Anxiety disorders are the most prevalent disorders of the central nervous system imposing
a high social burden to individuals as well as to the society (Andlin-Sobocki et al.,
2005). While physiological anxiety is essential for the survival under changing environmental
conditions, persistent generalized anxiety or exaggerated inappropriate fear are pathological
manifestations that severely reduce quality of life (Belzung and Griebel, 2001; Norrholm
and Ressler, 2009). Little is known, however, about the molecular mechanisms leading
to anxiety-related conditions.
There is considerable evidence indicating a role of altered GABA-ergic transmission
in human anxiety disorders (Millan, 2003). Thus, benzodiazepines acting through GABAA
receptors are among the most widely prescribed anxiolytic drugs (Nemeroff, 2003).
Changes in GABAA receptors were observed in limbic brain areas of panic disorder patients
by [14C]flumazenil positron emission tomography (Hasler et al., 2008) and single nucleotide
polymorphisms in the glutamate decarboxylase65 (GAD65) gene were associated with increased
susceptibility to anxiety disorders (Hettema et al., 2006).
GABA released from axon terminals can act on two classes of receptors, GABAA and GABAB
receptors. GABAA receptors are ligand-gated chloride channels (Olsen and Sieghart,
2009) consisting of five subunits derived from more than 15 different genes. Most
GABAA receptors, however, consist of two α-, two β- and one γ- or δ-subunit (Sieghart
and Sperk, 2002). The subunit composition determines the physiological and pharmacological
properties of individual receptor subtypes. In particular α2- and γ2-subunits of the
GABAA receptor have been implicated in mediating the anxiolytic effects of benzodiazepines
(Crestani et al., 1999; Low et al., 2000; Morris et al., 2006).
GABAB receptors are G-protein coupled, inhibitory receptors that are generally formed
by heteromeric assembly of two receptor molecules, one GABAB1 (GBBR1) with a GABAB2
(GBBR2) subunit (Ulrich and Bettler, 2007). GABAB receptors may also play a role in
the integration of anxiety, since mice deficient in GABAB receptors (GBBR1 or GBBR2)
are more anxious in several behavioral paradigms (Mombereau et al., 2004, 2005).
The GABA-ergic system in the amygdala does not only modulate fear and anxiety under
physiological conditions, but alterations in this system may also predispose individuals
to pathological anxiety traits (Shen et al., 2010). The aim of our present study was
therefore to elucidate the role of the GABA-erigc system in anxiety-related behavior
in HAB and NAB mouse lines. These two mouse lines expressing either high (HAB) or
normal (NAB) levels of inborn anxiety-related behavior were selected by breeding according
to their anxiety-related behavior displayed on the elevated plus maze (EPM) (Kromer
et al., 2005). The amygdala has a key role in the altered phenotypes of HAB mice,
since mild emotional challenge of HAB mice results in differential activation of the
amygdala and other brain areas (Muigg et al., 2009). We performed in situ hybridization
and immunohistochemistry for GABA synthesizing enzymes, glutamate decarboxylase 65
(GAD65) and 67 (GAD67), as well as for GABAA (α1–5, β1–3, γ1–2) and for GABAB (GBBR1,
GBBR2) receptor subunits in the amygdala under conditions of chronically increased
trait-anxiety of HAB compared to NAB mice.
Experimental procedures
Animals
All procedures involving animals and animal care were conducted in accordance with
international laws and policies (EEC Council Directive 86/609, OJ L 358, 1, December
12, 1987; Guide for the Care and Use of Laboratory Animals, U.S. National Research
Council, 1996) and were approved by the Austrian Ministry of Science. All effort was
taken to minimize the number of animals used and their suffering.
Experiments were carried out on adult male HAB (n=20) and NAB (n=23) mice at 12–15
weeks of age bred in the animal facilities of the Department of Pharmacology and Toxicology,
University of Innsbruck, Austria. As previously described in more detail (Kromer et
al., 2005; Muigg et al., 2009), the two lines were derived from a Swiss CD-1 outbred
population selectively inbred for either high or normal anxiety-related behavior displayed
on the EPM, with HAB mice spending less than 15% of the testing time on its open arms,
compared with approximately 35% for NAB mice with no overlapping between the lines.
The high anxiety phenotype of HAB mice was confirmed in the light/dark test and ultrasonic
vocalization, previously (Kromer et al., 2005). In the current experiments the behavioral
phenotype of each mouse was tested by an EPM test at 7 weeks of age. Animals were
housed under standard laboratory conditions (12:12 h light/dark cycle with lights
on at 7:00) with food and water available ad libitum. Mice were housed in groups of
four to five animals per cage.
Histochemistry
Tissue preparation
In order to avoid detection of possible immediate changes of gene expression due to
acute stress exposure of the mice during EPM testing, examination of the GABA-ergic
system was performed 4 weeks after behavioral experiments. Mice were killed by carbon
dioxide gas inhalation or by injecting an overdose of thiopental (Thiopental, Sandoz,
Austria) for in situ hybridization and immunohistochemistry, respectively. Brains
were either snap frozen (isopentane, −70 °C, 3 min) for in situ hybridization or perfused
with 4% paraformaldehyde (PFA) for immunohistochemistry (Tasan et al., 2010). Using
a cryostat, coronal sections of 20 μm and 40 μm were cut for in situ hybridization
and immunohistochemistry, respectively. Every seventh section was Nissl stained and
series of matching sections were selected for subsequent histochemistry.
In situ hybridization
In situ hybridization was performed as described previously in detail (Tsunashima
et al., 1997; Tasan et al., 2010). Oligonucleotide probes targeting diverse markers
of the GABA-ergic system are listed in Table 1. They were custom synthesized (Microsynth,
Balgach, Switzerland, purified by HPLC). Oligonucleotides (2.5 pmol) were 3′ end-labeled
by incubation with [35S]α-dATP (50 μCi; 1300 Ci/mmol, Hartmann Analytic GmbH, Braunschweig,
Germany) and terminal transferase (Roche Diagnostics, Basel, Switzerland), as described
previously in detail (Tsunashima et al., 1997). Hybridization was performed in 50%
formamide, 4× SSC (1× SSC is 150 mM NaCl, 15 mM sodium citrate, pH 7.2), 500 μg/ml
salmon sperm DNA, 250 μg/ml yeast tRNA, 1× Denhardt's solution (0.02% Ficoll, 0.02%
polyvinylpyrrolidone, and 0.02% bovine serum albumin), 10% dextran sulfate, and 20
mM dithiothreitol (all from Sigma) at 42 °C for 18 h. The slides were washed at stringent
conditions (50% formamide in 2× SSC, 42 °C) and briefly rinsed in water followed by
70% ethanol, and dried. Slides were exposed to BioMax MR films (Amersham Pharmacia
Biotech, Buckinghamshire, UK) together with [14C]-microscales for 7–14 days. For evaluation
of the hybridization signal at the cellular level, some slides were dipped in Kodak
NTB-2 photosensitive emulsion (Kodak, Rochester, NY, USA; diluted 1:1 with distilled
water) at 42 °C, air dried over night, and then exposed for 4–6 weeks at 4 °C. The
BioMax MR films and the dipped slides were developed with Kodak D19 developer. Sections
were counterstained with Cresyl Violet, dehydrated, cleared in butyl acetate, and
covered with a coverslip using Eukitt (Merck, Darmstadt, Germany).
Quantitative evaluation of in situ hybridization was done using digitized images of
the autoradiographs (eight bit digitized picture, 256 gray values). Gray values were
measured by the public domain program ImageJ 1.38x (NIH, USA; 255=white; 0=black)
and converted to relative optical density (ROD). ROD values obtained from autoradiographic
images were plotted against standard curves obtained from images of [14C]-microscales
exposed to the same film to insure that signal values are within the linear range
of radioactivity, and then converted to percentage of control. Each value was obtained
from the innermost 90% of the respective brain area. Adjacent background levels with
no hybridization signal (internal capsule) were obtained separately for each brain
section and side. The anatomical level was verified in sections counterstained with
Cresyl Violet using a mouse brain atlas (Franklin and Paxinos, 2007).
Immunohistochemistry
Immunohistochemical analysis was performed on free-floating, PFA-fixed, 40 μm thick
coronal sections using indirect peroxidase labeling, as described previously (Furtinger
et al., 2001). The following antisera were used: polyclonal rabbit anti-FosB (1:2000
SC-48; Santa Cruz Biotechnology), polyclonal rabbit anti-GABA (1:2000 A2052, Sigma),
monoclonal mouse anti-GAD67 (1:15,000 MAB 5406; Chemicon), monoclonal mouse anti-CaMKII
(1:5000 MAB, clone 6G9, Millipore), polyclonal rabbit anti-β2 subunit (1:100, residues
351–404) and polyclonal rabbit anti-γ2 subunit (1:300, residues 319–366) (both gifts
from W. Sieghart, Vienna). A mouse Ig blocking reagent (VECTOR® M.O.M. immundetection
kit, Vector laboratories, Inc., Burlingame, USA) was used for monoclonal mouse antibodies
(GAD67, CaMKII). In brief, free floating coronal sections were incubated in 10% normal
goat serum (Biomedica, Vienna, Austria) in Tris–HCl buffered saline (TBS; 50 mM, pH
7.2) for 90 min, followed by incubation with primary antiserum. The resulting complex
was visualized by incubation with horseradish peroxidase-coupled secondary antibody
(1:250 P0448; Dako, Vienna, Austria) at room temperature for 150 min. Sections were
further incubated in a solution containing 0.03% 3,3′-diaminobenzidine tetrahydrochloride
(Sigma, St. Louis, MO, USA) and 0.005% H2O2 in TBS for 6 min. They were then mounted
on slides, air-dried, dehydrated and coverslipped. After each incubation step (with
the exception of preincubation with 10% normal goat serum), sections were washed three
times for 5 min each with TBS. All buffers and antibody dilutions, except the buffer
used for washing after peroxidase treatment and the diaminobenzidine reaction buffer,
contained 0.4% Triton X-100. Normal goat serum (10%) was included in all buffers containing
antibodies. In each experiment, sections without primary antibody were included as
a control. No immuno-positive elements were detected in these control sections.
In addition, dual labeling of FosB with either CaMKII as marker for pyramidal neurons
or GAD67 as marker for non-pyramidal interneurons in the basolateral amygdala were
performed to determine the type of FosB expressing neurons. Sections were incubated
overnight at room temperature with rabbit anti-FosB antibody (1:500, SC-48; Santa
Cruz Biotechnology) together with either a mouse monoclonal anti-CaMKII antibody (1:5000,
Millipore, Billerica, MA, USA) or with a mouse monoclonal anti-GAD67 antibody (1:20,000,
Chemicon, USA). Sections were rinsed in TBS-Triton (0.4%) and then incubated with
a Vectastatin ABC-kit (Mouse IgG, Vector laboratories, Inc., Burlingame, USA) and
donkey anti-rabbit Alexa Fluor 488 (1:500, Invitrogen, Eugene, USA) for 60 min at
room temperature. After washing in TBS, sections were incubated in a tyramide amplification
solution (1:100, TSA Plus Cyanine 3 System, PerkinElmer, MA, USA) for 2 min, mounted
on slides and covered using Vectashield mounting medium (Vector laboratories, Inc.,
Burlingame, USA).
Number of FosB, GAD67 and GABA immunoreactivity positive cells was obtained for each
marker bilaterally from two matched sections per animal (four NAB and four HAB mice)
in the basolateral amygdala (BLA) at a magnification of 400 times in multiple separate
fields and mean values were calculated for each mouse. Results are presented as number
of immunoreactivity positive cells/mm2 and expressed as mean±SEM.
Analysis of dual labeling immunofluorescence was done as described elsewhere (McDonald
and Mascagni, 2010). In brief, two matched sections per animal (four NAB and four
HAB mice) containing the BLA were processed for either FosB/CaMK or FosB/GAD67 dual
localization. Identification of dual labeled cells was performed at 400 times magnification
in multiple separate fields within the BLA in each section.
Behavioral testing
Elevated plus maze
This test was used as a widely accepted test for anxiety-like behavior as described
in detail previously (Lister, 1987; Tschenett et al., 2003).
Statistical analysis
Data are presented as means±SEM. Data were analyzed for normal distribution and equal
variances using GraphPad Prism software (Prism 5 for Macintosh, GraphPad Software
Inc., San Diego, CA, USA). Statistical analysis of behavior was done by Student's
t-test for data with normal distribution or Mann–Whitney test as a nonparametric test.
ANOVA was used to analyze overall changes in percentage of mRNA ROD values in the
CEA, BLA and MEA of HAB and NAB mice, with a Bonferroni post hoc test comparing HAB
and NAB of the respective subregion. Linear regression and a Spearman correlation
were used to investigate a correlation between mRNA expression and anxiety-related
parameters as well as locomotor-related parameters on the EPM at 7 weeks of age.
Results
HAB mice (n=20) showed a significantly reduced percentage of time spent in the open
arms compared to NAB controls (n=23), (HAB: 5.2±0.89, NAB: 28.5±1.32, P<0.0001) and
percentage of open arm entries (HAB: 21.1±2.11, NAB: 44.5±1.65, P<0.0001), indicating
increased anxiety-related behavior and thus confirming the extremely anxious phenotype
of each HAB mouse on the EPM. In addition, HAB mice exhibited reduced motor activity
under stressful conditions as reflected by a decreased number of closed arm entries
(HAB: 23.3±1.66, NAB: 33.9±1.53, P<0.0001) and reduced total distance traveled (HAB:
9.1±0.82 m, NAB: 14.3±0.46 m, P<0.0001) on the EPM during the 5 min testing period.
Histochemical results
Increased expression of GAD in the amygdala of HAB mice
As shown in Fig. 1A, the mRNAs encoding for the GABA synthesizing enzymes GAD67 and
GAD65 (not shown) were particularly concentrated in the central amygdala (CEA) while
their expression was lower both in the medial (MEA) and basolateral nucleus of the
amygdala (BLA). In HAB mice, a pronounced up-regulation of mRNA abundance of the GABA
synthesizing enzymes GAD65 and GAD67 was evident in the amygdala (Fig. 1A, B, Table
2), but not in other brain regions, including the granule cell layer of the dentate
gyrus in the dorsal (% of control NAB: 100.0±7.00 and HAB: 107.0±6.15; P=0.47, Student's
t-test, mean±SEM) (Fig. 1A, C) as well as in the ventral hippocampus (% of control
NAB: 100.0±6.07 and HAB: 85.4±7.09; P=0.14, Student's t-test, mean±SEM). To investigate
whether the increased mRNA expression translates into higher protein levels we performed
immunohistochemistry for GAD67 and GABA (Fig. 2). Indeed immunoreactivity for GAD67
(Fig. 2A, C) as well as for GABA (Fig. 2B, C) was enhanced in the amygdala of HAB
mice.
Since increased GAD67 mRNA expression may be caused by sustained neuronal activity
due to chronically enhanced stress levels, we performed immunohistochemistry for ΔFosB,
a marker of long-term adaptive changes in the brain. The number of FosB positive cells
was significantly higher in the BLA of HAB mice than of NAB controls (Fig. 3A, B).
In both, HAB and NAB mice, FosB labeling was co-localized with CaMKII labeling in
the BLA (Fig. 3C), but was absent from GAD67 positive neurons (Fig. 3C), indicating
chronic activation of principal neurons rather than of interneurons.
Expression of mRNA and immunoreactivity of GABAA and GABAB receptor subunits in the
amygdala of NAB mice
Control NAB mice revealed a similar distribution of GABAA receptor subunit mRNAs in
the amygdaloid nuclei as previously described for C57BL/6J mice (Heldt and Ressler,
2007a,b). High expression of GABAA receptor subunit α2, α3, β3 and γ2 mRNAs was observed
in the BLA while GABAA receptor subunits α4 and β1 were moderately and α1, α5, β2
and γ1 subunit mRNAs only weakly expressed. In the CEA, there was a predominant expression
of α2, α3, β3 and γ2 mRNAs and a moderate expression of β1 and γ1 subunit mRNAs. The
MEA was characterized by high expression of GABAA receptor subunit α2, α3, β1, β3,
γ1 and γ2 mRNAs. Regarding GABAB receptors, GBBR1 subunit mRNA was equally high expressed
in the BLA, CEA and MEA, whereas the mRNA for the GBBR 2 subunit was restricted mainly
to the BLA and MEA.
Altered expression of GABAA receptor subunits in the amygdala of HAB mice
The expression profile of the different GABAA receptor subunit mRNAs in the amygdala
of HAB mice was distinct from NAB controls. In HAB mice, there was a significant increase
of GABAA receptor subunit β1 mRNA in all amygdala nuclei investigated (Table 2). In
addition, the percentage of β2 mRNA was higher in the BLA, but not in the CEA or MEA
of HAB mice (Fig. 4E–H, Table 2). Among the α-subunits only expression of α5 mRNA
was altered in the amygdala. Although basal levels of α5 mRNA were already low throughout
the amygdala of NAB controls, they were further reduced in the CEA and MEA of HAB
mice (Table 2). Interestingly, in HAB mice the γ1 mRNA levels were reduced in all
amygdaloid nuclei (Fig. 4A–D, Table 2), whereas those of γ2 mRNA were specifically
increased in the BLA and CEA (Table 2). As shown in Fig. 5, β2 and γ2 subunit immunoreactivities
were also clearly increased in the BLA of HAB mice.
No difference in the mRNA abundance of both GABAB receptor subunits GBBR1 and GBBR2
was detected in any of the amygdaloid nuclei of HAB mice compared to NAB controls.
Correlation of GAD expression in the amygdala with EPM behavior
To investigate whether increased expression of GADs may be associated with altered
anxiety-like and/or locomotor-related behavior, we performed a correlational analysis
of GAD67 and GAD65 mRNA levels with the behavioral data obtained from the EPM (percentage
open arm time and total distance traveled). Indeed, the expression of GAD67 mRNA in
the BLA of HAB mice, was positively correlated with the percentage of open arm time
(r=0.58, P=0.019; Fig. 1D), but not with the total distance traveled by the mice (r=0.40,
P=0.139). In contrast, no correlation of GAD67 mRNA levels with percentage open arm
time (r=0.21, P=0.383; Fig. 1E) or total distance traveled (r=−0.005, P=0.983) was
observed in NAB mice. These findings imply that the up-regulation of the GAD67 may
be a compensatory reaction to the high anxiety behavior phenotype of HAB mice. In
contrast, when we included both groups, HAB and NAB mice, into the analysis there
was a negative correlation of GAD65 and GAD67 mRNAs expression in all amygdaloid nuclei
(BLA, CEA and MEA) with both, anxiety-like and locomotor-related behavior on the EPM
(data not shown). These findings were not surprising, since they reflect largely the
differences in the preselected phenotypes.
Altered GABAA receptor subunit mRNA expression correlated with anxiety- and motor
activity-related behavior when both groups, HAB and NAB mice, were included. There
was no correlation, however, at the level of the individual groups (not shown).
Discussion
The present study identified prominent changes of the GABA system in the amygdala
of HAB mice, an animal model of increased trait anxiety. These include markedly increased
expression of the GABA synthesizing enzymes GAD65 and GAD67 together with increased
expression of GABAA receptor subunit β1, β2, γ2 mRNAs, whereas subunit α5 and γ1 mRNA
levels were decreased.
In the BLA, most non-pyramidal neurons use GABA as a principal neurotransmitter and
exert an inhibitory action on glutamatergic principal cells (Pare et al., 2003). This
inhibition is predominantly generated through GABAA receptors that mediate either
a phasic response when located within the synapse or maintain tonic inhibition at
extrasynaptic sites (Semyanov et al., 2003, 2004). Spontaneous firing rates in the
lateral and central amygdala are among the lowest in the brain, indicating potent
tonic inhibition suppressing neuronal activity in the amygdala (Quirk and Gehlert,
2003). On the other hand, repeated stimulation of the BLA, leads to long-term synaptic
facilitation and a behavioral state of chronic anxiety (Sajdyk and Shekhar, 2000).
GABA synthesizing enzymes (GAD65 and GAD67)
GAD65 and GAD67 are catalyzing GABA synthesis and their activity is tightly related
to the tone of GABA neurons. They are abundantly expressed in the amygdala (Fig. 1A).
In the BLA both enzymes are primarily expressed in interneurons (McDonald and Pearson,
1989) whereas in the CEA they are also contained in GABA-ergic projection neurons
targeting forebrain and brain stem areas (LeDoux et al., 1988; Bourgeais et al., 2001).
Impaired GABA-ergic transmission in GAD65 knockout mice results in increased anxiety-related
behavior in the open field and elevated zero maze (Kash et al., 1999). Considerable
evidence suggests that increased GABA-ergic transmission in the BLA is associated
with reduced anxiety, while increasing the excitability of BLA projection neurons
by inhibition of GABA transmission tends to be anxiogenic (Davis et al., 1994; Shekhar
et al., 2003). In contrast, the high anxiety level in HAB mice is associated with
an increased expression of GAD65 and GAD67. This increase in GABA synthesis may be
part of a compensatory mechanism balancing the persistent over-stimulation of limbic
brain areas, as indicated by the positive correlation of GAD mRNA expression with
percentage open arm time. The increased expression of GADs may be driven by a sustained
activation of the amygdala in HAB mice. On the other hand recent evidence suggests
also excitatory actions of GABA in the basal amygdala (Woodruff et al., 2006), resulting
in rapid activation of basolateral pyramidal neurons that may significantly contribute
to increased anxiety-related behavior.
Activation of amygdala nuclei in response to stress has been well documented by increased
expression of immediate early genes c-Fos and Zif-268 (Hoffman and Lyo, 2002; Singewald,
2007; Kovacs, 2008) and has also been demonstrated in limbic areas of HAB mice (Muigg
et al., 2009). Recently, Carta et al. (2008) reported increased expression of GAD67
mRNA in the CEA in response to stressful stimuli, reflecting long-term changes in
neuronal activity (Carta et al., 2008). In the present study, the pronounced increases
of GAD65 and GAD67 mRNAs in the amygdala of HAB mice may reflect chronic activation
in amygdala nuclei due to consistently higher stress levels in these mice. This is
supported by the increase in FosB-positive principal (glutamatergic) neurons in the
BLA of HAB mice indicating chronic activation of the amygdala in the highly anxious
HAB line already under baseline, non-stressful conditions.
This idea is further supported by a study in rats demonstrating that low exploratory
activity on the EPM is associated with increased expression of GAD65/67 mRNA in the
amygdala (Nelovkov et al., 2006). Notably, low exploratory behavior, besides reduced
general motor activity may be related to increased anxiety, particularly when displayed
in stressful situations like EPM exposure (Nelovkov et al., 2006). Our correlation
analysis revealed lower anxiety in HAB mice correlating with high GAD67 mRNA levels
(Fig. 1D). While this correlation provides informative evidence, further experiments,
however, are needed to obtain a clear causal relationship between an altered GABA-ergic
system and anxiety-related behavior in these mice.
GABAA receptor α subunits
Receptors containing the α5 subunit are often located extrasynaptically, predominantly
mediating tonic inhibition (Scimemi et al., 2005; Zarnowska et al., 2009). Mice with
reduced expression of α5 subunit in the hippocampus display facilitated trace fear
conditioning, a special form of associative learning that requires hippocampal processing
(Crestani et al., 2002). Although most of the investigations on α5 subunits have been
performed in the hippocampus, a similar yet more anxiety-related role may be integrated
in the amygdala. Since HAB mice displayed a decreased abundance of α5 subunit mRNAs
in the CEA and MEA, a reduced inhibitory tonus in the amygdala of HAB mice may facilitate
emotional activation and propagate a behavioral state of enhanced trait anxiety. Indeed,
over-excitation after mild emotional challenge has been demonstrated recently in the
amygdala of HAB compared to NAB mice (Muigg et al., 2009).
GABAA receptor β subunits
Given that β subunits contain part of the binding pocket for GABA, any change in their
expression level is of physiological significance. In situ hybridization experiments
indicate an increase of β1 subunit mRNA in all amygdaloid nuclei investigated (BLA,
CEA and MEA) and a selective increase of β2 mRNA restricted to the BLA of HAB mice
(Table 2). Recently, Heldt et al. (2007a) demonstrated an up-regulation of β2 subunit
in the amygdala after fear conditioning in mice after unpaired tone-shock exposure.
They speculated that this reflected a state of hyperexcitability, similar to increased
expression levels of β subunits seen in seizure-evoked animal models of epilepsy (Tsunashima
et al., 1997; Pirker et al., 2003; Nishimura et al., 2005). Therefore also in HAB
mice the selective increase in β2 subunit mRNA may be due to over-excitation of the
BLA as indicated by increased number of FosB positive neurons under baseline conditions.
GABAA receptor γ subunits
In HAB mice, we observed a reduction of the γ1 subunit mRNA levels in all amygdaloid
nuclei investigated (Table 2), whereas those of the γ2 subunit were increased specifically
in the BLA and CEA (Table 2). The mRNA levels for the δ subunit were below detection
limit in the amygdala. The γ2 subunit mediates postsynaptic clustering and is required
for normal channel functioning (Crestani et al., 1999). Knockout mice, heterozygous
for the γ2 subunit, display increased anxiety-related behavior and reduced benzodiazepine
binding sites. Although the total number of GABAA receptors is not altered, synaptic
clustering is significantly reduced (Crestani et al., 1999). The increased expression
of the γ2 subunit mRNA in HAB mice may reflect an accelerated turnover of the γ2 subunit
containing GABAA receptors due to chronically high stress conditions and may indicate
a compensatory mechanism counteracting increased amygdala activity. Following EPM
exposure, Chacur et al. (1999) observed increased [3H]-flunitrazepam but not [3H]-muscimol
binding specifically in the CEA and BLA, indicating a higher sensitivity of GABAA
receptors to benzodiazepines although the total number of receptors may not be altered
(Chacur et al., 1999).
Increased sensitivity to the anxiolytic effect of diazepam is generally observed in
rodents with low explorative activity and changes in the subunit composition of GABAA
receptors due to altered subunit expression may be an attractive explanation for this
phenomenon (Liebsch et al., 1998; Bert et al., 2001; Nelovkov et al., 2006). In the
present study, also HAB mice exhibit low explorative activity during EPM stress, presumably
related to the high trait anxiety of these mice. Thus, the high sensitivity of HAB
mice to the anxiolytic effect of benzodiazepines (Kromer et al., 2005) may, at least
in part, be related to the higher number of γ2 containing GABAA receptors, known to
mediate the action of benzodiazepines in the amygdala.
Interestingly, whereas γ2 subunits are increased in the amygdala of HAB mice, γ1 subunits,
particularly enriched in the CEA and MEA, are significantly reduced in these mice
(Table 2). Esmaeili et al. (2009) recently demonstrated that GABA-ergic synapses formed
by lateral inputs to the CEA, probably originating from intercalated cell masses,
contain γ1 subunits (Esmaeili et al., 2009). These connections are important for extinction
of conditioned fear (Likhtik et al., 2008). Deficits in extinction learning have been
described for HAB rats (Muigg et al., 2008) and mice (Yen et al., unpublished observation)
and could be associated with decreased γ1 subunit expression.
Conclusion
We demonstrated differential alterations of the GABA-ergic system in the amygdala
of HAB mice, a model of pathological trait anxiety. Expression of GAD65 and GAD67
was highly up-regulated on mRNA as well as on protein level, suggesting enhanced GABA
synthesis and release. Notably the increases in GAD65/67 and γ2 subunit expression,
but also that of subunits β1 and β2, point towards a facilitated GABA transmission,
aiming to compensate the high-anxiety state of HAB mice. On the other hand, the reduced
expression of the α5 subunit in the CEA and MEA may be part of the molecular mechanisms
leading to the anxious phenotype of HAB mice. A reduced number of α5 subunit containing
receptors may cause reduced tonic inhibition, favoring activation of the amygdala
even under non-stressful conditions as illustrated by the increase in FosB expression.
Both mechanisms may be causatively related to the compensatory increase in GAD65 and
GAD67 expression.