Download Distribution of GABA‐like immunoreactivity in the rat amygdaloid

THE JOURNAL OF COMPARATIVE NEUROLOGY 266:45-55 (1987)
Distribution of GABA-Like
Immunoreactivity in the Rat Amygdaloid
Complex
L. NITECKA AND Y. BEN-ARI
INSERM U-29, HGpital de Port-Royal, 75014 Paris, and LPN, CNRS, Gif sur Yvette
91190, France
ABSTRACT
The distribution of GABA-like (GABA-Li)immunoreactivity in the rat
amygdaloid complex was studied by using a n anti-GABA antibody. GABALi positive neurons and processes were present in every nucleus of the
complex. Three patterns of immunoreactivity were revealed: (1)the intercalated masses and the lateral olfactory tract nucleus exhibited the most
intense staining of the neuropil, and virtually every neuron was labeled, (2)
the central and medial nuclei contained intensely labeled neuropil and
moderately labeled neurons, and (3) in the remaining nuclei, the neuropil
was weakly labeled, and relatively numerous GABA-Li neurons were present. Our results suggest that: (1)the intercalated masses and lateral olfactory tract nucleus consist of large aggregates of GABA-Li immunoreactive
neurons, and (2) the lateral, basal dorsal, and the posterior cortical nuclei
may constitute a significant source of GABAergic connections to other amygdaloid nuclei, in particular to the medial and central nuclei.
Key words: immunocytochemistry, GABAergic system, amygdala
The mechanism by which amygdaloid nuclei influence Druga, '70; Jacobson and Trojanowski, '75; Herzog and Van
the activity of other limbic centers and coordinate motiva- Hoesen, '76; Kosmal, '76; Krettek and Price, '77; Hall, '63;
tional states and emotions is poorly understood (Elefther- Aggleton et al., '80; Kosmal and Dabrowska, '80; Mueson
iou, '72; Ben-Ari, '81). Amygdaloid nuclei control important et al., '81; Ottersen, '82). The basolateral region also has
biological functions; they influence alimentary, defensive, axonal projections to the centromedial zone (Krettek and
aggressive, neurosecretory, and sexual functions as well as Price, '78a; Nitecka et al., '81a,b; Ottersen, '82; Price and
memory and learning processes (Eleftheriou, '72; Ben-Ari, Amaral, '84). Furthermore, direct connections emerging
'81). Experiments performed two decades ago had shown from the basolateral part and extending to the hypothalathe existence of two functional regions within the amygda- mus and other brainstem centers, which may mediate its
loid complex: an excitatory (centromedial) and a n inhibi- inhibitory effect, are less numerous than the projections
tory (basolateral) part Kaada, '72; Fonberg, '81). The emerging from the centromedial (excitatory) region (Cowan
anatomical basis of these opposite activities has not been et al., '65; Leonard and Scott, '71; De Olmos, '72; Krettek
fully elucidated.
and Price, '78b; Aggleton et al., '80; Post and Mai, '80;
The inhibitory trasmitter a-aminobutyric acid (GABA) Mehler, '80; Nitecka, '81; Price, '81b). This raises the quesplays an important role in the modulation of neuronal tion of whether the amygdala mediates its inhibitory effect
activity. However, the organization of the GABAergic ~ Y S - on the hypothalamus and other subcortical limbic centers
tem in the amygdala has not been investigated in detail. by means of direct long axonal connections or by means of
Thus, at present, it is not clear how GABA-containing neu- a n inhibition of the centromedial zone.
rons are involved in mediating the inhibitory or excitatory
actions of the amygdaloid stimulation. The basolateral (inAccepted July 2, 1987.
hibitory) part of amygdaloid complex is known to have
massive, reciprocal connections with differentcortical fields, Address reprint requests to Y. Ben-Ari, INSERM U-29, HBpital
and during evolution it has increased in size dramatically de Port-Royal, 123 Bd de Port-Royal, 75014 Paris, FRANCE.
in parallel with the extensive development of the neocortex
L. Nitecka is now at the Academy of Medicine, Department of
(Johnston, '23; Koikegami, '63; Whitlock and Nauta, '66; Anatomy, Gdansk 80210, Poland.
0 1987 ALAN R. LISS, INC.
L. NITECKA AND Y. BEN-ARI
46
We describe in the present study the distribution of camera lucida. The general distribution of GABA-Li neuGABA-like (GABA-Li) inimunoreactive neurons and pro- rons and neuropil in the amygdaloid nuclei are shown in
the photomicrographs of Figures 3-5. It is evident from
cesses in the amygdala.
these figures that every amygdaloid nucleus contained
MATERIALS AND METHODS
GABA-Li material. However, the pattern and location of
The distribution of GABA-like immunoreactivity in the the neurons varied, as did their shapes, the density of their
amygdaloid complex has been studied by using specific distribution, and the intensity of the staining in the
anti-GABA antibodies (Seguela et al., '84; Geffard et al., neuropil.
Relying on the organization of GABA-Li neurons, we
'85). Male Wistar rats (n = 9) were anesthetized with pentobarbital, and perfusion-fixation was performed by intra- have differentiated three groups of nuclei: group I (lateral
cardiac injection of saline followed by 5% glutaraldehyde in olfactory tract nucleus and intercalated nuclei) with in0.1 M cacodylate buffer (pH 7.6). After postfixation (c. 1 h) tensely stained neurons and heavy labeling in the neuropil
in the same fixative, 50-pm vibratome sections were cut (Figs. 3-7); group I1 (central and medial nuclei and anterior
and used for the immunohistochemical procedure (Seguela amygdaloid area) with moderately labeled neurons and inet al., '84). The concentration of the antisera was 1/3,000 to tensely labeled neuropil (Figs. 3 4 8 ) ; group I11 (remaining
1/5,000. The peroxidase-antiperoxidase (PAP)method was nuclei) with weakly labeled neuropil and scattered clusters
used (Sternberger, '79). The PAP complex, FAB-fragment, of GABA-Li neurons (Figs. 3-5,8, 9).
or biodin-avidin peroxidase complex were used as a last
Nuclei of group I
step in the immunoreaction. The use of the latter proceIntercalated nuclei (Figs. 3-6). The small neurons of the
dures enabled tissue incubation time to be shortened and
background staining to be reduced, this was particularly intercalated masses were intensely labeled. Relying on the
useful in order to examine the labeling in cell bodies in counterstained sections, it was conspicuous that virtually
structures (e.g., intercalated masses) in which there is an every neuron contained GABA-Li material (Figs. 3-6). The
intense GABA-Li labeling in the neuropil. Alternate sec- neuropil was also intensely stained. This corresponded
tions were counterstained with Nissl. Histochemical con- probably to fine branches of the dendrites originating from
trols of the antibody specificity included incubation of GABA-Li perikarya located in the intercalated nuclei. The
sections in solutions in which the specific antiserum was intensely labeled neuropil often extended beyond the rereplaced by the primary antiserum preabsorbed with a n gions of aggregated perikarya (Figs. 5,6). As a consequence
excess of immunogen or nonimmune serum; other sections of this organization, histological sections made through the
were incubated without the primary antiserum. The control periphery of the intercalated nuclei showed spots of insera at a concentration of 1:3,000 or 1:5,000 stained all tensely labeled neuropil within adjacent amygdaloid nuclei
tissue elements weakly and unselectively. The sections in- (Fig. 5 , big arrows).
Lateral olfactory tract nucleus (Fig. 7). Three subdivicubated without primary serum did not show the stain.
sions of the lateral olfactory tract nucleus could be identiRESULTS
fied dorsal, intermediate, and ventral areas (Fig. 7). In the
The terminology of the amygdaloid nuclei established by dorsal area, scattered GABA-Li perikarya were conspicuJohnston ('23) has been used with modifications resulting ous and the neuropil was poorly labeled. The intermediate
from more recent morphological studies and acetylcholin- area contained an intensely stained neuropil and numerous
esterase stains (Nitecka, '75; Price, '81a). The simplified large, pyramidal neurons with intense GABA-Li material.
diagrams in Figure 1show the amygdaloid nuclei of the rat Virtually every neuron of the large intermediate zone was
in frontal sections. Diagrams in Figure 2 illustrate the labeled (Fig. 7A,B). The dendrites of these neurons were
pattern of distribution of GABA-Li neurons in amygdaloid always directed ventrally and the initial segments of the
nuclei; the location of every GABA-Li neuron is depicted by apical dendrites were often conspicuous (Fig. 7B). In contrast, very few GABA-Li neurons were observed in the
ventral area, but the neuropil was intensely labeled
(Fig. 7A).
A bbreuiations
AA
Bd
Bst
BV
C
Coa
COP
c-P
d
I
i
L
M
OT
P
Ped
To1
V
X,Y?
anterior amygdala
basal dorsal nucleus of the amygdala
bed nucleus of the stria terminalis
basal-ventral nucleus of the amygdala
commissural component of the stria terminalis
anterior part of the amygdaloid cortical nucleus
posterior part of the amygdaloid cortical nucleus
caudate-putamen
dorsal component of the stria terminalis
dorsal zone
intercalated masses
intermediate zone
lateral nucleus of the amygdala
medial nucleus of the amygdala
optic tract
pallidum
cerebral peduncle
lateral olfactory tract nucleus; d, i, v = dorsal, intermediate,
and ventral zones of TOL
ventral component of the stria terminalis
ventral zone
subdivisions of the central nucleus
Nuclei of group I1
Central nucleus (Figs. 3-5, 8A,B). The most prominent
feature of this nucleus was the presence of an intensely
labeled neuropil and densely packed GABA-Li neurons.
The labeling of the neuropil was not homogeneous; small,
very intensely stained areas were conspicuous especially in
the medial part of the nucleus (Fig. 5 , big arrow).
Three areas showing different arrangement of GABApositive neurons could be identified in frontal sections. Two
regions marked as "x" and "y" in Figure 5 correspond to
the medial and central part of the nucleus, respectively
(Johnston, '23; Hall and Geneser-Jensen, '71; Hall, '72;
Nitecka, '75; Price, '81a). In region "x", which probably
corresponds to a caudal extension of the anterior amygdaloid nucleus (Johnston, '23; Halland Geneser-Jensen, '71;
Hall, '72; Nitecka, '75; Price, '81a), clusters of medium-size
neurons and single larger neurons were arranged in irregular rows. These are probably situated along the bundles of
GABAERGIC SYSTEM OF THE AMYGDALOID COMPLEX
47
Fig. 1. Schematic diagrams of the boundaries between the nuclei in the rat amygdaloid body. A-D. Frontal
sections in the rostrocaudal order. The interclated cell masses are indicated by the hatched zones.
Fig. 2. Schematic diagrams showing the distribution of GABA-like immunorective (GABA-Li) neurons in
frontal serial sections. The location of every GABA-Li neuron is indicated on the camera lucida drawings. The
intensely labeled GABA neurons of the intercalated cell masses are indicated by the dotted circles.
fibers of the stria terminalis (Fig. 5). In region “y”, large
clusters (c. 10 cells) of medium-size, moderately stained
perikarya were arranged irregularly (Figs. 5, 8A-open arrows). Initial segments of dendrites emerging from these
neurons were often stained (Fig. 8B). In some cases because
of the intense labeling of the neuropil, it was difficult to
observe the moderately labeled cell bodies. GABA-Li punctate structures, presumably boutons, outlined GABA-Li
negative (Fig. 8B, small arrow) or GABA-Li positive perikarya or dendrites (Fig. 8B, large arrow). In region “z”, the
arrangement of GABA-Li positive perikarya is reminiscent
to that seen in the adjacent striatum (Fig. 5). Labeled,
medium-size perikarya were arranged in small clusters (45 neurons), which were less densely packed than in region
“y”. Acetylcholinesterase histochemistry also suggests similar subdivisions of the central nucleus (Hall and Geneser-
Fig. 3. Photomontage to illustrate the pattern of GABA-Li material in a frontal section the anterior part of
the amygdala. The arrows indicate the intercalated masses. Scale bars=2OO pm.
Fig. 4. Photomicrograph illustrating the pattern of GABA-Li material in the caudal part of the amygdala. The
arrows indicate the intercalated masses. Asterisks indicate artifacts. Scale bar=2OO urn.
Fig. 5. Photomontage of GABA-Li and acetylcholinesterase activity pat- crograph. Small arrows indicate the intercalated masses and large arrows
terns in the central portion of the amygdala. x,y,z indicates the three areas the neuropil with intense GABA-Li neurons. Scale bars=200 pm; 1 mm for
of the central nucleus with different patterns of GABA-Li material; the the boxed photomicrograph.
pattern of the acetylcholinesterase activity is shown in the boxed photomi-
Jensen, '71; Hall, '72; Nitecka, '75; Ben Ari et al., '77; also
see Fig. 5). Other lines of evidence suggest that these subdivisions of the central nucleus have also different patterns
of connections (Jones and Burton, '76;Norgen, '76; McBride
and Sutin, '77; Ottersen and Ben-Ari, '78; Veening, '78;
Nitecka et al., '79, '80; Saper and Loevy, '80). Areas "y"
and "z" are thought to be connected, respectively, with the
viscerosensory brainstem centers and with the somatosensory nuclei of the posterior thalamic region.
Medial nucleus (Figs. 3-5). The neuropil of the medial
nucleus contained relatively high levels of GABA-Li material, particularly in the caudal regions of the nucleus (Fig.
4). As in the central nucleus, small areas with intense
labeling of neuropil were observed notably in the vicinity
of the intercalated masses (Fig. 5, large arrow). Also, clusters of small, intensely labeled perikarya were scattered in
Fig. 6. Photomicrographs to illustrate GABA-Li perikarya and neuropil
of the intercalated masses (I). Note the location of the largest aggregates of
intercalated masses and the numerous large GABA-Li perikarya of the
anterior pole of the basal dorsal nucleus. Note the densely packed, small
GABA-Li perikarya of the intercalated masses. Higher magnification to
illustrate GABA-positive neurons in the intercalated masses is shown in
the insert. Scale bars=100 pm.
50
Fig. 7. Photomicrographs showing GABA-Li material in the lateral olfactory tract nucleus. A. Topographic distribution of the labeling in the dorsal
intermedial and ventral parts of the nucelus. B. Note the pyramidal shape
of the labeled neurons; all their dendrites are directed downward to the
ventral zone and brain surface. The shape and arrangement of the GABA-
L. NITECKA AND Y. BEN-ARI
Li profiles in the ventral area suggest that they represent obliquely cut,
fine branches of the dendrites. Large arrow indicates the boundary between
the intermediate and ventral part of the lateral olfactory tract nucleus.
Scale bars=100 pm.
Basal dorsal nucleus (Figs. 3-5, 8C,D). The distribution
the nucleus, usually along the bundle of fibers arising from
of GABA-Li neurons and neuropil was comparable to that
the stria terminalis.
Anterior amygdaloid area (Fig. 7). The distribution of seen in the lateral nucleus. Large, intensely labeled neuGABA-immunoreactivity in this region was similar to that rons were expecially numerous in the anterior part of the
seen in the medial nucleus. The neuropil was intensely nucleus (Fig. 6A). They were sparsely distributed and were
stained with few GABA-positive cell bodies.
not concentrated in clusters (Fig. 9A). Large pyramidalshaped neurons with labeled initial dendrites were often
conspicuous (Fig. 8C, arrowhead). GABA-Li punctate strucNuclei of group I11
tures were occasionally noted in contact with GABA-Li
perikarya (big arrow in Fig. 8C). Also, GABA-positive puncLateral nucleus (Figs. 3-5). GABA-Li neurons were ho- tate structures often outlined the shape of unlabeled, largemogeneously distributed throughout the entire extent of size cell bodies (Fig. 8C).
the nucleus. Their labeling varied in intensity from moderPosterior part of the cortical nucleus (Figs. 4,
ate to strong. Most of the GABA-Li neurons were of me- 9B). Scattered clusters of medium-size labeled neurons
dium size (15-20 pm), a few large neurons (30 pm) were, were prominent features of this region. Large, heavily
however, present. Medium-sue, labeled neurons were stained neurons with a pyramidal shape were also conspicgrouped in clusters of 2-4 neurons (Fig. 5). The neuropil uous; the initial segments of the dendrites were often also
was labeled weakly or moderatelv.
labeled (Fig. 9B, open arrow).
GABAERGIC SYSTEM OF THE AMYGDALOID COMPLEX
51
Fig. 8. Photomicrographs to show GABA-Li material in various nuclei.
A and B. Area Y of the central nucleus. C and D. Basal dorsal nucleus. A.
Clusters of GABA-Li neurons (open arrows). B: Higher magnification to
show GABA-Li perikarya and their dendrites (large arrows). Small arrow
indicates an unlabeled cell body surrounded by GABA-Li punctate struc-
tures. C. A large GABA-Li pyramidal neuron with positively stained initial
dendrite (arrowhead);the small arrow indicates boutonlike structures outlining the contours of GABA-Li negative perikarya (also conspicuous in D).
Note also in D the typical cluster of medium-size GABA positive neurons.
Scale hars=50 pm.
Basal ventral nucleus (Figs. 3, 9A). GABA-Li perikarya
of medium size and a few large ones were conspicuous in
this nucleus. Clusters of medium-size neurons were distributed within the nucleus and the neuropil was poorly labeled. This distribution is similar to that seen in layer I11
of the adjacent piriform cortex. Interestingly, the border
between the basal ventral nucleus and this layer of the
piriform cortex was ill-defined (Fig. 3).
Anteriorpart of the cortical nucleus (Figs. 3, 9A). The
distribution and intensity of GABA-Li labeling was similar
to that noted in the adjacent layers I and I1 of the piriform
cortex (Fig. 3). In the most superficially located zone, only
a few neurons with GABA-Li immunoreactivity were observed; a similar pattern was present in layer I of the
piriform cortex. Like layer I1 of the piriform cortex, the
more deeply located neuropil was more heavily labeled and
contained clusters of medium-size, GABA-positive neurons
(Figs. 3,9A).
The stria terminalis and its bed nucleus (Fig. 10). GABALi neurons of medium size were consipicuous in the bed
nucleus of the stria terminalis, i.e., both in its rostra1 parts
(Fig. 10A) and in the more caudal part where the stria
terminalis and its bed nucleus are located above the thalamus (Fig. 10A,B). Bundles of GABA-Li fibers were present
in every component of the fiber tract. They were evenly
distributed throughout the commissural component of the
stria terminalis. In the other components of the stria terminalis, GABA-Li fibers occupied mainly the dorsolateral
and ventromedial parts of the fiber tract (Fig. 10B).
DISCUSSION
Earlier studies suggest that GABA-like immunoreactivity is a good marker of neurons thought to use GABA as a
transmitter. Thus, a recent study using antisera raised
against GABA conjugated to bovine serum albumin with
glutaraldehyde (Storm-Mathisen and Ottersen, '86) indicates that the distribution of GABA-Li immunoreactivity
closely matches that of the GABA synthesizing enzyme,
glutamic acid decorboxylase (GAD) (Ottersen and StormMathisen, '84). The GABA antisera labeled most of the
neurons of the reticular nucleus of the thalamus, the medium-size cells of the caudate-putamen, and the stellate and
52
L. NITECKA AND Y. BEN-ARI
unpublished observations). Therefore, the immunoreactive
material probably reflects GABAergic systems, although
the use of this term is necessarily provisional, depending
on the specificity of the antibodies.
Organization of GABA-Li material in the amygdala
Fig. 9. A. GABA-Li material in the basal ventral nucleus and the anterior part of the cortical nucleus. B. Posterior part of the cortical nucleus.
The arrow indicates a group of cells reminiscent of the intercalated masses;
open arrow, a GABA-Li large neuron. Scale bars= 100 pm.
basket cells of the cerebellum. A similar parallelism between GAD and GABA-Li material is also conspicuous with
the anti-GABA antibodies used in the present study (Gamrani et al., '86; Seguela et al, '84; Nitecka and Ben-Ari,
The distribution of GABA-Li immunoreactivity shown in
the present study is in agreement both with brief recent
reports using anti-GABA antibodies from another source
(McDonald, '85; Ottersen et al., '86) and with earlier biochemical measurement of the levels of GAD and GABA in
microdissected nuclei (Ben-Ari et al., '76). Furthermore,
this distribution is in general agreement with that obtained
with GAD immunocytochemistry (Mugnaini and Oertel,
'85). The principal features of this distribution can be summarized as follows:
1. The central and medial nuclei, as well as the anterior
amygdaloid area, are the main targets of the GABA-Li
inputs to the amygdaloid nuclei. They show very strong
GABA-Li labeling in the neurqpil with a high density of
GABA-Li punctate structures - presumably boutons (Ottersen et al., '86; present study). As mentioned earlier,
these nuclei contain the highest amount of GAD and GABA
(Ben-Ari et al., '76) and a dense plexus of GAD immunoreactive material is conspicuous in the neuropil (Mugnaini
and Oertel, '85). In the cat, numerous symmetrical axodendritic synapses have been observed in these nuclei (Hall,
'72; Juraniec and Narkiewicz, '77; Narkiewicz et al., '77).
The majority of these synapses are likely to be GABAergic,
although assymetrical synapses can also be GAD-positive
(e.g., Sotelo et al., '86).
Three sources could be responsible for this GABAergic
innervation: (1)Anatomical observations suggest that the
medial and central nuclei receive most of the intra-amygdaloid (and interamygdaloid) connections (Krettek and
Price, '78a; Nitecka et al., '81a,b; Ottersen, '82; Price and
Amaral, '84). These fibers arise primarily from the basolatera1 region (Hall, '72; Kamal and Tombol, '75; Krettek and
Price, '78a; Nitecka et al., '81a,b; Price and Amaral, '84;
Smith and Milhouse, '85). Since the lateral and basal dorsal
nuclei also showed a high density of labeled neurons (Ottersen et al., '85; McDonald, '85; present study), they constitute a possible source of the GABA-Li innervation to the
central and medial nuclei. This is in keeping both with
lesion studies, which do not support the existence of a major
GABA input to the amygdaloid complex from external
sources (Le Gal La Salle, '78), and with electrophysiological
studies, which suggest the existence of inhibitory intraamygdaloid connections (Le Gal La Salle, '76). It bears
stressing that following blockade of the axonal transport by
local injections of colchicine, there is a considerable enhancement of the proportion of cells labeled in the central
nucleus. The use of colchicine, however, raises several problems and the significance of the enhancement of labeling
produced by this treatment is not fully understood at present (e.g., Mugnaini and Oertel, '85). (2)Neurochemical data
suggest that the bed nucleus of the stria terminalis constitutes another possible source of GABAergic innervation to
the central nucleus. Thus, transection of the stria terminalis produces a small but significant reduction of GAD in
the central, but not in other, amygdaloid nuclei (Le Gal La
Salle et al., '78). Furthermore, the content of GABAergic
markers in the microdissected stria terminalis is two times
higher than that found in other fiber tracts (Ben-Ari et al.,
'76). Furthermore, transection of the stria terminalis pro-
GABAERGIC SYSTEM OF THE AMYGDALOID COMPLEX
53
Fig. 10. Photomicrographs to illustrate the pattern of the GABA-Li material in the bed nucleus of the stria
terminalis (A) and in the caudal suprathalamic segment of the stria terminalis (B). Note the presence of GABALi perikarya in the bed nucleus and within the stria terminalis bundle. GABA-Li positive fibers are present in
every component of the stria terminalis. Scale bars=2OO pm.
duces a highly significant ( > 50%)reduction of GABAergic
markers in the central and medial nuclei (Le Gal La Salle
et al., '78). This observation is in keeping with the presence
of numerous GABA-Li fibers in the stria terminalis (Ottersen et al., '86; present study). (3) The intensely GABApositive neurons of the intercalated masses and lateral
olfactory tract nucleus constitute a third possible source of
GABAergic afference to the central and medial nuclei.
These nuclei send their axons to the ipsi- and contralateral
centromedial region of the amygdala (Kamal and Tombol
'75; Nitecka et al., '81: Millhouse '86; see also below).
2. The lateral olfactory tract nucleus and the intercalated
masses show an intense labeling in the neuropil and the
highest density of GABA-Li cell bodies in the amygdala.
Ottersen et al. ('86) also found a high labeling in the neuropil (but see also Ottersen and Storm-Mathisen, '84); these
authors, however, did not comment on a special reactivity
of the cell bodies. This difference may be due to the use in
the present study of secondary antibodies, which amplify
the immunoreactive reaction and facilitate the visualization of GABA-Li cell bodies in regions in which the neuropil
is also densely labeled (see Methods). It also bears stressing
that neurons of the lateral olfactory tract nucleus are heavily labeled by retrograde transport of 3H aspartate (injected
in the lateral nucleus), raising the possibility that the neurons are putatively glutamatergic or aspartergic (Price, '86;
Ottersen and Storm-Mathisen, '86). However, the neurons
labeled with (3H) aspartate are mainly concentrated in the
dorsal part of the nucleus and in the boundary of the intermediate region; i.e., there is little overlap with the regions
enriched in GABA-Li material. Further histochemical studies should be performed to better comprehend the organization of this nucleus.
Although the functional significance of the intercalated
cell masses has not been clarified, several observations
raise the possibility that they modulate the sensory information that converges to the lateral and central nuclei of
the amygdaliae. Earlier studies have shown that the lateral
and central nuceli receive direct connections, respectively,
from the somatosensory nuclei of the posterior brainstem
nuclei (Jones and Burton, '76; Jones et al., '76; Ottersen
and Ben-Ari, '78,'79; Nitecka, '79; Nitecka et al., '79,'80;
Ottersen, '81). Electrophysiological studies have also shown
the convergence of sensory modalities or neurons of the
lateral nucleus (Ben-Ari et al., '74) and the important alterations in unit activity that occur during sensory habituation and sensory-sensory conditioning procedures (Ben-Ari
and Le Gal La Salle, '72,'74). Interestingly, the intercalated
cell masses project to the central nucleus and to the basolateral region (De Olmos, '72; Kamal and Tombol, '75;
Nitecka et al., '81a,b; Millhouse, '86). Furthermore, fibers
originating in the peripeduncular nucleus, which is considered to be the subcortical auditory center, terminate in the
intercalated cell masses (Jones et al., '76). It is thus possible
that by means of these connections, the intercalated cell
masses modulate the elaboration in the central and lateral
nuclei of appropriate responses to exteroceptive stimuli.
3. There is a close similarity of the anterior part of the
cortical nucleus and the basal medial nucleus to the adjacent piriform cortex. Similar relationships are suggested
from the observations of their cytoarchitecture and the
location of the acetylcholinesterase activity in the piriform
lobe (Johnston, '23; Koikegami, '63; Hall and Geneser-Jensen, '71; Hall, '72; Nitecka, '75). These regions also share
in common the input from olfactory regions (Cowan et al.,
'65; Price and Powell, '70; Lammers, '72).
Summing up, the general pattern of GABA immunoreactivity in amygdaloid nuclei suggests that (1) GABA is located to a large extent in neurons giving rise to intraamygdaloid connections, (2) GABA-containing terminals are
L. NITECKA AND Y. BEN-ARI
54
enriched in the central and medial (excitatory) nuclei; and
(3) a large number of GABA-Liperikarya are present in the
basolateral (inhibitory) region. This raises the possibility
that by sending GABAergic connections to the central and
medial nuclei, the basal and lateral nuclei are able to exert
an important inhibitory effect on drives and emotions.
ACKNOWLEDGMENTS
We are grateful to G. Ghilini and G. Charton for technical
assistance and S. Bahurlet for typing the manuscript. The
GABA antibodies were kindly donated in part by M. Geffard. L. Nitecka received financial support from INSERM
and CNRS.
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