Endocrinology Vol. 139, No. 4 1810-1814
Copyright © 1998 by The Endocrine Society
Regulation of Glucocorticoid Receptor and Mineralocorticoid Receptor Messenger Ribonucleic Acids by Selective Agonists in the Rat Hippocampus
Helen M. Chao,
Li Yun Ma,
Bruce S. McEwen and
Randall R. Sakai
The Rockefeller University (H.M.C., B.S.M.), Laboratory of
Neuroendocrinology, New York, New York 10021; University of
Pennsylvania (L.Y.M., R.R.S.), Department of Animal
Biology, Philadelphia, Pennsylvania 19104
Address all correspondence and requests for reprints to: Helen M. Chao, The Rockefeller University, 1230 York Avenue, Box 165, New York, New York 10021. E-mail: chaoh{at}rockvax.rockefeller.edu
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Abstract
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Adrenal steroids can have prodigious effects on the structure,
function, and survival of hippocampal neurons. In the rat hippocampus,
the actions of adrenal steroids are mediated by two receptor types, the
glucocorticoid receptor (GR) and the mineralocorticoid receptor (MR).
Using in situ hybridization, we have examined the
regulation of the messenger RNAs (mRNAs) encoding the glucocorticoid
and mineralocorticoid receptors, by aldosterone, which acts selectively
through MR, and by RU28362, which acts selectively through GR. Our
results demonstrate that there is autoregulation of each receptor
subtype, such that activation of GR regulates GR mRNA levels and MR
activation regulates MR mRNA expression. In addition, there is evidence
that aldosterone, acting through MR, can affect the expression of GR
mRNA. The extent to which a specific agonist can produce a significant
change in the expression of a particular steroid receptor mRNA varies
between the different subfields of the hippocampus.
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Introduction
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THE HIPPOCAMPUS is highly sensitive to
glucocorticoid action by virtue of the prominence in this brain region
of two distinct adrenal steroid receptor subtypes. The
mineralocorticoid receptor (MR or Type I receptor) has a high affinity
for corticosterone and aldosterone (1, 2). In the hippocampus, MR
messenger RNA (mRNA) is most abundant in CA2 pyramidal cells and is
expressed at moderate levels in CA1, CA3, and the granule cells of the
dentate gyrus (3), a pattern that reflects that observed for receptor
binding (4, 5). The glucocorticoid receptor (GR or Type II receptor),
which is expressed in most brain regions, has a relatively low affinity
for aldosterone, a high affinity for dexamethasone, and is selectively
activated by the synthetic agonist RU28362 (6, 7). In the hippocampus,
the highest level of GR mRNA expression is in CA1 and lowest in CA3,
with an intermediate level expressed in the dentate gyrus (3), a
distribution similar to that for hippocampal GR protein as observed by
immunocytochemistry (5, 8).
Receptor binding studies indicate that hippocampal GR and MR are
sensitive to changes in the available levels of steroid ligand.
Withdrawal of endogenous steroids via bilateral adrenalectomy results
in an increase in the number of adrenal steroid receptor binding sites,
which can be reversed or prevented by steroid treatment (6, 9, 10).
These results suggest that adrenal steroids may regulate their target
receptors at the level of gene expression. Our studies sought to
investigate whether changes in circulating steroid levels would affect
the expression of the mRNAs encoding GR and MR in the rat hippocampus.
By using ligands that were specific for each receptor subtype in the
replacement regimen, it was possible to distinguish between
autoregulation and reciprocal regulation of GR and MR, in the different
hippocampal subfields.
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Materials and Methods
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Experimental animals
Adult male Sprague-Dawley rats (CD strain, Harlan, Indianapolis,
IN) were maintained on a 12-h light, 12-h dark cycle and had access to
both water and 0.5 M NaCl from 7 days before surgery, to
the time when the rats were euthanized.
Exp 1. Animals were 1) sham-operated and implanted with mock
minipumps (Sham); 2) adrenalectomized and implanted with mock minipumps
(ADX); 3) ADX and implanted with Alzet no. 2001 minipumps delivering
aldosterone at 1 µg/h (ADX + Aldo); and 4) ADX and implanted with
minipumps delivering corticosterone at 10 µg/h (ADX + CORT); n =
56 per treatment group. The aldosterone (Sigma Chemical Co., St.
Louis, MO) and corticosterone (Steraloids, Witton, NH) were delivered
in propylene glycol. Daily fluid intakes were monitored following
surgery. Animals were euthanized 7 days after surgery, and brains and
trunk blood were collected. Plasma corticosterone levels were assessed
by RIA (11).
Exp 2. Animals were 1) sham-operated and implanted with mock
minipumps (Sham); 2) adrenalectomized and implanted with mock minipumps
(ADX); 3) ADX and implanted with Alzet no. 2001 minipumps delivering
aldosterone at 1 µg/h (ADX + Aldo); 4) ADX and implanted with
minipumps delivering RU28362 at 10 µg/h (ADX + RU); and 5) ADX and
implanted with minipumps delivering aldosterone at 1 µg/h and RU28362
at 10 µg/h (ADX + Aldo+ RU); n = 56 per treatment group. The
aldosterone (Sigma) and RU28362 (Roussel-Uclaf, Romainville, France)
were delivered in propylene glycol. Daily fluid intakes were monitored,
and animals exhibiting aberrant intake levels were eliminated from the
study. Seven days after surgery, body weights were recorded, animals
were euthanized, and brains and trunk blood were collected. Plasma
steroid levels were assessed by RIA (Diagnostic Products Corporation,
Los Angeles, CA).
In situ hybridization
Brains were removed, immediately frozen, and stored at -70 C.
Sixteen-micron sections were prepared on a cryostat microtome,
collected on gelatin-coated slides, and stored frozen until
hybridization. Before hybridization, sections were fixed in 4%
formaldehyde in PBS, acetylated in a solution of 0.25% acetic
anhydride in 0.1 M triethanolamine-HCl, pH 8.0, rinsed in
2 x SSC, and allowed to air-dry.
Antisense riboprobes radioactively labeled with 35S-UTP
were transcribed from complementary DNA clones corresponding to the rat
glucocorticoid and mineralocorticoid receptors as previously described
(12). The hybridization mix (50% formamide; 10% dextran sulfate; 600
mM NaCl; 1 x Denhardts solution; 10 mM
Tris-HCl, pH 7.5; 1 mM EDTA, pH 8; 100 µg/ml denatured
salmon testis DNA; 10 mM DTT; radiolabeled probe) was added
at 0.2 ml per slide, the slides were coverslipped, and the sections
were incubated overnight at 55 C. Following hybridization, the
coverslips were floated off, and the sections were rinsed in 2 x
SSC. The sections were treated with 10 µg/ml RNAse A, washed in RNase
A buffer and in 2 x SSC at room temperature, followed by 0.5
x SSC at 55 C. The sections were allowed to air dry and then were
apposed to x-ray film for autoradiography.
The optical densities of the autoradiographic images were determined on
the Imaging Research (St. Catherines, Ontario, Canada) image analysis
system. The value of the low hybridization signal in CA1 stratum
radiatum was taken (by definition) as background and subtracted from
the optical density values for the hippocampal cell layers. The data
were expressed as optical density (means ± SEM). For
each animal, the optical density value represented the average of
measurements from two to three brain sections per slide and two to
three slides per animal. Statistical analysis was by one-way ANOVA
followed by Tukeys posthoc test, with P
0.05 as
the criterion for statistical significance.
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Results
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Exp 1
Steroid replacement of adrenalectomized animals with aldosterone or
corticosterone. Daily saline and water intake (Fig. 1
) and plasma corticosterone levels were
measured as indicators of the efficacy of the bilateral adrenalectomy
and of the steroid replacement. The replacement of aldosterone to ADX
animals restored their saline intake to levels comparable with Sham
animals, whereas corticosterone replacement did not appreciably alter
saline intake compared with the ADX animals (Fig. 1A
). Plasma
corticosterone levels were measurable in the Sham group (7.48 ±
0.71 µg%) and the ADX + CORT group (10.88 ± 1.35 µg%), but
for the ADX and ADX + Aldo groups these levels were below the limit of
detection for this RIA.

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Figure 1. Daily saline intake (A) and daily water intake (B)
in Exp 1. Daily intake levels were monitored from the day of surgery
(Day 1) until the day the rats were euthanized.
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In the adrenalectomized animals, there was a significant induction in
GR mRNA expression in all hippocampal subfields examined, relative to
the Sham animals (Fig. 2
). Replacement
with aldosterone resulted in a significant decrease in GR mRNA levels
compared with ADX animals. Replacement with corticosterone completely
reversed the ADX effect in all hippocampal subfields.

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Figure 2. Hippocampal GR mRNA expression in Exp 1. Levels of
GR mRNA expression were assessed in the CA1 pyramidal cell layer (CA1),
CA3 pyramidal cell layer (CA3), and granule cell layer of the dentate
gyrus (DG). Statistical analysis indicated that the ADX animals were
significantly different from the Sham animals in all regions examined
and that the ADX + Aldo group was different from the Sham group in the
CA3 subregion (*, P 0.05).
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Adrenalectomy resulted in a significant increase in MR mRNA levels in
pyramidal cells but not in the granule cells of the dentate gyrus (Fig. 3
). In aldosterone-replaced animals,
pyramidal cell MR levels were significantly decreased compared with ADX
animals. Corticosterone replacement restored the MR mRNA expression in
CA1, CA2, and CA3 to levels comparable with adrenal-intact animals.

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Figure 3. Hippocampal MR mRNA expression in Exp 1. Levels of
MR mRNA expression were assessed in the CA1 pyramidal cell layer (CA1),
CA2 pyramidal cell layer (CA2), CA3 pyramidal cell layer (CA3), and
granule cell layer of the dentate gyrus (DG). Statistical analysis
indicated that the ADX animals were significantly different from the
Sham animals in CA1, CA2, and CA3, and that the ADX + Aldo group was
different from the Sham group in CA2 and CA3 (*, P
0.05).
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Exp 2
Steroid replacement of adrenalectomized animals with aldosterone
and/or RU28362. Daily saline and water intake (Fig. 4
) and plasma steroid levels and body
weight (Fig. 5
) were measured as
indicators of the efficacy of the surgery and of the steroid
replacement. Plasma corticosterone levels were measurable in the Sham
group (30.37 ± 4.7 µg%), whereas plasma aldosterone levels
were detectable in the Sham group and in the ADX groups that received
aldosterone (Fig. 5A
). The profile of saline intake was comparable with
that observed in Exp 1 for the Sham, ADX, and aldosterone-replaced
animals; however, the ADX + RU group exhibited an anomalous pattern
(Fig. 4A
). Moreover, the ADX + RU animals showed the lowest body weight
gain during the study (Fig. 5B
).

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Figure 4. Daily saline intake (A) and daily water intake (B)
in Exp 2. Intake levels were monitored from 2 days before surgery until
the rats were euthanized, with the day of surgery designated as Day
1.
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Figure 5. Plasma aldosterone levels (A) and body weight
measurements (B) in Exp 2. Plasma aldosterone levels in the ADX and ADX
+ RU animals were below the limit of detection by this RIA.
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Adrenalectomy resulted in an induction of GR mRNA expression in CA1 and
CA3, relative to Sham animals (Fig. 6
).
In CA1 pyramidal cells, the ADX-induced increase was reversed by
RU28362, whereas in CA3 pyramidal cells replacement with aldosterone
was more effective in decreasing GR mRNA levels.

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Figure 6. Hippocampal GR mRNA expression in Exp 2. Levels of
GR mRNA expression were assessed in the CA1 pyramidal cell layer (CA1),
CA3 pyramidal cell layer (CA3), and granule cell layer of the dentate
gyrus (DG). Statistical analysis indicated that the ADX animals were
significantly different from the Sham animals in CA1 and CA3 and that
the ADX+RU group was different from the Sham group in the CA3 region
(*, P 0.05).
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In the adrenalectomized animals, there was a significant increase in MR
mRNA levels in hippocampal pyramidal cells but not in the granule cells
of the dentate gyrus, compared with Sham animals (Fig. 7
). The ADX-induced increase was reversed
by aldosterone replacement, in the presence or absence of RU28362, in
CA2 and CA3.

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Figure 7. Hippocampal MR mRNA expression in Exp 2. Levels of
MR mRNA expression were assessed in the CA1 pyramidal cell layer (CA1),
CA2 pyramidal cell layer (CA2), CA3 pyramidal cell layer (CA3), and
granule cell layer of the dentate gyrus (DG). Statistical analysis
indicated that the ADX animals were significantly different from the
Sham animals in CA1, CA2, and CA3, and that the ADX + RU group was
different from the Sham group in CA2 and CA3 (*, P
0.05).
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Discussion
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Glucocorticoids can cause dramatic changes in the morphology,
electrophysiology, and survival of neurons in the hippocampus, which
may underlie their role in modulating cognition. In many of these
processes, there are markedly different consequences to activation of
each receptor subtype. Steroid agonists specific for MR facilitate
long-term potentiation and reduce calcium currents in hippocampal
neurons, whereas GR-specific agonists have the opposite effect on these
parameters of neuronal excitability. The survival of granule cells in
the dentate gyrus is dependent upon activation of MR, whereas GR
activation can potentiate neurodegenerative and neurotoxic damage to
hippocampal pyramidal cells (13).
Adrenal steroid receptors act as transcriptional regulators of a
variety of gene products affecting hippocampal function including
neurotransmitters, neurotransmitter receptors, and neurotrophic factors
(13, 14, 15, 16). In the classical model of genomic regulation by GR and MR,
ligand binding causes a conformational change in the receptor that
results in nuclear translocation and association as homodimers with
response elements in the target gene promoters (17, 18). In
vitro studies have demonstrated that GR and MR are also able to
form a heterodimeric complex, with transcriptional activator properties
distinct from the homodimers (19, 20). The coexpression of GR and MR in
hippocampal neurons (21, 22) suggests that such heterodimers may form
in vivo as well.
Receptor binding studies have provided evidence that, in the
hippocampus, the number of GR and MR binding sites is susceptible to
regulation by adrenal steroid ligands (6, 9, 10). In situ
hybridization provides the anatomical resolution required to examine
the regional specificity of this regulatory mechanism within the
hippocampus. In the pyramidal cell layers of the hippocampus, we have
found the expression of GR and MR to be responsive to adrenal steroid
regulation, whereas such regulation in the granule cells of the dentate
gyrus was equivocal. Because adrenalectomy has been shown to result in
granule cell death (23), the possibility remains that, in measurements
of the entire granule cell layer, neuronal loss could be obscuring
increases in steroid receptor expression in the cells that survive, a
question that might be resolved by single-cell analysis of receptor
mRNA expression.
Previous studies have demonstrated that RU28362 is selective for GR,
whereas aldosterone selectively activates MR (6, 7, 23). The results
obtained through receptor activation with these specific agonists
indicate that in the hippocampus there is autoregulation of each
adrenal receptor subtype. In the CA1 hippocampal subfield where GR mRNA
is most abundant, the level of GR expression is regulated by RU28362,
the GR-specific ligand. Similarly, in CA2 where levels of MR mRNA are
highest, MR expression is regulated by the MR-activating ligand
aldosterone but is unaffected by activation of GR by RU28362. In
addition, we have found evidence that activation of MR can affect
expression of GR mRNA. In CA3 pyramidal cells (where levels of GR are
relatively low), aldosterone can act through MR to reverse the
ADX-induced increase of GR mRNA as well as MR mRNA.
These studies provide evidence that steroid receptor activation can
regulate the expression of the GR and MR genes. In hippocampal cells
that coexpress GR and MR, changes in receptor levels will alter the
ratio between GR homodimers, MR homodimers, and the putative GR/MR
heterodimers, which will in turn determine the extent to which target
genes are activated. Adrenal steroid regulation of neurotransmitters
and neurotransmitter receptors can influence hippocampal activity, and
changes in neurotrophin expression can affect the morphology and
survival of hippocampal neurons. Future studies will seek to correlate
target gene regulation by a specific ligand with the known differential
effects of GR and MR activation on the structure and function of
hippocampal neurons.
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Acknowledgments
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The authors would like to thank Dr. James P. Herman (University
of Kentucky Medical Center) for providing the GR and MR complementary
DNA clones and Roussel-Uclaf (Romainville, France) for providing
RU28362.
Received September 9, 1997.
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