Vol. 279, Issue 1, E33-E43, July 2000
Energetic response to repeated restraint stress in rapidly
growing mice
Kevin D.
Laugero and
Gary P.
Moberg
Stress Research Unit, Department of Animal Science, University of
California, Davis, California 95616
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ABSTRACT |
There is a cost of stress that may result in the loss of
normal biological function (e.g., growth). Repeated, and even single, applications of stressors have been shown to induce negative energy balance in rodents. However, here we addressed whether this energetic response changes during multiple stress exposure and whether there is
complete recovery subsequent to the cessation of stress exposure. These
questions were addressed in growing C57Bl/6 mice (31 day) by
determining at different times the energetic and endocrine responses
after the exposure to restraint (R) stress for 4 h applied once
(R1), repeatedly over 3 days (R3), or repeatedly over 7 days (R7).
Compared with control values, R elevated (P < 0.05)
plasma corticosterone and reduced plasma insulin-like growth factor I on all days of exposure to the stressor. Seven days, but not 1 or 3 days of R, decreased the net growth (126%, P < 0.05)
and deposition of fat (71%, P < 0.05) and lean (60%,
P < 0.05) energy over the 7 days. Only R7 depressed
the 7-day metabolizable energy intake (P < 0.05), and
R7, but not R1 or R3, increased the overall energy expenditure (10%,
P < 0.05). Our results demonstrate that repeated
episodes of stress are energetically costly to the rapidly growing
animal, but compensatory mechanisms mitigate this cost of repeated
stress exposure and permit complete recovery of energy balance after
the cessation of stress application.
growth; energy partitioning; feed intake; corticosterone
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INTRODUCTION |
STRESS
CAUSES A SHIFT in biological function, a shift that comes at a
biological cost to the individual (47). During stress, changes in metabolism help support the biological defenses an individual uses to maintain homeostasis (13,
21, 48, 57). These metabolic
alterations during stress often result in the mobilization of energy
away from energy-sensitive functions, such as growth in adolescents and
maintenance of body weight in adults (12, 21,
57). As a result, stress can cause negative energy balance. For example, stress characteristically results in depressed body weight and food intake in rodents (16,
27, 36).
Previous reports have demonstrated that the stress-induced depression
in body weight is maintained over time, even after the exposure to
stress has ceased (27, 52). Although repeated stress (3 daily episodes) was reported to reduce food intake during and
subsequent to the last day of stress exposure, one episode only of the
stressor depressed food consumption on the day of stress. When the
three daily episodes of restraint were applied to young (3-wk-old)
rats, reductions in food intake and especially body weight were not as
marked or as persistent as those observed in adult rats exposed to the
repeated stressor. Results from a study by Marty et al.
(42) suggest that chronic, intermittent restraint and
immobilization maintain their inhibitory effects on growth and food
intake in adult rats over the entire 4-wk experiment.
These findings, and previous results from studies conducted in our
laboratory (39), raise two interesting questions about rapidly growing mice: 1) is the energetic response altered
over the course of daily exposure to restraint stress? and
2) is there recovery of energy and body weight after the
stress-induced depression of growth? To address these questions, we
evaluated the energetic response to repeated restraint stress and
poststress recovery in young, postweanling mice.
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MATERIALS AND METHODS |
Male C57Bl/6 mice (B&K Universal, Fremont, CA) were individually
housed in hanging wire mesh cages in a room maintained at 23 ± 2°C on a 14:10-h lighting schedule. Before each experiment, mice were
acclimated for 1 wk, and all experiments began when mice were 31 days
of age. Each experiment lasted 7 days, beginning at 0700 on day
1 and ending at 0700 on day 8. Each day at 0700, body
weight was recorded, feed was removed, and feed intake (corrected for
spillage) was measured. All mice were then returned to their home cage.
On day 1 of each experiment, all mice were weighed and the
treated group was exposed to 4 h of restraint stress by placing each mouse into a well-ventilated restraint tube. During the 4-h restraint period, both experimental and control (nonrestrained) mice
had no access to food or water. The restraint tube was a well-ventilated 50-ml plastic centrifuge tube. The tube was tied to the
bottom of the cage with Silastic tubing, which was not accessible to
the mouse. Several hole punches were evenly distributed throughout the
length of each tube, and a bottom section of each tube was removed to
allow free drainage of urine and feces. Once the tube had been fastened
to the cage, the mouse was placed in the tube and a paper towel plug
was used to prevent the mouse from escaping. Mice could move freely
back and forth but could not turn around while in the tube. After the
4 h of restraint, mice were removed from the tube, and the tube
was removed from the cage.
Immediately after the 4-h restraint period, both restrained and control
mice were fed a semipurified test diet (PMI AIN-76A, PMI Feeds, St.
Louis, MO) having a guaranteed analysis of 18.4% protein, 5.0% fat,
65.0% carbohydrate, and 5.0% fiber. With the exception of the 4-h
restraint period, all mice had ad libitum access to feed and water. All
experiments were approved by the University of California, Davis Animal
Care and Use Committee.
Experiment 1a: Effect of repeated behavioral stress on growth and
energetics.
To evaluate the cost of repeated behavioral stress, we quantified
growth and energetics of growing mice repeatedly exposed (R,
n = 6) once/day for 7 days, or not exposed (Con,
n = 8) to restraint stress. A comparative slaughter
experimental design was employed to quantify any changes in 7-day
protein (lean) and fat tissue energy. On the 1st day of experimentation
and before the initiation of treatment, an initial group of mice (31 day) was decapitated and dissected into eviscerated carcass (C),
gastrointestinal tract (contents removed) + liver (G), and
remaining viscera (V). These components were analyzed for water
(freeze-drying to constant weight), fat (difference in weight of dried
component before and after ether-acetone extraction), and protein
(Kjeldahl N × 6.25) content. Carcass lean energy (CLE),
GI/Liver lean energy (GLE), and viscera lean energy (VLE) were
determined by multiplying the dry gram protein content by the energy
content of protein (assumed to be 5.4 kcal/g). Carcass fat energy
(CFE), GI/Liver fat energy (GFE), and viscera fat energy (VFE) were
determined by multiplying the dry gram content by the energy content of
fat (assumed to be 9.0 kcal/g). Regression equations were generated
from these data to express C, G, and V water, FE (fat energy), and LE
(lean energy) as linear functions of body weight at age 31 days (Table 1). These regression equations were used
to predict the initial state (i.e., FE, LE) of experimental mice.
On the final day of experimentation, experimental mice were
decapitated, dissected, and analyzed as described for 31-day-old mice.
Changes in C, G, and V water, fat energy (FE
), and lean energy
(LE) were determined by the difference between the final (measured)
and the initial (predicted from regression equations) values. Total
body change in lean and fat energy content was taken as the sum of C,
G, and V energy changes. Change in total (lean + fat) body energy
(BE) was taken as the sum of LE and FE. The 7-day
metabolizable energy intake (MEI) was calculated as the gram intake
multiplied by the metabolizable energy content of the diet (3.79 kcal/g
at maintenance, PMI Feeds). For mice gaining protein, 1.4 kcal/g
protein gain was added to the value of total MEI. Energy expenditure
(heat energy) was estimated by taking the difference between MEI and
BE.
Experiment 1b: Effect of repeated behavioral stress on
corticosterone and insulin-like growth factor I.
To obtain sufficient blood for hormone analyses and to avoid any stress
associated with blood collection, it was necessary to kill mice by
decapitation (within 30 s of removal from their home cage).
Therefore, this experiment paralleled experiment 1a to
examine the effects of repeated behavioral stress on circulating corticosterone and insulin-like growth factor I (IGF-I) over the 7-day
experimental period. In experiment 1b, mice were randomly assigned to 1 of 8 day/treatment combinations, and blood was collected 4 h after the initiation of restraint on days 1, 3, 6, and 7. On each day of blood collection, four mice from each
treatment group were decapitated. Restraint was initiated as described
for experiment 1a, and at the selected times, a
predetermined group of mice was decapitated for the collection of trunk
blood into heparinized (15 U/ml blood) tubes kept on ice until plasma
was separated by centrifugation at 1,000 g at 4°C for 30 min. After centrifugation, plasma was collected and stored at 
70°C
until assayed for corticosterone and IGF-I.
Experiment 2a: Growth and energetics of poststress recovery.
In this experiment, we examined our young mice to determine if they
were capable of recovering from the stress-induced inhibition of
growth. To address this question, we first compared the effect of 1 (n = 9), 3 (n = 10), or 7 (n = 11) daily episodes of restraint stress only on
growth over 7 days. Controls (n = 10) were not exposed
to restraint. Subsequently, we replicated this experiment in a parallel
study to quantify the effects of these three stressors on energy
partitioning among energy expenditure and lean and fat tissues, as
described in experiment 1a. With the exception of R7
(n = 6), each of the Con, R1, and R3 treatment groups
represented an n of 5 in this parallel study of energy partitioning.
Experiment 2b: Effect of 1, 3, or 7 episodes of behavioral stress
on corticosterone and IGF-I.
To address the possibility of a prolonged or carryover effect of 1, 3, or 7 daily episodes of restraint on circulating corticosterone and
IGF-I, we examined the plasma concentration of these hormones at 4, 28, 76, and 168 h after the initiation (0700, day 1) of restraint. At the selected times, mice from each treatment group were
decapitated, and their trunk blood was collected for the analyses of
corticosterone and IGF-I. On each of the selected times, five or six
animals from each treatment group were decapitated for blood collection.
Hormone assays.
Plasma corticosterone concentrations were determined by RIA with a
corticosterone 125I system (ICN, Costa Mesa, CA). Samples
were run in duplicate, and the intra- and interassay coefficients of
variation were determined as 3.1 and 8.1%, respectively. Plasma IGF-I
was extracted using an acid-ethanol procedure with cryoprecipitation
(10). Human recombinant IGF-I (R&D Systems, Minneapolis,
MN) was iodinated and purified as described by Hodgkinson et al.
(29). IGF-I concentrations were determined by a
nonequilibrium RIA (10). The polyclonal IGF-I antiserum
(UB3-189), kindly provided by Drs. J. J. Van Wyk and L. Underwood, was obtained through the NIDDK National Hormone and
Pituitary Program. Samples were run in duplicate, and the intra- and
interassay coefficients of variation were determined as 4.2 and 8.2%, respectively.
Statistical analyses.
All data were analyzed using least squares ANOVA procedures (SAS/STAT
User's Guide, 1990). Comparisons between treatment groups for 7-day
MEI, changes in body weight, lean energy (LE), fat energy (FE),
body energy (BE), and energy expenditure were analyzed in a
statistical model that included the effect of treatment. For hormone
data, differences between treatment groups were analyzed in a model
that included the effects of treatment, day of collection, and their
interaction. Where indicated, as a means of adjusting energetic
responses to a common MEI, analysis of covariance was employed. Data
from experiments evaluating daily body weight, daily body weight gain,
daily relative body weight gain [body wt gain (g)/body wt (g) or body
wt (g.75)], feed intake, daily relative feed intake
[feed intake/body wt (g) or body wt (g.75)], and
daily feed conversion [body wt gain (g)/feed intake (g)] were
evaluated using a repeated-measures model (22). If a
significant treatment × day interaction was present in the
repeated-measures analysis, data were stratified by day and differences
between treatment groups were analyzed using least squares procedures (SAS/STAT User's Guide, 1990) in a statistical model that included the
effect of treatment. For all analyses, differences in means were
determined with the PDIFF option in PROC GLM (SAS/STAT User's Guide,
1990), and a level of P < 0.05 was considered
statistically significant. A P < 0.10 was considered
to indicate tendencies.
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RESULTS |
Experiment 1a: Effect of repeated behavioral stress on growth and
energetics.
Although the initial body weights did not differ before the initiation
of treatment (P > 0.10), repeated restraint stress significantly (P < 0.0002) depressed final body weight
and total body weight gain over the 7 days of repeated restraint (Fig.
1A). The repeated
behavioral stressor also reduced (P < 0.0005) daily body weight, daily body weight gain (Fig. 1B), and daily
feed intake (Fig. 1C). However, the effects of repeated
restraint on daily body weight and daily body weight gain were
dependent on the day (Pstress × day
interaction < 0.0001). Relative body weight gain and relative
feed intake were also reduced by repeated restraint stress
(P < 0.0001). Although repeated restraint stress
suppressed absolute feed intake on all 7 days, this stressor reduced
absolute and relative body weight gain and relative feed intake only on
days 1-4. To determine whether the stress-induced reduction in daily body weight was strictly due to differences in feed
intake between stressed and nonstressed mice, daily feed conversion was
calculated. Repeated restraint stress depressed (P < 0.0001) daily feed conversion on only the first 2 days (Fig. 1D).
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Fig. 1.
Experiment 1a: Effect of repeated
behavioral stress on daily body weight (A), body weight gain
(B), feed intake (C), and feed efficiency
(gain/feed) (D). Values are least-squares means ± SE.
C, control (n = 8,  ); R7, repeated
restraint (n = 6,  ).
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Repeated behavioral stress reduced (P < 0.0002) lean
and fat energy deposition (Fig. 2, A and
B), MEI (Fig.
3A), and total energy
expenditure (Fig. 3B) over the 7-day experiment. However, stress can increase basal expenditure of energy (1,
24, 51, 59), and because energy
expenditure depends on MEI (which was also reduced by the repeated
stressor), it seemed reasonable that the reduction in MEI offset a
stress-induced increase in basal energy expenditure in the present
experiment. To address this possibility, we also examined energy
expenditure after adjusting for differences in MEI between repeatedly
stressed and nonstressed mice. When the effect of MEI was removed,
energy expenditure was increased (P < 0.009) by the
repeated stressor (Fig. 3C).
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Fig. 2.
Experiment 1a: Effect of repeated behavioral
stress on change in total body (A) and eviscerated carcass
(B) lean energy (LE) and fat energy (FE) over 7 days. Values
are least-squares means ± SE. C, control (n = 8);
R7, repeated restraint over 7 days (n = 6).
* Significantly different from C (P < 0.05).
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Fig. 3.
Experiment 1a: Effect of repeated behavioral
stress on metabolizable energy intake (MEI, A), heat energy
(B), and heat energy adjusted to a common MEI
(C). Values are least-squares means ± SE. C, control
(n = 8); R7, repeated restraint (n = 6). * Significantly different from C (P < 0.05).
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Experiment 1b: Effect of repeated behavioral stress on
corticosterone and IGF-I.
On each day of determination, restraint stress increased
(P < 0.0001) circulating corticosterone (Fig.
4A) and reduced
(P < 0.0001) circulating IGF-I (Fig. 4B).
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Fig. 4.
Experiment 1b: Effect of repeated behavioral
stress on plasma corticosterone (A) and insulin-like growth
factor I (IGF-I, B) 4 h after initiation of stressor.
Values are least-squares means ± SE. C, control
(n = 4); R7, repeated restraint (n = 4). * Significantly different from C (P < 0.05).
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Experiment 2a: Growth and energetics of poststress recovery.
Initial body weights did not differ (P > 0.10) between
treatment groups. One, three, or seven episodes of restraint stress initially depressed body weight (P < 0.0001), body
weight gain (P < 0.0001), and feed intake
(P < 0.0001) in all animals (Fig. 5). R1, R3, and R7 equally depressed each
of these parameters from day 1 to day 2 (P > 0.1). Likewise, R3 and R7 depressed body weight,
weight gain, and feed intake to the same degree over the first 3 days
(P > 0.1). As in experiment 1, only those
mice that were exposed to 7 days of repeated restraint stress had
depressed final body weight (P < 0.0001) and total
body weight gain (P < 0.0001) over the 7-day
experiment. However, body weight was restored to control levels by
day 3 in animals exposed to one episode of restraint and by
day 7 in mice exposed to three episodes of the stressor.
Mice exposed to one or three daily episodes of restraint stress
appeared to respond with enhanced feed intake, feed conversion, and
growth after the last episode of restraint. To answer this question,
the growth and feed intake data were analyzed from day 2 to
day 8 for those animals that experienced one episode of
restraint and from day 4 to day 8 for mice
exposed to three episodes of restraint. From day 2 to
day 8, separate analyses were made between control mice and
mice exposed to a single episode of restraint and between mice exposed
to one or seven episodes of restraint. From day 4 to
day 8, we compared the data, in two separate analyses, between control mice and mice exposed to three episodes of restraint and between mice exposed to three or seven episodes of restraint.
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Fig. 5.
Experiment 2a: Effect of 1 episode (R1), 3 episodes (R3), or 7 episodes (R7) of behavioral stress on daily body
weight (A), body weight gain (B), feed intake
(C), and feed conversion (gain/feed) (D). Values
are least-squares means ± SE. C, control (n = 10); R1 (n = 9); R3 (n = 10); R7
(n = 11).
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Compared with seven daily episodes of restraint stress and controls,
one episode of the stressor altered (P < 0.0001) daily body weight, daily body weight gain, daily feed intake, and daily feed
conversion from day 2 to day 8. However, these
effects of a single episode of restraint on body weight, body weight
gain, daily feed intake, and daily feed conversion depended on the
day (Pstress × day interaction < 0.0001). Exposure to one restraint episode increased body weight gain
only on day 2 (P < 0.0011) and day
3 (P < 0.0358). Only on day 3 was the
relative feed intake of mice exposed to a single episode of stress
higher (P < 0.007) than the relative feed intake of
nonrestrained mice. However, one episode of restraint increased
(P < 0.005) the relative feed intake above that of
mice exposed to seven episodes of the stressor on days
2-4. One episode of restraint increased (P < 0.0033) the feed conversion on day 2, relative to controls,
and on days 2 and 3 compared with mice repeatedly
restrained over 7 days (P < 0.0004).
From day 4 to day 8, exposure to three daily
episodes of behavioral stress altered (P < 0.0001)
daily body weight, daily body weight gain, and daily feed conversion,
and with the exception of daily body weight
(Pstress × day interaction < 0.0001),
three daily episodes of the stressor increased daily body weight gain
and daily feed conversion on each of the 4 days after the 3 days of
stress exposure (Pstress × day interaction
> 0.10). Relative to nonrestraint, three restraint episodes
increased (P < 0.003) relative feed intake on
days 4-6. Compared with seven restraint episodes, three
episodes of the stressor increased (P < 0.003)
relative feed intake on days 4-6.
Subsequent to this experiment, which compared the effect of one, three,
or seven daily episodes of restraint stress only on growth, a parallel
study examined the energetic response to one, three, or seven daily
episodes of the stressor. Only seven daily episodes of repeated
restraint stress altered (P < 0.05) energy partitioned
into lean and fat tissues over the 7 days (Fig.
6). Compared with nonrestraint and one or
three episodes of restraint, seven daily episodes of the stressor
depressed (P < 0.05) the deposition of energy into
total and carcass lean and fat tissues. Relative to controls and a
single restraint episode, three and seven episodes of the stressor
suppressed (P < 0.05) MEI. However, the MEI of mice
exposed to seven daily episodes of behavioral stress was even lower
(P < 0.05) than the MEI of mice exposed to three
stress episodes. Overall (7-day) energy expenditure was not affected by
restraint (P > 0.10), but when heat energy was adjusted for differences in MEI, only mice repeatedly exposed to seven
daily episodes of restraint had a higher (P < 0.05)
overall energy expenditure than nonstressed mice and mice exposed to
one or three daily restraint episodes.
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Fig. 6.
Experiment 2a: Effect of 1 episode (R1), 3 episodes (R3), or 7 episodes (R7) of behavioral stress on total and
eviscerated carcass LE change (A), total and eviscerated
carcass FE change (B), heat energy adjusted to a common MEI
(C), and MEI (D). Values are least-squares
means ± SE. C, control (n = 5); R1
(n = 5); R3 (n = 5); R7
(n = 6). Groups with different letters are
significantly different (P < 0.05).
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Experiment 2b: Effect of one, three, or seven daily episodes of
behavioral stress on corticosterone and IGF-I.
The behavioral stressor elevated (P < 0.0001)
circulating corticosterone on each of the designated sampling days,
when sampling occurred 4 h after the initiation of the stressor
(Fig. 7). The corticosterone
concentrations of mice experiencing a single episode of restraint
stress on day 1 did not differ from control levels on
days 2, 4, and 8. Likewise, the plasma
concentrations of corticosterone in mice exposed to only three episodes
of the stressor did not differ from control concentrations on
days 4 and 8.
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Fig. 7.
Experiment 2b: Effect of 1 episode (R1,
A), 3 episodes (R3, B), or 7 episodes (R7,
C) of behavioral stress on plasma corticosterone 4 h
after initiation of the stressor on days 1, 2, and
4. On day 8, blood was collected 24 h after
initiation of the stressor on day 7. Values are
least-squares means ± SE. C, control (n = 5); R1
(n = 5); R3 (n = 5); R7
(n = 6). * Significantly different from C
(P < 0.05).
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Each daily episode of restraint stress decreased
(P < 0.0001) circulating IGF-I 4 h after the
initiation of the stressor (Fig. 8).
Twenty-four hours after the initiation of the stressor on day
7, IGF-I tended (P < 0.10) to be lower in mice
exposed to seven daily episodes of restraint. On days 2, 4, and 8, there were no differences in circulating IGF-I
concentrations between controls and mice exposed to a single restraint
episode. Mice exposed to three episodes of the stressor had comparable
IGF-I concentrations to controls on days 4 and 8.
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Fig. 8.
Experiment 2b: Effect of 1 episode (R1,
A), 3 episodes (R3, B), or 7 episodes (R7,
C) of behavioral stress on plasma IGF-I 4 h after
initiation of stressor on days 1, 2, and 4. On
day 8, blood was collected 24 h after initiation of
stressor on day 7. Values are least-squares means ± SE. C, control (n = 5); R1 (n = 5); R3
(n = 5); R7 (n = 6). * Significantly
different from C (P < 0.05). ** Tended to be
different from C (P < 0.07).
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DISCUSSION |
We found that repeated exposure to an acute episode of restraint
stress induced a cost that significantly suppressed normal growth in
the growing mouse. This cost to normal growth resulted, at least in
part, from altered deposition of energy into lean and fat tissues.
Although the overall effect of the repeated stressor was inhibitory,
the energetic response to daily restraint was not constant over the
7-day study. Interestingly, although there was an attenuated energetic
response to repeated restraint, there were comparable and repeated
increases in circulating corticosterone and decreases in IGF-I each
time mice were exposed to the stressor. These results support the view
that stress can lead to a shift of energy from growth to those
functions that prepare the individual for survival (12,
21, 25, 53, 58).
Furthermore, our results demonstrate that there is a shift in the
energetic response to repeated stress from energy mobilization to
energy preservation in young, rapidly growing mice.
Although the body weights of mice exposed to one or three episodes of
restraint were initially depressed, the body weights returned to
control levels because of a rapid enhancement of growth after the last
exposure to stress. In fact, mice exposed to only one or three episodes
of the stressor failed to show any net impairment of lean and fat
energy deposition, which was in contrast to the animals exposed to
seven daily episodes of stress. We previously found (40)
that a single episode of restraint significantly reduced the energy
deposited into lean and fat tissue 24 h after the initiation of
restraint. Therefore, those results and our present work suggest that 1 or 3 days of restraint stress initially impaired energy deposition, but
during the poststress period, this energy was replenished by increased
rates of energy deposition into both lean and fat tissues. As a result,
the mice overcame the inhibitory effects of stress on growth by a
period of compensatory growth that enabled them to fully recover energy
and normal body weight.
Although other investigators (27, 52) have
demonstrated a sustained reduction of body weight after 3 days of
restraint stress, this very interesting finding was in adult rats. The
drive for growth and the attainment of a mature body weight may have been so great in our mice that compensatory mechanisms to ensure such a
body weight, after stress, are intact in such young animals. In
contrast, repeated exposure to behavioral stress on all 7 days prevented any compensatory growth, recovery of energy, or the recovery
of normal body weight by the final day after seven daily episodes of
stress. It is quite possible that, given a recovery period after the 7 days of repeated stress, these mice would also have recovered. However,
the longer an individual is incapable of reaching normal stature, the
greater that individual is at risk for developing pathologies related
to abnormal growth. For example, psychosocial short stature resulting
from chronic stress is related to delayed puberty, eating disorders,
and/or vulnerability to disease (58).
Although determining the mechanisms that lead to altered energy
deposition and growth was not the primary focus of this study, we did
evaluate factors known to influence these functions. Feed and
metabolizable energy consumption were significantly reduced by repeated
behavioral stress, and this response is consistent with the reduced
feed or energy intake that results from other types of stress
(18, 33, 34, 37).
On the other hand, feeding was enhanced during the recovery period
after either one or three episodes of stress. Growth depends on
nutrient and energy availability, as dictated by intake and its
partitioning among tissues (25). Therefore, it is possible
that, in this study, the stress-induced reduction in feed and energy
intake was responsible for impaired growth, whereas increased intake
may have stimulated catch-up growth seen in recovering mice. However,
the depressed feed conversion in mice repeatedly exposed to seven
episodes of behavioral stress and the enhanced feed conversion in
recovering mice indicate that some factor(s) other than altered feed
intake affected growth. However, the total effect of the repeated
stress was not completely independent from this altered feed
consumption, as suggested by the treatment × day interaction
effect on daily feed conversion in repeatedly stressed mice and mice
recovering from one episode of restraint. Daily feed conversion in mice
recovering from three episodes of stress was, however, significantly
enhanced on each of the 4 days after the last day of stress, suggesting
that mechanisms controlling the efficiency of energy deposition were
necessary for rapid recovery of a normal body weight.
Changes in energy expenditure may also explain the altered growth and
energy deposition in our repeatedly stressed and recovering mice. In
this study, both MEI and total heat energy production were reduced by
the repeated exposure to stress. However, this does not preclude the
possibility that stress increased basal heat energy production
(1, 20, 24, 51,
59). In fact, when differences in MEI between stressed and
control mice were removed, only repeated exposure to seven episodes of
restraint caused significantly higher net energy expenditure. Thus,
although the total energy expenditure may have been suppressed by the
significant reduction in MEI (7, 8,
17, 20, 35), the examination of
energy expenditure, independent from the effects of MEI, suggests that
mice repeatedly exposed to stress had an elevated basal energy expenditure. This stress-induced increase in basal energy expenditure probably caused the overall expenditure to be higher than one would
expect from an equivalent reduction of MEI in nonstressed mice.
Therefore, the stress-induced elevation in basal expenditure of energy
is a cost that increases the quantity of energy partitioned into heat
as opposed to growth, and may, in part, have accounted for the impaired
growth in the repeatedly stressed mice.
Whereas increased basal energy expenditure may have inhibited normal
energy deposition in mice exposed to seven episodes of restraint, mice
recovering from stress may have incurred a greater capacity for energy
deposition because of lowered basal energy expenditure over the period
of recovery. Although our measure of energy expenditure reflects the
net sum of all 7 days, the energy expended over the specific period of
recovery may have been reduced. A reduced expenditure of energy has
been shown to occur in animals experiencing compensatory growth after a
period of nutritional stress (19). Because, for a given
intake, a lower basal energy expenditure can lead to a greater
efficiciency of energy deposition (4), the enhanced feed
conversion in our recovering mice may reflect a depressed energy
expenditure during the recovery period.
Finally, the hormonal status of an individual dictates energy
partitioning, and thus growth capacity (21,
25, 53). Thus stress-induced alterations in
hormones known to affect this partitioning may affect growth. We
measured circulating concentrations of corticosterone and IGF-I in the
present study. IGF-I is an important growth factor (41,
50, 55), and its circulating levels are
positively correlated with body size and growth (5,
6, 45, 46). Therefore, repeated
inhibition of this growth factor may have contributed to the impaired
growth in mice exposed repeatedly to the behavioral stress.
Additionally, it has been reported that elevated circulating
corticosterone, as seen in this study, has catabolic effects on both
protein and fat tissue (30) and inhibits growth and feed
efficiency (15, 16, 56).
Therefore, the repeated increases in circulating corticosterone in mice
exposed repeatedly to restraint stress may have contributed to the
alterations in energy partitioning and growth in these behaviorally
stressed mice.
It remains uncertain whether changes in the growth hormone/IGF-I and
adrenal-cortical systems play a role in stimulating compensatory growth. Our results showed that circulating concentrations of corticosterone and IGF-I in mice exposed to restraint only on day
1 were similar to the concentrations of experimental controls on
day 2, and mice exposed to restraint on days
1-3 had circulating concentrations of corticosterone and
IGF-I similar to those in experimental controls when the levels of
these hormones were determined on day 4. However, our
morning determination of corticosterone and IGF-I is not necessarily
indicative of the animal's hormonal status at any given time
throughout the period of recovery, and thus we cannot exclude these
hormones from those factors regulating the enhanced growth of our
recovering mice. Furthermore, other factors, such as the receptors and
binding proteins for corticosterone and IGF-I, may alter the impact of
these circulating hormones on growth-related processes.
We demonstrated that repeated episodes of stress are energetically
costly to the rapidly growing animal. However, the results suggest that
there are counterregulatory mechanisms that become engaged to attenuate
the detrimental effects of repeated stress on growth. Furthermore,
there exist poststress mechanisms that induce rapid recovery of energy
in young mice. Thus there appear to be mechanisms that serve to
preserve excessive energy loss during repeated stress and to enhance
poststress recovery of body weight in the young, rapidly growing animal.
| |
ACKNOWLEDGEMENTS |
We thank Dr. Chris Calvert for expert advice on growth and
energetics and guidance in many areas of the experimentation. Our thanks go to Dr. Anita M. Oberbauer for help with the IGF-I assay and
to Dr. Edward J. DePeters and Scott Taylor for technical help with
nutritional analyses.
| |
FOOTNOTES |
Deceased 13 August 1999.
Address for reprint requests and other correspondence: K. D. Laugero, Department of Physiology, University of California, San
Francisco, CA 94143-0444 (E-mail: laugero{at}itsa.ucsf.edu).
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. §1734 solely to indicate this fact.
Received 11 May 1999; accepted in final form 22 February 2000.
| |
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