Download Thalamocortical projection from the ventral posteromedial nucleus

Neuroscience Letters 367 (2004) 394–398
Thalamocortical projection from the ventral posteromedial nucleus sends
its collaterals to layer I of the primary somatosensory cortex in rat
Satoko Odaa,∗ , Kiyoshi Kishia , Junli Yanga , Shaoyun Chena , Junko Yokofujitaa ,
Hiroaki Igarashia , Sachiko Tanihatab , Masaru Kurodaa
a
Department of Anatomy, Toho University School of Medicine, 5-21-16 Ohmorinishi, Ohta-ku, Tokyo 143-8540, Japan
b Department of Pharmacology, Toho University School of Medicine, Ohta-ku, Tokyo 143-8540, Japan
Received 24 April 2004; received in revised form 9 June 2004; accepted 16 June 2004
Abstract
Here we examined quantitatively axonal projections originating from the ventral posteromedial thalamic nucleus (VPM) to layer I of the
primary somatosensory cortex (SI) by extracellular and intracellular injections of biocytin as an anterograde tracer. Following the extracellular
injections, two types of VPM afferents with different arborization patterns in SI were observed. The type I extended vertically, forming dense
plexus in layers IV and VI, and projected collaterals to layer I. The type II rarely branched in SI, converged in the plexus formed by the type
I, and projected no collaterals to the supragranular layers. The labeled fibers in layer I derived from the first type ran parallel to the brain
surface, and their mean length was 339.7 ± 87.5 ␮m. Intracellular injection into VPM neurons bearing both types of afferent demonstrated
the full axonal arborization in both the reticular thalamic nucleus (Rt) and SI. The total length of the axon of a neuron bearing the type I was
86,968.8 ␮m, and the length of axonal collaterals in layer I of SI was 433.1 ␮m. The total axonal length of a neuron bearing the type II was
very small. The present study is the first to demonstrate substantial projections from VPM to layer I of SI, and provide quantitative data on
the entire extent of the axonal arborization of thalamocortical projections from single VPM neurons.
© 2004 Elsevier Ireland Ltd. All rights reserved.
Keywords: Somatosensory system; Thalamocortical pathway; VPM; Rat
The thalamocortical pathways, which relay trigeminal somatosensory information from subcortical structures to the
primary somatosensory cortex (SI), consist of a specific pathway originating from the ventral posteromedial thalamic nucleus (VPM) and a nonspecific pathway from the posterior
nucleus (Po) [9]. Afferent fibers from VPM form dense plexus
in both layers IV and VI in cortical columns in SI, whereas
those from Po distribute to all layers in the septa between
cortical columns and layer I in cortical columns. Thus, it has
been considered that these projections are complementary to
each other [14,19]. Using extracellular injections of an anterograde tracer, Lu and Lin [13] first demonstrated that VPM
∗ Corresponding author. Tel.: +81 3 5493 5411x2323;
fax: +81 3 5493 5437.
E-mail address: [email protected] (S. Oda).
0304-3940/$ – see front matter © 2004 Elsevier Ireland Ltd. All rights reserved.
doi:10.1016/j.neulet.2004.06.042
also projects to layer I of SI. This study, however, could not
rule out the possibility that the tracer was absorbed by Po
axons in VPM, where Po thalamocortical axons pass mediolaterally. Cauller et al. [4] also found retrogradely labeled
VPM neurons after a retrograde tracer application in layer I
of the SI. Previous studies using single-fiber labeling methods, however, have shown no substantial projections to layer
I [2,3,11,18,21]. Recent studies have shown that specific thalamocortical pathways from the medial geniculate nucleus in
the auditory system and the ventral lateral nucleus in the motor system send their fibers to layer I of the primary auditory
cortex and the frontal/premotor cortices, respectively [6,15].
Because layer I of SI receives many afferents from various
regions, it seems that it is an important site for cortical information processing [4].
The purpose of the present study was to examine
quantitatively specific thalamocortical projections of the
S. Oda et al. / Neuroscience Letters 367 (2004) 394–398
somatosensory system from VPM to SI, particularly layer
I, using extracellular and intracellular labeling methods.
Ten male Sprague–Dawley rats (400–620 g) were used in
this study. Two of these animals were used for extracellular
injection experiments, and the others were for intracellular injection experiments. All injections were administered bilaterally. All animal treatments and care were in accordance with
the European Communities Council Directive of 24 November 1986.
The animals were anesthetized with 20% urethane or 5.6%
chloral hydrate (i.p.) and placed in a stereotaxic apparatus.
Cerebrospinal fluid was drained from the cisterna magna to
maintain brain stability. The stereotaxic coordinates were obtained from the brain atlas of Paxinos and Watson [17]. For
intracellular injections, recording glass micropipettes filled
with 2.5% biocytin in 0.05 M Tris buffer (pH 7.3) containing
0.5 M KCl were inserted into VPM with a low-depolarizingcurrent pulse (0.1 nA: 500 ms duration at 1 Hz) to detect silent
cells [20]. After the cells were impaled, biocytin was injected
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by passing 2 or 3 nA depolarizing-current pulses (500 ms duration at 1 Hz) for 10–30 min. Extracellular injections were
administered using glass micropipettes (10–15 ␮m in tip diameter) for 10–15 min at a positive current of 5 ␮A in a cycle of 7 s on and 7 s off. The survival times were 6–8 h for
intracellular injections and 45–48 h for extracellular injections. The methods for the preparation of 80 ␮m thick serial
sections and the visualization of the tracer were according to
Chen et al. [7]. The sections were mounted on slides and then
counterstained with thionin. The images of biocytin-labeled
somata, dendrites, and axons observed under a microscope
(Olympus) equipped with a drawing tube were traced. They
were reconstructed in coronal plane serial sections viewed
with the aid of a 40× objective. For quantitative analysis,
axonal segment length was measured using a pen-type map
meter and estimated from the measured length and 80 ␮m
section thickness.
Fig. 1A shows an example of extracellular injection into
VPM. Two cortical columns in SI were labeled by this
Fig. 1. Example of extracellular injection into VPM. (A) Photomicrograph of site of injection into VPM. (B) Photomicrograph of a labeled fiber plexus in SI.
This plexus is mainly formed by vertically extending fibers (indicated by arrows). Two transversally running fibers (indicated by arrowheads) toward the plexus
is also observed. (C) Photomicrograph of labeled fiber in layer I of SI. (D) Drawing of reconstructed fibers in layer I (1–5) and superficial region of layer II (6,
7). eml, external medullary lamina; Rt, reticular thalamic nucleus; VPL, ventral posterolateral thalamic nucleus; VPM, ventral posteromedial thalamic nucleus.
Scale bars, 1 mm (A), 0.1 mm (B and C), and 0.2 mm (D).
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S. Oda et al. / Neuroscience Letters 367 (2004) 394–398
Fig. 2. Example of intracellular injection into VPM. (A) Photomicrograph of labeled VPM neurons, A-cell and B-cell. (B) Drawing of axonal trunks from
A-cell and B-cell. They branched into collaterals in Rt (a1 and b1) and SI (a2 and b2). Drawings of each axonal arborization in Rt (C) and SI (D). CPu, caudate
putamen; ic, internal capsule; LGP, lateral globus pallidus; Rt, reticular thalamic nucleus; VPM, ventral posteromedial thalamic nucleus; and WM, white matter.
Scale bars, 0.1 mm (A), 1 mm (B), and 0.2 mm (C and D).
S. Oda et al. / Neuroscience Letters 367 (2004) 394–398
injection. In these columns, labeled axonal fibers formed
dense plexus in layers IV and VI. Most of these fibers extended vertically in the columns and were highly branched
(indicated by arrows in Fig. 1B). Different types of axon
were also present. They entered SI distant from target sites,
extended toward the brain surface, then sharply turned toward
the plexus in layers IV and VI, and converged in the plexus
(indicated by arrowheads in Fig. 1B). The axons had few
branches and did not project to the supragranular layers. In
two of four extracellular injections, a few labeled fibers were
observed in layer I (Fig. 1C). Labeled fibers in layer I and
the adjacent region in a labeled column were reconstructed
(Fig. 1D). The mean length ± S.D. of labeled fibers in layer
I was 339.7 ± 87.5 ␮m (n = 6 from two injections; range,
218.3–432.8 ␮m). Some of these fibers originated from the
plexus in layer IV of the underlying column.
We injected 12 neurons intracellulary; however, both the
neuronal dendrites and distal ends of cortical collaterals could
be traced completely in two neurons. Fig. 2A shows a pair
of intracellulary labeled VPM neurons with overlapping dendritic fields, hereafter called “A-cell” and “B-cell”. They were
located in the dorsomedial part of VPM. The A-cell soma was
ovoid and its long and short axes were 26.6 and 12.5 ␮m, respectively. This neuron had thick dendrites, and the total dendritic length was 9494.1 ␮m. The B-cell soma was similar to
A-cell soma in shape and its long and short axes were 23.4
and 10.9 ␮m, respectively. It had rather slender dendrites and
the total dendritic length was 7086.7 ␮m. The morphological features of both neurons agreed with the report of Ohara
and Havton [16] in which the dendritic architecture of rat
somatosensory thalamic neurons was detailed. Both axonal
trunk pathways are shown in Fig. 2B. Their axonal collaterals branched only in the reticular thalamic nucleus (Rt) and
SI (Fig. 2C and D). The arborization patterns of collaterals
from each axonal trunk in Rt differed. A-cell axonal collaterals (a1) were highly branched and extended long distances,
whereas B-cell axonal collaterals (b1) were short and less
branched. Their terminal fields in Rt did not overlap. The
arborization patterns of collaterals from each axonal trunk
in SI illustrated in Fig. 2D also differed. A-cell axonal collaterals (a2) were thick and highly branched, and extended
to a cortical column. They formed dense plexus in layer IV
and in the superficial part of layer VI. Some axonal branches
from the plexus extended to the supragranular layers including layer I. B-cell axonal collaterals (b2) were rarely branched
and transversely converged in the plexus of A-cell fiber arborization. They did not project to the supragranular layers.
The estimated axonal lengths of A-cell, a standard type of
VPM neuron, were examined. The total axonal length was
86,968.8 ␮m, and the lengths of axonal collaterals in Rt and
the different layers of SI were 3548.4 ␮m (Rt), 433.1 ␮m (I),
8600.6 ␮m (II/III), 45,096.8 ␮m (IV), 10,862.6 ␮m (V), and
14,757.3 ␮m (VI). The ratios of fiber length of the layers
were 0.5 (I), 10.8 (II/III), 56.5 (IV), 13.6 (V), and 18.5%
(VI). The estimated lengths of B-cell axonal collaterals were
also examined. Its total axonal length was 10,933.9 ␮m, and
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the lengths of axonal collaterals in Rt and the different layers of SI were 297.8 ␮m (Rt), 0 ␮m (I–III), 783.1 ␮m (IV),
1276.2 ␮m (V), and 3533.5 ␮m (VI).
The present study using extracellular and intracellular labeling methods is the first to demonstrate the few but substantial projections to layer I of SI from VPM. Lu and Lin [13]
reported that VPM fibers in layer I are short and run vertically
to the brain surface, whereas Po fibers run transversely. The
present study shows that most of the VPM fibers in layer I
are longer, and run parallel to the brain surface. Layer I of
SI receives various afferents from the secondary somatosensory cortex (SII), agranular insular cortex, homotopic area
of contralateral SI, and Po. These excitatory afferents have
been considered as effective elements that act on the pyramidal neurons of layers II/III. Thus, layer I seems to be one of
the synaptic convergence sites of the somatosensory system
[4,5]. VPM afferent fibers in layer I were short and seemed to
be restricted to the width of the underlying cortical columns.
On the other hand, Po afferents, which are also thalamic afferents, were long and extended horizontally across the underlying columns. Thus, these two types of thalamocortical
afferent seem to participate in the synaptic convergence in
layer I in a different manner. The present study also showed
that the proportion of VPM fibers in layer I of SI is much
smaller than that in other thalamocortical systems, such as
in the auditory system [6,8]. This finding supports the physiological evidence that somatosensory information received
in layer IV is transmitted to the supragranular layers polysynaptically within a column [1].
Second, two types of VPM afferent with different arborization patterns in SI were observed in both experiments.
The first type was the standard type of VPM afferent, which
extended vertically forming dense plexus in layers IV and
VI, and projected collaterals to layer I. The second type was
rarely branched in both Rt and SI, transversely converged
in the fiber plexus formed by the first type of afferent, and
projected no collaterals to the supragranular layers. Lorente
de Nó classified somatosensory cortical afferent fibers that
apparently to originate from the thalamus into three types.
The second type of fibers in this study seems to be included in the third type (k in Fig. 25) of his classification
on the basis of its branching patterns [12]. However, our
second type sends no collaterals to the supragranular layers, whereas his third type sends fibers to layer III. Because
about 40% of VPM axons in Rt have short, simple branches,
the second type of afferent to SI may constitute a significant proportion of VPM thalamocortical projections [10].
At present, it is not known whether or not VPM neurons
bearing different types of afferent have distinct physiological
properties.
Acknowledgment
This work was supported by the Project Research Grant
(no. 15-9) from Toho University School of Medicine.
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S. Oda et al. / Neuroscience Letters 367 (2004) 394–398
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