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The Journal of Neuroscience, November 15, 2000, 20(22):8637-8642
Hypothalamic Melanin-Concentrating Hormone and Estrogen-Induced
Weight Loss
Paul
Mystkowski1,
Randy
J.
Seeley5,
Tina M.
Hahn1,
Denis G.
Baskin1,
Peter J.
Havel3,
Alvin M.
Matsumoto4,
Charles W.
Wilkinson2, 4,
Kimberly
Peacock-Kinzig5,
Kathleen A.
Blake5, and
Michael W.
Schwartz1
Departments of 1 Medicine and 2 Psychiatry
and Behavioral Sciences, University of Washington and Veterans Affairs
(VA) Puget Sound Health Care System, Seattle, Washington 98104, 3 Department of Nutrition, University of California at
Davis, Davis, California 95616, 4 Geriatric
Research/Education and Clinical Center, VA Puget Sound Health Care
System, Seattle, Washington, and 5 Department of
Psychiatry, University of Cincinnati Medical Center, Cincinnati, Ohio
45267
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ABSTRACT |
Melanin-concentrating hormone (MCH) is an orexigenic neuropeptide
produced by neurons of the lateral hypothalamic area (LHA). Because
genetic MCH deficiency induces hypophagia and loss of body fat, we
hypothesized that MCH neurons may represent a specific LHA pathway
that, when inhibited, contributes to the pathogenesis of certain
anorexia syndromes. To test this hypothesis, we measured behavioral,
hormonal, and hypothalamic neuropeptide responses in two models of
hyperestrogenemia in male rats, a highly reproducible anorexia
paradigm. Whereas estrogen-induced weight loss engaged multiple systems
that normally favor recovery of lost weight, the expected increase of
MCH mRNA expression induced by energy restriction was selectively and
completely abolished. These findings identify MCH neurons as specific
targets of estrogen action and suggest that inhibition of these neurons
may contribute to the hypophagic effect of estrogen.
Key words:
MCH; estrogen; body weight; energy balance; lateral
hypothalamus; anorexia; pair-feeding
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INTRODUCTION |
The lateral hypothalamus plays a
critical role in the regulation of energy intake and expenditure.
Initial work performed >50 years ago demonstrated that bilateral
lesions of the lateral hypothalamic area (LHA) result in profound
hypophagia in rodents (Stellar, 1954 ), and recent progress has begun to
identify candidate neuronal subpopulations that may mediate these LHA
feeding effects. Neurons producing melanin concentrating hormone (MCH)
represent one such subset. In rats and mice, intracerebroventricular
administration of MCH induces hyperphagia, whereas MCH deficiency
induced by targeted gene deletion leads to a hypophagia syndrome and
loss of body fat (Qu et al., 1996; Shimada et al., 1998; Tritos et al.,
1998). Thus, selective inhibition of MCH neurons might be expected to
induce an anorexic response.
Converging evidence suggests that the hypothalamic arcuate nucleus
(ARC), in addition to the LHA, plays a key role in the transduction of
peripheral signals reflecting negative energy balance into behavioral,
autonomic, and metabolic responses that favor the recovery of lost
weight (Elmquist et al., 1998; Woods et al., 1998; Schwartz et al.
2000). Accordingly, hormonal responses to negative energy balance
(declining plasma levels of insulin and leptin in concert with
increasing levels of corticosterone) induce the ARC to increase
biosynthesis and release of the coexpressed orexigenic molecules
neuropeptide Y (NPY) and agouti-related protein (AgRP) and to decrease
expression of pro-opiomelanocortin (POMC), the precursor of the
anorexic melanocortin  -MSH (Schwartz et al., 1997; Hahn et al.,
1998; Havel et al. 2000). Changes in the release of these neuropeptides
from ARC neurons are hypothesized to influence energy homeostasis via
effects on "second order" neurons situated in adjacent hypothalamic
areas, such as the LHA and the paraventricular nucleus (PVN) (Broberger
et al., 1998; Elias et al., 1998; Sawchenko, 1998). The abundance of
ARC projections to MCH neurons in the LHA (Elias et al., 1998) supports
the hypothesis that the MCH system lies "downstream" of ARC neurons
involved in the adaptive response to weight loss. Therefore, under
conditions in which MCH signaling is selectively disrupted, anorexia
might be expected to persist despite intact upstream signaling events (e.g., low levels of plasma leptin and ARC POMC and high levels of
plasma corticosterone and ARC NPY).
One anorexia paradigm of particular utility for investigating this
hypothesis is chronic hyperestrogenemia in male rats (Wade, 1972, 1986;
Mordes and Rossini, 1981; Mordes et al., 1984; Bernstein et al., 1986).
This model provides reliable and sustained decreases in food intake and
body weight without overt toxicity, is easily used and reversible, and
allows quantification of the anorexic stimulus. Furthermore, estrogen
was recently shown to inhibit hypothalamic MCH expression in
ovariectomized female rats (Murray et al., 2000). We therefore
hypothesized that anorexia because of hyperestrogenemia in male rats is
associated with selective inhibition of MCH. To investigate this
hypothesis, we measured changes in food intake, body weight, plasma
hormone levels, and hypothalamic levels of MCH, NPY, AgRP, POMC, and
corticotropin-releasing hormone (CRH) mRNA in two rat models of
hyperestrogenemia: Leydig cell tumor implantation (whose anorexic
effects are proposed to be primarily mediated by estrogen secretion)
(Mordes and Rossini, 1981; Mordes et al., 1984; Bernstein et al., 1986)
and chronic subcutaneous estrogen administration.
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MATERIALS AND METHODS |
Study animals and protocols
Animals. Adult male Wistar-Furth rats (Simonsen
Laboratories, Gilroy, CA) weighing 200-300 gm were housed individually
and maintained on a 12 hr light/dark cycle (lights on at 7:00 A.M.) with ad libitum access to drinking water and pelleted Purina
lab chow (#5001), except as noted. All study protocols were approved by
the University of Washington Animal Care Committee.
Experiment 1: effects of Leydig cell tumor implantation on food
intake, plasma hormones, and hypothalamic neuropeptide levels. Body weight and food intake was measured daily for each rodent. One day
before tumor implantation surgery, 30 animals were weight-matched and
placed into one of three groups (n = 10/group):
tumor-bearing or sham-implanted controls fed ad libitum, or
sham-implanted controls that were pair-fed to the food intake of the
tumor bearing group. The pair-fed group received two daily meals of
equal size (given at lights out and 4 hr into the dark cycle). The
total amount of chow provided daily to pair-fed animals was equal to
that consumed on the previous day by a previously assigned partner in
the tumor-bearing group. This feeding regimen continued for 22 d.
Tumor implantation procedure. A LTW(m) Leydig cell tumor
(graciously provided by Dr. Ilene Bernstein) was removed from a donor rat anesthetized with ketamine and xylazine (6.5 ml/10 ml, 1 ml/kg, i.p. body weight for each drug), and the excised tumor was cut into
5-10 mg fragments and placed in 0.9% sterile saline. Under sterile
conditions, a tumor fragment was inserted into a small subcutaneous
pocket created in the right flank of similarly anesthetized recipient
rats. Ad libitum fed and pair-fed animals underwent the same
procedure, but tumor cells were not implanted.
Blood and tissue analyses. On day 22 after implantation (1 hr after lights on), tumor-bearing and sham-implanted rats were individually briefly exposed to CO2 and
decapitated, followed by rapid brain removal and collection of trunk
blood in cooled heparinized tubes. Brains and plasma were frozen at
 80°C. Tumors were weighed and stored at 20°C. The mean
postmortem tumor weight was 0.82 ± 0.46 gm, and in no case did
tumor weight exceed 1% of total body weight at the time of
killing. Given the 24 hr lag required for pair feeding,
collection of blood and tissue from the pair-fed group occurred 24 hr
later. Plasma levels of leptin (Linco, St. Louis, MO) and
corticosterone (ICN Biomedicals, Costa Mesa, CA) were measured by RIA.
Plasma insulin was assayed by a modification of the double-antibody
method of Morgan and Lazarow (1963). Serum estradiol was measured using
a solid phase, competitive, time-resolved immunofluorometric assay
(DELFIA; Wallac Oy, Oulansalo, Finland) using europium-labeled
estradiol, polyclonal anti-estradiol antibodies derived from rabbit,
and a second antibody directed against rabbit IgG coated onto 96 well
plates. The intra-assay coefficient of variation of this assay was
3.8%, and the lower limit of sensitivity was 0.05 nmol/l (13.6 pg/ml).
Plasma glucose levels were determined by the glucose oxidase method.
Experiment 2: dose-effect of estrogen administration on food
intake, body weight, and plasma hormone levels. To identify a dose
of estradiol that yields plasma levels comparable with those achieved
in tumor-bearing rats of experiment 1, rats were anesthetized with
ketamine and xylazine and implanted (subcutaneously) with a
pellet containing either vehicle only (n = 15) or 2.5 mg (n = 7), 7.5 mg (n = 8), or 15 mg
(n = 7) of 17 -estradiol (Innovative Research of
America, Sarasota, FL). Of the vehicle-treated rats, eight were
pair-fed, as in experiment 1, to a rat of similar initial body weight
in the group receiving the 15 mg estradiol dose. Food intake and body
weights were measured daily for 29 d and plasma was obtained for
analysis on day 29.
Experiment 3: effect of chronic 17-estradiol administration on
hypothalamic neuropeptide mRNA levels. Rats were implanted subcutaneously with a pellet containing either vehicle
(n = 10) or 7.5 mg (n = 8) of
17-estradiol, a dose selected on the basis of results from
experiment 2 to approximate plasma levels of tumor-bearing rats in
experiment 1, and food intake and body weights were measured daily for
22 d. Plasma and brains were obtained and analyzed as described
for experiment 1.
In situ hybridization
Tissue preparation. Frozen brains were sectioned in a
coronal plane at 14 µm with a cryostat, mounted on RNase-free slides, and treated with 4% paraformaldehyde, acetic anhydride, ethanol, and
chloroform. For each animal, six slides (12 brain sections) of
hypothalamus in the region 2.0-3.5 mm posterior to bregma were selected for hybridization for each riboprobe. Sections for
quantitation of POMC mRNA were selected from the ARC rostral to the
ventromedial nucleus (VMN), whereas those used for NPY and AgRP mRNA
hybridization contained the VMN and corresponded to the midregion of
the ARC. Sections for MCH hybridization were selected from an area just rostral to slides chosen for POMC. Particular care was taken to anatomically match sections among animals for each probe by an investigator blinded to study conditions. For each experiment, all
brain slices were concurrently prepared for hybridization and used in
the same assay. Riboprobes from cDNA templates for NPY, AgRP, POMC, and
MCH mRNAs were transcribed in the presence of 33P-UTP. Unincorporated
label was separated using a QIAquick Nucleotide Removal Kit (Qiagen,
Santa Clarita, CA). The hybridization signal on film autoradiograms in
the ARC of each brain slice was quantified using an MCID image
analysis system (Imaging Research, St. Catherine's, Ontario, Canada),
as described previously (Schwartz et al., 1997). Both the hybridization
density and image area were measured, the product of which was
calculated as an index of mRNA levels from the mean of six to eight
sections per animal (Schwartz et al., 1997).
Statistical analysis. Group mean values were compared by
one-way ANOVA for experiments 1 and 2, using Fisher's test for
multiple comparisons of between-group differences. For Experiment 3, group mean values were compared with a two-tailed unpaired Student's t test. The null hypothesis of equal means was rejected at
the p = 0.05 level of significance. Data are expressed
as mean ± SEM.
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RESULTS |
Experiment 1: effects of Leydig cell tumor implantation on food
intake, plasma hormones, and hypothalamic neuropeptide levels
To investigate the mechanisms underlying anorexia induced by
subcutaneous implantation of estrogen-secreting Leydig cell tumor cells, we studied three groups of male Wistar rats for 22 d:
Leydig cell tumor-bearing, sham-implanted controls that were pair-fed to the intake of tumor-bearing group, and sham-implanted controls fed
ad libitum. Relative to the ad libitum fed
controls, mean daily food intake and final body weight of the
tumor-bearing rats were reduced by 34 and 23%
(p < 0.05 for both), respectively (Table 1). Whereas food consumption was designed
to be comparable in pair-fed and tumor-bearing groups, final body
weights were significantly lower in tumor-bearing rodents, suggesting
an increase in energy expenditure relative to pair-fed animals. Mean
plasma levels of 17-estradiol were ~10-fold greater in the
tumor-bearing rats than those measured in either sham-implanted group.
As expected, leptin levels declined in both tumor-bearing and pair-fed
groups relative to ad libitum fed controls (4.5 ± 0.3 ng/ml), but were significantly lower in tumor-bearing (1.7 ± 0.3 ng/ml; p < 0.05 vs ad libitum fed controls)
than in pair-fed rats (3.3 ± 0.3 ng/ml; p < 0.05 vs ad libitum fed controls and tumor-bearing rodents). Plasma insulin levels were also significantly decreased in
tumor-bearing rodents, but not in pair-fed animals, whereas
corticosterone levels were significantly elevated in both tumor-bearing
and pair-fed animals. Nonfasting plasma glucose levels were similar
across groups (Table 1).
The results of in situ hybridization performed on coronal
brain sections are shown in Figures 1 and
2. Relative to ad libitum fed
controls, NPY and AgRP mRNA expression were comparably increased in the
pair-fed and tumor-bearing groups (Figs. 1a, 2), whereas POMC and CRH levels were reduced in these groups (Fig. 1b).
As with body weight and plasma leptin levels, POMC mRNA levels in the
tumor-bearing group were reduced to a significantly greater extent than
was observed in the pair-fed group. The direction of changes of each of
these neuropeptide mRNAs was similar in tumor-bearing and pair-fed
groups and consistent with a compensatory response to chronic energy
restriction. In contrast, expression of MCH mRNA in the LHA differed
markedly between tumor-bearing and pair-fed groups. Thus, whereas MCH
gene expression was markedly increased by pair-feeding (585% ± 117 of
ad libitum fed controls, p < 0.05 vs
ad libitum fed controls and tumor-bearing rodents), MCH
levels in the tumor-bearing group were nonsignificantly lower than in
ad libitum fed rats (74% ± 17 of ad libitum fed
rodents; p = 0.19 for ad libitum fed
controls vs tumor-bearing animals) (Figs. 1c, 2). MCH mRNA
levels in the LHA of pair-fed rats were therefore eightfold higher than
in the tumor-bearing rats (p < 0.05).
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Figure 1.
Hypothalamic neuropeptide mRNA levels as measured
by in situ hybridization from animals in experiment 1. Results from Leydig cell tumor-bearing (open bar),
pair-fed (diagonal hatching), and sham-implanted (Sham;
solid bar) rodents presented as a percentage of mRNA
expression relative to sham-implanted rodents fed ad
libitum. Pair-fed rodents were sham-implanted, and their food
intake was matched to that of the tumor-bearing group.
NPY, Neuropeptide Y; AgRP, Agouti-related
protein; POMC, pro-opiomelanocortin; CRH,
corticotropin-releasing hormone; MCH, melanin
concentrating hormone. *p < 0.05 versus Sham;
**p < 0.05 versus tumor bearing.
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Figure 2.
Representative autoradiograms of hypothalamic
neuropeptide Y (NPY) and melanin-concentrating
hormone (MCH) mRNA by in situ
hybridization in Leydig cell tumor-bearing (TB),
pair-fed (PF), and sham-implanted
(Sham) rodents of experiment 1. NPY mRNA levels were
visibly increased in tumor-bearing and pair-fed controls relative to
Sham animals, whereas MCH mRNA expression was reduced in tumor-bearing
compared with pair-fed animals. Arc, Arcuate nucleus of
the hypothalamus; ZI, zona incerta; LHA,
lateral hypothalamic area; 3v, third cerebral
ventricle.
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Experiment 2: dose-effect of estrogen administration on food
intake, body weight, and plasma hormone levels
To identify a dose of estradiol that achieves plasma levels
comparable with those detected in tumor-bearing rats in experiment 1 and to assess its effects on food intake and body weight, rats were
monitored for 29 d after subcutaneous implantation with
sustained-release pellets containing either vehicle or 17-estradiol
in one of three doses (2.5, 7.5, and 15 mg). Relative to
vehicle-implanted, ad libitum fed controls, mean daily food
intake in each of the estrogen-treated groups was reduced by ~25%,
an effect detected on the first day of treatment (Table
2). The magnitude of the food intake and body weight changes did not differ across the three estradiol groups
and approximated that seen in the tumor-bearing group of experiment 1. As in experiment 1, the mean final body weight of vehicle-treated,
pair-fed rats in experiment 2 was higher than that of any of the
estrogen-treated groups, suggesting increased energy efficiency of the
pair-fed rats relative to those receiving estradiol. Plasma estradiol
levels in rats receiving the 2.5 mg pellet (169 ± 42 pg/ml) were
lower than those of tumor-bearing rats in experiment 1 (305 ± 77 pg/ml), whereas the 7.5 and 15 mg pellets yielded plasma estradiol
levels higher than in tumor-bearing rats (624 ± 46 and 822 ± 149 pg/ml, respectively). As in experiment 1, estradiol-induced
weight loss was associated with reduced plasma leptin and insulin
levels and increased corticosterone levels (Table 2), and these effects
(like those on food intake and body weight) were not dose-related.
Unlike the response of tumor-bearing or pair-fed rats of experiment 1, plasma glucose levels decreased by 7-10% in each of the
estradiol-treated groups.
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Table 2.
Dose effect of 17-estradiol administration on food
intake, body weight, and plasma hormone levels from experiment 2
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Experiment 3: effect of chronic 17- estradiol administration on
hypothalamic neuropeptide mRNA levels
To investigate effects on hypothalamic gene expression of chronic
hyperestrogenemia at levels comparable with those detected in the
tumor-bearing group in experiment 1, male Wistar rats were implanted
subcutaneously with pellets containing either vehicle or 7.5 mg of
17-estradiol (selected on the basis of data from experiment 2).
After 22 d, estradiol-treated rats had sustained a 33% reduction
in final body weight relative to vehicle-implanted controls
(p < 0.05) (Table
3). As in experiment 2, estradiol levels
were markedly elevated in the treated group relative to controls
(1046 ± 105 vs 8.5 ± 0.5 pg/ml; p < 0.05),
whereas plasma leptin and glucose levels were significantly reduced.
Plasma corticosterone levels were increased in the estradiol-treated
group, but the effect did not reach statistical significance. As
measured by in situ hybridization, NPY and AgRP mRNA
expression in the ARC of estradiol-treated rats increased by 69.7 ± 10.9% (p < 0.05) and 124 ± 18%
(p < 0.05) relative to controls, whereas POMC
mRNA hybridization was reduced by 47.6% (p < 0.05) (Fig. 3a,b). This profile of NPY, AgRP, and POMC mRNA expression was similar to that seen
in the tumor-bearing rats of experiment 1. Similarly, MCH mRNA levels
in the LHA of estradiol-treated animals were nonsignificantly below
control values, despite pronounced weight loss (Fig. 3c). Thus, the 7.5 mg dose of estradiol elicited behavioral, hormonal, and
hypothalamic neuropeptide responses similar to those seen in experiment
1, including selective blockade of the effect of weight loss to
increase hypothalamic expression of MCH mRNA.
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Table 3.
Effect of chronic 17-estradiol administration on body
weight and plasma hormone levels from experiment 3
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Figure 3.
Hypothalamic mRNA levels as measured by in
situ hybridization in rodents bearing a subcutaneous pellet
releasing 17-estradiol (E2) or vehicle
(Veh) (experiment 3). Results (mean ± SEM) are
presented as a percentage of mRNA expression relative to Veh. The
pellets released 7.5 mg of E2 over 21 d.
*p < 0.05 versus Veh;
#p = 0.19.
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DISCUSSION |
A critical role for LHA neurons, particularly those producing MCH,
in energy homeostasis is suggested by the hypophagic and lean phenotype
arising from targeted gene deletion of this neuropeptide (Shimada et
al., 1998). We therefore hypothesized that decreased MCH signaling
contributes to the pathogenesis of certain forms of anorexia. Here we
report the complete abolition of the effect of chronic negative energy
balance to increase LHA MCH expression in two rat models of
estrogen-mediated anorexia, despite the preservation of key hormonal
(decreased plasma leptin and insulin levels with increased plasma
levels of corticosterone) and hypothalamic (increased expression of NPY
and AgRP mRNA along with decreased expression of CRH and POMC mRNA)
responses to weight loss. These findings identify MCH neurons as
specific targets of estrogen action that may contribute to its anorexic effects.
Hyperestrogenemic rats displayed similar or exaggerated hormonal
responses to weight loss as compared with pair-fed controls. Plasma
leptin and insulin concentrations were significantly below those of
both ad libitum-fed and pair-fed controls in experiment 1, whereas corticosterone elevations paralleled those seen in pair-fed
rodents. Thus, hyperestrogenemia had no apparent adverse affect on the
recruitment of adaptive hormonal responses to energy deficit. Moreover,
the ability of these hormonal cues to engage ARC and PVN neuronal
responses that accompany energy deficit appeared to be intact in the
hyperestrogenemic groups. Tumor-bearing rodents of experiment 1 displayed 45-100% increases in NPY and AgRP mRNA relative to ad
libitum fed control rodents, an effect similar to that observed in
the pair-fed group, whereas POMC and CRH mRNA expression was inhibited
to an even greater extent in hyperestrogenemic versus pair-fed rodents.
This combination of hypothalamic neuropeptide responses was also
observed in estradiol-treated rodents in experiment 3. Thus, anorexia
persisted in hyperestrogenemic animals, despite the apparent
preservation of a set of compensatory hormonal and medial hypothalamic
responses expected to potently stimulate food intake.
Although the PVN, like the LHA, contains second-order neurons involved
in energy homeostasis (such as those containing CRH), the finding that
electrolytic lesions of the PVN do not attenuate estrogen-induced
anorexia (Butera et al., 1992) suggests that key neurons that mediate
the effects of estrogen on energy homeostasis must be located
elsewhere. Our demonstration that hyperestrogenemia selectively and
completely suppressed the expected increase of MCH mRNA expression in
response to progressive weight loss, whereas important ARC and PVN
responses were unimpeded, is consistent with the model illustrated in
Figure 4, in which estrogen-sensitive MCH
neurons in the LHA are "downstream" in a pathway used by ARC neurons to trigger feeding responses (Elias et al., 1998; Elmquist et
al., 1998; Sawchenko, 1998; Schwartz et al., 2000). According to this
model, interventions that disrupt MCH signaling can cause sustained
anorexia by interrupting the pathway used by ARC neurons to control
feeding behavior.
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Figure 4.
Proposed model of the effects of estrogen on
hypothalamic neuronal pathways involved in the regulation of energy
balance. A, In response to energy restriction,
circulating leptin and insulin levels decrease, resulting in increased
gene expression of orexigenic peptides (e.g., NPY) and decreased gene
expression of anorexic peptides (e.g., POMC) in neurons of the ARC.
These neuronal responses are proposed to increase expression of the
orexigenic neuropeptide MCH in neurons of the LHA, which in turn
promote increased food intake. B, Estrogen-mediated
weight loss and anorexia also lower plasma leptin and insulin, but the
expected activation of MCH neurons fails to occur in the presence of
estrogen, despite the preservation of "upstream" ARC NPY and POMC
neuronal responses to reduced adiposity signaling. This inhibition of
MCH neurons by estrogen is hypothesized to contribute to the sustained
anorexia observed with chronic estrogen exposure in male rodents.
3rd vent, Third
cerebral ventricle.
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This hypothesis is supported by the finding that MCH-deficient mice
have reduced ARC POMC production, yet continue to undereat, and by
growing evidence that ARC neurons can directly influence LHA activity
(Shimada et al., 1998). Recent work has demonstrated rich connections
between NPY- and POMC-containing neurons of the ARC and MCH neurons in
the LHA (Broberger et al., 1998; Elias et al., 1998) and that
pharmacological blockade of melanocortin-4 receptors, which are
concentrated in the LHA, increases MCH mRNA expression (Hanada et al.,
2000). Furthermore, leptin-deficient ob/ob mice display
increased MCH mRNA expression (Qu et al., 1996), despite relatively low
expression of leptin receptor in the LHA (Schwartz et al., 1996). Thus,
leptin appears to influence MCH neurons via an indirect mechanism. The
recent demonstration that ARC-derived leptin-responsive neurons project
to the LHA provides a plausible mechanism, whereby leptin-deficiency
increases MCH gene expression (Elias et al., 1999).
Our data are in agreement with the recently demonstrated effect of
estrogen administration to decrease hypothalamic MCH mRNA levels in
female rats rendered estrogen-deficient by ovariectomy (Murray et al.,
2000). Thus, estrogen deficiency in female rats is associated with
increased hypothalamic MCH gene expression that may, in turn,
contribute to the accompanying hyperphagia and weight gain in this
setting (Wade and Gray, 1979). We found in male rats that, in contrast
to the sixfold increase of MCH mRNA expression observed in pair-fed
controls, hyperestrogenemic animals mounted MCH responses no different
from ad libitum fed controls. Together, these observations
establish MCH neurons as targets of estrogen action. Our data further
suggest that supraphysiological levels of estrogen can override the
ability of weight loss and decreased leptin signaling to induce MCH
expression. These findings also suggest, but do not establish, a
specific contribution of MCH signaling to the alterations of energy
balance observed in settings of estrogen excess and deficiency. Further
work is warranted to test the hypotheses that MCH neurons mediate the
feeding effects observed with physiological, as well as
supraphysiological, alterations in estrogen status.
The hypothesis that estrogen receptors mediate the effects of estrogen
on energy balance is supported by the observation that these receptors
are expressed at high levels in medial and lateral hypothalamic areas
implicated in energy homeostasis (Couse et al., 1997). However, whereas
Sar et al. (1990) reported the colocalization of NPY and estrogen
receptor in a subpopulation of ARC neurons, there is a paucity of data
regarding the presence of estrogen receptor on other neuronal systems
pertinent to energy homeostasis (Sar et al., 1990). Interestingly,
deficiency of the isoform of the estrogen receptor (ER), which
is concentrated in both the ARC and the LHA, induces obesity in mice,
whereas mice deficient in estrogen receptor (which is not expressed
in the LHA) have a normal body weight (Couse and Korach, 1999),
suggesting a specific role for ER in mediating the effects of
estrogen on energy balance. Whereas evidence that ER is expressed in
the LHA raises the possibility that estrogen may directly inhibit MCH
neurons, the hypothesis that MCH neurons express estrogen receptor is
untested (Couse et al., 1997).
The significantly higher body weights of pair-fed controls in
experiments 1 and 2 (relative to hyperestrogenemic animals) suggest
that hyperestrogenemia, in addition to its hypophagic effects, also
impairs the ability of rodents to manifest the decrease in energy
expenditure that normally accompanies weight loss. In concert with
reduced food intake, this defect appears to contribute to the ability
of estrogen to induce negative energy balance and deplete body fuel
stores (Laudenslager et al., 1980). The hypothesis that increased
hypothalamic MCH signaling contributes to adaptive decreases of energy
expenditure during weight loss is a possibility that warrants
additional study.
One mechanism implicated in the pathogenesis of estrogen-induced
anorexia is the development of taste aversions because of illness or
malaise (Bernstein et al., 1986). Whereas taste aversion may have
contributed to the anorexia observed in our studies, there are few
models in which sustained, marked weight loss occurs via this
mechanism. For example, acute central administration of glucagon-like
peptide-1 (GLP-1) reduces food intake by a mechanism that includes the
development of taste aversion, but repeated administration of GLP-1
does not induce sustained reductions in food intake or body weight (van
Dijk et al., 1997; Donahey et al., 1998). Thus, while taste aversions
may have developed, they seem unlikely to fully explain the sustained
anorexia induced by hyperestrogenemia in male rats.
The association between decreased MCH signaling and hypophagia may
prove relevant to the pathogenesis of clinical anorexia syndromes,
which are common and deleterious comorbidities of chronic diseases such
as cancer and AIDS (Grunfeld and Feingold, 1992). Such anorexia
syndromes are difficult to treat, perhaps because of our poor
understanding of their pathophysiology. Although the significance of
MCH in human anorexia syndromes remains uncertain, progress toward
characterizing its role and that of other CNS effector systems in
models of disordered food intake may eventually lead to novel
treatments for these syndromes.
In conclusion, the observation that estrogen-induced anorexia in
rodents is associated with decreased MCH signaling represents a first
step toward identifying a potentially important role for MCH in
estrogen-mediated alterations in energy homeostasis. Additional characterization of this effect will help to delineate the roles of
discrete neuronal circuits involved in energy homeostasis, clarify the
mechanism by which estrogen affects this process, and improve our
understanding of the pathogenesis of anorexia syndromes.
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FOOTNOTES |
Received May 23, 2000; revised Aug. 21, 2000; accepted Sept. 5, 2000.
This work was supported by Grants HL07028 (P.M.), NS-32273, DK-12829,
DK-52989-01 (M.W.S.), DK-50129, DK-35747 (P.J.H.), DK-54080, and
DK-54890 (R.J.S.) from the National Institutes of Health, the American
Diabetes Association, the United States Department of Agriculture
(P.J.H.), the Department of Veterans Affairs (A.M.M. and C.W.W.), and
by the Diabetes Endocrine Research Center and Clinical Nutrition
Research Unit of the University of Washington. We recognize the
excellent technical assistance of Hong Nguyen, Vicki Hoagland, Kimber
Stanhope, Elizabeth Colasurdo, Robert Beckham III, and Dr. Supriya
Kelkar, and we thank Linda Walters for assistance in manuscript preparation.
Correspondence should be addressed to Dr. Michael W. Schwartz, Division
of Endocrinology/Metabolism, Harborview Medical Center, 325 Ninth
Avenue, Box 359757, Seattle, WA 98104. E-mail:
mschwart{at}u.washington.edu.
| |
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TOP
ABSTRACT
INTRODUCTION
