Geriatric Research, Education and Clinical Center, Department of Veterans Affairs Medical Center, Gainesville 32608-1197; and Department of Pharmacology and Therapeutics, University of Florida College of Medicine, Gainesville, Florida 32610
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ABSTRACT |
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To investigate the role of aging on the fasting-induced suppression of leptin gene expression and increase in hypothalamic neuropeptide Y (NPY) gene expression, we fasted or fed ad libitum male F-344xBN rats aged 3, 24, and 31 mo for 2 days. We examined leptin mRNA levels in retroperitoneal, inguinal, and epididymal white adipose tissue (WAT); serum leptin levels; and NPY mRNA levels in the hypothalamus. We found that leptin mRNA levels were increased from 3 to 24 mo and leveled off between 24 and 31 mo in both retroperitoneal WAT and inguinal WAT but were unchanged with age in epididymal WAT. Serum leptin levels increased with age, whereas hypothalamic NPY mRNA levels did not change with age. Fasting suppressed leptin gene expression in all three WATs and serum leptin. Moreover, this suppression of serum leptin and of leptin message in retroperitoneal WAT was less in aged rats. Conversely, fasting increased hypothalamic NPY message, again to a lesser extent in aged rats. In both fed (ad libitum) and fasted rats, there was a strong correlation between serum leptin and hypothalamic NPY mRNA levels in the young but not in either of the two aged groups. These data suggest that aged F-344xBN rats are leptin resistant and that the fasting regulation of serum leptin, leptin mRNA, and hypothalamic NPY mRNA is impaired in aged rats.
leptin mRNA; obesity gene; inguinal, retroperitoneal, or epididymal white adipose tissue; Lee index; food intake; serum; F-344xBN rats
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INTRODUCTION |
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LEPTIN IS ONE REGULATOR of food intake and energy expenditure. It is the protein product of the ob gene (32), which is synthesized by white adipose tissue (WAT). Leptin is an afferent signal molecule that interacts with the appetite and satiety centers in the brain to regulate body weight (17). In addition, leptin appears to be the signal that indicates the size of the fat depot in the body (32). When injected into mice or rats, leptin reduces food intake and increases energy expenditure, resulting in a loss of body weight (5, 10, 19, 22). Obesity results from an imbalance of food intake over energy expenditure. Leptin is involved in both of these processes.
Adults tend to gain weight and become obese as they age until early senescence, after which body weight declines (4, 8, 16). The F-344xBN rat strain is a useful model for human obesity because it demonstrates a steady increase in body fat into early senescence (25). In our recent study (14), we found that adiposity in F-344xBN rats increased with age, which could not be attributed to either increased food intake, impaired leptin gene expression, or impaired peripheral leptin production. In fact, serum leptin levels and WAT leptin mRNA levels were actually increased with age in these rats. It is not known, however, whether a physiological challenge such as fasting would expose any impairment in the dynamic regulation of leptin gene expression. Because fasting suppresses leptin message (15, 21, 30), we hypothesized that the fasting-induced suppression of leptin message may be impaired in aged rats.
Another goal of the present study was to investigate the relationship among leptin, fasting, and hypothalamic neuropeptide Y (NPY) message in aged rats. NPY is an important feeding stimulant in the brain (7, 13, 26). Two known factors regulating hypothalamic NPY message are leptin and fasting. Leptin inhibits NPY synthesis, and this may be the mechanism by which leptin reduces food intake (24, 27). In contrast, fasting increases hypothalamic NPY levels (18, 31) and in turn stimulates appetite. Although NPY is only one of many redundant pathways regulating food intake, it is a potent feeding stimulant. Because serum leptin increases with age in F-344xBN rats (14), one might expect NPY mRNA levels to be suppressed. If NPY levels were reduced with age, and other factors being equal, one might expect that less food would be consumed by aged F-344xBN rats. We were surprised to find that aged rats consumed the same amount of food as their young, lean counterparts (14). Thus the elevated serum leptin levels with age suggest that NPY mRNA levels in the hypothalamus of aged F-344xBN rats may be elevated, whereas the food consumption data suggest that NPY mRNA levels may be unchanged with age.
To answer these questions, we examined leptin mRNA levels in three WAT depots, serum leptin levels, and NPY mRNA levels in the hypothalamus after a 48-h fasting or ad libitum feeding in F-344xBN rats of three different ages: 3 mo, the age of sexual maturation; 24 mo, the age of peak leptin gene expression and leptin serum level (14); and 31-mo-old aged rats.
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METHODS |
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Animals. Male F-344xBN rats aged 3, 24, and 31 mo (n = 16/age group) were obtained from Harlan Sprague Dawley (Indianapolis, IN). They were housed individually under a reversed light-dark 12:12-h cycle, with lights on from 6:30 PM to 6:30 AM, at a room temperature of 26°C, which is the thermoneutrality for these rats (23). Food (Purina rat chow) and water were provided ad libitum.
Eight rats of each age were either fed ad libitum or fasted for 48 h. Thus we had a factorial design with age as one factor (3, 24, and 31 mo) and feeding status (ad libitum vs. fasting) as the other. There were a total of six treatment groups, with eight rats in each group except the 31-mo-old fasted group, which had only seven because one rat died before the experiment. Average daily food intake was measured in all rats by the difference in weight between the amount of food provided and the amount remaining over a 2-day period before the fasting treatment. Daily food intake values were presented in terms of whole animal and independent of body mass, i.e., (g food intake)/(kg body wt)0.67. Use of the exponent 0.67 for body weight is based on Heusner's (11) observations that energy metabolism within a species is related to mass by this power function. After exactly 48 h of either ad libitum feeding or fasting treatment, the rats were killed from 8:30 AM to 11 AM in a rotating order among the six treatment groups. Rats were weighed and anesthetized with pentobarbital (90 mg/kg), and the naso-anal length was measured. Blood (4-5 ml) was collected in Vacutainer SST tubes (Becton Dickinson, Franklin Lakes, NJ) via cardiac puncture using an 18-gauge needle, followed by perfusion of the circulatory system with 60 ml of 0.9% saline. Epididymal WAT (EWAT), inguinal WAT (IWAT), and retroperitoneal WAT (RTWAT) and the hypothalamus were collected, weighed, and rapidly frozen in liquid nitrogen. The tissues were stored atDetermination of adiposity levels. Adiposity was determined by the Lee index, which is the cubic root of body weight in grams divided by the naso-anal length in millimeters times 104 (2, 12). The Lee index is highly correlated with the percentage of body fat (2, 12, 28).
RNA extraction and dot blotting. Total cellular RNA was extracted using Tri reagent (Molecular Research Center, St. Louis, MO), a modified procedure of the single-step method reported by Chomczynski and Sacchi (6) for total RNA isolation. The integrity of the isolated RNA was verified using agarose gels (1%) stained with ethidium bromide. The RNA was quantified by spectrophotometric absorption at 260 nm using multiple dilutions of each sample.
The probe to detect leptin mRNA was a 33-mer antisense oligonucleotide (5'-GGTCTGAGGCAGGGAGCAGCTCTTGGAGAAGGC) that has been fully characterized by Trayhurn et al. (29). It was end labeled using terminal deoxynucleotidyl transferase (Promega, Madison, WI). The oligonucleotide was based on a region of the mRNA downstream from the site of the primary mutation in ob/ob mice and synthesized at the University of Florida core facility. The leptin oligonucleotide probe was end labeled using terminal deoxynucleotidyl transferase (Promega). This probe binds to a single mRNA species of 4.1 kb in perirenal WAT and EWAT (15). Rat preproNPY cDNA probe, obtained from Dr. Janet Allen (Univ. of Glasgow, UK), was random-prime labeled using Prime-a-Gene (Promega). Northern analysis indicated that this NPY probe binds to a single mRNA species of mRNA of 0.9 kb in brain tissue (data not shown). The full-length humanSerum leptin RIA. Plasma leptin levels were measured using a commercial leptin RIA kit (Linco Research, St. Charles, MO). The antibody was raised against highly purified rat leptin. Both the standard and tracer were prepared with rat leptin. Leptin concentrations were calculated from standard curves generated for each assay.
Statistical analysis. Two-way ANOVA was applied to collected raw data. When there was no interaction, only main effect was examined, with Fishers protected least significant differences test applied to post hoc comparisons. If there was significant interaction, further one-way ANOVA was applied, and Fishers protected least significant differences test was used to examine post hoc comparisons. Paired t-test and Pearson linear correlation were calculated where applicable. Regression analysis was applied to produce simple fit for scatter plots. We chose a 0.05 level of statistical significance.
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RESULTS |
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Effect of aging, feeding, and fasting on body
weights.
Body weight increased significantly from 3 to 24 mo and leveled off
between 24 and 31 mo (Table 1). During the
two-day ad libitum feeding period, weight gain (difference between pre-
and post-ad libitum feeding or fasting weight) occurred in both
3-mo-old (4.8 ± 0.7 g; P < 0.0001, paired t-test) and
24-mo-old (4.3 ± 0.6 g; P < 0.0001) but not in 31-mo-old rats (0.3 ± 1.1 g).
Thus F-344xBN rats continued to gain weight at 24 mo under ad libitum feeding conditions. In contrast, for the fasted animals, comparable weight loss was found across three ages (3 mo, 28.6 ± 1.15 g, P < 0.0001; 24 mo, 30.9 ± 1.13 g, P < 0.0001; 31 mo, 30.6 ± 1.5 g, P < 0.0001). As a result, the
rats fed ad libitum had higher body weights than the 2-day-fasted rats
(Table 1).
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Tissue weights. The weights of EWAT, RTWAT, and IWAT all increased from 3 to 24 mo and leveled off between 24 and 31 mo (Table 1). In addition, RTWAT weight decreased with fasting at each rat age (Table 1). The weights of the three WATs strongly correlated with the following obesity-related measures: serum leptin (r = 0.73-0.77, P < 0.0001), body weight (r = 0.92-0.96, P < 0.0001), and Lee index (r = 0.70-0.74, P < 0.001).
Adiposity. In both fed (ad libitum) and fasted animals, the Lee index increased between 3 mo (fed, 298.5 ± 2.2; fasted, 293.2 ± 2.9) and 24 mo (fed, 319.4 ± 2.3; fasted, 311.5 ± 2.6) and then leveled off between 24 and 31 mo (fed, 313.1 ± 2.8; fasted, 311.3 ± 1.5; P < 0.0001 for the main effect of age). These data are similar to our previous report indicating an increase in adiposity with age up to 24 mo of age (13). In addition, the Lee index was less in the fasted rats than in the rats fed ad libitum (P < 0.02 for the main effect of feeding status).
Daily food intake for animals fed ad libitum. As shown in Table 2 and similar to our previous study (14), the daily food intake was not significantly different among the ages (main effect for age, P < 0.16). When food intake was calculated independent of body mass by dividing by (body wt)0.67, there was a significant decrease between the 3- and 24-mo-old rats (P < 0.0001) and between the 3- and 31-mo-old rats (P < 0.0001) but not between the 24- and 31-mo-old rats.
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Leptin mRNA in RTWAT.
There was a significant increase in leptin gene expression from 3 to 24 mo and no change between 24 and 31 mo for controls fed ad libitum (Fig.
1). Fasting significantly suppressed leptin mRNA in all three age groups. Although the magnitude of the decrease was greater in the older ages, because baseline leptin mRNA levels (i.e., fed ad libitum) increased with age, the percent decrease in
leptin mRNA induced by fasting was greater in the 3-mo-old (70 ± 11.2%) than in either the 24-mo-old (48 ± 5.3%,
P < 0.04) or the 31-mo-old rats (58 ± 4.0%, P < 0.05). Moreover, in
the fasted rats, similar to rats fed ad libitum, leptin gene expression was significantly greater in 24- and 31- compared with 3-mo-old rats
(P < 0.0001). In contrast to the
age-related increase in leptin mRNA, there was no change in -actin
mRNA level in RTWAT with age (Table 3).
However, there was a significant decrease in
-actin mRNA level in
RTWAT in the fasted rats.
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Leptin mRNA in IWAT.
Similar to the results in RTWAT, leptin mRNA levels increased with age
from 3 to 24 mo and leveled off between 24 and 31 mo for both ad
libitum fed and fasted animals (Fig. 2).
Leptin mRNA in IWAT was suppressed by fasting. In particular, the
fasting-induced suppression was highly significant at 24 mo
(P < 0.007). In contrast to RTWAT,
the extent of the fasting-induced suppression of leptin gene expression
was significantly greater in 24-mo-old rats (60 ± 7.3% decrease,
P < 0.04) but not in 31-mo-old rats
(45 ± 10.8%, P < 0.2) compared
with the young rats (27 ± 11.8%). There was no change in
-actin mRNA levels in IWAT with either age or fasting (Table 3).
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Leptin mRNA in EWAT.
Similar to our previous findings (14), leptin mRNA levels were
unchanged across the three ages in EWAT
(P < 1.0 for main effect of age;
Fig. 3). Fasting significantly suppressed
leptin mRNA levels in all three ages (Fig. 3). The extent of the
fasting-induced suppression of leptin gene expression was less in the
24-mo-old rats (41 ± 7.8% decrease) compared with either the
3-mo-old (61 ± 6.2%, P < 0.04)
or the 31-mo-old rats (69 ± 5.5%,
P < 0.007). Similar to RTWAT, there
was no change in -actin mRNA levels in IWAT with age (Table 3),
whereas
-actin mRNA levels in IWAT decreased with fasting.
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Serum leptin levels. Because the effects of fasting and age on leptin mRNA levels were variable in the three WATs investigated and because the size of each WAT changed with age, for a better representation of the effect of fasting and age on whole body leptin synthesis, serum leptin was determined. Serum leptin levels increased with age (Fig. 4A). Fasting suppressed serum leptin levels at each age. Furthermore, the percent decreases in serum leptin in the 3-mo-old rats (80 ± 2.7%) were significantly greater than in either the 24-mo-old (45 ± 5.1%, P < 0.0001) or the 31-mo-old rats (51 ± 5.0%, P < 0.0001).
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NPY mRNA levels in the hypothalamus.
Despite the elevated serum leptin levels with age, there were no
significant age-related differences in NPY mRNA levels (Fig. 4B). As expected, fasting
significantly increased hypothalamic NPY levels at each age
(P < 0.0001 for main effect of
feeding status). In addition, the percent increase in NPY mRNA levels was the greatest in the 3-mo-old (76 ± 21.0%) compared with the 24-mo-old (36 ± 12.1%,
P < 0.05) or the 31-mo-old rats (31 ± 9.6%, P < 0.04). There was no
change in -actin mRNA levels in the hypothalamus with either age or
fasting (Table 3).
Correlation between serum leptin and hypothalamic
NPY mRNA with age.
Under ad libitum fed conditions, aged rats had elevated leptin levels
and unchanged NPY mRNA levels (Fig. 4). Similar results were observed
for fasting conditions (Fig. 4). These data imply an impairment in the
leptin suppression of hypothalamic NPY synthesis with age. To examine
this further, the correlation between serum leptin and hypothalamic NPY
was determined at each age in ad libitum fed and fasted rats. In the
3-mo-old rats, there was a strong inverse correlation
(r = 0.70,
P < 0.004; Fig.
5), whereas no significant correlation was
found in either 24-mo-old (r =
0.15, P < 0.6) or 31-mo-old
rats (r =
0.49,
P < 0.07).
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DISCUSSION |
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In the present study, we found that the fasting-induced suppression of leptin mRNA was variable with age in EWAT, RTWAT, and IWAT. In RTWAT, there was a decrease with age; in IWAT, there was an increase with age; and in EWAT, there was no consistent change with age. In addition, leptin mRNA levels were unchanged in EWAT with age, whereas in both RTWAT and IWAT, levels of leptin mRNA increased with age from 3 to 24 mo and were unchanged between 24 and 31 mo. These results were similar to the changes with age in the leptin mRNA in different WAT depots as reported previously (14). Because of these inconsistencies in the fasting regulation of leptin gene expression with age, we examined serum leptin levels, which reflect the sum of all peripheral leptin production. It was not surprising to find that fasting suppressed serum leptin levels at each age. What was surprising, however, was that this suppression was more severe at 3 mo compared with either of the older ages, paralleling the findings with respect to leptin mRNA in RTWAT. Thus there was an age-related impairment in the ability of fasting to suppress serum leptin levels.
Similarly, we observed an age-related impairment in the ability of fasting to increase NPY mRNA levels, i.e., fasting induced a greater increase at 3 mo compared with either of the two older ages. Leptin is known to inhibit NPY synthesis in the hypothalamus (24, 27). The present data indicated a close correspondence between the fasting-induced suppression in serum leptin and the fasting-induced increase in hypothalamic NPY message: at 3 mo, fasting induced an 80% decrease in leptin and a 76% increase in NPY; at 24 mo, a 45% decrease in leptin and a 36% increase in NPY; and at 31 mo, a 51% decrease in leptin and a 31% increase in NPY. This close correspondence suggests that NPY gene expression is responsive to serum leptin levels, although to a lesser degree in the aged rats. Moreover, the impairment with age in the fasting-induced increase in NPY mRNA may be the result of deficient fasting suppression of serum leptin.
Our results regarding the effect of aging on leptin gene expression and serum leptin levels were consistent with what we have reported previously (14). Leptin mRNA levels per unit of white fat, i.e., per microgram total RNA, were increased with age in IWAT and unchanged in EWAT. In addition, changes in leptin mRNA levels with age in RTWAT were similar to the changes with age in IWAT, increasing with age from 3 to 24 mo and unchanging between 24 and 31 mo. With contribution from both the increased adiposity with age and the increased leptin mRNA expression per unit of white fat, serum leptin levels increased with age and then leveled off between the two older ages.
Adiposity measured by the Lee index increased from 3 to 24 mo and leveled off at 24 and 31 mo, conforming to the results found earlier (14). It is also interesting to note that the Lee index was sensitive enough to detect the decrease in adiposity by fasting.
EWAT, RTWAT, and IWAT weights all increased from 3 to 24 mo and leveled off between 24 and 31 mo. These data confirm what we reported previously: that IWAT and EWAT increased their weights from 3 mo and peaked at 24 mo, followed by a slight decline at 30 mo (14). These increases in the sizes of WAT depots with age contribute to the increase in adiposity and the elevation of serum leptin with age. Strong correlations were found between these fat weights and both Lee index and serum leptin levels. Additionally, RTWAT differed from the other two WATs in that its weight decreased with fasting at each age.
By measuring weight change during the period of ad libitum feeding or fasting, we found that F-344xBN rats continued to gain at 24 mo but stopped at 31 mo under ad libitum feeding conditions. This result is in line with the finding that the F-344xBN rat strain demonstrates a steady increase in body fat into early senescence (25). On the other hand, the ability to lose weight by fasting was similar across all ages.
In contrast to serum leptin levels, NPY levels in the hypothalamus were unchanged with age. This may explain why aged, more obese F-344xBN rats consume the same amount of food per day as their young, lean counterparts. Similar results in daily food consumption were found in the present study and in our recent study (14). These data are also similar to those of Gruenewald et al. (9) in that the daily food intake of middle-aged rats (12 mo) was not different from that of the old rats (24 mo). When food intake independent of body mass was considered, we found a decrease between 3 and 24 mo and no change between 24 and 30 mo. This was again in line with the findings of Gruenewald et al. However, in contrast to our finding, Gruenewald et al. found an age-related decrease in NPY gene expression. There were several major methodological differences between the two reports, which may explain the differences in NPY mRNA levels with age. One of the main differences is the time at which rats were killed with respect to the light-dark cycle: we killed the rats 2 h after lights-off, whereas Gruenewald et al. presumably killed their rats in the light phase. NPY mRNA demonstrates a circadian rhythm with a sustained increase in the second one-half of the light phase and then a sharp decline around lights-off (1). With age, the most commonly reported change in circadian rhythm is a diminished amplitude, i.e., the difference between the peak and the trough of the rhythm (20). Thus it is possible that young and aged rats may have a difference in their NPY mRNA levels in the light phase, when NPY mRNA level is at its peak, but not during the dark phase, when NPY mRNA level is at its baseline. In another study (unpublished data) from this laboratory, when we killed the rats during the light phase, we found a decrease in NPY mRNA with age similar to that reported by Gruenewald et al. Because we are examining the effect of fasting on leptin mRNA and NPY mRNA and because the normal feeding period for rodents is in the dark phase, assessment of NPY mRNA during dark phase may be more appropriate. There are other differences between the two studies as well. Different rat strains were used: we used F-344xBN rats, and they used BN rats. Whereas our rats were individually housed at thermoneutral temperature (26°C), at which level both young and aged rats maintain minimal oxygen consumption (23), their animals were doubly housed at an unspecified temperature (presumably room temperature). These housing conditions may affect the animal's energy balance and food intake, which may in turn influence NPY gene expression. Gruenewald et al. did not measure leptin mRNA or serum leptin levels; thus the relationship between NPY gene expression and serum leptin was not determined in that study.
With age, the increased serum leptin levels and unchanged hypothalamic NPY mRNA levels present a dilemma (24, 27). The inability of high peripheral leptin levels in aged rats to suppress NPY mRNA levels suggests that there is a leptin resistance with age. Further supporting this conclusion is the relationship between serum leptin and hypothalamic NPY mRNA levels within each age group of rats. There was a strong negative correlation between serum leptin and hypothalamic NPY mRNA in 3-mo-old rats, whereas the two aged groups demonstrated no significant correlation. Thus the aged rats demonstrated an impaired ability for leptin to suppress hypothalamic NPY message. One possibility for this impairment is that leptin levels in the cerebral spinal fluid do not parallel serum levels, such that leptin levels are not elevated in the cerebral spinal fluid with age. Another possibility is that there is impairment in the leptin receptor binding and/or the leptin signal transduction pathway (i.e., leptin resistance) with age that results in insufficient suppression of NPY.
In summary, the regulation of leptin gene expression, leptin serum levels, and hypothalamic NPY gene expression by fasting is impaired in aged rats. These data suggest that aged rats are leptin resistant, i.e., they have both increased adiposity and elevated serum leptin levels, whereas NPY mRNA levels in the hypothalamus are unchanged.
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ACKNOWLEDGEMENTS |
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This research was supported by the Medical Research Service of the Department of Veterans Affairs (P. J. Scarpace) and National Institute on Aging Grant AG-11465 (P. J. Scarpace) and Training Grant AG-00196-08 (H. Li).
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FOOTNOTES |
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Address for reprint requests: P. J. Scarpace, Geriatric Research, Education and Clinical Center (182), Dept. of Veterans Affairs Medical Center, Gainesville, Florida 32608-1197.
Received 29 December 1997; accepted in final form 12 May 1998.
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REFERENCES |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
1.
Akabayashi, A.,
N. Levin,
X. Paez,
J. T. Alexander,
and
S. F. Leibowitz.
Hypothalamic neuropeptide Y and its gene expression: relation to light/dark cycle and circulating corticosterone.
Mol. Cell. Neurosci.
5:
210-218,
1994[Medline].
2.
Bernardis, L. L.,
R. Luboshitsky,
L. L. Bellinger,
and
G. McEwen.
Nutritional studies in the weaning rat with normophagic hypothalamic obesity.
J. Nutr.
112:
1441-1455,
1982[Medline].
3.
Bernardis, L. L.,
and
B. D. Patterson.
Correlation between "Lee index" and carcass fat content in weanling and adult female rats with hypothalamic lesions.
J. Endocrinol.
40:
527-528,
1968[Medline].
4.
Brozek, J.
Changes in body composition in man during maturity and their nutritional implications.
Federation Proc.
11:
784-793,
1952.
5.
Campfield, L. A.,
F. J. Smith,
Y. Guisez,
R. Devos,
and
P. Burn.
Recombinant mouse ob protein: evidence for a peripheral signal linking adiposity and central neural networks.
Science
269:
546-549,
1995[Medline].
6.
Chomczynski, P.,
and
N. Sacchi.
Single-step method of RNA isolation by acid guanidinium thiocyanate-phenol-chloroform extraction.
Anal. Biochem.
162:
156-159,
1987[Medline].
7.
Clark, J. T.,
P. S. Kalra,
W. R. Crowley,
and
S. P. Kalra.
Neuropeptide Y and human pancreatic polypeptide stimulate feeding behavior in rats.
Endocrinology
115:
427-429,
1984[Abstract].
8.
Epstein, F. H.,
and
M. Higgins.
Epidemiology of obesity.
In: Obesity, edited by P. Bjorntorp,
and B. N. Brodoff. Philadelphia, PA: Lippincott, 1992, p. 330-342.
9.
Gruenewald, D. A.,
B. T. Marck,
and
A. M. Matsumoto.
Fasting-induced increases in food intake and neuropeptide Y gene expression are attenuated in aging male brown Norway rats.
Endocrinology
137:
4460-4467,
1996[Abstract].
10.
Halaas, J. L.,
K. S. Gajiwala,
M. Maffei,
S. L. Cohen,
B. T. Chait,
D. Rabinowitz,
R. L. Lallone,
S. K. Burley,
and
J. M. Friedman.
Weight-reducing effects of the plasma protein encoded by the obese gene.
Science
269:
543-546,
1995[Medline].
11.
Heusner, A.
Body size and energy metabolism.
Annu. Rev. Nutr.
5:
267-293,
1985[Medline].
12.
Lee, M. O.
Determination of the surface area of the white rat with its application to the expression of metabolic results.
Am. J. Physiol.
89:
24-31,
1929.
13.
Levine, A. S.,
and
J. E. Morley.
Neuropeptide Y: a potent inducer of consummatory behavior in rats.
Peptides
5:
1025-1029,
1984[Medline].
14.
Li, H.,
M. Matheny,
M. Nicolson,
N. Tümer,
and
P. J. Scarpace.
Leptin gene expression increases with age independent of increasing adiposity in rats.
Diabetes
46:
2035-2039,
1997[Abstract].
15.
Li, H.,
M. Matheny,
and
P. J. Scarpace.
3-Adrenergic-mediated suppression of leptin gene expression in rats.
Am. J. Physiol.
272 (Endocrinol. Metab. 35):
E1031-E1036,
1997
16.
Malina, R. M.
Quantification of fat, muscle and bone in man.
Clin. Orthop.
65:
9-38,
1969[Medline].
17.
Meier, C. A.
Advances in the understanding of the molecular basis of obesity.
Eur. J. Endocrinol.
133:
761-763,
1995[Medline].
18.
O'Shea, R. D.,
and
A. L. Gundlach.
Preproneuropeptide Y messenger ribonuceic acid in the hypothalamic arcuate nuceus of the rat is increased by food deprivation or dehydration.
J. Neuroendocrinol.
3:
11-14,
1991.
19.
Pelleymounter, M.,
M. J. Cullen,
M. B. Baker,
R. Hecht,
D. Winters,
T. Boone,
and
F. Collins.
Effects of the obese gene product on body weight regulation in ob/ob mice.
Science
269:
540-543,
1995[Medline].
20.
Richardson, G. S.
Circadian rhythms and aging.
In: Handbook of the Biology of Aging, edited by E. Schneider,
and G. Rowe. New York: Academic, 1990, p. 275-305.
21.
Saladin, R.,
P. De Vos,
M. Guerre-Millo,
A. Leturque,
J. Girard,
B. Staels,
and
J. Auwerx.
Transient increase in obese gene expression after food intake or insulin administration.
Nature
377:
527-529,
1995[Medline].
22.
Scarpace, P. J.,
and
M. Matheny.
Leptin increases uncoupling protein expression and energy expenditure.
Am. J. Physiol.
273 (Endocrinol. Metab. 36):
E226-E230,
1997
23.
Scarpace, P. J.,
M. Matheny,
S. Borst,
and
N. Tümer.
Thermoregulation with age: roll of thermogenesis and uncoupling protein expression in brown adipose tissue.
Proc. Soc. Exp. Biol. Med.
205:
154-161,
1994[Abstract].
24.
Schwatz, M. W.,
D. G. Baskin,
T. R. Bukowski,
J. L. Kuijper,
D. Foster,
G. Lasser,
D. E. Prunkard,
D. Porte,
S. C. Woods,
R. J. Seeley,
and
D. S. Weigle.
Specificity of leptin action on elevated blood glucose levels and hypothalamic neuropeptide Y gene expression in ob/ob mice.
Diabetes
45:
531-535,
1996[Abstract].
25.
Sprott, R. L.,
and
S. N. Austad.
Animals models for aging research.
In: Handbook of the Biology of Aging, edited by E. L. Schneider,
and J. W. Rowe. San Diego: Academic, 1996, p. 3-23.
26.
Stanley, B. G.,
and
S. F. Leibowitz.
Neuropeptide Y injected into hypothalamus: a powerful neurochemical inducer of hyperphagia and obesity.
Proc. Natl. Acad. Sci. USA
82:
3940-3943,
1984.
27.
Stephens, T. W.,
M. Basinski,
P. K. Bristow,
J. M. Bue-Valleskey,
S. G. Burgett,
L. Craft,
J. Hale,
J. Hoffmann,
H. M. Hsiung,
A. Kriauciunas,
W. MacKellar,
P. R. Rosteck, Jr.,
B. Schoner,
D. Smith,
F. C. Tinsley,
X.-Y. Zhang,
and
H. Heiman.
The role of neuropeptide Y in the antiobesity action of the obese gene product.
Nature
377:
530-532,
1995[Medline].
28.
Taylor, B. A.,
and
S. J. Phillips.
Detection of obesity QTLs on mouse chromosomes 1 and 7 by selective DNA pooling.
Genomics
34:
389-398,
1996[Medline].
29.
Trayhurn, P.,
J. S. Duncan,
and
D. V. Rayner.
Acute cold-induced suppression of ob (obese) gene expression in white adipose tissue of mice: mediation by the sympathetic system.
Biochem. J.
311:
729-733,
1995[Medline].
30.
Trayhurn, P.,
M. E. A. Thomas,
J. S. Duncan,
and
D. V. Rayner.
Effects of fasting and refeeding on ob gene expression in white adipose tissue of lean and obese (ob/ob) mice.
FEBS Lett.
368:
488-490,
1995[Medline].
31.
White, J. D.,
and
M. Kershaw.
Increased hypothalamic neuropeptide Y expression following food deprivation.
Mol. Cell. Neurosci.
1:
41-48,
1990.
32.
Zhang, Y.,
R. Proenca,
M. Maffei,
M. Barone,
L. Leopold,
and
J. Friedman.
Positional cloning of the mouse obese gene and its human homologue.
Nature
372:
425-432,
1994[Medline].