Chronic central leptin infusion enhances insulin-stimulated glucose metabolism and favors the expression of uncoupling proteins

Abstract

Continuous (4 days) intracerebroventricular leptin infusion (12 microg/day) was performed in lean rats, and its hormonometabolic effects were determined. Intracerebroventricular leptin administration did not result in leakage of the hormone into the peripheral circulation. Thus, its effects were elicited by its presence within the central nervous system. Intracerebroventricular leptin infusion produced marked decreases in food intake and body weight gain relative to vehicle-infused fed ad libitum rats. Because decreases in food intake alter hormonometabolic homeostasis, additional control rats pair-fed to the amount of food consumed by leptin-infused ones were included in the study. Intracerebroventricular leptin-infused and vehicle-infused pair-fed rats were characterized, relative to vehicle-infused ad libitum-fed animals, by decreases in body weight and insulinemia and by increases in insulin-stimulated overall glucose utilization and muscle and brown adipose tissue glucose utilization index. Brown adipose tissue uncoupling protein (UCP)1, UCP2, and UCP3 mRNA levels were markedly decreased in pair-fed animals relative to those of fed ad libitum control animals, as were liver and white adipose tissue UCP2 and muscle UCP3 mRNA levels. In marked contrast, intracerebroventricular leptin administration was accompanied by the maintenance of high UCP1, UCP2, and UCP3 expression in all these tissues. Thus, despite analogies between leptin's effects and those of pair-feeding with regard to glucose handling, their respective underlying mechanisms differ. While leptin maintains or favors energy-dissipating mechanisms (UCP1, UCP2, and UCP3), the latter are markedly depressed in pair-fed rats. This effect of leptin may prevent subsequent excessive storage processes, thereby maintaining normal body homeostasis.

Full text

1014 DIABETES, VOL. 47, JULY 1998
Chronic Central Leptin Infusion Enhances
Insulin-Stimulated Glucose Metabolism and Favors
the Expression of Uncoupling Proteins
Isabelle Cusin, Katerina E. Zakrzewska, Olivier Boss, Patrick Muzzin, Jean-Paul Giacobino,
Daniel Ricquier, Bernard Jeanrenaud, and Françoise Rohner- J e a n r e n a u d
Continuous (4 days) intracerebroventricular leptin infu-
sion (12 µg/day) was performed in lean rats, and its hor-
monometabolic effects were determined. Intracere-
broventricular leptin administration did not result in
leakage of the hormone into the peripheral circulation.
Thus, its effects were elicited by its presence within the
central nervous system. Intracerebroventricular leptin
infusion produced marked decreases in food intake and
body weight gain relative to vehicle-infused fed ad libi-
tum rats. Because decreases in food intake alter hor-
monometabolic homeostasis, additional control rats
p a i r-fed to the amount of food consumed by leptin-
infused ones were included in the study. Intracere-
broventricular leptin-infused and vehicle-infused pair- f e d
rats were characterized, relative to vehicle-infused ad
libitum–fed animals, by decreases in body weight and
insulinemia and by increases in insulin-stimulated over-
all glucose utilization and muscle and brown adipose tis-
sue glucose utilization index. Brown adipose tissue
uncoupling protein (UCP)1, UCP2, and UCP3 mRNA lev-
els were markedly decreased in pair-fed animals rela-
tive to those of fed ad libitum control animals, as were
liver and white adipose tissue UCP2 and muscle UCP3
mRNA levels. In marked contrast, intracerebroventricu-
lar leptin administration was accompanied by the main-
tenance of high UCP1, UCP2, and UCP3 expression in all
these tissues. Thus, despite analogies between leptin’s
effects and those of pair-feeding with regard to glucose
handling, their respective underlying mechanisms dif-
fer. While leptin maintains or favors energy-dissipating
mechanisms (UCP1, UCP2, and UCP3), the latter are
markedly depressed in pair-fed rats. This effect of leptin
may prevent subsequent excessive storage processes,
thereby maintaining normal body homeostasis. D i a b e t e s
47:1014–1019, 1998
L
eptin is a hormone produced by adipose tissue (1)
that, after its release into the blood, acts within
the hypothalamic area where specific long-form
leptin receptors Ob-Rb are located (2,3). After
binding to the receptors, leptin ultimately decreases food
intake and presumably increases energy dissipation (4–7).
Besides these effects, leptin has been reported to have direct
ones on peripheral tissues. In isolated white adipocytes, it has
been shown to decrease insulin-induced glucose uptake and
lipogenic activity, while increasing lipolysis (8). More gener-
a l l y, leptin has been shown, in vivo and in vitro, to markedly
deplete fat content of both adipocytes and non-adipocytes
(i.e., muscles and liver) (9). In contrast, leptin was found to
have insulin-like effects on glucose uptake and glycogen syn-
thesis in myotubes, without affecting the insulin action per se
(10). Leptin was also reported not to have any effect on basal
or insulin-stimulated glycogen synthesis in isolated soleus
muscle of wild-type mice, while it inhibited these processes
in muscles of o b / o b mice (11).
Because of the observation that the main action of leptin is
to decrease food intake and presumably increase energy
expenditure by acting within the hypothalamus, the purpose
of the present experiments was to determine whether hor-
monometabolic changes as well as alterations in the expres-
sion of different uncoupling proteins (UCPs) could be elicited
by the presence of leptin within the central nervous system.
To do this, leptin was continuously infused intracerebroven-
tricularly for 4 days, and insulin-stimulated glucose metabo-
lism, as well as the expression of UCP1, UCP2, and UCP3 in
different tissues, was measured. It was hypothesized that lep-
tin could influence carbohydrate metabolism as well as the
expression of not only UCP1 in brown adipose tissue (BAT )
(12) but also UCP2 (13) and UCP3 (14). Indeed, all such pro-
teins, via their properties of generating heat, may be implicated
in the regulation of body temperature and body weight. Fur-
thermore, the presence of UCP2 in tissues other than the
rodent BAT (e.g., in white adipose tissue [WAT], lung, kidney,
and liver) (13) and the presence of UCP3 in BAT as well as in
skeletal muscles, including human muscles (14), make UCP2
and UCP3 potential candidates for regulating body weight in
large mammals, which usually lack BAT UCP1.
RESEARCH DESIGN AND METHODS
A n i m a l s .Heterozygote lean (f a / f a) male rats of the Zucker strain from our breed-
ing colony, weighing about 220 g, were housed in individual cages under condi-
tions of controlled temperature (23°C) and illumination (7:00 A.M. –7:00 P.M. ) .
From the Laboratoires de Recherches Métaboliques (I.C., K.E.Z., B.J., F. R . - J . )
and the Département de Biochimie Médicale (O.B., P.M., J.-P.G.), Geneva Uni-
v e r s i t y, Geneva, Switzerland; and Centre National de la Recherche Scien-
tifique/CEREMOD (D.R.), Meudon, France.
Address correspondence and reprint requests to Dr. Isabelle Cusin, Lab-
oratoires de Recherches Métaboliques, 24 Rue Micheli-du-Crest, 1211
Geneva 14, Switzerland.
Received for publication 5 December 1997 and accepted in revised form
19 March 1998.
B AT, brown adipose tissue; HGP, hepatic glucose production; R
d
, rate of
glucose disappearance; RG, red gastrocnemius; RQ, red quadriceps; UCP,
uncoupling protein; WAT, white adipose tissue; WG, white gastrocnemius;
WQ, white quadriceps.

DIABETES, VOL. 47, JULY 1998 1015
I. CUSIN AND ASSOCIATES
Although theseanimals are heterozygous for a mutation of the leptin receptor gene
(f a), they represent an adequate model of lean rats. Indeed, it has recently been
demonstrated that the normal leptin receptor functions as a dominant negative
toward the mutated f areceptor (15). They were allowed ad libitum access to water
and standard laboratory food (Provimi Lacta, Cossonay, Switzerland) unless oth-
erwise stated. Food intake and body weight were measured daily.
Chronic intracerebroventricularinfusion of leptin. Rats were anesthetized
with intramuscular ketamin/xylasine used at 45 mg/kg and 9 mg/kg, respectively
(Parke-Davis and Bayer, Leverkusen, Switzerland), and equipped with a cannula
positioned in the right lateral ventricle. After 1 week of recovery, osmotic
minipumps (model 2001; Alza, Palo Alto, CA) delivering 12 µg of leptin (recom-
binant mouse leptin provided by Novartis, Basel, Switzerland) per day for 4
days or its vehicle (Tris 0.1 mol/l, pH 9) were connected to the intracere-
broventricular infusion cannula via a polyethylene catheter under ether anesthesia
(16). Three groups of rats were investigated: 1) rats infused with leptin; 2) con-
trol rats infused with the vehicle and allowed to eat ad libitum; and 3) control
rats infused with the vehicle but pair-fed to the amount of food consumed by lep-
tin-infused animals. The pair-feeding regimen was performed as follows: average
daily food intake for the leptin-treated group was calculated; one-third of this
amount of food was given in the morning (8:00 A.M.), while the remaining two-
thirds were given before the extinction of the light (6:00 P.M.), based on a pre-
liminary study of food consumption during the day and the night. To ensure that
leptin was present only within the cerebral ventricles, plasma leptin levels were
measured after intracerebroventricular application.
Measurement of in vivo glucose utilization during euglycemic-hyperin-
sulinemic clamps. After 4 days of intracerebroventricular leptin infusion, 5-h
fasted rats were anesthetized with sodium pentobarbital (55 mg/kg i.p.), and
euglycemic-hyperinsulinemic clamps were performed as previously described (17).
Mean steady-state values of insulinemia and glycemia during the clamps were,
r e s p e c t i v e l y, 3,480 ± 135 pmol/l and 6.6 ± 0.1 mmol/l. These insulinemia values
enabled study of the half-maximal stimulation of glucose utilization by insulin (17).
The in vivo insulin-stimulated glucose utilization index by individual tissues was
measured during euglycemic-hyperinsulinemic clamps associated with the
labeled 2-deoxy-D-glucose technique (2-deoxy-D- [ 1 -
3
H]glucose; Amersham, Ay l e s-
b u r y, U.K.), as previously described and validated (18,19). Different fib e r-type (red
or white) muscles, such as white quadriceps (WQ), red gastrocnemius (RG), and
white gastrocnemius (WG), and epididymal WAT and BAT were tested. Hepatic
glucose production (HGP) and total glucose metabolism (rate of glucose disap-
pearance [R
d
]) were measured in basal and insulin-stimulated states during the
clamps by infusing D- [ U -
1 4
C]glucose (50 µCi/rat; Amersham), as previously
described (17).
Analytical procedures relative to clamp studies. 2 - D e o x y -D- [ 1 -
3
H]glucose and
D- [ U -
1 4
C ] g l u c o s e - s p e c i fic activities were determined in deproteinized blood sam-
ples as described elsewhere (20). 2-Deoxy-D- [ 1 -
3
H]glucose-6-phosphate, determined
in tissues as described previously, allowed us to calculate the in vivo glucose uti-
lization index by individual tissues and was expressed in nanograms per minute
per milligram tissue (20). For the measurement of in vivo insulin-stimulated over-
all glucose turnover rate, supernatant of deproteinized blood was submitted to an
ion-exchange resin (AG2X8; Bio-Rad, Richmond, CA) to avoid concomitant lac-
tate measurement.
Plasma hormones and metabolites measurements. Plasma glucose was
determined by the glucose oxidase method (Beckman Glucose Analyzer II;
Fullerton, CA). Plasma insulin and corticosterone levels were measured by
radioimmunoassays (21). Plasma leptin levels were determined using a com-
mercial kit for rat leptin having 100% cross-reactivity with mouse leptin (Linco
Research, St Louis, MO).
Northern blot. In additional experiments, the three groups of rats mentioned
above (see A n i m a l s ) were continuously infused with vehicle or leptin for six days,
and at the end of the respective infusions, tissues were removed and total RNA
was extracted (22) from WAT and BAT, as well as from liver and muscle (RQ).
Aliquots of 10 µg were size-fractionated on 1.5% agarose gels, and blots were
hybridized (Quikhyb; Stratagene) to random primed labeled cDNAs for UCP1 (12),
UCP2 (13), UCP3 (GenBank Accession U92069), and -actin (Clontech Labora-
tory) (23). Autoradiographs (X-OMAT-AR film; Kodak, Rochester, NY) were quan-
t i fied by densitometry with Image Quant Software. Abundance of UCP1, UCP2,
and UCP3 mRNA relative to that of -actin was expressed as a percentage of cor-
responding fed ad libitum vehicle-infused control animals. Only signals obtained
on the same Northern blot were compared.
Statistical analysis. For daily measurements of changes in body weight and
plasma insulin concentrations, statistical analyses were made using one-way
analysis of variance for repeated measures followed by multiple Bonferroni com-
parisons. For all other analyses, two-tailed Student’s t test for unpaired data
were used. Values of P< 0.05 were accepted as being statistically signific a n t .
R E S U LT S
Comparisons were made among three intracerebroventricu-
larly infused groups of rats: 1) vehicle-infused control rats who
had free access to laboratory food; 2) leptin-infused rats; and
3) vehicle-infused rats pair-fed to the amount of food con-
sumed by the leptin-infused group.
Impact of intracerebroventricular leptin infusion on
body weight and food intake. Leptin-infused animals had,
from experimental day 2, a significant decrease in food
intake that remained 39.5 ± 3.4% lower than that of ad libi-
tum–fed control rats throughout the study. By defin i t i o n ,
vehicle-infused pair-fed rats had the same food intake as that
of leptin-infused animals. The changes in body weight relative
to the respective initial body weight for the three groups of
rats are illustrated in Fig. 1. As can be seen, control rats
gained weight during the 4-day experimental period, while
both the leptin-infused rats and their respective pair-fed con-
trol rats lost weight during the same time. Body weight loss
was identical in leptin-infused and in pair-fed control rats
compared with ad libitum–fed control rats.
Impact of intracerebroventricular leptin infusion onh o r-
monal changes and glucose metabolism. As can be seen in
Table 1, plasma leptin levels were not significantly different in
leptin-infused rats and pair-fed control rats, but they were signi-
ficantly lower than those of ad libitum–fed control rats. This indi-
cated that the leakage of intracerebroventricular leptin into
the circulating blood was nil and that the plasma leptin levels
were correlated with body weight. Corticosteronemia and gly-
cemia were identical in the three groups of rats (Table 1).
Throughout the study, basal insulinemia was and remained
within normal values in the ad libitum–fed control group. In con-
trast, leptin-infused rats, as well as pair-fed control rats, had low
plasma insulin levels that, at the end of the experiment, were
less than half those of the ad libitum–fed animals (Table 1).
Glucose handling was assessed during euglycemic-hyper-
insulinemic clamps (Table 1). Basal R
d
was the same in the
three groups of rats. More importantly, and relative to ad
libitum–fed control rats, both the leptin-infused rats and the
respective pair-fed control rats had significantly higher rates
of insulin-stimulated R
d
during the clamps, indicating the
presence of an increased overall insulin sensitivity in these
two groups. Additional data on glucose handling, as assessed
during euglycemic-hyperinsulinemic clamps, showed that
the HGP was the same and was normally suppressed by
insulin in the three groups of rats (data not shown).
Glucose uptake by individual tissues (referred to as glucose
utilization index) was measured using euglycemic-hyperinsu-
linemic clamps associated with the labeled 2-deoxyglucose
technique. For various muscles (including white and red type
muscles), it was observed, as illustrated in Fig. 2, that com-
pared with values of insulin-stimulated glucose utilization index
obtained in ad libitum–fed control rats, those of either leptin-
infused rats or their respective pair-fed control rats were signi-
ficantly higher. An analogous pattern was observed in BAT, the
glucose utilization index of which was higher in leptin-infused
or pair-fed rats than in the ad libitum–fed control rats (Fig. 3).
The situation was different in WAT. Indeed, as can be seen in Fig.
3, the insulin-stimulated glucose utilization index was lower in
WAT of leptin-infused animals than in that of pair-fed control rats.
Impact of intracerebroventricular leptin infusion on
the expression of UCP1, UCP2, and UCP3. C o m p a r i s o n s
among vehicle-infused fed ad libitum, leptin-infused, and

1016 DIABETES, VOL. 47, JULY 1998
LEPTIN: GLUCOSE METABOLISM AND UCPs
vehicle-infused pair-fed rats also bore on the status of the UCP
mRNA levels in different tissues. In separate experiments, it
was observed that in BAT of control animals, the mRNA lev-
els of UCP1 and UCP3 were similar, whereas those of UCP2
were about twofold lower (data not shown). As can be seen
in Fig. 4, the pair-feeding regimen resulted, relative to the fed
ad libitum situation, in very marked decreases in UCP1,
UCP2, and UCP3 mRNA levels in BAT. In contrast, intra-
cerebroventricular leptin infusion prevented the occurrence
of such decreases in BAT UCP1 and UCP2 expression and
resulted in a doubling of that in UCP3. As depicted in Fig. 5,
UCP2 mRNA levels measured in the liver and WAT were also
markedly decreased in vehicle-infused pair-fed animals. Such
a decrease in liver UCP2 expression was prevented by intra-
cerebroventricular leptin infusion. Additionally and relative
to UCP2 expression in WAT, leptin had a stimulatory effect on
UCP2 expression in this tissue. As shown in Fig. 6, muscle
UCP3 mRNA levels were markedly decreased by the pair- f e e d-
ing regimen, a decrease that was prevented by intracere-
broventricular leptin infusion and replaced, when compared
with values observed in ad libitum–fed control rats, by an
increased expression, although the latter failed to reach sta-
tistical signific a n c e .
D I S C U S S I O N
In vivo metabolic studies pertaining to the effects of leptin are
still relatively scarce. Acute intravenous leptin administration
to normal rats was shown to be followed by an increased sen-
sitivity of glucose utilization to insulin, as assessed by eugly-
cemic-hyperinsulinemic clamps (24). Also, acute (5-h) intra-
venous or intracerebroventricular leptin administration to
normal mice has been shown to increase basal (i.e., not
insulin-stimulated) R
d
and to stimulate basal glucose utiliza-
tion index (as assessed by the labeled 2-deoxyglucose tech-
nique) of both BAT and muscles. It was further suggested that
the effects of leptin on glucose utilization were mediated by
the central nervous system (25).
The aim of the present study was to investigate the effects
of a continuous intracerebroventricular infusion of leptin for
4 days on body weight homeostasis, basal hormonal output,
glucose handling, and the expression of UCP1, UCP2, and
UCP3 in different tissues (12–14).
This study has been carried out in lean rats of the Zucker
nondiabetic strain, heterozygous (FA / f a) for a mutation of
the leptin receptor gene (f a ), which might have interfered
with the results. However, these animals seem to represent an
adequate model of lean rats, since it has recently been demon-
strated that the normal leptin receptor functions as a dominant
negative toward the mutated f a receptor (15). This does not
seem to be the case in heterozygous lean rats (FA / f a) of the
Zucker diabetic strain (ZDF), which display an impaired fatty
acid–induced proinsulin mRNA response when compared
with homozygous lean animals (FA / FA) (26).
Intracerebroventricular leptin infusion was not accompa-
nied by any detectable increase in plasma leptin levels, indi-
cating that, for the dosage used (12 µg per rat per day), there
was no leakage of the hormone into the circulating blood.
Thus, the hormonometabolic changes observed were due to
genuine action of leptin within the central nervous system,
presumably within the hypothalamic area containing the
known long-form (Ob-Rb) receptor isoform (2,3).
Central leptin infusion was shown to markedly decrease
food intake, body weight, and basal insulinemia. These three
parameters were similarly decreased by a pair-feeding regi-
men whereby vehicle-infused control rats were pair-fed to the
same amount of food consumed by the leptin-infused group.
The similar decreases in basal plasma insulin levels in leptin-
infused rats and vehicle-infused pair-fed control rats are
likely related to decreased substrate availability to the
endocrine pancreas. It should be mentioned that while vehi-
FIG. 1. Body weight changes over initial respective body weights of
intracerebroventricular vehicle-infused control rats fed ad libitum,
leptin-infused (12 µg/day) rats, and vehicle-infused rats pair-fed to
the amount of food consumed by the leptin-infused group. Continu-
ous vehicle or leptin infusion over 4 days. Data are means ± SE of 5–6
animals per group. *P < 0.05 for the two test groups vs. fed ad libi-
tum control rats.
TABLE 1
Basal hormonal and metabolic parameters in lean rats chronically intracerebroventricularly infused with leptin or vehicle
Ve h i c l e - i n f u s e d
Vehicle-infused rats Leptin-infused rats p a i r-fed rats
Leptinemia (ng/ml) 2.5 ± 0.2 1.4 ± 0.4* 1.3 ± 0.1*
Insulinemia (pmol/l) 237 ± 10 81 ± 12* 99 ± 6*
Corticosteronemia (nmol/l) 162 ± 65 165 ± 23 187 ± 46
Glycemia (mmol/l) 6.7 ± 0.3 6.1 ± 0.2 6.0 ± 0.2
Basal R
d
(mg · kg
– 1
· min
– 1
) 6.4 ± 0.3 5.8 ± 0.2 6.0 ± 0.2
Insulin-stimulated R
d
(mg · kg
– 1
· min
– 1
) 16 ± 0.5 21.6 ± 2* 19.4 ± 0.6*
Data are means ± SE of 5–6 animals per group. Infused leptin was 12 µg per rat per day. *P at least <0.05.

DIABETES, VOL. 47, JULY 1998 1017
I. CUSIN AND ASSOCIATES
cle-infused pair-fed rats had marked (though not quantifi-
able) behavioral changes characteristic of intense food seek-
ing, such was not the case for leptin-infused rats.
Experiments carried out under both basal conditions and
during euglycemic-hyperinsulinemic clamps showed that
continuous intracerebroventricular leptin administration did
not alter basal HGP, nor did it change the effect of insulin in
suppressing HGP. It is therefore concluded that central lep-
tin infusion to normal rats fails to alter the in vivo liver glu-
cose output. This is in contrast with data obtained by others
in awake mice in which acute leptin infusion has been
demonstrated to increase basal HGP (25).
A d d i t i o n a l l y, this study demonstrates that continuous cen-
tral leptin administration was able to further increase the in
vivo insulin stimulatory effect on overall R
d
measured during
euglycemic-hyperinsulinemic clamps. The further increase in
R
d
brought about by intracerebroventricular leptin adminis-
tration was also observed after the pair-feeding regimen. The
potentiation of the insulin effect on R
d
by both leptin and pair-
feeding was measured in specific insulin-responsive tissues,
such as muscles and BAT. In these tissues, the glucose uti-
lization index was increased beyond the values reached by the
insulin stimulus alone, both by intracerebroventricular leptin
infusion and by the pair-feeding regimen. The only tissue in
which the effect of leptin was not identical to that of the
p a i r-feeding regimen was WAT. In this tissue, while chronic
intracerebroventricular leptin brought about a decrease in the
insulin-stimulated glucose utilization index, the pair- f e e d i n g
regimen failed to do so.
FIG. 2. In vivo glucose utilization index of various muscles measured
by the labeled 2-deoxyglucose technique during euglycemic-hyperin-
sulinemic clamps in intracerebroventricular vehicle-infused control
rats fed ad libitum, leptin-infused (12 µg/day) rats, and vehicle-
infused rats pair-fed to the amount of food consumed by the leptin-
infused group. Continuous vehicle or leptin infusion over 4 days. Data
are means ± SE of 5–6 animals per group. *P < 0.05 vs. fed ad libitum
control rats.
FIG. 3. In vivo glucose utilization index of BAT and epididymal WAT
measured by the labeled 2-deoxyglucose technique during euglycemic-
hyperinsulinemic clamps in intracerebroventricular vehicle-infused
control rats fed ad libitum, leptin-infused (12 µg/day) rats, and vehi-
cle-infused rats pair-fed to the amount of food consumed by the lep-
tin-infused group. Continuous vehicle or leptin infusion over 4 days.
Data are means ± SE of 5–6 animals per group. *P < 0.05 vs. fed ad libi-
tum control rats in BAT and vs. pair-fed control rats in WAT.
FIG. 4. UCP1, UCP2, and UCP3 mRNA levels in BAT in intracere-
broventricular vehicle-infused control rats fed ad libitum, leptin-
infused (12 µg/day) rats, and vehicle-infused rats pair-fed to the
amount of food consumed by the leptin-infused group. Continuous
vehicle or leptin infusion over 4 days. A: Data are means ± SE of 5 ani-
mals per group. *P at least <0.05 vs. fed ad libitum control rats. B: Rep-
resentative UCP1, UCP2, UCP3, and -actin mRNA signals under the
various conditions studied.
FIG. 5. UCP2 mRNA levels in liver and WAT in intracerebroventricu-
lar vehicle-infused control rats fed ad libitum, leptin-infused (12
µg/day) rats, and vehicle-infused rats pair-fed to the amount of food
consumed by the leptin-infused group. Continuous vehicle or leptin
infusion over 4 days. Data are means ± SE of 5 animals per group. *P
at least <0.05 vs. fed ad libitum control rats.
A
B

1018 DIABETES, VOL. 47, JULY 1998
LEPTIN: GLUCOSE METABOLISM AND UCPs
Although all the changes in glucose handling elicited by the
continuous intracerebroventricular infusion of leptin cannot
yet be explained mechanistically, it is worthwhile noting that
they are almost the exact opposite of those elicited by
chronic intracerebroventricular infusion of the orexigenic
neuropeptide, neuropeptide Y (23). This substantiates the
notion that leptin and neuropeptide Y belong to a tight loop
system connecting the hypothalamus and the periphery
aimed at regulating glucose handling together with body
weight homeostasis.
Despite the apparent analogy between leptin’s effects and
those of pair-feeding, their respective underlying mecha-
nisms differ. Indeed, the pair-feeding regimen seems to be
accompanied by a decrease in sympathetic tone. This is in
keeping with the observed marked decrease in BAT UCP1,
UCP2, and UCP3 mRNA levels, the decreased liver and WAT
UCP2 mRNA levels, and those of muscle UCP3, suggesting the
occurrence of a decrease in energy dissipation as heat in the
p a i r-fed group. The decreased expression of UCP3 mRNA
expression in skeletal muscle induced by food restriction
(50%) has already been reported (27) and is in contrast with
the effect of fasting (28) or severe food restriction (29).
In contrast, intracerebroventricular leptin administration
is characterized by the maintenance of adequate UCP expres-
sion in BAT (UCP1 and UCP2) and liver (UCP2), i.e., a main-
tenance at levels similar to those measured in fed ad libitum
control rats. Intracerebroventricular leptin infusion results in
an actual increase in UCP2 expression in WAT, in keeping with
data reported by others (30), and in an increase in UCP3
expression in BAT and muscle. This may indicate that leptin
brings about a state of fuel depletion while maintaining the
activity of thermogenic processes, as reported in mice via the
measurement of actual energy expenditure (7). Such leptin-
controlled processes (i.e., UCPs status and actual energy
expenditure derived thereof) are potentially sympathetic
nerve–mediated and occur at the level of the several target tis-
sues mentioned above (31). Thus, although the thermogenic
activity favored by intracerebroventricular leptin does not
appear to affect the fuel refurbishment of most tissues, as
measured during the euglycemic-hyperinsulinemic clamps, it
may be of importance to prevent excessive fuel storage after
cessation of leptin administration, while such may not be
the case after cessation of the pair-feeding regimen.
In conclusion, it is proposed that 1) the chronic intracere-
broventricular leptin-elicited behavioral, hormonal, and
metabolic effects are the direct consequences of the presence
of this hormone within the central nervous system, presum-
ably acting at the hypothalamic Ob-Rb receptor level; 2) both
intracerebroventricular leptin administration and the pair-
feeding regimen result in increased insulin-stimulated glucose
turnover in all tissues except for WAT, which indicates that
intracerebroventricular leptin does not affect insulin-stimu-
lated glucose utilization when its effect on food intake is
controlled for; 3) leptin’s effects differ from those of a pair-
feeding regimen, as it maintains a normal and even a supra-
normal expression of UCPs in BAT, liver, WAT, and muscle,
while this expression is markedly decreased by the pair- f e e d-
ing regimen; and 4) the observation that leptin decreases
WAT glucose utilization while it produces an increase in
UCP2 expression might be in keeping with a role of leptin in
favoring lipolysis in this tissue.
A C K N O W L E D G M E N T S
This research has been supported by grants 32–40806.94 and
31–43405.95 of the Swiss National Science Foundation
(Berne, Switzerland) and by a grant-in-aid from Novartis
(Basel, Switzerland). O.B. has been supported by the Swiss
Institute of Sport Sciences. This work has been made possi-
ble by the financial support of the Centre National de la
Recherche Scientifique (Meudon, France).
We wish to thank Dr. N. Levens and Dr. F. Cumin of Novar-
tis for useful comments and Dr. M. Chiesi of Novartis for
providing recombinant mouse leptin. We are indebted to P.
Arboit, P. Germann, D. Châtelain, and A. Volery for excellent
technical assistance.
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