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The Journal of Neuroscience, August 1, 2001, 21(15):5764-5772
Segregation and Convergence of Information Flow through the
Cortico-Subthalamic Pathways
B. P.
Kolomiets1,
J. M.
Deniau1,
P.
Mailly2,
A.
Ménétrey1,
J.
Glowinski1, and
A. M.
Thierry1
1 Institut National de la Santé et de la
Recherche Médicale U114, Chaire de Neuropharmacologie,
Collège de France, 75231 Paris Cedex 05, France, and
2 NPA-NSI, Centre National de la Recherche
Scientifique, Université Pierre et Marie Curie, 75230 Paris Cedex
05, France
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ABSTRACT |
Cortico-basal ganglia circuits are organized in parallel channels.
Information flow from functionally distinct cortical areas remains
segregated within the striatum and through its direct projections to
basal ganglia output structures. Whether such a segregation is
maintained in trans-subthalamic circuits is still questioned. The
effects of electrical stimulation of prefrontal, motor, and auditory
cortex were analyzed in the subthalamic nucleus as well as in the
striatum of anesthetized rats. In the striatum, cells
(n = 300) presenting an excitatory response to
stimulation of these cortical areas were located in distinct striatal
territories, and none of the cells responded to two cortical
stimulation sites. In the subthalamic nucleus, both prefrontal and
motor cortex stimulations induced early and late excitatory responses
as a result of activation of the direct cortico-subthalamic pathway and
of the indirect cortico-striato-pallido-subthalamic pathway,
respectively. Stimulation of the auditory cortex, which does not send
direct projection to the subthalamic nucleus, induced only late
excitatory responses. Among the subthalamic responding cells
(n = 441), a few received both prefrontal and motor
cortex (n = 19) or prefrontal and auditory cortex
(n = 10) excitatory inputs, whereas a larger number
of cells were activated from both motor and auditory cortices
(n = 48). The data indicate that the segregation of
cortical information flow originating from prefrontal, motor, and
auditory cortices that occurred in the striatum is only partly
maintained in the subthalamic nucleus. It can be proposed that the
existence of specific patterns of convergence of information flow from
these functionally distinct cortical areas in the subthalamic nucleus allows interactions between parallel channels.
Key words:
prefrontal cortex; motor cortex; auditory cortex; striatum; subthalamus; basal ganglia; rat; ketamine
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INTRODUCTION |
The striatum and the subthalamic
nucleus (STN) are the two main structures through which cortical
signals are transmitted to the output nuclei of the basal ganglia,
i.e., the substantia nigra pars reticulata (SNR) and the internal
segment of the globus pallidus (GPi) (Albin et al., 1989 ). Both the
striatum and the STN receive direct excitatory inputs from the cerebral
cortex. Through its direct GABAergic projections, the striatum exerts an inhibitory influence on the SNR and the GPi whereas, via its glutamatergic projection neurons, the STN provides a major excitatory drive to these structures (Kitai and Kita, 1987 ; Smith et al., 1998 ).
The STN also receives cortical information through a multisynaptic circuit involving the striatum and the external segment of the globus
pallidus (GPe) (Albin et al., 1989 , Smith et al., 1998 ). Via this
indirect circuit, the cerebral cortex activates the STN through a
disinhibitory process (Maurice et al., 1998 ). Thus, the direct striatal
inputs and the trans-subthalamic pathways exert opposite effects on the
output structures of the basal ganglia (Maurice et al., 1999 ;
Nambu et al., 2000 ).
Supporting the concept that cortico-basal ganglia circuits are
organized in parallel channels, growing evidence indicates that signals
originating from functionally distinct cortical areas are processed in
separate striatal territories and remain segregated in the direct
striatopallidal and striatonigral pathways (Alexander al., 1986 ;
Groenewegen and Berendse, 1994 ; Deniau and Thierry, 1997 ; Kitano et
al., 1998 ). This organization in parallel circuits does not exclude the
existence of convergence because each of these parallel circuits
integrates cortical information originating from functionally related
cortical areas (Yeterian and Van Hoesen, 1978 ; Flaherty and Graybiel,
1991 ; Deniau et al., 1996 ). Whether the principle of a parallel
processing of information originating from functionally distinct
cortical areas also applies to the trans-subthalamic basal ganglia
circuits remains to be established. Indeed, it has been shown that the
direct cortical inputs to the STN only originate from the prefrontal
and motor cortices and innervate the entire STN with a mediolateral
topography (Afshapour, 1985 ; Canteras et al., 1990 ; Berendse and
Groenewegen, 1991 ; Nambu et al., 1996 ). In addition, the indirect
cortico-striato-pallido-subthalamic pathways that are derived from the
whole cerebral cortex also innervate the entire STN in a topographical
manner (for review, see Joel and Weiner, 1997 ; Smith et al., 1998 ).
The aim of the present study was to determine to what extent the
segregation of information flow from functionally distinct cortical
areas that occurs in the striatum is also maintained within the STN.
For this purpose, using anatomical and electrophysiological approaches,
the organization of the projections to the striatum and the STN of
three functionally distinct cortical areas [the prelimbic-medial orbital areas of the prefrontal cortex
(PL-MO) and the orofacial-forelimb areas of the motor cortex and the
auditory cortex] were analyzed in anesthetized rats.
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MATERIALS AND METHODS |
Experiments were performed on 23 adult male Sprague Dawley rats
(270-300 gm; Charles River, Saint Aubin-les-Elbeuf, France). Efforts
were made to prevent animal suffering and to reduce the number of
animals used.
Anatomical experiments. Before surgical procedure,
animals were anesthetized by an initial injection of sodium
pentobarbital (40 mg/kg, i.p.) that was supplemented by administration
of ketamine (30 mg/kg, i.m.; Imalgène 500, Rhône-Mérieux, France). In four rats, microiontophoretic
injections of wheat germ agglutinin conjugated with horseradish
peroxidase (WGA-HRP; Sigma, St. Louis, MO; 2.5% in 0.9%
saline) were performed within prefrontal [anterior (A), 12.5; lateral
(L), 0.4 from the interaural line; height (H), 3.5 mm from the cortical
surface], motor (orofacial area: A,12.5; L, 3.8; H, 1.5; rostral
forelimb area: A, 12; L, 2.3; H, 1.5), and auditory areas (A, 3.5; L,
7,2; H, 2) of the cerebral cortex, using glass micropipettes (internal
tip diameter, 15 µm) and a iontophoretic delivery method (positive
current pulses of 5 µA for 5-15 min). After a survival period of
36-48 hr, the animals were deeply anesthetized with pentobarbital (160 mg/kg, i.p.) and perfused through the left ventricle with 100 ml of
0.9% saline, 500 ml of 3% glutaraldehyde in 0.1 M phosphate buffer, pH 7.4, and 200 ml of a 10%
sucrose solution in 0.1 M phosphate buffer, pH
7.4. After dissection the brains were immersed in 10% sucrose phosphate buffer at 4°C until sectioned. Frozen sections were cut at
50 µm and processed for HRP histochemistry using the
tetramethylbenzidine method of Mesulam (1978) . The sections were
mounted onto chrome-alum-coated slides, counterstained with safranin,
and dehydrated through alcohol to xylene for light microscopic examination.
The local diffusion of the marker, the distribution of retrogradely
labeled cells within the thalamus as well as anterograde and retrograde
labeling of cortico-cortical connections were analyzed with bright- and
dark-field microscopy and reconstructed using a camera lucida.
Electrophysiological experiments. Electrophysiological
experiments were performed in 19 rats. Animals were anesthetized with ketamine (100 mg/kg, i.p., supplemented by injections of 50 mg/kg, i.m.; Imalgène 500) and fixed in a conventional stereotaxic
apparatus (Horsley Clark apparatus; Unimécanique,
Epinay-sur-Seine, France). Body temperature was monitored with a rectal
thermometer and maintained at 37°C with a homeothermic warming
blanket (Harvard Apparatus, Kent, UK).
Single-unit activity of cells located in the striatum (five rats) or in
the STN (14 rats) was recorded extracellularly using glass
micropipettes (7-10 M ) filled with 4% Pontamine sky blue dissolved
in a 0.6 M sodium chloride solution. Action potentials of
single neurons were amplified with a differential preamplifier (DAM-5A;
World Precision Instruments, Hertfordshire, UK) and displayed on a
memory oscilloscope. Spikes were separated from noise using a window
discriminator and sampled on-line by a computer connected to a CED 1401 interface (Cambridge Electronic Design, Cambridge, UK). Because
striatal cells have very low activity, cortical stimulation was applied
continuously during penetration and when a response was observed to one
of the cortical stimulation site, the two other stimulation sites were tested.
Peristimulus time histograms (PSTHs) were generated from 50-100
stimulation trials using a binwidth of 1 msec and plotted on a Hewlett
Packard plotter. The criterion used to establish the existence of an
excitatory response was an increase >50% in the number of spikes
compared with the prestimulus frequency for at least three consecutive
bins. The duration of an inhibitory response corresponds to the time
interval during which no spike was observed.
Electrical stimulations of PL-MO areas of the prefrontal cortex (A,
12.5; L, 0.5; H, 3.5 mm from the cortical surface), the orofacial-forelimb motor cortex (A: 12.5; L: 4.0; H: 1.2), and the
auditory cortex (A: 4.7; L: 7; H: 2.0) ipsilateral to the recording
sites in the striatum and the STN were made with bipolar coaxial
stainless steel electrodes (diameter 400 µm, tip-barrel distance, 300 µm) positioned stereotaxically according to the atlas of Paxinos and
Watson (1986) . Correct positioning of the stimulation site within the
orofacial-forelimb motor cortex was achieved by observing the jaw
movement elicited by a brief train of pulses. Recording of an evoked
potential to an acoustic stimulus (click) ensured correct positioning
of the stimulating electrode in the auditory cortex. Electrical stimuli
consisted of monopolar pulses of 0.3-0.6 msec width and 200-600 µA
intensity delivered at a frequency of 0.5 to1.4 Hz.
At the end of each recording session, the tip of the stimulating
electrodes was marked by an electrical deposit of iron (15 µA anodal,
20 sec) and observed on histological sections after a ferriferrocyanide
reaction. The tip of the recording electrode was marked by
iontophoretic application of Pontamine sky blue (8 µA cathodal, 20 min), which allowed the determination of the position of recorded
cells. Brains were removed, fixed in a 10% formalin solution, and
positions of electrodes were microscopically identified on serial
frozen sections (100 µm) stained with safranin.
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RESULTS |
Anatomical organization of projections from prefrontal, motor, and
auditory cortical areas within the striatum and the subthalamic
nucleus
Single unilateral injections of WGA-HRP were performed in four
rats either in the prefrontal, motor, or auditory cortex. In the
prefrontal cortex the injection site included the PL-MO areas and
extended to the infralimbic area (Fig.
1A). For the motor cortex, the injection site, in one rat, was centered in the orofacial motor area (Fig. 1B), and in the other rat the
injection site was centered in the forelimb motor area (Fig.
1C) and overlapped partly the adjacent precentral medial
area. In the last rat, the injection site was located in the auditory
cortex (Fig. 1D) and overlapped partly the adjacent
secondary visual area.

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Figure 1.
Localization of retrogradely labeled cells in the
thalamus after WGA-HRP injection in the PL-MO areas of the prefrontal
cortex (A), in the orofacial motor cortex
(B), in the forelimb motor cortex
(C), and in the auditory cortex
(D). Stippled areas, Injection
sites. Dots, Retrogradely labeled cells. CG,
Central gray; CL, centrolateral thalamic nucleus;
CPu, caudate putamen; DpMe, deep
mesencephalic nuclei; fmi, forceps minor corpus
callosum; Fr2, frontal cortex area 2; ic,
internal capsule; MD, mediodorsal thalamic nucleus;
Oc1B, occipital cortex area1; Par1,
parietal cortex area 1; Po, posterior thalamic nuclear
group; SC, superior colliculus; SN,
substantia nigra; VM, ventromedial thalamic nucleus;
VP, ventroposterior thalamic nucleus.
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For each injection site, retrogradely labeled cells were observed in
distinct thalamic regions known to be related with these cortical areas
(Fig. 1). In the case of prefrontal cortex injection, retrogradely
labeled cells were observed in the mediodorsal nucleus, the medial part
of the ventromedial nucleus and adjacent midline thalamic nuclei, the
parataenial nucleus, the paraventricular nucleus, and the caudomedial
part of the parafascicular nucleus. After injection in the orofacial
motor cortex, retrogradely labeled cells were located in the central
medial nucleus, the paracentral nucleus, the dorsocaudal part of the
ventromedial nucleus, the ventral lateral nucleus, and the rostral part
of parafascicular nucleus. Retrogradely labeled cells from the forelimb
sensorimotor area were located in ventrolateral, central medial, and
lateral nuclei, ventral posterolateral nucleus, the ventromedial
nucleus except its medial part, and the rostrolateral part of the
parafascicular nucleus. After injection in the auditory cortex
retrogradely labeled cells were located in the medial geniculate
nucleus and the lateral posterior nucleus.
Finally, confirming the lack of direct cortico-cortical
connections between the PL-MO areas of the prefrontal cortex, the orofacial-forelimb motor areas, and the auditory cortex, no
anterograde or retrograde labeling was observed in these cortical areas.
Within the striatum, the projection fields of anterogradely labeled
fibers originating from PL-MO areas, motor (orofacial and forelimb)
areas, and auditory cortex are localized in distinct striatal
territories (Fig. 2). After WGA-HRP
injection in the prefrontal cortex, the projection field of
anterogradely labeled fibers occupied the whole rostrocaudal extent of
the nucleus accumbens and the adjacent medial part of the dorsal
striatum. The projection field of labeled fibers originating from the
orofacial motor cortex was located ventrolaterally in the dorsal
striatum, and its rostrocaudal extension is restricted to the central
part of the structure. After injection in the forelimb motor cortex,
anterogradely labeled fibers were distributed in the dorsolateral part
of the dorsal striatum and form a rostrocaudal band sparing the most
caudal part of the structure. Finally, anterogradely labeled fibers
from the auditory cortex were distributed in the most caudal part of the striatum where they form a narrow dorsoventral band adjacent to the
external pallidum.

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Figure 2.
Reconstruction of projection fields of
anterogradely labeled fibers within the striatum after WGA-HRP into
the PL-MO areas of the prefrontal cortex, the forelimb motor cortex,
the orofacial motor cortex, and the auditory cortex. Corresponding
injection sites are shown in Figure 1. Projection fields are
represented in gray on coronal sections of the striatum.
Numbers indicate the corresponding atlas coordinates.
CPu, Caudate putamen; GP, globus pallidus.
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Within the STN, the projection field of anterogradely labeled fibers
originating from the PL-MO areas was restricted to the medial third of
the structure, whereas projection fields of the orofacial and forelimb
motor areas were located more laterally (Fig.
3). Projections from the orofacial motor
cortex were distributed in the rostral half of the STN where they
occupied the central part of the mediolateral extension of the
structure (Fig. 3). The projection field of the fibers originating from
the forelimb motor area was located more ventrally and caudally,
sparing the medial and the most lateral part of the STN (Fig. 3).
Finally, no labeling was observed within the STN after WGA-HRP
injection in the auditory cortex.

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Figure 3.
Reconstruction of projection fields of
anterogradely labeled fibers within the STN after WGA-HRP injections
into the PL-MO areas of the prefrontal cortex, the forelimb motor
cortex, and the orofacial motor cortex. Corresponding injection sites
are shown in Figure 1. Projection fields are represented in
gray on coronal sections. Numbers
indicate the corresponding atlas coordinates. cp,
Cerebral peduncle; STN, subthalamic nucleus.
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Characteristics of the responses induced in the striatum by
cortical stimulation
Among the 300 striatal cells tested to electrical stimulation of
the prefrontal, the motor, and the auditory cortex (each of these cells
being tested to stimulation of the three sites), an excitatory response
was observed in 122, 115, and 63 cells, respectively. The mean
latencies of the excitatory responses were 11.4 ± 0.3 msec for
the prefrontal cortex, 9.2 ± 0.2 msec for the motor cortex, and
8.5 ± 0.4 msec for the auditory cortex stimulation. None of
the recorded cells presented response to more than one stimulation site.
Cells responding to electrical stimulation of these cortical regions,
observed in 84 from 123 penetrations, were located in distinct striatal
territories that did not overlap (Fig.
4). Cells activated from the prefrontal
cortex were located medially, in the "core" of the nucleus
accumbens and the dorsally adjacent part of the dorsal striatum,
whereas those activated from the motor cortex were located in the
lateral part of the dorsal striatum. Cells responding to the auditory
cortex were found in a medial and caudal striatal region adjacent to
the external pallidum. Finally, in the 39 penetrations performed in
other regions of the striatum, no responsive cells were observed.

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Figure 4.
Localization within the striatum of the cells
presenting an excitatory response to the stimulation of PL-MO areas of
the prefrontal cortex, the orofacial-forelimb areas of the motor
cortex, and the auditory cortex. Gray areas delineate
the territories where responding cells were located.
Numbers indicate the corresponding atlas coordinates.
CPu, Caudate putamen.
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Characteristics of responses induced in the subthalamic nucleus by
cortical stimulations
Responses evoked by electrical stimulations of the PL-MO areas,
the orofacial-forelimb motor cortex, and the auditory cortex were
investigated in 466 STN cells recorded from 14 rats. Most of these
cells (n = 441) presented excitatory responses to the stimulation of at least one of these sites.
Prefrontal cortex
The electrical stimulation of PL-MO areas evoked excitatory
responses in 77 STN cells, but these responses exhibited different patterns (Fig. 5, Table 1). In 26 of
these cells, responses consisted of two excitatory peaks with short
(L = 6.8 ± 0.3 msec) and long (19.2 ± 0.8 msec)
latencies to onset, the duration of the late excitatory response being
in most cases longer than that of the early excitatory peak (Fig.
5A,C). In 9 of these 26 cells (35%), the two excitatory
peaks were separated by a brief inhibition (D = 6.7 ± 0.8 msec) (Fig. 5C). A prolonged excitation (D = 24.8 ± 1.1 msec) with an early latency to onset (L = 8.1 ± 0.5 msec) was observed in 15 cells (Fig. 5B). A single late
excitation (L = 18.5 ± 0.5 msec) was recorded in 33 STN
cells (Fig. 5D), whereas a single short-duration excitatory
peak with an early onset (L = 7.5 ± 0.9 msec) was recorded
in only three STN cells (Fig. 5E).

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Figure 5.
Patterns of responses evoked by PL-MO stimulation
in STN cells. A, Two excitatory peaks with short- and
long-latency onsets; B, prolonged excitatory response
with a short-latency onset; C, two excitatory peaks
separated by a brief inhibition; D, late excitatory
response only; E, Early excitatory response only.
A-E, Arrows indicate the artifact of
stimulation. Peristimulus time histograms were generated from 60 stimulation trials. n, Number of cells. Bottom
right, Schematic representation of the position of the
stimulating electrode in the prefrontal cortex. Number
indicates the distance, in millimeters, from the interaural line.
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Motor cortex
When the electrical stimulation was applied in the
orofacial-forelimb motor cortex, excitatory responses were recorded in 277 STN cells (Fig. 6, Table
1). In a large proportion of these cells
(161 cells), the responses consisted of two excitatory peaks with short
(L = 4.0 ± 0.1 msec) and long (L = 13.2 ± 0.3 msec) latencies to onset, the duration of the late peak being in most cases longer than that of the early peak (Fig. 6A,C).
In 26 of these 161 cells, the two excitatory peaks were separated by a brief inhibition (D = 5.5 ± 0.7 msec) (Fig. 6C).
A prolonged excitation (D = 20.1 ± 0.6 msec) with an early
latency to onset (L = 4.4 ± 0.2 msec) was observed in 67 cells (Fig. 6B). A single late excitation (L = 14.2 ± 0.6 msec) was recorded in 39 STN cells (Fig.
6D), whereas an early excitatory peak (L = 3.5 ± 0.4 msec) of short duration only was recorded in 10 cells
(Fig. 6E).

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Figure 6.
Patterns of responses evoked by motor cortex
stimulation in STN cells. A, Two excitatory peaks with
short- and long-latency onsets; B, prolonged excitatory
response with a short-latency onset; C, two excitatory
peaks separated by a brief inhibition; D, late
excitatory response only; E, early excitatory response
only. A-D, Arrows indicate the artifact
of stimulation. Peristimulus time histograms were generated from 60 stimulation trials. n, Number of cells. Bottom
right, Schematic representation of the position of the
stimulation electrode in the motor cortex. Number
indicates the distance, in millimeters, from the interaural line.
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Auditory cortex
When the electrical stimulation was made in the auditory cortex,
excitatory responses were recorded in 87 STN cells (Fig. 7, Table 1). In all cases, the response
consisted of an excitation with a late onset (L = 12.2 ± 0.4 msec).

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Figure 7.
Pattern of response evoked by auditory cortex
stimulation in STN cells. Left, The only pattern of
response was an excitation with a late latency. Arrow
indicates the artifact of stimulation. Peristimulus time histograms
were generated from 60 stimulation trials Right,
Schematic representation of the position of the stimulating electrode
in the auditory cortex. Number indicates the distance,
in millimeters, from the interaural line.
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Localization of responding cells
Cells presenting excitatory responses to the stimulation of either
the prefrontal, the motor, or the auditory cortex showed distinct
distributions within the STN but with overlapping areas (Fig.
8A). Cells responding
to the PL-MO stimulation were located in the medial part of the STN,
whereas most of the cells responding to the orofacial-forelimb motor
cortex stimulation were found more laterally. Cells responding to the
auditory cortex stimulation were distributed within the ventral part of
the STN throughout most of its mediolateral extent, the localization of
these cells overlapping partly with that of cells responding to either
PL-MO or motor cortex stimulations. Finally, no obvious difference in the topographical distribution of cells responding with different patterns could be observed among the cells that responded to the stimulation of either PL-MO areas or the motor cortex.

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Figure 8.
Localization of responding cells within the STN.
A, Localization of cells presenting excitatory responses
to PL-MO, motor, or auditory cortex stimulation. B,
Localization of cells presenting responses to two stimulation sites:
PL-MO and motor cortex, PL-MO and auditory cortex, motor, and
auditory cortex. A, B,
Left to right, Posterior to anterior
coronal sections of the STN. cp, Cerebral
peduncle.
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Convergent responses induced in single STN cells by stimulations of
the prefrontal, motor, and auditory cortices
Excitatory responses to two distinct cortical stimulation sites
were observed in 77 of the 441 responding STN cells (Fig. 8B), however, no cells were found that responded to
stimulation of all three sites. Among the 77 STN cells that responded
to PL-MO stimulation and the 277 STN cells that responded to
orofacial-forelimb motor cortex stimulation, 19 of these cells
presented excitatory responses to both stimulation sites (24 and 7% of
cells responding to the PL-MO and orofacial-forelimb motor cortex
stimulations, respectively). All these 19 cells presented a late
excitation (Fig. 9) that was preceded by
an early excitation in 13 neurons. Another population of 10 cells
responded to both PL-MO (10/77cells; 12%) and auditory cortex (10/87;
11%) stimulations. In all cases, the response consisted of a late
excitation (Fig. 10). An additional early response to the prefrontal cortex stimulation was observed in one
of these cells. Finally, 48 cells responded to both auditory (48/87;
55%) and motor cortex (48/277; 17%) stimulations. All these cells
exhibited late excitations (Fig.
11A,B), and 29 of them presented an additional early excitation to the motor cortex stimulation (Fig. 11A). The distribution of the
convergent neurons within the STN is shown in Figure
8B.

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Figure 9.
Converging influence of prefrontal and motor
cortex in a single STN cell. Responses evoked by PL-MO
(A) and motor cortex (B)
stimulations in a single STN cell. Arrows indicate the
stimulation artifact. Peristimulus time histograms were generated from
60 stimulation trials.
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Figure 10.
Converging influence of prefrontal and auditory
cortex in a single STN cell. Responses evoked by PL-MO
(A) and auditory cortex (B)
stimulations in a single STN cell. Arrows indicate the
stimulation artifact. Peristimulus time histograms were generated from
60 stimulation trials.
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Figure 11.
Converging influence of motor and auditory cortex
stimulation in a single STN cell. Responses evoked by motor
(A) and auditory (B) cortex
stimulations in a single STN cell. Arrows indicate the
stimulation artifact. Peristimulus time histograms were generated from
60 stimulation trials.
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DISCUSSION |
The present study indicates that the segregation of cortical
information originating from the prefrontal, the motor, and the auditory cortex observed in the striatum is only partly maintained in
the STN. Indeed, among the overall population of STN recorded cells, a
small number of cells responded to both prefrontal and motor cortex
stimulations or to prefrontal and auditory cortex, whereas a larger
number of cells were activated from both motor and auditory cortices.
Segregation of inputs from the prefrontal, motor, and auditory
cortices in the striatum
The present data provide further evidence for a segregation in the
rat striatum of cortical inputs originating from the PL-MO areas of
the prefrontal cortex, the orofacial-forelimb areas of the motor
cortex, and the auditory cortex. Indeed, these three functionally
distinct cortical regions, which receive inputs from distinct thalamic
regions, are not interrelated through direct cortico-cortical
connections (Sesack et al., 1989 ; Van Eden et al., 1992 ; present study)
and innervate different striatal territories with no overlap.
Accordingly, striatal neurons did not receive convergent excitatory
inputs from these cortical regions. A topographical organization of the
projections from the prefrontal, motor, and auditory cortices similar
to that observed in the present study has previously been described
(McGeorge and Faull, 1989 ; Roger and Arnault, 1989 ; Deniau et al.,
1996 ; Kincaid and Wilson, 1996 ). However, after injections of WGA-HRP
in the rat striatum McGeorge and Faull (1989) suggested that striatal
projection fields from prefrontal, motor, and auditory cortex are
partly overlapping. This discrepancy with our data is likely
attributable to the location of striatal injection sites that
encompassed the specific projection fields of these cortical areas.
The direct and indirect cortico-STN pathways
The prefrontal and the motor cortices are related to the STN
through direct and indirect pathways. In agreement with previous electrophysiological studies (Ryan and Clark, 1991 ; Fujimoto and Kita,
1993 ; Kita, 1994 ; Maurice et al., 1998 ; Nambu et al., 2000 ), the
responses of STN cells to stimulation of prefrontal or motor cortex
consisted of either two excitatory peaks or one prolonged excitation
with an early onset. It has been established that the early excitatory
responses result from the activation of the direct cortico-subthalamic
pathway (Kitai and Deniau, 1981 ), whereas the origin of the late
excitatory responses has been controversial. Implication of the
indirect striato-pallido-subthalamic circuit has been questioned
because the pattern of excitatory responses evoked in the STN by
cortical stimulation was not markedly modified after excitotoxic
lesions of the striatum or of the external pallidum (Ryan and Clark,
1992 ; Fujimoto and Kita, 1993 ; Kita, 1994 ). Several explanations for
these excitatory responses have been proposed: a prolongation of the
early excitation caused by activation of the NMDA receptors, the
intrinsic membrane properties of STN cells, or finally a spread of
excitation through local recurrent collaterals (Kita, 1994 ). However,
the present study shows that stimulation of the auditory cortex, a
cortical area that lacks direct projections to the STN, induced only a
late excitatory response that was not preceded by an early excitatory
peak. This observation is in accordance with recent
electrophysiological and pharmacological data in rat and monkey
indicating that the late excitatory responses actually result from the
activation of the striato-pallido-subthalamic circuit, which operates
through a disinhibitory process (Maurice et al., 1998 ; Nambu et al.,
2000 ).
In the rat, the prefrontal, premotor, and motor cortical areas
innervate the STN with a topographical organization (Afshapour, 1985 ;
Canteras et al., 1990 ; Berendse and Groenewegen, 1991 ). Accordingly, in
the present study anterogradely labeled fibers from the prefrontal
cortex were confined to the medial part of the STN, whereas those from
the orofacial and forelimb areas of the motor cortex were located more
laterally. The indirect pathway that connects the whole cerebral cortex
to the STN is also topographically organized. Indeed, as previously
discussed, the functional mosaic of the cerebral cortex is orderly
mapped onto the striatum, and striatal projections to the pallidum as
well as pallidal projections to the STN are topographically organized
(Haber et al., 1985 ; Smith et al., 1989 , 1998 ; Groenewegen and
Berendse, 1990 ; Zahm and Brog, 1992 ; Bevan et al., 1997 ; Maurice et
al., 1997 ). In agreement with the anatomical organization of the
cortico-STN pathways, the cells that presented excitatory responses to
stimulation of either prefrontal, motor, or auditory cortex were found
in defined territories of the STN. Cells responding to prefrontal cortex stimulation were located medially, and their responses consisted
in most cases of an early and a late excitation, indicating a
functional convergence of the direct and indirect pathways derived from
this cortical region. STN cells responding to stimulation of the motor
cortex were located more laterally, and their responses also consisted
of an early and a late excitation. Finally, cells responding to the
auditory cortex were distributed within a large mediolateral extension
of the ventral STN. Their responses consisted only of a late excitation
that is attributable to the activation of the indirect pathway.
Patterns of convergence of information from the prefrontal, motor,
or auditory cortex in the STN
Although cells responding to prefrontal, motor, or auditory
cortices were located in defined territories of the STN, these territories presented areas of overlap where converging responses to
two stimulation sites were observed. Although a large number of cells
received converging influence from the auditory cortex and the
orofacial-forelimb areas, smaller subpopulations of cells presented
excitatory responses to PL-MO and orofacial-forelimb areas or to
PL-MO and auditory cortex. It can be excluded that these convergent
responses resulted from the activation of cortico-cortical circuits as
well as from an integration of cortical information at the level of the
striatum. Indeed, PL-MO, orofacial-forelimb, and auditory cortical
areas are not related through direct cortico-cortical connections
(Sesack et al., 1989 ; Van Eden et al., 1992 ; Condé et al., 1995 ;
present study) and, as shown in the present study, no convergent
responses were observed in the striatum after stimulation of these
areas. Because the STN cells that received a converging influence from
the prefrontal and the motor cortex through the direct and indirect
pathways were located at the interface of the projection territories of
the prefrontal and motor cortex, it can be proposed that the large
dimension of the dendritic field of STN cells provides an anatomical
substrate of this synaptic convergence (Hammond and Yelnik, 1983 ; Kita
et al., 1983 ; Bevan et al., 1997 ). Similarly, the convergence of
information flow originating from the auditory cortex and from the
motor or prefrontal cortex resulted from an integration of cortical
information at the level of the STN. Because projections issuing from
prefrontal and motor cortex together innervate the entire extent of the
STN, they were expected to overlap those originating from other
cortical areas that innervate the STN via the indirect circuit.
However, it cannot be excluded that integration of striatal information also occurs in the pallidum and thus participates in this convergence.
Finally, in agreement with the existence of a topographical
organization in the direct and indirect cortico-STN pathways, the
patterns of convergent information flow from the prefrontal, motor and
auditory cortices were observed in cells located in defined regions of
the STN. Because the STN receives topographically organized afferents
from the whole cerebral cortex via the indirect striatopallidal
pathway, further patterns of convergence are likely to occur within the STN.
Functional considerations
Confirming the concept of a parallel organization of basal ganglia
circuits in the rat, it was found that information from prefrontal,
motor, and auditory cortices remain segregated within the striatum. In
addition, the present study revealed, at the level of the STN, defined
patterns of convergence of information flow originating from these
functionally distinct cortical areas that indicates the existence of an
additional level of organization allowing specific interactions between
parallel circuits.
It is well established that the cerebral cortex and the basal ganglia
are functionally related through multisynaptic loop circuits (Alexander
et al., 1986 ). Within these circuits, the striatum exerts an inhibitory
influence on basal ganglia output neurons that results in an activation
of their thalamic target cells through a disinhibitory process
(Chevalier and Deniau, 1990 ). This process is considered as the basic
mechanism through which signals delivered to the striatum finally
increase the excitability of selected regions of the prefrontal and
motor cortices and consequently contribute to initiate movements.
However, through its prominent excitatory drive on basal ganglia output
neurons, the STN counteracts the direct inhibitory influence of the
striatum on basal ganglia output neurons. It has been proposed that STN
participates in the spatiotemporal shaping of the disinhibitory process
and thus contributes to the scaling of movement and suppression of
competing motor programs (Mink and Thach, 1993 ; Nambu et al., 2000 ). In agreement with this hypothesis, our data reveals that the
trans-subthalamic circuits allow specific interactions between
information from functionally distinct cortical areas. Understanding
the physiological significance of such interactions in the selection of
behaviors is necessary to determine how the cortical information flow
processed through the trans-subthalamic and the trans-striatal pathways interacts at the level of basal ganglia output neurons.
 |
FOOTNOTES |
Received Feb. 26, 2001; revised May 4, 2001; accepted May 11, 2001.
This work was supported by Institut National de la Santé et de la
Recherche Médicale. B. K. is a recipient of a fellowship from the Ministère de l'Education Nationale, de la Recherche et
de la Technologie. We thank S. Slaght for critical reading of the
manuscript and A. M. Godeheu and M. Saffroy for histological assistance.
Correspondence should be addressed to Anne-Marie Thierry, Institut
National de la Santé et de la Recherche Médicale U114, Chaire de Neuropharmacologie, Collège de France, 11 place M. Berthelot, 75231 Paris Cedex 05, France. E-mail:
anne-marie.thierry{at}college-de-france.fr.
 |
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