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Departments of Anatomy/Neurobiology and Pediatrics (S.A.-E., M.E.-A., K.L.B., T.Z.B.), University of California at Irvine, Irvine, California 92697-4475; and Hebrew University (S.A.-E., E.T.), Jerusalem and Kaplan Medical Center, Rehovoth, Israel 76100
Address all correspondence and requests for reprints to: Tallie Z. Baram, M.D., Ph.D., Med Sci I; ZOT 4475, University of California at Irvine, Irvine, California 92697-4475. E-mail: tallie{at}uci.edu
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Abstract |
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To elucidate the critical primary genes governing this molecular cascade, we determined the sequence of changes in GR-mRNA levels and in hypothalamic and amygdala CRH-mRNA expression at three developmental ages, and the temporal relationship between each of these changes and the emergence of reduced hormonal stress-responses.
Down-regulation of hypothalamic CRH-mRNA levels in daily-handled rats was evident already by postnatal day 9, and was sustained through postnatal days 23 and 45, i.e. beyond puberty. In contrast, handling-related up-regulation of hippocampal GR-mRNA expression emerged subsequent to the 23rd postnatal day, i.e. much later than changes in hypothalamic CRH expression. The hormonal stress response of handled rats was reduced starting before postnatal day 23. These findings indicate that early, rapid, and persistent changes of hypothalamic CRH gene expression may play a critical role in the mechanism(s) by which early-life experience influences the hormonal stress-response long-term.
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Introduction |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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In adult rats that were handled early in life, increased expression of GR in hippocampus and frontal cortex and reduced expression of the hypothalamic stress-mediating neuropeptide, CRH, have been considered to underlie their attenuated stress response compared with that of rats raised undisturbed (10, 11). It is generally recognized that complex sensory and integrative pathways must be involved in transducing altered maternal-derived sensory input into permanent modulation of the stress response (12). However, within the immature rat HPA, an early, critical effect of this sensory input on hippocampal neuronal plasticity, manifesting as increased GR expression, has been proposed (4, 6, 7, 8). These increased GR levels, in turn, more efficiently transmit negative glucocorticoid feedback to the HPA axis (13), down-regulating hypothalamic CRH and the subsequent responses to stress.
It may be reasoned that if a primary effect of handling/maternal input on the immature rat HPA axis involves up-regulation of GR-messenger RNA (mRNA) expression in hippocampus, then this enhancement of GR-mRNA levels should precede modulation of hypothalamic CRH-mRNA. Thus, there should be a period, during the early evolution of handling effects on the hippocampal-HPA axis, when these neuroplastic hippocampal changes are already apparent, whereas the consequent functional alterations of hypothalamic CRH expression have not yet emerged. Alternatively, other genes governing the extent and magnitude of the hormonal stress response may be involved in mechanism for the long-lasting effects of early-life handling. For example, modulation of hypothalamic CRH expression, found to result from several early life experiences (14, 15, 16), may be considered. If the effects of handling/maternal input on long-term HPA function involve early down-regulation of CRH-mRNA levels in hypothalamus, then these changes in CRH gene expression should precede alterations in GR-mRNA levels. Thus, this study aimed to gain insight about the mechanisms for the effect of early-life experience on immature rat HPA axis by determining the precise timing and sequence of handling-induced alterations of CRH-mRNA and GR-mRNA expression in specific brain regions involved in hippocampal-HPA axis regulation.
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Materials and Methods |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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Expression of CRH and of GR was evaluated on postnatal days 9, 23, and 45 in rats handled daily during postnatal days 2–8, compared with those left undisturbed. The first time-point was chosen to coincide with the end of the daily handling regimen. Day 23 was chosen as a prepubertal time-point that is comfortably beyond the established critical period for neuroplasticity induced by the handling experience (1, 17), i.e. a time-point when the effects of early-life experience should be stably ingrained. Day 45 was chosen as a postpubertal time-point, allowing evaluation of potential changes in GR-mRNA or CRH-mRNA expression that may emerge with puberty. In addition, the magnitude of the hormonal stress-response to age-appropriate stressful stimuli was determined on the same developmental days in handled and in undisturbed rat groups.
Animals
Immature rats were offspring of timed-pregnancy Sprague Dawley
rats (Zivic-Miller Laboratories, Inc., Zelienople, PA),
and maintained in NIH approved, uncrowded, temperature-controlled
animal facilities, on a 12-h light, 12-h dark cycle with unlimited
access to lab chow and water. Cages were inspected for presence of pups
at 12-h intervals, and the date of birth was considered day 0. All
experimental manipulations were conducted at 0800–0900, to minimize
potential diurnal variability in HPA-associated gene expression and
responses to stress (18, 19). All experiments were carried
out according to the NIH guidelines for the care of experimental
animals with approval by the Institutional Animal Care Committee. Group
size (n) was 6–10 rats per treatment per time-point. Both genders were
evaluated on day 9, and males only were used at the 23 and 45-day
time-points.
Early-life manipulations
1) Daily handling was performed on days 2–8 (the rationale for
this relatively short handling paradigm is provided in the experimental
design, above). Handling consisted of removing the mother followed by
the pups from their home cages, and placing the pups in a cage lined
with bedding for 15 min. Pups were returned to their home cage first,
followed by the mother (10, 20, 21). 2) Undisturbed rats were
inspected once on the first day of life. During postnatal days 2–8,
litters were completely undisturbed in a quiet corner of the animal
facility, without bedding changes. Water and food were gently applied
to the top of the cage. Starting on postnatal day 9, all surviving
experimental groups were housed under routine lab conditions, including
cage changes twice a week and weaning on day 21.
Methods of age-appropriate stress
Restraint, a powerful stress in adult rats, is a poor stress for
neonatal rats who are accustomed to confinement by their mother’s
body. In contrast, cold is a poor stress for furred adults with fully
developed metabolic responses. Therefore, cold was chosen for 9-
day-old rats and restraint for older ages.
For all experiments, time 0 was considered the onset of age-appropriate stressors, and these were imposed rapidly after removing rats from home cages. For 9-day-old rats, cold stress, extensively characterized previously (14, 16, 22), was used. Briefly, rats were placed in individual glass jars containing ice, in a cold-room (4 C) for 12 min. Following cold stress, pups were placed on a euthermic pad (33–34 C) and killed by decapitation at 30 or 60 min following the onset of cold stress. Older rats were subjected to restraint stress for 20 min (10) and were killed at the time points coinciding with those chosen by these authors: before stress (time 0 here), at the termination of restraint (20 min here) and 1 h after the end of restraint (80 min here). For 23-day-old rats, restrainers were fashioned via saran-wrapping the animals inside the larger containers; for 45-day-old rats, tubular plastic restrainers were used.
Tissue handling and hormonal assays
Brains were rapidly dissected onto powdered dry ice and stored
at -80 C. Coronal sections (20 µm) spanning prefrontal cortex to
ventral hippocampus (23) were cut using a cryotome.
Sections were sorted into matched series for in situ
hybridization histochemistry as described in detail previously
(14, 16, 22, 24). Trunk blood was collected for analysis
of plasma ACTH and corticosterone (CORT) using commercial RIA kits
(INCSTAR Corp., Stillwater, MN, and
ICN, Irvine, CA) as previously described
(25). Assay sensitivities were 15 pg/ml for ACTH and 0.5
µg/dl for CORT; interassay variabilities averaged 5–10%.
In situ hybridization histochemistry (ISH) and probe
preparation
ISH and probe labeling were performed as described previously
for oligonucleotide probes (14, 16, 22) or complementary
RNA probes (25, 26). Briefly, for CRH-mRNA analysis,
sections were brought to room temperature, air dried, and fixed in
fresh 4% buffered paraformaldehyde for 20 min, followed by dehydration
and rehydration through graded ethanols. Sections were exposed to
0.25% acetic anhydride in 0.1 M triethanolamine (pH 8) for
8 min and were dehydrated through graded ethanols. Prehybridization and
hybridization steps were performed in a humidified chamber at 40 C in a
solution of 50% formamide, 5x SET, 0.2% SDS, 5x Denhardt’s, 0.5
mg/ml salmon sperm DNA, 0.25 mg/ml yeast transfer RNA, 100
mM dithiothreitol, and 10% Dextran sulfate. Following a
1-h prehybridization, sections were hybridized overnight with 0.5
x 106 cpm of 35S-labeled
oligonucleotide probe. Post hybridization, sections were washed, most
stringently at 0.3 x SSC. For detection of GR-mRNA, sections were
hybridized overnight at 55 C with 1 x 106
cpm of 35S labeled ribonucleotide probe. After
hybridization, the sections were washed in 2 x SSC for 5 min at
room temperature and were digested with RNase (200 µg/ml RNase A;
Calbiochem, La Jolla, CA) in a 10 M Tris HCl
(pH = 8)/NaCl for 30 min at 37 C. Sections underwent serial washes
of increasing stringency at 55 C, the most stringent being at 0.03
x SSC for 1 h. For both procedures, sections were then dehydrated
through 100% ethanol, air dried and apposed to film (Hyperfilm
ß-Max, Amersham Pharmacia Biotech, Arlington Heights,
IL) for 7–14 days. Representative sections were also dipped in NTB-2
nuclear emulsion (Eastman Kodak Co., Rochester, NY) and
exposed for 4–6 weeks.
Acquisition and quantitative analysis of ISH signal, and
statistical considerations
Semiquantitative analyses of CRH-mRNA and GR-mRNA were performed
following in situ hybridization without knowledge of
treatment, as described in detail previously (16, 24, 25).
Digitized images of each brain section were analyzed using the
ImageTool software program (University of Texas Health Science Center,
San Antonio, TX). Densities were calibrated using
14C standards and are expressed in nCi/g, after
correcting for background by subtracting the density of the
hybridization signal over the corpus callosum. Hippocampal formation
sections were analyzed at coronal levels corresponding to
2.0–2.9 mm, 2.6–3.5 mm, and 3.2–4.1 mm anterior to bregma in
9-, 23-, and 45-day-old rats, respectively.
In hippocampus of the 9-day-old rat, GR-mRNA signal was analyzed
only in the CA1 region (see Fig. 3B
), based on the established
distribution of GR-mRNA in the developing rat hippocampus (26, 27). At older age-groups, GR-mRNA was also measured over the
granule cell layer of the dentate gyrus (DG; 13, 27). Hypothalamic PVN
was sampled at levels including the dorsomedial parvocellular cell
group expressing CRH and GR-mRNA (3.5–3.8, 4.4–5.0, and
5.0–4.4 mm anterior to bregma for 9-, 23-, and 45-day-old rats,
respectively). Frontal cortex sections were analyzed at coronal levels
corresponding to 2.3–3.2 mm anterior to the bregma in the 9-day-old
rat, 2.6–4.4 mm in the 23-day-old rat, and 3.2–5.6 in the 45-day-old
rat. ACe was analyzed at levels corresponding to 2.3–3.2 mm, 3.8–5.0,
and 4.4–5.6 mm anterior to bregma for 9-, 23-, and 45-day-old rats,
respectively. Optical densities from three optimal sections were
averaged to generate an expression value for each region. These values
were then used to calculate group means (n = animal number).
Statistical significance (P < 0.05) of observed
quantitative differences among experimental groups at each time-point
and brain region was evaluated using unpaired Student’s t
test, with Welch’s correction for unequal variance when indicated
(16, 24). Two-way ANOVA was used to evaluate the effects
of early-life experience and of stress on plasma ACTH levels.
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Results |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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, CRH-mRNA levels
in PVN of the 9-day-old, daily-handled rats were significantly lower
(233.3 ± 8.8 nCi/g) than those of undisturbed pups (545.0 ±
25.0 nCi/g, P < 0.01). Decreased CRH-mRNA signal over
PVN is evident in a dark-field photomicrograph derived from a handled
rat, compared with a matched section from an undisturbed animal (Fig. 1B).
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shows that, on postnatal
day 9, GR-mRNA levels in hippocampal CA1 did not differ between rats
that were handled daily on postnatal days 2–8 and those raised
undisturbed (65.0 ± 2.9 nCi/g vs. 67.3 ± 8.96
nCi/g). Figure 2B demonstrates robust GR-mRNA signal in CA1 at this
age, but (in contrast to older animals) little GR expression in the
dentate gyrus granule cell layer.
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, CRH-mRNA
levels in PVN were persistently reduced in handled rats, when measured
on postnatal day 23 (475.8 ± 35.4 nCi/g handled vs.
622.5 ± 24.8 nCi/g in undisturbed animals) and on day 45
(446.0 ± 92.0 nCi/g vs. 860.0 ± 140.0 nCi/g). In
contrast, analysis of CRH-mRNA levels in ACe did not reveal significant
effects of early-life handling. Thus, CRH-mRNA levels in ACe in handled
and undisturbed rats averaged 69.0 ± 5.2 nCi/g and 57.1 ±
9.1 nCi/g, respectively on postnatal day 23. By day 45, CRH-mRNA levels
averaged 82.5 ± 7.5 nCi/g in handled pups, and 76.7 ± 3.3
nCi/g in the undisturbed cohort.
GR gene expression in hippocampal CA1 of rats subjected to
early-life handling is up-regulated on the 45th, but not on the 23rd
postnatal day
To better resolve the timing of GR up-regulation, two additional
ages were examined. Figure 4A shows that
GR-mRNA levels in hippocampal CA1 did not differ between daily-handled
and undisturbed rats immediately following the handling paradigm (day
9) or 2 weeks later (for day 23: 72.1 ± 5.3 nCi/g in handled
vs. 71.8 ± 8.9 nCi/g in undisturbed rats). However, by
the 45th postnatal day, following puberty, GR-mRNA levels were higher
in rats experiencing early-life handling (102.3 ± 7.4 nCi/g
vs. 54.7 ± 2.8 nCi/g, P < 0.01). In
the 9-day-old rat, little GR-gene expression was found over the DG
granule cell layer (Fig. 2B). Therefore, the early effects of the
experimental manipulations described here on this transcript’s
expression could not be determined. Comparison of GR-mRNA signal over
DG in 23- and 45-day old rats revealed an up-regulation of GR
expression on the 45th postnatal day in the handled group (Fig. 4B, 106.7 ± 9.5 nCi/g vs. 70.7 ± 9.4 nCi/g), as
shown also in Fig. 4C. This figure depicts coronal brain sections at
the level of the dorsal hippocampus, following in situ
hybridization for GR-mRNA. Similar expression magnitude and pattern in
handled (A) and undisturbed (B) rats are evident on postnatal day 23.
By postnatal day 45, GR signal is enhanced in a rat subjected to
early-life handling (C) compared with one raised undisturbed (D).
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Early-life experiences influence the hormonal stress response by
the 23rd and 45th postnatal days, but not on postnatal day 9
Analyses of the hormonal stress responses of rats handled early in
life compared with the undisturbed groups were carried out on postnatal
days 9, 23, and 45. On day 9, robust elevations of both plasma ACTH and
CORT in response to age-appropriate stress were observed. Figure 5A shows significant increases of plasma
ACTH at both time points after stress onset. Whereas a robust effect of
stress was noted (F2,15 = 11; P =
0.0016), no effect of the early life experience was revealed by two-way
ANOVA (P = 0.59). CORT levels were in line with ACTH
values: basal AM levels were 0.99 ± 0.12 and 1.14 ± 0.15
µg/dl in the handled and undisturbed groups, respectively. In
response to stress, CORT levels of both groups rose, to 2.67± 0.41 and
2.59 ± 0.35 µg/dl at the 20 min time-point and 4.49 ±
0.40 and 3.51 ± 0.26 µg/dl at 60 min. Two-way ANOVA revealed a
robust effect of stress (F2,30 =47.68;
P < 0.0001), but not of the early-life experience
(F1,30 = 1.54; P = 0.22). By the
23rd day of life, (i.e. before alteration of hippocampal
GR-mRNA) significant effects of the early-life handling experience on
plasma ACTH and CORT responses to stress were evident. Figure 5B
demonstrates a diminished induction of plasma ACTH by stress in the
handled group: two-way ANOVA revealed both an effect of stress
(F2,35 = 20.30 P < 0.001) and a
robust effect of the early life treatment (F = 6.32;
P = 0.017). Plasma CORT response to stress and
early-life treatment was similar, showing significant effects of stress
(F2,16 = 29.55) and of handling
(F1,16 = 12.77). As shown in Fig. 5C, this effect
of early life treatment on the hormonal stress response persisted at
least to postnatal day 45 when, in addition to robust effects of stress
(P = 0.0046), two-way ANOVA showed a strong handling
effect (F = 7.7; P = 0.018). As evident from the
figure, basal AM levels of ACTH were not significantly different in
handled and undisturbed groups at all three ages. Similarly, for CORT
(see 9 day levels, above) basal values were 2.39 ± 0.44 and
4.42 ± 0.44 µg/dl on postnatal day 23, and 3.49± 0.86
vs. 4.81 ± 0.41 µg/dl on postnatal day 45.
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Discussion |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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Early-life experience influences the stress response profoundly and
permanently
Modulation of early-life experience of the neonatal rat may
permanently influence the expression of stress-mediating molecules and
the hormonal responses to stress (1, 8). Thus, exposure of
neonatal rats to age-appropriate physiologic or psychologic stressors
(14, 16) or to prolonged or repeated maternal separation
(10, 15, 22, 28) has resulted in immediate and/or
long-term enhancement of sensitivity to further stress. In contrast,
compared with animals raised without any disturbance, daily handling of
rat pups during the first 3 weeks of life has led to robust reduction
in the stress response that persists throughout the animal’s life
(1, 4). More recently, changes in maternal behavior
(increased grooming and licking), observed on returning pups to their
cages, have been implicated in mediating these effects of handling on
the neonatal rat (6, 7, 8). Sensory input derived from
altered maternal behavior is generally believed to influence the pup’s
HPA-axis at the molecular level, leading eventually to significantly
reduced responses to subsequent stress (4, 6). Thus,
enhanced GR expression in hippocampus and frontal cortex (29, 30) and decreased levels of hypothalamic CRH expression
(10, 11) of adult rats handled early in life have been
considered to underlie their diminished stress response compared with
that of those raised undisturbed.
The set-point and magnitude of the hormonal responses to stress are under tight and intricate regulation (31, 32, 33), and are influenced by both hippocampal GR and hypothalamic CRH function. In both mature and developing rat, CRH is released from hypothalamic peptidergic neurons within seconds of stress onset, to influence pituitary ACTH secretion and release of adrenal glucocorticoids (33, 34, 35). These hormones interact with GRs in hippocampus, PVN and pituitary (36, 37, 38) to negatively feed back onto the hormonal stress response (39). In addition, glucocorticoids regulate GR expression, thus further influencing the magnitude of HPA axis suppression (40, 41). Because of the known influence of glucocorticoids on hippocampal neuroplasticity—an effect requiring GR activation—alteration of GR expression has been proposed as a key mechanism for the effects of handling-induced changes in maternal-derived sensory input on HPA axis tone (4).
Down-regulation of hypothalamic CRH expression is an early, key
consequence of the early-life handling experience, preceding alteration
of GR-mRNA expression
The present study demonstrates that modulation of CRH-mRNA levels
by neonatal handling occurs relatively rapidly and is detectable
already by the 9th postnatal day. These early changes amplify previous
data, showing that daily handling during the first 5–7 postnatal days
is sufficient to influence the HPA-axis and the stress response
permanently (1, 2, 17), data that provided the rationale
for using an 8-day handling regimen in the current studies.
The early down-regulation of hypothalamic CRH expression persisted at least through early adulthood In contrast to the rapid modulation of hypothalamic CRH-mRNA expression, early (postnatal day 9) hippocampal GR-mRNA levels were not influenced by early-life handling. This finding is consistent with the notion that modulation of CRH-expression by early-life experience precedes (and is thus not a consequence of) GR-mediated changes in HPA axis tone. The age of onset of altered GR-mRNA levels after daily handling is not fully resolved. Although early (postnatal day 7) changes in hippocampal glucocorticoid binding have been observed (17), binding studies may not fully reflect changes in GR expression (42); indeed, some have failed to demonstrate any alteration of GR binding in adult rats handled as neonates (9). Published GR-mRNA data are limited to studies of adult animals, handled as neonates (29). The current study, focusing on molecular changes at the mRNA level, suggests that regulation of hypothalamic CRH expression may be an early—and thus key—target of the altered maternal-derived sensory input related to the handling maneuver.
How do sensory signals resulting from early-life handling alter CRH
expression in PVN permanently?
It is generally established that daily handling during a critical
postnatal period in the immature rat leads to augmented maternal
sensory input to handled pups. Previous work from this laboratory has
pointed to a chemically defined sensory signal-transduction pathway
integrating maternal sensory input to modulate the central components
of the HPA axis (25). This pathway involves a series of
intercommunicating structures that relay and integrate sensory
information: somatosensory signals from the spinal cord reach the
nucleus of the solitary tract (NTS). NTS efferents heavily innervate
the CRH-rich PVN (43), and via the parabrachial nucleus,
the ACe, a key stress integrator (44, 45), thus
positioning this pathway as a candidate for mediating HPA activation in
response to diverse stimuli (43, 46, 47). Earlier work
from this laboratory suggested that, in processing stress-modulating
signals, specificity in this sensory-integrative pathway is derived
from activation of CRH-expressing neurons signaling via the CRH
type 2 receptor (CRF2). Thus, altered maternal
sensory input (e.g. licking and grooming) was found to
influence CRH-CRF2 signaling in the hypothalamic
ventromedial nucleus (25), a region shown previously to
regulate stress-related PVN function (48, 49). Further
evidence for modulation of CRH expression in structures comprising this
sensory integration pathway (e.g. in parabrachial nucleus)
has been emerging (50, 51). Clearly, this proposed,
chemically specified (CRH-CRF2) pathway functions
within the broader context of neuroanatomical circuits using several
neurotransmitters to influence HPA function. For example, direct
noradrenergic input from NTS to PVN has been described
(52), conveying stress-related signals, and likely
stimulating PVN-CRH neurons (53, 54, 55). In addition, CRH is
expressed in—and interacts with—noradrenergic locus ceruleus neurons
(56, 57, 58). Indeed, early-life handling has been shown to
modulate intercommunicating noradrenergic and CRH-expressing neurons in
this circuit (7, 59). Other neurotransmitters, such as
serotonin, have also been implicated in regulating CRH expression and
release both in and outside the hypothalamus (56, 60, 61).
Processing of sensory signals to influence stress-response magnitude and set-points likely involves the ACe, a key amygdaloid stress-integrator (12, 58, 62). This region, highly expressing CRH (50, 63), promotes PVN-mediated stress functions. Therefore, the current study examined for potential down-regulation of CRH expression in ACe following early-life handling. The absence of such changes indicates that if handling influences ACe to decrease positive input to PVN, this effect does not involve alteration of ACe-CRH expression.
Potential mechanisms for enhancement of hippocampal GR
expression
The current study demonstrated an early reduction of hypothalamic
CRH expression with subsequent changes in the stress response that are
followed by increased hippocampal GR-mRNA levels. The mechanisms
mediating this chain of events may be divided into two general
alternatives: First, reduced hypothalamic CRH may influence hippocampal
GRs directly, via a neuroendocrine feedback loop. Thus, diminished CRH
release during stress, with consequent reduction of glucocorticoid
secretion, disinhibits (up-regulates) hippocampal GR expression
(13, 37). This molecular cascade leads to a new
steady-state, consisting of the reduced HPA tone observed in adult rats
handled during early-life. Alternatively, early-life handling/sensory
input may influence as yet unknown targets in the stress-integration
circuit, via complex multineurotransmitter mechanisms. This primary
modulation would then alter hippocampal GR and hypothalamic CRH with
different velocities or at different time-points. The actual interval,
days 23–45, when GR-mRNA up-regulation occurs also coincides with
puberty. Thus, potential interactions of these processes with sex
hormones, or their down-stream effects, may be considered.
Reduced stress response of handled rats arises subsequent to
down-regulation of CRH expression, but precedes changes in hippocampal
GR-mRNA levels
Although basal ACTH and CORT levels of the two groups did not
differ, the magnitude of hormonal stress response of the handled group
was smaller than that of the undisturbed one, starting on postnatal day
23. The emergence of this hormonal difference before up-regulation of
hippocampal GR expression indicates that the latter is unlikely to play
a mechanistic role in handling-related reduction of the stress
response. Indeed, the data are consistent with an alternative scenario
in which diminished levels of circulating glucocorticoids lead to
enhanced, disinhibited hippocampal GR expression. An apparent reduction
of the overall magnitude of the stress response of handled rats was
observed between days 23 and 45. Whether the restraint challenge was
not as powerful a stressor on day 45, or whether the up-regulation of
GR-mRNA—present by day 45—contributed to this observation, cannot be
resolved in the current study. Baseline ACTH levels during the two ages
did not differ and are consistent with levels found using the
INCSTAR Corp. assay.
In summary, this study centered on the hypothesis that determination of early and persistent molecular changes following neonatal handling would provide important clues about the mechanism(s) by which neonatal experience influences the hormonal stress-response long-term. The results of this study demonstrate that regulation of CRH expression is an early and sustained event in the molecular cascade resulting from neonatal handling. In addition, these data indicate that CRH regulation may provide an important target for clinically relevant intervention in the processes by which early-life experience modulates adult responses to subsequent stress.
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Acknowledgments |
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Footnotes |
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Received June 27, 2000.
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References |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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1b adrenoreceptor mRNA in
corticotropin-releasing hormone-containing cells of the rat
hypothalamus and its regulation by corticosterone. J Neurosci 19:10098–10106This article has been cited by other articles:
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  C. J. Rice, C. A. Sandman, M. R. Lenjavi, and T. Z. Baram A Novel Mouse Model for Acute and Long-Lasting Consequences of Early Life Stress Endocrinology, October 1, 2008; 149(10): 4892 - 4900. [Abstract] [Full Text] [PDF]
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B. C. Reeb and K. G. Akers Is Neuroplasticity of the Hypothalamic-Pituitary-Adrenal Axis Maternally Mediated? J. Neurosci., May 24, 2006; 26(21): 5589 - 5590. [Full Text] [PDF]
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K. A. Fenoglio, K. L. Brunson, S. Avishai-Eliner, B. A. Stone, B. J. Kapadia, and T. Z. Baram Enduring, Handling-Evoked Enhancement of Hippocampal Memory Function and Glucocorticoid Receptor Expression Involves Activation of the Corticotropin-Releasing Factor Type 1 Receptor Endocrinology, September 1, 2005; 146(9): 4090 - 4096. [Abstract] [Full Text] [PDF]
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 F.-C. Hsu, G.-J. Zhang, Y. S. H. Raol, R. J. Valentino, D. A. Coulter, and A. R. Brooks-Kayal Repeated neonatal handling with maternal separation permanently alters hippocampal GABAA receptors and behavioral stress responses PNAS, October 14, 2003; 100(21): 12213 - 12218. [Abstract] [Full Text] [PDF]
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 C. R. Pryce, D. Ruedi-Bettschen, A. C. Dettling, and J. Feldon Early Life Stress: Long-Term Physiological Impact in Rodents and Primates Physiology, August 1, 2002; 17(4): 150 - 155. [Abstract] [Full Text] [PDF]
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