Division of Neuroscience, John Curtin School of Medical Research,
Australian National University, Canberra, ACT 2601, Australia
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INTRODUCTION |
-Aminobutyric acid (GABA) is
the principal inhibitory transmitter in the CNS and activates
ionotropic GABAA and GABAC
receptors, and metabotropic GABAB receptors.
Ionotropic GABA receptors are heterooligomeric proteins of which 20 subunits have so far been identified:
1-6,
1-4,
1-3,
,
,
,
, and
1-3. Of these, the
subunits assemble to form
GABAC receptors, while
subunits are only
found outside the CNS. The other subunits coassemble as heteromultimers
to form a number of different types of GABA receptor (Barnard et
al. 1998
). The subunit composition of GABA receptors determines
their biophysical and pharmacological properties (MacDonald and
Olsen 1994
). Thus considerable GABA receptor heterogeneity is
expected to be present within the CNS. Immunohistochemical studies have
shown that different subunits can be targeted to different regions of
the neuronal membrane (Nusser et al. 1996
, 1998
). It has
therefore been suggested that functionally different receptors may be
targeted to different synapses. However, there is little direct
evidence to support this proposal.
The amygdala is intimately involved in emotional behavior
(Kluver and Bucy 1939
; LeDoux 2000
), and
its role in the generation of anxiety and conditioned fear is well
known (Davis 1992
; LeDoux 1995
). Studies
of Pavlovian fear conditioning have suggested that the basolateral
amygdala is the site for convergence of neural pathways conveying
information about conditioned and unconditioned stimuli. This
information is processed locally and then transmitted to the central
amygdala. Neurons in the central nucleus project to the hypothalamus
and brain stem regions, which are important in the behavioral,
hormonal, and autonomic aspects of the fear response (LeDoux
2000
). Neurons in the central amygdala express two types of
ionotropic GABA receptor (Delaney and Sah 1999
). One is
a typical GABAA-receptor that is blocked by low
doses of bicuculline and positively modulated by benzodiazepines,
anesthetics, and barbiturates. The other is a novel type of receptor,
similar to GABAC receptors first described in the
retina (Enz et al. 1995
; Qian and Dowling
1994
). These receptors are less sensitive to bicuculline and
are little affected by anesthetics and barbiturates. However, unlike
GABAC receptors, they are inhibited by 1,4 benzodiazepines such as diazepam (Delaney and Sah 1999
).
In this study, we examine the distribution of these two types of
ionotropic GABA receptors. We find that
GABAC-like receptors are present on dendritic
synapses where they colocalize with GABAA
receptors. These synapses are innervated by axons of the intercalated
cell masses (ICMs). In contrast, somatic synapses express only
GABAA receptors and are innervated by axons of
different origin.
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METHODS |
Wistar rats (17- to 20-days old) were anesthetized with
intraperitoneal pentobarbitone (50 mg/kg) and decapitated, and the brain was removed and immersed in ice-cold Ringer. Coronal brain slices
(400 µm) were prepared using standard methods. All procedures were in
accordance with the Institutional Animal Care and Ethics Committee
guidelines. Slices were superfused at 200 ml/h with oxygenated external
solution containing (in mM) 118 NaCl, 2.5 KCl, 25 NaHCO3, 10 glucose, 1.2 NaH2PO4, 1.3 MgCl2, and 2.25 CaCl2 in a
bath volume of 1 ml. Kynurenic acid (2 mM) was included in the external
solution to block excitatory glutamatergic transmission. Tetrodotoxin
(0.5 µM) was added to block synaptic transmission when recording
mIPSCs. Recordings were made from neurons in the lateral division of
central amygdala (CeL) using the whole cell patch-clamp method with
either the blind approach or infrared differential interference
contrast techniques. Borosilicate glass electrodes (3-5 M
) were
filled with high chloride internal solution containing (in mM) 130 CsCl2, 1 MgCl2.6H2O, 10 EGTA, 10 HEPES, 2 Mg2ATP, and 0.2 Na3GTP (pH 7.3 with CsOH, 290 mOsM). In
experiments where the sucrose solution was applied to the soma and
dendrites, Lucifer yellow was also added to the internal solution.
Neurons were held in voltage-clamp mode at
60 mV.
Drugs used were bicuculline methiodide,
(1,2,5,6-tetrohydropyridine-4-yl) methylphosphinic acid (TPMPA; RBI
Research Chemicals), kynurenic acid (Sigma), tetrodotoxin (Alamone),
and diazepam (gift from Professor P. Gage). Inhibitory postsynaptic
currents (IPSCs) were evoked electrically using stainless steel bipolar
stimulating electrodes (Frederick Haer) placed laterally along the edge
of CeL or medially in central nucleus medial sector (CeM). Stimuli were
50 µs in duration. Sucrose stimulation of miniature IPSCs (mIPSCs)
was performed by pressure ejection of external solution containing 0.5 M sucrose through a 3 M
patch pipette to the surface of the slice or
by low pressure injection through a 3-5 M
patch pipette under
visual guidance. Lucifer yellow was included in the sucrose solution
for visualization of the spread of the ejected sucrose solution.
Signals were recorded using an Axopatch 200B amplifier (Axon
Instruments), filtered at 5 kHz, and digitized at 10 kHz (Instrutech,
ITC 16). Data were acquired with Axograph (Axon Instruments) on a
Macintosh G3 computer. Series resistance (10-30 M
) was monitored
on-line throughout the experiment, and experiments were rejected if
resistance changed by 10%. No series resistance compensation was
used. IPSC peak amplitude, 10-90% rise time, half-peak width, and
decay time constants were analyzed using Axograph 4.0 and compiled and
statistically analyzed using Microsoft Excel. All values are expressed
as means ± SE, and all statistical comparisons were done using
Student's t-test. Spontaneous mIPSC were detected using the
variable-amplitude template event-detection program included in
Axograph. Measurements of amplitude and kinetics and tracer of mIPSCs
shown are averages of between 4 and 10 events for large mIPSCs and
between 30 and 50 events for the small-amplitude mIPSCs under control
conditions. All experiments were done at room temperature (21-24°C).
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RESULTS |
Whole cell recordings were made from neurons in the lateral
division of the central amygdala (Jolkkonen and Pitkanen
1998
). We have shown that locally evoked inhibitory synaptic
inputs to these neurons activate both GABAA and
GABAC-like ionotropic receptors (Delaney
and Sah 1999
). To test if both types of GABA receptor were
colocalized at all inhibitory synapses, we examined spontaneously occurring mIPSCs that reflect the response to single quanta of transmitter. At a holding potential of
60 mV, mIPSCs in CeL neurons showed a wide variation in amplitude (Fig.
1). Most responses were of small
amplitude (<50 pA; mean amplitude, 19 ± 2 pA; n = 8). However, a small number of large-amplitude events (>100 pA; mean
amplitude, 123 ± 9 pA; n = 7) were also detected.
These large-amplitude events had significantly faster
(P < 0.01) 10-90% rise times (0.91 ± 0.09 ms)
compared with the rise times of the smaller events (1.51 ± 0.12 ms; n = 4; Fig. 1, C and D).

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Fig. 1.
Spontaneous GABAergic inhibitory postsynaptic currents in the central
amygdala show wide variation in amplitude. A and
B: spontaneous miniature inhibitory postsynaptic
currents (mIPSCs) recorded in the presence of tetrodotoxin are shown
from 1 cell in A and their peak amplitudes are plotted
as a histogram in B. C: examples of
events with small amplitude (<50 pA, top traces) and
those with large amplitude (>100 pA, bottom traces).
D: average data for small- and large-amplitude events.
Events with large amplitudes had faster rise times than the smaller
events.
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To increase the frequency of large-amplitude events, we applied a
hypertonic Ringer solution containing 0.5 M sucrose, which causes
asynchronous release of minis (Fatt and Katz 1952
).
Application of hypertonic solution to the slice caused a large increase
in mIPSC frequency (Fig. 2A)
and revealed a much larger proportion of fast large-amplitude mIPSCs.
The frequency of large-amplitude events increased from 0.02 ± 0.01 to 0.31 ± 0.01 Hz (n = 6; P < 0.01). Large-amplitude mIPSCs evoked in sucrose had amplitudes and
rise times identical to those of large-amplitude events under control
conditions (Fig. 2B). The higher frequency of
large-amplitude events following application of sucrose increased the
amplitude of the average mIPSC from 19.4 ± 1.5 to 35.5 ± 3.4 pA (n = 9; Fig. 3,
A and B). Application of the
GABAA receptor antagonist bicuculline methiodide
(BIC, 10 µM) markedly reduced the amplitude (71 ± 6%
reduction; n = 6) and frequency of small mIPSCs but
completely abolished the large events (Fig. 3A). Furthermore
in the presence of BIC, application of sucrose increased the frequency
of mIPSCs with no change in their average amplitude (Fig.
3A). In contrast, application of the
GABAC-receptor antagonist TPMPA (60 µM)
(Ragozzino et al. 1996
) reduced the amplitude of the
small events (P < 0.01) while having no significant
effect (P > 0.05) on the large-amplitude IPSCs (Fig.
3, C and D; n = 3). Application
of hypertonic Ringer in the presence of TPMPA increased the frequency
of the large-amplitude events (Fig. 3B). In the presence of
TPMPA, mIPSCs were completely blocked by 10 µM BIC and application of
sucrose did not evoke any IPSCs (data not shown). These results
indicate that two types of GABAergic mIPSCs are present in CeL neurons.
One consists of large-amplitude fast-rising events that are fully
blocked by low doses of BIC but are unaffected by TPMPA, suggesting
that they are due to activation of pure GABAA
receptors. The second population consists of small-amplitude
slow-rising events that are partially blocked by BIC and TPMPA,
indicating that both GABAA receptors and
GABAC-like receptors (Delaney and Sah
1999
) are present at these synapses.

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Fig. 2.
Application of hypertonic Ringer increases the frequency of large
amplitude mIPSCs. A: application of hypertonic (0.5 M)
sucrose increased the frequency of large-amplitude events.
B: events recorded before (left) and
during sucrose application (right) are shown as
histograms. Note the increase in number of large-amplitude events in
sucrose. C: large-amplitude events evoked by sucrose
application have properties identical to those recorded under control
conditions (n = 6).
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Fig. 3.
Large fast mIPSCs are due to activation of purely GABAA
receptors, but small mIPSCs are due to activation of 2 types of GABA
receptor. A: effect of 10 µM bicuculline application
on spontaneous mIPSCs evoked by sucrose. Left: traces
are in control condition and average mIPSCs before and after sucrose
application have been superimposed in Ai. Note the
increase in mIPSC amplitude by sucrose due to the presence of more
large-amplitude events. Right: traces show the effects
of sucrose application in the presence of bicuculline. The average
amplitude of the control mIPSCs is smaller in bicuculline, but there is
no increase in their amplitude by sucrose (Aii).
B and C: the GABAC antagonist
(1,2,5,6-tetrohydropyridine-4-yl) methylphosphinic acid (TPMPA) reduces
the amplitude of small sIPSCs but has no effect on large sIPSCs.
B, left: traces show the action of sucrose under control
conditions; right: traces show the effect of sucrose in
the presence of 60 µM TPMPA. B, i and
ii: average mIPSCs before and after sucrose have been
superimposed. C: small-amplitude events are reduced in
amplitude by TPMPA (P < 0.01). Average data
(n = 3) are shown in the histograms on the
right. The IPSC recorded in control Ringer is marked
(*). D: large-amplitude mIPSCs evoked by sucrose before
and after application of TPMPA have been superimposed. Note that these
events are not significantly affected by TPMPA (P > 0.05). Average data are shown in the histograms on the right.
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What explains the rapid rising phase and large amplitude of the pure
GABAA mIPSCs? One possibility is that these
synapses are located electrotonically closer to the soma resulting in
less filtering of their kinetic properties (Bekkers and Stevens
1996
; Maccaferri et al. 2000
). To test this
idea, we focally applied hypertonic sucrose solution to the soma and
dendrites of CeL neurons. Focal somatic application of sucrose evoked
mostly large-amplitude mIPSCs (Fig. 4,
A-C). These events had a peak amplitude of 144 ± 13 pA and 10-90% rise time of 0.92 ± 0.04 ms (n = 4). Activation of these large-amplitude events led to a large increase
in the average mIPSC (Fig. 4D). In contrast, dendritic
application of sucrose evoked mIPSCs that had amplitudes and rise times
indistinguishable from those of the small events present before
application of sucrose (Fig. 4, E-G). Thus the average
mIPSC was not affected by the dendritic application of sucrose (Fig.
4H). In addition, large-amplitude mIPSCs evoked by somatic
application of sucrose were fully blocked by 10 µM BIC
(n = 3) but unaffected by TPMPA (Fig.
5; 13% reduction in amplitude,
n = 2). In contrast, mIPSCs evoked by dendritic sucrose
application were reduced in amplitude by TPMPA (Fig. 5; 25% reduction
in amplitude, n = 2). Together these results show that
fast-rising large-amplitude mIPSCs arise from synapses on or near the
soma of CeL neurons that express pure GABAA
receptors. In contrast, the smaller-amplitude slower-rising mIPSCs
arise from synapses of more distal dendritic origin that express both GABAA and GABAC-like
receptors. We next tested if these synapses are innervated by axons of
the same or different origin.

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Fig. 4.
Large fast events are due to somatic synapses, while smaller-amplitude
events are due to dendritic synapses. Whole cell recordings were made
with the patch pipette solution and the sucrose-containing electrode
containing Lucifer yellow. A and B:
somatic application of sucrose evokes large-amplitude fast-rising
mIPSCs. A, top: the control condition with the recording
electrode attached to the soma. Bottom: application of
sucrose to the soma. Scale bar: 5 µm. C: the amplitude
and rise time of mIPSCs recorded before ( ) and after
( ) application of sucrose are shown. D:
average mIPSC before and after sucrose application have been
superimposed and show that there is a large increase in peak amplitude.
E and F: dendritic application of sucrose
evokes only small-amplitude IPSCs. E, top: the control
condition with the soma out of view to the right of the panel.
Bottom: sucrose application is restricted to the
dendrite. Scale bar: 5 µm. G: the amplitude and rise
time of mIPSCs recorded before ( ) and after ( )
application of sucrose are shown. H: average mIPSC
before and after sucrose have been superimposed and show that there is
no change in peak amplitude.
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Fig. 5.
Somatic large IPSCs are fully blocked by bicuculline. A:
mIPSCs evoked by somatic application of sucrose are shown before and
after application of 10 µM bicuculline. B: averaged
large ( 100 pA) IPSCs in control and all IPSCs in the presence of
bicuculline have been superimposed. The average IPSC in the presence of
bicuculline is marked (*). C: mIPSCs evoked by somatic
application of sucrose are shown before and after application of 100 µM TPMPA. D: averaged large IPSCs in control and in
the presence of TPMPA have been superimposed. E:
averaged small IPSCs ( 50 pA) before and after application of TPMPA
have been superimposed.
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We have shown previously that IPSCs evoked by stimulation on the
border of the CeL and the lateral amygdala activates IPSCs that contain
both GABAA and GABAC-like
receptors (Delaney and Sah 1999
). These inhibitory
inputs are thought to arise from GABAergic neurons within the
intercalated cell masses of the amygdala (Paré and Smith
1993
; Royer et al. 1999
). To test if both types
of GABA receptors were present at all synapses made by these inputs, we evoked single-fiber IPSCs by using minimal stimulation (Raastad et al. 1992
). Glutamatergic inputs were blocked by inclusion of 2 mM kynurenic acid to the perfusing Ringer. Stimulation electrodes were placed on the lateral border of the CE to activate inputs arising
from the ICMs (Royer et al. 1999
) and medial to the CE to activate inputs that form part of the extended central amygdala (Sun and Cassell 1993
) (Fig.
6A). IPSCs evoked from the
ICMs had a peak amplitude of 19.3 ± 3.8 pA and a 10-90% rise
time of 3.2 ± 0.4 ms (n = 10; Fig.
6B). In contrast, when the stimulating electrode was placed
medially, minimal stimulation evoked a more rapidly rising (10-90%
rise time 1.7 ± 0.1 ms) large-amplitude (130.8 ± 13.4 pA)
IPSCs (Fig. 6, C and D). Laterally evoked IPSCs were partially inhibited by BIC (10 µM, 71 ± 4% block;
n = 8;) and TPMPA (100 µM, 47 ± 10% block;
n = 4), indicating that both types of GABA receptors
are present at these synapses (Fig. 7, A and B). Medially evoked IPSCs in the same cells
were fully blocked by 10 µM BIC (98 ± 3% block;
n = 7; Fig. 7A) and not affected by TPMPA
(n = 10; Fig. 7B). This result shows that
IPSCs evoked by medial stimulation activate a population of pure
GABAA receptors. In contrast, IPSCs evoked from
the ICMs activate both GABAA and GABAC-like receptors. In further confirmation of
this, application of diazepam potentiated the amplitude of minimally
evoked medial IPSCs from 99 ± 6 to 108 ± 10 pA, whereas it
reduced the amplitude of laterally evoked IPSCs from 23 ± 7 to
19 ± 6 pA (n = 6) (Delaney and Sah
1999
) (Fig. 8).

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Fig. 6.
Laterally evoked IPSCs activate dendritic inhibitory synapses, whereas
medially evoked IPSCs activate somatic synapses. A:
schematic diagram illustrating medial and lateral sites of stimulation.
EC, external capsule; LA, lateral amygdala; BLA, basolateral amygdala;
CeL, central nucleus lateral sector; CeM, central nucleus medial
sector. Lateral stimulation activates cells that form the interacalated
cell masses (ICM), whereas medial stimulation activates axons coursing
through the medial division of the central amygdala. B:
minimal stimulation of ICM cells evokes a smal-amplitude slowly rising
IPSC. Right: trace shows the average of 4 sweeps at a
stimulus of 4 V. C: minimal stimulation of medially
located fibers evoked large-amplitude fast-rising IPSC.
Right: trace shows the average of 4 sweeps at a stimulus
of 3.5 V. D: average data (n = 6)
illustrating the difference in the evoked IPSC from medial and lateral
inputs.
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Fig. 7.
Laterally evoked IPSCs activate synapses that contain both
GABAA and GABAC-like receptors, but medially
evoked IPSCs activate synapses with only GABAA receptors.
A: laterally evoked IPSCs were partially blocked by
bicuculline, while medially evoked IPSCs were fully blocked by
bicuculline. Average data are shown in the histogram. B:
laterally evoked IPSCs were partially blocked by TPMPA, while medially
evoked IPSCs were unaffected by TPMPA. Average data are shown in the
histogram (n = 3 for TPMPA and
n = 7 for bicuculline). All recording of 2 inputs
are from the same cells and are averages of 6 sweeps.
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Fig. 8.
Diazepam potentiates medially evoked IPSCs but reduces laterally evoked
IPSCs. A: IPSCs were evoked by minimal stimulation in
the region of the ICMs, and diazepam was bath applied at a
concentration of 10 µM. The IPSC shows reduction in peak amplitude
and a slowing of the remaining IPSC as expected for a synapse
expressing both GABAC-like and GABAA receptors.
B: IPSC evoked by minimal medial stimulation evokes a
large-amplitude event that is potentiated by diazepam as expected for a
pure GABAA synapse.
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DISCUSSION |
We have shown that two types of GABAergic mIPSCs are present on
neurons in the CeL. One type of mIPSC is of large amplitude and has a
fast rise time. These mIPSCs are fully blocked by low concentrations of
BIC but are unaffected by the GABAC antagonist TPMPA, indicating that they are purely due to activation of
GABAA receptors. The frequency of these mIPSCs
was greatly potentiated by somatic application of hypertonic sucrose
solution, indicating that they result from activation of somatic
synapses. The other type of mIPSC is of smaller amplitude, has a slower
rise time, and is partially blocked by both BIC and TPMPA, indicating
that these synapses contain both GABAA and
GABAC-like receptors. The frequency of these
mIPSCs is potentiated by dendritic application of sucrose, showing that
they are due to activation of dendritic synapses. Minimal electrical
stimulation of inputs that have a medial-origin-evoked IPSC whose
amplitude and rise time was similar to those of large mIPSCs evoked by
somatic sucrose application. Medially evoked IPSCs were fully blocked
by bicuculline but unaffected by TPMPA. These results indicate that
inputs that have a medial origin make synapses on the soma of CeL
cells, and these synapses contain only GABAA
receptors. In contrast, stimulation of lateral inputs arising from the
intercalated cell masses evoked IPSCs with kinetics and pharmacology
similar to those of dendritically located mIPSCs. These IPSCs were
partially blocked by BIC and TPMPA, indicating that both
GABAA and GABAC-like
receptors are present at these synapses. Together, these data show that
neurons in the lateral division of the central amygdala have two types of fast inhibitory synapses. GABAergic inputs arising from the intercalated cell masses make dendritic synapses at which both GABAA and GABAC-like
receptors are present. In contrast, inhibitory inputs arising medial to
the CeL (Sun and Cassell 1993
) make somatic synapses at
which only GABAA receptors are present. Thus
while GABAA receptors are present at both somatic
and dendritic synapses, GABAC-like receptors
appear to be selectively targeted to different synapses.
Ionotropic GABA receptors are formed from 19 different subunits that
can be assembled in a variety of combinations with distinct biophysical
and pharmacological properties (MacDonald and Olsen 1994
; Verdoon et al. 1990
). In some neurons,
different subunits have been shown to be targeted to different cellular
compartments (Nusser et al. 1996
, 1998
), and this has
led to the suggestion that different inhibitory synapses may express
kinetically and pharmacologically distinct types of receptor. Indeed,
GABAA receptors present at synapses have been
shown to have properties different from those present in the
extrasynaptic membrane (Banks and Pearce 2000
;
Brickley et al. 1999
). In hippocampal pyramidal neurons, two kinetically and pharmacologically distinct
GABAA receptor-mediated IPSCs have also been
described (Pearce 1993
).
The mechanisms that underlie this selective targeting of different
receptors are as yet not known. It has been suggested that a
subsynaptic matrix of receptor-associated proteins may play a key role
in selecting and anchoring particular receptor subtypes to particular
synapses (Kannenberg et al. 1997
; Sheng
1997
). GABAA and
GABAC-like receptors present in CeL neurons are
pharmacologically distinct, indicating that their subunit composition
is likely to be different (Delaney and Sah 1999
). Thus
one possibility is that the two types of receptor have different
anchoring proteins. In this model, anchoring proteins that bind
GABAC-like receptors would have to be present
only at dendritic synapses while those for GABAA
receptors would be present at somatic as well as dendritic locations.
The amygdala forms a key element of the circuit involved in emotional
processing and Pavlovian fear conditioning (LeDoux
2000
). Anatomically, the amygdala is divided into a number of
subnuclei (McDonald 1999
). Cortical (McDonald
1999
) and thalamic (Turner and Herkenham 1991
)
inputs enter the amygdala at the level of the basolateral nuclei. These
nuclei then send information to the central nucleus, which projects to
hypothalamic and brain stem regions that control the expression of
fear-related responses (Krettek and Price 1978
;
Maren and Fanselow 1996
). The intercalated cells are a
population of GABAergic neurons that lie in clusters between the
basolateral and central amygdala (Millhouse 1986
; Nitecka and Ben-Ari 1987
). These cells form an
inhibitory interface between the basolateral and central amygdala
(Royer et al. 1999
). Connections between medial and
lateral groups of ICMs, which receive inputs from different regions of
the basolateral complex, have been proposed to modulate the traffic of
information reaching different regions of the basolateral amygdala
(Royer et al. 2000
). Our results indicate that the axons
of ICM cells make synapses on the dendrites of CeL neurons.
We cannot be certain of the origin of the medial input to CeL neurons.
It is unlikely to be a projection originating from CeM as anatomical
studies have found this to be rather meager (Jolkkonen and
Pitkanen 1998
). The axons of GABAergic neurons in the CeL have
been found to make synapses on local cells (McDonald 1982
), raising the possibility that the medial input may be due to retrograde activation of neurons projecting out of the CeL. However,
against this proposal, in our extensive recordings from neurons in the
CeL, we have never recorded from neurons that were antidromically
activated by medical stimulation (unpublished observations). The
central amygdala also receives a number of extraamygdaloid inputs
(Pitkänen 2000
). However, as the exact nature of
these inputs has not been determined, it is not possible to speculate on the identity of medially originating GABAergic inputs.
The presence of GABAC-like receptors at these
synapses leads to a reduction in the amplitude of the IPSC by
benzodiazepines (Fig. 7), which enhance the IPSC at most GABAergic
synapses. This result suggests that processing of information between
the input and output stages of the amygdala will be differently
modulated by agents that modulate GABA receptors. The axons of neurons
in the CeL have been found to have spines in the axon initial segment that are innervated by axons of passage (McDonald 1982
).
While the nature of this synapse has not been determined, it is
tempting to speculate that the medial inhibitory inputs we have
recorded may in part be due to the activation of these axonic synapses. The large-amplitude of these inputs would make them potent modulators of the output of CeL neurons. Agents such as benzodiazephines, which
amplify these medial inputs would therefore have much larger effects on
the output of these cells as compared to its effects on the integration
of dendritic inputs.
Disorders of the storage or expression of fear responses are thought to
underlie such mental disorders as panic attacks, anxiety, and post
traumatic stress disorder. Benzodiazepines, which are commonly used for
the treatment of anxiety, are thought to act by enhancing the action of
the inhibitory transmitter GABA (Costa and Guidotti
1996
; Tallman and Gallager 1985
). The central
amygdala has a particularly high density of benzodiazepine binding
sites (Sibille et al. 2000
). Our results show that
benzodiazepines have distinct actions at different GABAergic circuits
within the amygdala and highlight the complex nature of the inhibitory
control over amygdaloid function. The implications of having two types
of inhibitory synapse on GABAergic lateral central amygdaloid neurons
must be taken into account when considering the mechanisms of action of benzodiazepines in the treatment of anxiety disorders.
We thank L. Faber, J. Power, and M. De Armentia for comments on the
manuscript. P. Sah is a Charles and Sylvia Viertel Senior Medical
Research Fellow.
This work was supported by grants from the National Health and Medical
Research Council of Australia.
Address for reprint requests: P. Sah, Div. of Neuroscience,
John Curtin School of Medical Research, GPO Box 334, Canberra, ACT
2601, Australia (E-mail: pankaj.sah{at}anu.edu.au).