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MICROSCOPY RESEARCH AND TECHNIQUE 41:224–233 (1998)
Localization of Neurotrophin-3-Like Immunoreactivity
in the Rat Cochlear Nucleus
A. BURETTE,1 G. BELLIOT,2 E. ALBUISSON,3 AND R. ROMAND1*
1Laboratoire
de Neurobiologie, Université Blaise Pascal, 63177 Aubière Cedex, France
of Viral and Rickettsial Diseases, National Center for Infection Diseases, Centers for Disease Control and Prevention
Atlanta, Georgia 30329
3Laboratoire de Biostatistiques, Faculté de Médecine, Université d’Auvergne, 63000 Clermont-Ferrand, France
2Division
KEY WORDS
neurotrophic factor; central auditory system; glial cells
ABSTRACT
Immunohistochemistry as well as immunohistofluorescence were used to investigate the distribution of the neurotrophin-3 (NT3) in the adult rat cochlear nucleus. We found a
widespread distribution of NT3 immunolabeled neurons throughout the three divisions of this
nucleus. NT3-like immunoreactivity was clearly population-specific, with some cell groups heavily
(various small neurons and granule cells) or moderately (large neurons of the ventral cochlear
nucleus) stained, while others remained negative (a major fraction of medium and large neurons of
the dorsal cochlear nucleus).
Double-labeling experiments were performed using antibody against the glial fibrillary acid
protein, a classic marker for mature astrocytes. This colocalization study revealed that NT3
immunoreactivity was also present in a subpopulation of astrocytes, particularly in the glia limitans
and their projections. Numerous small cells also colocalized NT3 together with the glial marker in
the granule cell domain and in the molecular cell layer of the dorsal cochlear nucleus.
These results suggest that NT3 may exist in widespread populations of adult cochlear nucleus
neurons as well as in glial cells. This abundant distribution of NT3-like immunoreactivity implies
that this neurotrophin may have an important role in the continued maintenance of mature cochlear
nucleus and makes it an attractive candidate for playing a role in regulation or stabilization of
neuronal circuits in this nucleus. Microsc. Res. Tech. 41:224–233, 1998. r 1998 Wiley-Liss, Inc.
INTRODUCTION
Neurotrophin-3 (NT3) is a member of the neurotrophin family including nerve growth factor (NGF),
brain-derived neurotrophic factor (BDNF), and neurotrophin-4/5 (NT4/5) (Hohn et al., 1990; Maisonpierre et
al., 1990). The expression of NT3 is widely distributed
throughout the central nervous system (CNS). In particular, mRNANT3 is found at the highest concentrations in hippocampus and cerebellum (Lauterborn et
al., 1994). It is believed that NT3 controls survival,
differentiation, and maintenance of specific populations
of vertebrate cells (Davies, 1994; Skup, 1994; Snider,
1994). Besides these actions, there is increasing evidence
that NT3, like the other neurotrophins, is involved in
processes of neuronal plasticity. In particular, in adult
CNS, NT3 may participate in synaptic plasticity in an
activity-dependent manner (Kang and Schuman, 1995).
NT3 mediates these effects by interacting with highand low-affinity receptors (Bothwell, 1995). The lowaffinity NGF receptor (LNGFR) binds all known neurotrophins with similar affinity. However, it may not be
able to mediate the effect of neurotrophins (Chao,
1994). The high-affinity receptors are transmembrane
tyrosine kinases encoded by the trk proto-oncogene
family. Whereas TrkA is a receptor for NGF and TrkB is
a receptor for BDNF and NT4/5, TrkC serves as a
receptor for NT3. NT3 also binds to TrkA and TrkB, but
at much lower efficiency than NGF and BDNF or
NT4/5, respectively.
r 1998 WILEY-LISS, INC.
A recent study showed a widespread distribution of TrkC
receptors throughout the central auditory system (Hafidi
et al., 1996). This observation provides evidence for a
possible role of this neurotrophin in auditory nucleus. In
this context, the study of the expression of the NT3 in this
system and particularly in the cochlear nuclei (CN) is of
interest.As its cytoarchitecture as well as its pharmacological and physiological properties are well known (Merchan
et al., 1993; Romand and Avan, 1997), the CN, which is the
first relay station in the ascending auditory pathway,
seems very appropriate for this study.
Thus, the aim of the present study is to investigate, for
the first time, the expression of NT3 protein in the CN, by
using an affinity-purified polyclonal IgG. Furthermore, we
tested the possibility that NT3-like immunoreactivity (NT3LI) could be present in glial cells by double-labeling experiments using antibody against the glial fibrillary acid
protein (GFAP), a marker of mature astrocytes. Results
indicated that NT3-LI was present in both neurons and
glial cells throughout the three divisions of the CN.
MATERIALS AND METHODS
Tissue Fixation and Processing
Four male Sprague-Dawley rats about 4 months old
and raised in our facilities were used. Rats were
*Correspondence to: R. Romand, Laboratoire de Neurobiologie, Université
Blaise Pascal, 63177 Aubière Cedex, France. E-mail: romand@cicsun.univbpclermont.fr
Received 20 June 1997; Accepted in revised form 2 January 1998
NEUROTROPHIN-3 IN THE MATURE COCHLEAR NUCLEUS
anesthetized with sodium pentobarbital (50 mg/kg body
weight, intraperitoneally) and perfused transcardially
with a flush of 0.1 M phosphate buffered saline (PBS),
pH 7.4, containing 4% paraformaldehyde. The brainstems were then removed and postfixed at 4°C for 24
hours in the same fixative. After washing in PBS at 4°C
overnight, all tissues were routinely embedded in paraplast (Sherwood Medical St. Louis, MO). Serial transverse sections through the CN were cut at 5 µm, and
mounted onto gelatinized glass slides. In each series,
one out of every 10 sections was Nissl stained. Thus,
sections for immunohistochemistry were chosen in relation
to the structures revealed on Nissl stained sections.
Immunohistochemistry
Immunodetection was carried out using a commercial antibody raised against amino acids 139–158 of
human NT3 (Sc-547, Santa Cruz Biotechnology, Santa
Cruz, CA). This sequence has no high sequence homology with any other known proteins. This antibody
recognized the rat NT3 with no NGF, BDNF, or NT4
cross-reactivity. In rat, the signal detected by Western
blot and immunohistochemistry can be competed off
with the specific blocking peptide (Santa Cruz Biotechnology, personal communication).
Brainstem sections containing CN were deparaffinized in xylene, hydrated, and washed for 5 minutes
in distilled water. The sections were treated for 30
minutes at room temperature in methanol/0.3% H2O2
to exhaust endogenous peroxidase activity. Then a
short Tris buffer saline (TBS) wash followed and the
sections were incubated for 30 minutes at room temperature in 10% fetal bovine serum (FBS) and 0.1% Tweeny
20 in TBS to suppress background. Tissue sections were
incubated overnight at 4°C in primary antibody diluted
to 1:200 in TBS containing 5% FBS. After extensive
washes in TBS, the sections were reacted for 1 hour at
37°C with secondary antibody (biotinylated horse antirabbit IgG, Vector Laboratories, Burlingame, CA) at
1:400. Sections were again washed thoroughly as above
and reacted with avidin-biotin complex (Vectastain
ABC-Elite, Vector) for 45 minutes at 37°C, following the
manufacturer’s indications. The immunocomplexed sections were carried through several TBS washes. The
resulting immune complexes have peroxidase activity
that yields a brown reaction product when reacted with
0.3% hydrogen peroxide, 0.034 mg/ml diaminobenzidine in TBS, pH 7.8. After peroxidase reaction, the
sections were dehydrated through graded ethanol solutions, dipped in xylene, and mounted with coverslips
and Eukitty mounting medium (Kindler Gmbh fco).
Finally, for each immunostained section (approximately one section out of every 40 µm), the adjacent
section was Nissl stained to allow comparison with
immunostained sections.
Distribution of NT3 Immunostained Cells
The CN complex of the rat is conventionally divided
into ventral CN (VCN) with two divisions: the anteroventral CN (AVCN), the postero-ventral CN (PVCN),
and the dorsal CN (DCN) (Merchan et al., 1993; Romand and Avan, 1997). For detailed parcellation and
cell types, we refer to the work done on rats (Harrison
and Irving, 1965, 1966), mice (Webster and Trune,
1982), and cats (Brawer et al., 1974; Osen, 1969).
225
For each of the three divisions of the CN (i.e., AVCN,
PVCN, and DCN), three positions were mapped in the
rostro-caudal axis: at 15, 50, and 85%. Percentages
were determined by dividing the section number by the
total number of sections per division. Only cells containing a nucleus were considered.
The cartographic analysis of NT3-immunoreactive
cells was performed using an image analysis workstation (Historag and Imaginia software by Biocom) coupled
to a microscope by a video camera. Briefly, this system
makes morphometric and cartographic measurements
of objects selected by users at high magnification from a
regional context, after first plotting the areas of interest
at low magnification. All cells resulting from exploration of a cross-section are recorded on a map in highresolution cartography. Assembling maps were made
with Vector drawing software.
Immunofluorescence Colocalization of NT3
and GFAP
Tissue Processing. The study was carried out in
two 4-month-old male Sprague-Dawley rats. Animals
were anesthetized with sodium pentobarbital (50 mg/
kg) and perfused transcardially with a flush of 25 ml
physiological saline, followed by 800 ml of a fixative
solution, containing 4% paraformaldehyde in PBS 0.1
M. After perfusion, brainstems were removed and
postfixed for 2 hours at 4°C in the same fixative
solution; then they were transferred for 3 hours in cold
PBS. Brainstems were cryoprotected in 15% sucrose
solution overnight and then sectioned in the transversal plane on a cryostat microtome at a thickness of 16
µm before being collected in cold PBS.
Immunohistochemistry. All the following procedures were performed on free-floating sections. After
washing, the sections were incubated overnight at 4°C
with the mixture of antibodies, i.e., rabbit anti-NT3
diluted 1:200 and monoclonal anti-GFAP (clone GA5,
Sigma, St. Louis, MO) diluted 1:400. In the second step,
a mixture of an FITC goat anti-rabbit IgG and TRICT
goat anti-mouse IgG, was applied overnight at 4°C.
After a washing procedure, sections were mounted on
gelatin-coated slides, air dried, and cleared with xylene
before being coverslipped with Fluoromounty (Gurr,
BDH Chemicals, Toronto, Canada).
Controls were obtained by processing the tissue as
described above except that an irrelevant IgG was
substituted in place of each primary antibody.
RESULTS
NT3-like immunoreactivity (NT3-LI) was detected
within the cerebellum (Fig. 1) and CN (Figs. 2–4). In
contrast, no specific staining was observed in sections
in which the primary antibody was deleted or substituted. In the cerebellum, intense NT3-LI was localized
in the granular cell layer, while a moderate immunoreactivity was observed in Purkinje cells, as discrete
punctate labeling in the cytoplasm (Fig. 1). In each
division of the CN, numerous cells were stained with
different intensities, showing an abundant and widespread distribution of NT3-LI (Figs. 2–4). By contrast
with others brainstem areas, where intense NT3-IL
was observed in the cell bodies as well as in dendrites
and axons (Fig. 1), in the CN the immunoreactivity was
226
A. BURETTE ET AL.
Fig. 1. Localization of NT3-LI in transversal sections of cerebellum
and raphe pontis nucleus. A, B: Low- and high-power photomicrographs of cerebellum. An immunoreactivity in the granule cell layer is
shown. Additionally, discrete punctate labeling is demonstrated in the
cell bodies of the Purkinje neurons (arrows). Scale bar: A, 100 µm; B,
25 µm. C: Cells of the raphe pontis nucleus. Note that intense NT3-LI
is visible in the cell bodies as well as in processes in the large neurons
(arrows). Scale bar: 25 µm.
mainly restricted to the cytoplasm. With prolonged
incubation, reactive product could be detected in the
beginning cell processes.
fraction of the multipolar cell population appeared
negative. Octopus cells of the PVCN do not seem to be
labeled. By contrast, most, if not all, spherical and
globular cells seem to be reactive. In fact, in line with
this interpretation, in the very rostroventral pole of the
AVCN where spherical cells occur as a homogeneous
population in relative isolation (Harrison and Irving,
1965), nearly all large cells were immunoreactive (Fig.
2D,E). Just as in the region close to the nerve root,
where globular cells could easily be distinguished (Tolbert and Morest, 1982), all cells of this type were
labeled. Additionally, a group of distinctive immunolabeled cells, characterized by their large, oval soma and
their eccentric nucleus, was found in the cochlear nerve
root. From their distribution and morphological features, they were identified as root neurons (Merchan et
al., 1988), classified as b cells by Harrison and Irving
(1965). In addition to the principal neurons of the VCN,
numerous small cells containing intense NT3-LI were
evenly distributed throughout the VCN. These cells
Neurotrophin-3-Like Immunoreactivity
in the Adult Ventral Cochlear Nucleus
A large number of NT3 immunoreactive cells was
observed within both anterior and posterior parts of the
VCN (Figs. 2, 3). A sample distribution map is shown in
Figure 4. Positive cells were more or less homogeneously distributed throughout the nucleus, except in
the granule cell domain where the highest density of
NT3-LI could be seen.
Inspection of NT3 immunostained sections showed
that the large neurons in the VCN, which presumably
represent spherical, globular, and multipolar cells contained NT3-LI in their cell bodies as discrete punctate
labeling (Figs. 2C,D, 3C,D). However, some large cells
remained negative (Fig. 3D). As judged by comparing
adjacent NT3 and Nissl-stained sections, a significant
Fig. 2. Localization of NT3-LI in transversal sections of anterior
ventral cochlear nucleus. A–C: Section throughout the middle of the
AVCN. One can observe a large and homogeneous distribution of
labeled cells (A, B). However, a larger number of immunostained cells
in the granule cell domain (arrows) can be observed (A, B). NT3-LI is
visible in the cell bodies as discrete punctate labeling (C). Scale bar: A,
200 µm; B, 100 µm; C, 10 µm. D, E: Section through the very
rostroventral pole of the VCN. Most spherical cells appeared immunopositive (D) with discrete punctate labeling in their cell bodies (E).
Scattered throughout this area small, highly reactive cells can be
observed (arrows). Numerous cells are labeled in the small cell cap (D,
arrowheads). Scale bar: D, 50 µm; E, 25 µm.
228
A. BURETTE ET AL.
Fig. 3. Localization of NT3-like immunoreactivity in a transversal
section of posterior ventral cochlear nucleus. A: Low-power photomicrograph illustrating the overall distribution of NT3-LI in the middle of
the PVCN. Scale bar: 200 µm. B: Higher magnification of the granule
cell domain. Note that intense NT3-LI is observed in the granule cell
domain (arrows). Scale bar: 50 µm. C: Higher magnification of central
region of the PVCN. Several immunopositive large cells can be
observed (large arrows). More or less homogeneously distributed
throughout this area, a large number of highly reactive small cells can
be also seen (small arrows). Scale bar: 100 µm. D: Two large cells
containing NT3-LI in their cell bodies as discrete punctate labeling.
Scale bar: 10 µm.
NEUROTROPHIN-3 IN THE MATURE COCHLEAR NUCLEUS
229
Fig. 4. Localization of NT3-like immunoreactivity in dorsal cochlear nucleus. A: Low-power photomicrograph illustrating the overall distribution of NT3-LI throughout the three layers of the DCN.
Numerous small cells, evenly distributed, are highly reactive (small
arrows). Some larger size cells are lightly stained (large arrows). m:
molecular cell layer; f: fusiform cell layer; p: polymorphic cell layer.
Scale bar: 100 µm. B: Two immunopositive cells from the fusiform cell
layer (arrows). Scale bar: 10 µm. C: A lightly labeled large cell (arrow)
from the fusiform cell layer. Scale bar: 10 µm. D: An immunonegative
large cell (arrows) from the polymorphic cell layer. Scale bar: 10 µm.
could represent small neurons as well as glial cells (see
Colocalization Studies). A part of these immunoreactive
small cells could correspond to granule cells as suggested by the large number of stained cells in the
granule cell domain where they represent the main
population (Fig. 3A,B) (Mugnaini et al., 1980). Lastly,
in the small cell cap, numerous cells also appeared NT3
positive (Fig. 2D).
230
A. BURETTE ET AL.
Fig. 5. Distribution of NT3 immunoreactive cells throughout a
transversal sections of the three parts of the cochlear nucleus. A large
number of immunopositive cells can be observed within the three
subnuclei, showing a more or less homogeneous distribution. However,
a higher concentration of labeled cells can be seen in the granule cells
domain. The three maps were made through the middle of the
rostro-caudal axis of each subnucleus.
Neurotrophin-3-Like Immunoreactivity
in the Adult Dorsal Cochlear Nucleus
NT3-LI was observed in cells within all layers of the
DCN (Figs. 4, 5), but unlike the VCN, where numerous
large cells were positive, only scattered medium and
large cells appeared immunolabeled. At the transition
between the molecular and the fusiform cell layer,
rounded medium-size cell bodies were immunolabeled.
From their distribution and morphological features,
they may represent cartwheel neurons (Mugnaini et al.,
1987; Ryugo and Willard, 1985). In the fusiform cell
layer some large cells containing NT3-LI were observed. The distribution of these cells as well as their
size and shape lead us to think that they may correspond to pyramidal cells. In the polymorphic cell layer,
large cells that presumably represent giant, turberculoventral and radiate stellate cells appeared most of the
time negative or occasionally containing only some rare
immunoreactive puncta (Fig. 4B–D).
As observed in the VCN, numerous small cells were
highly reactive (Fig. 4A). They were evenly distributed
throughout the DCN and represented the principal
fraction of immunostained cells. These cells could correspond to interneurones like granule cells and glial cells
(see Colocalization Studies).
with GFAP antigens (Fig. 6). This cytoskeletal protein
is a specific marker for mature astroglial cells (Bignami
et al., 1972) and has been used with success in the CN
(Jalenques et al., 1995). The immunofluorescence procedure demonstrated a NT3-LI similar to the immunohistochemical staining with the exception that the reactivity was more intense and processes of numerous small
cells were labeled. In both VCN and DCN, a significant
fraction of GFAP-positive astrocytes colocalized also
NT3-LI. In particular, NT3-LI was present in the glia
limitans and their projections. Furthermore, numerous
small cells colocalized NT3 together with the glial
marker in the granule cell domain and in the molecular
cell layer of the DCN (Fig. 6). In the others part of the
CN, double staining cells were less frequently observed
but were evenly distributed.
Colocalization Studies
The distinction between small neurons on the one
hand and glial cells on the other hand was difficult in
our immunostained sections (morphology and size of
small neurons and glial cells can be very similar). Thus,
we examined the biochemical phenotype of small cells
using a double-fluorescence procedure colocalizing NT3
DISCUSSION
In the present study, we report on the regional and
cellular distribution of NT3-LI in the mature CN of the
rat. We found a widespread distribution of NT3-LI,
present in both glia and neurons, throughout the three
divisions of this nucleus.
Significance of the Present Results With Respect
to the Previous Investigation on NT3
Distribution
In the cerebellum, we found an intense level of
NT3-LI in granule cells and a lower one in the Purkinje
cells, with discrete punctate labeling. This observation
appears to conflict with previous investigations, where
NT3-LI was only found in Purkinje cells (Zhou and
Rush, 1994). The discrepancy may result from the
NEUROTROPHIN-3 IN THE MATURE COCHLEAR NUCLEUS
231
Fig. 6. Localization of NT3-like immunoreactivity in GFAP positive glial cells. Arrows point toward
two glial cells, located in the molecular cell layer of the DCN, which showed GFAP (A) and NT3 (B)
colocalization. The two photomicrographs were taken from the same field using TRITC (A) or FITC (B)
filter cubes. Scale bar: 5 µm.
difference of the titer of the antibodies used. However,
previous in situ localization of NT3 mRNA showed that
in the cerebellum, NT3 transcripts were present in
granule cells (Lauterborn et al., 1994; Rocamora et al.,
1993). One hypothesis is that NT3 could act in Purkinje
cells and granule cells itself by an autocrine route
(Lauterborn et al., 1994; Rocamora et al., 1993; Segal et al.,
1992). The localization of NT3-LI observed in the present
study within the cerebellum mimics quite well the distribution of NT3 mRNA and was consistent with the potential
function of this neurotrophin in this part of the CNS.
Immunoreactivity of Neuronal Cell Bodies
Our results showed that NT3-LI found in neuronal
cells was predominantly distributed throughout the
cytoplasm as small punctate accumulations (Figs. 2B,E,
3D, 4B,C). However, we could not eliminate the possibility that some of the punctate labeling was present in
presynaptic boutons at the surface of neuron cell bodies. Further electron microscopic evaluation will be
needed to clarify this hypothesis. This distribution of
immunoreaction products may correspond to a vesicular localization of NT3-like antigen. A discrete cytoplamic punctate labeling was reported for other neurotrophins (i.e., NGF and BDNF) particularly in
neurotrophin-responsive neurons (Nishio et al., 1994;
Wetmore et al., 1991). In PC12 cells, the most extensively studied and well-understood NGF-responsive
cells, the same immunoreactive pattern for TrkA was
shown (Grimes et al., 1996). They also demonstrated
that these punctae represented an endosomal localization of active TrkA-NGF complex after internalization.
The same phenomena might apply for the TrkC-NT3.
Significance of NT3-LI Cells: NT3-Synthesizing
Cells or NT3-Responsive Cells
The NT3-LI-positive neurons could be NT3-synthesizing
cells or, alternatively, NT3-responsive cells. Unfortunately,
to our knowledge the expression of mRNANT3 has not yet
been studied in the central auditory system, so the localization of NT3-synthesizing cells remains unknown. However,
much information is available on NT3-responsive cells.
These cells should express the NT3 receptors: a low-affinity
receptor, LNGFR, and a high-affinity receptor, TrkC. Previous reports have shown that LNGFR is present in the CN
during development (Després et al., 1991) as well as in
adult nucleus (Pioro et Cuello, 1990). The neurons expressing LNGFR may be the neurons responsive to NT3 (Dechant
et al., 1997). However, the role of LNGFR in neurotrophin
responsiveness is currently controversial (Kaplan and
Miller, 1997). An abundant and widespread distribution of
the high-affinity receptor (TrkC) has been observed in the
adult CN (Burette et al., in press; Hafidi et al., 1996),
predicting that NT3 may exert significant effects on a large
proportion of neurons. Additionally, from our observations,
at least some neuronal populations may be immunoreactive to both NT3 and TrkC. For instance, in the granule cell
domain, many granule cells appear immunoreactive in
both NT3 and TrkC stained sections. Therefore, it seems
reasonable to speculate that this cell type colocalized both
proteins. The same observations could be made for spherical and globular cells in the very rostroventral pole of the
VCN and in the region close to the nerve root, respectively.
For the other cell populations of the VCN and in the DCN,
where the different cell types could not be observed in
relative isolation, colocalization study will be required to
address this issue. All of these observations together with
the immunoreactive pattern (i.e., labeling as discrete punctate) lead us to think that many NT3-LI neurons observed
in the CN may be NT3-responsive neurons.
Implications for NT3 Functions in the CN
NT3-LI is clearly population-specific, with some cell
groups heavily (various small cells and granule cells) or
moderately (large neurons of the VCN) labeled, while
others remain negative (a large fraction of large neu-
232
A. BURETTE ET AL.
rons of the DCN). The explanation for selectivity in NT3
distribution across neuronal groups is not immediately
obvious. However, the presence of NT3-LI in granule,
globular, and spherical cells, together with the limited
number of immunoreactive large cells in the DCN,
could lead us to think that this selectivity may be
related to the use by these cells of an excitatory
amino-acid as a neurotransmitter. It is possible that
NT3 supports the survival of these specific neurons and
stabilizes neural homeostasis by regulating calcium
concentration. This neurotrophin could also be involved
in synaptic plasticity in an activity-dependent manner
as suggested in the hippocampus (Kang and Schuman,
1995). However, destruction of the cochlea does not
seem to affect NT3-LI in the ipsilateral CN at any
survival time from 24 hours to 30 days (personal
observations). These findings could indicate that continued expression of NT3 is not controlled by afferent
activity. This raises the possibility that NT3 could act
by an autocrine route (Davies and Wright, 1995). By
analogy with the granule cells of the cerebellum, this
hypothesis is particularly tempting for granule cells of
the CN. These last cells have some features in common
with the granule cells of the cerebellum including
ontogenetic origin (Mugnaini et al., 1980).
Localization of NT3-IR in Glia
Our results indicate that a subpopulation of GFAPpositive glial cells were stained with NT3 antibody. A
similar result was also reported by Zhou and Rush in
other parts of the CNS (Zhou and Rush, 1994). The
significance of NT3-LI in astrocytes is not clear. First,
astrocytes could be considered as sites of action for
NT3. It is well known that astrocytes could express, in
vivo and in vitro, the truncated forms of Trk receptors
(Rudge et al., 1992, 1994; Zhou et al., 1994). However,
the functional status of these receptors on astrocytes is
not fully understood, but it has been proposed that
truncated Trks may have important functions such as
sequestration or presentation of neurotrophins within
the CNS (Rudge et al., 1994). Second, astrocytes could
synthesize NT3 and may have the capacity to provide
their partner neurons with this neurotrophin. Whether
or not NT3 is also produced by astrocytes in vivo is not
known, but in vitro mRNANT3 have been detected in
hippocampal astrocytes (Rudge et al., 1992). In addition, glial cells express mRNANT3 following colchicine
treatment (Ceccatelli et al., 1991). Thus, it is possible
that glial cells in vivo can also produce NT3. The
presence of NT3-LI in astrocytes of the CN suggests
that this neurotrophin may play a trophic role for some
CN neurons, like granule cells, in view of the number of
NT3 reactive astrocytes in the granule cell domain.
In conclusion, NT3-LI has been localized to a number
of both neuronal and glial cell groups in the mature CN
by immunohistochemical techniques. This abundant
and widespread distribution of NT3-LI implies that this
neurotrophin might play an important role in the
continued maintenance of adult CN and makes it an
attractive candidate for playing a role in regulation or
stabilization of specific neuronal circuits in the CN.
However, full understanding of the functional role of
NT3 in the CN will require ultrastructural localization
of NT3 protein as well precise distribution of its mRNA.
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