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1 Hannah Research Institute, Ayr KA6 5HL; and 2 Appetite and Energy Balance Division, Rowett Research Institute, Bucksburn, Aberdeen AB21 9SB, United Kingdom
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ABSTRACT |
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 TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
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Peripheral and hypothalamic mechanisms underlying the hyperphagia of lactation have been investigated in sheep. Sheep were fed ad libitum and killed at 6 and 18 days of lactation; ad libitum-fed nonlactating sheep were killed as controls. Despite increased food intake, lactating ewes were in negative energy balance. Lactation decreased plasma leptin and adipose tissue leptin mRNA concentrations. OB-Rb gene expression, determined by in situ hybridization, was increased in the hypothalamic arcuate nucleus (ARC) and ventromedial hypothalamic nucleus (VMH) at both stages of lactation. Neuropeptide Y (NPY) was increased by lactation in both the ARC and dorsomedial hypothalamus (DMH), although increased gene expression in the DMH was only apparent at day 18 of lactation. Gene expression was decreased for cocaine- and amphetamine-regulated transcript (CART) in the ARC and VMH and for proopiomelanocortin in ARC during lactation. Agouti-related peptide gene expression was increased in the ARC, and melanocortin receptor expression was unchanged in both the ARC and VMH with lactation. Thus the hypoleptinemia of lactation may activate NPY orexigenic pathways and attenuate anorexigenic melanocortin and CART pathways in the hypothalamus to promote the hyperphagia of lactation.
hypothalamic neuropeptides; appetite
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INTRODUCTION |
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TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
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NUTRIENT REQUIREMENTS ARE markedly increased during lactation to meet the demands for milk production (7, 70). In most mammals, these additional requirements are met primarily by increasing food intake (7, 20, 70), but factors regulating the hyperphagia of lactation are not well understood. In addition, in many species, the increased intake is insufficient to meet the metabolic demands, resulting in a state of negative energy balance during early lactation when body lipid reserves are mobilized (7, 69). In some species such as the rat, the degree of negative energy balance is usually slight, with animals mobilizing ~1 g fat/day (7). Domestic ruminants, however, usually exhibit a greater degree of negative energy balance during early lactation (7, 11), which can impact on their welfare and productivity (28), yet the regulatory mechanisms have not been elucidated.
It is postulated that leptin, a peptide hormone secreted by adipose tissue (75), could play a role in the hyperphagia of lactation. Leptin acts on hypothalamic neuronal systems to regulate energy balance and neuroendocrine function; in particular, a reduction in circulating leptin is a potent signal of negative energy balance, activating compensatory orexigenic pathways in the hypothalamus (3-5, 35, 64). Key targets are neurons in the hypothalamic arcuate nucleus (ARC) expressing the signaling form of the leptin receptor (OB-Rb); these are the orexigenic peptides neuropeptide Y (NPY) and agouti-related peptide (AGRP) and the anorexigenic peptides proopiomelanocortin (POMC, precursor for melanocortins) and cocaine- and amphetamine-regulated transcript (CART) (4, 5, 31, 62, 64, 72). Adipose leptin gene expression and serum leptin concentrations are reported to be reduced during lactation in the monogastric rat in most (16, 36, 39, 56, 65, 74), but not all (17, 19, 61), studies. NPY (18, 21, 43, 44, 54, 55, 63, 71) and AGRP (18) are upregulated, and POMC is downregulated (54, 63) in the hypothalamus during lactation in the rat. However, there are no reports of gene expression for CART or for the hypothalamic melanocortin receptor (MC3-R) during lactation.
Little is known about the leptin-hypothalamic axis in lactating ruminants, despite their markedly different nutritional physiology and greater degree of lactational negative energy balance compared with laboratory rats. This study examined the hypothesis that leptin signaling plays a role in the hyperphagia of lactation in a ruminant. Specifically, we investigated the effects of lactation in sheep on leptin gene expression in adipose tissue, plasma leptin concentrations, and the gene expression of leptin-sensitive hypothalamic neuropeptides and receptors.
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MATERIALS AND METHODS |
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TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
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Animals. Twenty-four multiparous Finn × Dorset Horn crossbred ewes from the same flock were used. Eight had not been mated and were nonlactating, nonpregnant controls, whereas the remaining 16 sheep suckled either two or three lambs. Animals were given a fixed daily ration of concentrates comprising 18% protein, 12.5 MJ metabolizable energy (ME)/kg dry matter (DM) (Davidson Brothers, Lanarkshire, UK) two times daily at 0700 and 1600, at 1.2 kg/day for lactating sheep and 0.5 kg/day for controls. Sheep were also fed "hay saver" pellets ad libitum; these comprised sodium hydroxide-treated straw and dried grass (9.4 MJ ME/kg DM; N. S. Milling, Heck Hall Farm, East Yorkshire, UK). Ewes were killed on day 6 or day 18 of lactation and on close calendar dates for the controls (n = 8/group). The number of animals suckling three lambs was equally distributed between the two groups. Food intake was measured daily throughout lactation and daily for 1 wk before killing for controls. Body weight of ewes and their lambs was measured at days 2, 6, 10, 14, and 18 of lactation and 1 wk before and immediately before the control animals were killed. All procedures involving animals were approved by the Hannah Research Institute Ethical Review Committee.
Blood sampling.
Blood samples were collected via a jugular catheter at 1200, 1400, and
1600 (just before feeding) on the day before killing and at 0900 immediately before the animal was killed. Plasma was prepared and
stored at 
20°C until further analysis.
Measurement of hormones and metabolites. Plasma glucose concentrations were determined in deproteinized samples as described by Bergmeyer (14). Free fatty acid (FFA) concentrations were measured colorimetrically (38). Insulin and thyroxine were determined by RIA using commercial kits (ICN Pharmaceuticals, Hampshire, UK, and IDS, Tyne and Wear, UK). Plasma leptin concentrations were determined by RIA (47), with a detection limit of 0.45 ng/ml and intra- and interassay coefficients of variation 12 and 16%, respectively.
Adipose tissue.
Sheep were anesthetized with 30 ml pentobarbital sodium (Sagatal; Rhone
Merieux) and were killed by exsanguination. Subcutaneous adipose tissue
samples were removed from the flank fat pads anterior to the hindlimbs.
Tissue for leptin RNA determination was snap-frozen in liquid
N2 and stored at 80°C before assay. Adipose tissue for
measurement of lipogenesis and mean cell volume was placed in medium
199 with Earle's salt and 25 mM HEPES, pH 7.3 (Life Technologies,
Paisley, UK), with 2 mM acetate and antibiotics (57) at
37°C for transportation to the laboratory (~5 min).
RNase protection assay.
A 252-bp ovine leptin cDNA in pCRII-TOPO (a kind gift from R. Ehrhardt
and Y. Boisclair, Cornell University, Ithaca, NY) was used to generate
sense transcripts using SP6 RNA polymerase and antisense transcripts
with [
-32P]CTP and T7 RNA polymerase (41,
52).
Hypothalamic gene expression.
Brains were removed and immediately frozen in isopentane and were
stored at 80°C until further analysis. Coronal brain sections (20 µm) were collected through the hypothalamus and mounted on gelatin-
and poly-L-lysine-coated slides. mRNAs for a number of leptin-sensitive neuropeptides and receptors were determined by semiquantitative in situ hybridization using methods described in
detail by Adam et al. (1) and Mercer et al.
(49).
1 · ml1, slides
were hybridized overnight at 58°C. After incubation, slides were
treated with RNase A, desalted in saline-sodium citrate with increasing
stringency, dried, and apposed to Hyperfilm 
-max (Amersham Pharmacia
Biotech, Little Chalfont, Buckinghamshire, UK). Autoradiographic images
were quantified using the Image-Pro Plus software system, which
measures the intensity and area of the hybridization signal on the
basis of set parameters. A standard curve was generated using
14C microscales (Amersham Pharmacia Biotech), and the
integrated intensity of the hybridization signal was then computed
against this standard curve. CART, OB-Rb, and MC3-R were expressed in both the ARC and ventromedial hypothalamus (VMH; adjacent regions of
the hypothalamus). Subdivision of the hypothalamus into ARC and VMH was
made by a comparison with other sections from the same region from the
same sheep but was hybridized for POMC, AGRP, and NPY, which are
expressed in the ARC but not in the VMH.
Statistical analysis. All data were analyzed by ANOVA using genstat (Genstat 5 Release 4.1; Lawes Agricultural Trust, Rothamsted Experimental Station); factors were animal and physiological state and time of day. Where no effect was found for "time of day," the average for the four plasma samples taken at different times was calculated, and the data were reanalyzed using animal and physiological state as factors.
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RESULTS |
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TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
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Total DM intake (concentrates plus hay saver pellets)
increased (P < 0.01) over the first 2 wk of lactation
and then appeared to plateau (Fig. 1).
Food intake of ewes at 6 and 18 days of lactation was greater than that
of nonlactating sheep (P < 0.01); in terms of ME,
controls consumed ~20 MJ/day, whereas lactating ewes consumed ~27
MJ/day at day 6 and ~29 MJ/day at day 18. The
body weight of control ewes was maintained at 81 ± 2.8 kg for the
week before killing. The body weight of the two lactating groups at
day 2 postpartum was 70.0 ± 2.1 and 73 ± 3.0 kg
for the groups killed on day 6 and day 18 of
lactation, respectively, and in both cases was significantly
(P < 0.05) less than that of controls. The body weight
at time of death was 68.7 ± 2.0 and 70.9 ± 2.6 kg for ewes killed on day 6 and day 18; in both cases, there
was a tendency for body weight to decrease between day 2 postpartum and the day of killing (P = 0.10 and 0.11, respectively, for day 6 and day 18 lactating
ewes). In the early lactating group, the lambs gained 2.8 ± 0.37 kg between days 2 and 6 of lactation, whereas in
the peak lactating group the lambs gained 7.5 ± 0.84 kg between
days 2 and 18 of lactation.
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Plasma insulin concentrations were lower at day 6 of
lactation compared with control animals, whereas lactation had no
effect on plasma thyroxine concentration (Table
1). There was no difference in the
concentration of plasma glucose between any of the groups (Table 1).
FFA concentrations were significantly higher (P < 0.001) in lactating animals than nonlactating animals (Fig.
2). The time of day affected plasma
levels of FFA in the lactating animals. Concentrations were lowest
2 h after the concentrate meal given at 0700 and increased until
the next meal of concentrates given at 1600. Furthermore, the plasma
concentration of FFA was higher at day 6 of lactation than
day 18, although this difference was only significant at
0900 and 1200 (Fig. 2). Plasma leptin concentration was four to five
times higher (P < 0.001) in nonlactating than in
lactating animals (Fig. 3). No difference
was evident in leptin concentration between day 6 and
day 18 of lactation, and there was no effect of time of day.
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The mean cell volume of subcutaneous adipocytes was significantly lower
during lactation (P = 0.01), but no difference was found between day 6 and day 18 of lactation
(Table 2). The rate of fatty acid
synthesis was markedly reduced during lactation (P < 0.001), and again no significant difference was evident between day 6 and day 18 of lactation (Table 2). Leptin
mRNA concentration of adipose tissue was significantly reduced during
lactation (P < 0.001; Table 2), and there was a
tendency for the concentration to be lower at day 6 than at
day 18 of lactation (P = 0.07). Leptin plasma levels correlated significantly with leptin mRNA levels in
adipose tissue (R = 0.72, P < 0.001);
the relationship was not altered by lactation.
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Hypothalamic neuropeptides and receptor gene expression.
OB-Rb gene expression in the hypothalamus was localized in the ARC and
VMH. Lactating animals tended to have higher expression of OB-Rb in
both ARC (P < 0.08) and VMH (P < 0.10) compared with nonlactating control animals, and no difference was
found between days 6 and 18 of lactation (Fig.
4). Values for lactating ewes were very
variable; when results were log transformed and then reanalyzed by
ANOVA, a significant increase was revealed in OB-Rb expression in the
ARC and VMH of lactating ewes (P = 0.002 and 0.049, respectively).
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DISCUSSION |
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TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
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The effects of pregnancy and lactation on Finn × Dorset Horn sheep have been studied in some detail previously. The empty carcass weight decreases during late pregnancy, mostly because of loss of adipose tissue lipid (58, 59); hence, the decreased body weight of the ewes at day 2 postpartum was not unexpected. Food intake increases after birth, reaching a plateau between 2 and 3 wk postpartum (24, 60). Nevertheless, ewes suckling two or more lambs lose ~5 kg in body weight, mostly because of loss of fat from adipose tissue during the first 5-6 wk of lactation (22, 23, 24, 60). Ewes suckling a single lamb do not appear to lose adipose tissue lipid, whereas ewes suckling three lambs lose an amount of adipose tissue lipid similar to those suckling two lambs (66). A net loss of adipose tissue lipid indicates that the ewes suckling two or more lambs were in negative energy balance during the first 5-6 wk of lactation. Consistent with this, serum nonesterified fatty acid concentrations are elevated during the first 3 wk of lactation, then decline gradually to levels found in fed, nonlactating ewes by ~6 wk after birth (23, 66). The changes in body weight, food intake, adipocyte size, and plasma nonesterified fatty acid concentration found with lactation in the present study are thus consistent with previous findings. Furthermore, the elevated serum nonesterified fatty acid levels and the very low rates of fatty acid synthesis of the adipocytes from the lactating ewes show that adipocytes are in a catabolic state. Hence the ewes were in negative energy balance for at least the first 18 days after birth, despite having food available ad libitum.
Plasma leptin concentrations and adipocyte leptin mRNA concentrations were both markedly decreased by lactation in sheep. A very recent study suggests that serum leptin concentrations were not decreased in Karakul ewes suckling a single lamb (30). However, although the lactating ewes in the study were fed ad libitum, food intake of the control, nonlactating ewes was restricted to maintain a constant body weight over the previous 24 wk (30), which could have decreased their serum leptin concentrations (26, 47). Previous studies with rats have also shown that serum leptin is decreased by lactation (16, 36, 39, 56, 65, 74). However, in the daytime at least, the fall in leptin concentration is much smaller in rats than in sheep. This species difference may reflect a greater degree of negative energy balance during lactation in sheep than in rats. Rats differ from sheep in that they are mainly nocturnal feeders, and this pattern of intake is reflected in an increase in serum leptin at night, which is markedly suppressed during lactation (56). Fed sheep show no evidence for a circadian rhythm of plasma leptin but do exhibit low-amplitude postprandial peaks (47) that could have remained undetected in the present trial given the low sampling frequency. Undernutrition or fasting in sheep results in a fall in serum leptin (26, 47) and adipose tissue leptin mRNA concentration (42), as in other species (3, 5, 37, 68). Thus the hypoleptinemia of lactating sheep suckling two or more lambs is not surprising in view of the negative energy balance of the animal and would be expected to increase appetite and so contribute to the hyperphagia of lactation.
Leptin acts on a number of neuropeptides and receptors in the hypothalamus to regulate appetite and energy balance; systems involved include the NPY, CART, and melanocortin pathways (3, 4, 31, 62, 64, 72). In this study, we examined the effect of lactation on four key neuropeptides (NPY, CART, POMC, and AGRP) and also on OB-Rb receptor and MC3-R. In our study, gene expression of OB-Rb in both the ARC and VMH increased during lactation in sheep. In the rat, there are complex changes in OB-Rb expression with lactation, with an increase in expression in the supraoptic nucleus, a decrease in expression in the VMH, and no change in the ARC, DMH, or PVN (15). By contrast, fasting or feed restriction increases OB-Rb expression in the ARC and VMH in both rats (9, 13) and sheep (29), suggesting that leptin suppresses expression of its receptor in the ARC and VMH. Our data for lactating sheep are consistent with this reciprocal relationship between leptin and OB-Rb expression. The reason for the rather different findings for the lactating rat (15) may be because of lactation inducing a more modest degree of negative energy balance and a smaller decrease in serum leptin in rats than in sheep. The increased gene expression of OB-Rb also suggests that the hypothalamus may become more sensitive to leptin during lactation, as shown during fasting in rats (10).
Leptin increases expression of the anorexigenic neuropeptides POMC and CART and decreases expression of the orexigenic neuropeptides NPY and AGRP in the hypothalamus (3, 4, 31, 62, 64, 72). The increased gene expression for NPY and AGRP and the decreased gene expression for POMC and CART found during lactation in the present sheep are thus consistent with the concurrent hypoleptinemia and should act to drive hyperphagia. A rise in gene expression for NPY (18, 21, 43, 44, 54, 55, 63, 71) and AGRP (18) and a fall in POMC gene expression (54, 63) have been reported previously for lactating rats, but this is the first report of decreased CART gene expression during lactation. In keeping with these findings, negative energy balance induced by fasting in sheep also decreases serum leptin concentrations (26, 47), increases hypothalamic gene expression for NPY and AGRP, and decreases expression of POMC and CART (C. L. Adam, Z. A. Archer, and P. A. Findlay, unpublished observations).
Lactation increased expression of NPY mRNA in both the ARC and the DMH in sheep; however, the increased expression in the DMH developed more slowly, being apparent at 18 but not 6 days of lactation. NPY gene expression in the rat is also increased in both the ARC and DMH around peak lactation (43, 44). However, the regulation of NPY neuronal activity apparently differs between the ARC and DMH; for example, NPY neurons in the DMH are not activated under conditions of food restriction, which activate NPY neurons in the ARC of rats (48). Conversely, in both MC4-R knockout mice and agouti mice, which lack a functional MC4-R because of functional blockade by endogenous agouti protein, NPY is upregulated in the DMH but not in the ARC, suggesting that signaling via MC4-R acts to attenuate NPY expression in the DMH (40). Li et al. (43) showed that, in lactating rats, NPY expression in the DMH could be induced within 3 h by the suckling stimulus, whereas 24 h of suckling were required to increase NPY expression in the ARC. During lactation, signaling through MC4-R is likely to be markedly suppressed by the large increase in the ratio of AGRP to POMC in both rats and sheep, and this may facilitate increased gene expression for NPY in the DMH in sheep, at least at 18 days of lactation. It is possible that the lack of upregulation of NPY mRNA in the DMH at 6 days as opposed to 18 days of lactation in sheep may be because of a lower suckling stimulus at this time. It is not known if NPY expression is increased in the rat DMH during early lactation.
The melanocortin system is a major anorexic system of the hypothalamus;
the system is stimulated by -melanocyte-stimulating hormone, which
is derived from POMC, and is inhibited by AGRP. These
neuropeptides act via MC3-R in the ARC and VMH and MC4-R in the PVN
(6, 53). In the ARC, MC3-R mRNA is expressed in both AGRP
and POMC neurons, which has led to the suggestion that MC3-R mediates
the antagonistic interaction between AGRP and POMC (6).
Localization of MC3-R mRNA in the ovine ARC and VMH is consistent with
its hypothalamic localization in rodents (2, 6). As
described above, expression of POMC is decreased, whereas that of AGRP
is increased, during lactation in both sheep and rats, and this, by
diminishing signaling through the melanocortin system, should promote
the hyperphagia of lactation. However, despite the changes in AGRP and
POMC gene expression, MC3-R gene expression was not affected in either
the ARC or VMH in lactating sheep. There are no published data
available on MC3-R gene expression in lactating rodents. In contrast to
lactating sheep, food-restricted hamsters, with reduced circulating
leptin, exhibit decreased gene expression for both MC3-R and POMC in
the ARC and increased MC3-R gene expression in the VMH
(50). It is open to speculation whether these differences
are attributable to a species difference in sensitivity of the
melanocortin pathway to negative energy balance or to a difference in
response to negative energy balance induced by food restriction as
opposed to lactation.
Thus lactation in sheep suckling two or more lambs induced a state of negative energy balance, low circulating leptin, and changes in hypothalamic gene expression consistent with responses to the diminished leptin feedback: the orexigenic NPY pathway was apparently activated, and the activities of the anorexic melanocortin and CART pathways were downregulated. These findings are consistent with reduced leptin signaling playing a role in the hyperphagia of lactation in ruminants. The changes in gene expression of leptin-sensitive hypothalamic neuropeptides were very similar to those observed during fasting or food restriction. However, during lactation, the negative energy balance is not the result of a lack of availability of food (the sheep in the present study were fed ad libitum); rather it was because of an inadequate appetite. Lactating rats with ad libitum food also show negative energy balance (7), and, for this species at least, it would appear that physical constraints such as gut size are not responsible for limiting intake (27). Furthermore, high-yielding dairy cows during early lactation were induced to increase both milk yield and food intake by increased milking frequency (8); this suggests that a physical constraint on appetite is also unlikely to be responsible for the negative energy balance that occurs during early lactation in dairy cows. Indeed, a period of negative energy balance appears to be a widely used strategy, possibly to increase metabolic efficiency, during lactation (69). Resolving the molecular basis for this self-imposed period of negative energy balance during lactation should provide useful insight into the regulation of appetite control in sheep and other species.
It is important to note, of course, that lactation can occur in animals in energy balance, so other factors must also drive the hyperphagia in addition to "negative energy balance." The suckling stimulus is important for hyperphagia in rats (33, 46, 74), but the mechanism is unresolved. Suckling increases secretion of both prolactin and oxytocin (25). Prolactin can increase food intake in nonlactating rats, and hypothalamic prolactin receptor mRNA expression is increased during lactation in the rat (34). Nevertheless, the role of prolactin in the hyperphagia of lactation is still unresolved (34). Oxytocin secretion is also increased during lactation, but oxytocin in fact decreases food intake in the rat (12). Thus the mechanisms driving the hyperphagia of lactation remain unresolved. Hypoleptinemia, probably arising from a self-induced state of negative energy balance, would appear to be an important stimulus of hyperphagia during the initial stages of lactation at least in sheep, as also appears to be the case in rats.
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ACKNOWLEDGEMENTS |
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We thank Dr. N. Hoggard for help in preparing the ovine leptin antiserum and Drs R. A. Ehrhardt, Y. R. Boisiclair, S. Sabol, J. Mercer, A. Ross, P. Barrett, and N. Hoggard for riboprobes. We thank I. Nevison (Biological and Biomathematical Sciences, Scotland) for statistical advice and the Scottish Executive Environment and Rural Affairs Department for financial support.
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FOOTNOTES |
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Present address for M. Marie: Sciences Animales, ENSAIA-INPL, BP 172, 54505, Vandoeuvre lès Nancy, France.
Address for reprint requests and other correspondence: R. G. Vernon, Hannah Research Institute, Ayr KA6 5HL, Scotland, UK (E-mail: vernonr{at}hri.sari.ac.uk).
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
10.1152/ajpregu.00595.2001
Received 28 September 2001; accepted in final form 12 December 2001.
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REFERENCES |
|---|
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TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
|
|---|
1.
Adam, CL,
Findlay PA,
Kyle CE,
Young P,
and
Mercer JG.
Effect of chronic food restriction on pulsatile luteinizing hormone secretion and hypothalamic neuropeptide Y gene expression in castrate male sheep.
J Endocrinol
152:
329-337,
1997.
2.
Adam, CL,
Moar KM,
Logie TJ,
Ross AW,
Barrett P,
Morgan PJ,
and
Mercer JG.
Photoperiod regulates growth, puberty and hypothalamic neuropeptides and receptor gene expression in female siberian hamsters.
Endocrinology
141:
4349-4356,
2000.
3.
Ahima, RS.
Leptin and the neuroendocrinology of fasting.
Frontiers Horm Res
26:
42-56,
2000.
4.
Ahima, RS,
and
Flier JS.
Adipose tissue as an endocrine organ.
Trends Endocrinol Metab
11:
327-332,
2000.
5.
Ahima, RS,
Saper CF,
Flier JS,
and
Elmquist JK.
Leptin regulation of neuroendocrine systems.
Front Neuroendocrinol
21:
263-307,
2000.
6.
Bagnol, D,
Lu XY,
Kaelin CB,
Day HE,
Ollmann M,
Gantz I,
Akil H,
Barsh GS,
and
Watson SJ.
Anatomy of an endogenous antagonist: relationship between agouti-related protein and proopiomelanocortin in brain (Abstract).
J Neurosci
19:
RC26,
1999.
7.
Barber, MC,
Clegg RA,
Travers MT,
and
Vernon RG.
Lipid metabolism in the lactating mammary gland.
Biochim Biophys Acta
1347:
101-126,
1997.
8.
Bar-Peled, U,
Aharoni Y,
Robinzon B,
Bruckental I,
Lehrer R,
Maltz E,
Knight C,
Kali J,
Folman Y,
Voet H,
Gacitua H,
and
Tagari H.
The effect of enhanced milk yield of dairy cows by frequent milking or suckling on intake and digestibility of the diet.
J Dairy Sci
80:
1420-1427,
1998.
9.
Barrett, P,
Morris MA,
Moar KM,
Mercer JG,
Davidson JA,
Findlay PA,
Adam CL,
and
Morgan PJ.
The differential regulation of CART gene expression in a pituitary cell line and primary cell cultures of ovine pars tuberalis cells.
J Neuroendocrinol
13:
347-352,
2001.
10.
Baskin, DG,
Seeley RJ,
Kuijper JL,
Lok S,
Weigle DS,
Erickson JC,
Palmiter RD,
and
Schwartz MW.
Increased expression of mRNA for the long form of the leptin receptor in the hypothalamus is associated with leptin hypersensitivity and fasting.
Diabetes
47:
538-543,
1998.
11.
Bauman, DE,
and
Currie B.
Partitioning of nutrients during pregnancy and lactation: a review of mechanisms involving homeostasis and homeorhesis.
J Dairy Sci
63:
1514-1529,
1980.
12.
Benelli, A,
Bertolini A,
and
Arletti R.
Oxytocin-induced inhibition of feeding and drinking: no sexual dimorphism in rats.
Neuropeptides
20:
57-62,
1991.
13.
Bennet, PA,
Lindell K,
Karlsson C,
Robinson IC,
Carlson LM,
and
Carlson B.
Differential expression and regulation of leptin receptor isoforms in the rat brain: effects of fasting and oestrogen.
Neuroendocrinology
67:
29-36,
1998.
14.
Bergmeyer, HU.
Determination of glucose with glucose oxidase and peroxidase.
In: Methods of Enzymatic Analysis, edited by Bergmeyer HU.. London: Academic, 1974, p. 1206-1213.
15.
Brogan, RS,
Grove KL,
and
Smith MS.
Differential regulation of leptin receptor but not orexin in the hypothalamus of the lactating rat.
J Neuroendocrinol
12:
1077-1086,
2000.
16.
Brogan, RS,
Mitchell SE,
Trayhurn P,
and
Smith MS.
Suppression of leptin during lactation: contribution of the suckling stimulus versus milk production.
Endocrinology
140:
2621-2627,
1999.
17.
Carmen-Garcia, MD,
Casanueva FF,
Dieguez C,
and
Senaris RM.
Gestational profile of leptin messenger ribonucleic acid (mRNA) content in the placenta and adipose tissue in the rat, and regulation of the mRNA levels of the leptin receptor subtypes in the hypothalamus during pregnancy and lactation.
Biol Reprod
62:
698-703,
2000.
18.
Chen, P,
Li C,
Haskell-Luevano C,
Cone RD,
and
Smith MS.
Altered gene expression of agouti-related protein and its colocalization with neuropeptide Y in the arcuate nucleus of the hypothalamus during lactation.
Endocrinology
140:
2645-2649,
1999.
19.
Chien, EK,
Hara M,
Rouard M,
Yano H,
Phillipe M,
Polonsky KS,
and
Bell GI.
Increase in serum leptin and uterine leptin receptor messenger RNA levels during pregnancy in rats.
Biochem Biophys Res Commun
237:
476-480,
1997.
20.
Chilliard, Y.
Variations quantitative et métabolisme des lipides dans les tissus adipeux et le foie au cours du cycle gestation-lactation: chez la brebis et la vache.
Reprod Nutr Dev
27:
327-398,
1986.
21.
Ciofi, P,
Fallon JH,
Croix D,
Pollak JM,
and
Tramu G.
Expression of neuropeptide Y precursor-immunoreactivity in the hypothalamic dopaminergic tubero-infundibular system during lactation in rodents.
Endocrinology
128:
823-834,
1991.
22.
Cowan, RT,
Robinson JJ,
Greenhalgh FD,
and
McHattie I.
Body composition changes in lactating ewes estimated by serial slaughter and deuterium dilution.
Anim Prod
29:
81-90,
1979.
23.
Cowan, RT,
Robinson JJ,
McDonald I,
and
Smart R.
Effects of body fatness at lambing and diet in lactation on body tissue loss, feed intake and milk yield of ewes in early lactation.
J Agric Sci
95:
497-514,
1980.
24.
Cowan, RT,
Robinson JJ,
McHattie I,
and
Pennie K.
Effects of protein concentration in the diet on milk yield, change in body composition and the efficiency of utilization of body tissue for milk production in ewes.
Anim Prod
1981:
111-120,
1981.
25.
Cowie, AT,
Forsyth IA,
and
Hart IC.
Hormonal Control of Lactation. Berlin, Germany: Springer-Verlag, 1980.
26.
Delavaud, C,
Bocquier F,
Chilliard Y,
Keisler DH,
Gertler A,
and
Kann G.
Plasma leptin concentration determination in ruminants: effect of nutritional status and body fatness on plasma leptin concentration assessed by a specific RIA in sheep.
J Endocrinol
165:
519-526,
2000.
27.
Denis, RGP,
Williams G,
and
Vernon RG.
Why are lactating rats in negative energy balance?
J Endocrinol Suppl
167:
p52,
2000.
28.
Domecq, JJ,
Skidmore AL,
Lloyd DW,
and
Kaneene JB.
Relationship between body condition scores and conception rate at first artificial insemination in a large dairy herd of high yielding Holstein cows.
J Dairy Sci
80:
113-120,
1997.
29.
Dyer, CJ,
Simmons JM,
Matteri RL,
and
Keisler DH.
Leptin receptor mRNA is expressed in ewe anterior pituitary and adipose tissues and is differentially expressed in hypothalamic regions of well-fed and feed restricted ewes.
Domest Anim Endocrinol
14:
119-128,
1997.
30.
Ehrhardt, RA,
Slepetis RM,
Bell AW,
and
Boisclair YR.
Maternal leptin is elevated during pregnancy in sheep.
Domest Anim Endocrinol
21:
85-96,
2001.
31.
Elmquist, JK,
Elias CF,
and
Saper CB.
From lesions to leptin: hypothalamic control of food intake and body weight.
Neuron
22:
221-232,
1999.
32.
Firestein, GS,
Gardner SM,
and
Roeder WD.
Quantitative molecular hybridisation with unfractionated, solubilised cells using RNA probes and polyacrylamide gel electrophoresis.
Anal Biochem
167:
381-386,
1987.
33.
Flemming, AS.
Control of food intake in the lactating rat: role of suckling and hormones.
Physiol Behav
17:
841-848,
1976.
34.
Grattan, DR,
Pi XJ,
Andrews ZB,
Augustine RA,
Kokay IC,
Summerfield MR,
Todd B,
and
Bunn SJ.
Prolactin receptors in the brain during pregnancy and lactation: implications for behaviour.
Horm Behav
40:
115-124,
2001.
35.
Havel, PJ.
Role of adipose tissue in body-weight regulation: mechanisms regulating leptin production and energy balance.
Proc Nutr Soc
59:
359-371,
2000.
36.
Herrera, E,
Lasunción Huerta L,
and
Martín-Hidalgo A.
Plasma leptin levels in rat mother and offspring during pregnancy and lactation.
Biol Neonate
78:
315-320,
2000.
37.
Ingvartsen, KL,
and
Andersen JB.
Integration of metabolism and intake regulation: a review focusing on periparturient animals.
J Dairy Sci
83:
1576-1597,
2000.
38.
Itaya, K,
and
Ui M.
Colorimetric determination of free fatty acids in biological fluids.
J Lipid Res
6:
16-20,
1965.
39.
Kawai, M,
Yamaguchi M,
Murakami T,
Shima K,
Murata Y,
and
Kishi K.
The placenta is not the main source of leptin production in pregnant rat: gestational profile of leptin in plasma and adipose tissues.
Biochem Biophys Res Commun
240:
798-802,
1997.
40.
Kersterson, RA,
Huszar D,
Lynch CA,
Simerly RB,
and
Cone RD.
Induction of neuropeptide Y gene expression in the dorsal medial hypothalamus nucleus in two models of the agouti obesity syndrome.
Mol Endocrinol
11:
630-637,
1997.
41.
Kreig, PA,
and
Melton DA.
In vitro RNA synthesis with SP6 RNA polymerase.
Methods Enzymol
155:
397-415,
1987.
42.
Kumar, B,
Francis SM,
Suttie JM,
and
Thompson MP.
Expression of obese mRNA in genetically lean and fat selection lines of sheep.
Comp Biochem Physiol A Physiol
120:
543-548,
1998.
43.
Li, C,
Chen P,
and
Smith MS.
The acute suckling stimulus induces expression of neuropeptide Y (NPY) in cells in the dorsomedial hypothalamus and increases NPY expression in the arcuate nucleus.
Endocrinology
139:
1645-1652,
1998.
44.
Malabu, U,
Kilpatrick A,
Ware M,
Vernon RG,
and
Williams G.
Increased neuropeptide Y concentrations in specific hypothalamic regions of lactating rats: possible relationships to hyperphagia and adaptive changes in energy balance.
Peptides
15:
83-87,
1994.
45.
Maniatis, T,
Jeffrey A,
and
van de Sand H.
Chain length determination of small double and single-stranded DNA molecules by polyacrylamide gel electrophoresis.
Biochemistry
14:
3787-3794,
1975.
46.
Mann, MA,
Miele JL,
Kinsley CH,
and
Svare B.
Postpartum behaviour in the mouse: the contribution of suckling stimulation to water intake, food intake and body weight regulation.
Physiol Behav
31:
633-638,
1983.
47.
Marie, M,
Findlay PA,
Thomas L,
and
Adam CL.
Daily patterns of plasma leptin in sheep: effects of photoperiod and food intake.
J Endocrinol
170:
277-286,
2001.
48.
Marks, JL,
Li M,
Schwartz M,
Porte D, Jr,
and
Baskin DG.
Effect of fasting on regional levels of neuropeptide Y mRNA and insulin receptor in the rat hypothalamus an autoradiographic study.
Mol Cell Neurosci
3:
199-205,
1992.
49.
Mercer, JG,
Lawrence CB,
Beck B,
Burlet A,
Atkinson T,
and
Barrett P.
Hypothalamic NPY and preproNPY mRNA in Djungarian hamsters: effects of food deprivation and photoperiod.
Am J Physiol Regulatory Integrative Comp Physiol
269:
R1099-R1106,
1995.
50.
Mercer, JG,
Moar KM,
Logie TJ,
Findlay PA,
Adam CL,
and
Morgan PJ.
Seasonally-inappropriate body weight induced by food restriction: effect on hypothalamic gene expression in male Siberian hamsters.
Endocrinology
142:
4173-4181,
2001.
51.
Mercer, JG,
Moar KM,
Ross AW,
and
Morgan PJ.
Regulation of leptin receptor, POMC and AGRP gene expression by photoperiod and food deprivation in the hypothalamic arcuate nucleus of the male Siberian hamster.
Appetite
34:
109-111,
2000.
52.
Neilsen, DA,
and
Shapiro DJ.
Preparation of capped RNA transcripts using T7 RNA-polymerase (Abstract).
Nucleic Acids Res
14:
5936,
1986.
53.
Ollmann, MM,
Wilson BD,
Yang YK,
Kerns JA,
Chen Y,
Gantz I,
and
Barsch GS.
Antagonism of central melanocortin receptors in vitro and in vivo by agouti-related protein.
Science
278:
135-138,
1997.
54.
Pape, JR,
and
Tramu G.
Suckling-induced changes in neuropeptide Y and proopiomelanocortin gene expression in the arcuate nucleus of the rat: evaluation of a putative intervention of prolactin.
Neuroendocrinology
63:
540-549,
1996.
55.
Pelletier, G,
and
Tong Y.
Lactation but not prolactin increases the levels of pre-proNPY mRNA in the rat arcuate nucleus.
Mol Cell Neurosci
3:
286-290,
1992.
56.
Pickavance, L,
Tadayyon M,
Williams G,
and
Vernon RG.
Lactation suppresses diurnal rhythm of serum leptin.
Biochem Biophys Res Commun
248:
196-199,
1998.
57.
Robertson, JP,
Faulkner A,
and
Vernon RG.
Regulation of glycolysis and fatty acid synthesis from glucose in sheep adipose tissue.
Biochem J
206:
577-586,
1982.
58.
Robinson, JJ,
McDonald I,
Fraser C,
and
Gordon JG.
Studies on reproduction in prolific ewes. The efficiency of energy utilization for conceptus growth.
J Agric Sci
94:
331-338,
1980.
59.
Robinson, JJ,
McDonald I,
McHattie I,
and
Pennie K.
Studies on reproduction in prolific ewes. Sequential changes in the maternal body during pregnancy.
J Agric Sci
91:
291-304,
1978.
60.
Robinson, JJ,
McHattie I,
Calderon Cortes JF,
and
Thompson JL.
Further studies on the response of lactating ewes to dietary protein.
Anim Prod
29:
257-269,
1979.
61.
Rubio-Aliaga, I,
and
Trayhurn P.
Expression of the ob (obese) gene during lactation in mice (Abstract).
Biochem Soc Trans
24:
157S,
1996.
62.
Schwartz, MW,
Woods SC,
Porte D, Jr,
Seeley RJ,
and
Baskin DG.
Central nervous system control of food intake.
Nature
404:
661-671,
2000.
63.
Smith, MS.
Lactation alters neuropeptide Y and proopiomelanocortin gene expression in the arcuate nucleus of the rat.
Endocrinology
133:
1258-1265,
1993.
64.
Spiegelman, BM,
and
Flier JS.
Obesity and the regulation of energy balance.
Cell
104:
531-543,
2001.
65.
Terada, Y,
Yamakawa K,
Sugaya A,
and
Toyoda N.
Serum leptin levels do not rise during pregnancy in age-matched rats.
Biochem Biophys Res Commun
253:
841-844,
1998.
66.
Vernon, RG,
Clegg RA,
and
Flint DJ.
Metabolism of sheep adipose tissue during pregnancy and lactation; adaptation and regulation.
Biochem J
200:
307-314,
1981.
67.
Vernon, RG,
and
Finley E.
Roles of insulin and growth hormone in the adaptations of fatty acid synthesis in white adipose tissue during the lactation cycle in sheep.
Biochem J
256:
873-878,
1988.
68.
Vernon, RG,
and
Houseknecht KL.
Adipose tissue: beyond an energy reserve.
In: Ruminant Physiology: Digestion, Metabolism, Growth and Reproduction, edited by Cronjé PB.. Wallingford, UK: CABI, 2000, p. 171-186.
69.
Vernon, RG,
and
Pond CM.
Adaptations of maternal adipose tissue to lactation.
J Mammary Gland Biol Neo
2:
231-241,
1997.
70.
Wade, GN,
and
Schneider JE.
Metabolic fuels and reproduction in female mammals.
Neurosci Behav Rev
16:
235-275,
1992.
71.
Wilding, JPH,
Ajala MO,
Lambert PD,
and
Bloom SR.
Additive effect of lactation and food restriction to increase hypothalamic neuropeptide Y mRNA in rats.
J Endocrinol
152:
365-369,
1997.
72.
Williams, G,
Harrold JA,
and
Cutler DJ.
The hypothalamus and the regulation of energy homeostasis: lifting the lid on a black box.
Proc Nutr Soc
59:
385-396,
2000.
73.
Williams, LM,
Adam CL,
Mercer JG,
Moar KM,
Slater D,
Hunter L,
Findlay PA,
and
Hoggard N.
Leptin receptor and neuropeptide Y gene expression in the sheep brain.
J Neuroendocrinol
11:
165-169,
1999.
74.
Woodside, B,
Abizaid A,
and
Walker CD.
Changes in leptin levels during lacation: implications for lactational hyperphagia and anovulation.
Horm Behav
37:
353-365,
2000.
75.
Zhang, Y,
Proenca R,
Barone M,
Leopold L,
and
Friedman JM.
Positional cloning of the mouse obese gene and its human homologue.
Nature
372:
425-432,
1994.
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) or day
18 (
) when the animals were killed. Animals were
offered hay saver pellets ad libitum plus 1,200 g (lactating ewes) or
500 g (control ewes) proprietary sheep concentrates given daily in
two meals at 0700 and 1600. Values are means ± SE of 8 observations.
) and day 6 (
