Heterogeneous expression of melatonin receptor MT1 mRNA in the rat intestine under control and fasting conditions.

Heterogeneous expression of melatonin receptor MT1 mRNA in
the rat intestine under control and fasting conditions
Introduction
Melatonin, the so-called �hormone of darkness�, is an
indolic substance whose function in vertebrates is widely
attributed to the mediation of circadian and circannual
rhythm sensing [1]. It is synthesized in pineal gland as well
as in retina and perhaps some peripheral organs. Recently,
large body of evidence has accumulated suggesting that
melatonin is abundant in the gastrointestinal tract (GIT)
and may be synthesized at least partially in situ. Melatonin
is stored and probably also synthesized by the entero-
chromaffin cells and its distribution along the rat aliment-
ary tract is not homogeneous [2]. Highest amount of rat
gastrointestinal melatonin was found in the duodenum
(mostly in the epithelium) while the jejunum was almost
melatonin free. Melatonin was also detected in the ileum
and the quantity of this indole increased towards the
rectum (again mostly in the epithelium) [2]. Like the plasma
concentration of melatonin, the gastrointestinal level of
melatonin is variable. In some species (chicken), gastroin-
testinal melatonin oscillates diurnally while in others (rat)
the nutritional status seems to control melatonin release [3].
Fasting was reported to increase the level of gastrointestinal
melatonin and transient elevation of peripheral blood
melatonin concentration was observed in animals re-fed
after fasting [reviewed in 4]. The increase of melatonin after
fasting might be related to the higher amount of the
enterochromaffin cells observed in the small intestine
mucosa of fasted rats [5].
Although the estimated amount of melatonin in the GIT
surpasses the amount of melatonin in pineal gland by more
than 400 times [6], the role of melatonin in this �ever-dark
place� has only recently started to be uncovered. The wide
spectrum of biological actions of melatonin in the gut
comprises antioxidant effects [7], regulation of enterocyte
mitotic activity [8], interaction with immune system [9, 10],
modulation of intestinal motility [11–13] and epithelial ion
transport regulation [14–18].
A variety of the melatonin effects is exerted via specific
melatonin receptors. Using binding assays with radioac-
tively labeled ligand, specific melatonin binding sites were
detected in the GIT of various vertebrate species [reviewed
in 4]. Maximum melatonin binding in the mammalian GIT
was usually found in the mucosa, with less binding sites
present in the submucosa and muscularis externa [4].
However the distribution of melatonin binding sites differs
markedly among species suggesting that melatonin may
serve different functions in the gut of different species [19].
Two pharmacologically distinct binding sites were
Abstract: Melatonin is found in mammalian central nervous system and
various peripheral tissues including gastrointestinal tract (GIT) where it
participates in the regulation of intestinal motility, blood flow,
immunomodulation, ion transport, cell proliferation and scavenging of free
radicals. Some of these effects are achieved via melatonin binding to specific
receptors, MT1 and MT2. As no thorough study on the expression of these
receptors in the GIT has yet been done, the aim of this study was to
determine the MT1 mRNA expression in the rat intestine under both control
and fasting conditions. Our results suggest that MT1 mRNA is present in
epithelial as well as subepithelial layer, with higher expression in the latter in
all intestinal segments studied. The highest signal of the MT1 transcript
along the rostro–caudal intestinal axis was found both in epithelial and
subepithelial layers of the duodenum. Nevertheless, duodenal MT1 mRNA
expression did not reach the level found in pituitary gland. In a 12:12-hr
light:dark cycle a MT1 receptor expression in the subepithelial layer of rat
distal colon did not manifest a significant diurnal rhythm. Short-term fasting
increased the expression of MT1 transcript in the subepithelial layer of both
the small and large intestine. During long-term fasting the increase persisted
only in distal colon while a return to control levels was observed in small
intestinal segments. In conclusion we demonstrated heterogeneous
expression of MT1 receptor in the rat intestine and showed that its
expression is up-regulated by nutritional deprivation.
Matu´sˇ Sota´k1,2, Libor Mrnka1 and
Jirˇı´ Pa´cha1
1Institute of Physiology, Academy of Sciences
of the Czech Republic, Prague; 2Faculty of
Science, Charles University in Prague, Czech
Republic
Key words: circadian, fasting, gastrointestinal
tract, melatonin, MT1, undernutrition
Address reprint requests to Jirˇı´ Pa´cha, Institute
of Physiology, Academy of Sciences of the
Czech Republic, Videnska 1083, Prague
4-Krc, 142 20 Czech Republic.
E-mail: pacha@biomed.cas.cz
Received March 5, 2006;
accepted May 8, 2006.
J. Pineal Res. 2006; 41:183–188
Doi:10.1111/j.1600-079X.2006.00355.x
� 2006 The Authors
Journal compilation � 2006 Blackwell Munksgaard
Journal of Pineal Research
183

observed; high affinity binding site (ML1) and low affinity
binding site (ML2). Strong expression of high affinity
binding sites was found in the mucosa of the jejunum and
the colon of human digestive system [20] as well as in the
mouse digestive system [21]. ML1 was later identified with
cloned MT1 and MT2 receptors and ML2 was reclassified
to MT3. Moreover, as a lipophilic compound easily
penetrating the cell membrane, melatonin appears to
activate nuclear �orphan� receptors from the RZR/ROR
family.
Several studies report the presence of MT1 and/or MT2
in the GIT. The MT1 mRNA was detected in rat
duodenum and colon [22]. MT1 receptor both at mRNA
and protein level was also found in human gallbladder
epithelia [23]. Finally, Sallinen et al. [24] detected the
presence of both MT1 and MT2 transcripts in the rat small
intestine. However, none of these studies dealt with the
distribution of MT1 and MT2 receptor expression along
the rostro–caudal and mucosal–serosal axes, nor did they
follow the effects of physiological stimuli such as circadian
and nutritional changes that were reported to influence the
melatonin levels in the alimentary tract [4].
As the knowledge of the location and abundance of
melatonin receptors is important for understanding mela-
tonin’s role in the GIT, we decided (a) to assess quantita-
tively the expression of MT1 receptor mRNA alongside the
rat intestine and (b) to examine the possible effect of
physiological stimuli such as the light/dark cycle and
nutritional deprivation on the MT1 receptor transcript
level. Considering the existence of melatonin binding sites
in thymus [25], the interaction of melatonin with the
immune system [26] and the influence of food intake on
immune cells and thymus [27, 28], we checked, moreover,
the effect of fasting on MT1 receptor transcript in thymus.
Materials and methods
Animals
Experiments were performed on adult Wistar rats (60–
90 days old). The animals were maintained at 21�C on a
12:12-hr light:dark cycle. Standard rat diet and water were
provided ad libitum. Rats were killed by decapitation and
pituitary gland, thymus lobe and specific segments of gut
were removed for subsequent analysis. Four intestinal
segments were isolated: duodenum (DUO), jejunum (JEJ; a
segment 5 cm long was removed 10 cm apart from duode-
num), ileum (ILE; a segment 5 cm long was removed 10 cm
apart from cecum) and colon (DC; the distal part above the
pelvic brim). Fasting animals were deprived of food for 2
(short-term fasting group) or 7 (long-term fasting group) -
days while retaining the free access to water. The institu-
tional animal ethics committee approved the study.
Tissue preparation, RNA extraction and reverse
transcription
After killing, the distal colon was removed, rinsed to
remove its content and cut longitudinally. Gentle scraping
of the mucosal layer yielded material rich in epithelial cells
(denoted as epithelial tissue). After subsequent thorough
scraping of submucosa, only muscularis externa together
with serosa remained (for the lack of appropriate term
denoted simply as subepithelial tissue). Both preparations
were used for total RNA isolation using RNA Blue method
(TopBio, Prague, Czech Republic) according to the manu-
facturer’s instructions. The quantity and quality of RNA
was assessed spectrophotometrically at 260 nm and 260/
280 nm, respectively. RNA was stored at )80�C. cDNA
synthesis was carried out using Superscript III reverse
transcriptase (Invitrogen, BV, Groningen, The Nether-
lands) and a mix of random hexamers (50 ng/lL; Sigma-
Aldrich, St Louis, MO, USA) together with MT1-specific
reverse primer (100 nM; for the sequence see Table 1).
Approximately 2 lg of total RNA were used for reverse
transcription run at 25�C for 10 min followed by 50 min at
45�C. Heating the samples at 95�C for 2 min and chilling
them at 4�C terminated the reaction.
Real-time RT-PCR
Real-time quantitative PCR was performed by the Light-
Cycler rapid thermal cycler system (Roche Diagnostic Ltd,
Lewes, UK) using the LightCycler Fast Start DNA master
SYBR Green I mix (ibid) according to the manufacturer’s
instructions. The LightCycler Fast Start DNA master
SYBR Green I mix was used for measurement of all
samples but the samples collected for circadian rhythm
estimation that were measured by QuantiTect SYBR Green
PCR Kit (Quiagen Inc., Valencia, CA, USA). All forward
and reverse primers were added in a 0.25 lm concentration.
The sequences of primers are shown in Table 1. The PCR
reaction started by a 7-min activation of hot-start DNA
polymerase followed by 45 cycles of target cDNA ampli-
fication. Amplification of MT1 cDNA was carried out as
follows (if different for QuantiTect kit, parameters are given
in parentheses): 45 cycles (55), hot-start DNA polymerase
activation at 95�C for 7 min (15), denaturation at 95�C for
15 s (15), annealing at 56�C for 10 s (15), elongation at
72�C for 6 s (10) and acquisition of SYBR Green fluores-
cence at 85�C (80) for 3 s (5). A different protocol was used
for amplification of b-actin: 45 cycles (55), hot-start DNA
polymerase activation at 95�C for 7 min (15), denaturation
at 95�C for 15 s (15), annealing at 54�C for 10 s (15),
elongation at 72�C for 7 s (10) and acquisition of fluores-
cence signal at 89�C (84) for 3 s (5). The mathematical
model of Pfaffl was used for relative quantification of MT1
to b-actin using the equation: ratioMT1=b�actin ¼
½EMT1^DCPðcontrol�sampleÞMT1 �=½Eb�actin^DCP
ðcontrol�sampleÞ
b�actin � where
E is the real time RT-PCR efficiency and DCP is the
crossing point deviation of internal control-sample [29].
The internal control was used for diminution of differences
caused by variation among the runs.
Table 1. Sequences of MT1 and b-actin primers used in the study
Primer 5¢-sequence-3¢
MT1 forward AGG CGG CGG GGA AAT AAG
MT1 reverse TGC CAC AGC TAA ACT CAC CAC AAA
b-Actin forward TAC AAC CTC CTT GCA GCT CC
b-Actin reverse TTC TGA CCC ATG CCC ACC A
Sota´k et al.
184

Statistics
The presented data are expressed as mean ± S.E.M. The
outlying values were determined by Grubbs test and
excluded. We used repeated measures two-way ANOVA
model with Greenhouse-Geisser and Huynh-Feldt adjust-
ment to evaluate the data. Two factors were defined: factor
�rostro–caudal� with four levels (DUO, JEJ, ILE, DC) and
�mucosal–serosal� with two levels (epitelial and subepithelial
layers). All post hoc comparisons were conducted using the
Tukey’s multiple range test. A probability level of P < 0.05
was considered as significant for all statistical analyses.
The putative circadian cycle of the MT1 mRNA expres-
sion was detected using simple cosinor analysis [30].
Cosinor calculates the acrophase, mesor, and amplitude
of the single cosine curve that approximates the data using
linear regression techniques. The data were fitted with an
equation of the form Y ¼ A + B1*cos(2p*t) + B2*-
sin(2p*t), where: Y, the actual data values; A, the intercept
constant estimated by the regression; B1, the cosine
coefficient estimated by the regression; B2, the sine coeffi-
cient estimated by the regression; t, the time for each Y,
expressed as t/tau (tau being the circadian period length).
The statistical significance of the model was determined
from critical values of analyses of variance (F-ratio).
Nonparametric Kruskal–Wallis test was used to assess
the differences among control and fasting groups in
separate intestinal subepithelial segments as well as in
thymus and pituitary gland. Holm’s method was used to
adjust the significance level of Mann–Whitney post hoc
pairwise group comparison [31].
Results
MT1 receptor mRNA was detected in all studied intestinal
segments (DUO, JEJ, ILE and DC) but its distribution was
not homogeneous in either the rostro–caudal or the
mucosal–serosal planes (Fig. 1). The nonhomogeneous
MT1 expression in the rostro–caudal axis was manifested
by significantly higher values of MT1 mRNA in duodenum
than in all other segments. Significant differences in MT1
mRNA expression were also found along the mucosal–
serosal axis. Material obtained by mucosal scraping (epi-
thelial tissue) contained a significantly lower amount of
MT1 mRNA compared with the material from the layers
lying beneath it (subepithelial tissue) in all intestinal
segments studied. In many cases there was no detectable
signal of MT1 in epithelial tissue at all. In comparison with
the pituitary gland, the level of MT1 mRNA in intestinal
segments was significantly lower. The MT1 mRNA level
found in thymus was similar to intestinal epithelial samples
(0.12 ± 0.05 compared with the epithelial values in Fig. 1).
The diurnal variation in MT1 receptor mRNA expres-
sion was studied in the subepithelial layer of the rat distal
colon. Rats were kept in standard light/dark conditions
(12:12 hr LD cycle) for 2 wk and killed at different
timepoints throughout the last 24-hr cycle of continuous
darkness. Maximal difference of MT1 mRNA levels at
different time points was less than twofold and did not
reach significance (Fig. 2). Simple cosinor analysis provided
the following characteristics: mesor 0.68, amplitude 0.15
and acrophase 7.7 hr (115.6� 360�/24 hr; 0�: light onset).
The goodness-of-fit (q) of the curve to the raw data reached
0.8 with F-ratio equal to 3.44 on 2 and 4 df (P-value for
significance of the fitted curve being 0.135). Thus, although
there is an indication of circadian rhythm of the MT1
expression in rat distal colon it is not significant. Because of
high data variation larger collection of samples would be
necessary to validate possible circadian rhythm. Log10-
transformation of the data previously shown to improve the
detection of circadian rhythm components [32] did not
significantly affect the cosinor analysis and was thus not
used.
Fig. 2. Daily profile of the MT1 mRNA expression in the rat distal
colon subepithelial tissue. For definition of the term �subepithelial
tissue� see the section �Tissue preparation, RNA extraction and
reverse transcription� in Materials and methods. Each point repre-
sents the mean ± S.E.M. of six to seven animals. Rats were kept in
standard light/dark conditions (12:12 hr LD cycle) for 2 wk and
killed at different timepoints throughout the last 24-hr cycle of
continuous darkness. Light period is indicated by white box, dark
period by black box. Circadian time (hours) is indicated below
boxes.
Fig. 1. Expression of the melatonin receptor MT1 mRNA in the
rat pituitary gland and intestine. HPF, pituitary gland; DUO,
duodenum; JEJ, jejunum; ILE, ileum; DC, distal colon. For defi-
nition of the terms �epithelial tissue� and �subepithelial tissue� see the
section �Tissue preparation, RNA extraction and reverse transcrip-
tion� in Materials and methods. Values representing mean ± S.E.M.
are expressed as the ratio of MT1 to b-actin. Samples with no
detectable level of MT1 were given the zero value. Six to seven
animals were used for each intestinal segment as well as pituitary
gland. *Significantly different from other intestinal segments at
P < 0.05, #significantly different from all intestinal segments at
P < 0.05.
MT1 receptor expression in the rat intestine
185

To study the effect of nutritional status on MT1
expression in the intestine, the rats were fasted for 2 or
7 days. The survival rate of the animals was 100% both in
short-term (2 days) and long-term (7 days) fasting rats.
Mean body weights (mean ± S.E.M.) dropped from
302 ± 7 g in the control group to 189 ± 7 g in the long-
term fasting rats. Only subepithelial level of MT1 was
measured due to low expression of MT1 mRNA in
epithelial tissue. Short-term fasting (2 days) caused 3- to
6-fold increase of MT1 mRNA expression compared with
the control group in all intestinal segments (Fig. 3). This
increase was significant at P < 0.05 in all intestinal
segments studied (with probability level of DUO and DC
being marginal – reaching in both cases P ¼ 0.052).
Prolonged fasting (7 days) lead to diminution of the MT1
mRNA increase in proximal intestinal segments (DUO, JEJ
and ILE), while the increase of the MT1 expression towards
the distal part of the intestine was augmented (DC). MT1
mRNA level in pituitary gland remained unchanged after
both short- and long-term fasting. Long-term fasting but
not short-term fasting increased the MT1 mRNA level in
thymus (P ¼ 0.053).
Discussion
Several studies reported the presence of melatonin and its
binding sites in the GIT of the rat [2, 19, 32, 33]. In the
central nervous system (CNS) the high-affinity melatonin
binding sites were identified with recently cloned receptors
MT1 and MT2. As gastrointestinal and CNS melatonin
binding sites have similar pharmacological profiles and are
blocked by the same inhibitors, it is probable that at least
some of these binding sites in the GIT might be assigned to
MT1 and/or MT2 [4, 20]. Indeed, previous reports sugges-
ted the presence of MT1 mRNA in the rat duodenum and
colon [22] and both MT1 and MT2 mRNA in rat small
intestine [24]. Nevertheless, as far as we know a detailed
study on distribution of these receptors in the intestine has
not yet been performed.
In this study, we were able to quantitatively estimate the
MT1 mRNA level in the rat intestine, pituitary gland and
thymus. MT1 mRNA expression in the pituitary gland
was several fold higher compared with the gut (and
thymus) levels. There was no significant correlation
between the expression of MT1 mRNA in the intestinal
segments and pituitary gland in individual animals.
Nevertheless, all between tissue comparisons of particular
gene expression suffer from the lack of universal referen-
tial value. Our data are expressed as a ratio of MT1 signal
to b-actin signal. It cannot be excluded that the higher
ratio of MT1/b-actin in pituitary gland relative to the
intestine is caused by the difference in b-actin expression
between pituitary and gut tissues rather than by the
difference in MT1 expression. However, lower absolute
crossing point values (CP) of MT1 in pituitary gland
compared with the gut suggest that there is truly a higher
amount of MT1 transcript in this tissue. The very small
variation of b-actin CP among intestinal segments (<5%)
justifies the usage of b-actin as house-keeping gene in this
study.
Based on our data, the MT1 transcript is expressed in all
studied intestinal segments (DUO, JEJ, ILE and DC)
though not homogeneously. Segmental distribution (cor-
responding to rostro–caudal axis) revealed the MT1
mRNA signal to be the highest in duodenum, lower in
jejunum and ileum and increasing again towards the distal
part of the large intestine both in epithelial as well as in
subepithelial tissues. Such expression patterns correspond
very well to the distribution of melatonin itself and its gut
stores, the enterochromaffin cells [2]. The distribution of the
MT1 mRNA along the vertical (i.e. mucosal–serosal) axis
was assessed in the material obtained by mucosal scraping.
Using this method the material rich in epithelial cells and
that devoid of them were separated. In contrast to the
prevalent melatonin location in epithelium [2], a higher
amount of MT1 mRNA was found in the subepithelial
tissue suggesting a paracrine mode of melatonin action.
Subepithelial melatonin receptors might also be activated
by melatonin from the circulation. As binding studies
revealed a higher amount of melatonin-binding sites in
epithelium compared with subepithelial layers [4], other
melatonin receptor types different from MT1 are probably
present in the epithelial layer. Sallinen et al. [24] were able
to detect MT2 receptor in small rat intestine and pharma-
cological data suggests the presence of MT3 receptor in
guinea pig colon [34]. In contrast to Sallinen et al. [24] we
did not succeed in quantitative estimation of MT2 receptor
mRNA in the rat intestine (data not shown). This might be
due to a different real-time RT-PCR technique (SYBR
Green I versus TaqMan probes) or due to a different rat
strain (Wistar versus Sprague–Dawley). In our experiments
only nested PCR was able to detect MT2 mRNA in the rat
intestine. Thus, we suppose that MT2 mRNA is more rare
than MT1 mRNA in the rat intestine. Further studies will
be necessary to clarify the melatonin receptor identity in
various layers and/or segments of GIT.
Fig. 3. Relative expression of the melatonin receptor MT1 mRNA
in pituitary gland, thymus and subepithelial tissue of several rat
intestinal segments under control conditions and fasting. HPF,
pituitary gland; THY, thymus; DUO, duodenum; JEJ, jejunum;
ILE, ileum; DC, distal colon. F2, short-term fasting rats (2 days);
F7, long-term fasting rats (7 days). For definition of the term
�subepithelial tissue� see the section �Tissue preparation, RNA
extraction and reverse transcription� in Materials and methods.
Values representing mean ± S.E.M. are expressed as the percent-
age of MT1 to b-actin expression in fasted to control rats. Six to
seven animals were used for each segment and each experimental
group. Significant difference at P < 0.05: *controls versus F7
group, +controls versus F2 group, #F2 group versus F7 group.
Sota´k et al.
186

Although a steady-state level of MT1 mRNA informs us
about its expression at the time of obtaining the samples a
periodic change in the expression of many proteins was
observed. Among various periodic changes the circadian
rhythm is the most important one. Not only intestinal
hormones but also their receptors might be subjected to
circadian variation as observed for the intestinal guanylin
cyclase and its ligands [35]. The diurnal variation in MT1
mRNA expression occurring both in CNS and periphery was
reported by several studies [24, 36–38], but not in the
intestine. As far as we know, only Sallinen et al. [24]
attempted to estimate diurnal variation of MT1 expression
in rat intestinal tissue.Noon andmidnight values of the small
intestinal MT1 mRNA expression were not significantly
different in their study. Also in our study the measurements
did not reveal significant changes ofMT1mRNA level in the
subepithelial layer of rat distal colon during the 24-hr period
(12:12 light:dark). Thus, it seems that in mammals not only
gastrointestinal melatonin [3, 4] but also gastrointestinal
MT1 receptor levels do not depend much on photic stimuli.
Once released, melatonin may exert a wide range of
functions in the GIT by activating its receptors. However,
while melatonin’s role has been convincingly established in
the regulation of duodenal bicarbonate secretion [15–18]
and the small intestine motility [11, 12], its function in the
distal intestinal segments remains largely unknown. Experi-
ments of Chan et al. [14] on colonic adenocarcinoma
derived cell line T84 showed that high concentration of
melatonin may influence ion transport but no data on the
native tissue has so far been available. A more detailed
study on the melatonin receptor distribution in subepithel-
ial layers using various approaches could provide links to
melatonin function in this region. As a vanguard of future
studies, we confirmed the presence of MT1 mRNA on
myenteric neurons (Sota´k M., unpublished observation)
from intestinal tissue of several days old rats. High density
of melatonin binding sites was also reported in the blood
vessels of rodent and human colonic sections suggesting its
potential vascular function [19]. Recently, yet another
potential melatonin function has been revealed. It has been
shown that melatonin may influence the energy homeostasis
by both peripheral and central regulation [39]. Energy
homeostasis is closely related to the feeding behavior.
Seasonal animals such as sheep and Siberian hamsters
(Phodopus sungorus) exhibit annual photoperiod-driven
cycles of appetite and body weight [40] and several findings
indicate melatonin may be involved. Melatonin was also
shown to induce seasonal changes in the voluntary food
intake in the blue fox (Alopex lagopus) [41] and to influence
the level of two potent metabolic hormones, leptin and
ghrelin [42]. However, it is not known whether GIT
melatonin takes part in these regulations. It was shown
that both undernutrition and re-feeding increase the level of
GIT melatonin [43, 44]. Our study suggests that nutrition-
ally induced changes in the melatonin levels are accom-
panied by changes in melatonin receptor density. We
observed that fasting affects MT1 mRNA expression in
both spatial and temporal manner. After short-term fasting
a general increase of MT1 mRNA level in all studied
intestinal segments was observed while prolonged fasting
led to diminution of the increase in the proximal part of the
intestine and to exaggeration of the increase in the distal
part of the intestine.
The reason for these changes remains unknown. It was
shown that melatonin counteracts the stimulatory effect of
serotonin on GIT motility [4]. It is plausible that enhanced
release of melatonin during fasting concomitantly with the
increased sensitivity of GIT to melatonin caused by higher
expression of melatonin receptors retards propagation of
chymus in the intestine and thus maximizes the nutritional
uptake. Nevertheless we do not know what cell type is
responsible for the higher MT1 mRNA expression observed
during fasting. Apart from smooth cells and neurons,
immune cells might be the candidates. The presence of
melatonin receptors was reported on various populations of
immune cells [45]. Taking into account that the enteric
immune system undergoes changes during undernutrition
[27], one might speculate whether a particular population of
immune cells may be responsible for the increased expres-
sion of MT1 mRNA in the rat intestine. The enhancement
of MT1 mRNA expression in the cells of the immune
system is indicated by our finding that long-term fasting
arises the MT1 mRNA level in thymus. This tissue
possesses melatonin-binding sites [25] and our results thus
suggest that melatonin is involved in regulation of thymus
activity at the time of changed nutritional status. Further
studies should elucidate which cell populations express
MT1 and/or other melatonin receptors in the GIT and what
is the melatonin function in this complex organ.
In summary, we have observed heterogeneous expression
of MT1 receptor in the rat intestine along the rostro–caudal
as well as the mucosal–serosal planes with a higher
expression in subepithelial tissue than in mucosa and with
the expression reaching its peak in duodenum. Expression
of MT1 mRNA was not associated with diurnal rhythm but
was up-regulated by starvation.
Acknowledgments
We are grateful to Dr M. Diener (Inst. Vet. Physiol., Univ.
Giessen) for isolated neurons from myenteric plexus. This
work was supported by a grant (305/03/D140) from Grant
Agency of the Czech Republic and by AVOZ 5011922.
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