www.elsevier.com / locate / bres
Research report
Localization of long-form leptin receptor in the
somatostatin-containing neurons in the sheep hypothalamus
a a b a ,
*
Javed Iqbal , Sueli Pompolo , Takashi Murakami , Iain J. Clarke
a
Prince Henry’s Institute of Medical Research (J.I., S.P., I.J.C), P.O. Box 5152, Clayton, Victoria-3168, Australia
b
Department of Laboratory Medicine(T.M.), University of Tokushima, Tokushima 770-8503, Japan
Accepted 22 August 2000
Abstract
Reduction in the adiposity or dietary restriction increases plasma growth hormone (GH) concentrations, and in sheep this appears to be due, at least in part, to a reduction in the concentrations of somatostatin (SRIF) in hypophyseal portal blood. Leptin is a hormone secreted by the adipocytes and it is possible that the effects of altered adiposity or fasting on GH secretion could be due to regulation of SRIF neurons by leptin. To ascertain the extent to which leptin may act on these neurons, we have used immunohistochemistry to examine co-localization of long-form of the leptin receptor (OB-Rb) and SRIF in the sheep hypothalamus. In the hypothalamic periventricular area (PeriV), 44.5610% of SRIF cells were found to co-stain for OB-Rb. In the dorsomedial hypothalamic, ventromedial hypothalamic and arcuate nuclei, 100% of SRIF immunoreactive neurons expressed OB-Rb. These findings provide a basis for the direct action of leptin on SRIF neurons. Thus, it is possible that leptin stimulates the secretion of SRIF in relatively obese individuals. The significance of the lower number of SRIF cells in the PeriV co-localizing OB-Rb expression is not clear at present. 2000 Elsevier Science B.V. All rights reserved.
Keywords: Growth hormone; Obesity; Immunohistochemistry; Somatostatin; Hypothalamus
1. Introduction strated inhibitory effects of leptin on SRIF mRNA levels and SRIF secretion [24], but no in vivo studies have been
Food restriction or fasting leads to elevated plasma conducted in this regard. The central modulatory actions of
levels of GH in man, pig, cow, and sheep [2,3,5,19,29,30], leptin are thought to be mediated via leptin receptors,
whereas the reverse is true in rodents [1,8]. Studies in this which are expressed by various neuropeptidergic neurons
laboratory have demonstrated that food restriction in- [11–13,22] localized in different hypothalamic nuclear
creases plasma GH concentrations in female sheep groups. The leptin receptor has widespread distribution
[3,29,30] in association with a reduction in the secretion of throughout the brain [6,14]. Using immunohistochemistry,
somatostatin (SRIF) into the hypophyseal portal blood Hakansson et al. [14] have reported co-staining for SRIF
[30]. The mechanism whereby an effect on SRIF secretion and leptin receptor in the rat hypothalamus, but the degree
is effected in vivo is not known, but it could be related to of co-localization is not known for any species.
Further-alteration in plasma levels of leptin, which are reduced more, it is not known whether the SRIF neurons possess
with decreasing adiposity [17]. Leptin is a secreted product the short (OR-Ra) or long form (OR-Rb) of leptin receptor.
of the adipocytes, and is involved in the regulation of OB-Rb is considered to be the major signaling form of the
energy homeostasis and neuroendocrine function receptor, activating the JAK / STAT pathway [4,11,12]. On
[7,15,23,31]. the other hand OB-Ra is unable to activate the JAK / STAT
An in vitro study using rat hypothalami has demon- pathway [4,11,12] and is thought to be involved in the
transport of leptin across the blood brain barrier [28]. The aim of the present study was to quantify the extent
*Corresponding author. Tel.: 161-39-594-4387; fax: 1
61-39-594-of co-localization 61-39-594-of OB-Rb to SRIF neurons in the sheep
6125.
E-mail address: iain.clarke@med.monash.edu.au (I.J. Clarke). hypothalamus. To this end, we used dual-labelling
2 J. Iqbal et al. / Brain Research 887 (2000) 1 –6
immunofluorescence histochemistry and guinea pig poly- incubated with guinea pig polyclonal antiserum specific for
clonal antibody, which specifically recognizes the long OB-Rb (1:1000) for 48 h at 48C. After washing, sections
isoform of the leptin receptor [21]. were incubated with biotinylated goat anti-guinea pig IgG
(1: 250, Vector) in PB for 1 h at 32 E C, washed and then reacted with Avidin-FITC conjugate (1:500, Pierce,
Rock-2. Materials and methods ford, IL) for 1 h at 328C. For double labelling, the sections were blocked again with normal goat serum and were then
2.1. Animals and tissue collection incubated with a rabbit polyclonal anti-SRIF antiserum (1:
2500; kind gift from Dr. J.R Oliver, Flinders University,
All procedures and tissue collections were carried out Adelaide, South Australia) overnight at 48C. The sections
with the prior approval of the Animal Experimentation were washed and incubated in goat anti-rabbit Alexa 546
Ethics Committee of Monash University and Victoria (1:500, Molecular Probes Inc., Eugene, OR) for 1 h. The
Institute of Animal Science. sections were washed and counter-stained with 1% Sudan
Four adult Corriedale castrated (male) sheep of similar Black to block non-specific autofluorescence, washed,
age and weight were used and the tissue was collected as mounted and cover-slipped with antifade medium (Dako
previously reported [20]. Briefly, the sheep were adminis- Corp., Australia). Specificity of the immunostaining was
tered 25 000 IU of heparin iv and killed 5 min later by an confirmed using preabsorption of antiserum with the
over-dose of sodium pentobarbital (Lethabarb; May & antigen, preimmune guinea pig serum and omission of the
Baker Pty Ltd, Melbourne, Australia). The heads were primary antiserum from the incubating medium as
de-flushed via the carotid arteries with 2 liters of heparinized scribed elsewhere [21]. Use and specificity of the
anti-OB-(12,500 IU / litre) 0.9% saline followed by 2 litres of Rb and anti-SRIF antisera has been reported previously
Zamboni’s fixative in 0.1 M phosphate buffer (PB, pH 7.4) [21,32].
and 1.5 liters of Zamboni’s fixative containing 20% sucrose. The hypothalamic blocks were cut from the brains
2.3. Tissue analysis and post-fixed in fixative containing 30% sucrose for 24 h
and transferred to 30% sucrose for an additional 4 days at
Tissue sections were viewed using an Olympus BMX 50
48C. The blocks were frozen on powdered dry ice and kept
microscope attached with mercury light source and
Texas-at2208C until used. Coronal sections of 20mm were cut
red and FITC band filter systems. The immunostaining on a cryostat and stored in cryoprotectant solution until
with OB-Rb antiserum (green) and / or SRIF antiserum used.
(red) was visualized in individual cells by switching between the Texas-red and FITC filter systems. To esti-2.2. Immunohistochemical procedures
mate degree of co-localization, clearly dual-labelled cells were visually counted for each region for each animal and Four representative sections from the rostral to caudal
presented as mean (6S.E.M.)% of SRIF cells co-staining
limits of the hypothalamic periventricular area
(periven-for OB-Rb. tricular nucleus; Pe, paraventricular hypothalamic nucleus;
PVN, and anterior hypothalamic area; AHA), dorsomedial hypothalamic nucleus (DMH), ventromedial hypothalamic
nucleus (VMH) and arcuate nucleus (ARC) from each 3. Photography
animal were processed for dual-labelling
immunofluores-cence immunostaining using the free-floating technique Photographs were produced by capturing the images
[20]. Briefly, the sections were washed in 0.05 M phos- using a digital camera (Fuji3HC–2000) directly mounted
phate buffered saline (PBS) and treated with 1% sodium to the microscope and using Analytical Imaging Station
borohydride (Sigma-Aldrich Corp., Australia) in 0.01 M (AIS; Imaging Research, Inc.) software. The images were
phosphate buffer (PB) for 20 min. After blocking (10% combined into a plate using Adobe Photoshop 5.2 software
normal goat serum, 0.3% Triton-x 100 and 1% bovine and printed on Epson Stylus Photo 700 printer. In all cases
serum albumin in PB) for 30 min, the sections were only brightness and contrast were adjusted.
4 J. Iqbal et al. / Brain Research 887 (2000) 1 –6
4. Results a very large secretory pulse of GH [2]. We have found, however, icv infusion of leptin over 3 days either does not
OB-Rb immunoreactivity was widespread throughout affect GH levels [18] or can have a small stimulatory
the hypothalamus of the sheep with a variable intensity of effect [16]. The stimulation could be effected either by the
immunostaining within different regions and nuclear release of GHRH or the inhibition of secretion of SRIF.
groups (Fig. 1A, C, E, G). No specific OB-Rb immuno- Thus localization of OB-Rb in the SRIF neurons in the
staining was seen after the preabsorption of either an- present study and the earlier results [30] offers support for
tiserum with the relevant antigenic peptide; blocking of the latter.
OB-Rb immunostaining is seen in Fig. 1B. No staining We have also provided evidence that 70% of pituitary
was seen with preimmune guinea pig serum or following somatotropes express the leptin receptor [20], so there is
omission of primary antiserum from the incubating likely to be a dual-level regulation of GH secretion by
medium (data not shown). All autofluorescence was com- leptin. At the level of the pituitary, it was found that leptin
pletely blocked after counter-staining of the tissue with inhibited the GHRH stimulated secretion of GH in sheep
Sudan Black B. pituitary cells in vitro [26], which would act against a
The distribution of SRIF immunoreactive cells and mechanism whereby leptin reduced the secretion of SRIF
fibers was essentially similar to that reported earlier by (predisposing to an increase in GH secretion). These latter
Willoughby et al. [32]. The examples of immunostaining studies were, however, conducted in the absence of SRIF
and co-localization of OB-Rb and SRIF are shown in Fig. and it is possible that a different result would be obtained
1(C–H). in vitro with varying levels of GHRH and SRIF. In both
In the hypothalamic periventricular area (PeriV) includ- normally fed and starved rats, leptin stimulates the GH
ing Pe, AHA and PVN (parvicellular division), 44.5610.0 secretion [8,27], which could be partly due to the
inhib-of SRIF positive cells co-stained for OB-Rb. In contrast, itory effects of leptin on the SRIF secretion as shown by in
100% of SRIF–immunopositive cells co-localized OB-Rb vitro studies [24]. The present study provides an evidence
immunoreactivity in the DMH, VMH, and ARC nuclei. that leptin may stimulate SRIF to reduce GH secretion in
the fat animal. This concept is in accord with the recog-nized lipolytic actions of GH [8,9]; thus elevated GH
5. Discussion levels in the starved animals would favour enhanced lipolysis, providing more fuel to the metabolic functions
This study demonstrates that SRIF neurons in the sheep and decrease hunger. The possibility that leptin acts on
hypothalamus contain OB-Rb, but there is a difference in GHRH neurons to regulate GH secretion remains to be
the number of cells that expressing the receptor in the investigated fully, although Hakansson et al. [14] have
PeriV compared to other nuclei in which SRIF-containing reported that at least some of this cell type contain leptin
cells are found. The difference in the number of OB-Rb receptors in the rat hypothalamus.
immunoreactive cells and levels of expression suggest that Only a sub-population of SRIF neurons (45%) contained
leptin may have varying effects on metabolic and endo- OB-Rb immunoreactivity in the PeriV, the exact
signifi-crine functions depending on the localization within the cance of low degree of co-localization being unclear at this
hypothalamus. The variable levels of leptin receptor im- stage. It is possible that only a subset of the SRIF neurons
munostaining within different hypothalamic regions and in the PeriV project to the median eminence and that those
nuclear groups also has been reported for the rat but never which express OB-Rb are involved in the regulation of GH
quantified [14]. secretion. On the other hand, those which are devoid of
The leptin receptor has widespread distribution through- OB-Rb immunoreactivity may be involved in a multitude
out the brain [6,14] but it exists as multiple splice variants of other functions since PeriV neurons project to the ARC
that differ in the length of the intracellular domain [28]. and other regions [25]. The situation could be clarified by
OB-Rb is considered to be the form of the receptor that determining the extent of projections from the PeriV to the
signals via the JAK / STAT pathway [4,11,12]. Thus, median eminence by retrograde tracing, predicating their
presence of OB-Rb within the neurons is indicative of a hypophysiotrophic function.
potential regulatory function. The present study therefore A high number of SRIF immunoreactive neurons are
suggests that all SRIF neurons in the basal hypothalamus localized in the VMH with moderate numbers in the DMH
and a smaller proportion of those in PeriV are direct targets and a low number in the ARC in the sheep ([32], present
for leptin. study). The results of the present study demonstrate that
GH secretion is under dual regulatory control by growth 100% of the SRIF neurons in these areas express OB-Rb.
hormone releasing hormone (GHRH) and SRIF, but only These nuclei are considered to be important feeding
the secretion of the latter is altered with changing adiposity centers and leptin may have varying effects on metabolic
in the sheep [30]. Leptin has been shown to modulate GH and endocrine functions depending on localization of cells
secretion, although the exact mechanism is not clear. In the which it targets. GHRH and SRIF, in addition to the
[11] N. Ghilardi, R.C. Skoda, The leptin receptor activates janus kinase 2
regulation of feeding (reviewed in [10]). Colocalization of
and signals for proliferation in a factor dependent cell line, Mol.
SRIF and OB-Rb in the feeding centers further suggests a
Endocrinol. 11 (1997) 393–399.
role for SRIF in this regard. [12] N. Ghilardi, S. Ziegler, A. Wiestner, R. Stoffel, M.H. Heim, R.C.
In summary, the present study demonstrates expression Skoda, Defective STAT signaling by the leptin receptor in diabetic
of OB-Rb in the SRIF-containing neurons of the sheep mice, Proc. Natl. Acad. Sci. USA 93 (1996) 6231–6235.
[13] S.R. Glaum, M. Hara, V.P. Bindokas, C.C. Lee, K.S. Polonksy, G.I.
hypothalamus, providing evidence that leptin may have
Bell, R.J. Miller, Leptin, the obese gene product, rapidly modulates
direct action on this cell type. This could have implications
synaptic transmission in the hypothalamus, Mol. Pharmacol. 50
for the regulation of GH secretion and some of the SRIF- (1996) 230–235.
containing cells in the feeding centers could also be [14] M.L. Hakansson, H. Brown, N. Ghilardi, R.C. Skoda, B. Miester,
involved in the regulation of feeding behavior / satiety and Leptin receptor immunoreactivity in chemically defined target
neurones of the hypothalamus, J. Neurosci. 18 (1998) 559–572.
other endocrine functions.
[15] J. Halaas, J.M. Friedman, Leptin and the regulation of body weight in mammals, Nature 395 (1998) 763–770.
[16] B.A. Henry, J.W. Goding, A.J. Tilbrook, F.R. Dunshea, A. Rao, I.J.
Acknowledgements Clarke, Intracerebroventricular infusion of leptin restores the secre-tion of luteinizing hormone with out affecting food intake in Thin animals, but increases growth hormone irrespective of bodyweight,
We thank Mr. Bruce Doughton and Ms. Karen Perkins
J. Endocrinology, in press.
for animal care and Dr. J.R. Oliver for SRIF antiserum. We
[17] B.A. Henry, A.J. Tilbrook, F.R. Dunshea, A. Rao, D. Blache, G.B.
also appreciate the technical assistance of Sue Pankridge in Martin, I.J. Clarke, Long term alterations in the adiposity affect the
preparing the figures. Supported by the National Health expression of melanin-concentrating hormone and enkephalin but
not proopiomelanocortin in the hypothalamus of ovariectomized
and Medical Research Council of Australia.
ewes, Endocrinology 141 (2000) 1506–1514.
[18] B.A. Henry, J.W. Goding, W.S. Alexander, A.J. Tilbrook, B.J. Canny, F.R. Dunshea, A. Rao, A. Mansell, I.J. Clarke, Central
References administration of leptin to ovariectomized ewes inhibits food intake
without affecting the secretion of hormones from the pituitary gland: evidence for a dissociation of effects on appetite and neuroendocrine [1] S. Ahima, D. Prabakaran, C. Mantzoros, D. Qu, B. Lowell, E.
function, Endocrinology 140 (1999) 1175–1182. Maratos-Flier, J.S. Flier, Role of leptin in the neuroendocrine
[19] K.Y. Ho, J.S. Veldhius, M.L. Johnson, R. Furlonetto, W.S. Evans, response to fasting, Nature 382 (1996) 250–252.
K.G. Alberite, M.O. Thorner, Fasting enhances growth hormone [2] C.R. Barb, X. Yan, M.J. Azin, P.R. Kraeling, G.B. Rampacek, T.G.
secretion and amplifies the complex rhythms of growth hormone Ramsay, Recombinant porcine leptin reduces feed intake and
secretion in man, J. Clin. Invest. 81 (1988) 968–975. stimulates growth hormone secretion in swine, Dom. Anim.
Endo-[20] J. Iqbal, S. Pompolo, R.V. Considine, I.J. Clarke, Localization of crinol. 15 (1998) 77–86.
leptin receptor-like immunoreactivity in the corticotropes, somatot-[3] M.L. Barker-Gibb, I.J. Clarke, Increased galanin and neuropeptide Y
ropes, and gonadotropes in the ovine anterior pituitary, Endocrinolo-imunoreactivity within the hypothalamus of ovariectomized ewes
gy 141 (2000) 1515–1520. following a prolonged period of reduced body weight correlates with
[21] J. Matsuda, I. Yokota, Y. Tsuruo, T. Murakami, K. Ishimura, K. changes in plasma growth hormone levels but not gonadotropin
Shima, Y. Kuroda, Developmental changes in long-form leptin levels, Neuroendocrinology 64 (1996) 194–207.
receptor expression and colocalization in rat brain, Endocrinology [4] H. Baumann, K.K. Morella, D.W. White, M. Dembski, P.S. Bailon,
140 (1999) 5233–5238. H.K. Kim, C.F. Lai, L.A. Tartaglia, The full-length leptin receptor
[22] K.C. McCowen, J.C. Chow, R.J. Smith, Leptin signaling in the has signaling capabilities of interleukin 6-type cytokine receptor,
hypothalamus of normal rats in vivo, Endocrinology 139 (1998) Proc. Natl. Acad. Sci. USA 93 (1996) 8374–8378.
4442–4447. [5] J.W. Blum, W. Schnyder, P.L. Kunz, A.K. Blom, M.H. Bickel, A.
[23] M.A. Pelleymounter, M.J. Cullen, M.B. Baker, R. Hecht, D. Schurch, Reduced and compensatory growth: endocrine and
meta-Winters, T. Boone, F. Collins, Effects of the obese gene product on bolic changes during food restriction and refeeding in steers, J. Nutr.
body weight regulation in ob /ob mice, Science 269 (1995) 540– 115 (1985) 417–424.
543. [6] B. Burguera, M.E. Couce, J. Long, J. Lamsam, K. Laakso, M.D.
[24] M. Quintela, R. Senaris, M.L. Heiman, F.F. Casanueva, C. Dieguez, Jensen, J.E. Parisi, R.V. Lloyd, The long form of the leptin receptor
Leptin inhibits in vitro hypothalamic somatostatin secretion and (OB-Rb) is widely expressed in the human brain,
Neuroendocrinolo-somatostatin mRNA levels, Endocrinology 138 (1997) 5641–5644. gy 61 (2000) 187–195.
[25] V. Rettori, L. Melenkovic, M.C. Aguila, S.M. McCann, Physiologi-[7] L.A. Campfield, F.J. Smith, Y. Guisez, R. Devos, P. Burn,
Recombi-cally significant effect of neuropeptide Y to suppress growth nant mouse OB protein: evidence for peripheral signal linking
hormone release by stimulating somatostatin discharge, Endocrinol-adiposity and central neural networks, Science 269 (1995) 546–549.
ogy 126 (1990) 2296–3001. [8] E. Carro, R. Senaris, R.V. Considine, F.F. Casanueva, C. Dieguez,
[26] S.G. Roh, I.J. Clarke, R.W. Xu, J.W. Goding, K. Loneragan, C. Regulation of in vivo growth hormone secretion by leptin,
Endo-Chen, The in vitro effects of leptin on the basal and growth crinology 138 (1997) 2203–2206.
hormone-releasing hormone-stimulated growth hormone secretion [9] N.J. Fain, J.H. Ihle, S.W. Bahouth, Stimulation of lipolysis but not of
from the ovine pituitary gland, Neuroendocrinology 68 (1998) leptin release by growth hormone is abolished in adipose tissue from
361–364. Stat5a and b knockout mice, Biochem Biophys. Res. Commun. 263
[27] G.S. Tannenbaum, W. Gurd, M. Lapointe, Leptin is a potent (1999) 201–205.
stimulator of spontaneous pulsatile growth hormone (GH) secretion [10] D. Feifel, F.J. Vaccarino, Growth hormone-releasing peptides
and the GH response to GH-releasing hormone, Endocrinology 139 (GHRH and somatostatin) and feeding: a model for the integration
(1998) 3871–3875. of central and peripheral function, Neurosci. Biobehav. Rev. 18
6 J. Iqbal et al. / Brain Research 887 (2000) 1 –6
Devos, G.J. Richards, L.A. Campfield, F.T. Clark, M.C. Deeds, S. McCloud, I.J. Clarke, Effect of restricted feeding on the relationship Sanker, A. Moriarty, K.J. Moore, J.S. Smutko, G.G. Mays, E.A. between hypophyseal portal concentrations of growth hormone Woolf, C.A. Monroe, R.I. Tepper, Identification and expression (GH)-releasing factor and somatostatin, and jugular concentrations cloning of a leptin receptor OB-R, Cell 83 (1995) 1263–1271. of GH in ovariectomized ewes, Endocrinology 128 (1991) 1151– [29] G.B. Thomas, J.E. Mercer, T. Karalis, A. Rao, J.T. Cummins, I.J. 1158.
Clarke, Effects of restricted feeding on the concentration of growth [31] W.H. Yu, M. Kimura, A. Walczewska, S. Karanth, S.M. McCann, hormone (GH) gonadotropin and prolactin (PRL) in plasma and on Role of leptin in hypothalamic-pituitary function, Proc. Natl. Acad. the amounts of messenger ribonucleic acid concentration for GH, Sci. USA 94 (1997) 1023–1028.
