Max-Planck-Institut für Physiologische und Klinische Forschung, W. G. Kerckhoff-Institut, D-61231 Bad Nauheim, Germany
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ABSTRACT |
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Suckling-age rats display endogenous circadian rhythmicity of metabolic rate (MR) with energy-saving, torpor-like decreases, which are sympathetically controlled and suppressed by leptin treatment. We investigated whether neonatal monosodium glutamate (MSG) treatment, known to cause arcuate nucleus damage and adult-age obesity, alters energy balance in the first two postnatal weeks. Continuously recorded MR and core temperatures (Tc) show that MSG treatment disinhibits the periodic, sympathetically controlled, energy-saving drops of Tc and MR. Increased energy expenditure thus explains reduced body fat at normal lean body mass found in MSG-treated pups artificially nourished identically to controls. In MSG-treated mother-reared pups, lean body mass is additionally reduced, suggesting that MSG also reduces suckling. Plasma leptin levels are similar in controls and MSG-treated pups but higher per unit of fat mass in the latter. We conclude that the postweaning development of MSG obesity and depressed thermogenesis are preceded by an early phase of increased energy expenditure with decreased fat deposition during suckling age and hypothesize cell damage in the arcuate nucleus to be involved in both.
monosodium glutamate; juvenile rat; arcuate nucleus; obesity; leptin; torpor
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INTRODUCTION |
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RODENTS TREATED POSTNATALLY with monosodium glutamate (MSG) develop into obese, stunted adults (4, 29, 37) and have been studied frequently as models for obesity. Postnatal MSG treatment produces central nervous (CNS) neurotoxic cell destruction, with the arcuate nucleus (ARC) as the most sensitive structure (35). With MSG doses sufficient to induce adult-age obesity but low enough to avoid extensive destruction such as retinal lesioning and tail automutilation (3, 22, 29), the CNS damage is seen in the ventromedial hypothalamus (VMH) and especially the ARC, two brain regions involved in the control of food intake. ARC damage is associated with impaired secretion of the releasing hormones for growth hormone (GHRH) and luteinizing hormone (LHRH) (2, 32), and the resulting changes in adenohypophysial growth hormone (GH) and gonadotropin secretion would explain retarded lean body mass growth and aberrations of gonadal development (19, 36). Moreover, in the ARC, neuropeptide Y (NPY) production is almost completely, and proopiomelanocortin (POMC) production distinctly, reduced (1, 16). With these alterations of two peptide systems contributing importantly to the control of food intake and energy expenditure, mice (29, 37) and rats (22) are more or less hypophagic but ultimately become obese. MSG-induced damage of the ARC as the origin of these peptidergic pathways further implies impairment of the neuronal targets for circulating leptin as a hormone reducing energy stores, and as a consequence, leptin sensitivity is diminished (4), whereas plasma leptin levels are distinctly increased in postnatally MSG-treated, adult-age-obese rodents (11, 22).
Because adult-age obesity develops without overt hyperphagia in postnatally MSG-treated mice and rats, reduced energy expenditure is assumed to be fueling the massive increases of fat depots. Adult neonatally MSG-treated mice show a lower core temperature (Tc) during the dark period and beginning of the light period than control mice (8, 37, 41). Increased uncoupling protein-1 mRNA expression in the brown adipose tissue (BAT) as a typical response to acute cold exposure is impaired (39). Their normal thermogenic response to norepinephrine (NE) injection (23), however, suggests that sympathetic activation of BAT, rather than its thermogenic capacity, is deficient (39). Reduced sympathetic activation of thermoregulatory energy expenditure, with the BAT as the main effector organ, may also account for increased energy efficiency in adult neonatally MSG-treated rats, as suggested by the observations that NE depletion (7) and reduced GLUT4 levels (22) are especially prominent in the BAT.
Postnatal MSG treatment is followed by manifest obesity only weeks later. Early postweaning mice and rats are not obese, and local fat stores may be smaller than in age-matched controls (22, 41). Total body mass is lower, too, for some time after the end of the postnatal MSG treatment period, and the body mass level of age-matched control animals is reached only well after weaning (22, 29, 41). Thus early effects of the MSG treatment on energy efficiency should be opposite those seen in adults. However, the primary metabolic disturbances occurring early during ongoing postnatal MSG treatment have not yet been studied. Suckling-age rats offer interesting access to the analysis of these early disturbances for three reasons. First, reduced thermoregulatory energy expenditure, as seems to be typical for MSG-induced adult-age obesity, is a response normally occurring in mother-reared suckling-age rats as well as in artificially reared rats during the morning phase of their endogenous circadian cycle (26, 27, 33). Second, the resulting torpor-like decrease of Tc is caused by a temporary decrease of metabolic rate (MR) as a consequence of depressed thermoregulatory thermogenesis due to reduced sympathetic activation of BAT (25, 27, 31). Third, at this age, leptin reduces energy stores exclusively by disinhibiting the reduction of the sympathetic drive for BAT thermogenesis during the circadian phase of depressed thermogenesis (33, 34). Because each of these control components of metabolic energy expenditure is known to deviate from normal in adult-age obesity after postnatal MSG treatment, we have studied their primary alterations as they were evolving during ongoing MSG treatment in suckling-age rats.
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MATERIALS AND METHODS |
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Animals. Lean (+/+) Zucker rat pups were used, because the juvenile Tc rhythm is especially pronounced in this strain (28). The animals were from the outbred Zucker rat colony at the W. G. Kerckhoff-Institute, founded in 1985 with a breeding stock donated by Harry Carlisle (University of California, Santa Barbara, CA). The breeding colony was maintained at 22°C and 60% relative humidity and with lights on from 2100 to 0900.
MSG treatment and experimental groups. On the day of birth, the litter size was reduced to 10 pups each. During the middle of the dark phase on postnatal days 1-9, one-half of the pups from eight litters were injected subcutaneously in the nape of the neck with 3 mg/g body mass L-MSG (MSG; Sigma, Taufkirchen, Germany) or the same volume of 0.9% saline. The MSG solution was freshly prepared in a concentration of 400 mg/ml saline. Treatment and control groups in each litter were balanced for starting body mass and sex.
On postnatal day 10, pups from five of these litters were removed from their mothers at the beginning of the dark phase. The majority of MSG-treated and control pups were decapitated for blood sampling and analysis of body composition, and the remaining randomly selected pups were used for histological examination of hypothalamic MSG lesions. For artificial rearing as a means to provide identical food supplies to MSG-treated and control pups, two litters were isolated for continuous feeding and measurements of Tc and MR from postnatal days 10-13 and a third litter from postnatal days 4-9.Artificial rearing. Artificial rearing in constant dim light starting on postnatal day 4 or 10 was carried out as previously described in detail (20, 26). Briefly, littermates were continuously fed identical amounts of synthetic rat milk (via esophageal catheters) at rates increasing from 2.0 ml/day on day 4 to 4.5 ml/day on day 13. Ambient temperature was adjusted to 33.2°C on day 4 and progressively decreased to 28°C on day 13, thereby establishing moderate cold-load conditions comparable to those experienced by pups huddling in the absence of their mothers (26, 33). Tc and oxygen consumption were measured continuously, as repeatedly described (20, 27). In short, the Tc of all pups (n = 10 in each litter) was continuously recorded with thin thermocouples inserted 1.8-2.5 cm beyond the anal sphincter. An electrochemical oxygen analyzer (Ametek, S-3A/II, AEI Technologies, Pittburgh, PA) and a mass flowmeter (Tylan, San Diego, CA) in an open-flow system successively measured flow rate and oxygen content of the air drawn from seven animal chambers and one reference chamber for 5-min periods. Thus ~36 measurements per day were obtained for each animal. Water manometers were used to set the bypass airflows equal to the recorded airflows. All data were recorded on a multichannel point printer and simultaneously stored on a personal computer.
MR was estimated with the assumption of a respiratory quotient of 0.8 and thus a caloric equivalent of 0.33 W · ml O2Plasma leptin determinations.
At the beginning of the dark phase of postnatal day 10,
mother-reared pups were decapitated, and trunk blood was sampled on ice
in tubes containing heparin (10 KIU/ml) as anticoagulant. Dilution was
determined by weighing, and concentrations were appropriately corrected. Plasma was collected after centrifugation and stored at
80°C until leptin was assayed with a commercial leptin RIA kit
(Linco, St. Charles, MO).
Body composition analysis. After the stomach and gut had been removed and the bladder opened, carcass mass and body composition [fat content, fat-free dry mass (FFDM), water content] were determined by drying to constant weight and whole body chloroform extraction in a Soxhlet apparatus, as previously described (20).
Histological analysis.
At the age of 10 days, MSG-treated and control pups (n = 14 each) were anesthetized with isoflurane (Pharmacia & Upjohn,
Erlangen, Germany) during the dark phase and subsequently perfused
transcardially with ice-cold 4% paraformaldehyde diluted 1:1 in 0.1 M
phosphate-buffered saline (PBS). Brains were removed, and tissue blocks
encompassing the hypothalamus were prepared, postfixed for 60 min, and
stored overnight in 20% saccharose dissolved in 0.1 M PBS. The series of MSG-treated animals contained seven female and seven male pups and
the control series eight female and six male pups. Not later than
24 h after storage at 40°C, 20-µm serial coronal sections were cut on a cryomicrotome (HM 500-O, Microm International, Walldorf, Germany) and Nissl-stained with cresyl violet to verify cell damage qualitatively by comparing corresponding sections from MSG-treated and
control animals. For the ARC, differences between treated and
nontreated animals were quantified by counting undamaged cells at four
ARC levels: I, rostral (section 26); II, rostromedial (section 28); III, caudomedial (section 31); and
IV, caudal (section 34), corresponding to the coordinates
caudal to the bregma of
2.12,
2.56,
3.30, and
4.16 defined for
adult rats (30).
Statistical analysis. The program Statistica 5.5 (StatSoft, Tulsa, OK) was used for multifactorial analysis of variance (ANOVA) including factor repetition if required. In the analysis of the course of body mass development, the factors considered were treatment and age (repeated measurements), and in addition litter and sex, to take into account variabilities introduced by these parameters according to preceding investigations (18, 38). Average data values were described by least square means (±SE) provided by ANOVA.
Periodicity of MR and Tc due to the juvenile endogenous circadian rhythmicity was analyzed as previously described for the assessment of potential modulatory influences (6, 34). Briefly, diurnal amplitudes were estimated for each day as the average deviation between the three consecutive highest and lowest MR measurements and between the highest and lowest 30-min mean value levels of Tc, and t-statistics were applied to analyze the amplitude differences determined for control and MSG-treated pups. ![]() |
RESULTS |
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Effect of neonatal MSG treatment on growth of
mother-reared pups.
Figure 1 demonstrates the average courses
of body mass for MSG-treated pups and their control littermates
(n = 25 each) reared by their mothers from days 1 to 10. It is obvious that MSG treatment retarded growth.
Multifactorial ANOVA with one-factor repetition for age showed a highly
significant difference in body mass development between treated and
control pups (P < 0.001), becoming apparent first on
day 3 (P < 0.03) as determined by multiple
comparisons (Bonferroni). A highly significant influence
(P < 0.001) was also found for the factor
"litter." The factor "sex" was not found to influence body mass
growth significantly (P = 0.42) of either MSG-treated
or control pups. Least square means obtained by ANOVA of body mass
measurements on day 10 showed for the controls that, in line
with previous studies (18, 38), average body mass of the
females tended to be smaller (19.6 ± 0.6 g) than that of the
males (20.7 ± 0.7), but MSG treatment reduced average body masses
to virtually identical levels (17.4 ± 0.5 g in females and
17.4 ± 0.8 g in males). Similar to studies investigating
other treatment factors and discussed previously (18),
there were no significant interactions between treatment and the
factors litter and sex.
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Effects of MSG treatment in artificially reared pups on MR and
Tc periodicities.
During artificial rearing in continuous dim light from postnatal
day 10 to day 13, Tc and MR of
individual pups from two litters were continuously recorded, and their
courses for the first 24 h are illustrated in Fig.
2. Saline-treated control pups showed the
typical circadian decreases of Tc and MR during subjective mornings and their increases during subjective evenings, i.e., at the
times at which the lights in the colony of origin had been turned on
and off, respectively. In contrast, the circadian decreases of MR and
Tc were attenuated or even abolished in the MSG-treated pups, whereas their levels during the maximum phase of the daily cycle
were only slightly influenced by the MSG treatment. During postnatal
days 10-13, comparison of mass-specific MR of control and MSG-treated pups (n = 6 and 8, respectively) during
the circadian minimum showed a level of 11.0 ± 1.0 W/kg in the
control pups and a level of 17.1 ± 1.0 W/kg in the MSG-treated
pups (P < 0.01). The corresponding average amplitudes
of the circadian rhythm of mass-specific MR were 13.1 ± 1.0 W/kg
for the control pups and only 8.3 ± 0.1 W/kg for the MSG-lesioned
pups (P < 0.03). The average differences in the
amplitudes of the circadian Tc rhythm during postnatal
days 10-13 of control and MSG-treated pups
(n = 8 and 10, respectively) were even more pronounced,
with 4.5 ± 0.3°C for control pups and only 2.6 ± 0.2°C
for the MSG-lesioned pups (P < 0.001). Minimum
Tc levels were 32.2 ± 0.3°C for control and
34.5 ± 0.3°C for MSG-treated pups and differed significantly (P < 0.001).
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Effects of MSG treatment on body composition of mother- and
artificially reared rat pups.
For those mother-reared litters that were not scheduled for artificial
rearing, the changes in carcass mass, FFDM, fat mass, and water mass on
postnatal day 10 were evaluated by expressing the deviations
in FFDM, fat, and water content measurements obtained for each treated
pup as the percentage of the corresponding averages determined for the
control littermates. To investigate whether lowered food intake in the
mother-reared pups might have contributed to the retardation of body
mass growth, one litter was artificially reared from postnatal
day 4 to day 9, providing identical food supply
to MSG-treated animals and their control littermates, and body
composition was determined on day 10. In both mother-reared and artificially reared pups, the MSG treatment reduced final body fat
content significantly and to nearly identical degrees (30%) compared
with control littermates (Fig. 3).
However, the changes in FFDM and body water induced by the MSG
treatment relative to the controls were different, depending on the
rearing conditions. Identical food supply to control and MSG-treated
pups during artificial rearing prevented significant reductions of FFDM
and body water relative to their controls as the result of MSG
treatment. In contrast, in the mother-reared pups suckling milk, both
FFDM and body water had decreased significantly by ~15% in the
MSG-treated pups relative to their controls. This difference suggests
that reduced food intake by suckling had contributed to growth
retardation in mother-reared, MSG-treated pups. In contrast, similar
degrees of reduction in fat deposition in the two groups may be
attributed to decreased energetic efficiency following from the
abolished circadian decrease in energy expenditure during subjective
mornings by the MSG treatment, as shown in the artificially reared pups and as assumed to occur also in the mother-reared pups.
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Plasma leptin levels of mother-reared rat pups.
The influence of MSG treatment on plasma leptin was determined by
analyzing blood samples obtained on day 10 from MSG-treated (9 female, 6 male) and control (10 female, 7 male) rat pups. Absolute plasma leptin levels of MSG-treated and control pups were not different
from each other (4.2 ± 0.4 vs. 4.1 ± 0.5 ng/ml,
respectively). However, plasma leptin levels per unit of fat mass of
3.7 ± 0.4 ng · ml1 · g
1 in the
control pups and 5.1 ± 0.5 ng · ml
1 · g
1 in the
MSG-treated pups differed significantly (P < 0.03) by ~40%.
Histological examination of cell rarification after MSG treatment.
To illustrate the degree of cell damage having developed by day
10, Fig. 4 compares coronal
hypothalamic tissue sections showing the periventricular neuropil at
the medial level of the ARC. It is obvious that cell density is
markedly reduced in the MSG-treated pups compared with the controls. To
quantify the degree of cell rarification, undamaged cells were counted
visually at 40-fold magnification in a field determined by a
superimposed translucent grid consisting of 100 squares (25 × 25 µm) in eight slides from MSG-treated and control specimens that were
distributed according to the four defined rostrocaudal ARC levels. At
each level, the cell counts were significantly lower in the MSG-treated
pups compared with controls. In the rostral ARC (level I), the number
of 151 ± 15 cells counted per field in MSG-treated pups was
reduced by ~50% compared with the number of 318 ± 11 counted
in the controls. In the rostromedial ARC (level II), the largest
difference was found (P < 0.001), with the average
cell count of 125 ± 8 in MSG-treated pups being reduced by 60%
compared with the average cell count of 317 ± 12 in the control
pups. In the caudomedial ARC (level III), the number of 161 ± 20 cells per field in the MSG-treated pups was 50% lower than the count
of 322 ± 11 cells in the controls. At the caudal level of the ARC
(level IV), the difference was smallest (P < 0.05),
the number of 323 ± 40 cells per field in the MSG-treated pups
being 20% smaller than the 412 ± 22 cells per field in the
control pups. Not shown in Fig. 4 is that the loss of cell elements had
resulted in a noticeable dilatation of the third cerebral ventricle at
the level of the ARC in the MSG-treated compared with the control pups
and in a ventral dislocation of the adjacent VMH, where cell density
was not distinctly reduced.
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DISCUSSION |
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This study shows that, in 10-day-old MSG-lesioned rat pups, energy expenditure is enhanced relative to their control littermates, and as a result, their fat content is reduced at that time by ~30%. Suckling-age rats raised by their mothers under the usual laboratory conditions require some degree of metabolic cold defense (26, 33), which typically follows an endogenous circadian periodicity involving a phasic depression of thermoregulatory thermogenesis in BAT at the beginning of the light phase (27). This pattern is of crucial importance for maintenance of normal energy balance (25), and as demonstrated in several studies, manipulations of sympathetically mediated thermoregulatory energy expenditure result in changes of body fat content but not of FFDM in both mother-reared and artificially reared suckling-age pups (33, 34). As an early effect of MSG treatment, we demonstrated here, by artificially rearing pups at thermal conditions mimicking those in the nest, that the normal circadian depression of sympathetically controlled thermogenesis is attenuated in 2-wk-old MSG-treated pups. Because in contrast to their control littermates Tc and MR of MSG-treated pups are thus maintained at high levels throughout the day, their fat deposition is decreased similarly, as previously shown for leptin-treated pups (34).
Reduced food intake is an early and persisting effect of postnatal MSG treatment. Artificial rearing provides identical supply with synthetic milk to control as well as to MSG-treated pups. In this condition, the MSG-induced decrease in energy efficiency retards only fat deposition but not growth of lean body mass (=FFDM + water mass). However, in MSG-treated mother-reared pups, not only is fat deposition similarly reduced, but lean body mass growth is also retarded. This difference strongly suggests that, in mother-reared MSG-treated pups, enhanced energy expenditure is accompanied by reduced suckling milk intake, resulting in reduced growth of lean body mass in addition to reduced fat deposition compared with control littermates. Although the control of suckling differs markedly from the control of independent feeding (13), reduced food intake obviously persists into adulthood, as animals developing adult-age obesity after postnatal MSG treatment are reported to be hypophagic or at least not hyperphagic (22, 29, 37).
Factors possibly contributing to the change from increased to decreased energy expenditure after postnatal MSG treatment. As pointed out in the introduction, rodents having become obese at adult age after postnatal MSG treatment display impaired metabolic cold defense. In contrast, the present study provides the first evidence that the primary effect of MSG treatment in suckling-age rats consists of the abolition of their ability to save energy by periodically entering a torpor-like state with reduced Tc and MR (25). Reduction of body mass is consistently reported as an early effect of MSG treatment in juvenile, postweaning mice (29) and rats (36) and is associated first with reduced, rather than enhanced, fat deposition. Primary sympathoexcitatory effects of centrally acting MSG preceding its destructive action as a neurotoxin were tentatively considered to explain the decrease in sympathetic tone developing later, as observed after weaning in rats treated postnatally with MSG (22). According to what is shown in this study for suckling-age rats, the early condition after MSG treatment of a temporarily enhanced sympathetic drive for thermogenesis seems to be restricted to the disinhibition of the energy-saving, torpor-like drops of thermoregulatory energy expenditure occurring normally at this age, i.e., a response pattern closely resembling the effect of leptin treatment in suckling-age pups (34). This similarity strongly suggests, as a common cause of the observed reductions in body fat content, a disinhibition of the circadian decrease in the sympathetic drive for BAT thermogenesis, which was clearly documented as the dominant action of leptin in pups of the same age (33).
The juvenile circadian cycle of MR in rats is known to cease during the 3rd wk of life (26), and around the same time, leptin treatment of rat pups starts to reduce food intake rather than only enhancing energy expenditure (10, 34). At the same age, leptin receptor-deficient (fa/fa) rat pups, which show a markedly depressed energy expenditure during the first 2 wk of life, start to become normometabolic (15). It therefore seems likely that the increased energy expenditure of MSG-treated pups described in the present study also ceases, as the torpor-like, energy-saving circadian phases of reduced MR vanish during the 3rd wk of life. Only after the MSG-induced neurodegenerative processes have become fully effective does a more positive energy balance seem to develop in MSG-treated animals. Transition from the preobese to the obese state was found to occur in mice between the 3rd and 6th wk (41) and in rats between the 30th and 90th day (22). The switch from decreased to increased energy efficiency, however, most likely occurs long before the total body mass of the MSG-lesioned animals starts to rise above that of untreated animals.Relevance of MSG-induced endocrine disturbances for its effect on growth during suckling age. Cell damage in the ARC is one of the most distinct neurotoxic effects of early postnatal MSG application and occurs within a few days, as the present study has shown. Depletion of LHRH is one of the consequences of MSG-induced ARC damage and, in particular, depletion of GHRH and a subsequently attenuated GH secretion pattern (2, 19, 32). In the suckling-age animals of the present study, these alterations were developing while the effects of ongoing MSG-treatment were being analyzed; however, in these prepubertal animals, any possible influence of altered LHRH secretion on body mass seems to have been obscured by the dominant influence of altered metabolic control, because no sex difference was apparent. With regard to possible influences of reduced GH secretion and GHRH depletion, respectively, lean body mass growth was found not to be reduced in MSG-treated pups, provided they were fed the same amount of milk as their control littermates. Connectivities between the median eminence and ARC may still be developing for the neuroendocrine GHRH system between days 5 and 20 in rat sucklings (5), and its impairment by MSG might affect growth only inasmuch as it means the loss of a feeding-stimulatory neuropeptide (40). Little influence of GH and secondary growth factors like insulin-like growth factor I (IGF-I) on early postnatal growth is suggested by the observation that, in mice transgenic for the GH gene, the rise of body mass above that of control mice commenced only after 2 wk of age despite overexpression of GH mRNA and enhanced and rising IGF-I plasma level immediately after birth (21).
Early influences of MSG treatment on leptin and on the ARC as the central leptin target. The MSG-treated juvenile rats of the present study display the apparent paradox that morphological damage is pronounced on postnatal day 10 in the ARC as the site of highest intrahypothalamic leptin receptor density, but the MSG-induced suppression of the diurnal, torpor-like drops of MR and Tc resemble those induced by treatment with high doses of recombinant leptin during postnatal days 7-16 in rat pups (34) as well as in small adult mammals as different as mice (6) and marsupials (12). In view of the known effects of leptin on sympathetic control of BAT thermogenesis, it is suggestive to assume that the neurotoxic action of MSG on the ARC as the leptin target might initially disinhibit reductions of sympathic activation of energy expenditure in suckling-age rats and only later cause impaired sympathetic activation of BAT thermogenesis as the precondition for adult-age obesity. The latter assumption would be in line with the observation that the MSG-induced reduction of sympathetic control in adults is specifically expressed in the BAT (7, 22), presumably as the consequence of leptin insensitivity having developed in MSG-treated adult animals (4), in which leptin levels are elevated 5- to 10-fold (11).
Plasma leptin levels of the MSG-lesioned rat pups, although not different from those of their control littermates, are significantly elevated in relation to body fat mass. In view of the developing damage of the ARC as the leptin target, it is not clear whether or not the observed disinhibition of phasically reduced BAT thermogenesis by the MSG treatment in the suckling-age rats is due to their relative hyperleptinemia. Hypercorticosteronism as a direct stress response to MSG treatment in suckling-age rats (14) may account for their disproportionately higher plasma leptin, when it is considered that glucocorticoids are potent stimulators of leptin secretion (24).Perspectives: time courses of MSG-induced depletion of hypothalamic NPY and POMC. The damage of ARC neurons seen early in the MSG-treated suckling-age rats of this study persists into adulthood and was shown to be associated with early and sustained disturbances of GH and gonadotropin secretion. The biphasic MSG effect with initial disinhibition and subsequent impairment of sympathetically controlled metabolic cold defense might be attributable to ARC damage as well. This working hypothesis draws attention to the destruction of those ARC neurons that produce NPY and POMC and constitute antagonistic pathways downstream of the leptin target, mediating the effects of leptin on both control of energy expenditure and food intake.
The ARC is the prevailing site of hypothalamic NPY synthesis, and MSG treatment results in an almost total loss of NPY immunoreactivity in the ARC of adult animals (16, 22). NPY transported to and released at fiber terminals in the paraventricular nucleus (PVN) is known to stimulate food intake and to reduce sympathetic outflow to BAT in adult rats (9). Indeed, our own immunocytochemical studies in 10-day-old rat pups clearly showed an already existing intense NPY-ergic innervation of the PVN (T. Hübschle and C. Schoelch, unpublished results). However, effects of changes in leptin and NPY levels on suckling milk intake seem to be missing during the first two postnatal weeks, whereas effects on juvenile energy expenditure are well established (10, 17, 33, 34). The destruction of NPY-synthesizing cells in the medial ARC might therefore contribute to a disinhibition of energy expenditure in suckling-age rats, whereas the observed reduction in suckling milk intake seems to be caused by other factors. Theoretically, the effect of NPY depletion might be compensated for if the POMC system were simultaneously destroyed to a similar degree. However, compared with the NPY system, the POMC system seems to be less susceptible to MSG damage according to the observation that adrenocorticotropic hormone and corticosterone levels and POMC mRNA levels of 10-day-old rats were significantly higher after injection of MSG (4 mg/g) on postnatal days 2, 4, 6, 8, and 10 than in controls injected with hypertonic saline (14). At the hypothalamic level, early postnatal MSG treatment seems to reduce POMC mRNA expression less readily than NPY mRNA expression (1). In summary, elucidating the time courses of inactivation of the antagonistic POMC and NPY pathways after neonatal MSG treatment might offer insights into the biphasic development of the disturbances in energy expenditure in this frequently used adiposity model. ![]() |
ACKNOWLEDGEMENTS |
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We are indebted to Eckhart Simon for critically reviewing the manuscript and contributing experience of many years of hormonal research.
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FOOTNOTES |
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This work was supported by the Deutsche Forschungsgemeinschaft (Schm 680/2).
Present addresses: C. Schoelch, Aventis Pharma Deutschland GmbH, DG Metabolic Diseases, D-65926 Frankfurt, Germany; T. Hübschle, Veterinärphysiologie der Justus-Liebig-Universität Giessen, Frankfurter-Str. 100, D-35392 Giessen, Germany; B. Nuesslein-Hildesheim, Novartis Pharma AG, Metabolic and Cardiovascular Diseases, CH-4002 Basel, Switzerland.
Address for reprint requests and other correspondence: I. Schmidt, Max-Planck-Institut für Physiologische und Klinische Forschung, W. G. Kerckhoff-Institut, Parkstr. 1, D-61231 Bad Nauheim, Germany (E-mail: I.Schmidt{at}kerckhoff.mpg.de).
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.
April 23, 2002;10.1152/ajpendo.00439.2001
Received 1 October 2001; accepted in final form 12 April 2002.
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