenal tissue was only available from a fraction of the animals
used for measurement of melatonin in plasma, pineal gland and hypothalamus [9]. An
additional meta-analysis on data from Bratislava on the quail [10] is beyond our scope herein,
even if, as originally stated, it represents an important qualification of any generalization.
3. Materials and methods
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One hundred and three Wistar (Amsterdam) rats (52 males and 51 females) had been randomly
assigned to two rooms kept for a standardization span of 1 month on opposite LD12:12
regimens, to be sampled for 7 days only during working hours at 4-h intervals at six circadian
stages should the adjustment to the antiphase regimen be complete. This adjustment was
validated by the agreement of data on plasma and pineal with those reported earlier. After
bleeding, the animals were killed by cervical dislocation; the tissues, including the duodenum,
were removed and placed on dry ice. Blood and tissues were kept frozen at −18 °C until required
for analysis [9]. Melatonin was determined by radioimmunoassay [11] directly in plasma and
after methanol and chloroform extraction in pineal glands and duodenum, respectively. The
assay and extraction procedures were previously validated in our lab for rats [12]. Efficiency
of extraction was tested by adding 3H-MEL into each sample and was about 80%. Sheep MEL
antiserum (G/S/704–8485, Stockgrand Ltd., Guildford, UK) and 3H-labeled MEL (specific
activity of 0.925–1.85 TBq/mmol; NEN Life Science Products, Bad Homburg, Germany) were
used in the radioimmunoassay. Duodenal melatonin could be determined in only 31 samples.
The data were log10-transformed to normalize their distributions, but only for didactic purposes
the analyses of the original data are also shown. All data were analyzed by cosinor [13,14].
Parameter tests were used to test the equality of circadian acrophases [15].
4. Results
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Fig. 1 shows results as original mean values to indicate that the naked eye may find a peak
both in the light and in the dark span, in keeping with the original statement in an early
investigation, a quarter-century ago [16]. By the unaided eye, with the large standard errors, a
rhythm may well be questioned. A log10-transformation (Fig. 1b) more closely approximates
a circadian pattern with the peak at 08:00 h after light onset (HALO) seen with the original
data being much less prominent after log10-transformation. Fig. 2a, b shows the data separately
for male and female rats, again as original values and after log10-transformation, respectively.
In Fig. 2a, a rhythm cannot be detected for the males, whereas in Fig. 2b, after log10transformation, the P-value from the zero-amplitude test for males is 0.057, whereas the Pvalue derived from the data on females remains below the 5% level of statistical significance.
When data from both sexes are combined, statistical significance is reached. The percentage
rhythm, PR, representing the proportion of the variance accounted for by the fitted 24-h cosine
model is 26% for the duodenum and the corresponding P-value from the zero-amplitude
(norhythm) test is 0.016.
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Acrophases, expressed in negative degrees, with 360 degrees equated to 24 h and light onset
used as reference, are estimated to be −252°, −280° and −303° for duodenum, plasma and
pineal, respectively. They are found to differ ( F = 4.078, P = 0.007), Fig. 3. Duodenal melatonin
leads pineal melatonin. The findings of this study are in keeping with a meta-analysis of data
collected earlier in Germany from rats and chickens [5,17,18], but differ from very close
acrophases found in quail [10].
5. Discussion
Credit for the first demonstration of dramatic day-night differences in the melatonin
concentration of the intestine, as well as the pineal, eye, plasma, hypothalamus and Harderian
gland goes unquestionably to Vakkuri et al. [19], who studied the pigeon, Columba livia. A
two-timepoint approach was a lucky strike, encountering remarkably different peaks and
troughs in all tissues examined [19]. The conclusion, in 1985 [19] is pertinent two decades
later: “In all tissues [melatonin] had a clear diurnal variation with low levels at midday and
high levels at midnight. The highest amounts of melatonin were found in the eyes, duodenum,
and the pineal so that extrapineal melatonin exceeded pineal melatonin. Further studies are
needed to evaluate the significance of extrapineal melatonin” [19,20].
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Species differences in this case are an as yet unsolved challenge. Denser sampling is indicated,
even if the phase differences were found with 4-h sampling in rats but not found with about 3h sampling in the quail [10]. The peak of melatonin in mouse pineal was missed because of
sparse sampling that may suffice on some but not all occasions [7].
A gut-brain connection via melatonin would be in keeping with a lead of the gut versus the
hypothalamus. A phase difference between hypothalamus and pineal was much larger in the
spontaneously hypertensive Okamoto rat (SHR) [7,8] than in this investigation. A modulation
of central neural pathways by melatonin has earlier been suggested by one of us in the context
of the effect of melatonin on growth and growth hormone [4]. But time relations, although
suggestive, are never necessarily causal. They can serve as hints and would be much more
conclusive in our case on starving organisms.
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Melatonin is often regarded as a solely pineal product, even though it is detected in many parts
of the body and has been called a vitamin. Whereas alternatives, such as lags of pineal melatonin
to reach other sites, remain to be examined [5,18], the results herein and elsewhere [9] indicate
that pineal melatonin has a later phase as compared to the duodenum and other extrapineal
sites in the rat. It is also important to note that the results herein are confirmed for the rat and
extended to the chicken albeit on an asymmetrical LD15:9 regimen [17,18]. Generalizations
to other species are unwarranted [10,21]. While the peaks are close and all in the scotophase,
it must not be forgotten that in human disease the circulating melatonin peak can occur during
the daily light span [6], as can the hypothalamic and pituitary peaks in the SHR rat [8].
6. Conclusion
A cosinor-documented circadian rhythm characterizes the duodenum of rats kept on an
antiphasic LD12:12 regimen for 1 month before study in Pecs, Hungary, as it does on an
asymmetrical lighting regimen in Goettingen, Germany, where this finding has been extended
to chickens.
Acknowledgements
VEGA1/1294/04 and APVT-20-022704 (M.Z.); ETT 82/2003 (R.J.) GM-13981 (F.H.) and Dr. h.c. Dr. h.c. Earl
Bakken Fund (F.H., G.C.).
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Figure 1.
Fig. 1a. Original data of Michal Zeman on duodenal melatonin concentration of Wistar
(Amsterdam) rats of both sexes, standardized in antiphasic lighting regimens for 1 month prior
to blood collection, summarized as mean and standard error of the mean seemingly reveal
(albeit with great dispersion indices; see SEs) two peaks to the naked eye, one in darkness and
perhaps another in light. An analysis of variance does not allow the detection of a significant
time effect at the 5% level (P = 0.078). Time scale is given in HALO. © Halberg.
Fig. 1 b. After log10-transformation of the data in Fig. 1a, a peak at 08:00 h after light onset
(HALO) is less prominent and an analysis of variance allows demonstration of a time effect
below the 1% level. © Halberg.
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Figure 2.
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Fig. 2a. Original data of Michal Zeman separately assessed for males and females allow cosinor
demonstration below the 5% level for the data on males ■ but not on those for females □. ©
Halberg.
Fig. 2b. After log10-transformation of the data in a, the P-values from the zero-amplitude test
are 0.057 for the males and 0.026 for the females. © Halberg.
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Fig. 3.
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Parameter tests yield P-values that characterize differences among the acrophases found in this
study on the duodenum vs. acrophases found in Pecs by Rita Jozsa in other tissues [9],
documenting the extrapineal lead in phase vs. the pineal, also specifically the lead in phase of
the hypothalamus vs. the pineal. © Halberg.
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